Nanomechanical infrared spectroscopy systems and methods using gated peak force IR

Abstract

An apparatus and method for sample characterization using an AFM and a pulsed IR laser directed at the tip of a probe of the AFM. In peak force tapping operations, gating laser pulses and gating detection based on lock-in amplifiers, car integrators, or FFT may be employed. Nanospectral measurements with resolutions below 20 nm, even below 10 nm can be performed with nanomechanical and other property measurements.

Description

Technical Field

[0001] Preferred embodiments involve performing nanomechanical and spectroscopic measurements, and more specifically, using peak force tap (PFT) as the oscillation mode for AFM operation and gated multipulse excitation and detection to obtain nanoscale local spectroscopic and / or imaging of a sample and its nanomechanical and other properties. Background Technology

[0002] Infrared spectroscopy and scanning probe microscopy (SPM) have been combined to perform a spectroscopic method that integrates an infrared light source (e.g., a tunable free-electron laser, an optical parametric oscillator, or a quantum cascade laser) with atomic force microscopy (AFM), which has a sharp probe to measure the local absorption of infrared light by a sample. Conventional techniques in this area are based on contact-mode AFM, extracting the absorption signal from contact resonant oscillations that occur when the sample expands (or contracts) during light absorption. Recently, a tapping-mode AFM technique using IR illumination has been shown to produce spatial resolutions as low as 10 nm. More recently, PeakForce IR, an IR spectroscopy and imaging mode based on peak force tapping AFM operating mode, has been developed to achieve a resolution of 10 nm.

[0003] Generally, the interaction between the sample being measured and electromagnetic energy can be monitored to obtain information about the sample. In spectrophotometry, the transmission or reflection of light through or from the sample produces a sample-characteristic map of transmission or reflection intensity as a function of wavelength. This spectrophotometric information allows users to determine the physical properties of the sample, such as its chemical composition or temperature.

[0004] It is worth noting that techniques for spectroscopic measurements with nanometer-scale spatial resolution are constantly being improved. However, despite the ongoing progress in developing imaging techniques with spatial resolution exceeding diffraction limitations, simultaneous spectroscopic implementations that provide chemical specificity and sensitivity at the molecular level remain challenging.

[0005] SPM is driving improvements in this area. AFM is a device that typically uses a probe with a tip and applies appropriate force to interact the tip with the sample surface, thereby characterizing the surface at the atomic scale. Generally, the probe is introduced into the surface of the sample to detect changes in the sample's characteristics. By providing relative scanning movement between the tip and the sample, surface feature data of specific regions of the sample can be acquired, and a corresponding sample map can be generated.

[0006] A typical AFM system is schematically illustrated in Figure 1. The probe device 12 employed in the AFM 10 includes a probe 14 with a cantilever 15. A scanner 24 generates relative motion between the probe 14 and the sample 22 while measuring the probe-sample interaction. In this way, images of the sample or other measurements can be obtained. The scanner 24 typically includes one or more actuators that typically generate motion in three orthogonal directions (XYZ). Typically, the scanner 24 is a single integrated unit comprising one or more actuators for moving the sample or probe along all three axes, for example, piezoelectric actuators. Alternatively, the scanner can also be a component consisting of multiple independent actuators. Some AFMs divide the scanner into multiple parts, for example, an XY scanner for moving the sample and a separate Z-actuator for moving the probe. Therefore, this instrument can establish relative motion between the probe and the sample while measuring the morphology or some other surface properties of the sample, as described, for example, in the following patents: U.S. Patent No. RE 34,489 to Hansma et al.; U.S. Patent No. 5,266,801 to Elings et al.; and U.S. Patent No. 5,412,980 to Elings et al.

[0007] In common configurations, probe 14 is often coupled to an oscillating actuator or driver 16 to drive probe 14 at or near the resonant frequency of cantilever 15. Alternative arrangements can measure deflection, torsion, or other motion of cantilever 15. Probe 14 is often a microfabricated cantilever with an integrated tip 17.

[0008] Typically, under the control of the SPM controller 20, an electronic signal from an AC signal source or driver 18 is applied to cause the actuator 16 to drive the probe 14 to oscillate (and / or drive the scanner 24 to oscillate the sample, for example). The probe-sample interaction is typically controlled by the controller 20 via feedback. It is worth noting that the actuator 16 may be coupled to the scanner 24 and the probe 14, but it may also be integrally formed with the cantilever 15 of the probe 14 as part of a self-actuating cantilever / probe.

[0009] Typically, the selected probe 14 oscillates and contacts the sample 22. As described above, sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of the probe 14. In this regard, a deflection detection device 25 is typically used to guide the beam towards the back of the probe 14 and then reflect the beam towards the detector 26. As the beam passes through the detector 26, an appropriate signal is processed in block 28, for example, to determine the RMS deflection and transmit it to the controller 20, which processes the signal to determine the change in the oscillation of the probe 14. Generally, the controller 20 generates a control signal to maintain a relatively constant interaction between the tip and the sample (or the deflection of the rod 15), typically to maintain the setpoint characteristics of the oscillation of the probe 14. More specifically, the controller 20 may include a PI gain control block 32 and a high-voltage amplifier 34 to adjust for errors obtained by comparing the signal corresponding to the probe deflection caused by the tip-sample interaction with the setpoint through circuit 30. For example, the controller 20 is often used to maintain the oscillation amplitude at the setpoint value A. S This ensures a substantially constant force between the tip and the sample. Alternatively, a setpoint phase or frequency can be used.

[0010] In the controller 20 and / or in a system of independent, connected, or standalone controllers, a workstation 40 is also provided, which receives data collected by the controller and processes the data acquired during the scanning process to perform point selection, curve fitting, and distance determination operations.

[0011] AFM can be designed to operate in multiple modes, including contact and oscillation modes. Operation is accomplished by moving the sample or probe assembly up and down relatively perpendicular to the sample surface, in response to the deflection of the probe assembly's cantilever as it scans the surface. Scanning typically occurs in the "xy" plane, at least generally parallel to the sample surface, while vertical movement occurs in the "z" direction, perpendicular to the xy plane. It should be noted that many samples have roughness, curvature, and tilt that deviate from a flat plane; hence the term "generally parallel." In this way, data associated with this vertical movement can be stored and then used to construct an image of the sample surface corresponding to the measured sample features (e.g., surface topography). One mode of AFM operation (called Tapping Mode) TM AFM (Tapping Mode) TMIn this (which is a trademark of the assignee) method, the tip oscillates at or near the resonant frequency of the associated probe cantilever. The feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” that is, the force generated by the tip / sample interaction. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample. It should be noted that in this document, “SPM” and the acronym for a particular type of SPM may be used to refer to a microscope apparatus or associated technology, such as “atomic force microscope.” The ubiquitous Tapping Mode discussed in U.S. Patent Nos. 8,739,309, 9,322,842, and 9,588,136, which are expressly incorporated herein by reference, is described in reference. TM (called Peak Force) In the latest improvements to the (PFT) mode, the feedback is based on the force measured in each oscillation cycle (also known as the transient probe-sample interaction force).

[0012] Regardless of the operating mode, AFM can achieve atomic-level resolution on various insulating or conductive surfaces in air, liquids, or vacuum using a piezoelectric scanner, an optical lever deflection detector, and an extremely small cantilever fabricated using photolithography. Due to its resolution and versatility, AFM is an important measurement instrument in numerous fields, from semiconductor manufacturing to biological research.

[0013] Infrared (IR) spectrometry is a useful tool in many analytical fields, such as polymer science and biology. However, conventional IR spectrometry and microscopy are limited by optical diffraction, achieving resolutions only on scales of many micrometers. It is evident that IR spectrometry at highly localized scales (equivalent to biological organelle scales or smaller) at various points on a sample surface would be particularly useful. In this way, information about the sample's composition can be obtained, such as the location of different materials or molecular structures.

[0014] Conventional far-field infrared (IR) spectrometry is a widely used technique for measuring the characteristics of materials. In many cases, the unique properties of IR spectroscopy can be used to identify unknown materials. IR spectrometry is performed on bulk samples and can provide compositional information, but not nanoscale structural information. This is because, as mentioned earlier, IR spectrometry only allows the collection of many micrometer-scale IR spectra with limited resolution. Far-field localization techniques can achieve spatial resolution as low as approximately 20 nm through point spread function reconstruction, but typically rely on the fluorescence of discrete molecules or quantum dot emitters, limiting their chemically specific information.

[0015] Scattering scanning near-field optical microscopy (s-SNOM) has been applied to some extent for infrared spectroscopy and imaging. In s-SNOM, external illumination of a sharp (metallic or semiconductor) AFM probe tip results in detectable light scattering from the near-field probe-sample interaction region—a scattering characteristic of the material beneath the tip. Alternative methods, such as coherent anti-Stokes Raman spectroscopy (CARS) or tip-enhanced Raman scattering (TERS), are also chemically sensitive but rely on the detection of sample-scattered light.

[0016] Despite ongoing progress in the development of imaging techniques with spatial resolution exceeding diffraction limitations, spectroscopic implementations that provide chemical specificity and sensitivity at the molecular level remain challenging. The following discussion will focus on techniques that rely on mechanical detection of IR absorption, distinct from optical detection implemented, for example, in s-SNOMs, CARS, or TERS.

[0017] A technique based on using AFM to generate such local spectra was described in Optics Letters (Vo. 30, No. 18, September 5, 2005) in an article entitled “Local Infrared Microspectroscopy with Sub-wavelength Spatial Resolution with an Atomic Force Microscope Tip Used as a Photo-thermal Sensor” (PTIR). This technique is also discussed in U.S. Patent No. 8,402,819 ('819 patent). Details of this technique can be found in publications by those skilled in the art, but for clarity, only a brief description will be given here. Mathurin et al. provided a more recent review of AFM-IR techniques based on photothermal detection in the Journal of Applied Physics (2022, 131, 010901).

[0018] Referring to the '819 patent, in PTIR, infrared radiation is incident on a region of a sample. At wavelengths absorbed by the sample, absorption typically causes a localized temperature rise and rapid thermal expansion of the sample. A probe is arranged to interact with the sample and a transducer to generate a signal associated with the IR energy absorbed in the region below the probe tip. "Interaction" means placing the probe tip close enough to the sample that a probe response can be detected in response to the absorption of IR radiation. For example, the interaction can be a contact mode, a tapping mode, or a non-contact mode. Associated detectors can be used to read one or more probe responses to the absorbed radiation. The induced probe response can be probe deflection, resonant oscillations of the probe (flexure, torsion, lateral, etc.), and / or a thermal response of the probe (e.g., temperature change). For probe deflection and / or resonant oscillations of the probe, suitable detectors may include a beam-bounce arrangement with segmented photodiodes, and any associated amplification and signal conditioning electronics. Regarding the thermal response, a suitable detector may include, for example, a Wheatstone bridge, current and / or voltage amplifiers, and / or other associated electronics, to sense, amplify, and modulate the thermal signal from the probe. The probe response is then measured as a function of the wavelength of the incident radiation, thereby establishing an absorption spectrum. Based on the spectrum, the material in the sample can be characterized and / or identified.

[0019] As described in the '819 patent, the AFM setup employs a bottom-up illumination scheme, in which the sample is mounted on a ZnSe prism and light is transmitted from below. A pulsed IR light source (in this case, a free-electron laser (FEL) beam) is guided into the prism and strikes the sample at an angle that causes total internal reflection, so that the beam propagates within the sample and evanizes in the air. Therefore, only the sample is explicitly exposed to the laser radiation, while the AFM probe is minimally exposed to the beam. The scanner places the probe at a point on the sample and holds it at an average height via feedback electronics. Both vertical and lateral deflection signals, as well as the feedback signal, can be monitored.

[0020] When an FEL pulse occurs, the sample may absorb some energy, causing rapid thermal expansion, as described in the '819 patent. Figure 3As shown. This will produce a rapid impact effect on the cantilever, and if the cantilever's response to this impact is slower than the impact itself, resonant oscillations will be excited within the cantilever. Since the absorbed energy is ideally contained within the sample, and since the cantilever itself absorbs very little IR energy, this impact is mainly due to rapid sample expansion. Although the probe remains in contact with the surface via feedback electronics, the resonant signal is too fast for the feedback electronics to be observed directly from the photodetector. Therefore, ringing down occurs while the cantilever is still in contact with the surface; this effect is called "contact resonance." The absolute deflection, amplitude, and frequency characteristics of contact resonance vary with the amount of absorption and other properties of the local area around the probe tip (such as local hardness), for example, by analyzing the ringing down and / or the Fourier transform (FFT) of the ringing down event. Similarly, depending on the direction of expansion, vertical resonance, lateral or torsional resonance, or all of them, can be excited.

[0021] As described in U.S. Patent No. 8,869,602 and the article by Lu et al. (“Tip-enhanced infrared nanospectroscopy via molecular expansion force detection”, Nature Photonics 8, 307 (2014)), resonant-enhanced PTIR is a state-of-the-art method that provides improved signal levels and spatial resolution. Improved sensitivity and spatial resolution are demonstrably achieved by using field enhancement (also present in s-SNOM or TERS) and resonant excitation of cantilever modes (e.g., bending modes or contact resonance modes) at the AFM tip. The latter can be used with 2 nd The cantilever bending mode is achieved using IR laser pulses of the same frequency, while AFM operates in contact mode and photodilation is detected. A spatial resolution of 25 nm was observed, although only on films up to ~2 nm in size. Furthermore, the film deposition on an Au substrate results in significant field enhancement in the substrate-tip cavity occupied by the sample. This approach clearly requires substrate reinforcement and therefore limits its applicability to films deposited on those substrates. Moreover, the AFM contact mode has significant drawbacks compared to intermittent contact (e.g., tapping) or peak force tapping modes, manifesting as tip / sample contamination, tip or sample abrasion, and poor performance on soft, sticky, or loose samples. Tip contamination or abrasion, in particular, can be a serious disadvantage, as any change in tip geometry or surface can affect the field distribution and field enhancement at the apex. Another improvement is the use of a benchtop QCL instead of an FEL (a large user facility laser). Other lasers, such as optical parametric oscillators operating in the 2–10 μm wavelength range, can also be used to supplement the typical QCL's 5–12 μm range.

[0022] Another recently developed technique is light-induced force microscopy (PiFM), described in U.S. Patent No. 8,739,311, which is essentially the same as tapping-mode IR. Here, AFM operates in tapping mode on a mechanical resonance of the cantilever (typically in the 500-1500 kHz range), while PiFM detection is performed on different cantilever modes tuned to the difference frequency of the IR laser. Similar to PTIR, PiFM detects mechanical motion, but unlike PTIR which detects photo-induced expansion, PiFM is claimed to be driven by a light-induced dipole-dipole force between the tip and the sample. High sensitivity and spatial resolution down to 10 nm have reportedly been observed. Currently, the requirement to match the laser frequency with the difference frequency of certain cantilever modes limits the use of QCL lasers for IR sources, as is the case with resonant-enhanced PTIR. Similarly, the frequency of the cantilever mode can shift depending on the material under the tip, thus requiring a tracking mechanism to adjust the laser frequency accordingly. Importantly, PiFM relies on resonant tapping (which will be described below), which is different from the peak force-based approach in the preferred embodiment of the invention. The method differs from that of the present invention. In the preferred embodiment of the invention, the probe oscillation occurs at a level much lower than that of cantilever resonance (at least 5 times).

[0023] In another technology known as “Peak Force IR” and described in U.S. Patents Nos. 8,955,161, 9,207,16, 9,719,916, and 10,520,426, which are expressly incorporated herein by reference, PeakForce The method combines modal AFM with directing light over the sample and locally exciting a photothermal response at the tip-sample interface. Resolution is improved and sample preparation is minimized. The method is based on the directing step to identify modulus changes, providing an indicator of the sample's IR absorption. Measurement techniques sensitive to modulus changes, such as peak force tap (PFT) AFM mode or contact resonance mode, can be employed.

[0024] The acronym "Peak Force IR (PFIR)" typically refers to a relatively new related technique. PFIR is an AFM-IR method based on peak force tapping, in which the sample response induced by an IR laser is detected during a PFT cycle, allowing the acquisition of chemical and nanomechanical information about the sample. The operating principle is described in U.S. Patent No. 10,845,382, or more recently, in an article by Wang et al. published in *Chemical Society Reviews* (Chem. Soc. Rev., 2022, 51, 5268-5286) or in an article by Mathurin et al. published in *Journal of Applied Physics* (2022, 131, 010901). In short, when the laser pulse is absorbed by the sample during the tip-sample contact time within the PFT cycle, the cantilever deflection is modified, and this change is detected. Typically, this change is an oscillation or shift in the deflection signal. Initially, a single laser pulse was used in the first PFT cycle, while IR laser irradiation was not used in subsequent cycles to eliminate the slowly varying cantilever deflection background by subtracting cycle two from cycle one. A later implementation (Wang et al., Nano Letters, 20, 3986, (2020)) eliminated the slowly varying background via a fitting process prior to the FFT for signal extraction. However, despite successful demonstrations, the adoption of PFIR microscopy is limited by the complexity of its customized signal generation and processing routines. Typical PFIR microscopy requires hardware or software-level programming of signal acquisition and processing for background fitting, background subtraction, and FFT extraction, often involving programming using LabVIEW or equivalent platforms.

[0025] This peak force tapping-based IR technology combines nanoscale chemical recognition and nanomechanical characterization, and requires continuous improvement for wider application and better performance. Summary of the Invention

[0026] Using Peak Force The (PFT) mode AFM, in its preferred embodiment, overcomes the shortcomings of the prior art. In the preferred embodiment, the gate pulse during the contact time and the gate detection using a lock-in amplifier do not require background subtraction, thereby reducing complexity.

[0027] In a preferred embodiment, an apparatus for spectroscopically examining a submicron region of a sample using atomic force microscopy (AFM) includes: a driver that generates an oscillating drive signal to cause a probe of the AFM to interact with the sample through multiple probe-sample interaction cycles, thereby generating a transient probe-sample interaction force, wherein the oscillating drive signal has a frequency lower than the resonant frequency of the probe. Additionally, the AFM includes: at least one controller for controlling the transient probe-sample interaction force; and a tunable light source for illuminating the tip-sample region with light pulses to induce sample modification. A detector is provided for measuring probe deflection at least partially due to the induced sample modification. At least one of a lock-in amplifier and a signal integrator is provided for extracting the sample response to the light pulses from the measured probe deflection.

[0028] According to another aspect of this embodiment, at least one of the lock-in amplifier and the signal integrator is a lock-in amplifier, and the sample response is phase-sensitive, wherein the phase-sensitive sample response is averaged.

[0029] According to another aspect of this embodiment, the at least one controller uses the sample response to establish a spatially resolved image indicating the absorbed infrared radiation, wherein the image is established over a region having at least 100x100 pixels within 5 minutes.

[0030] In another aspect of this embodiment, the frequency of the oscillation drive signal is at least 5 times lower than the lowest resonant frequency of the probe.

[0031] According to another aspect of this embodiment, the controller times the pulses between probe-sample interaction cycles, thereby causing a 180-degree phase change in the light-induced probe deflection between at least two cycles, and subtracting the probe deflection corresponding to the at least two cycles. The sample response is then extracted from the subtracted probe deflection.

[0032] According to other aspects of this embodiment, at least one of the optical pulse and the extracted sample response is gated during the probe-sample contact time. Furthermore, at least one of the optical pulse and the extracted sample response can be gated in each cycle of the probe-sample interaction. Additionally, the controller can extract at least one of nanomechanical properties and nanoelectrical properties from the sample response.

[0033] In another aspect of this embodiment, the averaging of the phase-sensitive lock-in amplifier output differs from the averaging of the amplitude, resulting in reduced noise and thus improved signal-to-noise ratio.

[0034] Specifically, a method is provided for spectroscopically examining a submicron region of a sample using atomic force microscopy (AFM). In a first step, the AFM probe interacts with the sample through multiple probe-sample interaction cycles, thereby generating a transient probe-sample interaction force. In this example, the oscillating drive signal has a frequency lower than the resonant frequency of the probe. Next, the method includes controlling the transient probe-sample interaction force and then illuminating the tip-sample region with a pulse of a tunable light source to induce sample modification, preferably within the tip-sample contact time. Next, the method includes measuring the probe deflection caused at least partially by the induced sample modification. Finally, the method includes extracting the sample response, which may or may not be measured within the tip-sample contact time.

[0035] Additionally, the extraction step may include providing at least one of a lock-in amplifier and a signal integrator to extract the sample response only during the tip-sample contact time. For example, if the extraction step employs a lock-in amplifier, the sample response is phase-sensitive, and the method may further include averaging the phase-sensitive sample response.

[0036] According to another aspect of this embodiment, the method further includes using the sample response to establish a spatially resolved image indicating the absorbed infrared radiation, wherein the image is established over a region of a sample having at least 100x100 pixels within 5 minutes.

[0037] In an additional preferred embodiment, a method for spectroscopically spectroscopying a submicron region of a sample using an AFM (Analuminous Fluid Dynamics) is again provided. The AFM probe interacts with the sample through multiple probe-sample interaction cycles, thereby generating a transient probe-sample interaction force. In this example, the oscillating drive signal has a frequency lower than the resonant frequency of the probe. Next, the method includes controlling the transient probe-sample interaction force and illuminating the tip-sample region with a pulse of a tunable light source to induce sample modification within the tip-sample contact time. Then, the probe deflection caused at least partially by the induced sample modification is measured. Finally, the sample response to the light pulse is extracted from the measured probe deflection, wherein this extraction step can be gated.

[0038] The extraction step may further include providing at least one of a lock-in amplifier, an FFT block, and a signal integrator to extract the sample response during the tip-sample contact time. The sample response may be phase-sensitive, such that the phase-sensitive sample response can be averaged.

[0039] In this additional preferred embodiment, the method may further include: (i) timing the pulses between probe-sample interaction cycles, thereby causing a 180-degree phase change in photoinduced probe deflection between at least two cycles; (ii) subtracting the probe deflection corresponding to the two cycles; and (iii) extracting the sample response from the subtraction step. Furthermore, a spatially resolved map using the sample response to indicate the absorbed infrared radiation can be constructed. The map can be constructed over a region of a sample having at least 100x100 pixels within 5 minutes, and the resolution of the sample response map can be less than 20 nm.

[0040] In another preferred embodiment, a method for spectroscopic analysis using an AFM (Anaerobic Fiber Optics) is provided. The method includes causing the probe of the AFM to interact with the sample for multiple cycles using an oscillating drive signal, thereby generating a probe-sample interaction force. The method also involves providing a pulsed light source to generate multiple pulses with pulse widths and directing the pulses to the sample where the probe is located, eliciting an induced sample response. The pulses may substantially coincide with the probe-sample contact time. Probe deflection caused at least in part by the induced sample response is measured. At least one characteristic of the sample response is derived. Furthermore, the directing step of the method can be gated during the probe-sample contact time, and the measurement step can be gated during the probe-sample contact time in each cycle of the eliciting step.

[0041] This other preferred embodiment may further include using the sample response to establish a spatially resolved image indicating the absorbed infrared radiation. The image can be established over a region of a sample having at least 100x100 pixels within 5 minutes. The resolution of the sample response can be less than 20 nm or even less than 10 nm. According to another feature, at least one of the irradiation step and the extraction step is gated during the probe-sample contact time. Furthermore, at least one of the irradiation step and the extraction step can be gated in each cycle of the initiating step.

[0042] These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description and accompanying drawings. However, it should be understood that while the detailed description and specific examples indicate preferred embodiments of the invention, they are for illustrative purposes only and not for limitation. Many changes and modifications can be made within the scope of the invention without departing from its spirit, and the invention includes all such modifications. Attached Figure Description

[0043] Preferred exemplary embodiments of the present invention are illustrated in the accompanying drawings, in which similar reference numerals represent similar parts, and wherein:

[0044] Figure 1 is a schematic diagram of existing atomic force microscopy (AFM);

[0045] Figure 2 This is a schematic diagram of the gate control peak force IR (PFIR) setting in a preferred embodiment;

[0046] Figure 3 This is a graph showing the PFT deflection versus time relationship according to a preferred embodiment of the laser-driven probe response during a gated pulse;

[0047] Figures 4A-4D A pulse and readout scheme for PFT deflection and time based on laser pulses synchronized with the PFT deflection cycle is described. According to a preferred embodiment, no phase change occurs in the pulse sequence during successive PFT cycles;

[0048] Figures 5A-5D The diagram shows the relationship between PFT deflection and time according to the preferred embodiment, illustrating the 180-degree phase change between laser pulses in a continuous PFT cycle, and the measurement that mutual contact oscillations can be eliminated in the preferred embodiment.

[0049] Figures 6A-6C This is a diagram illustrating an alternative gate control detection scheme according to a preferred embodiment;

[0050] Figures 7-10 The negative impact of continuous pulses and continuous detection compared to the gated pulses and detection of the preferred embodiment is shown. Figure 7 This is a graph showing the relationship between laser repetition rate and IR signal. Figure 8 This is a schematic AFM block diagram of the setup used to obtain an intensity image in a typical alignment step in PFIR. Figure 9 Depicting the use Figure 8 The image settings, and Figure 10 This is a graph showing the relationship between wavenumber and normalized locked amplitude;

[0051] Figures 11A-11F Imaging, spectroscopic, and related nanomechanical measurements performed using the instrument according to a preferred embodiment are demonstrated.

[0052] Figures 12A-12C High-resolution nano-IR absorption imaging data with a resolution below 10 nm obtained using the instrument according to a preferred embodiment are shown; and

[0053] Figure 13 This is a flowchart of a method according to a preferred embodiment. Detailed Implementation

[0054] Go to Figure 2The experimental setup 200 of an embodiment of the invention is described. A probe 201 with a cantilever 202 terminated with a sharp tip 203 is attached to a sample 204 of interest. The IR probe 201 preferably has a metallic coating (such as Au or PtIr) to provide field enhancement through a lightning rod effect and optical localization under the tip 203. The sample 204 is mounted on a stage 206 of an atomic force microscope (AFM), which includes a three-dimensional piezoelectric scanner. A piezoelectric element 208 may be attached to the cantilever 202. The AFM is capable of peak force tapping operations (e.g., Bruker's Dimension). or AFM). The sample platform 206 and / or piezoelectric sensor 208 provide relative vertical motion between the tip and the sample, while 206 also provides in-plane XY motion for sample scanning. The vertical deflection of the probe 201 is detected using conventional beam-bounce optics via a diode laser 210 and a position sensor 212 (e.g., a 4-quadrant photodetector). The vertical deflection is measured and routed to the controller 214 for AFM feedback, for example, the controller 214 uses the platform / xyz scanner 206 or the piezoelectric sensor 208 to control the z-position of the sample 204. Notably and importantly, the atomic force microscope controller 214 features PeakForce. Pattern capabilities, such as those described, for example, in US10,845,382.

[0055] Controller 214 also controls a frequency- and wavelength-tunable light source 216. Light source 216 can provide a wide range of wavelengths from UV to far-infrared. In one embodiment, light source 216 provides infrared radiation (IR) that matches the vibrational resonances of molecules in the material under test (i.e., sample 204). Laser 216, such as a quantum cascade laser (e.g., MIRcat, Daylight Photonics) or an optical parametric oscillator (OPO), emits laser pulses 218 at a frequency defined by controller 214. The beam 222 is focused onto the tip-sample region, i.e., the tip-sample interaction region, via a focusing element 220, such as an off-axis paraboloid with a focal length of 25 mm, or any other optical focusing element, such as a lens. Spatial scan results 224 at different wavelengths (λ1, λ2, λ3) and wavelength-dependent nanoscale local spectra 226 indicating IR absorption are processed and displayed on the workstation screen or saved as data via controller 214 or the workstation. Such IR imaging data can be obtained before, after, or during the acquisition of other sample property data (e.g., mechanical (modulus, adhesion), electrical (surface potential or current in KPFM or TUNA), or other measurement data that can be provided together with the AFM operating mode of PFT).

[0056] Preferably, during IR data acquisition, the relative position between the focal point of the infrared beam 222 and the tip 203 is constant; that is, the optical alignment with the tip remains unchanged during IR absorption mapping across the sample and during point beam splitting at a fixed sample location. This ensures that the light intensity at which surface modification occurs in the probe-sample interaction region is constant during IR scanning of the surface at a single IR wavelength, allowing for quantitative comparison of the surface response to IR light at different locations.

[0057] In different embodiments, the IR laser spot can be much larger than the AFM scanning area, so that when scanning the probe, the change in light intensity relative to the IR-illuminated spot can remain sufficiently constant during the scanning process, for example, within 10%. Therefore, in this example, due to the variation in laser power, the IR data at different locations on probe 201 can only be accurate to within 10%. In another embodiment, the relative motion effect between the probe and the IR-illuminated area can be compensated for. One approach is to use the IR-illuminated spot to follow the probe position during scanning. Another approach is to measure the spatial variation of the IR signal on samples with homogeneous IR responses. Once the 3D PFIR response is obtained for different xyz positions of probe 201 relative to the IR-illuminated spot when the probe contacts the sample, measurements of other samples can be corrected based on the spatial IR light variation.

[0058] Controller 214 includes a frequency generator for causing the laser source 216 to generate pulses. A QCL, for example, allows pulse generation to follow an applied TTL signal. Alternatively, the IR pulses can also be selected within the laser output beam 222 via optical devices (e.g., via an acousto-optic modulator, electro-optic modulator, or Pockels cell). A mechanical pulse selector (chopper) or rotating mirror can also allow only selected pulses to be directed to the tip while blocking unwanted pulses. It will be understood that these elements can be inserted into the IR output of the IR source, or they can be part of the IR light generation process within the laser system itself. In this case, for example, a Pockels cell can be used as a pulse selector to select the pump-laser pulse in an optical parametric oscillator or amplifier, thereby driving the IR light generation process. Ultimately, it is important that the tip 203 is irradiated by the laser pulses at a pulse repetition rate controlled by controller 214. The IR beam 222 is linearly polarized along the tip 203, resulting in field enhancement at the apex of a typical conductive or metal-coated (e.g., PtIr, Pt, or Au) tip 203 (similar to the experimental conditions of TERS or s-SNOM). A non-conductive tip and perpendicular polarization relative to tip 203 would cause signal attenuation.

[0059] In PFT, the vertical position of sample 204 on platform 206 can be sinusoidally modulated by a suitable drive signal provided by controller 214 at a low frequency of several kilohertz (significantly lower than the cantilever resonant frequency (i.e., at least 5 times)). Probe 201 can then remain stationary. Alternatively, probe 201 can be sinusoidally modulated in its vertical position, for example, using piezoelectric 208 (or an equivalent actuator that drives the cantilever using magnetic, electrostatic, thermal, or optical forces). Essentially, relative oscillation between the probe and the sample is necessary. Assuming only sample oscillation, near the upper turning point of the oscillation, the radius (typically on a 1-50 nm scale) of the sample and the apex of tip 203 of probe 201 makes controlled contact. The maximum deflection of the cantilever during contact, i.e., the peak force, is used by controller 214 as a setpoint for feedback to maintain the average distance between sample 204 and tip 203. By analyzing the time-varying trajectory of the vertical deflection of the cantilever recorded by the deflection sensor 212, mechanical properties such as modulus, dissipation, and adhesion can be extracted in the PFT.

[0060] Figure 3 As shown, in a preferred embodiment, in Peak Force The tip vertical deflection data 300 is presented as a function of time in the (PFT) cycle. In this example, the vertical deflection is given with IR laser pulse perturbation (deflection 302) and without perturbation (i.e., no IR laser pulse occurs during the contact time), or IR-no-pulse (deflection 304 - dashed curve, vertically deflected for clarity). In the PFT cycle, the probe approaches the sample and snaps into contact with the sample at a snap-in contact time 306 (approximately 160 microseconds in this case). Any free-space oscillations 308 of the cantilever prior to this snap-in time are subsequently suppressed. However, it should be noted that the snap-in contact will excite cantilever oscillations at one of the contact resonances of the cantilever. This is evident in the first ~10 microseconds after the snap-in contact point 306 in one or two oscillation cycles in deflection 304. After point 306, tip 203 and sample 204 continue to move relative to each other, and probe 201 and sample 204 further approach each other until the maximum force is reached at peak force tap control point 310 (that is, the point where feedback keeps the force constant in the PFT cycle). The location of the feedback point can be adjusted in software, but preferably coincides with the maximum value of the PFT deflection curve. Then, the relative motion of the tip and sample is reversed, and at adhesion point 312 (approximately 240 microseconds in this case), the tip lifts off the surface. This event may again cause strong free-space oscillations of the cantilever (245-270 microseconds), which decay until the probe contacts the sample surface again in a subsequent PFT cycle. In the given example, laser pulse 316 only occurs in the probe-sample interaction cycle or contact time t. c Within this timeframe, that is, the time interval (160-240 microseconds) during the PFT cycle when the tip contacts the sample, which is the time between the snap-in contact time 306 and the adhesion point 312. The start and end timestamps for the IR pulse train can be set by the peak force tapping control point 310, or, less preferably, by the snap-in contact point 306 or adhesion point 312, as discussed below, and the laser start-up delay time can be increased to offset the pulses accordingly to the contact time t. c Inside. It should be noted that the contact time is generally between 1 millisecond and 1 microsecond, depending on the Peak Force. The operating frequency, while the pulse width of IR lasers is typically between 1 picosecond and several hundred nanoseconds.

[0061] In this example, a typical polymer sample, such as PMMA or polystyrene (PS), will induce several cantilever responses when the sample absorbs IR laser radiation during laser pulse generation. Generally, the sample can be modified in several ways depending on the material; for example, photoinduced effects can cause surface motion, charge accumulation / displacement, and / or sample polarization, resulting in mechanical or electromagnetic surface pulse forces in response to light. In the case of PS, the infrared surface pulse force originates from the sample expansion when light is absorbed, and most notably causes oscillations in the cantilever 314, which, for clarity, are synchronously averaged over several PFT cycles. This oscillation may occur at the contact resonance of the cantilever, a condition similar to a resonant-driven oscillator, where the IR signal (i.e., the oscillation amplitude) is enhanced by the q-factor of the contact resonance mode. It should be noted that the force changes during PFT cycles; therefore, both the contact resonance and the “Q” are expected to change slightly during PFT cycles. The oscillations may also occur off-resonantly with any cantilever mode, or induce oscillations at frequency f when the laser repetition rate within pulse train 316 is a fraction of f (1 / n, where n = 1, 2, 3… integers). That is, lower resonances drive the excitation of higher harmonics that match the contact resonance of the probe. Next, in a preferred embodiment, a lock-in amplifier or other device (such as an FFT block or a boxcar-like signal integrator) is used to analyze the oscillations where the laser-induced effect is most pronounced. Typically, the contact resonances used fall within the range of 50–3000 kHz, and the typical average laser power used to excite the probe resonance is less than 1 mW.

[0062] Here, a lock-in amplifier is a physical device and / or algorithm that demodulates the system's response at a reference frequency. Lock-in amplifiers can be electronic components, including analog electronics, digital electronics, and combinations thereof. They can also be computational algorithms implemented on digital electronic devices such as microprocessors, field-programmable gate arrays (FPGAs), digital signal processors (DSPs), and personal computers. Lock-in amplifiers analyze oscillating systems and output different signals, including amplitude, phase, in-phase (X) and quadrature (Y) components, or any combination thereof. In this paper, lock-in amplifiers can also produce such measurements at the reference frequency and its higher harmonics.

[0063] In a preferred embodiment of the invention, the controller 214 will essentially be in contact time t during the PFT cycle. c Within a limited repetition rate f laserThis triggers laser emission in the form of laser pulses 316, but does not occur outside of the probe-sample interaction cycle. The peak force tap control point 310 serves as the synchronization point in the PFT cycle and is used to select the start and stop of the laser pulses.

[0064] In a preferred embodiment, the detection of the laser-induced IR signal relies on gated detection 318 based on a lock-in amplifier, caravan signal integration, or similar techniques. In lock-in detection, the vertical deflection signal 302 with laser-induced deflection change 314 is demodulated at a reference frequency given by the laser repetition rate. The controller 214 that determines the laser repetition rate may have a built-in lock to provide the reference frequency. Preferably, data acquisition and lock-in demodulation are initiated at the start of the laser pulse and terminated after the last laser pulse, i.e., within a contact time t. c Or slightly shorter than the contact time t c The acquisition is gated and signal integrated to avoid actual events such as jamming at contact 306 and adhesion point 312. The contact time depends on the sample properties and PFT operation settings. During imaging, the operable PFT properties may remain largely constant, while the mechanical properties of the sample under the tip (e.g., adhesion) may change significantly, causing t... c Changes occur. In this case, the gate length and position are kept constant relative to the PFT cycle, and remain less than the minimum t during scanning at different sample locations. c This could be beneficial. In this way, noise and instability in the PFT operation near the contact time and adhesion point will not affect data acquisition. Using a large gate window for data acquisition outside the contact time, or running lock continuously in many PFT cycles, will add noise to the signal of interest because the time period during which the probe is separated from the sample does not carry local information from the tip-sample interaction region.

[0065] To improve the signal-to-noise ratio, the acquired data can be averaged within or between PFT loops. Data averaging means summing or integrating, for example, data from the lock-in amplifier output, and normalizing it by the acquisition time, the number of analog-to-digital converter (ADC) samples acquired, or the number of PFT loops. In a preferred embodiment, averaging the data in more than one PFT loop requires averaging the locked phase-sensitive output acquired within the gate detection window. This means that the amplitude and phase outputs acquired from each PFT loop are averaged as complex values ​​with the next PFT loop. This differs from averaging only the amplitude output while ignoring phase information. In phase-sensitive averaging, noise in the signal is lower because, for example, two complex values ​​(representing noise) with the same amplitude but opposite phase can cancel each other out when added, but their sum of amplitudes cannot. Therefore, in this example, noise is suppressed in the first case but not in the second. If the increased noise and elevated baseline / offset are acceptable, phase-insensitive averaging of the amplitude can also be performed in one embodiment.

[0066] As described above, the laser pulse and detection are preferably limited to the probe-sample interaction cycle. The user can select the start and end of the detection and pulse window relative to the peak force control point 310 to confine the pulse 316 within the boundaries given by the snap-in contact point 306 and the adhesion point 312. The synchronization point can also be the snap-in contact 306 or the adhesion point 312 instead of point 310. However, it should be noted that the latter two vary depending on the sample, tap amplitude, peak force, and other effects; therefore, the peak force tap control point 310 is preferred. Due to the contact time t... c This will vary with the aforementioned influences (and particularly the sample location); therefore, it may be beneficial to dynamically adjust the window length of the laser pulse based on the measured points 306 and 312. Alternatively, the pulse length can be calculated from scanning parameters (such as peak force) or can be kept constant so that it always remains within points 306 and 312 during the scan. It should be noted that the window used for the laser pulse is greater than t. c It is acceptable, and the effect of continuous pulses in all PFT cycles is as good as discussed. Limiting the pulses to the contact time reduces sample heating and unwanted effects, such as free oscillations of the cantilever induced once the probe is lifted off the surface after the adhesion point 312. Gated detection only during the contact time improves the signal-to-noise ratio, which would otherwise only occur during the tip-sample non-contact process. c They enter the testing channel from outside.

[0067] In PFIR, a long tip-sample contact time is preferred, as this increases the duty cycle and thus improves the signal-to-noise ratio, while the tip still needs to be able to leave the surface without sticking to it. The contact time is controlled by the PFT tap amplitude, which is typically in the range of 30-150 nm for PFIR.

[0068] In this example, other less obvious features in the vertical deflection data 300 can also serve as signals indicating IR absorption or IR-induced surface pulse forces. Besides the strong oscillations in the deflection trajectory mentioned earlier (vertical oscillations here, but generally vertical and / or horizontal oscillations), IR laser absorption can also cause changes in the mechanical properties of the sample. This can cause, for example, a shift in the adhesion point 312 in time (towards a later or earlier time point) and / or in magnitude (i.e., towards a higher / lower deflection value). The same applies to the locking contact time 306 or the maximum force (as opposed to...). Figure 3 The slope between the peak force setpoint 310 (overlapping) and the adhesion point 312 (indicating a change in modulus) in example data 300. Free-space oscillations 308 may also be modified by IR-induced surface pulse forces, possibly caused by sample expansion or contraction. This may include changes in amplitude, frequency, or phase compared to an IR-free PFT cycle. Such changes compared to an IR-free trajectory may persist for hundreds of microseconds after the IR pulse until the next probe-sample interaction causes the free-space cantilever oscillations to stop at the snap-in contact time 306.

[0069] Other samples, particularly inorganic samples, may exhibit PFIR signal generation processes distinct from mechanical probe-sample forces derived from surface expansion or contraction or shock waves. Inorganic materials with low coefficients of thermal expansion may exhibit electromagnetic probe-sample forces, such as charge accumulation, displacement, or polarization induced by IR light. An example is graphene, which supports surface plasmons or localized collective charge oscillations in the IR, and the IR light can be effectively coupled with the momentum provided by the probe tip 203 (similar to s-SNOM). Surface and bulk plasmons can be detected in PFIR via the associated electromagnetic probe-sample forces. Similar forces are expected to be generated by other quasi-particle excitations, such as phonon-polarized phonons in boron nitride.

[0070] It should be noted that the preferred embodiment does not use pulseless PFT cycles, but each cycle is exposed to IR laser pulses to maximize the working cycle and signal-to-noise ratio. Other sample properties derived during the PFT operation, such as electrical properties (current or surface potential) or nanomechanical properties (adhesion, modulus), can be acquired during laser irradiation. Crosstalk or interference may occur between IR absorption measurements and, for example, modulus measurements, for example, when the sample softens and melts under IR radiation. In this case, it is advantageous to separate the PFT cycle used to extract IR absorption from the PFT cycle used to extract other properties, for example, by not pulsed the IR laser during the acquisition of nanomechanical data and not extracting such data during IR absorption measurements. Such separation may occur within scan lines in imaging, or alternate between scan lines, trajectory lines and backtracking lines, or alternate between PFT cycles. Even within a single PFT cycle, laser pulses and signal detection can be separated from the extraction of nanomechanical data; for example, IR absorption may be acquired in the first half of the PFT probe-sample interaction cycle, while modulus data may be acquired in the second half.

[0071] Interference can also occur between laser-induced oscillations and the PFT force feedback mechanism, which is essential for ensuring stable AFM operation. To avoid this, in a preferred embodiment, the force feedback at the peak force tap control at point 310 is based on a low-pass filtered deflection signal. For example, in the range of 100 kHz to several MHz, a 40 kHz low-pass filter suppresses laser-induced deflection oscillations on the absorbing sample when the laser pulse occurs at one of the contact resonances. On the other hand, the low-pass filter needs to transmit deflection changes at a lower frequency to allow the feedback to maintain a constant peak force setpoint during AFM operation, whether the sample is being scanned or stationary (e.g., when acquiring infrared absorption spectra). Without a low-pass filter, the peak force setpoint feedback is disturbed, resulting in increased peak force noise, high noise in the morphology, and general noise increases for any AFM modes that depend on stable AFM operation, including infrared absorption. Such a low-pass filter can be removed if the interference is small, or when the peak force tap feedback is only applied in pulseless PFT cycles. Alternatively, the time window around the peak force tapping control point 310 can be excluded from the laser pulse, so that the AFM feedback is not disturbed.

[0072] Figure 4 illustrates an embodiment based on continuous laser pulses. In this example, two consecutive PFT cycles are shown. Figure 4A f is given PFT The vertical deflection signal versus time during a PFT cycle of ~4.5 kHz, where the contact time t between the tip and the sample is ~80 microseconds.c Total PFT cycle time t PFT It is ~220 microseconds.

[0073] Figure 4B It shows a repetition rate f that is clearly defined. laser =1 / T laser Continuously running laser pulses 400. These pulses are synchronized with the PFT cycle, meaning that each laser pulse occurs at the same relative time point within the PFT cycle. For example, the vertical marker 402 of two consecutively displayed PFT cycles overlaps with the peak force tap control point 310 in each cycle. This point 310 can serve as a defined time marker within the PFT cycle, and it is synchronized with the relative PFT motion between the cantilever and the sample; that is, for sinusoidal PFT motion, the maximum value of the PFT oscillation occurs at a fixed phase or time delay relative to the peak force tap control point 310. The snap-in contact time 306 or the adhesion point 312 is less suitable as a timestamp because they change dynamically relative to point 310 and, therefore, relative to the PFT motion. This dynamic change is based on sample properties and AFM operation, and points 306 and 312 will need to be derived from measurements. Marker 402 indicates that the laser pulses 400 are synchronized with the PFT cycle, and in a given example, there is always one laser pulse that overlaps with the peak force tap control point. In other words, the laser repetition rate f laser It is an even multiple of the peak force tapping frequency, i.e., f laser =n*f PFT , where n = even.

[0074] In a typical example, the PFT frequency could be f PFT =2kHz, and the laser repetition rate can be f laser =1300kHz, 1302kHz, or 1304kHz. The laser pulse repetition rate and PFT frequency are synchronized, for example, by sharing a common clock. As previously stated in Figure 3 As described, the laser pulse will induce IR-induced mechanical or electromagnetic surface pulse forces, which may originate from surface motion (sample expansion or contraction), charge accumulation / displacement, and / or sample polarization. Figure 4C As highlighted, such a force will cause oscillation 404 (in the otherwise undisturbed tip deflection 406). Here, only the contact time t is shown. cOr the deflection during the probe-sample interaction cycle, as this is the time window of interest. For illustrative purposes, the frequency and amplitude of the oscillations are exaggerated. For times other than the contact time, the laser will not induce large oscillations in the cantilever unless the free-space resonance of the cantilever is nearby, which will be discussed later. It should be noted that, as mentioned above, synchronization of the laser pulse repetition rate with the PFT cycle frequency results in oscillations during the tip deflection during the contact time, which are now also synchronized with the PFT cycle. This means that, as indicated by vertical label 402, the minimum and maximum values ​​of the laser-induced oscillations occur at the same time points within the PFT cycle compared to reference points (such as peak force tap control point 310).

[0075] Figure 4D A lock-in reference signal 408 is provided. This reference signal is synchronized with the laser repetition rate, as both originate from controller 214. In one embodiment, the lock-in amplifier is gated and demodulates the laser-induced signal only within the probe-sample interaction cycle indicated by time window 410. Such gated locking of the lock-in operation can be triggered relative to a time marker (such as the peak force tap control point 310 described above) in each PFT cycle. Such lock-in-based demodulation is repeated in subsequent PFT cycles, for example, in window 412 in the next PFT cycle. The amplitude and phase (or in-phase and quadrature components) obtained using the lock-in amplifier in each cycle can then be averaged to complex values ​​between successive cycles, as described above. In another embodiment, lock-in demodulation is performed over the entire PFT cycle time t. PFT The process is performed within a range, for example, from 150 microseconds to 370 microseconds, as shown in 414. A drawback of this method is that the deflection signal from the lock-on demodulation only contains noise from the portion of the PFT cycle where the tip is not in contact with the sample. This additional noise is added to the portion that exists only during the contact time t. c In the real signal within, the overall signal-to-noise ratio in the laser-induced signal is reduced. Furthermore, if the laser repetition rate is close to the free resonance of the cantilever (or at 1 / n of the cantilever resonance, where n = 1, 2, 3… integers, such that the cantilever resonance is close to the harmonic of the laser repetition rate), the cantilever will be driven by at least two possible effects: (1) sample absorption, and the resulting surface pulse force causing acoustic waves (if the AFM is operating in an environment other than air or vacuum), can effectively drive the cantilever, which is typically located a few micrometers to tens of micrometers above the surface in the PFT. For example, the tip 203 can be 3-18 micrometers long, and the PFT amplitude can be 10-200 nm, such that the cantilever is generally located approximately 3-18 micrometers above the sample; and (2) at the contact time t cAnother possible cause of unwanted cantilever oscillations is the absorption of laser light by the cantilever material or its coating. In either case, the resulting cantilever oscillations can lead to a noticeable laser-induced signal that does not originate from a localized surface pulse force beneath the tip, but may instead be an artifact caused by cantilever absorption, unrelated to any sample feature, or it may be sample-specific but originating from a large, non-local area of ​​the sample, far from the tip. Such non-local artifacts are not the desired laser-induced signal of interest from the tip-sample region.

[0076] In summary, it is preferable not to perform detection outside of the probe-sample interaction cycle. This can be achieved by only performing detection at t c This is achieved through locking detection within the gate. Alternatively, the laser pulse can be limited or gated, causing it to only be activated at time t. c This occurs within the time frame, as described above. In this case, locking can still occur throughout the entire PFT cycle time t. PFT The signal is demodulated within the contact time t, but due to the contact time t c There are no laser pulses outside, therefore, no acoustic effects or cantilever absorption to drive the cantilever mode. The lock-in amplifier demodulates continuously in several PFT cycles 416 until the desired integration time is reached. However, in this case, if the free oscillation 308 ( Figure 3 If the frequency is close to the lock-in reference frequency (i.e., the laser repetition rate), the lock may still see strong unwanted signals. Therefore, the gated laser pulse (i.e., only at t) c Internal pulse generation) and gate lock detection (i.e., only at t c (In-line locked signal demodulation) is preferred and has several advantages: when the cantilever is not in contact with the sample, noise or signals from artifacts (such as acoustic effects or cantilever absorption) do not enter the signal demodulation, and therefore, the signal-to-noise ratio is not reduced. Furthermore, fewer laser pulses on the sample mean less sample heating, which may be beneficial for some samples (such as biological or polymeric samples).

[0077] Since the oscillations 404 between consecutive PFT deflection trajectories in this embodiment are synchronized with a fixed zero-phase change, the deflection trajectories (e.g., deflection trajectories of PFT cycles 1 and 2 or 1-4, etc.) can be synchronously averaged in the time domain before further processing to extract the laser-induced sample response. A timestamp or marker (e.g., peak force tap control point 310) serves as the synchronization time point, relative to which the deflection trajectories are averaged. Such averaging improves the signal-to-noise ratio before analyzing the oscillations to extract, for example, the IR absorption of the sample causing the deflection oscillations. When locking is used on such averaged time-domain data, the locked amplitude typically represents the IR absorption signal, but other channels (e.g., phase or in-phase and quadrature components) can also indicate IR absorption.

[0078] In another embodiment, laser pulse 400 is triggered by an event within each PFT cycle. Within each PFT cycle, a timestamp / marker (such as peak force tap control point 310) can act as a trigger to release the laser pulses after an adjustable positive or negative time delay following a specific number of laser pulses within the pulse train. During laser emission, the pulses within the pulse train are controlled by a constant laser repetition rate f. laser To define this. In such an operation, the laser pulse emission is triggered in the first PFT cycle, and the contact time t with the first cycle is... c Basically overlapping (e.g., in) Figure 4A In the context of time intervals from 160 microseconds to 375 microseconds, or for gated laser pulse emission, it is entirely limited to t c (For example, within a time frame of 160 microseconds to 240 microseconds). Then, the timestamp / marker of the second PFT cycle will trigger the next laser pulse train, which can begin as follows: at point 418, the time delay between the last laser pulse triggered by the first PFT cycle and the first laser pulse in the pulse train triggered by the second PFT cycle is defined by the laser pulse period T. laser Separate, that is, the two independent pulse trains triggered by the first PFT cycle and the second PFT cycle are analogous to a time delay T between all pulses. laser Equally spaced continuous pulse trains.

[0079] Alternatively, at point 418, the time delay between the last pulse and the first pulse in the pulse trains of the first and second PFT cycles can be greater than or less than the laser pulse period T, respectively. laserHowever, since the laser pulses remain synchronized with each PFT cycle, the oscillation 404 in the deflection still does not exhibit a phase delay between different PFT cycles. Due to this synchronization and locking, the deflection signal can still be averaged in the time domain, and other extraction methods can operate on the averaged data. Since the phase difference between the reference oscillation 408 and the laser-induced deflection oscillation 404 is constant at any given time, a lock-in amplifier or caravan signal integrator can also operate continuously on real-time data and average over multiple cycles, even with the presence of discontinuities 418.

[0080] In Figure 5, based on the contact time t in PFT c The vertical deflection signal within illustrates another embodiment of the invention. Figure 5A A similar situation to Figure 4 is depicted: the displayed PFT vertical deflection trajectory appears as 502 without laser illumination, but exhibits a distinctly different oscillation 500 driven by laser pulse 504. Specifically, in the displayed PFT cycle, the vertical time marker 402 is shown to overlap with the laser pulse in 504. In contrast, Figure 5B The case of laser pulse 508 is shown, where the pulse is offset by half the time period between laser pulses, meaning that marker 402 is no longer overlapping with the laser pulse, but is located at the center position between pulses in 508. In other words, Figure 5A Pulse train 504 and Figure 5B A 180-degree phase shift exists between pulse trains 508. Compared to 500, this phase shift means that the laser-induced oscillation 506 has undergone a 180-degree phase shift. Such a phase shift is not observed in Figure 4, where the oscillations within all PFT cycles are synchronized to a defined time point within each PFT cycle, e.g., the peak force tap control point 310. However, in the embodiment of Figure 5, the laser pulse repetition rate causes the oscillation phase to flip by 180 degrees between successive PFT cycles. (See previous reference...) Figure 3 The laser-induced real-time signal extraction method described in Figure 4 remains effective, meaning that non-gated or gated locked demodulation or boxcar integration of gated or non-gated laser pulses is also applicable in Figure 5.

[0081] However, averaging the signal in the time domain is different now. Adding consecutive pulses synchronously in the time domain to subsequently extract the laser-induced sample response in a vertical or horizontal cantilever deflection results in oscillation cancellation because the laser-induced oscillations in consecutive PFT cycles 500 and 506 are not in phase at all. In this case, it is necessary to subtract the two (2) consecutive PFT deflection trajectories. Figure 5CThis demonstrates the operation of subtracting trajectory 506 from 500 to obtain trajectory 510, in sub... 1-2 It is noteworthy that after this subtraction process, the amplitude of oscillation 510 doubles, while the slowly varying PFT deflection trajectory 502 is canceled out, meaning that in the absence of laser radiation, the background 512 will result in near-zero deflection. This means that this operation effectively eliminates the slowly varying PFT deflection signal. If the frequency of the slowly varying deflection is close enough to the laser pulse repetition rate, such a slowly varying signal 502 will add noise to the data extraction method. Previous methods for eliminating this slowly varying signal include subtracting the continuous PFT deflection trajectory between pulsed PFT cycles and pulseless PFT cycles, which means a reduction in the number of duty cycles. The embodiment shown in Figure 5 does not rely on pulseless PFT cycles, and therefore, when each PFT cycle is irradiated by a laser pulse, this twice-higher number of duty cycles is expected to improve the signal-to-noise ratio by sqrt(2) compared to some prior art where a laser pulse is received only once every other PFT cycle. Another known method to eliminate the slowly changing curvature 502 of the PFT cantilever deflection is to fit a polynomial function and then subtract it (H. Wang et al., Anal. Chem. 93, 3567 (2021)). The disadvantage of this fitting method is that real-time fitting is computationally expensive and it is only an approximation, whereas the embodiment in Figure 5 eliminates the measured (unfitted) background with a simple subtraction process. After obtaining the subtracted signal 510 using the described method, the curve is further processed, for example, using locking or boxcar-based methods, to extract the laser-induced signal intensity. An FFT algorithm can also be applied to such time-domain data: the integral of the spectral amplitude around the laser repetition rate, or possibly the integral of the spectral amplitude of higher harmonics around the laser repetition rate, or the average of the fundamental and harmonics, will serve as the laser-induced sample response signal, for example, indicating IR absorption.

[0082] based on Figure 5D The advantages of subtracting PFT cycles using the differential method in suppressing artifacts and noise were discussed. When consecutive PFT cycles are subtracted, random noise superimposed on the deflection signal is partially eliminated. Artifacts synchronized with the PFT cycles are also reduced. Figure 5DThe deflection measured without laser irradiation is shown. In a single deflection trajectory 514, a noticeable oscillation is excited after the snap-in contact time 306. The oscillation at the cantilever contact resonance decays in amplitude. Averaging over 4800 PFT cycles in 516 shows that these oscillations still exist: they are synchronous, and therefore, their sum is coherent when averaged over thousands of cycles (Note: for clarity, 516 is vertically biased compared to 514). Such oscillations, which are present even without laser radiation, can interfere with any laser-induced signal. This is especially true if the frequencies are close or overlap, since both are contact resonances. Furthermore, higher harmonics of the oscillations caused by rapid snap-in contact can also overlap with laser-induced changes in frequency space. Since 516 shows that the artifact oscillations are perfectly coherent over many PFT cycles, the aforementioned difference method can eliminate these artifact oscillations and reduce their influence on the true laser-induced deflection changes.

[0083] Figure 5 illustrates, for example, a 180-degree phase shift between consecutive PFT cycles. Such a phase shift can occur in either consecutive pulses or gated pulses. As an example, in consecutive pulses, pulse frequencies of 1301 kHz or 1303 kHz will result in a 180-degree phase reversal every other PFT cycle at a PFT frequency of 2 kHz. At 1300 kHz or 1302 kHz, such a reversal will not occur, and the deflection oscillations will remain in phase between PFT cycles. Generally, for a 180-degree phase shift to occur between consecutive PFT cycles, the laser pulse frequency needs to be an integer multiple of the PFT frequency plus or minus half of the PFT frequency, i.e., fp. laser =(n±1 / 2)*f PFT Where n = 1, 2, 3, etc. In the gated pulse, a time delay of half a laser pulse cycle can be introduced between the first and second PFT cycles, before the start of the pulse train. This results in a 180-degree phase shift between the two cycles, which can then be repeated in subsequent cycles. Delaying the laser pulse cycle by 1.5, 2.5, 3.5, etc., is also feasible. However, in 2... nd Before the laser pulse begins to occur in the cycle, 1 st The oscillations of the PFT cycle do not have a corresponding 2 nd The oscillation is offset by 180 degrees in the cycle. This causes a sudden change in the amplitude of the resulting oscillation after subtraction. In addition, the duty cycle is reduced due to the use of fewer laser pulses.

[0084] Although the flow in Figure 5 is described based on a 180-degree phase shift of the laser pulse between two consecutive PFT cycles, other embodiments can be directly deduced. For example, such a phase shift can be set to occur between PFT cycle 1 and cycle 3. In this case, oscillations 506 and 500 differ by 180 degrees in phase between cycle 1 and cycle 3, and this can be subtracted to obtain the sub... 1-3 In this process, the slowly changing baseline is eliminated again. The same applies to loops 2 and 4, which can be subtracted accordingly to obtain the sub... 2-4 Then, the resulting curve can be sub... 1-3 and sub 2-4 Simultaneous averaging, or separate averaging for each sub 1-3 and sub 2-4 Laser-induced data extraction is applied, and the results are averaged. In this example, the phase shift of the laser-induced oscillations 500 or 506 between consecutive PFT cycles is not 180 degrees, but 90 degrees (or 270 degrees, etc., i.e., more generally, n*90 degrees, where n = 1, 3, 5, ... odd numbers). If the oscillation phases of cycles 1 and 3 differ by 180 degrees, then the same would certainly apply to cycles 1 and 5 or cycles 1 and 7, since in the preferred embodiment, the two frequencies involved, the PFT frequency and the laser repetition rate, are fixed. Therefore, other background elimination combinations can also be used. However, it should be noted that for imaging where the XY sample positions change over time, it is preferable to combine and average only the temporally close PFT cycles so that the signal deduced from the laser-induced sample modifications (e.g., absorption) can be attributed to a small number of spatial XY pixels. Otherwise, the spatial resolution of the laser-induced sample modifications would be poor. For example, if a typical PFIR line scan rate is 1 Hz, the trajectory time for each pixel in a 256-pixel line is ~2 ms, and the backtrack time is ~2 ms. Typically, only the trajectory or backtrack is recorded; therefore, 2 ms is the longest time it takes to average a PFT cycle without first averaging multiple spatial pixels. If the PFT frequency is 2 kHz, only 4 PFT cycles will occur within a 2 ms pixel time.

[0085] During imaging, it may be desirable to avoid averaging across too many spatial pixels to prevent reducing the spatial resolution to a desired minimum of 2 nm, 5 nm, or 10 nm. However, a similar problem arises when performing nanospectral analysis at a fixed spatial location. In this analysis, the laser wavelength is swept, and the sample response is recorded to derive a wavelength-dependent sample response curve, such as a nano-IR absorption spectrum. This means that the wavelength remains constant for a short, specific time period; for example, when performing a 1000 cm⁻¹ wide sweep at a resolution of 2 cm⁻¹ over 5 seconds, the wavelength remains constant for 10 ms. If the averaging time used to extract the laser-induced sample response is much longer than 10 ms, the spectral resolution will be degraded because the data is now averaged across several wavelengths.

[0086] The scenarios in Figures 4 and 5 illustrate the laser-induced oscillations during the contact time t of the PFT cycle. c The embodiment shows an internal phase that is constant (Figure 4) or shifted by 180 degrees (Figure 5) between successive PFT cycles. In a more general implementation, this phase change between successive probe-sample interaction cycles may not be read as 0 degrees or 180 degrees. In this case, only PFT cycles that are far apart from each other will exhibit a phase change close to 180 degrees in the deflection oscillation, thus allowing Figure 5C The subtraction shown is of a substantially structural disturbance 510 with a substantially flat (i.e., zero) background 512 and two selected deflection trajectories. Since the PFT cycles are now more distant from each other, averaging multiple spatial pixels in the spectral steps of imaging or nanoscale spectral dispersion becomes problematic and reduces spatial or spectral resolution. Here, it is preferable to have a contact time t with gate control. c Lock-in demodulation or boxcar integration can be performed based on a single PFT deflection trajectory (such as 404 in Figure 4), or lock-in demodulation or boxcar integration can be performed throughout the entire PFT cycle 414 without gate control. This means that phase-sensitive averaging can be applied between data in a single PFT cycle, or lock-in demodulation or boxcar integration can be performed continuously in multiple cycles until the desired integration time is reached.

[0087] It should be noted that in this operating mode, the laser repetition rate is not synchronized with the PFT cycle frequency. For continuous pulses, it is preferable to gate the signal extraction, and the gated extraction window needs to overlap with the probe-sample interaction cycle to obtain the highest duty cycle and signal-to-noise ratio. In another embodiment, the laser pulse can be gated for continuous or gated signal extraction. In one such embodiment, the laser receives a TTL signal at a reference frequency of a lock-in or boxcar integrator, but this TTL signal occurs only in the tip-sample interaction cycle. This causes the laser to emit pulses synchronized with the reference frequency, while limiting the pulse train to the tip-sample interaction cycle. The lock-in amplifier or boxcar signal integration can then occur continuously over several PFT cycles; for example, for a 2 kHz PFT frequency, averaging over 20 PFT cycles results in an integration time of 10 ms. Alternatively, the signal extraction is also gated and limited to the probe-sample interaction cycle.

[0088] Gated detection can also be achieved by continuously reading data, such as data used for locking the amplifier, and selecting which data to hold and which to discard in controller 214. Furthermore, deflection data entering lock, FFT, or boxcar detection can be partially exchanged for data that does not increase noise in continuous detection. As an example, lock integration within time window 414 typically includes noise in the 250-375 microsecond time interval shown in Figure 4, during which the tip has already left the surface. To prevent this, deflection data within this range can be replaced by a suitable constant value, such as the average value of the lock output within window 414. In this case, averaging only signal 414 instead of signal 410 does not change the lock read value or noise. Similar methods can be used for other extraction methods; for example, the FFT algorithm can be used for the subtracted deflection data 510 in Figure 5, where deflections outside the probe-sample interaction cycle are filled with zeros. However, for FFT, to save computation time, it is preferable to reduce the number of data points for which the FFT algorithm is performed. This also facilitates the extraction of signals for gate control only during the contact time.

[0089] Although the laser-induced signal is shown as an oscillation in the deflection channel in the above description, more complex signal shapes are possible. Figure 6 illustrates these shapes, along with more optimized signal extraction methods to achieve the best signal-to-noise ratio. Figure 6A A typical laser pulse train 600 is shown, where the laser pulse start time is t. laser,0 The pulse length is t laser And the period is T laser . t laser and f laser =1 / Tlaser Typical values ​​are 1 picosecond to 1 microsecond and 50-3000 kHz, respectively. It should be noted that the rectangular shape of the laser pulse is only an approximation, and other rise and fall times are also possible.

[0090] exist Figure 6B The figure shows the contact time t. c The laser-induced deflection trajectory 602 is shown after subtracting the slowly varying mutual contact PFT deflection. In this image, the result without laser excitation would be a flat line. At laser pulse t... laser Within the sample, absorption and expansion, for example, can cause a linear change in the deflection signal over time.604 Such a change can typically begin from the laser pulse itself, i.e., t0. defl,0 =t laser,0 Once the laser pulse ends, no more energy is deposited into the sample, and thermal diffusion typically leads to sample cooling and a weakening of the deflection signal. This weakening can be exponentially reduced until the next laser pulse begins the heating and cooling cycle again. Based on the time scale T... laser Given the sample relaxation time (given by sample properties such as thickness and thermal coupling to the substrate), the sample may cool between pulses to near its temperature exhibited without any laser pulses. Curve 608 depicts such a case, showing a rapid exponential decay towards the "undisturbed" sample temperature, which is the same as the sample temperature when un-pulsed. If the cooling time is longer than T... laser The longer the sample temperature, the greater the deflection change caused by sample absorption and thermal expansion, which will also increase with the first laser pulse in the pulse train at the start of the PFT contact window. After the initial laser pulse, an equilibrium will be reached, meaning that the sample temperature will cycle between a minimum and a maximum temperature before and after each laser pulse, but for pulses within the pulse train, the minimum and maximum temperatures will be the same and will not continue to increase. Therefore, after equilibrium is established, the deflection baseline or offset will be higher than in the case of a pulseless PFT cycle, i.e., curve 602 will be offset vertically with a higher baseline and a small deflection change caused by the top laser pulse. Depending on the thermal properties of the sample, the laser repetition rate, its relationship with the contact resonant frequency of the probe, and the properties of the probe (e.g., its q-factor), the laser pulse may cause other probe responses besides more pronounced oscillations of 602 or 314, such as a more gradual increase of 604, or even a decrease (step) of 604 (for samples with negative thermal expansion), or a deflection change starting at t. defl,0 With the start of the laser pulse t laser,0 The delay between them.

[0091] exist Figure 6CIn addition to, for example, the lock-based detection described previously, different detection methods will be described. Curve 610 gives an example of integration and averaging of the boxcar signal. At time t g0 The deflection signal can be at time t g1 Integration, and in the dead time t g2 Within this range, integration does not occur. In a preferred embodiment, the method is applied to eliminate the deflection signal 602 after slowly varying mutual contact PFT deflection, for example, the deflection signal 602 obtained as shown in FIG. 5, or the deflection signal 602 obtained by using a pulseless PFT cycle as a reference for subtraction. Generally, this method is also applicable to the original PFT deflection in 304. Relative to laser pulse t laser,0 To select the starting point t g0 This causes the integrating gate to coincide with the signal of interest (here, deflection increase 604 and attenuation 606). When the gate is at t g1 When the gate is opened, the integrator detects a signal, and when the gate is at t g2 When the internal circuit is closed, no noise or interference can be detected. This corresponds to the input signal (i.e., deflection) being multiplied by the boxcar function, and at t g1 Integration is performed. Then, a boxcar averaging of the gated integration of many laser pulses is performed before potential averaging with the next PFT cycle. Curve 612 provides an example of a narrower signal integration gate, which may be more suitable for detecting the rapidly decaying curve 608. Otherwise, continuous measurement in this case would result in a low signal-to-noise ratio because the time interval after 608 has fully decayed helps to capture noise but does not carry the laser-induced signal. It should be noted that, in a preferred embodiment, any signal integration or accumulation is normalized by the acquisition time, the number of laser pulses, or the number of ADC samples being integrated.

[0092] Other gate control functions may be more suitable. Curve 614 illustrates such an optimized gate control function: during the linear rise 616, the deflection signal is integrated, but the result is weighted according to the gate control function; that is, the data points obtained at and near the peak of the gate control function have the highest weight. This also applies, for example, to the exponential decay portion 618 of the gate or weighting function. Such a gate control function can be adapted based on specific samples and excitation conditions (e.g., laser pulse length) to optimize the rise time 616 and fall time 618 while minimizing the integral of noise.

[0093] While lock-in amplifiers and FFT-based signal extraction require equidistant laser pulse periods (i.e., a fixed laser repetition rate) within the PFT probe-sample interaction cycle, non-equidistant pulses can be achieved using a more general gating function in caravan integration. As long as the gating used for caravan integration substantially coincides with the laser-induced deflection change, the laser pulse distance can vary from one laser pulse emission to another within and between PFT probe-sample interaction cycles. This means that the gating length t of 610... g1 and t g2 The timing can vary, but even if the period between pulses changes, they must still be synchronized with the laser pulse. Such a pulse scheme may require a light source in which, although the timing of light emission is not well controlled, the emission time can be determined (e.g., via a photodiode) and can be used to trigger a boxcar detection.

[0094] It should be noted that other methods exist for obtaining the laser-induced signal from the deflection curve 602 or the more oscillating motion 314 or 510. For example, the RMS of the signal can be determined, or a sin or cos curve can be fitted, where the amplitude of the fitted curve represents the laser-induced signal, or the minimum and maximum values ​​of the oscillation amplitude can be read. A bandpass filter around the laser repetition rate or a high-pass filter lower than it can also be applied to the deflection signal before further data extraction.

[0095] When the probe contacts the sample, the laser pulse does not need to occur at the contact resonance of the probe. The pulse may also occur at the out-of-resonance. In particular, when performing continuous pulses and continuous detection, it is beneficial to avoid excitation and detection at the free cantilever resonance or at its higher modes at ~6.3 or at 1 / n (n = 1, 2, 3… integers) of these modes, to avoid driving the cantilever as it lifts off the surface. After the cantilever lifts off the surface, it will oscillate at its free resonance anyway, so detection at only these frequencies will increase unwanted noise and background. In a particular embodiment, the laser pulse may occur at approximately twice the free cantilever resonance, i.e., at 2. In this case, if the laser pulse is continuous and ungated / limited to the PFT contact time, excitation at twice the free cantilever resonance will not excite the cantilever: even if it is absorbing or experiencing acoustic waves from the sample, such a drive at 2 will "kick" the cantilever oscillates to in-phase, then out-of-phase, and so on, thus effectively suppressing oscillations. However, the cantilever may still exhibit fundamental free resonance and higher harmonics after leaving the surface after the adhesion point, but these are not actively driven by the laser.

[0096] It should be noted that if contact resonance is selected for the laser pulse, the contact resonance will generally shift with changes in the mechanical tip-sample interaction. This can be caused by changes in AFM parameters (such as the peak force setpoint), or most commonly by changes in the nanomechanical sample properties (especially when scanning samples of different textures). This can lead to artifacts and ambiguity because, for example, a decrease in the IR signal over the sample region may be due to less IR absorption or a modulus change that shifts the contact resonance. To compensate for this, it is preferable to employ frequency tracking (e.g., via a phase-locked loop of the IR signal), which is well compatible with lock-based signal processing and its amplitude and phase channels. For example, such a tracking mechanism dynamically adjusts the laser repetition rate to overlap with the contact resonance. Alternatively, the effects of the contact resonance shift can be minimized by operating away from the resonance, that is, by pulse and detect the laser far from the cantilever resonance (where the laser-induced signal is less sensitive to the mechanical tip-sample interaction).

[0097] Signal extraction of the deflection trajectory can be performed using lock-in amplifiers, van-mounted signal integrators, FFT, or similar techniques, and can also occur at higher harmonics of the laser repetition frequency. This occurs when the cantilever is excited by the sample at the laser repetition rate during the PFT contact time. Figure 6B The deflection response 602 in the image may be asymmetric and not purely oscillatory; therefore, it generally carries higher harmonics of the fundamental frequency. This is useful in embodiments where the cantilever itself absorbs laser energy, causing nonlocal laser-induced oscillations that compete with oscillations generated by local surface pulse forces from the tip-sample region. Such laser-excited cantilever heating is typically associated with approximately sinusoidal cantilever motion. Detection with higher harmonics can suppress nonlocal cantilever motion at the fundamental frequency and recover the local signal from the tip. A drawback of this method is that the signal strength decreases at higher harmonics.

[0098] Figures 7-10 This highlights the importance of laser pulse and signal detection being ungated and limited to PFT contact time t. c (That is, potential problems that may occur when the entire PFT cycle is evaluated and exposed to continuous pulses.) Figure 7The IR response 700 of a continuous pulse obtained by continuously operating a lock-in amplifier is shown. The IR wavelength was selected as 1730 cm⁻¹ to match the carbonyl absorption peak of the PMMA thin film sample studied. While tuning the IR laser repetition rate, the lock-in-extracted signal amplitude was recorded at signal maximums 702 and 704 at data points of 1394.6 kHz and 1402.7 kHz. It should be noted that although the discrete points are shown at random frequencies, the system can be swept at a continuous repetition rate. The two peaks have been fitted with two Lorentzian curves. When the tip is withdrawn from the sample by several hundred micrometers, the broad peak disappears, while the narrow peak at 1394.6 kHz remains at the same frequency (Lorentzian fit 2). In a control experiment involving re-contact with the sample in PFT mode, the IR laser pulse was limited or gated to only be applied during the contact time t. c In this case, the narrow peak disappears, leaving only a broader peak, as shown in the fitting (Lorentz fitting 1). This demonstrates that the narrow peak originates from the free resonance of the cantilever, while the broad peak originates from the localized surface pulse force that the tip only experiences locally when in contact with the sample. Only this broad peak contains the local sample signal of interest desired when measuring nanoscale IR absorption. As mentioned earlier, the narrower free cantilever resonance can affect the detection of the desired nanoscale IR signal. Here, we present further experimental evidence of the negative impact of continuous laser pulses and ungated detection.

[0099] exist Figure 8 The text describes the methods used to obtain... Figure 9 The intensity maps are set in the nano-IR instrument during the alignment step before any IR absorption measurements are performed. When a new AFM probe is installed, the IR laser beam focus of beam 222 needs to be realigned in the xyz coordinates to the new position of tip 203 on sample 204 in order to be able to measure any IR signal from the tip-sample region. To do this, a visible light alignment laser collinear with the IR beam is first focused near the tip. Then, the IR beam focus is scanned over the tip, and the IR signal is recorded, with the maximum signal value only displayed when the IR beam focus overlaps with the tip-sample region. The beam focus can be scanned by different means. One method is to scan the focusing element 220 (such as an off-axis parabolic mirror or lens) in the xyz coordinates while acquiring the IR signal. Another method is to scan the angle of the incident IR beam 222.

[0100] Figure 9 The image shows the intensity map resulting from such a signal search. Continuous pulse and continuous lock detection at contact resonance 704 at ~1403 kHz results in numerous signal maxima in 706 related to the cantilever shape. When only during contact time t... cWhen pulsed internally, such gating results in a single bright spot in 708 corresponding to the tip-sample position; the artifacts, while greatly reduced, still exist. The remaining artifacts are due to ungated detection, which remains sensitive to the free-space oscillations 702 of the cantilever. As previously mentioned, these oscillations are excited after the cantilever is lifted (even without any laser pulses), and here their frequencies are close to the detection frequency. In 709, the detection is additionally gated, and the previous cantilever-related artifacts are absent. Clearly, for alignment, the case 709, where pulses and detection are performed only during contact, is preferred: the optimal alignment of the laser beam focus with the tip-sample position is clear. With ungated pulses and detection, the various signal maxima in image 706 can obscure the optimal alignment and may ultimately optimize the signal incorrectly, for example, optimizing the overlap between the cantilever 202 (instead of the tip 203) and the beam focus.

[0101] Figure 10 As shown, on a PMMA film sample, Figure 9 The different IR responses of different intensity images in the PFIR are functionally related to the IR laser wavenumber. For clarity, all curves are normalized to I(1) and vertically offset. The bottom curve 710 shows the IR laser-induced signal obtained by demodulating the vertical deflection signal during the gated laser pulse by a gated locked amplifier (i.e., limited to the probe-sample interaction cycle). The extracted amplitude signal is proportional to the sample absorption and shows a carbonyl absorption peak near 1730 cm⁻¹. It should be noted that the presented signal is not normalized by the laser power spectrum as is usually required to obtain nanoscale IR absorption spectra. Such normalization by the laser background eliminates the influence of the non-constant laser power spectrum on the absorption signal and eliminates water lines. It should be noted that the dip in the locked amplitude signal 710, for example, is at 1716 cm⁻¹. -1 Or 1732cm -1 The absorption at this point is due to water absorption in the IR beam path (which would cancel out if the signal were normalized from the laser power spectrum). Importantly, this single absorption line 710 is located where the IR laser beam is aligned. Figure 9 The curve 709 was obtained when a single bright spot was detected. In contrast, curve 712 was obtained when the IR beam was continuously pulsed and detected, and then aligned with the target. Figure 9 The image 706 was obtained at the center. Carbonyl absorption is still visible, but it is superimposed on a broader background, meaning the baseline is raised. The spectrum obtained at the more distinct hot spot at the center of 708 falls between the extreme cases of spectra 710 and 712 (not shown), i.e., the background is small but raised and broader. Depending on the selected laser repetition rate, Figure 9Other hot spots or bright spots in image 706 (besides the center) may show signals similar to curve 714. No or only a few carbonyl resonances are visible, but the background is broad (with numerous water absorption lines). Curves 712 and 714 (continuous pulses and detections) are caused by the effects discussed earlier: the cantilever is excited by its absorption of IR radiation, and / or driven by long-path acoustic waves originating from the sample absorption beneath the cantilever. In both cases, the feature of interest in nano-IR absorption, i.e., in this example, the absorption peak of the carbonyl resonance, may be masked or severely contaminated by these artifacts.

[0102] It should be noted that, in Figures 7-10 The visible effects are independent of the detection method; that is, FFT-based signal extraction and lock-based signal extraction exhibit the same behavior, which also applies to boxcar integration. The magnitude of artifacts depends on experimental conditions (e.g., IR spot size) and the nature of the AFM probe. For example, the free-space resonant frequency of the cantilever may not overlap much with the contact resonant frequency of cantilevers with different geometries. Alternatively, the tip 203 can be spatially more separated from the cantilever 202 so that the IR spot does not overlap much with it when the tip-sample region is excited. Or, the contact time can account for a large proportion of the PFT cycle, so that any artifacts originating outside the contact time are not very noticeable.

[0103] The method described is a true multimodal spectrophotometry technique that can simultaneously acquire IR and nanomechanical data, as both require PFT AFM operation. Figures 11A to 11D This illustrates the combination of PFIR with gated IR pulses and detection based on sequential locking for measuring adhesion and modulus. In this embodiment, the laser repetition rate was not selected at the contact resonance but set at 662 kHz, where the IR signal has decreased by 30% compared to the peak at 789 kHz. The advantage of such off-resonance driving is that it minimizes mechanical artifacts in the IR image, which would otherwise result in higher imaging contrast and more imaging artifacts due to the offset from the contact resonance: at selected frequencies far from the resonance peak, the IR signal may be fairly constant with frequency. Frequency tracking during the scan, for example based on PLL, will follow these offsets, but is not shown. The sample was PMMA beads in epoxy resin, scanned at 512 x 128 pixels at 1.5 μm at 0.5 Hz. All images were acquired simultaneously to prevent thermal drift during sequential imaging. The height image in panel (a) shows a central feature that reveals lower adhesion (b) and lower modulus (c) compared to the rest of the image. Panel (d) shows the view at 1730cm -1 IR absorption at the location. The PMMA beads are bright in the lower right, while the central feature is not very bright.

[0104] Figure 11E The PMMA beads within the epoxy resin matrix were shown at a 1730 cm⁻¹ value corresponding to the carbonyl absorption of PMMA. -1 Large-scale IR scanning at wavelengths of [wavelength missing]. Circular absorbing PMMA beads emit light within a non-absorbing matrix. Scanning is performed at 0.5 Hz with a 10-micron scan size and 512 pixels. For different probes with a spring constant of 0.4 N / m, at contact resonance, a laser repetition rate of 1400 kHz is used, employing a force of 1.5 nN and a PFT amplitude of 30 nm. In this embodiment, laser pulses and lock-in detection are limited to the contact time within the PFT cycle. Lock-in is implemented within the FPGA. The corresponding spectra of the PMMA beads and the epoxy matrix are [missing information]. Figure 11F The image shows a spectral resolution of 2 cm⁻¹. -1 In this case, ten (10) spectra were averaged, with each spectrum taking ten (10) seconds. As usual, nano-RI absorption was obtained from the wavelength-dependent tip response, which was normalized by the laser output power (which was measured by a power meter under similar experimental conditions). The setup was purged with dry air to remove water lines.

[0105] Figure 12 highlights the spatial resolution achievable by the embodiments of the present invention. Imaging the PS-b-PMMA bulk copolymer reveals a resolution at 1730 cm⁻¹. -1 The absorption of PMMA in the lower region and at 1599 cm -1 The distribution of the PS domain under absorption. Images at a 400 nm scan size were acquired at 512 x 256 pixels at a scan rate of 1 Hz, with each scan lasting 5 minutes. A probe with a spring constant of 5 N / m was used with a PFT tap amplitude of 30 nm and a PFT force setpoint of 3 nN, within a typical PFIR range of tens of piconet to several nanonewtons. Figure 12A In this method, a gate-controlled pulse (approximately 961 kHz contact resonance) and gate-controlled locking detection are used only during the contact time, and from... Figure 12B As can be seen from the linecut image, the spatial IR resolution of <8nm is displayed. Figure 12C In this process, after subtracting the deflection of the continuous PFT cycle according to Figure 5, an FFT readout routine is employed to eliminate slowly changing background and improve the signal-to-noise ratio. Here, the laser repetition rate is 961 kHz, close to the contact resonance of the probe. Images are acquired at 512 x 256 pixels at a scan rate of 1 Hz. The FFT amplitude is integrated within a window of ±20 kHz around the laser repetition rate, and this routine is implemented in the controller DSP. The IR spatial resolution remains the same as before.

[0106] It should be noted that the preferred embodiment of pulse and detection (sample response extraction) gated via lock-in amplifier, caravan integration, FFT, or similar methods offers a speed advantage compared to existing technologies. While previous single-pulse or multi-pulse excitation PFIR reports scan rates of 0.1–0.2 Hz, the preferred embodiment allows scan rates exceeding 0.5 Hz or 1 Hz, representing an improvement of 3–10 times. This improvement is attributable to a larger duty cycle associated with the pulse throughout the contact time and the absence of a pulseless PFT cycle in the absence of IR irradiation. Furthermore, synchronization between the pulse and phase-sensitive detection enables phase-sensitive averaging, where noise and artifacts are reduced, and noise is not attenuated by localized destructive interference. Background cancellation according to Figure 5 further reduces noise and the amplitude of any contact resonant oscillations caused by the “shock” of rapid contact engagement at the start of the PFT probe-sample interaction cycle.

[0107] It should be understood that, in alternative embodiments, the wavelength region may extend beyond the infrared of the preferred embodiment, for example, into the ultraviolet, visible, near-infrared, and terahertz or far-infrared spectral regions. QCLs and optical parametric oscillators exist as pulsed and modulated light sources in the infrared. UV, visible, and near-IR are covered by laser sources such as solid-state lasers, fiber lasers, diode lasers, optical parametric oscillators, or gas lasers, as well as laser sources based on nonlinear frequency conversion methods including optical parametric generation, frequency generation, harmonic generation, frequency combing, and related methods. In the terahertz spectral region, terahertz quantum cascade lasers are emerging, while terahertz gas lasers, terahertz antennas, or free-electron lasers already exist, covering this range. In the extended wavelength range from UV to terahertz, the surface pulse force during laser pulses may originate from several effects. In the terahertz region, plasmon polarons in graphene or copper-pair polarons in superconductors, under photoexcitation induced by charge redistribution and charge oscillations, may induce electromagnetic forces between the probe and the sample. Another example is phonon resonances that lead to absorption and photoexpansion in the terahertz range. In the UV, visible, and near-infrared ranges, for example in metallic nanostructures, plasmon resonances exist that absorb energy through charge oscillations or charge redistribution to induce photoexpansion or alter the electromagnetic field, thereby applying surface pulsed forces to the probe.

[0108] In another embodiment, the sample is illuminated from the bottom, rather than... Figure 2The diagram illustrates top-down illumination. Bottom illumination requires a transparent sample or a sufficiently thin film (within a few wavelengths) within the wavelength range of interest to allow light to transmit to the volume being probed. The advantage of bottom illumination is that it reduces the exposure of the tip 203 and probe 201 to the laser pulse, thus reducing artifacts that may occur when the probe itself absorbs light and is heated. Another advantage is that bottom illumination can use a higher aperture value than top illumination because, in top illumination, the probe blocks some light, while in bottom illumination, the entire half of the space below the probe can be used for light focusing. Therefore, the smaller the focal point, the lower the power requirement of the laser and the less sample heating. A major advantage of bottom illumination is that it allows for PFIR of samples in liquid environments, which will be described below.

[0109] For bottom irradiation, the sample can be placed or spin-coated, for example, onto a prism of a transparent material in the wavelength range of interest, such as ZnSe, ZnS, diamond, or germanium. The laser beam undergoes total internal reflection so that it propagates within the sample while fading in the air. In this way, only the sample is exposed to radiation, resulting in strong light-matter interactions.

[0110] This bottom-up configuration is most useful for measurements in liquids. The tip and sample region are surrounded by fluid to study, for example, the condition of a biological sample in its natural environment or the electrochemical reactions that occur. Water can be used as a liquid to study the near-infrared absorption of biological material in its natural environment because water absorption is minimized in the UV to near-infrared spectral region compared to the infrared region. Other suitable liquids with little or no absorption in the wavelength range of interest, such as heavy water, can be used to extend the wavelength range. Water absorption is minimized for bottom radiation compared to top-down illumination of longer light rays through the liquid.

[0111] In bottom-up illumination using a prism, total internal reflection of light establishes an effusive field, inducing light absorption within the sample's IR mode or due to electronic transitions. Electronic transitions can occur in ultraviolet, visible, or near-infrared light, for example, using fluorophores employed in fluorescence microscopy. Absorption of electronic transitions and subsequent nonradiative conversion generally result in thermal effects or pulsed forces, a mechanical response detected in PFIR. In another embodiment, a prism and total internal reflection are not necessary, but light is focused directly from the bottom onto the tip-sample interaction region. The sample and its substrate (e.g., ZnSe) need to be transparent so that light can transmit to the tip-sample interaction volume.

[0112] Figure 13 The flowchart outlines some preferred embodiments of method 1300. Method 1300 includes oscillating the probe-sample distance in step 1302, for example, by peak force tapping (PFT mode) at a frequency below the probe's resonant frequency (e.g., 2 kHz). In step 1304, during the tip-sample contact time, the probe interacts with the sample at a certain xy position. In step 1306, during the peak force tapping, the maximum probe-sample interaction force is controlled, for example, to avoid sample damage while ensuring good tip-sample contact for sensing the laser-induced sample response in subsequent steps of method 1300. Next, in step 1308, the tip-sample region in contact with the sample is illuminated with pulses of a light source of interest at a wavelength of interest, for example, in a preferred embodiment, an infrared laser. The pulse repetition rate is preferably set at or near the contact resonance of the probe, but pulses away from the resonance are also acceptable, albeit at the cost of a lower signal-to-noise ratio.

[0113] In step 1310, the laser pulses can be selected to occur continuously or to be gated, for example, restricted to a series of several pulses that substantially overlap with the probe-sample interaction cycle limited to the tip-sample contact time. In the latter “gated” case, the laser pulses preferably, but not necessarily, are synchronized with the probe-sample interaction cycle and arrive at the same relative time point in each cycle. In step 1312, continuous detection or gated detection is selected within the probe-sample interaction cycle. In step 1314, the background of slow changes in probe deflection repeated in each probe-sample interaction cycle can be subtracted. For this purpose, the laser pulse repetition rate needs to be set in step 1316 such that the deflection change induced by the light pulse has a phase difference of 180 degrees between any two probe-sample interaction cycles, preferably between consecutive cycles. As an example, for a PFT frequency of 2 kHz, a laser repetition rate of 751 kHz can ensure phase reversal between consecutive cycles. In step 1318, the deflection of a cycle with a phase offset of 180 degrees is subtracted synchronously (meaning the same time point is subtracted relative to the synchronization timestamp, such as the peak force tap control point). Alternatively, a pulseless probe-sample interaction cycle can be used to eliminate background, or a slowly changing background can be fitted as a reference for subtraction. If background elimination is not desired, the subtracted deflection in step 1318 is not used; instead, the probe deflection in step 1320 is used for further processing. Next, in step 1322, the sample response is extracted from the deflection obtained in step 1320 or the processed deflection 1318. Before or as part of extraction step 1322, the temporal deflection data can be synchronously averaged in steps 1318 or 1320 to improve the signal-to-noise ratio.

[0114] In step 1322, the sample response is extracted, preferably based on the use of a lock-in amplifier, a box-type signal integrator, or an FFT routine / algorithm. Averaging is also optional, and if the output of the extraction step provides phase information (e.g., for a lock-in amplifier or FFT implementation), it is preferred to average the phase-sensitive response. It should be noted that the sample response can be extracted from the deflection after at least one complete probe-sample interaction cycle deflection measurement, or in some implementations in real-time, particularly when using a lock-in or signal integrator, i.e., during the probe-sample interaction cycle, even before the cycle is complete. Once the sample response has been extracted, step 1322 can be repeated to collect sample responses at more wavelengths of the light source in step 1324. The resulting spectrum of the sample response as a function of wavelength can be established in step 1326, and in a preferred embodiment, the absorption spectrum can be represented after normalization by wavelength-dependent laser power. Alternatively, in step 1328, the wavelength can be kept constant while the sample position is changed, and step 1322 can be repeated. In this scenario, a spatial map can be established in step 1330 to indicate, for example, position-dependent infrared absorption. Steps 1326 and 1330 can also be combined to establish hyperspectral data: a spatial map containing position-dependent spectra. It should be noted that, in another embodiment, the laser repetition rate may need to be adjusted in step 1308 to follow contact resonance in the frequency-tracking step during spatial scanning when the xy position changes, or during spectral acquisition when the wavelength changes.

[0115] Although the optimal mode for carrying out the invention as envisioned by the inventors has been disclosed above, the practice of the invention is not limited thereto. It will be apparent that various additions, modifications, and rearrangements can be made to the features of the invention without departing from the spirit and scope of the basic inventive concept.

Claims

1. An apparatus for spectroscopic analysis of a submicron region of a sample using an atomic force microscope, the apparatus comprising: A driver generates an oscillating drive signal to cause the probe of the AFM to interact with the sample through multiple probe-sample interaction cycles, thereby generating a transient probe-sample interaction force, wherein the oscillating drive signal has a frequency lower than the resonant frequency of the probe; At least one controller is provided for controlling the transient probe-sample interaction force; A tunable light source is used to illuminate the tip-sample region with light pulses to induce sample modification; A detector for measuring probe deflection caused at least in part by induced sample modification; and At least one of a lock-in amplifier and a signal integrator is used to extract a sample response to the optical pulse from the measured probe deflection.

2. The apparatus according to claim 1, wherein, At least one of the lock-in amplifier and the signal integrator is a lock-in amplifier, and the sample response is phase-sensitive, wherein the phase-sensitive sample response is averaged.

3. The apparatus according to claim 1, wherein, The at least one controller uses the sample response to build a spatially resolved image indicating the absorbed infrared radiation, wherein the image is built over a region of samples having at least 100x100 pixels within 5 minutes.

4. The apparatus according to claim 1, wherein, The frequency of the oscillation driving signal is at least 5 times lower than the lowest resonant frequency of the probe.

5. The apparatus according to claim 1, wherein, The at least one controller: The pulses are timed between probe-sample interaction cycles, thereby causing a 180-degree phase change in the light-induced probe deflection between at least two cycles; Subtract the probe deflection corresponding to the at least two cycles; as well as Extract the sample response from the subtracted probe deflection.

6. The method according to claim 5, wherein, The sample response is extracted by at least one of the lock-in amplifier, the signal integrator, and the FFT algorithm.

7. The apparatus according to claim 1, wherein, At least one of the optical pulse and the extracted sample response is gated during the probe-sample contact time.

8. The apparatus according to claim 7, wherein, At least one of the optical pulse and the extracted sample response is gated in each cycle of the probe-sample interaction.

9. The apparatus according to claim 1, wherein, The controller extracts at least one of the nanomechanical properties and nanoelectric properties from the sample response.

10. A method for spectroscopically analyzing a sub-micron region of a sample using an atomic force microscope, the method comprising the following steps: An oscillating drive signal with a frequency lower than the resonant frequency of the probe causes the probe of the AFM to interact with the sample through multiple probe-sample interaction cycles, thereby generating transient probe-sample interaction forces. Controlling the transient probe-sample interaction force; A light pulse from a tunable light source is used to illuminate the tip-sample region to induce sample modification during the tip-sample contact time. The probe deflection is measured at least in part due to induced sample modification; The pulses are timed between probe-sample interaction cycles, thereby causing a 180-degree phase change in the light-induced probe deflection between at least two cycles; Subtract the probe deflection corresponding to the at least two cycles; as well as Extract the sample response from the subtraction step.

11. The method according to claim 10, wherein, At least one of the irradiation step and the extraction step is gated during the probe-sample contact time.

12. The method according to claim 11, wherein, The at least two cycles are consecutive cycles.

13. The method according to claim 10, wherein, The sample response is extracted using at least one of a lock-in amplifier, a signal integrator, and an FFT algorithm to generate the output.

14. The method of claim 10, wherein, The extraction step employs at least one of a lock-in amplifier and an FFT algorithm, and also includes averaging the phase-sensitive output.

15. The method according to claim 10, wherein, The frequency of the oscillation driving signal is at least 5 times lower than the lowest resonant frequency of the probe.

16. A method for spectroscopic observation using an atomic force microscope, the method comprising the following steps: The probe of the AFM interacts with the sample multiple times using an oscillating drive signal, thereby generating a probe-sample interaction force. Controlling the probe-sample interaction force; Provide a pulsed light source to generate multiple light pulses, each with its own pulse width; The pulse is directed to the sample where the probe is located, causing an induced sample response; The probe deflection is measured at least in part due to the induced sample response; as well as Extracting a sample response to the light pulse from the measured probe deflection, wherein the extraction step employs at least one of a lock-in amplifier and a signal integrator.

17. The method according to claim 16, wherein, At least one of the guiding step and the extraction step is gated during the probe-sample contact time.

18. The method according to claim 17, wherein, At least one of the guiding step and the extracting step is gated in each cycle of the initiating step.

19. The method of claim 16, wherein, The initiation step is performed in PFT mode.

20. The method of claim 16, wherein, The resolution of the sample response is less than 20 nm.

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