Scanning probe microscopy / spectroscopy and scanning probe spectroscopic microscope
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2026-02-03
- Publication Date
- 2026-08-13
Smart Images
Figure JP2026003734_13082026_PF_FP_ABST
Abstract
Description
Scanning probe microscopy / spectroscopy and scanning probe spectroscopy
[0001] This invention relates to scanning probe microscopy / spectroscopy and scanning probe spectroscopic microscopy.
[0002] Scanning probe microscopes, such as scanning tunneling microscopes (STM) and atomic force microscopes (AFM), have been able to reveal the arrangement and electronic state of various molecules on a sample surface with atomic and molecular-level spatial resolution. However, identifying the composition of unknown materials using STM or AFM is difficult. On the other hand, vibrational spectroscopy, such as infrared spectroscopy and Raman spectroscopy, which utilize the resonance of molecular and atomic group vibrations with respect to light, has accumulated a large amount of vibrational spectral data over many years, and vibrational spectroscopy has proven to be extremely powerful for sample identification and molecular structure analysis. However, vibrational spectroscopy requires a considerable amount of sample, and the spatial resolution is limited to about 3-10 μm due to the diffraction limit of light.
[0003] In recent years, probe-enhanced Raman spectroscopy (TERS), which combines the advantages of AFM and near-field optical microscopy (SNOM), has been developed. TERS utilizes Raman scattering enhancement due to the near-field effect in a local region at the probe tip, and has a spatial resolution on the order of 10 nm, exceeding the diffraction limit of infrared light. Furthermore, AFM-IR, which combines AFM and infrared spectroscopy (IR), achieves a resolution on the order of 10 nm by capturing local thermal expansion due to infrared absorption of sample molecules and atomic groups localized at the nanoscale using AFM. For example, Patent Document 1 discloses such AFM-IR technology.
[0004] International Publication No. 2018 / 080868
[0005] There is a further need to obtain spectral information with a spatial resolution at the atomic or molecular level that is higher than that of the TERS or AFM-IR mentioned above. The present invention aims to provide a scanning probe spectroscopic microscope and scanning probe microscopy / spectroscopy method that can obtain spectral information, i.e., elemental composition (information), simultaneously with sample surface shape information with spatial resolution at the atomic or molecular level.
[0006] According to one aspect of the present invention, a scanning probe spectroscopic microscope includes: an actuator drive circuit configured to drive an actuator that vibrates a vibrator equipped with a probe in the height direction of a sample; a vibration detector configured to detect the vibration of the vibrator; a light irradiator configured to irradiate the sample with light; an excitation signal generation unit configured to output an excitation signal generated so that the vibrator vibrates at its resonant frequency and constant amplitude to the actuator drive circuit; a light source control unit configured to control the operation of the light irradiator so that the light flashes in synchronization with the upward or downward movement of the vibrator due to vibration; a frequency change detection unit configured to detect frequency changes in the vibration of the vibrator based on the detection result of the vibration detector; and a dissipated energy calculation unit configured to calculate the dissipated energy in the vibration of the vibrator based on the operation of the excitation signal generation unit and to obtain spectral information caused by the irradiation of the light at the position where the frequency change was detected.
[0007] According to the present invention, a scanning probe spectroscopic microscope and a scanning probe microscopy / spectroscopy method can be provided that can obtain spectral information with spatial resolution at the atomic or molecular level.
[0008] Figure 1 is a schematic diagram of an example configuration of a scanning probe spectrometer according to one embodiment. Figure 2 is an example of a time chart relating to probe vibration and infrared light flickering. Figure 3 is a schematic diagram showing polarization molecules (atomic groups) arranged vertically and periodically on the sample surface and the probe. Figure 4 is a schematic diagram of an example of a force-distance curve showing the relationship between the electrostatic attraction between the probe and the sample and the distance between the probe and the sample when infrared light irradiation is turned off when the oscillator is descending and turned on when the oscillator is ascending. Figure 5 is a diagram showing the IR spectrum of muscovite. Figure 6 is a diagram showing an example of an image that can be obtained with a scanning probe spectrometer. Figure 7 is a diagram showing an example of a force sensor that can be used with a scanning probe spectrometer.
[0009] One embodiment will be described with reference to the drawings. This embodiment relates to a scanning probe spectroscopic microscope and a scanning probe microscopy / spectroscopy method. The scanning probe spectroscopic microscope according to this embodiment is a frequency-modulated atomic force microscope (FM-AFM) with atomic resolution, equipped with spectroscopic functions such as infrared spectroscopy. The scanning probe spectroscopic microscope according to this embodiment uses a method that can also be called tip-oscillation synchronized detection of infrared spectroscopy (TS-IR), in which pulsed light, such as infrared light, is irradiated onto the sample in synchronization with the oscillation period of the AFM probe to exhibit spectroscopic functions. The scanning probe spectroscopic microscope according to this embodiment utilizes the dissipation of the mechanical energy of the oscillator's vibration caused by the change in electrostatic force acting between the AFM probe and the sample when pulsed light is flashed. According to the scanning probe spectroscopic microscope according to this embodiment, molecules and bonding groups (atomic groups) on the sample surface can be identified with atomic-level spatial resolution.
[0010] [Configuration of Scanning Probe Spectroscopy Microscope] The general configuration of the scanning probe spectroscopy microscope according to this embodiment will be described below. Figure 1 is a schematic diagram of an example configuration of the scanning probe spectroscopy microscope 1 according to this embodiment. The scanning probe spectroscopy microscope 1 according to this embodiment is based on technology related to frequency-modulated atomic force microscopy (FM-AFM).
[0011] The FM-AFM-based scanning probe spectrometer 1 excites a transducer 80 equipped with a sharp probe 82 at its resonant frequency. The transducer 80 vibrates in the height direction of the sample 90. The scanning probe spectrometer 1 obtains a shape image of the sample based on the change in resonant frequency due to the conservative interaction force between the sharp probe 82 at the tip of the transducer 80 and the sample 90. That is, when a force acts between the probe 82 of the transducer 80 and the sample 90, the resonant frequency f changes. Specifically, when an attractive force acts, the resonant frequency f decreases, and when a repulsive force acts, the resonant frequency f increases. The change in resonant frequency (Δf) mainly reflects the distance derivative of the conservative force acting between the probe 82 and the sample 90. In the scanning probe spectrometer 1, the distance between the transducer 80 and the sample 90, i.e., the height position of the sample 90, can also be adjusted based on the change in resonant frequency (Δf). The scanning probe spectrometer 1 can use this information to obtain surface shape information of the sample 90. Generally, FM-AFM is considered to be capable of detecting force with higher sensitivity than amplitude-modulated atomic force microscopes (AM-AFM) that perform measurements based on changes in the vibration amplitude of the oscillator 80.
[0012] Furthermore, the FM-AFM-based scanning probe spectroscopic microscope 1 can simultaneously detect the mechanical energy dissipated by non-conservative interaction forces during the above-described measurement. This dissipated energy can be obtained based on an excitation signal used to keep the amplitude of the oscillator 80 vibration constant. Generally, the main sources of dissipated energy detected by FM-AFM may be atomic displacement accompanied by hysteresis when the probe and the sample are in extremely close proximity, adhesion, Joule heating due to displacement currents associated with the oscillator vibration, etc. The scanning probe spectroscopic microscope 1 of this embodiment irradiates the sample 90 with light, such as infrared light, by flashing it in synchronization with the vibration of the probe 82. The scanning probe spectroscopic microscope 1 obtains the change in polarization of molecules and atomic groups on the surface of the sample 90 at this time as the value of the dissipated energy described above, and acquires a spectral image.
[0013] The scanning probe spectroscopic microscope 1 includes an actuator 12 that holds the transducer 80 at the end opposite to the probe 82 and excites the transducer 80. The actuator 12 is, for example, a piezo actuator. The scanning probe spectroscopic microscope 1 includes an actuator drive circuit 14 that operates the actuator 12. In this embodiment, the vibration direction of the transducer 80 is the Z-axis direction.
[0014] Furthermore, the scanning probe spectroscopic microscope 1 includes a vibration detector 20 configured to detect the displacement of the transducer 80, that is, the vibration of the transducer 80. The vibration detector 20 detects the displacement of the side of the transducer 80 on which the probe 82 is attached, for example, using an optical lever method. In this case, the vibration detector 20 includes, for example, a laser light source 21, a photodetector 22, various optical elements, an amplifier, etc. That is, the laser light source 21 irradiates the transducer 80 with laser light, and the photodetector 22 detects the displacement of the laser light reflected by the transducer 80.
[0015] Alternatively, the vibration detector 20 may be as follows: for example, a piezoresistive cantilever may be used for the oscillator 80. In this case, the warping of the oscillator 80 can be measured electrically. The vibration detector 20 may be an electrical circuit that detects the warping of such an oscillator 80.
[0016] Furthermore, the scanning probe spectroscopic microscope 1 includes a scanner 16 that changes the relative position between the transducer 80 and the sample 90. The scanner 16 displaces the relative position between the transducer 80 and the sample 90 in the Z-axis direction, and in the X-axis and Y-axis directions which are mutually orthogonal to the Z-axis. In Figure 1, the scanner 16 is depicted as holding the sample 90 and displacing the sample 90. However, it is not limited to this configuration. The sample 90 may be fixed, and the scanner 16 may displace the transducer 80 in the X-axis, Y-axis, and Z-axis directions. The scanner 16 may, for example, have a piezo actuator that operates in each axial direction. The scanning probe spectroscopic microscope 1 includes a scanner drive circuit 18 that operates the scanner 16.
[0017] The position of the scanner 16 in the Z-axis direction may be feedback-controlled based on the change in the resonant frequency (Δf) of the vibration of the oscillator 80 described above, so that the change in the resonant frequency (Δf) remains constant. In this case, the displacement of the scanner 16 in the Z-axis direction represents the height of the sample 90 in the Z-axis direction. The scanner 16 also changes its position in the XY direction to change the planar position of the sample 90 under the probe 82. In this way, the scanning probe spectroscopic microscope 1 can acquire information on each part of the sample 90 in the XY direction, i.e., in the planar direction.
[0018] Furthermore, the scanning probe spectroscopic microscope 1 includes a light irradiator 30 configured to irradiate the sample 90 with light. The irradiated light is not limited to this, but is typically infrared light corresponding to the energy of molecular vibrations. The light irradiator 30 includes, for example, a tunable laser light source, a light source driving circuit, various optical elements, etc. The light irradiator 30 is configured to irradiate the sample 90 directly below the probe 82 with irradiation light 39. The irradiation diameter of the irradiation light 39 may be, for example, about 10 μm. For the infrared light source, for example, a high-brightness pulsed quantum cascade laser (QCL) can be used. A QCL is a type of semiconductor laser. A QCL is 1 cm －１ Because it can perform high wavenumber sweeping at high wavenumber resolution, it is suitable for scanning probe spectroscopic microscopes 1.
[0019] Furthermore, the scanning probe spectrometer 1 is equipped with a control device 40 that controls the operation of each part of the scanning probe spectrometer 1 and performs various calculations. The control device 40 includes, for example, a computer that includes various integrated circuits.
[0020] The control device 40 has functions such as an excitation signal generation unit 42, a scanner control unit 44, a light source control unit 48, a vibration signal acquisition unit 52, a phase shifter 53, an amplitude calculation unit 54, a frequency change detection unit 55, a shape image generation unit 56, a dissipated energy calculation unit 57, and a spectral image generation unit 58.
[0021] The excitation signal generation unit 42 generates an excitation signal, such as a sine wave, to cause the vibrator 80 to vibrate at its resonant frequency. The excitation signal generation unit 42 outputs an excitation signal generated so that the vibrator 80 vibrates at its resonant frequency and with a constant amplitude. The excitation signal generation unit 42 outputs the excitation signal to the actuator drive circuit 14. The actuator drive circuit 14 operates the actuator 12 based on this excitation signal to vibrate the vibrator 80.
[0022] The scanner control unit 44 generates control signals to control the operation of the scanner 16 and outputs them to the scanner drive circuit 18. The scanner drive circuit 18 operates the scanner 16 based on these control signals. The scanner control unit 44 includes a Z control unit 45 that controls the movement of the scanner 16 in the Z-axis direction and an XY control unit 46 that controls the movement of the scanner 16 in the XY directions. The XY control unit 46 outputs information related to the operation of the scanner 16 to the shape image generation unit 56 and the spectral image generation unit 58. Based on this information, the position of the probe 82 on the sample 90 that is being measured at that time can be determined.
[0023] The light source control unit 48 controls the operation of the light irradiator 30. Specifically, the light source control unit 48 controls the operation of the light irradiator 30 in relation to the wavelength and intensity of the irradiated light, and the timing of the blinking to be on or off in synchronization with the vibration of the oscillator 80.
[0024] The vibration signal acquisition unit 52 acquires a signal related to the vibration of the vibrator 80 detected by the vibration detector 20. The vibration signal acquisition unit 52 outputs the acquired signal to the light source control unit 48. The light source control unit 48 causes the light irradiator 30 to flash light onto the sample 90 in synchronization with the vibration of the vibrator 80.
[0025] Furthermore, the vibration signal acquisition unit 52 inputs the acquired signal to the excitation signal generation unit 42 via the phase shifter 53. The phase shifter 53 advances the phase of the vibration signal by, for example, +π / 2 in order to efficiently excite the vibrator 80. The amount of phase shift of the phase shifter 53 may be any angle. For example, the amount of phase shift that causes the vibrator 80 to vibrate most efficiently may be determined in the initial stage of operation. The vibration signal acquisition unit 52 also inputs the acquired signal to the amplitude calculation unit 54. The amplitude calculation unit 54 outputs a voltage value corresponding to the amplitude of the vibrator 80 to the excitation signal generation unit 42. The excitation signal generation unit 42 determines the excitation signal, for example, by automatic gain control (AGC), in order to keep the vibration amplitude of the vibrator 80 constant.
[0026] Furthermore, the vibration signal acquisition unit 52 outputs the acquired signal to the frequency change detection unit 55. Based on the signal acquired by the vibration signal acquisition unit 52, the frequency change detection unit 55 detects changes in the vibration frequency of the vibrator 80, for example, using a phase-locked loop (PLL). The frequency change detection unit 55 outputs a signal to the Z control unit 45 to adjust the height position of the sample 90 so that this frequency change becomes a separately defined value.
[0027] By scanning the sample 90 in a planar direction with the probe 82 using the scanner 16, frequency changes in each part of the sample 90 are identified. In this mode, also called constant frequency shift mode measurement, the frequency change detection unit 55 outputs a signal to the Z control unit 45 to adjust the height position of the sample 90 so that the frequency change becomes a constant value in each part of the sample 90. The shape image generation unit 56 generates a shape image representing the surface shape of the sample 90, such as irregularities, based on the height of the sample 90 in the Z direction based on the frequency change identified by the frequency change detection unit 55 and the information obtained from the scanner control unit 44 regarding the planar position of the probe 82 on the sample 90 that is being measured. With the FM-AFM-based scanning probe spectroscopic microscope 1, a shape image with atomic-level resolution can be obtained.
[0028] Alternatively, in a mode also called constant height mode, the probe 82 scans the sample 90 at a constant height in a planar direction using the scanner 16, thereby identifying frequency changes in each part of the sample 90. The shape image generation unit 56 can also generate a frequency change image of the surface of the sample 90 based on the frequency change value output from the frequency change detection unit 55 and information obtained from the scanner control unit 44 regarding the planar position of the probe 82 on the sample 90 that is being measured. In the following description, this frequency change image will also be described as a type of shape image.
[0029] The dissipated energy calculation unit 57 calculates the energy supplied to the oscillator 80 to vibrate at a constant amplitude, based on the excitation signal determined by the excitation signal generation unit 42. In other words, it calculates the energy dissipated by the oscillator 80, including the energy dissipated by the interaction between the sample 90 and the probe 82. The spectral image generation unit 58 generates a dissipated energy image of the sample 90 based on the dissipated energy calculated by the dissipated energy calculation unit 57 and information obtained from the scanner control unit 44 regarding the planar position of the probe 82 on the sample 90 being measured. In measurements performed while light is flashed by the light irradiator 30 in synchronization with the vibration of the oscillator 80, the vibration of molecules or atomic groups near the surface of the sample 90 is resonantly excited by the irradiation of light of a specific wavenumber, causing a change in the polarization of those molecules or atomic groups. The interaction force between the sample 90 and the probe 82 changes when light is irradiated and when it is not. This causes hysteresis in the relationship between the distance between the sample 90 and the probe 82 and the interaction force during one period of vibration (up and down) of the oscillator 80. The area enclosed by this hysteresis curve (which has the dimension of energy) can be captured as the dissipated energy value. Therefore, in measurements performed while light is flashing from the light irradiator 30 in synchronization with the vibration of the oscillator 80, a spectral image of the sample 90 is obtained as this dissipated energy image. With the FM-AFM-based scanning probe spectroscopic microscope 1, a spectral image with atomic-level resolution can be obtained. Furthermore, the shape image and spectral image described above can be obtained simultaneously, and the correspondence between the shape image and the spectral image can also be obtained.
[0030] In addition, although not shown in the figures, the scanning probe spectroscopic microscope 1 may have the following configuration. That is, the chamber in which the sample 90 and the oscillator 80 etc. are placed may be configured to achieve ultra-high vacuum (UHV). Furthermore, this chamber may be configured so that measurements can be performed in any environment from ultra-high vacuum to atmospheric pressure. The chamber may be irradiated with light such as infrared light, using a material with high transmittance of infrared light, such as zinc selenide (ZnSe) or barium fluoride (BaF ２ Windows or the like may be provided. To suppress the influence of thermal expansion during measurement on the measurement, it is preferable that the device mechanism involved in the measurement be highly rigid and symmetrical. When measuring under high humidity conditions, noise due to leakage current around the actuator 12 is likely to occur. It is preferable that exposed parts of the device's electrodes and wiring are treated to prevent moisture.
[0031] [Principle of acquiring spectral images] The principle of acquiring spectral images using the scanning probe spectroscopic microscope 1 of this embodiment will be explained. The scanning probe spectroscopic microscope 1 flashes, for example, infrared light in synchronization with the vibration of the oscillator while performing frequency-modulated atomic force microscopy (FM-AFM) image observation. More specifically, infrared light is irradiated onto the oscillator undergoing simple harmonic motion with a phase shift of 90° and switched on and off.
[0032] The scanning probe spectroscopic microscope 1 excites the oscillator 80 at its resonant frequency (f) and measures the change in resonant frequency (Δf) caused by the interaction (conservative force) between the probe 82 and the sample 90 to obtain a shape image of the sample 90. Simultaneously, it measures the change in amplitude of the excitation signal used to keep the oscillation amplitude of the oscillator 80 constant, and detects the change in energy dissipated due to the generation of non-conservative forces between the probe 82 and the sample 90 caused by the oscillation of the oscillator 80, surface atomic transitions, excitation of lattice vibrations, etc. Solving the equation of motion for the oscillation of the oscillator 80 reveals that the change in resonant frequency (Δf) caused by the interaction (conservative force) between the probe 82 and the sample 90 is obtained by the Fourier cosine integral of the interaction force with respect to the oscillation, and the change in dissipated energy is obtained by the Fourier sinine integral, and both are obtained as independent measured quantities.
[0033] Figure 2 is an example of a time chart related to the vibration of the probe 82 and the flickering of infrared light. The topmost figure represents the vibration of the vibrator 80. The vibrator 80 vibrates at a frequency of 10 - 1000 kHz. The upper side shows the state where the probe 82 of the vibrator 80 rises and moves away from the sample 90, and the lower side shows the state where the probe 82 descends and approaches the sample 90. The second figure from the top shows the on or off state of the infrared light synchronized with the vibration of the vibrator 80. In this example, when the vibrator 80 is descending, the infrared light irradiation is turned off, and when the vibrator 80 is ascending, the infrared light irradiation is turned on.
[0034] Figure 3 is a diagram schematically showing polarized molecules periodically arranged vertically on the sample surface and the probe 82. There is a local electric field in the vicinity of the polarized molecules. This electric field appears uniform when viewed from a distance, but changes rapidly when viewed closely. From electrostatics, a long-range force inversely proportional to the cube of the distance acts between a point charge and a single dipole. When the molecules are arranged with a period d, the electric field lines are confined between adjacent polarized molecules, and the electric field change becomes large in the vicinity. When the point charge of the probe 82 approaches closer than the period d, it can be seen that using the Poisson sum formula, the electrostatic force becomes a short-range force represented by an exponential function with an attenuation length of d / 2π. In FM-AFM, the closest distance between the vibrating probe 82 and the sample 90 can be approximated to a few Å, so by capturing the spatial change of this electrostatic force, a spatial resolution at the molecular level can be achieved.
[0035] Molecules or atomic groups on the surface of the sample 90 resonate at a specific wave number to excite the vibration of the molecules or atomic groups, and their polarization changes. For example, when irradiated with infrared light at a wave number resonant with the vibration of the target molecule or atomic group, as shown in the third figure from the top of Figure 2, its polarization increases, and the local electric field in the vicinity of the molecule or atomic group becomes stronger.
[0036] This change in the electric field changes the electrostatic force exerted on the probe 82 (approximated by a point charge in FIG. 3). That is, as shown in the fourth figure from the top in FIG. 2, the electrostatic attraction acting between the probe 82 and the sample 90 becomes stronger only during the time when infrared light is irradiated. In the measurement by the scanning probe spectroscope 1, the change in the electrostatic force acting between the probe 82 and the sample 90 when the infrared light is irradiated and when it is not irradiated causes a change in the mechanical vibration energy of the oscillator. As shown in FIG. 2, the electrostatic attraction becomes stronger while the probe is rising, and if the electrostatic attraction becomes weaker while the probe is descending than when it is rising, energy is dissipated. Conversely, if infrared light is irradiated while the probe is descending to strengthen the electrostatic attraction, and the electrostatic attraction is weaker while the probe is rising than when it is descending, the mechanical vibration energy of the oscillator increases (the dissipated energy value has a negative value). The scanning probe spectroscope 1 quantitatively and highly sensitively detects this dissipated energy value.
[0037] Thus, due to the blinking of the infrared light synchronized with the vibration of the oscillator 80, a difference occurs in the electrostatic attraction acting between the probe 82 and the polarized molecules of the sample 90 between the rising and falling of the vibration of the probe 82. FIG. 4 shows an example of a schematic of a force-distance curve showing the relationship between the electrostatic attraction between the probe 82 and the sample 90 and the distance between the probe 82 and the sample 90 when the infrared light irradiation is turned off when the oscillator 80 is descending and the infrared light irradiation is turned on when the oscillator 80 is rising, as shown in FIG. 2.
[0038] The difference in the electrostatic attraction acting between the probe 82 and the polarized molecules or atomic groups of the sample 90 between the rising and falling of the vibration of the probe 82 is slight, but as shown in FIG. 4, it appears as hysteresis in the force-distance curve. The cyclic integral of the probe vibration (the area of the lightly shaded part in FIG. 4, which is the difference in force × the total amplitude of the probe vibration) of this difference (hysteresis) repeated every vibration cycle represents the amount of mechanical dissipated energy of the vibration of the oscillator 80 due to the interaction with the polarized molecules or atomic groups. A high S / N ratio is obtained by this integration process, and the background signal is also removed. If the vibration amplitude of the oscillator 80 is controlled by a self-excited oscillation circuit so as to be constant, the change in the excitation electrical signal input from the circuit to the oscillator 80 is proportional to the change in this dissipated energy.
[0039] By fixing the wavenumber of the infrared light and obtaining a shape image using FM-AFM, while simultaneously mapping the change in dissipated energy in two dimensions, a spectral image can be obtained in which the values change at the locations of molecules or atomic groups that resonate with the irradiated infrared light. By stopping the scanning of the probe 82 at each observation point while acquiring a shape image using FM-AFM and sweeping the wavenumber of the infrared light, intensity (spectral) peaks of the dissipated energy values appear at the wavenumbers that resonate with the infrared light for individual molecules or atomic groups on the surface of the sample 90. These intensity spectra of dissipated energy values can be obtained as different spectra with atomic-level spatial resolution.
[0040] In the example above, the infrared light was shown to turn on when the probe 82 moved away from the sample 90 and turn off when it approached, but it may also turn on when approaching and turn off when moving away. Furthermore, the pulse width of the infrared light that turns on when the probe 82 moves away from the sample 90 may be shorter than the time it takes to move away (half the period of the oscillator 80). Also, the state in which the infrared light is on may be a repetition of on and off of short-pulse infrared light pulses.
[0041] [Operation of Scanning Probe Spectroscopy] The general operation of the scanning probe spectroscopy microscope 1 according to this embodiment will be described below. The measurement environment for the scanning probe spectroscopy microscope 1 can range from ultra-high vacuum to atmospheric pressure gas atmosphere. The transducer 80 is installed on the actuator 12, the sample 90 is installed on the scanner 16, and the measurement of the sample 90 by the scanning probe spectroscopy microscope 1 is started.
[0042] The excitation signal generation unit 42 of the control device 40 outputs an excitation signal, for example a sinusoidal wave, to the actuator drive circuit 14 so as to cause the probe 82 to vibrate at the resonant frequency. Based on the input excitation signal, the actuator drive circuit 14 operates the actuator 12 to cause the vibrator 80 to vibrate at the resonant frequency.
[0043] The displacement of the vibrating transducer 80 is detected by the vibration detector 20. The vibration signal acquisition unit 52 of the control device 40 acquires an output signal from the vibration detector 20 indicating the displacement of the transducer 80. Due to the positive feedback loop of the control device 40, the transducer 80 vibrates at the resonant frequency (self-oscillation). The signal acquired by the vibration signal acquisition unit 52 is input to the excitation signal generation unit 42 via the phase shifter 53, and the phase difference between the vibration of the transducer 80 and the excitation is adjusted to -π / 2 (-90°). The amplitude calculation unit 54 also identifies the voltage value of the signal acquired by the vibration signal acquisition unit 52. This voltage value represents the amplitude value corresponding to the amplitude of the transducer 80. The amplitude calculation unit 54 outputs the identified amplitude value to the excitation signal generation unit 42. When the amplitude value is input to the excitation signal generation unit 42, the excitation signal generation unit 42 adjusts the excitation gain by feedback control, and the vibration amplitude is adjusted to be constant.
[0044] The frequency change detection unit 55 detects changes in the vibration frequency of the vibrator 80 based on the vibration signal related to the vibration of the vibrator 80 acquired from the vibration signal acquisition unit 52. The frequency change detection unit 55 may output a feedback signal to the Z control unit 45 based on this frequency change. Based on these operations, height information of the sample 90 is acquired. The shape image generation unit 56 generates a shape image representing the surface shape of the sample 90 based on this height information and the planar position information used by the XY control unit 46.
[0045] The light source control unit 48, based on the displacement of the vibrator 80 acquired from the vibration signal acquisition unit 52, causes the light irradiator 30 to irradiate the sample 90 with infrared light in synchronization with the vibration of the vibrator 80. For example, the light irradiator 30 does not irradiate the sample 90 with infrared light when the vibrator 80 is descending, and irradiates the sample 90 with infrared light when the vibrator 80 is ascending. The light source control unit 48 may also acquire a trigger signal synchronized with the excitation signal from the excitation signal generation unit 42.
[0046] When molecules or atomic groups on the surface of the sample 90 are excited by the irradiation of infrared light, and the polarization of those molecules or atomic groups changes, the electrostatic attraction between the probe 82 and the sample 90 also changes. Electrostatic attraction is a conservative force. In other words, the electrostatic force between the probe 82 and the sample 90 is the same if the distance between them is the same. However, if infrared light is irradiated only when the oscillator 80 is rising or falling, the electrostatic attraction will be different when the oscillator 80 is rising and falling, even if the distance between the probe 82 and the sample 90 is the same. That is, hysteresis occurs in the force-distance curve between the probe 82 and the sample 90, and the mechanical vibration energy of the oscillator changes. At this time, the excitation signal generation unit 42 adjusts the excitation gain so as to keep the vibration amplitude of the oscillator 80 constant. The dissipated energy calculation unit 57 calculates the dissipated energy based on the amount of vibration energy input due to this gain adjustment. Since the origin of this dissipated energy is the excitation of surface molecules or atomic groups of the sample 90 by the infrared light irradiation described above, spectral information can be obtained based on the dissipated energy. The spectral image generation unit 58 generates a spectral image of the sample 90 based on this spectral information and the planar position information used by the XY control unit 46.
[0047] Furthermore, the difference in the electrostatic attraction between the probe 82 and the polarized molecules or atomic groups of the sample 90 during the upward and downward oscillations of the probe 82 is slight. Therefore, this difference has almost no effect on the shape image generated by the shape image generation unit 56, which reflects the average value of the distance derivative of the force acting between the probe 82 and the sample 90 over the oscillation range of the probe 82.
[0048] By fixing the wavenumber of the infrared light and having the oscillator 80 scan the sample 90, a spectral image corresponding to that wavenumber can be obtained with atomic-level spatial resolution. Furthermore, by fixing the positional relationship between the sample 90 and the oscillator 80 and sweeping the wavenumber of the infrared light, an atomic-level spectral spectrum can be obtained for that position on the sample 90. Through these combinations, spectral information for each position in the planar direction of the sample 90 can be obtained with atomic-level spatial resolution.
[0049] [Measurement Example] The measurement example will be described by taking the identification of Si-O-Si and Si-O-Al on the mica cleavage surface as an example. The cleavage layer material mica (muscovite, KAl ２ (AlSi ３ )O １０ (OH) ２ ) has a (001) cleavage surface which is a SiO ２ tetrahedral sheet, and SiO with a tetrahedral structure ２ is arranged in a honeycomb pattern. One out of four Si atoms of the SiO ２ is replaced by Al. The positions of the replaced Al are random, and pairs of Si-O-Si and Si-O-Al irregularly exist on the surface.
[0050] Let a QCL with variable wave number in the range of about 900 to 1500 cm -1 be used as the infrared light source. Synchronously turn on and off the infrared light while observing the FM-AFM image (shape image). Fig. 5 shows the FT-IR spectrum of bulk muscovite published in the database. The arrows indicate the stretching vibrations of Si-O-Al (988 cm -1 ) and Si-O-Si (1023 cm -1 ). If the probe 82 is scanned with the wave number of the pulsed irradiation light fixed at 1023 cm -1 , an image of the dissipation energy related to Si-O-Si (spectroscopic image) can be obtained simultaneously with the FM-AFM image (shape image). If the probe 82 is scanned with the wave number of the pulsed irradiation light fixed at 988 cm -1 , an image of the dissipation energy related to Si-O-Al (spectroscopic image) can be obtained simultaneously with the FM-AFM image (shape image).
[0051] Fig. 6 shows images and the like that can be acquired by the scanning probe spectroscope 1. (a) shows an example of the shape image obtained by FM-AFM. (b) is a figure obtained by superimposing the structure model of the mica cleavage (001) surface on the figure of (a). In the structure model, large circles indicate Si atoms or Al atoms, and small circles indicate O atoms. The mica cleavage surface is SiO ４The sheet is arranged in a distorted tetrahedron honeycomb structure, with two Si atoms sharing one O atom. One-quarter of the Si sites are replaced by Al. The Al atoms are not adjacent to each other and are arranged irregularly. In the shape image obtained by FM-AFM shown in (a), it is impossible to distinguish between Si and Al.
[0052] (c) is 1023 cm -1 This is a predicted spectral image obtained when infrared light is flashed in synchronization with the vibration of the oscillator 80. When infrared light is absorbed, the polarization increases, and the electrostatic attraction between the probe 82 and the sample 90 becomes stronger. 1023 cm -1 Light is absorbed by the Si-O-Si structure, and that region is obtained as a dissipation energy image; in other words, a spectral image is obtained. In this figure, the large circles represent Si atoms, and the small circles represent O atoms.
[0053] (d) is 988 cm -1 This is a predicted spectral image obtained by flashing infrared light. When infrared light is absorbed, the polarization increases, and the electrostatic attraction between the probe 82 and the sample 90 becomes stronger. 988 cm -1 The light is absorbed by the Si-O-Al structure, and that region is obtained as a dissipation energy image. In other words, a spectral image is obtained. In this figure, the large circles represent Al atoms, and the small circles represent O atoms.
[0054] In this way, by simultaneously acquiring FM-AFM images (shape images) and energy dissipation images (spectroscopic images) while flashing light of a specific wave frequency synchronized with the probe vibration, it is possible to identify the location of Al, which could not be determined from the FM-AFM image (shape image) alone.
[0055] By sweeping the wavenumber of infrared light and measuring the dissipated energy without scanning the probe 82, an IR spectrum like the one in Figure 5 can be obtained. Here, depending on the position, there is a 3:1 probability that a Si-O-Si spectrum of 1023 cm⁻¹ is obtained. -1 A sharp peak is measured, or a 988 cm⁻¹ point indicating Si-O-Al is observed. -1 A sharp peak will be measured.
[0056] The measurement may be performed, for example, as follows: UHV(10 -10After preparing a clean sample in a Torr stage, high-purity gas at atmospheric pressure and water vapor are introduced to set the humidity range to 0-80%RH. Measurements can then be performed under these conditions.
[0057] In experiments conducted at room temperature, atmospheric pressure, and high humidity, the time-dependent change (drift) of the relative position between the probe 82 and the sample 90 is larger compared to experiments conducted at low temperatures and under UHV (ultra-high heat). Furthermore, the sample 90 may undergo thermal expansion due to irradiation with infrared light. This thermal expansion of the sample 90 is on a micrometer scale and can be distinguished from local polarization changes on an atomic scale. The thermal expansion of the sample 90 can also be an important experimental indicator. To identify this, it is preferable to make the apparatus mechanism highly rigid and symmetrical in order to suppress drift caused by the apparatus mechanism.
[0058] [About the Scanning Probe Spectroscopy Microscope] The scanning probe spectroscopy microscope 1 according to this embodiment can obtain spectral information such as infrared spectroscopy of the sample surface with atomic-level spatial resolution. Furthermore, with this scanning probe spectroscopy microscope 1, measurements can be performed in various environments, that is, from ultra-high vacuum to atmospheric pressure, and not only at extremely low temperatures but also at room temperature and other temperatures.
[0059] One of the important challenges common to a wide range of materials science, regardless of whether they involve inorganic, organic, or biomaterials, is to elucidate which types of atoms and molecules are arranged where and in what state on a solid surface, which is also the site of chemical reactions. The scanning probe spectrometer 1 according to this embodiment makes it possible to elucidate which types of atoms and molecules are arranged where and in what state. Compared to conventional techniques, the scanning probe spectrometer 1 according to this embodiment dramatically improves the spatial resolution of spectroscopic methods on solid surfaces.
[0060] The scanning probe spectroscopic microscope 1 of this embodiment can measure the sample 90 in an ultra-high vacuum, in air, or in other environments.
[0061] Conventionally, a method is known for obtaining spectral information using amplitude-modulated AFM (AM-AFM) based on differences in AM-AFM images that can change simply by the presence or absence of infrared light irradiation. In this method, the surface shape deformed by infrared light irradiation is acquired by AM-AFM. Unlike this method, the scanning probe spectroscopic microscope 1 of this embodiment changes the presence or absence of infrared light irradiation in synchronization with the vibration of the oscillator 80, and detects the resulting hysteresis between the forward and return paths of the electrostatic attraction between the probe 82 and the sample 90 as dissipated energy with high sensitivity and high resolution. As a result, while conventional methods have a resolution of, for example, about 10 nm, the scanning probe spectroscopic microscope 1 of this embodiment can obtain atomic-level resolution. Furthermore, much higher sensitivity can be obtained compared to conventional methods.
[0062] [Regarding the vibrator] The scanning probe spectroscopic microscope 1 of this embodiment may use a vibrator 80 having a high Q value. With a vibrator 80 having a high Q value, spectral information obtained by infrared light irradiation can be obtained with high sensitivity.
[0063] Figure 7 shows an example of a high-Q oscillator 80 that can be used in the scanning probe spectroscopic microscope 1 of this embodiment. This example of an oscillator, a force sensor 85, functions as both an oscillator 80 and an actuator 12 in the scanning probe spectroscopic microscope 1. The force sensor 85 is a tuning fork-type quartz oscillator having a first arm 86 and a second arm 87. A probe 82 is provided at the end of the first arm 86. In the force sensor 85, the natural frequencies of the first arm 86 and the second arm 87, including the probe 82, are matched.
[0064] An example of a method for manufacturing the force sensor 85 shown in Figure 7 will be described. This force sensor 85 can also be called a resonant retuning force sensor. That is, a tuning fork type quartz crystal oscillator can be used to manufacture the force sensor 85. The tip of the first arm 86 of the tuning fork type quartz crystal oscillator is filed down. Then, a probe 82 is formed on the tip of the first arm 86. The weights of the first arm 86 and the second arm 87 are adjusted so that their natural frequencies match.
[0065] When the force sensor 85 is in use, an AC voltage is applied to one of the two electrodes formed on the quartz crystal oscillator, and the first arm 86 and the second arm 87 are excited in opposite phases using the inverse piezoelectric effect. In this way, the force sensor 85 functions as an actuator 12, and the first arm 86, which is equipped with a probe 82, functions as the oscillator 80 described above. A current amplifier is connected to the other electrode, and the displacement of the first arm 86 and the second arm 87 is detected from the output due to the piezoelectric effect. That is, the force sensor 85 is connected to a vibration detector 20 which has a current amplifier.
[0066] With a force sensor 85 having such a configuration, good vibration characteristics (high Q value) can be obtained with low loss due to the characteristics of the tuning fork structure.
[0067] 1: Scanning probe spectroscopic microscope 12: Actuator, 14: Actuator drive circuit, 16: Scanner, 18: Scanner drive circuit, 20: Vibration detector, 21: Laser light source, 22: Photodetector, 30: Light irradiator 40: Control device, 42: Excitation signal generation unit, 44: Scanner control unit, 45: Z control unit, 46: XY control unit, 48: Light source control unit, 52: Vibration signal acquisition unit, 53: Phase shifter, 54: Amplitude calculation unit, 55: Frequency change detection unit, 56: Shape image generation unit, 57: Dissipated energy calculation unit, 58: Spectroscopic image generation unit 80: Oscillator, 82: Probe, 85: Force sensor, 86: First arm, 87: Second arm 90: Sample
Claims
1. A scanning probe spectroscopic microscope comprising: an actuator drive circuit configured to drive an actuator that vibrates a vibrator equipped with a probe in the height direction of a sample; a vibration detector configured to detect the vibration of the vibrator; a light irradiator configured to irradiate the sample with light; an excitation signal generation unit configured to output an excitation signal generated so that the vibrator vibrates at its resonant frequency and constant amplitude to the actuator drive circuit; a light source control unit configured to control the operation of the light irradiator so that the light flashes in synchronization with the upward or downward movement of the vibrator due to vibration; a frequency change detection unit configured to detect frequency changes in the vibration of the vibrator based on the detection result of the vibration detector; and a dissipated energy calculation unit configured to calculate the dissipated energy in the vibration of the vibrator based on the operation of the excitation signal generation unit and to obtain spectral information caused by the irradiation of the light at the position where the frequency change was detected.
2. The scanning probe spectroscopic microscope according to claim 1, wherein the light source control unit is configured to control the operation of the light irradiator such that the light is irradiated to the oscillator during either its upward or downward movement, but not during the other movement.
3. The scanning probe spectroscopic microscope according to claim 1 or 2, further comprising: a scanner for changing the relative position between the sample and the oscillator; a scanner control unit configured to scan the relative position between the sample and the oscillator in a plane perpendicular to the height direction; a shape image generation unit configured to generate a shape image of the sample based on the frequency change and the scanned position; and a spectral image generation unit configured to generate a spectral image of the sample based on the dissipated energy and the scanned position.
4. The scanning probe spectroscopic microscope according to claim 1 or 2, wherein the light irradiator is configured to irradiate the sample with light while sweeping the wavenumber of the light, and the dissipation energy calculation unit is configured to calculate the dissipation energy for each wavenumber and acquire spectral information.
5. A force sensor used as the oscillator and actuator in a scanning probe spectroscopic microscope according to claim 1 or 2, comprising a tuning fork type quartz oscillator including a first arm and a second arm, and a probe provided at the end of the first arm, wherein the natural frequencies of the first arm including the probe and the second arm are matched.
6. Scanning probe spectroscopic microscopy / spectroscopy method comprising: vibrating a vibrator equipped with a probe in the height direction of a sample at its resonant frequency and constant amplitude; detecting the vibration of the vibrator; flashing a light in synchronization with the upward or downward movement of the vibrator due to the vibration and irradiating the sample with the light; detecting a change in the frequency of the vibrator's vibration based on the detection result of the vibrator's vibration; and calculating the dissipated energy in the vibration of the vibrator based on the operation of vibrating the vibrator, and obtaining spectral information caused by the irradiation of the light at the position where the frequency change was detected.
7. The scanning probe spectroscopic microscopy / spectroscopy method according to claim 6, wherein the irradiation of the light includes irradiating the oscillator with the light while it is rising or falling and not irradiating it with the light while it is falling.
8. Scanning probe spectroscopic microscopy / spectroscopy according to claim 6 or 7, further comprising: scanning the relative position of the sample and the oscillator in a plane perpendicular to the height direction; generating a shape image of the sample based on the frequency change and the scanned position; and generating a spectral image of the sample based on the dissipated energy and the scanned position.
9. Scanning probe spectroscopic microscopy / spectroscopy according to claim 6 or 7, further comprising irradiating the sample with the light while sweeping the wavenumber of the light, and calculating the dissipated energy for each wavenumber and obtaining spectral information.