Characterization method of embedded carbon nanotube carrier properties

Abstract

The invention discloses a characterization method for the property of a carrier embedded with a carbon nano tube. The characterization method comprises the following steps: executing one-time scanning to obtain a terrain scanning line; upwards moving a fixed distance to obtain a secondary scanning line, and executing secondary scanning to obtain a dielectric power signal diagram; the probe voltage applied in the secondary scanning comprises a direct-current voltage and an alternating-current voltage, and a plurality of different voltage values are applied to the direct-current voltage in a positive-negative symmetry manner; and according to the positive and negative symmetry of the dielectric power signal along with the change of the direct current voltage, the carrier characteristics are judged. On the basis of the traditional EFM technology, the cantilever beam can detect the signal change of frequency doubled (2 omega) by simultaneously applying the alternating voltage and the direct voltage during the second scanning, so that an image with higher spatial resolution is obtained, and by applying different direct voltages and obtaining corresponding images, the detection accuracy is improved. The carrier characteristics of the embedded carbon nanotubes can be further analyzed, and more accurate spatialized carrier characteristic characterization of the electrical characteristics under the nanoscale is provided.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of micro-nano manufacturing technology, and particularly relates to a characterization method of embedded carbon nanotube carrier properties. BACKGROUND

[0002] Single-walled carbon nanotubes (SWCNTs) are nanoscale tubular structures formed by rolling a single layer of graphene. Due to their outstanding electrical, mechanical, and thermal properties, SWCNTs occupy an important position in the field of nanotechnology. SWCNTs have extremely high electrical and thermal conductivity, as well as excellent strength and flexibility, making them have wide application prospects in electronic devices, energy storage, biomedicine, and composite materials. Their high aspect ratio and quasi-one-dimensional structure endow them with unique physical and chemical properties. These properties can be optimized by controlling the diameter, chirality, and length of carbon nanotubes, and can be further enhanced and regulated by filling other materials inside the carbon nanotubes (i.e. embedding).

[0003] Embedded structures utilize the nanoscale space inside carbon nanotubes to stably encapsulate atoms, molecules, or other nanomaterials, thereby endowing carbon nanotubes with new functions. Such embedded carbon nanotubes have great application potential in quantum computing, nanoelectronic devices, catalytic reactions, and drug delivery. By precisely controlling the type and morphology of the embedded material, the electrical, optical, and magnetic properties of the carbon nanotubes can be adjusted, making them irreplaceable in cutting-edge scientific research and advanced technology development.

[0004] In particular, the electrical properties of carbon nanotubes are greatly dependent on carrier properties. However, existing carrier concentration measurement methods, such as field effect, Hall effect, and capacitance-voltage curve, although highly accurate, cannot achieve nanoscale spatial resolution and can only know the overall carrier properties of the embedded carbon nanotube material itself, and cannot accurately locate and identify the carrier property distribution in the carbon nanotube. Therefore, it is of great significance to realize high spatial resolution characterization of the carrier properties of single embedded carbon nanotubes for in-depth understanding of the physical properties of embedded carbon nanotubes and expanding their applications.

[0005] In order to characterize the carrier properties of single embedded carbon nanotubes, a document Angewandte Chemie International Edition, 2017, 56, 12240-12244 reports a method of electrical testing by preparing single carbon nanotube field effect transistor (FET) devices. The preparation of electrodes uses alternating current dielectrophoresis (AC-DEP) technology, that is, after dispersing carbon nanotubes in tetrachloroethane solution, the electrodes are immersed therein and an alternating current field is applied. By optimizing the voltage, frequency and time parameters, the efficiency of capturing single carbon nanotubes between each pair of electrodes is improved. Subsequently, whether the carbon nanotube successfully bridges the electrodes is confirmed by scanning electron microscope (SEM) imaging. Finally, the I-V curve of single carbon nanotube is measured by this method to verify its carrier properties.

[0006] There are two main problems in this technical solution: first, the contact between the electrode and the carbon nanotube is uncontrollable, which may cause unstable contact resistance and affect the performance of the device; second, the structural defects of the carbon nanotube itself may also have a significant impact on the experimental results. In addition, this method is difficult to realize the accurate characterization of the local electrical properties of the embedded carbon nanotube.

[0007] Another document Carbon, 2010, 48, 3287-3292 introduces a single carbon nanotube metallic and semiconductor testing characterization method based on electrostatic force microscope (EFM). First, single carbon nanotubes are grown on a silicon oxide substrate by chemical vapor deposition (CVD). Then, the two-step scanning mode of EFM is used for characterization: in the first step, the surface topography of the sample is recorded by tapping mode first; in the second step, the probe is lifted to a certain height, and a direct current bias voltage V is applied at the needle tip, and the path is scanned again along the original topography. In the second scanning path, the induced charge and dipole in the SWCNTs interact with the AFM probe, thereby affecting the resonant vibration of the cantilever and recording the phase shift of the cantilever deflection signal as our EFM signal. Because the polarizability of metallic carbon tube is large in EFM, it shows a monotonous V-shaped signal, while the polarizability of semiconductor carbon tube is low and is affected by the substrate dielectric, forming a W-shaped signal with "bump", so as to distinguish metallic and semiconductor carbon nanotubes.

[0008] The traditional electrostatic force microscope (EFM) test mode used in the above technical solution has the problem of low resolution, mainly because the signal collected is only single frequency (ω) signal. In addition, the limitation of traditional EFM technology enables it to distinguish between metallic and semiconductor, but it is difficult to reveal the key properties of single-walled carbon nanotubes (SWCNTs), such as carrier type. SUMMARY

[0009] In view of the deficiencies of the prior art, the present application aims to provide a method for characterizing the carrier properties of embedded carbon nanotubes.

[0010] To achieve the above-mentioned application purposes, the technical solutions adopted by the present application include:

[0011] In the first aspect, the present application provides a method for characterizing the carrier properties of embedded carbon nanotubes, which includes:

[0012] Performing a first scan with the probe of an atomic force microscope to obtain a topographic scan line of the embedded carbon nanotubes;

[0013] Obtaining a second scan line by moving a fixed interval on the topographic scan line, performing a second scan along the second scan line to obtain a dielectric force signal graph; wherein the probe voltage applied on the probe during the second scan includes a direct current voltage and an alternating current voltage, and the direct current voltage is symmetrically applied with multiple different voltage values;

[0014] According to the positive and negative symmetry of the change relationship of the dielectric force signal with respect to the direct current voltage, the carrier properties of the embedded carbon nanotubes are determined.

[0015] Based on the above technical solutions, compared with the prior art, the present application has at least the following beneficial effects:

[0016] The technical solutions provided by the present application, on the basis of the traditional EFM technology, can detect the signal change of double frequency (2ω) by simultaneously applying an alternating current voltage and a direct current voltage during the second scan, thereby obtaining an image with higher spatial resolution. By applying different direct current voltages and obtaining the corresponding images, the carrier properties of the embedded carbon nanotubes can be further analyzed, and a more accurate spatialized carrier property characterization of the electrical properties at the nanoscale can be provided.

[0017] The above description is only a summary of the technical solutions of the present application. In order for those skilled in the art to more clearly understand the technical means of the present application and can implement it according to the content of the description, the following describes the preferred embodiments of the present application with reference to the detailed description of the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a schematic diagram of the working principle of the characterization method provided by a typical embodiment of the present application;

[0019] Figure 2a is a topographic scan line graph of metallic carbon nanotubes provided by a typical embodiment of the present application;

[0020] Figure 2b is a dielectric force signal graph (gate voltage-4V) of metallic carbon nanotubes provided by a typical embodiment of the present application;

[0021] Figure 2c is a dielectric force signal graph of the metallic carbon nanotube provided by a typical embodiment of the present application (gate voltage -2V);

[0022] Figure 2d is a dielectric force signal graph of the metallic carbon nanotube provided by a typical embodiment of the present application (gate voltage 0V);

[0023] Figure 2e is a dielectric force signal graph of the metallic carbon nanotube provided by a typical embodiment of the present application (gate voltage +2V);

[0024] Figure 2f is a dielectric force signal graph of the metallic carbon nanotube provided by a typical embodiment of the present application (gate voltage +4V);

[0025] Figure 3a is a topographic scanning line graph of the p-type semiconductive carbon nanotube provided by a typical embodiment of the present application;

[0026] Figure 3b is a dielectric force signal graph of the p-type semiconductive carbon nanotube provided by a typical embodiment of the present application (gate voltage -4V);

[0027] Figure 3c is a dielectric force signal graph of the p-type semiconductive carbon nanotube provided by a typical embodiment of the present application (gate voltage -2V);

[0028] Figure 3d is a dielectric force signal graph of the p-type semiconductive carbon nanotube provided by a typical embodiment of the present application (gate voltage 0V);

[0029] Figure 3e is a dielectric force signal graph of the p-type semiconductive carbon nanotube provided by a typical embodiment of the present application (gate voltage +2V);

[0030] Figure 3f is a dielectric force signal graph of the p-type semiconductive carbon nanotube provided by a typical embodiment of the present application (gate voltage +4V);

[0031] Figure 4 is a signal intensity change rule test graph of the filled carbon nanotube and the non-filled carbon nanotube provided by a typical embodiment of the present application. DETAILED DESCRIPTION

[0032] In view of the deficiencies in the prior art, the present inventors have long studied and practiced to come up with the technical solution of the present application. The technical solution, its implementation process and principles will be further explained as follows.

[0033] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to one skilled in the art that the present application can be practiced without the specific details set forth in this description.

[0034] The embodiment of the present application provides a characterization method of embedded carbon nanotube carrier properties, which comprises the following steps:

[0035] A probe of an atomic force microscope is used to perform a first scan to obtain a topographic scan line of the embedded carbon nanotube;

[0036] A second scan line is obtained by moving a fixed interval on the topographic scan line, and a second scan is performed along the second scan line to obtain a dielectric force signal diagram; wherein a probe voltage applied to the probe in the second scan includes a direct current voltage and an alternating current voltage, and the direct current voltage is symmetrically applied to a plurality of different voltage values;

[0037] According to the positive and negative symmetry of the change relationship of the dielectric force signal with the direct current voltage, the carrier characteristics of the embedded carbon nanotube are determined.

[0038] Based on the above technical solution, the embodiment of the present application can obtain an image with higher spatial resolution by simultaneously applying an alternating current voltage and a direct current voltage in the second scan, so that the cantilever can detect a double frequency (2ω) signal change. By applying different direct current voltages and obtaining corresponding images, the present application can further analyze the carrier characteristics of the embedded carbon nanotube, and provide more accurate characterization of electrical characteristics at the nanoscale.

[0039] Some existing technologies use DFM technology to measure biological macromolecules, and by additionally adding an alternating current voltage to a direct current voltage, the imaging resolution of biological macromolecules is improved; but the above-mentioned prior art and the present application have obvious differences in principle and technical effects. The fundamental reason why the present application can realize accurate carrier characterization is not the improvement of resolution, but the electrical interaction between the carrier and the probe. The principle can be referred to the specific embodiments described later.

[0040] As for some specific technical details, in some embodiments, the direct current voltage includes a gate voltage and a bias voltage, the bias voltage is a fixed value, and is used to compensate for the contact potential difference between the probe and the substrate; the gate voltage is set to have a plurality of pairs of positive and negative voltage values, and the signs of any pair of positive and negative voltage values are opposite, and the absolute values are equal.

[0041] In some embodiments, the gate voltage is also set to have a zero voltage value.

[0042] In some embodiments, the probe voltage is expressed as a positive voltage with the potential of the probe being higher than the potential of the substrate, as:

[0043] V tip = V g + V cp + V ac sin(ωt)

[0044] wherein V tip represents the probe voltage, V g represents the gate voltage, V cp represents the bias voltage, V ac represents the AC peak voltage, ω represents the angular frequency of the AC (ω = 2πf in terms of frequency f), and t represents time.

[0045] In some embodiments, the dielectric force signal is expressed as:

[0046]

[0047] wherein F2ωrepresents the dielectric force signal, dC / dz represents the capacitance gradient between the tip of the probe and the embedded carbon nanotube sample, A2ωand represent the amplitude and phase, respectively.

[0048] As to how to determine the carrier property of the embedded carbon nanotube, in some embodiments, when the trend of the dielectric force signal with respect to the gate voltage exhibits positive-negative voltage symmetry, the embedded carbon nanotube exhibits metallic property.

[0049] In some embodiments, when the trend of the dielectric force signal with respect to the gate voltage exhibits positive-negative voltage asymmetry, the embedded carbon nanotube exhibits semiconductive property.

[0050] In some embodiments, when the dielectric force signal exhibits cumulative rise in the positive voltage interval of the gate voltage (cumulative rise with respect to the absolute value of the gate voltage, the same below) and flat depletion in the negative voltage interval, the carrier property of the embedded carbon nanotube is p-type.

[0051] In some embodiments, when the dielectric force signal exhibits cumulative rise in the negative voltage interval of the gate voltage and flat depletion in the positive voltage interval, the carrier property of the embedded carbon nanotube is n-type.

[0052] As to the specific parameter interval, in some embodiments, the value interval of the gate voltage is [-4V, 4V], and the value interval of the AC peak voltage is [2.5V, 5V].

[0053] Generally, the larger V ac is, the larger the amplitude A2 ωThe larger, the V ac The larger, the V AC +V DC The value of V AC The value of V

[0054] In some embodiments, the frequency of the AC voltage is 40-60 kHz, usually set at 50 kHz, and the specific value can be set by the tester, which is slightly smaller than half of the frequency of the vibration of the needle in the air, for example, between 40-50% of the frequency of the needle in the air.

[0055] In some embodiments, the fixed distance is 5-200 nm, more preferably 10-100 nm. The lifting height is set by the tester, which is related to the height of the sample surface topography and needs to be estimated. The lifting distance is usually in the range of greater than 0 nm to 200 nm, and too low may cause the needle to hit in the second scan due to the amplitude, and too high may result in a weak measurement signal and low signal-to-noise ratio. Therefore, whether the parameter value of the distance is reasonably set directly affects the imaging quality and the reliability of the characterization results.

[0056] The technical solutions of the present application are further described in detail below through several embodiments in combination with the accompanying drawings. However, the selected embodiments are only used to illustrate the present application, and do not limit the scope of the present application.

[0057] Example 1

[0058] This example illustrates the characterization experiment of the carrier properties of a single embedded carbon nanotube, as shown below:

[0059] Figure 1 This example is a schematic diagram of the working principle of EFM and DFM. The sample is imaged by a two-step scanning imaging method. In the first scan, standard AC mode AFM imaging is performed to obtain the terrain scan line. In the second scan, the probe is lifted to a constant height on the terrain profile obtained in the first scan. The probe only applies DC voltage (EFM) or DC + AC voltage (DFM) in the second scan.

[0060] Experiments were performed using a Cyp h er atomic force microscope (Asylum Research). The conductive probe used in the experiment has a resonance frequency of about 75 kHz and a spring constant of about 2.8 N / m. Continue as Figure 1The DFM imaging system is developed based on the working principle of EFM. The DFM measurement is performed in two scanning passes. In the first scanning pass, a standard tapping mode scan is used to acquire the surface topography. In the second scanning pass, the cantilever scans at a constant height of 30 nm above the surface topography acquired in the first scanning pass, while an AC voltage V ac of 3 V at a frequency of 50 kHz, a gate voltage V g (-4 to +4 V with a 2 V step for multi-point sampling), and an offset voltage V cp (typically ~0.1 V) to compensate for the contact potential difference between the probe and the substrate are applied to the conductive probe. The 2ω phase shift component of the cantilever deflection signal is finally used as the DFM signal.

[0061] EFM (ω signal) is sensitive to the electrostatic force, which is a long-range interaction (~hundreds of nm), thus the signal spatially spreads more and the resolution is relatively low. However, DFM (2ω signal) comes from the capacitance gradient dC / dz, which is equivalent to a more localized probing, making it more sensitive to the region underneath the probe and less sensitive to the long-range coupling from far away regions, thus the resolution is usually better than EFM and can be modulated by the different carriers.

[0062] The dielectric force F2ω, which is determined by the interaction between the oscillating tip charge and the induced sample carriers, can be expressed as:

[0063]

[0064] where dC / dz is the tip-sample capacitance gradient, A2ωand are the amplitude and phase of DFM, respectively, and t represents time. The value of A2ωcan be changed by adjusting the gate voltage V g , which is attributed to the change of the accumulation / depletion state of the induced charge carriers in the sample underneath the tip, thus leading to the change of the value of dC / dz. In DFM mode, the signal curves of p-type and n-type semiconductors are mirror images of each other (with the zero gate voltage as the symmetry axis), while metallic carbon nanotubes show symmetric signal curves, thus DFM can be used to distinguish the carrier types.

[0065] Figures 2a to 2f is a topography image of a carbon nanotube-embedded sample and DFM signal images at different gate voltages. It can be seen that at gate voltages V g =−4 V and +4 V, the A values of the SWCNTs are similar, and there is no modulation effect on the sample, indicating that the measured SWCNTs become metallic after filling.

[0066] Figures 3a to 3f is a topography image of another carbon nanotube-embedded sample and DFM signal images at different gate voltages. It can be seen that at gate voltages Vg As the negative voltage increases, the signal increases, and at positive voltage the signal tends to be flat. This shows the tunability of the gate voltage to the dielectric force signal, which means that the sample is n-type carrier characteristic.

[0067] Figure 4 are the intensity plots of the dielectric force signal of filled and non-filled carbon nanotubes as a function of gate voltage. For DFM, the nature of the sample can be distinguished from the effect of the gate voltage tuning, the signal curves of p-type and n-type semiconductors are mirror images, while metallic carbon nanotubes show symmetric signals. For p-type carrier characteristic of embedded carbon nanotubes: keep rising (accumulation) on the negative voltage side, and let the curve approach flat (depletion) on the positive voltage side. For n-type carrier characteristic of embedded carbon nanotubes: keep rising (accumulation) on the positive voltage side, and approach flat (depletion) on the negative voltage side. Metallic keeps symmetric, the response under positive and negative voltage does not change.

[0068] Example 2

[0069] This example is generally the same as Example 1, the main difference is that:

[0070] The range of the gate voltage is adjusted to [-4, 4V], and is sampled at intervals of 2V; the AC peak voltage is adjusted to 2V, and the frequency is 50kHz; the fixed interval is adjusted to 10nm.

[0071] Example 3

[0072] This example is generally the same as Example 1, the main difference is that:

[0073] The range of the gate voltage is adjusted to [-4, 4V], and is sampled at intervals of 2V; the AC peak voltage is adjusted to 2V, and the frequency is 50kHz; the fixed interval is adjusted to 10nm.

[0074] Based on the above examples, it can be clear that the technical scheme provided by the embodiments of the present application, on the basis of the traditional EFM technology, by simultaneously applying an alternating voltage and a direct voltage in the second scanning, the cantilever can detect the signal change of double frequency (2ω), thereby obtaining an image with higher spatial resolution. By applying different direct voltages and obtaining the corresponding images, the carrier characteristics of the embedded carbon nanotubes can be further analyzed, and a more accurate spatialized carrier characteristic characterization of the electrical characteristics at the nanoscale can be provided.

[0075] It should be understood that the above examples are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and to implement it, and it cannot limit the protection scope of the present application. Any equivalent changes or modifications made in accordance with the spirit and essence of the present application should be covered within the protection scope of the present application.

Claims

1. A method for characterizing the carrier properties of embedded carbon nanotubes, characterized by, The method comprises: performing a first scan by a probe of an atomic force microscope to obtain a topographic scan line of a carbon nanotube embedded in a substrate; obtaining a second scan line by moving the topographic scan line by a fixed interval, performing a second scan along the second scan line to obtain a dielectric force signal map; wherein a probe voltage applied to the probe during the second scan comprises a direct current voltage and an alternating current voltage, and the direct current voltage comprises a plurality of different voltage values symmetrically positive and negative; judging a carrier characteristic of the carbon nanotube embedded in the substrate according to a positive and negative symmetry of a change relationship of the dielectric force signal with respect to the direct current voltage.

2. The characterization method of claim 1, wherein, The direct current voltage comprises a gate voltage and a bias voltage, the bias voltage is a fixed value for compensating a contact potential difference between the probe and the substrate, and the gate voltage is set to have a plurality of pairs of positive and negative voltage values, the sign of any pair of the positive and negative voltage values is opposite, and the absolute values are equal.

3. The characterization method of claim 2, wherein, The gate voltage is also set to have a zero voltage value.

4. The characterization method of claim 2, wherein, The probe voltage is set to have a positive voltage when a potential of the probe is higher than a potential of the substrate, and is represented as: V tip = V g + V cp + V ac sin(ωt) where V tip represents the probe voltage, V g represents the gate voltage, V cp represents the bias voltage, V ac represents the AC peak voltage, ω represents the angular frequency of the AC voltage, and t represents time.

5. The characterization method of claim 4, wherein, The dielectric force signal is represented as: where F2 ω represents the dielectric force signal, dC / dz represents the capacitance gradient between the tip of the probe and the embedded carbon nanotube sample, A2 ω and represent the amplitude and phase, respectively.

6. The characterization method of claim 4, wherein, When a change trend of the dielectric force signal with respect to the gate voltage presents a positive and negative voltage symmetry, the carbon nanotube embedded in the substrate presents a metallic property.

7. The characterization method of claim 4, wherein, When the change trend of the dielectric force signal with respect to the gate voltage presents a positive and negative voltage asymmetry, the carbon nanotube embedded in the substrate presents a semiconductor property.

8. The characterization method of claim 7, wherein, When the dielectric force signal presents a cumulative rise in a positive voltage interval of the gate voltage and a flat depletion in a negative voltage interval of the gate voltage, the carrier characteristic of the carbon nanotube embedded in the substrate is p-type.

9. The characterization method of claim 7, wherein, When the dielectric force signal presents a cumulative rise in a negative voltage interval of the gate voltage and a flat depletion in a positive voltage interval of the gate voltage, the carrier characteristic of the carbon nanotube embedded in the substrate is n-type.

10. The characterization method of claim 4, wherein, The value interval of the gate voltage is [-4V, 4V], and the value range of the alternating current peak voltage is [2V, 5V]; and / or, the frequency of the alternating current voltage is 40-60 kHz; and / or, the fixed interval is 5-200 nm.

Images