CN112748260B - STM (scanning tunneling microscope) needle tip enhanced spectrum acquisition device and acquisition method thereof - Google Patents

Abstract

The present invention provides an STM tip enhancement spectrum acquisition device and an acquisition method thereof, wherein the method includes: S1, by moving a test sample horizontally, while a laser emits a pulsed laser to focus on the surface of the test sample to form a circular or elliptical light spot; S2 , Drive the needle tip piezoelectric stage through the drive signal to drive the STM needle tip to make a step displacement movement perpendicular to the center of the light spot, and trigger the frequency multiplier to output a double-frequency pulse signal to the spectrometer, so that the drive signal is on the rising edge and falling edge respectively. Trigger the spectrometer synchronously; S3, collect the spectral signals induced on the surface of the test sample when the STM tip is at the lowest point and the highest point in a movement cycle through the spectrometer; S4, use the signal acquisition system to collect the two spectral signals collected in each movement cycle. Subtraction processing to obtain a subtracted spectrum, and perform linear superposition based on multiple subtracted spectra to obtain a spectrum with the background signal removed. The invention can eliminate the background light signal and realize the extraction of the enhanced light signal.

Description

STM tip-enhanced spectrum acquisition device and acquisition method thereof

technical field

The invention relates to the technical field of atomic-level resolution spectral imaging, in particular to an enhanced spectral acquisition device and an acquisition method based on the periodic displacement of the STM needle tip.

Background technique

Scanning Tunneling Microscope (STM) can realize atomic-scale detection of material electron distribution and obtain super atomic-scale spatial resolution, which provides useful information for understanding and mastering single molecules, nanostructures and low-dimensional materials. Provide powerful means for understanding nanoscale electronic states and quantization effects. The tip of the STM probe is generally made of precious metal material and has a very small radius of curvature. Under the induction of the laser, the plasma coating on the surface of the tested material can be localized, and the tip-enhanced fluorescence and Raman scattering can be generated on the surface of the tested material. , which provides a basis for obtaining atomic-scale resolved Raman scattering or fluorescence, but since the fluorescence or Raman scattering signals generated in the atomic-scale space are often masked in the surrounding large-scale average signals, it is difficult to distinguish them. Optical signals in the nanoscale range are particularly critical.

Due to the limitation of the diffraction limit, the spatial resolution of optical signals cannot be broken through for a long time. It was not until the rise of new technologies such as stimulated emission quenching and radial light field that the optical diffraction limit was broken, and super-resolution fluorescence microscopy was realized. To directly observe the spatial information of molecules and atoms at the nanometer and even sub-nanometer scales based on optical information is extremely important for a deeper understanding of atomic or molecular behaviors at the microscopic scale. In addition to intensity information, the emission or scattering of light can also provide energy distribution, momentum and dynamic phase, etc., which can grasp the spatial characteristics of atoms and molecules in materials when realizing device functionalization. In recent years, the concept of nanocavity and probe technology have also been introduced into the field of fluorescence and light scattering imaging. The design of the combination of nanocavity concept and probe technology has greatly improved the enhancement of optical signals (Raman and fluorescence) in nanoscale and microscopic regions. As a result, a Raman image of the spatial structure at the molecular level was observed. The design idea is to place a single molecule horizontally on the metal surface. When the probe scans near the molecule, a sub-surface will be generated between the tip of the probe and the metal surface. Nanoscale locally enhanced electric field, if located above certain atomic groups in the molecule, local enhanced electric field and tip/intermolecular charge transfer will increase vibrational mode Raman scattering signals or dipole-coupled fluorescence by orders of magnitude , while above certain positions between the coupled atomic groups, the field-enhanced Raman attenuates due to the change in distance from the tip, resulting in signal contrast, obtaining molecular-scale spatially distributed Raman scattering images or on-molecular fluorescence images.

So far, based on the spatial scanning imaging of STM tip-enhanced light, the tip-enhanced Raman and fluorescence signals are directly extracted optical signals, and the light scattering or fluorescence background has not been completely eliminated, that is, the tip-enhanced optical signal and the background light signal (such as The portion around the material under the tip is also irradiated by the excitation laser beam, resulting in scattering of light or emitting fluorescence), which is mixed and captured by the collection system and converted into an electrical signal. In addition, the spatial scanning imaging of the tip-enhanced light (including light scattering and light emission) adopts the tip flat scanning mode, and the optical signal changes caused by the difference of the spatial structure of the atomic distribution of the detected object are used to directly compare the contrast to realize the spatial image. Such a design cannot improve the signal-to-noise ratio of the light signal of the enhanced light part of the needle tip, so the objects tested by the current technology or the research work reported now can only target a single type of molecules, and they are placed on a substrate that can eliminate the background. For example, an epitaxial layer of NaCl is placed on the gold film, so as to eliminate the fluorescence background of the gold plasma effect. If faced with a material system with light scattering or fluorescent background, such as fluorescence or Raman in the point defect area of a semiconductor film, the prior art is completely unable to effectively extract the enhanced light signal from the tip, because the light emission and light scattering of the semiconductor material itself It is very strong. Since the ratio of the number of atomic groups under the effective action area of the needle tip to the number of atomic groups under the irradiated surface of the laser beam spot is more than 10 ^-8 , the part of the enhanced optical signal of the needle tip will be greatly affected by the background of this laser irradiation surface. So for some research subjects, this is an unavoidable question.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the defects of the prior art, and propose an STM needle tip enhanced spectrum acquisition device and an acquisition method thereof. The STM needle tip is used to perform a step displacement movement above the surface of the test sample, and the collection of The spectral signal induced by the highest point and the lowest point on the surface of the test sample, through subtraction processing, eliminates the background light signal of the test sample, realizes the acquisition of the enhanced light signal at the tip, and obtains the spatial image of the photoresponse mode at the atomic level.

For achieving the above object, the present invention adopts following concrete technical scheme:

The STM needle tip enhanced spectrum acquisition device provided by the present invention includes a laser, an STM needle tip, a needle tip piezoelectric displacement stage, a sample stage, a spectrometer, a frequency multiplier and a signal processing system; wherein,

The sample stage is used to carry the test sample and drive the test sample to move in the horizontal direction;

The laser is used to emit pulsed laser light and focus on the surface of the test sample to form a spot;

The STM tip is perpendicular to the surface of the test sample and points to the center of the spot;

The needle tip piezoelectric displacement stage is used to drive the STM needle tip to perform a step displacement movement perpendicular to the surface of the test sample under the drive of the driving signal;

The frequency multiplier is used to output a double frequency pulse signal to the spectrometer under the triggering of the driving signal, so that the driving signal can trigger the spectrometer synchronously on the rising edge and the falling edge respectively;

The spectrometer is used to collect the spectral signals induced on the surface of the test sample when the STM tip is at the lowest point and the highest point in a movement cycle; when the STM tip is at the lowest point in the movement cycle, an enhanced optical signal is excited on the surface of the test sample;

The signal processing system is used to perform subtraction processing on the two spectral signals collected in each motion cycle to obtain a subtracted spectrum, and perform linear superposition based on multiple subtracted spectra at the same position of the test sample to obtain an enhanced spectrum.

Preferably, the lowest point position of the STM needle tip in the movement cycle is 0.1-1 nm away from the surface of the test sample, and the highest point position of the STM needle tip in the movement cycle is 3-100 nm away from the test sample surface.

Preferably, the holding time of the STM tip at the lowest point position and the highest point position in the movement cycle is respectively greater than the acquisition time of the spectrometer.

Preferably, the movement frequency of the STM needle tip is 10 Hz to 500 Hz.

Preferably, the holding time of the STM needle tip at the lowest point position and the highest point position in the movement cycle is 3-10 ms.

Preferably, the waveform for driving the needle tip piezoelectric stage is a square wave or a trapezoidal wave.

The STM needle tip enhancement spectrum acquisition method provided by the present invention includes the following steps:

S1. Drive the test sample to move in the horizontal direction through the sample stage, and at the same time, the laser emits a pulsed laser and focuses on the surface of the test sample to form a circular or elliptical light spot;

S2. Drive the needle tip piezoelectric stage through the drive signal to drive the STM needle tip to make a step displacement movement perpendicular to the center of the light spot, and at the same time trigger the frequency multiplier to output a double-frequency pulse signal to the spectrometer, so that the drive signal is on the rising and falling edges. Trigger the spectrometer synchronously, respectively;

S3. Collect the spectral signals induced on the surface of the test sample when the STM tip is at the lowest point and the highest point in a movement cycle through the spectrometer;

S4. The signal acquisition system performs subtraction processing on the two spectral signals collected in each motion cycle to obtain a subtracted spectrum, and performs linear superposition based on a plurality of subtracted spectra to obtain a spectrum from which the background signal is removed.

Preferably, the holding time of the STM needle tip at the lowest point position and the highest point position in the movement cycle is respectively longer than the acquisition time of the spectrometer; The position of the highest point in the motion cycle is 3 to 100 nm away from the surface of the test sample.

Preferably, the background light signal is removed by a subtraction algorithm with the spectrum obtained by the STM tip at the highest point position in the motion cycle as the background, and the spectrum obtained by the STM tip at the lowest point position in the motion cycle as the response.

Preferably, the spectral signals induced by N groups of STM needle tips at the highest point position in the motion cycle are pre-determined, and the background spectral reference and mean square deviation are determined by statistical distribution and weighted average processing of the N groups of spectral signals. Spectral reference and mean square deviation The spectral signal obtained at the highest point position of the STM tip in the motion cycle is discriminated each time, and when the spectral signal deviates from the background spectral reference by more than 1 times the mean square deviation, it is regarded as an invalid spectrum; among them, 5<N <10.

The present invention can achieve the following technical effects:

1. Using the step displacement movement of the needle tip, the induced spectral signals are collected at the highest point and the lowest point respectively, and the background light signal is removed by subtracting the spectral signal at the highest point and the spectral signal at the lowest point. The enhanced optical signal is extracted.

2. By performing multiple subtraction spectra for each position of the test sample, and then performing linear superposition, the signal-to-noise ratio of the enhanced optical signal is improved.

3. Obtaining the spectrum at the highest and lowest points of the needle tip displacement can reduce the noise caused by the random position of the needle tip, avoid the system noise caused by the signal floating caused by the phase and distance fluctuations of the discrete points, and improve the reliability of the enhanced optical signal. .

Description of drawings

1 is a schematic structural diagram of an STM tip-enhanced spectrum acquisition device according to an embodiment of the present invention;

2 is a schematic diagram of a driving voltage output waveform of a needle tip piezoelectric stage according to an embodiment of the present invention;

3 is a first schematic flow chart of a method for acquiring a STM tip-enhanced spectrum according to an embodiment of the present invention;

FIG. 4 is a second schematic flowchart of a method for acquiring an STM tip-enhanced spectrum according to an embodiment of the present invention.

The reference signs therein include: laser 1 , STM tip 2 , tip piezoelectric stage 3 , spectrometer 4 , CCD detector 41 , frequency multiplier 5 , and test sample 6 .

Detailed ways

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are denoted by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, its detailed description will not be repeated.

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention.

The coupling between the STM tip and the optical field will enhance the light scattering and light emission intensity of the material in the near field of the tip. It is theoretically predicted that the localized plasmon effect on the metal surface of the tip will induce an enhancement of the Raman scattering intensity of the material by six to eight orders of magnitude. Mann and fluorescence detection or Raman spectroscopy detection of vibrational modes of single point defects in materials provides the conditions. However, under the condition of optical excitation, due to the limitation of the diffraction limit of light, the spot size of the excitation light (whether it is a laser or a conventional light source) acting on the irradiated material, whether on the surface or in the depth, is of the same order of magnitude as the wavelength. For the optical design requirements that can be used in the actual equipment, especially the space limitation of the STM system and the needle tip movement and observation requirements, the distance between the lens of the light convergence and signal collection system and the material test point should not be too close. Generally, this distance should be If it is larger than 10mm, the numerical aperture of the lens cannot be too large, and the actual size of the excitation spot is several times or even ten times the wavelength. The ratio of the number of atoms in the region of the material excited by the light to the number of atoms in the effective active area of the tip differs by six to seven orders of magnitude. The light scattering and fluorescence generated by the material region excited by light is a huge background light signal. If the spectrum of the enhanced light at the tip of the needle does not overlap with the spectrum of the surrounding area in energy, the extraction of the enhanced light signal is convenient, but in practice In situations, especially when testing materials with a matrix, often the spectrum of the tip-enhanced light and the spectrum of the surrounding area overlap in energy distribution. The effect of this overlap on signal observation is like observing trees on a mountain with a magnifying glass in the distance. It is extremely difficult to obtain details and accurate information. You must climb to the side of the trees on the mountain and observe with a magnifying glass to erase the background of the mountain. Clear specific details.

Based on this idea, the present invention drives the STM needle tip to perform periodic movement in the vertical direction up and down within a certain height range above the surface of the test sample, so as to ensure that an enhanced optical signal of sufficient multiplication level is induced at the lowest point. By collecting the STM needle tip The spectral signal induced by laser excitation at the highest point and the lowest point of the motion cycle is subtracted to remove the intrinsic light emission and light scattering of the material induced by laser excitation, that is, to eliminate the background light signal, and extract the STM tip at the lowest point. Induced enhanced optical signal, obtaining atomic scale photoresponse mode spatial images.

The specific solutions of the embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 shows the structure of an STM tip-enhanced spectrum acquisition device according to an embodiment of the present invention.

As shown in FIG. 1 , the STM tip-enhanced spectrum acquisition device provided by the embodiment of the present invention includes: a laser 1 , an STM tip 2 , a tip piezoelectric displacement stage 3 , a spectrometer 4 , a frequency multiplier 5 , a sample stage and a signal processing system.

The sample stage is actually a two-dimensional moving platform, which is used to carry the test sample 6 and drive the test sample 6 to move in the X-axis and Y-axis directions along the horizontal direction to realize two-dimensional space imaging. Since the two-dimensional mobile platform is the prior art, it will not be repeated here.

The laser 1 is used to emit pulsed laser light and focus on the surface of the test sample 6 to form a circular or elliptical light spot. The wavelength of the laser 1 is selected in the visible light band and the near-infrared band, and the output mode of the laser is steady state or high pulse repetition frequency (>1MHz).

The STM tip 2 is a metal tip, which is located above the surface of the test sample 6 and perpendicular to the surface of the test sample 6 , and points to the center of the spot formed by the pulsed laser emitted by the laser 1 on the surface of the test sample 6 .

The needle tip piezoelectric displacement stage 3 is used to drive the STM needle tip 2 to perform periodic displacement movement perpendicular to the surface of the test sample 6 . When the STM needle tip 2 is at the lowest point in a movement cycle (that is, the position where the STM needle tip 2 is closest to the surface of the test sample 6), an enhanced optical signal is induced on the surface of the test sample 6; when the STM needle tip 2 is at the highest point in a movement cycle (ie, the position where the STM needle tip 2 is farthest from the surface of the test sample 6 ), the light enhancement effect of the STM needle tip 2 on the surface of the test sample 6 is reduced. The periodic displacement movement of the STM tip 2 is actually a step displacement movement performed at a fixed frequency perpendicular to the surface of the test sample 6 .

It can be seen from this that in the embodiment of the present invention, the STM needle tip 2 is driven by the needle tip piezoelectric displacement stage 3 to perform a step-type displacement movement perpendicular to the surface of the test sample 6, in order to ensure that sufficient increments can be induced at the lowest point of the step-type displacement movement. The enhanced optical signal of the highest level can not only eliminate the enhancement effect of the STM tip 2 on the surface of the test sample 6 at the highest point, but also ensure the reliability of the mechanical high-frequency displacement and the adaptation with the STM device (including the drive response bandwidth, function consumption and thermal effects, etc.).

Since the molecular fluorescence intensity enhanced by the STM tip 2 decays exponentially with the increase of the distance between the STM tip 2 and the molecule, the fluorescence intensity of the tip attenuates by nearly 7 orders of magnitude in the variation range of 5 nm. Based on experimental data and theoretical analysis, considering that the difference in the curvature radius of the tip and the dielectric properties of the test material leads to changes in the evolution of the enhancement effect, in order to optimize the background effect and system response, the present invention changes the step displacement of the STM tip 2 The position of the highest point of motion is set in the range of 3-100nm above the surface of the test sample 6, and the lowest point is the best point of the STM image scanning, which is about 0.1nm-1nm above the surface of the test sample 6.

When the STM tip 2 is displaced upward perpendicular to the surface of the test sample 6, the enhancement effect of the STM tip 2 will be attenuated. If the position of the highest point of the STM tip 2 is set too low, when it is less than 3 nm, the fluorescence or fluorescence caused by the movement of the STM tip 2 will be reduced. The difference in light scattering intensity will overlap the background fluorescence and light scattering fluctuation noise induced by the laser, resulting in a low signal-to-noise ratio, which is not enough to extract accurate, reliable and clear signals. When it is higher than 100nm, the driver and system of the reciprocating motion of the needle tip will greatly increase the response bandwidth requirements and the difficulty of ensuring the linear shape of the needle tip movement, which will increase unnecessary control risks. Therefore, the present invention controls the range of the position of the highest point of the STM needle tip 2 to be 3-100 nm above the surface of the test sample 6 .

In the present invention, the repetition frequency of the up and down displacement of the needle tip piezoelectric stage 3 is not lower than 10 Hz, and the optimal selection is 200 Hz. If the signal is strong enough, the high repetition frequency can be selected as 500 Hz. The repetition frequency of the needle tip piezoelectric stage 3 is not high, and the control at low frequency is mainly to obtain highly accurate and reliable spectral signals.

The spectrometer 4 is used to collect the spectral signals excited by the STM needle tip 2 at the lowest point and the highest point when the STM needle tip 2 moves to the lowest point position and the highest point position. The spectrum acquisition and analysis of the spectrometer 4 uses a CCD detector spectrometer equipped with a narrow-band edge or notch filter. The spectral resolution of the spectrometer 4 should be higher than 5cm ^-1 , and the response wavelength range of the spectrometer 4 should cover the laser wavelength and extend to the laser Beyond 4000cm ^-1 on both sides of the wavelength or on the long-wave side. The CCD detector 41 can also be replaced by a CMOS detector.

Since the change of the displacement of the STM tip 2 has a strong correlation with the optical signal enhanced by the STM tip 2, the random displacement position of the STM tip 2 when the shutter of the CCD detector 41 is opened is not advisable, and the spectrometer has a gate in the spectroscopic process. The acquisition of a spectral signal can only be completed by controlling time, CCD reading time, dead time and other time sequences. During certain periods of the process, the movement of displacement will bring a lot of fluctuation noise, which requires STM. The needle tip 2 remains stationary at a certain position to complete the acquisition of a spectral signal. The present invention adopts the sequential driving voltage/current of trapezoidal wave or square wave to modulate the movement of the STM needle tip 2 , so that the holding time of the STM needle tip 2 at the lowest point position and the highest point position is longer than the acquisition time of the CCD detector 41 . In order to reduce the background noise signal intensity of the CCD detector 41, the acquisition time of the CCD detector 41 should not be too short, and the acquisition time of the CCD detector 41 is usually 1-3 ms. In this way, the modulation period of the driving voltage of the needle tip piezoelectric stage 3 is controlled within 500 Hz, and a synchronous frequency multiplier 5 is triggered by the displacement driving modulator to generate a double-frequency pulse signal to trigger the spectrometer 4 and the CCD detector 41 gate. When the control is turned on, the phase of the frequency multiplier 5 is locked with the phase of the gated pulse of the spectrometer 4 and can be optimally adjusted to maximize the response signal while suppressing the noise background intensity.

Since the STM needle tip 2 performs a step-type displacement motion, the driving signal of the needle-tip piezoelectric stage 3 must be a step type in which the voltage/current changes with time. As shown in FIG. 2 , the driving signal of the needle tip piezoelectric stage 3 is a trapezoidal wave or a square wave.

In a specific embodiment of the present invention, the holding time of the STM needle tip 2 at the highest point and the lowest point is 3-10 ms to ensure that the CCD detector 41 of the spectrometer 4 completes the collection of the spectrum. The rising edge and falling edge of the driving signal of the needle tip piezoelectric stage 3 are synchronized with the shutter trigger of the CCD detector 41 through the frequency multiplier 5, and the time ratio of the rising edge and the falling edge is not higher than 5%.

The signal processing system is used to perform subtraction processing on the two spectral signals collected by the spectrometer 4 in each motion cycle to obtain a subtracted spectrum. In this way, the same position of the test sample 6 is repeatedly collected to obtain multiple sets of differentially subtracted spectra. Group subtraction spectra were linearly superimposed to obtain enhanced spectra.

The above content describes in detail the structure and working principle of the STM tip-enhanced spectrum acquisition device provided by the present invention. Corresponding to the above-mentioned STM tip-enhanced spectrum acquisition device, the present invention also provides a method for acquiring an enhanced spectrum by using the STM needle-tip enhanced spectrum device.

FIG. 3 shows a first flow of a method for acquiring a STM tip-enhanced spectrum according to an embodiment of the present invention.

As shown in FIG. 3 , the STM needle tip enhancement spectrum acquisition method provided by the embodiment of the present invention includes the following steps:

S1. Drive the test sample to move in the horizontal direction through the sample stage, and at the same time, the laser emits a pulsed laser to focus on the surface of the test sample to form a circular or elliptical light spot.

S2. Drive the needle tip piezoelectric stage through the drive signal to drive the STM needle tip to make a step displacement movement perpendicular to the center of the light spot, and at the same time trigger the frequency multiplier to output a double-frequency pulse signal to the spectrometer, so that the drive signal is on the rising and falling edges. The spectrometers are triggered synchronously, respectively.

When the STM tip moves to the lowest point, an enhanced optical signal is induced on the surface of the test sample.

S3. Collect the spectral signals induced on the surface of the test sample when the STM tip is located at the lowest point and the highest point in a movement cycle through a spectrometer.

In order to ensure that the spectrometer completes the acquisition of the spectrum, the holding time of the STM tip at the lowest point and the highest point in the motion cycle is respectively longer than the acquisition time of the spectrometer, and the spectrum acquisition of the STM tip at the lowest point and the highest point is completed.

S4. Perform subtraction processing on the two spectral signals collected in each motion cycle through the signal acquisition system to obtain a subtracted spectrum, and perform linear superposition based on multiple subtracted spectra to obtain a spectrum from which the background signal is removed; wherein, 5< N<10.

Taking the spectrum obtained by the STM tip at the highest point as the background, and taking the spectrum obtained by the STM tip at the lowest point in the motion cycle as the response, subtract the spectrum obtained at the highest point from the spectrum obtained at the lowest point to remove the background light Signal.

When obtaining the spectrum of the highest point position, it needs to be screened to determine whether it meets the conditions. The specific process is as follows: pre-determine the spectral signals induced by N groups of STM needle tips at the highest point position in the motion cycle, and determine the background spectral reference and mean square deviation through statistical distribution and weighted average processing of the N groups of spectral signals. Background spectral reference and mean square deviation The spectral signal obtained at the highest point position of the STM tip in each motion cycle is discriminated. When the spectral signal deviates from the background spectral reference by more than 1 times the mean square deviation, it is regarded as an invalid spectrum.

FIG. 4 shows a second flow of a method for acquiring an STM tip-enhanced spectrum according to an embodiment of the present invention.

As shown in FIG. 4 , the STM needle tip enhancement spectrum acquisition method provided by the embodiment of the present invention includes the following steps:

S1`, the STM tip moves to the surface of the test sample, and the tip position of the tunnel current is optimized.

S2`, adjust the light spot where the laser converges to overlap with the STM needle tip.

S3`, the STM tip is moved up, and the background spectral reference and the mean square deviation are determined.

After the laser and the STM system are stabilized, the needle tip is moved away from the sample surface, and the spectrum at 5-10 high points is measured first, and the background spectrum reference and mean square deviation are obtained through statistical distribution and weighted average processing. Based on this background spectrum reference, we can identify whether the spectrum obtained every time the STM tip is displaced to the highest point is a usable spectrum.

S4`, drive the tip piezoelectric stage to control the STM tip to move up to the highest position, and collect the spectrum through the spectrometer.

S5', discriminate whether the deviation of the spectrum from the background spectrum reference exceeds the mean square deviation, if it exceeds, execute step S4', if not, execute step 6'.

After obtaining the background spectral reference and the mean square deviation, the spectrum obtained when the STM tip is displaced to the highest point is normally tested. If the spectrum deviates from the background spectral reference by more than 1 times the mean square deviation, the spectrum is regarded as invalid data, thereby reducing the environment, laser Beam-induced background and effects of laser fluctuations.

S6`, drive the tip piezoelectric stage to control the STM tip to move down to the lowest position, and collect the spectrum through the spectrometer.

When the STM tip is moved down to the lowest point, an enhanced light signal on the surface of the test sample can be induced.

S7`, determine the subtraction spectrum, and store the spectrum data.

A set of spectral signals was obtained in each motion cycle, with the spectrum obtained at the highest point as the background and the spectrum obtained at the lowest point as the response, and the background light scattering or fluorescence induced by laser irradiation was deducted through a subtraction algorithm.

S8', judging whether the stored spectral data reaches the set value, if so, go to step S9', if not, go to step S4'.

S9`, superimpose multiple sets of spectral data to obtain enhanced spectra.

At each scanning point of the test sample, 10-50 subtracted spectra are repeatedly obtained. By screening these 10-50 spectra, no less than 10 qualified spectra are selected for superposition processing, thereby eliminating the environment , The effect of laser beam-induced background and laser fluctuations. According to the spectral signal-to-noise ratio characteristics, the number of spectral groups repeated for each test point can be selected and optimized between 5-50 times.

In the description of this specification, description with reference to the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples", etc., mean specific features described in connection with the embodiment or example , structure, material or feature is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the particular features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, those skilled in the art may combine and combine the different embodiments or examples described in this specification, as well as the features of the different embodiments or examples, without conflicting each other.

Although the embodiments of the present invention have been shown and described above, it should be understood that the above-mentioned embodiments are exemplary and should not be construed as limiting the present invention. Embodiments are subject to variations, modifications, substitutions and variations.

The above specific embodiments of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made according to the technical concept of the present invention shall be included in the protection scope of the claims of the present invention.

Claims (10)

1. an STM tip enhancement spectrum acquisition device, is characterized in that, comprises laser, STM tip, tip piezoelectric displacement stage, sample stage, spectrometer, frequency multiplier and signal processing system; Wherein, The sample stage is used to carry the test sample and drive the test sample to move in the horizontal direction; The laser is used to emit pulsed laser light and focus on the surface of the test sample to form a light spot; The STM tip is perpendicular to the surface of the test sample and points to the center of the light spot; The needle tip piezoelectric displacement stage is used to drive the STM needle tip to perform a step displacement movement perpendicular to the surface of the test sample under the driving of a driving signal; The frequency multiplier is used for outputting a double frequency pulse signal to the spectrometer under the triggering of the driving signal, so that the driving signal synchronously triggers the spectrometer on the rising edge and the falling edge respectively; The spectrometer is used to collect the spectral signals induced on the surface of the test sample when the STM needle tip is at the lowest point and the highest point in a movement cycle; wherein, when the STM needle tip is at the lowest point in the movement cycle, at the The surface of the test sample excites an enhanced optical signal; The signal processing system is used to perform subtraction processing on the two spectral signals collected in each motion cycle to obtain a subtracted spectrum, and perform linear superposition based on multiple subtracted spectra at the same position of the test sample to obtain an enhanced spectrum . 2 . The STM needle tip enhancement spectrum acquisition device according to claim 1 , wherein the lowest point position of the STM needle tip in the movement cycle is 0.1-1 nm away from the surface of the test sample, and the STM needle tip is in the movement cycle. 3 . The position of the highest point is 3-100 nm away from the surface of the test sample. 3 . The STM tip-enhanced spectrum acquisition device according to claim 1 or 2 , wherein the holding time of the STM tip at the lowest point position and the highest point position in the movement cycle is respectively longer than the acquisition time of the spectrometer. 4 . 4 . The STM needle tip enhancement spectrum acquisition device according to claim 3 , wherein the movement frequency of the STM needle tip is 10 Hz to 500 Hz. 5 . 5 . The STM needle tip enhancement spectrum acquisition device according to claim 3 , wherein the holding time of the STM needle tip at the lowest point position and the highest point position in the movement cycle is 3-10 ms. 6 . 6 . The STM needle tip enhancement spectrum acquisition device according to claim 1 , wherein the waveform driving the needle tip piezoelectric stage is a square wave or a trapezoidal wave. 7 . 7. A method for acquiring STM tip enhancement spectrum, characterized in that, comprising the steps: S1. Drive the test sample to move in the horizontal direction through the sample stage, and at the same time, the laser emits a pulsed laser and focuses on the surface of the test sample to form a circular or elliptical light spot; S2. Drive the needle tip piezoelectric displacement stage through the drive signal to drive the STM needle tip to perform a step displacement movement perpendicular to the center of the light spot, and at the same time trigger the frequency multiplier to output a double-frequency pulse signal to the spectrometer, so that the drive signal is rising The spectrometer is triggered synchronously by the edge and the falling edge respectively; S3, collecting the spectral signals induced on the surface of the test sample when the STM needle tip is located at the lowest point and the highest point in one motion cycle by a spectrometer; S4. The signal acquisition system performs subtraction processing on the two spectral signals collected in each motion cycle to obtain a subtracted spectrum, and performs linear superposition based on multiple subtracted spectra to obtain a spectrum from which the background signal is removed. 8. The STM needle-tip enhancement spectrum acquisition method according to claim 7, wherein the holding time of the STM needle-tip at the lowest point position and the highest point position in the motion cycle is respectively greater than the acquisition time of the spectrometer; and, The lowest point position of the STM needle tip in the movement cycle is 0.1-1 nm away from the surface of the test sample, and the highest point position of the STM needle tip in the movement cycle is 3-100 nm away from the test sample surface. 9 . The method for acquiring enhanced spectrum of STM needle tip according to claim 7 , wherein the spectrum obtained at the highest point position of the STM needle tip in the movement cycle is used as a background, and the lowest value of the STM needle tip in the movement cycle is used as a background. 10 . The spectrum obtained at the point position is used as the response, and the background light signal is removed through the subtraction algorithm. 10 . The STM needle tip enhancement spectrum acquisition method according to claim 9 , wherein the spectral signals induced by the N groups of the STM needle tips at the highest point position in the motion cycle are pre-determined, and the spectral signals of the N groups are obtained by comparing the spectral signals of the N groups. 11 . Statistical distribution and weighted average processing to determine the background spectral reference and mean square deviation, according to the background spectral reference and the mean square deviation, the spectral signal obtained at the highest point position of the STM tip in the movement cycle is screened each time. When the deviation from the background spectrum reference exceeds 1 times the mean square deviation, it is regarded as an invalid spectrum; wherein, 5<N<10.

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