CN112748260B - STM (scanning tunneling microscope) needle tip enhanced spectrum acquisition device and acquisition method thereof - Google Patents
STM (scanning tunneling microscope) needle tip enhanced spectrum acquisition device and acquisition method thereof Download PDFInfo
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- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
- G01Q60/10—STM [Scanning Tunnelling Microscopy] or apparatus therefor, e.g. STM probes
- G01Q60/12—STS [Scanning Tunnelling Spectroscopy]
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
- G01Q60/10—STM [Scanning Tunnelling Microscopy] or apparatus therefor, e.g. STM probes
- G01Q60/16—Probes, their manufacture, or their related instrumentation, e.g. holders
Abstract
The invention provides an STM (scanning tunneling microscope) needle tip enhanced spectrum acquisition device and an acquisition method thereof, wherein the method comprises the following steps: s1, horizontally moving the test sample, and simultaneously, emitting pulse laser by the laser to focus on the surface of the test sample to form a circular or elliptical light spot; s2, driving the needle tip piezoelectric displacement platform through a driving signal to drive the STM needle tip to make a step displacement motion perpendicular to the center of the light spot, and simultaneously triggering a frequency multiplier to output a frequency doubled pulse signal to the spectrometer so that the driving signal synchronously triggers the spectrometer on a rising edge and a falling edge respectively; s3, collecting spectral signals induced on the surface of the test sample when the STM needle point is located at the lowest point and the highest point in a motion period through a spectrometer; and S4, performing subtraction processing on the two spectrum signals acquired in each motion period through the signal acquisition system to obtain a subtraction spectrum, and performing linear superposition on the basis of the plurality of subtraction 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
Technical Field
The invention relates to the technical field of atomic level resolution spectral imaging, in particular to an enhanced spectrum acquisition device based on STM (scanning tunneling microscope) needle point periodic displacement and an acquisition method thereof.
Background
The Scanning Tunneling Microscope (STM for short) can realize the electron distribution detection of the material with the atomic scale, obtain the super atomic scale spatial resolution capability, provide useful information for understanding and mastering monomolecular, nano-structure and low-dimensional material, and provide a powerful means for knowing the electronic state and the quantization effect of the nano scale. The tip of the STM probe is generally made of a noble metal material and has a very small curvature radius, plasma cover on the surface of a detected material can be localized under the induction of laser, and tip-enhanced fluorescence and Raman scattering are generated on the surface of the detected material, which provides a basis for obtaining atom-scale resolved Raman scattering or fluorescence, but because fluorescence or Raman scattering signals generated in an atom-scale space are often masked in surrounding large-scale average signals and are difficult to distinguish, the extraction of optical signals in a nanoscale range is particularly critical.
Due to the limitation of diffraction limit, the spatial resolution of optical signals cannot be broken through for a long time, and the super-resolution fluorescence microscopy is not realized until the optical diffraction limit is broken through by the emergence of new technologies such as stimulated emission quenching and radial optical field. Direct observation of molecular atomic spatial information on a nanometer or even sub-nanometer scale based on optical information is extremely important for a more profound understanding of atomic or molecular behavior on a microscopic scale. Besides intensity information, the emission or scattering of light can provide energy distribution, momentum, dynamic phase and the like, so that the spatial characteristics of atomic molecules in the material can be grasped when the device is functionalized. In recent years, the nano-cavity concept and the probe technology are also introduced into the field of fluorescence and light scattering imaging, the design of the combination of the nano-cavity concept and the probe technology greatly improves the enhancement effect of optical signals (Raman and fluorescence) in a nano microscopic region, and a molecular level spatial structure Raman image is observed, the design idea is that a single molecule is horizontally placed on a metal surface, when the probe scans near the molecule, a sub-nanometer scale local enhanced electric field is generated between the needle tip of the probe and the metal surface, if the probe is positioned above certain atomic groups in the molecule, the local enhanced electric field and the needle tip/intermolecular charge transfer can increase the vibration mode Raman scattering signals or dipole coupled fluorescence by orders of magnitude, and above certain positions between the coupled atomic groups, the field enhanced Raman can be attenuated due to the distance change between the probe tip, so that the signal contrast is generated, and obtaining a molecular-scale spatial distribution Raman scattering image or a molecular fluorescence image.
STM tip based up to nowThe spatial scanning imaging of the enhanced light, the enhanced Raman of the needle tip and the fluorescence signal are both optical signals extracted directly, and the light scattering or fluorescence background is not completely eliminated, namely, the enhanced Raman of the needle tip and the background light signal (for example, the part around the material under the needle tip can be irradiated by the excitation laser beam to generate the scattering of light or emit fluorescence) are mixed and captured by the collecting system and converted into electric signals. In addition, the spatial scanning imaging of the enhanced light (including light scattering and light emission) of the needle tip adopts a needle tip flat scanning mode, and contrast is directly performed by using the change of optical signals caused by the difference of the atomic distribution spatial structure of the detected object, so that a spatial image is realized. Such a design cannot improve the signal-to-noise ratio of the optical signal of the portion of the light enhanced by the tip, so that the object of the current technical test or the research work reported at present can only aim at a single type of molecules, and is placed on a substrate capable of eliminating the background, for example, a thin NaCl layer is further extended on a gold film, so as to eliminate the fluorescence background of the gold plasma effect. If faced with material systems having a background of light scattering or fluorescence, such as fluorescence or Raman of point defect regions of semiconductor films, the prior art is completely unable to effectively extract a tip-enhanced optical signal because the semiconductor material itself has strong light emission and light scattering due to the ratio of the number of radicals in the effective active area of the tip to the number of radicals in the irradiated surface of the laser beam spot being 10-8In the above, the portion of the tip-enhanced optical signal is greatly affected by the background of the laser irradiated surface. This is an inevitable problem for some subjects.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides an STM (scanning tunneling microscope) needle point enhanced spectrum acquisition device and an acquisition method thereof.
In order to achieve the purpose, the invention adopts the following specific technical scheme:
the invention provides an STM needle tip enhanced spectrum acquisition device, which comprises a laser, an STM needle tip, a needle tip piezoelectric displacement table, a sample table, a spectrometer, a frequency multiplier and a signal processing system, wherein the STM needle tip is arranged on the sample table; wherein the content of the first and second substances,
the sample table is used for bearing a test sample and driving the test sample to move along the horizontal direction;
the laser is used for emitting pulse laser and focusing on the surface of the test sample to form light spots;
the STM needle point is vertical to the surface of the test sample and points to the center of the light spot;
the needle point piezoelectric displacement platform is used for driving the STM needle point to make step displacement motion vertical to the surface of the test sample under the drive of a drive signal;
the frequency multiplier is used for outputting a frequency-doubled pulse signal to the spectrometer under the trigger of the driving signal, so that the driving signal synchronously triggers the spectrometer on a rising edge and a falling edge respectively;
the spectrometer is used for acquiring spectral signals induced on the surface of the test sample when the STM needle point is positioned at the lowest point and the highest point in a motion period; when the STM needle point is positioned at the lowest point in the motion period, exciting an enhanced optical signal on the surface of the test sample;
the signal processing system is used for carrying out subtraction processing on the two spectral signals acquired in each motion period to obtain a subtraction spectrum, and carrying out linear superposition on the multiple subtraction spectra at the same position of the test sample to obtain an enhanced spectrum.
Preferably, the distance between the lowest point position of the STM needle point in the motion period and the surface of the test sample is 0.1-1 nm, and the distance between the highest point position of the STM needle point in the motion period and the surface of the test sample is 3-100 nm.
Preferably, the holding time of the STM tip at the lowest point position and the highest point position in the motion cycle is respectively longer than the acquisition time of the spectrometer.
Preferably, the frequency of movement of the STM tip is between 10Hz and 500 Hz.
Preferably, the holding time of the STM needle tip at the lowest point position and the highest point position in the motion cycle is 3-10 ms.
Preferably, the waveform driving the tip piezoelectric displacement stage is a square wave or a trapezoidal wave.
The invention provides an STM (scanning tunneling microscope) needle tip enhanced spectrum acquisition method, which comprises the following steps of:
s1, driving the test sample to move along the horizontal direction through the sample stage, and simultaneously, emitting pulse laser by the laser to focus on the surface of the test sample to form a circular or elliptical light spot;
s2, driving the needle tip piezoelectric displacement platform through a driving signal to drive the STM needle tip to make a step displacement motion perpendicular to the center of the light spot, and simultaneously triggering a frequency multiplier to output a frequency doubled pulse signal to the spectrometer so that the driving signal synchronously triggers the spectrometer on a rising edge and a falling edge respectively;
s3, collecting spectral signals induced on the surface of the test sample when the STM needle point is located at the lowest point and the highest point in a motion period through a spectrometer;
and S4, performing subtraction processing on the two spectrum signals acquired in each motion period through the signal acquisition system to obtain a subtraction spectrum, and performing linear superposition on the basis of the plurality of subtraction spectra to obtain a spectrum with the background signal removed.
Preferably, the keeping time of the STM needle tip at the lowest point position and the highest point position in the motion period is respectively longer than the acquisition time of the spectrometer; and the distance between the lowest point position of the STM needle point in the motion period and the surface of the test sample is 0.1-1 nm, and the distance between the highest point position of the STM needle point in the motion period and the surface of the test sample is 3-100 nm.
Preferably, the background light signal is removed by a subtraction algorithm with the spectrum obtained by the STM tip at the highest point in the movement period as the background and the spectrum obtained by the STM tip at the lowest point in the movement period as the response.
Preferably, spectral signals induced by N groups of STM needle points at the highest point position in the motion period are measured in advance, a background spectral standard and a mean square deviation are determined through statistical distribution and weighted average processing of the N groups of spectral signals, the spectral signals obtained at the highest point position of the STM needle point in the motion period every time are discriminated according to the background spectral standard and the mean square deviation, and the spectral signals are used as invalid spectra when the deviation of the spectral signals from the background spectral standard exceeds 1 time of the mean square deviation; wherein N is more than 5 and less than 10.
The invention can obtain the following technical effects:
1. and (3) acquiring induced spectral signals at the highest point and the lowest point respectively by utilizing the step-type displacement motion of the needle point, removing background light signals by carrying out subtraction processing on the spectral signals at the highest point and the spectral signals at the lowest point, and extracting enhanced light signals.
2. The signal-to-noise ratio of the enhanced optical signal is improved by carrying out multiple subtraction spectrums on each position of the test sample and then carrying out linear superposition.
3. The spectrum is acquired at the highest point and the lowest point of the needle point displacement, so that the noise caused by random positions of the needle points can be reduced, the system noise generated by signal floating caused by phase and distance fluctuation of discrete points is avoided, and the reliability of the enhanced optical signal is improved.
Drawings
FIG. 1 is a schematic structural diagram of an STM tip enhanced spectral acquisition apparatus according to one embodiment of the present invention;
FIG. 2 is a schematic diagram of a driving voltage output waveform of a tip piezoelectric displacement stage according to one embodiment of the invention;
FIG. 3 is a first flow diagram of a STM tip enhanced spectroscopy acquisition method according to one embodiment of the invention;
FIG. 4 is a second flow diagram of a STM tip enhanced spectral acquisition method according to one embodiment of the invention.
Wherein the reference numerals include: the device comprises a laser 1, an STM needle tip 2, a needle tip piezoelectric displacement table 3, a spectrometer 4, a CCD detector 41, a frequency multiplier 5 and a test sample 6.
Detailed Description
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same reference numerals are used for the same blocks. In the case of the same reference numerals, their names and functions are also the same. Therefore, detailed description thereof will not be repeated.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not to be construed as limiting the invention.
The coupling of the STM needle tip and the optical field can enhance the light scattering and light emission intensity of the material under the near field of the needle tip, theoretically, the metal surface local plasma effect of the needle tip is predicted to induce the enhancement of the Raman scattering intensity of the material by six to eight orders of magnitude, and thus, conditions are provided for monomolecular Raman and fluorescence detection or Raman spectrum detection in a single point defect vibration mode of the material. However, under the condition of optical excitation, due to the limitation of the diffraction limit of light, the dimension of a light spot acted by the excitation light (whether the laser light or the conventional light source) on the irradiated material, whether the surface or the depth, is in the same order of magnitude as the wavelength. The distance between the lens of the light gathering and signal collecting system and the material test point cannot be too close, the distance of the general equipment is more than 10mm, the numerical aperture of the lens cannot be too large, and the actual size of the excitation light spot is several times or even more than ten times of the wavelength. The ratio of the number of atoms in the material region excited by light to the number of atoms in the effective active area of the tip is six to seven orders of magnitude different. The light scattering and fluorescence generated by the material region excited by light is a huge background light signal, and if the spectrum of the tip enhanced light is not overlapped with the spectrum of the surrounding region in energy, the extraction of the enhanced light signal is convenient, but in practical situations, particularly when testing the material with the substrate, the spectrum of the tip enhanced light is overlapped with the spectrum of the surrounding region in energy distribution. The overlapping influences on signal observation just like observing trees on the mountain with a magnifying glass at a distance, details and accurate information are extremely difficult to obtain, and the mountain background can be erased to see specific details by climbing to the vicinity of the trees on the mountain and observing with the magnifying glass.
Based on the thought, the STM needle point is driven to do vertical periodic motion with fixed height up and down in a certain height range above the surface of a test sample, so that an enhanced optical signal with enough multiplication magnitude is induced at the lowest point, the spectral signals induced by laser excitation of the STM needle point at the highest point and the lowest point of the motion period are collected to be subjected to subtraction processing so as to remove intrinsic light emission and light scattering of materials induced by the laser excitation, namely background optical signals are eliminated, the enhanced optical signal induced by the STM needle point at the lowest point is extracted, and an atomic magnitude light response mode spatial image is obtained.
The following detailed description of embodiments of the invention will be made with reference to the accompanying drawings.
FIG. 1 shows the structure of an STM tip enhanced spectral acquisition apparatus according to one embodiment of the present invention.
As shown in fig. 1, an STM tip enhanced spectrum acquisition apparatus provided by an embodiment of the present invention includes: the system comprises a laser 1, an STM needle tip 2, a needle tip piezoelectric displacement table 3, a spectrometer 4, a frequency multiplier 5, a sample table and a signal processing system.
The sample stage is actually a two-dimensional moving platform, and is used for bearing the test sample 6 and driving the test sample 6 to move in the X-axis direction and the Y-axis direction along the horizontal direction, so that two-dimensional space imaging is realized. Since the two-dimensional mobile platform is the prior art, it is not described herein again.
The laser 1 is used for emitting pulse laser and focusing 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 a visible light band and a near infrared band, and the output mode of the laser is a steady state or high pulse repetition frequency (>1 MHz).
The STM tip 2 is a metal tip that is positioned above and perpendicular to the surface of the test sample 6 and is directed towards the center of the spot formed on the surface of the test sample 6 by the pulsed laser light emitted by the laser 1.
The needle tip piezoelectric displacement platform 3 is used for driving the STM needle tip 2 to do periodic displacement motion perpendicular to the surface of the test sample 6. When the STM tip 2 is at its lowest point within one cycle of motion (i.e. the position at which the STM tip 2 is closest to the surface of the test sample 6), an enhanced optical signal is induced at the surface of the test sample 6; when the STM tip 2 is at the highest point within one period of motion (i.e. the position at which the STM tip 2 is furthest from the surface of the test sample 6), the light enhancing effect of the STM tip 2 on the surface of the test sample 6 is reduced. The periodic displacement motion of the STM tip 2 is actually a stepwise displacement motion performed at a fixed frequency perpendicular to the surface of the test sample 6.
Therefore, in the embodiment of the invention, the STM needle point 2 is driven by the needle point piezoelectric displacement platform 3 to make the step-type displacement motion vertical to the surface of the test sample 6, so that the enhanced optical signal with enough multiplication magnitude can be induced at the lowest point of the step-type displacement motion, and the enhanced effect of the STM needle point 2 on the surface of the test sample 6 can be eliminated at the highest point, and the reliability of mechanical high-frequency displacement and the adaptation (including the bandwidth of driving response, power consumption, thermal effect and the like) with the STM device can be ensured.
As the molecular fluorescence intensity enhanced by the STM needle tip 2 becomes exponential attenuation along with the increase of the distance between the STM needle tip 2 and molecules, the needle tip fluorescence intensity is attenuated by nearly 7 orders of magnitude in the variation range of 5 nm. Based on experimental data and theoretical analysis, considering the change of enhancement effect evolution caused by the difference of different needle point curvature radiuses and dielectric properties of test materials, in order to optimize background effect reduction and system response, the invention sets the highest point position of step-type displacement motion of the STM needle point 2 within the range of 3-100 nm above the surface of the test sample 6, and sets the lowest point position as the optimal point of STM image scanning, which is about 0.1-1 nm above the surface of the test sample 6.
When the STM needle point 2 is displaced upwards in a direction perpendicular to the surface of the test sample 6, the enhancement effect of the STM needle point 2 is attenuated, and if the highest point position of the STM needle point 2 is set too low and is less than 3nm, the difference of fluorescence or light scattering intensity caused by the movement of the STM needle point 2 can be used for substituting background fluorescence and light scattering fluctuation noise induced by laser, so that the signal-to-noise ratio is low, and accurate, reliable and clear signals cannot be extracted; and if the highest point position of the STM needle point 2 is set to be too high and is larger than 100nm, the requirements of the driver and the system for the reciprocating motion of the needle point on the response bandwidth and the difficulty of ensuring the moving line type of the needle point are greatly improved, and unnecessary control risks are increased. Therefore, the invention controls the range of the highest point position of the STM needle point 2 to be 3-100 nm above the surface of the test sample 6.
In the invention, the repetition frequency of the up-and-down displacement of the needle tip piezoelectric displacement table 3 is not lower than 10Hz, the optimal selection is 200Hz, and if the signal is strong enough, the high repetition frequency can be 500 Hz. The repetition frequency of the needle tip piezoelectric displacement table 3 is not high, and the low frequency is mainly controlled to obtain a highly accurate and reliable spectrum signal.
The spectrometer 4 is used to collect the spectral signals excited by the STM tip 2 at the lowest and highest points as the STM tip 2 moves to the lowest and highest point positions. The spectrum collection and analysis of the spectrometer 4 adopts a CCD detector spectrometer provided with a narrow-band edge or notch filter plate, and the spectral resolution of the spectrometer 4 is higher than 5cm-1The response wavelength range of the spectrometer 4 covers the laser wavelength and extends to 4000cm on both sides or long wave side of the laser wavelength-1And (c) other than. The CCD detector 41 may also be replaced with a CMOS detector.
Because the change of the displacement of the STM needle point 2 generates strong relevance to the optical signal enhanced by the STM needle point 2, the random STM needle point 2 displacement position is not acceptable when the shutter of the CCD detector 41 is opened, and the spectrum shooting process of the spectrometer has time sequences such as gating time, CCD reading time, dead time and the like, so that the acquisition of a spectrum signal can be completed, and in some time periods in the process, the movement of the displacement can bring great fluctuation noise, so that the acquisition of a spectrum signal is completed by keeping the STM needle point 2 at a certain position. The invention adopts time sequence driving voltage/current of trapezoidal wave or square wave to modulate the movement of the STM needle point 2, so that the retention time of the STM needle point 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 intensity of background noise signal 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. Therefore, the modulation period of the driving voltage of the needle tip piezoelectric displacement table 3 is controlled within 500Hz, meanwhile, the displacement driving modulator triggers a synchronous frequency multiplier 5 to generate a frequency-doubled pulse signal to trigger the gate control opening of the spectrometer 4 and the CCD detector 41, the phase of the frequency multiplier 5 is locked with the phase of the gate control pulse of the spectrometer 4 and can be optimally adjusted, the maximum response signal is achieved, and the background intensity of noise is suppressed.
Because the STM needle point 2 makes step-type displacement motion, the driving signal of the needle point piezoelectric displacement platform 3 must be in a step type of voltage/current change along with time. As shown in FIG. 2, the driving signal of the tip piezoelectric displacement table 3 is trapezoidal wave or square wave.
In one embodiment of the 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 finishes the collection of the spectrum. The rising edge and the falling edge of a driving signal of the pinpoint piezoelectric displacement table 3 are synchronous with the shutter trigger of the CCD detector 41 through the frequency multiplier 5, and the time ratio of the rising edge to the falling edge is not higher than 5%.
The signal processing system is used for carrying out subtraction processing on two spectrum signals collected by the spectrometer 4 in each motion period to obtain a subtraction spectrum, so that the same position of the test sample 6 is repeatedly collected to obtain a plurality of groups of subtraction spectra, and the plurality of groups of subtraction spectra are linearly superposed to obtain an enhanced spectrum.
The above details explain the structure and the working principle of the STM tip enhanced spectrum acquisition apparatus provided by the present invention. Corresponding to the STM needle tip enhanced spectrum acquisition device, the 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 STM tip enhanced spectral acquisition method according to one embodiment of the invention.
As shown in fig. 3, the method for acquiring an STM tip enhanced spectrum according to an embodiment of the present invention includes the following steps:
and S1, driving the test sample to move along the horizontal direction through the sample stage, and simultaneously enabling the laser to emit pulse laser to be focused on the surface of the test sample to form a circular or elliptical light spot.
And S2, driving the needle tip piezoelectric displacement platform through the driving signal to drive the STM needle tip to make a step displacement motion perpendicular to the center of the light spot, and simultaneously triggering the frequency multiplier to output a frequency doubled pulse signal to the spectrometer so that the driving signal synchronously triggers the spectrometer on the rising edge and the falling edge respectively.
When the STM tip moves to the lowest point, an enhanced optical signal is induced at the surface of the test sample.
And S3, collecting spectral signals induced on the surface of the test sample when the STM needle point is at the lowest point and the highest point in one motion period through the spectrometer.
In order to ensure that the spectrometer finishes spectrum collection, the retention time of the STM needle point at the lowest point position and the highest point position in the motion period is respectively longer than the collection time of the spectrometer, and the spectrum collection of the STM needle point at the lowest point position and the highest point position is finished.
S4, carrying out subtraction processing on the two spectrum signals collected in each motion period through a signal collection system to obtain a subtraction spectrum, and carrying out linear superposition on the basis of a plurality of subtraction spectrums to obtain a spectrum with background signals removed; wherein N is more than 5 and less than 10.
And taking the spectrum obtained by the STM needle point at the highest point position as a background, taking the spectrum obtained by the STM needle point at the lowest point position in the motion period as a response, and performing a subtraction algorithm on the spectrum obtained by the highest point and the spectrum obtained by the lowest point to remove background light signals.
When the spectrum at the highest point position is obtained, the spectrum needs to be screened and judged whether to meet the condition. The specific process is as follows: the spectrum signal induced by N groups of STM needle points at the highest point position in the motion period is measured in advance, the background spectrum reference and the mean square deviation are determined through the statistical distribution and weighted average processing of the N groups of spectrum signals, the spectrum signal obtained by the STM needle point at the highest point position in the motion period every time is screened according to the background spectrum reference and the mean square deviation, and the spectrum signal is used as an invalid spectrum when the spectrum signal deviates from the background spectrum reference and exceeds the mean square deviation by 1 time.
FIG. 4 shows a second flow of a STM tip enhanced spectral acquisition method according to one embodiment of the invention.
As shown in fig. 4, the method for acquiring an STM tip enhanced spectrum provided by the embodiment of the present invention includes the following steps:
and S1', moving the STM needle tip to the surface of the test sample, and optimizing the position of the needle tip of the tunnel current.
And S2', adjusting the laser converged light spot to overlap with the STM needle tip.
And S3', moving the STM needle tip upwards, and determining the background spectrum reference and the mean square deviation.
After the laser and STM system are stabilized, the needle tip is moved away from the surface of the sample, the spectra at 5-10 high points are measured, and the background spectrum reference and the mean square deviation are obtained through statistical distribution and weighted average processing. And screening whether the spectrum obtained when the STM needle point is shifted to the highest point is a usable spectrum or not according to the background spectrum standard.
And S4', driving the needle tip piezoelectric displacement table to control the STM needle tip to move upwards to the highest position, and collecting the spectrum through a spectrometer.
S5 ', judging whether the deviation of the spectrum from the background spectrum reference exceeds the mean square deviation, if so, executing a step S4 ', and if not, executing a step 6 '.
And after obtaining the background spectrum reference and the mean square deviation, normally testing the spectrum obtained when the STM needle point is shifted to the highest point, and if the spectrum deviates from the background spectrum reference by more than 1 time of the mean square deviation, taking the spectrum as invalid data, thereby reducing the influence of environment, laser beam induced background and laser fluctuation.
And S6', driving the needle tip piezoelectric displacement table to control the STM needle tip to move downwards to the lowest point position, and collecting the spectrum through a spectrometer.
When the STM needle point moves downwards to the lowest point position, an enhanced optical signal of the surface of the test sample can be induced.
And S7', determining a difference subtraction spectrum, and storing the spectrum data.
And obtaining a group of spectrum signals in each motion period, and subtracting background light scattering or fluorescence induced by laser irradiation by using the spectrum obtained at the highest point as a background and the spectrum obtained at the lowest point as a response through a subtraction algorithm.
S8 ', judging whether the stored spectrum data reaches the set value, if yes, proceeding to step S9 ', if not, proceeding to step S4 '.
And S9', overlapping the multiple groups of spectral data to obtain an enhanced spectrum.
Repeatedly obtaining 10-50 difference reduction spectra at each scanning single point of the test sample, selecting not less than 10 spectra meeting conditions by screening the 10-50 spectra, and performing superposition processing, thereby eliminating the influence of environment, laser beam induced background and laser fluctuation. The number of spectral groups which are repeatedly iterated at each test point can be selected and optimized between 5 and 50 times according to the spectral signal-to-noise ratio characteristics.
In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer 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, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.
The above embodiments of the present invention should not be construed as limiting the scope of the present invention. Any other corresponding changes and modifications made according to the technical idea of the present invention should be included in the protection scope of the claims of the present invention.
Claims (10)
1. An STM needle tip enhanced spectrum acquisition device is characterized by comprising a laser, an STM needle tip, a needle tip piezoelectric displacement table, a sample table, a spectrometer, a frequency multiplier and a signal processing system; wherein the content of the first and second substances,
the sample table is used for bearing a test sample and driving the test sample to move along the horizontal direction;
the laser is used for emitting pulse laser and focusing on the surface of the test sample to form a light spot;
the STM needle point is vertical to the surface of the test sample and points to the center of the light spot;
the needle point piezoelectric displacement platform is used for driving the STM needle point to make step displacement motion vertical to the surface of the test sample under the drive of a drive signal;
the frequency multiplier is used for outputting a frequency-doubled pulse signal to the spectrometer under the trigger of the driving signal, so that the driving signal synchronously triggers the spectrometer on a rising edge and a falling edge respectively;
the spectrometer is used for acquiring spectral signals induced on the surface of the test sample when the STM needle point is located at the lowest point and the highest point in a motion period; wherein, when the STM needle point is at the lowest point in the movement period, an enhanced optical signal is excited on the surface of the test sample;
the signal processing system is used for carrying out subtraction processing on the two spectral signals collected in each motion period to obtain a subtraction spectrum, and carrying out linear superposition on the multiple subtraction spectra at the same position of the test sample to obtain an enhanced spectrum.
2. The STM tip enhanced spectrum acquisition device of claim 1, wherein the position of the lowest point of the STM tip in the motion period is 0.1-1 nm away from the surface of the test sample, and the position of the highest point of the STM tip in the motion period is 3-100 nm away from the surface of the test sample.
3. An STM needle tip enhanced spectral acquisition apparatus as claimed in claim 1 or claim 2 in which the holding time of the STM needle tip at the lowest and highest point positions respectively within a movement cycle is greater than the acquisition time of the spectrometer.
4. An STM tip enhanced spectral acquisition apparatus according to claim 3 in which the frequency of movement of the STM tip is between 10Hz and 500 Hz.
5. An STM needle tip enhanced spectral acquisition apparatus 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 motion cycle is 3-10 ms.
6. An STM tip enhanced spectral acquisition apparatus according to claim 1, wherein the waveform driving the tip piezoelectric displacement stage is a square wave or a trapezoidal wave.
7. An STM needle tip enhanced spectrum acquisition method is characterized by comprising the following steps:
s1, driving the test sample to move along the horizontal direction through the sample stage, and simultaneously enabling the laser to emit pulse laser to be focused on the surface of the test sample to form a circular or elliptical light spot;
s2, driving the needle tip piezoelectric displacement platform through a driving signal to drive the STM needle tip to make a step displacement motion perpendicular to the center of the light spot, and simultaneously triggering a frequency multiplier to output a frequency doubled pulse signal to a spectrometer so that the driving signal respectively synchronously triggers the spectrometer on a rising edge and a falling edge;
s3, collecting spectral signals induced on the surface of the test sample when the STM needle point is located at the lowest point and the highest point in a motion period through a spectrometer;
and S4, performing subtraction processing on the two spectrum signals acquired in each motion period through the signal acquisition system to obtain a subtraction spectrum, and performing linear superposition on the basis of the plurality of subtraction spectra to obtain a spectrum with the background signal removed.
8. The method for acquiring the enhanced spectrum of the STM needle tip as claimed in claim 7, wherein the retention time of the STM needle tip at the lowest point position and the highest point position in the motion period is respectively longer than the acquisition time of the spectrometer; and the distance between the lowest point position of the STM needle point in the motion period and the surface of the test sample is 0.1-1 nm, and the distance between the highest point position of the STM needle point in the motion period and the surface of the test sample is 3-100 nm.
9. An STM tip enhanced spectral acquisition method according to claim 7 in which background light signals are removed by a subtraction algorithm with the spectrum obtained by the STM tip at the highest point in the movement period as the background and the spectrum obtained by the STM tip at the lowest point in the movement period as the response.
10. The method for acquiring the enhanced spectrum of the STM needle tip according to claim 9, wherein spectral signals induced by N groups of STM needle tips at the highest point in the motion period are measured in advance, a background spectral reference and a mean square deviation are determined by statistical distribution and weighted average processing of the N groups of spectral signals, the spectral signals acquired by the STM needle tip at the highest point in the motion period are screened according to the background spectral reference and the mean square deviation, and the spectral signals are used as invalid spectra when the spectral signals deviate from the background spectral reference by more than 1 time of the mean square deviation; wherein N is more than 5 and less than 10.
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