CN116297337A - Method for judging number of layers of two-dimensional material by using dark field optical imaging technology - Google Patents
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- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 24
- 229910052710 silicon Inorganic materials 0.000 claims description 24
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- 229910052594 sapphire Inorganic materials 0.000 claims 1
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- 229910052723 transition metal Inorganic materials 0.000 claims 1
- -1 transition metal sulfide Chemical class 0.000 claims 1
- 230000005540 biological transmission Effects 0.000 abstract description 9
- 238000001237 Raman spectrum Methods 0.000 abstract description 4
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/47—Scattering, i.e. diffuse reflection
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
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Abstract
The invention relates to a method for judging the number of layers of a two-dimensional material by using a dark field optical imaging technology, which comprises the following steps: light generated by the light source is obliquely incident on the observed two-dimensional material sample and the substrate through a light path of the dark field optical imaging technology system, and part of scattered light is in a light collecting range of optical information receiving equipment of the dark field optical imaging technology system; collecting scattered light of a layer number change area of a two-dimensional material sample, and converting the scattered light into an optical photo; reading the brightness value of each pixel in the optical photo, and drawing a position-brightness curve; calculating dark field contrast or relative contrast; and calculating the number of the two-dimensional material layers according to the correlation between the number of the two-dimensional material layers and the dark field contrast or the relative contrast. The invention is far higher in precision than the existing optical contrast method. The equipment cost and the time cost of the invention are far lower than those of the existing atomic force microscope, raman spectrum and scanning electron transmission microscope methods. The invention has the adjustable range and the adjustable precision which are not possessed by the existing optical method.
Description
Technical Field
The invention relates to a method for judging the number of layers of a two-dimensional material by using a dark field optical imaging technology, and belongs to the technical field of measurement and characterization of the two-dimensional material.
Background
Dark field optical imaging techniques refer to optical imaging techniques that use oblique incidence of light and that utilize the observed physical edge reflection. Dark field optical imaging techniques are generally classified as transmissive dark fields and reflective dark fields. Among them, the transmission type dark field is often used for detection in the biological and medical fields. The advantage of dark field optical imaging techniques is that the profile of the sample being observed can be clearly resolved. Dark field optical imaging images have the advantage of high contrast compared to bright field (bright field) images. Reflective dark fields often need to be used with metallographic microscopy systems. Are often used to observe powders or metal particles.
A two-dimensional material refers to a material in which electrons in the material can move in only two dimensions in a plane. The thickness is only one atomic layer to several tens of atomic layers. Two-dimensional materials tend to have superior properties compared to the corresponding bulk materials. A prominent representation of two-dimensional materials is graphene found in 2004 and acquired the nobel prize in 2010. The properties of two-dimensional materials tend to be closely related to thickness. Therefore, accurate measurement of the thickness of two-dimensional materials is a prerequisite for further research.
However, since two-dimensional materials have only atomic thicknesses, there remains a challenge to accurate characterization of their thickness. The existing detection method mainly comprises the following steps: optical contrast, raman spectroscopy, atomic force microscopy, and scanning transmission electron microscopy. Among them, the optical contrast method is the mainstream method at present, has fast, advantage with low costs. There are national standards based on the optical contrast method: GB/T40071-2021. When the number of observation layers is less than 5, the Raman spectroscopy has the characteristic of obvious characteristic peaks. However, raman spectroscopy is difficult to characterize two-dimensional material layers with a high number of layers. There are national standards based on raman spectroscopy: GB/T40069-2021. The atomic force microscope has higher theoretical precision, but is easily affected by the measurement parameter setting and the environment. Compared with the optical contrast method, the atomic force microscope equipment has higher price and longer scanning time. Scanning transmission electron microscopy has the most intuitive microstructure image to obtain the layer number, but the scanning transmission electron microscopy equipment is extremely expensive, the sample preparation process is very complex, and additional large-scale equipment is needed to assist in sample preparation. At the same time, the scanning transmission electron microscope measurement can generate irreversible damage to the tested sample.
In summary, the main problems of the conventional optical contrast method are as follows: the accuracy is low, and when the refractive index of the observed sample is similar to that of the substrate, the contrast is low, so that the method is difficult to be applied to a transparent substrate. Only qualitative analysis can be performed when samples with more layers are characterized; the main problems with raman spectroscopy are: the laser light source and the related light path are expensive to build, the thicker sample is difficult to characterize, and the time consumption is long when the surface scanning is performed. The main problems of atomic force microscopy are: the equipment is expensive and is easily influenced by the substrate and the environment; scanning transmission microscopy has the problems: the equipment is extremely expensive, the measurement requires complex sample preparation operations, and the measurement can generate irreversible damage to the sample.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a method for judging the number of layers of a two-dimensional material by using a dark field optical imaging technology;
the invention has the advantages of high contrast, low cost, low time consumption, adjustability, no damage and high universality. The most remarkable characteristic is that the contrast index is far higher than the prior national standard: the optical contrast method for measuring the number of layers of the graphene related two-dimensional material by using the GB/T40071-2021 nanotechnology.
The technical scheme of the invention is as follows:
a method for discriminating the number of layers of a two-dimensional material using dark field optical imaging techniques, comprising:
light generated by the light source is obliquely incident on the observed two-dimensional material sample and the substrate through a light path of the dark field optical imaging technology system, the incident light irradiated on the layer number change area of the two-dimensional material sample is subjected to Rayleigh scattering due to the difference between electric dipole moment and the internal area of the two-dimensional material sample, and part of scattered light is positioned in a light collecting range of optical information receiving equipment of the dark field optical imaging technology system;
collecting scattered light of a layer number change area of a two-dimensional material sample by using optical information receiving equipment of a dark field optical imaging technology system, and converting the scattered light into an optical photo by using imaging equipment in the dark field optical imaging technology system;
reading the brightness value of each pixel in the optical photo, and drawing a position-brightness curve;
calculating dark field contrast, which is the difference between the brightness of the two-dimensional material sample and the brightness of the substrate; the brightness of a two-dimensional material sample refers to the maximum brightness value in the position-brightness curve; the brightness of the substrate is the average of the brightness values of the photographs read at the locations where there is no two-dimensional material sample;
and calculating the number of the two-dimensional material layers according to the correlation between the number of the two-dimensional material layers and the dark field contrast.
According to the invention, the process for obtaining the correlation between the two-dimensional material layer number and dark field contrast is as follows:
light generated by the light source is obliquely incident on a two-dimensional material sample and a substrate with known layers through a light path of a dark field optical imaging technology system, incident light irradiated on a layer number change area of the two-dimensional material sample is subjected to Rayleigh scattering at the position of the layer number change area of the two-dimensional material sample due to the difference between electric dipole moment and an internal area of the sample, and part of scattered light is positioned in a light collecting range of optical information receiving equipment of the dark field optical imaging technology system;
collecting scattered light of a layer number change area of a two-dimensional material sample by using optical information receiving equipment of a dark field optical imaging technology system, and converting the scattered light into an optical photo by using imaging equipment in the dark field optical imaging technology system;
reading the brightness value of each pixel in the optical photo, and drawing a position-brightness curve;
calculating dark field contrast, which is the difference between the brightness of the two-dimensional material layer number change area and the brightness of the substrate;
and establishing the correlation between the layer number of the two-dimensional material and the dark field contrast.
According to the present invention, preferably, reading the luminance value of each pixel in the optical photograph and plotting the position-luminance curve includes: and reading the brightness value of each pixel in the optical photo by a corresponding function of a picture reading function in image processing software, and drawing a position-brightness curve by taking the position as an X axis and the brightness value as a Y axis.
According to the invention, when the two-dimensional material is graphene, the correlation between the number of layers of the two-dimensional material and the dark field contrast is shown as the formula (1):
m＝n*t+s (1)
in the formula (1), m is dark field contrast, n is a contrast contribution value of each layer, t is the number of two-dimensional material layers, and s is a correction parameter.
It is further preferable that when the illumination intensity of the light source is 36.6. Mu.W/cm 2 When the integration time of the imaging device is 2000ms, the contrast contribution value of the graphene of each layer on the 1133 rd line is 40.3+/-4.3.
According to the invention, the dark field optical imaging technical system comprises a light source, an optical information receiving device and an imaging device; the model of the dark field optical imaging technology system is Eclipse LV150N, nikon; the model of the optical information receiving device is CFI TU plan EPI ELWD, and the model of the imaging device is Ds-Ri2, nikon.
Further preferably, the resolution of the obtained photo is 2048 pixels by 1536 pixels, the single pixel brightness of the obtained photo is 0 at the lowest value, 255 at the highest value, and the brightness step length is 1, 1 column pixel or 1 row pixel passing through the two-dimensional material sample is selected, the brightness value of each pixel is read, and the position-brightness curve is drawn; or selecting a plurality of columns of pixels or a plurality of rows of pixels where the two-dimensional material sample is located, reading the brightness value of each pixel, calculating the average value of the brightness values of the plurality of columns of pixels or the plurality of rows of pixels, and drawing a position-brightness curve.
According to the invention, the wavelength of light emitted by the light source covers all optical bands;
further preferably, the optical wavelength of the light emitted by the light source covers an optical band of 156-1000nm.
Most preferably, the wavelength of light emitted by the light source covers an optical band of 300-800nm.
According to a preferred embodiment of the present invention, the dark field optical imaging technique system employs a dark field mode.
According to the invention, the silicon substrate with different silicon dioxide layer thicknesses of 0-2000nm is adopted as the substrate;
further preferably, the substrate is a transparent substrate; the thickness of the substrate is 100 μm to 5000 μm.
The beneficial effects of the invention are as follows:
1. the invention is far higher in precision than the traditional optical contrast method. Taking the example of detecting a single layer of graphene sample, the dark field contrast obtained using this method is about 45%, whereas the highest value of the existing method is about 10%. Compared with the traditional optical method, the dark field method provided by the invention is based on the Rayleigh scattering of the sample, is less influenced by the substrate, and therefore, can be suitable for various substrates, and can be typically used for silicon substrates or transparent substrates with different silicon dioxide layer thicknesses.
2. The invention is far lower than the methods of atomic force microscope, raman spectrum, scanning electron transmission microscope, etc. in equipment cost and maintenance cost. The equipment cost of the method is similar to that of the existing optical method, and is about 5 ten thousand yuan (RMB, the same applies below). Atomic force microscopes are hundreds of thousands to millions of yuan in price and probes need to be replaced frequently, with each probe being about 1000 yuan in price. Raman spectroscopy is priced at millions of yuan or more. Scanning electron transmission microscopes are priced at millions of yuan or more. And the sample preparation of the focusing ion beam equipment is needed, and the equipment cost of the focusing ion beam equipment is millions of yuan or more. The maintenance costs for each of the scanning electron transmission microscopes and the focused ion beam apparatus are about hundreds of thousands of yuan per year. The invention has remarkable advantages in equipment cost and maintenance cost.
3. The invention is far lower than the methods of atomic force microscope, raman spectrum, scanning electron transmission microscope, etc. in time cost. The time cost of the method is similar to that of the existing optical method, and the test time of a single sample is only a few minutes. To obtain a scan range comparable to the size of an optical microscope, the single sample test time of an atomic force microscope is at least about half an hour. The single point test of the raman spectrometer takes only a few seconds, but requires about half an hour to perform laser preheating and substrate calibration before use. In order to obtain a scanning range comparable to the size of an optical microscope, the surface scanning mode of a raman spectrometer requires at least 1 hour. The observation of the scanning electron transmission microscope system needs a plurality of steps such as vacuumizing, sample searching, amplifying, focusing, photographing and the like, and takes about 1 hour. And also requires about 1 hour to use in making a sample using a focused ion beam apparatus. The invention has significant advantages in terms of time costs.
4. The invention has an adjustable range and adjustable precision which are not possessed by the traditional optical method. The existing optical method based on the reflection principle has a fixed detection range and detection precision on the premise of not changing the wavelength of incident light (greatly increasing the cost if a monochromatic light source with adjustable wavelength is used). Only qualitative results were given when facing samples of thicker thickness. On the premise of not increasing the cost additionally, the contrast contribution value of each layer of two-dimensional material sample can be changed simply and quickly by changing the light intensity of the incident light, and further, the proper precision and the proper measurement range are selected according to the thickness of the measured sample.
Drawings
FIG. 1 is a schematic diagram of an apparatus setup for a method of discriminating the number of layers of a two-dimensional material using dark field optical imaging technique, implemented in accordance with the present invention;
FIG. 2 is a schematic overall flow chart of a method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to the present invention;
FIG. 3 is a detailed flow chart of a method for determining the number of layers of a two-dimensional material using dark field optical imaging techniques according to the present invention;
FIG. 4 is a schematic diagram of a gray-scale format picture with a resolution of 2048 pixels by 1536 pixels and a cut-away position thereof;
FIG. 5 is a graph showing the relationship between the brightness and the position of the horizontal pixel corresponding to the 1133 rd line, i.e. a position-brightness curve;
FIG. 6 (a) is a disulfideMolybdenum sample SiO 2 Dark field photomicrographs one on a silicon substrate having a layer thickness of 285 nm;
FIG. 6 (b) is a sample of molybdenum disulfide SiO 2 Dark field photomicrographs II on a silicon substrate having a layer thickness of 285 nm;
FIG. 6 (c) is a sample of molybdenum disulfide SiO 2 Dark field photomicrographs III on a silicon substrate having a layer thickness of 285 nm;
FIG. 7 (a) is a graph of graphene sample at SiO 2 Dark field photomicrographs one on a silicon substrate having a layer thickness of 285 nm;
FIG. 7 (b) is a graph of graphene sample at SiO 2 Dark field photomicrographs II on a silicon substrate having a layer thickness of 285 nm;
FIG. 7 (c) is a graph of graphene sample at SiO 2 Dark field photomicrographs III on a silicon substrate having a layer thickness of 285 nm;
FIG. 8 (a) is a graph of graphene sample at SiO 2 Dark field photomicrographs first on a silicon substrate having a layer thickness of 100 nm;
FIG. 8 (b) is a graph of graphene sample at SiO 2 Dark field photomicrographs II on a silicon substrate with a layer thickness of 100 nm;
FIG. 9 (a) is a graph of graphene sample without SiO 2 Dark field photomicrographs one on a bare silicon substrate of a layer;
FIG. 9 (b) is a graph of graphene sample in the absence of SiO 2 Dark field photomicrographs II on the bare silicon substrate of the layer;
FIG. 10 (a) shows the same boron nitride sample at SiO 2 Dark field photomicrograph one at 250ms exposure integration time on a silicon substrate with a layer thickness of 285 nm;
FIG. 10 (b) shows the same boron nitride sample at SiO 2 Dark field photomicrographs two at 500ms exposure integration time on a silicon substrate with a layer thickness of 285 nm;
FIG. 10 (c) shows the same boron nitride sample at SiO 2 Dark field photomicrographs three at 1000ms exposure integration time on a silicon substrate with a layer thickness of 285 nm;
FIG. 10 (d) shows the same boron nitride sample at SiO 2 Dark field photomicrographs four at 2000ms exposure integration time on a silicon substrate with a layer thickness of 285 nm;
FIG. 11 (a) is a dark field photomicrograph of a graphene sample on a transparent quartz substrate at an exposure integration time of 200 ms;
FIG. 11 (b) is a dark field photomicrograph II of a graphene sample on a transparent quartz substrate at an exposure integration time of 500 ms;
FIG. 11 (c) is a dark field photomicrograph III of a graphene sample on a transparent quartz substrate at an exposure integration time of 1000 ms;
FIG. 12 is a sample of SiO without two-dimensional material 2 Dark field photomicrographs on a silicon substrate having a layer thickness of 285 nm.
Detailed Description
The invention is further defined by, but is not limited to, the following drawings and examples in conjunction with the specification.
Example 1
A method for discriminating the layer number of a two-dimensional material by using a dark field optical imaging technology, as shown in fig. 2 and 3, comprises the following steps:
as shown in fig. 1, the light generated by the light source is obliquely incident on the observed two-dimensional material sample and the substrate through the optical path of the dark field optical imaging technology system, typically, the two-dimensional material prepared by the mechanical stripping method is randomly distributed on the substrate, and the shape, the size and the thickness distribution of the two-dimensional material are random, so that the two-dimensional material with the required thickness needs to be screened out by a certain method. The optical path of the dark field optical imaging technology system should meet the condition that the optical information receiving device is not positioned at the specular reflection angle of the incident light direction. Light obliquely incident on a substrate or planar area of a two-dimensional material follows the law of reflection of light, the angle of reflection being equal to the angle of incidence, such light reflected in a single direction exceeding the light collection range of an optical information receiving device of a dark-field optical imaging technique system and thus contributing no brightness to imaging. Incident light irradiated on a two-dimensional material layer number change area is different from internal electric dipole moment at the edge, rayleigh scattering occurs, the direction of scattered light is spindle-type distribution in space, and part of scattered light is positioned in the light collecting range of optical information receiving equipment of a dark field optical imaging technology system; the part of scattered light is the main light which can be received by optical information receiving equipment of the dark field optical imaging technology system;
collecting scattered light of a layer number change area of a two-dimensional material sample by using optical information receiving equipment of a dark field optical imaging technology system, and converting the scattered light into an optical photo by using imaging equipment in the dark field optical imaging technology system;
reading the brightness value of each pixel in the optical photo, and drawing a position-brightness curve; fig. 5 is a schematic diagram of a position-brightness curve, which is a relationship between brightness and position of horizontal pixels corresponding to the 1133 rd line. The upper right hand corner inset is a partial enlarged view.
Calculating dark field contrast, which is the difference between the brightness of the two-dimensional material layer number change area and the brightness of the substrate; the brightness of the two-dimensional material layer number change region refers to a brightness peak value in a position-brightness curve; the brightness of the substrate is the average of the brightness values of the photographs read at the locations where there is no two-dimensional material sample; the average value of the substrate brightness as in fig. 4 is 88;
and calculating the number of layers of the two-dimensional material according to the correlation between the change area of the number of layers of the two-dimensional material and the contrast of the dark field.
The process for obtaining the correlation between the two-dimensional material layer number and the dark field contrast is as follows: the method requires a calibration process. It is necessary to obtain the dark field contrast of a sample of known thickness of the layers by the above method. And a correlation between the number of layers and dark field contrast is established.
Light generated by the light source is obliquely incident on a two-dimensional material sample and a substrate with known layers through a light path of a dark field optical imaging technology system, incident light irradiated on a layer number change area of the two-dimensional material is subjected to Rayleigh scattering under the action of electric dipole moment at the edge of the two-dimensional material sample, and part of scattered light is positioned in a light collecting range of optical information receiving equipment of the dark field optical imaging technology system;
collecting scattered light of a layer number change area of a two-dimensional material sample by using optical information receiving equipment of a dark field optical imaging technology system, and converting the scattered light into an optical photo by using imaging equipment in the dark field optical imaging technology system;
reading the brightness value of each pixel in the optical photo, and drawing a position-brightness curve;
calculating dark field contrast, which is the difference between the brightness of the two-dimensional material layer number change area and the brightness of the substrate;
and establishing a correlation between the two-dimensional material layer number change area and the dark field contrast.
In the subsequent characterization, only the dark field contrast is obtained through the position-brightness curve, and then the correlation between the two-dimensional material layer number change area obtained in the calibration process and the dark field contrast can be utilized to calculate the two-dimensional material layer number.
The scattered light intensity can be expressed as
I 0 Is the incident light intensity, c is the vacuum light velocity, x 0 For electric dipole moment, the angle between the incident light alpha and the receiving area of the optical information receiving device is epsilon 0 Is the vacuum dielectric constant, λ is the wavelength of incident light, and d is the distance between the sample and the optical information receiving device. The scattered light intensities within the acceptance range of the optical information acceptance device are accumulated (integrated), i.e. the scattered light intensities at the corresponding edges.
Example 2
A method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to embodiment 1 is different in that:
the optical path of the dark field optical imaging technology system should be such that the optical information receiving device should be in a proper position such that light specularly reflected by the substrate and the two-dimensional material sample cannot be received by the optical information receiving device.
Reading the brightness value of each pixel in the optical photo and drawing a position-brightness curve, including: and reading the brightness value of each pixel in the optical photo through a corresponding function in software, and drawing a position-brightness curve by taking the position as an X axis and the brightness value as a Y axis.
The correlation between the two-dimensional material layer number and dark field contrast is shown in the formula (1):
m＝n*t+s (1)
in the formula (1), m is dark field contrast, n is a contrast contribution value of each layer, t is the number of two-dimensional material layers, and s is a correction parameter. The contrast contribution value n of each layer is given by a fitting value in the calibration process; the correction parameters are given by the result of the fitting. The correction parameters are affected by the scattering intensity of the whole edge of the sample.
The number of graphene layers can be verified by the existing methods such as atomic force microscope, raman spectrum and the like, and the correlation between the number of graphene layers and dark field contrast can be established.
A dark field photograph of a sample of unknown thickness was taken with the same exposure conditions and equipment level, with the external environment unchanged. Substituting the dark field contrast value of the two-dimensional material layer number change region into the correlation between the graphene layer number and the dark field contrast, and correspondingly obtaining the layer numbers of each edge of the tested sample.
The invention also supports the representation of the thickness of the two-dimensional material with adjustable range and precision. By changing the light intensity of the incident light, the contribution value of the contrast of each layer can be correspondingly changed, and then a proper observation range and proper observation precision can be selected. This was not the case with previous optical contrast methods.
When the illumination intensity of the light source is 36.6 mu W/cm 2 When the integration time of the imaging device is 2000ms, the contrast contribution value of the graphene of each layer on the 1133 rd line is 40.3+/-4.3.
The dark field optical imaging technical system comprises a light source and imaging equipment; the model of the dark field optical imaging technical system is Eclipse LV150N, nikon; the model of the optical information receiving device of the dark field optical imaging technology system is CFI TU plan EPI ELWD, and the model of the imaging device is Ds-Ri2, nikon.
The obtained image can adopt gray scale mode or other brightness calculation modes, and the brightness maximum value, minimum value and step size are specifically determined by the selected mode. The resolution of the obtained optical photo is 2048 pixels by 1536 pixels, the single pixel brightness of the obtained picture is 0 at the lowest value, 255 at the highest value and 1 in brightness step length, 1 column pixel or 1 row pixel passing through the two-dimensional material sample is selected, the brightness value of each pixel is read, and a position-brightness curve is drawn. Or selecting a plurality of columns of pixels or a plurality of rows of pixels where the two-dimensional material sample is located, reading the brightness value of each pixel, calculating the average value of the plurality of columns of pixels or the plurality of rows of pixels, and drawing a position-brightness curve.
The light source covers all optical bands, typically 156-1000nm, and more typically 300-800nm.
The number of layers of the two-dimensional material is 1 monoatomic layer to 127 monoatomic layers.
The two-dimensional sample to be observed is not limited in size in the horizontal plane, but a suitable movable carrier platform is required so that the edge of the sample to be observed can be located in the observable area of the optical information receiving device.
Dark field optical imaging technology systems employ a dark field mode.
In the present invention, the brightness of the substrate is caused by the scattered light and the multiple reflections of the lens of the optical information receiving apparatus. If the same contrast calculation method as other conventional optical methods is used, the acquired contrast becomes dark-field relative contrast. The specific calculation formula is as follows:
under the above conditions, the average value of the substrate brightness is 88, and the brightness contribution value of each layer of graphene is (45.7% ± 4.8%), which is higher than the existing national standard in contrast: the optical contrast method for measuring the number of layers of the graphene related two-dimensional material by using the GB/T40071-2021 nanotechnology. It is worth noting that the dark field relative contrast varies with the intensity of the incident light. In the invention, dark field relative contrast is only used as a parameter compared with traditional optical contrast, and is not used as a main method for judging the layer number.
Example 3
A method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to embodiment 2 is different in that:
FIGS. 6 (a), 6 (b) and 6 (c) are a plurality of molybdenum disulfide samples SiO 2 Dark field photomicrographs on a silicon substrate having a layer thickness of 285 nm. Wherein the illumination intensity of the light source is 36.6 mu W/cm 2 The integration time of the imaging device is 50ms.
Example 4
A method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to embodiment 2 is different in that:
FIGS. 7 (a), 7 (b) and 7 (c) are the SiO of the graphene samples 2 Dark field photomicrographs on a silicon substrate having a layer thickness of 285 nm. The light intensity of the light source was 36.6. Mu.W/cm 2 The integration time of the imaging device is 2000ms.
Example 5
A method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to embodiment 2 is different in that:
FIGS. 8 (a) and 8 (b) are the SiO of the graphene samples 2 Dark field photomicrographs on a silicon substrate with a layer thickness of 100 nm. The light intensity of the light source was 36.6. Mu.W/cm 2 The integration time of the imaging device is 2000ms.
Example 6
A method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to embodiment 2 is different in that:
FIGS. 9 (a) and 9 (b) are the SiO-free of the graphene samples 2 Dark field photomicrographs on bare silicon substrate of layer. The light intensity of the light source was 36.6. Mu.W/cm 2 The integration time of the imaging device is 1000ms.
Example 7
A method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to embodiment 2 is different in that:
FIGS. 10 (a), 10 (b), 10 (c) and 10 (d) show the same boron nitride sample on SiO 2 Dark field photomicrographs at different exposure integration times on a silicon substrate with a layer thickness of 285 nm. The light intensity of the light source was 36.6. Mu.W/cm 2 The integration times of the imaging devices were 250, 500, 1000, 2000ms, respectively.
Example 8
A method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to embodiment 2 is different in that:
FIGS. 11 (a), 11 (b) and 11 (c) show transparent quartz substratesDark field photomicrographs of the graphene sample at different exposure integration times; the light intensity of the light source was 36.6. Mu.W/cm 2 The integration time of the imaging device is divided into 200, 500, 1000ms.
FIG. 12 is a sample of SiO without two-dimensional material 2 Dark field photomicrographs on a silicon substrate having a layer thickness of 285 nm. The light intensity of the light source was 36.6. Mu.W/cm 2 The integration times of the imaging devices were 2000ms, respectively.
Dark field optical pictures of the same graphene sample under different magnification factors are sequentially 50 times, 20 times and 10 times from left to right. The multiple of the eyepiece is 10 times. The light intensity of the light source was 36.6. Mu.W/cm 2 The integration time of the imaging device is 2000ms.
Claims (10)
1. A method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique, comprising:
light generated by the light source is obliquely incident on the observed two-dimensional material sample and the substrate through a light path of the dark field optical imaging technology system, the incident light irradiated on the layer number change area of the two-dimensional material sample is subjected to Rayleigh scattering due to the difference between electric dipole moment and the internal area of the two-dimensional material sample, and part of scattered light is positioned in a light collecting range of optical information receiving equipment of the dark field optical imaging technology system;
collecting scattered light of a layer number change area of a two-dimensional material sample by using optical information receiving equipment of a dark field optical imaging technology system, and converting the scattered light into an optical photo by using imaging equipment in the dark field optical imaging technology system;
reading the brightness value of each pixel in the optical photo, and drawing a position-brightness curve;
calculating dark field contrast, which is the difference between the brightness of the two-dimensional material sample and the brightness of the substrate; the brightness of a two-dimensional material sample refers to the maximum brightness value in the position-brightness curve; the brightness of the substrate is the average of the brightness values of the photographs read at the locations where there is no two-dimensional material sample;
and calculating the number of the two-dimensional material layers according to the correlation between the number of the two-dimensional material layers and the dark field contrast.
2. The method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique according to claim 1 wherein the process of obtaining the correlation with the dark field contrast of the area where the number of layers of the two-dimensional material changes is:
light generated by the light source is obliquely incident on a two-dimensional material sample and a substrate with known layers through a light path of a dark field optical imaging technology system, incident light irradiated on a layer number change area of the two-dimensional material sample is subjected to Rayleigh scattering at the position of the layer number change area of the two-dimensional material sample due to the difference between electric dipole moment and an internal area of the sample, and part of scattered light is positioned in a light collecting range of optical information receiving equipment of the dark field optical imaging technology system;
collecting scattered light of a layer number change area of a two-dimensional material sample by using optical information receiving equipment of a dark field optical imaging technology system, and converting the scattered light into an optical photo by using imaging equipment in the dark field optical imaging technology system;
reading the brightness value of each pixel in the optical photo, and drawing a position-brightness curve;
calculating dark field contrast, which is the difference between the brightness of the two-dimensional material layer number change area and the brightness of the substrate;
and establishing the correlation between the layer number of the two-dimensional material and the dark field contrast.
3. A method of discriminating a number of layers of a two-dimensional material using a dark field optical imaging technique as defined in claim 1 wherein reading a luminance value of each pixel in an optical photograph and plotting a position-luminance curve includes: and reading the brightness value of each pixel in the optical photo by a corresponding function of a picture reading function in picture processing software, and drawing a position-brightness curve by taking the position as an X axis and the brightness value as a Y axis.
4. The method for discriminating the number of layers of the two-dimensional material by using the dark field optical imaging technique according to claim 1 wherein when the two-dimensional material is graphene, the correlation between the number of layers of the two-dimensional material and the dark field contrast is as shown in formula (1):
m＝n*t+s (1)
in the formula (1), m is dark field contrast, n is a contrast contribution value of each layer, t is the number of two-dimensional material layers, and s is a correction parameter.
5. The method for discriminating a two-dimensional material layer using a dark field optical imaging technique as defined in claim 1 wherein said dark field optical imaging technique system includes a light source, an optical information receiving device, an imaging device.
6. The method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique as defined in claim 1 wherein the wavelength of light emitted by said light source covers all optical bands;
further preferably, the optical wavelength coverage of the light emitted by the light source is 156-1000nm;
most preferably, the wavelength of light emitted by the light source covers an optical band of 300-800nm.
7. The method for discriminating a two-dimensional material layer using a dark field optical imaging technique as defined in claim 1 wherein an optical path of said dark field optical device is adapted to satisfy a reflected optical path direction in which an optical information receiving device is not in an incident light direction; so that light specularly reflected by the substrate and the two-dimensional material sample is not received by the optical information receiving device.
8. The method for discriminating the number of layers of a two-dimensional material using a dark field optical imaging technique as defined in claim 1 wherein 1 column of pixels or 1 row of pixels passing through a two-dimensional material sample are selected, the luminance value of each pixel is read and a position-luminance curve is plotted; or selecting a plurality of columns of pixels or a plurality of rows of pixels where the two-dimensional material sample is located, reading the brightness value of each pixel, calculating the average value of the brightness values of the plurality of columns of pixels or the plurality of rows of pixels, and drawing a position-brightness curve.
9. The method of claim 1, wherein the two-dimensional material comprises graphene, hexagonal boron nitride, transition metal sulfide, black phosphorus.
10. A method for discriminating two-dimensional material layer number using dark field optical imaging technique as defined in any one of claims 1-9 wherein silicon substrate with different silicon dioxide layer thickness of 0-2000nm is adopted as the substrate;
further preferably, the substrate is a transparent sapphire substrate; the thickness of the substrate is 100 μm to 5000 μm.
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