CN108871890B - Method for preparing TEM sample by using graphene as protective layer - Google Patents
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- G01N1/00—Sampling; Preparing specimens for investigation
- G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
- G01N1/44—Sample treatment involving radiation, e.g. heat
Abstract
The invention discloses a method for preparing a TEM sample by using graphene which is transferred or directly grown by chemical vapor deposition as a protective layer, which is mainly characterized in that the graphene is transferred or directly grown on the surface of a material needing to prepare the TEM sample, the graphene is used for avoiding the damage and pollution of the material needing to be observed in the sample preparation process, and the TEM sample containing the graphene protective layer is obtained, if the protective layer does not influence the experimental observation, the TEM sample can be directly placed into a transmission electron microscope for observation, and if the protective layer is to be removed, the TEM sample is calcined at a high temperature (550 ℃) for a period of time (2h) and then is removed. The invention has great significance for manufacturing the TEM sample with complete structure and zero damage.
Description
Technical Field
The invention belongs to the field of materials, and particularly relates to a method for preparing a TEM sample by using a graphene protective layer.
Background
Any material has a surface contacting with the outside, and compared with an internal body, the material has obvious difference in structure and chemical composition, the most main means for researching the surface of the material is STM at present, but based on the principle of STM, the method is only limited to research on the surface of non-insulator materials, and the method is not reasonable for some insulator materials with great application prospects. The Transmission Electron Microscope (TEM) can well solve the problem, and the properties of the material can be explored through the TEM regardless of an insulating material or a non-insulating material, so that the relevant material can be more intuitively regulated and controlled on an atomic scale. However, in the process of preparing the TEM sample, the surface of the sample is easily contaminated and damaged, so that the sample loses its intrinsic crystal structure, and the structure-activity relationship between further observation and the material is inevitably affected. The graphene is a hexagonal honeycomb plane two-dimensional ultrathin film material formed by hybridizing carbon atoms in sp2, except protons, atoms and the like cannot penetrate through the graphene, the surface structure of a sample can be well protected in the thinning process, the graphene is not damaged by other external particles, and when external force is applied to the sample, the carbon atom surface of the graphene can be bent and deformed to relax the applied force; in addition, the graphene is calcined at high temperature, and can be oxidized into CO2 gas to be volatilized. The characteristics that graphene is ultrathin and flexible, can shield external particles and can be removed at high temperature are utilized, the graphene growing on a metal substrate is transferred or directly grows to the surface of a material to be observed to serve as a protective layer, the surface of a sample is protected in the sample preparation process, and a TEM sample with a complete structure and zero damage is further obtained, so that irreplaceable effects are achieved for researching material properties and developing research methods.
Disclosure of Invention
The invention provides a method for preparing a TEM sample by using graphene directly grown by transfer or chemical vapor deposition as a protective layer, aiming at the technical problems in the preparation process of the existing TEM sample, such as the fact that the surface of a cross-section sample is damaged in the ion thinning process, so that the actual condition of the surface of the sample cannot be judged, the pollution of the surface of the sample (the phenomenon that impurity elements are segregated to an interface in the sample preparation process), the stress generated on the surface of the sample and the like.
Specifically, the transfer steps are as follows:
(1) growing a continuous graphene film on a metal foil (copper foil or nickel foil) substrate by using a chemical vapor deposition method, and spin-coating a 4% PMMA solution on the surface of a metal foil substrate on which graphene grows;
(2) setting the temperature of a hot table to be 70 ℃, placing the graphene/metal foil spin-coated with PMMA on the hot table, and heating for 3-5 min to completely volatilize the solvent in the PMMA solution;
(3) placing the graphene/metal foil into a ferric chloride solution in a manner that the surface coated with PMMA faces upwards, and standing until the substrate is separated from the graphene/PMMA layer;
(4) fishing the graphene/PMMA layer into pure deionized water by using a clean filter paper sheet, and dropwise adding 1-2 drops of isopropanol to enable the graphene/PMMA layer to stretch and move sufficiently so as to remove ferric chloride adsorbed on the surface of the graphene/PMMA layer;
(5) sequentially ultrasonically cleaning a sapphire substrate (not limited to the sapphire substrate, and any other material needing to be protected) for 5min by using ultrapure water, isopropanol and acetone, and drying by using nitrogen after cleaning is finished, wherein the power of ultrasonic is 90W;
(6) fishing out graphene/PMMA (polymethyl methacrylate) from a cleaned TEM sample to be sampled (preferably 5mm in length and width) with required size, and heating at 70 ℃ to enable the film to be tightly attached to the surface of the sample;
(7) adding a proper amount of acetone into a beaker, heating to boiling, clamping PMMA-graphene/sapphire by using a pair of tweezers, placing the surface transferred with the graphene downwards at a position 1-2cm above the liquid level of the acetone, standing for 10-15min to enable the acetone to be condensed and refluxed on the surface of the sapphire to remove PMMA, and then taking out and drying by using nitrogen;
specifically, the parameters for directly growing graphene on the high-temperature resistant material to be sampled are as follows:
in the graphene chemical vapor deposition, the chemical vapor deposition method may be specifically Atmospheric Pressure Chemical Vapor Deposition (APCVD), Low Pressure Chemical Vapor Deposition (LPCVD), or Plasma Enhanced Chemical Vapor Deposition (PECVD).
(1) Specifically, in the atmospheric pressure chemical vapor deposition, the carbon source is methane or ethylene;
the deposition temperature is 1000-1100 ℃, and is 1050 ℃;
the deposition pressure is normal pressure;
the carrier gas is a mixed gas consisting of argon and hydrogen, wherein the flow ratio of the argon to the hydrogen is 1-10:1, specifically 5: 1; the flow rate of the argon gas is specifically 100-1000sccm, specifically 500 sccm; the flow rate of the hydrogen is 50-500sccm, specifically 100 sccm;
the flow rate of the carbon source is 10-50sccm, and specifically can be 20 sccm;
the deposition time is 0.5h-5h, specifically 3 h;
(2) in the low-pressure chemical vapor deposition, a carbon source is ethanol vapor;
the deposition temperature is 1000-1100 ℃, and can be 1080 ℃;
the deposition pressure is 200-5000Pa, in particular 250 Pa;
the carrier gas is a mixed gas composed of argon and hydrogen, wherein the flow ratio of argon to hydrogen is 1-10:1, and specifically can be 5: 1; the flow rate of argon gas can be specifically 100-1000sccm, specifically 500 sccm; the flow rate of the hydrogen is 50-500sccm, specifically 100 sccm;
the flow rate of the carbon source is 500 sccm; the partial pressure of the carbon source is 250 Pa;
the deposition time is 0.5h-5h, specifically 1 h;
(3) in the plasma enhanced chemical vapor deposition, a carbon source is methane or ethylene;
the deposition temperature is 500-800 ℃, in particular 600 ℃;
the deposition pressure is 100-2000Pa, in particular 500 Pa;
the flow rate of the carbon source can be 5-50sccm, specifically 18 sccm;
the power of the plasma generator is 60-200W, specifically 120W;
the deposition time is 0.5h-2h, specifically 1 h.
The method further comprises the following steps: before the chemical vapor deposition step, pretreating a high-temperature-resistant sample preparation material to be protected;
specifically, the pretreatment comprises the steps of sequentially ultrasonically cleaning the sample material to be protected for 5min by ultrapure water, isopropanol and acetone respectively, and drying by using nitrogen after cleaning;
in the ultrasonic step, the power of the ultrasonic wave is 90W.
The method is mainly characterized in that graphene is transferred or directly grows to the surface of a material needing to prepare a TEM sample, the graphene is utilized to avoid damage and pollution of the material needing to be observed in the sample preparation process, the TEM sample containing the graphene protective layer is obtained, if the protective layer does not affect experimental observation, the TEM sample can be directly placed into a transmission electron microscope for observation, and if the protective layer is to be removed, the TEM sample is calcined at high temperature (550 ℃) for a period of time (2h) and then removed. The invention has great significance for manufacturing the TEM sample with complete structure and zero damage.
Drawings
The first picture is a TEM sample spherical aberration electron microscope HADDF picture manufactured by the method, and the contrast shows that the sample with the graphene protective layer has a complete expression structure; and the surface structure without the graphene protective layer is damaged, and the structure cannot be judged.
Detailed Description
The present invention will be further illustrated with reference to the following specific examples, but the present invention is not limited to the following examples. The method is a conventional method unless otherwise specified. The starting materials are commercially available from the public unless otherwise specified.
Example 1
A preparation method of a surface structure prepared by using graphene to protect a TEM sample comprises the following steps:
(1) growing graphene on a metal foil (copper foil or nickel foil) substrate by using a chemical vapor deposition method, and spin-coating 0.5mL of 4% PMMA solution on the surface of a metal foil substrate on which the graphene is grown;
(2) setting the temperature of a hot table to be 70 ℃, placing the graphene/metal foil spin-coated with PMMA on the hot table, and heating for 3-5 min to completely volatilize the solvent in the PMMA solution;
(3) placing the graphene/metal foil into a ferric chloride solution in a manner that the surface coated with PMMA faces upwards, and standing until the substrate is separated from the graphene/PMMA layer;
(4) fishing the graphene/PMMA layer into pure deionized water by using a clean filter paper sheet, and dropwise adding 1-2 drops of isopropanol to enable the graphene/PMMA layer to stretch and move sufficiently so as to remove ferric chloride adsorbed on the surface of the graphene/PMMA layer;
(5) sequentially ultrasonically cleaning a sapphire substrate (not limited to the sapphire substrate, and any other material needing to be protected) for 5min by using ultrapure water, isopropanol and acetone, and drying by using nitrogen after cleaning is finished, wherein the power of ultrasonic is 90W;
(6) fishing out graphene/PMMA (polymethyl methacrylate) from a cleaned TEM sample to be sampled (preferably 5mm in length and width) with required size, and heating at 70 ℃ to enable the film to be tightly attached to the surface of the sample;
(7) adding a proper amount of acetone into a beaker, heating to boiling, clamping PMMA-graphene/sapphire by using a pair of tweezers, placing the surface transferred with the graphene downwards at a position 1-2cm above the liquid level of the acetone, standing for 10-15min to enable the acetone to be condensed and refluxed on the surface of the sapphire to remove PMMA, and then taking out and drying by using nitrogen;
(8) ultrasonically cleaning a silicon wafer with required size for 5min by using acetone, and drying by using nitrogen after cleaning;
(9) preparing G1 glue according to a corresponding proportion, stirring uniformly by using a toothpick, immediately coating on a cleaned silicon wafer, uniformly flattening and thinning the glue by using the toothpick as much as possible, then oppositely adhering the glue to one side of the sapphire with graphene, pressing by using a pair of tweezers to enable the glue and the sapphire to be in close contact, clamping the glue on a spring seat, and heating for 3 hours on a hot table at the heating temperature of 70 ℃;
(10) after heating and natural cooling, contacting the iron block with a cutting fixed iron block coated with paraffin wax to solidify the iron block, and cutting the iron block into 2 x 3mm (based on sapphire) by using a slow saw;
(11) taking a cut sample, making a cross section sample, grinding the thickness to be below 100 micrometers by using a tripod sample grinding tool table, then adhering a molybdenum ring, heating for 20min, taking down, soaking in acetone for 30min, and taking out to perform ion thinning to obtain a final transmission sample;
(12) if the graphene protective layer needs to be removed, the sample can be upwards placed into a crucible, then the crucible is placed into a tube furnace (or any other instrument capable of finishing high-temperature treatment), the temperature is raised to 550 ℃, the high-temperature treatment is carried out for 2 hours, the crucible is slowly taken out, glue (paraffin) is supplemented under a microscope, the sample is firmly adhered to the molybdenum ring, and the sample with the graphene protective layer removed is obtained; this step can be omitted if the graphene protection layer does not need to be removed.
As a result, it was found that, in the TEM sample prepared by this method, a non-damaged sapphire surface was observed in the graphene-protected region, and the stop layer of sapphire was clearly seen as an Al atomic layer (fig. 1 f); in the areas without graphene protection, the samples all had different degrees of damage (fig. 1 a-e).
Example 2
The procedure is the same as that of example 1, except that in the step (5), the TEM sample to be prepared is not limited to the sapphire substrate, but other materials such as AlN, GaN, InGaN, AlGaN, SrTiO3 and other materials requiring sample preparation may be used, and the TEM sample is not limited to a single material, a composite or a stack of two or more materials, and a functional device made of any material, regardless of size, not only a material with a smooth surface without special treatment, but also a substrate material with various surface patterns, and the like.
Example 3
The steps are the same as the embodiment 1, except that the high-temperature-resistant TEM sample can be directly grown on the surface of the high-temperature-resistant TEM sample by using a CVD method, specifically, the steps 1-7 are replaced by the following two steps (1), the sapphire substrate is sequentially cleaned by ultra-pure water, isopropanol and acetone for 5min by ultrasonic waves respectively, and the sapphire substrate is dried by using nitrogen after being cleaned, wherein the ultrasonic power is 90W;
(2) growing the graphene film: and (2) putting the clean sapphire glass substrate obtained in the step (1) into an APCVD cavity, setting Ar and H2 gas flow meters to be 500sccm and 300sccm respectively, heating the furnace body to 1060 ℃ after the gas washing is finished, and keeping the flow rates of Ar and H2 unchanged in the heating process. And after the furnace temperature is increased to 1060 ℃, stabilizing for 15min, setting a CH4 flowmeter to be 30sccm, setting the growth time to be 5h, and naturally cooling after the growth is finished to obtain the sapphire covered by the graphene.
Finally, a TEM sample with an undamaged surface and protected by graphene is obtained.
Example 4
For the material of the TEM sample to be prepared in example 2, if it is resistant to high temperature, steps 1-7 can be replaced by growing graphene directly on its surface by CVD. Specifically, two steps 1 and 2 in example 3 were substituted.
Example 5
The procedure of example 1 was followed, except that LPCVD was used instead of APCVD, and the deposition environment was a low pressure environment; the deposition temperature is 1080 ℃; the carrier gas is a mixed gas consisting of argon and hydrogen, wherein the flow ratio of the argon to the hydrogen is 5: 1, specifically, the flow rate of argon is 500sccm, and the flow rate of hydrogen is 100 sccm; the carbon source is ethanol steam, the flow rate is set to be 500sccm, and the partial pressure is 250 Pa; the deposition time was 1 h. Finally, a nondestructive TEM sample is obtained.
Example 6
According to the steps of the embodiment 1, only PECVD is used for replacing APCVD, the deposition environment is a low-pressure environment, and the pressure is 100 Pa; the deposition temperature is 600 ℃; the carbon source is methane, the flow rate is 18sccm, the power of the plasma generator is 120W, and the deposition time is 1 h. Finally, a nondestructive TEM sample is obtained.
The above examples are general embodiments of the present invention, and many other embodiments can be adopted, including graphene protection for any other material to be used as TEM sample and other microscopic samples, and all equivalent changes or modifications made according to the claims of the present invention shall fall within the scope of the present invention.
Claims (1)
1. A method for preparing a TEM sample by using graphene as a protective layer is characterized by comprising the following steps:
A) transferring graphene to or directly growing graphene on the surface of a TEM sample material to be prepared by chemical vapor deposition, avoiding surface damage and pollution caused by the material to be observed in the sample preparation process by using the graphene, protecting the surface structure of the sample in the thinning process, preventing the sample from being damaged by external particles, and when external force is applied to the sample, enabling the carbon atom surface of the graphene to be bent and deformed to relax the received force, thereby obtaining a lossless TEM sample, wherein the method specifically comprises the following steps:
a) the method for transferring the graphene to the surface of the TEM sample material to be prepared comprises the following specific steps:
1) growing a continuous graphene film on a metal foil substrate by using a chemical vapor deposition method, and spin-coating a 4% PMMA solution on the surface of a metal foil substrate on which graphene is grown;
2) setting the temperature of a hot table to be 70 ℃, placing the graphene/metal foil spin-coated with PMMA on the hot table, and heating for 3-5 min to completely volatilize the solvent in the PMMA solution;
3) placing the graphene/metal foil into a ferric chloride solution in a manner that the surface coated with PMMA faces upwards, and standing until the substrate is separated from the graphene/PMMA layer;
4) fishing the graphene/PMMA layer into pure deionized water by using a clean filter paper sheet, and dropwise adding 1-2 drops of isopropanol to enable the graphene/PMMA layer to stretch and move sufficiently so as to remove ferric chloride adsorbed on the surface of the graphene/PMMA layer;
5) sequentially ultrasonically cleaning the sapphire by ultrapure water, isopropanol and acetone for 5min, and blow-drying by using nitrogen after cleaning is finished, wherein the ultrasonic power is 90W;
6) fishing out graphene/PMMA (polymethyl methacrylate) from a cleaned TEM sample material with a required size, and heating at 70 ℃ to enable the film to be tightly attached to the surface of the sample;
7) adding a proper amount of acetone into a beaker, heating to boiling, clamping PMMA-graphene/sapphire by using a pair of tweezers, placing the surface transferred with the graphene downwards at a position 1-2cm above the liquid level of the acetone, standing for 10-15min to enable the acetone to be condensed and refluxed on the surface of the sapphire to remove PMMA, and then taking out and drying by using nitrogen;
b) the parameters for directly growing graphene on the high-temperature-resistant TEM sample material needing to be prepared are as follows: in the chemical vapor deposition, the chemical vapor deposition method is specifically atmospheric pressure chemical vapor deposition, low pressure chemical vapor deposition or plasma enhanced chemical vapor deposition, and the specific steps include:
1) in the atmospheric pressure chemical vapor deposition, a carbon source is methane or ethylene; the deposition temperature is 1000-1100 ℃, and is 1050 ℃; the deposition pressure is normal pressure; the carrier gas is a mixed gas consisting of argon and hydrogen, wherein the flow ratio of the argon to the hydrogen is 1-10: 1; the flow rate of argon is 100-1000 sccm; the flow rate of the hydrogen is 50-500 sccm; the flow rate of the carbon source is 10-50 sccm; the deposition time is 0.5h-5 h;
2) in the low-pressure chemical vapor deposition, a carbon source is ethanol vapor; the deposition temperature is 1000-1100 ℃; the deposition pressure is 200-5000 Pa; the carrier gas is a mixed gas consisting of argon and hydrogen, wherein the flow ratio of the argon to the hydrogen is 1-10: 1; the flow rate of argon is 100-1000 sccm; the flow rate of the hydrogen is 50-500 sccm; the flow rate of the carbon source is 500 sccm; the partial pressure of the carbon source is 250 Pa; the deposition time is 0.5h-5 h;
3) in the plasma enhanced chemical vapor deposition, a carbon source is methane or ethylene; the deposition temperature is 500-800 ℃; the deposition pressure is 100-2000 Pa; the flow rate of the carbon source is 5-50 sccm; the power of the plasma generator is 60-200W; the deposition time is 0.5h-2 h;
B) manually grinding a sample and thinning an ion beam to obtain a TEM sample containing a graphene protective layer, directly placing the TEM sample into a transmission electron microscope for observation, and calcining the TEM sample at a high temperature for a period of time to remove the protective layer, wherein:
a) the manual sample grinding comprises the following specific steps:
1) ultrasonically cleaning a silicon wafer with required size for 5min by using acetone, and drying by using nitrogen after cleaning;
2) preparing G1 glue according to a corresponding proportion, stirring uniformly by using a toothpick, immediately coating on a cleaned silicon wafer, uniformly flattening and thinning the glue by using the toothpick as much as possible, then oppositely adhering the glue to one side of the sapphire with graphene, pressing by using a pair of tweezers to enable the glue and the sapphire to be in close contact, clamping the glue on a spring seat, and heating for 3 hours on a hot table at the heating temperature of 70 ℃;
3) after heating and natural cooling, contacting the iron block with a cutting fixed iron block coated with paraffin wax to solidify the iron block, and cutting the iron block into 2 x 3mm size by using a slow saw;
4) taking a cut sample, making a section sample, grinding the thickness to be less than 100 micrometers by using a tripod sample grinding tool table, adhering a molybdenum ring to the section of the ground sample by using AB glue, enabling the inner hole of the molybdenum ring to be just positioned at the interface of sapphire and a silicon wafer, then placing a sample seat on a heating table, heating for 20min, taking down a glass sheet and the sample together, soaking in acetone for 30min, and taking out the sample to carry out ion thinning;
b) the sample thickness was reduced to around 50nm with an ion thinning instrument according to the following parameters:
1) high pressure: 4.5kv, angle: 6 DEG, left gun stream: 25uA, right gun beam: 26uA, time: 40 min;
2) high pressure: 4kv, angle: 6 DEG, left gun stream: 20uA, right gun beam: 21uA, time: 90 min;
3) high pressure: 3.5kv, angle: 6 degrees, the left gun beam is 15uA, the right gun beam is: 16uA, time: 65 min;
4) high pressure: 1.5kv, angle: 3 DEG, left gun stream: 5uA, right gun beam: 6uA, time: 5 min;
besides sapphire, the material for preparing TEM sample can also be AlN, GaN, InGaN, AlGaN, SrTiO3(ii) a The material is a bulk material, a thin film material, a layered material or nanoparticles.
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