CN108871890A - A method of TEM sample is prepared using graphene as protective layer - Google Patents

Abstract

The invention discloses a method for preparing a TEM sample by transferring or directly growing graphene by chemical vapor deposition as a protective layer. Graphene is used to avoid damage and pollution caused by the material to be observed during the sample preparation process, and a TEM sample containing a graphene protective layer is obtained. If the protective layer does not affect the experimental observation, it can be directly put into the transmission electron microscope for observation. If If you want to remove this protective layer, you can calcine it at high temperature (550°C) for a period of time (2h) to remove it. The invention has great significance for the manufacture of TEM samples with complete structure and zero damage.

Description

A method of preparing TEM samples using graphene as a protective layer

technical field

The invention belongs to the field of materials, and in particular relates to a method for preparing a TEM sample by using a graphene protective layer.

Background technique

Any material has a surface that is in contact with the outside world. Compared with its internal body, there are obvious differences in structure and chemical composition. At present, the most important method for studying the surface of materials is STM, but based on the principle of STM, this This method is limited to the research on the surface of non-insulator materials, but it is helpless for some insulator materials with great application prospects. However, transmission electron microscopy (TEM) can solve this problem very well. Regardless of insulating or non-insulating materials, we can explore their properties through TEM, and can more intuitively regulate related materials at the atomic scale. However, in the process of making TEM samples, the surface of the sample is easily polluted and damaged, causing the sample to lose its intrinsic crystal structure, which has an inevitable impact on our further observation and construction of the structure-activity relationship of the material. Graphene is a hexagonal honeycomb planar two-dimensional ultra-thin film material formed by sp2 hybridization of carbon atoms. Except for protons, atoms cannot pass through. It can well protect the surface structure of the sample during the thinning process, making it It is not damaged by other foreign particles, and when an external force is applied to the sample, the carbon atomic surface of graphene will bend and deform to relax the force received; in addition, calcining graphene at high temperature can make it oxidize into CO2 gas and volatilize it. We take advantage of the properties of graphene that is ultra-thin, flexible, capable of shielding external particles, and can be removed at high temperature, to transfer or directly grow the graphene grown on the metal substrate to the surface of the material to be observed as a protective layer to protect the sample during the sample preparation process. The surface of the sample, and then obtain a structurally complete, zero-damage TEM sample, which plays an irreplaceable role in our exploration of material properties and development of research methods.

Contents of the invention

In view of some technical problems in the current TEM sample preparation process, such as the surface of the cross-section sample will be damaged during the ion thinning process, making it impossible to judge the actual situation of the sample surface, and the contamination of the sample surface (there will be impurities in the sample preparation process The phenomenon of segregation of elements to the interface), and problems such as stress on the surface of the sample, the present invention provides a method for preparing a TEM sample using graphene directly grown by transfer or chemical vapor deposition as a protective layer.

Specifically, the transfer steps are as follows:

(1) Utilize chemical vapor deposition method to grow continuous graphene film on metal foil (copper foil or nickel foil) substrate, 4% PMMA solution is spin-coated to the metal foil substrate surface that has graphene;

(2) The temperature of the hot stage is set to 70°C, and the graphene/metal foil spin-coated with PMMA is placed on the hot stage and heated for 3 to 5 minutes to evaporate the solvent in the PMMA solution;

(3) Graphene/metal foil is put into ferric chloride solution with the side that is spin-coated with PMMA facing up, and leaves standstill until the substrate is separated from the graphene/PMMA layer;

(4) Fish the graphene/PMMA layer into pure deionized water with a clean filter paper, add 1 to 2 drops of isopropanol to make the graphene/PMMA fully stretch and move, to remove the chlorination adsorbed on its surface iron;

(5) Clean the sapphire substrate (here not limited to the sapphire substrate, any other material that needs to be protected) with ultrapure water, isopropanol, and acetone for 5 minutes, and dry it with nitrogen gas after cleaning. The ultrasonic power is 90W;

(6) Use the cleaned TEM sample of the required size (the length and width are 5×5 mm respectively) to scoop up graphene/PMMA, and heat at 70°C to make the film tightly adhere to the surface of the sample;

(7) Take a beaker and add an appropriate amount of acetone to heat to boil, clamp the PMMA-graphene/sapphire with tweezers, place the graphene-transferred side down at 1-2cm above the liquid surface of the acetone, and let it stand for 10-15min. Make acetone condense and reflux on the sapphire surface to remove PMMA, then take it out and dry it with nitrogen;

Specifically, the parameters for growing graphene directly on the high temperature resistant material required for sample preparation are as follows:

In the graphene chemical vapor deposition, the method of chemical vapor deposition can specifically be 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°C-1100°C, specifically 1050°C;

Deposition pressure is normal pressure;

The carrier gas is a mixed gas composed of argon and hydrogen, wherein the flow ratio of argon to hydrogen is 1-10:1, specifically 5:1; the flow rate of argon is specifically 100-1000 sccm, specifically 500 sccm; The flow rate is 50-500sccm, specifically 100sccm;

The flow rate of the carbon source is 10-50 sccm, specifically 20 sccm;

The deposition time is 0.5h-5h, specifically 3h;

(2) In the low-pressure chemical vapor deposition, the carbon source is ethanol vapor;

The deposition temperature is 1000°C-1100°C, specifically 1080°C;

The deposition pressure is 200-5000Pa, specifically 250Pa;

The carrier gas is a mixed gas composed of argon and hydrogen, wherein the flow ratio of argon to hydrogen is 1-10:1, specifically 5:1; the flow rate of argon can be 100-1000 sccm, specifically 500 sccm; The flow rate of hydrogen is 50-500 sccm, specifically 100 sccm;

The flow rate of the carbon source is 500sccm; the partial pressure of the carbon source is 250Pa;

The deposition time is 0.5h-5h, specifically 1h;

(3) In the plasma enhanced chemical vapor deposition, the carbon source is methane or ethylene;

The deposition temperature is 500°C-800°C, specifically 600°C;

The deposition pressure is 100-2000Pa, specifically 500Pa;

The flow rate of the carbon source can be specifically 5-50 sccm, specifically 18 sccm;

The power of the plasma generator is 60-200W, specifically 120W;

The deposition time is 0.5h-2h, specifically 1h.

The method also includes: before the chemical vapor deposition step, pretreating the high temperature resistant sample preparation material to be protected;

Specifically, the pretreatment includes ultrasonically cleaning the material to be protected with ultrapure water, isopropanol, and acetone for 5 minutes in sequence, and blowing dry with nitrogen after cleaning;

In the ultrasonic step, the power of ultrasonic is 90W.

The main feature of the present invention is to transfer or directly grow graphene to the surface of the material that needs to be prepared for TEM samples, use graphene to avoid the damage and pollution caused by the material to be observed during the sample preparation process, and obtain a graphene-containing protective material. If the protective layer does not affect the experimental observation, it can be directly put into the transmission electron microscope for observation. If you want to remove this protective layer, you can calcine it at high temperature (550°C) for a period of time (2h) to remove it. That's it. The invention has great significance for the manufacture of TEM samples with complete structure and zero damage.

Description of drawings

Figure 1 is the spherical aberration electron microscope HADDF picture of the TEM sample produced by this method. By comparison, it can be seen that the sample with the graphene protective layer has a complete structure; while the surface structure without the graphene protective layer is destroyed, and the structure cannot be judged.

Detailed ways

The present invention will be further described below in conjunction with specific examples, but the present invention is not limited to the following examples. The methods are conventional methods unless otherwise specified. All raw materials can be obtained from public commercial channels unless otherwise specified.

Example 1

A preparation method utilizing graphene to protect the surface structure of TEM sample preparation, the steps are as follows:

(1) Utilize chemical vapor deposition method to grow graphene on metal foil (copper foil or nickel foil) substrate, 0.5mL4% PMMA solution is spin-coated to the metal foil substrate surface that has graphene;

(2) The temperature of the hot stage is set to 70°C, and the graphene/metal foil spin-coated with PMMA is placed on the hot stage and heated for 3 to 5 minutes to evaporate the solvent in the PMMA solution;

(3) Graphene/metal foil is put into ferric chloride solution with the side that is spin-coated with PMMA facing up, and leaves standstill until the substrate is separated from the graphene/PMMA layer;

(4) Fish the graphene/PMMA layer into pure deionized water with a clean filter paper, add 1 to 2 drops of isopropanol to make the graphene/PMMA fully stretch and move, to remove the chlorination adsorbed on its surface iron;

(5) Clean the sapphire substrate (here not limited to the sapphire substrate, any other material that needs to be protected) with ultrapure water, isopropanol, and acetone for 5 minutes, and dry it with nitrogen gas after cleaning. The ultrasonic power is 90W;

(6) Use the cleaned TEM sample of the required size (the length and width are 5×5 mm respectively) to scoop up graphene/PMMA, and heat at 70°C to make the film tightly adhere to the surface of the sample;

(7) Take a beaker and add an appropriate amount of acetone to heat to boil, clamp the PMMA-graphene/sapphire with tweezers, place the graphene-transferred side down at 1-2cm above the liquid surface of the acetone, and let it stand for 10-15min. Make acetone condense and reflux on the sapphire surface to remove PMMA, then take it out and dry it with nitrogen;

(8) Ultrasonic cleaning of silicon wafers of required size with acetone for 5 minutes, and drying with nitrogen after cleaning;

(9) Configure the G1 glue according to the corresponding proportion, stir it evenly with a toothpick, and immediately apply it on the cleaned silicon wafer, and use the toothpick to make the glue as even and thin as possible, and then connect it with the graphene side of the sapphire For the glue, press it with tweezers to make the two closely contact, clamp it on the spring seat and place it on the heating platform for 3 hours, and the heating temperature is 70°C;

(10) After heating and cooling down naturally, put it in contact with a cut and fixed iron block coated with paraffin to solidify it on the iron block, and cut it to a size (based on sapphire) of 2×3mm with a slow speed saw ;

(11) Take a cut sample, make a cross-section sample, use a tripod grinding tool table to grind the thickness to less than 100 microns, then stick a molybdenum ring, heat it for 20 minutes, remove it, soak it in acetone for 30 minutes, take it out Ion thinning can be performed to obtain the final transmission sample;

(12) If it is necessary to remove the graphene protective layer, the sample can be placed upwards into the crucible, and then the crucible is placed in a tube furnace (or any other instrument that can complete high-temperature treatment), and the temperature is raised to 550 ° C. Just 2 hours, take out the crucible slowly, and fill the glue (paraffin) under the microscope to make the sample firmly stick to the molybdenum ring, and get the sample with the graphene protective layer removed; if there is no need to remove the graphene protective layer, this step can be ignored .

It was found that the TEM samples prepared by this method can observe the undamaged sapphire surface in the area protected by graphene, and it can be clearly seen that the termination layer of sapphire is an atomic layer of Al (Fig. 1f); In the region, the samples were damaged to varying degrees (Fig. 1a-e).

Example 2

The steps are the same as in Example 1, the difference is that in the step (5), the TEM sample to be prepared is not limited to the sapphire substrate, other materials such as AlN, GaN, InGaN, AlGaN, SrTiO Any material that needs to be sampled can be prepared, not only Limited to a single material, composites, stacks of two or more materials, and functional devices made of any material, regardless of size, not only limited to materials with flat surfaces without special treatment, but also various surface patterned substrate materials , the above and so on are also applicable.

Example 3

The steps are the same as in Example 1, except that the CVD method can be used to directly grow graphene on the surface of the high-temperature-resistant TEM sample, specifically, steps 1-7 are replaced by the following two steps (1) the sapphire substrate Ultrapure water, isopropanol, and acetone were used to ultrasonically clean for 5 minutes, and then dried with nitrogen gas, and the ultrasonic power was 90W;

(2) Graphene film growth: put the clean sapphire glass substrate obtained in step (1) into the APCVD cavity, set the Ar and H2 gas flowmeters to 500 sccm and 300 sccm respectively, and after the gas scrubbing, heat up the furnace body Up to 1060°C, keep the flow rate of Ar and H2 constant during the heating process. After the furnace temperature rose to 1060°C and stabilized for 15 minutes, the CH4 flow meter was set to 30 sccm, and the growth time was 5 hours. After the growth was completed, the temperature was naturally lowered to obtain graphene-covered sapphire.

Finally, a TEM sample with a non-destructive surface protected by graphene is also 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 using CVD to directly grow graphene on its surface. Specifically replaced by two steps 1 and 2 in Example 3.

Example 5

According to the steps of Example 1, only LPCVD is used instead of APCVD, the deposition environment is a low-pressure environment; the deposition temperature is 1080 ° C; the carrier gas is a mixture of argon and hydrogen, and the flow ratio of argon to hydrogen is 5:1 , specifically, the flow rate of argon gas is 500 sccm, the flow rate of hydrogen gas is 100 sccm; the carbon source is ethanol vapor, the flow rate is set to 500 sccm, the partial pressure is 250 Pa; the deposition time is 1 h. Finally, a non-destructive TEM sample is also obtained.

Example 6

According to the steps of Example 1, only PECVD is used instead of APCVD, the deposition environment is a low-pressure environment, and the pressure is 100 Pa; the deposition temperature is 600 ° C; the carbon source is methane, the flow rate is 18 sccm, the power of the plasma generator is 120 W, and the deposition time is 1h. Finally, a non-destructive TEM sample is also obtained.

The above production examples are general implementations of the present invention, and there are many other implementations that can be taken in practice, including graphene protecting any other material to do TEM samples and other microscopic samples, all equal according to the claims of the present invention Any change or modification shall fall within the scope of the present invention.

Claims (7)

1. a kind of using graphene as the method for protective layer preparation TEM sample, which is characterized in that transfer graphene to or sharp The material surface of preparation TEM sample needed for directly being grown into chemical vapor deposition, is observed needed for being avoided using graphene Material in sample preparation procedure caused by surface damage and pollution, and by manual sample grinding, ion beam be thinned or utilize Fib (focused ion-electronics double-beam system) obtains the TEM sample containing graphene protective layer, if this protective layer does not influence experiment and sees Examine, transmission electron microscope observing can be directly placed into, if wanting to remove this layer of protective layer, at high temperature (550 DEG C) calcining one section when Between (2h) be removed.

2. as described in claim 1 using graphene as the method for protective layer preparation TEM sample, which is characterized in that described Graphene transfer method, the specific steps are：

(1) continuous graphite alkene film is grown using chemical vapour deposition technique on metal foil (copper foil or nickel foil) substrate, by 4% PMMA solution is spun to the metal foil-based bottom surface with graphene；

(2) thermal station temperature is set as 70 DEG C, and graphene/metal foil of spin coating PMMA is placed in thermal station and heats 3~5min, makes PMMA Solvent volatilization is clean in solution；

(3) there is the one side of PMMA to be put into ferric chloride solution in a manner of upward by spin coating graphene/metal foil, stand to substrate It is separated with graphene/PMMA layers；

(4) graphene/PMMA layers is fished in pure deionized water with clean filter paper, 1~2 drop isopropanol is added dropwise, Graphene/PMMA is set sufficiently to unfold, move, to remove the iron chloride of its adsorption；

(5) successively by Sapphire Substrate (being not limited only to Sapphire Substrate herein, other any need materials to be protected) Respectively it is cleaned by ultrasonic 5min with ultrapure water, isopropanol, acetone, cleaning finishes use and is dried with nitrogen, wherein the power of ultrasound is 90W；

(6) with the TEM sample to be sampled (length and width are respectively that 5 × 5mm is advisable) after the cleaning of required size fish for graphene/ PMMA, 70 DEG C of heating make film tightly be attached to sample surfaces；

(7) it takes a beaker that proper amount of acetone is added to be heated to boiling, clamps PMMA- graphene/sapphire with tweezers, transfer has graphite The one of alkene is placed face down at acetone ullage 1-2cm, stand 10-15min, make acetone sapphire surface be condensed back with PMMA is removed, is taken out later and with being dried with nitrogen.

3. as described in claim 1 using graphene as the method for protective layer preparation TEM sample, which is characterized in that directly The parameter that graphene is grown on the material resistant to high temperature of required sample preparation is as follows：In the graphene chemical vapor deposition, chemistry The method of vapor deposition concretely aumospheric pressure cvd (APCVD), low-pressure chemical vapor deposition (LPCVD) or plasma Body enhances chemical vapor deposition (PECVD).

(1) specifically, in the aumospheric pressure cvd, carbon source is methane or ethylene；

Depositing temperature is 1000 DEG C -1100 DEG C, specially 1050 DEG C；

Deposition pressure is normal pressure；

Carrier gas is the gaseous mixture being made of argon gas and hydrogen, and wherein the flow-rate ratio of argon gas and hydrogen is 1-10：1, specially 5：1； The flow of argon gas is specially 100-1000sccm, specially 500sccm；The flow of hydrogen is 50-500sccm, specially 100sccm；

The flow of carbon source is 10-50sccm, concretely 20sccm；

Sedimentation time is 0.5h-5h, specially 3h；

(2) in the low-pressure chemical vapor deposition, carbon source is alcohol vapour；

Depositing temperature is 1000 DEG C -1100 DEG C, concretely 1080 DEG C；

Deposition pressure is 200-5000Pa, specially 250Pa；

Carrier gas is the gaseous mixture being made of argon gas and hydrogen, and wherein the flow-rate ratio of argon gas and hydrogen is 1-10:1, concretely 5： 1；The flow of argon gas concretely 100-1000sccm, specially 500sccm；The flow of hydrogen is 50-500sccm, specially 100sccm；

The flow of carbon source is 500sccm；The partial pressure of carbon source is 250Pa；

Sedimentation time is 0.5h-5h, specially 1h；

(3) in the plasma enhanced chemical vapor deposition, carbon source is methane or ethylene；

Depositing temperature is 500 DEG C -800 DEG C, specially 600 DEG C；

Deposition pressure is 100-2000Pa, specially 500Pa；

The flow of carbon source concretely 5-50sccm, specially 18sccm；

Plasma generator power is 60-200W, specially 120W；

Sedimentation time is 0.5h-2h, specially 1h.

4. as described in claim 1 using graphene as the method for protective layer preparation TEM sample, which is characterized in that described Manual sample grinding the specific steps are：

(1) sapphire that there is graphene on surface is obtained using graphene transfer method as claimed in claim 2；

(2) silicon wafer of required size acetone is cleaned by ultrasonic 5min, cleaning finishes use and is dried with nitrogen；

(3) G1 glue is configured by corresponding proportion, after mixing evenly with toothpick, is applied on silicon wafer after cleaning at once, and by glue Water is even flat, even thin as far as possible with toothpick, then has the side of graphene to gluing with sapphire, is pressed with tweezers, keeps the two tight Contiguity touching, and be sandwiched to be placed in thermal station on spring base and heat 3h, heating temperature is 70 DEG C；

(4) heating finishes and after Temperature fall, it is contacted with the fixed iron block of the cutting for filling paraffin, it is made to be solidificated in iron block On, and it is cut to 2 × 3mm size (on the basis of sapphire) with saw at a slow speed；

(5) one piece of sample cut is taken, section sample is made, thickness is ground to 100 microns hereinafter, benefit with tripod sample grinding tool platform Molybdenum ring (molybdenum annular aperture there are the plurality of specifications such as 0.6mm, 0.8mm, 1.0mm, 1.5mm) is adhered in ground sample in cross section with AB glue, Make molybdenum ring inner hole just in the interface of sapphire and silicon wafer, then specimen holder is put into thermal station and heats 20min, by sheet glass It is removed together with sample, puts and steep 30min in acetone, sample, which is taken out, can carry out ion milling.

5. it is as described in claim 1 using graphene as the method for protective layer preparation TEM sample, it is characterized in that, uses ion Instrument is thinned, thickness of sample obtained in claim 4 is reduced into 50nm or so according to parameter once：

(1) high pressure：4.5kv, angle：6 °, left rifle line：25uA, right rifle line：26uA, time：40min；

(2) high pressure：4kv, angle：6 °, left rifle line：20uA, right rifle line：21uA, time：90min；

(3) high pressure：3.5kv, angle：6 °, left rifle line 15uA, right rifle line：16uA, time：65min；

(4) high pressure：1.5kv, angle：3 °, left rifle line：5uA, right rifle line：6uA, time：5min.

6. as described in claim 1 using graphene as the method for protective layer preparation TEM sample, which is characterized in that can benefit With Fib according to the acquisition of following general steps with a thickness of the TEM sample of 50nm or so：

(1) needle point initializes；

(2) needle point is repaired；

(3) sample is moved to the operating position FIB；

(4) needle point is encountered on the sample cut；

(5) sample cut is welded on needle point；

(6) cutting sample right edge connection；

(7) lift sample；

(8) sample will be cut and is moved to V-groove；

(9) sample on needle point is moved to V-groove；

(10) sample cut is soldered on V-groove；

(11) needle point is cut off；

(12) continued to reduce to thickness of sample into 50nm or so with the ion beam of Fib, or use thickness of sample according to claim 5 Ion Beam Thinner reduces to 50nm or so.

7. as described in claim 1 using graphene as the method for protective layer preparation TEM sample, which is characterized in that required system The material of standby TEM sample, not only the block materials such as including Al2O3, AlN, SiO2, GaN, further include thin-film material, stratiform material The material of any need TEM characterization such as material and nano particle.