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Efficient and robust fabrication of microscale graphene drums Peng Yin, and Ming Ma ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01347 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 13, 2018

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ACS Applied Nano Materials

Efficient and Robust Fabrication of Microscale Graphene Drums Peng Yin†,‡, Ming Ma*,†,‡ †State

Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua

University, Beijing 100084, China ‡Center

for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China

*Email: [email protected]

Abstract Few-layer graphene drums have been extensively studied as bulging devices to analyze their mechanical and optical properties. A controllable and efficient fabrication method of such drums is of great significance for related studies. We propose a onestep approach to fabricate graphene drums through modified mechanical exfoliation. Graphene sheets are annealed on the pre-patterned SiO2 substrate with micrometersized holes. With such an approach, one can fabricate graphene drums with larger height using higher annealing temperature or larger holes. Larger strain within the graphene drums could also be achieved using higher annealing temperature. The strain of the graphene drums gradually decreases over time within 24 hours. The mechanism underlying is attributed to the diffusion of gas molecules initially trapped at the graphite/substrate interface. This self-consistent mechanism prevents the drums from being destroyed by the pressure difference across graphene layers in principle. Compared with the existing method, our method provides an efficient and robust way of studying the mechanical, electric and optical properties of graphene, especially for the effects of strain and curvature. KEYWORDS: graphene drum, strain, annealing, gas diffusion, friction

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Introduction Graphene, a monolayer of carbon atoms in a honeycomb configuration, has attracted broad interests in research and development for its unique mechanical1-3, electrical4-5, thermal6, and optical7-8 properties. Great efforts have been made in its fabrication and characterization techniques9-10. Many potential applications, from electronic devices1112,

water filtration13-14, biomedical applications15-16 to self-assembled graphene

composites17-19 and micro or nanoelectromechanical systems (MEMS/NEMS)20 have been proposed. One typical example is the few-layer graphene (FLG) drum, where single- or few-layer graphene sieving a hole thus inducing tensile strain within the graphene. This system has been extensively studied as a bulging device. Such system, also called blister test, has been used to analyze the statistical variation of mechanical properties 1, 3, 21-22 by Raman spectroscopy and atomic force microscopy, and enabled electromechanical actuation for pressure sensing23-24. The unusual optical properties of graphene drums enable strong nonlinear light-matter interaction in the application to multifunctional nonlinear devices25, and to characterize the mechanical properties and permeance of suspended graphene micro-devices8. Other properties of graphene drums such as the control and probing of their shape26-27 have also been studied. In principle, the mechanism for preparing graphene drums is to establish a pressure difference across the graphene layer. There are two kinds of methods being used at present, and both include structures where a graphene layer covering the substrate with holes of which the diameter is of micrometer scale. The only difference is whether the hole is a blind hole or through hole. For the former, the whole system is put in a highpressure chamber for a few hours to a few days1, 8, 21-22, 28, then stabilized at ambient condition. Due to the osmosis of gas molecules, a pressure drop is established across the membrane, resulting in graphene drums. For the latter, gas molecules are directly pumped into the through holes by attaching the substrates onto an air pump23, 29. While both methods have their advantages, their drawbacks are also evident. For both methods, careful control of external pressure is required, or the systems could be easily destroyed either due to the collapse28 of the graphene layer or the rupture of the substrate, i.e. 2


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being lack of robustness. Furthermore, for each method, it either suffers from low efficiency (with blind holes) or expensive and complicated fabrication of the substrate (with through holes). In this work, we introduce a new method to fabricate graphene drums through mechanical exfoliation of graphene with annealing on the pre-patterned SiO2 substrate with micrometer-sized holes. Blind holes are used, and the whole process can be finished within an hour. Compared with existing methods, no external pressure is needed. By controlling annealing temperature or the size of the holes, graphene drums with different heights could be fabricated. The strain of the graphene drums could also be controlled by using annealing temperature. With comparative experiments, we propose a mechanism of our method, which is due to the diffusion of gas molecules at the graphene/substrate interface. Experiments We fabricated the micro graphene drums using a modified mechanical exfoliation method as shown in Figure 1. The substrate was prepared by thermally oxidizing the Si(100) surface to induce a 300nm-thick SiO2 layer (named as SiO2/Si(100) for short). Holes with radii from 3 to 5µm and depth of 250nm were introduced into a SiO2 layer using standard optical lithography together with reactive ion etching methods (Fig.1a). Afterward, the SiO2/Si(100) substrate was cleaned with a base piranha solution to remove inorganic impurities caused by the reactive ion etching, and ultrasonically cleaned in acetone, isopropyl alcohol, deionized (DI) water, and dried with streaming N2. This was followed by oxygen plasma cleaning to remove ambient adsorbates from the surfaces. An ordinary water-soluble tape (3M) was used to peel small graphite flakes from a natural graphite block (NGS, flaggy graphite), and then was put into contact with the clean SiO2/Si(100) surface treated (Fig.1b). The substrate with the attached tape was then annealed for 30 min at an elevated temperature (from 50-150℃) in the air on a conventional laboratory hot plate (Fig.1c). After the sample cooled to room temperature (Fig.1d) naturally, we used tweezers to firm the silicon substrate on 3



the table and pull the tape up slowly from one end. The adhesive tape was removed (Fig.1e), leaving the graphene or few-layer graphene transferred onto the SiO2/Si substrate, sealing the micro-holes. The whole experiment was done in the clean room of Class 1000, at an atmospheric environment with temperature ~25℃ and relative humidity of 30% - 60%.

Figure 1. Schematic Illustration of the production process for graphene drums on the SiO2/Si(100) substrates. (a) Pre-patterned SiO2/Si(100) substrate with micrometer-sized holes and ordinary water-soluble tapes (3M) adhering small graphite flakes. (b) Forming contact between the tape adhering graphite flakes and the substrate surface. (c) Heating the substrate with tape on a hot plate in air at an elevated temperature from 50-150℃ for 30 min. (d) Removing the substrate from the hot plate and peeling off the tape. (e) The graphene layers sealing the microchambers form graphene drums.

Results The optical micrographs of the transferred region on the SiO2/Si substrate (Fig.2a) after annealing at 150℃ clearly show the presence of both graphene and few-layer graphene. By measuring local Raman spectra (Horiba, LabRam HR Evolution) as shown in Fig.2b, we found that for supported graphene, the G band is located at 1584cm−1, which is slightly larger than the standard value (1580cm−1). This is attributed to the presence of charge transfer doping30 between graphene and substrate. Both the G band and the 2D band have significant migration and the frequency of the G band at the center of the suspended graphene over a hole with a diameter of 4μm is ~8cm−1 smaller 4


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than the standard value, demonstrating a tensile strain within the graphene31-33. We further characterized the morphology of the suspended graphene using atomic force microscope (Oxford Instruments, MFP-3D Infinity) as shown in Fig.2c. For the graphene layer covering the substrate, we found it to be single crystal (Fig.S1). A parabolic shape was observed with a height of 57.4 nm across the 4μm width. For such parabolic-shaped graphene, with the tensile strain measured at the center of the suspended graphene, intuitively there should be a non-uniform distribution of the strain1, 34.

This is confirmed by measuring the frequency shift of the G band across the

suspended region as well as the supported graphene nearby (Fig.2d). Here, the frequency of the G band was the position of the center of a single Lorentzian fit to the Raman spectra. Furthermore, we converted the frequency shift into strain as29 𝜀 = (𝜔 ― 𝜔0)/(2𝛾𝜔0), where the Gruneisen parameter γ is 1.8, ω and ω0 are the Raman frequencies at finite strain and zero strain, respectively. A tensile strain with the maximum of 1.3‰ is found near the center of the suspended graphene and gradually decreases to zero near the edge of the holes.

Figure 2. Characterization of the graphene layers prepared by the modified exfoliation method. (a) Optical image of monolayer and few-layer graphene on the substrate, the monolayer graphene is marked with a red dotted box. (b) Raman spectra of suspended 5



graphene and the SiO2/Si substrate supported graphene, taken on the regions as indicated in (a). (c) The cross topography of monolayer graphene drums ~4μm in diameter marked with a blue line in (a), measured by AFM in tapping mode. (d) The frequency shift of the Raman G band as a function of position for line scan across a single 4μm diameter monolayer sealed the micro hole.

Given that the drums resulted from the high pressure exerted by the gas confined within the holes1, 24, 28 with respect to ambient condition, an intuitive question is where the gas came from. Considering the fabrication process as shown in Fig.1, there could be two possibilities. The first is the gas confined within the holes during the attaching process (Fig.1a-1b), and the second is through the annealing process (Fig.1b-1d). For the attaching process, it is possible that a certain amount of gas was sealed within the holes, forming the drums. In this case, the annealing temperature should have no effects on the height of the graphene drums, since the volume expansion during the heating process would be compensated completely by the volume reduction during the cooling stage. By repeating the fabrication process (Fig.1a-e) at three different temperatures (50 ℃ , 100 ℃ and 150 ℃ ) for holes in different sizes (3μm, 4μm, and 5μm), and averaging the results over 3-4 different samples, however, we did see a monotonic dependence of the height of the drums on annealing temperature as shown in Fig.3. For the fabrication process without annealing, instead of forming a graphene drum, it will slightly collapse into the hole (Fig.S2). Specifically, for holes of the same size, the height of the drums increases as the temperature increases. Similar phenomena were also observed for two-layer graphene. The annealing temperature is limited by the requirement for the absence of tape melting. Thus, the temperature-dependent phenomenon of the height of the graphene drums clearly shows that the sealing of gas during the attaching process can’t be the reason for the formation of graphene drums, leaving the annealing process being the only possibility.

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Figure 3. The height of the graphene drums as a function of annealing temperature with the diameter of 3μm, 4μm, and 5μm respectively. The solid lines and dashed lines show the dependence of bilayers and monolayers, respectively.

The analysis above shows that the graphene drums are caused by the annealing process, thus there must be additional gas went into the holes during the annealing stage. As no external pressure was used, gas molecules in the atmosphere will not enter the confined region. Thus the only source of the additional gas is those gas molecules initially confined between the graphene and the substrate. This is supported by observing the phase map while measuring the morphology of the surface in tapping mode with AFM. In this case, the tape was removed and no annealing process was performed. The non-uniformity of the regions outside the holes (Fig.4a) clearly indicates the presence of gas molecules confined at the graphene-SiO2 interface. In other words, there are many gas bubbles at the interface, covered by graphene. Since the pressure drop across the graphene layer Δ𝑃 is inversely proportional to the radius of the graphene cap R as Δ𝑃~𝛾/𝑅 where γ is the line tension of graphene, the pressure for gas confined in small bubbles is larger than that in big bubbles, i.e. holes covered by graphene. Thus during the annealing process, the gas molecules will escape from 7



small bubbles into the holes, leading to an increase in the height of the suspended graphene and forming drums. This process resembles the well-known Oswald ripening process. A direct consequence of such process is the graphene layer should also become flattered due to the removal of gas bubbles, leading to a larger contact area between graphene and substrate. This is supported by the dramatic difference between the phase maps measured before and after annealing the substrate covered by a layer of graphene (Fig.4a and 4b), which reflect the elastic modulus and other mechanical properties of the base material. The bubbles observed in Fig.4a completely disappeared after annealing (Fig.4b), indicating a uniform contact between graphene and SiO2 surface. Quantitatively, we found that the volume of gas confined in the holes after annealing at 150℃ increased by about 1.11×108nm3 with detailed estimations shown in supporting information (SI). This is in agreement with the volume changes after annealing at 150℃ with tape attached as shown in Fig.1b-1e (1.24±0.34×108nm3) (see SI), indicating that the mechanism underlying these two processes should be the same.

Figure 4. The phase map measured with AFM in tapping mode before (a) and after (b) annealing the substrate covered by a layer of graphene.

With the experimental analysis shown above, our results suggest that the graphene layer on the substrate act in this process effectively as a one-way valve35 as illustrated 8


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in Fig.5. It allows the gas molecules at the graphite/substrate interface, e.g. confined within ripples, escape to micro holes during annealing, leading to a reduced distance between graphene and substrate. The enhanced van der Waals force between the surfaces induce a uniform and close contact between graphene and SiO2 surface, preventing the gas molecules escape from the holes during the cooling process. Thus the pressure in the micro-holes still keeps a higher level than outside, producing micro drums.

Figure 5. Schematic of the mechanisms for the formation of graphene drums. (a) The substrate in contact with graphite with gas trapped at the interface before annealing. (b) During the annealing process, the gas molecules initially trapped at the interface diffuse into the micro-holes. (c) Cooling to room temperature. A uniform and close contact between the graphene layer and SiO2 surface formed due to the removal of gas 9



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molecules during annealing. This contact serves as a valve to keep the pressure inside holes, forming graphene drums.

By comparing the morphology of suspended graphene after annealing with (Fig.2) and without tape (Fig.4), it is evident that the tape plays an important role in the formation of graphene drums. This can be understood by noticing that for suspended graphene before annealing without tape, it always collapsed into the hole due to the strong adhesion between the graphene and SiO2 (Fig.4) which has been observed before28. From this point of view, the tape serves as a support wall to prevent the graphene from collapsing before the gas molecules diffusing into the hole to form the drums during annealing. After being annealed, due to the existence of pressure difference across the graphene layer, the removing of tape will not cause collapse anymore.

Discussion With the mechanism revealed, the modified mechanical exfoliation method provides a rapid and robust approach to obtain graphene drums with a small strain on the surface. Since no external pressure is needed, the system is prevented from being destroyed by the pressure difference across the graphene layer either due to the breakage of the substrate or the collapse of the graphene layer. For the method proposed, we further studied the dependence of the height and strain of graphene drums on annealing temperature and the size of the holes to achieve controllable formation of graphene drums. As shown in Fig.6a, in all cases the height of graphene drums is proportional to the diameter of micro-holes. From the shape of the drums, it is possible to calculate the differential pressure as 𝛥𝑝 = 𝑓(𝜈)𝐸[ℎ3/

4

(𝐷2) ],

where 𝑓(𝜈) is a

function of the Poisson ratio 𝜈 for graphene, 𝐸 is Young's modulus of graphene, and 𝐷 is the diameter of the hole. We used E = 347 N/m and 𝜈 = 0.16; thus 𝑓(𝜈) = 3.09.21 Correspondingly the strain in the center of the graphene drums can be

10


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calculated as36 𝜀𝑐𝑒𝑛𝑡𝑒𝑟 =

[

2

𝐸

1 3

] , where

𝜓(𝜈)Δ2𝐷2

𝜓(𝜈) =

45(3 ― 𝜈)3(1 ― 𝜈2)

2

2 2

8(23 + 18𝜈 ― 3𝜈 )

. The results are

plotted in the Fig.6b. For the same temperature, the larger diameter of graphene drums leads to higher strain at the center. Meanwhile, the height and strain of graphene drums increase with the annealing temperature by keeping the duration of the annealing process the same. This is understandable since higher temperature leads to faster diffusion either due to higher pressure drop built between the gas confined in the holes and the small bubbles, or the faster diffusion rate based on Arrhenius law. Thus, the height of graphene drums is related to the amount of gas adsorbed on the substrate (e.g. ambient air). As a result, the dispersion of height inherently depends on the area of graphene covering the substrate around the hole and gas adsorption which shows certain variance. Plasma process can clean the impurities on the surface, and is beneficial to the adsorption of gas molecules. According to the mechanism revealed, within a given time, more gas molecules will diffuse into the holes, forming drums with a larger height.

Figure 6. (a) The height of the graphene drums as a function of the diameter of the holes with the annealing temperature of 50℃, 100℃ and 150℃ respectively. Solid lines and dashed lines show the dependence of bilayers and monolayers, respectively. (b) The strain in the center of graphene drums as a function of annealing temperature with the diameter of 3μm, 4μm, and 5μm respectively.

Besides the dependence of height and strain of the graphene drums on temperature and size of the holes, we’ve also studied the stability of the drums since it is an important property from the application point of view. Figure 7a shows the evolving 11



morphology of a graphene drum with a diameter of 4μm after annealed at 150℃ within 30 hours. Obviously, the height of the graphene drum gradually decreases with time. Due to the adsorption between graphene and the inner wall of the holes, the drum began to collapse into the holes from the edge rather than the center (Fig.7b) of the holes. The frequency deviation of the Raman G band in the center of the microchambers decreased from 11cm−1 to zero, demonstrating the strain of graphene gradually disappear over the time as shown in Fig.7c. Interestingly the collapse of the drums observed here could be regarded as a reverse process of the formation of graphene drums using a high-pressure reaction vessel8, 28, 34.

Figure 7. Change of the morphology of the graphene drum with time. (a) Evolution of morphology of graphene drums with ~4μm in diameter and annealed at a temperature ~150℃. (b) The change of crossing morphology of graphene drums obtained with a line scan using AFM with time. (c) Raman spectra in the center of the graphene drums. The inset shows the frequency shift of the Raman G band as a function of time after fabrication.

The gradual decrease of strain over time provides us a way to study the effects of drum formation/strain on the mechanical properties of graphene. To this end, we measured the friction during the unloading process after the graphene drum is formed (Fig.7). As shown in Fig.8, the friction decreases with the strain increases. Surprisingly, within a strain of 2‰, the friction decreases by one order. This result is in agreement with the simulated result37 qualitatively.

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Figure 8. Dependence of friction on the strain of the graphene measured during the unloading process for the drum.

Conclusion In summary, a one-step modified mechanical exfoliation method of graphene on the substrate configured with micro holes is proposed to fabricate graphene drums. With comparative experiments, the mechanism is revealed to be the diffusion of gas molecules confined at the interface between graphene and SiO2 surfaces during the annealing process. This is supported with the positive correlation between the height and strain of the graphene drums with the annealing temperature and diameter of the holes. Compared with existing methods8, 23, 28-29, this method has the advantages of being efficient and robust to obtain graphene drums with a small strain on the surfaces. Further improvement of our method may focus on increasing the strain (0.2% at present) to be comparable with other existing methods (~1%) or even higher. The approach proposed provides convenience and support on the follow-up studies about the effects of curvature and strain of graphene on its mechanical, optical and electrical properties.

Characterization. 13



The thickness of the graphene samples was first identified with Raman spectroscopy (Horiba, LabRam HR Evolution) with a laser wavelength of 532nm focused to a spot size of ~1μm and further confirmed with atomic force microscopy (Oxford Instruments, MFP-3D Infinity). Raman spectroscopy is also used to distinguish the difference between suspended graphene and supported graphene, and measure the strain of graphene drums changed over time through the frequency shift of the Raman G band. The laser power used was < 10 mW and the same absorption time was kept in the whole process when Raman spectra were collected with a resolution of ~0.1 cm−1. Atomic force microscope (AFM) was used for accurate characterization of the morphology of graphene layers. Tapping model was used in the morphology characterization. The stiffness of the probe is 40N/m, and the tip radius is ~9nm. An optical microscope (long working distance metallography microscope, OLYMPUS BXFM) was used for the optical characterization of thin graphene layers.

Micro-holes fabrication The SiO2/Si(100) substrate used in the experiment is prepared by thermal oxidation, in which, the cleaned Si(100) wafers were thermally oxidized in a kiln at 1050°C to prepare 300nm oxide films. Micro-holes were fabricated using standard optical lithographic methods to dene holes ranging from 3 to 5µm in diameter on the SiO2/Si(100) substrate, reactive ion etching to etch through the 250nm thermal oxide layer in order to form microchambers. Prior to mechanical exfoliation of graphene, the substrate configured with micro holes were cleaned with a base piranha solution of 3:1(v/v) ratio of concentrated H2SO4 and H2O2 (30%) at 85 °C for 30 minutes. The SiO2/Si(100) substrates were then rinsed with acetone, isopropyl alcohol, nanopure water, and dried with streaming N2. Before the graphene was transferred to seal the micro holes, the substrates were treated with oxygen plasma for 20 minutes at 300 sccm and 500 Watts to ensure hydrocarbon contaminants on the substrates were removed.

Acknowledgements 14


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M.M. wishes to acknowledge the financial support by Thousand Young Talents Program and the NSFC grant No. 11632009, 11772168.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at … It provides additional details about optical image and atomic structure of graphene on the substrate, the morphology of graphene covering the holes without annealing treatment, the roughness of the substrate before and after annealing, the calculation of the volume change before and after annealing, and the measurement of graphene height and strain over time.

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Efficient and robust fabrication of microscale graphene drums - ACS

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