Article pubs.acs.org/JPCC
Lattice Selective Growth of Graphene on Sapphire Substrate Gang Wang,†,‡ Yun Zhao,†,§ Ya Deng,†,‡ Wenbin Huang,† Xiaokun Fan,† Jian Zhang,†,‡ Ruifei Duan,§ and Lianfeng Sun*,† †
National Center for Nanoscience and Technology, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China § Institute of Semiconductor, Chinese Academy of Sciences, Beijing 100083, China ‡
S Supporting Information *
ABSTRACT: In this work, we report a systematic study of CVD synthesizing graphene on different crystal faces of sapphire (c-Plane, m-Plane, and r-Plane). Nickel films are deposited on sapphire substrates with c-Plane, m-Plane, and r-Plane to catalyzing the growth of graphene by CVD (Chemical Vapor Deposition) method. It is only on cPlane sapphire substrates that graphene can be found after growth and etching off nickel. In the case of m-Plane and r-Plane sapphire substrates, graphene forms only on the top surface of nickel films, but none at the interface between nickel and sapphire. Oxygen plasma treatment is introduced to certify that graphene on c-Plane sapphire does not originate from the top surface of nickel film. Selective formation of graphene is attributed to lattice structures of sapphire’s different faces. Moreover, influences of nickel’s thickness and growth time are studied by a series of control experiments.
1. INTRODUCTION
crystal faces are lattice selective in the growth of graphene by the CVD method.
Graphene, as an allotrope of carbon with sp2 bonding, has attracted great interest since its first isolation in 2004.1 Due to its unique physical properties, including extremely high carrier mobility and unique band structure, graphene exhibits a promising future for use in electronics.2−5 Much effort has been devoted to the fabrication of graphene, which is crucial for both fundamental research and applications. To date, several methods have been widely used to fabricate graphene, including micromechanical cleavage of natural graphite, CVD (chemical vapor deposition), and epitaxial growth on SiC,1,6−10 in which CVD has most possibility the largest application in electronic devices. Neverless, CVD synthesis of large-area graphene with high quality and its compatibility with current manufacturing processes are the main obstacles to large-scale application for electronics. Moreover, the detailed growth mechanism of CVD synthesis on different substrates is still elusive. Specifically, graphene is expected to play an important role in photoelectric devices as transparent conducting film (TCF),11−13 but existing manufacturing processes using graphene as TCF always need transfer procedures, which are a waste of time and would introduce chemical contaminations that may be hazardous to photoelectric devices. It would be much more convenient for graphene to be used as TCF if it could be directly integrated on the target substrate with a facial method that introduces no transfer process. In this work, we attempt to directly deposit graphene on sapphire, which is usually used as the substrate for semiconducting electronics, especially for the substrate of light emitting diodes (LED) like gallium nitride.14−16 During our experiments, we found that sapphire substrates with different © XXXX American Chemical Society
2. EXPERIMENTAL METHODS 2.1. Synthesis of Graphene. Figure 1 depicts the schematic procedure for direct growth of graphene on sapphire with different crystal faces by CVD. First of all, sapphire substrates (c-Plane, m-Plane, and r-Plane) were cleaned by sonication in acetone, alcohol, and DI water successively. Then several hundred nanometers thick nickel films (typically 300 nm) were deposited by electron beam evaporation. For graphene growth, the substrates with nickel films were placed in a horizontal quartz tube furnace and heated to 1050 °C. The pressure inside the furnace tube was kept at 10−100 Pa during the whole CVD process. The heating rate was controlled to be about 10 deg/min and the mixing flow of 100 sccm Argon and 50 sccm hydrogen was introduced during annealing. Once the temperature reached 1050 °C, methane was introduced into the furnace tube as the carbon source, with a flow of about 50 sccm. At the same time when introducing methane, argon was shut off and the flow of hydrogen was increased to 100 sccm as reductive gas. After growth (typically 30 min), the sample was fast-cooled to room temperature at the rate of 50−100 deg/ min. 2.2. Characterization. All Raman spectra were calibrated against a silicon wafer before experiments. A 100× objective lens was used in a Renishaw Raman Image spectrophotometer. The micro-Raman spectroscopy experiments were performed Received: October 7, 2014 Revised: December 16, 2014
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DOI: 10.1021/jp5101297 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Schematic procedure for the direct growth of graphene on sapphire by CVD method. (a) Sapphire substrates with different surfaces (cPlane, m-Plane, and r-Plane) are prepared to synthesize graphene. (b) Nickel is deposited on sapphire substrates by electron beam evaporation. (c) After growth, few-layered graphene forms on nickel’s top surface of three kinds of sapphire substrates. (d) After etching away the nickel, graphene can be found only on the c-Plane sapphire substrates.
Figure 2. Typical Raman spectrum of graphene (a) on the top surfaces of nickel films for c-Plane, m-Plane, and r-Plane sapphire substrates after growth and (b) on the c-Plane sapphire substrate after etching nickel (no graphene left on m-Plane and r-Plane sapphire after etching nickel).
three kinds of sapphire substrates’ nickel films, but only was found on c-Plane sapphire after etching off nickel. To testify the results after CVD growing, Raman spectra17−21 were used to characterize graphene left on the top side of nickel and sapphire substrate after etching nickel. Figure 2a shows the typical Raman characterization of graphene on the top side of nickel film for all three kinds of sapphire substrates (c-Plane, mPlane, and r-Plane). As exhibited in Figure 2a, three curves stand for the three kinds of typical Raman spectra signals for graphene obtained from the top side of the sample, which reveal the G peak at ∼1580 cm−1 and the 2D peak at ∼2700 cm−1. According to the shape of the 2D peak and I2D/IG, these three curves correspond to monolayer, bilayer, and few-layer graphene, respectively. It should be noted that almost no D peak can be found and full-widths at half-height maximum
under ambient condition with 514 nm excitation from an argon ion laser, the power of which was set to about 1.0 mW, and the size of the spot was about 1 μm.
3. RESULTS AND DISCUSSION Nickel is the metal catalyst that is usually used in CVD growth of graphene. During the growth period, under high temperature and low pressure atmosphere, methane, as the carbon source, decomposed and diffused in the nickel. Generally, carbon atoms diffused in the nickel would precipitate and then form graphene on both sides of the nickel film with fast cooling. In other words, graphene should be directly obtained on sapphire substrates after dissolving away the nickel by etchant. However, as shown in parts c and d of Figure 1, graphene formed on B
DOI: 10.1021/jp5101297 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C (fwhm) of the G peak and 2D peak are relatively narrow, which indicate high quality of the graphene on the nickel film.22 For the monolayer graphene as shown by the black curve, fwhm of the G peak is 16.6 cm−1, and fwhm of the 2D peak is 23.8 cm−1. For the bilayer graphene shown as the red curve, fwhm of the G peak is 19.9 cm−1, and fwhm of the 2D peak is 43.7 cm−1. For the few-layer graphene shown as the blue curve, fwhm of the G peak is 16.4 cm−1, and fwhm of the 2D peak is 65.2 cm−1. G peak positions for monolayer, bilayer, and few-layer graphene shown in Figure 2a are 1581.3, 1580.5, and 1580.7 cm−1, respectively. 2D peak positions for the three types of graphenes are 2720.7, 2705.3, and 2716.2 cm−1, respectively. There is about 20 cm−1 upshift of the 2D peak for monolayer graphene if we assume the 2D peak of the original monolayer graphene is around 2700 cm−1. According to ref 18, about 0.6% strain was estimated for the monolayer graphene. Similarly, Raman spectra were acquired to characterize the graphene on the sapphire substrate after etching the nickel by FeCl3 acid solution. Figure 2b exhibits the typical results of Raman spectra of graphene on the c-Plane sapphire substrate. As illustrated above, no graphene was found on r-Plane and m-Plane sapphire substrates after etching nickel. The graphene on the c-Plane sapphire substrate, similar to that on the nickel, also shows the G peak at ∼1580 cm−1, the 2D peak at ∼2700 cm−1, and the very weak D peak at ∼1350 cm−1 (hardly seen), which suggest high quality of obtained graphene. The three spectra in Figure 2b refer to the monolayer, trilayer, and few-layer graphene based on the shape of the 2D peak and I2D/IG. It should be noted that although graphene can be obtained on the upper surface of the nickel film on the m-/r-Plane sapphire, there is no graphene found at the interface for the mand r-Plane sapphire. This conclusion is confirmed by the following controlled experiment: Oxygen plasma (3 min, 100 W) was used to clear off the graphene on the upper side of nickel. No Raman signals of graphene can be detected after plasma treatment. Then the nickel film was dissolved with etchant. No graphene can be found in the etching solution during the etching process or on the m-/r-Plane sapphire after the nickel was removed (see Figure 1 in the Supporting Information). Figure 3, shown as the results of a control experiment, eliminated the concern that the obtained graphene on c-Plane sapphire substrate after etching nickel could originate from the top side of nickel. The black curve shows the typical Raman spectra of graphene on the top side of nickel, in which the D peak at ∼1350 cm−1 almost cannot be seen, corresponding with the result shown in Figure 2a,b. Then oxygen plasma treatment was carried out for 5 min at 10 Pa with 5 W of power. After plasma treatment, Raman spectra were acquired and the result was shown as the red curve in Figure 3. As illustrated by the red curve, significant D peak can be seen and ratio of I2D/IG increases sharply, indicating that graphene on the top side of nickel was badly damaged, which was consistent with our previous work.23 If graphene obtained on the c-Plane sapphire after dissolving away nickel originates from the top side of the nickel film, Raman spectra should show a similar feature as that shown by the red curve, which demonstrated many defects. Nevertheless, the graphene signal without the visible D peak was still obtained on the c-Plane sapphire substrate after etching, which was certified by Raman shown by the blue curve. It was evident that the defect-free graphene on the c-Plane sapphire substrate was not originating from the falling of
Figure 3. Raman spectra of graphene obtained from c-Plane sapphire substrate: on the nickel film before oxygen plasma treatment (black curve), after oxygen plasma treatment (red curve), and on sapphire after oxygen plasma treatment and etching away the nickel (blue curve). This indicates that graphene on the c-Plane sapphire after etching is not from the top side of nickel film.
graphene on the top side of nickel but was indeed from the nickel film underneath. Through the verification experiment presented above, it is seen that graphene obtained on the c-Plane sapphire after etching nickel originates from the interface between nickel and sapphire. It is intriguing that sapphire substrates with different crystal faces have distinct phenomenon during the CVD growth. The reason for this discrepancy is attributed to the crystal structures of sapphire with different lattice faces. Sapphire is composed of aluminum and oxygen in an hexagonal crystal system with R-mc space group. As shown in Figure 4a, a schematic picture of a unit cell of sapphire is presented, in which c-Plane, r-Plane, and m-Plane are indicated by specific labels, respectively. c-Plane sapphire, viewed from the c axis as marked in the figure, has an hexagonal structure that is similar to graphene, which is illustrated in Figure 4b. In contrast, crystal structures of m-Plane and r-Plane sapphire have no hexagonal structure. Lattice matching is usually important for epitaxial growth of 2D materials. However, for the epitaxial growth of graphene, the lattice mismatch can be much larger due to the van der Waals force between the substrate and graphene. This conclusion can be confirmed by the experimental reports of epitaxial growth of graphenes on the surfaces of sapphire.24,25 The growth mechanism in this work is different form that in refs 24 and 25 because of the nickel film, which acts as catalyst during the growth of graphene. For metalcatalyst CVD growth, formation of graphene mainly depends on the metal catalyst. In this work, the lattice selective growth of graphene can be attributed to the different size and morphologies of nickel film on c-Plane and m-/r-Plane sapphire. Optical images of nickel films are shown in Figure 4 after CVD growth and removing graphene by oxygen plasma on m-Plane (c), r-Plane (d), and c-Plane (e), respectively. Grain sizes and morphologies of nickel on c-Plane sapphire are quite different from those on m- and r-sapphire substrates. This results in the selective growth of graphene on the different surfaces of sapphire. To investigate the factors which have influence on the quality of graphene, two additional control experiments were designed C
DOI: 10.1021/jp5101297 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. (a) Unit cell of sapphire, in which c-Plane, r-Plane, and m-Plane are marked. (b) Front view of atomic structure of sapphire with c-Plane. Optical images of (c) m-Plane, (d) r-Plane, and (e) c-Plane sapphire after CVD growth and removing graphene by oxygen plasma. The grain sizes and morphologies of nickel on c-Plane sapphire are quite different from those on m- and r-sapphire substrates. This results in the selective growth of graphene on the different surfaces of sapphire. Scale bars in panels c−e are 20 μm.
Figure 5. (a) Raman spectrum of graphene obtained catalyzed by different thicknesses of nickel film. (b) Raman spectrum of graphene obtained with different growth time.
to explore the effects of thickness of nickel film and growth time. Nickel films with different thickness, specifically 400, 300, and 200 nm, were deposited on sapphire substrate by electron beam evaporation and then were positioned in the furnace to grow graphene for 30 min under the conditions which were mentioned above. After etching nickel, Raman spectra were used to characterize the quality of graphene obtained on sapphire substrates. As illustrated in Figure 5a, the graphene on the substrate with 400 nm nickel has the best quality and is almost defect-free because of no visible D peak. As the thickness of the nickel film decreased, the quality of graphene also declined. When the thickness of the nickel was below 200 nm, obvious D peak, as well as much wider full widths at halfheight maximum (fwhm) appeared, indicating significant decline of graphene’s quality. Various growth times have been
tried for c-Plane sapphire substrates with 400 nm nickel film (10, 20, and 30 min as shown in Figure 5b). Figure 5b demonstrates that the quality of the obtained graphene has no obvious diversity when growth time was above 10 min. However, once growth time was below 10 min, a large D peak at ∼1350 cm−1 arose and the 2D peak at ∼2700 cm−1 almost disappeared, indicating the quality of graphene declined dramatically, which should be attributed to the reason that growth time was too short for the nickel film to absorb atoms and arrange them to form a regular structure.
4. CONCLUSIONS To conclude, via CVD growth, graphene can be obtained on the top side of nickel films for all three kinds of sapphire substrates but only on the sapphire with c-Plane after etching D
DOI: 10.1021/jp5101297 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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nickel. To prove the graphene on sapphire substrate was not falling from the top side of the nickel film, we used oxygen plasma to damage the graphene and then characterized the graphene on sapphire substrate with Raman spectra. The reason why different sapphire substrates have distinct results after CVD growth is attributed to the crystal structure of sapphire. Moreover we investigated the impact of growth conditions on the quality of graphene.
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ASSOCIATED CONTENT
S Supporting Information *
Figures showing the optical images of m-/r-Plane sapphire substrates after CVD growth and dissolving the nicke l(Figure S1), the optical and SEM images of graphene obtained on cPlane sapphire (Figure S2), and TEM images of graphene obtained on c-Plane sapphire (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86 010 82545584. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (Grant Nos. 10774032 and 90921001), Key Knowledge Innovation Project of the Chinese Academy of Sciences on Water Science Research, Instrument Developing Project of the Chinese Academy of Sciences (Grant No. Y2010031).
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