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Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition
Carbon 42 (2004) 2867–2872 www.elsevier.com/locate/carbon
Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition Jianjun Wang *, Mingyao Zhu, Ron A. Outlaw, Xin Zhao, Dennis M. Manos, Brian C. Holloway Department of Applied Science, College of William and Mary, 325 McGlothin-Street Hall, Williamsburg, VA 23187, USA Received 29 May 2004; accepted 23 June 2004 Available online 14 August 2004
Abstract An ultrathin sheet-like carbon nanostructure, carbon nanosheet, has been effectively synthesized with CH4 diluted in H2 by an inductively coupled radio-frequency plasma enhanced chemical vapor deposition. Nanosheets were obtained without catalyst over a wide range of deposition conditions and on a variety of substrates, including metals, semiconductors and insulators. Scanning electron microscopy shows that the sheet-like structures stand on edge on the substrate and have corrugated surfaces. The sheets are 1 nm or less in thickness and have a defective graphite structure. Raman spectra show typical carbon features with D and G peaks at 1350 and 1580 cm1, respectively. The intensity ratio of these two peaks, I(D)/I(G), increases with methane concentration or substrate temperature, indicating that the crystallinity of the nanosheets decreases. Infrared and thermal desorption spectroscopies reveal hydrogen incorporation into the carbon nanosheets. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Graphite, Graphitic carbon; C. Chemical vapor deposition
1. Introduction While the remarkable properties and applications of fullerenes, nanotubes, and nanofibers have been well documented, another carbon allotrope, carbon nanosheet, characterized by its nanoscale thickness and open geometry (surfaces and edges), has not been thoroughly investigated. Carbon nanosheet provides an important model of a two-dimensional graphite structure with strong anisotropy in physical properties. For example, recent theory predicts an unconventional electronic structure of edge-inherited non-bonding states [1–5].
*
Corresponding author. Tel.: +1 757 221 1890; fax: +1 757 221 2050. E-mail address: [email protected] (J. Wang). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.06.035
The effects of the electron–electron interactions in a graphene layer have also been theoretically investigated. Gonzales et al. [6] indicate that short-range couplings are irrelevant and scale towards zero at low energies, due to the vanishing of density of states at the Fermi level. Topological disorder enhances the density of states and can lead to instabilities. In the presence of sufficiently strong repulsive interactions, p-wave superconductivity can emerge. From an applications perspective, carbon nanosheets have a very high surface-to-volume ratio and sharp edges which are attractive for fuel cells and microelectronic technologies. Free-standing and vertically oriented graphite sheets could serve as an ideal material for catalyst support or an efficient edge emitter for electron field emission. Isolated, sheet/petal-like carbon structures were previously reported as byproducts during fullerene and nanotube preparations by arc discharge [7–9] and laser ablation [9]. However, in all cases,
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J. Wang et al. / Carbon 42 (2004) 2867–2872
the produced carbon sheets coexisted with other carbon forms and had low controllability. Subsequent efforts were made by chemical vapor deposition [10,11] and chemical exfoliation associated with the GIC technique [12,13], but only thick graphite sheets (10 nm or greater) were achieved. In this work, free-standing, 1 nm thick carbon nanosheets, were effectively synthesized by inductively coupled radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) without any catalyst or special substrate pretreatment, and their basic properties were investigated.
2. Experimental Carbon nanosheets were grown in an RF PECVD system schematically shown in Fig. 1. RF (13.56 MHz) energy was inductively coupled into the deposition chamber with a planar-coiled RF antenna (20 cm in diameter) through a quartz window. The plasma density of this inductive plasma is 10 times greater than that that in a capacitive mode at the same RF power input [14]. Before deposition, neither catalyst nor special substrate treatment was needed. Substrates were simply cleaned by sonicating in ethanol for several minutes and then dried in air. The resistively heated sample stage was positioned 4–8 cm below the quartz window (although not used for this study, the stage is capable of DC bias). The substrate temperature was measured by a thermocouple on the upper surface. Mass flow controllers (MFC, MKS 1259B) were used to control the gas flow. During deposition, the RF power, total gas flow rate and gas pressure were kept at 900 W, 10 sccm (standard cubic centimeters per minute) and 12 Pa for all experiments, respectively. Methane was used as the carbon source with a volume concentration range of
5–100% in an H2 atmosphere. Substrate temperature was varied from 600 to 900 °C. Depositions were run for durations of 5–40 min. Substrates used in this study include Si, SiO2, Al2O3, Mo, Zr, Ti, Hf, Nb, W, Ta, Cu and 304 stainless steel. The surface morphology of carbon nanosheets was observed by scanning electron microscope (SEM, Hitachi S-4700) operating at 5 and 30 kV in order to achieve better surface image and resolution, respectively. The film cross-sections of as-deposited samples grown on Si substrates were also measured with SEM in order to obtain growth rates of nanosheet planes. The growth rate is defined as the height/length of nanosheet divided by deposition duration. A high resolution transmission electron microscope (HRTEM, Jeol JEM 2010F, operating at 200 kV) was employed to characterize the internal structure. Nanosheets were extracted from a Si substrate by a lacey carbon grid. Raman (Renishaw inVia, laser excitation 514 nm), Fourier transform infrared (FTIR, Nicolet Nexus 670) spectrometers were used to study the structural information and chemical environment. Thermal desorption spectroscopy (TDS) was also involved for chemical characterization.
3. Results and discussion 3.1. Synthesis of carbon nanosheets Fig. 2 shows SEM images of carbon nanosheets grown at different CH4 concentrations on Si substrates. From 10% to 100% CH4, all samples show the same highly corrugated sheet-like feature with almost the same thickness, standing on edges on the substrate. The sheet size decreases with the increase of CH4 concentration probably because of the higher nucleation tendency. The overall translucent appearance of these
Fig. 1. Schematic of the inductively coupled RF PECVD system used for carbon nanosheet deposition.
J. Wang et al. / Carbon 42 (2004) 2867–2872
Fig. 2. SEM images of carbon nanosheets grown at different CH4 concentrations on Si substrates: (a) 10% CH4; (b) 40% CH4; (c) 100% CH4. Other deposition conditions are RF 900 W, 680 °C, 12 Pa, 20 min.
nanosheets under the low energy electron beam (5 kV) suggests a thin, uniform thickness along the entire plane, similar to that observed for nanotubes. Fig. 3 shows SEM images of the carbon nanosheets grown at different substrate temperatures from 630 to 830 °C. It can be seen that the substrate temperature has a much stronger effect on the carbon nanosheet growth. At 630 °C, the sheet density is much lower, indicating that both nucleation and growth rates are slow. Carbon nanosheets actually can not be obtained if the substrate temperature is less than 600 °C in this study. At 730 °C, the morphology is similar to that at 680 °C as shown in Fig. 2b, but the sheet surface is much smaller and less smooth. The nanosheets interlace together and form a nest-like structure. At 830 °C, the sheet
2869
Fig. 3. SEM images of carbon nanosheets grown at different substrate temperatures on Si substrates: (a) 630 °C; (b) 730 °C; (c) 830 °C. Other deposition conditions are RF 900 W, 40% CH4, 12 Pa, 20 min.
planes become more corrugated and agglomerated, but still keep the sheet-like feature and do not increase in thickness. Fig. 4 shows that the growth rate of sheet plane obviously increases with CH4 concentration and/or substrate temperature, though their thicknesses do not change as shown in Figs. 2 and 3. Higher CH4 concentration and substrate temperature usually offer more carbon species or the driving force for nanosheet growth, however, its specific two-dimensional growth mode is still a question. A detailed dynamical model of this growth is under development, in which we propose that a high flux of fast atom and ion bombardment associated with the inductively coupled RF plasma will yield growth along the strongly bonded planes of graphene sheets, rather than in the more weakly bonded stacking direction.
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Growth Rate ( µ m/h)
2.1
(a)
1.8 1.5 1.2 RF 900 W 660 oC ~12 Pa
0.9 0.6
20
40
60
80
100
CH4 concentration (%) 5.0
Growth Rate ( µm/h)
(b) 4.0 3.0 2.0
RF 900 W 40% CH4 12 Pa
1.0 650
700
750
800
850
900
Substrate Temperature (oC) Fig. 4. (a) Growth rate of carbon nanosheet with CH4 concentration. (b) Growth rate of carbon nanosheet varies with substrate temperature. Other deposition conditions are listed in the figure.
Carbon nanosheets can be readily grown on a variety of substrates. Fig. 5 shows two representative nanosheets that were grown on 304 stainless steel and alumina in the same deposition experiment. It can be seen that nanosheet does not change its basic morphology with variation of substrate material. In this study, carbon nanosheets have reproducibly been grown on Si, SiO2, Al2O3, Mo, Zr, Ti, Hf, Nb, W, Ta, Cu and 304 stainless steel substrates and they all show the same general morphology. This non-selective growth property, as well as its flexible growth conditions, gives carbon nanosheet great potential for technological applications. 3.2. Thickness characterization on typical carbon nanosheets Fig. 6 shows a high magnification SEM image (operating at 30 kV) and HRTEM analysis (operating at 200 kV) of individual nanosheets grown under the same conditions as Fig. 2b. In Fig. 6a, the upper-right nanosheet is rolled up and shows the cross-section of the edge, which is about 1 nm thick. The image could not be more clearly focused because of the resolution limit of the SEM. The lower-left nanosheet is corrugated with a ridge in the middle. The ridge width, which is at least double-sheets thick (there has to be a spacing in between
Fig. 5. SEM images of carbon nanosheets grown on various substrates: (a) nanosheets on 304 stainless steel; (b) nanosheets on alumina. Deposition conditions are RF 900 W, 40% CH4, 680 °C, 12 Pa, 20 min.
as the sheet folds into a corrugation), is measured to be 2 nm at the narrowest position, indicating that the thickness of a single-sheet is 1 nm or less. With the higher electron accelerating voltage, the SEM also reveals much more internal structure besides surface information. Fig. 6b is a HRTEM image that displays a similar corrugated nanosheet with a dark folded ridge in the middle of the piece and two dark fringes (arrow points out) at the edge. The number of dark fringes at the edge suggests an even thinner sheet, which may consist of only two graphene layers in stacking. The electron diffraction pattern from nanosheets (Fig. 6b inset) is indexed to graphite. The intensive 002 spots indicate a well-stacked structure. The characteristic of this ultrathin sheet provides an ideal two-dimensional model of graphite structure for fundamental studies. 3.3. Raman investigation of carbon nanosheets Raman spectra taken on carbon nanosheets, as shown in Fig. 7, are similar to those observed for carbon nanotubes. Fig. 7a shows Raman spectra of carbon nanosheets grown at different CH4 concentrations (same samples as in Fig. 2). All samples show the D band (1350 cm1), the G band (1580 cm1) and so-called D 0 band (a shoulder at 1620 cm1), which are previously seen in microcrystalline graphite [15–17], indicating that carbon
J. Wang et al. / Carbon 42 (2004) 2867–2872
(a)
2871
D
G'
G D'
RF 900 W 680 oC ~12 Pa 20 min
Intensity (a.u.)
100%CH4
40%CH4
10%CH4
1000
1500
2000
2500
3000
3500
-1
Raman Shift (cm ) (b)
D
G'
G
Intensity (a.u.)
D'
RF 900 W 40% CH4 12 Pa 20 min
830 oC
730 oC
630 oC
1000
1500
2000
2500
3000
3500
-1
Raman Shift (cm ) Fig. 6. Typical individual nanosheets of the sample shown in Fig. 2b: (a) a higher magnification SEM image; (b) an HRTEM image; inset is the electron diffraction pattern indexed on graphite spacing.
nanosheet has a crystalline, but defective graphite structure. It can also be seen that the intensity ratio of the D and G peaks, I(D)/I(G), increases with CH4 concentration, which usually indicates a more nanocrystalline structure and the presence of a large number of defects. The defects may include vacancies and strained hexagonal/non-hexagonal (pentagon or heptagon) distortions that lead to the non-uniformity, corrugation and twisting shown in the electron microscope images. Secondorder modes in the range of 2000–3500 cm1 are also present in Fig. 7. The strong peak at 2700 cm1, socalled G 0 band, is the overtone of the D band. The medium peak at about 2950 cm1 is attributed to the combination of the D and G bands, and the small peak at 3250 cm1 is the overtone of D 0 band. Fig. 7b shows the Raman spectra taken from nanosheets grown at different substrate temperatures (same samples as in Fig. 3), which displays the same features as in Fig. 7a. According to the discussion above, the lower temperature sample shows a better crystallinity, however, spectra from the other two samples grown at 730 and 830 °C are almost the same though their morphologies are quite different.
Fig. 7. (a) Raman spectra of carbon nanosheets grown at different CH4 concentrations. (b) Raman spectra of carbon nanosheets grown at different substrate temperatures. Other deposition conditions are listed in the figure.
3.4. Brief FTIR and TDS results on typical carbon nanosheets Since CH4 and H2 were used in the growth process for carbon nanosheets, it is expected that hydrogen will incorporate into nanosheets. Typical carbon nanosheet samples (same samples as in Fig. 2b) were analyzed with FTIR and TDS on their hydrogen component. FTIR transmission spectra show obvious absorptions at 2900 cm1 due to C–H stretching vibrations. TDS data also reveals a very large release of H2 from carbon nanosheets when they were heated up to 800 °C.
4. Conclusions A sheet-like carbon nanostructure, carbon nanosheet, has been synthesized by inductively coupled RF PECVD without catalyst or special surface pretreatment. Nanosheets oriented to stand on edge can be readily grown over a wide range of deposition conditions on various substrates, including metals, semiconductors and insulators. These carbon sheets are 1 nm or less in thickness and have a defective graphite structure. The sheet-like
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carbon nanostructure offers new possibilities for both fundamental studies and technological applications. Acknowledgments The authors gratefully thank Kerry Siebein for assistance with HRTEM at the University of Florida, and Office of Naval Research for financial support (Grant No. N00014-02-1-0711). References [1] Nakada K, Fujita M, Dresselhaus G, Dresselhaus MS. Edge state in graphene ribbons: nanometer size effect and edge shape dependence. Phys Rev B 1996;54:17954–61. [2] Wakabayashi K, Fujita M, Ajiki H, Sigrist M. Electronic and magnetic properties of nanographite ribbons. Phys Rev B 1999;59:8271–82. [3] Andersson OE, Prasad BLV, Sato H, Enoki T, Hishiyama Y, Kaburagi Y, et al. Structure and electronic properties of graphite nanoparticles. Phys Rev B 1998;58:16387–95. [4] Prasad BLV, Sato H, Enoki T, Hishiyama Y, Kaburagi Y, Rao AM, et al. Heat-treatment effect on the nanosized graphite [p]electron system during diamond to graphite conversion. Phys Rev B 2000;62:11209–18. [5] Affoune AM, Prasad BLV, Sato H, Enoki T, Kaburagi Y, Hishiyama Y. Experimental evidence of a single nano-graphene. J Chem Lett 2001;348:17–20.
[6] Gonzalez J, Guinea F, Vozmediano MAH. Electron–electron interactions in graphene sheets. Phys Rev B 2001;63:134421/1–1/8. [7] Ando Y, Zhao X, Ohkohchi M. Production of petal-like graphite sheets by hydrogen arc discharge. Carbon 1997;35:153–8. [8] Ebbesen TW, Ajayan PM. Large-scale synthesis of carbon nanotubes. Nature 1992;358:220–2. [9] Iijima S, Wakabayashi T, Achiba Y. Structures of carbon soot prepared by laser ablation. J Phys Chem 1996;100:5839–43. [10] Shang NG, Au FCK, Meng XM, Lee CS, Bello I, Lee ST. Uniform carbon nanoflake films and their field emissions. J Chem Lett 2002;358:187–91. [11] Obraztsov AN, Volkov AP, Zakhidov AA, Lyashenko DA, Petrushenko YV, Satanovskaya OP. Field emission characteristics of nanostructured thin film carbon materials. Appl Surf Sci 2003;215:214–21. [12] Viculis LM, Mack JJ, Kaner RB. A chemical route to carbon nanoscrolls. Science 2003;299:1361. [13] Chen G, Weng W, Wu D, Wu C, Lu J, Wang P, et al. Preparation and characterization of graphite nanosheets from ultrasonic powdering technique. Carbon 2004;42:753–9. [14] Lieberman MA, Lichtenberg AJ. In: Principles of plasma discharges and materials processing. New York: Wiley; 1994. p. 387–411. [15] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53:1126–30. [16] Nemanich RJ, Solin SA. First- and second-order Raman scattering from finite-size crystals of graphite. Phys Rev B 1979;20:392–401. [17] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000; 61:14095–107.
Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition Jianjun Wang *, Mingyao Zhu, Ron A. Outlaw, Xin Zhao, Dennis M. Manos, Brian C. Holloway Department of Applied Science, College of William and Mary, 325 McGlothin-Street Hall, Williamsburg, VA 23187, USA Received 29 May 2004; accepted 23 June 2004 Available online 14 August 2004
Abstract An ultrathin sheet-like carbon nanostructure, carbon nanosheet, has been effectively synthesized with CH4 diluted in H2 by an inductively coupled radio-frequency plasma enhanced chemical vapor deposition. Nanosheets were obtained without catalyst over a wide range of deposition conditions and on a variety of substrates, including metals, semiconductors and insulators. Scanning electron microscopy shows that the sheet-like structures stand on edge on the substrate and have corrugated surfaces. The sheets are 1 nm or less in thickness and have a defective graphite structure. Raman spectra show typical carbon features with D and G peaks at 1350 and 1580 cm1, respectively. The intensity ratio of these two peaks, I(D)/I(G), increases with methane concentration or substrate temperature, indicating that the crystallinity of the nanosheets decreases. Infrared and thermal desorption spectroscopies reveal hydrogen incorporation into the carbon nanosheets. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Graphite, Graphitic carbon; C. Chemical vapor deposition
1. Introduction While the remarkable properties and applications of fullerenes, nanotubes, and nanofibers have been well documented, another carbon allotrope, carbon nanosheet, characterized by its nanoscale thickness and open geometry (surfaces and edges), has not been thoroughly investigated. Carbon nanosheet provides an important model of a two-dimensional graphite structure with strong anisotropy in physical properties. For example, recent theory predicts an unconventional electronic structure of edge-inherited non-bonding states [1–5].
*
Corresponding author. Tel.: +1 757 221 1890; fax: +1 757 221 2050. E-mail address: [email protected] (J. Wang). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.06.035
The effects of the electron–electron interactions in a graphene layer have also been theoretically investigated. Gonzales et al. [6] indicate that short-range couplings are irrelevant and scale towards zero at low energies, due to the vanishing of density of states at the Fermi level. Topological disorder enhances the density of states and can lead to instabilities. In the presence of sufficiently strong repulsive interactions, p-wave superconductivity can emerge. From an applications perspective, carbon nanosheets have a very high surface-to-volume ratio and sharp edges which are attractive for fuel cells and microelectronic technologies. Free-standing and vertically oriented graphite sheets could serve as an ideal material for catalyst support or an efficient edge emitter for electron field emission. Isolated, sheet/petal-like carbon structures were previously reported as byproducts during fullerene and nanotube preparations by arc discharge [7–9] and laser ablation [9]. However, in all cases,
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J. Wang et al. / Carbon 42 (2004) 2867–2872
the produced carbon sheets coexisted with other carbon forms and had low controllability. Subsequent efforts were made by chemical vapor deposition [10,11] and chemical exfoliation associated with the GIC technique [12,13], but only thick graphite sheets (10 nm or greater) were achieved. In this work, free-standing, 1 nm thick carbon nanosheets, were effectively synthesized by inductively coupled radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) without any catalyst or special substrate pretreatment, and their basic properties were investigated.
2. Experimental Carbon nanosheets were grown in an RF PECVD system schematically shown in Fig. 1. RF (13.56 MHz) energy was inductively coupled into the deposition chamber with a planar-coiled RF antenna (20 cm in diameter) through a quartz window. The plasma density of this inductive plasma is 10 times greater than that that in a capacitive mode at the same RF power input [14]. Before deposition, neither catalyst nor special substrate treatment was needed. Substrates were simply cleaned by sonicating in ethanol for several minutes and then dried in air. The resistively heated sample stage was positioned 4–8 cm below the quartz window (although not used for this study, the stage is capable of DC bias). The substrate temperature was measured by a thermocouple on the upper surface. Mass flow controllers (MFC, MKS 1259B) were used to control the gas flow. During deposition, the RF power, total gas flow rate and gas pressure were kept at 900 W, 10 sccm (standard cubic centimeters per minute) and 12 Pa for all experiments, respectively. Methane was used as the carbon source with a volume concentration range of
5–100% in an H2 atmosphere. Substrate temperature was varied from 600 to 900 °C. Depositions were run for durations of 5–40 min. Substrates used in this study include Si, SiO2, Al2O3, Mo, Zr, Ti, Hf, Nb, W, Ta, Cu and 304 stainless steel. The surface morphology of carbon nanosheets was observed by scanning electron microscope (SEM, Hitachi S-4700) operating at 5 and 30 kV in order to achieve better surface image and resolution, respectively. The film cross-sections of as-deposited samples grown on Si substrates were also measured with SEM in order to obtain growth rates of nanosheet planes. The growth rate is defined as the height/length of nanosheet divided by deposition duration. A high resolution transmission electron microscope (HRTEM, Jeol JEM 2010F, operating at 200 kV) was employed to characterize the internal structure. Nanosheets were extracted from a Si substrate by a lacey carbon grid. Raman (Renishaw inVia, laser excitation 514 nm), Fourier transform infrared (FTIR, Nicolet Nexus 670) spectrometers were used to study the structural information and chemical environment. Thermal desorption spectroscopy (TDS) was also involved for chemical characterization.
3. Results and discussion 3.1. Synthesis of carbon nanosheets Fig. 2 shows SEM images of carbon nanosheets grown at different CH4 concentrations on Si substrates. From 10% to 100% CH4, all samples show the same highly corrugated sheet-like feature with almost the same thickness, standing on edges on the substrate. The sheet size decreases with the increase of CH4 concentration probably because of the higher nucleation tendency. The overall translucent appearance of these
Fig. 1. Schematic of the inductively coupled RF PECVD system used for carbon nanosheet deposition.
J. Wang et al. / Carbon 42 (2004) 2867–2872
Fig. 2. SEM images of carbon nanosheets grown at different CH4 concentrations on Si substrates: (a) 10% CH4; (b) 40% CH4; (c) 100% CH4. Other deposition conditions are RF 900 W, 680 °C, 12 Pa, 20 min.
nanosheets under the low energy electron beam (5 kV) suggests a thin, uniform thickness along the entire plane, similar to that observed for nanotubes. Fig. 3 shows SEM images of the carbon nanosheets grown at different substrate temperatures from 630 to 830 °C. It can be seen that the substrate temperature has a much stronger effect on the carbon nanosheet growth. At 630 °C, the sheet density is much lower, indicating that both nucleation and growth rates are slow. Carbon nanosheets actually can not be obtained if the substrate temperature is less than 600 °C in this study. At 730 °C, the morphology is similar to that at 680 °C as shown in Fig. 2b, but the sheet surface is much smaller and less smooth. The nanosheets interlace together and form a nest-like structure. At 830 °C, the sheet
2869
Fig. 3. SEM images of carbon nanosheets grown at different substrate temperatures on Si substrates: (a) 630 °C; (b) 730 °C; (c) 830 °C. Other deposition conditions are RF 900 W, 40% CH4, 12 Pa, 20 min.
planes become more corrugated and agglomerated, but still keep the sheet-like feature and do not increase in thickness. Fig. 4 shows that the growth rate of sheet plane obviously increases with CH4 concentration and/or substrate temperature, though their thicknesses do not change as shown in Figs. 2 and 3. Higher CH4 concentration and substrate temperature usually offer more carbon species or the driving force for nanosheet growth, however, its specific two-dimensional growth mode is still a question. A detailed dynamical model of this growth is under development, in which we propose that a high flux of fast atom and ion bombardment associated with the inductively coupled RF plasma will yield growth along the strongly bonded planes of graphene sheets, rather than in the more weakly bonded stacking direction.
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Growth Rate ( µ m/h)
2.1
(a)
1.8 1.5 1.2 RF 900 W 660 oC ~12 Pa
0.9 0.6
20
40
60
80
100
CH4 concentration (%) 5.0
Growth Rate ( µm/h)
(b) 4.0 3.0 2.0
RF 900 W 40% CH4 12 Pa
1.0 650
700
750
800
850
900
Substrate Temperature (oC) Fig. 4. (a) Growth rate of carbon nanosheet with CH4 concentration. (b) Growth rate of carbon nanosheet varies with substrate temperature. Other deposition conditions are listed in the figure.
Carbon nanosheets can be readily grown on a variety of substrates. Fig. 5 shows two representative nanosheets that were grown on 304 stainless steel and alumina in the same deposition experiment. It can be seen that nanosheet does not change its basic morphology with variation of substrate material. In this study, carbon nanosheets have reproducibly been grown on Si, SiO2, Al2O3, Mo, Zr, Ti, Hf, Nb, W, Ta, Cu and 304 stainless steel substrates and they all show the same general morphology. This non-selective growth property, as well as its flexible growth conditions, gives carbon nanosheet great potential for technological applications. 3.2. Thickness characterization on typical carbon nanosheets Fig. 6 shows a high magnification SEM image (operating at 30 kV) and HRTEM analysis (operating at 200 kV) of individual nanosheets grown under the same conditions as Fig. 2b. In Fig. 6a, the upper-right nanosheet is rolled up and shows the cross-section of the edge, which is about 1 nm thick. The image could not be more clearly focused because of the resolution limit of the SEM. The lower-left nanosheet is corrugated with a ridge in the middle. The ridge width, which is at least double-sheets thick (there has to be a spacing in between
Fig. 5. SEM images of carbon nanosheets grown on various substrates: (a) nanosheets on 304 stainless steel; (b) nanosheets on alumina. Deposition conditions are RF 900 W, 40% CH4, 680 °C, 12 Pa, 20 min.
as the sheet folds into a corrugation), is measured to be 2 nm at the narrowest position, indicating that the thickness of a single-sheet is 1 nm or less. With the higher electron accelerating voltage, the SEM also reveals much more internal structure besides surface information. Fig. 6b is a HRTEM image that displays a similar corrugated nanosheet with a dark folded ridge in the middle of the piece and two dark fringes (arrow points out) at the edge. The number of dark fringes at the edge suggests an even thinner sheet, which may consist of only two graphene layers in stacking. The electron diffraction pattern from nanosheets (Fig. 6b inset) is indexed to graphite. The intensive 002 spots indicate a well-stacked structure. The characteristic of this ultrathin sheet provides an ideal two-dimensional model of graphite structure for fundamental studies. 3.3. Raman investigation of carbon nanosheets Raman spectra taken on carbon nanosheets, as shown in Fig. 7, are similar to those observed for carbon nanotubes. Fig. 7a shows Raman spectra of carbon nanosheets grown at different CH4 concentrations (same samples as in Fig. 2). All samples show the D band (1350 cm1), the G band (1580 cm1) and so-called D 0 band (a shoulder at 1620 cm1), which are previously seen in microcrystalline graphite [15–17], indicating that carbon
J. Wang et al. / Carbon 42 (2004) 2867–2872
(a)
2871
D
G'
G D'
RF 900 W 680 oC ~12 Pa 20 min
Intensity (a.u.)
100%CH4
40%CH4
10%CH4
1000
1500
2000
2500
3000
3500
-1
Raman Shift (cm ) (b)
D
G'
G
Intensity (a.u.)
D'
RF 900 W 40% CH4 12 Pa 20 min
830 oC
730 oC
630 oC
1000
1500
2000
2500
3000
3500
-1
Raman Shift (cm ) Fig. 6. Typical individual nanosheets of the sample shown in Fig. 2b: (a) a higher magnification SEM image; (b) an HRTEM image; inset is the electron diffraction pattern indexed on graphite spacing.
nanosheet has a crystalline, but defective graphite structure. It can also be seen that the intensity ratio of the D and G peaks, I(D)/I(G), increases with CH4 concentration, which usually indicates a more nanocrystalline structure and the presence of a large number of defects. The defects may include vacancies and strained hexagonal/non-hexagonal (pentagon or heptagon) distortions that lead to the non-uniformity, corrugation and twisting shown in the electron microscope images. Secondorder modes in the range of 2000–3500 cm1 are also present in Fig. 7. The strong peak at 2700 cm1, socalled G 0 band, is the overtone of the D band. The medium peak at about 2950 cm1 is attributed to the combination of the D and G bands, and the small peak at 3250 cm1 is the overtone of D 0 band. Fig. 7b shows the Raman spectra taken from nanosheets grown at different substrate temperatures (same samples as in Fig. 3), which displays the same features as in Fig. 7a. According to the discussion above, the lower temperature sample shows a better crystallinity, however, spectra from the other two samples grown at 730 and 830 °C are almost the same though their morphologies are quite different.
Fig. 7. (a) Raman spectra of carbon nanosheets grown at different CH4 concentrations. (b) Raman spectra of carbon nanosheets grown at different substrate temperatures. Other deposition conditions are listed in the figure.
3.4. Brief FTIR and TDS results on typical carbon nanosheets Since CH4 and H2 were used in the growth process for carbon nanosheets, it is expected that hydrogen will incorporate into nanosheets. Typical carbon nanosheet samples (same samples as in Fig. 2b) were analyzed with FTIR and TDS on their hydrogen component. FTIR transmission spectra show obvious absorptions at 2900 cm1 due to C–H stretching vibrations. TDS data also reveals a very large release of H2 from carbon nanosheets when they were heated up to 800 °C.
4. Conclusions A sheet-like carbon nanostructure, carbon nanosheet, has been synthesized by inductively coupled RF PECVD without catalyst or special surface pretreatment. Nanosheets oriented to stand on edge can be readily grown over a wide range of deposition conditions on various substrates, including metals, semiconductors and insulators. These carbon sheets are 1 nm or less in thickness and have a defective graphite structure. The sheet-like
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