In Situ High-Temperature NEXAFS Study on Carbon Nanotube and

Nov 2, 2015 - Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-PF), Tsukuba 305-0801, Japa...
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In Situ High-Temperature NEXAFS Study on Carbon Nanotube and Graphene Formation by Thermal Decomposition of SiC Takahiro Maruyama,*,† Shigeya Naritsuka,‡ and Kenta Amemiya∥ †

Department of Applied Chemistry and ‡Department of Materials Science and Engineering, Meijo University, Nagoya 468-8502, Japan ∥ Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-PF), Tsukuba 305-0801, Japan S Supporting Information *

ABSTRACT: Carbon nanotubes (CNTs) and epitaxial graphene on SiC substrates formed by thermal decomposition are of great interest for electronic and optoelectronic applications. The initial decomposition process of a SiC surface is critical to the formation of subsequent nanocarbon materials, such as CNTs or graphene. We present here an in situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy study of the initial formation processes of CNTs on the SiC C-face and graphene on the Si-face by thermal decomposition at high temperature. On both surfaces, desorption of Si atoms and subsequent graphitization of the remaining carbon atoms were observed above 1000 °C by carbon K-edge NEXAFS measurements in real time, but the incidence angle dependence of NEXAFS spectra showed marked difference between the SiC C-face and Si-face; on the SiC Si-face, the orientations of graphene layers are kept parallel to the surface during the graphene growth. In contrast, on the SiC C-face, graphene layers are initially oriented parallel to the SiC surface, but their orientations change toward the surface normal during the progression of CNT formation above 1300 °C. In addition, at the very initial stage of thermal decomposition, aromatic fragments composed of a few carbon hexagons are present parallel to the surface. Our NEXAFS results are consistent with previous density-functional tight-binding molecular dynamic simulations for CNT growth on the SiC C-face by thermal decomposition.

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increase rate (typically below 100 °C/min) above 1200 °C in a vacuum.23,24 It has been reported that the initial growth process is much different between the graphene formation on the SiC Si-face and that of CNTs on the SiC C-face; low-energy electron microscopy studies have shown that on the SiC Si-face, the formation of graphene layers starts at the step edges of the SiC surface by desorption of Si atoms and proceeds in a layer-bylayer growth mode.22,25 In contrast, on the SiC C-face, caplike structures composed of hemispherical graphene layers, that is, “carbon nanocaps”, were formed at the beginning; then, as Si atoms were desorbed, cylindrical parts of CNTs grew into the SiC, followed by the formation of vertically aligned MWCNTs films.15,23,26 However, the formation mechanism of the carbon nanocaps on the SiC C-face remains poorly understood. Several studies have been reported regarding the formation of carbon nanocaps on the SiC C-face using transmission electron microscopy (TEM),15,27 scanning tunneling microscopy,27−30 and X-ray photoelectron spectroscopy (XPS).28 However, most of those measurements were performed after the sample was

INTRODUCTION Recently, nanometer-scale carbon materials, “nanocarbons”, such as graphene1−3 and carbon nanotubes (CNTs),4,5 have attracted much interest from both basic scientists and exploratory device technologists because they show various unusual and exotic electronic properties.6−11 Among various growth techniques for nanocarbon materials, thermal decomposition of the SiC surface is unique because both graphene12,13 and CNTs14,15 can be obtained by annealing only single-crystal SiC substrates in a vacuum without any catalysts. The resultant structures, which are composed of either CNTs or graphene on SiC substrates, are suitable for subsequent device processing. It has been reported that both the polarity of the SiC surface and the heating conditions, such as the temperature increase rate, the ambient gas species, and the ambient gas pressure, are crucial to the formation of CNTs and graphene by the thermal decomposition of SiC. In general, the epitaxial graphene has been obtained by heating a suitably cleaned SiC (0001) Si-face above 1200−1300 °C in an ultrahigh vacuum (UHV)16−19 or above 1400−1600 °C in an ambient inert gas flow,20−22 because the decomposition rate is dependent on the ambient pressure. On the other hand, high-density, vertically aligned multiwalled CNTs (MWCNTs) films are formed by heating the SiC (0001̅) C-face with a sufficiently slow temperature © 2015 American Chemical Society

Received: June 18, 2015 Revised: November 2, 2015 Published: November 2, 2015 26698

DOI: 10.1021/acs.jpcc.5b05854 J. Phys. Chem. C 2015, 119, 26698−26705

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The Journal of Physical Chemistry C cooled to room temperature, and the real crystallization process to graphene or CNTs at each temperature has never been directly investigated. Only Watanabe’s group has carried out in situ TEM measurements for the initial formation process of CNTs using 3C-SiC(111) Si-face films above 1000 °C. They observed the formation of graphite layers parallel to the SiC surface at 1360 °C and presumed that they were formed on SiC(1̅1̅1̅) C-face of the thin crystal film.27 However, it was not clear whether the graphite layers were formed on SiC C-face or Si-face, because of the insufficient TEM resolution. Also, the details of the CNT growth process including the formation of sp2 bonding remain unclear. In this study, we have carried out in situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements at 800−1400 °C to investigate the initial formation process of CNTs on the SiC (0001̅) C-face and graphene on the SiC (0001) Si-face by thermal decomposition. NEXAFS is a powerful tool to investigate the chemical states of carbon materials; it can distinguish between graphitic-like and amorphous carbon, estimating the degree of graphitization.31,32 Additionally, by using linearly polarized incident X-rays, the orientations of graphene layers with respect to an underlying substrate can be determined.33−35 By performing in situ carbon K-edge NEXAFS measurements at the instant that CNTs and graphene are being formed, the real formation processes are clarified, and the difference in growth mechanisms between CNTs and graphene by thermal decomposition of SiC is discussed.

Figure 1. (a) Schematic for the temperature rise process for the NEXAFS measurements. (b) Schematic of the geometry of the sample and the analyzer in the NEXAFS chamber, and registration of angular dependence of NEXAFS, where E and θ are an electric field vector and an incidence angle of X-ray beam, respectively. The incidence angle, θ, is measured as an angle between the direction of X-ray and the sample surface and is set at 30° or 90° (normal incidence) in this experiment. The angle between the incident X-ray and the electron energy analyzer is fixed at 60°.

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EXPERIMENTAL PROCEDURE The carbon K-edge NEXAFS measurements were carried out at BL-7A of the Photon Factory in the High Energy Accelerator Research Organization (KEK PF). Commercial single-crystal wafers of 6H-SiC (0001̅) C-face and (0001) Si-face (CREE Research, Inc.) cut into 4.0 × 7.0 mm2 were used as samples to grow CNTs and graphene, respectively. After being dipped in a 10% hydrofluoric acid solution for 15 min, they were introduced into the analysis chamber for NEXAFS measurements. The chamber was then evacuated by thermal bake-out for 1 day. After the pressure in the chamber was below 1 × 10−5 Pa, the samples were heated to an intended temperature and NEXAFS measurements were carried out. The temperature of the sample surface was monitored with an infrared pyrometer. To investigate the growth process of CNTs and graphene on the SiC surfaces in real-time, all NEXAFS spectra were performed keeping the samples at the heating temperature. A schematic temperature rise diagram for XPS and NEXAFS measurements is shown in Figure 1a. Previous studies showed that decomposition rate of SiC surface depends on both heating temperature and ambient pressure, and in the case of heating in a vacuum, it has been reported that carbon nanocaps were formed on SiC(0001)̅ C-face above around 1200 °C15,30 and that monolayer graphene were formed on SiC(0001) Si-face after heating above 1100−1200 °C.16−19 Therefore, to investigate the initial formation process of CNTs and graphene, the sample temperature was gradually increased to 1380 °C for SiC (0001̅) C-face, and 1280 °C for SiC(0001) Si-face, respectively. For the SiC (0001̅) C-face substrates, the temperature increase rate was set to be below 10 °C/min, as a sufficiently slow temperature increase is necessary to form CNTs by this method. Before the NEXAFS measurements, high-purity hydrogen gas of 6N (99.999%) with a pressure of 1 × 10−2 Pa was flowed for 30 min to suppress the oxidation of

the SiC surface. After the H2 gas was exhausted, in situ carbon K-edge NEXAFS measurements were performed under UHV. The NEXAFS spectra were obtained in the Auger electron yield detection mode monitoring the KLL carbon Auger peak by a hemispherical electron energy analyzer (Gammadata Scienta SES-200). This is because the Auger electron yield mode is highly surface sensitive and suitable for the detection of thin carbon layers on the SiC surface. In addition, there is a sufficient distance (more than 10 cm) between the sample and the analyzer in our NEXAFS measurement system, which enables NEXAFS measurements at high temperatures. To correct the transmission function of the monochromator, all Auger electron yield was normalized to photon flux, obtained from the current yield of a clean Au mesh placed before the samples. The monochromator energy scale was calibrated using carbon K edge π* resonance peak of CNTs and graphene, located at 285.4 eV. The schematic for the location of the sample relative to the synchrotron radiation beam is shown in Figure 1b. The angle between the incident X-ray and the analyzer was fixed at 60°, and the incident angle, θ, which is defined as the angle between the direction of the light source and the sample surface, was changed to investigate the orientation of the C−C bonds. In this experiment, we carried out the NEXAFS measurements with two incidence angles (θ), 30° and 90°. At θ = 90°, the electric field vector of the light source, E, is parallel to the sample surface (normal incidence), and at θ = 30°, E contains components both parallel and perpendicular to the surface. The X-ray irradiated areas were not the same between the two incidence angles, because the beam size of about 1 mm2 was 26699

DOI: 10.1021/acs.jpcc.5b05854 J. Phys. Chem. C 2015, 119, 26698−26705

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The Journal of Physical Chemistry C

Figure 2. Intensity ratio of Si 2p to C 1s for the SiC C-face (a) and the SiC Si-face (b) in the XPS spectra versus heating time, which are shown with red open circles with lines. The heating times are defined as the time passed after the sample temperature reached 820 °C. All XPS spectra were measured at normal emission (θ = 30°). Thickness variations of the CNT films on the SiC C-faces and graphene layers on the SiC Si-faces against the heating time, which are estimated from the Si 2p to C 1s intensity ratio in XPS spectra, are also shown in panels a and b with black solid and open circles with lines. Black solid and open circles correspond to thicknesses estimated based on the Strohmeier equation and the Beer−Lambert Law, respectively. In the lower part of each panel, the relationships between the heating time and the sample temperature are shown. Schematics of the cross-sectional images for the SiC Si-face (c) and the SiC C-face (d) before and after the CNT film and graphene layer are also shown.

at 1300 °C for 30 min in a vacuum.15 Therefore, it is considered that the decrease of the Si 2p/C 1s intensity ratio at 1380 °C was caused by the elongation of the CNT length on the SiC surface. In the CNT growth by surface decomposition of the SiC Cface, high-density, vertically aligned CNT films were formed on the SiC surface,15 in which the thickness of the CNT film is homogeneous, as is schematically shown in Figure 2c. Therefore, we estimated the thickness of the CNT films on SiC, based on the Strohmeier equation.36 In this estimation, we used the values of the Si 2p/C 1s intensity ratios in XPS spectra measured at θ = 30° and 90°, because the deconvolution of C 1s spectra was difficult because of the low-energy resolution at high temperature and because Si 2p peak intensity was dependent on the sample position. For CNT films with thickness of more than 4 nm, Si 2p peak intensity became negligible at θ = 90°. Therefore, we estimated the thickness using the Si 2p/C 1s intensity ratio at θ = 30°, assuming that the Si 2p peak intensity of a SiC substrate under the CNT film, I, follows the Beer−Lambert Law and is represented by

unchanged under both the polarization conditions. Even though the position was not exactly the same, we set the irradiated areas at the center of the samples for each measurements. After the NEXAFS measurement, the sample temperature was raised and NEXAFS measurements were performed at the higher temperature. In addition, XPS measurements were carried out just before or soon after each NEXAFS measurement with a photon energy of 430 eV to investigate the surface composition, estimating the thickness of the CNT and graphene layers on the SiC surface. XPS spectra were also measured at both θ = 30° and 90°.

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RESULTS AND DISCUSSION First, to investigate the CNT formation process on the SiC Cface, we carried out in situ XPS and NEXAFS measurements of the SiC C-face during heating. Figure 2a shows the variation of the intensity ratio of the Si 2p peak to the C 1s peak (Si 2p/C 1s intensity ratio) in the XPS spectra for the SiC C-face against heating time. In this process, the heating temperature was increased in a stepwise fashion with time, and the temperature for each time is shown in the lower part of Figure 2a. In this figure, the heating time is defined as the duration after the sample temperature reached 820 °C, and the Si 2p/C 1s intensity ratios are normalized to that of the SiC substrates at 820 °C, because desorption of Si atoms was negligible at 820 °C (see below). Above 820 °C, the temperature increase rate was kept below 5 °C/min because a sufficiently slow rate of temperature increase is necessary to form CNTs on the SiC Cface. As the heating temperature increased and the heating time became longer, the Si 2p/C 1s intensity ratio decreased, indicating the desorption of Si atoms. When the heating temperature reached 1320 °C, desorption of Si was enhanced. After the sample was heated to 1380 °C, the Si 2p/C 1s intensity ratio was 0.002. Previous studies showed that CNTs of more than 5 nm in length were formed after heating SiC C-face

I = I0 exp( −d /λ)

(1)

where I0 is the Si 2p peak intensity of the SiC substrate before the formation of a CNT film, d the thickness of a CNT film on the SiC substrate, and λ the inelastic mean free path (IMFP) of the Si 2p photoelectron in a CNT film.37 In our XPS measurements, the excitation photon energy was 430 eV; therefore, the kinetic energy of the Si 2p photoelectron was about 330 eV. Taking into account the table of IMFP values calculated by Tanuma et al.,38 the IMFP of an electron with a kinetic energy of 330 eV in graphite is 0.81 nm. However, owing to the hollow nature of the CNTs, the density of the CNTs and graphite should be different, even though both are composed of graphene sheets. Taking into account that the 26700

DOI: 10.1021/acs.jpcc.5b05854 J. Phys. Chem. C 2015, 119, 26698−26705

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The Journal of Physical Chemistry C volume of CNTs remains the same as that of SiC before the transformation to CNTs,39 the density of the CNT films formed by thermal decomposition of SiC is about 0.96 g/cm3, which is less than half of that for graphite (2.0−2.2 g/cm3). Therefore, the IMFP in the CNT films should be larger than that in graphite, if the kinetic energy of the photoelectrons is the same. With this presumption, we argue that the λ for the CNT films is similar to that for glassy carbon, 1.20 nm,38 because the density of the glassy carbon is 1.4−1.8 g/cm3, which is lower than that of graphite. On the basis of eq 1, we estimated the thickness of the CNT films at each heating time (details of this estimation are shown in the Supporting Information). The resultant CNT film thickness is shown in Figure 2a as solid and open circles with black lines, which were estimated using the Strohmeier equation (d < 4 nm) and the Beer−Lambert Law (d > 4 nm), respectively. In general, it is well-known that the calculated IMFP values are often 10−20% larger than the measured ones, so there is a possibility that the thickness in Figure 2 might be somewhat overestimated. Below 1120 °C, the CNT film thickness was less than 0.5 nm, but when the sample temperature was raised to 1320 °C, the thickness of CNT films gradually increased and became approximately 4 nm. After heating to 1380 °C, the thickness finally reached over 7 nm. We also investigated the graphene formation process on the SiC Si-face by the same procedure. The variation of the Si 2p/C 1s intensity ratio in the XPS spectra for the SiC Si-face with heating time and heating sequence are shown in Figure 2b. The graphene thickness should be homogeneous (Figure 2d);40 therefore, we estimated the graphene thickness from the Si 2p peak intensity again using the Strohmeier equation,36 where the value of d now represents the graphene thickness. In this estimation, we assumed the IMFP of a photoelectron in the graphene layers to be the same as that of graphite, 0.812 nm,38 because the density of graphene layers is the same as that of graphite. The estimated thickness for each heating time is shown by black circles with black lines in Figure 2b. In the case of graphene formation on the SiC Si-face, after preheating at 820 °C, the heating temperature was raised to 970, 1060, 1170, and 1280 °C in a stepwise manner and the graphene thickness gradually increased, and finally reached approximately 0.6 nm. The in situ carbon K-edge NEXAFS spectra for the SiC Cface, which were measured at 820, 960, 1120, 1320, and 1380 °C, are shown in Figure 3a−e. To evaluate the orientation of the graphene layers, NEXAFS spectra were obtained with two different incidence angles, θ = 30° and 90°, for each temperature. To measure the spectra at θ = 30°, we took 30−60 min, but 90−150 min was necessary to obtain spectra at θ = 90° with sufficiently low signal-to-noise (S/N) ratio, because of the decrease of the Auger electron emission intensity at the lower emission angle. During the heating, the spectral shapes were slowly changed, but the accumulated spectrum for each temperature is shown in Figure 3. The spectra at 820 °C were similar to that of SiC and were almost the same for the two incident angles (Figure 3a).37 After the sample was heated to 960 °C, shoulder peaks, corresponding to the π* resonance of graphene layers, appeared at approximately 285.5 eV34,35,37 for both incidence angles, and the intensity of this shoulder measured at θ = 30° seemed to be higher than that at θ = 90°. In addition, broad features appeared at around 292.0 eV (Figure 3b), which correspond to σ* resonance of graphene layers.41−43 These spectra show that desorption of Si atoms has occurred and

Figure 3. C K-edge NEXAFS spectra of SiC C-face during heating at (a) 820 °C, (b) 960 °C, (c) 1120 °C, (d) 1320 °C, and (e) 1380 °C, according to the diagram shown in Figure 2a. (f) C K-edge NEXAFS spectra of the SiC C-face during heating at 1050 °C in another heating sequence. The difference spectra after subtraction of the spectra of the pristine SiC C-face are shown in the inset of panel f. All spectra were measured at two incidence angles, θ = 30° and 90°.

carbon materials composed of sp2 orbitals had accumulated on the SiC surface. As the temperature was increased to 1120 °C, the π* resonance intensities were enhanced for both incidence angles, and that for θ = 30° remained higher than that for θ = 90° (Figure 3c). It should be noted that the spectra at 1120 °C are similar to those observed for graphite,33 indicating that crystallization to graphene-like layers mostly lying parallel to the surface has occurred at around this temperature, although previous studies reported that carbon nanocaps were observed after heating above 1200 °C.15,28,30 At 1320 °C, the spectral shapes became approximately the same for the two incidence angles (Figure 3d), indicating a transition to a situation in which roughly equal amount of sp2 bonded carbon are both parallel to, and perpendicular to, the surface, within the sampling depth of the NEXAFS measurement. When the sample temperature rose to 1380 °C (Figure 3e), the intensity of the π* resonance peak at θ = 90° was stronger than that at θ = 30°, indicating a majority of the graphene-like carbon is now oriented perpendicular to the surface. To investigate the formation process of graphene layers on the SiC Si-face, in situ NEXAFS measurements were also carried out for the SiC Si-face. Carbon K-edge NEXAFS spectra of the SiC Si-face are shown in Figure 4 a−e, which were measured at 820, 970, 1060, 1170, and 1280 °C. At 820 °C, the NEXAFS spectra were almost the same for both incidence angles, which were similar to the spectra of the SiC C-face in Figure 3a. When the temperature increased to 970 °C, the absorption edge of the spectrum for θ = 30° was about 1.2 eV 26701

DOI: 10.1021/acs.jpcc.5b05854 J. Phys. Chem. C 2015, 119, 26698−26705

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Figure 5. Variation of intensity ratio of the π* peak to σ* in the NEXAFS spectra for (a) the SiC C-face versus the CNT film thickness and (b) the SiC Si-face against graphene layer thickness, estimated by the Si 2p/C 1s intensity ratio in the XPS spectra. For both SiC faces, the π*/σ* intensity ratios measured at θ = 30° and 90° are shown.

were estimated by extrapolation from the thicknesses estimated from the XPS results performed before and after each NEXAFS measurement, assuming that the desorption of Si atoms proceeded linearly with the heating time. In the case of CNT growth on the SiC C-face, when the thickness was below 1 nm, the intensity ratio at θ = 90° increased from 0.43 to 0.7, and it was smaller than that at 30°, which increased from 0.84 to 1.1. Taking into account that the π-orbitals of carbon nanocaps are oriented continuously all the way from parallel to perpendicular to the SiC surface, this indicates that the orientation of the initial graphene layers are fairly inclined or almost parallel to the SiC surface. As the CNT films became thicker, the π*/σ* intensity ratio at θ = 90° increased gradually, while that at θ = 30° reached its maximum, 1.1, at 0.41 nm and subsequently decreased above 1 nm, becoming smaller than that at θ = 90°. As a result, the π*/σ* intensity ratio at θ = 90° was larger than that at θ = 30° above 2.0 nm. This shows that the orientation of the graphene layers changed from predominantly parallel to overall perpendicular to the SiC surface with the CNT growth. Simplified images of the growth process of CNTs on the SiC C-face are shown in Figure 6. In CNT growth by thermal decomposition of the SiC C-face, carbon nanocaps have been shown to be formed on the SiC surface at the beginning; then, cylindrical parts of CNTs are formed toward the inside of the

Figure 4. C K-edge NEXAFS spectra of the SiC Si-face during heating at (a) 820 °C, (b) 970 °C, (c) 1060 °C, (d) 1170 °C, and (e) 1280 °C according to the diagram shown in Figure 2b. All spectra were measured at two incidence angles, θ = 30° and 90°. (f) Difference spectra between the NEXAFS spectra at 970 °C and those of the pristine SiC Si-face measured at θ = 30° and 90°.

lower than that at θ = 90°, owing to the appearance of the π* resonance component of the accumulated carbon layers. This indicates that graphene flakes formed at the beginning are highly oriented parallel to the SiC surface. When the temperature was increased to 1060 °C, the π* resonance peaks appeared at 285.5 eV for both incidence angles, but the π* resonance peak at θ = 30° was larger than that at θ = 90° (Figure 4c). As the temperature was increased further, the π* resonance peaks became stronger for both incidence angles, but the π* resonance peak at θ = 30° remained larger than that at θ = 90°, irrespective of the temperature (Figure 4d,e). At 1170 and 1280 °C, the NEXAFS spectra and their incidence angle dependence were quite similar to those of highly ordered pyrolytic graphite,33 indicating that graphitization had occurred and that graphene layers were formed parallel to the surface. On the basis of the in situ XPS and NEXAFS results, we are able to quantitatively summarize the relationships between the π*/σ* intensity ratio in the NEXAFS spectra and the thicknesses of the CNT films and graphene layers. The relationships between the π*/σ* intensity ratios and the thicknesses of CNTs and graphene layers for both incidence angles are shown in Figure 5. To determine the π* and σ* intensities, after the background spectrum corresponding to the bare SiC substrate was subtracted from each spectrum, arctangent step functions were subtracted from the pre- and postedge normalized spectra to simulate the edge jump, followed by fitting the π* and σ* peaks with Gaussian functions.33,35 Because desorption of Si atoms gradually proceeded during the NEXAFS measurements, the thicknesses

Figure 6. Growth process images of CNTs on the SiC C-face by thermal decomposition. (a) Formation of small aromatic fragments on the SiC C-face, (b) formation of a top layer of a carbon nanocap, (c) carbon nanocap formation, and (d) CNT growth. For simplicity, images of one single-walled CNT growth are shown. 26702

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The Journal of Physical Chemistry C SiC substrates with desorption of Si atoms.15,23,28 The carbon nanocaps have hemispherical structures with diameters of 2−6 nm, which are typically composed of about two to three graphene layers (0.70−1.05 nm in thickness), taking into account the distance between graphene layers.40 Therefore, the graphene layers parallel to the surface observed below 1 nm correspond to the formation of the top layers of carbon nanocaps (Figure 6b), and carbon nanocaps are formed with further progression of desorption of Si atoms (Figure 6c). Taking into account that the π-orbitals of carbon nanocaps are oriented continuously all the way from parallel to perpendicular to the SiC surface, similar π*/σ* intensity ratios between θ = 30° and θ = 90° at thicknesses of around 2.0 nm should arise from the formation of carbon nanocaps. As the CNT formation proceeds, the cylindrical parts of the CNTs become longer, and the ratio of graphene sheets perpendicular to the surface increases (Figure 6d). As a result, the π*/σ* intensity ratio at θ = 90° became larger than that at θ = 30° above 2 nm. In our experiments, we carried out carbon K-edge NEXAFS measurements using the Auger electron yield mode, in which carbon KLL Auger electrons were detected. Considering that the kinetic energy of the Auger electrons is 272 eV, their IMFP is 1.1 nm,38 if the IMFP in the CNT films is assumed to be similar to that in glassy carbon. In this estimation, 95.0% of the Auger electrons come from within the depth of 3.3 nm from the CNT surface. Therefore, the contribution of the carbon nanocaps to the NEXAFS spectra remains almost constant above 4.0 nm, even though cylindrical parts of the CNTs grow under the carbon nanocaps. To investigate the initial stage of CNT formation, we carried out additional in situ NEXAFS measurements for the SiC Cface. Figure 3f shows the raw NEXAFS spectra measured at the incidence angles of θ = 30° and 90° for the SiC C-face just after the sample temperature was increased to 1050 °C with a temperature rise rate below 10 °C/min. The inset shows the difference spectra in which the NEXAFS spectra for the SiC Cface at 820 °C were subtracted from the raw data. In the raw spectra, a shoulder peak derived from the π* resonance peak was observed at θ = 30°. It should be noted that in the difference spectra, a sharp π* resonance peak with a negligibly weak σ* resonance feature is observed at θ = 30°, while the former disappeared at θ = 90°. To date, many NEXAFS spectra have been reported for graphene22,37,41−46 and molecules with conjugated bonds47−51 on metal or semiconductor surfaces. In those systems, one π-orbital overlaps with another across the intervening σ-bond. As a result, σ* resonance peaks are generally observed with the π* resonance peaks in the NEXAFS spectra. Even in NEXAFS spectra of monolayer graphene, a distinct σ* resonance peak appeared.22,41−43 However, for small aromatic molecules, such as benzene47,48 or perylene,51 the σ* resonance peak in the carbon K-edge NEXAFS spectra is comparatively weak or structureless under the condition that the electric field vector, E, is parallel to their π-orbitals. Therefore, we consider that the fairly weak σ* resonance feature suggests the formation of aromatic fragments composed of a few carbon hexagons parallel to the SiC surface at the very early stage (Figure 6a). The sharp π* resonance peak at θ = 30° in the difference spectra (Figure 3f) indicates that the interaction between those fragments and the SiC substrate is weak because the hybridization of π-orbitals with the dangling bonds of the SiC surface would induce the splitting or broadening of the π* resonance peak.48

So far, only a few theoretical studies have been reported regarding the initial stage of CNT formation on the SiC C-face. On the basis of ab initio calculations and photoemission results, Levita et al. proposed that open-ended CNTs initially grow vertically as a C-sp2 network structure, followed by closure of the CNT tips after the tube walls have reached sufficient length.46 Irle and Kusunoki’s group performed densityfunctional tight-binding molecular dynamics (DFTB/MD) simulations for graphene and CNT formation on the SiC surface by thermal decomposition.52−54 In contrast to the former model, they demonstrated that a large carbon network is formed at the terrace site on the SiC C-face at 1500 K by both removing Si atoms and subsequent carbon rearrangement, resulting in carbon nanocap formation at the initial stage. In their simulation, a carbon pentagon accompanied by carbon chains appears just after the decomposition of the first layer of the SiC surface and, during the second layer decomposition, two-dimensional networks composed of carbon hexagons are formed, resulting in carbon nanocaps.54 Our NEXAFS results, demonstrating the formation of small aromatic fragments at the initial stage and the subsequent formation of carbon nanocaps, are consistent with their simulations. In the case of graphene growth on the SiC Si-face, the π*/σ* intensity ratio at θ = 30° is always larger than that at θ = 90° (Figure 5b). It is well-established that graphene layers grow flat on the SiC Si-face, keeping the layers parallel to the SiC surface.22,25 In this process, the π-orbitals of graphene are always perpendicular to the SiC surface; as a result, the π* resonance peak became fairly weak at θ = 90° and the π*/σ* intensity ratio at θ = 30° is always observed to be larger than that at θ = 90°. It should be noted that the π*/σ* intensity ratio at θ = 30° was enhanced below 0.4 nm. This also suggests the formation of small aromatic fragments at the early stage of graphene growth, in which the contribution from σ* orbitals is relatively weak, as discussed. In our estimation, the graphene thickness at 970 °C was 0.1−0.2 nm, which corresponds to the thickness of the buffer layer on SiC.40 Figure 4f shows the difference NEXAFS spectra at 970 °C, which were obtained by the subtraction of the SiC spectra from the raw data for each incidence angle. In Figure 4f, the π* resonance peak at 285.5 eV is distinct at θ = 30°, while the π* peak disappears and broad σ* resonance is observed above 294 eV at θ = 90°. Taking into account the NEXAFS spectra of graphene,22,41−43 the incidence angle dependence in Figure 4f again indicates that small aromatic fragments or graphene flakes are formed by decomposition of the SiC (0001) face, which are kept parallel to the surface during the formation of the buffer layer. Previous DFTB simulations55 and TEM observations56 showed that nucleation of graphene occurs at the SiC step and that graphene growth proceeds toward the terrace site with covering of the step edge. Our NEXAFS results suggest that graphene flakes are present on the surface at the beginning, which are enlarged to form graphene during the formation of the first layer.

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CONCLUSIONS We carried out in situ NEXAFS measurements above 1000 °C to investigate the formation process of CNTs and graphene on the SiC surface by thermal decomposition. Using the polarized light source of a synchrotron, we directly detected the orientations of the graphene layers during CNT and graphene growth. On the SiC Si-face, graphene layers were initially formed parallel to the SiC surface at approximately 1000 °C, 26703

DOI: 10.1021/acs.jpcc.5b05854 J. Phys. Chem. C 2015, 119, 26698−26705

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The Journal of Physical Chemistry C

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followed by the layer-by-layer growth by heating to higher temperatures. This growth process is consistent with previous studies reporting graphene formation on the SiC Si-face. In contrast, on the SiC C-face, graphene flakes parallel to the SiC surface were initially formed on the SiC C-face, but their orientation changed to perpendicular to the surface as CNT growth proceeded by heating above 1300 °C. In addition, at the very early stage of sublimation of the SiC C-face, aromatic fragments composed of a few carbon hexagons were observed parallel to the surface. Our NEXAFS results are consistent with a recent DFTB/MD simulation for CNT formation on the SiC C-face by thermal decomposition.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05854. Further theory regarding the thickness of CNTs formed by thermal decomposition on the SiC C-face and thickness of graphene formed by thermal decomposition on the SiC Si-face (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. S. Sakakibara and Mr. T. Yajima for their assistance during the NEXAFS experiments. A part of this work was supported by JSPS KAKENHI Grants 25600031 and 25400326. This work was also partly performed in the Institute for Molecular Science, supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The NEXAFS measurements were performed under the approval of the Photon Factory Program Advisory Committee (Proposals 2009G530 and 2011G539).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b05854 J. Phys. Chem. C 2015, 119, 26698−26705