Near-Edge X-Ray Absorption Fine Structure Study of Vertically Aligned Carbon Nanotubes Grown by the Surface Decomposition of SiC

Abstract

Vertically aligned carbon nanotubes (CNTs) grown by the surface decomposition of SiC were studied by angular-dependent C K-edge near-edge X-ray absorption fine structure spectroscopy (NEXAFS) with a linearly polarized X-ray beam. The NEXAFS spectra measured in total electron yield mode showed a distinct angular dependence on à and à resonances and the orientation parameter was tentatively estimated to be 0.38, which is fairly larger than those reported for other vertically aligned CNTs grown by chemical vapor deposition. The high order of the vertical alignment of CNTs grown by the surface decomposition of SiC was demonstrated by NEXAFS measurements for the first time.

Accounting for Nanometer-Thick Adventitious Carbon Contamination in X-ray Absorption Spectra of Carbon-Based Materials

“…For carbon K-edge NEXAFS spectroscopy, in particular, the resolvable energy difference between the resonant X-ray excitations of a core-level (1s) electron to unoccupied molecular orbitals (either π* or σ*) allows the identification of the bonding configuration and hybridization state of carbon atoms in the near-surface region for many materials, including diamond, ,,,,,,− diamond-like carbon, − graphene, ,, and polymers, − as well as the determination of the surface molecular orientation of nanomaterials ,,, and adsorbates. − …”

Band alignment of a carbon nanotube/n-type 6H-SiC heterojunction formed by surface decomposition of SiC using photoelectron spectroscopy

“…Using this method, aligned zigzag-type double-or triple-walled CNTs with fairly uniform tube diameters can be selectively produced normal to the SiC surface simply by heating SiC substrates at sufficiently high temperature (!1300 C) under vacuum. [11][12][13][14][15] In addition, because this method does not use any catalysts, the CNT walls form directly from the constituent atoms in the SiC substrate without any interlayers. 16 Therefore, at present, surface decomposition of SiC is the only method to form an "intrinsic" CNT/semiconductor heterojunction similar to those realized in conventional semiconductor heterojunction systems.…”

In Situ High-Temperature NEXAFS Study on Carbon Nanotube and Graphene Formation by Thermal Decomposition of SiC

“…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 eV 34,35,37 for both incidence angles, and the intensity of this shoulder measured at θ = 30°s eemed 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.…”

“…The relationships between the π*/σ* intensity ratios and the thicknesses of CNTs and graphene layers for both incidence angles are shown in Figure . 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. , Because desorption of Si atoms gradually proceeded during the NEXAFS measurements, the thicknesses 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 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. , Additionally, by using linearly polarized incident X-rays, the orientations of graphene layers with respect to an underlying substrate can be determined. − 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.…”