Observation of Nanosized Cap Structures on 6H–SiC(0001) Substrates by Ultrahigh-Vacuum Scanning Tunneling Microscopy

Abstract

Nanosized cap structures on a thermally treated 6H-SiC(000 1 1) substrate were investigated using atomic-resolution ultrahighvacuum scanning tunneling microscopy (UHV-STM). Hexagonal carbon networks, partly composed of pentagons, were clearly observed on the surface of the cap structures for a sample annealed at 1250 C, in which carbon nanotubes (CNTs) had negligibly grow into the SiC substrate. Comparing their sizes and shapes with those annealed at 1350 C, the cap structures were considered to be the initial state of carbon nanotube (CNT) growth which determines the tube diameters of the final grown CNTs.

Carbon Nanotubes Grow on the C Face of SiC (0001̄) during Sublimation Decomposition: Quantum Chemical Molecular Dynamics Simulations

“…As of yet, it is not at all clear how Si atoms leave C atoms behind in high-temperature vacuum decomposition of SiC, 24 although it is known that the higher vapor pressure of Si with respect to C is somehow related to the selective Si sublimation. 50,51 The fact that aligned CNTs can grow on the C face in experiment [18][19][20][21][22]25,26 suggests that the Si removal follows an ordered evaporation layer by layer, otherwise, perfect vertical tube alignment would not be possible. However, since nobody has analyzed the evaporated species immediately after leaving the surface, it is unknown in what chemical form the Si atoms are extracted from the SiC crystal.…”

“…15 Most severely, it is still impossible to synthesize tubes with specific chirality, which of course determines their electronic conductivity properties. 16,17 TM catalyst-free CNT synthesis has been reported in the exceptional case where SiC is used as a carbon source material and thermally decomposed in vacuum sublimation at high temperatures, either by means of heating [18][19][20][21][22][23][24][25][26] or by laser vaporization. [27][28][29] This "carbide-derived-carbon" 30 method produces typically MWCNTs with a very minor fraction of SWCNTs, but preferential SWCNT formation by the SiC decomposition method has also been claimed.…”

“…TM catalyst-free CNT synthesis has been reported in the exceptional case where SiC is used as a carbon source material and thermally decomposed in vacuum sublimation at high temperatures, either by means of heating − or by laser vaporization. − This “carbide-derived-carbon” method produces typically MWCNTs with a very minor fraction of SWCNTs, but preferential SWCNT formation by the SiC decomposition method has also been claimed . Interestingly, Kusunoki et al − ,, found that CNTs are preferentially grown in a perpendicular direction on the C face of the crystalline (0001̄) SiC surface (see Figure ) with 1−2 nm diameter of the inner tubes 22,25,26 and preference for zigzag chirality, whereas Derycke et al claimed to have observed SWCNT growth on the Si face parallel to the crystal surface with 1.2−1.6 nm diameter. Nagano et al observed CNT growth in perpendicular direction on the Si face with larger CNT diameters and 65% shorter lengths than C face grown tubes.…”

Effect of Annealing in Hydrogen Atmosphere on Carbon Nanocap Formation in Surface Decomposition of 6H-SiC(000-1)

“…As we have reported, the diameter of CNTs grown by this method is determined by the size of the carbon nanocaps. 7 As a result, the reduction of native oxides by annealing in a hydrogen atmosphere should be an effective way of obtaining CNTs with uniform diameters. We also note that one of the significant advantages of CNTs grown by surface decomposition of SiC is their chirality property.…”

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

“…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-by-layer growth mode. , 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. ,, 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), , scanning tunneling microscopy, − and X-ray photoelectron spectroscopy (XPS) . However, most of those measurements were performed after the sample was cooled to room temperature, and the real crystallization process to graphene or CNTs at each temperature has never been directly investigated.…”