Different stacking sequences of graphene are investigated a using combination of experimental and theoretical methods. The high-resolution transmission electron microscopy (HRTEM) of the stacking sequence of several layers of graphene, formed on the C-terminated 4H-SiC(0001) surface, was used to determine the stacking sequence and the interlayer distances. These data prove that the three metastable configurations exist: ABAB, AAAA, ABCA. In accordance to these findings, those three cases were considered theoretically, using Density Functional Theory calculations comparing graphene sheets, freestanding and positioned on the SiC(0001) substrate. The total energies were calculated, the most stable structure was identified and the electronic band structure was obtained. The four graphene layer electron band structure depends crucially on the stacking: for the ABAB and ABCA stacking, the bands, close to the K point, are characterized by the hyperbolic dispersion relation while the AA stacking the dispersion in this region is linear, similar to that of a single graphene layer. It was also shown that the linear dispersion relation is preserved in the presence of the SiC substrate, and also for different distances between adjacent carbon layers.
Spontaneous formation of smectic and columnar structures was observed when spherical gold nanoparticles were functionalized with mesogenic thiols (see layered structure and X-ray pattern of a sample in smectic phase). The particle ordering is stimulated by softening of the interparticle potential and flexibility for deformation of the grafting layer.
An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.
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