We report on total-energy electronic structure calculations in the density-functional theory performed for the ultra-thin atomic layers of Si on Ag(111) surfaces. We find several distinct stable silicene structures: $sqrt{3}timessqrt{3}$, $3times3$,
$sqrt{7}timessqrt{7}$ with the thickness of Si increasing from monolayer to quad-layer. The structural bistability and tristability of the multilayer silicene structures on Ag surfaces are obtained, where the calculated transition barriers infer the occurrence of the flip-flop motion at low temperature. The calculated STM images agree well with the experimental observations. We also find the stable existence of $2times1$ $pi$-bonded chain and $7times7$ dimer-adatom-stacking fault Si(111)-surface structures on Ag(111), which clearly shows the crossover of silicene-silicon structures for the multilayer Si on Ag surfaces. We further find the absence of the Dirac states for multilayer silicene on Ag(111) due to the covalent interactions of silicene-Ag interface and Si-Si interlayer. Instead, we find a new state near Fermi level composed of $pi$ orbitals locating on the surface layer of $sqrt{3}timessqrt{3}$ multilayer silicene, which satisfies the hexagonal symmetry and exhibits the linear energy dispersion. By examining the electronic properties of $2times1$ $pi$-bonded chain structures, we find that the surface-related $pi$ states of multilayer Si structures are robust on Ag surfaces.
We report on total-energy electronic-structure calculations in the density-functional theory performed for both monolayer and bilayer silicene on Ag(111) surfaces. The rt3 x rt3 structure observed experimentally and argued to be the monolayer silicen
e in the past [Chen et al., Phys. Rev. Lett. 110, 085504 (2013)] is identified as the bilayer silicene on the Ag(111) surface. The identification is based on our accurate density-functional calculations in which three approximations, the local density approximation, the generalized-gradient approximation, and the van-der-Waals-density-functional approximation, to the exchange-correlation energy have been carefully examined. We find that the structural tristability exists for the rt3 x rt3 bilayer silicene. The calculated energy barriers among the three stable structures are in the range of 7 - 9 meV per Si atom, indicating possible flip-flop motions among the three. We have found that the flip-flop motion between the two of the three structures produces the honeycomb structure in the STM images, whereas the motion among the three does the 1 x 1 structure. We have found that the electron states which effectively follow Dirac equation in the freestanding silicene couple with the substrate Ag orbitals due to the bond formation, and shift downwards deep in the valence bands. This feature is common to all the stable or metastable silicene layer on the Ag(111) substrate.
We report first-principles calculations that clarify stability and electronic structures of silicene on Ag(111) surfaces. We find that several stable structures exist for silicene/Ag(111), exhibiting a variety of images of scanning tunneling microsco
py. We also find that Dirac electrons are {em absent} near Fermi energy in all the stable structures due to buckling of the Si monolayer and mixing between Si and Ag orbitals. We instead propose that either BN substrate or hydrogen processing of Si surface is a good candidate to preserve Dirac electrons in silicene.
We study the effect of SiC substrate on thermal conductivity of epitaxial graphene nanoribbons (GNRs) using the nonequilibrium molecular dynamics method. We show that the substrate has strong interaction with single-layer GNRs during the thermal tran
sport, which largely reduces the thermal conductivity. The thermal conductivity characteristics of suspended GNRs are well preserved in the second GNR layers of bilayer GNR, which has a weak van der Waals interaction with the underlying structures. The out-of-plane phonon mode is found to play a critical role on the thermal conductivity variation of the second GNR layer induced by the underlying structures.
Using non-equilibrium molecular dynamics method(NEMD), we have found that the thermal conductivity of multilayer graphene nanoribbons monotonously decreases with the increase of the number of layers, such behavior can be attributed to the phonon reso
nance effect of out-of-plane phonon modes. The reduction of thermal conductivity is found to be proportional to the layer size, which is caused by the increase of phonon resonance. Our results are in agreement with recent experiment on dimensional evolution of thermal conductivity in few layer graphene.
