Silicene, the silicon equivalent of graphene, is most commonly grown on Ag(111) substrates where it undergoes reconstruction due to the strong interaction between the Si and Ag atoms. We demonstrate ...through first-principles density functional theory for eight reconstructions that the Raman spectrum is unique for each configuration. We argue that the reconstructions can, in fact, be identified by their Raman spectra and suggest key features within the spectra as points of reference to be used for identification.
We apply a multiscale modeling approach to study lattice reconstruction in marginally twisted bilayers of transition metal dichalcogenides (TMD). For this, we develop density functional theory ...parametrized interpolation formulae for interlayer adhesion energies of MoSe2, WSe2, MoS2, and WS2, combine those with elasticity theory, and analyze the bilayer lattice relaxation into mesoscale domain structures. Paying particular attention to the inversion asymmetry of TMD monolayers, we show that 3R and 2H stacking domains, separated by a network of dislocations develop for twist angles θ∘<θP∘∼2.5° and θ∘<θAP∘∼1° for, respectively, bilayers with parallel (P) and antiparallel (AP) orientation of the monolayer unit cells and suggest how the domain structures would manifest itself in local probe scanning of marginally twisted P and AP bilayers.
We report density-functional-theory calculations of the electronic band structures and optical absorption spectra of two-dimensional crystals of Ga sub(2)X sub(2) (X = S, Se, and Te). Our ...calculations show that all three two-dimensional materials are dynamically stable indirect-band-gap semiconductors with a sombrero dispersion of holes near the top of the valence band. We predict the existence of Lifshitz transitions-changes in the Fermi-surface topology of hole-doped Ga sub(2)X sub(2)- at hole concentrations n sub(s) = 7.96 x 10 super(13) cm super(-2), n sub(Se) = 6.13 x 10 super(13) cm super(-2), and n sub(Te) = 3.54 x 10 super(13) cm super(-2).
Electrically tunable band gap in silicene Drummond, N. D.; Zólyomi, V.; Fal'ko, V. I.
Physical review. B, Condensed matter and materials physics,
02/2012, Letnik:
85, Številka:
7
Journal Article
Recenzirano
Odprti dostop
We report calculations of the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si ...atoms. The electric field produces a tunable band gap in the Dirac-type electronic spectrum, the gap being suppressed by a factor of about eight by the high polarizability of the system. At low electric fields, the interplay between this tunable band gap, which is specific to electrons on a honeycomb lattice, and the Kane-Mele spin-orbit coupling induces a transition from a topological to a band insulator, whereas at much higher electric fields silicene becomes a semimetal.
The electronic band structure of van der Waals (vdW) layered crystals has properties that depend on the composition, thickness and stacking of the component layers. Here we use density functional ...theory and high field magneto-optics to investigate the metal chalcogenide InSe, a recent addition to the family of vdW layered crystals, which transforms from a direct to an indirect band gap semiconductor as the number of layers is reduced. We investigate this direct-to-indirect bandgap crossover, demonstrate a highly tuneable optical response from the near infrared to the visible spectrum with decreasing layer thickness down to 2 layers, and report quantum dot-like optical emissions distributed over a wide range of energy. Our analysis also indicates that electron and exciton effective masses are weakly dependent on the layer thickness and are significantly smaller than in other vdW crystals. These properties are unprecedented within the large family of vdW crystals and demonstrate the potential of InSe for electronic and photonic technologies.
We use density functional theory to calculate the electronic band structures, cohesive energies, phonon dispersions, and optical absorption spectra of two-dimensional In sub(2) X sub(2) crystals, ...where X is S, Se, or Te. We identify two crystalline phases ( alpha and beta ) of monolayers of hexagonal In sub(2) X sub(2), and show that they are characterized by different sets of Raman-active phonon modes. We find that these materials are indirect-band-gap semiconductors with a sombrero-shaped dispersion of holes near the valence-band edge. The latter feature results in a Lifshitz transition (a change in the Fermi-surface topology of hole-doped In sub(2) X sub(2)) at hole concentrations n sub(S) = 6.86 x 10 super(13) cm super(-2), n sub(Se) = 6.20 x 10 super(13) cm super(-2) and n sub(Te) = 2.86 x 10 super(13) cm super(-2) for X=S, Se, and Te, respectively, for alpha -In sub(2) X sub(2) and n sub(S) = 8.32 x 10 super(13) cm super(-2), n sub(Se) = 6.00 x 10 super(13) cm super(-2) and n sub(Te) = 8.14 x 10 super(13) cm super(-2) for beta -In sub(2) X sub(2).
We use dispersion-corrected density functional theory to determine the relative energies of competing polytypes of bulk layered hexagonal post transition metal chalcogenides to search for the most ...stable structures of these potentially technologically important semiconductors. We show that there is some degree of consensus among dispersion-corrected exchange-correlation functionals regarding the energetic orderings of polytypes, but we find that for each material there are multiple stacking orders with relative energies of less than 1 meV per monolayer unit cell, implying that stacking faults are expected to be abundant in all post transition metal chalcogenides. By fitting a simple model to all our energy data, we predict that the most stable hexagonal structure has the P 63/ m m c space group in each case but that the stacking order differs between GaS, GaSe, GaTe, and InS, on the one hand, and InSe and InTe, on the other. At zero pressure, the relative energies obtained with different functionals disagree by around 1–5 meV per monolayer unit cell, which is not sufficient to identify the most stable structure unambiguously; however, multigigapascal pressures reduce the number of competing phases significantly. At higher pressures, an AB ′ -stacked structure of the most stable monolayer polytype is found to be the most stable bulk structure.
We present a tight-binding (TB) model and k·p theory for electrons in monolayer and few-layer InSe. The model is constructed from a basis of all s and p valence orbitals on both indium and selenium ...atoms, with tight-binding parameters obtained from fitting to independently computed density functional theory (DFT) band structures for mono- and bilayer InSe. For the valence and conduction band edges of few-layer InSe, which appear to be in the vicinity of the Γ point, we calculate the absorption coefficient for the principal optical transitions as a function of the number of layers, N. We find a strong dependence on N of the principal optical transition energies, selection rules, and optical oscillation strengths, in agreement with recent observations D. A. Bandurin et al., Nat. Nanotechnol. (2016). Also, we find that the conduction band electrons are relatively light (m∝0.14–0.18me), in contrast to an almost flat, and slightly inverted, dispersion of valence band holes near the Γ point, which is found for up to N∝6.
Double wall carbon nanotubes were prepared by vacuum annealing of single wall carbon nanotubes filled with C60. Strong evidence is provided for a highly defect free and unperturbed environment in the ...interior of the tubes. This is concluded from unusual narrow Raman lines for the radial breathing mode of the inner tubes. Lorentzian linewidths scale down to 0.35 cm(-1) which is almost 10 times smaller than linewidths reported so far for this mode. A splitting is observed for the majority of the Raman lines. It is considered to originate from tube-tube interaction between one inner tube and several different outer tubes. The highest RBM frequency detected is 484 cm(-1) corresponding to a tube diameter of only 0.50 nm. Labeling of the Raman lines with the folding vector is provided for all inner tubes. This labeling is supported by density functional calculations.
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