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By performing first-principles electronic structure calculations in frames of density functional theory we study the dependence of the valence band shape on the thickness of few-layer III-VI crystals (GaS, GaSe, and InSe). We estimate the critical thickness of transition from the bulklike parabolic to the ring-shaped valence band. Direct supercell calculations show that the ring-shaped extremum of the valence band appears in β-GaS and β-GaSe at a thickness below 6 tetralayers (∼4.6 nm) and 8 tetralayers (∼6.4 nm), respectively. Zone-folding calculations estimate the β-InSe critical thickness to be equal to 28 tetralayers (∼24.0 nm). The origin of the ring-shaped valence band maximum can be understood in terms of k·p theory, which provides a link between the curvature of the energy bands and the distance between them. We explain the dependence of the band shape on the thickness, as well as the transition between two types of extremes, by the k-dependent orbital composition of the topmost valence band. We show that in the vicinity of critical thickness the effective mass of holes in III-VI compounds depends strongly on the number of tetralayers.

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We developed a six-band k · p model that describes the electronic states of monolayer transition metal dichalcogenides (TMDCs) in K-valleys. The set of parameters for the k · p model is uniquely determined by decomposing tight-binding (TB) models in the vicinity of K ± -points. First, we used TB models existing in literature to derive systematic parametrizations for different materials, including MoS2, WS2, MoSe2 and WSe2. Then, by using the derived six-band k · p Hamiltonian we calculated effective masses, Landau levels, and the effective exciton g-factor g X 0 in different TMDCs. We showed that TB parameterizations existing in literature result in small absolute values of g X 0 , which are far from the experimentally measured g X 0 ≈ −4. To further investigate this issue we derived two additional sets of k · p parameters by developing our own TB parameterizations based on simultaneous fitting of ab-initio calculated, within the density functional (DFT) and GW approaches, energy dispersion and the value of g X 0 . We showed that the change in TB parameters, which only slightly affects the dispersion of higher conduction and deep valence bands, may result in a significant increase of |g X 0 |, yielding close-to-experiment values of g X 0 . Such a high parameter sensitivity of g X 0 opens a way to further improvement of DFT and TB models.

Above a critical diameter, single- or few-walled carbon nanotubes spontaneously collapse as flattened carbon nanotubes. Raman spectra of isolated flattened and cylindrical carbon nanotubes have been recorded. The collapse provokes an intense and narrow D band, despite the absence of any lattice disorder. The curvature change near the edge cavities activates a D band, despite framework continuity. Theoretical calculations based on Placzek approximation fully corroborate this experimental finding. Usually used as a tool to quantify defect density in graphenic structures, the D band cannot be used as such in the presence of a graphene fold. This conclusion should serve as a basis to revisit materials comprising structural distortion where poor carbon organization was concluded on a Raman basis. Our finding also emphasizes the different visions of a defect between chemists and physicists, a possible source of confusion for researchers working in nanotechnologies.

Stannous selenide is a layered semiconductor that is a polar analogue of black phosphorus, and of great interest as a thermoelectric material. Unusually, hole doped SnSe supports a large Seebeck coefficient at high conductivity, which has not been explained to date. Angle resolved photo-emission spectroscopy, optical reflection spectroscopy and magnetotransport measurements reveal a multiplevalley valence band structure and a quasi two-dimensional dispersion, realizing a Hicks-Dresselhaus thermoelectric contributing to the high Seebeck coefficient at high carrier density. We further demonstrate that the hole accumulation layer in exfoliated SnSe transistors exhibits a field effect mobility of up to 250 cm 2 /Vs at T = 1.3 K. SnSe is thus found to be a high quality, quasi twodimensional semiconductor ideal for thermoelectric applications. arXiv:1802.08069v1 [cond-mat.mtrl-sci]

Molybdenum borides were studied theoretically using first-principles calculations, parameterized lattice model, and global optimization techniques to determine stable crystal structures. Our calculations reveal the structures of known Mo–B phases, attaining close agreement with experiment. Following our developed lattice model, we describe in detail the crystal structure of boron-rich MoB x phases with 3 ≤ x ≤ 9 as the hexagonal P63/mmc-MoB3 structure with Mo atoms partially replaced by triangular boron units. The most energetically stable arrangement of these B3 units corresponds to their uniform distribution in the bulk, which leads to the formation of a disordered nonstoichiometric phase, with ordering arising at compositions close to x = 5 because of a strong repulsive interaction between neighboring B3 units. The most energetically favorable structures of MoB x correspond to the compositions 4 ≲ x ≤ 5, with MoB5 being the boron-richest stable phase. The estimated hardness of MoB5 is 37–39 GPa, suggesting that the boron-rich phases are potentially superhard.

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.