Two dimensional materials, befitting nanoscale electronics, can benefit strain-tunable applications due to their ultrathin and flexible nature. Based on the first-principles calculations within the generalized gradient approximation, GeSe monolayer with a distorted NaCl-type structure is predicted. The GeSe monolayer is found to be a direct semiconductor with a band gap of (1.16 ± 0.13) eV against the bulk counterpart. The electronic responses of the GeSe monolayer to strain are found to be sensitive and anisotropic, and the transitions between direct and indirect band gap are repeatedly met in the course of energy engineering by uniaxial and biaxial strains. The direct band gap of the GeSe monolayer is tunable by small strain within a large energy range (0.95–1.48 eV). The carrier effective masses in the GeSe monolayer are also tunable by strain in a low mass range (0.03–0.61 m0). These intriguing properties make GeSe monolayer a promising two-dimensional material for nanomechanics, thermoelectrics, and ...

GeSe monolayer semiconductor with tunable direct band gap and small carrier effective mass

Two-dimensional (2D) layered materials, transition-metal dichalcogenides, and black phosphorus have attracted considerable interest from the viewpoints of fundamental physics and device applications. The establishment of new functionalities in anisotropic layered 2D materials is a challenging but rewarding frontier, owing to the remarkable optical properties of these materials and their prospects for new devices. Herein, we report the anisotropic and thickness-dependent optical properties of a 2D layered monochalcogenide of germanium sulfide (GeS). Three Raman-scattering peaks corresponding to the B3g, A
 g
 1
 , and A
 g
 2
 modes with a strong polarization dependence are demonstrated in the GeS flakes, which validates polarized Raman spectroscopy as an effective method for identifying the crystal orientation of anisotropic layered GeS. Photoluminescence (PL) is observed with a peak at ~1.66 eV that originates from the direct optical transition in GeS at room temperature. The polarization-dependent characteristics of the PL, which are revealed for the first time, along with the demonstration of anisotropic absorption, indicate an obvious anisotropic optical transition near the band edge of GeS, which is supported by density functional theory calculations. The significantly thickness-dependent PL is observed and discussed. This anisotropic layered GeS presents opportunities for the discovery of new physical phenomena and will find applications that exploit its anisotropic properties, such as polarization-sensitive photodetectors.

Anisotropic optical and electronic properties of two-dimensional layered germanium sulfide

SnS is a metal monochalcogenide suitable for use as absorber material in thin film photovoltaic cells. Its structure is an orthorhombic crystal of weakly coupled layers, each layer consisting of strongly bonded Sn-S units. We use first-principles calculations to study model single-layer, double-layer, and bulk structures of SnS in order to elucidate its electronic structure. We find that the optoelectronic properties of the material can vary significantly with respect to the number of layers and the separation between them: the calculated band gap is wider for fewer layers (2.72 eV, 1.57 eV, and 1.07 eV for single-layer, double-layer, and bulk SnS, respectively) and increases with tensile strain along the layer stacking direction (by ∼55 meV/1% strain).

https://scholar.harvard.edu/files/efthimios_kaxiras/files/japplphys_113_233507_2013.pdf

Optoelectronic properties of single-layer, double-layer, and bulk tin sulfide: A theoretical study

Based on the first-principles calculations, we demonstrated that a GeSe/SnSe heterostructure has type-II band alignment and a direct band gap, which can effectively prevent the recombination of photogenerated electron–hole pairs. Moreover, the GeSe/SnSe heterostructure also exhibits strong optical absorption intensity, which can reach the order of 105 cm−1. Our predicted photoelectric conversion efficiency (PCE) for the GeSe/SnSe heterostructure reaches 21.47%. We also found that the hole carrier mobility of the GeSe/SnSe heterostructure along the x direction has been significantly improved to 6.42 × 104 cm2 V−1 s−1, which is higher than that of black phosphorus (1 × 104 cm2 V−1 s−1). By applying a vertical external electric field, we found that the band gap and band offset of the GeSe/SnSe heterojunction can be effectively tuned. The revealed type-II band alignment, strong optical absorption, superior PCE and superior hole carrier mobility of the GeSe/SnSe heterostructure imply that this new proposed material has broad application prospects in solar cells.

A two-dimensional GeSe/SnSe heterostructure for high performance thin-film solar cells

Using the ab initio method within the local-density approximation with ultrasoft pseudopotentials and plane-wave basis, calculating the Hellmann-Feynman forces and applying the direct method, the phonon-dispersion relations for ferroelectric and paraelectric rhombohedral phases of ${\mathrm{LiNbO}}_{3}$ crystal have been derived. The calculated phonon frequencies for the ferroelectric phase at the $\ensuremath{\Gamma}$ point are compared with Raman, infrared, and neutron-scattering measurements, and they agree very well. The phonon frequencies at the Z, L, and F high-symmetry points are also calculated. It is disclosed that the force constants involving lithium ions are an order of magnitude smaller than the force constants between niobium and oxygen ions. As a consequence, the lithium ions do not contribute to high-frequency optical vibrations. In the paraelectric phase the phonon-dispersion curves show a soft branch with the soft mode of symmetry ${A}_{2u}$ at the $\ensuremath{\Gamma}$ point. This soft mode leads to the observed paraelectric-ferroelectric phase transition. The LO/TO splitting is considered by assuming formal values of ionic charges, and by fitting the longitudinal infrared active modes to the experimental data.

Ab initio calculations of phonons in LiNbO 3

The structural and vibrational properties of the prototypical layered semiconductor germanium sulfide (GeS) have been studied under pressure using a combination of high-resolution x-ray powder diffraction, Raman scattering, and ab initio simulation. The theoretically and experimentally determined pressure response of the static and dynamical properties are in good agreement with each other. No structural phase transformation is found up to 94 kbar. Inspection of the calculated eigenvectors of zone center phonons at several pressures indicates that the validity of the rigid-layer mode approximation is appropriate only at near-ambient pressure conditions and breaks down under compression. \textcopyright{} 1996 The American Physical Society.

Vibrational properties of the layered semiconductor germanium sulfide under hydrostatic pressure: Theory and experiment.

We report the results of ab initio total-energy pseudopotential calculations, high-pressure angle-dispersive powder x-ray diffraction, and vibrational spectroscopy measurements on the IV-VI layered semiconductor germanium selenide (GeSe). The calculated hydrostatic pressure dependence of the crystal structure is found to be in good agreement with the results of high-resolution x-ray structural studies. In contrast to previous reports, no evidence of a pressure-induced first-order structural phase transition is found up to 130 kbar. However, a metallization transition is predicted in our electronic structure calculations. This is consistent with reports of a large resistivity drop at high pressure. The rigid-layer vibrational shear mode is also investigated by ab initio methods within the rigid-layer mode approximation and the frequency is found to be in fair agreement with our Raman scattering results at ambient pressure. At higher pressures the calculated frequency is substantially overestimated in the calculations.

High-pressure effects in the layered semiconductor germanium selenide

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Vibrational properties of the layered semiconductor germanium sulfide under hydrostatic pressure: Theory and experiment.

High-pressure effects in the layered semiconductor germanium selenide