Optical properties of orthorhombic germanium sulfide: unveiling the anisotropic nature of Wannier excitons

To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in anisotropic materials. Here, we investigate the optical properties of anisotropic excitons in GeS encapsulated by h-BN using different approaches that combine polarization- and temperature-dependent photoluminescence (PL) measurements, ab initio calculations, and effective mass approximation (EMA). Using the Bethe-Salpeter Equation (BSE) method, we found that the optical absorption spectra in GeS are significantly affected by the Coulomb interaction included in the BSE method, which shows the importance of excitonic effects besides it exhibits a significant dependence on the direction of polarization, revealing the anisotropic nature of bulk GeS. By combining ab initio calculations and EMA methods, we investigated the quasi-hydrogenic exciton states and oscillator strength (OS) of GeS along the zigzag and armchair axes. We found that the anisotropy induces lifting of the degeneracy and mixing of the excitonic states in GeS, which results in highly non-hydrogenic features. A very good agreement with the experiment is observed.

Electronic property modulation in two-dimensional lateral superlattices of monolayer transition metal dichalcogenides

Fabricating lateral heterostructures (HSs) and superlattices (SLs) provides a unique degree of freedom for modulating the physical properties of two-dimensional (2D) materials by varying the chemical component, geometric size and interface structure in the ultra-thin atomic thickness limit. While a variety of 2D lateral HSs/SLs have been synthesized, especially for transition metal dichalcogenides (TMDs), how such structures affect quantitatively the physical properties of 2D materials has not yet been established. We herein explore electronic property modulation in 2D lateral SLs of monolayer TMDs through first-principles high-throughput calculations. The dependence of the electronic structure, bandgap, carrier effective masses, charge density overlap on chemical components, interface type, and sub-lattice size of lateral TMD-SLs are investigated. We find that by comparison with their random alloy counterparts, the lateral TMD-SLs exhibit generally type-II band alignment, a wider range of bandgap tunability, larger carrier effective masses, and stronger electron-hole charge separation tendency. The bandgap variation with a sub-lattice size shows larger bowing parameters for the SLs with heterogeneous anions, by comparison with the homogeneous anion cases. A similar behavior is observed for the SLs with an armchair-type interface, by comparison with the zigzag-type interface cases. Further analyses reveal that the underlying physical mechanism can be attributed to the synergistic interplay among the band offset of sub-lattices, quantum confinement effect, and existing internal strain.

Colossal 3D Electrical Anisotropy of MoAlB Single Crystal

3D anisotropic functional properties (such as magnetic, electrical, thermal, and optical properties, etc.) in a single material are not only beneficial to the multipurpose of a material, but also helpful to enrich the regulatory dimensionality of functional materials. Herein, a colossal 3D electrical anisotropy of layered MAB-phase MoAlB single crystal is introduced and dissected. Using high-temperature metal-solution method, high-quality MoAlB single crystals are obtained and a surprisingly strong out-of-plane (σ_a /σ_b  = 1.43 × 10⁵ , at 2 K) and in-plane (σ_a /σ_c  = 12.12, at 2 K) electrical anisotropies are first observed. After a series of experimental and theoretical investigations, it is demonstrated that the 3D anisotropic crystal structure and chemical bond of MoAlB result in its 3D anisotropic phonon vibration and electronic structure, influence the corresponding electron-electron as well as electron-phonon interactions, and finally give rise to its colossal 3D anisotropy of electrical conductivity. This work experimentally and theoretically proves MoAlB single crystal possessing the 3D anisotropies of crystal structure, chemical bond, phonon vibration, electronic structure, and electrical transport, but also provides a promising platform for the future design of functionalized electronic devices as well as synthesis of new and large-sized in-plane anisotropic 2D material (MoBene).

Solid-source vapor growth and optoelectronic properties of arsenic-based layered group-IV monopnictides

We investigated the preparation of SiAs and GeAs films using solid-source vapor growth. The shapes of the SiAs and GeAs films were rectangular, reflecting the thermally stable monoclinic phase (C2/m)...

Investigation of the sublimation mechanism of GeSe and GeS

GeSe and GeS have emerged as promising light-harvesting materials for photovoltaics due to their attractive optoelectronic properties, non-toxic and earth-abundant constituents, and excellent stability. Here we unveil the diatomic molecule sublimation mechanism of GeSe and GeS that directly guides the optimization of GeSe and GeS solar-cell fabricated via the close-space sublimation method.

Interfacial Strain Engineering in Wide-Bandgap GeS Thin Films for Photovoltaics

Wide-bandgap semiconductors exhibiting a bandgap of ∼1.7-1.9 eV have generated great interest recently due to their important applications in tandem solar cells as top cells and emerging indoor photovoltaics. However, concerns about the stability and toxicity especially in indoor application limit the choice of these materials. Here we report a new member of this family, germanium monosulfide (GeS); this material displays a wide bandgap of 1.7 eV, nontoxic and earth-abundant constituents, and high stability. We find that the little success of GeS solar cells to date is primarily attributed to the challenge in fabricating high-quality polycrystalline GeS films, wherein the high thermal expansion coefficient (α = 3.1 × 10-5 K-1) combined with high crystallization temperature (375 °C) of GeS induces large tensile strain in the GeS film that peels off GeS from the substrate. By introducing a high-α buffer layer between GeS and substrate, we achieve a high-quality polycrystalline GeS thin film that compactly adheres to substrate with no voids. Solar cells fabricated by these GeS films show a power conversion efficiency of 1.36% under AM 1.5G illumination (100 mW cm-2). The unencapsulated devices are stable when stored in ambient atmosphere for 1500 h. Their efficiencies further increase to 3.6% under indoor illumination of 1000 lux.