2D Free-Standing GeS1-xSex with Composition-Tunable Bandgap for Tailored Polarimetric Optoelectronics

Germanium-based monochalcogenides (i.e., GeS and GeSe) with desirable properties are promising candidates for the development of next-generation optoelectronic devices. However, they are still stuck with challenges, such as relatively fixed electronic band structure, unconfigurable optoelectronic characteristics, and difficulty in achieving free-standing growth. Herein, it is demonstrated that two-dimensional (2D) free-standing GeS1-xSex (0 ≤ x ≤ 1) nanoplates can be grown by low-pressure rapid physical vapor deposition (LPRPVD), fulfilling a continuously composition-tunable optical bandgap and electronic band structure. By leveraging the synergistic effect of composition-dependent modulation and free-standing growth, GeS1-xSex-based optoelectronic devices exhibit significantly configurable hole mobility from 6.22 × 10-4 to 1.24 cm2V-1s⁻1 and tunable responsivity from 8.6 to 311 A W-1 (635 nm), as x varies from 0 to 1. Furthermore, the polarimetric sensitivity can be tailored from 4.3 (GeS0.29Se0.71) to 1.8 (GeSe) benefiting from alloy engineering. Finally, the tailored imaging capability is also demonstrated to show the application potential of GeS1-xSex alloy nanoplates. This work broadens the functionality of conventional binary materials and motivates the development of tailored polarimetric optoelectronic devices.

Fabrication of (Ge0.42Sn0.58)S Thin Films via Co-Evaporation and Their Solar Cell Applications

In this study, as a novel approach to thin-film solar cells based on tin sulfide, an environmentally friendly material, we attempted to fabricate (Ge, Sn)S thin films for application in multi-junction solar cells. A (Ge0.42 Sn0.58)S thin film was prepared via co-evaporation. The (Ge0.42 Sn0.58)S thin film formed a (Ge, Sn)S solid solution, as confirmed by X-ray diffraction (XRD) and Raman spectroscopy analyses. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) of (Ge0.42 Sn0.58)S thin-film solar cells were 0.29 V, 6.92 mA/cm2, 0.34, and 0.67%, respectively; moreover, the device showed a band gap of 1.42-1.52 eV. We showed that solar cells can be realized even in a composition range with a relatively higher Ge concentration than the (Ge, Sn)S solar cells reported to date. This result enhances the feasibility of multi-junction SnS-system thin-film solar cells.

Stacking Fault Induced Symmetry Breaking in van der Waals Nanowires

While traditional ferroelectrics are based on polar crystals in bulk or thin film form, two-dimensional and layered materials can support mechanisms for symmetry breaking between centrosymmetric building blocks, e.g., by creating low-symmetry interfaces in van der Waals stacks. Here, we introduce an approach toward symmetry breaking in van der Waals crystals that relies on the spontaneous incorporation of stacking faults in a nonpolar bulk layer sequence. The concept is realized in nanowires consisting of Se-rich group IV monochalcogenide (GeSe_1-xS_x) alloys, obtained by vapor-liquid-solid growth. The single crystalline wires adopt a layered structure in which the nonpolar A-B bulk stacking along the nanowire axis is interrupted by single-layer stacking faults with local A-A' stacking. Density functional theory explains this behavior by a reduced stacking fault formation energy in GeSe (or Se-rich GeSe_1-xS_x alloys). Computations demonstrate that, similar to monochalcogenide monolayers, the inserted A-layers should show a spontaneous electric polarization with a switching barrier consistent with a Curie temperature above room temperature. Second-harmonic generation signals are consistent with a variable density of stacking faults along the wires. Our results point to possible routes for designing ferroelectrics via the layer stacking in van der Waals crystals.

Extended anisotropic phonon dispersion and optical properties of two-dimensional ternary SnSSe

The phonon dispersion and optical properties of mechanically exfoliated SnSSe were investigated with the aid of high-resolution Raman scattering and photoluminescence (PL) spectroscopies along with first-principles calculations.

Low-Pressure CVD of GeE (E = Te, Se, S) Thin Films from Alkylgermanium Chalcogenolate Precursors and Effect of Deposition Temperature on the Thermoelectric Performance of GeTe

The homologous series [GeⁿBu₃(EⁿBu)] (E = Te, Se, S; (1), (3) and (4)) and [GeⁿBu₂(TeⁿBu)₂] (2) have been synthesized as mobile oils in excellent yield (72-93%) and evaluated as single-source precursors for the low-pressure chemical vapor deposition (LPCVD) of GeE thin films on silica. Compositional and structural characterizations of the deposits have been performed by grazing-incidence X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, and Raman spectroscopy, confirming the phase purity and stoichiometry. Electrical characterization via variable-temperature Hall effect measurements is also reported. Given the strong interest in GeTe and its alloys for thermoelectric applications, variable-temperature Seebeck data were also investigated for a series of p-type GeTe films. The data show that it is possible to tune the thermoelectric response through intrinsic Ge vacancy regulation by varying the deposition temperature, with the highest power factor (40 μW/K²cm@629 K) and effective ZT values observed for the films deposited at higher temperatures.

Ternary Chalcogenides GeSb2Se3 and Ge3Sb4Se7 Containing a ∞1[Sb2Se2]2- 1D Chain and a 2D Structure Related to SnSe

Ternary chalcogenides, GeSb₂Se₃ and Ge₃Sb₄Se₇, were synthesized and characterized. These chalcogenides are the first ternary selenides in a ternary Ge-Sb-Se system that feature a layer structure related to black phosphorus and SnSe-type structures. Both compounds contain a _∞¹[Sb₂Se₂]^2- unit with Sb⁺ cations in a zigzag Sb-Sb chain structure, and Sb³⁺ cations in a distorted NaCl¹⁰⁰-type of _∞¹[Ge_n-2Sb₂Se_n]²⁺ unit (n = 4, 5). These materials exhibit n-type semiconducting properties with thermal conductivity significantly lower than that of GeSe and Sb₂Se₃, which could be correlated to the 1D Sb⁺ chain and disordered sites with different Ge/Sb compositions. It is anticipated that these newly discovered ternary chalcogenides may provide unique properties with enhanced thermoelectric properties.

Synthesis and thermal characterization of novel phase change materials (PCMs) of the Se-Te-Sn-Ge (STSG) multi-component system: calorimetric studies of the glass/crystal phase transition

According to recent literature, germanium-containing chalcogenide glasses (ChGs) show improvement in thermal stability and glass-forming ability because of the self-organization of the glass network towards a more rigid structure. The Ge-containing ChGs play a potential role as PCMs in phase-change optical memory (PCOM) applications. This endeavor reports the synthesis of some novel PCMs with Ge as the chemical modifier to improve the kinetic parameters of glass/crystal phase transition. The compositional variation of the various kinetic parameters in the present STSG chalcogen-rich non-oxide glasses Se78-yGeyTe20Sn2 (0 ≤ y ≤ 6) has been studied by means of the state-of-the-art differential scanning calorimetric (DSC) technique in the non-isothermal mode. The thermally assisted glass transition and crystallization phenomena have been investigated by examining the variation in various kinetic parameters like the characteristic kinetic temperatures (glass transition temperature Tg, on-set crystallization temperature To and peak crystallization temperature Tc), the activation energies involved in both phenomena, the thermal stability factor S and the glass-forming ability (GFA). The thermal stability factor S and GFA increase appreciably at higher concentrations of Ge as a signature of stiffness transition followed by the self-organization of the corner-sharing and the edge-sharing arrangements of the GeSe4 phase.

Cubic SnGe nanoalloys: beyond thermodynamic composition limit

Tin-germanium alloys are increasingly of interest as optoelectronic and thermoelectric materials as well as materials for Li/Na ion battery electrodes. However, the lattice incompatibility of bulk Sn and Ge makes creating such alloys challenging. By exploiting the unique strain tolerance of nanosized crystals, we have developed a facile synthetic method for homogeneous SnxGe1-x alloy nanocrystals with composition varying from essentially pure Ge to 95% Sn while still maintaining the cubic structure.

Composition-tuned SnxGe1−xS nanocrystals for enhanced-performance lithium ion batteries

Complete composition-tuned Sn_xGe_1−xS alloy nanocrystals exhibit excellent cycling performances in lithium ion batteries, with the greatest rate capability for Sn-rich compositions.