π-Phase Tin and Germanium Monochalcogenide Semiconductors: An Emerging Materials System

Prediction of novel ground-state structures and analysis of phonon transport in two-dimensional GexSy compounds

We conducted this study to explore the ground-state structures of two-dimensional (2D) variable-composition GexSy compounds, driven by the polymorphic characteristics of bulk germanium sulfides and the promising thermoelectric performance of 2D GeS (Pmn21). To accomplish this, we utilized the highly successful evolutionary-algorithm-based code USPEX in conjunction with VASP for total energy calculations, leading to the discovery of three previously unexplored structures of Ge2S (P2/c), GeS (P3̄m1), and GeS2 (P21/c). These 2D materials exhibit significantly lower formation energies compared to their reported counterparts. We thoroughly scrutinized the structural stability and subsequently analyzed their electronic structures. Our analysis reveals a nearly direct band gap of 0.12/0.84 eV with the PBE/HSE06 functional for 2D Ge2S and an indirect band gap for 2D GeS and GeS2. Their semiconducting nature highlights the crucial importance of lattice thermal conductivity (κl), which we determined by solving the Boltzmann transport equation for phonons. Importantly, we predict a room temperature κl value of 6.82 W m-1 K-1 for GeS, lower than its 2D orthorhombic counterpart. In the case of GeS2, we observed an anisotropic κl value of 16.95/10.68 W m-1 K-1 along the zigzag/armchair directions at 300 K, with an in-plane anisotropy ratio of 1.59, surpassing that of 2D IV-VI compounds. We delve into detailed discussions regarding the role of lattice anharmonicity, group velocities, phonon lifetimes, and three-phonon weighted phase space in the overall thermal conductivity analysis.

Electronic, mechanical and gas sensing properties of two-dimensional γ-SnSe

Two-dimensional (2D) materials are excellent candidates for advanced flexible electronics and gas sensors. Herein, we systematically investigate the layer-dependent electronic structures, mechanical properties and gas sensing characteristics of the newly synthesized γ-SnSe based on first-principles calculations. Bulk γ-SnSe is a typical van der Waals layered material with an indirect narrow band gap, while monolayer and multilayer γ-SnSe can be obtained through mechanical exfoliation due to its low cleavage energy. The band gap of γ-SnSe gradually increases with decreasing layers, reaching a value of 2.25 eV for the monolayer due to weakened interlayer coupling. Mechanical analysis reveals strong anisotropy in multilayer γ-SnSe, whereas the monolayer exhibits a negative Poisson's ratio (-0.023/-0.025). Additionally, based on the analysis of electronic structures, adsorption energies and charge transfer of the host materials after adsorption of various gases, it is found that the γ-SnSe monolayer demonstrates enhanced sensitivity and selectivity towards NO, NO2, and SO2 compared to CO, CO2, H2S and NH3. These findings highlight the potential of γ-SnSe as an excellent gas-sensitive material for the detection of nitrogen oxides and sulfur dioxide.

Electrical and Optical Properties of γ-SnSe: A New Ultra-narrow Band Gap Material

We describe the unusual properties of γ-SnSe, a new orthorhombic binary phase in the tin monoselenide system. This phase exhibits an ultranarrow band gap under standard pressure and temperature conditions, leading to high conductivity under ambient conditions. Density functional calculations identified the similarity and difference between the new γ-SnSe phase and the conventional α-SnSe based on the electron localization function. Very good agreement was obtained for the band gap width between the band structure calculations and the experiment, and insight provided for the mechanism of reduction in the band gap. The unique properties of this material may render it useful for applications such as thermal imaging devices and solar cells.

'Beneficial impurities' in colloidal synthesis of surfactant coated inorganic nanoparticles

Colloidal synthesis of nanoparticles (NP) has advanced tremendously over the past 25 years, with an increasing number of research papers introducing nanomaterials with a variety of compositions, shapes, sizes, and phases. Although much progress has been achieved, commonly used synthetic procedures often fail to reproduce results, and the fine details of the syntheses are often disregarded. Reproducibility issues in synthesis can be ascribed to the effects of impurities, trace amounts of chemical moieties which significantly affect the reaction products. Impurities in NP synthesis are rarely reported or regularly studied, despite their impact, deleterious, or beneficial. This topical review discusses several case studies of colloidal NP synthesis where the sources and the chemistry of impurities are highlighted, and their role is examined.

Chemical epitaxy of π-phase cubic tin monosulphide

Epitaxial thin films of cubic tin monosulfide (π-SnS), a recently discovered new binary phase, were deposited from solution on GaAs substrates and on GaAs with intermediate PbS layers.

Beneficial Impurities and Phase Control in Colloidal Synthesis of Tin Monoselenide

The effect of impurities in colloidal synthesis of SnSe in oleylamine surfactant was investigated. Specifically, it was found that impurities such as water, hydrochloric acid, and carbon dioxide stabilize the recently discovered cubic phase of tin monoselenide, π-SnSe. We describe the reaction that releases HCl to the reaction medium through reaction of SnCl₂ with moisture and its subsequent reaction with oleylamine, transforming it from neutral to charged surfactant. A similar path occurs in the case of CO₂, which reacts with oleylamine to give charged oleylammonium-oleylcarbamate molecular pairs. By exposing the reaction to controlled concentrations of "beneficial contaminants", hitherto a major source of irreproducibility in this synthesis, we demonstrate phase and shape control from π-SnSe cubes to α-SnSe rods.

Distorted structures in half-filled p-band materials

Many half-filled p-band materials form complex, semiconducting or semi-metallic crystallographic structures, which are commonly conceived of as distortions of simpler, higher-symmetry structures. This distortion is conventionally attributed to the energy gained by the opening of a band gap in the vicinity of the Fermi level, which was assumed to lower the overall energy of the lattice. Applying DFT calculations of the total energy and its component terms to both elemental and binary half-filled p-band materials, we show that the energy gain from distortion arises from the Coulombic interactions. Furthermore, we demonstrate that although the distortion is followed by an opening of a band gap, there may be other changes of the same order of magnitude in lower energy levels of the electrons. These results are demonstrated to apply both in the distortion parameter space of a specific phase and between different phases with different symmetries. It is therefore our conclusion that, in contrast to the prevailing concept, the metal-semiconductor or metal-semimetal transitions of such materials are the consequence of the distortion rather than its cause. This may suggest a more general mechanism of high-to-low symmetry transitions, relevant also to other distorted structures which do not demonstrate the same electronic transitions.

Stepwise Conversion from GeO2 to [MGe4S10]n3n- (M = Cu, Ag) Polymer via Isolatable [Ge2S6]4- and [Ge4S10]4- Anions by Virtue of Templating Technique

Rational synthesis of inorganic matter remains a great challenge encountered with modern synthetic chemistry. Here we reported the stepwise solvothermal conversion from GeO₂ to [MGe₄S₁₀]_n^3n- (M = Cu, Ag) polymer via isolatable [Ge₂S₆]^4- and [Ge₄S₁₀]^4- anions by virtue of templating technique. The facile sulfuration of GeO₂ resulted in the methylammonium-templated dimeric thiogermanate [CH₃NH₃]₄Ge₂S₆ (1). This was used subsequently as a precursor for the formation of adamantane-like [Ge₄S₁₀]^4- cluster, which was isolated as a mixed methylammonium/ethylammonium salt [CH₃CH₂NH₃]₃[CH₃NH₃]Ge₄S₁₀ (2). Compound 2 was then successfully used as a precursor to react with Cu⁺ and Ag⁺ cations in the presence of tetraethylammonium, resulting in alternating copolymeric products [(CH₃CH₂)₄N]₃MGe₄S₁₀ (M = Cu (3), Ag (4)), whose anionic moieties feature a novel zigzag chainlike structure constructed by [Ge₄S₁₀]^4- clusters via two-coordinate Cu⁺/Ag⁺ linkers. Mixed amine/ethanol or deep eutectic solvents were applied as media for the syntheses of 1-4, and all the products were characterized in the solid state and solution. Crystal structural analysis of the title compounds revealed significant templating roles of the alkylammonium cations as both space-filling agents and hydrogen-bonding donors, suggesting the structure-directing mechanism for the species formation and crystal growth. The design and optimization of multistep structural conversion upon templating effects would be beneficial for drawing rational, predictable pathways for inorganic synthesis.

Stability of cubic tin sulphide nanocrystals: role of ammonium chloride surfactant headgroups

New semiconducting metastable cubic phases have been recently discovered in the tin monosulfide and monoselenide systems. Surface energy calculations and experimental studies indicate that this cubic π-phase is stabilized by specific ligand adsorption on the surface. In this work, it is shown experimentally that the synthesis carried out using mixtures of oleylamine and oleylammonium chloride (OACl) surfactants results in the cubic phase, transforming the growth from orthorhombic to cubic nanoparticles with increasing OACl concentration up to a limiting point. Complementary ab initio calculations find that adsorbed ligands lower the surface energies for both the cubic phase and the orthorhombic phase, relative to the pristine surfaces. The decrease in the surface energy increases with ligand coverage. Stronger binding energies to the cubic phase suggest a higher coverage, and therefore preferential stabilization of this phase. Upon further increasing the coverage, the surface energy becomes negative, effectively destabilizing the particles in agreement with experimental observations. Bonding analysis shows that Cl bonds to Sn and replaces missing Sn-S bonds at the surface of the cubic structure. In the competing orthorhombic layered phase, Cl also bonds to a Sn atom but at the expense of one of the Sn-S bonds of this Sn atom. This observation can explain the trends of the surface energies. This combined experimental and computational analysis sheds light on the stabilization processes of these nano-materials and indicates the path to improve synthetic routes.