n-type conversion of SnS by isovalent ion substitution: Geometrical doping as a new doping route

Defect pairing in Fe-doped SnS van der Waals crystals: a photoemission and scanning tunneling microscopy study

We investigate the effect of low concentrations of iron on the physical properties of SnS van der Waals crystals grown from the melt. By means of scanning tunneling microscopy (STM) and photoemission spectroscopy we study Fe-induced defects and observe an electron doping effect in the band structure of the native p-type SnS semiconductor. Atomically resolved and bias dependent STM data of characteristic defects are compared to ab initio density functional theory simulations of vacancy (VS and VSn), Fe substitutional (FeSn), and Fe interstitial (Feint) defects. While native SnS is dominated by acceptor-like VSn vacancies, our results show that Fe preferentially occupies donor-like interstitial Feint sites in close proximity to VSn defects along the high-symmetry c-axis of SnS. The formation of such well-defined coupled (VSn, Feint) defect pairs leads to local compensation of the acceptor-like character of VSn, which is in line with a reduction of p-type carrier concentrations observed in our Hall transport measurements.

Realizing high-ranged thermoelectric performance in PbSnS2 crystals

Great progress has been achieved in p-type SnS thermoelectric compound recently, while the stagnation of the n-type counterpart hinders the construction of thermoelectric devices. Herein, n-type sulfide PbSnS₂ with isostructural to SnS is obtained through Pb alloying and achieves a maximum ZT of ~1.2 and an average ZT of ~0.75 within 300–773 K, which originates from enhanced power factor and intrinsically ultralow thermal conductivity. Combining the optimized carrier concentration by Cl doping and enlarged Seebeck coefficient through activating multiple conduction bands evolutions with temperature, favorable power factors are maintained. Besides, the electron doping stabilizes the phase of PbSnS₂ and the complex-crystal-structure induced strong anharmonicity results in ultralow lattice thermal conductivity. Moreover, a maximum power generation efficiency of ~2.7% can be acquired in a single-leg device. Our study develops a n-type sulfide PbSnS₂ with high performance, which is a potential candidate to match the excellent p-type SnS.

Direct and reversible conversion between heat and electricity can be achieved in thermoelectric materials. Here, the authors realize high thermoelectric performance in PbSnS₂ crystals enabled by multiple bands convergence.

Doped 2D SnS materials derived from liquid metal-solution for tunable optoelectronic devices

Gas-liquid reaction phenomena on liquid-metal solvents can be used to form intriguing 2D materials with large lateral dimensions, where the free energies of formation determine the final product. A vast selection of elements can be incorporated into the liquid metal-based nanostructures, offering a versatile platform for fabricating novel optoelectronic devices. While conventional doping techniques of semiconductors present several challenges for 2D materials. Liquid metals provide a facile route for obtaining doped 2D semiconductors. In this work, we successfully demonstrate that the doping of 2D SnS can be realized in a glove box containing a diluted H₂S gas. Low melting point elements such as Bi and In are alloyed with base liquid Sn in varying concentrations, resulting in the doping of 2D SnS layers incorporating Bi and In sulphides. Optoelectronic properties for photodetectors and piezoelectronics can be fine-tuned through the controlled introduction of selective migration doping. The structural modification of 2D SnS results in a 22.6% enhancement of the d₁₁ piezoelectric coefficient. In addition, photodetector response times have increased by several orders of magnitude. Doping methods using liquid metals have significantly changed the photodiode and piezoelectric device performances, providing a powerful approach to tune optoelectronic device outputs.

Tunability of the bandgap of SnS by variation of the cell volume by alloying with A.E. elements

We clarified that the bandgap of inorganic materials is strongly correlated with their effective coordination number (ECoN) via first-principles calculations and experimental confirmations. Tin mono-sulphide (Pnma) and germanium mono-sulphide (Pnma) were selected as model cases since these materials successively alter the ECoN as the cell volume changes and show an uncommon relationship between cell volume and bandgap. Contrary to the common semiconductors, the bandgaps of SnS (Pnma) and GeS (Pnma) have a positive relationship with respect to cell volume. This unique phenomenon was explained by incorporating the concept of ECoN into the theoretical studies. The theory proposed in this study is widely applicable to semiconductors with low-symmetry structures. Further, we experimentally demonstrated that the bandgap of SnS (Pnma) can be broadly tuned by changing the unit cell volume via alloying with alkali-earth (A.E.) metals, which could allow SnS to be applied to Si-based tandem photovoltaics. Alloying with A.E. elements also stabilised Cl as an n-type donor, which enabled n-type conduction in the bandgap-widened SnS film in the SnS-based semiconductors.

Single source precursor route to nanometric tin chalcogenides

Low-temperature solution phase synthesis of nanomaterials using designed molecular precursors enjoys tremendous advantages over traditional high-temperature solid-state synthesis. These include atomic-level control over stoichiometry, homogeneous elemental dispersion and uniformly distributed nanoparticles. For exploiting these advantages, however, rationally designed molecular complexes having certain properties are usually required. We report here the synthesis and complete characterization of new molecular precursors containing direct Sn-E bonds (E = S or Se), which undergo facile decomposition under different conditions (solid/solution phase, thermal/microwave heating, single/mixed solvents, varying temperatures, etc.) to afford phase-pure or mixed-phase tin chalcogenide nanoflakes with defined ratios.

SnS nanosheets for harmonic pulses generation in near infrared region

Two-dimensional materials have attracted increasing attention because of their excellent mechanical, thermodynamic, magnetic, electrical and optical properties. Here, a new two-dimensional material of tin sulfide (SnS) is experimentally prepared. It is layered like black phosphorus and owns distinct optoelectronic properties, but eliminates the disadvantage of instability. The nonlinear saturable absorption characteristics of the SnS nanosheets is investigated at 1563.3 nm by the double-balanced detection method. The obtained modulation depth and saturation intensity are 5.4% and 66.3 MW/cm², respectively. A passively harmonic mode-locked erbium-doped fiber laser based on the SnS saturable absorber (SA) has been demonstrated. The results show that mode-locking with fundamental frequency of 5.47 MHz is realized at pump power of 28.38 mW. With the increase of pump power, the laser can operate from fundamental frequency to high-order harmonic mode-locking. The maximum repetition rate of 412.73 MHz has been obtained, which is equivalent to the 76th harmonic mode-locking. This work reveals that SnS nanosheets is a novel and efficient SA with high damage threshold, which will find potential applications in optical communication, photoelectric detection, laser medicine, etc.

Computational Design of Mixed-Valence Tin Sulfides as Solar Absorbers

Binary tin sulfides, such as SnS and SnS₂, are appealing because of their simple stoichiometry and semiconducting properties and are, therefore, being pursued as potentially cost-effective materials for optoelectronic applications. The multivalency of Sn, that is, Sn(+2) and Sn(+4) allows yet more intermediate compositions, Sn_xS_y, whose structures and properties are of interest. Sn₂S₃ is already under consideration as a mixed-valence semiconductor. Other intermediate compositions, for example, Sn₃S₄ and Sn₄S₅ have remained elusive, although their existences have been alluded to in literature. Here we report a comprehensive study of phase stability of the Sn_xS_y series compounds, utilizing swarm-intelligence crystal structure search method combined with first-principles energetic calculations. We find that the stability of mixed-valence Sn_xS_y compounds with respect to decomposition into pure-valence SnS and SnS₂ is in general weaker than the Sn_xO_y counterparts, likely due to differences in chemical bonding. Besides identifying the experimentally discovered stable phases of Sn₂S₃, our calculations indicate that the Sn₃S₄ phase is another mixed-valence composition which shows marginal stability with respect to decomposition into SnS and SnS₂. Other studied compositions may be metastable under ambient conditions, with slightly positive formation enthalpies. We find two structures of Sn₃S₄ having comparably low energies, both of which feature one-dimensional chain-like fragments obtained by breaking up the edge-connected octahedral layers of SnS₂. Both structures indicate lattice phonon stability and one shows quasi-direct band gap with a calculated value of 1.43 eV, ideal for solar absorbers. A further analysis of the composition-structure-property relationship supports the notion that low-dimensional Sn-S motifs and van der Waals interaction may lead to diverse structure types and chemical compositions, having functional properties that are yet to be identified in the Sn_xS_y series with mixed valency.

Mitigating Reasons for the Poor Performance of n-CdS/p-SnS Solar Cells

In the present work, indium tin oxide (ITO)/n-CdS/p-SnS/Au structured solar cells are fabricated with best conversion efficiency of 0.005%. A detailed investigation is made into the cause of the poor conversion efficiency and the cause is narrowed down to defects in p-SnS which effect the junction and the neutral region of the cell. The junctions performance is quantified using the ideality factor which is found to be related to the band misalignment. The paper also investigates into literature and discusses efforts made to overcome the problems with this structure.

Single-Crystal Growth of Cl-Doped n-Type SnS Using SnCl2 Self-Flux

SnS is a promising photovoltaic semiconductor owing to its suitable band gap energy and high optical absorption coefficient for highly efficient thin film solar cells. The most significant carnage is demonstration of n-type SnS. In this study, Cl-doped n-type single crystals were grown using SnCl₂ self-flux method. The obtained crystal was lamellar, with length and width of a few millimeters and thickness ranging between 28 and 39 μm. X-ray diffraction measurements revealed the single crystals had an orthorhombic unit cell. Since the ionic radii of S^2- and Cl^- are similar, Cl doping did not result in substantial change in lattice parameter. All the elements were homogeneously distributed on a cleaved surface; the Sn/(S + Cl) ratio was 1.00. The crystal was an n-type degenerate semiconductor with a carrier concentration of ∼3 × 10¹⁷ cm^-3. Hall mobility at 300 K was 252 cm² V^-1 s^-1 and reached 363 cm² V^-1 s^-1 at 142 K.

Multiple states and roles of hydrogen in p-type SnS semiconductors

The states and roles of hydrogen in p-type SnS are studied by hydrogen plasma treatment and density functional theory calculations.

Two-dimensional SnS nanoflakes: synthesis and application to acetone and alcohol sensors

SnS nanoflakes were synthesized using a solid state reaction method at 600 °C and their gas sensing properties were investigated.

Screening limited switching performance of multilayer 2D semiconductor FETs: the case for SnS

Gate tunable p-type multilayer tin mono-sulfide (SnS) field-effect transistor (FET) devices with SnS thickness between 50 and 100 nm were fabricated and studied to understand their performance. The devices showed anisotropic inplane conductance and room temperature field effect mobilities ∼5-10 cm² V^-1 s^-1. However, the devices showed an ON-OFF ratio ∼10 at room temperature due to appreciable OFF state conductance. The weak gate tuning behavior and finite OFF state conductance in the depletion regime of SnS devices are explained by the finite carrier screening length effect which causes the existence of a conductive surface layer from defect induced holes in SnS. Through etching and n-type surface doping by Cs₂CO₃ to reduce/compensate the not-gatable holes near the SnS flake's top surface, the devices exhibited an order of magnitude improvement in the ON-OFF ratio, and a hole Hall mobility of ∼100 cm² V^-1 s^-1 at room temperature is observed. This work suggests that in order to obtain effective switching and low OFF state power consumption, two-dimensional (2D) semiconductor based depletion mode FETs should limit their thickness to within the Debye screening length of the carriers in the semiconductor.

Photovoltaic Properties of Two-Dimensional (CH3NH3)2Pb(SCN)2I2 Perovskite: A Combined Experimental and Density Functional Theory Study

We explore the photovoltaic-relevant properties of the 2D MA2Pb(SCN)2I2 (where MA = CH3NH3(+)) perovskite using a combination of materials synthesis, characterization and density functional theory calculation, and determine electronic properties of MA2Pb(SCN)2I2 that are significantly different from those previously reported in literature. The layered perovskite with mixed-anions exhibits an indirect bandgap of ∼2.04 eV, with a slightly larger direct bandgap of ∼2.11 eV. The carriers (both electrons and holes) are also found to be confined within the 2D layers. Our results suggest that the 2D MA2Pb(SCN)2I2 perovskite may not be among the most promising absorbers for efficient single-junction solar cell applications; however, use as an absorber for the top cell of a tandem solar cell may still be a possibility if films are grown with the 2D layers aligned perpendicular to the substrates.

Defect properties of the two-dimensional (CH3NH3)2Pb(SCN)2I2 perovskite: a density-functional theory study

To optimize the photovoltaic performance, the 2D (CH₃NH₃)₂Pb(SCN)₂I₂ perovskite absorber layers should be synthesized under Pb-poor and I-rich conditions so that the dominant defects are V_Pb, which create shallow defect transition levels and making the absorber layers intrinsically p-type.