Monolayer SnS Research Articles

Fabrication of Ag embedded−SnS films via the RF approach: First study on NO2 gas–sensing performance

We report on the synthesis of 5-nm thick-Ag embedded−SnS (Ag/SnS) films by the radio frequency (RF) sputtering method as an absorbent layer to detect NO2 gas. Structure, morphology, elements, and optical properties of the Ag/SnS films were studied by X − ray diffraction, scanning electron microscopy, energy−dispersive spectroscopy, Raman and photoluminescence techniques, respectively. We firstly investigated the sensitivity of 5 – 20 ppm concentration of the Ag/SnS films to NO2 gas at an operating temperature ranging 30 – 100 °C. It was indicated that the Ag/SnS film–based sensor got good response and recovery at an operating temperature of 80 °C for NO2 gas. The 100 nm thick SnS film−based sensor made a faster response time (40 s) and recovery time (1320 s) to 20 ppm NO2 gas at 30 °C than those of other SnS films−based sensors. The result shows that the Ag/SnS film might be suitable for improving NO2 sensor properties due to the enhanced hole mobility, and more defectiveness of the SnS monolayer (Sn–vacancy). Based on these findings, we propose the fabrication of nanometals on SnS film for the high sensitive and selective NO2 gas−sensing a lower detection limit and at room temperature.

Theoretical study of SnS2 encapsulated in graphene as a promising anode material for K-ion batteries

Rechargeable batteries with superior electronic conductivity, large capacity, low diffusion barriers and moderate open circuit voltage have attracted amount attention. Due to abundant resources and safety, as well as the high voltage and energy density, potassium ion batteries (KIBs) could be an ideal alternative to next-generation of rechargeable batteries. Based on the density functional theory calculations, we find that the SnS2 monolayer expands greatly during the potassiumization, which limits its practical application. The construction of graphene/SnS2/graphene (G/SnS2/G) heterojunction effectively prevents SnS2 sheet from deformation, and enhances the electronic conductivity. Moreover, the G/SnS2/G has not only a high theoretical special capacity of 680 mAh g−1, but an ultra-low K diffusion barrier (0.08 eV) and an average open circuit voltage (0.22 V). Our results predict that the G/SnS2/G heterostructure could be used as a promising anode material for KIBs.

Interface controlled band alignment type in Janus SnS2/SSnSe and SnS2/SeSnS van der Waals heterojunctions

Two-dimensional (2D) Janus structures have been employed significantly considering their asymmetrical layout and multiple applications in different areas. A novel van der Waals (vdW) heterojunction is constructed based on the SnS2 monolayer and Janus SnSSe monolayer in the current study. Due to different atoms in two sides of the SnSSe monolayer, two kinds of heterojunctions, including SnS2/SSnSe and SnS2/SeSnS, can be constructed. Both heterojunctions are indirect band gap semiconductors. The kind of interface atom can affect the electronic properties of heterojunctions, resulting in interface-controlled band alignment (BA) type. Since the conduction band minimum states of the SnS2/SSnSe system contain the contribution of both SSnSe and SnS2 layers, this system is not suitable for efficient electron-hole separation. The SnS2/SeSnS heterojunction constructs a standard type-II BA, in which the holes and electrons are placed within the layers of SeSnS and SnS2, respectively. The external electric field, in-plane strain, and the adsorption of nonmetal atoms are also considered to modulate the electronic properties and BA type of the heterojunctions. According to these findings, the interface atom is an important factor influencing the electronic properties of those Janus vdW heterojunctions.

Tunable electronic properties of GaS–SnS2 heterostructure by strain and electric field

Vertically stacked heterostructures have received extensive attention because of their tunable electronic structures and outstanding optical properties. In this work, we study the structural, electronic, and optical properties of vertically stacked GaS–SnS2 heterostructure under the frame of density functional theory. We find that the stacked GaS–SnS2 heterostructure is a semiconductor with a suitable indirect band gap of 1.82 eV, exhibiting a type-II band alignment for easily separating the photo-generated carriers. The electronic properties of GaS–SnS2 heterostructure can be effectively tuned by an external strain and electric field. The optical absorption of GaS–SnS2 heterostructure is more enhanced than those of the GaS monolayer and SnS2 monolayer in the visible light region. Our results suggest that the GaS–SnS2 heterostructure is a promising candidate for the photocatalyst and photoelectronic devices in the visible light region.

Low thermal conductivity and interface thermal conductance in SnS2

After the discovery of graphene, there have been tremendous efforts in exploring various layered two-dimensional (2D) materials for their potential applications in electronics, optoelectronics, as well as energy conversion and storage. One of such 2D materials, SnS2, which is earth abundant, low in toxicity, and cost effective, has been reported to show a high on/off current ratio, fast photodetection, and high optical absorption, thus making this material promising for device applications. Further, a few recent theoretical reports predict high electrical conductivity and Seebeck coefficient in its bulk counterparts. However, the thermal properties of SnS2 have not yet been properly explored, which are important to materialize many of its potential applications. Here, we report the thermal properties of SnS2 measured using the optothermal method and supported by density functional theory (DFT) calculations. Our experiments suggest very low in-plane lattice thermal conductivity (\k{appa} = 3.20 +- 0.57 W m-1 K-1) and cross-plane interfacial thermal conductance per unit area (g = 0.53 +- 0.09 MW m-2 K-1) for monolayer SnS2 supported on a SiO2/Si substrate. The thermal properties show a dependence on the thickness of the SnS2 flake. Based on the findings of our DFT calculations, the very low value of the lattice thermal conductivity can be attributed to low group velocity, a shorter lifetime of the phonons, and strong anharmonicity in the crystal. Materials with low thermal conductivity are important for thermoelectric applications as the thermoelectric power coefficient goes inversely with the thermal conductivity.

Exact Long-Range Dielectric Screening and Interatomic Force Constants in Quasi-Two-Dimensional Crystals

We develop a fundamental theory of the long-range electrostatic interactions in two-dimensional crystals by performing a rigorous study of the nonanalyticities of the Coulomb kernel. We find that the dielectric functions are best represented by 2×2 matrices, with nonuniform macroscopic potentials that are two-component hyperbolic functions of the out-of-plane coordinate z. We demonstrate our arguments by deriving the long-range interatomic forces in the adiabatic regime, where we identify a formerly overlooked dipolar coupling involving the out-of-plane components of the dynamical charges. The resulting formula is exact up to an arbitrary multipolar order, which we illustrate in practice via the explicit inclusion of dynamical quadrupoles. By performing numerical tests on monolayer BN, SnS2, and BaTiO3 membranes, we show that our method allows for a drastic improvement in the description of the long-range electrostatic interactions, with comparable benefits to the quality of the interpolated phonon band structure.1 MoreReceived 14 December 2020Revised 27 July 2021Accepted 8 September 2021DOI:https://doi.org/10.1103/PhysRevX.11.041027Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLattice dynamicsOptical phononsPhononsPhysical Systems2-dimensional systemsThin filmsTransition metal dichalcogenidesTechniquesDensity functional theoryFirst-principles calculationsCondensed Matter, Materials & Applied Physics

First-principles investigation on electronic structure, magnetic states and optical properties of Mn-doped SnS2 monolayer via strain engineering

Two-dimensional magnetic materials are high desirable for realizing advanced optoelectronic and spintronic devices. In this paper, we report systematic studies the effects of strain on the electronic structure, magnetic states and optical properties of Mn-doped SnS2 monolayer (ML) by means of first-principles calculations. Ab-initio molecular dynamics simulations and formation energy reveal that the Mn-doped SnS2 ML is stable at 500 K and can be fabricated under the S-rich condition. The substitution of a Mn for a Sn atom induces 3 μB magnetic moment in nonmagnetic SnS2 ML, which is consistent with the Hund's rule and Aufbau principle. Based on classical Heisenberg model and mean-field approximation the Curie temperature is calculated to be 476.13 K. Due to symmetry reserved crystal structure, the net magnetic moment of Mn-doped SnS2 ML is robust even if considerable strains (−10 to 10%) are applied. The blue-shift and red-shift of the absorption peaks are observed in the optical spectrum by biaxial compressive and tensile strains. The biaxial strain effectively improves the optical properties of Mn-doped SnS2 ML, particularly in the visible light region. This study provide useful guidance for practical application in magnetic and optoelectronic devices.

Tuning electronic and optical properties of two–dimensional vertical van der waals arsenene/SnS2 heterostructure by strain and electric field

The structure and electronic properties of two-dimensional vertical van der Waals arsenene/SnS2 heterostructure are investigated based on the first-principles calculations. The results show that the arsenene/SnS2 bilayer forms a type-II heterostructure with the indirect band gap of 0.870 eV, which is conducive to separating the photogenerated electron-hole pairs. Importantly, the electronic properties of the arsenene/SnS2 heterostructure can be effectively adjusted by applying an external electric field and biaxial strain. Both compressive and tensile strains can decrease the band gap of the arsenene/SnS2 heterostructure, while compressive strain reduces the band gap faster, and the band alignment will have a transition from type-II to type-I when the tensile strain increases to 8%. The band gap of the arsenene/SnS2 heterostructure changes linearly under the external electric field from -1.0 to 1.0 V/Å, while the type-II property is robust in the arsenene/SnS2 heterostructure, not changed by the electric field we consider. Compared to the isolated arsenene and SnS2 monolayers, the arsenene/SnS2 heterostructure has the enhanced optical abaorption from visible to ultraviolet region, especially possessing the absorption peak up to 35.1% in the ultraviolet. Interestingly, there exhibits red shift under tensile strain or negative electric field, and blue shift under compressive strain or positive electric field. Our proposed arsenene/SnS2 heterostructure has great potential applications in electronic and optoelectronic devices.

Carrier and magnetism engineering for monolayer SnS2 by high throughput first-principles calculations* *Project supported by the National Natural Science Foundation of China (Grant No. 91833302).

Two-dimensional (2D) semiconducting tin disulfide (SnS2) has been widely used for optoelectronic applications. To functionalize SnS2 for extending its application, we investigate the stability, electronic and magnetic properties of substitutional doping by high throughput first-principles calculations. There are a lot of elements that can be doped in monolayer SnS2. Nonmetal in group A can introduce p-type and n-type carriers, while most metals in group A can only lead to p-type doping. Not only 3d, but also 4d and 5d transition metals in groups VB to VIIIB9 can introduce magnetism in SnS2, which is potentially applicable for spintronics. This study provides a comprehensive view of functionalization of SnS2 by substitutional doping, which will guide further experimental realization.

Gas selectivity regulation of monolayer SnS by introducing nonmetallic dopants: A combined theoretical and experimental investigation

The group-IV monochalcogenide of SnS is explored promising for gas sensor applications due to its unique two-dimensional puckered monolayer and superior oxidation resistance. This work focuses on the gas selectivity regulation of SnS monolayer with adjustable doping. To this end, detailed studies for the effects of nonmetallic dopants (X) of Si, P, or Cl in SnS lattice on the selective adsorption of NO2, NH3, SO2 are first performed by first-principles calculations. It reveals that the SnS monolayers with Si and P doping exhibit stronger adsorption capability and rapid recovery characteristic towards NH3 and SO2 at room temperature, respectively, while Cl-doped SnS could realize the reversible detection of NO2 at elevated temperature. These theoretical findings were further verified in experiment. Gas-sensing evaluations show the obvious gas selectivity of SnS with different doping of Si, P and Cl towards NH3, SO2, and NO2, respectively. The response magnitude exhibited by the doped SnS is revealed to be positively correlated with the calculated electron transfer. In terms of recovery performance of the doped SnS, intrinsic recovery characteristic could be predicted by the calculation on recovery time, while the measured recovery time reflects the combined effect of material characteristic and environmental factors.

Adsorption and gas-sensing properties of C2H4, CH4, H2, H2O on metal oxides (CuO, NiO) modified SnS2 monolayer: A DFT study

Detection of oil dissolved gases is of great significance to diagnose the diverse insulation faults in oil-immersed transformers. Therefore, the Metal oxides (CuO, NiO) doped SnS2 monolayer were exploited to reveal their gas-sensing properties to the typical oil dissolved gases (C2H4, CH4, H2) and H2O. Based on density functional theory calculations, the most stable modified substrate and adsorption structures were selected according to adsorption energy and geometry optimization. In addition, the gas-sensing mechanisms were obtained through the analysis of the adsorption structure, molecular orbit, charge transfer, total density of states and projected density of states. For gases adsorption on CuO-SnS2 the adsorption capacity is ranked as: C2H4 > H2O > H2 > CH4. For NiO-SnS2 the adsorption capacity is ranked as: H2O > C2H4 > H2 > CH4. By contrast, CuO-SnS2 shows higher superiority towards C2H4 adsorption, while the adsorption energy of CH4 ,H2 and H2O on NiO-SnS2 is more prominent than that of CuO-SnS2. Considering the ameliorative electronic properties and appropriate adsorption energy, NiO-SnS2 can be a potential candidate sensor for C2H4.

Thermoelectric properties of the SnS monolayer: Fully ab initio and accelerated calculations

An energetic and dynamical stability analysis of five candidate structures—hexagonal, buckled hexagonal, litharge, inverted litharge, and distorted-NaCl—of the SnS monolayer is performed using density functional theory. The most stable is found to be a highly distorted-NaCl-type structure. The thermoelectric properties of this monolayer are then calculated using the density functional theory and the Boltzmann transport equation. In terms of phonon scattering, there is a sharp contrast between this monolayer and bulk materials, where normal processes are more important. The calculations reveal that the SnS monolayer has enhanced electrical performance as compared to the bulk phase. As a consequence, high figures of merit ZT∼5 and ZT∼1.36 are predicted at 600 and 300 K, respectively, for the monolayer, ∼33 times higher than the ZT of its bulk analog. Therefore, this structure is an interesting candidate for room-temperature thermoelectric applications. A comparison between the fully ab initio results and simpler models based on relaxation times for electrons and phonons highlights the efficiency of computationally inexpensive models. However, ab initio calculations are found to be very important for the prediction of thermal transport properties.

Realization of Large Scale, 2D van der Waals Heterojunction of SnS2 /SnS by Reversible Sulfurization.

2D van der Waals heterojunction provides an attractive opportunity for realizing novel electronic or optoelectronic devices. It remains challenging to realize high-quality heterostructures through scalable methods such as molecular epitaxy growth (MBE). Here, growth of few-layer SnS thin films is reported on mica and Nb-doped SrTiO3 (100) substrates by MBE. Then the top layer of SnS film is uniformly sulfurized to monolayer SnS2 in a sulfur atmosphere, resulting in a high-quality SnS2 /SnS 2D heterojunction. Furthermore, the SnS2 layer can be recovered to SnS by annealing SnS2 /SnS without sulfur supply, indicating the heterojunction formation is reversible. The scanning tunneling spectroscopy measurements on SnS2 /SnS heterostructure indicate the type-II band alignment in SnS2 /SnS. The work provides a promising approach to construct artificial 2D heterojunctions with desired properties, which could be extended to other sulfide and selenide systems.

Strain-controlled Rashba spin-orbit coupling effect in SnS and SnSe monolayers

The miniaturization of transistors and the high density of integrated circuits makes conventional transistors reach a size limit, causing leakage currents, unstable performance, and increasing production cost. Spin field effect transistors (SFETs) based on spintronics use electron spin as an information carrier and regulate electron spin degree of freedom for storing and processing information. As a class of spintronics, Rashba spin–orbit coupling (SOC) effect attracts increasing attention because of its electric tunability. SFETs based on Rashba SOC and two-dimensional materials will make a qualitative leap in future semiconductor devices. Based on first-principles calculations, it is found that the applied strains along zigzag (ZZ), armchair (AC), and biaxial direction make the bandgaps of SnS and SnSe monolayers (ML) have an indirect-direct-zero transition. SnS and SnSe ML have a more stable electron spin and stronger Rashba spin splitting than InGaAs/InAlAs heterojunction and Au(111) surface. For SnS ML, 6% tensile strain along ZZ direction not only stabilizes the electron spin but also strengthens Rashba spin splitting to a maximum of 0.76 eV Å. For SnSe ML, 2% tensile strain along biaxial direction makes Rashba spin splitting strength reach a maximum of 1.33 eV Å.

Effects of gallium and arsenic substitution on the electronic and magnetic properties of monolayer SnS

Monolayer tin sulphide (SnS) is an extraordinary two-dimensional material with semiconductor nature. We have explored the doping effects of Gallium (Ga) and Arsenic (As) atoms on the electronic and magnetic properties of monolayer SnS using first-principles calculations. We find that the doped system are energetically stable due to high binding energies. Both the dopants retain the semiconductor nature of monolayer SnS with a tuneable band gap. Interestingly, spin-polarization with magnetic moment of 1.00 μ B has been induced in both Ga- and As-doped monolayer SnS. Moreover, the realization of magnetic anisotropy energy (MAE) could pave a way to utilize Ga- and As-doped monolayer SnS for applications in magnetic semiconductor devices.

Electric field controlled type-I and type-II conversion of BP/SnS van der Waals heterostructure

Type-I heterostructure, in which electrons and holes are confined in same region, is widely used in light emitting diodes and semiconductor lasers. Type-II heterostructure is widely used in photovoltaic devices because of its excellent spatial separation property of electrons and holes. Can we integrate photovoltaic, photoelectric properties with luminescent property in one device? Here we report a van der Waals heterostructure formed by black phosphorus (BP) and SnS monolayers. It is expected to realize these functions in one device. By first-principles methods, the structural stability, electronic properties and optical properties are investigated. It was found that the BP/SnS bilayer is type-II heterostructure with an indirect bandgap of 0.56 eV. The p-like character of the band edge in BP/SnS vdW heterostructure makes it to be an excellent optoelectronic material. The type-II stability of the system can be improved by applying a negative electric field. However, when the positive electric field is bigger than 0.1 V Å−1, the system begins to transform from type-II to type I. Therefore, by adding a gate voltage the bandgap and band alignment of this system can be controlled. The photovoltaic and photoelectric properties can be integrated in one device based on this heterostructure.

SnS2 monolayer and SnS2/graphene heterostructure as promising anchoring materials for lithium-sulfur batteries: A computational study

• Interactions between polysulfides and SnS 2 were studied by DFT calculations. • SnS 2 /G heterostructure was considered to improve electrical conductivity. • Both SnS 2 and SnS 2 /G are promising anchoring materials for Li-S batteries. Lithium-sulfur (Li-S) batteries have attracted widespread attention as high energy storage systems. However, the shuttle of intermediate polysulfides is still one of the unresolved obstacles for their practical application. In this work, the potential of SnS 2 monolayer as polysulfides anchoring material for suppressing the “shuttle effect” were investigated through density functional theory (DFT) calculations. We found that SnS 2 monolayer can immobilize polysulfides with moderate binding strength and preserve their structural integrity well. The low diffusion barriers of long-chain polysulfides on SnS 2 surface suggest the improved electrode reaction efficiency. Furthermore, SnS 2 /graphene heterostructure with improved electrical conductivity was selected to analyze the applicability, which also shows outstanding anchoring performance and good diffusion property for polysulfides. Combined with the small reduction free energies of S 8 to Li 2 S and low decomposition barriers of Li 2 S during the discharge/charge process, we can conclude that SnS 2 -based composites are promising anchoring materials for advanced Li-S batteries.

Nondegenerate P‐Type In‐Doped SnS2 Monolayer Transistor

AbstractDoping can change the intrinsic properties of 2D materials by tuning their electronic structure. In this work, high‐quality 2D In‐doped SnS2 (In‐SnS2) monolayer is reported through chemical vapor transport method and following mechanical cleavage process. The In content of In‐SnS2 is ≈0.9%, and doping results in the downward shift of the Fermi level compared with the undoped one. Transmission electron microscopy images show that doping is uniform in the In‐SnS2 nanosheets with high quality. The In‐SnS2 monolayer field effect transistors (FETs) show p‐type feature which is different from the n‐type feature of undoped SnS2. The average hole produced by one In atom is estimated as 0.37 from FETs measurement. Density functional theory calculations show that the decreasing of hole concentration results from the hole killing mechanism induced by S vacancy. The results suggest that changing the polarity of 2D semiconductor by doping is successful, and In‐SnS2 monolayer has great potential in the applications of electronics and optoelectronics.

Optoelectronic properties of van der Waals stacked homo- and hetero-bilayers of tin-monochalcogenides: A first-principles study

In recent years, the van der Walls (vdW) stacking of two-dimensional (2D) materials has been widely practiced to improve their physical properties and functionalities. In this work, we constructed vdW homo- and hetero-bilayers by stacking α- and δ-types of SnS and SnSe monolayers and investigated their physical properties from the first-principles approach. The designed vdW bilayers exhibited the highest binding energies for interlayer separation (Δy) equivalent to ~3.75 Å, whereas, the vdW coupling between the constituting monolayers has been realized for Δy as large as 7 Å. The weakening of the vdW interactions driven by the increase in Δy has resulted in the widening of the energy bandgaps. The α-type vdW bilayers exhibited indirect bandgap whereas nearly direct bandgap has been realized for δ-type of vdW bilayers. The high probability of photogenerated electronic transitions in electronic structures of these homo- and hetero-bilayers has resulted in substantial optical absorption (of the order of ~105 cm−1). Moreover, the constructed vdW bilayers exhibited interesting optical properties and transparent behavior in the infrared, visible, and ultraviolet ranges below 5 eV. This study indicates the vdW stacking as an effective route to engineer the physical properties of 2D tin-monochalcogenides for diverse electronic, optoelectronic, and photovoltaic applications.

Vertical ferroelectricity in two-dimensional mixed-valence tin sulfide system: Unprecedented piezoelectricity, efficient nanogenerator and facile control of morphotoropic phase transformations

Abstract For the recently emerging two-dimensional van der Waals ferroelectrics, their promising prospects in nanoelectronic applications are hindered by their low vertical polarizations. Despite recent experimental breakthroughs and theoretical high throughput screening of material database, the obtained vertical polarizations of two-dimensional ferroelectrics are still limited. Here we propose a strategy of constructing two-dimensional mixed-valence compounds. We show first-principles evidence that Sn2S3 monolayer can be synthesized via epitaxial growth of SnS on SnS2 monolayer with degenerate mixed-valence bi-states. Such a system possesses a room-temperature robust vertical polarization higher than 10 pC/m, which can be switched via interchange of oxidation states between two layers crossing an energy-saving low barrier, either by applying an electric field or mechanical bending. If such Sn2S3 monolayer is utilized as a nanogenerator, an unprecedented alternating voltage of ~130 V can be generated by applying an oscillating driving force that repeatedly reverses its polarization. Moreover, it possesses multiple metastable phases with distinct electronic properties. The transformations between them are not only strain-tunable, but can also be controlled via external electric field or low frequency linearly polarized light. Due to the small energy difference between polar and nonpolar states with distinct thicknesses, akin to morphotoropic phases, an ultra-high piezoelectric coefficient (> 2700 pm/V) can be obtained in the phase transformation under pressure, which can be further greatly enhanced via critical doping.