Layered SnSe2 Research Articles

Microstructural Instability and Its Effects on Thermoelectric Properties of SnSe and Na-Doped SnSe.

Effects of thermal cycling on the microstructure and thermoelectric properties are studied for the undoped and Na-doped SnSe samples using X-ray computed tomography and property measurements. It is observed that thermal cycling causes significant cracks to develop, which decrease both the electrical and lattice thermal conductivities but do not affect the thermopower. The zT values are drastically reduced after the repeated heat treatment. It is important to account for density changes during cycling to obtain accurate values of the thermal conductivity. Even before thermal cycling, the spark-plasma sintered (SPS) samples have a significant number of microcracks. The orientation of cracks within the SPS pellets and their effect on the microstructure are influenced by the presence of a Na-rich impurity. The SnSe and Sn0.995Na0.005Se samples without the impurity develop cracks and exhibit grain growth parallel to the pellet surface, which is also the plane of the 2D SnSe layers. The Sn0.97Na0.03Se sample containing the impurity develops cracks that are orthogonal to the pellet surface. Such an orientation of cracks in Sn0.97Na0.03Se inhibits grain growth. All samples appear mechanically unstable after thermal cycling.

In Situ Formation of SnSe2/SnSe Vertical Heterostructures toward Polarization Selectable Band Alignments.

2D van der Waals (vdW) layered semiconductor vertical heterostructures with controllable band alignment are highly desired for nanodevice applications including photodetection and photovoltaics. However, current 2D vdW heterostructures are mainly obtained via mechanical exfoliation and stacking process, intrinsically limiting the yield and reproducibility, hardly achieving large-area with specific orientation. Here, large-area vdW-epitaxial SnSe2/SnSe heterostructures are obtained by annealing layered SnSe. These in situ Raman analyses reveal the optimized annealing conditions for the phase transition of SnSe to SnSe2. The spherical aberration-corrected transmission electron microscopy investigations demonstrate that layered SnSe2 epitaxially forms on SnSe surface with atomically sharp interface and specific orientation. Optical characterizations and theoretical calculations reveal valley polarization of the heterostructures that originate from SnSe, suggesting a naturally adjustable band alignment between type-II and type-III, only relying on the polarization angle of incident lights. This work not only offers a unique and accessible approach to obtaining large-area SnSe2/SnSe heterostructures with new insight into the formation mechanism of vdW heterostructures, but also opens the intriguing optical applications based on valleytronic nanoheterostructures.

Nonlinear optical absorption and ultrafast carrier dynamics in layered tin diselenide quantum dots

Quantum dots (QDs) derived from typical two-dimensional materials present attractive unique chemical and physical properties because of the quantum-confinement effect. Herein, high-quality layered tin diselenide (SnSe2) QDs with controllable size and thickness were prepared from layered bulk SnSe2 crystals using a simple, effective, and economical mechanical and liquid exfoliation technique. The resulting SnSe2 QDs were subsequently incorporated into chemically stable transparent silica-gel glasses using a sol–gel method. The nonlinear optical (NLO) absorption of the SnSe2 QDs was systematically explored using a combination of open-aperture Z-scan and pump–probe technologies. The derived NLO parameters and ultrafast carrier dynamics of the QDs were comparable to those of reported low-dimensional materials. Interestingly, the layered SnSe2 QDs exhibited thickness/layer-dependent NLO properties and pulse duration-dependent saturable absorption and reverse saturable absorption in both dimethylformamide suspensions and solid silica-gel glasses. Such unique NLO characteristics make layered SnSe2 QDs a promising candidate for technological innovations in areas including optoelectronics and nonlinear optics.

Defect induced Raman shifts and bandgap engineering in layered SnSe2+δ bulks

In the context of the extensive application prospect of two-dimensional (2D) chalcogenides, we synthesized layered SnSe2+δ bulks with defects employing a hybrid chemical vapor transport-melt approach. Both the Eg and A1g Raman characteristic peaks in SnSe2+δ are dominated by cubic anharmonicity, coupled with nonlinear temperature dependencies below 140 K. Notably, the reduction in phonon energy observed in these vibrational modes can be ascribed to defect-mediated Raman scattering, irrespective of deficient or excess Se defects. However, the lower consistency in the Raman shifts of the in-plane Eg vibrations compared to the out-of-plane A1g modes suggests that the defects predominantly entail the absence of Se atoms and the substitutions of Sn by Se, delineating a continuum of Se-deficient and Se-enriched compositions. Furthermore, Se defects induce the contraction of the indirect bandgaps, facilitating a transition from medium to narrow bandgap semiconductors in SnSe2+δ, which underscores the tunable nature of the bandgaps through the incorporation of Se defects. These discoveries present an avenue for bandgap engineering and foster a deeper comprehension of the phonon and thermal properties of layered chalcogenides for further advanced technologies.

Preparation of vertical structure SnS2/SnSe2/H–TiO2 ternary heterojunction and its high performance ultraviolet/visible photodetector

Developing highly efficient heterojunction photodetector, especially ternary heterojunctions of vertical structure, is a promising strategy to enhance the migration of photo-generated electron for exploit potentialities of optoelectronic devices. In this study, we successfully deposited two-dimensional layered SnSe2 NSs and SnS2 NSs vertically on TiO2 NTs arrays using chemical vapor deposition (CVD), constructing a novel ternary SnS2/SnSe2/H–TiO2 heterojunction. The morphology, structure, and composition of the samples were analyzed, and highly sensitive ultraviolet/visible light photodetectors were fabricated. Photocurrent measurements demonstrated that the prepared SnS2/SnSe2/H–TiO2 ternary heterojunction exhibited superior light detection capability at different wavelengths (370 nm, 450 nm, 520 nm) compared to the pristine H–TiO2 and its binary heterojunctions (SnS2/H–TiO2, SnSe2/H–TiO2). Under a bias voltage of 1 V and 370 nm illumination, it achieved a high photocurrent of 7.45 mA cm−2, a switch ratio of 7.924, as well as high responsivity (9.39 A/W), high EQE (3.15 × 103 %), and large detectivity (9.53 × 1010 Jones). The excellent performance of the SnS2/SnSe2/H–TiO2 ternary heterojunction reveals its enormous potential in the development of wide-band, high-performance photodetectors.

Growth and characterization of superconducting bulk crystal [(SnSe)1+δ]m(NbSe2) misfit layer compounds

[(SnSe)1+δ]m(NbSe2) (m = 1–6, 8, and 12) highly orientated crystals 1–2 mm in size and well-defined c-planes were successfully grown using CsCl/KCl flux, including the first growth of crystals with m = 12. The stacked layers along the c axis in the obtained crystals were directly observed by transmission electron microscopy as m alternating layers of SnSe and single layers of NbSe2. The superconducting transition temperature of the obtained [(SnSe)1+δ]m(NbSe2) crystals decreased with an increase in the number of SnSe layers per unit cell. As the superconducting anisotropy parameters increase, a significant increase is observed between m = 4 and 5. This indicates that the superconducting dimensionality becomes more two-dimensional with an increasing m.

Modeling and efficiency enhancement of SnSe thin film solar cell with a thin CIS layer

Research has been conducted on a solar cell utilizing Tin Selenide (SnSe) as the absorber material having a structure of n-CdS/p-SnSe/p+-CuInSe2/p++-WSe2 with CuInSe2 (CIS) current augmenting and WSe2 back surface field (BSF) layers. The various parameters in CdS, SnSe, CIS, and WSe2 layers have been adjusted methodically for the optimum output. Under the optimum conditions, the proposed n-CdS/p-SnSe solar cell showcases a power conversion efficiency (PCE) of 26.12 % with VOC = 0.788 V, JSC = 38.62 mA/cm2, and FF = 85.78 %. The PCE of the device enhances to 33.88 % with VOC = 1.09 V, JSC = 38.64 mA/cm2, and FF = 80.24 % owing to the incorporation of the WSe2 BSF layer. However, a conjunction of the CIS layer with the SnSe achieves a higher JSC of 42.54 mA/cm2 and a PCE of 36.45 %. These in-depth simulation findings demonstrate the enormous potential of the SnSe absorber with the CIS current boosting layer for the creation of a thin film solar cell that is both affordable and highly efficient.

Phase-dependent microstructure modification leads to high thermoelectric performance in n-type layered SnSe2

Microstructure control is crucial for thermoelectrics since it is intimately related to the scattering mechanism of both electrons and phonons. Herein, we propose a new strategy to modify microstructure by phase regulation that simultaneously induces high carrier mobility and low lattice thermal conductivity. As demonstrated in layered SnSe2, the addition of Cu can induce a phase transition from space group P3¯m1 to P63mc. Due to the enlarged formation energy of stacking faults in the later phase, the stacking fault density is greatly reduced after heat treatment that leads to an increased grain size. Accordingly, the carrier mobility of SnSe1.97Br0.03–3 % Cu sample is enhanced by 100 % at room temperature. Furthermore, the reduction of stacking fault density is accompanied by the formation of pores in the matrix, which results in low lattice thermal conductivity. As a result, a record peak zT of 1.13 for SnSe2-based materials is achieved at 773 K, and the attained ZTave of 0.62 is a record-high value among n-type polycrystalline layered materials working in intermediate-to-high temperature region. This work provides a feasible strategy to decouple the electron and phonon transport in layered thermoelectric compounds by phase-dependent microstructure modification.

1D Electronic Flat Bands in Untwisted Moiré Superlattices.

After the preparation of 2D electronic flat band (EFB) in van der Waals (vdW) superlattices, recent measurements suggest the existence of 1D electronic flat bands (1D-EFBs) in twisted vdW bilayers. However, the realization of 1D-EFBs is experimentally elusive in untwisted 2D layers, which is desired considering their fabrication and scalability. Herein, the discovery of 1D-EFBs is reported in an untwisted in situ-grown two atomic-layer Bi(110) superlattice self-aligned on an SnSe(001) substrate using scanning probe microscopy measurements and density functional theory calculations. While the Bi-Bi dimers of Bi zigzag (ZZ) chains are buckled, the epitaxial lattice mismatch between the Bi and SnSe layers induces two 1D buckling reversal regions (BRRs) extending along the ZZ direction in each Bi(110)-11×11 supercell. A series of 1D-EFBs arises spatially following BRRs that isolate electronic states along the armchair (AC) direction and localize electrons in 1D extended states along ZZ due to quantum interference at a topological node. This work provides a generalized strategy for engineering 1D-EFBs in utilizing lattice mismatch between untwisted rectangular vdW layers.

Tuning metal/superconductor to insulator/superconductor coupling via control of proximity enhancement between NbSe2 monolayers

The interplay between charge transfer and electronic disorder in transition-metal dichalcogenide multilayers gives rise to superconductive coupling driven by proximity enhancement, tunneling and superconducting fluctuations, of a yet unwieldy variety. Artificial spacer layers introduced with atomic precision change the density of states by charge transfer. Here, we tune the superconductive coupling between monolayers from proximity-enhanced to tunneling-dominated. We correlate normal and superconducting properties in tailored multilayers with varying SnSe layer thickness (). From high-field magnetotransport the critical fields yield Ginzburg–Landau coherence lengths with an increase of cross-plane (), trending towards two-dimensional superconductivity for . We show cross-overs between three regimes: metallic with proximity-enhanced coupling (), disordered-metallic with intermediate coupling () and insulating with Josephson tunneling (). Our results demonstrate that stacking metal mono- and dichalcogenides allows to convert a metal/superconductor into an insulator/superconductor system, prospecting the control of two-dimensional superconductivity in embedded layers.

Density functional theory study of SnSe2 as anode material for Mg ion battery application

In this work we report, an effort to move beyond lithium ion battery, using density functional theory (DFT) approach to explore the possibility of SnSe2 as an anode material for Mg ion battery. Structural stability and changes in electronic structure on Mg insertion and related charge transfer mechanism has been examined. Results indicate three different types of reaction mechanism on Mg+2 addition to SnSe2. The possible reaction stages are; (a) Mg+2 intercalation in van der Waals gap of SnSe2 structure in the Sn–Se layers, (b) conversion reaction initiated in the semiconducting phase MgSe and metallic Sn, (c) alloying reaction occurring via formation of metallic alloy Mg2Sn. The estimated voltage for these three distinct reaction mechanisms are; (a) ∼1.1 V during Mg+2 intercalation, (b) 0.65 V during conversion reaction, (c) ∼0.26 V during alloying that sustains ultimately. The volume strain is minimal during intercalation (∼10.54%–13.38%) while maximum (∼253.3%–267.7%) during alloying-dealloying reaction. Formation of semiconducting MgSe phase causes loss of capacity during the first cycle operating within voltage window <1.5 V and it accounts for the irreversible capacity loss. The maximum achievable capacity is estimated to be ∼774.8 mAh/g while after first discharge it is likely to stabilize close to ∼640.7 mAh/g for alloying-dealloying reaction. Results indicate feasibility of Mg ion battery in near future.

Wafer-Scale Production of Two-Dimensional Tin Monoselenide: Expandable Synthetic Platform for van der Waals Semiconductor-Based Broadband Photodetectors.

A synthetic platform for industrially applicable two-dimensional (2D) semiconductors that addresses the paramount issues associated with large-scale production, wide-range photosensitive materials, and oxidative stability has not yet been developed. In this study, we attained the 6 in. scale production of 2D SnSe semiconductors with spatial homogeneity using a rational synthetic platform based on the thermal decomposition of solution-processed single-source precursors. The long-range structural and chemical homogeneities of the 2D SnSe layers are manifested using comprehensive spectroscopic analyses. Furthermore, the capability of the SnSe-based photodetectors for broadband photodetection is distinctly verified. The photoresponsivity and detectivity of the SnSe-based photodetectors are 5.89 A W-1 and 1.8 × 1011 Jones at 532 nm, 1.2 A W-1 and 3.7 × 1010 Jones at 1064 nm, and 0.14 A W-1 and 4.3 × 109 Jones at 1550 nm, respectively. The minimum rise times for the 532 and 1064 nm lasers are 62 and 374 μs, respectively. The photoelectrical analysis of the 5 × 5 SnSe-based photodetector array reveals 100% active devices with 95.06% photocurrent uniformity. We unequivocally validated that the air and thermal stabilities of the photocurrent yielded from the SnSe-based photodetector are determined to be >30 d in air and 160 °C, respectively, which are suitable for optoelectronic applications.

Self-limiting stoichiometry in SnSe thin films.

Unique functionalities can arise when 2D materials are scaled down near the monolayer limit. However, in 2D materials with strong van der Waals bonds between layers, such as SnSe, maintaining stoichiometry while limiting vertical growth is difficult. Here, we describe how self-limiting stoichiometry can promote the growth of SnSe thin films deposited by molecular beam epitaxy. The Pnma phase of SnSe was stabilized over a broad range of Sn : Se flux ratios from 1 : 1 to 1 : 5. Changing the flux ratio does not affect the film stoichiometry, but influences the predominant crystallographic orientation. ReaxFF molecular dynamics (MD) simulation demonstrates that, while a mixture of Sn/Se stoichiometries forms initially, SnSe stabilizes as the cluster size evolves. The MD results further show that the excess selenium coalesces into Se clusters that weakly interact with the surface of the SnSe particles, leading to the limited stoichiometric change. Raman spectroscopy corroborates this model showing the initial formation of SnSe2 transitioning into SnSe as experimental film growth progresses. Transmission electron microscopy measurements taken on films deposited with growth rates above 0.25 Å s-1 show a thin layer of SnSe2 that disrupts the crystallographic orientation of the SnSe films. Therefore, using the conditions for self-limiting SnSe growth while avoiding the formation of SnSe2 was found to increase the lateral scale of the SnSe layers. Overall, self-limiting stoichiometry provides a promising avenue for maintaining growth of large lateral-scale SnSe for device fabrication.

Ultralow In-Plane Thermal Conductivity in 2D Magnetic Mosaic Superlattices for Enhanced Thermoelectric Performance.

Lowering thermal conductivity via introducing heterointerfaces of heterophase fillings (HPFs) is a common strategy for optimizing thermoelectric performance, but it is always accompanied by deterioration of electrical conductivity. Here we report an ordered magnetic HPF system in a CoSe2-SnSe mosaic heterostructure superlattice synthesized by van der Waals confined epitaxial growth (vdWCEG), which realizes a maximized filling amount to decrease in-plane thermal conductivity of SnSe layers and maintain the intact in-plane carrier transport path. The in-plane thermal conductivity of CoSe2-SnSe superlattice reaches the lowest range among SnSe-based materials with a value of 0.27 W m-1 K-1 at 850 K, which can be attributed to abundant interfaces between CoSe2 nanocrystals and SnSe layers. Moreover, the CoSe2 nanocrystals show superparamagnetic behavior, by which the rotation of magnetic domains provides additional phonon scattering to further decrease in-plane thermal conductivity. By combination with the preserved in-plane electrical conductivity of SnSe layers, an enhanced in-plane ZT value of 0.62 is achieved at 850 K. This vdWCEG approach can also be generally applied to fabricate various other two-dimensional (2D) mosaic heterostructures, providing an avenue for artificial 2D heterostructures with desired functionalities.

Pipe-diffusion-enriched dislocations and interfaces in SnSe/PbSe heterostructures

Dislocations provide fast diffusion pathways for atoms in semiconductors which can alter compositional profiles of finely tuned heterostructures. We show in model lattice-mismatched IV-VI semiconductor heterostructures of SnSe/PbSe on GaAs substrates that threading dislocations are highly enriched with group IV species of the neighboring layer, with artifacts that reflect the dynamic nature of dislocations when growing dissimilar materials as well as altered bonding properties at the dislocation core. Using atom probe tomography, we characterize one-dimensional nanometer-wide Sn-enriched filaments, which extend downward from SnSe through threading dislocations in the PbSe layer and infiltrate the remote PbSe/GaAs interface through the misfit dislocation network in the short time of growth. Local Sn compositions of only 6%--8% around the dislocations are significantly lower than the 30% Sn expected in bulk PbSe. We estimate the diffusivity of Sn atoms along threading dislocations to be ${10}^{--14}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{--1}$ at 300 \ifmmode^\circ\else\textdegree\fi{}C, approximately three orders of magnitude larger than the lattice diffusivity. In contrast, Pb atoms from PbSe either do not diffuse upward into the orthorhombic SnSe layer or do so only a short distance as one-dimensional filaments before abruptly stopping, likely due to the nature of SnSe on PbSe epitaxy involving lattice mismatch. Beyond compositional anomalies, we detect elevated multiple-atom evaporation events that are spatially correlated to the dislocation, over and above an already high baseline in the matrix. In mixed-bonded IV-VI materials, where previous work has linked such events to the nature of bonding, we find that dislocations show up as distinct from the matrix.

First principles predictions of Na and K storage in layered SnSe2

Recent years have witnessed a flow of research in two-dimensional (2D) materials for the development of new rechargeable metal-ion battery systems due to their large surface areas and plenty accommodating sites for the adsorption of metal ions. Inspired from the experimental synthesis of 2D SnSe2 for Na ion batteries, we speculate the adsorption and diffusion mechanism of Na and K on SnSe2 by means of computational approach. Our results disclose strong adsorption strength with Na and K loading, confirming the favorable electrochemical reactions between adsorbates (Na/K) and the host SnSe2. The electronic structure of the host material is displaying the conductive behavior with Na and K loading. Additionally, the low average voltages for Na (0.68 V) and K (0.48 V), and the high theoretical capacity (387.5 mAhg−1) captivate SnSe2 monolayer for Na- and K- ion batteries. Finally, the activation barriers and the energy pathways for Na and K are calculated. The simulated activation barriers for Na and K are as low as 0.104 eV and 0.113 eV, respectively. The current study endows that Na and K storage for SnSe2 sheet could be industrious and productive for the commercialization of Na- and K- ion batteries.

Fabrication of Highly Textured 2D SnSe Layers with Tunable Electronic Properties for Hydrogen Evolution.

Hydrogen is regarded to be one of the most promising renewable and clean energy sources. Finding a highly efficient and cost-effective catalyst to generate hydrogen via water splitting has become a research hotspot. Two-dimensional materials with exotic structural and electronic properties have been considered as economical alternatives. In this work, 2D SnSe films with high quality of crystallinity were grown on a mica substrate via molecular beam epitaxy. The electronic property of the prepared SnSe thin films can be easily and accurately tuned in situ by three orders of magnitude through the controllable compensation of Sn atoms. The prepared film normally exhibited p-type conduction due to the deficiency of Sn in the film during its growth. First-principle calculations explained that Sn vacancies can introduce additional reactive sites for the hydrogen evolution reaction (HER) and enhance the HER performance by accelerating electron migration and promoting continuous hydrogen generation, which was mirrored by the reduced Gibbs free energy by a factor of 2.3 as compared with the pure SnSe film. The results pave the way for synthesized 2D SnSe thin films in the applications of hydrogen production.

Anisotropic thermoelectric and superconducting properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound

We present the thermoelectric and superconducting properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound with the critical temperature (Tc ~ 3.8 K). From XRD and electrical resistivity, the anisotropic properties of the misfit-layered compound by the preferred orientation were observed. The zero-temperature-limit upper critical field Hc2(0) and coherence length ξ are obtained by Hc2(0) = 8.94 T and ξ = 6.1 nm. The electron-phonon coupling constant (λe-p = 0.735) and the specific heat jump (ΔCe/γTc ~ 1.4) imply that the (SnSe)1.17(TaSe2) compound is a weak coupled conventional s-wave superconductor. Density functional theory (DFT) calculation results show that some electronic charge transfers from the SnSe layers into the TaSe2 layers. In addition, the thermoelectric properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound show significant anisotropic properties.

Effect of Br substitution on thermoelectric transport properties in layered SnSe2

The effect of Br substitution on enhancing the thermoelectric properties of n-type layered SnSe2 is investigated characterizing SnSe2-xBrx synthesized with various Br contents (x = 0, 0.004, 0.008, 0.01, and 0.02). The electrical conductivity systematically increases with an increase in x owing to the simultaneous increase in both the carrier concentration (n) and mobility. As x increases, n increases by ~100 times; however, the magnitude of the Seebeck coefficient (|S|) decreases from 600 to 229 μV/K, resulting in an enhanced power factor at room temperature. The density functional theory calculations support that the valley degeneracy at the conduction band minimum increases with the Br substitution in SnSe2, while the conduction band minimum becomes flatter. This finding indicates that the increased density-of-state effective mass leads to the relatively large |S| even under a considerably increased n. Combined with the slight decrease in the thermal conductivity with an increase in x without significant additional phonon scattering, a thermoelectric figure of merit (zT) of 0.62 at 750 K is obtained for SnSe1.98Br0.02 (x = 0.02), which is 50% higher than that for non-doped SnSe2.

Layer dependence of electronic structure in SnSe using first principle study

Recently, two-dimensional (2D) materials have been studied due to its unique properties and potentials for electronic devices. Tin Selenide (SnSe) is a promising material to be developed in many fields by identifying its electronic structure. In this study, we investigate the effect of layer-dependent electronic properties of SnSe using first-principles calculations based on density functional theory (DFT). We firstly optimized layer dependent of the lattice constant and atomic distortion and then calculate the electronic structure-related parameter including band structure and density of electron (DOS). We find that the calculated band gap decreases with increasing the layers of SnSe which is not dependent on fully relativistic calculation by turning spin orbit coupling (SOC). However, we identify substantial spin splitting in the band structure under the presence of the SOC, making this multilayer is promising for spintronics.