SnSe Films Research Articles

Bi doping-induced ferromagnetism of layered material SnSe2 with extremely large coercivity

Ferromagnetism of two-dimensional layered materials is of considerable interest in spintronics. In the present work, we report the ferromagnetism of the layered material SnSe2 induced by Bi doping. The pure SnSe2 and Bi-doped SnSe2 film samples are prepared by chemical vapor transport method. X-ray diffractions show that both the SnSe2 and Bi-doped SnSe2 have CdI2-type layered structure grown perfectly along the c-axis. Magnetic measurements indicate that the pure SnSe2 is a diamagnet. The doping of nonmagnetic Bi introduces ferromagnetism in SnSe2 with Curie temperature of 65 K. A coercivity as high as 4.4 kOe is observed at T = 10 K. Such high coercivity has never been reported in d0 two-dimensional layered materials. First-principles calculations indicate that the Bi-doping leads to the spin splitting of degenerate px and py orbital states and gives rise to a spin order aligned in ab plane. Calculations also show that the Bi-doped SnSe2 is a half metal. All these features make Bi-doped SnSe2 a potential candidate for spintronic material.

2D SnSe/Si heterojunction for self-driven broadband photodetectors

Van der Waals heterojunctions based on atomically thin 2D materials have opened up new realms in modern semiconductor industry. However, it is still challenging to fabricate large-area ultrathin 2D films. Herein, we successfully fabricate wafer-size 2D SnSe films on Si substrate by magnetron sputtering technique, enabling the formation of SnSe/Si van der Waals (vdWs) heterojunction device. The high-resolution transmission electron microscopy is employed to character the structure of SnSe film and SnSe/Si heterojunction with ideal orthorhombic structure and atomically abrupt interface, respectively. The energy diagram of SnSe/Si heterojunction is constructed, exhibiting similar barrier heights for electron and hole carrier. The SnSe/Si heterojunction shows obvious diode behavior with rectification ratio of ~1.6 × 104, forward current of ~194.5 mA cm−2 at ±1.0 V. Furthermore, owing to the high crystalline orientation, specific energy-band alignment, as well as the strong built-in electrical field, the SnSe/Si heterojunction illustrates a broadband photodetecting properties with the wavelength ranging from ultraviolet to near-infrared light, showing a high detectivity of 4.4 × 1012 cmHz1/2 W−1, a high responsivity of 566.4 mA mW−1 and an ultrafast response/recovery time of ~1.6/47.7 µs under zero external bias. This work provides a new strategy for fabrication of low cost 2D optoelectronic devices with high-performance.

Preparation and photoelectrochemical properties of SnS/SnSe and SnSe/SnS bilayer structures fabricated via electrodeposition

In this work, electrodeposition was employed to deposit a series of thin films, including SnS, SnSe, SnS/SnSe, and SnSe/SnS, onto ITO conductive glass, which were then characterized by XRD, EDS, XPS, SEM, and UV–Vis absorption spectrophotometry. The XRD and SEM results verified the successful preparation of these films, while the EDS and XPS results suggested that the atomic ratio approached 1 for the SnS and SnSe films. According to the UV–Vis absorption spectra, the optical absorption properties were greatly improved for the SnS/SnSe and SnSe/SnS bilayer films compared with those of the monolayer films. For SnS and SnSe, direct band gaps of 1.82 and 1.29 eV and indirect band gaps of 1.03 and 0.89 eV, respectively, were separately obtained from the calculations. The photoelectrochemical properties of the as-fabricated films were further investigated under simulated sunlight, and excellent photoresponses and photostabilities were exhibited by all the samples. The photocurrent densities of SnS, SnSe, SnS/SnSe and SnSe/SnS films were 22, 19, 26, and 20 μA/cm2, respectively.

Self-power position-sensitive detector with fast optical relaxation time and large position sensitivity basing on the lateral photovoltaic effect in tin diselenide films

Layered tin diselenide (SnSe2) is of high interest to material scientists because it is an n-type semiconductor with a narrow bandgap. In this letter, we prepared single-phase SnSe2 films and investigated the lateral photovoltaic effect (LPE) in SnSe2/p-Si junctions. The XRD, XPS, and Raman spectra confirm that the SnSe2 film is of high quality. The LPE shows a linear dependence on the position of the laser spot. The observed position sensitivity is as high as 364.5 mV mm−1. The optical relaxation-time for the LPE is about 3.07 μs. The dependence of the LPE on laser power and laser wavelength suggest that the LPE originates from the photoelectric effect and not from the thermoelectric effect. The nonlinearly current-voltage curve on the surface of the SnSe2 film indicates that most of the laser-excited electrons diffuse laterally within the inversion layer at the SnSe2/p-Si interface. This results in a large LPE with a fast relaxation time. The self-power, fast optical relaxation time and large position sensitivity of the LPE opens new opportunities for optical position-sensitive detector of SnSe2.

The effects of ion implantation on rhenium and tin dichalcogenide ultrathin films

Investigations were carried out on the effects of rhenium and tin chalcogenide ultrathin films using an ion implantation method for specific characteristics and wide applications. ReS2, ReSe2, SnS2, and SnSe2 films were implanted with 300 keV O ions at a fluence of 1 × 1014 ions/cm2. The research focused on the evolution of the morphology, crystallinity and structure of the films through atomic force microscopy, X-ray diffraction, Raman scattering and photoluminescence before and after ion implantation. The results showed diverse changes in different films, such as thickness decreases in the ReS2, ReSe2, and SnS2 films and smoothing effects in the ReS2, ReSe2, and SnSe2 films. In terms of crystallinity, small decrements were observed only in ReSe2 and SnSe2 films. The Raman modes of ReS2 film exhibited redshifts which also appeared in the photoluminescence peaks of ReS2, ReSe2, SnS2, and SnSe2 films. The results indicate feasible modification of film properties by ion implantation, which also bring new possibilities and options to ultrathin material research.

Electronic structure of SnSe2 films grown by molecular beam epitaxy

SnSe2 is a layered main-group metal dichalcogenide that has exhibited gate-tunable interfacial superconductivity as well as promising optoelectronic applications. Here, we synthesize SnSe2 films by molecular beam epitaxy and investigate their electronic structure with angle-resolved photoemission spectroscopy (ARPES). A comparison between density functional theory calculations and ARPES data from a thick film reveals the importance of spin-orbit coupling and out-of-plane dispersion in the SnSe2 valence bands, which were neglected in previous studies of its electronic structure. We conclude that the conduction band minimum lies along the M-L direction in momentum space, while the valence band maximum lies along Γ−K.

Study on thermoelectric properties of co-evaporated Sn-Se films with different phase formations

In this study, polycrystalline thin films of 330 nm in thickness with different SnSe and SnSe2 contents were prepared by co-evaporation of Sn and Se through adjusting source temperatures and substrate temperature. The result illustrates that the film with pure SnSe phase is p-type and consists by bulk-like nanoparticles, its resistivity is high up to 139 mΩ·m, which results in a low thermoelectric performance. The films with the coexistence of SnSe and SnSe2 phases are comprised of nanosheets. The existence of SnSe2 phase changes the conductivity type of the films to n-type and reduces resistivity. At the same time, the nanosheets perpendicular to the substrate can improve the Seebeck coefficient by filtering the low energy carrier. When the SnSe2 phase content is 34%, the Seebeck coefficient and the power factor of the film can reach the maximum, 255 μV/K and 155 μW/(m·K2) at 250 °C, respectively.

Passively Q-switched self-frequency doubling Nd3+:ReCa4O(BO3)3 (Re = Y, Gd) lasers with tin diselenide as a saturable absorber

With tin diselenide (SnSe2) film as a saturable absorber (SA), the passively Q-switched self-frequency doubling (SFD) lasers were realized in Nd3+:ReCa4O(BO3)3 (Re = Y, Gd) crystals. For Nd:YCa4O(BO3)3 crystal, the maximum average output power at 532 nm was 19.6 mW, and the corresponding pulse repetition frequency, pulse duration, single pulse energy, and peak power were 17.6 kHz, 91.9 ns, 1.1 μJ, and 12.1 W, respectively. For Nd:GdCa4O(BO3)3 crystal, these values were 14.5 mW, 22.1 kHz, 48.7 ns, 0.66 μJ, and 13.5 W.

Enhanced thermoelectric properties of SnSe thin films grown by single-target magnetron sputtering

SnSe thin films were successfully grown using single-target magnetron sputtering. The SnSe thin film annealed at 700 K exhibits superior thermoelectric performance compared with previously reported SnSe films and polycrystalline SnSe bulk material.

Effect of stress layer on thermal properties of SnSe2 few layers

Layered SnSe2 have much attention due to their potential novel applications in electronic devices areas, such as photo-transistors, solar cells and memory devices. We investigate the thermal properties of few layer SnSe2 with capping of Al2O3 layer using the temperature and power-dependent Raman spectroscopy. The first order temperature coefficient of A1g modes of SnSe2 layers on a SiO2/Si substrate was found to be −0.01757 cm−1/K, which is much higher than the corresponding value −0.01270 cm−1/K of SnSe2 with Al2O3 layer. The difference in temperature coefficients is attributed to compressive stress introduced by the Al2O3 stress layer. Meanwhile, the Al2O3 layer significantly increases the thermal conductivity to 3.102 W/mK, which is larger than that of the SnSe2 films without Al2O3 (∼2.378 W/mK). The first principle calculation using CASTP demonstrated that this improvement of thermal conductivity is attributed to the compressive strain. This work suggests an effective way to improve the performances of SnSe2 based devices, and also need to understand the vibrational properties and electron-phonon interactions at the interface of Al2O3 and SnSe2 layers.

Observation of interface superconductivity in a SnSe2 /epitaxial graphene van der Waals heterostructure

We report on the direct observation of interface superconductivity in single-unit-cell SnSe2 films grown on graphitized SiC(0001) substrate by means of van der Waals epitaxy. Tunneling spectrum in the superconducting state reveals rather conventional character with a fully gapped order parameter. The occurrence of superconductivity is further confirmed by the presence of vortices under external magnetic field. Through interface engineering, we unravel the mechanism of superconductivity that originates from a two-dimensional electron gas formed at the interface of SnSe2 and graphene. Our finding opens up novel strategies to hunt for and understand interface superconductivity based on van der Waals heterostructures.

Enhanced Cross-Plane Thermoelectric Transport of Rotationally Disordered SnSe2 via Se-Vapor Annealing.

We report cross-plane thermoelectric measurements of SnSe and SnSe2 films grown by the modulated element reactant (MER) approach. These materials exhibit ultralow cross-plane thermal conductivities, which are advantageous for thermoelectric energy conversion. The initially grown SnSe films have relatively low cross-plane Seebeck coefficients (-38.6 μV/K) due to significant unintentional doping originating from Se vacancies when annealed in nitrogen, as a result of the relatively high vapor pressure of Se. By performing postgrowth annealing at a fixed Se partial pressure (300 °C for 30 min using SnSe2 as the Se source in a sealed tube), a transition from SnSe to SnSe2 is induced, which is evidenced by clear changes in the X-ray diffraction patterns of the films. This results in a 16-fold increase in the cross-plane Seebeck coefficient (from -38.6 to -631 μV/K) after Se annealing due to both the SnSe-to-SnSe2 transition and the mitigation of unintentional doping by Se vacancies. We also observe a corresponding 6-fold drop in the electrical conductivity (from 3 to 0.5 S/m) after Se annealing, which is consistent with both a drop in the carrier concentration and an increase in band gap. The power factor S2σ increased by 44× (from 4.5 nW/m·K2 to 0.2 μW/m·K2) after Se annealing. We believe that these results demonstrate a robust method for mitigating unintentional doping in a promising class of materials for thermoelectric applications.

Structural characterisation and thermal stability of SnSe\\GaSb stacked films

ABSTRACTWe have investigated the effect of thermal annealing on the structure of single and stacked phase change memory films based on SnSe and GaSb. Samples were prepared by pulsed laser deposition and investigated by X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) methods. Electrical resistance versus temperature investigations showed crystallisation temperatures of 292°C and 198°C for SnSe and GaSb single films, respectively. Above the transition temperature, GaSb crystallises into a face-centered cubic structure, whereas SnSe has an orthorhombic arrangement. Annealing at three temperatures (160°C, 250°C and 350°C) of the SnSe\\GaSb stacked films promotes bond breaking, atom diffusion between the two layers and formation of new phases. At 160°C, GaSb films crystallise partially and no effect is observed on the crystallinity of SnSe films. After 250°C, rhombohedral SnSb emerges in addition to GaSb complete crystallisation. A major, completely new, body-centered orthorhombic unindexed quaternary Ga-Sn-Sb-Se phase formation was observed in the samples annealed at 350°C. The GaSb crystallites are fully dissolved and we have observed the formation of a minor hexagonal SnSe2 phase. The analysis of EXAFS data, measured at Se and Ga K-edges, revealed changes in the local atomic environment as a function of the annealing temperature. A tetrahedral configuration is obtained for the Ga atoms in both as-deposited and annealed samples, whereas Se is mostly bivalent in the amorphous samples and has an octahedral arrangement in crystalline SnSe. Our results show that inter-layer diffusion should always be considered and evaluated when designing memory cells composed of stacked phase change chalcogenide films.

Investigation on Structural, Optical and Morphological Properties of the SnSe Films with Function of Annealing Temperatures

The work, studies on the impact of annealing on Tin Selenide thin films prepared by thermal evaporation technique. The films were annealed at different temperatures (150 °C–350 °C) inside the vacuum chamber under the pressure of 10−5 to 10−6 torr. The effects of annealing on the SnSe films were studied by various characterization techniques. The X-ray diffraction study reveals the as prepared SnSe films have orthorhombic crystal structure and annealing enhance the crystalline nature. The grain size is found to be 29 nm to 35 nm. The FE-SEM images observed that both uniform shaped grains of the annealing. The Selenium (Se) content has reduced after annealing due to re-evaporation of Se at higher annealing temperatures. The optical band gap values are found to be varying between 1.1 eV and 2.79 eV.

Controllable Colloidal Synthesis of Tin(II) Chalcogenide Nanocrystals and Their Solution-Processed Flexible Thermoelectric Thin Films.

A systematic colloidal synthesis approach to prepare tin(II, IV) chalcogenide nanocrystals with controllable valence and morphology is reported, and the preparation of solution-processed nanostructured thermoelectric thin films from them is then demonstrated. Triangular SnS nanoplates with a recently-reported π-cubic structure, SnSe with various shapes (nanostars and both rectangular and hexagonal nanoplates), SnTe nanorods, and previously reported Sn(IV) chalcogenides, are obtained using different combinations of solvents and ligands with an Sn4+ precursor. These unique nanostructures and the lattice defects associated with their Sn-rich composition allow the production of flexible thin films with competitive thermoelectric performance, exhibiting room temperature Seebeck coefficients of 115, 81, and 153 μV K-1 for SnS, SnSe, and SnTe films, respectively. Interestingly, a p-type to n-type transition is observed in SnS and SnSe due to partial anion loss during post-synthesis annealing at 500 °C. A maximum figure of merit (ZT) value of 0.183 is achieved for an SnTe thin film at 500 K, exceeding ZT values from previous reports on SnTe at this temperature. Thus, a general strategy to prepare tin(II) chalcogenide nanocrystals is provided, and their potential for use in high-performance flexible thin film thermoelectric generators is demonstrated.

Enhanced photoresponse of graphene oxide functionalised SnSe films

The successful synthesis of large-area SnSe films on SiO2/Si substrate through vapor transport is reported. Based on this we fabricated photodetectors that exhibited a fast response from ultraviolet (UV) to near infrared (NIR) wavelengths. By decorating the SnSe film with graphene oxide quantum dots (GO QDs) the photocurrent and photoresponse time were enhanced significantly over the whole spectrum. Our results indicate that GO functionalised SnSe is a promising material for broadband photodetection.

Thin Film Tin Selenide (SnSe) Thermoelectric Generators Exhibiting Ultralow Thermal Conductivity

Tin selenide (SnSe) has attracted much attention in the field of thermoelectrics since the discovery of the record figure of merit (ZT) of 2.6 ± 0.3 along the b-axis of the material. The record ZT is attributed to an ultralow thermal conductivity that arises from anharmonicity in bonding. While it is known that nanostructuring offers the prospect of enhanced thermoelectric performance, there have been minimal studies in the literature to date of the thermoelectric performance of thin films of SnSe. In this work, preferentially orientated porous networks of thin film SnSe nanosheets are fabricated using a simple thermal evaporation method, which exhibits an unprecedentedly low thermal conductivity of 0.08 W m-1 K-1 between 375 and 450 K. In addition, the first known example of a working SnSe thermoelectric generator is presented and characterized.

Electrical characterization of nanocrystalline SnSe and ZnSe thin films: effect of annealing

In this paper, experimental data on the electrical properties of as deposited and annealed nanocrystalline SnSe and ZnSe thin films are reported. The thin films of SnSe and ZnSe are deposited on glass substrate by chemical bath deposition method. The films are studied before and after thermal annealing at temperatures 473 K for 1 h. This annealing is done in vacuum of 2 × 10−3 mbar. The various electrical parameters like dark conductivity, photoconductivity, activation energy, photosensitivity and carrier life time have been measured on these films before and after annealing.

Electrochemically synthesis and optoelectronic properties of Pb- and Zn-doped nanostructured SnSe films

In this study, electrodeposition technique was applied to deposit un-, lead (Pb), and zinc (Zn)-doped SnSe films. X-ray diffraction (XRD) patterns of the films showed a polycrystalline SnSe phase with orthorhombic crystalline lattice. SEM images revealed ball-shaped, rod-shaped, and wire-shaped morphologies for SnSe films. Moreover, optical measurements indicated incorporation of dopant in the crystalline lattice of films by varying the optical energy band gap. Electrical characterization of Pb- and Zn-doped SnSe films showed their p-type nature. Finally, the solar cell device fabricated using the Zn-doped SnSe films reveal a higher efficiency because of their higher carrier concentration.

Thermoelectric prospects of chemically deposited PbSe and SnSe thin films

Thin films of PbSe of 400–600 nm in thickness, were obtained via chemical deposition from a solution containing lead nitrate, thiourea and selenosufate. SnSe thin films of 90–180 nm in thickness, were also obtained by chemical deposition from a solution containing selenosulfate. Optical and electrical properties of these thin films were significantly altered by heating them in selenium vapor at 300 °C. Thin film PbSe has a bandgap (Eg) of 1.17 eV (direct gap, forbidden transitions), which decreases to 0.77 eV when it has been heated. Its electrical conductivity (σ) is p-type: 0.18 Ω−1 cm−1 (as-prepared), and 6.4 Ω−1 cm−1 when heated. Thin film SnSe is of orthorhombic crystalline structure which remains stable when heated at 300 °C, but its Eg increases from 1.12 eV (indirect) in as-prepared film to 1.5 eV (direct, forbidden transitions) upon heating. Its electrical conductivity is p-type, which increases from 0.3 Ω−1 cm−1 (as-prepared) to 1 Ω−1 cm−1 when heated (without Se-vapor). When SnSe film is heated at 300 °C in the presence of Se-vapor, they transform to SnSe2, with Eg of 1.5 eV (direct, forbidden) with n-type electrical conductivity, 11 Ω−1 cm−1. The Seebeck coefficient for the PbSe films is: +0.55 mV K−1 (as prepared) and +0.275 mV K−1 (heated); for SnSe films it is: +0.3 mV K−1 (as prepared) and +0.20 mV K−1 (heated); and for SnSe2 film, − 0.35 mV K−1. A five-element PbSe-SnSe2-PbSe-SnSe2-PbSe thermoelectric device demonstrated 50 mV for a temperature difference ΔT = 20 °C (2.5 mV K−1). For SnSe-SnSe2-SnSe-SnSe2-SnSe device, the value is 15 mV for ΔT = 20 °C (0.75 mV K−1). Prospect of these thin films in thermoelectric devices of hybrid materials, in which the coatings may be applied on distinct substrate and geometries is attractive.