Anisotropic thermoelectric transport properties in polycrystalline SnSe_2

It is reported that SnSe_2 consisting of the same elements as SnSe, is a new promising thermoelectric material with advantageous layered structure. In this work, the thermoelectric performance of polycrystalline SnSe_2 is improved through introducing SnSe phase and electron doping(Cl doped in Se sites). The anisotropic transport properties of SnSe_2 are investigated. A great reduction of the thermal conductivity is achieved in SnSe_2 through introducing SnSe phase, which mainly results from the strong SnSe_2–SnSe inter-phase scattering. Then the carrier concentration is optimized via Cl doping, leading to a great enhancement of the electrical transport properties, thus an extraordinary power factor of ～5.12 μW·cm~(-1)·K~(-2) is achieved along the direction parallel to the spark plasma sintering(SPS) pressure direction( P). Through the comprehensive consideration on the anisotropic thermoelectric transport properties, an enhanced figure of merit ZT is attained and reaches to ～ 0.6 at 773 K in SnSe_2-2% SnSe after 5% Cl doping along the P direction, which is much higher than ～ 0.13 and ～ 0.09 obtained in SnSe_2-2% SnSe and pristine SnSe_2 samples, respectively.     

Synthesis and thermoelectric properties of Bi-doped SnSe thin films

Bi doped n-type SnSe thin films were prepared by chemical vapor deposition (CVD) and their structure and thermoelectric properties were studied. The x-ray diffraction patterns, x-ray photoelectron spectroscopy, and microscopic images show that the prepared SnSe thin films were composed of pure SnSe crystals. The Seebeck coefficients of the Bi-doped SnSe were greatly improved compared to that of undoped SnSe thin films. Specifically, Sn_0.99Bi_0.01Se thin film exhibited a Seebeck coefficient of -905.8μV·K^-1 at 600 K, much higher than 285.5 μV·K^-1 of undoped SnSe thin film. Further first-principles calculations reveal that the enhancement of the thermoelectric properties can be explained mainly by the Fermi level lifting and the carrier pockets increasing near the Fermi level due to Bi doping in the SnSe samples. Our results suggest the potentials of the Bi-doped SnSe thin films in thermoelectric applications.     

Observation of pseudogap in SnSe2 atomic layers grown on graphite

Superconducting metal dichalcogenides (MDCs) present several similarities to the other layered superconductors like cuprates. The superconductivity in atomically thin MDCs has been demonstrated by recent experiments, however, the investigation of the superconductivity intertwined with other orders are scarce. Investigating the pseudogap in atomic layers of MDCs may help to understand the superconducting mechanism for these true two-dimensional (2D) superconducting systems. Herein we report a pseudogap opening in the tunneling spectra of thin layers of SnSe₂ epitaxially grown on highly oriented pyrolytic graphite (HOPG) with scanning tunneling microscopy/spectroscopy (STM/STS). A significant V-shaped pseudogap was observed to open near the Fermi level (E_F) in the STS. And at elevated temperatures, the gap gradually evolves to a shallow dip. Our experimental observations provide direct evidence of a pseudogap state in the electron-doped SnSe₂ atomic layers on the HOPG surface, which may stimulate further exploration of the mechanism of superconductivity at 2D limit in MDCs.     

二维InSe/SnSe2范德华异质结的电子结构和光学特性研究

本文构建了三种不同堆叠形式下的二维InSe/SnSe₂范德华异质结模型,利用基于密度泛函理论的第一性原理方法综合考察了二维InSe/SnSe₂三种不同堆叠情况下的几何构型及稳定性,在此基础上选取具有最稳定性能的构型.该异质结呈现出Ⅱ型能带对齐特征,带隙值为1.118 eV,可以实现电子-空穴的有效分离.另外,相比与单层二维InSe/SnSe₂范德华异质结的光吸收能力达到明显提升,在紫外光范围内吸收系数达到10~6 cm^-1.研究结果将为相关物理实验及机理研究提供理论基础,对二维InSe/SnSe₂范德华异质结在光电器件中的应用具有重要的物理意义.     

Electrodeposition and growth mechanism of SnSe thin films

Tin selenide (SnSe) thin films were electrochemically deposited onto Au(1 1 1) substrates from an aqueous solution containing SnCl₂, Na₂SeO₃, and EDTA at room temperature (25 °C). The electrochemical behaviors and the codeposition potentials of Sn and Se were explored by cyclic voltammetry. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and UV-vis absorption spectroscopy were employed to characterize the thin films. When the electrodeposition potential increased, the Se content in the films decreased. It was found that the stoichiometric SnSe thin films could be obtained at −0.50 V. The as-deposited films were crystallized in the preferential orientation along the (1 1 1) plane. The morphologies of SnSe films could be changed from spherical grains to platelet-like particles as the deposition potential increases. The SEM investigations show that the film growth proceeds via nucleation, growth of film layer and formation of needle-like particles on the overlayer of the film. The optical absorption study showed the film has direct transition with band gap energy of 1.3 eV.     

Understanding the role of defects in influencing the thermoelectric properties of SnSe

As compared to single crystals, polycrystalline SnSe shows a considerable decline in its ZT value. Optimization of carrier concentration by the way of chemical doping is useful but creates point defect and vacancies that are often overlooked. Here we study polycrystalline Sn_0.95M_0.05Se (M = Co, Ni, In) with an aim to understand the role of defects. The overall crystal structure and microstructure of SnSe is not much affected with substitution as evident from X-ray diffraction and scanning electron microscopy study. Rietveld refinement confirms the single phase nature of the all compositions and provides unit cell parameters. Analysis of the stoichiometry reveals the presence of cation vacancies. Optical spectroscopy indicates a degradation of the in-direct gap and Urbach band tail-width fitting confirms the presence of localized states within the gap. Electrical resistivity and Seebeck coefficient are adversely affected by defects, but thermal conductivity decreases by almost 50% of SnSe value.     

Local probe of the interlayer coupling strength of few-layers SnSe by contact-resonance atomic force microscopy

The interlayer bonding in two-dimensional (2D) materials is particularly important because it is not only related to their physical and chemical stability but also affects their mechanical, thermal, electronic, optical, and other properties. To address this issue, we report the direct characterization of the interlayer bonding in 2D SnSe using contact-resonance atomic force microscopy (CR-AFM) in this study. Site-specific CR spectroscopy and CR force spectroscopy measurements are performed on both SnSe and its supporting SiO₂/Si substrate comparatively. Based on the cantilever and contact mechanic models, the contact stiffness and vertical Young’s modulus are evaluated in comparison with SiO₂/Si as a reference material. The interlayer bonding of SnSe is further analyzed in combination with the semi-analytical model and density functional theory calculations. The direct characterization of interlayer interactions using this non-destructive methodology of CR-AFM would facilitate a better understanding of the physical and chemical properties of 2D layered materials, specifically for interlayer intercalation and vertical heterostructures.     

The effect of Te doping on the electronic structure and thermoelectric properties of SnSe

SnSe_1−xTe_x (x=0, 0.0625) bulk materials were fabricated by melting Sn, Se and Te powders and then hot pressing them at various temperatures. The phase compositions of the materials were determined by X-ray diffraction (XRD) and the crystal lattice parameters were refined by the Rietveld method performed with DBWS. XRD analysis revealed that the grains in the materials preferentially grew along the (l 0 0) directions. The structural behavior of SnSe_1−xTe_x (x=0, 0.0625) was calculated using CASTEP package provided by Materials Studio. We found that the band gap of SnSe reduced from 0.643 to 0.608 eV after Te doping. The calculated results were in good agreement with experimental results. The electrical conductivity and the Seebeck coefficient of the as-prepared materials were measured from room temperature to 673 K. The maximum power factor of SnSe is ∼0.7 μW cm⁻¹K⁻² at 673 K.     

High thermoelectric performance and low thermal conductivity in K-doped SnSe polycrystalline compounds

SnSe single crystal showed a high thermoelectric zT of 2.6 at 923 K mainly due to an extremely low thermal conductivity 0.23 W m⁻¹ K⁻¹. It has anisotropic crystal structure resulting in deterioration of thermoelectric performance in polycrystalline SnSe, providing a low zT of 0.6 and 0.8 for Ag and Na-doped SnSe, respectively. Here, we presented the thermoelectric properties on the K-doped K_xSn_1−xSe (x = 0, 0.1, 0.3, 0.5, 1.5, and 2.0%) polycrystals, synthesized by a high-temperature melting and hot-press sintering with annealing process. The K-doping in SnSe efficiently enhances the hole carrier concentration without significant degradation of carrier mobility. We find that there exist widespread Se-rich precipitates, inducing strong phonon scattering and thus resulting in a very low thermal conductivity. Due to low thermal conductivity and moderate power factor, the K_0.001Sn_0.999Se sample shows an exceptionally high zT of 1.11 at 823 K which is significantly enhanced value in polycrystalline compounds.     

Modulation of magnetic properties of bilayer SnSe with transition-metals doping in the interlayer

We investigate the spin-polarized electronic and magnetic properties of bilayer SnSe with transition-metal (TM) atoms doped in the interlayer by using a first-principles method. It shows that Ni dopant cannot induce the magnetism in the doped SnSe sheet, while the ground state of V, Cr, Mn, Fe and Co doped systems are magnetic and the magnetic moment mainly originates from 3d TM atom. Two types of factors, which reduce the magnetic moment of TM atoms doped in bilayer SnSe, are identified as spin-up channel of the 3d orbital loses electrons to SnSe sheet and spin-down channel of the 3d orbital gains electrons from 4s orbital. The spin polarization is found to be 100% at Fermi level for the Mn and Co atoms doped system, while the Ni-doped system is still a semiconductor with a gap of 0.26 eV. These results are potentially useful for development of spintronic devices.     

激光分离卵磷脂混合物的实验研究

对于沸点相近并且分子自由程相差不大的物质,常规分离方法难以实现有效的分离,激光分离有效地弥补了常规分离方法的不足.用脉冲CO2激光器和连续CO2激光器对大豆卵磷脂混合物进行了激光分离实验,用高效液相色谱对激光辐照前后的样品进行了分析.经192.31 W/cm2的脉冲CO2激光辐照,卵磷脂的质量分数相对提高了26.7%,...     

Structural characterisation and thermal stability of SnSe\GaSb stacked films

We 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.     

快重离子辐照对MoS2热导率的影响研究

利用0.97 GeV的209Bi离子辐照二硫化钼（MoS₂）晶体,辐照注量范围为1×1010～1×1012 ions/cm2,结合原子力显微镜（AFM）观测和Raman光谱分析研究了快重离子辐照对MoS₂热导率的影响。实验结果显示,快重离子辐照在MoS₂中产生了潜径迹,较高激光功率下的Raman测试使样品局部温度升高,导致E_1/2g和A_1g峰随注量增加向低波数方向移动,且峰形展宽。引入了通过改变激光功率测量Raman光谱得到MoS₂热导率的计算方法,获得了不同辐照注量下MoS₂的热导率的定量分析结果,随注量增加,热导率不断降低,从未辐照样品的563 W/mK下降到1×1012 ions/cm2辐照时的132 W/mK。Molybdenum disulphide (MoS₂) was irradiated by 0.97 GeV 209Bi ions with the fluence of 1×1010 to 1×1012 ions/cm2. The irradiation effect on the thermal conductivity of MoS₂ was analyzed by atomic force microscope (AFM) and Raman spectroscopy. The experimental results show that hillock-like latent tracks are observed on irradiated MoS₂ by AFM. The measurement of MoS₂ by Raman spectrometer with high laser power results in the increase of local temperature of MoS₂, which cause the downshift of peaks position and broadening of E_1/2g and A_1g peak. Furthermore, according to Raman spectra measured at different laser power, thermal conductivity of MoS₂ before and after irradiation was calculated, which show that the thermal conductivity of MoS₂ decreases with increasing fluence, from 563 to 132 W/mK for pristine and 1×1012 ions/cm2 irradiated MoS₂, respectively.     

Structural instability of PbSe/SnSe superlattices under pressure

Abstract

PbSe/SnSe superlattice, phase transition, high pressure, SR x-ray diffraction)

Synchrotron x-ray diffraction experiments have revealed successive phase transitions in epitaxially-grown PbSe/SnSe superlattices. The transition pressures from the low-pressre cubic B1- to the high-pressure orthorhombic B16-type structures are observed to vary systematically depending upon thickness of the PbSe layer. For example, a [PbSe(36A)/SnSe(12A)]₁₉, with the B1 structure in both layers stabilized in its asgrown state, undergoes the [B1/B1]-to-[B1/B16] and [B1/B16]-to-[B16/B16] structural transitions at 1.9 and 3.8GPa, respectively. This result is in contrast to their bulk data that the B1-to-B16 transition takes place at 5.3GPa in PbSe while the B16 phase is stable in SnSe at atmospheric pressure.     

利用Er3+光学温度传感特性研究Tm3+/Yb3+掺杂NaY(WO4)2微米球的激光辐照热效应

采用微波水热方法合成了Er³⁺/Yb³⁺及Tm³⁺/Yb³⁺两个共掺杂的绣球花状NaY（WO₄）₂微米球样品。XRD结果表明所获得的产物为纯相体心结构的NaY（WO₄）₂,利用SEM观察发现产物粒子结构为纳米片组装成的绣球花状。考虑到红外激光辐照对样品产生加热效应,采用380 nm激发下不同温度的发射光谱获得了Er³⁺/Yb³⁺共掺杂NaY（WO₄）₂微米球的温度传感特性曲线和灵敏度曲线。把Er³⁺/Yb³⁺共掺杂NaY（WO₄）₂与Tm³⁺/Yb³⁺共掺杂的NaY（WO₄）₂按质量比1：10进行混合,采用980 nm激发测量了混合物的上转换发光光谱,研究了激光持续辐照对Tm³⁺/Yb³⁺掺杂NaY（WO₄）₂样品的加热效应和Tm³⁺的¹G₄→³H₆ 跃迁发光强度随激光辐照时间的变化。实验发现980 nm激光辐照使Tm³⁺/Yb³⁺掺杂NaY（WO₄）₂样品的温度持续升高并达到某一平衡温度,Tm³⁺的蓝色上转换发光也随着激光辐照时间的延长而增强,最后达到饱和。此外,在相同条件下,Tm³⁺/Yb³⁺共掺杂样品的激光辐照热效应比Er³⁺/Yb³⁺共掺杂样品的热效应更为显著。     

CO2激光局域辐照对熔石英损伤特性的影响

研究了CO₂激光局域辐照对熔石英损伤特性的影响, 发现当辐照中心温度较低时(1139 K), 辐照对损伤阈值没有明显影响, 但辐照中心温度较高时(1638 K), 辐照对损伤阈值有明显的影响, 损伤阈值随距离辐照中心间距的增大而减小, 在残余应力产生光程差最大处附近, 损伤阈值降到最小, 随着与辐照中心间距的进一步增加, 损伤阈值略有上升. 对导致此现象的原因做了分析. 由于残余应力的存在, 在辐照中心发生再损伤产生的裂纹后, 裂纹先沿径向扩展, 在残余应力产生光程差最大处附近, 裂纹转而向切向扩展, 这可能与径向和环向张应力随半径的变化有关. 在采用热处理炉退火消除残余应力时, 必须注意元件的洁净处理, 否则退火会出现析晶现象, 对损伤阈值和透射率造成不良影响. 关键词： 2激光局域辐照')" href="#">CO₂激光局域辐照 熔石英 损伤特性     

Absolute spectral distribution of soft X-ray emission from laserproduced gold plasma

Quantitative measurement of spectral distribution of soft X-ray emission from laser produced plasmas and estimation of X-ray conversion efficiency are reported. The X-ray emission from planar gold target irradiated by an Nd:glass laser was recorded using a high resolution transmission grating spectrograph. Spectral distribution of X-ray intensity was derived using calibrated film data and a deconvolution procedure to account for contribution of higher diffraction orders. Results of conversion efficiency per unit solid angle, at a laser intensity of ~4×10¹² W/cm² (λ_L = 1.054 μm), for 10< λ <80 Å and in the water window spectral region (23< λ< 44 Å) are presented. A three-fold increase in conversion efficiency was observed for second harmonic laser irradiation (λ_L = 0.527 μm) at an intensity of 8×10¹² W/cm²     

Microstructure and hardening effect of pure tungsten and ZrO2 strengthened tungsten under carbon ion irradiation at 700℃

Microstructure evolution and hardening effect of pure tungsten and W-1.5%ZrO₂ alloy under carbon ion irradiation are investigated by using transmission electron microscopy and nano-indentation. Carbon ion irradiation is performed at 700 ℃ with irradiation damages ranging from 0.25 dpa to 2.0 dpa. The results show that the irradiation defect clusters are mainly in the form of dislocation loop. The size and density of dislocation loops increase with irradiation damages intensifying. The W-1.5%ZrO₂ alloy has a smaller dislocation loop size than that of pure tungsten. It is proposed that the phase boundaries have the ability to absorb and annihilate defects and the addition of ZrO₂ phase improves the sink strength for irradiation defects. It is confirmed that the W-1.5%ZrO₂ alloy shows a smaller change in hardness than the pure tungsten after being irradiated. From the above results, we conclude that the addition of ZrO₂ into tungsten can significantly reduce the accumulation of irradiated defects and improve the irradiation resistance behaviors of the tungsten materials.     

SnSe分子外场下的基态性质和激发态性质

对Sn原子使用SDB-cc-pVTZ基组, Se原子采用6-311++G^**基组, 利用密度泛函中的B3LYP方法研究了电场强度为-0.04–0.04 a.u.的外电场对SnSe基态分子的几何结构、 电荷布居分布、 HOMO能级、 LUMO能级、 能隙、 费米能级、 谐振频率和红外光谱强度的影响. 继而使用含时密度泛函(TD-B3LYP) 方法研究了SnSe分子在外场下的激发特性. 结果表明, 外电场的大小和方向对SnSe分子基态的这些性质有明显影响. 在所加的电场范围内(-0.04 a.u.–0.04 a.u.), 随着正向电场的增大, 核间距先减小后增大, 在F=0.03 a .u.时取得最小值0.2317 nm; 分子电偶极矩μ近似线性地增大; E_L, E_H、 费米能级E_F和能隙E_g均减小. 随着正向电场逐渐增大, 分子总能量和谐振频率均先增大后减小; 红外谱强度则先减小后增大, 在F=0.03 a.u.时, 取得最小值 0.1138 km·mol^-1. 由基态到第1–10个单重激发态的波长均随着正向电场的增大而增大. 激发能均随着正向电场的增大而减小. 电场的引入可改变SnSe分子激发态出现的顺序并使得一些禁止的跃迁变得可能. 关键词： SnSe 外电场 能隙 激发特性     

激光辐照VO2薄膜温度场分布及透射特性研究

为了探究VO₂薄膜受激光辐照的温度场分布,以及1 064 nm激光直接辐照100 s内至相变的激光功率密度阈值,并比较近红外和中红外波段透过率调制特性差异。首先基于COMSOL建立了薄膜受激光辐照的模型并进行了温度场仿真,然后分别测试了薄膜正反面被不同功率密度的1 064 nm激光辐照100 s内激光透过率随时间响应特性。实验中的VO₂薄膜利用分子束外延法在Al₂O₃基底上制备得到。仿真结果表明,激光功率密度为25 W·mm^-2时,50 nm厚薄膜在被辐照1 ms时间内即达到相变温度。经激光辐照实验发现:50 nm厚的VO₂薄膜正反面受1 064 nm激光直接辐照100 s内至相变的功率密度阈值分别为4.1 W·mm^-2和5.39 W·mm^-2。30 nm厚VO₂薄膜对1 064 nmn激光的透过率调制深度约为13%,对3 459 nm激光透过率调制深度约62%,说明VO₂薄膜对近红外透过率调制特性不明显。