Enhanced thermoelectric performance through homogenously dispersed MnTe nanoparticles in p-type Bi_(0.52)Sb_(1.48)Te_3 nanocomposites

In this work,we report that the thermoelectric properties of Bi_(0.52)Sb_(1.48)Te_3alloy can be enhanced by being composited with Mn Te nano particles(NPs)through a combined ball milling and spark plasma sintering(SPS)process.The addition of Mn Te into the host can synergistically reduce the lattice thermal conductivity by increasing the interface phononscattering between Bi_(0.52)Sb_(1.48)Te_3 and MnTe NPs,and enhance the electrical transport properties by optimizing the hole concentration through partial Mn~(2+)acceptor doping on the Bi~(3+)sites of the host lattice.It is observed that the lattice thermal conductivity decreases with increasing the percentage of Mn Te and milling time in a temperature range from 300 Kto 500 K,which is consistent with the increasing of interfaces.Meanwhile,the bipolar effect is constrained to high temperatures,which results in the figure of merit z T peak shifting toward higher temperature and broadening the z T curves.The engineering z T is obtained to be 20%higher than that of the pristine sample for the 2-mol%Mn Te-added composite at a temperature gradient of 200 K when the cold end temperature is set to be 300 K.This result indicates that the thermoelectric performance of Bi_(0.52)Sb_(1.48)Te_3 can be considerably enhanced by being composited with Mn Te NPs.     

Point-contact tunneling spectroscopy between a Nb tip and an ideal topological insulator Sn-doped Bi_(1.1)Sb_(0.9)Te_2S

The lightly Sn-doped Bi_(1.1)Sb_(0.9)Te_2S is a good material to investigate the pure topological surface state because the bulk bands are far away from the Fermi level. By measuring point-contact tunneling spectra on the topological insulator Bi_(1.08)Sn_(0.02)Sb_(0.9)Te_2S samples with a superconducting Nb tip, we observed the suppression of differential conductance near zero bias, instead of the enhancement due to Andreev reflection on the spectra. The fitting to the measured spectrum results in a superconducting gap of more than 4 meV, and this value is much larger than the superconducting gap of the bulk Nb. The gaped feature exists at temperatures even above the critical temperature of bulk Nb, and is visible when the magnetic field is as large as 9 T at 3 K. We argue that such behaviors may be related to the pressure induced superconductivity by the tip in the junction area, or just some novel phenomena arising from the junction between an s-wave superconductor and an ideal topological insulator.     

Fundamental and progress of Bi_2Te_3-based thermoelectric materials

Thermoelectric materials,enabling the directing conversion between heat and electricity,are one of the promising candidates for overcoming environmental pollution and the upcoming energy shortage caused by the over-consumption of fossil fuels.Bi_2Te_3-based alloys are the classical thermoelectric materials working near room temperature.Due to the intensive theoretical investigations and experimental demonstrations,significant progress has been achieved to enhance the thermoelectric performance of Bi_2Te_3-based thermoelectric materials.In this review,we first explored the fundamentals of thermoelectric effect and derived the equations for thermoelectric properties.On this basis,we studied the effect of material parameters on thermoelectric properties.Then,we analyzed the features of Bi_2Te_3-based thermoelectric materials,including the lattice defects,anisotropic behavior and the strong bipolar conduction at relatively high temperature.Then we accordingly summarized the strategies for enhancing the thermoelectric performance,including point defect engineering,texture alignment,and band gap enlargement.Moreover,we highlighted the progress in decreasing thermal conductivity using nanostructures fabricated by solution grown method,ball milling,and melt spinning.Lastly,we employed modeling analysis to uncover the principles of anisotropy behavior and the achieved enhancement in Bi_2Te_3,which will enlighten the enhancement of thermoelectric performance in broader materials     

Effects of Thickness and Temperature on Thermoelectric Properties of Bi_2Te_3-Based Thin Films

Bi_2Te_3 thin films and GeTe/B_2Te_3 superlattices of different thicknesses are prepared on the silicon dioxide substrates by magnetron sputtering technique and thermally annealed at 573 K for 30 min. Thermoelectric(TE)measurements indicate that optimal thickness and thickness ratio improve the TE performance of Bi_2Te_3 thin films and GeTe/B_2Te_3 superlattices, respectively. High TE performances with figure-of-merit(ZT) values as high as 1.32 and 1.56 are achieved at 443 K for 30 nm and 50 nm Bi_2Te_3 thin films, respectively. These ZT values are higher than those of p-type Bi_2Te_3 alloys as reported. Relatively high ZT of the GeTe/B_2Te_3 superlattices at 300-380 K were 0.62-0.76. The achieved high ZT value may be attributed to the unique nano-and microstructures of the films,which increase phonon scattering and reduce thermal conductivity. The results indicate that Bi_2Te_3-based thin films can serve as high-performance materials for applications in TE devices.     

Transport properties of doped Bi_2Se_3 and Bi_2Te_3 topological insulators and heterostructures

In this review article, the recent experimental and theoretical research progress in Bi_2Se_3-and Bi_2Te_3-based topological insulators is presented, with a focus on the transport properties and modulation of the transport properties by doping with nonmagnetic and magnetic elements. The electrical transport properties are discussed for a few different types of topological insulator heterostructures, such as heterostructures formed by Bi_2Se_3-and Bi_2Te_3-based binary/ternary/quaternary compounds and superconductors, nonmagnetic and magnetic metals, or semiconductors.     

Electrically Tunable Wafer-Sized Three-Dimensional Topological Insulator Thin Films Grown by Magnetron Sputtering

Three-dimensional(3 D) topological insulators(TIs) are candidate materials for various electronic and spintronic devices due to their strong spin-orbit coupling and unique surface electronic structure.Rapid,low-cost preparation of large-area TI thin films compatible with conventional semiconductor technology is the key to the practical applications of TIs.Here we show that wafer-sized Bi_2 Te_3 family TI and magnetic TI films with decent quality and well-controlled composition and properties can be prepared on amorphous SiO_2/Si substrates by magnetron cosputtering.The SiO_2/Si substrates enable us to electrically tune(Bi_(1-x)Sb_x)_2 Te_3 and Cr-doped(Bi_(1-x)Sb_x)_2 Te_3 TI films between p-type and n-type behavior and thus study the phenomena associated with topological surface states,such as the quantum anomalous Hall effect(QAHE).This work significantly facilitates the fabrication of TI-based devices for electronic and spintronic applications.     

Enhanced thermoelectric performance in p-type Mg_3Sb_2 via lithium doping

The Zintl compound Mg_3Sb_2 has been recently identified as promising thermoelectric material owing to its high thermoelectric performance and cost-effective,nontoxicity and environment friendly characteristics.However,the intrinsically p-type Mg_3Sb_2 shows low figure of merit(z T = 0.23 at 723 K) for its poor electrical conductivity.In this study,a series of Mg_(3-x)Li_xSb2 bulk materials have been prepared by high-energy ball milling and spark plasma sintering(SPS) process.Electrical transport measurements on these materials revealed significant improvement on the power factor with respect to the undoped sample,which can be essentially attributed to the increased carrier concentration,leading to a maximum z T of0.59 at 723 K with the optimum doping level x = 0.01.Additionally,the engineering z T and energy conversion efficiency are calculated to be 0.235 and 4.89%,respectively.To our best knowledge,those are the highest values of all reported p-type Mg_3Sb_2-based compounds with single element doping.     

Se substitution and micro-nano-scale porosity enhancing thermoelectric Cu_2Te

Binary Cu-based chalcogenide thermoelectric materials have attracted a great deal of attention due to their outstanding physical properties and fascinating phase sequence.However,the relatively low figure of merit z T restricts their practical applications in power generation.A general approach to enhancing z T value is to produce nanostructured grains,while one disadvantage of such a method is the expansion of grain size in heating-up process.Here,we report a prominent improvement of z T in Cu_2Te_(0.2)Se_(0.8),which is several times larger than that of the matrix.This significant enhancement in thermoelectric performance is attributed to the formation of abundant porosity via cold press.These pores with nano-to micrometer size can manipulate phonon transport simultaneously,resulting in an apparent suppression of thermal conductivity.Moreover,the Se substitution triggers a rapid promotion of power factor,which compensates for the reduction of electrical properties due to carriers scattering by pores.Our strategy of porosity engineering by phonon scattering can also be highly applicable in enhancing the performances of other thermoelectric systems.     

Pressure-Induced Topological and Structural Phase Transitions in an Antiferromagnetic Topological Insulator

Recently,natural van der Waals heterostructures of(MnBi_2 Te_4)_m(Bi_2 Te_3)_n have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states.We systematically investigate both the structural and electronic responses of MnBi_2 Te_4 and MnBi_4 Te_7 to external pressure.In addition to the suppression of antiferromagnetic order,MnBi_2 Te_4 is found to undergo a metalsemiconductor-metal transition upon compression.The resistivity of MnBi_4 Te_7 changes dramatically under high pressure and a non-monotonic evolution of p(T) is observed.The nontrivial topology is proved to persist before the structural phase transition observed in the high-pressure regime.We find that the bulk and surface states respond differently to pressure,which is consistent with the non-monotonic change of the resistivity.Interestingly,a pressure-induced amorphous state is observed in MnBi_2 Te_4,while two high-pressure phase transitions are revealed in MnBi_4 Te_7.Our combined theoretical and experimental research establishes MnBi_2 Te_4 and MnBi_4 Te_7 as highly tunable magnetic topological insulators,in which phase transitions and new ground states emerge upon compression.     

Femtosecond Tm-Ho co-doped fiber laser using a bulk-structured Bi_2Se_3 topological insulator

We experimentally demonstrate a femtosecond mode-locked thulium-holmium(Tm-Ho) co-doped fiber laser incorporating a saturable absorber(SA) based on a bulk-structured bismuth selenide(Bi_2Se_3) topological insulator(TI). The SA was prepared by depositing a mechanically exfoliated Bi_2Se_3 TI layer onto a side-polished optical fiber platform. Unlike high-quality nano-structured Bi_2Se_3 TI-based SA, bulk-structured Bi_2Se_3 with non-negligible oxidation was used as a saturable absorption material for this experimental demonstration due to its easy fabrication process. The saturation power and modulation depth of the prepared SA were measured to be ~ 28.6 W and ~13.4%, respectively. By incorporating the prepared SA into a Tm-Ho co-doped fiber ring cavity, stable soliton pulses with a temporal width of ~ 853 fs could be generated at 1912.12 nm. The 3-dB bandwidth of the mode-locked pulse was measured to be ~4.87 nm. This experimental demonstration reaffirms that Bi_2Se_3 is a superb base material for mid-infrared passive mode-locking even under oxidation.     

Visualizing the in-Gap States in Domain Boundaries of Ultra-Thin Topological Insulator Films

Ultra-thin topological insulators provide a platform for realizing many exotic phenomena such as the quantum spin Hall effect, and quantum anomalous Hall effect. These effects or states are characterized by quantized transport behavior of edge states. Experimentally, although these states have been realized in various systems,the temperature for the edge states to be the dominating channel in transport is extremely low, contrary to the fact that the bulk gap is usually in the order of a few tens of milli-electron volts. There must be other in-gap conduction channels that do not freeze out until a much lower temperature. Here we grow ultra-thin topological insulator Bi_2Te_3 and Sb_2Te_3 films by molecular beam epitaxy and investigate the structures of domain boundaries in these films. By scanning tunneling microscopy and spectroscopy we find that the domain boundaries with large rotation angles have pronounced in-gap bound states, through which one-dimensional conduction channels are suggested to form, as visualized by spatially resolved spectroscopy. Our work indicates the critical role played by domain boundaries in degrading the transport properties.     

Raman spectra and XPS studies of phase changes in Ge2Sb2Te5 films

Ge_2Sb_2Te_5 film was deposited by RF magnetron sputtering on Si (100) substrate. The structure of amorphous and crystalline Ge_2Sb_2Te_5 thin films was investigated using XRD, Raman spectra and XPS. XRD measurements revealed the existence of two different crystalline phases, which has a FCC structure and a hexagonal structure, respectively. The broad peak in the Raman spectra of amorphous Ge_2Sb_2Te_5 film is due to the amorphous －Te－-Te－ stretching. As the annealing temperature increases, the broad peak separates into two peaks, which indicates that the heteropolar bond in GeTe_4 and the Sb－Sb bond are connected with four Te atoms, and other units such as (TeSb) Sb－Sb (Te_2) and (Sb_2) Sb－Sb (Te_2), where some of the four Te atoms in the above formula are replaced by Sb atoms, remain in crystalline Ge_2Sb_2Te_5 thin film. And from the results of Raman spectra and XPS, higher the annealing temperature, more Te atoms bond to Ge atoms and more Sb atoms substitute Te in (Te_2) Sb－Sb (Te_2).     

Growth and transport properties of topological insulator Bi_2Se_3 thin film on a ferromagnetic insulating substrate

Exchange coupling between topological insulator and ferromagnetic insulator through proximity effect is strongly attractive for both fundamental physics and technological applications. Here we report a comprehensive investigation on the growth behaviors of prototype topological insulator Bi_2Se_3 thin film on a single-crystalline LaCoO_3 thin film on SrTiO_3 substrate, which is a strain-induced ferromagnetic insulator. Different from the growth on other substrates, the Bi_2Se_3 films with highest quality on LaCoO_3 favor a relatively low substrate temperature during growth. As a result, an inverse dependence of carrier mobility with the substrate temperature is found. Moreover, the magnetoresistance and coherence length of weak antilocalization also have a similar inverse dependence with the substrate temperature, as revealed by the magnetotransport measurements. Our experiments elucidate the special behaviors in Bi_2Se_3/LaCoO_3 heterostructures,which provide a good platform for exploring related novel quantum phenomena, and are inspiring for device applications.     

Gate Tunable Supercurrent in Josephson Junctions Based on Bi_2Te_3 Topological Insulator Thin Films

We report transport measurements on Josephson junctions consisting of Bi_2Te_3 topological insulator(TI) thin films contacted by superconducting Nb electrodes.For a device with junction length L=134 nm,the critical supercurrent I_c can be modulated by an electrical gate which tunes the carrier type and density of the TI film.I_c can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state.In the p-type regime the Josephson current can be well described by a short ballistic junction model.In the n-type regime the junction is ballistic at 0.7 K T 3.8 K while for T 0.7 K the diffusive bulk modes emerge and contribute a larger I_c than the ballistic model.We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p-n junctions.Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices.     

Antimony Selenide Thin Film Solar Cells with an Electron Transport Layer of Alq_3

We fabricated Sb_2 Se_3 thin film solar cells using tris(8-hydroxy-quinolinato) aluminum(Alq_3) as an electron transport layer by vacuum thermal evaporation.Another small organic molecule of N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine(NPB) was used as a hole transport layer.We took ITO/NPB/Sb_2Se_3/Alq_3/Al as the device architecture.An open circuit voltage(V_(oc)) of 0.37 V,a short circuit current density(J_(sc)) of 21.2 mA/cm~2,and a power conversion efficiency(PCE) of 3.79% were obtained on an optimized device.A maximum external quantum efficiency of 73% was achieved at 600 nm.The J_(sc),V_(oc),and PCE were dramatically enhanced after introducing an electron transport layer of Alq_3.The results suggest that the interface state density at Sb_2 Se_3/Al interface is decreased by inserting an Alq_3 layer,and the charge recombination loss in the device is suppressed.This work provides a new electron transport material for Sb_2Se_3 thin film solar cells.     

Site Preference of Se and Te in Bi_2Se_(3-x)Te_x Thin Films

The ternary topological insulators Bi_2Se_(3-x)Te_x have attracted a great deal of attention due to their exotic physical and chemical properties.While most of the studies focus on the properties of these ternary TIs,limited research was performed to investigate the dynamic atomic stack of its crystal structure.We prepared highquality Bi_2Se_(3-x)Te_x thin films on Ga As(111)B substrates using molecular beam epitaxy,characterized with Raman spectroscopy,x-ray diffraction and photoelectron spectroscopy.It is found that when Se is replaced by Te,the preferred substituting sites are the middle layer at 0x1,and this is also valid for Se substituting Te at 2x3.In the middle region,the substituting atoms prefer to go to the first and the fifth layer.     

Thermoelectric effects and topological insulators

The recent discovery of topological insulators(TIs) offers new opportunities for the development of thermoelectrics,because many TIs(like Bi_2Te_3) are excellent thermoelectric(TE) materials.In this review,we will first describe the general TE properties of TIs and show that the coexistence of the bulk and boundary states in TIs introduces unusual TE properties,including strong size effects and an anomalous Seebeck effect.Importantly,the TE figure of merit zT of TIs is no longer an intrinsic property,but depends strongly on the geometric size.The geometric parameters of twodimensional TIs can be tuned to enhance zT to be significantly greater than 1.Then a few proof-of-principle experiments on three-dimensional TIs will be discussed,which observed unconventional TE phenomena that are closely related to the topological nature of the materials.However,current experiments indicate that the metallic surface states,if their advantage of high mobility is not fully utilized,would be detrimental to TE performance.Finally,we provide an outlook for future work on topological materials,which offers great possibilities to discover exotic TE effects and may lead to significant breakthroughs in improving zT.     

Anomalous Hall Effect in Layered Ferrimagnet MnSb_2Te_4

We report on low-temperature electron transport properties of MnSb_2Te_4,a candidate of ferrimagnetic Weyl semimetal.Long-range magnetic order is manifested as a nearly square-shaped hysteresis loop in the anomalous Hall resistance,as well as sharp jumps in the magnetoresistance.At temperatures below 4K,a lnT-type upturn appears in the temperature dependence of longitudinal resistance,which can be attributed to the electron-electron interaction(EEI),since the weak localization can be excluded by the temperature dependence of magnetoresistance.Although the anomalous Hall resistance exhibits a similar lnT-type upturn in the same temperature range,such correction is absent in the anomalous Hall conductivity.Our work demonstrates that MnSb_2Te_4 microflakes provide an ideal system to test the theory of EEI correction to the anomalous Hall effect.     

GeSe2-Sb2Se3-CsCl玻璃的光学性质与析晶动力学研究

采用熔融淬冷法制备了20GeSe₂-（80-x）Sb₂Se₃-xCsCl（x=2,4,8,10 mol%）硫卤玻璃系统,测试了样品在可见至中远红外区域的透过光谱,研究了硫卤玻璃中CsCl对玻璃短波吸收限的影响;并测试了典型的20GeSe₂-76Sb₂Se₃-4CsCl样品在不同升温速率下的DTA曲线,利用非等温方法研究了其析晶动力学; 关键词： 硫卤玻璃 非等温过程 析晶动力学     

Multinary diamond-like chalcogenides for promising thermoelectric application

Thermoelectric(TE)materials have been considered as a strong candidate for recovering the waste heat from industry and vehicles due to the ability to convert heat directly into electricity.Recently,multinary diamond-like chalcogenides(MDLCs),such as Cu In Te_2,Cu_2Sn Se_3,Cu_3Sb Se_4,Cu_2ZnSnSe_4,etc.,are eco-friendly Pb-free TE materials with relatively large Seebeck coefficient and low thermal conductivity and have aroused intensive research as a popular theme in the TE field.In this review,we summarize the TE performance and device development of MDLCs.The features of crystalline and electronic structure are first analyzed,and then the strategies that have emerged to enhance the TE figure of merits of these materials are illustrated in detail.The final part of this review describes the advance in TE device research for MDLCs.In the outlook,the challenges and future directions are also discussed to promote the further development of MDLCs TE materials.