Characteristics of electrodeposited bismuth telluride thin films with different crystal growth by adjusting electrolyte temperature and concentration

Bismuth telluride (Bi₂Te₃) thin films were prepared with various electrolyte temperatures (10°C–70 °C) and concentrations [Bi(NO₃)₃ and TeO₂: 1.25–5.0 mM] in this study. The surface morphologies differed significantly between the experiments in which these two electrodeposition conditions were separately adjusted even though the applied current density was in the same range in both cases. At higher electrolyte temperatures, a dendrite crystal structure appeared on the film surface. However, the surface morphology did not change significantly as the electrolyte concentration increased. The dendrite crystal structure formation in the former case may have been caused by the diffusion lengths of the ions increasing with increasing electrolyte temperature. In such a state, the reactive points primarily occur at the tops of spiked areas, leading to dendrite crystal structure formation. In addition, the in-plane thermoelectric properties of Bi₂Te₃ thin films were measured at approximately 300 K. The power factor decreased drastically as the electrolyte temperature increased because of the decrease in electrical conductivity due to the dendrite crystal structure. However, the power factor did not strongly depend on the electrolyte concentration. The highest power factor [1.08 μW/(cm·K²)] was obtained at 3.75 mM. Therefore, to produce electrodeposited Bi₂Te₃ films with improved thermoelectric performances and relatively high deposition rates, the electrolyte temperature should be relatively low (30 °C) and the electrolyte concentration should be set at 3.75 mM.     

Optical and thermoelectric characterizations of electroplated n-Bi2(Te0.9Se0.1)3

Bismuth selenotelluride (Bi₂(Te_0.9Se_0.1)₃) films were electrodeposited at constant current density from acidic aqueous solutions with Arabic gum in order to produce thin films for miniaturized thermoelectric devices. X-ray fluorescence spectroscopy determined film compositions. X-ray diffraction pattern shows that the films as deposited are polycrystalline, isostructural to Bi₂Te₃ and covered by crystallites. Mueller-matrix analysis reveals that the electroplated layers are optically like an isotropic medium. Their pseudo-dielectric functions were determined using mid-infrared spectroscopic ellipsometry. Tauc-Lorentz combined with Drude dispersion relations were successfully used. The energy band gap E_g was found to be about 0.15 eV. Moreover, the fundamental absorption edge was described by an indirect optical band-to-band transition. From Seebeck coefficient measurement, films exhibit n-type charge carrier and the value of thermoelectric power is about −40 μV/K.     

Bi-Sb-Te基热电薄膜温差电池离子束溅射制备与表征

本文采用离子束溅射Bi/Te和Sb/Te二元复合靶,直接制备n型Bi₂Te₃热电薄膜和p型Sb₂Te₃热电薄膜.在退火时间同为1 h的条件下,对所制备的Bi₂Te₃薄膜和Sb₂Te₃薄膜进行不同温度的退火处理,并对其热电性能进行表征.结果表明,在退火温度为150 ℃时,制备的n型Bi₂Te₃关键词： 薄膜温差电池 2Te₃薄膜')" href="#">Sb₂Te₃薄膜 2Te₃薄膜')" href="#">Bi₂Te₃薄膜 离子束溅射     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

Electrodeposition of BixSb2−xTey thermoelectric thin films from nitric acid and hydrochloric acid systems

The electrochemical behaviors of Bi^III, Te^IV and Sb^III single ions and their mixtures were investigated in nitric acid and hydrochloric acid system separately. Based on which, Bi_xSb_2−xTe_y thermoelectric films were prepared by potentiostatic electrodeposition from the solutions with different concentrations of Bi^III, Te^IV and Sb^III in the two acid systems. The morphologies, compositions, structures, Seebeck coefficients and resistivities of the deposited thin films were characterized and compared by ESEM (or FESEM), EDS, XRD, Seebeck coefficient measurement system and four-probe resistivity measuring device respectively. The results show that although Bi_xSb_2−xTe_y thermoelectric thin film which structure is consistent with the standard pattern of Bi_0.5Sb_1.5Te₃ can be gained in both of the two acid solutions by adjusting the deposition potential, their morphologies and thermoelectric properties have big differences in different acid solutions.     

Effect of annealing temperature on photoelectrochemical properties of nanocrystalline MoBi2(Se0.5Te0.5)5 thin films

Nanocrystalline MoBi₂(Se_0.5Te_0.5)₅ thermoelectric thin films have been deposited on ultrasonically cleaned glass and FTO-coated glass substrates by Arrested Precipitation Technique. The change in properties of MoBi₂(Se_0.5Te_0.5)₅ thin films were examined after annealing at the temperature 473 K for 3 h. The structural, morphological, compositional and electrical properties of thin films were characterized by X-ray Diffraction, Scanning Electron Microscopy, Energy Dispersive Spectroscopy, etc. Thermoelectric properties of the thin films have been evaluated by measurements of electrical conductivity and Seebeck coefficient in the temperature range 300–500 K. Our aim is to investigate the effect of annealing on behaviour of MoBi₂(Se_0.5Te_0.5)₅ thin films along with photoelectrochemical properties.     

Annealing effects on the structural and electrical transport properties of n-type Bi2Te2.7Se0.3 thin films deposited by flash evaporation

N-type Bi₂Te_2.7Se_0.3 thermoelectric thin films with thickness 800 nm have been deposited on glass substrates by flash evaporation method at 473 K. Annealing effects on the thermoelectric properties of Bi₂Te_2.7Se_0.3 thin films were examined in the temperature range 373-573 K. The structures, morphology and chemical composition of the thin films were characterized by X-ray diffraction, field emission scanning electron microscope and energy dispersive X-ray spectroscopy, respectively. Thermoelectric properties of the thin films have been evaluated by measurements of the electrical resistivity and Seebeck coefficient at 300 K. The Hall coefficients were measured at room temperature by the Van der Pauw method. The carrier concentration and mobility were calculated from the Hall coefficient. The films thickness of the annealed samples was measured by ellipsometer. When annealed at 473 K, the electrical resistivity and Seebeck coefficient are 2.7 mΩ cm and −180 μV/K, respectively. The maximum of thermoelectric power factor is enhanced to 12 μW/cm K².     

Formation of highly textured (1 1 1) Bi2O3 films by anodization of electrodeposited bismuth films

Highly textured bismuth oxide (Bi₂O₃) thin films have been prepared using anodic oxidation of electrodeposited bismuth films onto stainless steel substrates. The Bi₂O₃ films were uniform and adherent to substrate. The Bi₂O₃ films were characterized for their structural and electrical properties by means of X-ray diffraction (XRD), electrical resistivity and dielectric measurement techniques. The X-ray diffraction pattern showed that Bi₂O₃ films are highly textured along (1 1 1) plane. The room temperature electrical resistivity of the Bi₂O₃ films was 10⁵ Ω cm. Dielectric measurement revealed normal oxide behavior with frequency.     

在预刻蚀的衬底上通过分子束外延直接生长出拓扑绝缘体薄膜的微器件

在利用光刻将拓扑绝缘体外延薄膜加工成微米尺寸结构的过程中,所用的各种化学物质会导致薄膜质量的下降.在实验中,通过在钛酸锶衬底上预先光刻出Hall bar形状的凸平台并以此为模板进行拓扑绝缘体(Bi x Sb1-x)2Te3薄膜的分子束外延生长,直接获得了薄膜的Hall bar微器件,从而避免了光刻过程对材料质量的影响.原子力显微镜和输运测量结果均显示该微器件保持了(Bi x Sb1-x)2Te3外延薄膜原有的性质.这种新的微器件制备方法有助于在拓扑绝缘体中实现各种新奇的量子效应,并可推广于其他外延生长的低维系统.     

Effect of heat treatment on the structure and the thermoelectric properties of Sb<Subscript>0.9</Subscript>Bi<Subscript>1.1</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> thin films and composites based on them

This work considers the effect of vacuum annealing on the thermoelectric properties of Sb_0.9Bi_1.1Te_2.9Se_0.1 thin film and Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites with various carbon contents produced by ion-beam deposition in an argon atmosphere. The electrical resistivity and the thermopower of Sb_0.9Bi_1.1Te_2.9Se_0.1–C nanocomposites are found to be dependent on not only the carbon concentration but also the type and the concentration of intrinsic point defects of the Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution, which determine the type of conductivity of Sb_0.9Bi_1.1Te_2.9Se_0.1 granules. The power factors are estimated for films of Sb_0.9Bi_1.1Te_2.9Se_0.1 solid solution and films of Sb_0.9Bi_1.1Te_2.9Se_0.1–C composites and found to have values comparable with the values for nanostructured materials on the basis of (Bi,Sb)₂(Te,Se)₃ solid solutions.     

Preferential growth of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> films with a nanolayer structure: enhancement of thermoelectric properties induced by nanocrystal boundaries

Preferential growth of different crystal planes in layered Bi₂Te₃ thin films with each layer <40 nm has been achieved by a simple magnetron co-sputtering method. The preferential growth of (015) plane or (001) was achieved at special depositing conditions due to the more sufficient growth along the in-plane direction induced by the enhanced diffusion of atoms and lower deposition rate. The Bi₂Te₃ film with preferential growth of (001) plane possesses about two times higher electrical conductivity and Seebeck coefficient as compared to the film with preferential growth of (015) plane, due to the greatly enhanced carrier mobility. Furthermore, the thermal conductivity has been suppressed due to more phonon scattering at grain boundaries, compared with ordinary Bi₂Te₃ alloys and films.     

Electronic structures and thermoelectric properties of La or Ce-doped Bi2Te3 alloys from first principles calculations

Thermoelectric properties of La or Ce-doped Bi₂Te₃ alloys were systematically investigated by ab initio calculations of electronic structures and Boltzmann transport equations. The Seebeck coefficient of p-type LaBi₇Te₁₂ and La₂Bi₆Te₁₂ was larger than that of Bi₂Te₃, because La doping increased the effective mass of carriers. On the other hand, the electrical conductivity of LaBi₇Te₁₂ and La₂Bi₆Te₁₂ decreased, which caused a reduction of power factor of these La-doped Bi₂Te₃ alloys in comparison with Bi₂Te₃. The influence of Ce doping on the band structure and thermoelectric properties of Bi₂Te₃ was similar to that of La doping. The theoretical calculation provided an insight into the transport properties of La or Ce-doped Bi₂Te₃-based thermoelectric materials.     

Laser-induced forward transfer of intact chalcogenide thin films: resultant morphology and thermoelectric properties

We present a laser-based transfer method for the novel application of fabricating elements for planar thermoelectric devices. Thin films of thermoelectric chalcogenides (Bi₂Te₃, Bi₂Se₃ and Bi_0.5Sb_1.5Te₃) were printed via laser-induced forward transfer (LIFT) onto polymer-coated substrates over large areas of up to ～15 mm² in size. A morphological study showed that it was possible to partially preserve the polycrystalline structure of the transferred films. The films’ Seebeck coefficients after LIFT transfer were measured and resulted in ?49±1 μV/K, ?93±8 μV/K and 142±3 μV/K for Bi₂Te₃, Bi₂Se₃ and Bi_0.5Sb_1.5Te₃, respectively, which were found to be ～23±6 % lower compared to their initial values. This demonstration shows that LIFT is suitable to transfer sensitive, functional semiconductor materials over areas up to ～15 mm² with minimal damage onto a non-standard polymer-coated substrate.     

The effect of (00l) crystal plane orientation on the thermoelectric properties of Bi2Te3 thin film

Highly (00l)-oriented pure Bi₂Te₃ films with in-plane layered grown columnar nanostructure have been fabricated by a simple magnetron co-sputtering method. Compared with ordinary Bi₂Te₃ film and bulk materials, the electrical conductivity and Seebeck coefficient of such films have been greatly increased simultaneously due to raised carrier mobility and electron scattering parameter, while the thermal conductivity has been decreased due to phonon scattering by grain boundaries between columnar grains and interfaces between each layers. The power factor has reached as large as 33.7 μW cm⁻¹ K⁻², and the out-of-plane thermal conductivity is reduced to 0.86 W m⁻¹ K⁻¹. Our results confirm that tailoring nanoscale structures inside thermoelectric films effectively enhances their performances.     

A significant enhancement in thermoelectric power factor of In2Te3 thin films by Se doping and Te composition tuning

Thermoelectric power factor of a material significantly relies on its electrical conductivity, thermal conductivity, and Seebeck coefficient. Herein, an attempt has been made to enhance the thermoelectric power factor of In₂Te₃ thin films by tuning their Te composition and via Se doping. The optimum Se-doping concentration and Te composition enhanced the power factor of pristine In₂Te₃ films by 14 and 7.4 times, respectively. The modified chemical composition, structural characteristics, and surface morphological features of In₂Te₃ films are observed to be pivotal in improving their thermoelectric power factor. Overall, this study offers a facile approach to control the thermoelectric power factor of In₂Te₃ thin films which is significant for their futuristic applications.     

Pulsed laser deposition of Bi2Te3 thermoelectric films

Bi₂Te₃ is one of the most used materials for thermoelectric applications at ambient temperature. An improvement of thermoelectric performances through a suitable modification of electron and phonon transport mechanisms is predicted for low dimensional or nanostructured systems, but this requires a control of the material structure down to the nanoscale. We show that pulsed laser deposition provides control on film composition, phase and structure, necessary for a comprehension of the relationship between structure and thermoelectric properties. We have explored the role of deposition temperature, background inert gas type and pressure, laser fluence and target-to-substrate distance and we found the experimental condition ranges to obtain crystalline films containing the Bi₂Te₃ phase only, by comparing energy dispersive X-ray spectroscopy, Raman spectroscopy and X-ray diffraction analysis. Variations of substrate temperature and deposition gas pressure prove to be crucial also for the control of film morphology and crystallinity. Substrate type has no influence on film stoichiometry and crystallinity, but highly oriented growth can be achieved on mica due to van der Waals epitaxy.     

Hydrothermal synthesis and microstructure investigation of nanostructured bismuth telluride powder

Nanostructured thermoelectric Bi₂Te₃ powders with various morphologies were hydrothermally synthesized using different precursors and routes to give an experimental comprehension on the formation of the nanopowders. It was found that the polyhedral Bi₂Te₃ particles are formed by surface nucleation in a continuous nucleation process, the hexagonal Bi₂Te₃ thin sheets are formed in a nucleus saturation process due to the anisotropic growth of the crystals, and the mono-atom reaction model leads to irregular bent thin Bi₂Te₃ sheets. Some quasi one-dimensional nanorods and nanotubes were also found in the synthesized Bi₂Te₃ powders. PACS 81.05.Hd; 81.20.Ka; 61.46.+w     

Thermoelectric properties of nano-bulk bismuth telluride prepared with spark plasma sintered nano-plates

The pursuit for a high-performance thermoelectric n-type bismuth telluride-based material is significant because n-type materials are inferior to their corresponding p-type materials in highly efficient thermoelectric modules. Herein, to improve the thermoelectric performance of an n-type Bi₂Te₃, we prepared Bi₂Te₃ nano-plates with a homogeneous sub-micron size distribution and thickness range of about a few tens of nanometers. This was achieved using a typical nano-chemical synthetic method, and the prepared materials were then spark plasma sintered to fabricate n-type nano-bulk Bi₂Te₃ samples. We observed a significant enhancement of the anisotropic electrical transport properties for the nano-bulk sample with a higher power factor along the in-plane direction (24.3?μW?cm^?1?K^?2 at 300?K) than that along the out-of-plane direction (8.1?μW?cm^?1?K^?2 at 300?K). However, thermal transport properties were insensitive along the measured direction for the nano-bulk sample. We used a dimensionless figure of merit ZT to calculate the thermoelectric performance. The results showed that the maximum ZT value of 0.69 was achieved along the in-plane direction at 440?K for the nano-bulk n-type Bi₂Te₃ sample, which was however smaller than that of the previously reported n-type samples (ZT of 1.1). We believe that a further enhancement of the ZT value in the fabricated nano-bulk sample could be accomplished by effectively removing the surface organic ligand of the Bi₂Te₃ nano-plate particles and optimizing the spark plasma sintering conditions, maintaining the nano-plate morphology intact.     

Fermi surface and thermoelectric power of (Bi1−x Sbx)2Te3<Ag, Sn> mixed crystals

The Fermi surface anisotropy of (Bi_1?x Sb_x)₂Te₃ single crystals (0.25 ≤ x ≤ 1) was studied by analyzing the angular dependence of the frequency of Shubnikov-de Haas oscillations and the effect of tin and silver doping on the thermoelectric power in these crystals in the temperature range 77 ≤ T ≤ 300 K. It was shown that silver doping of (Bi_1?x Sb_x)₂Te₃ mixed crystals produces acceptors, while silver in Bi₂Te₃ acts as a donor. Tin also exhibits acceptor properties. Both tin and silver doping of p-(Bi_1?x Sb_x)₂Te₃ mixed crystals decrease the thermoelectric power due to an increase in the hole concentration.     

粗糙界面对Bi2Te3/PbTe超晶格热电优值影响的理论分析

以Bi₂Te₃/PbTe超晶格薄膜为例,分析电子在Bi₂Te₃量子阱中的输运过程,综合了薄膜的经典散射效应和理想量子效应,并以此混合效应为基础,在PbTe障碍层厚度一定时,模拟计算了两种混合效应中量子效应占不同比例时,Bi₂Te₃/PbTe超晶格热电优值的变化.在镜面反射占混合效应的0.3时,得到的热电优值与当前报道的量子阱超晶格的实验值接近. 关键词： 超晶格 粗糙界面 热电优值