Narrow Results

An interest in the study of the structural and optical properties of topological insulators, led to the selection of Bismuth telluride thin films. Among the topological insulators, Bismuth telluride is one of the promising materials that exhibit applications in the field of thermoelectric power generation. The films were prepared using the thermal evaporation method under vacuum conditions of ${10}^{-5}$ mbar on glass substrates maintained at a temperature of 120^∘C. The thickness of the film samples ranges from 400 Å to 1400 Å. Pure phases of Bi₂Te₃ with a rhombohedral structure are confirmed using X-ray Diffraction. The crystallite size was calculated using Debye–Scherrer’s formula. Structural morphology and compositions are examined using SEM and EDAX. The UV–Vis study reflects the optical behavior of the thin film samples. The absorption spectra gives the direct and allowed transition band gap of the Bi₂Te₃ thin film samples for various thicknesses. The band gap decreases with the increase in the thickness of the Bi₂Te₃ thin films, suggesting that the material has significant absorption towards the visible spectrum. The Photoluminescence (PL) emissions for Bi₂Te₃ thin film samples of various thicknesses are examined and analysed. Raman study reveals the presence of two active modes.

Bi₂Te₃ has attracted attention due to its potential applications in the microfabrication of integrated thermoelectric devices. It is also interesting to study the metallization process of this compound. Metallic nanostructures were deposited by means of an electron gun evaporator in ultra high vacuum (UHV) conditions (10^-8 Pa) on the freshly cleaved 0001 surface of the crystal Bi₂Te₃. Measurements were conducted using the commercially available Omicron UHV scanning tunneling microscope (STM). Scanning tunneling spectroscopy (STS) measurements were performed using current imaging tunneling spectroscopy (CITS), and subsequent calculation of the dI/dV maps. Metallic characteristics were observed on nickel islands since early stages of the growth. CITS and dI/dV maps showed distinct contrast between the substrate and metallic islands. Similar contrast was not observed in the case of titanium, most probably due to an intercalation process. Occurring of such a process was confirmed by the appearance of the superlattice structure.

Bismuth Telluride (Bi₂Te₃) films were deposited by a simple and cost-effective galvanostatic electrodeposition process from a solution containing bismuth tri-nitrate penta-hydrate and tellurium dioxide of different concentration ratios in 1 M nitric acid onto gold sputtered silicon substrate at various current densities. The effect of distinct current densities, electrolyte concentrations and electrodes distances on the microstructure and the thermoelectric properties of Bi₂Te₃ films were investigated. X-Ray diffraction (XRD) and field emission scanning electron microscope (FESEM) analysis ensured a high density, homogenous and near stoichiometric film. The surface morphology, crystalline structure and grain size were correlated with the applied current density. A prominent orientation (110) was observed for all the films and the grain size was acquired from 21 to 45 nm. The Seebeck measurement affirmed n-type semiconductor behavior of the deposited films. Enhancement in carrier mobility without significant change of the carrier concentration and Seebeck coefficient was achieved by tuning the electrodes distance. The thermoelectric film has a maximum measured Seebeck coefficient of –61.215 μV/K and a very high electrical conductivity of 2.13 × 10³ Ω^-1 ⋅ cm^n-1. The maximum calculated power factor was 8.2 μW⋅K^-2 ⋅ cm^-1.

Grain boundary engineering is considered an effective approach to improve the performance of thermoelectric materials. Herein, by introducing KOH into the grain boundary of Bi₂${\text{Te}}_{2.7}$${\text{Se}}_{0.3}$ (BTS) via liquid phase sintering strategy, the thermoelectric performances are improved significantly. The melting KOH spreads over the grain boundaries during the high temperature sintering process, which could be used to optimize the carrier/phonon transport behavior. The maximum ZT reaches up to 0.97 for the sample incorporated with 0.5%Wt of KOH at 425K, which achieves 30% improvement over the pure BTS. The homogeneous distribution of KOH layer on the grain boundaries forms efficient potential barrier scattering, which increases power factor and reduces thermal conductivity simultaneously. Particularly, it is found that the maximum ZT can be tuned gradually in the temperature range from 450K to 375K by tuning the weight percent of KOH, demonstrating a possibility in adjusting the thermoelectric properties of BTS using a relatively simple method.

Bismuth telluride-based materials are the most celebrated thermoelectric (TE) materials near room temperature. However, for the n-type bismuth telluride-based materials, the traditional powder metallurgy (PM) method, such as grinding and ball milling, can induce strong donor-like effect. The strong donor-like effect boosts the carrier concentration $n$ and the carrier thermal conductivity ${\kappa }_e$, leading to the deterioration of both electrical and thermal performance. Herein, the basal deformation (BD) has been performed in the n-type Bi₂Te₃-based materials to introduce weak donor-like effect. The BD crystalline bulks were obtained through sintering the stacked crystals, which had been heavily deformed on the (00$l)$ plane. The BD process optimizes the carrier concentration $n$ and improves the power factor (PF). Meanwhile, the lattice thermal conductivity ${\kappa }_L$ is suppressed due to the enhanced grain boundary scattering. Consequently, a peak ZT of 1.0 at 380 K has been achieved in Bi₂Te${}_{2.79}$Se${}_{0.21}$, which is 24% and 30% higher than that of the original zone-melting (ZM) ingot and the polycrystalline sample by PM. This study sheds light on the further TE performance enhancement of bismuth telluride-based materials via introducing the weak donor-like effect.

High-performance flexible Bi₂Te₃-based thin films carry significant promise for future portable and wearable thermoelectric devices. In this study, a series of Bi${}_{0.5}$Sb${}_{1.5}$Te₃ thin films were deposited on polyimide substrates under different RF magnetron sputtering powers from 60 W to 140 W. The crystallinity, (00$l$) preferential orientation and atomic composition can be effectively modulated by varying the sputtering power. Benefiting from the synchronous enhancements of the electrical conductivity (mostly due to the enhanced carrier mobility) and Seebeck coefficient, the film deposited under the sputtering power 100 W presents the best PF of 12.86 $\mu$W cm${}^{-1}$K${}^{-2}$ at 360 K and the highest average PF of 11.25 $\mu$W cm${}^{-1}$K${}^{-2}$ in the temperature range of 300–560 K, which are much better than those of the films deposited under other sputtering powers.