Bismuth Telluride Films Research Articles

Growth Mechanism during the Early Stages of electrodeposition of Bismuth telluride films

An optical-electrochemical study on the initial stages of bismuth telluride (Bi2Te3) electrodeposition onto gold substrate is presented in this work. Numerical methods were applied on both electrochemical and optical data to obtain the relevant kinetic parameters from electrocrystallization theoretical equations and ellipsometric models in order to describe this system in the relevant 0-15seconds range. The in-situ ellipsometry and electrodeposition analysis reveals a three-step mechanism. After an induction time including the double layer charge, the formation of a monolayer of tellurium is highlighted, followed by the adatom surface diffusion. Then, the growth of three-dimensional Bi2Te3 crystallites is observed according a progressive nucleation and a linear growth rate. Experimental and theoretical details about each technique are given and analyzed.

Rapid bespoke laser ablation of variable period grating structures using a digital micromirror device for multi-colored surface images.

A digital micromirror device has been used to project variable-period grating patterns at high values of demagnification for direct laser ablation on planar surfaces. Femtosecond laser pulses of ∼1 mJ pulse energy at 800nm wavelength from a Ti:sapphire laser were used to machine complex patterns with areas of up to ∼1 cm2 on thin films of bismuth telluride by dynamically modifying the grating period as the sample was translated beneath the imaged laser pulses. Individual ∼30 by 30μm gratings were stitched together to form contiguous structures, which had diffractive effects clearly visible to the naked eye. This technique may have applications in marking, coding, and security features.

Thermoelectric properties of electrodeposited tellurium films and the sodium lignosulfonate effect

The effect of the addition of a surfactant, sodium lignosulfonate (SLS), on the thermoelectric properties of tellurium films prepared by electrochemical deposition is studied. The growth mechanism is found to have an important role in the thermoelectric properties since the grain size of the films is sharply reduced when the surfactant is added to the solution. For this reason, the electrical resistivity of the tellurium films when the surfactant is not added is 229μΩ·m, which is lower than 798μΩ·m with SLS. The Seebeck coefficient values are not influenced, with values in the vicinity of 285μV/K for both solutions. The power factor resulted higher values than previous works, reaching values of 280μW/m·K2 (without SLS) and 82μW/m·K2 (with SLS) at room temperature. Finally, the thermal conductivity was measured by means of the Photoacoustic technique, which showed values of the order of 1W/m·K for both solutions, which is a factor of 3 less than the bulk value of tellurium. A notable observation is that the power factor and the thermal conductivity of electrodeposited tellurium films have the same order of magnitude of bismuth telluride films grown by electrodeposition. The figure of merit is estimated to be approximately one order of magnitude higher than the bulk value, 0.09 without SLS and 0.03 with SLS, both at room temperature.

Structural and Thermoelectric Properties of Nanocrystalline Bismuth Telluride Thin Films Under Compressive and Tensile Strain

To investigate the effect of strain on bismuth telluride films, we applied different compressive and tensile strains to thin films by changing the bending radius of a flexible substrate so the strain ranged from −0.3% (compressive) to +0.3% (tensile). The structural properties of the strained thin films, composed of nanosized grains, were analyzed by x-ray diffraction and scanning electron microscopy. For all samples the main peak was the (015) diffraction peak; crystal orientation along the (015) growth direction was slightly enhanced by application of compressive strain. The thermoelectric properties of strained bismuth telluride thin films were evaluated by measurement of electrical conductivity, Seebeck coefficient, and power factor. The magnitude and direction of the applied strain did not significantly affect the power factor, because when the strain changed from compressive to tensile the electrical conductivity increased and the absolute Seebeck coefficient decreased.

Use of a Soluble Anode in Electrodeposition of Thick Bismuth Telluride Layers

Integration of thermoelectric devices within an automotive heat exchanger could enable conversion of lost heat into electrical energy, contributing to improved total output from the engine. For this purpose, synthesis of thick bismuth telluride (Bi2Te3) films is required. Bismuth telluride has been produced by an electrochemical method in nitric acid with a sacrificial bismuth telluride anode as the source of cations. The binary layer grows on the working electrode while the counter-electrode, a Bi2Te3 disk obtained by high frequency melting, is oxidized to BiIII and TeIV. This process leads to auto-regeneration of the solution without modification of its composition. The thickness of films deposited by use of the Bi2Te3 anode was approximately 10 times that without. To demonstrate the utility of a soluble anode in electrochemical deposition, we report characterization of the composition and morphology of the films obtained under different experimental conditions. Perfectly dense and regular Bi2Te3 films (∼400 μm) with low internal stress and uniform composition across the cross-section were prepared. Their thermoelectric properties were assessed.

Polymer-coated compliant receivers for intact laser-induced forward transfer of thin films: experimental results and modelling

In this study, we investigate both experimentally and numerically laser-induced forward transfer (LIFT) of thin films to determine the role of a thin polymer layer coating the receiver with the aim of modifying the rate of deceleration and reduction of material stress preventing intact material transfer. A numerical model of the impact phase during LIFT shows that such a layer reduces the modelled stress. The evolution of stress within the transferred deposit and the substrate as a function of the thickness of the polymer layer, the transfer velocity and the elastic properties of the polymer are evaluated. The functionality of the polymer layer is verified experimentally by LIFT printing intact 1- \(\upmu \)m-thick bismuth telluride films and polymeric light-emitting diode pads onto a layer of 12-\(\upmu \)m-thick polydimethylsiloxane and 50-nm-thick poly(3,4-ethylenedioxythiophene) blended with poly(styrenesulfonate) (PEDOT:PSS), respectively. Furthermore, it is demonstrated experimentally that the introduction of such a compliant layer improves adhesion between the deposit and its substrate.

Design of a real-time spectroscopic rotating compensator ellipsometer without systematic errors

We describe a spectroscopic ellipsometer in the visible domain (400–800nm) based on a rotating compensator technology using two detectors. The classical analyzer is replaced by a fixed Rochon birefringent beamsplitter which splits the incidence light wave into two perpendicularly polarized waves, one oriented at +45° and the other one at −45° according to the plane of incidence. Both emergent optical signals are analyzed by two identical CCD detectors which are synchronized by an optical encoder fixed on the shaft of the step-by-step motor of the compensator. The final spectrum is the result of the two averaged Ψ and Δ spectra acquired by both detectors. We show that Ψ and Δ spectra are acquired without systematic errors on a spectral range fixed from 400 to 800nm. The acquisition time can be adjusted down to 25ms. The setup was validated by monitoring the first steps of bismuth telluride film electrocrystallization. The results exhibit that induced experimental growth parameters, such as film thickness and volumic fraction of deposited material can be extracted with a better trueness.

EFFECT OF DEPOSITION CONDITIONS ON THE MICROSTRUCTURE AND THE THERMOELECTRIC PROPERTIES OF GALVANOSTATICALLY ELECTRODEPOSITED Bi2Te3 FILM

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 μ W ⋅ K -2 ⋅ cm -1.

Phosphorus-doped bismuth telluride films by electrodeposition

Phosphorus-doped Bi2Te3 films were synthesized on a stainless-steel electrode by electrochemical deposition. X-ray diffraction, scanning electron microscopy and transmission electron microscopy confirmed that the films are single-phased Bi2Te3 solid solutions with a rhombohedral structure. The as-prepared films exhibit n-type characteristics with the Hall coefficient −1.76E−2 m3 C−1 and the electrical conductivity 280 S cm−1. The thermal conductivity is 0.47 W m−1 K−1, which is as low as one-third of the value observed in the bulk material. The doped P atoms occupy the interstitial positions between the two adjacent Te(1) layers connected by Van der Waals interaction in Bi2Te3.

Effects of annealing temperature on thermoelectric properties of Bi2Te3 films prepared by co-sputtering

N-type thermoelectric bismuth telluride (Bi2Te3) films were grown on SiO2/Si substrates by RF magnetron co-sputtering without intentional substrate heating. The effects of annealing temperature (150–350°C) on surface morphology, crystal structure and thermoelectric properties of the Bi2Te3 films were investigated. Its crystallization was significantly improved by increasing the annealing temperature, giving rise to the enhanced charge carrier mobility. It is found that the optimum of Seebeck coefficient and power factor was about −242μV/K and 21μW/K2cm, respectively, obtained at the annealing temperature of 300°C.

MOCVD of C‐oriented Bi2Te3 Films on SiO2 Substrates using Triethyl Bismuth and Di‐tertiarybutyl Tellurium

AbstractMetal‐organic (MO)CVD of bismuth telluride thin films is investigated by using a combination of optimized bismuth and tellurium precursors along with H2 as a reactant gas. Good quality films are obtained, even at 300 °C. The Bi precursors are trimethyl bismuth (Bi(CH3)3, Bi(Me3)) and triethyl bismuth (Bi(C2H5)3, Bi(Et)3), and the Te precursors are diethyl tellurium (Te(C2H5)2, Te(Et)2), di‐isopropyl tellurium (Te(C3H7)2, Te(iPr)2), and di‐tertiarybutyl tellurium (Te(C4H9)2, Te(tBu)2). The integrated absorbance area of C‐H stretching vibrations in the range 2800–3200 cm−1 is estimated to determine the decomposition rate of the Bi and Te precursors. C‐oriented Bi2Te3 films are obtained at 300 °C and under 2.5 Torr using Bi(Et)3 and Te(tBu)2 and characterized. Further, the microstructure, crystallinity, and surface morphology of the films are determined. The appropriate stoichiometry of the bismuth telluride films is obtained as Bi/Te = 2:3. The combination of Bi(Et)3 and Te(tBu)2 is the most effective for low‐temperature deposition because, compared to the other precursors, the decomposition of these precursors starts at a lower temperature. Investigations of the precursor decomposition behavior show that the absorbance of C‐H stretching vibrations of ligands in precursors in the gas phase is sensitive to the analysis conditions.

Temperature and size effects on electrical properties and thermoelectric power of Bismuth Telluride thin films deposited by co-sputtering

N-type Bismuth telluride thin films of different thicknesses were deposited on cleaned glass substrate at room temperature by co-sputtering technique. The films were annealed at 300°C for 12h in nitrogen atmosphere to improve their properties. The thermoelectric power and electrical properties measurements were carried out on the films with thickness from 70nm to 480nm in the temperature range 300–430K. The thickness dependence of electrical resistivity and Seebeck coefficient of annealed films was analyzed using the effective mean free path model. Some physical parameters such as effective mean free path of charge carriers in hypothetical bulk, the exponent of the energy term of mean free path, activation energy, and the Fermi energy were calculated. Both the electrical conductivity and the Seebeck coefficient of the bismuth telluride films increased with increasing of film thickness and grain size. Films with fewer grain boundaries and defects have longer effective mean free path of carriers and the mean free path decreases with the increase of temperature. The electron–phonon interaction was considered as the main scattering mechanism in the annealed bismuth telluride thin films.

Особенности формирования субмикронных пленок теллурида висмута методом импульсного лазерного осаждения

Особенности формирования субмикронных пленок теллурида висмута методом импульсного лазерного осаждения

Electrodeposition of bismuth, tellurium, and bismuth telluride thin films from choline chloride–oxalic acid ionic liquid

This article presents a series of preliminary results regarding the electrodeposition of bismuth, tellurium, and bismuth telluride films at 60 °C from ionic liquids, containing a mixture of choline chloride and oxalic acid (ChCl–OxA). Ten millimolar concentration solutions of BiCl3 and TeO2 were used as precursors in this supporting electrolyte. Cyclic voltammetry and electrochemical impedance spectroscopy techniques were used to demonstrate the deposition processes on Pt and Cu electrodes. Long-time electrolyses (30–120 min) performed at 60 °C with potential control (between −0.22 and −0.37 V vs. Ag reference electrode) have resulted in films deposited on copper substrate. Film surfaces were studied by scanning electron microscopy and analyzed by energy dispersive X-ray spectroscopy. The results of this study show that ChCl–OxA ionic liquid may be considered as a promising substitute of aqueous baths for Bi, Te or Bi2Te3 film plating.

Electrochemically Deposited Thermoelectric Bismuth-Telluride Films

Thermoelectric bismuth-telluride films were successfully prepared from nitric acid aqueous solution with ethylene glycol on Ti-sputtered silicon substrates by electrochemical deposition techniques. Cyclic voltammograms (CV), X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were used to investigate nucleation process, the crystal structures, morphologies and compositions of the bismuth-telluride films. The results show that the materials are Bi-rich, exhibiting preferred (110) orientation with rhombohedral structure. The crystallite size of films was calculated based on Scherrer’s equation and found to be 18-20 nm.

Surfactant-Mediated Electrodeposition of Bismuth Telluride Films and Its Effect on Microstructural Properties

We report the synthesis of highly crystallographically textured films of stoichiometric bismuth telluride (Bi(2)Te(3)) in the presence of a surfactant, sodium lignosulfonate (SL), that resulted in the improved alignment of films in the (110) plane and offered good control over the morphology and roughness of the electrodeposited films. SL concentrations in the range 60-80 mg dm(-3) at a deposition potential of -0.1 V vs SCE (saturated calomel electrode) were found to yield the most improved crystallinity and similar or superior thermoelectric properties compared with results reported in the literature.

N-type thermoelectric recycled carbon fibre sheet with electrochemically deposited Bi2Te3

An N-type thermoelectric recycled carbon fibre sheet with bismuth telluride coating has been successfully synthesised through an electro-deposition technique. The Seebeck coefficient and electrical properties of the combined recycled carbon fibre sheet and bismuth telluride films are reported. Classification of the crystal structure, surface morphology and the elemental composition of the resulting deposits are methodically characterised by XRD, SEM and EDX. Cyclic voltammetry is also carried out in nitric acid solutions to investigate the right range of deposition potential. The synthesis N-type thermoelectric sheet has a highest attainable Seebeck coefficient of −54μVK−1 and an electrical resistivity of 8.9×10−5Ωm. The results show slight differences in morphologies and thermoelectric properties for the films deposited at varying deposition potential. The increase in thermoelectrical properties of the recycled carbon fibre is in line with the development of using coated recycled fibre for thermoelectrical applications.

Growth and Thermoelectric Properties of Multilayer Thin Film of Bismuth Telluride and Indium Selenide via RF Magnetron Sputtering

A bismuth telluride (BT)/indium selenide (IS) multilayer film was deposited at room temperature by rf magnetron sputtering on a sapphire substrate in order to investigate how the multilayered structure affects the microstructure and thermoelectric properties. The effect of annealing at different temperatures was also studied. The results were compared with those from a BT film with the same thickness. A TEM study showed that the interface between the BT and IS layers became vague as the annealing temperature increased, and the BT layer crystallized while the IS layer did not. The presence of thin IS layers can help to limit the evaporation of Te from the BT/IS multilayer film, thus increasing the amount of Bi2Te3 phase in the multilayer film as compared with that of the BT film. An abrupt increase in the Seebeck coefficient of the multilayer film was observed when annealed at 300 degrees C, and the resistivity of the annealed multilayer film was high compared to that of the BT film. This result can also be explained by the proposed role of the IS layer, which limits the evaporation of Te at high temperature. The highest power factor of -3.9 x 10(-6) W/K2 cm was obtained at room temperature from the multilayer film annealed at 300 degrees C.

Growth and thermoelectric properties of Bi2Te3 films deposited by modified MOCVD

Bismuth telluride films were prepared by a modified metal organic chemical vapor deposition (MOCVD) on (001) GaAs substrates, and their thermoelectric properties were investigated. In the modified MOCVD system used in this study, metal organic sources of bismuth and tellurium were mixed with hydrogen gas in a graphite mixing room which can be heated by radio frequency induction and transferred to the substrate through a planar gap of mixing room. The effect of deposition parameters such as Te/Bi ratio in the reactor and mixing room temperature on the surface morphologies and the thermoelectric properties of the films was investigated. It was found that the grain size and the growth rate of the films can be controlled by adjusting the mixing room temperatures. Growth rates 2–3 times faster than that of the conventional MOCVD were obtained. A maximum growth rate of 7μm/h was achieved at the mixing room temperature of 300°C. The highest Seebeck coefficient was about −225μV/K. Reduction of thermal conductivity can be expected when the size of grain can be controlled to nano-scale. The results in this study suggest that the fabrication of Bi2Te3 films with high thermoelectric performance using a high throughput process can be achieved by the modified MOCVD system used in this work.

Electrodeposition of bismuth telluride thermoelectric films from a nonaqueous electrolyte using ethylene glycol

Ethylene glycol was studied as an electrolyte for the electrodeposition of thermoelectric bismuth telluride films by cyclic voltammetry, rotating ring disk electrode and electrochemical quartz crystal microbalance (EQCM). The reduction of both Bi3+ and Te4+ ions proceeds in one step without the formation of intermediates at potentials more negative than +0.2V and +0.55V vs. SHE, respectively. The diffusion coefficients and the rate constants for reduction were found to be similar for Bi and Te. Stoichiometric Bi2Te3 films with a uniform composition profile were obtained from solutions containing up to 1M of Bi(NO3)3 and TeCl4, at current densities up to 5Adm−2 (∼102μmh−1). Both p- and n-type bismuth telluride films could be obtained, as confirmed by Seebeck coefficient measurements.