Cassinelli, M.

Gronefeld, J.

Müller, Sven

Picht, O.

Toimil-Molares, M. E.

Trautmann, C.

GSI Repository

Influence of Electrolyte Concentration on Crystal Orientation and Seebeck Co-efficient of Bi2Te3 Nanowires Arrays O. Picht1, J. Gronefeld1, M. Cassinelli1, S. Mueller1, C. Trautmann1,2, M.E. Toimil-Molares1,#1GSI, Darmstadt, Germany, 2 Technische Universität Darmstadt, Germany.Bulk materials based on bismuth telluride (Bi2Te3) ex-hibit high thermoelectric efficiency at room temperature,being thus common constituents of thermoelectric devicesoperating at ambient conditions. The efficiency of a ther-moelectric device is described by the figure of merit ZT = S2ıT/Ȝ, where S denotes the Seebeck coefficient, ı electri-cal, Ȝ thermal conductivity, and T the absolute tempera-ture. Theoretical calculations predicted an enhancementof the thermoelectric efficiency in low-dimensionalBi2Te3 structures, such as nanowires (NWs), due to finite-and quantum-size effects [1]. For NWs, the diameterclearly represents one of the crucial parameters for theenhancement of ZT. Bi2Te3 NWs had been synthesized byelectrodeposition mainly in porous alumina membranes[2] displaying in most cases diameters above 50 nm. Re-cently, we have reported the electrodeposition of Bi2Te3NWs with diameters as small as ~17 nm in the pores ofetched ion-track membranes [3]. In addition, Bi2Te3 has ahighly anisotropic crystal structure and the electrodeposi-tion parameters can thus influence its crystal orientationand physical properties. Here we present systematic stud-ies on the influence of electrolyte concentration on thecrystallographic characteristics and Seebeck coefficient of 25-nm diameter Bi2Te3 NW arrays [4].Polycarbonate foils (thickness 30 μm) were irradiatedwith ~GeV U ions at the UNILAC accelerator. The ion tracks were etched in 6M NaOH solution at 50 °C for 60 sto fabricate channels with diameter ~25 nm. Bi2Te3 NWs were electrodeposited in the channels using a thermo-stated three-electrode cell and a saturated calomel refer-ence (SCE) electrode. The deposition was performed ap-plying U = 0 mV vs. SCE at T = 4°C. The electrolytes were based on 1 M nitric acid, 5 mM tellurium powderand three different bismuth nitrate pentahydrate concen-trations, namely 10 mM, 7.5 mM, and 3.5 mM Bi.Bi2Te3 NW arrays synthesized with different electro-lytes were characterized by x-ray diffraction (XRD) tostudy the preferential crystallographic orientation (Fig. 1).The XRD spectra display reflections assigned to the poly-carbonate membrane, the Au cathode layer and the (101),(105), and (205) reflections from the Bi2Te3 NWs. The(101) reflection is small in all cases, while the NW array deposited with lowest Bi:Te ratio (green) displays thestrongest, but still relatively weak, signal for (015) orien-tation. Finally, with increasing Bi:Te ratio in the electro-lyte, an increase of the (205) reflection, i.e. (205) planesparallel to membrane surface, is noticeable.Fig. 1: XRD patterns of Bi2Te3 NW arrays deposited withelectrolytes of Bi:Te ratio 3.75:5 (green), 7.5:5 (blue), and10:5 (red) at T=4°C applying U=0 mV vs. SCE.All samples show negative S values, indicating n-typebehaviour, with a linear T dependence. The sample syn-thesized with the lowest Bi:Te electrolyte ratio exhibitshighest S in the whole range, achieving S = -55.0 μV/K at270°C, while NWs deposited with 7.5 mM Bi and 10 mMBi, displayed S ~ -49.6 μV/K and -38.6 μV/K at 270 °C,respectively.Fig. 2: Seebeck coefficient vs. temperature.In conclusion, Bi2Te3 NWs arrays were electrodepos-ited in etched ion-track membranes using electrolytes with three different Bi:Te ratios. XRD measurements re-vealed an increasing (205) preferred crystallographic ori-entation for higher Bi concentrations in the electrolyte.Seebeck measurements indicate only a weak dependenceof the S coefficient on the different Bi:Te ratio of the elec-trolyte.[1] L.D. Hicks and M.S. Dresselhaus, Phys. Rev. B 47,12727 (1993).[2] S.A. Sapp et al., Adv. Mater. 11, 402 (1999).[3] O. Picht et al., J.of Phys. Chem.. C 116, 5367 (2012).[4] J. Gronefeld, Bachelor Thesis, TU-Darmstadt.* Work supported by DFG (No. SPP 1386).# M.E.Toimilmolares@gsi.de[5] S. Mueller, PhD Thesis, Univ. of Heidelberg.GSITemplate2007GSI SCIENTIFIC REPORT 2012 PNI-INHOUSE-EXP-46415

Influence of Electrolyte Concentration on Crystal Orientation and Seebeck Co-efficient of Bi$_{2}$Te$_{3}$ Nanowires Arrays

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Influence of Electrolyte Concentration on Crystal Orientation and Seebeck Co-efficient of Bi $_{2}$ Te $_{3}$ Nanowires Arrays

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Influence of Electrolyte Concentration on Crystal Orientation and Seebeck Co-efficient of Bi2_{2}2​Te3_{3}3​ Nanowires Arrays

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Influence of Electrolyte Concentration on Crystal Orientation and Seebeck Co-efficient of Bi $_{2}$ Te $_{3}$ Nanowires Arrays