Letter www.acsami.org
Tailoring Electrical Transport Across Metal−Thermoelectric Interfaces Using a Nanomolecular Monolayer Thomas Cardinal,† Devender,† Theodorian Borca-Tasciuc,‡ and Ganpati Ramanath*,† †
Department of Materials Science and Engineering and ‡Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States ABSTRACT: We report a 13-fold increase in electrical contact conductivity Σc upon introducing a 1,8-octanedithiol (ODT) monolayer at Cu−Bi2Te3 interfaces. In contrast introducing ODT at Ni−Bi2Te3 interfaces results in a 20% decrease in Σc. Rutherford backscattering spectrometry, X-ray diffraction and electron spectroscopy analyses indicate that metal−sulfur and sulfur-Bi2Te3 bonds at metal-Bi2Te3 interfaces inhibit chemical mixing, curtail metal-telluride formation, and suppress oxidation. Suppressing p-type Cu2Te favors electrical transport across Cumetallized n-type Bi2Te3, whereas inhibiting the formation of Ohmic-contact-promoting NixTey compromises the electrical conductance at Ni−Bi2Te3 interfaces. Our findings illustrate that molecular nanolayers could be attractive for manipulating interface chemistry and phase formation for tailoring electrical transport across metal−thermoelectric interfaces for solid-state refrigeration applications. KEYWORDS: thermoelectrics, contact conductivity, self-assembled monolayers, phase formation, interface chemistry, diffusion barrier
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work function of the metal.14 A recent study reported improvements in electrical contact conductivity Σc upon introducing an organosilane NML at Ni-metallized Bi2Te3−xSex interfaces.15 However, the property enhancement mechanism is not yet studied and understood in terms of interface chemistry and phase formation. Here, we report that introducing an octanedithiol (ODT) monolayer inhibits interfacial diffusion and increases Σc across Cu−Bi2Te3 interfaces, while the same treatment at Ni−Bi2Te3 interfaces decreases Σc. Our studies of interface chemistry and structure using spectroscopy and diffraction techniques demonstrate that the efficacy of NMLs in inhibiting interfacial mixing and phase formation is a key determinant of Σc. Our findings suggest that NMLs composed of appropriately chosen terminal moieties could offer new possibilities for realizing thermoelectric device metallization with desired electrical properties. Cylindrical 0.25-in. diameter pellets of nanostructured bulk Bi2Te3 were prepared by cold-compaction of nanocrystals and subsequently annealed at 350 °C for 1 h in a 1 × 10−7 Torr vacuum. The nanocrystals were synthesized by a microwavestimulated solvothermal process, as described in detail elsewhere.2 The pellets were polished with 5-μm-diameter SiC particles using a 1200 grit surface, and sonicated in pure ethanol for 10 min. We therefore expect that our pellet surfaces have microscale roughness. We obtained 1,8 octanedithiol (ODT) from Sigma-Aldrich and used without further purification. A 5 mM solution of ODT in ethanol was prepared in a glovebox held
ncreasing the efficiency of thermoelectric devices is necessary to realize the potential of solid-state refrigeration and electricity generation from waste heat for a variety of applications, such as night-vision infrared cameras, climate control in cars and buildings, and cooling nanoelectronics devices. The materials used to fabricate the active elements for energy conversion must have a high thermoelectric figure of merit Z = α2σ/κ, predicated on a high Seebeck coefficient α, a high electrical conductivity σ, and a low thermal conductivity κ. The state-of-the-art thermoelectric devices for near-roomtemperature applications are built from group V−VI compounds and their alloys1 due to the high intrinsic Z of these materials. Recent works have demonstrated approaches to obtain high Z materials through nanostructuring, doping and compositional control.2 The thermoelectric device efficiency, however, depends on the effective Z that includes contributions from thermal and electronic transport characteristics of the metallized thermoelectric interfaces.3 Hence, there is a great deal of interest in controlling the interfacial transport properties by understanding and manipulating the diffusion and phase formation processes at metal-thermoelectric interfaces. Many metallization schemes based on solder alloys,4 Ni, Co and multicomponent alloys5,6 have been studied for thermoelectrics devices, but the relationships between interfacial chemistry and transport properties are not yet understood. Recently, we showed7 that interfacial electrical transport across metallized V−VI thermoelectric materials is strongly dependent on interdiffusion and phase formation, and the majority carriers. Prior works have shown that introducing a nanomolecular layer (NML) at metal−insulator interfaces can inhibit diffusion,8,9 influence phase formation pathways,10 enhance interfacial toughness11 and thermal conductance,12,13 and alter the effective © 2016 American Chemical Society
Received: September 30, 2015 Accepted: February 4, 2016 Published: February 4, 2016 4275
DOI: 10.1021/acsami.5b08990 ACS Appl. Mater. Interfaces 2016, 8, 4275−4279
Letter
ACS Applied Materials & Interfaces
Figure 1. (a) Bi 4f, (b) Te 3d, and (c) S 2s core-level spectra from Bi2Te3 surfaces treated with ODT. Baseline spectra from a freshly polished Bi2Te3 surface, and a surface immersed in ethanol for 1 h are also shown for comparison. (d) Semilog plots capturing the attenuation of the intensities of the oxidized and unoxidized Bi 4f sub-bands as a function of photoelectron takeoff angle α. The former was used for determining the ODT NML thickness dODT (triangles) on Bi2Te3. The latter was used for determining the oxide thickness dox on Bi2Te3 surface modified with ODT (squares), or ethanol (circles).
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