Multifold Electrical Conductance Enhancements at MetalâBismuth

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Letter

Multifold electrical conductance enhancements at metal-bismuth telluride interfaces modified using an organosilane monolayer Thomas Cardinal, Matthew Kwan, Theodorian Borca-Tasciuc, and Ganpati Ramanath ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12488 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 17, 2016

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ACS Applied Materials & Interfaces

Multifold electrical conductance enhancements at metal-bismuth telluride interfaces modified using an organosilane monolayer Thomas Cardinal1, Matthew Kwan1, Theodorian Borca-Tasciuc2, Ganpati Ramanath1* 1

Department of Materials Science and Engineering and 2Department of Mechanical, Aerospace and

Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180. *

Email: [email protected]

Keywords: Thermoelectrics, contact conductivity, self-assembled monolayers, phase formation, interface chemistry, diffusion barrier, surface oxide reduction

Abstract Controlling electrical transport across metal-thermoelectric interfaces is key to realizing high efficiency devices for solid state refrigeration and waste-heat harvesting. We obtain up to 17-fold increases in electrical contact conductivity Σc by inserting a mercaptan-terminated organosilane monolayer at Cu-Bi2Te3 and Ni-Bi2Te3 interfaces, yielding similar Σc for both metals by offsetting an otherwise 7-fold difference. The Σc improvements are underpinned by silane-moiety-induced inhibition of Cu diffusion, promotion of high-conductivity interfacial nickel telluride formation, and mercaptaninduced reduction of Bi2Te3 surface oxides. Our findings should enable incorporating nanomolecular layers with appropriately chosen terminal moieties in thermoelectric device metallization schemes without metal diffusion barriers.

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Thermoelectric materials offer promise for advanced applications1 such as solid-state cooling of optoelectronic devices and computer chips, and harvesting electricity from waste heat, e.g., in automobiles and power plants. High efficiency thermoelectric devices require p-n junctions to be created from materials with a high thermoelectric figure of merit Z = α2σ/κ, where α is the Seebeck coefficient, σ the electrical conductivity, and κ the thermal conductivity. The complex crystal, electronic, and phononic structures of many pnictogen chalcogenides result in an inherently high Z, which can be further enhanced by strategies such as nanostructuring, compositional control,2 and dilute doping3. Recent works have highlighted the importance of appropriate metallization to transmute the benefits of high Z thermoelectrics to high conversion efficiency in devices4,5. This recognition has spawned studies of pnictogen chalcogenides metallized with Ni alloys6,7 and Sn-based solders8, and investigations on the roles of ion implantation9 and surface preparation10. We recently showed that electrical and thermal transport across metal-thermoelectric interfaces11 are determined by interfacial diffusion and phase formation. We also showed12 that introducing a nanomolecular monolayer (NML) can be an unobtrusive means to tailor interface chemistry, and hence, the properties. In particular, we showed that functionalizing Cu-Bi2Te3 interfaces with an alkanedithiol NML increased electrical conductance by a factor of 13. The approach of using NMLs at interfaces has been previously demonstrated to enhance the mechano-chemical stability, electronic structure, and thermal transport at metal-ceramic interfaces13–17. Here, we demonstrate that introducing 3-mercaptopropyl trimethoxysilane (MPTMS) NMLs at CuBi2Te3 and Ni-Bi2Te3 interfaces yields multifold increases in the contact conductivity Σc, but due to completely different mechanisms. The observed enhancements are greater than that seen for alkane dithiol nanolayers, underscoring the importance of the silane moiety. We find that MPTMS suppresses Cu diffusion due to Cu-silane interactions, and fosters the chemical reduction of bismuth and tellurium surface oxides at Ni-Bi2Te3 interfaces. Our findings pave a way for tailoring the contact properties across metal-thermoelectric interfaces via nanomolecular functionalization, e.g., by exposing the thermoelectric surfaces to wet-chemical or vapor-phase fluxes of nanolayer-forming molecules prior to metallization.


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Nanocrystalline n-type Bi2Te3 powders prepared by a microwave-solvothermal process2 were coldcompacted into 0.25-inch-diameter cylindrical pellets, and annealed at 350 °C for 1 hour in a 1 x 10-7 Torr vacuum. The conductivity of the pellets was σ = 3 x 104 Ω-1 m-1, consistent with our previous reports11,12. The pellets were polished with 5-µm-diameter SiC sandpaper and sonicated in 200-proof ethanol for 10 min. MPTMS molecules purchased from Gelest were used without further purification. An MPTMS NML was formed on the Bi2Te3 pellets by immersing the pellets into a 10 mM MPTMS solution in toluene for 10 minutes in a glovebox held at 70% (Figs. 4a-b). However, the Si 2p signature remained unchanged (Fig. 4c). A


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concomitant increase in the Ni 2p peak shoulder at ~856 eV20 corresponding to Ni-O bonds (Fig. 4d), suggests NiO formation by Ni reduction of the surface oxides of Bi2Te3. This reaction is thermodynamically feasible, as seen from the higher NiO formation free energy23,24 (-8.1 eV/molecule) than that of Bi2O3 and TeO2. In addition, the presence of a 853 eV Ni 2p shoulder suggests NixTey formation. Spectra from sputter-cleaned (for oxide removal) Bi2Te3 surfaces exposed to Ni flux, also show the Ni-Te shoulder (Fig. 4c), confirming NixTey formation. The unoxidized Bi 4f and Te 3d subbands remain unchanged before and after Ni deposition. These results suggest that interfacial oxygen scavenging via NiO and NixTey formation contributes to high Σc at Ni-Bi2Te3 interfaces. Delivering sub-monolayer Cu fluxes onto MPTMS-modified Bi2Te3 surfaces result in essentially identical spectra seen from unmodified Bi2Te3 surfaces. This suggests that MPTMS does not significantly alter the interface bonding and/or phase formation paths upon Cu metallization (Fig. 3). In contrast, submonolayer Ni exposure onto MPTMS-modified Bi2Te3 surfaces result in ~50-70% decrease in BiOx and TeOy sub-band intensities. This diminution is only half as much for BiOx, and ~5% less for TeOy than seen upon Ni deposition on non-functionalized Bi2Te3 surfaces, which exhibit large BiOx and TeOy signatures to begin with. The lower extent of surface oxides reduction by Ni at MPTMS-modified surfaces is probably because the surface oxides are already partially reduced during MPTMS monolayer formation, and MPTMS inhibits Ni diffusion, as discussed later below. Additionally, the higher Ni 2p tail seen on MPTMS-modified Bi2Te3 indicates that MPTMS fosters NiTe2,25 and NiSi226 formation. We annealed blanket Cu-Bi2Te3 and Ni-Bi2Te3 structures with and without interfacial MPTMS to accentuate and capture the role of the NML on interfacial diffusion, bonding, and phase formation. Nearly identical X-ray diffractograms from both Cu-Bi2Te3 and Cu-MPTMS-Bi2Te3 structures (Fig. 5a) show a broad Bragg peak attributable12,27 to p-type Cu2-xTe28, indicating that MPTMS does not significantly alter interfacial structure. RBS depth profiles remain unchanged for Tanneal ≤ 100 °C irrespective of MPTMS functionalization indicating that Cu-Bi2Te3 and Cu-MPTMS-Bi2Te3 structures have similar interfacial mixing and structure at these temperatures. However, Cu diffusion is suppressed across MPTMS-modified interfaces for Tanneal ≥ 125 °C (Fig. 5b), as seen from the retention of the Cu


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surface peak in Cu-MPTMS-Bi2Te3 structures, and the lack thereof in structures without MPTMS. For Tanneal = 125 °C, MPTMS results in the retention of twice as much surface Cu than retained by ODT, reported previously12, and nearly 8-fold higher than that seen for an non-functionalized interface (Fig. 5c). Thus, MPTMS is a more effective barrier to Cu diffusion than an ODT. At Tanneal = 150 °C, MPTMS inhibits 17% more Cu than ODT did for Tanneal = 125 °C. Since our in vacuo XPS results (Fig. 3) rule out Cu-Si and Cu(II) oxide formation, we are persuaded to infer that inhibited Cu diffusion arises from the chelation of Cu(I) with the methoxy groups of the silane moieties, similar to our report on Cu–COOH chelation13. X-ray diffractograms from Ni-MPTMS-Bi2Te3 structures annealed to 150 °C (Fig. 5d) show a 10-fold stronger NiTe2(011) Bragg reflection29 than that seen from untreated Ni-Bi2Te3 interfaces, indicating that MPTMS promotes trigonal NiTe2 formation. This result is consistent with our in vacuo XPS experiments showing increased Ni-Te bonding at MPTMS-modified interfaces. While XPS spectral resolution precludes us from unequivocally assigning the Ni oxidation state, X-ray diffraction reveals interfacial NixTey formation11,12 known to promote ohmic contacts25,30. Since n-type NiTe2 fosters ohmic behavior30 with n-Bi2Te3, the increased Σc seen for Ni-MPTMS-Bi2Te3 is not unexpected. RBS spectra show that MPTMS inhibits Ni diffusion at Tanneal > 100 °C (Fig. 5e). The MPTMS-induced diminution of the Ni surface peak is smaller than observed for Cu, but similar to that observed for ODT modified Ni-Bi2Te3 structures12. Thus, MPTMS inhibits both Cu and Ni diffusion into Bi2Te3 with a greater effectiveness for Cu. The difference in the barrier properties of MPTMS to Cu and Ni diffusion probably arises due to differences in the metal-methoxy chelation interactions31 for the two metals. In conclusion, organosilane functionalization of Cu-Bi2Te3 and Ni-Bi2Te3 interfaces yields significantly higher Σc than dithiol functionalization12, underscoring the key role of the silane moiety. Our results obtained with blanket as well as sub-monolayer-thick metallization of Bi2Te3 show that silaneinduced Σc increases are metal-specific, and are underpinned by changes in interfacial bonding, metal diffusion, and phase formation. The MPTMS molecule is more effective than the longer ODT in inhibiting Cu diffusion due to strong Cu-methoxy chelation. In contrast, the weaker Ni-methoxy bonding


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is less effective in curtailing Ni diffusion, thereby fostering the formation of high-conductivity interfacial NixTey via Bi2Te3 oxide reduction. In addition, both MPTMS and ODT increase Σc due to mercaptaninduced Bi2Te3 surface oxides reduction. These results thus demonstrate the versatility of molecular nanolayers with appropriately chosen chemical termini to tune metal-thermoelectric interfacial transport properties through multiple interfacial chemical mechanisms. Our results showing comparably high Σc values for both Cu- and Ni-metallization of MPTMS-treated Bi2Te3 offer the potential to pave way for alternative thermoelectric device metallization schemes. Present schemes use high electrical conductivity Cu with a several-nm-thick Ni interlayer to inhibit Cu diffusion. Our results indicate that the thick Ni barrier can be obviated through the use of a sub-nm-thick organosilane layer that can be formed easily on Bi2Te3 surfaces from vapor-phase or wet-chemical fluxes. These findings should be relevant for designing thermoelectric device metallization schemes to realize high efficiency devices for solid-state refrigeration and waste-heat harvesting applications.

Associated contents Supporting information.

Figure S1 showing Bi 4f and Te 3d core-level photoelectron spectra

obtained by XPS from Bi2Te3 surfaces before, and after, functionalization with either MPTMS or ODT.

Acknowledgements We gratefully acknowledge funding from the National Science Foundation under grants ECCS 1002282/301 and MRI 0923181. We thank Rob Planty for assistance with the in vacuo metal deposition experiments.


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Figure 1. Contact conductivity Σc plotted for Cu-Bi2Te3 and Ni-Bi2Te3 interfaces with and without a MPTMS NML. Σc is also shown for interfaces modified with an ODT NML for comparison.


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Figure 2. (a) Bi 4f, and (b) Te 3d core-level spectra obtained by XPS from MPTMS-functionalized Bi2Te3 surfaces. Baseline spectra from non-functionalized Bi2Te3 surfaces are also shown. (c) Semilog plots of oxidized (squares) and unoxidized (circles) Bi 4f sub-band intensities as a function of surface-detector takeoff angle α. (d) S 2s and (e) Si 2p core-level spectra from MPTMS-modified Bi2Te3 surfaces. (f) S/Si intensity ratio plotted as a function of α.


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Figure 3.

(a) Bi 4f (b) Te 3d and (c) Cu 2p core-level bands from MPTMS-modified and non-

functionalized Bi2Te3 surfaces before (circles), and immediately after exposure to sub-monolayer Cu fluxes (squares) without a vacuum break. (d) Si 2p core-level spectra from non-functionalized (circles), and MPTMS-modified (squares) Bi2Te3 surfaces after exposure to sub-monolayer Cu flux.


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Figure 4. (a) Bi 4f (b) Te 3d and (c) Si 2p core-level bands from sputter-cleaned, oxidized, and MPTMSmodified Bi2Te3 surfaces obtained by XPS before (circles), and immediately after (squares) exposure to sub-monolayer Ni fluxes. (d) Ni 2p core-level spectra from unmodified (triangles), sputter-cleaned (circles), and MPTMS-modified (squares) Bi2Te3 surfaces, after exposure to Ni.


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Figure 5. (a) X-ray diffractograms and (b) RBS spectra from Cu-metallized Bi2Te3 pellets. Superscripts “S” and “B” denote surface and bulk, respectively. (c) Surface Cu peak intensities (normalized with the intensity at Tanneal = 50 °C) plotted vs. Tanneal for Cu-Bi2Te3 interfaces with MPTMS (squares), ODT (circles) or without an NML (triangles). (d) Diffractograms and (e) RBS spectra from Ni-Bi2Te3 pellets with (triangles) and without (squares) MPTMS for different Tanneal.


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Multifold Electrical Conductance Enhancements at MetalâBismuth

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Multifold Electrical Conductance Enhancements at MetalâBismuth