Silicide Nanowires and Nanoparticles for Thermoelectrics
General chemical synthesis of metal silicide nanowires
Nanomaterials extensively studied so far are usually made of elements or prototypical compound semiconductors with simple stoichiometries. In contrast, intermetallic compounds such as metal silicides have multiple and unpredictable stoichiometries and complex phase behavior, making them challenging to synthesize and rarely explored so far. However, novel nanowire (NW) materials of silicides have many new physical properties and significant applications in nanoelectronics, nanophotonics, nanospintronics (see below), and thermoelectrics. We are developing general synthetic approaches to silicide NWs to overcome such previously unaddressed complexity. Our first approach utilizes chemical vapor deposition (CVD) of single source organometallic precursors (SSPs) to reproducibly deliver both silicon and metals with stoichiometric control. For example, using SSPs Fe(SiCl3)2(CO)4 and Co(SiCl3)(CO)4, FeSi (Fig. B) and CoSi nanowires are produced respectively on silicon substrates covered with a thin (1-2 nm) layer of silicon oxide without any catalyst seeds (see Figure 2).
Figure 2. A) Our unique synthetic route to silicide NWs using SSP; B) SEM of FeSi NWs we synthesized; C) HRTEM of a FeSi nanowire.
We have also developed a complementary chemical vapor transport (CVT) method to synthesize NWs of silicides for which suitable organometallic precursors are not readily accessible, such as CrSi2 and Ni2Si. We discovered a new and general nanowire growth mechanism that is different from the typical vapor-liquid-solid (VLS) NW growth and critically depends on the oxide thickness. Taking advantage of such unique oxide dependence, we have developed a simple method to pattern the location of NW growth by modulating the surface oxide without the use of metal catalysts. We are systematically investigating families of metal-silicon organometallic complexes as SSPs and working on elucidating the detailed NW growth mechanism and the chemical rules governing the formation of the nanoscale intermetallic phases towards the goal of rational synthesis of nanomaterials of any pure or alloyed metal silicides.
Figure 3. CVT synthesis of silicide nanowires using Ni2Si nanowires as an example.
Nanoscale semiconduction silicides as efficient thermoelectric materials
Thermoelectric (TE) materials convert heat to electricity and can improve energy efficiency by harvesting waste heat for power generation or improve refrigeration. The performance of TE materials needs to be improved beyond the current state-the-art as benchmarked by the TE figure of merit (ZT) value of 1 for doped Bi2Te3. Dimensional reduction of semiconductors enhances the ZT relative to the bulk values due to surface phonon scattering and/or quantum confinement. Many semiconducting silicides are known as robust, stable, and inexpensive thermoelectric materials, with the most notable being MnSi1.8 and ReSi1.8 that have reported ZT values of 0.7 to 0.8. These silicides are really homologous family of compounds with uniquely complex crystal structures that belong to the “Nowotny chimney ladder” (NCL) phases with one lattice constant as large as tens of nanometers (Fig. 4A). Phonon confinement will occur more readily in nanostructures of these NCL structures with a level of complexity rarely observed in common semiconductors, which together with surface roughness should result in pronounced reduction of the lattice thermal conductivity. If other properties are maintained for single crystal NWs made of NCL silicides with controlled doping, their ZT may be enhanced from the already high bulk values to well above 1 and that of many bulk and nanostructured TE materials.
1) Schmitt, A. L.; Higgins, J. M.; Szczech, J. R.; Jin, S. Synthesis and Applications of Metal Silicide Nanowires J. Mater. Chem. 2010, 20, 223-235
2) Schmitt, A.L.; Bierman, M.J.; Schmeisser, D.; Himpsel, F.J.; Jin, S.; Synthesis and Properties of Single-Crystal FeSi Nanowires, Nano Lett. 2006, 6, 1617-1621
3) Schmitt, A.L.; Zhu, L.; Schmeisser, D.; Himpsel, F.J.; Jin, S.; Metallic Single-Crystal CoSi Nanowires via Chemical Vapor Deposition of Single-Source Precursor, J. Phys. Chem. B 2006, 110, 18142-18146
4) Schmitt, A.L.; Jin, S.; Selective Patterned Growth of Silicide Nanowires without the Use of Metal Catalysts, Chem. Mater. 2007, 19, 126-128
5) Song, Y.; Schmitt, A.; Jin, S.; Ultralong Single-Crystal Metallic Ni2Si Nanowires with Low Resistivity, Nano Lett. 2007, 7, 965-969
6) Song, Y.; Jin, S.; Synthesis and Properties of Metallic ß3-Ni3Si Nanowires, Appl. Phys. Lett. 2007, 90, 173122
7) Szczech, J; Schmitt, A.L.; Bierman, M.J.; Jin, S.; Single-Crystal Semiconducting Chromium Disilicide Nanowires Synthesized via Chemical Vapor Transport, Chem. Mater. 2007, 19, 3238-3243
8) Zhou, F; Szczech, J.; Pettes, M.T.; Moore, A.L.; Jin, S.; Shi, L.; Determination of Transport Properties in Chromium Disilicide Nanowires via Combined Thermoelectric and Structural Characterizations, Nano Lett. 2007, 7, 1649-1654
9) Schmitt, A. L.; Higgins, J. M.; Jin, S.; Chemical Synthesis and Magnetotransport of Magnetic Semiconducting Fe1-xCoxSi Alloy Nanowires, Nano. Lett. 2008, 8, 810-815
10) Szczech, J. R.; Jin, S.; Mg2Si Nanocomposite Converted from Diatomaceous Earth as a Potential Thermoelectric Nanomaterial, J. of Solid State Chem. 2008, 181, 1565-1570
11) Higgins, J. M.; Schmitt, A. L.; Guzei, I. A.; Jin, S.; Higher Manganese Silicide Nanowires of Nowotny Chimney Ladder Phase, J. Am. Chem. Soc. 2008, 130, 16086-16094
12) Szczech, J. R.; Jin, S.; Epitaxially-hyperbranched FeSi Nanowires Exhibiting Merohedral Twinning J. Mater. Chem. 2010, 20, 1375-1382
13) Higgins, J. M.; Carmichael, P.; Schmitt, A. L.; Lee, S.; Degrave, J. P.; Jin, S.; Mechanistic Investigation of the Growth of Fe1−xCoxSi (0 ≤ x ≤ 1) and Fe5(Si1−yGey)3 (0 ≤ y ≤ 0.33) Ternary Alloy Nanowires, ACS Nano 2011, 5, 3268-3277
14) Yan, C.; Higgins, J. M.; Faber, M. S.; Lee, P. S.; Jin, S.; Spontaneous Growth and Phase Transformation of Highly Conductive Nickel Germanide Nanowires, ACS Nano, 2011, 5, 5006-5014
15) Higgins, J. M.; Ding, R.; Jin, S.; Synthesis and Characterization of Manganese-Rich Silicide (α-Mn5Si3, β-Mn5Si3, and β-Mn3Si) Nanowires, Chem. Mater. 2011, 23, 3848-3853
16) DeGrave, J. P.; Schmitt, A. L.; Selinsky, R. S.; Higgins, J. M.; Keavney, D. J.; Jin, S.; Spin Polarization Measurement of Homogeneously Doped Fe1-xCoxSi Nanowires by Andreev Reflection Spectroscopy, Nano Lett., 2011, ASAP, DOI 10.1021/nl2026426
17) Szczech, J; Schmitt, A.L.; Bierman, M.J.; Jin, S.; Single-Crystal Semiconducting Chromium Disilicide Nanowires Synthesized via Chemical Vapor Transport, Chem. Mater. 2007, 19, 3238-3243
18) Zhou, F; Szczech, J.; Pettes, M.T.; Moore, A.L.; Jin, S.; Shi, L.; Determination of Transport Properties in Chromium Disilicide Nanowires via Combined Thermoelectric and Structural Characterizations, Nano Lett. 2007, 7, 1649-1654
19) Szczech, J. R.; Jin, S.; Mg2Si Nanocomposite Converted from Diatomaceous Earth as a Potential Thermoelectric Nanomaterial, J. of Solid State Chem. 2008, 181, 1565-1570
20) Higgins, J. M.; Schmitt, A. L.; Guzei, I. A.; Jin, S.; Higher Manganese Silicide Nanowires of Nowotny Chimney Ladder Phase, J. Am. Chem. Soc. 2008, 130, 16086-16094
21) Szczech, J. R.; Higgins, J. M.; Jin, S.; Enhancement of the Thermoelectric Properties in Nanoscale and Nanostructured Materials, J. Mater. Chem., 2011, 21, 4037-4055
22) Ankit Pokhrel, Zachary Degregorio, Jeremy Higgins, Steven Girard, and Song Jin; Vapor Phase Conversion Synthesis of Higher Manganese Silicide (MnSi1.75) Nanowire Arrays for Thermoelectric Applications Chem. Mater. 2013, Just Accepted Manuscript, DOI: 10.1021/cm3040032.