Graduate Student, Physical Chemistry
Department of Chemistry
University of Wisconsin-Madison
B.S. DePauw University, Chemistry, 2004
PhD University of Wisconsin-Madison, 2004-2009
Postdoc at UW-Madison with Mahesh Mahanthappa, 2009-present
My thesis research involves synthesizing transition metal silicide nanowires using chemical vapor deposition of single source precursors and utilizing them as building blocks for functional nanoelectronic and nanospintronic devices. Such a bottom up approach to functional devices has enjoyed prominent attention and success because nanowires can be chemically synthesized in single-crystalline form with precisely controlled structures, diameters, and chemical compositions.
Additionally, the intermetallic transition metal silicides have diverse physical properties that are both very practical and fundamentally significant. Currently, metallic silicides provide ohmic contacts, interconnects, and gates to CMOS microelectronic transistors, the semiconducting silicides have been extensively investigated for silicon-based photoelectronics such as LEDs and IR detectors, and the narrow bandgap semiconducting silicides have been used for robust, stable, and inexpensive thermoelectric materials.
One interesting subclass of silicides, the so called B20 metal monosilicides (MSi, M = Fe, Co, Mn), are highly correlated electron systems that have fascinated physicists for years and continue to turn up surprises at the frontiers of theoretical and experimental condensed matter physics. These monosilicides and their alloys, Fe 1-x-yCo xMn ySi (0 < x,y <1) display a myriad of magnetic behaviors including unusual helical magnetic ordering and half-metallicity, with certain phases displaying magnetic semiconducting behavior. These properties make them promising candidates to facilitate CMOS compatible silicon-based spintronics, a field that seeks to exploit the spin properties instead of or in addition to charge degrees of freedom in electronic and photonic devices.
Figure 1. A) FeSi NW growth from single source precursor by chemical vapor deposition without using metal catalysts. B) Representative SEM images of FeSi NWs C) SEM of patterned growth and D) MR of alloy nanowires.
Thus far we have discovered a new nanowire growth mechanism that has enabled the synthesis of freestanding nanowires of FeSi, CoSi, CrSi 2, Ni 2Si, and Ni 3Si, through both chemical vapor transport of the target source materials and more rationally through carefully picked single source precursor molecules that pyrolyze via chemical vapor deposition to form nanowires. In a typical reaction, 100 mg of trans-Fe(CO) 4(SiCl 3) 2 will decompose at 750°C in a homebuilt chemical vapor deposition setup onto a Si substrate that has been oxidized with a HCl/H 2O 2 solution (Figure 1 A,B). By masking the substrate to produce regions with different oxide thicknesses, we can spatially control where nanowires nucleate and grow (Figure 1 C). This growth mechanism is different from the popular vapor-liquid-solid growth of nanowires and its generality has been demonstrated by application to several other silicides. We have also preliminarily succeeded in making nanowires of the alloy Fe 1-xCo xSi, although careful structural analysis is needed to prove homogeneity and composition. Even so these nanowires will be promising building blocks for nanospintronics as we have already measured magnetoresistance effects of 5% (Figure 1 D). It is the goal of this thesis research to understand and utilize this new growth mechanism to synthesize Fe 1-xCo xSi nanowires, carefully characterize their microstructure, and fabricate proof of concept spintronic devices.
1)Bierman, Matthew J.; Lau, Y.K. Albert; Kvit, Alexander V.; Schmitt, Andrew L.; Jin, Song; Dislocation-Driven Nanowire Growth and Eshelby Twist, Science, 320 (5879) (2008) 1060-106
2)Schmitt, Andrew L.; Higgins, Jeremy M.; Jin, Song; Chemical Synthesis and Magnetotransport of Magnetic Semiconducting Fe1-xCoxSi Alloy Nanowires, Nano Lett, 2008.
3)Szczech, J.R., Schmitt, A.L., Bierman, M.J., and Jin, S. “ Single-Crystal Semiconducting Chromium Disilicide Nanowires Synthesized via Chemical Vapor Transport,” Chem. Mater., 19, 13, 3238 – 3243, 2007.
4) Song, Y., Schmitt, A., Jin, S. “ Ultralong Single-Crystal Metallic Ni 2Si Nanowires with Low Resistivity,” Nano Lett. , 7, 965 – 969, 2007.
5) Schmitt, A.L. , Jin, S. “ Selective Patterned Growth of Silicide Nanowires without the Use of Metal Catalysts,” Chem. Mater., 19, 126, 2007.
6) Jin, S., Schmitt, A.L., Song, Y. “Metal Silicide Nanowires and Methods for Their Production,” US Patent applied for 2006.
7) 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., 110, 18142-18146, 2006.
8) Schmitt, A.L. , Bierman, M.J., Schmeißer, D., Himpsel, F.J., Jin, S. “ Synthesis and Properties of Single-Crystal FeSi Nanowires,” Nano Lett., 6, 1617 – 1621, 2006.