Jeremy Higgins
Research Assistant
Song Jin Research Group
Department of Chemistry
University of Wisconsin-Madison
1101 University Avenue
Madison, WI 53706

Biographical Sketch
B.S. University of Pittsburgh, Chemistry, 2005
PhD University of Wisconsin-Madison, 2005-Present

Résumé
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Research
My thesis research involves the synthesis, characterization, and physical property measurement of transition metal silicide nanowires as building blocks for bottom-up nanoscale electronic, thermoelectric, and spintronic devices.

Transition metal silicides are a large family of refractory materials that have found success in many applications including CMOS devices, photovoltaics, and thermoelectrics. Semiconducting silicides, such as MnSi1.8, are known for being robust and inexpensive thermoelectric materials with reported figure of merit (ZT) values up to 0.7 and 0.8 in the bulk. This compound is a “Nowotny chimney ladder” phase having one lattice constant as large as tens of nanometers (Figure 2). It has been predicted and experimentally confirmed that dimensional reduction of semiconductors enhances ZT relative to the bulk values due to surface phonon scattering and/or quantum confinement. Beyond their numerous technological applications, metal silicides have fascinated physicists for over 70 years continuing to turn up surprises at the frontiers of theoretical and experimental condensed matter physics.  For example, the metal monosilicides that share the B20 structure type (MSi, M = Fe, Co, and Mn) have some very unusual properties. MnSi, once thought of as a classical itinerant ferromagnet, has recently been discovered to exhibit non-Fermi liquid behaviour without a quantum critical transition. The B20 monosilicides and their alloys, Fe1-x-yCoxMnySi (0 < x, y <1) also display a myriad of magnetic behaviours, including unusual helical magnetic ordering (Figure 3) and even more exotic skyrmion magnetic phases. Furthermore, FexCo1-xSi alloys were recently discovered to be magnetic semiconductors bringing exciting prospects of CMOS compatible silicon-based spintronics, a growing field that seeks to exploit the electronic spin degree of freedom in lieu of or in addition to the electon’s charge in electronic and photonic devices.

Figure 1 – Left – Binary Mn-Si phase diagram (ASM Phase Diagram Center) typifying the complex phase behavior and numerous stoichimetries present in transition metal silicides.  Right  - ­Journal of Materials Chemistry cover highlighting our recent review of transition metal silcide nanowire synthesis (Schmitt, A.L., et.al., J. Mater. Chem., 2010, 20, 223–235).

One-dimensional silicide nanomaterials, such as nanowires, are currently being explored to improve performance in these and new applications. However, synthesis of silicide nanowires via chemical methods is more complicated than in many other classes of nanomaterials due to the complex phase behaviour between metals and silicon and the numerous stoichiometries and structures of their resulting compounds. (Figure 1) The rapid development of semiconducting NWs, such as group IV elements and normal valence compounds II-VI and III-V compounds, that exhibit simple phase behaviour with low melting eutectics have been greatly served by the VLS growth scheme, but the lack of a similar mechanistic scheme has severely limited the concurrent development of free standing transition metal silicide nanowire syntheses. Several synthetic strategies have been developed to overcome this challenge resulting in increasing reports of silicide nanowires in the literature which we have recently reviewed. (Figure 1)

Figure 2 – Left – Commensurate Nowotny chimney ladder structures known from X-ray diffraction studies of MnSi1.7 bulk crystals.  Center and Right – Transmission electron micrographs and diffraction patterns displaying the features of the “chemical superlattice” present in MnSi1.7 compounds.  Careful analysis of diffraction patterns reveals the exact structure to be Mn19Si33.

My thesis project has focused on the diverse Mn-Si phase diagram (Figure 1) with an eye open to making connections between the work done in the Jin group and worldwide on the synthesis of transition metal silicide nanowires.  Along these lines we have successfully synthesized and characterized MnSi1.8 and MnSi nanowires, two previously unknown nanowire phases.  The use of advanced electron microscopy and diffraction has enabled the determination of the exact commensurate structure as Mn19Si33 (Figure 2).  This functional nanowire material is currently under further characterization for its potential in thermoelectric technology.  Further, magnetotransport studies of MnSi nanowires, a B20 silicide with known unusual magnetic behavior, has revealed the first signature of helimagnetism in nanowires of any kind (Figure 3).  Studies of the growth mechanism of these and other transition metal silicide nanowires are currently underway.

Figure 3 – Left – Graphic showing the relative alignment of planes of spins in normal and conical helimagnets. Right – Characterization of the transverse magnetotransport behavior of a typical MnSi NW. The critical helimagnetic ordering temperature (TC) is observed as a negative peak in plot of MR vs T, and the critical fields for the transition from regular to conical helimagnet (Hc) and from conical helimagnet to field induced ferromagnet (Hd) are observed MR vs H plots.

Selected Publications and Presentations

1)   Higgins, J.M., Ding, R., DeGrave, J., Jin, S. “Signature of Helimagnetic Ordering in MnSi NWs,” Nano Lett., 2010, 10 (5), 1605-1610.

2)   Higgins, J.M., Schmitt, A.L., Jin, S. “Higher Manganese Silicide Nanowires of Nowotny Chimney Ladder Phase,” J. Am. Chem. Soc., 2008, 130 (47), 16086-16094.

3)   Schmitt, A.L., Higgins, J.M., Szczech, J., Jin, S. “Synthesis and Applications of Transition Metal Silicide Nanowires”, J. Mater. Chem., 2010, 20, 223-235.

4)   Schmitt, A.L., Higgins, J. M., Jin, S. ‘‘Chemical Synthesis and Magneto-transport of Magnetic Semiconducting Fe1-xCoxSi Alloy Nanowires,’’ Nano Lett., 2008, 8, 810- 816.

5)   Higgins, J.M., Ding, R., Jin, S. “Selective Synthesis of Manganese Rich Silicide Nanowire Phases,” 2010 (in preparation).

6)   Higgins, J.M., Carmichael, P., Jin. S. “Insights into the Growth Mechanism and Doping of Transition Metal Silicide Nanowires”, 2010 (in preparation).

7)   Higgins, J.M., Jin. S. “Transition Metal Silicide Nanowires – Synthetic Methods and Applications,” In Processing, Properties, and Applications of Silicon Nanowires and Silicides; Huang, Y. , Tu, K.-N., Eds.; 2010 (in preparation).

Selected Publications and Presentations

1)  MRS Spring National Meeting; “Synthesis and Thermoelectric Properties of MnSi1.75 Nanowires – Nanostructured Complex Crystals as Thermoelectric Materials” (oral presentation); San Francisco, CA, April 5-9, 2010.

2)   MRS Spring National Meeting; “Growth and Physical Properties of Manganese Silicide Nanowires - Signature of Helimagnetism” (poster); San Francisco, CA, April 5-9, 2010.

3)   American Conference on Crystal Growth and Epitaxy; “Mechanistic Insights of the Growth and Doping of Transition Metal Silicide Nanowires” (poster); Lake Geneva, WI Aug 9-14, 2009.

4)   MRS Spring National Meeting; “Semiconducting Nanowires of MnSi1.7: the First Nowotny Chimney Ladder Phase Nanowires” (poster); San Francisco, CA, March 24-28, 2008.

5)   3M Science and Engineering Faculty Day; “General Syntheses, Properties, and Applications of Metal Silicide Nanowires” (poster); 3M Campus, St Paul, MN, June 17-18, 2008.

6)   UW Energy Hub Conference; “Semiconducting Nanowires of MnSi1.7: the First Nowotny Chimney Ladder Phase Nanowires” (poster); Madison, WI, Nov 7, 2008.

7)   ACS Spring National Meeting; “Transition Metal Silicide NWs: Novel Materials and Heterostructures” (oral presentation); Chicago, IL, March 25-29, 2007.