Picraux Research Group

Research Overview

With expertise in materials science, physics, chemistry, and electrical engineering, our group studies the synthesis, properties, and integration of semiconducting nanowires, and their application in electronics, photonics, and energy harvesting and storage. A current emphasis is on Si/Ge nanowire and nanowire heterostructure growth mechanisms, growth kinetics, strain distributions, chemical doping, electrical and optical properties and devices, bandstructure engineering, photovoltaics and sensing, Li ion storage, and nanowire integration into devices.

Nanowire synthesis

We utilize the chemical vapor solid (CVD) growth of semiconducting nanowires by the vapor-liquid-solid (VLS) technique as an effective bottom-up approach for the synthesis of quasi-one-dimensional materials with significant potential for electronic, optical, energy, and sensing applications.

We specialize in CVD growth of Si and Ge nanowires and their alloys and heterostructures using VLS growth for axial structures and vapor-solid (VS) growth for radial control. Our growth system is a cold wall CVD reactor with computer control of gas and metallorganic species, as well as with remote plasma excitation capability. We seek to understand the basic materials science and control of nanowire nucleation, diameter, structure, axial and radial heterostructure formation, strain distributions, and electrical doping concentrations and profiles. High resolution transmission and scanning electron microscopy are used to relate the microscopic structure of the nanowires to growth conditions and properties.









Selected publications

  • Direct Observation of Nanoscale Size Effects in Ge Semiconductor Nanowire Growth, S. A. Dayeh, S. T. Picraux, Nano Letters 10, 4032 (2010).
  • Silicon and Germanium Nanowires: Growth, Properties and Integration, S. T. Picraux, S. Dayeh, P. Manandhar D. E. Perea and S. G. Choi, JOM, 62, (4) p.  (2010).
  • Strain distributions and electronic property modifications in Si/Ge axial nanowire heterostructures, J. G. Swadener and S. T. Picraux, J. Appl. Phys. 105, 044310 (2009).

Devices and electrical/optical properties

We are investigating the electrical and optical properties of our nanowires and nanowire heterostructures. Impurity doping and the electrical and optical response of the nanowires are studied by conventional transport and by conducting nanoprobe measurements. In collaboration with other scientists, ultrafast optical pump-probe techniques are used to measure electron and hole excited state lifetimes of individual and nanowire ensembles. Photoconductivity and Raman scattering are employed to understand the effect of strain and defect states on the nanowire electronic and optical properties. Nanowire device structures including p-n, p-i-n, and field effect transistor devices are being investigated to understand basic properties, such as the role of surface and interface recombination, and to explore novel heterostructured device effects not possible in conventional microscale planar devices.



Selected publications

  • Synthesis, Fabrication, and Characterization of Ge/Si Axial Nanowire Heterostructure Tunnel FETs, Shadi A. Dayeh, Jianyu Huang, Aaron Gin, and S. T. Picraux, Proceedings of the IEEE Nano 2010 Intl. Conference, Seoul, Korea, 2010.
  • Diameter-dependent electronic transport of Au-catalyst/Ge-nanowire Schottky diodes, F. Leonard, A.A. Talin, B.S. Swartzentruber, S.T. Picraux, Phys. Rev. Lett. 102, 106805 (2009).
  • Ultrafast electron and hole dynamics in germanium nanowires, R.P. Prasankumar, S. Choi, S.A. Trugman, S.T. Picraux, A.J. Taylor, Nano Lett. 8, 1619 (2008).

Energy-related research

Applications of nanowires to solar cells and Li ion battery anodes for energy harvesting and storage are being studied. Arrays of vertical nanowires are synthesized as radial p-i-n structures to examine their optical absorption and photovoltaic response. Optical absorption, carrier separation, and surface/interface recombination are of particular interest. Growth of nanowires on non-crystalline substrates is also being pursued. In the area of energy storage, heterogeneous Si- and C-based quasi 1D structures are being fabricated as well as high density arrays. Li cycling storage, mechanical properties, electrical transport, and stability are also being studied. This latter work is primarily done in collaboration with colleagues at the DOE Energy Frontier Research Center, Nanostructures for Electrical Energy Storage (NEES) lead by the University of Maryland.












Nanomaterials integration

We are investigating combined top-down and bottom-up synthesis, directed assembly, and novel fabrication approaches for the integration of nanomaterials. A current area of emphasis is the formation of vertical and lateral nanowire arrays integrated onto various substrates. Methods include electron beam lithography, nano-biotemplating, directed assembly, fill and planarization for top contacting, etc. We consider this area an essential part of our investigations since in order to realize the potential of nanoscale materials, methods are needed to incorporate these building blocks into micro- or macro- scale systems. Thus advances of nanotechnology will ultimately be enabled by the invention, development, and refinement of methods for the integration of nanomaterials into useful architectures and systems. Nanowire integration will require diverse materials to be combined across length scales and into nanosystems to achieve novel properties and performance.














Selected publications

  • Epitaxy of Ge Nanowires from Biotemplated Au Nanoparticle Catalysts, Y. Sierra-Sastre, S. A. Dayeh, S. T. Picraux, C.A. Batt, ACS Nano 4, 1209 (2010).
  • Integration of nanowire devices in out-of-plane geometry, P Manandhar, E A Akhadov, C Tracy, S T Picraux, Nano Letters 10, 2126 (2010).
  • Vertical growth of Ge nanowires from biotemplated Au nanoparticle catalysts, Yajaira Sierra-Sastre, Sukgeun Choi, S. T. Picraux, Carl A. Batt, J. of the Amer. Chem. Soc. 130, 10488 (2008).
  • Assembly and magnetic properties of nickel nanoparticles on Si nanowires, S. Ingole, P. Manandhar, J.A. Wright, E. Nazaretski, J.D. Thompson, S.T.Picraux, Appl. Phys. Lett 94, 223118 (2009).