AVS2004 Session NS2-ThA: Nanowires II

Thursday, November 18, 2004 2:00 PM in Room 213D

Thursday Afternoon

Start	Invited?	Item
2:00 PM	Invited	NS2-ThA-1 Gaede-Langmuir Award Presentation: Ultra-high Vacuum Electron Microscopy for Surface Analysis and Nanomaterials K. Takayanagi (Tokyo Institute of Technology, Japan) Nanostructures are investigated increasingly by ultra-high vacuum (UHV) electron microscopy in the last decades. In recent years scanning tunneling microscope (STM) and/or atomic force microscope (AFM) is incorporated in UHV electron microscope apparatus to study not only structures but also properties in-situ. Also a great advance in the optics of the electron microscope, aberration free lens, allows us to image individual atoms (or atomic columns) within nanostrucutres. I present recent development of our UHV high-resolution electron microscopy that has an aberration free lens and a scanning probe microscope, and results on conductance quantization of metal nanowires and forces acting on the free-standing nanowire that spans between electrodes
2:40 PM		NS2-ThA-3 VLS Epitaxy of Si Nanowires: Kinetics and Morphology T. Clement, J.L. Taraci, J.W. Dailey, D.J. Smith, J. Drucker, S.T. Picraux (Arizona State University) The high aspect ratios and novel electronic and chemical properties of silicon nanowires have made them increasingly interesting for applications ranging from flexible electronics to chemical sensors to microfluidic surfaces. However the mechanisms behind vapor-liquid-solid (VLS) crystal growth have received only limited attention since the seminal work of Wagner for whisker growth more than 30 years ago.¹ Here we study the VLS kinetics and growth modes dominating CVD grown Si nanowires (SiNWs) as a function of growth pressure and temperature. In contrast to our previous studies of Ge nanowires, we observe SiNW growth throughout a wide range of pressures and temperatures. Au nanodots with a nominal thickness of 2.5 nm and average lateral separation of 30 nm are evaporated in UHV on hydrogen terminated Si(111) and Si(100) surfaces; these Au nanodots function as the seeding species for VLS nanowire growth. Subsequent growth using disilane (Si₂H₆) gas at pressures ranging from 1x10^-2 to 1x10^-6 Torr and temperatures ranging between 300°C and 700°C produces highly uniform SiNW structures at low temperatures and unstable structures at higher temperatures, specifically kinks and nonuniformity in SiNW diameter. The initiation of nanowire growth is a nucleation-limited process, and we also report on new in situ optical scattering studies of SiNW nucleation times. Finally, characterization via field-emission SEM, RBS, and TEM will establish the SiNW growth rate dependence on substrate temperatures and pressures. Examples of the dramatic superhydrophobic and superhydrophilic behavior of functionalized SiNW surfaces will be given. ¹ R.S. Wagner, in Whisker Technology, Levit, A.P. ed, (Wiley-Interscience, New York, 1970) pp 47-119.
3:00 PM		NS2-ThA-4 Growth Of ZnO Nanowires and Their Applications in Dye Sensitized Solar Cells J.B. Baxter (University of California Santa Barbara); M. Reichman (University of Texas-Austin); E.S. Aydil (University of California Santa Barbara) ZnO is a wide band gap semiconductor (Eg = 3.37 eV) with applications in UV optoelectronics, varistors, piezoelectronics, and photovoltaics. Nanostructured ZnO can enable applications that require high semiconductor surface area such as sensors and dye sensitized solar cells. We have grown single crystal ZnO nanowires ~80 nm in diameter and several microns long by chemical vapor deposition (CVD) using the organometallic precursor zinc acetylacetonate (Zn(acac)₂) in the presence of oxygen. Dense arrays of ZnO nanowires exhibit large surface areas and can be grown on various oxide substrates, making them suitable as the mesoporous semiconductor in dye sensitized solar cells. Single crystal ZnO nanowires offer improved conduction pathways compared to sintered nanoparticles used currently, where electron transport occurs by a hopping mechanism. The surface area for dye adsorption can be increased significantly by extending the growth time to allow for the nucleation and growth of smaller secondary nanowires from the primary nanowires, improving current densities in the cells. We have used these types of nanowires to produce initial solar cells with short circuit current densities of 75 µA/cm², open circuit voltages of 0.63 V, and fill factors of 39 % when illuminated with 100 mW/cm² simulated solar light. A limitation of the initial solar cells is poor light harvesting, with less than 10% of incident light absorbed by the dye. We are currently investigating methods for seeding nanowire growth to improve the nucleation density, which will increase nanowire surface area and dye adsorption. Transport properties of the nanowires can be significantly enhanced by treatment in hydrogen plasma at room temperature. H atoms passivate defects and increase conductivity by increasing carrier densities. H exposure improves the UV photoluminescence of the nanowires and both the fill factor and the open circuit voltage of the solar cells.
3:20 PM	Invited	NS2-ThA-5 Progress Towards Silicon Nanowire-based Complementary Logic T.S. Mayer, Y. Wang, T.-T. Ho, K.-K. Lew, L. Pan, E.C. Dickey, J.M. Redwing (Penn State University) There has been considerable interest in bottom-up assembly of semiconductor nanowires for their application in future logic, memory, and sensor circuits. In this talk, we will present results of recent research showing that p- and n-type dopants can be intentionally incorporated into silicon nanowires (SiNWs) during template-directed vapor-liquid-solid (VLS) growth to produce complementary field effect devices. In this work, Au metal particles electrodeposited within 80-nm diameter pores of anodized alumina templates serve to catalyze SiNW growth at temperatures of 500° C using 10% silane (SiH₄) in H₂ as the silicon gas source, trimethylboron (TMB) as the p-type dopant, and phosphine (PH₃) as the n-type dopant. Transmission electron microscopy (TEM) studies of individual SiNWs show that approximately two-thirds of the SiNWs are single crystal, while the remaining one-third are bicrystals. Additionally, the surfaces of all of the p- and n-type SiNWs investigated were free of amorphous layers that were observed previously when diborane was used as a p-type dopant gas. Secondary ion mass spectroscopy (SIMS) on bundles of SiNWs indicate that B- and P-concentrations increase with increasing TMB:SiH₄ or PH₃:SiH₄ gas ratios between 10^-5 and 10^-2, and can exceed 10¹⁹ cm^-3 for the highest gas ratios investigated. Gate-dependent conductance measurements of individual B- and P-doped SiNWs show complementary characteristics that are consistent with depletion mode device operation, where the threshold voltage is adjusted by changing the dopant:SiH₄ gas ratio during VLS growth. Independent measurements of four-point resistivity also show a clear decrease in resistivity with increasing TMB:SiH₄ or PH₃:SiH₄ gas ratios. These results confirm that p- and n-type dopants can be effectively incorporated during SiNW synthesis to produce complementary field effect devices in the same material system.
4:00 PM		NS2-ThA-7 The Role of Electrodeposited Metal Nanowires in Gas Sensing B.J. Murray, E.C. Walter, R.M. Penner (University of California, Irvine) It in unclear what role, if any, metal nanowires have to play in chemical sensing. While the literature is ripe with examples of sensors based on semiconductor nanowires, there have been very few examples using metal nanowires. For these investigations - silver, copper, gold, and platinum nanowires were prepared by Electrochemical Step Edge Decoration (ESED) on a graphite surface. These nanowires were polycrystalline, consisted of a 1-D array of fused particles 50 nm to 950 nm in diameter, and had lengths of 100 µm or more. The resistance of these metal nanowires was probed as a function of the concentration of a chemisorbing gas. Upon exposure to ammonia (NH₃), arrays of these "fused particle" wires showed a resistance increase, ΔR/R₀, that was fast, large (up to 1,000%), and reversible. Compared to literature on thin metal film sensors, the response of these nanowire arrays was much larger than expected. We propose that the elements responsible for this response were concentrated at a small minority of locations along axis of the wires. This proposed model, the Chemically Responsive Interparticle Boundary (CRIB) model, will be discussed. Finally, the chemical structure of these elements has been investigated to determine the role, if any, of air oxidation on sensor function.
4:20 PM		NS2-ThA-8 1-D Metal Oxide Sensor and Catalyst: the Comparative Study of Pristine and Surface Doped Individual Nanowire A.A. Kolmakov (University of California, Santa Barbara); S.V. Kalinin (Oak Ridge National Laboratory); Y. Lilach, M. Moskovits (University of California, Santa Barbara) We investigated transport properties of individual metal oxide single crystal nanowires and nanobelts operating in high vacuum and under â?oreal worldâ? conditions for sensing and catalysis applications. Using impedance measurements under different gas environment in conjunction with scanning probe microscopy we were able to determine the major factors contributing to charge transport in nanowire. We found that when nanowire radius is comparable with its Debye length, the adsorption/desorption of donor/acceptor molecules on the surface of the nanowire and in its proximity alters the bulk electron density inside the nanowire what sensitively modulates conductivity of the nanowire. In vivo conductometric measurements on individual nanowire during its surface doping with metal particles reveals the formation of nano-Schottky barriers which drastically enhance of the reactivity/selectivity of the nanowire as gas sensors and catalyst.
4:40 PM		NS2-ThA-9 Thermoelectric Nanowire Arrays for Waste Heat Conversion E. Menke, R.M. Penner (University of California at Irvine) No longer relegated to specialized roles like powering satellites, thermoelectric materials are garnering interest for more mundane uses like solid-state refrigerators and collecting waste heat in automobiles. A number of research groups are currently working on a variety of methods to improve the efficiency of thermoelectric materials. Our attempts to create more efficient thermoelectric devices have focused on fabricating arrays of high aspect ratio nanowires of bismuth telluride, presently the best conventional thermoelectric material for room temperature applications. I will present our method for preparing bismuth telluride and doped bismuth telluride nanowires on highly oriented pyrolytic graphite via electrochemical step-edge decoration. This will be followed by the characterization of these nanowires by scanning electron microscopy, x-ray diffraction, and energy dispersive x-ray analysis. Finally, I will end by briefly discussing our attempts to measure the thermoelectric figure of merit for these nanowires.
5:00 PM		NS2-ThA-10 Devices Formed Using Deposited Polymeric Nanofibers J. Kameoka (Cornell University); H. Liu (Cornell Univeristy); D. Czaplewski, R. Mathers, S. Verbridge, G. Coates, H.G. Craighead (Cornell University) We have used deposited polymer nanofibers for the formation of electrical, optical and mechanical devices. We used a microfabricated tip as a controlled scanning source for electrostatically driven deposition of oriented nanofibers and for interfacing the fibers to lithographically defined surface structures. Because of the ability to deposit these materials as individual oriented fibers with diameters in the range of 50 nm to ~1µ, they can be utilized in new device architectures. In this talk we describe the use of the deposited polymer nanofibers as chemical sensors and as templates for the formation of mechanical and nanofluidic devices composed of inorganic materials. Utilizing the properties of a conducting polyaniline polymer we have formed ammonia sensors comprising a single oriented fiber deposited on gold electrodes. We created mechanical devices such as silicon nitride mechanical oscillators with dimensions on the order of 100 nm, formed using deposited poly(methylmethacrylate) fibers. The oscillators were defined in a silicon nitride layer by using the fiber as a mask for reactive ion etching, followed by removal of a sacrificial underlying layer. After releasing the devices, the frequencies of the modes of oscillation of the beams were determined by laser interference techniques. We fabricated nanofluidic channels of elliptical cross-section, without the use of high resolution lithography. The sacrificial template fiber consisted of a heat-decomposable polycarbonate that was deposited on a substrate and encapsulated by a spin-on glass. The channels were formed by thermal removal of the sacrificial polymer nanofibers. The oriented nanofiber deposition method, used in these experiments, offers an approach for the rapid formation of self-assembled nanoscale devices, connected to microfabricated structures, which would be difficult to form using a completely self-assembled or completely lithographic approach.