AVS2004 Session NS2-ThM: Nanowires I

Thursday, November 18, 2004 8:20 AM in Room 213D

Thursday Morning

Start	Invited?	Item
8:20 AM		NS2-ThM-1 MOCVD Synthesis of Group III Nitride Nanowires and Heterostructure Nanowires G.T. Wang, J.R. Creighton, P.P. Provencio, W. Pan (Sandia National Laboratories) Nanowires based on the direct bandgap semiconductor Group III nitride (AlGaInN) materials system are attractive due to their potential in novel optoelectronic applications, including LEDs, lasers, high power transistors, and sensors. To date, the primary growth methods used to synthesize GaN nanowires have been thermal evaporation or chemical vapor deposition techniques using Ga metal or GaN powder source materials in hot-wall tube reactors. These evaporation-based techniques suffer from a general lack of control, reproducibility, scalability, and the ability to produce complex heterostructures. Metal-organic chemical vapor deposition (MOCVD) has achieved widespread commercial adoption for the growth of III-nitride films and devices, with the demonstrated ability to produce complex heterostructures and doping. We have employed a MOCVD process to synthesize GaN nanowires in a standard cold-wall rotating disk reactor on 2-inch diameter wafer substrates coated with Ni catalysts. TEM, EDS, and photoluminescence studies indicate that the nanowires are single-crystalline GaN with Ni clusters at the tips, indicating growth via the vapor-liquid-solid (VLS) mechanism. The nanowires have tip diameters typically from 20-100 nm and lengths of up to tens of microns. We have also been able to synthesize core-shell heterostructure nanowires consisting of a GaN cores and various III-nitride shell materials, including AlN, InN, and AlGaN, and InGaN. The growth processes and reactor environment employed in this study are typical of those used to synthesize device-quality III-nitride films and should be scalable to larger commercial reactors and substrates. The optical and electrical properties of single nanowires and heterostructure nanowires along with the challenges of the MOCVD nanowire growth process will also be discussed.
8:40 AM		NS2-ThM-2 Ultralong and Portable Semiconductor Nanowire Arrays Q. Li, E.C. Walter, W. van der Veer, R.M. Penner (University of California, Irvine) Long semiconductor nanowires, organized into parallel arrays, are desirable for a variety of nanoelectronic applications. Most of the current synthesis methods produce nanowires that are randomly distributed and effort must be expended to organize the nanowires onto solid surface for electronic applications. Here we propose a hybrid electrochemical/chemical method for the synthesis of millimeter-long semiconductor nanowires that are organized into arrays on solid surface. Our method involves two steps: First, electrochemical step edge decoration was adopted to obtain the precursor nanowires on highly oriented pyrolytic graphite (HOPG). Second, the as-deposited nanowires were chemically converted to semiconductor nanowires. MoS₂ nanowire arrays, a semiconductor material stable in moist air up to 80°C, were synthesized by heating electrodeposited MoO₂ nanowires in H₂S at elevated temperatures. The nanowires were characterized by TEM, SEM and XRD. Two discrete structures were observed depending on the conversion temperature. For nanowires annealed at or below 700°C, the MoS₂ nanowires were composed of randomly distributed 5-10 atomic layer thick MoS₂ ribbons. For nanowires annealed at 800°C, MoS₂ atomic layers oriented parallel to the HOPG basal plane. Their diameters were easily controlled by their precursor MoO₂ nanowires. The electronic and optical properties were probed by transferring the nanowires onto suitable surfaces. Conductivity in both types of wires was thermally activated and the thermal activation energy was tuned from 125meV (700°C annealing) to 25meV (800°C annealing), lower than the reported MoS₂ thin films. The optical adsorption spectra showed two excitons, which blueshifted as a function of nanowire thickness due to the quantum confinement. Such organized and portable nanowire arrays are promising for nanoelectronics applications.
9:00 AM		NS2-ThM-3 Fabrication of Gold Nanowires by Non-contact Atomic Force Microscopy M.E. Pumarol, Y. Miyahara, P. Grutter (McGill University, Canada) Interfacing nanostructures to the macroscopic world is fundamental for their study and possible electronic applications. SPM-based metal deposition techniques are an attractive approach for this goal: due to their easy implementation and the possibility of a maskless direct modification of the surface. These techniques exploit the very intense electric field that appears when an SPM tip is in close proximity with a surface and a potential difference is applied. Here we use a commercial AFM operated in a dynamic mode and in ambient conditions for direct writing /patterning of gold nanowires. For their fabrication, voltage pulses of 20 â?" 30V are applied to a gold coated AFM tip and an insulating surface with a finite tip-surface gap of several nanometers. This produces gold dots with lateral dimensions from under 10 nm to 100 nm, and by increasing the deposition duty cycle dots are overlapped to form a nanowire. Control of the tip sample separation is critical to ensure the reliability and reproducibility of the deposition process. In this work, we use an innovative non-contact based technique to precisely control this separation and by extension the electric field. An advantage of this method of deposition is the ability of locating the part of the sample to which the nanowire will be contacted / attached. Here, we bridge gold macro-electrodes, deposited by EBL, by forming a nanowire between them. A new way for overcoming proximity effects of the AFM tip with a protruding electrode is presented. In the future, this technique will be useful for attaching contact leads to nanostructures like q-dots, nanodots, nanoparticles, and others.
9:20 AM		NS2-ThM-4 Controlled Polymerization of Substituted Diacetylene Self-assembled Monolayers Confined in Molecule Corrals T.P. Beebe, Jr. (University of Delaware); A. Schnieders (ION-TOF USA, Inc.); S.P. Sullivan (University of Delaware) The ever growing need to further miniaturize integrated circuits has lead to an increase in research on nanoelectronics. We have shown that it is possible to directly polymerize self-assembled 10, 12-tricosadiynoic acid (TCDA) adsorbed on highly oriented pyrolytic graphite (HOPG) at the solid/liquid interface using a Scanning Tunneling Microscope (STM) tip. Polymerized oligomers are formed at a predefined point where a voltage pulse is applied while operating in Scanning Tunneling Spectroscopy (STS) mode. The oligomers can be confined and controlled on the nanometer scale using molecule corrals created on the substrate via ToF-SIMS Cs⁺ ion bombardment. In over ~ 150 observations polymerized oligomers never extended over domain boundaries or corral edges, providing natural connection points to possibly test the electrical properties of the nanowires. The quasi-infinite supply of diacetylene molecules remaining in the covering solution enables a dynamic exchange of molecules to the surface. This exchange occurred on approximately the same time scale (10^-1 s) as it does to collect one image, and depends weakly on the length of the desorbing oligomer. The desorption is thus likely influenced by tip-surface interactions as is often the case in STM experiments. A theoretical model is currently being developed to further our understanding of the effect of oliogmer length on the rate of oligomer desorption from the HOPG surface.
9:40 AM	Invited	NS2-ThM-5 Semiconducting Nanowires - Synthesis, Characterization and Novel Properties S.-T. Lee (City University of Hong Kong, China) Oxide-assisted growth (OAG) via thermal evaporation is introduced to produce large-quantity, high-purity (no metal contamination) silicon nanowires. OAG is a generic synthetic method that can produce a host of one-dimensional semiconducting nanowires, including those of Group VI (Ge, C, SiC), III-V (GaN, GaAs, GaP) and II-VI (ZnO, ZnS, ZnSe) elements. Silicon nanowires are produced with controlled diameter, desired orientation or pattern, and morphology (wire, chain, ribbon, cable). The structural, optical, electronic, and chemical properties of silicon nanowires have been characterized. Atomically-resolved STM images revealed detailed atomic structure of Si nanowires, while STS measurements demonstrated quantum size effect in the bandgap of Si nanowires. Regular arrays of intramolecular junctions in Si nanowires are shown to exhibit sharp conductivity changes across junctions. Si nanowires give strong polarized green-red emission, and exhibit interesting chemical and sensing properties. We further show properly assembled nanowires possess strong photoluminescence and lasing properties. The results offer exciting opportunities for research and applications in nanoscience and nanotechnology.
10:20 AM		NS2-ThM-7 Strain Mapping in Nanowire Heterostructures J.L. Taraci (Arizona State University); M.J. Hÿtch (Centre National de Recherche Scientifique, France); T. Clement, J.W. Dailey, D.J. Smith, P. Peralta, J. Drucker, S.T. Picraux (Arizona State University) A new method for the detailed strain analysis of nanowires and nanowire heterostructures will be discussed. This technique enables strain mapping of nanowires based on the combination of high resolution electron microscopy and image analysis. The accuracy to which strain may be determined using this method is better than 0.3%, which allows for the accurate strain mapping of core-shell and heterostructure nanowires. The technique is applied to nanowires grown by vapor-liquid-solid CVD using disilane and digermane. We will show how this technique can be used to obtain detailed distributions of ε_xx, ε_yy, ε_xy, mean dilatation, and rotation maps within individual nanowire heterostructures. We first demonstrate the method by analysis of a single Ge nanowire which displayed a linear rotation along the growth axis, with the nanowire in compression and tension on either side of the central axis. The measured results are shown to be in agreement with a nanomechanics description of the nanowire for a bending moment applied at the end of the cantilevered nanowire beam. We then present preliminary results for Si-Ge core-shell and heterointerface nanowires. This technique allows for the direct strain mapping at heterostructure interfaces due to the lattice mismatch. In these studies the Si/Ge growth is carried out at 400°C and below to minimize any chemical interdiffusion effects on the strain profiles. The resulting strain maps of nanowire heterostructures can then be directly compared with Stillinger Weber modeling of the anticipated Si-Ge strain distributions, assuming chemically abrupt interfaces. The large aspect ratio nanowire structures allow rapid lateral relaxation with distance from the interface and thus provide an interesting contrast to conventional strained layer heterostructures.
10:40 AM		NS2-ThM-8 Direct Atomically Resolved Imaging inside a Nanowire A. Mikkelsen, N. Skold, L. Ouattara, M. Borgstrom, J.N. Andersen, L. Samuelson, W. Seifert, E. Lundgren (Lund University, Sweden) Semiconductor nanowires are perceived as future components in nanoelectronics and photonics. Applications, such, bio/chemical sensors , n- p- type diode logic and single nanowire lasers have already been realized in the laboratory. Because of the extremely small dimensions of a nanowire, atomic scale structural features can have a significant impact on their properties. The large surface to bulk ratio of tailor-made nano-crystallites and low dimensional systems as compared to usual bulk crystals can result in new crystal structure and morphology not found in bulk equivalents. Therefore, structural methods that address these issues are highly desirable. One such method is Scanning Tunneling Microscopy (STM) that has revolutionized our perception nano-scale objects and low-dimensional systems. In this study we demonstrate a new powerful method to image individual atoms inside freestanding III-V semiconductor nanowires using a combination of Cross-Sectional Scanning Tunneling Microscopy and a novel embedding scheme. We image areas of the nanowire with atomic resolution both along the wire, and through the face of the wire. Utilizing this method we for example image the individual atoms in planar twin segments of the wire and show that individual atomic impurities in a GaAs nanowire can be imaged. Finally we image the GaAs nanowire at the substrate interface revealing intriguing details about the initial growth of the nanowire.
11:00 AM		NS2-ThM-9 Time-Resolved X-Ray Excited Optical Luminescence Studies of Semiconductor Nanowires¹ R.A. Rosenberg, G.K. Shenoy (Argonne National Laboratory); S.T. Lee (University of Hong Kong, China); F. Heigl (Canadian Synchrotron Radiation Facility); P.-S.G. Kim, X.-T. Zhou, T.K. Sham (University of Western Ontario, Canada) Due to quantum confinement effects nanostructures often exhibit unique and intriguing fluorescence behavior. X-ray excited optical luminescence (XEOL) provides the capability to chemically map the sites responsible for producing low energy (1-6 eV) fluorescence. By taking advantage of the time structure of the x-ray pulses at the Advanced Photon Source (APS, ~80 ps wide, 153 ns separation) it also possible to determine the dynamic behavior of the states involved in the luminescence. In this presentation we show how this technique can be utilized to understand the XEOL from silicon nanowires (~14 nm diameter) and show preliminary results from studies of II-VI nanoribbons. Previous XEOL studies of silicon nanowires have revealed luminescence in the 400-700 nm region.²^,³ The lower wavelength part of the spectrum is associated with the oxide shell while longer wavelength emission is due to the silicon core. The present results support these findings. In addition we find that the longer wavelength, silicon core emission has a relatively short lifetime (<10 ns) while the oxide shell fluorescence has a much longer lifetime. These results will be discussed in terms of prior time-resolved work on porous silicon and related systems. In addition we plan to present initial results from studies of ZnS, ZnTe, CdSe and CdS nanoribbons. ¹Work supported by U.S. Department of Energy, Office of Basic Energy Sciences under Contract No. W-31-109-ENG-38. ²X.-H. Sun, Y.-H. Tang, R. Zhang, S.J. Naftel, R. Sammynaiken, T.K. Sham, H. Y. Peng, Y.-F. Zhang, N.B. Wong, and S.T. Lee, J. Appl. Phys. 90, 6379 (2001). ³T.K. Sham, S.J. Naftel, P.-S. G. Kim, R. Sammynaiken, Y.H. Tang, I. Coulthard, A. Moewes, J.W. Freeland, Y.-F. Hu, S.T. Lee, Phys. Rev. B, to be published.
11:20 AM		NS2-ThM-10 Mechanical and Electromechanical Behaviour of Li⁺(Mo₃Se₃)^- Nanowires and Nanowire Bundles A. Heidelberg (Trinity College Dublin, Ireland); J.W. Schultze (Heinrich-Heine-Universität Düsseldorf, Germany); J.G. Sheridan, B. Wu, J.J. Boland (Trinity College Dublin, Ireland) Li⁺(Mo₃Se₃)^- forms quasi-1D crystals and is structurally related to the Chevrel phases ¹. It can be viewed as a condensation polymer of (Mo₃Se₃)^- units. In the crystal the {(Mo₃Se₃)^-}_n strands are separated by Li⁺ counterions. The crystals dissolve in polar solvents with ε > 45 yielding conductive polyelectrolytes. From solution conductive single nanowires with a diameter of 0.6 nm and bundles of nanowires were deposited on substrate surfaces. The bundle height is typically between 10 and 100 nm and the length exceeds 5 μm ². Mechanical measurements on Li⁺(Mo₃Se₃)^- nanowire bundles with a height range between 25 and 200 nm have been carried out using a SPM-nanomanipulator. For the experiments nanowires were deposited out of solution across trenches on SiO₂. The trench depth was typically between 100 and 300 nm and the width between 1 and 3 μm. To prevent any slippage of the nanowires during the manipulation, they were pinned down by E-beam induced deposition of Pt at the trench edges in a dual beam FIB/SEM system. The size of the Pt lines varied depending on the size of the wire of interest. Lateral manipulations on nanowire bundles yielded force traces. Taking into account the wire shape and dimensions as well as the AFM cantilever dimensions, the Youngs modulus, the yield strength and the maximum bending strength of the nanowires can be obtained from the force traces. The Youngs modulus for Li⁺(Mo₃Se₃)^- nanowires has been measured to be in the range of 500 to 600 GPa. The electromechanical properties of nanowire bundles under mechanical stress were also measured. ¹ ¹ R. Chevrel, M. Sergent, J. Prigent, J. Solid State Chem 3 (1971) 515 ² A.Heidelberg, J. W. Schultze, C. J. Booth, E. T. Samulski, J. J. Boland, Z. Phys. Chem. 217 (2003) 573.
11:40 AM		NS2-ThM-11 Strong Field Emission of Taper-Like and Rod-like Si Nanowires Grown on Si_XGe_1-X Substrate Y.-L. Chueh, L.J. Chou, S.L. Cheng, J.H. He, W.W. Wu, L.J. Chen (National Tsing Hua University, Taiwan) Taper-like and rod-like Si nanowires (SiNWs) have been synthesized on Si and Si_0.8Ge_0.2 substrate annealed at 1200 °C in N₂ ambient. The tip regions of taper-like SiNWs are about 5-10 nm in diameter. The average length of the taper-like SiNWs is about 6 µm with aspect ratios is around 150-170. On the other hand, the rod-like is 5-100 nm in diameter, and 4-5 µm in length. The proposed growth models of there nanowires are oxide-assisted growth (OAG) and vapor-liquid-solid (VLS) growth. The taper-like morphology may be created by the passivation of the SiO₂ coating layer, and resulted in the different levels of absorption of SiO along the nanowires. The formation of metal-catalyst free rod-like SiNW is due to creation of unstable thin SiO_x layer, which vaporized easily during the annealing process. The optical and field emission characterization of these SiNWs have been investigated and present. Taper-like Si nanowires exhibit a superior field emission with a turn-on field of 6.3-7.3 V/µm and a threshold field of 9-10 V/µm. The β value are estimated to be 700 and 1000 at low and high fields, respectively. The excellent field emission characteristics are attributed to the perfect crystalline structure and taper-like geometry of the Si nanowires.