AVS2001 Session PN-MoA: Nanostructures from 0 to 3 Dimensions

Monday, October 29, 2001 2:00 PM in Room 133

Monday Afternoon

Start	Invited?	Item
2:00 PM	Invited	PN-MoA-1 Inorganic Nanorods: Synthesis, Alignment, and Properties A.P. Alivisatos (University of California, Berkeley) Inorganic nanocrystals with well defined shapes are important for understanding basic size-dependent scaling laws, and may be useful in a wide range of applications. Methods for controlling the shapes of inorganic nanocrystals are evolving rapidly. This talk will focus on a strategy that involves pyrolysis of organometallic precursors in mixtures of hot organic surfactants. The surfactant mixtures can be used to control the growth rates of different facets of the nanocrystals, allowing for wide tunability of shape. This will be illustrated with CdSe and Co nanocrystals. Both of these materials show pronounced variation of fundamental properties with aspect ratio. The nanorods can be aligned in a variety of ways. For instance, monolayers of surfacatant coated rod-like nanocrystals of these materials display a very rich phase diagram, analogous to the phases of liquid crystals. Block copolymers can be used to orient the rods. Finally, very special inorganic structures, tetrapods consisting of four rods at the tetrahedral angle, will always spontaneously align perpendicular to a surface. The possible application of these aligned nanorods in biological detection, photovoltaics, and light emitting diodes will be described briefly.
2:40 PM		PN-MoA-3 Interfacial Controlled, Self-assembled Copper-Oxide Nano-dots and Rings on SrTiO₃(001) Y. Liang, D. McCready, S. Lea (Pacific Northwest National Laboratory) Since oxides exhibit a much wider range of novel electronic, optical, magnetic, and dielectric properties than semiconductors and metals, oxide nano-structures have the potential to provide systems with much higher functionality and richer behaviors. We have successfully synthesized copper oxide nano-dots and rings on SrTiO₃ substrates using oxygen-plasma assisted molecular beam epitaxy. The morphological, structural, chemical, and electronic properties were examined by XPS, AFM, scanning Auger microscopy, and high-resolution XRD. Four types of nanostructures were found under different synthesis conditions: truncated dots, square pyramids, multifaceted domes, and corral-like rings. XPS measurements on band offsets show that Cu₂O/SrTiO₃ exhibits a type-II heterojunction with both the valance and conduction bands of Cu₂O higher than that of SrTiO₃, i.e quantum dot for electrons but anti-quantum dot for holes. Consequently the photo-excited electrons and holes are spatially separated with holes being confined to Cu₂O quantum dots and electrons confined to SrTiO₃, a property important for photocatalysis and other applications. We are currently using the surface-potential AFM and magnetic-force microscopy to elucidate the nano-scale spatial charge separation from the nanodots and the induced magnetic property from the nano-rings. Pacific Northwest Laboratory is a multiprogram national laboratory operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC06-76RLO 1830.
3:00 PM		PN-MoA-4 Charge Mediated Selective Reaction and Assembly of Functional Nanostructures T. Alvarez, S.V. Kalinin, D.A. Bonnell (University of Pennsylvania) A primary challenge in fabricating functional devices with nanometer scale components is to position and connect a number of dissimilar electroactive device elements in a predefined scheme. We will demonstrate the use of local electric fields to constrain selective chemical reactions that produce and position metallic nano contacts and metallic wires. The local electric fields are induced by atomic polarization in a semiconducting ferroelectric substrate. Nanometer scale (30-80 nm) metal nanocontacts are deposited by photoreduction and the size dependence of interface contact potential quantified. Subsequent reactions similar to conventional self-assembly allow functional elements such as organic molecules that act as diodes or NDR devices to develop into complex structures. Atomic polarization, charge compensation in the substrate, and local properties of nanocontacts are characterized with variants of scanning probe microscopy. The approach will be demonstrated with Ag nanocontacts on BaTiO3 substrates. The generalization of the approach to broader materials sets and to 3-D structures will be discussed.
4:00 PM		PN-MoA-7 Size Selective Electrochemical Growth of Molybdenum Disulfide Nanowires and Nanoparticles K.H. Ng, K. Inazu, R.M. Penner, J.C. Hemminger (University of California, Irvine) MoS₂ nanoparticles and nanowires having mean diameters ranging from 2 to 200 nm were synthesized on the basal plane and step edges of highly oriented pyrolytic graphic surface using an electrochemical/chemical (E/C) method. This method involves the following steps: (1) electrochemical deposition of molybdenum dioxide precursor nanostructures consisting of nanowires or nanoparticles onto an electrode surface, (2) displacement of oxygen in MoO₂ with sulfide by heating the sample at 500°C in H₂S. Each metal oxide nanostructure was converted into a semiconductor nanoparticle of the same shape and size as the original MoO₂ precursor nanostructure. Selected-area electron diffraction (SAED), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) confirm the formation of MoS₂ nanostructure. For MoS₂ nanostructure having a critical dimension larger than 20 nm, photoluminescence spectroscopic analysis showed a strong, room temperature emission peak around 2 eV which is consistent with the direct bandgap of this material. As the critical dimension was reduced from 20 to 2 nm, the energy of this emission shifted to higher energies qualitatively as predicted by the effective mass, strong confinement model.
4:20 PM		PN-MoA-8 Generation of Self Organized Nanometer Structures on Surfaces of Ionic Solids Generated by Electron and Laser Beam Irradiation - The Role of Water K. Nwe, S.C. Langford, J.T. Dickinson (Washington State University) Radiation effects on hydrated single crystals are poorly understood. We find that dense ordered arrays of nanoscale conical structures, with aspect ratios greater than 200, are produced when single crystal brushite (CaHPO4.2H2O) is exposed to energetic electrons (2 keV). All cones are pointed directly towards the incident electron beam. Other three dimensional nanostructures are generated by exposing brushite to excimer laser irradiation. We show that the mechanism involves: (a) photo/electron stimulated decomposition of the matrix, and (b) thermally stimulated migration of water (in this case, crystalline) and ionic material. We have isolated these factors and present plausible mechanisms for structure formation where water plays a dominant role. In addition, we have recently exposed non-hydrated ionic crystals to radiation in the presence of background water at partial pressures of 10-7 Torr, which yields exceedingly fine structures (sub-10 nm). These materials show very interesting optical and luminescence properties which we will present. Finally, we show that in the presence of water vapor, the rates of both laser and electron beam induced decomposition/desorption increase by as much as an order of magnitude. We present evidence that this is a highly localized, defect mediated mechanism.
5:00 PM		PN-MoA-10 Optical Behavior In Semiconductor-Transparent Conducting Oxide Nanocomposite Thin Films Fabricated by RF Magnetron Sputtering T.J. Bukowski, K. Simmons-Potter (Sandia National Laboratories); P. Lucas, J.H. Simmons (University of Arizona) Semiconductor nanocrystals were fabricated within a transparent conducting oxide matrix using a dual gun rf magnetron sputtering system. The resulting nanocomposite film is composed of a multilayer structure with controllably thin semiconductor layers forming isolated clusters. Through manipulation of the deposition conditions and subsequent anneals, the size of the nanocrystals can be tailored. This work examines the optical behavior of germanium quantum dots surrounded by indium tin oxide (ITO). ITO has transmittance as high as 90% and resistivities in the range of 7x10^-5-5x10^-4 Ωcm. The presence of a transparent conducting oxide matrix surrounding the semiconductor nanoclusters can lead to interesting optical and electronic properties, having exciting photoconductive and photovoltaic applications. Raman and absorption spectroscopies are used to study the development and structure of the multilayer films and to observe changes due to the quantum confinement effect. Photoconductivity is also reported in order to gain insight into the electronic properties of the nanocomposite.