AVS1997 Session NS-MoA: Nanowires and Electronic Properties of Materials

Monday, October 20, 1997 2:00 PM in Room K

Monday Afternoon

Start	Invited?	Item
2:00 PM		NS-MoA-1 Conductance and Stability of Atomic-Scale Gold Contacts under High Bias Voltages A. Sakai, H. Yasuda (Kyoto University, Japan) Contacts of commercial relays with gold-coated electrodes have been used to study the quantum conductance as a function of bias voltage. When these relay contacts are broken, their transient conductance traces show marked plateaus at and near the integer multiples of the conductance quantum G₀=2e²/h. The conductance histogram constructed from 10,000 traces exhibits a pronounced peak at 1G₀ and broad peaks near 2G₀ and 3G₀. These results are obtained at low bias voltages less than 0.5 V. With increasing the bias voltage, the peaks in the conductance histogram start to be suppressed. The 2G₀ and 3G₀ peaks disappear at around 1.5 V, while the large 1G₀ peak survives up to 1.9 V at room temperature. At liquid nitrogen temperature, the 1G₀ peak disappears at 2.2 V. If the contact at the 1G₀ state consists of just one atom, as is generally assumed, our results would indicate that the single-atom gold contact can carry a current of 0.137 mA at room temperature and 0.165 mA at 77 K. The absence of smearing of the conductance steps at high biases suggest that the disappearance of the conductance peaks is caused not by the thermal broadening due to Joule heating but by the contact instability due to the electromigration of contact atoms, induced by the high current density.
2:20 PM		NS-MoA-2 Doping of Semiconducting Atomic Chains T. Yamada (MRJ-NASA Ames Research Center) Due to the rapid progress in atom manipulation technology, atomic chain electronics¹ would not be a dream, where foreign atoms are placed on a substrate to form a chain, and its electronic properties are designed by controlling the lattice constant d. It has been shown theoretically that a Si atomic chain is metallic regardless of d and that a Mg atomic chain is semiconducting or insulating with a band gap modified with d². For electronic applications, it is essential to establish a method to dope a semiconducting chain, which is to control the Fermi energy position without altering the original band structure. If we replace some of the chain atoms with dopant atoms randomly, the electrons will see random potential along the chain and will be localized strongly in space (Anderson localization). However, if we replace periodically, although the electrons can spread over the chain, there will generally appear new bands and band gaps reflecting the new periodicity of dopant atoms. This will change the original band structure significantly. In order to overcome this dilemma, we may place a dopant atom beside the chain at every N lattice periods (N > 1). Because of the periodic arrangement of dopant atoms, we can avoid the unwanted Anderson localization. Moreover, since the dopant atoms do not constitute the chain, the overlap interaction between them is minimized, and the band structure modification can be made smallest. Some tight- binding results will be discussed to demonstrate the present idea. ¹T. Yamada, Y. Yamamoto, and W. A. Harrison, J. Vac. Sci. Technol. B 14, 1243 (1996).
2:40 PM	Invited	NS-MoA-3 Mechanical and Electronic Properties of Nanowires N. Agrait, G. Rubio, C. Untiedt, S. Vieira (University Autonoma de Madrid, Spain) Metallic constrictions of atomic dimensions can be formed and characterized experimentally using STM and AFM techniques. In these nanoscopic structures plastic deformation occurs as a sequence of structural transformations involving elastic and yielding stages. Mechanically these structures are much stronger (typically more than one order of magnitude) than macroscopic structures due to the fact that dislocations are energetically unfavorable. Their strength attains values of the order of those expected for a perfect crystal. The electronic conductance measured during these deformation presents abrupt changes correlated to the mechanical relaxations, since the conductance is determined by the atomic configuration at the contact ¹. In the smallest constrictions, consisting of just one atom, quantum effects become evident since the Fermi wavelength for metals is of the order of the atomic diameter. Long structures of nanoscopic dimensions (nanowires) can be also fabricated. Probing their structure at low temperatures by phonon point contact spectroscopy shows that they are crystalline and electron transport is ballistic. ¹Agrait et al, Phys. Rev. Lett. 74, 3995 (1995); Rubio et al, Phys. Rev. Lett. 76, 2302 (1996)
3:20 PM		NS-MoA-5 Single Electron Charging Effect at Ag Droplets Grown on Sb-terminated Silicon Surface at Room Temperature K.-H. Park, J.S. Ha, M. Shin, W.S. Yun, E.-H. Lee (Electronics and Telecommunications Research Institute, Republic of Korea) We have investigated the room temperature single electron charging and tunneling effects at the nano-sized Ag droplets grown on Sb-terminated Si(100) surface with scanning tunneling microscopy (STM). A well ordered Sb-terminated Si(100) surface was made by deposition of Sb on a clean Si(100) at 375 degree C, and subsequent annealing at 450 degree C. On the Sb-terminated Si(100), the dangling bonds of the surface Si atoms were saturated through bonding with the overlayer Sb dimers. The Sb layer eliminated the interaction between Ag and surface Si atoms and caused the significant change of growth behavior of Ag on silicon. In particular, the Ag islands were formed in a manner very close to the atomic defect structure of Si(100). Averge sizes of Ag droplets were controlled from 1 to 10 nm with variation of Ag coverage, and the single electron tunneling effects were studied with the local current-voltage (I-V) measurements on the various sized Ag droplets. The I-V characteristics were very sensitive to the size of Ag islands and the measurement position within the same droplet. Those behaviors were analyzed in terms of the metal(tip)-vacuum-metal(Ag)-semiconductor(Si) double junction structure. We have considered the charging and transfer mechanism of tunneling electrons from the tungsten tip in Ag-Si Shottky junction to describe I-V characteristics at room temperature.
3:40 PM		NS-MoA-6 Spectroscopy and Imaging of Supported Gold Nanocrystals at Cryogenic Temperatures L.E. Harrell, T.P. Bigioni, D.K. Guthrie, W.G. Cullen, J.T. Khoury, R.L. Whetten, P.N. First (Georgia Institute of Technology) Cryogenic scanning tunneling microscopy and spectroscopy have been used to study the electronic properties of isolated gold nanocrystals supported on Au(111) substrates. Each nanocrystal has a mass of 28k amu, and is passivated by a self assembled monolayer of alkane-thiol molecules. To promote adhesion to the substrate, the passivation layer is partially substituted with alkane-dithiol molecules. Current-voltage spectra over the nanocrystals show clear Coulomb staircases along with additional structure which is likely due to quantum size effects in the nanocrystals.¹ ¹Supported by ONR and NSF
4:00 PM	Invited	NS-MoA-7 Magnetic Nanostructures with 2D and 1D Entities C.L. Chien (Johns Hopkins University) The novel properties of nanostructured materials arise from the low dimensional entities (2D as in multilayers, 1D as in nanowire arrays, and 0D as in granular solids), the small length scales, and the interplay of the constituent materials. These materials, with controlled heterogeneity, are well suited for exploring physics in small structures and interfaces, as well as for certain technological applications. We describe several examples of magnetic nanostructures. Using layer geometry, we explore characteristics of the exchange coupling between a ferromagnet (FM) and an antiferromagnet (AF), which is featured prominently in the spin-valve giant magnetoresistance (GMR) devices. We explore the angular dependence, the finite-size effects, the interfacial nature, and other aspects of the exchange coupling. Arrays of nanowires, with wire diameter as small as 30 nm, and wire density up to 10⁶/mm², can now be fabricated using electrodeposition via special templates. These nanowires may be magnetic, non-magnetic, elemental, alloy, or multilayered. Some of the unusual magnetic, transport, and magnetoresistance properties exhibited by nanowires will be described.
4:40 PM		NS-MoA-9 Imaging Current Flow in High Temperature Bi-Superconductors F. Král (ETH Zürich, Switzerland); D.A. Bonnell (The University of Pennsylvania); G. Kostorz, L.J. Gauckler (ETH Zürich, Switzerland) Thick films (thickness of several hundreds µm) of high temperature Bi-Sr-Ca-Cu-O -based superconductors with a dominance of the 2212 phase have several orders of magnitude lower critical currents than expected. The morphology of the superconducting phase is sub micron sized platelets oriented in the plane of the film. The temperature dependence of critical current suggests that the limitation is related to microstructural weak links. Optical, atomic and magnetic force microscopy were used to relate microstructure and current distribution at variable temperature. The key to elucidating this phenomenon is to measure magnetic fields at nm spatial scales in the presence of variable current flow and at variable temperature. We present such measurements and show a strong localization of current distribution on the nm length scale, which is correlated to grain misorientation, i.e. defects.
5:00 PM		NS-MoA-10 Direct Measuring the Conductance Distributions of Conducting Paths in Percolating Ni_x(SiO₂)_1-x Thin Films by Conducting Atomic Force Microscopy E.Z. Luo, I.H. Wilson, J.B. Xu (The Chinese University of Hong Kong); X. Yan (Hong Kong University of Science & Technology) Electron transport in percolating systems has been the fundamental problem for both theorists and experimentalists. Studying this problem at nano-meter scale, such as the electron transport in individual conducting paths (CPs), is very much desired and helpful in understanding the macroscopic properties. By using conducting atomic force microscopy (C-AFM) we have imaged the conducting paths in Ni_x(SiO₂)_1-x nano-composite thin films and observed measurable changes for just below and above the percolation threshold x_c. This system was chosen since it exhibits Giant Hall Effect (GHE) when x approaching x_c. In this paper, we will present systematic and quantitative studies on conductance of individual CPs, statistic distributions and fluctuations in this material near x_c. Our experiments show that electron transport is dominated by Ohm's law for individual CPs, average in an ensemble and as well as their distributions. The late was characterized by the factor that conductance distribution is independent of the bias. Not only the average conductance shows divergence at x_c, but also there are considerable changes in the conductance distributions. There are well defined peaks in the conductance distribution for x>x_c but those peaks smeared out as x decreases to xc, which implies that there exist preferred CPs for x>x_c. Discussions on the relation between conductance fluctuation and GHE will also be given in this paper.