Session U25: Nanowires II: Metals and Oxides

{\it Ab initio} molecular dynamics study of pure and contaminated gold nanowires

Gold nanowires have the ability to form linear chains that are one atom wide and that have just a few atoms in length. One of the unexpected features of these wires is that before rupture quite large interatomic distances of $\simeq$ 3.6 {\AA} have been observed, which are most likely due to the presence of impurities. In view of these facts we recently studied [1] the effect of H, B, C, N, and S impurities on the breaking of Au nanowires, in particular how they affect the maximum Au-Au bond length. Out of all these impurities, under quasi-static pulling conditions the only one that produced an Au-X-Au close to 3.6 {\AA} was hydrogen. All the others produced distances of the order of 3.9 {\AA} or larger. As the calculations were all performed at zero temperature, it is not obvious if, or how, the vibrational motion of the atoms could change these conclusions. In order to investigate these issues we will present results of {\it ab initio} molecular dynamics for pure and contaminated (H atoms) nanowires. In particular, temperature effects cannot rule out the presence of H atoms in Au nanowires, as recently claimed by Legoas {\it et al.} [2]. [1] F. D. Novaes {\it et al.}, Phys. Rev. Lett. {\bf 90}, 036101 (2003). [2] S. B. Legoas {\it et al.} Phys. Rev. Lett. {\bf 93}, 216103 (2004). We acknowledge support from FAPESP, Capes and CNPq.   

Martensitic phase transitions in metallic nanowires

We use the NRL tight binding method (TB) and modified embedded atom method (MEAM) to investigate the spontaneous phase transition from fcc to bct for Au nanowires oriented in the (001) direction. Employing density functional theory (DFT) calculations of the energy of bulk Au under uniaxial strain along the Bain path, we find that bulk Au has a minimum along this path for a bct structure, which, under uniaxial stress of greater than $\sim $2 GPa, would become energetically favorable to the fcc structure. This state, however, is unstable with respect to shear deformation. The TB method predicts the same behavior. TB simulations of Au nanowires smaller than 2.0 nm diameter, however, indicate that that these nanowires will spontaneously relax from an original fcc structure to a bct structure even at 0 K and zero external pressure. The driving force for the transition is the surface stress along the outer boundary of the nanowire, which provides the necessary 2 GPa of total stress to effect the phase transition. Furthermore, the surface stress stabilizes the bct structure with respect to shear. Its stability is verified by TB simulated annealing and large-scale MEAM simulated annealing simulations. We will also discuss a TB model for the shape-memory NiTi alloy and its use in nanowire simulations.   

The comparison of metal coating growth on nanofibers with metal film growth on flat surfaces

Recent experiments showed that physical vapor deposition is a powerful technique to form novel one-dimensional nanostructures such as metal coated nanofibers and metallic hollow nanowires. In order to have a better understanding of metal coating growth on nanofibers and to determine it's differences with metal film growth on flat surfaces, molecular dynamics simulations are performed. Adsorption, reflection and etching events are analyzed and corresponding reaction probabilities are calculated for both flat and cylindrical coating surfaces with different radii. Our investigations showed that reaction probabilities for metal coating growth on nanofibers are very different from the reaction probabilities for metal film growth for higher kinetic energies or for large off-normal angles of incidence of Al atoms. If one considers only the reaction rates, diffusive transport of Al ions in the plasma of physical vapor deposition is found to be more favorable than ballistic transport of Al ions for the growth of Al coatings on nanofibers. These investigations provide us important insights for the growth of metal coatings on nanofibers and for the formation of hollow nanowires with different surface morphologies.   

Structural and Electronic Properties of Mo$_{6}$S$_x$I$_{9-x}$ Nanowires.

We investigate the equilibrium geometry and electronic structure of recently synthesized Mo$_{6}$S$_x$I$_{9-x}$ nanowires using {\em ab initio} Density Functional calculations. Our structure optimization calculations suggest a well-defined atomic structure within these nanowires, which are energetically unusually stable in view of their sub-nanometer diameter. For particular stoichiometries, we find the Mo$_{6}$S$_x$I$_{9-x}$ nanowires to be rather soft with respect to axial compression, and also to be metallic. We characterize the quantum conductance in these nanowires using a self-consistent nonequilibrium Green's function approach within the Landauer-Buttiker formalism. We find the charge density near the Fermi level to be delocalized along the wires, suggesting a high polarizability. For particular metastable geometries, the nanowires also exhibit a magnetic instability. Combination of atomic-scale perfection with a high structural stability and unusual electronic and transport properties lends itself to potential applications of these nanowires as unique building blocks in hierarchically assembled electronic nanocircuits.   

Non-linear current-voltages character of Au quantum point contact

In this study, we simultaneously observed the configuration and the non-linear current-voltages character (I-V) of gold quantum point contacts (Au-QPC). UHV Transmission Electron Microscope (UHV-TEM) which combined with Scanning Tunneling Microscope (STM) enabled us to observe the configuration of QPC. TEM images were synchronized with the measured I-V. The bias voltage to Au-QPC swept from 0V to 0.3V at room temperature in UHV($\sim $1$\times $10$^{?|7}$[Pa]). The Au-QPC with short length($<\sim $1nm) showed the non-linear I-V which were fitted to a cubic function ( I=aV+cV$^{3 })$. The value of c/a in our results ($\sim $20[1/V$^{2}$]) was lager than that of previous reports (0.3$\sim $2[1/V$^{2}$]). Simultaneous TEM images revealed a changed of the width of Au-QPC. The width was found to increase from 1.1nm (0.02V) to 1.9 nm (0.27V). On the other hand, the Au-QPC with long length (nanowire $>\sim $1nm) showed the linear I-V, and the width was kept constant. We suggested that the changing of the width caused the non-linear I-V. The mechanism of increasing the width should be solved by further investigation.   

First-principles simulation of the field emission from noble metal nanowires

We carry out a theoretical study on the field emission from the nanowire which is composed of noble metal elements such as silver or gold. Our calculations are based on the first-principles density functional theory within a localized basis scheme using the SIESTA package. Electronic states and the potential of the system under finite applied voltages are determined self-consistently. Through explicit time evolution of the wavefunction, we obtain the emission current and the shape of the charge density distribution of the emitted electrons from noble metal nanowire tip.   

Free-Standing Vertical Gold Nanowires from Template Synthesis

Gold nanowires are electrochemically grown in a track-etched polycarbonate membrane inside a Teflon cell containing gold plating solution. Using this method we have grown gold nanowires with diameters in the range of 20 - 200 nm and lengths on the order of 1 - 10 um. By controlling the membrane-dissolving process, we can deposit randomly oriented nanowires with the length in the plane of a substrate, or we can leave the nanowires vertically free-standing with one end still attached to a conducting base. We are currently exploring the vertical configuration in order to study the physics of individual nanowires or groups of nanowires. Quantities of interest include the cantilever mechanical resonance frequencies and the mechanical quality factor.   

Time-resolved x-ray excited optical luminescence studies of II-VI semiconductor nanowires

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, 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 ZnS, ZnTe, and ZnO nanowires. Time-gated optical spectra show that the high-energy, band-edge states have a short lifetime while the lower-energy, deep-levels have a relatively long lifetime. X-ray excitation curves are obtained using the relevant optical photons as signals and compared to the corresponding x-ray absorption spectra. We will show how these results enable us to determine the local structure of the luminescent site(s).   

Synthesis, transport studies and applications of In2O3 nanowires

Single-crystalline indium oxide nanowires were synthesized using a laser ablation method and characterized using various techniques. Precise control over the nanowire diameter down to 10 nm was achieved by using monodisperse gold clusters as the catalytic nanoparticles. In addition, field effect transistors with on/off ratios as high as 10$^{4 }$were fabricated based on these nanowires. Detailed electronic measurements confirmed that our nanowires were n-type semiconductors with thermal emission as the dominating transport mechanism, as revealed by temperature-dependent measurements. Furthermore, we studied the chemical sensing properties of our In$_{2}$O$_{3 }$nanowire transistors at room temperature. Upon exposure to a small amount of NO$_{2}$, the nanowire transistors showed a decrease in conductance of up to six orders of magnitude, in addition to substantial shifts in the threshold gate voltage. Our devices exhibit significantly improved chemical sensing performance compared to existing solid-state sensors in many aspects, such as the sensitivity, the selectivity, the response time and the lowest detectable concentrations. We have also demonstrated the use of UV light as a ?gas cleanser? for In$_{2}$O$_{3 }$nanowire chemical sensors, leading to a recovery time as short as 80seconds.   

Structure of Nanocrystals by the Atomic Pair Distribution Function Technique

Knowledge of the atomic-scale structure is an important prerequisite to understand and predict the properties of materials. In the case of crystals it is obtained from the positions and the intensities of the Bragg peaks in the diffraction data. However, many materials of technological importance, in particular nanophase materials, are not perfect crystals. The diffraction patterns of such materials show only a few Bragg peaks and a pronounced diffuse component. This poses a real challenge to the usual techniques for structure determination. The challenge can be met by employing the so-called atomic pair distribution function technique. The basic features of the technique will be introduced and its potential demonstrated with results from recent structure studies of V$_{2}$O$_{5}$ nanotubes. \textbf{Acknowledgements: }This work was supported by NSF through grant DMR-0304391.   

Gallium oxide nanostructures

Crystalline $\beta $-Ga$_{2}$O$_{3}$ nanowires with two distinct morphologies have been synthesized through simple physical evaporation of Te doped GaAs powder in argon atmosphere. Nanowires as long as hundreds of micrometers with diameters in the range of 10-100 nm have been produced with a high yield. Absence of Tellurium in the nanowires indicates that the growth mechanism is not VLS based. Substitution of sulfur in place of tellurium resulted in similar nanostructures. Some of the nanowires exhibit herringbone structure morphology and the TEM images showed hexagonal crystallites ordered in regular spacing along the nanowires axis and the crystal planes of the nanowires were parallel to one of the facets of the crystallite. The other nanowires morphology is essentially single crystalline nanoribbons. The structures of the nanowires were characterized by SEM, TEM, XRD, EDX, and Raman spectroscopy.   

A Generic Synthesis of Transition Metal Oxide Core-Shell Nanowires

A generic nonequilibrium synthesis technique has been developed to produce novel transition metal oxide nanowires, including YBa$_{2}$Cu$_{3}$O$_{6.66}$, La$_{0.67}$Ca$_{0.33}$MnO$_{3}$, PbZr$_{0.58}$Ti$_{0.42}$O$_{3}$ and Fe$_{3}$O$_{4}$. Key to our success is the growth of vertically aligned single-crystalline MgO nanowires, which worked as excellent templates for epitaxial deposition of the desired transition metal oxides and led to high-quality core-shell nanowires. Transport studies on La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ nanowires have revealed the remarkable persistence of metal-insulator transition and magnetoresistance down to nanometer scale. Our technique will enable various in-depth studies such as phase transition in nanoscale oxides and may pave the way for novel applications of these fascinating materials.   

Formation of super arrays of periodic nanoparticles and aligned ZnO nanorods - simulation and experiments

It had been demonstrated that large-scale honeycomb-like nanoparticle arrays could be fabricated inexpensively by the process of monolayer nanosphere self-assembly. Here we report that a double-layer masking procedure can be effectively used to overcome the restriction of honeycomb order in an array resulted from a monolayer mask. By varying the relative angle between the two layers, different arrangement of nanoparticles could be obtained. The relative angle can be directly controlled with the aid of diffraction patterns from illuminating the layers by a laser beam. Experimental results were fully confirmed by computer simulations. Using these nanoparticles as catalysts, we have grown arrays of aligned ZnO nanorods with various orders.