Session A31: Metallic Nanostructures

Gold Nanorod/Single-Wall Carbon Nanotube Heterojunctions Formed Directly on Surfaces

In this presentation we describe work on the formation of heterojunctions between single-wall carbon nanotubes (SWNTs) and one-dimensional (1D) gold nanorods (AuNRs) assembled directly on surfaces. The chemical procedure is a simple benchtop method using commercially available reagents. Au NRs are grown directly and selectively on surface-attached SWCNTs by depositing hexanethiolate-terminated Au monolayer protected clusters (MPCs) on the SWCNTs and growing Au MPCs into NRs by seed-mediated growth, involving reduction of AuCL$_{4}^{-}$ onto Au MPCs in the presence of cetyltrimethylammonium bromide (CTAB). UV-vis, AFM, and SEM show highly selective growth of Au on SWCNTs only. Different combinations of junctions are possible, including AuNRs connecting two CNTs or two AuNRs attached to one CNT. We also show that Au nanostructures enhance Raman scattering of SWNTs. Several NRs and CNTs integrated into a connected assembly were studied. Electrical measurements with contacts on the AuNRs of these heterostructures will be presented. Finally carbon-supported metal structures with unique size and shape may be useful in electrocatalysis or electrochemical sensing applications.   

Formation of Silver Nanostructures (nanowires, nanoshells, and nanorods) on MgZnO hexagonal and Cubic Alloys

With potential applications in many fields from fundamental science to engineering technology, multi-dimensional (MD) metal nanostructures such as nanowires, nanoshells, and nanotubes have been very popular topics of research. They have been used to experimentally probe the effects of quantum confinement on electronic, magnetic, and other related properties, and they could be used as active components or interconnects in fabricating electronic, photonic, and sensing devices. Silver nanowires are particularly interesting to explore because bulk silver exhibits the highest electrical and thermal conductivities among all metals. Other modern applications of silver nanowires have also been discovered in many fields including catalysis, electronics, photonics, and photography. In this study, we present the formation of silver nanowires, shells and rods by the thermal reduction of an aqueous silver nitrate solution on the surface of Magnesium Zinc Oxide (Mg$_{x}$Zn$_{1-x}$O) multiphase alloy thin films. In this context, the formation of silver nanostructures on MgZnO alloy films is technologically important as it would lead in to the potential area of nano metallic contacts to emerging oxide electronic materials.   

Oxygen clamps in gold nanowires.

We investigate how the insertion of an oxygen atom in an atomically thin gold nanowire can affect its rupture. We find, using \textit{ab initio} total energy density functional theory calculations, that O atoms when inserted in gold nanowires form not only stable but also very strong bonds, in such a way that they can extract atoms from a stable tip, serving in this way as a clamp that could be used to pull a string of gold atoms (see Novaes et al., Phys. Rev. Lett. 2006). In all calculations we have used a GGA-PBE approximation and norm-conserving pseudopotentials. All calculations were performed using the SIESTA code, and we employ a DZP basis function. We observe that the presence of O atoms increases the stability of a local configuration composed of the O atom and its two nearest neighbors at each side (an Au-Au-O-Au-Au structure). This indicates that in an oxygen reach atmosphere it may be possible to pull a longer string of gold atoms (an effect as predicted here has been observed by W. H. A. Thijssen et al., cond-mat/0509376). Finally, local vibrational modes will be investigated.   

Phase transitions in escape processes of metal nanowires

Thermally induced conductance jumps of metal nanowires are modeled using stochastic Ginzburg-Landau field theories. The activation rate displays nontrivial dependence on nanowire length, and undergoes first- or second-order-like transitions, with a critical length proportional to the wire radius. Several experimental manifestations of the predicted phase transition are identified, including the temperature- and radius-dependence of peak heights in conductance histograms, and the length dependence of I-V characteristics of gold nanowires.   

Stability of metal nanowires with multipolar cross sections

A linear stability analysis of metal nanowires with arbitrary cross sections is performed within the nanoscale free-electron model, focusing on quadrupolar, hexapolar, and octopolar deformations. Families of stable wires with similar cross sections and various dimensions are found, which are stabilized semiclassically in the vicinity of bifurcations of important classical periodic electron orbits. For large deformations, quadrupolar cross sections are found to be most favorable. The predicted highly-deformed stable structures are compared to experimental data for Aluminum nanowires.   

Mechanical testing of slanted and isolated copper nanorods using Atomic Force Microscope*

Recently, the study of mechanical properties of sub-micron and nano-scale specimen has drawn a renewed attention. With the atomic force microscope (AFM) it is possible to probe the mechanics of these structures. Here we report one such test to find the force constant, Young's modulus and yield stress of a new type of nanostructures - polycrystalline Cu slanted nanorods of approximately rectangular cross section, fixed at one end to the substrate. These were grown by oblique angle physical vapor deposition. An AFM was used to image, locate the rod and apply force to get deflection at its free end as a function of applied force. Samples with different dimensions and rise angles were tested in bending using this technique. The method employed by our group for fabrication as well as the mechanical testing will be discussed and a comparison of the mechanical properties of the bulk and nano-scale specimen will be made based on our results. *Supported by NSF grant No. CMS-0324490.   

Electron Transport through curved and rolled Two Dimensional Electron Gas

We report on topology dependent electron transport in tubular shaped two-dimensional electron gases. These micron-sized tubes are realized in a strained InGaAs quantum well. This is the first step towards investigating geometric potentials in low dimensional quantum systems. We investigate magneto-resistance of the tubular systems in a perpendicularly applied magnetic field. At low magnetic field, an increased zero field magneto resistance followed by a negative magneto resistance is observed. We ascribe this effect to an increase in electron scattering along the curved regions due to newly formed dangling bonds. At high magnetic fields we observe a linear increase in resistance of the curved region as compared to planar regions.   

Intensity-Intensity correlations of waves propagating in disordered quasi-one-dimensional geometries

Spatial intensity correlations between waves transmitted through random media are analyzed within the framework of the random matrix theory of transport. Assuming that the statistical distribution of transfer matrices is isotropic, we found that the spatial correlation function of the normalized intensity can be expressed as the sum of three terms, with distinctive spatial dependences. This result coincides with the one obtained in the diffusive regime from perturbative calculations, (Patrick Sebbah et al in Phys. Rev. Lett. 88, 123901,(2002)) but holds all the way from quasi-ballistic transport to localization. Only the specific value of the prefactors depends on the transport regime. Their values obtained from the Monte Carlo solution of the Dorokhov, Mello, Pereyra, and Kumar (DMPK) scaling equation are in full agreement with microscopic numerical calculations of bulk disordered wires. The experimental and numerical results are recovered in the large-N (number of propagating channels)limit in Random Matrix theory. While correlations are positive in the diffusive regime, we predict a transition to negative correlations as the length of the system decreases.   

Manipulation of nanowires in suspension by ac electric fields

While highly desirable for nanoscale devices, manipulation of nanoentities in suspension has been a formidable problem because of the extremely low Reynolds number at the level of $10^{-5}$. In this work, we show that nanowires a few µm in length can be efficiently manipulated by ac electric fields applied to strategically designed microelectrodes. The nanowires, both magnetic and non-magnetic, can be driven to align, to chain, to accelerate in directions parallel or perpendicular to the nanowires orientation. The nanowires can also be patterned into desired structures with high efficiency. This versatile method of manipulation has also been applied to other small elongated entities such as carbon nanotubes.   

On the Formation of Monatomic Metal Wires

The formation of monatomic metal wires under stress has been observed with HRTEM. It has been revealed that gold chains can be obtained if the stress is applied on the (111) and (100) orientations, while not on the (110) orientation. Other experiments have evidenced that some metals are unable to exhibit this monatomic wire formation. Theoretical efforts addressed to determine what are the geometrical and chemical conditions for such monatomic chain formations is still lacking in the literature. We have implemented Tight-Binding Molecular Dynamics simulations for the formation of metallic wires under stress, making special attention to the dynamics at the end of the contact breakage. Different geometrical orientations and chemical elements were considered. In order to determine the evolution of the electronic structure the simulations were complemented with \textit{ab initio} calculations of the atomic configurations. The study allows us to give some insights about the required conditions for the formation of such monatomic metal wires.   

Quantum confinement between self-organized Pt nanowires on Ge(001)

Annealing of Pt covered Ge(001) surface leads to formation of one atom thick, hundreds of nanometers long, literally defect free chains of Pt atoms, hereafter named as Pt nanowires. By using scanning tunneling spectroscopy (STS) we have discovered one dimensional (1D) electronic states, confined between these Pt nanowires. The nanowires are separated by either 1.6 or 2.4 nm. The Pt atoms create a potential barrier for the surface state electrons on modified Ge-terraces located just below the Fermi level. The peak positions obtained from I-V spectroscopy experiments are in good agreement with the eigenvalues of a quantum mechanical particle in box problem. The spatial mapping of the differential conductivity of the 1D states reveals that the states are confined in the troughs between the nanowires. As an additional proof, we performed careful analysis of the regions, where either the nanowires or the underlying substrate have defects. This analysis clearly shows that the confined states around the defect sites fade away, i.e. the electron states ``leak'' out of the trough via the defects.   

Hall of Mirrors Scattering from Impurities in Quantum Waveguides

We develop a scattering theory to examine how a point impurity affects transport through multimode quantum wires. While some of our new results apply specifically to hard-walled wires, others are more general; for example, an effective optical theorem which we derive for two- dimensional waveguides. Using the method of images, we examine the hard-walled guide, explicitly showing the effect of each reflection from the impurity on the wire's conductance. We express the effective cross section of a confined s-wave scatterer entirely in terms of the empty waveguide's Green's function, suggesting a way in which to use semiclassical methods to understand transport properties of smooth wires. In addition to predicting some new phenomena, our approach provides a simple physical picture for previously observed effects such as conductance dips and confinement-induced resonances. We discuss generalizations of this work to include higher partial waves, as well as the case of two interacting particles confined in other geometries, such as nanotubes and tori.   

A fine-scale nanostructure in $\gamma$-alumina

Despite the preeminent industrial importance of $\gamma $-alumina in catalysis, details of the structure remain unresolved due to its 15-30$\sim $nm domain nanocrystalline nature. Diffraction patterns are broad and single crystals are not available making accurate structural solution difficult using conventional crystallographic methods. We have applied a local structural technique, the atomic pair distribution function (PDF) analysis of powder diffraction, to obtain a quantitative structure. This is a total scattering technique that incorporates both Bragg and diffuse scattering information in the PDF, allowing all diffracted intensities from the XRD pattern to be equally considered. Surprisingly, we find a previously unknown fine-scale nanostructure with a domain size $\sim $ 1 nm. Within these nanodomains the oxygen sublattice is modified from the average structure and retains aspects of the boehmite precursor. This results in a novel and unexpected view of the $\gamma $-alumina structure since earlier controversies about it centered on the arrangement of Al ions among different cation sites, whereas the oxygen sublattice arrangement was not usually questioned.   

Conversion of CDW TaS$_{3}$ to superconducting TaS$_{2}$ nanowires

The synthesis of nanowires has attracted considerable interest for their potential applications in many areas of advanced nanotechnologies. Recently we have developed a simple method to fabricate nanowires of a transition metal dichalcogenide through a nondestructive reduction from one-dimensional (1D) trichalcogenide nanostructures.$^{1}$ In this report, we present results on synthesis and characterization of TaS$_{2}$ nanowires. Our approach includes the synthesis of 1D charge-density-wave (CDW) TaS$_{3}$ nanostructure precursors followed by the nondestructive and controlled adjustment of the S composition. The nanowires, as identified with scanning electron microscopy, have a rectanglelike cross section with widths of 20 to 700 nm and lengths of up to a few millimeters. TaS$_{3}$ nanowires show the canonical CDW behaviors. However, the converted TaS$_{2}$ nanowires show superconducting behavior with Tc $\sim$~ 4 K, which is different from the bulk property. $^{1}$ Appl. Phys. Lett. 87, 142506 (2005).