Session H16: Nanowires: Synthesis, Structural Properties and Growth Kinetics

Characteristics of Coalesced Planar GaN Layers on GaN/Si (111) Nanowires Grown By Plasma-AssistedMolecular Beam Epitaxy

GaN-based quantum well and quantum dot light emitting diodes are being investigated and developed for solid state lighting. This development has, in general, been impeded due to the lack of suitable substrates with low defect density. It has been demonstrated that dislocation free GaN nanowires can be grown on silicon substrates due to the large surface-to-volume ratio of the nanowire structure. The growth of catalyst-free GaN nanowires on (111) silicon substrates by plasma-assisted MBE has been investigated in the present study. It is seen that the average nanowire diameter is determined by the growth temperature, T$_{g}$. The diameter can be increased by lowering T$_{g}$ and the nanowires can coalesce into a planar film by continued growth. Utilizing the coalesced GaN film as a substrate, InGaN/GaN quantum wells with emission wavelength of 530 nm were grown and the photoluminescence spectra of these exhibit significantly narrower linewidth ($\sim $26 nm at 300 K) than those reported for wells grown on commercially available GaN templates ($\sim $40-60 meV). Detailed structural and optical characteristics of the nanowires and planar GaN films on top of them will be presented.   

Synthesis and Photochemical Properties of Au-Attached Titania Nanotubes Arrays

Modification of titania nanostructures by various functional groups has been a subject of active research interest. It has been reported by Haruta et al. [1] that Au particles attached to oxide surfaces, such as TiO2 have enhanced catalytic properties, such as in water splitting, CO oxidation, etc. It is expected that attachment of the Au particles to titania nanotubes will have additional advantages with respect to catalysis due to the enhanced surface area. Au nanoparticles have been attached to the surface of titania nanotube arrays by means of a deposition-precipitation method. We demonstrate a high deposition density of the gold particles over the nanotube surface and also good control over the size and coverage density of the gold nanoparticles. Photocurrent measurements using such Au attached titania nanotubes as photoanodes have been measured and compared with blank titania nanotubes. References: 1. M. Haruta, M. Date, Applied Catalysis A General 222, (2001)   

Synthesis and Characterization of Templated Si-based Nanowires via Vapor-liquid-solid (VLS) Growth for Electrical Transport

Silicon nanowires have attracted substantial attention for a variety of nanoelectronic and optoelectronic applications. Recent research has demonstrated excellent conductance and conductance scaling in NiSi nanowires further highlighting the potential applicability for SiNW-based systems. In this paper, we report investigations of controlled growth of VLS-grown SiNWs as templates for the surface formation of metal silicide to investigate confinement of electron transport at the nanowire surface by analyzing compositional and electrical characterization. Conduction channel formation utilized W and Ni silicidation. TEM analysis confirmed VLS SiNWs is crystalline. Ni evaporation and Ni and W atomic layer deposition (ALD) and post-deposition thermal processing were carried out for silicide formation. TEM-EDS results showed that ALD W was conformally deposited on the surface of SiNWs. In contrast, e-beam evaporated Ni was asymmetrically deposited on the template nanowire although the resultant silicide was nearly symmetric. Conformal Ni deposition and silicidation was successfully performed, however, using Ni ALD processing. Silicide nanowires were exhibited an improvement in electrical conductivity of eight orders of magnitude compared with that of as-grown VLS SiNWs.   

Analysis of Electric Field Alignment of PbSe Nanowires for Transport Studies

Colloidal PbSe nanowires have excellent electronic and thermoelectric properties including a direct band gap, small effective mass, high carrier mobilities, large static dielectric constant, and a high thermoelectric figure of merit. Such properties make them attractive for a wide array of applications including transistors, detectors/solar cells and thermoelectrics, among others. Recently, the ability to selectively deposit and align PbSe nanowires between patterned metal electrodes using a static electric field has been demonstrated. This is a critical step towards functional integration of these materials into devices; however, larger scale assemblies and deposition of PbSe onto substrates free of metal contact are still needed. In this presentation, the use of dc and ac electric fields to pattern large arrays of aligned PbSe nanowires is demonstrated. The effects of electrode separation, field intensity, and solvent type are discussed. This technique is then extended to patterning of PbSe nanowires onto dielectric materials using the fringing fields of buried contacts to induce the alignment. Simulations have been developed to calculate and illustrate the fields induced within the solvent for the various biasing conditions, geometries, and solvents used. This talk will conclude with a discussion of the transport properties of the aligned PbSe devices.   

Effects of carbon dopant on the morphology of GaAs nanowires

Carbon is a well known dopant in the growth of planar III-V materials such as GaAs, however its use has not yet been investigated for nanowire growth by the vapor-solid-liquid growth mechanism. In this work we show that the morphology of gold catalyzed GaAs nanowires is significantly modified by the presence of CBr$_4$ vapor during growth by metalorganic vapor phase epitaxy (MOCVD). Nanowires grown in the presence of CBr$_4$ exhibit negligible tapering and much lower levels of Au catalyst migration than nanowires grown in the absence of CBr$_4$ under the same conditions. Increasing concentrations of CBr$_4$ lead to increased linear growth rate and reduced wire diameter. Nanowires grown with CBr$_4$ show no detectable stacking faults, in contrast to wires grown in the absence of CBr$_4$ which show a high density of stacking faults. We propose a simple model in which adsorbed carbon blocks the surface diffusion of Au down the wire. This work suggests a simple method to control the degree of tapering which should be applicable to other III-V materials. Preliminary Raman measurements indicate substantial red shifts in the lattice phonon modes with increasing carbon doping, indicating that carbon is actually incorporated into the nanowires.   

Morphology of $<$111$>$-oriented Si nanowire and its physical origin

Si nanowires with a $<$111$>$ orientation, synthesized by vapor-liquid-solid process with low silane partial pressure reactant and gold as the catalyst, are known to exhibit sawtooth facets containing gold adsorbates. We report herein the study of the nanowire morphology by means of transmission electron microscopy and scanning tunneling microscopy. The nanowire sidewalls are found to have two different widths depending on the three equivalent $<$-1-12$>$ or $<$11-2$>$ orientations. In addition the direct incorporation of Si atoms onto the sidewalls occurs at the growth temperature of 550 degrees and shows two different growth rates for the two types of sidewalls. By investigating the atomic structures of the sidewalls, the STM topographic images reveal that the $<$111$>$ facets of the sidewalls have a 6 x 6 phase at room temperature. Because this phase and its counterpart, the $\beta -\surd $3 x $\surd $3 phase that occurs at the nanowire growth temperature, consists of two unit cell with two inequivalent parts, we explain the trigonal symmetry as well as the higher lateral growth rate on the widest sidewalls by the barrier energy that exists between both unit cells.   

N and N$_{2}$ Enhancing Atomic Chain Formation in Copper Nanowires

The trend towards nano-miniaturization, with the development of nanodevices, will require the understanding of the behavior of metals at the nanoscale, especially gold and copper. Au and Cu can form very thin nanowires (NWs), as thin as linear atomic chains (LAC). As NWs are produced they can get contaminated. One important question is the effect of light impurities in the mechanical and electronic properties of Cu NWs. We use \textit{ab initio} calculations based in the density functional theory to study the contamination of a linear atomic chain (LAC) of a thin Cu NW with H, C, O, N N$_{2}$ and S. In this study we calculate forces before the NW's rupture, binding energies and LAC distances. We show that N and N$_{2}$ produce special effects to the LAC as compared with the other impurities. They form strong $p-d$ bonds enhancing LAC formation through rearrangement of tips and inclusion of Cu atoms into the LAC. This effect can be used to produce longer Cu LACs.   

Synthesis of and mesoscopic charge transport in magnetically-doped Bi$_{2}$Se$_{3}$ nanoribbons

A simple band structure and a large bulk band gap have allowed Bi$_{2}$Se$_{3}$ to become a reference material for the newly discovered three-dimensional topological insulators, which exhibit topologically-protected conducting surface states that reside inside the bulk band gap. Theoretically, introducing magnetic impurities in Bi$_{2}$Se$_{3}$ is predicted to open a small gap in the surface states by breaking the time-reversal symmetry. We present synthesis of magnetically doped Bi$_{2}$Se$_{3}$ nanoribbons using the vapor-liquid-solid growth method. Studying Bi$_{2}$Se$_{3}$ in nanostructures is useful because of the high surface-to-volume ratio. Low-temperature magneto-transport measurements in the mesoscopic regime show clear differences between the undoped and doped Bi$_{2}$Se$_{3}$ nanoribbons, confirming the presence of magnetic impurities in the Bi$_{2}$Se$_{3}$ nanoribbons.   

Growth kinetics nano-scale faceted solid-liquid interfaces

In this talk we investigate kinetics of a faceted solid-liquid interface during growth of nanowires. To this end we have undertaken Molecular-Dynamics simulations of systems with nanowire-like geometries to probe the kinetics of their solid-liquid interfaces. These simulations has been undertaken for thermally driven systems of elemental material as well as for chemically driven alloys. In both cases the temperature was below the roughening value which assures that the solid-liquid interface remains faceted. However, as these simulations demonstrate the interface can be non-planar, remaining faceted in the middle and bounded by rough parts at the edges of the system. The solid-liquid interface propagates on a layer by layer growth with a growth rate depending on the size of the wire. Based on these insights we develop a kinetic theoretical model for the nucleation of crystal nuclei on the interface. The kinetic theory that we present for nanowire growth is applicable to both pure systems and alloys with non-planar interfaces. The above theoretical model predicts the same form for the dependence of growth velocity on driving force as in MD simulations. In a general context, this study establishes that system size-dependent growth rates are an intrinsic feature of systems with non-planar interfaces.   

First-principles study of bulk MnSe and its narrow nanowires

Density-functional pseudo-potential calculations are employed to study the structural stability, electronic, and magnetic properties of bulk MnSe in NaCl and NiAs type structures as well as MnSe[001] nanowires in NaCl type structure. The bulk computations indicate that the LSDA+U scheme with an effective U value of about 2eV is necessary to reproduce the measured electronic and magnetic properties. Such U value is used throughout our nanowire calculations. The calculated total energies of nanowires are combined with a phenomenological model to describe the energy of broken bonds at surfaces and edges. The obtained cohesive energies are fitted by appropriate equations of state to determine the edge and surface energies of various MnAs nanowires. We conclude that occurrence of (110) facets in MnSe [001] nanowires is not favorable and rather these nanostructures prefer to have (100) and (010) facets. The significant role of edge atoms on the stability of MnSe nanowires is confirmed in all parts of our theoretical investigation.   

A study of Au adsorption on yttrium disilicide nanowires on Si (001) substrate from first principles

Core-shell nanoclusters with Au coating layer on uniformly dispersed rear-earth disilicide nanowires are very promising for various applications such as nanocatalysis. Using the first principles approach, we studied the structure and electronic properties of a single Au atatom on yttrium disilicide nanowires on the Si(100) substrate. A series of possible adsorption sites were explored and we found that the ``hollow'' site on the YSi2 nanowire is the most preferential one, where the Au adatom binds to two Si atoms and an Y atom underneath. The most stable site for Au on Si(001) is the ``cave'' site, but the binding energy is 0.11 eV higher than that on the wire. This indicates that the Au atoms tend to aggregate toward nanowires, which leads to growth of core-shell metallic structures. Electrons deplete from Au adatom and accumulated in regions between Au and Si. The chemical activity of Au/YSi2 will be discussed in light of charge density, density of states and adsorption energy of CO molecules.   

Atomistic Mechanism of Catalyzed Growth and Termination of Silicon Nanowire

Understanding the growth mechanism of semiconductor nanowire from catalyzed droplet is important for better control of the shape and growth speed of nanowires by chemical vapor deposition through the Vapor-Liquid-Solid (VLS) process. To accurately describe the interatomic interaction between gold and silicon atoms, we developed a Au-Si binary potential based on modified embedded-atoms method (MEAM), which is benchmarked against the experimental binary phase diagram and mixing enthalpy. Advanced sampling method is employed to obtain the critical island at the liquid (Si Au alloy) - solid (silicon nanowire) interface. The dependence of the nucleation rate of the critical island on temperature and Si supersaturation is compared with experimentally observed nanowire growth rate. The termination of Si nanowire growth is investigated from Au-crystal nucleation inside the liquid droplet at the tip of nanowire.   

Structural ordering, electronic and magnetic properties of bundled Mo$_6$S$_{9-x}$I$_x$ nanowires

We use {\em ab initio} density functional theory calculations to determine the effect of bundling on the equilibrium structure, electronic and magnetic properties of Mo$_6$S$_{9-x}$I$_x$ nanowires with $x=0,3,4.5,6$. Each unit cell of these systems contains two S- and I-decorated Mo$_6$ clusters that are connected by S$_3$ linkages to form an ordered linear array. Due to the bi-stability of the sulfur linkages, the total energy of the nanowires exhibits typically many minima as a function of the wire length. We find the optimum inter-wire distance to depend sensitively on the orientation of the wires, but only weakly on $x$. Structural order is expected in bundles with $x=0$ and $x=6$, since there is no disorder in the decoration of the Mo clusters. In bundles with other stoichiometries we expect structural disorder to occur. We find that the nanowires can be switched from metallic to semiconducting behavior by applying axial stress. Selected stable and many more unstable atomic arrangements cause ferromagnetic behavior in nanowire bundles.   

Theoretical investigation of the InN/In2O3 interface

Indium Nitrides (InN) nanowires (NWs) have attracted lot of attention because of their potential applications in optoelectronics and solar cells. Recent XPS measurements on collections of InN NWs showed that a thin amorphous oxide layer (In2O3) at the InN NW surface is formed upon exposure to air, altering the electronic properties of the NWs [1]. A deep understanding of the electronic properties of amorphous In2O3 and of the InN/In2O3 interface assume great relevance in the clarification on the NWs electronic properties and it can shed light on the surface contribution to their conductivity. Motivated by this experimental evidence, we have undertaken a combined classical and ab initio theoretical investigation of amorphous In2O3 and of a realistic In2O3/InN interface. Here we discuss the properties of these systems evidencing the difference and similarities between the In2O3 amorphous and crystalline structure first and than presenting how the level alignment at the In3O3/InN heterostructure may affect InN NWs features. [1] F. Werner et al., Nano Letters 9, 1567 (2009   

Accelerated Molecular Dynamics Simulation of the Tensile Behavior of Silver Nanowires on Experimentally Accessible Timescales

We present the result of accelerated molecular dynamics simulations of silver [110] nanowires stretched at a fixed velocity. By implementing the parallel-replica dynamics method on the peta-scale Roadrunner super-computer, experimentally-relevant pulling velocities as low as $10^{-6}$ m/s were directly simulated with full atomistic accuracy in both space and time. Using this tool, we study the tensile behavior of the nanowires as a function of temperature and pulling velocity. We observe that plasticity is initially mediated by the formation of a zig-zag network of stacking faults along various [111] planes. This mechanism is very robust and is observed in almost all of the cases. The next phase of the deformation is however extremely sensitive to external conditions: while fast pulling (on typical molecular dynamics timescales) lead to the accumulation of defects, necking and ultimately failure, slower drives allows for the annihilation of the stacking faults and the formation of thinner, defect-free wires. However, in agreement with experimental observations, this mechanism does not lead to the formation of atomic chains but typically to failure when the thickness of the wires drops below 3 atomic layers.