Session X7: Electronic, Structural and Chemical Properties of Non-Carbon Nanotubes and Nanowires

Electronic properties of fluorine passivated silicon nanowires: density functional calculations

Arrays of silicon nanowire(SiNWs) have recently gained attention as a promising new photovoltaic technology. Previous studies show that halogen passivated SiNWs have good chemical stability and also form a critical pathway towards organic group functionalization. Yet, surface halogens are known to alter the SiNW electronic structure, most notably by reducing the band gap. We explore the fundamental physics behind this effect through first principles calculations on hydrogen and fluorine passivated (110) SiNWs. Electronic structure analysis reveals that the highly electronegative fluorine passivation modifies the quantum confinement potential and induces changes in the energy level ordering. Furthermore, we show how a modified cylindrical potential well model can demonstrate a link between this quantum confinement modification and the shift in energy levels responsible for the band gap reduction.   

Molecular Hydrogen and Oxygen Interactions with Armchair Si Nanotubes

First principles calculations based on hybrid density functional theory have been used to study the electronic structure properties of armchair silicon nanotubes from (3, 3) to (12, 12). Full geometry and spin optimizations have been performed without any symmetry constraints with an all electron 3-21G* basis set and the B3LYP functional. The largest silicon nanotube studied has a cohesive energy of 3.47eV/atom. Molecular hydrogen and oxygen adsorptions on a (6, 6) tube have been studied by optimizing the distances of the admolecules from both inside and outside the tube. The molecule is originally placed perpendicular or parallel to the tube axis. The on-top external site is the most preferred site for the hydrogen molecule with adsorption energy of 3.71eV and an optimized distance of 3.31 {\AA} when it is perpendicular to the tube axis. For oxygen, the molecule dissociates into two atoms with adsorption energy of 7.45eV, the optimized distances being 1.65/1.68{\AA}. The buckling of the nanotubes increased significantly indicating structural deformation and an increase of sp$^{3}$ structure. The band gap increases from 0.98eV of bare nanotube to 1.26eV after adsorption of hydrogen molecule. For oxygen molecule, the band gap slightly increases from 0.98eV to 1.01eV.   

Modeling Earle-Stage Kinking during VLS Ge Nanowire Growth

The catalyzed growth of Ge nanowires from gold nanoparticles via the vapor-liquid-solid (VLS) mechanism has been the subject of intense research worldwide, due to their potential applications in nanotechnology. Understanding the fundamental mechanisms underlying kinking during Ge nanowire growth, especially at the early-stage, is helpful for better control of Ge nanowire growth for technological applications. We report an investigation of wire morphology before and during Ge nanowire kinking in early stage growth under typical nucleation conditions. The Ge nanowires grew either along the vertical [111] direction or kinked away onto inclined $<$111$>$ axes early on during their growth. We found that most kinked Ge nanowire deposited under these conditions kinked at similar height, and had similar sidewall facet structure in the kinked region. High-resolution transmission and scanning electron microscopy investigations also showed that the typical kinking-structure was bounded by (111) and other relatively stable Ge surface facets. We construct 3D phase field model of the nanowire based on the transmission and scanning electron microscopy and compare the evolution of the droplet and nanowire with experimental observations.   

Detection of bio-molecules of different polarities by Boron Nitride Nanotoube

The effect of molecular polarity on the interaction between a boron nitride nanotube (BNNT) and amino acids is investigated with density functional theory. Three representative amino acids, namely, tryptophane (Trp), a nonpolar aromatic amino acid, and asparatic acid (Asp) and argenine (Arg), both polar amino acids are considered for their interactions with BNNT. The polar molecules, Asp and Arg, exhibit relatively stronger binding with the tubular surface of BNNT. The binding between the polar amino acid molecules and BNNT is accompanied by a charge transfer, suggesting that stabilization of the bioconjugated complex is mainly governed by electrostatic interactions. The results show modulation of the BNNT band gap by Trp. Interestingly, no change in band gap of BNNT is seen for the polar molecules Asp and Arg. The predicted higher sensitivity of BNNTs compared to carbon nanotubes (CNTs) toward amino acid polarity suggests BNNTs to be a better substrate for protein immobilization than CNTs.   

Quantum Confinement and Surface Relaxation Effects in Rutile TiO$_{2}$ Nanowires

Recent developments in synthesis of TiO$_{2}$ nanowires with diameters on the atomic scale have opened up new grounds for studying structure-property relationships in the regime where quantum confinement effects are important. In the sub-nanometer range, the properties of nanowires can be sensitive to atomic-level control of surface morphology, functionalization, and nanowire stoichiometry during the growth and fabrication processes, thereby opening the way for new applications. First-principles density functional theory calculations have been applied to investigate the size- and shape-dependent properties of [001]-oriented rutile TiO$_{2}$ nanowires of rectangular cross section. We find that the pronounced oscillation in the formation energy and band structure of the nanowires as a function of the number of TiO$_{2}$ trilayers is largely connected to the presence or absence of a mirror Ti-O plane along each confinement direction. We demonstrate that the relative stability and the indirect or direct character of the band structure of the rutile TiO$_{2}$ nanowires arise from the interplay between surface relaxation and quantum confinement effects that depend on the even-odd parity of the number of TiO$_{2}$ trilayers.   

Simulation of Nanowires on Metal Vicinal Surfaces: Effect of Growth Parameters and Energetic Barriers

Growing one-dimensional metal structures is an important task in the investigation of the electronic and magnetic properties of new devices. We used kinetic Monte-Carlo (kMC) method to simulate the formation of nanowires of several metallic and non-metallic adatoms on Cu and Pt vicinal surfaces. We found that mono-atomic chains form on step-edges due to energetic barriers (the so-called Ehrlich-shwoebel and exchange barriers) on step-edge. Creation of perfect wires is found to depend on growth parameters and binding energies. We measure the filling ratio of nanowires for different chemical species in a wide range of temperature and flux. Perfect wires were obtained at lower deposition rate for all tested adatoms, however we notice different temperature ranges. Our results were compared with experimental ones [\textit{Gambardella }\textit{et al.}, Surf. Sci.449, 93-103 (2000), PRB \textbf{61}, 2254-2262, (2000)]. We review the role of impurities in nanostructuring of surfaces [Hamouda \textit{et al.}, Phys. Rev. B \textbf{83}, 035423, (2011)] and discuss the effect of their energetic barriers on the obtained quality of nanowires. Our work provides experimentalists with optimum growth parameters for the creation of a uniform distribution of wires on surfaces.   

Escape of an axial dislocation from a thin rod

Whiskers, nanowires and nanorods have been supposed to grow by preferential attachment to the atomic step formed from a screw dislocation intersecting the surface. This is expected to leave behind an axial screw dislocation, as has been observed in ionic nanowires such as NaCl, PbS and PbSe. However, we are not aware of any studies that have directly observed axial dislocations in pure FCC metal nanowires. To explain this, we speculate that thermal vibrations will be enough to kick out dislocations due to their high mobility. We consider two models of how a dislocation might escape from a nanowire. The first model is that the dislocation vibrates inside the nanowire. The second is that the nanowire itself vibrates, causing deformations of the nanowire that push the dislocation out. Analysis of these models imply that dislocations in thin nanowires are remarkably thermally stable. We test this prediction with molecular dynamics calculations on Cu nanowires, and find that the preparation of these systems is critical to the dislocation stability. Preparing the sample by simply raising the MD temperature will cause the dislocation to run out.   

Ensemble Measurements of Temperature Dependence in High-Q GaN Nanowire Mechanical Resonators

We report on measurements of c-axis oriented, single crystal, gallium nitride nanowire (GaN NW) mechanical resonators. Our measurements use a capacitively-coupled, microwave homodyne reflectometer that allows for simultaneous detection of large ensembles of the as-grown, GaN NW resonators. The NWs - grown via molecular beam epitaxy - behave as singly clamped beams, have lengths near 15 microns, and radii near 100 nm. We observe, respectively, typical resonance frequencies and mechanical quality factors (defined as the ratio of resonance frequency to full width at half maximum power), Q, near 1 MHz and above 10,000, near room temperature [1]. At lower temperatures, we observe increases in resonance frequencies consistent with temperature-dependent elastic moduli. Measured Q factors demonstrate a minimum near 150 K and typically increase an order of magnitude - to above 100,000 - below 100 K.\\[4pt] [1] S.W. Hoch et al., Appl. Phys. Lett. 99(5), 053101 (2011).   

Low-frequency noise in gallium nitride nanowire mechanical resonators

We report on the low-frequency 1/f (flicker) parameter noise displayed by the resonance frequency and resistance of doubly clamped c-axis gallium nitride nanowire (NW) mechanical resonators. The resonators are electrostatically driven and their mechanical response is electronically detected via NW piezoresistance. With an applied dc voltage bias, an NW driven near its mechanical resonance generates a dc and Lorentzian rf current that both display 1/f noise. The rf current noise is proportional to the square of the derivative of the Lorentzian lineshape with a magnitude highly dependent on NW dc bias voltage conditions, consistent with noise in the NW's resistance leading to temperature noise from local Joule heating, which in turn generates resonance frequency noise. An example device with a 27.8 MHz resonance frequency and 220 k$\Omega$ resistance experiences an approximate resonance frequency shift of -5.8 Hz/nW. In terms of NW resistance change, this corresponds with shifts of 0.1 Hz/$\Omega$ and 2.6 Hz/$\Omega$ at 1 V bias and 4 V bias, respectively, with an average resistance fluctuation of 1 k$\Omega$ in a 1-second bandwidth.   

Unusual Germanium Nanowire Crystal Structures Formed During Low Temperature Catalytic Growth

Implementing bottom-up grown semiconductor nanowires (NW) in technological applications will require a detailed understanding of the electronic/physical properties of the NWs, which are closely related to their crystal structure. We present environmental transmission electron microscopy video data of Au catalyzed Ge NW growth under digermane exposure at 240 -- 320 \r{ }C. The catalyst particles are initially liquid after gas exposure [1], and Ge NWs temporarily grow by step flow from the liquid AuGe alloy [2] before the liquid catalyst crystallizes into a metastable solid phase. Nucleating solid Ge at low temperatures requires a considerable supersaturation in the liquid catalyst, which drives the formation of a NW with a non-equilibrium crystal structure [3]. We explore how nanoscale systems with high supersaturations lead to the formation of metastable phases whose properties can be dramatically different from those of the common bulk phases. [1] A. D. Gamalski \textit{et al.}, Nano Lett., 10, 2972 (2010) [2] A. D. Gamalski \textit{et al.}, J. Phys. Chem. C, 115, 4413 (2011) [3] A. D. Gamalski \textit{et al.}, in preparation   

Strong Spin-Orbit Interaction and Helical Hole States in Ge/Si Nanowires

We study theoretically the low-energy hole states of Ge/Si core/shell nanowires. The low-energy valence band is quasidegenerate, formed by two doublets of different orbital angular momenta, and can be controlled via the relative shell thickness and via external fields. We find that direct (dipolar) coupling to a moderate electric field leads to an unusually large spin-orbit interaction of Rashba type on the order of meV which gives rise to pronounced helical states enabling electrical spin control. The system allows for quantum dots and spin qubits with energy levels that can vary from nearly zero to several meV, depending on the relative shell thickness [1].\\[4pt] [1] C. Kloeffel, M. Trif, and D. Loss, Phys. Rev. B {\bf 84}, 195314 (2011).   

Spontaneous Formation of A Nanotube From A Square Ag Nanowire: An Atomistic View

We have performed molecular static calculations to investigate the recently observed phenomenon of the spontaneous formation of a nanotube from a regular, square Ag nanowire[1]. In the simulations, atoms are allowed to interact via the model potential obtained from the modified embedded atom method. Our simulations predict that this particular type of structural phase transformation is controlled by the nature of applied strain, length of the wire and initial cross-sectional shape. For such a perfect structural transformation, the $<$100$>$ axially oriented fcc nanowire needs (1) to be formed by stacking A and B layers of an fcc crystal, both possessing the geometry of two interpenetrating one-lattice-parameter-wide squares, containing four atoms each, (2) to have an optimum length of eight layers, and (3) to be exposed to a combination of low and high stress along the length direction. The results further offer insights into atomistic nature of this specific structural transformation into a nanotube with the smallest possible cross-section. [1] M.J. Lagos et al., Nature Nanotech. \textbf{4}, 149 (2009).   

Crossed Andreev Reflection in Quantum Wires with Strong Spin-Orbit Interaction

We theoretically study tunneling of Cooper pairs from an s-wave superconductor into two semiconductor quantum wires with strong spin-orbit interaction under magnetic field, which approximate helical Luttinger liquids. The entanglement of the two electrons within a Cooper pair can be detected by the electric current cross correlations in the wires. By controlling the relative orientation of the wires, either lithographically or mechanically, on the substrate, the ensuing current correlations can be tuned, as dictated by the initial spin entanglement. This proposal of a spin-to-charge readout of quantum correlations is alternative to a recently proposed utilization of the quantum spin Hall insulator.   

A Microscopic View of Repeated Polytypism in Self-organization of Hierarchical Nanostructures

We demonstrate that the stacking-fault-induced repeated polytypism of wurtzite (WZ) and zinc-blend (ZB) phases may lead to a hierarchical nanostructure of ZnO possessing a hexagonal central trunk decorated with thin blades. Each blade keeps a fixed angle to the central trunk, resembling propellers with seemingly six-fold symmetry. The blades epitaxially nucleate on the ZB stripe assisted by the defect-induced reentrant corners at the interface of ZB-WZ phases. Our experiments reveal a unique approach to assemble hierarchical nanostructures, and the mechanism could be general to many materials.