Session W30: Nanotubes and Nanowires III: Other Properties II

Nanotube Radio

We have constructed a fully functional, fully integrated radio receiver from a single carbon nanotube. The nanotube serves simultaneously as all essential components of a radio: antenna, tunable band-pass filter, amplifier, and demodulator. A direct current voltage source, as supplied by a battery, powers the radio. Using carrier waves in the commercially relevant 40-400 MHz range and both frequency and amplitude modulation techniques, we demonstrate successful music and voice reception.   

Template-Based Electroless deposition and TEM Analysis of TiO$_{2}$ Nanotubes

We report on the fabrication of TiO$_{2}$ nanotubes using a template-based electroless deposition method. We used anodic aluminum oxide membranes of pore diameter $\sim $ 200 nm as the templates. The TiO$_{2}$ nanotubes were fabricated by first dipping the membranes into a titanium tetrafluoride solution at 60 \r{ }C for 30 minutes, and then annealing at 225 \r{ }C for 2 hours. The composition of the nanotubes was verified using the energy dispersive spectroscopy. We used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the surface morphology of the nanotubes extracted after the membranes were dissolved. The nanotubes were found to be uniform along the length of the nanotube. For TEM analysis, an ion-milling technique was also used to produce a thin sample region so that the cross-section of the nanotubes remaining in their original pore channels could be directly imaged. The thickness of the TiO$_{2}$ nanotubes is about 50 nanometers, and could be controlled by varying the deposition time. Potential applications of these semiconducting TiO$_{2}$ nanotubes for forming nanostructured semiconducting interfaces will be discussed.   

Electron Microscopy Analyses of Nanowires Electrochemically Deposited into Porous Membranes

We fabricated single crystal nanowires of Zn, Sn and Pb by electrochemically depositing materials into the pores of porous anodic aluminum oxide membranes and polycarbonate membranes. We applied an \textit{in situ} self-contacting technique to electrically contact single nanowires with macroscopic electrodes of Au, Sn, and Pb pre-fabricated on the membrane surfaces. We observed an anomalous long-range proximity effect in this nanowire/electrode system. In this talk, we describe electron microscopy methods we used to analyze the structure and the composition of the nanowires/electrode system. These included analyses of extracted nanowires using the scanning electron microscope (SEM) and the transmission electron microscope (TEM). Nanowires remaining in their original pore channels were also analyzed with TEM using samples prepared by ion-milling and ultramicrotomy. These analyses revealed that the nanowires were single crystalline. Furthermore, the interface between the nanowires and the electrodes were directly imaged. The chemical compositions of the nanowires were also confirmed by the energy dispersive spectroscopy (EDS) analyses and mappings.   

Characterization of individual SnO$_{2}$ nanowires with surface science techniques

Tin oxide is widely used as a solid-state gas sensor for detection of combustible and toxic gases. Recently, the use of nanobelts and nanoribbons has been suggested as novel materials for gas sensing applications. Large surface-to-volume ratio of the semiconducting metal oxide nanobelts and the congruence of the carrier screening length with their lateral dimensions make them highly sensitive and efficient transducers of surface chemical processes into electrical signal. The surface morphology of an individual nanobelts (NB) was studied with STM. Atomically resolved STM images of NBs reveal an 1X1 (101) SnO$_{2}$ structure on the top surface of the NB. The thermal stability of the NBs was studied with SEM. The critical temperatures were determined, where structural changes occur in UHV, O$_{2}$, and air. XPS was used to characterize chemical composition and monitor the cleanness of the NB material. Ca and C contamination was detected on as-grown SnO$_{2}$ nanobelts. O plasma, ozone treatment, and annealing in oxygen were used to remove the contaminants.   

Structure and Stability of Metal Oxide Nanowires

We present a comprehensive theoretical study --- within the framework of {\it ab initio} density functional theory method --- of the structural and stability properties of metal oxide nanowires. We consider nanowires with $\langle$100$\rangle$ growth direction with several diameters and surface facet configurations. A stability analysis of the results obtained for theses nanowires is used to determine the most stable geometries. We show that the perimeter of the nanowires is a meaningful dimensional parameter, and that the surface facets play a central role on the energetics of the nanowires. The results are compared to available experimental data.   

First principles investigation of InN non-polar surfaces and nanowires

In the last years InN nanostructures have been proposed for application in solar cells, because of the outstanding electronic properties of this nitride compound. An increase of solar energy conversion in these kinds of cells requires a deep knowledge of surfaces properties, and on the effect of confinement on the electronic properties of the material. Here we present an investigation of the structural and electronic properties of InN nanowires as obtained by means of \textit{ab initio} Density Functional calculations. First we discuss the results for the clean (1-100) and (11-20) faces, which are usually exposed in nanostructures, then we show how InN electronic properties change in nanowires due to confinement effects. We will finally present a possible InN functionalization pathway based on the use of molecules containing thiol groups. Our results show that thiols may attach to the surface following an exothermic process (dissociation energy is about 2.5 eV/mol for the (11-20) surface), thus they represent an effective anchoring group for the realization of hybrid InN based devices.   

Van Der Waals Interaction between Two Parallel Radially Deformed Single Wall Carbon Nanotubes

The van der Waals potential energy is calculated between two parallel infinitely long radially deformed single walled carbon nanotubes within the pairwise Lennard-Jones approximation for extended systems. The nanotubes will undergo different geometrical radial shape transitions if an external hydrostatic pressure with an increasing strength is applied. We describe these shapes with analytically in order to facilitate the calculations. The most preferred mutual orientations are determined in all considered cases in terms of their potential well depths, equilibrium distances, and geometrical parameters. We find that the interaction evolves in such a way as to keep the distance between the interacting surfaces comparable to the graphene-graphene distance in graphite. In addition, the universal graphitic potential concept is extended to radially deformed carbon nanotubes. These results can be used as a guide for future experiments to investigate interactions between deformed carbon nanotubes.   

Atomic Structure of the Si(111)-4x1-In System

The indium-induced 4 x 1 reconstruction on silicon (111) has been extensively studied due to its unique physical structure and electronic properties, which indicate so-called ``Quantum Wire Behaviour''. The likely crystallographic structure of this system has been hotly debated since 1999. The structure was mathematically modeled by Tsay using a planewave calculation method in 2005, which yielded results of atomic positions that matched closely to the Bunk model. In this study we model the this system using the molecular dynamics (MD) package Fireball, taking as a starting point the atomic positions of the Bunk model. This ab-initio tight-binding MD method has the advantage of being able to operate with a very large number of atoms per cell in a single calculation, thus allowing for a very large superslab in the calculation model. This allows for greater surface area and thus is expected to produce a more accurate surface characteristic calculation result. The results of this calculation are compared to the recent results of Tsay.   

Vapor-solid-solid growth mechanism driven by epitaxial match between solid AuZn alloy catalyst particle and ZnO nanowire at low temperature

The present paper provides a comprehensive picture of the precise mechanism of ZnO vapor-solid-solid nanowire growth at low temperatures and gives the fundamental reasons responsible [1]. We demonstrate by using a combination of synchrotron XRD and high resolution TEM that the growth dynamics at low temperatures is not governed by the well-known VLS mechanisms. Based on the Au-Zn phase diagram, temperature measurement and temperature size effects, we show that growth occurs via VSS. The precise composition of the Au-Zn catalyst nanoparticle has been determined to be $\gamma $-AuZn. Furthermore, we experimentally observe that there is an indication of a epitaxial relationship between the ZnO nanowires and the $\gamma $-AuZn seed particle. A critical new insight on the driving factor of VSS growth is proposed in which the VSS process occurs by a solid diffusion mechanism that is driven by a preferential oxidation process of the Zn inside the alloy catalyst induced by an epitaxial match between the ZnO(10-10) plane and the $\gamma $-AuZn(222) plane. [1] L. C. Campos et al, Adv. Materials (accepted for publication).   

Temperature effects in the stability of pure and doped gold nanowires

Gold nanowires have attracted a great deal of attention, one of the reasons being the ability to form linear gold chains that are one atom wide and that have just a few atoms in length. One of the amazing characteristics of these wires is their stability, even at ambient temperatures. Therefore, it is very important to understand why they are stable and why they break. Moreover, inserted impurities, such as H and C, can dramatically affect their stability. In the present work we use state-of-the-art ab initio molecular dynamics to perform simulations of pure and doped Au nanowires. We propose a general mechanism that helps to explain the stability of these wires and the rupture process. We show that triplets of Au atoms in the neck of atomically thin nanowires have an instability towards rupture at a length around 6.0-6.1 angstroms. At this length, the potential energy surface (PES) describing the motion of the central atom changes from a single minimum at the middle of the triplet, to a double minimum profile. This provides a consistent picture of the mechanism of rupture of metallic nanowires. Impurities such as C and H also have this behavior. However, as they have stiffer bonds, these PES instabilities always happen in pure Au bonds.   

Surface Effects on the Electronic, Magnetic and Structural Properties of Free-standing InP nanowires

Nanowires due to their large surface-to-volume ratio of particular interest is the understanding of the surface structure and the electronic effects due to surface states. In this work we investigate the structural and electronic properties of InP nanowires aligned along the [111] direction, in which the surface have been studied by passivating with hydrogens or OH radicals and also oxidized. The magnetic properties of Mn doped InP nanowires with different surface termination have been also investigated. Our {\it ab initio} density functional calculations show that hydrogen passivation removes the surface states, opening up the band gap. Our results for oxygen adsorbed on the hydrogen passivated InP nanowires, show that there are many configurations where the oxygens are chemisorbed processes. For Mn doped InP nanowires our results reveal that the surface of the nanocrystals play a fundamental role on the impurity stability and on the magnetic properties of InP nanowires. The formation energy of pairs of Mn impurities in unpassivated nanowires are lower than that of the bulk InP. Most of the Mn pair configurations present FM coupling and their prefer to be inside the nanowire and not on the surface.   

Shift- and injection-current optical response of BN nanotubes

We present the frequency dependence of the electrical shift and injection currents optically generated on BN nanotubes. The shift and injection currents are a second order effect that is associated to a divergence of the nonlinear susceptibility at zero frequency. The shift current is generated with linearly polarized light and the injection current can be generated with circularly polarized light. We make use of density functional theory and pseudopotentials to calculate the nanotube structures and their electronic states. The current-coefficient calculations are done in optical rectification, within the independent particle approximation and within a full band structure scheme. We also obtain the frequency dependence for the shift distance that the center of electron charge moves in the shift-current process and for the swarm velocity or maximum velocity acquired by the electrons in the injection process.   

An ab initio study of the interaction of transition metal atoms with single-wall armchair SiC nanotubes

A systematic study of Fe atom encapsulated and adsorbed in armchair SiC nanotubes has been performed using hybrid density functional theory calculations within finite cluster approximation. A detailed comparison of the binding energies, equilibrium positions, Mulliken charges and spin magnetic moments of Fe atoms has been performed for three types of nanotubes. The electronic states, HOMO-LUMO gaps, and changes in gaps with respect to the bare nanotube gaps have been investigated as well. Binding energies of the encapsulated and adsorbed Fe atoms indicate that these structures are stable and show site dependence. For both cases significant band gap decrease is observed for type 1 nanotubes enabling band gap tailoring. This decrease is not observed for the other two types in both cases of interactions. All the structures are found to have magnetic ground states with very high magnetic moments indicating the possibility of them being used as nanomagnets.   

Energetics of Cu Nanowires

We have calculated activation energies for several single atom and vacancy diffusion mechanisms on the $<$100$>$ and $<$110$>$ axially oriented, rectangular Cu nanowires with a particular interest in determining the effect of varying cross-sectional area on the activation barriers for the investigated processes. The calculations are performed using the nudged elastic band technique based on the interaction potential obtained from the embedded atom method. Our results on activation barriers for adatom diffusion mechanisms indicate a clear dependence on the cross-sectional area of the nanowires. We, furthermore, find that the energy barrier for single vacancy diffusion is decreasing drastically near the outer wall compared to barriers for single vacancy diffusions taking place in the interior region of the nanowire.   

{\it Ab Initio} Computational Studies of the Electronic and Optical Properties of SiC Nanotubes

Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima, carbon and other nanotubes have attracted considerable interest worldwide because of their unusual properties and also great potentials for technological applications. Though CNTs continue to attract great interest, other nanotubes such as BN nanotubes (BN-NTs) may offer different opportunities that CNTs cannot provide. We have carried out systematic computational studies of various physical properties of SiC nanotubes. In this talk, we will present ab initio calculations of electronic, linear and nonlinear optical properties of SiC nanotubes [1]. \newline [1] I.J. Wu and G.Y. Guo, Phys. Rev. B {\bf 76}, 035343 (2007).