### Session P30: Nanowires & Nanotubes: Growth & Absorption Kinetics

Sponsoring Units: DCMP
Chair: Latika Menon, Northeastern University
Room: C147/154

 Wednesday, March 23, 2011 8:00AM - 8:12AM P30.00001: Covalent functionalization of ZnO nanowires Andreia Luisa da Rosa , Ney Moreira , Adriel Garcia , Thomas Frauenheim Understanding the interaction of organic species with inorganic nanostructures constitutes a step forward in the development of~semiconductor based biosensors. In this work we have used density functional theory to investigate ZnO-(1010) nanowire surfaces modified with substituted methane molecules (Me-X, with X= OH, NH2, SH, COOH, and CN). We have found three relevant stabilization mechanisms acting on the surface stabilization: passivation of surface oxygen lone-pairs via dissociative chemisorption processes, electrostatic adsorbate-interations involving Zn surface sites and hydrogen bonding interactions involving oxygen surface sites. Covalent adsorbate-substrate interactions were found to play only a marginal role on the surface stabilization. Contradicting the usual chemical intuition, we have found no significant evidence for the formation of classical Lewis acid-base adducts on Zn surface sites. Finally we suggest that the functionalization with Me-COOH is also expected to be stable under ordinary laboratory conditions or in aqueous media. Wednesday, March 23, 2011 8:12AM - 8:24AM P30.00002: Growth and Characterization of Serrated GaN Nanowires Zheng Ma , Dillon McDowell , Mohamed Abd Elmoula , Eugen Panaitescu , Dalmau Reig , Latika Menon We describe our results on the growth of single crystalline GaN nanowires on catalyst-patterned substrates by means of chemical vapor deposition. The growth is carried out in a horizontal quartz tube inside a tube furnace wherein gallium oxide powder is used as reactor source and a mixture of ammonia and hydrogen gas is used as precursor. Growth of GaN nanowires are demonstrated on both Au and Ni-catalyst patterned substrates (either sapphire or silicon). The growth temperature is maintained at around 960\r{ }C. We show that by controlling the deposition parameters, specifically the size of the catalyst and amount of gallium oxide GaN nanowires grow in a serrated'' pattern. The serrated nanowires maintain a stable, single crystalline state with very regular periodic serrations. The wires have been characterized by means of scanning electron microscopy, transmission electron microscopy and energy dispersive x--ray scattering measurements. Preliminary electrical transport measurements on single serrated GaN nanowires released onto a Si substrate show that the wires exhibit improved electron transport capabilities in comparison with regular GaN nanowires. Wednesday, March 23, 2011 8:24AM - 8:36AM P30.00003: GaAs nanowires and GaAs/AlGaAs core/shell nanowires synthesized by MOCVD Brian Peters , Nicholas Minutillo , John Carlin , Fengyuan Yang Nanowires made by the bottom-up'' approach can be used in a variety of electrical and optoelectronic devices as well as in the study of low dimensional transport physics. We have grown GaAs nanowires using Au catalysts in a closed couple showerhead MOCVD system. A number of growth parameters, including the substrate temperature, growth rate, and Arsine/TMGa ratio, are explored to identify optimal conditions for growth of GaAs nanowires with large aspect ratio and minimal tapering. Higher substrate temperatures result in larger tapering and lower temperature leads to kinks.'' Meanwhile, large V/III source ratio gives large tapering as well. We have found that our optimal conditions are at a substrate temperature of 420\r{ }C and V/III ratio of $\sim$25, which gives a tapering of less than 1 nm increase in diameter per micron in length. In addition, GaAs/AlGaAs core/shell structured nanowires were also grown to minimize the surface states. Characterizations by SEM and photoluminescence will be presented. Wednesday, March 23, 2011 8:36AM - 8:48AM P30.00004: Fe doped ZnO nanotubes synthesized by low temperature electrochemical process Gopal Sapkota , Karol Gryczynski , Arup Neogi , Usha Philipose We report the synthesis of Fe doped ZnO nanotubes (NTs) with tube diameter of about 60-100 nm and wall thickness of about 20nm. To the best of our knowledge, this is the first report on Fe doped ZnO NTs, that could possibly be ferromagnetic. Fe doping will enable us to tune the electrical, optical and magnetic properties of the NTs which are crucial for practical applications (spintronics and optoelectronics). The morphology of the NTs was found to be very sensitive to concentration and temperature of the electrolyte and growth time. Structural and compositional analysis revealed that Fe was incorporated into the ZnO lattice. High Resolution Transmission Electron Microscopy and X-ray diffraction shows good crystalline quality of the NTs with preferential growth along the wurtzite c-axis. Room temperature photoluminescence (PL) measurement of the NTs exhibit strong UV emission around 370nm whereas low temperature PL of the NTs exhibits the optical signature of Fe doping. Wednesday, March 23, 2011 8:48AM - 9:00AM P30.00005: Growth and Characterization of ZnMgO Nanowires by Thermal Chemical Vapor Deposition Gang Shen , Shawn David Wilbert , Nick Harris , Nabil Dawahre , William Baughman , Lee Butler , Joseph Brewer , Seongsin Margaret Kim , Patrick Kung Zinc oxide (ZnO) nanowires are promising structures for nano-optoelectronic devices and applications ranging from solid-state lighting to photovoltaics because of the wide bandgap and large exciton binding energy of ZnO, in addition to serving as template matrices for nanoscale sensors. Alloying ZnO with MgO to achieve ternary ZnMgO compounds represents a potential approach for future optoelectronic heterostructure devices. Well-aligned ZnMgO nanowires (NWs) were grown on a sapphire substrate by thermal Chemical Vapor Deposition (CVD). The alignment of the ZnMgO NWs was confirmed by x-ray diffraction and electron microscopy along with elemental composition information through EDS analysis. The optical and vibrational properties of the ZnMgO NWs were studied by micro-Raman and micro-photoluminescence (PL) measurement. Through the combination of confocal laser scanning microscopy and the micro-Raman and micro-PL, the ZnMgO NWs were imaged at sub-micron resolution. Wednesday, March 23, 2011 9:00AM - 9:12AM P30.00006: Coating of Multi-walled Carbon Nanotubes with Inorganic - Organic Silicas Purnatosh Saha , Brian Grady Silica-coated multi-walled carbon nanotubes (MWCNTs) have been prepared by sol-gel techniques. An inorganic silica layer has been formed by hydrolyzation of tetraethoxy silane (TEOS) on surfactant-treated nanotubes. Additionally, a secondary layer has been deposited using organosilane precursors. Coating thicknesses have been measured by transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) has also been used to determine the amount of silicates coating the nanotubes. The thickness of the final coating can be tailored by controlling reaction conditions and the number of layers. It is expected that carefully controlling the inorganic to organic ratio in the coatings will allow for the variation of its stiffness. Wednesday, March 23, 2011 9:12AM - 9:24AM P30.00007: Characterization of Co$_{2}$FeAl nanowires Keshab R. Sapkota , I.L. Pegg , J. Philip Heusler alloy, Co$_{2}$FeAl (CFA) is a potentially useful material in the field of spintronics due to its high spin polarization. The CFA nanowires are grown for the first time by the electrospinning method. The diameters of the wires formed are ranging from 80 -- 100 nm. The structural characterization of the nanowires is done using X-Ray diffraction and Raman spectroscopy. The nanowires exhibit cubic structure with a lattice constant, $a$ = 2.44 {\AA}. Parallel arrays of nanowires are grown for magnetic characterization using electric field applied at the collector plate. The nanowires exhibit ferromagnetic behavior with a Curie temperature higher than 400 K. Nanoscale devices are fabricated with single CFA nanowire to understand the magnetotransport properties. Wednesday, March 23, 2011 9:24AM - 9:36AM P30.00008: A study of the effect of nitrogen doping in TiO$_2$ Lisa DeBeer-Schmitt , Xiaofeng Qiu , Larry Anovitz , William Heller , Ken Littrell , M. Parans Paranthaman TiO$_2$ nanotube arrays have great potential as photovoltaics due to its unique chemical and physical properties associated with highly ordered tubular geometry. Small-angle neutron scattering (SANS) can characterize the specific nitrogen doping impact to the nanotube array structures. N-doping holds the promise of overcoming the large intrinsic bandgap barrier, which prevent TiO$_2$ from utilizing larger portion of solar energy. Combining with nanotube structures, N-doping could further promote the energy conversion efficiency of TiO$_2$. The SANS data demonstrate that the nitrogen doping during the nanotube growth alters their structure. This stabilized structure is evident in the data via extra peaks in the nitrogen doped sample as compared to the pure. The results demonstrate that the nanotube array morphology can be manipulated by varying the growth conditions, making it possible to tailor the arrays to specific purposes. Wednesday, March 23, 2011 9:36AM - 9:48AM P30.00009: Spontaneous Formation of a Nanotube from a Square Ag Nanowire Sondan Durukanoglu , Mine Konuk The recently observed phenomenon of spontaneous formation of a tube from a regular, square Ag nanowire has been investigated through molecular static and dynamic simulations based on the interaction potentials obtained from the embedded atom method. With molecular static calculations, we investigate the effect of strain on this particular type of transformation by focusing specifically on square Ag nanowires. Our results demonstrate that the formation of hollow structures requires a combination of minimum basis size and high gradient stress. Using molecular dynamic simulation, we also discuss the effect of temperature on the evolution of silver nanowire during the elongation. Wednesday, March 23, 2011 9:48AM - 10:00AM P30.00010: Effect of catalyst preparation on diameter of single-walled carbon nanotubes synthesized by alcohol CVD Theerapol Thurakitseree , Erik Einarsson , Rong Xiang , Shinya Aikawa , Shohei Chiashi , Junichiro Shiomi , Shigeo Maruyama We investigated the effect of various aspects of the catalyst preparation procedure on the diameters of SWNTs synthesized by the alcohol CVD method. Prior to nanotube growth, a Co/Mo binary catalyst was deposited by dip-coating, and then reduced under flowing Ar/H$_{2}$ at temperatures ranging from 300 to 800\r{ }C. We found that the mean SWNT diameter depends on both reduction time and temperature, with lower reduction temperature or short reduction time resulting in smaller diameter SWNTs. The morphology of SWNTs changed from vertically aligned for reduction temperatures above 500\r{ }C to randomly aligned when reduction occurred below 500\r{ }C. Introducing small amount of water during heating, the mean diameter of the SWNTs was also reduced despite synthesis at 800\r{ }C. Small diameter SWNTs were synthesized with this new cobalt/rhodium (Co/Rh) catalyst. The average diameter SWNT is similar to that from Co/Fe catalysts and slightly smaller than HiPco. Wednesday, March 23, 2011 10:00AM - 10:12AM P30.00011: Water monomer interaction with H-passivated Si nanowires from density functional theory Abraham Hmiel , Yongqiang Xue A molecular-level understanding of the interaction between water and nanomaterials is essential for such important phenomena as corrosion, catalysis, electrochemistry and biology. In this talk we present density functional (DFT) study of the structure and energetics of water monomer binding to the surfaces of hydrogen terminated silicon nanowires (SiNWs), which represents the initial phase of interfacial water. We present results on the binding site and orientation of individual water molecules on the surfaces of [112] and [110] oriented SiNWs and analyze the energetics of water adsorption through potential energy surface scan along selected degrees of freedom. Wednesday, March 23, 2011 10:12AM - 10:24AM P30.00012: In-situ dynamical study of capillary absorption of molten silver nanodroplets by multiwall carbon nanotubes Yen-Song Chen , Yuan-Chih Chang , Tung Hsu , Chia-Seng Chang Since the discovery of carbon nanotubes (CNTs), they have been widely investigated for their properties. Due to the large aspect ratio and the uniform diameters, the inner cavities of the CNTs are used as nano test tubes, siphons, catalyst carriers, and so on. Based on recent molecular dynamic simulations, a CNT with open end might act as a capillary pipette'' which can absorb nonwetting metal nanoparticles. In our study, the in-situ dynamical process of nonwetting Ag nanodroplets drawn into the hollow cores of multiwall carbon nanotubes (MWCNTs) was observed in an ultrahigh-vacuum transmission electron microscope equipped with a scanning tunneling microscopy probe. We discover this capillary absorption of melted Ag nanodroplets can occur only when the ratio of the Ag nanodroplet size to inner diameter of MWNTs is below a critical value, which is dependent on the inner diameter of MWCNTs. With continuous operations of capillary absorption for Ag nanodroplets, the one-dimensional Ag nanowires with a specific length could be fabricated inside the MWNTs for NEMS electronics or other applications. Wednesday, March 23, 2011 10:24AM - 10:36AM P30.00013: O$_{2}$ dissociation on nitrogen doped carbon nanotubes (10, 0) from first principles simulation Shizhong Yang , Guang-Lin Zhao , Ebrahim Khosravi Reducing the amount of precious platinum (Pt) loading by identifying non-precious metal catalyst is essential for large-scale applications of fuel cells, which provide a cleaning energy technology. Recent experimental, theoretical, and simulation works accelerate the advance in the research area of doped carbon nanotubes acting as an alternate non-precious metal catalyst for dioxygen reduction in the fuel cells. First principles spin polarized density functional theory(DFT) simulations have been performed to understand O$_{2}$ dissociation on nitrogen doped carbon nanotubes. We have studied nitrogen substitutional doping of carbon nanotubes (CNTs) for dioxygen adsorption, reduction, and dissociation. The calculated results show that nitrogen prefers to stay at the open-edge of short CNTs. Two O$_{2}$ chemisorption sites are found, the carbon-nitrogen complex (Pauling site) and carbon-carbon long bridge (long bridge) sites. The spin polarized DFT calculations using the nudged elastic band (NEB) method show that O$_{2}$ dissociation at the Pauling site has a reaction energy barrier of about 0.55 eV. The unique open-edge structure and charge redistribution are crucial to the novel properties of nitrogen-doped CNTs as a new non-precious metal catalyst for fuel cells. Wednesday, March 23, 2011 10:36AM - 10:48AM P30.00014: Adsorption kinetics of polyatomic molecules on a heterogeneous surface Jared T. Burde , M. Mercedes Calbi We study the kinetics of adsorption of diatomic and triatomic molecules on the external surface of a carbon nanotube bundle. The Kinetic Monte Carlo algorithm is employed to track the number of particles adsorbed on the bundle and the orientation of those particles with respect to the surface at any given time. Our model is further complicated by the inclusion of a more complex surface geometry; a two dimensional, heterogeneous lattice better models the reality of groove between adjacent nanotubes on the outside of the bundle. This allows us to see interesting kinetic effects in the adsorption process, as the adsorbates have multiple transitional states through which they can pass as they evolve towards equilibrium. Wednesday, March 23, 2011 10:48AM - 11:00AM P30.00015: Simulations of adsorption on a single carbon nanotube Hye-Young Kim , Silvina Gatica , Milton Cole Using the grand canonical Monte Carlo method, we have evaluated the adsorption isotherms of simple gases (Ar, Kr, Xe) on a variety of carbon nanotubes. The adsorption potential is a sum of anisotropic atom-C interactions, dependent on the angle between the outward normal and the atom-C separation vector. For varying gas species and nanotube chirality, different commensurate phases are seen than on the surface of graphite. Comparison is made with recent experiments of Wang, et al, Science 327, 552 (2010).