Session X18: Growth and Properties of Nanoparticles and Nanowires

Electronic structures and adsorption configurations of gold nanoclusters on cerium oxide defect surfaces

Fluorite-structured cerium oxide (or ceria, CeO$_{2-x}$, 0 $\le x\le $ 0.5) has been shown to be an important material in catalysis, yet few study has investigated the effect of non-dopant introduced oxygen vacancy defect (OVD). In addition, we found experimentally that when doped with Au nanoclusters, the catalytic ability of ceria enhanced. In this work, we modeled and optimized an (111) fluorite-structured slab model of defective ceria with a chemical formula corresponding to CeO$_{1.5}$. The optimized surface structure of this model was found to contain both surface and sub-surface OVDs, similar to those observed in our HRTEM data for low pressure activated nanoceria. Significantly, the model captures comparable reduction in the average Ce-O bond distance and also atomic coordination numbers observed in our EXAFS data. To explore the roles of Au nanoclusters, we adsorbed flat clusters of 3, 4, 9, 10, and 19 Au atoms on ceria slabs, optimized their configurations, and computed the corresponding electronic structures applying first-principle approach. Consequently, we present the density of states results to elucidate the experimentally observed optical property change and $s-d$ hybridization.   

The Kirkendall Effect in Copper Nanocrystals

The study of copper nanocrystals, unlike nanocrystals of other noble metals such as gold and silver, has been limited due to challenges in the synthesis of monodisperse copper nanoparticles and their reactivity at ambient conditions. Copper is a material of broad interest, however, because of its unique optical and catalytic properties, as well as its non-toxic nature and relative abundance. Copper nanocrystals in the process of being oxidized are subject to the Kirkendall effect, which describes independent diffusion rates in a binary system (Cu and oxygen in this case) and that causes the formation of voids at the core of spherical nanocrystals. Previous studies have attributed the Kirkendall effect in Cu/Cu2O nanocrystals to interactions between the organic passivation layer and the solvent. Here we will present our results on the kinetics of oxidation of Cu nanocrystals in solvent-less conditions as a function of temperature and show that void formation occurs at a relatively narrow range of temperatures. In-situ UV-vis spectroscopy, x-ray diffraction, and electron microscopy have been used to monitor the oxidation process in Cu nanocrystals and a model has been developed to describe hollow particle formation in the Cu/Cu2O system.   

Systematic Study of Supported Ni, Pd, Pt Metal Nanocrystals for Catalytic Energy Conversion

Ni, Pd, and Pt nanocrystals ranging from 4-12 nm have been synthesized with great control in size and shape using a high-temperature chemical synthetic method for gas-phase catalytic testing. The tunability in size allows for controlled catalytic study of model reactions such as CO oxidation, CO hydrogenation and methane oxidation. The monodispersity of the particles allow for a systematic correlation between size and catalytic activity. There is consistent size dependence of CO oxidation for each of the metal systems on CeO2 support as indicated by the lower temperatures needed for full conversion. Our calculated activation energy using the Arrhenius equation for each size and material correlated with surface-area-to-volume ratio. Consistent with the light-off studies, the smallest particles were the most catalytically active under differential conditions. The same trend was observed for CO hydrogenation amongst each system. This trend was inverted for the oxidation of methane. This current study aims to elucidate the size dependence of catalytic activity in model systems with supported uniform nanocrystals.   

Ab Initio Investigation of the Structures of Fe-Doped Carbon Clusters

We continue our interest in the theoretical study of carbon clusters to examine the effects of the doping of small carbon clusters (C$_{n}$, n = 2 - 15) with iron atoms. This work applies the hybrid ab initio methods of quantum chemistry to derive the different Fe$_{m}$C$_{n}$ (m = 1-3) geometries. Of particular interest are linear, fan, and cyclic geometries. Electronic energies, rotational constants, dipole moments, and vibrational frequencies for these geometries are calculated. Exploration of the singlet, triplet, quintet, and septet potential energy surfaces is performed. The type of bonding in terms of competition between sp$^{2}$ and sp$^{3}$ hybridization is examined, with a view to addressing the possibility of the stabilization of the doped carbon nano-particles in a diamond type structure. The potential for the existence of new pathways to the fabrication of nanotubes is explored.   

Electrical Detection of Mechanical Resonance of ZnO Nanowhiskers

Here, we present the fundamental mechanism for the observation of electrically actuated resonances in semi-conducting ZnO nanowhiskers (NWs). Previous studies have claimed that various mechanisms including charge induction lead to a mechanical resonance in NWs. Many of such studies employ an electron beam to visualize the resonance of NWs. However, we find that the use of an electron beam changes the electrical character of the NWs making it difficult to understand fundamental actuation mechanism. In this article, we developed a novel, fully electrical harmonic detection of resonance (HDR) method that enables us to probe mechanical resonances of NWs even in the absence of an electron beam. In contrast to the traditional optical detection scheme, the HDR method allows us to successfully decouple the effects of the electron probe beam from the actual driving force. Interestingly, we find that the observed mechanical resonance of ZnO NWs is dominated by their interactions with the electron probe beam. Importantly, ZnO NWs exhibit strong (weak) mechanical resonance only in presence (absence) of the electron probe beam suggesting that the observed behavior originates from dynamically induced (static) charges.   

Direct growth of vertically aligned long ZnO nanowires on FTO substrates and their application for dye sensitized solar cells

In this research we report a direct growth of vertically aligned ZnO nanowires on fluorine-doped-tin-oxide (FTO) coated substrates for dye sensitized solar cell (DSSC) applications. ZnO nanowires with length of more than 30 $\mu $m were synthesized using a fine-tuned chemical vapor deposition method at temperatures as low as 550 $^{\circ}$C. The nanowires grew along the [0001] direction and exhibited a need-like shape with a wurtzite single crystal structure. Compared to the ZnO nanowires fabricated by solution based methods, the nanowires synthesized in this study have much longer length, thus could increase the amount of dye loading and improve DSSC performance. Furthermore, since the nanowires were directly synthesized on FTO substrates, DSSCs could be fabricated using the as-grown nanowires without the usual wire transfer processing that caused nanowire breaking and created additional transport barriers and recombination possibilities for photo-generated carriers. DSSCs fabricated in this study showed attractive performance with short-circuit current density of 5.1 mA/cm$^{2}$ and power conversion efficiency of 1.66{\%}. Dependence of the DSSC performance on nanowire length and annealing processing was also examined in this research.   

Competing gas phase reactions during vapor transport deposition of ZnO nanowires

We present results illuminating some of the major chemical processes in the vapor deposition of ZnO nanowires. The analysis of our deposition experiments indicates that carbon dioxide is a major oxidizing agent rather than carbon monoxide as previously thought. Additionally, we present evidence that carbon monoxide will etch zinc oxide at high temperatures. Zinc oxide nanowires have been prepared by using chemical vapor deposition on silicon (100) substrates with a 10-15nm layer of gold as a catalyst. Zinc oxide and graphite powders were heated to approximately 1000$^{\circ}$C in a tube furnace in a flow of argon. We have delivered oxygen gas specifically in the growth zone to facilitate the formation of high aspect ratio nanowire growth. Thermodynamics calculations were used to justify the growth and etching processes. Imaging of samples was performed with scanning electron microscopy. Chemical composition was determined by energy dispersive x-ray spectroscopy. Photoluminescence spectroscopy was used to characterize the emission properties of the zinc oxide samples.   

Study on Pure Phase Formation of Lead Oxide Nanowires by Oxidation of Lead Nanowires

Lead-oxide nanowires were synthesized by oxidizing lead metal nanowires. The phase structures, sizes and morphologies of the nanowires were investigated by atomic force microscopy and x-ray diffraction, and the band gap of the nanowires was determined by UV-Vis-NIR reflectance diffusion spectrums. The thermodynamic environment for the pure phase formation has been studied. The first-principle computation has been done to help understand the phase formation. Our results reveal that the pure phase formation strongly relies on both the process temperature and the oxygen flow/oxygen partial pressure, and the pure phase ?-PbO nanowires can be obtained only in a narrow, low temperature range under a low oxygen flow.   

Formation of metallic gold chain on patterned hydrogen terminated Si(001)-2$\times $1 surface: Density functional study

Metal adsorption on silicon surface for the formation of linear metallic chain is one of the important research areas for the advancement of nanotechnology. Due to the presence of dangling bonds all over the surface of bare Si(001), metals when deposited, generally do not tend to form stable wire structures. However, patterned hydrogen terminated Si-surface may be a good choice for the formation of atomic chain structures of metals. Since patterned hydrogen terminated Si(001):2x1 surface is very stable, we consider patterning it by removing desired hydrogen atoms and adsorbing gold atoms. We have examined the structure, energetic and electrical properties of such gold adsorbed surface by varying gold coverage. We have found that linear gold chain structures may be formed by controlling gold coverage. Some of these gold chain structures are metallic in nature. We hope that our results will motivate synthesis of gold chains on patterned hydrogen terminated Si(001): 2$\times $1 surface.   

Catalytic role of Au nanoparticle in GaAs nanowire growth

The energetics of Ga, As and GaAs species on the Au(111) surface (employed as a model for Au nanoparticles) is investigated by means of density-functional calculations. Apart from formation of the compound Au$_7$Ga$_2$, Ga is found to form a surface alloy with Au, with comparable $\Delta H \sim 0.5$~eV for both processes. Dissociative adsorption of As$_2$ is found to be exothermic by more than 2~eV on both clean Au(111) and AuGa surface alloys. The As-Ga species formed by reaction of As with the surface alloy is sufficiently stable to cover the surface of an Au particle {\it in vacuo} in contact with a GaAs substrate. Concerning the Au-catalysed growth of GaAs nanowires, we conclude that impingement of As$_2$ or As$_4$ molecules on the Au particle suffices as supply of arsenic to the growth zone. We identify a regime of temperatures and As$_2$ partial pressures suitable for Au-catalysed nanowire growth in molecular beam epitaxy.   

Engineering Enhanced Optical Properties of Near-IR Upconverting Nanoparticles

Due to their unique properties in converting low energy light into higher energy electronic transitions, upconverting nanoparticles (UCNPs) have garnered considerable interest in bio-imaging, photovoltaic, and opto-electronic applications. In particular, lanthanide-doped UCNPs have demonstrated a host of functionalities due to their nanoscale dimensions, wide range in transition-metal doped compounds, and high photostability in both aqueous and ambient environments.\footnote{P.J. Schuck, et al \textit{Proc. Nat. Acad. Sci.} \textbf{106}, 10917, 2009} In addition, their mixed electric and magnetic dipole transitions make them ideal materials for study of plasmon-enhanced properties with metal nanostructures in which tunable surface properties can mediate energy transfer processes. Here we report on the luminescence properties of Er$^{3+}$, Yb$^{3+}$-doped NaYF$_{4}$ UCNPs with diameters ranging from 5 -- 50 nm in both core and core-shell architectures. Optical characterization of the luminescence lifetime and spectral emission from both UCNP films and single particles reveal a strong dependence on particle size and surface functionalization. Furthermore, by utilizing the large shift (anti-stokes) in absorption energy versus transition energy, we investigate the interaction of energy transfer across metal-semiconductor nano-interfaces whereby the intrinsic luminescence lifetimes are probed for Purcell enhancement and emission rate modification.   

Tuning the parameters to establish Quenching and enhancement regimes in hybrid Gold nanoparticles and CdSe Quantum dots monolayer

The multi-component nanomaterials combine the individual properties and give rise to emergent phenomenon. Optical excitations in such hybrid nonmaterial's ( for example Exciton in semiconductor quantum dots and Plasmon in Metal nanomaterials) undergo strong \ weak electromagnetic coupling. Such exciton-plasmon interactions allow design of absorption and emission properties, control of nanoscale energy-transfer processes, and creation of new excitations in the strong coupling regime.This Exciton plasmon interaction in hybrid nanomaterial can lead to both enhancement in the emission as well as quenching. In this work we prepared close-packed hybrid monolayer of thiol capped CdSe and gold nanoparticles. They exhibit both the Quenching and enhancements the in PL emission.The systematic variance of PL from such hybrid nanomaterials monolayer is studied by tuning the Number ratio of Gold per Quantum dots, the surface density of QDs and the spectral overlap of emission spectrum of QD and absorption spectrum of Gold nanoparticles. Role of Localized surface Plasmon which not only leads to quenching but strong enhancements as well, is explored.   

Enhanced Free Exciton and Direct Band-Edge Emissions at Room Temperature in Ultrathin Zn0 Films Grown on Si Nanopillars by Atomic Layer Deposition

Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films, which were grown on silicon nanowires (Si-NWs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at $200^\circ$ C, an intensive UV emission corresponding to free-exciton recombination (~3.31eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at $25^\circ$ C, albeit the appearance of the abovementioned defect-associated broad visible emission, the UV emission peak was observed to shift by ~60meV to near the direct band edge (3.37 eV) recombination emission. The high resolution transmission electron microscopy (HRTEM) examinations showed that, indeed, the microstructure of the obtained ZnO films for the former case was of continuous crystalline nature, while that for the latter displayed a microstructure consisting of ZnO nanocrystals with a mean diameter of $4$nm embedded in a largely amorphous matrix. The blue shift of the UV emission peak in the latter films, thus, might have been due to the effects of quantum confinement on the free-exciton recombination.   

Growth of Molybdenum Oxide Nano-Micro Structures by Thermal Annealing Process

The needle and planar molybdenum oxide structures on molybdenum foil were grown by thermal annealing process. The effects of different parameters such as oxygen flow rate, presence of oxygen, annealing temperatures and annealing time on structures, grain size and aspect ratio of nano/micro structures were studied. It is found that the density of structures is only function of oxygen flow rate and annealing temperature. The maximum density of MoO3 nanorods were observed at annealing temperature 530$^{\circ}$C in air. The mechanism of the crystals growth was found for synthesized nanorods.