Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U15: Focus Session: Synthesis and Doping of Nanostructures |
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Sponsoring Units: FIAP Chair: Tom Kennedy, NRL Room: LACC 405 |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U15.00001: X-ray Analysis of Erbium Doping in Group IV Nanocrystalline Materials Robert Meulenberg, Tony van Buuren, Trevor Willey, Jonathan Lee, Louis Terminello We have produced erbium-doped germanium nanoparticles using a new two cell physical vapor deposition system. Doped nanoparticles are fabricated using two methods: 1) by co-evaporation of Er and Ge and 2) by Er deposition on the surface of undoped Ge nanoparticles. Using elemental specific x-ray techniques [x-ray absorption (XAS) and photoemission (PES) spectroscopy], we are able to monitor band edge shifts as a function of both particle size and Er concentration. In addition, we have used XAS and PES to probe the chemical environment of Er in Ge nanoparticles. We find that large Er/Ge ratios lead to strong spectroscopic signatures in the core level PES spectra. Lower Er/Ge ratios show very little effects in the core level spectra; however, the valence band density of states is altered which allows PES to probe dilute concentrations of Er in Ge nanoparticles. Impact of Er doping on the Ge nanoparticle electronic structure will be discussed. This work was supported by the Division of Materials Sciences, Office of Basic Energy Science, and performed under the auspices of the U. S. DOE by LLNL under contract No. W-7405-ENG-48. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U15.00002: Doping semiconductor nanocrystals: Theory S.C. Erwin, M.I. Haftel, Al.L. Efros, T.A. Kennedy, L. Zu, D.J. Norris The intentional introduction of impurities into semiconductor nanocrystals (NCs) is a poorly understood process whose phenomenology remains largely unexplained. For example, Mn can easily be incorporated into ZnSe NCs using simple precursors, but not into CdSe NCs---despite comparable solubility limits of 50-60 percent in the two bulk crystals. The conventional wisdom is that NCs can ``self-purify'' by expelling impurity atoms to the nearby surface (consistent with the fact that very small NCs cannot generally be doped). Here we propose a very different view: namely, that doping is controlled by the initial adsorption of impurities on the NC surface. For NCs with sufficiently well-defined facets, this view leads to several striking predictions. (i) Dopant incorporation should depend on crystallography. For example, we predict that Mn incorporation will be generally allowed in zinc-blende NCs (such as ZnSe) but suppressed or absent in wurtzite NCs (such as CdSe). (ii) Very small NCs are often known to form cage clusters with strongly reconstructed surfaces. We find that these surfaces do not permit strong dopant adsorption, thus precluding incorporation. (iii) If the equilibrium NC shape can be controlled, doping may be externally tunable or even switchable. Hence, for II-VI NCs grown colloidally, we predict that dopant incorporation will vary with the II:VI concentration ratio in solution. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U15.00003: Doping semiconductor nanocrystals: Experiment David Norris, Lijun Zu, Thomas Kennedy, Steven Erwin, Michael Haftel, Alexander Efros The intentional introduction of impurities into semiconductor nanocrystals is a poorly understood process whose phenomenology remains largely unexplained. The preceeding talk outlines a theoretical model that addresses this doping problem. Here we experimentally test some of the predictions of this model. In particular, we use photoluminescence (PL), electron paramagnetic resonance (EPR), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) to examine how the doping efficiency in ZnSe:Mn nanocrystals is influenced by various experimental parameters. In agreement with the theoretical model, the doping concentration can be enhanced by increasing the anion (Se) to cation (Zn) ratio in the growth solution. Finally, we exploit the predictions of the model to incorporate individual Mn impurities into previously undopable CdSe nanocrystals. This success indicates that doping is not intrinsically problematic in nanocrystals and a variety of doped particles should be experimentally realizable. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U15.00004: Shape Manipulation of II-VI Semiconductor Nanocrystals and Heterostructures by Controlled Reactant Injection. Aaron Saunders, Felice Shieh, Brian Korgel We have developed a general solution-phase method for inducing CdS, CdSe, and CdTe nanorod growth. The shape can be tuned from spheres to rods with aspect ratios up to 20 simply by sequentially injecting precursor solutions to promote epitaxial elongation of the wurtzite crystal structure in the [001] direction (i.e., along the c-axis). Under the appropriate conditions, homogeneous particle nucleation can be avoided upon subsequent precursor injections and the fast growth kinetics in the [001] direction extends the nanorods without increasing the diameter. We have employed this sequential injection approach to form both Type I (Nested) and Type II (Offset) heterostructures of CdS/CdSe/CdS and CdTe/CdSe/CdTe nanorods. Consistent with the Type I band offset, addition of CdS extensions to the ends of the CdSe nanorods significantly enhances the photoluminescence (PL); whereas, the extension of CdTe off the ends of the CdSe rods quenches the PL emission as electrons and holes separate at the CdSe/CdTe interface in the rod. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U15.00005: EPR Study of Manganese-Doped TiO2 Nanorods Tijana Rajh, Zoran Saponjic, Nada Dimitrijevic Titanium dioxide nanoparticles and nanorods were prepared by a hydrothermal method using scrolled anatase nanotubes as the starting material. The addition of manganese ions to scrolled nanotube precursors was found to unroll the nanotubes into sheet-like structures indicating a strong adsorption of manganese ions at undercoordinated sites that terminate layers of scrolled titanium dioxide nanotubes. Hydrothermal treatment of Mn$^{2+}$ unscrolled nanotubes was found to result in the formation of doped anatase nanorods (30 x 300 nm). Upon 1 {\%} doping, the optical properties of nanorods change, resulting in the appearance of a broad absorption band at $\sim $650 nm. X-band EPR spectra show unusual eleven line spectrum with 72 G hyperfine splitting and a g factor of 2.007. The total width of the spectrum was 890 G due to the appearance of second order effects. The same hyperfine coupling was found in samples having doping levels in the range 0.1-2 {\%} of Mn$^{2+}$, indicating specific interaction of manganese ions within the TiO$_{2}$ lattice. Adsorption of Mn$^{2+}$ ions onto the surface of nanorods did not change their optical properties and exhibits the typical six line spectrum of Mn$^{2+}$ with 90 G hyperfine splitting in addition to a broad, unresolved solution spectrum of Mn$^{2+}$. The effects of the size and shape of titanium dioxide nanoobjets on the spin multiplicity of manganese dopants are being investigated. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U15.00006: Synthesis of One-dimensional GaN Nanostructures and Their Implications for Formation Mechanisms Hsi-Lien Hsiao, A.B. Yang, M.C. Lu, Y.L. Chueh, L.J. Chen, L.J. Chou One-dimensional GaN nanostructures were successfully synthesized by gold-catalyzed metal-organic vapor phase approach. It was found that GaN whiskers of various morphologies could be synthesized on Si substrates by just controlling the temperature. Amorphous Ga/nitride nanowires formed at 450C with tadpole-like structures. GaN tubular nanostructures synthesized were observed at 600C. Wavy-like hollow interiors with single crystalline wurtzite phase were seen from high-resolution transmission electron microscope images. With increasing the catalytic temperature, crystalline GaN nano-pyramids, and straight nanowires were formed. It is proposed that the morphologies evolution of GaN whiskers was attributed to the competitions of TMG surface diffusion to the Ga-Au eutectic droplets, Ga bulk diffusion into the catalyst, and GaN seeding. While at low temperature, only Ga atoms at surface react with ammonia and form amorphous Ga@nitride nanowires. With increasing the temperature, GaN seeding and the subsequent growth along the circumferential edges of these seeds leads to the evolution of nanotube morphology. Further increasing the temperature, promoted the nitridation efficiency and axial growth rate and lead to the nanowires and nanopyramids growth. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U15.00007: Synthesis and Electrical Transport Studies of Zn-doped Ga$_2$O$_3$ Nanowires Pai-chun Chang, Z. Fan, W. Tseng, D. Wang, A. Rajagopal, Jia G. Lu Ga$_{2}$O$_{3}$ is a wideband gap material ($E_{g}=4.9eV$). Its one dimensional nanostructures have attracted much research effort. Ga$_{2}$O$_{3}$ nanowire is a promising material in the applications such as blue light emitter, transparent conducting oxide, and chemical sensor. However, the electronic device application of Ga$_{2}$O$_{3}$ nanowire is difficult due to its low electrical conductivity. In this work, $\beta $-Ga$_{2}$O$_ {3}$ nanowires were synthesized via catalytic chemical vapor deposition method. The diameter of the as-grown nanowires ranges from 20 to 80nm. In order to improve the electrical properties, zinc was used as a dopant. A series of material characterizations were performed to study the properties. Electron microscopy shows the morphology and crystal structure, while X-ray diffraction provides the crystal information and composition. In addition, photoluminescence spectra and photoconductivity measurements show trapping states located within the bandgap. The nanowires were also fabricated into field-effect-transistors for transport measurements. And $I-V$ and $I-V_{g}$ curves manifest QTR{it}{p}-type semiconducting behavior, and carrier concentration and mobility are estimated. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U15.00008: Synthesis and structural analysis of $\gamma $--Fe2O3/CdS nanocrystal heterodimers Kwan-Wook Kwon, Moonsub Shim Inorganic nanocrystal oligomers with two or more distinct chemical compositions open up interesting avenues of developing building block materials for a variety of research directions. For example, asymmetric dimers, trimers, etc. can provide chemically programmable assembly of nanostructures. Unique properties arising at the nanoscale may also be juxtaposed in a controlled manner (e.g. ferromagnetic behavior with optical properties governed by quantum confinement). $\gamma $--Fe$_{2}$O$_{3}$/CdS heterodimers have been synthesized in solution by annealing Cd and S reagents adsorbed on $\gamma $--Fe$_{2}$O$_{3}$ nanocrystals. While the large lattice mismatch between $\gamma $--Fe$_{2}$O$_{3}$ and CdS leads to dewetting, TEM analysis reveals that certain junction planes can lead to minimized strain allowing dimers to form. Furthermore, both wurtzite and zinc blend structures are observed to grow on the close-packed (1 1 1) plane of $\gamma $--Fe$_{2}$O$_{3}$. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U15.00009: Fabrication and characterization of SnO2 nanobelt field effect transistors Yi Cheng, S. von Molnar, P. Xiong, Lenwood Fields, J.P. Zheng, R. Yang, Z.L. Wang Single-crystalline SnO$_{2}$ nanobelts have been produced by thermal evaporation of oxide powders in a tube furnace without any chemical catalyst. Individual SnO$_{2}$ nanobelts with thicknesses of 30nm$\sim $300nm and lengths as long as several hundred $\mu $m were dispersed onto a doped Si/SiO$_{2}$ substrate, and multi-terminal metal electrodes were defined on a nanobelt using photolithography. An individual nanobelt was then characterized by measuring current--voltage characteristics as a function of temperature using 4-probe measurement. Temperature dependence of the resistivity is characteristic of a doped semiconductor. A field effect transistor (FET) is formed using a nanobelt as the channel and doped Si as the gate. Electrical measurements revealed characteristic behavior of an n-channel depletion-mode FET, with well-defined linear and saturation regimes, a threshold voltage of $\sim $-15V, and on/off ratio as high as 10$^{3}$. The channel mobility is estimated to be 25 cm$^{2}$/V$\cdot $s, and carrier concentration about 6x10$^{15}$ cm$^{-3}$. The results demonstrate the potential of using SnO$_{2}$ nanobelt to construct high performance nanoFET with possible applications as chemical and biological sensors. This work is supported by NSF NIRT grant ECS-0210332. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U15.00010: Superparamagnetic Core-shell Silica- Polypeptide Composite Particles Paul Russo, Sibel Turksen, Erick Soto-Cantu, Jianhong Qiu Core-shell composite particles have been prepared, each consisting of a silica-coated cobalt center to which a homopolypeptide shell, either poly ($\varepsilon $-carbobenzyloxy-L-lysine) or poly ($\gamma $-benzyl-L-glutamate), is attached covalently. Core particles were decorated with a mixture of amino groups and passivating groups through silylation reactions. The amino groups initiated the polymerization, with attachment, of $N$-carboxyanhydride monomers, resulting in a homopolypeptide shell. Characterization by dynamic light scattering confirmed the helix-coil transition of the particles through repeated heating and cooling cycles in an organic solvent. The living nature of the polypeptide shell has also been confirmed. The particles have a size and uniformity that leads to formation of colloidal crystals. Magnetometer measurements suggest the particles are superparamagnetic. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U15.00011: Synthesis of monodisperse nanocrystals via green chemistry William Yu, Vicki Colvin Novel strategy for the synthesis of monodisperse nanocrystals was developed. This new method is cheap, reliable, safe and environmentally benign. The nanocrystals synthesized by this new method, including semiconductor nanocrystals (quantum dots) CdS........$^{1}$, CdSe, CdTe........$^{2}$, PbSe...........$^{3}$, and magnetic nanocrystals, Fe$_{3}$O$_{4}$.()$^{4}$ (magnetite), have wider size range, and narrower size distribution (less than 10{\%}). Through this new method, one can control the size, shape, and crystal structure of the aimed nanocrystals by simply changing the ligands used in the synthesis. With the high quality nanocrystals, some basic physical constants, such as extinction coefficients of semiconductor nanocrystals were accurately measured........$^{5}$. A simple method was also developed to transfer the above-mentioned organic-media synthesized high quality nanocrystals to aqueous media (pure or buffered water). The water-soluble nanocrystals keep their original properties in organic media. For example, water-soluble semiconductor nanocrystals have the same absorption and emission spectra, the same quantum yield, and the same size and size distribution as the ones dispersed in chloroform. The water-soluble nanocrystals are stable in pure water and conventional biological buffers. .$^{1 }$W. W. Yu and X. Peng, \textit{Angew. Chem. Int. Ed.}, \textbf{41}, 2368 (2002). $^{2 }$W. W. Yu, Y. A. Wang and X. Peng, \textit{Chem. Mater.}, \textbf{15}, 4300 (2003). $^{3 }$W. W. Yu, J. C. Falkner, B. S. Shih and V. L. Colvin, \textit{Chem. Mater.}, \textbf{16}, 3318 (2004). $^{4 }$W. W. Yu, J. C. Falkner, C. Yovuz and V. L. Colvin, \textit{Chem. Commun}, 2306 (2004). $^{5 }$W. W. Yu, L. Qu, W. Guo and X. Peng, \textit{Chem. Mater.}, \textbf{15}, 2854 (2003). [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U15.00012: Formation and Properties of Self-Assembled Ni Nanodots and VLS Growth of GaN Nanowires D. Aurongzeb, G. Kipshidze, B. Yavich, A. Chandolu, J. Yun, V. Kuryatkov, I. Ahmad, H. Temkin, M. Holtz We report the formation of Ni nanodots and subsequently use them to grow GaN nanowires. For the Ni nanodots both Si(111) and sapphire substrates are used. Layers of Ni are deposited with different thickness (1 to 5 nm) on these substrates using UHV electron-beam evaporation. The layers are annealed ex situ and the nanodot formation is studied for different anneal temperatures and durations. For nanodot formation on Si(111) the process is self limiting at high temperature with distinct facets. Activation energies are consistent with Ni surface diffusion as the primary formation mechanism. Nanostructures on sapphire are droplet shaped, we observe no distinct faceting. The nanodot size can be controlled with initial Ni thickness. Based on these studies, we have grown GaN nanowires on sapphire substrates covered by Ni layers having various thickness. Vapor- liquid-solid growth mechanism is demonstrated using a pulsed metal-organic chemical vapor deposition approach. The GaN nanowires are straight, vertically oriented, and of constant diameter following the same trend as the Ni nanodot diameter with initial thickness. We report control of GaN nanowire diameter and length up to 1 micron. [Preview Abstract] |
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