AVS2010 Session NS-ThM: Nanowires and Nanoparticles

Thursday, October 21, 2010 8:00 AM in Room La Cienega
Thursday Morning

Time Period ThM Sessions | Abstract Timeline | Topic NS Sessions | Time Periods | Topics | AVS2010 Schedule

Start Invited? Item
8:00 AM NS-ThM-1 Enantioselective Separation on Chiral Au Nanoparticles
Nisha Shukla, Melissa Bartel, Nathaniel Ondeck, Andrew J. Gellman (Carnegie Mellon University)

The surfaces of chemically synthesized Au nanoparticles have been modified with D- or L- cysteine to render them chiral and enantioselective for adsorption of chiral molecules. Their enantioselective interaction with chiral compounds has been probed by optical rotation measurements when exposed to racemic propylene oxide. The ability of optical rotation to detect enantiospecific adsorption arises from the fact that the specific rotation of polarized light by R- and S-propylene oxide is enhanced by interaction Au nanoparticles. The enhancement of the specific optical rotation of polarized light by R- and S-propylene oxide is sensitive to excitation wavelength. Longer the excitation wavelength, smaller is the specific rotation of polarized light. This effect is related to previous observations of enhanced circular dichroism by Au nanoparticles modified by chiral adsorbates. More importantly, chiral Au nanoparticles modified with either D- or L- cysteine enantioselectively adsorb one enantiomer of propylene oxide from a solution of racemic propylene oxide, thus leaving an enantiomeric excess in the solution phase. Au nanoparticles modified with L- cysteine ( D- cysteine) selectively adsorb the R-propylene oxide (S-propylene oxide). A simple model has been developed that allows extraction of the enantiospecific equilibrium constants for R- and S-propylene oxide adsorption on the chiral Au nanoparticles.

8:20 AM NS-ThM-2 Unique Optical Properties in Wet - Chemically Etched Silicon Nanowires
Vladimir Sivakov, Bjoern Hoffmann, Felix Voigt, Gerald Broenstrup, Florian Talkenberg (Institute of Photonic Technology, Germany); Gottfried Bauer (University of Oldenburg, Germany); Silke H. Christiansen (Max Planck Institute for the Science of Light, Germany)
Silicon nanowire (SiNW) ensembles with vertical and zig-zag architectures have been realized using wet chemical etching of bulk silicon wafers (p-Si(111) and p-Si(100)) with an etching hard mask of silver nanoparticles that are deposited by wet electroless deposition on polystyrene pattered silicon surfaces.
 
Strong visible (red-orange) room temperature photoluminescence has been observed in wet chemically etched heavily (1020 cm-3) and lowly (1015 cm-3) doped SiNWs. Our observations strongly suggest that visible light emission at room temperature of SiNWs is a result of the rough sidewall structure that can be such that nanoscale features form that make quantum confinement most probable. Significant light absorption (over 90% in a range between 300-2000 nm) was observed in the SiNWs covered by the TCO (Al doped ZnO) thin layers performed via Atomic Layer Deposition. The strong absorption, less reflection of visible and infra-red light and room temperature photoluminenscence of the SiNW ensembles strongly suggest that such a material has a real potential to be applied in the fields of opto-electronics, photonics, sensoric and photovoltaics. The morphology, crystallographic and surface structure, and optical properties of SiNWs will be presented and discussed in details.
 
 
 
 
8:40 AM NS-ThM-3 Size, Composition and Support Effects in Nanocatalysis: I. Bridging the Sub-Nanometer and Nanometer Size Range & II. Coupling the Studies of Model and “Real” Catalysts
Stefan Vajda (Argonne National Laboratory)
The elucidation of the size/composition/shape/structure and function correlation is instrumental for the design of new catalysts. Uniform particles are prerequisites for such studies, making size-selected clusters of few atoms to several nm in size as ideal model systems. The experiments are based on 1) size-selected cluster deposition, 2) electron microscopy and 3) in situ synchrotron X-ray characterization under working conditions (scattering and absorption), combined with 4) mass spectroscopy of products. DFT calculations performed by our collaborators are instrumental at the understanding of the catalytic properties of these materials. In this presentation, examples will be given on bridging the size gap between the sub-nanometer and nanometer cluster size regime and on coupling studies of model size-selected [1-3] and "real"- with wet chemical methods prepared [4-6] catalysts. Processes discussed will include dehydrogenation, Fischer-Tropsch synthesis and partial oxidation of alkenes. For example, our studies led to the identification of a new class of silver-based direct propylene epoxidation catalyst which works at considerably lower temperatures than existing ones [1]. The role of the size in catalyst's activity and the evolving morphology of silver nanoclusters under epoxidation conditions will be addressed [1-3], followed by the discussion of strong size, composition and support effects in dehydrogenation [4,5], hydrogenation [6] and Fischer–Tropsch reactions.
  
[1] Y. Lei, F. Mehmood, S. Lee, J. P. Greeley, B. Lee, S. Seifert, R. E. Winans, J. W. Elam, R. J. Meyer, P. C. Redfern, D. Teschner, R. Schlögl, M. J. Pellin, L. C. Curtiss, and S. Vajda, Science 328, 224 (2010)
[2] S. Vajda, S. Lee, K. Sell, I. Barke, A. Kleibert, V. von Oeynhausen, K.-H. Meiwes-Broer, A. Fraile-Rodriguez, J. W. Elam, M. J. Pellin, B. Lee, S. Seifert, R. E. Winans , J. Chem. Phys., 131, 121104 (2009),
[3] L. M. Molina, S. Lee, K. Sell, G. Barcaro, A. Fortunelli, B. Lee, S. Seifert, R. E. Winans, J. W. Elam, M. J. Pellin, I. Barke, A. Kleibert, V. von Oeynhausen, Y. Lei, R. J. Meyer, J. A. Alonso, A. Fraile-Rodríguez, S. Giorgio, C. R. Henry, K.-H. Meiwes-Broer, and S. Vajda, Catal. Today, invited, under review
[4] M. Di Vece, S. Lee, X. Wang, B. Lee, S. Seifert, R.E. Winans, M. Neurock, G. Haller, L. D. Pfefferle, and S. Vajda, in preparation
[5] M. Di Vece, S. Lee, R. Si, B. Ricks, S. Seifert, R.E. Winans, M. Flytzani-Stephanopoulos, and S. Vajda, in preparation
[6] S. A. Wyrzgol, S. Schäfer, S. Lee, B. Lee, M. Di Vece, X. Li, S. Seifert, R. E. Winans, M. Stutzmann, J. A. Lercher, and S. Vajda, 2010, Phys. Chem. Chem. Phys. feature article, on-line April 27, 2010
9:20 AM Invited NS-ThM-5 Single-nanoparticle Catalysis at Single-turnover Resolution
Peng Chen (Cornell University)

Metal nanoparticles can catalyze many chemical transformations for energy conversion, petroleum processing, and pollutant removal. Charactering their catalytic activity is important, but challenging in ensemble measurements due to their morphology dispersions and variable surface active sites. Using single-molecule microscopy of fluorogenic reactions, we monitor the redox catalytic reactions on the surface of individual Au-nanoparticles in an aqueous environment in real time at single-turnover resolution. We find that for catalytic product generation, all Au-nanoparticles follow a Langmuir-Hinshelwood mechanism, but individual nanoparticles show drastically different reactivity. And for product dissociation, three nanoparticle subpopulations are present that show differential selectivity between parallel dissociation pathways. Individual nanoparticles show large temporal activity fluctuations, attributable to both catalysis-induced and spontaneous dynamic surface restructuring that occurs at different timescales at the surface catalytic and product docking sites. Individual Au-nanoparticles also show reactant-concentration dependent dynamic surface switching between a low reactivity state and high reactivity state. Strong size dependences are also observed in the catalytic activity, selectivity, and dynamics of these Au-nanoparticles. Smaller particles are more reactive but bind the reactant weaker. Larger particles are less selective in the parallel reaction pathways. The smaller particles are more prone to dynamic surface restructuring, whose activation energies and timescales are quantified. The results exemplify the power of the single-molecule approach in revealing the interplay of catalysis, heterogeneous reactivity, and surface structural dynamics in nanocatalysis.

10:00 AM BREAK - Complimentary Coffee in Exhibit Hall
10:40 AM NS-ThM-9 Size Effects in the Synthesis of Ge and Ge/Si Nanowire Heterostructures
Shadi A. Dayeh, S. Tom Picraux (Los Alamos National Laboratory)

Progress in the synthesis of semiconductor nanowires has prompted intensive discussions of the science of their growth and the technological applications they promise. Fundamental aspects of their growth have been postulated for nearly five decades for larger diameter nanowires and debated more recently in detailed growth studies for different materials systems. Here, we exploit an extreme level of control over diameter, morphology, and placement in VLS synthesized germanium nanowires to establish systematic size effects on their growth at small diameters, down to sub-10 nm, where quantum effects become relevant. We observe reproducible reduction in Ge nanowire growth rates with decreased diameter coupled to a measured increase in the Ge equilibrium solubility1 for the same wires, validating the role of the Gibbs-Thomson effect in nanowire growth at small diameters. We show how this sets a practical thermodynamic limit on the lowest achievable nanowire diameters (~ 3 nm) and present comprehensive studies of the effects of temperature, pressure, and the introduction of dopant precursors on the size dependences and cutoff diameters for nanowire growth. We also discuss methods to control and eliminate Au diffusion during the growth of Ge/Si core/shell heterostructures. Single crystal core/multi-shell Ge/p+Ge/Si nanowires were grown using such a process and their transport properties benchmarked. Using field-effect transistors as a test-bed for their transport properties, we observe up to 2X mobility enhancement in such heterostructured nanowires without Au diffusion and obtain record geometry-normalized on-currents for p-type FET devices of up to 430 µA/V. These studies provide an in-depth understanding for the control of the growth of Ge/Si nanowires and for exploiting their bandgap engineering possibilities for unique nanoscale device performance.

1 E. Sutter et al., 2010 (to be published).

11:00 AM NS-ThM-10 Formation and Characterization of Metallic Glass Nanowire
Koji Nakayama, Yoshihiko Yokoyama, Takahito Ono, Ming Wei Chen, Kotome Akiyama, Toshio Sakurai, Akihisa Inoue (Tohoku University, Japan)
Metallic glasses have exciting potential for structural, chemical, and magnetic applications with the sizes ranging from micrometer to centimeter, but the fabrication and characterization down to nanoscale remains an important challenge. Progress has been hindered by the lack of bottom-up methodologies to produce amorphous nanostructures. Recently, we show the self-organized amorphous nanowires that are formed on the fracture surfaces of bulk metallic glasses [Nakayama et al., Nano Lett. 8, 516-519 (2008)]. However, it is difficult to control their morphologies because they were created by instantaneous fracture processes. Here we report the controlled formation and mechanical characterization of individual amorphous nanowires. We find that they have a high strength with the excellent flexibility where the elastic modulus is much smaller than that of the bulk owing to the hyper-excess free volume in nanowire. The nanowire composed of amorphous materials leading to outstanding mechanical properties would offer a new paradigm for development in nanotechnology and materials science.
11:20 AM NS-ThM-11 Diameter Dependence of the Minority Carrier Diffusion Length in Semiconductor Nanowires
Afsoon Soudi, Yi Gu (Washington State University)
Carrier transport in semiconductors is of both fundamental and technological significance, as it not only reflects fundamental aspects such as electron-phonon interactions, but also controls electronic and opto-electronic device characteristics. Minority carrier transport is particularly important, as it determines the performance of p-n junction based devices. A fundamental understanding of carrier transport properties, especially those of minority carriers, provides a critical basis for material engineering and device design efforts.
In advancing semiconductor nanowire-based device technologies, a quantitative knowledge of carrier transport parameters, such as the carrier diffusion length, is required for a rational design of devices with controlled performance. From a fundamental perspective, in semiconductor nanowires, the one-dimensional confinement of carriers and phonons, together with the high surface-to-volume ratio, can render carrier transport characteristics significantly different from those in the bulk. Here, using the near-field scanning photocurrent microscopy technique, we have directly measured the minority carrier diffusion length in single ZnO nanowires. In particular, a near-field scanning optical microscope was used to locally generate minority carriers in single nanowire Schottky diodes; the spatial variations of the resulting photocurrent images near the Schottky contact were used to obtain the minority carrier diffusion length, LD. The diameter dependence of LD suggests a diameter-dependent surface electronic structure, particularly an increase in the density of mid-bandgap surface states with the decreasing diameter. This diameter dependence of the surface electronic structure might be a universal phenomenon in wurtzite-type nanostructures, and is critical in interpreting and understanding the effects of surfaces on various material properties.
11:40 AM NS-ThM-12 Photocatalytic Deposition of Pt or Ag Nanoparticles on Ordered Linear Arrays of TiO2 Nanoparticles
James Taing, John C. Hemminger (University of California, Irvine)

TiO2 nanoparticles are generated on step edges of highly oriented pyrolitic graphite (HOPG) via physical vapor deposition of Ti followed by air oxidation. Deposition of Ti on HOPG while the substrate is held at 900 K results in nanoparticle growth exclusively at the graphite step edges. Since the steps on high quality HOPG are long (>1 micron) and very parallel, the result is highly ordered arrays of nanoparticles. Photodeposition of Pt on the TiO2 nanoparticles results in the decoration of the TiO2 with Pt nanoparticles (≤ 5 nm). Photodeposition of Pt is accomplished by submersion in an aqueous solution of 0.25 mM K2PtCl4 and 0.5 mM trisodium citrate followed by photolysis with TiO2 bandgap radiation (365 nm radiation from a 200 W Mercury lamp). Similarly, Ag nanoparticles can be deposited by photolysis of the sample in an aqueous solution of 0.25 mM Ag(NO)3 and 0.5 mM trisodium citrate. Scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), x-ray dispersive spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS) characterize the morphology, crystal structure, and chemical identity of the nanoparticles. Images of decorated TiO2 nanoparticles are included in the supplement.

Time Period ThM Sessions | Abstract Timeline | Topic NS Sessions | Time Periods | Topics | AVS2010 Schedule