AVS2010 Session NS-ThM: Nanowires and Nanoparticles

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.

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.

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).

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.