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Spectroscopy, Imaging, and Modeling of Individual Gold Decahedra

Jan 2 11

Jessica Rodrıguez-Fernandez, Carolina Novo, Viktor Myroshnychenko, Alison M. Funston,Ana Sanchez-Iglesias, Isabel Pastoriza-Santos, Jorge Pe´rez-Juste, F. Javier Garcıa de Abajo, Luis M. Liz-Marzan, and Paul Mulvaney

J. Phys. Chem. C 113, 18623–18631 (2009)

Abstract Gold nanodecahedra (pentagonal bipyramids) exhibit striking optical properties spanning the visible and near- IR spectral regions, which together with a high electric-field enhancement at their tips, makes them very promising materials for sensing-related applications. The effective design of biosensors based on these nanocrystals requires a detailed investigation and understanding of their surface plasmon resonances. For that purpose, we have used correlated electron microscopy imaging and optical dark-field spectroscopy on individual decahedra to investigate their size and orientation-dependent optical properties at the single particle level. We have also investigated the effects of tip truncation and local refractive index changes on single particle plasmon resonances. The experimental results are supported by theoretical modeling based on a fully 3D, boundary element method (BEM-3D).

Electrochemical Charging of Single Gold Nanorods

Jan 2 11

Carolina Novo, Alison M. Funston, Ann K. Gooding, and Paul Mulvaney

J.Am.Chem. Soc. 131, 14664 (2009).

Gold rods have been electrochemically charged and the resultant spectral shifts studied by dark field spectroscopy. It is shown that electron transfer to single nanocrystals can be studied by surface plasmon spectroscopy. Although several thousand electrons are transferred in these experiments, the possibility is opened up for the optical detection of single electrons by this method.

 

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Mapping the Optical Properties of CdSe/CdS Heterostructure Nanocrystals: The Effects of Core Size and Shell Thickness

Jan 2 11

Joel van Embden, Jacek Jasieniak and Paul Mulvaney

J.Am.Chem. Soc., 131, 14299-309 (2009)

Abstract: Here we present the first comprehensive report on CdSe/CdS heterostructure nanocrystals. The effects of core size and shell thickness on the optical properties of CdSe/CdS heterostructure nanocrystals are investigated. We report a reliable synthetic method to grow thick CdS shells on CdSe cores with sizes ranging from 2.5-4.7 nm.We provide a calibration curve, which enables determination of CdS shell thickness ((0.1 nm) over a wide range of core sizes, circumventing the need for time-consuming HRTEM analyses. Epitaxial growth of the shells was verified by HRTEM, XRD, and SAED. In-situ reaction measurements revealed the average per particle (p) deposition rates for cadmium and sulfur to be kCd ) 5.38 °— 10-25 mol s-1 p-1 and kS ) 4.83 °— 10-24 mol s-1 p-1. Faster sulfur deposition rates are attributed to the absence of strong sulfur binding ligands in the growth medium. Through the rigorous use of high resolution transmission electron microscopy, a direct link between the dimensions of the heterostructures and their band-edge transition energies, quantum yields, and excited state lifetimes is established. The experiments show that the band-edge transition energies of the core samples, which initially span approximately 431 meV, condense to span only 163 meV after the growth of a 6 monolayer-thick CdS shell. Furthermore, shifts in the band-edge transition energies were found to be extremely sensitive to core size. The QY of the as-prepared core/shells ranged from 25 to 60%. The QYs and band-edge lifetimes of the core/shells were found to depend upon the ligands adsorbed to the particle surface. These data prove that one or both of the charge carriers still has access to the particle surface despite the presence of a 2.2 nm thick CdS shell.

 

Exciton-Trion Transitions in Single CdSe–CdS Core–Shell Nanocrystals

Jan 2 11

Daniel E. Gomez, Joel van Embden, Paul Mulvaney, Mark J. Fernee and Halina Rubinsztein-Dunlop

ACS Nano 3 ,  2281–2287 ( 2009)

ABSTRACT We report on the observation of an intermediate state in the blinking of single CdSe/CdS core-shell nanocrystals. This state has a low quantum yield and connects the “on” and “off” states commonly observed in the photoluminescence blinking of individual nanocrystals. We find that the transitions between these two emitting states follow nearly single-exponential statistics. The transitions from the “on” state to this intermediate state result from changes in the surface passivation of the nanocrystal. The data are consistent with photoinduced, adsorption/desorption events that take place at the surface of the nanocrystals. The trion state leads to a reduction in photoluminescence in nanocrystals.

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Re-examination of the Size-Dependent Absorption Properties of CdSe Quantum Dots

Oct 23 10

Jacek Jasieniak, Lisa Smith, Joel van Embden, and Paul Mulvaney and Marco Califano

J. Phys. Chem. C 2009, 113, 19468 19474

Abstract:

We investigate the size-dependent optical absorption coefficients of CdSe nanocrystals at both the band-edge and high within the absorption profile. The absorption properties in both of these regions must be selfconsistent to ensure accuracy of the measured coefficients. By combining transmission electron microscopy and inductively coupled plasma-optical emission spectroscopy, we map out the optical absorption properties and establish reliable size-dependent band-edge calibration curves. The measured absorption properties are compared to a simple 0D confinement model, to classical theory based on light absorption by small particles in a dielectric medium and to state-of-the-art atomistic semiempirical pseudopotential modeling. The applicability of these newly established calibration curves is demonstrated by analyzing the nucleation and growth kinetics of CdSe nanocrystals in solution.

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Gold Nanoparticles: Past, Present, and Future

Oct 23 10

Rajesh Sardar, Alison M. Funston, Royce Murray and Paul Mulvaney

The development of surface plasmon spectroscopy and gold cluster electrochemistry are reviewed in this Langmuir Perspective.

Langmuir review

Evolution of Colloidal Nanocrystals: Theory and Modeling of their Nucleation and Growth

Oct 23 10

Joel van Embden, John E. Sader, Malcolm Davidson and Paul Mulvaney

J. Phys. Chem. C 113, 16342 16355 (2009).

Abstract

Through the use of a population balance equation (PBE), the nucleation and growth of nanocrystals evolving under various initial reaction conditions are simulated. Simulations of nanocrystal (NC) growth in both the diffusion and reaction limits are presented, and it is concluded that NC growth proceeds under strongly reaction limited kinetics. The particle size distributions obtained in the asymptotic diffusion and reaction limits were found to be slightly narrower than the stationary distributions predicted by LSW and Wagner theories, respectively. There is strong experimental evidence indicating that the early stages of NC synthesis involve simultaneous nucleation, growth and coarsening. The simulations performed here are able to replicate these conditions, providing insight into the factors that govern these early time processes, as well as the consequences they have at longer reaction times.

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Plasmon Coupling of Gold Nanorods at Short Distances and in Different Geometries

Oct 23 10

Alison M. Funston, Carolina Novo, Tim J. Davis, and Paul Mulvaney

Nano Lett., 9 (4), 1651-1658 (2009).

Abstract

The experimentally determined scattering spectra of discrete, crystalline, gold nanorod dimers arranged side to-side, end-to-end, at right angles in different orientations and also with longitudinal offsets are reported along with the electron micrographs of the individual dimers. The spectra exhibit both red- and blue-shifted surface plasmon resonances, consistent with the plasmon hybridization model. However, the plasmon coupling constant for gold dimers with less than a few nanometers separation between the particles does not obey the exponential dependence predicted by the Universal Plasmon Ruler equation. The experimentally determined spectra are compared with electrodynamic calculations and the interactions between the individual rod plasmons in different dimer orientations are elucidated.

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Nanoletters

Hydrogen-Bond-Selective Phase Transfer of Nanoparticles across Liquid/Gel Interfaces

Oct 10 10

Zhengwei Mao, Jinshan Guo, Shuo Bai, Tich-Lam Nguyen, Haibing Xia, Yubin Huang, Paul Mulvaney, and Dayang Wang

Angew.Chem. Int Ed. 48, 4953 -56 (2009).

We have successfully directed hydrophobic colloidal NPs coated with PLA and PEG to transfer from the organic to the aqueous phase across not only liquid/liquid but also gel/liquid interfaces during the PLA degradation. Crossing the interface for NPs is exceedingly fast, and the transfer kinetics is limited by the NP diffusion in the bulk phases and the NP attachment at the interfaces.

AngewChem 2009

Electrodynamic ratchet motor

Oct 7 10

Jiufu Lim, John E. Sader, and Paul Mulvaney

PHYSICAL REVIEW E 79, 030105R  (2009).

Brownian ratchets produce directed motion through rectification of thermal fluctuations and have been used for separation processes and colloidal transport. We propose a flashing ratchet motor that enables the transduction of electrical energy into rotary micromechanical work. This is achieved through torque generation provided by boundary shaping of equipotential surfaces. The present device contrasts to previous implementations that focus on translational motion. Stochastic simulations elucidate the performance characteristics of this device as a function of its geometry. Miniaturization to nanoscale dimensions yields rotational speeds in excess of 1 kHz, which is comparable to biomolecular motors of similar size.