82 research outputs found
Electron beam nanosculpting of suspended graphene sheets
We demonstrate high-resolution modification of suspended multi-layer graphene
sheets by controlled exposure to the focused electron beam of a transmission
electron microscope. We show that this technique can be used to realize, on
timescales of a few seconds, a variety of features, including nanometer-scale
pores, slits, and gaps that are stable and do not evolve over time. Despite the
extreme thinness of the suspended graphene sheets, extensive removal of
material to produce the desired feature geometries is found to not introduce
long-range distortion of the suspended sheet structure
Collective Fluorescence Enhancement In Nanoparticle Clusters
Many nanoscale systems are known to emit light intermittently under continuous illumination. In the fluorescence of single semiconductor nanoparticles, the distributions of bright and dark periods (\u27on\u27 and \u27off\u27 times) follow Levy statistics. Although fluorescence from single-quantum dots and from macroscopic quantum dot ensembles has been studied, there has been little study of fluorescence from small ensembles. Here we show that blinking nanorods (NRs) interact with each other in a cluster, and the interactions affect the blinking statistics. The on-times in the fluorescence of a NR cluster increase dramatically; in a cluster with N NRs, the maximum on-time increases by a factor of N or more compared with the combined signal from N well-separated NRs. Our study emphasizes the use of statistical properties in identifying the collective dynamics. The scaling of this interaction-induced increase of on-times with number of NRs reveals a novel collective effect at the nanoscale
Levy statistics and anomalous transport in quantum-dot arrays
A novel model of transport is proposed to explain power law current
transients and memory phenomena observed in partially ordered arrays of
semiconducting nanocrystals. The model describes electron transport by a
stationary Levy process of transmission events and thereby requires no time
dependence of system properties. The waiting time distribution with a
characteristic long tail gives rise to a nonstationary response in the presence
of a voltage pulse. We report on noise measurements that agree well with the
predicted non-Poissonian fluctuations in current, and discuss possible
mechanisms leading to this behavior.Comment: 7 pages, 2 figure
Imaging the charge transport in arrays of CdSe nanocrystals
A novel method to image charge is used to measure the diffusion coefficient
of electrons in films of CdSe nanocrystals at room temperature. This method
makes possible the study of charge transport in films exhibiting high
resistances or very small diffusion coefficients.Comment: 4 pages, 4 jpg figure
Characterization of Memory and Measurement History in Photoconductivity of Nanocyrstal Arrays
Photoconductivity in nanocrystal films has been previously characterized, but memory effects have received little attention despite their importance for device applications. We show that the magnitude and temperature dependence of the photocurrent in CdSe/ZnS core-shell nanocrystal arrays depends on the illumination and electric field history. Changes in photoconductivity occur on a few-hour timescale, and subband gap illumination of nanocrystals prior to measurements modifies the photocurrent more than band gap illumination. The observed effects can be explained by charge traps within the band gap that are filled or emptied, which may alter nonradiative recombination processes and affect photocurrent
Fluorescence Blinking Statistics From CdSe Core And Core/Shell Nanorods
We report fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratios ranging from 3 to 11. This study also included core/shell CdSe/ZnSe NRs and core NRs with two different surface ligands producing different degrees of surface passivation. We compare the findings for NRs to our measurements of blinking statistics from spherical CdSe core and CdSe/ZnS core/shell nanocrystals (NCs). We find that, for both NRs and spherical NCs, the off-time probability distributions are well described by a power law, while the on-time probability distributions are best described by a truncated power law, P(tau(on)) similar to tau(-alpha)(on)e(on)(-tau)(/tau c). The measured crossover time, tau(c), is indistinguishable within experimental uncertainty for core and core/shell NRs, as well as for core NRs with different ligands, for the same core size, indicating that surface passivation does not affect the blinking statistics significantly. We find that, at fixed excitation intensity, 1/tau(c) increases approximately linearly with increasing NR aspect ratio; for a given sample, 1/tau(c) increases very gradually with increasing excitation intensity. Examining 1/tau(c) versus the single-particle photon absorption rate for all samples indicates that the change in NR absorption cross section with sample size can account for some but not all of the differences in crossover time. This suggests that the degree of quantum confinement may be partially responsible for the aspect ratio dependence of the crossover time
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