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Richard D Schaller

Publications and source records attributed to Richard D Schaller.

8 recordsLinked to original sources
PubMed2006

High-efficiency carrier multiplication and ultrafast charge separation in semiconductor nanocrystals studied via time-resolved photoluminescence.

We demonstrate novel methods for the study of multiple exciton generation from a single photon absorption event (carrier multiplication) in semiconductor nanocrystals (or nanocrystal quantum dots) that are complementary to our previously reported transient absorption method. By monitoring the time dependence of photoluminescence (PL) from CdSe nanocrystals via time-correlated single photon counting, we find that carrier multiplication is observable due to the Auger decay of biexcitons. We compare these data with similar studies using transient absorption and find that the two methods give comparable results. In addition to the observation of dynamical signatures of carrier multiplication due to the Auger decay, we observe spectral signatures of multiple excitons produced from the absorption of a single photon. PL spectra at short times following excitation with high-energy photons are red-shifted compared to the single-exciton emission band, which is consistent with previous observations of significant exciton-exciton interactions in nanocrystals. We then show using a combination of transient absorption and time-resolved PL studies that charge transfer between a nanocrystal and a Ru-based catalyst model compound takes place on a time scale that is faster than Auger recombination time constants, which points toward a possible design of donor-acceptor assemblies that can be utilized to take advantage of the carrier multiplication process.

Journal Article↗
PubMed2006

Non-Poissonian exciton populations in semiconductor nanocrystals via carrier multiplication.

We analyze distributions of exciton populations in PbSe nanocrystal (NC) ensembles as a function of excitation wavelength. For photon energies that result in carrier multiplication, these distributions are non-Poissonian and are characterized by two dominant exciton multiplicities that are determined by the ratio of photon energy to NC energy gap. For certain photon energies, we produce photoexcited NC ensembles with a nearly pure single multiplicity that can be tuned from 1 to 7. This result can find applications ranging from lasing and nonlinear optics to photovoltaics and photocatalysis.

Journal Article↗
PubMed2006

Seven excitons at a cost of one: redefining the limits for conversion efficiency of photons into charge carriers.

The performance of photovoltaic and photochemical devices is directly linked to the efficiency with which absorbed photons are converted into electron-hole pairs (excitons). A usual assumption is that one photon produces a single exciton, while the photon energy in the excess of the material's energy gap (the gap that separates the conduction from the valence band) is wasted as heat. Here we experimentally demonstrate that using semiconductor nanocrystals we can reduce this energy loss to a nearly absolute minimum allowed by energy conservation by producing multiple excitons per single photon. Specifically, we generate seven excitons from a photon with an energy of 7.8 energy gaps, which corresponds to only approximately 10% energy loss, while in the normal scenario (one photon produces one exciton) approximately 90% of the photon energy would be dissipated as heat. Such large yields of charge carriers (photon-to-exciton conversion efficiency up to 700%) has the potential to dramatically increase the performance of photovoltaic cells and can greatly advance solar fuel producing technologies.

Journal Article↗
PubMed2005

Breaking the phonon bottleneck in semiconductor nanocrystals via multiphonon emission induced by intrinsic nonadiabatic interactions.

We observe ultrafast 1P-to-1S intraband relaxation in PbSe and CdSe nanocrystals (NCs) that have distinct energy spectra. While ultrafast dynamics in CdSe NCs has typically been interpreted in terms of electron-hole energy transfer, this mechanism is not active in PbSe NCs because of sparse densities of states in the conduction and valence bands. Our observations of temperature activation and confinement-enhanced relaxation in PbSe NCs can be explained by efficient multiphonon emission triggered by nonadiabatic electron-phonon interactions and are indicative of large, size-dependent, intraband Huang-Rhys parameters.

Journal Article↗
PubMed2004

Pushing the band gap envelope: mid-infrared emitting colloidal PbSe quantum dots.

Efficient mid-infrared sources are of considerable general interest for gas analysis, remote sensing, and atmospheric monitoring, but existing technologies are limited. Here, we report the synthesis of the first colloidal QDs having photoluminescence (PL) in the mid-infrared. We show particle-size-tunable mid-infrared emission for large (10-17 nm), but quantum-confined, colloidal PbSe QDs, with efficient, narrow-bandwidth PL at energies as low as 0.30 eV (4.1 mum). Applying two new synthetic routes, we have achieved fine control of QD size and size distribution, allowing us to provide the first systematic correlation of QD size with PL energy for PbSe QDs emitting at wavelengths longer than 2 mum, results which are compared with a literature model. For the entire spectral range reported, we provide measured quantum yields in emission, showing a marked decrease with increasing QD size, for which we include a possible explanation. Finally, we present very promising preliminary results for overcoating PbSe with CdSe, a wider-gap semiconductor. We show PL enhanced by approximately 6-fold for such core/shell samples.

Journal Article↗
PubMed2003

Intermolecular coupling in nanometric domains of light-harvesting dendrimer films studied by photoluminescence near-field scanning optical microscopy (PL NSOM).

Near-field scanning optical microscopy (NSOM) has been used to investigate the photophysical characteristics of first- to fourth-generation (G1 to G4) light-harvesting dendrimer thin films containing coumarin-343 and coumarin-2 as the core and peripheral chromophores, respectively. Thin film photoluminescence (PL) spectra exhibit a significant red shift in the lower generations (G1, G2, and G3) as compared to their respective solution PL spectra, implying the formation of excimers. Spatially resolved PL NSOM images exhibit pronounced nanoscopic domains in G1, which become more homogeneous in higher generations due to site-isolation of the core chromophore. G4 exhibits complete site-isolation for these light-harvesting dendrimer films.

Journal Article↗
PubMed2002

Single gallium nitride nanowire lasers.

There is much current interest in the optical properties of semiconductor nanowires, because the cylindrical geometry and strong two-dimensional confinement of electrons, holes and photons make them particularly attractive as potential building blocks for nanoscale electronics and optoelectronic devices, including lasersand nonlinear optical frequency converters. Gallium nitride (GaN) is a wide-bandgap semiconductor of much practical interest, because it is widely used in electrically pumped ultraviolet-blue light-emitting diodes, lasers and photodetectors. Recent progress in microfabrication techniques has allowed stimulated emission to be observed from a variety of GaN microstructures and films. Here we report the observation of ultraviolet-blue laser action in single monocrystalline GaN nanowires, using both near-field and far-field optical microscopy to characterize the waveguide mode structure and spectral properties of the radiation at room temperature. The optical microscope images reveal radiation patterns that correlate with axial Fabry-Perot modes (Q approximately 10(3)) observed in the laser spectrum, which result from the cylindrical cavity geometry of the monocrystalline nanowires. A redshift that is strongly dependent on pump power (45 meV microJ x cm(-2)) supports the idea that the electron-hole plasma mechanism is primarily responsible for the gain at room temperature. This study is a considerable advance towards the realization of electron-injected, nanowire-based ultraviolet-blue coherent light sources.

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