The excited state of colloidal nanoheterostructures consisting of a spherical CdSe nanocrystal with an epitaxially attached CdS rod can be perturbed effectively by electric fields. Field-induced fluorescence quenching coincides with a conversion of the excited state species from the bright exciton to a metastable trapped state (dark exciton) characterized by a power-law luminescence decay. The conversion is reversible so that up to 10% of quenched excitons recombine radiatively post turn-off of a 1 micro s field pulse, increasing the delayed luminescence by a factor of 80. Excitons can be stored for up to 10(5) times the natural lifetime, opening up applications in optical memory elements.

In-situ synchrotron X-ray diffraction (XRD) was used to monitor the thermal decomposition (thermolysis) of Cd thiolates precursors embedded in a polymer matrix and the nucleation of CdS nanoparticles. A thiolate precursor/polymer solid foil was heated to 300 degrees C in the X-ray diffraction setup of beamline W1.1 at Hasylab, and the diffraction curves were each recorded at 10 degrees C. At temperatures above 240 degrees C, the precursor decomposition is complete and CdS nanoparticles grow within the polymer matrix forming a nanocomposite with interesting optical properties. The nanoparticle structural properties (size and crystal structure) depend on the annealing temperature. Transmission electron microscopy (TEM) and photoluminescence (PL) analyses were used to characterize the nanoparticles. A possible mechanism driving the structural transformation of the precursor is inferred from the diffraction features arising at the different temperatures.

We study the interplay between Auger effects and ionization processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence decay rate on excitation density. Our results suggest that Auger recombination accelerates ionization processes that lead to the occupation of dark, nonemissive nanocrystal states. A model is proposed in the quantized Auger regime describing these experimental observations and providing an estimate of the Auger assisted ionization rates.The optoelectronic properties of semiconductor nanocrystal quantum dots (NQDs) bear great relevance to fundamental research in semiconductor physics. 1,2 Moreover, the tunability of their photoluminescence (PL) from ultraviolet to near-infrared wavelengths and the large absorption cross-section under nonresonant excitation render these nanostructures promising elements of future applications, e.g., in light-emitting diodes, biophysical fluorescence markers, and for biomedical sensitization purposes. [3][4][5] It is therefore important to understand the nature of spontaneous emission and competing nonradiative decay channels in these materials.The most significant nonradiative processes that reduce the quantum efficiency of NQDs are thermal ionization or tunneling of a carrier to a trap state and Auger effects. In semiconductor nanostructures with strong quantum confinement, the Auger effects are greatly enhanced by increased carrier-carrier interactions, 6 whereas thermal ionization is usually hindered by the increased spacing between successive energy states. The energy released from an Auger recombination process is given to another carrier in a single scattering event, usually with the participation of a phonon, whereas thermal ionization is a multiscattering process of a carrier with several phonons. 7 In the case of Auger autoionization or other ionization processes, the excess of a charge carrier in the NQD quenches the radiative recombination of the remaining electron-hole (e-h) pairs. 8 The general importance of Auger effects and ionization processes on the fluorescence and photoinduced absorption dynamics of NQDs has been discussed in several studies using different timeresolved techniques and rather different excitation densities. [9][10][11][12][13][14][15][16] Auger effects have been invoked to explain fluorescence and bleaching dynamics in ultrafast spectroscopy measurements. [17][18][19][20] Ionization processes are often referred to in the context of low densities and slow dynamics, such as the reversible PL quenching of single NQDs, manifested by "off" periods in the fluorescence under continuous-wave excitation. 8,21,22 However, the effect of the fast Auger recombination on the comparatively slow ionization processes and the implication on the PL dynamics of NQDs have not been treated so far, neither experimentally nor theoretically.In this letter, we study the correlation between ion...

Spectrally resolved fluorescence decay measurements reveal a non-Auger like dependence of the photoluminescence decay rate with excitation density. Our results suggest that Auger recombination accelerates phonon assisted ionisation that lead to the formation of dark, non-emissive nanocrystal states.The optoelectronic properties of semiconductor nanocrystal quantum dots (NQDs) bear great relevance to fundanental research in semiconductor physics [1,2]. Moreover, the tunability of their photoluminescence (PL) from ultraviolet to near infrared wavelengths and the large absorption cross-section under non-resonant excitation render these nanostructures promising elements of future applications, e.g. light-emitting diodes, biophysical fluorescence markers and for biomedical sensitisation purposes [3][4][5]. It is therefore important to understand the nature of spontaneous emission and competing non-radiafive decay channels in these materials. Fig. 1. Elementary non-radiative effects in core/shell NQDs with smaller band gap energy (Eg) than carrier ionisation energy (Es). A carrier from the ground state can be scattered to a trap by phonon assisted tunneling (left sketch) or to a higher energy state via an Auger recombination process (middle sketch). An excited carrier can relax energetically, or alternatively be scattered to a trap via phonon assisted tunnelling (right sketch). We show that the correlation between Auger recombination and phonon assisted tunnelling dominates the fluorescence decay dynamics in the high excitation density regime.In this paper, we present the experimental results of our study [6] on the interplay between Auger effects and ionisation processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots (NQDs). We show that cooperative non-radiative effects dominate the fluorescence decay dynamics in the high excitation density regime. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence (PL) decay rate with excitation density. In the usually studied regime of low excitation densities, PL decay rates remain virtually unaffected, whereas under high density excitation, in the multi e-h pair regime, PL decay rates clearly increase with increasing density. A kinetic analysis, pictorially described in Fig. 1, suggests that phonon assisted tunnelling is accelerated in the presence of the comparatively faster Auger recombination.In semiconductor nanostructures Auger effects strongly depend on the degree of spatial confinement [7]. Therefore, the distribution of NQD sizes in an ensemble impedes a quantitative interpretation of the recombination dynamics. Spectral filtering of the PL can substantially reduce the size distribution of NQDs involved in the dynamics. The distribution of sizes in the ensemble is mirrored in the inhomogeneously broadened PL specta of the film, as shown by the solid curve in the inset of Fig.2. Interferometric filters are usedto spectrally narrow the PL 546

Things are what they are TO THE EDITOR: I finished residency in 1996. I'm not sure this qualifies me to respond to Dr. Lansdale's article, but I will anyway. In 12 years, I have witnessed what he describes, even though I work in a not-for-profit military hospital (medical center). Yet I am uncertain that things are worse than they were then, even though it seems like the house staff spend thrice the time typing on a keyboard in the team room than they do at the bedside. Things are what they are. Patients are living longer-I have seen this with my own eyes. Some of them are seeing children graduate, get married, and have babies and spending final holidays with other loved ones. I often feel a sense of helplessness at exactly the sort of obstacles to true excellence Dr. Lansdale points out. However, in the spirit of evidence-based medicine, it remains to be established that spending less time touching the patient doesn't reduce nosocomial infections. We were putting Swan-Ganz catheters in 12 years ago, and I am pretty sure in retrospect we were hurting patients-we don't do that much any more. When I struggle with these difficulties and I try to figure out how to emulate my mentors from what seems like a better time, I remember what my mom told me when I was a second-grader: "Just do your best, and no one will fault you." While I understand burnout, I think a more productive approach would be to redouble efforts at preserving humanistic traditions, valuable clinical skills, and a sense of what we were, rather than to retreat.