Joanna L. Casson

Utilizing the lability of lead selenide to produce heterostructured nanocrystals with bright, stable infrared emission.

Infrared-emitting nanocrystal quantum dots (NQDs) have enormous potential as an enabling technology for applications ranging from tunable infrared lasers to biological labels. Notably, lead chalcogenide NQDs, especially PbSe NQDs, provide efficient emission over a large spectral range in the infrared, but their application has been limited by instability in emission quantum yield and peak position on exposure to ambient conditions. Conventional methods for improving NQD stability by applying a shell of a more stable, wider band gap semiconductor material are frustrated by the tendency of lead chalcogenide NQDs toward Ostwald ripening at even moderate reaction temperatures. Here, we describe a partial cation-exchange method in which we take advantage of this lability to controllably synthesize PbSe/CdSe core/shell NQDs. Critically, these NQDs are stable against fading and spectral shifting. Further, these NQDs can undergo additional shell growth to produce PbSe/CdSe/ZnS core/shell/shell NQDs that represent initial steps toward bright, biocompatible near-infrared optical labels.

Effect of shell thickness and composition on blinking suppression and the blinking mechanism in ‘giant’ CdSe/CdS nanocrystal quantum dots

We recently developed an inorganic shell approach for suppressing blinking in nanocrystal quantum dots (NQDs) that has the potential to dramatically improve the utility of these fluorophores for single-NQD tracking of individual molecules in cell biology. Here, we consider in detail the effect of shell thickness and composition on blinking suppression, focusing on the CdSe/CdS core/shell system. We also discuss the blinking mechanism as understood through profoundly altered blinking statistics. We clarify the dependence of blinking behavior and photostability on shell thickness, as well as on interrogation times. We show that, while the thickest-shell systems afford the greatest advantages in terms of enhanced optical properties, thinner-shell NQDs may be adequate for certain applications requiring relatively shorter interrogation times. Shell thickness also determines the sensitivity of the NQD optical properties to aqueous-phase transfer, a critical step in rendering NQDs compatible with bioimaging applications. Lastly, we provide a proof-of-concept demonstration of the utility of these unique NQDs for fluorescent particle tracking.

Suppressed blinking and auger recombination in near-infrared type-II InP/CdS nanocrystal quantum dots.

Nonblinking excitonic emission from near-infrared and type-II nanocrystal quantum dots (NQDs) is reported for the first time. To realize this unusual degree of stability at the single-dot level, novel InP/CdS core/shell NQDs were synthesized for a range of shell thicknesses (~1-11 monolayers of CdS). Ensemble spectroscopy measurements (photoluminescence peak position and radiative lifetimes) and electronic structure calculations established the transition from type-I to type-II band alignment in these heterostructured NQDs. More significantly, single-NQD studies revealed clear evidence for blinking suppression that was not strongly shell-thickness dependent, while photobleaching and biexciton lifetimes trended explicitly with extent of shelling. Specifically, very long biexciton lifetimes-up to >7 ns-were obtained for the thickest-shell structures, indicating dramatic suppression of nonradiative Auger recombination. This new system demonstrates that electronic structure and shell thickness can be employed together to effect control over key single-dot and ensemble NQD photophysical properties.

Cascaded electro-optic scanning of laser light over large angles using domain microengineered ferroelectrics

We present a device concept based on cascaded electro-optic deflection in a domain microengineered ferroelectric chip. In our design, large deflection angles are achieved by cascading several smaller scanners in a single ferroelectric chip, such that each successive scanner stage builds upon the deflection of the previous stage. We demonstrate the basic concept using a two-stage device fabricated in a single crystal wafer of ferroelectric LiTaO3. By operating the device using a specially designed programmable multichannel driver that provides ±1.1 kV per stage, a total scan angle of 25.4° at 5 kHz was demonstrated. Even larger angles of deflection are possible with additional scanner stages.

Elucidation of Two Giants: Challenges to Thick-shell Synthesis in CdSe/ZnSe and ZnSe/CdS Core/Shell Quantum Dots

Core/thick-shell giant quantum dots (gQDs) possessing type II electronic structures exhibit suppressed blinking and diminished nonradiative Auger recombination. We investigate CdSe/ZnSe and ZnSe/CdS as potential new gQDs. We show theoretically and experimentally that both can exhibit partial or complete spatial separation of an excited-state electron-hole pair (i.e., type II behavior). However, we reveal that thick-shell growth is challenged by competing processes: alloying and cation exchange. We demonstrate that these can be largely avoided by choice of shelling conditions (e.g., time, temperature, and QD core identity). The resulting CdSe/ZnSe gQDs exhibit unusual single-QD properties, principally emitting from dim gray states but having high two-exciton (biexciton) emission efficiencies, whereas ZnSe/CdS gQDs show characteristic gQD blinking suppression, though only if shelling is accompanied by partial cation exchange.

Integrated high-power electro-optic lens and large-angle deflector

We present a theoretical discussion and experimental demonstration of what to our knowledge is a novel integrated electro-optic lens and beam deflector fabricated in lithium tantalate. The cylindrical lens collimates Gaussian beams as small as 4 mum in diameter, whereas the independently controlled deflector is capable of scanning the collimated beam through an angular range of nearly 20 degrees .

Experimental isotherms of HCl on H2O ice under stratospheric conditions: Connections between bulk and interfacial thermodynamics

The adsorption of HCl on the surface of H(2)O ice has been measured at temperatures and pressures relevant to the upper troposphere and lower stratosphere. The measured HCl surface coverage is found to be at least 100 times lower than currently assumed in models of chlorine catalyzed ozone destruction in cold regions of the upper atmosphere. Measurements were conducted in a closed system by simultaneous application of surface spectroscopy and gas phase mass spectrometry to fully characterize vapor/solid equilibrium. Surface adsorption is clearly distinguished from bulk liquid or solid phases. From 180 to 200 K, submonolayer adsorption of HCl is well described by a Bragg-Williams modified Langmuir model which includes the dissociation of HCl into H(+) and Cl(-) ions. Furthermore, adsorption is consistent with two distinct states on the ice substrate, one in which the ions only weakly adsorb on separate sites, and another where the ions adsorb as an H(+)-Cl(-) pair on a single site with adsorption energy comparable to the bulk trihydrate. The number of substrate H(2)O molecules per adsorption site is also consistent with the stoichiometry of bulk hydrates under these conditions. The ionic states exist in equilibrium, and the total adsorption energy is a function of the relative population of both states. These observations and model provide a quantitative connection between the thermodynamics of the bulk and interfacial phases of HCl/H(2)O, and represent a consistent physicochemical model of the equilibrium system.

NANOSECOND INTERFEROMETRIC STUDIES OF SURFACE DEFORMATIONS OF DIELECTRICS INDUCED BY LASER IRRADIATION

Transient surface deformations in dielectric materials induced by laser irradiation were investigated with time-resolved interferometry. Deformation images were acquired at various delay times after exposure to single pulses (100 ps at 1.064 micrometer) on fresh sample regions. Above the ablation threshold, we observe prompt ejection of material and the formation of a single unipolar compressional surface acoustic wave propagating away from the ablation crater. For calcite, no deformation -- either transient or permanent -- is discernable at laser fluences below the threshold for material ejection. Above and below-threshold behavior was investigated using a phosphate glass sample with substantial near infrared absorption (Schott filter KG3). Below threshold, KG3 exhibits the formation of a small bulge roughly the size of the laser spot that reaches its maximum amplitude by approximately 5 ns. By tens of nanoseconds, the deformations become quite complex and very sensitive to laser fluence. The above-threshold behavior of KG3 combines the ablation-induced surface acoustic wave seen in calcite with the bulge seen below threshold in KG3. A velocity of 2.97 +/- 0.03 km/s is measured for the KG3 surface acoustic wave, very close to the Rayleigh wave velocity calculated from material elastic parameters. Details of the transient interferometry system are also given.

Using shape to turn off blinking for two-colour multiexciton emission in CdSe/CdS tetrapods

Semiconductor nanostructures capable of emitting from two excited states and thereby of producing two photoluminescence colours are of fundamental and potential technological significance. In this limited class of nanocrystals, CdSe/CdS core/arm tetrapods exhibit the unusual trait of two-colour (red and green) multiexcitonic emission, with green emission from the CdS arms emerging only at high excitation fluences. Here we show that by synthetic shape-tuning, both this multi-colour emission process, and blinking and photobleaching behaviours of single tetrapods can be controlled. Specifically, we find that the properties of dual emission and single-nanostructure photostability depend on different structural parameters—arm length and arm diameter, respectively—but that both properties can be realized in the same nanostructure. Furthermore, based on results of correlated photoluminescence and transient absorption measurements, we conclude that hole-trap filling in the arms and partial state-filling in the core are necessary preconditions for the observation of multiexciton multi-colour emission.

Nanosecond interferometric studies of surface deformation induced by laser irradiation

Transient surface morphology changes in dielectric materials induced by laser irradiation were investigated with time- resolved interferometry. Deformation images were acquired at various delay times after exposure to single pulses on fresh sample regions. Above the ablation threshold, we observe prompt ejection of material and the formation of a single unipolar compressional surface acoustic wave propagating away from the ablation crater. For calcite, no deformation - either transient or permanent - is discernable at laser fluences below the threshold for material ejection. Below- threshold behavior was investigated using a phosphate glass sample with substantial near IR absorption. KG3 exhibits the formation of a small bulge roughly the size of the laser spot that reaches its maximum amplitude by approximately 5 ns. At lower laser fluences, diffusion of thermal energy away from that region causes a much weaker and boarder bulge to appear on a slower time scale. At higher laser fluences, a pair of strong, unipolar rarefaction surface acoustic waves is launched, separating from the central region at roughly 17 and 22 ns. Details of the transient interferometry system will also be given.