Donald J. Werder

Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission

We report an efficient synthesis of copper indium sulfide nanocrystals with strong photoluminescence in the visible to near-infrared. This method can produce gram quantities of material with a chemical yield in excess of 90% with minimal solvent waste. The overgrowth of as-prepared nanocrystals with a few monolayers of CdS or ZnS increases the photoluminescence quantum efficiency to > 80%. On the basis of time-resolved spectroscopic studies of core/shell particles, we conclude that the emission is due to an optical transition that couples a quantized electron state to a localized hole state, which is most likely associated with an internal defect.

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.

Highly aligned, template-free growth and characterization of vertical GaN nanowires on sapphire by metal–organic chemical vapour deposition

We report the growth of exceptionally well aligned and vertically oriented GaN nanowires on r-plane sapphire wafers via metal–organic chemical vapour deposition. The nanowires were grown without the use of either a template or patterning. Transmission electron microscopy indicates the nanowires are single crystalline, free of threading dislocations, and have triangular cross-sections. The high degree of vertical alignment is explained by the crystallographic match between the oriented nanowires and the r-plane sapphire surface. We find that the degree of alignment and size uniformity of the nanowires are highly dependent on the nickel nitrate catalyst concentration used, with the highest degree of uniformity and alignment occurring at concentrations much more dilute than typically employed for vapour–liquid–solid-based nanowire growth. Additionally, we report here a strong dependence of the optical and electrical properties of the nanowires on the growth temperature, which we hypothesize is due to increased carbon incorporation at lower growth temperatures.

Colloidal synthesis of infrared-emitting Germanium nanocrystals

In this study, we synthesized Ge nanocrystals and studied the effects of variables such as solvents, reducing agents, reaction temperature, and capping ligands. The resulting nanocrystals showed infrared photoluminescence with quantum yields as high as approximately 8% and enhanced resistance to oxidation. Size analysis of the samples by transmission electron microscopy revealed that the size dependence of the emission is consistent with the effects of quantum confinement.

CdSe Quantum-Dot-Sensitized Solar Cell with ∼100% Internal Quantum Efficiency

We have constructed and studied photoelectrochemical solar cells (PECs) consisting of a photoanode prepared by direct deposition of independently synthesized CdSe nanocrystal quantum dots (NQDs) onto a nanocrystalline TiO(2) film (NQD/TiO(2)), aqueous Na(2)S or Li(2)S electrolyte, and a Pt counter electrode. We show that light harvesting efficiency (LHE) of the NQD/TiO(2) photoanode is significantly enhanced when the NQD surface passivation is changed from tri-n-octylphosphine oxide (TOPO) to 4-butylamine (BA). In the PEC the use of NQDs with a shorter passivating ligand, BA, leads to a significant enhancement in both the electron injection efficiency at the NQD/TiO(2) interface and charge collection efficiency at the NQD/electrolyte interface, with the latter attributed mostly to a more efficient diffusion of the electrolyte through the pores of the photoanode. We show that by utilizing BA-capped NQDs and aqueous Li(2)S as an electrolyte, it is possible to achieve ∼100% internal quantum efficiency of photon-to-electron conversion, matching the performance of dye-sensitized solar cells.

Hybrid photovoltaics based on semiconductor nanocrystals and amorphous silicon

Semiconductor nanocrystals (NCs) are promising materials for applications in photovoltaic (PV) structures that could benefit from size-controlled tunability of absorption spectra, the ease of realization of various tandem architectures, and, perhaps, increased conversion efficiency in the ultraviolet region through carrier multiplication. The first practical step toward utilization of the unique properties of NCs in PV technologies could be through their integration into traditional silicon-based solar cells. Here, we demonstrate an example of such hybrid PV structures that combine colloidal NCs with amorphous silicon. In these structures, NCs and silicon are electronically coupled, and the regime of this coupling can be tuned by altering the alignment of NC energy states with regard to silicon band edges. For example, using wide-gap CdSe NCs we demonstrate a photoresponse which is exclusively due to the NCs. On the other hand, in devices comprising narrow-gap PbS NCs, both the NCs and silicon contribute to photocurrent, which results in PV response extending from the visible to the near-infrared region. The hybrid silicon/PbS NC solar cells show external quantum efficiencies of approximately 7% at infrared energies and 50% in the visible and a power conversion efficiency of up to 0.9%. This work demonstrates the feasibility of hybrid PV devices that combine advantages of mature silicon fabrication technologies with the unique electronic properties of semiconductor NCs.