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.

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.

Solution-liquid-solid growth of ternary Cu-In-Se semiconductor nanowires from multiple- and single-source precursors.

Ternary CuInSe(2) nanowires were synthesized for the first time by the solution-liquid-solid (SLS) mechanism. Here, both metal-organic multiple- and single-source molecular precursors were thermally decomposed in the presence of molten metal nanoparticles and coordinating ligands. The nature of the precursor-multiple- compared to single-source (wherein Cu-Se-In bonds are effectively preformed)-as well as the choice of coordinating ligands, reaction temperature, and reactant order-of-addition strongly affected the morphology and composition of the reaction product obtained. Crystalline, straight, and nearly stoichiometric CuInSe(2) nanowires were most readily achieved using the single-source precursor; however, careful tuning of reaction conditions could also be used to obtain high-quality nanowires from multiple-source precursor systems. The CuInSe(2) nanowires are strong light absorbers from the near-infrared through the visible and ultraviolet spectral regions and, thereby, comprise new soluble and processable "building blocks" for applications in solar-light harvesting.

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.

"Giant" multishell CdSe nanocrystal quantum dots with suppressed blinking.

Semiconductor nanocrystal quantum dots (NQDs) comprise an important class of inorganic fluorophores for applications from optoelectronics to biology. Unfortunately, to date, NQD optical properties (e.g., their efficient and particle-size-tunable photoluminescence) have been susceptible to instabilities at the bulk and single-particle levels. Specifically, ensemble quantum yields (QYs) in emission are dependent upon NQD surface chemistry and chemical environment, while at the single-particle level, NQDs are characterized by significant fluorescence intermittency (blinking) that hinders applications as single-photon light sources for quantum informatics and biolabels for real-time monitoring of single biomolecules. Furthermore, while NQDs are significantly more photostable than their organic dye counterparts, traditional NQDs photobleach over periods of seconds to many minutes. Here, we demonstrate for the first time that by encapsulating the NQD core in a sufficiently thick inorganic shell, we are able to divorce NQD function from NQD surface chemistry and chemical environment. We show that our "giant" NQDs (g-NQDs) are functionally distinct from standard core-only, core/shell and even core/multishell NQDs. g-NQDs are substantially less sensitive to changes in surface chemistry. They do not photobleach under continuous laser excitation over periods of several hours repeated over several days, and they exhibit markedly different blinking behavior; >20% of the g-NQDs do not blink, while >40% have on-time fractions of >80%. All of these observations are in stark contrast with control samples comprising core-only and standard, thinner core/multishell NQDs.

Flow-based solution-liquid-solid nanowire synthesis.

Discovered almost two decades ago, the solution-liquid-solid (SLS) method for semiconductor nanowire synthesis has proven to be an important route to high-quality, single-crystalline anisotropic nanomaterials. In execution, the SLS technique is similar to colloidal quantum-dot synthesis in that it entails the injection of chemical precursors into a hot surfactant solution, but mechanistically it is considered the solution-phase analogue to vapour-liquid-solid (VLS) growth. Both SLS and VLS methods make use of molten metal nanoparticles to catalyse the nucleation and elongation of single-crystalline nanowires. Significantly, however, the methods differ in how chemical precursors are introduced to the metal catalysts. In SLS, precursors are added in a one-off fashion in a flask, whereas in VLS they are carried by a flow of gas through the reaction chamber, and by-products are removed similarly. The ability to dynamically control the introduction of reactants and removal of by-products in VLS synthesis has enabled a degree of synthetic control not possible with SLS growth. We show here that SLS synthesis can be transformed into a continuous technique using a microfluidic reactor. The resulting flow-based SLS ('flow-SLS') platform allows us to slow down the synthesis of nanowires and capture mechanistic details concerning their growth in the solution phase, as well as synthesize technologically relevant axially heterostructured semiconductor nanowires, while maintaining the propensity of SLS for accessing ultrasmall diameters below 10 nm.

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.

Multicolor light-emitting diodes based on semiconductor nanocrystals encapsulated in GaN charge injection layers.

Numerous technologies including solid-state lighting, displays, and traffic signals can benefit from efficient, color-selectable light sources that are driven electrically. Semiconductor nanocrystals are attractive types of chromophores that combine size-controlled emission colors and high emission efficiencies with excellent photostability and chemical flexibility. Applications of nanocrystals in light-emitting technologies, however, have been significantly hindered by difficulties in achieving direct electrical injection of carriers. Here we report the first successful demonstration of electroluminescence from an all-inorganic, nanocrystal-based architecture in which semiconductor nanocrystals are incorporated into a p-n junction formed from GaN injection layers. The critical step in the fabrication of these nanocrystal/GaN hybrid structures is the use of a novel deposition technique, energetic neutral atom beam lithography/epitaxy, that allows for the encapsulation of nanocrystals within a GaN matrix without adversely affecting either the nanocrystal integrity or its luminescence properties. We demonstrate electroluminescence (injection efficiencies of at least 1%) in both single- and two-color regimes using structures comprising either a single monolayer or a bilayer of nanocrystals.