Stephen G. Hickey

Size and shape control of colloidally synthesized IV-VI nanoparticulate tin(II) sulfide.

We report the synthesis and characterization of monodisperse SnS nanocrystals and demonstrate shape control by varying the ratio of ligands present in the reaction mixture. The nanoparticles are subsequently linked to conducting transparent substrates, and their optoelectronic response is probed. Values of the photocurrent for this system, without attempts to optimize, in the range of 6-8 nA cm(-2) were obtained.

Quantum-Dot-Based Photoelectrochemical Sensors for Chemical and Biological Detection

Quantum-dot-based photoelectrochemical sensors are powerful alternatives for the detection of chemicals and biochemical molecules compared to other sensor types, which is the primary reason as to why they have become a hot topic in nanotechnology-related analytical methods. These sensors basically consist of QDs immobilized by a linking molecule (linker) to an electrode, so that upon their illumination, a photocurrent is generated which depends on the type and concentration of the respective analyte in the immediate environment of the electrode. The present review provides an overview of recent developments in the fabrication methods and sensing concepts concerning direct and indirect interactions of the analyte with quantum dot modified electrodes. Furthermore, it describes in detail the broad range of different sensing applications of such quantum-dot-based photoelectrochemical sensors for inorganic and organic (small and macro-) molecules that have arisen in recent years. Finally, a number of aspects concerning current challenges on the way to achieving real-life applications of QD-based photochemical sensing are addressed.

Bright White-Light Emitting Manganese and Copper Co-Doped ZnSe Quantum Dots†

White-light emission (WLE) from semiconductor nanostructures is presently a research area of intense interest, especially where the primary objective is to replace conventional light sources in order to minimize energy costs and therefore global energy consumption for lighting. Presently the general methods to achieve white-light emission are either by coating a yellow phosphor or by combining green and red phosphors on a background consisting of a blue-light emitting diode (LED) or by employing nanocrystals (NCs) of the three primary colors (red, green, blue) using multilayer structures in LEDs. However, when one simply mixes these nanocrystal quantum dots (QDs) of different colors together to generate white light, the efficiencies are often observed to decrease due to the re-absorption of light and subsequent undesired energy transfer (ET). This may lead to undesirable changes in the chromaticity coordinates and photometric performance due to the different relative temporal stabilities of the components. Hence the use of a single-emitting component offers many advantages over multiple component systems for whitelight emitting sources such as LEDs, amongst which are: greater reproducibility, low cost preparation, ease of modification, and simpler fabrication processes. Therefore, it is of great importance for many applications to find high-quality single source white-light emitters through low-cost chemical synthesis approaches that will allow the production of white light while meeting the needs of industry, such as satisfactory Commission International d Eclairage (CIE) coordinates. One route that offers the possibility by which such materials may be accessed is that of the colloidal synthesis of doped semiconductor nanocrystals, which has already proven itself to be an interesting field for future nanotechnologies as it can presently provide highly efficient emission sources for various applications. Although various attempts have been made towards the growth of doped QDs in solution, it still remains a challenge to dope all of the nanocrystals present in the reaction mixture simultaneously with the different dopants, as the host matrix tends to expel the dopant ions from the internal crystal lattice to the surface, in a sort of “self-purification” process. Therefore, even in the most favorable cases of the dopant ions having the same valence state and similar ionic radius as those of the corresponding host, successful doping remains difficult to achieve by the simple addition of a small amount of dopant precursors during the synthesis of the host NCs. To overcome this, a number of doping strategies, such as nucleation–doping and growth–doping, where the doping is decoupled from the nucleation and/or growth, provide ample possibilities to selectively introduce dopants at desired positions within the host materials to generate different emission centers inside of a single quantum dot. Usually, in a Mn-doped ZnSe system there is a dominant yellow/orange emission present at 585 nm which results from the T1– A1 transition of the Mn 2+ impurity excited by energy transfer from the host lattice. Therefore, if one can supply a source of blue and/or green emission within such a system, then white-light emission is likely to result. In fact, Mn-doped CdS and ZnS 19] NCs with white-light emission have been successfully prepared by the combination of the orange emission of the Mn impurity and the blue and/or green emission of the surface defect states of the NCs. There are also a number of reports which describe the synthesis of semiconductor NCs with white-light emission such as “magic-sized” CdSe NCs, trap-rich CdS-QDs and onion-like CdSe/ZnS/CdSe/ZnS-QDs, alloyed ZnxCd1 xSe quantum dots, ZnS:Pb, ZnS incorporated into porous Silicon, and ZnSe. However, all the above-mentioned systems rely on the manipulation of surface-state emission from the NCs, which is notoriously difficult to control and/or reproduce and, in addition, the temporal stability of these states varies with the environmental conditions in a manner which is presently still not fully understood. Also the intrinsic toxicity of cadmium and lead sheds a doubt on the future applicability of these NCs, particularly in view of recent environmental regulations. Herein we report a method which overcomes these difficulties through the successful synthesis of doubly doped QDs using a versatile hot-injection colloidal synthesis to produce Mn and Cu co-doped ZnSe QDs (Cu:Mn-ZnSe), where white-light emission can be readily realized and its characteristics tuned. The two dopants have been introduced into the host material in a two-step process such that the dopants retain their individual emission properties which cover most of the visible spectral range. Also we demonstrate versatility of the tuning of the white-light generation with [*] Dr. S. K. Panda, Dr. S. G. Hickey, Prof. A. Eychm ller Physical Chemistry/Electrochemistry, TU Dresden Bergstrasse 66b, 01062 Dresden (Germany) Fax: (+ 49)351-463-37164 E-mail: s.hickey@chemie.tu-dresden.de

Progress in the Light Emission of Colloidal Semiconductor Nanocrystals

Some 25 years ago it was found that semiconductor nanocrystals emitted light. Since then tremendous progress has been made with respect to increasing the emission quantum yields, extending the spectral range that may be addressed, from the UV across to the near infrared, and improving the color purity. Here some major lines in these developments are reviewed, touching on milestones as well as on the principles of the most successful preparative approaches.

Light Energy Conversion by Mesoscopic PbS Quantum Dots/TiO2 Heterojunction Solar Cells

Solid state PbS quantum dots (QDs)/TiO(2) heterojunction solar cells were produced by depositing PbS QDs on a 500 nm thick mesoscopic TiO(2) films using layer-by-layer deposition. Importantly, the PbS QDs act here as photosensitizers and at the same time as hole conductors. The PbS QDs/TiO(2) device produces a short circuit photocurrent (J(sc)) of 13.04 mA/cm(2), an open circuit photovoltage (V(oc)) of 0.55 V and a fill factor (FF) of 0.49, corresponding to a light to electric power conversion efficiency (η) of 3.5% under AM1.5 illumination. The electronic processes occurring in this device were investigated by transient photocurrent and photovoltage measurements as well as impedance spectroscopy in the dark and under illumination. The investigations showed a high resistivity for the QD/metal back contact, which reduces drastically under illumination. EIS also indicated a shift of the depletion layer capacitance under illumination related to the change of the dipole at this interface.

High Efficiency Quantum Dot Heterojunction Solar Cell Using Anatase (001) TiO2 Nanosheets

This is the first report of using anatase TiO(2) nanosheets with exposed (001) facets in a high-efficiency PbS quantum dot/TiO(2) heterojunction solar cell. The TiO(2) nanosheets have higher conduction band, and surface energy compared to normal anatase (101) TiO(2) nanoparticles. This PbS QD/TiO(2) heterojunction solar cell produces power conversion efficiency of 4.7% which is one of the highest reported in literature.

Amplified spontaneous emission of surface plasmon polaritons and limitations on the increase of their propagation length

Amplified spontaneous emission of surface plasmon polaritons (SPPs) at the interface of a resonant gain medium has been observed. The amplification is accompanied by significant spectral narrowing and limits the gain available for compensation of SPP propagation losses. The effect is similar to the deteriorating influence of amplified spontaneous emission in laser resonators.

Large-area (over 50 cm × 50 cm) freestanding films of colloidal InP/ZnS quantum dots.

We propose and demonstrate the fabrication of flexible, freestanding films of InP/ZnS quantum dots (QDs) using fatty acid ligands across very large areas (greater than 50 cm × 50 cm), which have been developed for remote phosphor applications in solid-state lighting. Embedded in a poly(methyl methacrylate) matrix, although the formation of stand-alone films using other QDs commonly capped with trioctylphosphine oxide (TOPO) and oleic acid is not efficient, employing myristic acid as ligand in the synthesis of these QDs, which imparts a strongly hydrophobic character to the thin film, enables film formation and ease of removal even on surprisingly large areas, thereby avoiding the need for ligand exchange. When pumped by a blue LED, these Cd-free QD films allow for high color rendering, warm white light generation with a color rendering index of 89.30 and a correlated color temperature of 2298 K. In the composite film, the temperature-dependent emission kinetics and energy transfer dynamics among different-sized InP/ZnS QDs are investigated and a model is proposed. High levels of energy transfer efficiency (up to 80%) and strong donor lifetime modification (from 18 to 4 ns) are achieved. The suppression of the nonradiative channels is observed when the hybrid film is cooled to cryogenic temperatures. The lifetime changes of the donor and acceptor InP/ZnS QDs in the film as a result of the energy transfer are explained well by our theoretical model based on the exciton-exciton interactions among the dots and are in excellent agreement with the experimental results. The understanding of these excitonic interactions is essential to facilitate improvements in the fabrication of photometrically high quality nanophosphors. The ability to make such large-area, flexible, freestanding Cd-free QD films pave the way for environmentally friendly phosphor applications including flexible, surface-emitting light engines.

Synthesis and characterization of cadmium phosphide quantum dots emitting in the visible red to near-infrared.

The synthesis of high-quality cadmium phosphide quantum dots with emission wavelength maxima in the range from 1200 to approximately 760 nm are reported. The results demonstrate that the nucleation and growth linked with the optical properties can be controlled by the temperature, the growth time, and the addition of ligands such as oleylamine and trioctylphosphine. Photoelectrochemical investigations revealed that the cadmium phosphide QD-derivatized electrodes show an optical response and that photocurrents of several nanoamperes per square centimeter can be obtained upon illumination.

Ultrasmall SnO 2 Nanocrystals: Hot-bubbling Synthesis, Encapsulation in Carbon Layers and Applications in High Capacity Li-Ion Storage

Ultrasmall SnO2 nanocrystals as anode materials for lithium-ion batteries (LIBs) have been synthesized by bubbling an oxidizing gas into hot surfactant solutions containing Sn-oleate complexes. Annealing of the particles in N2 carbonifies the densely packed surface capping ligands resulting in carbon encapsulated SnO2 nanoparticles (SnO2/C). Carbon encapsulation can effectively buffer the volume changes during the lithiation/delithiation process. The assembled SnO2/C thus deliver extraordinarily high reversible capacity of 908 mA·h·g−1 at 0.5 C as well as excellent cycling performance in the LIBs. This method demonstrates the great potential of SnO2/C nanoparticles for the design of high power LIBs.