ELECTROLUMINESCENT devices have been developed recently that are based on new materials such as porous silicon1 and semiconducting polymers2,3. By taking advantage of developments in the preparation and characterization of direct-gap semiconductor nanocrystals4–6, and of electroluminescent polymers7, we have now constructed a hybrid organic/inorganic electroluminescent device. Light emission arises from the recombination of holes injected into a layer of semiconducting p-paraphenylene vinylene (PPV)8–10 with electrons injected into a multilayer film of cadmium selenide nanocrystals. Close matching of the emitting layer of nanocrystals with the work function of the metal contact leads to an operating voltage11 of only 4V. At low voltages emission from the CdSe layer occurs. Because of the quantum size effect19–24 the colour of this emission can be varied from red to yellow by changing the nanocrystal size. At higher voltages green emission from the polymer layer predominates. Thus this device has a degree of voltage tunability of colour.

Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer

ELECTROLUMINESCENT devices have been developed recently that are based on new materials such as porous silicon' and semiconducting polymers 2,3 . By taking advantage of developments in the preparation and characterization of direct-gap semiconductor nanocrystals 4-6 , and of electroluminescent polymers7, we have now constructed a hybrid organic/inorganic electroluminescent device. Light emission arises from the recombination of holes injected into a layer of semiconducting p-paraphenylene vinylene (PPV) 2-10 with electrons injected into a multilayer film of cadmium selenide nanocrystals. Close matching of the emitting layer of nanocrystals with the work function of the metal contact leads to an operating voltage" of only 4 V. At low voltages emission from the CdSe layer occurs. Because of the quantum size effect 19-24 the colour of this emission can be varied from red to yellow by changing the nanocrystal size. At higher voltages green emission from the polymer layer predominates. Thus this device has a degree of voltage tunability of colour.

We describe the synthesis of ZnS-capped CdSe semiconductor nanocrystals using organometallic reagents by a two-step single-flask method X-ray photoelectron spectroscopy, transmission electron microscopy and optical absorption are consistent with nanocrystals containing a core of nearly monodisperse CdSe of 27−30 A diameter with a ZnS capping 6 ± 3 A thick The ZnS capping with a higher bandgap than CdSe passivates the core crystallite removing the surface traps The nanocrystals exhibit strong and stable band-edge luminescence with a 50% quantum yield at room temperature

Synthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals

The synthesis and study of so-called “nanoparticles”, particles with diameters in the range of 1−20 nm, has become a major interdisciplinary area of research over the past 10 years. Semiconductor nanoparticles promise to play a major role in several new technologies. The intense interest in this area derives from their unique chemical and electronic properties, which gives rise to their potential use in the fields of nonlinear optics, luminescence, electronics, catalysis, solar energy conversion, and optoelectronics, as well as other areas. The small dimensions of these particles result in different physical properties from those observed in the corresponding macrocrystalline, “bulk”, material. As particle sizes become smaller, the ratio of surface atoms to those in the interior increase, leading to the surface properties playing an important role in the properties of the material. Semiconductor nanoparticles also exhibit a change in their electronic properties relative to that of the bulk material; as th...

/pdf/nanocrystalline-semiconductors-synthesis-properties-and-5b7rvpaf0d.pdf

Nanocrystalline semiconductors: Synthesis, properties, and perspectives

In semiconductor particles of nanometer size, a gradual transition from solid-state to molecular structure occurs as the particle size decreases. Consequently, a splitting of the energy bands into discrete, quantized levels occurs. Particles that exhibit these quantization effects are often called “Q-particles” or, generally, quantized material. The optical, electronic and catalytic properties of Q-particles drastically differ from those of the corresponding macrocrystalline substance. The band gap, a substance-specific quantity in macrocrystalline materials, increases by several electron volts in Q-particles with decreasing particle size. In Q-particles there are approximately as many molecules on the surface as in the interior of the particle. Therefore, the nature of the surface as well as the particle size is also largely responsible for the physico-chemical properties of the particle. Q-particles of many materials can be prepared in the form of colloidal solutions or embedded in porous matrices and are stable over a long period of time. In sandwich colloids, in which Q-particles of different materials are coupled, as well as in porous semiconductor electrodes containing Q-particles in the pores, very efficient primary charge separation is observed. As a result, sandwich colloids have greatly enhanced photocatalytic activity relative to the individual particles, while electrodes modified with Q-particles show high photocurrents. This article deals with the size quantization effect, the synthesis and characterization of Q-particles, as well as with the spectroscopic, electrochemical, and electron-microscopic investigation of these particles.

Colloidal Semiconductor Q‐Particles: Chemistry in the Transition Region Between Solid State and Molecules

Detection of shallow electron traps in quantum sized CdS by fluorescence quenching experiments

The fluorescence properties of surface modified quantum-sized CdS colloids were investigated by means of static and time-resolved low temperature fluorescence spectroscopy. All samples investigated had high fluorescence quantum yields already at room temperature and exhibited either exclusively excitonic or a mixture of excitonic and trapped fluorescence. Mechanisms are presented which explain both the static and time-resolved behaviour of both the excitonic and trapped fluorescence. The excitonic fluorescence is described as delayed fluorescence occuring by detrapping of electrons. Extremely shallow traps having trap depths of a few meV are discussed.

Photochemistry of Semiconductor Colloids. 36. Fluorescence Investigations on the Nature of Electron and Hole Traps in Q‐Sized Colloidal CdS Particles

Fluorescence mechanism of highly monodisperse Q-sized CdS colloids

Quantum-sized HgS in contact with quantum-sized CdS colloids

We describe the preparation of highly monodisperse, quantum-size CdS particles. In these samples a band splitting into discrete exciton-like states is observed. A mechanism for the excitonic fluorescence is presented involving shallow traps as a reservoir for electrons which finally recombine with free positive holes across the bandgap. Quantum-size CdS and PbS particles prepared in situ on highly porous, polycrystalline TiO2 electrodes work as highly efficient sensitizers. Photocurrent yields of more than 70% are reported. In the case of PbS, it is shown that an efficient electron transfer is possible only with particles in the size of the quantization regime.

A. Hässelbarth

Papers

Detection of shallow electron traps in quantum sized CdS by fluorescence quenching experiments

Photochemistry of Semiconductor Colloids. 36. Fluorescence Investigations on the Nature of Electron and Hole Traps in Q‐Sized Colloidal CdS Particles

Fluorescence mechanism of highly monodisperse Q-sized CdS colloids

Quantum-sized HgS in contact with quantum-sized CdS colloids

Synthesis and Photochemistry of Quantum-Size Semiconductor Particles in Solution and in Modified Layers