Size-dependent Band Gap Research Articles

Nanocrystalline semiconductor LEDs with simple structure and high efficiency

AbstractNanocrystalline semiconductor particles exhibit a size dependent bandgap emission, due to size quantisation effects. These particles are derived from solution chemistry and can be made monodisperse under the right synthesis conditions. Compared to organic materials, the inorganic nanoparticles show much higher stability against oxidation and degradation, which makes them an interesting candidate for LEDs and displays. So far, LEDs based on semiconductor nanoparticles typically included low stability organic materials to provide charge injection. The talk will present a new class of nanoparticle LEDs, made without sensitive organic materials. These LEDs show high efficiencies, well defined color throughout the red to green part of the visible spectrum and improved stability under ambient conditions without excessive encapsulation. Using high quality monodisperse suspensions, high color purity is achieved for the emission which paves the road to cheap, high quality displays based on inorganic semiconductor nanoparticles.

Energy Levels in Embedded Semiconductor Nanoparticles and Nanowires

A finite-depth square-well model is used to show that the band gap of a nanoparticle for a given size can be tuned by embedding the nanoparticle in different matrices. This may find a technological importance in different branches of physics. The band gaps calculated using this model are found to be consistent with reported experimental band gaps of semiconductor materials (PbS and CdS) embedded in different matrices. A new wave function for a one-dimensional semiconductor has been deduced to evaluate the size-dependent band gap of a semiconductor wire, explaining the variation of the band gap of porous Si with the ratio of Si−O and Si−H bond concentrations.

Grain size dependent optical band gap of CdI2 films

The thermally evaporated stoichiometric CdI2 films show goodc-axis alignment normal to substrate plane for film thickness up to 200 nm. The optical absorption data indicate an allowed direct interband transition across a gap of 3.6 eV in confirmation with earlier band structure calculations. However, part of the absorption data near band edge can be fitted to an indirect band gap of 3 eV. The dependence of band gap on film thickness (> 200 nm) can be explained qualitatively in terms of decreasing grain boundary barrier height with grain size.

Structural and electronic properties of bare and hydrogenated silicon clusters

We have investigated the structural and electronic properties of both bare and hydrogenated Si m ( m=5,17,29,35,47) clusters using density functional theory. It has been found that the total binding energy decreases as the cluster size increases for both bare and hydrogenated silicon clusters. Moreover, optimization contracts the clusters; optimized geometry of bare clusters is quite different from those of hydrogenated clusters. The electronic structure calculations give the size-dependent band gap for the hydrogenated silicon clusters and the band gap increases as the cluster size decreases.

Catalytic Photooxidation of Pentachlorophenol Using Semiconductor Nanoclusters

Pentachlorophenol (PCP) is a toxic chlorinated aromatic molecule widely used as a fungicide, a bactericide, and a wood preservation, and thus is ubiquitous in the environment. We report photooxidation of PCP using a variety of nanosize semiconductor metal oxides and sulfides in both aqueous and polar organic solvents and compare the photooxidation kinetics of these nanoclusters to widely studied bulk powders such as Degussa P25 TiO2 and CdS. We study both the light- intensity dependence of PCP photooxidation for nanosize SnO2 and the size dependence of PCP photooxidation for both nanosize SnO2 and MoS2. We find an extremely strong size dependence for the latter which we attribute to its size-dependent band gap and the associated change in redox potentials due to quantum confinement of the hole−electron pair. We show that nanosize MoS2 with a diameter of d = 3.0 nm and an absorbance edge of ∼450 nm is a very effective photooxidation catalyst for complete PCP mineralization, even when using only visible-lig...

Self-organized type-II In0.55Al0.45As/Al0.50Ga0.50As quantum dots realized on GaAs(311)A

Self-organized In0.55Al0.45As/Al0.50Ga0.50As quantum dots are grown by the Stranski–Krastanow growth mode using molecular beam epitaxy on the GaAs(311)A substrate. The optical properties of type-II InAlAs/AlGaAs quantum dots have been demonstrated by the excitation power and temperature dependence of photoluminescence spectra. A simple model accounting for the size-dependent band gap of quantum dots is given to qualitatively understand the formation of type-II In0.55Al0.45As/Al0.50Ga0.50As quantum dots driven by the quantum-confinement-induced Γ→X transition. The results provide new insights into the band structure of InAlAs/AlGaAs quantum dots.

Manganese doped zinc sulphide nanoparticles by aqueous method

Zinc sulphide nanoparticles in the size range ∼10–40 Å diameter have been synthesized using the aqueous chemical method. Scanning tunneling microscopy showed that particles are indeed nanosize particles. The size dependent band gap could be varied from a bulk value of 3.68 to 4.5 eV. X-ray diffraction indicated that nanoparticles are crystalline except for those with band gap ∼4.5±0.1 eV. Nanoparticles with particle size ∼21×2 Å diameter or energy gap 4.1×0.1 eV were doped with manganese. The photoluminescence peak at ∼600 nm corresponding to yellow light emission was observed. Atomic absorption studies show that maximum luminescence intensity is achievable with 0.12 at. wt % of Mn doping.

Improved CdS buffer/window layers for thin film solar cells

A dip method to fine tune the absorbance of CdS thin polycrystalline films using the size-dependent band gap of this semiconductor is presented. The method consists of preparing pure and anisotropically capped CdS nanocrystals in an organic solvent. These particles have the property to self-connect on solid surfaces, leading to a textured homogeneous coating. The film can be converted thermally into a pure CdS film. The band gap of the thin film can be fine tuned through the size of the particle precursor or by controlling the conversion temperature during the capping ligand thermolysis. These films should provide a good buffer-window layer for thin film solar cells. In fact, the optical cut-off can be adjusted to avoid interfering with light collection. These films are compared with others made by classical chemical bath techniques.