Chun-Chuen Yang

Formation of self-assembled ZnTe quantum dots on ZnSe buffer layer grown on GaAs substrate by molecular beam epitaxy

Self-assembled ZnTe quantum dot structures were grown by molecular beam epitaxy on GaAs substrates with a 200 nm ZnSe buffer layer. Surface morphology was studied by atomic force microscopy. A three-dimensional Volmer– Weber growth mode was identified. Two types of dots were observed. Strong photoluminescence observed at 1.9–2.2 eV was attributed to emission from the large type II ZnTe quantum dots. Emission from the smaller ZnTe quantum dots was observed at an energy of around 2.26 eV. The density of the larger and smaller dots was approximately 10 8 /cm 2 and 10 9 /cm 2 , respectively. r 2002 Elsevier Science B.V. All rights reserved.

Structural and electrical characterization of polyanilines synthesized from chemical oxidative polymerization via doping/de-doping/re-doping processes

Structural characterization of various emeraldine salt (ES) and emeraldine base (EB) polyaniline (PANI) powders synthesized from chemical oxidative polymerization and then via de-doping and re-doping processes is first presented. Partial crystallinity with well-separated x-ray diffraction peaks is observed in the as-prepared ES PANI powder and an amorphous characteristic is identified in the de-doped EB PANI powder. From scanning electron microscopy, both the nanostructures of the as-prepared ES and de-doped EB PANI precipitated powders possess similar morphology. This finding reveals that the de-doping process to remove the counter-ions intercalated between the as-prepared ES PANI chains makes the de-doped EB PANI pieces become soft but does not change the nanostructures significantly. Then, the electrical conductivity perpendicular to the surface of the pressed PANI disc pellets is studied. The re-doped ES PANI pellets possess less electrical conductivity than their as-prepared counterparts, which suggests that during the re-doping process the counter-ions (Cl− or ) are re-adsorbed predominantly on the surface region of the PANI nanostructures. Besides, the re-doping of larger counter-ions (with respect to Cl−) intercalated between the PANI chains increases the d-spacing of (1 0 0) planes and hence reduces the corresponding π–π inter-chain interaction. Finally, from the temperature dependence of the electrical conductivity together with the structural information, the charge carrier hopping mechanisms perpendicular to the surface of the pressed pellets corresponding to various PANI samples are compared and discussed.

Type-II Zn1-xMnxSe/ZnSe1-yTey quantum wells

Abstract Zn 1− x Mn x Se/ZnSe 1− y Te y ( x =0.03, y =0.08) multiple-quantum-well structures were grown on GaAs substrates by molecular-beam epitaxy. Strong photoluminescence associated with iso-electronic Te traps was observed. A type-II band alignment was proposed for this class of quantum-well structures. Electrons were confined in the Zn 1− x Mn x Se layers. Holes were localized in the ZnSe 1− y Te y layers. The respective valence and conduction band offset were determined as 350±50 and 205±50 meV.

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

We report on the optical and structural characterization of rubrene polycrystalline films fabricated from vacuum deposition with various substrate temperatures (Tsub). Depending on Tsub, the role of twisted and planar rubrene conformational isomers on the properties of rubrene films is focused. The temperature (T)-dependent inverse optical transmission (IOT) and photoluminescence (PL) spectra were performed on these rubrene films. The origins of these IOT and PL peaks are explained in terms of the features from twisted and planar rubrene molecules and of the band characteristics from rubrene molecular solid films. Here, two rarely reported weak-peaks at 2.431 and 2.605 eV were observed from IOT spectra, which are associated with planar rubrene. Besides, the T-dependence of optical bandgap deduced from IOT spectra is discussed with respect to Tsub. Together with IOT and PL spectra, for Tsub > 170 °C, the changes in surface morphology and unit cell volume were observed for the first time, and are attributed to the isomeric transformation from twisted to planar rubrenes during the deposition processes. Furthermore, a unified schematic diagram in terms of Frenkel exciton recombination is suggested to explain the origins of the dominant PL peaks performed on these rubrene films at 15 K.