Route to the Smallest Doped Semiconductor: Mn(2+)-Doped (CdSe)13 Clusters.

Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn(2+)-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure.

High prevalence of Opisthorchis viverrini infection in a riparian population in Takeo Province, Cambodia.

Opisthorchis viverrini infection was found to be highly prevalent in 3 riverside villages (Ang Svay Chek A, B, and C) of the Prey Kabas District, Takeo Province. This area is located in the southern part of Cambodia, where the recovery of adult O. viverrini worms was recently reported. From May 2006 until May 2010, fecal examinations were performed on a total of 1,799 villagers using the Kato-Katz thick smear technique. In the 3 villages, the overall positive rate for helminth eggs ranged from 51.7 to 59.0% (av. 57.4%), and the percentage positive for O. viverrini was 46.4-50.6% (47.5%). Other helminths detected included hookworms (13.2%), echinostomes (2.9%), Trichuris trichiura (1.3%), Ascaris lumbricoides (0.6%), and Taenia spp. (0.06%). The prevalence of O. viverrini eggs appeared to reflect a lower infection in younger individuals (<20 years) than in the adult population (>20 years). Men (50.4%) revealed a significantly higher (P=0.02) prevalence than women (44.3%). The Ang Svay Chek villages of the Prey Kabas District, Takeo Province, Cambodia have been confirmed to be a highly endemic area for human O. viverrini infection.

Interaction of D2 with Ti-adsorbed polyaniline and implication for hydrogen storage.

Interaction of D2 with Ti-adsorbed polyaniline (PANI) and TiO2 at low Ti coverages has been investigated by temperature programmed desorption, Auger electron spectroscopy, and H2-D2 exchange in a high pressure cell. In contrast to recent DFT calculations (Lee et al. Phys. Rev. Lett. 2006, 97, 56104-56107) that have shown a multiple number of molecular hydrogen chemisorption on Ti-decorated PANI, only 0.52 D2 molecules per Ti atomically adsorb on Ti-deposited PANI at 87 K at a nominal Ti coverage of 2 ML. Broad TPD spectra of D2, HD, and H2 were observed, which showed a common peak at 250 K and another isotope-dependent peak at higher temperature. Ti deposited on TiO2 forms clusters with a size distribution at 87 K, on which D2 atomically adsorbs. As the Ti coverage increases, the D2 desorption peak gradually shifts from 200 to 260 K and the number of D2 molecules adsorbed per Ti also increases from 0.26 at 0.1 ML to 0.87 at 2 ML. We attribute this to the fact that D2 adsorbs on larger clusters with a greater adsorption energy. TPD spectra of D2 desorbing from Ti (2 ML)/PANI and Ti (1 ML)/TiO2 look very similar to each other. This led us to conclude that Ti also adsorbs on PANI in clusters. We suggest that no molecular D2 adsorption on Ti/PANI at 87 K is due to a reduced electron backdonation ability of Ti upon clustering. The small number of adsorbed D2 per Ti on both substrates was ascribed to irreversible D2 adsorption only on relatively large Ti clusters, while D2 adsorbs reversibly on small clusters. This was indirectly confirmed by the observation that continuous H2-D2 isotope exchange occurs to produce HD on Ti/PANI as well as on Ti/TiO2 at 106 K in a high pressure cell at 5.7 mTorr. Desorption mechanisms involving H atom abstraction from PANI by Ti clusters are proposed to account for the complex isotope-dependent TPD spectra. Implication of the present results for hydrogen storage based on Ti-dispersed polymers is briefly addressed.

Colloidal synthesis of ultrathin two-dimensional semiconductor nanocrystals.

2D semiconductor quantum wells have been recognized as potential candidates for various quantum devices. In quantum wells, electrons and holes are spatially confined within a finite thickness and freely move in 2D space. Much effort has focused on shape control of colloidal semiconductor nanocrystals(NCs), and synthesis of 2D colloidal NCs has been achieved very recently. Here, recent advances in colloidal synthesis of uniform and ultrathin 2D CdSeNCs are highlighted. Structural and optical property characterization of these quantum-sized 2D CdSe NCs is discussed. Additionally, 2D CdSe NCs doped with Mn 2+ ions for dilute magnetic semiconductors (DMS) are presented.These 2D CdSe-based NCs can be used as model systems for studying quantum-well structures.