Dramatic band gap reduction incurred by dopant coordination rearrangement in Co-doped nanocrystals of CeO2.

A dramatic band gap narrowing of 1.61 eV has been observed in Co-doped nanocrystals of CeO2 (ceria), as a result of thermal annealing, without changing the ceria crystal structure and the Co concentration. As demonstrated by x-ray absorption fine structures, thermal annealing incurs an oxygen coordination rearrangement around Co atoms from an octahedral coordination to a square-planar coordination. First principle calculation using density functional theory reveals two stable oxygen coordination types surrounding Co, consistent with the experimental observation. The band gap values calculated for the two stable coordination types differ dramatically, reproducing the experimentally observed band gap narrowing. These prominent effects due to local structure rearrangement around dopant atoms can lead to unprecedented methods for band gap engineering in doped nanocrystal oxides.

Unconventional interplay between heterovalent dopant elements: Switch-and-modulator band-gap engineering in (Y, Co)-Codoped CeO2 nanocrystals.

We report the experimental observation and theoretical explanation of an unconventional interplay between divalent Co and trivalent Y dopants, both of which incur oxygen vacancies in the CeO2 host that has predominantly tetravalent Ce cations. The Co dopant atoms were experimentally found to act as a switch that turns on the dormant effect of Y-modulated band-gap reduction. As revealed by density functional theory (DFT) calculations with structures verified by synchrotron-radiation x-ray measurements, a Co 3d band that hybridizes with Ce 4f band was lowered due to reduced O 2p repulsion arising from oxygen vacancies incurred by Y doping and therefore gave rise to the observed band-gap narrowing effect. Such switch-and-modulator scheme for band-gap engineering in nanocrystal materials can lead to important applications in environmental protection and solar energy harvesting technologies.

Studies of impurities in magnetic semiconductors: an example of important XAFS applications.

The x-ray absorption fine structure (XAFS) technique has been employed to investigate the local structure and valency about Mn and Fe ions in the III-V diluted magnetic semiconductors In(1-x)Mn(x)As and Ga(1-x)Fe(x)As, prepared by molecular-beam-epitaxy under various growth conditions. These new systems are promising magnetic materials of considerable current interest and with important technical applications including photo-carrier induced magnetism and spin-polarized current devices. The local structure around the magnetic ions can play a pivotal role in affecting the magnetic properties of these semiconductors. Local structure information obtained from XAFS has provided the first direct evidence that the magnetic impurities can indeed substitute for the cation host atoms in samples prepared under appropriate conditions.