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Lippmaa Group

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The band gap and transport behavior of oxide semiconductors is generally determined by point defects and doping. In many doped oxide systems, the dominant site substitution mechanism can be assumed from the dopant valence and ionic radius, but explicitly determining the dopant site structure is generally impossible. X-ray fluorescence holography is a useful technique in this regard, as it produces directly the atomic positions of the nearest neighbors surrounding a dopant atom. The method is based on measuring the angular distribution of the dopant atom’s fluorescence x-ray intensity (Fig. 1) and solving for the positions of the nearest-neighbor scatterer atoms. We have analyzed the structure of Rh-doped SrTiO3 photocatalysts and found that while the Rh4+ dopant substitutes at the Ti site without lattice distortion, a Rh3+ dopant forms clusters with oxygen vacancies or substitutes at the oxygen site (Fig. 2). Such clustering has a detrimental effect on photocarrier dynamics in Rh:SrTiO3 photocatalysts and photoelectrodes used for the solar-powered water splitting reaction.

Fig. 1. Angular distribution of Rh fluorescence x-ray intensity for (a) Rh3+:SrTiO3 and (b) Rh4+:SrTiO3 thin films. The difference in the patterns indicates that the arrangement of the Rh nearest-neighbor atoms is different. The pattern in (b) corresponds to a perfect perovskite structure.
Fig. 2. Two main defect cluster types in a Rh3+:SrTiO3 film grown at low oxygen pressure: (upper) Rh cluster with an oxygen vacancy, (lower) Rh substitution at the anion site.

Research Subjects

  1. Growth of thin oxide films and heterostructures by pulsed laser deposition
  2. Development of oxide photoelectrode materials for photocatalytic water splitting
  3. Polar oxides and multiferroic coupling
  4. Synthesis of nanostructures and nanocomposite thin films