Study for d-electronic states in titanium dioxide by doping of transition metal and control of oxygen defects

采用化学传递反应法制备锐钛矿单晶。为了控制氧空位，进行了光谱和x射线衍射测量，建立了在氧气或氢气气氛下热处理的实验条件。测量了生长、氢离子注入、在氧或氢气氛下退火的锐钛矿单晶的光吸收和ESR光谱。经氧退火后，晶体的颜色由浅蓝色（生长状态）到黄色转变为无色。通过氢退火使无色晶体还原为深蓝色晶体。在氧气气氛下反复热处理，再次变为深绿色、黄色和无色。由于晶体的颜色差异与缺陷状态的颜色差异相对应，因此建立了对缺陷状态的控制。生长样品、氢热处理样品和氢离子注入样品的光吸收光谱和ESR光谱相同。得到的向[001]方向的各向异性g张量可以很好地解释为以Ti为中心的电子的存在。以金红石型固溶体为原料，成功地合成了v或nb掺杂锐钛矿单晶。它们的尺寸分别为2×2×2mm^3，透明，颜色分别为紫色和深蓝色。与未掺杂晶体相比，它们的基本吸收边位于能量较低的侧。掺铌单晶在2.5 eV以下的低能区有较强的吸收。掺铌单晶经氢退火后的吸收光谱与未掺铌单晶相似。这表明了它们在电子结构上的相似性。

Single-crystals of anatase form were grown by means of chemical transport reaction method. For the control of oxygen vacancies, the measurements of spectra and X-ray diffraction were performed to establish the experimental condition for heat-treatments under the oxygen or hydrogen atmosphere.Optical absorption and ESR spectra were measured for anatase single crystals as-grown, implanted by H ions, annealed under oxygen or hydrogen atmosphere at desired temperature. The color of the crystal changed from pale-blue (as-grown) through yellow to colorless ones on oxygen annealing. The colorless crystal was reduced to dark-blue one by hydrogen annealing. On repetitive heat treatment under oxygen atmosphere, it becomes dark-green, yellow and colorless, again. Since the difference in color of the crystal corresponds to that in the state of defects, it was established to control the defect states. Optical absorption and ESR spectra were same for the as-grown, hydrogen-heat-treated and H ion implanted samples. The obtained anisotropic g tensor to [001] direction is well interpreted by the presence of the delectrons centered at Ti.V-or Nb-doped anatase single crystals were successfully synthesized by the use of rutile form solid solution as raw material. They were 2×2×2mm^3 in size and transparent with purple or dark-blue color, respectively. The fundamental absorption edges of them were located at lower energy side than undoped crystal. The Nb-doped single crystal has strong absorption in lower energy region from 2.5 eV.The absorption spectrum of the Nb-doped single crystal resembles to that of undoped crystal with hydrogen annealing. This indicates the similarity in electronic structure each other.