Relaxation of photoexcited charge in semiconductors proceeds through “bulk” and surface midgap states which significantly affect operation of electronic and optoelectronic devices, rates of photocatalytic, catalytic, surface chemical reactions, etc. The pathways and rates of relaxation of excited charge in semiconductors are routinely determined by photoluminescence (PL) spectroscopy. However, the “conventional” PL emission spectra of nanocrystalline semiconductors are quite broad and featureless at 25 °C which prevents a reliable and robust assignment of electronic states. For the first time, we determined the energy band diagram for excitation and relaxation of electric charge in a benchmark semiconductor nanocrystalline titanium dioxide (anatase vs. rutile) using a newly developed technique of synchronous luminescence spectroscopy of pure solids under ambient conditions. Strongly trapped exciton (STE) or free exciton were successfully used in synchronous luminescence spectroscopy experiments as probe quasi-particle in combination with water as probe molecule to learn about the number, energy and localization of bulk, subsurface and surface midgap states in anatase and rutile. Synchronous luminescence spectra at 25 °C are much narrower than “conventional” PL emission spectra and superior in resolving characteristic emission peaks in visible range due to three distinct midgap states. For example, we have used free exciton in rutile under the subbandgap photoexcitation at 430 nm in combination with water adsorbate to assign luminescence bands of “green light” (ca. 520 nm) and “green−yellow light” (ca. 560 nm) at 25 °C to surface and subsurface oxygen vacancies. In addition, synchronous luminescence spectra are significantly more convenient than “conventional” PL emission spectra in precise determination of bandgap in nanocrystalline rutile. Beyond titanium dioxide, synchronous luminescence spectra of nanocrystalline strontium titanate are much more accurate than “conventional” PL spectra for determination of diagram of relaxation of excited charge.

Charge transfer through surface and bulk midgap states in titanium dioxide and metal titanates: Exciton and water as probes in novel synchronous versus "conventional" luminescence spectroscopy

255th National Meeting (2018)

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