Evidence for photogenerated intermediate hole polarons in ZnO

Abstract

Despite their pronounced importance for oxide-based photochemistry, optoelectronics and photovoltaics, only fairly little is known about the polaron lifetimes and binding energies. Polarons represent a crucial intermediate step populated immediately after dissociation of the excitons formed in the primary photoabsorption process. Here we present a novel approach to studying photoexcited polarons in an important photoactive oxide, ZnO, using infrared (IR) reflection-absorption spectroscopy (IRRAS) with a time resolution of 100 ms. For welldefined (10-10) oriented ZnO single-crystal substrates, we observe intense IR absorption bands at around 200 meV exhibiting a pronounced temperature dependence. On the basis of first-principles-based electronic structure calculations, we assign these features to hole polarons of intermediate coupling strength.

First-principles supercell calculations of small polarons with proper account for long-range polarization effects

“…In contrast, small polarons are mainly localized on one atomic site and form when the el-ph interaction is strong. Intermediate polarons [5] cover the size range in between. Pioneering work on small polarons was performed by Holstein [8], but taking only short-range interactions into account.…”

IR spectroscopy applied to metal oxide surfaces: adsorbate vibrations and beyond

“…Using the so-called rapid scan method [123,124], the temporal evolution of these species could be determined and allowed to obtain an effective activation energy of −246 meV. In this case, a rather detailed theoretical analysis was carried out which provided strong evidence that the observation seen in the experimental data is in fact due to the excitation of hole polarons of intermediate coupling strength [125]. According to the theoretical analysis, these polarons are particular in the sense that they correspond to an intermediate size with a diameter of about 15 Å. Interestingly, the rather detailed analysis of the data allowed concluding that from the difference of the spectra seen for p-and s-polarized light the feature at 1288 cm −1 can actually be ascribed to a surface polaron.…”

Small Polaron Hopping in Fe:LiNbO3 as a Function of Temperature and Composition

“…On the other hand, in photorefractive optics and holography, those mechanisms allow for low-intensity optical interaction and for recording of high fidelity optical gratings and real-time holograms [3]. In analogous materials, polaron formation and transport play a key role as well in a number of important applications such as ferroelectric photovoltaics [4][5][6] and in oxide catalysis [7][8][9].…”