Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite (α-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 10 fs at 2.2 eV to 350 30 fs at 2.6 eV, whereas the polaron formation probability (85 10%) remains constant. By comparison to hematite (α-Fe2O3), an oxide analog, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides. TOC GRAPHICSKEYWORDS XUV; extreme-ultraviolet; high-harmonic generation; ligand-to-metal charge transfer; light harvesting; photocatalysis.
Metal-oxide-semiconductor junctions are central to most electronic and optoelectronic devices. Here, the element-specificity of broadband extreme ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and recombination kinetics in each layer of a Ni-TiO 2 -Si junction. After photoexcitation of silicon, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in spectral shifts in the Ni M 2,3 XUV edge that are characteristic of holes and the absence of holes initially in TiO 2 . Meanwhile, the electrons are observed to remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO 2 , shifting the Ti spectrum to higher oxidation state, followed by electron-hole recombination at the Si-TiO 2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO 2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.
Transient extreme ultraviolet (XUV) spectroscopy probes core level transitions to unoccupied valence and conduction band states. Uncertainty remains to what degree the core-hole created by the XUV transition modifies the measurement of photoexcited electron and hole energies. Here, the Si L2,3 edge is measured after photoexcitation of electrons to the Δ, L, and Γ valleys of Si(100). The measured changes in the XUV transition probability do not energetically agree with the increasing electron photoexcitation energy. The data therefore experimentally confirm that, for the Si L2,3 edge, the time-dependent electron and hole energies are partially obscured by the core-hole perturbation. A model based on many-body approximations and the Bethe-Salpeter equation is successfully used to predict the core-hole's modification of the final transition density of states in terms of both electronic and structural dynamics. The resulting fit time constants match the excited state electron thermalization time and the inter-valley electronphonon, intra-valley electron-phonon, and phonon-phonon scattering times previously measured in silicon. The outlined approach is a more comprehensive framework for interpreting transient XUV absorption spectra in photoexcited semiconductors.
Silicon nanoparticles have the promise to surpass the theoretical efficiency limit of single-junction silicon photovoltaics by the creation of a "phonon bottleneck," a theorized slowing of the cooling rate of hot optical phonons that in turn reduces the cooling rate of hot carriers in the material. Verifying the presence of a phonon bottleneck in silicon nanoparticles requires simultaneous resolution of electronic and structural changes at short timescales. Here, extreme ultraviolet transient absorption spectroscopy is used to observe the excited-state electronic and lattice dynamics in polycrystalline silicon nanoparticles following 800 nm photoexcitation, which excites carriers with 0.35 ± 0.03 eV excess energy above the Δ 1 conduction band minimum. The nanoparticles have nominal 100 nm diameters with crystalline grain sizes of about ∼16 nm. The extracted carrier−phonon and phonon−phonon relaxation times of the nanoparticles are compared to those for a silicon (100) single-crystal thin film at similar carrier densities (2 × 10 19 cm −3 for the nanoparticles and 6 × 10 19 cm −3 for the film). The measured carrier− phonon and phonon−phonon scattering lifetimes for the polycrystalline nanoparticles are 870 ± 40 fs and 17.5 ± 0.3 ps, respectively, versus 195 ± 20 fs and 8.1 ± 0.2 ps, respectively, for the silicon thin film. The reduced scattering rates observed in the nanoparticles are consistent with the phonon bottleneck hypothesis.
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