Midgap levels for wide gap TiO 2 have become increasingly important because they can be used to capture solar light efficiently for photocatalysis as demonstrated by black TiO 2 in a recent paper [Chen, X., et al. Science 2011, 331 (6018), 746−750]. However, a method for systematically characterizing the midgap state energy levels is still lacking. We proposed an optical method, i.e., transient infrared (IR) absorption − excitation energy scanning spectrum, by recording nanosecond time-resolved transient IR absorption from the excited electrons either in the conduction band or at the excited localized states below the conduction in combination with midgap excitation energy scanning. We demonstrate that both the electron trap states beneath the Fermi level and those excited localized states below the conduction band as well as the Fermi level of TiO 2 nanoparticles can unambiguously be determined by this method, which has great potential for characterizing the midgap trap states of various semiconductor nanomaterials other than TiO 2 . ■ INTRODUCTIONWide bandgap oxide semiconductor TiO 2 is still considered one of the best materials for photocatalysis and solar energy conversion, 1 while the anatase form is usually considered to be more active in photocatalytic and photovoltaic applications. The large bandgap of TiO 2 ensures the photogenerated electrons within the conduction band (CB) have a strong reducing ability and the holes in the valence band have a strong oxidizing ability. 2 Either in photocatalysis or in the photovoltaic process, nanophase TiO 2 has been used for its highly effective surface area as well as its large number of surface binding sites. However, the number of surface and interstitial defects known as trap states also increases substantially in the nanoparticles versus the number in the single crystals. These trap states with their energy levels lying in the bandgap act as carrier traps in competition with the fast carrier recombination in the bulk during photoexcitation, which enhances the photoactivity of the nanoparticles. On the other hand, the deep trap states reduce the photocatalytic activities when their chemical potentials are considered. 3 Because of its wide bandgap, TiO 2 absorbs light only in the UV range, which accounts for only 3−5% of the total sunlight; 4 this leads to a low light conversion efficiency in the solar spectral region. Therefore, extending the absorption of TiO 2 to the visible range would be an effective means of increasing its overall efficiency. One way to increase solar energy absorption efficiency is to narrow the bandgap by anion doping such as nitrogen doping. 5 At present, nitrogen-doped TiO 2 exhibits the strongest response to solar radiation, 6 but its absorption in the visible and infrared region remains insufficient.In contrast to the cation-doped TiO 2 , the surface and interstitial defects usually known in a form of Ti 3+ such as selfdoping 7 act as color centers; these color centers in principle would not narrow the bandgap but would provide a cha...

Confirmation of direct photogeneration of intrinsic delocalized free carriers in small-molecule organic semiconductors has been a long-sought but unsolved issue, which is of fundamental significance to its application in photo-electric devices. Although the excitonic description of photoexcitation in these materials has been widely accepted, this concept is challenged by recently reported phenomena. Here we report observation of direct delocalized free carrier generation upon interband photoexcitation in highly crystalline zinc phthalocyanine films prepared by the weak epitaxy growth method using ultrafast spectroscopy. Transient absorption spectra spanning the visible to mid-infrared region revealed the existence of short-lived free electrons and holes with a diffusion length estimated to cross at least 11 molecules along the π−π stacking direction that subsequently localize to form charge transfer excitons. The interband transition was evidenced by ultraviolet-visible absorption, photoluminescence and electroluminescence spectroscopy. Our results suggest that delocalized free carriers photogeneration can also be achieved in organic semiconductors when the molecules are packed properly.

Periodic zinc oxide rod arrays were fabricated on patterned templates by electrochemical deposition and were employed as field emitters. The morphology and crystal structure of the zinc oxide array were examined by scanning electron microscopy and x-ray diffraction, respectively. The dependence of the field emission current density J and the applied electric field E presented a two-stage slope behavior in ln(J∕E2)−1∕E plot according to Fowler-Nordheim equation. The mechanism of the electron emission is attributed to the defects in the electrochemically deposited zinc oxide rods.

This work shows a novel artificial donor-catalyst-acceptor triad photosystem based on a mononuclear C5 H5 -RuH complex oxo-bridged TiO2 hybrid for efficient CO2 photoreduction. An impressive quantum efficiency of 0.56 % for CH4 under visible-light irradiation was achieved over the triad photocatalyst, in which TiO2 and C5 H5 -RuH serve as the electron collector and CO2 -reduction site and the photon-harvester and water-oxidation site, respectively. The fast electron injection from the excited Ru(2+) cation to TiO2 in ca. 0.5 ps and the slow backward charge recombination in half-life of ca. 9.8 μs result in a long-lived D(+) -C-A(-) charge-separated state responsible for the solar-fuel production.

Wurtzite structural ZnO microneedles with hexagonal cross section were fabricated by vapor-phase transport method and an individual microneedle was employed as a lasing microcavity. Under excitation of a femtosecond pulse laser with 800 nm wavelength, the ultraviolet (UV) laser emission was obtained, which presented narrow linewidth and high Q value. The UV emission, resonant mechanism, and laser mode characteristics were discussed in detail. The results demonstrated that the UV laser originated from the whispering-gallery mode induced by two-photon absorption assisted by Rabi oscillation.

The femtosecond time-resolved multiplex coherent anti-Stokes Raman scattering (CARS) technique has been performed to investigate intramolecular vibrational redistribution (IVR) through vibrational couplings in 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) molecules. In the multiplex CARS experiment, the supercontinuum (SC) was used as broad-band Stokes light to coherently and collectively excite multiple vibrational modes, and quantum beats arising from vibrational couplings among these modes were observed. The IVR of RDX is visualized by a topological graph of these vibrational couplings, and with analysis of the topological graph, two vibrational modes, both of which are assigned to ring bending, are confirmed to have coupling interactions with most of the other vibrational modes and are considered to have a tendency of energy transfer with these vibrational modes. We suggest that the mode at 466 cm is a portal of energy transfer from outside to inside of the RDX molecule and the mode at 672 cm is an important transit point of energy transfer in the IVR.

Understanding the interfacial charge transfer of the photoinduced transients of all-inorganic cesium lead halide perovskites (CsPbX 3 ; X = Cl, Br, I) is critical for their photovoltaic applications. Ultrafast dynamics can provide comprehensive information about the transient behavior of the carriers and their transfer mechanism in the materials. In this work, the interfacial charge transfer of CsPbX 3 films assembled with TiO 2 with different halogen doping ratios was studied using femtosecond transient absorption spectroscopy combined with global analysis. Four subsequent decay processes after photoexcitation were carried out, including hot carrier cooling, free exciton formation, electron transfer, and charge recombination. The results indicate that the time constant of the interfacial electron transfer varies with the location of the trap state of these perovskites and the relative energy of conduction bands in the perovskite and TiO 2 and that the time constant of the charge recombination can be attributed to the electron−hole interactions. These interpretations are supported by calculations based on first-principles density functional theory. Higher iodine doping in such perovskite CsPbX 3 /TiO 2 systems increases the time constants of the electron transfer and charge recombination, which suggests that all-inorganic perovskite CsPbX 3 with a high iodine content is favorable for improving the power conversion efficiency of solar cells.

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