Comparative study of adsorptions, reactions and electronic properties of U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces.

The mysterious properties of individual U atoms on transition metal surfaces play indispensable parts in supplementing our understanding of uranium-transition metal systems, which are important subjects for both nuclear energy applications and fundamental scientific studies. By using scanning tunneling microscopy and density functional theory calculations, the adsorptions, reactions and electronic properties of individual U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces were comparatively studied for the first time in this work. Upon the deposition of a small amount of U onto Cu(111) or Ag(111) at 8 K, individual U atoms show relatively high activity and can either be adsorbed on intact substrate surfaces or induce various surface vacancies surrounded by clusters of substrate atoms. By contrast, the majority of U atoms tend to dispersedly adsorb on intact surfaces of Au(111) and Ru(0001) rather than producing surface vacancies at the same temperature. In all cases, Kondo resonance manifested as asymmetric dip feature around Fermi energy is only observed in the differential tunneling conductance spectra of single U adatoms on Ag(111).

Observation and Manipulation of Visible Edge Plasmons in Bi2Te3 Nanoplates.

Noble metals, like Ag and Au, are the most intensively studied plasmonic materials in the visible range. Plasmons in semiconductors, however, are usually believed to be in the infrared wavelength region due to the intrinsic low carrier concentrations. Herein, we observe the edge plasmon modes of Bi2Te3, a narrow-band gap semiconductor, in the visible spectral range using photoemission electron microscopy (PEEM). The Bi2Te3 nanoplates excited by 400 nm femtosecond laser pulses exhibit strong photoemission intensities along the edges, which follow a cos4 dependence on the polarization state of incident beam. Because of the phase retardation effect, plasmonic response along different edges can be selectively exited. The thickness-dependent photoemission intensities exclude the spin-orbit induced surface states as the origin of these plasmonic modes. Instead, we propose that the interband transition-induced nonequilibrium carriers might play a key role. Our results not only experimentally demonstrate the possibility of visible plasmons in semiconducting materials but also open up a new avenue for exploring the optical properties of topological insulator materials using PEEM.

Electronic structure and photoabsorption of Ti3+ ions in reduced anatase and rutile TiO2.

We have used two-photon photoemission (2PPE) spectroscopy and first-principles density functional theory calculations to investigate the electronic structure and photoabsorption of the reduced anatase TiO2(101) and rutile TiO2(110) surfaces. 2PPE measurements on anatase (101) show an excited resonance induced by reduced Ti3+ species centered around 2.5 eV above the Fermi level (EF). While this state is similar to that observed on the rutile (110) surface, the intensity of the 2PPE peak is much weaker. The computed oscillator strengths of the transitions from the occupied gap states to the empty states in the conduction band show peaks between 2.0 and 3.0 eV above the conduction band minimum (CBM) on both surfaces, confirming the presence of empty Ti3+ resonances at these energies. Although the crystal field environment of Ti ions is octahedral in both rutile and anatase, Ti3+ ions exhibit distinct d orbital splittings due to different distortions of the TiO6 units. This affects the directions of the transition dipoles from the gap states to the conduction band, explaining the polarization dependence of the 2PPE signal in the two materials. Our results also show that the Ti3+ induced states in the band gap are shallower in anatase than in rutile. The d → d transitions from the occupied gap states to the empty Ti3+ excited states in anatase can occur at energies well below 3 eV, consistent with the observed visible-light photocatalytic activity of Ti3+ self-doped anatase.

Photocatalytic chemistry of methanol on rutile TiO2(011)-(2 × 1).

Photocatalytic chemistry of methanol on the reconstructed rutile TiO2(011)-(2 × 1) surface upon 266 nm and 400 nm light excitation has been investigated quantitatively using the post-irradiation temperature-programmed desorption (TPD) method. Photochemical products such as formaldehyde, methyl formate and water, which result from the recombination of surface bridging hydroxyls through the abstraction of lattice oxygen atoms, have been identified under both 266 nm and 400 nm light irradiation. However, ethylene is detected only under 266 nm light irradiation. Through an analogy experiment, ethylene production is attributed to the photochemistry and the following thermochemistry of formaldehyde. The absence of the ethylene signal under 400 nm light is consistent with the significantly lower conversion at this wavelength compared with 266 nm. The photocatalytic reaction rate of methanol is also wavelength dependent. Possible reasons for the photon energy dependent phenomena have been discussed. This work not only provides a detailed characterization of the photochemistry of methanol on the rutile TiO2(011)-(2 × 1) surface, but also indicates the importance of photon energy in the photochemistry on TiO2 surfaces.

Localized Excitation of Ti(3+) Ions in the Photoabsorption and Photocatalytic Activity of Reduced Rutile TiO2.

In reduced TiO2, electronic transitions originating from the Ti(3+)-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the material's blue color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO2(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti(3+) species is identified at 2.5 ± 0.2 eV above the Fermi level (EF) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below EF, as they both result from the Jahn-Teller induced splitting of the 3d orbitals of Ti(3+) ions in reduced TiO2. Localized excitation of Ti(3+) ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO2 photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO2 through Ti(3+) self-doping. Our work reveals the physical origin of the Ti(3+) related photoabsorption and visible light photocatalytic activity in prototypical TiO2 and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides.

Physical properties and field-induced metamagnetic transitions in UAu0.8Sb2.

We have successfully synthesized single crystals of UAu0.8Sb2 using a flux method and present a comprehensive study of its physical properties by measuring the magnetic susceptibility, electrical resistivity and specific heat. Evidence for at least three magnetic phases is observed in the field-temperature phase diagram of UAu0.8Sb2. In zero field, the system undergoes an antiferromagnetic transition at 71 K, and upon further cooling it passes through another antiferromagnetic phase with a ferromagnetic component, before reaching a ferromagnetic ground state. A complex magnetic field-temperature phase diagram is obtained for fields along the easy c-axis, where the antiferromagnetic order eventually becomes polarized upon applying a magnetic field.