NaV2O4: a quasi-1D metallic antiferromagnet with half-metallic chains

NaV2O4 crystals were grown under high pressure using a NaCl flux, and the crystals were characterized with x-ray diffraction, electrical resistivity, heat capacity, and magnetization. The structure of NaV2O4 consists of double chains of edge-sharing VO6 octahedra. The resistivity is highly anisotropic, with the resistivity perpendicular to the chains more than 20 times greater than that parallel to the chains. Magnetically, the intrachain interactions are ferromagnetic and the interchain interactions are antiferromagnetic; 3D antiferromagnetic order is established at 140 K. First-principles electronic structure calculations indicate that the chains are half-metallic. Interestingly, the case of NaV2O4 seems to be a quasi-1D analogue of what was found for half-metallic materials.     

Adsorbate-induced pinning of a charge-density wave in a quasi-1D metallic chains: Na on the In/Si(111)-(4x1) surface

We find that foreign adsorbates acting as local impurities can induce a metal-insulator transition by pinning a charge-density wave (CDW) on the quasi-1D metallic In/Si(111)-(4x1) chain system. Our scanning tunneling microscopy image clearly reveals the presence of a new local 4x2 structure nucleated by Na adatoms at room temperature, which turns out to be insulating with a doubled periodicity along the chains. We directly determine a CDW gap energy Delta = 105+/-8 meV by identifying a characteristic loss peak in our high-resolution electron-energy-loss spectra. We thus report the first observation of a local impurity-derived Peierls-like reconstruction of a quasi-1D system.     

Adsorption of L-cysteine on rutile TiO2(110)

We have used X-ray photoelectron spectroscopy to study the adsorption of L-cysteine on a rutile TiO₂(110) surface at room temperature and ? 65 °C. For the molecules in direct contact with the surface our results suggest that the molecules bind dissociatively to the fivefold-coordinated Ti atoms of the surface through their deprotonated carboxylic groups. A second, dissociative interaction occurs between the molecular thiol groups and the surface. It is attributed to a dissociative bond to the bridging oxygen vacancies. Most likely, the thiol groups are deprotonated and a bond is formed between the thiolates and defects. In an alternative scenario, the C–S bond is cleaved and atomic sulfur binds to the defects. With regard to the molecular amino groups, they remain neutral at the lowest investigated coverages (0.3–0.5 ML), but already starting from around 0.7 ML nominal coverage protons are being transferred to them. The fraction of protonated amino groups increases with coverage and becomes dominating in multilayers prepared at room temperature and ? 65 °C. In these multilayers the carboxylic groups are deprotonated.     

Elastic constants of semiconducting Ti2O3 and metallic (Ti1−xVx)2O3 at 1.5 K

Ultrasonic measurements at 1.5 K reveal that the elastic constants of metallic (Ti_1-xV_x)₂O₃ differ from those of semiconducting Ti₂O₃. This is explained in terms of how vanadium influences the electronic energy band structure and the lattice parameters. The electronic effect is calculated as the strain-induced shift of Van Zandt's vanadium impurity band relative to the titanium valence band. The lattice parameter effect is compared with how pressure influences the elastic constants of Al₂O₃.     

Structure and local-equilibrium thermodynamics of the c(2x2) reconstruction of rutile TiO2 (100)

We resolve the structure of a c(2x2) reconstruction of the rutile TiO2 (100) surface using a combination of transmission electron diffraction, direct methods analysis, and density functional theory. The surface structure contains an ordered array of subsurface oxygen vacancies and is in local thermodynamic equilibrium with bulk TiO2, but not the with oxygen gas-phase environment. The transition into a bulklike (1x1) reconstruction offers insights into the time-dependent local thermodynamics of TiO2 surface reconstruction under global nonequilibrium conditions.     

Stability of gold nanostructures on rutile TiO2(110) surface

The adsorption of gold atoms and formation of nanostructures on the rutile TiO₂(110) surface with different degree of oxygen reduction was studied from first principles. The Au atoms adsorb strongest at oxygen vacancy sites. Starting from a very low coverage limit the potential energy profiles or diffusion paths of the adsorbed Au monomers and dimers were calculated. Stable structures of two to nine Au atoms arranged in finite and infinite rows and in the shape of finite-size clusters were determined. All these structures are found to bind to the reduced surface stronger than 2 eV/atom. The elongated Au row-like structures bind by about 0.1 eV stronger than 3D clusters, suggesting a preference for the 1D-like Au growth mode on the missing-row reconstructed TiO₂(110).     

Vacancy-assisted diffusion of alkoxy species on rutile TiO2(110)

Using scanning tunneling microscopy (STM) and density functional theory simulations, we have studied the diffusion of alkoxy species formed by the dissociation of alcohols on bridge-bonded oxygen (BBO) vacancies (BBO(V)'s) on TiO2(110). At elevated temperatures (>or=400 K) the sequential isothermal STM images show that mobile BBO(V)'s mediate the diffusion of alkoxy species by providing space for alkyl-group-bearing BBO atom to diffuse into. The experimental findings are further supported by simulations that find that BBO(V) diffusion is the rate limiting step in the overall diffusion mechanism.     

Methylene bromide chemistry and photochemistry on rutile TiO2(110)

The chemistry and photochemistry of methylene bromide (CD₂Br₂) on the rutile TiO₂(110) surface was probed using temperature programmed desorption (TPD). CD₂Br₂ desorbed in three desorption states at 145, 160 and 250 K tentatively assigned to desorption from the multilayer, from an η¹-CD₂Br₂ species and a bridging η²-CD₂Br₂ species, respectively. The latter two TPD states presumably involve binding of CD₂Br₂ molecules to the surface through Br coordination at five-coordinate Ti⁴⁺ surface sites. The 160 and 250 K TPD states saturated at coverages of 1.0 and 0.33 ML, respectively, where 1 ML is equivalent to the surface Ti⁴⁺ site density (5.2 × 10¹⁴ cm^? 2). No thermal decomposition of CD₂Br₂ was observed on either the clean surface or with preadsorbed O₂. UV irradiation of CD₂Br₂ on TiO₂(110) resulted in predominately photodesorption, with trace amounts of photodecomposition evidenced in TPD. The rate of CD₂Br₂ photodesorption from TiO₂(110) occurred with a low cross section (~ 2 × 10^? 21 cm²) similar to that expected from direct optical excitation of CD₂Br₂. This observation suggests that charge carriers generated in TiO₂(110) were no more effective in activating adsorbed CD₂Br₂ molecules than would be expected through direct molecular excitation. These findings suggest that photocatalytic destruction of halocarbons such as CD₂Br₂ on TiO₂ may preferentially occur though indirect processes (such as OH radical attack) as opposed to direct electron transfer processes involving charge carriers generated in TiO₂ by bandgap excitation.     

Direct evidence for ethanol dissociation on rutile TiO2(110)

We have studied the interaction of ethanol with reduced TiO(2)(110)-(1 × 1) by high-resolution scanning tunneling microscopy (STM) measurements and density functional theory calculations. The STM data revealed direct evidence for the coexistence of molecularly and dissociatively adsorbed ethanol species on surface Ti sites. In addition, we found evidence for dissociation of ethanol at bridge-bonded O vacancies. The density functional theory calculations support these findings and rationalize the distinct diffusion behaviors of molecularly and dissociatively adsorbed ethanol species, as revealed in time-lapsed STM images.     

Coupled mode behavior of the soft E(TO) phonon of Pb(Ti1-x,Zrx)O3 and (Pb1-3x2, Lax)TiO3

We report the temperature dependence of the low frequency part (ω < 100 cm^?1) of the Raman spectra of the Pb(Ti_1-x, Zr_x)O₃ and $\text{(}\text{Pb}{}_{\mathrm{<mtext>1-</mtext><mtext>3x</mtext><mtext>2</mtext>}}$, La_x)TiO₃ systems. The spectra indicate a coupled mode behavior between the soft E(TO) phonon and a mode which produces an additional band in the spectra. We discuss the possibility that the additional band might be due to zone boundary tranverse acoustical phonons which couple to the q ～ 0 soft E(TO) mode because of the disorder existing in the systems.     

Tracer impurity diffusion in single-crystal rutile (TiO2−x)

By means of the radioactive-tracer sectioning technique, the tracer diffusion of the impurity ions, ⁴⁶Sc, ⁵¹Cr, ⁵⁴Mn, ⁵⁹Fe, ⁶⁰Co, ⁶³Ni and ⁹⁵Zr, in rutile single crystals was measured as functions of crystal orientation, temperature, oxygen partial pressure and Al impurity content. The diffusion coefficients are very sensitive to the electric charge of the impurity ions. Divalent impurities (e.g., Co and Ni) diffuse extremely rapidly in TiO₂, compared to cation self-diffusion, and exhibit an extreme anisotropy in diffusion behavior, divalent-impurity diffusion parallel to the c-axis is much larger than it is perpendicular to the c-axis. Trivalent impurity ions (Sc and Cr) and tetravalent impurity ions (Zr) diffuse similar to cation self-diffusion, both as functions of temperature and oxygen partial pressure. The divalent impurity ions Co and Ni apparently diffuse as interstitial ions along open channels parallel to the c-axis. The results suggest that Sc, Cr and Zr ions diffuse by an interstitialcy mechanism involving the simultaneous and cooperative migration of tetravalent interstitial titanium ions and the tracer-impurity ions. Iron ions diffuse both as divalent and as trivalent ions. The impurity diffusion as functions of oxygen partial pressure and Al-impurity content are consistent with calculations of point-defect concentrations in rutile.     

Surface structure of TiO2(011)-(2x1)

A combined experimental and first principles study of the (2x1)-reconstructed rutile TiO2(011) surface is presented. Our results provide evidence that the surface structure is described by a model that includes onefold coordinated (titanyl) oxygen atoms giving rise to double bonded Ti=O species. These species should play a special role in the enhanced photocatalytic activity of the TiO2(011) surface.     

Pyrocatechol as a surface capping molecule on rutile TiO2 (110)

A ‘cap and dip’ method of adsorbing ruthenium di-2,2′-bipyridyl-4,4′-dicarboxylic acid diisocyanate (N3 dye) on a rutile TiO₂ (110) surface was investigated using pyrocatechol as a capping molecule. This method involves cleaning the rutile surface in ultra-high vacuum (UHV), depositing pyrocatechol onto the surface to ‘cap’ the adsorption sites, removing from vacuum, ‘dipping’ in an N3 dye solution and returning to vacuum. Photoemission measurements following the return of the crystal to vacuum suggest that the pyrocatechol keeps the surface free from contamination on exposure to atmosphere. Photoemission spectra also indicate that the pyrocatechol capping molecules are replaced by the N3 dye in solution and that the N3 dye is adsorbed intact on the rutile TiO₂ (110) surface. This technique may allow other large molecules, which are thermally unstable to evaporation in UHV, to be easily deposited onto TiO₂ surfaces.     

Atomic resolution noncontact atomic force and scanning tunneling microscopy of TiO2(110)-(1 x 1) and - (1 x 2): simultaneous imaging of surface structures and electronic states

We present simultaneous imaging of TiO2(110)-(1 x 1) and - (1 x 2) using noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM). The surface topography was imaged under NC-AFM feedback, while the surface electronic states were imaged by STM. The image contrasts of NC-AFM and STM were antiphase in (1 x 1) and in phase in (1 x 2). The uppermost oxygen and Ti atoms underneath were, respectively, imaged by NC-AFM and STM. The NC-AFM image contrast was close to the true surface topography in (1 x 2), but reduced in (1 x 1).     

Bonding of gold nanoclusters to oxygen vacancies on rutile TiO2(110)

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile TiO2(110) surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the TiO2(110) system involving vacancy-cluster complex diffusion is presented.