Ultrathin films of barium oxide were grown on Ag(001) and Ag(111) using the evaporation of Ba metal in an O2 atmosphere by molecular beam epitaxy. Ultraviolet photoemission spectroscopy reveals that ...films consisting of predominantly BaO or BaO2 result in Ag(001) work function reductions of 1.74eV and 0.64eV, respectively. On the Ag(001) surface, Ba oxide growth is initiated by two-dimensional nucleation of epitaxial BaO, followed by a transition to three-dimensional dual-phase nucleation of epitaxial BaO and BaO2. Three-dimensional islands of primarily BaO2(111) nucleate epitaxially on the Ag(111) substrate leaving large patches of Ag uncovered. We find no indication of chemical reaction or charge transfer between the films and the Ag substrates. These data suggest that the origin of the observed work function reduction is largely due to a combination of BaO surface relaxation and an electrostatic compressive effect.
•Work function reductions determined for BaO/Ag(001) and BaO2/Ag(001)•SK growth mode for BaO/Ag(001) while deposition on Ag(111) is Volmer-Weber•X-ray photoelectron spectroscopy determination for BaO and BaO2•Electrostatic compression and surface relaxation origin for work function reduction•Ultrathin barium oxide films grown on Ag(001) and Ag(111) using MBE in an O2 ambient
We show that the HF acid etch commonly used to prepare SrTiO3(001) for heteroepitaxial growth of complex oxides results in a non-negligible level of F doping within the terminal surface layer of ...TiO2. Using a combination of x-ray photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction, we determine that on average ~13% of the O anions in the surface layer are replaced by F, but that F does not occupy O sites in deeper layers. Despite this perturbation to the surface, the Fermi level remains unpinned, and the surface-state density, which determines the amount of band bending, is driven by factors other than F doping. The presence of F at the STO surface is expected to result in lower electron mobilities at complex oxide heterojunctions involving STO substrates because of impurity scattering. Unintentional F doping can be substantially reduced by replacing the HF-etch step with a boil in deionized water, which in conjunction with an oxygen tube furnace anneal, leaves the surface flat and TiO2 terminated.
► F substitutes for O in the surface layer of HF-etched and tube-furnace-annealed SrTiO3(001). ► The F is quite stable against annealing up to at least 850K. ► Substitutional F does not dope the surface n-type. ► The surface Fermi level remains unpinned.
Interfacial intermixing and electronic structure were investigated at thin (3–5 unit cells.), epitaxial La1-xAl1+xO3/SrTiO3(001) heterojunctions for x = 0 and ± 0.05. Angle-resolved X-ray ...photoelectron spectroscopy reveals rather extensive cation intermixing for all films, independent of composition. The valence band offset for the nominally stoichiometric (x = 0) film is 0.16 ± 0.10 eV, with the valence band maximum of SrTiO3 being deeper in binding energy than that of LaAlO3. Similar values are obtained for x = ± 0.05. There is no measurable band bending in either the LaAlO3 or the SrTiO3 near the interface. These results are at odds with first principles theoretical predictions based on perfect stoichiometry and an abrupt interface model. Finally, however, inclusion of intermixing in the compositional description of the interface results in successful prediction of the valence band offset and absence of band bending.
The question of stability against diffusional mixing at the prototypical LaAlO3/SrTiO3(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical ...methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using onaxis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as potential energy gradients and valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO3 or SrTiO3, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Significantly, we find it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface – electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO3 below the interface as being the cause of the observed conductivity.
Photoemission electron microscopy (PEEM) excited by X‐ray and UV sources is used to investigate epitaxial anatase thin films with embedded rutile nanocrystals, a model system for the study of ...heterocatalysis on mixed‐phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases; however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X‐ray PEEM images but brighter in UV‐PEEM images. We observe that topography‐induced contrast is dominant in X‐ray PEEM imaging, whereas work function and density‐of‐state‐based contrast, dominates in UV‐PEEM. This assertion is confirmed by UPS and conducting AFM data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density‐of‐states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed‐phase TiO2 interfaces and grain boundaries.
Photoemission electron microscopy (PEEM) image of a mixed‐phase TiO2 thin film grown by oxygen‐plasma‐ assisted molecular beam epitaxy. The bright rutile nanocrystals are embedded in the dark anatase film. Femtosecond laser pulses (3.1 eV) are used to induce photoemission.
The flux of atomic oxygen generated in an electron cyclotron resonance microwave plasma source was quantified by two different methods. The commonly applied approach of monitoring the frequency ...change of a silver-coated quartz crystal microbalance (QCM) deposition rate monitor as the silver is oxidized was found to underestimate the atomic oxygen flux by an order of magnitude compared to a more direct deposition approach. In the mixed-phase Ag/Ag2O deposition method, silver films were deposited in the presence of atomic oxygen such that the films were partially oxidized to Ag2O; X-ray photoelectron spectroscopy was utilized for quantification of the oxidized fraction. The inaccuracy of the QCM oxidation method was tentatively attributed to efficient catalytic recombination of O atoms on the silver surface.
We show that the HF acid etch commonly used to prepare SrTiOsub3(001) for heteroepitaxial growth of complex oxides results in a non-negligible level of F doping within the terminal surface layer of ...TiOsub2. Using a combination of x-ray photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction, we determine that on average ~ 13% of the O anions in the surface layer are replaced by F, but that F does not occupy O sites in deeper layers. Despite this perturbation to the surface, the Fermi level remains unpinned, and the surface-state density, which determines the amount of band bending, is driven by factors other than F doping. The presence of F at the STO surface is expected to result in lower electron mobilities at complex oxide heterojunctions involving STO substrates because of impurity scattering. Unintentional F doping can be substantially reduced by replacing the HF-etch step with a boil in deionized water, which in conjunction with an oxygen tube furnace anneal, leaves the surface flat and TiOsub2 terminated.
Over the past five years, considerable work has been carried out in the exploration of candidate diluted oxide magnetic semiconductors with high Curie temperatures. Fueled by early experimental ...results and theoretical predictions, claims of ferromagnetism at and above room temperature in doped oxides have abounded. In general, neither the true nature of these materials nor the physical causes of the magnetism have been adequately determined. It is now apparent that these dilute magnetic systems are deceptively complex. We consider two well-characterizedn-type magnetically doped oxide semiconductors and explore the relationship between donor electrons and ferromagnetism.
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