The p-type semiconductor Cu 5 Ta 11 O 30 has been investigated for the effect of Cu extrusion on its crystalline structure, surface chemistry, and photoelectrochemical properties. The Cu 5 Ta 11 O 30 phase was prepared in high purity using a CuCl-mediated flux synthesis route, followed by heating the products in air from 250 to 750 °C in order to investigate the effects of its reported film preparation conditions as a p-type photoelectrode. At 650 °C and higher temperatures, Cu 5 Ta 11 O 30 is found to decompose into CuTa 2 O 6 and Ta 2 O 5 . At lower temperatures of 250 to 550 °C, nanosized Cu II O surface islands and a Cu-deficient Cu 5−x Ta 11 O 30 crystalline structure (i.e., x ∼ 1.8(1) after 450 °C for 3 h in air) is found by electron microscopy and Rietveld structural refinement results, respectively. Its crystalline structure exhibits a decrease in the unit cell volume with increasing reaction temperature and time, owing to the increasing removal of Cu(I) ions from its structure. The parent structure of Cu 5 Ta 11 O 30 is conserved up to ∼50% Cu vacancies but with one notably shorter Cu−O distance (by ∼0.26 Å) and concomitant changes in the Ta−O distances within the pentagonal bipyramidal TaO 7 layers (by ∼0.29 Å to ∼0.36 Å). The extrusion and oxidation of Cu(I) to Cu(II) cations at its surfaces is found by X-ray photoelectron spectroscopy, while magnetic susceptibility data are consistent with the oxidation of Cu(I) within its structure, as given by Cu I(5−2x) Cu II x Ta 11 O 30 . Polycrystalline films of Cu 5−x Ta 11 O 30 were prepared under similar conditions by sintering, followed by heating in air at temperatures of 350 °C, 450 °C, and 550 °C, each for 15, 30, and 60 min. An increasing amount of copper deficiency in the Cu 5−x Ta 11 O 30 structure and Cu II O surface islands are found to result in significant increases in its p-type visiblelight photocurrent at up to −2.5 mA/cm 2 (radiant power density of ∼500 mW/cm 2 ). Similarly high p-type photocurrents are also observed for Cu 5 Ta 11 O 30 films with an increasing amount of CuO nanoparticles deposited onto their surfaces, showing that the enhancement primarily arises from the presence of the CuO nanoparticles which provide a favorable band-energy offset to drive electron−hole separation at the surfaces. By contrast, negligible photocurrents are observed for Cu-deficient Cu 5−x Ta 11 O 30 without the CuO nanoparticles. Electronic structure calculations show that an increase in Cu vacancies shifts the Fermi level to lower energies, resulting in the depopulation of primarily Cu 3d 10 -orbitals as well as O 2p orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and Cu II O surface islands on the p-type photoelectrode films for solar energy conversion systems.

Evidence for the coexistence of both Bi and Se terminations of the topological insulator Bi2Se3 is presented that is connected with details of sample storage and cleaving procedures. X-ray photoelectron spectroscopy of the Bi 4f core levels show a lower binding energy component indicative of metallic Bi near the sample surface. Single crystals stored and cleaved in high vacuum predominantly show the usual Se surface termination while those stored in air for long periods of time have a high probability for Bi termination. The different terminations have very different electronic structures as measured by angle resolved photoelectron spectroscopy. Our photoemission studies show the Se-terminated electronic structure can be recovered after annealing at 400 °C.

Recent electrical measurements have accessed transport in the topological surface state band of thin exfoliated samples of Bi2Se3 by removing the bulk n-type doping by contact with thin films of the molecular acceptor F4-TCNQ. Here we report on the film growth and interfacial electronic characterization of F4-TCNQ grown on Bi2Se3. Atomic force microscopy shows wetting layer formation followed by 3D island growth. X-ray photoelectron spectroscopy is consistent with this picture and also shows that charge transferred to the molecular layer is localized on nitrogen atoms. Ultraviolet photoelectron spectroscopy shows a work function increase and an upward shift of the valence band edge that suggest significant reduction in carrier density at the Bi2Se3 surface.

The evolution of electronic structure of manganese phthalocyanine on Bi 2 Te 3 shows a transition to a bulk-like aspect abruptly after completion of the first layer. This allows the inference that, in the first layer, there is charge transfer and electronic hybridization involving the occupied Mn-derived d orbitals of the molecule into the conduction band of the substrate. The charge transfer coupling is seen using angle-resolved ultraviolet photoelectron spectroscopy by monitoring the evolution of work function and band structure with increasing molecular film thickness. The electronic structure in the second layer is more bulk-like as indicated by the reappearance of well-known low energy d orbitals that were depopulated in the first layer. Scanning tunneling microscopy shows that the transition to bulk like behavior is also reflected in film structure as a transition from a unique disordered monolayer to a locally ordered and dense second layer. These observations are relevant to ongoing efforts to control topological insulator interfaces especially for spintronics applications.