[en] The presence of different surface charges and thus different surface reactivities offers the possibility of exploiting domain-structured ferroelectrics as templates for nanostructure assembling. Recent work has reported on the photochemical adsorption of noble-metal nanoparticles to such surfaces serving as pin-points for attaching desired organic molecules. Here, we report on the formation of domain structures of μm-sizes in 5 mol% Mg-doped congruent LiNbO₃ single crystals by (a) UV-induced poling using a HeCd laser beam (λ=325 nm) focused through liquid electrodes, as well as (b) the application of high voltage between a contact mode AFM tip and a back electrode. Subsequently, the deposition of wire-like metal structures at the domain wall positions was stimulated by illuminating the sample dipped into aqueous solutions of AgNO₃ or Pt(NO₃)₂ by super bad-gap illumination (λ<310 nm) using a Hg spectral lamp. Structural properties of the resulting nano- and mesowires were examined using non-contact AFM and Kelvin probe force microscopy. Electrical conductivity of 300 μm long Pt wires between macroscopic electrodes could be demonstrated by performing I-V-scans

[en] Deposition routes for metallic and molecular nanowires on the 180 domain walls of lithium niobate through ferroelectric lithography are well established. However, a detailed understanding of the underlying processes is still missing, which implies the necessity of further systematic studies of specific domain wall properties. Here, we report on conductive-AFM (c-AFM) measurements on 5 mol% Mg-doped congruent LiNbO₃ single crystals. We compare the c-AFM results with the domain distribution recorded by piezoresponse force microscopy (PFM). Surprisingly, we find an enhanced current density at room temperature and in air along the 180 domain walls under UV-illumination above the band gap of 4 eV. This domain wall conductivity was then studied as a function of wavelength and light intensity. Implications for ferroelectric lithography on LiNbO₃ templates are discussed.

[en] The energy distribution of electronic defect states within the bandgap of oxygen-reduced n-type strontium titanate has been analyzed by means of wavelength-dependent surface photovoltage (SPV) measurements using a Kelvin probe set-up. As a further result, the parameters of several surface states, i.e. the trap state density and the thermal and optical cross sections, were extracted from SPV transients. Finally, the appropriateness of the method as a general tool for electrical characterization of complex oxide surfaces is illustrated by applying the SPV technique to comparable oxide crystals such as undoped SrTiO₃, Nb-doped SrTiO₃, or BaTiO₃.

[en] Ferroelectric 180 domain walls (DWs) may play an important role in future electronic devices. The size and position of domains and DWs can be easily controlled. In addition, DWs in lithium niobate (LiNbO₃: LNO) can be functionalized through depositing metallic and/or molecular nanowires by means of ferroelectric lithography. Moreover, the same DWs become conductive under super-bandgap illumination, hence manifesting switchable, tunable, and nanoscale conductive channels that extend across the whole insulating bulk material. However, a detailed understanding of the underlying processes that lead to DW conductivity is still missing to date. Here, we report on conductive-AFM (c-AFM) measurements on 5 mol% Mg-doped congruent LNO single crystals. We compare the c-AFM results with the domain distribution recorded by piezoresponse force microscopy (PFM). Surprisingly, we find an enhanced current flow inside the 180 DWs under super-bandgap UV-illumination even at ambient conditions. The DW conductivity was studied as a function of wavelength, light intensity and doping concentration. Our results suggest a hopping transport being relevant for the reported DW conductivity.

[en] The question whether electron-doped mixed-valence manganites, such as La_0.7Ce_0.3MnO₃, can be synthesized as single-phase compounds has been under debate for a decade. Meanwhile it has become clear that electron doping can indeed be achieved in epitaxial thin films. However, as-prepared films often suffer from overoxygenation and concomitant hole doping, which can be overcome by deoxygenation through a post-deposition annealing procedure. Disappointingly, those reduced samples do not exhibit the typical metal-insulator transition (MIT) any longer. In the present work, we show that the MIT of La_0.7Ce_0.3MnO_3-δ films can be recovered by exposition to visible light. Our films turn out to be highly photoconductive: Laser illumination at 514 nm with a power of 400 mW gave rise to a dramatic resistance drop of around seven orders of magnitude at 100 K as compared to the dark state, while illumination had no impact on the conductivity of an as-prepared reference film

[en] We report on local measurements of the surface potential and quantum capacitance in single layer graphene as well as multilayers thereof as a function of the carrier density by using frequency-modulated Kelvin probe force microscopy. We find excellent agreement to tight-binding calculations reported for graphene monolayers and extract the minimum quantum capacitance from density sweeps at room temperature. The surface potential of graphene multilayers is found to depend linearly on the carrier density, which suggests treating them as two-dimensional electron gases. In addition, we demonstrate that the simultaneously detected second harmonic of the Kelvin modulation, proportional to |∂²C/∂z²|, is directly sensitive to local changes in the quantum capacitance of graphene.

[en] The electric resistivity of stoichiometric and oxygen-deficient epitaxial 10-nm-thick La_0.7Ca_0.3MnO₃ thin films on SrTiO₃ under photoexcitation has been investigated systematically. In contrast to the as-prepared film, the oxygen-deficient one exhibits a pronounced photoinduced decrease of the resistivity of up to five orders of magnitude at low temperatures. A detailed analysis of the resistivity as a function of illumination intensity and wavelength (visible to ultraviolet range) is presented for the bare substrate as well as for the film/substrate heterostructure. The roles of carrier generation in the film and carrier injection from the substrate, which both contribute to the observed effects, are discussed.

[en] The preparation of epitaxial La_0.7Ce_0.3MnO₃ (LCeMO) films by pulsed-laser deposition is very challenging - especially the oxygen partial pressure during deposition plays a crucial role to suppress the presence of a CeO₂ secondary phase. As-prepared films typically suffer from overoxygenation and concomitant hole doping instead of the nominal and desired electron-doping. Post-deposition annealing however, using a reducing atmosphere seems to solve this problem. These samples though are insulating and do not exhibit a phase transition from a paramagnetic insulating to a ferromagnetic metallic phase any more. Here, we show the possibility to drive the Mn valence towards 2+ by photoexcitation, which renders LCeMO thin films conductive and recovers their phase transition. Possible mechanisms behind the photoconductivity effect are presented, especially the role of the SrTiO₃ substrate and the influence of interface defect states.

[en] The question whether electron-doped mixed-valence manganites, such as La_0.7Ce_0.3MnO₃, can be synthesized as single-phase compounds has been under debate for more than a decade. By now there is some agreement that it is possible to prepare high-quality single-phase material through epitaxial thin-film growth, but as-prepared films often suffer from overoxygenation and concomitant hole doping instead of the nominal and desired electron doping. Deoxygenation through post-deposition annealing in vacuum seems to solve this problem. However, oxygen-reduced samples are insulating and do not exhibit a phase transition from a paramagnetic insulating to a ferromagnetic metallic phase any longer. In the present work, we show that photoexcitation renders such thin films conductive and recovers the phase transition. Possible mechanisms behind the pronounced photoconductivity effect are presented, especially the role of the SrTiO₃ substrate. Several indications that strongly point towards an electron-doped nature of the illuminated films are discussed.

[en] Tetravalent-ion-doped lanthanum manganite films typically suffer from overoxygenation in the as-prepared state, which in turn leads to an effective hole doping instead of the nominal and desired electron doping. This problem can be overcome by post-deposition annealing in a reducing atmosphere, which, however, removes the typical phase transition from a paramagnetic insulating to a ferromagnetic metallic phase and makes the films insulating in the whole temperature range. Such electron-doped La_0.7Ce_0.3MnO_3-δ thin films were investigated with respect to their transport characteristics under photoexcitation. While the films are insulating in the dark, even exposure to diffuse daylight dramatically decreases the low-temperature resistance and recovers the insulator-metal transition (IMT). Exposure to continuous visible laser light further decreases the resistance by up to seven orders of magnitude and shifts the IMT towards higher temperatures. Investigations of the spectral, transient, and intensity-dependent behaviour of the photoconductivity suggest that (i) both photogeneration of carriers in the film as well as charge injection from the substrate contribute to the effect, and that (ii) the excess carriers are electrons.