Towards dielectric relaxation at a single molecule scale.

Dielectric relaxation lies at the heart of well-established techniques of dielectric spectroscopy essential to diverse fields of research and technology. We report an experimental route for increasing the sensitivity of dielectric spectroscopy ultimately towards the scale of a single molecule. We use the method of radio frequency scanning tunneling microscopy to excite a single molecule junction based on a polar substituted helicene molecule by an electric field oscillating at 2-5 GHz. We detect the dielectric relaxation of the single molecule junction indirectly via its effect of power dissipation, which causes lateral displacement. From our data we determine a corresponding relaxation time of about 300 ps-consistent with literature values of similar helicene derivatives obtained by conventional methods of dielectric spectroscopy.

1D tungsten oxide nanostructures on a Cu(1 1 0) surface.

Thin epitaxial layers of tungsten oxide on metal substrates are suitable as model systems for investigation of chemical reactivity and catalytic properties. However, the ability to prepare epitaxial tungsten oxide model system in situ is quite rare. Here we present a method to prepare highly ordered tungsten oxide thin film on a Cu(1 1 0) single crystal substrate using physical vapor deposition in a reactive atmosphere of atomic oxygen. The oxygen induced reconstruction of the copper substrate gives rise to unique self-organized 1D structures of tungsten oxide parallel with the Cu[1 -1 0] crystallographic direction. Utilizing a combination of photoemission spectroscopy and density functional theory calculations we reveal emergent physicochemical properties related to the low-dimensionality of the system. Specifically, we observe a support mediated charge redistribution at the interface and a momentum dependent modulation of the valence-band electronic structure. The unusual character of the 1D oxide nanostructures on Cu(1 1 0) surface opens up a unique avenue for preparation of tungsten oxide-based functionalized nanostructures and provides options for further investigation of the fundamental properties of tungsten oxide.

Counting electrons on supported nanoparticles.

Electronic interactions between metal nanoparticles and oxide supports control the functionality of nanomaterials, for example, the stability, the activity and the selectivity of catalysts. Such interactions involve electron transfer across the metal/support interface. In this work we quantify this charge transfer on a well-defined platinum/ceria catalyst at particle sizes relevant for heterogeneous catalysis. Combining synchrotron-radiation photoelectron spectroscopy, scanning tunnelling microscopy and density functional calculations we show that the charge transfer per Pt atom is largest for Pt particles of around 50 atoms. Here, approximately one electron is transferred per ten Pt atoms from the nanoparticle to the support. For larger particles, the charge transfer reaches its intrinsic limit set by the support. For smaller particles, charge transfer is partially suppressed by nucleation at defects. These mechanistic and quantitative insights into charge transfer will help to make better use of particle size effects and electronic metal-support interactions in metal/oxide nanomaterials.

Maximum noble-metal efficiency in catalytic materials: atomically dispersed surface platinum.

Platinum is the most versatile element in catalysis, but it is rare and its high price limits large-scale applications, for example in fuel-cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst's surface. To maximize the noble-metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria "nanopocket", which binds Pt(2+) so strongly that it withstands sintering and bulk diffusion. On model catalysts we experimentally confirm the theoretically predicted stability, and on real Pt-CeO2 nanocomposites showing high Pt efficiency in fuel-cell catalysis we also identify these anchoring sites.

Epitaxial Cubic Ce2O3 Films via Ce-CeO2 Interfacial Reaction.

Thin films of reduced ceria supported on metals are often applied as substrates in model studies of the chemical reactivity of ceria based catalysts. Of special interest are the properties of oxygen vacancies in ceria. However, thin films of ceria prepared by established methods become increasingly disordered as the concentration of vacancies increases. Here, we propose an alternative method for preparing ordered reduced ceria films based on the physical vapor deposition and interfacial reaction of Ce with CeO2 films. The method yields bulk-truncated layers of cubic c-Ce2O3. Compared to CeO2 these layers contain 25% of perfectly ordered vacancies in the surface and subsurface allowing well-defined measurements of the properties of ceria in the limit of extreme reduction. Experimentally, c-Ce2O3(111) layers are easily identified by a characteristic 4 × 4 surface reconstruction with respect to CeO2(111). In addition, c-Ce2O3 layers represent an experimental realization of a normally unstable polymorph of Ce2O3. During interfacial reaction, c-Ce2O3 nucleates on the interface between CeO2 buffer and Ce overlayer and is further stabilized most likely by the tetragonal distortion of the ceria layers on Cu. The characteristic kinetics of the metal-oxide interfacial reactions may represent a vehicle for making other metastable oxide structures experimentally available.