We have studied cleaning procedures of Nb(110) by verifying the surface quality with low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy and spectroscopy. Our results show that the formation of a surface-near impurity depletion zone is inhibited by the very high diffusivity of oxygen in the Nb host crystal which kicks in at annealing temperatures as low as a few hundred degree Celsius. Oxygen can be removed from the surface by heating the crystal up to T = 2400 • C. Tunneling spectra measured on the clean Nb(110) surface exhibit a sharp conductance peak in the occupied states at an energy of about −450 meV. Density functional theory calculations show that this peak is caused by a d z 2 surface resonance band at theΓ point of the Brillouin zone which provides a large density of states above the sample surface. The clean Nb(110) surface is superconducting with a gap width and a critical magnetic field strength in good agreement to the bulk value. In an external magnetic field we observe the Abrikosov lattice of flux quanta (vortices). Spatially resolved spectra show a zero-bias anomaly in the vortex core.

Envisioned extremely scaled, high-performance memory devices request to conduct the step from thin semiconductor films to nanoscale structures and the use of promising high-k materials such as hafnium oxide (HfO 2 ). HfO 2 is well suited for use in resistive random-access memory (ReRAM) devices based on the valence change mechanism. Here, we provide a decidedly scaled system, namely, HfO 2 nanoislands that are grown by van der Waals epitaxy on highly oriented pyrolytic graphite (HOPG). The electronic and structural properties of these wellseparated, crystalline HfO 2 nanoislands are investigated by scanning probe methods as well as ab initio methods. The topography reveals homogeneously formed HfO 2 nanoislands with areas down to 7 nm 2 and a thickness of one unit cell. They exhibit several acceptor-and donor-like in-gap states in addition to the bulk band gap, implying bulk properties. X-ray photoelectron spectroscopy indicates hafnium carbide formation as one possible origin for defect states. Going further to the crystal nucleation of HfO 2 , nanocrystals with a diameter of 2.7−4.5 Å are identified next to carbon vacancies in the topmost HOPG layer, indicating that carbon is incorporated into the islands at early nucleation stages. A precise description of these nuclei is accomplished by the simulation of small Hf m O n (:C) clusters (m = 3 to 10; n = 3 to 22) with and without carbon incorporation using ab initio methods. The comparison of the theoretically determined lowest-energy clusters and electronic states with the experimental results allows us to identify the structure of the most relevant HfO 2 sub-nanometer crystals formed during the first nucleation steps and the nature of the in-gap states found at the surfaces of HfO 2 nanoislands. That way, a model system is derived that consists of distinct structural units, related to surface states or defect states, respectively, that will promote the tailoring of in-gap states of smallest HfO 2 structures and thus the scalability of memory devices.

The enduring demand for ever-increasing storage capacities inspires the development of new few nanometer-sized, high-performance memory devices. In this work, tri-n-octylphosphine oxide (TOPO)-stabilized sub-10 nm monoclinic HfO2 nanocrystals (NC) with a rod-like and spherical shape are synthesized and used to build up microscale and nanoscale test devices. The electrical characterization of these devices studied by cyclic current–voltage measurements reveals a redox-like behavior in ambient atmosphere and volatile threshold switching in vacuum. By employing a thorough spectroscopic and surface analysis (FT-IR and NMR spectroscopy and XPS), the origin of this behavior was elucidated. While the redox behavior is enabled by residual moisture present during clean-up of the NC and thin film preparation, which leads to a partial desorption of TOPO from the NC surface, threshold switching is obtained for dry TOPO-stabilized HfO2 NC in microchannel as well as in nanoelectrode devices addressing only a few sub-10 nm TOPO-stabilized HfO2 NC. The results show that integration of sub-10 nm HfO2 NC in nanoscale devices is feasible to build up switching elements.