Igor Beinik

This thesis discusses the chemistry and physics of Cu and H2O on ZnO surfaces, based primarily on results from quantum chemical calculations. The underlying context is heterogeneous catalysis, where Cu/ZnO-mixtures are used in the industrial synthesis of methanol and in the water gas shift reaction. Electron transfer between small Cu clusters and ZnO is central to this thesis, as are the design and use of models that can describe realistic and very large-scale ZnO surface structures while still retaining the electronic nature of the system. Method and model enhancements as well as tests and validations constitute a large part of this thesis.The thesis demonstrates that the charges of small Cu clusters, adsorbed on the non-polar ZnO(10-10) surface, depend on whether the Cu clusters contain an even or odd number of atoms, and whether water is present (water can induce electron transfer from Cu to ZnO). On the polar Zn-terminated ZnO(0001) surface, Cu becomes negatively charged, which causes it to attract positively charged subsurface defects and to wet the ZnO(0001) surface at elevated temperatures.When a Cu cluster on a ZnO surface becomes positively charged, this happens because it donates an electron to the ZnO conduction band. Hence, it is necessary to use a method which describes the ZnO band gap correctly, and we show that a hybrid density functional, which includes a fraction of Hartree-Fock exchange, fulfills this requirement. When the ZnO conduction band becomes populated by electrons from Cu, band-filling occurs, which affects the adsorption energy. The band-filling correction is presented as a means to extrapolate the calculated adsorption energy under periodic boundary conditions to the zero coverage (isolated adsorbate, infinite supercell) limit.A part of this thesis concerns the parameterization of the computationally very efficient SCC-DFTB method (density functional based tight binding with self-consistent charges), in a multi-scale modeling approach. Our findings suggest that the SCC-DFTB method satisfactorily describes the interaction between ZnO surfaces and water, as well as the stabilities of different surface reconstructions (such as triangularly and hexagonally shaped pits) at the polar ZnO(0001) and ZnO(000-1) surfaces.

The sulfidation of a MoO3 precursor into MoS2 is an important step in the preparation of catalysts for the hydrodesulfurization process that is widely utilized in oil refineries. Molybdenum oxides are also the most commonly used precursors for MoS2 growth in, e.g., the synthesis of novel two-dimensional materials. In the present study, we investigate the transformation of MoOx into MoS2 on a model Au(111) surface through sulfidation in H2S gas atmosphere using in situ scanning tunneling microscopy and X-ray photoemission spectroscopy. We find that progressive annealing steps of physical vapor deposited MoO3 powder allow us to control the stoichiometry and oxidation state of the precursor oxide. Subsequently, we investigate the sulfidation of the compounds ranging from pure low-oxygen Mo to fully oxidized MoO3 oxide sulfidation using two different methods. We find that the prerequisite for the efficient formation of MoS2 is that Mo stays in the highest Mo(6+) state before sulfidation...

Metal adhesion on metal oxides is strongly controlled by the oxide surface structure and composition, but lack of control over the surface conditions often limits the possibilities to exploit this in opto- and micro-electronics applications and heterogeneous catalysis where nanostructural control is of utmost importance. The Cu/ZnO system is among the most investigated of such systems in model studies, but the presence of subsurface ZnO defects and their important role for adhesion on ZnO have been unappreciated so far. Here we reveal that the surface-directed migration of subsurface defects affects the Cu adhesion on polar ZnO(0001) in the technologically interesting temperature range up to 550 K. This leads to enhanced adhesion and ultimately complete wetting of ZnO(0001) by a Cu overlayer. On the basis of our experimental and computational results we demonstrate a mechanism which implies that defect concentrations in the bulk are an important, and possibly controllable, parameter...

The growth of ultra-thin KCl films on the Si(111)7 × 7 reconstructed surface has been investigated as a function of KCl coverage and substrate temperature. The structure and morphology of the films were characterized by means of scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions. Detailed analysis of the atomically resolved STM images of islands grown at room and high temperatures (400 K-430 K) revealed the presence of KCl(001) and KCl(111) islands with the ratio between both structures depending on the growth temperature. At room temperature, the growth of the first layer, which covers the initial Si(111)7 × 7 surface, contains double/triple atomic layers of KCl(001) with a small fraction of KCl(111) islands. The high temperature growth promotes the appearance of large KCl(111) areas, which are built up by three atomic layers. At room and high temperatures, flat and atomically well-defined ultra-thin KCl films can be grown on the Si(111)7 × 7 substrate. Th...

ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, photoconductive atomic force microscopy (PC-AFM) has been applied to investigate transient photoconductivity and photocurrent spectra of upright-standing ZnO nanorods (NRs). With a view to evaluate the electronic properties of the NRs and to get information on recombination kinetics, we have also performed time-resolved photoluminescence measurements macroscopically. Persistent photoconductivity from single ZnO NRs was observed for about 1800 s and was studied with the help of photocurrent spectroscopy, which was recorded locally. The photocurrent spectra recorded from single ZnO NRs revealed that the minimum photon energy sufficient for photocurrent excitation is 3.1 eV. This value is at least 100 meV lower than the band-gap energy determined from the photoluminescence experiments. The obtained results suggest that the photorespons...

Conductive atomic-force microscopy (C-AFM), where a conductive, biased probe is scanned in contact mode across the surface under investigation is one of the most prominent scanning probe microscopy based techniques to study electrical properties of dielectric and semiconducting thin films on the nanometer scale. The technique, originally developed to evaluate the homogeneity in gate dielectrics is also successfully applied to study electrical and electronic properties of semiconductor nanostructures. The chapter starts with the discussion of the technical implementation of the technique (both under ambient conditions and in ultra-high vacuum) and the experimental peculiarities due to contact mode. The concepts of two-dimensional current maps acquired at constant tip-to-sample bias and local current voltage maps will be introduced for the example of thin silicon gate oxide and high-k dielectric thin films. Applicability of C-AFM to semiconductor nanostructures is demonstrated for supported semiconductor nanowires and free standing nanorods. Characterization of antiphase defects in ternary alloys and ZnO based multilayer varistor films show the technique’s potential for device evaluation. An outlook is devoted to the so-called photoconductive AFM where photocurrents are detected under simultaneous illumination with monochromatic light.

Surface planarization and masked ion-beam structuring (MIBS) of high-Tc superconducting (HTS) YBa2Cu3O7-δ (YBCO) thin films grown by pulsed-laser deposition (PLD) method is reported. Chemical–mechanical polishing, plasma etching, and oxygen annealing of YBCO films strongly reduce the particulate density (~10–2 ×) and surface roughness (~10–1 ×) of as-grown PLD layers. The resistivity, critical temperature Tc≈90K and critical current density Jc (77K)>1

We apply scanning probe microscopy (SPM) to study the morphology and electrical properties of vertical zinc oxide nanorods grown by hydrothermal methods on silicon substrates. It is demonstrated that - against the intuition - SPM techniques can indeed be used to study such fragile high-aspect ratio semiconductor nanorods. Atomic-force microscopy (AFM) operating in tapping mode yields - via the analysis of the height histograms calculated from AFM images - easy access to the height fluctuations in the nanorod ensemble. High-resolution AFM images reveal the three-dimensional shape of the nanorods including transition facets between the (0001) top terrace and the {10-10} side facets. Further, we were able to acquire current-voltage curves of individual nanorods by conductive atomic force microscopy (CAFM) operating in contact mode.

ZnO nanostructures are of special interest for device applications. However, their structural characterization remains an ongoing challenge. This paper reviews recent efforts and latest achievements in this direction. Results comprise PAS in the form of Slow Positron Implantation Spectroscopy (SPIS) and Pulsed Low Energy Positron Lifetime Spectroscopy (PLEPS), Nuclear Reaction Analysis (NRA), Atomic Force Microscopy (AFM), conductive AFM (C-AFM), Nuclear Magnetic Resonance (NMR), Electron Spin Resonance (ESR), Photoluminescence (PL) spectroscopy, and latest theoretical investigations of structure-related and positron properties of selected defects. The fundamental importance of a relationship between fabrication conditions, native defect formation, and resulting optical and electronic properties is demonstrated by getting either inferior (nanorods) or significantly improved (tetrapods) optical properties compared to single crystal samples, depending on the nanostructure fabrication method. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Irradiation of thin films of the cuprate high-temperature superconductor YBa2Cu3O7-δ (YBCO) with 75keV He+ ions leads to an exponential increase of the resistivity and a non-linear decrease of the critical temperature. At a fluence above 3×1015cm-2 the material becomes semiconducting. Calculations of ion-target interactions using the MARLOWE code indicated that these effects are due to the creation of point defects,