Sexiphenyl on Cu(100): nc-AFM tip functionalization and identification

The contrast obtained in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images is determined by the tip termination and symmetry. Functionalizing the tip with a single metal atom, CO molecule or organic species has been shown to provide high spatial resolution and insights into tip-surface interactions. A topic where this concept is utilized is the adsorption of organic molecules at surfaces. With this work we aim to contribute to the growing database of organic molecules that allow assignment by intra-molecular imaging. We investigated the organic molecule para-sexiphenyl (C36H26, 6P) on Cu(100) using low-temperature STM and non-contact AFM with intra-molecular resolution. In the sub-monolayer regime we find a planar and flat adsorption with the 6P molecules rotated 10{\deg} off the directions. In this configuration, four of the six phenyl rings occupy almost equivalent sites on the surface. The 6P molecules are further investigated with CO- functionalized tips, in comparison to a single-atom metal and 6P-terminated tip. We also show that the procedure of using adsorbed CO to characterize tips introduced by Hofmann et al. Phys. Rev. B 112 (2014) 066101 is useful when the tip is terminated with an organic molecule

Interplay between adsorbates and polarons: CO on rutile TiO2_2(110)

Polaron formation plays a major role in determining the structural, electrical and chemical properties of ionic crystals. Using a combination of first principles calculations and scanning tunneling microscpoy/atomic force microscopy (STM/AFM), we analyze the interaction of polarons with CO molecules adsorbed on the rutile TiO$_2$(110) surface. Adsorbed CO shows attractive coupling with polarons in the surface layer, and repulsive interaction with polarons in the subsurface layer. As a result, CO adsorption depends on the reduction state of the sample. For slightly reduced surfaces, many adsorption configurations with comparable adsorption energies exist and polarons reside in the subsurface layer. At strongly reduced surfaces, two adsorption configurations dominante: either inside an oxygen vacancy, or at surface Ti$_{5c}$ sites, coupled with a surface polaron

Controlling the Electrical Properties of Undoped and Ta-doped TiO2 Polycrystalline Films via Ultra-Fast Annealing Treatments

We present a study on the crystallization process of undoped and Ta doped TiO2 amorphous thin films. In particular, the effect of ultra-fast annealing treatments in environments characterized by different oxygen concentrations is investigated via in-situ resistance measurements. The accurate examination of the key parameters involved in this process allows us to reduce the time needed to obtain highly conducting and transparent polycrystalline thin films (resistivity about $6 \times 10^{-4}$ {\Omega}cm, mean transmittance in the visible range about $81\%$) to just 5 minutes (with respect to the 180 minutes required for a standard vacuum annealing treatment) in nitrogen atmosphere (20 ppm oxygen concentration) at ambient pressure. Experimental evidence of superficial oxygen incorporation in the thin films and its detrimental role for the conductivity are obtained by employing different concentrations of traceable 18O isotopes during ultra-fast annealing treatments. The results are discussed in view of the possible implementation of the ultra-fast annealing process for TiO2-based transparent conducting oxides as well as electron selective layers in solar cell devices; taking advantage of the high control of the ultra-fast crystallization processes which has been achieved, these two functional layers are shown to be obtainable from the crystallization of a single homogeneous thin film.Comment: 30 pages (including Supporting Information and graphical TOC), 4 figure

Small Polarons in Transition Metal Oxides

The formation of polarons is a pervasive phenomenon in transition metal oxide compounds, with a strong impact on the physical properties and functionalities of the hosting materials. In its original formulation the polaron problem considers a single charge carrier in a polar crystal interacting with its surrounding lattice. Depending on the spatial extension of the polaron quasiparticle, originating from the coupling between the excess charge and the phonon field, one speaks of small or large polarons. This chapter discusses the modeling of small polarons in real materials, with a particular focus on the archetypal polaron material TiO2. After an introductory part, surveying the fundamental theoretical and experimental aspects of the physics of polarons, the chapter examines how to model small polarons using first principles schemes in order to predict, understand and interpret a variety of polaron properties in bulk phases and surfaces. Following the spirit of this handbook, different types of computational procedures and prescriptions are presented with specific instructions on the setup required to model polaron effects.Comment: 36 pages, 12 figure

Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Perovskite oxides attract increasing attention due to their broad potential in many applications. Understanding their surfaces is challenging, though, because the ternary composition of the bulk allows for multiple stable surface terminations. We demonstrate a simple procedure for preparing the bulk-terminated (001) surface of SrTiO3, a prototypical cubic perovskite. Controlled application of strain on a SrTiO3 single crystal results in a flat cleavage with micrometer-size domains of SrO and TiO2. Distribution of these two terminations is dictated by ferroelectric domains induced by strain during the cleavage process. Atomically-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) measurements reveal the presence of point defects in a well-defined concentration of (14+-2)%; Sr vacancies form at the SrO termination and complementary Sr adatoms appear at the TiO2 termination. These intrinsic defects are induced by the interplay between ferroelectricity, surface polarity, and surface charge

Formation and dynamics of small polarons on the rutile TiO2_2(110) surface

Charge trapping and formation of polarons is a pervasive phenomenon in transition metal oxide compounds, in particular at the surface, affecting fundamental physical properties and functionalities of the hosting materials. Here we investigate via first-principle techniques the formation and dynamics of small polarons on the reduced surface of titanium dioxide, an archetypal system for polarons, for a wide range of oxygen vacancy concentrations. We report how the essential features of polarons can be adequately accounted in terms of few quantities: the local structural and chemical environment, the attractive interaction between negatively charged polarons and positively charged oxygen vacancies, and the spin-dependent polaron-polaron Coulomb repulsion. We combined molecular dynamics simulations on realistic samples derived from experimental observations with simplified static models, aiming to disentangle the various variables at play. We find that depending on the specific trapping site, different types of polarons can be formed, with distinct orbital symmetries and different degree of localization and structural distortion. The energetically most stable polaron site is the subsurface Ti atom below the undercoordinated surface Ti atom, owing to a small energy cost to distort the lattice and a suitable electrostatic potential. Polaron-polaron repulsion and polaron-vacancy attraction determine the spatial distribution of polarons as well as the energy of the polaronic in-gap state. In the range of experimentally reachable oxygen vacancy concentrations the calculated data are in excellent agreement with observations, thus validating the overall interpretation