Orientation Dependence of Step Stiffness: Failure of SOS and Ising Models to Describe Experimental Data

We have investigated the step stiffness on Cu(001) surfaces as a function of step orientation by two independent methods at several temperatures near 300 K. Both sets of data agree well and show a substantial dependence of the stiffness on the angle of orientation. With the exception of steps oriented along $$, the experimental stiffness is significantly larger than the stiffness calculated within the solid-on-solid (SOS) model and the Ising-model, even if next nearest-neighbor interactions are taken into account. Our results have considerable consequences for the understanding and for the theoretical modeling of equilibrium and growth phenomena, such as step meandering instabilities.Comment: 5 pages, 2 figure

Kink Energy and Kink Dipole Moment on Vicinal Au(001) in Halide Electrolytes

Using electrochemical scanning tunnelling microscopy, we measured the potential-dependent kink energy and the corresponding dipole moments for kinks at step edges on vicinal Au(001) surfaces in chloride and bromide containing electrolyte. Combining the results for the potential dependence of the kink energy with impedance spectroscopy data for the surface charge, we can directly deduce the dipole moment of kinks at the Au steps with co-adsorbed Cl-, respectively Br-. We find μ^Cl= (6.0±0.7) × 10^(-3) eÅ and μ^Br= (10.1±0.6) × 10^(-3) eÅ

Principal component analysis: Reveal camouflaged information in x-ray absorption spectroscopy photoemission electron microscopy of complex thin oxide films

Principal component analysis (PCA) has become a standard tool in spectromicroscopy and hyperspectral imaging to handle large spectral data sets and to decompose raw data into relevant and residual information. In particular in studies of complex compounds, PCA can be used to disentangle chemical information and thereby deepen the understanding of chemical and physical material properties. Surprisingly, in photoemission electron spectromicroscopy (PEEM), PCA is rarely used. This paper serves to demonstrate how powerful PCA can be to detect hidden chemical information in PEEM data. We demonstrate the capability of PCA in PEEM spectromicroscopy for the case of a thin film of a complex quaternary oxide, Pr0.5Ba0.5CoO3-δ (PBCO) which is a main contender catalyst material for electrocatalytic water splitting. Upon annealing in air, PBCO decomposes into different phases at the surface. Two of them become obvious from the raw PEEM images, but one is revealed only after PCA

Step Dipole Moment and Step Line Tension on Au(100) in aqueous KBr electrolyte

Here, we demonstrate a new method to measure step dipole moments for electrode surfacesin the presence of specifically adsorbed anion adlayers. The method is based on potential-dependentstudies of the equilibrium shape and the equilibrium fluctuations of monatomic high Au islands asobserved in scanning tunneling microscopy data.Furthermore we measure the angular anisotropy of the absolute step line tension for an Au(100)electrode in contact with KBr solution. A method previously introduced for surfaces in vacuum is nowextended for electode surfaces in contact with electrolyte

Repulsive Interactions Induced by Specific Adsorption: Anomalous Step Diffusivity and Inadequacy of Nearest-Neighbor Ising Model (Part II Theory)

This is Part II of two closely related papers, where we show that the strong repulsive interaction caused by specifically adsorbed anions leads to a failure of the nearest-neighbor Ising model to describe structures on electrode surfaces. In this part, an analytical form of the step diffusivity is derived in terms of nearest and next-nearest neighbor interactions for steps with a mean direction along 〈110〉〈110〉 (the close-packed direction). With the help of a further analytical expression for the diffusivity of steps with 〈100〉〈100〉 mean orientation a simple scheme is developed whereby the nearest and next-nearest interaction energies can be extracted from the experimental values for the diffusivity along the 〈110〉〈110〉- and 〈100〉〈100〉-directions. The method is applicable to repulsive and attractive next-nearest neighbor interactions, both for surfaces in vacuum and in contact with an electrolyte. An example is presented in Part I, where we apply our approach to Au(100) in Br--, Cl-- and SO42—containing electrolytes

Repulsive interactions induced by specific adsorption: Anomalous step diffusivity and inadequacy of nearest-neighbor Ising model

This is Part I of two closely related papers, where we show that the specific adsorption of anions leads to a failure of the nearest-neighbor Ising model to describe island perimeter curvatures on Au(100) electrodes in dilute KBr, HCl and H2SO4 electrolytes and the therewith derived step diffusivity vs. step orientation. This result has major consequences for theoretical studies aiming at the understanding of growth, diffusion and degradation phenomena. Part I focuses on the experimental data.As shown theoretically in detail in Part II (doi:10.1016/j.susc.2016.03.022), a set of nearest-neighbor and next-nearest-neighbor interaction energies (ϵNN, ϵNNN) can uniquely be derived from the diffusivity of steps along 〈100〉 and 〈110〉. We find strong repulsive next-nearest neighbor (NNN) interaction in KBr and HCl, whereas NNN interaction is negligibly for H2SO4. The NNN repulsive interaction energy ϵNNN therefore correlates positively with the Gibbs adsorption energy of the anions. We find furthermore that ϵNNN increases with increasing Br− and Cl− coverage.The results for ϵNN and ϵNNN are quantitatively consistent with the coverage dependence of the step line tension. We thereby establish a sound experimental base for theoretical studies on the energetics of steps in the presence of specific adsorption