Extending holographic LEED to ordered small unit cell superstructures

Following on the success of the recent application of holographic LEED to the determination of the 3D atomic geometry of Si adatoms on a SiC(111) p(3x3) surface, which enabled that structure to be solved, we show in this paper that a similar technique allows the direct recovery of the local geometry of adsorbates forming superstructures as small as p(2x2), even in the presence of a local substrate reconstruction.Comment: 10 pages, 5 figures postscript included, revtex, Phys. Rev. B in pres

Holographic Image Reconstruction from Electron Diffraction Intensities of Ordered Superstructures

We report on a novel holographic reconstruction of well resolved atomic images from discrete spot intensities appearing in low-energy electron diffraction (LEED) from crystalline surfaces. This opens holographic LEED to the wide field of ordered systems giving access to rather complex surface structures

Direct reconstruction of three-dimensional atomic adsorption sites by holographic LEED

We report on the application to measured data of an algorithm for holographic low-energy electron diffraction (LEED), which overcomes the two most important limitations of the technique to date: the ‘‘searchlight’’ effect, which tends to highlight only atoms forward scattered by the adsorbates, and the distorting effects on diffuse LEED intensities due to possible long-range order among the adsorbates. The only experimental input required is a set of the most reliably measured diffuse LEED patterns from normally incident electrons. The algorithm is applied to a set of 11 measured diffraction patterns from a K/Ni(001) surface. A fully three-dimensional image is reconstructed from these data by compensating for the anisotropy of the reference wave by an appropriate scattered-wave kernel. © 1996 The American Physical Society

Influence of the data base and algorithmic parameters on the image quality in holographic diffuse LEED

A detailed examination of the influence of different parameters used in diffuse low-energy-electron-diffraction holography is presented. Effects due to the finite and discrete character of the data that are input to the method's reconstruction algorithm are investigated, as well as the influence of additional parameters introduced both for the removal of experimental Bragg-spots and for the recently proposed correction for the anisotropy of the reference wave. As a test case, we applied the reconstruction algorithm to simulated data of the disordered adsorption system O/Ni(001). Guidelines for the width and resolution of the data base to be used, as well as for the proper selection of algorithmic parameters are deduced, which should hold also for other and unknown systems of disordered atomic adsorption. Using a parameter configuration optimized according to our results, a well-resolved and fully three-dimensional image of the local adsorption geometry is reconstructed from the data

Novel Reconstruction Mechanism for Dangling-Bond Minimization: Combined Method Surface Structure Determination of SiC(111)-(3×3)

The SiC(111)−(3×3) phase was analyzed by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) holography, density functional theory (DFT), and conventional LEED. A single adatom per unit cell found in STM acts as a beam splitter for the holographic inversion of discrete LEED spot intensities. The resulting 3D image guides the detailed analyses by LEED and DFT which find a Si tetramer on a twisted Si adlayer with cloverlike rings. This twist model with one dangling bond left per unit cell represents a novel (n×n)-reconstruction mechanism of group-IV (111) surfaces

Approaching the low-temperature limit in nucleation and two-dimensional growth of fcc (100) metal films Ag/Ag(100)

We analyze the formation of two-dimensional Ag islands following deposition of about 0.1 ML of Ag on Ag(100) over a temperature regime ranging from classical nucleation and growth behavior to almost immobile adatoms, from 300 to 125 K. Particular emphasis is placed on the post-deposition dynamics at the lower end of the temperature range, where the saturation island density is not reached at the end of the deposition, and nucleation and aggregation processes continue with adatoms from the remaining adatom gas. Our analysis combines VT scanning tunneling microscopy experiments with kinetic Monte Carlo simulation of appropriate atomistic models. The only adjustable parameters in the model are the terrace diffusion barrier and prefactor, which can be determined from island density behavior near room temperature. Other processes such as rapid edge diffusion, and “easy” nucleation and aggregation of diagonally adjacent adatoms, are treated as instantaneous. The model excellently reproduces all aspects of behavior at low temperatures, demonstrating that nucleation and growth processes can be described in one consistent scheme, down to the regime of almost immobile adatoms