The Formation of Self-Assembled Nanowire Arrays on Ge(001): a DFT Study of Pt Induced Nanowire Arrays
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The Formation of Self-Assembled Nanowire Arrays on Ge(001): a DFT Study of Pt Induced Nanowire Arrays Danny E. P. Vanpoucke1 and Geert Brocks1 1 Computational Materials Science, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ABSTRACT Nanowire (NW) arrays form spontaneously after high temperature annealing of a sub monolayer deposition of Pt on a Ge(001) surface. These NWs are a single atom wide, with a length limited only by the underlying beta-terrace to which they are uniquely connected. Using ab-initio density functional theory (DFT) calculations we study possible geometries of the NWs and substrate. Direct comparison to experiment is made via calculated scanning tunneling microscope (STM) images. Based on these images, geometries for the beta-terrace and the NWs are identified, and a formation path for the nanowires as function of increasing local Pt density is presented. We show the beta-terrace to be a dimer row surface reconstruction with a checkerboard pattern of Ge-Ge and Pt-Ge dimers. Most remarkably, comparison of calculated to experimental STM images shows the NWs to consist of germanium atoms embedded in the Ptlined troughs of the underlying surface, contrary to what was assumed previously in experiments. INTRODUCTION Microelectronics industry is driven by the ever further miniaturization of electronic components and devices. Monatomic nanowires present the physical limits of miniaturization for wires, and much research interest is directed toward the construction of wires closer to this physical limit and understanding the associated physical phenomena. In 2003, Gürlü et al. [1] observed self-assembled nanowire arrays after deposition of 0.25 monolayer (ML) of Pt on a clean Ge(001) surface and subsequent high temperature annealing. The NWs are equally spaced at 1.6 nm, virtually defect and kink free, and only a single atom wide while being hundreds of nanometers long. The NWs, dubbed Pt-nanowires, turn out not to be conductive. Instead the regions between the NWs show conductive behavior [2]. Using ab-initio DFT calculations we study possible geometries for the beta-terrace and the NW arrays. Direct comparison to experiment was made using calculated STM images, allowing for the identification of the geometry of the experimentally observed structures. THEORETICAL METHOD DFT calculations are performed using the projector augmented wave (PAW) method and local density approximation (LDA) functional as implemented in the Vienna Ab-initio Simulation Package (VASP) [3-6]. A plane wave basis set with kinetic energy cutoff of 345 eV is applied and the surface Brillouin zone is sampled using an 8x4x1 Monkhorst-Pack special kpoint mesh. Symmetric slabs of 12 layers, separated by 1.55 nm of vacuum, are used with reconstructions on both surfaces. The geometries are optimized using the conjugate gradient method. The center two layers of the slab are kept fixed to represent bulk conditions.
STM images are calculated using the Tersoff-Hamann method [7], which states that the tunne
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