154 research outputs found
Ab initio study of Pt induced nanowires on Ge(001)
No full textThe aim of this thesis: “Ab Initio Study of Pt Induced Nanowires on Ge(001)”, is to model the experimentally observed ‘Pt nanowires’ on Ge(001). These one-atom-thick wires can be hundreds of nanometers long while remaining defect and kink free, providing the ultimate wire any chip designer dreams of. However, experiments show the wires not to be conducting; on the contrary, one-dimensional states are discovered between the wires. To model these nanowires, we combine state of the art density functional calculations with calculated scanning tunneling microscope (STM) images. First, the beta-terrace substrate is modeled, showing a checkerboard pattern of Pt-Ge and Ge-Ge surface dimers in a Ge(001)-reconstructed surface. Starting from this substrate model, different models with increasing Pt density are developed in an iterative fashion showing increasing agreement with the experimentally observed nanowires. We show that, contrary to previous assumptions, the observed wires are not Pt atoms but Ge atoms, explaining the lacking conductivity. The germanium nanowires consist of Ge dimers located in a Pt-lined trough. In addition, the 4x1 periodicity observed in the nanowire-arrays is traced back to the bonds of the Ge nanowire dimers to an extra Pt atom at the bottom of the trough, resulting in the buckling of the nanowires dimers. In the last part of the thesis we investigate the adsorption of CO on the Ge nanowires under study. The observed adsorption of CO seems to contradict our proposed model due to the high sticking probability of CO on Pt, where it is low on Ge. We show that no contradiction exists. The CO molecules bind to the Pt atoms in the surface, but because they are tilted toward the nanowires, the resulting STM images give the impression that they are located on top of the nanowire giving rise to the apparent contradiction. In this last study, we also discover a very stable CO adsorption configuration in which the CO molecules remain invisible for STM, but could allow for the formation of one-dimensional molecular chains. This would open the door to one-dimensional molecular electronics
1D metallic states at 2D transition metal dichalcogenide semiconductor heterojunctions
No full textTwo-dimensional (2D) lateral heterojunctions between different transition metal dichalcogenides (TMDCs) have been realized in recent years. Homogeneous semiconducting TMDC layers are characterized by a topological invariant, their in-plane electric polarization. It suggests the possibility of one-dimensional (1D) metallic states at heterojunctions where the value of the invariant changes. We study such lateral 2D TMDC junctions by means of first-principles calculations and show that 1D metallic states emerge even in cases where the different materials are joined epitaxially. We find that the metallicity does not depend on structural details, but, as the invariant is protected by spatial symmetry only, it can be upset by breaking the symmetry. Indeed, 1D charge- and spin-density wave instabilities appear spontaneously, making 2D TMDC heterojunctions ideal systems for studying 1D systems
The formation of self-assembled nanowire arrays on Ge(001) : a DFT study of Pt induced nanowire arrays
No full textNanowire (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 Pt-lined troughs of the underlying surface, contrary to what was assumed previously in experiments
Spin/charge density waves at the boundaries of transition metal dichalcogenides
No full textOne-dimensional grain boundaries of two-dimensional semiconducting MX2 (M=Mo,W;X=S,Se) transition metal dichalcogenides are typically metallic at room temperature. The metallicity has its origin in the lattice polarization, which for these lattices with D3h symmetry is a topological invariant, and leads to one-dimensional boundary states inside the band gap. For boundaries perpendicular to the polarization direction, these states are necessarily 1/3 occupied by electrons or holes, making them susceptible to a metal-insulator transition that triples the translation period. Using density-functional-theory calculations, we demonstrate the emergence of combined one-dimensional spin density/charge density waves of that period at the boundary, opening up a small band gap of ∼0.1eV. This unique electronic structure allows for soliton excitations at the boundary that carry a fractional charge of ±1/3e
Edge reconstruction of 2D-Xene (X = Si, Ge, Sn) zigzag nanoribbons
No full textBy performing first principles calculations, we investigate the edge reconstruction in free-standing 2D-Xene (X = Si, Ge, Sn) zigzag nanoribbons. Three different periodicities of edge reconstruction (2a,3a,4a) are found, in which the reconstruction with 3a periodicity has the lowest energy and shows non-magnetic ground state. The edge reconstruction can be understood by the reconfiguration of the dangling bond states and edge states at the zigzag edges. Due to the structural buckling, extra bonding states are formed between edge atoms and inner atoms, accompanied with charge transfer from the edge states to the dangling bond states. This results in one-third occupied dangling bond states and a Peierls-like structural reconstruction with 3a periodicity at the edge which opens a small band gap. With a tight binding model, the reconstruction of the electronic structures at the edges are revealed by the hopping integrals between different edge X-p orbitals.</p
Ohmic Contacts to 2D Semiconductors through van der Waals Bonding
No full textHigh contact resistances have blocked the progress of devices based on MX2 (M = Mo, W; X = S, Se, Te) 2D semiconductors. Interface states formed at MX2/metal contacts pin the Fermi level, leading to sizable Schottky barriers for p-type contacts in particular. It is shown that i) one can remove the interface states by covering the metal by a 2D layer, which is van der Waals-bonded to the MX2 layer, and ii) one can choose the buffer layer such that it yields a p-type contact with a zero Schottky barrier height. Possible buffer layers are graphene, a monolayer of h-BN, or an oxide layer with a high electron affinity, such as MoO3. The most elegant solution is a metallic M′ X′2 layer with a high work function. A NbS2 monolayer adsorbed on a metal yields a high work function contact, irrespective of the metal, which gives a barrierless contact to all MX2 layers
Surface dipoles and work functions of alkylthiolates and fluorinated alkylthiolates on Au(111)
No full textWe study the dipole formation at the surface formed by −CH3 and −CF3 terminated short-chain alkylthiolate monolayers on Au(111). In particular, we monitor the change in work function upon chemisorption using density functional theory calculations. We separate the surface dipole into two contributions, resulting from the gold−adsorbate interaction and the intrinsic dipole of the adsorbate layer, respectively. The two contributions turn out to be approximately additive. Adsorbate dipoles are defined by calculating dipole densities of free-standing molecular monolayers. The gold−adsorbate interaction is, to a good degree, determined by the Au−S bond only. This bond is nearly apolar and its contribution to the surface dipole is relatively small. The surface dipole of the self-assembled monolayer is then dominated by the intrinsic dipole of the thiolate molecules. Alkylthiolates increase the work function of Au(111), whereas fluorinated alkylthiolates decrease it
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