Mechanical, Electrical and Chemical Properties of Metal Nanowires

Abstract

The intensive research in the field of nanotechnology in its theoretical, experimental and industrial branches has led, in the last two decades, to impressive advances in the control and design of electronic devices at the nanoscale limit. In particular, the mechanical, electrical and chemical properties of metallic nanowires have received a lot of attention. The gentle control of the distance between two metals, using a scanning tunneling microscope (STM) or a mechanically controlled breaking junction (MCBJ) [10], has allowed the experimental characterization of atomic contacts and nanowires, as well as the observation of quantum effects in both their conductance and their forces. On the other hand, nanowires are a good example of systems whose small size enhances their reactivity with molecules considerably. For example, gold surfaces are chemically inert and are regarded as poor catalysts at variance with other metal surfaces; however, Au is considered an exceptional catalyst when prepared as nanoparticles on a variety of support materials. This is also the case for Au nanowires and, in this paper we also discuss the chemical properties of these systems. It is also of interest to stress that the adsorption of molecules like H2 on Au-nanowires seems to introduce important changes in the gold conductance, an issue that we discuss in more detail later on. The formation of necks and atomic contacts in stretched metallic nanowires has been analyzed theoretically using different approaches. Classical or effective-medium theory potentials are an effective way of studying the nanowire deformation. However, first-principles calculations based on plane waves-density functional theory (DFT) provide a very accurate description of the mechanical properties and the electronic structure needed for the calculation of the conductance, but the large computational demands restricts most of the applications to an analysis of the model geometries for the contact. Leaving apart these very demanding, computationally, first-principles calculations, one can resort to local orbital DFT-methods, especially those devised with the aim of computational efficiency, which allow first-principles studies of much more complex systems at the prize of reducing somehow the accuracy of the calculations. This formulation in terms of local orbitals has an added value, as the transport properties can be appropriately calculated from the resulting Kohn-Sham tight-binding electronic Hamiltonian, as discussed below. Thus, efficient local-orbital DFT methods are probably the best available tools for a first-principles analysis of complex nanowires; this is the point of view taken in our group and, in this paper, we present a short overview of the main results obtained for some nanowires using this approach. In section II, we discuss our methods for analyzing the structural, mechanical and chemical properties of metal nanowires; in section III we present some results for Al, a typical normal metal, and in section IV we discuss the case of Au, a case presenting long nanowires with interesting chemical properties.