Tunneling Spectroscopy of short atomic chains.
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G7.7.1
Tunneling Spectroscopy of short atomic chains. Jeremy Stein, Fredy R. Zypman1 Yeshiva University, Department of Physics, 2495 Amsterdam Ave., New York, NY, USA
ABSTRACT In this paper we present results on transmission-energy and current-voltage curves for a Scanning Tunneling Microscopy probe in the presence of an atomic chain, operating in spectroscopy mode. We compare the results with independently calculated density of sates. Finally, we propose algorithms to process this experimental information to obtain chemical specificity and position of impurities in the chain. INTRODUCTION With the current interest in nanotechnology, it has been necessary to develop wires of nanometric dimensions. Although the techniques of producing and characterizing nanowires are far from being completely developed, much advance has been obtained in the last three years. It is now possible to produce doped carbon nanotubes and measure their electronic and elastic properties with STM and AFM. Nevertheless, the exact positioning of such nanotubes on a substrate has not been dominated thus far. In addition, very recently it has been possible to create metallic chains down to seven-atom long [1]. This has two implications. Experimentally it is necessary to have models that allow converting measured data into microscopic information as, for example, processing a current-voltage curve to obtain electronic density of states (DOS). From a theoretical standpoint, we have a new framework to test extant theories and to further develop them. Within that context, we present a new theoretical solution to the problem of detecting sample properties from current-voltage curves. Our model is based on a self-consistent solution to the quantum problem of electrons in the presence of an array of attractive centers. The problem, being simpler than its three dimensional counterparts is solved exactly for DOS and for the current-voltage curves. We have shown in the past [2] that the current-voltage peaks at the position of the 1
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energies of the electron in the chain. In this paper, we extend the results to show that the current-voltage curves are sensitive to subtle changes in chemical composition of atomic chains. The main open problem in scanning tunneling spectroscopy (STS) has been the inability to create clear cut, general rules, between sought physical quantities and measured ones. All the efforts have concentrated on finding relationships between the density of states (DOS) or the local density of states (LDOS) and current-voltage (I-V) or conductance-voltage (-V) curves [3,4,5,6,7,8,9,10,11]. Experiments have been carried out on semiconductors [12,13,14], superconductors [15], and metals [16]. Arguments have been given [17,18] that support a correlation between DOS and -V curves for semiconductor samples. However, a full theoretical description of such program, encompassing any sample, has not been yet developed. On the other hand, general qualitative agreements for particular kinds of materials have emerged, a
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