Small polaron formation in dangling - bond wires on the Si ( 001 ) surface

From electronic structure calculations, we find that carriers injected into dangling-bond atomic wires on the Si(001) surface will self-trap to form lo-calised polaron states. The self-trapping distortion takes the form of a local suppression of the buckling of the dimers in the wire, and is qualitatively different for the electron and hole polarons. This result points to the importance of polaronic effects in understanding electronic motion in such nanostructures. 1 Developments in nanotechnology are placing increasing demands on our understanding of electron transport in confined systems; this applies particularly to coherent quantum transport and tunnelling phenomena. Among the most promising one-dimensional structures that have been produced by nanolithography are 'dangling-bond wires', formed by the selective removal of H atoms from the H-saturated Si(001) surface with a scanning tunnelling microscope [1,2]. In this letter we show that carriers injected into these dangling-bond wires self-localise to form polarons; this process is crucial in determining the transport properties of the wires. There is good experimental evidence that a Peierls distortion occurs at low teperatures in a dangling-bond wire [4], although there is still debate about the extent of the underlying structural distortion [2,4]; this distortion removes the one-dimensional metallic density of states and produces a narrow-gap one-dimensional semiconductor [3]. In systems with Peierls distortions, charge transport is dominated by polaron species [5,6]; this is true both when charge is permanently introduced into the wire by doping, and also when the charge enters the chain only transiently, as part of a quantum mechanical tunnelling event [7]. However , recent calculations of the transport properties of dangling-bond wires do not include polaronic effects [8]. It is therefore of great importance to determine whether such polarons are, in fact, formed in this system, where the electrons in the wire are strongly coupled to a three-dimensional substrate and so a purely one-dimensional picture is not applicable. In this letter we provide clear theoretical evidence that polarons will form when holes (or, as discussed later, electrons) are injected into a dangling-bond wire. We calculate the binding energy, atomic distortion, and wavefunction localisation of the polarons, and provide realistic estimates of the effective mass and of the effect that polaron formation will have on the coherent transport of charge along the wire. We use three different methods to perform our calculations. The first is density functional theory in the local density approximation (LDA), with a plane-wave basis set and …