The electronic origin of contrast reversal in bias-dependent STM images of nanolines J.M. MacLeod a , R.H. Miwa b , G.P. Srivastava c , A.B. McLean a, * a Department of Physics, Queen’s University, Kingston, Ont., Canada K7L 3N6 b Faculdade de Fı ´sica, Universidade Federal de Uberla ˆndia, C.P. 593, 38400-902 Uberla ˆndia, MG, Brazil c School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK Received 10 August 2004; accepted for publication 19 November 2004 Available online 24 December 2004 Abstract Self-organized Bi lines that are only 1.5 nm wide can be grown without kinks or breaks on Si(001) surfaces to lengths of up to 500 nm. Constant-current topographical images of the lines, obtained with the scanning tunneling microscope, have a striking bias dependence. Although the lines appear darker than the Si terraces at biases below j1.2j V, the contrast reverses at biases above j1.5j V.Betweenthesetworangesthelinesandterracesareofcompa- rable brightness. It has been suggested that this bias dependence may be due to the presence of a semiconductor-like energy gap within the line. Using ab initio calculations it is demonstrated that the energy gap is too small to explain the experimentally observed bias dependence. Consequently, at this time, there is no compelling explanation for this phenomenon. An alternative explanation is proposed that arises naturally from calculations of the tunneling current, using the Tersoff–Hamann approximation, and an examination of the electronic structure of the line. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Silicon; Bismuth; Density functional calculations; Scanning tunneling microscopy; Growth; Low index single crystal surfaces; Self-assembly; Surface electronic phenomena 1. Introduction RecentstudiesofBigrowthonSi(001)havere- vealed that, if the substrate is heated to above the Bi desorption temperature (T d  500 °C), Bi lines assembleinparallelarraysthathaveaperiodicline spacing(Fig.1).Thelinespossessanexceptionally high degree of structural order and they can be 0039-6028/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.11.045 * Corresponding author. Tel.: +1 613 533 2709; fax: +1 613 533 6463. E-mail address: mclean@physics.queensu.ca (A.B. McLean). Surface Science 576 (2005) 116–122 www.elsevier.com/locate/susc