University of Groningen NaCl multi-layer islands grown on Au ( 111 )-( 22 x root 3 ) probed by scanning tunneling microscopy

The growth of multi-layer NaCl islands on Au(111)-(22× √ 3) surfaces was investigated using scanning tunneling microscopy (STM). We observed that the aspect of the NaCl islands drastically differs depending on the tunneling conditions. It is therefore possible to observe the layers forming an NaCl island or to image the gold reconstruction below the first NaCl layer. Atomically resolved STM images obtained on the first NaCl layer demonstrate that NaCl grows as an epitaxial crystalline film on Au(111)-(22× √ 3). STM images also suggest that some NaCl layers can be non-crystalline. (Some figures in this article are in colour only in the electronic version) Intense research efforts have been focused on the preparation of oxide surfaces and insulating thin films for potential applications in the field of nanotechnology and nanoelectronics [1]. These materials are promising substrates to support nanostructures when the aim is to decouple their electronic states from those of the support and to reduce charging effects. Insulating thin films have been successfully employed to explore properties of nanostructures: for example, it has been possible to address and probe the electronic states of a single molecule [2] and of a single metal particle [3], to control the charge state of a single adatom [4], to observe the interactions between spins in individual nanostructures [5], and to flip the spin of a single adsorbed atom [6]. These oxide and insulating surfaces are also interesting for technological applications. They have been used to control the growth of fcc metal nanocrystals [7] and five-fold twinned structures [8, 9], and to enhance the catalytic activity of metal nanoparticles [10]. Silica [11, 12], vanadium oxide [13], alumina [2, 12], ceria [14], iron oxide [15, 16], magnesium oxide [17], titania [10], strontium titanate [18–20], and sodium chloride [4, 21–27] are some of the promising materials to support and electronically isolate nanostructures for technological application. Among these materials, sodium chloride is not only very versatile, it also has the great advantage of growing as layers. NaCl islands have indeed been successfully grown on numerous crystalline metal surfaces, such as Cu(111) [4, 21, 22], Cu(110) [4, 21], Cu(311) [21], Ag(111) [23], Ag(100) [24], Ge(100) [25], Al(111) [26], and Al(100) [27]. Bulk sodium chloride is an ionic crystal with cubic symmetry consisting of positively charged sodium ions, Na+, and negatively charged chloride ions, Cl−, figure 1(a). Each of these ions is surrounded by six ions of opposite charge. This crystallographic structure maximizes the contact between ions of opposite charge. The lattice parameter of the NaCl unit cell is 5.64 Å. Sodium atoms donate their single valence electron to chlorine atoms; crystalline NaCl is therefore an insulator at low temperature. The different faces of an ionic crystal have different electronic properties and this may strongly influence the growth and shape of ionic nanocrystals or thin films. In the case of crystalline NaCl, the (100) facet is electrically neutral, whereas the (111) facet would be polar because it consists of only one type of ion, which can be either Na+ or Cl−. 0957-4484/08/495307+05$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1 Nanotechnology 19 (2008) 495307 X Sun et al