Growth of aligned Mo 6 S 6 nanowires on Cu(111) Duy Le a , Dezheng Sun b, c , Wenhao Lu b , Maral Aminpour a , Chen Wang b , Quan Ma b , Talat S. Rahman a, ⁎, Ludwig Bartels b, ⁎ a Department of Physics, University of Central Florida, Orlando, FL 32816, USA b Pierce Hall, University of California, Riverside, CA 92521, USA c Department of Physics, Columbia University, New York, NY 10027, USA abstract article info Article history: Received 24 November 2012 Accepted 31 December 2012 Available online 9 January 2013 Keywords: Density functional calculations Scanning tunneling microscopy Molybdenum disulfide Cu(111) Nanowires Based on density functional theory (DFT) predictions and scanning tunneling microscopy (STM) measure- ments we report the possibility of using the Cu(111) surface for growth of molybdenum sulfide nanowires (Mo 6 S 6 ). Strong substrate interactions coupled with small lattice mismatch lead to epitaxial growth of the nanowires parallel to a set of substrate high symmetry directions. We observe a propensity for creation of aligned and equally spaced arrays of nanowires and use DFT to elucidate interaction strength both in the ab- sence and presence of the substrate. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Molybdenum-sulfide compounds have recently attracted consid- erable attention for a broad range of applications beyond MoS 2 's established function as a lubricant and as a hydrodesulfurization cat- alyst. Mechanical exfoliation allows access to single layer and few layer MoS 2 films which have properties that are complementary to graphene [1,2] including a native band gap [3–6] and valleytronic properties [7–11]. Beyond its catalytic use in controlling the sulfur contents of petrochemical products, its ability to catalyze the forma- tion of alcohols from syngas is also being investigated for production of fuels from biomass. In this context we recently explored the bind- ing of oxygenate species on a new Mo 2 S 3 monolayer phase [12]. Work on hybrid compounds such as MoO 3 /MoS 2 with a core–shell structure shows promise for electrocatalytic application in hydrogen evolution [13–16]. As is the case with carbon, compounds of molybdenum and sulfur may assume a number of nanoscale forms such as nanotubes [17], nanorods [18] and nanoparticles [19,20]. Here we describe the forma- tion of one-dimensional (1D) Mo 6 S 6 features, similar to nanowires reported earlier by Kibsgaard et al. [21] with the difference that our structures are not bundled but well separated from each other. These Mo 6 S 6 nanowires display 60°-rotated AB stacking of Mo 3 S 3 building blocks consisting of a triangle of Mo atoms decorated with three sulfur atoms at the perimeter (Fig. 1a,b). Kibsgaard and coworkers [21] grew molybdenum–sulfur nanowire bundles on highly ordered pyrolytic graphite (HOPG); they suggest that their bundles consist of several Mo 6 S 6 units side by side. The wires were found to grow in a disordered fashion: substrate interac- tions appear to be insufficiently strong to align them with the HOPG crystallographic directions. Here we show, theoretically and experi- mentally, that the use of a more interactive substrate, Cu(111), permits the growth of molybdenum sulfide nanowires that consist of a single stack of Mo 6 S 6 units only and that are aligned with the substrate direc- tions. Moreover, we find the preferred spacing between adjacent Mo 6 S 6 wires to be slightly larger than their van-der-Waals (vdW) separation distance in the gas phase, highlighting the importance of substrate in- teractions and suggesting the latter's ability to both align and space the wires evenly. Following Ref. [21], we refer to the features as nanowires even though they are found in the presence of an interactive substrate. 2. Methods Density functional theory (DFT) simulations are carried out using the projector-augmented wave (PAW) method [22] and a plane-wave basis set which are both implemented in the Vienna Ab Initio Simula- tion Package [23]. In order to take into account vdW interactions, which are expected to play an important role in the attraction between Mo 6 S 6 nanowires, we use the optB88-vdW version [24] of the van der Waals density functional (vdW-DF) [25,26] to describe the exchange– correlation interaction of the electrons, as implemented in the code of Ref. [27]. The Román-Pérez and Soler algorithm [28] is used for speed- ing up the evaluation of the non-local energy. The pseudo-potentials Surface Science 611 (2013) 1–4 ⁎ Corresponding authors. E-mail addresses: talat.rahman@ucf.edu (T.S. Rahman), bartels@ucr.edu (L. Bartels). 0039-6028/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.susc.2012.12.016 Contents lists available at SciVerse ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc