One-dimensional organic nanostructures: A novel approach based on the selective adsorption of organic molecules on silicon nanowires

Scanning tunneling microscopy (STM) is used to investigate the formation of one-dimensional organic nanostructures by chemisorption of specific molecules on silicon nanowires. STM data demonstrate that depending on the molecular functional groups, the molecules adsorb either randomly on the substrate or preferentially on the nanowires. In the latter case, chemisorption of suitable organic molecules on the nanowires leads to a well-defined one-dimensional aggregation and changes the metallic character of the nanowires to a semi-conducting one. 2008 Elsevier B.V. All rights reserved. One-dimensional (1D) metal, semiconductor and insulator structures have attracted a great deal of interest from the scientific community because of their potential for nanotechnology and the opportunity they provide to understand the fundamentals of the physics of low-dimensional systems. Fabricating these structures with controlled pattern and dimensions remains experimentally challenging for inorganic materials, and even more so for organic ones. One method for growing 1D organic structures consists of adsorbing organic molecules on a template surface presenting a well-defined 1D-pattern. In that regard, it has been recently demonstrated that 16 Å wide silicon nanowires (NWs), heretofore referred to as SiNWs, can be grown by adsorbing silicon atoms onto a single crystal of silver oriented with a (110) surface (Ag(110)) [1,2]. This system has been thoroughly studied by means of scanning tunneling microscopy (STM), low energy electron diffraction and photoelectron spectroscopies (PESs) [1–3]. These SiNWs were shown to form massively parallel assemblies with a density of states consistent with quantized states dispersing along the NWs. This system has also been theoretically investigated by He, who proposed an arrangement of the Si atoms on the surface [4]. In the present work, we use this template of SiNWs and investigate the ability to form organic 1D-structures by exposing it to two different molecules, i.e., tris{2,5-bis(3,5-bis-trifluoromethyl-phenyhl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenylene (THAP) and 9,10-phenanthrenequinone (PQ). The results are a ll rights reserved. : +1 609 258 6279. on). non-selective adsorption of THAP across the SiNWs/Ag(110) sample, but a clearly selective adsorption of PQ on the SiNWs, leading to the formation of 1D organic structures. In addition, the electronic structure of the NWs undergoes a significant transition from metallic to semi-conducting upon adsorption of 4 L of PQ. STM and scanning tunneling spectroscopy (STS) studies of the adsorption of PQ and THAP on the SiNWs were performed in a series of interconnected ultra-high vacuum (UHV) chambers (base pressure p 5 10 11 Torr) equipped with sputtering and annealing facilities, a Si evaporator for SiNWs formation, organic molecules sublimation stations and a room-temperature Omicron STM. STM images were recorded in constant current topographic mode and processed with the WSxM software [5]. We present the STS results as both the current–voltage I(V) and the normalized differential conductance (dI/dV)/(I/V) curves, the latter serving as a good approximation of the local density of states (LDOSs) [6–8]. A single crystal Ag(110) purchased from Mateck was prepared by several cycles of Ar-ion sputtering (500 eV) and annealing (690 K). This procedure produced an atomically clean surface with 80 nm wide terraces exhibiting a rectangular (1 1) Ag unit cell. The NWs were obtained by deposition of Si atoms on the clean Ag(110) surface at room-temperature at a typical rate of 0.7 Å/ min. This process led to the formation of a 1D-template surface consisting of SiNWs oriented along the 1⁄2 110 direction of the Ag(110) surface. The THAP molecules, synthesized and purified by the group of Marder [9], were deposited on the SiNWs/ Ag(110) surface by thermal evaporation ( 480 K) from a quartz crucible at a rate of 0.5 Å/min. Deposition rate and thickness were estimated using a calibrated quartz crystal microbalance. The Fig. 1. (a) Chemical structure of the THAP molecule and R stands for a trifluoromethyl group (–CF3). (b) Chemical structure of the PQ molecule. L80 E. Salomon, A. Kahn / Surface Science 602 (2008) L79–L83 S U RF A CE SC IE N CE