Appl. Phys. A 59, 135-138 (1994) Applied so.,.s PhysicsA an. Surfaces © Springer-Verlag 1994 Selective imaging of self-assembled monolayers by tunneling microscopy J. P. Bucher, L. Santesson*, K. Kern Institut de Physique Exp6rimentale, EPF Lausanne, CH-1015 Lausanne, Switzerland (Fax: + 41-21/693-3604) Received 18 January 1994/Accepted 24 March 1994 Abstract. The properties of thin organic films offer many challenging opportunities for science and technology. A crucial requirement for the advancement of molecular film technology is the selective characterization and mod- ification on an atomic level. Local proximal probes like Scanning Tunneling Microscopy (STM) or Atomic Force Microscopy (AFM) bear certainly the potential for this purpose. So far, however, mainly adsorbed organic mole- cules lying flat on a smooth substrate have been imaged with near atomic resolution. Here, we demonstrate the ability of STM to selectively image self-assembled mono- layers of long-chain molecules (hexanethiol) oriented upright on the substrate Au(111) with molecular resolu- tion. Upon proper choice of the tunneling parameters we can image the molecular head-group anchored at the substrate and/or the molecular tail group. PACS: 68.55,61.16.Ch A particular promising method for the controlled forma- tion of thin organic films is the self-assembly technique [1]. Compared to the sophisticated Lagmuir-Blodgett technique [2], the self-assembly process is characterized by its ease of operation and the non-restricted sample geometry. In particular the co-functionalized long-chain alkanethiols have been found to form highly ordered and stable Self-Assembled Monolayers (SAMs) on noble- metal substrates [3]. While the sulfur head-group assures, via a strong covalent bonding to the substrate, high mechanical and thermal stability, the tail-group serves for functionalization purposes to control the interfacial properties of the monolayer. On Au(111) numerous stud- ies have revealed the characteristic brush-like ordering (Fig. la) of the alkanethiols [3]. The alkyl chains are ordered in a high-density crystal-like packing with the sulfur head group adopting a (~ x ~)R30 structure * Present address: Institut de Physique Appliqu6e, Universit6 de Genbve, CH-1211 Gen~ve, Switzerland (Fig. lb). The chains are in all-trans conformation with an average tilt of ~ ~ 30° of the molecular axes from the surface normal. Recent structural studies of the self-assembled al- kanethiols did, however, reveal the presence of a high density of defects in the monolayer [4-6]. In thermal helium- and X-ray-diffraction a substantial diffraction peak broadening was noticed indicating a coherence length of less than 100~ [4]. This behavior was associated with the pressence of domain boundaries separating neighboring anti-phase ~ domains. In addition, STM [5] and AFM [6] images revealed the presence of small de- pressions (1-3 nm in diameter) randomly distributed in the molecular film. The nature of the holes was a matter of controversial discussion. The explanations which have been put forward include electronic artifacts of the STM [6], the accumulation of"gauche" defects within the thiol layer [7] and defects in the topmost gold substrate layer [5]. The substrate origin of the defects has recently con- vincingly been demonstrated by studying their thermal evolution with STM [8, 9]. The substrate holes are formed during the self-assembly process via chemical erosion of gold atoms and are confined to the first substrate layer. In our in-situ STM [8] study the substrate holes have been found to migrate and coalesce to large vacancy islands upon thermal annealing. These vacancy islands, several 10 nm in size, exhibit a characteristic quasi-triangular equilibrium shape, characteristic for vacancy islands within a fcc(lll) surface [8, 10]. In Fig. lc, d we demonstrate that the thermal mass transport can be used to order the hexanethiol/gold interface. Upon extensive annealing at 350 K the vacancy islands diffuse towards preexisting substrate steps where they are annihilated leaving perfectly flat depression-free SAMs. The STM experiments shown in Fig. 1 and reported below have been done with a home-built "beetle-type" microscope. Due to its inherent thermal self-compensa- tion, this microscope is particularly suited vor variable temperature studies. For UHV applications we have re- cently developed a system for a sample temperature