Effect of Irradiation Dose in Making an Insulator from a Self-Assembled Monolayer Yian Tai, Andrey Shaporenko, Michael Grunze, ² and Michael Zharnikov* Angewandte Physikalische Chemie, UniVersita ¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ReceiVed: June 21, 2005; In Final Form: August 25, 2005 A combination of functionalization and irradiation-induced cross-linking allows fabrication of stable metal film on top of an aromatic self-assembled monolayer, [1,1′;4′,1′′-terphenyl]-4,4′′-dimethanethiol (TPDMT) on Au. Using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and ion-scattering spectroscopy the optimal irradiation dose for producing a stable metal overlayer was estimated to be 40-45 mC/cm 2 . This dose is necessary for complete 2D-polymerization and closure of transient channels, which would otherwise allow metal penetration into the SAM. What is also important, the majority of the thiol tail groups, responsible for 2D growth and chemical adherence of the metal film, remains intact even at this high dose. The optimal dose corresponds to a crossover in the response of the TPDMT film to ionizing radiation: the irradiation-induced processes progress fast at lower doses and saturate at higher doses. 1. Introduction The fabrication of thin metallic films and nanostructures (e.g. nanowires) on self-assembled monolayers (SAMs) is an im- portant scientific and technological goal, e.g. for the preparation of top electrodes in molecular electronic devices and for the fabrication of monomolecular insulator layers to provide an alternative to commonly used oxide dielectrics in future electronic and spintronic devices (see e.g. refs 1 and 2). However, the formation of stable metal film on the SAM surface (i.e. at the SAM-ambient interface) is a nontrivial task, since the metal adsorbates are not stable on the top of a SAM but penetrate into the monomolecular layers and diffuse to the SAM-substrate interface. 3-5 Diffusion occurs via structural defects in the film and is enhanced by formation of additional transient channels via dynamic hopping of the SAM constituents across the substrate. 6,7 This process can, however, be partly suppressed by improving the SAM quality and by the introduc- tion of chemically reactive tail groups, which bind the adsorbate atoms at the SAM-ambient interface and provide nucleation centers for the growth of the metal overlayer. Following this strategy, partial or temporary stabilization of some metal adsorbates, such as Ti, Cr, Pd, and Al, on the SAM surface has been achieved using HOOC-, CO 2 CH 3 -, CH 2 OH-, thiophene-, pyridine-, and SH-terminated films. 3,6-11 Recently, we suggested a new approach to form a stable metal layer on the top of a SAM, combining functionalization of the monomolecular film with its 2D-polymerization by electron irradiation. With nickel as a test metal adsorbate we succeeded to form a stable metal film on the surface of [1,1′;4′,1′′- terphenyl]-4,4′′-dimethanethiol (TPDMT) SAM on Au 12 and showed that the polymerized TPDMT film in the metal/SAM/ substrate sandwich has good insulating properties. 12,13 Note that this particular SAM substrate was specially designed for metal evaporation experiments. It possesses a reactive functional group, high packing density, high orientational order, and high resistance to ionizing radiation. 14,15 The degree of 2D-polymerization is of major importance to stabilize the metal film on top of the TPDMT SAM. Here we analyze the properties and performance of the TPDMT films as a diffusion barrier as a function of irradiation dose. For this purpose, we applied X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectros- copy, and ion-scattering spectroscopy (ISS). In the following section our experimental procedure will be described. The results are presented in section 3, followed by a discussion and a summary in sections 4 and 5, respectively. 2. Experimental Section The synthesis of TPDMT is described elsewhere. 14 The gold substrates were prepared by thermal evaporation of 100 nm of gold onto Si(100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer. In some cases, 20 nm of Au and 9 nm of Ti were used, which resulted in a smother gold film. The gold films are polycrystalline, with a grain size of 20-50 nm. The grains predominantly exhibit an (111) orientation. The SAMs were prepared by immersion of the freshly prepared substrates into a 1 mM solution of TPDMT in THF at 55 °C for 24 h. After immersion, the samples were carefully rinsed with pure solvent and blown dry with argon. No evidence for impurities or oxidative degradation products was found. The irradiation of the TPDMT films was performed with low- energy electrons (10 eV). The current density was ≈2.5 μA/ cm 2 . The dose was calibrated by a Faraday cup. Ni (Goodfellow, 99.999% purity) was evaporated from a commercial e-beam evaporator (Omicron) with a deposition rate of 0.2 nm/min; the rate was calibrated by a commercial quartz crystal microbalance sensor (Inficon). The Ni coverage of the SAM samples (θ Ni ) was estimated by the multiplication of the calibrated deposition rate by the evaporation time and related to the Au(111) surface. The sticking coefficient of Ni was assumed to be 1 for both quartz sensor and SAM samples. The evaporation was per- formed at a base pressure better than 1.5 × 10 -9 Torr; the substrates were kept at room temperature. * To whom correspondence should be addressed. E-mail: Michael.Zharnikov@urz.uni-heidelberg.de. ² Second address: Institute for Molecular Biophysics, University of Maine, Orono, ME. 19411 J. Phys. Chem. B 2005, 109, 19411-19415 10.1021/jp053340l CCC: $30.25 © 2005 American Chemical Society Published on Web 09/23/2005