AVS2008 Session BO+SS+AS-FrM: Self Assembled Ultrathin Organic Interfaces

Friday, October 24, 2008 8:40 AM in Room 201

Friday Morning

Start	Invited?	Item
8:40 AM		BO+SS+AS-FrM-2 Photo-Patterned SAMs on Au(111) and SiO_x: An XPS Investigation A.M. Track (University of Graz, Austria); A. Lex, T. Höfler, T. Griesser, P. Pacher, H.-G. Flesch (Graz University of Technology, Austria); G. Hlawacek (University of Leoben, Austria); R. Schenach, G. Trimmel (Graz University of Technology, Austria); W. Kern (University of Leoben, Austria); G. Koller, M.G. Ramsey (University of Graz, Austria) Self-assembled monolayers (SAMs) can provide a simple, flexible, highly ordered and convenient system to tailor and functionalize surface and interface properties of metals, metal oxides and semiconductors. For instance a simple way to change the chemical reactivity of the substrates is to use organic photosensitive SAMs which will also allow patterning with UV illumination. We focus on the characterization of the reactions occurring in SAMs and thin interfacial layers using X-ray photoemission spectroscopy (XPS). To increase the contrast between the non-illuminated and illuminated SAMs (or thin films) we perform a selective post modification of the patterned films with organic reagents containing heteroatoms. The chemical differences of the structured film have been followed with spatially resolved XPS. Further chemical and structural information has been obtained with reflection absorption infrared spectroscopy (RAIRS), friction force microscopy (FFM), contact angle measurements and X-ray reflectivity (XRR). Two different types of photo-reactive functional groups have been investigated: a benzyl thiocaynate and a phenylester. First we present thin films of silanes on SiO_x, which bear the photoreactive benzyl thiocyanate (-SCN) unit. This photoisomerizes to the corresponding isothiocyanate (-NCS). The photoisomerization can be detected by a significant shift of the S2s core level. Additionally, we can prove the change of chemical reactivity due to the illumination via a post modification with fluorinated amines introduced in gas phase. Here more fluorine could be detected with the spatially resolved XPS on the illuminated areas of the sample.¹ In the second example the SiO_x substrates are modified with photosensitive silanes of different chain length containing the phenylester group. The phenylester can undergo the Photo-Fries rearrangement upon UV illumination forming a hydroxyphenyl keton. The different chemical reactivity of these two groups is again shown with a selective post modification with, e.g., fluorinated acid chlorides. The corresponding reaction has again been followed by the appearance of the F1s signal only on the illuminated areas of the samples. Finally, results of investigations extended to patterned photo-reactive thiol-SAMs on Au(111) will be discussed. ¹ A. Lex et. al., Chem. Mater. 2008, 20, 2009-2015.
9:00 AM		BO+SS+AS-FrM-3 Odd-Even Efect in Molecular Packing and Stability of ω-Biphenyl-Alkane-Selenol SAMs on Au(111) P. Cyganik, K. Szelagowska-Kunstman (Jagiellonian University, Poland); M. Zharnikov (Universität Heidelberg, Germany); A. Terfort (Marburg University, Germany) To fabricate aromatic self-assembled monolayers (SAMs) of practical importance for molecular electronics and other applications, high level of control over the SAMs properties should be achieved. In particular, besides monitoring the electronic properties, control of structure, i.e., crystallinity and defects, is an issue of equal importance. Our experimental approach to gain a general concept for the rational design of high-quality SAMs includes a systematic modification of the chemical composition of the SAM constituents and detailed investigation of the structure of the resulting films. Following this general idea, we studied the influence of the substitution of the headgroup atom (S versus Se, which binds SAMs constituent to the substrate) on the microscopic structure of hybrid aromatic-aliphatic SAMs formed on Au(111). The talk focuses on STM experiments¹ performed for the homologue series of BPnSe (CH₃-(C₆H₄)₂-(CH₂)_n-Se-, n =2-6) molecules. Obtained data are discussed and analysed in view of the spectroscopic results reported by us recently for these systems²^,³, as well as compared to the previously reported STM data⁴^,⁵ for the corresponding thiol-based analogues, i.e. BPnS/Au(111) . The main conclusions are: (1) higher structural quality of the Se based SAMs, (2) drastic changes in the BPnSe film structure depending on either odd or even value of the parameter n, accompanied by (3) the respective odd-even changes in their thermal stability. ¹ P. Cyganik et al. J. Phys. Chem. C, submited. ² A. Shaporenko et al. 2007 J. Am. Chem. Soc. 129, 2232. ³ T. Weidner et al., J. Phys. Chem. C, submited. ⁴ P. Cyganik et al. 2006 J. Am. Chem. Soc. 128, 13868. ⁵ P. Cyganik et al. 2007 J. Phys. Chem. C. 111, 16909. .
10:20 AM		BO+SS+AS-FrM-7 Self-Assembly of Methanethiol on the Reconstructed Au(111) Surface G. Nenchev, B. Diaconescu (University of New Hampshire); F. Hagelberg (East Tennessee State University); K. Pohl (University of New Hampshire) The molecular self-assembly of alkanethiols (CH₃(CH₂)_n-1SH) on Au(111) surface has been studied extensively in the last 20 years. Despite the abundance of experimental and theoretical data, the true nature of the processes involved in the monolayer formation is still not fully established. We will present a combined UHV VT-STM and DFT study of the adsorption of the simplest alkanethiol, methanethiol (CH₃SH), on the reconstructed Au(111) surface. Our findings challenge the established notion that methanethiol is too short to form ordered structures even at low temperature. At sub-monolayer coverage, dimer chains are resolved on the FCC areas of the reconstruction pattern. At higher coverage the monolayer evolves into two continuous self-assembled phases: a rectangular c(4√3 x 2) phase, which coexists with the substrate reconstruction network, and a close-packed p(√3 x √3)R30° hexagonal phase. Our DFT calculation, which takes the reconstruction of the surface into account, confirms the non-dissociative character of the methanethiol adsorption and derives the bonding geometry of the molecular dimers - a sequence of shifted hollow-top and hollow-bridge bonding positions. The numerical calculation reveals that, in stark contrast to longer alkanethiols, at low temperature the self-assembly process of methanethiol is not driven by Van der Waals forces, but by a surface-mediated interaction. These novel results clearly demonstrate the unique nature of the methanethiol adsorption and self-assembly. This work is supported by the National Science Foundation under Award #0425826 for the Center for High-Rate Nanomanufacturing and under Grant No. DMR-0134933. The computations are performed on the CRAY XT3 machine Sapphire at US Army/Engineer Research and Development Center (ERDC, Vicksburg, MS) in collaboration with Jackson State University, and supported by the DoD through Contract #W912HZ-06-C-005.
10:40 AM		BO+SS+AS-FrM-8 Thermo Scientific Theta Probe : Measuring the Quality of Self-Assembled Monlayers on Gold P. Mack (Thermo Fisher Scientific, UK); D.J. Graham (Asemblon Inc); J. Wolstenholme, R.G. White (Thermo Fisher Scientific, UK) Self assembled monolayers (SAMs) are becoming increasingly important as a means to functionalise surfaces and to control surface properties or reactivity. The attributes of angle resolved X-ray photoelectron spectroscopy (ARXPS), such as surface specificity, chemical selectivity and non-destructive depth profiling, make it the ideal technique for characterising these layers. The Thermo Scientific Theta Probe was used to characterise the quality of self assembled monolayers on gold surfaces. For alkanethiol layers with high coverage, bonding was almost entirely via the thiol group (forming Au-S bonds). Mixed bonding modes were observed, however, with lower coverage, in agreement with a proposed mechanism for the formation of these films. The influence of the head group of the self-assembly precursor was investigated by analysing hydroxyalkanethiol and ethylene glycol monolayers.
11:00 AM		BO+SS+AS-FrM-9 Morphology and Bonding in Alkene and Alkyne Based Monolayers Chemomechanically Formed on Si Surfaces T.M. Willey (Lawrence Livermore National Laboratory); M.V. Lee (Materials Nanoarchitectonics, Japan); J.R.I. Lee (Lawrence Livermore National Laboratory); M.R. Linford (Brigham Young University) One of the most simple and economical methods for attaching and patterning alkenes or alkynes on silicon surfaces is through chemomechanical modification. Scribing the Si removes the oxide passivation layer, allowing the alkene or alkyne to chemisorb to the exposed and highly reactive Si surface. Near-edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy reveals chemically homogeneous films with nearly monolayer coverage. Both alkene and alkyne precursors show no orientational order; however, more rigid perfluoronated molecules do have some incident angle dependence in NEXAFS resonances. Both alkenes and alkynes retain significant sp/sp² content. Hypothesized mechanisms of chemisorption are presented based on comparing the intensity of the sp/sp² features in the NEXAFS acquired from monolayers to measurements of precursor alkenes and alkynes in the gas-phase.