Papers

AVS2001 Session SS+BI-TuA: Poirier Memorial Session: Self-Assembled Monolayers II

Tuesday, October 30, 2001 2:00 PM in Room 120

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		SS+BI-TuA-1 Self-Assembling Monolayers and Thin-Film Templates - Driven Assembly of Functional Organic Structures R.G. Nuzzo (University of Illinois at Urbana-Champaign) The properties of complex matter and assemblies follow from structural elements embedded at a variety of length scales. These influences begin at the most microscopic level--that is at sizes associated with the atomic or molecular constituents--and extend upwards through increasingly more complex hierarchies of organization. While it is common in research to focus on organizations that arise via a thermodynamically directed assembly, it is now possible to design and study assemblies that form solely as a result of the temporal evolution of the underlying dynamics of the system. In this talk I will explore these ideas, focusing on the important role played by dynamics in prototypical complex organic thin-film assembly systems. Specific attention will be given to thin film systems that develop interesting mesoscopic organizations as a result of driven processes occurring at a substrate surface.
2:40 PM		SS+BI-TuA-3 Adsorption and Photoinitiated Chemistry on Organic-functionalized Semiconductor Surfaces N. Camillone III, K. Adib, R.M. Osgood, Jr. (Columbia University) Greg Poirier's pioneering imaging work had a specific and important role in the scientific and intellectual development of one of the authors (N.C.), and played an instrumental part in the growth of our understanding of the structure and dynamics of self-assembled monolayers. Many experiments have been conceived based upon this understanding. We report results of one such experiment, which characterizes the growth and photoinitiated chemistry of methyl bromide monolayers adsorbed at the surface of short-chain thiolate monolayers on GaAs(110) surfaces. The thiolate monolayers are grown in UHV by exposure of the GaAs to alkyl disulfides. Temperature programmed desorption reveals that the CH₃Br-surface interactions are strongly modified by the presence of the thiolate spacer layer. With increasing chain length, the CH₃Br becomes progressively less tightly-bound. The simple layer-by-layer growth behavior observed on bare GaAs(110) is lost, and CH₃Br appears to form clusters at critical coverages. Exposure to 248 nm light results in the dissociation of the CH₃Br, most likely due to the dissociative attachment of excited substrate electrons. The cross section for the reaction is strongly enhanced in the case of the ethyl disulfide spacer layer. The implications of the results will be discussed within the context of electron transport through molecular spacers, adsorbate affinity level shifts, and deexcitation mechanisms.
3:00 PM		SS+BI-TuA-4 Photoinduced Processes in Self-Assembled Monolayers on Semiconductor Surfaces T. Ye, E. McArthur, E. Borguet (University of Pittsburgh) The photoreactivity of ODS(Octadecylsiloxane) SAMs on semiconductor surfaces under UV illumination in air, has been investigated by a combination of contact angle, FTIR and AFM. This work provides strong evidence that SAM degradation does not result from ozone alone. A combination of UV and oxygen is necessary for monolayer degradation to proceed. AFM measurements on submonolayer coverage SAMs provide direct evidence of the degradation of ODS SAMs and reveal the role of defects in the degradation process. FT-IR and AFM results suggest that the hydrocarbon chain is the reactive site of the monolayers. A microscopic mechanism of the degradation involving hydrogen abstraction is suggested based on the mechanism of gas-phase oxidation of alkanes. These results have implications for photoresist micropatterning and nanotechnology applications that require high spatial resolution.
3:20 PM		SS+BI-TuA-5 Organic Monolayers on Silicon and Silica Surfaces Via Covalent Linkages X.-Y. Zhu (University of Minnesota) Monolayer assemblies on silicon and silica surfaces are of interest for a number of technological reasons, such as substrates in micro-arrays, surface passivation in MEMS, and monolayer mask in soft-lithography. I present novel approaches for the formation of molecular monolayers on silicon and silica surfaces. These approaches rely on a two-step strategy: in the first step, a silicon or silica surface is activated to give reactive functional groups; in the second step, the activated surface reacts with simple organic molecules for monolayer assembly. Examples of alkyl, aromatic, and PEG monolayer assemblies will be presented and their applications in MEMS and micro-arrays will be demonstrated.
4:00 PM		SS+BI-TuA-7 Study of Alkylsilane Monolayers Islands on Mica in the Presence of Water M. Luna (Lawrence Berkeley National Laboratory); I. Diez, F. Teran, F. Sanz (Universitat de Barcelona, Spain); D.F. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory) The increasing interest in the study of self-assembled monolayers for the last two decades is related to their potential in different fields of science and technology such as biological sensors, ¹ optoelectronics,² tribology³ and corrosion inhibition. ⁴ The knowledge of the stability of the packing, the electrostatic and dielectric properties, and the understanding of the formation process of SAMs constitute crucial phenomena to be investigated in the presence of a changing relative humidity atmosphere. Thus we have performed several series of experiments on alkylsilane monolayer islands vs. relative humidity by means of Polarization Force Microscopy and Dynamic Force Microscopy. The results show that the autoorganizacion is achieved starting from a layer where the molecules are lying down. Also, due to the hydrophilic properties of the alkylsilane monolayer islands, water adsorbs only on mica. This produces an important increase of the dielectric constant on the mica which, in polarization force microscopy, reverses the topographic contrast at around 30% and reaches a maximum at 80%. In addition, by measuring the relative topographic distance between the mica and the islands we are able to quantify the amount of water adsorbed on mica at high relative humidity. . ¹ K. Shierbaum, T. Weiss, E. T. van Velzen, J. Engbersen, D. Reinhoudt and W. Göpel, Science 265, 1413 (1994). ² A. Ulman, Thin Films: Self-Assembled Monolayers of Thiols, Academic Press, New York, 1998. ³ J. I. Siepmann and I. R. McDonald, Phys. Rev. Lett. 70, 453 (1993). foornote 4 F. Zamborini and R. Crooks, Langmuir 14, 3279 (1998).
4:20 PM		SS+BI-TuA-8 "Interphase" Liquid Structure and Interfacial Forces H.I. Kim, J.E. Houston (Sandia National Laboratories) There is growing evidence that liquids behave differently within nanometer-proximity to surfaces due to the formation of quasi-ordered "interphase." Understanding the correlation between the surface chemistry, the interphase structure and the resulting in terfacial forces between neighboring surfaces is of fundamental importance to various aspects of nano-scale materials research, including biomaterials, nanotribology and microelectromechanical (MEMs) devices. We use interfacial force microscopy (IFM) to q uantitatively measure interfacial bonding and frictional forces in order to identify the correlation between surface chemistry and interphase sturcture. We outline the results from two systems. In the first, we measure forces between a gold tip and sample, both coated with oligo(ethylene glycol)-terminated SAMs, in water. We observe anomalous interfacial forces due to the interphase water layer, which dominates the interfacial properties. It has been suggested that the interphase water is responsible for the protein-resistant nature of this SAM surface. In the second, we use a long-chain hydrocarbon liquid, hexadecane, which is known to form an ordered layer on a flat metal surface (e.g., gold). We find that this ordered layer passivates the interfacial i nteraction of a tungsten tip with a gold surface even under high stresses. The structure of this layer can be perturbed by applying an electric field and results in extended frictional forces without significant changes in the behavior of the normal-force. These results will be discussed in terms of the interfacial chemistry and the structure of the interphase liquid. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under a Contract DE-AC04-94AL85000.H. H.
4:40 PM		SS+BI-TuA-9 Alkanethiols on Copper Single Crystal Surfaces S. Vollmer, G. Witte, C. Wöll (Ruhr-Universität Bochum, Germany) Ultrathin organic films of self-assembled molecules (SAMs) have attracted considerable interest because of their promising technical applications. Whereas the vast majority of related studies have been focused on alkanethiolate (C_n) films on gold surfaces, which are considered as prototype systems, the structure and properties of SAMs on transition metal surfaces are by far less intense studied. Here we present the results of a comprehensive study of the formation and structure of alkanethiol monolayers of various chain length (n ≤ 12) on the low indexed copper surfaces [i.e. (111), (100) and (110)]. The films which have been prepared by gas phase adsorption under UHV conditions were characterized by means of XPS, TDS, LEED and HAS. At low temperatures alkanethiol molecules are physisorbed with their molecular axis parallel to the surface without any long range ordering. With increasing temperature (110-250K) a conversion into a copper thiolate species takes place depending on the alkyl chain length which is accompanied by an upright tilting of the molecules. Upon adsorption at room temperature highly ordered thiolate monolayers are observed of which structures depend only on the substrate orientation but not on the chain length. Heating the films above 380K causes a S-C bond cleaving and desorption of the entire alkyl chains leaving the S-atoms on the surface. Surprisingly, the saturation structures are metastable and decompose already at about 250K upon partial desorption of the alkyl chains into a thermodynamically stable mixture of thiolate and sulfide species. The presently observed differences to the case of SAMs on gold surfaces are attributed to the much stronger S-Cu interaction.
5:00 PM		SS+BI-TuA-10 Electro-Rheology of Liquid Hexadecane Near a Au Surface J.E. Houston, H.I. Kim (Sandia National Laboratories) In an earlier study we showed that C10 and C12 alkanethiol self-assembled monolayers on Au surfaces conduct only with significant applied stress and that the friction increases along with the increasing current, indicating significant changes in both structural and electronic properties of the film. In this presentation we outline results from a similar interfacial force microscope (IFM) study of hexadecane films near a Au(111) surface immersed in liquid. At room temperature, hexadecane forms mechanically stable surface multi-layers, which also inhibit conduction. However, under an applied electric field, the film structure is perturbed causing friction forces at extended separations but with no significant repulsion. We discuss these findings in terms of both the molecular and dielectric properties of the films as well as the effect of liquid temperature. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin company, for the DOE under Contract DE-AC04-94AL85000.