研究领域

Surface and nanomaterials chemistry of metals, semiconductors and polymers.

Research Interests of the Ferguson Group Our research group is active in the following areas of research: 1) the relationship between bulk and interfacial properties in polymeric systems; 2) the use of sequential adsorption from solution to build well-defined thin films of organic, inorganic, and composite materials; 3) the use of electrochemistry to direct the regioselective formation of self-assembled monolayers; and 4) room-temperature welding between thin metal films on elastomeric supports. Dynamics and Thermodynamics at Polymer Surfaces Our research focuses on the connection(s) between the bulk properties of a polymer and its interfacial behavior, and in the use of those connections to build functional systems. These studies have revealed the need to view polymers as integrated systems, with correlated bulk andsurface properties. These bulk-surface connections, in turn, provide fascinating opportunities, such as the design of responsive surfaces and interfaces. Our approach has been to study the interfacial behavior —wettability and adhesion— of elastomers, using rubber elasticity to control composition and properties at polymer/water, polymer/metal, and polymer/air interfaces. We have designed and prepared systems in which enthalpically favorable interactions, such as hydrogen bonding or other Lewis acid-base interactions, at an interface require entropically unfavorable chain extension. As a result, there is an inherent competition between surface activity and chain coiling that depends on temperature. Most of our work has focused on model elastomers (e.g., polybutadiene, polyisoprene) that can easily be crosslinked and surface-modified. These materials have allowed a systematic study of interfacial behavior as a function of molecular weight between crosslinks (Mc) and of the degree and type of surface functionalization Stepwise Approaches to Multilayered Thin Films We have developed methods for the preparation of multilayered thin films based on the stepwise adsorption of molecules, polymers, and nanoparticles from solution. This area has grown tremendously over the past decade. Our first article in the area, published in Science in 1994, described the sequential adsorption of an organic polyelectrolyte and single sheets of a silicate clay to build multilayers with controlled thickness and sufficient order in the stacking direction to diffract x-rays. Our continuing work in this area focuses on two key features of this method: nanometer-scale control of thickness and flexibility in choice of materials. Our interest in these materials is motivated, in part, by potential applications in sensor design and optical coatings. In more recent work, we have extended this method to allow controlled growth of titanium (hydr)oxide films and silica films. Titanium oxide is a material of intense current interest because it is a robust large-bandgap semiconductor and because it has found utility in the photochemical remediation of toxic waste. Silicon dioxide is a ubiquitous optical and lowdielectric material. Our method for building thin films of SiO2 allows simultaneous control of both thickness and refractive index over wide ranges, thereby controlling the optical pathlength within the film. Other specific projects have focused on: incorporation of uncharged and conducting polymers into polyelectrolyte multilayers, to greatly extend the types materials usable with this method; sol-gel processing of polymer/clay multilayers to enhance film rigidity and barrier properties; and developing a method for analysis of single sheets of a silicate clay using scanning electron microscopy (SEM). Self-Assembled Monolayers: Selective Synthesis and Application Simple chemisorption of alkanethiols or disulfides on gold provides a facile method for uniform coverage of the surface with an ordered self-assembled monolayer (SAM) of the organosulfur species, but it provides no selectivity for substrates containing more than one gold feature (e.g., a microelectrode array). This limitation seriously mitigates its usefulness for differentiating interfacial structure, for example in electrode arrays for sensor or device applications. To address the problem of selective functionalization, we recently demonstrated an alternative approach for the formation of SAMs on gold –electrochemical oxidation of alkyl thiosulfates at gold electrodes. Because chemisorption only occurs at electrodes held at a sufficiently high potential (~ 1 V vs. Ag/AgCl), a given electrode can be functionalized selectively in presence of a nearby electrode. We recently demonstrated this method by preparing a SAM selectively from an alkyl thiosulfate onto one microelectrode in the presence of nearby microelectrode on a test chip (~ 80-μm spacing). On-going studies focus on extending the method to accommodate a variety of different functional groups, mechanistic aspects of the process, and application to microelectrode sensors, in collaboration with Professor Michael Freund at the University of Manitoba in Canada. Cold-Welding of Gold and Its Use in Device Technology Even the surface of gold is noble, forming no stable oxide under ordinary conditions. As a consequence, clean gold surfaces spontaneously weld to one another at room temperature when placed into contact. Of course when gold is exposed to the ambient atmosphere, it immediately adsorbs a layer of contamination and therefore does not generally weld on contact. A fascinating exception is thin (~10-200 nm) films of gold supported on an elastomer (i.e., rubber). In this case, cold-welding does occur under ambient conditions, and with little or no applied load! In collaboration with Professor Svetlana Tatic-Lucic in the Electrical Engineering Department at Lehigh, we are exploring the usefulness of cold-welding for building microelectro- mechanical system (MEMS) devices. The fabrication of MEMS and NEMS (nanoelectro- mechanical systems) devices typically involves series of sequential processing steps of several types: bonding, articulation of device features, and the application and removal of encapsulation (first-level packaging). Although manipulations of packaging add no value directly, they are necessary because they allow new features to be etched into a wafer without damaging preexisting features that are already in place. For this strategy to succeed, the techniques used to add and remove encapsulating structures must be gentle enough not to damage these preexisting features, as well as convenient enough to allow processing within practical periods of time. Our work focuses both on the use of cold-welding for the formation of first-level packaging, and on development of a gentle and fast method for removal of this coldwelded packaging.

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"Relative lability of gold-oxide thin films in contact with air, solvents, or electrolyte solutions," Cook, K.M.;Ferguson, G.S. J,. Vac. Sci. Technol. A 2013, 31(2), 021508 (1-5) “Gold oxide as a protecting group for regioselective surface chemistry,” Cook, K.M.; Ferguson, G.S. Chemical Communications 2011, 47(46), 12550- 12552 “Determination of the Wavelength-Dependent Refractive Index of a Gold-Oxide Thin Film,” Cook, K.M.; Ferguson, G.S. Journal of Physical Chemistry C 2011, 115(46), 22976–22980 “Electrochemically directed two-component monolayers on gold,” Labukas, J.P.; Ferguson, G.S. Journal of Materials Research 2011, 26(2), 262-267 “Mechanism of Spontaneous Formation of Monolayers on Gold from Alkyl Thiosulfates,” Fealy, R.J.; Ackerman, S.R.; Ferguson, G.S. Langmuir 2011, 27(9), 5371-5376 “Direct Route to Well-Defined, Chemically Diverse Electrode Arrays,” Labukas, J.P.; Ferguson, G.S. Langmuir 2011, 27(7), 3219-3223 “Compatibility of ω-Functionality in the Electrochemically Directed Self-Assembly of Monolayers on Gold from Alkyl Thiosulfates,” Labukas, J.P.; Drake, T.J.H.; Ferguson, G.S. Langmuir 2010, 26(12), 9497-9505 "An Alternative Method for Fabricating Microcontact Printing Stamps," Jing, G.; Labukas, J.P.; Zhang, W.; Perry, S.F.; Lu, S-F.; Ferguson G.S.; Tatic-Lucic, S. Microelectronic Engineering 2009, 86(11), 2334-2341 "Does water swell the ordered domains in polyelectrolyte/clay multilayers?," Rouse, J.H.; Ferguson, G.S. Clays and Clay Minerals 2007, 55(2), 160-164 "Temperature-Actuated Changes in Wettability at Elastomer/Water Interfaces," Khongtong, S.; Ferguson, G.S. Langmuir 2004, 20(23), 9992-10000 "A Method for Imaging Single Clay Platelets by Scanning Electron Microscopy," Rouse, J.H.; White, S.T.; Ferguson, G.S. Scanning 2004, 26(3), 131-134 "Electrochemical Self-Assembly of Monolayer from Alkylthiosulfate on Gold," Lee, M-T.; Hsueh, C-C.; Freund, M.S.; Ferguson, G.S. Langmuir 2003, 19(13), 5246-5253 "Preparation of Thin Silica Films with Controlled Thicknes and Tunable Refractive Index," Rouse, J.H.; Ferguson, G.S. Journal of the American Chemical Society 2003, 125(50), 15529-15536 "Stepwise Incorporation of Nonpolar Polymers within Polyelectrolyte Multilayers," Rouse, J.H.; Ferguson, G.S. Langmuir 2002, 18(20), 7635-7640 "A Smart Adhesive Joint: Entropic Control of Adhesion at a Polymer/Metal Interface," Khongtong, S.; Ferguson, G.S. Journal of the American Chemical Society 2002, 124(25), 7254-7255 "Integration of Bulk and Interfacial Properties in a Polymeric System. 2. Rubber Elasticity at Polyisoprene/Water Interfaces," Khongtong, S. Ferguson G.S. Macromolecules 2002, 35(10), 4023-4029 "Stepwise Formation of Ultrathin Films of a Titanium (Hyr)oxide by Polyelectrolyte-assisted adsorption," Rouse, J.H.; Ferguson, G.S. Advanced Materials 2002, 14(2), 151-154 "Coupled, Consecutive Reconstructions at a Polymer/Air Interface," Grunzinger, S.J.; Ferguson G.S. Journal of the American Chemical Society 2001 , 123(51), 12927-12928 "Integration of Bulk and Interfacial Properties in a Polymeric System: Rubber Elasticity at a Polybutadiene/Water Interface," Khongtong, S.; Ferguson, G.S. Journal of the American Chemical Society 2001, 123(15), 3588-3594 "Stepwise Synthesis of a Well-Defined Silicon (Oxide)/Polyimide Interface," Lee, M-T.; Fersuson, G.S. Langmuir 2001,17(3), 762-767.