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Interfacial adhesion at the molecular level

Description: Interfacial adhesion is of extraordinary technological importance and has long been of intense scientific interest. However, the study of the adhesive bond and its failure is made difficult by the complexity of the interfacial interaction and the problems involved with establishing carefully characterized and controlled interfacial surfaces and that of quantitatively evaluating the bonding after its formation. In the present work, we outline the results of studies using Interfacial Force Microscopy (IFM) to study the adhesive bond formation and failure between (1) differing end-group combinations on self-assembling monolayer (SAM) films covering Au surfaces and (2) between clean surfaces of a W probe and a Au single-crystal sample. The IFM is a scanning probe technique distinguished by its use of a mechanically stable, zero-compliance force sensor. This sensor permits the study of the interfacial force as a function of separation without the mechanical instability giving rise to the {open_quotes}jump-to-contact{close_quotes} seen in all presently used displacement-based sensors, such as the surface forces apparatus and the atomic force microscope. Thus, information can be obtained concerning the details of the adhesive bond formation and failure over the entire range of the interfacial interaction. We demonstrate that such measurements yield valuable quantitative information concerning the individual bond strengths between chemically distinct SAM end groups and show that the clean metal-surface interaction is dominated by surface roughness and plastic deformation.
Date: December 31, 1997
Creator: Houston, J.E.; Michalske, T.A. & Crooks, R.M.
Partner: UNT Libraries Government Documents Department

Interactions between self-assembled monolayers and an organophosphonate: A detailed study using surface acoustic wave-based mass analysis, polarization modulation-FTIR spectroscopy, and ellipsometry

Description: Self-assembled monolayers (SAMs) having surfaces terminated in the following functional groups: -CH{sub 3}, -OH, -COOH, and (COO{sup -}){sub 2}Cu{sup 2+} (MUA-Cu{sup 2+}) have been prepared and examined as potential chemically sensitive interfaces. Mass measurements made using surface acoustic wave (SAW) devices indicate that these surfaces display different degrees of selectivity and sensitivity to a range of analytes. The response of the MUA-Cu{sup 2+} SAM to the nerve-agent simulant diisopropyl methylphosphonate (DIMP) is particularly intriguing. Exposure of this surface to 50%-of-saturation DIMP yields a surface concentration equivalent to about 20 DIMP monolayers. Such a high surface concentration in equilibrium with a much lower-than-saturation vapor pressure has not previously been observed. Newly developed analytical tools have made it possible to measure the infrared spectrum of the chemically receptive surface during analyte dosing. Coupled with in-situ SAW/ellipsometry measurements, which permit simultaneous measurement of mass and thickness with nanogram and Angstrom resolution, respectively, it has been possibly to develop a model for the surface chemistry leading to the unusual behavior of this system. The results indicate that DIMP interacts strongly with surface-confined Cu{sup 2+} adduct that nucleates growth of semi-ordered crystallites having substantially lower vapor pressure than the liquid.
Date: June 24, 1997
Creator: Crooks, R.M.; Yang, H.C. & McEllistrem, L.J.
Partner: UNT Libraries Government Documents Department

Single-monolayer in situ modulus measurements using a SAW device: Photocrosslinking of a diacetylenic thiol-based monolayer

Description: We report direct measurement of the modulus change that accompanies the crosslinking of a single molecular monolayer. We measured a change in elastic modulus of 5 x 10{sup 10} dyn/cm{sup 2} as a result of ultraviolet-induced photocrosslinking of a single surface-confined monolayer of the conjugated diacetylenic thiol HS(CH{sub 2}){sub 10}C{triple_bond}CC{triple_bond}C(CH{sub 2}){sub 10}COOH, designated {open_quotes}DAT{close_quotes} hereafter. The modulus measurement was made on a monolayer of DAT chemisorbed upon a gold film on the surface of a 97-MHz ST-quartz surface acoustic wave delay line. The ratio of the changes recorded in SAW velocity and attenuation, approximately 4:1, suggests that the measured effect is mainly a change in the elastic (real) component of the complex shear modulus, viscous changes playing a lesser role. In relation to typical polymer modulus values, the change of 5 x 10{sup 10} dyn/cm{sup 2} is consistent with a change from a rubbery material (G{prime} {approximately} 10{sup 7} - 10{sup 8} dyn/cm{sup 2}) to a fairly rigid, glassy material (G{prime} {approximately} 10{sup 10} dyn/cm{sup 2}), reasonable for comparison of the monolayer in its as-adsorbed and crosslinked forms. This report of the direct SAW-based measurement of the modulus change associated with the crosslinking of a single molecular monolayer is complementary to and consistent with previous in-situ measurements of this process using thickness-shear mode resonators.
Date: October 1, 1997
Creator: Ricco, A.J.; Staton, A.W.; Crooks, R.M. & Kim, Taisun
Partner: UNT Libraries Government Documents Department

Chemical class specificity using self-assembled monolayers on SAW devices

Description: We have studied the chemical selectivity and sensitivity of surface acoustic wave (SAW) sensors covered by (COO{sup {minus}}){sub 2}/Cu{sup 2+}-terminated interfaces by examining the response of self-assembled monolayer (SAM) films formed from the solution phase for 36, 84, and 180 h adsorption times. These organomercaptan SAMs were prepared on thin-film Au surfaces having variable, controlled grain size. The SAW response from the carboxylate coordinated Cu{sup 2+}-terminated SAM is compared to that from methyl-terminated SAM, as these films interact with a vapor-phase organophosphonate analyte and the vapors of common organic solvents. Results have implications for designing and reliably fabricating chemical sensors that respond to specific organic analytes.
Date: June 1, 1996
Creator: Thomas, R.C.; Ricco, A.J.; Yang, H.C.; Dermody, D. & Crooks, R.M.
Partner: UNT Libraries Government Documents Department

Chemical sensors based on surface-confined dendrimers

Description: The use of dendrimers for preparing chemically sensitive interfaces for detecting volatile organic compounds (VOCs) using surface acoustic wave (SAW) device transducers is described. Specifically, the synthesis of the dendrimers and the means by which they are affixed to SAW devices is discussed, followed by a detailed spectroscopic analysis of the surface-confined dendrimers and a discussion of their interaction with different VOCs. Most of these preliminary experiments focus on dendrimer surface modification using benzoylchloride, which leads to phenyl terminal groups linked to the dendrimer via amide groups. The results of this study lead us to conclude that dendrimers: (1) provide general specificity towards classes of functional groups and are therefore suitable for array-based sensing schemes; (2) are intermediate in structure between monolayers and polymers and exhibit the desirable properties of both; (3) can be straightforwardly attached to the surfaces of acoustic wave devices.
Date: October 1, 1997
Creator: Tokuhisa, Hideo; Crooks, R.M.; Ricco, A.J. & Osbourn, G.C.
Partner: UNT Libraries Government Documents Department

Chemical class specificity using self-assembled monolayers on SAW devices: Effects of adsorption time and substrate grain size

Description: The authors report selectivity and sensitivity for 97-MHz SAW (surface acoustic wave) sensors functionalized with (COO{sup {minus}}){sub 2}/Cu{sup 2+}-terminated, organomercaptan-based, self-assembled monolayers (SAMs). Responses were obtained as a function of SAM formation time on thin Au films of controlled grain size. The authors find that the SAM films (1) preferentially adsorb classes of organic analytes according to simple chemical interaction concepts, (2) reversibly adsorb multilayers of some analytes well below their saturation vapor pressure, (3) adsorb more diisopropylmethylphosphonate (DIMP) at a given partial pressure as SAM solution-phase adsorption time increases, and (4) adsorb more DIMP at a given partial pressure as the grain size of the supporting Au film decreases.
Date: July 1, 1997
Creator: Thomas, R.C.; Ricco, A.J.; DiRubio, C.R.; Yang, H.C. & Crooks, R.M.
Partner: UNT Libraries Government Documents Department

SAW arrays using dendrimers and pattern recognition to detect volatile organics

Description: chemical sensor arrays eliminate the need to develop a high-selectivity material for every analyte. The application of pattern recognition to the simultaneous responses of different microsensors enables the identification and quantification of multiple analytes with a small array. Maximum materials diversity is the surest means to create an effective array for many analytes, but using a single material family simplifies coating development. Here the authors report the successful combination of an array of six dendrimer films with mass-sensitive SAW (surface acoustic wave) sensors to correctly identify 18 organic analytes over wide concentration ranges, with 99.5% accuracy. The set of materials for the array is selected and the results evaluated using Sandia`s Visual-Empirical Region of Influence (VERI) pattern recognition (PR) technique. The authors evaluated eight dendrimer films and one self-assembled monolayer (SAM) as potential SAW array coatings. The 18 organic analytes they examined were: cyclohexane, n-hexane, i-octane, kerosene, benzene, toluene, chlorobenzene, carbon tetrachloride, trichloroethylene, methanol, n-propanol, pinacolyl alcohol, acetone, methyl isobutyl ketone, dimethylmethylphosphate, diisopropylmethylphosphonate, tributylphosphate, and water.
Date: August 1, 1998
Creator: Ricco, A.J.; Osbourn, G.C.; Bartholomew, J.W.; Martinez, R.F.; Crooks, R.M.; Garcia, M.E. et al.
Partner: UNT Libraries Government Documents Department

Detection of Volatile Organics Using a Surface Acoustic Wave Array System

Description: A chemical sensing system based on arrays of surface acoustic wave (SAW) delay lines has been developed for identification and quantification of volatile organic compounds (VOCs). The individual SAW chemical sensors consist of interdigital transducers patterned on the surface of an ST-cut quartz substrate to launch and detect the acoustic waves and a thin film coating in the SAW propagation path to perturb the acoustic wave velocity and attenuation during analyte sorption. A diverse set of material coatings gives the sensor arrays a degree of chemical sensitivity and selectivity. Materials examined for sensor application include the alkanethiol-based self-assembled monolayer, plasma-processed films, custom-synthesized conventional polymers, dendrimeric polymers, molecular recognition materials, electroplated metal thin films, and porous metal oxides. All of these materials target a specific chemical fi.mctionality and the enhancement of accessible film surface area. Since no one coating provides absolute analyte specificity, the array responses are further analyzed using a visual-empirical region-of-influence (VERI) pattern recognition algorithm. The chemical sensing system consists of a seven-element SAW array with accompanying drive and control electronics, sensor signal acquisition electronics, environmental vapor sampling hardware, and a notebook computer. Based on data gathered for individual sensor responses, greater than 93%-accurate identification can be achieved for any single analyte from a group of 17 VOCs and water.
Date: October 14, 1999
Partner: UNT Libraries Government Documents Department

Versatile materials for use as chemically sensitive interfaces in SAW-based sensor arrays

Description: The primary research objective of the work described here is to design, synthesize, and characterize new materials for use as chemical sensor interfaces, integrate these materials, using appropriate transducers, into sensor arrays, and then develop appropriate mathematical algorithms for interpreting the array response. In this paper, we will discuss two new types of materials we have developed that are ideally suited for use as chemically sensitive interfaces for array-based chemical sensing applications, since they: (1) provide general specificity towards classes of functional groups rather than individual compounds; (2) are intermediate in structure between monolayers and polymers; (3) exhibit both endo- and exo-recognition. The first class of materials is surface-confined dendrimers and the second is hyperbranched polymers.
Date: July 1, 1996
Creator: Crooks, R.M.; Bergbrieter, D.E.; Bruening, M.L.; Wells, M.; Zhou, Yuefen; Ricco, A.J. et al.
Partner: UNT Libraries Government Documents Department