Effects of Surface Functionality and Humidity on the Adhesion Force And Chemical Contrast Measured with AFM
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Effects of Surface Functionality and Humidity on the Adhesion Force And Chemical Contrast Measured with AFM Tinh Nguyen, Xiaohong Gu, Lijiang Chen, Duangrut Julthongpiput, Michael Fasolka, Kimberly Briggman, Jeeseong Hwang, and Jon Martin National Institute of Standards and Technology, Gaithersburg, MD 20899 Introduction The ability to probe chemical heterogeneity with nanometer scale resolution is essential for developing a molecular–level understanding of a variety of phenomena occurring at surfaces of materials. One area that could benefit greatly from nanoscale chemical measurement is an understanding of the degradation mechanisms of polymeric materials exposed to the environment. For example, the degradation (photo and hydrolytic) of polymers and polymeric materials has been observed to occur non-uniformly in which nanometer pits form locally, which deepen and enlarge with exposure (1,2). The pitting has been postulated to initiate in the hydrophilic degradation-susceptible regions of the films (3). However, due to the lack of spatial resolution of the most current surface analytical techniques, the chemical nature of the degradation-initiated locations has not been identified. The use of a chemically-functionalized probe in an AFM (chemical force microscopy CFM) (4) has been shown to be capable of discriminating chemically-different domains of self-assembled monolayer (SAM) surfaces at the nanoscale spatial resolution. This study provides data to demonstrate that, by using proper RH at the tip-sample environment, the contrast between the hydrophilic and hydrophobic domains in SAM and polymer samples can be discerned, and presents results on the effects of RH on tipsample adhesion forces for different substrates. Materials and Experimental Procedures Samples used included non-gradient and gradient-patterned SAMs, homogeneous hydrophilic and hydrophobic SAMs, SiO2-covered Si (SiO2-Si), Au-coated Si, and hydrophilic/hydrophobic block copolymer. Except for one patterned SAM where silane on Si was used, other SAM samples were prepared using thiol chemistry on Au-coated Si substrates. The gradient patterned SAM samples consisted of alternating hydrophilic (COOH) and hydrophobic (CH3) stripes with gradient surface free energy along the perpendicular direction of the stripes. The preparation of this sample has been described elsewhere (5). The SiO2/hydrophobic patterned SAM was fabricated using the vapor-mediated soft lithography technique. In this approach, a polydimethylsiloxane stamp having raised and recessed pattern was applied onto a SiO2-Si substrate. The stamp/substrate was exposed to saturated vapor of noctyldimethylchlorosilane (ODS). In this way, the substrate areas that were under the stamp recessed domains reacted with ODS to form a CH3-terminated SAM, while the areas under the raised domains were masked by the stamp, leaving the hydrophilic SiO2 layer on the substrate unreacted. After two hours of exposure, the stamp was removed and the treated substrates were washed thoroughly with toluene to r
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