Effect of Relative Humidity on Chemical Heterogeneity Imaging with Atomic Force Microscopy
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Effect of Relative Humidity on Chemical Heterogeneity Imaging with Atomic Force Microscopy Xiaohong Gu1, Lijiang Chen1, Chang Xu2, Duangrut Julthongpiput2, Michael Fasolka2, and Tinh Nguyen1 1 BFRL, NIST, 100 Bureau Drive, Stop 8615, Gaithersburg, MD, 20899 2 Polymer Division, NIST, 100 Bureau Drive, Stop 8615, Gaithersburg, MD, 20899 ABSTRACT A well-controlled humidity system is used to enhance the sensitivity of atomic force microscopy (AFM) for characterizing surface chemical heterogeneity of patterned selfassembled monolayers (SAMs) and hydrophilic-hydrophobic polymeric brush specimens. Dependence of the AFM friction contrasts on surface energy differences between the hydrophilic regions and hydrophobic regions of the chemically heterogeneous samples has been investigated as a function of relative humidity (RH). Effects of RH and surface chemistry on tip/-sample adhesion are also investigated. Both AFM image contrast and tipsample adhesion forces between the hydrophilic and hydrophobic regions are highly dependent on RH and follow similar trends as a function of RH. Results clearly demonstrate that, by using proper RH at the tip-sample environment, chemically heterogeneous regions can be distinguished with the AFM. INTRODUCTION Nanoscale spatial chemical information is essential for developing fundamental understanding of a variety of surface phenomena, such as adhesion and degradation. Therefore, the ability to probe and image surface chemical heterogeneity with nanometer scale spatial resolution is highly desirable. Over the last 20 years, atomic force microscopy (AFM) has played an increasingly important role in characterizing surface morphology and surface properties (such as elasticity, friction and adhesion) of materials. However, the ability to identify and map the surface chemical heterogeneity has remained an unfulfilled opportunity in the field of AFM. Chemical force microscopy (CFM) [1] is a successful technique for enhancing the chemical sensitivity of AFM. In this method, AFM probes with controlled surface chemistry (usually modified with a self-assembled monolayer or SAM) are used to measure the tip-sample adhesion or the frictional contrast between different regions of the sample. The key to the success of this method is ensuring tip/sample interactions are dominated by the chemical species on the tip and the sample surface. CFM has usually been conducted in liquid instead of air to eliminate capillary effects. However, for specimens sensitive or susceptible to liquid, different tip-sample environments must be used [2, 3]. In this study, a well-controlled humidity system was used to enhance the sensitivity of AFM in characterizing surface chemical heterogeneity. The relative humidity of the tip-sample environment was provided using a humidity generator and a patented small-volume environmental chamber designed and fabricated at the National Institute of Standards and Technology [4]. The relative humidity (RH)in the chamber was controlled from nearly 0 % up to 95 % at room temperature.
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