Surface Science Reports 64 (2009) 233–254 Contents lists available at ScienceDirect Surface Science Reports journal homepage: www.elsevier.com/locate/surfrep Surface modification in microsystems and nanosystems Shaurya Prakash a,b,∗,1, M.B. Karacor a, S. Banerjee a a Department of Mechanical & Aerospace Engineering, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA b Institute for Advanced Materials, Devices, and Nanotechnology, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA article info a b s t r a c t Article history: Phenomena in microsystems and nanosystems are influenced by the device walls due to the high surface- Accepted 5 May 2009 area-to-volume ratios that are a characteristic feature of these systems. The role of surfaces in these editor: W.H. Weinberg small-scale systems has led to natural interest in developing methods to manipulate surface-mediated phenomena toward improving device performance, developing next generation systems, and mitigat- Keywords: ing problems that arise due to interfacial interactions between surfaces and materials within microscale Surface modification Chemical and nanoscale systems. This report presents a critical review of the existing literature as it relates to role Physical of surfaces and surface modification in microsystems and nanosystems. In addition, this report strives Polymer to present this literature review with an eye on the tutorial aspect of surface modification for new re- Glass searchers. Toward the dual goal of presenting a tutorial review with a critical analysis of literature many Plasma open scientific questions are discussed. Both chemical and physical surface modification methods are dis- Self-assembled monolayer cussed with several examples, applications, and a brief description of underlying theory. The importance Fluid of surfaces in microsystems and nanosystems and the applicability of controlling surface properties in a Micro- systematic manner for both fundamental science and applied studies is also discussed. The readers are Nano- Water purification pointed to several pioneering research efforts over the years that have made surface modification and Energy generation surface science a rich, diverse, and multi-disciplinary research field. It is hoped that this report will as- sist researchers from diverse fields by providing a collection of varied references and encourage the next generation of surface scientists and engineers to significantly advance the state of knowledge. ' 2009 Elsevier B.V. All rights reserved. Contents 1. Introduction........................................................................................................................................................................................................................234 2. Surface modification methods ..........................................................................................................................................................................................235 2.1. Overview ................................................................................................................................................................................................................235 2.2. Physical methods ...................................................................................................................................................................................................235 2.3. Chemical methods .................................................................................................................................................................................................235 3. Thermodynamics of surfaces.............................................................................................................................................................................................236 3.1. Overview ................................................................................................................................................................................................................236 3.2. Surface layers .........................................................................................................................................................................................................237 4. Surface characterization ....................................................................................................................................................................................................240 4.1. Indirect methods....................................................................................................................................................................................................240 4.2. Direct methods.......................................................................................................................................................................................................241 5. Applications........................................................................................................................................................................................................................243 5.1. Photocatalysts for organic decontamination .......................................................................................................................................................243 5.2. Dye modified surfaces for energy generation ......................................................................................................................................................244 5.3. Microfluidic and nanofluidic separation systems................................................................................................................................................244 5.4. Biosensor platforms...............................................................................................................................................................................................247 5.5. Water purification .................................................................................................................................................................................................248 6. Summary ............................................................................................................................................................................................................................248 Acknowledgements............................................................................................................................................................................................................249 References...........................................................................................................................................................................................................................249 ∗ Corresponding address: Department of Mechanical & Aerospace Engineering, The State University of New Jersey, Rutgers, B240 Engineering Building, 98 Brett Road, Piscataway, NJ 08854, United States. Tel.: +1 732 445 3797; fax: +1 732 445 1 Current address: Department of Mechanical Engineering, 201 W. 19th Avenue, 3124. Columbus, OH 43210, USA. E-mail address: [email protected] (S. Prakash). 0167-5729/$ – see front matter ' 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfrep.2009.05.001 234 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 Nomenclature θ Fraction of surface sites occupied c Concentration of adsorbing species near the surface in solution or gas-phase b Constant related to adsorbent t Adsorption time ka Adsorption rate constant kd Desorption rate constant N0 Surface adsorbate concentration at full coverage γSV Interfacial free energy between solid and vapor phases γLV Interfacial free energy between liquid and vapor phases γSL Interfacial free energy between solid and liquid Fig. 1. Comparison of papers published in the broad subject area of surface phases modification in microsystems and nanosystems as compared through two commonly used scientific databases. The databases used are Elsevier's Scopus and θc Contact angle ISI Thomson's Web of Science. σs Surface charge density ρ Counter-ion charge density in the bulk solution s In light of the definition presented above, surfaces provide a Surface potential s unique set of challenges and opportunities in microsystems and " Dielectric function of medium nanosystems as these systems are inherently characterized by high " Permittivity of free-space 0 surface-area-to-volume (SA=V ) ratios varying from about 103 m−1 k Boltzmann constant b to as high as 109 m−1 in some cases. Given these SA=V ratios T Absolute temperature the influence of device and system walls in affecting phenomena ρ Charge density in solution-phase near a surface 0 within confined spaces can no longer be ignored since the walls ζ Zeta potential of surface the of the devices and systems interact directly with the species e Elementary charge in the surface-region. These wall-species interactions that impart λ Debye length or electric double layer thickness D several important and useful characteristics, as discussed later, µ Electroosmotic mobility eo to transport and reaction phenomena in confined microscale and v Electroosmotic velocity eo nanoscale systems. The well-documented [7–13] advantages of φ Applied electric potential microsystems and nanosystems have spawned great interest