title: Highly Stable Bonding of Thiol Monolayers to Hydrogen-Terminated Si via Supercritical Carbon Dioxide: Toward a Super Hydrophobic and Bioresistant Surface.
abstract: Oxide-free silicon chemistry has been widely studied using wet-chemistry methods, but for emerging applications such as molecular electronics on silicon, nanowire-based sensors, and biochips, these methods may not be suitable as they can give rise to defects due to surface contamination, residual solvents, which in turn can affect the grafted monolayer devices for practical applications. Therefore, there is a need for a cleaner, reproducible, scalable, and environmentally benign monolayer grafting process. In this work, monolayers of alkylthiols were deposited on oxide-free semiconductor surfaces using supercritical carbon dioxide (SCCO2) as a carrier fluid owing to its favorable physical properties. The identity of grafted monolayers was monitored with Fourier transform infrared (FTIR) spectroscopy, high-resolution X-ray photoelectron spectroscopy (HRXPS), XPS, atomic force microscopy (AFM), contact angle measurements, and ellipsometry. Monolayers on oxide-free silicon were able to passivate the surface for more than 50 days (10 times than the conventional methods) without any oxide formation in ambient atmosphere. Application of the SCCO2 process was further extended by depositing alkylthiol monolayers on fragile and brittle 1D silicon nanowires (SiNWs) and 2D germanium substrates. With the recent interest in SiNWs for biological applications, the thiol-passivated oxide-free silicon nanowire surfaces were also studied for their biological response. Alkylthiol-functionalized SiNWs showed a significant decrease in cell proliferation owing to their superhydrophobicity combined with the rough surface morphology. Furthermore, tribological studies showed a sharp decrease in the coefficient of friction, which was found to be dependent on the alkyl chain length and surface bond. These studies can be used for the development of cost-effective and highly stable monolayers for practical applications such as solar cells, biosensors, molecular electronics, micro- and nano- electromechanical systems, antifouling agents, and drug delivery.
authors.affiliation: Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research) , 2 Fusionopolis Way, Innovis, #08-32, Singapore 138634.|Department of Chemical and Biomolecular Engineering, National University of Singapore , 4 Engineering Drive 4, Singapore 117585.
authors.forename: Bhavesh
authors.initials: B
authors.lastname: Bhartia
authors.affiliation: Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research) , 2 Fusionopolis Way, Innovis, #08-32, Singapore 138634.
authors.forename: Sreenivasa Reddy
authors.initials: SR
authors.lastname: Puniredd
authors.affiliation: Environmental and Water Technology Centre of Innovation, Ngee Ann Polytechnic , 535 Clementi Road, Singapore 599489.
authors.forename: Sundaramurthy
authors.initials: S
authors.lastname: Jayaraman
authors.affiliation: Centre for Nanofibers and Nanotechnology, Nanoscience and Nanotechnology Initiative, National University of Singapore , Singapore 117576.
authors.forename: Chinnasamy
authors.initials: C
authors.lastname: Gandhimathi
authors.affiliation: Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research) , 2 Fusionopolis Way, Innovis, #08-32, Singapore 138634.
authors.forename: Mohit
authors.initials: M
authors.lastname: Sharma
authors.affiliation: National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan.
authors.forename: Yen-Chien
authors.initials: YC
authors.lastname: Kuo
authors.affiliation: National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan.
authors.forename: Chia-Hao
authors.initials: CH
authors.lastname: Chen
authors.affiliation: Centre for Nanofibers and Nanotechnology, Nanoscience and Nanotechnology Initiative, National University of Singapore , Singapore 117576.
authors.forename: Venugopal Jayarama
authors.initials: VJ
authors.lastname: Reddy
authors.affiliation: Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research) , 2 Fusionopolis Way, Innovis, #08-32, Singapore 138634.
authors.forename: Cedric
authors.initials: C
authors.lastname: Troadec
authors.affiliation: School of Engineering, Royal Melbourne Institute of Technology , Melbourne, Victoria 3001, Australia.
authors.forename: Madapusi Palavedu
authors.initials: MP
authors.lastname: Srinivasan
mesh_terms: D002245:Carbon Dioxide / Q000737:chemistry*; D006859:Hydrogen; D012666:Semiconductors; D012825:Silicon; D013438:Sulfhydryl Compounds; D013499:Surface Properties
chemical_list: D013438:Sulfhydryl Compounds; D002245:Carbon Dioxide; D006859:Hydrogen; D012825:Silicon
keywords: alkylthiol; bio-resistant; cell proliferation; hydrogen-terminated silicon; monolayer; supercritical carbon dioxide (SCCO2); superhydrophobic surface