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By Chun-Chun Hsu and Shu-Ping Lin


Silicon nanowire-based metal-oxide-semiconductor field-effect transistors (SiNW MOSFETs) havebeen demonstrated excellent sensitivity and stability after surface modification and functionalization ofnanowires. Chemical molecules have been applied to functionalize the surface of silicon surface. Silanecoupling agents are good candidates for forming self-assembled monolayers (SAMs) by chemicallyinteracting with silicon oxide. Those chemically modified SAMs can provide a functional surface tofurther conjugate biomolecules on SiNW MOSFETs. After functionalization, SiNW MOSFETs withtunably biocompatible surface can sustain a functional biointerface for biological tests. In this work,SiNW MOSFETs were fabricated using the standard I-line stepper of MOS semiconducting process andthen visualized by scanning electron microscopy (SEM). The n-type SiNW MOSFETs devices werefabricated after the process of trimming, the scale of nanowire was down to a level of approximate 165 nm.3-aminopropyl trimethoxysilane (APTMS) and 3-mercaptopropyl trimethoxysilane (MPTMS) SAMs wereindependently used to modify the surface of SiNW MOSFETs for pH sensing in biological buffer solution.Atomic force microscopy (AFM) and electron spectroscopy for chemical analysis (ESCA) were appliedto characterize before and after surface modification. AFM found APTMS and MPTMS were successfullymodified on silicon substrates. The average vertical length of APTMS and MPTMS SAMs from our AFMobservation was around 2.628 nm and 2.698 nm, respectively. ESCA showed the specifically functionalamino (-NH2) groups and mercapto (-SH) groups on each APTMS and MPTMS modified silicon substrates.The specific amine functional group at 399.4 eV occurred after the modification of APTMS on siliconsubstrate in N1s spectra. S2p spectra showed the specific binding at 163.6 eV (C-SH) and 165.8 eV (-C-SS-C-) after the modification of MPTMS on silicon substrate. Those disulfide bonds further influenced theorganization of MPTMS-SAM on the surface; therefore, the APTMS had better SAM performance on oursilicon substrate. On the other hand, electrical measuring system was used for elucidating that the suitablesurface modification would have great impact on the sensing response and sensitivity. Varied biologicalPBS solutions at different pH values showed that unmodified SiNW MOSFETs were sensitive to theH+ ion change. When the pH level of the solution increased, the drain current of the unmodified SiNWMOSFETs decreased accordingly. In comparison with unmodified nanowires in current measurement,the changes of current of APTMS or MPTMS modified nanowires were enhanced in sensing of differentpH solutions. Our results also showed that amino and mercapto groups of APTMS and MPTMS canimprove the protonation and deprotonation reactions in different pH solutions. Both APTMS and MPTMSmodified SiNW MOSFETs in pH sensings possessed good electrical sensing response and sensitivity incontrast with unmodified one. Moreover, in consequence of lower mercaptal groups of MPTMS on NWs,the relatively minor signal responses to varied pH solutions in MPTMS modified SiNW MOSFETs. Theelectrical measurement showed that the amino groups of APTMS significantly improve the sensitivity ofSiNW MOSFET in different pH sensings. Our results showed that adequate modification could provide afunctionable surface for SiNW MOSFETs. We inferred the APTMS modified SiNW MOSFETs could be areal-time sensor for different pH levels detection and further applied in monitoring biological environmentin the future

Topics: MOSFET, silicon nanowire, surface modification, sensitivity
Year: 2014
DOI identifier: 10.6287/JENCHU.2013.2403.04
OAI identifier:

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