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Highly stable bonding of thiol monolayers to HydrogenTerminated Si via supercritical carbon dioxide: Towards a super hydrophobic and bio-resistant surface Bhavesh Bhartia, Sreenivasa Reddy Puniredd, Sundaramurthy Jayaraman, Chinnasamy Gandhimathi, Mohit Sharma, Yen-Chien Kuo, Chia-Hao Chen, Venugopal Jayarama Reddy, Cedric Troadec, and Madapusi P. Srinivasan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06018 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 21, 2016

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ACS Applied Materials & Interfaces

Highly stable bonding of thiol monolayers to Hydrogen-Terminated Si via supercritical carbon dioxide: Towards a super hydrophobic and bioresistant surface Bhavesh Bhartia†‡, Sreenivasa Reddy Puniredd†*, Sundaramurthy Jayaraman§, Chinnasamy Gandhimathi¶, Mohit Sharma†, Yen-Chien Kuo#, Chia-Hao Chen#, Venugopal Jayarama Reddy¶, Cedric Troadec†* and Madapusi Palavedu Srinivasanǁ* †

‡

§

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08-32, Singapore 138634, Singapore. Department of Chemical and Biomolecular Engineering, National University of Singapore,4 Engineering Drive 4, Singapore 117585, Singapore. Environmental and Water Technology, Centre of Innovation, Ngee Ann Polytechnic, 535 Clementi Rd, Singapore 599489, Singapore.

¶

#

ǁ

Centre for Nanofibers and Nanotechnology, Nanoscience and Nanotechnology Initiative, National University of Singapore, Singapore 117576, Singapore. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan. School of Engineering, Royal Melbourne Institute of Technology, Melbourne, Vicotria 3001, Australia.

Abstract Oxide-free silicon chemistry has been widely studied using wet chemistry methods, but for emerging applications such as molecular electronics on silicon, nanowires based sensors and biochips, these methods may not be suitable as it 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 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 favourable physical properties. The identity of grafted 1 ACS Paragon Plus Environment


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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 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 significant decrease in cell proliferation owing to their superhydrophobicity combined with the rough surface morphology. Furthermore, tribological studies showed a sharp decrease in coefficient of friction which was found to be dependent on 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, bio-sensors, molecular electronics, micro- and nanoelectromechanical systems, antifouling agents and drug delivery. Keywords: Supercritical Carbon dioxide (SCCO2), Hydrogen Terminated Silicon, Alkylthiol, Monolayer, Superhydrophobic Surface, Cell Proliferation, and Bio-Resistant.

Introduction Monolayer grafting presents a facile, effective and versatile method to control and modulate the surface properties of semiconductor surfaces.1 Silicon is the most important material in the microelectronics industry, in part, due to its abundance and ease of processing but mainly because of electrical and chemical stability of silicon oxide (SiO2). To a large extent, silanes have been employed to form dense and well-ordered monolayers on semiconductors and are widely used to functionalize SiO2 via reaction with surface –OH groups.2 However, silane monolayer deposition is affected by a number of factors such as water content in solvent,3 2 ACS Paragon Plus Environment

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ambient humidity,4 surface –OH group concentration5 along with unfavourable cross-linking and limited stability.6 Current and future applications and devices that demand high performance require avoidance of the intervening insulating oxide between silicon and the functional surface.7 Field effect transistors based on oxide-free silicon nanowires (SiNWs) have been shown to exhibit better electrical and sensing performance by avoiding charge trap states associated with poor quality Si/SiO2 interface.7-8

Similarly, molecular electronics devices require

electrically active and direct contact with silicon and thus necessitate the use of oxide-free silicon without intervening insulating SiO2.9 Presence of a well-ordered and dense monolayer on oxide-free silicon provides much needed stability and passivation against oxide formation by neutralization of the surface dangling bonds.10 In addition to physical stability, monolayer functionalization presents a versatile method to control and tune the surface chemistry.8 Because of these advantages, monolayer grafted oxide-free silicon has found applications in 3D porous silicon,11 2D devices,12 1D silicon nanowires13 and 0-D silicon quantum dots.14 Many methods are commonly used to passivate and functionalize bare silicon via hydrogen terminated surfaces such as UV or white light irradiation,15 halogenation/alkylation using

Grignard

reagent,16

thermal

energy,17

radical

initiator,18

electrochemical

functionalization.19 However, the need for an inert processing environment due to the affinity of silicon for oxygen in ambient atmosphere, coupled with the long processing times (typically 10-24 h) required for these methods, poses challenges in scaling of these processes. Furthermore, the use of liquid organic solvents as the carrier fluid or pure liquid molecular solution (for monolayers of linear alcohols17) makes the surface vulnerable to trace amounts of water and oxygen present either in solvent or from ambient atmosphere.20 From an

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engineering point of view, if a carrier fluid itself acts as an inert environment for monolayer deposition on oxide-free silicon or any other air sensitive surface for that matter, it would be the best possible solution for the problem associated with liquid organic solvents, thus allowing easy processing and reproducible scalability. From previous studies, SCCO2 is known as an excellent carrier fluid for monolayer deposition,21 and was found to graft better quality monolayers compared to conventional organic solvent based methods on metal and metal oxide surfaces.22-23 In case of oxide-free silicon, apart from favourable physical properties of SCCO2 i.e. high diffusivity, absence of surface tension, low viscosity, the inert nature of CO2 provides the ideal contamination free isolated processing environment for grafting better quality monolayers.24 Recently, surface functionalization of oxide-free silicon using silicon-sulphur surface bond by direct grafting of thiol monolayer has received much attention. The strong and nonpolar Si-S surface bond has been the chief motivation for researchers’ interest in grafting thiol monolayers on oxide-free silicon.25-28 Alkylthiol monolayers are deposited by either ultra-high vacuum processes on bare silicon29 or exposing the hydrogen terminated silicon surface to white light/UV light/microwave,15, 30 high temperature31 or radical initiators32 in the presence of alkylthiol solution. The use of high vacuum conditions for monolayer deposition hinders the extension of techniques for practical applications at large scale. The liquid processing medium employed in other methods suffers from aforementioned disadvantages. Effect of these disadvantages is evident in the lower contact angle (~ 100° as opposed to the expected value of 110-113°) of the grafted monolayer surface that has been observed; these results hint towards poor monolayer quality and thus hinder the use of thiol grafted surface for practical applications. Also, the harsh processing conditions used in these methods make it difficult to translate these processes on brittle and fragile 1D Si nanostructures. Hence, there is a need to develop a process that can reliably and reproducibly 4 ACS Paragon Plus Environment

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deposit highly stable monolayers of thiol on oxide-free silicon for practical applications with easy scalability, low cost and benign environmental impact. In this work, we have demonstrated the applicability of SCCO2 processing for grafting of molecules on 2D Si (111), Ge (111) and 1D SiNWs. The similar experimental conditions were used to modify different surfaces demonstrates the versatility of SCCO2 for monolayer deposition - a necessary requirement for process scalability and versatility in practical applications. Monolayers of alkylthiols ranging from 1-heptanethiol to 1octadecanethiol (C7SH-C18SH) were deposited on oxide-free Si (111) using SCCO2 process. The inert nature of SCCO2 along with advantageous physical properties facilitated grafting of dense and well-ordered monolayers without oxide formation. The quality and stability of the monolayers were assessed for their passivation abilities against ambient atmosphere and aqueous conditions with the objective of deploying the monolayer in devices. Grafting of alkythiol on SiNWs resulted in complete reversal of surface wettability from superhydrophilic to superhydrophobic. This was further investigated with cell proliferation studies to observe the interaction of SiNWs with biological entities and their controlling factors. The monolayer grafted superhydrophobic SiNWs surface was observed to inhibit the growth of liver cells and can be employed in future for designing self-cleaning and antibacterial surfaces. The role of surface binding of monolayers on tribological properties was also carried out by means of a comparative study between alkoxy (Si-O-C) and alkylthiol (Si-S-C) monolayers. The SCCO2 process developed in this work provides an efficient route to deposit high quality thiol monolayers on oxide-free semiconductors which is a prerequisite for successful incorporation of new monolayer system in practical applications.



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Materials and Methods Materials Silicon (111) and (100) wafers (from Silicon Valley, Microelectronics Inc.) were 0.65 mm thick, n-doped (doped with phosphorus: 1016/cm3 and resistivity: 1-10 Ω-cm), polished on one side with native oxide. Ge (111) wafers were 0·5mm thick, n-doped (doped with antimony, resistivity: 1-10 Ω-cm), polished on one side with native oxide. Chemicals: alkylthiols (>98% purity, from Sigma-Aldrich), HPLC grade tetrahydrofuran (THF) (from Tedia), analytical grade acetone, isopropanol (from Fisher chemical), hydrogen peroxide (from Merck), sulphuric acid (from J.J. Baker), hydrofluoric acid (HF), ammonium fluoride (both from Sigma-Aldrich), CO2 (>99.8% purity, from SOXAL Code P40J purified grade with SiNW-H > SiNW-S-C18. Also, adhered cells were segregated and not colonized on all the SiNW surfaces when compared to TCP and Si/SiO2 (see, Figure 11C-11E). As shown in Figure 8A, the separated SiNWs bundles restrict the inter cell interaction, hence resisting cell colony formation. Furthermore, the round cell morphology on SiNW-H and SiNW-S-C18 surfaces were observed while flat cell morphology was observed on Si/SiO2, TCP, Si/SiO2NW surfaces.58, 63

These results were analogous to water droplet behaviour on these surfaces. In case of

SiNW-H and SiNW-S-C18 surfaces, the sharp, stiff and individually protruding SiNW with super hydrophobic surface provides very limited area for cell to adhere to the surface and as a result cells assume the round shape. The cells rest on the surface according to Cassie-Baxter model, with air pockets residing between the cells and the surface.63 To summarize, it was observed that both physical and chemical properties of the surface play an important role in defining the surface response to biological species. In this work, oxide-free SiNWs functionalized with long chain alkylthiol molecules were able to successfully resist cell proliferation with controlled cell morphology. These significant results can be utilized for fine tuning the surface properties by varying the terminal group of thiol molecules to vary the surface properties appropriately.


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Figure 11: CMFDA dye expression for different surfaces: (A) TCP, (B) Si/SiO2, (C) Si/SiO2NW, (D) SiNW-H, (E) SiNW-S-C18

Tribology of Alkylthiol Monolayers In addition to air and aqueous stability of monolayers, mechanical stability plays an important role in defining the monolayer’s overall quality.64 In nano- and micro- electromechanical systems (MEMS/NEMS), where surfaces are in close contact with each other or involve a relative movements of surfaces, tribological properties such as wear, friction and adhesion properties of surfaces plays a critical role in determining devices lifetime.64



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Previous studies on monolayer tribology have shown that the frictional properties are a function of monolayer chain length and terminal group.65 Increasing chain length and low energy terminal group have shown better lubrication properties. Methyl terminated or fluorine terminated monolayer were found to be more lubricating compared to polar terminal group such as -OH, -COOH.66 However, very few studies have been conducted to study the

effect of surface binding of monolayer on frictional properties and even fewer have been carried out on oxide-free silicon. The effect of surface binding on monolayer’s frictional properties have shown to be a function of strength, polarity and binding of surface atom with monolayer head group.67 The alkylthiol monolayer deposited on hydrogen terminated silicon were studied for micro-macro frictional properties using pin on disk tribometer. High sliding speed, greater loading capability with low contact pressure used in microtribology mimic the conditions of practical working devices compared to other techniques used such as surface force apparatus (SFA) or atomic force microscopy (AFM).65-66 Comparative studies were carried out on 1octadecanethiol and 1-octadecanol monolayers having Si-S-C and Si-O-C surface binding respectively. 1-octadecanol monolayer was deposited using SCCO2 processing method published elsewhere.24, 68


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Figure 12: Coefficient of friction on oxide-free Si (111) surface for (A) alkylthiol monolayers of different chain length (C(n)SH, n refers to alkyl chain length), (B) C18SH (1-octacanethiol) and C18OH (1-octadecanol) monolayer.

As shown in Figure 12A, a decrease of 50% in coefficient of friction was observed in presence of C8 monolayer compared to freshly etched silicon and a further reduction of 37% was noticed with an increase in chain length to C18. The variation of coefficient of friction with respect to chain length can be attributed to the increase in order and density of the monolayer with increase in alkyl chain length. The results obtained are in agreement with similar studies carried out on silanes on silicon dioxide/silicon interface,67 mica69 and thiols on gold using different measurement techniques.65 The effect of head group binding was observed by difference in the coefficient of friction for C18SH (0·120) and C18OH (0·300) monolayers (see, Figure 12B). According to previous reports, the alkylthiol monolayers were reported to be tilted at an angle of 57°, while alkoxy monolayers were reported to be more upright with tilt angle of 5-15°.18, 25, 30 Also, from our analysis of tilt and twist angles using non-polarized FTIR spectra as shown in Figure S10, 1-octadecanol monolayer was found to be more upright (tilt angle ~ 9°) as compared to C18SH (tilt angle ~ 15°) (for further details, see, SI).40 Due to difference in tilt angle, 1-octadecanol monolayer was observed to be more rigid vis-à-vis high coefficient of friction than flexible 1-octadecanethiol monolayer which allowed easy slipping under applied load. Two more contributing factors for the difference in coefficient factors could be at play: (i) the electronegativity difference between 31 ACS Paragon Plus Environment


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Si (1·9) and S (2·5) is lesser than Si (1·9) and O (3·5), hence, it is possible that the difference in µ can be explained by partial ionic nature of Si-O bond compared to covalent Si-S bond.70 (ii) in addition, the difference in monolayer ordering and density of 1-octadecanol and 1octadecanethiol can also explain the observed variation in µ values, although in future, more detailed analysis is needed to confirm the determining factor.

Conclusion In this work, we presented a simple and versatile method to graft stable and dense alkylthiol monolayer on different oxide-free 1D and 2D semiconductors using SCCO2 as a processing fluid with similar reaction conditions. Deposited monolayer exhibited excellent surface passivation capability and were able to resist oxide formation on surface for more than 50 days upon exposure to ambient atmosphere. The stability of grafted monolayer is attributed to strong and non-polar Si-S-C surface bond of alkylthiol monolayer, and also clean and inert environment of SCCO2 processing. Furthermore, to extent the purview of herein developed SCCO2 process, alkylthiol monolayers were also deposited on oxide free Ge (111) and 1D SiNWs surfaces. Monolayer grafting on SiNWs resulted in complete change of surface wettability from superhydrophilic to superhydrophobic. Consequently, the alkylthiol modified SiNWs surface were able to resist cell proliferation on surface for more than 15 days. These results suggest that monolayer grafted oxide-free SiNWs can be a viable material for future biological applications and the cell growth can be further modulated for many biological applications by tuning the surface chemistry of oxide-free SiNWs in SCCO2. Alkylthiol monolayers on oxide-free 2D silicon surface were studied for their tribological performance. Coefficient of friction was found to be inversely proportional to chain length; upon further preliminary investigation, coefficient of friction was found to vary as a function 32 ACS Paragon Plus Environment

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of head group binding. The results obtained in the study can be exploited in the future for practical application such as ultra-thin lubrication coating for NEMS devices. So far the reported literature has been focused on the variation of tail group to modulate the work function and fine tune the electronic properties for molecular electronics and bio sensing applications. The herein developed method to deposit stable and dense alkylthiol monolayer on oxide-free semiconductors opens up new avenues for advanced studies on Si-S surface bond and its effect on surface electronic structure such as band alignment, fermi-level positioning and electron transport for molecular electronics. The Si-S bonds of alkylthiol monolayers, unlike Si-C and Si-O-C bonds of alkyl and alkoxy monolayers, would be an excellent candidate for monolayer doping of semiconductors and can be employed in fabrication of ultra-shallow junctions and also doping of 1D semiconductor structures which are difficult to dope using conventional techniques.

Supporting Information Supporting Information: Schematic for SCCO2 process, FTIR spectra for Si-H absorbance peak of all alkylthiol monolayer, HRXPS spectra of C 1s peak for C18SH, C12SH and C10SH monolayer, XPS spectra of Si 2p with log scale intensity for all alkylthiol monolayers, AFM images for freshly etched Si-H, C7SH, C12SH and C18SH monolayers, evolution of XPS spectra of C 1s during the passivation studies of C18SH monolayer, sequential images for water droplet spreading on SiNWs, SEM images for effect of drying technique on SiNWs are available free of charge at http://pubs.acs.org.

Author Information Corresponding Authors



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*Email: [email protected] *Email: [email protected] *Email: [email protected]

Notes Authors declare to competing financial interest.

Acknowledgement The authors are grateful to National University of Singapore for financial support to B.B. in the form of research scholarship. S.R.P. and C.T. acknowledge funding provided by IMRE, A*STAR. C.-H.C. thanks the financial support of the Ministry of Science and Technology, Taiwan, and the skilful assistance of NSRRC staff.

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