Polymers, Plasmons, and Patterns: Mechanism of Plasmon-Induced

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Polymers, Plasmons & Patterns - Mechanism of Plasmon-induced Hydrosilylation on Silicon Fenglin Liu, Tate C Hauger, Brian C. Olsen, Erik J. Luber, and Jillian M. Buriak Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04504 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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Chemistry of Materials

Polymers, Plasmons & Patterns - Mechanism of Plasmon-induced Hydrosilylation on Silicon Fenglin Liu†,‡, Tate C. Hauger†,‡, Brian C. Olsen†,‡, Erik J. Luber†,‡* & Jillian M. Buriak†,‡* † Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, AB T6G 2G2, Canada. ‡ National Institute for Nanotechnology, National Research Council Canada, 11421 Saskatchewan Drive, Edmonton, AB T6G 2M9, Canada.

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ABSTRACT Directed-assembly for nanopatterning on semiconductor surfaces is of interest as a costeffective approach for lithography on silicon, that is complementary to photolithography. In this work, self-assembly of block copolymers is used to produce nanoscale hexagonal arrays of gold hemispheroids, which are then incorporated into an optically transparent, flexible PDMS stamp. These ‘plasmonic stamps’ can then be used to drive hydrosilylation of alkenes and alkynes on hydride-terminated silicon surfaces upon illumination with low intensity green light (which corresponds with the absorption of the localized surface plasmon, LSPR, resonance of the gold nanostructures). The resulting hexagonal arrays of nanoscale alkyl or alkenyl patches mirror the spacing of gold nanoparticles in the parent plasmonic stamp. Close examination of the hydrosilylated patches reveal that they are not continuous across the 20-30 nm-diameter patches, but instead display an annular motif, which closely resembles the plasmonic electric field (Efield) distribution of the gold hemispheroids embedded within the stamp. The localized surface plasmon appears to drive the hydrosilylation reaction on the silicon surface via formation of electron-hole pairs within the silicon, or injection of hot holes. The yield of hydrosilylation is, however, strongly influenced by the doping of the silicon, and the distance between the plasmonic stamp and the silicon surface. A more nuanced mechanism is thus proposed, involving band bending at the metal-insulatorsemiconductor junction, where plasmonically injected/generated holes are swept towards the surface. The accumulation of holes at the silicon surface is the key element of the mechanism, as this step is followed by nucleophilic attack of the alkene or alkyne, to produce the silicon-carbon bond.


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INTRODUCTION Large scale nanopatterning of well-defined organic molecules on semiconductor surfaces allows for engineering of these interfaces at the molecular level, for applications in the areas of organic electronics,1 sensors,2 molecular recognition,3,4 tissue interfacing,5 cell growth,6 and others.7–10 Interest in bottom-up lithographic methods, based upon selfassembly, is of increasing interest for sub-20 nm patterning due to the low-cost nature of this approach when compared to photolithography.11–13 The directed self-assembly of dior triblock copolymers is highlighted by the International Technology Roadmap for Semiconductors (ITRS) as a potentially viable route towards the generation of nanopatterned templates on semiconductor surfaces.14–20 As is typical, a thin film (~30-50 nm thick) of a self-assembled di- or triblock copolymer acts as a nanoscale template on the surface of interest, and the chemical differences between the blocks can be harnessed to enable selective removal of one the blocks.21–24 In the example of a self-assembled thin film of polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) diblock copolymer on a single crystalline Si wafer, the PMMA block can be selectively removed to leave a nanopatterned PS template on the surface, which can then be used as an etch stop to produce complex, three-dimensional silicon nanostructures.25–28 In another example, one block in a self-assembled thin film can be loaded with a reagent such as a metal ion and subsequently converted into a metal nanopattern that mirrors that of the parent block copolymer.19,29–32


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Figure 1. a) Scheme of a typical procedure for plasmonic stamp fabrication: gold nanopatterns obtained from self-assembly and metallization of block copolymers were firstly coated with cured PDMS, and transferred to the surface of the PDMS layer by a subsequent peel-off step, resulting in the formation of plasmonic stamp with Au nanopatterns on the surface; b, c) SEM and AFM micrographs of Au nanopatterns on the silicon surface; (PS-b-P2VP (125k-58.5k)) Scale bar: 200 nm. d) Optical picture of a plasmonic stamp carrying Au nanopatterns on the surface. Scale bar: 1 cm. It was demonstrated earlier that block copolymer self-assembly could be performed independently of the surface functionalization via use of a catalytic stamp.33–36 A metal nanoparticle array was prepared via block copolymer self-assembly, and then incorporated into a flexible, optically transparent PDMS stamp (Figure 1). Metal nanoparticle arrays were produced from a self-assembled thin film of the diblock copolymer, polystyrene-block-poly(vinyl-2-pyridine) (PS-b-P2VP), on a native-oxide capped silicon surface, and then lifted-off from the surface as the PDMS-based stamp. Catalytic stamps with embedded palladium nanoparticle arrays were tested as the first generation of stamps, and were applied towards patterned stamping of localized heterogeneous catalysis on monolayers on silicon surfaces, including hydrogenation of terminal azido groups to amines and Heck coupling reactions,36 as well as direct hydrosilylation of alkenes, alkynes, and aldehydes on Si(100)-Hx.34,35


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Recently, it was demonstrated that the localized surface plasmon resonance (LSPR) of colloidal gold nanoparticles,37 and arrays of gold hemispheroids within a PDMS stamp,33 could also drive hydrosilylation of alkenes on silicon surfaces via completely different mechanisms than those typical of heterogeneous metal nanoparticle catalysis. The mechanism proposed for the observed plasmon-driven hydrosilylation invoked local generation of electron-hole pairs in the silicon due to the enhanced local electric field of the plasmon. The generation of holes at the silicon surface was the lynchpin of the mechanism - the positive charge at the hydride-terminated silicon was attacked by the alkene, leading to the silicon-carbon bond formation event, as has been postulated in other related mechanisms.38–42 In this paper, the mechanism of action of gold plasmons on the chemical reactivity of a proximal silicon surface was reevaluated and refined in light of recent work in the area of plasmonics, and new data obtained, as described here. To broaden the scope of the plasmon-driven surface chemistry and to further interrogate the mechanism via chemistry, the hydrosilylation reaction was generalized to alkynes as well as alkenes, and the recyclability/reusability of the plasmonic stamp was investigated to demonstrate that the stamps could be reused.

Figure 2. Scheme of plasmonic stamping on flat Si(111)-H surface: White light was filtered by a green optical filter (center wavelength 526 nm, FWHM: 180 nm), which then passes through a transparent PDMS stamp carrying Au nanopatterns. After plasmonic stamping, the pattern can be transferred from the stamp to a Si(111)-H surface via the LSPR-assisted localized hydrosilylation of 1-alkenes or 1-alkynes.


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RESULTS Gold hemispheroid arrays, formed via diblock copolymer self-assembly and embedded within a PDMS stamp, were prepared as previously described (Figure 1). By changing the molecular weight of the BCP, the center-to-center spacings can be modulated. For instance, self-assembly of PS-b-P2VP (125k-58.5k) resulted in pseudo-hexagonal arrays of gold hemispheroids with diameters of ~20 nm and center-to-center spacings of ~70 nm; PS-b-P2VP (190k-190k) yielded arrays of gold hemispheroids with diameters of ~35 nm and center-to-center spacings of ~150 nm (Figures 3a,b). The gold nanoparticle arrays, still on the original native-oxide capped silicon surface, were then embedded within a 3 mm-thick layer of PDMS precursors, allowed to cure, and then peeled off to yield the free plasmonic stamp. The plasmonic stamp absorbs in the visible region at ~530 nm due to the local surface plasmon resonance of the embedded gold nanoparticles (Figure S1a).

Figure 3. a) and b): AFM height micrographs of gold hemispheroid nanopatterns on a flat silicon surface from PS-b-P2VP (125k-58.5k) and PS-b-P2VP (190k-190k), respectively. c) and d) are the corresponding AFM height images of p-type Si(111)-H surfaces after 60 min of stamping with 1-dodecene c) and 1-dodecyne d). Scale bar: 400 nm.


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Application of the plasmonic stamp to drive silicon surface-mediated chemistry involved sandwiching a thin layer of the selected alkene or alkyne between the plasmonic stamp and a freshly prepared Si(111)-H surface, as shown in Figure 2. Illumination of the silicon surface, through the PDMS stamp and alkene/alkyne layer with low-intensity green light (50 mW/cm2, 526 nm bandpass filter with FWHM of 180 nm, Figure S1b) initiated localized hydrosilylation on the silicon surface. After 60 min of illumination, the silicon surface was rinsed with dichloromethane to reveal the pseudo-hexagonal nanopatterned array of alkyl or alkenyl patches on the silicon surface. Figures 3a and b show AFM micrographs of the pattern of the parent gold hemispheroids on flat silicon substrates, and Figures 3c and d show the corresponding hydrosilylated domains produced on the Si(111)-H surface with 1-dodecene and 1-dodecyne, respectively. Two other examples demonstrating the hydrosilylation of alkynes with chlorine- and fluorinetags are shown in Figure S2 and Table S1 in the Supplementary Information, with corresponding AFM micrographs, XPS spectra, and water contact angle measurements. Reusability would be important for a ‘real world’ application to minimize the necessity to fabricate new stamps. As shown in Figure S3, the plasmonic stamp could be reused three times after cleaning with dichloromethane and hexane after each use. Contact angle measurements reveal a minor degradation in the stamping process, which drop from 96°, to 95° to 93° after successive stamping and cleaning cycles.

Figure 4. a) AFM micrographs of 1-dodecyl patterned p-type Si(111)-H surface after plasmonic stamping; b) Zoomed-in AFM micrograph shows ring-like dodecyl patterns on Si(111)-H surfaces. BCP: PS-b-P4VP (20k-19k) Scale bars: 200 nm for a); 20 nm for b).


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Upon close inspection of the AFM micrographs of the patterned Si(111)-H surfaces with hydrosilylated alkyl or alkenyl groups, the hydrosilylated patches appear to be more akin to rings, as opposed to uniform regions, particularly those produced with the smaller gold hemispheroids, as shown in Figure 4. Other examples are shown in Figure S4. These thin (97%), trichloro(1H,1H,2H,2H-perfluorooctyl)silane (98%), 1,7-octadiene(>99.0%), 1-dodecene (>99.0%), 1H,1H,2H-perfluoro-1-decene (99.0%), 1-dodecyne (99.0%), 4(Trifluoromethoxy)phenyl acetylene (99.0%), 3,4-dichlorophenylacetylene (99.0%), ethanethiol (97.0%), 1-butanethiol (99.0%), 1-hexanethiol (>98.0%), 1-dodecanethiol (99.0%), and 1-hexadecanethiol (99.0%) were obtained from Sigma-Aldrich. 1-dodecene, 1,7-octadiene, 1H,1H,2H-perfluoro-1-decene, and 4-(Trifluoromethoxy)phenyl acetylene were passed through a hot alumina (dried at 100 °C for over 24 h and used while still hot) column to remove water residues and peroxides, and then deoxygenated with a stream of nitrogen gas. Optical filters (CW526 (centre band wavelength: 526 nm); FWHM (full width at half max: 180 nm) were purchased from Edmund Optics Inc. Characterization. Plan-view scanning electron microscopy (SEM) was carried out with a field emission scanning electron microscope (S-4800, Hitachi), and cross-section and


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tilted SEM images were obtained using a field emission scanning electron microscope (S5500, Hitachi); the working pressure for SEM imaging was < 10-8 Torr. Tapping mode atomic force microscopy (AFM) height micrographs were captured using a Digital Instruments/Vecco Nanoscope IV with silicon PPP-NCHR cantilevers (Nanosensors). Sessile drop contact angle measurements were measured on a contact-angle goniometer (Model 100-00, Rame-Hart) using a 5 μL water droplet after the droplet reached the equilibrium state. X-ray photoelectron spectroscopy (XPS) spectra were recorded using an X-ray photoelectron spectrometer (Kratos Axis 165) with binding energies calibrated to the C(1s) (285.0 eV). Ellipsometric measurements were performed using a J.A. Woollam M-2000V variable angle spectroscopic ellipsometer (VASE). Prior to determination of silicon dielectric constants, the substrates were hydrogen terminated as previously described, then immediately transfer into a nitrogen glovebox and sealed sealed in a vial. Ellipsometry was performed in atmospheric conditions, however, less than 10 minutes elapsed between opening the sealed vials and then end of data acquisition for each substrate. Ellipsometric scans were performed at an incident angle of =70°, Δ = 2 nm and 20 revolutions per data point. The substrate dielectric constants are calculated using53 tan

(cos 2 − sin 2 sin (1 + sin2 cos )

= sin

[1 +

= sin

tan sin4 sin (1 + sin2 cos )

Where

and

)

]

are the usual ellipsometric angles,

is the angle of incidence and tan =

1/tan . Silicon wafer cleaning. Silicon chips (1 cm x 1 cm) were fabricated by cutting with a dicing saw (Disco DAD 321). After 15 min of sonication in methanol and drying with a nitrogen gas stream, the wafers underwent a standard RCA cleaning procedure by sequentially immersing in base [volume ratio of 1:6:1 for 30% NH4OH (aq): DI water: 30% H2O2 (aq)] and acid [volume ratio of 1:5:1 for 37.5% HCl (aq): water: 30% H2O2 (aq)] at 80 °C. Finally, the wafers were rinsed with DI water and dried with a nitrogen stream.


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Preparation of Au nanopatterns on Si/SiOx. Typically, the BCP solution of PS-b-P4VP (20k-19k), (or PS-b-P2VP (190k-190k)) with a concentration of 1.0 wt.% in toluene was spin cast at 4000 rpm for 15 s on a clean, native oxide-capped silicon wafer to allow for the BCP self-assembly on the surface. As for PS-b-P2VP (125k-58.5k), an additional annealing procedure was applied by placing the BCP-coated silicon wafer into a customized chamber for 60 min.65 Then, the wafer was immersed in 10 mL of a 10 mM KAuCl4 (aq) solution for 10 min to allow for the metal ion loading of the PVP blocks. Finally, an oxygen plasma was applied to the sample using a plasma cleaner (Harrick PDC 32G, 18 W) at 0.8 Torr for 10 min to remove the BCP and obtain the gold nanopatterns. Another 2 min of H2/Ar plasma was followed so as to completely remove polymer residues. Plasmonic stamp preparation. The preparation of plasmonic stamps was described in our previously published work.33 Briefly, the Au patterned silicon wafer was exposed in trichloro(1H,1H,2H,2H-perfluorooctyl)silane vapor in a sealed desiccator under vacuum (1 Torr, 60 min), and then rinsed with ethanol, and dried with a stream of nitrogen gas. The precursors to h-PDMS, a mixture of VDT-731, HMS 301, 2,4,6,8-tetramethyl2,4,6,8-tetravinylcyclotetra-siloxane, and SIP6831.2, were spin-coated and cured on the gold-patterned silicon surface. Then, the 184 PDMS, degassed by applying a vacuum to the PDMS prepolymer three times (volume ratio of polymer base: curing agent is 10:1) were added to the h-PDMS-coated wafers in a Teflon mold with the final PDMS layer thickness of ~4 mm. Another curing step was applied in a vacuum oven at 65 °C for at least 3 hours. The cured PDMS stamp with embedded Au nanopatterns was slowly and carefully peeled off from silicon surface and then cleaned with Soxhlet extraction in hexane for 6 hours, rinsed with ethanol/water, and stored under vacuum. Plasmonic stamping of hydrogen terminated Si(111) surfaces. The hydrogen terminated silicon surfaces are obtained by soaking the Si(111) wafers in degassed 40% NH4F (aq) for 5 min, and then in deoxygenated water for 10 s. The wafer was then dried with a stream of argon gas prior to use; the silicon surfaces were then used immediately. The plasmonic stamping was conducted in a nitrogen-filled glovebox. Typically, 30 μL of neat alkene or alkyne was carefully dropped on the hydrogen-terminated Si(111)


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wafer, and quickly covered by the PDMS plasmonic stamp with the Au pattern facing down on the silicon surface. For the solid alkyne, 3,4-dichlorophenylacetylene, a solution needed to be used: 30 μL of a 100 mM 3,4-dichlorophenylacetylene solution in a hexane:toluene (v:v = 3:1) was used for stamping. A glass slip was placed on top of the plasmonic stamp and another below the silicon wafer to form a glass slip/plasmonic stamp/alkene or alkyne/Si(111)-H wafer/glass slip sandwich, with two customized paper clamps applied at both sides of the glass slips (Figure S7). White light (300 W ELH bulb) was focused through a PCX lens and filtered through the CW526 band-pass filter, and shone onto the sandwiched sample for 60 min, with an incident intensity of 50 mW/cm2 (measured with a metrologic radiometer). Upon completion of the reaction, the wafer was rinsed repeatedly with dichloromethane and dried with a stream of argon gas. Thiol-ene coupling and gold electroless deposition. The procedure for visualization of an alkyl-patterned surface via gold electroless deposition closely follows the method previously described,66 but was slightly modified here. Briefly, a hydrogen terminated Si(111) wafer was first stamped with 1,7-octadiene to form omega-terminated alkeneterminated groups on the silicon surface. Then, 100 μL of neat 1,4-butanedithiol was dropped on the surface under the illumination of ultraviolet visible light (254 nm, UV pen lamp, Model 11SC-1) for 30 min, and then rinsed with dichloromethane to remove unreacted molecules. Lastly, the thiol-patterned sample was immersed in a beaker containing 10 mL of 0.1 mM HAuCl4 (aq) for 30 min, followed by the addition of 100 μL of 0.1 mM ascorbic acid. The processed surface was finally rinsed with DI water and dried with a stream of nitrogen gas. FDTD simulations. Simulations were carried out using a the commercial software Lumerical FDTD Solutions. The gold nanoparticles were modelled as hemispheroids with a 10 nm radius. Center-to-center spacing of the particles was set to be 70 nm. A nonprimitive rectangular unit cell was used for the simulation, and further refined using symmetric and periodic boundary conditions and the x and y directions (Figure S8). In the x-direction, the 70 nm-wide unit cell was broken into 160 mesh units of 0.438 nm, and in the y-direction the 120 nm tall unit cell was broken up into 280 mesh units of 0.429 nm. For the z-direction, the simulation had a span of 2000 nm centered around the


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gold nanoparticles. The maximum mesh was set to 0.2 nm for a range of 100 nm on each above and below the particles, and the remainder were free to mesh dynamically. The boundary conditions were set as perfectly matched layers at the z extremes. To simulate unpolarized light, two completed simulations were run with x- polarized and y- polarized light. The resultant time averaged electric field from each polarization was then added together, and then divided in half. The apparent four-fold symmetry located at the extremes in the top view of Figure 6b results from the discretization of the spheroid into mesh cells. To confirm this, a simulation was carried out with light polarized at ± 45°, the time averaged electric field of each polarization was added together and divided in half. ASSOCIATED CONTENT Supporting Information UV-vis spectra of plasmonic stamps, AFM micrographs and XPS spectra of Si surfaces with fluorinated alkynes, stamp reusability data, additional AFM micrographs of ring-like nanopatterns, dielectric constants of Si substrates with different doping densities, photograph of plasmonic stamping setup, scheme of FDTD simulation cell and tables of contact angle measurements. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This work was supported by grants from the Natural Sciences and Engineering Research Council (NSERC, grant number RGPIN-2014-05195), Alberta Innovates Technology Futures (fellowship to CJ, and grant number AITF iCORE IC50-T1 G2013000198), and the Canada Research Chairs program (CRC 207142). FL thanks the China Scholarship Council (CSC) for a scholarship. Electron microscopy was carried out at NRC-NINT.


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Polymers, Plasmons, and Patterns: Mechanism of Plasmon-Induced

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