Deposition and Characterization of Roughened Surfaces

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Deposition and Characterization of Roughened Surfaces Hagit Aviv, Shirly Berezin, Ortal Agai, Miri Sinwani, and Yaakov R. Tischler Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04392 • Publication Date (Web): 03 Feb 2017 Downloaded from http://pubs.acs.org on February 4, 2017

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Deposition and Characterization of Roughened Surfaces Hagit Avivʇ,a,b, Shirly Berezinʇ,a,c, Ortal Agaia,b, Miri Sinwania,b, and Yaakov R. Tischler*a,b

a.

Bar-Ilan Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Israel

b.

Department of Chemistry, Bar-Ilan University, Israel

c.

Department of Physics, Bar-Ilan University, Israel

* Corresponding author: E-mail: [email protected] Phone number: +972 50 4168008

ʇ

These authors contributed equally to this work.

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ABSTRACT Phase separation occurs whenever a solvent leaves a solution of strongly incompatible polymers. This can happen in a bulk and in films. Films can be tailored as substrates for multiple applications, such as solar cells, surface catalysis, and anti-reflection coatings. In this research, polystyrene (PS) was dissolved with polyvinyl acetate (PVAc) in a few different ratios using chlorobenzene as the solvent. Thin films of the different ratios were deposited on glass via spin coating. The deposited films were investigated for morphology, strain, surface area and Raman scattering. The incompatibility between the two polymers leads to growth of roughened PVAc islands supported by the PS matrix. Down shift in the Raman PVAc signal was observed in the combined film as compared to a 100% PVAc film, which was attributed to the high strain of PVAc grown as tips. As PVAc concentration in the polymer blend increases, the porous regions in the film expand and the amount and height of PVAc tips increases as well, up to the point where the pores merge to create a uniform surface. The optimal ratio for the deposition of a uniformly roughened surface is 75% PVAc and 25% PS. For demonstrating a possible application, we applied the partially roughened surface as a substrate for surface enhanced Raman scattering (SERS) and demonstrated at least 500% increase in the signal intensity measured in roughened areas. This is explained by the rod effect given by PVAc tips.

Keywords: Polymer blend, Phase separation, Raman scattering, strain, SERS.


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1. INTRODUCTION Phase separation occurs whenever a solvent leaves a solution of strongly incompatible polymers. The interesting physical and chemical properties of the deposited polymers in bulk and films have attracted many research groups during the last few decades1–3. The films can be tailored as substrates for multiple applications, such as solar cells4, antireflection coatings2, and lithography processing3. Granular substrates were also reported as a waveguide surface for random lasing5. Surface enhanced Raman scattering (SERS) provides a significant enhancement in the Raman signal that is attributed to an enhancement in the electric field of the metallic nanostructures on the substrate’s surface6–8. For certain applications, robust coatings are required; this is obtained by covalently binding the molecules to the surface. Attachment of coating molecules to a surface by one end can lead to the formation of densely packed, well-defined layers. Two such coatings are self-assembled monolayers (SAMs) and polymer brushes9–11. The different methods for phase separation presented by the different studies have motivated further investigations of morphological control of phase separating polymer mixtures. Many applications require control in films' deposition, in order to create surfaces with different properties. It has been shown that the phase morphology depends on polymer molecular structures, solvent, composition, molecular weights, and the method of blend preparation, it can also be influenced by the substrate surface free energy1,12,13. The surface structures of the incompatible polymer blend films, obtained by means of spin-coating, are not in a thermodynamic equilibrium state9–11. To reduce the interfacial


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tension, the coarsening of the domains near to the surface towards equilibrium will take place upon annealing. Previous studies have proposed a kinetic model for the phase separation process based on rate equations of processes such as adsorption and polymer growth in the film14,15. Modeling proved to fit film growth of a variety of nanostructures. The modeling results showed that the thin film structure depends on the ratio of diffusion coefficient near the surface over the growth rate15. In this work we present the deposition of thin films containing different ratios of polystyrene (PS) and polyvinyl acetate (PVAc) using chlorobenzene as the solvent. The films were investigated for morphology, strain, surface area and Raman scattering. The incompatibility between the two polymers leads to growth of roughened PVAc islands supported by the PS matrix. As PVAc concentration in the polymer blend increases, the porous regions in the film expand and the amount and height of PVAc tips increase accordingly, up to the point where the pores merge to create a uniform rough surface. High correlation was observed between Raman scattering and the strain of different areas in the films. We also demonstrate that using the bumpy surface achieves additional enhancement over the regular gold SERS substrate.

2. MATERIALS AND METHODS Materials. The following analytical-grade chemicals were purchased from AldrichSigma and used without further purification: Micro-90 semiconductor grade detergent, acetone (99.9%), isopropanol (99.5%), chlorobenzene (99.8%), anhydrous chloroform


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(99%), poly (vinyl acetate) (PVAc, average Mw: 140,000), polystyrene (PS, average Mw: 192,000), Fullerene-C60 (99.5%), gold beads (99.999%). Deionized (DI) water was obtained by purifying water through a Barnstead EASY Pure II osmosis system (Thermo Fisher Scientific Inc.). Thin films preparation via spin coating. A total of 50 mg/mL of the following polymers/polymer-blends was dissolved in chlorobenzene: 100% PVAc, 100% PS, 25% PVAc & 75% PS, 50% PVAc & 50% PS, and 75% PVAc & 25% PS. A clear solution was achieved after one hour of sonicating followed by 10 hours of stirring at room temperature. Thin films of approximately 400 nm thick were deposited from the above solutions on glass slides by spin-coating technique. The polymers dissolved in chlorobenzene were spin-coated at 1000 rpm for 90 seconds, with a ramp acceleration of 1000 rpm/s, using a Headway Research PWM32 spin-coater. Before spin-coating, the pre-cleaned substrates were exposed to oxygen-plasma to promote adhesion of the film. The thicknesses of the spin-coated films were measured by a stylus profilometer (Dektak 150, Veeco). Surface topography was probed using AFM and Amplitude features. All measurements were performed using a MultiProbe system with a dual microscope from Nanonics Imaging which was free-space coupled to a LabRam HR Micro-Raman microscope from Horiba. The probe diameter was 20 nm and the scans were performed at phase feedback mode.


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Topography images were processed by WSXM program using "flatten" feature. Profile curves and surface area calculations were performed by Gwyddion, SPM data visualization and analysis. Raman scattering measurements were taken using a micro-PL set-up (HORIBA Scientific LabRAM HR) in air at room temperature. The polymeric thin films were excited by a laser with an excitation wavelength λex = 532 nm at 30 mW with an acquisition time of 10 seconds and a grating density of 1800 g/mm. The C60 film was excited by a laser with an excitation wavelength λex = 784 nm with an acquisition time of 10 seconds and a grating density of 1800 g/mm. Thin films preparation via thermal evaporation. A thin film of gold with a thickness of 15 nm was deposited on top of the substrates at a rate of 0.5 Å/sec using thermal evaporation system (Nano 36, Kurt J. Lesker) with base pressure of 10-6 Torr. On top of that, a thin film of 8 nm Fullerene-C60 was thermally evaporated on the substrates at a rate of 1 Å/sec.

3. RESULTS AND DISCUSSION In this research, several different ratios of polymers were prepared for thin film deposition via spin coating. Figure 1 indicates that the incompatibility between the highly hydrophobic polymer, PS and the somewhat hydrophilic polymer, PVAc, leads to heterogeneously deposited films. Figure 1a presents the thin film prepared with a solution containing 25% PVAc and 75% PS. This image shows that PVAc grows as tips on top of a thin layer of PS, and as the solvent continues leaving the polymers, PS grows on top of


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the substrate, surrounding PVAc-rich areas and eventually creates pores. This growth pattern occurs during phase separation due to the high degree of incompatibility between the two polymers. Figure 1b presents a thin film prepared with a solution containing 50% PVAc and 50% PS. In this figure the trend continues, as the higher concentration of PVAc within the film leads to bigger pores that contain significantly more tips. Figure 1c presents a thin film prepared with a solution containing 75% PVAc and 25% PS; here the pores merge to create a uniform roughened surface.

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Figure 1: AFM topography images of thin films containing PS and PVAc in different ratios. a- Presents 25% PVAc and 75% PS, b- Presents 50% PVAc and 50% PS, and cPresents 75% PVAc and 25% PS. The solvent of all three solutions is Chlorobenzene.

Figure 2 presents profile curves extracted from AFM topography images of thin films containing PS and PVAc in different ratios. The pores' depth in the thin film containing 50% PVAc and 50% PS was measured by drawing a profile line from the top PS layer to the bottom of the pore; Figure 2a presents a profile curve of 50% PVAc and 50% PS. This curve demonstrated that the pores' depth was 300 nm while PVAc tips inside the pores reached a height of only 100 nm, for the above ratio. Figure 2c presents a profile


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curve for 75% PVAc and 25% PS that produced a uniformly roughened surface. As the pores merge, the profile curve presented a surface that appears as crowded micro particles, and the tips' height reached 350 nm.

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Figure 2: Profile curves extracted from AFM topography images of thin films containing PS and PVAc in different ratios. Figure 2a presents a profile curve of 50% PVAc and 50% PS (the profile line is marked in Figure 2b). Figure 2c presents a profile curve of 75% PVAc and 25% PS (the profile line is marked in Figure 2d).

Raman spectroscopy was used for material identification in order to verify that PVAc tips grew out of PS pores, and not the other way around. For that, characterizing each


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polymer by its' Raman spectrum was essential. Therefore, thin films with a thickness of 400 nm were deposited via spin coating from solutions containing each one of the polymers in chlorobenzene. The films were measured for their Raman scattering and these spectra were used to analyze the combined film Raman mapping. Figure 3 presents Raman spectra of a thin film of PS and PVAc. The Raman shifts at the region of 3000 cm-1 were chosen for mapping analysis: For PS, the shift is at 3054 cm-1, which is attributed to the C-H vibrations of the aromatic rings16. For PVAc, the shift is at 2940 cm1

, which is attributed to C-H asymmetric stretching in CH217.

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Figure 3: Raman spectra of thin films containing 100% PS and 100% PVAc.

In order to verify the presence of the two polymers in different areas in the combined film, a Raman mapping of the film was produced by measuring an area of 100 X 100 µm. Figure 4 presents Raman mapping of a thin film containing 50% PVAc and 50% PS and typical spectra for both areas. The map includes an area of 1002 µm, and a Raman


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spectrum was taken every 2 µm (502 pixels). A red cursor was chosen to mark areas with a high intensity of Raman shift at 3054 cm-1 (which corresponds with PS rich areas), and a blue cursor was chosen to mark areas with a high intensity of Raman shift at 2940 cm-1 (which corresponds with PVAc rich areas). Typical Raman spectra for the different areas are presented in Figure 4b. Figure 4a presents a clear distribution of the polymers, which correlates with the surface morphology presented at Figure 1b. The presented distribution supports the claim that PVAc rich areas are found in the islands of PS porous areas. Figure 1c shows that the increase in PVAc concentration led to pores merging and a significantly higher amount of tips. Along with the Raman image of Figure 4a, we concluded that the tips within the pores are most likely composed of PVAc.

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Figure 4: Raman mapping of a thin film containing 50% PVAc and 50% PS (a), and typical spectra for each area that present the shifts used for the map analysis, 2940 cm-1 for PVAc and 3054 cm-1 for PS (b).

For further verification, we have performed two additional measurements. The first is a Raman scan along z-axis within a pore (in a tip-rich area). In this measurement, the focused beam excited different cross-sectional layers of the film, moving down from the


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air towards the glass substrate. Figure 5 presents Raman spectra of 4 different layers of the film; the top layer is described by spectrum “a”, while the bottom layer is described by spectrum “d”. The changes in the ratio between the PVAc Raman shift (2940 cm-1) and the PS Raman shift (3054 cm-1) demonstrate which material is predominant on top (i.e. the tips) and which is predominant at the bottom. In spectrum “a”, the primary Raman shift belongs to PVAc while in spectrum “d” it belongs to PS. The second measurement is based on the principle of Tip-Enhanced Raman Spectroscopy (TERS). In TERS, an incident laser light strikes the nanosize apex of a functionalized or modified AFM probe which increases the electromagnetic field underneath the end of the probe. As a result, locally enhanced Raman scattering is generated from materials underneath the probe which provides high lateral spatial resolution18. Considering the vertical extent of the TERS effect, the enhancement in the Raman signal occurs on the top layer of the substrate to a depth of about 20 nm, depending on the excitation conditions19–21. Thus, the TERS principle can be applied to obtain depth resolved material composition. For the thin film whose AFM scan is presented in Figure 6a, Figures 6b and 6c show the Raman spectra using a TERS probe when it is in contact with the sample (dashed line) and when it is retracted (solid line) for two different areas. Figure 6b presents the spectra of the smooth surface area (marked by the green asterisk on Figure 6a), in which TERS is generated for the PS Raman shift (3054 cm-1) since for this area PS is found on the surface. In contrast, Figure 6c presents the Raman spectra of a tip inside a pore (marked by the red asterisk on Figure 6a) where TERS is generated only for the PVAc Raman shift (2940 cm-1). The calculated enhancement in the Raman signal is about 25% for both


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areas. Thus, both measurements that vertically resolve the Raman spectra confirm that the tips within the pores are composed of PVAc.

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the sample and when retracted for outside a pore, green asterisk (b), and for a tip inside a pore, red asterisk (c).

This growth pattern can be explained by the incompatibility of the two polymers in solid state. PS is slightly less miscible in Chlorobenzene than PVAc, thus is first to start leaving the solvent and deposit on the substrate. Next, PVAc starts leaving the solvent and is forced to deposit as tips in order to minimize contact with PS, due to high incompatibility between them. Further removal of solvent causes the PVAc-rich phase to continue to phase separate inside the pores while PS continues to grow on top of the substrate and outside the pores. This model for the process of phase separation is generally referred to as secondary phase separation22.

The sample’s morphology was characterized by AFM using a tuning fork as force feedback. This technique possesses two advantages: 1. there is no optical interference with the Raman signal as there is with a typical beam bounce feedback mechanism. This enables to acquire a less noisy Raman signal. 2. The tuning fork has a very high spring constant which allows very high force measurements without the 'jump to contact' issue23. The tuning fork is oscillating at its resonance frequency. When the probe is in the near vicinity of the sample, a shift in the resonance frequency occurs. In Phase Feedback, the phase shift of the oscillation is used for force feedback in order to characterize the topography, while leaving the amplitude of the probe’s oscillation to monitor the strain of the sample24. When scanning in Phase Feedback the probe’s amplitude will alter due to energy dissipation from the sample25.


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An amplitude image of a roughened surface describes low strain areas as opposed to high strain areas. Figure 7 presents amplitude images of 752 µm from thin films composed of: 25% PVAc and 75% PS (a), 50% PVAc and 50% PS (b), and 75% PVAc and 25% PS (c). As presented in this figure, grey areas that correlate to PVAc islands describe higher strain. The grey areas increase with PVAc concentration. Figure 7c shows that in a ratio of 75% PVAc and 25% PS, a higher strain is observed in most of the surface. The higher strain is attributed to the way PVAc grows from PS as tips. These results are supported by the Raman scattering of thin films composed of 100% PVAc compares with 50% PVAc and 50% PS. A strong Raman shift of a thin film composed of 100% PVAc is observed at 2940 cm-1, while for a thin film composed of 50% PVAc and 50% PS, the same Raman shift is found at 2937.5 cm-1. It is important to mention that in figure 7b, clear grey rings are observed around PS pores. As mentioned before, the grey color describes higher amplitude and thus, a higher strain. When PS supports PVAc tips islands by creating pores around it, PS also increases its' strain. Raman scattering supports these results by showing a down shift of PS from 3054.5 cm-1 to 3052.5 cm-1.

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Figure 7: AFM amplitude images of thin films containing PS and PVAc in different ratios. a- Presents 25% PVAc and 75% PS, b- Presents 50% PVAc and 50% PS, and cPresents 75% PVAc and 25% PS. The solvent of all three solutions is Chlorobenzene.


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The uniformly roughened surface prepared with a concentration of 75% PVAc and 25% PS exhibited a relatively high surface area of 508.55 µm2 for a projected area of 4 µm2. When PVAc or PS is deposited as thin film alone, a perfectly smooth surface is produced with a surface area that equals to the projected area. To conclude, this measurement demonstrates a significant increase in the surface area; the roughened film has 127 times larger surface area than a smooth film. We chose to demonstrate how the described roughened surface can be used for SERS substrate as a possible application. The roughened areas of the substrate are expected to produce higher enhancement, rather than the smooth areas, due to an additional plasmonic resonance caused by the rod effect18 from PVAc tips.

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Figure 8: Raman mapping of Fullerene-C60 on 15 nm gold deposited on 50% PVAc and 50% PS substrate. Fullerene-C60 Raman spectrum (a) and the map created from its strong Raman shift at 1465 cm-1 (b).


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We thermally evaporated 15 nm of gold on top of a film composed of 50% PVAc and 50% PS. The substrate is composed of both rough areas and smooth areas and enabled better evaluation of the achieved enhancement. On top of the gold, 8 nm of Fullerene-C60 were deposited. Figure 8 presents the Raman spectrum of Fullerene-C60 (a), and the Raman mapping of the thin film deposited on the partially roughened surface (b). Fullerene-C60's strong Raman shift at 1465 cm-1 was chosen to produce the Raman mapping image. The map shows a clear difference between the roughened areas and the smooth areas, the roughened areas demonstrated at least 5 times higher peak intensity probably due to additional plasmonic resonance caused by the rod effect18 from the sharp PVAc tips.

4. CONCLUSIONS This study presents the deposition of thin films containing different ratios of PS and PVAc using chlorobenzene as solvent. The films were deposited on glass via spin coating and were investigated for morphology, strain, surface area and Raman scattering. As PVAc concentration in the polymer blend increases, the porous regions of PS in the film expand and the amount and height of PVAc tips increase as well, up to the point where the pores merge to create a uniformly roughened surface. Higher strain was measured in PVAc tips and in PS rings surrounding the pores, and a high correlation was found between Raman scattering and strain in different areas in the films. The surface was examined for SERS substrate as a possible useful application and demonstrated significant Raman signal enhancement.


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ACKNOWLEDGEMENT We gratefully acknowledge the professional support of Yirmi Bernstein from Nanonics Imaging Ltd.


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Deposition and Characterization of Roughened Surfaces

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