Water Droplet Self-Assembly to Au Nanoporous ... - ACS Publications

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Water Droplets Self-assembly to Au Nanoporous Films with Special Light Trapping and Surface Electromagnetic Field Enhancement Vanessa H Fragal, Elizangela H Fragal, Adley Forti Rubira, and Rafael Silva Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01794 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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Langmuir

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Water Droplets Self-assembly to Au Nanoporous Films with Special

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Light Trapping and Surface Electromagnetic Field Enhancement

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Vanessa H. Fragal, Elizângela H. Fragal, Adley F. Rubira and Rafael Silva*

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Departamento de Química, Universidade Estadual de Maringá. Avenida Colombo 5790, 87020-

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900, Maringá, Paraná, Brazil.

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* Email: [email protected]

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KEYWORDS: Electromagnetic Field Enhancement; Nanofabrication; Nanopores arrays; Breath

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Figure Method; Plasmonics.

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ABSTRACT. Gold nano “breath figure” films are for the first time reported and their function as

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ideal systems for plasmonics demonstrated. Metal nano breath figure substrates are metal thin

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films containing nanoholes arrays supported on a solid substrate. Au nanoholes arrays are

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prepared from the dynamic breath figure phenomenon, in which the pores formation is controlled

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to provide holes smaller than 100 nm. Au layer is deposited on polymer substrates containing

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breath figure topology. The breath figure topology can be fully translated to the Au layer. The

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nanofabrication process is completed within few minutes. Simplified preparation process but

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very impressive light trapping and surface electromagnetic field enhancement are related to the

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Au breath figure films obtained in this work. The porous films demonstrated higher absorbance

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in the region of 500 to 1100 nm than non-porous Au films. In the case of 10 nm Au film the

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plasmon absorbance becomes more intense than the electronic band absorbance. The EM

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enhancement is proved by SERS effect, which is found to be very close to the maximum possible

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value predicted for non-resonant species.

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INTRODUCTION

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Nanoscience and nanotechnology have been burgeoning the knowledge of human kind

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about the nature’s more intriguing features to create marvelous man-made systems to improve

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human development level. The main principle of nanoscience is to take advantage of properties

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that emerge from dimension confinement effects.1-4 A perfect example is the Surface Plasmon

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Resonance (SPR) effect that appears due to quantum confinement of free electrons in metallic

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systems.5-7 Plasmonics become a very broad research field once the capabilities of SPR effects

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are often interpolated in the development of cutting edge technologies, for instance enhanced

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biosensors, energy harvesting by sensitized solar cells, heterogeneous photocatalysis,

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microelectronics, and nanomedicines.8-12

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SPR can be used as a powerful analytical tool capable of providing information regarding

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the structure of a variety of analytes in a nondestructively away.13 Then, it has been applied to

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various fields including environmental science, health care, biology, and even against terroristic

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threats because of its ability to detect trace-level analytes such as pesticides, heavy metals,

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explosives, proteins, pathogens, and other chemical and biological contaminants. 14-21

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Macroscopic solid substrate with SPR effects are demanded to bring many of the

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plasmonic promises to real cases applications.22 In SPR, the selection of suitable substrate

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determines the sensitivity, reproducibility and stability of the signal. 23 Although the fabrication

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of SPR substrates is no longer viewed as an challenging barrier, the key impediment for the

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practical use of SPR-based sensors is still the lack of robust and facile fabrication strategies for


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reproducible SPR substrates with economic fabrication of large-scale and stable Raman

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enhancement.24

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Experimental and theoretical studies demonstrate that substrates with ordered structures

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show several advantages over disordered nanostructures; periodic arrays of nanoparticles possess

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maximized specific surface density of hot spots and higher surface enhancement factors (EFs).25-

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26

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mature research area with thousands of different approaches.27 However, an ideal system is still a

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challenge.28-29 The idealization of a perfect surface for plasmonics requests:30-32 (i)

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nanostructures that lead to maximum electromagnetic (EM) field enhancement areas known as

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“hot spots”; (ii) densely packed “hot spots” throughout macroscale dimensions; and (iii) a

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fabrication methodology that could be carried out quickly using only routine laboratory facilities.

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Up to now, the most commonly used method to prepare ordered SPR substrates is still dependent

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on lithography techniques. However, these methods usually require complicated synthesis

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procedures and expensive equipment. Currently the best alternative, honeycomb-patterned films

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via the breath figure (BF) method,

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uses condensed water droplets as dynamic templates40-41, have also been used as SERS

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substrates.42-44

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Noble metals such as Ag and Au are best substrates for SERS 23, 45. Than additional functionality

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can be achieved when metal films deposited over a monolayer of nano or microholes sustain

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both photonic and plasmonic resonances that can synergistically interact to create new

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capabilities that reach beyond those of the individual components. Depending on the periodicity

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and refractive index of the 2D array, it is possible transmit, scatter, reflect or laterally guide the

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normally incident light. 46-48

Therefore, the development of nanostructured substrate for plasmonic application is a very

33-39

which is a simple, fast and template - free method that


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In this work, we demonstrate the use of a phenomenon known as “breath figure” as a pathway to

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create ordered metallic surface containing hexagonal nanoholes’ arrays. Even though it consists

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of straightforward procedure, the obtained materials demonstrate to be extraordinary superior to

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nanostructured surfaces built by elaborated nanofabrication tools. Breath figure method has been

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only used for porous polymers thin layers supported on difference substrates since the beginning

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of 90’s.49-50 For a long time, this method was restricted to micrometer scale construction, but new

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perspectives to the method emerged from the control of the breath figure pore size at the

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nanoscale dimension. We recently reported the use of breath figure phenomenon to generate

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modified surface with ordered porous structures with pores size within nanoscale dimensions

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using breath figure method assisted by spin coating deposition.51 Here, we expand the idea to

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obtain Au 2D hexagonal close packed nanoholes arrays. A base substrate was covered with

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polymer thin layer by modified breath figure method, in which the pore size can be modulated,

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and then a thin Au layer was deposited by sputtering transferring the breath figure topology to

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the Au layer.

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Experimental section

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Spin coating and humidity control. The experiments were carried out using a SCS G3 Spin

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Coater Model G3P-8. Humidity was controlled using a saturated solution of ammonium sulfate.

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A beaker containing a saturated solution of ammonium sulfate is placed inside the spin coating

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chamber two days prior it uses to warranty the stabilization of the humidity. A hygrometer is also

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placed inside the chamber to measure the humidity. The humidity was kept constant at 81%. The

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spin coater chamber temperature was kept at 25 oC.


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Synthesis of nanoporous films of polystyrene (PS) on polyethylene (PE). On a PE film placed

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in the spin coater was added 150 µL of a solution of 10% (w/v) PS in tetrahydrofuran (THF).

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The system was run for 10 s at 1000 rpm. Similar procedure was used to produce samples with

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different spin speed of 3000 and 9000 rpm.51

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The sputtering of Au film on all nanoporous PS/PE substrate was carried out with SCD 050,

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Sputter coater, BAL-TEC. The sputtering was carried out using a constant current of 60 mA. The

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Au thicknesses were varied according to the total sputtering time to generate sputtered films with

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Au average thickness of 10, 30 and 80 nm (Figure S1).

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Scanning Electron Microscope (SEM) and Atomic Force Microscopic (AFM): SEM images

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were obtained by a Shimadzu model SS 550 Superscan, with accelerating voltage of 10 kV.

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AFM images and average depth roughness (Rz) surfaces values were obtained from Shimadzu

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SPM-9500J3 equipment.

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Total Reflectance and Total Transmittance. UV-Vis-NIR spectra were acquired in an

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equipment Perkin Elmer Lambda 1050 with integrating sphere accessory for both reflectance and

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transmittance measurements. The transmittance spectra were corrected to remove the effect

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polymer substrate absortance, using the total transmission spectrum of the bare PE film.

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Raman Measurement. Raman spectra were obtained by a dispersive Raman microscope Bruker

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Optics SENTERRA using a Renishaw laser diode (785 nm) for excitation. The focused laser

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beam was done using 20-fold increase lens. As a parameter for the acquisition, we selected the

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region of 500 - 2000 cm-1. Each sample was analyzed using at 4 different acquisition points and

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60 accumulations at each point and 1 second accumulation time. The experiments was carried

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out with triplicate samples. Therefore, all the Raman spectra presented in this work are the

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average of 12 measurements (3 samples and 4 points per sample).


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For the SERS measurements, as-prepared Au/breath figure substrates were cut into small

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square pieces (5 mm × 5 mm). On the substrate piece was added 0.25 mL (a drop) of 4-

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mercaptopyridine (4-Mpy) aqueous solutions. 4-Mpy solution concentration was varied in the

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range of 10−1 to 10−9 mol/L. For the comparison of SERS activity, the non-porous Au/PS/PE was

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used as standard, then 0.25 mL of 1 × 10−1 mol/L of 4-Mpy aqueous solution was uniformly

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distributed onto 5 mm × 5 mm pieces of the standard substrate. 4-Mpy solution were allowed to

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dry under room temperature. Raman measurements for these substrates were carried out under

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identical experimental conditions (such as laser, wavelength and power, microscope

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objective/lenses, integration duration, etc).

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RESULTS AND DISCUSSION

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Breath figure method involves water condensation on the surface of polymer solutions

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during polymer casting process at high humidity level, Figure 1a.52-55 Water condensation due to

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high humidity level of the surrounds leads to water droplets growth on the polymer surface. The

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water droplet growth process determines the final shape of the structured surface. During the

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growth process the droplets sink into the underneath polymer solution when the droplets reach a

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certain size that enables the droplets to overcome the surface tension of the polymer solution.

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Water droplets sink and dislodge polymer solution from specific spots.

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The polymer solution evaporation is essential in the formation of breath figure structure,

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therefore, a volatile solvent is used. Once the polymer solution is spread on the substrate surface

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the polymer solution evaporates in a short time period, then the water droplet size is limited by

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the evaporating time.


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Figure 1. (a) Mechanism of formation of porous thin film by breath figure phenomenon. (b)

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SEM micrographs of pristine PE and after the deposition of porous PS layer at different rotation

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(1000, 3000 and 9000 rpm) and 30 nm of Au. (c) Schematic representation of ordered Au/PS

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nanoporous films for SERS substrates.

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When breath figure method is performed with the aid of spin coating, the thickness of the

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polymer solution spread over the substrate depends on the spin speed. Therefore, very thin film


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of the polymer solution can be obtained at higher speeds and polymer solidify within few

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seconds due to the fast solvent evaporation. As a result, the formation of the breath figure

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structure depends on the underlying equilibrium between water condensation kinetics and

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solvent evaporation kinetics. A short evaporation time limits the size of the water droplets

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leading to small pores. In general, the procedure to breath figure fabrication takes less than 10

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seconds.

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PE was used as substrate, since it is inert, cheap, and flexible.56-58 Nonetheless, it is worth

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to point out that any solid substrate could be potentially used. In addition, previous surface

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treatment is not necessary to start the nanofabrication process. Although any substrate can be

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applied, we previously reported the pore size obtained changes accordingly to the surface

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wettability.51

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PS breath figure structures were deposited on PE surface using different spin speed.

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Nanoporous PS films deposited on PE were obtained at 1000, 3000, and 9000 rpm. Pore size

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decreases with the increase of the spin speed, Figure 1b and Figure S2, the average pore diameter

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are 303 ± 68, 123 ± 23 and 80 ± 23 nm, for 1000, 3000, and 9000 rpm, respectively.

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Au film was deposited on breath figure structure through sputtering process. Au thickness

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was controlled by the sputtering time. AFM images of the breath figure structures, before and

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after Au deposition, demonstrated that breath figure topology can be retained after Au

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deposition, Figure 2. Rz determined by the AFM analysis is presented in function of the Au

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thickness, Figure 2g. Rz values is used to demonstrate the attenuation of the surface breath figure

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structure after Au deposition. For 10 nm Au layer, the surface roughness is just slightly

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attenuated. Increasing the Au film thickness to 30 nm, a further attenuation of the surface

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roughness is verified, but Rz values is still close of the prepared PS breath figure substrate. The


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AFM images of the samples prepared with 30 nm Au layer evidenced the holes formed by breath

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figure is still present. An intense variation of Rz values is verified for 80 nm Au layer (Figure S4,

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S5 and S6), which a severe attenuation of the surface occurs and the nanoholes are deteriorated.

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Figure 2. AFM images of nanoporous PS on PE at 1000 (a,b); 3000 (c,d) and 9000 rpm (e,f);

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without Au deposition (a, c and e) and with 30 nm film (b, d and f). Rz vs Au layer thickness (g).

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The interaction of the light with the Au porous films are studied to evaluate the effect of

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the nanopores arrays in absorb and scatter light. Plasmonic effects are generated by the free

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electron confinement in the nanostructures that arise from the pore size and shape. The light

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trapped by the nanopores is measured and the absortance of the films calculated to a broad

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spectral region from UV to near IR region. The absortance of the films is determined by the

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difference among the incident light with total reflected light and total transmitted light, Figure 3.

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The total reflected and total transmitted light were measured by collecting the reflected or

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transmitted light with an integrating sphere. Some trivial aspects are observed in the system. The

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increase in the reflectance follows the increase of the surface roughness and the total transmitted

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light decreases greatly with the increase of the Au thickness.

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On the other hand, some aspect as worthy to note, the transmission of light occurs even in

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the case of flat Au films deposited. For instance, for the film with 10 nm it is observed

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transmittance around or higher than 40% for the whole analyzed spectral region with a maximum

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peak at the Au band edge (500 nm). A small reduction on the transmitted light in the regions

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between 500 to 1100 nm is the only difference observed to the Au flat film in relation to the

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porous films. Increasing Au thickness to 30 nm the transmittance of the film decrease abruptly

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but it is still observed. For the flat Au films, a peak at 450 nm and 10% of transmittance is

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verified. The data for the flat substrates are in good agreement with the literature reports about

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the extraordinary optical transmission through thin Au films.59-60

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The extraordinary optical transmission occurs as consequence of the light coupling with

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absorption modes present in the Au film.61 For the system with 30 nm Au film, optical

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transmission increases for the porous substrates with the burgeoning of a new transmission band

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in the Au longitudinal SPR region. The peak initially observed at 450 nm for the flat Au film,


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that is located close the Au band edge, undergoes a red shift of 50 nm and the peak intensity

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increases for all porous substrate reaching 20% of transmittance. This shift and increase is

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believed to be due to the overlapping of the light couled with the Au electronic band absorption

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and transverse SPR effect that is generated by the nanoholes. In addition, a second and broad

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peak is observed for the sample with larger pores around 700 nm with transmittance around 10%.

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Therefore, the increase of the transmittance in the region above 500 nm is due to the many

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different modes of longitudinal SPR band of Au structure due to the increase of the Au thickness.

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The effect of the pores in the Au films is perceived in the absortance pattern of the films.

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For the flat Au films, the absortance are higher in the regions with wavelength lower than the

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band edge and it decreases for wavelength higher than 500 nm. For porous Au films the

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absortance spectra changes abruptly. The absortance in the region with wavelength lower than

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band edge is drastically increased. In fact, the absortance for the porous films can be even higher

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in the region between 500 to 1100 than bellow 500 nm, as it is observed for the Au film with 10

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nm. The absortance in the 10 nm Au thickness is higher in smaller pores (3000 and 9000 rpm,

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black and red lines, respectively) than in the big ones. It could be correlated to the higher surface

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density of surface plasmon polaritons on the film with smaller pores. Therefore, the increase of

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the absorption in this range is a measurement of the trapped light due the nanoporous arrays

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absorptivity. The behavior of the absortance for the porous Au films with 30 nm evidenced that

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the higher increment in the light trapping is observed for the film prepared at 1000 nm. The

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reason is due to the attenuation of the porous structure in the smaller pores with 30 nm of Au.


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Figure 3. Scheme of the interaction of light with the Au breath figure substrate and experimental

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data for: (i) total reflectance measured by integrating-sphere reflectance spectra; (ii) transmitted

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light spectra measured in transmission mode; and (iii) absortance spectra determined by the

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difference of the incident light intensity with the process (i) and (ii). The different samples are

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identified by colored lines: non-porous substrate (green); breath figure substrate at 1000 rpm

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(blue); breath figure substrate prepared at 3000 rpm (black); breath figure substrate prepared at

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9000 rpm (red).


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The ability of the nanoholes arrays to EM field by surface plasmon effect is probed using

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surface enhanced Raman spectroscopy (SERS).62-63 The increase of the Raman signal’s intensity

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for probe molecules occurs by the enhancement of incident field intensity caused by the local

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electric-field and also due to the enhancement in polarizability due to chemical effects. In

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theoretical and semiempirical studies of nanoparticles arrays, the average EM enhancement

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factors are found to be correlated to the particles shape and size, particles arrangement and

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distance between particles. For instance, 2D square-lattice arrays of Au nanospheres can

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theoretically provide a maximum average enhancement of 5 x 108.64 Higher average EM

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enhancement factors are observed for Ag, around 2x109.64 However, Au is preferred in

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comparison to Ag due to the higher chemical stability of Au, which is important to the

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development of more reliable devices. SERS enhancement factors close to 108 or even higher

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have been reported in many experimental works, but these reports use resonant molecules, i.e.

15

rhodamine 6G, as molecular probes. The enhancement factor of resonant molecular probes, that

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can be as higher as 1014, is much higher than non-resonant species.

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The use of nanoholes formed in metal thin film to enhanced electromagnetic (EM) field

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have been previously reported. Au thin films having nanoholes arrays produced by focused ion

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been (FIB) milling or anodization process followed by chemical etching are able to enhance

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molecular fluorescence and also Raman scattering.65-68 Zheng et al. reported the enhancement

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factor of Au nanoholes arrays coupled with Au nanostar core@shell particles carrying molecular

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probe could be as high as 4.5 x 106.69 Honeycomb-patterned films with pore diameter around 2


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microns prepared by breath figure were covered with Ag nanoparticles and enhancement factor

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as high as 4 x 108 were reported, using rhodamine 6G, a resonant specie.25

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Au breath figure substrate were used as based substrate for SERS. 4-mercaptopyridine (4-

4

Mpy) was applied over the substrates as a non-resonant molecular probe. Strong Raman

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scattering signals of probe molecules are observed over Au breath figure substrates, Figure 4a,

6

when a diluted 1 µM 4-Mpy solution was applied to Au breath figure substrate. In the Raman

7

spectra it is possible to observe the intense bands characteristic of 4-Mpy at 1440 (v(C=C/C=N)),

8

1295(β (CH)/δ(NH)), 1130 and 1060 cm-1 (vibration ring CS).70-72

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The prove of the effect of the breath figure topology on the enhancement of the 4-Mpy

10

bands is demonstrated with a non-porous sample as blank. The blank sample consists of PE

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substrates covered with a non-porous PS layer and recovered with Au layer by the same

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sputtering method and condition (See Support information, Figure S3 for details on the

13

characterization of the non-porous substrate). There is a keen difference in the Raman spectra

14

between the sample obtained using the non-porous Au/PS/PE substrate and the Au/breath figure

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substrates. In fact, 4-Mpy signals can only be observed by the use of concentrated 4-Mpy

16

solution, 0.1 M. However, even applying a more concentrated solution on the non-porous Au/PS

17

surface, the 4-Mpy peaks appears with low intensity. The non-porous Au/PS/PE substrate was

18

used to calculate the enhancement factor (EF) assuming it does not provide any enhancement

19

effect. Therefore, the EF values described, Figure 4d and in the Figure S7, are calculated by the

20

difference of intensity of the signal at the Au/breath figure topology in comparison to the non-

21

porous Au/PS/PE substrate surface, taking in consideration the difference on the total amount of

22

molecular probe available at the substrates surface. The EF that Au/Breath figure topology

23

generated were calculated from the Raman spectra using Equation 1.73


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EF= (ISERS/IRAMAN). (NBULK/NADS)

(Equation (1))

2

where ISERS and IRAMAN are the intensities of the vibrational modes in the SERS and normal

3

Raman spectra, respectively. NBULK is the number of 4-Mpy molecules adsorbed on the non-

4

porous Au/PS/PE and NADS is the number of molecules of 4- Mpy adsorbed on the Au/Breath

5

figure.

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Figure 4. (a) Raman spectra of 4-Mpy 10-6 mol/L (i) 9000; (ii) 3000; (iii) 1000 rpm and (iv) non-

8

porous standard 0.1 mol/L; (b) 3000 rpm and 10-6 mol/L (i) 30 nm Au layer and (ii) 10 nm Au

9

layer; (c) 3000 rpm and 10 nm Au layer (i) 0.1 mol/L; (ii) 10-6 mol/L; (iii) 10-9 mol/L and (iv)

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non-porous standard 0.1 mol/L (d) EF calculated to Raman peaks at 1130 cm-1.


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Even though all the samples provided strong enhancement effects, some trends among

3

the sample fabrication parameters are verified. For instance, the pore diameter has linear

4

influence on the EF value. In the Figure 4a is possible to verified an increase in the Raman signal

5

in function of the increase of the spin speed. These trend is observed for almost all peaks and Au

6

thickness, Figure S8. In the Figure 4d, EF varies almost linearly with the spin speed. At 1130 cm-

7

1

8

rpm. The polymer breath figure pattern underneath Au in the samples prepared with spin speed

9

of 1000 rpm and 9000 rpm have pore diameter of 303 and 80 nm, respectively. Although, the

10

pore diameter affects the EF, there is a disparity between the pore diameter decrease and the

11

magnitude of the EF increase. The effect of the pore diameter can be understood considering the

12

pore cavities are not the “hot spots” region in the porous structures, but the rims along the pores

13

are, as demonstrated by finite-difference time-domain (FDTD) simulation carried out by Zheng

14

et al. using gold nanoholes array.69 Therefore, the variation of the rim shape and dimensions does

15

not direct follow the magnitude of the changes in the pore diameter.

, the sample prepared at 9000 rpm have EF 2 times higher than the sample prepared at 1000

16

Au layer thickness is also able to cause changes on the EF values. Considering 4-Mpy

17

signal at 1130 cm-1, the samples prepared with 10 nm Au layer provide generally higher EF

18

values than samples prepared with 30 nm Au layer. However, the intensity of the EF variations

19

for distinct Au layer thickness conditions depends on the vibrational mode and it can as high as

20

3-fold difference. Structural difference arises from the change on the Au layer thickness, like the

21

observed attenuation of the breath figure topology. The attenuation of the breath figure topology

22

is likely to be the main factor on the EF values, since the changes on the EF values is not similar

23

for all 4-Mpy vibration mode, therefore, it is caused by the change on geometric effect of EM

24

enhanced field.


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1

The sensibility of the SERS is verified on the analysis of diluted solution of 4-Mpy,

2

Figure 4c. The vibration modes of 4-Mpy can be identified by the analysis of a single drop (250

3

µL) of a 1 nM 4-Mpy solution by the use of the Au breath figure substrate, demonstrating the

4

limit detection of the analyte is better than 1 ppt (1 part per trillion). Two important facts have to

5

be noted at this point, substrate stability and non-resonant analyte. The substrate was prepared

6

with Au, that normally provide lower EF than Ag, but the use of Au substrates has stability as its

7

main advantage. In addition, the molecular probe used is a non-resonant specie and the most

8

common SPR applications deals with non-resonant species. Therefore, the EF value of ca. 2.4 x

9

108 EF for Au and a non-resonant molecular probe has to be considered a new milestone for

10

surface enhancement Raman scattering.

11 12

CONCLUSION

13

In conclusion, we showed here the use of breath figure phenomenon (modified breath

14

figure method) to produce metallic porous topography with pore diameter lower than 100 nm.

15

Substrates with pore size in the range of 80 to 300 nm scale were obtained by the modified

16

breath figure method. The pore size is easily controlled by adjusting the spin speed in the spin

17

coating process. The methodology can be easily extended to other metals like copper and silver.

18

The growth of the Au layer preserves the underneath breath figure topology when thin Au layer

19

are deposited (10 and 30 nm), however, increasing the Au thickness to 80 nm cause severe

20

attenuation of the breath figure topology.

21

Plasmonic effects of the nanostructured surface were observed as the high efficiency in

22

trapping light in the region with wavelength higher than the Au band edge. Strong SERS effect is

23

also demonstrated. In fact, it was observed the highest EF factor ever reported for an Au


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1

substrate and a non-resonant species, to the best of our knowledge. All the substrates studies

2

demonstrate EF around 108. These results are near to the maximum EF estimated by theoretical

3

studies, 5 x 108 for Au nanosphere arrays.64 Thus, Au breath figure substrates are very promising

4

since substrates previously reported have EF values in the range of 103-105.70-72, 74. The present

5

results strongly encourage the employment of metal nano breath figure substrates for solar cells,

6

photocatalysis, biosensor, and other plasmonic devices.75

7

ASSOCIATED CONTENT

8

Supporting Information. More information about materials and preparation of non-porous

9

standard; histogram size distribution of nanopores; addicional SEM and AFM images and Raman

10

spectra.

11

AUTHOR INFORMATION

12

Corresponding Author

13

* E-mail: [email protected]

14

Funding Sources

15

The authors declare no competing financial interest.

16

ACKNOWLEDGMENT

17

VHF and EHF thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior

18

(CAPES-Brasil) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico

19

(CNPq-Brasil) for doctorate fellowships. AFR and RS acknowledges the financial supports given

20

by CNPq, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-Brasil) and

21

Fundação Araucária-Brasil.


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1

ABBREVIATIONS

2

SPR, surface plasmon resonance; Au, gold; AFM, atomic force microscope; SEM, scanning

3

electron microscope; Rz, depth roughness; PS, polystyrene; PE, polyethylene; rpm, rotation per

4

minute; EM, electromagnetic; SERS, surface enhanced Raman spectroscopy; FIB, focused ion

5

been;

6

Mercaptopyridine.

7

REFERENCES

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

FDTD,

finite-difference

time-domain;

EF,

enhancement

factor;

4-Mpy,

4-

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75. Wu, B.; Zhang, W.; Gao, N.; Zhou, M.; Liang, Y.; Wang, Y.; Li, F.; Li, G., Poly (ionic liquid)-Based Breath Figure Films: A New Kind of Honeycomb Porous Films with Great Extendable Capability. Scientific Reports 2017, 7 (1), 13973.

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