Surface Plasmon Excited on Imprintable Thin Film Metallic Glasses for

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Surface Plasmon Excited on Imprintable Thin Film Metallic Glasses for Surface-enhanced Raman Scattering Applications Cheng Wang, Li-Wei Nien, Hsin-Chia Ho, Yi-Chen Lai, and Chun-Hway Hsueh ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00305 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Nano Materials

Surface Plasmon Excited on Imprintable Thin Film Metallic Glasses for Surface-enhanced Raman Scattering Applications Cheng Wang, Li-Wei Nien, Hsin-Chia Ho, Yi-Chen Lai, Chun-Hway Hsueh* Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan

ABSTRACT Metallic glasses (MGs) are a class of amorphous alloys in contrast with crystalline metals and provide a challenge of engineering applications for its unique structure and properties. However, plasmonic applications remain a virgin area for MGs. In this work, we discovered that certain compositions of gold-based MGs possessed negative dielectric constants and could be used as plasmonic materials. Furthermore, with a low glass transition temperature of gold-based thin film MGs (TFMGs), we were able to fabricate large dimensions of nanostructures using an inexpensive thermal imprint method in air instead of other costly lithography methods. We performed both measurements and simulations to demonstrate that our designed nanostructures were suitable for surface-enhanced Raman scattering (SERS) applications. In addition, in the absence of grain boundaries in amorphous TFMGs, damping due to increased scattering at grain boundaries does not occur and SERS could be improved. Also, compared to regular SERS substrates of textured Si with deposited Au films, imprinted Au-based TFMGs provided complete coverage of Si underneath and the vibrational signal of Si lattice would not show in Raman spectra to possibly overlap signals of analyte and decrease the accuracy of sensing. Our results suggested new avenues for applying a low-cost and high-throughput method on TFMGs to fabricate large dimensions of substrates for plasmonic applications.

KEYWORDS: metallic glass, thin film, dielectric constant, nano-imprint, SERS

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INTRODUCTION The discovery of surface-enhanced Raman scattering (SERS) helps to improve the efficiency of Raman spectroscopy,1 which has been widely applied to molecular analysis applications. The SERS technique is referred to the phenomenon that the pristine Raman signal from the probed molecules is amplified by electromagnetic (EM) field enhancement resulting from localized surface plasmon resonance (LSPR).2-3 Another accepted mechanism to contribute to the SERS phenomenon is the chemical enhancement (CE),4 which involves the charge transfer between adsorbed molecules and the metal surface. The key point of SERS is surface plasmon generation that depends on the surrounding medium and metallic nanostructure of SERS substrates. To excite LSPR, the plasmonic materials are required to have a negative real component and a small, positive imaginary component of the dielectric constant.5 In general, Au, Ag and Cu are the most commonly used plasmonic materials. Other materials such as Al, In, Pt, graphene, metallic oxides and metal alloys have also been explored as plasmonic substrates for SERS,6-10 although some of them do not adhere strictly to the traditional definitions of SERS substrates.5 In addition to materials investigation, researchers have committed to work on the substrate structures to increase the enhancement factor of SERS. Therefore, numerous morphologies of substrates have been fabricated to achieve strong EM field enhancements, including (i) random nanostructures with a large range of wave absorption, such as nano-particles,11 nano-wires,12 nano-discs13 and 3D nano structures with complex morphologies,14-16 and (ii) periodic arrays with one or more specific resonances of surface plasmon for SERS substrates realized by advanced nano-manufacturing techniques.17-20 Nanolithography techniques, such as optical lithography, electron-beam lithography, scanning probe lithography, nanosphere lithography and nanoimprint lithography have been used to fabricate periodic nano-structures on semiconductors and metals. Compared with other lithography techniques, imprint is a low-cost, productive and simple method. The most conventional material used for imprinting is polymer. Although some polymers can support SERS effects,21 the commonly used polymers for imprinting, such as


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polydimethylsiloxane (PDMS) and polymethyl methacrylate (PMMA), are unable to support SERS effects. In recent years, bulk metallic glasses (BMGs) have been explored for imprinting due to their low viscosities and excellent thermal forming ability in their supercooled liquid regions (SCLR, !T).22-24 The unique amorphous structure of BMGs gives them viscous behavior above their glass transition temperature (Tg). Compared with polymers, the superiority of metallic glasses (MGs) is that they resume metallic behavior such as high strength, photoemission and electric conductivity after thermal imprinting. However, the temperatures used for imprinting MGs were commonly above 250 ºC, which would result in oxidation in the atmosphere. In this work, we bridged the gap between MGs and SERS in terms of materials and manufacture process to highlight the convenience of thermal imprint method and feasibility of applying MGs as plasmonic materials. Au-based MGs, AuCuSi,25 were selected which includes two excellent plasmonic materials, Au and Cu, and Si atoms with a small size to facilitate the formation of amorphous structure. While thermal imprinting only change the surface morphology of imprinted material, Au-based thin film metallic glasses (TFMGs) with thickness of ~1 µm were fabricated in our work to reduce the material cost. In this case, AuCuSi TFMGs were deposited on the Si wafer by magnetron co-sputtering process. The as-deposited TFMGs with the compositions of Au35Cu28Si37, Au49Cu22Si29, Au61Cu19Si20 and Au65Cu17Si17 were named R30, R40, R50 and R55, respectively, based on the power used in sputtering.26

RESULTS AND DISCUSSION The significant parameters for thermal imprinting, such as Tg, crystallization temperature (Tx) and viscosity in the SCLR were measured in our previous work.26 Taking the advantage of thermal forming ability, designed topological structures were fabricated by nanoimprint lithography method on R55 as it had the lowest Tg of ~60 ºC among the Au-based TFMGs fabricated in our work. The imprinting process was performed at 68 oC. Figure 1 shows a schematic illustration of the imprinting process and scanning electron microscope (SEM) images



of several patterns fabricated on R55 TFMGs. The first step was embossing R55 against a stamper at 68 oC under a pressure of 20–50 MPa with holding time of about 10 min. Then, the temperature was deceased to below 50 oC for solidification and the stamper was retracted. In the present work, the designed stampers were fabricated from Si wafers using e-beam lithography (EBL),27 and the commercial anodic aluminum oxide (AAO) template was also used as the stamper. The nanoimprint process was conducted at relative low temperature in air and took about 15 min, and various patterns could be achieved by Si stampers (Figure 1(c), (d), (e) and (f)) and AAO template (Figure 1(g)). A common feature for all the imprinted patterns was the curved cap and obtuse contact angle indicating the un-wetting front between the glassy R55 and stamper (schematically shown in Figure 1(b)) and it was beneficial for stamper retracting. This un-wetting phenomenon was also observed during imprinting of BMGs and has been discussed elsewhere.22 In addition, the films kept amorphous after the imprinting process, which were demonstrated by the TEM results (shown in Figure S1).


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Figure 1. Thermal imprint method for nano-structure fabrication on AuCuSi TFMG. Schematic illustrations showing (a) nano-imprinting on R55 TFMG to fabricate SERS substrate and (b) un-wetting front leading to the curved cap and obtuse contact angle. SEM images of periodic (c) nano-pillars, (d) triangle holes, (e) nano-walls and (f) hemispheres fabricated by designed Si stampers, and (g) hexagonal prisms imprinted by AAO template. The enlarged images are shown in insets. The prerequisite of the negative dielectric constant for plasmonic materials was examined to check the feasibility of using AuCuSi TFMGs as SERS substrates. As shown in Figure 2, as-deposited AuCuSi TFMGs have negative real components (!r) of dielectric constant in the visible and near-infrared wavelengths like Au and Cu. As the content of Au increased in AuCuSi TFMGs, the value of !r became closer to that of Au. In particular, the strongest electric field enhancement would occur when the dielectric constants of substrate (!s) and external medium (!m) satisfy a certain functional relation for a certain nanostructure. For instance, for spherical nanoparticles, the extinction electric field will be the largest when the real part of !s (i.e., !r) equals –2!m based on the Mie theory.28-29 Depending upon the incident wavelength, the geometry of the nanoparticle and the surrounding medium, !r could be determined to yield the maximum field enhancement, and the corresponding SERS substrate material could be selected based on Figure 2(a). On the other hand, Figure 2(b) shows that the imaginary components (!i) of AuCuSi TFMGs were larger than those of Au and Cu, indicating the higher energy loss within the material; however, it was reasonably small in the visible range. It is worth noting !i of AuCuSi TFMGs were similar to those of platinum group materials. The 632.8 nm laser was used to obtain the Raman spectra (will be shown in Figure 4). At 632.8 nm wavelength, !i in our materials was ~10 that was lower than 19.35 for Pt. Also, although Pt is not as SERS-active as Au, it still has been used as SERS substrates.5 Although every specific geometry of a plasmonic material would have a different quality factor, it is convenient to define the quality factor by considering both the applicability of electrostatics and the loss. For comparison, the quality factor, Q = –!r/!i, for LSPR applications



is shown in Figure S2 for R55 TFMG and Au. Compared to Au, although R55 TFMG did not have the better quality factor, it is readily imprintable. The dielectric constants of other metallic glasses, such as Fe-based and Zr-based metallic glasses, were also measured and found to have positive dielectric constants in the present study. Therefore, like metals, not every metallic glass is suitable for SERS applications.

Figure 2. Dielectric constants of AuCuSi TFMGs. (a) Real component, !r, and (b) imaginary component, !i, of the dielectric constants of as-deposited AuCuSi TFMGs, Au and Cu measured by ellipsometer.

The imprinted substrates with periodic arrays of nano-pillars (Figure 1(c)) and hexagonal prisms (Figure 1(g)) were chosen to measure the reflectance and scattering spectra and the results are shown in Figure 3(a) and (b), respectively, to find the LSPR resonance wavelength. The resonance wavelength from the minimum reflectance (Figure 3(a)) agreed with that from the scattering peak (Figure 3(b)). The measured resonance wavelengths of nano-pillar and hexagonal prism substrates were 578 nm and 617 nm, and the corresponding reflectances were 5.1 % and 1.9 %, respectively. To make the comparison, the reflectance and scattering spectra of nano-pillars and hexagonal prisms were simulated by finite-different time-domain (FDTD) using the measured dielectric function (including !r and !i) of R55 (shown in Figure 2) as the material


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parameters. The dimensional parameters used in simulations are shown in Figure S3. The simulation results shown in Figure 3(a) were close to the measurements with the minimum reflectance at 584 nm for nano-pillars and 615 nm for hexagonal prisms, and the corresponding reflectances were 5.1 % and 0.5 %, respectively. Also, the simulation results corresponded well with the measurements for the scattering spectra shown in the Figure 3(b). The enhanced electric fields excited by the light with wavelength of 584 nm for nano-pillars and 615 nm for hexagonal prisms are shown in the Figure 3(c), (d) and (e), and hot spots were revealed in the simulation results. Specifically, EM field was strongly enhanced in the small gap region for hexagonal prisms (Figure 3(c) and (d)), while maximum enhancement occurred at the top edges of pillars (Figure 3(e)). Hence, both the experimental and simulation results provided a strong evidence of EM field enhancement of the imprinted R55 substrates and the feasibility of using R55 as a plasmonic material. The SERS substrates shown in Fig. 1 would have different resonance wavelengths. Depending upon the probed molecules, different substrates could be used to yield the optimum results. Generally, smaller gap sizes between nanoparticles would yield stronger enhancements at its resonance wavelength. Assuming the same nano-pillars and hexagonal prisms geometries, the simulated reflectance and scattering spectra as well as the enhanced electric fields are shown in Figure S4 for Au. Because of the better quality factor, Q, of Au shown in Figure S2, stronger enhancements were expected using Au instead of R55 substrate. However, Au cannot be readily imprinted to achieve the nanostructures shown in Figure 1. For Raman spectrum measurements, Figure 3(f) shows the schematic drawing of a drop of the aqueous solution containing probed molecules on the SERS substrate and the droplet leaves a circular area for Raman spectra collection after drying.




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1620 cm–1, 1178 cm–1 and 917 cm–1, which were assigned to the C¦C stretching, in-plane vibration of C¦H and skeletal vibration along radical orientation, versus CV concentration are plotted in Figure 4(c) and the linear relationships were maintained for CV concentrations from 10–7 M to 10–5 M. The calculation of enhancement factor (EF) is shown in Experimental Section and was evaluated to be 1.45"105 based on the CV concentration and the intensity ratio of the characteristic peak at 1620 cm–1 of CV between the R55 SERS substrate and the bare Si substrate. Another extensively used molecule, p-aminothiolphenol (p-ATP, CAS: 1193-02-8, Sigma-Aldrich), was also adopted to study the SERS effect of imprinted R55 TFMGs. In this case, three substrates were used: imprinted R55 with nano-walls (Figure 2(e)), deposition of ~40 nm Au on the Si stamper with inverted nano-walls structure, and planar R55 on Si substrate. The substrates were immersed in the analyte solution (10–3 M p-ATP in ethanol) for 12 h to obtain homogenous adsorption of molecules. As shown in Figure 4(d), while planar R55 showed negligible SERS effect, imprinted R55 exhibited significant SESR effect for p-ATP. For Au deposited on textured Si substrate, a strong vibrational signal of Si lattice at 900–1100 cm–1 was revealed in the Raman spectrum because of the incomplete side-wall coverage of Si by Au during film deposition,31 and this signal might overlap the signals of analyte and decrease the accuracy of sensing. Although more complete coverage of Si could be achieved by increasing the thickness of deposited Au, it would flatten the designed nano-structures and it has always been a dilemma for Si-based SERS substrates. In contrast, the vibrational signal of Si lattice did not show in the Raman spectrum of imprinted R55 substrate because Si was fully covered by R55 using imprinting. It is worth noting that p-ATP had the strongest Raman signal at 1493 cm–1 for imprinted R55 and at 1583 cm–1 for Au on textured Si. It might result from the difference in dielectric constants between R55 and Au (Figure 2 (a) and (b)) and different geometries (i.e., nano-walls versus inverted nano-walls structure) used in SERS measurements.



Figure 4. (a) and (b) Raman spectra showing the enhanced Raman characteristic signatures of CV on the imprinted R55 substrate with hexagonal prisms compared with that on the bare Si wafer. (c) Plots of the peak intensities at 1178 cm–1, 1620 cm–1 and 917 cm–1 as functions of CV concentration. (d) Raman spectra of p-ATP on three substrates: imprinted R55, Au deposited on textured Si, and planar R55 on Si.

CONCLUSION Au-based metallic glass, AuSi, was the first metallic glass reported in 1960.32 Since then, studies on metallic glass have been mainly focused on the mechanical properties of this amorphous alloy. In this work, we demonstrated that AuCuSi TFMG with a negative dielectric constant (Figure 2(a)) could be combined with LSPR for SERS substrates applications, and thin film instead of bulk would decrease the material cost significantly. In addition, by using the


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thermal forming ability of AuCuSi TFMG, a high throughput and low-cost method of thermal imprint could be applied to fabricate large dimensions of nanostructures (Figure 1). The volume of thin films was utilized efficiently, where few hundreds nanometers thick nano-structures were imprinted from the films with thickness of ~1 µm. We performed both measurements and simulations to demonstrate that our designed nanostructures were suitable for surface-enhanced Raman scattering (SERS) applications (Figures 3 and 4). Also, in the absence of grain boundaries in the amorphous TFMG, damping due to increased scattering at grain boundaries does not occur and SERS could be improved. The imprinted nanostructure could enhance Raman signal efficiently and the enhancement factor of 1.4"105 for CV was achieved from the SERS substrate shown in Figure 1(g). Compared to the regular SERS substrates of textured Si with deposited Au films, imprinted Au-based TFMGs provided complete coverage of Si underneath and the vibrational signal of Si lattice would not show in Raman spectra to possibly overlap the signals of analyte and decrease the accuracy of sensing (Figure 4(d)). Thermal-imprinting on TFMGs to fabricate substrates for plasmonic applications in this work is by no means limited to the specific AuCuSi TFMGs in our study, and it is feasible for any TFMG with the required dielectric properties for plasmonic materials and low glass transition temperature. Therefore, our work suggested new avenues for applying a low-cost and high-throughput method on TFMGs to fabricate large-dimension substrates for plasmonic applications.

EXPERIMENTAL SECTION Characterization. The refractive index n and extinction coefficient k of as-deposited TFMGs, R30, R40, R50 and R55, were measured by ellipsometry (M-2000 Ellipsometer, J.A. Woolam) with polarized incident light with wavelength in the range of 370 to 1200 nm. The real and imaginary components of dielectric constant could be obtained from !!!!!! ! !! ! ! ! !

(1a)

!! ! !!".

(1b)



The morphologies of the imprinted substrates were viewed by scanning electron microscope (SEM, NOVA 450, FEI). The microstructures of films were confirmed using transition electron microscopy (TEM, G2 F20, FEI). The reflectance and scattering spectra were measured using micro-spectrometer system (HR4000CG, Ocean Optics) equipped with dark-field module. The scattering spectra for nano-pillars and hexagonal prisms were collected by 20" and 50", 0.5-NA objective lenses, respectively. Raman measurements were performed by micro-Raman spectrometer (HR 600i, Horiba) with a 632.8 nm laser. Different laser powers were used in measurements to ensure that the probe molecules would not be decomposed. However, the laser used in the present study was excited by HeNe gas and it had the most stable power at 19 mW. To obtain the best result, the data were recorded at the power of 19 mW, and the SERS efficiency as a function of the laser power was not measured. In this case, the spot size of the laser beam condensed by a 50", 0.5-NA objective lens was ~1.2 µm in diameter and the incident power on the sample was 19 mW. The data acquisition time was 2 s for one accumulation and averaged 5 times. FDTD simulations. The simulations were conducted using the commercial software of Lumerical FDTD solutions to obtain the optical properties of imprinted nano-pillars on the R55 substrate. The materials parameters of R55 measured by ellipsometry were used as the input for simulations. As shown in Figure S3 (a) and (b), the periodicity of the FDTD boundary along xand y- directions were 600 nm and

3"600 nm, respectively, to match the morphology of the

imprinted nano-pillars. The diameter and height of each pillar was 200 nm and 400 nm, respectively, and a spheroid cap with a height of 65 nm was set on the top of pillar to simulate the curved cap. For the hexagonal prisms shown in Figure S3(c) and (d), the side length and height of prisms were 270 nm and 415 nm, respectively, and the gap between two prisms was 8 nm. A curved cap with height of 165 nm on each prism was simulated. A plane wave of 300– 1200 nm in wavelength polarized along the x-direction was illuminated in the direction perpendicular to the substrate in simulations for both nano-pillars and hexagonal prisms. Enhancement factor calculations. CV aqueous solution with a concentration of 10–7 M and a volume of 1 µL was dropped on the imprinted R55 substrate with hexagonal prisms (see Figure 2(g)) and left a circular area with a radius of ~500 µm after it dried. For the bare Si


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substrate, CV aqueous solution with a concentration of 10–2 M and a volume of 1 µL was dropped and a circular area with a radius of ~390 µm was obtained after drying. The Raman enhancement factor (EF) was evaluated according to the following equation: EF = (ISERS/NSERS)/(IS/NS).

(2)

where NSERS and NS are, respectively, the average numbers of adsorbed molecules on the imprinted SERS substrates and Si wafer within the Raman laser spot area (~0.6 µm in radius) and ISERS and IS represent the intensities at 1620 cm–1 collected from CV with the concentration of 10–7 M on the imprinted substrate and 10–2 M on the Si wafer under the same experimental setup, and ISERS and IS are 210 cnts and 241 cnts, respectively (see Figure 4(b)). Assuming the uniform distribution of CV molecules on substrates in the droplet area, NSERS and NS could be calculated and were about 8.7 " 104 molecules (1 µL " 10–7 mol/L " 6.02 " 1023 " #(0.6 µm)2/#(500 µm)2 ) and 1.4 " 1010 molecules (1 µL " 10-2 mol/L " 6.02 " 1023 " #(0.6 µm)2/#(390 µm)2), respectively. Using Equation (2), the EF for CV molecules on the imprinted SERS substrate with hexagonal prisms (Figure 2(g)) could be calculated and was about 1.4 " 105.

ASSOCIATED INFORMATION Supporting Information TEM images, the quality factors of as-deposited R55 and Au, FDTD simulation parameters, and simulated reflectance and scattering spectra as well as EM field enhancements of textured Au substrates with nano-pillars and hexagonal prisms.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENTS The work was supported by the Ministry of Science and Technology, Taiwan under Contract no.



MOST 106-2221-E-002-072-MY2. We are grateful to the National Center for High-Performance Computing, Taiwan, for providing us with the computation time and facilities and Prof. Jinn P. Chu for providing ellipsometer and Raman spectroscopy. Portion of the experiments in this work were performed at the National Taiwan University NEMS Research Center and is gratefully acknowledged.

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Figure 1. Thermal imprint method for nanostructure fabrication on AuCuSi TFMG. Schematic illustrations showing (a) nano-imprinting on R55 TFMG to fabricate SERS substrate and (b) un-wetting front leading to the curved cap and obtuse contact angle. SEM images of periodic (c) nano-pillars, (d) triangle holes, (e) nano-walls and (f) hemispheres fabricated by designed Si stampers, and (g) hexagonal prisms imprinted by AAO template. The enlarged images are shown in insets. 114x76mm (300 x 300 DPI)



Figure 2. Dielectric constants of AuCuSi TFMGs. (a) Real component, εr, and (b) imaginary component, εi, of the dielectric constants of as-deposited AuCuSi TFMGs, Au and Cu measured by ellipsometer. 52x19mm (600 x 600 DPI)


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Figure 3. Optical properties of imprinted SERS substrates. (a) Reflectance and (b) scattering spectra of imprinted R55 substrates with nano-pillars and hexagonal prisms (H. prism). E and S represent experiment and simulation for short, respectively. FDTD simulation results showing the EM field enhancements monitored from (c) top and (d) cross section of hexagonal prisms, and (e) cross section of nano-pillars at their resonance wavelengths. (f) Schematic drawing showing the probed molecular CV dropped on the hexagonal prisms and the droplet leaves a circular area for Raman spectra collection after drying. 80x33mm (600 x 600 DPI)



Figure 4. (a) and (b) Raman spectra showing the enhanced Raman characteristic signatures of CV on the imprinted R55 substrate with hexagonal prisms compared with that on the bare Si wafer. (c) Plots of the peak intensities at 1178 cm–1, 1620 cm–1 and 917 cm–1 as functions of CV concentration. (d) Raman spectra of p-ATP on three substrates: imprinted R55, Au deposited on textured Si, and planar R55 on Si. 134x101mm (600 x 600 DPI)


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Table of Contents Graphic 35x15mm (300 x 300 DPI)


Surface Plasmon Excited on Imprintable Thin Film Metallic Glasses for

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