Large-Area, Highly Sensitive SERS Substrates with Silver Nanowire Thin Films Coated by Microliter-Scale Solution Process
A microliter-scale solution process was used to fabricate large-area, uniform films of silver nanowires (AgNWs). These thin films with cross-AgNWs were deposited onto Au substrates by dragging the meniscus of a microliter drop of a coating solution trapped between two plates. The hot spot density was tuned by controlling simple experimental parameters, which changed the optical properties of the resulting films. The cross-AgNW films on the Au surface served as excellent substrates for surface-enhanced Raman spectroscopy, with substantial electromagnetic field enhancement and good reproducibility.
KeywordsSilver nanowires Thin film coating Microliter-scale solution process Surface-enhanced Raman spectroscopy
Surface plasmon resonance (SPR) is the collective oscillation of conduction band electrons on a metal surface excited by incident light at a metal-dielectric interface [1, 2, 3]. For nanostructures of noble metals such as gold and silver, the SPR absorption band is present in the visible region, and its exact wavelength is very sensitive to particle size, shape, spacing, and the surrounding dielectric medium [4, 5]. In particular, when two nanoparticles are close to each other with a nanoscale gap, the electromagnetic field is confined in this gap [6, 7], also known as a “hot spot.” Many efforts have been studied to reliably produce surface-enhanced Raman spectroscopy (SERS) hot spots, through the use of metal nanoparticle aggregates [8, 9], patterned arrays of nanostructures [10, 11], and metal films over nanospheres [12, 13]. This allows for highly sensitive SERS sensing systems, but their application is limited by the ability to fabricate structures with regular gap dimensions, which is a current challenge in nanofabrication.
Silver nanowires (AgNWs) have been studied as an ideal SERS candidate due to their large surface area, high phase purity, and good crystallinity . For single nanowire studies, surface etching of AgNWs  and decorated metallic nanoparticles on AgNWs  have been shown to increase the amount of SERS active “hot spots.” To further increase these enhancements, AgNWs have been paired (crossed and parallel) [17, 18] and bundled  to create gaps between neighboring nanowires, increasing the electromagnetic fields present. AgNWs have been assembled into large surface area parallel arrays [20, 21], which showed strong SERS enhancements in the gaps between parallel AgNWs. While parallel arrays of AgNW films have been extensively studied, large-scale crossed AgNW assemblies have received less attention.
Homogeneous SERS substrate can provide uniform distributions of hot spots for single molecule detection. Many routes have been proposed to fabricate SERS-active nanostructures, such as Langmuir-Blodgett assembly , layer-by-layer assembly [22, 23, 24, 25], convective assembly [26, 27], and electron-beam lithography [28, 29, 30]. However, some of these techniques are expensive, complex, and time-consuming, whereas others are not suitable for large-scale production of uniform SERS substrates.
Herein, we present a simple and scalable approach to fabricate high-density cross-patterned AgNW films on Au surfaces by utilizing a meniscus-dragging deposition (MDD) method. AgNWs were aligned in the coating direction while the deposition plate was moved back and forth, dragging the meniscus of a microliter of AgNW solution injected into the gap between the moving deposition plate (on top) and the Au substrate (on the bottom). To produce a large number of SERS hot spots, we fabricated cross-junctions between the nanowires by rotating the pre-coated substrate by 90° and repeating the process, resulting in uniform cross-AgNW films. In this study, we demonstrated that the cross-AgNW films show higher Raman intensity than drop-AgNW films of the same surface density. In particular, the cross-AgNW films on Au films shows 1.8 times stronger SERS enhancement than drop-AgNW films.
Fabrication of Cross-AgNW Films
Characterization of Cross-AgNW Films
The fabricated Au/cross-AgNW films were characterized using digital photography (Lumix DMC-LX5, Panasonic), field emission scanning electron microscopy (FE-SEM, Carl Zeiss SIGMA), and UV-vis-NIR spectrophotometry (V-670, Jasco). To perform SERS using the prepared substrates, Au/cross-AgNW films were heated on a hot-plate at 110 °C for 10 min to remove the polyvinylpyrrolidone (PVP) layer on the AgNW surface. The SERS substrates were then dipped in 100 mM benzenethiol in ethanol (Sigma Aldrich) for 15 min, rinsed with ethanol, and then dried under N2. Raman spectra of benzenethiol were collected using a confocal Raman microscope (Alpha 300, WITec) with a 785 nm excitation laser. The integration time was 0.5 s, and the laser power was ~ 15 mW. Raman spectral images (40 × 40 μm2) were obtained under 15 mW laser power and 0.2 s integration times.
Results and Discussion
To fabricate cross-patterned AgNW assemblies on Au film substrate, we used a MDD method as shown in Fig. 1. The concentrated AgNW/IPA suspension was injected between the deposition plate and the Au film contacted at an angle of θ = 30°, and a meniscus was formed between the end of deposition plate and the Au surfaces due to capillary action (Fig. 1a). When the deposition plate moved back and forth, the shear stress applied to the AgNWs in the meniscus causes them to assemble parallel to each other and align along the direction of the shearing force. After this process, the AgNW film substrate was rotated 90° (Fig. 1b), and another layer of AgNW was assembled on top of it (Fig. 1c). This process was repeated to form a high density of cross-AgNW assemblies with 8–18 layers. Using multiple deposition steps, we fabricated high density cross-AgNWs on Au film substrates, where 8, 10, 14, and 18 deposited layer samples are denoted as C-8, C-10, C-14, and C-18, respectively. The photograph in Fig. 1d shows the high-density AgNW assemblies on Au film with 18 deposition number, covering a relatively large area (2 × 2 cm2).
In summary, we have presented solution-based fabrication of extremely enhanced and reproducible large-area SERS substrates with uniform cross-arrays of AgNWs on Au; these arrays were produced using microliter volumes of AgNW suspension. The AgNWs were aligned by the shear stress applied to the meniscus of a droplet of AgNW suspension injected between the deposition plate and the coating plate. The regularly assembled AgNW films demonstrated better structural homogeneity and SERS intensity 1.8–36-fold higher than random, drop-casted AgNW films. The increased SERS intensity was attributed to an increase in SERS multiple plasmon couplings among AgNWs (crossed and parallel gaps) and between the Au film and the AgNWs. We have demonstrated that the SERS enhancement brought about by the cross-AgNW films was optimized at C-14 (Au/cross-AgNW films). Therefore, the cross-AgNW-based SERS substrate is sufficient to fabricate a highly sensitive SERS system. This approach has great potential for use in a wide range of applications in optoelectronics, nanoelectronics, and sensors.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2016R1A2B4012992) and the Chung-Ang University Graduate Research Scholarship in 2017.
SJ and JL equally contributed to this work. HK and SC developed the idea of the Au/cross-AgNW film for a highly sensitive SERS substrate. SJ and SN prepared the samples and performed the SEM images measurements. JL and SJ measured the UV-vis and Raman spectra. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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