An optimal substrate design for SERS: dual-scale diamond-shaped gold nano-structures fabricated via interference lithography

Exploring SERS from complex patterns fabricated by multi-exposure laser interference lithography

Designing uniform plasmonic surfaces in a large area is highly recommended for surface-enhanced Raman scattering (SERS). As periodic morphologies exhibit uniform SERS and optical tunability, diverse fabrication methods of periodic nanostructures have been reported for SERS applications. Laser interference lithography (LIL) is one of the most versatile tools since it can rapidly fabricate periodic patterns without the usage of photomasks. Here, we explore complex interference patterns for spatially uniform SERS sensors and its cost-effective fabrication method termed multi-exposure laser interference lithography (MELIL). MELIL can produce nearly periodic profiles along every direction confirmed by mathematical background, and in virtue of periodicity, we show that highly uniform Raman scattering (relative standard deviation <6%) can also be achievable in complex geometries as the conventional hole patterns. We quantitatively characterize the Raman enhancement of the MELIL complex patterns after two different metal deposition processes, Au e-beam evaporation and Ag electroplating, which results in 0.387 × 10⁵and 1.451 × 10⁵in enhancement factor respectively. This alternative, vacuum-free electroplating method realizes an even more cost-effective process with enhanced performance. We further conduct the optical simulation for MELIL complex patterns which exhibits the broadened and shifted absorption peaks. This result supports the potential of the expanded optical tunability of the suggested process.

Gradient wettability induced by deterministically patterned nanostructures

We report a large-scale surface with continuously varying wettability induced by ordered gradient nanostructures. The gradient pattern is generated from nonuniform interference lithography by utilizing the Gaussian-shaped intensity distribution of two coherent laser beams. We also develop a facile fabrication method to directly transfer a photoresist pattern into an ultraviolet (UV)-cured high-strength replication molding material, which eliminates the need for high-cost reactive ion etching and e-beam evaporation during the mold fabrication process. This facile mold is then used for the reproducible production of surfaces with gradient wettability using thermal-nanoimprint lithography (NIL). In addition, the wetting behavior of water droplets on the surface with the gradient nanostructures and therefore gradient wettability is investigated. A hybrid wetting model is proposed and theoretically captures the contact angle measurement results, shedding light on the wetting behavior of a liquid on structures patterned at the nanoscale.

Fabrication of highly sensitive and reproducible 3D surface-enhanced Raman spectroscopy substrates through in situ cleaning and layer-by-layer assembly of Au@Ag nanocube monolayer film

A highly sensitive and uniform three-dimensional (3D) surface-enhanced Raman spectroscopy (SERS) substrate has been fabricated by in situ ultraviolet ozone cleaning and layer-by-layer self-assembly. The SERS properties and the structural changes of the substrates were systematically studied by adjusting the cleaning time upon the in situ and post cleaning strategy. Under the optimal cleaning condition, the cleaning technology could give rise to clean and optimal surfaces for SERS analysis, thus obtaining efficient plasmonic films populated with a large number of homogeneous 'hot-spots'. Then with the optimal monolayer film, the SERS performance derived from plasmon coupling in multilayers of the Au@Ag nanocubes substrates was explored. It demonstrated that the plasmon coupling between layers (out-of-plane) contributed much to the SERS intensity, leading a more superior SERS enhancement from the 3D SERS substrates than that from the conventional two-dimensional SERS substrates. Also the in situ cleaning 3D SERS substrates displayed a nice uniformity and excellent time stability. With this method, the optimized substrates were further used to detect prohibited pigments in drink with an excellent linear relationship between characteristic peak intensity and analytes concentration over wide concentration ranges. Our experimental results clearly show that the in situ cleaning 3D SERS substrates provide an ideal candidate for SERS applications in food safety.

Fabrication and SERS Performances of Metal/Si and Metal/ZnO Nanosensors: A Review

Surface-enhanced Raman scattering (SERS) sensors are very powerful analytical tools for the highly sensitive detection of chemical and biological molecules. Substantial efforts have been devoted to the design of a great number of hybrid SERS substrates such as silicon or zinc oxide nanosystems coated with gold/silver nanoparticles. By comparison with the SERS sensors based on Au and Ag nanoparticles/nanostructures, higher enhancement factors and excellent reproducibilities are achieved with hybrid SERS nanosensors. This enhancement can be due to the appearance of hotspots located at the interface between the metal (Au/Ag) and the semiconducting substrates. Thus, in this last decade, great advances in the domain of hybrid SERS nanosensors have occurred. In this short review, the recent advances of these hybrid metal-coated semiconducting nanostructures as SERS sensors of chemical and biological molecules are presented.

Influence of gold nanoholes and nanoslits arrays on Raman spectra and optical reflectance of graphene oxide

We report the effect of gold nanostructured substrates, fabricated by interference lithography technique (IL), on the Raman spectra and optical reflectance of graphene oxide (GO) layers. For purposes of comparison two gold nanostructured substrates, nanoslits (AuNSs) and circular nanoholes (AuNHs) were compared with a non-nanostructured gold substrate. Effects induced by the gold nanostructured substrates are discussed in terms of the I_D/I_G ratio and the FWHM of the G band (FWHM(G)) as a function of the G band intensity (I_G), showing that both I_D/I_G and FWHM(G) parameters are highly sensitive to the number of GO layers (n_GO), which would allow to identify the number of GO layers in a reliable way. Optical reflectance spectra (R(λ)) reveal that plasmons are generated on the surface of nanostructured substrates by the incident radiation. Dips in R(λ) are ascribed as coupling by surface plasmon polaritons described by Bloch waves (BW-SPP). A peak in R(λ) is also observed and it is ascribed to visible radiation produced by Förster resonance energy transfer and Purcell effect. The relevance of these results lies in the possibility of designing colorimetric plasmonic sensors, based on few layers of GO with an excellent control of n_GO and with potential in detection of molecules by fluorescent absorption.

Controlling surface morphology and sensitivity of granular and porous silver films for surface-enhanced Raman scattering, SERS

The design of efficient substrates for surface-enhanced Raman spectroscopy (SERS) for large-scale fabrication at low cost is an important issue in further enhancing the use of SERS for routine chemical analysis. Here, we systematically investigate the effect of different radio frequency (rf) plasmas (argon, hydrogen, nitrogen, air and oxygen plasma) as well as combinations of these plasmas on the surface morphology of thin silver films. It was found that different surface structures and different degrees of surface roughness could be obtained by a systematic variation of the plasma type and condition as well as plasma power and treatment time. The differently roughened silver surfaces act as efficient SERS substrates showing greater enhancement factors compared to as prepared, sputtered, but untreated silver films when using rhodamine B as Raman probe molecule. The obtained roughened silver films were fully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS and Auger) and ultraviolet-visible spectroscopy (UV-vis) as well as contact angle measurements. It was found that different morphologies of the roughened Ag films could be obtained under controlled conditions. These silver films show a broad range of tunable SERS enhancement factors ranging from 1.93 × 102 to 2.35 × 105 using rhodamine B as probe molecule. The main factors that control the enhancement are the plasma gas used and the plasma conditions, i.e., pressure, power and treatment time. Altogether this work shows for the first time the effectiveness of a plasma treatment for surface roughening of silver thin films and its profound influence on the interface-controlled SERS enhancement effect. The method can be used for low-cost, large-scale production of SERS substrates.

Tunable laser interference lithography preparation of plasmonic nanoparticle arrays tailored for SERS

The facile preparation of arrays of plasmonic nanoparticles over a square centimeter surface area is reported. The developed method relies on tailored laser interference lithography (LIL) that is combined with dry etching and it offers means for the rapid fabrication of periodic arrays of metallic nanostructures with well controlled morphology. Adjusting the parameters of the LIL process allows for the preparation of arrays of nanoparticles with a diameter below hundred nanometers independently of their lattice spacing. Gold nanoparticle arrays were precisely engineered to support localized surface plasmon resonance (LSPR) with different damping at desired wavelengths in the visible and near infrared part of the spectrum. The applicability of these substrates for surface enhanced Raman scattering is demonstrated where cost-effective, uniform and reproducible substrates are of paramount importance. The role of deviations in the spectral position and the width of the LSPR band affected by slight variations of plasmonic nanostructures is discussed.

Toward highly sensitive surface-enhanced Raman scattering: the design of a 3D hybrid system with monolayer graphene sandwiched between silver nanohole arrays and gold nanoparticles

We report a novel graphene-metal hybrid system by introducing monolayer graphene between gold nanoparticles (Au NPs) and silver nanohole (Ag NH) arrays. The design incorporates three key advantages to promote the surface-enhanced Raman scattering (SERS) sensing capacity: (i) making full use of the single-atomic feature of graphene for generating uniform sub-nanometer spaces; (ii) maintaining the bottom layer of Ag nanoarrays with an ordered manner for facilitating the transfer of graphene films and assembly of the top layer of Au NPs; (iii) integrating the advantages of the strong plasmonic effect of Ag, the chemical stability of Au, as well as the mechanical flexibility and biological compatibility of graphene. In this configuration, the plasmonic properties can be fine-tuned by separately optimizing the horizontal or vertical gaps between the metal NPs. Exactly, sub-20 nm spaces between the horizontally patterned Ag tips constructed by adjacent Ag NHs, and sub-nanometer scale graphene gaps between the vertically distributed Au NP-Ag NH have been achieved. Finite element numerical simulations demonstrate that the multi-dimensional plasmonic couplings (including the Au NP-Au NP, Au NP-Ag NH and Ag NH-Ag NH couplings) promote for the hybrid platform an electric field enhancement up to 137 times. Impressively, the as-prepared 3D Au NP-graphene-Ag NH array hybrid structure manifests ultrahigh SERS sensitivity with a detection limit of 10^-13 M for R6G molecules, as well as good reproducibility and stability. This work represents a step towards high-performance SERS substrate fabrication, and opens up a new route for graphene-plasmonic hybrids in SERS applications.

"Atomic Force Masking" Induced Formation of Effective Hot Spots along Grain Boundaries of Metal Thin Films

We present an interesting phenomenon, "atomic force masking", which is the deposition of a few-nanometer-thick gold film on ultrathin low-molecular-weight (LMW) polydimethylsiloxane (PDMS) engineered on a polycrystalline gold thin film, and demonstrated the formation of hot spot based on SERS. The essential principle of this atomic force masking phenomenon is that an LMW PDMS layer on a single crystalline grain of gold thin film would repel gold atoms approaching this region during a second cycle of evaporation, whereas new nucleation and growth of gold atoms would occur on LMW PDMS deposited on grain boundary regions. The nanostructure formed by the atomic force masking, denoted here as "hot spots on grain boundaries" (HOGs), which is consistent with finite-difference time-domain (FDTD) simulation, and the mechanism of atomic force masking were investigated by carrying out systematic experiments, and density functional theory (DFT) calculations were made to carefully explain the related fundamental physics. Also, to highlight the manufacturing advantages of the proposed method, we demonstrated the simple synthesis of a flexible HOG SERS, and we used this substrate in a swabbing test to detect a common pesticide placed on the surface of an apple.

Dendrimer-Capped Gold Nanoparticles for Highly Reliable and Robust Surface Enhanced Raman Scattering

Dendrimer-stabilized gold nanoparticles (Au-Den) were prepared by a facile solution based method for a highly reliable and robust surface enhanced Raman scattering (SERS) substrate. Au-Den was selectively attached on the surface of reduced graphene oxide (rGO) by noncovalent interactions between the Au capping dendrimer and the graphene surface. Au-Den/rGO exhibits the outstandingly stable and highly magnified Raman signal with an enhancement factor (EF) of 3.9 × 10(7) that enables detection of R6G dyes with concentration as low as 10 nM, retaining 95% of the Raman signal intensity after 1 year. The remarkable stability and enhancement originated not only from a simple combination of the electromagnetic and chemical mechanism of SERS but also from intensified packing density of stable Au-Den on the graphene substrate due to the firm binding between the dendrimer capped metal nanoparticles and the graphene substrate. This method is not limited to the gold nanoparticles and G4 dendrimer used herein, but also can be applied to other dendrimers and metal nanoparticles, which makes the material platform suggested here superior to other SERS substrates.

Highly effective and uniform SERS substrates fabricated by etching multi-layered gold nanoparticle arrays

Gold nanoparticle multilayers printed on silicon substrates layer by layer were etched by a gold etchant to form highly effective and uniform substrates for surface-enhanced Raman scattering (SERS). The performance of the SERS substrates was systematically studied by adjusting the number of nanoparticle layers and the etching time. The optimized enhancement factor (EF) and the detection limit of the substrates were determined to be 8.6 × 10(6) and 1 × 10(-12) M, respectively. The high EF and low detection limit were attributed to the high density of "hot-spots" and the facile accession of probe molecules to these spots. Moreover, the SERS substrates exhibited a nice uniformity with a small spot-to-spot variation and a good sample-to-sample reproducibility as well. The experimental results were supported by finite-difference time domain (FDTD) simulations. Our study suggests that low-cost, large-scale, and uniform SERS substrates with a high EF and low detection limit can be achieved by using bottom-up chemical methods.

Fabrication of flexible silicon nanowires by self-assembled metal assisted chemical etching for surface enhanced Raman spectroscopy

Flexible silicon nanowires fabricated by nano spheres lithography and metal assisted chemical etching for surface enhanced Raman spectroscopy.

Dimer-on-mirror SERS substrates with attogram sensitivity fabricated by colloidal lithography

Nanoplasmonic substrates with optimized field-enhancement properties are a key component in the continued development of surface-enhanced Raman scattering (SERS) molecular analysis but are challenging to produce inexpensively in large scale. We used a facile and cost-effective bottom-up technique, colloidal hole-mask lithography, to produce macroscopic dimer-on-mirror gold nanostructures. The optimized structures exhibit excellent SERS performance, as exemplified by detection of 2.5 and 50 attograms of BPE, a common SERS probe, using Raman microscopy and a simple handheld device, respectively. The corresponding Raman enhancement factor is of the order 10(11), which compares favourably to previously reported record performance values.

Development of Ag nanopolyhedra based fiber-optic probes for high performance SERS detection

Tapered fiber probes based on silver nanopolyhedra were fabricated using a direct and effective autoclave reaction method and exhibited high SERS activity.

Highly sensitive surface-enhanced Raman scattering based on multi-dimensional plasmonic coupling in Au-graphene-Ag hybrids

We report an efficient surface-enhanced Raman scattering (SERS) substrate by utilizing the multi-dimensional plasmonic coupling in Au nanoparticle (NP)-graphene-Ag NP hybrid structures. An ultrasensitive SERS detection with a limit of down to 10(-13) M has been achieved when the sandwiched hybrid film is fabricated on an Ag substrate.

Self-assembly of large-scale gold nanoparticle arrays and their application in SERS

Surface-enhanced Raman scattering is an effective analytical method that has been intensively applied in the field of identification of organic molecules from Raman spectra at very low concentrations. The Raman signal enhancement that makes this method attractive is usually ascribed to the noble metal nanoparticle (NMNP) arrays which can extremely amplify the electromagnetic field near NMNP surface when localized surface plasmon resonance (LSPR) mode is excited. In this work, we report a simple, facile, and room-temperature method to fabricate large-scale, uniform gold nanoparticle (GNP) arrays on ITO/glass as SERS substrates using a promoted self-assembly deposition technique. The results show that the deposition density of GNPs on ITO/glass surface increases with prolonging deposition time, and nanochain-like aggregates appear for a relatively longer deposition time. It is also shown that these films with relatively higher deposition density have tremendous potential for wideband absorption in the visible range and exhibit two LSPR peaks in the extinction spectra because the electrons simultaneously oscillate along the nanochain at the transverse and the longitudinal directions. The SERS enhancement activity of these GNP arrays was determined using 10-6 M Rhodamine 6G as the Raman probe molecules. A SERS enhancement factor as large as approximately 6.76 × 106 can be obtained at 1,363 cm-1 Raman shift for the highest deposition density film due to the strong plasmon coupling effect between neighboring particles.

Towards visible light hydrogen generation: quantum dot-sensitization via efficient light harvesting of hybrid-TiO2

We report pronounced enhancement of photoelectrochemical hydrogen generation of a quantum dot-sensitized hybrid-TiO2 (QD/H-TiO2) electrode that is composed of a mesoporous TiO2 layer sandwiched by a double sided energy harvesting layer consisting of a surface-textured TiO2 inverse opals layer on the bottom and a patterned mesoporous TiO2 layer on the top. CdSe/H-TiO2 exhibits a maximum photocurrent density of ~16.2 mA/cm(2), which is 35% higher than that of the optimized control sample (CdSe/P25), achieved by matching of the bandgap of quantum dot-sensitization with the wavelength where light harvesting of H-TiO2 is observed. Furthermore, CdSe/H-TiO2 under filtered exposure conditions recorded current density of ~14.2 mA/cm(2), the greatest value in the visible range. The excellent performance of the quantum dot-sensitized H-TiO2 suggests that alteration of the photoelectrodes to suitable nanostructures with excellent light absorption may offer optimal strategies for attaining maximum efficiency in a variety of photoconversion systems.

Optimization for visible light photocatalytic water splitting: gold-coated and surface-textured TiO2 inverse opal nano-networks

A gold nanoparticle-coated and surface-textured TiO2 inverse opal (Au/st-TIO) structure that provides a dramatic improvement of photoelectrochemical hydrogen generation has been fabricated by nano-patterning of TiO2 precursors on TiO2 inverse opal (TIO) and subsequent deposition of gold NPs. The surface-textured TiO2 inverse opal (st-TIO) maximizes the photon trapping effects triggered by the large dimensions of the structure while maintaining the adequate surface area achieved by the small dimensions of the structure. Au NPs are incorporated to further improve photoconversion efficiency in the visible region via surface plasmon resonance. st-TIO and Au/st-TIO exhibit a maximum photocurrent density of ∼0.58 mA cm(-2) and ∼0.8 mA cm(-2), which is 2.07 and 2.86 times higher than that of bare TIO, respectively, at an applied bias of +0.5 V versus an Ag/AgCl electrode under AM 1.5 G simulated sunlight illumination via a photocatalytic hydrogen generation reaction. The excellent performance of the surface plasmon-enhanced mesoporous st-TIO structure suggests that tailoring the nanostructure to proper dimensions, and thereby obtaining excellent light absorption, can maximize the efficiency of a variety of photoconversion devices.