Reusable three-dimensional nanostructured substrates for surface-enhanced Raman scattering
© Zhu et al.; licensee Springer. 2014
Received: 12 October 2013
Accepted: 26 December 2013
Published: 13 January 2014
To date, fabricating three-dimensional (3D) nanostructured substrate with small nanogap was a laborious challenge by conventional fabrication techniques. In this article, we address a simple, low-cost, large-area, and spatially controllable method to fabricate 3D nanostructures, involving hemisphere, hemiellipsoid, and pyramidal pits based on nanosphere lithography (NSL). These 3D nanostructures were used as surface-enhanced Raman scattering (SERS) substrates of single Rhodamine 6G (R6G) molecule. The average SERS enhancement factor achieved up to 1011. The inevitably negative influence of the adhesion-promoting intermediate layer of Cr or Ti was resolved by using such kind of 3D nanostructures. The nanostructured quartz substrate is a free platform as a SERS substrate and is nondestructive when altering with different metal films and is recyclable, which avoids the laborious and complicated fabricating procedures.
KeywordsThree-dimensional (3D) nanostructure Nanosphere lithography Surface-enhanced Raman scattering (SERS)
Raman spectroscopy is a powerful and label-free tool for identifying molecular species because the signals of re-emitted Raman photons address for all molecular species and correspond to a particular set of vibration modes. However, the Raman signal is very weak because Raman scattering is an inelastic scattering process of photon, only one in every 107 photon incidence on a molecule undergoing Raman scattering, and it has a second-order dipole transition nature. Fortunately, it was discovered that the signals of Raman scattering could be amplified enormously by molecules contacting with a textured or patterned special noble metal surface, termed as surface-enhanced Raman scattering (SERS) [1, 2]. Commonly, the origins of this enhancement [3–6] are believed to have contributions from both electromagnetic enhancement (EM) and chemical enhancement mechanisms. The latter involves charge transfer (CT) excitation [4–6] between detecting molecules and metal particles, whereas the former originates from a resonance between the incidence and scattered radiation fields associated to the excitation of localized surface plasmon resonances (LSPR) , termed as 'hot-spots’ [2, 7, 8] around nanoscale metal particles or artificial architectures. The sensitivity and reproducibility [9–11] of SERS signal strongly relies on different fabricated hot-spots, in which a vital role is played by a SERS substrate. In general, SERS substrate can be divided into two fundamental classes, random and artificial substrates . Both of them should possess enough surface area to absorb more molecules to contribute to the Raman scattering and abundant hot-spots to enhance the local electromagnetic field. However, random substrate, such as colloidal, is proved to be limited because of weak reproducibility and fractal nanoparticle aggregation, leading their enhancement factors to decrease with increasing fractal size . For the artificial nanostructure, the fourth power of local electromagnetic field of the hot-spots contributes to the signals of SERS and is sensitive to the critical dimension of artificial nanostructure [5, 13]. To date, however, it is a challenge to control the nanostructures with extremely small size. Typically, previous engineering nanostructures were resorted to lithography-based nanotechnologies, involving electron-beam lithography (EBL), nanoimprint (NIL), nanosphere lithography (NSL), electrochemical lithography , and so on. For example, some arbitrary two-dimensional (2D) dimer nanostructures with small gaps such as bowties and nano-antennas, were proposed and prepared by EBL [15–25]. Some nanostructures were fabricated by NIL such as nanograting  and nanopost  as uniform SERS hot substrate. However, the major limitation lies in the sophistication of the fabrication processes and the inevitable defect.
Triangular noble nanoparticle arrays were fabricated by NSL [24, 27]. Recently, nanocrescent [28, 29] as a quasi-three-dimensional (3D) and tuning resonance SERS substrate was fabricated by NSL, which resorted by glancing angular metal deposition onto nanospheres. However, it is difficult to fabricate large-area and uniform 3D nanostructures with small gaps between adjacent patterns because lithography-based techniques are isotropic and the resolution is limited. Previous investigations depended on wet etching and electrochemical method, a typical example is pyramidal pits [30, 31]; these engineering structures had large pitches which are much larger than the excitation laser probe spot size and lead to SERS enhancement with poor reproducibility and sensitivity. It is of crucial importance to develop 3D metal nanostructures with controllable nanogap sizes for the generation of strongly localized field. Van Duyne  and Fang  proposed metal films over nanosphere (MFON) electrodes as SERS active substrates in order to improve the surface nanostructure stability and suppress the inherent loss, where nanocavities with hot-spots are presented. However, the MFON structures are disposable substrates. Therefore, it is demanded to investigate reusable and high-sensitivity SERS substrates.
Here, we developed an NSL technique to produce large-area subwavelength 3D nanostructures performed as SERS substrates with high sensitivity, the SERS enhancement factor up to 1011, with high reproducibility, and especially free with adhesive layer. Hexagon-close-packed (hcp) 3D nanostructure arrays were fabricated with precise nanogaps. Three types of nanostructures were obtained by controlling etching parameters, involving hemispherical nanostructure (HS), hemi-ellipsoidal nanostructure (HE), and pyramidal pits. We proved the detrimental influences of the adhesion layer between noble metal layer and quartz substrate to the SERS enhancement. Such kind of SERS substrate is a reusable substrate which can be reused simply by removing and redepositing the metal thin film.
Monolayer of long-range-ordered polystyrene (PS) polystyrene as mask
Two hundred-nanometer monodispersed polystyrene (PS) nanospheres were synthesized by emulsifier-free emulsion polymerization, which would perform as colloidal mask of quartz substrate. The diameter of PS nanosphere was 200 nm with a standard deviation within 2 nm. A monolayer, long-range-ordered, large-area (more than 2 cm2), and hcp PS nanosphere was coated onto a cleaned quartz substrate by self-assembly. All quartz substrates were pre-treated with hydrophilic solution (H2O2/NH3.H2O/H2O 1:1:5 (v/v/v)) at 70°C for improving the stability of long-range-ordered nanosphere. The samples of surface-assembled PS nanospheres were baked on hotplate at 70°C for 5 min to remove some solvents.
Assembly of detecting molecules
After etching the quartz substrates, all samples should be cleaned in butanone under ultrasonication for 2 min to remove organic residues and other particles. Consequently, a desirable noble metal (Ag, Au, Al, or Pt) thin film was directly deposited onto the surface by electron-beam evaporation. However, it was not necessary in the additional coated adhesive layer between the noble metal and quartz substrate, such as Cr or Ti. The samples with deposited metal thin film were soaked overnight in Rhodamine 6G (R6G)/methanol solutions. Two kinds of concentrations were used for nanopatterned samples and unpatterned for contrast samples, 10-9 and 10-3 mMol/L, respectively. The R6G-coated samples were rinsed three times in 10 mL of deionized (DI) water and blow-dried in nitrogen.
The top morphologies and the cross section of the samples were characterized by a FEI Sirion 200 field scanning electron microscope (SEM; Hillsboro, OR, USA) with acceleration voltages ranging from 5 to 10 kV. The SERS spectra were collected in backscattering mode by a JY LabRAM HR Raman spectrum (Horiba, Kyoto, Japan) with a laser wavelength of 633 nm. In order to achieve comparable Raman signal intensities on various samples with significantly different Raman signal enhancements, we fixed exciting laser with the pump power 0.6 mW and the integration time 20 s. In each sample, we measured 10 points to obtain average Raman intensity as the reference used in the SERS enhancement factor calculation. The Raman peaks fitted from the baseline-removed Raman spectra using a Guassian-Lorentzian lineshape.
Calculation of SERS enhancement factors
where EF was the enhancement factor, ISERS and Ibulk are the Raman signal intensities at 1,365 cm-1 band, which is a characteristic representative vibration wave number of R6G molecules adsorbed on the 3D nanostructure and from the bulk R6G, respectively; Nsurf and Nbulk are the numbers of the R6G molecules absorbed on the 3D nanostructures and the bulk R6G molecules exposed to the laser spot, respectively.
Results and discussion
The quartz substrate was directly nanopatterned by RIE. The tailored PS nanosphere performed as the sacrificial mask. The final geometries, sizes, and nanogaps between two adjacent architectures were changed with the mixture of the etching gases, involving CF4/CHF3/SF6/Ar/O2. We first characterized the etching rate of PS nanosphere and quartz substrate under each individual pure etching gas (CF4/CHF3/SF6/Ar/O2) at a RF power of 40 W and a typical gas pressure of 2 Pa. And then according to the etching results of the above individual gases, we designed several reasonable etching recipes with the mixture of the above gases. It was found that the scale of PS nanosphere was gradually reduced, and therefore, the gap of two adjacent nanospheres was also gradually increased. The quartz substrate was nanopatterned and kept the same, gradually changing with the gradual change of PS nanosphere mask.
Above fabrication procedures, providing a simple and spatially controllable method on the nanoscale structures according to rational etching parameters, are instrumental in developing SERS substrates. The motivations for the 3D noble metallic nanostructural substrates are to create large-surface area and high-surface dense hot-spots to contribute to SERS with a large enhancement factor, to improve enhancement reproducibility, and to resolve the problem of adhesion layer. The 3D nanostructures would cause the incident light to converge, amplify the total absorption of excitation light and increase the effective cross section of Raman scattering. The geometries, sizes, and gaps of these 3D nanostructures all affect the surface plasmons (SPs). In this article, SERS spectra were collected at 633-nm laser wavelength. The R6G molecules were employed as detection target. Before the R6G molecules were dosed onto the nanostructures, a desirable noble metal (Ag or Au) was directly deposited onto the surface by electron-beam evaporation on the fabricated three types of 3D nanostructures and unpatterned substrate, and then the samples were soaked overnight in R6G/methanol solutions. Two kinds of bulk concentrations were used for nanopatterned samples and unpatterned for contrast samples, 10-9 and 10-3 mM, respectively. The R6G coated samples were rinsed several times in 10 mL of DI water and blow-dried in nitrogen.
We addressed a prompting nanosphere lithography method for fabricating spatially controllable 3D nanostructures, and successfully achieved hemisphere, hemiellipsoid, and pyramidal pit-shaped nanostructures. We demonstrated that the kernel factor was to precisely control the ratio of the lateral and vertical etching rate to achieve the desirable geometries. Effective and extreme tailoring of the diameter of the PS nanosphere mask played a crucial role in achieving the controllable nanogaps between these nanostructures, which could be below 10 nm or even at point contact between two adjacent nanostructures. Applying the reliable 3D nanostructures as tunable SERS substrates, we extensively study influences of geometries, nanogaps, and the adhesion layer between the desirable noble metal and the underlying quartz substrate on SERS enhancement effect. Negative contribution of adhesive layer was demonstrated according to the results of SERS enhancement factors. The tunable SERS substrates possess great advantages: (1) achieving strong average SERS enhancement factor up to 1011; (2) free-adhesion layer; (3) a platform for any desirable metal, and can be reused by simply removing and redepositing the metal film while not destructing the 3D nanostructures or repeating the tedious fabricating procedures. Due to the increase in damping plasmonic resonance with increasing the thickness of the adhesion, we suggest the suitable adhesion of Ti layer below 5 nm and of Cr below 2 nm.
This work was supported by the Chinese National Science and Technology Plan 973 with Grant No. 2007CB935301.
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