Design, fabrication and optimization of large area chemical sensor based on Surface-enhanced Raman Scattering (SERS) mechanism

Abstract

In recent years there has been an increasing interest in analysis and identification of complex molecules for the medical diagnostics, pharmaceutical research and homeland security applications. If these molecules are present in high concentration, a technique known as Raman spectroscopy can be utilized. Unfortunately, only one in every 10¹² photon incidence on molecule undergoes Raman scattering resulting in weak Raman absorption. An efficient technique to overcome this limitation is to utilize surface-enhanced Raman scattering (SERS) whereby molecules are placed on the surface of nanostructured metallic substrate which performs the function of transducting photons into and out of the molecules. SERS extends the scope of Raman scattering to detect molecules at low concentrations to few/single molecule level. Previously the ‘KlariteTM’ substrate consisting of an inverted array of square based pyramidal nanostructures patterned onto a Silicon substrate has been demonstrated to afford highly reproducible SERS signals with approximately 10⁷ enhancement factor.

In this report, the effect of geometrical parameters associated with the inverted pyramidal array on SERS effect for sensing applications was investigated. Geometrical parameters studied include pitch length, pit size, aspect ratio of the base of pyramid and fill factor. 3D computational modelling based on Rigorous Coupled Wave Analysis (RCWA) is used to bridge with theory. From these observations, the geometrical parameters of inverted pyramid nanostructures have been optimized for better sensing ability. A test chip is fabricated for the purpose of performing a matrix experiment, allowing deconvolution of geometrical variables: lattice pitch (1000nm-3000nm), pit size (500nm-2500nm), pit aspect ratio [width to length]. Fabrication steps include electron-beam lithography, anisotropic wet etching and metallization.

Computational and experimental reflectometry systems were applied to enable the identification and analysis of a variety of dispersive features including propagating surface plasmons, localized surface plasmons and diffraction dispersion. From the study of inverted pyramid, plasmonic behaviors are observed: 1. the surface plasmon polariton depends strongly on polarization. 2. Highly dispersive features arising from simple surface diffraction effects appear insensitive to polarization state. 3. As the pit size gets bigger, the diffraction efficiency decreases but the wavelength/angular position remain the same. 4. Diffractive features are relatively sharp and clearly defined (narrow bandwidth), and are highly dependent on lattice pitch. Hence they move in wavelength and angle (e.g. highly dispersive) as pitch is varied. These features relate to the coupling of light into or out of the sensor chip. 5. Localized surface plasmons have characteristic of small wavelength shift over wide angular range (low dispersion), and are generally broader in bandwidth. Plasmon features can conclusively be identified over diffractive features by making comparisons between simulations ‘with’ and ‘without’ the top metal coating.

In order to derive the optimal geometry for SERS sensor, a highly stable test molecule which is known to form a monolayer coating on gold is required. For this purpose benzenethiol was used as standard in this work. Devices were tested using a Renishaw Invia Raman system. The main wavelength of interest here is 785nm where this laser is readily available and compatible with the end user Raman system. Full details of the optical and Raman measurements are carried out on the silicon test platform. Results show that the averaged SERS enhancement factor was only slightly dependent upon lattice pitch, but was highly dependent on pit size and aspect ratio. Density of the pits plays a further role simply by increasing the number of pits/unit area and so provides extra increase in SERS signal. The experimental data shows this is not simply a surface area dependent effect, but the optimal SERS signal can be obtained by close packing as tightly as possible pits of the optimal size. Minimum spacing (between adjacent pits) of 250nm is found to give the highest SERS enhancement. The optimal aspect ratio was found to be 1:1.2 and the optimal pit size determined to be 1000nm. This new optimized design shows 10-fold improvement in sensitivity compared to current available benchmark Klarite. The study has also explored the possibility of replicating the optimized design to a cost effective and disposable polymer for the purpose of mass production. This was carried out using nanoimprint lithography. The replicated plastic sensor is comparable to the benchmark silicon Klarite. As a proof of principle, the qualitative performance in two demonstrator molecules such as ibuprofen and melamine has been carried out. The disposable plastic sensor was demonstrated for the possibility of dual sensing mechanisms such as surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS). The sensitivity of plastic sensor using SPR mechanism is 225.83nm/RIU on thiol molecule. The work has also been carried out for an alternative SERS sensor design by changing the sidewall profile to 90º angle from (100) silicon etched plane. Changing the sidewall profile makes impact on the plasmonic behaviour. The straight sidewalls are favourable to the localized plasmon mode. The structures with slope sidewalls are favourable to both localized and propagating plasmons inside the cavities. This work was conducted as part of the FP7 "PHOTOSENSE" consortium project.

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