Abstract
This paper attempts to compare the main features of random and highly ordered gold nanostructure arrays (NSA) prepared by thermally annealed island film and nanoimprint lithography (NIL) techniques, respectively. Each substrate possesses different morphology in terms of plasmonic enhancement. Both methods allow such important features as spectral tuning of plasmon resonance position depending on size and shape of nanostructures; however, the time and cost is quite different. The respective comparison was performed experimentally and theoretically for a number of samples with different geometrical parameters. Spectral characteristics of fabricated NSA exhibited an expressed plasmon peak in the range from 576 to 809 nm for thermally annealed samples and from 606 to 783 nm for samples prepared by NIL. Modelling of the optical response for nanostructures with typical shapes associated with these techniques (parallelepiped for NIL and semi-ellipsoid for annealed island films) was performed using finite-difference time-domain calculations. Mathematical simulations have indicated the dependence of electric field enhancement on the shape and size of the nanoparticles. As an important point, the distribution of electric field at so-called ‘hot spots’ was considered. Parallelepiped-shaped nanoparticles were shown to yield maximal enhancement values by an order of magnitude greater than their semi-ellipsoid-shaped counterparts; however, both nanoparticle shapes have demonstrated comparable effective electrical field enhancement values. Optimized Au nanostructures with equivalent diameters ranging from 85 to 143 nm and height equal to 35 nm were obtained for both techniques, resulting in the largest electrical field enhancement. The application of island film thermal annealing method for nanochips fabrication can be considered as a possible cost-effective platform for various surface-enhanced spectroscopies; while the NIL-fabricated NSA looks like more effective for sensing of small-size objects.
Highlights
Plasmonic phenomena are widely used in optical devices [1], imaging microscopy [2], biosensing [3,4,5], and medical diagnostics [6,7,8]
It was found that the peak position in the unpolarized light extinction spectrum, which corresponds to the occurrence of localized surface plasmon resonance (LSPR), shifts towards longer wavelengths with an increase in the initial gold island film thickness
Considered plasmonic enhancement (PE) nanochips fabrication technology based on gold island films with subsequent thermal annealing can be exploited while taking into account inherent technological limitations that hinder the preparation of geometrically ordered nanoparticle arrays
Summary
Plasmonic phenomena are widely used in optical devices [1], imaging microscopy [2], biosensing [3,4,5], and medical diagnostics [6,7,8]. One of the possible ways to obtain general sensitivity enhancement for multiple applications is to Lopatynskyi et al Nanoscale Research Letters (2015) 10:99 nanometer dimensions (‘hot spots’) [18] phenomenon, which depends on nanostructure size, shape, and material properties [19,20]. Nanostructures enabling the PE effect can be fabricated using a multitude of methods [21]. A simple and commonly used approach involves highly conductive, continuous film possessing surface roughness as effective plasmonic amplifiers; this method does not yield the ability for spectral tuning, and, the process of enhancement cannot be applied for matching with molecular resonances that is preferable for a number of spectroscopic techniques. The benefit of spectrally controlled nanostructured PE surfaces is obvious as only uniform surface-bound nanostructure arrays (nanochips) with known surface 3D geometry can provide a real possibility to perform preliminary estimation of final PE parameters when using this technique and ensure their stability and reproducibility
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