Coupled noble metal nanostructures for high-performance surface-enhanced spectroscopies
Date of Issue2016
School of Materials Science and Engineering
Noble metal nanoparticles (NPs) have been widely explored in surface-enhanced spectroscopies (SES) because of their ability to generate intense electric fields originating from the surface plasmon resonances. Particularly, coupled metallic NPs have attracted a lot of attentions in SES because they can generate much higher electric field enhancement than the single ones. In this study, two kinds of coupled structures, the dimer and the metal NP over the metal film (MNOMF), are examined using finite-difference time-domain method. Hot spots in these structures are optimized for high-performance SES. Hot spots in dimers are usually confined within small volumes, limiting the sensitivity and reproducibility of SES. In this study, we show that Au spiky NP dimers (SNPDs) can provide large-volume hot spots, arising from the synergistic effect of “lightning rod effect” in spiky NPs and the interparticle coupling. The “hot spots volume” (defined as the whole volumes of all the regions with electric field enhancement larger than 1E4) in T–T SNPD is almost 5 times and 7 times larger than in the sphere dimer and spike dimer with the same 2 nm gap, respectively. In addition, further increased “hot spots volume” is achieved in crossed T–T SNPD, being ~1.5 times of that in T–T SNPD. At the same time, the electric field enhancement (|E|^2/|E0|^2) in the tip-to-tip (T–T) SNPD with a 2 nm gap is as large as 1.21E6. The results indicate a strategy to obtain huge-volume hot spots with large electric field enhancement in dimers by adding a bulky core at one end of the spindly building blocks. We have shown SNPDs could provide hot spots with large volumes. Nevertheless, it is hard to fabricate SNPDs in experiments. Alternatively, spiky disk dimers (SDDs) on glass substrate may be fabricated using top-down lithography. We demonstrate crossed T-T SDD can generate much larger “hot spots volume” and much higher electric field enhancement than disk dimer. The results demonstrate the possibility to achieve large-volume hot spots in experiments. In both SNPDs and SDDs, two adjacent particles are coupled in lateral direction. In experiments, it is difficult to achieve precisely controlled small gaps in dimers along lateral direction and hence limiting the enhancement of electric fields. On the contrary, MNOMF which offers coupling in vertical direction can provide even sub-1 nm gap, exhibiting great potentials for SES. In this study, we demonstrate a method to optimize the hot spots in MNOMF by exciting the localized surface plasmon resonances (LSPRs) using the surface plasmon polaritons (SPPs). We find the largest electric field enhancement and the highest surface-enhanced fluorescence intensity are obtained when the SPPs of the metal film are excited. The results show that exciting LSPRs using SPPs can generate 1-3 orders higher electric field intensity than direct exciting the LSPRs using incidence from free space. The ultrahigh enhancement is attributed to the strong confinement of the SPP waves in the vertical direction. The drastically intensified electric fields generated from excitation of LSPRs using SPP waves are extremely essential for sensitive refractive index sensing, high-performance SES, and enhancing the efficiency of optoelectronic devices.
DRNTU::Engineering::Materials::Photonics and optoelectronics materials