By: Mohamed Elsayed, American University in Cairo
In this work, we proposed surface enhanced Raman scattering as a sensing tool. The details of the work are in the attached paper and one example simulation file is attached. I am awaiting the community’s comments to start a discussion, and I can share other example files.
SERS 2 SiNW 100nm AgNP (2) water.fsp (367.3 KB)
[jlt.2017.2707476] Elsayed, Mohamed Y.; Gouda, Abdelaziz M.; Ismail, Yehea; Swillam – Silicon-based SERS substrates fabricated by Elect.pdf|attachment](upload://8G1Itb8ZWBKETIQ6fEik2DNKmy6) (1.3 MB)
Simulation methodology and results:
Finite Difference Time Domain (FDTD) simulations were used to predict the enhancement factor of different substrates. Lumerical 3D FDTD simulations were used to study the electric field amplification at different locations. The material definitions of silicon and silver are based on curve fitting to the data of Palik and Johnson & Christy, respectively. Source was total-field scattered-field.
Fig. 2 (a) and (b) show the electric field enhancement between the AgNP and Si. There is a drastic amplification of the electric field due to surface plasmons. Raman enhancement factor is given by the equation
Enhancement factor = Eexcitation^2 Eemission^2
where Eexcitation is the electric field at the excitation wavelength (532 nm) and Eemission is the emission wavelength at v1 peak of pyridine around 1050 cm⁻1 (563 nm)
Simulations suggested that increasing the nanoparticle size increases the enhancement factor, figure 2 (d) because the electromagnetic field is more strongly localized in the gap. Figure 2 (b) shows the electric field amplification in SiNWs decorated with AgNPs and it explains the large enhancement factor in the metallized nanowires. This is strongly related to the direction of light propagation in relation to the hot spot, the spacing between the SiNW and AgNP. The hot spot is vertically aligned and is thus oriented with the direction of light polarization. Figure 2 (e) shows that by depositing these nanoparticles on silicon nanowires, enhancement factors around 10^7 to 10^8 and as high as 10^9 is possible. Coupling between nanoparticles can also occur when the nanowires bring the nanoparticles very close to one another. Fig. 2© shows the electric field amplification caused by coupling two silver nanoparticles with 1 nm gap in between. Fig. 2(f) highlights the effect that the coupled nanoparticles have; enhancement factor increased by an order of 2.
Advanced notes and tips:
A simulation time of 40 fs was needed to observe the electric field amplification with dt = 0.00017 fs, i.e. over 200,000 time steps were required and the computation time using 4 cores typically took 18 to 24 hours. Due to the spherical shape, very fine meshing was needed and the mesh size was 0.1 nm at the hot spots.