FDTD for plasmonics:Applications in enhanced Raman spectroscopy

The exact electromagnetic enhancement mechanism behind SERS,TERS,HERS and SHINERS is one of the issues focused on in the study of enhanced Raman spectroscopy.The three dimensional finite difference time domain method(3D-FDTD),which is widely used in nanoplasmonic simulations,not only provides us with a powerful numerical tool for theoretical studies of the ERS electromagnetic enhancement mechanism,but also serves as a useful tool for the design of ERS-active systems with higher sensitivities and spectral spatial resolution.In this paper,we first introduce the fundamental principles of FDTD algorithms,and then the size-dependent dielectric function of dispersive metallic material is discussed.A comparative study of FDTD and rigorous Mie evaluations of electromagnetic fields in the vicinity of a system of self-similar nanospheres shows an excellent correlation between the two computational methods,directly confirming the validity and accuracy of 3D-FDTD simulations in ERS calculations.Finally,we demonstrate,using a TERS calculation as an example,that the non-uniform mesh method can be more computationally efficient without loss of accuracy if it is applied correctly.

SERS mechanism of nickel electrode

Based on the theoretical model for the two-dimensional arrays, the dependence of the surface-enhanced Raman scattering (SERS) effect of nickel electrode, especially the ordered two-dimensional nanowires, on the incident photon energy in the range of 0.6-4.0 eV are analyzed, and most of the works are focused on the effect of the shape of nano-particles. The theoretical analysis shows that nickel can exhibit weak surface-enhanced Raman scattering effect when the surface is roughened properly, and the enhancement factor is about 102-104. Compared to the typical highly SERS-active Ag substrate, the SERS of nickel does not show the character of surface plasma resonance of the metal. The calculated result shows that the lightning-rod effect contributes the most to the SERS of Ni nanowires in the EM mechanism. The theoretical prediction is in good agreement with the experimental result qualitatively and may be instructive to finding a new method to fabricate the SERS-active transition-metal substrate.

Surface bonding on silicon surfaces as probed by tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been used to obtain the Raman signal of surface species on silicon single crystal surfaces without the necessity for surface enhancement by addition of Ag nanoparticles. By illuminating the hydrogen terminated silicon surface covered with a droplet of 4-vinylpyridine with UV light, a 4-ethylpyridine modified silicon surface can be easily obtained. By bringing a scanning tunneling microscope (STM) Au tip with a nanoscale tip apex to a distance of ca. 1 nm from the modified silicon surface, enhanced Raman signals of the silicon phonon vibrations and the surface-bonded 4-ethylpyridine were obtained. The Raman enhancement factor was estimated to be close to 107. By comparing the surface enhanced Raman scattering (SERS) signal obtained after surface enhancement with Ag nanoparticles and the TERS signal of the surface, the advantage of TERS over SERS for characterizing the surface species on substrates becomes apparent: TERS readily affords vibrational information about the system without disturbing it by surface enhancement. In this sense, TERS can be considered a truly non-invasive tool which is ideal for characterizing the actual surface species on substrates.

Preface

The definition or concept of the nanostructure has two levels of implication [1]. The first refers to nanostructures, including nanoparticles, nanoclusters,