Effects of quantum tunneling in metal nanogap on surface-enhanced Raman scattering

Abstract

The quantum tunneling effects between two silver plates are studied using the time dependent density functional theory. Our results show that the tunneling depends mainly on the separation and the initial local field of the interstice between plates. The smaller separation and larger local field, the easier the electrons tunnels through the interstice. Our numerical calculation shows that when the separation is smaller than 0.6 nm the quantum tunneling dramatically reduces the enhancing ability of interstice between nanoparticles.PACS numbers: 33.20.Fb, 03.65.Xp, 78.67.Bf Metal nano-gaps offering strong surface plasmon couplings have very rich physical properties. The related studies have been very hot topics in the field of plamonics, e.g., single molecule surface-enhanced Raman spectroscopy [1,2], optical nano-antennas [3], high-harmonic generation [4]. The electromagnetic (EM) enhancement near the metal surface, which is caused by the resonant excitation of surface plasmon [5], is the dominating reason for the surface-enhanced Raman scattering (SERS) [1,6]. Huge SERS with single molecule sensitivity can be obtained when molecules are located in the nano-gap between two metallic nano-structures [1,2,7,8,9]. A lot of efforts have been made to seek extreme sensitive SERS substrates [10,11,12].Theoretically, people have used many methods based on the classical electrodynamics [13,14,15] to estimate the SERS enhancement. These classical results indicate that the smaller the nano-gap, the higher the enhancement. However, as the separation decreases to 1 nm, the displacive current would partly become electron tunneling current which can reduce the EM enhancement substantially [16]. A recent experiment on the four-wave mixing at coupled gold nanoparticles clearly demonstrated that the quantum tunneling (QT) effect becomes significant for the distance smaller than 0.2 nm [17], and a recent study of the plasmon resonance of a nanoparticle dimer gave quantum description of such a phenomenon [18]. It is well known that the EM enhancement is the main contribution to SERS. Its enhancement factor is proportional to the fourth power of the local field enhancement, i.e. M 4 , where M =|E loc |/|E 0 | with E loc and E 0 being the local enhanced electric field and the incident electric field, respectively. Therefore, even for small QT effects on M , after a fourth power, the influence to SERS could be huge. In this Letter we investigate the effects of QT on SERS with the time dependent density functional theory [19]. Our studies are able to quantify these effects and point out at exactly what conditions the QT has to be taken into account.As the "hot spot", where the SERS is strongest, is lo- * Electronic address: bwu@aphy.iphy.ac.cn † Electronic address: hxxu@aphy.iphy.ac.cn FIG. 1: Schematic drawing of the "hot spot" between two silver nano-spheres. As the "hot spot" (shaded area) is small, its local field is almost identical to the one computed by replacing the sphere with a plate. E0 is the incident laser field.calized i...

A classical treatment of optical tunneling in plasmonic gaps: extending the quantum corrected model to practical situations

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