Peña-Rodríguez and Pal Nanoscale Research Letters 2011, 6:279 http://www.nanoscalereslett.com/content/6/1/279 Page 4 of 5 Figure 4 Simulated extinction efficiency for Au-Au and Ag-Au DCN structures. Simulated extinction efficiency as a function of the wavelength for (a) Au-Au and (b) Ag-Au DCN structures, having t0 = t2 = 5 nm, while t1 and t3 are varied simultaneously (t1 = 2, ..., 14 nm; t3 = 16 nm - t1). is almost entirely controlled by the ω−o ( ωi ) mode. For − this reason the bonding mode (the one we are interested in) is virtually identical for both compositions and the gain obtained in this case is quite small. The intermediate region, which includes thicknesses of the outer layer ranging from 5 to 10 nm, is the most interesting because appreciable increases in the intensity of the ω− mode are obtained for the bimetallic parti- − cles, at the expense of a small blue-shift. When both effects are weighted, a net gain of intensity is obtained, revealing the advantage of using bimetallic structures. The improvement in intensity can be explained by con- sidering the characteristics of the geometry as well as the differences between the SPRs of silver and gold. Firstly, in this configuration the internal and external energy modes are closer to each other, resulting in a greater coupling between them, and consequently, both have some influence over the two hybridized modes. Moreover, the SPR of silver is considerably more intense than that of gold, and appears at higher energy (lower wavelength). Thus, the changes in the ω− mode are − driven by the increased influence of the inner metallic layer. Fortunately, the increase in intensity dominates over the red-shift, improving the optical response. On reducing the value of t3, the ωo mode red-shifts, get- ting away from the ω−i mode a−nd reducing their cross- influence over the hybridized modes, and then the differences in the ω− mode between the Au-Au and − Ag-Au configurations are erased (first region). For the third region (t3 > 10 nm), again there are few differences between the two compositions; here the bonding hybridized mode keeps blue-shifting until it almost disappears, “absorbed” by the antibonding one. This effect is produced by the shielding caused by the thick outer layer, through which the light does not “see” the inner region and, therefore, the particle behaves almost like a single Au nanoshell. Finally, in Figure 5, the variations of the extinction effi- ciency at the SPR maximum (QeSxPtR) as a function of its wavelength (lSPR), for the considered bimetallic structures are summarized. The region of transparency of the human tissues (700-1300 nm) is marked with vertical dashed lines. As can be seen, in spite of the differences in the intensity and red-shifts, the dependence of the ω−− energy mode on the geometrical parameters is essentially the same for both the MDM and DCN structures. Moreover, the intensity of their SPR bands is greater than the same for monometallic (Au-Au) structures in the 700-1300 nm spectral range. The gain in SPR intensity is particularly important in the region between 700 and 900 nm, which is up to 20 and 60 percent for the MDM and DCN, respectively. There- fore, these bimetallic nanoshell structures could be excel- lent replacements for the Au-only ones for bio-medical applications such as biosensors and cancer treatments [3]. Figure 5 Summary of the variations of QSexPtR as a function of lSPR. Summary of the variations of the extinction efficiency at the SPR maximum (QeSxPtR) as a function of its wavelength (lSPR), for the studied MDM and DCN structures. The region of transparency of the human tissues (700-1300 nm) is marked with vertical dashed lines.