## 3 Results  
### Characterization of Au Nanopeanuts  
Au nanorods (Au NRs) with an aspect ratio of -4 (length: -40 nm, width: -10 nm) were first synthesized using the seedless method (Figure 2a) [17]. Following centrifugation and wash processes, Au NRs were collected and redispersed in H\({}_{2}\)O and then subjected to the formation of Ag nanolayers on their surface by the reduction of AgNO\({}_{3}\) in a mixture of ascorbic acid (AA), cetyltrimethylammonium bromide (CTAB), and polyvinylpyrrolidone (PVP) to yield Au NR@Ag [18]. The TEM image (Figure 2b) clearly shows differential contrast with darkened Au nanorods embedded in the Ag nanolayer. The thickness of the Ag nanolayer could be tuned relative to the change in AgNO\({}_{3}\) amount, and 7-8 nm Ag nanoshells were prepared in this study. Au NR@Ag products were collected and subjected to centrifugation and wash procedures for further nanopeanut formation. Au NR@Ag was converted to a nanopeanut structure with Au/Ag nanoshells in a reaction mixture containing HAuCl\({}_{4}\) (700 uL), CTAB, and AA rather than by pure addition of HAuClalone, which is a common approach for galvanic exchange. After a short reaction period of 16 min, the nanopeanut formed with an Au NR embedded in a hollow Ag/Au shell (Figure 2c). Typically, nanopeanuts have a size of ~53 nm in length and ~22 nm in width with ~6 nm in the thickness of Au/Ag nanoshells. We estimated the gap between Au NRs and Au/Ag nanoshells as 2 nm from TEM images (Figure S1). An energy dispersive X-ray analysis (EDX) of a single nanopeanut gives an average atomic ratio of Au/Ag of 47.5/52.5 in the shell (Figure S1). The most interesting observation is the evolution of UV-Vis-NIR absorption behavior in the course of preparation. Initially, Au NRs display transverse and longitudinal absorbance maxima at 521 and 800 nm, respectively (Figure 2a). Once the gold nanorod was epitaxially coated with an 8 nm silver shell, the transverse and longitudinal maxima blueshifted to 393 nm and 606 nm (Figure 2b), respectively, due to the different dielectric functions of Ag and Au [18,22]. In addition, a small shoulder appeared at 360 nm, which could be attributed to the anisotropic growth mode [23], indicating that Ag nanoshells were not evenly deposited on Au NRs. Upon replacement with gold, this thin silver shell grew into a rough dented Au/Ag shell around the core rod with a distinct dielectric gap between them. The optical behavior of nanopeanuts exhibits wide absorbance scaling from UV wavelength to NIR 1350 nm. As demonstrated in our previous work [14], this broadened absorption is due to the gap structure enhanced surface plasmon resonance. Interestingly, two separate bands stand up at 1110 and 1300 nm. The colloidal solutions changed from reddish (Au NRs) to grayish blue (nanopeanuts). The as-prepared Au NRs, Au NR@Ag, and nanopeanuts were all washed with H\({}_{2}\)O and centrifuged, and then redispersed in H\({}_{2}\)O. The absorbance bands appearing at NIR have driven us to tune the gap between Au NRs and Au/Ag nanoshells. Our preliminary examination showed that the additional amount of HAuCl\({}_{4}\) tunes the gap distance in the replacement reaction. Figures S2 and S3 show the UV-Vis-NIR, colloidal color, TEM, HRTEM, and EDX analysis of nanopeanuts prepared with 140, 280, and 420 \(\mu\)L in addition to 700 \(\mu\)L of HAuCl\({}_{4}\) for replacement reactions. For 140 \(\mu\)L HAuCl\({}_{4}\), we hardly observed the gap appearance, but the creation of the pinhole could be recognized (Figure S3). Increasing the HAuCl\({}_{4}\) volume to 280 \(\mu\)L resulted in a distinct dielectric gap of ~2.6 nm with a shell thickness of ~6 nm. When the gold ion volume was further increased to 420 \(\mu\)L, the gap increased to ~3.8 nm, whereas the shell thickness decreased to ~3.7 nm. Consequently, the change in shell width and gap tailored the UV-Vis-NIR behavior.