(397bj) Comparative Localized Surface Plasmon Resonance Between Silver Nanocubes and Nanospheres On the Massed Silver Surfaces

Strong localized surface plasmon resonance (LSPR) can be derived from silver nanocubes and nanospheres self-assembling on massed silver surfaces. Ag surfaces were linked in a nano-scale gap to become hot spots for incurring the substantial increase in plasmonic interaction and lead to high performance with several orders of magnitude in SERS. Since the LSPR is sensitive to the geometry of nanoparticles, we synthesized Ag nanocubes and nanospheres by polyol method and self-assembled them homogeneously on the massed Ag surface via 1, 2-ethanedithiol to used for robust SERS substrates. With a 1, 2-ethanedithiol monolayer as the dielectric spacer, nanoparticles are separated from a continuous metal surface. The existence of the dielectric spacer offered space between two metallic surfaces that provided more opportunity for the generation of localized surface plasmon to prevail the strong plasmonic interaction at the nanogaps. In order to elucidate the plasmon coupling in our substrates, the electric field simulations were performed using finite-difference time-domain (FDTD) method.

The induced electric field of the substrate without dielectric spacer occurs near the vertex on the top of Ag nanocube and at the bottom with the decay strength because of the complete contact. Figure 2c and 2d show the EM fields for the substrate with spacer. The strong induced electric field appears in nanogaps between the Ag nanocubes and the massed Ag surface rather than the sharp corner, indicating strong plasmon coupling happened in nanogaps between two Ag surfaces. Otherwise, the hot region of the substrate made by self-assembling Ag nanocubes on the massed Ag surface is much larger than the substrate with nanosphere, showing the importance of the dielectric spacer.

Due to the low surface energy, nanosphere is the commonly used nanoparticle. In order to creat a roubst SERS substrate, we use nanocubes to replace the commonly used nanosphere as our nanoparticle. Figure 3 shows the electric field simulations around an Ag nanocube and nanosphere placed on Ag surface with 2 nm spacer. The strength of induced electric field in the nanogap between two Ag surfaces are huge different. The substrate with nanocubes shows stronger induced electric field than the former the substrate with nanospheres and larger hot spot area, indicating the powerful interaction happened when Ag nanocubes were assembled on the massed Ag surface with the ultrathin spacer, being a suitable material to form strong plasmon coupling. Since the intensity of Raman is proportional to the fourth power of the electric field strength, the huge changes in the electromagnetic field of Ag nanocubes and nanospheres on massed Ag surface would result in the substantial increase with several orders in magnitude in SERS signals. The rhodamine 6G (R6G) is used as a model compound to confirm SERS results.

Three substrates were used for SERS measurments: (a)10^|1 M R6G solution on a microscope slide, (b) 10^|6 M R6G solution on the substrate by assembling Ag nanospheres on massive Ag surface, and (c) for 10^|9 M R6G solution on the substrate by assembling Ag nanocubes on massed Ag surface. The substrate by assembling Ag nanocubes on massed Ag surface shows about 100 times EF of nanospheres (2.9×10⁶), being attributable to the geometry effect of nanoparticles. The SERS results corresponded to the calculation results from finite-difference time-domain method, indicating strong LSPR can be derived from silver nanoparticles self-assembling on massed silver surfaces, especially using nanocubes as nanoparticles.

It is very difficult to arrange hot spots in a large scale on a substrate in the general case. For efficiently and promptly analysis, nanoparticles are regarded as SERS substrates owing to the rapidly, easily, and low-cost production. Our robust substrate, prepared by assembling Ag nanocubes on the massed Ag surface represents the homogeneous distribution of nanocubes, possessing the high EF and the low standard deviation, generating strong LSPR, indicating highly potential application in SERS measurements and quantitative analysis.