Nanosphere lithography: Effect of substrate on the localized surface plasmon resonance spectrum of silver nanoparticles

In this paper, we explore the optical contributions of the substrate to the localized surface plasmon resonance (LSPR) spectrum of surface confined Ag nanoparticles produced by nanosphere lithography (NSL). We present optical extinction spectra of Ag nanoparticles fabricated on the following substra tes: fused silica, borosilicate optical glass, mica, and SF-10-a high refra ctive index specialty glass. For all the experiments discussed here, the Ag nanoparticles were approximately 100 nm in in-plane width and 25 nm in out -of-plane height. In a controlled N-2 environment, the wavelength correspon ding to the extinction maximum lambda (max), shifts to the red with increas ing refractive index of the substrate, n(substrate). The sensitivity factor , Delta lambda (max)/Deltan(substrate), was measured to be 87 nm per refrac tive index unit (RIU). Experimental extinction spectra were modeled using t he discrete dipole approximation (DDA). The DDA theory qualitatively predic ts the experimentally observed trend that lambda (max) is linearly dependen t on n(substrate); however, the theory overestimates the sensitivity by app roximately a factor of 2. The sensitivity of the LSPR to the refractive ind ex of bulk external solvent, n(external), was also examined for each of the four substrates listed above. For all the cases, the change in II,,, in re sponse to bulk external solvent was linearly dependent upon n(external). Va lues of the sensitivity factors, Delta lambda (max)/Deltan(external), range d from 206 nm RIU-1 for mica to 258 nm RIU-1 for SF-10, a difference of onl y 25%. From the results presented here, we conclude that there is no system atic dependence, or at most a weak dependence, which correlates the bulk so lvent sensitivity of the LSPR to n(substrate). The DDA theory overestimates the LSPR sensitivity to bulk external environment, but the ratio of solven t to substrate sensitivity factors is correct within experimental uncertain ty. This ratio has a value of approximately 2, which indicates that there i s greater sensitivity in the optical response to the solvent than to the su bstrate. This ratio is within 10% of the ratio of areas of the particles th at are exposed to solvent and substrate. We suggest that chemical interacti ons at the interfaces between the nanoparticle and the substrate and/or the nanoparticle and the bulk environment contribute significantly to the obse rved difference between experimental and theoretical sensitivity factors.