As such, we interpret the shifting peak positions with composition (x 6% ) as arising from a changing average number of spectrally resonant neighboring nanocrystals (Fig. ). For instance, compared to a pure 1% Sn superlattice (x 6% = 0), the lower frequency extinction peak blueshifts with increasing x 6% as each 1% Sn nanocrystal has fewer 1% Sn neighbors with which to couple. The same is true of the higher frequency peak moving away from x 6% = 1. A third peak, around 5100 cm -1 , arises only for intermediate compositions, ascribed to "isolated" 6% Sn nanocrystals with relatively few 6% Sn neighbors.