Positive and negative contrast lithography on silver quantum dot monolayers

Scanning nonlinear optical microscopy, at a resolution of ̃2 μm, was utilized to examine monolayers of alkylthiol-passivated silver nanocrystals. Selected regions of the monolayers were irradiated with a pulse train of picosecond 1064 nm laser pulses, and then the second harmonic generation (SHG) response of those monolayers was recorded in a second scan. Two lithographic processes - one leading to a negative contrast SHG image (observed for particles <4 nm diameter) and a second leading to a positive contrast SHG image (observed for larger particles)-were found. These processes are explained within the context of literature models that account for particle size dependent energy partitioning.