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 processesone leading to a negative contrast SHG image (observed for particles <4 nm diameter) and a second leading to a positive contrast SH G 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.