Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

deposit as the droplet snaps away suddenly as evaporation proceeds [57–60].This evaporation process deposits a series of concentric rings, as shown inFig. 12.Solvent evaporation can occur under conditions of diffusive mass andheat transfer, or convective transfer. It is generally difficult to avoid convectionin the system; however, on local length scales, solvent evaporation canoccur in a controlled fashion, particularly when the solvent viscosity is highand volatility is low. In this case, solvent dewetting from the substrate can leadto the formation of ring patterns of nanocrystals [54,55]. An evaporatingwetting film will reach a critical thickness when holes open up in the film in anattempt to maintain the equilibrium film thickness of the solvent layer, t e =(3A H /S) 1/2 , which depends on the Hamaker constant A H and the spreadingcoeffcient S [61]. The opening hole drags the nanocrystals with the solventfront, collecting more nanocrystals as it continues to widen. Once the holecollects a critical number of nanocrystals, it becomes pinned due to thefrictional force resulting from the interactions between the particles and thesubstrate, and the nanocrystals deposit as a ring. Figure 13A shows a TEMimage of evaporated rings of gold nanocrystals. Note the relatively narrowsize distribution indicative of diffusion-limited ring growth. If several holesare opening within close proximity to each other, it is possible for these holesto merge together and make a loose network of nanoparticles.Figure 12 Nanocrystals drop-cast onto a mica substrate exhibit concentric ringscharacteristic of stick–slip evaporative deposition of colloidal particles resulting frommass, thermal, and surface tension gradients in the droplet.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.