Fracture in electrophoretically deposited CdSe nanocrystal films

Abstract

We have studied the fracture, strain, and stress of electrophoretically deposited (EPD) films of CdSe nanocrystals as a function of the film thickness, nanocrystal size, and drying method. Fracture results from the film stress that develops with the loss of residual solvent after EPD when the film exceeds a threshold thickness that increases with nanocrystal core diameter from ∼300 to 1200 nm for core diameters from 2.3 to 5.0 nm, respectively. A hierarchical pattern of wider first generation and then narrower higher-generation cracks is observed after drying and this generational crack formation and a preferred direction for film drying are observed in real time. Delamination is seen to initiate from wider cracks, mostly between the bulk of the film and a very thin layer of nanocrystals strongly bound to the Au-coated silicon substrate. Estimates of the film toughness are made for channel cracking and delamination.