Viscoplastic and Granular Behavior in Films of Colloidal Nanocrystals

Nanoindentation measurements of electrophoretically deposited films of colloidal CdSe nanocrystals, capped by organic ligands, show the films have an elastic stiffness modulus of $\sim 10\mathrm{GPa}$ and exhibit viscoplasticity. This mechanical response suggests polymeric features that are attributable to the ligands. After particle cross-linking and partial ligand removal, the films exhibit more features of granularity.

Figure 1

Force vs displacement curves of 1000 nm-thick CdSe nanocrystal EPD films with (a) $5\mathrm{nm}/\mathrm{s}$ or $10\mathrm{nm}/\mathrm{s}$ displacement rate and 20 s hold time at peak load, and (b) $5\mathrm{nm}/\mathrm{s}$ displacement rate and 10 s or 20 s hold times at peak load. Average displacement is plotted for a given force for each load cycle of a curve (increasing displacement, constant load, decreasing displacement), with the value of the displacement at constant load given on the plot. Error bars indicate the standard deviation in displacement at a given force.

Figure 2

AFM image and scan of an indent in a 600 nm-thick film.

Figure 3

Simulation results for a 1000 nm-thick CdSe nanocrystal film on the $\mathrm{Au}/\mathrm{Cr}/\mathrm{Si}$ substrate loaded at $5\mathrm{nm}/\mathrm{s}$ with a hold time of 20 s, are compared to experiment for (a) force-displacement curves, (b) average apparent $E$ vs displacement, and (c) average $H$ vs displacement. In (b) and (c), vertical bars represent standard deviation of average apparent $E$ and $H$, and simulations are also shown for a semi-infinite medium of CdSe nanocrystals (no substrate, “bulk”); the difference between apparent $E$ from experiment and simulation can be attributed to uncertainties in modeling the nanoindenter tip.