High Strength, Molecularly Thin Nanoparticle Membranes

The unique strength observed in molecular thin films consisting of assemblies of nanoparticles encoded with short organic chains opens an intriguing new realm of controllable materials. Here the fundamental mechanisms underlying this unique mechanical strength are probed by molecular dynamics simulations. Using nanoparticles encoded with short hydrocarbon chains, we show that the mechanical response and failure of single nanoparticle thick membranes depend on subtle changes of the coating. Extremely high moduli were observed in agreement with experiment. We calculate Young’s modulus for the membrane system based on properties of the individual components and find that ligand end-group interactions explain the observed changes in mechanical properties. Specifically, for dodecanethiol chains on 6 nm diameter gold cores, Young’s modulus is 2.5 GPa for ${\mathrm{CH}}_3$ terminated chains and increases by 50% when end groups are replaced by COOH.

Figure 1

(a) Angled side view of ${\mathrm{CH}}_3$-terminated dry membrane prior to deformation. Hydrogen atoms are represented in white, carbon in cyan, gold in brown, and sulfur in yellow. (b) A close-up view of COOH terminated (left) and ${\mathrm{CH}}_3$-terminated (right) NPs in a dry membrane prior to deformation. Hydrogen atoms have been omitted except in the COOH end group. In both cases carbon atoms on the left NP are colored cyan while those on the right are colored purple.

Figure 2

Typical two-dimensional stress-strain curves for samples undergoing pure shear deformation. Data have been time averaged over 0.4 ps, and curves for the $6\times 6$ membranes have been further smoothed as described in the text.

Figure 3

Radial density $\rho (r)$ versus carbon-carbon separation $r$. Inset: top view of membranes showing oxygen atoms from COOH end groups (red, left) or carbon atoms from ${\mathrm{CH}}_3$ (cyan, right) within a 5 nm slice centered at the average NP height.

Figure 4

${\mathrm{CH}}_3$ (top) and COOH (bottom) terminated samples shown at (from left to right) 10%, 26%, and 40% strain. Failure in the ${\mathrm{CH}}_3$ sample begins at about 26% strain. At this strain the COOH sample exhibits only void formation, and does not begin to tear until $\approx 40\%$ strain. The boundary in the lateral direction is open.