Nanomechanical and structural study of Au nanocluster Langmuir-Blodgett films using bimodal atomic force microscopy and X-ray reflectivity.

HYPOTHESIS

Langmuir-Blodgett (LB) technique allows the deposition of gold nanoparticles and nanoclusters (atomically precise nanoparticles below 2 nm in diameter) onto solid substrates with an unprecedented degree of control and high transfer ratios. Nanoclusters are expected to follow the crinkle folding mechanism, which promotes the formation of trilayers of nanoparticles but kinetically disfavors the formation of the fourth layer.

EXPERIMENTS

LB films of Au(SCHPh) nanocluster were prepared at a range of surface pressures in the bilayer/trilayer regime and their internal structure was analyzed with X-ray Reflectivity (XRR) and Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). Bimodal atomic force microscopy (AFM) imaging was used to quantify the elastic modulus, which can be correlated with the topography at the same point on the surface.

FINDINGS

Nanocluster bilayers and trilayers exhibited the elastic moduli of ca. 1.2 GPa and 0.9 GPa respectively. Films transferred in the 20-25 mN/m surface pressure regime displayed a particular propensity to form highly vertically organized trilayers. Further compression resulted in disorganization of the layers. Crucially, the use of two cantilevers of contrasting stiffness for bimodal AFM measurements has demonstrated a new approach to quantify the mechanical properties of ultrathin films without the use of deconvolution algorithms to remove the substrate contribution.