Monolayer/bilayer transition in Langmuir films of derivatized gold nanoparticles at the gas/water interface:An x-ray scattering study

摘要

The microscopic structure of Langmuir films of derivatized gold nanoparticles has been studied as a function of area/particle on the water surface.The molecules (AuSHDA) consist of gold particles of means core diameter D approx 22 A that have been stabilized by attachment of carboxylic acid termianted alkylthiols,HS-(CH_2)_(15)-COOH.Compression of the film results in a broad plateau of finite pressure in thes urface pressure versus area/particle isotherm that is consistent with a first-order monolayer/bilayer transition.X-ray specular reflectivity (XR) and grazing incidence diffraction show that when first spread at large area/particle,AuSHDA particles aggregate two dimensionally to form hexagonally packed monolayer domains at the nearest-neighbor distance of a=34A.The lateral positional correlations associated with the two-dimensional (2D) hexagonal order are of short range and extend over only a new interparticle distances;this appears to be a result of the polydispersity in particle size.Subsequent compression of the film increases the surface coverage by the monolayer but has little effect on the interparticle distance in the close-packed domains.The XR and off-specular diffuse scattering (XOSDS) results near the onset of the monolayer/bilayer coexistence palteau are consistent with complete surface coverage by a laterally homogeneous monolayer of AuSHDA particles.On the high-density side of the palteau,the electron-density profile extracted from XR clearly shows the formation of a bilayer in which the newly formed second layer on top is slightly less dense than the first layer.In contrast to the case of the homogeneous monolayer,the XOSDS intensitie observed from the bilayer are higher than the prediction based on the capillary wave model and the assumption of homogeneity,indicating the presence of lateral density inhomogeneities in the bilayer.According to the results of Bragg rod measurements,the 2D hexagonal order in the two layers of the bilayer are only partially correlated.