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

Abstract

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 mean core diameter D∼22 Å that have been stabilized by attachment of carboxylic acid terminated alkylthiols, HS–(CH2)15–COOH. Compression of the film results in a broad plateau of finite pressure in the surface 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 a nearest-neighbor distance of a=34 Å. The lateral positional correlations associated with the two-dimensional (2D) hexagonal order are of short range and extend over only a few 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 plateau are consistent with complete surface coverage by a laterally homogeneous monolayer of AuSHDA particles. On the high-density side of the plateau, 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 intensities 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.

Self-Alignment of Plasmonic Gold Nanorods in Reconfigurable Anisotropic Fluids for Tunable Bulk Metamaterial Applications

“…The unidirectional self-ordering of gold nanorods with high 3D order parameter values approaching 0.9 gives rise to the strong polarization sensitivity of surface plasmon resonance effects, showing the potential for LC-mediated oriented self-assembly of bulk optical metamaterials with tunable and switchable properties. Our method of device-scale largevolume bulk alignment of nanorods extends the capabilities provided by other alignment methods [8][9][10][11][12][13]23,24,[26][27][28][29] and can be used for a broad range of rodlike particles with different length-to-diameter aspect ratios and chemical compositions. Furthermore, the observed strong orientation dependence of SPR spectra may allow for novel orientation sensors 30 and plasmonic versions of 3D label-free imaging of director structures in liquid crystals.…”

“…5-7 Althoughthemetamaterial self-assembly from nanoparticles remains a significant challenge, recent advances in colloidal science show that it may be realized and the emerging nanoscale alignment and assembly approaches utilize surface monolayers, 8,9 stretched polymer films, 10,11 and functionalized nanoparticles 12,13 but are usually restricted to only short-range ordering, twodimensional rather than three-dimensional assembly, and limited switching.…”

“…Mass production of bulk optical metamaterials from self-aligning and self-assembling nanoparticles is poised to revolutionize scientific instruments, technologies,andconsumerdevices. [5][6][7] Althoughthemetamaterial self-assembly from nanoparticles remains a significant challenge, recent advances in colloidal science show that it may be realized and the emerging nanoscale alignment and assembly approaches utilize surface monolayers, 8,9 stretched polymer films, 10,11 and functionalized nanoparticles 12,13 but are usually restricted to only short-range ordering, twodimensional rather than three-dimensional assembly, and limited switching. 7 Tunable metamaterials may potentially be obtained by nanoparticle self-assembly in liquid crystals (LCs) 14 through the LC-mediated realignment and rearrangement of incorporated nanoparticles in response to applied fields.…”

Prolonged reorganization of thiol-capped Au nanoparticles layered structures

“…[13][14][15][16] Monolayer formed by the Au nanoparticles exhibits reversible buckling on water surface up to a certain surface pressure (π ) but due to more compression irreversible monolayer-to-bilayer transformation occurs. 17,18 A trilayer structure has also been observed due to the irreversible transition of the monolayer 14 and with the compression of the monolayer the folding, wrinkling and then wrinkling to folding transitions have been observed. 19 Moreover, layer-by-layer assembly of thiol-capped Au nanoparticles has also been observed with the compression of the monolayer.…”

Molecular Interactions at Interfaces

“…They observed a quantum interference between the particles which was governed by interparticle distances. There are few studies on Langmuir monolayer of functionalized metal nanoparticles at the A-W interface Heath et al (1997); Swami et al (2003); Greene et al (2003); Brown et al (2001);Fukuto et al (2004). However, the particles studied so far did not show a stable Langmuir monolayer.…”