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Journal of Chemical Physics / Volume 120 / Issue 7 / ARTICLES / Surfaces, Interfaces, and Materials

J. Chem. Phys. 120, 3446 (2004); http://dx.doi.org/10.1063/1.1640334 (14 pages)

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

Masafumi Fukuto1, Ralf K. Heilmann1, Peter S. Pershan1, Antonella Badia2, and R. Bruce Lennox3

1Department of Physics and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138
2Department of Chemistry, Université de Montréal, Montreal, Canada
3Department of Chemistry, McGill University, 801 Sherbrooke St. W., Montreal H3A 2K6, Canada

(Received 3 September 2003; accepted 17 November 2003)

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. © 2004 American Institute of Physics.

© 2004 American Institute of Physics

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KEYWORDS and PACS

Keywords

Langmuir-Blodgett films, gold compounds, organic compounds, nanoparticles, water, surface chemistry, interface phenomena, monolayers, aggregation, capillarity, X-ray reflection, X-ray scattering

PACS

  • 68.18.Fg

    Liquid thin film structure: measurements and simulations

  • 68.18.Jk

    Phase transitions in liquid thin films

  • 61.46.-w

    Structure of nanoscale materials

  • 78.70.Ck

    X-ray scattering

  • 82.70.-y

    Disperse systems; complex fluids

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.1640334

PUBLICATION DATA

ISSN

0021-9606 (print)  
1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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