The assembly of nanoparticles into large, two-dimensional structures provides a route for the exploration of collective phenomena among mesoscopic building blocks. We characterize the structure of Langmuir monolayers of dodecanethiol-ligated gold nanoparticles with in situ optical microscopy and X-ray scattering. The interparticle spacing increases with thiol concentration and does not depend on surface pressure. The correlation lengths of the Langmuir monolayer crystalline domains are on the order of five to six particle diameters. Further compression of the monolayers causes wrinkling; however, we find that wrinkled monolayers with excess thiol can relax to an unwrinkled state following a reduction of surface pressure. A theoretical model based on van der Waals attraction and tunable steric repulsion is adopted to explain this reversibility.

The mechanical response to compression of a self-assembled gold nanoparticle monolayer and trilayer at the air-liquid interface is examined. Analysis of the film's buckling morphology under compression reveals an anomalously low bending rigidity for both the monolayer and the trilayer, in contrast with continuum elastic plates. We attribute this to the spherical geometry of the nanoparticles and poor coupling between layers, respectively. The elastic energy of the trilayers is first delocalized in wrinkles and then localized into folds, as predicted by linear and nonlinear elastic theory for an inextensible thin film supported on a fluid.

BioCARS, a NIH-supported national user facility for macromolecular time-resolved X-ray crystallography at the Advanced Photon Source (APS), has recently completed commissioning of an upgraded undulator-based beamline optimized for single-shot laser-pump X-ray-probe measurements with time resolution as short as 100 ps. The source consists of two in-line undulators with periods of 23 and 27 mm that together provide high-flux pink-beam capability at 12 keV as well as first-harmonic coverage from 6.8 to 19 keV. A high-heat-load chopper reduces the average power load on downstream components, thereby preserving the surface figure of a Kirkpatrick-Baez mirror system capable of focusing the X-ray beam to a spot size of 90 µm horizontal by 20 µm vertical. A high-speed chopper isolates single X-ray pulses at 1 kHz in both hybrid and 24-bunch modes of the APS storage ring. In hybrid mode each isolated X-ray pulse delivers up to ~4 × 10(10) photons to the sample, thereby achieving a time-averaged flux approaching that of fourth-generation X-FEL sources. A new high-power picosecond laser system delivers pulses tunable over the wavelength range 450-2000 nm. These pulses are synchronized to the storage-ring RF clock with long-term stability better than 10 ps RMS. Monochromatic experimental capability with Biosafety Level 3 certification has been retained.

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