Nanoparticle suspensions studied by x-ray photon correlation spectroscopy
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1027-D04-04
Nanoparticle suspensions studied by x-ray photon correlation spectroscopy Xinhui Lu1, Simon G. J. Mochrie1, S. Narayanan2, Alec R. Sandy2, and Michael Sprung2 1 Department of Physics, Yale University, Sloane Physics Laboratory, New Haven, CT, 06520 2 Advanced Photon Source, Argonne National Laboratory, South Cass Avenue, Argonne, IL, 60439 ABSTRACT Multispeckle x-ray photon correlation spectroscopy measurements, carried out at beamline 8-ID at the Advanced Photon Source at Argonne National Laboratory, of opaque suspensions of silica nanoparticles in water and lutidine-water binary mixtures are presented. INTRODUCTION Understanding the glass transition remains a grand challenge for condensed matter science (Götze and Sjögren, 1992). In this effort, studies of model systems continue to play a pivotal role. The goal of this paper is to show that multispeckle x-ray photon correlation spectroscopy (XPCS) is a valuable new method for elucidating the dynamical behavior near the glass transitions of suspensions of nanoparticles. XPCS in particular, and photon correlation methods in general, directly determine the sample’s intermediate scattering function (ISF), i.e. its timeand wavevector-dependent density-density correlation function. This is a quantity of key interest for any condensed matter system, especially near the glass transition, where the relaxation of density fluctuations becomes slower and slower. Specifically, photon correlation techniques measure the intensity autocorrelation function [g2(Q,t)], as a function of wavevector (Q) and delay time (t), which is related to the ISF [f(Q,t)] via the Siegert relation. In many cases, the mesoscopic size of nanoparticles facilitates the application of a variety of powerful optical methods including confocal microscopy, optical tweezers, and dynamic light scattering (DLS), which is the optical analogue of XPCS. It is therefore pertinent to ask: Why study nanoparticle systems with XPCS? An important part of the answer is that light scattering experiments on concentrated nanoparticle suspensions, in which interesting transitions can occur, suffer from multiple scattering, because of their large light scattering cross-sections, making many optical experiments challenging. [Exceptionally, optical diffusing wave spectroscopy (DWS) relies on multiple scattering. See below.] This difficulty certainly exists for DLS on nanoparticle systems near their glass transition. Therefore, to reduce multiple scattering as far as possible, in DLS experiments, it is essential to employ suspensions, in which the particles and the suspending fluid have essentially the same refractive index. Even in this case, it is necessary to employ so-called “two-color” DLS, or similar methods, in which the cross-correlation of light from two lasers of different wavelengths, but scattered at exactly the same Q, is used to isolate single scattering. In fact, two-color DLS is technically extremely demanding and has only been realized in a few laboratories around the world, and only then with dedic
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