Hopping Diffusion of Gold Nanoparticles Observed with Liquid Cell TEM See Wee Chee 1,2 , Duane Loh 1 , Zhaslan Baraissov 1,2 , Paul Matsudaira 1 and Utkur Mirsaidov 1,2,3 1. Center for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Singapore 2. Centre for Advanced 2D Materials and Graphene Research Centre, Department of Physics, National University of Singapore, Singapore 3. Nanocore, Faculty of Engineering, National University of Singapore, Singapore The diffusion of nanoparticles in the microfluidic cells used for liquid cell transmission electron microscopy (TEM) have always been found to be much slower [1-6], often by several orders of magnitude, when compared with bulk diffusion. While this highly suppressed motion is serendipitous for the atomic resolution imaging of nanoparticle nucleation and coalescence events, we still lack a compelling explanation for this anomalous phenomena. Here, we report results from our experiments tracking the motion of Au nanoparticles in water, using a combination of energy filtered imaging and image acquisition at frame rates of 100 Hz. The Au nanoparticles (~20-70 nm in diameter) are dispersed in water and sandwiched between 30 nm thick SiN x windows in a Hummingbird Scientific liquid flow holder. The holder is loaded into a JEOL 2200FS TEM with an Omega filter, where zero loss imaging (using a 20 eV energy slit) was used to mitigate the resolution loss from imaging through the liquid layer. Movies are recorded on a Direct Electron DE-12 camera system at 100 frames per second. The field of view is 819.2 nm by 819.2 nm and electron dose rate used was between 30-100 electrons/Å 2 /s. The field of view and frame rate were chosen so that it was possible, in principle, to capture nanoparticles moving at bulk velocities, while minimizing the electron dose rate. The liquid layer thickness in each liquid cell was also measured using electron energy loss spectroscopy. In these experiments, we observed that nanoparticles moved via surface hops. This motion is illustrated in Figure 1 as an image sequence where the nanoparticle made a series of such transient displacements over 0.10 seconds; 1 between the second and third frame, and at least 8 between the fourth and tenth frame, as inferred from the remnant contrast of the nanoparticle. We further deduce from the discontinuous motion blur that these displacements are discrete. The nanoparticle also appear to re-visit a few of the same locations. Figure 2 shows the entire recorded trajectory and the displacements between frames as a function of time. In general, the nanoparticles move in a pattern where intermittent hops that are tens of nanometers long are interspaced between lengthier segments of short displacements that are only a few nanometers long. Our analysis indicates that the short, stuck motion is similar to the reported suppressed diffusion, whereas during hops, the nanoparticles have mobility only two to three orders of magnitude slower than bulk values calculated from the Stokes-Einstein equation. We propose that the observed motion is due to surface potential wells, where a nanoparticle can make larger displacements when it escapes a trapping well. This motion is, however, short-lived because the nanoparticle is quickly trapped again. The implications of this study will be discussed. [7] References: [1] H.M. Zheng et al., Nano Lett. 9 (2009) p. 2460. [2] E.A Ring and N. de Jonge, Micron 43 (2012) p. 1078. [3] White et al. Langmuir 28 (2012) p. 3695. [4] J.Y. Lu et al. Nano Lett. 14 (2014) p. 2111. 750 doi:10.1017/S1431927616004608 Microsc. Microanal. 22 (Suppl 3), 2016 © Microscopy Society of America 2016