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45995-G6
Real-Time Investigation of the Dynamic Structural Changes of Metal Oxide Nanocrystals using Time-Resolved X-ray Spectroscopy

Dong Hee Son, Texas A&M University

As a first step towards understanding the dynamic property relationship in metal oxide nanocrystals, we focused on the two tasks: (1) instrumental development for time-resolved structural study of nanocrystals using short x-ray pulses and (2) initial time-resolved optical measurement of the dynamic electronic and magnetic properties of iron oxide nanocrystals.

For the time-resolved study of the structural dynamics of transition metal oxide nanocrystal in colloidal solutions, X-ray absorption spectroscopy is an optimal tool. While the off-site synchrotron ultrafast X-ray facility will be used initially and perhaps more often for the time-resolved X-ray absorption measurements, a laser-based ultrafast X-ray source is also being constructed in the laboratory. The laser-based X-ray source, which is a part of the pre-existing laser system for the optical experiment, provides practical convenience despite its weakness in intensity. So far, we have built the X-ray source chamber and characterized the stability of Hg jet-flow and recirculation inside a vacuum chamber and laser beam focusing characteristics. Figure 1 shows a diagram of X-ray generation chamber (top) and a picture of currently completed part of the X-ray generation chamber (bottom). The vacuum chamber contains optics focusing the laser beam onto the X-ray generation target, a circulation system for liquid mercury used as X-ray generation target and an X-ray focusing optics. A stable jet of Hg (100 mm-thick) is formed using nitrogen gas as a pressurizing medium, with only 5 minutes of interruption every ~2 hours for recirculation. Laser beam size on Hg jet is ~15 mm from an off-axis parabolic mirror, which will be further minimized to deliver the highest intensity to Hg target.  Recently, a similar system operating in another group produced X-ray pulses of 5×10⁹ photon/4p×s within 1 keV width in 5-10 keV range, using the laser system operating at 5 kHz and 15 W. Our initial target performance will be in a similar range.

We also investigated the electronic relaxation dynamics and demagnetization and recovery dynamics in Fe₃O₄ nanocrystals, which can be modulated due to optically induced coherent lattice motion. Pump-probe transient absorption and Faraday rotation techniques, both with sub 100 fs temporal resolution, were employed to obtain time-dependent electronic and magnetic properties respectively. Figure 2 shows pump-probe transient absorption and transient magnetization data of colloidal Fe₃O₄ nanocrystals of various sizes. At the pump wavelength of 780 nm, metal-metal charge transfer transitions involving d-electrons of Fe ions were excited. Electronic excitation also excited coherent acoustic phonon in the lattice and also resulted in demagnetization via spin-orbit coupling. Modulation of transition absorption by coherent lattice motion was clearly visible, while magnetization was not noticeably affected. Another notable feature in Figure 2 is the different size dependence of the electronic and magnetization dynamics. While the former is not very sensitive to the size of nanocrystals, the latter exhibit strong size dependence. Weaker sensitivity of the electronic relaxation dynamics to the size may be explained in terms of the relatively localized nature of d-electrons, which will experience weak spatial confinement. On the other hand, magnetization dynamics clearly reveal stronger size dependence of spin degrees of freedom. The data can be interpreted in terms of size-dependent cooperativeness of spins, i.e. spin correlation: weaker spin correlation in smaller nanocrystals results in more prompt and larger recovery of magnetization. This is the first experimental result showing the size effect on the dynamics of the magnetization induced by an ultrashort optical excitation in transition metal oxide nanocrystals of <10 nm length scale. 

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