Lobastov Vladimir A, Weissenrieder Jonas, Tang Jau, Zewail Ahmed H
Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA.
Nano Lett. 2007 Sep;7(9):2552-8. doi: 10.1021/nl071341e. Epub 2007 Jul 10.
Four-dimensional (4D) imaging during structural changes are reported here using ultrafast electron microscopy (UEM). For nanostructures, the phase transition in the strongly correlated material vanadium dioxide is our case study. The transition is initiated and probed in situ, in the microscope, by a femtosecond near-infrared and electron pulses (at 120 keV). Real-space imaging and Fourier-space diffraction patterns show that the transition from the monoclinic (P21/c) to tetragonal (P42/mnm) structure is induced in 3 +/- 1 ps, but there exists a nonequilibrium (metastable) structure whose nature is determined by electronic, carrier-induced, structural changes. For the particles studied, the subsequent recovery occurs in about 1 ns. Because of the selectivity of excitation from the 3d parallel-band, and the relatively low fluence used, these results show the critical role of carriers in weakening the V4+-V4+ bonding in the monoclinic phase and the origin of the nonequilibrium phase. A theoretical two-dimensional (2D) diffusion model for nanoscale materials is presented, and its results account for the observed behavior.
本文报道了利用超快电子显微镜（UEM）在结构变化过程中进行的四维（4D）成像。对于纳米结构，强关联材料二氧化钒中的相变是我们的案例研究对象。在显微镜中，通过飞秒近红外和电子脉冲（120 keV）原位引发并探测该相变。实空间成像和傅里叶空间衍射图样表明，从单斜（P21/c）到四方（P42/mnm）结构的转变在3±1皮秒内发生，但存在一种非平衡（亚稳）结构，其性质由电子、载流子诱导的结构变化决定。对于所研究的粒子，随后的恢复过程在约1纳秒内发生。由于来自3d平行带的激发具有选择性，且使用的能量密度相对较低，这些结果表明了载流子在削弱单斜相中V4+-V4+键合方面的关键作用以及非平衡相的起源。提出了一种用于纳米尺度材料的理论二维（2D）扩散模型，其结果解释了所观察到的行为。
