Université Pierre et Marie Curie, UMR 7070, LM2N, 4 place Jussieu, 75005 Paris, France.
Nano Lett. 2013 Oct 9;13(10):4914-9. doi: 10.1021/nl4028704. Epub 2013 Sep 11.
We use broadband picosecond acoustics to detect longitudinal acoustic phonons with few-gigahertz frequency in three-dimensional supracrystals (with face-centered cubic lattice) of 7 nm cobalt nanocrystal spheres. In full analogy with atomic crystals, where longitudinal acoustic phonons propagate with the speed of sound through coherent movements of atoms of the lattice out of their equilibrium positions, in these supracrystals atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains) that act like mechanical springs holding together the nanocrystals. By repeating the measurements at different laser angles of incidence it was possible to accurately determine both the index of refraction of the supracrystal (n = 1.26 ± 0.03) and the room-temperature longitudinal speed of sound (v(s)= 1235 ± 12 m/s), which is quite low due to the heavy weight of the spheres (with respect to atoms in a crystal) and the soft carbon chains (with respect to atomic bonds). Interestingly, the speed of sound inside the supracrystal was found to dramatically increase by decreasing the sample temperature due to a change in the stiffness of the dodecanoic acid chains which coat the Co nanocrystals.
我们使用宽带皮秒声学技术来探测三维超晶格（面心立方晶格）中 7nm 钴纳米晶球中具有几 GHz 频率的纵声学声子。与原子晶体完全类似，在原子晶体中，晶格中原子通过相干运动离开平衡位置，从而以声速传播纵声学声子，在这些超晶格中，原子被（不可压缩）纳米晶取代，原子键被涂层剂（碳链）取代，涂层剂充当机械弹簧，将纳米晶保持在一起。通过在不同激光入射角重复测量，可以准确确定超晶格的折射率（n=1.26±0.03）和室温下的纵声速（v(s)=1235±12m/s），由于球体的重量（相对于晶体中的原子）和软碳链（相对于原子键）较大，因此声速相当低。有趣的是，由于涂覆 Co 纳米晶的十二烷酸链的刚度发生变化，发现随着样品温度的降低，超晶格内部的声速会显著增加。
