Analogy Between 3D Superlattices and Atomic Crystals: Physical Properties

Abstract

In this chapter, we describe some analogies between atomic crystals and 3D superlattices. The 3D superlattice atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains) that act like mechanical springs holding together the nanocrystals. The magnetic properties of ordered and disordered films of Co nanoparticles having same average diameter are studied. Ordered or disordered films reach the same saturation. However, the coercive field of ordered is larger than that of disordered and that of the latter saturates at slightly lower fields. This is explained that the flipping of the spins requires higher fields when the nanoparticles are ordered in a long-range superlattice compared with the disordered system, where very short-range order exists. Longitudinal acoustic phonons with few-GHz frequency are detected in fcc 3D superlattices of cobalt nanocrystal. A broadband picosecond acoustic experiment is performed: a first femtosecond pump pulse induced a strain wave (i.e. a traveling phonon wavepacket), and a second white-light pulse monitored the propagation of the coherent acoustic pulse inside the 3D superlattice. The index of refraction of the 3D superlattice ( n = 1.26 ± 0.03) and the room-temperature longitudinal speed of sound ( v s = 1235 ± 12 m s −1 ) are estimated. The speed of sound is quite low. This is attributed to the heavy weight of the spheres (with respect to atoms in a crystal) and the soft carbon chains (with respect to atomic bonds). Nanocrystals vibrate coherently when they are self-organized in fcc 3D superlattices. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a 3D superlattice. This vibrational coherence can be observed by a substantial change of the quadrupolar low-frequency Raman scattering peak. With Au 3D superlattices dispersed in aqueous solution, the mechanical oscillation period attributed to the breathing mode of the whole assembly is estimated to 300 ps.