Dynamical charge and structural strain in MoS2 and MnO nanoparticles*

We measured the far infrared vibrational properties of bulk and nanoscale MoS$_2$ and MnO in order to investigate finite length scale effects and chemical bonding in these materials. From an analysis of frequencies, oscillator strengths, and the high frequency dielectric constants, we extract Born and local effective charges for both materials. In the intralayer direction of MoS$_2$, we find that the Born effective charge of the nanoparticles is decreased significantly compared to the layered bulk, a result that we attribute to structural strain (and resulting change in polarizability). Preliminary results on the 7 nm MnO nanoparticles suggest that structural strain impacts both polarizability and depolarization field.