Charge, Bonding, and Magneto-Elastic Coupling in Nanomaterials

Phonons are exquisitely sensitive to finite length scale effects in a wide variety of materials because they are intimately connected to charge, structure, and magnetism, and a quantitative analysis of their behavior can reveal microscopic aspects of chemical bonding and spin-phonon coupling. To investigate these effects, we measured infrared vibrational properties of bulk and nanoscale MoS₂ [molybdenum disulfide], MnO [manganese(II) oxide], and CoFe₂O₄ [cobalt iron oxide]. From an analysis of frequencies, oscillator strengths, and high-frequency dielectric constants, we extracted Born and local effective charges, and polarizability for MoS₂ and MnO. For MoS₂ nanoparticles, in the intralayer direction, Born effective charge of the nanoparticles decreases significantly compared to the layered bulk, a difference that we attribute to the structural strain and resulting change in polarizability in nanoparticles. For MnO nanoparticles, our analysis reveals that Born effective charge decreases by ∼20%, compared to the bulk material. Moreover, this change impacts both ionicity and polarizability. Specifically, MnO nanoparticles are ∼12% less ionic than the corresponding bulk. We also studied magnetoelastic coupling driven by the spin-ordered phase in MnO. Effective plasma frequency, Born and local effective charge, and force constant split through the 118 K Néel transition. The spin-lattice coupling drops from ∼5 N/m in the single crystal to ∼0.5 N/m in the nanoparticles. We attribute this result to a shorter phonon lifetime and reduced antiferromagnetic proportion. For CoFe₂O₄, the spectroscopic response is sensitive to the size-induced crossover ferrimagnetic → superparamagnetic state, which occurs between 7 and 10 nm. A spin-phonon coupling analysis supports the core-shell model. Moreover, it provides an estimate of the magnetically disordered shell thickness, which increases from 0.4 nm in the 14 nm particles to 0.8 nm in the 5 nm particles, demonstrating that associated local lattice distortions take place on the length scale of the unit cell. Taken together, these findings demonstrate that the properties of nanomaterials, such as ionicity and polarizability, are quite different than the corresponding bulk. These ideas and concepts are important for understanding finite length scale effects in nanoscale materials and may benefit on-going work on nanodevices.