High Pressure Vibrational Properties of WS2 Nanotubes

We bring together synchrotron-based infrared and Raman spectroscopies, diamond anvil cell techniques, and an analysis of frequency shifts and lattice dynamics to unveil the vibrational properties of multiwall WS2 nanotubes under compression. While most of the vibrational modes display similar hardening trends, the Raman-active A1g breathing mode is almost twice as responsive, suggesting that the nanotube breakdown pathway under strain proceeds through this displacement. At the same time, the previously unexplored high pressure infrared response provides unexpected insight into the electronic properties of the multiwall WS2 tubes. The development of the localized absorption is fit to a percolation model, indicating that the nanotubes display a modest macroscopic conductivity due to hopping from tube to tube

Pressure-driven high-to-low spin transition in the bimetallic quantum magnet [Ru2(O2CMe)4]3[Cr(CN)6]

Synchrotron-based infrared and Raman spectroscopies were brought together with diamond anvil cell techniques and an analysis of the magnetic properties to investigate the pressure-induced high → low spin transition in [Ru2(O2CMe)4]3[Cr(CN)6]. The extended nature of the diruthenium wave function combined with coupling to chromium-related local lattice distortions changes the relative energies of the π ∗ and δ ∗ orbitals and drives the high → low spin transition on the mixed-valence diruthenium complex. This is a rare example of an externally controlled metamagnetic transition in which both spin-orbit and spin-lattice interactions contribute to the mechanism

High-pressure spectroscopic investigation of multiferroic Ni3TeO6

We combined diamond anvil cell techniques, infrared and Raman spectroscopies, and lattice dynamics calculations to explore the high pressure properties of multiferroic Ni3TeO6. Using a frequency trend analysis, we trace a subtle decrease in compressibility near 4 GPa to a minimum in the O-Ni2-O bond angle. This unique behavior emanates from the proximity of the Ni2 center in the Ni3-Ni2-Ni1-Te chain to a flexible pocket that is intrinsic to the crystal structure. At the same time, predicted trends in the superexchange pathways are consistent with greater antiferromagnetic character under compression, in line with both phase stability calculations and direct susceptibility measurements. These findings highlight opportunities for local structure control of corundumlike materials

Vibronic coupling and band gap trends in CuGeO3 nanorods

We measured the optical response of CuGeO3 nanorods in order to reveal size effects on the electronic properties. The vibronically activated d-to-d color band excitations are activated by the 131 and 478 cm−1 phonons, with the relative contribution of the lower frequency O-Cu-O bending mode increasing with decreasing size until it dominates the process. We also uncover trends in the direct band gap, with the charge transfer edge hardening with decreasing size. These findings advance the understanding of size effects in low-dimensional copper oxides

Electronic and Magnetic Materials Under External Stimuli

The interaction between spin, charge, and lattice degrees of freedom leads to exotic and useful properties in multifunctional materials. This delicate balance of energy scales allows external stimuli such as temperature, magnetic field, or pressure to drive to novel phases. As a local probe technique, spectroscopy can provide insight into the microscopic mechanism of the phase transitions. In this dissertation I present spectroscopic studies of functional materials under extreme conditions. Nanomaterials have attracted attention because nanoscale confinement affects various material properties and often reduces energy scales or suppress phase transitions. Combining Raman and infrared spectroscopies reveals that the breakdown mechanism of tungsten disulfide nanowires, a dichalcogenide widely used as a solid state lubricant, under compression is mainly driven by a breathing mode as revealed by its high sensitivity to compression. The optical properties of nanoscale hematite, a model antiferromagnet, reveal a size-dependent vibronic coupling behind the activation of the iron on-site excitation. Moreover, spin-charge coupling is enhanced below a critical size until the superparamagnetic limit is reached. Molecule-based magnets offer opportunities to probe coupling processes due to their soft lattices and overall low energy scales. As an example, I reveal how the antiferromagnetic to ferromagnetic crossover of a copper halide coordination polymer originates from the formation of hydrogen bonds with applied pressure that increase the dimensionality of the copper-copper magnetic superexchange network. Finally, combining temperature and pressure spectroscopy techniques with theoretical calculations of manganese dicyanimide revealed a temperature-pressure-magnetic field phase diagram that contains many competing magnetoelastic phases, and indicates possible quantum behavior despite the typical classical treatment of Mn(II). Together, these studies provide insight on structure-property relations, spin-charge-lattice coupling, and phase transitions in nanomaterials and molecule-based magnets, and by extension higher energy scale materials like bulk oxides

Pressure-induced structural transition in copper pyrazine dinitrate and implications for quantum magnetism

We combined synchrotron-based infrared and Raman spectroscopies, diamond anvil cell techniques, and first principles calculations to unveil pressure-induced distortions in quasi-one-dimensional Cu(pyz)(NO3)2. The crossover at 0.7 GPa is local in nature whereas the transition at 5 GPa lowers symmetry fromPmna to P2221 and is predicted to slightly increase magnetic dimensionality. Comparison with prior magnetoinfrared results reveals the striking role of out-of-plane bending of the pyrazine ligand, a finding that we discuss in terms of the possibility of using pressure to bias the magnetic quantum critical transition in this classic S = 1/2 antiferromagnet

Charge and Bonding in CuGeO3 Nanorods

We combine infrared and Raman spectroscopies to investigate finite length scale effects in CuGeO3 nanorods. The infrared-active phonons display remarkably strong size dependence whereas the Raman-active features are, by comparison, nearly rigid. A splitting analysis of the Davydov pairs reveals complex changes in chemical bonding with rod length and temperature. Near the spin-Peierls transition, stronger intralayer bonding in the smallest rods indicates a more rigid lattice which helps to suppress the spin-Peierls transition. Taken together, these findings advance the understanding of size effects and collective phase transitions in low-dimensional oxides

Size-dependent vibronic coupling in α-Fe2O3

We report the discovery of finite length scale effects on vibronic coupling in nanoscale α-Fe2O3 as measured by the behavior of vibronically activated d-d on-site excitations of Fe3+ as a function of size and shape. An oscillator strength analysis reveals that the frequency of the coupled symmetry-breaking phonon changes with size, a crossover that we analyze in terms of increasing three-dimensional character to the displacement pattern. These findings demonstrate the flexibility of mixing processes in confined systems and suggest a strategy for both enhancing and controlling charge-lattice interactions in other materials

Spin−Lattice Coupling in [Ni(HF2)(pyrazine)2]SbF6 Involving the HF2 − Superexchange Pathway

Magnetoelastic coupling in the quantum magnet [Ni(HF2)- (pyrazine)2]SbF6 has been investigated via vibrational spectroscopy using temperature, magnetic field, and pressure as tuning parameters. While pyrazine is known to be a malleable magnetic superexchange ligand, we find that HF2 − is surprisingly sensitive to external stimuli and is actively involved in both the magnetic quantum phase transition and the series of pressureinduced structural distortions. The amplified spin−lattice interactions involving the bifluoride ligand can be understood in terms of the relative importance of the intra- and interplanar magnetic energy scales

Magnetochromic sensing and size-dependent collective excitations in iron oxide nanoparticles

We combine optical and magneto-optical spectroscopies with complementary vibrational and magnetic property measurements to reveal finite length scale effects in nanoscale α-Fe2O3. Analysis of the d-to-d on-site excitations uncovers enhanced color contrast at particle sizes below approximately 75 nm due to size-induced changes in spin-charge coupling that are suppressed again below the superparamagnetic limit. These findings provide a general strategy for amplifying magnetochromism in α-Fe2O3 and other iron-containing nanomaterials that may be useful for advanced sensing applications.We also unravel the size dependence of collective excitations in this iconic antiferromagnet