The scientific relevance and potential for technological applications of complex materials have made them the focus of active investigation in order to fully charac- terize the competition and interactions between their electronic, structural, orbital, and spin degrees of freedom. Optical and terahertz (THz) spectroscopy provide ac- cess to electronic and low frequency quasiparticle responses, and therefore play a key role in understanding the fundamental mechanisms which dictate the macroscopic properties of complex materials. Time-resolved experiments, in turn, have the po- tential to disentangle the various coexisting energy scales through a careful selection of the pump and probe characteristics. This work investigates the role played by the electronic, structural and magnetic excitations in the insulator-to-metal transi- tions (IMT) of VO2, V2O3 and NdNiO3, through studies under different conditions of temperature, strain, doping and photoexcitation. Our work shows that a complete understanding of the IMT in VO2 requires sev- eral length scales and time scales to be considered. Indeed, epitaxial strain leads to anisotropy in the IMT characteristics of thin films of (100) and (110) VO2/TiO2, measured using THz spectroscopy, which can be explained by strain induced modi- fications both in the (microscopic) V3d orbitals and in the geometry of mesoscopic metallic domains. On the other hand, ultrafast studies which track, with femtosecond resolution, the electronic and structural dynamics of VO2 thin films following THz excitation reveal a delay in the onset of the structural response with respect to the electronic one, lending support to the correlation rather than Peierls driven picture of the IMT in this material. As for V2O3, the IMT is seen to occur via nucleation and growth of metallic domains, as previously reported in VO2. However, a scaling of the photoinduced conductivity dynamics rise time is further identified, which reveals the temperature and fluence dependence of the nucleation and growth process. Finally, strained NdNiO3 films exhibit a two step dynamical conductivity response following optical excitation, different from that of the vanadates with which they share a complex, albeit more tunable, phase diagram. This hints at a significant role being played by the magnetic structure during the IMT in NdNiO3.

<p>The traditional semiconductor device scaling based on Moore’s law is reaching its physical limits. New materials hosting rich physical phenomena such as correlated electronic behavior may be essential to identify novel approaches for information processing. The tunable band structures in such systems enables the design of hardware for neuromorphic computing. Strongly correlated perovskite nickelates (ReNiO3) represent a class of quantum materials that possess exotic electronic properties such as metal-to-insulator transitions. In this thesis, detailed studies of NdNiO3 thin films from wafer-scale synthesis to structure characterization and to electronic device demonstration will be discussed.</p> <p>Atomic layer deposition (ALD) of correlated oxide thin films is essential for emerging electronic technologies and industry. We reported the scalable ALD growth of neodymium nickelate (NdNiO3) with high crystal quality using Nd(iPrCp)3, Ni(tBu2-amd)2 and ozone (O3) as precursors. By controlling various growth parameters such as precursor dose time and reactor temperature, we have optimized ALD condition for perovskite phase of NdNiO3­. We studied the structure and electrical properties of ALD NdNiO3 films epitaxially grown on LaAlO3 and confirmed their properties were comparable to those synthesized by physical vapor deposition methods. </p> <p>ReNiO3 undergoes a dramatic phase transition by hydrogen doping with catalytic electrodes independent of temperature. The electrons from hydrogen occupy Ni 3<em>d</em> orbitals and create strongly correlated insulating state with resistance changes up to eight orders of magnitudes. At room temperature, protons remain in the lattice locally near catalytic electrodes and can move by electrical fields due to its charge. The effect of high-speed voltage pulses on the migration of protons in NdNiO3 devices is discussed. After voltage pulses were applied with changing the voltage magnitude in nanosecond time scale, the resistance changes of the nickelate device were investigated. </p> <p>Reconfigurable perovskite nickelate devices were demonstrated and a single device can switch between multiple electronic functions such as neuron, synapse, resistor, and capacitor controlled by a single electrical pulse. Raman spectroscopy showed that differences in local proton distributions near the Pd electrode leads to different functions. This body of results motivates the search for novel materials where subtle compositional or structural differences can enable different gaps that can host neuromorphic functions.</p>