Publications

The high-temperature spin and electronic transitions in LaCoO₃ have recently been leveraged to create neuromorphic (brain-inspired) devices. While these devices have shown the potential for impactful functionality in next-generation computing systems, the nanoscale dynamics of the spin and electronic transitions that underlie their operation are not well understood. Inhomogeneities related to interfaces, electrode contacts, strain, and crystal defects can all affect device performance, making nanoscale characterization of the transitions essential for producing consistent and reliable devices. Here, we demonstrate the first nanoscale in situ measurement of the spin transition in LaCoO₃ at device-relevant temperatures (25–325 °C) over length scales of tens of nanometers using STEM-EELS. This measurement is enabled by an Al₂O₃ coating, which prevents unwanted reduction of the LaCoO₃ specimen at high temperature and vacuum. The detailed understanding of LaCoO₃ transition dynamics enabled by such measurements will be crucial for optimizing LaCoO₃-based neuromorphic devices and increasing reliability for real-world application.

Vanadium oxides cystallize in a diverse array of structures and compositions arising from the redox versatility of vanadium, variable covalency of V–O bonds, and myriad coordination geometries. Their open frameworks present abundant interstitial sites that enable insertion of guest-ions. In such compounds, V3d electron and spin localization and disorder couple strongly to structural preferences. The rich structural diversity manifests as a “rugged” free energy landscape with multiple interconvertible polymorphs. Such a landscape sets up structural, electronic, and magnetic transitions that underpin the promise of these materials as ion-insertion battery electrodes; compact primitives for brain-inspired computing, and heterogeneous catalysts. Here, we examine the structural and compositional diversity, electronic instabilities, defect dynamics, structure transformations, mechanical properties, and surface structure of vanadium oxides using single crystals as a distinctive lens. Single crystals enable the measurement of structure–function correlations without the ensemble and orientational averaging inevitable in polycrystalline materials. Their well-defined surfaces further enable examination of facet-dependent reactivity toward molecular adsorbates, ion fluxes, and lattice (mis)matched solids. We provide a comprehensive account of vanadium-oxide single-crystal studies, from delineation of common structural motifs to single-crystal growth techniques, topochemical modification strategies, mechanisms underpinning electronic instabilities, and implementation as electrothermal neurons and battery electrode materials.

The insertion of electron-donating ions has emerged as a powerful technique to manipulate the electronic structure of correlated oxides. However, the resulting electronic structure remains poorly understood, with challenges in quantifying dopant concentration, unexplained differences with substitutionally doped films, and a poor understanding of how dopant atoms interact with insulator–metal transitions (IMTs). Here, these issues are addressed in the context of the rare earth nickelates, a prototypical correlated oxide family with widely tunable electronic behavior under the insertion of protons and alkali metals as interstitial dopants. RNiO₃ (R = Pr, Nd) epitaxial thin films are synthesized, lithium dopants are introduced and quantified using electrochemical and synchrotron-based techniques, and the resulting electronic structure is studied. From electronic transport measurements of Li_xLiRNiO₃, lithium is found to affect the metal–insulator transition, causing more than an order of magnitude reduction in ground-state resistivity at fractions x_Li < 0.18, a systematic lowering of transition temperature, and successively smaller ON/OFF ratios over 0.00 < x_Li < 0.25. At larger fractions x_Li > 0.25, the transition is destroyed, and insulating behavior is observed over T = 5–300 K. Angle-resolved photoemission (ARPES) confirms transport results and reveals band renormalization occurring over 0.10 < x_Li ≤ 0.71. ARPES and X-ray absorption spectroscopy (XAS) combined with density functional theory indicate that rigid band filling models are generally insufficient to explain doping from lithium, especially at low temperatures, but could approximate room temperature effects in the low doping regime (x_Li < 0.10). Broadly, the results indicate that interstitial dopants lead to complex interactions with metal–insulator transitions and the emergence of an exciting family of correlated electronic phases.