Metal/Insulator Transitions in V2O3 Systems Investigated at the Nanoscale by Spectromicroscopy Techniques under Cryo-conditions

Exploiting abrupt phase transitions in transition metal oxides is a promising approach for the development of advanced information processing and storage systems. V2O3 is considered as a prototypical system of the metal-to-insulator transitions (MITs) which can be activated under external stimuli such as temperature (T), pressure, or chemical doping [1] but are difficult to exploit technologically. Recent experiments under electric pulses in (V1-xCrx)2O3 yield transitions between the Mott insulator and the strongly-correlated metal phases with real potential capabilities for non-volatile memories and neuromorphic applications [2]. All these electronic transitions rely on their relationship between structural and electronic degrees of freedom. For instance, when cooled below 160 K, V2O3 presents a symmetry breaking (from hexagonal to monoclinic), associated with (i) a magnetic transition to an antiferromagnetic (AFI) ordered state, (ii) a large volume change (+1.4%) and (iii) an IMT yielding a resistivity change of 7 orders of magnitude. This T-driven MIT has been extensively studied at the macroscopic scale [1], but still provides a perfect arena to probe in situ the V2O3 structural and electronic evolutions at the finest scale. Recently, the microscopic electronic coexistence of insulator/metallic (I/M) domains has been mapped within both (V1-xCrx)2O3 crystals and thin films by in situ scanning photoemission spectroscopy [3], PEEM [4] and nano-IR [5] with 25nm spatial resolution at best. In addition, combined micro-XR and nano-IR experiments have highlighted competitive mechanisms between structural and electronic contributions during this temperature-induced MIT [6]. These investigations also raised possible experimental limitations when it comes to accurately mapping the dynamics of mechanisms within regions of interest of a few tens of nm by combining different instruments, hence the need to perform structural and electronic experiments within the same instrument. Understanding the V2O3 electronic phase separation and its local mechanisms governing the I/M domains dynamics across the IMTs remains of key interests. Advanced monochromated electron spectromicroscopes emerged this last decade as real game-changers for nanomaterials characterization. Here we performed in situ monochromated STEM/EELS experiments on the NION CHROMATEM 200 MC with variable-T options under cryo-conditions thanks to a double-tilt HennyZ cryo-holder using MEMS to vary continuously the temperature conditions across the IMTs. For each temperature, we associated mapping of relevant spectroscopic electronic excitations (from IR to soft X-ray) with an ultra-high EELS resolution at the nm scale and the local structural features (symmetry and lattice parameters) determined by 4D STEM nano- and microdiffraction. The T-activated V2O3 transition was investigated in V2O3 single crystals to give a direct, and local access to the structural and electronic evolution. During low-T thermal cycling through the resistive transition, EELS spectra acquired in the low-loss regime display a characteristic signature at ∼1eV only in the metallic phase. Upon cooling, an abrupt MIT was observed while the coexistence of I/M nanodomains was evidenced upon heating over few degrees yielding the propagation of the electronic I/M domain wall. Remarkably, periodic HAADF contrast inhomogeneities appeared solely in the insulating state. 4DSTEM nanodiffraction experiments reveal the local distribution of monoclinic/rhombohedral phases coexisting in the insulating domains. The observation of the insulating hexagonal phase at low-T suggests the presence an paramagnetic insulating (PI) -like phase of large volume, as confirmed by jumps in the lattice parameters observed at the transition. This PI-like phase emerges also at the I/M domain wall, as a precursor of the metallic phase [7]. a) Henny-Z’s tip cryo-holder including the b) heating/biasing MEMS where c) the V2O3 lamella (inset Figure) is supported within the heating area. d) and e) nanodiffraction (left) and microdiffraction (right) patterns acquired at 139 K (metallic state) and 138 K (insulating state), respectively, and f) lattice parameters of the monoclinic phases determined upon cooling from 4DSTEM microdiffraction. g) Series of low-loss EELS spectra probed upon heating to track the metallic characteristic signature at 1eV across the IMT. h) HAADF image acquired at 152 K within the I/M nanodomains coexistence and i) and j) the corresponding electronic maps where the I/M domain wall propagates here between 152 K and 152.5 K, respectively.