A d‑Band Electron Correlated Thermoelectric Thermistor Established in Metastable Perovskite Family of Rare-Earth Nickelates

The d-band electron correlations shed a light on bridging multiple functionalities within one material system, and this further extends the horizon in material designs and their emerging device applications. Herein, we demonstrate the combination of thermoelectric and thermistor functionalities within the perovskite family of correlated rare-earth nickelates (ReNiO3) having small rare-earth elements (i.e., YNiO3 and DyNiO3), in addition to their already known metal-to-insulator transitions. In contrast to conventional semiconductive materials, the electronic band structure of ReNiO3 split within the hybridized Ni3d–O2p is closely coupled to the structure of NiO6 octahedron. Based on such a distinguished feature, it is possible to achieve the coexistence of a large magnitude of thermopower (S) and negative temperature coefficient of resistance (NTCR) in the insulating phase of ReNiO3 with small Re and more distorted NiO6 octahedron. This establishes a thermoelectric thermistor that can be used for sensing the thermal perturbations by integrating the two distinguished detection modes within one system: the active mode utilizing the high NTCR, and the passive mode utilizing the large S. It is worth noticing that as-achieved S-NTCR relationship in ReNiO3 differs form the one for conventional semiconductors, in which cases enlarging the band gap enlarges S but reduces NTCR. As achieved thermoelectric thermistor combing thermistor and thermoelectric functionalities via electron correlation opens up a new direction to explore emerging energy/electronic devices for sensing the thermal perturbations. The temperature range that keeps a high thermoelectric thermistor performance (i.e., |TCR | >2%K–1 and meanwhile S > 100 μVK–1) of ReNiO3 with a small rare-earth radius is possible to cover most of the outdoor conditions on earth (i.e., −50 to 150 °C)

Revealing the Anisotropy in Protonation-Induced Electronic Phase Transitions of Rare-Earth Nickelates within a Marine Environment

Although the discovery of the electrochemical protonation-induced electronic phase transition of rare-earth nickelates (ReNiO3) enables potential application in sensing the ocean electric field that simulates the working principle of the ampullary organ of marine animals, whether such a functionality is anisotropic is previously overlooked. Herein, we demonstrate the anisotropy in the protonation-induced electronic phase transition in ReNiO3 (Re = Sm, Nd, and Eu) thin films as electrochemically triggered in an ocean environment. A larger elevation in the material resistivity triggered by an electric field within an ocean environment is observed for ReNiO3/LaAlO3(110), compared to ReNiO3/LaAlO3(001) and ReNiO3/LaAlO3(111). This is attributed to the orientation-related in-plane oxygen atomic density that results in more effective in-plane proton diffusion along the adjacent oxygen position, as further confirmed by the electrochemical cyclic voltammetry characterization. In addition, the larger activation energy associated to the anisotropic in-plane electronic structures of ReNiO3/LaAlO3(110) is also expected to promote the formation of electron-localized orbital configurations upon hydrogenation. As demonstrated, anisotropy sheds light on another possibility that can be further introduced to regulate the protonation-induced electronic phase transition properties of ReNiO3 for its potential applications such as ocean electric field sensing or biosensing

Rare-Earth Regulation in the Crystal Structure, Electronic Structure, and Metal-To-Insulator Transitions of the High Oxygen Pressure-Annealed <i>Re</i>_0.33Sr_0.67FeO₃

The complex electromagnetic phase diagram of iron-based perovskites, e.g., Re0.33Sr0.67FeO3 (Re stands for rare earth), exhibits a charge/spin ordering transition that enables metal insulator transitions (MIT) beyond conventional oxide semiconductors. While the previous investigations focused on Re0.33Sr0.67FeO3 with light or middle rare-earth compositions, Re0.33Sr0.67FeO3 containing heavy rare-earth elements beyond Gd have not yet been synthesized owing to their larger intrinsic metastability. Herein, we effectively synthesize Re0.33Sr0.67FeO3 covering a large variety of rare-earth elements (e.g., Re = La–Dy) by first forming an oxygen-deficient Re0.33Sr0.67FeO3‑δ framework via conventional solid-state reactions in air and afterward high-oxygen-pressure post annealing that compensates the oxygen composition. Compared to the ones with light or middle rare-earth compositions (e.g., Nd–Eu), the crystal structures of Re0.33Sr0.67FeO3 containing heavy rare-earth compositions (e.g., Dy) change from the space group Imma to R3̅c, while a larger amount of oxygen vacancy is also expected. Consequently, the potentially more distorted FeO6 octahedron is expected to be balanced by the tendency of generating the oxygen vacancy within Re0.33Sr0.67FeO3 containing heavy rare-earth compositions. The heavy rare-earth compositions elevate the Mott temperature T0 and activation energy EA in their carrier transportations and eliminate their MIT property. Further, on combining with the near-edge X-ray absorption fine-structure analysis, an abrupt variation is observed in the Fe-L edge and O-K edge across Re = Sm, and this reflects the boundary of MIT in the material family of Re0.33Sr0.67FeO3 when varying the rare-earth compositions. Therefore, pronounced MIT performance is achieved in Re0.33Sr0.67FeO3 with light rare-earth compositions and a low oxygen vacancy

Non-equilibrium Spark Plasma Reactive Doping Enables Highly Adjustable Metal-to-Insulator Transitions and Improved Mechanical Stability for VO₂

Although vanadium dioxide (VO2) exhibits the most abrupt metal-to-insulator transition (MIT) properties near room temperature, the present regulation of their MIT functionalities is insufficient owing to the high complexity and susceptibility associated with V4+. Herein, we demonstrate a spark plasma-assisted reactive sintering approach to simultaneously achieve in situ doping and sintering of VO2 within a largely short period (∼10 min). This enables high convenience and flexibility in regulating the electronic structure of VO2via dopant elements covering Ti, W, Nb, Mo, Cr, and Fe, leading to a wide adjustment in their MIT temperature (TMIT) and basic resistivity (ρ). Furthermore, the mechanical strength of the doped VO2 is meanwhile largely improved via the compositing effect of the high-melting-point dopant oxide. The high adjustability in MIT properties and improved mechanical properties further pave the way toward practical applications of VO2 in power electronics, thermochromism, and infrared camouflage

Revealing the Role of the Tetragonal Distortion in the Metal–Insulator Transition of Co- and Fe-Doped NiS

Although the NiS exhibits the most widely adjustable metal-to-insulator (MIT) properties among the chalcogenides, the mechanisms, with respect to the regulations in their critical temperatures (TMIT), are yet unclear. Herein, we demonstrate the overlooked role associated with the structurally tetragonal distortion in elevating the TMIT of NiS; this is in distinct contrast to the previously expected hybridization and bandwidth regulations that usually reduces TMIT. Compared to the perspective of structure distortions, the orbital hybridization and band regulation of NiS are ∼19 times more effective adjustment in TMIT. As a result, the respective abruptions in both the electrical and thermal resistive switches across the TMIT of NiS can be better preserved in the low-temperature range (<273 K), shedding light on their optimum usage at cryogenic temperatures

Superlow Thermal Conductivity 3D Carbon Nanotube Network for Thermoelectric Applications

Electrical and thermal transportation properties of a novel structured 3D CNT network have been systematically investigated. The 3D CNT net work maintains extremely low thermal conductivity of only 0.035 W/(m K) in standard atmosphere at room temperature, which is among the lowest compared with other reported CNT macrostructures. Its electrical transportation could be adjusted through a convenient gas-fuming doping process. By potassium (K) doping, the original p-type CNT network converted to n-type, whereas iodine (I₂) doping enhanced its electrical conductivity. The self-sustainable homogeneous network structure of as-fabricated 3D CNT network made it a promising candidate as the template for polymer composition. By in situ nanoscaled composition of 3D CNT network with polyaniline (PANI), the thermoelectric performance of PANI was significantly improved, while the self-sustainable and flexible structure of the 3D CNT network has been retained. It is hoped that as-fabricated 3D CNT network will contribute to the development of low-cost organic thermoelectric area