Regulation Of The Correlated Electron State Of VO₂ Nanobeams And Their Response Functionalities

In strongly correlated eletron systems, the charge, spin, lattice and orbital degrees of freedom are strongly coupled and competing, presenting a rich phase diagram and many exotic physical phenomena, such as superconductivity, charge density wave, colossal magnetoresistance and metal-insulator transition (MIT). They have great significance for fundamental research and enormous potential application, being the hottest research topic in condensed matter physiscs. The study of strongly correlated electron systems is of great significance not only to understanding the microscopic origin of these complex physical phenomena, but also to the exploration of the physical laws of complex many-body system and the new material phase. VO2 is one of the most widely studied binary oxides of correlated materials, and has a dramatic metal insulator transition near room temperature (340 K) due to the interplay of charge, orbital and lattice. The understanding of the mechanism of the metal-insulator transition and tremendous applications has been actively pursued by scientists. Regulating the correlated electron states of VO2 is not only beneficial to reveal the VO2 phase transition mechanism, also induce new quantum behavior and new phase. Meanwhile, it can also help understand the physica of various mutual effects in strongly correlated systems and direct the design of new functional materials.In this dissertation, based on the well-defined single crystalline VO2 nanobeam as the as the research model, we realize the regulation of the spin and electronic structure of VO2 nanobeams through controllable chemical and physical means, and investigate systematically investigated the correlation among the correlated electron state, metal insulator transition and tunnable functionalities. The dissertation includes the following several aspects:1. In this chapter, we present a hydrogen modulation stratergy to regulate the spin structure of ID V-V atomic chains in VO2 nanobeam, resulting in an extraordinarily large negative magnetoresistance (MR) in monoclinic VO2 (M1) nanobeams. The hydrogen-treated VO2 (M1) nanobeams successfully introduce V3+ (3d2) ions into the zigzag V-V atomic chains, triggering ferromagnetic-coupled V3+-V4+ dimers to produce 1D superparamagnetic chain structure, which result in a large intrinsic negative MR at room temperature and under magnetic fields as high as -23.9%(0.5 T, 300 K). The large MR effect is ascribed to the spin-polarized electron hopping between the ferromagnetic V3+-V4+ dimers. In addition, the MR effect in metal-insulator transition materials behaves as a novel magnetotransport model of positive-negative magnetoresistance transition, exhibiting a strong coupling of structure, spin and charge in a 1D electron transport system. We hope that 1D atomic chain provides an efficient platform for investigating the underlying physics behind the MR effect in low-dimensional TMO systems.2. In this chapter, we highlight an interfacial charge transfer effect to reorganize the 1D charge density of V 3d orbitals in VO2 nanobeam, and successfully stabilize a metal-like monoclinic VO2 metastable phase at room temperature. In the experiment, we utilize the surface coordination of L-ascorbic acid molecules (AA) on VO2 nanobeam and caused the hybrid of the 3d-orbits of V ions and frontier orbits of AA species, leading to a charge-transfer process from AA to nanobeam. Combination with the dielectric force microscopy (DFM) and scanning near-field infrared microscopy (SNIM), we observed a metastable monoclinic VO2 with high electron density and having metal-like electronic structure. The external electrons inject could increase the electron density of 3d orbitals and increase the filling of dxz subshells, resulting in the 1D charge density reorganization of t2g orbitals along the octahedral chains and exhibiting the metal-like electronic state. However, the increased electron density could not suppress the Peierls distortion and transform to the monoclinic structure. Thus, the reorganization of 1D charge density due to the electron injection induced the new metal-like monoclinic VO2 metastable phase. This new metastable phase indicate that the MIT of VO2 results from the competition between the charge density, orbital-selective occupation and the Peierls instability, giving a deeper understanding of the MIT mechanism. Meanwhile, our results are beneficial to reveal the relationship between the interplay of charge, orbital and lattice and the exotic physical phenomenon. We hope surface charge transfer provides an efficient platform to control the electronic phases and investigate the underlying physics of phase competition in low-dimensional correlated system, with consequences for the design of new materials and electronic devices.3. In this chapter, based on the narrow bandgap of VO2 (0.58 eV) and suitable design of interface heterojunction, we construct the first example of photoconductive near infrared (NIR) detector based on heterostructure in transition metal oxides (TMOs) family. The VO2/V2O5 core/shell nanobeam heterostructures (CSNHs) was achieved through in situ surface oxidation of the VO2 nanobeam. The type Ⅱ heterojunction could form at VO2/V2O5 interface by the theoretical calculation, promoting the efficient separation of photo-induced excitons. In the as-established CSNHs-based photoconductive NIR detector, the responsivity (Rλ) of 2873.7 A W-1 and specific detectivity (D*) of 9.23 × 1012 Jones are achieved at room temperature with the 990 nm wavelength irradiation and a power density of 0.2 mW cm-2, recording the best IR performance compared with those reported IR detectors based on the heavy-metal-free material systems, and even comparable to the reported ones based on heavy-metal-including materials. The effective separation of the photo-generated electron-hole using heterojunction opens a new way to design and optimize new type photodetector.