Electron–Proton Co‐doping‐Induced Metal–Insulator Transition in VO₂ Film via Surface Self‐Assembled l‐Ascorbic Acid Molecules

Abstract

Charge doping is an effective way to induce the metal–insulator transition (MIT) in correlated materials for many important utilizations, which is however practically limited by problem of low stability. An electron–proton co‐doping mechanism is used to achieve pronounced phase modulation of monoclinic vanadium dioxide (VO2) at room temperature. Using l‐ascorbic acid (AA) solution to treat VO2, the ionized AA− species donate electrons to the adsorbed VO2 surface. Charges then electrostatically attract surrounding protons to penetrate, and eventually results in stable hydrogen‐doped metallic VO2. The variations of electronic structures, especially the electron occupancy of V 3d/O 2p hybrid orbitals, were examined by synchrotron characterizations and first‐principle theoretical simulations. The adsorbed molecules protect hydrogen dopants from escaping out of lattice and thereby stabilize the metallic phase for VO2.

A Room‐Temperature Verwey‐type Transition in Iron Oxide, Fe5O6

“…Strongly correlated oxides, with their unusual optoelectronic and magnetic properties related to interacting electrical charges, are of considerable interest from both fundamental and applied perspectives . The existence of unique phase transitions or crossover points in these materials at temperatures close to room temperature would open a way to their use in various innovative applications, where properties of materials can be switched by external stimuli like heat, stress (strain), the electric field, and others. However, such cases are rather rare, particularly among simple oxides of inexpensive transition metals that are commercially most attractive.…”

“…This leads to a pronounced dimerization within these chains and to significant changes in the electrical resistivity and other physical properties. Even though the phase transition in Fe 5 O 6 is not as abrupt as the metal–insulator transition in VO 2 , it demonstrates novel features that can stimulate the development of atomic‐scale switches. For example, by using appropriate optical or other techniques, one can manipulate individual Fe−Fe dimers in thin single‐crystalline films of Fe 5 O 6 whose orientation coincides with the dimerization direction.…”

“…However, such cases are rather rare, particularly among simple oxides of inexpensive transition metals that are commercially most attractive. Presently, vanadium dioxide (VO 2 ), demonstrating an abrupt metal–insulator transition whose temperature point can be moderately tuned about 340 K, is considered as a basic candidate for the use in prospective appliances using controlled switching of its electrical properties. A number of novel devices utilizing the metal–insulator transition in VO 2 has been already reported, including field‐effect transistors, various ultrafast optoelectronic switches, and memory elements .…”

Modelling local structural and electronic consequences of proton and hydrogen-atom uptake in VO2 with polyoxovanadate clusters

“…Spectroscopic analysis of H-doped VO2 via XPS suggests that vanadium atoms bound to hydrogenated oxygen atoms also undergo a localised reduction from V IV to V III . 8,10,15 Although comparison of 2-V IV 6O6(OSiMe3) 1and 3-V III V IV 5O6(OSiMe3) 2is useful for gaining insight into the perturbations of the assembly upon addition of electron density, a more useful analysis from the perspective of modelling H-atom doping in VO2 lies in the comparison of the structural data of 3-V III V IV 5O6(OSiMe3) 2to 1-V IV 6O7 2-. In analogy to spectroscopic data reported for H-doped VO2, the general positions of vanadium ions within the Lindqvist lattice do not change substantially upon formation of 3-V III V IV 5O6(OSiMe3) 2-; V=Ot and V-Oc lengths of vanadyl ions composing 1-V IV 6O7 2and 3-V III V IV 5O6(OSiMe3) 2are approximately identical.…”

“…Electron-proton co-doping of VO2(M) takes advantage of acidic protons and an applied voltage as the source of H-atoms, and is proposed to occur through the initial injection of an electron, followed by protonation of the lattice. 8,15 Mechanistic analysis is complicated by poor resolution of structural perturbations that occur upon hydrogen uptake; 8,13,15 the small size of the hydrogen nucleus precludes definitive resolution of changes to the proximal lattice structure of the material. As such, insight into the mechanism of H-atom uptake is largely based on computational modelling, which does not explicitly take into account the role that the coupled uptake of protons and electrons might play in lowering the kinetic barrier of the reduction of this insulating material at room temperature.…”