Metallic monoclinic phase in VO ₂ induced by electrochemical gating: In situ Raman study

Abstract

We report in-situ Raman scattering studies of electrochemically top gated VO2 thin film to address metal-insulator transition (MIT) under gating. The room temperature monoclinic insulating phase goes to metallic state at a gate voltage of 2.6 V. However, the number of Raman modes do not change with electrolyte gating showing that the metallic phase is still monoclinic. The high frequency Raman mode Ag(7) near 616 cm −1 ascribed to V-O vibration of bond length 2.06Å in VO6 octahedra hardens with increasing gate voltage and the Bg(3) mode near 654 cm −1 softens. This shows that the distortion of the VO6 octahedra in the monoclinic phase decreases with gating. The time dependent Raman data at fixed gate voltages of 1 V (for 50 minute, showing enhancement of conductivity by a factor of 50) and 2 V (for 130 minute, showing further increase in conductivity by a factor of 5) show similar changes in high frequency Raman modes Ag(7) and Bg(3) as observed in gating. This slow change in conductance together with Raman frequency changes show that the governing mechanism for metalization is more likely to the diffusion controlled oxygen vacancy formation due to the applied electric field.PACS numbers: 78.30. Am, 71.30.+h, 71.27.+a The metal-insulator based transistor is a most promising candidate for surpassing limits of classical electrical charge based device with the potential of higher speed and low power consumptions 1-3 . Vanadium dioxide (VO 2 ) is a well-known strongly correlated material that undergoes metal insulator-transition (MIT) with five orders of magnitude change in resistance near T M I ≈ 340 K 4,5 . This thermally driven MIT is accompanied by structural phase transition (SPT), from the high temperature metallic rutile (R) structure (P4 2 /mnm) to insulating ( band gap ∼ 0.6 eV) with monoclinic (M) structure (P2 1 /c) 6,7 . The MIT of VO 2 can be controlled not only by temperature but also by light and electrical current 4,8,9 . This unique characteristic makes VO 2 a compelling candidate for microelectronic devices, terahertz devices, energy harvesting systems etc 2,10-12 . Controlling and tailoring the conductivity of VO 2 by electric field have a potential for realizing next-generation hybrid multifunctional, low power logic and non-volatile memory devices13 . This type of switching process requires high electric field, which cannot be achieved by present day dielectric gates and hence calls for electrolytic top gating 14-16 as has been done using ionic liquid (IL) by two groups of Iwasa et al. 8,17 and Jeong et al. 18,19 . Both the groups showed that above a certain positive gate voltage, the MIT temperature is suppressed. However, different mechanisms have been proposed for electric field induced metalization in VO 2 films. Iwasa el.8,17 suggested that the three dimensional metallic ground state emerges due to the collective bulk delocalization of electrons driven by electrostatic charging at the surface of VO 2 . On the other hand, Jeong et al. 18,19 proposed that the metalization of VO 2 is due to...

The Role of Ionic Liquid Breakdown in the Electrochemical Metallization of VO2: An NMR Study of Gating Mechanisms and VO2 Reduction

“…The experiments were performed on bulk VO 2 , using ∼15 mg free-standing composite films made with standard battery/supercapacitor electrode preparation techniques. Unlike previous potentiostatic electrolyte gating experiments, − , here galvanostatic reduction was used, so that the energetics of different processes could be inferred from the potential, which was measured relative to a silver wire pseudo-reference electrode (Figure a, right). The use of the reference electrode avoids electrode polarization effects and allows the potential of the insertion reaction to be measured relative to a known potential, which is particularly important in this case because the reaction that occurs at the counter electrode has not been established definitively.…”

“…Jeong et al later showed that the metallization was in fact due to the electrochemical reduction of the vanadium and consequent introduction of electrons into the band structure. This reduction must be charge balanced, and Jeong et al established the simultaneous creation of oxygen vacancies on the basis of 18 O secondary ion mass spectrometry (SIMS) data, which showed an excess of 18 O at the surface of devices that had been gated and reverse gated in an 18 O 2 atmosphere; this is the generally accepted mechanism in the literature. − The same group later showed that oxygen plays a role in ionic liquid gating of several other oxides including WO 3 , again by 18 O SIMS . Most recently they directly observed oxygen vacancies, using in situ transmission electron microscopy, in SrCoO 2.5 produced by electrolyte gating of SrCoO 3 , which was accompanied by dramatic structural and magnetic changes .…”

“…In this work, multinuclear NMR spectroscopy ( 1 H, 2 H, 17 O, and 51 V) is utilized to study the crystal and electronic structure of electrochemically metallized VO 2 and explore the mechanism of electrochemical metallization, using the imidazolium-based ionic liquids that are commonly used for electrolyte gating experiments; − these NMR results are supported by X-ray diffraction (XRD), resistivity, and magnetic susceptibility measurements. First, the NMR of pristine micrometer-sized VO 2 particles are investigated above and below the MIT temperature, before the magnetic and electronic properties of catalytically hydrogenated VO 2 are examined.…”

Emergence of metallic monoclinic states of VO2 films induced by K deposition

“…Finally, we discuss the other possibility that the metallization states in K/VO2 are caused by oxygen deficiency due to K deposition. There are several studies that have claimed that metallization in EDLTs originates from electric-field-induced oxygen vacancy formation and not electrostatic surface charge accumulation [21,45,46]: The metallization states remain unchanged after washing out of the ion liquid (removing the electrostatic gating) [21,47]. These results drive us to the question as to whether the deposition of K atoms onto the VO2 surface leads to oxygen vacancy owing to the high chemical activity of K. To verify the possibility of oxygen-vacancy-induced metallization in K/VO2, we oxidized the absorbed K atoms by exposing the sample to air [48]; the corresponding results are shown in Fig.…”

A Monoclinic V1-x-yTixRuyO2 Thin Film with Enhanced Thermal-Sensitive Performance

“…It is well-known that the SMT of VO 2 is associated with structural transformation from monoclinic phase to tetragonal phase [51]. Compared with the tetragonal phase, monoclinic VO 2 has remarkably lowered symmetry, which is characterized by zigzag V-V chains with two V-V distances (2.65 and 3.12 Å) [51,52]. As the temperature rises across the SMT temperature, zigzag V-V chains in the monoclinic phase are transformed into linear V-V chains with a unique V-V distance of about 2.85 Å in the tetragonal phase.…”