ИСТИНА

Liquid ionic gating of an ultra-thin VO2 film the film induces an insulator to metal transition, which is accompanied by a structural phase transition from monoclinic to tetragonal symmetry and by an expansion of the c-lattice parameter by about 3% [1]. It was argued that for the liquid gating induced metallization of the film, the formation of oxygen defects plays a major role. [1,2]. In order to study in detail the effect of liquid gating and backgating on the VO2 film struc-ture we have carried out a precise structure analysis of a 10 nm thick gated and the back-gated film using synchrotron radiation at the ESRF. For the gated film we find that the structure is purely tetragonal of rutile type. In total, 79 independent structure factor amplitudes, |Fhkl| were collected and fitted (Ru=8%) using the well-known rutile structure by allowing the oxygen atom located at position 4f (xx0) with x=0.297(1) of the space group P42/mnm to vary. The calculated the z-projected charge density [(x,y,z)] is shown in the figure below: It shows four unit cells in which the strong peaks are related to the V atom at (000) and (½ ½ ½) as well as to the regular oxygen atom at (x,x,0) with x=0.297(1) and the symmetrically equivalent ones. In addition, the density map shows a faint charge density at an aysmmetric site at about (x,y,z)=(0.38, 0.00, 0.00) which is related to an about 2% site occupancy of an oxygen intersti-tial. Simultaneously, the occupancy of the lattice site (xx0) is about 98% only. The considera-tion of this modification of the rutile structure improved the fit by 10%. To gain insight into the formation of these oxygen interstitials, we perform the first-principles calculations based on density functional theory and pseudopotential method. The rutile struc-ture of VO2 with oxygen interstitial sites was treated by means of supercell approach with peri-odic boundary conditions. The supercell structural relaxations and total energy calculations reveal that the formation of oxygen interstitials could be promoted by oxygen vacancies, which can be induced by liquid gating process of VO2 film [4]. [1] J. Jeong, N. Aetukuri, D. Passarello et al., PNAS 112, 1013 (2015) [2] S. G. Altendorf, J. Jeong, D. Passarello, et al., Adv. Mater. 28, 5284 (2016) [3] K. D. Rogers, Powder Diffraction 8, 240 (1993) [4] J. Jeong, et al., Science 339, 1402 (2013).