Artificially Controlled Nanoscale Chemical Reduction via Electron Beam Illumination

Chemical reduction is one of the most generic and essential reactions in solid state chemistry, which could drastically change the physical and chemical properties of functional materials [1,2].In complex oxides, this reaction is usually dominated by the formation of oxygen vacancy [3], and the incorporation of oxygen vacancies through chemical reduction forms an important pathway to manipulate the ground state of strongly correlated materials, as every oxygen vacancy would donate two electrons into the lattice.Furthermore, the oxygen vacancy forms an indispensable ingredient for the manipulation of ionic conductivity [4] and catalysis [5] properties, as well as demonstrates novel approaches to design advanced piezoelectric [6] and magnetoelectric [7] couplings.Conventionally, the thermal treatment within the reducing environment or chemical agents forms the most accessible method to achieve this purpose as it can dramatically modify the thermodynamic phase diagram of the bulk compounds.However, this approach faces its fundamental limitation for functional miniaturization, as the whole sample would undergo the identical environment.Alternative approaches, using electric-field [8, 9], ultraviolet light [10] or scanning tip [11-13], etc., have been proposed and demonstrated to manipulate the thermodynamic stability at the microscale, while nanoscale control is still of great challenge, as these methods are constrained by the corresponding intrinsic length scales.Here, we directly image the atomistic behavior of correlated disorder at low temperature and reveal an inverse transition from correlated disorder to random.Here, we exploit e-beam to induce chemical reduction in VO 2 with an accompanied insulator to metal transition (IMT) at the nanoscale lateral resolution [14].The electron beam induces both surface oxygen desorption through radiolytic process and positively charged background through secondary electrons, which contribute cooperatively to facilitate the vacancy migration from the surface toward the sample bulk (Fig. 1a).As the chemical reduction of oxides is strongly correlated with the surface facets, we prepared two different single-crystalline VO 2 thin films with non-polar and polar facets (termed as c-facet and a-facet).By systematically investigating the conductivity (Fig. 1b,c), we find that the e-beam induced chemical reduction of VO 2 shows an interesting facet-dependence with the pronounced transformation observed for the c-facet as compared with the a-facet.Furthermore, combining the Raman spectra and STEM measurement, we reveal the phase transformation from VO 2 to V 2 O 3 driven by e-beam when the c-facet VO 2 is exposed to e-beam (Fig. 1d-f).Remarkably, we readily achieve a lateral resolution of tens nanometer (50 nm) for the controlled structural transformation with a commercial scanning electron microscope (Fig. 2a,b).We note that this featured size is much smaller than the other methods, as summarized in Fig. 2c.Given the vast applications of VO 2 and its associated IMT for extensive applications, the manipulation of IMT at nanoscale forms a promising pathway to achieve improved performances.Our work highlights the capability of nanoscale control of chemical reduction assisted by the e-beam and offers possibilities to design functions through sketchable chemical reaction [15].