Tailoring the Local Conductivity of TiO₂ by X‐Ray Nanobeam Irradiation

Abstract

It is well known that intense synchrotron beams can alter the state of materials, but this effect is generally considered undesired radiation damage. The effect of local irradiation of TiO2 rutile single crystals is investigated by a 56 × 57 nm2 synchrotron X‐ray nanobeam at 17.4 keV. Aside from a transient increase of conductivity due to a photovoltaic‐like process, a nonvolatile localized change of resistance by about 4 orders of magnitude is measured after X‐ray exposure. This effect can be ascribed to the local generation of oxygen vacancies by the X‐ray nanoprobe, which are subsequently ordered by the electric field applied during the acquisition of I–V curves. These results demonstrate that intense synchrotron beams can create oxygen vacancies in materials with tightly bound oxygen atoms, highlighting that X‐ray nanoprobes could become an effective tool for oxide nanofabrication, able to locally tune the material resistivity. For instance, since the localized presence and migration of oxygen vacancies is an essential requisite for redox‐based memristive devices, the possibility to locally induce oxygen vacancies could represent a novel tool for the production of oxide‐based memristive devices, replacing the problematic electroforming step.

Functional Modifications Induced via X‐ray Nanopatterning in TiO2 Rutile Single Crystals

“…This behavior further supports the hypothesis of a local increase in the density of oxygen vacancies and significantly differs with respect to previous XNP experiments carried out on rutile single crystals with two Au electrodes, where no evident change in surface conductivity was highlighted after irradiation. [ 38 ] This was attributed to the formation of subsuperficial conductive channels. By comparing these two experiments, we can observe that in the present case, the use of an ohmic electrode (i.e., Ta) has resulted in a direct‐polarization current in the IV curves that is one order of magnitude larger than the previous one.…”

“…[ 37 ] Following these indications, we have applied the XNP method to nonannealed TiO 2 rutile single crystals with two Au electrodes and shown that controlled irradiation can open a subsurface conduction channel in the insulating region between two electrodes. [ 38 ] This experiment proved that XNP can be applied also to materials where the binding energy for oxygen is much greater than the one corresponding to the interstitial atoms present in the superconducting oxides. However, we were neither able to prove the ability of XNP to locate and pin the formation of conducting channels nor to observe which kind of changes could be associated in our case to the device‐resistance transition.…”

New reflections on hard X-ray photon-in/photon-out spectroscopy

“…Modulation of the sample under an X-ray beam is not always unwanted. Desired effects of X-rays can be local changes of the conductivity, 66 emulating the effects of heavy ion impact and combining cause with diagnose when X-rays reproduce modifications that are otherwise achieved using optical or thermal excitations. 67 We show in Figure 7 an example for Eu in BaMgAl 10 O 17 .…”

Mapping of Structural Changes Induced by X-ray Nanopatterning via Nano-X-ray Diffraction and Corresponding Electrical Effects

“…Even though we have already extended this approach also to a semiconducting material like TiO 2 , nevertheless the microscopic mechanisms responsible for the material modifications are not known yet. Among the ones that we have already investigated, it has become increasingly clear that both the oxygen knock-on and the local heating induced by the photoelectron cascades can only play quite a limited role. , …”