Formation of Au nanoparticles in SiO$_2$–TiO$_2$ films by local electrochemical reduction using an atomic force microscope probe

Background. The purpose of this work is to experimentally investigate the features of the Au nanoparticles (NPs) formation in SiO$_2$–TiO$_2$ films by the method of local electrochemical reduction using an atomic force microscope (AFM) probe. The additional aim of the study has been also to establish the modes of the Au NPs formation, which provide controlled production of NPs with specified parameters. The created scientific and technical products are intended for use in nanoelectronics, integrated optics, optoelectronics and plasmonics to create new nanoelectronic devices based on MNP arrays embedded in dielectric films, metal nanoantennas of arbitrary shape embedded in optical dielectric waveguides based on thin-film structures, etc. Related to this is the relevance of ongoing research. Materials and methods. An experimental study of the formation processes of individual Au NPs in the thickness of SiO$_2$–TiO$_2$ films has been carried out by the method of local electrochemical reduction of Au (III) ions using an atomic force microscope (AFM) probe. The formation of Au NPs in SiO$_2$–TiO$_2$ films has been carried out using a SolverPro atomic force microscope manufactured by “Nanotechnologiya-MDT” (Zelenograd, Russian Federation) in the contact mode. We have used AFM cantilevers made of Si with Pt coating by “Nanotechnologiya-MDT” CSG-01. Before the formation of Au NPs, AFM images of a selected area of the gel film surface have been measured: z(x, y), where x, y are the coordinates of the AFM probe tip in the sample surface plane, z is the surface height at the point with coordinates x, y. In addition, simultaneously with AFM images, images of current for selected areas of the sample surface have been measured. Results. The processes of the Au NPs formation in SiO$_2$–TiO$_2$ gel films containing Au (III) ions deposited on glass substrates with an ITO sublayer by the sol-gel method, have been studied in the course of local electrochemical reduction of Au(III) ions using a conducting AFM probe. It is shown that after the modification of gel films by applying positive voltage pulses to the AFM probe relative to the ITO sublayer, the images of current for the modified regions show channels of current associated with the formation of Au NPs at the interface between the ITO sublayer and of the gel film as a result of local electrochemical reduction of Au (III) in the area under the contact of the AFM probe to the surface of the gel film. It has been established that the formation of Au NPs also manifests itself in the appearance of hysteresis in the cyclic CVC of the contact between the AFM probe and the surface of the gel film measured during the formation of NPs. It was found that, upon modification of the SiO$_2$–TiO$_2$ gel film by applying a negative voltage pulse to the AFM probe relative to the ITO sublayer, the formation of toroidal Au nanostructures has been observed, associated with the electrochemical reduction of Au (III) ions near the contact of the AFM probe with the surface of the gel film. Conclusions. The results of the carried out studies are planned to be used in the future in the development of methods for the controlled formation of MNPs in thin dielectric films using AFM.