In the presented article, surfaces with highly resistivity anisotropic have been produced. In the process indium tin oxide (ITO) sheets have been irradiated with a laser beam pulsed in femtosecond and operating at 1030nm. The irradiation causes a process of self-organization at the nanoscale producing alternating stripes of large and small thickness. Anisotropy is caused by the formation of these periodic laser-induced surface structures (known as LIPSS) that extend over regions around 1 cm² in size. Two types of optimized structures have been obtained. With high-energy laser pulses, ablation (removal of material) in the valleys of the LIPSS is almost complete, while there is significant loss of material in the ridges. This results in an insulating structure in the transverse direction to the LIPSS and conductive in the longitudinal direction. In addition, a strong decrease in the content of Indium is observed in the remaining material, which leads to a ρL ≈ 1.0 Ω-cm. With lower pulse energy, the material in the ridges of the LIPSS remains essentially unchanged while partial ablation is observed in the valleys. The structures show a longitudinal conductivity twice as high as the transverse one, and a resistivity similar to that of the untreated ITO surface (ρ≈ 5 × 10 ^-4 Ω-cm).

The article presents the exhaustive characterization of these transparent and conductive structures at the same time. The composition changes induced as the laser pulses accumulate, condition the evolution of the LIPSS and, therefore, the result of the structuring process. Strategies are also presented to further improve the achieved anisotropic resistivity results.

Transparent conductive oxides (TCO) are materials with low optical absorption in the visible region of the spectrum (they are transparent), which makes them especially suitable as transparent electrodes for applications in information technology (organic light emitting diodes, flat screens, etc.) and also in energy harvesting (photovoltaic, low-emissivity coatings, etc.). They are produced by creating effects of degeneration in the electronic states of a broadband oxide, either by introducing "non-stoichiometry" (solid solutions) and / or appropriate dopants, such as Sb or F. This is usually achieved in mixtures of different oxides of the Group III with metal oxides, in which the metal can be part of the semiconductor oxide or act as a dopant. This results in different families of transparent conducting oxides (TCOs), including AZO (zinc aluminum oxide), IZO (indium zinc oxide), ITO (indium tin oxide), GZO (zinc oxide). gallium-zinc), etc. Among them ITO (typically ≈90% wt% In2O3 + 10 wt% SnO2) plays a key role, especially to produce transparent conductive electrodes (ECT) for flat and flexible displays.

Regarding the structuring of ITO films, although wet etching is suitable for producing micrometer width electrodes, the need to be able to produce patterns quickly and without masking over large areas led to research on the use of structuring. by laser already at the end of the 90's. An equally important application of lasers for the processing of TCO is its use to sinter films formed by ITO nanoparticles by the "spin-coating" method, especially on flexible substrates.

On the other hand, the use of ultra-fast lasers in the pulse duration regimes of ps- or fs has been analyzed by various research groups, which has led in some cases to better performance. This approach, when using multipulse processing, has the additional consequence of generating laser-induced periodic surface structures (LIPSS), an effect that has recently received attention due to its high potential to produce optical or electrical anisotropies. However, due to its high penetration depth in the visible - near infrared, it is difficult to fabricate continuous LIPSS over macroscopic regions in the ITO, which is a necessary requirement for many practical applications.

In this work we present a novel technique for the fabrication of strongly anisotropic resistivity surfaces in ITO films. It is possible to achieve electrical isolation in one direction and electrical conductance in the transverse direction. This strong anisotropy effect is caused by the formation of coherently extended LIPSS over the irradiated zone with the fs laser at 1030nm at a high processing speed (m / s). Following optimization of laser processing parameters, two main types of structures with anisotropic resistivity have been produced based on either a process of a strong blation with changes in the composition of the remaining material material, or in mild ablation. The structures have been thoroughly characterized in terms of their morphology, composition, structure, optical properties and resistivity. The origin of its electrical and optical response has been examined based on the compositional and structural evolution of the irradiated material. In addition, feasible routes are presented to further improve the obtained anisotropy results.

With all this, the foundations for the development of anisotropic surfaces of ultra-high performance TCEs based on ITO have been established. There are two aspects that are worth highlighting at this point. The first is the fact that the approach used, consisting of the fabrication of anisotropic TCE surfaces by laser-induced self-organizing structuring, could be applied to other transparent conductors based on materials less scarce than In. The second is the fact that, even in the case of ITO, there are application niches (electrochemical sensors, optical electroswithching, etc.) where the consumption of ITO (through the use of a subtractive process) would be comparatively very small and where the cost (due to the shortage of supply) would be perfectly acceptable in the price of the final product. For some of these applications, the chemical stability of ITO in water and in aqueous solutions (eg in biological fluids) is a decisive factor for the use of ITO.

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The work is a collaboration between the Institute of Optics and the group of Nanotechnology in Surfaces of the Institute of Materials Science of Seville (ICMS-US-CSIC) and the Department of Atomic, Molecular and Nuclear Physics of the Faculty of Physics of the University of Sevilla