A new publication from Opto-Electronic Science; DOI 10.29026/oes.2023.220002 considers femtosecond laser induced periodic surface structures.
The laser induced periodic surface structures (LIPSS) is a universal phenomenon, and has been demonstrated on the surfaces of metals, semiconductors and transparent materials irradiated by linearly polarized lasers. The period of the LIPSS depends on the laser conditions and the nature of the material, varying from close to the incident laser wavelength to less than a tenth of the wavelength. These periodic nanostructures can be used to efficiently modify the properties of materials and have many applications in surface coloring, optoelectronic characteristic modulation, birefringence optical elements, and surface wettability.
Indium-tin oxide (ITO) films with a wide bandgap of 3.5–4.3 eV provide high electrical conductivity and transmittance in the visible and near-infrared (NIR) wavelengths. Because of their unique optoelectronic properties, ITO films have been widely used as transparent electrodes for solar cells and liquid-crystal displays, and as anode coatings for organic light-emitting diodes. It has been a very important research topic to develop new methods to prepare ITO thin films and to regulate the optoelectronic properties of ITO thin films, and in the field of laser processing, using laser to induce LIPSS in ITO thin films is an effective and simple method.
The group of Prof. Tianqing Jia at the State Key Laboratory of Precision Spectroscopy of East China Normal University has investigated a method to process LIPSS on the surface of ITO films by direct femtosecond laser writing and analyzed in detail the variation of transmittance and anisotropic conductivity of ITO films with LIPSS structure in the visible to infrared wavelengths under different laser parameters. Suitable laser parameters can effectively process large-area low spatial frequency LIPSS on ITO films, and these LIPSS can exhibit the properties of independent nanoconductors with good uniformity in electrical properties. The results show that the material properties are not changed during the direct femtosecond laser writing process, and the average transmittance of ITO films with regular LIPSS is increased by 197% in the infrared band compared with the pristine ITO films. This has important application implications for processing regular LIPSS on the surface of ITO films as transparent electrodes for optoelectronic devices in the near-infrared band.
As shown in Fig. 1. The surface resistance of the original ITO film is isotropic. With the increase of laser fluence, the surface resistance increases rapidly in the transverse and longitudinal directions of LIPSS and the gradient of change is different, showing the anisotropic conductivity in each direction.
By adjusting the laser fluence, nanowires with different morphologies (LIPSS) can be prepared in a wide range. Figure 2(a) shows the SEM images of nanowires processed by femtosecond laser with different fluences. The width of the nanowires decreases from 537 nm to 271 nm as the fluence increases. the height of the nanowires decreases from an average of 220 nm to 142 nm, as shown in Figure 2(b). The unit resistance of the nanowires increases from 15 kΩ/mm to 73 kΩ/mm as the fluence rises, which is caused by the simultaneous decrease in both the width and height of the nanowires, as shown in Fig. 2(c).
As shown in Fig. 3, the average transmittance of the ITO film with the original thickness of 185 nm is 21.31% in the NIR spectral range of 1200 - 2000 nm. After the femtosecond laser direct writing, the transmittance of the ITO film to the NIR band is significantly increased. When the fluence was in the range of 0.510 - 0.637 J/cm2, the transmittance of ITO films reached 54.48 - 63.38% for NIR, which was an increase of 156 - 197% compared with the original ITO films. At the same time, the transmission rate of the ITO film in the visible band was slightly increased and the curve was smoother after the femtosecond laser direct writing. By adjusting the energy density of the laser, the transmission rate of the ITO film in the near infrared can be significantly improved and the conductivity can be maintained.
Keywords: transparent nanowires / periodic surface nanostructures / femtosecond laser direct writing / ITO film / anisotropic electrical conductivity
