Femtosecond Laser texturing of CVD Diamond surface for Solar Conversion Application

Nanoscale periodic texturing on diamond surface has been performed by femtosecond laser and optimized on CVD polycrystalline diamond to obtain a significant increase of optical absorbance and, specifically, of responsivity to visible and infrared radiation (Fig. 1, left). The horizontal polarized laser beam has been focused perpendicularly to the diamond plate surface with a fluence of 20 mJ/cm2 so that plates have been treated with an intensity higher than the diamond damage threshold. Fs-laser ultrashort pulses induced a controlled periodicity of ripples of 170 nm and length of several ?m (Fig. 1, right), able to dramatically increase the diamond absorption[1], which is intrinsically visible-blind due the wide bandgap of 5.47 eV. The surface treatment has been performed in a high vacuum chamber (<10-7 mbar) by moving the X axis of the X-Y translational stage with different speed resulting in a variation of the effective treatment dose on the sample. A study of the influence of dose on absorbance and responsivity has been performed, highlighting an optimum condition. A defect engineering strategy is at the basis of the surface texturing, supported by the evidence of an enhanced responsivity (about 100 times and 10 times at 350 nm and 500 nm, respectively, higher than the untreated one, shown in Fig. 2). The operating mechanisms of the first optimized surface-textured diamond is discussed and explained by disentangling the optical enhancement from an electronic increased density of states within the diamond bandgap. The enhanced diamond sensitivity to solar radiation, demonstrated for the first time, can open the path for development of high performance wide spectrum detectors as well as solar energy converters based on PETE (photon-enhanced thermionic emission) energy converters[2].