Femtosecond Laser Synthesis of Polymorphic Diamond from Highly Oriented Pyrolytic Graphite

We synthesized polymorphic diamond directly from highly oriented pyrolytic graphite (HOPG) using femtosecond laser driven shock wave without catalyst. A femtosecond laser pulse (wavelength: 800 nm, pulse width: 120 fs, intensity: 2×10 W/cm) was irradiated onto the HOPG surface in air. Crystalline structures of HOPG after the laser irradiation were analyzed using the synchrotron X-ray at the BL13XU in the SPring-8. We found that the hexagonal diamond exists in the HOPG which was irradiated by the femtosecond laser normal to the basal plane. Introduction A stable phase of carbon is graphite in the lower pressure regime and cubic diamond in the higher pressure regime [1]. The cubic diamond is one of the most important materials in the field of the manufacturing industry due to its high hardness and thermal conductivity. Hexagonal diamond was discovered in the Canyon Diablo iron meteorite [2] and is considered the metastable high-pressure phase of carbon. The hexagonal diamond is synthesized artificially using the shock compression method from graphite with catalyst [3] and from cubic diamond without catalyst [4], the static compression method [5], and the chemical vapor deposition method [6]. There still exist many unresolved subjects on the synthesis mechanism of cubic and hexagonal diamonds. Metallization of carbon is predicted theoretically under more than 1.1 TPa [7]. The metallic carbon is the lightest metal on earth if we can synthesis and retain it under the atmospheric pressure. Therefore, exploration of the high-pressure polymorphic carbon is significantly important for not only the industry but the materials and planetary sciences. A solid material transits directly to the plasma, which expands to the vacuum explosively, after an intense femtosecond laser pulse is irradiated to the solid. The shock wave is driven by the recoil pressure during the plasma expansion and propagates into the solid. There is no interaction of the femtosecond laser pulse with the expanding plasma because the plasma expansion occurs after the whole femtosecond laser pulse injection, which is the definitively different point from the nanosecond laser pulse irradiation. The expansion velocity of the plasma is faster than that of the nanosecond laser induced plasma. Therefore, the disturbance due to the heating by the radiation from the plasma is negligible. The femtosecond laser-driven shock wave is used for the investigation of the materials dynamics under pressure [8]. A high pressure ε phase of iron, which has not been synthesized using conventional compression methods, was synthesized using the femtosecond laser-driven shock wave [9,10]. The femtosecond laser-driven shock wave is the potential tool to synthesize high-pressure materials. Materials Science Forum Vols. 561-565 (2007) pp 2349-2352 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007/Oct/02 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-17/04/08,10:43:09) The purpose of this study is to investigate the synthesized phase of carbon after the femtosecond laser irradiation to the graphite. We expect that diamond will be synthesized if the strong shock wave is driven. But we cannot imagine whether the structure of the diamond is cubic or hexagonal because the rapidly cooling effect is significant for the femtosecond laser-driven shock wave compression compared to the conventional compression methods such as the flyer impact method and the nanosecond laser shock method. Experimental method The target used in this study was the highly oriented pyrolytic graphite (HOPG, NT-MDT Co., Russia, 10 mm × 10 mm × t 1.5 mm) with the ZYA quality, the mosaic spread of 0.4 – 0.7 deg, and the mass density of 2.267 g/cm. A femtosecond laser pulse (Spitfire, Spectra-Physics Inc.) with the wavelength of 800 nm, the pulse width of 120 fs, and the spatial energy distribution of the near Gaussian firstly passed through an aperture of 6 mm diameter, and secondly was focused onto a target surface in air using a plano-convex lens with the focal length of 70 mm. The femtosecond laser pulse was irradiated normally to the basal plane of the HOPG target. A single pulse was shot at a point, and the shot to shot interval was 100 μm on the surface. Two kinds of the laser pulse energy, 0.7 and 5.5 mJ, was used. The spatially averaged laser intensity was approximately 3×10 and 2×10 W/cm for the laser pulse energy of 0.7 and 5.5mJ, respectively, because the laser spot size was approximately 50 μm. The HOPG surface was divided into two regions, the laser irradiated and unirradiated regions. Crystalline structures were analyzed using the grazing incident XRD method at the BL13XU in the SPring-8[11]. The synchrotron X-ray with the wavelength of 1.000 Å passed through a slit with the horizontal width of 410 μm and the vertical width of 50 μm, and was irradiated to the sample. The incident angle α was fixed and the detector angle δ was varied. A solar slit was located in front of the detector, which is the YAP scintillation counter. Results and discussions SEM images of the laser irradiated HOPG surface are shown in Fig. 1 for the laser pulse energy of (a) 0.7 mJ and (b) 5.5 mJ. The laser irradiated area was explosively removed and the deep crater was created for the laser pulse energy of 5.5 mJ, while the significant crater was not created for the laser pulse energy of 0.7 mJ. The deep crater created explosively indicates that the explosive ablation occurs and that the strong shock wave was driven. Fig. 1. SEM images of the femtosecond laser irradiated HOPG surface. The laser pulse energy: (a) 0.7 and (b) 5.5 mJ. Grazing incidence XRD patterns are shown in Fig. 2 for the laser pulse energy of (a) 0.7 and (b) 5.5 mJ. The incident angle of the X-ray to the sample surface was 0.1 deg, the angular resolution of the detector was 0.02 deg/step, and the exposure time was 5.0 s/step. The critical angle of carbon for the light wavelength of 1.000 Å is 0.13 deg. Therefore, there is no scattered X-ray if the HOPG 2350 PRICM 6