Synthesis of diamond nanostructures from carbon nanotube and formation of diamond-CNT hybrid structures

We report direct conversion of multiwall carbon nanotubes (CNTs), synthesized by chemical vapor deposition, into diamond by nanosecond pulsed laser melting process at ambient temperature and pressure in air without any catalysts. The Raman spectroscopy of the CNTs after the laser irradiation showed the characteristic diamond peak at around 1324-1325 cm(-1). The downshift of this peak from its theoretical position (at 1332 cm(-1)) is explained by phonon confinement in nanostructured diamond. The SEM and TEM images show the formation of diamond mostly at the tip and bends of the CNTs. The grain size distribution and the shape of the converted nanodiamonds suggest that the transformation takes place by melting of the CNTs in a super undercooled state by nanosecond laser pulses, and subsequent rapid quenching to convert it into phase-pure diamond. The EBSD analysis illustrates the phase-pure single crystal diamond formation at the tips and bends of the CNTs. The high-resolution electron energy-loss spectrum in the STEM contains characteristic sigma* peak at 292 eV for sp(3) bonding of diamond. This study on the laser-induced direct conversion of CNTs to diamond marks a major breakthrough in the formation of diamond nanostructures and diamond-CNT hybrid for a variety of potential applications. (C) 2019 Published by Elsevier Ltd.