Month: August 2019

Covalent Diamond–Graphite Bonding: Mechanism of Catalytic Transformation

Covalent Diamond–Graphite Bonding: Mechanism of Catalytic Transformation

Posted on by Mario Kotlár

Semir Tulić, Thomas Waitz, Mária Čaplovičová, Gerlinde Habler, Marián Varga, Mário Kotlár, Viliam Vretenár, Oleksandr Romanyuk, Alexander Kromka, Bohuslav Rezek, Viera Skákalová

ACS Nano, 2019, 1344621-4630

doi.org/10.1021/acsnano.9b00692

Abstract

Aberration-corrected transmission electron microscopy of the atomic structure of diamond–graphite interface after Ni-induced catalytic transformation reveals graphitic planes bound covalently to the diamond in the upright orientation. The covalent attachment, together with a significant volume expansion of graphite transformed from diamond, gives rise to uniaxial stress that is released through plastic deformation. We propose a comprehensive model explaining the Ni-mediated transformation of diamond to graphite and covalent bonding at the interface as well as the mechanism of relaxation of uniaxial stress. We also explain the mechanism of electrical transport through the graphitized surface of diamond. The result may thus provide a foundation for the catalytically driven formation of graphene–diamond nanodevices.

Interference-enhanced Raman scattering in SiO2/Si structures related to reflectance

Interference-enhanced Raman scattering in SiO2/Si structures related to reflectance

Posted on by Lubomír Vančo

Ľubomír Vančo, Mário Kotlár, Magdaléna Kadlečíková, Viliam Vretenár, Marian Vojs, Jaroslav Kováč

In Journal of raman spectroscopy. Vol. 50, iss. 10 (2019)

https://doi.org/10.1002/jrs.5666

Abstract

Enhancement of Raman scattering due to optical interference may act as a source of error in the issues necessitating a determination of Raman intensity. Its dependence on thin film thickness is the conventional way how to examine the effects of optical interference in Raman signal. To provide a new platform for evaluation of signal coming from substrate in the presence of a transparent capping, we investigate its relation to reflectance (R) instead of the capping thickness. We derived a theoretical model, which was experimentally tested on simple structures consisting of SiO2 deposited on mono-Si substrates. In agreement between the model and the experiment, interference enhancement is proportional to the product of (1 − R) terms taken at excitation and scattered light wavenumbers. We experimented with two different Raman bands in Si on two different Raman systems. The model was valid regardless of excitation, Raman band, and grating. Constructed for normal incidence, it was in agreement with experiment using objectives with numerical apertures up to 0.25 (0.32). The model was valid also in ultraviolet region, where imaginary part in refractive index of Si considerably rises.