The pressure evolution of the Raman spectrum of graphene grown by chemical vapour deposition on polycrystalline copper is investigated with the use of a polar and a non-polar pressure transmitting medium (PTM). The G and 2D Raman bands exhibit similar pressure slopes for both PTM irrespectively of any unintentional initial doping and/or strain of the samples. Our analysis suggests that any pressure-induced charge transfer effects are negligibly small to influence the pressure response of graphene. This is determined by the mechanical stress due to the pressure-induced substrate contraction and the transfer efficiency of the latter to the graphene layer, as well as the PTM-graphene interaction. For the non-polar PTM, a peculiar pressure behavior of graphene is observed in the PTM solidification regime, resembling that of free standing graphene.
Single- and bi-layer MoS2 are two-dimensional semiconductors able to withstand very large deformations before failure, standing out as suitable templates for strain engineering applications and flexible electronics. It is imperative, for the proper integration of this material in practical applications, that the relationship between material property and strain is well understood. Two dimensional MoS2 crystals fabricated by chemical vapor deposition or micromechanical exfoliation are transferred onto flexible substrates and subjected to biaxial tension on a carefully designed and assessed loading stage with high accuracy and control. The successful stress transfer from substrate to the overlying 2D crystal is identified by in-situ monitoring of the strain-induced phonon frequency and photoluminescence peak shifts. Reliable values for the mode Grüneisen parameters and exciton deformation potentials were obtained by studying a significant number of crystals. The experimental results are backed by density functional theory calculations and are in good agreement with the experiments. This work highlights the potential of these materials in strain engineering applications and gives accurate values for single- and bi-layer MoS2 thermomechanical parameters.
This study reports a novel approach for growing multilayer thin films consisting of alternate layers of carbon nanotubes (CNT) and nickel on Si (1 0 0) substrates and justifies their use in thin film temperature sensors. A low pressure chemical vapor deposition system was employed for synthesizing CNT films, while Ni films were deposited by electrodeposition. Porous-Si was used as substrate to increase adhesion between the layers of the multilayer structure. The structure of the multilayer films and the quality of the CNT grown were analyzed using several characterization methods, including scanning electron microscopy, x-ray photoelectron spectroscopy, x-ray auger electron spectroscopy and Raman spectroscopy. The electrical characteristics were investigated using a van der Pauw setup and the effect of the increasing number of CNT layers in the multilayer structure was studied. The sensitivity of the multilayer film was found to increase with increasing number of CNT layers, despite the decrease of the temperature coefficient of resistance. On the other hand, the initial resistance was found to increase. Results indicated that these multilayer structures are appropriate for fabricating highly sensitive thin film gauges that can detect lower heat fluxes with more accuracy.
Debo Ajagunna Adebowale Olufunso (Debo) Ajagunna-Brief Biography Adebowale Olufunso (Debo) Ajagunna, a native of Nigeria moved to Greece and obtained his PhD in Physics from the University of Crete in 2011. His thesis was entitled "Heteroepitaxy of InN on silicon (111) and R-Plane sapphire substrates [1] " and it was performed under the supervision of Prof. Alexandros Georgakilas. [2] At the time he also served a church in Heraklion. The church is composed of expatriate converts, many of them from Nigeria. Afther his PhD Debo return back to Nigeria, to be with his wife and three children. On May 1, 2012 he was praying with members of his former congregation in Ikare-Akoko, a city in the southwestern Nigerian state of Ondo, when armed robbers entered the church building."Everyone ran for their life," said Eleni Melirrytos, a member of the Omonia Church of Christ in Athens, Greece, and longtime friend of Ajagunna. "There was a pregnant lady who got shot and was asking for help. Debo had already left the room but came back to help her. That is when he got shot." Ajagunna died from his injuries. The pregnant woman (and her unborn child) survived the attack.
Chemical vapor deposited WS 2 monolayers are subjected for the first time to controlled pure biaxial tensile strain up to 0.7%. From photoluminescence (PL) spectroscopy, the trion and neutral exciton deformation potentials are found to be similar, approximately −130 meV/%. It is shown that the excess carrier concentration as well as residual strain in WS 2 samples can be determined from the PL spectra. The experimental Gruneisen parameter of the in-plane E′ Raman mode for 1L-WS 2 is found to be equal to the corresponding mode (E 2g ) mode in bulk WS 2 . The impact of mechanical strain on the electronic and phonon band structures is also calculated in the framework of density functional theory. The theoretically obtained deformation potential for the direct transition is in very good agreement with the experiment. The reduced dimensionality of the monolayer enables the visualization over the entire Brillouin zone of both the calculated phonon dispersions and the Gruneisen parameters, which are compared with the experimentally accessible ones. This work contributes to the experimental implementation of mechanical strain engineering applications in semiconducting two-dimensional transition metal dichalcogenides.
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