F. Ay

Vibrational and mechanical properties of single layer MXene structures: a first-principles investigation.

MXenes, carbides, nitrides and carbonitrides of early transition metals are the new members of two dimensional materials family given with a formula of [Formula: see text] X n . Recent advances in chemical exfoliation and CVD growth of these crystals together with their promising performance in electrochemical energy storage systems have triggered the interest in these two dimensional structures. In this work, we employ first principles calculations for n = 1 structures of Sc, Ti, Zr, Mo and Hf pristine MXenes and their fully surface terminated forms with F and O. We systematically investigated the dynamical and mechanical stability of both pristine and fully terminated MXene structures to determine the possible MXene candidates for experimental realization. In conjunction with an extensive stability analysis, we report Raman and infrared active mode frequencies for the first time, providing indispensable information for the experimental elaboration of MXene field. After determining dynamically stable MXenes, we provide their phonon dispersion relations, electronic and mechanical properties.

Focused ion beam nano-structuring of

In this work, we report on utilization and optimization of the focused-ion-beam technique for the fabrication of nanostructures on Al2O3 waveguides for applications in integrated photonic devices. In particular, the investigation of the effects of parameters such as ion-beam current, dwell time, and scanning strategy is addressed. As a result of optimizing these parameters, excellent quality gratings with smooth and uniform sidewalls are reported. The effects of redeposition are minimized and good control of the nanostructuring process is reported. The effect of Ga+ ion implantation during the milling process on the optical performance of the devices is discussed.

Al_2O_3

Single-layer, large-scale two-dimensional material growth is still a challenge for their wide-range usage. Therefore, we carried out a comprehensive study of monolayer MoS2 growth by CVD investigating the influence of growth zone configuration and precursors ratio. We first compared the two commonly used approaches regarding the relative substrate and precursor positions, namely, horizontal and face-down configurations where face-down approach is found to be more favorable to obtain larger flakes under identical growth conditions. Secondly, we used different types of substrate holders to investigate the influence of the Mo and S vapor confinement on the resulting diffusion environment. We suggest that local changes of the S to Mo vapor ratio in the growth zone is a key factor for the change of shape, size and uniformity of the resulting MoS2 formations, which is also confirmed by performing depositions under different precursor ratios. Therefore, to obtain continuous monolayer films, the S to Mo vapor ratio is needed to be kept within a certain range throughout the substrate. As a conclusion, we obtained monolayer triangles with a side length of 90 µm and circles with a diameter of 500 µm and continuous films with an area of 850 µm × 1 cm when the S-to-Mo vapor ratio is optimized.

dielectric layers for photonic applications

The inherent low lattice thermal conductivity (TC) of semiconductor nanowires (s-NW) due to one-dimensional phonon confinement might provide a solution for the long-lasting figure-of-merit problem for highly efficient thermoelectric (TE) applications. Standalone diameter modulation or alloying of s-NW serve as a toolkit for TC control, but realizing the full potential of nanowires requires new atomic-scale designs, growth, characterization, and understanding of the physical mechanisms behind the structure–property (TC) relationship. Before undertaking time-consuming and expensive experimental work, molecular dynamics (MD) simulations serve as an excellent probe to investigate new designs and understand how nanostructures affect thermal transport properties through their capability to capture various phenomena such as phonon boundary scattering, phonon coherence resonance, and phonon backscattering. On the other hand, because different research groups use different structural and MD parameters in their simulations, it is rather difficult to make comparisons between different nanostructures and select appropriate ones for potential TE applications. Therefore, in this work, we systematically investigated pristine, core–shell (C–S), holey (H-N), superlattice (SL), sawtooth (ST), and superlattice sawtooth (SL-ST) nanowires with identical structural parameters. Specifically, we aim to compare the relative TC reduction achieved by these nanostructures with respect to pristine nanowires in order to propose the best structural design with the lowest lattice TC, using Green–Kubo method-based equilibrium molecular dynamics simulations at 300xa0K. Our results show that the TC can be minimized by changing specific parameters such as the core diameter and monolayer separation for C–S, H-N, and ST structures. In the case of SL structures, the TC is found to be independent of these parameters. However, surface roughness in the form of a ST morphology provides a TC value below 2xa0W/m-K, approaching the amorphous limit.

CVD growth of monolayer MoS2: Role of growth zone configuration and precursors ratio

The operation dynamics of end-pumped solid-state lasers are investigated by means of a spatially resolved numerical rate-equation model and a time-dependent analytical thermal model. The rate-equation model allows the optimization of parameters such as the output coupler transmission and gain medium length, with the aim of improving the laser output performance. The time-dependent analytical thermal model is able to predict the temperature and the corresponding induced thermal stresses on the pump face of quasi-continuous wave (qcw) end-pumped laser rods. Both models are found to be in very good agreement with experimental results.

Thermal Conductivity Suppression in Nanostructured Silicon and Germanium Nanowires

In this study, optical and electronic transport properties of chemical vapor deposition (CVD) grown 2D WS2 and MoS2 based transistors and photodetectors are investigated and compared in ambient air by using 2D flakes grown with the same CVD system. To assess the performance variations between these two materials and understand the underlying mechanisms, it is essential to utilize identical growth methods (i.e. using the same CVD system), identical substrate and dielectric materials with the identical device fabrication methods and geometries. Transistor devices fabricated out of these flakes are examined in terms of their field effective mobility, current ON/OFF ratio, and photoresponsivity. Our results show that the MoS2 based devices have higher mobility and photoresponsivity than the WS2 based devices. However, the hysteresis curve of WS2 based transistors is smaller when compared to that of MoS2 based transistors. The mobilities of MoS2 and WS2 are estimated from measurements as 1.45 and 0.98xa0cm2xa0V−1xa0s−1, respectively. The electronic transport performance of MoS2 based devices (FETs and photodetectors) are found to be unexpectedly better than the WS2 based devices in terms of effective carrier mobility and photoresponsivity at ambient atmosphere and temperature. Our results suggest that WS2 is more sensitive to ambient conditions in comparison to MoS2, in spite of its theoretically estimated superior performance.