Motivated by a recent experiment that reported the successful synthesis of hexagonal (h) AlN [Tsipas et al., Appl. Phys. Lett. 103, 251605 (2013)], we investigate structural, electronic, and vibrational properties of bulk, bilayer, and monolayer structures of h-AlN by using first-principles calculations. We show that the hexagonal phase of the bulk h-AlN is a stable direct-band-gap semiconductor. The calculated phonon spectrum displays a rigid-layer shear mode at 274 cm −1 and an E g mode at 703 cm −1 , which are observable by Raman measurements. In addition, single-layer h-AlN is an indirect-band-gap semiconductor with a nonmagnetic ground state. For the bilayer structure, AA -type stacking is found to be the most favorable one, and interlayer interaction is strong. While N -layered h-AlN is an indirect-band-gap semiconductor for N = 1 − 9, we predict that thicker structures (N 10) have a direct band gap at the point. The number-of-layer-dependent band-gap transitions in h-AlN is interesting in that it is significantly different from the indirect-to-direct crossover obtained in the transition-metal dichalcogenides.

Defect-free graphene is impermeable to gases and liquids [1][2][3][4] but highly permeable to thermal protons [5][6][7][8] . Atomic-scale defects such as vacancies, grain boundaries and Stone-Wales defects are predicted [9][10][11] to enhance graphene's proton permeability and may even allow small ions through, whereas larger species such as gas molecules should remain blocked. These expectations have so far remained untested in experiment. Here we show that atomically thin carbon films with a high density of atomic-scale defects continue blocking all molecular transport, but their proton permeability becomes ~1,000 times higher than that of defect-free graphene. Lithium ions can also permeate through such disordered graphene. The enhanced proton and ion permeability is attributed to a high density of 8-carbon-atom rings. The latter pose approximately twice lower energy barriers for incoming protons compared to the 6-atom rings of graphene and a relatively low barrier of ~0.6 eV for Li ions. Our findings suggest that disordered graphene could be of interest as membranes and protective barriers in various Li-ion and hydrogen technologies.Despite being a one-atom-thick material, no more than a few gas atoms per hour can permeate through micrometer-sized defect-free graphene membranes, as proven experimentally 3 . Even the smallest ions are blocked by the crystal 4 . These phenomena arise because the dense electron clouds of graphene's crystal lattice impose energy barriers of several eV to incoming molecular and ionic species [9][10][11] , which forbids their permeation under ambient conditions. In contrast, it has been shown experimentally that protons, nuclei of hydrogen atoms, can transport through defect-free graphene relatively easily, overcoming an energy barrier of only ≲1 eV (refs 3-6 ). In this context, theory predicts

Layered transition metal trichalcogenides (TMTCs) are a new class of anisotropic two-dimensional materials that exhibit quasi-1D behavior. This property stems from their unique highly anisotropic crystal structure where vastly different material properties can be attained from different crystal directions. Here, we employ density functional theory predictions, atomic force microscopy, and angle-resolved Raman spectroscopy to investigate their fundamental vibrational properties which differ significantly from other 2D systems and to establish a method in identifying anisotropy direction of different types of TMTCs. We find that the intensity of certain Raman peaks of TiS, ZrS, and HfS have strong polarization dependence in such a way that intensity is at its maximum when the polarization direction is parallel to the anisotropic b-axis. This allows us to readily identify the Raman peaks that are representative of the vibrations along the b-axis direction. Interestingly, similar angle resolved studies on the novel TiNbS TMTC alloy reveal that determination of anisotropy/crystalline direction is rather difficult possibly due to loss of anisotropy by randomization distribution of quasi-1D MX chains by the presence of defects which are commonly found in 2D alloys and also due to the complex Raman tensor of TMTC alloys. Overall, the experimental and theoretical results establish non-destructive methods used to identify the direction of anisotropy in TMTCs and reveal their vibrational characteristics which are necessary to gain insight into potential applications that utilize direction dependent thermal response, optical polarization, and linear dichroism.

By performing density functional theory-based ab-initio calculations, Raman active phonon modes of novel single-layer two-dimensional (2D) materials and the effect of in-plane biaxial strain on the peak frequencies and corresponding activities of the Raman active modes are calculated.Our findings confirm the Raman spectrum of the unstrained 2D crystals and provide expected variations in the Raman active modes of the crystals under in-plane biaxial strain. The results are summarized as follows; (i) frequencies of the phonon modes soften (harden) under applied tensile (compressive) strains, (ii) the response of the Raman activities to applied strain for the in-plane and out-of-plane vibrational modes have opposite trends, thus, the built-in strains in the materials can be monitored by tracking the relative activities of those modes, (iii) in particular, the A-peak in single-layer Si and Ge disappear under a critical tensile strain, (iv) especially in mono and diatomic single-layers, the shift of the peak frequencies is stronger indication of the strain rather than the change in Raman activities, (v) Raman active modes of single-layer ReX 2 (X=S, Se) are almost irresponsive to the applied strain. Strain-induced modifications in the Raman spectrum of 2D materials in terms of the peak positions and the relative Raman activities of the modes could be a convenient tool for characterization.

Motivated by the recent synthesis of layered hexagonal aluminum nitride (h-AlN), we investigate its layerand strain-dependent electronic and optical properties by using first-principles methods. Monolayer h-AlN is a wide-gap semiconductor, which makes it interesting especially for usage in optoelectronic applications. The optical spectra of 1-, 2-, 3-, and 4-layered h-AlN indicate that the prominent absorption takes place outside the visible-light regime. Within the ultraviolet range, absorption intensities increase with the number of layers, approaching the bulk case. On the other hand, the applied tensile strain gradually redshifts the optical spectra. The many-body effects lead to a blueshift of the optical spectra, while exciton binding is also observed for 2D h-AlN. The possibility of tuning the optoelectronic properties via thickness and/or strain opens doors to novel technological applications of this promising material.