Buckling effects in AlN monolayers: Shifting and enhancing optical characteristics from the UV to the near visible light range

The structural, electronic, and optical properties of flat and buckled AlN monolayers are investigated using first-principles approaches. The band gap of a flat AlN monolayer is changed from an indirect one to a direct one, when the planar buckling increases, primarily due to diminishing sp$^2$ overlapping and bond symmetry breaking in the conversion to sp$^3$ bonds. The sp$^3$ hybridization thus results in a stronger $\sigma\text{-}\pi$ bond rather than a $\sigma\text{-}\sigma$ covalent bond. The calculations of the phonon band structure indicates that the buckled AlN monolayers are structurally and dynamically stable. The optical properties, such as the dielectric function, the refractive index, and the optical conductivity of an AlN monolayer are evaluated for both flat systems and those impacted with planar buckling. The flat AlN monolayer has outstanding optical characteristics in the Deep-UV and absorbs more effectively in the UV spectrum due to its large band gap. The results reveal that optical aspects are enhanced along different directions of light polarization, with a considerable shift in the optical spectrum from Deep-UV into the visible range. Additionally, depending on the polarization direction of the incoming light, increased planar buckling enhances the optical conductivity in both the visible and the Deep-UV domains. The ability to modify the optical and electronic properties of these essential 2D materials using planar buckling technique opens up new technological possibilities, particularly for optoelectronic devices.Comment: RevTeX - pdfLaTeX, 9 pages with 7 included pdf figure

Exploring electronic, optical, and phononic properties of MgX (X=C, N, and O) monolayers using first principle calculations

The electronic, the thermal, and the optical properties of hexagonal MgX monolayers (where X=C, N, and O) are investigated via first principles studies. Ab-initio molecular dynamic, AIMD, simulations using NVT ensembles are performed to check the thermodynamic stability of the monolayers. We find that an MgO monolayer has semiconductor properties with a good thermodynamic stability, while the MgC and the MgN monolayers have metallic characters. The calculated phonon band structures of all the three considered monolayers shows no imaginary nonphysical frequencies, thus indicating that they all have excellent dynamic stability. The MgO monolayer has a larger heat capacity then the MgC and the MgN monolayers. The metallic monolayers demonstrate optical response in the IR as a consequence of the metal properties, whereas the semiconducting MgO monolayer demonstrates an active optical response in the near-UV region. The optical response in the near-UV is beneficial for nanoelectronics and photoelectric applications. A semiconducting monolayer is a great choice for thermal management applications since its thermal properties are more attractive than those of the metallic monolayer in terms of heat capacity, which is related to the change in the internal energy of the system.Comment: RevTeX - pdfLaTeX, 9 pages with 8 included pdf figure

Optical conductivity enhancement and thermal reduction of BN-codoped MgO nanosheet: Significant effects of B-N atomic interaction

We investigate the electronic, the thermal, and the optical properties of BN-codoped MgO monolayers taking into account the interaction effects between the B and the N dopant atoms. The relatively wide indirect band gap of a pure MgO nanosheet can be changed to a narrow direct band gap by tuning the B-N attractive interaction. The band gap reduction does not only enhance the optical properties, including the absorption spectra and the optical conductivity, but also the most intense peak is shifted from the Deep-UV to the visible light region. The red shifting of the absorption spectra and the optical conductivity are caused by the attractive interaction. In addition, both isotropic and anisotropic characteristics are seen in the optical properties depending on the strength of the B-N attractive interaction. The heat capacity is reduced for the BN-doped MgO monolayer, which can be referred to changes in the bond dissociation energy. The bond dissociation energy decreases as the difference in the electronegativities of the bonded atoms decreases. The lower difference in the electronegativities leads to a weaker endothermic process resulting in reduction of the heat capacity. An ab initio molecular dynamics, AIMD, calculation is utilized to check the thermodynamic stability of the pure and the BN-codoped MgO monolayers. We thus confirm that the BN-codopant atoms can be used to gain control of the properties of MgO monolayers for thermo- and opto-electronic devices.Comment: RevTeX - pdfLaTeX, 10 pages with 8 included pdf figure

Role of planar buckling on the electronic, thermal, and optical properties of Germagraphene nanosheets

We report the electronic, the thermal, and the optical properties of a Germagraphene (GeC) monolayer taking into account buckling effects. The relatively wide direct band gap of a flat GeC nanosheet can be changed by tuning the planar buckling. A GeC monolayer has an sp$^2$ hybridization in which the contribution of an $s$-orbital is half of the contribution of a $p$-orbital leading to stronger $\sigma\text{-}\sigma$ bonds compared to the $\sigma\text{-}\pi$ bonds. Increasing the planar buckling, the contribution of an $s$-orbital is decreased while the contribution of a $p$-orbital is increased resulting in a sp$^3$-hybridization in which the $\sigma\text{-}\pi$ bond becomes stronger than the $\sigma\text{-}\sigma$ bond. As a result, the band gap of a buckled GeC is reduced and thus the thermal and the optical properties are significantly modified. We find that the heat capacity of the buckled GeC is decreased at low values of planar buckling, which is caused by the anticrossing of the optical and the acoustic phonon modes affecting phonon scattering processes. The resulting optical properties, such as the dielectric function, the refractive index, the electron energy loss spectra, the absorption, and the optical conductivity show that a buckled GeC nanosheet has increased optical activities in the visible light region compared to a flat GeC. The optical conductivity is red shifted from the near ultraviolet to the visible light region, when the planar buckling is increased. We can thus confirm that the buckling can be seen as another parameter to improve GeC monolayers for optoelectronic devices.Comment: RevTeX - pdfLaTeX, 10 pages with 12 included pdf figure

Properties of BC6N monolayer derived by first-principle computation : Influences of interactions between dopant atoms on thermoelectric and optical properties

© 2021 Elsevier Ltd. All rights reserved.The properties of graphene-like BC6N semiconductor are studied using density functional theory taking into account the attractive interaction between B and N atoms. In the presence of a strong attractive interaction between B and N dopant atoms, the electron charge distribution is highly localized along the B-N bonds, while for a weaker attractive interaction the electrons are delocalized along the entire hexagonal ring of BC6N. Furthermore, when both B and N atoms are doped at the same site of the hexagon, the breaking of the sub-lattice symmetry is low producing a small bandgap. In contrast, if the dopant atoms are at different sites, a high sub-lattice symmetry breaking is found leading to a large bandgap. The influences of electron localization/delocalization and the tunable bandgap on thermal behaviors such as the electronic thermal conductivity, the Seebeck coefficient, and the figure of merit, and optical properties such as the dielectric function, the excitation spectra, the refractive index, the electron energy loss spectra, the reflectivity, and the optical conductivity are presented. An enhancement with a red shift of the optical conductivity at low energy range is seen while a reduction at the high energy range is found indicating that the BC6N structure may be useful for optoelectronic devices in the low energy, visible range.Peer reviewe

Range determination of the influence of carrier concentration on lattice thermal conductivity for bulk Si and nanowires

Mathematical modeling has been extended to simulate some physical systems to calculate some parameters that may need a sophisticated cost or may have some obstacles to be measured directly with an experimental method. In this study, the Modified Callaway Model has been used to calculate size dependence lattice thermal conductivity (LTC), and the influence of carrier concentration for bulk Si and its nanowires (NWs) with diameters of 22, 37, 56, and 115 nm has been investigated. Calculations were performed from 3K to 1600K for all cases. The effects of carrier concentration on LTC has found to begin from (1016 cm-1 ) for the bulk state and that increased to (1024 cm-1 ) for the NW with a diameter of 22 nm. The temperature that the maximum effect of carrier concentration can occur, has found to be at (10 K) for the bulk, and that increased to (340 K) for the (22 nm) Si NW