Scaling of energy gaps in phosphorene nanoflakes

Electronic structure of phosphorene nanoflakes which consist of hundreds of phosphorus atoms are studied in the framework of unrestricted Hartree-Fock approach. On the base of Pariser-Parr-Pople model for electron-electron interactions, a simplified Bethe-Salpeter formalism is established for the calculation of excitation states of the system. Taking into account the electron-hole interaction in various dielectric environments, the optical gap of a triangular phosphorene nanoflake is shown to increase as the screening effect becomes stronger while its graphene counterpart exhibits just the opposite dependence. After confirming an exponential dependence of the optical gap on the effective dielectric constant, the quasiparticle and optical gaps are also found to obey an exponential scaling rule against the total number of atoms in the nanoflakes, respectively. By extrapolating the dependence on the size of the system, one is able to estimate the exciton binding energy of a monolayer phosphorene sheet on a SiO2 substrate to be 0.894 eV. The result is found to agree well with the previous experimental result of $ 0.9 eV.

Epoxy/Silicone Blend Loaded with N-Doped CNT Composites: Study on the Optoelectronic Properties

Thin films of epoxy/silicone loaded with N-CNT were prepared by a method of sol-gel and deposited on ITO glass substrates at room temperature. The properties of the loaded monolayer samples (0.00, 0.07, 0.1, and 0.2 wt% N-CNTs) were analyzed by UV-visible spectroscopy. The transmittance for the unloaded thin films is 88%, and an average transmittance for the loaded thin film is about 42 to 67% in the visible range. The optical properties were studied from UV-visible spectroscopy to examine the transmission spectrum, optical gap, Tauc verified optical gap, and Urbach energy, based on the envelope method proposed by Swanepoel (1983). The results indicate that the adjusted optical gap of the film has a direct optical transition with an optical gap of 3.61 eV for unloaded thin films and 3.55 to 3.19 eV for loaded thin films depending on the loading rate. The optical gap is appropriately adapted to the direct transition model proposed by Tauc et al. (1966); its value was 3.6 eV for unloaded thin films and from 3.38 to 3.1 eV for loaded thin films; then, we determined the Urbach energy which is inversely variable with the optical gap, where Urbach’s energy is 0.19 eV for the unloaded thin films and varies from 0.43 to 1.33 eV for the loaded thin films with increasing rate of N-CNTs. Finally, nanocomposite epoxy/silicone N-CNT films can be developed as electrically conductive materials with specific optical characteristics, giving the possibility to be used in electrooptical applications.

Structural, Electronic and Optical Properties of ScxGa1-xP Alloys An: Ab Initio Study

The structural, electronic and optical properties of the of ScxGa1-xP alloys have been investigated by using the full-potential plane-wave FP-LAPW method as implemented in the Wien2k code. The exchange-correlation (XC) energy of electrons was treated using the Perdewe-Burke-Ernzerhof parametrization (PBEGGA), and the Tran-Blaha modified Beck-Johnson potential (TB–mBJ). The lattice constant and the bulk modulus have been calculated and analyzed where a deviation from Végard’s law is observed for both. The calculation of the band structure of binary GaP shows that there is an indirect gap of 2.27 eV, while for the ScxGa1-xP compounds there are direct gaps with values of 1.91 eV, 1, 39 eV, 2.04 eV and 1.849 eV for x = 0.25, 0.5, 0, 75 and 1, respectively. At ambient pressure, the refractive index and the dielectric constant are in good agreement with the experimental results. The extinction coefficient does not begin to increase until a threshold, which represents the optical gap. This threshold is equal to 1.224 eV and it starts to increase to reach a maximum at an energy of 3.551 eV.

Mechano-Chemical Synthesis, Structural Features and Optical Gap of Hybrid CH3NH3CdBr3 Perovskite

Hybrid methyl-ammonium (MA:CH3NH3+) lead halide MAPbX3 (X = halogen) perovskites exhibit an attractive optoelectronic performance that can be applied to the next generation of solar cells. To extend the field of interest of these hybrid materials, we describe the synthesis by a solvent-free ball-milling procedure, yielding a well crystallized, pure and moisture stable specimen of the Cd tribromide counterpart, MACdBr3, which contains chains of face-sharing CdBr6 octahedra in a framework defined in the Cmc21 (No 36) space group. The details of the structural arrangement at 295 K have been investigated by high angular resolution synchrotron x-ray diffraction (SXRD), including the orientation of the organic MA units, which are roughly aligned along the c direction, given the acentric nature of the space group. UV-vis spectra unveil a gap of 4.6 eV, which could be useful for ultraviolet detectors.

The Effect of Particle Size on the Optical and Electronic Properties of MgO Nanoparticles

The (band) edge states, fundamental gaps, optical gaps, exciton binding energies and UV-Vis spectra for a series of cuboidal nanoparticles of the prototypical oxide magnesium oxide (MgO), the largest of with has 216 atoms and edges of 1 nm, were predicted using many-body perturbation theory (ev<i>GW</i>-BSE). The evolution of the properties with particle size was explicitly studied. It was found that while the edge states and fundamental gap change with particle size, the optical gap remains essentially fixed for all but the smallest nanoparticles, in line with what was previously observed experimentally. The explanation for these observations is demonstrated to be that while the optical gap is associated with an exciton that is highly localised around the particle’s corner atoms, the edge states, while primarily localised on the magnesium corner atoms ­(electron) and oxygen corner atoms (hole), show significant delocalisation along the edges. The strong localisation of the exciton associated with the optical gap on the corner atoms is argued to also explain why the nanoparticles have a much smaller optical gaps and red-shifted spectra than bulk MgO. Finally, it is discussed how this non-quantum confinement behaviour, where the properties of the nanoparticles arise from surface defects rather than differences in localisation of edge or exciton states, appears typical of alkaline earth oxide nanoparticles, and that the true optical gap of bulk crystals of such materials is also probably the result of surface defects, even if unobservable experimentally. <br>