Optical Properties Of Two-dimensional Superlattices: A First-principles Study

Graphene has attracted numerous interest since its successful synthesis due to its remarkable properties and vast applications, To further expand its application, finite band gap at room temperature is needed. Graphane(GNR’s) and monolayer boron nitride(BN) sheet are typical semiconductor, moreover, they show similar lattices parameters and crystal structure to graphene. The hybrid systems consisting of these materials such as graphene/graphane superlattices(GG’SLs) and graphene/BN superlattices(GBNSLs), provide efficient approach to modulate the optical and electronic properties of pristine graphene, These SLs has attracted lots of interest due to their novel electronic properties and potential in nano-technology. Optical anisotropy of GG’SLs and GBNSLs with zigzag interface and its dependence on the size of the superlattices are investigated with ?rst-principles density functional calculations. We get the main conclusions as follows:(1) The results of the Gibbs free energy indicate that the Gibbs free energy decreases as the width of SLs increase, negative Gibbs free energy indicating their stability. When we ?x the total width of GG’SLs with 14 zigzag atom chains, the increase of the proportion of graphane induces further decrease of the Gibbs free energy indicating that more wide of graphane segment more stable of the systems. For all GG’SLs, we calculate their dielectric function. The results indicate that in the high-energy region there are two prominent peaks around 7.0 e V and 13.0 e V for E//Z corresponding to σ-π* transition, there is one prominent peak around 11.0 e V as for E//X and E//Y corresponding to σ-σ* transition, all peaks have red-shift along with the increase in width. In low-energy region, the optical properties of the GG’SLs below 6 e V are transparent for E//Z, all SLs display considerable optical anisotropy for E//X and E//Y when the incident light is perpendicularly incident to the plane of GG’SLs. Using the calculated imaginary part of the dielectric function, a unit incident photon energy, and classical electromagnetic wave theory, we calculate the optical constants of all GG’SLs, absorptivity, reflectivity, transmissivity, reflection polarization, and transmission polarization. The results indicate that the adsorption in the low energy region shows two step adsorption characteristics that E//X direction ?rstly appears and followed by the absorption in E//Y direction. However, the absorptivity for both direction are very low(less than 3%) for all systems. The re?ectivity is very low for all SLs which is around 1% according to our calculations. However, interestingly, the re?ection polarizability is more than 80% and even reaches 100%.(2) We calculated the Gibbs free energy, band structures and partial density of states(PDOS) of GBNSLs. From the Gibbs free energy we find that except for 1Gr-1BN and 1Gr-1BN-1Gr-1NB whose Gibbs free energy is relatively large, the Gibbs free energy decreases as the width of SLs increase CBNSLs without inversion symmetry is more stable than their counterpart. For all CBNSLs, we calculate their dielectric function, The anisotropy is not only shown between the imaginary part of dielectric function under E//X(E//Y) and E//Z but also shown between the under E//X and E//Y. When the width of the systems is even or odd, the optical properties for i Gr-j BN-i Gr-j NB are different. Two prominent peaks appear for E//Z corresponds to σ-π* transition and one peak as for E//X and E//Y, corresponds toσ-σ* transition. In lower energy region, all the peaks are derived fromπ-π* transition. When the incident light is perpendicular to the CBNSLs, the adsorption for E//X and E//Y is coexistence. The anisotropy of E//X and E//Y might induce polarization of the light of reflectivity and transmissivity. When incident light is linearly polarized with a unit photon energy, the adsorption in X direction ?rstly appears and followed by the absorption in Y direction, especially, the width of the systems is odd. The reflectivity is also very low for all SLs according to our calculations, but interestingly the reflection polarization is more than 80%, even reaches 100%. The results indicate that high polarized re?ection of CBNSLs is hoping to be excellent polarized ultraweak photon emission source.