Photothermal response of plasmonic nanofillers for membrane distillation.

Light-to-heat conversion in plasmonic nanoparticles (NPs) inside polymeric membranes is beneficial for improving the efficiency of membrane distillation for seawater desalination. However, the physical mechanisms ruling photothermal membrane distillation are unclear yet. Here, we model the plasmonic photothermal light-to-heat conversion from Ag, Au, and Cu nanofillers in polymeric membranes for membrane distillation. Photothermal effects in the cases of isolated metallic NPs and their assembly are investigated considering size effects and excitation sources. The increasing content of metallic NPs improves the efficiency of the light-to-heat conversion. For a polymeric membrane, filled with 25% Ag NPs, our model well reproduces the experimental temperature increase of 10 K. Specifically, we find that Ag NPs with a radius of around 30-40 nm are favorite candidates for membrane heating with excitation energy in the visible/near-UV range. The incorporation of a term associated with heat losses into the heat transfer equation well reproduces the cooling effect associated with vaporization at the membrane surface. Compared to Ag NPs, Au and Cu NPs show a broadened absorption cross section and their resonance has a nonlinear behavior with varying the excitation energy, better matching with sunlight radiation spectrum.

Excitonic complexes in GaN/(Al,Ga)N quantum dots.

Here we report a theoretical investigation of excitonic complexes in polar GaN/(Al,Ga)N quantum dots (QDs). A sum rule between the binding energies of charged excitons is used to calculate the biexciton binding energy. The binding energies of excitonic complexes in GaN/AlN are shown to be strongly correlated to the QD size. Due to the large hole localization, the positively charged exciton energy is found to be always blueshifted compared to the exciton one. The negatively charged exciton and the biexciton energy can be blueshifted or redshifted according to the QD size. Increasing the size of GaN/AlN QDs makes the identification of charged excitons difficult, and the use of an Al0.5Ga0.5N barrier can be advantageous for clear identification. Our theoretical results for the binding energy of exciton complexes are also confronted with values deduced experimentally for InAs/GaAs QDs, confirming our theoretical prediction for charged excitonic complexes in GaN/(Al,Ga)N QDs. Finally, we realize that the trends of excitonic complexes in QDs are significantly related to competition between the local charge separation (whatever its origin) and the correlation effect. Following our findings, entangled photons pairs can be produced in QDs with careful control of their size in order to obtain zero exciton-biexciton energy separation.