Crystal Facet Engineering in Ga-Doped ZnO Nanowires for Mid-Infrared Plasmonics

The metal–organic chemical vapor deposition growth of various Ga-doped ZnO nanostructures for plasmonics is investigated, with a particular focus on the nanowire facet transformations induced by the addition of trimethylgallium in the gas phase. For nonintentionally doped spontaneous ZnO nanowires, the aspect ratio is strongly decreased due to residual Ga in the reactor, and the shape evolves rapidly toward Christmas-tree-like and hierarchical structures upon intentional Ga doping. Regarding ZnO/ZnO:Ga core–shell structures, a change of the smooth initial M-oriented facets occurs, with the development of {202̅1} surfaces, and further {101̅1} and {0001} surfaces. Interestingly, a similar evolution of the lateral roughness is observed in Au-catalyzed doped nanowires. High concentrations of Ga in the grown nanostructures are revealed by photoluminescence and confirmed by Rutherford backscattering spectrometry. First photoacoustic measurements show an optical absorption at 6 μm, evidencing that the degenerated material is suitable for plasmonics applications in the infrared range. The influence of Ga doping on the facet transformations and the occurrence of unexpected {0001} polar surfaces are discussed. The results can be mainly understood by a Ga surfactant effect (at least partial) responsible for the modification of the surface energies and kinetics. Density functional calculations support the floating behavior of the negatively charged Ga^– ion on the growing surface

Efficient Pump Photon Recycling via Gain-Assisted Waveguiding Energy Transfer

We propose a new concept for enhancing the fluorescence of ultrathin nanolayers. In this article, we address the issue of efficient absorption of polymer thin films with nanometer characteristics. For many applications, such as sensing, but also for lighting or photovoltaics, devices require the use of nanometer-sized films of a specific polymer or a luminescent nanolayer in general. Usually, most studies are geared toward enhancing the emission of such luminescent films via Bragg mirror-type cavities, for instance, but little attention is paid for optimizing the absorption of the thin films. We show the principle of gain-assisted waveguiding energy transfer (G-WET) by inserting a gain-active layer between an active nanometer-scale layer (a luminescent polymer in our case) and the passive substrate. Efficient absorption via “recycling” of the pumping photons is ensured by the waveguiding effect due to this high-index active layer. To demonstrate the G-WET effect, two kinds of samples were studied. They consist of extremely thin (∼10 nm) polymer nanolayers spin-coated either on quartz, referred as the passive case, or on a ZnO active thin film (∼170 nm, acting as a gain medium) grown on sapphire, referred as the active case. Samples were characterized by room-temperature photoluminescence (PL) spectroscopy under various pumping intensities. Compared to the quartz substrate, the ZnO thin film induces a remarkable enhancement of a factor ∼8 on the fluorescence of the polymer nanolayer. Observations show that, for the passive quartz substrate case, the PL of the spin-coated polymer rapidly saturates, defining a luminescence limit; whereas, with the active ZnO layer, the polymer presents a nonlinear PL intensity surpassing the saturation level. This new photonic system revealed that the polymer luminescence enhancement is the result of both an efficient energy transfer and a geometrical effect ensured by an evanescent coupling of the waveguided ZnO stimulated emission. Although our work discusses the specific organic–inorganic case of fluorescent polymer and ZnO, the G-WET concept can be generalized to any hybrid layered sample verifying the necessary energy transfer conditions discussed in this article, thus, demonstrating that this is of a special interest for efficient absorption and efficient recycling of the excitation photons for any nanometer scale fluorescent layer