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Given the recent increase in the demand for gallium nitride (GaN) in different markets like optoelectronics and power devices, the request for epitaxially grown GaN will further increase. To meet this high demand, higher throughput and more economical manufacturing technologies must be advanced. In this work, GaN thin films are deposited by reactive sputter deposition from a liquid gallium target at a substrate temperature of 900 °C. The layers are grown epitaxially on c-plane oriented sapphire in an industrial-scale sputter tool from Evatec AG. Due to the growth rate of >1 nm/s and the fast substrate heat-up time, the throughput in a production setup can be increased compared to other GaN growth techniques. The resistivity of the intrinsic insulating GaN can be lowered by intentional Si doping during the sputter deposition process by three orders of magnitude. Thereby, conductive n-type GaN can be grown with different dopant amounts. The carrier mobility of the sputter deposited film is 45 cm2 V−1 s−1 at a carrier concentration of 1.1 × 1020 cm−3 based on room temperature Hall measurements using a van der Pauw geometry. The lowest resistivity reaches 1300 μΩ cm, which is confirmed by sheet resistance measurements. Undoped films exhibit an x-ray diffraction rocking curve full width at half maximum of 0.2°, which increases up to 0.5° for highly Si-doped layers. The presented results show that GaN prepared by reactive sputter deposition from a liquid gallium source is a viable alternative to conventional deposition techniques for GaN.

The photon upconversion based on triplet–triplet annihilation (TTA) is a mechanism that converts the absorbed low-energy electromagnetic radiation into higher energy photons also at extremely low excitation intensities, but its use in actual technologies is still hindered by the limited availability of efficient annihilator moieties. We present here the results obtained by the synthesis and application of two new fluorinated chromophores based on phenazine and acridine structures, respectively. Both compounds show upconverted emission demonstrating their ability as TTA annihilator. More interesting, the acridine-based chromophore shows an excellent TTA yield that overcomes the one of some of best model systems. By correlating the experimental data and the quantum mechanical modeling of the investigated compound, we propose an alternative efficient pathway for the generation of the upconverted emissive states involving the peculiar high-energy triplet levels of the dye, thus suggesting a new development strategy for TTA annihilators based on the fine tuning of their high-energy excited states properties.Graphical abstract

The conversion of low-energy light into photons of higher energy based on sensitized triplet-triplet annihilation (sTTA) upconversion is emerging as the most promising wavelength-shifting methodology because it operates efficiently at excitation powers as low as the solar irradiance. However, the production of solid-state upconverters suited for direct integration in devices is still an ongoing challenge owing to the difficulties concerning the organization of two complementary moieties, the triplet sensitizer, and the annihilator, which must interact efficiently. This problem is solved by fabricating porous fluorescent nanoparticles wherein the emitters are integrated into robust covalent architectures. These emitting porous aromatic framework (ePAF) nanoparticles allow intimate interaction with the included metallo-porphyrin as triplet sensitizers. Remarkably, the high concentration of framed chromophores ensures hopping-mediated triplet diffusion required for TTA, yet the low density of the framework promotes their high optical features without quenching effects, typical of the solid state. A green-to-blue photon upconversion yield as high as 15% is achieved: a record performance among annihilators in a condensed phase. Furthermore, the engineered ePAF architecture containing covalently linked sensitizers produces full-fledge solid-state bicomponent particles that behave as autonomous nanodevices.

Long-life excimer-like structures Metal quantum clusters have ideal properties for medical applications such as imaging. The challenge is to prolong their transient properties for the fabrication of useful devices. Santiago-Gonzalez et al. arranged gold clusters in a supramolecular lattice held together by hydrogen bonding and showed that this material can be used for imaging of fibroblast cells. In the superstructure, the gold molecules can come together in the excited state as excimers and then dissociate to emit radiation. Because they are within a lattice, this behavior shows long-term stability. Furthermore, the lattice superstructure scavenges reactive oxygen species and reduces cell damage. Science , this issue p. 571

Lead halide perovskite nanocrystals (NCs) are emerging as optically active materials for solution-processed optoelectronic devices. Despite the technological relevance of tracing rational guidelines for optimizing their performances and stability beyond their intrinsic resilience to structural imperfections, no in-depth study of the role of selective carrier trapping and environmental conditions on their exciton dynamics has been reported to date. Here we conduct spectro-electrochemical (SEC) experiments, side-by-side to oxygen sensing measurements on CsPbBr3 NCs for the first time. We show that the application of EC potentials controls the emission intensity by altering the occupancy of defect states without degrading the NCs. Reductive potentials lead to strong (60%) emission quenching by trapping of photogenerated holes, whereas the concomitant suppression of electron trapping is nearly inconsequential to the emission efficiency. Consistently, oxidizing conditions result in minor (5%) brightening due to suppressed hole trapping, confirming that electron traps play a minor role in nonradiative decay. This behavior is rationalized through a model that links the occupancy of trap sites with the position of the NC Fermi level controlled by the EC potential. Photoluminescence measurements in controlled atmosphere reveal strong quenching by collisional interactions with O2, which is in contrast to the photobrightening effect observed in films and single crystals. This indicates that O2 acts as a scavenger of photoexcited electrons without mediation by structural defects and, together with the asymmetrical SEC response, suggests that electron-rich defects are likely less abundant in nanostructured perovskites than in the bulk, leading to an emission response dominated by direct interaction with the environment.

We investigate light-matter interactions in periodic silicon microcrystals fabricated combining top-down and bottom-up strategies. The morphology of the microcrystals, their periodic arrangement, and their high refractive index allow the exploration of photonic effects in microstructured architectures. We observe a notable decrease in reflectivity above the silicon bandgap from the ultraviolet to the near-infrared. Finite-difference time-domain simulations show that this phenomenon is accompanied by a ∼2-fold absorption enhancement with respect to a flat sample. Finally, we demonstrate that ordered silicon microstructures enable a fine tuning of the light absorption by changing experimentally accessible knobs as pattern and growth parameters. This work will facilitate the implementation of optoelectronic devices based on high-density microcrystals arrays with optimized light-matter interactions.