Deep-Level Emission Tailoring in ZnO Nanostructures Grown via Hydrothermal Synthesis.

Zinc oxide (ZnO) nanostructures are widely used in various fields of science and technology due to their properties and ease of fabrication. To achieve the desired characteristics for subsequent device application, it is necessary to develop growth methods allowing for control over the nanostructures' morphology and crystallinity governing their optical and electronic properties. In this work, we grow ZnO nanostructures via hydrothermal synthesis using surfactants that significantly affect the growth kinetics. Nanostructures with geometry from nanowires to hexapods are obtained and studied with photoluminescence (PL) spectroscopy. Analysis of the photoluminescence spectra demonstrates pronounced exciton on a neutral donor UV emission in all of the samples. Changing the growth medium chemical composition affects the emission characteristics sufficiently. Apart the UV emission, nanostructures synthesized without the surfactants demonstrate deep-level emission in the visible range with a peak near 620 nm. Structures synthesized with the use of sodium citrate exhibit emission peak near 520 nm, and those with polyethylenimine do not exhibit the deep-level emission. Thus, we demonstrate the correlation between the hydrothermal growth conditions and the obtained ZnO nanostructures' optical properties, opening up new possibilities for their precise control and application in nanophotonics, UV-Vis and white light sources.

Flexible Perovskite CsPbBr3 Light Emitting Devices Integrated with GaP Nanowire Arrays in Highly Transparent and Durable Functionalized Silicones.

The architecture of transparent contacts is of utmost importance for creation of efficient flexible light-emitting devices (LEDs) and other deformable electronic devices. We successfully combined the newly synthesized transparent and durable silicone rubbers and the semiconductor materials with original fabrication methods to design LEDs and demonstrate their significant flexibility. We developed electrodes based on a composite GaP nanowire-phenylethyl-functionalized silicone rubber membrane, improved with single-walled carbon nanotube films for a hybrid poly(ethylene oxide)-metal-halide perovskite (CsPbBr3) flexible green LED. The proposed approach provides a novel platform for fabrication of flexible hybrid optoelectronic devices.

XRD Evaluation of Wurtzite Phase in MBE Grown Self-Catalyzed GaP Nanowires.

Control and analysis of the crystal phase in semiconductor nanowires are of high importance due to the new possibilities for strain and band gap engineering for advanced nanoelectronic and nanophotonic devices. In this letter, we report the growth of the self-catalyzed GaP nanowires with a high concentration of wurtzite phase by molecular beam epitaxy on Si (111) and investigate their crystallinity. Varying the growth temperature and V/III flux ratio, we obtained wurtzite polytype segments with thicknesses in the range from several tens to 500 nm, which demonstrates the high potential of the phase bandgap engineering with highly crystalline self-catalyzed phosphide nanowires. The formation of rotational twins and wurtzite polymorph in vertical nanowires was observed through complex approach based on transmission electron microscopy, powder X-ray diffraction, and reciprocal space mapping. The phase composition, volume fraction of the crystalline phases, and wurtzite GaP lattice parameters were analyzed for the nanowires detached from the substrate. It is shown that the wurtzite phase formation occurs only in the vertically-oriented nanowires during vapor-liquid-solid growth, while the wurtzite phase is absent in GaP islands parasitically grown via the vapor-solid mechanism. The proposed approach can be used for the quantitative evaluation of the mean volume fraction of polytypic phase segments in heterostructured nanowires that are highly desirable for the optimization of growth technologies.

Structural and Optical Properties of Self-Catalyzed Axially Heterostructured GaPN/GaP Nanowires Embedded into a Flexible Silicone Membrane.

Controlled growth of heterostructured nanowires and mechanisms of their formation have been actively studied during the last decades due to perspectives of their implementation. Here, we report on the self-catalyzed growth of axially heterostructured GaPN/GaP nanowires on Si(111) by plasma-assisted molecular beam epitaxy. Nanowire composition and structural properties were examined by means of Raman microspectroscopy and transmission electron microscopy. To study the optical properties of the synthesized nanoheterostructures, the nanowire array was embedded into the silicone rubber membrane and further released from the growth substrate. The reported approach allows us to study the nanowire optical properties avoiding the response from the parasitically grown island layer. Photoluminescence and Raman studies reveal different nitrogen content in nanowires and parasitic island layer. The effect is discussed in terms of the difference in vapor solid and vapor liquid solid growth mechanisms. Photoluminescence studies at low temperature (5K) demonstrate the transition to the quasi-direct gap in the nanowires typical for diluted nitrides with low N-content. The bright room temperature photoluminescent response demonstrates the potential application of nanowire/polymer matrix in flexible optoelectronic devices.

Biological Kerker Effect Boosts Light Collection Efficiency in Plants.

Being the polymorphs of calcium carbonate (CaCO3), vaterite and calcite have attracted a great deal of attention as promising biomaterials for drug delivery and tissue engineering applications. Furthermore, they are important biogenic minerals, enabling living organisms to reach specific functions. In nature, vaterite and calcite monocrystals typically form self-assembled polycrystal micro- and nanoparticles, also referred to as spherulites. Here, we demonstrate that alpine plants belonging to the Saxifraga genus can tailor light scattering channels and utilize multipole interference effect to improve light collection efficiency via producing CaCO3 polycrystal nanoparticles on the margins of their leaves. To provide a clear physical background behind this concept, we study optical properties of artificially synthesized vaterite nanospherulites and reveal the phenomenon of directional light scattering. Dark-field spectroscopy measurements are supported by a comprehensive numerical analysis, accounting for the complex microstructure of particles. We demonstrate the appearance of generalized Kerker condition, where several higher order multipoles interfere constructively in the forward direction, governing the interaction phenomenon. As a result, highly directive forward light scattering from vaterite nanospherulites is observed in the entire visible range. Furthermore, ex vivo studies of microstructure and optical properties of leaves for the alpine plants Saxifraga "Southside Seedling" and Saxifraga Paniculata Ria are performed and underline the importance of the Kerker effect for these living organisms. Our results pave the way for a bioinspired strategy of efficient light collection by self-assembled polycrystal CaCO3 nanoparticles via tailoring light propagation directly to the photosynthetic tissue with minimal losses to undesired scattering channels.