Stimulated Emission in Vertically Aligned Hexagonal ZnO Microcrystals Synthesized by Magnetron Sputtering Method

This study is devoted to the luminescence and stimulated emission properties of the ZnO hybrid structure, which is vertically aligned microcrystals with the [0001] crystallographic orientation and a pronounced hexagonal shape formed on a continuous layer of micron thickness. These microcrystals are up to 10 µm high and up to 8 µm in diameter and form the main part of the structure’s thickness. The structure was synthesized on the M(101¯0) plane of sapphire using the magnetron sputtering method. Luminescence of the structure, represented only by conventional near-UV and green components under low-intensity continuous photoexcitation, confirms its high structural and optical quality. Under pulsed photoexcitation with relatively high intensity, stimulated emission (SE) was observed from the structure in the near-UV region at room temperature. The threshold power density for SE was 0.1–0.2 MW/cm2. Exceeding the threshold leads to a significant increase in the emission intensity compared to the control film without [0001] microcrystals, also grown on M(101¯0) sapphire. It was assumed that the optical gain is provided by the whispering gallery modes of individual [0001] microcrystals as a result of inelastic exciton–electron scattering, at least at near-threshold excitation intensities

Excitonic Mechanisms of Stimulated Emission in Low-Threshold ZnO Microrod Lasers with Whispering Gallery Modes

Whispering gallery mode (WGM) ZnO microlasers gain attention due to their high Q-factors and ability to provide low-threshold near-UV lasing. However, a detailed understanding of the optical gain mechanisms in such structures has not yet been achieved. In this work, we study the mechanisms of stimulated emission (SE) in hexagonal ZnO microrods, demonstrating high-performance WGM lasing with thresholds down to 10–20 kW/cm2 and Q-factors up to ~3500. The observed SE with a maximum in the range of 3.11–3.17 eV at room temperature exhibits a characteristic redshift upon increasing photoexcitation intensity, which is often attributed to direct recombination in the inverted electron-hole plasma (EHP). We show that the main contribution to room-temperature SE in the microrods studied, at least for near-threshold excitation intensities, is made by inelastic exciton-electron scattering rather than EHP. The shape and perfection of crystals play an important role in the excitation of this emission. At lower temperatures, two competing gain mechanisms take place: exciton-electron scattering and two-phonon assisted exciton recombination. The latter forms emission with a maximum in the region near ~3.17 eV at room temperature without a significant spectral shift, which was observed only from weakly faceted ZnO microcrystals in this study

Synthesis of Superhydrophobic Barium Hexaferrite Coatings with Low Magnetic Hardness

Using the multifunctional material barium hexaferrite as an example, the prospects for treatment at a quasi-equilibrium low temperature in an open atmosphere to form superhydrophobic magnetic coatings with pronounced crystalline and magnetic anisotropy have been demonstrated for the first time. The relationship between plasma treatment conditions, structural-phase composition, morphology, and superhydrophobic properties of (0001) films of barium hexaferrite BaFe12O19 on C-sapphire is studied. X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), as well as magnetometry and moisture resistance analysis, were used as research methods. During plasma treatment with a mass-average temperature of 8–10 kK, intense evaporation and surface melting were observed, and texturing of the deposit along (0001) is found. When the treatment temperature was reduced to 4–5 kK, the evaporation of the material was minimized and magnetic and crystal anisotropy increased. However, the increase in the size of crystallites was accompanied by the transition of oxygen atoms from lattice nodes to interstitial positions. All samples exhibited low coercive fields below 500 Oe, associated with the frustration of the magnetic subsystem. Features of growth of materials with a wurtzite structure were used to form a superhydrophobic coating of barium hexaferrite. Plasma treatment regimes for obtaining self-cleaning coatings are proposed. The use of magnetically hard barium hexaferrite to radically change the properties of a coating is demonstrated herein as an example

Interference Phenomena and Stimulated Emission in ZnO Films on Sapphire

We studied the texturing, roughness, and morphology features of ZnO films grown on the R (11¯02)-, M (101¯0)-, A (112¯0)-, and C (0001)-planes of sapphire, as well as their optical and luminescent properties. We showed that the growth conditions, substrate orientation, and the presence of a buffer layer significantly affected the structure and morphology of the growing films, which was reflected in their optical and radiative properties. In particular, films grown on the A- and M- planes of sapphire showed the highest UV radiation brightness values and exhibited stimulated emissions upon pulsed photoexcitation. The dependence of the topography of the film surface on the substrate orientation allowed the formation of a smooth continuous film with pronounced interference properties using the R- and M- planes of sapphire. A change in the crystallographic orientation, as well as a significant enhancement in crystallinity and luminescence, were observed for ZnO films grown on R-plane sapphire substrates with a gold buffer layer as compared to films grown on bare substrates. At the same time, the use of gold facilitates a significant smoothing of the film’s surface, retaining its interference properties. The sensitivity of interference and laser properties to changes in the external environment, as well as the ease of fabrication of such structures, create prospects for their application as key elements of optical converters, chemical and biological sensors, and sources of coherent radiation

Stimulated Emission in Vertically Aligned Hexagonal ZnO Microcrystals Synthesized by Magnetron Sputtering Method

This study is devoted to the luminescence and stimulated emission properties of the ZnO hybrid structure, which is vertically aligned microcrystals with the [0001] crystallographic orientation and a pronounced hexagonal shape formed on a continuous layer of micron thickness. These microcrystals are up to 10 µm high and up to 8 µm in diameter and form the main part of the structure’s thickness. The structure was synthesized on the M(101¯0) plane of sapphire using the magnetron sputtering method. Luminescence of the structure, represented only by conventional near-UV and green components under low-intensity continuous photoexcitation, confirms its high structural and optical quality. Under pulsed photoexcitation with relatively high intensity, stimulated emission (SE) was observed from the structure in the near-UV region at room temperature. The threshold power density for SE was 0.1–0.2 MW/cm2. Exceeding the threshold leads to a significant increase in the emission intensity compared to the control film without [0001] microcrystals, also grown on M(101¯0) sapphire. It was assumed that the optical gain is provided by the whispering gallery modes of individual [0001] microcrystals as a result of inelastic exciton–electron scattering, at least at near-threshold excitation intensities

Influence of Plasma Treatment Parameters on the Structural-Phase Composition, Hardness, Moisture-Resistance, and Raman-Enhancement Properties of Nitrogen-Containing Titanium Dioxide

The paper shows, for the first time, the prospects of treatment with a quasi-equilibrium low-temperature nitrogen plasma in an open atmosphere for the formation of super-hard, super-hydrophobic TiN/TiO2 composite coatings with pronounced Raman-enhancement properties. X-ray diffractometry (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy, as well as the analysis of hardness and moisture-resistance properties, are used as analytical research methods. During plasma treatment of titanium films on sapphire with a mass average temperature of 4–6 kK, an X-ray amorphous hydrophilic titanium oxide film with a low nitrogen content is formed. The nitrogen content in titanium oxide films increases with increasing treatment temperature up to 6–7 kK. In this case, an X-ray amorphous hydrophobic film is formed. With a further increase in temperature to 7–10 kK, a TiN/TiO2 composite structure based on polycrystalline rutile is formed with increased hydrophobicity and pronounced Raman enhancement properties due to the effective excitation of surface plasmon polaritons. The presence of the crystalline phase increases the dephasing time, which determines the quality of the resonance and the achievable amplification of the electromagnetic field near the TiN inclusions. All treated films on sapphire have a super-hardness above 25 GPa (Vickers hardness test) due to high grain size, the presence of nitrogen-containing inclusions concentrated along grain boundaries, and compressive stresses

Fabrication of Segments for ZnO-Based Tube Ceramic Targets by the Spark Plasma Sintering Method

In this article, problems associated with the fabrication of ZnO-based ceramics in the form of large-diameter hollow cylinders with a large ratio of height h to wall thickness ∆r (h/∆r ≥ 3) by the spark plasma sintering (SPS) method were studied. The design of the press-form is proposed, which ensures the reduction in temperature gradients along the inner and outer surfaces of the hollow cylindrical sintered body and, as a result, the achievement of a high-density uniformity of the sintered body over its volume. A hollow cylindrical ZnO-based ceramic sample considered as segments of TCO tube targets with outer diameter D = 72 mm, inner diameter d = 47 mm, and height h = 36 mm were fabricated. The sample had a relative density of more than 98.5% of the theoretical density for a given composition with a minimal density inhomogeneity along the height of the sample. The microstructure of the obtained ceramics was studied using the SEM and XRD methods

A Porous Nanostructured ZnO Layer for Ultraviolet Sensing with Quartz Crystal Microbalance Technique

Porous films of metals and metal oxides have gained growing attention as potential materials for use in applications that require large, specific surface areas, such as sensors, supercapacitors, and batteries. In this study, a “black-metal”-like porous Zn–ZnO composite layer was grown by room temperature co-sputtering of Zn metal and ZnO:Ga (3 at/%) ceramic targets. Following deposition, a porous ZnO layer was obtained by a subsequent thermal annealing process at 400 °C in air. The morphology and structural properties of the obtained porous layered objects were analyzed. The porosity and chemical characteristics of the nanostructured ZnO layer obtained with the method herein described make it suitable to be used as a sensitivity-enhancing active layered element in quartz crystal microbalance (QCM)-based ultraviolet (UV) sensors. The prepared resonant ZnO/QCM sensors under UV radiation exhibited maximum shift up to 35 Hz for several “on-off” UV cycles, excellent response, and recovery times of 11 and 12 s, respectively

On the Effect of the Co-Introduction of Al and Ga Impurities on the Electrical Performance of Transparent Conductive ZnO-Based Thin Films

In this study, a set of ZnO-based thin films were prepared on glass substrates at various substrate temperatures via the direct current magnetron sputtering of ceramic targets with the following compositions: pure ZnO, Al-doped ZnO with doping levels of 1 and 2 at.%, Ga-doped ZnO with doping levels of 1 and 2 at.%, and (Al, Ga)-co-doped ZnO with doping levels of 1 and 2 at.% for each impurity metal. The dependencies of sheet resistance, carrier concentration, and Hall mobility on the substrate temperature were studied for the deposited films. The results of evaluating the electrical performances of the films were compared with the data of their XRD study. According to the XRD data, among all the deposited ZnO films, the maximum crystallinity was found in the co-doped thin film with doping levels of 2 at.% for each impurity metal, deposited at a substrate temperature of 300 °C. It was revealed that the observed increase in the Hall mobility and carrier concentration for the co-doped films may, in particular, be due to the difference in the preferred localization of Ga and Al impurities in the ZnO film: the Ga ions were mainly incorporated into the crystal lattice of ZnO nanocrystallites, while the Al impurity was mostly localized in the intercrystalline space at the grain boundaries

Morphology and Luminescence of Flexible Free-Standing ZnO/Zn Composite Films Grown by Vapor Transport Synthesis

A method for fabricating flexible free-standing ZnO/Zn composite films from the vapor phase using a regular array of silicon microwhiskers as a substrate is presented. The structural and morphological peculiarities, as well as luminescent properties of the films, were studied. The films have a hybrid structure consisting of two main microlayers. The first layer is formed directly on the tops of Si whiskers and has a thickness up to 10 µm. This layer features a polycrystalline structure and well-developed surface morphology. The second layer, which makes up the front side of the films, is up to 100 µm thick and consists of large microcrystals. The films show good bending strength—in particular, resistance to repeated bending and twisting—which is provided by a zinc metallic part constituting the flexible carrier of the films. ZnO photoluminescence was observed from both surfaces of the films but with conspicuous spectral differences. In particular, a significant weakening of ZnO green luminescence (more than 10 times) at an almost constant intensity of UV near-band edge emission was found for the polycrystalline side of the films as compared to the microcrystalline side. A high degree of homogeneity of the luminescent properties of the films over their area was demonstrated. The results obtained emphasize the relevance of further studies of such ZnO structures—in particular, for application in flexible devices, sensors, photocatalysis and light generation