Enhancing GaN Nanowires Performance Through Partial Coverage with Oxide Shells.

Core-shell gallium nitride (GaN)-based nanowires offer noteworthy opportunities for innovation in high-frequency opto- and microelectronics. This work delves deeply into the physical properties of crystalline GaN nanowires with aluminum and hafnium oxide shells. Particular attention is paid to partial coverage of nanowires, resulting with exceptional properties. First, the crystal lattice relaxation is observed by X-ray diffraction, photoluminescence, and Raman spectroscopy measurements. A high potential of partial coverage for optoelectronic applications is revealed with photo- and cathodoluminescence spectra along with an exploration of their temperature dependency. Next, the study focuses on understanding the mechanisms behind the observed enhancement of the luminescence efficiency. It is confirmed that nanowires are effectively protected against photoadsorption using partial coatings. This research advances the frontiers of nanotechnology, investigating the benefits of partial coverage, and shedding light on its complex interaction with cores.

Correction to "Synthesis Attempt and Structural Studies of Novel A2CeWO6 Double Perovskites (A2+ = Ba, Ca) in and outside of Ambient Conditions".

[This corrects the article DOI: 10.1021/acsomega.2c00669.].

Role of Metallic Adlayer in Limiting Ge Incorporation into GaN.

Atomically thin metal adlayers are used as surfactants in semiconductor crystal growth. The role of the adlayer in the incorporation of dopants in GaN is completely unexplored, probably because n-type doping of GaN with Si is relatively straightforward and can be scaled up with available Si atomic flux in a wide range of dopant concentrations. However, a surprisingly different behavior of the Ge dopant is observed, and the presence of atomically thin gallium or an indium layer dramatically affects Ge incorporation, hindering the fabrication of GaN:Ge structures with abrupt doping profiles. Here, we show an experimental study presenting a striking improvement in sharpness of the Ge doping profile obtained for indium as compared to the gallium surfactant layer during GaN-plasma-assisted molecular beam epitaxy. We show that the atomically thin indium surfactant layer promotes the incorporation of Ge in contrast to the gallium surfactant layer, which promotes segregation of Ge to the surface and Ge crystallite formation. Understanding the role of the surfactant is essential to control GaN doping and to obtain extremely high n-type doped III-nitride layers using Ge, because doping levels >1020 cm−3 are not easily available with Si.

Synthesis Attempt and Structural Studies of Novel A2CeWO6 Double Perovskites (A2+ = Ba, Ca) in and outside of Ambient Conditions.

This comprehensive work showcases two novel, rock-salt-type minerals in the form of amphoteric cerium-tungstate double perovskite and ilmenite powders created via a high-temperature solid-state reaction in inert gases. The presented studies have fundamental meaning and will mainly focus on a detailed synthesis description of undoped structures, researching their possible polymorphism in various conditions and hinting at some nontrivial physicochemical properties like charge transfer for upcoming optical studies after eventual doping with selectively chosen rare-earth ions. The formerly mentioned, targeted A2BB'X6 group of compounds contains mainly divalent alkali cations in the form of XIIA = Ba2+, Ca2+ sharing, here, oxygen-arranged clusters (IIX = O2-) with purposely selected central ions from f-block VIB = Ce4/3+ and d-block VIB' = W4/5/6+ since together they often possess some exotic properties that could be tuned and implemented into futuristic equipment like sensors or energy converters. Techniques like powder XRD, XPS, XAS, EPR, Raman, and FTIR spectroscopies alongside DSC and TG were involved with an intent to thoroughly describe any possible changes within these materials. Mainly, to have a full prospect of any desirable or undesirable phenomena before diving into more complicated subjects like: energy or charge transfer in low temperatures; to reveal whether or not the huge angular tilting generates large enough dislocations within the material's unit cell to change its initial properties; or if temperature and pressure stimuli are responsible for any phase transitions and eventual, irreversible decomposition.

Optical and structural properties of europium doped Y-Al-O compounds grown by microwave driven hydrothermal technique.

Perovskites, garnets, monoclinic forms, and lately also oxyhydroxides doped with rare-earth ions have been drawn large attention due to their beneficial optical and photovoltaic properties. In this work, we have shown that several forms of crystals from Y-Al-O family can be synthesized using microwave driven hydrothermal technique using different pH and post-growth annealing at different temperatures. The structural and optical properties of these crystals were investigated as a function of hydrothermal crystallization conditions. For this purpose, x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, transmission electron microscopy, photoluminescence, and photoluminescence excitation studies were performed. All the structures have been doped with Eu3+ions which are known as a local symmetry sensor because various symmetries generate different crystal fields and thus affect their luminescence spectra. The optical properties of the obtained nanoparticles in correlation with their structure and chemical composition are discussed.

Near-infrared emission from spatially indirect excitons in type II ZnTe/CdSe/(Zn,Mg)Te core/double-shell nanowires.

ZnTe/CdSe/(Zn, Mg)Te core/double-shell nanowires are grown by molecular beam epitaxy by employing the vapor-liquid-solid growth mechanism assisted with gold catalysts. A photoluminescence study of these structures reveals the presence of an optical emission in the near infrared. We assign this emission to the spatially indirect exciton recombination at the ZnTe/CdSe type II interface. This conclusion is confirmed by the observation of a significant blue-shift of the emission energy with an increasing excitation fluence induced by the electron-hole separation at the interface. Cathodoluminescence measurements reveal that the optical emission in the near infrared originates from nanowires and not from two-dimensional residual deposits between them. Moreover, it is demonstrated that the emission energy in the near infrared depends on the average CdSe shell thickness and the average Mg concentration within the (Zn, Mg)Te shell. The main mechanism responsible for these changes is associated with the strain induced by the (Zn, Mg)Te shell in the entire core/shell nanowire heterostructure.

GaN Nanowire Array for Charge Transfer in Hybrid GaN/P3HT:PC71BM Photovoltaic Heterostructure Fabricated on Silicon.

We demonstrate that a GaN nanowire array can be used for efficient charge transfer between the organic photovoltaic layer and silicon in a Si/GaN/P3HT:PC71BM inverted hybrid heterostructure. The band alignment of such a material combination is favorable to facilitate exciton dissociation, carrier separation and electron transport into Si. The ordered nature of the GaN array helps to mitigate the intrinsic performance limitations of the organic active layer. The dependence of photovoltaic performance enhancement on the morphology of the nanostructure with nanowire diameters 30, 50, 60, 100 and 150 nm was studied in detail. The short circuit current was enhanced by a factor of 4.25, while an open circuit voltage increase by 0.32 volts was achieved compared to similar planar layers.

Indium Incorporation into InGaN Quantum Wells Grown on GaN Narrow Stripes.

InGaN quantum wells were grown using metalorganic chemical vapor phase epitaxy (vertical and horizontal types of reactors) on stripes made on GaN substrate. The stripe width was 5, 10, 20, 50, and 100 µm and their height was 4 and 1 µm. InGaN wells grown on stripes made in the direction perpendicular to the off-cut had a rough morphology and, therefore, this azimuth of stripes was not further explored. InGaN wells grown on the stripes made in the direction parallel to the GaN substrate off-cut had a step-flow-like morphology. For these samples (grown at low temperatures), we found out that the InGaN growth rate was higher for the narrower stripes. The higher growth rate induces a higher indium incorporation and a longer wavelength emission in photoluminescence measurements. This phenomenon is very clear for the 4 µm high stripes and less pronounced for the shallower 1 µm high stripes. The dependence of the emission wavelength on the stripe width paves a way to multicolor emitters.

Growth and optical properties of ZnO/Zn1-xMgxO quantum wells on ZnO microrods.

While synthesis methods for pure ZnO nanostructures are well established, an efficient technique for the growth of ZnO-based nanowires or microrods that incorporate any type of quantum structure is yet to be established. Here, we report on the fabrication and optical properties of axial Zn1-xMgxO/ZnO/Zn1-xMgxO quantum wells that were deposited by molecular beam epitaxy on ZnO microrods obtained using a hydrothermal method. Using the emission energy results found in cathodoluminescence measurements and the results of a numerical modeling process, we found the quantum well width to be 4 nm, as intended, at the growth stage. The emission of quantum well-confined excitons persists up to room temperature. We used the fabricated structures to determine the carrier diffusion length (>280 nm) in ZnO using spatially resolved cathodoluminescence. The micro-photoluminescence results suggest an increase in the electron-phonon coupling strength with increasing microrod size.

Defect-free SnTe topological crystalline insulator nanowires grown by molecular beam epitaxy on graphene.

SnTe topological crystalline insulator nanowires have been grown by molecular beam epitaxy on graphene/SiC substrates. The nanowires have a cubic rock-salt structure, they grow along the [001] crystallographic direction and have four sidewalls consisting of {100} crystal planes known to host metallic surface states with a Dirac dispersion. Thorough high resolution transmission electron microscopy investigations show that the nanowires grow on graphene in the van der Waals epitaxy mode induced when the catalyzing Au nanoparticles mix with Sn delivered from a SnTe flux, providing a liquid Au-Sn alloy. The nanowires are totally free from structural defects, but their {001} sidewalls are prone to oxidation, which points out the necessity of depositing a protective capping layer in view of exploiting the magneto-electric transport phenomena involving charge carriers occupying topologically protected states.

Facile synthesis of core/shell ZnO/ZnS nanofibers by electrospinning and gas-phase sulfidation for biosensor applications.

This study describes a new method of passivating ZnO nanofiber-based devices with a ZnS layer. This one-step process was carried out in H2S gas at room temperature, and resulted in the formation of core/shell ZnO/ZnS nanofibers. This study presents the structural, optical and electrical properties of ZnO/ZnS nanofibers formed by a 2 nm ZnS sphalerite crystal shell covering a 5 nm ZnO wurtzite crystal core. The passivation process prevented free carriers from capture by oxygen molecules and significantly reduced the impact of O2 on nanostructure conductivity. The conductivity of the nanofibers was increased by three orders of magnitude after the sulfidation, the photoresponse time was reduced from 1500 s to 30 s, and the cathodoluminescence intensity increased with the sulfidation time thanks to the removal of ZnO surface defects by passivation. The ZnO/ZnS nanofibers were stable in water for over 30 days, and in phosphate buffers of acidic, neutral and alkaline pH for over 3 days. The by-products of the passivation process did not affect the conductivity of the devices. The potential of ZnO/ZnS nanofibers for protein biosensing is demonstrated using biotin and streptavidin as a model system. The presented ZnS shell preparation method can facilitate the construction of future sensors and protects the ZnO surface from dissolving in a biological environment.

Low-Temperature Cathodoluminescence Investigations of High-Quality Zinc Oxide Nanorods.

We present results of cathodoluminescence (CL) investigations of high-quality zinc oxide (ZnO) nanorods obtained by an extremely fast hydrothermal method on a silicon substrate. A scanning electron microscopy (SEM) system equipped with CL allows direct comparison of SEM images and CL maps, taken from exactly the same areas of samples. Investigations are performed at a temperature of 5 K. An interlink between sample microstructure and emission properties is investigated. CL confirms a very high quality of ZnO nanorods produced by our method. In addition, the presence of super radiation effects in ZnO nanorod arrays is suggested.

Pressure control of magnetic clusters in strongly inhomogeneous ferromagnetic chalcopyrites.

Room-temperature ferromagnetism in Mn-doped chalcopyrites is a desire aspect when applying those materials to spin electronics. However, dominance of high Curie-temperatures due to cluster formation or inhomogeneities limited their consideration. Here we report how an external perturbation such as applied hydrostatic pressure in CdGeP2:Mn induces a two serial magnetic transitions from ferromagnet to non-magnet state at room temperature. This effect is related to the unconventional properties of created MnP magnetic clusters within the host material. Such behavior is also discussed in connection with ab initio density functional calculations, where the structural properties of MnP indicate magnetic transitions as function of pressure as observed experimentally. Our results point out new ways to obtain controlled response of embedded magnetic clusters.

The mechanism of Bi nanowire growth from Bi/Co immiscible composite thin films.

Single crystalline Bi nanowires were grown by extrusion from Bi/Co thin films. The films were obtained by thermal evaporation in high vacuum. The average diameter, length and density of obtained nanowires were 100 nm, 30 microm and 6.5 x 10(5) cm(-2), respectively. The non-catalyzed self-organized process of whisker formation on the surface of immiscible composite thin film was exploited for nanowire growth. It was shown that the whiskers had formed during and after a thin film deposition. The value of residual stresses in a whole thin film coating as well as in its bismuth component was measured using X-ray diffraction technique. It was revealed that local compressive stresses, that had induced the whisker growth, had been formed by a segregation of Bi layers into Bi globules. A simple model of the whisker formation to minimize free energy in the Bi/Co system was proposed taking into account interfacial and elastic deformation energies. The obtained results can be utilized for growing of nanowires of other low-melting-point metals and semiconductors from immiscible composite thin films.