Controllable synthesis and morphology-dependent light emission efficiency of Zn2GeO4 nanophosphors.

Zn2GeO4 is considered a very promising alternative to current luminescent semiconductors. Previous results suggest that its emitted wavelength may depend on different variables, such as particle size and morphology, among others. In this work, we have prepared pure and highly homogeneous Zn2GeO4 nanorods under hydrothermal synthesis conditions with a willemite-like structure. Their luminescent properties have been explored and their band gap is estimated, which are distinct from those of previously reported Zn2GeO4 bulk particles. Therefore, our results identify particle morphology as a crucial factor for maximizing and fine-tuning the luminescence of Zn2GeO4 nano-phosphors.

Temperature-Dependent and Time-Resolved Luminescence Characterization of γ-Ga2O3 Nanoparticles.

The temperature-dependent luminescence properties of γ-Ga2O3 nanoparticles prepared by a precipitation method are investigated under steady-state and pulsed-light excitation. The main photoluminescence (PL) emission at room temperature consists of a single blue band centered around 2.76 eV, which hardly undergoes a blueshift of 0.03 eV when temperature goes down to 4 K. The emission behaves with a positive thermal quenching following an Arrhenius-type curve. The data fitting yields two non-radiative levels affecting the emission band with activation energies of 7 meV and 40 meV. On the other hand, time-resolved PL measurements have also been taken and studied as a function of the temperature. The data analysis has resulted in two lifetimes: one of 3.4 ns and the other of 32 ns at room temperature, which undergo an increase up to 4.5 ns and 65 ns at T = 4 K, respectively. Based on both stationary and dynamic PL results, a model of radiative and non-radiative levels associated with the main emission bands of γ-Ga2O3 is suggested. Finally, by using PL excitation measurements, an estimation of the bandgap and its variation with temperature between 4 K and room temperature were obtained and assessed against O'Donnell-Chen's law. With this variation it has been possible to calculate the average of the phonon energy, resulting in ⟨hω⟩ = 10 ± 1 meV.

Temperature-Dependent Anisotropic Refractive Index in ß-Ga2O3: Application in Interferometric Thermometers.

An accurate knowledge of the optical properties of ß-Ga2O3 is key to developing the full potential of this oxide for photonics applications. In particular, the dependence of these properties on temperature is still being studied. Optical micro- and nanocavities are promising for a wide range of applications. They can be created within microwires and nanowires via distributed Bragg reflectors (DBR), i.e., periodic patterns of the refractive index in dielectric materials, acting as tunable mirrors. In this work, the effect of temperature on the anisotropic refractive index of ß-Ga2O3n(λ,T) was analyzed with ellipsometry in a bulk crystal, and temperature-dependent dispersion relations were obtained, with them being fitted to Sellmeier formalism in the visible range. Micro-photoluminescence (µ-PL) spectroscopy of microcavities that developed within Cr-doped ß-Ga2O3 nanowires shows the characteristic thermal shift of red-infrared Fabry-Perot optical resonances when excited with different laser powers. The origin of this shift is mainly related to the variation in the temperature of the refractive index. A comparison of these two experimental results was performed by finite-difference time-domain (FDTD) simulations, considering the exact morphology of the wires and the temperature-dependent, anisotropic refractive index. The shifts caused by temperature variations observed by µ-PL are similar, though slightly larger than those obtained with FDTD when implementing the n(λ,T) obtained with ellipsometry. The thermo-optic coefficient was calculated.

Room Temperature Polymorphism in WO3 Produced by Resistive Heating of W Wires.

Polymorphous WO3 micro- and nanostructures have been synthesized by the controlled Joule heating of tungsten wires under ambient conditions in a few seconds. The growth on the wire surface is assisted by the electromigration process and it is further enhanced by the application of an external electric field through a pair of biased parallel copper plates. In this case, a high amount of WO3 material is also deposited on the copper electrodes, consisting of a few cm2 area. The temperature measurements of the W wire agrees with the values calculated by a finite element model, which has allowed us to establish the threshold density current to trigger the WO3 growth. The structural characterization of the produced microstructures accounts for the γ-WO3 (monoclinic I), which is the common stable phase at room temperature, along with low temperature phases, known as δ-WO3 (triclinic) on structures formed on the wire surface and ϵ-WO3 (monoclinic II) on material deposited on external electrodes. These phases allow for a high oxygen vacancies concentration, which is interesting in photocatalysis and sensing applications. The results could help to design experiments to produce oxide nanomaterials from other metal wires by this resistive heating method with scaling-up potential.

Nanostructural Characterization of Luminescent Polyvinyl Alcohol/Graphene Quantum Dots Nanocomposite Films.

This study focuses on the fabrication of polymer nanocomposite films using polyvinyl alcohol (PVA)/graphene quantum dots (GQDs). We investigate the relationship between the structural, thermal, and nanoscale morphological properties of these films and their photoluminescent response. Although according to X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and differential thermal analysis (DTA), the incorporation of GQDs does not significantly affect the percentage crystallinity of the PVA matrix, for a range of added GQD concentrations, atomic force microscopy (AFM) showed the formation of islands with apparent crystalline morphology on the surface of the PVA/GQD films. This observation suggests that GQDs presumably act as nucleating agents for island growth. The incorporation of GQDs also led to the formation of characteristic surface pores with increased stiffness and frictional contrast, as indicated by ultrasonic force microscopy (UFM) and frictional force microscopy (FFM) data. The photoluminescence (PL) spectra of the films were found to depend both on the amount of GQDs incorporated and on the film morphology. For GQD loads >1.2%wt, a GQD-related band was observed at ~1650 cm-1 in FT-IR, along with an increase in the PL band at lower energy. For a load of ~2%wt GQDs, the surface morphology was characterized by extended cluster aggregates with lower stiffness and friction than the surrounding matrix, and the PL signal decreased.

Contactless doping characterization of [Formula: see text] using acceptor Cd probes.

Finding suitable p-type dopants, as well as reliable doping and characterization methods for the emerging wide bandgap semiconductor [Formula: see text]-[Formula: see text] could strongly influence and contribute to the development of the next generation of power electronics. In this work, we combine easily accessible ion implantation, diffusion and nuclear transmutation methods to properly incorporate the Cd dopant into the [Formula: see text]-[Formula: see text] lattice, being subsequently characterized at the atomic scale with the Perturbed Angular Correlation (PAC) technique and Density Functional Theory (DFT) simulations. The acceptor character of Cd in [Formula: see text]-[Formula: see text] is demonstrated, with Cd sitting in the octahedral Ga site having a negative charge state, showing no evidence of polaron deformations nor extra point defects nearby. The possibility to determine the charge state of Cd will allow assessing the doping type, in particular proving p-type character, without the need for ohmic contacts. Furthermore, a possible approach for contactless charge mobility studies is demonstrated, revealing thermally activated free electrons for temperatures above [Formula: see text] 648 K with an activation energy of 0.54(1) and local electron transport dominated by a tunneling process between defect levels and the Cd probes at lower temperatures.

Wide Dynamic Range Thermometer Based on Luminescent Optical Cavities in Ga2 O3 :Cr Nanowires.

Remote temperature sensing at the micro- and nanoscale is key in fields such as photonics, electronics, energy, or biomedicine, with optical properties being one of the most used transducing mechanisms for such sensors. Ga2 O3 presents very high chemical and thermal stability, as well as high radiation resistance, becoming of great interest to be used under extreme conditions, for example, electrical and/or optical high-power devices and harsh environments. In this work, a luminescent and interferometric thermometer is proposed based on Fabry-Perot (FP) optical microcavities built on Cr-doped Ga2 O3 nanowires. It combines the optical features of the Cr3+ -related luminescence, greatly sensitive to temperature, and spatial confinement of light, which results in strong FP resonances within the Cr3+ broad band. While the chromium-related R lines energy shifts are adequate for low-temperature sensing, FP resonances extend the sensing range to high temperatures with excellent sensitivity. This thermometry system achieves micron-range spatial resolution, temperature precision of around 1 K, and a wide operational range, demonstrating to work at least in the 150-550 K temperature range. Besides, the temperature-dependent anisotropic refractive index and thermo-optic coefficient of this oxide have been further characterized by comparison to experimental, analytical, and finite-difference time-domain simulation results.

Near-UV optical cavities in Ga2O3 nanowires.

In this Letter, we report optical confinement in the near-ultraviolet (near-UV) range in Ga2O3 nanowires (NWs) by distributed Bragg reflector (DBR) nanopatterned cavities. High-contrast DBRs, which act as the end mirrors of the cavities of the desired length, are designed and fabricated by focused ion beam etching. The resonant modes of the cavities are analyzed by micro-photoluminescence measurements, analytical models, and simulations, which show very good agreement between each other. Experimental reflectivities up to 50% are obtained over the 350-410 nm region for the resonances in this wavelength range. Therefore, Ga2O3 NW optical cavities are shown as good candidates for single-material-based near-UV light emitters.

Chiral Microneedles from an Achiral Bis(boron dipyrromethene): Spontaneous Mirror Symmetry Breaking Leading to a Promising Photoluminescent Organic Material.

Supramolecular self-assembly of a highly flexible and achiral meso bis(boron dipyrromethene) [bis(BODIPY)] dye straightforwardly yields fluorescent microfibers, exhibiting an intriguing anisotropic photonic behavior. This performance includes the generation of chiroptical activity owing to spontaneous mirror symmetry breaking (SMSB). Repetition of several self-assembly experiments demonstrates that the involved SMSB is not stochastic but quasi deterministic in the direction of the induced chiral asymmetry. The origin of these intriguing (chiro)photonic properties is revealed by fluorescent microspectroscopy studies of individual micrometric objects, combined with X-ray diffraction elucidation of microcrystals. Such a study demonstrates that J-like excitonic coupling between bis(BODIPY) units plays a fundamental role in their supramolecular organization, leading to axial chirality. Interestingly, the photonic behavior of the obtained fibers is ruled by inherent nonradiative pathways from the involved push-pull chromophores, and mainly by the complex excitonic interactions induced by their anisotropic supramolecular organization.

Probing surface states in C60 decorated ZnO microwires: detailed photoluminescence and cathodoluminescence investigations.

ZnO microwires synthesised by the flame transport method and decorated with C60 clusters were studied in detail by photoluminescence (PL) and cathodoluminescence (CL) techniques. The optical investigations suggest that the enhanced near band edge recombination observed in the ZnO/C60 composites is attributed to the reduction of the ZnO band tail states in the presence of C60. Well-resolved free and bound excitons recombination, as well as 3.31 eV emission, are observed with increasing amount of C60 flooding when compared with the ZnO reference sample. Moreover, a shift of the broad visible emission to lower energies occurs with increasing C60 content. In fact, this band was found to be composed by two optical centres peaked in the green and orange/red spectral regions, presenting different lifetimes. The orange/red band exhibits faster lifetime decay, in addition to a more pronounced shift to lower energies, while the peak position of the green emission only shows a slight change. The overall redshift of the broad visible band is further enhanced by the change in the relative intensity of the mentioned optical centres, depending on the excitation intensity and on the C60 flooding. These results suggest the possibility of controlling/tuning the visible emission outcome by increasing the C60 amount on the ZnO surface due to the surface states present in the semiconductor. An adequate control of such phenomena may have quite beneficial implications when sensing applications are envisaged.

Direct observation of tunnelled intergrowth in SnO2/Ga2O3 complex nanowires.

ß-Ga2O3 intergrowths have been revealed in the SnO2 rutile structure when SnO2/Ga2O3 complex nanostructures are grown by thermal evaporation with a catalyst-free basis method. The structure is formed by a Ga2O3 nanowire trunk, around which a rutile SnO2 particle is formed with [001] aligned to the [010] Ga2O3 trunk axis. Inside the SnO2 particle, ß-Ga2O3 units occur separated periodically by hexagonal tunnels in the (210) rutile plane. Orange (620 nm) optical emission from tin oxide, with a narrow linewidth indicating localised electronic states, may be associated with this ß-Ga2O3 intergrowth.

Shape Engineering Driven by Selective Growth of SnO2 on Doped Ga2O3 Nanowires.

Tailoring the shape of complex nanostructures requires control of the growth process. In this work, we report on the selective growth of nanostructured tin oxide on gallium oxide nanowires leading to the formation of SnO2/Ga2O3 complex nanostructures. Ga2O3 nanowires decorated with either crossing SnO2 nanowires or SnO2 particles have been obtained in a single step treatment by thermal evaporation. The reason for this dual behavior is related to the growth direction of trunk Ga2O3 nanowires. Ga2O3 nanowires grown along the [001] direction favor the formation of crossing SnO2 nanowires. Alternatively, SnO2 forms rhombohedral particles on [110] Ga2O3 nanowires leading to skewer-like structures. These complex oxide structures were grown by a catalyst-free vapor-solid process. When pure Ga and tin oxide were used as source materials and compacted powders of Ga2O3 acted as substrates, [110] Ga2O3 nanowires grow preferentially. High-resolution transmission electron microscopy analysis reveals epitaxial relationship lattice matching between the Ga2O3 axis and SnO2 particles, forming skewer-like structures. The addition of chromium oxide to the source materials modifies the growth direction of the trunk Ga2O3 nanowires, growing along the [001], with crossing SnO2 wires. The SnO2/Ga2O3 junctions does not meet the lattice matching condition, forming a grain boundary. The electronic and optical properties have been studied by XPS and CL with high spatial resolution, enabling us to get both local chemical and electronic information on the surface in both type of structures. The results will allow tuning optical and electronic properties of oxide complex nanostructures locally as a function of the orientation. In particular, we report a dependence of the visible CL emission of SnO2 on its particular shape. Orange emission dominates in SnO2/Ga2O3 crossing wires while green-blue emission is observed in SnO2 particles attached to Ga2O3 trunks. The results show that the Ga2O3-SnO2 system appears to be a benchmark for shape engineering to get architectures involving nanowires via the control of the growth direction of the nanowires.

Crossed Ga2O3/SnO2 multiwire architecture: a local structure study with nanometer resolution.

Crossed nanowire structures are the basis for high-density integration of a variety of nanodevices. Owing to the critical role of nanowires intersections in creating hybrid architectures, it has become a challenge to investigate the local structure in crossing points in metal oxide nanowires. Thus, if intentionally grown crossed nanowires are well-patterned, an ideal model to study the junction is formed. By combining electron and synchrotron beam nanoprobes, we show here experimental evidence of the role of impurities in the coupling formation, structural modifications, and atomic site configuration based on crossed Ga2O3/SnO2 nanowires. Our experiment opens new avenues for further local structure studies with both nanometer resolution and elemental sensitivity.

Assessment of waveguiding properties of gallium oxide nanostructures by angle resolved cathodoluminescence in a scanning electron microscope.

Cathodoluminescence (CL) of Ga(2)O(3) nanowires and planar microstructures has been studied in a scanning electron microscope, as a function of the orientation angle of the structures relative to the position of the light detection system in the microscope chamber. CL contrast shows a marked dependence on the detection angle due to the waveguiding behaviour of the structures. The angle resolved cathodoluminescence (ARCL) measurements enable to evaluate the optical losses of guided blue-ultraviolet light in nanowires with diameters in the sub-wavelength range, deposited on graphite tape or silicon. In planar, branched feather-like microstructures, ARCL images demonstrate the directional-dependent light guiding behaviour of the nano-branches.