Edge-emitting polariton laser and amplifier based on a ZnO waveguide

Ultra-Confined Phonon Polaritons and Strongly Coupled Microcavity Exciton Polaritons in Monolayer MoSi2N4 and WSi2N4

The 2D semiconductors are an ideal platform for exploration of bosonic fluids composed of coupled photons and collective excitations of atoms or excitons, primarily due to large excitonic binding energies and strong light-matter interaction. Based on first-principles calculations, it is demonstrated that the phonon polaritons formed by two infrared-active phonon modes in monolayer MoSi2N4 and WSi2N4 possess ultra-high confinement factors of around ≈105 and 103, surpassing those of conventional polaritonic thin-film materials by two orders of magnitude. It is observed that the first bright exciton possesses a substantial binding energies of 750 and 740 meV in these two monolayers, with the radiative recombination lifetimes as long as 25 and 188 ns, and the Rabi splitting of the formed cavity-exciton polaritons reaching 373 and 321 meV, respectively. The effective masses of the cavity exciton polaritons are approximately 10-5me, providing the potential for high-temperature quantum condensation. The ultra-confined and ultra-low-loss phonon polaritons, as well as strongly-coupled cavity exciton polaritons with ultra-small polaritonic effective masses in these two monolayers, offering the flexible control of light at the nanoscale, probably leading to practical applications in nanophotonics, meta-optics, and quantum materials.

A path towards single molecule vibrational strong coupling in a Fabry-Pérot microcavity

Interaction between light and molecular vibrations leads to hybrid light-matter states called vibrational polaritons. Even though many intriguing phenomena have been predicted for single-molecule vibrational strong coupling (VSC), several studies suggest that these effects tend to be diminished in the many-molecule regime due to the presence of dark states. Achieving single or few-molecule vibrational polaritons has been constrained by the need for fabricating extremely small mode volume infrared cavities. In this theoretical work, we propose an alternative strategy to achieve single-molecule VSC in a cavity-enhanced Raman spectroscopy (CERS) setup, based on the physics of cavity optomechanics. We then present a scheme harnessing few-molecule VSC to thermodynamically couple two reactions, such that a spontaneous electron transfer can now fuel a thermodynamically uphill reaction that was non-spontaneous outside the cavity.

Femtosecond laser writing of waveguides in zinc oxide crystals: fabrication and mode modulation

We report for the first time on optical waveguides in zinc oxide (ZnO) crystals fabricated by femtosecond laser direct writing. The confocal Raman microscopy under 488 nm laser excitation is used to investigate the micro-modifications of the laser irradiation, and guiding properties are studied via the end-face coupling at 632.8 nm. The mode modulation has been achieved by the adjustment of laser writing parameters. A minimum propagation loss of ∼6 dB/cm is obtained for the double-line waveguide structures. A Y-branch waveguide beam splitter is also fabricated, reaching a splitting ratio of nearly 1:1. The original optical properties in the guiding region have been well preserved, according to the confocal Raman investigation, which suggests potential applications of the ZnO waveguides for integrated photonics and nonlinear optics.

Electrically driven whispering-gallery-mode microlasers in an n-MgO@ZnO:Ga microwire/p-GaN heterojunction

In emerging miniaturized applications, semiconductor micro/nanostructures laser devices have drawn great public attentions of late years. The device performances of micro/nanostructured microlasers are highly restricted to the different reflective conditions at various side surfaces of microresonators and junction interface quality. In this study, an electrically driven whispering-gallery-mode (WGM) microlaser composed of a Ga-doped ZnO microwire covered by a MgO layer (MgO@ZnO:Ga MW) and a p-type GaN substrate is illustrated experimentally. Incorporating a MgO layer on the side surfaces of ZnO:Ga MWs can be used to reduce light leakage along the sharp edges and the ZnO:Ga/GaN interface. This buffer layer incorporation also enables engineering the energy band alignment of n-ZnO:Ga/p-GaN heterojunction and manipulating the current transport properties. The as-constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction device can emit at an ultraviolet wavelength of 375.5 nm and a linewidth of about 25.5 nm, achieving the excitonic-related recombination in the ZnO:Ga MW. The broadband spectrum collapsed into a series of sharp peaks upon continuous-wave (CW) operation of electrical pumping, especially for operating current above 15.2 mA. The dominant emission line was centered at 378.5 nm, and the line width narrowed to approximately 0.95 nm. These sharp peaks emerged from the spontaneous emission spectrum and had an average spacing of approximately 5.5 nm, following the WGM cavity modes. The results highlight the significance of interfacial engineering for optimizing the performance of low-dimensional heterostructured devices and shed light on developing future miniaturized microlasers.

Electron-hole plasma Fabry-Perot lasing in a Ga-incorporated ZnO microbelt via Ag nanoparticle deposition

In this work, individual ZnO via Ga-doped (ZnO:Ga) microbelts with excellent crystallinity and smooth facets can enable the realization of lateral microresonator Fabry-Perot (F-P) microlasers, and the F-P lasing action originates from excitonic state. Interestingly, introducing Ag nanoparticles (AgNPs) deposited on the microbelt can increase F-P lasing characteristics containing a lower threshold and enhanced lasing output. Especially for the large size AgNPs (the diameter d is approximately 200 nm), the lasing features also exhibit a significant redshift of each lasing peak and an observable broadening of the spectral line width with an increase of the excitation fluence. And the remarkable lasing characteristics are belonging to the electron-hole plasma (EHP) luminescence. The behavior and dynamics of the stimulated radiation in an AgNPs@ZnO:Ga microbelt are studied, suggesting the Mott-transition from the excitonic state to EHP state that is responsible for the F-P lasing. These features can be attributed to the working mechanism that the hot electrons created by the large size AgNPs through nonradiative decay can fill the conduction band of nearby ZnO:Ga, leading to a downward shift of the conduction band edge. This novel filling influence can facilitate bandgap renormalization and result in EHP emission. The results provide a comprehensive understanding of the transition between excitonic and EHP states in the stimulated emission process. More importantly, it also can provide new scheme to developing high efficiency and ultra-low threshold microlasing diodes.

Motional narrowing, ballistic transport, and trapping of room-temperature exciton polaritons in an atomically-thin semiconductor

Monolayer transition metal dichalcogenide crystals (TMDCs) hold great promise for semiconductor optoelectronics because their bound electron-hole pairs (excitons) are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, excitons can hybridise with cavity photons to form exciton polaritons, which inherit useful properties from their constituents. The ability to manipulate and trap polaritons on a microchip is critical for applications. Here, we create a non-trivial potential landscape for polaritons in monolayer WS₂, and demonstrate their trapping and ballistic propagation across tens of micrometers. We show that the effects of dielectric disorder, which restrict the diffusion of WS₂ excitons and broaden their spectral resonance, are dramatically reduced for polaritons, leading to motional narrowing and preserved partial coherence. Linewidth narrowing and coherence are further enhanced in the trap. Our results demonstrate the possibility of long-range dissipationless transport and efficient trapping of TMDC polaritons in ambient conditions.

Synthesis of ZnO sol-gel thin-films CMOS-Compatible

Zinc oxide (ZnO) is a II–VI group semiconductor with a wide direct bandgap and is an important material for various fields of industry and high-technological applications. The effects of thickness, annealing process in N₂ and air, optical properties, and morphology of ZnO thin-films are studied. A low-cost sol–gel spin-coating technique is used in this study for the simple synthesis of eco-friendly ZnO multilayer films deposited on (100)-oriented silicon substrates ranging from 150 to 600 nm by adjusting the spin coating rate. The ZnO sol–gel thin-films using precursor solutions of molarity 0.75 M exhibit an average optical transparency above 98%, with an optical band gap energy of 3.42 eV. The c-axis (002) orientation of the ZnO thin-films annealed at 400 °C were mainly influenced by the thickness of the multilayer, which is of interest for piezoelectric applications. These results demonstrate that a low-temperature method can be used to produce an eco-friendly, cost-effective ZnO sol–gel that is compatible with a complementary metal-oxide-semiconductor (CMOS) and integrated-circuits (IC).

A low-cost sol–gel spin-coating technique is used in this study for the simple synthesis of eco-friendly ZnO multilayer films deposited on (100)-oriented silicon substrates ranging from 150 to 600 nm by adjusting the spin coating rate.

An electrically driven whispering gallery polariton microlaser

Near-infrared micro/nanolaser devices utilizing low-dimensional semiconductors can provide essential building blocks to achieve integrated optoelectronic devices and circuitry for advanced functionalities and are compatible with on-chip technologies. Although significant progress has been made through using narrow-band semiconductor micro/nanostructures to realize near-infrared stimulated radiation at room temperature, severe challenges still remain involving much lower quantum efficiencies and higher auger recombination. Herein, we report an experimental realization of a current-injection semiconductor polariton device made of a ZnO microwire via Ga-doping (ZnO:Ga MW) and p-type GaAs template. The device can emit polaritonic illumination directly from sharp edges of the hexagonal MW. The experimental results of angle-resolved electroluminescence measurements reveal a typical anticrossing feature between excitons and cavity modes, unambiguous evidence of the strong exciton-polariton coupling, with corresponding Rabi splitting energy extracted to be about 195 meV. As the applied bias goes above a certain value, electrically driven whispering gallery lasing action was achieved in the near-infrared spectrum, and the lasing features can be assigned to the exciton-polariton effect. The results not only can afford insights into the development of low-threshold coherent light sources via the exciton-polariton effect, but also can expand the fabrication of low-dimensional, near-infrared microlaser devices.

A single microwire near-infrared exciton-polariton light-emitting diode

Exciton-polaritons, which originate from the strong coupling between photon modes of microresonators and excitons in semiconductor micro-/nanostructures, have drawn much attention due to their significance for fabricating coherent light sources which possess considerably lower emission thresholds. In this study, an exciton-polariton light-emitting diode (LED), made from a Ga-doped ZnO microwire (ZnO:Ga MW) and a p-GaAs template serving as the hole supplier, is fabricated. The n-ZnO:Ga MW/p-GaAs heterojunction device can emit light with a near-infrared wavelength of 880 nm and a narrow line width of about 60 nm. Due to the high quality whispering gallery mode (WGM) microcavities which are naturally self-constructed by the hexagon-shaped MW, the electroluminescence (EL) spectrum resolves into a series of resonance peaks which can be assigned to exciton-polariton features, leading to the strong coupling of the exciton and the WGM photon in the as-fabricated LED. The strong exciton-photon coupling is clearly evidenced via angle-resolved EL measurements, with the Rabi splitting energy extracted as 160 meV. Furthermore, by adjusting the size of the WGM microcavity structure naturally formed by the hexagonal MWs, particularly by adjusting the diameter of the wires, the exciton-polariton coupling strength in the single MW based LEDs can be tuned, with the as-extracted Rabi splitting energy varying in the range of 92-294 meV. The realization of a single MW based LED, which shows exciton-polariton behavior from a built-in optical microresonator, can enable a promising route for the future fabrication of polariton emitters, where the device performance no longer suffers from obstacles including the need for additional optical resonators, large lattice mismatch, and template availability.

Plasmon-enhanced strong exciton-polariton coupling in single microwire-based heterojunction light-emitting diodes

Manipulating the strong light-matter coupling interaction in optical microresonators that are naturally formed by semiconductor micro- or nanostructures is crucial for fabricating high-performance exciton-polariton devices. Such devices can function as coherent light sources having considerably lower emission threshold. In this study, an exciton-polariton light-emitting diode (LED), made of a single ZnO microwire (MW) and a p-GaN substrate, serving as the hole injector, was fabricated, and its working characteristics, in the near-ultraviolet region, were demonstrated. To further improve the quality of the single ZnO MW-based optical microresonator, Ag nanowires (AgNWs) with ultraviolet plasmonic response were deposited on the MW. Apart from the improvement of the electrical and optical properties of the hexagonal ZnO MW, the optically pumped whispering-gallery-mode lasing characteristics were significantly enhanced. Furthermore, a single ZnO MW not covered, and covered by AgNWs, was used to construct a heterojunction LED. Compared with single bare ZnO MW-based LED, significant enhancement of the device performance was achieved, including a significant enhancement in the light output and a small emission band blueshift. Specifically, the exciton-polariton emission was observably enhanced, and the corresponding Rabi splitting energy (∼ 495 meV) was significantly higher than that of the bare ZnO MW-based LED (∼ 370 meV). That ultraviolet plasmons of AgNWs enhanced the exciton-polariton coupling strength was further confirmed via angle-resolved electroluminescence measurements of the single MW-based polaritonic devices, which clearly illustrated the presence of Rabi splitting and subband anti-crossing characteristics. The experimental results provide new avenues to achieve extremely high coupling strengths, which can accelerate the advancements in electrically driven high-efficiency polaritonic coherent emitters and nonlinear devices.

Continuous-wave operation of an electrically pumped single microribbon based Fabry-Perot microlaser

Fabry-Perot (FP) mode microlasers have been popularized and applied widely in on-chip coherent light sources because of the unique advantages of directional output emission. In this work, a heterojunction light-emitting diode (LED) made of a Ga-doped ZnO (ZnO:Ga) microribbon and p-GaAs template is fabricated. And its electroluminescence characteristics of strong coupling of exciton-photon and polariton lasing, in the blue-violet spectrum, were demonstrated under continuous-wave operation of an electrical injection. In the device structure, a single microribbon with a rectangular-shaped cross section can achieve the FP-mode lasing action by the optical oscillation between the two lateral sides of the microcrystals in the ultraviolet spectrum. As the reverse-current is below the threshold value, the device can have radiative polaritonic lighting directly from bilateral sides of the microribbon, yielding strong coupling between excitons and FP-mode microresonator. And the exciton-polariton coupling strengths characterized by a Rabi splitting energy were extracted to be 500 meV. Further, when the input current increased more than a certain value, strong laser illuminating developed as two sharp peaks at the lower energy shoulder of the spontaneous emission peak, and these oscillating modes can dominate the waveguide EL spectra. The experimental results can provide us with further unambiguous evidence that the lasing is originated from the polariton resonances for the microribbon with strong exciton-polariton coupling. Since single microribbon based optical FP-mode microresonators do not require additional feedback mirrors, their compact size and resulting low thresholds make them a powerful candidate to construct on-chip coherent light sources for future integrated nanophotonic and optoelectronic circuitry.

An electrically driven single microribbon based near-infrared exciton–polariton light-emitting diode

An electrically driven exciton–polariton NIR-LED involving an n-ZnO:Ga microribbon/p-GaAs heterojunction was achieved. The Rabi splitting is measured to be 109 meV.

A Review of Microwave Synthesis of Zinc Oxide Nanomaterials: Reactants, Process Parameters and Morphoslogies

Zinc oxide (ZnO) is a multifunctional material due to its exceptional physicochemical properties and broad usefulness. The special properties resulting from the reduction of the material size from the macro scale to the nano scale has made the application of ZnO nanomaterials (ZnO NMs) more popular in numerous consumer products. In recent years, particular attention has been drawn to the development of various methods of ZnO NMs synthesis, which above all meet the requirements of the green chemistry approach. The application of the microwave heating technology when obtaining ZnO NMs enables the development of new methods of syntheses, which are characterised by, among others, the possibility to control the properties, repeatability, reproducibility, short synthesis duration, low price, purity, and fulfilment of the eco-friendly approach criterion. The dynamic development of materials engineering is the reason why it is necessary to obtain ZnO NMs with strictly defined properties. The present review aims to discuss the state of the art regarding the microwave synthesis of undoped and doped ZnO NMs. The first part of the review presents the properties of ZnO and new applications of ZnO NMs. Subsequently, the properties of microwave heating are discussed and compared with conventional heating and areas of application are presented. The final part of the paper presents reactants, parameters of processes, and the morphology of products, with a division of the microwave synthesis of ZnO NMs into three primary groups, namely hydrothermal, solvothermal, and hybrid methods.

Determination of absorption coefficients and Urbach tail depth of ZnO below the bandgap with two-photon photoluminescence

In this article, we demonstrate a novel approach to determine the absorption coefficient of ZnO below the bandgap via measuring the self-absorption (SA) effect on the two-photon luminescence (TPL) spectrum of the ZnO bulk crystal rod at cryogenic temperature. Under a geometric configuration of side-excitation and front-detection, the intensities of several major spectral components of TPL spectra of ZnO can be decisively tuned by precisely varying the transmitting distance of luminescence signal, so that the absorption coefficients at different wavelengths can be determined on the basis of Beer-Lambert law. Furthermore, the peak position of donor bound exciton luminescence exhibits a unique redshift tendency with increasing the transmitting distance. Starting from the product of Lorentzian lineshape function and exponential absorption edge of Urbach tail, an analytical formula is derived to quantitatively interpret the experimental redshift characteristic with the transmitting distance. The energy depth of Urbach tail of the studied ZnO crystal is deduced to be ∼13.3 meV. In principle, this new approach can be used to determine absorption coefficient of any luminescent solids as long as the SA effect happens.

Hybrid quadrupole plasmon induced spectrally pure ultraviolet emission from a single AgNPs@ZnO:Ga microwire based heterojunction diode

Ultraviolet light-emitting materials and devices with high-efficiency are required for many applications. One promising way to enhance the ultraviolet luminescence efficiency is by incorporating plasmonic nanostructures. However, a large energy mismatch between the plasmons and the light emitters greatly limits the direct realization of light enhancement. In this work, a single Ga-doped ZnO microwire prepared with large-sized Ag nanoparticle (the diameter d ∼ 200 nm) deposition (AgNPs@ZnO:Ga MW) was utilized to construct a high-performance heterojunction diode, with p-GaN serving as the hole injection layer. In addition to enhanced optical output, the emission spectra also revealed that typical near-band-edge (NBE) emission with higher wavelength stability centered around 378.0 nm was achieved, accompanied by narrowing of the spectral linewidth to around 10 nm. Thus, the interfacial and p-GaN emissions were successfully suppressed. The spectral profile of the emission spectra of the heterojunction diodes precisely matched the photoluminescence spectrum of the single ZnO:Ga MW, which indicates that the single ZnO:Ga MW can act as the active region for the radiative recombination of electrons and holes in the diode operation. In the emission mechanism, hybrid quadrupole plasmons induce the generation of hot electrons, which are then injected into the conduction band of the neighboring ZnO:Ga and are responsible for the NBE-type emission of the single MW based heterojunction diode. This novel emission enhancement and modulation principle can aid in the design and development of new types of luminescent materials and devices with high-efficiency, spectral stability and spectral purity.

Tailoring Dispersion of Room-Temperature Exciton-Polaritons with Perovskite-Based Subwavelength Metasurfaces

Exciton-polaritons represent a promising platform for studying quantum fluids of light and realizing prospective all-optical devices. Here we report on the experimental demonstration of exciton-polaritons at room temperature in resonant metasurfaces made from a sub-wavelength two-dimensional lattice of perovskite pillars. The strong coupling regime is revealed by both angular-resolved reflectivity and photoluminescence measurements, showing anticrossing between photonic modes and the exciton resonance with a Rabi splitting in the 200 meV range. Moreover, by tailoring the photonic Bloch mode to which perovskite excitons are coupled, polaritonic dispersions are engineered exhibiting linear, parabolic, and multivalley dispersions. All of our results are perfectly reproduced by both numerical simulations based on a rigorous coupled wave analysis and an elementary model based on a quantum theory of radiation-matter interaction. Our results suggest a new approach to study exciton-polaritons and pave the way toward large-scale and low-cost integrated polaritonic devices operating at room temperature.

Employing rhodium tripod stars for ultraviolet plasmon enhanced Fabry–Perot mode lasing

Rhodium tripod stars serving as ultraviolet plasmons can provide a highly competitive platform to achieve high-performance Fabry–Perot lasing of quadrilateral ZnO microwires.

Vertically-aligned ZnO microrod for high-brightness light source

ZnO-microrod array with well-aligned orientation prepared on p-GaN template can be utilized to construct high-performance near-ultraviolet emitters due to desired high optical quality and well-defined geometries.

Tailoring the electroluminescence of a single microwire based heterojunction diode using Ag nanowires deposition

A single Ga-doped ZnO microwire covered by Ag nanowires (AgNWs@ZnO:Ga MW) was utilized to construct a promising ultraviolet light source, with p-GaN serving as a hole injection layer.

Two Hybrid Au-ZnO Heterostructures with Different Hierarchical Structures: Towards Highly Efficient Photocatalysts

A new paradigm for photocatalysts based on two different hierarchically structured honeycomb and porous cylindrical Au-ZnO heterostructures was successfully developed via a straightforward and cost-effective hydrothermal method under different preparation conditions, which can be promising for industrial applications. The photocatalytic performance of all as-prepared samples under the illumination of sunlight was evaluated by the photocatalytic degradation of rhodamine B (RhB) and malachite green (MG) aqueous solutions. The results show that the photocatalytic degradation efficiency of RhB and MG was 55.3% and 40.7% for ZnO, 95.3% and 93.4% for the porous cylindrical Au-ZnO heterostructure, and 98.6% and 99.5% for the honeycomb Au-ZnO heterostructure, respectively. Compared with those from the ZnO, the results herein demonstrate an excellent reduction in the photoluminescence and improvement in the photocatalysis for the Au-ZnO hybrids with different morphologies. These results were attributed not only to the greatly improved sunlight utilization efficiency due to the surface plasmon resonance (SPR) absorption of Au nanoparticles in the visible region coupled with the UV light utilization by the ZnO nanostructures and multi-reflections of the incident light in the pore structures of their interior cavities but also to the high charge separation efficiency and low Schottky barrier generated by the combination of Au nanoparticles and ZnO micromaterials. Moreover, the honeycomb Au-ZnO heterostructure had a high Au content, surface area and surface oxygen vacancy (O_V), which enabled photocatalytic properties that were higher than those of the porous cylindrical Au-ZnO heterostructures. In addition, two different formation mechanisms for the morphology and possible photocatalytic mechanisms are proposed in this paper.

Fluorescent incandescent light sources from individual quadrilateral ZnO microwire via Ga-incorporation

By means of nanophotonics principle, the thermal radiation can be tailored, thus, traditional tungsten lamp light source can glow the vitality and the vigor due to the low-efficiency approaching to commercial fluorescent or light-emitting diode bulbs. However, too far by demanding exacting terms, such as high-temperature thermal radiation (∼ 3000 K), high-vacuum encapsulation technology, restricted spectrally controllable source and so on, tungsten-based incandescent lamp filament has greatly limited the application in lighting, diagnosis and treatment, communication, imaging, etc. Herein, individual Ga-doped ZnO microwires (ZnO:Ga MWs) were successfully synthesized, which can be utilized to construct typical incandescent sources. By adjusting the Ga-incorporation, lighting colors are tuned in the visible spectral band. Especially, by incorporating Au quasiparticle nanofilms, the incandescent lighting features can further be modulated, such as the emission peaks, the modulation of lighting regions. Therefore, individual ZnO:Ga MWs based incandescent emitters can undertake a new function of the oldest, affordable and easily prepared light sources. While preliminary, individual ZnO:Ga MWs being treated as efficient incandescent light sources, can also open up intriguing scientific questions, and possible applications of linear, transparent, flexible displays and optical interconnects with electronic circuits.

Amplification of nonlinear polariton pulses in waveguides

Using a sub-millimeter exciton-polariton waveguide suitable for integrated photonics, we experimentally demonstrate nonlinear modulation of pico-Joule pulses at the same time as amplification sufficient to compensate the system losses. By comparison with a numerical model we explain the observed interplay of gain and nonlinearity as amplification of the interacting polariton field by stimulated scattering from an incoherent continuous-wave reservoir that is depleted by the pulses. This combination of gain and giant ultrafast nonlinearity operating on picosecond pulses has the potential to open up new directions in low-power all-optical information processing and nonlinear photonic simulation of conservative and driven-dissipative systems.

Wavelength-Tunable Waveguide Emissions from Electrically Driven Single ZnO/ZnO:Ga Superlattice Microwires

Because of the superlattice structures comprising periodic and alternating crystalline layers, one-dimensional photon crystals can be employed to expand immense versatility and practicality of modulating the electronic and photonic propagation behaviors, as well as optical properties. In this work, individual superlattice microwires (MWs) comprising ZnO and Ga-doped ZnO (ZnO/ZnO:Ga) layers were successfully synthesized. Wavelength-tunable multipeak emissions can be realized from electrically driven single superlattice MW-based emission devices, with the dominant wavelengths tuned from ultraviolet to visible spectral regions. To illustrate the multipeak character, single superlattice MWs were selected to construct fluorescent emitters, and the emission wavelength could be tuned from 518 to 562 nm, which is dominated by Ga incorporation. Especially, by introducing Au quasiparticle film decoration, emission characteristics can further be modulated, such as the red shift of the emission wavelengths, and the multipeaks were strongly modified and split into more and narrower subbands. In particular, electrically pumped exciton-polariton emission was realized from heterojunction diodes composed of single ZnO/ZnO:Ga superlattice MWs and p-GaN layers in the blue-ultraviolet spectral regions. With the aid of localized surface plasmons from Au nanoparticles, which deposited on the superlattice MW, significant improvement of emission characteristics, such as enhancement of output efficiencies, blue shift of the dominant emission wavelengths, and narrowing of the spectral linewidth, can be achieved. The multipeak emission characteristics would be originated from the typical optical cavity modes, but not the Fabry-Perot mode optical cavity formed by the bilateral sides of the wire. The resonant modes are likely attributed to the coupled optical microcavities, which formed along the axial direction of the wire; thus, the emitted photons can be propagated and selected longitudinally. Therefore, the novel ZnO/ZnO:Ga superlattice MWs with a quadrilateral cross section can provide a potential platform to construct multicolor emitters and low-threshold exciton-polariton diodes and lasers.

Engineering Charge Transfer Characteristics in Hierarchical Cu₂S QDs @ ZnO Nanoneedles with p⁻n Heterojunctions: Towards Highly Efficient and Recyclable Photocatalysts

Equipped with staggered gap p-n heterojunctions, a new paradigm of photocatalysts based on hierarchically structured nano-match-shaped heterojunctions (NMSHs) Cu₂S quantum dots (QDs)@ZnO nanoneedles (NNs) are successfully developed via engineering the successive ionic layer adsorption and reaction (SILAR). Under UV and visible light illumination, the photocatalytic characteristics of Cu₂S@ZnO heterojunctions with different loading amounts of Cu₂S QDs are evaluated by the corresponding photocatalytic degradation of rhodamine B (RhB) aqueous solution. The results elaborate that the optimized samples (S3 serial specimens with six cycles of SILAR reaction) by means of tailored the band diagram exhibit appreciable improvement of photocatalytic activities among all synthesized samples, attributing to the sensitization of a proper amount of Cu₂S QDs. Such developed architecture not only could form p⁻n junctions with ZnO nanoneedles to facilitate the separation of photo-generated carries but also interact with the surface defects of ZnO NNs to reduce the electron and hole recombination probability. Moreover, the existence of Cu₂S QDs could also extend the light absorption to improve the utilization rate of sunlight. Importantly, under UV light S3 samples demonstrate the remarkably enhanced RhB degradation efficiency, which is clearly testified upon the charge transfer mechanism discussions and evaluations in the present work. Further supplementary investigations illustrate that the developed nanoscale Cu₂S@ZnO heterostructures also possess an excellent photo-stability during our extensive recycling photocatalytic experiments, promising for a wide range of highly efficient and sustainably recyclable photocatalysts applications.