Edge Emission Research Articles

Studies on structural and optical characterization of Na:ZnS nanocomposites

In this work, we report the structural, morphological and optical characterization of pure and sodium (Na) mixed ZnS nanocomposites. The Na:ZnS nanocomposites were synthesized by the co-precipitation method using the Na2SO4. The powder XRD pattern reveals a mixed phase of cubic and hexagonal structure. The crystalline size was found to be ∼6 nm based on the full width at half maximum (FWHM) by using the Debye Scherrer formula. Moreover, the diffraction peaks were shifted towards a higher angle compared to pure ZnS. This can be due to the difference in the ionic radii of Na+ and Zn2+ ions. The SEM images show the spherical shape of nanocomposites. The UV-visible absorption spectra of pure and Na:ZnS show an absorption cut-off wavelength of 278 nm (4.46 eV) compared to bulk band gap of ZnS (3.78 eV). The blue shift can be attributed to the quantum confinement effect. In addition to that, the optical absorption increases with increasing Na concentration. The functional groups are identified by FTIR analysis. The photoluminescence (PL) study reveals the near band edge emission at 468 nm (2.64 eV). The broad and high relative intensity of defect level emission is observed in the lower energy side of the PL spectra. The intensity of luminescence increases with increasing Na concentration due to the radiative recombination process. The Raman spectra show the formation of nanocomposite through change in phonon frequency, structural disorder and width for LO phonon mode. A change of 1.7 cm−1 phonon width suggests the formation of disorder.

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

High photocurrent performance of nebulizer sprayed Bi2S3 thin films by a novel Cu-La doping strategy

Nowadays, chalcogenide-based thin layers are proving a promising light-absorbing material with low cost and are technologically suitable for light to current conversion applications. In this study, for the first time, pristine Bi2S3, Bi2S3:Cu2%, Bi2S3:La2%, and Bi2S3:Cu2%:La2% thin films were prepared by nebulizer spray pyrolysis technique on a glass substrate at 300 °C. The obtained XRD pattern confirms the orthorhombic structure of Bi2S3 and possesses a maximum crystallite size of 38 nm for Bi2S3:Cu: La co-doped sample. Images taken with a FESEM analysis showed the unformed distribution of round shaped grains without any cracks and co-doping leads to the development of slight cluster formation. The optical absorbance spectra of the thin films were observed from the wavelength of 350 nm to 1000 nm, and the co doping process highly strengthen the optical absorbance, which is prerequisite for an efficient photodetector. A lower band gap value of 1.7 eV is achieved for Bi2S3: Cu: La thin film. From PL studies, defect emission bands around ∼325 and 525 nm related to band edge emission and sulfur vacancies are obtained. Further, UV photosensing properties of each sample were evaluated. Commendable performance with maximum responsivity (1.24 × 10−1 AW−1), detectivity (1.13 × 1010 Jones), and EQE (40%) were achieved for Bi2S3:Cu:La-sample. The enhanced figures-of-merit are attributed large crystalline size, good crystallinity, lower bandgap and crack free surface. The photo sensing results may widen the field of photosensing application.

Double-sided asymmetric metasurfaces achieving sub-microscale focusing from a GaN green laser diode.

We proposed and demonstrated a highly efficient sub-microscale focusing from a GaN green laser diode (LD) integrated with double-sided asymmetric metasurfaces. The metasurfaces consist of two nanostructures in a GaN substrate: nanogratings on one side and a geometric phase based metalens on the other side. When it was integrated on the edge emission facet of a GaN green LD, linearly polarized emission was firstly converted to the circularly polarized state by the nanogratings functioning as a quarter-wave plate, the phase gradient was then controlled by the metalens on the exit side. In the end, the double-sided asymmetric metasurfaces achieve a sub micro-focusing from linearly polarized states. Experimental results show the full width at half maximum of the focused spot size is about 738 nm at the wavelength 520 nm and the focusing efficiency is about 72.8%. Our results lay a foundation for the multi-functional applications in optical tweezers, laser direct writing, visible light communication, and biological chip.

Photoluminescence from Cu / Mn: ZnSe quantum dots and their phase transformation in silicate glass

To realize the polychromatic and tunable luminescence of ZnSe quantum dots (QDs) in glass, Cu and Mn co-doped ZnSe QDs (Cu/Mn: ZnSe QDs) glass were prepared by melt quenching and subsequent heat treatment method. For the ZnSe QDs glass without Cu and Mn dopants, there is only intrinsic luminescence from ZnSe QDs observed. For Cu/Mn: ZnSe QDs glass, the band edge emission from ZnSe QDs is quenched and the broadband emission from Cu2+/Mn2+ ions is observed, which is resulted by the effective energy transfer from ZnSe QDs to Cu2+/Mn2+ ions. With the increase of heating temperature, the emission spectra of Cu/Mn: ZnSe QDs glass change from green to orange light, which is characteristic luminescence wavelengths of Cu2+ and Mn2+ ions, respectively. For the ZnSe QDs glass without Cu2+ and Mn2+ dopants, ZnSe QDs undergo a phase transformation from the cubic to the hexagonal structure at higher heating temperature (≥610 °C) than those of Cu/Mn: ZnSe QDs glass at 600 °C and Cu-doped ZnSe QDs glass at 580 °C, in addition, For Cu/Mn: ZnSe QDs glass, the phase transformation temperature drops to 600 °C. For Cu-doped ZnSe QDs glass, the phase transformation temperature is further reduced to 580 °C. However, for Mn-doped ZnSe QDs glass, there is no phase transformation. It indicated that Cu2+ promotes the phase transformation of ZnSe QDs.

Tailoring the Structural, Optical and Electrical Properties of Mn Doped ZnO Thin Films for Gas Sensing Response

This manuscript presents a study on the photoinduced and gas sensing response of Mn-doped ZnO thin films (Zn1−xMnxO, x = 5, 10 mol %) synthesized using the spin coating method. The fabricated thin films were characterized to investigate their structural, bonding, optical, surface morphology, electrical, and gas sensing properties. SEM images displayed a homogeneous surface morphology across the fabricated films with typical grain size ranging from 25 to 30 nm. Optical absorption spectra demonstrated a variation in the optical band gap, ranging from 3.41 eV to 3.87 eV, indicating the tunability of the optical properties with the Mn doping concentration. Photoluminescence (PL) spectra exhibited Near Band Edge emission, as well as blue and green emission peaks, which can be attributed to the presence of defects and impurities in the Mn-doped ZnO thin films. The photoinduced effect was observed through the variation in I–V characteristics due to the excitation of electron-hole pairs, highlighting the influence of Mn doping on the charge transport properties of the thin films. Additionally, the gas sensing response of the Mn-doped ZnO thin films to hydrogen gas was investigated. The results indicated an improved gas sensing response with increasing Mn concentration in the doped ZnO thin films, suggesting the active role of Mn in enhancing the sensitivity of ZnO to hydrogen gas. Based on these findings, Mn-doped ZnO thin films show promise for application in gas sensors.

Effect of aging on microstructural and optical properties of sol-gel dip coated BaTiO3 thin films

This paper reports the effect of aging, thickness, and annealing temperature on microstructural and optical properties of sol-gel dip-coated BaTiO3 (BTO) thin films. The absence of any sharp peaks in the XRD pattern confirmed the amorphous nature of thin films regardless of the aging time of the solution and the thickness. Annealed BTO thin films showed the polycrystalline structure of films, which indicated the formation of a single-phase compound, and increased the intensity with respect to the annealing temperature from 550 to 650 °C. The crystallite size of annealed films increased from 19.44 nm to 23.6 nm, while the lattice strain and dislocation density decreased from 6.63 × 10−3 to 5.65 × 10−3 and 2.65 × 1011 to 1.79 × 1011 cm−2 as the annealing temperature increased from 550 to 650 °C, respectively. The bandgap of as-deposited (solution aged from 48 - 96 h) films decreased from 3.90 to 3.87 eV, while the bandgap of annealed films increased from 3.19 to 3.86 eV as the annealing temperature increased from 550 to 650 °C owing to releasing the defects of films. The near band edge emission (NBE) of all annealed samples was observed to be 361 nm which is in close agreement with the estimated bandgap. The variation of aging, thickness, and annealing temperature of BTO thin films can significantly improve the physical properties of optoelectronic applications.

Encapsulated Co-ZnO nanospheres as degradation tool for organic pollutants: Synthesis, morphology, adsorption and photo luminescent investigations

Zinc oxide (ZnO) nanostructures, both undoped and Co-doped, were synthesized through the solution combustion process. The diffraction patterns from powder XRD revealed that the materials were crystalline. The morphology of the spherically formed nanoparticles was visualized in SEM micrographs. FTIR spectra verified the existence of a defect-associated peak in Co-encapsulated ZnO (Zn0.98Co0.02O) NPs. Photoluminescence studies are undertaken. Malachite Green (MG) dye is used as a representative organic pollutant to study the adsorptive degradation of Co-doped ZnO nanomaterial. Moreover, the adsorption properties, including isotherm and kinetics, are investigated by analyzing the degradation of MG dye. Experimental parameters, such as the concentration of the MG dye, dosage and pH, were varied to ascertain favorable conditions for the degradation study. The results indicate that the MG dye is 70% degraded. After Co-doping, near-band edge emission in undoped ZnO changed into intense red defect emission and was directly correlated with changes in PL emission.

Effect of deposition time on copper incorporation of ZnS thin films by low-cost technique

The copper-doped zinc sulphide (CuZnS) thin films were prepared by low-cost chemical bath deposition (CBD). The deposited films were varied by copper doping concentration and preparation time. The structural, optical, and electrical properties of the films were analyzed. The XRD patterns of the films exhibit polycrystalline nature with a prominent peak in the (111) plane. The improvement of the crystallinity was observed in the variation of deposition time. The lattice constant of the CuZnS films slightly deviates from that of pure ZnS. The deposited films have shown uniform substrate coverage without pinholes. EDAX results confirmed the presence of copper, zinc, and sulphur in the deposited films. The absorbance was found to be high in the ultraviolet region. The bandgap of the films obtained was 3.7 eV and marginally reduced with deposition time. The Urbach energy of the CuZnS films varied with deposition time. The PL study confirms the incorporation of copper and band edge emission. The electrical properties of the films were investigated by Hall effect measurement.

Electrically driven amplified spontaneous emission from colloidal quantum dots

Colloidal quantum dots (QDs) are attractive materials for realizing solution-processable laser diodes that could benefit from size-controlled emission wavelengths, low optical-gain thresholds and ease of integration with photonic and electronic circuits1–7. However, the implementation of such devices has been hampered by fast Auger recombination of gain-active multicarrier states1,8, poor stability of QD films at high current densities9,10 and the difficulty to obtain net optical gain in a complex device stack wherein a thin electroluminescent QD layer is combined with optically lossy charge-conducting layers11–13. Here we resolve these challenges and achieve amplified spontaneous emission (ASE) from electrically pumped colloidal QDs. The developed devices use compact, continuously graded QDs with suppressed Auger recombination incorporated into a pulsed, high-current-density charge-injection structure supplemented by a low-loss photonic waveguide. These colloidal QD ASE diodes exhibit strong, broadband optical gain and demonstrate bright edge emission with instantaneous power of up to 170 μW.

Raman silicon nanocavity laser with efficient light emission from the edge of an adjacent waveguide.

A Raman nanocavity laser can emit light into free space and into a properly designed waveguide adjacent to the cavity by mode coupling. In common device designs, the emission from the edge of this waveguide is relatively weak. However, a Raman silicon nanocavity laser with strong emission from the waveguide edge would be advantageous for certain applications. Here we investigate the increase in the edge emission that can be achieved by adding photonic mirrors to the waveguides adjacent to the nanocavity. We experimentally compare devices with and without photonic mirrors: the edge emission for devices with mirrors is 4.3 times stronger on average. This increase is analyzed using coupled-mode theory. The results indicate that the control of the round-trip phase shift (between the nanocavity and the mirror) and an increase of the quality factors of the nanocavity are important for further enhancement.

Efficient Band-Edge Emission from Indirect Bandgap Semiconductor Quantum Dots upon Shell Engineering

Environmentally friendly colloidal quantum dots (QDs) of groups III-V are in high demand for next-generation high-performance light-emitting devices for display and lighting, yet many of them (e.g., GaP) suffer from inefficient band-edge emission due to the indirect bandgap nature of their parent materials. Herein, we theoretically demonstrate that efficient band-edge emission can be activated at a critical tensile strain γc enabled by the capping shell when forming a core/shell architecture. Before γc is reached, the emission edge is dominated by dense low-intensity exciton states with a vanishing oscillator strength and a long radiative lifetime. After γc is crossed, the emission edge is dominated by high-intensity bright exciton states with a large oscillator strength and a radiative lifetime that is shorter by a few orders of magnitude. This work provides a novel strategy for realizing efficient band-edge emission of indirect semiconductor QDs via shell engineering, which is potentially implemented employing the well-established colloidal QD synthesis technique.

Noticeable effect of coating temperature on Co3O4 thin films developed through low-cost nebulizer spray pyrolysis for photo-sensing applications

Cobalt oxide thin films are synthesized at various temperatures from 275 to 375 °C using a nebulizer spray pyrolysis technique. Different characterization approaches are used to investigate the effects of coating temperature on the characteristics of coated thin films. The deposited films have a cubic structure with a preferred orientation along the (111) direction. Field Emission Scanning Electron Microscope (FESEM) reveals the grain size of the samples as the coating temperature rises. The energy dispersive x-ray analysis (EDS) spectra confirm that the growing film is primarily made up of Co and O. The optical absorption spectra show an increase in absorption with increasing coating temperature. The band-gap energy values are lowered by 2.12, 2.10, 2.09, 2.01, and 2.05 eV with respect to the coating temperatures of 275, 300, 325, 350, and 375 °C, respectively. The observed photoluminescence(PL) spectra exhibit the band edge and defect-based emissions for all the films. Studies of current (vs) voltage revealed ohmic nature up to 350 °C coating temperature. At coating temperature 375 °C hysteretic behavior of film is observed because of the increasing number of vacancies and defective sites, which act as trap of charge carriers. The photo-detection capabilities of these coated Co3O4 thin films were investigated using optoelectronic characteristics. The optical and electrical properties have also been controlled by the fusing temperature. The Co3O4 (350 °C) produce high photocurrent (8.84 × 10−7A), external quantum efficiency (11.6%), detectivity (3.5 × 109 Jones), responsivity (4.99 × 10−2 AW−1) and has the shortest rise/decay time among the constructed photo-detectors. These findings show the tuning of the properties of Co3O4 by optimizing the coating temperature for the fabrication of effective photo-detectors.

Optical characterization of high-quality ZnO (0002) / Cu (111) epilayers grown by electrodeposition

Wurtzite ZnO thin films were grown on Cu substrates of three orientations – (111), (100), and (130), respectively – by a simple electrodeposition technique. Among them, the films deposited on the (111) and (130) substrates were epilayers with different crystallinity. Optical characterizations were therefore conducted for the (0002)ZnO/(111)Cu epilayers showing promising crystallinity while having different morphologies. Photoluminescence (PL) spectra of epilayers show a sharp UV near band edge (NBE) emission band, owing to excitonic transitions in ZnO layer, whereas a wide feature in visible spectral region is related to deep defect (O-vacancy) states in ZnO. The strong excitonic ZnO band (NBE) suppresses the defect (DEF) band intensity by ∼100 times, indicating good optical quality of ZnO epilayers. Using the Arrhenius-type equation, activation energies were determined as Ea = 23 meV and 27 meV, which appeared to be related with the Stokes shift between emission and absorption spectra. From the excitation-dependent (80–700 kW/cm2) PL measurements at 3 K, a linear behavior of ratio NBE:DEF was deduced, which indicated that excitonic- and defect-related radiative recombination paths are independent. Furthermore, from the dependence of the PL intensity on the laser power density, characterized with exponential factor of k ≃ 1.4, the donor-bound nature of the excitonic transitions (DoX) in the ZnO/Cu epilayers is suggested. Corroborative structural and optical findings with the bandgap values estimated (3.362 eV and 3.371 eV) in a very good agreement with the literature data for ZnO single crystal, confirm high-quality of the studied ZnO/Cu (111) epilayers.

Efficient tunability of size and optical properties of reduced graphene oxide-ZnO composite nanocrystallites on solid substrates

Growth of ultra-thin luminescent reduced graphene oxide (rGO)-zinc oxide (ZnO) composite nanostructures was accomplished by Langmuir-Blodgett (LB) transfer of GO-Zn composite layers onto solid substrates and subsequent oxidation. Layer structure of LB transferred composites and oxidation procedure determine the size distribution, density, structural order, optical quality of grown ZnO and the extent of GO reduction. Prolonged oxidation (120 min) of composite monolayers produce ZnO nanocrystallites nearly deprived of defects and exhibiting substantially enhanced band edge emission. Prior anchoring of Zn2+ ions on GO layers and oxidation facilitate controlled growth of ZnO nanocrystallites and nanoclusters (20–100 nm), offer tunability of absorption edge (325–355 nm) and assists in amendment of surface defects/band edge emission (376–384 nm). Conductivity of precursor and oxidized composite layers was mapped by recording STM images as well as STS spectra and, the obtained differential conductivity spectra substantiate the changes in GO conductivity and provide range of bandgap values of grown ZnO nanocrystallites (3.1–3.4 eV). The multiple GO-Zn compositing procedures and optimum oxidation conditions introduced in present work facilitate development of rGO-ZnO composite nanostructures with high optical, crystalline quality and conductive characteristics, which pave path for fabrication of such nanostructures for optoelectronic device applications.

Role of Boron in Assisting the Super-Enhancement of Emissions from Carbon-Implanted Silicon.

The super enhancement of silicon band edge luminescence when co-implanted with boron and carbon is reported. The role of boron in the band edge emissions in silicon was investigated by deliberately introducing defects into the lattice structures. We aimed to increase the light emission intensity from silicon by boron implantation, leading to the formation of dislocation loops between the lattice structures. The silicon samples were doped with a high concentration of carbon before boron implantation and then annealed at a high temperature to activate the dopants into substitutional lattice sites. Photoluminescence (PL) measurements were performed to observe the emissions at the near-infrared region. The temperatures were varied from 10 K to 100 K to study the effect of temperature on the peak luminescence intensity. Two main peaks could be seen at ~1112 and 1170 nm by observing the PL spectra. The intensities shown by both peaks in the samples incorporated with boron are significantly higher than those in pristine silicon samples, and the highest intensity in the former was 600 times greater than that in the latter. Transmission electron microscopy (TEM) was used to study the structure of post-implant and post-anneal silicon sample. The dislocation loops were observed in the sample. Through a technique compatible with mature silicon processing technology, the results of this study will greatly contribute to the development of all Si-based photonic systems and quantum technologies.

The role of boron related defects in limiting charge carrier lifetime in 4H–SiC epitaxial layers

One of the main challenges in realizing 4H–SiC (silicon carbide)-based bipolar devices is the improvement of minority carrier lifetime in as-grown epitaxial layers. Although Z1/2 has been identified as the dominant carrier lifetime limiting defect, we report on B-related centers being another dominant source of recombination and acting as lifetime limiting defects in 4H–SiC epitaxial layers. Combining time-resolved photoluminescence (TRPL) measurement in near band edge emission and 530 nm, deep level transient spectroscopy, and minority carrier transient spectroscopy (MCTS), it was found that B related deep levels in the lower half of the bandgap are responsible for killing the minority carriers in n-type, 4H–SiC epitaxial layers when the concentration of Z1/2 is already low. The impact of these centers on the charge carrier dynamics is investigated by correlating the MCTS results with temperature-dependent TRPL decay measurements. It is shown that the influence of shallow B acceptors on the minority carrier lifetime becomes neutralized at temperatures above ∼422 K. Instead, the deep B related acceptor level, known as the D-center, remains active until temperatures above ∼570 K. Moreover, a correlation between the deep level concentrations, minority carrier lifetimes, and growth parameters indicates that intentional nitrogen doping hinders the formation of deep B acceptor levels. Furthermore, tuning growth parameters, including growth temperature and C/Si ratio, is shown to be crucial for improving the minority carrier lifetime in as-grown 4H–SiC epitaxial layers.

Annealing-induced phase conversion on spray pyrolyzed cubic-SnS thin films

The cubic-tin sulfide (SnS) compound material is optimal for the absorber layer in photovoltaic technology. In this study, the role of annealing temperature on the physical properties of cubic-SnS thin film has been determined. The spray pyrolyzed SnS thin films were post-annealed, using the chemical vapor deposition system, at the temperature range between 350 and 500 °C The annealed films have been analyzed using a comprehensive range of characterization techniques i.e., X-ray diffraction (XRD), Raman spectroscopy, UV–Vis spectroscopy, Photoluminescence spectroscopy (PL), Field-emission scanning electron microscopy (FESEM), Energy dispersive spectroscopy (EDS) and Hall measurements. The XRD results discovered the phase deterioration of cubic-SnS at higher annealing temperatures i.e., > 350 °C. Further, the Raman analysis confirmed the cubic-SnS phase deterioration, along with the formation of SnS2 and Sn2S3 secondary phases, at higher annealing temperatures. Besides that, a band gap in the range of 1.63–1.68 eV has been obtained for the SnS thin films. The films exhibit the near-band edge emission peak in the PL spectra. Moreover, the SEM micrographs show the needle-shaped grains, and their size and distribution were increased with respect to the enhancement in annealing temperature. A considerable amount of sulfur inclusion was observed in EDS analysis and the films annealed at 450° exhibit the near stoichiometric composition ratio of Sn/S = 1.01. The hall measurement studies showed resistivity, carrier concentration, and mobility of 29.4–376.5 Ω cm, 4.2 × 1014–3.0 × 1016 cm−3 and 13.1–66.1 cm2/Vs, respectively.

Investigations of Sidewall Passivation Using the Sol-Gel Method on the Optoelectronic Performance for Blue InGaN Micro-LEDs

The optoelectronic effects of sidewall passivation on micro-light-emitting diodes (Micro-LEDs) were investigated using sol-gel chemical synthesis. Blue InGaN/GaN multi-quantum well (MQW) Micro-LEDs, ranging in size from 20 × 20 μm to 100 × 100 μm and with high EQE, were fabricated and distinguished by the passivation method used, including no passivation, sol-gel SiO2, and plasma-enhanced chemical vapor deposition (PECVD) SiO2. Impressively, the sol-gel method is advantageous in improving the optoelectronic performance of Micro-LEDs. The fabricated 20 × 20 μm Micro-LEDs showed an EQE of 27.7% with sol-gel passivation, which was a 14% improvement compared to devices without sidewall passivation. Sol-gel sidewall passivation allows Micro-LEDs to effectively achieve sharper edge emission, superior surface luminous uniformity, and intensity, providing the possibility for the fabrication of low-cost and high-efficiency Micro-LEDs.

Structural, spectroscopic, and electrical studies of spin-coated ZnO-ZTO thin films for their potential application in photocatalysis and optoelectronics

In this paper, we synthesized and investigated the structural, optical, and electrical properties of spin-coated zinc-tin-oxide thin films with varying molar ratios and total precursor concentrations for photocatalytic applications. The structural studies were carried out using XRD and Raman spectroscopy. XRD and Raman analysis confirms that all the polycrystalline films are composed of ZnO and Zn2SnO4 mixed phases. UV–Vis and fluorescence spectroscopy were used to investigate the optical properties and estimate the band gap of the thin films. Dual absorption edges are observed in the UV–Vis absorption spectra of films prepared with a Zn/Sn = 2 M ratio. Photoluminescence studies confirm the dominated near band edge (NBE) emission for films prepared with a Zn/Sn = 2 M ratio. The electrical resistivity and Hall Effect measurement were carried out on all the studied compositions. The Haacke's figure of merit (FOM) is found to be in the range of 3 × 10−6 to 9 × 10−7 for the studied samples. The as-grown films were subjected to photocatalytic degradation studies using methyl orange (MO) dye under UV light irradiation. We achieved MO degradation efficiency as high as 80% in less than 90 min.