Enhanced UV Emission Research Articles

Enhanced UV emission and photocatalytic activity due to morphology evolution of ZnO thin films

Tuning the optical properties of nanomaterials through morphology evolution has attracted extensive attention. In this study, ZnO films were deposited on substrates using sol-gel approach, and ZnO nanocolumn arrays were formed on the film surface induced by doped ions. XRD results show that the nanocolumns all have a growth orientation along the c-axis and good crystallization quality. Compared with pure ZnO film, the ones with nanocolumn arrays on the film surface exhibit higher UV emission efficiency and photocatalytic activity. The enhanced UV emission efficiency is attributed to the higher crystalline quality and fewer surface defects of the nanocolumn arrays. The improved photocatalytic activity is mostly ascribed to two factors: (1) nanocolumns are more conducive to the migration of photogenerated electrons and holes to the photocatalyst surface to participate in redox reactions; (2) the formation of nanocolumn arrays improves light absorption of the ZnO films.

Solution processed highly transparent nitrogen-doped carbon quantum dots/ZnO hybrid thin films: A study on structural and enhanced UV emission

We deposited the highly transparent nitrogen-doped carbon quantum dots (N-CQDs) incorporated ZnO (N-CQDs/ZnO) hybrid thin films by sol–gel spin-coating process. TEM studies show a narrow size distribution of hydrothermally synthesized N-CQDs in the range of 2.5 to 3 nm. The SEM images ascertain the formation of ZnO nanoparticles upon incorporation of N-CQDs in the hybrid films. Raman studies show the hexagonal wurtzite structure of spin-coated ZnO thin film and ascertain the presence of incorporated N-CQDs adsorbed on the surface of ZnO in the hybrid films. The incorporated N-CQDs get adsorbed onto the surface of ZnO crystallites and control its size at nanoscale in the hybrid thin films. The XPS study also confirms the incorporation of nitrogen-doped CQDs and the existence of chemical interaction (chemisorption) between N-CQDs and ZnO nanoparticles, making the pathways for favorable energy transfer between N-CQDs and ZnO in the hybrid thin films. The band-edge emission (383 nm) of ZnO film increases for the increasing concentration of N-CQDs upon UV excitation (325 nm), showing the non-radiative UV energy transfer from N-CQDs to ZnO in the hybrid thin films along with the contribution of photoluminescence emission of N-CQDs in the visible region.

Enhanced UV emissions in Polypyrrole/PVA composite for smart apparels

A cost-effective, scalable, eco-friendly, flexible smart apparel was fabricated by casting method employing polypyrrole(PPy) with biodegradable polyvinyl alcohol on cotton fabric. Different concentration of PPy is embedded in PVA. The X-ray diffraction pattern affirmed the interaction between PVA and PPy. Optical absorption spectra showed two peaks corresponding to PVA and the excitonic peak of PPy higher concentration of GCN exhibited a low bandgap harvesting the visible and IR part of the solar regime. High optical (0.104 S/cm) and electrical conductivity(2.3928 × 10−3) were attained for the higher concentration of PPy which might be attributed to the availability of the more defect centers in between the HUMO and LUMO of PVA. The room temperature photoluminescence exhibited enhanced blue emission at 379 nm for a higher concentration of PPy in PVA thin film due to the charge transfer mechanism. As 5PPy possesses, higher PL emission and higher electrical conductivity, it was taken further to fabricate smart apparel in cotton cloth. The fabricated smart apparels can be used to manufacture flexible, lightweight, eco-friendly optoelectronic devices.

Defects suppression and Co–O–Co bonding mediated influences to enhance the luminescence characteristics of Co doped ZnO nanoparticles

• Co–O–Co bonding influences are observed on Co doped ZnO nanoparticles. • Suppression of Zinc interstitials, Zn i and oxygen vacancies, V o is noted with increased doping. • Exceptionally low level of BMP density (3.22 × 10 7 /cm 3 ) is noted in 5 mol% doped nanoparticles. • Enhanced luminescence characteristics justify the nanoparticles suitability for optical devices. Co–O–Co bonding mediated influences are discussed on the structural, defect content, magnetization and luminescence characteristics of Co doped ZnO nanoparticles. X-ray diffraction (XRD) studies indicated an increased fraction of secondary phase while varying Co doping levels from 0.45 to 5 mol%. Contrarily, a reverse trend is noted on the concentration of Zinc interstitials, Zn i and Oxygen vacancies, V o defects through Photoluminescence (PL) spectroscopy. For 5 mol% doping level, Bound Magnetic Polaron (BMP) model fitting of Magnetization versus magnetic field, M(H, 300 K) curve yields an exceptionally low level of BMP density i.e ., 3.22 × 10 7 /cm 3 thereby governing the dominance of Co–O–Co exchange interactions over V o based Co– V o –Co interactions; while PL result displays an enhanced UV emission efficiency suitable for short wavelength optical devices.

Enhanced UV emission and supercapacitor behavior of Zn doped CeO2 quantum dots

The impact of Zn dopant on the phase, energy gap, functional group, microstructure, cyclic voltmeter and GCD behavior of CeO2 quantum dots prepared via a simple precipitation method is discussed in this article. The XRD analysis indicates that Zn doping doesn’t alter the basic FCC structure of CeO2. The crystalline size and lattice constant were found to decrease with increasing the Zn dopant. The optical studies disclose that Zn2+ doping increases the bandgap (Eg) of CeO2 from 3.39 to 3.86 eV. The blue shift in the absorption edge confirmed the quantum confinement effect. The photoluminescence spectra displayed strong UV emission due to Zn doping in CeO2 quantum dots. FESEM and TEM study reveals that the synthesized samples consist of spherical to nanoclusters morphology and the particle size varies between 11 and 4 nm. The presence of Ce-Zn-O bonds has been identified via FT-IR and EDS analysis. The electrochemical analysis suggests that the reported samples exhibit better electrochemical behavior. GCD study confirmed the 5 wt% Zn doped CeO2 quantum dots exhibit a higher specific capacitance of 358Fg−1 at a current density of 10Ag−1. The electrode stability was tested for Zn doped CeO2 electrode it shows 96% of cyclic stability even after 1000 continuous charge-discharge cycles. The cyclic voltmeter, GCD, power density, energy density and cyclic stability analysis suggest that the Zn doped CeO2 quantum dots could be utilized as electrode material for supercapacitor application.

Enhanced UV emission of solution processed highly transparent Alq3/ZnO hybrid thin films

We deposited the highly transparent tris-(8-hydroxyquinoline)aluminum(Alq3)/ZnO hybrid thin films by spin-coating process. The hybrid thin film with Alq3/ZnO molar ratio of 0.5 × 10−3 exhibits the higher transparency of 90% at 550 nm and it is slightly reduced for increasing Alq3 molecules due to the adsorption of Alq3 on the ZnO grain boundaries. The adsorption of incorporated Alq3 molecules on grain boundaries of ZnO creates Alq3 capped ZnO nanoparticles in the hybrid film. The X-ray diffraction studies reveal the textured crystalline property of the films with decreased crystallite size upon increasing Alq3 content. Fourier transform infrared studies confirm the increasing content of luminescent quencher (carbonyl group) formed in Alq3 molecule for increasing Alq3 in the films. The photoluminescence study shows a fourfold enhancement in the band-edge emission (387 nm) of ZnO upon increasing Alq3 content in the hybrid films, attributing the energy transfer from Alq3 molecule to ZnO when excited by 324 nm wavelength. These studies show that the excited state energy of adsorbed Alq3 molecule may be non-radiatively transferred to ZnO via luminescence quencher, enhancing a fourfold UV emission (387 nm) of ZnO in the hybrid thin films.

Effect of Mg doping on the improvement of photoluminescence and magnetic properties of NiO nanoparticles

NiO nanoparticles doped with different concentration of Mg were synthesized by a low cost wet chemical method. XRD characterization indicated that the fcc structure of NiO is not affected with Mg doping. TEM analysis further reveals the Mg doping induced reduction in grain agglomeration along with the spherical shape transformation in NiO. Significant enhancement of UV emission in Mg doped NiO nanoparticles is evident from photoluminescence characterization. Magnetic characterization of the samples showed that all the samples show weak ferromagnetism along with the dominance of background antiferromagnetism at room temperature. The perseverance of background antiferromagnetic order is further probed through Raman characterization. All the samples show first order antiferromagnetic to weak ferromagnetic transition along with the presence of shape memory effect as evident by the negative slope in Arrott plot. The Ni0.9Mg0.1O sample with much enhanced UV emission and magnetization could find the usefulness in UV optoelectronic device, spintronic device etc.

Enhanced UV Emission of GaN Nanowires Functionalized by Wider Band Gap Solution-Processed p-MnO Quantum Dots.

GaN-based UV light-emitting devices suffer from low efficiency. To mitigate this issue, we hybridized GaN nanowires (NWs) grown on Si substrates by plasma-assisted molecular beam epitaxy with solution-processed p-type MnO quantum dots (QDs) characterized by a wider band gap (∼5 eV) than that of GaN. Further investigations reveal that the photoluminescence intensity of the GaN NWs increases up to ∼3.9-fold (∼290%) after functionalizing them with p-MnO QDs, while the internal quantum efficiency is improved by ∼1.7-fold. Electron energy loss spectroscopy (EELS) incorporated into transmission electron microscopy reveals an increase in the density of states in QD-decorated NWs compared to the bare ones. The advanced optical and EELS analyses indicate that the energy transfer from the wider band gap p-MnO QDs to n-GaN NW can lead to substantial emission enhancement and greater radiative recombination contribution because of the good band alignment between MnO QDs and GaN NWs. This work provides valuable insights into an environmentally friendly strategy for improving UV device performance.

Hot Dust-obscured Galaxies with Excess Blue Light

Abstract Hot dust-obscured galaxies (Hot DOGs) are among the most luminous galaxies in the universe. Powered by highly obscured, possibly Compton-thick, active galactic nuclei (AGNs), Hot DOGs are characterized by spectral energy distributions that are very red in the mid-infrared yet dominated by the host galaxy stellar emission in the UV and optical. An earlier study identified a subsample of Hot DOGs with significantly enhanced UV emission. One target, W0204–0506, was studied in detail and, based on Chandra observations, it was concluded that the enhanced emission was most likely due to either extreme unobscured star formation (star formation rate > 1000 M ⊙ yr−1) or to light from the highly obscured AGN scattered by gas or dust into our line of sight. Here, we present a follow-up study of W0204–0506 as well as two more Hot DOGs with excess UV emission. For the two new objects we obtained Chandra/ACIS-S observations, and for all three targets we obtained Hubble Space Telescope/WFC3 F555W and F160W imaging. The analysis of these observations, combined with multiwavelength photometry and UV/optical spectroscopy suggests that UV emission is most likely dominated by light from the central highly obscured, hyperluminous AGN that has been scattered into our line of sight, by either gas or dust. We cannot decisively rule out, however, that star formation or a second AGN in the system may significantly contribute to the UV excess of these targets.

Enhancement of the UV emission from gold/ZnO nanorods exhibiting no green luminescence

Large reflection losses at interfaces in light-emitting semiconductor devices cause a significant reduction in their light emission and energy efficiencies. Metal nanoparticle (NP) surface coatings have been demonstrated to increase the light extraction efficiency from planar high refractive index semiconductor surfaces. This emission enhancement in Au NP-coated ZnO is widely attributed to involvement of a green (∼ 2.5 eV) deep level ZnO defect exciting localized surface plasmons in the NPs. In this work, we achieve a 6 times enhancement of the ultra-violet excitonic emission in ZnO nanorods coated with 5 nm Au NPs without the aid of ZnO defects. Cathodoluminescence (CL) and photoluminescence (PL) spectroscopy revealed that the increased UV emission is due to the formation of an additional fast excitonic relaxation pathway. Concurrent CL-PL measurements ruled out the presence of charge transfer mechanism in the emission enhancement process. While time-resolved PL confirmed the existence of a new excitonic recombination channel that is attributed to exciton relaxation via the excitation of rapid non-radiative Au interband transitions that increases the UV spontaneous emission rate. Our results establish that ZnO defect levels ∼ 2.5 eV are not required to facilitate Au NP induced enhancement of the ZnO UV emission.

Tailoring the optical properties of ZnO thin film by Citrus limon doped Polypyrrole

In this paper we demonstrate an approach to synthesize composite thin film comprising of polypyrrole (PPy) and zinc oxide (ZnO) by Solution casting method to meet the demand of LEDs. Commercial citric acid doped PPy (D1-PPy) and Citrus limon doped PPy (D2-PPy) has been synthesized via oxidative polymerization method. The effects of dopants on the physicochemical properties were investigated in detail. Interestingly, D2-PPy revealed lower bandgap when compared to D1-PPy due to the presence of polyphenols and terpenes in citrus limon. Due to the synergistic effect of ZnO and PPy, the absorption spectra extended to the visible region of the solar spectrum. The Room Temperature photoluminescence (RTPL) measurements reveal the enhancement of UV emission for D2-PPy/ZnO/PVA thin film which might be due to electronic passivation effect. The presence of –OH group in both the PVA and citric acid blocks the oxygen vacancy in ZnO, and exhibits enhanced near band edge emission. Thus, opens up the possibility of the D2-PPy/ZnO/PVA as a suitable candidate for UV based LEDs.

Interaction of ZnO nanorods with plasmonic metal nanoparticles and semiconductor quantum dots

We model the enhancement of near band edge emission from ZnO nanorods using plasmonic metal nanoparticles and compare it with emission enhancement from ZnO with semiconducting quantum dots. Selected CdSe quantum dots with absorption energies close to those of Ag and Au nanoparticles are chosen to construct model systems with ZnO to comprehend the role of ZnO's intrinsic defects and plasmonic excitation in realizing the spectrally selective luminescence enhancement. Excitation wavelength dependent photoluminescence spectra along with theoretical models quantifying the related transitions and plasmonic absorption reveal that a complex mechanism of charge transfer between the ZnO nanorods and metal nanoparticles or quantum dots is essential along with an optimal energy band alignment for realizing emission enhancement. The theoretical model presented also provides a direct method of quantifying the relative transition rate constants associated with various electronic transitions in ZnO and their change upon the incorporation of plasmonic nanoparticles. The results indicate that, while the presence of deep level defect states may facilitate the essential charge transfer process between ZnO and the plasmonic nanoparticles, their presence alone does not guarantee UV emission enhancement and strong plasmonic coupling between the two systems. The results offer clues to designing novel multicomponent systems with coupled plasmonic and charge transfer effects for applications in charge localization, energy harvesting, and luminescence enhancement, especially in electrically triggered nanophotonic applications.

Enhanced self-powered UV sensing performance of ZnO/Au/Al2O3 photodetector with the decoration of Au nanoparticles

In this work, self-powered ZnO/Au/Al2O3 photodetectors (PDs) with enhanced UV sensing performance were successfully fabricated. The effects of Au nanoparticles on optical and photodetect ability of ZnO/Au/Al2O3 nanorods were studied in detail. The results showed that the ZnO nanorods exhibited enhanced UV emission and reduced visible emission after the Au decoration. The best UV sensing performance was achieved from the ZnO/Au/Al2O3-based PD, of which the size of the Au nanoparticles and the thickness of the Al2O3 were 3–4 nm and 6 nm, respectively. The on/off ratio, UV sensing responsibility, and detectivity were 805, 6.8 mA/W, and 1.7 × 109 Jones, respectively, which were 8.5, 8.6, and 8.5 times to that of ZnO/Al2O3-based PD without Au nanoparticle decoration (@ light intensity of 0.6 mW/cm2; 0 bias). It was believed that the surface plasmonic effect, carrier confinement, and the surface passivation co-contributed to the enhanced UV photodetect ability. The work may provide an insight into the design and synthesis of 1D nano-material-based self-powered UV detectors for next-generation low cost, large area, and energy-efficient optoelectronic devices.

Plasmon-enhanced photoluminescence from SnO2 nanostructures decorated with Au nanoparticles

We report significantly enhanced photoluminescence from hybrid nanostructured Au/SnO2 thin films due to surface plasmons excited in Au nanoparticles decorating SnO2 nanostructured thin film. Nanostructured thin films with Au nanoparticles decorated SnO2 nanostructures were prepared by sputter deposition of ultrathin Au films following thermal evaporation of SnO2 film. The effects of deposition of ultrathin film of Au nanoparticles on the morphology, structure and optical response of nanostructured SnO2 film were investigated. Nanocrystalline SnO2, Au, Sn and Au-Sn alloy were revealed by XRD analysis. PL studies revealed that Au nanoparticle decoration of nanostructured SnO2 film led to highly enhanced UV emission. The mechanism of strong enhancement of PL from nanostructured SnO2 film is tentatively proposed. The coupling of excitonic emission from SnO2 nanostructures and excited surface plasmons in Au nanoparticles led to the observed strong PL enhancement. The hybrid nanostructured Au/SnO2 thin films with strong UV emission are promising for designing efficient optical and photoelectrical devices.

Enhanced UV emission of Li–Y co-doped ZnO thin films via spray pyrolysis

Pure ZnO and ZnO: 2%Y: x%Li (x = 0, 3, 5 and 7 at.%) thin films have been successfully prepared onto glass substrates under optimized conditions by spray pyrolysis technique at 450 °C and their suitability for the fabrication of efficient optoelectronic devices is demonstrated. The samples have been characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–Visible absorption spectroscopy photoluminescence (PL) and Raman spectroscopy (RS), in order to investigate the effect of Y–Li co-doping on the structure, surface morphology, and optical features of the thin films. The films crystallized into a hexagonal structure, with a preferred orientation along the c-axis. No additional phases have been observed. SEM micrographs showed that Y and Li co-doping plays a key role in the grain size and morphology of the films. The optical study via transmittance and absorption measurements within the UV–vis region revealed that the films are highly transparent (82–90%). The optical bandgap (Eg) depends on the concentration of lithium added, which is explained by the Burstein-Moss (BM) effect. The PL measurements at room temperature under excitation with 325 nm wavelength, showed an appreciable improvement of ultraviolet emission by increasing the Li co-doping concentration. This enhancement reaches a maximum at 5 at.% Li content, and decreases after further increase in Li content. Raman scattering spectra were also carried out and revealed the presence of the wurtzite phase of ZnO exclusively.

Tuning ZnO nanorods photoluminescence through atmospheric plasma treatments

Room temperature atmospheric plasma treatments are widely used to activate and control chemical functionalities at surfaces. Here, we investigated the effect of atmospheric pressure plasma jet (APPJ) treatments in reducing atmosphere (Ar/1‰ H2 mixture) on the photoluminescence (PL) properties of single crystal ZnO nanorods (NRs) grown through hydrothermal synthesis on fluorine-doped tin oxide glass substrates. The results were compared with a standard annealing process in air at 300 °C. Steady-state photoluminescence showed strong suppression of the defect emission in ZnO NRs for both plasma and thermal treatments. On the other side, the APPJ process induced an increase in PL quantum efficiency (QE), while the annealing does not show any improvement. The QE in the plasma treated samples was mainly determined by the near band-edge emission, which increased 5–6 fold compared to the as-prepared samples. This behavior suggests that the quenching of the defect emission is related to the substitution of hydrogen probably in zinc vacancies (VZn), while the enhancement of UV emission is due to doping originated by interstitial hydrogen (Hi), which diffuses out during annealing. Our results demonstrate that atmospheric pressure plasma can induce a similar hydrogen doping as ordinarily used vacuum processes and highlight that the APPJ treatments are not limited to the surfaces but can lead to subsurface modifications. APPJ processes at room temperature and under ambient air conditions are stable, convenient, and efficient methods, compared to thermal treatments to improve the optical and surface properties of ZnO NRs, and remarkably increase the efficiency of UV emission.

ZnO-doped KCl single crystal with enhanced UV emission lines

The embedded bulk KCl single crystal with the ZnO nanocrystal was grown using Czochralski technique. Various structural and optical analyses were employed to characterize the grown sample. X-ray diffraction confirmed the diffusion of ZnO NCs in the KCl bulk lattice. The crystal structure and the lattice parameters were estimated for both ZnO NCs and KCl matrix by Rietveld analysis. The bond formation of KCl:ZnO was demonstrated by Fourier transform infrared (FTIR) spectroscopy. FE-SEM was carried out for the morphological study of ZnO NCs and estimation of particle size distributions. Energy-dispersive X-ray (EDX) analysis was used to investigate the elemental composition of the prepared sample. The experimental results confirmed the improvement in optical properties of KCl single crystal was found when it was doped by ZnO. The second derivative of absorption spectrum confirmed the band gap of 6.06 eV for KCl single crystal. In PL spectrum, several intense UV light emission peaks at 297 and 360 nm were observed in which the emission peaks of KCl:ZnO samples were very sharp in comparison with the pure KCl single crystal. It seems that the UV peaks at 297 and 360 nm emission lines of ZnO-doped KCl crystals originated from electron capturing from F center.

Effect of surfactants on photoluminescence properties of ZnO synthesized by hydrothermal method

The effects of different surfactants on structure, morphology and PL properties of ZnO synthesized by hydrothermal method are investigated. SEM results show that the size and the morphology of ZnO can be effectively controlled by surfactants. Compared to other XRD results, the diffraction peak of ZnO in the presence of surfactant monoethanolamine (MEA) has better crystal quality, smaller FWHM and stronger intensity. Furthermore, the PL spectra of ZnO in the presence of sufactant MEA show an increase in the sharp UV emission band and the nearly visible broad emission band. Gaussian curve deconvolution for experimental PL spectrum of ZnO indicates that the enhancement of UV emission is due to the improvement on crystallinity and the mechanism of visible emission is ascribed to several intrinsic defects or hydroxyl groups on the surface of ZnO, which is in consistent with the FTIR results. The best PL properties of ZnO can be found in the presence of MEA (the molar ratio of MEA to Zn2+ is 1:1) with annealing temperature at 120 °C for 8 h annealing time. In conclusion, the introduction of surfactant MEA with both amino group and hydroxyl group in the synthesis of ZnO is beneficial to crystallinity and intrinsic defects on the surface of ZnO, which can enhance the PL properties.

Enhanced UV emission of TiO2–ZnO nanocomposite films synthesized by simplified sol–gel dip-coating method

Enhanced UV emission of TiO₂–ZnO nanocomposite films synthesized by simplified sol–gel dip-coating method

Preparation and characterization of ZnO/ZnS core/shell nanocomposites through a simple chemical method

In this paper, we reported the preparation of ZnO/ZnS core/shell nanocomposites by sulfidation of ZnO nanostructures via a simple hydrothermal method. The precursors of bare ZnO nanoparticles and ZnO nanorods were synthesized by a surfactant-assisted hydrothermal growth. The structural, morphological, and element compositional analysis of bare ZnO nanostructures and ZnO/ZnS core/shell nanocomposites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy techniques. The XRD results indicated that the phase of bare ZnO nanoparticles and ZnO nanorods was wurtzite structure, and the phase of coated ZnS nanoparticles on the surface of bare ZnO nanostructures was sphalerite structure with the size of about 8 nm. Photoluminescence measurement was carried out, and the PL spectra of ZnO/ZnS core/shell nanocomposites revealed an enhanced UV emission and a passivated orange emission compared to that of bare ZnO nanostructures. In addition, the growth mechanism of ZnO/ZnS core/shell nanostructures through hydrothermal method was preliminarily discussed.