Near-band Edge Emission Research Articles

The surface-plasmon-resonance and band bending effects on the photoluminescence enhancement of Ag-decorated ZnO nanorods

Doping with noble metal nanoparticles (NPs) is an effective method to tune the photoluminescence (PL) properties of semiconductor materials. The mechanism is widely attributed to the localized surface-plasmon-resonance (SPR) effect of the metal NPs, while the contribution of potential barrier at the interface between metal and semiconductor is less discussed. Taking ZnO nanorods on fused silica substrates as a model system, we present a facile low-temperature fabrication method to derive Ag metal NPs coated ZnO nanorods. Great enhancement of the near-band-edge (NBE) ultraviolet emission of the ZnO nanorods was achieved by the surface decoration with Ag NPs, accompanied with a decrement of deep-level (DL) emission intensity. Since, the Ag NPs are active even at ambient conditions, spontaneous changes in PL properties have been observed with aging time. The influence of oxidation state of Ag on the contacting potential and subsequently on the PL properties of ZnO nanorods were studied by employing a subsequent thermal annealing process. The role of contacting behavior and the varied potential barrier are comprehensively discussed. It was demonstrated that both the SPR effect and band bending effect can enhance the NBE emission and suppress the DL emission. In the Ag-decorated ZnO nanorods, the SPR effect is the main reason for PL enhancement of the metallic silver, while the potential barrier enhancement is the main reason for the oxidized silver.

Investigation of structural and photoluminescence properties of gas and metal ions doped zinc oxide single crystals

We report the structural and photoluminescence properties of annealing dependent and gas and metal ion implanted zinc oxide (ZnO) single crystals. Gd, Na and N ions were implanted into ZnO at an average implantation depth of 12, 24, and 38nm, respectively from the surface layer. The samples were annealed under high vacuum or oxygen at 650°C and their effects were studied. Raman spectra of Gd implanted and oxygen annealed ZnO revealed the A1 longitudinal optical, A1(LO) mode, usually assigned to intrinsic defects, which suggested a partial recovery of implantation induced disorders, unlike ion implanted and vacuum annealed ZnO. Low temperature photoluminescence spectra from unimplanted and Gd implanted ZnO revealed transitions from neutral bound exciton, two electron satellites (TES) and LO phonon replicas. Deep level transitions in unimplanted and Gd implanted ZnO were affected by the annealing atmosphere, and oxygen annealing reduced the deep level emission, suggesting oxygen vacancy related emissions. A small red-shift in the near-band edge emission and a blue-shift in deep level emissions were observed in N and Na implanted and annealed ZnO, indicating slight changes in their band gap energies.

Influence of annealing temperature on ZnO thin films grown by dual ion beam sputtering

We have investigated the influence of in situ annealing on the optical, electrical, structural and morphological properties of ZnO thin films prepared on p-type Si(100) substrates by dual ion beam sputtering deposition (DIBSD) system. X-ray diffraction (XRD) measurements showed that all ZnO films have (002) preferred orientation. Full-width at half-maximum (FWHM) of XRD from the (002) crystal plane was observed to reach to a minimum value of 0.139° from ZnO film, annealed at 600 °C. Photoluminescence (PL) measurements demonstrated sharp near-band-edge emission (NBE) at ~ 380 nm along with broad deep level emissions (DLEs) at room temperature. Moreover, when the annealing temperature was increased from 400 to 600 °C, the ratio of NBE peak intensity to DLE peak intensity initially increased, however, it reduced at further increase in annealing temperature. In electrical characterization as well, when annealing temperature was increased from 400 to 600 °C, room temperature electron mobility enhanced from 6.534 to 13.326 cm2/V s, and then reduced with subsequent increase in temperature. Therefore, 600 °C annealing temperature produced good-quality ZnO film, suitable for optoelectronic devices fabrication. X-ray photoelectron spectroscopy (XPS) study revealed the presence of oxygen interstitials and vacancies point defects in ZnO film annealed at 400 °C.

Effect of annealing on lattice strain and near-band-edge emission of ZnO nanorods

The effect of air and oxygen annealing on the structural and the optical properties of hydrothermally synthesized ZnO nanorods was investigated. After hydrothermal synthesis, the resulting ZnO nanorods were annealed in air and under an oxygen atmosphere at 370°C for 1 h. X-ray diffraction results revealed that the oxygen-annealed nanorods possessed high crystallinity with a hexagonal-wurtzite crystal structure in the (002) plane. Evaluation of strain showed a tensile lattice strain of 0.426% resulting from oxygen annealing. The photoluminescence measurements showed that the relative intensity ratio of the near-band-edge emission (NBE) to the green emission (INBE/IGE) increased from ~2.6 for the as-grown ZnO nanorods to ~68.7 when the nanorods were annealed under oxygen. After annealing, a red shift of ~30 and ~44 meV in the NBE was observed for the nanorods that were annealed in air and under oxygen, respectively. This shift is attributed to the interaction between the neutral acceptors and the adsorbed oxygen atoms.

Enhanced near-band edge emission from ZnO nanorods by V2Os coating and subsequent thermal annealing.

V2O5-coated ZnO 1D nanostructures were prepared by using a two step process: thermal evaporation of a mixture of ZnO and graphite powders (ZnO:C = 1:1) in an oxidative atmosphere and sputter-deposition of V2O5. Scanning electron microscopy revealed that the nanostructures had a rod-like morphology with the thickness diminishing gradually from an end to the other. The thicknesses and lengths of the nanorods range from a few tens to a few hundreds of nanometers and from a few to a few tens of micrometers, respectively. Transmission electron microscopy and X-ray diffraction analyses revealed that the ZnO cores and V2O5 shells of the core-shell nanorods were wurtzite-type hexagonal close-packed structured single crystal and amorphous, respectively. The intensity ratio of the near-band edge (NBE) emission to the deep-level emission was increased about three times by coating the ZnO nanorods with a V2o5 thin film about 10 nm thick. The NBE emission enhancement may be mainly attributed to two sources: the effects of suppression of capturing of carriers by surface states and suppression of visible emission and nonradiative recombination by depletion regions formed in the ZnO cores. In addition, it was found that postannealing of V2O5-coated ZnO nanorods is not desirable, whereas post annealing makes a positive effect on the NBE emission enhancement in uncoated ZnO nanorods.

Synthesis at the nanoscale of ZnO into poly(methyl methacrylate) and its characterization

In this paper, PMMA/ZnO nanocomposites have been prepared by a very simple, facile and versatile chemical approach. The prepared PMMA/ZnO nanocomposites possess no color, high transparency, good thermal stability, UV-shielding capability, luminescence and homogeneity. The chemical process involved solution mixing of ZnO nanoparticles dispersed in DMAc with the Polymethylmethacrylate (PMMA) matrix dissolved in the same solvent. The effect of ZnO content on the physical properties of the PMMA matrix is investigated by X-ray diffraction, field emission scanning electron microscopy, thermogravimetric analysis, UV–Vis absorption and photoluminescence spectroscopy. It was found that pure hexagonal ZnO nanoparticles with an average particle size of 4–8 nm were homogeneously dispersed in the PMMA matrix. A significant improvement in thermal properties was observed with the incorporation of 1.0 wt% ZnO nanoparticles. The prepared nanocomposite films are highly transparent and a clear excitonic peak is observed in their absorption spectra. Measurement of room temperature photoluminescence spectra shows intensive near-band edge emission peak at 3.28 eV without any structural defects for a nanocomposite film with a filler content of 1.0 wt%.

Deep-level emission in ZnO nanowires and bulk crystals: Excitation-intensity dependence versus crystalline quality

The excitation-intensity dependence of the excitonic near-band-edge emission (NBE) and deep-level related emission (DLE) bands in ZnO nanowires and bulk crystals is studied, which show distinctly different power laws. The behavior can be well explained with a rate-equation model taking into account deep donor and acceptor levels with certain capture cross sections for electrons from the conduction band and different radiative lifetimes. In addition, a further crucial ingredient of this model is the background n-type doping concentration inherent in almost all ZnO single crystals. The interplay of the deep defects and the background free-electron concentration in the conduction band at room temperature reproduces the experimental results well over a wide range of excitation intensities (almost five orders of magnitude). The results demonstrate that for many ZnO bulk samples and nanostructures, the relative intensity R = INBE/IDLE can be adjusted over a wide range by varying the excitation intensity, thus, showing that R should not be taken as an indicator for the crystalline quality of ZnO samples unless absolute photoluminescence intensities under calibrated excitation conditions are compared. On the other hand, the results establish an all-optical technique to determine the relative doping levels in different ZnO samples by measuring the excitation-intensity dependence of the UV and visible luminescence bands.

K-doping effects on the characteristics of ZnO thin films synthesized by using a spin-coating method

Zinc-oxide (ZnO) thin films doped with various amounts of potassium were synthesized on quartz substrates by using the sol-gel method. The effects of K-doping on the structural and optical properties of the resultant ZnO thin films were investigated using X-ray diffraction (XRD), Ultravioletvisible (UV) spectroscopy, and photoluminescence (PL) spectroscopy. From the XRD results, the full width at half maximum (FWHM) for ZnO (002) and the grain size was measured. In the transmittance spectra, all the K-doped ZnO thin films were found to exhibit a high transmittance in the visible region. The optical band gap of the thin films was also determined from the transmittance results. The PL spectra showed near-band-edge (NBE) emission peaks in the UV region and deep-level emission (DLE) peaks of around 640 nm. The PL peak intensities of both the NBE emission and the DLE were found to change with varying concentrations of K.

Synthesis, characterization and catalytic properties of palladium–cupric oxide–aluminum oxide nanocomposites

Cupric oxide–aluminum oxide (CuO–Al2O3) composites were prepared by co-precipitation of carbonates from aqueous solutions of metal nitrates followed by calcination. A novel palladium-oxide (Pd·Cu1+xAl2−xO4) composite has been synthesized by heating Pd (acetylacetonate)3 and CuO·Al2O3 in a mixture of oleic acid and oleyl amine. The sample was characterized by XRD, SEM, EDS, TEM and PL measurements. Both XRD and TEM indicate that the product is Pd dispersed in Cu1+xAl2−xO4 matrix. Emission spectra have been recorded at 220 to 470nm excitations. The sharp PL peak observed in each case in the UV region (360–371nm) when excited at 220, 250, 270 and 300nm, respectively is due to the Near Band Edge Emission (NBE) of Pd·Cu1+xAl2−xO4. It has been found that the PL behavior of Pd·Cu1+xAl2−xO4 depends on the excitation wavelength. This kind of excitation wavelength dependent PL behavior is a violation of Kasha׳s rule of excitation wavelength dependence of emission spectrum. The major excitation peak was observed at 356nm when monitored at 470nm. Catalytic activity of Pd·Cu1+xAl2−xO4 was tested for Suzuki-Coupling reactions.

Tunable enhancement of exciton emission from MgZnO by hybridized quadrupole plasmons in Ag nanoparticle aggregation

Mg0.2Zn0.8O/metal nanoparticle systems have been fabricated and investigated. The photoluminescence results indicate that Al and Au nanoparticles could slightly enhance the near-band-edge (NBE) emission from Mg0.2Zn0.8O. In contrast, a giant and tunable NBE emission enhancement could be induced by Ag nanoparticles based on the coupling interaction between the hybridized quadrupole plasmon in Ag nanoparticle aggregation and the excitons of Mg0.2Zn0.8O. Interestingly, the intensity and position of the narrow quadrupole resonance could be controlled by tuning the interspace gap and size of Ag nanoparticles, which was clearly demonstrated both experimentally and theoretically. Our findings may pave the way for further development of high-efficiency UV light-emitting devices.

Effects of magnetite nanoparticles on optical properties of zinc sulfide in fluorescent-magnetic Fe3O4/ZnS nanocomposites

Magnetic-fluorescent Fe3O4/ZnS nanocomposites (NCs) with varying concentration of the constituents have been prepared by a simple chemical route. XRD and TEM measurements confirmed the phase purity and crystallinity of the NCs. Thermal analysis shows that the NCs are stable up to about 500°C. Mössbauer analysis reveals that magnetite in the NCs is nonstoichiometric having cation vacancies at the octahedral sites. UV–vis absorption spectra of the NCs show a shift of the absorption peak towards shorter wavelength indicating modification of the band gap of ZnS after formation of the NCs. Photoluminescence emission (PLE) spectra of the NCs display strong near band edge emission (NBE) in the UV region along with moderately strong emission bands in the visible range caused by electronic transitions involving defect-related energy levels in the band gap of ZnS. FTIR and Raman studies confirm presence of only ZnS and Fe3O4 in the NCs. Dc magnetization measurements show a reduction of magnetization value with increase of ZnS content in the NCs. The irreversibility noticed in the ZFC–FC curves has been attributed to the presence of spin-glass like phase in the NCs caused by relaxing spin moments near the surface of Fe3O4 nanoparticles.

Fabrication and multi-band photoluminescence of vertically aligned ZnO/ZnSe core/shell nanorod arrays

Vertically aligned ZnO/ZnSe core/shell nanorod (NR) arrays were fabricated on Si (100) using combined approach of hydrothermal growth of ZnO followed by pulsed laser deposition of ZnSe. The obtained ZnO/ZnSe core/shell NRs are composed of wurtzite ZnO cores and zinc blende ZnSe shells with the hydrothermally grown ZnO core nanorods preferentially c-axis oriented. Upon excitation by 325 nm light at room temperature, the ZnO/ZnSe core/shell NR arrays emit multi-band photoluminescence (PL) including the near-band-edge (NBE) emissions of ZnO and ZnSe as well as the defect-related emission and the emission associated with the effective band gap formed due to the staggered band alignment of the heterojunction constructed from ZnO and ZnSe, while the NBE emission of the ZnO cores is greatly suppressed by the ZnSe shells. The PL shows a strong dependence on temperature. Post-fabrication annealing of the samples in N2 results in an improvement in crystal structure and an enhancement of PL intensity.

Extended photoresponse and multi-band luminescence of ZnO/ZnSe core/shell nanorods

Aligned ZnO/ZnSe core/shell nanorods (NRs) with type-II energy band alignment were fabricated by pulsed laser deposition of ZnSe on the surfaces of hydrothermally grown ZnO NRs. The obtained ZnO/ZnSe core/shell NRs are composed of wurtzite ZnO cores and zinc blende ZnSe shells. The bare ZnO NRs are capable of emitting strong ultraviolet (UV) near band edge (NBE) emission at 325-nm light excitation, while the ZnSe shells greatly suppress the emission from the ZnO cores. High-temperature processing results in an improvement in the structures of the ZnO cores and the ZnSe shells and significant changes in the optical properties of ZnO/ZnSe core/shell NRs. The fabricated ZnO/ZnSe core/shell NRs show optical properties corresponding to the two excitonic band gaps of wurtzite ZnO and zinc blende ZnSe and the effective band gap between the conduction band minimum of ZnO and the valence band maximum ZnSe. An extended photoresponse much wider than those of the constituting ZnO and ZnSe and a multi-band photoluminescence including the UV NBE emission of ZnO and the blue NBE emission of ZnSe are observed.

Heterogeneous nucleation for synthesis of sub-20 nm ZnO nanopods and their application to optical humidity sensing

We present a novel method for colloidal synthesis of one-dimensional ZnO nanopods by heterogeneous nucleation on zero-dimensional ZnO nanoparticle ‘seeds’. Ultra-small ZnO nanopods, multi-legged structures with sub-20nm individual leg diameters, can be synthesized by hydrolysis of a Zn2+ precursor growth solution in presence of ∼4nm ZnO seeds under hydrothermal conditions via microwave-assisted heating in as little as 20min of reaction time. One-dimensional ZnO nanorods are initially generated in the reaction mixture by heterogeneous nucleation and growth along the [0001] direction of the ZnO crystal. Growth of one-dimensional nanorods subsequently yields to an ‘attachment’ and size-focusing phase where individual nanorods fuse together to form multi-legged nanopods having diameters ∼15nm. ZnO nanopods exhibit broad orange-red defect-related photoluminescence in addition to a near-band edge emission at 373nm when excited above the band-gap at 350nm. The defect-related photoluminescence of the ZnO nanopods has been applied towards reversible optical humidity sensing at room temperature. The sensors demonstrated a linear response between 22% and 70% relative humidity with a 0.4% increase in optical intensity per % change in relative humidity. Due to their ultra-small dimensions, ZnO nanopods exhibit a large dynamic range and enhanced sensitivity to changes in ambient humidity, thus showcasing their ability as a platform for optical environmental sensing.

β-Sialon nanowires, nanobelts and hierarchical nanostructures: morphology control, growth mechanism and cathodoluminescence properties

Morphology control of one dimension (1D) nanomaterials is a pivotal issue in the field of nanoscience research to exploit their novel properties. Herein, we report the morphology controlled synthesis of 1D β-Sialon nanowires, nanobelts and hierarchical nanostructures via a thermal-chemical vapour deposition process using an appropriately selected catalyst and optimized temperature schedule. Vapour-solid (VS), a combination of vapour-liquid-solid (VLS)-based and VS-tip, and a combination of VS for one-generation nanowires with nucleation, growth and coalescence of two-generation nanobranches (NGCB) are used to explain the growth of β-Sialon nanowires, nanobelts and hierarchical nanostructures, respectively. Cathodoluminescence measurements show that the individual β-Sialon 1D nanostructures with different morphologies have different luminescent properties. All nanostructures exhibit two distinct emission peaks, the violet/blue emission centered at ~390 nm (3.18 eV), attributable to the near band edge (NBE) emission, and the red emission centered at ~728 nm (1.70 eV), assigned to the deep level (DL) emission. However, the DL emission is the ruling emission in the case of an individual β-Sialon nanowire, whereas the NBE emission becomes dominant in the case of an individual nanobelt as well as a hierarchical nanostructure due to the size and surface effects. The as-synthesized β-Sialon with controlled nanostructures and various morphologies can find potential applications in future nanodevices with tailorable or tunable photoelectric properties.

Substrate-induced effects on the optical properties of individual ZnO nanorods with different diameters

We present the influence of a substrate on the properties of well-dispersed individual ZnO nanorods (NRs) with different diameters, especially on the photoluminescence (PL) properties. The studied ZnO NRs were partially supported by the quartz substrate and partially suspended in air. Continuous redshift and intensity decrease of the near band-edge emission (NBE) were observed along the suspended segment of the ZnO NRs due to the increasing temperature under UV laser excitation, suggesting that the presence of the substrate can effectively enhance the heat-sinking capability of ZnO NRs. Based on the PL measurements on individual suspended ZnO NRs with diameters from 86 nm to 2.35 μm, the redshift of NBE along the suspended segment was more obvious for ZnO NRs with a smaller diameter, indicating that the thermal conductive ability increases as diameter increases. Additionally, by combining the experimental and simulation results, we found that the presence of the substrate also quenched the whispering gallery modes (WGMs) of the ZnO NRs with a diameter above about 350 nm due to the symmetry breaking induced by the quartz substrate which has a larger refractive index compared with air. Our studies confirm that the substrate significantly influences the properties of ZnO NRs. To fully utilize the potential properties of nanomaterials for applications in nanodevices, the substrate-induced effects should be considered thoughtfully.

Hydrothermally grown boron-doped ZnO nanorods for various applications: Structural, optical, and electrical properties

The structural, optical, and electrical properties of ZnO and BZO nanorods were investigated using fieldemission scanning electron microscopy, x-ray diffraction (XRD), photoluminescence (PL), and van der Pauw Hall-effect measurements. All the nanorods had grown well on the ZnO seed layers and were hexagonal. The BZO nanorods were shorter than the undoped ZnO nanorods, and the BZO nanorods grew shorter with increasing concentration of B to 2.0 at. % while the average length of the nanorods doped with 2.5 at. % B increased from 1620 to 1830 nm. The XRD patterns suggest that the amount of residual stress in the nanorods decreased with increasing concentration of B in the nanorods. The PL spectra showed near-bandedge and deep-level emissions, and B doping also varied the PL properties of the ZnO nanorods. The Halleffect data suggest that B doping also varied the electrical properties such as the carrier concentration, mobility, and resistivity of the ZnO nanorods.

Effects of N2O addition on the properties of ZnO thin films grown using high-temperature H2O generated by catalytic reaction

ABSTRACTThe effects of N2O gas addition on the properties of zinc oxide films grown on a-plane (11-20) sapphire (a-Al2O3) substrates were investigated, using a chemical vapor deposition method based on the reaction between dimethylzinc and high-energy H2O produced by a Pt-catalyzed H2-O2 reaction. By employing an optimal N2O gas pressure, both the film crystallinity and crystal orientation were improved. Subsequent to treatment with N2O, the electron mobility of films at room temperature increased from 207 to 234 cm2/Vs while the electron concentration decreased at low temperatures. In addition, the photoluminescence peak intensity of the nearband-edge emission was increased.

Strong Enhancement of Near-Band-Edge Photoluminescence of ZnO Nanowires Decorated with Sputtered Metallic Nanoparticles

The effect of the Ar plasma during metal deposition on the photoluminescence (PL) of metal-coated ZnO nanowires (NWs) has been investigated. Strong enhancement of near-band-edge emission (NBE) is observed for ZnO NWs coated with Al and Ni nanoparticles (NPs) by radiofrequency magnetron sputtering, while the samples coated with NPs by e-beam evaporation show quenching of the PL intensity. A model is proposed that satisfies the observed experimental results and assigns the strong enhancement of the NBE PL of ZnO NWs to excitons bound to structural defects in the surface layer of the ZnO NWs.

Growth of Low-dimensional ZnO Materials on Graphite Substrate

Low-dimensional ZnO materials, (1-dimensional ZnO nanorods (NRs) and 2-dimensional ZnO films) were grown perpendicularly on graphite substrates. The crystalline structure, morphology, and optical properties of the as-grown low-dimensional ZnO materials were investigated with X-ray diffraction (XRD), field effect scanning electron microscope (FESEM), photoluminescence (PL), and the relative reflection spectra measurements. The high optical qualities of borth ZnO NRs and ZnO films on graphite substrates were demonstrated by the dominant near-band edge emission and nearly undetectable deep level emissions of photoluminescence spectra under room temperature. The extremely low average reflectance was obtained from the low-dimensional ZnO materials/graphite structures in the spectra range from 300 nm to 800 nm, indicating that the obtained low-dimensional ZnO materials/graphite structures have significant opportunity for potential application in high-performance photovoltaic devices.