Excitons In ZnO Research Articles

Enhanced Fluorescence and Local Vibrational Mode in Near‐White‐Light‐Emitting ZnO:Mg Nanorods System

We report exciton and phonon properties of ZnO:Mg nanorods of different Mg doping concentration. X‐ray diffraction studies (XRD) confirm the growth of wurtzite phase ZnO nanostructures. XRD reveals doping‐induced shift in peaks and formation of secondary phase related to Mg. Optical properties of the prepared nanorods are investigated by using UV‐Visible absorption and photoluminescence spectroscopic techniques. Optical absorption studies show strong free excitonic absorption of ZnO and extra absorption bands related to the defect centers of the secondary phase (MgO) formed after Mg doping. Photoluminescence studies show sharp band in UV region and defects‐related broad band emission in the visible range. Gaussian‐fitted photoluminescence spectra show that the emission is composed of free exciton recombination and its longitudinal optical (LO) phonon replica. In addition, Mg‐related local vibrational mode observed in Raman and FTIR spectra after Mg doping, indicates the incorporation of Mg into the lattice positions of wurtzite ZnO.

Improved UV photoresponse of ZnO nanorod arrays by resonant coupling with surface plasmons of Al nanoparticles.

In this study, localized surface plasmon resonance mediated by aluminium nanoparticles (Al NPs) was employed to enhance the ultraviolet (UV) response of ZnO nanorod array (NRA) photodetectors grown vertically on a Quartz substrate using a simple vapor transport method. The responsivity of the ZnO NRA photodetector decorated with Al NPs was enhanced from 0.12 to 1.59 A W(-1) and the sensitivity and response rate have been improved greatly compared with those of the bare one. The measurement results in the transmittance spectra and time-resolved photoluminescence spectra suggest that the improved photoresponse and the enhanced spontaneous emission of the ZnO NRA photodetector with Al NP decoration are both attributed to resonant coupling between the excitons in ZnO and the localized surface plasmons (LSPs) in the Al NPs. Our results demonstrated that the plasmon-enhanced ZnO NRA photodetector has a great potential for application in building sensors with a fast response and reset time, high sensitivity, and good signal-to-noise ratio for photoelectric sensing.

Zinc Oxide Exciton Luminescence Dependence on Photoexitation Level in Presence of Surface Plasmon Resonance

For the analysis of surface plasmon resonance influence on ZnO exciton (ultraviolet) luminescence the set of rate equations (SRE) containing a set of parameters that characterizes processes participating in luminescence was proposed. It is shown that utilizing SRE experimental ZnO microstructure radiation intensity dependence on photoexcitation level can be approximated. Thus, the values of SRE parameters can be estimated and used for luminescence analysis. This approach allows treating experimental results for Ag-covered ZnO microfilms exciton luminescence.

溶液法制备氧化锌晶体的微结构及发光特性研究

Micro/ nano鄄sized ZnO crystals with different shapes were fabricated by a simple hydro/ solvothermal method using zinc acetate and methenamine as precursors. The morphology and size of ZnO crystals are strongly de鄄 pendent on synthetic conditions. The growth of the polar plane can be suppressed due to the ethylenediaminetetraace鄄 tic acid disodium salt and citric acid compound selective adsorbing on the polar plane facets by the chemical adsorp鄄 tion. The selective adsorption makes the shape of ZnO crystal be tunable. Raman spectra confirm that grain size is smaller as ZnO synthesized in citric acid solution. The photoluminescence shows the transitions of exciton and defects in ZnO.

Plasmon-Enhanced Whispering Gallery Mode Lasing from Hexagonal Al/ZnO Microcavity

Wurtzite structural ZnO microrods with hexagonal cross section were fabricated by a simple vapor transport method and an individual ZnO microrod was employed as a whispering gallery microcavity. The obviously enhanced spontaneous and stimulated emissions were observed as the ZnO microrod was decorated with Al nanoparticles (NPs). Based on the absorption spectrum of Al NPs, the underlying mechanism was proposed and can be attributed to the resonant coupling between excitons of ZnO and surface plasmons (SPs) of Al NPs at the metal/ZnO interface on the surface of microcavity. In addition, the threshold of the ZnO microcavity decorated with Al NPs has reduced by half compared with that of the bare one, which provides additional evidence to support the energy coupling between the ZnO microrod and Al NPs. The controllability and improved emission properties of these types of microcavities are potentially important for designing highly efficient optoelectronic devices.

Enhanced photoluminescence of Co2+ ions in ZnCoO/ZnMgO multiple quantum wells and the fluorescence energy transfer mechanism

Using a pulsed laser deposition system, ZnCoO/ZnMgO multiple quantum well (MQW) samples were grown on c-plane sapphire substrate with a ~20nm thick ZnO buffer layer. Compared with monolayer ZnCoO film, the MQW samples exhibited obviously enhanced Co2+ photoluminescence (PL) at ~1.80eV and multiple-phonon resonant Raman scattering (RRS). The enhancement in multiple-phonon RRS was due to the introduction of ZnMgO barrier layer. The enhanced Co2+ PL was assigned to the quantum confinement effect (QCE) of MQW samples. However, QCE was found not helpful to prevent the band-gap PL quenching. Co2+ 3d electronic states were proved to be highly localized and a mechanism of fluorescence resonance energy transfer (FRET) between ZnO excitons and the localized Co2+ 3d states was proposed.

Spin dynamics of isoelectronic bound excitons in ZnO

Time-resolved optical spin orientation is employed to study spin dynamics of ${I}^{*}$ and ${I}_{1}^{*}$ excitons bound to isoelectronic centers in bulk ZnO. It is found that spin orientation at the exciton ground state can be generated using resonant excitation via a higher lying exciton state located at about 4 meV from the ground state. Based on the performed rate equation analysis of the measured spin dynamics, characteristic times of subsequent hole, electron, and direct exciton spin flips in the exciton ground state are determined as being ${\ensuremath{\tau}}_{h}^{s}$ = 0.4 ns, ${\ensuremath{\tau}}_{e}^{s}\ensuremath{\ge}$ 15 ns, and ${\ensuremath{\tau}}_{eh}^{s}\ensuremath{\ge}$ 15 ns, respectively. This relatively slow spin relaxation of the isoelectronic bound excitons is attributed to combined effects of (i) weak $e$-$h$ exchange interaction, (ii) restriction of the exciton movement due to its binding at the isoelectronic center, and (iii) suppressed spin-orbit coupling for the tightly bound hole.

Growth of ZnS-coated ZnO nanorod arrays on (1 0 0) silicon substrate by two-step chemical synthesis

In this study, ZnS coated ZnO nanorods were synthesized using a simple, cost effective two-step chemical method. A continuous coating of ZnS on a ZnO nanorod, having a uniform thickness, is demonstrated using high resolution transmission electron microscopy, electron energy loss spectroscopy and selected area diffraction (SAD). These core–shell structures can be produced at relatively low temperatures (75°C) and within relatively short times (3h). The ZnS coating exhibits a polycrystalline structure with a lattice parameter of 5.35Å, which is 1.1% smaller than the unstrained cubic zinc-blende structure. The SAD pattern taken at the ZnO–ZnS interface exhibits a partial epitaxial relationship, where (10–10) ZnO//(111) ZnS. Our detailed analysis shows that the ZnS shell comprises two different regions: a ZnS rich inner shell region is produced via the first sulphidation process, followed by a mixture of ZnO and ZnS in the outer shell region during the second treatment. From the detailed microscopy results a growth mechanism is proposed for each step of the sulphidation process. The results are complemented by room temperature photoluminescence spectroscopy. Strong emission from free excitons in ZnO is observed at 3.27eV before ZnS coating, while a composite band peaking at 2.9eV is measured after sulphidation. The origin of the latter will be discussed.

Optical and phonon properties of ZnO:CuO mixed nanocomposite

Optical and phonon properties of ZnO:CuO nanocrystals which are prepared through sol-gel method are reported here. From X-ray diffraction studies, observed that Cu doping replaces the Zn and also forms secondary phase. Optical absorption spectral studies shows that the exciton and plasmon related bands of ZnO and CuO phase, respectively. Fluorescence studies of the prepared samples shows that green emission from ZnO is completely depleted and the same is attributed to CuO Plasmon. Raman spectral studies reveal that secondary phase (impurity) induced profile changes in 1LO and E2High modes. Asymmetry in peak shape is analyzed using Fano profile with the combination of Lorentzian profile. Moreover, the monotonic increase of Fano factor and full width at half maxima is hopefully attributed to the continuum arises by the plasmons of Cu-O phase in ZnO nanosystem.

Interplay of Cu and oxygen vacancy in optical transitions and screening of excitons in ZnO:Cu films

We study room temperature optics and electronic structures of ZnO:Cu films as a function of Cu concentration using a combination of spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Mid-gap optical states, interband transitions, and excitons are observed and distinguishable. We argue that the mid-gap states are originated from interactions of Cu and oxygen vacancy (Vo). They are located below conduction band (Zn4s) and above valence band (O2p) promoting strong green emission and narrowing optical band gap. Excitonic states are screened and its intensities decrease upon Cu doping. Our results show the importance of Cu and Vo driving the electronic structures and optical transitions in ZnO:Cu films.

High spatial resolution ZnO scintillator for an in situ imaging device in EUV region

A single shot image of a ZnO crystal excited by the EUV laser of Kansai Photon Science Institute was captured. The evaluated EUV beam waist radii from the ZnO emission pattern along the horizontal and vertical axes are 5.0 and 4.7μm, respectively. The expected focal spot size of EUV laser and the spatial resolution of the magnifier (including the Schwarzschild objectives and lenses) are however 1 and 4μm, respectively. The discrepancy on the spatial resolutions is attributed to exciton diffusion. We estimated the ZnO exciton diffusion length from the effective decay time which is shortened by exciton–exciton collision quenching and which is dependence on excitation energy density. Our results indicate that the short lifetime of ZnO is required to improve the spatial resolution.

Second-harmonic generation spectroscopy of excitons in ZnO

Nonlinear optics of semiconductors is an important field of fundamental and applied research, but surprisingly the role of excitons in the coherent processes leading to harmonics generation has remained essentially unexplored. Here we report results of a comprehensive experimental and theoretical study of the three-photon process of optical second-harmonic generation (SHG) involving the exciton resonances of the noncentrosymmetric hexagonal wide-band-gap semiconductor ZnO in the photon energy range of $3.2$--$3.5$ eV. Resonant crystallographic SHG is observed for the $1s(A,B)$, $2s(A,B)$, $2p(A,B)$, and $1s(C)$ excitons. We show that SHG signals at these exciton resonances are induced by the application of a magnetic field when the incident and the SHG light wave vectors are along the crystal $z$ axis where the crystallographic SHG response vanishes. A microscopic theory of SHG through excitons is developed, which shows that the nonlinear interaction of coherent light with excitons has to be considered beyond the electric-dipole approximation. Depending on the particular symmetry of the exciton states SHG can originate from the electric- and magnetic-field-induced perturbations of the excitons due to the Stark effect, the spin as well as orbital Zeeman effects, or the magneto-Stark effect. The importance of each mechanism is analyzed and discussed by confronting experimental data and theoretical results for the dependences of the SHG signals on photon energy, magnetic field, electric field, crystal temperature, and light polarization. Good agreement is obtained between experiment and theory proving the validity of our approach to the complex problem of nonlinear interaction of light with ZnO excitons. This general approach can be applied also to other semiconductors.

Origin of ultraviolet electroluminescence in n -ZnO/p -GaN and n -MgZnO/p -GaN heterojunction light-emitting diodes

In this work, a series of n-ZnO/p-GaN and n-MgZnO/p-GaN heterojunctions are designed and fabricated. The carrier transport and recombination mechanism is discussed based on electroluminescence (EL) and photoluminescence (PL) spectra, current–voltage (I–V) characteristics as well as energy band diagram. For ZnO device, the near-ultraviolet (UV) emission at ∼400 nm is attributed to the spatially-indirect, interfacial transition from ZnO conduction band minimum to GaN acceptor level. While for MgZnO diodes, their UV EL is independent on Mg composition, is thought to origin from the donor–acceptor pair (DAP) recombination in GaN layer. Our experiment results suggest that pure ZnO or MgZnO emission can hardly be achieved in n-(Mg)ZnO/p-GaN heterojunctions, rational device design towards (Mg)ZnO exciton emission is more important in the further work. EL spectra of different n-MgxZn1−xO/p-GaN diodes and the schematic carrier transport and recombination process.

Enhancing multi-photon induced excitonic emission of ZnO single crystals by shaping fs laser pulses

We report on the control of multi-photon excited emission of the ZnO exciton band via spectral phase modulation of the femtosecond excitation pulses. It was observed that the optimum spectral phase that enhances the exciton emission results in a pulse-train temporal profile, whose separation corresponds to an energy of 82 meV, which is close to the energy of LO phonon sidebands in ZnO emission. Such a result suggests that exciton–LO phonon coupling can be explored to coherently enhance the exciton emission in ZnO single crystals with respect to the defect luminescence band.

Two-electron-satellite transition of donor bound exciton in ZnO: Radiative Auger effect

Two-electron-satellite (TES) transition of donor bound exciton (D0X) is an interesting many-body quantum process. In this letter, precise luminescence spectra and temperature behaviors of the TES transition of aluminum bound exciton in two kinds of zinc oxide single crystals were investigated in detail. It is found that the TES transition can be treated as a radiative Auger process in which the temperature dependence of the emission intensity of the transition can be fitted very well with a model taking into account the temperature dependent Auger term and thermal dissociation of the D0X excitons.

Tuning the influence of metal nanoparticles on ZnO photoluminescence by atomic-layer-deposited dielectric spacer

Abstract There is increasing interest in tuning the optical and optoelectronic properties of semiconductor nanostructures using metal nanoparticles in their applications in light-emitting and detection devices. In this work we study the effect of a dielectric Al2O3 gap layer (i.e., spacer) on the interaction of ZnO nanowires with metal nanoparticles. The Al2O3 spacer thickness is varied in the range of 1–25 nm using atomic layer deposition (ALD) in order to tune the interaction. It is found that ~5 nm is an optimum spacer thickness common for most metals, although the enhancement ratio of the near-bandedge emission differs among the metals. Consistent results are obtained from both photoluminescence (PL) and cathodoluminescence (CL) spectroscopies, with the latter being applied to the optical properties of individual semiconductor/metal nanoheterostructures. The interaction is primarily proposed to be related to coupling of ZnO excitons with local surface plasmons of metals, although other mechanisms should not be ruled out.

Dynamics of donor bound excitons in ZnO

Comprehensive time-resolved photoluminescence measurements are performed on shallow neutral donor bound excitons (D0Xs) in bulk ZnO. It is found that transients of the no-phonon D0X transitions (I6-I9 lines) are largely affected by excitation conditions and change from a bi-exponential decay with characteristic fast (τf) and slow (τs) time constants under above-bandgap excitation to a single exponential one, determined by τs, under two-photon excitation. The slow decay also dominates transients of longitudinal optical phonon-assisted and two-electron-satellite D0X transitions, and is attributed to “bulk” D0X lifetime. The fast component is tentatively suggested to represent effects of surface recombination.

Enhanced ultraviolet emission and improved spatial distribution uniformity of ZnO nanorod array light-emitting diodes via Ag nanoparticles decoration

Localized surface plasmon (LSP) enhanced ultraviolet (UV) light-emitting diodes (LEDs) were fabricated by embedding a ZnO nanorod array/p-GaN film heterostructure into a Ag-nanoparticles/PMMA composite. By optimizing the concentration of Ag nanoparticles in PMMA, two distinct changes in electroluminescence (EL) spectra were observed: (1) the UV EL component from ZnO excitons was selectively enhanced more than 13-fold and the entire spectral lineshape was changed and (2) the spatial uniformity of the output photon intensity was improved and the linewidth of an angular distribution curve was increased by ∼2 times. These observations can be attributed to near-field optical coupling between Ag LSPs and ZnO excitons. Time-resolved luminescence measurements and a model calculation reveal that the optical coupling results in the increase of the spontaneous emission rate and internal quantum efficiency of Ag-nanoparticles-decorated ZnO nanorod arrays. Moreover, the LSP-exciton interaction allows the device's EL to be coupled out of the nanorod waveguide and to be isotropically scattered into every direction, thus broadening the angular distribution of the EL intensity.

Band edge emission enhancement by quadrupole surface plasmon–exciton coupling using direct-contact Ag/ZnO nanospheres

Periodic Ag nanoball (NB) arrays on ZnO hollow nanosphere (HNS) supporting structures were fabricated in a large area by a laser irradiation method. The optimized laser power and spherical supporting structure of ZnO with a certain size and separation were employed to aggregate a sputtering-deposited Ag nano-film into an ordered, large-area, and two dimensional Ag NB array. A significant band edge (BE) emission enhancement of ZnO HNSs was achieved on this Ag NB/ZnO HNS hybrid structure and the mechanism was revealed by further experimental and theoretical analyses. With successfully fabricating the direct-contact structure of a Ag NB on the top of each ZnO HNS, the highly localized quadrupole mode surface plasmon resonance (SPR), realized on the metal NBs in the ultraviolet region, can effectively improve the BE emission of ZnO through strong coupling with the excitons of ZnO. Compared with the dipole mode SPR, the quadrupole mode SPR is insensitive to the metal nanoparticle's size and has a resonance frequency in the BE region of the wide band gap materials, hence, it can be potentially applied in related optoelectronic devices.

ZnO nanowires as effective luminescent sensing materials for nitroaromatic derivatives

We report on the efficient room-temperature photoluminescence (PL) quenching of ZnO in the presence of 2,4-dinitrotoluene (DNT) vapor and for concentration as low as 180 ppb. Compared to ZnO thin films, ZnO nanowires exhibit a strong (95%) and fast (41 s) quenching of the PL intensity in the presence of DNT vapor. Assuming that the PL quenching is due to a trapping of the ZnO excitons by adsorbed DNT molecules, Monte-Carlo calculations show that the nanometric dimensions as well as the better crystallographic quality (longer mean free path) of the ZnO nanowires result in an enhanced trapping process at the origin of the improved sensing properties of the nanowires. The results demonstrate the importance of nanostructures in improving the sensitivity of ZnO. The study also reveals the sensing capability of ZnO nanowires and paves the path towards the potential realization of low-cost sub-ppb nitroaromatic derivative sensors.