Exciton Region Research Articles

Mapping of two-photon luminescence amplification in zinc-oxide microstructures

The mapping of two-photon excited luminescence in aggregations of free-standing zinc oxide microrods has been carried out at room temperature. Two-photon luminescence spectra in the excitonic region for individual microrods have been recorded. The luminescence intensity exhibits a power-law dependence on the optical pump power with the exponent n > 2. This fact, along with the existence of a threshold power above which the dependence in the exciton region deviates from a quadratic one, indicates the onset of light amplification in individual ZnO microrods and the conditions preceding laser oscillation.

三波段白光有机电致发光器件及其色稳定性的控制

White organic light-emitting diodes based on three small molecular fluorescence materials were designed and prepared.The emitting layers consisted of a red dye,4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetra-methyljulolidin-4-yl-vinyl)-4H-pyran(DCJTB),doped in a green layer of 8-hydroxyquinoline aluminum salt(Alq3),which further formed a heterojunction with a blue layer of 2,2′-(1,3-phenylene)-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole](OXD-7).N,N′-bis-(naphthalene-1-yl)-N,N′-bis(phenyl)-benzidine(NPB) was used as a hole transport layer.Voltage dependent emission spectra and their colorimetric characteristics were manipulated using different device structures.After adjusting the carrier injection/transporting properties,the exciton recombination region was confined and the color stability was improved against the driving voltages.An improved white light-emitting device has a wide spectrum in visible range and a good color stability,its color coordinates change slightly from CIE(x,y)=(0.364,0.314) to CIE(x,y)=(0.332,0.291) when the driven voltage changes from 9.1 V to 12.9 V.

Magnetooptical Properties of (Ga,Fe)N Layers

Numerous works on diluted magnetic semiconductors carried out over the last decade have aimed in examining intricate interplay of ferromagnetic and semiconducting properties [1]. Wide band gap semiconductors such as GaN and ZnO doped with magnetic ions are particularly interesting in view of their wide ranging applications. Here we present results of magnetic circular dichroism (MCD) and Magneto-optical Kerr effect (MOKE) measurements in the excitonic spectral region performed on (Ga,Fe)N layers. In contrast to previous magnetooptical studies [2], we focus here on possible magnetoptical signatures of Fe-rich nanocrystals that appear in (Ga,Fe)N films obtained under suitable growth conditions [3]. We compare magnetoptical properties of two layers with similar concentrations of paramagnetic Fe ions (310cm and 210cm, respectively), one containing Fe-rich nanocrystals, the other not, according to extensive SQUID, XRD, and HRTEM characterization. As found in SQUID measurements, the sample with Fe-rich nanocrystals reveals a ferromagnetic response persisting up to above room temperature for two orientations of the magnetic field in respect to the c-axis. Reflectivity and MOKE measurements are performed in the Faraday configuration with the c-axis parallel to the incident beam in the magnetic field up to 7 T at 2 K. The sample is illuminated by a Xe lamp. We find that the field dependent magnitudes of MCD and Kerr rotation angle in the excitonic region are well descried by the paramagnetic Brillouin function in both samples. This means that ferromagnetic features, clearly seen in SQUID magnetometry, are not contributing to the magnetooptical response. We conclude that either character of nanocrystal magnetoptical properties or the nanocrystal distribution and density are not optimized to result in sizable magnetoptical phenomena. Instead, the presence on Fe-rich nanocrystals reduces the magnitude of the paramagnetic-like magnetooptical signals. In line with this observation, we find that a large magnetooptical response can be recovered by Ar sputtering of the sample surface, the finding consistent with the location of nanocrystals near the film surface [3].

Birefringence of CuGaxAl1 − xSe2 crystals

Excitonic reflection spectra of CuAlSe2 crystals were studied at 10 K. The spectral dependences of refractive indicesno andne were calculated. The isotropic wavelength (the point of intersection ofno andne) in the excitonic region was determined. The spectral dependence ofΔn = no − ne was studied on the long-wavelength and short-wavelength sides of the isotropic wavelength ofCuGaxAl1 − xSe2 crystals. Up to 15 narrow transmission (absorption) lines were revealed in the interferencespectra of thin crystals, which can be used for the development of comb filters.

Investigation of low-temperature excitonic and defect emission from Ni-doped ZnO nanoneedles and V-doped ZnO nanostructured film

We report the growth and low-temperature photoluminescent characteristics of well-aligned Ni-doped ZnO nanoneedles and V-doped ZnO nanostructured thin film grown by a modified pulsed laser deposition technique. Low-temperature photoluminescence spectra of the as-grown films show the presence of free excitonic as well as bound excitonic transitions, whose relative intensity changes with increasing temperature. Ni-doped ZnO films show a characteristic fine structure in the visible range (2.6–2.9 meV) attributed to either exciton–polariton longitudinal–transverse splitting or the splitting caused by electron–hole exchange interaction. The excitonic and visible region emission can be clearly seen as can the phonon replicas produced from longitudinal optical phonons. Different possible attributions of the various peaks in the emission band at low temperature have been discussed. The as-grown nanostructures of Ni- and V-doped ZnO thin films also clearly show the effect of doping on the microstructure of ZnO.

Contactless electroreflectance and photoluminescence characterization of Zn0.68Be0.06Mg0.26Se crystalline alloys

Abstract In this report, results of optical characterization of Bridgman-grown Zn 0.68 Be 0.06 Mg 0.26 Se crystalline alloys in the near-band-edge interband transitions region using temperature-dependent contactless electroreflectance (CER) and photoluminescence (PL) in the temperature range 10–300 K and time-resolved PL measurements at 10 K are presented. Two features with separation energy of 40 meV are observed in the near-band-edge CER spectra. The origins of these two features are attributed to the fundamental band-edge excitonic transitions of the coexisted different crystal phases in accordance with the X-ray diffraction and transmission electron microscope results. The increase of the broadening parameter of the excitonic feature of the PL spectra with increasing temperature has been attributed to the electron–longitudinal optical phonon interactions. In addition, the transient PL spectrum at 10 K in the excitonic emission region exhibits the asymmetric lineshape and can be deconvoluted into two Gaussian features with one order of magnitude difference in the decay time. These two components are assigned to the recombination of free excitons and localized excitons in this quaternary compound.

Optical and structural properties of ZnO nanorods grown by pulsed laser deposition without a catalyst

Pulsed laser deposition without a catalyst is used to grow ZnO nanorods less than 10 nm in diameter. The structure of the rods is studied by Raman scattering during excitation in the visible and UV regions. The temperature dependences of exciton spectra and the behavior of green luminescence are investigated in the temperature range 10–280 K. At room temperature, the luminescence intensity of the ZnO nanorods in the exciton region is higher than the green luminescence intensity by a factor of 7.8.

Theoretical Characterization of Electronic Transitions in Co2+- and Mn2+-Doped ZnO Nanocrystals

Linear response time-dependent hybrid density functional theory has been applied for the first time to describe optical transitions characteristic of Co2+- and Mn2+-doped ZnO quantum dots (QDs) with sizes up to 300 atoms (∼1.8 nm diam) and to investigate QD size effects on the absorption spectra. Particular attention is given to charge-transfer (CT or “photoionization”) excited states. For both dopants, CT transitions are calculated to appear at sub-band-gap energies and extend into the ZnO excitonic region. CT transitions involving excitation of dopant d electrons to the ZnO conduction band occur lowest in energy, and additional CT transitions corresponding to promotion of ZnO valence band electrons to the dopant d orbitals are found at higher energies, consistent with experimental results. The CT energies are found to depend on the QD diameter. Analysis of excited-state electron and hole density distributions shows that, for both CT types, the electron and hole are localized to some extent around the impurity ion, which results in “heavier” photogenerated carriers than predicted from simple effective mass considerations. In addition to CT transitions, the Co2+-doped ZnO QDs also exhibit characteristic d−d excitations whose experimental energies are reproduced well and do not depend on the size of the QD.

Quenching of excitonic luminescence of alkaline earth fluorides excited by VUV harmonics of femtosecond laser

We studied fast luminescence of barium, calcium and strontium fluoride crystals in the excitonic emission region. For excitation we used high harmonics of Ti:sapphire laser generated in the rare gas cells with photon energies from 15 to 50 eV. The usage of 600 fs excitation pulses with 1 kHz repetition rate coupled with conventional registration system allows us to measure the luminescence decay curves in the wide time domain from 50 ps up to 1 ms. Decay curves demonstrate the strong deviations from single and/or two-exponential decay laws. The decay curve profiles depend both on excitation and emission photon energies. We attribute the subnanosecond component to the fast dissociation of exciton to the F–H pair at 300 K. The energy transfer from exciton to the F-center or another exciton also contributes to the non-exponential decay.

Decay kinetics of the defect-based visible luminescence in ZnO

Photoluminescence spectra and decays under pulsed N 2 (337 nm) laser excitation were measured for hydrothermally grown bulk and liquid-phase epitaxy (LPE)-grown film ZnO samples within 9–300 K. Temperature dependence of integrated spectra over the exciton and visible spectral regions was evaluated using a model involving standard energy barrier processes. Decay curves measured within a broad time window (10 ns–1 ms) and with extreme signal/background ratio (five orders of magnitude) point to complex decay mechanism in which the exponential and inverse power-law processes can coexist. There is no straightforward interconnection between the observed temperature dependence of integrated visible photoluminescence intensity and its decay shape over the 9–300 K temperature interval.

Energy Dissipation in Impurity DopedAlkaline-Earth Fluorides

Excitation spectra of CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , SrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> crystals doped by CdF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , LaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , YF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> in the vacuum ultraviolet region were investigated at liquid-helium temperature and at room temperature. Spectra of doped BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at low temperature and CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , SrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at room temperature show significant decrease of excitation intensity against those in undoped crystals while the intensities in exciton regions remain nearly the same. The common reason of the suppression of exciton emission by impurity in alkaline-earth fluorides is unradiative electron-hole recombination on impurity sites.

Subdiffraction Far-Field Imaging of Luminescent Single-Walled Carbon Nanotubes

Far-field near-infrared fluorescence microscopy of single-walled carbon nanotubes (SWNTs) has been hampered by the diffraction limit to resolution. A new analysis method is presented that allows subwavelength (<lambda/10) mapping of single-molecule chemical reaction sites on semiconducting SWNTs, enabling precise localization of excitonic luminescence regions along the nanotube axis through a nonperturbing, far-field optical measurement. This method is applied to reveal the subdiffraction lengths, curvatures, and defects of luminescent SWNTs in unprecedented detail.

Study of the recombination around the excitonic region of MBE ZnSe:Cl thin films

The recombination processes around the excitonic region of undoped ZnSe and chlorine doped ZnSe thin films were studied by continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (TRPL) spectroscopies. Samples with different chlorine concentration were obtained by varying the temperature of the Cl source. The evolution of the PL signal and its decay time were analyzed as a function of temperature. Activation energy (Ea) values associated to the quenching of the D0X and band-to-band emission were obtained from the temperature dependent cw-PL experiments. The activation energy was lower for the film with higher Cl content. The characteristic exponential decay time (τPL) of the PL signal decreased with increasing Cl concentration.

Tunneling recombination processes in PbWO4 crystals

AbstractTime‐resolved emission and excitation spectra and luminescence decay kinetics were studied at 160‐300 K for undoped and Mo‐doped PbWO4 crystals under XeCl (4.02 eV), N2 (3.67 eV) and KrF (5.0 eV) pulsed excimer laser excitation. The G(II) emission was found to be responsible for the slow (µs‐ms) luminescence decay. Under excitation or after irradiation in the exciton region (Eexc = 4.02 eV), this emission accompanies the monomolecular tunneling recombination in genetic pairs of electron and hole centers produced at the photo‐thermally stimulated decay of localized excitons, which occurs without release of free charge carriers. Under excitation or after irradiation in the host lattice (Eexc = 5.0 eV) and defect‐related (Eexc = 3.67 eV) regions, the bimolecular tunneling recombination takes place in chaotic pairs of the electron and hole centers created at the trapping of optically released free charge carriers. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Vibronic spectra of charge transfer excitons and of mixed charge transfer and Frenkel excitons

The linear absorption spectra in the excitonic and vibronic regions in the case of mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) have been theoretically studied for the exciton parameters of the crystals of MePTCDI and PTCDA. Two coupling parameters for the exciton–phonon coupling are introduced: the FE–phonon coupling and the CTE–phonon coupling. The main features of the vibronics in the absorption spectra are the following: (a) the existence of a doublet structure in the vibronic spectra of CTEs; (b) the vibronic levels of the FE at intermediate values of both coupling parameters are located in the continuum of the many-particle exciton–phonon states which makes its absorption line wide and flat; (c) in the case of strong coupling (coupling constants larger than 1) a doublet of bound states appears above this continuum; (d) in the case of vanishing CTE–phonon coupling the vibronics of the charge transfer excitons practically disappear in the absorption spectra.

Vibronic spectra of mixed Frenkel and charge transfer excitons

The excitonic and vibronic spectra of a molecular chain and of a crystal of $N{N}^{\ensuremath{'}}$-dimetyl 3,4,9,10-perylene tetracarboxylic diimide (MePTCDI) are studied in the case when: (i) a Frenkel exciton (FE) and charge transfer excitons (CTEs) mix strongly; and (ii) two mechanisms of coupling between these mixed excitons and intramolecular vibrations, notably linear and quadratic coupling, are acting. Using a convenient canonical transformation and the Green function method, we calculate the linear optical susceptibility: (a) in the exciton region taking into account the contribution of the transition dipoles of FE; and (b) in one-phonon vibronic regime. The spectra of linear absorption in the excitonic and vibronic regions have been calculated introducing the exciton parameters of MePTCDI. These spectra exhibit: (i) a relative separation of vibronics of FE and CTEs; (ii) a stronger impact of the linear coupling on the intensity of the excitonic and vibronic spectra; and (iii) the appearance of a spectral doublet of vibronics of the CTEs---its splitting depends on the parameters of linear and quadratic exciton--phonon coupling in the neutral excited molecule and ions. Moreover, in the case of weak linear exciton--phonon coupling the vibronic line of FE, being wide and flat, lies in many-particle continuum while in the case of intermediate and strong linear coupling the linear absorption is dominated by the bound exciton--phonon states and their narrow Lorentzian maxima depend strongly on the quadratic coupling.

Luminescence of ZnO nanorods grown by chemical vapor deposition on (111) Si substrates

ZnO nanorods have been grown on (111) Si substrates by chemical vapor deposition in a horizontal reactor, with no catalyst. The nanorods grown far from the outlet end of the reactor are larger in size, have a higher structural perfection, and exhibit more efficient room-temperature edge luminescence in comparison with the nanorods grown at the outlet end. The low-temperature cathodoluminescence spectrum of the nanorods also depends on their position in the reactor during growth, which is interpreted in terms of the density of native defects. The nanorods exhibit room-temperature stimulated emission in the excitonic spectral region.

Photoelasticity effects in superlattices with quantum wells having a sloping bottom near the interband resonances

An analytical expression is obtained for the linear coefficients of photoelasticity in superlattices with quantum wells having a sloping bottom in the vicinity of the interband resonances. It is shown that these coefficients considerably decrease when the slope of the quantum-well bottom is induced by an electric field applied to the superlattice and remain almost unchanged if the slope is produced by the variband method. A new mechanism providing the emergence of linear photoelasticity near the interband resonances is revealed for superlattices based on piezoelectric crystals. This mechanism is associated with the modulation of the band gap of the quantum-well structure by the generated piezoelectric field. Numerical estimates demonstrate that the contribution to the linear photoelasticity in the region of the interband resonances is substantially greater than the corresponding contribution in the exciton region of the spectrum and is comparable in magnitude to the contribution made by the deformation potential.

Excitonic and vibronic structure of absorption spectra of Me-PTCDI and PTCDA crystals

The excitonic and vibronic spectra (exciton + one quantum of intramolecular vibration) of Me-PTCDI and PTCDA crystals are studied in the case of strong mixing of a Frenkel exciton (FE) and charge-transfer excitons (CTEs). The linear optical susceptibility is calculated in the framework of dynamical theory of vibronic spectra. The absorption spectra of both crystals have been modelled. The positions, maximal values and integral intensity of the absorption peaks which correspond to the bound exciton–phonon states and to unbound (many-particle) states have been calculated using FE and CTEs’s parameters of Me-PTCDI and PTCDA crystals. The calculated spectra show: (i) the possible recovering of excitonic and vibronic regions; (ii) bigger integral intensity of many-particle states in the case of weak exciton–phonon coupling; (iii) the dominant role of the bound states in the case of intermediate and strong exciton–phonon coupling.

High efficiency white organic light-emitting devices by effectively controlling exciton recombination region

An efficient white organic light-emitting device was fabricated. The used device was a multilayer structure of ITO/N,N-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) (30 nm)/NPB: DCJTB: C545T (10 nm)/NPB (4 nm)/DNA (8 nm)/2,9-dimethyl, 4,7-diphenyl, 1,10-phenanthroline (BCP) (9 nm)/tris-(8-hydroxyquinoline) aluminium (Alq3) (30 nm)/LiF (1 nm)/Al (100 nm). In our devices, a red dye 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyl julolidyl-9-enyl)-4H-pyran and a green dye 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7–8-i,j)quinolizin-11-one were co-doped into NPB and sandwiched between two pure NPB layers. The introduction of the co-doping layer as well as the multilayer structure permits us to control the exciton recombination region well, thereby producing stable white light emission in a wide range of voltage. The device shows a turn-on voltage of 5.9 V and a maximum luminance of 13 600 cd m−2. The power efficiency and current efficiency reach 4.4 lm W−1 and 12.3 cd A−1, respectively.