Luminescence Profiles Research Articles

Synthesis, structure and preliminary spectral properties of K4RE0.01W10.99O35 hexatungstate bronze-like crystals (RE = Er, Eu)

The structure of potassium hexatungstate has been studied using X-ray diffraction method and Er3+ and Eu3+ ions doped absorption and luminescence investigations. The optically active erbium and europium ions replace the potassium atoms forming two-site system. A rich level structure of RE3+ appears both at room and 77K temperature. The effective emission transitions are observed in both systems studied. Temperature dependence of luminescence and decay profiles show structural anomalies at low and high temperatures.

Photoluminescence and ultra-fast decay profiles of rare-earth (III) ion-doped semiconductor nanocrystals encapsulated in silica nanotube AMCM-41/Cd xZn 1− xS: Eu 3+, Tb 3+

Luminescence, time-resolved luminescence and decay profiles are measured in order to investigate quantum-confinement effects in alumina(Al 2O 3)-modified MCM-41 silica nanotubes doped with Zn x Cd 1− x S:Eu 3+, Tb 3+ nanocrystals at temperatures down to 10 K. In comparison with bulk materials, in the presence of vibronic luminescence bands, luminescence spectral characteristics are found associated with SiO 2 centers, a new f–f transition mechanism and very short (600 ps) lifetime features due to surface-bound centers.

Luminescent Lanthanide Complexes with Stereocontrolled Tris(2-pyridylmethyl)amine Ligands: Chirality Effects on Lanthanide Complexation and Luminescence Properties

A series of tris(2-pyridylmethyl)amines including one and two asymmetric centers were synthesized in a stereo-controlled fashion as potential ligands of lanthanide cations. The reaction of chiral pyridylethyl methanesulfonates and bis(pyridylmethyl)amines occurred via an S(N)2 mechanism with complete inversion of asymmetric centers and gave the stereocontrolled tris(2-pyridylmethyl)amines, the stereochemical purity of which was ascertained by GPC, NMR, X-ray, and polarimetry experiments. They formed stable Tb(3+) and Eu(3+) complexes having 1:1, 1:2, and 1:3 stoichiometry (metal:ligand) in CH(3)CN solutions. NMR and UV titration experiments revealed that their complexation behaviors were rarely influenced by ligand chirality but significantly affected by the nature of the counteranion and the concentration ratio of metal to ligand. The Tb(3+) and Eu(3+) complexes with these tripodal ligands exhibited characteristic luminescence spectra upon excitation for pyridine chromophores (260 nm), the intensities of which were largely dependent on the ligand chirality. The meso isomer of the disubstituted tripods particularly exhibited the enhanced terbium luminescence ca. three times more than its diastereomer and un- and monosubstituted tripods. Direct excitation at the lanthanide center had similar chirality effects on the luminescence profiles, indicating that the stereochemistry of the employed ligand largely influenced the lanthanide emitting processes. Since the ligand chirality finely modified the local coordination environments around the lanthanide center, the use of stereocontrolled ligands is applicable in design of the luminescent lanthanide complexes.

Luminescence and circularly polarized luminescence of macrocyclic Eu(III) and Tb(III) complexes embedded in xerogel and sol–gel SiO 2 glasses

Luminescence, time-resolved luminescence, circularly polarized luminescence (CPL) and decay profiles of Ln(III)(15-crownether-5) ( Ln= Ce, Sm, Eu, Tb ) and Tb(III)-(R),(S)-cyclen derivative complexes doped in xerogel and sol–gel silica glasses are measured at temperatures down to 10 K to characterize luminescence properties and the electronic structure in the excited states. Luminescence spectral profiles and calculation of crystal field parameters ( B 0 (2), B 2 (2)) in the 5 D 0→ 7 F J(J=1,2) transition give evidence of the fact that the pentagonal and planar structure of Eu(III) (15-crownether-5) does hold in xerogel and sol–gel glasses prepared at temperatures below 100°C. As annealing temperatures are increased from 80°C to 750°C, Eu(III) complexes in sol–gel glasses are found to decompose gradually to SiO 2:Eu 3+. Tb(III)-(R) and (S)-cyclen derivative complexes in xerogel reveal at room temperature and 10 K sharp CPL spectra with luminescence dissymmetry factors g lum=−0.1 and 0.1, respectively. These complexes doped in sol–gel glasses represent luminescence characteristics of rare earth ions encapsulated in the nano-porous host.

The red to near-infrared luminescence in alkali feldspar

Results are presented for the radio-, cathodo- and ionoluminescence (RL, CL and IL) of an ordered, exsolved alkali feldspar from South Greenland (BM2). BM2, a microperthite typical of many evolved rocks, has a broad red near-infrared luminescence, which, using lifetime-resolved CL, reveals four overlapping emissions, of which two are dominant. These derive from Fe3+-activated luminescence from the low albite and maximum microcline components of the microperthite in which the Fe is on the most ordered tetrahedral site. The temperature dependence of the emission shows a smooth change from longer wavelengths at 20 K to shorter values at room temperature. There is a broad relationship between Fe–O bond distance and the energy of the luminescence. Comparisons of RL, CL and IL allow features in the luminescence associated with surface states to be identified. The dependence of red RL intensity versus temperature is divided into four regions for which the activation energies and temperature ranges are estimated. Variations in RL intensity between 67 and 225 K are interpreted as a phase transition in microcline. RL intensity indicates a multi-stage transformation, proceeding via a first step between 67 and ~100 K in which luminescence intensity increases, followed by a region between 100 and ~225 K in which intensity falls. A discontinuity in intensity at ~225 K marks the end of the second region. These variations may result from changes in the bond angles of bridging oxygens. IL dose dependence studies have been performed. As implantation progresses, the red/IR emission profile skews towards short wavelengths, reflecting amorphisation of the structure and local variations in Fe–O bond distances. If the sample recovers for 24 h, the luminescence profile partly returns to its original state, but remains skewed, reflecting partial resumption of short-range order. Such profiles are also observed from natural shocked feldspars, and it may be that ion beam damage is akin to structural modifications associated with impact events. Ion implantation also causes the formation or enhancement of UV, blue and yellow emissions. Recovery of the sample over 24 h causes the UV and blue emissions to be greatly reduced, whereas the yellow emission may be enhanced. Tentative interpretations of this behaviour are presented.

Cathodoluminescence Depth Profiling in SiO 2 :Ge Layers

For investigation of the luminescent center profile cathodoluminescence measurements are used under variation of the primary electron energy E 0 = 2…30 keV. Applying a constant incident power regime (E 0·I 0 = const), the depth profiles of luminescent centers are deduced from the range of the electron energy transfer profiles dE/dx. Thermally grown SiO2 layers of thickness d = 500 nm have been implanted by Ge+-ions of energy 350 keV and doses (0.5–5)1016 ions/cm2. Thus Ge profiles with a concentration maximum of (0.4 – 4) at% at the depth of dm≅240 nm are expected. Afterwards the layers have been partially annealed up to T a = 1100 °C for one hour in dry nitrogen. After thermal annealing, not only the typical violet luminescence (λ = 400 nm) of the Ge centers is strongly increased but also the luminescent center profiles are shifted from about 250 nm to 170 nm depth towards the surface. This process should be described by Ge diffusion processes, precipitation and finally Ge nanocluster formation. Additionally, a Ge surface layer is piled-up extending to a depth of roughly 25 nm.

Interaction of tert -butyl Hydroperoxide with Hypochlorous Acid. A Spin Trapping and Chemiluminescence Study

The formation of radical species during the reaction of tert-butyl hydroperoxide and hypochlorous acid has been investigated by spin trapping and chemiluminescence. A superposition of two signals appeared incubating tert-butyl hydroperoxide with hypochlorous acid in the presence of the spin trap f -(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). The first signal (aN = 1.537mT, agH = 0.148mT) was an oxidation product of POBN caused by the action of hypochlorous acid. The second spin adduct (aN = 1.484mT, agH = 0.233mT) was derived from a radical species that was formed in the result of reaction of tert-butyl hydroperoxide with hypochlorous acid. Similarly, a superposition of two signals was also obtained using the spin trap N-tert-butyl- f -phenylnitrone (PBN). tert-Butyl hydroperoxide was also treated with Fe2+ or Ce4+ in the presence of POBN. Using Fe2+ a spin adduct with a N= 1.633mT and agH = 0.276mT was observed. The major spin adduct formed with Ce4+ was characterised by αN = 1.480mT and agH = 0.233mT. The reaction of tert-butyl hydroperoxide with hypochlorous acid was accompanied by a light emission, that time profile and intensity were identical to those emission using Ce4+. The addition of Fe2+ to tert-butyl hydroperoxide yielded a much smaller chemiluminescence. Thus, tert-butyl hydroperoxide yielded in its reaction with hypochlorous acid or Ce4+ the same spin adduct and the same luminescence profile. Because Ce4+ is known to oxidise organic hydroperoxides to peroxyl radical species, it can be concluded that a similar reaction takes place in the case of hypochlorous acid.

Photoluminescence properties and ultra-fast decay profiles of nanoparticles in sol–gel zirconia thin films xZrO 2·100− x Cds : Mn 2+ and Eu 3+

Thin zirconia films doped with semiconductor nanocrystals xZrO 2·100− x CdS : Mn 2+ and Eu 3+ ( x=20, 50, 80) were prepared by sol–gel method to classify emission centers by measurements of luminescence, time-resolved luminescence, excitation spectra and decay profiles in the temperature range between 10 and 300 K. The broad emission band over the 24–13×10 3 cm −1 is found to be composed of four spectral components due to band-edge emission (CdS), ligand to metal charge transfer (LMCT) center (ZrO 2), surface-bound deep centers, and lattice-bound d–d (Mn 2+) or f–f transition (Eu 3+) centers. Luminescence of Mn 2+ and Eu 3+ showed x-composition-dependent spectral profiles and shortening of lifetimes to 5–50 μs at 10 K. Ultra-fast decay components of 0.5–1.5 ns were also detected in thin films at room temperature. Luminescence properties of doped-CdS nanocrystals are characterized by dependence on annealing temperatures, very fast decay times and low thermal activation energy of 1300 cm −1 in comparison with bulk crystals.

Explaining the luminescence profile of erbium in silicon under short excitation pulses

A mechanism for the luminescence of erbium in silicon has been developed. Erbium atoms in silicon have been considered as recombination centers with specific values of capture and emission coefficients. Electron–hole recombination through these levels has been considered to be the origin of erbium excitation. Capture and emission processes of photo generated excess carriers in the erbium related level have been equated for non-steady-state conditions. The extended rise of erbium luminescence after termination of short excitation pulses of micro/nano second durations has been explained by the model.

Near-infrared luminescence and spectral anomaly in PLZT ceramics doped with Nd 3+, Er 3+, Yb 3+ and Cr 5+ ions at low temperatures

Luminescence spectral properties of PLZT(9l/65/35) ceramics doped with various rare earth (III) ions and chromium (III) ions in the near-infrared spectral region are reported in order to investigate structural disorder and interaction with the ferroelectric host. PLZT pellets doped with Nd 3+, Er 3+, and Yb 3+ are found to give sharp luminescence at room temperature and lower temperatures down to 10 K. Temperature dependence of luminescence and decay profiles of PLZT: Nd 3+ show a structural anomaly at 160 K. Luminescence spectra of PLZT: Nd 3+ and Pr 3+ are perturbed by reabsorption and visible emission band of the PLZT host. Crystal field splitting in Yb 3+ ions is considered to elucidate site occupancy in the amorphous ceramics. NIR luminescence form PLZT:Cr 5+ is found at 10,000 cm −1 as a broad band. Near-infrared luminescence due to f–f and d–d transitions are found to have a high efficiency up to room temperature.

Analysis for charged spacers in FED

Charged spacers in the field emission display (FED) are analyzed with the Monte Carlo method. The spacer is made of an insulator, which has generally a high secondary electron emission property. Under electron bombardment, the secondary electron emission induces charge on the spacer. We show that the surface of the spacer is charged positively in FED operation, which would cause an image distortion. We analyze the effect of charging on the spacer in terms of the electron density profile and luminescence profile of a dot near the spacer. Simulation results show that the image of a dot near the spacer is darker and smaller than that of a dot away from the spacer, though electrons are crowded near spacers. The results are confirmed by experiments. Finally, we suggest a way to reduce the effect of spacer charging by introducing a metal strip.

Two Photon Excitation Spectrum of a Twisted Quadruple Bond Metal−Metal Complex

Two Photon Excitation Spectrum of a Twisted Quadruple Bond Metal−Metal Complex

Response characteristics of optical sensors for oxygen: models based on a distribution in τo or kq

The typical response characteristics of most optical oxygen sensors include downward curving Stern–Volmer plots and often multi-exponential luminescence decay profiles in the absence and presence of oxygen. The wide range of different response features exhibited by optical oxygen sensors is usually attributed to different degrees of heterogeneity in the sensor films. This heterogeneity is described in this paper by a log-Gaussian distribution in the natural luminescent lifetime of the oxygen-quenchable lumophore, τo, and the quenching rate constant, kq. A ‘log-Gaussian distribution in τo and kq’ model is used to generate theoretical response profiles which exhibit the same disparate range of features as real optical oxygen sensors. However, unlike other models, such as the ‘two-site’ model, the ‘log-Gaussian distribution in τo and kq’ model generates model parameter values which are physically plausible and consistent at all partial pressures of oxygen, pO2. The model is used to fit successfully the Stern–Volmer plots and luminescent decay profiles reported for a number of different optical oxygen sensors. For each of the real sensors examined, the values of the model parameters KSV,mdl, ρ1, ρ2 and τo,mdl which give the best fit to the observed data are reported. The latter data allows prediction of the response features of the associated optical oxygen sensor at any value of pO2

Surface effects in luminescence and decay profiles of Cr 6+-V 5+ and Cr 6+-Ag + codoped nanostructured sol-gel SiO 2 glass

Luminescence and decay profiles of codopant (Cr 6+. V 5+) and (Cr 6+, Ag + ) ions in sol- gel silica glasses are investigated at temperatures between 10 and 300 K. The closed-shell electronic structures of these ions reveal luminescence and decay profiles affected sensitively by energy migration processes, surface effects and preparation methods of the nano-structured matrices.

Ru(phen)(2)dppz(2+) Luminescence: Dependence on DNA Sequences and Groove-Binding Agents.

Emission of Delta-Ru(phen)(2)dppz(2+) bound to nucleic acid polymers of different sequence has been investigated by time-resolved luminescence spectroscopy and the effect of major and minor groove DNA binding agents on the luminescence profile of the complex evaluated. In the presence of a 1:1 mixture of poly d(AT) and poly d(GC), the excited-state decay of Delta-Ru(phen)(2)dppz(2+) can be described by a linear combination of the decay profiles in the presence of poly d(AT) and poly d(GC) independently. This analysis indicates that approximately 85% of the complexes are bound to poly d(AT) and that the metallointercalator preferentially occupies AT sites in mixed-sequence polymers such as calf thymus or T4 DNA. When rac-Ru(phen)(2)dppz(2+) bound to [d(5'-GAGTGCACTC-3')(2)] is titrated with the major groove intercalator Delta-alpha-[Rh[(R,R)-Me(2)trien]phi](3+), the ruthenium emission yield decreases while the absorbance of the pi-pi transition centered on the dppz ligand increases, until saturation behavior is observed at a 1:1 Rh/duplex ratio. These titrations indicate that Ru(phen)(2)dppz(2+) is displaced from the major groove by the rhodium complex. In contrast, for rac-Ru(phen)(2)dppz(2+) bound to poly d(AT), addition of the minor groove binding agent distamycin produces an increase in ruthenium emission which saturates at approximately 1 distamycin/5 bp, consistent with the double helix being able to accommodate major and minor groove binders simultaneously. Distamycin has no effect on the emission of Ru(phen)(2)dppz(2+) emission bound to poly d(GC). These photophysical studies establish a sequence preference in binding to DNA by Ru(phen)(2)dppz(2+) as well as providing support for the original assignment by NMR of ruthenium intercalation from the major groove side of the DNA helix.

A Study on Delocalization of MLCT Excited States by Rigid Bridging Ligands in Homometallic Dinuclear Complexes of Ruthenium(II)

For the structurally rigid homometallic dinuclear complexes (ttp)Ru(tpy-tpy)Ru(ttp)4+ and (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+, we have obtained ground-state absorption spectra and transient-absorption difference spectra at room temperature and luminescence spectra and lifetimes in the temperature interval from room temperature to the rigid matrix (90 K); the solvent was acetonitrile or butyronitrile (tpy is 2,2‘:6‘,2‘‘-terpyridine, ttp is 4‘-p-tolyl-2,2‘:6‘,2‘‘-tpy, and ph is 1,4-phenylene). The gathered spectroscopic data indicate that after absorption of visible light, formation of the luminescent metal-to-ligand charge transfer (MLCT) excited states takes place, which involves the bridging ligand (BL). Since we found that (ttp)Ru(tpy-tpy)Ru(ttp)4+ is a good luminophore (λmax = 720 nm, Φ = 4.7 × 10-3, and τ = 570 ns) while both (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+ (λmax = 656 nm, Φ = 1.1 × 10-4, and τ = 4 ns) and the reference mononuclear complex Ru(ttp)22+ (λmax = 640 nm, Φ = 3.2 × 10-5, and τ = 0.9 ns) are not, we have explored the effects brought about by the delocalization and energy content of the luminescent state. The study of the temperature dependence of the luminescence lifetimes indicates that two main nonradiative paths, i and ii, are responsible for deactivation of the luminescent state. Path i directly connects the luminescent and ground states; within the frame of the “energy-gap law”, vibronic analysis of low-temperature luminescence profiles enables one to correlate the delocalization of the M → BL CT state and the extent of structural distortions occurring at the accepting ligand. Thermally activated decay via a metal-centered (MC, of dd orbital origin) excited state characterizes path ii, with a MLCT−MC energy separation ΔE = 3800, 2300, and 1600 cm-1 for (ttp)Ru(tpy-tpy)Ru(ttp)4+, (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+, and Ru(ttp)22+, respectively. At room temperature, for this limited series of complexes it is found that nonradiative processes governed by the “energy-gap law” play a minor role as compared to thermally activated processes, the ratios of the rate constants being knract/knrdir ≈ 16, 1900, and 7000 for (ttp)Ru(tpy-tpy)Ru(ttp)4+, (ttp)Ru(tpy-ph-tpy)Ru(ttp)4+, and Ru(ttp)22+, respectively.

Investigation of distribution of defects and impurities in boron-doped CVD diamond film by cathodoluminescence spectroscopy

Distribution of defects and impurities was investigated by cathodoluminescence spectroscopy on as-grown boron-doped diamond film synthesized by microwave plasma chemical vapor deposition method. Luminescence profiles in a boron-doped diamond film were observed by cross-section CL images. Intense emission of 535 nm (~2.32 eV) band was found in a thin layer of the growth surface and in a rather thick layer of interface to the silicon substrate. Complementary to the 535-nm emission, the edge emission appeared in the bulk region excluding the surface layer. The origination of the 535-nm band can be explained by a boron-vacancy center.

Femtosecond site-selective probing of energy relaxing excitons in poly(phenylenevinylene): Luminescence dynamics and lifetime spectra

Exciton relaxation in poly(phenylenevinylene), PPV, has been probed by femtosecond luminescence-up-conversion. We report on excitonic luminescence profiles that depend on the spectral position of the detection window (ε̂=2.7,2.6,2.5, and 2.4 eV, respectively). In an attempt to reveal the transient steps implied in fs relaxation, we present a quantitative forward reconvolution fit procedure that is based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations. Special emphasis has been placed (i) on the analysis of luminescence lifetime distributions 〈φ(τ;ε̂,τ0)〉 which directly map out the spectra of hopping modes of energy-cascading neutral excitations, and (ii) on the rigorous evaluation of (radiationless) transfer population from high-energy subensembles to low-energy tail states of the DOS. We quantitatively show that the absence of significant rise terms in the S0ν=0←S1ν=0 luminescence transition of low-lying tail states is due to the spectral superposition of the S0ν=1←S1ν=0 luminescence decay from states near the center of the DOS which, owing to the predominantly positive amplitudes of their ultrafast decay channels, destructively interfere with the negative amplitudes (luminescence rise terms) of migratively prepared bottom states. Finally, the asymptotic branch of the luminescence lifetime distribution 〈φ(τ;ε̂=2.4 eV,τ0)〉 has been reconstructed in an unbiased fashion by inverting the high-precision, picosecond, single-photon counting luminescence data at ε̂=2.4 eV with the help of a regularized exponential series methodology. The measurements combined by numerical computing are in accord with the molecular picture of exciton relaxation in PPV in which neutral, localized excitations undergo predominantly incoherent excitation energy transfer among sites of fluctuating self-energies related to segmental subunits with distributed π-bond conjugation lengths.

Optical measurement of the ambipolar diffusion length in a ZnCdSe–ZnSe single quantum well

We describe a straightforward technique for the measurement of carrier diffusion in semiconductors. Using an optical microscope we can spatially image luminescence with a resolution of ∼500 nm. We measured the ambipolar diffusion length in a Zn0.75Cd0.25Se–ZnSe single quantum well by fitting the spatially resolved luminescence profile with the solution of the two-dimensional diffusion equation. The ambipolar diffusion length was found to be 498 nm at a carrier density of ∼1×1018 cm−3 and we deduce an ambipolar diffusion constant of 1.7 cm2 s−1.

Theory of nonradiative decay dynamics in intensely pumped solid-state laser media via laser photothermal diagnostics

A rigorous analytical treatment of nonradiative deexcitation and thermal-conduction transient evolution in solid-state laser media, resulting from intense optical pumping with rectangular pulses from a time-gated laser source, is presented. This situation arises in rate-window photothermal detection from laser rods with bulk and surface absorptions, the latter being due to polishing during the manufacturing of the rod. Numerical simulations of the theory show that the surface nonradiative (optical-to-thermal) energy conversion term is likely to dominate even at absorptances on the order of 1%–2%. Therefore, polishing optimization appears to be necessary in order to minimize laser losses at the surfaces when laser rods are active in a resonator cavity. The present theory also provides physical insights into the very different nature of the bulk- and surface-originating heat-conduction transients, as well as on the profile of the superposition photothermal wave form and its dependence on the optical, thermal, and metastable-state relaxation parameters. As a prelude for input to the photothermal theory, a treatment of the excited-state dynamics of a typical laser manifold pumped by an intense laser beam, away from or near saturation, is developed, and the luminescence and photothermal energy source profiles are calculated analytically.