Individual Emission Bands Research Articles

Intrinsic emissions and formation of electron-hole trapping centres in excited Li2SO4

Vacuum ultraviolet and thermal activation spectroscopies were used to study the nature of intrinsic emission and the mechanisms of the formation of electron and hole trapping centres in irradiated Li2SO4 powders and crystals. The obtained results along with excitation spectra showed that recombination emission at 3.8–4.2 eV and 4.5–5.5 eV occurs when an electron passes from the second and third sub-bands of the valence band formed from the 2p state of oxygen to the conduction band. It has been determined that Li2SO4 electron and hole trapping centres are created when electrons are trapped in the SO42– anionic sites. The holes are localised in the form of SO4 – radicals. The spectral position and intensity of individual emission bands are dependent on the crystallographic arrangement of localised holes.

Crystal structure and photoluminescence of ZnSe and ZnSe:Mn nanocrystals obtained by combustion synthesis

ZnSe and ZnSe:Mn nanocrystals were obtained by combustion synthesis (self-propagating high-temperature synthesis) using current pulses to initiate a reaction with amplitudes of ∼35 A and ∼40 A. The magnitude of the amplitude of the current pulse affects the size of the nanocrystals, their phase composition, the ratio of the cubic and hexagonal phases, the degree of microstresses and the density of dislocations. The inclusion of Mn dopants into ZnSe has little effect on the nanocrystal morphology and strongly influences the morphology of polycrystals. An EPR spectrum of Mn2+ ions with a hyperfine structure constant A = 6.55 mТ and a g-factor g = 2.0055, which is due to Mn2+ ions in a cubic environment, was found in self-activated and doped ZnSe and ZnSe: Mn nanocrystals. It was found that increasing the amplitude of the current pulse, which initiates the combustion synthesis reaction, increases the intensity of the diffusion processes and more effective isovalent substitution of Zn2+ ions by Mn2+ ions in the crystal lattice of ZnSe nanocrystals. The photoluminescence spectra of ZnSe and ZnSe:Mn nanocrystals were investigated, and individual emission bands were detected in the integral spectra. There were three such individual bands in the photoluminescence spectrum of ZnSe nanocrystals. Their maxima were characterized using the following parameters: λ max = 592 nm (E = 2.095 eV), λ max = 543 nm (E = 2.282 eV), and λ max = 505 nm (E = 2.455 eV). Six individual emission bands were detected in the photoluminescence spectra of ZnSe:Mn nanocrystals with the parameters: λ max = 675. 5 nm (E = 1.835 eV), λ max = 642.5 nm (E = 1.929 eV), λ max = 613 nm (E = 2.022 eV), λ max = 583.5 nm (E = 2.124 eV), λ max = 550 nm (E = 2.255 eV), λ max = 528.5 nm (E = 2.345 eV). This paper discusses the nature of the centers of radiative recombination of individual bands.

Optical thermometric properties in Tb3+ and Eu3+-coactivated dual-emissive fluorophosphate phosphors

Lanthanide (Ln) coactivated phosphors have received considerable attention in temperature sensing. Accordingly, Ln/Ln co-doped dual-emitting phosphors have been proposed as a novel optical thermometry scheme as a result of selected dual emission peaks from different luminescent centers rather better than individual emission bands from a single luminescent center. Here, dual-emissive Tb3+-Eu3+ lanthanide coactivated Sr3GdNa(PO4)3F fluorophosphate phosphors have been exploited as a new type of optical thermometer via fluorescence intensity ratio (FIR) technique. The threefold energy transfer phenomenon from Gd3+ to both Tb3+ and Eu3+, along with from Tb3+ to Eu3+ have been corroborated in Sr3GdNa(PO4)3F host by the photoluminescence spectra. On account of different thermal responses between Tb3+ to Eu3+ and the energy transfer process of Tb3+ to Eu3+, the FIR of Tb3+ to Eu3+ in Sr3GdNa(PO4)3F phosphor displays good temperature sensing property in the temperature range from 303 K to 483 K. Moreover, the monitored main emission peaks of the Tb3+ (5D4–7F5 transition) at 545 nm and the Eu3+ (5D0–7F4 transition) at 700 nm respectively, being commendably separated to afford a superior signal detectability. The maximum relative sensitivity reaches as high as 0.16% K−1, revealing that this phosphor might have potential application as a thermometric material for optical thermometry.

Triplet emission of atomic ytterbium isolated in a xenon matrix

The electronic transitions of ytterbium atoms in a solid Xe matrix grown at 4.8 K are investigated. Absorption bands are detected in the regions of the gas-phase 6s2 1S0 → 4f135d6s2 and 6s2 1S0 → 6s6p 1P1 transitions. Both bands indicate that Yb atoms occupy multiple trapping sites, of which three are identified. Emission induced by the 6s2 1S0 → 6s6p 1P1 excitation is found to be concentrated entirely in the region of the 6s6p 3PJ → 6s2 1S0 decay, whereas the singlet emission is completely quenched. Multiple emission peaks are observed and the effects of annealing and prolonged irradiation on their amplitudes are found to be significant and are interpreted as a consequence of Yb population transfer from one type of site to another. Modeling of the ground-state site structure and stability predicts three Yb/Xe occupation types, substitutional (ss), tetravacancy (tv) and hexavacancy (hv), in order of decreasing stability. Their tentative associations with observed absorption and emission features are discussed. Time correlated single photon spectroscopy is used to determine the lifetimes of the individual emission bands. They are found to be different from each other with indications of a mixture of short- and long- lived 6s6p 3PJ fine-structure components and demonstrate distinct temperature dependencies. A dramatic decrease in the lifetime of the emission peak tentatively assigned to the most stable site with temperature is explained by a competition between the radiative and non-radiative decay paths of the 6s6p 3P1 state. The mechanism of the latter can be attributed to electron–phonon coupling as confirmed by a model of the temperature-dependence of the lifetime.The electronic transitions of ytterbium atoms in a solid Xe matrix grown at 4.8 K are investigated. Absorption bands are detected in the regions of the gas-phase 6s2 1S0 → 4f135d6s2 and 6s2 1S0 → 6s6p 1P1 transitions. Both bands indicate that Yb atoms occupy multiple trapping sites, of which three are identified. Emission induced by the 6s2 1S0 → 6s6p 1P1 excitation is found to be concentrated entirely in the region of the 6s6p 3PJ → 6s2 1S0 decay, whereas the singlet emission is completely quenched. Multiple emission peaks are observed and the effects of annealing and prolonged irradiation on their amplitudes are found to be significant and are interpreted as a consequence of Yb population transfer from one type of site to another. Modeling of the ground-state site structure and stability predicts three Yb/Xe occupation types, substitutional (ss), tetravacancy (tv) and hexavacancy (hv), in order of decreasing stability. Their tentative associations with observed absorption and emission features are discu...

Analysis of excited state proton transfer dynamics of HPTS in methanol-water mixtures from time-resolved area-normalised emission spectrum (TRANES)

Although excited state proton transfer (ESPT) dynamics has been extensively investigated in several aqueous-organic mixtures in the literature, but those are based on analysis of single wavelength transient representing either of the protonated or the deprotonated emission. Single wavelength decay, in principle, contains all necessary dynamic information but usually susceptible to potential error, if overlap of different excited species are not properly considered. Moreover, analysis of these decays are less intuitive and more challenging, since various dynamic parameters operate together. Recently, we proposed a simple and intuitive analysis to extract ESPT dynamics of 8-hydroxypyrene-1,3,6-trisulphonic acid (HPTS) from time-resolved area-normalised emission spectrum (TRANES). Here, we further refined the analysis by inducting contact ion-pair into the ESPT scheme and applied to elucidate ESPT dynamics of HPTS in methanol-water mixtures at different mole fractions of methanol. The time constants were extracted from the decay and rise of the overlap-corrected protonated or deprotonated intensities (or areas of individual emission bands) of TRANES by either a global fit or from the ratio of their intensities (or areas). We found an exponential correlation between the deprotonation times and the mole fraction of methanol, which leads to the conclusion that the activation energy of ESPT in the mixture increases linearly with increase of the mole fraction of methanol.

Absolute upconversion quantum yields of blue-emitting LiYF4:Yb3+,Tm3+ upconverting nanoparticles

The upconversion quantum yield (ΦUC) is an essential parameter for the characterization of the optical performance of lanthanoid-doped upconverting nanoparticles (UCNPs). Despite its nonlinear dependence on excitation power density (Pexc), it is typically reported only as a single number. Here, we present the first measurement of absolute upconversion quantum yields of the individual emission bands of blue light-emitting LiYF4:Yb3+,Tm3+ UCNPs in toluene. Reporting the quantum yields for the individual emission bands is required for assessing the usability of UCNPs in various applications that require upconverted light of different wavelengths, such as bioimaging, photocatalysis and phototherapy. Here, the reliability of the ΦUC measurements is demonstrated by studying the same batch of UCNPs in three different research groups. The results show that whereas the total upconversion quantum yield of these UCNPs is quite high-typically 0.02 at a power density of 5 W cm-2-most of the upconverted photon flux is emitted in the 794 nm upconversion band, while the blue emission band at 480 nm is very weak, with a much lower quantum yield of ∼6 × 10-5 at 5 W cm-2. Overall, although the total upconversion quantum yield of LiYF4:Yb3+,Tm3+ UCNPs seems satisfying, notably for NIR bioimaging, blue-light demanding phototherapy applications will require better-performing UCNPs with higher blue light upconversion quantum yields.

Caesium hydride: MS-CASPT2 potential energy curves and A1Σ+→X1Σ+ absorption/emission spectroscopy.

Correlated ab initio methods (CASPT2 and CCSD(T)) in conjunction with the ANO-RCC basis sets were used to calculate potential energy curves (PECs) of the ground, valence, and Rydberg electronic states of CsH with the inclusion of the scalar relativistic effects. The spectroscopic constants of bound states were calculated from the PECs and compared with previous theoretical and/or available experimental data. Absorption and emission spectra arising from the transition between X1Σ+ and A1Σ+ states were modelled using vibrational and rotational energy levels and corresponding nuclear wave functions obtained via the direct numerical integration of one-dimensional rovibrational Schrödinger equation in the CASPT2/ANO-RCC electronic potentials. The anharmonic shape of the A1Σ+ potential and the shape of the pertinent vibrational wave functions have an interesting impact on the final shape of the spectrum and result in the complicated fine structure of individual emission bands.

Structural and optical characterization of self-assembled Ge nanocrystal layers grown by plasma-enhanced chemical vapor deposition

We present a structural and optical study of solid-state dispersions of Ge nanocrystals prepared by plasma-enhanced chemical vapor deposition. Structural analysis shows the presence of nanocrystalline germanium inclusions embedded in an amorphous matrix of Si-rich SiO2. Optical characterization reveals two prominent emission bands centered around 2.6 eV and 3.4 eV, and tunable by excitation energy. In addition, the lower energy band shows an excitation power-dependent blue shift of up to 0.3 eV. Decay dynamics of the observed emission contains fast (nanosecond) and slow (microseconds) components, indicating contributions of several relaxation channels. Based on these material characteristics, a possible microscopic origin of the individual emission bands is discussed.

Cover Picture: Phys. Status Solidi C 6/S2

AbstractLight‐emitting diodes (LED) with vertical stacking geometry were fabricated and characterized, as reported by K. N. Hui et al. on p. S902. Red, green and blue LED chips are stacked on top of each other to form a color‐adjustable light emitter. Color‐tunable and white‐light emission have been demonstrated. The emission of different colors, ranging from green‐blue over pink‐purple to yellow‐orange as well as white light emission have been achieved by adjusting the relative intensity of two or three individual emission bands, which in turn are controlled by adjusting the voltage supply (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vertically‐stacked color‐tunable LEDs

AbstractLight‐emitting diodes with vertical stacking geometry were fabricated and characterized. Color‐tunable emission and white‐light emission has been demonstrated in Stacked‐LED (SLED). Emissions of different colors, ranging from green‐blue, to pink‐purple, to yellow‐orange and white light emission have be achieved by adjusting the relative intensity of two or three individual emission bands, which in turn are controlled by adjusting voltage supply. Compared to conventional lateral side‐by‐side geometry, the shift of white color chromaticity with respect to viewing angles is less in the case of SLED device. The farthest shift of white CIE coordinate point away from the pure white CIE coordinate is (0.29, 0.29) in SLED compared to (0.37, 0.35) in the commerical available lateral packaged structure. This vertically‐stacked LED is poised to open new frontiers in high performance color‐tunable LEDs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of the Photoluminescence of Individual Emission Bands of Mn2+ Ions in ZnS Plastically Strained Single Crystals

The photoluminescence spectra of ZnS : Mn plastically deformed single crystals are investigated. It is established that the plastic strain causes the relative concentrations of individual manganese emission centers to change together with conditions of excitation of the centers whose nearest environment has been rearranged upon plastic strain.

Luminescence of Thin Films of Lead and Bismuth Tungstates.

The luminescence spectra of thin films of PbWO4 and Bi2WO6 were investigated. It is shown that these spectra are similar and that they consist of three individual bands in the blue (2.80 eV PbWO4 and 2.93 eV Bi2WO6), green (2.35 eV PbWO4 and Bi2WO6), and red (1.75 eV PbWO4 and 1.90 eV Bi2WO6) spectral regions. The differences in the nature of the absorption centers of excitation energy are established. The distinguishing features displayed by the temperature dependences of the individual emission bands in the PbWO4 films are explained by energy migration between emission centers via transfer of free carriers through the conduction and valence bands.

Siting of the Cu + ions in dehydrated ion exchanged synthetic and natural chabasites: a Cu + photoluminescence study

Cu + emission spectra of Cu 2+ ion exchanged and reduced natural and synthetic CuNa-, CuCa-, CuCs- and CuBa-chabasites were used to identify cationic sites of the Cu + luminescence centres in this zeolite. Two different Cu + emission bands with maxima at 500 and 540 nm were observed; the emission at 540 nm prevailed at higher Cu loadings. The concentration dependence of the intensity of 500 and 540 nm emission bands indicated that both bands correspond to the single Cu + ions. The effect of the presence of co-cations in Cu-chabasites on the luminescence spectra was employed for the identification of the cationic sites corresponding to the individual emission bands. Cu + emission at 500 nm reflects the Cu + ion located at the cationic site in the eight-membered ring. Cu + emission at 540 nm is attributed to the Cu + ion coordinated to three oxygen atoms of the regular six-membered ring. The Cu + ion in this site is suggested to be located above the six-ring plane, similar to the siting of Cu + ions in Y zeolite.

Ba2ErCl7—a new near IR to near UV upconversion material

New near IR to near UV upconversion (UC) materials are required for the development of solid state lasers pumped by laser diodes. Two materials are reported: an air‐stable and efficient UC material emitting mostly in the yellowish‐green and a very promising UC compound, Ba2ErCl7, which is being further characterized spectroscopically. THe general requirements for UC materials are briefly discussed, and the preparation of Ba2ErCl7 detailed. The intensity of individual emission bands is compared for known and new UC materials.

The Emission Spectrum of Manganese-Activated Magnesium Germanate***

An unusual fine structure of emission has been reported1 for some manganese-activated magnesium germanate phosphors. In order to determine the precise nature of the transitions involved, the temperature dependence of the actual intensities, half-widths and peak positions of the individual emission bands have been measured from −190°C to +90°C using ultraviolet excitation. The measurements were made in a constant temperature vacuum sample chamber and with a Littrow type spectrograph containing calibrated photographic plates. Five emission bands were clearly resolved between 6200 and 6800A. The intensity of each band increases with temperature up to a critical temperature above which the efficiency decreases rapidly. Also, the short wave-length bands practically disappear at −190°C, and the individual bands shift slightly to shorter wave-lengths and decrease in half-width at low temperatures. The experimental results are qualitatively interpreted, and tentative correlations with atomic spectra are proposed.