eV Absorption Band Research Articles

Regulation of CN-related optical transitions and non-radiative capture cross-section by biaxial strain in AlN

Carbon-related 4.7 eV absorption band and small in-plane strains in AlN may have some significant effects on its application in optoelectronic devices. Based on the accurate hybrid density functional calculation, we investigate the transition energy levels, photo-transition processes, and hole capture cross-sections of CN defect. We propose that the transition from −1 to 0 charge states of CN defect may be responsible for the 4.7 eV absorption band in AlN. In addition, the CN defect-related absorption and emission peaks are linearly dependent on the biaxial strain in the range of −3% to +3%, and the hole non-radiative capture rate by the CN center at the −3% biaxial strain is only 3.65% of that at the +3% biaxial strain. This work provides an effective approach for regulating the charge carrier capture ability of the defect center and improving device performance.

Photoluminescence Study of Silver Nanoparticles Decorated on Multiwall Carbon Nanotubes (MWCNTs) by Spectroflourometer

Multiwall carbon nanotubes (MWCNTs) have recently attracted much attention due to their appealing properties in several domains. Multiwall carbon nanotubes (MWCNTs) were functionalized in this research study and then decorated with silver nanoparticles. Fourier transform Infrared Spectroscopy (FTIR) was used to check the successful attachment of hydroxyl (OH) and carboxyl (C=O) groups with MWCNTs. XRD analysis was used to check the crystallite size of silver nanoparticles and the decoration of silver nanoparticles on MWCNTs. Pure Carbon nanotubes (CNTs) show luminescence in an infrared region having approximately 1.3 eV absorption band. At room temperature, our hybrid material's photoluminescence (PL) spectra indicate only one peak in the UV region and many high-intensity peaks in the visible region. These PL results show the change in the band structure of Ag/MWCNTs composite compared to pure silver nanoparticles and carbon nanotubes. Therefore, it unlocks the possibilities to use this hybrid material for bio-sensing and bio-imaging devices, chemical sensing devices, optoelectronics devices, drug delivery devices, cancer cell detection, and environment detection devices.

Platinum(II)-containing donor-acceptor dimesitylborane-based complexes: Synthesis, characterization, photophysical and photovoltaic properties

• Platinum(II)-containing donor-acceptor (D-A) dimesitylborane-based complexes were synthesized. • Photophysical, electrochemical and photovoltaic properties of the complexes were investigated. • Complexes have narrow energy bandgaps and broad absorption bands. • Bulk heterojunction solar cells show the power conversion efficiency reaching 3.07%. A new series of platinum(II)-containing donor-acceptor (D-A) dimesitylborane-based complexes PT1−PT4 have been successfully synthesized in reasonable yields. The photophysical and electrochemical properties have also been investigated. Complexes PT1−PT4 have narrow energy bandgaps within 1.69−1.74 eV and broad absorption bands within 240−700 nm, which would be suitable candidates for the fabrication of devices in bulk heterojunction (BHJ) solar cells. The devices based on PT4 , which has the longest π-conjugated framework, showed the highest PCE of 3.07% among all the devices with high open-circuit voltage ( V oc ) of 0.92 V, short-circuit photocurrent density ( J sc ) of 7.95 mA cm −2 and fill factor ( FF ) of 0.42 at the optimized active layer thickness of 85 nm. These complexes illustrate the potential of solution-processible metallo-organic D-A complexes for efficient power generation in photovoltaic implementation.

Origins of the coloration from structure and valence state of bismuth oxide glasses

We report the origins of the coloration of Bi2O3–SiO2, Bi2O3–B2O3, and Bi2O3–P2O5 glasses. The glasses were quantitatively analyzed to reveal the Bi valence state and network connectivity of silicate, borate, and phosphate. The electronic structure was correlated with Bi valence states, giving two types of signals: those related to optical bandgaps with Bi3+s–p intra-transition states at 3.6–4.7 eV and visible absorption bands at 2.5–2.7 eV originating from the charge transfer between Bi3+ and Bi5+ and/or plasmonic Bi0 clusters in the colored glasses. These Bi oxide glasses with tvisible transparent and high refractive index are attractive to replace toxic Pb-containing optical glasses.

Optical property in colorless AlN bulk crystals: investigation of native defect-induced UV absorption

The 4.7 eV absorption band of colorless AlN crystal with the carbon concentration level of 107 cm−3 is investigated. An analysis of the absorption characteristics of the annealed AlN crystal indicates that the native defect is the origin of 4.7 eV absorption band and gives rise to an infrared absorption band at 2850 cm−1. The theory calculations predict two absorption bands at 4.65 eV and 0.35 eV of nitrogen vacancy, and the defect explains the increased and decreased intensity of infrared absorption band at 2850 cm−1 as well as a similar increased-decreased ultraviolet absorption band at 4.7 eV.

Radiation Effects on Aluminosilicate Optical Fibers: Spectral Investigations From the Ultraviolet to Near‐Infrared Domains

Online X‐ray radiation induced attenuation (RIA) has been performed in aluminosilicate optical fibers having different Al concentrations. The studied UV‐visible spectral range revealed the presence of absorption bands related to Al defects. Their generation is shown to be not noticeably dependent on the dose rate. Furthermore, the Al content (2–4 wt%), the core sizes, and the manufacturing processes (SPCVD or MCVD) of the preforms have no significant influence on the RIA levels and kinetics, as well as the drawing parameters within the range used for specialty fiber production. The Aluminum–Oxygen Hole Center (Al–OHC) presence was proved by their 2.3 eV absorption band and by their electron paramagnetic resonance (EPR) signature. The growth kinetic of their concentration versus dose is linear up to 2–3 kGy (SiO2). While, at higher doses, EPR data highlight a saturation, suggesting that the Al–OHC generation results from a process involving precursors. To explain the E'Si growth kinetic with the dose, two processes are necessary, the first is from precursors and the second by breaking SiOSi links. The study of the RIA induced in the NIR demonstrates that the tail of the 2.3 eV Al–OHC band cannot explain the fiber degradation and that additional defects contribute to the induced losses.

Optical properties of LiGaSe2 noncentrosymmetric crystal

Large single crystals of LiGaSe2 were grown and their spectroscopic properties were studied. As-grown LiGaSe2 crystals are transparent in the 0.5–14 μm spectral region as well as at λ > 200 μm in the THz domain. A large blue shift of the fundamental absorption edge was established after high-temperature annealing in the presence of Li. Intense low-temperature photoluminescence (PL) in the 1.77 eV band is excited in the 3.0–3.6 eV absorption band near the fundamental absorption edge; PL intensity decreases by two orders of magnitude as temperature grows to 300 K. The X-ray irradiation results in crystal darkening, whereas the original state is restored after heating to 500 K or by visible light illumination. This effect is due to six traps that capture charge carriers: these traps were revealed using the thermoluminesence technique and their parameters were calculated. Pyroelectric luminescence in the 100–270 K range confirms the noncentrosymmetric structure of this crystal. DFT calculations were carried out to simulate the phonon density of states, optical absorption and Raman spectra. Calculations show that the intense 3.0–3.6 eV absorption band and corresponding 1.77 eV PL band may be associated with the cation antisite defects (GaLi), i.e. gallium in the Li site. The 2.15 and 2.4 eV emissions are likely due to the recombination of self-trapped excitons.

Synthesis and photovoltaic properties of new ruthenium(II)-bis(aryleneethynylene) complexes

A new series of small-molecular ruthenium(II)-diynes trans-Ru(dppe)2(C≡CAr)2 (D1−D4) (dppe = Ph2CH2CH2Ph2; Ar = aromatic moiety) have been successfully designed, synthesized and characterized by photophysical, electrochemical and computational methods, and complexes D1 and D3 were crystallographically characterized. The optical and time-dependent density functional theory studies showed that the absorption ability of these complexes was significantly enhanced by incorporating the stronger electron-donor groups. The effect of different electron-donor groups in these metallo-organic complexes on the optoelectronic and photovoltaic properties was also examined. In this work, benzothiadiazole as the electron acceptor and triphenylamine and/or thiophene as the electron donor were introduced in these complexes, which were found to have optimal energy bandgaps spanning from 1.70 to 1.83 eV and broad absorption bands within 300–700 nm, rendering them good electron donor materials to blend with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) in the fabrication of the solution-processed bulk heterojunction (BHJ) solar cells. The best power conversion efficiency (PCE) of 0.66% was achieved, which is the highest PCE in ruthenium(II)-containing BHJ solar cells to date.

Study of the effect of optical bleaching at selected photon energies on the optical absorption and thermoluminescence of LiF: Mg, Ti (TLD-100)

A new interpretation of the optical absorption energy spectrum of LiF:Mg,Ti (TLD-100) is presented in which the 4.0 eV band is composed of two-sub-bands at 3.84 eV and 4.3 eV. The new interpretation is of central importance in the understanding of the geminate recombination mechanisms giving rise to glow peak 4 and composite glow peak 5 in the thermoluminescence (TL) of TLD-100. Photon bleaches at energies of 3.6 eV and 4.2 eV on samples of gamma irradiated TLD-100 were carried out in order to investigate the correlation of the 4.3 eV absorption band with the electron-hole configuration giving rise to glow peak 5a and the 3.84 eV absorption band with the e-only configuration giving rise to glow peak 5. The increase in peak 4 (correlated with the hole-only configuration) induced by the 4.2 eV bleach is enhanced by 90% over the increase induced by the 3.6 eV bleach. The decrease in peaks 5a and 5 induced by the 4.2 eV bleach are enhanced by 400% and 50% respectively over the 3.6 eV bleach. The results are consistent with the suggested structures giving rise to the 4.3 eV and 3.84 eV bands observed in the optical absorption energy spectrum.

The effect of optical bleaching at selected photon energies on the thermoluminescence of LiF:Mg,Ti

A new interpretation of the optical absorption energy spectrum of LiF:Mg,Ti (TLD-100) is presented in which the 4.0 eV band is composed of two-sub-bands at 3.8 eV and 4.3 eV. The new interpretation is of central importance in the understanding of the geminate recombination mechanisms giving rise to composite glow peak 5 in the thermoluminescence (TL) of TLD-100. Photon bleaches at energies of 3.6 eV and 4.3 eV on samples of gamma irradiated TLD-100 were carried out in order to investigate the correlation of the 4.3 eV absorption band with the electron-hole configuration giving rise to glow peak 5a and the 3.8 eV absorption band with the e-only configuration giving rise to glow peak 5. The 4.3 eV bleach decreases peak 5a dramatically and increases peak 4 (a hole-only configuration). No such effect is observed following the 3.6 eV bleach. The results strongly support the suggested structures giving rise to the 4.3 eV and 3.8 eV bands observed in the optical absorption energy spectrum.

Effect of Mg2+ ions co-doping on luminescence and defects formation processes in Gd3(Ga,Al)5O12:Ce single crystals

Photo- and radioluminescence and thermally stimulated luminescence characteristics of Ce3+ - doped and Ce3+, Mg2+ co-doped Gd3(Ga,Al)5O12 (GAGG) single crystals of similar composition are investigated in the 9–500 K temperature range. The Ce3+ - related luminescence spectra and the photoluminescence decay kinetics in these crystals are found to be similar. Under photoexcitation in the Ce3+ - and Gd3+ - related absorption bands, no prominent rise of the photoluminescence intensity in time is observed neither in GAGG:Ce,Mg nor in GAGG:Ce crystals. The afterglow is strongly reduced in GAGG:Ce,Mg as compared to GAGG:Ce, and the afterglow decay kinetics is much faster. Co-doping with Mg2+ results in a drastic decrease of the thermally stimulated luminescence (TSL) intensity in the whole investigated temperature range and in the appearance of a new complex Mg2+ - related TSL glow curve peak around 285 K. After irradiation in the Ce3+ - related 3.6 eV absorption band, the TSL intensity in GAGG:Ce,Mg is found to be comparable with that in the GAGG:Ce epitaxial film of similar composition. The Mg2+ - induced changes in the concentration, origin and structure of the crystal lattice defects and their influence on the scintillation characteristics of GAGG:Ce,Mg are discussed.

UV laser photoactivation of hexachloroplatinate bound to individual nucleobases in vacuo as molecular level probes of a model photopharmaceutical.

Isolated molecular clusters of adenine, cytosine, thymine and uracil bound to hexachloroplatinate, PtCl6(2-), have been studied using laser electronic photodissociation spectroscopy to investigate photoactivation of a platinum complex in the vicinity of a nucleobase. These metal complex-nucleobase clusters represent model systems for identifying the fundamental photochemical processes occurring in photodynamic platinum drug therapies that target DNA. This is the first study to explore the specific role of a strongly photoactive platinum compound in the aggregate complex. Each of the clusters studied displays a broadly similar absorption spectra, with a strong λmax ∼ 4.6 eV absorption band and a subsequent increase in the absorption intensity towards higher spectral-energy. The absorption bands are traced to ligand-to-metal-charge-transfer excitations on the PtCl6(2-) moiety within the cluster, and result in Cl(-)·nucleobase and PtCl5(-) as primary photofragments. These results demonstrate how selective photoexcitation can drive distinctive photodecay channels for a model photo-pharmaceutical. In addition, cluster absorption due to excitation of nucleobase-centred chromophores is observed in the region around 5 eV. For the uracil cluster, photofragments consistent with ultrafast decay of the excited state and vibrational predissociation on the ground-state surface are observed. However, this decay channel becomes successively weaker on going from thymine to cytosine to adenine, due to differential coupling of the excited states to the electron detachment continuum. These effects demonstrate the distinctive photophysical characteristics of the different nucleobases, and are discussed in the context of the recently recorded photoelectron spectra of theses clusters.

Interpretation of the photoelectron, ultraviolet, and vacuum ultraviolet photoabsorption spectra of bromobenzene by ab initio configuration interaction and DFT computations.

New photoelectron, ultraviolet (UV), and vacuum UV (VUV) spectra have been obtained for bromobenzene by synchrotron study with higher sensitivity and resolution than previous work. This, together with use of ab initio calculations with both configuration interaction and time dependent density functional theoretical methods, has led to major advances in interpretation. The VUV spectrum has led to identification of a considerable number of Rydberg states for the first time. The Franck-Condon (FC) analyses including both hot and cold bands lead to identification of the vibrational structure of both ionic and electronically excited states including two Rydberg states. The UV onset has been interpreted in some detail, and an interpretation based on the superposition of FC and Herzberg-Teller contributions has been performed. In a similar way, the 6 eV absorption band which is poorly resolved is analysed in terms of the presence of two ππ* states of (1)A1 (higher oscillator strength) and (1)B2 (lower oscillator strength) symmetries, respectively. The detailed analysis of the vibrational structure of the 2(2)B1 ionic state is particularly challenging, and the best interpretation is based on equation-of-motion-coupled cluster with singles and doubles computations. A number of equilibrium structures of the ionic and singlet excited states show that the molecular structure is less subject to variation than corresponding studies for iodobenzene. The equilibrium structures of the 3b13s and 6b23s (valence shell numbering) Rydberg states have been obtained and compared with the corresponding ionic limit structures.

Electronic State Spectroscopy of Halothane As Studied by ab Initio Calculations, Vacuum Ultraviolet Synchrotron Radiation, and Electron Scattering Methods.

We present the first set of ab initio calculations (vertical energies and oscillator strengths) of the valence and Rydberg transitions of the anaesthetic compound halothane (CF3CHBrCl). These results are complemented by high-resolution vacuum ultraviolet photoabsorption measurements over the wavelength range 115-310 nm (10.8-4.0 eV). The spectrum reveals several new features that were not previously reported in the literature. Spin-orbit effects have been considered in the calculations for the lowest-lying states, allowing us to explain the broad nature of the 6.1 and 7.5 eV absorption bands assigned to σ*(C-Br) ← nBr and σ*(C-Cl) ← n(Cl) transitions. Novel absolute photoabsorption cross sections from electron scattering data were derived in the 4.0-40.0 eV range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of halothane in the upper stratosphere (20-50 km).

Communication: Photoactivation of nucleobase bound platinum(II) metal complexes: probing the influence of the nucleobase.

We present UV laser action spectra (220-300 nm) of isolated nucleobase-bound Pt(II)(CN)4(2-) complexes, i.e., Pt(CN)4(2-)⋅M, where M = uracil, thymine, cytosine, and adenine. These metal complex-nucleobase clusters represent model systems for identifying the fundamental photophysical and photochemical processes occurring in photodynamic platinum (II) drug therapies that target DNA. This is the first study to explore the specific role of the nucleobase in the photophysics of the aggregate complex. Each of the complexes studied displays a broadly similar absorption spectra, with a strong λmax ∼ 4.7 eV absorption band (nucleobase localized chromophore) and a subsequent increase in the absorption intensity towards higher spectral-energy (Pt(CN)4(2-) localized chromophore). However, strikingly different band widths are observed across the series of complexes, decreasing in the order Pt(CN)4(2-)⋅Thymine > Pt(CN)4(2-)⋅Uracil > Pt(CN)4(2-)⋅Adenine > Pt(CN)4(2-)⋅Cytosine. Changes in the bandwidth of the ∼4.7 eV band are accompanied by distinctive changes in the photofragment product ions observed following photoexcitation, with the narrower-bandwidth complexes showing a greater propensity to decay via electron detachment decay. We discuss these observations in the context of the distinctive nucleobase-dependent excited state lifetimes.

Narrow Bandgap Platinum(II)-Containing Polyynes with Diketopyrrolopyrrole and Isoindigo Spacers

Two new solution-processible platinum(II) polyyne polymers P1 and P2 bridged by diketopyrrolopyrrole and isoindigo units have been synthesized via the CuI-catalyzed dehydrohalogenation reaction of the platinum(II) chloride precursors and the corresponding diacetylene ligands. The photophysical and electrochemical properties of P1 and P2 were investigated. These metallo-organic polymers have relatively narrow bandgaps of around 1.58–1.70 eV and broad absorption bands which are favorable for harvesting solar energy.

Luminescence of unfused 95%SiO2–5%GeO2 amorphous films with fluorine additive: No evidence for presence of GeODC(I) defects found

Photoluminescence (PL) of unfused amorphous germanosilicate films with fluorine additive is studied in 2–8.5eV spectral range. Experiments are based on films deposited on silica substrates by means of the surface-plasma chemical vapor deposition (SPCVD). Films of about 100μm in thickness with “high F” (~4.2wt.%) and “low F” (~0.5wt.%) fluorine content have been fabricated for the experiments. KrF (248nm), ArF (193nm) and F2 (157nm) excimer lasers are used to pump PL. It is found that absorption and luminescence associated with germanium oxygen deficient centers (GeODCs) in “high F” and “low F” films differ. In the “high F” unfused film absorption coefficient of the band at 5eV as well as intensity of the blue PL band at 3.1eV are significantly greater. This film proves features of the so called GeODCs(II), which symbolize twofold coordinated germanium defects in silica network. In the “low F” unfused film absorption band at 5eV is feebly marked. Poorly resolved PL intrinsic to GeODCs(II) can be detected in this film under the KrF laser pump. The most significant PL features are revealed under deeper UV pump by ArF and F2 lasers. Spectral positions of PL bands excited by these lasers correspond to GeODCs(II). However PL decay kinetics dramatically differs from that one intrinsic to GeODCs(II). Noticeable growth of PL intensity caused by permanent (half an hour and more) exposure to ArF and/or F2 lasers takes place, indicating GeODC(II) formation. It is found that considerable body of fluorine additive has the same effect as profusion for “low F” and/or fluorine free germanosilicate amorphous material synthesized by SPCVD. In the “high F” film yield of GeODC(II) PL pumped by the F2 laser remains high. This speaks for the suppression of the competitive 7.6eV absorption band associated with SiODCs(I) by fluorine additive indicating a decrease in the content of this type of defects in the material. High yield of GeODC(II) luminescence pumped by deep UV photons as well as hypothetical similarity of SiODC(I) and GeODC(I) permit one to conclude that GeODCs(II) are the only defects dominating in the materials under study.

Formation mechanisms of precursors of radiation-induced color centers during fabrication of silica optical fiber preform

Samples in the form of transverse slices of rods and optical fiber preforms made from the high-hydroxyl KU-1 and low-hydroxyl KS-4V silica by the plasma outside deposition (POD) method are γ-irradiated to a dose of ∼1 MGy (SiO2). Next, the radial dependences of the radiation-induced nonbridging oxygen hole center (NBOHC) and E′-center (three-coordinated silicon) in the samples are constructed by measuring the amplitudes of their 4.8 and 5.8 eV absorption bands, respectively. Based on the analysis of these radial dependences and considering the temperature and duration of the preirradiation heat treatment of the rods and preforms at the POD-installation, we determine the ratio of the oscillator strengths of the above bands and the microscopic thermoinduced processes occurring during preform fabrication and producing precursors of the radiation-induced NBOHC and E′-center. These processes are found to be associated with the escape of either H2 or H2O from neighboring hydroxyl groups, and, therefore, can occur in high-hydroxyl silica only. It is concluded that enhancement of the radiation resistance of high-hydroxyl silica optical fibers requires decreasing the temperature and duration of the preform fabrication process, in particular, changing from the POD-technology to the low-temperature plasmachemical vapor deposition (PCVD) or surface PCVD (SPCVD)-technology.

Luminescence of F+‐type centers in undoped Lu3Al5O12 single crystals

AbstractUndoped single crystals of Lu3Al5O12 annealed in the reduced atmosphere or additively colored in Al vapors were studied at 4.2–400 K by the time‐resolved spectroscopy methods. The 3.15 eV emission, excited in the 3.4, 5.3, and 6.1 eV absorption bands and having a decay time of 3.14 ns at 10 K, was ascribed to the F+‐type centers. It was suggested that these centers consist of an oxygen vacancy with one trapped electron which is perturbed by the neighboring antisite defect (Lu) or an impurity ion at the Al3+ site. The small Stokes shift and half‐width of the emission band indicate that the F+‐type centers are characterized by a weak electron–phonon interaction.

Solution processable low bandgap diketopyrrolopyrrole (DPP) based derivatives: novel acceptors for organic solar cells

Novel low bandgap solution processable diketopyrrolopyrrole (DPP) based derivatives functionalized with electron withdrawing end capping groups (trifluoromethylphenyl and trifluorophenyl) were synthesized, and their photophysical, electrochemical and photovoltaic properties were investigated. These compounds showed optical bandgaps ranging from 1.81 to 1.94 eV and intense absorption bands that cover a wide range from 300 to 700 nm, attributed to charge transfer transition between electron rich phenylene-thienylene moieties and the electron withdrawing diketopyrrolopyrrole core. All of the compounds were found to be fluorescent in solution with an emission wavelength ranging from 600 to 800 nm. Cyclic voltammetry indicated reversible oxidation and reduction processes with tuning of HOMO–LUMO energy levels. Bulk heterojunction (BHJ) solar cells using poly(3-hexylthiophene) (P3HT) as the electron donor with these new acceptors were used for fabrication. The best power conversion efficiencies (PCE) using 1 : 2 donor–acceptor by weight mixture were 1% under simulated AM 1.5 solar irradiation of 100 mW cm−2. These findings suggested that a DPP core functionalized with electron accepting end-capping groups were a promising new class of solution processable low bandgap n-type organic semiconductors for organic solar cell applications.