Recombination Of Self-trapped Excitons Research Articles

Broadband Emission with a Massive Stokes Shift from Sulfonium Pb–Br Hybrids

Similar to organoammonium cations that template the well-studied layered perovskites, organosulfonium cations template the layered Pb–Br hybrid (tms)4Pb3Br10 (1; tms = (CH3)3S+). Upon ultraviolet excitation, this colorless, organic-inorganic hybrid exhibits broad red/near-infrared photoluminescence (PL) with a massive Stokes shift of ca. 1.7 eV. A suite of PL measurements suggests that the broad emission is an intrinsic property of the material. We ascribe this emission to radiative recombination of self-trapped excitons in the inorganic sublattice, which features both face- and corner-sharing Pb–Br octahedra. We further demonstrate that complex sulfonium-based cations can expand this family of lead-halide hybrids while maintaining the broad, Stokes-shifted PL. The use of more exotic main-group cations affords an exciting opportunity for synthetic chemists to expand the phase-space of semiconducting hybrids with unusual properties for potential applications in optoelectronics.

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.

Self-trapped exciton emission from carbon dots investigated by polarization anisotropy of photoluminescence and photoexcitation.

Carbon dots have attracted tremendous attention because of their intrinsic advantages that open up opportunities to replace traditional fluorescent materials in various application fields. However, until now, the emission mechanism from carbon dots has been controversial, substantially hindering the extensive exploitation of these materials. Here, we explore systematically the essential emission behavior of carbon dots by using polarization anisotropy spectroscopy, electric-field modulation spectroscopy, and time-resolved photoluminescence measurements. We probe the momentum evolution dynamics and evaluate the decay process of the photoexcited hot carriers, which manifest characteristics that are distinct from band edge emission. We provide clear evidence that carbon dot emission originates from radiative recombination of self-trapped excitons, where the mobilization of the carriers is largely impeded due to the existence of a strong local potential field and thus the relaxation of the hot carriers is strongly suppressed. Based on the self-trapped exciton model, all the optical properties of carbon dots inferred from both steady-state and time-resolved optical spectroscopy can be interpreted consistently. Our investigation provides an alternative insight into the emission mechanisms of carbon dots, which may improve our understanding of these novel materials.

In Situ Luminescence Measurement from Silica Glasses Irradiated with 20 keV H− Ions

A new low-energy negative-ion induced luminescence setup was recently developed at the injector of the GIC4117 2 × 1.7 MV Tandem accelerator in Beijing Normal University. In situ luminescence measurements are performed on silica glass by using 20keV H− ions at room temperature. Gauss fitting of the spectra revealed six overlapping components at about 2.7 eV, 2.4eV, 1.9eV, 1.8eV, 4.2eV, and 3.6eV, which except for the new observed emission band at 3.6 eV are assigned to the creation of type II oxygen-deficient centers, E' centers, non-bridging oxygen hole centers with different precursor states, and type-I oxygen-deficient centers. The fitted results of the saturation concentration show that self-trapped exciton recombination at type-II oxygen-deficient centers is the main luminescence emission process. The evolution of the luminescence intensity and full width at half maximums as a function of ion fluence is also discussed. It is found that the number of recombination centers reaches its maximum at lower fluence, and the area ratio between blue bands and red bands is much lower than that under high energy H+ ion irradiation.

Luminescence and scintillation properties of Cs3BiCl6 crystals

The optical and scintillation properties of Cs3BiCl6 single crystals were characterized, and absorption properties were investigated for thin films. The thin films showed two absorption bands at ∼220 nm and 330 nm; these bands can be attributed to the 1A1g → 1T1u and 1A1g → 3T1u transitions of Bi3+ ions, respectively. A luminescence band was observed at ∼390 nm in the photoluminescence spectra; this band can be attributed to the 3T1u → 1A1g transition of Bi3+ ions. In the X-ray-induced radioluminescence spectrum, in addition to the luminescence band at 390 nm, another band was observed at 600–700 nm; this band was tentatively attributed to the radiative recombination of self-trapped excitons. The decay-time constants of photoluminescence and scintillation decay were of the order of nanoseconds. The scintillation light yield was 800 photons/MeV. The results indicate that Cs3BiCl6 has a fast scintillation decay and a relatively poor light yield.

Photoluminescence of monoclinic Li3AlF6 crystals under vacuum ultraviolet and soft X-ray excitations

Using Bridgman technique we have grown monoclinic β-LiAF crystals suitable for optical studies, performed XRD-identification and Rietveld refinement of the crystal structure and carried out a photoluminescence study upon vacuum ultraviolet (VUV) and extreme ultraviolet (XUV)-excitations, using the low-temperature (T=7.2K) time-resolved VUV-spectroscopy technique. The intrinsic PL emission band at 340–350nm has been identified as due to radiative recombination of self-trapped excitons. The electronic structure parameters were determined: bandgap Eg≈12.5eV, energy threshold for creation of unrelaxed excitons 11.8eV<En<12.5eV. The PL emission bands at 320–325 and 450nm were attributed to luminescence caused by lattice defects. We have discovered an efficient excitation of PL emission bands in the energy range of interband transitions (Eex>13.5eV), as well as in the energy range of core transitions at 130eV. We have revealed UV–VUV PL emission bands at 170 and 208nm due to defects. A reasonable assumptions about the origin of the UV–VUV bands were discussed.

Nanosecond light-induced transient absorption in As2S3: Self-trapped exciton recombination in amorphous chalcogenides

In this article, we report spectral and temporal dependence of nanosecond (ns) transient absorption (TA) in a-As2S3 thin film when illuminated with the above (355nm) and near (532nm) bandgap laser. Detailed temperature dependent study of TA by 355nm excitation demonstrates the decrease in TA together with faster relaxation kinetics at higher temperatures. Such observation of the change in kinetics and magnitude of TA with temperature is well consistent with the self-trapped exciton recombination mechanism. In addition to this, our experimental results also reveal that the long wavelength component of TA decays faster than the short wavelengths which provides interesting insights that the trapped exciton possesses longer lifetime in deep traps than in shallow traps. The excitation fluence dependent study further reveals that TA exhibits a quadratic dependence on laser dose which yields that the effects are originating from the two-photon process.

Laser-Excited Excitonic Luminescence of Nanocrystalline TiO2 Powder

Titanium dioxide (TiO2) nanocrystalline powders were prepared by the thermal hydrolysis method in the form of pure anatase or rutile and were investigated by X-ray diffraction, X-ray fluorescence, FT-Raman spectroscopy, optical absorption, and photoluminescence (PL) methods. PL spectra were studied under the intense UV laser excitation at 337.1 nm (3.68 eV) at room temperature. Some interesting features in the PL spectra including the well-resolved peaks of excitonic and band-band transitions in TiO2 were observed for the first time. It is shown that PL bands with peaks at 2.71–2.81 eV and its phonon replicas in anatase and rutile TiO2 arise from the excitonic e− − ℎ+ recombination via oxygen vacancies. The excitonic peak at 2.91 eV is attributed to the recombination of self-trapped excitons in anatase or free excitons in rutile TiO2. The PL peaks within 3.0–3.3 eV in anatase TiO2 are ascribed to indirect allowed transitions due to the band-band e− − ℎ+ recombination. The peaks at 3.03 and 3.26 eV are attributed to the free exciton emission near the fundamental band edge of rutile and anatase TiO2, respectively.

Optical properties of hierarchical-nanostructured TiO2 and its time-dependent photo-degradation of gaseous acetaldehyde

The TiO2 hierarchical nanostructures (HNs) composed of rutile TiO2 nanowires on anatase TiO2 nanofibers had higher photocatalytic activities of 62% and 48% than the commercial TiO2 nanoparticles (∼21 nm diameter) in the continuous flow mode and closed-circulation mode, respectively, leading to an efficient degradation of gaseous acetaldehyde under UV-light irradiation. This behavior may be attributed to the effective TiO2 HNs with specific surface area of 85.1 m2/g and lower radiative recombination of self-trapped excitons, enabling an effective electron-hole separation.

Optical spectroscopy of Pr3+-doped γ-BiB3O6 crystals

Single crystals of Pr3+-doped γ-BiB3O6 of optical quality were grown. Band gap is 4.16eV at 80K, which differs only slightly from that for undoped samples. Pr concentration is 2×1020cm−3 and the dopant is distributed uniformly along the crystal. Absorption cross-section for Pr3+ was estimated to be ∼1×10−20cm2. At X-ray and UV, band-to-band excitation the Pr:γ-BiB3O6 crystals demonstrate an intense emission in the 450nm broad band, which is related mainly to recombination of self-trapped excitons (STE). A part of energy is transferred radiatively to Pr3+ ions. The STE emission quenches as temperature increases to 250K with simultaneous decrease of the decay time from 11μs to 17ns. Intensity and decay time of the Pr3+ intracenter luminescence with τ∼50μs at 450nm excitation do not depend on temperature. Intensity of thermoluminescence in the 150–400K range quenches 2 orders at Pr doping, and parameters of the traps were estimated. As a result of relatively narrow band gap in γ-BiB3O6:Pr3+, there is no cascade luminescence typical of many wide-band-gap matrices.

Room temperature photoluminescence of anatase and rutile TiO2 powders

The optical absorption and photoluminescence of anatase and rutile TiO2 were studied at room temperature. TiO2 nanocrystalline powders were synthesized in the form of pure anatase or rutile. The samples were characterized by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, optical absorption and photoluminescence (PL) methods. The PL spectra were studied under the intensive UV (3.68eV) laser excitation. Some interesting features in the PL spectra including the well-resolved peaks of excitonic and band–band transitions in TiO2 were observed, to our knowledge, for the first time. It is shown that PL bands including peaks at 2.71–2.81eV and its phonon replicas in anatase and rutile TiO2 arise from the excitonic e−−h+ recombination via oxygen vacancies. The excitonic peak at 2.91eV is attributed to the recombination of self-trapped excitons in anatase or free excitons in rutile TiO2. The PL peaks within 3.0–3.3eV in anatase TiO2 are ascribed to indirect allowed transitions due to the band–band e−−h+ recombination. The peaks at 3.03eV and 3.26eV are attributed to the free exciton emission near the fundamental band edge of rutile and anatase TiO2, respectively. The influence of TiO2 crystal structure and calcination temperature on the PL spectra is discussed.

Radiative and nonradiative recombination of self-trapped exciton on silicon nanocrystal interface

The theory of the multiphonon and radiative recombination of a selftrapped exciton on the interface of a sil� icon nanocrystal in a SiO2 matrix is developed. Selftrapped excitons play a key role in the hot carrier dynam� ics in nanocrystals under photoexcitation. The ratio of the probabilities of the multiphonon and radiative recombination of the selftrapped exciton is estimated. The probabilities of exciton tunnel transition from the selftrapped state to a nanocrystal are calculated for na nocrystals of various sizes. The infrared range spec� trum of the luminescence of the selftrapped exciton is obtained.

Low temperature luminescence of ZnMoO4 single crystals grown by low temperature gradient Czochralski technique

ZnMoO4 single crystals of high optical quality were grown using low temperature gradient Czochralski technique and their luminescence properties were studied. At low temperatures two overlapping luminescence bands with the maxima 1.93eV and 2.45eV were detected at Eex>3.9eV. The bands are attributed to the radiative recombination of self-trapped excitons of MoO42- parentage. High-intensity non-elementary peak of thermostimulated luminescence (TSL) observed at 65K is due to the thermal release of self-trapped holes. The threshold of TSL excitation spectrum at 6.2eV exceeds the bandgap value by 1.9eV and corresponds to electronic transitions from the valence band to the upper subband of the conduction band.

Host Sensitization of Tb3+ Ions in Tribarium Lanthanide Borates Ba3Ln (BO3)3 (Ln = Lu and Gd)

The vacuum-ultraviolet (VUV) spectroscopic properties of undoped and Tb(3+)-doped borates Ba(3)Ln(BO(3))(3) (Ln = Lu and Gd) with different crystal structures were investigated by using synchrotron radiation. Ba(3)Lu(BO(3))(3) (BLB) crystallizes in a hexagonal structure, whereas Ba(3)Gd(BO(3))(3) (BGB) crystallizes in a trigonal structure. The maximum host absorption for BLB and BGB was found to locate at ~179 and ~195 nm, respectively. Upon host excitation, BLB exhibits an intrinsic broad UV emission centered at 339 nm, which is attributed to the recombination of self-trapped excitons that may presumably be associated with band-gap excitations or molecular transitions within the BO(3)(3-) group. In contrast to BLB, no broad emission but line emission ascribed to a Gd(3+)(6)P(J)-(8)S(7/2) transition was observed in the emission spectrum of BGB. Upon doping of Tb(3+) ions into the hosts of BLB and BGB, an efficient energy transfer from the host excitations to Tb(3+) via host/Gd(3+) emission was observed, showing that host sensitization of Tb(3+) occurs in these rare-earth borates.

Emission features of Ho 3+ ion in Nb 2O 5, Ta 2O 5 and La 2O 3 mixed Li 2O–ZrO 2–SiO 2 glasses

Li 2O–ZrO 2–SiO 2: Ho 3+ glasses mixed with three interesting d-block elemental oxides, viz., Nb 2O 5, Ta 2O 5 and La 2O 3, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb 2O 5 and Ta 2O 5 mixed Li 2O–ZrO 2–SiO 2 glasses (free of Ho 3+ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta 5+ and Nb 5+ ions in the glass network. The Judd–Ofelt theory was successfully applied to characterize Ho 3+ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β r and the radiative lifetime τ r, for 5S 2 emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for 5S 2 level of Ho 3+ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La 2O 3 mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho 3+ ions.

Self-Trapped Exciton Photoluminescence from Polycrystalline K2AgI3 Obtained by Solid-State Reaction Method

Polycrystalline K2AgI3 is synthesized by solid-state reaction at 403K using AgI and KI as starting materials. X-ray diffraction, photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra, electron paramagnetic resonance (EPR) and absorption spectra are employed to investigate the structure and photo-physical properties of such material. A broad luminescence band centered at about 608nm at 180K with large Stokes shift is found. When the temperature rises to about 300K, an obvious PL blue-shift, PL intersity weakening, and an emission band broadening are observed. It is suggested that the emission at about 608nm may originate from the recombination of self-trapped excitons.

Intrinsic photoluminescence of adamantane in the ultraviolet spectral region

We report intrinsic photoluminescence in the ultraviolet for adamantane $({\text{C}}_{10}{\text{H}}_{16})$, the smallest in a series of hydrogen-passivated diamond clusters (diamondoids). The luminescence is ascribed to recombination of self-trapped excitons. The inclusion of high amounts of nitrogen into the nanodiamond's crystal lattice, using the example of urotropine (hexamethylenetetramine), is found to quench the luminescence. The results show that diamondoids are promising semiconductor nanocrystals for nanophotonic applications in the ultraviolet spectral region.

Time-resolved photoluminescence of barium titanate ultrafine powders

Time-resolved photoluminescence (PL) near 540nm, taken from barium titanate (BaTiO3) ultrafine powders of ∼80, 90, and 110nm in size, was measured at 11K. Two-exponential functions were found to fit the decay curves well, and the decay lifetimes were composed of one short, about 10ns, and one long, about 200ns, time constants, respectively. Results also indicate that the PL peaks show a small blueshift with decreasing BaTiO3 particle size. The decay process consists of the recombination of self-trapped excitons (long lifetime decay) and of the electrons in the surface states with the holes in the valence band (short lifetime decay). The short lifetime component makes comparable contribution to the decay intensity of PL with the long lifetime one.

Fabrication and Luminescence Properties of Monoclinic Gallium Oxide (β-Ga2O3) Nanostructures

ABSTRACTβ-Ga2O3 nanostructures including nanowires, nanorods, nano and micro-pillars, nanosheets and nanobelts were successfully fabricated by simple and efficient thermal evaporation and condensation technique under argon flow.. The structures have been investigated by the electron microscope, XRD and EDX techniques and shown that nanostructures are predominatly β-Ga2O3 without other crystallographic phases. Cathodouminescence of as-grown nanostructures was investigated in the 10-300 K temperature range and exhibit luminescence band centered at 485 nm at 300 K due to donor-acceptor pair recombination. A new luminescence band centered at 387 nm developes at temperature below 150 K due to self-trapped exciton recombination.

Photoluminescence and photocatalytic properties of SrSnO 3 perovskite

Band structure calculation, UV–visible diffuse reflectance and photoluminescence spectroscopies were employed to investigate photophysical properties of SrSnO 3. The broad luminescence band centered at 425 nm with large Stokes shift was attributed to the recombination of self-trapped excitons. Photocatalytic H 2 and O 2 evolution from water containing sacrificial reagents indicated that SrSnO 3 is highly active. In particular, RuO 2-loaded SrSnO 3 can efficiently split pure water into H 2 and O 2 in a stoichiometric ratio under UV light irradiation. The distorted SnO 6 octahedral connection seems to be favorable for effective migration of electrons or holes. The strong electron–lattice interaction in SrSnO 3 might also be responsible for high photocatalytic activities.