Exciton Luminescence Research Articles

Рентгенолюминесценция нанопорошков SrF-=SUB=-2-=/SUB=-: Eu

Concentration series of single-phase SrF2:Eu solid solutions with a particle size of ≈50 nm were synthesized by precipitation from aqueous solutions. The bands of europium in the di- and trivalent states as well as a band of intrinsic exciton luminescence of strontium fluoride were identified in the X-ray luminescence spectra. The most intense luminescence band is the band of trivalent europium with a maximum at 585–590 nm which corresponding to the 5D0 →7F1 transition. The highest X-ray luminescence intensities upon excitation by an X-ray tube with silver and tungsten anodes are achieved for compositions containing about 15.0 mol.% and 7.5 mol.% europium, respectively.

Seven-photon absorption from Na+/Bi3+-alloyed Cs2AgInCl6 perovskites.

Nonlinear multi-phonon (2-7) absorption in the Na+/Bi3+-alloyed Cs2AgInCl6 lead-free double perovskites with ∼100% photoluminescence quantum yield and superior stability is observed for the first time, which can be pumped by a femtosecond laser in a wide spectral range (800-2600 nm). First-principles calculations verify that the parity-forbidden transition from the valence band maximum and conduction band minimum (at the Γ point) is not broken by Na+/Bi3+ doping, and strong optical band-to-band absorption occurs at the L&X points. Time-resolved emission spectra evidence that single-photon and multi-photon pumping leads to the same self-trapped exciton transition and high-order nonlinear absorption will not induce a remarkable thermal effect. Finally, we demonstrate that the Cs2Na0.4Ag0.6In0.99Bi0.01Cl6 DP shows great potential for next-generation wavelength-selective and highly sensitive multiphoton imaging applications.

FA+ and Mn2+ codoped CsPbCl3 perovskite quantum dots with super thermal stability

As a new generation of luminous material, perovskite quantum dots (PQDs) possess many excellent properties. However, PQDs suffer from dramatic decrease of optical performance when affected by high temperature. In this paper, methamidine acetate (FA + ) and manganese divalent cation (Mn 2+ ) codoped CsPbCl 3 quantum dots (QDs) were synthesized. The introduced FA + and Mn 2+ were designed to substitute Cs + (A sites) and Pb 2+ (B sites), respectively. By comparison with CsPbCl 3 :Mn 2+ QDs, the results show that the CsPbCl 3 :FA + ,Mn 2+ QDs improve the blue exciton luminescence by 15% and the red Mn 2+ ions luminescence by 37%. Furthermore, CsPbCl 3 :FA + ,Mn 2+ QDs demonstrate super thermal stability by maintaining 97% of Mn 2+ emission when the temperature is raised to 70 °C even after 6 thermal cycles. Simultaneously, the UV light and water stability of CsPbCl 3 :FA + ,Mn 2+ QDs have also been obviously enhanced. The powder of the coped PQDs have been fabricated into a white LED which exhibits Color Rendering Index (CRI) of 88.2 and Correlated Color Temperature (CCT) of 5745 K. It shows that CsPbCl 3 :FA + ,Mn 2+ QDs not only reduce the Pb 2+ ions toxicity problem but also boost the thermal stability and thermal cyclability, which may open up more possibilities for the applications in optoelectronic devices.

Effect of Intrinsic Defects on the Photoluminescence of Pristine and Doped CsPbBr3 Perovskite

We report the experimental results demonstrating the effect of either photoinduced or doping-induced intrinsic defects on excitonic photoluminescence in both pristine and doped CsPbBr3 halide perovskite. It is shown that an increase in the number of intrinsic defects leads to stronger photoluminescence quenching, which is confirmed by the direct negative correlations between the absorption of the intrinsic defects and excitonic luminescence intensity. The proposed mechanism indicates that the excitonic luminescence quenching originates from the exciton decay at the intrinsic defect sites. The efficiency of the exciton decay depends on the type and charge of the cations located near the defect sites and increases in the order Ag+ < Pb2+ < Bi3+.

In English

In review, deals with the theory of exciton quasimolecules (formed of spatially separated electrons and holes) in a nanosystems that consists of semiconductor and dielectric colloidal quantum dots (QDs) synthesized in a dielectric and semiconductor matrixs. It has been shown that the exciton quasimolecule formation is of the threshold character and possible in a nanosystem, where the distance D between the surfaces of QD is given by the condition (where and are some critical distance). We have shown that in such a nanoheterostructures acting as “exciton molecules” are the QDs with excitons localizing over their surfaces. The position of the quasimolecule state energy band depends both on the mean radius of the QDs, and the distance between their surfaces, which enables one to purposefully control it by varying these parameters of the nanostructure. It was found that the binding energy of singlet ground state of exciton quasimolecules, consisting of two semiconductor and dielectric QDs is a significant large values, larger than the binding energy of the biexciton in a semiconductor and dielectric single crystals almost two orders of magnitude. It is shown that the major contribution to tue binding energy of singlet ground state of exciton quasimolecule is made by the energy of the exchange interaction of electrons with holes and this contribution is much more substantial than the contribution of the energy of the Coulomb interaction between the electrons and holes. It is established that the position of the exciton quasimolecule energy band depends both on the mean radius of the QDs and the distance between their surfaces. It is shown that with increase in temperature above the threshold (), a transition can occur from the exciton quasimolecule to exciton state. It has been found that at a constant concentration of excitons (i.e. constant concentration of QD) and temperatures Т below , one can expect a new luminescence band shifted from the exciton band by the value of the exciton quasimolecule binding energy. This new band disappears at higher temperatures (). At a constant temperature below , an increase in exciton concentration (i.e. in QD concentration) brings about weakening of the exciton luminescence band and strengthening of the exciton quasimolecule. These exciton quasimolecules are of fundamental interest as new quasi-atomic colloidal nanostructures; they may also have practical value as new nanomaterials for nanooptoelectronics. The fact that the energy of the ground state singlet exciton quasimolecule is in the infrared range of the spectrum, presumably, allow the use of a quasimolecule to create new infrared sensors in biomedical research.

A theoretical study on the effects of intramolecular and intermolecular interactions on excited state properties of two NIR-TADF combined with AIE molecules

TADF emitters with both high exciton utilization and luminescence efficiency show promising applications in OLEDs, especially it with AIE characteristics. However, theoretical studies on emitters with amorphous structure with the consideration of the solid-state effect are limited. In this work, a multi-scale simulation is performed to study the reported compounds TBSMCN and O-TBSMCN, which both possess distinct features of NIR-AIE-TADF. The packing modes of the molecules in film and crystal are obtained by molecular dynamics simulations (MD) and experiments, and then the photo physical properties are studied by using the QM/MM method. The results show that geometrical changes between the ground state (S0) and the first singlet excited state (S1) are restricted in aggregate state. Thus, the non-radiative energy consumption process is hindered, this indicates the AIE mechanisms. Second, the molecules exhibit a smaller △ES1-T1 and a more significant SOC in aggregate state, which is more favorable for the RISC. Finally, by analyzing the NTO of the dimers in the film, the O-TBSMCN molecule undergoes intermolecular charge transfer, which may be one of the reasons for the significant reduction in its film efficiency. Our calculations could help one to better understand the NIR-AIE-TADF mechanism from a theoretical perspective.

Decoupling excitons behavior of two-dimensional Ruddlesden-Popper PEA2PbI4 nanosheets

Two-dimensional (2D) Ruddlesden-Popper hybrid organic-inorganic perovskites are the most promising candidates for highly efficient optoelectronic devices due to their remarkable exciton characteristics, adjustable band structure and long-term environmental stability. However, excitons emission behavior of 2D Ruddlesden-Popper perovskites is still unclear. Here we theoretically-experimentally decoupled the excitons behavior in 2D Ruddlesden-Popper PEA2PbI4 nanosheets. Theoretically, the Elliott theory was used to separate exciton and band edge absorptions of PEA2PbI4 nanosheets and determine an exciton binding energy of about 274 meV. Experimentally, spectral-dependent photoluminescence lifetime and excitation powder density-dependent photoluminescence spectra revealed that a distinct photoluminescence tail emission should be derived from localized excitons with lower emission energy than free excitons. Further, the luminescent component of the free excitons and localized excitons with their corresponding densities of states was theoretically decoupled by line-shape analysis of PEA2PbI4 nanosheets photoluminescence spectra, which is consistent with the experimental results demonstrating the existence of localized excitons. Hence, our work provides understanding on the exciton behavior and luminescence mechanism of 2D Ruddlesden-Popper perovskites, which would benefit the prediction of high-performance excitonic devices based on 2D Ruddlesden-Popper perovskites.

Peculiarities of erbium incorporation into ZnO microrods at high doping level leading to upconversion and the morphology change. Influence on excitonic as well as shallow donor states

Heavily Er-doped zinc oxide (ZnO) microrods with nominal compositions, ZnO:Er(2, 10, 30%), were prepared by the hydrothermal growth method. The crystallographic phases present in the as grown and annealed materialswere determined. The presence of the hexagonal Wurtzite ZnO phase in the form of the hexagonal shape nanoplatelets and microrods was confirmed. Moreover, the Er2O3phase in the form of nanosheets has been revealed as well. Erbium is partly incorporated into the ZnO hosts. It was thinning upon erbium doping level contributing to erbium oxide. This effect was even more pronounced upon annealing in air. Moreover, the influence of erbium doping and annealing on paramagnetic shallow donor centers has been studied. There were two types of these centers, both affected by the erbium presence. The increase of the neutral zinc vacancies amount upon erbium doping has been proven. Annealing in air moderated the charge distribution between them. Moreover, upconversion processes between the two erbium ions were observed and explained. The surface distribution of excitonic emission centers was studied. The larger the erbium content the weaker the exciton luminescence. The smaller the ZnO microrods the stronger the exciton emission.

Relation between Ga Vacancies, Photoluminescence, and Growth Conditions of MOVPE-Prepared GaN Layers.

A set of GaN layers prepared by metalorganic vapor phase epitaxy under different technological conditions (growth temperature carrier gas type and Ga precursor) were investigated using variable energy positron annihilation spectroscopy (VEPAS) to find a link between technological conditions, GaN layer properties, and the concentration of gallium vacancies (VGa). Different correlations between technological parameters and VGa concentration were observed for layers grown from triethyl gallium (TEGa) and trimethyl gallium (TMGa) precursors. In case of TEGa, the formation of VGa was significantly influenced by the type of reactor atmosphere (N2 or H2), while no similar behaviour was observed for growth from TMGa. VGa formation was suppressed with increasing temperature for growth from TEGa. On the contrary, enhancement of VGa concentration was observed for growth from TMGa, with cluster formation for the highest temperature of 1100 °C. From the correlation of photoluminescence results with VGa concentration determined by VEPAS, it can be concluded that yellow band luminescence in GaN is likely not connected with VGa; additionally, increased VGa concentration enhances excitonic luminescence. The probable explanation is that VGa prevent the formation of some other highly efficient nonradiative defects. Possible types of such defects are suggested.

Effect of In-substitution on luminescence properties of organic-inorganic lead bromide layered perovskite-type scintillators

(C6H5C2H4NH3)2PbBr4 single crystals substituted with various concentrations of In (5, 10, 25, and 50%) were grown by the solvent diffusion method, and the effect of In-substitution on photoluminescence (PL) and scintillation properties were investigated. PL and scintillation peaks due to excitons confined in inorganic layers were observed, and the decay times were consistent with those of the free excitonic luminescence. The photoluminescence quantum yields of the 5, 10, 25, and 50% In-substituted crystals were respectively 23.0, 24.7, 26.0, and 25.2%. The light yield of the 25% In-substituted crystal was estimated to be 14100 photons/MeV from the pulse height spectrum under 137Cs γ-rays irradiation.

Exciton Luminescence and Optical Properties of Nanocrystalline Cubic Y2O3 Films Prepared by Reactive Magnetron Sputtering.

This paper presents a comprehensive study of the energy structure, optical characteristics, and spectral-kinetic parameters of elementary excitations in a high-purity nanocrystalline cubic Y2O3 film synthesized by reactive magnetron sputtering. The optical transparency gaps for direct and indirect interband transitions were determined and discussed. The dispersion of the refractive index was established based on the analysis of interference effects. It was found that the refractive index of the Y2O3 film synthesized in this study is higher in order of magnitude than that of the films obtained with the help of other technologies. The intrinsic emission of Y2O3 film has been discussed and associated with the triplet–singlet radiative relaxation of self-trapped and bound excitons. We also studied the temperature behavior of the exciton luminescence of Y2O3 for the first time and determined thermal activation barriers. The optical energy and kinetic parameters of cubic Y2O3 films were analyzed in comparison with those of the monoclinic films of yttrium oxide. The main difference between the optical properties of cubic and monoclinic Y2O3 films was established, which allowed for a supposition of their application prospects.

Role of Octahedron Alloying in Photodynamics of Lead‐Free Halide Double Perovskite Nanoplatelets

AbstractThe lead‐free halide double perovskites have attracted great interest owing to their unique photophysical properties. Among them, the luminescence mechanisms of the Cs2AgInCl6 crystallites are still under debate. A hot‐injection method is developed to synthesize novel hollow and spatially symmetric Cs2AgInCl6 nanoplatelets (NPLs) and study the effect of the cation/octahedron alloying on their photodynamics by using the experimental characterizations in conjunction with the density functional theory calculations. The results reveal that the pure Cs2AgInCl6 NPLs exhibit wide‐band and double‐peaked photoluminescence originating separately from free and self‐trapped excitons. The Ag+/Na+ and In3+/Bi3+ alloying, that is, the partial substitution of the [AgCl6]5− and [InCl6]3− octahedra by [NaCl6]5− and [BiCl6]3− octahedra, leads to crystal symmetry breaking and strongly enhanced exciton localization. As a result, the self‐trapped exciton luminescence becomes predominant in the yielded Cs2AgInCl6:NaBi nanoplatelets, and the quantum yield is highly improved to 10.8%. These results improve the understanding of the role of octahedron alloying in the photophysics of the double perovskite nanocrystals and pave the way for their applications in solid‐state lighting.

Strong modulation limit of excitons and trions in moiré materials

The optical properties of weakly-doped two-dimensional materials are dominated by strong exciton and trion absorption and luminescence features. In this article we examine the influence of moir\'e patterns in semiconductor heterobilayers on exciton and trion states in the limit of strong moir\'e modulation potentials, commenting on similarities and differences compared to the case of excitons and trions in semiconductor quantum dots. We discuss strategies for using optical properties as quantitative probes of moir\'e materials, and the prospects for exploiting moir\'e materials to design unique light emitters.

(Invited) High Temperature Light Emission Spectra of Metallic Carbon Nanotubes Under Different Heating Methods

Semiconducting single-walled carbon nanotubes (SWCNTs) show exceptional optical properties such as excitonic thermal radiation under thermal equilibrium [1,2] as well as exciton luminescence under nonequilibrium conditions, which potentially provides various promising applications including photonic and thermo-optic devices. With regard to the counterpart, metallic SWCNTs, high-temperature light emission induced by Joule heating has been reported [3,4], but the effects of nonequilibrium induced by the current injection on the light emission remain to be clarified. Here, we report comparison of high-temperature light emission from an identical individual metallic SWCNT under laser- and Joule-heating conditions. The spectral shapes were qualitatively different depending on the heating methods. We will discuss these high-temperature light emission phenomena quantitatively in terms of the electron and phonon distribution. [1] T. Nishihara, A. Takakura, Y. Miyauchi, and K. Itami, Nat. Commun. 9, 3144 (2018). [2] S. Konabe, T. Nishihara, and Y. Miyauchi, Opt. Lett. 46, 3021 (2021). [3] D. Mann et al., Nat. Nanotechnol. 2, 33 (2007), [4] Z. Liu et al., ACS Nano 5, 4634 (2011).

Dopant compensation in p-type doped MAPb1−xCuxI3 alloyed perovskite crystals

Tuning the optical and electrical properties of semiconductors by designed doping is the basis of most energy-related semiconductor optoelectronic devices. In this Letter, we report the dopant compensation effect of P-type doped MAPb1−x CuxI3 alloyed perovskite crystals. MAPb1−xCuxI3 single crystals were prepared by the inverse temperature crystallization method using cupric chloride (CuCl2) as the doping source. By XRD, XPS, STEM, and photoluminescence (PL) spectra analyses, we demonstrate that the doped cupric (Cu2+) ions can partially substitute lead (Pb2+) ions and form Cu–Pb based crystal semiconductor alloys of MAPb1−xCuxI3 with tunable bandgap by controlling the Pb/Cu ratio. More detailed XPS analysis of the doped crystal shows that the Cu2+ ions in MAPb1−xCuxI3 are partially reduced by I− ions, and the coexistence of two valence states of Cu species (Cu2+ and Cu+) was observed in the doped crystals. Hall results of MAPb1−xCuxI3 semiconductors show that the presence of reduced Cu+ ions impels the change of conductive type from weak N-type to P-type obviously, while the resistivity of doped MAPb1−xCuxI3 increases significantly from 104 to 107 Ω cm. The defect-related optical fingerprints of cupric doped crystals were investigated in detail by temperature-dependent PL spectroscopy. The pristine MAPbI3 perovskite crystal exhibits intrinsic donor bound exciton (D0X) luminescence at low temperature (10 K), while the doped MAPb1−xCuxI3 perovskites exhibit donor-acceptor or bound exciton (A0X) peaks related to a Cu+ dopant in sequence with the increase in the Cu ion content. These results indicate that the doping of Cu2+/+ ions into the MAPb1−xCuxI3 crystal not only changes the semiconductor bandgap but also causes the dopant compensation.

Temperature-dependent luminescence of intrinsic defects and excitons in nanocrystalline monoclinic Y2O3 films

This work presents a comprehensive study of the optical properties of Y2O3 film with a monoclinic structure obtained by magnetron sputtering on a silica glass substrate. The parameters of band energy structure and emission characteristics of the film were determined by absorption, transmission, and photoluminescence spectroscopy. The dispersion of refractive index was established and the values of transparency gaps for direct and indirect interband transitions were determined. Optically active intrinsic centers were identified as vacancy (F - centers) and interstitial type (Oi - centers) defects, which are characterized by distribution of radiative states over activation energy of emission quenching. Luminescence in the UV and visible spectral ranges is interpreted as triplet-singlet radiative transitions in self-trapped and bound excitons. The values of thermally activated barriers for flare up and quenching of exciton luminescence are determined, and the configuration-coordinate diagram is proposed.

Systematic synthesis of different-sized AgInS2/GaS x nanocrystals for emitting the strong and narrow excitonic luminescence

This paper presents for the first time the systematic synthesis of AgInS2 (AIS) nanocrystals (NCs) with different sizes of 2.6–6.8 nm just by controlling only the reaction temperature. The synthesis of AIS core NCs was carried out in 2 steps: (i) synthesis of Ag2S NCs and then (ii) partial exchange of Ag+ with In3+ in the template Ag2S NCs. For step (i), Ag2S NCs of different sizes were synthesized by reaction of the Ag and S precursors at different temperatures of 30 °C to 130 °C, for the same reaction time of 30 min. For step (ii), AIS NCs were created by the exchange of Ag+ with In3+ at 120 °C for 60 min. Finally, GaS x was shelled on AIS core NCs to produce the AgInS2/GaS x core/shell structures. The synthesized AIS/GaS x NCs demonstrate the clear excitonic absorptions and strong, narrow excitonic luminescence peaking at 530–606 nm depending on the size of AIS core NCs.

Rare‐Earth‐Based Perovskite Cs2AgScCl6:Bi for Strong Full Visible Spectrum Emission

AbstractMost luminescent materials can only emit light with a particular wavelength. In general, various phosphors with complementary spectral ranges are mixed to realize full‐spectrum lighting. Therefore, the integration of luminescence at different wavebands in the single component matrix to obtain white emission is the optimal strategy for designing lighting materials. Thus, Sc‐based double perovskite Cs2AgScCl6:xBi with adjustable free exciton and self‐trapped exciton luminescence is synthesized by the vacuum evaporation assisted synthetic method. This perovskite can be used as white phosphor to assemble a high‐quality white lighting device. Density functional theory calculations have revealed that the introduction of Bi optimizes the electron transfer and suppresses the defects in Cs2AgScCl6, which leads to excellent luminescent performance in a broad wavelength range. This work provides a new concept and strategy for the design and preparation of rare‐earth optical perovskites in the future.

Observation of in-plane exciton–polaritons in monolayer WSe2 driven by plasmonic nanofingers

Abstract The transition metal dichalcogenides (TMDs) have drawn great research attention, motivated by the derived remarkable optoelectronic properties and the potentials for high-efficient excitonic devices. The plasmonic nanocavity, integrating deep-sub wavelength confinement of optical mode with dramatic localized field enhancement, provides a practical platform to manipulate light–matter interaction. In order to obtain strong exciton–plasmon coupling effects, it’s crucial to match the vibration direction of exciton to the available strong localized in-plane electric field. Herein, we demonstrate the coupling effect of in-plane exciton in monolayer tungsten diselenide (WSe2) to deterministic gap-plasmon field which is produced by nanometrically gapped collapsed nanofingers. The gap-plasmon field which is completely parallel to the in-plane excitons in WSe2 will drive a strong exciton–plasmon coupling at room temperature. More interestingly, it is experimentally observed that the luminescence of exciton–polariton cannot be influenced by the temperature in the range from 77 K to 300 K due to the presence of nanofingers. According to the theoretical analysis results, we attribute this finding to the dielectric screening effect arising from the extremely strong localized electric field of plasmonic nanofingers. This work proposes a feasible way to harness and manipulate the exciton of low-dimensional semiconductor, which might be potential for quantum optoelectronics.

Resonant defect recombination-localized surface plasmon energy transfer and exciton dominated fluorescence in ZnO-Au-ZnO multi-interfaced heteronanocrystals.

The semiconductor-metal heteronanocrystals (HNCs) that possess a perfect epitaxial interface can accommodate novel and interesting physical phenomena owing to the strong interaction and coupling between the semiconductor excitons and metal plasmons at the interface. Here, we fabricate the pyramidal ZnO-Au HNCs and study their unique photophysical properties. Several Au nanospheres are perfectly epitaxially bound with a single ZnO NC owing to the small lattice mismatch between them and there are also ZnO-Au-ZnO sandwiched HNCs. There is a strong coupling between the green defect-associated recombination in the ZnO NC and the localized surface plasmon resonance (LSPR) of the Au nanosphere at the interface of the HNC. This leads to resonant defect recombination-LSPR energy transfer and resultant nearly complete quenching of the green defect luminescence of the ZnO NCs in the HNCs, leaving only the UV exciton luminescence. The lifetimes of both the green and UV emission bands decrease significantly in the ZnO-Au HNCs relative to that of the pure ZnO NCs owing to the combined effect of resonance energy transfer and surface plasmon enhanced radiative transition. The exponent of the luminescence intensity-excitation intensity power function for the green emission band is remarkably smaller than unity, and this suggests that the involved defects have an intermediate concentration.