Free Exciton Research Articles

Exciton emissions of CdS nanowire array fabricated on Cd foil by the solvothermal method* *Project supported by the Natural Science Foundation of Henan Province, China (Grant No. 202300410304) and Key Research Project for Science and Technology of the Education Department of Henan Province, China (Grant No. 21A140021).

Nanowires have recently attracted more attention because of their low-dimensional structure, tunable optical and electrical properties for next-generation nanoscale optoelectronic devices. CdS nanowire array, which is (002)-orientation growth and approximately perpendicular to Cd foil substrate, has been fabricated by the solvothermal method. In the temperature-dependent photoluminescence, from short wavelength to long wavelength, four peaks can be ascribed to the emissions from the bandgap, the transition from the holes being bound to the donors or the electrons being bound to the acceptors, the transition from Cd interstitials to Cd vacancies, and the transition from S vacancies to the valence band, respectively. In the photoluminescence of 10 K, the emission originated from the bandgap appears in the form of multiple peaks. Two stronger peaks and five weaker peaks can be observed. The energy differences of the adjacent peaks are close to 38 meV, which is ascribed to the LO phonon energy of CdS. For the multiple peaks of bandgap emission, from low energy to high energy, the first, second, and third peaks are contributed to the third-order, second-order, and first-order phonon replica of the free exciton A, respectively; the fourth peak is originated from the free exciton A; the fifth peak is contributed to the first-order phonon replica of the excitons bound to neutral donors; the sixth and seventh peaks are originated from the excitons bound to neutral donors and the light polarization parallel to the c axis of hexagonal CdS, respectively.

Evaluation of Single-Reference DFT-Based Approaches for the Calculation of Spectroscopic Signatures of Excited States Involved in Singlet Fission

Singlet fission (SF) has the potential to dramatically increase solar cell efficiency by converting one singlet exciton to two free triplet excitons via a correlated triplet pair intermediate. Identification and characterization of excited states involved in SF are of great importance for understanding the fundamentals of SF. Despite their importance, it is still non-trivial to distinguish various species in transient absorption spectra due to their spectral overlaps and ultra-short lifetimes. Theoretical modeling of SF and its electronically excited state absorption (ESA) is generally challenging due to the multi-exciton nature of the correlated triplet pair, which usually requires description by expensive high-level ab initio methods. In this work, taking TIPS-pentacene monomer and its covalently linked dimer as representative examples, we demonstrate the use of single-reference DFT-based approaches to simulate the ESA spectra during SF. In particular, the singlet and triplet ESA are evaluated by TDDFT, QR-TDDFT, SLR-TDDFT, SF-TDDFT, and UTDDFT, in combination with ten different exchange-correlation functionals. The correlated triplet pair and its ESA are characterized by broken-symmetry DFT and TDDFT, and the role of orbital relaxation is highlighted. With a rational choice of exchange-correlation functionals, we found the resulting spectra to show good agreement with transient absorption experiments and certain improvements over high-order CI methods.

Defect-Related Broadband Emission in Two-Dimensional Lead Bromide Perovskite Microsheets.

Low-dimensional hybrid lead halide perovskites (LHPs) with broadband emission (BE) have been developed as promising candidates for single-source white-light-emitting diodes. However, the underlying origin of such BE is poorly understood. Herein, dual-emissive [NH3(CH2)8NH3]PbBr4 perovskite microsheets (PMSs) with good dispersibility are successfully prepared. Besides the general narrowband emission (NE) originating from free excitons, BE (∼522 nm) is generated under a Br-poor condition, which is not observed in the single-crystal sample. Unlike self-trapped exciton emission, the BE observed in PMSs is experimentally determined to be related to bromide vacancies (VBr), thereby exhibiting quasisaturation under high excitation intensity. Femtosecond transient absorption spectroscopy first shows that the trapping time of the photogenerated electrons by acceptor-like VBr- is ∼15 ps, slower than that by surface defects (<1 ps). This study provides new insight into the underlying mechanism of BE and an effective approach to manipulating the optical properties of 2D perovskites.

Biexcitons in highly excited CdSe nanoplatelets

We present the phase diagram of free charges (electrons and holes), excitons, and biexcitons in highly excited CdSe nanoplatelets that predicts a crossover to a biexciton-dominated region at easily attainable low temperatures or high photoexcitation densities. Our findings extend previous work describing only free charges and excitons by introducing biexcitons into the equation of state, while keeping the exciton and biexciton binding energies constant in view of the relatively low density of free charges in this material. Our predictions are experimentally testable in the near future and offer the prospect of creating a quantum degenerate, and possibly even superfluid, biexciton gas. Furthermore, we also provide simple expressions giving analytical insight into the regimes of photoexcitation densities and temperatures in which excitons and biexcitons dominate the response of the nanoplatelets.

Tuning the charge transfer character of the multiexciton state in singlet fission.

Intramolecular singlet fission (SF) produces the multiexciton correlated triplet pair state, (T1T1), prior to the formation of free triplet excitons. The nature of the multiexciton state is complex, as generation of the (T1T1) state may involve a charge transfer (CT) intermediate and has been shown to have both mixed electronic and spin characters. According to transient absorption spectroscopy, a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI2) exhibits solvent-dependent excited-state dynamics. As solvent polarity increases from 1,2,4-trichlorobenzene (ε = 2.2) to chlorobenzene (ε = 5.6) to 1,2-dichlorobenzene (ε = 9.9), the SF rate in TDI2 increases and the multiexciton state, which can be thought of as a linear combination of the 1(S1S0), CT, and (T1T1) states, gains more CT character. Eventually, the CT state becomes a trap state as indicated by symmetry-breaking charge separation in TDI2 in pyridine (ε = 12.3). The dielectric environment influences not only the SF rate and the relative contributions of the 1(S1S0), CT, and (T1T1) states to the overall multiexciton state but also the rate at which the state mixing evolves, with faster dynamics in higher polarity solvents. More importantly, the tunability and presence of strong CT character in the multiexciton state have implications for SF applications since they often rely on electron transfer from the free triplet excitons. This enhanced CT character in the (T1T1) state may assist with two-electron transfer directly from the (T1T1) state, allowing for facile extraction of charges in intramolecular SF systems whose (T1T1) states do not always efficiently dissociate to two triplet excitons.

Solution-State Long-Range Molecular Ordering in Poly(3-hexylthiophene)

A blend of poly(3-hexylthiophene) (P3HT) and poly(n-hexyl isocyanate-block-2-vinylpyridine) (PHIC-b-P2VP) in a common solvent shows the formation of long-range (micrometer-scale) nanowires of P3HT through hydrophobic interactions between the hexyl arms of P3HT and PHIC in a parallel way, which increase the planarity that leads to the generation of vibration bands with a lower free exciton bandwidth (W = 67 meV) in the solution state, which is further decreased to 9 meV after 48 h annealing of the blend film. The resulting nanowires of the P3HT show a 100-fold increase in current in comparison to pristine P3HT.

Hole and Electron Effective Masses in Single InP Nanowires with a Wurtzite-Zincblende Homojunction.

The formation of wurtzite (WZ) phase in III-V nanowires (NWs) such as GaAs and InP is a complication hindering the growth of pure-phase NWs, but it can also be exploited to form NW homostructures consisting of alternate zincblende (ZB) and WZ segments. This leads to different forms of nanostructures, such as crystal-phase superlattices and quantum dots. Here, we investigate the electronic properties of the simplest, yet challenging, of such homostructures: InP NWs with a single homojunction between pure ZB and WZ segments. Polarization-resolved microphotoluminescence (μ-PL) measurements on single NWs provide a tool to gain insights into the interplay between NW geometry and crystal phase. We also exploit this homostructure to simultaneously measure effective masses of charge carriers and excitons in ZB and WZ InP NWs, reliably. Magneto-μ-PL measurements carried out on individual NWs up to 29 T at 77 K allow us to determine the free exciton reduced masses of the ZB and WZ crystal phases, showing the heavier character of the WZ phase, and to deduce the effective mass of electrons in ZB InP NWs (me= 0.080 m0). Finally, we obtain the reduced mass of light-hole excitons in WZ InP by probing the second optically permitted transition Γ7C ↔ Γ7uV with magneto-μ-PL measurements carried out at room temperature. This information is used to extract the experimental light-hole effective mass in WZ InP, which is found to be mlh = 0.26 m0, a value much smaller than the one of the heavy hole mass. Besides being a valuable test for band structure calculations, the knowledge of carrier masses in WZ and ZB InP is important in view of the optimization of the efficiency of solar cells, which is one of the main applications of InP NWs.

Exciton Relaxation Dynamics in Perovskite Cs4PbBr6 Nanocrystals.

Lower-dimensional metal halide perovskites have been recognized as an efficient white light emitter. The broad band emission spectrum originates from the recombination of excited charge carriers through free excitons (FEs), self-trapped excitons (STEs), and defect-trapped excitons. However, the emission properties of zero-dimensional (0-D) perovskites have not been explored extensively. Here, in this work, we have performed low-temperature absorbance, photoluminescence (PL), PL excitation (PLE), PL lifetime, and Raman measurements to understand the exciton relaxation processes in Cs4PbBr6 NCs. Our experimental observations indicate that two distinct UV light spectra evolved from the photoexcited carrier recombination through FE and STE states. We emphasize that such UV light sources can be beneficial for various applications, like curing of materials, disinfection of viruses, hygiene control, etc.

Optical and photocatalytic properties of ZnO and ZnS structures formed as controlled calcination products of l-cysteine assisted aqueous precipitation

ZnO and ZnS structures were obtained by the calcination of the aqueous precipitation products of Zn(NO3)2, NaOH and l-cysteine (Cys). Initial Cys:Zn molar ratios were changed as 0.1:1, 0.5:1, 1:1 and 1.5:1. All the precursors were transformed into ZnO upon calcination at 700 °C. ZnS structures were obtained by calcining the precursors prepared at the Cys:Zn ratios of 1 and 1.5 at 350 °C. In addition to changing chemical composition of the precipitation products, calcination temperature and initial Cys:Zn ratio also affected morphology, surface area, photoluminescence and photocatalytic properties of the final products. Free exciton energy values of the ZnO samples were observed to be between 3.29 eV and 3.35 eV. PL spectra of the ZnO samples indicated blue and green emission centers. Zinc interstitials (Zni), revealed by the blue emissions in the PL spectra were also confirmed by Auger Zn L3M4.5M4.5 spectra. The samples calcined at 350 °C removed rhodamine B mainly by adsorption. All the samples calcined at 700 °C successfully degraded the dye under UV light. Among the samples calcined at 700 °C, ZnO sample prepared at Cys:Zn = 0.5, which has the highest surface area and unique photoluminescence spectrum exhibited the fastest photodegradation rate.

Optical and positron annihilation studies of structural defects in LiInSe2 single crystals

Abstract Lithium-indium di-selenide (LiInSe2) is a semiconductor material, which has been shown promising for applications in nonlinear optics and neutron detection. LiInSe2 crystals of optical quality, of different (from greenish to red) color were grown. Analysis of the fundamental absorption edge shows allowed direct band-to-band transitions and reveals structural disorder leading to the blurring of the edges of valence and conduction bands. Photoluminescence (PL) intensity is low in LiInSe2 of stoichiometric composition and increases after sample annealing in Se vapors. A narrow line at 408 nm is associated with free excitons. Analysis of PL and PL excitation spectra allows one to associate broad emission bands with point defects as well as with self-trapped excitons. The mean positron lifetime increases after annealing in Se vapor as a result of changes of the dominating defect type. For red crystals only big voids with lifetime of about 1021 ps are observed. Both methods suggest that greenish and red coloring of LiInSe2 are due to Se vacancies and interstitial Se atoms, respectively.

Fast and Anomalous Exciton Diffusion in Two-Dimensional Hybrid Perovskites.

Two-dimensional hybrid perovskites are currently in the spotlight of condensed matter and nanotechnology research due to their intriguing optoelectronic and vibrational properties with emerging potential for light-harvesting and light-emitting applications. While it is known that these natural quantum wells host tightly bound excitons, the mobilities of these fundamental optical excitations at the heart of the optoelectronic applications are barely explored. Here, we directly monitor the diffusion of excitons through ultrafast emission microscopy from liquid helium to room temperature in hBN-encapsulated two-dimensional hybrid perovskites. We find very fast diffusion with characteristic hallmarks of free exciton propagation for all temperatures above 50 K. In the cryogenic regime, we observe nonlinear, anomalous behavior with an exceptionally rapid expansion of the exciton cloud followed by a very slow and even negative effective diffusion. We discuss our findings in view of efficient exciton-phonon coupling, highlighting two-dimensional hybrids as promising platforms for basic research and optoelectronic applications.

Impact of excitation energy on the excitonic luminescence of cesium lead bromide perovskite nanosheets.

An excitation-energy-dependent luminescence phenomenon is reported in cesium lead bromide (CsPbBr3) perovskite nanosheets. At 10 K, the relative integrated luminescence intensity between the trapped exciton (TX) emission and the free exciton (FX) emission shows an interesting tendency with increasing the optical excitation energy from 2.431 eV (510 nm) to 3.758 eV (330 nm). To interpret such phenomenon, we develop a quantitative model on the basis of the biological population growth theory. A good agreement between experiment and theory is obtained. It is thus revealed that the lower capture coefficient of the TX level to the excited excitons via multiphonon emission relative to the FX level shall be the major cause of the observed phenomenon. These findings may help to deepen the current understanding of the complex luminescence mechanisms of these emerging light-emitting materials.

Photoluminescence efficiency of Al-rich AlGaN heterostructures in a wide range of photoexcitation densities over temperatures up to 550 K

Time-resolved photoluminescence and light-induced transient grating techniques were applied for temperature and excitation dependent internal quantum efficiency and carrier diffusion investigation in silicon-doped epilayer and AlxGa1-xN MQWs (x > 0.6). Decrease of photoluminescence efficiency at high excitations correlated with increase of diffusion coefficient as well as nonradiative recombination rate, indicating excitation and temperature dependent localized exciton density reduction. Excitation-dependent lifetime decrease was less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency dominated by thermal dissociation of excitons. At lowest excitations excitons were found captured to vacancy complexes. With increase of excitation exciton ionization appeared, leading to enhanced nonradiative recombination of holes on aluminum vacancies and simultaneous droop of PL efficiency due to saturated bimolecular free carrier plasma recombination. At initial recombination stages and high excitations diffusive recombination on dislocations was also revealed. Modelling provided free exciton binding energies of 104, 140 meV in the MQWs and in the layer; exciton localization energies in 12-35 meV range; localized exciton lifetimes in 2-4 ns range; free exciton and carrier radiative recombination rate coefficients of rex = (0.6 0.2)109(T/300)-1 1/s and Brad = (7 1)10-10(T/300)-3/2 cm3/s, respectively. Vacancy complex capture cross section for excitons was determined to be = (2 1)10-16(300/T)2 cm2. Electron and hole capture cross sections to aluminum vacancy were modeled (1.5 1)10-13 cm2 and (7 1)10-13 cm2, respectively. Dislocation Coulomb radius for free carrier recombination was found to be 12-15 nm.

Temperature-dependent photoluminescence and time-resolved photoluminescence study of monolayer molybdenum disulfide

We have investigated the static and dynamic photoluminescence (PL) of monolayer molybdenum disulfide (MoS2) from low-temperature (4 K) to room temperature (300 K). Free excitons (A excitons) and bound excitons (XL excitons) can be seen in the low-temperature PL spectra. The radiative rates and nonradiative rates of the two kinds of excitons as a function of temperature can be calculated through the parameters derived from the PL and time-resolved PL spectra. As the temperature increases, the observed phenomena, including the red-shift of the center wavelength of the PL spectrum, the decrease of the PL intensity, and the shorter of the relaxation time, all can be explained by the temperature-dependence of the band structure of MoS2 and the temperature-dependence of the nonradiation process. The radiation process originates from excitons recombination in monolayer MoS2. The nonradiation process is due to defect trapping and thermal energy generation. The temperature-dependence of the radiation and nonradiation processes significantly affects the optical properties of the monolayer MoS2.

Large enhancement in UV emission and photocatalytic performance of Al-doped Co3O4Nanostructures under visible light

The luminescence and photocatalytic performance of cobalt oxide (Co3O4) nanostructures have been widely discussed. However, the study of ultraviolet (UV) luminescence and visible photocatalytic activity of metal (Al) doped Co3O4 has not been considered till date. In this study, we produce a series of cubic Al-doped Co3O4 nanoparticles (NPs) synthesized by chemical route. The photoluminescence spectrum of pure Co3O4 sample shows UV emission as well as visible emission peaks while on the incorporation of Al3+ ion, only intense UV emission peak around 399 nm (3.10 eV) is observed with quenched DLEs in all doped Co3O4 samples. Though, the intensity of UV emission peak is found high as nine fold of the pure sample in all doped samples which have been affected by the density of free excitons induced by the Al doping. Also, photocatalytic degradation of reactive blue-171 (RB-171) dye was studied under visible light using pure and doped Co3O4 samples as a catalyst which possess extraordinarily enhanced visible photocatalytic performance. Therefore, due to considerable enhancement in UV emission and photocatalytic performance, we suggest that Al-doped Co3O4 NPs play a significant role in the applications of strong UV luminescence devices, electrochromic devices as well as photocatalytic utilizations.

Study of detailed balance between excitons and free carriers in diamond using broadband terahertz time-domain spectroscopy

A fundamental understanding of the photoexcited carrier system in diamond is crucial to facilitate its application in photonic and electronic devices. Here, we report the detailed balance between free carriers and excitons in intrinsic diamond by using a deep-ultraviolet pump in combination with broadband terahertz (THz) probe spectroscopy. We investigated the transformation of photoexcited carriers to excitons by using an internal transition of excitons, which is found to occur at a frequency of 16 THz. We determined the equilibrium constant in the Saha equation from the temperature dependence of the free-carrier density measured at chemical equilibrium. The derived exciton binding energy is larger than the conventional value, which indicates an energy shift due to the fine-structure splitting of the exciton states.

Systematic study of shockley-read-hall and radiative recombination in GaN on Al2O3, freestanding GaN, and GaN on Si

Here we study and correlate structural, electrical, and optical properties of three GaN samples: GaN grown by metalorganic chemical vapor deposition on sapphire (GaN/Al2O3), freestanding GaN crystals grown by the high nitrogen pressure solution method (HNPS GaN), and GaN grown by hydride vapor phase epitaxy on silicon (GaN/Si). Defect and impurity densities and carrier concentrations are quantified by x-ray diffraction, secondary mass ion spectroscopy, and Hall effect studies, respectively. Power-dependent photoluminescence measurements reveal GaN near-band-edge emissions from all samples having mixtures of free exciton and band-to-band transitions. Only the defect luminescence in the GaN/Si sample remains unsaturated, in contrast to those from the HNPS GaN and GaN/Al2O3 samples. Carrier lifetimes, extracted from time-resolved photoluminescence measurements, and internal quantum efficiencies, extracted from temperature-dependent photoluminescence measurements, are used to extract radiative and nonradiative lifetimes. Shockley–Read–Hall (A) and radiative recombination coefficients (B) are then calculated accordingly. Overall, the A coefficient is observed to be highly sensitive to the point defect density rather than dislocation density, as evidenced by three orders of magnitude reduction in threading dislocation density reducing the A coefficient by one order of magnitude only. The B coefficient, while comparable in the higher quality and lowly doped GaN/Al2O3 and HNPS GaN samples, was severely degraded in the GaN/Si sample due to high threading dislocation density and doping concentration.

Scintillation properties of organic–inorganic layered perovskite nanocrystals in glass

We fabricated organic–inorganic perovskite nanocrystals in nanoporous glasses (OIPiG) where (C6H5C2H4NH3)2PbBr4 (Phe) was incorporated into about 4 nm diameter pores and evaluated their photoluminescence and scintillation properties, compared with the Phe single crystal. Both the samples showed emissions due to the recombination of free excitons in the inorganic layer under 280 nm excitation light. In scintillation, a weak and broad scintillation peak possibly due to the free excitons and bound excitons was observed in the OIPiG, while the Phe single crystal showed efficient scintillation due to free exciton emissions with a peak approximately at 435 nm. The scintillation decay time constant (4.4 ns) due to the free excitons for the OIPiG was found to be faster than that (6.6 ns) for the Phe single crystal. In addition, the afterglow levels were confirmed to be 340 ppm for the OIPiG and 20 ppm for the Phe single crystal. The afterglow level of the OIPiG was higher than that of the Phe single crystal but was almost equivalent to that for the commonly used inorganic scintillator CsI:Tl.

Sn–Ga co-doping in sol-gel derived ZnO thin films: Studies of their microstructural, optical, luminescence and electrical properties

Sn–Ga co-doped ZnO films with different proportions synthesized by the sol-gel spin-coating technique have been investigated for their microstructural, optical, luminescence, and electrical properties using a variety of characterization techniques. All the films exhibit c-axis orientation along the (0 0 2) plane having a hexagonal wurtzite structure of zinc oxide. The crystallinity of the films improves upon doping and co-doping. Morphological images reveal that the films possess the granular morphologies with varying features. The co-doped films have a higher root mean square (RMS) roughness in comparison to the undoped and doped ZnO films. In general, relative to the undoped film, the transmittance of the film improves upon doping and co-doping. The direct energy band-gap has a maximum value of 3.26 eV for the undoped ZnO film, which decreases upon doping and co-doping. In the visible region, the refractive index exhibits a minimum value of 2.12 for the 1T3GZO film at the wavelength of 600 nm. The photoluminescence spectra of the films consist of one near-band edge (NBE) emission peak related to the recombination of free excitons, and a few broad emission peaks linked to the native defects in the films. Among the co-doped ZnO films, the 1 at% Sn–Ga co-doped film (1T1GZO) has the optimum transmittance and electrical conductivity.

Bright and fast scintillations of an inorganic halide perovskite CsPbBr3 crystal at cryogenic temperatures

Highly efficient scintillation crystals with short decay times are indispensable for improving the performance of numerous detection and imaging instruments that use- X-rays, gamma-quanta, ionising particles or neutrons. Halide perovskites emerged recently as very promising materials for detection of ionising radiation that motivated further exploration of the materials. In this work, we report on excellent scintillation properties of CsPbBr3 crystals when cooled to cryogenic temperatures. The temperature dependence of luminescence spectra, decay kinetics and light yield under excitation with X-rays and α-particles was investigated. It is shown that the observed changes of spectral and kinetic characteristics of the crystal with temperature can be consistently explained by radiative decay of free excitons, bound and trapped excitons as well as electron-hole pairs originating from their disintegration. It has been found that the crystal exhibits a fast decay time constant of 1 ns at 7 K. The scintillation light yield of CsPbBr3 at 7 K is assessed to be 50,000 ± 10,000 ph/MeV at excitation with 12 keV X-rays and 109,000 ± 22,000 ph/MeV at excitation with α-particles of 241Am. This finding places CsPbBr3 in an excellent position for the development of a new generation of cryogenic, efficient scintillation detectors with nanosecond response time, marking a step-change in opportunities for scintillator-based applications.