Exciton Radiation Research Articles

Spin radiation of electrons, excitons, and phonons

Spin radiation of electrons, excitons, and phonons

Hydrogen Bond‐Induced Flexible and Twisted Self‐Assembly of Functionalized Carbon Dots with Customized‐Color Circularly Polarized Luminescence

AbstractCircularly polarized luminescence (CPL) has numerous applications in optical data storage, quantum computing, bioresponsive imaging, liquid crystal displays, and backlights in three‐dimensional (3D) displays. In addition to their competitive optical properties, carbon dots (CDs) benefit from simple and low‐cost preparation, facile post‐modification, and excellent resistance to photo‐ and chemical bleaching after carbonization. Combining the superior optical performance with polarization peculiarities through hierarchical structure engineering is imperative for the development of CDs. In this study, hydrophobic interactions of aromatic ligands, which participate in the surface‐ligand post‐modification process on the ground‐state chiral carbon core, are employed to drive the oriented assembly. Furthermore, the residual chiral amides on CDs form multiple hydrogen bonds during gradual aggregation, causing the assembled materials to form an asymmetric bending structure. Superficial ligands interfere with the optical dynamics of the exciton radiation transition and stabilize the excited state of the assembled materials to achieve a circularly polarized signal. The linkage ligands overcome the frequent aggregation‐induced quenching phenomenon that present difficulties in conventional CDs, facilitate the assembly of self‐supporting films, and improve chiral optical expression. The full‐color and white CPL are manipulated by simply adjusting the functional groups of the ligands, which also illustrates the versatility of the post‐modification strategy. Finally, large chiral flexible films and multicolor chiral light‐emitting diodes based on the stable chiral powder phosphors were constructed, thereby providing feasible materials and technical support for flexible 3D displays.

Recent Advances in Electroluminescent Metallic Nanoclusters: From Materials to Devices.

Metallic nanoclusters (MNCs) were developed rapidly in recent decades, owing to their unique electronic structures and excited state characteristics, leading to their wide applications. Luminescence as one of the most important functions for MNCs has also been used to realize biodetection, displays, and lighting, through either electrochemiluminescence (ECL) or electroluminescence (EL). Both emissive properties and electrochemical activities of MNCs were utilized to enhance ECL and EL through facilitating exciton formation and radiation, rendering the rapid emerging of the latter in the last ten years. Through ligand modification, radiative excited-state components were increased to realize state-of-the-art photo- and electroluminescence efficiencies up to ∼100% and ∼30%, as well as ultralow biodetection limits. Nonetheless, material selection space and processing technologies are still limited. Herein, we overview and discuss recent advances of MNCs-based ECL and EL, through both aspects of materials/systems and devices, which would enlighten continuous innovations in optoelectronic MNCs.

Temperature-Dependent Excitonic Light Manipulation with Atomically Thin Optical Elements.

Monolayer 2D semiconductors, such as WS2, exhibit uniquely strong light-matter interactions due to exciton resonances that enable atomically thin optical elements. Similar to geometry-dependent plasmon and Mie resonances, these intrinsic material resonances offer coherent and tunable light scattering. Thus far, the impact of the excitons' temporal dynamics on the performance of such excitonic metasurfaces remains unexplored. Here, we show how the excitonic decay rates dictate the focusing efficiency of an atomically thin lens carved directly out of exfoliated monolayer WS2. By isolating the coherent exciton radiation from the incoherent background in the focus of the lens, we obtain a direct measure of the role of exciton radiation in wavefront shaping. Furthermore, we investigate the influence of exciton-phonon scattering by characterizing the focusing efficiency as a function of temperature, demonstrating an increased optical efficiency at cryogenic temperatures. Our results provide valuable insights into the role of excitonic light scattering in 2D nanophotonic devices.

Sustained robust exciton emission in suspended monolayer WSe2 within the low carrier density regime for quantum emitter applications

The development of semiconductor optoelectronic devices is moving toward low power consumption and miniaturization, especially for high-efficiency quantum emitters. However, most of these quantum sources work at low carrier density regions, where the Shockley–Read–Hall (SRH) recombination may be dominant and seriously reduce the emission efficiency. In order to reduce the effect of carrier trapping and sustain a strong photoluminescence (PL) emission under low power pumping conditions, we investigated the influence of “suspending” a monolayer of tungsten diselenide (WSe2), a novel two-dimensional quantum material. Not only the PL intensity but also the fundamental photoluminescence quantum yield (PLQY) has exhibited a huge, order-scale enhancement through suspending; even surprisingly, we found the PLQY improvement to be far significant under small pumping powers and observed an exponential increase in tendency toward an even lower carrier density region. With its strong excitonic effect, suspended WSe2 offers a solution to reduce carrier trapping and participate in non-radiative processes. Moreover, in the low-power range, where SRH recombination dominates, suspended WSe2 exhibited a remarkably higher percentage of excitonic radiation compared to contacted WSe2. Herein, we quantitatively demonstrate the significance of the suspended WSe2 monolayer in a low carrier density region, highlighting its potential for developing compact, low-power quantum emitters in the future.

Photoelectric and structural properties of super stable perovskite quantum dots glasses for contactless underwater electroluminescent LEDs

The photoelectric field has witnessed notable progress with the emergence of perovskite quantum dots. However, their practical utility is constrained by their susceptibility to degradation and malfunction when exposed to external environmental factors like light, heat, and humidity. To overcome these limitations, a potential solution lies in the growth of perovskite quantum dots within glasses, leveraging the dense network structure of inorganic glasses to achieve a seamless encapsulation of the quantum dots. This approach effectively addresses the challenges of quantum dot stability and the risk of lead contamination by isolating the quantum dots from the external environment. This study utilizes an in situ nanocrystallization approach to cultivate CsPbBr3 perovskite quantum dots within a matrix of glasses. The resulting CsPbBr3 perovskite quantum dots glasses (CsPbBr3 PQDs@glasses) demonstrate a vibrant green emission due to exciton recombination radiation. The investigation focused on the luminescent characteristics of CsPbBr3 PQDs@glasses subjected to pressures ranging from 0 to 7 GPa, which resulted in a significant red shift in the spectra, transitioning from 519 nm to 525 nm. Electronic structure, dispersion and vibrational properties of CsPbBr3 was investigated under varying pressure conditions through the utilization of first-principles methods. Furthermore, an examination of the pressure distribution within the diamond anvil cells was conducted from a macroscopic perspective employing the finite element method. Subsequently, contactless underwater electroluminescence devices incorporating CsPbBr3 PQDs@glasses were devised, employing the principle of electromagnetic induction. This study offers essential insights into the physics of perovskite materials subjected to high pressure, thereby establishing a groundwork for prospective applications.

(Invited) Coupling of Plasmonic Metal Nanostructures with Organic Molecules for Excitonic Radiation and Non-Radiation Processes

When plasmonic metal nanostructures are coupled with organic molecules, plasmon can transfer energy from the metal to the organic molecules, leading to exciton generation. In addition, plasmonic hot electrons can transfer from the metal to the adsorbate molecules, leading to chemical interface damping. Based on the above two transfer processes, photoluminescence, surface-enhanced Raman scattering and photocurrent of organic molecules can be enhanced by coupling of organic molecules with plasmonic metal nanostructure, which has implication in photodetection, sensing and solar energy conversion.

Exciton binding energy and radiation lifetime in CdTe/Cd1−xZnxS strained core/shell spherical quantum dots under pressure

In this paper, the exciton binding energy and radiation lifetime in type I and II structures of CdTe/Cd[Formula: see text]ZnxS strained core/shell spherical quantum dots under the hydrostatic pressure have been studied by using the variational method within the continuous dielectric model and effective mass approximation. The results show that for these two structures, the exciton binding energies with and without strain decrease with the increasing core radius but increase with the increasing pressure. The exciton binding energy with strain is smaller than that without strain. For type I structure, the effect of strain is small, and the radiation lifetime decreases monotonically with the increasing pressure. By contrast, for type II structure, the effects of shell radius and strain on the exciton binding energy are obvious, and the radiation lifetime increases first and then decreases with the increasing pressure.

<i>In situ</i> study of light emission from SiO<sub>2</sub> irradiated by 645 MeV Xe<sup>35+</sup> ions

Silicon dioxide (SiO<sub>2</sub>) is an important component of nuclear reactor optical fiber and is also a candidate material for wast solidification. Owing to its special physical and chemical characteristics, it is used in many different technology fields like optics, electronics, energy orspace. Swift heavy ion irradiation can modify the crystal structure and optical property of optical material SiO<sub>2</sub>. Swift heavy ions deposit their energy mainly by inelastic interaction. Highly ionized lattice atoms may be formed along the trajectory, and a fraction of their electrical energy can be converted directly into the kinetic energy of the ions. The irradiation experiment is performed with Xe<sup>q+</sup> ions at the irradiation terminal of the sector-focused cyclotron at heavy-ion research facility in Lanzhou (HIRFL). The on-line spectral measurement experiment is carried out during irradiation. In the darkroom, the UV-visible light emission from the target is focused into optical fiber by a collimating lens, and then is analyzed with the Sp-2558 spectrometer equipped with a 1200 g/mm optical grating blazed at 500 nm. In the present work, SiO<sub>2</sub> single crystals are irradiated with 93–609 MeV Xe<sup>q+</sup> ions with a dose in a range of 1×10<sup>11</sup>–3×10<sup>11</sup> ions/cm<sup>2</sup>. During irradiation, the emission spectra, in a range of 200–800 nm, from SiO<sub>2</sub> irradiated by 93, 245, 425 and 609 MeV Xe<sup>q+</sup> ions, are obtained. Two emission bands centered at 461 and 631 nm are observed. These emission bands are produced by Frenkel exciton radiation de-excitation and their intensities are closely related to the irradiated ion energy and radiation dose. The results show that the light intensity increases with the electron energy loss index increasing. And owing to crystal damage caused by ion irradiation, the intensity of emission spectrum decreases with the augment of irradiation dose. Ion loses its energy throughout the ion track via Sn and Se interacting with target atoms and electrons respectively, and the energy lost by the ion is estimated by using SRIM code. The SRIM simulated ion ranges and recoil atom distribution, target ionization (energy loss to target electrons), damage production in SiO<sub>2</sub> are presented. Based on the energy deposition process, the emission bands related to the crystal structure itself are discussed. It indicates that electron energy loss plays a leading role in the process of light emission. In-situ measurement of the optical emission is of great significance in studying the irradiation modification and can help to understand the process of crystal damage caused by ion irradiation.

High efficiency pure blue perovskite quantum dot light-emitting diodes based on formamidinium manipulating carrier dynamics and electron state filling

Achieving high efficiency and stable pure blue colloidal perovskite quantum dot (QD) light-emitting diodes (LEDs) is still an enormous challenge because blue emitters typically exhibit high defect density, low photoluminescence quantum yield (PLQY) and easy phase dissociation. Herein, an organic cation composition modification strategy is used to synthesize high-performance pure blue perovskite quantum dots at room temperature. The synthesized FA-CsPb(Cl0.5Br0.5)3 QDs show a bright photoluminescence with a high PLQY (65%), which is 6 times higher than the undoped samples. In addition, the photophysical properties of the FA cation doping was deeply illustrated through carrier dynamics and first principal calculation, which show lower defects, longer lifetime, and more reasonable band gap structure than undoped emitters. Consequently, pure blue FA-CsPb(Cl0.5Br0.5)3 QDs light-emitting devices were fabricated and presented a maximum luminance of 1452 cd m−2, and an external quantum efficiency of 5.01 % with an emission at 474 nm. The excellent photoelectric properties mainly originate from the enhanced blue QDs emitter and effective charge injection and exciton radiation. Our finding underscores this easy and feasible room temperature doping approach as an alternative strategy to blue perovskite QD LED development.

Energy and spin relaxation of excitons in CdMnTe/CdMgTe quantum wells

The energy and spin relaxation of excitons and trions is studied in the (Cd, Mn)Te/(Cd,Mg)Te quantum wells containing hole magnetic polarons by means of polarized photoluminescence technique. It was found that under circularly polarized photoexcitation with an energy higher than the ground state energy of the exciton, photoluminescence is also polarized. This fact points to the presence of the spin memory of the excitons in these systems. It is suggested that the long spin memory is due to the magnetic polaron formation stabilizing the spin of the exciton. Under quasiresonant excitation of the trion states an inverted sign of circular polarization degree is observed in the radiation spectrum. Such inversion of the polarization degree requires the special mechanism of the spin relaxation, associated with the spin flip of an electron in the trion magnetic polaron state. Application of the external magnetic field in the Voigt geometry leads to the depolarization of the excitonic radiation. The cause of this depolarization is the destabilization of the magnetic polaron and the shortening of the spin relaxation time of the exciton.

Modulation of Valley Dynamics in Hybrid H/T Phase Monolayer WSe2

AbstractTwo unequal K and K' valleys in transition metal dichalcogenides (TMDC) enable large and controllable polarization, which is the cornerstone of emerging valleytronic applications. Here, a phase engineering strategy aided by resonant plasmonic coupling is proposed to manipulate the valley degree of freedom. Compared with the pristine WSe2 monolayer, the hybrid H/T phase WSe2 exhibits an enhanced degrees of circular polarization (DCP) and valley polarization (DVP). As further aided by the designed Au plasmonic array, the T phase facilitates the excitons process and promotes the charge transfer in WSe2/Au interface under the plasmonic‐enhanced electromagnetic field. Consequently, both the DCP and DVP values are considerably enhanced to 38.5% (15.6%) and 15.1% (7.6%) at 13 K (room temperature), respectively. Through finite difference time domain simulations (FDTD), the near‐field excitation, exciton decay, and far‐field detection processes are systematically analyzed, and highly consistent polarizations are quantitatively achieved between the theoretical and the experimental results. Accordingly, the high polarizations are revealed to be contributed by the increased exciton generation and radiation efficiency, chiral electromagnetic field, and non‐equilibrium spin distribution in the hybrid phase. The research presented here illustrates a promising route to control the spin and valley degrees of freedom in TMDC materials.

(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).

Exciton and Phonon Radiative Linewidths in Monolayer Boron Nitride

Exciton and Phonon Radiative Linewidths in Monolayer Boron Nitride

Study of the multiband exciton spectrum of ZnSe in the range of 477-490 nm

The photoluminescence spectra of CVD (chemical vapour deposition) ZnSe polycrystalline grown with a large excess of selenium and containing \O*Se-Cu+i\ complexes at stacking faults are studied. Absorbance was measured to complement these data. The features of the PL spectra in comparison with cathodoluminescence are considered. It is shown that PL bands identical to CL are observed as somewhat shorter wavelengths. For the studied crystals, a band model is presented according to the data obtained in this work. The low-energy shift of the PL spectra with decreasing excitation energy corresponds to a shift on the energy scale of the band model with a corresponding change in the type of radiative transitions. It is shown that identical photoluminescence bands are observed as somewhat shorter wavelengths than cathodoluminescence bands. For the crystals under study, a band model is presented according to the data obtained in the given decreasing work. The low-energy shift of the photoluminescence spectra upon the excitation energy corresponds to a shift along the energy scale of the band model with a corresponding change in the type of radiative transitions. Changes have been introduced that characterize the nature of the group of equidistant bands 477-490 nm, which are characteristic of ZnSe samples with an excess of oxygen and Se. The results can be useful for a more complete study of the structure of multiphonon exciton spectra of photo and cathodoluminescence of AIIBVI crystals. Keywords: band model, narrow-line multi-photon spectra, exciton radiation, stacking faults, isoelectronic oxygen impurity, carrying effective negative charge.

Controlling exciton spontaneous emission of quantum dots by Au nanoparticles

As an ideal single-photon source, quantum dots (QDs) can play a unique role in the field of quantum information. Controlling QD exciton spontaneous emission can be achieved by anti-phase coupling between QD exciton dipole field and Au dipole field after QD film has been transferred onto the Si substrate covered by Au nanoparticles. In experiment, the studied InAs/GaAs QDs are grown by molecular beam epitaxy (MBE) on a (001) semi-insulation substrate. The films containing QDs with different GaAs thickness values are separated from the GaAs substrate by etching away the AlAs sacrificial layer and transferring the QD film to the silicon wafer covered by Au nanoparticles with a diameter of 50 nm. The distance <i>D</i> (thickness of GaAs) from the surface of the Au nanoparticles to the QD layer is 10, 15, 19, 25, and 35 nm, separately. A 640-nm pulsed semiconductor laser with a 40-ps pulse length is used to excite the QD samples for measuring QD exciton photoluminescence and time-resolved photoluminescence spectra at 5 K. It is found that when the distance <i>D</i> is 15–35 nm the spontaneous emission rate of exciton is suppressed. And when <i>D</i> is close to 19 nm, the QD spontaneous emission rate decreases to <inline-formula><tex-math id="M2">\begin{document}$ ~{10}^{-3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211863_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211863_M2.png"/></alternatives></inline-formula>, which is consistent with the theoretical calculations. The physical mechanism of long-lived exciton luminescence observed in experiment lies in the fact that Au nanoparticles scatter the light field of the exciton radiation in the QD wetting layer, and the phase of the scattered field is opposite to the phase of the exciton radiation field. Therefore, the destructive interference between the exciton radiation field and scattering field of Au nanoparticles results in long-lived exciton emission observed in experiment.

Effect of phononic and electron collisional interaction on temperature dependent exciton radiation dynamics of doped GaN

In ultraviolet light emission devices, excitons are a high-efficiency emission source. However, the mechanism of experimentally observed dependence of excitonic radiative lifetimes on temperature (T) has not been discovered. We present a numerical simulation based on the phonon-exciton-radiation system, which reveals that the dependence of the radiative lifetime of GaN excitons on T is dominated by the population distribution among discrete and continuum energy states. This finding is in contrast to an existing model considering the existence of 1S exciton only. This population distribution is determined by the temperature-dependent integrated effect of background carrier density and exciton energy broadening, which induces a combination of high-order excitonic states and the continuum. Various experimental results on the dependence on temperature, including the functions of T3/2 and higher or lower power of T, are interpreted by a model integrating the interactions with the electron and phononic fields. The proposed model elucidates the corresponding effects of electronic and phononic processes in this complex system and provides a platform for the discussion of Wannier exciton dynamics under various thermal conditions, including nonequilibrium cases beyond the Saha-Boltzmann relation in population distribution.

First-Principles Calculations of Exciton Radiative Lifetimes in Monolayer Graphitic Carbon Nitride Nanosheets: Implications for Photocatalysis

In this work, we report on the exciton radiative lifetimes of graphitic carbon nitride monolayers in the triazine- (gC$_3$N$_4$-t) and heptazine-based (gC$_3$N$_4$-h) forms, as obtained by means of ground- plus excited-state ab initio calculations. By analysing the exciton fine structure, we highlight the presence of dark states and show that the photo-generated electron-hole pairs in gC$_3$N$_4$-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC$_3$N$_4$-h in photocatalysis and makes it attractive for the emerging field of exciton devices. Although very long intrinsic radiative lifetimes are an important prerequisite for several applications, pristine carbon nitride nanosheets show very low quantum photo-conversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC$_3$N$_4$-t and gC$_3$N$_4$-h layers which shows a type-II band alignment and looks promising for achieving net charge separation.

Defects Dominated Regulation of Fluorescence Properties of Copper Nanoclusters

At present, many studies have suggested that during the luminescence process of Cu nanoclusters, the increase of metal defects can further enhance the emission, but the lack of understanding of the defect luminescence principle makes it tough to further adjust the emissions of luminescence and achieve multicolor luminescence. Herein, the 2D assembled sheet materials are modified with surface or deep defects. Metal defects affect the luminous energy level by regulating the optical properties of 2D assembled materials and regulating the detailed progress of exciton radiation action. Further more, research shows that continuous adjustment of the emission color can be achieved by only regulating the ingredient of surface defects and deep metal defects, and such surface metal defects have a lighter level and multiple sub-levels, which broadens the luminescent color. Finally, white light emitting diodes (WLEDs) with warm, cool and pure white color temperatures were achieved for illuminating applications used the full-color luminescence of Cu clusters.

Effect of Spectral Overlap and Separation Distance on Exciton and Biexciton Quantum Yields and Radiative and Nonradiative Recombination Rates in Quantum Dots Near Plasmon Nanoparticles

AbstractEfficient biexciton (BX) photoluminescence (PL) from quantum dots (QDs) paves the way to the generation of entangled photons and related applications. However, the quantum yield (QY) of BX PL is much lower than that for single excitons (EX) due to efficient Auger‐like recombination. In the vicinity of plasmon nanoparticles, the recombination rates of EX and BX may be affected by the Purcell effect, fluorescence quenching, and the excitation rate enhancement. Here, the effect of the plasmon resonance spectral position on the EX and BX PL is experimentally studied in two cases: when the plasmon band overlaps with the excitation wavelength and when it coincides with the QDs PL band. In the first case, the EX and BX excitation efficiencies are significantly increased but the EX QY reduced. As a result, the BX‐to‐EX QY ratio is higher than 1 at plasmon–exciton systems separations shorter than 40 nm. In the second case, the radiative recombination rates are enhanced by several orders of magnitude, which led to an increase in BX QY over distances of up to 90 nm. Finally, these two effects are obtained in the same hybrid structure, with the resultant increase in both excitation efficiency and QY of BX PL.