Polarized Photoluminescence Spectra Research Articles

Optical Microcavity‐Induced Moiré Exciton Localization in Twisted WSe2 Homobilayer

Abstract2D moiré superlattices offer a versatile platform for manipulating quantum materials, exploiting their spatially varying atomic registry to modify electronic structures, resulting in the emergence of flat electronic minibands, enhanced electronic interactions, and the onset of novel quantum phenomena. Despite extensive investigations into moiré excitons within twisted bilayer moiré superlattices, the interplay between moiré superlattices and optical microcavities remains elusive. Here, a WSe2 homobilayer with a small twist angle positioned on optical microcavities atop a SiO2 (285 nm)/Si substrate is investigated. Utilizing low‐temperature photoluminescence (PL) spectroscopy, it is observed that the twisted homobilayer at the edge of the optical cavity (Edge‐THB) exhibits enhanced localization of moiré excitons and a deeper moiré potential. This behavior is attributed to the combined influence of strain induced by the optical microcavity structure on the moiré superlattice and the amplification of the electromagnetic field. Furthermore, the existence of moiré excitons through a comprehensive analysis, encompassing temperature‐dependent PL spectra, circularly polarized PL spectra, and Landé g‐factor measurements for Edge‐THB is validated. These findings provide valuable insights into the intricate interaction between moiré superlattices and optical microcavities, paving the way for future applications in quantum technologies.

Planar Chiral Thermally Activated Delayed Fluorescence Materials Based on Di[2.2]paracyclophane for Circularly Polarized Electroluminescence

AbstractIn this study, featuring di[2.2]paracyclophane (DpCp) amine derivatives as electron donors and 4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl (Trz) as the electron acceptor, four thermally activated delayed fluorescence (TADF) molecules are disclosed. The C2 symmetric molecules, DpCpN‐Trz and DpCpCz‐Trz, bearing a chair‐like planar chiral di[2.2]paracyclophane moiety, are obtained as optically pure enantiomers (R/S)‐C‐DpCpN‐Trz and (R/S)‐C‐DpCpCz‐Trz. And the compounds B‐DpCpN‐Trz and B‐DpCpCz‐Trz with a bridge‐like di[2.2]paracyclophane skeleton are meso‐compounds, thus without optically activity. All emitters show blue emissions peaking from 464 to 485 nm, accompanied by high photoluminescence quantum efficiency (up to 93%). The enantiomers (R/S)‐C‐DpCpN‐Trz and (R/S)‐C‐DpCpCz‐Trz exhibit symmetric circularly polarized photoluminescence spectra with dissymmetry factors (|gPL|) ranging from 3.8 × 10−4 to 6.7 × 10−4 in toluene and films. The organic light‐emitting diodes (OLEDs) fabricated with these emitters achieve maximum external quantum efficiencies of up to 19.5%. Notably, the circularly polarized OLEDs (CP‐OLEDs) with (R/S)‐C‐DpCpN‐Trz and (R/S)‐C‐DpCpCz‐Trz demonstrate |gEL| factors of up to 7.6 × 10−4. This study demonstrates a feasible way to design TADF materials and enantiomers by incorporating DpCp derivatives for CP‐OLEDs.

Polarized photoluminescence from Sn, Fe, and unintentionally doped β-Ga2O3

In this work, we demonstrate that β-Ga2O3 shows orientation-dependent polarized photoluminescence (PL) emission and give a comprehensive insight into gallium oxide's PL spectral properties. We characterized the polarization and spectral dependencies of both the incident and emitted light for (−201) unintentionally doped (UID) as well as (−201) and (010) Sn-doped and Fe-doped crystals. We observed for UID and Sn-doped samples that the electron to self-trapped hole and native defect-related emission bands are linearly polarized with polarized emission intensities ordered as E || c (and c*) > E || a (and a*) > E || b. Furthermore, the spectral shape of emission does not change between the UID and Sn-doped samples; instead, the Sn-doping quenches the total PL spectral intensity. For Fe-doped samples, polarized red emission caused by unintentional Cr3+ doping generates emission intensities ordered E || b > E || c (and c*) > E || a (and a*). It is also observed that in some circumstances, for some doped crystals, the PL spectra can show variations not only in intensity but also in spectral shape along different polarization directions. As an example, the PL emission band for emission along c is blueshifted relative to that along a in Sn-doped β-Ga2O3.

Planar Chiral Multiple Resonance Thermally Activated Delayed Fluorescence Materials for Efficient Circularly Polarized Electroluminescence

AbstractChiral boron/nitrogen doped multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters are promising for highly efficient and color‐pure circularly polarized organic light‐emitting diodes (CP‐OLEDs). Herein, we report two pairs of MR‐TADF materials (Czp‐tBuCzB, Czp‐POAB) based on planar chiral paracyclophane with photoluminescence quantum yields of up to 98 %. The enantiomers showed symmetric circularly polarized photoluminescence spectra with dissymmetry factors |gPL| of up to 1.6×10−3 in doped films. Meanwhile, the sky‐blue CP‐OLEDs with (R/S)‐Czp‐tBuCzB showed an external quantum efficiency of 32.1 % with the narrowest full‐width at half‐maximum of 24 nm among the reported CP‐OLEDs, while the devices with (R/S)‐Czp‐POAB displayed the first nearly pure green CP electroluminescence with |gEL| factors at the 10−3 level. These results demonstrate the incorporation of planar chirality into MR‐TADF emitter is a reliable strategy for constructing of efficient CP‐OLEDs.

Planar Chiral Multiple Resonance Thermally Activated Delayed Fluorescence Materials for Efficient Circularly Polarized Electroluminescence.

Chiral boron/nitrogen doped multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising for highly efficient and color-pure circularly polarized organic light-emitting diodes (CP-OLEDs). Herein, we report two pairs of MR-TADF materials (Czp-tBuCzB, Czp-POAB) based on planar chiral paracyclophane with photoluminescence quantum yields of up to 98 %. The enantiomers showed symmetric circularly polarized photoluminescence spectra with dissymmetry factors |gPL | of up to 1.6×10-3 in doped films. Meanwhile, the sky-blue CP-OLEDs with (R/S)-Czp-tBuCzB showed an external quantum efficiency of 32.1 % with the narrowest full-width at half-maximum of 24 nm among the reported CP-OLEDs, while the devices with (R/S)-Czp-POAB displayed the first nearly pure green CP electroluminescence with |gEL | factors at the 10-3 level. These results demonstrate the incorporation of planar chirality into MR-TADF emitter is a reliable strategy for constructing of efficient CP-OLEDs.

Circularly Polarized Photoluminescence of Chiral 2D Halide Perovskites at Room Temperature.

Chiral halide perovskites have attracted considerable attention because of their chiroptical, second-harmonic generation, and ferroelectricity properties and their potential application in chiroptoelectronics and chiral spintronics. However, the fundamental research of these properties is insufficient. In this work, chiral perovskites were synthesized using precursor solutions with various stoichiometric ratios ⟨n⟩. The chiral perovskite film prepared from the solution with ⟨n⟩ = 1 is composed of (R-/S-/rac-MBA)2PbBr4, whereas the films prepared from the solutions with ⟨n⟩ larger than 1 are a mixture of (R-/S-/rac-MBA)2(CsMA)n-1PbnBr3n+1 with n = 1 and large n values. A photoluminescence quantum yield of approximately 90 was obtained. Symmetric circular dichroism (CD) spectra were observed without an external magnetic field. Under various magnetic fields, magnetic field-induced CD features are superimposed with the intrinsic chirality-induced CD features. For the ⟨n⟩ = 1 chiral perovskite film, the energy level splitting induced by chiral molecules are a few 10 μeV, whereas the energy level splitting induced by magnetic fields are at the range of ∼-250 to ∼250 μeV. Circularly polarized photoluminescence spectra were observed at room temperature and associated with the spin-preserved energy funneling from highly energetic phases to the lower energetic phases.

Optical and charge transport properties of 1,3,6,8-tetrakis(4′-methoxycarbonylphenyl)pyrene crystal

Crystals of 1,3,6,8-tetrakis(4′-methoxycarbonylphenyl)pyrene (383 µm × 36 µm) were successfully obtained on a substrate by a heat-melt and recrystallization method. The crystals showed the photoluminescence (PL) spectra with clear vibronic structures, which indicates a lack of direct π–π interaction between pyrene cores. Polarized PL spectra suggested strong reabsorption of the crystals. Field-effect transistors (FETs) having a bottom-gate bottom-contact structure were fabricated using the polycrystals as an active layer. These devices showed p-type characteristics with the maximum carrier mobility of 2.1 × 10−6 cm2 V−1 s−1 in the linear region and 5.1 × 10−6 cm2 V−1 s−1 in the saturation region. The fabricated FET showed light emission under applying higher voltages (VGS = −185 V, VDS = −162 V) just before breaking down.

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Polarized photoluminescence spectroscopy in WS2, WSe2 atomic layers and heterostructures by cylindrical vector beams* *Project supported by the National Natural Science Foundation of China (Grant Nos. 91850116, 51772084, 52022029, and U19A2090), Hunan Provincial Natural Science Foundation of China (Grant Nos. 2018RS3051 and 2018WK4004), and the Key Program of the Hunan Provincial Science and Technology Department, China (Grant No. 2019XK2001).

Due to the large exciton binding energy, two-dimensional (2D) transition metal dichalcogenides (TMDCs) provide an ideal platform for studying excitonic states and related photonics and optoelectronics. Polarization states lead to distinct light-matter interactions which are of great importance for device applications. In this work, we study polarized photoluminescence spectra from intralayer exciton and indirect exciton in WS2 and WSe2 atomic layers, and interlayer exciton in WS2/WSe2 heterostructures by radially and azimuthally polarized cylindrical vector laser beams. We demonstrated the same in-plane and out-of-plane polarization behavior from the intralayer and indirect exciton. Moreover, with these two laser modes, we obtained interlayer exciton in WS2/WSe2 heterostructures with stronger out-of-plane polarization, due to the formation of vertical electric dipole moment.

Anisotropic photoluminescence of nonpolar ZnO epilayers and ZnO/Zn1-xMgxO multiple quantum wells grown on LiGaO2 substrate.

The temperature-dependent polarized photoluminescence spectra of nonpolar ZnO samples were investigated by 263 nm laser. The degree of polarization (DOP) of m-plane quantum wells changes from 76% at 10 K to 40% at 300 K, which is much higher than that of epilayer. The strong anisotropy was presumably attributed to the enhanced confinement effect of a one-dimension confinement structure formed by the intersection of quantum well and basal stacking fault. The polarization of laser beam also has an influence on the DOP. It is assumed that the luminescence polarization should be affected not only by the in-plane strains but also the microstructural defects, which do modify the electronic band structure.

Clarification of the Molecular Doping Mechanism in Organic Single-Crystalline Semiconductors and their Application in Color-Tunable Light-Emitting Devices.

Organic single-crystalline semiconductors with long-range periodic order have attracted much attention for potential applications in electronic and optoelectronic devices due to their high carrier mobility, highly thermal stability, and low impurity content. Molecular doping has been proposed as a valuable strategy for improving the performance of organic semiconductors and semiconductor-based devices. However, a fundamental understanding of the inherent doping mechanism is still a key challenge impeding its practical application. In this study, solid evidence for the "perfect" substitutional doping mechanism of the stacking mode between the guest and host molecules in organic single-crystalline semiconductors using polarized photoluminescence spectrum measurements and first-principles calculations is provided. The molecular host-guest doping is further exploited for efficient color-tunable and even white organic single-crystal-based light-emitting devices by controlling the doping concentration. The clarification of the molecular doping mechanism in organic single-crystalline semiconductor host-guest system paves the way for their practical application in high-performance electronic and optoelectronic devices.

Photoanisotropic properties of luminescence media for polarization holography based on new-type dyes

The luminescence polarization properties of new recording media obtained by directed synthesis—disulfochlorides of luminescent dyes (homologues of 1,7-diamino-3,9-dihydrodibenzo-[1,2,3de:4,5,6- d'e']diquinoline-2.8-dione (1,5-diaminoanthradipyridone) series), with the general structure (ClSO2)2-1,5-di-AAP-di-R—are investigated. Polarized photoluminescence spectra are recorded, and spectral dependences of the degree of anisotropy of phosphor on its chemical structure at different acidities of the medium are plotted.

Valley Pseudospin with a Widely Tunable Bandgap in Doped Honeycomb BN Monolayer.

Valleytronics is a promising paradigm to explore the emergent degree of freedom for charge carriers on the energy band edges. Using ab initio calculations, we reveal that the honeycomb boron nitride (h-BN) monolayer shows a pair of inequivalent valleys in the vicinities of the vertices of hexagonal Brillouin zone even without the protection of the C3 symmetry. The inequivalent valleys give rise to a 2-fold degree of freedom named the valley pseudospin. The valley pseudospin with a tunable bandgap from deep ultraviolet to far-infrared spectra can be obtained by doping h-BN monolayer with carbon atoms. For a low-concentration carbon periodically doped h-BN monolayer, the subbands with constant valley Hall conductance are predicted due to the interaction between the artificial superlattice and valleys. In addition, the valley pseudospin can be manipulated by visible light for high-concentration carbon doped h-BN monolayer. In agreement with our calculations, the circularly polarized photoluminescence spectra of the B0.92NC2.44 sample show a maximum valley-contrasting circular polarization of 40% and 70% at room temperature and 77 K, respectively. Our work demonstrates a class of valleytronic materials with a controllable bandgap.

Large degree of polarization of photoluminescence caused by anisotropic strain in nonpolar a-plane MgxZn1-xO layers grown by plasma-assisted molecular beam epitaxy.

A large degree of polarization (ρ) of photoluminescence (PL) approximate to 1 is obtained in each nonpolar a-plane MgxZn1-xO layer grown by plasma-assisted molecular beam epitaxy (MBE) with x=0.01, 0.03, and 0.10, respectively. Anisotropic in-plane strains are selectively introduced by using foreign substrates and doping with different Mg contents, which strongly modify the valence band structures, leading to anisotropic optical properties. A polarized Raman measurement shows that anisotropic in-plane strains along the y and z axes increase with the increasing Mg contents. Polarized PL spectra show that ρ gradually increases to 0.97 with decreasing in-plane strains, resulting from an increasing difference in transition energy (ΔE) between E⊥c and E‖c caused by a lift of the degeneracy of valence band structures. The obtained highly polarized emission is close to linear polarized light, which is desirable in the backlighting of liquid crystal displays.

Strong Circularly Polarized Photoluminescence from Multilayer MoS2 Through Plasma Driven Direct-Gap Transition

We report circularly polarized photoluminescence spectra taken from few layer MoS2 after treatment with a remotely generated oxygen plasma. Here, the oxygen plasma decouples the individual layers in MoS2 by perturbing the weak interlayer van der Waals forces without damaging the lattice structure. This decoupling causes a transition from an indirect to a direct band gap material, which causes a strong enhancement of the photoluminescence intensity. Furthermore, up to 80% circularly polarized photoluminescence is observed after plasma treatment of few layer MoS2 flakes, consistent with high spin polarization of the optically excited carriers. A strong degree of polarization continues up to room temperature, further indicating that the quality of the crystal does not suffer degradation due to the oxygen plasma exposure. Our results show that the oxygen plasma treatment not only engineers the van der Waals separation in these TMDC multilayers for enhanced PL quantum yields, but also produces high quality mul...

Spatially resolved study of polarized micro-photoluminescence spectroscopy on single GaAs nanowires with mixed zincblende and wurtzite phases

Localized and polarized photoluminescence spectra are observed in single GaAs nanowires with mixed zincblende and wurtzite phases, grown using molecular beam epitaxy. For low excitation intensities, the photoluminescence emission exhibits narrow spectral features predominantly polarized perpendicular to the nanowire axis. For high excitation intensities, the photoluminescence spectra transform into dominant broadened features, which exhibit different peak energies and polarization properties. The strongly polarized emission at high excitation intensities is identified as being due to a spatially direct transition in wurtzite sections of the nanowires. The analysis, including band structure calculations suggests that carriers in the wurtzite sections diffuse into regions where the average low-temperature peak emission energy and crystal field parameter are 1.535 eV and 20 meV, respectively.

One-dimensional miniband formation in closely stacked InAs/GaAs quantum dots

We have studied the electronic states of closely stacked InAs/GaAs quantum dots (QDs) with a 4.0-nm spacer layer using linearly polarized photoluminescence (PL) and time-resolved PL measurements. An increase in the stacking-layer number (SLN) leads to an increase in the linear polarization anisotropy in the (001) plane; the [$\ensuremath{-}$110]-polarization component becomes dominant. These SLN-dependent polarization characteristics result from the valence-band mixing induced by the vertically coupled electronic states. The PL spectrum of the stacked QDs shows clear blueshifts with an increase in the excitation power because of the band filling. In addition, the radiative recombination lifetime has been found to obey the ${T}^{1/2}$ dependence, which directly confirms the one-dimensional translational motion of excitons in the closely stacked QDs.

Polarized photoluminescence characteristics of rubrene sheets with nanometer thickness

We report on the temperature dependence of the polarized photoluminescence (PL) exhibited by crystalline organic rubrene sheets with a thickness of \ensuremath{\sim}60 nm. To probe the correlations between the electronic states and the molecular stacking of rubrene molecules, the polarized PL spectrum of a single rubrene sheet was analyzed as a function of temperature from 10 to 250 K. Drastic changes were observed in PL peak positions and intensities \ensuremath{\sim}100 and \ensuremath{\sim}170 K, which were likely related to variations in molecular ordering with temperature. The changes in the PL characteristics of the sheet \ensuremath{\sim}170 K are similar to those observed for rubrene bulk crystals, in terms of the enhancement of mobility and crystallinity. In addition, \ensuremath{\sim}100 K, the reversal of the relative ratio of PL intensities corresponding to two crystalline axes, $a$ and $b$, in the rubrene sheets suggested an anisotropic variation in the molecular ordering along these two crystalline axes.

Synthesis and photophysical properties of polyethylene containing pyrenyl units in the side chain

Copolymerization of ethylene and diallyl-bis(pyren-1-yl)-silane (APyS) was investigated with zirconocene catalysts, rac-ethylenebis(indenyl)zirconium dichloride (1) and diphenylmethylene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (2), using methylaluminoxane as a cocatalyst. APyS was copolymerized via both 1,2-insertion and cyclization insertion, and cyclization selectivity, ratio of cyclized insertion unit, of APyS in the copolymers obtained with Catalyst 1 was higher than that obtained with Catalyst 2. Catalyst 2 showed a higher reactivity for APyS than Catalyst 1. Photophysical properties of the copolymer were investigated by UV–vis and photoluminescence (PL) spectroscopy, and absorption and fluorescence derived from pyrenyl groups were detected in the copolymers. Chloroform solution of the copolymer showed emission derived from both monomer and eximer of pyrenyl units. Only the emission derived from eximer of pyrenyl units was observed in the cast film. The polarized PL spectrum of an oriented film showed anisotropy, and the polarization excitation parallel to the drawing direction showed high fluorescence intensity. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Surface-bound-exciton emission associated with domain interfaces in m-plane ZnO films

Small amount of (101¯3)ZnO domains were found in the m-plane ZnO films grown on m-sapphire by pulsed laser deposition, which provide strain relaxation of the m-ZnO matrix behaving as a low strain layer. Through carefully correlating low-temperature polarized photoluminescence spectra with the x-ray diffraction peak intensity ratio of (101¯3)ZnO/(101¯0)ZnO of the samples grown at different temperature and after thermal treatment, we found that the broad-band emission around 3.17 eV may result from the interface defects trapped excitons at the boundaries between the (101¯3)ZnO domains and the m-ZnO matrix. The more (101¯3)ZnO domains in the m-ZnO layer cause the more surface boundary that makes the stronger surface-bound-exciton emission. And the a-axes of both the (101¯3)ZnO domains and the m-ZnO matrix are aligned with the c-axis of the sapphire (α-Al2O3) substrate. The c-axis of the (101¯3)ZnO domains rotates by about ±59° against the common a-axis of the m-ZnO.