Circularly Polarized Photoluminescence Research Articles

Nanoimprinted 2D-Chiral Perovskite Nanocrystal Metasurfaces for Circularly Polarized Photoluminescence.

The versatile hybrid perovskite nanocrystals (NCs) are one of the most promising materials for optoelectronics by virtue of their tunable bandgaps and high photoluminescence (PL) quantum yields. However, their inherent crystalline chemical structure limits the chiroptical properties achievable with the material. The production of chiral perovskites has become an active field of research for its promising applications in optics, chemistry, or biology. Typically, chiral halide perovskites are obtained by the incorporation of different chiral moieties in the material. Unfortunately, these chemically modified perovskites have demonstrated moderate values of chiral PL so far. Here, a general and scalable approach is introduced to produce chiral PL from arbitrary nanoemitters assembled into 2D-chiral metasurfaces. The fabrication via nanoimprinting lithography employs elastomeric molds engraved with chiral motifs covering millimeter areas that are used to pattern two types of unmodified colloidal perovskite NC inks: green-emissive CsPbBr3 and red-emissive CsPbBr1 I2 . The perovskite 2D-metasurfaces exhibit remarkable PL dissymmetry factors (glum ) of 0.16 that can be further improved up to glum of 0.3 by adding a high-refractive-index coating on the metasurfaces. This scalable approach to produce chiral photoluminescent thin films paves the way for the seamless production of bright chiral light sources for upcoming optoelectronic applications.

Enhanced Circularly Polarized Photoluminescence of Chiral Perovskite Films by Surface Passivation with Chiral Amines.

Hybrid organic-inorganic perovskites have shown promise in circularly polarized light source applications when chirality has been introduced. Circularly polarized photoluminescence (CPL) is a significant tool for investigating the chiroptical properties of perovskites. However, further research is still urgently needed, especially with regard to optimization. Here we demonstrate that chiral ligands can influence the electronic structure of perovskites, increasing the asymmetry and emitting circularly polarized photons in photoluminescence. After the modification of chiral amines, the defects of films are passivated, leading to enhanced radiation recombination for which more circularly polarized photons are emitted. Meanwhile, the modification increases the asymmetry in the electronic structure of perovskites, manifested by an increase in the magnetic dipole moment from 0.166 to 0.257 μB and an enhanced CPL signal. This approach offers the possibility of fabricating and refining circularly polarized light-emitting diodes.

Circularly Polarized Photoluminescence from Nanostructured Arrays of Light Emitters

Strategically designed metamaterials can influence the properties of light emitters in several ways, including shaping of the directionality and polarization of luminescence. These properties, however, are limited in systems where the luminophores uniformly coat the metamaterial. Here, we study and design metamaterials composed of both Au nanobars and nanopatterned light emitters. We systematically investigate the role of spatial averaging, dipole orientation, chirality, near-field effects, and other factors for these multimaterial systems. Finally, we discuss multiple design routes to create metasurfaces that can emit photoluminescence of any circular polarization at any arbitrary angle. These systems simultaneously exhibit high photoluminescence intensity and tailored, directional, and polarized photoluminescence.

Electrochemically Switchable Circularly Polarized Photoluminescence within Self-Assembled Conducting Polymer Helical Microfibers.

Achieving electrically and/or electrochemically controlled circularly polarized photoluminescence (CPL) is challenging due to the non-electroactive characteristics of most chiral materials and the non-electrosensitive feature of materials' chiroptical signals. Here we found that the CPL of self-assembled conducting polyaniline (PANI) helical microfibers could be reversibly switched by applying an alternating electrical bias. The conducting polymer is not the fluorophore but can transfer its chirality to the coassembled aggregation-induced emission (AIE) fluorescent molecules. The electrochemically switchable CPL is derived from the reversible transformation of the chirality of the polyaniline microfibers, which is probably due to the change in the molecular interchain distance upon doping/dedoping. Subsequently, we have demonstrated double-layer information encryption based on the electrochemically reversible CPL and conductance.

Circularly polarized deep-blue polymer light emitting diodes induced by a chiral small molecule dopant

The solution-processable molecule S-F4SO containing enantiomerically chiral side-chains was designed, and showed the weak circulardichroism (CD) and insignificant circularly polarized photoluminescence (CPPL) signal. Then the chiral perturbation strategy took effect when S-F4SO was blended with achiral polymer poly(9,9-dioctylfluorene) (PFO), the CD and CPPL signal can be enhanced dramatically. The circularly polarized light-emitting dioxidesshowed the deep blue circularly polarized electroluminescence with the maximal emission at 448 nm, |gEL|max value of 4.5 × 10−3, the LEmax of 1.8 cd A1 with a CIE coordinate of (0.16, 0.10) and good electroluminescence spectra stability.

Environmental-friendly lead-free chiral Mn-based metal halides with efficient circularly polarized photoluminescence at room temperature

In recent years, chiral organic-inorganic hybrid metal halides have attracted more and more attention from researchers. However, most chiral metal halides are based on toxic Pb elements. In this work, we first successfully prepared a pair of lead-free manganese-based metal halide enantiomers (R/S-1-PPA) 2 MnBr 4 (R/S-1-PPA=R/S-1-Phenylpropan-1-amine). Under the excitation of 365 nm UV light, (R/S-1-PPA) 2 MnBr 4 exhibited bright green emission, which can be attributed to the spin-forbidden d-d transition of Mn 2+ . In addition, the decay lifetimes were as high as 32.57 μs and 33.60 μs for (R-1-PPA) 2 MnBr 4 and (S-1-PPA) 2 MnBr 4 , respectively. (R/S-1-PPA) 2 MnBr 4 showed notable circular dichroism (CD) signals and displayed highly efficient circularly polarized photoluminescence (CPPL) at room temperature with the large g lum of − 0.01 and 0.008, respectively. The high g lum enables them to be a series of very promising materials for the development of chiral optoelectronic devices based on chiral hybrid metal halides. Finally, the white LED device constructed with (R-1-PPA) 2 MnBr 4 also showed a wide color gamut, as high as 107% of NTSC, and a "warm" white light-correlated color temperature of 6883 K, which indicated it has good competitiveness in the application of white light LED devices. • We synthesized a pair of Pb-free Mn-based chiral metal halides (R/S-1-PPA) 2 MnBr 4 . • The halides emitted green CPPL, with g lum of − 0.01 and 0.008 at room temperature. • The g lum represent the maximum of the reported Mn-based metal halides.

Multifunctional Chiral 2D Lead Halide Perovskites with Circularly Polarized Photoluminescence and Piezoelectric Energy Harvesting Properties.

Introducing the chiral spacers to two-dimensional (2D) lead halide perovskites (LHPs) enables them to exhibit circularly polarized photoluminescence (CPPL), which could have applications in chiral-optics and spintronics. Despite that a great deal of effort has been made in this field, the reported polarization degree of CPPL at ambient conditions is still very limited, and the integration of multiple functionalities also remains to be explored. Here we report the structures, CPPL, and piezoelectric energy harvesting properties of chiral 2D LHPs, [R-1-(4-bromophenyl)ethylaminium]2PbI4 (R-[BPEA]2PbI4) and [S-1-(4-bromophenyl)ethylaminium]2PbI4 (S-[BPEA]2PbI4). Our results show that these chiral perovskites are direct bandgap semiconductors and exhibit CPPL centered at ∼513 nm with a maximum degree of polarization of up to 11.0% at room temperature. In addition, the unique configurational arrangement of the chiral spacers is found to be able to reduce the interlayer π-π interactions and consequently result in strong electron-phonon coupling. Furthermore, the intrinsic chirality of both R-[BPEA]2PbI4 and S-[BPEA]2PbI4 enables them to be piezoelectric active, and their composite films can be applied to generate voltages and currents up to ∼0.6 V and ∼1.5 μA under periodic impacting with a strength of 2 N, respectively. This work not only reports a high degree of CPPL but also demonstrates piezoelectric energy harvesting behavior for realizing multifunctionalities in chiral 2D LHPs.

Glass-Forming Chiral Liquid Crystalline Dimers Based on an Oligo(phenylenevinylene) Unit Exhibiting Circularly Polarized Photoluminescence

Oligo(p-phenylenevinylene)-based chiral nematic liquid crystals were synthesized. Mixtures of dimeric and monomeric materials exhibited a glassy chiral nematic phase at room temperature, and the reflection band was tunable from near UV to near IR areas by the temperature and mixing ratio of the chiral nematic materials. The reflection band could be tuned to cover the emission band of the oligo(p-phenylenevinylene) chromophores completely, resulting in the circularly polarized fluorescence with the dissymmetric factor exceeding 1.4. The reflection band could be immobilized by cooling film samples of the mixtures in ice water. The dissymmetric factor of the glassy chiral nematic films exceeded 1.3 at room temperature. From the film samples of the S-form of the chiral nematic liquid crystals emitted left-handed circularly polarized light while right-handed circularly polarized light was enhanced at the edge of the reflection band due to the Purcell effect.

Spin-Selective Hole-Exciton Coupling in a V-Doped WSe2 Ferromagnetic Semiconductor at Room Temperature.

While valley polarization with strong Zeeman splitting is the most prominent characteristic of two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors under magnetic fields, enhancement of the Zeeman splitting has been demonstrated by incorporating magnetic dopants into the host materials. Unlike Fe, Mn, and Co, V is a distinctive dopant for ferromagnetic semiconducting properties at room temperature with large Zeeman shifting of band edges. Nevertheless, little known is the excitons interacting with spin-polarized carriers in V-doped TMDs. Here, we report anomalous circularly polarized photoluminescence (CPL) in a V-doped WSe2 monolayer at room temperature. Excitons couple to V-induced spin-polarized holes to generate spin-selective positive trions, leading to differences in the populations of neutral excitons and trions between left and right CPL. Using transient absorption spectroscopy, we elucidate the origin of excitons and trions that are inherently distinct for defect-mediated and impurity-mediated trions. Ferromagnetic characteristics are further confirmed by the significant Zeeman splitting of nanodiamonds deposited on the V-doped WSe2 monolayer.

Chiral Thermally Activated Delayed Fluorescence Emitters-Based Efficient Circularly Polarized Organic Light-Emitting Diodes Featuring Low Efficiency Roll-Off.

Direct emission of circularly polarized light from organic light-emitting diodes (OLEDs) is a research hotspot as it could increase the efficiency and significantly simplify device architecture of OLED-based 3D displays. In this study, R/S-OBS-Cz and R/S-OBS-TCz with axial chirality were efficiently prepared by using a stable chiral octahydro-binaphthol unit, carbazole/3,6-ditert-butylcarbazole donors, and a 5,5,10,10-tetraoxide acceptor. The chiral unit-acceptor-donor structure provides them not only thermally activated delayed fluorescence (TADF) characteristics with minor singlet-triplet energy gaps of 0.04 and 0.05 eV but also obvious circularly polarized photoluminescence (CPPL) phenomenon with dissymmetry factors of 8.7 × 10-4 and 6.4 × 10-4 in codoped films. Meanwhile, the CP-OLEDs prepared by enantiomers exhibit good device performances with the maximum external quantum efficiency reaching 20.3% and ideal efficiency roll-off as well as obvious CPEL properties with a |gEL| factor up to 1.0 × 10-3.

Multicolor Circularly Polarized Photoluminescence and Electroluminescence with 1,2-Diaminecyclohexane Enantiomers.

Development of simple chiral materials with tunable circularly polarized photoluminescence (CPPL) and circularly polarized electroluminescence (CPEL) for efficient circularly polarized organic light-emitting diodes (CP-OLEDs) is the key toward future 3D displays. In this study, four pairs of chiral 1,2-diaminocyclohexane-based fluorescence enantiomers were efficiently prepared with high yields (up to 92%) and enantiomeric excesses (ee >99%). By the introduction of N-methyl, carbazole, and diphenylamine-donating groups, these materials showed multicolor CPPL and CPEL from blue (420 nm) to red (610 nm) with good thermal and conformational stability. The multilayer CP-OLEDs based on these enantiomers show high external quantum efficiency of up to 5.5% with low-efficiency roll-off and microimage circularly polarized electroluminescence with a dissymmetry factor (gEL) of up to -1.4 × 10-3/+1.3 × 10-3. These results push forward the development of future multicolor circularly polarized electroluminescent materials.

Chiral 2D Perovskites with a High Degree of Circularly Polarized Photoluminescence.

Chiral materials are of particular interest and have a wide range of potential applications in life science, material science, spintronic, and optoelectronic devices. Two-dimensional (2D) hybrid organic-inorganic lead halide perovskites have attracted increasing attention. Incorporating the chiral organic ligands into the layered lead iodide frameworks would introduce strong chirality in pure 2D perovskites for potential applications in circularly polarized light (CPL) emission and detection; nonetheless, studies on those aspects are still in their infancy. Here, we report on the strong CPL emission and sensitive CPL detection in the visible-wavelength range in pure chiral ( R-/ S-MBA)2PbI4 (MBA = C6H5C2H4NH3) 2D perovskites, which are successfully synthesized with a needle shape and millimeter size by incorporating the chiral molecules. The chiral 2D perovskites ( R-MBA)2PbI4 and ( S-MBA)2PbI4 exhibit an average degree of circularly polarized photoluminescence (PL) of 9.6% and 10.1% at 77 K, respectively, and a maximum degree of the circularly polarized PL of 17.6% is achieved in ( S-MBA)2PbI4. The degree of circularly polarized PL dramatically decreases with increasing temperature, implying that the lattice distortion induced by the incorporated chiral molecules and/or temperature-dependent spin flipping might be the origin for the observed chirality. Finally, CPL detection has been achieved with decent performance in our chiral 2D perovskite microplate/MoS2 heterostructural devices. The high degree of the circularly polarized PL and excellent CPL detection together with the layered nature of pure chiral 2D perovskites enables them to be a class of very promising materials for developing and exploring spin associated electronic devices based on the chiral 2D perovskites.

Control of Circularly Polarized Electroluminescence in Induced Twist Structure of Conjugate Polymer.

An extremely high degree of circularly polarized photoluminescence (CPPL) and electroluminescence (CPEL) (dissymmetry factor values: |gPL | = 0.72 and |gEL | = 1.13) are generated from twisted stacking of achiral conjugated polymer induced by nonemitting chiral dopant of high helical twisting power for the first time. Using a theoretical analysis incorporating the Stokes parameter, the twisting angle and birefringence of the aligned conjugated polymer, and the degree of linear polarization in the emitted light are found to make a roughly equal contribution to the degree of CPEL as to the degree of CPPL. Moreover, it is also found that the location of the recombination zone within the emitting layer is a crucial parameter for determining the difference in the dissymmetry factor between CPEL and CPPL. This result is applied to an organic light-emitting display to improve the luminous efficiency by 60%.

Spin excitation and band-narrowing in Al xGa 1− xAs heterostructures

We studied the spin excitation in dependences of the applied electric field and lattice temperature (LT) via the measurements of the circularly polarized photoluminescence (CPPL) in Al x Ga 1− x As heterostructures (HSs). The intensity of CPPL was found to strongly depend on the electric field applied to the HSs. The CPPL was also found to enhance with decreasing LT. It was demonstrated that the observed LT dependence might be due to the LT-dependent band-gap shift of the HS materials.

Photoresponsive Glass‐Forming Butadiene‐Based Chiral Liquid Crystals with Circularly Polarized Photoluminescence

AbstractThe synthesis and study of the photo‐ and thermoresponsive behavior of a series of novel trimesogens consisting of a diphenylbutadiene core linked to cholesterol moieties on either side via flexible alkyl chains are reported. These molecules possess the combined glass‐forming properties of bischolesterols and the photochromic and luminescent properties of the butadiene moiety. The pitch of the cholesteric phase of these materials could be continuously varied thermally and photochemically, making it possible to tune the color of the film over the entire visible region. The color images thus generated could be stabilized by converting them to N* glasses. These materials were also highly photoluminescent, exhibiting circularly polarized characteristics in the glassy liquid‐crystalline state even by linearly polarized excitation.

TEMPORAL DYNAMICS OF EXCITON–TRION SYSTEM

Time-resolved and time-integrated circularly-polarized photoluminescence of excitons and trions have been studied in external magnetic fields up to 10 T. ZnSe-based quantum well structures of n-type with carrier densities varied from 5×109 to 1011 cm -2 were used in this study. Absence of the chemical equilibrium in the exciton–trion system has been demonstrated at low temperatures (<10 K ). The recovery of the equilibrium has been found at elevated temperatures (<15 K ).

Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1−xMnxTe/ZnTe Quantum Wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn1−xMn x Te/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn1−xMn x Te exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (Hc) of 4 T, due to the giant Zeeman shift of Zn1−xMn x Te. The PL intensities and lifetimes in each layer become gradually equal toward Hc, which shows the mixing of wavefunctions of the excitons generated in each layer. Above Hc, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn1−xMn x Te and ZnTe layers.

Chiroptical Properties of Chiral Substituted Polyfluorenes

Liquid-crystalline polyfluorene (PF) homopolymers substituted with chiral alkyl side chains were synthesized, and their chiroptical properties in the solid state were investigated by means of circular dichroism (CD), circularly polarized photoluminescence (CPPL), and circularly polarized electroluminescence (CPEL) measurements. Polarization-selective scattering of light is shown to cause artifacts in the circularly polarized absorption and emission spectra in the wavelength range near or above the absorption edge, and a measurement scheme to avoid these is presented. For all derivatives, significant chiroptical effects appeared only after the solid layers have been annealed at elevated temperatures, preferably into the liquid-crystalline state of the polymer. The largest anisotropy factors were measured for a polyfluorene substituted with chiral (R)-2-ethylhexyl side chains, yielding absolute values of up to 0.28 for CPPL and up to 0.25 for CPEL. These are among the highest ever reported for a chiral conj...

Thermotropic Chiral−Nematic Poly(p-phenylene)s as a Paradigm of Helically Stacked π-Conjugated Systems

As an emerging class of multifunctional optical materials, chiral−nematic poly(p-phenylene)s with cyanobiphenyl and (S)-1-phenylethyl pendants were synthesized and characterized. Nematic homopolymer was also synthesized for preparation into a uniaxially aligned film. Linearly polarized photoluminescence (LPPL) and FTIR dichroism revealed that both the conjugated backbone and the nematic pendant are oriented along the director with an orientational order parameter, S = 0.62 and 0.73, respectively. Films prepared with chiral−nematic copolymers were found to selectively reflect left-handed circularly polarized light in the ultraviolet, visible, and infrared region at a decreasing chiral content. The helically stacked poly(p-phenylene)s were further characterized in terms of circularly polarized photoluminescence (CPPL). On the basis of a recent theory, S = 0.67 was found for the orientational order of conjugated backbones within quasinematic layers comprising the chiral−nematic film. Within experimental uncertainty, the S values determined with the LPPL and CPPL techniques are in good agreement, thereby validating the proposed supramolecular structure on the basis of which theories governing light absorption, emission, and propagation in structured media were constructed.

Mechanistic insight into circularly polarized photoluminescence from a chiral-nematic film

Circularly polarized photoluminescence (CPPL) was characterized for rod-like molecules of Exalite 428 helically arranged in a chiral-nematic liquid crystalline film. With an unpolarized excitation at 370nm, CPPL intensities measured at 428nm, and a selective reflection wavelength ranging from 7.2 to 72.6mum, the observed dissymmetry factor, g, was found to be in excellent agreement with theoretical prediction, without resorting to adjustable parameters. As a result, new insight into liquid crystal-induced CPPL has emerged. Specifically, it was found that circular dichroism and circular polarization of the excitation beam prior to inducing linearly polarized photoluminescence (LPPL) at quasi-nematic layers play an insignificant role. The relatively large g value in the spectral region far removed from the selective reflection band was attributed to the circular polarization of LPPL emanating from all quasi-nematic layers comprising the chiral-nematic film. In the absence of a fluorescent dye, the propagation of unpolarized light through the chiral-nematic film under otherwise identical conditions results in no circular polarization. The present study has provided a foundation for generating circularly polarized light by way of photo-excitation with unpolarized light of a chiral-nematic film containing a fluorescent dye. e e