Highly Efficient Asymmetric Multiple Resonance Thermally Activated Delayed Fluorescence Emitter with EQE of 32.8 % and Extremely Low Efficiency Roll-Off.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters show great potentials for high color purity organic light-emitting diodes (OLEDs). However, the simultaneous realization of high photoluminescence quantum yield (PLQY) and high reverse intersystem crossing rate (kRISC ) is still a formidable challenge. Herein, a novel asymmetric MR-TADF emitter (2Cz-PTZ-BN) is designed that fully inherits the high PLQY and large kRISC values of the properly selected parent molecules. The resonating extended π-skeleton with peripheral protection can achieve a high PLQY of 96 % and a fast kRISC of above 1.0×105  s-1 , and boost the performance of corresponding pure green devices with an outstanding external quantum efficiency (EQE) of up to 32.8 % without utilizing any sensitizing hosts. Remarkably, the device sufficiently maintains a high EQE exceeding 23 % at a high luminance of 1000 cd m-2 , representing the highest value for reported green MR-TADF materials at the same luminescence.

Strong valley splitting ind0two-dimensional SnO induced by magnetic proximity effect.

Strong magnetic interfacial coupling in van der Waals heterostructures is important for designing novel electronic devices. Besides the most studied transition metal dichalcogenides (TMDCs) materials, we demonstrate that the valley splitting can be activated in two-dimensional tetragonald0metal oxide, SnO, via the magnetic proximity effect by EuBrO. In SnO/EuBrO, the valley splitting of SnO can reach ∼46 meV, which is comparable to many TMDCs and equivalent to an external magnetic field of 800 T. In addition, the valley splitting can be further enhanced by adjusting interlayer distance and applying uniaxial strains. A design principle of new spintronic device based on this unique electronic structure of SnO/EuBrO has been proposed. Our findings indicate that SnO is a promising material for future valleytronics applications.

Anisotropic Melting Path of Charge-Ordering Insulator in LSMO/STO Superlattice.

Multiple phases coexist in manganite with simultaneously active couplings, and the transition among them depends on the relative intensities of different interactions. However, the melting path with variable intensities is unclear. The concentration and the ordering of oxygen vacancy in previous work are found to induce ferromagnetic charge-ordering insulator phase in [(La0.7 Sr0.3 MnO3 )10 /(SrTiO3 )5 ]n superlattice, which translates into metallic phase with magnetic field H and temperature T. In the current work, the H-T phase diagram for current I//[100] and I//[110] shows a large difference with H normal to the film plane, which is ascribed to the response of a variable range of hopping process to H with the in-plane anisotropic hopping probability of charge carrier. With H rotating from the out-of-plane to the in-plane direction, the preferred occupancy of the 3 d z 2 - r 2 $3{d}_{{z}^2 - {r}^2}$ orbital causes a decrease of spin-orbital coupling and lowers the activation energy, inducing a gentler melting process of a charge-ordering insulator. This work shows that the melting path of a charge-ordering insulator phase can be largely modulated in manganite with anisotropy.

Magnetic Properties and Microstructure of FePt(BN, X, C) (X = Ag, Re) Films.

A sputtered FePt(BN, Re, C) film, here boron nitride (BN), was compared to a reference sample FePt(BN, Ag, C). Intrinsically, these films illustrate a high anisotropy field (Hk) and perpendicular magnetocrystalline anisotropy (Ku),although the reference sample shows a higher value (Hk = 69.5 kOe, Ku = 1.74 × 107 erg/cm3) than the FePt(BN, Re, C) film (Hk = 66.9 kOe, Ku = 1.46 × 107 erg/cm3). However, the small difference in the anisotropy constant (K2/K1) ratio presents a close tendency in the angular dependence of the switching field. Extrinsically, the out-of-plane coercivity for the reference sample is 32 kOe, which is also higher than the FePt(BN, Re, C) film (Hc = 27 kOe), and both films present lower remanence (Mr(parallel)/Mr(perpendicular) = 0.08~0.12), that is, the index for perpendicular magnetic anisotropy. The higher perpendicular magnetization for both films was due to highly (001) textured FePt films, which was also evidenced by the tight rocking width of 4.1°/3.0° for (001)/(002) X-ray diffraction peaks, respectively, and high-enough ordering degree. The reference sample was measured to have a higher ordering degree (S = 0.84) than FePt(BN, Re, C) (S = 0.63). As a result, the Ag segregant shows stronger ability to promote the ordering of the FePt film; however, the FePt(BN, Re, C) film still has comparable magnetic properties without Ag doping. From the surface and elemental composition analysis, the metallic Re atoms found in the FePt lattice result in a strong spin-orbital coupling between transition metal Fe (3d electron) and heavy metals (Re, Pt) (5d electron) and we conducted high magnetocrystalline anisotropy (Ku). Above is the explanation that the lower-ordered FePt(BN, Re, C) film still has high-enough Ku and out-of-plane Hc. Regarding the microstructure, both the reference sample and FePt(BN, Re, C) show granular structure and columnar grains, and the respective average grain size and distributions are 6.60 nm (12.5%) and 11.2 nm (15.9%). The average widths of the grain boundaries and the aspect ratio of the columnar grain height are 2.05 nm, 1.00 nm, 2.35 nm, and 1.70 nm, respectively.

Propagation Characteristics of Circular Airy Vortex Beams in a Uniaxial Crystal along the Optical Axis.

Circular airy vortex beams (CAVBs) have attracted much attention due to their "abruptly autofocusing" effect, phase singularity, and their potential applications in optical micromanipulation, communication, etc. In this paper, we numerically investigated the propagation properties of circular airy beams (CABs) imposed with different optical vortices (OVs) along the optical axis of a uniaxial crystal for the first time. Like other common beams, a left-hand circular polarized (LHCP) CAVB, propagating along the optical axis in a uniaxial crystal, can excite a right-hand circular polarized (RHCP) component superimposed with an on-axis vortex of topological charge (TC) number of 2. When the incident beam is an LHCP CAB imposed with an on-axis vortex of TC number of l = 1, both of the two components have an axisymmetric intensity distribution during propagation and form hollow beams near the focal plane because of the phase singularity. The phase pattern shows that the LHCP component carries an on-axis vortex of TC number of l = 1, while the RHCP component carries an on-axis vortex of TC number of l = 3. With a larger TC number (l = 3), the RHCP component has a larger hollow region in the focal plane compared to the LHCP component. We also studied cases of CABs imposed with one and two off-axis OVs. The off-axis OV makes the CAVB's profile remain asymmetric throughout the propagation. As the propagation distance increases, the off-axis OVs move near the center of the beam and overlap, resulting in a special intensity and phase distribution near the focal plane.

Topological properties of Xene tuned by perpendicular electric field and exchange field in the presence of Rashba spin-orbit coupling.

Xene (X=Si, Ge, Sn) is a typical and promising two-dimensional topological insulator with many novel topological properties. Here, we investigate the topological properties of Xene tuned by a perpendicularly applied electric field, exchange field, and Rashba spin-orbit coupling (RSOC) using the tight-binding (TB) method. We show that in the presence of RSOC, the system can be converted from a quantum spin Hall (QSH) insulator into a conventional band insulator (BI) by a weak perpendicular electric field or into a quantum anomalous Hall (QAH) insulator by a weak exchange field. Additionally, a suitable combination of electric and exchange fields can give rise to a valley-polarized metallic (VPM) state. Furthermore, we explore the competition between the electric field and exchange field in tuning the topological states owing to the Rashba coupling effect. When the electric field is stronger than the exchange field, the system tends to be in a topologically trivial BI state; otherwise, it will be a QAH insulator. More intriguingly, for a fixed exchange field and RSOC, as the perpendicular electric field increase continuously from zero, the system undergoes multiphase (e.g. QSH-VPM-BI) transitions. This paves the way for designing multiphase transition devices through external single-field regulation.

Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High-Efficiency Exciplex Light-Emitting Diodes.

Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge-transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light-emitting diodes are reported. Magneto-photoluminescence studies reveal that the triplet-to-singlet conversion in exciplexes involves an artificially generated spin-orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge-transfer occurs with forming electric dipoles (D+• →A-• ), providing the ionic polarization to generate SOC in exciplexes. By having different singlet-triplet energy differences (ΔEST ) in 9,9'-diphenyl-9H,9'H-3,3'-bicarbazole (BCzPh):3',3'″,3'″″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1'-biphenyl]-3-carbonitrile)) (CN-T2T) (ΔEST = 30 meV) and BCzPh:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methyl-pyrimidine (B3PYMPM) (ΔEST = 130 meV) exciplexes, the SOC generated by the intermolecular charge-transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQEmax (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔEST , and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D+• →A-• ) size, enhanced SOC, increased orbital polarization, and decreased ΔEST can simultaneously occur to cooperatively operate the triplet-to-singlet conversion.

Electronic and Optical Properties of Two-Dimensional Tellurene: From First-Principles Calculations.

Tellurene is a new-emerging two-dimensional anisotropic semiconductor, with fascinating electric and optical properties that differ dramatically from the bulk counterpart. In this work, the layer dependent electronic and optical properties of few-layer Tellurene has been calculated with the density functional theory (DFT). It shows that the band gap of the Tellurene changes from direct to indirect when layer number changes from monolayer (1 L) to few-layers (2 L-6 L) due to structural reconstruction. Tellurene also has an energy gap that can be tuned from 1.0 eV (1 L) to 0.3 eV (6 L). Furthermore, due to the interplay of spin-orbit coupling (SOC) and disappearance of inversion symmetry in odd-numbered layer structures resulting in the anisotropic SOC splitting, the decrease of the band gap with an increasing layer number is not monotonic but rather shows an odd-even quantum confinement effect. The optical results in Tellurene are layer dependent and different in E ⊥ C and E || C directions. The correlations between the structure, the electronic and optical properties of the Tellurene have been identified. Despite the weak nature of interlayer forces in their structure, their electronic and optical properties are highly dependent on the number of layers and highly anisotropic. These results are essential in the realization of its full potential and recommended for experimental exploration.

Strain-Induced Quantum Spin Hall Effect in Two-Dimensional Methyl-Functionalized Silicene SiCH3.

Quantum Spin Hall (QSH) has potential applications in low energy consuming spintronic devices and has become a researching hotspot recently. It benefits from insulators feature edge states, topologically protected from backscattering by time-reversal symmetry. The properties of methyl functionalized silicene (SiCH3) have been investigated using first-principles calculations, which show QSH effect under reasonable strain. The origin of the topological characteristic of SiCH3, is mainly associated with the s-pxy orbitals band inversion at Γ point, whilst the band gap appears under the effect of spin-orbital coupling (SOC). The QSH phase of SiCH3 is confirmed by the topological invariant Z2 = 1, as well as helical edge states. The SiCH3 supported by hexagonal boron nitride (BN) film makes it possible to observe its non-trivial topological phase experimentally, due to the weak interlayer interaction. The results of this work provide a new potential candidate for two-dimensional honeycomb lattice spintronic devices in spintronics.

Benzimidazobenzothiazole-based highly-efficient thermally activated delayed fluorescence emitters for organic light-emitting diodes: A quantum-chemical TD-DFT study.

Based upon two thermally activated delayed fluorescence (TADF) emitters 1 and 2, compounds 3-6 have been designed by replacing the carbazol group with the bis(4-biphenyl)amine one (3 and 4) and introducing the electron-withdrawing CF3 group into the acceptor unit of 3 and 4 (5 and 6). It is found that the present calculations predict comparable but relatively large energy differences (approximate 0.5eV) between the lowest singlet S1 and triplet T1 states (∆EST) for the six targeted compounds. In order to explain the highly-efficient TADF behavior observed in compounds 1 and 2, the"triplet reservoir" mechanism has been proposed. In addition, the fluorescence rates of all six compounds are very large, in 107-108 orders of magnitude. According to the present calculations, it is a reasonable assumption that the newly designed compounds 3-6 could be considered as the potential TADF emitters, which needs to be further verified by experimental techniques.

Magnetodielectric Response from Spin-Orbital Interaction Occurring at Interface of Ferromagnetic Co and Organometal Halide Perovskite Layers via Rashba Effect.

The spin on a ferromagnetic Co surface can interact with the asymmetric orbital on an organometal halide perovskite surface, leading to an anisotropic magnetodielectric effect. This study presents an opportunity to integrate ferromagnetic and semiconducting properties through the Rasbha effect for achieving spin-dependent electronic functionalities based on thin-film design.

Theoretical characterization and design of highly efficient iridium (III) complexes bearing guanidinate ancillary ligand.

A density functional theory/time-depended density functional theory was used to investigate the synthesized guanidinate-based iridium(III) complex [(ppy)2Ir{(N(i)Pr)2C(NPh2)}] (1) and two designed derivatives (2 and 3) to determine the influences of different cyclometalated ligands on photophysical properties. Except the conventional discussions on geometric relaxations, absorption and emission properties, many relevant parameters, including spin-orbital coupling (SOC) matrix elements, zero-field-splitting parameters, radiative rate constants (kr) and so on were quantitatively evaluated. The results reveal that the replacement of the pyridine ring in the 2-phenylpyridine ligand with different diazole rings cannot only enlarge the frontier molecular orbital energy gaps, resulting in a blue-shift of the absorption spectra for 2 and 3, but also enhance the absorption intensity of 3 in the lower-energy region. Furthermore, it is intriguing to note that the photoluminescence quantum efficiency (ΦPL) of 3 is significantly higher than that of 1. This can be explained by its large SOC value(n=3-4) and large transition electric dipole moment (µS3), which could significantly contribute to a larger kr. Besides, compared with 1, the higher emitting energy (ET1) and smaller (2) value for 3 may lead to a smaller non-radiative decay rate. Additionally, the detailed results also indicate that compared to 1 with pyridine ring, 3 with imidazole ring performs a better hole injection ability. Therefore, the designed complex 3 can be expected as a promising candidate for highly efficient guanidinate-based phosphorescence emitter for OLEDs applications.