Efficient Light-emitting Materials Research Articles

Rational Design of Tetrahedral Derivatives as Efficient Light-Emitting Materials Based on "Super Atom" Perspective.

Traditional semiconductor quantum dots of groups II-VI are key ingredients of next-generation display technology. Yet, the majority of them contain toxic heavy-metal elements, thus calling for alternative light-emitting materials. Herein, we have explored three novel categories of multicomponent compounds, namely, tetragonal II-III2-VI4 porous ternary compounds, cubic I2-II3-VI4 ternary compounds, and cubic I-II-III3-V4 quaternary compounds. This is achieved by judicious introduction of a "super atom" perspective and concurrently varying the solid-state lattice packing of involved super atoms or the population of surrounding counter cations. Based on first-principles calculations of 392 candidate materials with designed crystal structures, 53 highly stable materials have been screened. Strikingly, 34 of them are direct-bandgap semiconductors with emitting wavelengths covering the near-infrared and visible-light regions. This work provides a comprehensive database of highly efficient light-emitting materials, which may be of interest for a broad field of optoelectronic applications.

Design Principle for Tetrahedral Semiconductors and Their Functional Derivatives: Cation Stabilizing Charged Cluster Network

Colloidal quantum dots (QDs) of groups II-VI and III-V are key ingredients for next-generation light-emitting devices. Yet, many of them are heavy-element-containing or indirect bandgap, causing limited choice of environmental friendly efficient light-emitting materials. Herein, we resolve this issue by exploring potential derivatives of the parent semiconductors, thus expanding the material space. The key to success is the discovery of a principle for designing those materials, namely, cation stabilizing charged cluster network. Guided by this principle, three novel categories of cubic materials have been predicted, namely, porous binary compounds, I-II-VI ternary compounds, and I-II-III-V quaternary compounds. Using first-principles calculations, 65 realistic highly stable candidate materials have been theoretically screened. Their structural and compositional diversity enables a wide tunability of emitting wavelength from far-infrared to ultraviolet region. This work enriches the family of tetrahedral semiconductors and derivatives, which may be of interest for a broad field of optoelectronic applications.

4,4'-(Anthracene-9,10-diylbis(ethyne-2,1-diyl))bis(1-methyl-1-pyridinium) Lead Iodide C30H22N2Pb2I6: A Highly Luminescent, Chemically and Thermally Stable One-Dimensional Hybrid Iodoplumbate.

A new one-dimensional hybrid iodoplumbate, namely, 4,4'-(anthracene-9,10-diylbis(ethyne-2,1-diyl))bis(1-methyl-1-pyridinium) lead iodide C30H22N2Pb2I6 (AEPyPbI), is reported here for the first time with its complete characterization. The material exhibits remarkable thermal stability (up to 300 °C), and it is unreactive under ambient conditions toward water and atmospheric oxygen, due to the quaternary nature of the nitrogen atoms present in the organic cation. The cation exhibits strong visible fluorescence under ultraviolet (UV) irradiation, and when its iodide is combined with PbI2, it forms AEPyPb2I6, an efficient light-emitting material, with a photoluminescence emission intensity comparable to that of high-quality InP epilayers. The structure determination was obtained using three-dimensional electron diffraction, and the material was extensively studied by using a wide range of techniques, such as X-ray powder diffraction, diffuse reflectance UV-visible spectroscopy, thermogravimetry-differential thermal analysis, elemental analysis, Raman and infrared spectroscopies, and photoluminescence spectroscopy. The emissive properties of the material were correlated with its electronic structure by using state-of-the-art theoretical calculations. The complex, highly conjugated electronic structure of the cation interacts strongly with that of the Pb-I network, giving rise to the peculiar optoelectronic properties of AEPyPb2I6. The material, considering its relatively easy synthesis and stability, shows promise for light-emitting and photovoltaic devices. The use of highly conjugated quaternary ammonium cations may be useful for the development of new hybrid iodoplumbates and perovskites with optoelectronic properties tailored for specific applications.

3D Integrated Laser Attach Technology on a 300-mm Monolithic CMOS Silicon Photonics Platform

Enabling cost-effective and power-efficient laser source on a silicon photonics (SiPh) platform is a major goal that has been highly sought after. In the past two decades, tremendous effort has been made to develop various on-chip integration techniques to enhance SiPh circuits with efficient light-emitting materials. Here we review our recent advancements in hybrid flip-chip integration of III-V lasers on a 300-mm monolithic SiPh platform. By leveraging advanced complementary metal oxide semiconductor (CMOS) manufacturing processes, we have demonstrated wafer-scale laser attach based on a precisely controlled cavity formed on a silicon-on-insulator (SOI) substrate. The laser integration process is aided by precise mechanical alignment features on the SiPh wafer and high-precision fiducials on the laser. Efficient laser-to-SiPh-circuit butt-coupling with optical power up to 20mW was demonstrated through wafer- and module-level characterizations. Key performance metrics including side-mode suppression ratio, mode-hopping, and relative intensity noise were characterized after laser integration. In addition, early reliability assessments were performed on laser-attached SOI wafers and Si submount assemblies to understand the long-term performance stability of the lasers on the monolithic platform. To further enhance the performance of the laser-integrated chip, we explored alternative spot-size converters that could simultaneously enable improved coupling efficiency and relaxed fabrication tolerance, thus showing great promise over traditional designs.

Nanopatterning by Length-Dependent Self-Assembly from Fluorene-Terpyridine Derivatives

Nanopatterning of photoactive precursors by self-assembly is an efficient nanoscale approach to optimize materials with tailored photoluminescence properties. Ditopic ligands are designed that contain terpyridine terminals connected through one or two photoactive fluorene central units. The tpy-Fluo-tpy and tpy-bisFluo-tpy molecules are self-assembled on HOPG surfaces and are investigated by high-resolution scanning tunneling microscopy, where similar organizations are observed due to the intermolecular terpyridine-terpyridine interactions. By an increase in the number of central fluorenes (single to double), the molecular centers bridges appear as distinct “X” or “S” shapes under STM. Moreover, the adsorption spectra shift toward a longer wavelength and an obvious improvement of the quantum luminescence yield is observed. In the tpy-bisFluo-tpy molecules, the STM tip is functionalized to observe the different HOMO or LUMO electronic contrasts that are interpreted by density functional theory (DFT) calculations. Our work provides a novel nanopatterning approach to organize photoactive precursors with molecular-scale precision for designing efficient light-emitting materials.

Plasma enhanced light emission from the Si+-N+ co-implanted SOI in the violet-blue waveband

Silicon (Si) has been established for a long time as the footing stone of the whole microelectronics industry. This does not come into true in the photonics domain yet, where the limited degrees of freedom in material design and the indirect bandgap are the major constraints. Therefore, making Si as an efficient light-emitting material is an important approach for Si photonics, even for achieving the all-Si-based photoelectronic and microelectronic integration engineering. Here we reported a facile and effective method for enhancing the photoluminescence (PL) from the Si+/N+ ion co-implanted Si film by coating the polystyrene (PS)-sphere@metal complex core-shell structure upon the Si film on insulator (SOI) wafers. The PL enhanced phenomena have been demonstrated in the violet-blue waveband by the three kinds of single-layer PS sphere array capping with different metal films (namely Ag, Au, and Al), respectively. The experiments and theoretic calculation based on the finite difference time domain (FDTD) model indicate that the plasma resonance is responsible for the PL enhancement, and these core-shell structures take advantage of both metal plasma and photonic crystal in the PL enhancing process.

Enhanced luminescence through interface energy transfer in hierarchical heterogeneous nanocomposites and application in white LEDs

Highly efficient light-emitting materials are essential for achieving high-performance devices. Here, a novel composite system, as well as enhanced luminescence processes, was designed, where NaLn(MoO4)2 ultra-small nucleus can be effectively isolated by In(OH)3 to form NaLn(MoO4)2@In(OH)3 composite nanoclusters due to the different nucleation rate between NaLn(MoO4)2 and In(OH)3, and then these small composite clusters gradually self-assemble into hierarchical structures. As we expected, the enhanced luminescence was achieved from hierarchical NaLn(MoO4)2 nanostructures with adjusting the distance among NaLn(MoO4)2 ultra-small nucleus by inserting In(OH)3. A series of spectroscopy results show that the In(OH)3 not only acts as an energy transfer bridge from CTB Eu3+ → O2− (or MoO42− absorption) to Eu3+, but also can effectively alleviate the concentration quenching of Ln3+ and change the J-O parameters. The Raman peak at 134 cm−1 is helpful to populate the 5D0 level of Eu3+ or the excited states of Er3+, resulting in stronger up/down-conversion emissions. The use of NaLn(MoO4)2@In(OH)3 in white light-emitting diodes (LEDs) has been demonstrated. The combination of red emission from NaLn(MoO4)2@In(OH)3 with blue, green, and yellow emission from halide perovskites could achieve white light with excellent vision performance (an LER of 376 lm/W) and superior color quality (CRI > 92). The findings of this experiment provide a new idea for the design of composite interface materials.

High-Performance Perovskite Light-Emitting Diodes with Surface Passivation of CsPbBrxI3–x Nanocrystals via Antisolvent-Triggered Ion-Exchange

Inorganic lead halide perovskite nanocrystals (PeNCs) have intensively drawn the attention as efficient light emitting materials for optoelectronic applications due to fine optoelectronic properties with high photoluminescence quantum yield and easily tunable saturated emission color. However, the low stability of the red-emitting PeNCs have become an obstacle because of the uncontrollable iodine substitution from the PeNCs owing weak Pb-I bonding. In this work, we have demonstrated a ligand-mediated post-treatment (LMPT) method using halide ion-pair ligand, tridodecylmethyl ammonium iodide (TrDAI), for the air stable and high-quality red-emitting PeNCs. Through the LMPT method, the optoelectronic properties of red-emitting PeNCs are dramatically improved resulting in the PLQY of 88.7% at 637 ± 2 nm emission with increased carrier lifetime from 20.77 ns to 31.52 ns. We achieve highly efficient red perovskite light-emitting diodes exhibiting a maximum current efficiency of 7.69 cd A-1 and an external quantum efficiency of 6.36% at 637 ± 2 nm electroluminescence emission with a sharp full-width at half maximum of 31 nm.

Influence of the Anatase and Rutile phases on the luminescent properties of rare-earth-doped TiO2 films

For a long time, the development of new and more efficient light-emitting materials has been the driving force to investigate the presence of trivalent rare-earth (RE3+) ions into solid hosts. It happens because RE3+ ions provide efficient and well-defined optical transitions whose intensities are determined by phenomena regarding excitation−recombination mechanisms or, ultimately, by the RE3+ atomic surroundings. Motivated by these aspects, this paper presents a comprehensive study of the influence exerted by TiO2 (under the amorphous and crystalline forms) onto the light emission due to Sm3+ and Tm3+ ions. To this end, Sm-, Tm-, and SmTm-doped TiO2 films were prepared and their structural−electronic properties were investigated by optical spectroscopic techniques − following appropriate thermal annealing treatments. As a result of the deposition method and conditions, the as-deposited (stoichiometric) TiO2 films present an amorphous structure and single RE concentrations around 0.5 at.%. Also, whereas thermal annealing the films at increasing temperatures resulted in almost no changes in their elemental composition, extensive variations were observed in their atomic structure and RE3+-related light emission characteristics. Based on the experimental results, it became clear the role played by the amorphous or crystalline (Anatase or Rutile) structures of the TiO2 films onto the RE3+-related luminescence features, and the likely mechanisms behind the phenomenon are presented and discussed in detail.

The unsuspected influence of the pyridyl-triazole ligand isomerism upon the electronic properties of tricarbonyl rhenium complexes: an experimental and theoretical insight.

Two isomeric tricarbonyl rhenium(i) complexes, ReL1 and ReL2, that possess a 2-pyridyl-1,2,n-triazole (pyta) ligand (n = 4 and 3, respectively) connected to a 2-phenylbenzoxazole (PBO) moiety, were synthesized in good yields. The X-ray structures showed that in ReL1 the PBO moiety and the pyta ligand almost form a right angle hindering electron delocalization, while in ReL2 their nearly planar arrangement favors the electron delocalization in the whole organic ligand. Therefore, the nature of the ligand significantly influences the electron distribution in the two complexes, as indicated by the results of TD-DFT calculations. An electrochemical study highlighted that, by comparison with ReL2, the smaller HOMO-LUMO energy gap of ReL1 is in line with its lower first reduction potential. From a spectroscopic viewpoint, both complexes emitted phosphorescence in organic solvents, with distinct color and intensity. They also emitted in the solid state, but only ReL1 showed significant aggregation-induced phosphorescence emission (AIPE). This complete study sheds light on the crucial role of structural isomerism of the triazole group, which has been unsuspected for a long time although it can govern the geometry and electronic properties of rhenium complexes. It is shown for the first time that grafting a non-coordinated π-conjugated fragment on the N(4) atom of a 1,2,4-triazole group can be of high value for the design of efficient light-emitting materials based on rhenium complexes.

Broadband infrared photoluminescence in silicon nanowires with high density stacking faults.

Making silicon an efficient light-emitting material is an important goal of silicon photonics. Here we report the observation of broadband sub-bandgap photoluminescence in silicon nanowires with a high density of stacking faults. The photoluminescence becomes stronger and exhibits a blue shift under higher laser powers. The super-linear dependence on excitation intensity indicates a strong competition between radiative and defect-related non-radiative channels, and the spectral blue shift is ascribed to the band filling effect in the heterostructures of wurtzite silicon and cubic silicon created by stacking faults.

Synthesis and luminescent properties of star-burst D-π-A compounds based on 1,3,5-triazine core and carbazole end-capped phenylene ethynylene arms

Two new star-burst compounds based on 1,3,5-triazine core and carbazole end-capped phenylene ethynylene arms (1a and 1b) were synthesized and characterized. Their photophysical properties were investigated systematically via spectroscopic and theoretical methods. Both compounds exhibit strong 1π–π⁎ transitions in the UV region and intense 1π–π⁎/intramolecular charge transfer (1ICT) absorption bands in the UV–vis region. Introducing the carbazole end-capped phenylene ethynylene arm on the 1,3,5-triazine core causes a slight bathochromic shift and enhanced molar extinction coefficient of the 1π–π⁎/1ICT transition band. Both compounds are emissive in solution at room temperature and 77K, which exhibit pronounced positive solvatochromic effect. The emitting state could be ascribed to 1ICT state in more polar solvent, and 1π–π⁎ state in low-polarity solvent. The high emission quantum yields (Φem=0.90~1.0) of 1a and 1b (in hexane and toluene) make them potential candidates as efficient light-emitting materials. The spectroscopic studies and theoretical calculations indicate that the photophysical properties of these compounds can be tuned by the carbazole end-capped phenylene ethynylene arm, which would also be useful for rational design of photofunctional materials.

Unsymmetrical Dendrimers as Highly Efficient Light-Emitting Materials: Synthesis, Photophysics, and Electroluminescence

Highly efficient light-emitting materials can be archived by a rational design that involves rigid dendron and π-conjugated chromophores. This strategy is exemplified by three unsymmetrical dendrimers 5-(2′,3′,4′,5′-tetraphenyl)phenyl-5′-(9- anthracenyl)-2,2′-bithiophene (1), 5-(2′,3′,4′,5′-tetraphenyl)phenyl-5′-(1-pyrenyl)-2,2′ -bithiophene (2), and 5-(2′,3′,4′,5′-tetraphenyl)phenyl-5′-(1-pyrenylethynyl)-2,2′- bithiophene (3). Thermogravimetric analysis and differential scanning calorimetry suggest that unsymmetrical dendrimers 1 and 2 have good thermal stability (>400 °C) and high glass transition temperature (>130 °C) which make them promising candidates for highly stable organic light-emitting devices. The comparative study on their UV−vis absorption and fluorescence spectra in solution and as thin films indicates that the unsymmetrical dendrimers can form intermolecular π−π stacking in the solid state. Electroluminescence (EL) devices using them as emitters were made, and it was found that they emit ...

Light-Emitting Efficiency Tuning of Rod-Shaped π Conjugated Systems by Donor and Acceptor Groups

In view of the increasing importance of highly efficient light-emitting materials in chemistry, biological science, and materials science, we investigated the light-emitting efficiency tuning of rod-shaped oligo(p-phenylene ethynylene)s (OPEs, trimeric to pentameric systems) by donor and acceptor groups, so that they emit the very intense fluorescence (Phif approximately 1.0, log epsilon approximately 5) at 460 nm as the desired wavelength region. This goal was achieved by side modification by MeO (donor) groups and end modification by a CN-substituted benzene ring or CF3-substituted pyridine ring (acceptor) of tetrameric p-phenylene ethynylene rod-shaped molecules (Phif = 0.96, lambdaem = 458 nm, log epsilon = 4.96 for the former and Phif = 0.99, lambdaem = 459 nm, log epsilon = 4.92 for the latter). The high Phif values for 11 and 12 are interpreted in terms of kr (radiative rate constant) and kd (radiationless rate constant). The linear relationship with a positive slope between Phif and the Hammett sigma constant was found for the first time. It is found that kd rather than kr varies with sigmap-X. The photophysical properties (Phif, lambdaem, lambdaabs, log epsilon) were not so altered with the solvent polarity, which could be explained by the dipole moments in the excited and ground states. The results would be valuable for the molecular design of highly efficient light-emitting materials.

Synthesis of efficient blue and red light emitting phenanthroline derivatives containing both hole and electron transporting properties

Two phenanthroline derivatives containing a hole transporting triphenylamine and an electron transporting 1,3,4-oxadiazole unit have been prepared with high yield. UV–vis absorption and fluorescence measurement indicated they are high efficient light-emitting materials. The compounds are 6-(5-(4- N, N′-diphenylaminophenyl)-1,3,4-oxadiazol-2-yl) quinoxalino[2,3- f] phenanthroline ( 9 , λ max = 635 nm, 40% quantum yield), and 1-ethyl-2-(4-(5-(4- N, N′-diphenylaminophenyl)-1,3,4-oxadiazol-2-yl)phenyl)imidazo[4,5- f]phenanthroline ( 14 , λ max = 461 nm, 78% quantum yield). Preliminary study on electroluminescence for the two fluorescent dyes prepared from vacuum evaporation resulted in blue and red light emitting organic light emitting diodes (OLED).