Red-shifted Photoluminescence Research Articles

Luminescence redshift of thick InGaN/GaN heterostructures induced by the migration of surface adsorbed atoms

Nitride heterostructures with an extremely high number of InGaN/GaN multiple quantum wells (MQWs) were grown and studied by multiple techniques. Large redshift (282 meV) in photoluminescence (PL) spectra was observed with an increasing number of quantum wells (QWs) in the structure from 10 to 60. From the comparison of structures grown on sapphire and GaN substrates, the phenomenon of giant redshift is explained. A theory based on the surface migration of the adsorbed atoms from the semi-polar V-pit facets to the c-plane facets is suggested and proven by several experimental techniques including scanning transmission electron microscopy (STEM), secondary ion-mass spectroscopy (SIMS), secondary electron microscopy (SEM), white light interferometry (WLI), and spectrally and spatially resolved cathodoluminescence (SSRCL). Finally, a change in the surface morphology caused by the growing size of the V-pits and the transformation of the c-plane surface areas is discussed.

Self-assembly and phosphorescence of ionic<italic> </italic>Pd(II) <italic>N</italic>-heterocyclic allenylidene complexes in nonpolar solvents

<p indent=0mm>In sharp contrast with the rich phosphorescent Pt(II) complexes reported in the literatures, phosphorescent Pd(II) complexes in room temperature solutions are scarce, in spite of having the same d⁸ electronic configuration and square-planar coordination mode. Due to their low-lying d-d transition state, phosphorescence origin of the Pd(II) complexes are usually ligand-based, rather than much metal participation. Recently, it has been proved to be an effective strategy to construct self-assembled aggregates via metallophilic Pd···Pd interactions for Pd(II) complexes to achieve metal-metal-to-ligand charge transfer (MMLCT) and corresponding phosphorescent emission in room temperature fluid state. Herein, a cationic <italic>N</italic>-heterocyclic allenylidene (NHA) Pd(II) complex <bold>3</bold>·PF₆ has been synthesized via a facile two-step transmetalation reaction of NHA precursor 2-ethynyl-1,3-dimethyl-1H-imidazol-3-ium hexafluorophosphate (<bold>1</bold>·PF₆) and a pincer type [(C^N^N)PdCl] (HC^N^N = 6-phenyl-4-(3,4,5-tris(dodecyloxy)phenyl)-2,2′-bipyridine) <bold>2</bold> with long alkyl chains in the presence of excessive amount of Ag₂O. It is stable towards air, moisture and common organic solvents. The purity of the cationic NHA Pd(II) complex <bold>3</bold>·PF₆ was characterized via NMR, HR-MS. And IR spectra revealed its resonance structure between zwitterionic acetylide and <italic>N</italic>-heterocyclic allenylidene to some extent. Solid state complex <bold>3</bold>·PF₆ showed orange emission with microsecond lifetime at both room temperature and <sc>77 K.</sc> Temperature-dependent PXRD and TGA/DSC analysis demonstrated the changes of complex <bold>3</bold>·PF₆ from an amorphous phase to a stable crystalline one upon raising the temperature. Due to the lipophilic property of the long alkyl chains, ionic complex <bold>3</bold>·PF₆ can be soluble in both polar (such as dimethyl sulfoxide (DMSO), CH₃CN, and dichloromethane) and nonpolar (such as cyclohexane, methylcyclohexane, and decalin) organic solvent, despite its ionic nature. Complex <bold>3</bold>·PF₆ in degassed dilute DCM solution showed rather weak photoluminescence with peak maximum at <sc>562 nm</sc> (QY < 1%, <italic>τ</italic>: <sc>17 ns).</sc> By increasing the volumetric ratio of cyclohexane which lowered solvent polarity, UV-vis absorption spectra of <bold>3</bold>·PF₆ in mixed solvent of DCM/cyclohexane showed a distinct low-energy absorption band with peak maximum at approximately <sc>500 nm,</sc> and the corresponding phosphorescence emerged. The NMR spectra recorded in mixed solvent deuterium substituted DCM/cyclohexane also showed shifted ¹H chemical-shift and broaden peaks by increasing the volumetric ratio of cyclohexane, which revealed the deeper degree of aggregation of <bold>3</bold>·PF₆ by decreasing the solvent polarity. Meanwhile, enhancement of red-shifted photoluminescence reached its largest absolute QY of 35.4% with emission peak maximum at <sc>612 nm</sc> and lifetime of <sc>730 ns</sc> in pure cyclohexane. The photophysical data recorded for <bold>3</bold>·PF₆ aggregates in other nonpolar solvents (such as toluene and decalin) showed similar broad and featureless phosphorescent emission band with peak maximum at around <sc>620 nm.</sc> The origin of the phosphorescence for <bold>3</bold>·PF₆ aggregates in nonpolar solvents can be ascribed to MMLCT enabled by the metallophilic Pd···Pd interactions. To further explore the self-assembly mode of <bold>3</bold>·PF₆ in nonpolar solvents, the corresponding temperature-dependent UV-vis absorption spectra have been recorded. The distinct low-energy absorption bands represented the aggregation degrees of the <bold>3</bold>·PF₆ monomer in the fluid state. The plot of variable-temperature UV-vis absorbance at the distinct low-energy band (peak maximum at <sc>504 nm)</sc> against temperature well-fits the nucleation-elongation model for the self-assembly of <bold>3</bold>·PF₆ in nonpolar solvent (such as methylcyclohexane and decalin). In conclusion, we have synthesized an ionic Pd(II) <italic>N</italic>-heterocyclic allenylidene complex with lipophilic long alkyl chains and manipulated its self-assembly behavior in nonpolar solvents by controlling the polarity and temperature. Corresponding MMLCT phosphorescence was also recorded and correlated with metallophilic Pd···Pd interactions in the aggregates.

Compositional Inhomogeneity in AlGaN Multiple Quantum Wells Grown by Molecular Beam Epitaxy: Effect on Ultraviolet Light-Emitting Diodes

AlGaN-based multiple quantum well structures grown by plasma-assisted molecular beam epitaxy were investigated for their use in light-emitting diode (LED) structures. Initial testing was carried out on 20 pairs of AlxGa(1−x)N/AlyGa(1−y)N quantum well structures with different levels of compositional inhomogeneities in the alloy, introduced by appropriate choice of deposition conditions. It was observed that these conditions did not change the overall absorption edge, but generated a strong red-shifted photoluminescence peak. A series of LEDs with similar quantum well structures in the active region were grown, fabricated and tested. Our results indicate that deliberate introduction of compositional inhomogeneities in the quantum wells increased their electroluminescence intensity by an order of magnitude, but similar to the photoluminescence results, shifted the peak from 320 nm to 350 nm. Additionally, for these devices, no efficiency reduction was observed up to an injection of 60 mA for a device area of 0.25 mm2. These improvements may be attributed to carrier trapping mechanisms in the AlGaN well layers at deliberately introduced deep potential minima, which are likely to arrest both their overflow and their transport to extended defect states. The growth conditions also improve the p-doping efficiency and reduce the probability of leakage of carriers into the p-type layers through defect states. However, the increased efficiency and reduced droop comes at the cost of a red-shift of the electroluminescence peak.

Study the optoelectronic properties of PbI2 nanorods/Si photodetector prepared by magnetic field-assisted laser deposition route

Here, we report the synthesis of lead iodide (PbI2) nanorods (NRs) by using magnetic field-assisted pulsed laser deposition (PLD). The structural, morphological, and optical properties of the films were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL), Raman spectroscopy, and UV–Vis absorption. XRD results revealed that the deposited film was (001)-oriented polycrystalline with hexagonal structure (2H-polytype). The film crystallinity has been improved when it deposited under the effect of an external magnetic field. SEM showed that the film morphology changed completely from spherical nanosized to 450 nm long grain nanorods with 60 nm average diameter. The films’ optical energy gap before and after the application of magnetic field were 2.4 and 3.3 eV, respectively. The film exhibited red-shifted intense PL emission centered at 2.5 eV at a magnetic field of 0.4 T. The figures of merit of the photodetector were measured with and without the magnetic field. The responsivity of the PbI2/n-Si photodetector at λ = 560 nm increased from 0.22 A/W to 0.58 A/W at 7.5 V bias when magnetic field was applied during the deposition. The rise time of the photodetector prepared with magnetic field and without magnetic field was measured at 3 V bias.

2π + 2π] Photocycloaddition of Enones to Single-Walled Carbon Nanotubes Creates Fluorescent Quantum Defects.

Single-walled carbon nanotubes (SWCNTs) have been widely applied in biomedical fields such as drug delivery, biosensing, bioimaging, and tissue engineering. Understanding their reactivity with biomolecules is important for these applications. We describe here a photoinduced cycloaddition reaction between enones and SWCNTs. By creating covalent and tunable sp3 defects in the sp2 carbon lattice of SWCNTs through [2π + 2π] photocycloaddition, a bright red-shifted photoluminescence was gradually generated. The photocycloaddition functionalization was demonstrated with various organic molecules bearing an enone functional group, including biologically important oxygenated lipid metabolites. The mechanism of this reaction was studied empirically and using computational methods. Density functional theory calculations were employed to elucidate the identity of the reaction product and understand the origin of different substrate reactivities. The results of this study can enable engineering of the optical and electronic properties of semiconducting SWCNTs and provide understanding into their interactions with the lipid biocorona.

Green preparation of carbon dots with different surface states simultaneously at room temperature and their sensing applications

It is a considerable challenge to develop environmental friendly, low-cost methodology for green preparation of carbon dots (CDs). Herein, CDs with different surface states are prepared using o-phenylenediamine (o-PD) and hydroquinone (HQ) as precursors via oxidation/polymerization and Schiff base reaction at room temperature without additional oxidizing agents. Two CDs products (YCDs and GCDs) are obtained after separation with silica gel column chromatography based on their polarity differences. The different surface states endow these two CDs with different properties. The rich NO2 and OH groups on the surface of YCDs contribute to a narrow band gap, resulting in the red-shifted photoluminescence (PL) emission of this CDs product, making it a sensitive probe for the detection of toxic pollutant p-nitrophenol (p-NP) attributed to the inner filter effect, along with a detection limit of 0.08 μmol/L. GCDs are characterized with abundant surficial NH2 groups, and can be used as a potential probe to detect H2O content in D2O, giving a detection limit of 0.17 vol%.

Strain-mediated unusual bandgap bowing in continuous composition tuned monolayer Mo1−xWxS2 ternary alloys

Band gap engineering via 2D alloying is a vital strategy for three-atom-thick transition metal dichalcogenides based optoelectronics, valleytronics and nanophotonics. Here we demonstrate the growth of Mo1−xWxS2 ternary alloy monolayers and precise compositional tuning for the entire range of x from 0 to 1 using the gas-phase precursor approach. By means of Raman spectroscopy we show that W alloying in MoS2 lattice can lead to a tensile strain of ∼0.8%. The alloying-induced tensile strain plays a key role in observing redshift in optical absorption and photoluminescence (PL) bands and resulted an unusual bandgap bowing. The coupling of tensile strain and alloying effect allowed us to tune the overall PL emission energy to as large as 185 meV. Our optical spectroscopy results indicate three different phase-regions for the Mo1−xWxS2 alloy system. For x &amp;lt; 0.37, the alloys exhibit MoS2-like nature, whereas, WS2-like behavior is observed for x &amp;gt; 0.64, and a mixed behavior for 0.37 ≤ x ≤ 0.64.

Concentration‐Dependent Photoluminescence Properties of Graphene Oxide

Herein, a comprehensive examination of the effect of concentration on graphene oxide (GO) photoluminescence (PL) is presented. It is found that GO exhibits a concentration‐induced redshift and white light PL for GO aqueous dispersions, and displays visible–NIR PL for a 3D GO foam (3DGOF) corresponding to the high‐concentration‐limit of GO. It is deduced that the effect of the coupling between the GO sheets gives rise to the observed redshift, and hopping and tunneling effects result in the saturation of the PL redshift and then the PL quenching. Due to the intense interactions and coupling of the GO sheets, the modification of the GO surface state increases the energy of the π* state and/or makes the high‐energy level appear and therefore gives rise to the blue light component. Consequently, high concentrations of GO aqueous dispersion lead to the appearance of combined white light. For 3DGOF, electron–hole radiation recombination in laser‐induced plasma and GO bandgap may be responsible for the observed visible–NIR PL. Based on its tunable PL in the visible–NIR region of the spectrum and the induced white light PL, GO has huge potential for applications in graphene‐based optoelectronics and biomedicine.

MLCT Excited-State Behavior of Trinuclear Ruthenium(II) 2,2'-Bipyridine Complexes.

Four trinuclear ruthenium(II) polypyridyl complexes were synthesized, and a detailed investigation of their excited-state properties was performed. The tritopic sexi-pyridine bridging ligands were obtained via para or meta substitution of a central 2,2'-bipyridine fragment. A para connection between the 2,2'-bipyridine chelating moieties of the bridging ligand led to a red-shifted MLCT absorption band in the visible part of the spectra, whereas the meta connection induced a broadening of the LC transitions in the UV region. A convergent energy transfer from the two peripheral metal centers to the central Ru(II) moiety was observed for all trinuclear complexes. These complexes were in thermal equilibrium with an upper-lying 3MLCT excited state over the investigated range of temperatures. For all complexes, deactivation via the 3MC excited state was absent at room temperature. Importantly, the connection in the para position for both central and peripheral 2,2'-bipyridines of the bridging ligand resulted in a trinuclear complex (Tpp) that absorbed more visible light, had a longer-lived excited state, and had a higher photoluminescence quantum yield than the parent [Ru(bpy)3]2+, despite its red-shifted photoluminescence. This behavior was attributed to the presence of a highly delocalized excited state for Tpp.

Optical, electrical, and chemical characterization of nanostructured InxGa1-xN formed by high fluence In+ ion implantation into GaN

Abstract In this work, we have performed and analyzed a nanometric layer of InxGa1-xN to be used on a photodetector by the high fluence low energy In + ion implantation into GaN. Secondary ion mass spectrometry (SIMS) elemental analysis revealed a maximal In concentration around 1021 cm−3 for a fluence and energy of 4.48 × 1016 cm−2 and 50 keV respectively. After annealing, the implanted sample showed strong red shift photoluminescence centered at 2.1eV. Photoluminescence was attributed to the formation of InxGa1-xN nanocrystals with high In content around 42%. The formation of InxGa1-xN due to implantation of In + ions was demonstrated by Raman, X-ray photoelectron spectroscopy (XPS), photo-reflectance, and photoluminescence. Furthermore, the nanostructured material was used to fabricate a Schottky photo-diode with a leakage current about 4 × 10−8 A.

Density functional calculations to understand Ba2-xEuxSiO4 photoluminescence thermal stability

The green broad emission of Ba2-xEuxSiO4 can be deconvoluted into two peaks, which are related to Eu2+(I, 10-coordinated) and Eu2+(II, 9-coordinated) respectively. With Eu2+ concentration increasing (x = 0.01→0.05), the green emission become stronger, more thermally stable and red-shifted. The density functional theory (DFT) calculation was used to find the explanation for the phenomenon. The calculation results show that Eu2+ ions have site preference in Ba2SiO4 crystal, as more likely being Eu2+(I) than Eu2+(II). Compared Eu2+(II) with Eu2+(I), Edf (4f65d-4f7 electronic transition) is smaller but EdC (energy difference between 5d energy level and conductive bottom) is larger. It can be deduced that Eu2+ ion do-pant would start as Eu2+(I, shorter emission wavelength, less thermal stable), and then acts as Eu2+(II, longer emission wavelength, more thermal stable) when Eu2+ ions are more and more in the crystal. The calculation results provide a reasonable explanation for photoluminescence red-shift and thermal stability improvement with Eu2+ ion content increasing.

Short-wave infrared (SWIR) photodetection by InAs/GaAs quantum dot heterostructures grown on Ge (100) substrate without Migration Enhanced Epitaxy layer

Integration of III-V based optoelectronic devices with Silicon (Si) technology is the prime choice of researchers for the futuristic Si-photonics. Profusion research is going on for direct growth of III-V heterostructures on the Germanium (Ge) substrate. Also, monolithic integration of III-V based device structures on Si using Ge layer and Migration Enhanced Epitaxy (MEE) technique have been endeavored. In this study, InAs based quantum dot infrared photodetectors (QDIPs) are grown on Ge substrate without MEE layers. The GaAs/AlAs superlattice buffer (SLB) layers are used to suppress the defects and dislocations. Two different QDIPs with GaAs capped and In 0.15 Ga 0.85 As/GaAs capped InAs QD are grown and characterized using photoluminescence (PL), high-resolution X-Ray Diffraction (HRXRD), and Raman spectroscopy (RS). Red-shifted and narrow PL spectrum of the InGaAs/GaAs capped QDIP with respect to the GaAs capped QDIP attributes to formation of larger dots and homogeneous size distribution. Low strain and high activation energy in the InGaAs capped QDs signify better strain relaxation via the InGaAs strain reducing layer (SRL) and better carrier confinement. Interband-assisted near-infrared (NIR) photoresponse is obtained from both devices till room temperature. However, the InGaAs/GaAs capped QDIP shows shortwave infrared (SWIR) response, which promulgates the formation of QDs with better crystallinity due to the presence of InGaAs SRL. The room temperature (300 K) dark current density of the GaAs and InGaAs capped QDIP is 1.2 × 10 −1 A/cm 2 and 6.6 × 10 −3 A/cm 2 respectively with −2 V bias voltage. This work demonstrates the pathway to uncomplicated growth of high quality QDIP structures on Ge substrate. • A novel technique is used to grow defect free InAs QD structures on Ge substrate. • GaAs/AlAs SLB layers are used to suppress the defects and dislocations. • InAs QDs with GaAs and InGaAs/GaAs capping are investigated and compared. • Both structures show low dark current at room temperature. • Infrared photoresponse till room temperature is obtained for both QDIPs.

Pressure-Engineered Optical and Charge Transport Properties of Mn2+/Cu2+ Codoped CsPbCl3 Perovskite Nanocrystals via Structural Progression.

In this work, compared with the corresponding pure CsPbCl3 nanocrystals (NCs) and Mn2+-doped CsPbCl3 NCs, Mn2+/Cu2+-codoped CsPbCl3 NCs exhibited improved photoluminescence (PL) and photoluminescence quantum yields (PL QYs) (57.6%), prolonged PL lifetimes (1.78 ms), and enhanced thermal endurance (523 K) as a result of efficient Mn2+ doping (3.66%) induced by the addition of CuCl2. Furthermore, we applied pressure on Mn2+/Cu2+-codoped CsPbCl3 NCs to reveal that a red shift of photoluminescence followed by a blue shift was caused by band gap evolution and related to the structural phase transition from cubic to orthorhombic. Moreover, we also found that under the preheating condition of 523 K, such phase transition exhibited obvious morphological invariance, accompanied by significantly enhanced conductivity. The pressure applied to the products treated with high temperature enlarged the electrical difference and easily intensified the interface by closer packaging. Interestingly, defect-triggered mixed ionic and electronic conducting (MIEC) was observed in annealed NCs when the applied pressure was 2.9 GPa. The pressure-dependent ionic conduction was closely related to local nanocrystal amorphization and increased deviatoric stress, as clearly described by in situ impedance spectra. Finally, retrieved products exhibited better conductivity (improved by 5-6 times) and enhanced photoelectric response than those when pressure was not applied. Our findings not only reveal the pressure-tuned optical and electrical properties via structural progression but also open up the promising exploration of more amorphous all-inorganic CsPbX3-based photoelectric applications.

Synthesis of Carbazole-Fused Azaborines via a Pd-Catalyzed C-H Activation-Cyclization Reaction

Abstract Carbazole-fused azaborines were synthesized via a Buchwald-Hartwig amination followed by a Pd-catalyzed C-H activation-cyclization reaction. These azaborines exhibited red-shifted absorptions and photoluminescence emissions compared to those of dibenzoazaborines, suggesting the efficient HOMO-LUMO energy gap decrease by the carbazole-annulation. The carbazole-fused azaborines showed improved electrochemical stability compared with the previously reported dibenzoazaborine derivatives.

Anomalous photoluminescence quenching in DIP/MoS2 van der Waals heterostructure: Strong charge transfer and a modified interface

Due to the fascinating optoelectronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) and the flexible nature, vast library, and easy fabrication process of organic molecules, the combination of 2D TMD materials and organic molecules have been gaining enormous attention in recent years. The organic semiconductors can enhance the optoelectronic and improved transport properties by interfacing with atomically thin TMDs through vdW interaction. Here, we investigate the photoluminescence (PL) properties of type-II heterostructure made of monolayer molybdenum disulfide (ML MoS2) and a non-planar Diindenoperylene(DIP) molecules. A drastic quenching of PL intensity nearly 300% is observed with slightly redshift in DIP/MoS2as compared to the PL of isolated MoS2. The observed quenching behavior in DIP/MoS2heterostructure can be explained by the appeared ‘trap – like’ states in density of states (DOS), and suppression of exciton to trion ratio due to electron transfer from the lowest unoccupied molecular orbital (LUMO) level of DIP to the conduction band minima (CBM) of MoS2. Furthermore, the redshift in the PL of heterostructure could be due to weak coupling strength which is ~8 times smaller than that of the energy of A-exciton resonance of individual layers and dielectric screening effect induced by the non-planar DIP based organic layer. The expected strong interaction between the DIP molecule and MoS2 layer due to the permanent dipole moments (significant molecular quadruple moment 8.25D) of DIP molecules is another reason, which may slightly reduce the bandgap and results in red-shift of PL. We observed Raman (A1g phonon mode) is blue-shifted, which is supposed to stiffening due to the non-planar DIP molecule onto ML MoS2. The strong PL quenching behavior of DIP/MoS2heterostructure can be a potential candidate for tailoring the optoelectronic properties of TMDs that may demonstrate for light-harvesting and canserve as the next generation TMD/organic-based photovoltaic applications.

Controlled Size Growth of Thermally Stable Organometallic Halide Perovskite Microrods: Synergistic Effect of Dual-Doping, Lattice Strain Engineering, Antisolvent Crystallization, and Band Gap Tuning Properties.

Organometallic halide perovskites, as the light-harvesting material, have been extensively used for cost-effective energy production in high-performance perovskite solar cells, despite their poor stability in the ambient atmosphere. In this work, methylammonium lead iodide, CH3NH3PbI3, perovskite was successfully doped with KMnO4 using antisolvent crystallization to develop micrometer-length perovskite microrods. Thus, the obtained KMnO4-doped perovskite microrods have exhibited sharp, narrow, and red-shifted photoluminescence band, as well as high lattice strain with improved thermal stability compared to undoped CH3NH3PbI3. During the synthesis of the KMnO4-doped perovskite microrods, a low boiling point solvent, anhydrous chloroform, was employed as an antisolvent to facilitate the emergence of controlled-size perovskite microrods. The as-synthesized KMnO4-doped perovskite microrods retained the pristine perovskite crystalline phases and lowered energy band gap (∼1.57 eV) because of improved light absorption and narrow fluorescence emission bands (fwhm < 10 nm) with improved lattice strain (∼4.42 × 10–5), Goldsmith tolerance factor (∼0.89), and high dislocation density (∼5.82 × 10–4), as estimated by Williamson–Hall plots. Thus, the obtained results might enhance the optical properties with reduced energy band gap and high thermal stability of doped-perovskite nanomaterials in ambient air for diverse optoelectronic applications. This study paves the way for new insights into chemical doping and interaction possibilities in methylamine-based perovskite materials with various metal dopants for further applications.

Unusual Red-Shift and Enhanced Photoluminescence of BaMgAl10O17:Eu2+ Phosphor Under Ultraviolet A Excitation for Modern Lighting Systems.

An unconventional red-shift but enhanced photoluminescence (PL) under ultraviolet A (UV-A) irradiation of Eu2+ doped Barium Magnesium Aluminate (BAM) phosphor prepared in both bulk and nanoforms useful for modern lighting applications has been presented. The solid-state reaction and solution combustion approaches were used for the preparation of phosphors with post-annealing step in reduced atmosphere. A significant broad blue-green (˜500 nm) PL associated with the transition of Eu2+ from 4f6 5d1 excited state to the 4f7 ground state has been observed. The observed shifts and PL intensities were found to be extremely reliant on the thermal processing parameters during the synthesis of phosphor/nanophosphormaterials. It's also important to note that the size of the phosphor particles have significant role in defining the red-shift of PL due to quantum confinement effect. Detailed structural and morphological characterizations were also done in this paper. The results are promising and suggest that the BAM phosphor is highly desirable for enhancing the brightness levels in modern lighting and display systems.

Large-scale synthesis of CH3NH3BF4 crystal and its application on CH3NH3PbBrx(BF4)(3-x) perovskite thin films

A simple and environment friendly method for the synthesization of CH3NH3BF4, which exhibits high purity and yield. With close ionic radius, BF4− was doped into CH3NH3PbBr3 perovskite as substitution of Br− to improve the chemical stability of lead halide perovskite. The results show that the doped-BF4− can increase the charge carrier lifetime of CH3NH3PbBrx(BF4)(3-x) perovskite thin films, meanwhile decrease the rate of recombination. This is mainly due to the larger ionic radius of BF4−, which brings about a lower degree X-ray diffraction angle shift, wider ultraviolet visible spectrophotometer absorbance band, red-shift photoluminescence emission peak, and longer fluorescence lifetime.

Direct conjugation of distinct carbon dots as Lego-like building blocks for the assembly of versatile drug nanocarriers

As a promising drug nanocarrier, carbon dots (CDs) have exhibited many excellent properties. However, some properties such as bone targeting and crossing the blood–brain barrier (BBB) only apply to a certain CD preparation with limited drug loading capacity. Therefore, it is significant to conjugate distinct CDs to centralize many unique properties on the novel drug nanocarrier. Considering that CDs have abundant and tunable surface functionalities, in this study, a direct conjugation was initiated between two distinct CD models, black CDs (B-CDs) and gel-like CDs (G-CDs) via an amidation reaction. As a result of conjugation at a mass ratio of 5:3 (B-CDs to G-CDs) and a two-step purification process, the conjugate, black-gel CDs (B-G CDs) (5:3) inherited functionalities from both CDs and obtained an enhanced thermostability, aqueous stability, red-shifted photoluminescence (PL) emission, and a figure-eight shape with a width and length of 3 and 6 nm, respectively. In addition, the necessity of high surface primary amine (NH2) content in the CD conjugation was highlighted by replacing G-CDs with other CDs with lower surface NH2 content. Meanwhile, the carboxyl groups (COOH) on G-CDs were not enough to trigger self-conjugation between G-CDs. Moreover, the drug loading capacity was enhanced by 54.5% from B-CDs to B-G CDs (5:3). Furthermore, when the mass ratio of B-CDs to G-CDs was decreased from 5:30, 5:100 to 5:300, the obtained nanostructures revealed a great potential of CDs as Lego-like building blocks. Also, bioimaging of zebrafish demonstrated that various B-G CDs exhibited properties of both bone targeting and crossing the BBB, which are specific properties of B-CDs and G-CDs, respectively.

Concerted Photoluminescence of Electrochemically Self-Assembled CuSCN / Stilbazolium Dye Hybrid Thin Films

Whether they are inorganic or organic, the key words for creation of new functional materials should be (1) solution, (2) low temperature, and (3) rare-element-free for sustainable development. As a leading example, we explore electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films. In addition to the interests to study evolution of unique hybrid structures, concerted new functionalities are anticipated due to intimate interaction between the inorganic and organic constituents. Herein, we report ESA of CuSCN / 4-(N,N-dimethylamino)-4’-(N’-methyl)stilbazolium (DAS) hybrid thin films and its concerted photoluminescence (PL) behavior [1]. Hybrid thin films of crystalline CuSCN/DAS in three distinctively different nanostructures were obtained by ESA from a single pot containing all the chemical ingredients. Adsorption of DAS during the electrochemical precipitation of CuSCN result in significant change of film morphology, crystal structure and its orientation, as DAS “occluded” into crystal grains of β-CuSCN, DAS phase separated nano-“haircomb” shape β-CuSCN, and DAS phase-separated in nano-“scale” shape α-CuSCN, by simply increasing DAST concentration in the bath [2]. Their optical properties for UV-vis-NIR absorption, photoluminescence (PL), and PL excitation (PLE) spectra were examined between 77 and 298 K, in comparison with solution and solid powder of DAS tosylate (DAST). While all the fluorescent dyes we previously found to hybridize with CuSCN underwent quenching due to hole injection from dye excited state to the valence band of CuSCN, DAS presented the first and only example to exhibit PL when combined with CuSCN [3]. DAST exhibited a strong exciton-phonon coupling to weaken, broaden, and red shift PL at room temperature, so that it inversely is strongly enhanced, sharpened, and blue-shifted at 77 K. However, the PL of the same dye in the hybrid thin film shows a slight red shift and not much intensified. Dielectric environment as well as ordered alignment of DAS greatly stabilize exciton against thermalization loss by suppressing twisted intramolecular charge transfer (TICT) and exciton-phonon coupling. The smallest temperature dependence, thus to be interpreted as the strongest exciton stabilization, was found for the “haircomb” β-CuSCN/DAS hybrid. The PLE spectra of the “scale” α-CuSCN/DAS hybrid show a prominent sharp peak at 380 nm and another double-peak between 400 and 450 nm by cooling temperature. The former corresponds to the band-edge absorption of CuSCN, while the latter being similar to the LMCT absorption of [Cu(SCN)]+ complex. It has been confirmed that those peaks in PLE spectra indicate the contribution of CuSCN thin film without DAS for the generation of PL. These contributions of the light absorption by CuSCN to the PL from DAS clearly indicate energy transfer from CuSCN to DAS as a concerted PL mechanism. Concerted PL by energy transfer from CuSCN to DAS has also been found to occur efficiently, especially for the “scale” α-CuSCN/DAS with the largest contact area due to interpenetrating hybrid nanostructure in the smallest domain size. Those hybrid thin film having the concerted PL mechanism (Fig. 1) can possibly make these materials useful as LEDs.[1] K. Uda et al., ACS Omega, 2019, 4 (2), 4056-4062.[2] Y. Tsuda et al., Chem., 2017, 148, 845-854.[3] K. Uda et al., ECS Trans., 2019, 88, 323-333. Figure 1