Direct identification of interfacial degradation in blue OLEDs using nanoscale chemical depth profiling.

Understanding the degradation mechanism of organic light-emitting diodes (OLED) is essential to improve device performance and stability. OLED failure, if not process-related, arises mostly from chemical instability. However, the challenges of sampling from nanoscale organic layers and interfaces with enough analytical information has hampered identification of degradation products and mechanisms. Here, we present a high-resolution diagnostic method of OLED degradation using an Orbitrap mass spectrometer equipped with a gas cluster ion beam to gently desorb nanometre levels of materials, providing unambiguous molecular information with 7-nm depth resolution. We chemically depth profile and analyse blue phosphorescent and thermally-activated delayed fluorescent (TADF) OLED devices at different degradation levels. For OLED devices with short operational lifetimes, dominant chemical degradation mainly relate to oxygen loss of molecules that occur at the interface between emission and electron transport layers (EML/ETL) where exciton distribution is maximised, confirmed by emission zone measurements. We also show approximately one order of magnitude increase in lifetime of devices with slightly modified host materials, which present minimal EML/ETL interfacial degradation and show the method can provide insight for future material and device architecture development.

Facile and versatile ligand analysis method of colloidal quantum dot.

Colloidal quantum-dots (QDs) are highly attractive materials for various optoelectronic applications owing to their easy maneuverability, high functionality, wide applicability, and low cost of mass-production. QDs usually consist of two components: the inorganic nano-crystalline particle and organic ligands that passivate the surface of the inorganic particle. The organic component is also critical for tuning electronic properties of QDs as well as solubilizing QDs in various solvents. However, despite extensive effort to understand the chemistry of ligands, it has been challenging to develop an efficient and reliable method for identifying and quantifying ligands on the QD surface. Herein, we developed a novel method of analyzing ligands in a mild yet accurate fashion. We found that oxidizing agents, as a heterogeneous catalyst in a different phase from QDs, can efficiently disrupt the interaction between the inorganic particle and organic ligands, and the subsequent simple phase fractionation step can isolate the ligand-containing phase from the oxidizer-containing phase and the insoluble precipitates. Our novel analysis procedure ensures to minimize the exposure of ligand molecules to oxidizing agents as well as to prepare homogeneous samples that can be readily analyzed by diverse analytical techniques, such as nuclear magnetic resonance spectroscopy and gas-chromatography mass-spectrometry.

Increasing the Energy Gap between Band-Edge and Trap States Slows Down Picosecond Carrier Trapping in Highly Luminescent InP/ZnSe/ZnS Quantum Dots.

Non-toxic InP-based nanocrystals have been developed for promising candidates for commercial optoelectronic applications and they still require further improvement on photophysical properties, compared to Cd-based quantum dots (QDs), for better device efficiency and long-term stability. It is, therefore, essential to understand the precise mechanism of carrier trapping even in the state-of-the-art InP-based QD with near-unity luminescence. Here, it is shown that using time-resolved spectroscopic measurements of systematically size-controlled InP/ZnSe/ZnS core/shell/shell QDs with the quantum yield close to one, carrier trapping decreases with increasing the energy difference between band-edge and trap states, indicating that the process follows the energy gap law, well known in molecular photochemistry for nonradiative internal conversion between two electronic states. Similar to the molecular view of the energy gap law, it is found that the energy gap between the band-edge and trap states is closely associated with ZnSe phonons that assist carrier trapping into defects in highly luminescent InP/ZnSe/ZnS QDs. These findings represent a striking departure from the generally accepted view of carrier trapping mechanism in QDs in the Marcus normal region, providing a step forward understanding how excitons in nanocrystals interact with traps, and offering valuable guidance for making highly efficient and stable InP-based QDs.

Evolution and expansion of Li concentration gradient during charge-discharge cycling.

To improve the performance of Li-ion batteries (LIBs), it is essential to understand the behaviour of Li ions during charge-discharge cycling. However, the analytical techniques for observing the Li ions are limited. Here, we present the complementary use of scanning transmission electron microscopy and atom probe tomography at identical locations to demonstrate that the evolution of the local Li composition and the corresponding structural changes at the atomic scale cause the capacity degradation of Li(Ni0.80Co0.15Mn0.05)O2 (NCM), an LIB cathode. Using these two techniques, we show that a Li concentration gradient evolves during cycling, and the depth of the gradient expands proportionally with the number of cycles. We further suggest that the capacity to accommodate Li ions is determined by the degree of structural disordering. Our findings provide direct evidence of the behaviour of Li ions during cycling and thus the origin of the capacity decay in LIBs.

A safe and sustainable bacterial cellulose nanofiber separator for lithium rechargeable batteries.

Bacterial cellulose nanofiber (BCNF) with high thermal stability produced by an ecofriendly process has emerged as a promising solution to realize safe and sustainable materials in the large-scale battery. However, an understanding of the actual thermal behavior of the BCNF in the full-cell battery has been lacking, and the yield is still limited for commercialization. Here, we report the entire process of BCNF production and battery manufacture. We systematically constructed a strain with the highest yield (31.5%) by increasing metabolic flux and improved safety by introducing a Lewis base to overcome thermochemical degradation in the battery. This report will open ways of exploiting the BCNF as a "single-layer" separator, a good alternative to the existing chemical-derived one, and thus can greatly contribute to solving the environmental and safety issues.

Origins of High Performance and Degradation in the Mixed Perovskite Solar Cells.

The origins of the high device performance and degradation in the air are the greatest issues for commercialization of perovskite solar cells. Here this study investigates the possible origins of the mixed perovskite cells by monitoring defect states and compositional changes of the perovskite layer over the time. The results of deep-level transient spectroscopy analysis reveal that a newly identified defect formed by Br atoms exists at deep levels of the mixed perovskite film, and its defect state shifts when the film is aged in the air. The change of the defect state is originated from loss of the methylammonium molecules of the perovskite layer, which results in decreased JSC , deterioration of the power conversion efficiency and long-term stability of perovskite solar cells. The results provide a powerful strategy to diagnose and manage the efficiency and stability of perovskite solar cells.

Identification and Clinical Implications of Novel MYO15A Mutations in a Non-consanguineous Korean Family by Targeted Exome Sequencing.

Mutations of MYO15A are generally known to cause severe to profound hearing loss throughout all frequencies. Here, we found two novel MYO15A mutations, c.3871C>T (p.L1291F) and c.5835T>G (p.Y1945X) in an affected individual carrying congenital profound sensorineural hearing loss (SNHL) through targeted resequencing of 134 known deafness genes. The variant, p.L1291F and p.Y1945X, resided in the myosin motor and IQ2 domains, respectively. The p.L1291F variant was predicted to affect the structure of the actin-binding site from three-dimensional protein modeling, thereby interfering with the correct interaction between actin and myosin. From the literature analysis, mutations in the N-terminal domain were more frequently associated with residual hearing at low frequencies than mutations in the other regions of this gene. Therefore we suggest a hypothetical genotype-phenotype correlation whereby MYO15A mutations that affect domains other than the N-terminal domain, lead to profound SNHL throughout all frequencies and mutations that affect the N-terminal domain, result in residual hearing at low frequencies. This genotype-phenotype correlation suggests that preservation of residual hearing during auditory rehabilitation like cochlear implantation should be intended for those who carry mutations in the N-terminal domain and that individuals with mutations elsewhere in MYO15A require early cochlear implantation to timely initiate speech development.

A high-density array of size-controlled silicon nanodots in a silicon oxide nanowire by electron-stimulated oxygen expulsion.

Methods of producing Si nanodots embedded in films of silicon oxide and silicon nitride abound, but fabrication of Si nanodots in a nanowire of these materials is very rare despite the fact that nanowire architecture enhances the charge collection and transport efficiencies for solar cells and field-effect transistors. We report a novel fabrication method for a high-density array of size-controlled sillicon nanodots from a silicon oxide nanowire using electron-beam irradiation. Our results demonstrate that a highly dense phase of Si nanodots with a narrow size distribution can be made from a silicon oxide nanowire with a core-shell structure of crystalline silicon-rich oxide (c-SRO)/amorphous silicon oxide (a-SiO(2)). This new nanomaterial shows the carrier transport characteristics of a semiconductor. The initially produced amorphous Si nanodots can be readily turned into crystalline Si (c-Si) nanodots by thermal annealing. Key characteristics of c-Si nanodots such as their size, number density, and rate of nucleation and growth are easily controlled by varying the electron radiation dose and annealing temperature. Nanodot formation is mechanistically initiated by electron trapping at the c-SRO core as well as at the core-shell interface, which leads to out-diffusion of the negatively charged oxygen through Coulomb repulsion, fostering the aggregation of Si atoms.

Study on a set of bis-cyclometalated Ir(III) complexes with a common ancillary ligand.

Bis-cyclometalated iridium(III) complexes [Ir(F2ppy)2ZN] (FZN), [Ir(F2CNppy)2ZN] (FCZN), [Ir(DMAF2ppy)2ZN] (FDZN) and [Ir(MeOF2ppy)2ZN] (MeOFZN) (F2ppy = 4',6'-difluoro-2-phenylpyridinate, F2CNppy = 5-cyano-4',6'-difluoro-2-phenylpyridinate, DMAF2ppy = 4',6'-difluoro-4-dimethylamino-2-phenylpyridinate, MeOF2ppy = 4',6'-difluoro-4-methyl-2-phenylpyridinate and ZN = 3,5-dimethylpyrazole-N-carboxamide) emitting in the sky blue region were synthesized. We studied the effect of the ancillary ligand ZN and the substituents on the cyclometalating ligands on the crystal structures, photophysical and electrochemical properties and the frontier orbitals. Density functional theory (DFT) calculation results indicate that in FCZN and FDZN the cyclometalating ligands show negligible participation in the HOMO, the ancillary ligand ZN being the main participant along with the Ir(III) d-orbitals. MeOFZN exhibits the maximum photoluminescence quantum efficiency and radiative emission rates along with the dominant low frequency metal-ligand vibrations and maximum reorganization energy in the excited state. All the substituted complexes show more polar characteristics than FZN, FCZN possessing the highest dipole moment among the complexes. The performances of the solution-synthesised organic light emitting devices (OLEDs) of FZN, FCZN and FDZN doped in a blend of mCP (m-bis(N-carbazolylbenzene)) and polystyrene are studied.

Synthetic molecular machine based on reversible end-to-interior and end-to-end loop formation triggered by electrochemical stimuli.

We have designed and synthesized a novel [2]pseudorotaxane-based molecular machine in which the interconversion between end-to-interior and end-to-end loop structures is reversibly controlled by electrochemical stimuli. Cucurbit[8]uril (CB[8]) and the thread molecule 3(4+) with an electron-rich hydroxynaphthalene unit and two electron-deficient viologen units form the 1:1 complex 4(4+) with an end-to-interior loop structure, which is reversibly converted into an end-to-end structure upon reduction. Large changes in shape and size of the molecule accompany the reversible redox process. The key feature of the machine-like behavior is the reversible interconversion between an intramolecular charge-transfer complex and viologen cation radical dimer inside CB[8] triggered by electrochemical stimuli.

Metformin restores the penile expression of nitric oxide synthase in high-fat-fed obese rats.

Obesity is a well-known risk factor for erectile dysfunction, which is associated with reduced penile nitric oxide synthase (NOS) expression. Recently it was reported that metformin activates AMP-activated protein kinase (AMPK), which increases the expression of neuronal (n) NOS and endothelial (e) NOS. Thus, to evaluate whether metformin restores NOS expression in penile tissue, we measured penile expression of nNOS and eNOS after 4 weeks of metformin treatment (300 mg/kg/d) in 5-month-old high-fat-fed obese (HFO) rats. HFO rats have increased fat accumulation in visceral areas and marked suppression of nNOS and eNOS expression in penile tissue. However, metformin treatment decreased visceral fat deposition and restored nNOS and eNOS expression in penile tissue. The levels of AMPK and phosphorylated AMPK were also decreased in HFO rats but were subsequently elevated by metformin treatment. These results suggest that expression of NOS was suppressed by the high-fat diet but restored by metformin treatment. The effect of metformin on the expression of NOS may be associated with its activation of AMPK.

Substituent effect on the luminescent properties of a series of deep blue emitting mixed ligand Ir(III) complexes.

The syntheses of the bright deep blue emitting mixed ligand Ir(III) complexes comprising two cyclometalating, one phosphine and one cyano, ligands are reported. In this study, a firm connection between the nature of the excited states and the physicochemical behavior of the complexes with different ligand systems is elucidated by correlating the observed crystal structures, spectroscopic properties, and electrochemical properties with the theoretical results obtained by the density functional theory (DFT) methods. The cyclometalating ligands used here are the anions of 2-(4',6'-difluorophenyl)-pyridine (F2ppy), 2-(4',6'-difluorophenyl)-4-methyl pyridine (F2ppyM), and 4-amino-2-(4',6'-difluorophenyl)-pyridine (DMAF2ppy). The phosphine ligands are PhP(O-(CH2CH2O)3-CH3)2 and Ph2P(O-(CH2CH2O)n-CH3), where Ph = phenyl and n = 1 (P1), 3 (P3), or 8 (P350). The thermal stabilities of the complexes were enhanced upon increasing the "n" value. The crystal structures of the complexes, [(DMAF2ppy)2Ir(P1)CN], (P1)DMA, and [(F2ppyM)2Ir(P3)CN], (P3)F2M, show the cyano and phosphine groups being in a cis configuration to each other and in a trans configuration to the coordinating Cring atoms. The long Ir-Cring bond lengths are ascribed to the trans effect of the strong phosphine and cyano ligands. DFT calculations indicate that the highest occupied molecular orbital (HOMO) is mainly contributed from the d-orbitals of the iridium atom and the pi-orbitals of cyclometalating and cyano ligands, whereas the lowest unoccupied molecular orbital (LUMO) spreads over only one of the cyclometalating ligands, with no contribution from phosphine ligands to both frontier orbitals. Dimethylamino substitution increases the energy of the emitting state that has more metal-to-ligand-charge-transfer (MLCT) character evidenced by the smaller vibronic progressions, smaller difference in the 1MLCT and 3MLCT absorption wavelengths, and higher extinction coefficients (epsilon) than the F2ppy and F2ppyM complexes. However, the increase in the basicity of the dimethylamino group in the DMAF2ppy complexes in the excited states leads to distortions and consequent nonradiative depopulation of the excited states, decreasing their lower photoluminescence (PL) efficiency. The effect of the substituents in the phosphine ligand is more pronounced in the electroluminescence (EL) than in the PL properties. Multilayer organic light emitting devices (OLEDs) are fabricated by doping the Ir(III) complexes in a blend of mCP (m-bis(N-carbazolyl benzene)) and polystyrene, and their device characteristics are studied. The (P3)F2M complex shows a maximum external quantum efficiency (etaex) of 2%, a maximum luminance efficiency (etaL) of 4.13 cd/A at 0.04 mA/cm2, and a maximum brightness of 7200 cd/m2 with a shift of the Commission Internationale de L'Eclairage (CIE) coordinates from (0.14, 0.15) in film PL to (0.19, 0.34) in EL.

Complexation of ferrocene derivatives by the cucurbit[7]uril host: a comparative study of the cucurbituril and cyclodextrin host families.

The formation of inclusion complexes between cucurbit[7]uril (CB[7]) and ferrocene and its derivatives has been investigated. The X-ray crystal structure of the 1:1 inclusion complex between ferrocene and CB[7] revealed that the guest molecule resides in the host cavity with two different orientations. Inclusion of a set of five water-soluble ferrocene derivatives in CB[7] was investigated by 1H NMR spectroscopy and calorimetric and voltammetric techniques. Our data indicate that all neutral and cationic guests form highly stable inclusion complexes with CB[7], with binding constants in the 10(9)-10(10) M(-)(1) and 10(12)-10(13) M(-1) ranges, respectively. However, the anionic ferrocenecarboxylate, the only negatively charged guest among those surveyed, was not bound by CB[7] at all. These results are in sharp contrast to the known binding behavior of the same guests to beta-cyclodextrin (beta-CD), since all the guests form stable inclusion complexes with beta-CD, with binding constants in the range 10(3)-10(4) M(-1). The electrostatic surface potentials of CB[6], CB[7], and CB[8] and their size-equivalent CDs were calculated and compared. The CD portals and cavities exhibit low surface potential values, whereas the regions around the carbonyl oxygens in CBs are significantly negative, which explains the strong affinity of CBs for positively charged guests and also provides a rationalization for the rejection of anionic guests. Taken together, our data suggest that cucurbiturils may form very stable complexes. However, the host-guest interactions are very sensitive to some structural features, such as a negatively charged carboxylate group attached to the ferrocene residue, which may completely disrupt the stability of the complexes.

Stable pi-dimer of a tetrathiafulvalene cation radical encapsulated in the cavity of cucurbit[8]uril.

The first stable pi-dimer of a tetrathiafulvalene (TTF) cation radical encapsulated in the cavity of cucurbit[8]uril has been isolated at room temperature and fully characterized; it shows absorption bands at 400, 540 and 760 nm, characteristic of the TTF cation radical dimer.

Control of the stoichiometry in host-guest complexation by redox chemistry of guests: inclusion of methylviologen in cucurbit[8]uril.

The binding stoichiometry of a host-guest complex can be effectively controlled by the redox chemistry of the guest: a 1:1 inclusion complex of methylviologen dication (MV2+) in cucurbit[8]uril (CB[8]) converts completely and reversibly to a 2:1 inclusion complex of cation radical (MV+.) in CB[8] upon the reduction of the guest.

Inclusion of methylviologen in cucurbit[7]uril.

The inclusion behavior of methylviologen (N,N'-dimethyl-4,4'-bipyridinium, MV) dication in cucurbit[7]uril (CB[7]) has been studied by using various spectroscopic and electrochemical methods. The inclusion complex of MV dication in CB[7] is stable thermodynamically and kinetically. The electrochemical study reveals that unlike beta-cyclodextrin, CB[7] prefers the charged species, MV dication (MV(2+)), and cation radical (MV(+)*) to the fully reduced neutral (MV(0) species as guests. Dimerization of MV(+)* is suppressed effectively by forming a stable complex with CB[7] in aqueous solution as confirmed by spectroelectrochemical experiments. Furthermore, the first redox process (MV(2+)/MV(+)*) of the MV(2+)-CB[7] complex occurs predominantly via the direct electron transfer pathway, whereas the second redox process (MV(+)*/MV(0)) occurs via both the direct and indirect pathway because of the low affinity of the fully reduced species MV(0) to CB[7].