An oxygen-vacancy rich 3D novel hierarchical MoS2/BiOI/AgI ternary nanocomposite: enhanced photocatalytic activity through photogenerated electron shuttling in a Z-scheme manner.

An oxygen-vacancy rich, bismuth oxyiodide-based Z-scheme 3D hierarchical MoS2/BiOI/AgI ternary nanocomposite photocatalyst was fabricated using a simple precipitation process in ethylene glycol and water. The presence of oxygen-vacancies in BiOI and the two-dimensional nature of molybdenum disulfides in the composite prolongs the charge carrier lifetime through a Z-scheme system and enhances the performance of the photocatalyst for the degradation of rhodamine B. On the basis of efficient separation of photoexcited electron-hole pairs, a mechanism is proposed whereby MoS2 and oxygen vacancy states increase charge carrier lifetimes and improve the photocatalytic activity. The Z-scheme mechanism of the photocatalysis is consistent with the results of static and time-resolved photoluminescence, scavenging, and terephthalic acid photoluminescence experiments. Among the as-synthesized photocatalysts, the one containing 2 wt% of MoS2 in a composite of MoS2/BiOI/AgI exhibited the highest photocatalytic activity towards rhodamine B degradation, and its activity was 7 and 16 times higher than that of BiOI/AgI and BiOI, respectively. Degradation of phenol, the colorless model pollutant, was studied to confirm the visible-light photocatalytic performance of the MoS2/BiOI/AgI composite. This easily fabricated Z-scheme based MoS2/BiOI/AgI composite exhibits promising photocatalytic activity and will be useful for potential applications in energy and environmental areas.

Reversible Halide Exchange Reaction of Organometal Trihalide Perovskite Colloidal Nanocrystals for Full-Range Band Gap Tuning.

In recent years, methylammonium lead halide (MAPbX3, where X = Cl, Br, and I) perovskites have attracted tremendous interest caused by their outstanding photovoltaic performance. Mixed halides have been frequently used as the active layer of solar cells, as a result of their superior physical properties as compared to those of traditionally used pure iodide. Herein, we report a remarkable finding of reversible halide-exchange reactions of MAPbX3, which facilitates the synthesis of a series of mixed halide perovskites. We synthesized MAPbBr3 plate-type nanocrystals (NCs) as a starting material by a novel solution reaction using octylamine as the capping ligand. The synthesis of MAPbBr(3-x)Clx and MAPbBr(3-x)Ix NCs was achieved by the halide exchange reaction of MAPbBr3 with MACl and MAI, respectively, in an isopropyl alcohol solution, demonstrating full-range band gap tuning over a wide range (1.6-3 eV). Moreover, photodetectors were fabricated using these composition-tuned NCs; a strong correlation was observed between the photocurrent and photoluminescence decay time. Among the two mixed halide perovskite series, those with I-rich composition (x = 2), where a sole tetragonal phase exists without the incorporation of a cubic phase, exhibited the highest photoconversion efficiency. To understand the composition-dependent photoconversion efficiency, first-principles density-functional theory calculations were carried out, which predicted many plausible configurations for cubic and tetragonal phase mixed halides.

Blue luminescence of dendrimer-encapsulated gold nanoclusters.

Direct evidence for the blue luminescence of gold nanoclusters encapsulated inside hydroxyl-terminated polyamidoamine (PAMAM) dendrimers was provided by spectroscopic studies as well as by theoretical calculations. Steady-state and time-resolved spectroscopic studies showed that the luminescence of the gold nanoclusters consisted largely of two electronic transitions. Theoretical calculations indicate that the two transitions are attributed to the different sizes of the gold nanoclusters (Au8 and Au13). The luminescence of the gold nanoclusters was clearly distinguished from that of the dendrimers.

Band gap grading and photovoltaic performance of solution-processed Cu(In,Ga)S2 thin-film solar cells.

The photophysical properties of CuInxGa1-xS2 (CIGS) thin films, prepared by solution-based coating methods, are investigated to understand the correlation between the optical properties of these films and the electrical characteristics of solar cells fabricated using these films. Photophysical properties, such as the depth-dependent band gap and carrier lifetime, turn out to be at play in determining the energy conversion efficiency of solar cells. A double grading of the band gap in CIGS films enhances solar cell efficiency, even when defect states disturb carrier collection by non-radiative decay. The combinational stacking of different density films leads to improved solar cell performance as well as efficient fabrication because a graded band gap and reduced shunt current increase carrier collection efficiency. The photodynamics of minority-carriers suggests that the suppression of defect states is a primary area of improvement in CIGS thin films prepared by solution-based methods.

Enhancement mechanism of quantum yield in core/shell/shell quantum dots of ZnS-AgIn5S8/ZnIn2S4/ZnS.

To achieve a high quantum yield (QY) of nanomaterials suitable for optical applications, we improved the optical properties of AgIn5S8 (AIS) quantum dots (QDs) by employing an alloyed-core/inner-shell/outer-shell (ZAIS/ZIS/ZnS) structure. We also investigated the mechanism of optical transitions to clarify the improvement of QYs. In AIS, the low-energy absorption near the band edge region is attributed to the weakly allowed band gap transition, which gains oscillator strength through state intermixing and electron-phonon coupling. The main photoluminescence is also ascribed to the weakly allowed band gap transition with characteristics of self-trapped excitonic emission. With alloying/shelling processes, the weakly allowed transition is enhanced by the evolution of the electronic structures in the alloyed core, which improves the band gap emission. In shelled structures, the nonradiative process is reduced by the reconstructed lattice and passivated surface, ultimately leading to a high QY of 85% in ZAIS/ZIS/ZnS. These findings provide new insights into the optical transitions of AIS because they challenge previous conclusions. In addition, our work elucidates the mechanism behind the enhancement of QY accomplished through alloying/shelling processes, providing strategies to optimize nontoxic QDs for various applications using a green chemistry approach.

MODE-specific deactivation of adenine at the singlet excited states.

The deactivation process of adenine excited near the band origin of the lowest ππ* state ((1)L(b)) is investigated using picosecond (ps) time-resolved photoionization spectroscopy. The transients obtained with a ps pump pulse at the sharp vibronic bands, 36,105 cm(-1) (D) and 36,248 cm(-1) (E), in the resonant two-photon ionization spectrum exhibit a bi-exponential decay with two distinct time constants of τ1 ~ 2 ps and τ2 > 100 ps, whereas the transients with the pump at other wavenumbers in this energy region show a single exponential decay with τ = 1-2 ps. We suggest that the τ1 represents the lifetimes of the (1)nπ∗ energy levels near the D and E peaks, which are excited together by the ps pump pulse having a broad spectral bandwidth, and the τ2 shows the lifetimes of D and E peaks. The long lifetime of D level is attributed to a small barrier for internal conversion from the minimum of the (1)L(b) state to the (1)nπ* state. On the other hand, the long lifetime of E level is ascribed to the nuclear configuration of adenine at this level, which is unfavorable to reach the seam of the conical intersection leading to nearly barrierless deactivation to the electronic ground state. This study shows that the ps time-resolved spectroscopy provides a powerful tool to study mode- and energy-specific deactivation processes occurring in a multi-dimensional potential energy surface.

Infrared predissociation of ternary cluster cations: the solvent effects on the branching ratio.

Infrared (IR) predissociation of hydrogen-bonded ternary cluster ions such as aniline-water-ethanol (AWE(+)), aniline-water-isopropanol (AWP(+)), aniline-methanol-ethanol (AME(+)), aniline-water-pyrrole (AWPy(+)), and aniline-water-benzene (AWB(+)) was examined in the region of 2700-4000 cm(-1) to explore the key factors which determine the branching ratios in the concurrent unimolecular dissociation. The smaller solvent molecule was predominantly ejected when the binding energies of the two were not too different. On the other hand, when they were far off, the binding energy also acted significantly on the branching ratio. Besides, mode-selective IR predissociation was observed, while the selectivity was not quite distinct. The IR predissociation of ternary cluster ions bound via hydrogen bonding is considered to occur on a time scale much faster than intramolecular vibrational energy redistribution, which was proved by a statistical transition state theory.

Light-matter interaction and polarization of single ZnO nanowire lasers.

The optical properties of single zinc oxide (ZnO) nanolasers are investigated. ZnO nanowires with different diameters and lengths are prepared by chemical vapor transport. The diameter plays an important role in the stimulated emission process in nanowires. The spectral shift and spacing of Fabry-Pérot-type modes imply a strong light-matter interaction in the lasing nanowires, which is explained by the exciton-polariton model. The polarization of the electric field in the lasing nanowires is perpendicular to the long axis of the nanowire and parallel to the substrate plane. The coexistence of the transverse modes is distinguished by decomposing the peak shape and the degree of polarization. In addition to the transverse mode of the lasing with the polarization parallel to the substrate plane, the lasing mode with the polarization perpendicular to the substrate plane is observed.

Branching ratio in photodissociation of 1-bromo-3-chlorobenzene cation.

A new and convenient calculation method based on Rice-Ramsperger-Kassel-Marcus (RRKM) theory assuming an extremely loose transition state (LTS) has been attempted to predict the branching ratio in photodissociation. This method enables estimation of the branching ratios without detailed structural information on the transition state which is indispensable in conventional RRKM calculations. To evaluate our simple method through comparison to the experimental results, photodissociation of 1-bromo-3-chlorobenzene cation (3BCB+) was chosen as a model unimolecular reaction system which has two distinct photodissociation channels in ultraviolet region: 3BCB+ → Br-dissociated daughter ion (ClBz+) + Br and 3BCB+ → Cl-dissociated daughter ion (BrBz+) + Cl. The branching ratio was monitored with decreasing the internal energy using a linear tandem time-of-flight mass spectrometer, which clearly showed decreased branching ratios of 3BCB+ → ClBz+ + Br over 3BCB+ → BrBz+ + Cl in reasonable agreement with the calculation results employing the new method. Although there was some discrepancy in internal energy between the experimental and calculation results, the new calculation method is worth to be extended to other diverse systems considering its intuitive and simple nature.

Distinctive optical transitions of tunable multicolor carbon dots.

Three types of carbon dots (CDs) are synthesized from isomers of phenylenediamine to develop multicolor nanomaterials with low toxicity, high stability, and high quantum yield. The distinctive electronic structures of CDs lead to the characteristic optical transitions, such as three colors of blue, green, and red, which are primarily attributed to the difference in configurations, despite the similar basic structures of conjugated systems. The excitation-independent emission and the single exponential decay of CDs indicate the single chromophore-like nature in each type of CD. In addition, the two-photon luminescence of CDs exhibits a comparable shape and time profile to the typical photoluminescence with high photostability. Although the surface-related defect states are observed by intragap excitation, the contribution of defect states is barely observed in the emission profile upon band gap excitation. Consequently, the controllability of optical transitions in CDs enhances the potential of tunable multicolor nanomaterials for various applications as alternatives to quantum dots containing toxic elements.

Photofragmentation in selected tautomers of protonated adenine.

The photofragmentation by UV excitation of selectively prepared 1(+) and 3(+) tautomers of protonated adenine is studied after excitation at a 266 and 263 nm wavelengths with two different experimental set-ups located in Seoul and Orsay. While the production of 1(+) tautomers with an electrospray ion source is now well accepted, calculations were used to ascribe the preparation of 3(+) tautomers from cold adenine dimers. The fragmentation patterns are rather similar for both tautomers, suggesting similar mechanisms as a statistical fragmentation in the ground electronic state after internal conversion.

Anion clusters of naphthalene and solvents: structure, ion core, and intermolecular interactions.

We carried out a comparative study on the anion clusters of naphthalene with various solvents to understand the nature of intermolecular interactions involving an aromatic anion. Photoelectron spectra of mass-selected naphthalene anion clusters, (Np)(1)(-)(Sol)(n) (Sol = benzene, naphthalene, water, and acetone), were obtained, from which the electron affinities were estimated. The electron affinities were significantly different from the vertical detachment energies due to the geometry difference between the neutral and anion clusters along intermolecular coordinates. Theoretical calculations showed that the most stable structure of the naphthalene-acetone anion cluster tends to be a -shaped geometry because the intermolecular interaction is dominated by pi-hydrogen bonding. With the attachment of a second solvent, solvent-solvent interaction was found to compete with ion-solvent interaction for the stable geometries of the anion clusters.

Photoelectron spectroscopy of pyrene anion clusters: autodetachment via excited states of anion and intermolecular interactions in anion clusters.

This study examined the anion clusters of pyrene (Py) by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. The photoelectron spectra of Py(n)(-) (n=1-4) were obtained at various photon energies. A change in photodetachment wavelength resulted in a large change in the relative intensities of vibrational progression in the photoelectron spectra. It is proposed that the observed modulation of the Franck-Condon factors by the different photon energies reflects autodetachment via the excited states of anion. The photoelectron spectra of Py(n)(-) at 355 nm showed a broad band structure between the S(0) and T(1) states, which is also due to the autodetachment via a Feshbach resonance state. The photoelectron spectra of Py(2)(-) suggest the presence of a unique dimeric interaction between the two pyrene moieties, whereas the spectral features of Py(3)(-) are similar to those of Py(1)(-). The stable structures of Py(2)(-) and Py(3)(-) obtained by density functional theory calculations support the experimental findings, where different intermolecular interactions govern the stabilization of these two species.

Structures and infrared photodissociation of [(aniline)-(methanol)-(water)2].

The structures of [(aniline)-(methanol)-(water)2]+ were investigated by infrared spectroscopy coupled with linear tandem mass spectrometry. We suggest the most stable structure of [(aniline)-(methanol)-(water)2]+ through infrared photodissociation spectra supported by the density functional theory calculations at the level of ωB97X-D/cc-pVQZ. Methanol and one water molecule formed hydrogen bonding with the amino group of aniline, while the other water formed hydrogen bonding with methanol. Upon infrared excitation of [(aniline)-(methanol)-(water)2]+, the water molecule connected to methanol turned out to be preferentially ejected, although the total internal energy in the cluster ion was large enough to dissociate other solvent molecules. This unique dissociation feature was attributed to the significant difference in the dissociation rates as obtained by the Rice-Ramsperger-Kassel-Marcus theory calculations as well as structural restriction.

Dual wavelength lasing of InGaN/GaN axial-heterostructure nanorod lasers.

Optical confinement effects are investigated in InGaN/GaN axial-heterostructure nanolasers. Cylindrical nanorods with GaN/InGaN/GaN structures are prepared using combined processes of top-down and bottom-up approaches. The lasing of InGaN is observed at a low threshold (1 µJ cm-2), which is attributed to an efficient carrier transfer process from GaN to InGaN. The lasing of GaN is also found in the threshold range of 10-20 µJ cm-2 with a superlinear increase in emission intensity and high quality factors (Q = 1000), implying that dual wavelengths of lasing are tunable as a function of excitation intensity. The non-classical Fabry-Pérot modes suggest strong light-matter interactions in nanorods by optical confinement effects. The polarization of lasing indicates that the non-classical modes are in the identical transverse mode, which supports the formation of exciton-polaritons in nanorods. Polariton lasing in a single axial-heterostructure nanorod is observed for the first time, which proposes small-sized light sources with low threshold, polarized light, and tunable wavelengths in a single nanorod.

Density functional study of intradimer proton transfers in hydrated adenine dimer ions, A2(+)(H2O)n (n = 0-2).

Structures of mono- and dihydrated adenine dimers and their cations were calculated using B3LYP density functional theory with the 6-31+G(d,p) basis set, in order to help understand photofragmentation experiments of hydrated adenine dimers from the energetics point of view. Several important pathways leading to the major fragmentation product, protonated adenine ion (AH(+)), thermodynamically at minimum costs were investigated at the ground-state electronic potential surface of hydrated adenine dimer cations. Our calculations suggest that the proton transfer from one adenine moiety to the other in hydrated dimer ions readily occurs with negligible barriers in normal hydration conditions. In asymmetrically hydrated ions, however, the proton transfer to more hydrated adenine moieties is kinetically hindered due to heightened transition-state barriers, while the other way is still barrierless. Such directional preference in proton transfer may be characterized as a unique dimer ion property, stemming from the difference in basicity of the two nitrogen atoms involved in the double hydrogen bond that would be equivalent without hydration. We also found that dimer cleavage requires about 4 times larger energy than evaporation of individual water molecules, so it is likely that most solvent molecules evaporate before the eventual dimer cleavage when available internal energy is limited.

Structures of aniline(pyrrole)+, aniline(ethanol)+, and aniline-(benzene).

Molecular structures of aniline(pyrrole)+, aniline(ethanol)+, and aniline(benzene)+ produced via resonance two-photon ionization at 266 nm were analyzed by infrared predissociation spectroscopy coupled with tandem mass spectrometry. Structural optimization and frequency calculation using density functional theory were carried out to suggest the most probable isomers which are in good agreement with the observed infrared absorption spectra. Intermolecular bonds in the cluster ions were formed such that the electronegative oxygen atom of the solvent molecule or the pi electron of the aromatic ring forms a hydrogen bonding to NH of aniline.

Efficient and Stable CsPbBr3 Quantum-Dot Powders Passivated and Encapsulated with a Mixed Silicon Nitride and Silicon Oxide Inorganic Polymer Matrix.

Despite the excellent optical features of fully inorganic cesium lead halide (CsPbX3) perovskite quantum dots (PeQDs), their unstable nature has limited their use in various optoelectronic devices. To mitigate the instability issues of PeQDs, we demonstrate the roles of dual-silicon nitride and silicon oxide ligands of the polysilazane (PSZ) inorganic polymer to passivate the surface defects and form a barrier layer coated onto green CsPbBr3 QDs to maintain the high photoluminescence quantum yield (PLQY) and improve the environmental stability. The mixed SiN x/SiN xO y/SiO y passivated and encapsulated CsPbBr3/PSZ core/shell composite can be prepared by a simple hydrolysis reaction involving the addition of adding PSZ as a precursor and a slight amount of water into a colloidal CsPbBr3 QD solution. The degree of the moisture-induced hydrolysis reaction of PSZ can affect the compositional ratio of SiN x, SiN xO y, and SiO y liganded to the surfaces of the CsPbBr3 QDs to optimize the PLQY and the stability of CsPbBr3/PSZ core/shell composite, which shows a high PLQY (∼81.7%) with improved thermal, photo, air, and humidity stability as well under coarse conditions where the performance of CsPbBr3 QDs typically deteriorate. To evaluate the suitability of the application of the CsPbBr3/PSZ powder to down-converted white-light-emitting diodes (DC-WLEDs) as the backlight of a liquid crystal display (LCD), we fabricated an on-package type of tricolor-WLED by mixing the as-synthesized green CsPbBr3/PSZ composite powder with red K2SiF6:Mn4+ phosphor powder and a poly(methyl methacrylate)-encapsulating binder and coating this mixed paste onto a cup-type blue LED. The fabricated WLED show high luminous efficacy of 138.6 lm/W (EQE = 51.4%) and a wide color gamut of 128% and 111% without and with color filters, respectively, at a correlated color temperature of 6762 K.

Near-infrared electrochemiluminescence from orange fluorescent Au nanoclusters in water.

We report the unusual generation of near-infrared (near-IR) electrochemiluminescence (ECL) from water-soluble Au nanoclusters (NCs), of which the photoluminescence is primarily within the visible wavelength region. The near-IR ECL is ascribed to the Au(0)-glutathione motif in the Au NCs stabilized by glutathione in water.