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Halide perovskites, including CsPbX

Clarification of the morphology of bulk heterojunction (BHJ) is indispensable for true comprehension of the organic solar cells. Here, we performed scanning transmission X-ray microscopy (STXM) for a poly-(9,9-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blend film annealed at various temperatures (Tan). We found that the fullerene concentration within the polymer-rich domain decreases with Tan while the domain size (∼230 nm) is essentially unchanged. We will discuss the interrelation between the film morphology and the photovoltaic performance.

Time-resolved reflection imaging and spectroscopy were performed for single crystals of reduced-type titanium oxide Ti 4 O 7 . This shows two-step phase transition at 140 K from the low temperature (LT) phase to the intermediate (IT) phase and at 160 K from the IT phase to the high temperature (HT) phase. After the photo-irradiation in the IT phase, the time profiles of the reflection spectra and the excited area images showed spontaneous domain growth of the photo-induced HT phase. Analysis of the reflection images with the dielectric functions derived the growth velocity of the induced HT state. The value is close to that of the acoustic phonon in the core titanium dioxide.

Low-dimensional (low-D) luminescent materials have attracted significant attention due to the high photoluminescent quantum yields. However, it is unclear whether low-D materials are superior to 3D materials for electroluminescent (EL) devices given that low-D materials have poor charge transport nature due to their highly localized electronic structures. We noticed a significant phenomenon that EL performances for 3D materials, such as CsPbX3, are governed by adjacent charge transport layers, which is possibly due to nonradiative recombination resulting from the small exciton binding energy. This finding encouraged us to develop new electron transport layers (ETLs) that satisfy not only the energy alignment to confine excitons but also an efficient electron injection into 3D CsPbX3 layers. This strategy enables one to exploit the good charge transport nature of 3D CsPbX3. The proposed amorphous Zn-Si-O ETL has sufficiently shallow electron affinity (∼3.2 eV) to confine excitons and sufficiently high electron mobility (∼0.8 cm2/V s) to transport electrons. Furthermore, the controllable conductivity and electron affinity of amorphous Zn-Si-O enable fine-tuning of charge balance. Consequently, the very low operating voltage of 2.9 V at 10 000 cd/m2 and high power efficiency of 33 lm/W were achieved for a green perovskite (CsPbBr3) EL (PeLED). The obtained ultrahigh brightness of ∼500 000 cd/m2 demonstrates the effectiveness of the proposed strategy. We also extend this strategy into 3D CsPbBrI2 (red) and 3D CsPbBrCl2 (blue) PeLEDs, and demonstrate a record high brightness of 20 000 cd/m2 for the red PeLED. We believe this study provides new insight into the realization of practical PeLEDs.

We investigated the red afterglow process of CaTiO3:Pr, Al through photoluminescence and thermoluminescence measurements by using fabricated single crystals. The crystals with several cubic millimeter in size were obtained through a flux method. We found that the afterglow properties varied with the excitation energy. At room temperature, the red afterglow was observed for the charge transfer from Pr3+ to Ti4+ excitation and the direct excitation of Pr3+, while not observed for the interband excitation of CaTiO3 due to the relaxation process going through 4d15d1 state of Pr3+. For the direct excitation of Pr3+ at 80 K, the excited electrons did not reached to trap states for afterglow due to a potential barrier, which can overcome with thermal assistance at room temperature. For the charge transfer excitation at 80 K, the excited electrons could reach the traps with excess energy.

Measurements of electrical resistance at high pressure were performed up to 2.5 GPa for the insulating and the superconducting epitaxial Ti4O7 films of Magneli phase. For the insulating film, the m...

Effects of fluorine doping on the diffusion of interstitial oxygen molecules (O2) in amorphous SiO2 (a-SiO2) were compared to those obtained from a-SiO2 containing SiOH groups. Incorporation of moderate concentration ( ∼ 1 0 19 cm−3) of SiF groups gives rise to minor changes in diffusion parameters between 800 and 1100 ° C: only a slight decrease in solubility and an increase in the activation energy for diffusion can be detected. Incorporation of SiOH groups has similar weak effects on the solubility and activation energy for diffusion. These minor changes are most likely due to the enhancement of the flexibility of local Si–O network as a result of the dissociation of the network by SiOH and SiF groups. However, in contrast to the SiF doping, SiOH doping leads to a notable decrease in the diffusion coefficient. The heat of solution changes by ∼ 0.1 –0.2 eV at ∼ 1000 ° C and it is attributed to the glass transition of a-SiO2.

We have developed novel broad luminescent phosphors (La,Ce,Y)6Si4S17 including multiple substituted sites of Ce3+ . They have formed a triclinic structure (P − 1) with three kinds of coordination structures around Ce3+ ions. The broad photoluminescence (PL) spectra at the range from 430 nm to 700 nm were observed, and they can be respectively decomposed into three PL bands. Moreover, the correlation between the three PL bands and the three substitution sites for Ce3+ was confirmed through the PL spectra at 78 K and Van Uitert's universal equation.

We have applied a time-resolved absorption spectroscopy to investigate the charge transfer (CT) processes in the Prussian Blue-type cyanides MII[FeIII(CN)6]0.675H2O (M = Mn, Co) embedded in a Nafion film. In the Mn-Fe cyanide, we observed the bleaching of the CT band at 2.4 eV, similar to that found in the Co-Fe cyanide. We found that the lifetime of the photo-induced CT state in Mn-Fe cyanide shows significant dependence on the excitation photon energy. This is in sharp contrast to the case of the Co-Fe cyanide. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Au + ion implantation with fluences from 1 × 10 14 to 3 × 10 16 cm −2 into 12CaO · 7Al 2 O 3 (C12A7) single crystals was carried out at a sample temperature of 600 °C. The implanted sample with the fluence of 1 × 10 15 cm −2 exhibited photoluminescence (PL) bands peaking at ∼3.1 and ∼2.3 eV at ⩽150 K when excited by He–Cd laser (325 nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au − ion having the electronic configuration of 6 s 2 , judged from their similarities to those reported on Au − ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (∼2.3 × 10 21 cm −3 ), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au + implantation if an appropriate fluence is chosen.

The energy splitting of valence band in oxychalcogenides LaCuOCh (Ch=S, Se) was studied through degenerative four-wave mixing (DFWM) signals excited by femtosecond laser pulses and high-resolution absorption spectra at 4 K. The DFWM signals for LaCuOS exhibited a beat structure with a period of 480 fs just at the exciton peak energy, indicating that the lowest exciton states were split by 9 meV, while the corresponding splitting in LaCuOSe was 125 meV. The spin–orbit interaction of Ch ion accompanied by the hybridization of Cu 3d orbital causes the splitting of the exciton levels.

The interstitial oxygen molecules (O 2 ) in SiO 2 glass were detected down to ∼10 15 cm -3 by photoluminescence of O 2 at 1272nm excited at 765nm by a continuous-wave titanium sapphire laser. It was evidenced that SiO 2 glass thermally annealed in air between 800 and 1100°C spontaneously absorbs ∼10 16 cm -3 of O 2 from the ambient atmosphere. The time-dependent concentration change of the interstitial O 2 allows the determination of both the diffusion coefficient and the solubility of the interstitial O 2 .

We report distinct optical/electrical properties inherent to build-in nanostructures in transparent oxide based materials, oxychalcogenides LaCuOS and LaCuOSe having naturally formed multi-quantum well structures and 12CaO·7Al2O3 (C12A7) with a unique nanoporous structure. LaCuOSe doped with Mg ions exhibit degenerative p-type conduction presumable due to “modulation doping mechanism” with a high mobility of 4.0 cm2 V−1 s−1 even at high hole concentration (∼2×1020 cm−3). The room temperature exciton, having a large binding energy (∼50 meV) due to the confinement effect of carriers in the CuS or CuSe layer, induces a large optical nonlinearity at room temperature (RT) (χ(3) = 2–4×10−9 esu) near the absorption band. The H− ion entrapped in nanosize cages in C12A7 lattice causes an persistent conversion at RT from an electric insulator (

Surface dissolution and diffusion of oxygen molecules (O 2 ) in SiO 2 glass were studied by their photoluminescence at 1272 nm excited with a titanium sapphire laser oscillating at 765 nm. The dissolution of O 2 from ambient atmosphere at both surfaces was much faster than the following diffusion of O 2 into SiO 2 glass, indicating that the surface dissolution is not the rate-limiting step for the saturation of SiO 2 glass with O 2 . The time- and temperature-dependent concentration changes of O 2 allow to evaluate the diffusion coefficient and the saturation solubility of O 2 in SiO 2 glass.

We studied the quantum spin ladder system, sodium vanadate, by using a femtosecond impulsive excitation technique and report on the first observation of the lattice and spin dynamics. We found a coherent magnetic oscillatory mode (magnon) at 127 cm −1 (twice the spin gap energy) below T C (=34 K) with circularly polarized light, which means that the magnetic mode corresponds to a spin-flip excitation from the singlet ground state.

We have used reflective electro-optic sampling (REOS) technique combined with 20 fs laser pulses to detect coherent phonon signals. We observe the plasmon–phonon coupling mode and its excitation power dependence in InAs. In addition, we interpreted relative intensity of coupled mode L − to LO phonon in terms of the numerical calculation based on phenomenological equations of motion for polarization and lattice displacement.

We demonstrate Eu2+ ion activated sorosilicate Ca2ZnSi2O7 exhibits a broad emission band centered at 600 nm due to Eu2+ 4f65d1→4f7 transition, which is compatible for a blue light (∼450 nm) excitation. Orbital symmetry of the emitting state and the tight coordination of 8 oxygen ions in its layered structure, which enhance covalency of Eu2+-O bonding and ligand field splitting of 5d levels, are responsible for the long wavelength emission from Eu2+ ions. In addition, the formation of a photo-induced transient Eu3+ state from Eu2+ was observed by the irradiation of UV light with an energy being slightly larger than the band gap energy of the host. A recovery life time of the Eu3+ to Eu2+ ions is evaluated to be 11.0 ms at 77 K by a pump-probe technique.

We have studied the third-order optical nonlinearity (χ(3)) of epitaxial thin films of layered compounds LaCuOS and LaCuOSe at room temperature by a spectrally resolved degenerative four-wave mixing technique with femtosecond time resolution. The χ(3) values in both films are sharply resonant to optical absorption bands in the ultraviolet (UV) light region due to room-temperature exciton. The peak values are evaluated to be as large as 2–4×10−9 esu with a fast time response of 250–300 fs. These findings indicate that LaCuOS and LaCuOSe are promising materials for emerging optical nonlinear devices that operate in the UV light region compatible for GaN-based lasers.

The ultrafast photo-induced insulator-metal transition in a Ti4O7 single crystal is investigated with different excitation densities using an optical-pump terahertz (THz)-probe technique. The oxide shows a two-step phase transition at 130 K from a low temperature (LT) phase to an intermediate phase and to a high temperature metallic phase at 150 K. The photo-irradiation in the LT phase induces an ultrafast decrease in the transmittance of the THz probe within 3.6 ps, corresponding to the generation of a metallic domain. This is followed by an additional slow decrease when the excitation is above the threshold level. The nonlinear dependence of the amplitude and lifetime on the excitation density indicates that the metallic domains grow spontaneously and are stabilized depending on their resultant amount.

Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaics (OPV). In order to improve the incident photon-to-current efficiency (IPCE) of the BHJ solar cell, a comprehensive understanding of the ultrafast dynamics of excited species, such as singlet exciton (D*), interfacial charge-transfer (CT) state, and carrier (D+), is indispensable. Here, we performed femtosecond time-resolved spectroscopy of two prototypical BHJ blend films: poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend film and poly(9,9′-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) blend film. We decomposed differential absorption spectra into fast, slow, and constant components via two-exponential fitting at respective probe photon energies. The decomposition procedure clearly distinguished photoinduced absorptions (PIAs) due to D*, CT, and D+. Based on these assignments, we will compare the charge dynamics between the F8T2/PC70BM and P3HT/PCBM blend films.

Fine-pitched microgratings are encoded on fused silica surfaces by a two-beam laser interference technique employing UV femtosecond pulses from the third harmonics of a Ti:sapphire laser. A pump and prove method utilizing a laser-induced optical Kerr effect or transient optical absorption change has been developed to achieve the time coincidence of the two pulses. Use of the UV pulses makes it possible to narrow the grating pitches to an opening as small as 290 nm, and the groove width of the gratings is of nanoscale size. The present technique provides a novel opportunity for the fabrication of periodic nanoscale structures in various materials.

Amorphous ${\mathrm{SiO}}_{2}$ ($a$-${\mathrm{SiO}}_{2}$) thermally annealed in an oxygen atmosphere incorporates oxygen molecules (O${}_{2}$) in interstitial voids. When the thermal annealing is performed in $^{18}\mathrm{O}$${}_{2}$ gas, interstitial $^{18}\mathrm{O}$${}_{2}$ as well as interstitial $^{16}\mathrm{O}$$^{18}\mathrm{O}$ and $^{16}\mathrm{O}$${}_{2}$ are formed due to the oxygen exchange with the $a$-${\mathrm{SiO}}_{2}$ network. The ${a}^{1}{\ensuremath{\Delta}}_{g}(v=0)\ensuremath{\rightarrow}{X}^{3}{\ensuremath{\Sigma}}_{g}^{\ensuremath{-}}(v=1)$ infrared photoluminescence band of interstitial ${\mathrm{O}}_{2}$ was utilized to quantitatively analyze the oxygen exchange, taking into account the influences of common network modifiers in synthetic $a$-${\mathrm{SiO}}_{2}$ (SiOH, SiF, and SiCl groups). The presence of network modifiers does not significantly change the average rate of $^{18}\mathrm{O}$ transfer from interstitial ${\mathrm{O}}_{2}$ to the $a$-${\mathrm{SiO}}_{2}$ network and its activation energy, suggesting that the network modifiers themselves do not serve as preferential oxygen exchange sites. When the concentration of SiOH groups is low, the oxygen exchange rate is distributed, indicating that only a small part of the network oxygen atoms participates in the oxygen exchange. However, the distribution of the oxygen exchange rate is distinctly narrow in the sample with high SiOH concentration. It is attributed to the redistribution of the network $^{18}\mathrm{O}$ atoms and the modification of the $a$-${\mathrm{SiO}}_{2}$ network topology caused by reactions with mobile interstitial water molecules, which are transiently formed by dehydroxylation of paired SiOH groups. The activation energy for the average oxygen exchange rate is larger than that of the permeation of interstitial ${\mathrm{O}}_{2}$ in $a$-${\mathrm{SiO}}_{2}$. Furthermore, the average exchange-free diffusion length of interstitial ${\mathrm{O}}_{2}$ below 900 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C ($\ensuremath{\gtrsim}$1 $\ensuremath{\mu}$m) is far larger than the scale of the interstitial voids in $a$-${\mathrm{SiO}}_{2}$ ($\ensuremath{\lesssim}$1 nm). These observations confirm that the oxygen exchange is not necessarily involved in the permeation of interstitial ${\mathrm{O}}_{2}$.

Brilliant pulsed x‐ray synchrotron radiation (SR) is useful for pump‐probe experiment such as time‐resolved x‐ray diffraction, x‐ray absorption fine structure, and x‐ray spectroscopy. For laser pump‐SR x‐ray probe experiments, short pulsed lasers are generally synchronized to the SR master oscillator controlling the voltage for acceleration of electron bunches in an accelerator, and the interval between the laser and the SR pulses is changed around the time scale of target phenomenon. Ideal delay control produces any time delay as keeping the time‐precision and pointing‐stability of optical pulses at a sample position. We constructed the time delay control module using a continuous phase shifter of radio frequency signal and a frequency divider, which can produce the delayed trigger pulses to the laser without degradation of the time precision and the pointing stability. A picoseconds time‐resolved x‐ray diffraction experiment was demonstrated at SPring‐8 storage ring for fast lattice response by femtosecond pulsed laser irradiation, and suggested the possibility of accurate sound velocity measurement. A delay control unit operating with subpicosecond precision has also been designed for femtosecond pump‐probe experiments using a free electron laser at SPring‐8 campus.

The exchange between lattice and interstitial oxygen species in an oxide was studied by the $^{16}\mathrm{O}\mathrm{\text{\ensuremath{-}}}^{18}\mathrm{O}$ isotope shift of the ${a}^{1}{\ensuremath{\Delta}}_{g}(v=0)\ensuremath{\rightarrow}{X}^{3}{\ensuremath{\Sigma}}_{g}^{\ensuremath{-}}(v=1)$ infrared photoluminescence band of the oxygen molecules (${\mathrm{O}}_{2}$) incorporated into the interstitial voids of amorphous ${\mathrm{SiO}}_{2}$ ($a\mathrm{\text{\ensuremath{-}}}{\mathrm{SiO}}_{2}$) by thermal annealing in $^{18}\mathrm{O}_{2}$ gas. A large site to site variation of the oxygen exchange rate, originating from structural disorder of $a\mathrm{\text{\ensuremath{-}}}{\mathrm{SiO}}_{2}$, is found. The average exchange rate has an activation energy of $\ensuremath{\sim}2\text{ }\text{ }\mathrm{eV}$, which is much larger than that for the diffusion of interstitial ${\mathrm{O}}_{2}$ ($\ensuremath{\sim}0.8--1.2\text{ }\text{ }\mathrm{eV}$). The average exchange-free diffusion length of interstitial ${\mathrm{O}}_{2}$ exceeds $\ensuremath{\sim}1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ below $900\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, providing definite evidence that oxygen diffuses as interstitial molecules in $a\mathrm{\text{\ensuremath{-}}}{\mathrm{SiO}}_{2}$.

The exchange between lattice and interstitial oxygen species in an oxide was studied by the 16O-18O isotope shift of the a1Deltag(v=0)--X3Sigmag-(v=1) infrared photoluminescence band of the oxygen molecules (O2) incorporated into the interstitial voids of amorphous SiO2 (a-SiO2) by thermal annealing in 18O2 gas. A large site to site variation of the oxygen exchange rate, originating from structural disorder of a-SiO2, is found. The average exchange rate has an activation energy of approximately 2 eV, which is much larger than that for the diffusion of interstitial O2 (approximately 0.8-1.2 eV). The average exchange-free diffusion length of interstitial O2 exceeds approximately 1 microm below 900 degrees C, providing definite evidence that oxygen diffuses as interstitial molecules in a-SiO2.

Electronic phase diagram has been derived for the Prussian-Blue-type cyano-bridged transition-metal compound, ${\text{Na}}_{0.84\ensuremath{-}\ensuremath{\delta}}\text{Co}{[\text{Fe}{(\text{CN})}_{6}]}_{0.71}\ensuremath{\cdot}3.8{\text{H}}_{2}\text{O}$ $(0.0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.61)$, as a function of the hole concentration $\ensuremath{\delta}$ of the $d$-electron system. The mother compound $(\ensuremath{\delta}=0)$ takes the ${\text{Co}}^{2+}$ $({t}_{2g}^{5}{e}_{g}^{2}:S=3/2)$ and ${\text{Fe}}^{2+}$ $({t}_{2g}^{6}:S=0)$ configuration and is paramagnetic down to zero temperature. At room temperature, the holes are selectively introduced on the Fe site. A slight hole doping $(\ensuremath{\delta}=0.13)$ causes the charge-transfer (CT) transition, that is, cooperative electron transfer from the ${\text{Co}}^{2+}$ site to the ${\text{Fe}}^{3+}$ site, with a decrease in temperature below ${T}_{\text{CT}}\ensuremath{\approx}250\text{ }\text{K}$. With a further increase in $\ensuremath{\delta}$, ${T}_{\text{CT}}$ slightly decreases from $\ensuremath{\approx}230\text{ }\text{K}$ at $\ensuremath{\delta}=0.24$ to $\ensuremath{\sim}210\text{ }\text{K}$ at $\ensuremath{\delta}=0.61$. Accordingly, the nature of the transition changes from the second-order type to the first-order type. In all the concentration ranges, the high-temperature (HT) phase is metastable even at low temperature. In this metastable phase, the ${\text{Fe}}^{3+}$ $({t}_{2g}^{5}:S=1/2)$ species mediate the ferromagnetic exchange coupling between the adjacent ${\text{Co}}^{2+}$ spins. The ferromagnetic transition appears at $\ensuremath{\delta}=0.39$, and the transition temperature ${T}_{C}$ increase from 7 K at $\ensuremath{\delta}=0.39$ to 13 K at $\ensuremath{\delta}=0.61$. Based on these experimental data, we will discuss the significant roles of the coupling between the charge, spin, and lattice degrees of freedom in the transition-metal cyanides.

Dynamics of the charge-transfer (CT) pairs has been investigated for the cyano-bridged ${\mathrm{Co}}^{2+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ particles grown in a hydrophilic cavities of Nafion 117 film. We decomposed the differential absorption spectra into the fast and slow components. We ascribed the fast and slow components to the Frank-Condon state and the CT $({\mathrm{Co}}^{3+}\text{\ensuremath{-}}{\mathrm{Fe}}^{2+})$ pairs with lattice relaxation, respectively. We found that the lifetime ${\ensuremath{\tau}}_{\mathrm{CT}}$ of the pairs at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ increases with the absorbed photon density $n$ from ${\ensuremath{\tau}}_{\mathrm{CT}}\ensuremath{\approx}2\phantom{\rule{0.3em}{0ex}}\mathrm{ns}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}\ensuremath{\approx}6\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, reflecting the attractive interaction between the pairs.

An ultra-high-precision clock system for long time delay has been developed for picosecond time-resolved x-ray diffraction measurements using synchrotron radiation (SR) pulses and synchronized femtosecond laser pulses. The time delay control between pump laser pulse and the probe SR pulse was achieved by combining an in-phase quadrature modulator and a synchronous counter. This method allowed us to change the delay time by a nearly infinite amount while maintaining the precision of +/-8.40 ps. Time-resolved diffraction measurements using the delay control system were demonstrated for precise measurement of an acoustic velocity in a single crystal of gallium arsenide.

The dynamics of the photoinduced phase transition has been investigated for the $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}$ cyanide film, ${\mathrm{Na}}_{0.35}\mathrm{Co}[\mathrm{Fe}(\mathrm{CN}{)}_{6}{]}_{0.79}∙3.7{\mathrm{H}}_{2}\mathrm{O}$, by means of the transient absorption spectroscopy. We observed two characteristic transient bands: short-lived intermediate band (I band) at $\ensuremath{\sim}1.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and long-lived photoinduced band (PI band) at $\ensuremath{\sim}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We have found that the intensity of the I band transfers to the PI band as time exceeds, and interpreted the behavior in terms of the intersystem crossing from the photocreated low-spin ${\mathrm{Co}}^{2+}$ species to the high-spin species. We further have found that the excitation spectra of these bands are qualitatively different.

Implantation of ${\mathrm{Au}}^{+}$ ions into a single crystalline $12\mathrm{CaO}∙7{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (C12A7) was performed at high temperatures with fluences from $1\ifmmode\times\else\texttimes\fi{}{10}^{14}$ to $3\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. This material is composed of positively charged sub-nanometer-sized cages compensated by extra-framework negatively charged species. The depth profile of concentrations of Au species was analyzed using Rutherford backscattering spectrometry. The measured optical spectra and ab initio embedded cluster calculations show that the implanted Au species are stabilized in the form of negative ${\mathrm{Au}}^{\ensuremath{-}}$ ions below the fluences of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ (Au volume concentration of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{21}{\mathrm{cm}}^{\ensuremath{-}3}$). These ions are trapped in the cages and exhibit photoluminescence (PL) bands peaking at 3.05 and $2.34\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at temperatures below $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At fluences exceeding $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, the implanted Au atoms form nano-sized clusters. This is manifested in quenching of the PL bands and creation of an optical absorption band at $2.43\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ due to the surface plasmon of free carriers in the cluster. The PL bands are attributed to the charge transfer transitions$({\mathrm{Au}}^{0}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{Au}}^{\ensuremath{-}})$ due to recombination of photo-excited electrons $({e}^{\ensuremath{-}})$, transiently transferred by ultraviolet excitation into a nearby cages, with ${\mathrm{Au}}^{0}$ atoms.

This article reviews novel electrical and optical properties found for epitaxial thin films of wide-gap semiconductors, LnCuOCh (Ln = lanthanide, Ch = chalcogen). This material series has a two-dimensional crystal structure composed of alternately stacked (Ln 2 O 2 ) 2 + and (Cu 2 Ch 2 ) 2 - layers. Distinctive properties such as high hole mobility, degenerate p-type conduction, room temperature exciton, and large optical nonlinearity were found and these are attributed to two-dimensional electronic structure arising from the layered structure, i.e., a narrow-gaped and hole-conductive (Cu 2 Ch 2 ) 2 - layer is sandwiched by wide-gaped insulating (Ln 2 O 2 ) 2 + layers. In particular, the wide-gap p-type metallic conduction was the first demonstration among any class of wide-gap materials including GaN: Mg. Realization of epitaxial thin films for these materials by reactive solid-phase epitaxy led to these discoveries which make LnCuOCh promising materials for optoelectronic devices utilizing the high p-type conductivity and/or the room temperature exciton.

Degenerate four-wave mixing signals excited by femtosecond laser pulses were measured in LaCuOCh (Ch = S and Se) at 4 K. The signals for LaCuOS exhibit a beat structure with a period of 480 fs just at the exciton peak energy, indicating that the lowest exciton states are split by 9 meV, but no beat structure was observed in LaCuOSe, regardless of the laser's energy. The spin--orbit interaction of the Ch ion accompanied by the hybridization of Cu 3d orbitals causes splitting of the exciton levels. Furthermore, the contribution of Ch p orbitals in the valence band maximum is larger in LaCuOSe owing to increased covalency in the Cu--Ch bond when S is replaced with Se.

The ability of UV femtosecond laser pulse to fabricate the fine-pitched microgratings on fused silica or ZnO surfaces has been demonstrated through a two-beam laser interference technique. A pump and probe method has been developed to find the time coincidence of the two UV pulses through a laser-induced optical Kerr effect or transient transmission change. The UV pulses achieve to narrow the grating pitches as small as 290nm. The establishment of the technique provides a novel opportunity for the fabrication of periodic nanoscale structures in various materials.

We report the first observation of coherent magnetic excitations in a spin ladder system ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ by using femtosecond time-domain spectroscopy. A pronounced coherent oscillation is observed at $127{\mathrm{cm}}^{\ensuremath{-}1}$ (nearly twice the spin gap energy) and assigned to a two-magnon bound state, based on the temperature dependence of the intensity below the charge ordering phase transition at ${T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}34\mathrm{K}$. This mode can be observable only when circularly polarized light is used as a pump or a probe beam, suggesting that it corresponds to a spin-flip excitation from the singlet ground state. A phonon mode strongly coupled to the spin state is also found at $303{\mathrm{cm}}^{\ensuremath{-}1}$.

The nanolevel molecular structure of a bulk heterojunction (BHJ) with a donor (D) polymer and acceptor (A) fullerene derivative is indispensable for true comprehension of highly efficient organic photovoltaic devices. Here, we performed scanning transmission X-ray microscopy of a poly(9,9-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blend film with periodic nanostructure. The spatially resolved carbon K-edge absorption spectra revealed that the nanostructure consists of two types of domains with considerable molecular mixing. The fullerene mass fraction is 71 ± 1 and 33 ± 2 wt % for the PC71BM- and F8T2-rich domains, respectively.

By means of femto-second time-resolved spectroscopy, we investigated the carrier formation process against film morphology and temperature (T) in highly-efficient organic photovoltaic, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] (PTB7)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) solar cells. We found that the carrier formation efficiency (ϕCF) from an absorbed photon is nearly independent of the film morphology, indicating that the internal quantum efficiency (ϕIQ) is mainly governed by the carrier transfer efficiency (ϕCT) to the electrodes. The activation energy (Ea=0.5–0.8 meV) of ϕCF is significantly low, which suggests an extended charge-transfer state around the PTB7/PC70BM interface.

We performed femtosecond time-resolved spectroscopy in α-sexithiophene (6T)/C60 bilayer and mixed films. We observed sharp and intense photoinduced absorption (PIA) at 1.2 eV and ascribed it to the charge-transfer (CT) state. The spectral features are interpreted in terms of homogenous molecular stacking at 6T/C60 interface and quantum confinement effect in 6T.

A bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaic (OPV). We investigated femtosecond charge dynamics after D (at 633 nm) and A (at 400 nm) excitations in a prototypical low-band-gap and highly efficient OPV, i.e., poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM). The photoinduced absorption (PIA) spectra of the blend film were decomposed into three PIAs, i.e., donor exciton (D*), acceptor exciton (A*), and carrier (D+) components. The analysis revealed that D*→D+ conversion predominates in 633 nm excitation, while A*→D+ conversion predominates in 400 nm excitation. The carrier formation times of both processes are nearly the same (τrise∼0.2–0.3 ps).

We investigated carrier formation dynamics in a small-molecular bulk heterojunction solar cell, 2,5-di-(2-ethylhexyl)-3,6-bis-(5″-n-hexy-[2,2′,5′,2″]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrolo-1,4-dione/[6,6]-phenyl C71-butyric acid methyl ester, with low bandgap (Egap ≈ 1.5 eV). The photoinduced absorption (PIA) spectra of the blend film were decomposed into three PIAs, i.e., those due to donor exciton (D*), acceptor exciton (A*), and mobile carrier (D+). The analysis revealed carrier conversion from D* with a conversion time of ∼1.3 ps.

Charge formation on the donor–acceptor (D–A) interface is the key process in organic photovoltaics (OPVs). Here, we experimentally demonstrated that the charge formation dynamics in the hexabenzocoronene (HBC)–C60 dyad system crucially depends on the structure of the D–A interface. In the self-assembled (SA) film with well-defined tube-type interface, we observed a conversion process from the donor HBC exciton (D*) to the donor charge (D+). Reflecting the two-dimensional (2D) shape of the respective domains, the charge formation time (τrise = 7.9 ps) in the SA film is much slower than τrise (≤1 ps) in the nonassembled (NA) film.

Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaic. Here, we investigated the femtosecond charge dynamics after acceptor excitation in poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b '] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] (PTB7)/[6,6]-phenyl C71-butyric acid methyl ester (PC70BM), which is a low-gap (Egap≈1.6 eV) organic photovoltaic. We observed fast conversion from the acceptor exciton (A*) to the donor polaron (D+) within τrise = 0.45 ps. The fast carrier formation is ascribed to the prompt charge transfer of an electron from the D highest occupied molecular orbital (HOMO) to the A HOMO.

Bulk heterojunction (BHJ) based on a donor (D) polymer and acceptor (A) fullerene derivative is a promising organic photovoltaics. Here, we performed femtosecond time-resolved spectroscopy of poly(9,9'-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C70-butyric acid methyl ester (PC70BM) blend film with distinct D/A interfaces. We decomposed differential absorption spectra into fast, slow, and constant components via two-exponential fitting at respective probe photon energies. The decomposition distinguished photoinduced absorptions (PIAs) due to singlet exciton, donor polaron, and interfacial charge-transfer (CT) state. Based on these assignments, we discussed the charge dynamics in the F8T2/PC70BM blend film.

Simultaneous measurements of electron and ion transfers were performed in an all solid ion-transfer device made of transition metal cyanide films, indium–tin oxide (ITO)/Na x Ni[Fe(CN)6]0.68·5.1H2O (NNF68)/Na x Co[Fe(CN)6]0.90·2.9H2O (NCF90)/ITO, against time (t) and temperature (T). At 330 K, the current density (I) steeply decreases below 10 s, and then becomes nearly constant above 10 s. We found that there exists time-lag between the electron and ion transfers. We analyzed the I–t curves with an equivalent circuit, which consists of the external component (R ex) of resistance, the bulk component (R') resistance, the leak current component (R leak), and the capacitance (C). The large R' component was ascribed to suppressed current density owning to the local charge neutrality.

Spin-crossover compounds show many photoinduced phase transition (PIPT) phenomena, reflecting the strong coupling between the electron, spin, and lattice degrees of freedoms. In these compounds, the photoinduced electronic excitation triggers the intersystem of crossing in Fe2þ or Co3þ with a high probability, and alters the spin state of the transition metals (light-induced excited spin state trapping: LIESST). The spin state transition further causes local lattice deformation as a result of the variation in the ionic radii of Fe2þ or Co3þ.14) The resultant local lattice distortion modifies the electronic structure of the surrounding sites, causing a nonlinear response of the system. Ogawa et al. reported the threshold and incubation behaviors in the photoinduced phase transition of [Fe(2-pic)3]Cl2 EtOH (2-pic = 2-aminomethyl-pyridine) at 2.2K. Liu et al. reported a dynamical phase transition in Nafion–[Fe(Htrz)3] (Htrz = 1,2,4-4H-triazole) and an optical hysteresis in [Fe(pyz)6](BF4)2 (ptz = 1-propyltetrazole) at 77K. Several researchers theoretically investigated the photoinduced dynamics beyond the mean-field approximation, even though the approach had been successful in several situations. In this Note, we reported the mesoscale lattice effect on PIPT of a Co–Fe cyanide, Na0:27Co[Fe(CN)6]0:713.8H2O, at 200K by a microscopic spectroscopy. The compound shows a spin-crossover-type PIPT from the low-spin (LS) phase with LS Co3þ and LS Fe2þ to the high-spin (HS) phase with HS Co2þ and LS Fe3þ, accompanying a significant lattice expansion of 3%. We carefully investigated the PIPT dynamics at each position around the photoexcitation spot, which is 75 m in diameter. We found that the PIPT at each position takes place simultaneously within 0.2 s, even though the density (nHS) of the HS sites is widely distributed in the range of 0–0.6. The distribution of the linear expansion coefficient ( L=L) within the HS region suggests that the propagation of the PIPT region is mediated by a coherent lattice expansion. A thin film of Na0:27Co[Fe(CN)6]0:713.8H2O, 1310 nm in thickness, was electrochemically synthesized on an indium–tin oxide (ITO) transparent electrode. The chemical composition was determined by inductively coupled plasma (ICP) and CHN organic elementary analyses. The Xray diffraction pattern revealed that the compound is facecentered cubic with a lattice constant of a 1⁄4 10:34 A. The surface and cross-sectional scanning electron microscopy (SEM) images revealed that the film consists of rodlike crystals, 600 nm in length and 130 nm in area. The film shows a thermally induced LS–HS phase transition at 230K (1⁄4 Tc) in the warming run, and the film changes from violet (LS phase) to red (HS phase). The photoinduced dynamics was recorded with a laboratory-built microscopy system equipped with a charge-coupled device (CCD) camera. We used the second harmonics (532 nm) of the yttrium aluminum garnet (YAG) laser as the excitation light source. The spot size of the excitation light was 75 m, and the excitation density was 6.6 photon s 1 site . A notch filter was inserted to eliminate the scattering from the excitation light. The magnitudes of nHS at each time and position were determined by the chromaticity in the microscopy moving image. The spatial resolution of the system was 5 m. To increase the signal-to-noise ratio, the chromaticity was averaged within the 10 10 pixels around each position. Figure 1 shows examples of the microscopy images of the Na0:27Co[Fe(CN)6]0:713.8H2O film under photoirradiation at 200K at various times. The bright region corresponds to the HS region. No macroscopic PIPT is observed at (b) 41.7 s. The PIPT starts around the central region of the excitation spot at (c) 43.3 s, and then expands outside at (d) 43.5 s. A similar incubation behavior of the PIPT was observed in [Fe(2-pic)3]Cl2 EtOH. One may notice that the PIPT region at (d) 43.5 s extends beyond the excitation spot [broken-line circle in Fig. 1(a)]. The inset of Fig. 2(b) shows a prototypical example of the time dependence of nHS. (a) 0 .0 s

The Prussian blue lattice is known to show characteristic thermal response; the coefficient (β≡d a /d T ; a is lattice constant) of the thermal expansion changes its sign from positive to negative with increasing a . In order to comprehend the curious thermal response, we systematically investigated the pressure response of the lattice for A x M 2+ [Fe 3+ (CN) 6 ] y z H 2 O ( A = Rb and Cs, M = Ni, Zn, and Cd) by means of high-pressure Raman spectroscopy. We found that pressure coefficients (\(\alpha\equiv\mathrm{d}\hbar\omega/\mathrm{d}P\)) of the Raman shift (\(\hbar\omega\)) for the CN stretching modes are small in the M = Cd compounds. The small-α is interpreted in terms of the dynamical rotational displacement of the Fe(CN) 6 octahedra in the large- a region.

The dynamics of the photoinduced phase transition (PIPT) was systematically investigated in Co–Fe cyanide film, Na 0.16 Co[Fe(CN) 6 ] 0.71 5.8H 2 O, at 160 and 190 K above the melting temperature ( T m = 120 K) of the photoinduced phase. Below T m , the film shows a permanent photoinduced phase transition from a low-spin (LS) phase to a static high-spin (HS) phase. We found that the relaxation curve from the photo-excited state changes from the exponential type to Avrami's type as the excitation photon density ( n ) increases. We interpreted this behavior in terms of crystallization process of the photo-created HS sites.