Blue-Shifted and Broadened Fluorescence Enhancement by Visible and Mode-Selective Infrared Double Excitations.

Mode-selective vibrational excitations to modify the electronic states of fluorescein dianion in methanol solutions are carried out with a femtosecond visible pulse synchronized with a tunable high-power, narrow-band picosecond infrared (IR) pulse. In this work, simultaneous intensity enhancement, peak blueshift, and line width broadening of fluorescence are observed in the visible/IR double resonance experiments. Comprehensive investigations on the modulation mechanism with scanning the vibrational excitation frequencies, tuning the time delay between the two excitation pulses, theoretical calculations, and nonlinear and linear spectroscopic measurements suggest that the fluorescence intensity enhancement is caused by the increase of the Franck-Condon factor induced by the vibrational excitations at the electronic ground state. Various enhancement effects are observed as vibrations initially excited by the IR photons relax to populate the vibrational modes of lower frequencies. The peak blueshift and line width broadening are caused by both increasing the Franck-Condon factors among different subensembles because of IR pre-excitation and the long-lived processes induced by the initial IR excitation. The results demonstrate that the fluorescence from the visible/IR double resonance experiments is not a simple sum frequency effect, and vibrational relaxations can produce profound effects modifying luminescence.

Regulating the Electrical Double Layer with Supramolecular Cyclodextrin Anions for Dendrite-Free Zinc Electrodeposition.

The interfacial stability of a Zn battery is dependent on the electrical double layer (EDL) that forms at the interface between the electrolyte and the Zn metal anode. A fundamental understanding of the regulation of the EDL structure and stability on the Zn surface is highly desirable for practical applications of aqueous batteries. Herein, the interfacial chemistry of the EDL is regulated by the adsorption of supramolecular cyclodextrin anions in the inner Helmholtz plane (IHP). The nucleation overpotential and the charge transfer activation energy for Zn2+ to go through the OHP (Ea1) and IHP (Ea2) are increased, leading to slower Zn2+ transfer kinetics. The electric field distribution and Zn2+ flux in the proximity of the Zn metal surface are homogenized, thus suppressing the growth of dendrites. The mechanism is supported with theoretical and experimental analyses. Consequently, a Zn||Zn symmetric cell achieves an ultrahigh cumulative capacity of 10000 and 4250 mAh cm-2 at a respective current density of 10 and 50 mA cm-2, and an average Coulombic efficiency of 99.5% over 1000 cycles under harsh conditions (at a high current density of 10 mA cm-2 with a high capacity of 10 mAh cm-2). This work provides insight into the introduction of supramolecular anions to regulate the electrical double layer EDL structure and improve the interfacial stability.

Ultrabroadband hot-hole photodetector based on ultrathin gold film.

Hot carriers injected into semiconductor enables below-bandgap photodetection, thus attracting increasing interest. The performance of hot carrier-based device is directly related to the absorptivity of metal. Several strategies such as surface plasmons, metamaterials, and optical cavities are utilized to enhance the weak intrinsic absorption of the metal. However, the detection range is limited by their narrow resonance bandwidth alternatively. Impedance-matched absorbers, whose sheet resistance is equal to half of the free-space impedance (188 Ω), can achieve a wavelength-independent absorptivity up to 50%. Herein, we theoretically design a purely planar hot-hole photodetector based on ultrathin gold film, a new type of metallic impedance-matched absorber. Benefiting both from the efficient absorption and ultrathin nature of the film, we predict that the photoresponsivity of our device can reach 35.7 mA W-1 under zero bias at the wavelength of 1.3 µm, with a full width at half maximum (FWHM) of detection range reaching 1050 nm, setting a new record for the bandwidth of the hot carrier photodetectors. We also demonstrated that the device is robust to the incident angle and can be tuned through the external bias voltage. This work provides a pathway for broadband hot carrier detectors and other hot carrier-based applications.

Ultrahigh-rate and ultralong-life aqueous batteries enabled by special pair-dancing proton transfer.

The design of Faradaic battery electrodes with high rate capability and long cycle life comparable to those of supercapacitors is a grand challenge. Here, we bridge this performance gap by taking advantage of a unique ultrafast proton conduction mechanism in vanadium oxide electrode, developing an aqueous battery with untrahigh rate capability up to 1000 C (400 A g-1) and extremely long life of 0.2 million cycles. The mechanism is elucidated by comprehensive experimental and theoretical results. Instead of slow individual Zn2+ transfer or Grotthuss chain transfer of confined H+, the ultrafast kinetics and excellent cyclic stability are enabled by rapid 3D proton transfer in vanadium oxide via the special pair dance switching between Eigen and Zundel configurations with little constraint and low energy barriers. This work provides insight into developing high-power and long-life electrochemical energy storage devices with nonmetal ion transfer through special pair dance topochemistry dictated by hydrogen bond.

Restriction of crossing conical intersections: the intrinsic mechanism of aggregation-induced emission.

Elucidating the mechanism of aggregation-induced emission (AIE) is a prerequisite for designing more AIE-gens. The diphenylethylene (DPE) featured molecules are one of the most important AIE-gens due to their propeller structure. Three representative DPE-featured AIE-gens, triphenylethylene, cis-stilbene, and trans-stilbene, are explored via ultrafast ultraviolet/infrared (UV/IR) spectroscopy and theoretical calculations. Both experimental and computational results suggest that readily crossing conical intersections (CIs) with flexible structural evolutions in solutions significantly reduces fluorescence, whereas crossing CIs is restricted because of high energy cost, and therefore no fast nonradiative decay can compete with spontaneous emission in solids. The mechanism also well explains the different emission quantum yields and interconversion ratios between cis-stilbene and trans-stilbene after photoexcitation.

Direct observation of conformations of a high-mobility n-type low-bandgap copolymer in solutions and solid films.

The aggregation morphologies of conjugated polymers in solutions and solid films are important for their optoelectronic applications. Due to the amorphous state of the polymers, it remains a great challenge to determine their conformations in either liquids or solids. Herein, a ps/fs synchronized 2D IR technique is applied to investigate the molecular conformations of a high-mobility n-type low-bandgap copolymer, N2200, dissolved in CHCl3 and CCl4, and in solid films cast from both solutions by the vibrational cross-angle method. In CCl4, the polymer forms more aggregates and folds more and the backbone dihedral angle of C-C(NDI)/C-S(Thiophene) of its average conformation is about 10° more distorted than that in CHCl3 and the most stable conformation for a free molecule. Anti-intuitively, the solid films cast from both solutions have the same molecular conformation, and the conformation is similar to that of the polar CHCl3 rather than the conformation of the less polar CCl4. The results imply that the interaction between the polymer backbones is probably stronger than its interaction with CCl4, which can naturally guide the rearrangement of polymer chains during the evaporation of solvent molecules. This work also implies that the balance and competition between the polymer/polymer interaction and the polymer/solvent interaction seem to be the dominant factors responsible for what morphology can form in a solid film cast from solution. It is not always true that different molecular conformations must exist in solid films grown from different solutions with different polarity or different extents of aggregates with different conformations.

Graphene Coated Dielectric Hierarchical Nanostructures for Highly Sensitive Broadband Infrared Sensing.

Broadband infrared (IR) absorption is sought after for wide range of applications. Graphene can support IR plasmonic waves tightly bound to its surface, leading to an intensified near-field. However, the excitation of graphene plasmonic waves usually relies on resonances. Thus, it is still difficult to directly obtain both high near-field intensity and high absorption rate in ultra-broad IR band. Herein, a novel method is proposed to directly realize high near-field intensity in broadband IR band by graphene coated manganous oxide microwires featured hierarchical nanostructures (HNSs-MnO@Gr MWs) both experimentally and theoretically. Both near-field intensity and IR absorption of HNSs-MnO@Gr MWs are enhanced by at least one order of magnitude compared to microwires with smooth surfaces. The results demonstrate that the HNSs-MnO@Gr MWs support vibrational sensing of small organic molecules, covering the whole fingerprint region and function group region. Compared with the graphene-flake-based enhancers, the signal enhancement factors reach a record high of 103 . Furthermore, just a single HNSs-MnO@Gr MW can be constructed to realize sensitively photoresponse with high responsivity (over 3000 V W-1 ) from near-IR to mid-IR. The graphene coated dielectric hierarchical micro/nanoplatform with enhanced near-field intensity is scalable and can harness for potential applications including spectroscopy, optoelectronics, and sensing.

Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors.

Energy loss caused by exciton binding energy (Eb) has become a key factor that restricts further advancement of organic solar cells (OSCs). Herein, we used transient mid-IR spectroscopy to study direct photogeneration of free charge carriers in small-molecule acceptors (SMAs) Y6 and IDIC as well as polymerized SMAs (PSMAs) PYFT and PZ1. We found that free carrier concentration is higher in PSMAs than in their corresponding SMAs, indicating reduced exciton Eb, which is then confirmed by ultraviolet photoelectron spectroscopy, low-energy inverse photoemission spectroscopy, and film absorption spectra measurements. The measured Eb values of PYFT and PZ1 are 0.24 and 0.37 eV, respectively, smaller than those of Y6 (0.32 eV) and IDIC (0.47 eV). This work not only provides a method to directly monitor the photogenerated free carriers in OSC materials but also demonstrates that polymerization is an effective strategy to reduce the Eb, which is crucial to decrease the energy losses in high-performance OSCs.

Double crossing conical intersections and anti-Vavilov fluorescence in tetraphenyl ethylene.

Conical intersections (CIs) provide effective fast nonradiative decay pathways for electronic excitation, which can significantly influence molecular photoluminescence properties. However, in many cases, crossing a CI does not have direct observables, making studies of CIs experimentally challenging. Herein, the theoretically predicted double CIs by cis-trans twisting and cyclization in tetraphenyl ethylene, a well-known aggregation-induced emission molecule, are investigated with excitation dependent ultrafast UV/IR spectroscopy and fluorescence. Both the fluorescence quantum yield and the efficiency of cyclization are found to be smaller with a shorter excitation wavelength. An abrupt change occurs at about 300-310 nm. The results imply that crossing the twisting CI has a larger barrier than the cyclization CI, and the cis-trans twisting motion is probably involved with large solvation reorganization.

Two-atomic-layered optoelectronic device enabled by charge separation on graphene/semiconductor interface.

The Fermi level of graphene on different substrates usually changes significantly due to the interface difference between graphene and two-dimensional semiconductors. This feature opens many possibilities of manipulating optoelectronic devices by constructing graphene heterostructures through interface modification. Herein, we report the fabrication and optoelectronic response of an unconventional heterojunction device based on a graphene-MoSe2 hybrid interface. Different from the traditional three or more layered structure where the semiconductor is sandwiched between two electrodes, this device contains only two atomic layers: the MoSe2 layer serving as the photon absorber and the graphene layer functioning as the charge acceptor and both electrodes. This structure looks like short-circuited but shows an obvious photoelectric response, which is aided by electron transfers from MoSe2 to graphene. The photocurrent generation is explored quantitatively with electronic dynamics of graphene aided with ultrafast measurements. The two-layered architecture simplifies the fabrication of atomic-thick optoelectronic devices, allowing the as-grown semiconductors to be directly used and eliminating the damage-prone transfer process.

Photoluminescence of monolayer MoS2 modulated by water/O2/laser irradiation.

The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate their PL efficiencies. Herein, we use PL/Raman microspectroscopy and the corresponding in situ mapping, atomic force microscopy, and field-effect transistor (FET) characterization to investigate the changes in the structural and optical properties of monolayer MoS2. Relatively low power density (<4.08 × 105 W cm-2) of laser irradiation in ambient air can cause a slight PL suppression effect on monolayer MoS2, whereas relatively high power density (∼1.02 × 106 W cm-2) of laser irradiation brings significant PL enhancement. Experiments under different atmospheres reveal that the laser-irradiation-induced enhancement only occurs in the atmosphere containing O2 and is more remarkable in pure O2. In addition, physically adsorbed water can also induce PL enhancement of monolayer MoS2. FET devices suggest that the adsorbed water produces a p-doping effect on MoS2, and the laser irradiation in ambient air generates an n-doping effect, and both types of doping can enhance the PL intensity. The island-shaped defects caused by laser irradiation can be stabilized by oxygen atoms and act as trapping centers for excited trions or electrons, thus reducing the non-radiative recombination ratio and enhancing the PL intensity. The physically adsorbed water works in a similar way. A low power density of laser irradiation can sweep away the originally adsorbed H2O on the surface, thus reducing the PL.

Effectively Regulating More Robust Amorphous Li Clusters for Ultrastable Dendrite-Free Cycling.

A disordered phase in Li-deposit nanostructure is greatly attractive, but plagued by the uncontrollable and unstable growth, and the nanoscale characterization in the structure. Here, fully characterized in cryogenic transmission electron microscopy (cryo-TEM), more robust amorphous-Li (ALi) clusters are revealed and effectively regulated on heteroatom-activating electronegative sites and an advanced solid electrolyte interphase (SEI) layer. Heteroatom-activating electronegative sites capably enhance the electrostatic interaction of Li+ and heteroatom-doping graphene-like film (HDGs), meaning lower Li diffusion barrier and larger binding energy that is confirmed by small nucleation overpotentials of 13.9 and 10 mV at 0.1 mA cm-2 in the fluoroethylene carbonate-adding ester-based (FEC-ester) and LiNO3 -adding ether-based (LiNO3 -ether) electrolytes. Orderly multilayer SEI structure comprised of inorganic-rich components enables fast ion transports and durable capabilities to construct highly reversible and long-term plating/stripping cycling. ALi cluster anodes exhibit non-crystalline morphologies and perform ultrastable dendrite-free cycling over 2800 times. Stable ALi clusters are also grown in LiFePO4 (LFP) (LFP-ALi-HDGs-N||LiFePO4 [LFP]) full cells with advantageous capacities up to 165.5 and 164.3 mAh g-1 in these optimized electrolytes at 0.1 C; the remarkable capacity retentions maintain to 93% and 91% after 150 cycles at 0.2 C. Structure viability, electrochemical reversibility, and excellent performance in ALi clusters are effectively regulated.

What Leads to Aggregation-Induced Emission?

Aggregation-induced emission (AIE), usually referring to the phenomenon in which molecules emit more strongly in the aggregate state than in the solution state, is intriguing and promising in various optoelectronic and biosensing applications. In this Perspective, the basic principles that can lead to AIE and experimental evidence to reveal the AIE mechanism of tetraphenyl ethylene (TPE)-type molecules are discussed. AIE is the consequence of two factors: (1) the fast energy dissipation by crossing a conical intersection (CI) in solutions but not in solids results in low luminescence efficiencies in the solutions, and (2) the weak intermolecular coupling and thus slow intermolecular energy/charge transfers in the AIE solids effectively prevent quenching and result in relatively high luminescence efficiencies. The key to AIE is that the luminescence efficiency is tuned by controlling molecules to cross or not to cross a CI by changing the phase of molecules. How fast a molecule can cross a CI is dependent on the energy barrier of isomerization, which can be tuned in many ways, including mechanical or electrical stimuli, in addition to changing phases. Barrier-dependent crossing CI also results in a very important consequence: excitation-wavelength-dependent fluorescence yield within one electronic excited state, an anti-Vavilov's rule phenomenon. In principle, there can be an alternative way to tune luminescence efficiency by manipulating the formation of CIs instead of crossing or not crossing them. This approach relies on the fact that the electronic ground state and the excited state have many different properties, e.g., dipole moment. By tuning the environment, e.g., dielectric constant, to favor or disfavor one state, one may be able to lift or lower the potential surface of one state so that the potential surfaces of two states can vary between intersected and not contacted.

Second-harmonic generation divergence-a method for domain size evaluation of 2D materials.

Single-atomic-layered materials are important for future electronics. They allow optoelectronic devices to be fabricated at the single-atomic layer level. A single-atomic-layered two-dimensional (2D) transition metal dichalcogenide (TMD) film is usually composed of randomly orientated single-crystalline domains, and the size distribution of the domains on a large-area film has a significant impact on the applications of the film, but the impact is difficult to characterize. We report an approach to evaluate the size of the single-crystalline domains by measuring the second-harmonic generation divergence caused by the domains of different orientations. Using this method, domain size mapping on an 8×8mm2 region of a continuous MoS2 film is achieved. This method provides a fast and efficient way of domain size characterization across a large area in a non-destructive and transfer-free manner for single-atomic-layered TMD films.

Wide-Range Color-Tunable Organic Phosphorescence Materials for Printable and Writable Security Inks.

Organic materials with long-lived, color-tunable phosphorescence are potentially useful for optical recording, anti-counterfeiting, and bioimaging. Herein, we develop a series of novel host-guest organic phosphors allowing dynamic color tuning from the cyan (502 nm) to orange red (608 nm). Guest materials are employed to tune the phosphorescent color, while the host materials interact with the guest to activate the phosphorescence emission. These organic phosphors have an ultra-long lifetime of 0.7 s and a maximum phosphorescence efficiency of 18.2 %. Although color-tunable inks have already been developed using visible dyes, solution-processed security inks that are temperature dependent and display time-resolved printed images are unprecedented. This strategy can provide a crucial step towards the next-generation of security technologies for information handling.

Direct Observation of Aggregation-Induced Emission Mechanism.

The mechanism of aggregation-induced emission, which overcomes the common aggregation-caused quenching problem in organic optoelectronics, is revealed by monitoring the real time structural evolution and dynamics of electronic excited state with frequency and polarization resolved ultrafast UV/IR spectroscopy and theoretical calculations. The formation of Woodward-Hoffmann cyclic intermediates upon ultraviolet excitation is observed in dilute solutions of tetraphenylethylene and its derivatives but not in their respective solid. The ultrafast cyclization provides an efficient nonradiative relaxation pathway through crossing a conical intersection. Without such a reaction mechanism, the electronic excitation is preserved in the molecular solids and the molecule fluoresces efficiently, aided by the very slow intermolecular charge and energy transfers due to the well separated molecular packing arrangement. The mechanisms can be general for tuning the properties of chromophores in different phases for various important applications.

Synthesis of Lactams via Ir-Catalyzed C-H Amidation Involving Ir-Nitrene Intermediates.

x-membered lactams were synthesized via either an amidation of sp3 C-H bonds or an electrophilic substitution of arenes via Ir-nitrene intermediates. With the employment of a readily available iridium catalyst in dichloromethane or hexafluoro-2-propanol, a wide range of lactams were synthesized in good to excellent yields with high selectivity.

Ordered-to-Disordered Transformation of Enhanced Water Structure on Hydrophobic Surfaces in Concentrated Alcohol-Water Solutions.

The effects of hydrophobic solutes on the structure of the surrounding water have been a topic of debate for almost 70 years. However, a consistent description of the physical insight into the causes of the anomalous thermodynamic properties of alcohol-water mixtures is lacking. Here we report experimental results that combined temperature-dependent linear and femtosecond infrared spectroscopy measurements to explore the water structural transformation in concentrated alcohol-water solutions. Experiments show that the enhancement of water structure arises around microhydrophobic interfaces at room temperature in the solutions. As temperature increases, this ordered water structure disappears and a surface topography-dependent new disordered water structure arises at concentrated solutions of large alcohols. The water structural transformation is dependent on not only the length of the alkyl chain but also the clustering of the alcohols. A more-ordered-than-water structure can transform into a less-ordered-than-water structure.

Non-sedated functional imaging based on deep synchronization of PROPELLER MRI and NIRS.

BACKGROUND AND OBJECTIVE: Periodically rotated overlapping parallel lines with enhanced reconstruction-echo planar imaging (PROPELLER-EPI) is a promising technique for non-sedated functional imaging due to its unique advantage of motion correction. However, its multiple-blades sampling blood-oxygen-level dependent (BOLD) signal leads to low sampling rate and aliasing of higher frequency physiological signal components such as the cardiac pulsation. METHODS: In this study, we use near infrared spectroscopy (NIRS) synchronized with pulse sequences of PROPELLER-EPI, utilizing the fact that the optical sensing speed is inherently high. NIRS measures changes of oxyhemoglobin and deoxyhemoglobin to identify the transient states of on-BOLD and off-BOLD, and then labels each blade by temporal co-registration. The labeled blades from multiple epochs of a functional experiment are then used for the k-space data combination and subsequent image reconstruction. An eigenfunction model is proposed for temporal co-registration and to quantify the temporal resolution of the hemodynamic response. RESULT: The experiment of NIRS labeled PROPELLER-EPI was carried out with the optical sampling rate of 10 Hz and the magnetic pulses repetition time of 1000 ms, and the temporal resolution is 20 times better than that of the state-of-the-art sliding-window PROPELLER-EPI. We compared the functional imaging results against the conventional magnetic resonance echo planar imaging-measured activity and achieved an accuracy of 0.9. CONCLUSIONS: Using the synchronization of NIRS, the proposed imaging scheme provides an effective way to implement PROPELLER-EPI, which features motion free, high SNR, and enhanced spatial-temporal resolution.