All-Optical Switching Based on Sub-Bandgap Photoactivation of Charge Trapping in Metal Halide Perovskites.

Controllable optical properties are crucial for the application of light-emitting materials in optical devices. In this work, controllable photoluminescence in metal halide perovskite crystals is realized via photoactivation of their defects. It is found that under continuous excitation, the photoluminescence intensity of a CH3 NH3 PbBr3 crystal can be fully controlled by sub-bandgap energy photon illumination. Such optically controllable emission behavior is rather general as it is observed also in CsPbBr3 and other perovskite materials. The switching mechanism is assigned to reversible light-induced activation/deactivation of nonradiative recombination centers, the presence of which relates to an excess of Pb during perovskite synthesis. Given the success of perovskites in photovoltaics and optoelectronics, it is believed that the discovery of green luminescence controlled by red illumination will extend the application scope of perovskites toward optical devices and intelligent control.

Ultrasensitive detection of local acoustic vibrations at room temperature by plasmon-enhanced single-molecule fluorescence.

Sensitive detection of local acoustic vibrations at the nanometer scale has promising potential applications involving miniaturized devices in many areas, such as geological exploration, military reconnaissance, and ultrasound imaging. However, sensitive detection of weak acoustic signals with high spatial resolution at room temperature has become a major challenge. Here, we report a nanometer-scale system for acoustic detection with a single molecule as a probe based on minute variations of its distance to the surface of a plasmonic gold nanorod. This system can extract the frequency and amplitude of acoustic vibrations with experimental and theoretical sensitivities of 10 pm Hz-1/2 and 10 fm Hz-1/2, respectively. This approach provides a strategy for the optical detection of acoustic waves based on molecular spectroscopy without electromagnetic interference. Moreover, such a small nano-acoustic detector with 40-nm size can be employed to monitor acoustic vibrations or read out the quantum states of nanomechanical devices.

Nonblinking Colloidal Quantum Dots via Efficient Multiexciton Emission.

Nonblinking colloidal quantum dots (QDs) are significant to their applications as single-photon sources or light-emitting materials. Herein, a simple heat-up method was developed to synthesize high-qualityWZ-CdSe/CdS core-shell colloidal QDs, which achieved a near-unity photoluminescence quantum yield (PLQY). It was found that the blinking behavior of such QDs was completely suppressed at high excitation intensities, and ultra-stable PL emission was observed. For this reason, a systematic investigation was conducted, revealing that the complete blinking suppression was attributed mainly to the efficient multiexciton emission at high excitation intensities. Such high-quality QDs with nonblinking behaviors and nearly ideal PL properties at high excitation intensities have massive potential applications in various robust conditions, including QD display screens, single-particle tracks, and single-photon sources.

Rational Building of Nonblinking Carbon Dots via Charged State Recovery.

Carbon dots (CDs), as emerging luminescent nanomaterials, possess excellent but complex properties, and thus, they have attracted immense attention for their applications. Further practical application of CDs has been hindered by their limited photostability and photoluminescence intermittency. In this study, we demonstrated that an antioxidant (Trolox) can dramatically enhance the photostability and minimize the photoblinking of CDs without affecting their native spectral characteristics. Significant photoluminescence enhancement and stabilization were observed with the addition of Trolox in ensemble level. Meanwhile, strikingly stable emissions from individual CDs have been observed in the presence of Trolox in single-particle-level experiments. Our observations revealed that the charged state of CDs can be effectively recovered to a neutral state by Trolox via electron transfer. These results prove that the combination of antioxidants and CDs is a powerful means to improve their fluorescence robustness, which is crucial for applications that demand long-lived, nonblinking emission.

Localized quenching sites in MAPbI3 investigated by fluorescence and photothermal microscopy.

In this work, we developed a fluorescence and photothermal microscope with extremely large scanning range and high spatial resolution. We demonstrated the capability of this instrument by simultaneously measuring the photoluminescence and photothermal signals of the CH3NH3PbI3 (MAPbI3) film. After scanning the MAPbI3 film on the scale of centimeters, we can obtain information of both emissive and nonemissive processes with a resolution of 200 nm at any location of the large area. We can clearly see the localized photothermal signal while the photoluminescence signal is uniform. These results directly prove that the emissive recombination happens all over the materials, but the nonemissive recombination happens only at certain localized quenching sites. The fluorescence and photothermal microscope with both large scanning range and high spatial resolution can provide information of all the relaxation channels of the excitons, showing potential applications for investigation of photophysical mechanisms in photoelectric materials.

Electric-Field-Induced Ion Migration Behavior in Methylammonium Lead Iodide Perovskite.

Ionic movement inside organometal halide perovskites (OMHP) materials has been widely reported to be linked with stability issues in the perovskite-based optoelectronic devices. However, the dynamic processes of the ionic movement and how they influence the devices are still not well-understood. In this work, we applied an external electric field to the CH3NH3PbI3 crystal and simultaneously monitored the PL behaviors. Two successive PL responses were observed in the same location of the crystal. First, an irreversible PL quenching was observed caused by the photo-annealing effect under an electric field accompanied by a permanent morphology change. The annealed area also showed reversible PL variation, which was attributed to the activation-deactivation of the radiative recombination centers induced by the migration of the iodine ions. Such results can help us gain a deep insight into how the ionic movements in OMHPs influence the performance of the perovskite-based optoelectronic devices under working conditions.

Physical insights into protection effect of conjugated polymers by natural antioxidants.

Conjugated polymers (CPs) known as organic semiconductors have been broadly applied in photovoltaic and light emitting devices due to their easy fabrication and flexibility. However, one of the bottlenecks limiting the application of CPs is their poor photostability upon continuous excitation which is one of the crucial parameters of CPs. How to improve the photostability of CPs is always one of the key questions in this field. In this work, we found that the photostability of poly(3-hexylthiophene-2,5-diyl) (P3HT) molecules can be largely improved by addition of vitamin E (VE) in bulk solution, solid films and single molecules. In solution and films, VE can not only significantly retard the photodegradation of P3HT but also enhance the fluorescence intensity. For individual P3HT molecules, with increasing VE concentrations, the on-time duration increases and the off-time duration becomes shorter. VE as natural antioxidants can not only donate electrons to the long-lived charged species but also quench the triplet states of CPs via energy transfer accelerating the depopulation process back to the ground state. The short duration time of the charged species and the triplet states provides higher fluorescence intensity. Furthermore, VE can also directly react with singlet oxygen or other reactive oxygen species (ROS) preventing them from reacting with CPs. These results not only provide an efficient strategy for improving the photostability of conjugated polymers in solution and films, but also shed light on better understanding the photophysics of conjugated polymers at single-molecule level.

Excitonic Au4Ru2(PPh3)2(SC2H4Ph)8 cluster for light-driven dinitrogen fixation.

The surface plasmon resonance of metal nanoparticles has been widely used to improve photochemical transformations by plasmon-induced charge transfer. However, it remains elusive for the molecular-like metal clusters with non-metallic or excitonic behavior to enable light harvesting including electron/hole pair production and separation. Here we report a paradigm for solar energy conversion on an atomically precise Au4Ru2 cluster supported on TiO2 with oxygen vacancies, in which the electron-hole pairs can be directly generated from the excited Au4Ru2 cluster and the TiO2 support, and the photogenerated electrons can transfer to the Ru atoms. Importantly, the Ru atoms in the Au4Ru2 cluster are capable of injecting the electrons into adsorbed N2 to activate N2 molecules. The cooperative effect in the supported Au4Ru2 catalyst efficiently boosts the photocatalytic activity for N2 fixation in comparison with homogold (Au n ) clusters.