Bing-Rong Gao

Time-resolved fluorescence study of aggregation-induced emission enhancement by restriction of intramolecular charge transfer state.

Cyano-substituted oligo (alpha-phenylenevinylene)-1,4-bis(R-cyano-4-diphenylaminostyryl)-2,5-diphenylbenzene (CNDPASDB) molecules are studied in solution and aggregate state by time-resolved fluorescence techniques. CNDPASDB exhibits a strong solvent polarity dependent characteristic of aggregation-induced emission (AIE). By time-dependent spectra, the gradual transition from local excited state to intramolecular charge transfer state with the increasing solvent polarity is clearly resolved. The transition time in high polarity solvent DMF is very fast, around 0.5 ps, resulting in a low fluorescence quantum yield. While in aggregate state, the intramolecular torsion is restricted and the local environment becomes less polar. Thus, the intramolecular charge transfer state is eliminated and efficient AIE occurs.

Exciton diffusion and charge transfer dynamics in nano phase-separated P3HT/PCBM blend films.

Exciton quenching dynamics has been systematically studied in pristine P3HT and nano phase separated P3HT/PCBM blend films under various excitation intensities by femtosecond fluorescence up-conversion technique. The behaviors of excitons in the films can be well described by a three-dimensional diffusion model. The small diffusion length and large charge transfer radius indicate that excitons reach the interface most likely by the delocalization of the excitons in P3HT fibrillar at a range of 4.8-9 nm so that the excitons can quickly delocalize in the P3HT domain to reach the interface (instead of by diffusion).

The Role of Trap-assisted Recombination in Luminescent Properties of Organometal Halide CH3NH3PbBr3 Perovskite Films and Quantum Dots

Hybrid metal halide perovskites have been paid enormous attentions in photophysics research, whose excellent performances were attributed to their intriguing charge carriers proprieties. However, it still remains far from satisfaction in the comprehensive understanding of perovskite charge-transport properities, especially about trap-assisted recombination process. In this Letter, through time-resolved transient absorption (TA) and photoluminescence (PL) measurements, we provided a relative comprehensive investigation on the charge carriers recombination dynamics of CH3NH3PbBr3 (MAPbBr3) perovskite films and quantum dots (QDs), especially about trap-assisted recombination. It was found that the integral recombination mode of MAPbBr3 films was highly sensitive to the density distribution of generated charge carriers and trap states. Additional, Trap effects would be gradually weakened with elevated carrier densities. Furthermore, the trap-assisted recombination can be removed from MAPbBr3 QDs through its own surface passivation mechanism and this specialty may render the QDs as a new material in illuminating research. This work provides deeper physical insights into the dynamics processes of MAPbBr3 materials and paves a way toward more light-harvesting applications in future.

Study of aggregation induced emission of cyano-substituted oligo (p-phenylenevinylene) by femtosecond time resolved fluorescence

The aggregation induced emission (AIE) mechanism of the cyano-substituted oligo (p-phenylenevinylene)1,4-bis [1-cyano-2-(4-(diphenylamino) phenyl) vinyl] benzene (TPCNDSB) is investigated by time resolved fluorescence technique. By reconstructing the time resolved emission spectra (TRES), it is found that in solvent of low polarity, the emission is mainly from the local emission (LE) state with high quantum yield, but in high polarity solvent, the emission is mainly from the intramolecular charge transfer (ICT) state, which is a relatively dark state, with low quantum yield. In crystal form, the restriction of transfer from LE state to ICT state results in efficient AIE.

Transient Absorption Spectroscopic Study on Band-Structure-Type Change in CdTe/CdS Core-Shell Quantum Dots

We report on the band edge alignment in zinc-blended-type CdTe/CdS core-shell quantum dots, using femtosecond spectroscopy. Time-resolved transient absorption spectroscopy directly shows that the system gradually developed from quasi-type-I to type-II as the shell thickness increased. Following photoexcitation of CdS, the injection of a hole from CdS valence band into CdTe valence band is observed. The hole transfer occurs on the 1000 fs time scale in these heteromaterials. Furthermore, a strain ca. 150 meV is obtained by comparing the transient with steady-state spectra.

Surface plasmon enhanced absorption dynamics of regioregular poly(3-hexylthiophene)

An increased absorption of regioregular poly(3-hexylthiophene) due to surface plasmons (SPs) has been studied by steady state fluorescence spectra measurements and femtosecond transient absorption (TA) technique. The SPs were excited by periodic Ag grating structures transferred from the photoresist gratings which were fabricated by two-beam laser interference. The steady state fluorescence measurements show a maximum enhanced fluorecence of about 16 times larger arising from the absorption enhancement. Further investigation by TA technique reveals that the yield of the polaron pairs on Ag gratings significantly increase, much bigger than that of the pure sample, which also confirms the enhanced absorption efficiency as resulting from the excited SPs.

Hierarchical self-assembly of CdTe quantum dots into hyperbranched nanobundles: suppression of biexciton Auger recombination.

In this paper, we report a novel nanobundle structure formed by the hierarchical self-assembly of TGA-capped CdTe quantum dots. HR-TEM confirms the polycrystalline phase of the bundle structure, and that pristine quantum dots are the building units. The steady state absorption and luminescence properties of the pristine quantum dots can be well inherited by the nanobundles. In transient state observation, carrier quenching induced by Auger recombination is found to be remarkably suppressed. Electron delocalizing to close building units is considered to be the reason. Suppression of Auger recombination may earn much more time for charge separation, which makes the novel nanobundle structures suitable for the excellent donor material in solar cell applications.

Temporal dynamics of two-photon-pumped amplified spontaneous emission in slab organic crystals

We have studied the ultrafast dynamics of two-photon-pumped amplified spontaneous emission (ASE) from a single crystal by the time-resolved fluorescence upconversion technique. With the increase of two-photon pump intensities, the emission decay time is dramatically shortened by 30 times (from 3ns to approximately 87 ps), and the energy migration rate is acutely enhanced when ASE occurs. The stripe length is also found to play an important role in the formation of the ASE. Meanwhile, the gain coefficient is evaluated to be 15cm(-1) for 560nm at an excitation intensity of 2.3mJ/pulse/cm(2) by the variable stripe length technique.

Investigation of Polaron Pair Dynamics in Poly(3-Hexylthiophene) Film by Time Resolved Spectroscopy

Polaron pair dynamics in regioregular Poly (3-hexylthiophene) (RR-P3HT) film was studied by applying both femtosecond transient absorption and fluorescence up-conversion techniques. The comparison of the dynamics of the pure fluorescence, the ground state bleaching recovery and the polaron pair absorption reveals that the polaron pairs decay back to the ground state of certain morphological places with distinct absorption features, instead of the singlet excited state.

Evidence of concerted inversion for the photon-induced molecular switching of azobenzene using rotation-free azobenzene derivatives

The excited state relaxation of two types of azobenzene derivative excited at 350 nm in the short-wavelength (S2(π, π*)) band was studied by transient absorption spectroscopy. We compared the relaxation dynamics and the time-resolved anisotropic relaxation properties of these two molecules in liquid solution and in bulk film. The average relaxation life-time for Azo-2 in bulk film (13.2 ps) is much slower than that of Azo-2 (4.06 ps) and Azo-1(1.31 ps) in liquid solution. The time-resolved polarization anisotropy spectroscopy measurements of these samples also showed significant differences. The value of anisotropy for Azo-2 remained almost constant at about 0.2 in bulk film but decayed slowly from 0.3 in liquid solution. The value for Azo-1 in liquid solution decayed very fast from 0.4 to 0.2 within 6 ps. Our results gave new evidence for the existence of a converted inversion channel. We also concluded that the concerted inversion process and the conventional rotation process govern the relaxation for the photon-induced switches of azobenzene and its derivatives under short-wavelength band excitation.