Ultrafast Carrier Dynamics in CdSe/CdS/ZnS Quantum Dots

The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe technique. By selectively exciting the core and shell carrier, the dynamics are studied in detail. Carrier relaxation is found faster in the conduction band of the CdS shell （about 130 fs） than that in the conduction band of the CdSe core （about 400 fs）. From the experiments it is distinctly demonstrated the existence of the defect states in the interface between the CdSe core and the CdS shell, indicating that ultrafast spectroscopy might be a suitable tool in studying interface and surface morphology properties in nanosystems.

Kinetics and Dynamics of Photocatalyzed Dissociation of Ethanol on Ti02（110）

The kinetics and dynamics of photocatalyzed dissociation of ethanol on TiO2（110） sur- face have been studied using the time-dependent and time-resolved femtosecond two-photon photoemission spectroscopy respectively, in order to unravel the photochemical properties of ethanol on this prototypical metal oxide surface. By monitoring the time evolution of the photoinduced excited state which is associated with the photocatalyzed dissociation of ethanol on Ti5c sites of Ti02（ll0）, the fractal-like kinetics of this surface photocatalytic reaction has been obtgined. The measured photocatalytic dissociation rate on reduced TiO2（l10） is faster than that on the oxidized surface. This is attributed to the larger defect density on the reduced surface which lowers the reaction barrier of the photocatalytic reaction at least methodologically. Possible reasons associated with the defect electrons for the acceleration have been discussed. By performing the interferometric two-pulse corre- lation on ethanol/TiO2（l10） interface, the ultrafast electron dynamics of the excited state has been measured. The analyzed lifetime （24 fs） of the excited state is similar to that on methanol/TiO2（110）. The appearance of the excited state provides a channel to mediate the electron transfer between the TiO2 substrate and its environment. Therefore studying its ultrafast electron dynamics may lead to the understanding of the microscopic mechanism of photocatalysis and photoelectrochemical energy conversion on TiO2.

A Surface Femtosecond Two-Photon Photoemission Spectrometer for Excited Electron Dynamics and Time-Dependent Photochemical Kinetics

A surface femtosecond two-photon photoemission （2PPE） spectrometer devoted to the study of ultrafast excited electron dynamics and photochemical kinetics on metal and metal oxide surfaces has been constructed. Low energy photoelectrons are measured using a hemispherical electron energy analyzer with an imaging detector that allows us to detect the energy and the angular distributions of the photoelectrons simultaneously. A Mach-Zehnder interferom- eter was built for the time-resolved 2PPE （TR-2PPE） measurement to study ultrafast surface excited electron dynamics, which was demonstrated on the Cu（111） surface. A scheme for measuring time-dependent 2PPE （TD-2PPE） spectra has also been developed for studies of surface photochemistry. This technique has been applied to a preliminary study on the photochemical kinetics on ethanol/TiO2（110）. We have also shown that the ultrafast dynamics of photoinduced surface excited resonances can be investigated in a reliable way by combining the TR-2PPE and TD-2PPE techniques.

Doping Copper Ions in a Metal-Organic Framework(UiO-66-NH2):Location Effect Examined by Ultrafast Spectroscopy

We constructed two types of copper-doped metal-organic framework(MOF),i.e.,Cu@UiO-66-NH2 and Cu-UiO-66-NH2.In the former,Cu2+ions are impregnated in the pore space of the amine-functionalized,Zr-based UiO-66-NH2;while in the latter,Cu^2+ions are incorporated to form a bimetal-center MOF,with Zr^4+being partially replaced by Cu2+in the Zr-O oxo-clusters.Ultrafast spectroscopy revealed that the photoinduced relaxation kinetics associated with the ligand-to-cluster charge-transfer state is promoted for both Cudoped MOFs relative to undoped one,but in a sequence of Cu-UiO-66-NH2>Cu@UiO-66-NH2>UiO-66-NH2.Such a sequence turned to be in line with the trend observed in the visible-light photocatalytic hydrogen evolution activity tests on the three MOFs.These findings highlighted the subtle effect of copper-doping location in this Zr-based MOF system,further suggesting that rational engineering of the specific metal-doping location in alike MOF systems to promote the photoinduced charge separation and hence suppress the detrimental charge recombination therein is beneficial for achieving improved performances in MOF-based photocatalysis.

Unveiling Defect-Mediated Carrier Dynamics in Few-Layer MoS2 Prepared by Ion Exchange Method via Ultrafast Vis-NIR-MIR Spectroscopy

Defect-mediated processes in two-dimensional transition metal dichalcogenides have a significant influence on their carrier dynamics and transport properties,however,the detailed mechanisms remain poorly understood.Here,we present a comprehensive ultrafast study on defect-mediated carrier dynamics in ion exchange prepared few-layer MoS2 by femtosecond time-resolved Vis-NIR-MIR spectroscopy.The broadband photobleaching feature observed in the near-infrared transient spectrum discloses that the mid-gap defect states are widely distributed in few-layer MoS2 nanosheets.The processes of fast trapping of carriers by defect states and the following nonradiative recombination of trapped carriers are clearly revealed,demonstrating the mid-gap defect states play a significant role in the photoinduced carrier dynamics.The positive to negative crossover of the signal observed in the mid-infrared transient spectrum further uncovers some occupied shallow defect states distributed at less than0.24 e V below the conduction band minimum.These defect states can act as effective carrier trap centers to assist the nonradiative recombination of photo-induced carriers in few-layer MoS2 on the picosecond time scale.