Size dependence of spin dynamics in colloidal CdSe quantum dots (QDs) are investigated with circularly polarized pump-probe transmission spectroscopy at room temperature. The excitation energy is tuned to resonance with the lowest exciton (1S(h)1S(e)) energy of the CdSe QDs. The exciton spin dynamics of CdSe QD with the diameter of 5.2 nm shows monoexponential decay with a typical time constant of about 1-3 ps depending on the excitation energy. For the cases of CdSe QDs with smaller size (with the diameter of 4.0 and 2.4 nm), the exciton spin relaxation shows biexponential decay, a fast component with time constant of several ps and a slow one with time constant of hundreds of ps to nanosecond time scale. The fast spin relaxation arises from the bright-dark transition, i.e., J = ±1 ↔ -/+2 transition. This process is dominated by the hole spin flips, while the electron spin conserves. The slow spin relaxation is attributed to the intralevel exciton transitions (J = ±1 ↔ -/+1 transition), which is relevant to the electron spin flip. Our results indicate that the exciton spin relaxation pathways in CdSe QD are controllable by monitoring the particle size, and polarized pump-probe spectroscopy is proved to be a sensitive method to probe the exciton transition among the fine structures.
A nickel-catalyzed
Kumada coupling of aniline derivatives was developed
by selective cleavage of aryl C–N bonds under mild reaction
conditions. Without preinstallation of an ortho directing
group on anilines, the cross-coupling reactions of Boc-protected aromatic
amines with aryl Grignard reagents afforded unsymmetric biaryls. Mechanistic
studies by DFT calculations revealed that the nickel-mediated C–N
bond cleavage is the rate-limiting step.
The coherent longitudinal acoustic (LA) phonons in La and Nb codoped polycrystalline BiFeO3 (Bi0.8La0.2Fe0.99Nb0.01O3 (BLFNO)) films are photo-induced and detected by the ultrafast reflectance spectroscopy. The generation mechanism of LA phonons is strongly connected with the ferroelectric polarization and is attributed to the transient photostriction effect, which is a combination of the optical rectification effect and the electrostriction effect. The strain modulation of sound velocity and out-of-plane elastic properties are demonstrated in BLFNO film on SrTiO3, which gives the insight on the dynamical coupling between electrical polarization and lattice deformation. Our findings are desired for the design of BiFeO3-based photo-driven remote control micro/nano devices.
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