Spectroscopy and trapping dynamics in WS2 nanoclusters

WS2 nanoclusters have been synthesized using inverse micelle methods and ch aracterized by TEM, electron diffraction, and optical spectroscopy. The TEM images and rotational diffusion results show that the particles have avera ge diameters of approximately 4-7 nm and have the same crystal structure as bulk WS2. The electron diffraction results are compared with diffraction p atterns calculated as a function of the nanocluster diameter and thickness. This comparison shows that the particles are single trilayer disks. The ab sorption spectrum shows a large blue shift compared to bulk WS2, with the f irst absorption maximum shifting from about 680 nm to 364 nm. Polarized emi ssion is observed following photoexcitation. The emission polarization spec tra indicate that absorption occurs into several different low-lying states . The results also indicate that emission from the band-edge state is polar ized, while emission from trap states is not. The dynamics of these nanoclu sters in room temperature solutions have been examined using time-resolved emission and polarization spectroscopies. Trap-state emission exhibits mult iexponential distributed kinetics, while emission from the band-edge state follows a single exponential decay. In samples having a high density of sub band-gap trap states, the vast majority of the emission is from trap states . Tn samples in which most of the traps have been passivated, most of the e mission is from the band-edge state. Time-resolved emission polarization me asurements indicate that trapping takes place on the 100 ps time scale, and that the trapping rate depends on the density of trap states.