Spectroscopy and trapping dynamics in WS2 nanoclusters

WS2 nanoclusters have been synthesized using inverse micelle methods and characterized by TEM, electron diffraction, and optical spectroscopy. The TEM images and rotational diffusion results show that the particles have average diameters of approximately 4-7 nm and have the same crystal structure as bulk WS2. The electron diffraction results are compared with diffraction patterns calculated as a function of the nanocluster diameter and thickness. This comparison shows that the particles are single trilayer disks. The absorption spectrum shows a large blue shift compared to bulk WS2, with the first absorption maximum shifting from about 680 nm to 364 nm. Polarized emission is observed following photoexcitation. The emission polarization spectra indicate that absorption occurs into several different low-lying states. The results also indicate that emission from the band-edge state is polarized, while emission from trap states is not. The dynamics of these nanoclusters in room temperature solutions have been examined using time-resolved emission and polarization spectroscopies. Trap-state emission exhibits multiexponential distributed kinetics, while emission from the band-edge state follows a single exponential decay. In samples having a high density of subband-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 emission is from the band-edge state. Time-resolved emission polarization measurements indicate that trapping takes place on the 100 ps time scale, and that the trapping rate depends on the density of trap states.