Synthesis, optical properties and ultrafast electronic relaxation of metal (silver, gold, platinum) and manganese(2+)-doped zinc sulfide nanoparticles

Ultrafast transient absorption and time-dependent luminescence spectroscopy have been performed on metal (Ag, Au, Pt) and semiconductor (Mn2+ doped ZnS) nanoparticles respectively. In metal nanoparticles it was found that the decay dynamics exhibit a complex size and surface dependence. Specifically, the photoinduced dynamics show an exponential time constant that is longer than the same in bulk for particle sizes of 4–40 nm. When particle size is reduced further the plasmon band is drastically broadened and the relaxation time constant is similar to bulk. Upon reducing the size still further to only 13 atoms per cluster a much longer electronic relaxation is observed. These results are attributed to an intrinsic size dependent reduction in the electron-phonon coupling when particle size is reduced. This slows down the electronic relaxation compared to bulk. The increase in the number of surface collisions for very small particles increases the rate of electronic relaxation relative to larger particles. For particles on the order of 13 atoms the excitation is more singular in nature and the long relaxation is attributed to the fact that the excited species is a molecular as opposed to a fermi system.; Also, the synthesis and luminescence decay kinetics of 1.2 nm Mn 2+ doped ZnS nanoclusters grown in reverse micelles are reported. The preparation method produces small particles with narrow size distribution and fluorescence bands near 400–450 nm and 585 nm. Time-dependent fluorescence decay measurements using picosecond, nanosecond and millisecond techniques reveal relaxation processes on all three time scales. In the doped sample, the red emission detected at 600 nm exhibits an 1–2 ms decay in addition to faster decays with time constants on the order of hundreds of ps, a few ns and tens of μs. While the slow decay is the same as that of bulk Mn 2+ doped ZnS, the fast decays are present for both doped and undoped samples and are unique to nanocluster ZnS, which are attributed to deep trap state emission. The blue luminescence near 400–450 nm is attributed to shallow trap state emission of ZnS. These results help to resolve the controversy in the literature regarding the lifetime of the Mn2+ emission in Mn2+ doped ZnS nanoparticles.