Brian Gregory Alberding, Adam J. Biacchi, Angela R. Hight Walker and Edwin Jay Heilweil
Tin(II) monosulfide (SnS) is a semiconductor material with an intermediate band gap, high absorption coefficient in the visible range, and earth abundant, non-toxic constituent elements. For these reasons, SnS has generated much interest for incorporation into optoelectronic devices, but little is known concerning the charge carrier dynamics, especially as measured by optical techniques. Here, phase-pure, colloidal SnS has been synthesized by solution chemistry in three size regimes, ranging from the nanometer- to the micron-scale, and evaluated by time-resolved terahertz spectroscopy (TRTS), an optical, non-contact probe of the photoconductivity. Dropcast films of the SnS colloids were studied by TRTS both as-deposited and after thermal annealing. The TRTS results revealed that the micron-scale SnS crystals and all of the annealed films undergo decay mechanisms during the first 200 ps following photoexcitation at 800 nm assigned to hot carrier cooling and carrier trapping. The charge carrier mobility of both the dropcast and annealed samples depends strongly on the size of the constituent colloids. The mobility of the SnS colloidal films, following the completion of the initial decays, ranged from 0.14 cm2/V·s for the smallest SnS crystals to 20.3 cm2/V·s for the largest. Annealing these colloidal films resulted in a ~ 20 % improvement in mobility for the largest SnS samples and a ~ 5-fold increase for the smaller nanocrystals.
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