Charge Carrier Dynamics and Mobility Determined by Time-Resolved Terahertz Spectroscopy on Films of Nano-to-Micrometer-Sized Colloidal Tin(II) Monosulfide

Abstract

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, as opposed to prior studies of vapor deposited films, phase-pure colloidal SnS was synthesized by solution chemistry in three size regimes, ranging from nanometer- to micron-scale (SnS small nanoparticles, SnS medium 2D nanosheets, and SnS large 2D μm-sheets), 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 and compared to both thermally annealed films and dispersed suspensions of the same colloids. 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 the colloidal films resulted in a ~ 20 % improvement in mobility for the large SnS 2D μm-sheets and a ~ 5-fold increase for the small nanoparticles and medium nanosheets.

Direct comparison of time-resolved terahertz spectroscopy and Hall Van der Pauw methods for measurement of carrier conductivity and mobility in bulk semiconductors

“…The apparatus for terahertz spectroscopy is based on an amplified femtosecond Ti:sapphire kilohertz repetition rate laser system that has been previously described in detail [33]. Briefly, the amplified 800 nm output is split into three arms and used to generate the visible pump, THz probe, and gated electro-optic detection.…”

Ultrafast Transient Absorption and Terahertz Spectroscopy as Tools to Probe Photoexcited States and Dynamics in Colloidal 2D Nanostructures

“…Figure 5a depicts the quantity S(t) = Φ e (t)μ e + Φ h (t)μ h , as a function of time, t, after photoexcitation, which is the sum of the product of the quantum yield of electrons, Φ e (t) and holes, Φ h (t), and the real mobility component of their mobility, (μ e and μ h ). At early times, the real THz conductivity exhibits a fast decay at times ≤ 4 ps, which is assigned to relaxation of highly mobile hot charges to the band edge, where the mobility is lower [7,[22][23][24]. After 4 ps, S(t) is as high as 20 ± 2 cm 2 /Vs.…”

Static and Time-Resolved Terahertz Measurements of Photoconductivity in Solution-Deposited Ruthenium Dioxide Nanofilms

“…22,30,31 These increases in N can be on the order of 1×10 15 photons/cm 3 to 1×10 18 photons/cm 3 in TRTS experiments and therefore typically dominate the observed sign of ΔE. However, observation of ΔE < 0 does not necessarily identify the studied material as a semiconductor.…”