Ultrafast Electrical Measurements of Isolated Silicon Nanowires and Nanocrystals

Defect-mediated saturable absorption and carrier dynamics in tin (II) monosulfide quantum dots

Tin (II) monosulfide (SnS) has attracted considerable attention in emerging photonics and optoelectronics because of high carrier mobility, large absorption coefficient, anisotropic linear and nonlinear optical properties, and long-time stability. In this Letter, we report third-order nonlinear absorption and refraction of SnS quantum dots (QDs). Under excitation with 800-nm femtosecond pulses, QDs exhibit saturable absorption (saturation intensity ∼ 47.69 GW/cm2) and positive refractive nonlinearity (nonlinear refraction coefficient ∼ 1.24 × 10-15 cm2/W). Nonetheless, we investigate charge carrier dynamics using femtosecond transient absorption spectroscopy and propose the presence of midgap defect states which not only dictate carrier dynamics but also give rise to nonlinear optical properties in SnS QDs.

Ultrafast time-resolved carrier dynamics in tellurium nanowires using optical pump terahertz probe spectroscopy

We report carrier relaxation dynamics in semiconducting tellurium nanowires (average diameter ∼ 10 nm) using ultrafast time-resolved terahertz spectroscopy. After photoexcitation using an 800 nm pump pulse, we observed an initial increase in the THz conductivity due to the absorption of THz radiation by photoexcited carriers. The time evolution of the differential conductivity (Δσ(τ_pp) = σ_{pump on}(τ_pp) - σ_{pump off}) shows a bi-exponential relaxation with the initial fast decay time scale of τ₁ ∼ 25 ps followed by a longer relaxation time constant of τ₂ ∼ 100 ps. Interestingly, the two time scales depend on the amount of the capping agent present on the surface of TeNWs, showing a faster relaxation of the photoexcited carriers as the percentage of capping decreases. This is physically interpreted as the surface state mediated relaxation mechanism of the photo-pumped carriers depending on the density of available surface states. A quantitative understanding is obtained using a coupled rate equation model taking into account the decay mechanisms determined from the surface mediated relaxation rate (D_S) and direct recombination rate (D_R) of the electron-hole pairs. Furthermore, the measured lattice temperature (T_L) dependent dynamics, showing a faster relaxation at lower temperature, is understood using the same rate equation model, giving a power law dependence of the electron-hole recombination rate (D_R) on T_L as D_R ∝ T_L^-1/2. This is explained by estimating D_R using the van Roosbroeck-Shockley theory taking into account the density of states () of one-dimensional nanowires. Furthermore, to understand the measured frequency-dependent THz photoconductivity, we model Δσ(ω) using the Boltzmann transport equation taking into account the energy-dependent scattering rates showing the dominant role of short range (Γ_sr) and Coulomb scattering (Γ_C) rates in the relaxation process, which further provides a measure of the charged and neutral impurity concentrations.

Dirac surface plasmons in photoexcited bismuth telluride nanowires: optical pump-terahertz probe spectroscopy

Collective excitation of Dirac plasmons in graphene and topological insulators has opened new possibilities of tunable plasmonic materials ranging from THz to mid-infrared regions. Using time resolved Optical Pump-Terahertz Probe (OPTP) spectroscopy, we demonstrate the presence of plasmonic oscillations in bismuth telluride nanowires (Bi₂Te₃ NWs) after photoexcitation using an 800 nm pump pulse. In the frequency domain, the differential conductivity (Δσ = σ_{pump on}-σ_{pump off}) spectrum shows a Lorentzian response where the resonance frequency (ω_p), attributed to surface plasmon oscillations, shifts with photogenerated carrier density (n) as . This dependence establishes the absorption of THz radiation by the Dirac surface plasmon oscillations of the charge carriers in the Topological Surface States (TSS) of Bi₂Te₃ NWs. Moreover, we obtain a modulation depth, tunable by pump fluence, of ∼40% over the spectral range of 0.5 to 2.5 THz. In addition, the time evolution of Δσ(t) represents a long relaxation channel lasting for more than 50 ps. We model the decay dynamics of Δσ(t) using coupled second order rate equations, highlighting the contributions from surface recombination as well as from trap mediated relaxation channels of the photoinjected carriers.

Both Free and Trapped Carriers Contribute to Photocurrent of Sb2Se3 Solar Cells

Polycrystalline semiconductor films, such as methylammonium lead iodide, cadmium telluride, copper-indium-gallium selenide, etc., are being intensively studied because of their great potential for highly efficient and cost-effective solar cells. Among them, polycrystalline antimony chalcogenide films are also promising for photovoltaic applications because they are nontoxic, stable, and flexible and have a suitable band gap. Considerable effort has already been devoted to improving the power conversion efficiency of antimony chalcogenide solar cells, but their efficiency still lingers below 10% due in part to scarce fundamental optoelectronic studies that help guide their development. Here, we use a combination of time-resolved terahertz and transient absorption spectroscopies to interrogate the optoelectronic behavior of antimony selenide thin films. By combining these two techniques we are able to monitor both free and trapped carrier dynamics and then evaluate their respective diffusion lengths. Our results indicate that trapped carriers remain mobile and can reach charge-collecting interfaces prior to recombination, and therefore, both free and trapped carriers can contribute to the photocurrent of antimony selenide solar cells.

Room-Temperature Electron Transport in Self-Assembled Sheets of PbSe Nanocrystals with a Honeycomb Nanogeometry

It has been shown recently that atomically coherent superstructures of a nanocrystal monolayer in thickness can be prepared by self-assembly of monodisperse PbSe nanocrystals, followed by oriented attachment. Superstructures with a honeycomb nanogeometry are of special interest, as theory has shown that they are regular 2-D semiconductors, but with the highest valence and lowest conduction bands being Dirac-type, that is, with a linear energy-momentum relation around the K-points in the zone. Experimental validation will require cryogenic measurements on single sheets of these nanocrystal monolayer superstructures. Here, we show that we can incorporate these fragile superstructures into a transistor device with electrolyte gating, control the electron density, and measure the electron transport characteristics at room temperature. The electron mobility is 1.5 ± 0.5 cm² V^-1 s^-1, similar to the mobility observed with terahertz spectroscopy on freestanding superstructures. The terahertz spectroscopic data point to pronounced carrier scattering on crystallographic imperfections in the superstructure, explaining the limited mobility.

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

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.

Size-Dependent Exciton Formation Dynamics in Colloidal Silicon Quantum Dots

We report size-dependent exciton formation dynamics within colloidal silicon quantum dots (Si QDs) using time-resolved terahertz (THz) spectroscopy measurements. THz photoconductivity measurements are used to distinguish the initially created hot carriers from excitons that form at later times. At early pump/probe delays, the exciton formation dynamics are revealed by the temporal evolution of the THz transmission. We find an increase in the exciton formation time, from ∼500 to ∼900 fs, as the Si QD diameter is reduced from 7.3 to 3.4 nm and all sizes exhibit slower hot-carrier relaxation times compared to bulk Si. In addition, we determine the THz absorption cross section at early delay times is proportional to the carrier mobility while at later delays is proportional to the exciton polarizability, αX. We extract a size-dependent αX and find an ∼r(4) dependence, consistent with previous reports for quantum-confined excitons in CdSe, InAs, and PbSe QDs. The observed slowing in exciton formation time for smaller Si QDs is attributed to decreased electron-phonon coupling due to increased quantum confinement. These results experimentally verify the modification of hot-carrier relaxation rates by quantum confinement in Si QDs, which likely plays a significant role in the high carrier multiplication efficiency observed in these nanomaterials.