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Abstract Germanium sulfide (GeS) is a 2D semiconductor with potential for high-speed optoelectronics and photovoltaics due to its near-infrared band gap and high mobility of optically excited charge carriers. Here, we use time-resolved THz spectroscopy to investigate the differences in ultrafast carrier dynamics in GeS following near-band gap photoexcitation (1.55 eV), which penetrates deep into the multilayer GeS, and excitation with above-band gap photon energy (3.1 eV), which is absorbed within a sub-20 nm surface layer. We find that the photoexcited carriers in the bulk have significantly longer lifetimes and higher mobility, as they are less impacted by trap states that affect carrier behavior in the surface layer. These insights are important for designing GeS-based photodetectors, solar energy conversion devices, and sensors that leverage the sensitivity of surface-layer photoexcited carriers to trap states. 

Abstract Above‐band gap optical excitation of non‐centrosymmetric semiconductors can lead to the spatial shift of the center of electron charge in a process known as shift current. Shift current is investigated in single‐crystal SnS₂, a layered semiconductor with the band gap of ≈2.3 eV, by THz emission spectroscopy and first principles density functional theory (DFT). It is observed that normal incidence excitation with above gap (400 nm; 3.1 eV) pulses results in THz emission from 2H SnS₂() polytype, where such emission is nominally forbidden by symmetry. It is argued that the underlying symmetry breaking arises due to the presence of stacking faults that are known to be ubiquitous in SnS₂single crystals and construct a possible structural model of a stacking fault with symmetry properties consistent with the experimental observations. In addition to shift current, it is observed THz emission by optical rectification excited by below band gap (800 nm; 1.55 eV) pulses but it requires excitation fluence more than two orders of magnitude higher to produce same signal amplitude. These results suggest that ultrafast shift current in which can be excited with visible light in blue–green portion of the spectrum makes SnS₂a promising source material for THz photonics. 

Abstract Significant optical absorption in the blue–green spectral range, high intralayer carrier mobility, and band alignment suitable for water splitting suggest tin disulfide (SnS₂) as a candidate material for photo‐electrochemical applications. In this work, vertically aligned SnS₂nanoflakes are synthesized directly on transparent conductive substrates using a scalable close space sublimation (CSS) method. Detailed characterization by time‐resolved terahertz and time‐resolved photoluminescence spectroscopies reveals a high intrinsic carrier mobility of 330 cm²V⁻¹s⁻¹and photoexcited carrier lifetimes of 1.3 ns in these nanoflakes, resulting in a long vertical diffusion length of ≈1 µm. The highest photo‐electrochemical performance is achieved by growing SnS₂nanoflakes with heights that are between this diffusion length and the optical absorption depth of ≈2 µm, which balances the competing requirements of charge transport and light absorption. Moreover, the unique stepped morphology of these CSS‐grown nanoflakes improves photocurrent by exposing multiple edge sites in every nanoflake. The optimized vertical SnS₂nanoflake photoanodes produce record photocurrents of 4.5 mA cm⁻²for oxidation of a sulfite hole scavenger and 2.6 mA cm⁻²for water oxidation without any hole scavenger, both at 1.23 V_RHEin neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte. 

SnS2 is a two-dimensional (2D) layered semiconductor with a visible-range bandgap (~2.3eV), high charge carrier mobility, long carrier lifetimes, and good environmental stability. This study explores the impact of zero-valent metal intercalation into the van der Waals gaps of SnS2 on charge carrier dynamics. We demonstrate that metal intercalation enhances optical absorption in the yellow-to-IR range and induces metal-dependent bandgap shifts. Time-resolved THz spectroscopy reveals that different metals uniquely influence photoconductivity dynamics: We find that intercalation with Bi, Ni, and Fe shortens the photoconductivity decay times, whereas Rh intercalation results in a slower decay. These findings highlight the potential of metal intercalation to tailor SnS2 properties for diverse applications, from solar energy conversion to high-speed photodetectors. 

We report on THz emission in single-crystalline SnS2 in response to above bandgap excitation. Symmetry properties of THz generation suggest that its origin is an ultrafast surface shift current, a 2nd order nonlinear effect that can occur as a result of above-gap photoexcitation of a non-centrosymmetric semiconductor. Multilayer SnS2 can exist in several polytypes that differ in the layer stacking. Of those polytypes, 2H and 18R are centrosymmetric while 4H is not. While Raman spectroscopy suggests that the single crystalline SnS2 in our experiments is 2H, its THz emission has symmetry that are fully consistent with the P3m1 phase of 4H polytype. We hypothesize that the stacking disorder, where strain-free stacking faults that interrupt regions of 2H polytype, can break inversion symmetry and result in THz emission. These results lay the foundations for application of SnS2 as an efficient, stable, flexible THz source material, and highlight the use of THz spectroscopy as a sensitive tool for establishing symmetry properties of materials. 

GeS and GeSe are 2D semiconductors with band gaps in the near infrared and predicted high carrier mobility. We find that excitation with 800 nm pulses results in long-lived free photocarriers, persisting for hundreds of picoseconds, in GeS and GeSe noribbons. We also demonstrate that zerovalent Cu intercalation is an effective tool for tuning the photoconductive response. Intercalation of ~ 3 atomic % of zerovalent Cu reduces the carrier lifetime in GeSe and GeS. In GeS, it also shortens the photoconductivity rise and improves carrier mobility. 

Germanium sulfide (GeS) and germanium selenide (GeSe) are layered 2D van der Waals materials that belong to a family of group-IV monochalcogenides. These semiconductors have high carrier mobilities and moderate band gaps in the near infrared. Additionally, we have demonstrated that above gap photoexcitation results in ultrafast surface photocurrents and emission of THz pulses due to a spontaneous ferroelectric polarization that breaks inversion symmetry in the monolayer. Beyond the sub-picosecond time scales of shift currents, photoexcited carriers in both materials result in long-lived transient conductivity. We find that 800 nm excitation results in longer lived free photocarriers, persisting for hundreds of picoseconds to several nanoseconds, compared to tens to hundreds of picoseconds lifetimes for 400 nm excitation. Here, we report on tailoring the free photoexcited carrier lifetimes by intercalation of zero-valent Cu into the van der Waals gaps of GeS and GeSe. Density functional theory calculations predict that Cu atoms introduce mid-gap states. We demonstrate that intercalating only ∼3 atomic % of zero-valent Cu reduces the carrier lifetime by as much as two-to-four-fold, raising the prospects of these materials being used for high-speed optoelectronics.