Ferroelectric semiconductors have been predicted to exhibit strong zero-bias shift current, spurring the search for ferroelectric semiconductors with band gaps in the visible range as candidates for socalled shift current photovoltaics with efficiencies not constrained by the Schockley−Queisser limit. Recent theoretical works have predicted that two-dimensional IV−VI monochalcogenides are multiferroic and capable of generating significant shift currents. Here we present experimental validation of this prediction, observing ultrafast shift currents by detecting terahertz electromagnetic pulses emitted by the photoexcited GeS nanosheets without external bias. We explore excitation fluence, orientation, and excitation polarization dependence of the terahertz emission to confirm that shift currents are indeed responsible for the observed emission. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third-generation photovoltaics based on shift current, or bulk photovoltaic effect.

No 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 SnS2 nanoflakes 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 cm2 V−1 s−1 and 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 SnS2 nanoflakes 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 SnS2 nanoflake photoanodes produce record photocurrents of 4.5 mA cm−2 for oxidation of a sulfite hole scavenger and 2.6 mA cm−2 for water oxidation without any hole scavenger, both at 1.23 VRHE in neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte.

MXenes is an emerging class of 2D transition metal carbides, nitrides and carbonitrides which exhibit large conductivity, ultrahigh volumetric capacitance, high threshold for light-induced damage and nonlinear optical transmittance, making them attractive candidates for a variety of optoelectronic and electrochemical applications. Here, we report on equilibrium and non-equilibrium free carrier dynamics of Ti3C2Tx gleaned from THz spectroscopic studies for the first time. Ti3C2Tx showed high (~2  ×  1021 cm−3) intrinsic charge carrier density and relatively high (~34 cm2 V−1 s−1) mobility of carriers with an exceptionally large, ~46 000 cm−1 absorption in the THz range, which suggests that Ti3C2Tx is well suited for THz detection. We also demonstrate that Ti3C2Tx conductivity and THz transmission can be manipulated by photoexcitation, as absorption of near-infrared, 800 nm pulses is found to cause transient suppression of the conductivity that recovers over hundreds of picoseconds. The possibility of control over THz transmission and conductivity by photoexcitation suggests the promise for application of Ti3C2Tx Mxenes in THz modulation devices and variable electromagnetic shielding.

Replacing some Bi with In in Bi 2 Se 3 transforms it from a topological insulator to a band insulator. Here, we have used time-resolved terahertz spectroscopy to investigate photoexcited carrier dynamics in (Bi 1−x In x ) 2 Se 3 films with indium concentration x = 0%, 25%, and 50%. In Bi 2 Se 3 , optically excited carriers scatter from the bulk conduction band states into high mobility topological surface states within picoseconds after excitation. We demonstrate that a second set of Dirac surface states, located ∼1.5−1.8 eV above the conduction band minimum and accessible to carriers excited by 3.1 eV pulses, is characterized by a higher mobility than the surface states within the band gap that dominate equilibrium conductivity. In (Bi 0.75 In 0.25 ) 2 Se 3 and (Bi 0.50 In 0.50 ) 2 Se 3 , which are insulating without photoexcitation, the dynamics of photoexcited free carriers are affected by the twin domain boundaries and are sensitive to the disorder introduced by indium substitution. Transient conductivity rise time, as well as the mobility and lifetime of the photoexcited carriers in (Bi 1−x In x ) 2 Se 3 films, can be tuned by the indium content, enabling tailoring of band insulators that have the desired optoelectronic properties and are fully structurally compatible with the topological insulator Bi 2 Se 3 for applications in high-speed photonic devices based on topological insulator/band insulator heterostructures.

We enhanced the solar energy conversion efficiency of solution-deposited Bi2S3 by annealing in sulfur vapor at elevated temperature.

International audienceWe report on the realization of electrically tunable micro-arrays of space-variant optically anisotropic optical vortex generators. Each individual light orbital angular momentum processor consists of a microscopic self-engineered nematic liquid crystal q-plate made of a nonsingular topological defect spontaneously formed under electric field. Both structural and optical characterizations of the obtained spin-orbit optical interface are analyzed. An analytical model is derived and results of simulations are compared with experimental data. The application potential in terms of parallel processing of the optical orbital angular momentum is quantitatively discussed

Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.