Ultrafast Zero-Bias Photocurrent in GeS Nanosheets: Promise for Photovoltaics

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 so-called 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

Generation of Terahertz Radiation by Optical Excitation of Aligned Carbon Nanotubes

We have generated coherent pulses of terahertz radiation from macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs) excited by femtosecond optical pulses without externally applied bias. The generated terahertz radiation is polarized along the SWCNT alignment direction. We propose that top-bottom asymmetry in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized terahertz radiation by a transient photocurrent surge directed along the nanotube axis

Size <i>vs</i> Surface: Tuning the Photoluminescence of Freestanding Silicon Nanocrystals Across the Visible Spectrum <i>via</i> Surface Groups

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3–4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies

Dynamics of Photoexcited Carriers in Polycrystalline PbS and at PbS/ZnO Heterojunctions: Influence of Grain Boundaries and Interfaces

We investigate the impact of grain boundaries and interfaces on dynamics of photoexcited charge carriers in polycrystalline lead sulfide (PbS) films and at interfaces between polycrystalline PbS and ZnO by studying transient photoconductivity over sub-picoseconds to microseconds timescales using time-resolved terahertz spectroscopy and time-resolved microwave conductivity measurements. Narrow band gap bulk-like polycrystalline PbS with high absorption in the infrared paired with wide band gap metal oxide current collectors holds promise for infrared photodetectors and photovoltaics for converting infrared radiation to electricity. We find that grain boundaries in polycrystalline PbS suppress long-range conductivity and confine photoexcited carriers within individual crystallites. The mobility of photoexcited holes inside the ∼150 nm crystallites reaches 750 cm²/V s, and their lifetime exceeds hundreds of microseconds, while electrons get rapidly trapped at grain boundary states. The presence of PbS/ZnO interfaces dramatically reduces the lifetime of the photoexcited free holes in the PbS crystallites. Moreover, we detect no injection of free electrons from PbS to ZnO. Optimal transfer of photoexcited electrons, as is needed for optoelectronic devices with PbS/ZnO heterojunctions, may require engineering PbS/ZnO heterojunctions with buffer layers or organic ligands to passivate deleterious interface states

High Light Absorption and Charge Separation Efficiency at Low Applied Voltage from Sb-Doped SnO₂/BiVO₄ Core/Shell Nanorod-Array Photoanodes

BiVO₄ has become the top-performing semiconductor among photoanodes for photoelectrochemical water oxidation. However, BiVO₄ photoanodes are still limited to a fraction of the theoretically possible photocurrent at low applied voltages because of modest charge transport properties and a trade-off between light absorption and charge separation efficiencies. Here, we investigate photoanodes composed of thin layers of BiVO₄ coated onto Sb-doped SnO₂ (Sb:SnO₂) nanorod-arrays (Sb:SnO₂/BiVO₄ NRAs) and demonstrate a high value for the product of light absorption and charge separation efficiencies (η_abs × η_sep) of ∼51% at an applied voltage of 0.6 V versus the reversible hydrogen electrode, as determined by integration of the quantum efficiency over the standard AM 1.5G spectrum. To the best of our knowledge, this is one of the highest η_abs × η_sep efficiencies achieved to date at this voltage for nanowire-core/BiVO₄-shell photoanodes. Moreover, although WO₃ has recently been extensively studied as a core nanowire material for core/shell BiVO₄ photoanodes, the Sb:SnO₂/BiVO₄ NRAs generate larger photocurrents, especially at low applied voltages. In addition, we present control experiments on planar Sb:SnO₂/BiVO₄ and WO₃/BiVO₄ heterojunctions, which indicate that Sb:SnO₂ is more favorable as a core material. These results indicate that integration of Sb:SnO₂ nanorod cores with other successful strategies such as doping and coating with oxygen evolution catalysts can move the performance of BiVO₄ and related semiconductors closer to their theoretical potential

Evolution of the Ultrafast Photoluminescence of Colloidal Silicon Nanocrystals with Changing Surface Chemistry

The role of surface species in the optical properties of silicon nanocrystals (SiNCs) is the subject of intense debate. Changes in photoluminescence (PL) energy following hydrosilylation of SiNCs with alkyl-terminated surfaces are most often ascribed to enhanced quantum confinement in the smaller cores of oxidized NCs or to oxygen-induced defect emission. We have investigated the PL properties of alkyl-functionalized SiNCs prepared using two related methods: thermal and photochemical hydrosilylation. Photochemically functionalized SiNCs exhibit higher emission energies than the thermally functionalized equivalent. While microsecond lifetime emission attributed to carrier recombination within the NC core was observed from all samples, much faster, size-independent nanosecond lifetime components were only observed in samples prepared using photochemical hydrosilylation that possessed substantial surface oxidation. In addition, photochemically modified SiNCs exhibit higher absolute photoluminescent quantum yields (AQY), consistent with radiative recombination processes occurring at the oxygen-based defects. Correlating spectrally- and time-resolved PL measurements and XPS-derived relative surface oxidation for NCs prepared using different photoassisted hydrosilylation reaction times provides evidence the PL blue-shift as well as the short-lived PL emission observed for photochemically functionalized SiNCs are related to the relative concentration of oxygen surface defects