Photocatalytic hydrogen production by biomimetic indium sulfide using Mimosa pudica leaves as template

Biomimetic sulfur-deficient indium sulfide (In2.77S4) was synthesized by a template-assisted hydrothermal method using leaves of Mimosa pudica as a template for the first time. The effect of this template in modifying the morphology of the semiconductor particles was determined by physicochemical characterization, revealing an increase in surface area, decrease in microsphere size and pore size and an increase in pore volume density in samples synthesized with the template. X-ray photoelectron spectroscopy (XPS) analysis showed the presence of organic sulfur (S O/S C/S H) and sulfur oxide species ( SO2, SO32−, SO42−) at the surface of the indium sulfide in samples synthesized with the template. Biomimetic indium sulfide also showed significant amounts of Fe introduced as a contaminant present on the Mimosa pudica leaves. The presence of these sulfur and iron species favors the photocatalytic activity for hydrogen production by their acting as a sacrificial reagent and promoting water oxidation on the surface of the templated particles, respectively. The photocatalytic hydrogen production rates over optimally-prepared biomimetic indium sulfide and indium sulfide synthesized without the organic template were 73 and 22 μmol g−1, respectively, indicating an improvement by a factor of three in the templated sample

Efficiency droop in zincblende InGaN/GaN quantum wells

The decrease in emission efficiency with increasing drive current density, known as ‘droop’, of c-plane wurtzite InGaN/GaN quantum wells presently limits the use of light-emitting diodes based on them for high brightness lighting applications. InGaN/GaN quantum wells grown in the alternative zincblende phase are free of the strong polarisation fields that exacerbate droop and so were investigated by excitation-dependent photoluminescence and photoreflectance studies. Polarisation-resolved measurements revealed that for all excitation densities studied the emission from such samples largely originates from similar microstructures or combinations of microstructures that form within the quantum well layers. Emission efficiency varies significantly with excitation at 10 K showing that non-radiative recombination processes are important even at low temperature. The onset of efficiency droop, as determined by photomodulated reflection measurements, occurred at a carrier density of around 1.2 × 1020 cm−3 – an order of magnitude greater than the value reported for a reference wurtzite quantum well sample using the same method. The high carrier density droop onset combined with the much shorter carrier lifetime within zincblende InGaN/GaN quantum wells indicate they have the potential to effectively delay efficiency droop when used in GaN based light-emitting diodes. However, the material quality of the quantum well layers need to be improved by preventing the formation of microstructures within these layers, and the importance of the role played by non-radiative centres in the QW layer needs to be elucidated, to fully realise the material's potential

Transient absorption measurements on Zn3N2 colloidal quantum dots

These transient absorption data were acquired from 3 samples of Zn3N2 colloidal quantum dots with different diameters using a system comprising a Helios (Ultrafast Systems LLC) spectrometer, an ultrafast Ti:sapphire amplifier system (Spectra Physics Solstice Ace) and an optical parametric amplifier (Topas Prime) with an associated NIR-UV-Vis unit. This system generated 100 fs pump pulses at 375 nm with a beam diameter of 240 μm. The pulse energy could be reduced using a series of reflective neutral density filters to give pump fluences from 1×1014 to 3.5×1015 photons·per cm2 per pulse. A white light continuum generated by the same laser system which was used as the probe to record changes in absorption between 430 and 913 nm. The samples were magnetically stirred to avoid photocharging effects during the measurements. Steady state absorbance spectra were acquired periodically to monitor and account for changes in absorbance due to oxidation of the Zn3N2 QDs

Experimental data on optical characterization of CsPbCl3 perovskite nanocrystals

Experimental data obtained by static-state and transient absorption and photoluminescence techniques as well as scanning transmission electron microscopy. These data are associated with the manuscript: Emission properties and ultrafast carrier dynamics of CsPbCl3 perovskite nanocrystals. Full details about interpretation of these data are given in the publication

Transient absorption measurements on Zn3N2 colloidal quantum dots

These transient absorption data were acquired from 3 samples of Zn3N2 colloidal quantum dots with different diameters using a system comprising a Helios (Ultrafast Systems LLC) spectrometer, an ultrafast Ti:sapphire amplifier system (Spectra Physics Solstice Ace) and an optical parametric amplifier (Topas Prime) with an associated NIR-UV-Vis unit. This system generated 100 fs pump pulses at 375 nm with a beam diameter of 240 μm. The pulse energy could be reduced using a series of reflective neutral density filters to give pump fluences from 1×1014 to 3.5×1015 photons·per cm2 per pulse. A white light continuum generated by the same laser system which was used as the probe to record changes in absorption between 430 and 913 nm. The samples were magnetically stirred to avoid photocharging effects during the measurements. Steady state absorbance spectra were acquired periodically to monitor and account for changes in absorbance due to oxidation of the Zn3N2 QDs.THIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV

Origin of the two dimensional electron gas at the CdO 100 surface

Synchrotron-radiation angle-resolved and core-level photoemission spectroscopy are used together to investigate the origin of the two-dimensional (2D) electron gas on the surface of single-crystal CdO (100) films. A reduction in the two-dimensional electron density of the surface state is observed under the synchrotron beam during angle-resolved photoemission spectroscopy, which is shown to be accompanied by a concomitant reduction in the surface-adsorbed species (monitored through the O 1s core-level signal). This shows that surface adsorbates donate electrons into the surface accumulation layer. When the surface is cleaned, the surface conduction band state empties. A surface doped with atomic H is also studied. Here, interstitial H increases the two-dimensional electron density at the surface. This demonstrates that reversible donor doping is possible. The surface band-bending profiles, 2D electron densities, and effective masses are calculated from subband dispersion simulations

Effect of Micron-scale Photoluminescence Variation on Droop Measurements in InGaN/GaN Quantum Wells

Abstract Micro-photoluminescence maps reveal micron-scale spatial variation in intensity, peak emission energy and bandwidth across InGaN/GaN quantum wells. To investigate the effect of this spatial variation on measurements of the dependence of emission efficiency on carrier density, excitation power-dependent emission was collected from a bright and dark region on each of blue-and green emitting samples. The onset of efficiency droop was found to occur at a greater carrier density in the dark regions than in the bright, by factors of 1.2 and 1.8 in the blue and green-emitting samples, respectively. By spatially integrating the emission from progressively larger areas, it is also shown that collection areas greater than ∼50 μm in diameter are required to reduce the intensity variation to less than 10%.</jats:p

Disentangling the Impact of Point Defect Density and Carrier Localization-Enhanced Auger Recombination on Efficiency Droop in (In,Ga)N/GaN Quantum Wells.

The internal quantum efficiency of (In,Ga)N/GaN quantum wells can surpass 90% for blue-emitting structures at moderate drive current densities but decreases significantly for longer emission wavelengths and at higher excitation rates. This latter effect is known as efficiency "droop" and limits the brightness of light-emitting diodes (LEDs) based on such quantum wells. Several mechanisms have been proposed to explain efficiency droop including Auger recombination, both intrinsic and defect-assisted, carrier escape, and the saturation of localized states. However, it remains unclear which of these mechanisms is most important because it has proven difficult to reconcile theoretical calculations of droop with measurements. Here, we first present experimental photoluminescence measurements extending over three orders of magnitude of excitation for three samples grown at different temperatures that indicate that droop behavior is not dependent on the point defect density in the quantum wells studied. Second, we use an atomistic tight-binding electronic structure model to calculate localization-enhanced radiative and Auger rates and show that both the corresponding carrier density-dependent internal quantum efficiency and the carrier density decay dynamics are in excellent agreement with our experimental measurements. Moreover, we show that point defect density, Auger recombination, and the effect of the polarization field on recombination rates only limit the peak internal quantum efficiency to about 70% in the resonantly excited green-emitting quantum wells studied. This suggests that factors external to the quantum wells, such as carrier injection efficiency and homogeneity, contribute appreciably to the significantly lower peak external quantum efficiency of green LEDs

Ultrafast Trap State-Mediated Electron Transfer for Quantum Dot Redox Sensing

Quantum dots (QDs) conjugated to electron acceptor ligands are useful as redox sensors in applications ranging from chemical detection to bioimaging. We aimed to improve effectiveness of these redox-sensing QD conjugates, which depends on the initial charge separation and on the competing mechanisms of recombination, including luminescence and electron transfer to the conjugated redox molecules. In this study, ultrafast laser measurements were used to study the excited state dynamics in CdTe/CdS core/shell QDs with quinone/quinol acceptor (Q2NS) ligands attached to the surface (up to 40 per QD). Detailed analysis, along with computational modeling of the system, showed multiple electron-transfer pathways and identified an ultrafast electron transfer from a surface electron trap state to the quinone ligands (2–8 ps). We propose that this leads to high, redox-dependent, quenching efficiencies (98.7% with an average of 10 quinone/quinols on the surface). As only low populations of redox ligands are required, the colloidal properties of the QD are preserved, which allows for further functionalization. These new insights into the excited state properties and ultrafast charge transfer have important implications for fields exploring charge extraction from quantum dots, which range from bioimaging to solar energy technology