A Low-Temperature Synthetic Route Toward a High-Entropy 2D Hexernary Transition Metal Dichalcogenide for Hydrogen Evolution Electrocatalysis

High-entropy (HE) metal chalcogenides are a class of materials that have great potential in applications such as thermoelectrics and electrocatalysis. Layered 2D transition-metal dichalcogenides (TMDCs) are a sub-class of high entropy metal chalcogenides that have received little attention to date as their preparation currently involves complicated, energy-intensive, or hazardous synthetic steps. To address this, a low-temperature (500 °C) and rapid (1 h) single source precursor approach is successfully adopted to synthesize the hexernary high-entropy metal disulfide (MoWReMnCr)S₂ . (MoWReMnCr)S₂ powders are characterized by powder X-ray diffraction (pXRD) and Raman spectroscopy, which confirmed that the material is comprised predominantly of a hexagonal phase. The surface oxidation states and elemental compositions are studied by X-ray photoelectron spectroscopy (XPS) whilst the bulk morphology and elemental stoichiometry with spatial distribution is determined by scanning electron microscopy (SEM) with elemental mapping information acquired from energy-dispersive X-ray (EDX) spectroscopy. The bulk, layered material is subsequently exfoliated to ultra-thin, several-layer 2D nanosheets by liquid-phase exfoliation (LPE). The resulting few-layer HE (MoWReMnCr)S₂ nanosheets are found to contain a homogeneous elemental distribution of metals at the nanoscale by high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) with EDX mapping. Finally, (MoWReMnCr)S₂ is demonstrated as a hydrogen evolution electrocatalyst and compared to 2H-MoS₂ synthesized using the molecular precursor approach. (MoWReMnCr)S₂ with 20% w/w of high-conductivity carbon black displays a low overpotential of 229 mV in 0.5 M  H₂ SO₄ to reach a current density of 10 mA cm^-2 , which is much lower than the overpotential of 362 mV for MoS₂ . From density functional theory calculations, it is hypothesised that the enhanced catalytic activity is due to activation of the basal plane upon incorporation of other elements into the 2H-MoS₂ structure, in particular, the first row TMs Cr and Mn.

Synthesis and characterisation of Ga- and In-doped CdS by solventless thermolysis of single source precursors

We report a facile and low temperature synthesis of Ga- and In-doped CdS nanoparticles from molecular precursors. Diethyldithiocarbamate complexes of Cd(II), Ga(III), and In(III), were synthesised and decomposed in tandem through solventless thermolysis, producing Ga- or In-doped CdS. The resultant M_xCd_1-xS_1+0.5x (where M = Ga/In at x values of 0, 0.02, 0.04, 0.06, 0.08 and 0.1) particulate powder was analysed by powder X-ray diffraction, which showed that both Ga (through all doping levels) and In (at doping levels <8 mol%) were successfully incorporated into the hexagonal CdS lattice without any impurities. Raman spectroscopy also showed no significant change from CdS. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to investigate the morphology and elemental dispersion through the doped CdS materials, showing homogenous incorporation of dopant. The optical and luminescent properties of the doped M_xCd_1-xS_1+0.5x materials were examined by UV-Vis absorption and photoluminescence spectroscopies respectively. All materials were found to exhibit excitonic emission, corresponding to band gap energies between 2.7 and 2.9 eV and surface defect induced emission which is more prominent for Ga than for In doping. Additionally, moderate doping slows down charge carrier recombination by increasing the lifetimes of excitonic and surface state emissions, but particularly for the latter process.

Quantum Confined High-Entropy Lanthanide Oxysulfide Colloidal Nanocrystals

We have synthesized the first reported example of quantum confined high-entropy (HE) nanoparticles, using the lanthanide oxysulfide, Ln2SO2, system as the host phase for an equimolar mixture of Pr, Nd, Gd, Dy, and Er. A uniform HE phase was achieved via the simultaneous thermolysis of a mixture of lanthanide dithiocarbamate precursors in solution. This was confirmed by powder X-ray diffraction and high-resolution scanning transmission electron microscopy, with energy dispersive X-ray spectroscopic mapping confirming the uniform distribution of the lanthanides throughout the particles. The nanoparticle dispersion displayed a significant blue shift in the absorption and photoluminescence spectra relative to our previously reported bulk sample with the same composition, with an absorption edge at 330 nm and a λmax at 410 nm compared to the absorption edge at 500 nm and a λmax at 450 nm in the bulk, which is indicative of quantum confinement. We support this postulate with experimental and theoretical analysis of the bandgap energy as a function of strain and surface effects (ligand binding) as well as calculation of the exciton Bohr radiii of the end member compounds.

Synthesis of High Entropy Lanthanide Oxysulfides via the Thermolysis of a Molecular Precursor Cocktail

High entropy (HE) materials have received significant attention in recent years, due to their intrinsically high levels of configurational entropy. While there has been significant work exploring HE alloys and oxides, new families of HE materials are still being revealed. In this work we present the synthesis of a novel family of HE materials based on lanthanide oxysulfides. Here, we implement lanthanide dithiocarbamates as versatile precursors that can be mixed at the molecular scale prior to thermolysis in order to produce the high entropy oxysulfide. The target of our synthesis is the HE Ln₂SO₂ phase, where Ln = Pr, Nd, Gd, Dy, Er and where Ln = Pr, Nd, Gd, Dy for 5 and 4 lanthanide samples, respectively. We confirmed the structure of samples produced by powder X-ray diffraction, electron microscopy, and high-resolution energy dispersive X-ray spectroscopy. Optical spectroscopy shows a broad emission feature centered around 450 nm as well as a peak in absorption at around 280 nm. From this data we calculate the band gap and Urbach energies of the materials produced.

Synthesis, X-ray Single-Crystal Structural Characterization, and Thermal Analysis of Bis(O-alkylxanthato)Cd(II) and Bis(O-alkylxanthato)Zn(II) Complexes Used as Precursors for Cadmium and Zinc Sulfide Thin Films

This work investigates tuning of the molecular structure of a series of O-alkylxanthato zinc and cadmium precursor complexes to enhance production of ZnS and CdS materials. The structures of several bis(O-alkylxanthato) cadmium(II) complexes (8-13) and bis(O-alkyl xanthato)zinc(II) complexes (18 and 19) are reported based on single crystal X-ray diffraction data. CdS and ZnS films were produced by the spin-coating of these metal complexes followed by their thermal decomposition to the corresponding metal sulfides. Thin films of CdS were deposited by spin-coating the bis(O-alkylxanthato) cadmium(II) precursors (7-13) on glass substrates, followed by annealing at 300 °C for 60 min. Thin films of ZnS were deposited by spin-coating bis(O-alkylxanthato) zinc(II) (14-20), followed by annealing at 200 °C for 60 min. The molecular complexes and solid state materials are characterized using a range of techniques including single-crystal X-ray diffraction, pXRD, EDS and XPS, DSC and TGA, UV-vis and PL spectroscopies, and electron microscopy. These techniques provided information on the influence of alkyl chain length on the thermal conditions required to fabricate metal sulfide films as well as film properties such as film quality, and morphology. For example, the obtained crystallite size of metal sulfide films formed is correlated to the hydrocarbon chain length of xanthate ligands in the precursor. The behavior of the complexes under thermal stress was therefore studied in detail. DTA and TGA profiles explain the relationship between hydrocarbon chain length, decomposition temperatures, and the energies required for decomposition. A higher decomposition temperature for complexes with longer hydrocarbon chains is observed compared to complexes with shorter hydrocarbon chains. Band-gap energies calculated from the optical absorption spectra alongside steady state and time-resolved photoluminescence studies are reported for CdS films.

Confinement Effects and Charge Dynamics in Zn3N2 Colloidal Quantum Dots: Implications for QD-LED Displays

Zinc nitride (Zn₃N₂) colloidal quantum dots are composed of nontoxic, low-cost, and earth-abundant elements. The effects of quantum confinement on the optical properties and charge dynamics of these dots are studied using steady-state optical characterization and ultrafast fluence-dependent transient absorption. The absorption and emission energies are observed to be size-tunable, with the optical band gap increasing from 1.5 to 3.2 eV as the dot diameter decreased from 8.9 to 2.7 nm. Size-dependent absorption cross sections (σ = 1.22 ± 0.02 × 10^-15 to 2.04 ± 0.03 × 10^-15 cm²), single exciton lifetimes (0.36 ± 0.02 to 0.65 ± 0.03 ns), as well as Auger recombination lifetimes of biexcitons (3.2 ± 0.4 to 5.0 ± 0.1 ps) and trions (20.8 ± 1.8 to 46.3 ± 1.3 ps) are also measured. The degeneracy of the conduction band minimum (g = 2) is determined from the analysis of the transient absorption spectra at different excitation fluences. The performance of Zn₃N₂ colloidal quantum dots thus broadly matches that of established visible light emitting quantum dots based on toxic or rare elements, making them a viable alternative for QD-LED displays.

Erratum: Ultrafast Charge Dynamics in Trap-Free and Surface-Trapping Colloidal Quantum Dots

[This corrects the article DOI: 10.1002/advs.201500088.].

Ultrafast Charge Dynamics in Trap-Free and Surface-Trapping Colloidal Quantum Dots

Ultrafast transient absorption spectroscopy is used to study subnanosecond charge dynamics in CdTe colloidal quantum dots. After treatment with chloride ions, these can become free of surface traps that produce nonradiative recombination. A comparison between these dots and the same dots before treatment enables new insights into the effect of surface trapping on ultrafast charge dynamics. The surface traps typically increase the rate of electron cooling by 70% and introduce a recombination pathway that depopulates the conduction band minimum of single excitons on a subnanosecond timescale, regardless of whether the sample is stirred or flowed. It is also shown that surface trapping significantly reduces the peak bleach obtained for a particular pump fluence, which has important implications for the interpretation of transient absorption data, including the estimation of absorption cross-sections and multiple exciton generation yields.

Near-unity quantum yields from chloride treated CdTe colloidal quantum dots

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion. However, trap states associated with the CQD surface can produce non-radiative charge recombination that significantly reduces device performance. Here a facile post-synthetic treatment of CdTe CQDs is demonstrated that uses chloride ions to achieve near-complete suppression of surface trapping, resulting in an increase of photoluminescence (PL) quantum yield (QY) from ca. 5% to up to 97.2 ± 2.5%. The effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. This process also dramatically improves the air-stability of the CQDs: before treatment the PL is largely quenched after 1 hour of air-exposure, whilst the treated samples showed a PL QY of nearly 50% after more than 12 hours.

Effect of Chloride Passivation on Recombination Dynamics in CdTe Colloidal Quantum Dots

Colloidal quantum dots (CQDs) can be used in conjunction with organic charge-transporting layers to produce light-emitting diodes, solar cells and other devices. The efficacy of CQDs in these applications is reduced by the non-radiative recombination associated with surface traps. Here we investigate the effect on the recombination dynamics in CdTe CQDs of the passivation of these surface traps by chloride ions. Radiative recombination dominates in these passivated CQDs, with the radiative lifetime scaling linearly with CQD volume over τ_r=20–55 ns. Before chloride passivation or after exposure to air, two non-radiative components are also observed in the recombination transients, with sample-dependent lifetimes typically of less than 1 ns and a few ns. The non-radiative dynamics can be explained by Auger-mediated trapping of holes and the lifetimes of this process calculated by an atomistic model are in agreement with experimental values if assuming surface oxidation of the CQDs.

Ultrafast exciton dynamics in Type II ZnTe-ZnSe colloidal quantum dots

Ultrafast transient absorption spectroscopy is used to investigate the exciton dynamics of Type II ZnTe-ZnSe core-shell colloidal quantum dots. Surface-trapping is shown to occur within a few picosecond for hot electrons and with a few 10s of picoseconds for electrons cooled to the band-edge, and is the dominant process in the decay of the band-edge bleach for well-stirred samples pumped at moderate powers. The surface-trapped electrons produce a broad photo-induced absorption that spectrally overlaps with the band-edge, distorting and partially cancelling out the bleach feature. At high pump powers and for unstirred samples, these surface-trapped electrons can survive sufficiently long within the pumped volume to accumulate under repeated excitation of the sample, resulting in the formation of an additional exciton decay channel.

Ultrafast exciton dynamics in InAs/ZnSe nanocrystal quantum dots

Colloidal nanocrystal quantum dots with a band gap in the near infra-red have potential application as the emitters for telecommunications or in vivo imaging, or as the photo-absorbing species in next generation solar cells or photodetectors. However, electro- and photoluminescence yields and the efficiency with which photo-generated charges can be extracted from quantum dots depend on the total rate of recombination, which can be dominated by surface-mediated processes. In this study, we use ultrafast transient absorption spectroscopy to characterise the recombination dynamics of photo-generated charges in InAs/ZnSe nanocrystal quantum dots. We find that recombination is dominated by rapid, sub-nanosecond transfer of conduction band electrons to surface states. For the size of dots studied, we also find no evidence of significant multiple exciton generation for photon energies up to 3.2 times the band gap, in agreement with our theoretical modelling.

Enhanced photosynthetic output via dichroic beam-sharing

Microbial solar biofuels offer great promise for future sustainable food, fuels and chemicals but are limited by low productivities and a requirement for large land areas to harvest sunlight. A 71 % increase in combined photosynthetic activity was achieved by illuminating both Rhodobacter sphaeroides and Arthrospira (Spirulina) platensis from a single beam of simulated sunlight, divided using a dichroic mirror. Therefore, this technique is termed ‘dichroic beam-sharing’, in which the complementary action spectra of two different useful micro-organisms, belonging to green and purple groups, is exploited and allows a single beam of sunlight to be shared efficiently between separate photobioreactors. Because the action spectra of these two organisms are typical of large groups, this novel method could increase the productivity of photosynthetic micro-organisms in the production of diverse commodities.

Photorefractive performance of a CdSe/ZnS core/shell nanoparticle-sensitized polymer

We report the photorefractive performance of a polymer composite sensitized by CdSe/ZnS core/shell nanoparticles, and also comprising poly(N-vinylcarbazole) and an electro-optic chromophore. The nanoparticles are characterized by absorption and photoluminescence spectroscopy, elemental analysis, transmission electron microscopy, and powder x-ray diffraction. The electro-optic response of the composite is measured independently of the photorefractive effect by transmission ellipsometry. An asymmetric two-beam coupling gain of 30.6+/-0.4 cm(-1) is obtained, confirming photorefractivity. Degenerate four-wave mixing is used to assess photorefractive performance and, at a poling field of 70 V microm(-1), yields a diffraction efficiency of 4.21%+/-0.03%, a holographic contrast of 3.05 x 10(-4)+/-1 x 10(-6), a space-charge rise time of 25+/-2 s, and a sensitivity of 4.7 x 10(-5)+/-4 x 10(-6) cm3 J(-1). These results constitute a significant improvement on the performance of previous nanoparticle-sensitized photorefractive polymer composites.

Strongly coupled intracavity frequency-doubled pulsed lasers

Simple expressions for the peak power, pulse energy, and pulse length of strongly coupled intracavity frequency-doubled lasers are presented. For nonlinear coupling values that correspond to many common systems, these expressions agree well with the results of numerical integration of the rate equations. In contrast with weak nonlinear or linear coupling, pulsed lasers with strong nonlinear coupling only deplete the population inversion down to the threshold level.

Full geometry dependence of index contrast in photorefractive polymer composites

Detailed analysis of the relationship between the experimental geometry and the holographic contrast in photorefractive polymers is important for applications, such as angle multiplexing in holographic data storage. In this paper the field dependent photogeneration efficiency is introduced into the complete reorientational model to provide a full account of the electric field and geometrical dependence of the index contrast. The interaction of a local grating and the photorefractive grating is also considered. A simplification for acute angles between writing beams is described. Experimental verification by use of four-wave mixing and transmission ellipsometry reveals an excellent agreement between theory and measurement.

Frequency locking of a pulsed single-longitudinal-mode laser in a coupled-cavity resonator

The performance of a novel resonator that couples a grazing-incidence and a linear cavity is reported. The coupling secures single-longitudinal mode, TEM(00), higher-efficiency and lower-threshold operation. By use of Ti:sapphire as the gain medium, a slope efficiency of 23% and a 100-nm tuning range are reported. A model is explained that fully predicts the mode behavior of the resonator and that can be used to optimize the cavity for single-mode operation. We have developed computer control of the cavity, which is simple in design and is used to lock the <200-MHz bandwidth mode to +/-40 MHz. A 4.8-GHz scan has also been demonstrated.