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.

Influence of Multistep Surface Passivation on the Performance of PbS Colloidal Quantum Dot Solar Cells.

The performance of devices containing colloidal quantum dot (CQD) films is strongly dependent on the surface chemistry of the CQDs they contain. Multistep surface treatments, which combine two or more strategies, are important for creating films with high carrier mobility that are well passivated against trap states and oxidation. Here, we examine the effect of a number of these surface treatments on PbS CQD films, including cation exchange to form PbS/CdS core/shell CQDs, and solid-state ligand-exchange treatments with Cl, Br, I, and 1,2-ethanedithiol (EDT) ligands. Using laboratory-based and synchrotron-radiation-excited X-ray photoelectron spectroscopy (XPS), we examine the compositions of the surface layer before and after treatment, and correlate this with the performance data and stability in air. We find that halide ion treatments may etch the CQD surfaces, with detrimental effects on the air stability and solar cell device performance caused by a reduction in the proportion of passivated surface sites. We show that films made up of PbS/CdS CQDs are particularly prone to this, suggesting Cd is more easily etched from the surface than Pb. However, by choosing a less aggressive ligand treatment, a good coverage of passivators on the surface can be achieved. We show that halide anions bind preferentially to surface Pb (rather than Cd). By isolating the part of XPS signal originating from the topmost surface layer of the CQD, we show that air stability is correlated with the total number of passivating agents (halide + EDT + Cd) at the surface.

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 MxCd1-xS1+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 MxCd1-xS1+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.

Surface band bending and carrier dynamics in colloidal quantum dot solids.

Band bending in colloidal quantum dot (CQD) solids has become important in driving charge carriers through devices. This is typically a result of band alignments at junctions in the device. Whether band bending is intrinsic to CQD solids, i.e. is band bending present at the surface-vacuum interface, has previously been unanswered. Here we use photoemission surface photovoltage measurements to show that depletion regions are present at the surface of n and p-type CQD solids with various ligand treatments (EDT, MPA, PbI2, MAI/PbI2). Using laser-pump photoemission-probe time-resolved measurements, we show that the timescale of carrier dynamics in the surface of CQD solids can vary over at least 6 orders of magnitude, with the fastest dynamics on the order of microseconds in PbS-MAI/PbI2 solids and on the order of seconds for PbS-MPA and PbS-PbI2. By investigating the surface chemistry of the solids, we find a correlation between the carrier dynamics timescales and the presence of oxygen contaminants, which we suggest are responsible for the slower dynamics due to deep trap formation.

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.