Abstract The synthesis of Er- and Nd-doped MoS2 powders by the co-pyrolysis of molecular precursors is demonstrated by heating tetrakis-(diethyldithiocarbamato) molybdenum(IV) (Mo(DTC)4) and mono-1,10-phenanthroline tris-(diethyl dithiocarbamato) erbium(III) or mono-1,10-phenanthroline tris-(diethyl dithiocarbamato) neodymium(III) (Ln(DTC)3phen, where Ln = Er, Nd) to 500 °C for 1 h. Powder x-ray diffraction and Raman spectroscopy indicated that the resulting powders produced from this reaction are comprised of 0D nanoscale crystallites of the 2 H phase of MoS2. Spatial mapping of elements in these samples was performed in the scanning electron microscope at the microscale using energy dispersive X-ray spectroscopic mapping, which showed that the dopants are homogeneously distributed throughout the samples. Synchrotron radiation x-ray absorption measurements tentatively indicate that the dopants are incorporated within the MoS2 layers rather than being intercalated between them. Electron paramagnetic resonance spectroscopy and SQUID magnetometry demonstrated that rare earth doping of MoS2 significantly enhances the magnetic response of the material. Remarkably, the samples are found to remain paramagnetic down to temperature at least as low as 2 K.

Light emitting diodes based on c-plane (In,Ga)N/GaN quantum wells (QWs) can have > 90% emission efficiency at modest current densities but this drops significantly at higher excitation, an effect known as efficiency droop that limits device efficacy at high brightness. Several explanations for this have been proposed including the saturation of carrier localisation sites at high excitation densities, resulting in a greater exposure of carriers to defects and hence a significant increase in the associated non-radiative recombination processes. Here, power- and temperature-dependent photoluminescence spectroscopy of c-plane (In,Ga)N/GaN QWs is used to investigate the relationship between the saturation of localised states and emission efficiency. For the samples studied, we find that the saturation of localised sites broadly coincides with the onset of efficiency droop. However, it is also found that as the localised states saturate with increasing excitation, the relative contribution of defect-associated non-radiative processes to overall recombination decreases rather than increases. Based on these observations and on modelling of recombination processes in the QW, it is concluded that the saturation of localised states does not significantly contribute to the reduction in emission efficiency at high excitation. Our studies rather suggest that defect-related non-radiative recombination is out-competed by radiative and Auger-Meitner recombination at the carrier densities required for saturation.

Doped colloidal quantum dots (CQDs) are promising spin-photon interfaces, combining excellent optical properties and a substrate-free, nano-positionable platform. Here, we report the synthesis of InP/ZnSeS core-shell CQDs from zinc sulphide based seed clusters containing single Mn(II) ions. The resulting doped CQDs demonstrate photoluminescence quantum yields of up to 70%, a spin-lattice lifetime of 22 ms and a spin coherence lifetime of 2.7 μs, greater than and similar to the best yet reported for Mn-doped CQD systems, respectively. These spin lifetimes were achieved despite the high nuclear spin of InP, demonstrating the effectiveness of the ZnS cluster in shielding the Mn ion from its environment.

Singly doped colloidal quantum dots (CQDs) are promising spin-photon interfaces for quantum memories and quantum repeaters. Coupling between the dopant spin and the band edge CQD exciton substantially enhances the optical interaction cross-section for the former. As substrate-free nanocrystals, CQDs can be manipulated by pick-and-place nano-positioning techniques for straightforward device incorporation. They also benefit from a suite of well-developed synthetic techniques that enable their structure and composition to be controlled for enhanced spin lifetime and to tune their optical properties. Single doping of CQDs has relied on stochastic processes, resulting in a distribution of the number of dopants per CQD and the random positioning of the dopant within the CQD. However, the deterministic doping of InP/ZnSeS core/shell CQDs with a single Mn has recently been demonstrated via growth from molecular seed clusters including a single Mn atom. Here the optical properties of these deterministically doped CQDs are reported. The photoluminescence (PL) spectra of the doped and undoped CQDs are compared, enabling the contribution of the dopant to the emission to be identified. Further, the dependence of their PL transients on temperature and emission wavelength is used to study energy transfer between the band edge of the CQD and the dopant.

Conventional c-plane wurtzite InGaN/GaN quantum wells are subject to a large internal field that acts to separate electrons and holes and thereby lowers the rate of radiative recombination. This effect is exacerbated for higher indium contents and so may contribute to the lower efficiency of c-plane wurtzite InGaN/GaN QWs when emitting at green and amber wavelengths. In comparison, InGaN/GaN QWs grown in the cubic zincblende phase along the (001) direction are free of such fields and so exhibit recombination lifetimes that are shorter by two orders of magnitude and independent of indium content. Here, we report on zincblende QWs grown by metal-organic chemical vapor deposition at different temperatures. This results in different indium contents and thereby allows tuning of the emission band from blue to yellow. For each indium content, the spectrally integrated emission quenches as the temperature rises. However, the ratio of room temperature to low temperature emission improves for higher indium contents, increasing from 18 % to 34 % as the emission peak is tuned from 2.8 eV to 2.1 eV. This behavior is attributed to the thermal escape of carriers from the QWs playing an important role in the temperature dependent quenching of emission.

Ligand exchange has become the standard route for modifying the surface chemistry of colloidal nanocrystals (NCs), providing a mechanism to tailor their optoelectronic properties, solubility, and chemical functionalization. This enables NCs to be deployed in myriad applications where the surface-solution interface is key to performance or process compatibility. Ligand-passivated surfaces are generally considered stable and chemically inert, with incoming ligands assumed to fully replace native ones. Here, we demonstrate that inorganic ligand shells on CdS NCs are dynamic, chemically evolving over time, with transformation pathways strongly dependent on ligand identity. This is evaluated for representative oxoanions (o-phosphoric acid), chalcogenides (Na2Se, Se2-), and metal-chalcogenides (thiostannate). Using multinuclear (1H, 13C, 23Na, 31P, 113Cd, 119Sn) solid-state NMR spectroscopy and electron microscopy, we show that (i) surface-bound phosphates reorganize over time into cadmium phosphate domains, degrading NC quality; (ii) sodium polyselenide ligands remain chemically stable, but associated Na+ counterions exhibit dynamics that may suppress charge transport; and (iii) partial oxidation of thiostannate ligands to SnO2 occurs, along with surface reconstruction, which improves NC passivation. Across all systems studied, residual oleylamine is detected. These findings reveal that inorganic ligand exchange does not necessarily yield chemically uniform or stable surfaces. Instead, each ligand class exhibits distinct behavior, ranging from surface degradation (phosphates), to stable yet dynamic interfaces (selenides), to hydrophilic and partially oxidized surfaces (thiostannates). This challenges the static-surface model and positions multinuclear solid-state NMR spectroscopy as a key tool for designing future functional materials.

Deep-level defects in Ta-doped β-Ga2O3 single crystals grown using the optical floating zone method are investigated. Deep-level transient spectroscopy (DLTS) in conjunction with Laplace-DLTS (L-DLTS) and photoluminescence (PL) has been applied to (100) oriented β-Ga2O3:Ta bulk crystals. The temperature dependence of the bias capacitance of diodes indicates no significant sign of carrier freeze-out down to 20 K. This confirms the predicted shallow donor behavior of Ta impurity atoms in β-Ga2O3 samples with a carrier concentration of (1.0–1.2) × 1018 cm−3. DLTS and L-DLTS analysis show six traps with activation energies of electron emission of 0.28 (Es), 0.46 (E9), 0.52 (E1), 0.69 (E2a), 0.75 (E2b), and 0.97 (E3) eV, with trap concentrations in the range of 1015–1017 cm−3. In addition, temperature-dependent PL has been used to study the broad luminescence bands with their maxima at 3.10 and 3.40 eV. Subsequent Arrhenius analysis extracted activation energy values (EA of 20 ± 1 and 79 ± 4 meV for quenching of the PL peaks at 3.10 and 3.40 eV, respectively.

Cubic zincblende InGaN/GaN quantum wells are free of the electric fields that reduce recombination efficiency in hexagonal wurtzite wells.

AlxGa1−xN epilayers are used as the basis of ultraviolet LEDs and detectors. The trap states produced by defects and impurities can play a key role in the device performance. In this work, conventional deep-level transient spectroscopy, photoluminescence (PL), and secondary ion mass spectrometry have been used to characterize a deep-level trap termed as E3 in dilute AlxGa1−xN (x < 0.063) epilayers grown by metal-organic vapor phase epitaxy (MOVPE) on highly conductive ammono-GaN substrates. The AlxGa1−xN epilayers were doped with silicon to about 3 × 1016 cm−3. The electrical and the optical measurements were conducted on Ni/Au Schottky barrier diodes and virgin samples, respectively. First, we observed a general trend that the E3 (FeGa) electron trap concentration significantly changes along the wafers in AlxGa1−xN layers that is fully consistent with previously reported results for GaN materials grown by the MOVPE technique. Second, we report that the activation energies for electron emission for the E1 and E3 traps in dilute AlxGa1−xN exhibit linear variations with Al content. Moreover, low-temperature PL results show a proportional relation between the intensity of the line with its maximum at 1.299 eV and concentration of residual Fe impurity. Finally, we discuss how the presence of defects resulting from Fe contamination may result in degradation of AlxGa1−xN-based devices.

Perovskite‐based light‐emitting diodes (PeLEDs) with a mixed halide composition can be used to obtain the “pure red” emission, i.e., in the 620–650 nm range, required for high‐definition displays. However, fast halide ion migration induces phase separation in these materials under electric fields, resulting in poor spectral stability and low efficiency. Herein, a method for producing mixed halide CsPbI 3‐x Br x quantum dots (QDs) is reported in which ion migration is suppressed. The mixed halide composition is first achieved by anion exchange between CsPbI 3 QDs and hydrobromic acid (HBr), during that the bromine ions efficiently passivate the iodine vacancies of the QDs. The original oleic acid ligands are then exchanged for 1‐dodecanethiol (1‐DT), which suppresses halide ion migration via the strong binding of the sulfhydryl group with the QD surface. PeLEDs based on these QDs exhibit a pure‐red electroluminescence (EL) peak at 637 nm, a maximum external quantum efficiency (EQE) of 21.8% with an average value of 20.4%, a peak luminance of 2653 cd m −2 , and low EQE decease with increasing luminance. The EL spectrum of these devices is stable even at 6.7 V and they have an EQE half‐life of 70 min at an initial luminance of 150 cd m −2 .

Abstract 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 H2SO4 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.

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.

The internal quantum efficiency of (In,Ga)N/GaN quantumwells cansurpass 90% for blue-emitting structures at moderate drive currentdensities but decreases significantly for longer emission wavelengthsand at higher excitation rates. This latter effect is known as efficiency"droop" and limits the brightness of light-emittingdiodes (LEDs) based on such quantum wells. Several mechanisms havebeen proposed to explain efficiency droop including Auger recombination,both intrinsic and defect-assisted, carrier escape, and the saturationof localized states. However, it remains unclear which of these mechanismsis most important because it has proven difficult to reconcile theoreticalcalculations of droop with measurements. Here, we first present experimentalphotoluminescence measurements extending over three orders of magnitudeof excitation for three samples grown at different temperatures thatindicate that droop behavior is not dependent on the point defectdensity in the quantum wells studied. Second, we use an atomistictight-binding electronic structure model to calculate localization-enhancedradiative and Auger rates and show that both the corresponding carrierdensity-dependent internal quantum efficiency and the carrier densitydecay 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 onlylimit the peak internal quantum efficiency to about 70% in the resonantlyexcited green-emitting quantum wells studied. This suggests that factorsexternal to the quantum wells, such as carrier injection efficiencyand homogeneity, contribute appreciably to the significantly lowerpeak external quantum efficiency of green LEDs.

Colloidal quantum dots (CQDs) are isolated semiconductor nanocrystals with a size-tunable bandgap that can be prepared and processed by well-established solvent-based chemistry, and are currently used for a number of optoelectronic applications. When doped with a single atom, they also have great potential as a platform for optically addressable spin qubits. This perspective first describes the process by which doped CQDs can be made and the electronic structure produced in them by doping with a single atom. The properties that make them particularly well-suited as a spin-photon interface are identified: a local environment for the dopant that is free of unwanted spins; an optical cross-section for the dopant that can be enhanced by orders of magnitude via an exchange interaction with the band edge exciton of the dot; and, as an isolated nanocrystal, the scope for nano-positioning and hence precise incorporation into device structures. Lastly, two areas for development are discussed which would enhance the impact of singly doped quantum dots on quantum technology. The first of these is a synthetic method that ensures deterministic doping with single atoms and the second is to expand the range of dopants available.

Here, we develop an in situ photoluminescence (PL) system to monitor the nucleation and growth of perovskite nanocrystals and control the monomer supply rate to achieve strongly confined and monodispersed quantum dots (QDs) with average size of 3.4 nm. Pure-blue (460 nm wavelength) CsPbBr3 QDs with near unity PL quantum yield and narrow size distribution (small size dispersion of 9.6%) were thus produced. Light-emitting diodes (LEDs) based on these QDs were prepared by using an all-solution processing route, which showed narrow electroluminescence with full width at half-maximum of 20 nm and a high color purity of 97.3%. The device also had a high external quantum efficiency of 10.1%, maximum luminance of 11 610 cd m-2, and continuous operation lifetime of 21 h at the initial luminance of 102 cd m-2, corresponding to the state-of-art for pure-blue perovskite LEDs.

We present a combined theoretical and experimental analysis of Auger recombination in c-plane (In,Ga)N quantum wells. On the theoretical side we use an atomistic model that accounts for random alloy fluctuations to investigate the impact that temperature and carrier density has on the radiative and Auger recombination rate. Our calculations indicate a weak temperature dependence of the Auger rate compared to the temperature dependence of the radiative rate. However, with increasing carrier density the Auger rate increases more strongly when compared to the radiative rate. Our theory results indicate an onset of the efficiency drop at carrier densities ≳ 1×10 19 cm −3 , in very good agreement with our photoluminescence studies on similar (In,Ga)N quantum well samples. Overall, we find that alloy enhanced Auger recombination is sufficient to explain the experimental data investigated here.

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.

The extension of X-ray photoelectron spectroscopy (XPS) to measure layers and interfaces below the uppermost surface requires higher X-ray energies and electron energy analysers capable of measuring higher electron kinetic energies. This has been enabled at synchrotron radiation facilities and by using lab-based instruments which are now available with sufficient sensitivity for measurements to be performed on reasonable timescales. Here, we detail measurements on buried interfaces using a Ga Kα (9.25 keV) metal jet X-ray source and an EW4000 energy analyser (ScientaOmicron GmbH) in the Henry Royce Institute at the University of Manchester. Development of the technique has required the calculation of relative sensitivity factors (RSFs) to enable quantification analogous to Al Kα XPS, and here we provide further substantiation of the Ga Kα RSF library. Examples of buried interfaces include layers of memory and energy materials below top electrode layers, semiconductor heterostructures, ions implanted in graphite, oxide layers at metallic surfaces, and core-shell nanoparticles. The use of an angle-resolved mode enables depth profiling from the surface into the bulk, and is complemented with surface-sensitive XPS. Inelastic background modelling allows the extraction of information about buried layers at depths up to 20 times the photoelectron inelastic mean free path.

LEDs based on hexagonal InGaN/GaN quantum wells are dominant technology for many lighting applications. However, their luminous efficacy for green and amber emission and at high drive currents remains limited. Growing quantum wells instead in the cubic phase is a promising alternative because, compared to hexagonal GaN, it benefits from a reduced bandgap and is free of the strong polarization fields that can reduce the radiative recombination rate. Initial attempts to grow cubic GaN in the 1990s employed molecular beam epitaxy, but now, metal-organic chemical vapor deposition can also be used. Nonetheless, high phase purity requires careful attention to growth conditions and the quantification of any unwanted hexagonal phase. In contrast to hexagonal GaN, in which threading dislocations are key, at its current state of maturity, the most important extended structural defects in cubic GaN are stacking faults. These modify the optical properties of cubic GaN films and propagate into active layers. In quantum wells and electron blocking layers, segregation of alloying elements at stacking faults has been observed, leading to the formation of quantum wires and polarized emission. This observation forms part of a developing understanding of the optical properties of cubic InGaN quantum wells, which also offer shorter recombination lifetimes than their polar hexagonal counterparts. There is also growing expertise in p-doping, including dopant activation by annealing. Overall, cubic GaN has rapidly transitioned from an academic curiosity to a real prospect for application in devices, with the potential to offer specific performance advantages compared to polar hexagonal material.

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%.