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.

Achieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport.

Abstract.We report the optical measurement of the spin dynamics at elevated temperatures and in zero magnetic field, for two types of degenerately doped n-InSb quantum wells (QWs), one asymmetric (sample A) and one symmetric (sample B) with regards to the electrostatic potential across the QW. Making use of three directly determined experimental parameters: the spin lifetime, τ s , the sheet carrier concentration, n, and the electron mobility, , we directly extract the zero field spin splitting. For the asymmetric sample where the Rashba interaction is the dominant source of spin splitting, we deduce a room temperature Rashba parameter of  = 0.09 ± 0.1 eVÅ which is in good agreement with calculations and we estimate the Rashba coefficient α 0 (a figure of merit for the ease with which electron spins can be modulated via an electric field). We review the merits/limitations of this approach and the implications of our finding for spintronic devices.

Ultrafast charge dynamics in dispersions of MoS2 nanosheets in N-methyl-2-pyrrolidone are reported. Samples were prepared by the ultrasonication-assisted exfoliation of MoS2 powder, resulting in nanosheets that were predominantly monolayer and had an average sheet size of 32.4 ± 0.1 nm. These dispersions were characterized using absorption and photoluminescence spectroscopy, transient photoluminescence measurements, and atomic force microscopy before the ultrafast charge dynamics were studied via transient absorption spectroscopy. The transient absorption spectra exhibited bleach peaks and photoinduced absorption peaks in spectral regions corresponding to both the A and B excitons of MoS2. The growth and decay of the features in the B exciton region were determined largely by the dynamics of the exciton population, while the features in the A exciton region depend on the dynamics of both excitons and trions.

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 τr=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 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.

Time-resolved laser-pump X-ray-photoemission-probe spectroscopy of a ZnO (101¯0) substrate with and without PbS quantum dots (QDs) chemically linked to the surface is performed, using laser photon energies resonant with and below the band gap energy of the substrate (λ = 372 and 640 nm, hν = 3.33 and 1.94 eV). Charge injection from the photoexcited QDs to ZnO is demonstrated through the change in the surface photovoltage of the ZnO substrate observed when the heterojunction is illuminated with 1.94 eV radiation. The measured carrier dynamics are limited by the persistent photoconductivity of ZnO, giving dark carrier lifetimes of the order of 200 μs in a depletion layer at the interface. The chemical specificity of soft X-rays is used to separately measure the charge dynamics in the quantum dots and the substrate, yielding evidence that the depletion region at the interface extends into the PbS QD layer.

We report measurements of multiple exciton generation and recombination in InP/CdS (core/shell) colloidal quantum dots. Ultrafast transient absorption spectroscopy was used to measure the sub-nanosecond charge dynamics for a range of pump fluences and for different pump photon energies. Analysis of the resulting transients was used to determine the quantum yield of multiple exciton generation, which was found to be 1.22 ± 0.01 for a pump photon energy equivalent to three times the band gap.