Size-dependent photoionization in single CdSe/ZnS nanocrystals

Encapsulation of the Sulfur Compounds by Cucurbit[7]uril: A Quantum Chemistry Study

Benzothiophene (BT) and dibenzothiophene (DT) are the most important contaminants in the petroleum derivatives responsible for serious environmental and health problems. Therefore, we have investigated the absorption of these compounds for the first time by considering cucurbit[7]uril (CB[7]) as the host molecule and using the theoretical levels of density functional theory//B3LYP-D3/6-31G(d). BT and DT absorbed into CB[7] do not undergo a significant structural change in the CB[7] structure. The energy gap of the S-compounds@CB[7] in water and hexane solvents was approximately 5 eV, and this large value implies that the complexes have high chemical stability. Moreover, the absorption of the BT and DT into CB[7] in the water and hexane solvents is a favorable process, whereas the lowest binding energy was observed between the dibenzothiophene and CB[7] in the DT@CB[7] complex. The solvation enthalpy shows a preferential solvation of the complexes in water than in hexane solvent. This trend is confirmed by the AIM analysis that shows the highest stability for the DT@CB[7] system with the contribution of cooperative hydrogen bonding. The transfer free energy of S-compounds@CB[7] complexes from hexane to water are -66.12 and -59.56 kcal/mol for BT@CB[7] and DT@CB[7], respectively, implying the spontaneous transference of these complexes from hexane to water solvent. Overall, our results show that the cucurbiturils can be a new class of host molecules to be used in the removal of S-compounds from petroleum derivatives. Finally, a schematic flow diagram of the desulfurization process by cucurbiturils was proposed.

Sizing Up Excitons in Core-Shell Quantum Dots via Shell-Dependent Photoluminescence Blinking

Semiconductor nanocrystals or quantum dots (QDs) are now widely used across solar cell, display, and bioimaging technologies. While advances in multishell, alloyed, and multinary core-shell QD structures have led to improved light-harvesting and photoluminescence (PL) properties of these nanomaterials, the effects that QD-capping have on the exciton dynamics that govern PL instabilities such as blinking in single-QDs is not well understood. We report experimental measurements of shell-size-dependent absorption and PL intermittency in CdSe-CdS QDs that are consistent with a modified charge-tunnelling, self-trapping (CTST) description of the exciton dynamics in these nanocrystals. By introducing an effective, core-exciton size, which accounts for delocalization of charge carriers across the QD core and shell, we show that the CTST models both the shell-depth-dependent red-shift of the QD band gap and changes in the on/off-state switching statistics that we observe in single-QD PL intensity trajectories. Further analysis of CdSe-ZnS QDs, shows how differences in shell structure and integrity affect the QD band gap and PL blinking within the CTST framework.

Synthesis and thermoelectric properties of defect-containing PbSe–PbTe heterojunction nanostructures

Defect-containing and defect-free PbSe–PbTe heterogeneous (HNSs) nanostructures are synthesized. The strain distribution and defects in the interface of PbSe–PbTe HNSs affect the thermoelectric parameters of PbSe–PbTe HNSs.

Charge-tunnelling and self-trapping: common origins for blinking, grey-state emission and photoluminescence enhancement in semiconductor quantum dots

Understanding instabilities in the photoluminescence (PL) from light emitting materials is crucial to optimizing their performance for different applications. Semiconductor quantum dots (QDs) offer bright, size tunable emission, properties that are now being exploited in a broad range of developing technologies from displays and solar cells to biomaging and optical storage. However, instabilities such as photoluminescence intermittency, enhancement and bleaching of emission in these materials can be detrimental to their utility. Here, we report dielectric dependent blinking, intensity-"spikes" and low-level, "grey"-state emission, as well as PL enhancement in ZnS capped CdSe QDs; observations that we found consistent with a charge-tunnelling and self-trapping (CTST) description of exciton-dynamics on the QD-host system. In particular, modulation of PL in grey-states and PL enhancement are found to have a common origin in the equilibrium between exciton charge carrier core and surface-states within the CTST framework. Parameterized in terms of size and electrostatic properties of the QD and its nanoenvironment, the CTST offers predictive insight into exciton-dynamics in these nanomaterials.

Enhanced Luminescent Stability through Particle Interactions in Silicon Nanocrystal Aggregates

Close-packed assemblies of ligand-passivated colloidal nanocrystals can exhibit enhanced photoluminescent stability, but the origin of this effect is unclear. Here, we use experiment, simulation, and ab initio computation to examine the influence of interparticle interactions on the photoluminescent stability of silicon nanocrystal aggregates. The time-dependent photoluminescence emitted by structures ranging in size from a single quantum dot to agglomerates of more than a thousand is compared with Monte Carlo simulations of noninteracting ensembles using measured single-particle blinking data as input. In contrast to the behavior typically exhibited by the metal chalcogenides, the measured photoluminescent stability shows an enhancement with respect to the noninteracting scenario with increasing aggregate size. We model this behavior using time-dependent density functional theory calculations of energy transfer between neighboring nanocrystals as a function of nanocrystal size, separation, and the presence of charge and/or surface-passivation defects. Our results suggest that rapid exciton transfer from "bright" nanocrystals to surface trap states in nearest-neighbors can efficiently fill such traps and enhance the stability of emission by promoting the radiative recombination of slowly diffusing excited electrons.

Extension of the diffusion controlled electron transfer theory for intermittent fluorescence of quantum dots: inclusion of biexcitons and the difference of "on" and "off" time distributions

The equations for the diffusion controlled electron transfer (DCET) theory of quantum dot blinking are extended to include biexcitons. In contrast to excitons, which undego resonant light to dark transitions, the biexcitons, having a much larger total energy, undergo a Fermi's Golden rule type transfer (many acceptance states). The latter immediately gives rise to an exponential tail for the light state, and it is explained why the dark state power law behavior is unaffected. Results are given for both continuous and pulsed excitation. The typical -3/2 power law for the light state at low light intensities, and for the dark state at all intensities, as well as dependence of the exponential tail on the square of the light intensity, and a decrease of the power in the power law for the light state from -3/2 to less negative values with increasing light intensity are all consistent with the theory. The desirability of measuring the dependence of the spectral diffusion coefficient on light intensity at room temperature as a test of several aspects of the theory is noted.

Photocharging Artifacts in Measurements of Electron Transfer in Quantum-Dot-Sensitized Mesoporous Titania Films

Transient absorption and time-resolved photoluminescence measurements of high-performance mesoporous TiO2 photoanodes sensitized with CuInSexS2-x quantum dots reveal the importance of hole scavenging in the characterization of photoinduced electron transfer. The apparent characteristic time of this process strongly depends on the local environment of the quantum dot/TiO2 junction due to accumulation of long-lived positive charges in the quantum dots. The presence of long-lived photoexcited holes introduces artifacts due to fast positive-trion Auger decay (60 ps time constant), which can dominate electron dynamics and thus mask true electron transfer. We show that the presence of a redox electrolyte is critical to the accurate characterization of charge transfer, since it enables fast extraction of holes and helps maintain charge neutrality of the quantum dots. Although electron transfer is observed to be relatively slow (19 ns time constant), a high electron extraction efficiency (>95%) can be achieved because in well-passivated CuInSexS2-x quantum dots neutral excitons have significantly longer lifetimes of hundreds of nanoseconds.