Hot excitons are responsible for increasing photoluminescence blinking activity in single lead sulfide/cadmium sulfide nanocrystals.

The kinetics of photoluminescence blinking for isolated PbS/CdS nanocrystals changes with the photon excitation energy, with PL blinking increasing in frequency and changing from a two-state to a multistate on/off switching when the excitation energy changes from 1Sh-1Se (≈1.4 eV) to 1Ph-1Pe (≈2.4 eV). This increase in PL blinking activity with increased photon excitation energy is reasoned in terms of the formation of hot excitons leading to emission from both the band edge state and defect states, with defect states connected to traps that are otherwise inaccessible following low photon energy excitation at the band edge.

Electron transfer dynamics from single near infrared emitting lead sulfide-cadmium sulfide nanocrystals to titanium dioxide.

In this study we report the first successful demonstration of electron transfer between single near infrared emitting PbS/CdS nanocrystals and an external acceptor, titanium dioxide (TiO2). We demonstrate distance-dependent electron transfer from single nanocrystals to TiO2 and explore the effect of this process on the photoluminescence dynamics of these nanocrystals. Isolated PbS/CdS QDs are found to exhibit blinking dynamics similar to other nanocrystals like CdSe/ZnS; however, their photoluminescence follows a quasi two-state pattern with heterogeneous photoluminescence lifetimes which may be the result of their emission originating from different energy states. Electron transfer of these nanocrystals with an external acceptor inhibits their photoluminescence lifetime heterogeneity and biases their blinking dynamics in a manner similar to that observed for visible emitting CdSe/ZnS nanocrystals undergoing electron transfer with external acceptors. While the present study reconfirms the universality of quantum dot blinking among various types of nanocrystals, it also demonstrates that universality remains valid for the communication of various types of nanocrystals with the exterior world, here pictured as electron transfer with external acceptors.

Thick-Shell CuInS2/ZnS Quantum Dots with Suppressed "Blinking" and Narrow Single-Particle Emission Line Widths.

Quantum dots (QDs) of ternary I-III-VI2 compounds such as CuInS2 and CuInSe2 have been actively investigated as heavy-metal-free alternatives to cadmium- and lead-containing semiconductor nanomaterials. One serious limitation of these nanostructures, however, is a large photoluminescence (PL) line width (typically >300 meV), the origin of which is still not fully understood. It remains even unclear whether the observed broadening results from considerable sample heterogeneities (due, e.g., to size polydispersity) or is an unavoidable intrinsic property of individual QDs. Here, we answer this question by conducting single-particle measurements on a new type of CuInS2 (CIS) QDs with an especially thick ZnS shell. These QDs show a greatly enhanced photostability compared to core-only or thin-shell samples and, importantly, exhibit a strongly suppressed PL blinking at the single-dot level. Spectrally resolved measurements reveal that the single-dot, room-temperature PL line width is much narrower (down to ∼60 meV) than that of the ensemble samples. To explain this distinction, we invoke a model wherein PL from CIS QDs arises from radiative recombination of a delocalized band-edge electron and a localized hole residing on a Cu-related defect and also account for the effects of electron-hole Coulomb coupling. We show that random positioning of the emitting center in the QD can lead to more than 300 meV variation in the PL energy, which represents at least one of the reasons for large PL broadening of the ensemble samples. These results suggest that in addition to narrowing size dispersion, future efforts on tightening the emission spectra of these QDs might also attempt decreasing the "positional" heterogeneity of the emitting centers.

Using Perovskite Nanoparticles as Halide Reservoirs in Catalysis and as Spectrochemical Probes of Ions in Solution.

The ability of cesium lead halide (CsPbX3; X = Cl(-), Br(-), I(-)) perovskite nanoparticles (P-NPs) to participate in halide exchange reactions, to catalyze Finkelstein organohalide substitution reactions, and to colorimetrically monitor chemical reactions and detect anions in real time is described. With the use of tetraoctylammonium halide salts as a starting point, halide exchange with the P-NPs was performed to calibrate reactivity, stability, and extent of ion exchange. The exchange of CsPbI3 with Cl(-) or Br(-) causes a significant blue-shift in absorption and photoluminescence, whereas reacting I(-) with CsPbBr3 causes a red-shift of similar magnitudes. With the high local halide concentrations and the facile nature of halide exchange in mind, we then explored the ability of P-NPs to catalyze organohalide exchange in Finkelstein like reactions. Results indicate that the P-NPs serve as excellent halide reservoirs for substitution of organohalides in nonpolar media, leading to not only different organohalide products, but also a complementary color change over the course of the reaction, which can be used to monitor kinetics in a precise manner. The merits of using P-NP as spectrochemical probes for real time assaying is then expanded to other anions which can react with, or result in unique, classes of perovskites.

Nonradiative Energy Transfer from Individual CdSe/ZnS Quantum Dots to Single-Layer and Few-Layer Tin Disulfide.

The combination of zero-dimensional (0D) colloidal CdSe/ZnS quantum dots with tin disulfide (SnS2), a two-dimensional (2D)-layered metal dichalcogenide, results in 0D-2D hybrids with enhanced light absorption properties. These 0D-2D hybrids, when exposed to light, exhibit intrahybrid nonradiative energy transfer from photoexcited CdSe/ZnS quantum dots to SnS2. Using single nanocrystal spectroscopy, we find that the rate for energy transfer in 0D-2D hybrids increases with added number of SnS2 layers, a positive manifestation toward the potential functionality of such 2D-based hybrids in applications such as photovoltaics and photon sensing.

Charge trapping and de-trapping in isolated CdSe/ZnS nanocrystals under an external electric field: indirect evidence for a permanent dipole moment.

Single nanoparticle studies of charge trapping and de-trapping in core/shell CdSe/ZnS nanocrystals incorporated into an insulating matrix and subjected to an external electric field demonstrate the ability to reversibly modulate the exciton dynamics and photoluminescence blinking while providing indirect evidence for the existence of a permanent ground state dipole moment in such nanocrystals. A model assuming the presence of energetically deep charge traps physically aligned along the direction of the permanent dipole is proposed in order to explain the dynamics of nanocrystal blinking in the presence of a permanent dipole moment.

Surface-charge accumulation effects on open-circuit voltage in organic solar cells based on photoinduced impedance analysis.

The accumulation of dissociated charge carriers plays an important role in reducing the loss occurring in organic solar cells. We find from light-assisted capacitance measurements that the charge accumulation inevitably occurred at the electrode and photovoltaic layer interface for bulk-heterojunction ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al solar cells. Our results indicate, for the first time through impedance measurements, that the charge accumulation exists at the anode side of the device, and more importantly, we successfully identify the type of charge accumulated. Further study shows that the charge accumulation can significantly affect open circuit voltage and short circuit current. As a result, our experimental results from light assisted capacitance measurements provide a new understanding of the loss in open-circuit voltage and short-circuit photocurrent based on charge accumulation. Clearly, controlling charge accumulation presents a new mechanism to improve the photovoltaic performance of organic solar cells.

Core size dependent hole transfer from a photoexcited CdSe/ZnS quantum dot to a conductive polymer.

Photoinduced hole transfer from a CdSe/ZnS quantum dot to a conjugated polymer is tuned by varying the quantum dot core size. Hole transfer affects the photoluminescence blinking of the quantum dot, increasing the duration of the on-states and decreasing that of the off-states.

Spin radical enhanced magnetocapacitance effect in intermolecular excited states.

This article reports the magnetocapacitance effect (MFC) based on both pristine polymer MEH-PPV and its composite system doped with spin radicals (6R-BDTSCSB). We observed that a photoexcitation leads to a significant positive MFC in the pristine MEH-PPV. Moreover, we found that a low doping of spin radicals in polymer MEH-PPV causes a significant change on the MFC signal: an amplitude increase and a line-shape narrowing under light illumination at room temperature. However, no MFC signal was observed under dark conditions in either the pristine MEH-PPV or the radical-doped MEH-PPV. Furthermore, the magnitude increase and line-shape narrowing caused by the doped spin radicals are very similar to the phenomena induced by increasing the photoexcitation intensity. Our studies suggest that the MFC is essentially originated from the intermolecular excited states, namely, intermolecular electron-hole pairs, generated by a photoexcitation in the MEH-PPV. More importantly, by comparing the effects of spin radicals and electrically polar molecules on the MFC magnitude and line shape, we concluded that the doped spin radicals can have the spin interaction with intermolecular excited states and consequently affect the internal spin-exchange interaction within intermolecular excited states in the development of MFC. Clearly, our experimental results indicate that dispersing spin radicals forms a convenient method to enhance the magnetocapacitance effect in organic semiconducting materials.

Magneto-dielectric effects induced by optically-generated intermolecular charge-transfer states in organic semiconducting materials.

Traditionally, magneto-dielectric effects have been developed by combining ferroelectric and magnetic materials. Here, we show a magneto-dielectric effect from optically-generated intermolecular charge-transfer states in an organic semiconducting donor:acceptor (PVK:TCNB) system. We observe in magnetic field effects of photoluminescence that a magnetic field can change singlet/triplet population ratio in intermolecular charge-transfer states. Furthermore, our theoretical analysis and experimental evidence indicate that the singlets and triplets in charge-transfer states have stronger and weaker electrical polarizations, respectively. Therefore, the observed magneto-dielectric effect can be attributed to magnetically-dependent singlet/triplet ratio in intermolecular charge-transfer states. In principle, a magneto-dielectric effect can be generated through two different channels based on magneto-polarization and magneto-current effects when the singlet/triplet ratio in intermolecular charge-transfer states is changed by a magnetic field. We find, from the simulation of dielectric effects, that magneto-polarization and magneto-current effects play primary and secondary roles in the generation of magneto-dielectric effect.

Magneto-optical investigations on the formation and dissociation of intermolecular charge-transfer complexes at donor-acceptor interfaces in bulk-heterojunction organic solar cells.

The electron-hole pairs can be formed in intermolecular charge-transfer (CT) states between two adjacent molecules due to Coulomb interaction in organic semiconducting materials. In general, the exciton dissociation can experience the intermediate states: intermolecular CT states at the donor-acceptor interfaces to generate a photocurrent in organic solar cells. This article reports the magneto-optical studies on intermolecular CT states in the generation of photocurrent by using magnetic field effects of photocurrent (MFE(PC)) and light-assisted dielectric response (LADR). The MFE(PC) and LADR studies reveal that internal electrical drifting and local Coulomb interaction can largely change the formation and dissociation of CT states by changing internal charge-transport channels and local Coulomb interaction through morphological development upon thermal annealing. Therefore, the MFE(PC) and LADR can be used as effective magneto-optical tools to investigate charge recombination, separation, and transport in organic solar cells.