Cesium lead halide (CsPbX3) perovskite nanocrystals (NCs) possess the unique capability of post-synthesis anion exchange providing facile tunability of the optical properties, which is usually achieved by mixing NCs with reactive anion precursors. In this work, we show that the controllable anion exchange can be achieved in a dihalomethane solution of CsPbX3 NC in the absence of any spontaneously reacting anion source using photoexcitation of CsPbX3 NCs as the triggering mechanism for the halide ion exchange. The reaction begins with the photoinduced electron transfer from CsPbX3 NCs to dihalomethane solvent molecules producing halide ions via reductive dissociation, which is followed by anion exchange. The reaction proceeds only in the presence of excitation light and the rate and extent of reaction can be controlled by varying the light intensity. Furthermore, the asymptotic extent of reaction under continuous excitation can be controlled by varying the wavelength of light that self-limits the reaction when light becomes off-resonance with the absorption of NCs. The light-controlled anion exchange demonstrated here can be utilized to pattern the post-synthesis chemical transformation of CsPbX3 NCs, not readily achievable using typical methods of anion exchange.

Because of the absence of native dangling bonds on the surface of the layered transition metal dichalcogenides (TMDCs), the surface of colloidal quantum dots (QDs) of TMDCs is exposed directly to the solvent environment. Therefore, the optical and electronic properties of TMDCS QDs are expected to have stronger influence from the solvent than usual surface-passivated QDs due to more direct solvent-QD interaction. Study of such solvent effect has been difficult in colloidal QDs of TMDC due to the large spectroscopic heterogeneity resulting from the heterogeneity of the lateral size or (and) thickness in ensemble. Here, we developed a new synthesis procedure producing the highly uniform colloidal monolayer WS2 QDs exhibiting well-defined photoluminescence (PL) spectrum free from ensemble heterogeneity. Using these newly synthesized monolayer WS2 QDs, we observed the strong influence of the aromatic solvents on the PL energy and intensity of monolayer WS2 QD beyond the simple dielectric screening effect, which is considered to result from the direct electronic interaction between the valence band of the QDs and molecular orbital of the solvent. We also observed the large effect of stacking/separation equilibrium on the PL spectrum dictated by the balance between inter QD and QD-solvent interactions. The new capability to probe the effect of the solvent molecules on the optical properties of colloidal TMDC QDs will be valuable for their applications in various liquid surrounding environments.Keywords: Colloidal quantum dot; monolayer WS2; photoluminescence; solvent-quantum dot interaction; transition metal dichalcogenide;

Recent success in Mn2+ ion doping in cesium lead halide (CsPbX3) nanocrystals opened the door to exploring new optical, magnetic and charge carrier transport properties mediated via exciton–dopant exchange coupling in this new family of semiconductor nanocrystals. Here, we studied the dynamics of energy transfer from exciton to Mn2+ ions in Mn-doped CsPbCl3 nanocrystals to gain an insight into the relative strength of exciton–Mn exchange coupling compared to more extensively studied Mn-doped II–VI quantum dots. The comparison of exciton–Mn energy transfer times in CsPbCl3 nanocrystals and CdS/ZnS core/shell quantum dots suggests that exciton–Mn exchange coupling in CsPbX3 is not far behind that of CdS/ZnS despite the lack of quantum confinement. With further progress in the synthesis of Mn-doped CsPbX3 nanocrystals, such as imposing quantum confinement and expanding the range of host chemical composition, one could fully benefit from many properties of CsPbX3 superior to those of other semiconductor nanocrystals for hosting magnetic dopants.

We report the one-pot synthesis of colloidal Mn-doped cesium lead halide (CsPbX3) perovskite nanocrystals and efficient intraparticle energy transfer between the exciton and dopant ions resulting in intense sensitized Mn luminescence. Mn-doped CsPbCl3 and CsPb(Cl/Br)3 nanocrystals maintained the same lattice structure and crystallinity as their undoped counterparts with nearly identical lattice parameters at ∼0.2% doping concentrations and no signature of phase separation. The strong sensitized luminescence from d–d transition of Mn2+ ions upon band-edge excitation of the CsPbX3 host is indicative of sufficiently strong exchange coupling between the charge carriers of the host and dopant d electrons mediating the energy transfer, essential for obtaining unique properties of magnetically doped quantum dots. Highly homogeneous spectral characteristics of Mn luminescence from an ensemble of Mn-doped CsPbX3 nanocrystals and well-defined electron paramagnetic resonance spectra of Mn2+ in host CsPbX3 nanocrystal lattices suggest relatively uniform doping sites, likely from substitutional doping at Pb2+. These observations indicate that CsPbX3 nanocrystals, possessing many superior optical and electronic characteristics, can be utilized as a new platform for magnetically doped quantum dots expanding the range of optical, electronic, and magnetic functionality.Keywords: exciton-to-dopant energy transfer; Mn-doping; Perovskite; sensitized phosphorescence;

The benefits of the hot electrons from semiconductor nanostructures in photocatalysis or photovoltaics result from their higher energy compared to that of the band-edge electrons facilitating the electron-transfer process. The production of high-energy hot electrons usually requires short-wavelength UV or intense multiphoton visible excitation. Here, we show that highly energetic hot electrons capable of above-threshold ionization are produced via exciton-to-hot-carrier up-conversion in Mn-doped quantum dots under weak band gap excitation (∼10 W/cm2) achievable with the concentrated solar radiation. The energy of hot electrons is as high as ∼0.4 eV above the vacuum level, much greater than those observed in other semiconductor or plasmonic metal nanostructures, which are capable of performing energetically and kinetically more-challenging electron transfer. Furthermore, the prospect of generating solvated electron is unique for the energetic hot electrons from up-conversion, which can open a new door for long-range electron transfer beyond short-range interfacial electron transfer.Keywords: exciton-to-hot-carrier up-conversion; hot electron; Mn-doped quantum dot; photoemission;

Controlled lateral quantum confinement in single-layer transition-metal chalcogenides (TMCs) can potentially combine the unique properties of two-dimensional (2D) exciton with the size-tunability of exciton energy, creating the single-layer quantum dots (SQDs) of 2D TMC materials. However, exploring such opportunities has been challenging due to the limited ability to produce well-defined SQDs with sufficiently high quality and size control, in conjunction with the commonly observed inconsistency in the optical properties. Here, we report an effective method to synthesize high-quality and size-controlled SQDs of WSe2 via multilayer quantum dots (MQDs) precursors, which enables grasping a clear picture of the role of lateral confinement on the optical properties of the 2D exciton. From the single-particle optical spectra and polarization anisotropy of WSe2 SQDs of varying sizes in addition to their ensemble data, we reveal how the properties of 2D exciton in single-layer TMCs evolve with increasing lateral quantum confinement.

We report the in situ optical measurements of the rapid Li intercalation and deintercalation dynamics in 2-dimensional (2D) layered transition metal dichalcogenide (TMD) with a nanoscale lateral dimension using thin films fabricated with size-controlled colloidal TiS2 nanodiscs. The films exhibiting high optical homogeneity, where the interband absorption changes near-linearly to the amount of intercalated Li, enabled facile optical probing of the intercalation dynamics overcoming the shortcomings of amperometry susceptible to complications from non-Faradaic processes. The time scale of Li intercalation and deintercalation was on the order of seconds in the nanodiscs of ∼100 nm lateral dimension, indicating sufficiently rapid dynamic control of the intercalation-induced material properties with a reduced lateral dimension. The change in the rate and reversibility of the dynamics during the multiple intercalation/deintercalation cycles was also measured, providing a unique window to observe the effect of potential structural changes on the intercalation and deintercalation dynamics in 2D layered TMD structures with a nanoscale lateral dimension.

We show that hot electrons exhibiting the enhanced photocatalytic activity in H2 production reaction can be efficiently generated in Mn-doped quantum dots via the “upconversion” of the energy of two excitons into the hot charge carriers. The sequential two-photon-induced process with the long-lived Mn excited state serving as the intermediate state is considered as the pathway generating hot electrons. H2 production rate from doped quantum dots is significantly higher than that from undoped quantum dots and also exhibited the quadratic increase with the light intensity, demonstrating the effectiveness of the hot electrons produced in doped quantum dots in photocatalytic reaction. Due to the very long lifetime of Mn excited state (∼6 ms) in the doped quantum dots, the sequential two-photon excitation requires relatively low excitation rates readily achievable with a moderately concentrated solar radiation, demonstrating their potential as an efficient source of hot electrons operating at low excitation intensities.

Atomically thin layered transition metal dichalcogenides with highly anisotropic structure exhibit strong anisotropy in various material properties. Here, we report the anisotropic coupling between the interband optical transition and coherent acoustic phonon excited by ultrashort optical excitation in a colloidal solution of multilayered TiS2 nanodiscs. The transient absorption signal from the diameter- and thickness-controlled TiS2 nanodiscs dispersed in solution exhibited an oscillatory feature, which is attributed to the modulation of the interband absorption peak by the intralayer breathing mode. However, the signature of the interlayer acoustic phonon was not observed, while it has been previously observed in noncolloidal exfoliated sheets of MoS2. The dominance of the intralayer mode in modulating the interband optical transition was supported by the density functional theory (DFT) calculations of the optical absorption spectra of TiS2, which showed the stronger sensitivity of the interband absorption peak in the visible region to the in-plane strain than to the out-of-plane strain.

The dependence of the energy transfer rate on the content of sp2-hybridized carbon atoms in the hybrid structures of reduced graphene oxide (RGO) and Mn-doped quantum dot (QDMn) was investigated. Taking advantage of the sensitivity of QDMn’s dopant luminescence lifetime only to the energy transfer process without interference from the charge transfer process, the correlation between the sp2 carbon content in RGO and the rate of energy transfer from QDMn to RGO was obtained. The rate of energy transfer showed a strongly superlinear increase with increasing sp2 carbon content in RGO, suggesting the possible cooperative behavior of sp2 carbon domains in the energy transfer process as the sp2 carbon content increases.

We report an unusual response of colloidal layered transition metal dichalcogenide (TMDC) nanodiscs to the electric field, where the orientational order is created transiently only during the time-varying period of the electric field while no orientational order is created by the DC field. This result is in stark contrast to the typical electrokinetic response of various other colloidal nanoparticles, where the permanent dipole or (and) anisotropic-induced dipole creates a sustaining orientational order under the DC field. This indicates the lack of a sizable permanent dipole or (and) anisotropic-induced dipole in colloidal TMDC nanodiscs, despite their highly anisotropic lattice structure. While the orientational order is created only transiently by the time-varying field, a near-steady-state orientational order can be obtained by using an AC electric field. We demonstrate the utility of this method for the controlled orientation of colloidal nanoparticles that cannot be controlled via the usual interaction of the electric field with the nanoparticle dipole.Keywords: electric-field-induced orientation; transition metal dichalcogenide; two-dimensional nanostructure;

Colloidal 2-D layered transition metal dichalcogenide (TMDC) nanodiscs synthesized with uniform diameter and thickness can readily form the vertically stacked assemblies of particles in solution due to strong interparticle cohesive energy. The interparticle electronic coupling that modifies their optical and electronic properties poses a significant challenge in exploring their unique properties influenced by the anisotropic quantum confinement in different directions taking advantage of the controlled diameter and thickness. Here, we show that the assemblies of 2-D layered TiS2 nanodiscs are efficiently separated into individual nanodiscs via photoexcitation of the charge carriers by pulsed laser light, enabling the characterization of the properties of noninteracting TiS2 nanodiscs. Photoinduced separation of the nanodiscs is considered to occur via transient weakening of the interparticle cohesive force by the dense photoexcited charge carriers, which facilitates the solvation of each nanodisc by the solvent molecules.

Trapping of charge carriers is the major process competing with radiative recombination or transfer of charge carriers important in the application of semiconductor nanocrystals in photonics and photocatalysis. In typical semiconductor quantum dots, trapping of charge carriers usually leads to quenching of exciton luminescence. In this study, we present evidence indicating that thiol ligands on the surface that quench exciton luminescence can have an opposite effect on sensitized dopant luminescence in doped semiconductor nanocrystals by facilitating the recovery of the trapped exciton for sensitization. Despite the increase in hole trapping by the added octanethiol to the surface of Mn-doped CdS/ZnS nanocrystals, the sensitized Mn luminescence increased by the added octanethiol and the enhancement became stronger with increasing Mn doping concentration. While the role of octanethiol as the hole trap and the enhancement of Mn luminescence may seem contradictory, the thiol-induced enhancement of Mn luminescence is possible, since thiols play dual role as the hole trap and as the facilitator of the energy transfer from the trapped exciton to Mn, in contrast to the pre-existing hole traps that inhibit the energy transfer.

We show the suppression of luminescence quenching by metal nanoparticles (MNPs) in the plasmon enhancement of luminescence via fast sensitized energy transfer in Mn-doped quantum dots (QDs). The rapid intraparticle energy transfer between exciton and Mn, occurring on a few picoseconds time scale, separates the absorber (exciton) from the emitter (Mn), whose emission is detuned far from the plasmon of the MNP. The rapid temporal separation of the absorber and emitter combined with the reduced spectral overlap between Mn and plasmonic MNP suppresses the quenching of the luminescence while taking advantage of the plasmon-enhanced excitation. We compared the plasmon enhancement of exciton and Mn luminescence intensities in undoped and doped QDs simultaneously as a function of the distance between MNP and QD layers in a multilayer structure to examine the expected advantage of the reduced quenching in the sensitized luminescence. At the optimum MNP–QD layer distance, Mn luminescence exhibits stronger net enhancement than that of the exciton, which can be explained with a model incorporating fast sensitization along with reduced emitter–MNP spectral overlap. This study demonstrates that materials exhibiting fast sensitized luminescence that is sufficiently red-shifted from that of the sensitizer can be superior to usual luminophores in harvesting plasmon enhancement of luminescence by suppressing quenching.Keywords: doped semiconducting nanocrystals; plasmonics