Hot-carrier trapping preserves high quantum yields but limits optical gain in InP-based quantum dots.

Indium phosphide is the leading material for commercial applications of colloidal quantum dots. To date, however, the community has failed to achieve successful operation under strong excitation conditions, contrasting sharply with other materials. Here, we report unusual photophysics of state-of-the-art InP-based quantum dots, which makes them unattractive as a laser gain material despite a near-unity quantum yield.

Interplay between Mixed and Pure Exciton States Controls Singlet Fission in Rubrene Single Crystals.

Singlet fission (SF) is a multielectron process in which one singlet exciton converts into a pair of separated triplet excitons . SF is widely studied as it may help overcome the Shockley-Queisser efficiency limit for semiconductor photovoltaic cells. To elucidate and control the SF mechanism, great attention has been given to the identification of intermediate states in SF materials, which often appear elusive due to the complexity and fast time scales of the SF process.

Extending the defect tolerance of halide perovskite nanocrystals to hot carrier cooling dynamics.

Defect tolerance is a critical enabling factor for efficient lead-halide perovskite materials, but the current understanding is primarily on band-edge (cold) carriers, with significant debate over whether hot carriers can also exhibit defect tolerance. Here, this important gap in the field is addressed by investigating how intentionally-introduced traps affect hot carrier relaxation in CsPbX nanocrystals (X = Br, I, or mixture). Using femtosecond interband and intraband spectroscopy, along with energy-dependent photoluminescence measurements and kinetic modelling, it is found that hot carriers are not universally defect tolerant in CsPbX, but are strongly correlated to the defect tolerance of cold carriers, requiring shallow traps to be present (as in CsPbI).

Ultrafast vibrational control of organohalide perovskite optoelectronic devices using vibrationally promoted electronic resonance.

Vibrational control (VC) of photochemistry through the optical stimulation of structural dynamics is a nascent concept only recently demonstrated for model molecules in solution. Extending VC to state-of-the-art materials may lead to new applications and improved performance for optoelectronic devices. Metal halide perovskites are promising targets for VC due to their mechanical softness and the rich array of vibrational motions of both their inorganic and organic sublattices.

Alloying metal cations in perovskite nanocrystals is a new route to controlling hot carrier cooling.

Hot carrier cooling is slowed down upon alloying tin in lead-halide perovskite nanocrystals through the engineering of carrier-phonon and carrier-defect interactions.

Hot Carrier Cooling and Trapping in Atomically Thin WS Probed by Three-Pulse Femtosecond Spectroscopy.

Transition metal dichalcogenides (TMDs) have shown outstanding semiconducting properties which make them promising materials for next-generation optoelectronic and electronic devices. These properties are imparted by fundamental carrier-carrier and carrier-phonon interactions that are foundational to hot carrier cooling. Recent transient absorption studies have reported ultrafast time scales for carrier cooling in TMDs that can be slowed at high excitation densities via a hot-phonon bottleneck (HPB) and discussed these findings in the light of optoelectronic applications.

Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials.

The relaxation of the above-gap ("hot") carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier-carrier and carrier-phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump-push-probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials.

Photoexcitation Dynamics and Long-Lived Excitons in Strain-Engineered Transition Metal Dichalcogenides.

Strain-engineering in 2D transition metal dichalcogenide (TMD) semiconductors has garnered intense research interest in tailoring the optical properties via strain-induced modifications of the electronic bands in TMDs, while its impact on the exciton dynamics remains less understood. To address this, an extensive study of transient optical absorption (TA) of both W- and Mo-based single-crystalline monolayer TMDs grown by a recently developed laser-assisted evaporation method is performed. All spectral features of the monolayers as grown on fused silica substrates exhibit appreciable redshifts relating to the existence of strain due to growth conditions.

Quantum Shells Boost the Optical Gain of Lasing Media.

Auger decay of multiple excitons represents a significant obstacle to photonic applications of semiconductor quantum dots (QDs). This nonradiative process is particularly detrimental to the performance of QD-based electroluminescent and lasing devices. Here, we demonstrate that semiconductor quantum shells with an "inverted" QD geometry inhibit Auger recombination, allowing substantial improvements to their multiexciton characteristics.

Dynamical Interconversion between Excitons and Geminate Charge Pairs in Two-Dimensional Perovskite Layers Described by the Onsager-Braun Model.

Time-resolved photoluminescence (PL) and femtosecond transient absorption (TA) spectroscopy are employed to study the photoexcitation dynamics in a highly emissive two-dimensional perovskite compound (en)PbBr·3Br with the ethylene diammonium (en) spacer. We find that while the PL kinetics is substantially -dependent over the whole range of studied temperatures ∼ 77-350 K, the PL quantum yield remains remarkably nearly -independent up to ∼ 280-290 K, appreciably decreasing only at higher temperatures. Considerable differences are also revealed between the TA spectra and the responses to the excitation power at low and at room temperatures.

Ultrafast carrier dynamics of metal halide perovskite nanocrystals and perovskite-composites.

Perovskite nanocrystals (NCs), especially those based on cesium lead halides, have emerged in recent years as highly promising materials for efficient solar cells and photonic applications. The key to realization of full potential of these materials lies however in the molecular level understanding of the processes triggered by light. Herein we highlight the knowledge gained from photophysical investigations on these NCs of various sizes and compositions employing primarily the femtosecond pump-probe technique.

Biexciton Generation and Dissociation Dynamics in Formamidinium- and Chloride-Doped Cesium Lead Iodide Perovskite Nanocrystals.

Recent studies show that perovskites (ABX-type) comprising mixed A or B cation and/or mixed halide (X) are more stable and efficient materials for photovoltaic applications than their respective pure forms. Herein we report how doping of a small quantity of formamidinium and/or chloride ion influences the single and multiexciton dynamics of CsPbI nanocrystals (NCs). With the help of ultrafast pump-probe spectroscopic measurements, we show that chloride doping can enhance the biexciton lifetime of the system significantly by slowing down the Auger recombination (AR) process.

Functional Molecular System of Bis(pyrazolyl)pyridine Derivatives: Photophysics, Spectroscopy, Computation, and Ion Sensing.

A new series of conjugated donor-π-acceptor type of 2,6-bis(pyrazolyl)pyridine derivatives (compounds ) have been synthesized via Horner-Wadsworth-Emmons (HWE) reaction, starting from a common phosphonate precursor and diverse donor aromatic aldehydes and characterized by routine spectral analysis including elemental analysis. Compound , one of the starting precursors, and molecule , the first member of the donor-π-acceptor series, are additionally characterized by single-crystal X-ray structure determination. Compounds and are crystallized in 1̅ (triclinic) and 2/ (monoclinic) space groups, respectively.

All-inorganic perovskite nanocrystal assisted extraction of hot electrons and biexcitons from photoexcited CdTe quantum dots.

Excitation of semiconductor quantum dots (QDs) by photons possessing energy higher than the band-gap creates a hot electron-hole pair, which releases its excess energy as waste heat or under certain conditions (when hν > 2E) produces multiple excitons. Extraction of these hot carriers and multiple excitons is one of the key strategies for enhancing the efficiency of QD-based photovoltaic devices. However, this is a difficult task as competing carrier cooling and relaxation of multiple excitons (through Auger recombination) are ultrafast processes.

Luminescence tuning and exciton dynamics of Mn-doped CsPbCl nanocrystals.

Mn-Doped perovskite nanocrystals (NCs) are a new class of materials offering exciting opportunities to control over their optical and magnetic properties. Herein, we report a series of Mn-doped CsPbCl NCs exhibiting a tunable Mn photoluminescence (PL) band with a PL peak wavelength pushed up to 625 nm and tuned over a range of 40 nm, the largest achieved so far, by only varying the Mn content. The X-band EPR data and Mn PL decay behaviour of the NCs reveal that the exchange interaction between Mn ions is mainly responsible for a large shift of the Mn PL band.

Complete ultrafast charge carrier dynamics in photo-excited all-inorganic perovskite nanocrystals (CsPbX).

Understanding the nature and dynamics of the photo-induced transients of all-inorganic perovskite nanocrystals (NCs) is key to their exploitation in potential applications. In order to determine the nature of charge carriers, their deactivation pathways and dynamics, the photo-induced transients of CsPbBr, CsPbBrI, CsPbBrI and CsPbI NCs are spectrally and temporally characterized employing a combination of femtosecond transient absorption (TA) and photoluminescence (PL) up-conversion techniques and global analysis of the data. The results provide distinct identities of the excitons and free charge carriers and distinguish the hot charge carriers from the cold ones.

Photoinduced 2-way electron transfer in composites of metal nanoclusters and semiconductor quantum dots.

In order to explore the potential of nanocomposites comprising semiconductor quantum dots (QDs) and metal nanoclusters (NCs) in photovoltaic and catalytic applications, the interaction between CdTe QDs and gold NCs, Au10 and Au25, stabilized by histidine, bovine serum albumin (BSA) and glutathione, is studied by an ultrafast transient absorption (TA) technique. Temporal and spectral studies of the transients reveal photoinduced 2-way electron transfer between the two constituents of the nanocomposites, where Au NCs, which generally act as electron donors when used as photosensitizers, perform the role of the efficient electron acceptor. Interestingly, it is found that the electron transfer dynamics in these composites is governed not by the distance of separation of the constituents but by the nature of the surface capping ligands.

Fluorescence Blinking and Photoactivation of All-Inorganic Perovskite Nanocrystals CsPbBr3 and CsPbBr2I.

Study of the emission behavior of all-inorganic perovskite nanocrystals CsPbBr3 and CsPbBr2I as a function of the excitation power employing fluorescence correlation spectroscopy and conventional techniques reveals fluorescence blinking in the microsecond time scale and photoinduced emission enhancement. The observation provides insight into the radiative and nonradiative deactivation pathways of these promising substances. Because both blinking and photoactivation processes are intimately linked to the charge separation efficiency and dynamics of the nanocrystals, these key findings are likely to be helpful in realizing the true potential of these substances in photovoltaic and optoelectronic applications.

Hexaethylsubporphyrins: β-alkyl analogues in the subporphyrin family.

Two new subporphyrins were synthesized for the first time from a β-substituted pyrrole i.e. 3,4-diethylpyrrole via pyridine-tri-N-(3,4-diethylpyrrolyl)borane as building blocks.

Effect of the alkyl chain length on the rotational dynamics of nonpolar and dipolar solutes in a series of N-alkyl-N-methylmorpholinium ionic liquids.

Rotational dynamics of two dipolar solutes, 4-aminophthalimide (AP) and 6-propionyl-2-dimethylaminonaphthalene (PRODAN), and a nonpolar solute, anthracene, have been studied in N-alkyl-N-methylmorpholinium (alkyl = ethyl, butyl, hexyl, and octyl) bis(trifluoromethansulfonyl)imide (Tf2N) ionic liquids as a function of temperature and excitation wavelength to probe the microheterogeneous nature of these ionic liquids, which are recently reported to be more structured than the imidazolium ionic liquids (Khara and Samanta, J. Phys. Chem.