Electron-Phonon Coupling and Resonant Relaxation from 1D and 1P States in PbS Quantum Dots

Ultranarrow Deep-Blue Luminescence of Perovskite Nanocrystals by A-Site Cation Control

Metal-halide perovskite nanocrystals (NCs) are one of the most promising emitters for the application of display and nanolight sources. The full width at half-maximum (FWHM) of photoluminescence (PL) emission is essential for color purity, which however remains a difficulty to further reduce the FWHM of the perovskite NCs at room temperature. Here, we show the quasi-sphere perovskite NCs with narrow PL emission at a deep-blue wavelength of ∼430 nm; its PL FWHM reaches ∼11 nm at room temperature, owing to the monodispersion in size distribution as well as the symmetric quasi-sphere morphology of NCs releasing the fine structure splitting-induced inhomogeneous broadening. Through regulating A cations with respect to the ratio of FA (or MA)-to-Cs and Cs-to-Pb, the PL emission of the NCs could be tuned from ∼505 to ∼430 nm combined with varied morphologies from large cube to small quasi-sphere. Such spectroscopic and morphological discrepancies are supposed to be attributed to the different crystalline kinetics that is strongly dependent on the synthetic condition. To be specific, in the case of increasing FA (or MA)-to-Cs, the growth rate of CsPbBr3 and FAPbBr3 (or MAPbBr3) perovskites is determined by the reactivity of transient species, while in the case of decreasing the Cs-to-Pb ratio, the growth rate of perovskites is slowed down by the serious reduction of Cs+ in the precursor. This study provides an effective strategy to adjust the emission across from green to deep-blue color and promotes the perovskite NCs with a narrow FWHM, and tunable PL emission facilitates in application of optoelectronic devices.

Making and Breaking of Exciton Cooling Bottlenecks in Halide Perovskite Nanocrystals

Harnessing quantum confinement (QC) effects in semiconductors to retard hot carrier cooling (HCC) is an attractive approach for enabling efficient hot carrier extraction to overcome the Shockley-Queisser limit. However, there is a debate about whether halide perovskite nanocrystals (PNCs) can effectively exploit these effects. To address this, we utilized pump-probe and multipulse pump-push-probe spectroscopy to investigate HCC behavior in PNCs of varying sizes and cation compositions. Our results validate the presence of an intrinsic phonon bottleneck with clear manifestations of QC effects in small CsPbBr3 PNCs exhibiting slower HCC rates compared to those of larger PNCs. However, the replacement of inorganic Cs+ with organic cations suppresses this intrinsic bottleneck. Furthermore, PNCs exhibit distinct size-dependent HCC behavior in response to changes in the cold carrier densities. We attribute this to the enhanced exciton-exciton interactions in strongly confined PNCs that facilitate Auger heating. Importantly, our findings dispel the existing controversy and provide valuable insights into design principles for engineering QC effects in PNC hot carrier applications.

Controlling Electronic Coupling of Acene Chromophores on Quantum Dot Surfaces through Variable-Concentration Ligand Exchange

Controlling the binding of functional organic molecules on quantum dot (QD) surfaces and the resulting ligand/QD interfacial structure determines the resulting organic-inorganic hybrid behavior. In this study, we vary the binding of tetracenedicarboxylate ligands bound to PbS QDs cast in thin films by performing solid-state ligand exchange of as-produced bound oleate ligands. We employ comprehensive Fourier-transform infrared (FTIR) analysis coupled with ultraviolet-visible (UV-vis) spectrophotometric measurements, transient absorption, and Density Functional Theory (DFT) simulations to study the QD/ligand surface structure and resulting optoelectronic properties. We find that there are three primary QD/diacid structures, each with a distinct binding mode dictated by the QD-ligand and ligand-ligand intermolecular and steric interactions. They can be accessed nearly independently of one another via different input ligand concentrations. Low concentrations produce mixed oleate/tetracene ligand structures where the tetracene carboxylates tilt toward QD surfaces. Intermediate concentrations produce mixed oleate/tetracene ligand structures with ligand-ligand interactions through intramolecular hydrogen bonding with the ligands perpendicular to the QD surface and weaker QD/ligand electronic interactions. High concentrations result in full ligand exchange, and the ligands tilt toward the surface while the QD film compacts. When the tetracene ligands tilt or lie flat on the QD surface, the benzene ring π-system interacts strongly with the p-orbitals at the PbS surface and produces strong QD-ligand interactions evidenced through QD/ligand state mixing, with a coupling energy of ≈700 meV.

Ultrafast Carrier Drift Transport Dynamics in CsPbI3 Perovskite Nanocrystalline Thin Films

We study the early time carrier drift dynamics in CsPbI3 nanocrystal thin films with a sub 25 ps time resolution. Prior to trapping, carriers exhibit band-like transport characteristics, which is similar to those of traditional semiconductor solar absorbers including Si and GaAs due to optical phonon and carrier scattering at high temperatures. In contrast to the popular polaron scattering mechanism, the CsPbI3 nanocrystal thin film demonstrates the strongest optical phonon scattering mechanism among other inorganic-organic hybrid perovskites, Si, and GaAs. This ultrafast dynamics study establishes a foundation for understanding the fundamental carrier drift properties that drive perovskite nanocrystal optoelectronics.

Carrier Transport in Colloidal Quantum Dot Intermediate Band Solar Cell Materials Using Network Science

Colloidal quantum dots (CQDs) have been proposed to obtain intermediate band (IB) materials. The IB solar cell can absorb sub-band-gap photons via an isolated IB within the gap, generating extra electron-hole pairs that increase the current without degrading the voltage, as has been demonstrated experimentally for real cells. In this paper, we model the electron hopping transport (HT) as a network embedded in space and energy so that a node represents the first excited electron state localized in a CQD while a link encodes the Miller-Abrahams (MA) hopping rate for the electron to hop from one node (=state) to another, forming an "electron-HT network". Similarly, we model the hole-HT system as a network so that a node encodes the first hole state localized in a CQD while a link represents the MA hopping rate for the hole to hop between nodes, leading to a "hole-HT network". The associated network Laplacian matrices allow for studying carrier dynamics in both networks. Our simulations suggest that reducing both the carrier effective mass in the ligand and the inter-dot distance increases HT efficiency. We have found a design constraint: It is necessary for the average barrier height to be larger than the energetic disorder to not degrade intra-band absorption.

Quantized Exciton Motion and Fine Energy-Level Structure of a Single Perovskite Nanowire

The quantum-confinement effect profoundly influences the exciton energy-level structures and recombination dynamics of semiconductor nanostructures but remains largely unexplored in traditional one-dimensional nanowires mainly due to their poor optical qualities. Here, we show that in defect-tolerant perovskite material of highly luminescent CsPbBr₃ nanowires, the exciton's center-of-mass motion perpendicular to the axial direction is severely confined. This is reflected in the two sets of photoluminescence spectra emitted from a single CsPbBr₃ nanowire, each of which consists of doublet peaks with linear polarizations perpendicular and parallel to the axial direction. Moreover, different exciton states can be mixed by the Rashba spin-orbit coupling effect, resulting in two single photoluminescence peaks with linear polarizations both along the nanowire axis. The above findings mark the emergence of an ideal platform for the exploration of intrinsic one-dimensional exciton photophysics and optoelectronics, thus bridging the long-missing research gap between the well-studied two- and zero-dimensional semiconductor nanostructures.

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

Photon upconversion is a strategy to generate high-energy excitations from low-energy photon input, enabling advanced architectures for imaging and photochemistry. Here, we show that ultra-small PbS nanocrystals can sensitize red-to-blue triplet-fusion upconversion with a large anti-Stokes shift (ΔE = 1.04 eV), and achieve max-efficiency upconversion at near-solar fluences (I th = 220 mW cm-2) despite endothermic triplet sensitization. This system facilitates the photo-initiated polymerization of methyl methacrylate using only long-wavelength light (λ exc: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry. Time-resolved spectroscopy and kinetic modelling clarify key loss channels, highlighting the benefit of long-lifetime nanocrystal sensitizers, but revealing that many (48%) excitons that reach triplet-extracting carboxyphenylanthracene ligands decay before they can transfer to free-floating acceptors-emphasizing the need to address the reduced lifetimes that we determine for molecular triplets near the nanocrystal surface. Finally, we find that the inferred thermodynamics of triplet sensitization from these ultra-small PbS quantum dots are surprisingly favourable-completing an advantageous suite of properties for upconversion photochemistry-and do not vary significantly across the ensemble, which indicates minimal effects from nanocrystal heterogeneity. Together, our demonstration and study of red-to-blue upconversion using ultra-small PbS nanocrystals in a quasi-equilibrium, mildly endothermic sensitization scheme offer design rules to advance implementations of triplet fusion, especially where large anti-Stokes wavelength shifts are sought.

Influence of Ligand Structure on Excited State Surface Chemistry of Lead Sulfide Quantum Dots

The ligand-nanocrystal boundaries of colloidal quantum dots (QDs) mediate the primary energy and electron transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We use mid-infrared transient absorption spectroscopy to reveal the influence that ligand structure and bonding to nanocrystal surfaces have on the changes of the excited state surface chemistry of this boundary in PbS QDs and the corresponding impact on charge transfer processes between nanocrystals. We demonstrate that oleate ligands undergo marked changes in their bonding to surfaces in the excitonic excited states of the nanocrystals, indicating that oleate passivated PbS surfaces undergo significant structural changes following photoexcitation. These changes can impact the surface mobility of the ligands and the ability of redox shuttles to approach the nanocrystal surfaces to undergo charge transfer in photocatalytic reactions. In contrast, markedly different transient vibrational features are observed in iodide/mercaptoproprionic acid passivated PbS QD films that result from charge transfer between neighboring nanocrystals and localization of holes at the nanocrystal surfaces near MPA ligands. This ability to distinguish the influence that excitonic excited states vs charge transfer processes have on the surface chemistry of the ligand-nanocrystal boundary lays the groundwork for exploration of how this boundary can be understood and controlled for the design of nanocrystalline materials tailored for specific applications in solar energy harvesting and photocatalytic reactions.

Photoinduced Fano Resonances between Quantum Confined Nanocrystals and Adsorbed Molecular Catalysts

Interaction of surface adsorbate vibration and intraband electron absorption in nanocrystals has been reported to affect the photophysical properties of both nanocrystals and surface adsorbates and may affect the performance of hybrid photocatalysts composed of semiconductor nanocrystals and molecular catalysts. Here, by combining ultrafast transient visible and IR spectroscopic measurements, we report the observation of Fano resonances between the intraband transition of the photogenerated electrons in CdS and CdSe nanocrystals and CO stretching vibrational modes of adsorbed molecular catalysts, [Fe₂(cbdt)(CO)₆] (FeFe; cbdt = 1-carboxyl-benzene-2,3-dithiolate), a molecular mimic for the active site of FeFe-hydrogenase. The occurrence of Fano resonances is independent of nanocrystal types (rods vs dots) or charge transfer character between the nanocrystal and FeFe, and is likely a general feature of nanocrystal and molecular catalyst hybrid systems. These results provide new insights into the fundamental interactions in these hybrid assemblies for artificial photosynthesis.

Dynamic Ligand Surface Chemistry of Excited PbS Quantum Dots

The ligand shell around colloidal quantum dots mediates the electron and energy transfer processes that underpin their use in optoelectronic and photocatalytic applications. Here, we show that the surface chemistry of carboxylate anchoring groups of oleate ligands passivating PbS quantum dots undergoes significant changes when the quantum dots are excited to their excitonic states. We directly probe the changes of surface chemistry using time-resolved mid-infrared spectroscopy that records the evolution of the vibrational frequencies of carboxylate groups following excitation of the electronic states. The data reveal a reduction of the Pb-O coordination of carboxylate anchoring groups to lead atoms at the quantum dot surfaces. The dynamic surface chemistry of the ligands may increase their surface mobility in the excited state and enhance the ability of molecular species to penetrate the ligand shell to undergo energy and charge transfer processes that depend sensitively on distance.

Non-adiabatic Excited-State Molecular Dynamics: Theory and Applications for Modeling Photophysics in Extended Molecular Materials

Optically active molecular materials, such as organic conjugated polymers and biological systems, are characterized by strong coupling between electronic and vibrational degrees of freedom. Typically, simulations must go beyond the Born-Oppenheimer approximation to account for non-adiabatic coupling between excited states. Indeed, non-adiabatic dynamics is commonly associated with exciton dynamics and photophysics involving charge and energy transfer, as well as exciton dissociation and charge recombination. Understanding the photoinduced dynamics in such materials is vital to providing an accurate description of exciton formation, evolution, and decay. This interdisciplinary field has matured significantly over the past decades. Formulation of new theoretical frameworks, development of more efficient and accurate computational algorithms, and evolution of high-performance computer hardware has extended these simulations to very large molecular systems with hundreds of atoms, including numerous studies of organic semiconductors and biomolecules. In this Review, we will describe recent theoretical advances including treatment of electronic decoherence in surface-hopping methods, the role of solvent effects, trivial unavoided crossings, analysis of data based on transition densities, and efficient computational implementations of these numerical methods. We also emphasize newly developed semiclassical approaches, based on the Gaussian approximation, which retain phase and width information to account for significant decoherence and interference effects while maintaining the high efficiency of surface-hopping approaches. The above developments have been employed to successfully describe photophysics in a variety of molecular materials.

PbS Nanocrystals Made with Excess PbCl2 Have an Intrinsic Shell that Reduces Their Stokes Shift

The use of excess PbCl₂ in the synthesis of PbS nanocrystals has become a convenient route to produce narrow-line-width infrared emitters. However, these materials have found limited adoption in optoelectronic devices-even compared to PbS nanocrystals prepared with lead oleate. Here, using both transmission electron microscopy and small-angle X-ray scattering, we show that excess PbCl₂ results in larger-diameter PbS nanocrystals for the same excitonic features, which is consistent with the formation of an intrinsic insulating shell. We observe further differences in excess-lead-chloride nanocrystals consistent with a shell, including lattice strain and smaller Stokes shifts for intermediate sizes (⌀: 4.8-6.8 nm) that match the passivation/rigidification predicted for a chloride-terminate surface. Our results clarify and rationalize the divergent properties of PbS nanocrystals prepared using different synthetic methodologies, give guidance for device implementation, and offer a new target for synthetic control.