Photoemission of Energetic Hot Electrons Produced via Up-Conversion in Doped Quantum Dots

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

Precise Control of Quantum Confinement in Cesium Lead Halide Perovskite Quantum Dots via Thermodynamic Equilibrium

Cesium lead halide (CsPbX₃) nanocrystals have emerged as a new family of materials that can outperform the existing semiconductor nanocrystals due to their superb optical and charge-transport properties. However, the lack of a robust method for producing quantum dots with controlled size and high ensemble uniformity has been one of the major obstacles in exploring the useful properties of excitons in zero-dimensional nanostructures of CsPbX₃. Here, we report a new synthesis approach that enables the precise control of the size based on the equilibrium rather than kinetics, producing CsPbX₃ quantum dots nearly free of heterogeneous broadening in their exciton luminescence. The high level of size control and ensemble uniformity achieved here will open the door to harnessing the benefits of excitons in CsPbX₃ quantum dots for photonic and energy-harvesting applications

Photoinduced Separation of Strongly Interacting 2‑D Layered TiS₂ Nanodiscs in Solution

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

Anisotropic Electron–Phonon Coupling in Colloidal Layered TiS₂ Nanodiscs Observed via Coherent Acoustic Phonons

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

In Situ Observation of Chemical Reactions at Buried Solid/Solid Interfaces in Coextruded Multilayer Polymer Films

The characterization of chemical reactions at the buried interface is critical to understand interfacial molecular interactions and improve interfacial properties like adhesion. Interface-sensitive sum frequency generation (SFG) vibrational spectroscopy can probe the buried interface in situ nondestructively. While SFG has been used to study many model polymer interfaces, it has never been applied to study multilayer polymer films produced on commercial coextrusion lines. Here, we apply SFG to elucidate the molecular details of chemical reactions at the buried interface in multilayer cast films consisting of maleic anhydride (MAH)-modified Tie layers promoting the adhesion between polyamide and polyethylene. We demonstrated the utility of SFG to identify the reaction products from the interfacial reaction between MAH and polyamide with varying MAH concentrations and to monitor changes of the interfacial molecular orientation. The developed approach is generally applicable to probe chemical reactions and molecular interactions at buried interfaces in multilayer polymer films

[Ti₈Zr₂O₁₂(COO)₁₆] Cluster: An Ideal Inorganic Building Unit for Photoactive Metal–Organic Frameworks

Metal–organic frameworks (MOFs) based on Ti-oxo clusters (Ti-MOFs) represent a naturally self-assembled superlattice of TiO₂ nanoparticles separated by designable organic linkers as antenna chromophores, epitomizing a promising platform for solar energy conversion. However, despite the vast, diverse, and well-developed Ti-cluster chemistry, only a scarce number of Ti-MOFs have been documented. The synthetic conditions of most Ti-based clusters are incompatible with those required for MOF crystallization, which has severely limited the development of Ti-MOFs. This challenge has been met herein by the discovery of the [Ti₈Zr₂O₁₂­(COO)₁₆] cluster as a nearly ideal building unit for photoactive MOFs. A family of isoreticular photoactive MOFs were assembled, and their orbital alignments were fine-tuned by rational functionalization of organic linkers under computational guidance. These MOFs demonstrate high porosity, excellent chemical stability, tunable photoresponse, and good activity toward photocatalytic hydrogen evolution reactions. The discovery of the [Ti₈Zr₂O₁₂­(COO)₁₆] cluster and the facile construction of photoactive MOFs from this cluster shall pave the way for the development of future Ti-MOF-based photocatalysts