Vector properties in photodissociation: Quantum treatment of the correlation between the spatial anisotropy and the angular momentum polarization of the fragments

The dependence of the angular momentum polarization (orientation and alignment) of the fragments on the direction of ejection k, is studied quantum mechanically for molecular photodissociation into two fragments of which one carries an angular momentum j. Explicit expressions in terms of the transition matrix elements for electronic excitation into the final dissociative states are given in the axial-recoil limit and for different photon polarizations. The importance of interference effects due to coherent excitation of dissociative states with different helicity quantum numbers (the projection Ω of j on the recoil direction k) is stressed. It is shown that not only absolute magnitudes but also relative phases of individual transition matrix elements can be determined separately if the spatial anisotropy of the angular momentum polarization is measured. © 1994 American Institute of Physics.Peer Reviewe

Explaining observed stability of excitons in highly excited CdSe nanoplatelets

Two-dimensional electron-hole gases in colloidal semiconductors have a wide variety of applications. Therefore, a proper physical understanding of these materials is of great importance. In this paper we present a detailed theoretical analysis of the recent experimental results by Tomar et al. [J. Phys. Chem. C 123, 9640 (2019)1932-744710.1021/acs.jpcc.9b02085] that show an unexpected stability of excitons in CdSe nanoplatelets at high photoexcitation densities. Including the screening effects by free charges on the exciton properties, our analysis shows that CdSe nanoplatelets behave very differently from bulk CdSe, and in particular do not show a crossover to an electron-hole plasma in the density range studied experimentally, even though there is substantial overlap between the excitons at the highest densities achieved. From our results we also conclude that a quantum degenerate exciton gas is realized in the experiments, which opens the prospect of observing superfluidity in CdSe nanoplatelets in the near future

Efficient Steplike Carrier Multiplication in Percolative Networks of Epitaxially Connected PbSe Nanocrystals

Carrier multiplication (CM) is a process in which a single photon excites two or more electrons. CM is of interest to enhance the efficiency of a solar cell. Until now, CM in thin films and solar cells of semiconductor nanocrystals (NCs) has been found at photon energies well above the minimum required energy of twice the band gap. The high threshold of CM strongly limits the benefits for solar cell applications. We show that CM is more efficient in a percolative network of directly connected PbSe NCs. The CM threshold is at twice the band gap and increases in a steplike fashion with photon energy. A lower CM efficiency is found for a solid of weaker coupled NCs. This demonstrates that the coupling between NCs strongly affects the CM efficiency. According to device simulations, the measured CM efficiency would significantly enhance the power conversion efficiency of a solar cell

Generating Triplets in Organic Semiconductor Tetracene upon Photoexcitation of Transition Metal Dichalcogenide ReS2

[Image: see text] We studied the dynamics of transfer of photoexcited electronic states in a bilayer of the two-dimensional transition metal dichalcogenide ReS(2) and tetracene, with the aim to produce triplets in the latter. This material combination was used as the band gap of ReS(2) (1.5 eV) is slightly larger than the triplet energy of tetracene (1.25 eV). Using time-resolved optical absorption spectroscopy, transfer of photoexcited states from ReS(2) to triplet states in tetracene was found to occur within 5 ps with an efficiency near 38%. This result opens up new possibilities for heterostructure design of two-dimensional materials with suitable organics to produce long-lived triplets. Triplets are of interest as sensitizers in a wide variety of applications including optoelectronics, photovoltaics, photocatalysis, and photon upconversion

Observation of the quantized motion of excitons in CdSe nanoplatelets

We show that the finite lateral sizes of ultrathin CdSe nanoplatelets strongly affect both their photoluminescence and optical absorption spectra. This is in contrast to the situation in quantum wells, in which the large lateral sizes may be assumed to be infinite. The lateral sizes of the nanoplatelets are varied over a range of a few to tens of nanometers. For these sizes excitons experience in-plane quantum confinement, and their center-of-mass motion becomes quantized. Our direct experimental observation of the discretization of the exciton center-of-mass states can be well understood on the basis of the simple particle-in-a-box model

PIZZICATO: Picosecond Scintillator Timing with Superconducting Nanowire Single-Photon Detectors

Time-of-flight positron emission tomography (TOF-PET) visualizes molecular processes in vivo and is commonly used in the treatment of cancer. TOF-PET can be transformed into a tool for personalized medicine in a much wider range of clinical applications if we can improve the time-of-flight resolution to ~10 ps. The PIZZICATO consortium is developing a novel type of TOF-PET detector based on large-area superconducting nanowire single-photon detectors (SNSPD) optically coupled to ultrafast direct-bandgap semiconductor scintillators. Here, we present first proof-of-concept results, including the successful development of large-area SNSPDs with sub-10 ps single-photon time resolution and first measurements of scintillation signals using SNSPDs

Electrochemical Charging of CdSe Quantum Dot Films: Dependence on Void Size and Counterion Proximity

Films of colloidal quantum dots (QDs) show great promise for application in optoelectronic devices. Great advances have been made in recent years in designing efficient QD solar cells and LEDs. A very important aspect in the design of devices based on QD films is the knowledge of their absolute energy levels. Unfortunately, reported energy levels vary markedly depending on the employed measurement technique and the environment of the sample. In this report, we determine absolute energy levels of QD films by electrochemical charge injection. The concomitant change in optical absorption of the film allows quantification of the number of charges in quantum-confined levels and thereby their energetic position. We show here that the size of voids in the QD films (<i>i.e.</i>, the space between the quantum dots) determines the amount of charges that may be injected into the films. This effect is attributed to size exclusion of countercharges from the electrolyte solution. Further, the energy of the QD levels depends on subtle changes in the QD film and the supporting electrolyte: the size of the cation and the QD ligand length. These nontrivial effects can be explained by the proximity of the cation to the QD surface and a concomitant lowering of the electrochemical potential. Our findings help explain the wide range of reported values for QD energy levels and redefine the limit of applicability of electrochemical measurements on QD films. Finally, the finding that the energy of QD levels depends on ligand length and counterion size may be exploited in optimized designs of QD sensitized solar cells

Data about: Photogeneration, relaxation and many-body effects of excitons and charge carriers in MoS₂, WS₂, and the Mo₀.₆W₀.₄S₂ alloy, probed by transient optical absorption spectroscopy.

The samples studied in this dataset are of MoS2, WS2, and the Mo0.6W0.4S2 alloy. Transient absorption measurements are performed on these multilayered compounds. By varying the pump photon energies and pump photon fluences, we studied the generation and relaxation dynamics of the photogenerated charges and/or excitons. To understand the spectral dynamics in detail, fits are made to the obtained signal. The signal is probed in the visible region and the data set contains the relevant files. </p

A Phonon Scattering Bottleneck for Carrier Cooling in Lead-Chalcogenide Nanocrystals

Absorption of photons by a semiconductor with an energy exceeding the band gap transition results in the formation of hot electron-hole pairs that quickly dissipate their excess free energy, resulting in quasi-thermalized conduction band electrons and valence band holes. For photovoltaic energy conversion, this cooling of the hot electron-hole pair is a major loss channel that restricts the maximum conversion efficiency of a single junction solar cell. Harvesting this excess free energy by hot carrier charge transfer has proven challenging due to the high cooling rates, reaching a few ps-1 or more due to electron-phonon and/or carrier-carrier interactions1. Using colloidal quantum dots (QDs), it proved possible to harvest the excess energy either by hot carrier transfer2 or the generation of multiple excitons (MEG)3, processes involving truly hot carriers with energies far above the band edge states (such as 1S, 1P). However, most studies on carrier cooling focus on changing occupation of these band edge states which only gives an idea of the rate-limiting step of the entire hot carrier cooling process. The relevant rates however are those of hot electron-hole pairs with energies far above the rate limiting 1P-1S transition. Here, we analyzed the electron-hole pair cooling in PbX(X=S,Se) QDs after photo-excitation with high energy photons where, by means of femtosecond white light transient absorption spectroscopy, energy levels throughout the entire Brillouin zone are probed. In bulk, PbX has a direct band gap at the 4-fold degenerate L point and at higher energy a number of critical points are found, notably along the Σ and Δ – directions leading to discrete quantized states in QDs. We observe a transient accumulation of charge carriers at these high energy quantized states around Σ when the pump photon energy exceeds the energy of the Σ-states and link this cooling bottleneck, which slows down carrier cooling to a net rate of ∼ 1 ps-1, via tight binding calculations to the energy level structure around Σ: a specific set of longitudinal optical phonons is required to scatter hot carriers away from Σ to the L-point absolute energy minima. We show that cooling via the Σ-direction is the dominant pathway for hot carrier relaxation in lead-salts, indicating its importance for the study of hot carrier processes such MEG. The impact of our findings on understanding the high effiency of MEG in lead-chalcogenide QDs is discussed and implications for both theoretical4 and experimental5,6 work on MEG are discussed. The concept that high energy critical points slow down hot carrier cooling through a phonon scattering bottleneck is applicable to other materials than lead-chalcogenides and presents a new route to design high effiency photovoltaic materials for example through the deliberate introduction of such critical points in novel nanomaterials (e.g. twisted bilayer graphene)

Photogeneration and Ultrafast Dynamics of Excitons and Charges in P3HT/PCBM Blends

The photogeneration quantum yield and dynamics of charge carriers and excitons in thin films of neat regioregular poly(3-hexylthiophene) (P3HT) and blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were studied with ultrafast optical pump-probe spectroscopy. In neat P3HT the quantum yield for direct photogeneration of charge carriers amounts to 0.15 per absorbed photon. The remaining fraction of absorbed photons leads to formation of excitons. Recombination of charges reduces the quantum yield to about 25% of its initial value on a time scale of 100 ps followed by decay to a no longer observable yield after 1 ns. Addition of 50% PCBM by weight leads to ultrafast (<200 fs) formation of charge pairs with a total quantum yield of 0.5. The presence of 50% PCBM causes exciton decay to be about an order of magnitude faster than in neat P3HT, which is expected to be at least in part due to interfacial exciton dissociation into charge carriers. The yield of charges in the blend has decayed to about half its initial value after 100 ps, while no further decay is observed within 1 ns. The small fraction (~1%) of excitons in neat P3HT that is probed by photoluminescence measurements has a lifetime of 660 ps, which significantly exceeds the 200 ps lifetime of nonfluorescent excitons that are probed by transient absorption measurements. The nonfluorescent excitons have a diffusion coefficient of about 2 × 10-4 cm2/s, which is an order of magnitude smaller than reported values for fluorescent excitons. The interaction radius for second-order decay of photoexcitations is as large as 8-17 nm, in agreement with an earlier result in the literature.