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1. Effect of shape anisotropy in nanocrystals of semiconductors with small spin–orbit splitting

   https://doi.org/10.1063/5.0187728  

   Goupalov, S. V. (March 2024, The Journal of Chemical Physics)

   We derive an effective spin-Hamiltonian accounting for the shape anisotropy of the zinc blende semiconductor nanocrystals within the k · p formalism explicitly taking into account the spin–orbit split-off valence band. It is shown that, for small InP nanocrystals, neglect of the spin–orbit split-off band can lead to significant underestimation of one of the two parameters determining the exciton fine-structure splittings. This parameter is only important for nanocrystals with shape anisotropy. 

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2. Surface passivation extends single and biexciton lifetimes of InP quantum dots

   https://doi.org/10.1039/D0SC01039A  

   Yang, Wenxing; Yang, Yawei; Kaledin, Alexey L.; He, Sheng; Jin, Tao; McBride, James R.; Lian, Tianquan (June 2020, Chemical Science)

   Indium phosphide quantum dots (InP QDs) are nontoxic nanomaterials with potential applications in photocatalytic and optoelectronic fields. Post-synthetic treatments of InP QDs are known to be essential for improving their photoluminescence quantum efficiencies (PLQEs) and device performances, but the mechanisms remain poorly understood. Herein, by applying ultrafast transient absorption and photoluminescence spectroscopies, we systematically investigate the dynamics of photogenerated carriers in InP QDs and how they are affected by two common passivation methods: HF treatment and the growth of a heterostructure shell (ZnS in this study). The HF treatment is found to improve the PLQE up to 16–20% by removing an intrinsic fast hole trapping channel ( τ h,non = 3.4 ± 1 ns) in the untreated InP QDs while having little effect on the band-edge electron decay dynamics ( τ e = 26–32 ns). The growth of the ZnS shell, on the other hand, is shown to improve the PLQE up to 35–40% by passivating both electron and hole traps in InP QDs, resulting in both a long-lived band-edge electron ( τ e > 120 ns) and slower hole trapping lifetime ( τ h,non > 45 ns). Furthermore, both the untreated and the HF-treated InP QDs have short biexciton lifetimes ( τ xx ∼ 1.2 ± 0.2 ps). The growth of an ultra-thin ZnS shell (∼0.2 nm), on the other hand, can significantly extend the biexciton lifetime of InP QDs to 20 ± 2 ps, making it a passivation scheme that can improve both the single and multiple exciton lifetimes. Based on these results, we discuss the possible trap-assisted Auger processes in InP QDs, highlighting the particular importance of trap passivation for reducing the Auger recombination loss in InP QDs. 

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3. Exciton Fine Structure in Axially Symmetric Quantum Dots and Rods of III–V and II–VI Semiconductors

   https://doi.org/10.1021/acs.jpclett.4c02800  

   Goupalov, Serguei V (November 2024, The Journal of Physical Chemistry Letters)

   Both absorption and emission of light in semiconductor quantum dots occur through excitation or recombination of confined electron−hole pairs, or excitons, with tunable size-dependent resonant frequencies that are ideal for applications in various fields. Some of these applications require control over quantum dot shape uniformity, while for others, control over energy splittings among exciton states emitting light in different polarizations and/or between bright and dark exciton states is of key importance. These splittings, known as exciton fine structure, are very sensitive to the nanocrystal shape. Theoretically, nanocrystals of spheroidal shape are often considered, and their nonsphericity is treated perturbatively as stemming from a linear uniaxial deformation of a sphere. Here, we compare this treatment with a nonperturbative model of a cylindrical box, free of any restrictions on the cylinder’s aspect ratio. This comparison allows one to understand the limits of validity of the traditional perturbative model and offers insights into the relative importance of various mechanisms controlling the exciton fine structure. These insights are relevant to both colloidal nanocrystals and epitaxial quantum dots of III−V and II−VI semiconductors. 

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4. Coordinated Anions Tune Z-Type Ligand Displacement from Colloidal CdSe and InP Nanocrystal Surfaces

   https://doi.org/10.1021/acs.inorgchem.4c03534  

   Degbevi, Mawuli; Stoffel, Jonathan T; Tsui, Emily Y (December 2024, Inorganic Chemistry)

   Neutral metal salts coordinate to the surfaces of colloidal semiconductor nanocrystals (NCs) by acting as Lewis acid acceptors for the NC surface anions. This ligand coordination has been associated with increased emission due to passivation of surface hole traps. Here, variation of the anionic ligands of metal salts is used to study anion effects on metal complex Lewis acidity and surface coordination at CdSe and InP NCs. To resolve dynamic ligand exchange processes, the tetracarbonylcobaltate anion, [Co(CO)4]–, is used as a monoanionic ligand for which IR spectroscopy can readily identify displacement of neutral M[Co(CO)4]x species (M = Cd or In; x = 2 or 3, respectively) upon addition of neutral donor ligands. Notably, although Cd[Co(CO)4]2 is more Lewis acidic than cadmium oleate, the former is more readily displaced from the NC surfaces. Lewis acidity and X-type anion exchange are therefore factors to be considered when performing post-synthetic addition of metal salts for NC photoluminescence emission enhancement. 

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5. Spectroscopic investigations of electron and hole dynamics in MAPbBr ₃ perovskite film and carrier extraction to PEDOT hole transport layer

   https://doi.org/10.1039/D1CP00658D  

   Niedzwiedzki, Dariusz M.; Kouhnavard, Mojgan; Diao, Yifan; D'Arcy, Julio M.; Biswas, Pratim (June 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   Organometallic halide perovskite (MAPPbBr 3 ), Rust-based Vapor Phase Polymerization (RVPP)-PEDOT hole transporting layers and (RVPP-PEDOT)/MAPPbBr 3 dual-layer, deposited on fluorine doped tin oxide glass were studied at room temperature using steady-state absorption, time-resolved photoluminescence imaging and femtosecond time-resolved absorption spectroscopy. Application of these techniques in conjunction with diverse excitation intensities allowed determination of various optoelectronic properties of the perovskite film and the time constant of the hole extraction process. Spectral reconstruction of the bandedge absorption spectrum using Elliot's formula enabled separation of the exciton band. The binding energy of the exciton was determined to be 19 meV and the bandgap energy of the perovskite film was 2.37 eV. Subsequent time-resolved photoluminescence studies of the perovskite film performed using a very weak excitation intensity followed by a global analysis of the data revealed monomolecular recombination dynamics of charge carriers occurring with an amplitude weighted lifetime of 3.2 ns. Femtosecond time-resolved transient absorption of the film performed after excitation intensity spanning a range of over two orders of magnitude enabled determining the rate constant of bimolecular recombination and was found to be 2.6 × 10 −10 cm 3 s −1 . Application of numerous high intensity excitations enabled observation of band filling effect and application of the Burstein–Moss model allowed to determine the reduced effective mass of photoexcited electron–hole pair in MAPPbBr 3 film to be 0.19 rest mass of the electron. Finally, application of transient absorption on RVPP-PEDOT/MAPPbBr 3 enabled determination of a 0.4 ps time constant for the MAPPbBr 3 -to-PEDOT hole extraction process. 

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