62 research outputs found
Quantum simulation of multiple-exciton generation in a nanocrystal by a single photon
We have shown theoretically that efficient multiple exciton generation (MEG)
by a single photon can be observed in small nanocrystals (NCs). Our quantum
simulations that include hundreds of thousands of exciton and multi-exciton
states demonstrate that the complex time-dependent dynamics of these states in
a closed electronic system yields a saturated MEG effect on a picosecond
timescale. Including phonon relaxation confirms that efficient MEG requires the
exciton--biexciton coupling time to be faster than exciton relaxation time
Circular dichroism in non-chiral metal halide perovskites
We demonstrate theoretically that non-chiral perovskite layers can exhibit
circular dichroism (CD) in the absence of a magnetic field and without chiral
activation by chiral molecules. The effect is shown to be due to splitting of
helical excitonic states which can form in structures of orthorhombic or lower
symmetry that exhibit Rashba spin effects. The selective coupling of these
helical exciton states to helical light is shown to give rise to circular
dichroism. Polarization dependent absorption is shown to occur due to the
combined effect of Rashba splitting, in-plane symmetry breaking, and the effect
of the exciton momentum on its fine structure, which takes the form of Zeeman
splitting in an effective magnetic field. We calculate significant CD with an
anisotropy factor of up to 30% in orthorhombic perovskite layers under
off-normal top illumination conditions, raising the possibility of its
observation in non-chiral perovskite structures.Comment: 26 pages, 5 figures, 1 table. Jul5 revision, clarified description
after Eq. 11, added ref. Jul 13 revision, typo corrections; added ref
Circular dichroism in non-chiral metal halide perovskites
We demonstrate theoretically that non-chiral perovskite layers can exhibit circular dichroism (CD) in the absence of a magnetic field and without chiral activation by chiral molecules. The effect is shown to be due to splitting of helical excitonic states which can form in structures of orthorhombic or lower symmetry that exhibit Rashba spin effects. The selective coupling of these helical exciton states to helical light is shown to give rise to circular dichroism. Polarization dependent absorption is shown to occur due to the combined effect of Rashba splitting, in-plane symmetry breaking, and the effect of the exciton momentum on its fine structure, which takes the form of Zeeman splitting in an effective magnetic field. This phenomenon, which can be considered as a manifestation of extrinsic chirality, results in significant CD with an anisotropy factor of up to 30% in orthorhombic perovskite layers under off-normal, top illumination conditions, raising the possibility of its observation in non-chiral perovskite structures
Exciton fine structure in perovskite nanocrystals
The bright emission observed in cesium lead halide perovskite nanocrystals (NCs) has recently been explained in terms of a bright exciton ground state [Becker et al. Nature 2018, 553, 189−193], a claim that would make these materials the first known examples in which the exciton ground state is not an optically forbidden dark exciton. This unprecedented claim has been the subject of intense experimental investigation that has so far failed to detect the dark ground-state exciton. Here, we review the effective-mass/electron–hole exchange theory for the exciton fine structure in cubic and tetragonal CsPbBr_3 NCs. In our calculations, the crystal field and the short-range electron–hole exchange constant were calculated using density functional theory together with hybrid functionals and spin–orbit coupling. Corrections associated with long-range exchange and surface image charges were calculated using measured bulk effective mass and dielectric parameters. As expected, within the context of the exchange model, we find an optically inactive ground exciton level. However, in this model, the level order for the optically active excitons in tetragonal CsPbBr_3 NCs is opposite to what has been observed experimentally. An alternate explanation for the observed bright exciton level order in CsPbBr_3 NCs is offered in terms of the Rashba effect, which supports the existence of a bright ground-state exciton in these NCs. The size dependence of the exciton fine structure calculated for perovskite NCs shows that the bright–dark level inversion caused by the Rashba effect is suppressed by the enhanced electron–hole exchange interaction in small NCs
Double-Rashba materials for nanocrystals with bright ground-state excitons
While nanoscale semiconductor crystallites provide versatile fluorescent
materials for light-emitting devices, such nanocrystals suffer from the "dark
exciton"\unicode{x2014}an optically inactive electronic state into which the
nanocrystal relaxes before emitting. Recently, a theoretical mechanism was
discovered that can potentially defeat the dark exciton. The Rashba effect can
invert the order of the lowest-lying levels, creating a bright excitonic ground
state. To identify materials that exhibit this behavior, here we perform an
extensive high-throughput computational search of two large open-source
materials databases. Based on a detailed understanding of the Rashba mechanism,
we define proxy criteria and screen over 500,000 solids, generating 173
potential "bright-exciton" materials. We then refine this list with
higher-level first-principles calculations to obtain 28 candidates. To confirm
the potential of these compounds, we select five and develop detailed
effective-mass models to determine the nature of their lowest-energy excitonic
state. We find that four of the five solids (BiTeCl, BiTeI, GaTe, and
KIO) can yield bright ground-state excitons. Our approach thus reveals
promising materials for future experimental investigation of bright-exciton
nanocrystals.Comment: 19 pages, 4 figure
Quasicubic model for metal halide perovskite nanocrystals
We present an analysis of quantum confinement of carriers and excitons, and exciton fine structure, in metal halide perovskite (MHP) nanocrystals (NCs). Starting with coupled-band k · P theory, we derive a nonparabolic effective mass model for the exciton energies in MHP NCs valid for the full size range from the strong to the weak confinement limits. We illustrate the application of the model to CsPbBr₃ NCs and compare the theory against published absorption data, finding excellent agreement. We then apply the theory of electron-hole exchange, including both short- and long-range exchange interactions, to develop a model for the exciton fine structure. We develop an analytical quasicubic model for the effect of tetragonal and orthorhombic lattice distortions on the exchange-related exciton fine structure in CsPbBr₃, as well as some hybrid organic MHPs of recent interest, including formamidinium lead bromide (FAPbBr₃) and methylammonium lead iodide (MAPbI₃). Testing the predictions of the quasicubic model using hybrid density functional theory (DFT) calculations, we find qualitative agreement in tetragonal MHPs but significant disagreement in the orthorhombic modifications. Moreover, the quasicubic model fails to correctly describe the exciton oscillator strength and with it the long-range exchange corrections in these systems. Introducing the effect of NC shape anisotropy and possible Rashba terms into the model, we illustrate the calculation of the exciton fine structure in CsPbBr₃ NCs based on the results of the DFT calculations and examine the effect of Rashba terms and shape anisotropy on the calculated fine structure
Effects of shell thickness on the electric field dependence of exciton recombination in CdSe/CdS core/shell quantum dots
Here we examine the effects of shell thickness on the photophysical properties of CdSe/CdS core/shell quantum dots (QDs) in an electric field. Photoluminescence (PL) of QDs in an applied electric field is observed to decrease markedly with increasing shell thickness, with a thick-shelled (4.9 nm shell) sample exhibiting an order of magnitude greater PL suppression than a thin-shelled sample (1.25 nm shell) with the same core
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