Dopants in semiconductor nanostructures offer tremendous control over electronic, optical, and magnetic properties beyond what is achievable in bulk materials. We demonstrate that the broad dopant emission in semiconductor nanoplatelets effectively maps the electron wave function across the nanoplatelet thickness. Both the emission energy and lifetime of the dopant transition depend strongly on the depth of the dopant within the nanoplatelet. This dependence arises from the electrostatic self-interaction of the charged dopant, which varies with proximity to the dielectric discontinuity at the nanoplatelet surface. Through comprehensive single-particle spectroscopy of silver-doped CdSe nanoplatelets, we verify that acceptors near the center emit at higher energies with shorter lifetimes, while those near the surface emit at lower energies with longer lifetimes. This spatial mapping also reveals unusual two-color emission from individual nanoplatelets, with enhanced Auger recombination yielding exceptional photon antibunching (>90% purity) at room temperature, suggesting potential applications in quantum information technologies.

Semiconductor nanoplatelets possess exceptional optical properties that make them promising candidates for next-generation optoelectronic applications. However, unlike quantum dots where absorption spectroscopy alone can determine both size and concentration, nanoplatelets present a significant characterization challenge: the absorption peak position reveals only thickness, providing no information about lateral dimensions or concentration. This limitation forces researchers to rely on time-consuming elemental analysis techniques for complete sample characterization. Here, we present an experimentally verified theoretical framework that predicts the frequency-dependent absorption coefficient of randomly oriented CdSe, CdS, and CdTe nanoplatelets, enabling concentration determination from absorption measurements and lateral size estimates. Our model shows that the integrated absorption coefficient depends universally on nanoplatelet surface area and thickness, yielding a practical tool to extract concentrations without laborious elemental analysis. This approach bridges the characterization gap between quantum dots and nanoplatelets, offering a streamlined method for sample analysis that could accelerate nanoplatelet research and applications.

In this article, we developed a theory describing surface exciton polaritons (SEPs) that accounts for the spatial dispersion of the dielectric constant connected with exciton momentum. Due to strong coupling between light and bulk excitons in the frequency separation, & hbar;omega LT, between the longitudinal and transverse excitons, the SEP is formed and behaves as partially light and partially matter. The dispersion of the SEP was found through a combined solution of Maxwell's and Thomas-Hopfield's equations. The analytical theory describes SEPs at any bulk exciton/vacuum interface and provides its complete dispersion if one knows & hbar;omega LT, the exciton effective mass, M, and the high-frequency dielectric constant, kappa infinity. The presented theory is in excellent agreement with the only numerical modeling of this problem, which was conducted for SEPs at a ZnO/vacuum interface. Calculations show the spatial dispersion of the dielectric constant leads to rather small broadening of the photon-like quasi-particle and suggest using SEPs for long-range coherence transfer. The theory was used to describe SEP dispersion in CsPbCl3 and CsPbBr3 perovskites.

The spin properties of charge carriers confined in CuCl semiconductor nanocrystals (NCs) of different sizes (radius from 1.8 nm up to 28 nm) crystallized in a glass matrix are studied experimentally and theoretically. By means of photoluminescence, spin-flip Raman scattering, time-resolved Faraday ellipticity, and time-resolved differential transmission performed at temperatures in the range 1.6 - 120 K at magnetic fields up to 8 T, comprehensive information on the Land & eacute; g factors as well as the population and spin dynamics is obtained. The spin signals are contributed by confined electrons with a g factor close to 2, which shows a weak increase with decreasing NC size, i.e., increasing electron confinement energy. We revisit the theory of exciton confinement as a whole in spherical NCs within the six-band valence band model in order to describe the size dependence of the Z3 and Z1,2 exciton energies in CuCl NCs. We demonstrate theoretically that the stronger increase of the Z1,2 energy transitions with decreasing radius can be explained by the strong absorption from the excited exciton state caused by strong heavy hole-light hole mixing in the exciton. The parameters of the six-band Hamiltonian describing both the exciton and hole kinetic energies are estimated from the comparison of the calculated and experimental size dependencies of the exciton transitions. A theoretical model of the size-dependent Land & eacute; g factors for electrons and holes confined in spherical NCs of semiconductors with negative spin-orbit splitting of the valence band is developed.

In this paper, we have developed a theory describing surface exciton polariton (SEPs) that accounts for the spatial dispersion of the dielectric constant connected with exciton momentum. Due to strong coupling between light and bulk excitons in the frequency separation, ħω_LT, between the longitudinal and transverse exciton, the SEP is formed and behaves at partially light and partially matter. The dispersion of the SEP was found through a combined solution of Maxwell's and Thomas-Hopfield's equations. The analytical theory describes SEPs at any bulk exciton/vacuum interface and provides its complete dispersion if one knows ħω_LT, the exciton effective mass, M, and the high frequency dielectric constant, κ_∞. The presented theory is in excellent agreement with the only numerical modeling of this problem, which was conducted for SEPs at a ZnO/vacuum interface. Calculations show the spatial dispersion of the dielectric constant leads to rather small broadening of the photon-like quasi-particle and suggests using SEPs for long-range coherence transfer.

While semiconductor nanocrystals provide versatile fluorescent materials for light-emitting devices, their brightness suffers from the "dark exciton"─an optically inactive electronic state into which nanocrystals relax before emitting. Recently, a theoretical mechanism, the Rashba effect, was discovered that can overcome this limitation by inverting the lowest-lying levels and creating a bright excitonic ground state. However, no methodology is available to systematically identify materials that exhibit this inversion, hindering the development of superbright nanocrystals and their devices. Here, based on a detailed understanding of the Rashba mechanism, we demonstrate a procedure that reveals previously unknown "bright-exciton" nanocrystals. We first define physical criteria to reduce over 500,000 known solids to 173 targets. Higher-level first-principles calculations then refine this list to 28 candidates. From these, we select five with high oscillator strength and develop effective-mass models to determine the nature of their lowest excitonic state. We confirm that four of the five solids yield bright ground-state excitons in nanocrystals. Thus, our results provide a badly needed roadmap for experimental investigation of bright-exciton nanomaterials.

We discuss the electronic and optical properties of various semiconductor nanostructures: nanocrystals, nanorods, nanowires and nanoplatelets. The modification of the shapes of these structures leads to very different energy spectra and different densities of carrier states. We show that the optical properties of all these structures are controlled by excitons. Optical properties are also strongly affected by the large difference in dielectric constants of the semiconductor and the matrix in which the nanostructures are embedded. This phenomenon leads to the dielectric confinement effect that increases the energies of the electron and hole confined levels and, due to the small dielectric constant of the matrix, increases the exciton binding energy by up to 0.5 eV in the thinnest nanoplatelets. In the end, we discuss the exciton fine structures created by electron-hole spin-spin exchange interaction enhanced by the spatial confinement of carriers in nanoscale semiconductors. This fine structure, consisting of the dark (optically passive) and bright (optical active) exciton sublevels, strongly affects photoluminescence and polarization properties in various nanostructures.

The brightness of an emitter is ultimately described by Fermi’s golden rule, with a radiative rate proportional to its oscillator strength times the local density of photonic states. As the oscillator strength is an intrinsic material property, the quest for ever brighter emission has relied on the local density of photonic states engineering, using dielectric or plasmonic resonators 1 , 2 . By contrast, a much less explored avenue is to boost the oscillator strength, and hence the emission rate, using a collective behaviour termed superradiance. Recently, it was proposed 3 that the latter can be realized using the giant oscillator-strength transitions of a weakly confined exciton in a quantum well when its coherent motion extends over many unit cells. Here we demonstrate single-photon superradiance in perovskite quantum dots with a sub-100 picosecond radiative decay time, almost as short as the reported exciton coherence time 4 . The characteristic dependence of radiative rates on the size, composition and temperature of the quantum dot suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations. The results aid in the development of ultrabright, coherent quantum light sources and attest that quantum effects, for example, single-photon emission, persist in nanoparticles ten times larger than the exciton Bohr radius.

Semiconductor nanoplatelets are atomically flat nanocrystals which emit light with high spectral purity at wavelengths controlled by their thickness. Despite their technological potential, efforts to further sharpen the emission lines of nanoplatelets have generally failed for unknown reasons. Here, we demonstrate theoretically that the linewidth is controlled by surface chemistry-specifically, inhomogeneities in the ligand layer on the nanoplatelet surface lead to a spatially fluctuating potential that localizes excitons. This localization leads to increased scattering and optical broadening. Importantly, localization also reduces the rate of radiative emission. Our model explains the observed linewidth and predicts that a more uniform ligand layer will sharpen the lines and increase the emission rates. These findings demonstrate that light emission from nanoplatelets can be controlled by optimizing their surface chemistry, an important advantage for their eventual use in optical technologies. Semiconductor nanoplatelets emit light in narrow spectral ranges. Here, the authors establish a theoretical model showing this behavior is controlled by the inhomogeneities of the ligand layer on the nanoplatelet surface.

Exciton-polaritons confined in plasmonic cavities are hybridized light-matter quasiparticles, with distinct optical characteristics compared to plasmons and excitons alone. Here, we demonstrate the electric tunability of a single polaritonic quantum dot operating at room temperature in electric-field tip-enhanced strong coupling spectroscopy. For a single quantum dot in the nanoplasmonic tip cavity with variable dc local electric field, we dynamically control the Rabi frequency with the corresponding polariton emission, crossing weak to strong coupling. We model the observed behaviors based on the quantum confined Stark effect in the strong coupling regime.

The absorption edge of CdSe and CdSSe microcrystals has been observed to undergo a significant short-wavelength shift (of more than 30 meV) during excitation by light pulses at the second harmonic from a Nd:YAG laser. An optical-nonlinearity mechanism is proposed. This mechanism involves the filling of levels in the energy spectrum of a spatially bounded multiexciton system in microcrystals with a size comparable to the exciton radius.

The spectra of the interband absorption of microscopic CdS crystals, 15 to 30 Å in size, grown in a transparent insulating matrix, are analyzed. A structural feature caused by quantization of the energy spectrum of the hole in the adiabatic potential of its Coulomb interaction with the electron has been detected near the transitions to the lower level of the size quantization of the electron.

We have shown theoretically that giant magnetic circular dichroism (MCD) should be observed at the absorption band edge of two-dimensional (2D) semiconductors exhibiting Rashba splitting in the presence of a magnetic field perpendicular to the 2D layers. This feature has been found in calculated interband sigma+ and sigma- light absorption coefficients for transitions between the Landau levels (LLs) of the conduction and valence bands. The giant MCD signal is shown to be connected with an unusual magnetic field dependence of the lowest LL in the presence of the Rashba term. A second unique signature of Rashba splitting is a reduction of the exciton diamagnetic shift created by the Rashba terms. We show that these terms lead to the characteristic Rashba exciton dispersion with minima at nonzero momentum. The analysis of the exciton dispersion at nonzero momentum conducted by Gor'kov and Dzaloshinskii shows that variable separation leads to mixed magneto-impulse terms. These terms lead to a negative diamagnetic shift, which should be observed in the exciton photoluminescence that occurs from the dispersion minimum.

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, Ga$_2$Te$_3$, and KIO$_3$) can yield bright ground-state excitons. Our approach thus reveals promising materials for future experimental investigation of bright-exciton nanocrystals.

This article appears in the following collection:Emmanuel Rashba: Breaking New Ground in Solid-State ExplorationPhysical Review B is pleased to present the “Collection in Honor of Emmanuel I. Rashba and His Fundamental Contributions to Solid-State Physics” in the year of his 95th birthday, highlighting the many ways in which his work has changed the landscape of modern condensed matter physics. Papers belonging to this collection will be published through mid-2023. The contributed articles, and an editorial by Guest Editors Mark Dykman, Alexander Efros, Bertrand Halperin, Leonid Levitov, and Charles Marcus, are linked below.

We investigate theoretically the band transport of electrons and holes in a "quantum-dot-in-perovskite" solid, a periodic array of semiconductor nanocrystal quantum dots embedded in a matrix of lead halide perovskite. For concreteness we focus on PbS quantum dots passivated by inorganic halogen ligands and embedded in a matrix of CsPbI3. We find that the halogen ligands play a decisive role in determining the band offset between the dot and matrix and may therefore provide a straightforward way to control transport experimentally. The model and analysis developed here may readily be generalized to analyze band transport in a broader class of dot-in-solid materials.

We revisit the criteria of photovoltaic (PV) efficiency and current collection in X-ray detectors (RDs) for semiconductors with suppressed carrier recombination. These materials render the promise of ultimate performance as thin-film PV cells (for efficiency) and RDs (for current collection). We show that the device performance can be understood neglecting defect contributions to the uniform properties of semiconductors and demonstrate this in the case of perovskites RDs. We also show that the efficiencies of the best PV cells are very close to the ultimate PV limits calculated in this Letter.

This review traces nanocrystal quantum dot (QD) research from the early discoveries to the present day and into the future. We describe the extensive body of theoretical and experimental knowledge that comprises the modern science of QDs. Indeed, the spatial confinement of electrons, holes, and excitons in nanocrystals, coupled with the ability of modern chemical synthesis to make complex designed structures, is today enabling multiple applications of QD size-tunable electronic and optical properties.

The Rashba effect has been proposed to give rise to a bright exciton ground state in halide perovskite nanocrystals (NCs), resulting in very fast radiative recombination at room temperature and extremely fast radiative recombination at low temperature. In this paper we find the dispersion of the "Rashba exciton", i.e., the exciton whose bulk dispersion reflects large spin-orbit Rashba terms in the conduction and valence bands and thus has minima at non-zero quasi-momenta. Placing Rashba excitonsin quasi-2D cylindrical quantum dots, we calculate size-dependent levels of confined excitons and their oscillator transition strengths. We consider the implications of this model for two-dimensional hybrid organic-inorganic perovskites, discuss generalizations of this model to 3D NCs, and establish criteria under which a bright ground exciton state could be realized.