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The optoelectronic properties of type-II CdSe/CdTe colloidal nanoplatelets (NPLs) charged with neutral excitons (X0) have been intensively investigated in the last years. Motivated by the recent experimental progress, here we use effective mass simulations to study the effect of charging core/crown NPLs with a few extra electrons or holes. Emission spectra are calculated for charged excitons (Xn, with n = 2 to n = −3) and biexcitons (XX). The strong Coulomb interactions within the platelet lead to a number of remarkable properties. For excitons, varying the number of excess charges gives rise to band gap red- and blue-shifts spanning over 100 meV and widely tunable oscillator strength. For biexcitons, the binding energy can be tuned from nearly nonbonding to strongly antibonding (∼40 meV) by modulating the core/crown area ratio. We conclude that the number of excess carriers injected into type-II NPLs is a versatile degree of freedom to modulate the optoelectronic properties. Funding for open access charge: CRUE-Universitat Jaume I

International audience; Nanocrystals are now established light sources, and as synthesis and device integration have gained maturity, new functionalities can now be considered. So far, the emitted light from a nanocrystal population remains mostly driven by the structural properties (composition, size, shape) of the particle, and only limited post-synthesis tunability has been demonstrated. Here, we explore the design of light amplitude modulators using a nanocrystal-based light-emitting diode operated under reverse bias. We demonstrate strong photoluminescence modulations for devices operating in the visible and near telecom wavelengths using low bias operations (

The optoelectronic properties of metal chalcogenide colloidal nanoplatelets are often interpreted in terms of excitonic states. However, recent spectroscopic experiments evidence the presence of trion states, enabled by the slow Auger recombination in these structures. We analyze how the presence of an additional charge in trions modifies the emission energy and oscillator strength as compared to neutral excitons. These properties are very sensitive to dielectric confinement and electronic correlations, which we describe accurately using image-charge and variational Quantum Monte Carlo methods in effective mass Hamiltonians. We observe that the giant oscillator strength of neutral excitons is largely suppressedin trions. Both negative and positive trions are redshifted with respect to the exciton, and their emission energy increases with increasing dielectric mismatch between the platelet and its surroundings, which is a consequence of the self-energy potential. Our results are consistent with experiments in the literature, and assess on the validity of previous theoretical approximations., submitted

Biexciton properties in semiconductor nanostructures are highly sensitive to quantum confinement, relative electron–hole masses, dielectric environment and Coulomb correlations. Here we present a variational Quantum Monte Carlo model which, coupled to effective mass Hamiltonians, takes into account all of the above effects. The model is used to provide theoretical assessment on the biexciton ground state properties in colloidal CdSe nanoplatelets. A number of characteristic features is observed: (i) the finite thickness of these systems makes the biexciton geometry depart from the planar square expected in the two-dimensional (2D) limit, and form a distorted tetrahedron instead; (ii) the strong dielectric confinement enhances not only Coulomb attractions but also repulsions, which lowers the ratio of the biexciton-to-exciton binding energy down to EXXb/EXb = 0.07. (iii) EXXb is less sensitive than EXb to lateral confinement, and yet it can reach values above 30 meV, thus granting room temperature stability; (iv) the ratio of biexciton-to-exciton radiative rates, kradXX/kradX, decreases from 3.5 to ∼1 as the platelet area increases. These results pave the way for the rational design of biexciton properties in metal chalcogenide nanoplatelets.

Growing CdSe/CdS nanocrystals from a large CdSe core, and employing a giant CdS shell, a continuous, broadband gain spectrum, covering the spectral range between the CdSe and the CdS band edge, is induced. As revealed by k.p calculations, this feature is enabled by a set of closely spaced S-, P- and, for larger CdSe cores, D-state hole levels, which are thermally occupied at room temperature, combined with a sparse density of electron states. This leads to a range of bleach signals in the transient absorption spectra that persist up to a microsecond. By extending a state-filling model including relevant higher-energy states and a Fermi-Dirac distribution of holes at finite temperature, it is shown that thermal occupancy can lower the gain threshold for excited states. Inclusion of Gaussian broadening of discrete transitions also leads to a smoothening of the gain threshold spectrum. Next to a direct measurement of the gain threshold, a method is also developed to extract this from the gain lifetime, taking advantage that population inversion is limited by Auger recombination and recombination rates scale with the exciton density as < N >.(< N > - 1). The results should be readily extendable to other systems, such as perovskite or III-V colloidal nanocrystals.

Colloidal semiconductor nanoplatelets are excellent optical emitters, which combine a quasi-2D structure with strong in-plane Coulomb interactions. Here, we go beyond the photoexcitation regime and investigate theoretically the effect of charging nanoplatelets with a few interacting fermions (electrons or holes). This introduces severe Coulomb repulsions in the system, enhanced by the inherent dielectric confinement. We predict strong electronic correlations and electron-electron exchange energies (over 20 meV) in type-I (CdSe/CdS) and type-II (CdSe/CdTe) nanoplatelets, which give rise to characteristic physical phenomena. These include shell filling spectra deviating from the Aufbau principle, large addition energies which permit deterministic control of the number of charges at room temperature and paramagnetic electron spin configuration activated at cryogenic temperatures.

II–VI two-dimensional (2D) nanoplatelets (NPLs) exhibit the narrowest optical features among nanocrystals (NCs). This property remains true for Hg-based NPLs, despite a cation exchange procedure to obtain them from Cd-based NPLs, which leads to structural defects (poorly defined edges and voids) inducing inhomogeneous broadening. Here, we propose an optimized procedure for which a solvent, surface chemistry, and reaction conditions are rationally considered. The procedure is applied to the growth of alloyed HgSe1–xTex NPLs with various compositions. We report a bright photoluminescence for all compositions. Structural properties being now well defined, it is possible to study the electronic properties of these objects. To do so, we combine k·p modeling of quantum-confined structures with X-ray photoemission. In particular, we clarify the origin of the similarity between CdTe and HgTe NPLs absorption spectra despite their vastly differing bulk band structures. Finally, static- and time-resolved photoemission unveil a crossover from n- to p-type behavior in HgSe1–xTex NPLs while increasing the Te content. The project was supported by ERC starting grants Ne2DeM (grant no 853049) and blackQD (grant no 756225). The authors acknowledge the use of clean-room facilities at the “Centrale de Proximité Paris-Centre”. This work was supported by Region Ile-de-France in the framework of DIM Nano-K (grant dopQD). This work was also supported by French state funds managed by the ANR within the Investissements d’Avenir programme under reference ANR-11-IDEX-0004-02 and, more specifically, within the framework of the Cluster of Excellence MATISSE and by grants IPER-Nano2 (ANR-18CE30-0023-01), Copin (ANR-19-CE24-0022), Frontal (ANR-19-CE09-0017), Graskop (ANR-19-CE09-0026), NITQuantum (ANR-20-ASTR-0008-01), Bright (ANR-21-CE24-0012-02), and MixDferro (ANR-21-CE09-0029). A.C. thanks Agence Innovation Defense for Ph.D. funding. J.P. and J.I.C. acknowledge support from Prometeo Grant Q-Devices (Prometeo/2018/098).

The approaches to stabilize the perovskite structure of formamidinium lead iodide (FAPI) commonly result in a blue shift of the band gap, which limits the maximum photoconversion efficiency. Here, we report the use of PbS colloidal quantum dots (QDs) as a stabilizing agent, preserving the original low band gap of 1.5 eV. The surface chemistry of PbS plays a pivotal role by developing strong bonds with the black phase but weak ones with the yellow phase. As a result, a stable perovskite FAPI black phase can be formed at temperatures as low as 85 °C in just 10 min, setting a record of concomitantly fast and low-temperature formation for FAPI, with important consequences for industrialization. FAPI thin films obtained through this procedure reach an open-circuit potential (Voc) of 1.105 V, 91% of the maximum theoretical Voc, and preserve the efficiency for more than 700 h. These findings reveal the potential of strategies exploiting the chemi-structural properties of external additives to relax the tolerance factor and optimize the optoelectronic performance of perovskite materials.

PbS quantum dots and nanoplatelets (NPLs) are of enormous interest in the development of optoelectronic devices. However, some important aspects of their nature remain unclear. Recent studies have revealed that colloidal PbS NPLs may depart from the rock-salt crystal structure of bulk and form an orthorhombic (Pnma) modification instead. To gain insight into the implications of such a change over the optoelectronic properties, we have synthesized orthorhombic PbS NPLs and determined the lattice parameters by means of selected area electron diffraction measurements. We have then calculated the associated band structure using density functional theory with Perdew–Burke–Ernzerhof functional for solids and with the GW approximation, including spin–orbit interactions. An indirect band gap is found, which may explain the weak luminescence reported in experiments. We derive effective masses for conduction and valence bands and deduce that quantum confinement along the a crystallographic axis (short axis of the NPL) reinforces the indirect band gap but that along b and c axes favors a direct gap instead. Calculations for colloidal nanoplatelets of 1.8 nm thickness, carried out with k·p theory, show that excitonic effects are strong, with binding energies of about 150 meV.

The symmetry of the electronic ground state is of paramount importance in determining the magnetic, optical and electrical properties of semiconductor nanostructures. Here it is shown theoretically that non-trivial spin and orbital symmetries can be induced in colloidal nanoplatelets by applying out-of-plane magnetic fields. Two scenarios are presented. The first one deals with two electrons confined inside a platelet. Here, the strong electron-electron exchange interaction reduces the interlevel energy spacing set by lateral quantum confinement. As a result, relatively weak magnetic fields suffice to induce a singlet-to-triplet spin transition. The second one deals with type-II core/crown nanoplatelets. Here, the crown has doubly-connected topology, akin to that of quantum rings. As a result, the energy levels of carriers within it undergo Aharonov-Bohm oscillations. This implies changes in the ground state orbital symmetry, which switch the exciton and trion optical activity from bright to dark. Funding for open access charge: CRUE-Universitat Jaume I

[Image: see text] Current colloidal synthesis methods for CdSe nanoplatelets (NPLs) routinely yield samples that emit, in discrete steps, from 460 to 550 nm. A significant challenge lies with obtaining thicker NPLs, to further widen the emission range. This is at present typically achieved via colloidal atomic layer deposition onto CdSe cores, or by synthesizing NPL core/shell structures. Here, we demonstrate a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and increase the thickness in a two-step reaction that switches from 2D to 3D growth. The key feature is the enhancement of the growth rate of basal facets by the addition of CdCl(2), resulting in a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 and 8.5 ML. Optical characterization yielded emission peaks from 554 nm up to 625 nm with a line width (fwhm) of 9–13 nm, making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. The NPLs maintained a short emission lifetime of 5–11 ns. Finally, due to the increased red shift of the NPL band edge photoluminescence excitation spectra revealed several high-energy peaks. Calculation of the NPL band structure allowed us to identify these excited-state transitions, and spectral shifts are consistent with a significant mixing of light and split-off hole states. Clearly, chloride ions can add a new degree of freedom to the growth of 2D colloidal nanocrystals, yielding new insights into both the NPL synthesis as well as their optoelectronic properties.

International audience; Type-II heterostructures are key elementary components in optoelectronic, photovoltaic, and quantum devices. The staggered band alignment of materials leads to the stabilization of indirect excitons (IXs), i.e., correlated electron–hole pairs experiencing spatial separation with novel properties, boosting optical gain and promoting strategies for the design of information storage, charge separation, or qubit manipulation devices. Planar colloidal CdSe/CdTe core–crown type-II nested structures, grown as nanoplatelets (NPLs), are the focus of the present work. By combining low temperature single NPL measurements and electronic structure calculations, we gain insights into the mechanisms impacting the emission properties. We are able to probe the sensitivity of the elementary excitations (IXs, trions) with respect to the appropriate structural parameter (core size). Neutral IXs, with binding energies reaching 50 meV, are shown to dominate the highly structured single NPL emission. The large broadening linewidth that persists at the single NPL level clearly results from strong exciton–LO phonon coupling (Eph = 21 meV) whose strength is poorly influenced by trapped charges. The spectral jumps (≈10 meV) in the photoluminescence recorded as a function of time are explained by the fluctuations in the IX electrostatic environment considering fractional variations (≈0.2 e) of the noncompensated charge defects.

We show theoretically that carriers confined in semiconductor colloidal nanoplatelets (NPLs) sense the presence of neighbor, cofacially stacked NPLs in their energy spectrum. When approaching identical NPLs, the otherwise degenerate energy levels red-shift and split, forming (for large stacks) minibands that are several millielectronvolts in width. Unlike in epitaxial structures, the molecular behavior does not result from quantum tunneling but from changes in the dielectric confinement. The associated excitonic absorption spectrum shows a rich structure of bright and dark states, whose optical activity and multiplicity can be understood from reflection symmetry and Coulomb tunneling. We predict spectroscopic signatures that should confirm the formation of molecular states, whose practical realization would pave the way for the development of nanocrystal chemistry based on NPLs.

Recent experiments suggest that the photoluminescence line width of CdSe and CdSe/CdS nanoplatelets (NPLs) may be broadened by the presence of shakeup (SU) lines from negatively charged trions. We carry out a theoretical analysis, based on effective mass and configuration interaction (CI) simulations, to identify the physical conditions that enable such processes. We confirm that trions in colloidal NPLs are susceptible of presenting SU lines up to one order of magnitude stronger than in epitaxial quantum wells, stimulated by dielectric confinement. For these processes to take place trions must be weakly bound to off-centered impurities, which relax symmetry selection rules. Charges on the lateral sidewalls are particularly efficient to this end. We propose that the broad line width reported for core/shell CdSe/CdS NPLs may relate not only to SU processes but also to a metastable spin triplet trion state. Understanding the origin of SU processes opens paths to rational design of NPLs with narrower line width., submitted

Colloidal two-dimensional (2D) nanoplatelet heterostructures are particularly interesting as they combine strong confinement of excitons in 2D materials with a wide range of possible semiconductor junctions due to a template-free, solution-based growth. Here, we present the synthesis of a ternary 2D architecture consisting of a core of CdSe, laterally encapsulated by a type-I barrier of CdS, and finally a type-II outer layer of CdTe as so-called crown. The CdS acts as a tunneling barrier between CdSe- and CdTe-localized hole states, and through strain at the CdS/CdTe interface, it can induce a shallow electron barrier for CdTe-localized electrons as well. Consequently, next to an extended fluorescence lifetime, the barrier also yields emission from CdSe and CdTe direct transitions. The core/barrier/crown configuration further enables two-photon fluorescence upconversion and, due to a high nonlinear absorption cross section, even allows to upconvert three near-infrared photons into a single green photon. These results demonstrate the capability of 2D heterostructured nanoplatelets to combine weak and strong confinement regimes to engineer their optoelectronic properties.

The extraordinary low non-radiative recombination and band gap versatility of halide perovskites have led to considerable development in optoelectronic devices. However, this versatility is limited by the stability of the perovskite phase, related to the relative size of the different cations and anions. The most emblematic case is that of formamidinium lead iodine (FAPI) black phase, which has the lowest band gap among all 3D lead halide perovskites, but quickly transforms into the non-perovskite yellow phase at room temperature. Efforts to optimize perovskite solar cells have largely focused on the stabilization of FAPI based perovskite structures, often introducing alternative anions and cations. However, these approaches commonly result in a blue-shift of the band gap, which limits the maximum photo-conversion efficiency. Here, we report the use of PbS colloidal quantum dots (QDs) as stabilizing agent for the FAPI perovskite black phase. The surface chemistry of PbS plays a pivotal role, by developing strong bonds with the black phase but weak ones with the yellow phase. As a result, stable FAPI black phase can be formed at temperatures as low as 85°C in just 10 minutes, setting a record of concomitantly fast and low temperature formation for FAPI, with important consequences for industrialization. FAPI thin films obtained through this procedure preserve the original low band gap of 1.5 eV, reach a record open circuit potential (Voc) of 1.105 V -91% of the maximum theoretical Voc- and preserve high efficiency for more than 700 hours. These findings reveal the potential of strategies exploiting the chemi-structural properties of external additives to relax the tolerance factor and optimize the optoelectronic performance of perovskite materials.

We investigate the role of stress arising between core and shell materials in colloidal CdSe/X hetero-nanoplatelets (X = ZnS, CdS, CdTe). The resulting strain distribution is calculated within the linear elastic regime and also its influence on the electronic structure with k·p theory. We show that strain shifts the energy of electrons and that of holes by several tens of megaelectronvolts (meV). In structures with type I band alignment, the two shifts have opposite signs and the net effect on the exciton emission energy is small, but both these add up in type II systems. The strain response in colloidal nanoplatelets (NPLs) is found to exhibit some differences as compared to that of epitaxial quantum wells, including a sizable influence of lateral dimensions below 10 nm and a potentially relevant effect of coupled strain–momentum terms of the Hamiltonian. We further show that an asymmetric shell covering leads to bending of the nanoplatelet and tilted potential profiles along the strong confinement direction, analogous to a built-in electric field. We propose overcoating CdSe/CdS NPLs with an outer ZnS shell as a method to mitigate the tunneling-induced red shift of emission via strain engineering.

Seeded CdSe/CdS nanorods exhibit intense polarized emission along the rod main axis. The degree of linear polarization cannot be explained by dielectric effects alone, an additional electronic contribution is present whose nature has not been settled up to date. Using multi-band k.p theory, we analyse the potential influence of several factors affecting excitonic emission and show that shear strain is the main electronic mechanism promoting linear polarization. It favors energetically light hole excitons over heavy hole ones, via deformation potential, and makes their radiative recombination faster via piezoelectricity. Implications of this mechanism are that linear emission can be enhanced by growing long but thin CdS shells around large, prolate CdSe cores, which indeed supports and rationalizes recent experimental findings. Together with the well-known dielectric effects, these results pave the way for controlled degree of linear polarization in dot-in-rods through dedicated structural design., Includes supporting information

We demonstrate by the means of two dimensional two photon k-space spectroscopy that two photon absorption in CdSe nanoplatelets is highly directional. 85% of the concatenated absorption dipoles are in-plane oriented. We show that this strong anisotropy is related to competing two photon pathways with one inter- and one intra-band transition or one inter- and one inter-valence-band transition, as based on the crystal symmetry inter-, intra- and inter-valence- band transitions exhibit different transition dipole orientations. Based on the identified relevant transitions we explain the observed, up to now puzzling, quadratic volume scaling of two photon absorption cross sections in CdSe nanoplatelets, leading to record TPA cross sections. It is further shown, how excitonic correlation and coherence in these colloidal quantum wells lead to size tunable, directional, unprecedented TPA cross sections, opening up new perspectives for their usage e.g. in confocal two photon microscopy, orientation sensing or imaging.

Anisotropic single-phase wurtzite CdSe/CdS nanocrystals were synthesized by colloidal chemistry, introducing ZnCl2 to increase the shell growth in the radial direction. As a result, dot-in-giant-rod nanocrystals were obtained, with a core diameter that varied between 3.2 and 7.5 nm and an overall diameter between 15 and 22 nm, corresponding to a 14–26 ML CdS shell. In addition to an extended fluorescence lifetime, typical for CdSe/CdS heteronanocrystals, all samples also yielded a blue-shifted biexciton emission peak. This contrasts with existing data on CdSe/CdS dot-in-rod nanocrystals with a thin shell, which yield a type-I band offset and attractive biexciton interactions for CdSe/CdS with a core larger than about 2.8 nm. However, k·p calculations support the blue shift, with a significant electron delocalization into the CdS shell even for large core diameter. We assign this effect to the influence of strain at the CdSe/CdS interface and associated reduction of the conduction band offset, as well as the buildup of a piezoelectric field along the nanorod long axis. The strain-induced electron–hole separation is particularly effective in large-core nanocrystals, providing a tool to engineer electron and hole wave functions that is complementary to quantum confinement.

We show that electron-hole correlation can be used to tune interband and intraband optical transition rates in semiconductor nanostructures with at least one weakly confined direction. The valence-to-conduction band transition rate can be enhanced by a factor $(L/a_B)^{N}$ -- with $L$ the length of the weakly confined direction, $a_B$ the exciton Bohr radius and $N$ the dimensionality of the nanostructure -- while the rate of intraband and inter-valence-band transitions can be slowed down by the inverse factor, $(a_B/L)^{N}$. Adding a hitherto underexplored degree of freedom to engineer excitonic transition rates, this size dependence is of interest for various opto-electronic applications. It also offers an interpretation of the superlinear volume scaling of two-photon absorption (TPA) cross-section recently reported for CdSe nanoplatelets, thus laying foundations to obtain unprecedented TPA cross sections, well above those of conventional two-photon absorbers. Further, our concept explains the background of the validity of the universal continuum absorption approach for the determination of particle concentrations via the intrinsic absorption. Potential applications of our approach include low excitation intensity confocal two-photon imaging, two-photon autocorrelation and cross correlation with much higher sensitivity and unprecedented temporal resolution as well as TPA based optical stabilization and optimizing of inter-subband transition rates in quantum cascade lasers., Comment: submitted

The physics of semiconductor quantum rings near the band edge is often well described considering decoupled bands. There are however instances where band coupling leads to relevant changes in the electronic structure and derived properties. In this chapter we analyze two such cases. First, we focus on the heavy hole-light hole band mixing in self-assembled InAs/GaAs quantum rings, which is important for current endeavour to develop quantum information science using the spin of holes. In InAs/GaAs quantum dots, the hole ground state is known to be mainly formed by the heavy hole subband. However, there is a finite spin-orbit coupling with the light-hole subband which is critical in determining the hole spin properties. Based on k\(\cdot \)p theory, in this chapter we study the influence of hole subband mixing in quantum rings. It is shown that the inner cavity of the ring enhances the light hole component of the ground state. As the quasi-1D limit is approached, the light-hole character becomes comparable to that of the heavy hole. Strain reduces the coupling, but it is still larger than in quantum dots. Second, we study the electronic structure of monolayer MoS\(_2\) quantum dots subject to a magnetic field. Here, the coupling between conduction and valence band gives rise to mid-gap topological states which localize near the dot edge. These edge states are analogous to those of 1D quantum rings. We show they present a large, Zeeman-like, linear splitting with the magnetic field, anticross with the delocalized Fock-Darwin-like states of the dot, give rise to Aharonov-Bohm-like oscillations of the conduction (valence) band low-lying states in the K (K\(^{\prime }\)) valley, and modify the strong-field Landau levels limit form of the energy spectrum.

We show that two-photon absorption (TPA) is highly anisotropic in CdSe nanoplatelets, thus promoting them as a new class of directional two-photon absorbers with large cross sections. Comparing two-dimensional k-space spectroscopic measurements of the one-photon and two-photon excitation of an oriented monolayer of platelets, it is revealed that TPA into the continuum is a directional phenomenon. This is in contrast to one-photon absorption. The observed directional TPA is shown to be related to fundamental band anisotropies of zincblende CdSe and the ultrastrong anisotropic confinement. We recover the internal transition dipole distribution and find that this directionality arises from the intrinsic directionality of the underlying Bloch and envelope functions of the states involved. We note that the photoemission from the CdSe platelets is highly anisotropic following either one- or two-photon excitation. Given the directionality and high TPA cross-section of these platelets, they may, for example, find employment as efficient logic AND elements in integrated photonic devices, or directional photon converters.

Using semi-analytical models we calculate the energy, effective Bohr radius and radiative lifetime of neutral excitons confined in CdSe colloidal nanoplatelets (NPLs). The excitonic properties are largely governed by the electron-hole in-plane correlation, which in NPLs is enhanced by the quasi-two-dimensional motion and the dielectric mismatch with the organic environment. In NPLs with lateral size $L \gtrsim 20$ nm the exciton behavior is essentially that in a quantum well, with superradiance leading to exciton lifetimes of 1 ps or less, only limited by the NPL area. However, for $L < 20$ nm excitons enter an intermediate confinement regime, hence departing from the quantum well behavior. In heterostructured NPLs, different response is observed for core/shell and core/crown configurations. In the former, the strong vertical confinement limits separation of electrons and holes even for type-II band alignment. The exciton behavior is then similar to that in core-only NPL, albeit with weakened dielectric effects. In the latter, charge separation is also inefficient if band alignment is quasi-type-II (e.g. in CdSe/CdS), because electron-hole interaction drives both carriers into the core. However, it becomes very efficient for type-II alignment, for which we predict exciton lifetimes reaching $��s$., typographical errors fixed (with respect to v1 and PRB) in eqs. 9,12 and definition of overlap

We address theoretically differences and similarities on the electronic structure of CdSe/CdS dot-in-dot nanocrystals (NCs) with wurtzite/wurtzite (WZ/WZ), zinc-blende/zinc-blende (ZB/ZB) and polytype ZB/WZ crystalline phases, as they are currently being synthesized and used in optoelectronic devices. We show that the electronic structure of polytypic CdSe/CdS NCs closely resembles that of WZ or ZB NCs with regard to quantum confinement and strain, resulting in similar single-exciton wave functions. The main differences arise in the nature and magnitude of built-in electric fields. We predict that these fields are stronger in polytypes than in pure WZ or ZB NCs due to the sharp spontaneous polarization mismatch between the cubic core and the hexagonal shell lattices. Polarization in NCs is currently believed to be screened by several surface effects. In polytypical structures, however, the polarization mismatch at the interface may create effective charges that are sufficiently far from the outer surface to be quenched. To make a definitive assessment on this controversial issue, we propose experiments in polytypic ZB/WZ NCs where both core and shell are made of CdSe. In such a case, band offsets are small, strain is absent, and our calculations predict pyroelectricity should become the driving force, inducing transitions from type-I to type-II excitons with increasing core or shell size. We thank I. Moreels and P. Guyot-Sionnest for useful discussions. Support from MINECO project CTQ2014-60178-P, UJI project P1-1B2014-24 is acknowledged.

We investigate the spectral dependence of the linear and two-photon absorption of wurtzite CdS nanoparticles (dots and rods) by means of quantitative one- and two-photon photoluminescence excitation spectroscopy and effective mass theory modeling. Absolute two-photon absorption cross sections free from spectrally varying beam related uncertainties are obtained by means of a new reference dye-based method. The two-photon spectrum features of rods strongly differ from those of dots, due to the distinct energy structure of quasi-one-dimensional systems. The transversal confinement is found to dominate the energy of the absorption maxima while the longitudinal one dominates their absorption intensity. This suggests two-photon transition energy and intensity can be controlled independently in nanorods. For both geometries we observe a sizable spectral shift between the first one- and two-photon absorption maxima, which we conclude is inherent to the small rates of near-bandgap two-photon transitions rather than to the particular geometry of the absorber. The provided understanding of the spectral dependence of the two-photon absorption of CdS dots and rods is of strong interest for the design of nanocrystals with optimized two-photon absorption properties for bioimaging and phototherapy applications. .A. and U.W. acknowledge funding by DFG SPP 1165 Nanowires. A.B., J.L.M., J.I.C., and J.P. acknowledge support from MICINN project CTQ-2011-27324 and UJI-BANCAIXA project P1-1B2011-01. A.P. and A.A. acknowledge partial support from ELECTRONICS 2.2.19.3 project. M.A. acknowl- edges partial support from CHEMREAGENTS 1.33 project.

The spectral dependence of the two-photon absorption in CdSe/CdS core/shell nanocrystal heterorods has been studied via two-photon-induced luminescence excitation spectroscopy. We verified that the two-photon absorption in these samples is a purely nonlinear phenomenon, excluding the contribution from multistep linear absorption mediated by defect states. A large absorption cross section was observed for CdSe/CdS core/shell quantum rods, in the range of 10(5) GM (1 GM = 10(-50) cm(4) s phot(-1)), scaling with the total nanocrystal volume and thus independent of the core emission wavelength. In the two-photon luminescence excitation spectra, peaks are strongly blue-shifted with respect to the one-photon absorption peaks, for both core and shell transitions. The experimental results are confirmed by k·p calculations, which attribute the shift to both different parity selection rules that apply to one-photon and two-photon transitions and a low oscillator strength for two-photon transitions close to the ground-state one-photon absorption. In contrast with lead chalcogenide quantum dots, we found no evidence of a breakdown of the optical selection rules, despite the presence of band anisotropy, via the anisotropic hole masses, and the explicitly induced reduction of the electron wave function symmetry via the rod shape of the shell. The anisotropy does lead to an unexpected splitting of the electron P-states in the case of a large CdSe core encapsulated in a thin CdS shell. Hence, tuning of the core and shell dimensions and the concurrent transition from type I to quasi-type II carrier localization enables unprecedented control over the band-edge two-photon absorption.

Intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy-efficient display technology. Here, we propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic, directed photoluminescence. Analysis of the two-dimensional k-space distribution reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters.

We investigate the electron spin relaxation between Zeeman sublevels of fully three-dimensional quantum dots. By going beyond the usual two-dimensional description of Rashba and Dresselhaus spin–orbit interactions (SOI), we provide a general overview of the effect of the quantum dot shape. It is shown that, in spherical quantum dots, the Dresselhaus SOI is severely suppressed, leading to slow relaxation rates and a strong (B9) dependence on the magnetic field.

We show that the repulsive core of semiconductor quantum rings enhances the heavy hole-light hole mixing. Calculations for InAs/GaAs quantum rings show that the light hole character of the valence band ground state increases at least by a factor of two as compared to that of quantum dots. The light-hole character becomes comparable to the heavy hole one in quasi-1 D rings. Biaxial strain reduces the light hole influence, but it is still larger than in quantum dots. The effect of strain on the magnetic field response and wave function localization is studied.

We calculate the recombination energies and probabilities of neutral excitons (X), singly charged excitons (X±), and biexcitons (XX) in CdSe quantum dots of variable length. For spherical dots the relative position of the emission lines is determined by the confinement. As the dot is elongated, however, Coulomb correlation overcomes single-particle effects and the emission line of X becomes more energetic than that of any other excitonic complex. Likewise, the recombination probability (τ−1) of spherical dots is characteristic of the strong confinement regime: τ−1(XX) ∼ 2τ−1(X±) ∼ 4τ−1(X). However, these ratios are reduced with increasing length, as correlations enhance the emission of each excitonic complex at a different rate. Our results are compared with available experimental data.

Using multiband k·p calculations, we show that strain-engineered piezoelectricity is a powerful tool to modulate the electron−hole spatial separation in a wide class of wurtzite CdSe/CdS nanocrystals. The inherent anisotropy of the hexagonal crystal structure leads to anisotropic strain and, consequently, to a pronounced piezoelectric field along the c axis, which can be amplified or quenched through a proper design of the core−shell structure. The use of large cores and thick shells promotes a gradual departure from quantum confined nanocrystals to a regime dominated by piezoelectric confinement. This allows excitons to evolve from the usual type-I and quasi-type-II behavior to a type-II behavior in dot-in-dots, dot-in-rods, rod-in-rods, and dot-in-plates. Piezoelectric fields explain experimental observations for giant-shell nanocrystals, whose time-resolved photoluminescence reveals long exciton lifetimes for large cores, contrary to the expectations of standard quantum confinement models. They also explain the large differences in exciton lifetimes reported for different classes of CdSe/CdS nanocrystals. Support from MINECO project CTQ2014-60178-P, UJI project P1-1B2014-24 and a FPU grant (C.S.) is acknowledged. The present publication is further realized with the support of the Ministero degli Affari Esteri e della Cooperazione Internazionale (IONX-NC4SOL, I.M.).

Single and multi-band k⋅p Hamiltonians for GaAs crystal phase quantum dots are used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization,valence band mixing, and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorius et al., Nano Lett. 15, 2652 (2015)], it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al., Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture, and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes. We are grateful to P. Caroff, M. E. Pistol, and B. Loitsch for useful discussions. Support from UJI Project No. P1-1B2014-24, MINECO Project No. CTQ2014-60178-P, and a FPU grant (C.S.) is acknowledged.

We investigate theoretically the spin purity of single holes confined in vertically coupled GaAs/AlGaAs quantum dots (QDs) under longitudinal magnetic fields. A unique behavior is observed for triangular QDs, by which the spin is largely pure when the hole is in one of the dots, but it becomes strongly mixed when an electric field is used to drive it into molecular resonance. The spin admixture is due to the valence band spin-orbit interaction, which is greatly enhanced in C3h symmetry environments. The strong yet reversible electrical control of hole spin suggests that molecules with C3-symmetry QDs, like those obtained with [111] growth, can outperform the usual C2-symmetry QDs obtained with [001] growth for the development of scalable qubit architectures., Comment: 5-pages manuscript + supplementary information. Version to be published in PRB Rapid Communications

We study the electron momentum relaxation in single and vertically coupled quantum dots due to coupling with acoustic phonons. We consider both the single-electron and the few-electron regimes, using a Full Configuration Interaction approach to account for the electron-electron repulsion. Our findings suggest that electronic correlations tend to reduce intradot and interdot transition rates. We also show that external magnetic fields acting on properly designed quantum dots may strongly suppress electron-phonon coupling. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)