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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

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.

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

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

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.