We study the electronic structure of ultrathin zinc-blende two-dimensional (2D)-CdSe nanosheets both theoretically, by Hartree-renormalized k·p calculations including Coulomb interaction, and experimentally, by temperature-dependent and time-resolved photoluminescence measurements. The observed 2D-heavy hole exciton states show a strong influence of vertical confinement and dielectric screening. A very weak coupling to phonons results in a low phonon-contribution to the homogeneous line-broadening. The 2D-nanosheets exhibit much narrower ensemble absorption and emission linewidths as compared to the best colloidal CdSe nanocrystallites ensembles. Since those nanoplatelets can be easily stacked and tend to roll up as they are large, we see a way to form new types of multiple quantum wells and II-VI nanotubes, for example, for fluorescence markers.

A systematic variation of the exciton fine-structure splitting with quantum dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from -80 to as much as 520 µeV with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band k·p theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.The exchange interaction of electron-hole pairs (excitons) in semiconductor quantum dots (QDs) has been subject of a lively debate in recent years [1,2,3,4,5,6,7,8]. In such strongly confined systems it is supposed to be enhanced with respect to the bulk case due to the close proximity of electrons and holes. However, the influence of the exact geometry of the confining potential on the exchange interaction still needs to be clarified. A detailed understanding of the resulting exciton fine structure in quantum dots is of fundamental interest and of largest importance for potential applications of QDs in single-photon emitters and entangled two-photon sources for quantum cryptography [9].The total angular momentum M of heavy-hole excitons (X) in QDs is composed of the electron spin (s = ± 1 2 ) and the heavy hole angular momentum (j = ± 3 2 ), consequently producing four degenerate exciton states frequently denoted as dark (M = ±2) and bright (M = ±1) states indicating whether they couple to the photon field or not. Independent of the given confinement symmetry electron-hole exchange interaction causes a dark-bright splitting. Furthermore it mixes the dark states lifting their degeneracy and forming a dark doublet (|2 ±|−2 ). Likewise, additional lowering of the confinement symmetry to C 2v or lower mixes the bright states producing a nondegenerate bright doublet (|1 ± | − 1 ).While emission lines involving pure states are circularly polarized, the mixed states usually produce lines showing linear polarization along the [110] and [110] crystal directions, respectively (Fig. 1). The two bright states are thus directly observable as linearly polarized transitions in luminescence experiments. The energetic difference between these lines is called exciton fine-structure splitting (FSS).The biexciton (XX) ground state is not split by the exchange interaction, since the net spin of the involved electrons and holes is 0. However, the XX to X decay involves two allowed transitions with the final states being the bright states of the X. Therefore, the FSS is reproduced (yet inverted) in the XX to X decay (Fig. 1).Recently reported experimental values of the FSS in * Email: seguin@sol.physik.tu-berlin.de

We investigate the interface between core and shell in zinc blende CdSebased CdSe/CdS dot-in-dot heteronanocrystals. Using X-ray diffraction and transmission electron microscopy, we show that a CdS shell grows coherently around the CdSe core. A comparison of the Raman spectrum of bare CdSe nanocrystals and CdSe/CdS heteronanocrystals indicates that the difference in lattice constant leads to compressive and tensile strain in core and shell, respectively. Concomitant continuum mechanical calculations follow this result, yet the calculated strain exceeds the experimental values. Moreover, a detailed analysis of the CdSe/CdS Raman spectra reveals the appearance of additional features upon shell growth. A comparison with pure Cd(Se,S) alloyed nanocrystals relates these features to alloy vibrations. We show that these observations point toward the presence of a mixed Cd(Se,S) layer at the CdSe/CdS interface. In this way, this work provides an experimental framework based on Raman spectroscopy to analyze in detail interfacial alloying in heteronanocrystals.

The strain fields in and around self-organized In(Ga)As/GaAs quantum dots (QD) sensitively depend on QD geometry, average InGaAs composition and the In/Ga distribution profile. Piezoelectric fields of varying size are one result of these strain fields. We study systematically a large variety of realistic QD geometries and composition profiles, and calculate the linear and quadratic parts of the piezoelectric field. The balance of the two orders depends strongly on the QD shape and composition. For pyramidal InAs QDs with sharp interfaces a strong dominance of the second order fields is found. Upon annealing the first order terms become dominant, resulting in a reordering of the electron p-and d-states and a reorientation of the hole wavefunctions.

We evidence excited state emission from p states well below ground state saturation in CdSe nanoplatelets. Size-dependent exciton ground and excited state energies and population dynamics are determined by four independent methods: time-resolved PL, time-integrated PL, rate equation modeling, and Hartree renormalized k·p calculations-all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Depending on its detuning to the LO phonon energy, the PL decay of CdSe platelets is governed by a size tunable LO phonon bottleneck, related to the low exciton-phonon coupling, very large oscillator strength, and energy spacing of both states. This is, for instance, ideal to tune lasing properties. CdSe platelets are perfectly suited to control the exciton-phonon interaction by changing their lateral size while the optical transition energy is determined by their thickness.