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.

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 optical absorption properties of colloidal CdSe nanoplatelets and compare them to CdSe quantum dots. Starting from inductively coupled plasma−atomic emission spectroscopy (ICP-AES) measurements on their intrinsic absorption coefficients μ i , we compare these results with a theoretical approach by a continuum absorption Lorentz local field model. We show that the platelets' intrinsic absorption coefficients μ i are strongly thickness and aspect ratio dependent, which results in the possibility to tune the absorption properties of this material class by the lateral size and thickness. The continuum intrinsic absorbance of the platelets is considerably larger if compared with quantum dots making them more efficient absorbers with higher light−matter interaction that is essential for their use in, for example, solar cells. The obtained μ i values can be used for concentration determination of CdSe nanoplatelets in solution and solid films which is essential for all optical experiments with controlled generated population density upon optical excitation. ■ INTRODUCTIONIn recent years, the influence of size and shape on the linear absorption properties of semiconductor nanoparticles has gained growing interest with respect to nanoparticles in dispersion or polymeric matrices. Recent investigations 1−6 have shown that the absorption properties of II−VI semiconductor nanoparticles can be strongly altered not only by the electronic and dielectric carrier confinement but also by the dielectric environment of these particles. This is because the light−matter interaction (e.g., absorption or emission of radiation) is connected to local fields inside the nanoparticles and the effective medium dielectric constant. 7 Different kinds of CdSe, CdS, PbS, and PbSe 1−6,8,9 particles in the shape of dots, rods, and wires were investigated. They exhibited a strong shape, aspect ratio, and dielectric contrast (semiconductor vs surrounding medium) dependence of their optical properties such as a shape dependent intrinsic absorption coefficient, polarized emission and absorption, and a varying oscillator strength and exciton lifetime. 2,7 In recent years, a new class of colloidal nanoparticles has been synthesized, the colloidal II−VI nanoplatelets (NPLs). 10−13 These ultrathin and flat nanocrystals (made of CdSe, CdS, or CdTe) exhibit lateral dimensions of the order of tens of nanometers and thicknesses of several monolayers. 10−12,14−19 Strong anisotropic quantum confinement in the NPLs results in spectrally narrow absorption and photoluminescence peaks even at room temperature. Because lateral dimensions of NPLs are much larger than their thicknesses, NPLs can be considered as the colloidal analogue of ultrathin semiconductor quantum wells (QWs) prepared by molecular beam epitaxy (MBE). 20,21 For many, for example, optical investigations, linear absorption cross sections or intrinsic absorption coefficients are necessary to determine the absolute particle concentrations of nanoparticles in colloidal solutions. Ther...

We report a comprehensive study on the two-photon absorption cross sections of colloidal CdSe nanoplatelets, -rods, and -dots of different sizes by the means of z-scan and two-photon excitation spectroscopy. Platelets combine large particle volumes with ultra strong confinement. In contrast to weakly confined nanocrystals, the TPA cross sections of CdSe nanoplatelets scale superlinearly with volume (V(∼2)) and show ten times more efficient two-photon absorption than nanorods or dots. This unexpectedly strong shape dependence goes well beyond the effect of local fields. The larger the particles' aspect ratio, the greater is the confinement related electronic contribution to the increased two-photon absorption. Both electronic confinement and local field effects favor the platelets and make them unique two-photon absorbers with outstanding cross sections of up to 10(7) GM, the largest ever reported for (colloidal) semiconductor nanocrystals and ideally suited for two-photon imaging and nonlinear optoelectronics. The obtained results are confirmed by two independent techniques as well as a new self-referencing method.

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.