Two-dimensional CdSe nanoplatelets of magic thickness

• Applicant: Professorin Dr. Ulrike K. Woggon
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2013 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 235473865

Currently, low-dimensional colloidal nanocrystals appear in the form of quantum dots or quantum rods or as part of more complex, coupled hetero- and superstructures. Since 2008, the year of publication of successful synthesis of ideally two-dimensional ultrathin CdSe nanoplatelets of magic thickness of d = n 0.302 nm (n=1, 2, 3 etc.) a growing interest exists concerning their optical properties. The first optical data of atomically flat, n=3 to 7 monolayer thick, two-dimensional colloidal CdSe nanoplatelets showed spectrally well resolved electronic states in the UV to green spectral range with properties, which deviate surprisingly strong from those of spherical or one-dimensional CdSe nanocrystals and are likewise very much different from those of epitaxially grown (MBE) II-VI quantum dots and wells. The published observations concern the strength of the radiative transition probability (Ithurria et al., Nature Materials 2011, 10, 936), the low coupling strength to phonons and the large impact of the dielectric confinement on the electronic states (A. Achtstein et al. Nano Letters 2012, 12, 351). In the presented project fundamental optical and electronic properties of ideal two-dimensional, ultrathin CdSe nanoplatelets of magic-thickness, shall be studied and elucidated. The influence of Coulomb interaction and dielectric screening on the confined excitonic energy states shall be systematically studied as a function of monolayers, dielectric environment and lateral extension of magic-thickness CdSe nanoplatelets. For cubic, ultrathin CdSe nanoplatelets the existence and size of exchange splitting shall be uncovered by single-object spectroscopy and time-resolved luminescence. In these, strongly anisotropic ultrathin CdSe nanoplatelets, the strength of exciton-phonon interaction shall be investigated as a function of crystal structure and shape effects and the origin of its very low coupling strength analysed. The evidence of the optical Starkeffekt and the determination of its external electrical field dependence in ideal 2D-structures is a further goal of the project. Likewise the understanding of the microscopic origins of two-photon absorption and other nonlinear processes is an essential part of the project. Based on the results of these latter tasks, application concepts will be examined which exploit the small inhomogeneous spectral broadening and thus sharp resonances for electrooptic modulators, sensors, markers in microscopy and active components in hybrid structures. Experimental methods of linear and nonlinear optics at single CdSe nanoplatelets and at ensembles will be combined and completed by theoretical models.

• DFG Programme: Research Grants