Two-dimensional CdSe nanoplatelets of magic thickness

目前，低维胶体纳米晶以量子点或量子棒的形式出现，或者作为更复杂、耦合的异质结构和超结构的一部分出现。自2008年以来，成功地合成了魔术厚度为d=n 0.302 nm(n=1，2，3等)的理想二维超薄CdSe纳米片。人们对它们的光学性质越来越感兴趣。原子平坦，n=3到7单层厚度的二维胶体CdSe纳米片的第一个光学数据显示，在紫外到绿色光谱范围内具有良好的光谱分辨电子态，其性质与球形或一维CdSe纳米晶的性质偏离了令人惊讶的强烈，并且与外延生长的(MBE)II-VI量子点和量子井也有很大的不同。已发表的观测涉及辐射跃迁几率的强度(Ithurria等人，自然材料2011，10,936)，与声子的低耦合强度以及介电限制对电子态的巨大影响(A.Achtstein等人)。Nano Letters 2012，12,351)。在本项目中，将研究和阐明理想的二维、魔术厚度的超薄CdSe纳米片的基本光学和电学性质。库仑相互作用和介电屏蔽对受限激子能态的影响应作为单分子膜、介电环境和魔术厚度的CdSe纳米片横向伸展的函数来系统地研究。对于立方体、超薄的CdSe纳米小片，交换分裂的存在和大小应通过单目标光谱和时间分辨发光来揭示。在这些强各向异性的超薄CdSe纳米片中，激子-声子相互作用的强度应作为晶体结构和形状效应的函数进行研究，并分析其耦合强度很低的原因。光学Starkeffekt的证据以及在理想2D结构中确定其外部电场的依赖关系是该项目的另一个目标。同样，了解双光子吸收和其他非线性过程的微观起源也是该项目的重要组成部分。基于后一项任务的结果，将研究利用电光调制器、传感器、显微镜标记和混合结构中的有源组件的微小非均匀光谱展宽从而产生尖锐共振的应用概念。线性和非线性光学的实验方法将结合起来，并由理论模型完成。

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.