Two-dimensional CdSe nanoplatelets of magic thickness

目前，低维胶体纳米晶体以量子点或量子棒的形式出现，或者作为更复杂的、耦合的异质和超结构的一部分。自2008年成功合成理想的二维超薄CdSe纳米片（神奇厚度d = n 0.302 nm （n= 1,2,3等））以来，人们对其光学性质的兴趣日益浓厚。原子平面、n=3 ~ 7层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，电子态的分辨率很高，其性质与球形或一维CdSe纳米晶体的性质有惊人的强烈偏差，同样与外延生长（MBE） II-VI量子点和量子阱的性质也有很大不同。已发表的观察结果涉及辐射跃迁概率的强度（Ithurria et al., Nature Materials 2011,10, 936），与声子的低耦合强度以及介电约束对电子态的大影响（A. Achtstein et al.）。纳米材料，2012,12,351)。本课题将研究和阐明理想的二维、具有神奇厚度的超薄CdSe纳米片的基本光学和电子性质。系统地研究库仑相互作用和介电屏蔽对限制激子能态的影响，作为单层、介电环境和神奇厚度CdSe纳米薄片横向延伸的函数。对于立方的超薄CdSe纳米片，交换分裂的存在和大小需要通过单目标光谱和时间分辨发光来揭示。在这些具有强各向异性的超薄CdSe纳米薄片中，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，并分析其极低耦合强度的原因。在理想的二维结构中，光学斯塔克费特效应的证据和其外电场依赖性的确定是该项目的进一步目标。同样，了解双光子吸收和其他非线性过程的微观起源是该项目的重要组成部分。基于这些后一项任务的结果，将研究应用概念，利用小的非均匀光谱拓宽，从而为电光调制器、传感器、显微镜中的标记和混合结构中的主动元件提供尖锐的共振。线性和非线性光学的实验方法在单CdSe纳米片和在集成将结合和理论模型完成。

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.