Spectrally-Resolved Dielectric Functions of Solution-Cast Quantum Dot Thin Films

Quantum confinement is the divergence, at small crystallite size, of the electronic structure of semiconductor nanocrystals, or quantum dots, from the properties of larger crystals of the same materials. Although the extinction properties of quantum dots in the dispersed state have been extensively studied, many applications for quantum dots require the formation of a solid material which nonetheless retains a size-dependent electronic structure. The complex index of refraction (or complex dielectric function), including the extinction coefficient, is critical information for interpretation of optoelectronic measurements and use of quantum dot solids in optoelectronic devices. Here, spectroscopic ellipsometry is used to provide an all-optical method to determine the thickness, complex index, and extinction coefficient of thin films made of quantum-confined materials through the visible and near-infrared spectral ranges. The characteristic, size-dependent spectral features in the absorption of monodisperse quantum dots are readily translated into spectral variations of the index of refraction. The complex indices of refraction of CdSe and PbS quantum dot solids depend strongly on quantum dot size and the processing conditions of the thin film, including ligand exchange and annealing. The dielectric functions of quantum dot solids are dominated by the fill fraction of quantum dots, with only secondary influence from interparticle interaction.量子限域（quantum confinement）指当微晶尺寸较小时，半导体纳米晶，亦称量子点（quantum dots）的电子结构与同种材料的大尺寸晶体的电学性质产生偏离。尽管学界已对分散态量子点的消光特性开展了广泛研究，但诸多量子点应用场景需要制备仍保留尺寸依赖型电子结构的固态材料。复折射率（complex index of refraction，亦称复介电函数）包含消光系数（extinction coefficient），是解读光电子学测量结果、以及将量子点固态材料应用于光电子器件的关键信息。本研究采用光谱椭偏术（spectroscopic ellipsometry），建立了一种全光学方法，可在可见光及近红外光谱范围内测定量子限域材料制备的薄膜的厚度、复折射率与消光系数。单分散量子点的吸收光谱中具有特征性的尺寸依赖光谱特征，可直接转化为复折射率的光谱变化。硒化镉（CdSe）与硫化铅（PbS）量子点固态材料的复折射率，强烈依赖于量子点尺寸以及薄膜的制备工艺，包括配体交换与退火处理。量子点固态材料的介电函数主要由量子点的填充率决定，粒子间相互作用仅产生次要影响。