Passivating ligand and solvent contributions to the electronic properties of semiconductor nanocrystals
Passivating ligand and solvent contributions to the electronic properties of semiconductor nanocrystals
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DOI：
10.1039/c2nr11398h
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发表时间：
2012-01-01
期刊：
影响因子：
6.7
通讯作者：
Tretiak, Sergei
中科院分区：
文献类型：
--
作者：
Fischer, Sean A.;Crotty, Angela M.;Tretiak, Sergei
我们使用密度泛函理论（DFT）和时间依赖DFT （TDDFT）量子化学方法详细研究了钝化配体（即胺，磷化氢，磷化氢氧化物和吡啶）对Cd33Se33量子点（QDs）的电子和光学光谱的影响。大多数配体轨道都位于量子点的价带和导带的深处，吡啶是一个例外，它在导带边缘附近引入了新的态。重要的是，所有的配体都在量子点表面和配体上提供高度离域的状态，形成杂化轨道而不是配体局域的陷阱态。相比之下，靠近带隙的态仅在量子点原子上离域，并定义了较低能量的吸收光谱。其中一个配体从量子点表面随机脱离，导致在量子点的能隙内出现高度局域的未占据态。这种电子结构的变化与各自的qd配体结合能和空间配体-配体相互作用有关。极性溶剂显著降低了导致表面状态的离域和稳定的两种效应。因此，阱态和表面态基本上被溶剂消除了。极性溶剂也蓝移（例如，在乙腈中0.3-0.4 eV）计算的吸收光谱。这种位移随溶剂介电常数的增加而增加。我们还发现近似的单粒子Kohn-Sham （KS）方法足以计算连接量子点的吸收光谱。除了系统的蓝移外，KS光谱与用更精确的TDDFT方法计算的对应光谱非常吻合。
We examine in detail the impact of passivating ligands (i.e., amines, phosphines, phosphine oxides and pyridines) on the electronic and optical spectra of Cd33Se33 quantum dots (QDs) using density functional theory (DFT) and time-dependent DFT (TDDFT) quantum-chemical methodologies. Most ligand orbitals are found deep inside in the valence and conduction bands of the QD, with pyridine being an exception by introducing new states close to the conduction band edge. Importantly, all ligands contribute states which are highly delocalized over both the QD surface and ligands, forming hybridized orbitals rather than ligand-localized trap states. In contrast, the states close to the band gap are delocalized over the QD atoms only and define the lower energy absorption spectra. The random detachment of one of ligands from the QD surface results in the appearance of a highly localized unoccupied state inside the energy gap of the QD. Such changes in the electronic structure are correlated with the respective QD-ligand binding energy and steric ligand-ligand interactions. Polar solvent significantly reduces both effects leading to delocalization and stabilization of the surface states. Thus, trap and surface states are substantially eliminated by the solvent. Polar solvent also blue-shifts (e. g., 0.3-0.4 eV in acetonitrile) the calculated absorption spectra. This shift increases with an increase of the dielectric constant of the solvent. We also found that the approximate single-particle Kohn-Sham (KS) approach is adequate for calculating the absorption spectra of the ligated QDs. Besides a systematic blue-shift, the KS spectra are in very good agreement with their respective counterparts calculated with the more accurate TDDFT method.