合金化对TDDFT制备Ge $_{1-x}$ Si $_x$纳米晶光学性质的影响及与有效介质理论的比较
Alloying effects on the optical properties of Ge$_{1-x}$Si$_x$ nanocrystals from TDDFT and comparison with effective-medium theory
摘要：
采用含时密度泛函理论，在绝热局域密度近似（TDLDA）下，计算了Ge $_{1-x}$ Si $_x$合金纳米晶的光谱.光谱作为组成$x$的函数平滑地变化。在Ge侧的组成范围内，在吸收边的最低激发几乎是纯Kohn-Sham独立粒子HOMO-LUMO跃迁，而对于更高的Si含量，发现强烈的混合跃迁。然而，吸收的开始，特别是它的combustion依赖性是类似的独立粒子的结果。此外，经典的去极化效应是一个非常强的抑制的吸收强度负责。我们表明，他们可以考虑在一个更简单的方式使用麦克斯韦-加内特经典有效介质理论。发射光谱是通过计算激发态纳米晶体在其弛豫几何形状下的吸收来研究的。结构对斯托克斯位移的贡献约为0.5 eV。发射光谱在独立粒子渡越离子方面的分解与吸收的分解相似。对于发射，在富锗团簇中发现非常弱的跃迁，远低于强吸收起始。
英文摘要：
We present the optical spectra of Ge$_{1-x}$Si$_x$ alloy nanocrystals calculated with time-dependent density-functional theory in the adiabatic local-density ap proximation (TDLDA). The spectra change smoothly as a function of the compositio n $x$. On the Ge side of the composition range, the lowest excitations at the ab sorption edge are almost pure Kohn-Sham independent-particle HOMO-LUMO transitio ns, while for higher Si contents strong mixing of transitions is found. Within T DLDA the first peak is slightly higher in energy than in earlier independent-par ticle calculations. However, the absorption onset and in particular its composit ion dependence is similar to independent-particle results. Moreover, classical depolarization effects are responsible for a very strong suppression of the abs orption intensity. We show that they can be taken into account in a simpler way using Maxwell-Garnett classical effective-medium theory. Emission spectra are in vestigated by calculating the absorption of excited nanocrystals at their relaxe d geometry. The structural contribution to the Stokes shift is about 0.5 eV. Th e decomposition of the emission spectra in terms of independent-particle transit ions is similar to what is found for absorption. For the emission, very weak tra nsitions are found in Ge-rich clusters well below the strong absorption onset.
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