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首页> 外文期刊>The journal of physical chemistry, C. Nanomaterials and interfaces >Sub-Bandgap Photoinduced Transient Absorption Features in CdSe Nanostructures: The Role of Trapped Holes
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Sub-Bandgap Photoinduced Transient Absorption Features in CdSe Nanostructures: The Role of Trapped Holes

机译：CDSE纳米结构中的子带隙光漏瞬态吸收特征：截留孔的作用

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摘要

Transient absorption (TA) is widely used to study the dynamics of various processes, such as trapping, nonradiative decay, or transferring of photoexcited carriers in semiconductor nanocrystals. TA spectra of these systems show photoinduced absorption (PA) features that appear lower in energy than those of the band edge, which have been attributed to sub-bandgap absorptions of photoexcited electrons and holes. Here, we perform atomistic, semiempirical pseudopotential calculations in CdSe nanostructures to compute oscillator strengths for sub-bandgap transitions of conduction band electrons, valence band holes, and surface-trapped holes. We find that sharper peaks in the infrared (IR) range and broader features in the near-IR range (0.5-1.0 eV) are due to near-band-edge transitions of electrons and holes, respectively. Additionally, we focus on the region from 1.45 to 1.9 eV (850-650 nm), in which broad features have been observed and assigned to the PA of holes populating surface traps of nanocrystals. While there has been experimental justification of this assignment, there has been little theoretical investigation. We find that, in this region of interest from 1.45 to 1.9 eV, oscillator strengths for transitions of trapped holes are significantly larger than those of electrons or valence band holes. We conclude that the low symmetry of localized surface trap states and optimal spatial overlap with highly oscillatory states deep in the valence band lead to large electric dipole matrix elements and increased oscillator strengths. Our results are consistent for CdSe and CdS cores, CdSe-CdS core-shell quantum dots, and CdSe nanorods.
机译：瞬态吸收（TA）被广泛用于研究各种方法的动态，例如捕获，非辐射衰减或在半导体纳米晶体中的光透射载体转移。这些系统的TA光谱显示出比带边缘的能量低的光诱导的吸收（PA）特征，这归因于光屏蔽电子和孔的子带隙吸收。这里，我们在CDSE纳米结构中执行原子，半透明伪能量计算，以计算导电带电子，价带孔和表面捕获的孔的子带隙转变的振荡器强度。我们发现红外线（IR）范围和近红外范围（0.5-1.0eV）中更广泛的特征的峰值分别是由于电子和孔的近带边缘过渡。此外，我们专注于1.45至1.9eV（850-650nm）的地区，其中已经观察到广泛的特征，并分配给纳米晶体填充表面陷阱的孔Pa。虽然这项任务有实验性的理由，但有很少的理论调查。我们发现，在这个感兴趣的区域，从1.45到1.9eV，捕获孔过渡的振荡器强度明显大于电子或价带孔的振荡。我们得出结论，局部表面捕集状态的低对称性和具有高度振荡状态的偏振态在价带中的高度振荡状态导致大电偶极矩阵元件和增加的振荡器强度。我们的结果是CDSE和CDS核心，CDSE-CDS核心壳量子点和CDSE纳米棒一致。

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