Physics and Chemistry of Nanostructures, Ghent University, Ghent, Belgium.
ACS Nano. 2011 Jan 25;5(1):58-66. doi: 10.1021/nn102980e. Epub 2010 Dec 28.
We present a quantitative analysis of the absorption and luminescence of colloidal PbSe/CdSe core/shell quantum dots (QDs). In absorption, both the energy and the oscillator strength of the first exciton transition coincide with that of plain PbSe QDs. In contrast, luminescence lifetime measurements indicate that the oscillator strength of the emitting transition is reduced by at least a factor of 4 compared to PbSe core QDs. Moreover, the addition of an electron scavenger quenches the PbSe/CdSe emission, while a hole scavenger does not. This implies that the electron wave function reaches the QD surface, while the hole is confined to the PbSe core. These observations are consistent with calculations based on the effective mass model, which show that PbSe/CdSe QDs are at the boundary between the type-I and quasi-type-II regime, where the electron spreads over the entire nanoparticle and the hole remains confined in the PbSe core. However, as this only leads to a minor reduction of the oscillator strength, it follows that the drastic reduction of the oscillator strength in emission cannot be explained in terms of electron delocalization. In combination with the increased Stokes shift for PbSe/CdSe QDs, this indicates that the emission results from lower energy states that are fundamentally different from the absorbing states.
我们对胶体 PbSe/CdSe 核/壳量子点 (QD) 的吸收和发光进行了定量分析。在吸收中，第一激子跃迁的能量和振子强度都与纯 PbSe QD 一致。相比之下，发光寿命测量表明，与 PbSe 核 QD 相比，发射跃迁的振子强度至少降低了 4 倍。此外，添加电子清除剂会猝灭 PbSe/CdSe 发射，而空穴清除剂则不会。这意味着电子波函数到达 QD 表面，而空穴被限制在 PbSe 核内。这些观察结果与基于有效质量模型的计算结果一致，该模型表明 PbSe/CdSe QD 处于 I 型和准 II 型的边界，其中电子扩展到整个纳米颗粒，而空穴仍然局限在 PbSe 核内。然而，由于这只导致振子强度的轻微降低，因此在发射中振子强度的急剧降低不能用电子离域来解释。结合 PbSe/CdSe QD 较大的斯托克斯位移，这表明发射源自与吸收态根本不同的低能态。
