Surface-Induced Deprotonation of Thiol Ligands Impacts the Optical Response of CdS Quantum Dots
Surface-Induced Deprotonation of Thiol Ligands Impacts the Optical Response of CdS Quantum Dots
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DOI：
10.1021/acs.chemmater.0c03610
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发表时间：
2021-01-19
影响因子：
8.6
通讯作者：
Kilina, Svetlana
中科院分区：
文献类型：
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作者：
Lystrom, Levi;Roberts, Alyssa;Kilina, Svetlana
硫醇配体钝化的硫族镉量子点在照明、传感和成像技术等方面表现出许多优势。然而，它们的排放对硫醇浓度、pH条件和温度很敏感。利用基于密度泛函理论的计算，我们确定了硫醇/硫酸盐在CDS量子点表面平衡的条件，这些条件可以消除或引入光学不活跃的空穴陷阱态，有利于或不利于发射。我们的计算表明，QD和质子化物种(硫醇)之间的相互作用比它们的去质子化物种(硫醇酸盐)弱得多。此外，CDS量子点的表面有助于硫醇的部分去质子化，导致形成额外的稳定的网络构象，其中质子在配体和量子点表面之间共享。硫代酸盐极大地降低了CdS量子点中低能跃迁的光学强度，贡献了量子点带隙中的硫酸盐局域空穴陷阱态。然而，硫醇和表面之间的网络，以及伯胺等天然配体的存在，稳定了这种陷阱态，使最低的光学跃迁变得明亮。这解释了在中性或酸性溶液中，较低浓度的硫醇钝化量子点的发射增加的原因。表面对去质子化物种的偏置及其对光学非活化态的贡献也使高温或强激光脉冲作用下的CdSe/CDS量子点的不可逆发射猝灭和漂白合理化。
Cadmium chalcogenide quantum dots (QDs) passivated by thiol-based ligands exhibit several advantages in their applications in lighting, sensing, and imaging technologies. However, their emission is sensitive to thiol concentrations, pH conditions, and temperatures. Using calculations based on the density functional theory, we identify conditions for thiol/thiolate equilibrium at the CdS QD surface that either eliminate or introduce optically inactive hole trap states favoring or disfavoring the emission. Our calculations indicate much weaker interactions between the QD and protonated species (thiols), compared to their deprotonated counterparts (thiolates). Additionally, the surface of CdS QD facilitates the partial deprotonation of thiols, leading to the formation of an additional stable networking conformation where the proton is shared between the ligand and the QD surface. Thiolates strongly reduce the optical intensity of low-energy transitions in CdS QDs, contributing thiolate-localized hole trap states at the QD band gap. However, networking between the thiols and the surface, as well as the presence of native ligands such as primary amines, stabilize such trap states brightening the lowest optical transitions. This explains the increased emission of thiol-passivated QDs at lower concentrations in neutral or acidic solutions. Surface-mediated bias toward deprotonated species and their contribution to optically inactive states also rationalizes irreversible emission quenching and bleaching in the CdSe/CdS QDs exposed to high temperatures or intensive laser pulse.