Dozov Ivan, Goldmann Claire, Davidson Patrick, Abécassis Benjamin
Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Université Paris-Sud, UMR 8502, 91405 Orsay, France.
Nanoscale. 2020 May 28;12(20):11040-11054. doi: 10.1039/d0nr00884b.
Zinc-blende CdSe semiconducting nanoplatelets (NPL) show outstanding quantum confinement properties thanks to their small, atomically-controlled, thickness. For example, they display extremely sharp absorption peaks and ultra-fast recombination rates that make them very interesting objects for optoelectronic applications. However, the presence of a ground-state electric dipole for these nanoparticles has not yet been investigated. We therefore used transient electric birefringence (TEB) to probe the electric dipole of 5-monolayer thick zinc blende CdSe NPL with a parallelepipedic shape. We studied a dilute dispersion of isolated NPL coated with branched ligands and we measured, as a function of time, the birefringence induced by DC and AC field pulses. The electro-optic behavior proves the presence of a large dipolar moment (>245 D) oriented along the length of the platelets. We then induced the slow face-to-face stacking of the NPL by adding oleic acid. In these stacks, the in-plane dipole components of consecutive NPL cancel whereas their normal components add. Moreover, interestingly, the excess polarizability tensor of the NPL stacks gives rise to an electro-optic contribution opposite to that of the electric dipole. By monitoring the TEB signal of the slowly-growing stacks over up to a year, we extracted the evolution of their average length with time and we showed that their electro-optic response can be explained by the presence of a 80 D dipolar component parallel to their normal. In spite of the 4[combining macron]3m space group of bulk zinc blende CdSe, these NPL thus bear an important ground-state dipole whose magnitude per unit volume is twice that found for wurtzite CdSe nanorods. We discuss the possible origin of this electric dipole, its consequences for the optical properties of these nanoparticles, and how it could explain their strong stacking propensity that severely hampers their colloidal stability.
闪锌矿结构的CdSe半导体纳米片（NPL）由于其原子级精确控制的小厚度而展现出出色的量子限制特性。例如，它们具有极其尖锐的吸收峰和超快的复合速率，这使其成为光电子应用中非常有吸引力的研究对象。然而，这些纳米颗粒基态电偶极的存在尚未得到研究。因此，我们使用瞬态电双折射（TEB）来探测具有平行六面体形状的5单层厚闪锌矿结构CdSe NPL的电偶极。我们研究了涂有支化配体的孤立NPL的稀溶液，并测量了直流和交流场脉冲随时间诱导的双折射。电光行为证明了沿纳米片长度方向存在一个大的偶极矩（>245 D）。然后，我们通过添加油酸诱导NPL缓慢地面对面堆叠。在这些堆叠中，连续NPL的面内偶极分量相互抵消，而其法向分量相加。此外，有趣的是，NPL堆叠的过量极化率张量产生了与电偶极相反的电光贡献。通过监测长达一年时间内缓慢生长的堆叠的TEB信号，我们提取了它们平均长度随时间的变化，并表明它们的电光响应可以由平行于其法线的80 D偶极分量的存在来解释。尽管块状闪锌矿结构CdSe具有4[combining macron]3m空间群，但这些NPL因此具有一个重要的基态偶极，其单位体积的大小是纤锌矿CdSe纳米棒的两倍。我们讨论了这个电偶极的可能起源、其对这些纳米颗粒光学性质的影响，以及它如何解释它们强烈的堆叠倾向，而这种倾向严重阻碍了它们的胶体稳定性。
