Probing permanent dipoles in CdSe nanoplatelets with transient electric birefringence

Abstract

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 electrooptic 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 the43m 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.

Manipulation of the Optical Properties of Colloidal 2D CdSe Nanoplatelets

“…The authors attributed the origin of the out‐of‐plane dipole in CdSe NPL stacks to the distortion arising from the ligand‐induced stress at the NPL surface. [ 51 ] Then, the presence of out‐of‐plane dipole components in NPL aggregates was also confirmed via Fourier‐imaging techniques. [ 137 ] Previous study of intrinsic TDMs in ML CdSe NPL film via 2D k‐space spectra characterization has manifested the 3D isotropic orientation of absorption dipoles and 2D in‐plane distribution of emission dipoles.…”

“…However, superlattices with face‐down configuration in which a large lateral surface is parallel to the substrate have been more pursued due to the suppressed AR rates and directional emission properties with respect to edge‐up oriented NPL stacks. Until recently, liquid–liquid interfacial assembly was proposed to fabricate CdSe NPL superlattice films in either edge‐up alignment or face‐down configuration, followed by fundamental research on the orientation‐dependent properties [ 50,51 ] and the study of the remarkable optical gain properties [ 52,53 ] of these stacked NPL films with controlled orientation. The performance of state‐of‐the‐art colloidal nanocrystal‐based light emitting diodes (LEDs) [ 54,55 ] without light extraction structures is greatly hindered by the theoretical upper limit of peak external quantum efficiencies (EQE, determined as the rate of the amounts of radiated photons to the amounts of injected carriers) reaching 20%.…”

Polarization at the Nanoscale

“…Or it happens because surface DBs come into play. Most recently, permanent dipoles were also observed in zincblende CdSe nano-platelets by transient electric birefringence measurement [20]. Importantly, zincblende is a high-symmetry structure which should not have any spontaneous polarization due to bound charge.…”

Fourier-Imaging of Single Self-Assembled CdSe Nanoplatelet Chains and Clusters Reveals out-of-Plane Dipole Contribution

“…By a careful control of the ligands and solvents, NPLs can be assembled into linear chains of a few hundred nm to a few µm, with good photostability and brightness, excellent order and highly uniform inter-NPL distance 25 . Ligand-induced strains within platelets can create a twist of these NPL chains [26][27] and may be responsible for the recently-observed permanent dipole parallel to the chain 28 .…”

“…25 Ligand-induced strains within platelets can create a twist of these NPL chains 26,27 and may be responsible for the recently observed permanent dipole parallel to the chain. 28 A key characteristic of a fluorescent system is the orientation of its radiating dipole, with implications for orientation and conformation tracking in bioimaging 29 and super-resolution, 30 for directional emission and light extraction in optoelectronics 12 or for coupling and Purcell efficiency in nanophotonics and plasmonics. 31,32 A corollary question is how many dipoles contribute to the emission.…”