Nanocrystals (NCs) are now established building blocks for optoelectronics and their use as down converters for large gamut displays has been their first mass market. NC integration relies on a combination of green and red NCs into a blend, which rises post-growth formulation issues. A careful engineering of the NCs may enable dual emissions from a single NC population which violates Kasha’s rule, which stipulates that emission should occur at the band edge. Thus, in addition to an attentive control of band alignment to obtain green and red signals, non-radiative decay paths also have to be carefully slowed down to enable emission away from the ground state. Here, we demonstrate that core/crown/crown 2D nanoplatelets (NPLs), made of CdSe/CdTe/CdSe, can combine a large volume and a type-II band alignment enabling simultaneously red and narrow green emissions. Moreover, we demonstrate that the ratio of the two emissions can be tuned by the incident power, which results in a saturation of the red emission due to non-radiative Auger recombination that affects this emission much stronger than the green one. Finally, we also show that dual-color, power tunable, emission can be obtained through an electrical excitation.
In the semiconducting perovskite materials family, the
cesium-lead-chloride
compound (CsPbCl3) supports robust excitons characterized
by a blue-shifted transition and the largest binding energy, thus
presenting a high potential to achieve demanding solid-state room-temperature
photonic or quantum devices. Here we study the fundamental emission
properties of cubic-shaped colloidal CsPbCl3 nanocrystals
(NCs), examining in particular individual NC responses using micro-photoluminescence
in order to unveil the exciton fine structure (EFS) features. Within
this work, NCs with average dimensions ⟨L
α⟩ ≈ 8 nm (α = x, y, z) are studied with a level
of dispersity in their dimensions that allows disentangling the effects
of size and shape anisotropy in the analysis. We find that most of
the NCs exhibit an optical response under the form of a doublet with
crossed polarized peaks and an average inter-bright-state splitting,
Δ
BB
≈ 1.53 meV, but triplets
are also observed though being a minority. The origin of the EFS patterns
is discussed in the frame of the electron–hole exchange model
by taking into account the dielectric mismatch at the NC interface.
The different features (large dispersity in the Δ
BB
values and occasional occurrence of triplets) are
reconciled by incorporating a moderate degree of shape anisotropy,
observed in the structural characterization, by preserving the relatively
high degree of the NC lattice symmetry. The energy distance between
the optically inactive state and the bright manifold, Δ
BD
, is also extracted from time-resolved photoluminescence
measurements (Δ
BD
≈ 10.7
meV), in good agreement with our theoretical predictions.
Nanocrystals are now established light sources, and as synthesis and device integration have gained maturity, new functionalities can now be considered. So far, the emitted light from a nanocrystal population remains mostly driven by the structural properties (composition, size, and shape) of the particle, and only limited postsynthesis tunability has been demonstrated. Here, we explore the design of light amplitude modulators using a nanocrystalbased light-emitting diode operated under reverse bias. We demonstrate strong photoluminescence modulations for devices operating in the visible and near-telecom wavelengths using low bias operations (<3 V) compatible with conventional electronics. For a visible device based on 2D nanoplatelets, we demonstrate that the photoluminescence quenching is driven by the field-induced change of nonradiative decay rate and that the field is less involved than the particle charging. This work demonstrates that a simple diode stack can combine several functionalities (light-emitting diode, detector, and light modulator) simply by selecting the driving bias.
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