Given the current needs for lasers on flexible substrates or as disposable, low-cost appliances, solution-processable lasers based on tunable colloidal quantum dots (QD) could revolutionize the field ...of laser-based opto-electronics, much as these QD materials currently do for the growing markets of displays and lighting. In this perspective, we present the status of this rapidly advancing field, followed by a discussion of the remaining challenges and possible avenues for future research and valorization.
Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to ...nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. 1H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We find that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures. However, when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity. In addition, we find that a high amine content in the ligand shell increases the quantum yield due to the improved binding of the carboxylic acid.
Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV–vis absorption spectroscopy and transmission electron microscopy to determine the size, composition, and intrinsic ...absorption coefficient μi of 4 to 11 nm sized colloidal CsPbBr3 nanocrystals (NCs). The ICP-MS measurements demonstrate the nonstoichiometric nature of the NCs, with a systematic excess of lead for all samples studied. Rutherford backscattering measurements indicate that this enrichment in lead concurs with a relative increase in the bromide content. At high photon energies, μi is independent of the nanocrystal size. This allows the nanocrystal concentration in CsPbBr3 nanocolloids to be readily obtained by a combination of absorption spectroscopy and the CsPbBr3 sizing curve.
Colloidal core/shell InP/ZnSe quantum dots (QDs), recently produced using an improved synthesis method, have a great potential in life-science applications as well as in integrated quantum photonics ...and quantum information processing as single-photon emitters. Single-particle spectroscopy of 10 nm QDs with 3.2 nm cores reveals strong photon antibunching attributed to fast (70 ps) Auger recombination of multiple excitons. The QDs exhibit very good photostability under strong optical excitation. We demonstrate that the antibunching is preserved when the QDs are excited above the saturation intensity of the fundamental-exciton transition. This result paves the way toward their usage as high-purity on-demand single-photon emitters at room temperature. Unconventionally, despite the strong Auger blockade mechanism, InP/ZnSe QDs also display very little luminescence intermittency (“blinking”), with a simple on/off blinking pattern. The analysis of single-particle luminescence statistics places these InP/ZnSe QDs in the class of nearly blinking-free QDs, with emission stability comparable to state-of-the-art thick-shell and alloyed-interface CdSe/CdS, but with improved single-photon purity.
Abstract
2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance ...spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength
F
S
t
a
r
k
that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton’s center of mass and the total area, which yields a coherence area of a mere 6.1 nm
2
. Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that
F
S
t
a
r
k
is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems.
Hybrid silicon nitride (SiN)–quantum‐dot (QD) microlasers coupled to a passive SiN output waveguide with a 7 µm diameter and a record‐low threshold density of 27 µJ cm−2 are demonstrated. A new ...design and processing scheme offers long‐term stability and facilitates in‐depth QD material and device characterization, thereby opening new paths for optical communication, sensing, and on‐chip cavity quantum optics based on colloidal QDs.
We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over ...a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ∼10 ps) of “warm” electrons in the vicinity (within ∼0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps–1 carrier–1, increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron–phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.
The absorption cross section of colloidal quantum dots in close-packed monolayers shows a 4 (CdSe) to 5-fold (PbS) enhancement compared to quantum dots in a dilute dispersion. Quantitative agreement ...is demonstrated between the value and the size dependence of the enhancement and theoretical model predictions based on dipolar coupling between neighboring quantum dots. This collective optical behavior offers a new degree of freedom in the custom design of optical properties for electro-optical devices.
We determine the optical properties of PbS/CdS core/shell QDs, addressing the energy and the oscillator strength of the first exciton transition and the intrinsic absorption coefficient at short ...wavelengths. It follows that the intrinsic absorption coefficient at wavelengths of 350 and 400 nm agrees with theoretical numbers predicted using the Maxwell–Garnett model. Moreover, close correspondence is demonstrated between the energy and the oscillator strength of the first exciton transition of PbS/CdS core/shell and PbS QDs with a similar core size. This provides a straightforward way to estimate the PbS core diameter in PbS/CdS core/shell quantum dots by means of the PbS sizing curve, and it indicates that PbS/CdS QDs exhibit a type 1 band alignment.
Abstract
Colloidal quantum dots (QDs) are excellent optical gain materials that combine high material gain, a strong absorption of pump light, stability under strong light exposure and a suitability ...for solution-based processing. The integration of QDs in laser cavities that fully exploit the potential of these emerging optical materials remains, however, a challenge. In this work, we report on a vertical cavity surface emitting laser, which consists of a thin film of QDs embedded between two layers of polymerized chiral liquid crystal. Forward directed, circularly polarized defect mode lasing under nanosecond-pulsed excitation is demonstrated within the photonic band gap of the chiral liquid crystal. Stable and long-term narrow-linewidth lasing of an exfoliated free-standing, flexible film under water is obtained at room temperature. Moreover, we show that the lasing wavelength of this flexible cavity shifts under influence of pressure, strain or temperature. As such, the combination of solution processable and stable inorganic QDs with high chiral liquid crystal reflectivity and effective polymer encapsulation leads to a flexible device with long operational lifetime, that can be immersed in different protic solvents to act as a sensor.
PDF
