We report on the synthesis of silver (Ag)-doped CdSe nanoplatelets (NPLs) via postsynthesis cation exchange, using silver acetate as the Ag precursor. High-resolution transmission electron microscopy and X-ray diffraction confirmed that the NPLs maintain their morphology and crystal structure after doping when executing the exchange under reduced temperature in an ice bath. Spectroelectrochemistry and transient absorption spectroscopy revealed that Ag+ acts as an acceptor dopant. Ag doping results in an emission that is tunable from 609 to 880 nm, with a Stokes shift up to 1 eV and a photoluminescence quantum efficiency exceeding 50%. This is achieved by varying the Ag dopant concentration, which determines the hole energy level, and by controlling the electron energy level via quantum confinement in CdSe NPLs with varying core thickness or in CdSe/CdS core/shell NPLs. As highly fluorescent materials with a strongly suppressed emission reabsorption because of the large Stokes shift, Ag-doped colloidal two-dimensional NPLs offer new opportunities for the development of colloidal nanocrystal-based optoelectronic and photonic devices such as light-emitting diodes or luminescent solar concentrators.

Abstract2D 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 FStark 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 nm2. 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 FStark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems.

In recent years, hot injection synthesis has emerged as a promising route for the production of nanostructured transition metal dichalcogenides, in large due to its better control over the crystallinity and monodispersity compared to other solution based methods. Understanding the photophysics of excitons in the thus obtained colloidal nanosheets is of great importance to explore their potential for applications in optoelectronics. Here, we study the carrier dynamics in these few-layer colloidal WS2 nanosheets by use of broadband transient absorption spectroscopy. The dynamics of both the bleach, linewidth broadening and energy shift across the entire visible and near-infrared spectrum, allows us to identify subpicosecond electron trapping as the main carrier loss channel. A more quantitative analysis shows that the intrinsic properties of colloidally synthesized nanosheets are on par with other synthesis methods, paving the way for this method to produce high quality nanosheets.

Few-layered transition metal dichalcogenides (TMDs) are increasingly popular materials for optoelectronics and catalysis. Amongst the various types of TMDs available today, rheniumchalcogenides (ReX2) stand out due to their remarkable electronic structure, such as the occurrence of anisotropic excitons and potential direct bandgap behavior throughout multilayered stacks. In this letter, we have analyzed the nature and dynamics of charge carriers in highly crystalline liquid-phase exfoliated ReS2, using a unique combination of optical pump-THz probe and broadband transient absorption spectroscopy. Two distinct time regimes are identified, both of which are dominated by unbound charge carriers despite the high exciton binding energy. In the first time regime the unbound charge carriers cause an increase and a broadening of the exciton absorption band. In the second time regime, a peculiar narrowing of the excitonic absorption profile is observed, which we assign to the presence of built-in fields and/or charged defects. Our results pave the way to analyze spectrally complex transient absorption measurements on layered TMD materials and indicate the potential for ReS2 to produce mobile free charge carriers, a feat relevant for photovoltaic applications.

Colloidal quantum dots (QDs) are an attractive light source for visible photonics, in particular their widely tunable emission wavelength, inexpensive wet-chemical synthesis and straight-forward hybrid integration can make the difference. In this work, integrated light-emitting diodes are demonstrated based on CdSe/CdS QDs, with the emission directly coupled to a silicon nitride waveguide. The devices feature a record current density of up to 100 A cm −2 and a maximum on-chip power density of almost 1.5 W cm −2 in a single-mode waveguide. Operated as detectors, the photodiodes have a low dark current of 1.5 µA cm −2 . It is anticipated, that the devices will find an application in chip-based absorption spectroscopy and bio-sensing, as they can be post-processed on foundry-fabricated waveguide platforms, at a low cost. In addition, this approach provides the missing low-loss waveguide layer, necessary for building an electrically pumped laser using colloidal QDs.