Metal halide perovskites are a family of semiconductor materials with exciting properties such as long charge carrier diffusion lengths, ease of synthesis and composition tunability, and remarkable defect tolerance. Recently, methods have been developed to synthesize metal halide perovskites in the form of colloidal nanosheetsor nanoplateletswhich are only a few unit cells in thickness and, as a result, experience the effects of strong dielectric and quantum confinement. This leads to narrow and blue-shifted absorption/emission as compared to the bulk stateallowing lead bromide and lead iodide nanoplatelets to cover the entire visible range. In contrast to bulk crystals, nanoplatelets exhibit strongly excitonic properties and enhanced radiative recombination. In this article we present an overview of colloidal perovskite nanoplatelets: how they are made, what their capabilities are, why 2D is beneficial, and where these materials are headed. We draw analogues to solid phase layered perovskites, cadmium selenide nanoplatelets, and 2D transition metal dichalcogenides to emphasize some of the most promising attributes of 2D materials such as their penchant for directional emission, fast/directional energy transfer, strong exciton binding energy, and reduced dielectric screening effects. We discuss the interesting physics present in these materials, remaining stability issues, and the future applications for nanoplatelets in LEDs, photovoltaics, photodetectors, and lasers.

We report highly efficient non-radiative energy transfer from cadmium selenide (CdSe) quantum dots to monolayer and few-layer molybdenum disulfide (MoS 2 ). The quenching of the donor quantum dot photoluminescence increases as the MoS 2 flake thickness decreases, with the highest efficiency (>95%) observed for monolayer MoS 2 . This counterintuitive result arises from reduced dielectric screening in thin layer semiconductors having unusually large permittivity and a strong in-plane transition dipole moment, as found in MoS 2 . Excitonic energy transfer between a 0D emitter and a 2D absorber is fundamentally interesting and enables a wide range of applications including broadband optical down-conversion, optical detection, photovoltaic sensitization, and color shifting in light-emitting devices.

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters.ABSTRACT: Lateral heterostructures with planar integrity form the basis of two-dimensional (2D) electronics and optoelectronics. Here we report that, through a twostep chemical vapor deposition (CVD) process, highquality lateral heterostructures can be constructed between metallic and semiconducting transition metal disulfide (TMD) layers. Instead of edge epitaxy, polycrystalline monolayer MoS 2 in such junctions was revealed to nucleate from the vertices of multilayered VS 2 crystals, creating one-dimensional junctions with ultralow contact resistance (0.5 kΩ·μm). This lateral contact contributes to 6-fold improved field-effect mobility for monolayer MoS 2 , compared to the conventional on-top nickel contacts. The all-CVD strategy presented here hence opens up a new avenue for all-2D-based synthetic electronics. See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Corresponding SEM images. Inset in panel f is the zoomed-in SEM image of a lateral VS 2 −MoS 2 interface (scale bar 2 μm). (g, h) Raman and PL spectra on the central VS 2 region and the surrounding MoS 2 region (blue and red curves, respectively) for the MoS 2 −VS 2 heterostructures. (i) Schematic illustration of MoS 2 growth on presynthesized multilayered VS 2 .

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Here, we show that deep trapped "dark" exciton states are responsible for the surprisingly long lifetime of band-edge photoluminescence in acid-treated single-layer MoS 2 . Temperaturedependent transient photoluminescence spectroscopy reveals an exponential tail of long-lived states extending hundreds of meV into the band gap. These sub-band states, which are characterized by a 4 µs radiative lifetime, quickly capture and store photogenerated excitons before subsequent thermalization up to the band edge where fast radiative recombination occurs. By intentionally saturating these trap states, we are able to measure the "true" 150 ps radiative lifetime of the band-edge exciton at 77 K, which extrapolates to ∼600 ps at room temperature. These experiments reveal the dominant role of dark exciton states in acid-treated MoS 2 , and suggest that excitons spend > 95% of their lifetime at room temperature in trap states below the band edge. We hypothesize that these states are associated with native structural defects, which are not introduced by the superacid treatment; rather, the superacid treatment dramatically reduces non-radiative recombination through these states, extending the exciton lifetime and increasing the likelihood of eventual radiative recombination.

Atomically thin semiconductors such as monolayer MoS2 and WS2 exhibit nonlinear exciton–exciton annihilation at notably low excitation densities (below ∼10 excitons/μm2 in exfoliated MoS2). Here, we show that the density threshold at which annihilation occurs can be tuned by changing the underlying substrate. When the supporting substrate is changed from SiO2 to Al2O3 or SrTiO3, the rate constant for second-order exciton–exciton annihilation, k XX [cm2/s], is reduced by 1 or 2 orders of magnitude, respectively. Using transient photoluminescence microscopy, we measure the effective room-temperature exciton diffusion coefficient in bis­(trifluoromethane)­sulfonimide-treated MoS2 to be in the range D = 0.03–0.06 cm2/s, corresponding to a diffusion length of L D = 350 nm for an exciton lifetime of τ = 18 ns, which does not depend strongly on the substrate. We discuss possible mechanisms for the observed behavior, including substrate permittivity, long-range exciton–exciton or exciton–charge interactions, defect-mediated Auger recombination, and spatially inhomogeneous exciton populations arising from substrate-induced disorder. Exciton annihilation limits the overall efficiency of 2D semiconductor devices operating at high exciton densities; the ability to tune these interactions via the underlying substrate is an important step toward more efficient optoelectronic technologies featuring atomically thin materials.

Hybrid quantum dot (QD)/transition metal dichalcogenide (TMD) heterostructures are attractive components of next generation optoelectronic devices, which take advantage of the spectral tunability of QDs and the charge and exciton transport properties of TMDs. Here, we demonstrate tunable electronic coupling between CdSe QDs and monolayer WS using variable length alkanethiol ligands on the QD surface. Using femtosecond time-resolved second harmonic generation (SHG) microscopy, we show that electron transfer from photoexcited CdSe QDs to single-layer WS occurs on ultrafast (50 fs to 1 ps) time scales. Moreover, in the samples exhibiting the fastest charge transfer rates (≤50 fs) we observed oscillations in the time-domain signal corresponding to an acoustic phonon mode of the donor QD, which coherently modulates the SHG response of the underlying WS layer. These results reveal surprisingly strong electronic coupling at the QD/TMD interface and demonstrate the usefulness of time-resolved SHG for exploring ultrafast electronic-vibrational dynamics in TMD heterostructures.

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