Two-dimensional halide perovskite nanoplatelets (NPLs) have exceptional light-emitting properties, including wide spectral tunability, ultrafast radiative decays, high quantum yields (QY), and oriented emission. To realize efficient devices, it is imperative to understand how exciton 2 transport progresses in NPL thin films. Due to the high binding energies of electron-hole pairs, excitons are generally considered the dominant species responsible for carrier transfer. We employ spatially and temporally resolved optical microscopy to map exciton diffusion in perovskite nanocrystal (NC) thin films between 15 °𝐶 and 50 °𝐶. At room temperature (RT), we find the diffusion length to be inversely correlated to the thickness of the nanocrystals (NCs). With increasing temperatures, exciton diffusion declines for all NC films, but at different rates. This leads to specific temperature turnover points, at which thinner NPLs exhibit higher diffusion lengths. We attribute this anomalous diffusion behavior to the coexistence of excitons and free electron hole-pairs inside the individual NCs within our temperature range. The organic ligand shell surrounding the NCs prevents charge transfer. Accordingly, any time an electron-hole pair spends in the unbound state reduces the FRET-mediated inter-NC transfer rates and consequently the overall diffusion. These results clarify how exciton diffusion progresses in strongly confined halide perovskite NC films, emphasizing critical considerations for optoelectronic devices.

Trap states of the semiconductor/gate dielectric interface give rise to a pronounced subthreshold behavior in field‐effect transistors (FETs) diminishing and masking intrinsic properties of 2D materials. To reduce the well‐known detrimental effect of SiO2 surface traps, this work spin‐coated an ultrathin (≈5 nm) cyclic olefin copolymer (COC) layer onto the oxide and this hydrophobic layer acts as a surface passivator. The chemical resistance of COC allows to fabricate monolayer MoS2 FETs on SiO2 by standard cleanroom processes. This way, the interface trap density is lowered and stabilized almost fivefold, to around 5 × 1011 cm−2 eV−1, which enables low‐voltage FETs even on 300 nm thick SiO2. In addition to this superior electrical performance, the photoresponsivity of the MoS2 devices on passivated oxide is also enhanced by four orders of magnitude compared to nonpassivated MoS2 FETs. Under these conditions, negative photoconductivity and a photoresponsivity of 3 × 107 A W−1 is observed which is a new highest value for MoS2. These findings indicate that the ultrathin COC passivation of the gate dielectric enables to probe exciting properties of the atomically thin 2D semiconductor, rather than interface trap dominated effects.

Semiconductor nanocrystals could replace conventional bulk materials completely in displays and light-emitting diodes. However, the organic ligands enabling their unique optical properties, prevent current flow in nanocrystal films, leaving energy transfer as the only means of injecting or extracting carriers. Here, we investigate exciton diffusion in halide perovskite superlattices - nearly perfect 3D nanocrystal assemblies. This high degree of order is not as crucial as the individual nanocrystal size, which affects transport differently depending on temperature. Up to 70 K, a confinement-induced splitting of excitonic energies, especially for the smallest nanocrystals, traps excitons into dark levels, suppressing diffusion. At intermediate temperatures, the distance of individual FRET steps corresponding to nanocrystal size enhances diffusion in the larger nanocrystals. This trend is reversed, as exciton dissociation and carrier trapping in less strongly confined nanocrystals become dominant up to room temperature. Our results reveal that transport must be factored strongly into nanocrystal design strategies for future optoelectronic applications.