Wet-chemical syntheses for quasi two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as promising methods for straightforward solution-processing of these materials.However, photoluminescence properties of colloidal TMDs are virtually unexplored due to the typically non-emitting synthesis products. In this work, we demonstrate room temperature microphotoluminescence spectroscopy on delicate ultrathin colloidal WS 2 nanosheets synthesized from WCl 6 and elemental sulfur in oleic acid and oleylamine at 320 °C for the first time. Both, monoand multilayer photoluminescence is observed, revealing comparable characteristics to exfoliated TMD monolayers and underpinning the high quality of colloidal WS 2 nanosheets. In addition, a promising long-term air-stability of colloidal WS 2 nanosheets is observed and the control of their photodegradation under laser excitation is identified as a challenge for further advancing nanosheet monolayers. Our results render colloidal TMDs as easily synthesized and highly promising 2D semiconductors with optical properties fully competitive with conventionally fabricated ultrathin TMDs.

Colloidal two-dimensional (2D) lead chalcogenide nanoplatelets (NPLs) represent highly interesting materials for near- and short wave-infrared applications including innovative glass fiber optics exhibiting negligible attenuation. In this work, we demonstrate...

2D semiconducting transition metal dichalcogenides (TMDCs) are highly promising materials for future spin- and valleytronic applications and exhibit an ultrafast response to external (optical) stimuli which is essential for optoelectronics....

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Colloidal 2D PbSe nanoplatelets (NPLs) are promising near- and short wave-infrared emitters for optoelectronic applications at telecommunication wavelengths. However, their photoluminescence quantum yield (PLQY) is limited by the ubiquitous presence of surface-related trap states. Here, we apply a treatment of colloidal PbSe NPLs with different metal halides (MX2, M = Zn, Cd, Pb; X = F, Cl, Br, I) to improve their emission brightness. A surface passivation of the NPLs by PbI2 leads to the best results with a strongly increased PLQY (27% for PbSe NPLs emitting at 0.98 eV (1265 nm) and up to 61% for PbSe NPLs emitting at 1.25 eV (989 nm)). Simultaneously, the full width at half-maximum of the NPL photoluminescence decreased by 10% after the treatment. X-ray photoelectron spectroscopy and complementary surface treatment of PbSe NPLs with organic halides reveal the combined passivating role of both X-type binding halides X– and Z-type binding metal halides MX2 in enhancing the optical properties of the PbSe NPLs. Our results emphasize the potential of 2D PbSe NPLs for efficient emission tailored for the application in fiber optics.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202206379.

2D semiconducting transition metal dichalcogenides (TMDCs) are highly promising materials for future spin- and valleytronic applications and exhibit an ultrafast response to external (optical) stimuli which is essential for optoelectronics. Colloidal nanochemistry on the other hand is an emerging alternative for the synthesis of 2D TMDC nanosheet (NS) ensembles, allowing for the control of the reaction via tunable precursor and ligand chemistry. Up to now, wet-chemical colloidal syntheses yielded intertwined/agglomerated NSs with a large lateral size. Here, we show a synthesis method for 2D mono- and bilayer MoS2 nanoplatelets with a particularly small lateral size (NPLs, 7.4 nm ± 2.2 nm) and MoS2 NSs (22 nm ± 9 nm) as a reference by adjusting the molybdenum precursor concentration in the reaction. We find that in colloidal 2D MoS2 syntheses initially a mixture of the stable semiconducting and the metastable metallic crystal phase is formed. 2D MoS2 NPLs and NSs then both undergo a full transformation to the semiconducting crystal phase by the end of the reaction, which we quantify by X-ray photoelectron spectroscopy. Phase pure semiconducting MoS2 NPLs with a lateral size approaching the MoS2 exciton Bohr radius exhibit strong additional lateral confinement, leading to a drastically shortened decay of the B exciton which is characterized by ultrafast transient absorption spectroscopy. Our findings represent an important step for utilizing colloidal TMDCs, for example small MoS2 NPLs represent an excellent starting point for the growth of heterostructures for future colloidal photonics.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have a thickness-tunable band gap in the semiconducting crystal phase and – in monolayer form – exhibit a direct band gap. TMDCs are highly promising for spin- and valleytronics and show an ultrafast response to external (optical) stimuli, an essential feature for optoelectronics. However, up to now, colloidal synthesis routes for ultrathin TMDCs typically yield different shapes and crystal phases and a thorough understanding of the product guiding reaction mechanism is missing. We investigate the colloidal synthesis of ultrathin MoS2 nanoplatelets (8 nm ± 4 nm) and nanosheets (22 nm ± 9 nm) in terms of the evolution of crystal phase and shape over the course of their formation. The reaction is followed by X-ray photoelectron spectroscopy, showing that a mixture of the semiconducting 2H and the metallic 1T crystal phase is formed initially, regardless of the molybdenum oleate precursor concentration used for the reaction. A low precursor concentration however leads to the formation of MoS2 nanoplatelets, while a high concentration yields laterally larger MoS2 nanosheets. Both structures have undergone a full transition to the semiconducting 2H crystal phase by the end of the reaction. Phase pure semiconducting MoS2 nanoplatelets with a lateral size approaching the MoS2 exciton Bohr radius exhibit strong additional lateral quantum confinement leading to a drastically shortened decay of the B-exciton, which we characterize by ultrafast transient absorption spectroscopy. Our results offer a straight-forward synthesis strategy to phase pure semiconducting 2D MoS2 and represent an important starting point for chemically exploring upcoming colloidal TMDC heterostructures for optical applications.