Controlling Morphology and Excitonic Disorder in Monolayer WSe2 Grown by Salt-Assisted CVD Methods.

Chemical synthesis is a compelling alternative to top-down fabrication for controlling the size, shape, and composition of two-dimensional (2D) crystals. Precision tuning of the 2D crystal structure has broad implications for the discovery of new phenomena and the reliable implementation of these materials in optoelectronic, photovoltaic, and quantum devices. However, precise and predictable manipulation of the edge structure in 2D crystals through gas-phase synthesis is still a formidable challenge. Here, we demonstrate a salt-assisted low-pressure chemical vapor deposition method that enables tuning W metal flux during growth of 2D WSe2 monolayers and, thereby, direct control of their edge structure and optical properties. The degree of structural disorder in 2D WSe2 is a direct function of the W metal flux, which is controlled by adjusting the mass ratio of WO3 to NaCl. This edge disorder then couples to excitonic disorder, which manifests as broadened and spatially varying emission profiles. Our work links synthetic parameters with analyses of material morphology and optical properties to provide a unified understanding of intrinsic limits and opportunities in synthetic 2D materials.

Selective Monofunctionalization Enabled by Reaction-History-Dependent Communication in Catalytic Rotaxanes.

Selective monofunctionalization of substrates with distant, yet equally reactive functional groups is difficult to achieve, as it requires the second functional group to selectively modulate its reactivity once the first functional group has reacted. We now show that mechanically interlocked catalytic rings can effectively regulate the reactivity of stoppering groups in rotaxanes over a distance of about 2 nm. Our mechanism of communication is enabled by a unique interlocked design, which effectively removes the catalytic rings from the substrates by fast dethreading as soon as the first reaction has taken place. Our method not only led to a rare example of selective monofunctionalization, but also to a "molecular if function". Overall, the study presents a way to get distant functional groups to communicate with each other in a reaction-history-dependent manner by creating linkers that can ultimately perform logical operations at the molecular level.