Grains and grain boundaries in highly crystalline monolayer molybdenum disulfide

TL;DR

This study advances the synthesis of highly crystalline monolayer molybdenum disulfide, revealing detailed microstructural features and their effects on optical and electrical properties, crucial for atomically thin electronic applications.

Contribution

It refines chemical vapor deposition to produce large, high-quality monolayer MoS2 with detailed microstructural analysis linking defects to optical and electrical behavior.

Findings

Triangular islands are single crystals.

Boundaries influence photoluminescence significantly.

Defects induce localized mid-gap states.

Abstract

Recent progress in large-area synthesis of monolayer molybdenum disulfide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapor deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulfide up to 120 um in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology, and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror boundaries cause strong photoluminescence quenching while tilt boundaries cause strong enhancement. In contrast, the boundaries only slightly increase the measured in-plane electrical conductivity.