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ACS Nano. 2016 Feb 23;10(2):2399-405. doi: 10.1021/acsnano.5b07214. Epub 2016 Jan 15.

Biexciton Emission from Edges and Grain Boundaries of Triangular WS₂ Monolayers.

Kim MS1, Yun SJ1,2, Lee Y1,2, Seo C1,2, Han GH1, Kim KK3, Lee YH1,2, Kim J1,2.

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Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University , Suwon 440-746, Republic of Korea.
Department of Energy Science, Sungkyunkwan University , Suwon 440-746, Republic of Korea.
Department of Energy and Materials Engineering, Dongguk University-Seoul , Seoul 100-715, Republic of Korea.


Monolayer tungsten disulfides (WS2) constitute a high quantum yield two-dimensional (2D) system, and can be synthesized on a large area using chemical vapor deposition (CVD), suggesting promising nanophotonics applications. However, spatially nonuniform photoluminescence (PL) intensities and peak wavelengths observed in single WS2 grains have puzzled researchers, with the origins of variation in relative contributions of excitons, trions, and biexcitons to the PL emission not well understood. Here, we present nanoscale PL and Raman spectroscopy images of triangular CVD-grown WS2 monolayers of different sizes, with these images obtained under different temperatures and values of excitation power. Intense PL emissions were observed around the edges of individual WS2 grains and the grain boundaries between partly merged WS2 grains. The predominant origin of the main PL emission from these regions changed from neutral excitons to trions and biexcitons with increasing laser excitation power, with biexcitons completely dominating the PL emission for the high-power condition. The intense PL emission and the preferential formation of biexcitons in the edges and grain boundaries of monolayer WS2 were attributed to larger population of charge carriers caused by the excessive incorporation of growth promoters during the CVD, suggesting positive roles of excessive carriers in the PL efficiency of TMD monolayers. Our comprehensive nanoscale spectroscopic investigation sheds light on the dynamic competition between exciton complexes occurring in monolayer WS2, suggesting a rich variety of ways to engineer new nanophotonic functions using 2D transition metal dichalcogenide monolayers.


biexciton; chemical vapor deposition; grain boundary; monolayer; photoluminescence; tungsten disulfide


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