TY - JOUR
T1 - Spontaneous Formation of Atomically Thin Stripes in Transition Metal Dichalcogenide Monolayers
AU - Azizi, Amin
AU - Wang, Yuanxi
AU - Lin, Zhong
AU - Wang, Ke
AU - Elias, Ana Laura
AU - Terrones, Mauricio
AU - Crespi, Vincent H.
AU - Alem, Nasim
N1 - Funding Information:
The authors acknowledge the Center for 2-Dimensional and Layered Materials (2DLM) at the Pennsylvania State University and support from the National Science Foundation under EFRI 2-DARE awards 433378, 1433311, and 1542707 and the Materials Innovation Platform program under award DMR-1539916. Y.W., A.L.E., Z.L., M.T., and V.H.C. also acknowledge support from the U.S. Army Research Office under MURI grant W911NF-11-1-0362. The authors also would like to thank N. Perea, A. Castro-Beltran, and C. Zhou for technical assistance.
Publisher Copyright:
© 2016 American Chemical Society.
PY - 2016/11/9
Y1 - 2016/11/9
N2 - Whether an alloy is random or ordered can have profound effects on its properties. The close chemical similarity of W and Mo in the two-dimensional semiconductors MoS2 and WS2 has led to the expectation that WxMo1-xS2 is a random alloy. Here we report that triangular monolayer flakes of WxMo1-xS2 produced by sulfurization of MoO3/WO3 are not only nonrandom, but also anisotropic: W and Mo form atomically thin chains oriented parallel to the edges of the triangle, especially around x ∼ 0.5, as resolved by aberration-corrected transmission electron microscopy. First-principles calculations reveal that the binding energies of striped and random alloys are nearly identical but that phase segregation at the growth edge favors one metal over another depending on the local sulfur availability, independent of the composition deeper inside the monolayer. Thus, atomically thin striping is kinetically driven and controlled by fluctuations that couple the local chemical potentials of metals and chalcogenide. Considering the nearly identical electronic properties but very different atomic masses of Mo and W, the resulting striped alloy is electronically isotropic, but vibrationally anisotropic. Phonon anomalies associated with the stripe ordering are predicted, as is an anisotropic thermal conductivity. More generally, fluctuation-driven striping provides a mechanism to produce in-plane subnanometer superlattices within two-dimensional crystals, with broad implications for controlling the electronic, optical, and structural properties of these systems.
AB - Whether an alloy is random or ordered can have profound effects on its properties. The close chemical similarity of W and Mo in the two-dimensional semiconductors MoS2 and WS2 has led to the expectation that WxMo1-xS2 is a random alloy. Here we report that triangular monolayer flakes of WxMo1-xS2 produced by sulfurization of MoO3/WO3 are not only nonrandom, but also anisotropic: W and Mo form atomically thin chains oriented parallel to the edges of the triangle, especially around x ∼ 0.5, as resolved by aberration-corrected transmission electron microscopy. First-principles calculations reveal that the binding energies of striped and random alloys are nearly identical but that phase segregation at the growth edge favors one metal over another depending on the local sulfur availability, independent of the composition deeper inside the monolayer. Thus, atomically thin striping is kinetically driven and controlled by fluctuations that couple the local chemical potentials of metals and chalcogenide. Considering the nearly identical electronic properties but very different atomic masses of Mo and W, the resulting striped alloy is electronically isotropic, but vibrationally anisotropic. Phonon anomalies associated with the stripe ordering are predicted, as is an anisotropic thermal conductivity. More generally, fluctuation-driven striping provides a mechanism to produce in-plane subnanometer superlattices within two-dimensional crystals, with broad implications for controlling the electronic, optical, and structural properties of these systems.
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U2 - 10.1021/acs.nanolett.6b03075
DO - 10.1021/acs.nanolett.6b03075
M3 - Article
AN - SCOPUS:84994762001
VL - 16
SP - 6982
EP - 6987
JO - Nano Letters
JF - Nano Letters
SN - 1530-6984
IS - 11
ER -