In situ atomic level studies of thermally controlled interlayer stacking shifts in 2D transition metal dichalcogenide bi
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HETEROGENEITY IN 2D MATERIAL
In situ atomic level studies of thermally controlled interlayer stacking shifts in 2D transition metal dichalcogenide bilayers Si Zhou1, Jun Chen1, Jamie H. Warner1,a) 1
Department of Materials, University of Oxford, Oxford OX1 3PH, U.K. Address all correspondence to this author. e-mail: [email protected]
a)
Received: 24 October 2019; accepted: 4 December 2019
We show interlayer stacking shifts occur in transition metal dichalcogenides (TMD) bilayers due to the strain introduced during sample heating, and attributed to rippling of one layer relative to the other. The atomic structure of the interlayer stacking is studied using annular dark field scanning transmission electron microscopy with an in situ heating holder. Before heating, bilayers show uniform interlayer stacking of AA9 and AB. When heated, contrast change is seen and associated with interlayer stacking changes at the atomic scale due to ripples. When cooled down to room temperature, these contrast features disappear, confirming it is a reversible process that is not related to defects or vacancies. Because the bottom layer is attached to the in situ heating chip made from Si3N4 and the top layer is in contact with the underlying TMD layer with weak van der Waals interaction, the two layers experience different forces during thermal expansion.
Introduction Layered materials such as graphite and transition metal dichalcogenides (TMDs) are stacked through weak interlayer van der Waals interactions [1]. For 2D TMDs such as WS2 and MoS2, their bilayer structures offer an extra layer degree of freedom, which introduces performance enhancements compared with the monolayer counterpart [2, 3, 4]. For example, few-layer MoS2 shows higher carrier mobility and perform better in gas sensing [5, 6]. Interaction between MoS2 or WS2 layers and the specific stacking arrangement can significantly modify their electrical, optical, and vibrational properties [3, 7, 8, 9]. 2H stacking and 3R stacking are two predominantly existing stacking forms [3, 10]. Previous studies have shown that these differently stacked MoS2 have distinct properties in terms of Raman and photoluminescence (PL) [11, 12, 13]. The stacking alters the band gap, which can be engineered by rotating or sliding the layers [7, 14]. Therefore, the control of TMD bilayer stacking is significant when considering their future device applications [15]. In previous studies of layered materials, evidence for the coexistence of domains with different stacking orders was reported [16, 17, 18, 19]. For example, domains with mirrored AB and AC stacking were synthesized in bilayer graphene
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(BLG) [17]. An atomically sharp interface, which was denoted as an antiphase grain boundary, was observed between 2H and 3R stacked MoS2 bilayer regions [16]. However, the AB and AC stacking boundaries in graphene bilayer is not atomically sharp but instead nanometer-wide strained channels in the form of ripples [17], which have been intensi
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