Two-dimensional (2D) materials possess unique properties that make them suitable for implementation in various technological devices. Manipulating and modifying the stacking order between layers is a way to tune their properties.
Two-dimensional transition metal dichalcogenides (TMDs) are important candidates for transistor downscaling. These materials are typically stacked in an arrangement known as 2H, consisting of two monolayers rotated by 180°. Inducing an alternative artificial stacking (for example, the rhombohedral stacking known as 3R, in which three monolayers are shifted without rotation) imparts exotic properties such as ferroelectricity, superconductivity, among others. However, inducing this type of stacking is not straightforward, due to the high thermodynamic stability of the 2H stacking.
Scientists from Chinese universities have succeeded in obtaining large-scale (on the order of centimeters) molybdenum disulfide (MoS₂) with 3R stacking. Using chemical vapor deposition (CVD), they deposited a MoS₂ monolayer onto a sapphire substrate. Once the first monolayer was deposited, the nucleation process of the subsequent monolayer became crucial to achieving the desired stacking, since once formed, it cannot be rotated.
This study proposes, through density functional theory (DFT) calculations, that defects known as Mo antisites promote 3R stacking over 2H. Experimentally, the presence of such defects was demonstrated in samples with 3R stacking, while no such defects were observed in samples with 2H stacking, corroborating the proposed growth mechanism. The obtained samples were shown to exhibit ferroelectricity via piezoresponse force microscopy (PFM).
Since 3R-MoS₂ is a semiconductor with ferroelectric characteristics, it is proposed for the fabrication of ferroelectric semiconductor field-effect transistors (FeS-FETs). Moreover, the advantage of obtaining high-quality, large-area samples ensures device reproducibility.
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