Browsing by Author "Wakabayashi, Katsunori"

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Purely in-plane ferroelectricity in monolayer SnS at room temperature
(2020-05-15)
Higashitarumizu, Naoki
;
Kawamoto, Hayami
;
Lee, Chien-Ju
;
Lin, Bo-Han
;
Chu, Fu-Hsien
2D van der Waals ferroelectrics have emerged as an attractive building block with immense potential to provide multifunctionality in nanoelectronics. Although several accomplishments have been reported in ferroelectric switching for out-of-plane ferroelectrics down to the monolayer, a purely in-plane ferroelectric has not been experimentally validated at the monolayer thickness. Herein, an in-plane ferroelectricity is demonstrated for micrometer-size monolayer SnS at room temperature. SnS has been commonly regarded to exhibit the odd-even effect, where the centrosymmetry breaks only in the odd-number layers to exhibit ferroelectricity. Remarkably, however, a robust room temperature ferroelectricity exists in SnS below a critical thickness of 15 layers with both an odd and even number of layers, suggesting the possibility of controlling the stacking sequence of multilayer SnS beyond the limit of ferroelectricity in the monolayer. This work will pave the way for nanoscale ferroelectric applications based on SnS as a platform for in-plane ferroelectrics. Out-of-plane ferroelectricity is usually unstable in the two dimensional limit due to the presence of the depolarization field. Here, the authors successfully circumvent this issue by growing mu m-sized SnS monolayers that exhibit in-plane ferroelectricity that is stable at room temperature.
Strong Enhancement of Raman Scattering from a Bulk-Inactive Vibrational Mode in Few-Layer MoTe2
(2014-04-01)
Yamamoto, Mahito
;
Wang, Sheng Tsung
;
Ni, Meiyan
;
Lin, Yen-Fu
;
Li, Song-Lin
Two-dimensional layered crystals could show phonon properties that are markedly distinct from those of their bulk counterparts, because of the loss of periodicities along the c-axis directions. Here we investigate the phonon properties of bulk and atomically thin alpha-MoTe2 using Raman spectroscopy. The Raman spectrum of alpha-MoTe2 shows a prominent peak of the in-plane E-2g(1) mode, with its frequency upshifting with decreasing thickness down to the atomic scale, similar to other dichalcogenides. Furthermore, we find large enhancement of the Raman scattering from the out-of-plane B-2g(1) mode in the atomically thin layers. The B-2g(1) models Raman inactive in the bulk, but is observed to become active in the few-layer films. The intensity ratio of the B-2g(1) to E-2g(1) peaks evolves significantly with decreasing thickness, in contrast with other dichalcogenides. Our observations point to strong effects of dimensionality on the phonon properties of MoTe2.