Browsing by Author "Han, Yimo"

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Janus monolayers of transition metal dichalcogenides
(2017-08-01)
Lu, Ang-Yu
;
Zhu, Hanyu
;
Xiao, Jun
;
Chuu, Chih-Piao
;
Han, Yimo
Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers(1,2) or stacked van der Waals heterostructures(3,4). In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics(5,6). Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields(7,8) or, as theoretically proposed, with an asymmetric out-of-plane structural configuration(9). Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.
Metal-Guided Selective Growth of 2D Materials: Demonstration of a Bottom-Up CMOS Inverter
(2019-05-03)
Chiu, Ming-Hui
;
Tang, Hao-Ling
;
Tseng, Chien-Chih
;
Han, Yimo
;
Aljarb, Areej
2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large-area electronics and circuits strongly relies on wafer-scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal-guided selective growth (MGSG), is reported. The success of control over the transition-metal-precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p- and n-type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom-up complementary metaloxide-semiconductor inverter based on p-type WSe2 and n-type MoSe2 is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.