Zheng, Fangyuan; Guo, Deping; Huang, Lingli; Wong, Lok Wing; Chen, Xin; Wang, Cong; Cai, Yuan; Wang, Ning; Lee, Chun-Sing; Lau, Shu Ping; Ly, Thuc Hue; Ji, Wei and Zhao, Jiong
Abstract
Phase patterning in polymorphic two-dimensional (2D) materials offers diverse properties that extend beyond what their pristine structures can achieve. If precisely controllable, phase transitions can bring exciting new applications for nanometer-scale devices and ultra-large-scale integrations. Here, the focused electron beam is capable of triggering the phase transition from the semiconducting T” phase to metallic T’ and T phases in 2D rhenium disulfide (ReS2) and rhenium diselenide (ReSe2) monolayers, rendering ultra-precise phase patterning technique even in sub-nanometer scale is found. Based on knock-on effects and strain analysis, the phase transition mechanism on the created atomic vacancies and the introduced substantial in-plane compressive strain in 2D layers are clarified. This in situ high-resolution scanning transmission electron microscopy (STEM) and in situ electrical characterizations agree well with the density functional theory (DFT) calculation results for the atomic structures, electronic properties, and phase transition mechanisms. Grain boundary engineering and electrical contact engineering in 2D are thus developed based on this patterning technique. The patterning method exhibits great potential in ultra-precise electron beam lithography as a scalable top-down manufacturing method for future atomic-scale devices. DOI:10.1002/advs.202200702
Lukas Rogée, Lvjin Wang, Yi Zhang, Songhua Cai, Peng Wang, Manish Chhowalla, Wei Ji & Shu Ping Lau
Two-dimensional materials with out-of-plane (OOP) ferroelectric and piezoelectric properties are highly desirable for the realization of ultrathin ferro- and piezoelectronic devices. We demonstrate unexpected OOP ferroelectricity and piezoelectricity in untwisted, commensurate, and epitaxial MoS2/WS2 heterobilayers synthesized by scalable one-step chemical vapor deposition. We show d33 piezoelectric constants of 1.95 to 2.09 picometers per volt that are larger than the natural OOP piezoelectric constant of monolayer In2Se3 by a factor of ~6. We demonstrate the modulation of tunneling current by about three orders of magnitude in ferroelectric tunnel junction devices by changing the polarization state of MoS2/WS2 heterobilayers. Our results are consistent with density functional theory, which shows that both symmetry breaking and interlayer sliding give rise to the unexpected properties without the need for invoking twist angles or moiré domains.
Chen Ye, Cong Wang, Qiong Wu, Sheng Liu, Jiayuan Zhou, Guopeng Wang, Aljoscha Söll, Zdenek Sofer, Ming Yue, Xue Liu, Mingliang Tian, Qihua Xiong, Wei Ji & Xiao Renshaw Wang
Abstract
Magnetic van der Waals (vdW) materials possess versatile spin configurations stabilized in reduced dimensions. One magnetic order is the interlayer antiferromagnetism in A-type vdW antiferromagnet, which may be effectively modified by the magnetic field, stacking order, and thickness scaling. However, atomically revealing the interlayer spin orientation in the vdW antiferromagnet is highly challenging, because most of the material candidates exhibit an insulating ground state or instability in ambient conditions. Here, we report the layer-dependent interlayer antiferromagnetic spin reorientation in air-stable semiconductor CrSBr using magnetotransport characterization and first-principles calculations. We reveal an odd–even layer effect of interlayer spin reorientation, which originates from the competitions among interlayer exchange, magnetic anisotropy energy, and extra Zeeman energy of uncompensated magnetization. Furthermore, we quantitatively constructed the layer-dependent magnetic phase diagram with the help of a linear-chain model. Our work uncovers the layer-dependent interlayer antiferromagnetic spin reorientation engineered by magnetic field in the air-stable semiconductor. (DOI: 10.1021/acsnano.2c01151)
Rui Xu, Yingzhuo Lun, Lan Meng, Fei Pang, Yuhao Pan, Zhiyue Zheng, Le Lei, Sabir Hussain, Yanjun Li, Yasuhiro Sugawara, Jiawang Hong, Wei Ji & Zhihai Cheng*
Abstract
Strain engineering plays a crucial role in controlling the physical properties of two-dimensional (2D) materials. However, the mechanical behavior of stressed 2D crystals has not been fully understood. In this study, the fracture behavior and accompanying properties of a strained single-crystal monolayer WS2 of submicron scale were investigated using a theoretical–experimental joint study. After thermal strain, the WS2 monolayer was split into different forms by several cracks, with the cause of the crack formation being studied using finite element analysis (FEA). The cracks were initiated from the vertex of the nucleation center, extending along the stronger von Mises stress isolines and terminating at the edges of the monolayers. Within the separate sections, ripple regions were observed, forming several typical nanopatterns. The band gap, frictional, viscosity, and elasticity characteristics of the different strain regions were also investigated. The nanopattern should enable flexibility in the design of more sophisticated devices based on 2D materials.
Yutian Yang, Yingying Wang, Jingsi Qiao, Weiwei Zhao, Yuanfang Yu, Shaopeng Feng, Xuhong An, Jialin Zhang, Wei Ji, Xinran Wang, Junpeng Lu & Zhenhua Ni
Abstract
The functionality of 2D molecular crystal-based devices crucially depends on their intrinsic properties, such as molecular energy levels, light absorption efficiency, and dielectric permittivity, which are highly sensitive to molecular aggregation. Here, it is demonstrated that the dielectric permittivity of the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) molecular crystals on monolayer WS2 substrates can be tuned from 4.62 in the wetting layer to 2.25 in the second layer. Its origin lies in the different molecular orientations in the wetting layer (lying-down) and in the subsequently stacked layers (standing-up), which lead to a positive Coulomb coupling (JCoup) value (H-aggregation) and a negative JCoup value (J-aggregation), respectively. Polarized optical contrast spectroscopy reveals that the permittivity of C8-BTBT is anisotropic, and its direction is related to the underlying substrate. The study offers guidelines for future manipulation of the permittivity of 2D molecular crystals, which may promote their applications toward various electronic and optoelectronic devices.
