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.
Kui Huang (黄逵), Zhenxian Li (李政贤), Deping Guo (郭的坪), Haifeng Yang (杨海峰), Yiwei Li (李一苇), Aiji Liang (梁爱基), Fan Wu (吴凡), Lixuan Xu (徐丽璇), Lexian Yang (杨乐仙), Wei Ji (季威), Yanfeng Guo (郭艳峰), Yulin Chen (陈宇林)* and Zhongkai Liu (柳仲楷)*
Abstract
As a van der Waals ferromagnet with high Curie temperature, Fe5–xGeTe2 has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe5–xGeTe2 crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight kz evolution. Interestingly, across the ferromagnetic transition, we observed the merging of two split bands above the Curie temperature, suggesting the band splitting due to the exchange interaction within the itinerant Stoner model. Our results provide important insights into the electronic and magnetic properties of Fe5–xGeTe2 and the understanding of magnetism in a two-dimensional ferromagnetic system.
Haohan Li, Mykola Telychko, Linwei Zhou, Zhi Chen, Xinnan Peng, Wei Ji, Jiong Lu & Kian Ping Loh
Abstract
Formation of a two-dimensional (2D) supramolecular self-assembly and a 2D organometallic framework derived from a brominated N-heterocyclic aromatic molecule (4Br-TAP) on Au(111) and Ag(111) substrates were studied using chemical bond-resolved scanning tunneling microscopy (STM) and noncontact atomic force microscopy (ncAFM) techniques combined with density functional theory (DFT) calculations. The 4Br-TAP-based 2D molecular framework on Au(111) is constructed by diverse Br···Br and Br···N noncovalent interactions, which are resolved with sub-angstrom resolution using combined STM and ncAFM imaging with a CO-functionalized tip and further quantified using DFT calculations. The distortion of molecular backbones, triggered by a highly nonuniform bonding environment, leads to lifting of the degeneracy of the intrinsic resonance structures of tetraazapyrene (TAP) moieties and emergence of two chiral Kekulé-like structures. In contrast, debromination of 4Br-TAP on Ag(111) leads to the formation of an ordered 2D organometallic framework linked by C–Ag–C bonds. Our results underpin the tremendous potential of the tip-functionalized ncAFM technique for microscopic identification of a complex interplay of intermolecular interactions and their associated impact on the molecular resonance structures.
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.
Quantum confinement has remarkable effects on the band structures and optoelectronic performance of semiconducting materials. The confinement of electronic states developed along van der Waals (vdW) gaps in transition metal dichalcogenides (TMDs) has unique advantages compared with those of artificial quantum wells. Here, we detected the quantized electronic states of few-layered MoS2 in real space using scanning tunneling microscope/spectroscopy. Combined with density-functional theory calculations, the quantized states were attributed to quantum-well states (QWSs), and the number of the states was strictly determined by the MoS2 layer thickness. We further regulated the QWSs of few-layered MoS2 by tuning the strength of interlayer hybridization through directly adjusting the interlayer distance. More importantly, substitutional defects in few-layered MoS2 were introduced to control the energy eigenvalues of the QWSs. Our work proves the existence of the interlayer electronic hybridization in conventional weakly coupled vdW interfaces, and provides a way to manipulate the electronic states of few-layered TMD through controlling interlayer hybridization. It also suggests potential applications of quantum-well materials in subband transitions, spin splitting, photoexcitation, and electronic devices.
