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.
Zijing Zhao, Jian Zhou, Luhao Liu, Nanshu Liu, Jianqi Huang, Biao Zhang, Wei Li, Yi Zeng, Teng Zhang, Wei Ji, Teng Yang, Zhidong Zhang, Songlin Li & Yanglong Hou
Abstract
Two-dimensional (2D) magnetic materials have attracted significant attention for promising applications in energy-saving logic and robust memory devices. However, most 2D magnets discovered so far typically feature drawbacks for practical applications due to low critical temperatures. Herein, we synthesize ultrathin room-temperature (RT) magnetic Fe7Se8 nanoflakes via the space-confined chemical vapor deposition method. It is found that the appropriate supply and control of Se concentration in the reaction chamber is crucial for synthesizing high-quality nonstoichiometric Fe7Se8 nanoflakes. Cryogenic electrical and magnetic characterizations reveal the emergence of spin reorientation at ∼130 K and the survival of long-range magnetic ordering up to room temperature. The RT magnetic domain structures with different thicknesses are also uncovered by magnetic force microscopy. Moreover, theoretical calculations confirm the spin configuration and metallic band structure. The outstanding characteristics exhibited by Fe7Se8 nanoflakes, including RT magnetism, spin reorientation property, and good electrical conductivity, make them a potential candidate for RT spintronics.
Pingfan Gu, Yujia Sun, Cong Wang, Yuxuan Peng, Yaozheng Zhu, Xing Cheng, Kai Yuan, Chao Lyu, Xuelu Liu, Qinghai Tan, Qinghua Zhang, Lin Gu, Zhi Wang, Hanwen Wang, Zheng Han, Kenji Watanabe, Takashi Taniguchi, Jinbo Yang, Jun Zhang, Wei Ji, Ping-Heng Tan & Yu Ye
Abstract
Materials with a quasi-one-dimensional stripy magnetic order often exhibit low crystal and magnetic symmetries, thus allowing the presence of various energy coupling terms and giving rise to macroscopic interplay between spin, charge, and phonon. In this work, we performed optical, electrical and magnetic characterizations combined with first-principles calculations on a van der Waals antiferromagnetic insulator chromium oxychloride (CrOCl). We detected the subtle phase transition behaviors of exfoliated CrOCl under varying temperature and magnetic field and clarified its controversial spin structures. We found that the antiferromagnetism and its air stability persist down to few-layer samples, making it a promising candidate for future 2D spintronic devices. Additionally, we verified the magnetoelastic coupling effect in CrOCl, allowing for the potential manipulation of the magnetic states via electric field or strain. These virtues of CrOCl provide us with an ideal platform for fundamental research on spin-charge, spin-phonon coupling, and spin-interactions.
Jun Zhang, Linwei Zhou, Pengcheng Chen, Bingkai Yuan, Zhihai Cheng, Wei Ji & Xiaohui Qiu
Abstract
Catalytic bond cleavage and formation of transient intermediates on metal substrates play an essential role in surface synthesis and heterogeneous catalysis. Previous studies usually focus on the bond-breaking process, whereas the knowledge regarding the construction of dissociative moieties that lead to the final products is limited. Here, we investigate the facet-selective dissociation of dibenzotetrathiafulvalene (DBTTF) molecules on Cu(110) and Cu(100) surfaces using low-temperature scanning tunneling microscopy and first-principles calculations. Atomic resolution images enable the identification of various intermediates and reaction pathways on different facets. The dissociation of DBTTF molecules generated 1,2-BDT–Cu complex chains on Cu(110), while phenyl diradical superstructure islands were observed on Cu(100). The various chemical species found on different Cu facets were explained in the context of their formation energies related to the spatial inhomogeneity of surface electronic states. Our results address the effects of electronic and geometrical diversities on the surface mobility of intermediate products and subsequent on-surface reaction pathways.
