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Electronic contacts to two-dimensional (2D) semiconductors, e.g., MoS2, of both n- and p-type, are important for complementary metal-oxide–semiconductor logic circuitry. Here, via systematic first-principles density-functional theory calculations, we report that both n- and p-type ohmic contact to MoS2 can be obtained via different surfaces of the same material, the binary compound covellite (CuS). The weak metallicity is helpful to suppress the metal-induced gap states and hence suppress the Fermi-level pinning effect. Importantly, the work functions of different CuS surfaces varies a lot from 3.8 eV to 5.8 eV. The higher work function F(Cu–S) surface forms a p-type contact to MoS2, and the p-type Schottky barrier height (SBH) can be reduced by increasing the layer number of the MoS2. The origin of the p-type SBH reduction can be attributed to quasi-bonding both at the F(Cu–S)/MoS2 interface and between MoS2 layers, which synergistically shifts the valence band edge up. Due to the large work function variation of CuS surfaces and interface quasi-bonding, p-type ohmic contact to monolayer MoS2 can be obtained with the P(S) surface. Additionally, the P(Cu)/monolayer MoS2 junction forms an n-type ohmic contact because of the large work function variation. The widely tunable SBH and contact types of the binary compound CuS/MoS2 junctions make them promising for high-efficiency electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross‐conjugated π‐electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non‐contact atomic force microscopy, and time‐of‐flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction. [ABSTRACT FROM AUTHOR]

Recent studies have revealed that the interlayer interaction in two-dimensional (2D) layered materials is not simply of van der Waals character but could coexist with quasi-bonding character. Herein, we classify the interlayer quasi-bonding interactions into two main categories (I: homo-occupancy interaction; II: hetero-occupancy interaction) according to the occupancy of the involved energy bands near the Fermi level. We then investigate the quasi-bonding-interaction-induced band structure evolution of several representative 2D materials based on density functional theory calculations. Further calculations confirm that this classification is applicable to generic 2D layered materials and provide a unified understanding of the total strength of interlayer interaction, which is a synergetic effect of the van der Waals attraction and the quasi-bonding interaction. The latter is stabilizing in main category II and destabilizing in main category I. Thus, the total interlayer interaction strength is relatively stronger in category II and weaker in category I.

Ladder phenylenes (LPs) composed of alternating fused benzene and cyclobutadiene rings have been synthesized in solution with a maximum length no longer than five units. Longer polymeric LPs have not been obtained so far because of their poor stability and insolubility. Here, we report the synthesis of linear LP chains on the Au(111) surface via dehalogenative [2+2] cycloaddition, in which the steric hindrance of the methyl groups in the 1,2,4,5-tetrabromo-3,6-dimethylbenzene precursor improves the chemoselectivity as well as the orientation orderliness. By combining scanning tunneling microscopy and noncontact atomic force microscopy, we determined the atomic structure and the electronic properties of the LP chains on the metallic substrate and NaCl/Au(111). The tunneling spectroscopy measurements revealed the charged state of chains on the NaCl layer, and this finding is supported by density functional theory calculations, which predict an indirect bandgap and antiferromagnetism in the polymeric LP chains.