Two-dimensional (2D) materials with the atomically thin thickness have attracted great interest in the post-Moore's Law era because of their tremendous potential to continue transistor downscaling and offered advances in device performance at the atomic limit. However, the metal−semiconductor contact is the bottleneck in field-effect transistors (FETs) integrating 2D semiconductors as channel materials. A robust and tunable doping method at the source and drain region of 2D transistors to minimize the contact resistance is highly sought after. Here we report a stable carrier doping method via the mild covalent grafting of maleimides on the surface of 2D transition metal dichalcogenides. The chemisorbed interaction contributes to the efficient carrier doping without degrading the high-performance carrier transport. Density functional theory results further illustrate that the molecular functionalization leads to the mild hybridization and the negligible impact on the conduction bands of monolayer MoS 2 , avoiding the random scattering from the dopants. Differently from reported molecular treatments, our strategy displays high thermal stability (above 300 °C) and it is compatible with micro/nano processing technology. The contact resistance of MoS 2 FETs can be greatly reduced by ∼12 times after molecular functionalization. The Schottky barrier of 44 meV is achieved on monolayer MoS 2 FETs, demonstrating efficient charge injection between metal and 2D semiconductor. The mild covalent functionalization of molecules on 2D semiconductors represents a powerful strategy to perform the carrier doping and the device optimization.
Conspectus Two-dimensional (2D) layered materials have atomically thin thickness and outstanding physical properties, attracting intensive research in past years. To realize the applications in (opto)electronic devices, the strategies to engineer the properties of 2D materials have been widely explored, including defect engineering, in-plane strain engineering, surface modification, etc. Besides the in-plane bonding, the out-of-plane interlayer interaction is another unique degree of freedom to engineer the properties of 2D materials. Different from the well-accepted weak van der Waals interactions, some recently discovered 2D material systems display strong interlayer interaction with “covalent-like quasi-bonding”. The unusually strong interlayer interaction gives rise to the dramatic evolution of physical properties with the layer number, including electronic band structure, carrier mobility, optical absorption, photoluminescence, and mechanical interlayer vibration. These results unambiguously demonstrate that the tuning of interlayer interaction via material/structure design is a powerful method to engineer 2D materials with desired physical properties for electronic and optoelectronic applications. In this Account, we focus on our recent progress in the discovery of 2D materials with strong interlayer interaction, the modulation of interlayer interaction, and the application of 2D materials with strong interlayer interaction in advanced (opto)electronics. We provide a quantitative criterion to determine the strength of interlayer interaction and discover several classes of 2D materials with strong interlayer interaction. By tuning the layer number, these 2D materials exhibit dramatic evolution of electronic, optical, and vibrational properties. The in-depth understanding of the physical origins of strong interlayer interaction provides a guideline to discover and design the unique interlayer properties in 2D materials. Furthermore, we discuss the strategies to modulate the interlayer interaction in 2D materials/heterostructures and demonstrate their promising applications in electronic and optoelectronic devices. The interlayer interaction in 2D materials can be a unique degree of freedom to modulate the physical properties of 2D material and promote the development of 2D (opto)electronic devices in the post-Moore era.
Different from monolayer inorganic semiconductors, such as transition metal dichalcogenides, monolayer organic semiconductors derived from perylene have attracted much attention because of their strong absorption and bright photoluminescence (PL). Pressure has proved to be an effective tool in probing the exciton behavior in monolayer semiconductors. Here, by studying the high-pressure behavior of purely J-aggregated monolayer organic semiconductors experimentally and theoretically, we find a red shift of PL spectra due to a decrease of band gap, which is consistent with fluorescent images taken under pressure. The PL center dominates the perylene group and the band edges are flat, indicating Frenkel exciton in the monolayer organic semiconductor under ambient conditions. With increasing pressure, the band edges become more dispersive, suggesting the exciton transform to Wannier–Mott exciton, which is commonly observed in inorganic semiconductors.
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