Jialei Miao#, Linlu Wu#, Zheng Bian, Qinghai Zhu, Tianjiao Zhang, Xin Pan, Jiayang Hu, Wei Xu, Yeliang Wang, Yang Xu, Bin Yu, Wei Ji, Xiaowei Zhang*, Jingsi Qiao*, Paolo Samorì*, and Yuda Zhao*
Abstract
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 MoS2, 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 MoS2 FETs can be greatly reduced by ∼12 times after molecular functionalization. The Schottky barrier of 44 meV is achieved on monolayer MoS2 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.
Emerging functionalities in two-dimensional materials, such as ferromagnetism,superconductivity and ferroelectricity, open new avenues for promising nanoelectronic applications.Here, we report the discovery of intrinsic in-plane room-temperature ferroelectricity in two-dimensional Bi2TeO5 grown by chemical vapor deposition, where spontaneous polarization originates from Bi column displacements. We found an intercalated buffer layer consist ofmixed
Bi/Te column as 180° domain wall which enables facile polarized domain engineering, including continuously tunable domain width by pinning different concentration of buffer layers, and even ferroelectric-antiferroelectric phase transition when the polarization unit is pinned down to single atomic column. More interestingly, the intercalated Bi/Te buffer layer can interconvert to polarized Bi columns which end up with series terraced domain walls and unusual fan-shaped ferroelectric domain. The buffer layer induced size and shape tunable ferroelectric domain in two-dimensional Bi2TeO5 offer insights into the manipulation of functionalities in van der Waals materials for future nanoelectronics.
Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. Though the recently discovered Au-enhanced exfoliation technique provides an effective strategy for the preparation of large-scale 2D crystals, the high cost of gold hinders this method from being widely adopted in industrial applications. In addition, direct Au contact could significantly quench photoluminescence (PL) emission in 2D semiconductors. It is therefore crucial to find alternative metals that can replace gold to achieve efficient exfoliation of 2D materials. Here, the authors present a one-step Ag-assisted method that can efficiently exfoliate many large-area 2D monolayers, where the yield ratio is comparable to Au-enhanced exfoliation method. Differing from Au film, however, the surface roughness of as-prepared Ag films on SiO2/Si substrate is much higher, which facilitates the generation of surface plasmons resulting from the nanostructures formed on the rough Ag surface. More interestingly, the strong coupling between 2D semiconductor crystals (e.g., MoS2, MoSe2) and Ag film leads to a unique PL enhancement that has not been observed in other mechanical exfoliation techniques, which can be mainly attributed to enhanced light-matter interaction as a result of extended propagation of surface plasmonic polariton (SPP). This work provides a lower-cost and universal Ag-assisted exfoliation method, while at the same time offering enhanced SPP-matter interactions. DOI:10.1002/advs.202204247
Deping Guo#, Pengjie Guo#, Shijing Tan, Min Feng, Limin Cao, Zheng-Xin Liu*, Kai Liu*,
Zhong-Yi Lu, Wei Ji*
Abstract
Dirac nodal-line semimetals (DNLSMs) host novel quasiparticle excitations and intriguing transport properties, which are, however, easily perturbed under strong spin-orbit coupling (SOC), especially in low-dimensions. Two-dimensional (2D) layers have numerous advantages and are under continuous development; however, 2D-DNLSMs resistant to SOC are yet to be discovered. Here, we report the C_2v×Z_2^T little co-group, a non-symmorphic symmetry we found in 2D, guarantees a robust 2D-DNLSM against SOC, which could be imposed in three-atomic-layer (3-AL) Bismuth (the brick phase, a novel Bi allotrope) and other layered materials. Intriguingly, (4n+2) valence electrons fill the electronic bands in 3-AL Bi, such that the nodal line passes the Fermi level where other low-energy states are gapped, allowing feasible observation of DNLSM-induced phenomena without interference from other bands in future transport measurements. Thus, our study demonstrates an unprecedented category of layered materials, allowing for the exploration of nearly isolated DNL states in 2D.
Mismatched lattice constants at a van der Waals epitaxy interface often introduce in-plane strains to the lattice of the epitaxial layer, termed epitaxy strain, wherein the strains do not follow the intralayer Poisson’s relation. In this study, we obtained the magnetic phase diagrams of CrSe2 and CrTe2 mono- and bilayers under epitaxy strain up to 8%, as predicted using density functional theory calculations. The magnetic phase diagrams indicate that the in-plane epitaxy strain manipulates either the intra- or interlayer magnetism. The in-plane strain varies the interlayer distance, defined using an interlayer Poisson’s ratio, which determines whether the interlayer magnetism follows a Bethe–Slater curve-like (BSC-like) or a reversed BSC-like behavior, depending on the in-plane magnetism. The tunability of the intralayer magnetism is a result of competing intralayer Cr–Cr superexchange interactions. A graphene substrate was introduced to examine the validity of our diagrams in practice. This study also afforded a tentative explanation on the previously reported magnetizations in CrSe2 and CrTe2 epitaxial mono- or bilayers under epitaxy strains, which had given rise to some controversy.
