Grain boundaries in two-dimensional (2D) semiconductors generally induce distorted band alignment and interfacial charge, which impair their electronic properties for device applications. Here, we report the improvement of band alignment at the grain boundaries of PtSe2, a 2D semiconductor, with selective adsorption of a presentative organic acceptor, tetracyanoquinodimethane (TCNQ). TCNQ molecules show selective adsorption at the PtSe2 grain boundary with strong interfacial charge. The adsorption of TCNQ distinctly improves the band alignment at the PtSe2 grain boundaries. With the charge transfer between the grain boundary and TCNQ, the local charge is inhibited, and the band bending at the grain boundary is suppressed, as revealed by the scanning tunneling microscopy and spectroscopy (STM/S) results. Our finding provides an effective method for the advancement of the band alignment at the grain boundary by functional molecules, improving the electronic properties of 2D semiconductors for their future applications.
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.
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.
Yanping Guo, Linlu Wu, Jinghao Deng, Linwei Zhou, Wei Jiang, Shuangzan Lu, Da Huo, Jiamin Ji, Yusong Bai, Xiaoyu Lin, Shunping Zhang, Hongxing Xu, Wei Ji & Chendong Zhang
Abstract
Semiconducting heterojunctions (HJs), comprised of atomically thin transition metal dichalcogenides (TMDs), have shown great potentials in electronic and optoelectronic applications. Organic/TMD hybrid bilayers hold enhanced pumping efficiency of interfacial excitons, tunable electronic structures and optical properties, and other superior advantages to these inorganic HJs. Here, we report a direct probe of the interfacial electronic structures of a crystalline monolayer (ML) perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA)/ML-WSe2 HJ using scanning tunneling microscopy, photoluminescence, and first-principle calculations. Strong PTCDA/WSe2 interfacial interactions lead to appreciable hybridization of the WSe2 conduction band with PTCDA unoccupied states, accompanying with a significant amount of PTCDA-to-WSe2 charge transfer (by 0.06 e/PTCDA). A type-II band alignment was directly determined with a valence band offset of ∼ 1.69 eV, and an apparent conduction band offset of ∼ 1.57 eV. Moreover, we found that the local stacking geometry at the HJ interface differentiates the hybridized interfacial states.
A confined electronic system can host a wide variety of fascinating electronic, magnetic, valleytronic and photonic phenomena due to its reduced symmetry and quantum confinement effect. For the recently emerging one-dimensional van der Waals (1D vdW) materials with electrons confined in 1D sub-units, an enormous variety of intriguing physical properties and functionalities can be expected. Here, we demonstrate the coexistence of giant linear/nonlinear optical anisotropy and high emission yield in fibrous red phosphorus (FRP), an exotic 1D vdW semiconductor with quasi-flat bands and a sizeable bandgap in the visible spectral range. The degree of photoluminescence (third-order nonlinear) anisotropy can reach 90% (86%), comparable to the best performance achieved so far. Meanwhile, the photoluminescence (third-harmonic generation) intensity in 1D vdW FRP is strong, with quantum efficiency (third-order susceptibility) four (three) times larger than that in the most well-known 2D vdW materials (e.g., MoS2). The concurrent realization of large linear/nonlinear optical anisotropy and emission intensity in 1D vdW FRP paves the way towards transforming the landscape of technological innovations in photonics and optoelectronics.
