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Register a new userChemical trends of deep levels in van der Waals semiconductors
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Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels and their chemical trends in the bandgap of MoS2, WS2 and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different hosts, explaining a persistent photoconductivity above 400K.
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Electrical control of magnetism in a two-dimensional (2D) semiconductor is of great interest for emerging nanoscale low-dissipation spintronic devices. Here, we propose a general approach of tuning magnetic coupling and anisotropy of a van der Waals (vdW) 2D magnetic semiconductor via a built-in electric field generated by the adsorption of superatomic ions. Using first-principles calculations, we predict a significant enhancement of ferromagnetic (FM) coupling and a great change of magnetic anisotropy in 2D semiconductors when they are sandwiched between superatomic cations and anions. The magnetic coupling is directly affected by the built-in electric field, which lifts the energy levels of mediated ligands orbitals and enhances the super-exchange interactions. These findings will be of interest for ionic gating controlled ferromagnets and magnetoelectronics based on vdW 2D semiconductors.
We performed first-principles calculations aimed to investigate the role of an heteroatom like N in the chemical and the long-range van der Waals (vdW) interactions for a flat adsorption of several pi-conjugated molecules on the Cu(110) surface. Our study reveals that the alignment of the molecular orbitals at adsorbate-substrate interface depends on the number of heteroatoms. As a direct consequence, the molecule-surface vdW interactions involve not only pi-like orbitals which are perpendicular to the molecular plane but also sigma-like orbitals delocalized in the molecular plane.
We investigated low-frequency current fluctuations, i.e. noise, in the quasi-two-dimensional (2D) van der Waals antiferromagnetic semiconductor FePS3 with the electronic bandgap of 1.5 eV. The electrical and noise characteristics of the p-type, highly resistive, thin films of FePS3 were measured at different temperatures. The noise spectral density was of the 1/f - type over most of the examined temperature range but revealed well-defined Lorentzian bulges, and increased strongly near the Neel temperature of 118 K (f is the frequency). Intriguingly, the noise spectral density attained its minimum at temperature T~200 K, which was attributed to an interplay of two opposite trends in noise scaling - one for semiconductors and another for materials with the phase transitions. The Lorentzian corner frequencies revealed unusual dependence on temperature and bias voltage, suggesting that their origin is different from the generation - recombination noise in conventional semiconductors. The obtained results are important for proposed applications of antiferromagnetic semiconductors in spintronic devices. They also attest to the power of the noise spectroscopy for monitoring various phase transitions.
Recent technical progress demonstrates the possibility of stacking together virtually any combination of atomically thin crystals of van der Waals bonded compounds to form new types of heterostructures and interfaces. As a result, there is the need to understand at a quantitative level how the interfacial properties are determined by the properties of the constituent 2D materials. We address this problem by studying the transport and optoelectronic response of two different interfaces based on transition-metal dichalcogenide monolayers, namely WSe2-MoSe2 and WSe2-MoS2. By exploiting the spectroscopic capabilities of ionic liquid gated transistors, we show how the conduction and valence bands of the individual monolayers determine the bands of the interface, and we establish quantitatively (directly from the measurements) the energetic alignment of the bands in the different materials as well as the magnitude of the interfacial band gap. Photoluminescence and photocurrent measurements allow us to conclude that the band gap of the WSe2-MoSe2 interface is direct in k space, whereas the gap of WSe2/MoS2 is indirect. For WSe2/MoSe2, we detect the light emitted from the decay of interlayer excitons and determine experimentally their binding energy using the values of the interfacial band gap extracted from transport measurements. The technique that we employed to reach this conclusion demonstrates a rather-general strategy for characterizing quantitatively the interfacial properties in terms of the properties of the constituent atomic layers. The results presented here further illustrate how van der Waals interfaces of two distinct 2D semiconducting materials are composite systems that truly behave as artificial semiconductors, the properties of which can be deterministically defined by the selection of the appropriate constituent semiconducting monolayers.
Most of the studied two-dimensional (2D) materials have been obtained by exfoliation of van der Waals crystals. Recently, there has been growing interest in fabricating synthetic 2D crystals which have no layered bulk analogues. These efforts have been focused mainly on the surface growth of molecules in high vacuum. Here, we report an approach to making 2D crystals of covalent solids by chemical conversion of van der Waals layers. As an example, we use 2D indium selenide (InSe) obtained by exfoliation and converted it by direct fluorination into indium fluoride (InF3), which has a non-layered, rhombohedral structure and therefore cannot be possibly obtained by exfoliation. The conversion of InSe into InF3 is found to be feasible for thicknesses down to three layers of InSe, and the obtained stable InF3 layers are doped with selenium. We study this new 2D material by optical, electron transport and Raman measurements and show that it is a semiconductor with a direct bandgap of 2.2 eV, exhibiting high optical transparency across the visible and infrared spectral ranges. We also demonstrate the scalability of our approach by chemical conversion of large-area, thin InSe laminates obtained by liquid exfoliation into InF3 films. The concept of chemical conversion of cleavable thin van der Waals crystals into covalently-bonded non-cleavable ones opens exciting prospects for synthesizing a wide variety of novel atomically thin covalent crystals.
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