Electronic Structures of Germanene on MoS₂: Effect of Substrate and Molecular Adsorption

Germanene, a two-dimensional (2D) Dirac semimetal beyond graphene, has been recently synthesized on a nonmetallic substrate, which offers great opportunities for realization of germanene-based electronic devices. Understanding the effects of substrate and chemical modification on the electronic properties of germanene is thus crucial for tailoring this novel 2D material for future applications. Herein we investigate the structure, interlayer interaction, and electronic band structure of monolayer germanene supported on various transition metal dichalcogenide (TMD) substrates. A band gap of 38–57 meV can be opened by the TMD substrates due to breaking of lattice symmetry of the germanene sheet. An electron donor molecule, tetrathiafulvalene (TTF), is exploited to noncovalently functionalize the germanene on MoS₂ substrate. The electron transfer from TTF to germanene disturbs the Dirac cone of germanene and leads to an augment of the band gap up to 180 meV. Meanwhile, the charge carriers of the hybrid system are still mobile possessing small effective masses (≤0.16<i>m</i>₀). Applying a vertical electric field can increase the interface dipole of the hybrid system and further enhance the band gap up to 214 meV. These theoretical results provide an effective and reversible route for engineering the band gap and work function of germanene without severely affecting the transport properties of this material

Enhanced Intralayer Ferromagnetism in CrI₃ by Interfacial Super-Superexchange Interaction

Manipulating the interlayer exchange interaction in two-dimensional (2D) layered materials is crucial for achieving intrinsic long-range magnetic ordering for high-performance spintronic devices. In this work, we propose a general and experimentally feasible approach to enhance the ferromagnetism of a monolayer material in van der Waals (vdW) heterostructures by taking advantage of the interfacial super-superexchange interaction. As a proof of concept, we consider the CrI3/Fe3GeTe2 heterostructure with a strong Cr–I···Te–Fe super-superexchange interaction. Our first-principles calculations show that the interlayer distance and electronic coupling between CrI3 and Fe3GeTe2 sheets highly depend on their stacking geometry, exhibiting distinct weak and strong coupling regions. Specifically, a Cr–I–Te angle of ∼103° leads to the strongest interfacial coupling, robust ferromagnetism for the interlayer spin configuration, and enhanced Curie temperature of the CrI3 monolayer by nearly two fold

Possible Formation of Graphyne on Transition Metal Surfaces: A Competition with Graphene from the Chemical Potential Point of View

Graphyne (GY), a two-dimensional (2D) allotrope of carbon with mixed sp/sp² hybridization, is predicted to exist in many stable phases and has recently received great attention. However, it is energetically less stable than graphene and remains difficult to be synthesized in experiment to date. In this report, the possible environments for synthesis of graphyne on Ru(0001), Rh(111), and Pd(111) substrates are investigated by considering three typical phases of GY (α, β, and γ). Their structures, interactions with metal substrates, as well as thermodynamic stability are calculated using first-principles calculations. The chemical potential phase diagram of GYs and graphene on metal substrates is constructed. For all these substrates, the α phase of GY can form in the carbon-poor environment, while formation of graphene dominates in the carbon-rich condition

Oxidation Resistance of Monolayer Group-IV Monochalcogenides

Ridged, orthorhombic two-dimensional (2D) group-V elemental and group IV–VI compound analogues of phosphorene provide a versatile platform for nanoelectronics, optoelectronics, and clean energy. However, phosphorene is vulnerable to oxygen in ambient air, which is a major obstacle for its applications. Regarding this issue, here we explore the oxidation behavior of monolayer group-IV monochalcogenides (GeS, GeSe, SnS, and SnSe), in comparison to that of phosphorene and arsenene by first-principles calculations. We find superior oxidation resistance of the monolayer group-IV monochalcogenides, with activation energies for the chemisorption of O₂ on the 2D sheets in the range of 1.26–1.60 eV, about twice of the values of phosphorene and arsenene. The distinct oxidation behaviors of monolayer group-IV monochalcogenides and group-V phosphorene analogues originate from their different bond natures. Moreover, the chemisorption of a moderate amount of oxygen atoms does not severely deteriorate the electronic band structures of the monolayer group-IV monochalcogenides. These results shine light on the utilization of the monolayer group-IV monochalcogenides for next-generation 2D electronics and optoelectronics with high performance and stability

Nitrogen-Doped Graphene on Transition Metal Substrates as Efficient Bifunctional Catalysts for Oxygen Reduction and Oxygen Evolution Reactions

Composites of transition metal and carbon-based materials are promising bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and are widely used in rechargeable metal–air batteries. However, the mechanism of their enhanced bicatalytic activities remains elusive. Herein, we construct N-doped graphene supported by Co(111) and Fe(110) substrates as bifunctional catalysts for ORR and OER in alkaline media. First-principles calculations show that these heterostructures possess a large number of active sites for ORR and OER with overpotentials comparable to those of noble metal benchmark catalysts. The catalytic activity is modulated by the coupling strength between graphene and the metal substrates, as well as the charge distribution in the graphitic sheet, which is delicately mediated by N dopants. These theoretical results uncover the key parameters that govern the bicatalytic properties of hybrid materials and help prescribe the principles for designing multifunctional electrocatalysts of high performance

Structures and Magnetic Properties of MoS₂ Grain Boundaries with Antisite Defects

Monolayer molybdenum disulfide (MoS₂), a two-dimensional semiconductor, possesses extraordinary physical properties and holds great promise for electronics, optoelectronics, and optics. However, the synthetic MoS₂ samples usually comprise substantial structural defects, which greatly affect the device performance. Herein we comprehensively explore the atomic structures, energetic stability, and electronic and magnetic properties of grain boundaries (GBs) in monolayer MoS₂ as well as the GBs decorated by antisite defects by first-principles calculations. Eighteen types of GBs each carrying five kinds of antisite defects (a total of 108 defective systems) are constructed. The stability and magnetic properties of these defective monolayers are closely related to the type and number of homoelemental bonds. The GBs dominated by one type of homoelemental bond are ferromagnetic and have intrinsic magnetic moments up to 1.10 μ_B/nm. The GBs with equal number of defect rings that involve Mo–Mo and S–S bonds can exhibit antiferromagnetic behavior. Formation of antisite defects on the MoS₂ GBs is much more favored than that in perfect monolayer, and the antisite defects do not severely affect the magnetic properties of the GB systems. Our theoretical results provide vital guidance for modulating the magnetic properties of monolayer transition metal dichalcogenides by defect engineering

Interaction between Post-Graphene Group-IV Honeycomb Monolayers and Metal Substrates: Implication for Synthesis and Structure Control

Beyond graphene, other group IV monolayers with honeycomb lattice, including silicene, germanene, and stanene, have attracted much attention due to their peculiar physical properties and potential applications in future electronic devices. However, since sp³ hybridization is more favorable than sp² hybridization for Si, Ge, and Sn, these group IV monolayers have to be stabilized by metal surfaces during epitaxial synthesis. Using systematical first-principles calculations, here we investigate the interactions between these monolayers and various metal surfaces, i.e., Ag(111), Ir(111), Pt(111), Al(111), Au(111), and Cu(111). STM images, charge density difference, and partial density of states of these monolayer/metal systems have been calculated and discussed. In combination with the known experimental facts, we find that a moderate strength of interaction at 0.6–0.7 eV/atom is beneficial for the epitaxial growth of silicene and germanene without too much buckling or in-plane distortion. We further propose that the Al(111) substrate might be a good choice for synthesis of stanene with low-buckled structure

Optical Activity and Excitonic Characteristics of Chiral CdSe Quantum Dots

Introduction of chirality to colloidal semiconductor quantum dots (QDs) triggers a chiroptical effect. However, there remains a knowledge gap in the mechanism of chirality transfer and amplification from molecules to QDs. By time-dependent density functional theory calculations combined with a correlated electron–hole picture, we explored the chiroptical activity of CdSe QDs decorated with different chiral monocarboxylic acids from an excitonic perspective. Our calculations showed strong circular dichroism (CD) signals in the visible region for the chiral CdSe QDs. The excitonic states with large CD originate from QDs, while the chiral molecules break the orthogonality between electric and magnetic transition dipoles, which synergistically facilitates the prominent dissymmetric effect. The considered monocarboxylic acid chiral molecules all favor the bidentate adsorption configuration of the carboxyl group on the CdSe surface, endowing an identical CD signature but distinct excitonic characteristics. These findings are crucial for the regulation of chirality and excitons in semiconductor QDs to develop excitonic devices

Optical Activity and Excitonic Characteristics of Chiral CdSe Quantum Dots

Introduction of chirality to colloidal semiconductor quantum dots (QDs) triggers a chiroptical effect. However, there remains a knowledge gap in the mechanism of chirality transfer and amplification from molecules to QDs. By time-dependent density functional theory calculations combined with a correlated electron–hole picture, we explored the chiroptical activity of CdSe QDs decorated with different chiral monocarboxylic acids from an excitonic perspective. Our calculations showed strong circular dichroism (CD) signals in the visible region for the chiral CdSe QDs. The excitonic states with large CD originate from QDs, while the chiral molecules break the orthogonality between electric and magnetic transition dipoles, which synergistically facilitates the prominent dissymmetric effect. The considered monocarboxylic acid chiral molecules all favor the bidentate adsorption configuration of the carboxyl group on the CdSe surface, endowing an identical CD signature but distinct excitonic characteristics. These findings are crucial for the regulation of chirality and excitons in semiconductor QDs to develop excitonic devices

Atomic Structure and Dynamics of Defects in 2D MoS₂ Bilayers

We present a detailed atomic-level study of defects in bilayer MoS₂ using aberration-corrected transmission electron microscopy at an 80 kV accelerating voltage. Sulfur vacancies are found in both the top and bottom layers in 2H- and 3R-stacked MoS₂ bilayers. In 3R-stacked bilayers, sulfur vacancies can migrate between layers but more preferably reside in the (Mo–2S) column rather than the (2S) column, indicating more complex vacancy production and migration in the bilayer system. As the point vacancy number increases, aggregation into larger defect structures occurs, and this impacts the interlayer stacking. Competition between compression in one layer from the loss of S atoms and the van der Waals interlayer force causes much less structural deformations than those in the monolayer system. Sulfur vacancy lines neighboring in top and bottom layers introduce less strain compared to those staggered in the same layer. These results show how defect structures in multilayered two-dimensional materials differ from their monolayer form