Electronic Structure of Twisted Bilayers of Graphene/MoS₂ and MoS₂/MoS₂

Vertically stacked two-dimensional multilayer structures have become a promising prototype for functionalized nanodevices due to their wide range of tunable properties. In this paper we performed first-principles calculations to study the electronic structure of nontwisted and twisted bilayers of hybrid graphene/MoS₂ (Gr/MoS₂) and MoS₂/MoS₂. Both twisted bilayers of Gr/MoS₂ and MoS₂/MoS₂ show significant differences in band structures from the nontwisted ones with the appearance of the crossover between direct and indirect band gap and gap variation. More interestingly, the band structures of twisted Gr/MoS₂ with different rotation angles are very different from each other, while those of MoS₂/MoS₂ are very similar. The variation of band structure with rotation angle in Gr/MoS₂ is, indeed, originated from the misorientation-induced lattice strain and the sensitive band-strain dependence of MoS₂

Greatly Improved Methane Dehydrogenation via Ni Adsorbed Cu(100) Surface

Synthesizing large-area high-quality graphene at low temperature is crucial for graphene applications in electronics and spintronics. In this work, we demonstrate that adsorption of a single active metal atom into inactive matrix would remarkably improve the catalytic reactivity. Our first-principles calculations show that the reaction barrier of methane dehydrogenation is remarkably reduced from 1.76 eV on flat Cu (100) surface to 1.00 eV on a Ni atom adsorbed Cu (100) surface. Moreover, the adsorbed Ni atom is found to serve as the active reaction center, which might provide a possibility of manipulating the graphene nucleation position for controllable chemical vapor deposition growth. Additionally, different dehydrogenation behaviors are detected and well understood in terms of electronic structures involved in the reactions. This study shows the potential of synthesizing high-quality graphene at relatively low temperatures with the assistance of Ni adsorption on Cu foils, and it can be extended to other metal and substrates

Fully Electrically Controlled van der Waals Multiferroic Tunnel Junctions

The fully electrical control of the magnetic states in magnetic tunnel junctions is highly pursued for the development of the next generation of low-power and high-density information technology. However, achieving this functionality remains a formidable challenge at present. Here we propose an effective strategy by constructing a trilayer van der Waals multiferroic structure, consisting of CrI3-AgBiPSe6 and Cr2Ge2Te6-In2Se3, to achieve full-electrical control of multiferroic tunnel junctions. Within this structure, two different magnetic states of the magnetic bilayers (CrI3/Cr2Ge2Te6) can be modulated and switched in response to the polarization direction of the adjacent ferroelectric materials (AgBiPSe6/In2Se3). The intriguing magnetization reversal is mainly attributed to the polarization-field-induced band structure shift and interfacial charge transfer. On this basis, we further design two multiferroic tunnel junction devices, namely, graphene/CrI3-AgBiPSe6/graphene and graphene/Cr2Ge2Te6-In2Se3/graphene. In these devices, good interfacial Ohmic contacts are successfully obtained between the graphene electrode and the heterojunction, leading to an ultimate tunneling magnetoresistance of 9.3 × 106%. This study not only proposes a feasible strategy and identifies a promising candidate for achieving fully electrically controlled multiferroic tunnel junctions but also provides insights for designing other advanced spintronic devices

Theoretical Studies of Sandwich Molecular Wires with Europium and Boratacyclooctatetraene Ligand and the Structure on a H‑Ge(001)-2×1 Surface

The structural, electronic, and magnetic properties of two kinds of boron-doped europium cyclooctatetraene sandwich molecular wires (SMWs), [EuCOTB]_∞ and [Eu-COTB-Eu-COT]_∞ (Eu = europium, COT = cyclooctatetraene = C₈H₈, COTB = boratacyclooctatetraene), are investigated with spin-polarized density functional theory. Both SMWs are of high stability and ultrahigh magnetic moments, and the [Eu-COTB-Eu-COT]_∞ SMW even owns half-metallic characteristics. Our calculations further reveal that the [Eu-COTB-Eu-COT]_∞ SMW anchored on a semiconductor germanium surface is a quasi-half-metallic ferromagnet, and it can be tuned into full half-metallicity under a mild external electric field. The unveiled intriguing properties here suggest that the boron-doped europium cyclooctatetraene SMWs may be compelling candidates for future spintronics devices

Oxygen Intercalation of Graphene on Transition Metal Substrate: An Edge-Limited Mechanism

Oxygen intercalation has been proven to be an efficient experimental approach to decouple chemical vapor deposition grown graphene from metal substrate with mild damage, thereby enabling graphene transfer. However, the mechanism of oxygen intercalation and associated rate-limiting step are still unclear on the molecular level. Here, by using density functional theory, we evaluate the thermodynamics stability of graphene edge on transition metal surface in the context of oxygen and explore various reaction pathways and energy barriers, from which we can identify the key steps as well as the roles of metal passivated graphene edges during the oxygen intercalation. Our calculations suggest that in well-controlled experimental conditions, oxygen atoms can be easily intercalated through either zigzag or armchair graphene edges on metal surface, whereas the unwanted graphene oxidation etching can be suppressed. Oxygen intercalation is, thus, an efficient and low-damage way to decouple graphene from a metal substrate while it allows reusing metal substrate for graphene growth

Electronic and Optical Properties of Graphene Quantum Dots: The Role of Many-Body Effects

The electronic structure and optical properties of hexagonal armchair and zigzag-edged graphene quantum dots (GQDs) are investigated within the framework of many-body perturbation theory. Many-body effects are significant due to quantum confinement and reduced screening. The quasi-particle corrections and exciton binding energies can be several eV, much larger than those of other carbon allotropes with higher dimensionality. All the GQDs show similar absorption spectra when electron–hole interaction is included, with a prominent peak emerging below the absorption onset of the noninteracting spectrum. This peak is contributed by a pair of double-degenerate excited states originating from the transitions between degenerate frontier orbitals. The spin singlet–triplet splitting is closely related to the electron–hole overlap, which can be approximately measured by the overlap between frontier orbitals involved in the optical transitions. The strong many-body effects in GQDs should be of great importance in optoelectronic applications

Electronic and Optical Properties of Edge-Functionalized Graphene Quantum Dots and the Underlying Mechanism

We systematically investigate the electronic structure and optical properties of edge-functionalized graphene quantum dots (GQDs) utilizing density functional and many-particle perturbation theories. A mechanism based on the competition and collaboration between frontier orbital hybridization and charge transfer is proposed. The frontier orbital hybridization of the GQD moiety and functional group reduces the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), while the charge transfer from the GQD moiety to the functional group enlarges it. Contrarily, frontier orbital hybridization and charge transfer collaborate to shift down the energy of the first bright exciton, the former through activation of low-lying dark excitons and the latter via increased exciton binding energy. Functional groups containing a carbon–oxygen double bond (CO), namely, aldehyde (−CHO), ketone (−COCH₃), and carboxyl (−COOH), are more favorable for tailoring the electronic and optical properties of pristine GQD among all the functional groups investigated here. The amino group (−NH₂), although frequently employed in experiments, has a much weaker influence on electronic structure since the large charge transfer cancels out the effect of frontier orbital hybridization

Photoabsorption Tolerance of Intrinsic Point Defects and Oxidation in Black Phosphorus Quantum Dots

Black phosphorus quantum dots (BPQDs) exhibit excellent optical and photothermal properties and promising applications in optoelectronics and biomedicine. However, various intrinsic structural defects and oxidation are nearly unavoidable in preparation of BPQDs and how they affect the electronic and optical properties remains unclear. Here, by employing time-dependent density functional theory, we reveal that there are two types of photoabsorption in BPQDs for both point defects and oxidation. A close structure-absorption relation is unraveled: BPQDs are defect-tolerant and show excellent photoabsorption as long as the coordination number (CN) of defective P atoms is 3. By contrast, the unsaturated or oversaturated P atoms with CN ≠ 3 create in-gap-states (IGSs) and completely quench the optical absorption. An effective way to eliminate the IGSs and repair the photoabsorption of defective BPQDs via sufficient hydrogen passivation is further proposed

Molecular Self-Assembly on Two-Dimensional Atomic Crystals: Insights from Molecular Dynamics Simulations

van der Waals (vdW) epitaxy of ultrathin organic films on two-dimensional (2D) atomic crystals has become a sovereign area because of their unique advantages in organic electronic devices. However, the dynamic mechanism of the self-assembly remains elusive. Here, we visualize the nanoscale self-assembly of organic molecules on graphene and boron nitride monolayer from a disordered state to a 2D lattice via molecular dynamics simulation for the first time. It is revealed that the assembly toward 2D ordered structures is essentially the minimization of the molecule–molecule interaction, that is, the vdW interaction in nonpolar systems and the vdW and Coulomb interactions in polar systems that are the decisive factors for the formation of the 2D ordering. The role of the substrate is mainly governing the array orientation of the adsorbates. The mechanisms unveiled here are generally applicable to a broad class of organic thin films via vdW epitaxy

Searching for Highly Active Catalysts for Hydrogen Evolution Reaction Based on O‑Terminated MXenes through a Simple Descriptor

An efficient, earth-abundant, and low-cost catalyst for hydrogen evolution reaction (HER) is critical for sustainable hydrogen generation. In this work, we present a density-functional-theory-based screening among two-dimensional (2D) transition metal carbides (MXenes) with a fully O-terminated surface. The catalytic activity of 10 monometal carbides is first investigated, and Ti₂CO₂ and W₂CO₂ are found to be highly active catalysts for HER. Then, a volcano plot between the number of electron surface O atoms gains (<i>N</i>_e) and the absolute value of the free energy of hydrogen adsorption (Δ<i>G</i>_H) is established. A simple descriptor, <i>N</i>_e, is thus proposed to evaluate the HER performance of O-terminated MXenes. On this basis, TiVCO₂ is extracted with improved HER performance than Ti₂CO₂ and W₂CO₂ among 7 bimetal carbides. Our study provides new possibilities for cost-effective alternatives to Pt for HER, and, more importantly, develops a simple activity descriptor to efficiently search for highly active HER catalysts