Locally Induced Spin States on Graphene by Chemical Attachment of Boron Atoms

The role of local orbital hybridization in band gap opening and magnetism induced by single-atom doping in graphene

In this study, we employ first-principles density functional theory calculations to investigate the impact of single-atom doping on the electronic band structure of graphene. Our results demonstrate that specific dopants induce a local transition in graphene's hybridization from sp2 to sp3, which plays a crucial role in the opening of the band gap and the emergence of magnetism. We also found that electron-donating dopants cause significant electron localization near the Fermi level, resulting in the formation of nearly flat band states. Furthermore, when a dopant contributes an extra electron to the graphene lattice, the localized flat-band electrons drive band splitting, induce spin polarization, and generate a net spin magnetic moment in the system. These findings offer fundamental insights into how single-atom doping modifies graphene's electronic and magnetic properties, highlighting its potential for spintronic applications and tunable electronic devices.

Boron-Doped Single-Molecule van der Waals Diode

Single-molecule diode was the first proposed device in molecular electronics. Despite the great efforts and advances over 50 years, the reported rectification ratios, the most critical parameter of a diode, remain moderate for the single-molecule diode. Herein, we report an approach to achieve a larger rectification ratio by adopting the combined strategies of p-type boron doping, the single-layer graphene nodes, and the van der Waals layer-by-layer architecture. Measured current-voltage curves showed one of the as-fabricated single-molecule diodes hit an unprecedented large rectification ratio of 457 at ±1 V. Break junction operations and spectroscopic measurements revealed the three-atom-thick configuration of the single-molecule diodes. With the experimental and theoretical calculation results, we demonstrated the doped boron atoms induced holes to redistribute the electron density, making the asymmetric coupling at positive and negative biases, and the van der Waals interaction promoted asymmetric coupling and significantly boosted diode performance.

Large-scale 2D heterostructures from hydrogen-bonded organic frameworks and graphene with distinct Dirac and flat bands

The current strategies for building 2D organic-inorganic heterojunctions involve mostly wet-chemistry processes or exfoliation and transfer, leading to interface contaminations, poor crystallizing, or limited size. Here we show a bottom-up procedure to fabricate 2D large-scale heterostructure with clean interface and highly-crystalline sheets. As a prototypical example, a well-ordered hydrogen-bonded organic framework is self-assembled on the highly-oriented-pyrolytic-graphite substrate. The organic framework adopts a honeycomb lattice with faulted/unfaulted halves in a unit cell, resemble to molecular "graphene". Interestingly, the topmost layer of substrate is self-lifted by organic framework via strong interlayer coupling, to form effectively a floating organic framework/graphene heterostructure. The individual layer of heterostructure inherits its intrinsic property, exhibiting distinct Dirac bands of graphene and narrow bands of organic framework. Our results demonstrate a promising approach to fabricate 2D organic-inorganic heterostructure with large-scale uniformity and highly-crystalline via the self-lifting effect, which is generally applicable to most of van der Waals materials.

Enhancing the energetic and magnetic stability of atomic hydrogen chemisorbed on graphene using (non)compensated B-N pairs

In this pioneering study for identifying atomic scale magnetic moment, a single hydrogen atom chemisorbed on pristine graphene exhibits distinct spin polarization. Using first-principles calculations and analyses, we demonstrate that the binding between a H adsorbate and a C substrate is substantially enhanced via compensated B-N pairs embedded into graphene. Surprisingly, the interaction can be further enhanced via non-compensated B-N pair doping. Our established prototype of orbital intercoupling between H 1s and hybridized pz of gapped band edges gives an insight into the enhancing mechanism. For compensated B-N doping, the conduction band minimum (CBM) is pushed upward, which induces stronger interaction between the H 1s and hybridized pz orbitals of the CBM. For non-compensated B-N doping, the orbital interaction occurs between H 1s and hybridized pz orbitals of valence band maximum, thus further lowering the resulting bonding energy due to the enlarged gap. This significantly enhanced interaction between H and C atoms agrees well with the results of charge localization at the gapped band edges. More importantly, the corresponding magnetic moments can be well maintained or even enhanced in both doping; here, one more H atom is needed for non-compensated doping, where its electron occupies the empty CBM. Our findings might provide an effective and practical way to enhance the energetic and magnetic stability of atomic scale magnetic moment on graphene and extensively expand the conception of non-compensated doping.

Localized magnetic moment induced by boron adatoms chemisorbed on graphene

Inducing local spin-polarization in pristine graphene is highly desirable and recent experiment shows that boron adatom chemical attachment to graphene exhibits local high spin state. Using hybrid exchange-correlation functional, we show that boron (B) monomer chemisorbed on the bridge site of graphene is energically favorable, and indeed induces a weak local spin-polarization ∼0.56μB. The localized magnetic moment can be attributed to the charge transfer from boron atom to graphene, resulting in local spin charge dominantly surrounding to the adsorbed B and neighboring carbon (C) atoms. We also surprisingly find that boron dimer can even much more stable upright anchor the same site of graphene, giving rise to sizable spin magnetic moment 2.00μB. Although the apparent spin state remains mainly contributed by Bpand Cporbitals as the case of boron monomer, the delicate and substantial charge transfer of theintra-dimerplays a fundamental role in producing such sizable local spin-polarization. We employed various van der Waals corrections to check and confirm the validity of appeared local spin-polarization. In terms of the almost identical simulated scanning tunneling microscope between boron monomer and dimer, we might tend to support the fact that boron dimer can also be chemisorbed on graphene with much larger and stable localized spin magnetic moment.

Steering Large Magnetic Exchange Coupling in Nanographenes near the Closed-Shell to Open-Shell Transition

The design of open-shell carbon-based nanomaterials is at the vanguard of materials science, steered by their beneficial magnetic properties like weaker spin-orbit coupling than that of transition metal atoms and larger spin delocalization, which are of potential relevance for future spintronics and quantum technologies. A key parameter in magnetic materials is the magnetic exchange coupling (MEC) between unpaired spins, which should be large enough to allow device operation at practical temperatures. In this work, we theoretically and experimentally explore three distinct families of nanographenes (NGs) (A, B, and C) featuring majority zigzag peripheries. Through many-body calculations, we identify a transition from a closed-shell ground state to an open-shell ground state upon an increase of the molecular size. Our predictions indicate that the largest MEC for open-shell NGs occurs in proximity to the transition between closed-shell and open-shell states. Such predictions are corroborated by the on-surface syntheses and structural, electronic, and magnetic characterizations of three NGs (A[3,5], B[4,5], and C[4,3]), which are the smallest open-shell systems in their respective chemical families and are thus located the closest to the transition boundary. Notably, two of the NGs (B[4,5] and C[4,3]) feature record values of MEC (close to 200 meV) measured on the Au(111) surface. Our strategy for maximizing the MEC provides perspectives for designing carbon nanomaterials with robust magnetic ground states.

Giant g-factor in Self-Intercalated 2D TaS2

Central to the application of spintronic devices is the ability to manipulate spins by electric and magnetic fields, which relies on a large Landé g-factor. The self-intercalation of layered transitional metal dichalcogenides with native metal atoms can serve as a new strategy to enhance the g-factor by inducing ferromagnetic instability in the system via interlayer charge transfer. Here, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are performed to extract the g-factor and characterize the electronic structure of the self-intercalated phase of 2H-TaS₂ . In Ta₇ S₁₂ , a sharp density of states (DOS) peak due to the Ta intercalant appears at the Fermi level, which satisfies the Stoner criteria for spontaneous ferromagnetism, leading to spin split states. The DOS peak shows sensitivity to magnetic field up to 1.85 mV T^-1 , equivalent to an effective g-factor of ≈77. This work establishes self-intercalation as an approach for tuning the g-factor.

Spin Polarization-Induced Facile Dioxygen Activation in Boron-Doped Graphitic Carbon Nitride

Dioxygen (O₂) activation is a vital step in many oxidation reactions, and a graphitic carbon nitride (g-C₃N₄) sheet is known as a famous semiconductor catalytic material. Here, we report that the atomic boron (B)-doped g-C₃N₄ (B/g-C₃N₄) can be used as a highly efficient catalyst for O₂ activation. Our first-principles results show that O₂ can be easily chemisorbed at the B site and thus can be highly activated, featured by an elongated O-O bond (∼1.52 Å). Interestingly, the O-O cleavage is almost barrier free at room temperatures, independent of the doping concentration. It is revealed that the B atom can induce considerable spin polarization on B/g-C₃N₄, which accounts for O₂ activation. The doping concentration determines the coupling configuration of net-spin and thus the magnitude of the magnetism. However, the distribution of net-spin at the active site is independent of the doping concentration, giving rise to the doping concentration-independent catalytic capacity. The unique monolayer geometry and the existing multiple active sites may facilitate the adsorption and activation of O₂ from two sides, and the newly generated surface oxygen-containing groups can catalyze the oxidation coupling of methane to ethane. The present findings pave a new way to design g-C₃N₄-based metal-free catalysts for oxidation reactions.

In Situ Observation of Stepwise C-H Bond Scission: Deciphering the Catalytic Selectivity of Ethylbenzene-to-Styrene Conversion on TiO2

The conversion of light alkanes to olefins is crucial to the chemical industry. The quest for improved catalytic performance for this conversion is motivated by current drawbacks including: expensive noble metal catalysts, poor conversion, low selectivity, and fast decay of efficiency. The in situ visualization of complex catalysis at the atomic level is therefore a major advance in the rational framework upon building the future catalysts. Herein, the catalytic C-H bond activations of ethylbenzene on TiO₂(110)-(1 × 1) were explored with high-resolution scanning tunneling microscopy and first-principles calculations. We report that the first C-H bond scission is a two-step process that can be triggered by either heat or ultraviolet light at 80 K, with near 100% selectivity of β-CH bond cleavage. This work provides fundamental understanding of C-H bonds cleavage of ethylbenzene on metal oxides, and it may promote the design of new catalysts for selective styrene production under mild conditions.

Preparing dangling bonds by nanoholes on graphene oxide nanosheets and their enhanced magnetism

The effects of dangling bonds on the magnetic properties of graphene oxide (GO) were studied experimentally by creating nanoholes on GO nanosheets. GO with more nanoholes (MHGO) and less nanoholes (LHGO) on graphene oxide nanosheets were synthesized. Results showed that nanoholes brought new dangling bonds for GO and the increase of the dangling bonds on GO could be adjusted by the amounts of the nanoholes on GO. The magnetism of GO was enhanced with increased density of nanoholes on GO (MHGO > LHGO > GO). Furthermore, the increased dangling bonds induced magnetic coupling between the spin units and so converted paramagnetism GO to ferromagnetism (MHGO, LHGO). The easy generation and adjustment of GO dangling bonds by nanoholes on GO nanosheets will promote the applications of GO.

The effects of dangling bonds on the magnetic properties of graphene oxide (GO) were studied experimentally by creating nanoholes on GO nanosheets.