Hybrid Metal–Carbon 2D Materials

• Hybrid metal–carbon 2D materials are atomically thin systems that embed transition metals into carbon frameworks, yielding unique electronic and magnetic phenomena.
• They are synthesized via precise methods such as covalent embedding, MXene functionalization, and epitaxial growth to ensure ordered metal–carbon bonding.
• These materials offer practical applications in spintronics, energy storage, and flexible electronics through tunable band structures and emergent quantum phases.

Hybrid metal–carbon two-dimensional (2D) materials represent a class of atomically thin systems in which transition metal atoms or clusters are covalently or coordinatively integrated into extended carbon-based lattices, such as graphene or carbon frameworks. These materials exhibit complex interplays between electronic, magnetic, and chemical phenomena, enabled by the hybridization of carbon orbitals with metal d or sd states. Recent advances have outlined various synthetic methodologies, unique structural motifs, and emergent topological quantum phases, expanding the design space for functional nanomaterials in electronics, energy, and catalysis.

1. Structural Design Principles and Hybridization Schemes

Hybrid metal–carbon 2D materials typically employ strategies that integrate transition metals into carbon-rich matrices via covalent, coordination, or bond-centered mechanisms:

• sd₂ Graphene Model: Transition metal atoms (e.g., W, Co) form a hexagonal lattice, but instead of occupying the lattice points themselves, their sd hybridization (“sd₂” configuration) leads to bond-centered σ orbitals (Zhou et al., 2014). This transforms the underlying electronic connectivity, producing kagome-like band structures, where bond-centered hopping differentiates these systems from conventional sp²-carbon-based graphene.
• Covalent Metal Embedding: Synthesis protocols such as the three-stage method for cobalt-embedded graphene-like carbon (CoGLC) frameworks begin with metal–phthalocyanine intermediates, where cobalt is integrated at the molecular level. Controlled carbonization preserves metal–carbon (and often metal–nitrogen) linkages with long-range periodic order (Ryzhkova et al., 19 Oct 2025).
• Organic–Inorganic MXenes: Functionalization routes that covalently attach organic amido or imido ligands to transition metal carbides generate h-MXenes, expanding interlayer spacing and providing ordered interfaces reminiscent of self-assembled monolayers (SAMs) while maintaining the metallic backbone (Zhou et al., 2023).

2. Electronic Band Structure and Topological Phenomena

The electronic structure and topological phases in hybrid metal–carbon 2D materials arise from bond-centered hopping, strong spin–orbit coupling (SOC), and exchange-induced magnetism:

• Kagome Band Realization: In sd₂ graphene, bond-centered electronic hopping on the apparent hexagonal lattice mathematically maps to a tight-binding model (Eq. 1):

$$H = - t_1 \sum_{\langle ij \rangle, \sigma} c_{i\sigma}^\dagger c_{j\sigma} - t_2 \sum_{\langle\langle ij \rangle\rangle, \sigma} c_{i\sigma}^\dagger c_{j\sigma} - t_3 \sum_{\langle\langle\langle ij \rangle\rangle\rangle, \sigma} c_{i\sigma}^\dagger c_{j\sigma} + i\lambda \sum_{\langle ij \rangle} (\mathbf{E}_{ij} \times \mathbf{R}_{ij}) \cdot \mathbf{s}_{\alpha\beta} c_{i\alpha}^\dagger c_{j\beta} - M \sum_i c_{i\alpha}^\dagger \sigma_z^{\alpha\beta} c_{i\beta}$$

This leads to Dirac bands adjacent to nearly flat bands, characteristic of kagome physics (Zhou et al., 2014).

• Topological Quantum Anomalous Hall Effect: The interplay between the exchange field (M) and SOC (λ) gives rise to half-metallicity and the QAH state, evidenced by DFT-derived energy gaps (∼0.1–0.15 eV), Chern number $C = -1$, and quantized Hall conductance ($-e^2/h$), supporting robust chiral edge transport at room temperature (Zhou et al., 2014).
• Orbital Hybridization at Metal–Carbon Interfaces: In vertical junctions (e.g., Ni|Graphene|Ni), strong orbital overlap between Ni-d and C-p states results in significant density of states (DoS) enhancement at the Fermi level and formation of interface-induced states within bandgaps for semiconducting carbon frameworks (Bigeard et al., 5 Feb 2025).

3. Synthesis Approaches and Interface Engineering

Numerous synthetic routes have been developed for fabricating hybrid metal–carbon 2D materials, emphasizing control over atomic ordering and chemical coordination:

• Three-Stage Synthesis for CoGLC: Sequential thermal treatment (200 °C → 400 °C → 600 °C) transitions a cobalt–phthalocyanine polymer precursor into a carbonized, covalently metal-embedded graphene-like framework. Final products retain ordered Co–N/C arrangements and display distinctive XRD reflections at 5.6° and 7.9°, indicative of long-range metal ordering within the carbon layers (Ryzhkova et al., 19 Oct 2025).
• MXene Functionalization: Halide-terminated MXene surfaces (Ti₃C₂Br₂/Cl₂) are functionalized with deprotonated primary amines, forming amido and imido surface terminations. This expands interlayer spacing linearly with ligand length and enables ordered alkyl chain packing in analogy to SAMs, as confirmed by STEM, XRD, and NMR (Zhou et al., 2023).
• Epitaxial Metal Growth via Halogenated Templates: For sd₂ graphene, growth of W-atom hexagonal lattices on 1/3-monolayer Cl-covered Si(111) surfaces leverages strong chemical binding (2.4 Å bond length, adsorption energy –6.4 eV) and kinetic stability (barrier ~3.2 eV) (Zhou et al., 2014).

4. Functional Properties: Magnetic, Optical, and Transport Phenomena

The hybridization and atomic arrangements underpin unique electronic, magnetic, and optical properties:

• Magnetism and Spintronics: CoGLC features ordered cobalt atoms at 1.5 nm periodicity, generating localized magnetic moments and facilitating spin-dependent transport. The retention of pyridinic and pyrrolic nitrogen coordination enhances both electrical conductivity and catalytic efficiency (Ryzhkova et al., 19 Oct 2025).
• THz Optical Modulation: Graphene on hybrid metal–dielectric waveguides enables 100% THz modulation via electrical tuning of the Fermi level, exploiting maximized in-plane electric fields at the dielectric/air interface ($$\alpha_{wG} = -\frac{4\pi}{\lambda}\operatorname{Im}(n_\text{eff})$$). Multilayer graphene structures may reach gain conditions suitable for THz lasing, compatible with CMOS and quantum cascade laser architectures (Huang et al., 2018).
• Enhanced Electron and Ion Transport: Multi-walled carbon nanotube (MWCNT)/TiO₂ hybrids demonstrate improved conductivity and capacitance, with surface scan rates up to 89 mV/s (∼35× higher than non-hybrid electrodes), and current density quantified through $G_0 = 2e^2/h$ and $I = \int_S G\,dS$ (Marler, 2016).

5. Stability, Scalability, and Integration into Devices

Achieving practical device integration requires chemical and mechanical stability as well as scalable processability:

• Hydrolytic Resistance in h-MXenes: Amido/imido terminations confer superior resistance to hydrolysis in aqueous media compared to traditional MXenes (e.g., Ti₃C₂Tₓ), suppressing formation of deleterious TiO₂ phases under mild basic or heated conditions. This is attributed both to hydrophobic surface encapsulation and electronic passivation of Ti centers (Zhou et al., 2023).
• Surfactant-Free Ink Preparation: CoGLC can be exfoliated electrochemically into stable, surfactant-free inks (using tetrabutylammonium intercalation), which are suitable for printing conductive films with controlled resistivity and activation energy (~0.15 eV) (Ryzhkova et al., 19 Oct 2025).
• Epitaxial Templating for Lattice Ordering: Utilizing halogenated semiconductor surfaces for the ordered growth of transition metal lattices (e.g., Cl/Si(111) for sd₂ graphene) ensures both thermodynamic and kinetic stabilization of the hybrid 2D material, enabling scalable fabrication for electronics (Zhou et al., 2014).

6. Applications and Future Prospects

Hybrid metal–carbon 2D materials are positioned for advanced applications and further exploration:

• Spintronics: Ordered metal embedding in carbon matrices (e.g., Co in CoGLC) provides robust platforms for spin transport and manipulation, with periodicity enabling magnetic superlattice phenomena (Ryzhkova et al., 19 Oct 2025).
• Energy Storage and Catalysis: h-MXenes and MWCNT/TiO₂ hybrids improve charge storage dynamics and electrocatalytic activity in supercapacitors, batteries, and water splitting, benefiting from tailored surface chemistry and metal–carbon synergy (Marler, 2016, Zhou et al., 2023).
• Topological Electronics: Room-temperature QAH effect in sd₂ graphene (gap ~0.1 eV) enables quantized, dissipationless edge transport for spintronic and quantum device architectures (Zhou et al., 2014).
• Flexible Electronics: High processability into printable inks (CoGLC) and stability under operational conditions expand the utility of these materials for flexible, stretchable circuits and sensors (Ryzhkova et al., 19 Oct 2025).
• This suggests that ongoing innovations in synthesis, interface engineering, and atomic ordering will delineate the landscape for next-generation multifunctional nanodevices.

7. Comparative Analysis and Design Implications

Hybrid metal–carbon 2D materials differ fundamentally from purely carbon-based systems by virtue of their emergent quantum phases, tunable band structures, and functional interfaces:

System	Metal Species	Coordination/Ordering	Key Functionality
sd₂ Graphene	W	sd₂, bond-centered kagome	QAH effect, spin–orbit topology
CoGLC	Co	Covalent, 1.5 nm periodic	Flexible electronics, spintronics
h-MXene	Ti	Amido/imido, SAM ordering	Energy storage, hydrolytic stability
MWCNT/TiO₂ Hybrid	Ti	Coated nanotube network	Supercapacitance, charge transfer

Integrating transition metals into carbon matrices—via controlled hybridization schemes (e.g., sd₂ orbital physics, covalency, organic ligation)—enables access to properties not attainable in monoelemental 2D sheets such as pristine graphene. A plausible implication is that by rational design of lattice geometry, metal–carbon coordination, and functional surface groups, one can customize electronic, magnetic, and chemical behavior for specific device contexts.

In summary, hybrid metal–carbon 2D materials constitute an expansive domain in solid-state physics and nanochemistry, embodying tunable quantum phases, robust processability, and a wide spectrum of device functionalities. Continued research into precise synthesis, lattice engineering, and interfacial phenomena is poised to unlock their full potential across electronic, energy, and catalytic applications.