🔬 materials science

A Concise Review of Recently Synthesized 2D Carbon Allotropes: Amorphous Carbon, Graphynes, Biphenylene and Fullerene Networks

This paper provides a concise review of recently experimentally realized 2D carbon allotropes—including graphynes, biphenylene networks, fullerene networks, and amorphous carbon—by analyzing their structures, synthesis methods, and the interplay between theoretical predictions and experimental findings while highlighting gaps for future investigation.

Original authors: Ricardo Paupitz, Alexandre F. Fonseca, Mizraim Bessa, Guilherme S. L. Fabris, William F. da Cunha, Leonardo D. Machado, Marcelo L. Pereira Junior, Luiz A. Ribeiro Junior, Douglas S. Galvão

Published 2026-09-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ricardo Paupitz, Alexandre F. Fonseca, Mizraim Bessa, Guilherme S. L. Fabris, William F. da Cunha, Leonardo D. Machado, Marcelo L. Pereira Junior, Luiz A. Ribeiro Junior, Douglas S. Galvão

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Carbon is the ultimate shape-shifter of the material world. It is the element that builds the backbone of life, yet it also forms the hardest substance on Earth and the softest pencil lead. This versatility comes from carbon's ability to link with itself in different patterns, creating distinct forms called allotropes. For decades, scientists have studied these forms, ranging from hollow soccer-ball-shaped molecules to long, tube-like structures. The most famous of these is graphene, a single layer of carbon atoms arranged in a perfect honeycomb pattern. Since its discovery, graphene has been celebrated for being incredibly strong, flexible, and conductive, making it a star candidate for future electronics. However, it has a significant limitation: it conducts electricity so well that it cannot be easily turned off, which is a problem for making the switches and logic gates that power computers and phones.

This limitation has driven researchers to look beyond the perfect honeycomb. They have begun exploring other ways to arrange carbon atoms in flat, two-dimensional sheets. By changing the pattern of connections, scientists can create materials that still possess the strength of graphene but also have a "switchable" quality, allowing them to act as semiconductors. The quest has moved from purely theoretical ideas to actual laboratory creations. A new review paper brings together the latest achievements in this field, highlighting several recently synthesized carbon sheets that are not just variations of graphene, but entirely new structures with their own unique personalities. These materials range from completely disordered networks to highly organized lattices with holes, offering a diverse toolkit for future technology.

The review begins by examining a material that defies the usual expectation that order is required for strength: monolayer amorphous carbon. Unlike the neat, repeating honeycomb of graphene, this material is a single layer of carbon atoms arranged in a chaotic, disordered fashion. Despite this lack of a crystal pattern, researchers found that it remains remarkably stable and strong. When scientists created this material using a laser-assisted chemical process at relatively low temperatures, they discovered it formed a continuous, wrinkle-free film that could last for over a year without degrading. While it is not as stiff as graphene, it is still tough enough to withstand significant stretching. Crucially, this disorder gives it a small electronic bandgap, a feature graphene lacks, which makes it a promising candidate for optoelectronic devices that need to control the flow of electricity.

Moving from chaos to order, the paper explores a family of materials known as graphynes. These are not just random arrangements but are built by inserting chains of carbon atoms between the rings of a honeycomb structure. Imagine taking a standard honeycomb and replacing some of the direct connections between the hexagons with short, linear bridges. This creates a sheet that is naturally porous, filled with tiny holes. Researchers have successfully synthesized several versions of this material, including graphdiyne, which uses two-atom chains, and even more complex forms with longer chains. These materials are not just theoretical curiosities; they have been grown on copper surfaces using specific chemical reactions. The resulting sheets are predicted to have excellent electrical properties and could be used for filtering gases or storing energy, thanks to their built-in porosity.

Another significant discovery involves biphenylene-based networks. These structures are built from a repeating unit that looks like two hexagons connected by a four-sided ring, creating a pattern of four-, six-, and eight-membered rings. This arrangement is chemically stable and has been synthesized on gold surfaces using a process that links polymer chains together. What makes this material particularly interesting is its ability to change its electrical nature. In its sheet form, it behaves like a metal, but when cut into narrow strips or modified, it can act as a semiconductor. This flexibility, combined with its mechanical strength and the presence of regularly spaced pores, suggests it could be useful for separating gases or creating advanced sensors. A related material, graphenylene, features even larger pores and has been synthesized from simple organic molecules, showing promise for filtering water and storing hydrogen.

The final frontier discussed in the review involves two-dimensional networks made from fullerene molecules. Fullerenes are the hollow, cage-like carbon molecules often compared to soccer balls. For years, scientists knew they could link these cages together into three-dimensional solids, but creating a flat, single-layer sheet of them was a major challenge. In a breakthrough reported recently, researchers managed to peel apart a bulk crystal to reveal a monolayer of these linked cages. This material, which can exist in different geometric phases, behaves like a semiconductor and shows unique optical properties. It has already been tested as a catalyst for splitting water into hydrogen and oxygen, a key process for clean energy. Furthermore, scientists have found ways to create nanopores in these sheets, turning them into membranes that allow organic solvents to pass through at incredible speeds.

The review concludes by emphasizing that while these materials represent a significant leap forward, the journey is far from over. Synthesizing these sheets in large quantities without defects remains a difficult task, and their long-term stability in real-world devices is still being tested. However, the fact that these diverse structures—from the disordered amorphous carbon to the precise fullerene networks—have been successfully created in the lab proves that the landscape of carbon materials is expanding rapidly. These new allotropes offer a rich playground for scientists to design the next generation of electronic, energy, and sensing technologies, moving well beyond the limitations of the original graphene.

Technical Summary: A Concise Review of Recently Synthesized 2D Carbon Allotropes

Problem Statement
While graphene, discovered in 2004, remains a foundational material with exceptional mechanical, thermal, and electrical properties, its intrinsic zero-bandgap nature limits its applicability in active electronic devices such as organic light-emitting diodes (OLEDs), photovoltaics (OPVs), and field-effect transistors (OFETs). The search for alternative two-dimensional (2D) carbon allotropes is driven by the need to retain graphene's desirable traits while introducing a tunable electronic bandgap and other distinct topological features. Furthermore, a disconnect often exists between theoretical predictions and experimental realization; experimental studies sometimes overlook prior theoretical contributions, and many theoretically proposed structures remain unexplored. This review addresses the gap by critically examining recently experimentally realized 2D carbon allotropes, analyzing the interplay between their synthesis, structural characteristics, and theoretical predictions.

Methodology
This work employs a critical review methodology, synthesizing data from recent experimental literature and corresponding theoretical studies. The authors categorize the field into four primary classes of recently synthesized 2D carbon allotropes:

Monolayer Amorphous Carbon (MAC): Disordered single-layer carbon networks.
Graphynes and Graphdiynes: Structures featuring acetylenic chains connecting aromatic rings.
Biphenylene-Based Networks (BPN): Lattices constructed from biphenylene repeating units.
Fullerene Networks: Monolayers formed by polymerized $C_{60}$ molecules.

For each category, the review details structural characteristics, synthesis routes (e.g., chemical vapor deposition, mechanochemical synthesis, surface-assisted coupling), and physical properties. The authors explicitly compare experimental findings with theoretical predictions, highlighting instances where experiments validated theory, where theory guided synthesis, and where discrepancies or overlooked contributions exist. The review also analyzes publication trends using Scopus database metrics to contextualize the growth of the field relative to graphene research.

Key Contributions and Results

Monolayer Amorphous Carbon (MAC):
Synthesized in 2019 via laser-assisted chemical vapor deposition (CVD) at low temperatures (~~200°C), MAC consists of a disordered network of carbon atoms lacking long-range crystalline order. Unlike graphene, MAC exhibits a small electronic bandgap (~~2.1 eV optical gap) and a Young's modulus of ~323 GPa (compared to graphene's ~881 GPa). Theoretical modeling using kinetic Monte Carlo and density functional theory (DFT) suggests that MAC's disordered topology allows for gradual strain accommodation and the formation of linear atomic chains upon rupture. MAC demonstrates environmental stability and potential for optoelectronic applications due to its bandgap and transparency.
Graphynes and Graphdiynes:
Graphynes (GY), proposed in 1987, consist of aromatic rings linked by acetylenic chains ( $C \equiv C$ ). While early synthesis efforts yielded only oligomers, significant progress has been made recently.
- Graphdiyne (GDY, $n=2$ ): Successfully synthesized in 2010 on copper foils via Glaser-Hay coupling. It is the most investigated member, showing high charge carrier mobility and intrinsic porosity.
- Graphyne ( $n=1$ ): The densest form ( $\gamma$ -GY) was recently synthesized using Sonogashira crosslinking reactions. While mechanochemical methods were initially reported for synthesizing $n=1$ GYs, the validity of these achievements was contested, as identical conditions yielded disordered structures lacking triple bonds between carbon atoms. Consequently, Sonogashira reaction methods are regarded as the most suitable approach to obtain high-quality $\gamma$ -GY samples.
- Graphtriyne and Graphtetrayne: Synthesis of these higher-order structures ( $n=3, 4$ ) has been reported but remains less developed.
  Theoretical predictions suggest GYs possess tunable bandgaps, negative thermal expansion, and potential for gas separation and battery anodes. However, the large porosity of these materials can limit certain mechanical properties.
Biphenylene-Based Networks (BPN):
These networks utilize biphenylene molecules (two hexagons connected by a 4-membered ring) as repeating units.
- BPN Lattice: Synthesized in 2021 via interpolymer dehydrofluorination on gold surfaces. It features a rectangular lattice of 4-, 6-, and 8-membered rings. Theoretically predicted to be metallic in sheet form but semiconducting when cut into nanoribbons, BPN shows promise for lithium-ion batteries and gas separation.
- Graphenylene (GP): A porous lattice with 4-, 6-, and 12-membered rings, synthesized via surface-assisted coupling of macrocycles or dehydration of 1,3,5-trihydroxybenzene. GP is an intrinsic semiconductor with a direct bandgap (<1 eV) and high electron mobility. Its large dodecahedral pores enable efficient gas diffusion (e.g., $H_2$ separation) and water desalination. Theoretical studies also identify GP as a high-order topological insulator, suggesting applications in quantum computing.
2D Fullerene Networks:
These structures involve the polymerization of $C_{60}$ molecules into monolayers.
- Phases: Two primary phases have been experimentally realized: quasi-hexagonal (qHP) and quasi-tetragonal (qTP).
- Synthesis: Early attempts (1990s) produced 3D or mixed-phase solids. A breakthrough in 2022 by Hou et al. enabled the synthesis of pure carbon 2D qHP-C60 monolayers. This process involved the synthesis of a bulk crystal via Mg-intercalation to link adjacent layers, followed by an organic cation slicing procedure to exfoliate the 2D layers from the bulk material. Recent advancements (2023–2025) include scalable electrochemical synthesis (up to 5 g/day) and the creation of nanoporous fullerene membranes via depolymerization.
- Properties: Theoretical DFT simulations predict a direct electronic band gap of 1.55 eV for the qHP phase, a value noted as consistent with experimental results. Both qHP and qTP networks exhibit anisotropic mechanical and optoelectronic properties and show promise as photocatalysts for water splitting and hydrogen separation.

Significance and Claims
The paper positions the field of 2D carbon allotropes as having entered a "golden new era," moving beyond graphene to a diverse family of materials with tunable electronic, mechanical, and topological properties. The authors claim that the convergence of experimental synthesis and theoretical prediction is accelerating the discovery of functional materials.

Key significance points highlighted include:

Diversity of Functionality: Unlike graphene, these new allotropes offer intrinsic bandgaps, porosity, and topological features (e.g., high-order topological insulators) suitable for specific applications in electronics, energy storage, catalysis, and sensing.
Bridging Theory and Experiment: The review emphasizes the necessity of integrating theoretical insights with experimental efforts, noting that successful synthesis often follows theoretical guidance (e.g., GNRs, BPNs) but also that experimental discoveries (e.g., MAC) can prompt new theoretical investigations.
Scalability and Stability: Recent advances in synthesis methods (e.g., gram-scale electrochemical production of fullerene networks) address previous bottlenecks regarding yield and scalability, suggesting these materials are transitioning from laboratory curiosities to viable technological candidates.

The authors conclude that while challenges regarding large-scale, defect-free synthesis and long-term stability remain, the expanding landscape of 2D carbon allotropes provides a rich platform for both fundamental research and emerging technologies. They identify several theoretically predicted structures that remain unexplored experimentally, suggesting future directions for synthesis-driven investigations.

More like this