Patterned graphene-based carbon sheet is a tunable semiconductor and traps carbon monoxide

Dec 08, 2025

A new atomically precise carbon sheet combines nanoporous graphene and biphenylene stripes, offering controlled semiconducting behavior, tunable mechanics, and selective gas interaction for future electronic and sensing applications.

(Nanowerk Spotlight) Digital electronics rely on materials that can switch cleanly between a conducting state and a nonconducting state. Graphene, despite its excellent conductivity and stability, struggles with the second role. Its electrons move through a continuous range of energies without a bandgap, the small energy window that separates filled and empty states in a semiconductor. Without that gap, a transistor made from graphene cannot fully block current, which limits its usefulness in logic circuits and keeps two-dimensional carbon from behaving like a standard switchable semiconductor. The effort to engineer a bandgap in graphene has produced several strategies. Cutting graphene into nanoribbons can open a gap by confining electrons in a narrow strip. Adding foreign atoms or applying strain changes the local bonding and shifts electronic levels. Perforating the sheet with a regular pattern of nanometer-sized pores creates nanoporous graphene, which also modifies the band structure. A more radical idea is to change the ring pattern of the lattice itself. Graphene contains only six-membered rings. Theory suggests that inserting four- and eight-membered rings in a controlled way can yield new two-dimensional carbon networks with different bandgaps and strongly directional charge transport. On-surface synthesis has turned many of these ideas into atomically precise structures. In this approach, custom-designed molecules land on a metal surface and then couple and rearrange when heated. Chemists have used it to build armchair graphene nanoribbons and biphenylene nanoribbons whose width, edge structure, and ring pattern are set by the precursor design. That level of control makes it possible to imagine hybrid lattices that combine nanopores and biphenylene stripes in one continuous sheet. The difficulty is to achieve strict periodic order, since random defects or irregular connections would blur the very electronic features these designs aim to control. A study in Advanced Materials (“A Functional 2D Carbon Allotrope Combining Nanoporous Graphene and Biphenylene Segments”) addresses this challenge by constructing a two-dimensional carbon sheet that unites a graphene backbone, a regular lattice of nanopores, and parallel biphenylene stripes. The material behaves as a semiconductor with a tunable band structure, shows modified mechanical stiffness, and selectively traps carbon monoxide molecules in its pores while remaining stable in oxygen and in air. Structural models and DFT calculated band structures of 12-aGNR, 12-pGNR, NPG with a graphene-type fusion pattern, and NPG with a biphenylene-type fusion pattern. Bandgap values are indicated next to the red arrows denoting the direct or indirect transition. New bonds formed upon lateral fusion (resulting in 6- or 4- and 8-membered rings) are highlighted in red. The abbreviations used (e.g., VB, CB, etc.) to identify the main frontier valence and conduction bands are also included in the 12-aGNR band structure. (Image: Reprinted from DOI:10.1002/adma.202511706, CC BY) (click on image to enlarge) The design starts from an armchair graphene nanoribbon that is twelve carbon atoms wide. Calculations give this ribbon a bandgap of about 0.6 eV. The authors then model a porous version by removing rings of eighteen carbon atoms along the ribbon center to form a line of [18] annulene pores. The resulting porous ribbon, named 12-pGNR, has a calculated bandgap of 1.81 eV. The effective pore diameter, measured between hydrogen atoms at the pore edge, is about 3.7 Å. Each pore is only slightly wider than a benzene ring, yet the periodic row of such openings more than triples the bandgap. To realize this porous ribbon experimentally, the team synthesizes a precursor molecule, 7,10-dibromo-1,4-diphenyl-triphenylene, and deposits it onto a gold Au(111) surface under ultra-high vacuum at room temperature. Heating to 200 °C breaks the carbon-bromine bonds and links the molecules into chains through a surface-mediated coupling reaction. Heating further to 405 °C drives cyclodehydrogenation, which fuses the aromatic rings and flattens the structure into a conjugated framework. The product is a set of 12-pGNR ribbons with atomically precise armchair edges and a periodic row of [18] annulene pores. The spacing between neighboring pores is about 12.7 Å along the ribbon. Scanning tunneling microscopy and non-contact atomic force microscopy confirm the intended structure, and spectroscopy shows a widened bandgap consistent with the calculations. To turn ribbons into sheets, the researchers arrange the 12-pGNRs in dense arrays on Au(111) and anneal the surface at 550 °C. Under these conditions neighboring ribbons fuse laterally. High-resolution atomic force images reveal several distinct ways in which the ribbons can connect. Grouped by how the carbon rings join, these motifs give four kinds of nanoporous graphene networks. Two families use graphene-like junctions between pores and are referred to as NPG-G. Two use biphenylene-like junctions and are referred to as NPG-BP. In the graphene-junction case, the link between adjacent pores is a short chevron graphene nanoribbon segment. In the biphenylene-junction case, it is a chevron biphenylene nanoribbon segment that contains four-, six-, and eight-membered rings. The fully developed NPG-BP structure repeats this biphenylene segment between every pair of pores. The result is a continuous sheet with a graphene backbone, a regular pore lattice, and parallel biphenylene stripes. This arrangement defines a new two-dimensional carbon allotrope. It differs from graphdiyne and graphyne, which are also direct-bandgap carbon sheets based on sp- and sp²-hybridized carbon. Here the lattice remains purely sp², and the tuning elements are pores and non-hexagonal rings embedded in a graphene-type network. Electronic-structure calculations track how these design choices control conduction. As porous ribbons fuse into wider NPG-G or NPG-BP sheets, the bandgap decreases sharply when two ribbons first connect and then changes little as more ribbons are added and the system approaches a wide two-dimensional limit. For the NPG-G family, the extended material is a direct-bandgap semiconductor. The highest occupied and lowest unoccupied electronic states lie at the same point in the repeating lattice pattern, so light can promote electrons across the gap without changing their crystal momentum. For the NPG-BP family, the bandgap is smaller and indirect, with the band edges located at different points in momentum space. The calculations also identify where the states near the bandgap reside. They concentrate mainly on the chevron segments between pores. In NPG-G these states resemble those of an isolated chevron graphene nanoribbon. In NPG-BP they resemble those of an isolated chevron biphenylene nanoribbon. The pores couple these segments only weakly. This picture suggests a practical design rule. By choosing which chevron segment sits between pores, designers can decide whether the sheet has a direct or indirect bandgap and can tune the gap size over a useful energy range. Mechanical simulations provide a second axis of control. From computed stress-strain curves the authors extract the in-plane Young’s modulus, which measures how stiff a sheet is under tension. Both NPG-G and NPG-BP are less stiff than pristine graphene because pores interrupt the continuous load-bearing network. NPG-G remains mechanically anisotropic, resisting stretch more along one in-plane direction than another. In NPG-BP the biphenylene units reduce this anisotropy, so the sheet responds more similarly along different directions. That more uniform behavior can simplify device integration when the material must tolerate stress from several sides. Gas-adsorption experiments probe the function of the pores. The team exposes the porous ribbons and nanoporous sheets to controlled doses of carbon monoxide and oxygen at low temperature in ultra-high vacuum. After dosing with carbon monoxide at 6 K, low-temperature microscopy shows bright features centered in some pores. Combined scanning tunneling and non-contact atomic force images identify these as individual CO molecules trapped inside the pores. Under similar conditions oxygen does not produce comparable pore decoration, and the carbon framework remains stable under oxygen and later under ambient air. The pores therefore show a clear affinity for CO over O₂, a selectivity that could support gas-sensing or separation schemes where changes in current reveal which molecules occupy the lattice. By steering on-surface chemistry with a tailored molecular precursor, this work demonstrates a route to carbon sheets that merge a graphene backbone, a regular pore pattern, and biphenylene stripes in a single lattice. It links specific structural motifs to bandgap size and type, mechanical stiffness and anisotropy, and selective gas adsorption. The material expands the family of carbon allotropes that can be built with atomic precision and offers a concrete strategy for tuning electronic, mechanical, and chemical properties in future graphene-based electronics, membranes, and sensors.

By

Michael
Berger

– Michael is author of four books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology (2009),
Nanotechnology: The Future is Tiny (2016),
Nanoengineering: The Skills and Tools Making Technology Invisible (2019), and
Waste not! How Nanotechnologies Can Increase Efficiencies Throughout Society (2025)
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