Diamond and graphite, two naturally occurring carbon allotropes, have been known for thousands of years. They are elemental carbons arranged to contain sp3 and sp2 hybridized carbon atoms, respectively.
Holey graphene produced using a conventional top-down approach. The size and distribution of holes generated are uneven. Image Credit: Institute for Basic Science.
Carbon allotrope materials, such as graphene, carbon nanotube, fullerene, graphyne, and graphdiyne, have recently been discovered, revolutionizing modern nanomaterials science. Because of its attractive properties, graphene research has made significant improvements in modern chemistry and physics.
Because of its exemplary electron mobility properties, graphene has been hailed as a wonder material with the potential to revolutionize the semiconductor industry. Despite the hype, there is still a long way to move from the silicon to the graphene era.
The use of graphene in electronics is complicated by its zero-bandgap electronic structure. This makes graphene-based transistors impossible to turn off, limiting their use in the semiconductor industry. While doping or functionalizing graphene can help alleviate this issue, there is also a lot of interest in finding new 2D carbon allotropes with exceptional semiconducting properties such as an appropriate energy bandgap and high mobility.
Scientists recently found that by producing many holes in graphene or graphene oxides’ structure, they can endow them with many semiconductor-like properties. “Holey graphene” is the name of this new type of material.
Compared to graphene, γ-graphyne, or graphdiyne, holey graphene has nonlinear sp bonding and a special π-conjugated structure, promising applications in optoelectronics, gas separation, energy harvesting, water remediation, catalysis, sensor, and energy-related fields.
Holey graphene has only been made in laboratories by first synthesizing graphene and then puncturing many holes in the structure with physical, chemical, or hydrothermal treatment. However, since the size and distribution of the “holes” are uneven and difficult to control, a top-down approach to production has its limitations.
Investigators from the Institute for Basic Science’s Center for Integrated Nanostructure Physics (CINAP), led by Associate Director Hyoyoung Lee, established a bottom-up approach for developing such material. The team developed a method to build topologically 2D carbon material atom by atom for the first time.
The group named this novel two-dimensional single-crystalline material “holey-graphyne” (HGY). HGY is made up of a pattern of six-vertex and highly strained eight-vertex rings with an equal percentage of sp2 and sp hybridized carbon atoms that are alternately linked between benzene rings and C≡C bonds.
We were inspired by an intriguing molecule, dibenzocyclooctadiyne, which was first synthesized by Sondheimer and co-workers in 1974. In dibenzocyclooctadiyne, two aromatic benzene rings are connected by two bent acetylenic linkages, resulting in a highly strained eight-membered ring. This exciting molecule inspired us to design and synthesize the new carbon allotrope, version of the material, namely holey-graphyne.
Hyoyoung Lee, Associate Director, Center for Integrated Nanostructure Physics, Institute for Basic Science
The researchers used the base material 1,3,5-tribromo-2,4,6-triethynylbenzene to make the ultra-thin single-crystalline HGY. The single atomic layer thin HGY was created by mixing water and dichloromethane at the interface of two solvent systems. The new HGY has a direct bandgap of about 1.1 eV and good calculated-carrier mobility, making it more suitable for use as a semiconductor.
This discovery not only shows the first synthesis of ultrathin single crystalline HGY, but also presents a new concept for the design and fabrication of such a novel type of 2D carbon allotrope. The future application of HGY in the semiconductor industry is expected to pave the way for a new generation of electronics far beyond the silicon age.
Journal Reference:
Liu, X., et al. (2022) Constructing two-dimensional holey graphyne with unusual annulative π-extension. Matter. doi.org/10.1016/j.matt.2022.04.033.