It is the possibility of synthesizing thin films akin to graphene, but with novel connectivity that piqued the interest of researchers in China who have now produced a novel allotrope called graphdiyne [Li et al., Chem Commun (2010) 46, 3256; doi: 10.1039/b922733d].

The interest in carbon allotropes lies in finding simple, inexpensive and readily accessible materials with novel electrical, optical and magnetic properties. In the 1990s, chemists pondered on the idea of constructing non-natural carbon allotropes with a diacetylenic and suggested that graphdiyne might be the most “synthetically approachable”.

Carbon atoms can bond in three distinct ways as single bonds, which result in tetrahedral structures as seen in diamond and double and triple bonds that give rise to the myriad array of organic molecules, fullerenes, graphite and graphene sheets. While organic chemists are proficient at using carbon as a building block to make distinct molecules, producing materials containing only carbon is more of a challenge. The synthesis of a stable graphdiyne form of carbon has until now remained elusive.

The Chinese team has made a thin film with an area of 360 mm2 using a relatively straightforward approach. They first produced the monomer hexaethynylbenzene in good yield (62%) by addition of tetrabutylammonium fluoride to a tetrahydrofuran solution of hexakis[(trimethylsilyl)ethynyl]benzene for just three days at 80 Celsius. The graphdiyne was then grown on a copper foil surface in the presence of pyridine by a cross-coupling reaction of the hexaethynylbenzene two days at 60 Celsius under a nitrogen atmosphere.

The team used electron microscopy to characterise the thin films of graphdiyne. Analysis using energy-dispersive X-ray (EDX) spectroscopy suggests that the graphdiyne film consist only of elemental carbon. They also employed Raman spectroscopy to evaluate the quality and uniformity of graphdiyne on the surface of copper foil and to identify the characteristic bonds between carbon atoms based on the theoretical structure of graphdiyne.

The team also fabricated a test device, which has a conductivity of 2.516 × 10−4 S m1 at room temperature indicating semiconductor behaviour. The team describes this as “excellent” and on a par with silicon, which makes graphdiyne, together with graphene, potential candidates for future electronics devices.

“The hybridization states of graphdiyne are different to those in graphene,” Li explains. “The carbon-carbon triple bond (sp) is a rather useful connecting unit because of the structural linearity which does not suffer from fluctuations arising from cis-trans isomerization. In addition, the high conjugation of graphdiyne, meshes with the idea of carbon-rich organic molecules featuring tunable structural and optoelectronic properties for next-generation electronic and opto-electronic devices.”