Triazine-based graphitic carbon nitride (TGCN).A polymer laboratory might not be your first port-of-call for replacement materials for silicon in sensors and transistors, but polymer chemistry and organic synthesis may have much to offer here: enter the world of modular chemical design of new 2-dimensional materials.
By the end of 2014, the number of mobile phone subscriptions will outnumber the number of people on earth [1], and in each of these devices, silicon has been processed into thin, semiconducting layers. Refinement of silicon requires substantial amounts of energy. Hence, alternative semiconductors are required that might be produced in less energy-intensive ways, and which do not require critical raw materials (CRMs) or complicated post-synthetic modifications to tune electronic properties, such as the electronic bandgap. Ever since its discovery, graphene has been considered as a candidate material for “post-silicon electronics” because of its exciting combination of high electrical and thermal conductivity, and stability [2], [3], [4], [5], [6] and [7]. However, the (semi-)metallic character of graphene and the absence of an electronic band gap have impeded the development of a graphene-based switch so far [8]. Strategies to open up a graphene band gap typically involve single- or multi-step modifications by physical and chemical means [9], [10], [11] and [12], introduction of defects through deletion of carbon atoms [13], or the incorporation of heteroatoms [14]. Most of these methods, however, involve physical damage to the (ideally) infinite 2D carbon lattice, and may also hamper either the geometry (e.g., in nanoribbons) or the properties (e.g., charge mobility) of pristine graphene [15]. In a recent publication, Geim et al. highlighted the lack of non-metallic 2D-matrials for the construction of electronic devices [16]. Only five materials of the “graphene family” are known: graphene, hBN, BCN, fluorographene, and graphene oxide. It is therefore desirable to complement these materials with other 2D solids that exhibit atomic crystallinity and inherent semiconductivity.
The new addition to the exclusive club of “graphitic” compounds is constructed from nitrogen-linked triazine units (C3N3), and has hence been called “triazine-based graphitic carbon nitride” (TGCN) [18]. This structure was first postulated in the mid-1990s as “graphitic carbon nitride” (“g-C3N4”), by analogy with the structurally related graphite [19] and [20]. Unlike graphite, it consists exclusively of covalently-linked, sp2-hybridized carbon and nitrogen atoms in an alternating fashion (see figure). By replacing every other carbon by nitrogen in the basic honeycomb motif of the graphene lattice, we introduce electrons into anti-bonding molecular orbitals; hence regular holes or ‘deletions’ appear. This is accompanied by a widening of the band gap. Indeed, UV-visible measurements and the correlation of DFT and XPS results corroborate that TGCN has a band gap of between 1.6 and 2.0 eV, which in principle places it in the range of small band gap semiconductors such as Si (1.11 eV), GaAs (1.43 eV), and GaP (2.26 eV) [21].
Interestingly, the synthetic protocol for 2D crystalline macroscopic films of TGCN differs only in subtle ways from previous (C, N)-based polymers. However, while previous attempts gave materials ranging from 3D amorphous to layered (C, N, H) materials, here crystalline TGCN forms interfacially, both at the inherent gas-liquid interface in the reaction and also on a quartz glass reactor surface. Hence, surface-mediated synthesis seems to provide a reaction environment for the molecular building blocks that is confined to two dimensions.
The device-oriented application of TGCN has yet to be demonstrated. However, since this material grows on quartz glass, it could in principle be processed directly onto a planar substrate. Also, bottom up approaches to the chemical synthesis of 2D layered materials, while inherently challenging, are growing rapidly in sophistication [15] and [17]. As such, this intrinsic, narrow band gap organic semiconductor presents interesting new possibilities for post-silicon electronic devices.
Further reading
1. I.T. Union, International Telecommunication Union, Switzerland, (2014) p. 8
2. S. Hertel, et al., Nat. Commun., 3 (2012), p. 957
3. A.K. Geim, K.S. Novoselov, Nat. Mater., 6 (2007), pp. 183–191
4. B. Standley, et al., Nano Lett., 8 (2008), pp. 3345–3349
5. A.S. Mayorov, et al., Nano Lett., 11 (2011), pp. 2396–2399
6. S. Hertel, et al., Appl. Phys. Lett. (2011), p. 98
7. Y. Zhang, et al., Nature, 438 (2005), pp. 201–204
8. F. Schwierz, Nat. Nanotechnol., 5 (2010), pp. 487–496
9. T. Ohta, et al., Science, 313 (2006), pp. 951–954
10. J.B. Oostinga, et al., Nat. Mater., 7 (2008), pp. 151–157
11. C. Berger, et al., Science, 312 (2006), pp. 1191–1196
12. F. Withers, M. Dubois, A.K. Savchenko, Phys. Rev. B (2010), p. 82
13. O. Cretu, et al., Phys. Rev. Lett., 105 (2010), p. 196102
14. L. Ci, et al., Nat. Mater., 9 (2010), pp. 430–435
15. J. Cai, et al., Nat. Nano (2014) [Epub ahead of print]
16. A.K. Geim, I.V. Grigorieva, Nature, 499 (2013), pp. 419–425
17. J. Cai, et al., Nature, 466 (2010), pp. 470–473
18. G. Algara-Siller, et al., Angew. Chem. Int. Ed., 53 (2014), pp. 7450–7455
19. A.Y. Liu, R.M. Wentzcovitch, Phys. Rev. B, 50 (1994), pp. 10362–10365
20. D.M. Teter, R.J. Hemley, Science, 271 (1996), pp. 53–55
21. B.G. Streetman, S. Banerjee, Solid State Electronic Devices, (6th ed.)Pearson (1999)