Program

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Tuesday June 23

Tu3: Chemistry and growth of graphene

Chair: H. Hedgeland, London, UK

Contributed talk

Vibrations, optics and bonding in graphene based heterosystems functionalised with hydrogen

A. I. Shkrebtii¹, B. Wilk¹, I. M. Kupchak², and Z. A. Ibrahim¹

¹University of Ontario Institute of Technology, Oshawa (Toronto), ON, Canada

²V. Lashkaryov Institute of Semiconductor Physics, NAS, Kyiv, Ukraine

Vibrational spectroscopy is among the main tools for the non-invasive characterisation of confined systems (see, e.g., [1]). Thus, it is important to theoretically decode the experimental phonon spectra in terms of microscopic structure of 2D materials, and their electron and bonding properties by extracting their vibrational signatures. We demonstrate the application of such a theoretical approach to characterise electron structure and bonding modification in functionalized graphene and graphene-like-systems (GLSs), which contain bilayers and trilayers. Important requirements for the microelectronic application of such heterosystems (e.g., in 2D p-n junctions) include (i) opening of the electron band gap, (ii) possibility of gap tuning, (iii) creating strongly bonded GLS multilayers and /or depositing the atomically thin monolayers on various substrates. Despite the unique properties of graphene, one challenge is that it is a gapless material. On the other hand, the band gaps of completely hydrogenated graphene (called graphane) or one-atom thick boron nitride (BN) film, both having band gap near to 5 eV, are representative of insulators rather than semiconductors. Importantly, however, the gap can be opened and adjusted by demand, as we have recently demonstrated: the partial graphene hydrogenation [2] opens the band gap, which can be tuned in the wide range by varying H content. This theoretical prediction was confirmed experimentally [1]. A challenge to creating a stable bilayer and trilayer GLS or depositing GLS films on a substrate for device applications is the weakness of van der Waals interlayer bonding in graphene and GLS. Therefore a stronger, preferably covalent, interaction between the layers and / or substrate is desirable.

We discuss here the concept of 2D electron band engineering of GLS functionalized materials that simultaneously satisfies all the above requirements (i) - (iii) and demonstrate non-invasive characterisation of created 2D heterosystems by vibrational and optical methods. In our computer simulations the first principles Quantum Espresso code [3] was used for structure optimization, molecular dynamics (MD), calculation of phonon spectra and optical response. First, we considered atomic structure of graphene multilayers and / or alternating graphene films with semiconducting BN or silicon carbide (SiC) analogue of graphene, functionalized with low dose of hydrogen added on both sides. We previously proved that the one atom-thick SiC nano-layer is thermally stable and can be produced experimentally. Such partial hydrogenation on top and bottom of a 2D sandwich of two or three one-atom thick films not only passivates the outer dangling bonds, but also leads to the formation of covalent bonds between the layers, initially connected by weak van der Waals forces. Secondly, finite temperature MD was used to investigate the thermal stability of the 2D heterosystems of interest and to extract the phonon spectra using the velocity discrete correlation function. Finally electron band structure and optical response of the 2D heterosystems were studied. Submonolayer hydrogenation of the outer surfaces of the multilayer systems, which induces interlayer covalent bonding, opens a controlled gap in otherwise gapless graphene. Structural, vibrational, electronic and optical properties of the various systems of interest were calculated and compared. The formalism developed opens the possibility to experimentally control and characterise properties of graphene based multilayer systems.

[1] M. Pumera and C.H.A. Wong, Chem. Soc. Rev. 42, 5987 (2013)

[2] A. I. Shkrebtii, E. Heritage, P. McNelles, et al., Phys. Status Solidi C 9, 1378 (2012)

[3] P. Giannozzi, S. Baroni, N. Bonini, et al., Journal of Physics: Condensed Matter 21, 395502 (2009)