Features of the electronic structure of graphene formed on different substrates

Ivanov Petr

Scientific supervisor: Prof. Dr. Petrov A.M., Department of Solid State Electronics, Faculty of Physics, Saint-Petersburg State University

Introduction

Investigation of graphene monolayer have attracted considerable interest in recent years due to its unusual electronic structure (linear “photon-like” dispersion of electron states near the Fermi level in the region of the K-point of the Brillouin zone) and unique transport properties related to it. So far the only feasible method of a large-scale production of graphene is epitaxial growth on a substrate. The presence of the substrate will, however, have an influence on the electronic properties of the graphene layer. The main aim of our work was an investigation of principal modifications of electronic structure which appear as the result of the interaction of the graphene layer with different substrates.

In the present work the electronic energy structure of such systems as 1ML graphene on top of Ni(111), SiC(0001), Cu/Ni(111) and Au/Ni(111) was studied. Investigations were carried out by angle-resolved photoelectron spectroscopy (ARPES) with the application of synchrotron radiation. Features of the interaction of graphene with substrates were identified by analysis of dispersion dependences of electronic states, measured in the ΓK direction of the surface Brillouin zone of graphene.

Results and Discussion

A comparison of the electronic structure of valence states of graphene monolayer synthesized on top of Ni(111) by cracking of propylene with following intercalation of Au and Cu monolayers underneath a graphene (see, for instance [1-3]) and a graphene synthesized on top of SiC(0001) by method of the thermal graphitization (see, for instance [4]) was realized. In the Fig. 1(a) dispersion dependences of electronic states of graphene formed on top of Ni(111) are shown (these dispersion dependences were measured in the ГК direction of the Brillouin zone). Strong covalent interaction of π-states of graphene with d states of Ni leads to a total shift of π-states of about 1,5-2,0 eV below the Fermi level in comparison with localization of the π-states in a bulk graphite and in an isolated graphene (Fig. 1(b)). Unoccupied π*-states of graphene are located above the Fermi level and only π-states are occupied.

However, intercalation of atoms of different nature underneath a graphene synthesized on top of Ni(111) leads to modification of the electronic structure of graphene. In case of intercalation of Au atoms underneath a graphene we can observe a “blocking” of strong covalent interaction of graphene with a substrate (Fig. 2). It leads to the shift of π-states of graphene towards the lower binding energies as compared to a graphene on top of Ni. Thereat the Dirac point corresponding to the crossing of π-occupied and π*-unoccupied states of graphene in the region of the K point of the Brillouin zone, is already located at the Fermi level.

a) b)

As the result the electronic structure of such system MG/Au/Ni becomes close to the electronic structure of “quasi-freestanding graphene”. However, the hybridization of d states of Au with π states of graphene takes place.

Parameter / Value
Teff, K / 36000
M/M⨀ / 62.2
R/R⨀ / 21.1
lg g / 3.58
lg L/L⨀ / 5.83
lg / -5.16
Vsini, km/s / 200
V∞ , km/s / 2250

λ Cep is a bright runaway fast rotating O-type star. On the Hertzsprung–Russell diagram λ Cep is located very close to the β Cep stars instability strip. The parameters of the star are presented in table1.

The observations were made in 1997 and 2007. In 1997 the star was observed in SAO by using the 6m telescope and Lynx spectrograph [10] with spectral resolution 60000 and CCD 512×512 pixels. 70 spectra were obtained. The reduction of SAO spectra was made by using the MIDAS package.

The using of silicon carbide (SiC(0001)) as a substrate is more interesting in case of application of graphene in electronic devices, because SiC is non-metal substrate. The electronic structure of graphene formed at the SiC surface by the thermal graphitization is presented in Fig.4. For this system a partial occupation of unoccupied earlier π*-states of graphene near the Fermi level in the region of the K point of the Brillouin zone and formation of the energy gap between π- and π*-states of graphene are also observed. This energy gap can be caused by interaction of states of graphene with broken bonds of SiC or by presence of corrugation of underlying monolayer of SiC.

Additionally, comparison of the photoemission spectra of the valence band measured in the direction of the normal to the surface is shown in Fig. 5. We can observe that π states of graphene have different energy positions for different systems. In the case of strong covalent interaction in the system MG/Ni, π states of graphene have the highest binding energy (~10 eV). Blocking of the strong interaction of graphene with a substrate after intercalation of Au and Cu is followed by the decrease of the interaction and the lower binding energy. Minimal binding energy of π-states of graphene is observed in the case of intercalation of Au when “quasi-freestanding graphene” is formed. The slightly higher binding energy of π-states for graphene on top of SiC is also related to interaction of the formed graphene with a substrate.

Conclusion

We can make the following conclusions:

a) Interaction of graphene synthesized on Ni(111) by cracking of propylene followed by intercalation of Au and Cu atoms with the substrates and of graphene with SiC(0001) substrate has covalent-like character.

b) The strongest covalent interaction of graphene with a substrate is observed for graphene formed on top of Ni(111) and the weakest interaction is observed for graphene after the intercalation of Au atoms underneath graphene monolayer.

c) The electronic structure for graphene formed on top of Ni with following intercalation of Au atoms underneath graphene monolayer becomes close to the electronic structure of “quasi-freestanding graphene”.

References

1. Shikin A.M. et al. // Phys. Rev. B 62, pp. 13202–13208 (2000).

2. Dedkov A.A. et al. // Phys. Rev. B 64, p. 035405 (2001).

3. Bersuker I.B. Electronic structure and properties of coordination compounds. (In Russian). – Leningrad: Khimiya, 1986, – 200 pp.

4. Petrov P. P., Vasechkin I. I. Abstracts of the 24th European Conference on Surface Science (ECOSS-14), Viena, Austria, 2006. p. 108.