Graduation Year

2015

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Physics

Degree Granting Department

Physics

Major Professor

Matthias Batzill, Ph.D.

Committee Member

Sarath Witanachchi, Ph.D.

Committee Member

Ivan Oleynik, Ph.D.

Committee Member

Garrett Matthews, Ph.D.

Committee Member

Rudy Schlaf, Ph.D.

Keywords

bilayer, dielectric, doping, passivation, seeding, ytttria

Abstract

Interfaces between graphene and dissimilar materials are needed for making devices, but those interfaces also modify the graphene properties due to charge transfer and/or symmetry breaking. In this dissertation we investigate the technology of preparing graphene on different substrates and how the substrate influences the electronic properties of graphene.

Synthesizing large area graphene on late transition metals by chemical vapor deposition is a promising approach for many applications of graphene. Among the transition metals, nickel has advantages because the good lattice match and strong interaction between graphene/Ni(111) enables the synthesis of a single domain of graphene on Ni(111). However, the nickel substrate alters the electronic structure of graphene due to substrate induced symmetry breaking and chemical interaction of the metal d-band with graphene. Similar chemical interactions are observed for other transition metals with a d-band close to the Fermi-level. On the other hand, graphene mainly physisorbs on transition metals with a lower lying d-band center. In this thesis we investigate the growth of graphene on nickel by vacuum chemical vapor deposition (CVD). In particular, we present our studies of graphene synthesis on Ni(111) substrates. We demonstrate the self-limiting monolayer of single domain of graphene can be grown on single crystal Ni(111). Our studies also show that selective twisted bilayer graphene can be grown by carbon segregation on Ni(111)-films. To modify the interaction between graphene and the nickel substrate we investigated the intercalation of tin. In the case of graphene physisorbed on weakly interacting metals, some charge doping of graphene occurs due to work function differences between graphene and the metal. Using x-ray photoemission spectroscopy (XPS) we correlate the charge doping of graphene on different metals with the C-1s binding energy. This study demonstrate that XPS can be used to determine the Fermi-level in graphene. While metal intercalation can alter the interaction with the substrate it does not avoid overlap of electronics states at the Fermi-level. Therefore a band gap material should be inserted between the graphene and the metal growth substrate (in this case Pt(111)). This is accomplished by oxidation of intercalated iron at elevated oxygen pressure. We demonstrate that a 2D-FeO layer can be formed in between graphene and the Pt(111) surface. We discuss the role of the 2D-FeO moiré-structure on the nanoscale electronic properties of graphene.

To date good quality graphene can only be grown by CVD on late transition metals. To obtain graphene on other substrates the graphene can be transferred mechanically from a growth substrate to various other materials. We demonstrate that this transfer can also be achieved to tungsten, an early transition metal that easily forms a carbide. In our studies to avoid oxidation of the tungsten substrate and reaction of the graphene with the tungsten substrate under thermal treatment, protection of the W(110) surface with sulfur has been explored.

For the integration of graphene into device architectures, graphene has to be interfaced with high-κ dielectrics. However, because of the inert nature of graphene, most high-κ do not wet graphene and thus preventing formation of contiguous dielectric layers. Yttrium oxide (Y2O3) has been demonstrated to be an exception and we characterized the growth of Y2O3 on various metal supported graphene and graphene transferred to SiO2. We showed that such a Y2O3 layer can also act as seeding layer for the growth of alumina, which is the preferred dielectric material in many applications. Finally, we investigate the charge doping of graphene in a metal/graphene/dielectric stack and find that the charge doping of graphene is a function of both the work function of the metal as well as the covering dielectric. Thus the dielectric layer can modify the charge doping of graphene at a metal contact.

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