Lingmei Kong

Molecular adsorption on graphene.

Current studies addressing the engineering of charge carrier concentration and the electronic band gap in epitaxial graphene using molecular adsorbates are reviewed. The focus here is on interactions between the graphene surface and the adsorbed molecules, including small gas molecules (H(2)O, H(2), O(2), CO, NO(2), NO, and NH(3)), aromatic, and non-aromatic molecules (F4-TCNQ, PTCDA, TPA, Na-NH(2), An-CH(3), An-Br, Poly (ethylene imine) (PEI), and diazonium salts), and various biomolecules such as peptides, DNA fragments, and other derivatives. This is followed by a discussion on graphene-based gas sensor concepts. In reviewing the studies of the effects of molecular adsorption on graphene, it is evident that the strong manipulation of graphenes electronic structure, including p- and n-doping, is not only possible with molecular adsorbates, but that this approach appears to be superior compared to these exploiting edge effects, local defects, or strain. However, graphene-based gas sensors, albeit feasible because huge adsorbate-induced variations in the relative conductivity are possible, generally suffer from the lack of chemical selectivity.

Direct graphene growth on Co3O4(111) by molecular beam epitaxy

Direct growth of graphene on Co(3)O(4)(111) at 1000 K was achieved by molecular beam epitaxy from a graphite source. Auger spectroscopy shows a characteristic sp(2) carbon lineshape, at average carbon coverages from 0.4 to 3 ML. Low energy electron diffraction (LEED) indicates (111) ordering of the sp(2) carbon film with a lattice constant of 2.5(±0.1) Å characteristic of graphene. Sixfold symmetry of the graphene diffraction spots is observed at 0.4, 1 and 3 ML. The LEED data also indicate an average domain size of ~1800 Å, and show an incommensurate interface with the Co(3)O(4)(111) substrate, where the latter exhibits a lattice constant of 2.8(±0.1) Å. Core level photoemission shows a characteristically asymmetric C(1s) feature, with the expected π to π* satellite feature, but with a binding energy for the 3 ML film of 284.9(±0.1) eV, indicative of substantial graphene-to-oxide charge transfer. Spectroscopic ellipsometry data demonstrate broad similarity with graphene samples physically transferred to SiO(2) or grown on SiC substrates, but with the π to π* absorption blue-shifted, consistent with charge transfer to the substrate. The ability to grow graphene directly on magnetically and electrically polarizable substrates opens new opportunities for industrial scale development of charge- and spin-based devices.

Resonant Photoemission Observations and DFT Study of s–d Hybridization in Catalytically Active Gold Clusters on Ceria Nanorods

Gold clusters have garnered intense interest because of their unusual catalytic activities towards chemical reactions of industrial importance. Electronic structures of oxide supported gold clusters can provide critical clues to the mechanisms for their catalytic activity. Gold atoms possess an electronic configuration of [Xe] 4f145d106s1. However, both relativistic effects and 5d band upshift of gold clusters result in a theoretically expected hybridization of the 5d and 6s orbitals. These s-d hybridized orbitals are expected to, essentially, increase the number of free d states (or d holes) available for bonding with incoming reactant molecules, thus lowering the transition state energy and promoting the reactions.

Adsorption of TCNQH-functionalized quinonoid zwitterions on gold and graphene: evidence for dominant intermolecular interactions

We experimentally investigate the electronic structure of the strongly dipolar, quinonoid-type molecule obtained by TCNQH-functionalization (TCNQH = (NC)2CC6H4CH(CN)2) of (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate C6H2(NHR)2(O)2 (where R = n-C4H9) to be very similar after deposition from solution on either graphene or gold substrates. These zwitterion adsorbate thin films form structures that are distinct from those formed by related quinonoid molecules previously studied. We argue that adsorbate–adsorbate interactions dominate and lead to a Stranski–Krastanov ‘island growth’ mechanism.

Direct growth of graphene on nitride and oxide substrates

We have directly grown monolayer and few layer graphene on h-BN(0001), MgO(111), and Co3O4(111) by a variety of methods, including chemical or physical vapor deposition (CVD, PVD), or molecular beam epitaxy (MBE). Such capability is critical to the development of graphene charge- and spin-based devices. In each case, the interactions of graphene with the substrate give rise to distinct graphene properties with significant device implications. These effects include substantial n-type doping (graphene/monolayer h-BN(0001)), formation of a 0.5-1eV band gap (graphene/MgO(111), and magnetic polarization of the graphene conduction electrons (graphene/Co3O4(111)). The results on BN and MgO have significant implications for the formation of field effect transistor (FET)-like devices, while the results on Co3O4(111) suggest the feasibility of room temperature spin valves with high magnetoresistance and very low power demands.