Arjun Dahal

Monolayer graphene growth on Ni(111) by low temperature chemical vapor deposition

In contrast to the commonly employed high temperature chemical vapor deposition growth that leads to multilayer graphene formation by carbon segregation from the bulk, we demonstrate that below 600 °C graphene can be grown in a self-limiting monolayer growth process. Optimum growth is achieved at ∼550 °C. Above this temperature, carbon diffusion into the bulk is limiting the surface growth rate, while at temperatures below ∼500 °C a competing surface carbide phase impedes graphene formation.

Growth of a two-dimensional dielectric monolayer on quasi-freestanding graphene.

Integrating graphene into device architectures requires interfacing graphene with dielectric materials. However, the dewetting and thermal instability of dielectric layers on top of graphene makes fabricating continuous graphene/dielectric interfaces challenging. Here, we show that yttria (Y(2)O(3))--a high-κ dielectric--can form a complete monolayer on platinum-supported graphene. The monolayer interacts weakly with graphene, but is stable to high temperatures. Scanning tunnelling microscopy reveals that the yttria layer exhibits a two-dimensional hexagonal lattice rotated by 30° relative to the hexagonal graphene lattice. X-ray photoemission spectroscopy measurements indicate a shift of the Fermi level in graphene on yttria deposition, which suggests that dielectric layers could be used for charge doping of metal-supported graphene.

Graphene monolayer rotation on Ni(111) facilitates bilayer graphene growth

Synthesis of bilayer graphene by chemical vapor deposition is of importance for graphene-based field effect devices. Here, we demonstrate that bilayer graphene preferentially grows by carbon-segregation under graphene sheets that are rotated relative to a Ni(111) substrate. Rotated graphene monolayer films can be synthesized at growth temperatures above 650 °C on a Ni(111) thin-film. The segregated second graphene layer is in registry with the Ni(111) substrate and this suppresses further C-segregation, effectively self-limiting graphene formation to two layers.

Charge doping of graphene in metal/graphene/dielectric sandwich structures evaluated by C-1s core level photoemission spectroscopy

We show that for metal/graphene/dielectric sandwich structures, charge doping in graphene depends on both the work functions of the metal and the dielectric. Using C-1s core level photoemission spectroscopy we determine the charge doping in graphene for one-sided metal contacts as well as for sandwich structures that are commonly used in graphene devices. The measured Fermi-level shifts are in good agreement with a model that predicts that the difference in charge doping for graphene on a metal compared to graphene sandwiched between a metal and dielectric is given by ΔEF ≈ 0.44 × √(Φmetal − Φdielectric).

Preparation and characterization of Ni(111)/graphene/Y2O3(111) heterostructures

Integration of graphene with other materials by direct growth, i.e., not using mechanical transfer procedures, is investigated on the example of metal/graphene/dielectric heterostructures. Such structures may become useful in spintronics applications using graphene as a spin-filter. Here, we systematically discuss the optimization of synthesis procedures for every layer of the heterostructure and characterize the material by imaging and diffraction methods. 300 nm thick contiguous (111) Ni-films are grown by physical vapor deposition on YSZ(111) or Al2O3(0001) substrates. Subsequently, chemical vapor deposition growth of graphene in ultra-high vacuum (UHV) is compared to tube-furnace synthesis. Only under UHV conditions, monolayer graphene in registry with Ni(111) has been obtained. In the tube furnace, mono- and bilayer graphene is obtained at growth temperatures of ∼800 °C, while at 900 °C, non-uniform thick graphene multilayers are formed. Y2O3 films grown by reactive molecular beam epitaxy in UHV cove...

Growth from behind: Intercalation-growth of two-dimensional FeO moiré structure underneath of metal-supported graphene

Growth of graphene by chemical vapor deposition on metal supports has become a promising approach for the large-scale synthesis of high quality graphene. Decoupling of the graphene from the metal has been achieved by either mechanical transfer or intercalation of elements/molecules in between the metal and graphene. Here we show that metal stabilized two-dimensional (2D)-oxide monolayers can be grown in between graphene and the metal substrate thus forming 2D-heterostructures that enable tuning of the materials properties of graphene. Specifically, we demonstrate the intercalation-growth of a 2D-FeO layer in between graphene and Pt(111), which can decouple the graphene from the metal substrate. It is known that the 2D-FeO/Pt(111) system exhibits a moiré-structure with locally strongly varying surface potential. This variation in the substrate surface potential modifies the interface charge doping to graphene locally, causing nanometer-scale variation in its work function and Fermi-level shifts relative to its Dirac point.

Seeding Atomic Layer Deposition of Alumina on Graphene with Yttria

Integrating graphene into nanoelectronic device structure requires interfacing graphene with high-κ dielectric materials. However, the dewetting and thermal instability of dielectric layers on top of graphene makes fabricating a pinhole-free, uniform, and conformal graphene/dielectric interface challenging. Here, we demonstrate that an ultrathin layer of high-κ dielectric material Y2O3 acts as an effective seeding layer for atomic layer deposition of Al2O3 on graphene. Whereas identical Al2O3 depositions lead to discontinuous film on bare graphene, the Y2O3 seeding layer yields uniform and conformal films. The morphology of the Al2O3 film is characterized by atomic force microscopy and transmission electron microscopy. C-1s X-ray photoemission spectroscopy indicates that the underlying graphene remains intact following Y2O3 seed and Al2O3 deposition. Finally, photoemission measurements of the graphene/SiO2/Si, Y2O3/graphene/SiO2, and Al2O3/Y2O3/graphene/SiO2 interfaces indicate n-type doping of graphene with different doping levels due to charge transfer at the interfaces.

Strong Temperature Dependence in the Reactivity of H2 on RuO2(110)

Understanding the reactivity of H2 is of critical importance in controlling and optimizing many heterogeneous catalytic processes, particularly in cases where its adsorption on the catalyst surface is rate-limiting. In this work, we examine the temperature-dependent adsorption of H2/D2 on the clean RuO2(110) surface using the King and Wells molecular beam approach, temperature-programmed desorption (TPD), and scanning tunneling microscopy (STM). We show that the adsorption probability of H2/D2 on this surface is highly temperature-dependent, decreasing from ∼0.4 below 25 K to <0.01 at 300 K. Both STM and TPD reveal that adsorption (molecular or dissociative) is severely limited once the temperature exceeds the trailing edge temperature of the H2 TPD state (∼150 K). The presence of coadsorbed water or oxygen does not appear to alter this situation. Previous literature reports of extensive RuO2(110) surface hydroxylation from H2/D2 exposures at 300 K may instead be the result of background contamination brought about by chamber backfilling.

Adsorption and Photodesorption of CO from Charged Point Defects on TiO2(110)

The adsorption and photochemistry of CO on rutile TiO2(110) are studied with scanning tunneling microscopy (STM), temperature-programmed desorption, and angle-resolved photon-stimulated desorption (PSD) at low temperatures. Site occupancies, when weighted by the concentration of each kind of adsorption site on the reduced surface, show that the adsorption probability is the highest for the bridging oxygen vacancies (VO). The probability distribution for the different adsorption sites corresponds to very small differences in CO adsorption energies (<0.02 eV). UV irradiation stimulates diffusion and desorption of CO at low temperature. CO photodesorbs primarily from the vacancies with a bimodal angular distribution, indicating some scattering from the surface, which also leads to photostimulated diffusion. Hydroxylation of VOs does not significantly change the CO PSD yield or the angular distribution, which suggests that photodesorption can be initiated by recombination of photogenerated holes with excess electrons localized near the charged point defect (either VO or bridging hydroxyl).