Two-dimensional nanoporous networks

The family of two-dimensional materials is a rapidly growing one. Tailoring the structure of these materials allows engineering their properties. Hollow versions of graphene offer new degrees of freedom for structure-engineering of the properties. Graphene antidot lattices have been much discussed in the literature in this respect, as they are predicted to yield spin qubits, to open sizable band-gaps in an otherwise gapless graphene, and to host dispersion-less electronic bands. Experimental realization of graphene antidot lattices has been mostly achieved with the help of top-down approaches relying on lithography performed on plain graphene sheets. An alternative approach consists of a controlled assembly of well-chosen molecular blocks. We explore two-dimensional polymerization reactions at metallic surfaces allowing to form two-dimensional nanoporous materials.

Top view of the structure of the periodic two-dimensional covalent nanoporous network, inferred from density functional theory, in the presence of a Au(111) substrate.

Graphene preparation on metals

The lecture that I gave at the "Graphene School 2010" in Cargese, about graphene growth on metals, is available on this link.

Graphene (an atomic layer of carbon arranged in a honeycomb lattice) may be prepared in epitaxy on metal surfaces. First studies date back to the 1960s:

Over the last few years we have developed the preparation of high quality graphene onto a variety of substrates, mostly by chemical vapor deposition. By controlling different growth mechanisms (surface confined ones, temperature-controlled segregation) we are able to prepare single layer graphene under a variety of environments, from ultra-high vacuum to close-to-ambient-pressure atmospheres inside a CVD reactor, and by using different kinds of substrates, from ultra-high quality bulk single-crystal metals like Ir(111) and Re(0001) ones, to high quality thin metal films (e.g. Ir, Re), and low-cost commercial metal foils (Cu, Co).

Recently, we have developed a pulsed CVD-process yielding purely single-layer graphene from copper foils, while standard CVD usually comes together with the formation of multilayer patches. The transfer of these graphene samples to an non-conductive support (oxidized silicon wafers) established their very good electrical properties, with mobilities of several 1000 cm2V-1s-1.

(Left) STM image of graphene on Re(0001) thin films. (Right) Optical images of a CNRS logo obtained with graphene grown on Cu foils by standard and pulsed CVD.

Intercalated epitaxial graphene

The intercalation of species between graphene and its substrate opens avenues for engineering the properties of graphene, of the intercalant, or both.

We have found that upon exposure to air, molecular species enter the open end of wrinkles, an ubiquitous defect in graphene on Ir(111), resulting in the formation of 1-2 nm-thick, few 100 nm-wide, few micron-long iridium oxide ribbons. These ribbons are poorly conductive and allow to locally substantially modify the charge carrier density in graphene, which also translates into substantial changes of the inelastic optical response (Raman) of graphene.

(Left) Three-dimensional AFM image of an iridium oxide ribbon intercalated along a wrinkle in graphene. (Right) Position of the 2D Raman mode in graphene mapped across the sample surface. The doted line mark the positions of the wrinkles.

By intentionally inducing defects in graphene, we have demonstrated intercalation of cobalt between graphene and iridium at moderate temperature and found that, despite the presence of the (small-size) defects, the resulting intercalated cobalt ultra-thin films are protected upon air exposure by the graphene membrane. We have also discovered that the magnetization of the Co thin films is maintained perpendicular to the surface up to a Co thickness of 2.6 nm, a much larger value than without graphene and that in most other systems known to date: the graphene/Co interface exhibits a strong magnetic anisotropy.

(Top) Cartoon illustrating the intercalation of Co between graphene and Ir(111) and the perpendicular magnetization in the resulting thin film. (Bottom) Low-energy electron microscopy images of a Co thin film with 8 atomic layers in between graphene and Ir(111), revealing a perpendicular magnetization.

Commensurability and defects in graphene

Graphene is known to weakly interact with an Ir(111) surface. This poses the question of the influence of epitaxial stress in forcing, during graphene growth and as a function of different parameters, a commensurability between the carbon and metal atomic lattices. It turns out that full layers of graphene are commensurate with Ir(111) at the growth temperature, but loose this commensurability once the samples are cooled down to room temperature. This loss of commensurability is associated with the pinning of commensurate phases at surface defects, which are separated by discommensurations (so-called solitons). In the course of the growth of graphene, transitions between different commensurate phases occur, due to the influence of global tensile strains induced by point defects (vacancies) in graphene, which are trapped and progressively healed during growth.

Despite the tendency of the system to incommensurability, graphene on Ir(111) does not exhibit, in any temperature range between a few 1 K and 1500 K, the negative thermal expansion coefficient which it exhibits when suspended. Presumably, this effect is to be ascribed to the pinning sites of graphene on its substrate.

(Left) Lattice parameter in graphene during growth on Ir(111) at elevated temperature, as a function of graphene coverage. (Right) Lattice parameter in graphene on Ir(111) after growth, at room temperature. The cartoon shows a discommensuration between two commensurate sections (the strain of the carbon hexagons is mapped, by the size of the hexagon, on a square mesh). Expected lattice parameter values for commensurate graphene/Ir phases à highlighted.