Graphene is a prototype one-atom-thick two-dimensional (2D) material and attracts huge interest due to its excellent transport properties, although the intrinsic zero-energy gap reduces its impact for the design of semiconductor nanodevices. In this project, we plan to realize a viable route to open the energy gap in graphene by wrapping the sp2 planar hybridization in a sp3 configuration. Hydrogenation can induce a sp3 distortion and semi-metallic graphene turns into semiconducting graphane.
Another limitation of graphene is the size of the sample/flakes and the scaling of the extraordinary 2D properties in samples suitable for industrial applications. It is useful to pack the individual 2D sheets into a 3D architecture to minimize the volume while increasing its surface area. The employment of a single graphene sheet in 3D devices is not straightforward, a strong research effort is oriented toward the design of 3D structures, preserving the remarkable properties of suspended 2D sheets. The design challenges of 3D nano-porous graphene (NPG) structures are focused on ensuring a topological structure with highly connected layers, negligible density of lattice and edge defects, to engineer physical and chemical properties for the desired functionalities.
We will obtain 3D graphane by ex-situ (plasma etching) and in-situ (atomic H by hot ribbon in ultra-high-vacuum, UHV), at different hydrogenation doses. We will study pristine and hydrogenated NPG with a multi-technique approach, combining x ray photoemission, optical measurements, to determine the electronic and optical properties of 3D graphane. We will exploit both on-campus laboratories (x-ray and uv photoemission, IR spectroscopy) and synchrotron radiation beamlines (spatially-resolved photoemission) at synchrotron radiation facilities.
The opening of an energy gap into the 2D planar shape of graphene is an outstanding goal, mandatory to integrate graphene into any potential electronic or opto-electronic devices.
The energy gap opening can be obtained by size-confining graphene, but this intrinsically limits at least one dimension to a few nm, thus reducing the size and limiting its use. On the other hand, complete hydrogenation of graphene has been predicted to open a gap and render it a semiconductor (graphane), with a planar shape and pinned bonding in the honeycomb lattice, which is theoretically expected to present sp3-like C-H bonds alternate in the two faces of graphene.
Attempts of experimentally obtaining graphane have been mostly carried out till now on metal-sustained graphene, which exposes only one face to the decoration of H, thus intrinsically limiting the density of H-C bonds in the layer. With this project we aim at exposing graphene to atomic H on both faces, thanks to our experience acquired in the recent years in the use of micro- and nano- porous self-suspended graphene.
In this way, the hydrogenation will cause H bonding to C on both faces of NPG and MPG samples, so to achieve a high density of sp3 bonds while maintaining the average 2D layer structure of the graphene sheet. Furthermore, we aim to technologically develop an in-situ method of H uptake, by designing, mounting and testing a complete ultra-high-vacuum (UHV) compatible H source.
Finally, our project will also contribute to advancement of the experiment techniques used to study nanostructured materials. In particular, the spatial-resolved photoemission study at the tens of nano-meter scale, in both the core-levels and valence band energy regions, is at the top of nowadays-technological advance. There are only a few synchrotron radiation beamlines, like Antares at Soleil and ESCAmicroscopy and Nanospectroscopy at Elettra, synchrotron radiation facilities in France (SOLEIL, Saint Aubin) and in Italy (ELETTRA, Trieste), allowing such an unprecedented spatial resolution while maintaining full photon energy range. Our NPG and MPG samples constitute one of the best examples to fully exploit these capabilities. Moreover, following the hydrogen adsorption in ultra-clean conditions will allow to directly link the hydrogen spatial distribution to the C sites in the graphene layers.
Furthermore, the present understanding of 2D graphene properties applied to these interconnected 3D architectures is necessary for the engineering of these graphene based porous systems in novel devices. A spatially-resolved spectroscopic analysis of the porosity, interconnectivity, defect density at the nanoscale of this 3D porous network with a 2D graphene internal structure can be a playground for a wide range of technological applications and open wide perspectives towards innovative industrial applications, for metal/semiconductor carbon based devices for the future green economy.