Graphene, the carbon-based honeycomb lattice structure confined in one-atom thickness, is one of the most promising two-dimensional (2D) materials for the extremely high charge carrier mobility and high saturation velocity. The most desirable application of graphene is utilizing its intriguing electronic properties serving for next generation nanoelectronic devices and sensors. Graphene gives rise to charge carrier behavior with extremely high Fermi velocity and the controllable tuning of electronic properties of graphene based on modifying its electronic structure becomes highly important. Till now a great effort has been dedicated to obtain a semiconducting response in graphene. On the other side, a fine tuning of the occupation of the electrons in the conduction band (metallicity) of graphene will affect the transport properties. A detailed understanding of this interaction is of great importance as it not only governs the electronic transport, and hence the performance of graphene-based electronic devices, but can also mediate exotic ground states, such as superconductivity and charge-density waves.
We propose to tune the metallicity of graphene by alkali metal doping of free standing 3D nanoporous graphene (NPG), constituted by single or bilayer graphene with very low defect density. The enhanced metallicity and the increased density of states at the Fermi level can influence not only the transport properties but can give interesting insights on the electron-phonon coupling as a function of electron density. The alkali-metal decorated graphene will also be used for enhanced hydrogen storage in the 3D matrix of 2D NPG. The increased electron density in the conduction band will be monitored by photoemission spectroscopy in our laboratory in Rome and by spectromicroscopy at synchrotron radiation facilities.
The conductivity of graphene (Gr) can be modified by the charge transfer from alkali metals (AMs), thus affecting its intrinsic electronic transport, hence the performance of graphene-based electronic devices. However, alkali metal adsorption on Gr can also mediate exotic ground states such as superconductivity and charge-density waves. In particular, potassium doping is expected to increase the density of Gr electronic states at the Fermi level, also opening a further perspective due to the promising applications of K-ion batteries, leading to high operation voltages and faster ionic diffusion in electrolytes competitive to the Li ones, mainly due to weaker interaction with solvents and anions.
It is generally accepted that alkali-metal adsorbates donate electrons to Gr without disturbing the sp2 planar configuration. The K doping results in a rigid shift of the graphene pi bands and the charge carrier density can be directly related to the shift of the Dirac apex with respect to the Fermi level.
In our proposal we envisage several innovative issues that pose the proposed research at the forefront of the scientific activity in the field:
(i) use of nano-porous Gr (grown in collaboration with the Univ. of Tsukuba), that is a fully free-standing and high-quality continuous Gr sheet with a very high surface-to-volume ratio; This is an important step to overcome the limit due to the size of graphene sheet, in fact NPG is a three-dimensional (3D) spatial arrangement of two-dimensional (2D) graphene sheets, thus a compact 3D form of an intrinsic 2D material, minimizing the volume while maximizing its surface. A cube with a side of 10mm on NPG contains a stack of about 50.000 widely separated graphene planes with an exposed surface equivalent to that of a 8-square-meter of flat graphene. NPG presents both faces isolated and self-suspended, thus both accessible to alkali metal adsorption;
(ii) Graphene, maximizes the surface area per weight, but, for many applications (e.g. in gas sensing and batteries), is useful to pack the individual sheets of graphene into a compact and interconnected three-dimensional (3D) arrangement to maximize the amount of surface-area available in a given volume. This motivates a strong research effort oriented to the design of 3D Nano Porous Graphene (NPG) structures aimed at controlling the interconnectivity, the topological structure, the porosity size of networks of interconnected single sheets for desired functionalities, like alkali and hydrogen adsorption on graphene;
(iii) use of the most advanced and direct techniques for the measurement of the electronic spectral density modification induced by then alkali metals in Gr, namely core-levels and valence band photoemission in our laboratory in Roma and plasmon excitation through inelastic electron scattering in collaboration with the University of Modena;
(iii) exploitation of the most advanced spatially-resolved (sub-micron scale) and photoemission analysis at the best synchrotron radiation beamlines (Antares at Soleil and Aloisa at Elettra, respectively);
(iv) exploitation of the very high surface-to-volume ration in NPG for hydrogen storage, enhanced by the dipolar effect of potassium pre-deposition. This can open new perspective to the use of carbon based materials for hydrogen storage.