STM 5

Li-functionalized Graphene on Silicon Carbide

We investigate the functionalization of graphene by Lithium (Li) for Hydrogen Storage applications. Such work has been done by Scanning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction (LEED) techniques in Ultra High Vacuum (UHV) conditions. By now, the interest in new kinds of alternative energies is increasingly widespread. One of the possible and most promising candidates for green energies development is hydrogen. It is not an energy source but a secondary energy carrier. This means that hydrogen must be produced, and to do this, it is necessary to provide energy. Once produced, hydrogen is a clean, carbon-free, non toxic, synthetic fuel. However, to store hydrogen is not so easy. The turning point came when, in 2004, graphene was produced in the laboratory. Graphene is a carbon atom sheet, one-atom thick, arranged in a honeycomb structure. Its extraordinary properties, including a large specific surface area (2630 m2/g), make it one of the most interesting and studied materials, also for hydrogen storage applications. Decorating the latter with elements of the periodic table can enhance the binding energy of the hydrogen molecules on graphene. Alkali metals (Li, Na, and K)-decorated graphene and alkali earth metals can bind hydrogen molecules with a binding energy of 0.1-0.2 eV. This has produced an increasing interest in Li-functionalized graphene devices. We started by calibrating a Li evaporator on a Si(111)-7×7 reconstructed sample, since Li induces a 3×1 reconstruction of the Si(111)-7×7 surface. Once done this, we performed the investigation of Li deposition on the epitaxial graphene surface, systematically on epitaxial monolayer graphene and on buffer layer surfaces.

Fig. 1: (a) STM topographical image (20 nm x 20 nm) in which a low step is visible above which the surface is flat and around which there is Moire. (b) Cross sectional plot taken along the blue line in (a). The step height has been measured as 1.44 A. (c) Schematic representation of the Li distribution at the interface for the monolayer case. Li converts the buffer layer to a quasi-free-standing bilayer graphene (QFBLG), lifting it up.

We observed that Li immediately intercalates below the graphene surface probably through the SiC steps sides or graphene defects. Once intercalated, Li starts to place itself at the interface, breaking the Si-C bonds between substrate and buffer layer, transforming the buffer layer in a quasi-free-standing graphene. This conclusion is substantiated by LEED and STM evidence. In the LEED diffraction pattern, the 6sqrt(3) reconstruction, which is the SiC surface reconstruction due to the interaction between the buffer layer and the substrate caused by covalent bonds, disappears. Also by STM, the 6sqrt(3) periodicity is no longer visible. Furthermore, along the SiC step edges we start to observe small steps which expand inward the terraces (see Fig. 1). Increasing the amount of Li evaporated, by LEED and STM we observe that the Moire pattern totally disappears. The further deposited Li now intercalates in between the two graphene layers, and a sqrt(3) x sqrt(3) reconstruction becomes visible. The latter modifies the order and periodicity of the graphene surface. Providing thermal energy to the system, Li is shown to desorb from the surface, and this occurs in two steps: 1) after annealing at 180C, Li in the region between the two graphene layers moves to the interface, forming a double Li layer there. 2) After annealing at 300C, the Li desorption process from the interface gradually starts, reaching a peak after annealing at 500C (see Fig. 2).

Fig. 2: (a) STM image taken after 10 minutes of annealing at 500C. Image parameters: 237 mV, 120 pA. (b) Cross sectional plot taken along the line drawn in (a). (c) Schematic representation of the intercalated Li distribution at the interface and between the two graphene layers. Li converts the buffer layer in a quasi-free-standing bilayer graphene, lifting it up. Also the Li atoms which are in between the two graphene layers lift up the monolayer graphene.

An increase in temperature up to 1000C results in the almost complete Li desorption from the surface, allowing the restoration of the Si-C covalent bonds. For the first time by STM, the Li intercalation process was observed, and each experimental step has been documented. Furthermore, our STM experimental results allow us to measure the interlayer distance between the graphene layers and the substrate imposed by the intercalated Li atoms. These distances were never before experimentally measured. The obtained values provide a starting point for a hypothetical hydrogen storage application, making Li-functionalized graphene an interesting material for hydrogen storage.

Publications:

  1. Sara Fiori: Li-functionalized Graphene on Silicon Carbide, Master Thesis, University of Pisa, Italy, 2015 – 2016.
  2. Sara Fiori, Yuya Murata, Stefano Veronesi, Antonio Rossi, Camilla Coletti, Stefan Heun: Li-intercalated Graphene on SiC(0001): an STM study, arXiv:1706.01386 [cond-mat.mtrl-sci].
  3. Sara Fiori, Yuya Murata, Stefano Veronesi, Antonio Rossi, Camilla Coletti, and Stefan Heun: Li-intercalated graphene on SiC(0001): An STM study, Phys. Rev. B 96 (2017) 125429.
  4. Valentina Tozzini, Stefan Heun: Graphene Manipulation for Energy Applications, CNR Nano Activity Report 2018 [Page 59].
  5. Marion Bonhomme: Li intercalated Graphene on SiC(0001), Report on her work during her internship in Pisa. [Read]

Presented at:

  1. Sara Fiori: Li-functionalized Graphene on Silicon Carbide, Thesis Defense, University of Pisa, Italy, 13 March 2017. [Talk]
  2. S. Heun: Hydrogen storage in metal-functionalized graphene, 103º Congresso Nazionale della Società Italiana di Fisica, Trento, Italy, 11 – 15 September 2017 (invited). [Abstract] [Talk]
  3. S. Heun: Metal-functionalized graphene for hydrogen storage, SKKU, Seoul, S. Korea (Prof. Dongmok Whang), 21 September 2017 (invited). [Abstract] [Talk]
  4. S. Heun: The importance of surfaces for the properties of 2D materials: from graphene to phosphorene, University of Seoul, S. Korea (Prof. Jeil Jung), 22 September 2017 (invited). [Abstract] [Talk]
  5. S. Fiori, Y. Murata, S. Veronesi, A. Rossi, C. Coletti, and S. Heun: Li-intercalated graphene on SiC(0001): an STM study, Fismat 2017, Trieste, Italy, 1 – 5 October 2017 (oral). [Abstract] [Talk]
  6. Marion Bonhomme: Intercalation du Li dans du Graphène sur un substrat de SiC, Presentation on her work during her internship in Pisa, INP Phelma, Grenoble, France, 1 October 2018 (oral). [Talk]