Who: Vito Despoja, University of Zagreb, Croatia
Place: Donostia International Physics Center
Date: Friday, 15 September 2017, 12:00
Plasmonics in alkali-intercalated graphene
Vito Despoja1 and Leonardo Maru?i?2
1Department of Physics, University of Zagreb, Bijeni?ka 32, HR-10000 Zagreb, Croatia
2Maritime Department, University of Zadar, M. Pavlinovi?a 1, HR-23000 Zadar, Croatia
During the last decade, graphene and various graphene-like materials have been extensively studied because of the numerous possibilities for their application in various fields. This trend has been increasing as these materials have become easier to produce, making them applicable in fields like photonics, plasmonics or nanoelectronics. These application motivated accurate experimental and theoretical research of the single particle and collective electronic excitations, as well as their interaction with the crystal lattice vibrations, light, charged probes (EELS) , etc.
Pristine graphene supports only interband plasmons, and the dominant modes are ? and high energy ? + ? plasmons. The ? + ? plasmon strongly decays into the corresponding interband electron hole continuum and is only well defined in the optical long-wavelength limit, while the ? plasmon exists for higher wavevectors as well. The spectra and dispersions of these plasmons have been calculated using the DFT ab initio approach, as well as measured by numerous optical and EELS experiments , and the calculated values proved to be in good agreement with the measurements. In the doped graphene, in addition to the ? and ? + ? plasmons, the dominant mode is the low energy 2D Dirac plasmon, resulting from the intraband transitions within the Dirac cones around the K point .
Recently, graphene is being intercalated with various alkali and alkali earth metals, with very different motives. One is to explore the possible superconductivity of such compounds , the other is to restore the original properties of free-standing graphene which has been modified by the presence of a substrate (in which case low coverage is preferred), and the third one is to modify the electronic properties of the graphene (which is achieved for high coverage). This last feature is the most interesting from the point of view of electronic excitations and we therefore present it in detail, focusing especially on the lithium intercalated graphene.
The intercalated alkali metal donates electrons to the graphene causing the natural doping of the graphene and resulting in the formation of two quasi two-dimensional plasmas. It also adds new bands to the graphene band structure (e.g. Li(?) and Li(?) bands), opening possibilities for the intraband and interband transitions not possible in the pristine or doped graphene. For example, this system supports not one, but two intraband plasmons, acoustic and Dirac, with frequencies up to 4 eV, as well as various interband modes which occur at higher frequencies, and can be intra-layer and inter-layer . Because of the heavy doping, the Dirac plasmon is very strong, while the interband and inter-layer modes can be optically active in the visible and UV region and therefore interesting for optical applications. In addition to that, some of these modes can be manipulated, and even switched on and off, by a tiny doping.
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