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Plasmonics in alkali-intercalated graphene

Friday, September 15, 2017 - 12:00
Place: 
Donostia International Physics Center
Who: 
Vito Despoja, University of Zagreb, Croatia
Source Name: 
DIPC

Plasmonics
in alkali-intercalated graphene


Vito
Despoja1
and
Leonardo Marušić2



1Department
of Physics, University of Zagreb, Bijenička 32, HR-10000 Zagreb,
Croatia

2Maritime
Department, University of Zadar, M. Pavlinovića 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) [1], etc.

Pristine
graphene supports only interband plasmons, and the dominant modes are
π and high energy π + σ plasmons. The π + σ plasmon strongly
decays into the corresponding interband electron hole continuum and
is only well defined in the optical long-wavelength limit, while the
π 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 [2], and the calculated values proved to be in good
agreement with the measurements. In the doped graphene, in addition
to the π and π + σ plasmons, the dominant mode is the low energy
2D Dirac plasmon, resulting from the intraband transitions within the
Dirac cones around the K point [3].


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 [4], 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(σ) and Li(π) 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
[5]. 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.


[1] D.
Novko, M. Šunjić and V. Despoja, Phys. Rev B 93, 125413
(2016)

[2]
T. Eberlein et
al
,
Phys. Rev. B 77,
233406 (2008)

[3]
V. Despoja et
al,
Phys.
Rev. B 87,
075447 (2013)

[4]
S. Ichinokur et al, ACS Nano 10,
2761 (2016)

[5]
L. Marušić and V. Despoja, Phys. Rev. B 95,
201408(R) (2017)

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