posted on 2022-08-22, 09:01authored byTrevor P. Hardcastle, Fredrik S. Hage, Morten N. Gjerding, Demie M. Kepaptsoglou, Che R. Seabourne, Kirsten T. Winther, Recep Zan, Julian Alexander Amani, Hans C. Hofsaess, Ursel BangertUrsel Bangert, Kristian S. Thygesen, Quentin M. Ramasse
Single atom B or N substitutional doping in single-layer suspended graphene,
realised by low energy ion implantation, is shown to induce a dampening or enhancement
of the characteristic interband π plasmon of graphene through a high-resolution electron
energy loss spectroscopy study in the scanning transmission electron microscope. A relative
16% decrease or 20% increase in the π plasmon quality factor is attributed to the presence
of a single substitutional B or N atom dopant respectively. This modification is in both cases
shown to be relatively localised, with data suggesting the plasmonic response tailoring can
no longer be detected within experimental uncertainties beyond a distance of
approximately 1 nm from the dopant. Ab initio calculations confirm the trends observed
experimentally. Our results directly confirm the possibility of tailoring the plasmonic
properties of graphene in the ultraviolet waveband, at the atomic scale, a crucial step in the quest for utilising graphene’s properties towards the development of plasmonic and
optoelectronic devices operating at ultraviolet frequencies.
Due to its fascinating properties, graphene is emerging as a highly promising plasmonic
material for implementation in devices aimed at applications such as chemical and
molecular sensing, ultrafast optical modulation, non-linear optics, photo detection, light
sources and quantum optics.1-6 In the terahertz (THz) to mid-infrared (mid-IR) spectral range
graphene plasmons are associated with the collective excitation of free charge carriers and
exhibit a higher degree of tunability and mode confinement, as well as longer propagation
distances than noble metals.1-5 The graphene “charge carrier plasmon” frequency scales as
⁄ . , where EF is the Fermi energy and D is the size of the graphene sheet.1, 4 This
means that the plasmon frequency can be tuned by either varying the Fermi level (e.g.
through electrostatic gating4, 5, 7, 8 or chemical doping2, 4, 5), modifying the size of the
graphene sheet4 (e.g. by making micro to nanoscale graphene ribbons4, 5 or discs3, 5, 9) or a
combination of both. Using one or both these “methods” to push the charge carrier
plasmon frequency into the near-IR to visible spectrum is of significant current interest in the community.1, 3, 4, 10 Existing graphene-based IR plasmonic devices already exhibit
promising properties, such as gate tuneable switching and control of the plasmon
wavelength as well as a 40-60 times reduction in plasmon wavelength (as compared to the
incident IR illumination).7, 8 While these plasmons can propagate a distance on the order of
a few times their own wavelength,7, 8 on par with measurements of plasmons in Au,8, 11 this
falls short of that expected for high purity graphene.7, 8 This has been attributed to
disorder.12 Indeed, the graphene charge carrier plasmon mobility is expected to decrease to
various degrees depending on type and concentration of dopants12, 13 and other defects,14 as well as the specific edge structures of nanoscale ribbons and similar nanoscale
geometries.4 Possible strategies for realisation of graphene based plasmonics in the near-IR
to visible spectrum, while taking into consideration the above effects (among others), are
discussed in Ref. 4
At higher spectral frequencies, in the ultraviolet (UV) range, graphene exhibits interband
plasmons resonances attributed to the collective oscillation of π and σ valence electrons.15-
17 These interband plasmons show a remarkable degree of sensitivity to various nano- to
atomic-scale structures and defects in graphene: interband plasmon localisation has been
attributed to confinement induced by edge states of a ~1.3 nm graphene quantum disc18
and single substitutional Si atoms have been associated with a highly localised enhancement
of the interband plasmon response.19 In periodically rippled graphene (on a Ru(0001)
surface) the interband π plasmon is confined to ripple “hills” while being significantly
dampened in ripple “valleys”.20 Admittedly showing a more limited tunability compared to
the charge carrier plasmon,4 the interband π plasmon frequency is nonetheless predicted to
progressively red-shift with increasing graphene nanodisc diameter, being the most sensitive to disc diameters below 20 nm.21 However, with the exception of the above
studies, reports on other aspects of the interband plasmon response of graphene are lacking
in the literature. Such knowledge might open up avenues for future implementation of
graphene based plasmonic and optoelectronic devices operating in the UV waveband. With
this goal in mind, the present work investigates the modification of the interband plasmon
response of graphene associated with two key substitutional dopants, namely boron and
nitrogen atoms. The inclusion of B or N atoms in the graphene lattice is the focus of extensive study in the
scientific community, with the aim to modify the electronic structure of graphene.22-29
Substitutional B and N atoms have been predicted to induce a shift of the Fermi level,26, 29, 30
resulting in p or n doping akin to that routinely exploited in current semi-conductor
technology. Indeed a p and n character has recently been verified in suspended graphene
containing single substitutional B and N atom dopants.29 Under certain circumstances,
boron and nitrogen doping is also expected to induce a band gap in graphene.24, 25.
Key to these proof-of-principle studies, electron energy loss spectroscopy (EELS) in
combination with high angle annular dark field (HAADF) imaging in the aberration corrected
scanning transmission electron microscope (STEM) are considered particularly useful
techniques for identifying individual nano to atomic scale defects in a material and the
associated effects on its electronic structure and dielectric response.31 Due to the ideal
‘gentle STEM’32 combination of ultra-high vacuum conditions and low acceleration voltage
(which minimises any beam-induced damage to the samples), individual B23, 29 and N23, 29, 33,
34 atom dopants in graphene can be identified directly in an ADF image. So-called “core” EEL spectra (EEL >≈ 50 eV) contain information about the local electronic structure and bonding
in graphene,28, 29, 33-35 while “valence” EEL spectra (EEL <≈ 50 eV) contain information about
the graphene dielectric response18, 19, 36-41. In combination with simultaneous (STEM) ADF
imaging, EEL spectra allow for a direct correlation of defect-induced modifications of the
graphene electronic structure28, 29, 33-35 and dielectric response18, 19 with atomic scale
structure. These capabilities mean STEM-EELS is an excellent technique for investigating the
interband plasmon response induced by individual B and N atom dopants in graphene, as this information can be correlated directly with the atomic structure, all within the same
experiment.
In the valence loss spectrum of graphene the so-called “π peak” (~ 5 eV) is attributed
primarily to the excitation of the π interband plasmon,37, 39-43 superimposed on a sum over
π-π* interband transitions.37, 39-42 A recent controversy concerning the interpretation of the
π peak41, 44 was addressed in subsequent theoretical studies which show that (for STEMEELS)
the π loss peak is most appropriately interpreted as predominantly due to the
excitation of the interband π plasmon of both doped17 and dopant free15, 16 graphene. In
following with this, we use STEM-EELS to show that a single B or N substitutional atom
dopant induces dampening or enhancement of the graphene interband π plasmon,
respectively, with an estimated relative 16% decrease (B) or 20% increase (N) in quality
factor, and, a ~1 nm localisation in both cases. This effect was observed independently using
two separate STEM-EELS systems (with different yet complementary experimental parameters) and is significantly more pronounced than that previously reported for single Si
atoms.19
Ab initio calculations were carried out on the largest supercells possible whilst keeping the
computational costs tractable to validate the experimental results. While our theoretical
spectra broadly reproduce the trends observed experimentally, it is suggested that the
supercell sizes used in the present work (in practice limited by associated computational
costs) are simply too small to accurately predict the relevant properties of the
experimentally probed systems and faithfully reproduce all the details of the loss function.Nevertheless, our combination of state-of-the-art experimental and theoretical results
demonstrates that the plasmonic properties of graphene can be tailored at the atomic scale,
using an implantation technique already extensively used in semi-conductor industry.23, 29
Atomic scale plasmon engineering of graphene might prove valuable in the quest for
utilising graphene’s properties towards the development of plasmonic and optoelectronic
devices operating in the UV waveband. Indeed, recent reports propose utilising the π
interband plasmon response associated with graphene nanopores as a sensing mechanism
for DNA nucleotides.45, 46