Plasmons in graphene have unusual properties and offer promising prospects for plasmonic applications covering a wide frequency range, ranging from terahertz up to the visible. Plasmon modes have been recently studied in both free-standing and supported graphene. Here, we review plasmons in graphene with particular emphasis on plasmonic excitations in epitaxial graphene and on the influence of the underlying substrate on the screening processes. Although the theoretical comprehension of plasmons in supported graphene is still incomplete, several experimental results provide clues regarding the nature of plasmonic excitations in graphene on metals and semiconductors. Plasmon in graphene can be tuned by chemical doping and gating potentials. We show through selected examples that the adsorbates can be used to tune the plasmon frequency, while the intercalation of chemical species allows the decoupling of the graphene sheet from the substrate to recover the plasmon dispersion of pristine graphene. Finally, we also report intriguing effects due to many-body interaction, such as the excitations generated by electron-electron coupling (magnetoplasmons) and the composite modes arising from the coupling of plasmons with phonons and with charge carriers.

The dispersion and the damping of the sheet plasmon in a graphene monolayer grown on Pt(111) have been studied by using angle-resolved electron energy loss spectroscopy. We found that the dispersion relation of the plasmon mode confined in the graphene sheet is linear, as a consequence of the screening by the metal substrate. Present results demonstrate that the presence of an underlying metal substrate could have striking consequences on the plasmon propagation even in the case of a system which exhibits a weak graphene-substrate interaction. Moreover, we found that Landau damping essentially occurs via interband excitations starting above the Fermi wave vector. On the contrary, intraband transitions do not have a significant influence on the collective mode. Low-energy collective excitations in graphene are attracting much interest in recent years 1-8 as they influence many of the peculiar properties of graphene samples. In particular, the dispersion and damping of plasmons in epitaxial graphene have recently been studied for the case of graphene deposited on SiC(0001) (Refs. 2 and 4) and Ir(111). 6 The understanding of plasmonic excitations of graphene plays a key role in tailoring the properties of novel graphene-based devices. 9Among graphene systems, the epitaxial growth of monolayer graphene (MLG) on Pt(111) is particularly interesting 10-14 as a consequence of the weak graphene-Pt interaction, 11 in contrast with MLG grown on other transition-metal substrates. 15,16 In fact, the graphene-Pt distance (3.30Å) lies close to the c-axis spacing in graphite. The electronic structure of MLG on Pt(111) resembles that of isolated graphene. 10 In particular, the linear dispersion of π bands in the so-called Dirac cones, which gives rise to many manifestations of massless Dirac fermions, is preserved. Angle-resolved photoemission spectroscopy (ARPES) experiments 10 do not show any remarkable hybridization of graphene π states with metal d states. They just represent a superposition of graphene and metal-derived states, with minimal interaction between them. The MLG on Pt(111) is hole doped by charge transfer to the Pt substrate. Epitaxial graphene on Pt(111) thus behaves as an ideal 2D system, sustaining a purely 2D electron gas (2DEG) system whose collective excitations (plasmon modes) are able to propagate along the sheet. The dielectric response of the 2DEG system is determined by plasmon dispersion, which could be measured by high-resolution electron energy loss spectroscopy (HREELS).The 2D plasmon, characterized by its square-root-like dispersion, has been predicted 17 and observed in metal layers on semiconductors.18 On the other hand, the acoustic surface plasmon (ASP) with a linear dispersion was demonstrated to exist on semiconductor quantum wells with two interacting quantum well minibands.19 Successively, ASP has been experimentally revealed on Be(0001) (Ref. 20) and on noble-metal surfaces. 21,22 The acoustic-like dispersion is a consequence of the combination of the nonlocality of the 3D response and t...

We demonstrate that, in contrast to most two-dimensional materials, ultrathin flakes of InSe are stable under ambient conditions. Despite their ambient stability, InSe-based nanodevices show an environmental p-type doping, suppressed by capping InSe with hexagonal boron nitride. By means of transport experiments, density functional theory and vibrational spectroscopy, we attribute the p-type doping assumed by uncapped InSe under an ambient atmosphere to the decomposition of water at Se vacancies. We have estimated the site-dependent adsorption energy of O2, N2, H2O, CO and CO2 on InSe. A stable adsorption is found only for the case of H2O, with a charge transfer of only 0.01 electrons per water molecule.

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PtTe 2 is a novel transition-metal dichalcogenide hosting type-II Dirac fermions that displays application capabilities in optoelectronics and hydrogen evolution reaction. Here it is shown, by combining surface science experiments and density functional theory, that the pristine surface of PtTe 2 is chemically inert toward the most common ambient gases (oxygen and water) and even in air. It is demonstrated that the creation of Te vacancies leads to the appearance of tellurium-oxide phases upon exposing defected PtTe 2 surfaces to oxygen or ambient atmosphere, which is detrimental for the ambient stability of uncapped PtTe 2 -based devices. On the contrary, in PtTe 2 surfaces modified by the joint presence of Te vacancies and substitutional carbon atoms, the stable adsorption of hydroxyl groups is observed, an essential step for water splitting and the water-gas shift reaction. These results thus pave the way toward the exploitation of this class of Dirac materials in catalysis.

Transition-metal dichalcogenides showing type-II Dirac fermions are emerging as innovative materials for nanoelectronics. However, their excitation spectrum is mostly unexplored yet. By means of high-resolution electron energy loss spectroscopy and density functional theory, here, we identify the collective excitations of type-II Dirac fermions (3D Dirac plasmons) in PtTe_{2} single crystals. The observed plasmon energy in the long-wavelength limit is ∼0.5  eV, which makes PtTe_{2} suitable for near-infrared optoelectronic applications. We also demonstrate that interband transitions between the two Dirac bands in PtTe_{2} give rise to additional excitations at ∼1 and ∼1.4  eV. Our results are crucial to bringing to fruition type-II Dirac semimetals in optoelectronics.

Photoelectrochemical (PEC) systems represent powerful tools to convert electromagnetic radiation into chemical fuels and electricity. In this context, two-dimensional (2D) materials are attracting enormous interest as potential advanced photo(electro)catalysts and, recently, 2D group-IVA metal monochalcogenides have been theoretically predicted to be water splitting photocatalysts. In this work, we use density functional theory calculations to theoretically investigate the photocatalytic activity of single-/few-layer GeSe nanoflakes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in pH conditions ranging from 0 to 14. Our simulations show that GeSe nanoflakes with different thickness can be mixed in the form of nanoporous films to act as nanoscale tandem systems, in which the flakes, depending on their thickness, can operate as HER-and/or OER photocatalysts. On the basis of theoretical predictions, we report the first experimental characterization of the photo(electro)catalytic activity of single-/few-layer GeSe flakes in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H 2 SO 4 (pH 0.3), 1 M KCl (pH 6.5), and 1 M KOH (pH 14). The films of the GeSe nanoflakes are fabricated by spray coating GeSe nanoflakes dispersion in 2-propanol obtained through liquid-phase exfoliation of synthesized orthorhombic (Pnma) GeSe bulk crystals. The PEC properties of the GeSe nanoflakes are used to design PEC-type photodetectors, reaching a responsivity of up to 0.32 AW −1 (external quantum efficiency of 86.3%) under 455 nm excitation wavelength in acidic electrolyte. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching that of self-powered commercial UV−Vis photodetectors. The obtained results inspire the use of 2D GeSe in proof-of-concept water photoelectrolysis cells.