Plasmonic Nature Research Articles

Singular and Nonsingular Transitions in the Infrared Plasmons of Nearly Touching Nanocube Dimers.

Narrow gaps between plasmon-supporting materials can confine infrared electromagnetic energy at the nanoscale, thus enabling applications in areas such as optical sensing. However, in nanoparticle dimers, the nature of the transition between touching (zero gap) and nearly nontouching (nonzero gap ≲15 nm) regimes is still a subject of debate. Here, we observe both singular and nonsingular transitions in infrared plasmons confined to dimers of fluorine-doped indium oxide nanocubes when moving from touching to nontouching configurations depending on the dimensionality of the contact region. Through spatially resolved electron energy-loss spectroscopy, we find a continuous spectral evolution of the lowest-order plasmon mode across the transition for finite touching areas, in excellent agreement with the simulations. This behavior challenges the widely accepted idea that a singular transition always emerges in the near-touching regime of plasmonic particle dimers. The apparent contradiction is resolved by theoretically examining different types of gap morphologies, revealing that the presence of a finite touching area renders the transition nonsingular, while one-dimensional and point-like contacts produce a singular behavior in which the lowest-order dipolar mode in the touching configuration, characterized by a net induced charge in each of the particles, becomes unphysical as soon as they are separated. Our results provide valuable insights into the nature of dimer plasmons in highly doped semiconductors.

Self-assembly synthesis of two dimensional vanadium oxide nanotriangles for photocatalytic application

For the first time, we report a Two Dimensional Phase Transfer Reaction (2DPTR) between the three different phases of 2D nanosheets and their applications in catalysis and hydrogen evolution. Plasmonic two-dimensional (2D) nanomaterials have created a niche for themselves in a variety of applications due to their unique and tunable optical properties. In the present work, we report a simple method for the synthesis of three different phases of the 2D vanadium oxide nanosheets to convert to each other at room temperature (amorphous, nanoflakes and nanotriangles). We developed a two-step method that first involves probe ultrasonication to mediate the synthesis of 2D vanadium oxide nanoflakes (VNFs) at room temperature and secondly, their phase transformation to VNTs using hydrogen peroxide or Isopropyl Alcohol (IPA) as the phase transfer reagents. 2D VNTs have been studied for morphological, structural, and optical properties by using a variety of instruments, including TEM, UV–Vis, EPR, RAMAN, and XPS. Thus, after treatment with hydrochloric acid (HCl), the 2D vanadium oxide nanoflakes are easily converted to the 2D amorphous nanosheets (Vam) which then regain crystallinity in the form of VNTs after the addition of Hydrogen peroxide or IPA, with a plasmonic peak around 450 nm. Oxygen vacancies are created in the VNFs, after treatment with HCl, which results in the crystal lattice breakdown. These oxygen vacancies were filled up again when the nanoflakes were treated with hydrogen peroxide or IPA and then resulted in the formation of VNTs. The UV–vis spectroscopy was performed with different addition of concentrations of H2O2, the limit of detection 34.1 µM and linear range 0 to 1000 µM found. Also, a selectivity study proved H2O2 in these 2D VNTs. Vam and VNT also showed effective photocatalytic activity against KMnO4 which is a strong oxidizing reagent used in water treatment. The synthesis and plasmonic nature of the 2D VNTs and the photocatalytic activity of Vam and VNT present in this study is a pioneer report for these novel 2D nanomaterials and they can be further explored with many possibilities in many fields including sensing, catalysis, energy, biomedicine, and many other significant applications in the near future.

Modal Phase-Matched Bound States in the Continuum for Enhancing Third Harmonic Generation of Deep Ultraviolet Emission.

Coherent deep ultraviolet (DUV) light sources are crucial for various applications such as nanolithography, biomedical imaging, and spectroscopy. DUV light sources can be generated by using conventional nonlinear optical crystals (NLOs). However, NLOs are limited by their bulky size, inadequate transparency at the DUV regime, and stringent phase-matching requirements for harmonic generation. Recently, dielectric metasurfaces support high Q-factor resonances and offer a promising approach for efficient harmonic generation at short wavelengths. In this study, we demonstrated a crystalline silicon (c-Si) metasurface simultaneously exciting modal phase-matched bound states in the continuum (BIC) resonance at the fundamental wavelength of 840 nm with a higher degree of freedom for precise control of the BIC resonance and a plasmonic resonance at the wavelength of 280 nm in the DUV to enhance third harmonic generation (THG). We experimentally achieved a Q-factor of ∼180 owing to the relatively large refractive index of the c-Si and the geometric symmetry breaking of the structure. We realized THG at a wavelength of 280 nm with a power of 14.5 nW by using a peak power density of 15 GW/cm2 excitation. The measured THG power is 14 times higher than the state-of-the-art THG dielectric metasurfaces using the same peak power density in the DUV regime, and the maximum obtained THG power enhancement factor is up to 48. This approach relies on the significant third-order nonlinear susceptibility of c-Si, the interband plasmonic nature of the c-Si in the DUV, and the strong field confinement of BIC resonance to boost overall nonlinear conversion efficiency to 5.2 × 10-6% in the DUV regime. Our work shows the potential of c-Si BIC metasurfaces for developing efficient and ultracompact DUV light sources using high-efficacy nonlinear optical devices.

Intrinsic Visible Plasmonic Properties of Colloidal PtIn2 Intermetallic Nanoparticles.

Materials that intrinsically exhibit localized surface plasmon resonance (LSPR) in the visible region have been predominantly researched on nanoparticles (NPs) composed of coinage metals, namely Au, Ag, and Cu. Here, as a coinage metal-free intermetallic NPs, colloidal PtIn2 NPs with a C1 (CaF2 -type) crystal structure are synthesized by the liquid phase method, which evidently exhibit LSPR at wavelengths similar to face-centered cubic (fcc)-Au NPs. Computational simulations pointed out differences in the electronic structure and photo-excited electron dynamics between C1-PtIn2 and fcc-Au NPs; reduces interband transition and stronger screening with smaller number of bound d-electrons compare with fcc-Au are unique origins of the visible plasmonic nature of C1-PtIn2 NPs. These results strongly indicate that the intermetallic NPs are expected to address the development of alternative plasmonic materials by tuning their crystal structure and composition.

Single-Molecule Spectroscopy Reveals the Plasmon-Assisted Nanozyme Catalysis on AuNR@TiO2.

Gold nanoparticles are frequently employed as nanozyme materials due to their capacity to catalyze various enzymatic reactions. Given their plasmonic nature, gold nanoparticles have also found extensive utility in chemical and photochemical catalysis owing to their ability to generate excitons upon exposure to light. However, their potential for plasmon-assisted catalytic enhancement as nanozymes has remained largely unexplored due to the inherent challenge of rapid charge recombination. In this study, we have developed a strategy involving the encapsulation of gold nanorods (AuNRs) within a titanium dioxide (TiO2) shell to facilitate the efficient separation of hot electron/hole pairs, thereby enhancing nanozyme reactivity. Our investigations have revealed a remarkable 10-fold enhancement in reactivity when subjected to 530 nm light excitation following the introduction of a TiO2 shell. Leveraging single-molecule kinetic analyses, we discovered that the presence of the TiO2 shell not only amplifies catalytic reactivity by prolonging charge relaxation times but also engenders additional reactive sites within the nanozyme's intricate structure. We anticipate that further enhancements in nanozyme performance can be achieved by optimizing interfacial interactions between plasmonic metals and semiconductors.

Multilayer Model of Gold Nanoparticles (AuNPs) and Its Application in the Classical Molecular Dynamics Simulation of Citrate-Capped AuNPs.

Studies on the interaction between gold nanoparticles (AuNPs) and functional proteins have been useful in developing diagnostic and therapeutic agents. Such studies require a realistic computational model of AuNPs for successful molecular design works. This study offers a new multilayer model of AuNPs to address the inconsistency between its molecular mechanics' interpretation and AuNP's plasmonic nature. We performed partial charge quantum calculation of AuNPs using Au13 and Au55 models. The result showed that it has partial negative charges on the surface and partial positive charges on the inner part, indicating that the AuNP model should be composed of multiatom types. We tested the partial charge parameters of these gold (Au) atoms in classical molecular dynamics simulation (CMD) of AuNPs. The result showed that our parameters performed better in simulating the adsorption of Na+ and dicarboxy acetone in terms of consistency with surface charge density than the zero charges Au in the interface force field (IFF). We proposed that the multiple-charged AuNP model can be developed further into a simpler four-atom type of Au in a larger AuNP size.

Surfactant Layers on Gold Nanorods.

ConspectusGold nanorods (Au NRs) are an exceptionally promising tool in nanotechnology due to three key factors: (i) their strong interaction with electromagnetic radiation, stemming from their plasmonic nature, (ii) the ease with which the resonance frequency of their longitudinal plasmon mode can be tuned from the visible to the near-infrared region of the electromagnetic spectrum based on their aspect ratio, and (iii) their simple and cost-effective preparation through seed-mediated chemical growth. In this synthetic method, surfactants play a critical role in controlling the size, shape, and colloidal stability of Au NRs. For example, surfactants can stabilize specific crystallographic facets during the formation of Au NRs, leading to the formation of NRs with specific morphologies.The process of surfactant adsorption onto the NR surface may result in various assemblies of surfactant molecules, such as spherical micelles, elongated micelles, or bilayers. Again, the assembly mode is critical toward determining the further availability of the Au NR surface to the surrounding medium. Despite its importance and a great deal of research effort, the interaction between Au NPs and surfactants remains insufficiently understood, because the assembly process is influenced by numerous factors, including the chemical nature of the surfactant, the surface morphology of Au NPs, and solution parameters. Therefore, gaining a more comprehensive understanding of these interactions is essential to unlock the full potential of the seed-mediated growth method and the applications of plasmonic NPs. A plethora of characterization techniques have been applied to reach such an understanding, but many open questions remain.In this Account, we review the current knowledge on the interactions between surfactants and Au NRs. We briefly introduce the state-of-the-art methods for synthesizing Au NRs and highlight the crucial role of cationic surfactants during this process. The self-assembly and organization of surfactants on the Au NR surface is then discussed to better understand their role in seed-mediated growth. Subsequently, we provide examples and elucidate how chemical additives can be used to modulate micellar assemblies, in turn allowing for a finer control over the growth of Au NRs, including chiral NRs. Next, we review the main experimental characterization and computational modeling techniques that have been applied to shed light on the arrangement of surfactants on Au NRs and summarize the advantages and disadvantages for each technique. The Account ends with a "Conclusions and Outlook" section, outlining promising future research directions and developments that we consider are still required, mostly related to the application of electron microscopy in liquid and in 3D. Finally, we remark on the potential of exploiting machine learning techniques to predict synthetic routes for NPs with predefined structures and properties.

Surface plasmon resonance sensing in the advanced physics laboratory

We present a set of experiments and computations suitable for introducing upper-level undergraduate physics and engineering students to the interdisciplinary field of nanoplasmonics for periods ranging from a week-long advanced laboratory session to a summer research project. The end product is a tunable optofluidic device capable of detecting changes in a fluid medium as low as 0.002 refractive index units. The sensing element—a thin gold film on a glass prism coupled to a microfluidic cell—owes its sensitivity to the bound nature of the surface plasmon–polariton waves that are resonantly excited by evanescently coupled light at the gold–fluid interface. Pedagogically, surface plasmon resonance (SPR) sensing immerses students in the rich physics of nanoscale optics and evanescent waves in constructing and operating a precision apparatus and in developing theoretical, analytical, and numerical models to aid both in the physical understanding and engineering optimization of the SPR sensor.

Comparative study of GaAs nanostructures synthesized in air and distilled water by picosecond pulsed laser ablation and application in hazardous molecules detection

This work explored the fundamental differences/mechanisms between the GaAs substrates ablated in two different media of air and distilled water (DW). A scan area of 5 × 5 mm2 was ablated by a picosecond laser with a pulse duration of 30 ps, a repetition rate of 10 Hz, a wavelength of 1064 nm, and a pulse energy of 2 mJ. The spacing between raster scan lines was varied (0.05–0.35 mm), keeping the scan speed (0.15 mm/s) constant. The obtained GaAs nanostructures (NSs) were thoroughly analyzed using microscopy techniques. A clear increase in separation between the raster scan lines was observed with an increase in the scan spacing for the GaAs NSs fabricated in air, whereas the same result was not observed in DW. Moreover, structures with debris were formed in air irrespective of the spacing, unlike the formation of uniform quasiperiodic GaAs NSs throughout the sample in the case of DW ablation. To the best of our knowledge, there are no reports on the detailed studies involving DW in the fabrication of quasiperiodic NSs of GaAs. Further, these quasiperiodic GaAs NSs formed in DW were coated with a thin layer of gold using the thermal evaporation method, annealed at 400 °C for 1 h in an ambient atmosphere. As a consequence of annealing, Au NPs were uniformly decorated on the quasiperiodic NSs of GaAs imparting plasmonic nature to the whole structures. Subsequently, the Au NPs decorated GaAs NSs were utilized as surface enhanced Raman scattering substrates for the detection of methylene blue (dye molecule) and Thiram (pesticide molecule) at low concentrations.

Broadband Enhancement of Cherenkov Radiation Using Dispersionless Plasmons.

As one of leading technologies in detecting relativistic particles, Cherenkov radiation plays an essential role in modern high‐energy and particle physics. However, the limited photon yield in transparent dielectrics makes efficient Cherenkov radiation only possible with high‐energy particles (at least several MeV). This restriction hinders applications of Cherenkov radiation in free‐electron light source, bio‐imaging, medical therapy, etc. Broadband enhancement of Cherenkov radiation is highly desired for all these applications, but still widely acknowledged as a scientific challenge. To this end, a general approach is reported to enhance the photon yield of Cherenkov radiation using dispersionless plasmons. Broadband dispersionless plasmons can be realized by exploiting either the acoustic nature of terahertz plasmons in a graphene‐based heterostructure or the nonlocal property of optical plasmons in a metallodielectric structure. When coupled to moving electrons, such dispersionless plasmons give rise to a radiation enhancement rate more than two orders of magnitude (as compared with conventional Cherenkov radiation) over an ultrabroad frequency band. Moreover, since the phase velocity of dispersionless plasmons can be made as small as the Fermi velocity, giant radiation enhancements can be readily induced by ultralow‐energy free electrons (e.g., with a kinetic energy down to 3 eV), without resorting to relativistic particles.

Infrared nanoplasmonic properties of hyperdoped embedded Si nanocrystals in the few electrons regime

Abstract Using localized surface plasmon resonance (LSPR) as an optical probe we demonstrate the presence of free carriers in phosphorus doped silicon nanocrystals (SiNCs) embedded in a silica matrix. In small SiNCs, with radius ranging from 2.6 to 5.5 nm, the infrared spectroscopy study coupled to numerical simulations allows us to determine the number of electrically active phosphorus atoms with a precision of a few atoms. We demonstrate that LSP resonances can be supported with only about 10 free electrons per nanocrystal, confirming theoretical predictions and probing the limit of the collective nature of plasmons. We reveal the appearance of an avoided crossing behavior linked to the hybridization between the localized surface plasmon in the doped nanocrystals and the silica matrix phonon modes. Finally, a careful analysis of the scattering time dependence versus carrier density in the small size regime allows us to detect the appearance of a new scattering process at high dopant concentration, which can be explained by P clustering inside the SiNCs.

Visible light driven robust photocatalytic activity in vanadium-doped ZnO/SnS core-shell nanocomposites for decolorization of MB dye towards wastewater treatment

Vanadium metal oxide (V2O5) doped ZnO/SnS core-shell nanocomposites are fabricated by hydrothermal method at 2200C for 10 hours. All synthesized Nanocomposites were optically, structurally, morphologically and magnetically examined by different techniques such as UV-DRS, XRD, TEM, electron paramagnetic resonance (EPR), XPS and PL techniques. The photocatalytic activity (PCA) was studied for methylene blue (MB) model pollutant dye under 300 W visible light illumination. XRD peaks confirmed the mixed phase abundance with hexagonal ZnO and orthorhombic SnS. TEM micrographs inferred uneven surface with more agglomeration. XPS conclude V4+ oxidation state for vanadyl atoms along with constitutes elements. The UV-DRS inferred red shift in bandgap. EPR argued that vanadyl ions perform tetragonal compression distorted octahedral site symmetry. PL study described quantum confinement effect. The achieved MB dye degradation under visible light illumination is more than 96% in 2 hours. Enhanced PCA argued owing to plasmonic effect and tunable energy bandgap nature of heterogeneous semiconducting materials.

Low Cost and Easy Validation Anticounterfeiting Plasmonic Tags Based on Thin Films of Metal and Dielectric.

Multilevel anticounterfeiting Physical Unclonable Function (PUF) tags based on thin film of silver (Ag), Zinc Oxide (ZnO) and PolyVinylPyrrolidone (PVP), are experimentally demonstrated and validated. We exploit the low adhesion of silver to glass and consequent degradation during ZnO deposition to induce morphological randomness. Several photographs of the tag surfaces have been collected with different illumination conditions and using two smartphones of diverse brand. The photos were analyzed using an image recognition algorithm revealing low common minutiae for different tags. Moreover, the optical response reveals peculiar spectra due to labels of plasmonic nature. The proposed systems can be easily fabricated on large areas and represent a cost-effective solution for practical protection of objects.

Influencing surface phenomena by Au diffusion in buffered TiO2-Au thin films: Effects of deposition and annealing processing

Enhanced optical properties of TiO2 thin films can be achieved by doping or coating with metals. Metal-doped TiO2 can reduce electron-hole recombination and increase hydroxyl radical concentration on the surface of TiO2, resulting in increase of the photocatalytic activity. TiO2 and TiO2-Au thin films were obtained by DC magnetron sputtering of Ti target with Ar ions in O2 atmosphere. In the case of Au buffered TiO2 thin films, two different systems were deposited for comparison. Post deposition annealing at 400 °C was carried out in nitrogen atmosphere for 3 h. The structural and optical characteristics of the prepared films were investigated in detail by the combination of high-resolution microscopic and spectroscopic techniques. The photo-degradation rate was measured using Rhodamine B which simulated pollutant. Post deposition annealing resulted in diffusion of Au atoms through the layer as it was revealed by transmission electron microscopy and energy dispersive X-ray spectroscopy. The obtained TiO2 thin films were found to be amorphous after deposition and annealing at 400 °C leads to crystallization of anatase phase. Analysis of the binding energy in the corresponding X-ray photoelectron spectra has confirmed stoichiometry of TiO2 and that concentration of Au on the surface can be controlled by sputtering and annealing conditions, thus changing optical properties by influencing the surface plasmonic nature. All Au doped TiO2 thin films exhibited enhanced wettability and photo-degradation rates when compared to pristine titania films.

Tunable Near-Field Radiative Effect in a Td – WTe2 Single Layer

In contrast to artificial metamaterials, natural two-dimensional (2D) hyperbolic materials are able to support higher electromagnetic confinement and stronger photonic density of states. This means natural 2D hyperbolic materials have fascinating potential for applications in thermal photonics. In this work, we investigate mechanisms of near-field thermal radiation (NFTR) in a ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$ single layer at the nanoscale using the fluctuation-dissipation theorem and density-functional theory. The results show that NFTR of ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$ can be 3 orders of magnitude larger than the blackbody limit. By using the photonic tunneling coefficient and the plasmon dispersion, we present a comprehensive study of plasmonic properties of ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$. Moreover, our first-principles calculations predict that application of a certain amount of mechanical stress can trigger topological transition between the elliptic and the hyperbolic surface states by regulating the Fermi surface and interband excitation threshold. Lastly, we systematically exhibit the evolution of surface plasmon polaritons in ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$ under mechanical stress and analyze the performance of using mechanical stress to modulate the corresponding NFTR. Our work explores the great potential of tunable radiative heat flux in ${\mathrm{T}}_{d}$--${\mathrm{WTe}}_{2}$, which is explained here through the peculiar nature of hyperbolic surface plasmons.

Selective Defect Metal Sulfide for Localized Surface Plasmon Resonance

As a p-type semiconductor with metal vacancies in the lattice,coppersulfide Cu2-xS has been proved asan effective photocatalyst with its unique semiconducting character, non-toxic nature andexcellent optoelectronic properties. In this work, we have synthesized copper sulphide nanostructure via hydrothermal method by using cetyltrimethylammonium bromide (CTAB) as a morphology controlling agent. Due to plasmonic nature, results from copper vacancy Cu2-xS nanocrystals possess a property called localized surface plasmon resonance which makes them capable of utilizing the NIR region of solar spectrum. The aim of this study is to explore copper sulphide catalyst which is capable of absorbing NIR region of the solar spectrum. It is found that copper sulphide has significantly enhanced light absorption over the entire wavelength range (from ultra-violet to near-infrared).

Efficient Protein Transfection by Swarms of Chemically Powered Plasmonic Virus-Sized Nanorobots.

Transfection is based on nonviral delivery of nucleic acids or proteins into cells. Viral approaches are being used; nevertheless, their translational capacity is nowadays decreasing due to persistent fear of their safety, therefore creating space for the field of nanotechnology. However, nanomedical approaches introducing static nanoparticles for the delivery of biologically active molecules are very likely to be overshadowed by the vast potential of nanorobotics. We hereby present a rapid nonviral transfection of protein into a difficult-to-transfect prostate cancer cell line facilitated by chemically powered rectangular virus-sized (68 nm × 33 nm) nanorobots. The enhanced diffusion of these biocompatible nanorobots is the key to their fast internalization into cells, happening in a matter of minutes and being up to 6-fold more efficient compared to static nanorobots in a nonfueled environment. The Au/Ag plasmonic nature of these nanorobots makes them simply traceable and allows for their detailed subcellular localization. Protein transfection mediated by such nanorobots is an important step forward, challenging the field of nanomedicine and having potential in future translational medical research.

Tunable Size Dependence of Quantum Plasmon of Charged Gold Nanoparticles.

The quantum behavior of surface plasmons has received extensive attention, benefiting from the development of exquisite nanotechnology and the diverse applications. Blueshift, redshift, and nonshift of localized surface plasmon resonances (LSPRs) have all been reported as the particle size decreases and enters the quantum size regime, but the underlying physical mechanism to induce these controversial size dependences is not clear. Herein, we propose an improved semiclassical model for modifying the dielectric function of metal nanospheres by combining the intrinsic quantized electron transitions and surface electron injection or extraction to investigate the plasmon shift and LSPR size dependence of the charged Au nanoparticles. We experimentally observe that the nonmonotonic blueshift of LSPRs with size for Au nanoparticles is turned into an approximately monotonic blueshift by increasing the electron donor concentration in the reduction solution, and it can also be transformed to an approximately monotonic redshift after surface passivation by ligand molecules. Moreover, we demonstrate controlled blueshift and redshift for the electron and hole plasmons in Cu_{2-x}S@Au core-shell nanoparticles by injecting electrons. The experimental observations and the theoretical calculations clarify the controversial size dependences of LSPR reported in the literature, reveal the critical role of surface electron injection or extraction in the transformation between the different size dependences of LSPRs, and are helpful for understanding the nature of surface plasmons in the quantum size regime.

From single-particle-like to interaction-mediated plasmonic resonances in graphene nanoantennas.

Plasmonic nanostructures attract tremendous attention as they confine electromagnetic fields well below the diffraction limit while simultaneously sustaining extreme local field enhancements. To fully exploit these properties, the identification and classification of resonances in such nanostructures is crucial. Recently, a novel figure of merit for resonance classification has been proposed1 and its applicability was demonstrated mostly to toy model systems. This novel measure, the energy-based plasmonicity index (EPI), characterizes the nature of resonances in molecular nanostructures. The EPI distinguishes between either a single-particle-like or a plasmonic nature of resonances based on the energy space coherence dynamics of the excitation. To advance the further development of this newly established measure, we present here its exemplary application to characterize the resonances of graphene nanoantennas. In particular, we focus on resonances in a doped nanoantenna. The structure is of interest, as a consideration of the electron dynamics in real space might suggest a plasmonic nature of selected resonances in the low doping limit but our analysis reveals the opposite. We find that in the undoped and moderately doped nanoantenna, the EPI classifies all emerging resonances as predominantly single-particle-like and only after doping the structure heavily, the EPI observes plasmonic response.

Energy-Based Plasmonicity Index to Characterize Optical Resonances in Nanostructures

Resonances sustained by plasmonic nanoparticles provide extreme electric field confinement and enhancement into the deep subwavelength domain for a plethora of applications. Recent progress in nanofabrication made it even possible to tailor the properties of nanoparticles consisting of only a few hundred atoms. These nanoparticles support both single-particle-like resonances and collective plasmonic charge density oscillations. Prototypical systems sustaining both features are graphene nanoantennas. In pushing the frontier of nanoscience, traditional identification, and classification of such resonances is at stake again. We show that in such nanostructures, the concerted electron cloud oscillation in real space does not necessarily come along with collective dynamics of conduction band electrons in energy space. This unveils an urgent need for a discussion of how a plasmon in nanostructures should be defined. Here, we propose to define it relying on energy space dynamics. The unambiguous identification of the plasmonic nature of a resonance is crucial to find out whether desirable plasmon-assisted features, such as frequency conversion processes, can be expected from a resonance. We elaborate an energy-based figure of merit that classifies the nature of resonances in nanostructures, motivated by tight binding simulations with a toy model consisting of a linear chain of atoms. We apply afterward the proposed figure of merit to a doped hexagonal graphene nanoantenna, which is known to support plasmons in the near infrared and single-particle-like transitions in the visible.