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1. Gate-tunable plasmons in mixed-dimensional van der Waals heterostructures

   https://doi.org/10.1038/s41467-021-25269-0  

   Wang, Sheng; Yoo, SeokJae; Zhao, Sihan; Zhao, Wenyu; Kahn, Salman; Cui, Dingzhou; Wu, Fanqi; Jiang, Lili; Utama, M. Iqbal Bakti; Li, Hongyuan; et al (August 2021, Nature Communications)

   Abstract Surface plasmons, collective electromagnetic excitations coupled to conduction electron oscillations, enable the manipulation of light–matter interactions at the nanoscale. Plasmon dispersion of metallic structures depends sensitively on their dimensionality and has been intensively studied for fundamental physics as well as applied technologies. Here, we report possible evidence for gate-tunable hybrid plasmons from the dimensionally mixed coupling between one-dimensional (1D) carbon nanotubes and two-dimensional (2D) graphene. In contrast to the carrier density-independent 1D Luttinger liquid plasmons in bare metallic carbon nanotubes, plasmon wavelengths in the 1D-2D heterostructure are modulated by 75% via electrostatic gating while retaining the high figures of merit of 1D plasmons. We propose a theoretical model to describe the electromagnetic interaction between plasmons in nanotubes and graphene, suggesting plasmon hybridization as a possible origin for the observed large plasmon modulation. The mixed-dimensional plasmonic heterostructures may enable diverse designs of tunable plasmonic nanodevices. 

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2. Plasmonic gain in current biased tilted Dirac nodes

   https://doi.org/10.1038/s41467-022-35139-y  

   Park, Sang Hyun; Sammon, Michael; Mele, Eugene; Low, Tony (December 2022, Nature Communications)

   Abstract Surface plasmons, which allow tight confinement of light, suffer from high intrinsic electronic losses. It has been shown that stimulated emission from excited electrons can transfer energy to plasmons and compensate for the high intrinsic losses. To-date, these realizations have relied on introducing an external gain media coupled to the surface plasmon. Here, we propose that plasmons in two-dimensional materials with closely located electron and hole Fermi pockets can be amplified, when an electrical current bias is applied along the displaced electron-hole pockets, without the need for an external gain media. As a prototypical example, we consider WTe₂from the family of 1T$${}^{{\prime} }$$ ${}^{\prime }$-MX₂materials, whose electronic structure can be described within a type-II tilted massive Dirac model. We find that the nonlocal plasmonic response experiences prominent gain for experimentally accessible currents on the order of mAμm⁻¹. Furthermore, the group velocity of the plasmon found from the isofrequency curves imply that the amplified plasmons are highly collimated along a direction perpendicular to the Dirac node tilt when the electrical current is applied along it. 

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3. The role of the plasmon in interfacial charge transfer

   https://doi.org/10.1126/sciadv.adp3353  

   Ostovar, Behnaz; Lee, Stephen A; Mehmood, Arshad; Farrell, Kieran; Searles, Emily K; Bourgeois, Briley; Chiang, Wei-Yi; Misiura, Anastasiia; Gross, Niklas; Al-Zubeidi, Alexander; et al (July 2024, Science Advances)

   The lack of a detailed mechanistic understanding for plasmon-mediated charge transfer at metal-semiconductor interfaces severely limits the design of efficient photovoltaic and photocatalytic devices. A major remaining question is the relative contribution from indirect transfer of hot electrons generated by plasmon decay in the metal to the semiconductor compared to direct metal-to-semiconductor interfacial charge transfer. Here, we demonstrate an overall electron transfer efficiency of 44 ± 3% from gold nanorods to titanium oxide shells when excited on resonance. We prove that half of it originates from direct interfacial charge transfer mediated specifically by exciting the plasmon. We are able to distinguish between direct and indirect pathways through multimodal frequency-resolved approach measuring the homogeneous plasmon linewidth by single-particle scattering spectroscopy and time-resolved transient absorption spectroscopy with variable pump wavelengths. Our results signify that the direct plasmon-induced charge transfer pathway is a promising way to improve hot carrier extraction efficiency by circumventing metal intrinsic decay that results mainly in nonspecific heating. 

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4. Infrared plasmons in ultrahigh conductive PdCoO2 metallic oxide

   https://doi.org/10.1038/s42005-022-00924-0  

   Macis, Salvatore; Tomarchio, Luca; Tofani, Silvia; Piccirilli, Federica; Zacchigna, Michele; Aglieri, Vincenzo; Toma, Andrea; Rimal, Gaurab; Oh, Seongshik; Lupi, Stefano (December 2022, Communications Physics)

   Abstract PdCoO 2 layered delafossite is the most conductive compound among metallic oxides, with a room-temperature resistivity of nearly $$2\,\mu \Omega \,{{{{{\rm{cm}}}}}}$$ 2 μ Ω cm , corresponding to a mean free path of about 600 Å. These values represent a record considering that the charge density of PdCoO 2 is three times lower than copper. Although its notable electronic transport properties, PdCoO 2 collective charge density modes (i.e. surface plasmons) have never been investigated, at least to our knowledge. In this paper, we study surface plasmons in high-quality PdCoO 2 thin films, patterned in the form of micro-ribbon arrays. By changing their width W and period 2 W , we select suitable values of the plasmon wavevector q , experimentally sampling the surface plasmon dispersion in the mid-infrared electromagnetic region. Near the ribbon edge, we observe a strong field enhancement due to the plasmon confinement, indicating PdCoO 2 as a promising infrared plasmonic material. 

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5. Intrinsic electronic conductivity of individual atomically resolved amyloid crystals reveals micrometer-long hole hopping via tyrosines

   https://doi.org/10.1073/pnas.2014139118  

   Shipps, Catharine; Kelly, H. Ray; Dahl, Peter J.; Yi, Sophia M.; Vu, Dennis; Boyer, David; Glynn, Calina; Sawaya, Michael R.; Eisenberg, David; Batista, Victor S.; et al (December 2020, Proceedings of the National Academy of Sciences)

   Proteins are commonly known to transfer electrons over distances limited to a few nanometers. However, many biological processes require electron transport over far longer distances. For example, soil and sediment bacteria transport electrons, over hundreds of micrometers to even centimeters, via putative filamentous proteins rich in aromatic residues. However, measurements of true protein conductivity have been hampered by artifacts due to large contact resistances between proteins and electrodes. Using individual amyloid protein crystals with atomic-resolution structures as a model system, we perform contact-free measurements of intrinsic electronic conductivity using a four-electrode approach. We find hole transport through micrometer-long stacked tyrosines at physiologically relevant potentials. Notably, the transport rate through tyrosines (10⁵s⁻¹) is comparable to cytochromes. Our studies therefore show that amyloid proteins can efficiently transport charges, under ordinary thermal conditions, without any need for redox-active metal cofactors, large driving force, or photosensitizers to generate a high oxidation state for charge injection. By measuring conductivity as a function of molecular length, voltage, and temperature, while eliminating the dominant contribution of contact resistances, we show that a multistep hopping mechanism (composed of multiple tunneling steps), not single-step tunneling, explains the measured conductivity. Combined experimental and computational studies reveal that proton-coupled electron transfer confers conductivity; both the energetics of the proton acceptor, a neighboring glutamine, and its proximity to tyrosine influence the hole transport rate through a proton rocking mechanism. Surprisingly, conductivity increases 200-fold upon cooling due to higher availability of the proton acceptor by increased hydrogen bonding. 

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