Olga Lozan, Ravishankar Sundararaman, Buntha Ea-Kim, Jean-Michel Rampnoux, Prineha Narang, Stefan Dilhaire, and Philippe Lalanne. 2017. “Increased rise time of electron temperature during adiabatic plasmon focusing.” Nature Communications, 8, 1, Pp. 1656. Publisher's VersionAbstract
Decay of plasmons to hot carriers has recently attracted considerable interest for fundamental studies and applications in quantum plasmonics. Although plasmon-assisted hot carriers in metals have already enabled remarkable physical and chemical phenomena, much remains to be understood to engineer devices. Here, we present an analysis of the spatio-temporal dynamics of hot electrons in an emblematic plasmonic device, the adiabatic nanofocusing surface-plasmon taper. With femtosecond-resolution measurements, we confirm the extraordinary capability of plasmonic tapers to generate hot carriers by slowing down plasmons at the taper apex. The measurements also evidence a substantial increase of the ``lifetime''of the electron gas temperature at the apex. This interesting effect is interpreted as resulting from an intricate heat flow at the apex. The ability to harness the ``lifetime''of hot-carrier gases with nanoscale circuits may provide a multitude of applications, such as hot-spot management, nonequilibrium hot-carrier generation, sensing, and photovoltaics.
Bart de Nijs, Felix Benz, Steven J. Barrow, Daniel O. Sigle, Rohit Chikkaraddy, Aniello Palma, Cloudy Carnegie, Marlous Kamp, Ravishankar Sundararaman, Prineha Narang, Oren A. Scherman, and Jeremy J. Baumberg. 2017. “Plasmonic tunnel junctions for single-molecule redox chemistry.” Nature Communications, 8, 1, Pp. 994. Publisher's VersionAbstract
Nanoparticles attached just above a flat metallic surface can trap optical fields in the nanoscale gap. This enables local spectroscopy of a few molecules within each coupled plasmonic hotspot, with near thousand-fold enhancement of the incident fields. As a result of non-radiative relaxation pathways, the plasmons in such sub-nanometre cavities generate hot charge carriers, which can catalyse chemical reactions or induce redox processes in molecules located within the plasmonic hotspots. Here, surface-enhanced Raman spectroscopy allows us to track these hot-electron-induced chemical reduction processes in a series of different aromatic molecules. We demonstrate that by increasing the tunnelling barrier height and the dephasing strength, a transition from coherent to hopping electron transport occurs, enabling observation of redox processes in real time at the single-molecule level.
N. Jiang, E. Zarkadoula, P. Narang, A. Maksov, I. Kravchenko, A. Borisevich, S. Jesse, and S. V. Kalinin. 2017. “Atom-by-atom fabrication by electron beam via induced phase transformations.” MRS Bulletin, 42, 9, Pp. 653-659. Publisher's VersionAbstract
New developments in manufacturing and automation, from three-dimensional printing to the “Internet of things,” signify dramatic changes in our society. The push toward quantum materials is driving device fabrication toward atomic precision. Recent results suggest that scanning transmission electron microscopy (STEM) with sub-angstrom scale beams could offer a solution. However, a detailed theoretical understanding of the interaction of the electron beam with solids is needed to form a basis for new technology. This article summarizes the existing literature on electron-beam interactions with solids with a focus on irreversible transformation. We further suggest that the theoretical framework of a two-temperature model developed for fast ion damage in solids could be applicable to predicting the effects of fast electrons. Recent results from STEM-directed epitaxial growth on crystalline–amorphous interfaces are discussed in detail. Finally, perspectives on the development of this field in the near future are offered.
Prineha Narang, Litao Zhao, Steven Claybrook, and Ravishankar Sundararaman. 2017. “Effects of Interlayer Coupling on Hot Carrier Dynamics in Graphene-derived van der Waals Heterostructures.” Advanced Optical Materials. Publisher's VersionAbstract
Graphene exhibits promise as a plasmonic material with high mode confinement that could enable efficient hot carrier extraction. The lifetimes and mean free paths of energetic carriers have been investigated in free‐standing graphene, graphite, and a heterostructure consisting of alternating graphene and hexagonal boron nitride layers using ab initio calculations of electron–electron and electron–phonon scattering in these materials. It is found that the extremely high lifetimes (3 ps) of low‐energy carriers near the Dirac point in graphene, which are a 100 times larger than that in noble metals, are reduced by an order of magnitude due to interlayer coupling in graphite, but enhanced in the heterostructure due to phonon mode clamping. However, these lifetimes drop precipitously with increasing carrier energy and are smaller than those in noble metals at energies exceeding 0.5 eV. By analyzing the contribution of different scattering mechanisms and interlayer interactions, desirable spacer layer characteristics—high dielectric constant and heavy atoms—that could pave the way for plasmonic heterostructures with improved hot carrier transport have been identified.
Ana M. Brown, Ravishankar Sundararaman, Prineha Narang, Adam M. Schwartzberg, William A. Goddard, and Harry A. Atwater. 2017. “Experimental and Ab Initio Ultrafast Carrier Dynamics in Plasmonic Nanoparticles.” Phys. Rev. Lett., 118, Pp. 087401. Publisher's VersionAbstract
Ultrafast pump-probe measurements of plasmonic nanostructures probe the nonequilibrium behavior of excited carriers, which involves several competing effects obscured in typical empirical analyses. Here we present pump-probe measurements of plasmonic nanoparticles along with a complete theoretical description based on first-principles calculations of carrier dynamics and optical response, free of any fitting parameters. We account for detailed electronic-structure effects in the density of states, excited carrier distributions, electron-phonon coupling, and dielectric functions that allow us to avoid effective electron temperature approximations. Using this calculation method, we obtain excellent quantitative agreement with spectral and temporal features in transient-absorption measurements. In both our experiments and calculations, we identify the two major contributions of the initial response with distinct signatures: short-lived highly nonthermal excited carriers and longer-lived thermalizing carriers.
J. Mertens, M.E. Kleemann, R. Chikkaraddy, P. Narang, and J. J Baumberg. 2017. “How Light Is Emitted by Plasmonic Metals.” Nano Letters, 17, 4, Pp. 2568-2574. Publisher's VersionAbstract
The mechanism by which light is emitted from plasmonic metals such as gold and silver has been contentious, particularly at photon energies below direct interband transitions. Using nanoscale plasmonic cavities, blue-pumped light emission is found to directly track dark-field scattering on individual nanoconstructs. By exploiting slow atomic-scale restructuring of the nanocavity facets to spectrally tune the dominant gap plasmons, this correlation can be measured from 600 to 900 nm in gold, silver, and mixed constructs ranging from spherical to cube nanoparticles-on-mirror. We show that prompt electronic Raman scattering is responsible and confirm that “photoluminescence”, which implies phase and energy relaxation, is not the right description. Our model suggests how to maximize light emission from metals.
Emiliano Cortés, Wei Xie, Javier Cambiasso, Adam S. Jermyn, Ravishankar Sundararaman, Prineha Narang, Sebastian Schlücker, and Stefan A. Maier. 2017. “Plasmonic hot electron transport drives nano-localized chemistry.” Nature Communications, 8, Pp. 14880 EP . Publisher's VersionAbstract
Nanoscale localization of electromagnetic fields near metallic nanostructures underpins the fundamentals and applications of plasmonics. The unavoidable energy loss from plasmon decay, initially seen as a detriment, has now expanded the scope of plasmonic applications to exploit the generated hot carriers. However, quantitative understanding of the spatial localization of these hot carriers, akin to electromagnetic near-field maps, has been elusive. Here we spatially map hot-electron-driven reduction chemistry with 15 nm resolution as a function of time and electromagnetic field polarization for different plasmonic nanostructures. We combine experiments employing a six-electron photo-recycling process that modify the terminal group of a self-assembled monolayer on plasmonic silver nanoantennas, with theoretical predictions from first-principles calculations of non-equilibrium hot-carrier transport in these systems. The resulting localization of reactive regions, determined by hot-carrier transport from high-field regions, paves the way for improving efficiency in hot-carrier extraction science and nanoscale regio-selective surface chemistry.
Georgia T. Papadakis, Prineha Narang, Ravishankar Sundararaman, Nicholas Rivera, Hrvoje Buljan, Nader Engheta, and Marin Soljačić. 2017. “Ultralight Angstrom-Scale Optimal Optical Reflectors.” ACS Photonics. Publisher's VersionAbstract
High reflectance in many state-of-the-art optical devices is achieved with noble metals. However, metals are limited by losses and, for certain applications, by their high mass density. Using a combination of ab initio and optical transfer matrix calculations, we evaluate the behavior of graphene-based angstrom-scale metamaterials and find that they could act as nearly perfect reflectors in the mid–long-wave infrared (IR) range. The low density of states for electron–phonon scattering and interband excitations leads to unprecedented optical properties for graphene heterostructures, especially alternating atomic layers of graphene and hexagonal boron nitride, at wavelengths greater than 10 μm. At these wavelengths, these materials exhibit reflectivities exceeding 99.7% at a fraction of the weight of noble metals, as well as plasmonic mode confinement and quality factors that are greater by an order of magnitude compared to noble metals. These findings hold promise for ultracompact optical components and waveguides for mid-IR applications. Moreover, unlike metals, the photonic properties of these heterostructures could be actively tuned via chemical and/or electrostatic doping, providing exciting possibilities for tunable devices.
P. Narang, R. Sundararaman, A. S. Jermyn, W. A. Goddard, and H. A. Atwater. 2016. “Cubic Nonlinearity Driven Up-Conversion in High-Field Plasmonic Hot Carrier Systems.” J. Phys. Chem. C, 120, Pp. 21056. Publisher's VersionAbstract
Surface plasmon resonances confine electromagnetic fields to the nanoscale, producing high field strengths suitable for exploiting nonlinear optical properties. We examine the prospect of detecting and utilizing one such property in plasmonic metals: the imaginary part of the cubic susceptibility, which corresponds to two plasmons decaying together to produce high energy carriers. Here we present ab initio predictions of the rates and carrier distributions generated by direct interband and phonon-assisted intraband transitions in one and two-plasmon decay. We propose detection of the higher energy carriers generated from two-plasmon decays that are inaccessible in one-plasmon decay as a viable signature of these processes in ultrafast experiments.
Ana M. Brown, Ravishankar Sundararaman, Prineha Narang, William A. Goddard, and Harry A. Atwater. 2016. “Ab initio phonon coupling and optical response of hot electrons in plasmonic metals.” Physical Review B, 94, 7, Pp. 075120–. Publisher's VersionAbstract
Ultrafast laser measurements probe the nonequilibrium dynamics of excited electrons in metals with increasing temporal resolution. Electronic structure calculations can provide a detailed microscopic understanding of hot electron dynamics, but a parameter-free description of pump-probe measurements has not yet been possible, despite intensive research, because of the phenomenological treatment of electron-phonon interactions. We present ab initio predictions of the electron-temperature dependent heat capacities and electron-phonon coupling coefficients of plasmonic metals. We find substantial differences from free-electron and semiempirical estimates, especially in noble metals above transient electron temperatures of 2000 K, because of the previously neglected strong dependence of electron-phonon matrix elements on electron energy. We also present first-principles calculations of the electron-temperature dependent dielectric response of hot electrons in plasmonic metals, including direct interband and phonon-assisted intraband transitions, facilitating complete theoretical predictions of the time-resolved optical probe signatures in ultrafast laser experiments.
Ana M. Brown, Ravishankar Sundararaman, Prineha Narang, William A. Goddard, and Harry A. Atwater. 2016. “Nonradiative Plasmon Decay and Hot Carrier Dynamics: Effects of Phonons, Surfaces, and Geometry.” ACS Nano, 10, 1, Pp. 957–966. Publisher's VersionAbstract
The behavior of metals across a broad frequency range from microwave to ultraviolet frequencies is of interest in plasmonics, nanophotonics, and metamaterials. Depending on the frequency, losses of collective excitations in metals can be predominantly classical resistive effects or Landau damping. In this context, we present first-principles calculations that capture all of the significant microscopic mechanisms underlying surface plasmon decay and predict the initial excited carrier distributions so generated. Specifically, we include ab initio predictions of phonon-assisted optical excitations in metals, which are critical to bridging the frequency range between resistive losses at low frequencies and direct interband transitions at high frequencies. In the commonly used plasmonic materials, gold, silver, copper, and aluminum, we find that resistive losses compete with phonon-assisted carrier generation below the interband threshold, but hot carrier generation via direct transitions dominates above threshold. Finally, we predict energy-dependent lifetimes and mean free paths of hot carriers, accounting for electron–electron and electron–phonon scattering, to provide insight toward transport of plasmonically generated carriers at the nanoscale.
Narang Prineha, Sundararaman Ravishankar, and Atwater Harry A. 2016. “Plasmonic hot carrier dynamics in solid-state and chemical systems for energy conversion.” Nanophotonics, 5, Pp. 96. Publisher's VersionAbstract
Surface plasmons provide a pathway to efficiently absorb and confine light in metallic nanostructures, thereby bridging photonics to the nano scale. The decay of surface plasmons generates energetic ‘hot’ carriers, which can drive chemical reactions or be injected into semiconductors for nano-scale photochemical or photovoltaic energy conversion. Novel plasmonic hot carrier devices and architectures continue to be demonstrated, but the complexity of the underlying processes make a complete microscopic understanding of all the mechanisms and design considerations for such devices extremely challenging. Here,we review the theoretical and computational efforts to understand and model plasmonic hot carrier devices. We split the problem into three steps: hot carrier generation, transport and collection, and review theoretical approaches with the appropriate level of detail for each step along with their predictions. We identify the key advances necessary to complete the microscopic mechanistic picture and facilitate the design of the next generation of devices and materials for plasmonic energy conversion.
J. M Fitzgerald, P. Narang, R. V Craster, S. A Maier, and V. Giannini. 2016. “Quantum plasmonics.” Proceedings of the IEEE, 104, 12, Pp. 2307-2322. Publisher's VersionAbstract
Quantum plasmonics is an exciting subbranch of nanoplasmonics where the laws of quantum theory are used to describe light–matter interactions on the nanoscale. Plasmonic materials allow extreme subdiffraction confinement of (quantum or classical) light to regions so small that the quantization of both light and matter may be necessary for an accurate description. State-of-the-art experiments now allow us to probe these regimes and push existing theories to the limits which opens up the possibilities of exploring the nature of many-body collective oscillations as well as developing new plasmonic devices, which use the particle quality of light and the wave quality of matter, and have a wealth of potential applications in sensing, lasing, and quantum computing. This merging of fundamental condensed matter theory with application-rich electromagnetism (and a splash of quantum optics thrown in) gives rise to a fascinating area of modern physics that is still very much in its infancy. In this review, we discuss and compare the key models and experiments used to explore how the quantum nature of electrons impacts plasmonics in the context of quantum size corrections of localized plasmons and quantum tunneling between nanoparticle dimers. We also look at some of the remarkable experiments that are revealing the quantum nature of surface plasmon polaritons.
Vladimir Bochenkov, Jeremy Baumberg, Mikhail Noginov, Felix Benz, Hasan Aldewachi, Silvan Schmid, Viktor Podolskiy, Javier Aizpurua, Kaiqiang Lin, Thomas Ebbesen, Alexei A Kornyshev, James Hutchison, Katarzyna Matczyszyn, Samir Kumar, Bart de Nijs, Francisco Rodriguez Fortuno, James T. Hugall, Pablo de Roque, Niek van Hulst, Santhosh Kotni, Olivier Martin, F. Javier Garcia de Abajo, Michael Flatte, Andrew Mount, Martin Moskovits, Pavel Ginzburg, David Zueco, Anatoly Zayats, Sang-Hyun Oh, Yu Chen, David Richards, Alessandro Belardini, and Prineha Narang. 2015. “Applications of plasmonics: general discussion.” Faraday Discuss., 178, Pp. 435-466. Publisher's Version
Zeng-Hua Cai, Prineha Narang, Harry A. Atwater, Shiyou Chen, Chun-Gang Duan, Zi-Qiang Zhu, and Jun-Hao Chu. 2015. “Cation-Mutation Design of Quaternary Nitride Semiconductors Lattice-Matched to GaN.” Chemistry of Materials, 27, 22, Pp. 7757–7764. Publisher's VersionAbstract
The search for new direct bandgap, earth-abundant semiconductors for efficient, high-quality optoelectronic devices, as well as photovoltaic and photocatalytic energy conversion has attracted considerable interest. One methodology for the search is to study ternary and multiternary semiconductors with more elements and more flexible properties. Cation mutation such as binary → ternary → quaternary for ZnS → CuGaS2 → Cu2ZnSnS4 and ZnO → LiGaO2 → Li2ZnGeO4 led to a series of new quaternary chalcogenide and oxide semiconductors with wide applications. Similarly, starting with GaN, ternary nitrides such as ZnSnN2 and ZnGeN2 have been designed and synthesized recently. However, quaternary nitride semiconductors have never been reported either theoretically or experimentally. Through a combination of the Materials Genome database with the first-principles calculations, we designed a series of quaternary nitride compounds I–III–Ge2N4 (I = Cu, Ag, Li, Na, K; III = Al, Ga, In) following the GaN → ZnGeN2 → I–III–Ge2N4mutation. Akin to Li2ZnGeO4, these quaternary nitrides crystallize in a wurtzite-derived structure as their ground state. The thermodynamic stability analysis shows that while most of them are not stable with respect to phase separation, there are two key exceptions (i.e., LiAlGe2N4 and LiGaGe2N4), which are stable and can be synthesized without any secondary phases. Interestingly, they are both lattice-matched to GaN and ZnO, and their band gaps are direct and larger than that of GaN, 4.36 and 3.74 eV, respectively. They have valence band edges as low as ZnO and conduction band edges as high as GaN, thereby combining the best of GaN and ZnO in a single material. We predict that flexible and efficient band structure engineering can be achieved through forming GaN/LiAlGe2N4/LiGaGe2N4 heterostructures, which have tremendous potential for ultraviolet optoelectronics.
Prineha Narang, Shiyou Chen, Naomi C. Coronel, Sheraz Gul, Junko Yano, Lin-Wang Wang, Nathan S. Lewis, and Harry A. Atwater. 2014. “Bandgap Tunability in Zn(Sn,Ge)N2 Semiconductor Alloys.” Advanced Materials, 26, 8, Pp. 1235–1241. Publisher's VersionAbstract
ZnSn1‐xGexN2 direct bandgap semiconductor alloys, with a crystal structure and electronic structure similar to InGaN, are earth‐abundant alternatives for efficient, high‐quality optoelectronic devices and solar‐energy conversion. The bandgap is tunable almost monotonically from 2 eV (ZnSnN2) to 3.1 eV (ZnGeN2) by control of the Sn/Ge ratio.
Shiyou Chen, Prineha Narang, Harry A. Atwater, and Lin-Wang Wang. 2014. “Phase Stability and Defect Physics of a Ternary ZnSnN2 Semiconductor: First Principles Insights.” Advanced Materials, 26, 2, Pp. 311–315. Publisher's VersionAbstract
First‐principles calculations show that ZnSnN2 has a very small formation enthalpy, and the donor defects such as SnZn antisites and ON impurities have high concentration, making the material degenerately n‐type, which explains the observed high electron concentration. ZnSnN2 can be regarded as a new material that combines a metal‐like conductivity with an optical bandgap around 2 eV.
Andrew J. Leenheer, Prineha Narang, Nathan S. Lewis, and Harry A. Atwater. 2014. “Solar energy conversion via hot electron internal photoemission in metallic nanostructures: Efficiency estimates.” Journal of Applied Physics, 115, 13, Pp. 134301. Publisher's VersionAbstract

Collection of hot electrons generated by the efficient absorption of light in metallic nanostructures, in contact with semiconductor substrates can provide a basis for the construction of solar energy-conversion devices. Herein, we evaluate theoretically the energy-conversion efficiency of systems that rely on internal photoemission processes at metal-semiconductor Schottky-barrier diodes. In this theory, the current-voltage characteristics are given by the internal photoemission yield as well as by the thermionic dark current over a varied-energy barrier height. The Fowler model, in all cases, predicts solar energy-conversion efficiencies of <1% for such systems. However, relaxation of the assumptions regarding constraints on the escape cone and momentum conservation at the interface yields solar energy-conversion efficiencies as high as 1%–10%, under some assumed (albeit optimistic) operating conditions. Under these conditions, the energy-conversion efficiency is mainly limited by the thermionic dark current, the distribution of hot electron energies, and hot-electron momentum considerations.

Ravishankar Sundararaman, Prineha Narang, Adam S. Jermyn, William A. Goddard III, and Harry A. Atwater. 2014. “Theoretical predictions for hot-carrier generation from surface plasmon decay.” Nature Communications, 5, Pp. 5788 . Publisher's VersionAbstract
Decay of surface plasmons to hot carriers finds a wide variety of applications in energy conversion, photocatalysis and photodetection. However, a detailed theoretical description of plasmonic hot-carrier generation in real materials has remained incomplete. Here we report predictions for the prompt distributions of excited ‘hot’ electrons and holes generated by plasmon decay, before inelastic relaxation, using a quantized plasmon model with detailed electronic structure. We find that carrier energy distributions are sensitive to the electronic band structure of the metal: gold and copper produce holes hotter than electrons by 1–2 eV, while silver and aluminium distribute energies more equitably between electrons and holes. Momentum-direction distributions for hot carriers are anisotropic, dominated by the plasmon polarization for aluminium and by the crystal orientation for noble metals. We show that in thin metallic films intraband transitions can alter the carrier distributions, producing hotter electrons in gold, but interband transitions remain dominant.