Plasmonic response of metallic nanojunctions driven by single atom motion: Quantum transport revealed in optics

The correlation between transport properties across subnanometric metallic gaps and the optical response of the system is a complex effect that is determined by the fine atomic-scale details of the junction structure. As experimental advances are progressively accessing transport and optical characterization of smaller nanojunctions, a clear connection between the structural, electronic, and optical properties in these nanocavities is needed. Using ab initio calculations, we present here a study of the simultaneous evolution of the structure and the optical response of a plasmonic junction as the particles forming the cavity, two Na380 clusters, approach and retract. Atomic reorganizations are responsible for a large hysteresis of the plasmonic response of the system, which shows a jump-to-contact instability during the approach process and the formation of an atom-sized neck across the junction during retraction. Our calculations demonstrate that, due to the quantization of the conductance in metal nanocontacts, atomic-scale reconfigurations play a crucial role in determining the optical response of the whole system. We observe abrupt changes in the intensities and spectral positions of the dominating plasmon resonances and find a one-to-one correspondence between these jumps and those of the quantized transport as the neck cross-section diminishes. These results reveal an important connection between transport and optics at the atomic scale, which is at the frontier of current optoelectronics and can drive new options in optical engineering of signals driven by the motion and manipulation of single atoms.We acknowledge financial support from Projects FIS2013-41184-P and MAT2013-46593-C6-2-P from MINECO. M.B., P.K., F.M., and D.S.P. also acknowledge support from the ANR-ORGAVOLT project and the Euroregion Aquitaine-Euskadi program. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a Ph.D. grant. P.K. acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union. J.A. also acknowledges support from Grant 70NANB15H321, “PLASMOQUANTUM”, from the US Department of Commerce (NIST).Peer Reviewe

Atomistic near-field nanoplasmonics: Reaching atomic-scale resolution in nanooptics

Electromagnetic field localization in nanoantennas is one of the leitmotivs that drives the development of plasmonics. The near-fields in these plasmonic nanoantennas are commonly addressed theoretically within classical frameworks that neglect atomic-scale features. This approach is often appropriate since the irregularities produced at the atomic scale are typically hidden in far-field optical spectroscopies. However, a variety of physical and chemical processes rely on the fine distribution of the local fields at this ultraconfined scale. We use time-dependent density functional theory and perform atomistic quantum mechanical calculations of the optical response of plasmonic nanoparticles, and their dimers, characterized by the presence of crystallographic planes, facets, vertices, and steps. Using sodium clusters as an example, we show that the atomistic details of the nanoparticles morphologies determine the presence of subnanometric near-field hot spots that are further enhanced by the action of the underlying nanometric plasmonic fields. This situation is analogue to a self-similar nanoantenna cascade effect, scaled down to atomic dimensions, and it provides new insights into the limits of field enhancement and confinement, with important implications in the optical resolution of field-enhanced spectroscopies and microscopies.We acknowledge financial support from projects FIS2013-14481-P and MAT2013-46593-C6-2-P from MINECO. M.B., P.K., F.M., and D.S.P. also acknowledge support from the ANR-ORGAVOLT project and the Euroregion Aquitaine-Euskadi program. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a PhD grant, as well as from Euskampus and the DIPC at the initial stages of this work. R.E. and P.K. acknowledge financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union, “Una manera de hacer Europa”.Peer Reviewe

Ab-initio theoretical study of electronic excitations and optical properties in nanostructures

Thesis submitted to the University of the Basque Country for the degree of Doctor in Physics.El desarrollo de este proyecto de tesis está basado en el estudio de las propiedades ópticas de nano-estructuras de interés físico por medio de teorías ab initio. Las teorías ab initio nos permiten estudiar una amplia gama de sistemas cambiando solamente unos parámetros que dependen del sistema sin tener que reconsiderar la metodología de trabajo de la teoría. En particular, se ha usado la Teoría del Funcional de la Densidad (DFT) para obtener información sobre la estructura electrónica de los materiales estudiados. Mientras que la Teoría del Funcional de la Densidad Tiempo-Dependiente (TDDFT) nos ha permitido estudiar en detalle como los electrones de dichos materiales se comportan cuando son sometidos a un campo eléctrico. Las dos teorías ab initio, DFT y TDDFT han crecido en popularidad debido a la mayor potencia de los ordenadores y su favorable complejidad computacional, es decir, a que el coste computacional aumenta siguiendo potencias de exponentes relativamente bajos del número de átomo en el sistema. En este proyecto hemos empleado el método y código DFT SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms). SIESTA es un método y código DFT de licencia GPL que se empezó a desarrollar en los años noventa y que todavía tiene una comunidad de desarrolladores que siguen ampliando y mejorando el código. Con SIESTA hemos calculado las propiedades electrónicas de estado del estado de base de los sistemas. Estas propiedades incluyen las curvas de dispersión energía-momento, las energías de los niveles electrónicos, la energía total, la densidad electrónica, la geometría de equilibrio, etc. La propiedades ópticas que han sido el objeto principal de esta investigación han sido calculadas y estudiadas por medio del código MBPT-LCAO (Many Body Perturbation Theory with Linear Combination Atomic Orbitals), un código que todavía no se distribuye al público en general y que está en desarrollo, siendo su principal desarrollador el Dr. Peter Koval. A pesar de que el código MBPT-LCAO está diseñado para trabajar en conjunto con SIESTA, es decir que los ficheros de salida de un cálculo DFT de SIESTA son leídos por MBPT-LCAO como ficheros de entrada para el cálculo TDDFT, es posible usarlo por separado. El código, por medio de la teoría de pertubaciones de muchos-cuerpos y TDDFT, permite calcular el gap de energía, las energías de excitaciones, la densidad inducida, el potencial eléctrico inducido, la corriente inducida, etc. Específicamente, en este trabajo me he centrado en el cálculo y estudio de la polarizabilidad, de la sección óptica, de la densidad electrónica inducida y de la corriente inducida, para sistemas finitos y moléculas. En esta tesis el código MBPT-LCAO ha sido utilizado para calcular sistemas que contienen más de mil átomos.I have to thank the Centro de Fsica de Materiales (CFM-MPC) and the Donostia International Physics Center (DIPC) for the financial support they provided me.Peer reviewe

Nanoplasmonics driven by single-atom rearrangements

Resumen del trabajo presentado a la Spanish Conference on Nanophotonics (Conferencia Española de Nanofotónica-CEN), celebrada en Donostia-San Sebastián (España) del 3 al 5 de octubre de 2018.Dimers of metal clusters can serve as model systems for tip-enhanced, surfaceenhanced or cavity-enhanced spectroscopies. Classical modeling is based on the electrodynamics of continuous media, using the framework of dielectric functions. Classical models correctly account for basic features of the near field in the vicinity of large clusters, measuring 10 nm and more. However, there are many effects that elude the classical modeling, especially when it comes to small clusters or to small distances between the clusters in the metallic dimers. Small clusters containing less than hundred atoms must be treated using quantum mechanics, while the atomistic nature of metal clusters may reveal itself in much larger clusters, containing thousands of atoms. In this work, we present a detailed ab initio atomistic calculation of induced near fields in the vicinity of a sodium cluster dimer 2×Na380 forming a plasmonic cavity. Our model consists of two almost icosahedral clusters. Each of the clusters exhibits facets, edges and tips made by atoms. Positions of atoms were optimized at the level of density functional theory (DFT), using the SIESTA software package. The induced near fields were determined at the level of time-dependent DFT, assuming an optical external perturbation. Induced near fields depend strongly on such geometry-dependent features as orientation of the external field, the distance between clusters and the mutual orientation of clusters. Besides these anticipated dependencies, there are more intriguing effects such as jump to contact due to atomic bridging and the formation of metal necks, taking place in the cavity during the initial approach and the consecutive retraction of the clusters, correspondingly. These effects involve the motion of atoms and have profound consequences on the geometrical and optical properties of the metallic cluster dimer. In particular, we show how structural reorganizations involving a few atoms, or even a single atom, lead to dramatic changes in the optical response of the whole structure. This effect is related to the conductance quantization in metal contacts of atomic cross-sections. Beyond the relatively simple setup of two metal clusters approaching and retracting, there are more complex scenarios such as ligand-protected noble-metal clusters, modeling of which may bring unexpected physics into play. Many aspects of these organo-metallic compounds may be captured by modeling based on ab initio molecular dynamics and time-dependent DFT allowed by our efficient numerical tools.Peer reviewe

Optical response of metal nanojunctions driven by single-atom motion: influence of quantized electron transport on nanoplasmonics

Resumen del trabajo presentado al APS March Meeting, celebrado en Baltimore, Maryland (USA) del 14 al 18 de marzo de 2016.The correlation between transport properties across sub-nanometric metallic gaps and the optical response of the system is a complex effect that, similarly to the near-field enhancement, is determined by fine atomic-scale details in the junction structure. Using ab initio calculations, we present here a study of the simultaneous evolution of the structure and the optical response of a plasmonic junction as the two Naf380 clusters forming the cavity approach and retract. Atomic reorganizations are responsible for a large hysteresis of the optical response. The system exhibits a jump-to-contact instability during the approach, and the formation of an atom-sized neck across the junction during retraction. Due to the quantization of the conductance in metal nanocontacts, atomic-scale reconfigurations play a crucial role in determining the optical response. We observe abrupt changes in the intensities and spectral positions of the dominating plasmon resonances, and find a one-to-one correspondence between these jumps and those of the quantized transport across the neck. These results point out to an unforeseen connection between transport and optics at the atomic scale, which is at the frontier of current optoelectronics.We acknowledge support from MINECO (Grants FIS2013-14481-P and MAT2013-46593-C6-2-P), UPV/EHU and Gipuzkuako Foru Aldundia.Peer Reviewe

Subnanometric plasmonics: confining light to atomic-scale dimensions

Resumen del trabajo presentado al International Chemical Congress of Pacific Basin Societies, celebrado en Honolulu, Hawai (USA) del 15 al 20 de diciembre de 2015.Plasmonic fields have boosted field-enhanced spectroscopy and microscopy due to their effective antenna action improving both the field enhancement, as well as producing useful nanometric localization. As novel geometrical architectures are being developed in plasmonic gap configurations, subnanometric separation distances and features become a common situation that needs to be addressed to understand the performance of extreme field-enhanced spectroscopy. We theoretically analyze the optical response of ultranarrow plasmonic gaps, such as those in nanoparticle on mirror (NPoM) configurations, or in cavities of tip-enhanced Raman scattering (TERS), where a rich variety of complex optoelectronic processes emerge due to the subnanometric nature of the architectures involved. By a combination of classical electrodynamical and quantum mechanical calculations, we are able to reveal a complex distribution of modes in the optical response of the cavities with hybridizations and strong nonlinear effects affecting the ultimate limits of light confinement. We present state-of-the-art quantum mechanical calculations that reveal atomic-scale localization in plasmonic structures, given by the presence of crystallographic facets, vertices, and edges. These features produce an analogue to an atomistic lightning rod effect that further enhances the underlying nanometric plasmon near-field in a cascade effect, scaled down to atomic dimensions. The implications of atomic-scale localization of light in the optical resolution of field-enhanced spectroscopies, photochemistry and photoemission can be dramatic and might explain the tremendous variability of experimental findings.Peer Reviewe

Tunable molecular plasmons in polycyclic aromatic hydrocarbons

We show that chemically synthesized polycyclic aromatic hydrocarbons (PAHs) exhibit molecular plasmon resonances that are remarkably sensitive to the net charge state of the molecule and the atomic structure of the edges. These molecules can be regarded as nanometer-sized forms of graphene, from which they inherit their high electrical tunability. Specifically, the addition or removal of a single electron switches on/off these molecular plasmons. Our first-principles time-dependent density-functional theory (TDDFT) calculations are in good agreement with a simpler tight-binding approach that can be easily extended to much larger systems. These fundamental insights enable the development of novel plasmonic devices based upon chemically available molecules, which, unlike colloidal or lithographic nanostructures, are free from structural imperfections. We further show a strong interaction between plasmons in neighboring molecules, quantified in significant energy shifts and field enhancement, and enabling molecular-based plasmonic designs. Our findings suggest new paradigms for electro-optical modulation and switching, single-electron detection, and sensing using individual molecules. © 2013 American Chemical Society.This work has been supported in part by the Spanish MICINN (MAT2010-14885, FIS2010-19609-C02-00, and Consolider NanoLight.es), the European Commission (FP7-ICT-2009-4-248909-LIMA and FP7-ICT-2009-4-248855-N4E), and the Etortek program. A.M. acknowledges financial support through FPU from the Spanish MEC. P.K. aknowledges support from the CSIC JAE-doc program, cofinanced by the European Science Foundation. P.N. acknowledges support from the Robert A. Welch Foundation (C-1222) and the Office of Naval Research (N00014-10-1-0989).Peer Reviewe

Tunable molecular plasmons in polycyclic aromatic hydrocarbons

We show that chemically synthesized polycyclic aromatic hydrocarbons (PAHs) exhibit molecular plasmon resonances that are remarkably sensitive to the net charge state of the molecule and the atomic structure of the edges. These molecules can be regarded as nanometer-sized forms of graphene, from which they inherit their high electrical tunability. Specifically, the addition or removal of a single electron switches on/off these molecular plasmons. Our first-principles time-dependent density-functional theory (TDDFT) calculations are in good agreement with a simpler tight-binding approach that can be easily extended to much larger systems. These fundamental insights enable the development of novel plasmonic devices based upon chemically available molecules, which, unlike colloidal or lithographic nanostructures, are free from structural imperfections. We further show a strong interaction between plasmons in neighboring molecules, quantified in significant energy shifts and field enhancement, and enabling molecular-based plasmonic designs. Our findings suggest new paradigms for electro-optical modulation and switching, single-electron detection, and sensing using individual molecules. © 2013 American Chemical Society.This work has been supported in part by the Spanish MICINN (MAT2010-14885, FIS2010-19609-C02-00, and Consolider NanoLight.es), the European Commission (FP7-ICT-2009-4-248909-LIMA and FP7-ICT-2009-4-248855-N4E), and the Etortek program. A.M. acknowledges financial support through FPU from the Spanish MEC. P.K. aknowledges support from the CSIC JAE-doc program, cofinanced by the European Science Foundation. P.N. acknowledges support from the Robert A. Welch Foundation (C-1222) and the Office of Naval Research (N00014-10-1-0989).Peer Reviewe

Optical response of silver clusters and their hollow shells from linear-response TDDFT

We present a study of the optical response of compact and hollow icosahedral clusters containing up to 868 silver atoms by means of time-dependent density functional theory. We have studied the dependence on size and morphology of both the sharp plasmonic resonance at 3-4 eV (originated mainly from $sp$-electrons), and the less studied broader feature appearing in the 6-7 eV range (interband transitions). An analysis of the effect of structural relaxations, as well as the choice of exchange correlation functional (local density versus generalized gradient approximations) both in the ground state and optical response calculations is also presented. We have further analysed the role of the different atom layers (surface versus inner layers) and the different orbital symmetries on the absorption cross-section for energies up to 8 eV. We have also studied the dependence on the number of atom layers in hollow structures. Shells formed by a single layer of atoms show a pronounced red shift of the main plasmon resonances that, however, rapidly converge to those of the compact structures as the number of layers is increased. The methods used to obtain these results are also carefully discussed. Our methodology is based on the use of localized basis (atomic orbitals, and atom-centered- and dominant- product functions), which bring several computational advantages related to their relatively small size and the sparsity of the resulting matrices. Furthermore, the use of basis sets of atomic orbitals also brings the possibility to extend some of the standard population analysis tools (e.g., Mulliken population analysis) to the realm of optical excitations. Some examples of these analyses are described in the present work.Prédiction par calcul numérique intensif du potentiel à circuit ouvert au sein de cellules photovoltaïques organiques