nanoscale systems and structures: electronic, magnetic and ... · 1.1. magnetic properties of...

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AXIS 1 : MATERIALS AND EXTREME CONDITIONS | Nanoscale Systems and Structures: electronic, magnetic and optical properties

Nanoscale Systems and Structures: electronic, magnetic and optical properties

Identity Composition of the team (or participants) Team leader: H. Kachkachi (PR UPVD) Permanent personnel : N. Barros (MCF UPVD), R. Bastardis (MCF UPVD), F. Vernay (MCF UPVD), D. Schmool (PR UPVD), J. L. Déjardin (PR Emérite UPVD). Not permanent personnel : PhD students : (1) achieved thesis: Z. Sabsabi (date soutenance : 13/12/2013) (2) thesis in progress : E. Nadal (date début : 01/10/2014). Keywords Nanostructures, nanomagnetism, electrodynamics, transport phenomena, energy conversion Topics Interaction of light with nanostructured matter, role magnetism, transport phenomena, conversion and energy transfer Collaborations National - J. Laverdant (ILM, Lyon), M. Respaud (LPCNO, INSA, Toulouse), I. Lisiecki (MONARIS, UPMC, Paris), Ph. Ben-

Abdallah (Lab. Charles Fabry, Institut d'Optique, Paris), N. Guihéry (LCPS, Toulouse), G. de Loubens (IRAMIS, CEA, Saclay), S. Bégin-Colin, B. Pichon (IPCMS, Strasbourg), Ph. Cristol, Y. Cuminal (IES, Montpellier), N. Keller (GEMAC, Univ. de Versailles).

International - D. Garanin (Lehman College, New York, USA), G. Singh (Dept. Materials Science and Enginnering, NTNU, Trondheim,

Norvège), Th. Deveraux (SIMES, Stanford, USA), H. Crespo (Dept. de Physique, Univ. Porto, Portugal), R. Stamps (School of Physics and Astronomy, Glasgow, Ecosse), A. Garcia-Martin (Instituto de Microelectrónica de Madrid, CSIC, Madrid, Espagne), O. Chubykalo-Fesenko (ICMM, CSIC, Madrid, Espagne), K. Trohidou (Institute of Materials Science, NCSR, Athènes, Grèce), O. Iglesias (Institut de Nanociencia i Nanotecnologia (I2N), Univ. Barcelone, Espagne), R. Cuadrado Del Burgo (Institut Catal de Nanociencia i Nanotecnologia, Bercelone, Espagne), J. Garitaonandia (Université du Pays Basque, Bilbao, Espagne)

Contracts - MARVEL (ANR JC), fin novembre 2014, coordonné par G. de Loubens (CEA/Saclay) - PTI2014 (PEPS CNRS), en collaboration avec M. Respaud (LPCNO, Toulouse) - InPhyniti (PEPS CNRS), en collaboration avec J. Laverdant (ILM, Lyon) References 11, 24, 37, 40, 68, 99, 121, 126, 150, 202, 203, 204, 205, 211, 235, 236, 237, 259, 260, 271.

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Scientific report INTRODUCTION

National and international context In order to be part of and efficiently contribute to the fast development of nanoscience and nanotechnology, a research entity ought to adapt its research programs according to its skills, know-how and ability to execute them, while seeking complementarity in national and international collaborations. It is with this philosophy that our research group has built its own research projects. Our group benefits from a long-standing experience and an internationally acknowledged expertise in the area of magnetic systems and their dynamics, especially at the nanoscale. Our general approach consists in studying at the atomic scale the various elementary excitations and their interactions with the aim to infer the macroscopic properties observed in experiments. This fundamental approach allows us to investigate other physical phenomena related with spatial confinement in nanostructures. In particular, we are mostly interested in the interaction between light and nanostructured matter and its applications, especially in the area of conversion and transfer of electromagnetic energy. Accordingly, we think that (nano) magnetism can play a significant role in the optimization of the optical properties of hybrid nanostructures. Owing to its skills and precisely defined scientific projects, the activity of our group is clearly identified within the international community. Consequently, we have participated or coordinated various national and international collaborative research programs. Challenges Today nanostructured matter is of great interest owing to the high promises and big challenges it presents both for fundamental research and practical applications. The main reason resides in the fact that the small size of the constituting elements induces a huge enhancement of nearly all physical properties. Thus, reducing the spatial dimension to the nanoscale, and thereby the timescale to the nanosecond and even orders of magnitude smaller, endows nanosystems with extraordinary electronic, magnetic, optical and thermal properties with the promise of efficient applications in various areas, such as medicine, information storage and magnetic imaging, energy conversion and transfer, catalysis, etc. In the area of information storage on magnetic media, the use of nano-elements allows for very high storage densities and ultra-fast reading/writing processes. However, the small size also brings big challenges since the energy barriers separating the states of the two bits of information reduces with the size, thus impairing the thermal stability and thereby the lifetime of the stored information, especially at room temperature. As such, in order to use fine particles while maintaining reasonably high energy barriers and low switching fields, various routes have been explored including magnetization switching assisted by a laser or a microwave magnetic field, or still by the instabilities that are intrinsic to the nano-elements. For solar energy conversion, the basic technology producing solar cells from crystalline silicon, calls for new ideas in order to circumvent the bottleneck related, on one hand, with the production costs and the weak absorption of the thin films, on the other. In this context, nanostructured materials seem to offer a promising way-out. Indeed, nanostructuring allows for an increase in the optical path and absorption of electromagnetic radiation. Moreover, it has been shown that embedded nanoparticles make it possible to widen the absorption spectrum and thereby to optimize the solar energy absorption. In particular, in the visible part of the spectrum, nanostructured systems of noble metals (Au, Ag) exhibit greatly enhanced absorption owing to their ability to sustain plasmonic excitations. Furthermore, the latter can couple to various types of particles and excitations (magnons, phonons, excitons, etc) thus providing a multitude of channels for converting/conveying the absorbed electromagnetic energy into other forms of energy, be it electrical, thermal or chemical. These issues have also opened way to new activity in fundamental research and its applications related with the phenomena of transport and transfer of energy at the nanoscale or at the interface between a nanostructure and its immediate environment.

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Scientific objectives The group S2N-POEM studies systems of nanoparticles deposited on a substrate, embedded in a matrix or in a fluid suspension. In particular, we study magnetic and optical properties, and their mutual influences in hybrid nanostructures (magnetic and/or metallic), in the form of nanoparticles (core-shell) or layered thin films. Our work is focused on the following phenomena: - magneto-plasmonic coupling in hybrid nanostructures: magneto-optics, inelastic light scattering, ferromagnetic

resonance, - competition between intrinsic and collective effects in an assembly of magnetic and/or plasmonic nanoparticles, - mechanisms of conversion and transfer of energy by nanoparticle assemblies in their environments: magnetic

hyperthermia, photocatalysis plasmonics, plasmon-exciton coupling. Our main objectives are as follows: - Optical properties and role of magnetism: propose theories, along with experimental measurements, for investigating on

the microscopic level the influence of magnetism on the optical properties of nanostructures. In particular, we would like to propose, and check by experiment, a mechanism for the coupling between the magnetic and the metal components of the (hybrid) nanostructure.

- Conversion of electromagnetic energy: evaluate, by theory and experiments, the transfer rate of energy (electromagnetic → thermal, electromagnetic → electronic) through the nanostructure/environment interface.

Our research program is in line with the main research project of PROMES, namely « Développement de surfaces à propriétés optiques contrôlées », and that of Université de Perpignan Via Domitia (UPVD) which is: «Energies renouvelables, Procédés, Matériaux associés ».

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Summary

1. Interaction between light and nanostructured matter, role of magnetism

1.1. Magnetic properties of nanostructures

1.1.1. Ferromagnetic resonance: effects of shape and coupling

1.1.2. Surface effects in magnetic nanoparticles

1.2. Interaction between light and nanostructured matter: effect of magneto-plasmonic coupling

1.2.1. Experimental studies and applications

1.2.2. Theoretical developments: magnon – plasmon interaction

2. Transport phenomena, conversion and transfer of energy

2.1. Plasmonic nanoparticles for photo-catalysis

2.2. Conversion of electromagnetic energy

2.2.1. Conversion of electromagnetic energy into heat

2.2.2. Role of dipolar interactions in magnetic hyperthermia

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1. INTERACTION BETWEEN LIGHT AND NANOSTRUCTURED MATTER, ROLE OF MAGNETISM

1.1. Magnetic properties of nanostructures With the aim to understand the mutual influence of optical and magnetic properties, it is necessary to separately characterize the magnetic properties taking into account the shape and environment of the nano-objects. For this, work is being done on purely magnetic aspects. 1.1.1. Ferromagnetic resonance : effects of shape and coupling For characterizing our systems, we study ferromagnetic resonance of a magnetic dimer coupled by dipolar interactions [J. Appl. Phys. 116, 243905 (2014)]. We thus develop a general formalism of a dimer consisting of two magnetic elements (in horizontal or vertical configuration) taking account of their finite size and aspect ratio. In particular, we study the effect, in various configurations, on the resonance frequency (and resonance field) of the applied magnetic field (amplitude and direction), the coupling between the elements and the uniaxial anisotropy. Various regimes of the coupling between the magnetic nano-elements have been identified and analytical expressions have been obtained for the resonance frequency. In addition, the change in the resonance field has been evaluated in each regime. These theoretical results provide a useful means of comparison with the ferromagnetic resonance experiments. Indeed, existing experimental data on coupled FeV nanodiscs have allowed us to check on the behavior of the dipolar coupling as a function of the nano-elements separation. To generalize this work to assemblies of nano-elements we studied the ferromagnetic resonance of two (shifted or not) coupled parallel chains of Fe nanoparticles [work submitted to J. Appl. Phys.]. The dipolar interactions are treated here beyond the point-dipole approximation, thus taking into account the size and shape of the nano-elements and their spatial separation. Our analytical and numerical calculations have rendered the resonance frequency as a function of the amplitude of the applied magnetic field and also the resonance field as a function of the direction of the applied magnetic field, parallel and perpendicular to the chains axes. In another work in progress we deal with the resonance frequency of two-dimensional array of magnetic nanoparticles with both uniaxial anisotropy axis and applied magnetic field set normal to the assembly plane. We examine the case of weak dipolar interactions, which corresponds to dilute assemblies. We have been able to obtain an approximate expression for the correction to the resonance frequency of the array which is of second order in the intensity of the dipolar interactions. This correction turns out to be on the order of MHz.

Figure 1 : variation of the correction to the resonance frequency of a two-dimensional array of nanoparticles, as a function of the inter-particle distance.

1.1.2. Surface effects in magnetic nanoparticles The aim of this work is to understand the role of interactions (magnetic coupling, one-site anisotropies) and surface effects on the magnetization dynamics of an individual magnetic nanoparticle. For this purpose, we adopt a many-spin approach that allows for a distinction between the core and surface.

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Excitation spectrum of a single magnetic nanoparticle

Here we investigate the effect of surface anisotropy (Néel anisotropy) on the spin-wave spectrum in a cube-shaped magnetic nanoparticle with a simple cubic lattice. We have developed a computer program that makes it possible to disentangle the contributions of the surface and core spins to the magnetic excitation spectrum. The eigenvalues (energies of the spin-wave modes) and eigenvectors thus obtained allow to calculate the spectral weight of the different contributions of the surface and core spins. In Figure 2, we show the absorbed power as a function of the energy of the various modes to an AC field applied along the X axis, for two values of the Néel anisotropy (the core spins have a uniaxial anisotropy along the Z axis).

Figure 2 : absorbed power against the energy of spin-wave modes for an AC in the X direction and for two values of the surface anisotropy.

The results obtained for the X component are proportional to the absorbed power. It is therefore possible to assign each peak to the weight of the different spins. We note that for modes of low frequency the peaks have a dominant contribution from the surface (the uniform mode corresponds to an equal contribution to the spectral weight from the core and surface). This work is submitted to Phys. Rev. B (Feb. 2016); it has been done in the framework of the ANR project (MARVEL), in collaboration with G. Loubens (CEA / Saclay) and D. Garanin (Lehman College, New York).

Magnetization switching in a single magnetic nanoparticle

In a work in progress we examine the surface effects of a nanoparticle on the magnetization reversal subjected to an AC magnetic field, taking into account the size, the shape, the lattice and the boundary conditions. Using the Landau-Lifshitz equation we compute the phase diagram of the magnetization reversal for the critical values of the AC field a a function of the frequency. The results for a variable polarization of the magnetic field show that a linearly polarized field (applied perpendicularly to the easy axis of the magnetization) is more favorable to the magnetization switching. Moreover the sizes effects of the nanoparticle are significant when the number of surface spins exceeds that of core spins (size 5x5x7). On the other hand the phase diagram of the magnetization reversal becomes constant with the size. We have also shown that the Néel anisotropy, as compared to a uniaxial anisotropy (along the Z axis), allows for a magnetization reversal at lower frequencies and amplitudes of the AC field, but with a narrower reversal regime, as compared to that for a uniaxial anisotropy.

Figure 3 : Phase diagram of the (critical) frequency and amplitude of the AC magnetic field that lead to magnetization switching. Left: the system size is varied. Right: different anisotropies and boundary conditions are studied.

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1.2. Interaction between light and nanostructured matter: effect of magneto-plasmonic coupling As was pointed out earlier, the effect of magnetism on the optical properties of nanostructures and its use to optimize them holds a special place in our activities. Indeed, in a hybrid structure, with a magnetic core and a metallic shell, or a magnetic layer in contact with a metallic layer, the magnetic component may be used to control/adjust the optical and magneto-optical properties of the nanostructure with the help of an external magnetic field. It has already been shown by many researchers that the optical activity of various nanostructures strongly depends on the applied magnetic field or the underlying magnetic component. To date, these results are interpreted using macroscopic models based on solving Maxwell's equations. However, this does not provides us with a clear explanation of the coupling mechanisms between the magnetic and plasmonic excitations, especially that the scales of energies of these two types of excitations do not match. It is therefore essential to develop microscopic theories so as to offer clues as to the microscopic mechanisms of coupling and transport. This would, among other things, lead to a better targeting of materials and with optimized properties in regards to energy absorption and transfer. In this perspective, we are conducting several studies, theoretical and experimental, on hybrid structures made by our team or collaborators. The models we develop are both macroscopic and microscopic. The first ones use numerical approaches capable of treating an assembly of nano-objects deposited on a substrate or embedded in a matrix. The second type of model employs microscopic approaches of condensed matter physics and involves interactions between electrons, phonons and magnons. To support these efforts, we have participated in or coordinated several research programs, on the national (PEPS, ANR) and international (ANR network ICBP) levels, in collaboration with several experts in the field. 1.2.1. Experimental studies and applications In 2014, our group began the development of an experimental component by setting up facilities for fabrication, characterization and mostly measurements (optical and magnetic) on nanostructures with well chosen nanoparticles. As emphasized earlier, this project is well established in the priority themes of both the laboratory PROMES and the university UPVD. It has received financial support from both the laboratory through an “Action Incitative” and from UPVD through a PEPS and two BQR projects. These fundings have enabled us to achieve the following: - experimental setups allowing for the fabrication and optical characterization of nanocomposite thin layers consisting of

nanoparticles of gold or silver embedded in PMMA, - optical and magneto-optical characterization (Faraday effect) together with spectroscopy measurements under

magnetic field, necessary for the study of the interdependence between the optical and magnetic properties of hybrid nanostructures,

- spectroscopy measurements (absorption, reflection), in the absence of magnetic field, are also made, in collaboration with the PPCM group using a reflectometer.

Fabrication and characterization of assemblies of metallic nanoparticles

The originality of the fabrication of assemblies of nanoparticles that we propose is the use of light-sensitive polymers, which have the ability to migrate spontaneously to form Surface Relief Gratings (SRG) when illuminated by an appropriate coherent light. This allows us, for instances, to organize and orientate the nanoparticles, metallic (Au, Ag) and/or magnetic, in thin polymer films. This allows for a better control of the plasmonic (and/or magneto-plasmonic) properties of the nanostructures and thence to optimize the absorption of solar energy. The thesis of Elie Nadal (MESR fellowship of the UPVD doctoral school E2) is part of this project. Several studies conducted in collaboration with colleagues, chemists from UPVD and physicists from the laboratory LPCNO (INSA - Toulouse), has shown that an adjustment of the fabrication method was necessary. More precisely, now we prepare a solution containing the polymer and a metal precursor in an ionic form. We then make a thin layer of a few hundred nanometers of thickness of this polymer doped by the spin coating method. Finally, the nanoparticles are formed in situ from the precursor through an irradiation and annealing sequence. Nanocomposites thus obtained are characterized with regards to their optical and structural properties..

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The following experimental setups have been made: - spin-coater : thin film deposition, - setup of laser interferometry to perform irradiation, - spectroscopy setup: transmittance and absorptance measurements, - a specific measurement AFM mode called Atomic Force Acoustic Microscopy (AFAM) to image the nanoparticles on the

surface and also in the volume of the layer.

Study of the magneto-plasmonic coupling in hybrid nanostructures

This is about the study of hybrid nanoparticles which have both a metallic and a magnetic component. These nanoparticles which are cobalt nano-rods with or without gold nanospheres stuck at their ends (dumbbells) are made by our collaborators in LPCNO. Nanocomposites based on such nanoparticles embedded in a transparent polymer matrix are good candidates for a material with original magneto-optical properties. Indeed, the Faraday rotation (i.e. the rotation of the light polarization plane in the presence of a magnetic field) of these materials may exhibit a resonance (enhancement of the phenomenon at a given wavelength) caused by the presence of gold nanospheres and their plasmon activity.

Experimental setup: the sample (solid or liquid polymer containing Cobalt nano-rods) is placed in the gap of an electromagnet. We then measure the dependence on the wavelength of the rotation of light beam polarization that passes through the sample in the presence of a magnetic field. Finally, we compare the amplitude of this rotation for the nano-rods with and without gold nanoparticles.

Study of the magneto-plasmonic coupling by a network analyzer

In an ANR project ( MagPlas, submitted in 2015), in collaboration with Taiwan, we are targeting a thorough study of the inter-dependence of magnetic and optical properties of hybrid (metal/magnetic) well characterized nanostructures. In particular, we want to make optical and magnetic measurements at the plasmonic and ferromagnetic resonance in order to probe the influence of optical excitations on the ferromagnetic resonance and vice versa. The ferromagnetic resonance measurements will be performed using the technique of the vector network analyzer with variable frequency and field, since this allows for the observation of the ferromagnetic resonance on a transmission line and thereby makes it possible to illuminate the sample during the measurements of absorption (or dispersion). Nanostructured samples of Co/Ag will be be produced by lithography at the National Chung Hsing University in Taiwan. Experimental measurements of absorption and ferromagnetic resonance will be made in PROMES Laboratory. In parallel, a theoretical study will also be conducted in PROMES to accompany this experimental work.

Solar cells: chains of gold nanoparticles instead of contact grids

The objective of this project (ANR 2015, unsuccessful) is to use nanoparticles to enhance the optical absorption of solar cells. The main idea is to replace the cell contact gates with chains of gold nanoparticles. Thus, the absorption can be increased by exciting plasmons in nanoparticles and by eliminating the shadow that is usually caused by the electrical contacts. This is made possible by a near field coupling between the nanoparticles and the semiconductor, where the nanoparticles act as nano-antennas. The laboratory IPCMS (Strasbourg) is in charge of the fabrication of chains of gold nanoparticles, the solar cells were to be made and characterized at the institute IES (Montpellier), while optical measurements and theoretical calculations were part of the work-package of PROMES (Perpignan). For all magnetic aspects of our projects, an additional magnetic characterization is performed by ferromagnetic resonance at the University of the Basque Country (Bilbao) and the University of Glasgow where sophisticated equipment are available, including a vector network analyzer which allows for the study of ferromagnetic resonance with variable frequency and DC magnetic field. In addition, through collaboration with the University of Porto, it is possible to perform measurements of ultrafast magnetization dynamics in ferromagnetic thin films. This system is unique with respect to time resolution, namely of less than 8 femto seconds. The pump-probe spectrometer has been characterized using a new optical method (called d-scan) that can reconstruct the shape (amplitude and phase) of the incident light. This pump-probe technology was used to study a ferrimagnetic thin layer (GdFeCo).

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1.2.2. Theoretical developments: magnon – plasmon interaction In order to gain a better understanding of the fundamental physical phenomena induced by spatial confinement and highlighted by the interaction between light and matter at the nanoscale, we believe it is essential to develop appropriate models, taking into account the various aspects of these nanoscale systems, such as size, shape, (intra- and inter-objects) interactions and the environment. Our priority is given to microscopic models that are able to explain the basic interactions and mechanisms by which a nanostructure can absorb, convert and transfer energy to its environment. 1.2.2.1. Macroscopic approaches Before we began to develop our own microscopic models, we devoted due time to the study of various existing methods for investigating optical properties of pure metallic or hybrid nanomaterials. The Masters (M2) internship of Elie Nadal during 2014 was devoted to this task with the aim to identify the methods capable of handling the problems pertaining to nanoparticle assemblies. We have studied the effect of spatial separation and orientation on the absorption of elongated nanoparticles. Today, we conduct theoretical work both analytically and numerically to build models able to interpret our experimental results, first in a macroscopic approach. This includes the following tasks: - An analytical study of the coupling between the polarization of the metal component and the magnetization of the

magnetic component. This is applied to the particular case of cobalt nano-rods with ends gold. The ultimate goal here is to obtain analytical expressions of the Faraday rotation angle as a function of the magnetization, taking into account the effect of the excited plasmons in gold nanoparticles.

- A numerical study of plasmon coupling between elongated gold particles (ellipsoids) with the help of two methods: Boundary Element Method (BEM) and Discrete Dipole Approximation (DDA). Here we want to study an assembly of nanoparticles deposited on a magnetic layer as well as a metallic layer (gold, silver) on top of a magnetic layer.

More generally, the latter study aims to understand the coupling of magnetic and electric dipoles and its effect on the diffusion and absorption properties of electromagnetic waves by a nanostructure. The DDA method consists in meshing the system into multiple coupled electric and magnetic dipoles, on a periodic lattice, and to investigate the scattering and absorption of electromagnetic waves by the system. 1.2.2.2. Microscopic approach

Plasmon resonance controlled by nanostructures (PEPS CNRS, InPhyniti)

In this context, thanks to surface plasmons, metallic nanostructures offer the possibility to control the electromagnetic field locally. While plasmon resonances are well understood individually, it seems that their collective properties in assemblies have hardly been investigated, and very often only at the macroscopic scale. In collaboration with the “Institut Lumière Matière” in Lyon, we develop a solid-state theory approach, at the atomic scale, to investigate collective effects with the foreseen application of a better control of these resonances. Indeed, nowadays many potential applications related with electromagnetic energy conversion rely on plasmon resonances and nanostructures. The advantage of the latter lies in the fact that plasmonic properties of the materials can be controlled or even enhanced. For example, we consider a 2D square super-lattice made of noble metal nano-objects (silver nanodiscs for instance) and let us try to determine the effect of the nanostructure on the plasmon resonance. Taken individually, the plasmon resonance of a single nano-object can be computed, and it is well-known that its plasmonic properties depend not only on the metal but also on the size and shape of the considered nano-object. In the case of simple geometries (disks, spheres, …) the single particle resonance can be analytically computed. However, for more complex geometries one must resort to numerical calculations. If we focus on simple geometries, the main idea is then to analyze the modification of the plasmonic properties when the nano-objects are organized on a super-lattice, and thus to understand how the single-particle properties are affected by the interactions in the assembly. We develop a microscopic approach that describes electronic excitations by using many-body calculations and a systematic investigation of the effect of the assembly in the case of simple 2-dimensional lattices. The electronic properties that we are willing to extract are essentially linked to the density-density response function that describes the response to an external potential whose presence induces a variation of the electron density; this is strongly related to the dielectric function 𝜖𝜖(𝑞𝑞,𝜔𝜔). By way of example, for a free electron gas, the evaluation of

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the dielectric function for the nanostructure allows to simulate the optical reflectivity or Raman spectra expected for such systems. For the specific case of inelastic light scattering, it can be shown that the cross-section is given by

( 𝜔𝜔 ) 1 𝜖𝜖 (𝑞𝑞,𝜔𝜔) ). Hence, the effect of the interactions can be identified by taking the super-lattice parameter as an adjustable parameter, in addition to the shape and size of the nano-objects.

Magnetism-controlled plasmon resonance (project ANR 2015, unsuccessful)

Plasmons are often investigated for potential applications, especially for a better efficiency in solar cells with an increased absorption of photons. However, exploiting a broader part of the solar spectrum requires a means to fine-tune the plasmon resonance. One of these means could be provided by by magnetism. Indeed, this has already been demonstrated in the THz regime in graphene subjected to an external magnetic field. In order to increase the solar-cell efficiency in visible wave-lengths noble metals are more adequate. Yet, the typical energy scale of plasmons in these metals (from 1.5 eV to 2.5 eV) makes it difficult to achieve a coupling to magnetism as the typical energy scale of magnetic excitations lie at much lower energies. Thus, the control of the plasmon resonance via magnetism requires the use of some materials with novel magnetic properties. In this context, we investigate Gold/Transition Metal Oxide (TMO) bilayer materials. The main idea may be summarized as follows: TMO is an antiferromagnetic insulator whose low-energy excitations (magnons) are of the order of few hundreds meV. Hence, the magnon-plasmon coupling remains a priori a problem. Nonetheless, in this type of material a magnetic excitation results from a second-order process during which the system goes through a virtual state with a charge-transfer energy U of about 2eV. The charge-transfer energy should thus bridge the gap between the low-lying magnetic excitations and the higher-energy plasmons as sketched in Fig. 4. We have recently started our activity on this topic and are now carrying out calculations in order to clarify the existence of the magnon-plasmon coupling mediated by the charge-transfer process.

Figure 4 : Scheme of a magnetic excitation goeing through a virtual state with energy transfer U of circa 2eV. .

Charge and spin response to light in a hybrid nanostructure

Another configuration for studying, in a relatively simpler manner, the coupling between the degrees of freedom of charge and spin, consists of an assembly of metal nanoparticles (Au, Ag) deposited on an appropriate substrate, namely a ferromagnetic semiconductor (FMS). The basic idea here is to study the possibility of adjusting the optical properties of the entire structure by acting on the spin degrees of freedom that are coupled to the charge carriers (both belonging to the FMS), which are in turn coupled to the plasmons of the metallic nanoparticles. Therefore, by acting with an external magnetic field on the spin degrees of freedom, it should be possible to adjust the optical properties. This route could provide the solution to the problem mentioned above, namely the energy mismatch between plasmons and magnons. In addition, it is easier to identify those mechanisms by the very standard and simple technique of ferromagnetic resonance with variable frequency and magnetic field. Indeed, if the effect of plasmons is felt by the magnetic excitations, the change in spectrum of the latter should be detected by this technique.

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2. TRANSPORT PHENOMENA, CONVERSION AND TRANSFER OF ENERGY

2.1. Plasmonic nanoparticles for photo-catalysis A European regulatory framework, set up a decade ago, indicates qualitatively and quantitatively the criteria that have to be met for producing water of a better quality. These directives insist on the necessity to develop new treatments to prevent environmental risks related to the presence of organic micro-pollutants in the residual industrial or urban waters. In this context, advanced oxidation processes (AOPs) are investigated and developed. The processes are based on the production of strongly oxidating radical species, which induce the mineralization of the organic pollutants by attacking the chemical bonds. Among the AOPs, one promising process is photocatalysis using semiconductor nanoparticles. When these nanoparticles are exposed to an irradiation of a specific energy, electron-holes pairs are generated and react with water or dioxygen, leading to the formation of the radical species. However, this process requires the use of high energy ultraviolet radiation, which only makes for 5% of the solar spectrum. For improved efficiency, it would be necessary to conceive a photo-catalytic system allowing the use of visible light. For this purpose, we propose the use of metallic nanoparticles. These particles can absorb light by a resonant process, generating localized surface plasmons. The relaxation of these plasmons can then induce the creation of « hot electrons » which migrate to the nanoparticle surface and can react with adsorbed molecules. It has recently been observed that small gold or silver nanoparticles supported on oxide surfaces can be used as photo-catalysts for the degradation of organic pollutants in aqueous media. However, the mechanisms of the photocatalysis are complex and have not been thoroughly studied. The aim of our project is to investigate and understand the mechanisms of the photocatalysis in order to devise and fabricate composite materials including nanoparticles, which are able to photo-catalyze the degradation of organic micro-pollutants by visible irradiation. This project consists of a theoretical and an experimental part, the latter being carried out in collaboration with the SHPE group of PROMES.

Theorical developments

a) Generation of hot electrons In order to be transfered to adsorbed molecules, the generated hot electrons must have a high energy (more than 2eV) and be located in the vicinity of the particle surface. The first objective of our study is thus to compute, for a given nanoparticle in a given environment, the energy profile of the generated hot electrons (generation rate and localization in terms of energy). This can be carried out using a semi-classical model based on Fermi's golden rule, which requires two ingredients: (i) the wave functions and energies of the nanoparticle at rest and (ii) the electrostatic potential induced inside and outside the nanoparticle by the plasmonic resonance. We first computed this profile for spherical nanoparticles, for which both the wave functions and the potential can be semi-analytically calculated. This calculation will next be extended to various shapes of nanoparticles for which the wave functions and/or the potential must be numerically computed. This will be done using the quantum chemistry software OCTOPUS and the simulation program for metallic nanoparticles MNPBEM. These simulations will allow us to determine the ideal nanostructured material (shape, size and concentration of the nanoparticles, matrix) inducing the optimal generation of hot electrons. b) Transfer of the electrons to the adsorbed molecules To assess the possibility of an electron transfer between the nanoparticle and adsorbed molecules, it is first necessary to compute the potential energy surfaces of the molecules of interest adsorbed at the surface of a nanoparticle. For that, we model the nanoparticle by an infinite metallic surface and use a periodic DFT software such as GPAW. We will focus on several target molecules which may be implied in the mechanism, such as water, dissolved dioxygen, phenol or azobenzene. From these calculations, we should be able to conclude as to the possibility of a hot electron transfer resulting in the dissociation of the molecule.

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(a) (b) (c)

Figure 5 : Sample of results obtained within this project: (a) Absorption efficiency for various nanoparticles (NP) in water. (b) Plasmon potential induced by an excitation at 360 nm of a 10nm gold spherical NP. (c) Wave function of a 10nm gold spherical NP.

Experiments

a) Preparation of nanostructured films In this project our aim is to synthesize nanostructured films with a strong absorption of visible light, which may induce a strong generation of 'very hot electrons' with the appropriate energy for photocatalysis. We thus chose to prepare nanostructured polymer films including gold or silver nanoparticles, following the method developed by E. Nadal in his PhD thesis work. The films are then characterized with regards to their structure by Atomic Force Microscopy and their optical properties are probed through spectroscopic measurements. This synthesis method is very versatile and allows obtaining nanostructured films with a high concentration of nanoparticles with various sizes and shapes. By studying the effect of the various experimental parameters, we make nearly ideal nanostructures that could be reasonably compared with those consider in the theoretical work. b) Photocatalysis tests After the nanostructured films have been obtained, photocatalysis tests will be carried out in collaboration with the SHPE team. The samples will be introduced in solutions containing colored model organic compounds (methyl-orange, sulforhodamine B…) and irradiated by a solar simulator or a visible laser. Depending on the results thus obtained, kinetic and mechanistic studies could be planned in order to investigate the mechanisms involved and eventually extend the process to a larger scale. This project was initiated in April 2014 and received a funding from PROMES in the framework of the 'Actions Incitatives'. This made it possible to fund a Masters 2 internship during February - June 2016. 2.2. Conversion of electromagnetic energy We are interested in the possibilities of converting electromagnetic energy injected into the system via a time-dependent magnetic field with the aim to understand, from a microscopic point of view, the mechanisms and the conversion rate depending on the system parameters and external conditions. 2.2.1. Conversion of electromagnetic energy into heat In an ANR project coordinated by the LPCNO laboratory (INSA - Toulouse), we proposed an experimental as well as a theoretical study of the conversion of electromagnetic energy into thermal energy using magnetic nanoparticles. This ANR project is a follow up of the PEPS CNRS project PTI2014. Hydrogenating the CO or CO2 combines storage of intermittent energy and green production of hydrocarbons. A method has been patented on the ability to catalyze these reactions using nanoparticles (NPs) heated by induction. This method has many advantages, both in terms of yield and response time to variations in energy output. Nevertheless, the link between the intrinsic properties of NPs, temperature and catalytic activity is so far not understood. Our project aims to answer

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fundamental questions related to this new process, by combining experimental and theoretical approaches. This better understanding of the basics should allow us to synthesize nanoparticles that optimize this process. The theoretical work proposed by our team in this project first deals with an isolated magnetic nanoparticle represented as an atomic spin system on a crystal lattice. This approach treats the interactions between, on one hand, the magnons as excitations of the atomic spin and, on the other, the phonons as excitations of the underlying lattice. While this is well known for bulk systems, applications to the nanoscale have yet to be developed. This work has as main objectives: - understand and describe (microscopically) the mechanisms whereby a magnetic excitation caused by a time-dependent

magnetic field induces a temperature rise, - estimating the rate of the energy transfer and the corresponding temperature rise, depending on the alternating field and

the properties of the nanoparticle (size, shape, underlying material). 2.2.2. Role of dipolar interactions in magnetic hyperthermia Magnetic hyperthermia is an experimental treatment of cancer that needs to be further investigated from both a physiological and a physical point of view. Essentially, hyperthermia consists in injecting in tumors a solution that contains magnetic nanoparticles (NP) and then excite the NP's with an external high-frequency AC magnetic field. The resulting magnetic excitation is then transformed into a temperature increase of a few degrees which is enough to kill the targeted cells. It has already been shown that the specific absorption rate (SAR), which is related to the process efficiency, is directly proportional to the area of the hysteresis cycle of the magnetic sample. This cycle depends on various intrinsic and extrinsic parameters: type of material, size and shape of the NP's, overall shape of the sample, density of NP's. It is thus of paramount importance to understand the role of each of these parameters to optimize the whole process. Our activities on this topic aim at a general understanding of the underlying fundamental physics at play in these systems. The static and dynamic magnetic properties of an isolated magnetic NP are usually investigated with the help of one-spin models (OSP), i.e. models where the magnetic state of a NP is described by a macroscopic magnetic moment that takes account of intrinsic properties such as magnetic anisotropy. However, it can be shown that in the case of small NP's (2 to 10nm) OSP models are no longer valid. Indeed, the multi-spin aspect within a single NP becomes crucial when the surface contribution dominates. We then use an effective one-spin model where the surface effects are represented by an effective anisotropy which can be evaluated. Furthermore, experimental results in magnetic hyperthermia inherently result from assemblies of NP's. Hence, dipolar interactions between NP's play a central role. Taking into account the dipolar interactions on the same footing as surface effects is therefore important. We have shown that surface and dipolar effects may sometimes compete and screen each other and this is, for instance, seen in static observables such as the equilibrium magnetization curves. The dynamic properties, e. g. AC susceptibility, may themselves be obtained within Debye's formalism. In the framework of our model, this type of calculation leads to a microscopic understanding of what is usually treated as phenomenological parameters such as the Vogel-Fulcher temperature. In the same spirit, the SAR is written in term of the AC susceptibility, such that the analytical determination of the effect of the NP concentration or external static field on the SAR becomes possible (as shown in Figure 6).

Figure 6 : Specific absorption rate against the DC magnetic field for various assembly concentrations.

nanoscale systems and structures: electronic, magnetic and ... · 1.1. magnetic properties of...

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