And their relationship with structure and environment
Looking forward, the ability to determine crystal defects with high precision in nanoalloys and to functionalize hybrids at the nanoscale opens up the possibility for revolutionary new applications in many fields of science (catalysis, nanomedecine, high density data storage) and industry. There are many benefits from using nanoalloys to adjust their intrinsic catalytic, magnetic and optical properties depending on composition, shape and size, in order to improve their performance as fast time response to external solicitations or only to limit matter consumption. Nanoalloys thus need to be studied in their own environment to describe extrinsic properties of single nanoparticles or clusters assemblies and to optimize their use near real-life conditions.
- Catalytic properties: pressure gap, surface segregation, stability and chemical reactivity
State of the art
Perspectives for the next years
In preliminary to any progress in catalysis, the first requirement is to improve the preparation/synthesis method to obtain well-defined supported nanoalloys with homogeneous size, composition, etc. In order to comply with practical catalysis and bridge the so-called “materials gap”, these systems should not limit to planar systems (well-suited for surface-science studies) but should include high-surface-area porous supports.
In this context, several important challenges emerge: firstly, an in situ/operando/environmental characterization approach is crucial to understand catalytic processes, as it is now well documented that catalysts “at work” constantly evolve through atom diffusion. This especially holds true under realistic gas-phase or liquid-phase conditions at catalysis-relevant temperatures (with respect to ultrahigh-vacuum low-temperature conditions). In particular, nanoalloys may undergo severe restructuring, such as adsorption-induced surface segregation, phase transition, NP coalescence, etc  To this end, many modern techniques are now available: environmental aberration-corrected TEM/STEM, ambient-pressure X-ray photoelectron spectroscopy, operando X-ray absorption spectroscopy, operando infrared spectroscopy, etc.
Secondly, the fundamental understanding of catalytic phenomena can only be achieved through theoretical approaches. While the modeling of heterogeneous catalysis has made huge progress in recent years, especially through density functional theory-(DFT)-based computer simulations , to a large extent it still fails to be fully predictive (e.g. “what bimetallic catalyst should I select to efficiently catalyze a given reaction?”). Moreover, such recently evidenced catalytic phenomena as plasmon-assisted photocatalysis  are poorly understood and would benefit from theoretical investigations.
In summary, the design of next-generation multimetallic catalysts will require knowledge-based and multidisciplinary approaches. In this regard, the teams involved in the present proposal are ideally skilled.
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- Magnetic properties: morphologies, strain and coupling nanocomposites to improve magnetic moment and anisotropy
State of the art
For several decades, alloys made of iron or cobalt (Fe or Co) and platinum (Pt) have been of particular interest because they can yield one of the magnetically ‘hardest’ materials (magnetization is retained for a long time) as well as one of the magnetically softest . Nowadays, aberration-corrected transmission electron microscopy in combination with spectroscopic methods, are indispensable tools not only for atomic resolution structural characterization but also to the mere investigation of size-dependent phenomena related to magnetic moment and anisotropy . Very recently, Yang et al.  demonstrate a state-of-the-art approach at the single-atom level for determining the 3D atomic arrangement of a 8.4 nm FePt nanoparticle embedded in carbon by combining atomic electron tomography correlated to advanced DFT calculations. In one hand they quantitatively correlated local defects (in chemical ordering, composition…) and surface-relaxation effects to magnetic anisotropy energy (MAE) decrease. The average spin magnetic moment of the Fe atoms has been theoretically calculated as slightly larger than in bulk L10 FePt as previously experimentally obtained in such nanoparticles .
Perspectives for the next years
Considering the growing demand nowadays for high performance magnets, notably for green energy, one of solutions is to use rare earth (RE)-based ternary magnets. Until now, the breakthroughs in magnet research is based on NdFeB, but the monopoly of such compound causes a problem of heavy RE resources. One solution could be the use of RE nanometer-sized objects . Another promising route could be to fabricate nanocomposite alloyed materials made of fine mixture of a hard and a high magnetization phase in strong exchange magnetic coupling. Theoretical models of such nanocomposites predict superior performances than the current permanent magnets, provided the soft phase is confined in less than 10 nm-nanosized grains [6, 7].
A development will be to share the expertise of the partners on the control of the nanoalloy synthesis and characterization up to their nanostructuration, to realize model films and experimentally explore fine mechanisms that govern nanomagnet performances. To achieve that goal, we propose to develop alternative magnetic nanoalloys in interaction with their environment, and to study size effects, physico-chemical surface affinities, strain on monocrystalline template, matrix confinement from their extrinsic magnetic properties .
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- Optical properties: effect of the matrix or surrounding ligands, morphologies and coupling in nanohybrids to tailor plasmonic properties
State of the art
Multicomponent nanoparticles (NPs) may adopt various structures (alloys, Janus, core-shell…) and their optical properties have generated unceasing experimental and theoretical investigations for the last decades . Coinage metal based nanoalloys are of special interest as they display a strong resonance in their optical response (the Localized Surface Plasmon Resonance, LSPR) whose features depend not only on the morphology (size, shape) and environment of the NPs, but also on their internal structure. Therefore, the primary interest of studying nanoalloy optical properties was to shape their LSPR by playing with the relative proportion of both constituents and their chemical order. Conversely, linear and non-linear optics were shown to be a “soft” (nondestructive) method to probe the chemical structure of the NPs (segregation versus alloying) or structural modifications (oxidation/reduction), both in ensembles and single NPs.
Perspectives for the next years
Understanding the cross-correlation between size, shape, morphology and relative atomic composition effects on the structure and corresponding optical response of nanoalloys is not still incomplete. At present, a better understanding of the optical properties of such systems in relation with their fine structure can be expected, thanks to recent advances for elaborating very well controlled systems  and to developments in the modeling of the optical response in the low size range  (a few hundred atoms) as well as in the very low size range  (a few tenths atoms) which are now able to reliably account for the presence of an embedding matrix or of ligands . Moreover, the large size range (a few tens of nanometers), which may be investigated at the single particle level, is also rich of perspectives for its flexibility in design (with a possible plasmonic coupling between NPs of different materials) and its richness of applications . Another aspect that can be exploited is the LSPR sensitivity to the close environment of the NPs. In this respect, a “plasmonic” metal can be combined with another material of a given functionality (catalyst, chemically active, magnetic, fluorescent…). This will surely lead to new nanocomposite materials like passive metallo-hybrid NPs such as plasmonic sensing , or active metallo-hybrid with modified optical properties such as magneto-plasmonic , photocatalysis  or fluorescent enhancement . The development of in situ and in real-time optical spectroscopy combined with environmental electron microscopy techniques should permit a better understanding of the mechanisms involved during restructuring processes. All these trends constitute an exciting challenge that requires synergy between researchers with different backgrounds to develop multidisciplinary approaches in the field of nanoalloys.
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