Recent Developments in Plasmonic Alloy Nanoparticles: Synthesis, Modelling, Properties and Applications

Abstract

Despite the traditional plasmonic materials are counted on one hand, there are a lot of possible combinations leading to alloys with other elements of the periodic table, in particular those renowned for magnetic or catalytic properties. It is not a surprise, therefore, that nanoalloys are considered for their ability to open new perspectives in the panorama of plasmonics, representing a leading research sector nowadays. This is demonstrated by a long list of studies describing multiple applications of nanoalloys in photonics, photocatalysis, sensing and magneto-optics, where plasmons are combined with other physical and chemical phenomena. In some remarkable cases, the amplification of the conventional properties and even new effects emerged. However, this field is still in its infancy and several challenges must be overcome, starting with the synthesis (control of composition, crystalline order, size, processability, achievement of metastable phases and disordered compounds) as well as the modelling of the structure and properties (accuracy of results, reliability of structural predictions, description of disordered phases, evolution over time) of nanoalloys. To foster the research on plasmonic nanoalloys, here we provide an overview of the most recent results and developments in the field, organized according to synthetic strategies, modelling approaches, dominant properties and reported applications. Considering the several plasmonic nanoalloys under development as well as the large number of those still awaiting synthesis, modelling, properties assessment and technological exploitation, we expect a great impact on the forthcoming solutions for sustainability, ultrasensitive and accurate detection, information processing and many other fields.

Keywords: alloy nanoparticles; bimetallic nanoparticles; density functional calculations; photocatalysis; plasmon resonance.

Figure 1

Overview of the main motivations for the realization of plasmonic nanoalloys: achieving better plasmonic properties, better non‐plasmonic or hybrid plasmonic functions and better overall features compared to single element NPs.

Figure 2

Classification of plasmonic alloys: the metallic bond between atomic constituents is a prerequisite. Their structure can be homogeneous or heterogeneous. The homogeneous domains can be amorphous, intermetallic or solid solutions (interstitial or substitutional).

Figure 3

Hume‐Rothery rules prescribe that alloying depends on atomic radii, crystalline structure, electronegativity and valency of the pure elements.

Figure 4

Overview of the most frequently reported synthetic approaches for plasmonic nanoalloys and list of their advantages and limitations. “B”: bulk, “M”: molecular. For details see text.

Figure 5

Sketch of the different methodologies in the family of LSPC and overview of their main advantages. Adapted with permission from Ref. [23] under a Creative Commons (CC‐BY 4.0) license. Copyright (2020) The Authors. Published by Wiley‐VCH Verlag GmbH & Co. KGaA.

Figure 6

Sketch of Au−Fe NPs formation mechanism by LML for different initial gold content in the Au‐Fe₂O₃ educt nanoaggregates. For highest Au content, only alloy NPs are obtained. For low Au content, core‐shell morphologies are observed with a Fe‐rich shell. Reprinted with permission from Ref. [45] under a Creative Commons (CC‐BY 4.0) license. Copyright (2019) the Authors.

Figure 7

(A) Illustration of the amalgamation reaction, converting monometallic seeds into intermetallic NCs. As‐synthesized PdGa, PdIn, PdZn, AuGa₂, Cu₂Ga, Ni₂Ga₃, and Ag₃Ga intermetallic NPs are shown in TEM images (B to H) and STEM EDX maps (B to E and G). Line profile scans for AuGa₂ and Ni₂Ga₃ NPs (E and G) highlight the compositional uniformity of the intermetallic NPs. Reprinted with permission from Ref. [72] under a Creative Commons (BY‐NC 4.0) license. Copyright (2021) the Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.

Figure 8

Map of mainstream atomistic modelling approaches for specific aspects of plasmonic nanoalloys. Modelling of the plasmonic properties is possible starting from the optical constant. For details see text.

Figure 9

Sketch of atomic volume and d‐orbital extension for the coinage metals Cu, Ag and Au and the effect on d‐bond strength in alloys with Zn, according to Ref. [97]. The minimum is predicted for Ag−Zn alloys because of the worst compromise between atomic volume (which destabilize d‐bond formation) and d‐orbital extension (which favours d orbital hybridization).

Figure 10

(A) Comparison of the dielectric functions calculated for AuAl₂ with different functionals and experimental data (open circles). Reprinted from Ref. [115], Copyright (2020), with permission from Elsevier. (B) Comparison between calculated (PBE functional) and experimental reflectivity spectra for various Ag−Au alloys. Reprinted with permission from Ref. [116] under a Creative Commons (CC‐BY 4.0) license. Copyright (2020), The Author(s).

Figure 11

Measured (a) and calculated (b) extinction spectra of Au−Ag alloy NPs with different gold metal fractions. The interband contribution was modelled with the convolution of the jDOS with a single Lorentzian oscillator. Reprinted with permission from Ref. [126]. Copyright (2013) WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 12

(A) Energy and FWHM of the bulk plasmon in Au−Ag (a, c) and Au−Pd alloys (b, d) as a function of the Au content, measured by EELS in alloy nanodisks. Reprinted with permission from Ref. [140]. Copyright (2019) American Chemical Society. (B) Composition dependence of the surface plasmon resonance (SPR) wavelength for 20 nm spherical Au−Ag alloy nanoparticles with different compositions in water. The R² value corresponds to a third‐order polynomial fit. Inset corresponds to a linear regression of the composition dependence of the SPR wavelength at intermediate gold fractions. Reprinted with permission from Ref. [142]. Copyright (2013) American Chemical Society.

Figure 13

The real and imaginary parts of dielectric constant and plasmonic quality factor in Au−Ni thin films with different compositions (indicated in the legend) as deposited (left panels) and after annealing at 300 and 600 °C (middle and right panels). Insets in OCs components show the magnification for specific energy intervals. Insets in the quality factor show the dependence of quality factor maximum value and energy location versus Ni content. Reprinted from Ref. [153], Copyright (2019), with permission from Elsevier.

Figure 14

(A) Simulated EELS spectra of a 40 nm Au−Fe alloy NP with various compositions. The quenching of the LSP is clearly appreciable while increasing Fe content. (B) The plot of calculated LSP resonance (LSPR) maximum versus composition for the same alloy and different sizes and compositions, showing the blue‐shift. (C) DOS calculated with DFT for pure Au, Au₉₄Fe₆ and Au₇₅Fe₂₅ showing the appearance of VBS close to Fermi energy (E_F) in the alloys. (D) Calculated free electron density versus alloys composition. (E) Calculated and experimental cell volume versus alloy composition. Reprinted with permission from Ref. [40]. Copyright (2019) American Chemical Society.

Figure 15

Overview of problems observed in literature when plasmonic elements have been alloyed with non‐plasmonic elements, resulting in a decrease of the plasmonic quality factor.

Figure 16

Sketch of absorption in silica‐metal core‐shell NPs with size >100 nm, showing that at parity of geometrical shape, the Au−Fe alloy shell (with Fe content close to 20 at %) have much larger absorption in the red and NIR compared to a pure Au shell. Reprinted with permission from Ref. [155]. Copyright (2015) The Royal Society of Chemistry.

Figure 17

(A) LSP blue shift, intensity decrease and broadening in a single Au NR upon amalgamation with Hg. Reprinted with permission from Ref. [157]. Copyright (2022) American Chemical Society. (B) GIXRD recorded before and after annealing of Au−Al thin films with different Al content (x). The samples are shown as black lines before annealing and as red lines after annealing. (C) Measured OCs (real and imaginary components) for the thin films as‐deposited and after annealing at 500 °C. (B) and (C) reprinted with permission from Ref. [167]. Copyright (2018) American Chemical Society.

Figure 18

Overview of the main motivations for the realization of nanoalloys for plasmon‐enhanced catalysis. (A) Plasmon decay preferentially with intraband transitions. (B) The energy of hot carriers increases. (C) The metal work function decreases. (D) The Mott–Schottky barrier decreases. (D) The interface energy barrier with molecular adsorbates decreases. (F) The metal d‐band is shifted to facilitate the interaction with molecular adsorbates. (G) Hot‐carriers localization, mean free path and lifetime are improved. (H) Hot‐carriers generation rate increases.

Figure 19

The plot of the imaginary components of the OC in Au, Al, AuAl₂, and AuAl. (a) Contribution due to first‐order perturbation accounting for direct inter‐band and intraband electronic transitions. (b) The contribution due to second‐order perturbation theory due to phonon‐assisted indirect electronic transitions. Reprinted from Ref. [179], Copyright (2021), with permission from Elsevier.

Figure 20

(A) NPs: hysteresis loops (top, A) and zero‐field cooled (ZFC) and field‐cooled (FC) magnetization dependence on temperature (bottom, B) showing paramagnetic response at room temperature with a hysteresis loop at 3 K and ZFC‐FC behaviour typical of spin glass arrangement. Reprinted with permission from Ref. [29], Copyright (2013) The Royal Society of Chemistry 2013. (B) Magnetic properties of Ag−Fe thin films: (a) Saturation magnetization versus Ag composition. (b) M−T curves from the annealed samples. (c–d) Hysteresis loops for the 78 at % Fe sample (c) as deposited and (d) after annealing at various temperatures. Reprinted from Ref. [164], Copyright (2021), with permission from Elsevier.

Figure 21

Overview of the most frequent applications of plasmonic nanoalloys in recent literature: optical sensing based on LSP spectral position or evolution; photothermal effects for nanomedicine applications, towards hyperthermia effects or nanocavitation phenomena; plasmon‐enhanced catalysis and SERS for analytical or biolabeling purposes.

Figure 22

Overview of chemical processes reported in the recent literature about plasmon‐enhanced catalysis with nanoalloys.

Figure 23

MEF with Au−Ag nanoalloys in the paper substrate. (A) Optical extinction spectra showing a magnification of the LSP range for five Au−Ag NPs on cellulose substrates. (B) LSP wavelength calculated with FDTD simulations for the Au−Ag alloy NPs. (C) Fluorescence signal enhancement from three dyes (FITC, R6G and CR) showed twofold MEF from the substrates compared to the Whatman chromatography paper. The enhancement occurs when the LSP of Au−Ag alloy NPs matches the emission band of each specific dye. (D) Chromatographic MEF analysis. A three‐dye mixture was separated using paper chromatography, and then each component was selectively detected. Reprinted with permission from Ref. [225]. Copyright (2017), American Chemical Society.

Figure 24

Application of magnetic‐plasmonic nanoalloys as SERS substrates. (A) Ag−Fe NPs: (A) UV‐Vis spectra of the NPs coated with various ligands. (B) Photo of the magnetically assembled NPs on a glass substrate (distance between spots is 4 mm). (C) The intensity of the Raman band of malachite green (MG) at 1617 cm⁻¹ measured on Ag−Fe NPs with different surface coatings. (D) 2D Raman map of the major band of MG collected on NPs coated with MPS and GSH. Reprinted with permission from Ref. [160]. Copyright (2017) Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (B) Au−Co NPs: (A) A 4×4 Array of magnetic‐plasmonic dots obtained on glass by drop‐casting and evaporation at room temperature of an Au−Co NPs solution. During the evaporation of the solution, a squared array of 16 permanent cylindrical magnets (2 mm in diameter ×8 mm length) was placed below the glass slide. (B) Raman spectra were collected with 647 nm excitation by deposing 20, 2 or 0.2 pmol of MG on the magnetic‐plasmonic dots (and reference with 20 pmol MG on the bare glass). (C) 2‐D map of the Raman intensity at 1615 cm⁻¹ collected on 6 dots after deposition of 20 pmol of MG. Laser excitation at 532 nm. Reprinted with permission from Ref. [31]. Copyright 2021 Wiley‐VCH GmbH.

Figure 25

Summary of key advantages and open challenges in the field of plasmonic nanoalloys.