The Interplay between Surface Plasmon Resonance and Switching Properties in Gold@Spin Crossover Nanocomposites

TL;DR: Rod-like gold nanoparticles are directly embedded in a 1D-polymeric spin crossover (SCO) material leading to singular Au@SCO nanohybridウスρitectures.

Abstract: Rod-like gold nanoparticles are directly embedded in a 1D-polymeric spin crossover (SCO) material leading to singular Au@SCO nanohybrid architectures. The resulting architectures are designed to promote a synergetic effect between ultrafast spin-state photoswitching and photothermal properties of plasmonic nanoparticles. This synergy is evidenced by the strong modulation of the surface plasmon resonance of the gold nanorods through the spin-state switching of the SCO component and also the strong enhancement of the photoswitching efficiency compared to pure SCO particles. This remarkable synergy results from the large modulation of the dielectric properties of the SCO polymer upon its thermal switching and the enhancement of the heating of these hybrid nanostructures upon excitation of the surface plasmon resonance of the gold nanorods.

Summary

1. Introduction

• Spin CrossOver (SCO) is a versatile switching phenomenon that results in remarkable changes in magnetic, optical and mechanical properties of materials upon external perturbation such as light, temperature or pressure [1] .
• Upon excitation, the sample, initially set within its thermal hysteresis loop and in the LS state, is heated and driven to the HS state.
• The controlled synthesis of SCO nanomaterials makes it possible to introduce many other functionalities.
• To further reduce the laser fluence and switching time required to photoswitch AuNPs-SCO nanoensemble hereafter, the authors propose another approach.
• It is noteworthy to mention that directly coating of AuNRs with SCO will improve the contact and therefore the heat diffusion among the metallic particles (i.e. AuNRs) and the SCO complexes.

2.1. Methodology

• At first, the authors explore the conditions under which the crucial stability of the colloidal suspensions of the synthesized AuNRs [12] is kept during coating them with the 1D[Fe(Htrz)2(trz)](BF4) compound.
• Second addresses the direct synthesis of the nanocomposites through the polymerization of the [Fe(Htrz)2(trz)](BF4) chain around the AuNRs.
• To ensure that a large number of SCO NPs contains at least one AuNR, the authors explore several reactants’ concentrations.
• This makes it possible to compare the crystalline phase of the SCO shell, as well as the switching properties, with a reference compound.
• The latter is prepared using either a well-described reverse-micellar protocol [4f] or a direct synthesis approach detailed below.

2.2. Synthesis and stability of gold nanorods

• The AuNRs are synthesized according to the well-known and highly reproducible seedgrowth method . [12].
• The AuNRs dispersion, which is stable in CTAB (5 mM) aqueous solution, becomes unstable into a hydro-alcoholic mixture and in the presence of Fe(BF4)2.
• Therefore, different concentrations and volumes of Htrz solutions are tested.
• It is worth mentioning that the redshift of the transverse mode is weaker and about 5 nm.
• One cannot precisely indicate the level of substitution.

2.3. Synthesis of the composite nanomaterials

• Perfect control of the size can be achieved following micellar methods.
• A first synthesis is performed using the same conditions as for the synthesis of 2, adding a solution of [Fe(BF4)2] to the dispersion of the AuNRs previously mixed with Htrz ligand.
• After mixing and centrifugation a dark purple powder is obtained (compound 3).
• Summary of the studied compounds and their related characteristics, also known as Table1.

2.4. Morphological characterization of nanomaterials by electron microscopy

• Compounds 1-5 are first characterized by Transmission Electron Microscopy .
• Several conclusions can be drawn from these images.
• Second, from the TEM images, one can estimate that 75 to 90 % of the SCO particles are filled with at least one AuNR.
• The filling ratio (table 1) increases as [Fe] decreases.
• Therefore the well-defined morphology observed in 4 and 5 may result from a templating effect induced by the presence of AuNRs.

2.5. Structural characterization by X-ray powder diffraction

• Powder X-ray diffraction (PXRD) is performed using capillary sample holders and compared to a reference of polymorph I. [4f,13].
• The PXRD patterns are reported in Figure 3.
• It appears clear that compounds 2 and 3 do not exhibit the same pattern that all the other compounds or the reference pattern.
• The latter corresponds to gold, [16] in agreement with the observations made from the TEM images.

2.6. Evaluation of switching properties

• The switching properties of these composite architectures are investigated by means of magnetic measurements using a SQUID magnetometer and the related data are detailed in Table 2.
• Compounds 3-5 exhibit much wider thermal hysteresis loops (Table 2), up to 33 K large for compound 5.
• The experiments are carried out shining the sample with a broadband halogen light source and recording the spectra of the transmitted light with and without the sample with a compact spectrometer versus the temperature.
• Note that there is no obvious spectral shift around the transverse plasmon resonance centred at 515 nm.
• The authors have previously shown that the index of refraction of SCO decreases upon switching of the SCO compounds from LS to HS states. [22].

2.7. Photoswitching properties

• Since the authors aim at the synthesis of photoswitchable Au@SCO composites using low power irradiation, they performed photoswitching experiments using continuous-wave irradiation with a laser diode.
• The authors samples are prepared as for a classical photomagnetic measurement [23] , which is a thin layer inserted inside a SQUID magnetometer sample holder.
• Before sending the laser beam on the sample, three hysteretic cycles are recorded.
• After photo-saturation is reached, the laser is switched off and the temperature is decreased to recover the LS.
• Since the SCO compounds very weakly absorb in their HS state [22] , these experiments evidence the strong enhancement of the photoswitching effect for AuNRs doped SCO NPs.

3. Conclusion

• These nanocomposites exhibit several interesting and noticeable features.
• First, the AuNRs used seemed to bring a templating property during the synthesis of the composites, leading to welldefined architectures.
• Third, the authors have recorded a strong modulation of the SPR of the AuNRs upon the spin-state switching of the SCO shell.
• Among them, the authors would like to stress that since photoswitching of SCO nanoparticles can be achieved on a very short timescale (below 1 ns) [7] , such hybrid materials may exhibit an ultrafast photoswitching of their surface plasmon resonance.

Figures

Figure 1: Absorption spectra of gold nanorods dispersion in water, stabilized by 5 mM of CTAB, before (black curve) and after addition of 1.8 M of Htrz ligand aqueous solution. (Nota: the concentration of 1.8 M is further used for the synthesis of the nanocomposites).

Figure 5: (a) Optical spectra of compound 5 recorded at 313 K and 408 K, respectively. (b) Evolution of the peak of SPR upon temperature variation in both warming and cooling mode.

Table 2: Summary of the studied compounds and their switching features.

Figure 4: Thermal evolution of the χMT product of samples 1-5 at an applied magnetic field of 10 kOe and for a temperature sweep of 0.7 K/mn.

Figure 6: Photoswitching experiments performed on compounds 1 (top) and 5 (bottom), using a laser fluence of 15 mW/cm² centred at 830 nm. a) and c) report the hysteresis curve (o), the isothermal irradiations () and the following cooling curves (■). b) and d) report the converted HS fraction as function of time at two different temperatures.

Figure 3: Powder X-Ray Diffraction patterns of compounds 1-5, measured using capillary sample holders.

Figure 2: TEM images and size distributions of samples 1 ([Fe(Htrz)2(trz)](BF4) obtained from micellar synthesis), 2 ([Fe(Htrz)2(trz)](BF4) obtained from direct synthesis) and 4 and 5 (Au@[Fe(Htrz)2(trz)](BF4)).

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The Interplay between Surface Plasmon Resonance and
Switching Properties in Gold@Spin Crossover
Nanocomposites
Marlène Palluel, Ngoc Minh M Tran, Nathalie Daro, Sonia Buère, Stéphane
Mornet, Eric Freysz, Guillaume Chastanet
To cite this version:
Marlène Palluel, Ngoc Minh M Tran, Nathalie Daro, Sonia Buère, Stéphane Mornet, et
al.. The Interplay between Surface Plasmon Resonance and Switching Properties in Gold@Spin
Crossover Nanocomposites. Advanced Functional Materials, Wiley, 2020, 30 (17), 2000447 (9 p.).
�10.1002/adfm.202000447�. �hal-02493837�

1
DOI: 10.1002/ ((please add manuscript number))
Article type: Full Paper
The Interplay between Surface Plasmon Resonance and Switching Properties in Gold@Spin
Crossover Nano-Composites
Marlène Palluel, Ngoc Minh Tran, Nathalie Daro, Sonia Buffière, Stéphane Mornet,*
Eric
Freysz* and Guillaume Chastanet*
M. Palluel, Dr. N. Daro, Dr. S. Mornet, S. Buffière, Dr. G. Chastanet
CNRS, Univ. Bordeaux, ICMCB, UMR 5026, 87 Avenue du Dr A. Schweitzer, F-33608
Pessac, France.
E-mail: guillaume.chastanet@icmcb.cnrs.fr; stephane.mornet@icmcb.cnrs.fr
Dr. N. M. Tran, Dr. E. Freysz
Univ. Bordeaux, CNRS, UMR 5798, LOMA, 358 Cours de la libération, F-33405 Talence
cedex, France.
E-mail: eric.freysz@u-bordeaux.fr
Keywords: Spin crossover, surface plasmon resonance, gold nanoparticle, photoswitching,
SPR modulation.
Abstract: Rod-like gold nanoparticles were directly embedded in a 1D-polymeric Spin
CrossOver (SCO) material leading to singular Au@SCO nanohybrid architectures. The
resulting architectures are designed to promote a synergetic effect between ultrafast
spin-state photoswitching and photothermal properties of plasmonic nanoparticles.
This synergy has been evidenced by the strong modulation of the surface plasmon
resonance of the gold nanorods through the spin-state switching of the SCO component
and also the strong enhancement of the photoswitching efficiency compared to pure
SCO particles. This remarkable synergy results from the large modulation of the
dielectric properties of the SCO polymer upon its thermal switching and the
enhancement of the heating of these hybrid nanostructures upon excitation of the
surface plasmon resonance of the gold nanorods.

2
1. Introduction
Spin CrossOver (SCO) is a versatile switching phenomenon that results in remarkable
changes in magnetic, optical and mechanical properties of materials upon external
perturbation such as light, temperature or pressure
[1]
. Molecular-based SCO compounds offer
many interesting prospects in molecular electronics.
[2]
Along this line, lots of efforts have
been devoted to synthesize and design SCO nanoparticles (NPs)
[3,4]
with controlled shape and
size. Among SCO materials that can be processed at the nanometer scale, while keeping the
properties they exhibit at the macroscopic scale, the attention has been driven towards
polymeric triazol based iron(II) complexes. Thanks to an appropriate tuning of ligands and
anions, these complexes can exhibit a large thermal hysteresis loop between a high spin (HS)
and a low spin (LS) state of iron(II) centred at room temperature
[5]
. Such a hysteresis loop
makes it possible to record and store information in such compounds. The recording can be
induced by different means. For instance, it has been demonstrated that fast photoswitching
can be recorded using nanosecond laser pulses which central wavelength is centred around the
iron(II) d-d or metal to ligand charge transfer (MLCT) absorption bands of the complex.
[6,7]
Upon excitation, the sample, initially set within its thermal hysteresis loop and in the LS state,
is heated and driven to the HS state. The heating of the compound induced by the laser pulse
can be of several degrees according to the laser fluence and the switching time depends on its
size. Hence, the photoswitching time of the NPs was demonstrated to be drastically reduced.
[7]
The controlled synthesis of SCO nanomaterials makes it possible to introduce many
other functionalities. For instance, coating the SCO particles with silica offers a platform to
graft new functions at their surfaces, such as luminescent molecules
[8]
or gold nanoparticles
(AuNPs).
[9]
Combining the properties of SCO materials and AuNPs has been shown to open
promising properties, in particular because these metal nanoparticles display photothermal
heating properties in the visible and near-infrared range resulting from the non-radiative
decay of localized surface plasmons. For instance, it has been demonstrated that exciting the

3
plasmon resonance of 14 nm spherical AuNPs reduces significantly the power required to
photoswitch SCO complexes.
[9]
In this architecture, upon excitation of the plasmon resonance,
AuNPs act as efficient local nano heaters.
[10]
For such designed AuNPs-SCO nano ensemble,
a four-time reduction of the photoswitching laser intensity was demonstrated. To further
improve the heat transfer from the AuNPs to the SCO NPs avoiding the silica interphase, we
have recently reported a direct grafting of gold nanospheres onto SCO nanoparticles.
Following this approach, a well-controlled amount of gold nanospheres of various sizes were
grafted on [Fe(Htrz)
2
(trz)](BF
4
) (Htrz = 1H-1,2,4-triazole and trz = deprotonated triazolato(-)
ligand) nanoparticles.
[11]
To further reduce the laser fluence and switching time required to photoswitch
AuNPs-SCO nanoensemble hereafter, we propose another approach. It consists of a direct
coating of gold nanorods (AuNRs) by SCO material. AuNRs are of particular interest since
they exhibit a broad longitudinal surface plasmon resonance (SPR) band centred in the near-
infrared,
[10]
a spectral region where the SCO in the LS state is almost transparent. It is
noteworthy to mention that directly coating of AuNRs with SCO will improve the contact and
therefore the heat diffusion among the metallic particles (i.e. AuNRs) and the SCO complexes.
This is shown to drastically enhance the plasmonic assisted photoswitching of the as-prepared
nanocomposites.
Hereafter, we report on the coating of AuNRs by the 1D-polymeric iron(II) compound
of formula [Fe(Htrz)
2
(trz)](BF
4
).
[4f]
This original procedure was achieved developing a direct
coating of the AuNRs by the SCO compound. The latter is shown to be highly reproducible.
The Au@SCO NRs exhibit a widening of the thermal hysteresis loop compared to the pure
SCO NPs. Moreover, a drastic reduction of the laser fluence required to photoswitch the
nanocomposites is recorded. Furthermore, we evidence a large shift of the plasmon resonance
of the AuNRs upon switching of the SCO complex. This ensemble of results demonstrates the
interplay between the SCO phenomenon and the SPR of these original core-shell NPs.

4
2. Results and Discussions
2.1. Methodology
At first, we explore the conditions under which the crucial stability of the colloidal
suspensions of the synthesized AuNRs
[12]
is kept during coating them with the 1D-
[Fe(Htrz)
2
(trz)](BF
4
) compound. Second addresses the direct synthesis of the nanocomposites
through the polymerization of the [Fe(Htrz)
2
(trz)](BF
4
) chain around the AuNRs. To ensure
that a large number of SCO NPs contains at least one AuNR, we explore several reactants’
concentrations. [Fe(Htrz)
2
(trz)](BF
4
) nanoparticles without AuNRs are prepared as well. This
makes it possible to compare the crystalline phase of the SCO shell, as well as the switching
properties, with a reference compound. The latter is prepared using either a well-described
reverse-micellar protocol
[4f]
or a direct synthesis approach detailed below.
2.2. Synthesis and stability of gold nanorods
The AuNRs are synthesized according to the well-known and highly reproducible seed-
growth method
(Figure S1).
[12]
From the TEM images, the dimensions of these NRs are
estimated through a normal law fit of the histograms (Figure SI1b) to be 60  8 nm for the
length and 15  4 nm for the width. Accordingly, the mean value of the aspect ratio is 4  2.
The UV-visible spectrum of this colloidal suspension (Figure 1) exhibits an intense and large
absorption band at 780 nm associated to the longitudinal SPR mode of the AuNRs and an
additional band at 540 nm associated to the transverse mode.
[10]

Frequently asked questions

Q1. What are the contributions in "The interplay between surface plasmon resonance and switching properties in gold@spin crossover nanocomposites" ?

Palluel et al. this paper investigated the interaction between surface plasmon resonance and switching properties in Gold @ Spin Crossover Nanocomposites. 

Q2. How does the spr band of the gold nanoparticle change?

The shift of about 35 nm of the SPR band of the gold nanoparticle embedded in a SCO particle can be used to follow the spin crossover as a function of the temperature. 

Q3. What is the effect of the aqueous solution on the AuNRs?

the AuNRs dispersion, which is stable in CTAB (5 mM) aqueous solution, becomes unstable into a hydro-alcoholic mixture and in the presence of Fe(BF4)2. 

Q4. What is the effect of AuNRs on the SCO shell?

the AuNRs used seemed to bring a templating property during the synthesis of the composites, leading to welldefined architectures. 

Q5. What is the way to improve the heat transfer from AuNPs to SCO?

To further improve the heat transfer from the AuNPs to the SCO NPs avoiding the silica interphase, the authors have recently reported a direct grafting of gold nanospheres onto SCO nanoparticles. 

Q6. What is the effect of the AuNRs on the photoswitching effect?

Since the SCO compounds very weakly absorb in their HS state [22] , these experiments evidence the strong enhancement of the photoswitching effect for AuNRs doped SCO NPs. 

Q7. What is the spectral region where the SCO is almost transparent?

AuNRs are of particular interest since they exhibit a broad longitudinal surface plasmon resonance (SPR) band centred in the nearinfrared, [10] a spectral region where the SCO in the LS state is almost transparent. 

Q8. How many W/cm2 was required to photoswitch AuNRs?

It is worth mentioning that in a previous report, up to 2000 W/cm² (from a Raman spectrometer) was required to photoswitch SCO@SiO2@Au composite compounds. [9] 

Q9. What is the effect of AuNRs on the thermal hysteresis loop?

besides the templating effect previously hypothesized in the microscopy analysis, the synthesis using AuNRs seems to broaden the thermal hysteresis loop of both neat SCO and composite nanoparticles present in compounds 4 and 5, compared to compounds 1 and 2. 

Q10. How much water is used in the synthesis of the composite materials?

In consequence, the authors decided to perform the synthesis of the composite materials using a solvent mixture of 40 % of ethanol and 60 % of water. 

Q11. How much Htrz does the suspension have?

Up to 2 M of Htrz the dispersion remains stable below 40 % of ethanol, even after 24 hours (Table SI1) while above this ratio, the suspension is destabilized (Table SI2 and SI3 and Figure SI3).