The Interplay between Surface Plasmon Resonance and Switching Properties in Gold@Spin Crossover Nanocomposites
Summary (2 min read)
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.
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Frequently Asked Questions (11)
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).