Morphology and plasmonic properties of self-organized arrays of ...

98 CHAPTER 6. COMPOSITE MEDIA BASED ON AU/LIF ARRAYS3025Number [%]201510540 nma. b.01020Diameter [nm]3040 50Figure 6.1: Left panel: transmission electron microscopy image (courtesy of ICMM) ofFe 3 O 4 nanoparticles obtained by decomposition of an iron oleate complex in 1-octadecenand oleic acid (see text for details). Right panel: distribution of the particles hydrodynamicsize obtained from dynamic light scattering analysis on a colloidal suspension ofFe 3 O 4 /AO particles dispersed in hexane.ThetemplateforthedepositionoftheFe 3 O 4 /OAnanoparticleswaspreparedaccordingto the procedure described in the previous chapters. First, a nanopatterned LiF(110)substrate was grown following the homoepitaxial deposition of t LiF ≈ 250 nm LiF atT = 300 ◦ C, which induced the formation of a well-developed ripple structure with amean periodicity Λ ≈ 35 nm; an AFM image of the surface is shown in fig. 6.2(a). Then,the 2D array of gold nanoparticles was obtained by grazing deposition of t Au ≈ 5 nmgold at T = 100 ◦ C and subsequent annealing at T = 400 ◦ C. We report in fig. 6.2(b)a representative AFM image of the array, where we can distinguish parallel chains ofslightly elongated nanoparticles, oriented along the LiF ridges, and with mean in-planesemi-axes of a x ≈ 11 nm and a y ≈ 14 nm, respectively across and parallel the ripples.The corresponding Fourier spectrum, shown in the inset of the figure, confirms the orderedarrangement of the particles: the sharp intensity peak confined in the [1¯10] direction, andrapidly decaying in the [001] direction, shows that the periodicity of the underlying ripplestructure has been preserved within the Au NPs array, while the broader feature slowingdecaying in all directions, and slightly stretched in the [1¯10] direction, reflects the in-planeanisotropic shape of the particles.The deposition of the Fe 3 O 4 /OA NPs has been performed by immersing the Au/LiFsubstrate in a colloidal suspension of NPs dispersed in hexane with a concentration of≈ 6μg Fe per ml, for a period of time of about 5 minutes; then, the sample has beenwashed in pure hexane, in order to remove the particles weakly stuck to the surface,and dried under nitrogen flux. An AFM image measured after the deposition of themagnetic NPs is reported in fig. 6.2(c). The surface appears uniformly covered withFe 3 O 4 /OA nanoparticles, and no Au particle could be apparently singled out. From thesole inspection of the AFM image we cannot determine the exact thickness of the magneticNP layer. However earlier depositions on silicon substrates (not reported here) showedthat the Fe 3 O 4 /OA particles did not form more than a monolayer even after prolongedimmersions of several hours in solution; we can therefore expect that also in this case asingle monolayer of magnetic nanoparticles was deposited.