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

3.2. 2D ARRAYS OF GOLD NANOPARTICLES 57fig. 3.8(c) we report an AFM image measured on the same sample of fig. 3.7(b) after theannealingat400 ◦ C.Comparingthetwoimages, themaineffectoftheannealingprocedurewas a partial melting of the gold grains, which merged together forming larger particles,with a size comparable to the ripples width. The underlying ripple structure assisted theprocess, preventing the spreading of the grains across the ridges, and instead promotingthe formation of parallel chains of nanoparticles, preserving the same periodicity Λ of theripples.Interestingly, the LiF grooves not only guided the particles arrangement, but alsoinfluenced their sizes and shape. This effect can be observed looking at fig. 3.8, wherewe reported AFM images of several NP arrays, annealed at the same temperature of400 ◦ C but fabricated under different growth conditions. Let’s first consider panels (a)and (b). The samples have, respectively, periodicities Λ ≈ 30 nm and ≈ 40 nm, anda similar amount of gold were deposited, ≈ 4.5 nm and ≈ 4.8 nm. In both cases theparticles have an in-plane circular shape, and, although it is not possible to preciselydetermine the particles dimensions due to the convolution of the AFM tip, their typicalsize increases according to the variations of width of the LiF grooves. In panel (c) a samplewith Λ ≈ 45 nm, similar to the sample in panel (b), but t Au ≈ 5.4 nm of deposited goldis shown. In this case, the particles are still confined inside the ripples, and the higheramount of gold led to the formation of more elongated structures, with an average aspectratio of ≈ 1.5. A similar trend is observed increasing further the thickness of gold. Thesample in panel (d) has Λ ≈ 55 nm and t Au ≈ 7.2 nm.The majority of the particles is again as wide as the grooves, however we also observean increased dispersion of particles sizes with respect to the previous cases, both alongand across the ripples. This is probably due to the presence of morphological defects inthe underlying ripple structure, and it happens in concordance with the observations ofthe previous section, where an increasing disorder of the surface morphology was foundat the largest Λ (fig. 3.5(d)). Looking at the shape, the particles now have a maximumaspect ratio of ≈ 2, higher than for sample (c), but many less anisotropic ones are alsovisible; this could be due to the fact that the formation of structures with aspect ratiosgreater than 2 is not favoured, so in these cases a segregation to smaller particles occurs.Then, we can conclude that, for periodicities between 25 nm and 60 nm and thickness ofdeposited gold between 4 nm and 10 nm, the particles width is fixed by the grooves widthwhile the shape (aspect ratio) is determined by the amount of deposited gold.We can interpret this effects by considering the dynamics of the Au nanowires dewetting.If the NW evolve following a Rayleigh-type instability [192], then a linear relationshipbetween the NP spacing (and hence their volume) and the initial NW radiusr (r ∝ (Λ · t Au ) 1/2 ) is expected [193], showing that both the mean spacings along theripples and volume of the NP get larger in correspondence of the increasing Λ and t Au .The simultaneous increase in the aspect ratio can be ascribed to the lateral “constraint”effect exerted by the LiF nanopatterns, that triggers an anisotropic NP growth during thedewetting process.Another unexpected feature of the NP arrays is the relative positions of particlesbelonging to different grooves. As the dewetting induced by the annealing takes placeseparately in each ripple, one may expect no transversal correlation between the positionof the particles in adjacent rows; in contrast, many regions can be distinguished wherethe particles are locally arranged on a rectangular grid. These regions can be emphasizedby looking at the autocorrelations of the AFM images. In fig. 3.9 we report the 2Dautocorrelation plots for the samples of fig. 3.8(b,c); both the figures are characterizedby rectangular patterns of peaks and hollows, with sides oriented in the [1¯10] and [001]