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

58 CHAPTER 3. SELF-ORGANIZED NPS ARRAYS: MORPH. ASPECTS[001][001]a. b.Figure 3.9: 2-dimensional autocorrelations of the AFM images in fig. 3.8(b) and (c). Thesuperimposed grids are guide to the eye for the positions of the maxima of the correlation.directions; the typical spacing along the [1¯10] direction matches the ripples periodicity,while theseparations in the[001] direction areonthesamescale butmoreirregular. Thesestructures indicate that the NP arrays have a good translational symmetry not only inthe direction across the ripples, as imposed by the regular spacing of the grooves, butalso along the ripples, which is unexpectedly triggered by the annealing. The extensionof the rectangular NP meshes depends on the dispersion of the particles size; it can bequite large in case of monodispersed samples like in fig. 3.8(a,b), where the majority ofthe visible particles are orderly arranged, while for samples like in fig. 3.8(d) almost nointer-ridge correlation is found.Lastly, for less than ≈ 3 nm of deposited gold a different dewetting mode occurs. Dueto the low amount of gold, the particles cannot grow as wide as the ripples, and thereforethey form multiple chains inside each grooves, similar to the situation observed before theannealing (fig. 3.7(b,d)). We report an example in fig. 3.10; the sample has a periodicityΛ ≈ 30 nm and ≈ 2.7 nm of gold were deposited. We used dashed lines to delimit thegrooves in the figure, so that double or triple chains of particles can be distinguished.Furthermore, looking at the 2D autocorrelation plot in panel (b) we can identify again aweak rectangular pattern from the NP array, indicating that even in this case the thermaldewetting favoured a partial alignment of the particles.[001]Figure3.10: Panela: AFMimageofananopatternedLiF(110)sampleafterthedepositionof t Au = 2.7 nm Au at T ≈ 100 ◦ C and annealing at T = 400 ◦ C; the vertical dashedlines emphasizes the periodicity of the ripples. Panel b: 2D autocorrelation of the imagein panel a.