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

52 CHAPTER 3. SELF-ORGANIZED NPS ARRAYS: MORPH. ASPECTSThe deposition rate was acritical parameter for the achievement of well-ordered nanostructures.In fig. 3.3(b, c) we report AFM images measured ex-situ on two different samplesafter the deposition of LiF at a rate of 4 nm/min and 1 nm/min, respectively; inboth cases the substrate temperature was kept constant at ≈ 300 ◦ C and approximately250 nm of LiF were deposited. For the sample in panel (c) (1 nm/min) an ordered ripplemorphology is clearly visible, showing parallel and coherently aligned [001] macrostepswith an average spacing of ≈ 35 nm, and facets at an angle of 45 ◦ with respect to thesample plane (panel (c)). The morphology of the sample in panel (b) (4 nm/min), instead,is dramatically different. Here the surface is completely chaotic, consisting of randomlyoriented rectangular prisms with {100} facets: in this case the deposition rate was so highto prevent either long range ordering but also any local crystallographic matching withthe substrate, leading to the proliferation of stand-alone nanosized crystals. Therefore,we fixed the deposition rate at 1 nm/min for all the samples under scrutiny in this work.a.[001]b.[001]c.[001]d.[001]Figure 3.4: AFM images of a nanopatterned LiF(110) sample with Λ = 35 nm, as afunction of the deposited amount of LiF. Panel a: t LiF = 60 nm. Panel b: t LiF = 120 nm.Panel c: t LiF = 180 nm. Panel d: t LiF = 250 nm. Inset of the figures: Fourier spectra ofthe corresponding images.In order to investigate the evolution of the surface morphology from flat to nanostructured,we monitored the surface morphology at different stages of deposition. AFMimages measured in correspondence of increasing deposition times are shown in fig. 3.4,at thickness step of ≈ 60 nm. The [100] facets are already visible at the lowest coverageof 60 nm (panel (a)), forming elongated structures a few tens of nanometers long andoriented along the [001] direction; at this stage, however, the macrosteps are still relativelyrandomly distributed and no univocal long range order can be distinguished. The