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

[001][100]50 CHAPTER 3. SELF-ORGANIZED NPS ARRAYS: MORPH. ASPECTSab[010][110][100][001][001]c[010]daa√2[001][010][110]Figure 3.1: Panel a: 3-dimensional view of a cubic crystal of lithium fluoride; atoms oflithium and fluorine denoted with different colours. Panel b: 3-dimensional representationof a portion of ripple structure developed on LiF(110). Panel c: top view of the LiF(100)surface. Panel d: top view of the Lif(110) surface.forming parallel chains with alternating charge signs (fig. 3.1(d)). Therefore, LiF crystalscut along the {110} direction expose unstable surfaces, and during homoepitaxialdeposition of LiF they can undergo a spontaneous surface reconstruction driven by theproliferation of {100} facets; these facets are tilted of 45 ◦ with respect to the surface, sothat elongated islands with [001] macrosteps are formed, which eventually develop into aregularly spaced ridge-and-valley (ripple) structure (fig. 3.1(b)) [121].Mechanically-polished optical-grade LiF(110) substrates, with lateral sizes of 1 cm and1 mm thick, were acquired from MaTecK Gmbh. Some samples were manufactured withboth sides polished and were used for transmission measurements; the remaining ones,more suited to reflectivity measurements, were polished on only one side, the other onebeing rough in order to minimize the reflection from the backside.A representative AFM image of a LiF(110) substrate in the as-received state is reportedin fig. 3.2. As said before the (110) surface is not a preferred crystal plane, soit does not present extended flat terraces, but instead is uniformly rough. The straightscratches visible in the figure are due to the polishing procedure and were observed on allthe samples; in any case, they did not compromise the fabrication of the templates. Astatistical analysis of the surface characteristics, performed over several samples, yieldeda RMS surface roughness of about 6÷9 nm. These values were adequate to induce theformation of regular structures upon LiF homoepitaxy; in contrast, samples exhibitingRMS roughness values higher than 15 nm were found unsuitable for the fabrication ofordered nanostructures.