Virtually all STFs fabricated to date have been grown on flat substrates, with few exceptions [148, 154, 303]. Deposition is random in the xy plane, but not along the z axis. Since clusters of 1- to 3-nm size are the fundamental building blocks of the matchstick morphology, morphological changes along the z axis can be induced with transitions not exceeding similar dimensions. This is clearly evident from the SEM micrograph in Fig. 5.1.
By rotating the substrate about an axis in the xy plane, bent nematic or two-dimensionally shaped nanowires can be engineered to grow out of the xy plane, and by rotating the substrate about the z axis, helicoidal or three-dimensionally shaped nanowires can be engineered to grow similarly. Bent nematic and helicoidal are the two canonical morphologies  with distinct optical response characteristics, as detailed from Sec. 6.3 onwards. Combinations of both types of canonical morphologies are also possible, as exemplified by the recently introduced slanted chiral STFs [98, 127]. Whereas the following chapters are devoted to the optics of STFs, emerging thoughts on STF morphology and speculations on future trends are presented here.
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