# Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience
## 1 Introduction  
For many materials, the transition between the liquid and the solid phase is not a single-step process, but a range of various mesophases, which are called liquid crystals (LCs). LCs are self-organized anisotropic fluids that are thermodynamically located between the isotropic liquid and the crystalline phase, exhibiting the fluidity of liquids as well as the long-range lattice order that can only be found in crystalline solids [1; 2; 3]. Generally, LCs are composed of anisotropic building blocks (usually of rod or disc shape), which are spontaneously oriented along a specific direction, called the director \(\mathbf{n}\)[4]. Without an external alignment force, the director of a nematic LC, the simplest phase of LC, whose molecules are only orientationally ordered, is usually spatially changed continuously but randomly over large spatial extensions (except for defects, where the director may vary suddenly and drastically) [5; 6]. Normally, one can split LCs into two typical categories, i.e., thermotropic LCs and lyotropic LCs [7; 8; 9; 10]. Thermotropic LCs are usually further distinguished according to the shape of their constituent molecules, being called calamitic for rod-like, discotic for disk-like and sanidic for brick- or lath-like molecules (Figure 1a) [11]. A typical calamitic mesogen is generally composed of a rigid core, often incorporating phenyl and biphenyl groups, and two flexible endgroups, often alkyl or alkoxy chains. A common structure of discotic mesogens is a rigid, disk-like core to which six flexible endgroups are attached. Apart from these conventional mesogens, research attention has been recently focused on the so-called non-conventional LCs [12], which are neither rod- nor disk-shaped. Among them, bent-core LCs [13], in particular, have received attention due to their unique effects ofthe observation of chirality from achiral molecules, resulting from sterically induced packing of the bent-core mesogens [10], such as ferroelectricity or the formation of helical superstructures in the B7 phase [14]. Thermotropic LCs are commonly constituted by single organic entities or mixtures thereof, which exhibit various mesophases at different temperatures or pressures [15], illustrated in Figure 1b. As the temperature rises, a typical thermotropic LC passes through higher ordered phases, also called soft crystals, the hexatic smectic phases with positional order as well as bond orientational order, through the fluid smectic phases (SmC and SmA), which exhibit both positional and orientational order, and finally to the nematic phase (N) with purely orientational order, into the isotropic phase. The number of different phases observed depends on the chemical composition, symmetry and order of the LC molecules. About 25 different thermotropic phases are known to date, and they are still increasing in number. In contrast, lyotropic LCs consist of at least two different kinds of components: a collection of anisodiametric molecules and particles dispersed in a suitable solvent (Figure 2). These systems will thus always be mixtures, with the main control variable being the concentration. Unlike thermotropics, the phase transitions of lyotropic LCs are not merely dependent on the temperature of the system, but also, mainly, on the relative concentration of each component [15]. Being mixtures, there will always be two-phase regions at the phase boundaries between two different lyotropic phases. One can distinguish lyotropic LCs into three different kinds: (i) amphiphilic lyotropics, (ii) colloidal lyotropics, and (iii) chromonics, where the constituent molecules are dye molecules in a suitable solvent. As the name indicates, amphiphilic lyotropic LCs are usually composed of amphiphilic molecules upon addition of a solvent [16], often water. As Figure 2 shows, with increasing amphiphile concentration, due to the segregation of hydrophobic and hydrophilic regions, these molecules self-assemble into  
Figure 1: (**a**) The various shapes of LC molecules.