applied sciences Review Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience Yuan Shen and Ingo Dierking * School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK; [email protected] * Correspondence: [email protected] Received: 29 May 2019; Accepted: 14 June 2019; Published: 20 June 2019 Featured Application: Nanomaterial-doped liquid crystals. Abstract: The research field of liquid crystals and their applications is recently changing from being largely focused on display applications and optical shutter elements in various fields, to quite novel and diverse applications in the area of nanotechnology and nanoscience. Functional nanoparticles have recently been used to a significant extent to modify the physical properties of liquid crystals by the addition of ferroelectric and magnetic particles of different shapes, such as arbitrary and spherical, rods, wires and discs. Also, particles influencing optical properties are increasingly popular, such as quantum dots, plasmonic, semiconductors and metamaterials. The self-organization of liquid crystals is exploited to order templates and orient nanoparticles. Similarly, nanoparticles such as rods, nanotubes and graphene oxide are shown to form lyotropic liquid crystal phases in the presence of isotropic host solvents. These effects lead to a wealth of novel applications, many of which will be reviewed in this publication. Keywords: liquid crystal; ferroelectric nanoparticle; quantum dot; graphene oxide 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 [1–3]. 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 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 [7–10]. 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 (Figure1a) [ 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 of Appl. Sci. 2019, 9, 2512; doi:10.3390/app9122512 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 2512 2 of 47 Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 47 theunique observation effects of chiralitythe observation from achiral of chirality molecules, from resulting achiral from molecules, sterically resulting induced from packing sterically of the bent-coreinduced packing mesogens of [the10], bent-core such as ferroelectricity mesogens [10], or such the formationas ferroelectricity of helical or superstructures the formation of in helical the B7 phasesuperstructures [14]. Thermotropic in the B7 LCsphase are [14]. commonly Thermotropic constituted LCs byare single commonl organicy constituted entities or by mixtures single thereof,organic whichentities exhibit or mixtures various mesophasesthereof, which at di ffexhibiterent temperatures various mesophases or pressures at [15different], illustrated temperatures in Figure 1b.or Aspressures the temperature [15], illustrated rises, a in typical Figure thermotropic 1b. As the LCtemperature passes through rises, highera typical ordered thermotropic phases, alsoLC passes called softthrough crystals, higher the ordered hexatic phases, smectic also phases called with soft positional crystals, orderthe hexatic as well smectic as bond phases orientational with positional order, throughorder as thewell fluid as bond smectic orientational phases (SmC order, and through SmA), the which fluid exhibit smectic both phases positional (SmC andand orientationalSmA), which order,exhibit and both finally positional to the and nematic orient phaseational (N) order, with purelyand finally orientational to the nematic order, into phase the (N) isotropic with purely phase. Theorientational number of order, different into phases the isotropic observed phase. depends The onnumber the chemical of different composition, phases observed symmetry depends and order on ofthe the chemical LC molecules. composition, About 25symmetry different thermotropicand order of phases the LC are knownmolecules. to date, About and they25 different are still increasingthermotropic in number.phases are known to date, and they are still increasing in number. FigureFigure 1.1. ((aa)) TheThe variousvarious shapes shapes of of LC LC molecules. molecules. (b ()b A) A typical typical illustration illustration of diofff differenterent thermotropic thermotropic LC phasesLC phases observed observed on heating on heating from crystalline from crystalline state. Reproduced state. Reproduced with permission with permission from [7]. Copyright from [7]. (2017)Copyright MDPI. (2017) MDPI. InIn contrast,contrast, lyotropiclyotropic LCsLCs consistconsist ofof atat leastleast twotwo didifferentfferent kindskinds ofof components:components: aa collectioncollection ofof anisodiametricanisodiametric moleculesmolecules andand particlesparticles disperseddispersed inin aa suitablesuitable solventsolvent (Figure(Figure2 2).). These These systems systems will will thusthus alwaysalways be be mixtures, mixtures, with thewith main the control main variablecontrol beingvariable the concentration.being the concentration. Unlike thermotropics, Unlike thethermotropics, phase transitions the phase of lyotropic transitions LCs of are lyotropic not merely LCs are dependent not merely on thedependent temperature on the of temperature the system, butof the also, system, mainly, but on thealso, relative mainly, concentration on the relative of each concentration component of [15 each]. Being component mixtures, [15]. there Being will alwaysmixtures, be two-phasethere will regionsalways atbe the two-phase phase boundaries regions at between the phase two boundaries different lyotropic between phases. two different One can distinguishlyotropic phases. lyotropic One LCs can into distinguish three different lyotropic kinds: (i)LCs amphiphilic into three lyotropics,different (ii)kinds: colloidal (i) amphiphilic lyotropics, andlyotropics, (iii) chromonics, (ii) colloidal where lyotropics, the constituent and (iii) moleculeschromonics, are where dye molecules the constituent in a suitable molecules solvent. are dye As themolecules name indicates, in a suitable amphiphilic solvent. lyotropic As the LCsname are indicates, usually composed amphiphilic of amphiphilic lyotropic LCs molecules are usually upon additioncomposed of aof solvent amphiphilic [16], often molecules water. Asupon Figure addition2 shows, of witha solvent increasing [16], amphiphileoften water. concentration, As Figure 2 dueshows, to thewith segregation increasing of amphiphile hydrophobic concentration, and hydrophilic due regions, to the thesesegregation molecules of self-assemblehydrophobic intoand hydrophilic regions, these molecules self-assemble into spherical or rod-like micelles, leading to the formation of the hexagonal phase, cubic phase or lamellar phase. Nevertheless, the solute Appl. Sci. 2019, 9, 2512 3 of 47 sphericalAppl. Sci. 2019 or, rod-like 9, x FOR PEER micelles, REVIEW leading to the formation of the hexagonal phase, cubic phase or lamellar3 of 47 phase. Nevertheless, the solute component of a lyotropic LC need not always be molecular in nature, butcomponent may also of consist a lyotropic of much LC larger need (solid)not always particles be molecular with anisotropic in nature, shapes. but may It can also also consist be of colloidalof much sizelarger [7], (solid) as we particles will discuss with further anisotropic in Section shapes.3. SuchIt can materials also be of would colloidal then size be [7], known as we as will inorganic discuss liquidfurther crystals, in Section LC 3. clays, Such but materials also nanotubes, would then graphene be known oxide as orinorganic biological liquid structures crystals, such LC as clays, viruses but (Figurealso nanotubes,3). graphene oxide or biological structures such as viruses (Figure 3). FigureFigure 2.2.A A typical typical lyotropic lyotropic LC LC phase phase diagram diagram of amphiphilic of amphiphilic molecules molecules dissolved dissolved in a solvent. in a solvent. Cubic phasesCubic mayphases be observedmay be atobserved different positionsat different in thepo phasesitions diagram. in the Atphase very diagram.