Liquid crystalline order and magnetocrystalline anisotropy in magnetically doped semiconducting ZnO nanowires

A 2D material-based transparent hydrogel with engineerable interference colours

Transparent hydrogels are key materials for many applications, such as contact lens, imperceptible soft robotics and invisible wearable devices. Introducing large and engineerable optical anisotropy offers great prospect for endowing them with extra birefringence-based functions and exploiting their applications in see-through flexible polarization optics. However, existing transparent hydrogels suffer from limitation of low and/or non-fine engineerable birefringence. Here, we invent a transparent magneto-birefringence hydrogel with large and finely engineerable optical anisotropy. The large optical anisotropy factor of the embedded magnetic two-dimensional material gives rise to the large magneto-birefringence of the hydrogel in the transparent condition of ultra-low concentration, which is several orders of magnitude larger than usual transparent magnetic hydrogels. High transparency, large and tunable optical anisotropy cooperatively permit the magnetic patterning of interference colours in the hydrogel. The hydrogel also shows mechanochromic and thermochromic property. Our finding provides an entry point for applying hydrogel in optical anisotropy and colour centred fields, with several proof-of-concept applications been demonstrated.

Though transparent hydrogels with tunable optical anisotropy are attractive for soft robotics, wearable devices and optical applications, achieving large birefringence has been a challenge. Here, the authors report a transparent hydrogel with large, uniform and magnetically tunable birefringence.

Largely Tunable Magneto-Coloration of Monolayer 2D Materials via Size Tailoring

Magnetically influenced light-matter interaction provides a contactless, noninvasive and power-free way for material characterization and light modulation. Shape anisotropy of active materials mainly determines the sensitivity of magneto-optic response, thereby making magnetic two-dimensional (2D) materials suitable in achieving the giant magneto-birefringence effect as discovered recently. Consequently, relationship between magneto-birefringence response and shape anisotropy of 2D materials is critical but has remained elusive, restricting its widespread applications. Here, we report the highly sensitive and largely tunable magneto-coloration via manipulating the shape-anisotropy of magnetic 2D materials. We reveal a quadratic increasing relationship between the magneto-optic Cotton-Mouton coefficient and the lateral size of 2D materials and achieve a more than one order of magnitude tunable response. This feature enables the engineerable transmissive magneto-coloration of 2D materials by tailoring their shape anisotropy. Our work deepens the understanding of the tunability of magneto-optic response by size effect of active materials, offering various opportunities for their applications in vast areas where color is concerned.

Microcrystal alignment in drawn fiber over sub-meter to kilometer length scales

This paper describes a method for aligning stiff, high-aspect-ratio microcrystals over macro-length scales using a polymer fiber drawing process. A composite preform was constructed with an interfacial, liquid shell layer of grapeseed oil suspending ytterbium-doped potassium lutetium fluoride microcrystals (30% Yb:K2LuF5, KLF) between adjacent cylindrical surfaces of acrylic (polymethyl methacrylate, PMMA). The mean length of synthesized KLF microcrystals was 67 microns, and the mean aspect ratio, equivalent to crystal length divided by diameter, was eight. The acrylic-host preform was drawn into fiber, resulting in uniform reduction of all cross-sectional dimensions by a factor of approximately 20 in the final fiber. A corresponding width reduction of the interstitial liquid-filled gap, containing microcrystals between the polymer surfaces, constrains the microcrystals and causes alignment of the crystal long axes parallel to the axis of the drawn composite fiber. Alignment was best for clearly separated microcrystals and improved even further with the longest lengths, or highest aspect-ratio microcrystals.

Novel Trends in Lyotropic Liquid Crystals

We introduce and shortly summarize a variety of more recent aspects of lyotropic liquid crystals (LLCs), which have drawn the attention of the liquid crystal and soft matter community and have recently led to an increasing number of groups studying this fascinating class of materials, alongside their normal activities in thermotopic LCs. The diversity of topics ranges from amphiphilic to inorganic liquid crystals, clays and biological liquid crystals, such as viruses, cellulose or DNA, to strongly anisotropic materials such as nanotubes, nanowires or graphene oxide dispersed in isotropic solvents. We conclude our admittedly somewhat subjective overview with materials exhibiting some fascinating properties, such as chromonics, ferroelectric lyotropics and active liquid crystals and living lyotropics, before we point out some possible and emerging applications of a class of materials that has long been standing in the shadow of the well-known applications of thermotropic liquid crystals, namely displays and electro-optic devices.

Directed Assembly of Hybrid Nanomaterials and Nanocomposites

Hybrid nanomaterials are molecular or colloidal-level combinations of organic and inorganic materials, or otherwise strongly dissimilar materials. They are often, though not exclusively, anisotropic in shape. A canonical example is an inorganic nanorod or nanosheet sheathed in, or decorated by, a polymeric or other organic material, where both the inorganic and organic components are important for the properties of the system. Hybrid nanomaterials and nanocomposites have generated strong interest for a broad range of applications due to their functional properties. Generating macroscopic assemblies of hybrid nanomaterials and nanomaterials in nanocomposites with controlled orientation and placement by directed assembly is important for realizing such applications. Here, a survey of critical issues and themes in directed assembly of hybrid nanomaterials and nanocomposites is provided, highlighting recent efforts in this field with particular emphasis on scalable methods.

Stimuli-responsive hydroxyapatite liquid crystal with macroscopically controllable ordering and magneto-optical functions

Liquid crystals are mostly formed by self-assembly of organic molecules. In contrast, inorganic materials available as liquid crystals are limited. Here we report the development of liquid-crystalline (LC) hydroxyapatite (HAp), which is an environmentally friendly and biocompatible biomineral. Its alignment behavior, magneto-optical properties, and atomic-scale structures are described. We successfully induce LC properties into aqueous colloidal dispersions of rod-shaped HAp by controlling the morphology of the material using acidic macromolecules. These LC HAp nanorod materials are macroscopically oriented in response to external magnetic fields and mechanical forces. We achieve magnetic modulation of the optical transmission by dynamic control of the LC order. Atomic-scale observations using transmission electron microscopy show the self-organized inorganic/organic hybrid structures of mesogenic nanorods. HAp liquid crystals have potential as bio-friendly functional materials because of their facile preparation, the bio-friendliness of HAp, and the stimuli-responsive properties of these colloidal ordered fluids.

Hydrogen-Bonding-Directed Ordered Assembly of Carboxylated Poly(3-Alkylthiophene)s

Hydrogen-bonding-induced ordered assembly of poly(3-alkylthiophene)s derivatives bearing carboxylic acid groups has been investigated from diluted and concentrated solutions to solid films using ultraviolet-visible absorption spectroscopy, polarized optical microscopy, and four-point probe conductivity measurements. In dilute solutions, the polymer undergoes a spontaneous structural transition from disordered coil-like to ordered rodlike conformations, which is evidenced by time-dependent chromism. Many factors such as alkyl-chain length, types of solvents, and temperature are studied to understand the assembly behavior. Transition kinetics of the assembly process reveals a universal second-order rate law, indicating an intermolecular origin due to hydrogen bonding. When more concentrated, hydrogen bonding drives nematic liquid-crystalline gelation above a critical concentration and the gels are thermally reversible. Under an appropriate balance of mechanical and thermal stresses, uniform liquid-crystalline monodomains are obtained through the application of a mechanical shear force. The dried films made from the sheared solutions display both optical and electrical anisotropies, with a more than 200% increase in charge transport parallel to the direction of shear as opposed to that in the perpendicular one.

Lyotropic Liquid Crystal Phases from Anisotropic Nanomaterials

Liquid crystals are an integral part of a mature display technology, also establishing themselves in other applications, such as spatial light modulators, telecommunication technology, photonics, or sensors, just to name a few of the non-display applications. In recent years, there has been an increasing trend to add various nanomaterials to liquid crystals, which is motivated by several aspects of materials development. (i) addition of nanomaterials can change and thus tune the properties of the liquid crystal; (ii) novel functionalities can be added to the liquid crystal; and (iii) the self-organization of the liquid crystalline state can be exploited to template ordered structures or to transfer order onto dispersed nanomaterials. Much of the research effort has been concentrated on thermotropic systems, which change order as a function of temperature. Here we review the other side of the medal, the formation and properties of ordered, anisotropic fluid phases, liquid crystals, by addition of shape-anisotropic nanomaterials to isotropic liquids. Several classes of materials will be discussed, inorganic and mineral liquid crystals, viruses, nanotubes and nanorods, as well as graphene oxide.

Growth of a Novel Nanostructured ZnO Urchin: Control of Cytotoxicity and Dissolution of the ZnO Urchin

The applications of zinc oxide (ZnO) nanowires (NWs) in implantable wireless devices, such as diagnostic nanobiosensors and nanobiogenerators, have recently attracted enormous attention due to their unique properties. However, for these implantable nanodevices, the biocompatibility and the ability to control the behaviour of cells in contact with ZnO NWs are demanded for the success of these implantable devices, but to date, only a few contrasting results from their biocompatibility can be found. There is a need for more research about the biocompatibility of ZnO nanostructures and the adhesion and viability of cells on the surface of ZnO nanostructures. Here, we introduce synthesis of a new nature-inspired nanostructured ZnO urchin, with the dimensions of the ZnO urchin's acicula being controllable. To examine the biocompatibility and behaviour of cells in contact with the ZnO urchin, the Madin-Darby canine kidney (MDCK) epithelial cell line was chosen as an in vitro experimental model. The results of the viability assay indicated that, compared to control, the number of viable cells attached to the surface of the ZnO urchin and its surrounding area were reduced. The measurements of the Zn contents of cell media confirmed ZnO dissolution, which suggests that the ZnO dissolution in cell culture medium could lead to cytotoxicity. A purposeful reduction of ZnO cytotoxicity was achieved by surface coating of the ZnO urchin with poly(vinylidene fluorid-co-hexafluoropropylene) (PVDF-HFP), which changed the material matrix to slow the Zn ion release and consequently reduce the cytotoxicity of the ZnO urchin without reducing its functionality.

Revealing a room temperature ferromagnetism in cadmium oxide nanoparticles: an experimental and first-principles study

CdO nanoparticles obtained from a thermal decomposition, show a room temperature ferromagnetism due to the cadmium and oxygen defects.

Scalable high-fidelity growth of semiconductor nanorod arrays with controlled geometry for photovoltaic devices using block copolymers

Controlled density semiconducting oxide arrays are highly desirable for matching nanometer length scales specific to emerging applications. This work demonstrates a facile one-step method for templating hydrothermal growth which provides arrays with high-fidelity tuning of nanorod spacing and diameter. This solution-based method leverages the selective swelling of block copolymer micelle templates, which can be rationally designed by tuning molecular weight and volume fraction.

Directed self-assembly of block copolymers: a tutorial review of strategies for enabling nanotechnology with soft matter

Self-assembly of soft materials is broadly considered an attractive means of generating nanoscale structures and patterns over large areas. However, the spontaneous formation of equilibrium nanostructures in response to temperature and concentration changes, for example, must be guided to yield the long-range order and orientation required for utility in a given scenario. In this review we examine directed self-assembly (DSA) of block copolymers (BCPs) as canonical examples of nanostructured soft matter systems which are additionally compelling for creating functional materials and devices. We survey well established and newly emerging DSA methods from a tutorial perspective. Special emphasis is given to exploring underlying physical phenomena, identifying prototypical BCPs that are compatible with different DSA techniques, describing experimental methods and highlighting the attractive functional properties of block copolymers overall. Finally we offer a brief perspective on some unresolved issues and future opportunities in this field.

Fabrication of ZnCoO nanowires and characterization of their magnetic properties

Hydrogen-treated ZnCoO shows magnetic behavior, which is related to the formation of Co-H-Co complexes. However, it is not well known how the complexes are connected to each other and with what directional behavior they are ordered. In this point of view, ZnCoO nanowire is an ideal system for the study of the magnetic anisotropy. ZnCoO nanowire was fabricated by trioctylamine solution method under different ambient gases. We found that the oxidation of trioctylamine plays an essential role on the synthesis of high-quality ZnCoO nanowires. The hydrogen injection to ZnCoO nanowires induced ferromagnetism with larger magnetization than ZnCoO powders, while becoming paramagnetic after vacuum heat treatment. Strong ferromagnetism of nanowires can be explained by the percolation of Co-H-Co complexes along the c-axis.

Large area vertical alignment of ZnO nanowires in semiconducting polymer thin films directed by magnetic fields

We demonstrate the use of magnetic fields for the directed assembly of ZnO nanowires in semiconducting polymer films suitable for ordered bulk heterojunction photovoltaics. Using rotational field annealing, Co-doped ZnO nanowires with negative paramagnetic anisotropy were successfully aligned out-of-plane with respect to the substrate and polymer film.

Room-temperature synthesis of self-assembled Sb2S3 films and nanorings via a two-phase approach

The freestanding Sb(2)S(3) films were easily synthesized at the interface of water and toluene at room temperature, where Na(2)S and (C(2)H(5)OCS(2))(3)Sb (xanthate, O-ethyldithiocarbonate) acted as sulfur and antimony source, respectively. After 3 h of aging, the Sb(2)S(3) films with a flat surface toward organic side and rough surface toward aqueous side were assembled by sheaflike Sb(2)S(3) nanowires. The Sb(2)S(3) nanorings formed by end-to-end connection of the bundled nanowires appeared in the water layer when the reaction time reached 24 h. The Sb(2)S(3) nanorings showed higher photocatalytic activity for methyl orange degradation under visible light than the Sb(2)S(3) films owing to broader spectrum response and better aqueous dispersion.