A nanoring is a cyclic

nanostructure A nanostructure is a structure of intermediate size between microscopic and molecular structures. Nanostructural detail is microstructure at nanoscale. In describing nanostructures, it is necessary to differentiate between the number of dimen ...

with a thickness small enough to be on the

nanoscale Nanotechnology is the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing propertie ...

(10⁻⁹ meters). Note that this definition allows the diameter of the ring to be larger than the nanoscale.

Overview and history

The first nanorings were made of

gallium nitride Gallium nitride () is a binary III/ V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV af ...

in 2001. Nanorings have been prepared from

zinc oxide Zinc oxide is an inorganic compound with the Chemical formula, formula . It is a white powder which is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, Zinc metabolism, food supplements, rubbe ...

and cyclo-para-phenylenes as well as porphyrins. Although nanorings may have a diameter on the nanoscale, many of these materials have diameters that are larger than 100 nm, with many nanorings having a diameter on the

microscale The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...

(10⁻⁶ meters). As such, nanorings are considered to be members of a sub-class of

nanomaterials Nanomaterials describe, in principle, chemical substances or materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale). Nanomaterials research takes a materials science ...

called one-dimensional (1-D) nanomaterials. These are nanomaterials in which one of the three physical dimensions in a single unit of the material is on a

length scale In physics, length scale is a particular length or distance determined with the precision of at most a few orders of magnitude. The concept of length scale is particularly important because physical phenomena of different length scales cannot af ...

greater than the nanoscale. Other examples of one-dimensional nanomaterials are

nanowire file:SnSe@SWCNT.jpg, upright=1.2, Crystalline 2×2-atom tin selenide nanowire grown inside a single-wall carbon nanotube (tube diameter ≈1 nm). A nanowire is a nanostructure in the form of a wire with the diameter of the order of a nanometre ( ...

s, nanobelts,

nanotubes A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range (nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized: * ''Single-walled carbon nanotubes'' (''SWC ...

, and

nanosheet A nanosheet is a two-dimensional nanostructure with thickness in a scale ranging from 1 to 100 nm. A typical example of a nanosheet is graphene, the thinnest two-dimensional material (0.34 nm) in the world. It consists of a single layer ...

Mechanical

As with other nanomaterials, nanorings exhibit quantized phenomena. ZnO nanorings made from the spontaneous folding of a single nanobelt crystal can be extensively mechanically manipulated without breaking or fracturing, giving them a unique

mechanical advantage Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for ...

over other classes of ZnO nanostructures.

Synthesis

Generally, nanorings are synthesized using a bottom-up approach, as top-down syntheses are limited by the entropic barriers presented by these materials. Currently, the number of different synthetic techniques used to make these particles is almost as diverse as the number of different types of nanorings themselves. One common method for synthesizing nanorings involves first synthesizing nanobelts or nanowires with an uneven

charge distribution In electromagnetism, charge density is the amount of electric charge per unit length, surface area, or volume. Volume charge density (symbolized by the Greek letter ρ) is the quantity of charge per unit volume, measured in the SI system in co ...

focused on the edges of the material. If these criteria are met, these particles may naturally

self-assemble Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the ...

into ring structures such that

Coulomb repulsion Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that calculates the amount of force between two electrically charged particles at rest. This electric force is conventionally called the ''electrostatic f ...

forces are minimized within the resulting

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

. They cannot be grown on discrete

crystal growth Crystal growth is a major stage of a crystallization, crystallization process, and consists of the addition of new atoms, ions, or polymer strings into the characteristic arrangement of the crystalline lattice. The growth typically follows an ini ...

sites and thus, cannot be synthesized on a

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

with any crystallographic predictability. Therefore, nanorings are most commonly synthesized aqueously by creating entropically unique conditions which induce spontaneous nanoring self-assembly. Other approaches for nanoring synthesis include the assembly of a nanoring around a small

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

particle which is later removed or the expansion and twisting of

porphyrin Porphyrins ( ) are heterocyclic, macrocyclic, organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (). In vertebrates, an essential member of the porphyrin group is heme, w ...

-like structures into a hollow nanoring structure.

References

{{reflist Nanoparticles by morphology