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nanotechnology 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 ...
, nanorods are one morphology of nanoscale objects. Each of their dimensions range from 1–100 nm. They may be synthesized from metals or semiconducting materials. Standard
aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...
s (length divided by width) are 3-5. Nanorods are produced by direct chemical synthesis. A combination of
ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
s act as shape control agents and bond to different facets of the nanorod with different strengths. This allows different faces of the nanorod to grow at different rates, producing an elongated object. One potential application of nanorods is in display technologies, because the reflectivity of the rods can be changed by changing their orientation with an applied electric field. Another application is for microelectromechanical systems (MEMS). Nanorods, along with other noble metal nanoparticles, also function as theragnostic agents. Nanorods absorb in the near IR, and generate heat when excited with IR light. This property has led to the use of nanorods as cancer therapeutics. Nanorods can be conjugated with tumor targeting motifs and ingested. When a patient is exposed to IR light (which passes through body tissue), nanorods selectively taken up by tumor cells are locally heated, destroying only the cancerous tissue while leaving healthy cells intact. Nanorods based on semiconducting materials have also been investigated for application as energy harvesting and light emitting devices. In 2006, Ramanathan et al. demonstrated1 electric-field mediated tunable photoluminescence from ZnO nanorods, with potential for application as novel sources of near-ultraviolet radiation.


Synthesis


ZnO nanorods

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 ...
(ZnO) nanorod, also known as
nanowire file:[email protected], 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 ( ...
, has a direct bandgap energy of 3.37 eV, which is similar to that of GaN, and it has an excitation
binding energy In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...
of 60 meV. The optical bandgap of ZnO nanorod can be tuned by changing the morphology, composition, size etc. ZnO nanorods have been used to fabricate nano-scale electronic devices, including
field effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the current through a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). FETs have three termi ...
, ultraviolet
photodetector Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical ...
, Schottky diode, and ultra-bright
light-emitting diode A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corre ...
(LED). Various methods have been developed to fabricate the single crystalline,
wurtzite Wurtzite is a zinc and iron sulfide mineral with the chemical formula , a less frequently encountered Polymorphism (materials science), structural polymorph form of sphalerite. The iron content is variable up to eight percent.Palache, Charles, H ...
ZnO nanorods. Among those methods, growing from vapor phase is the most developed approach. In a typical growth process, ZnO vapor is condensed onto a solid substrate. ZnO vapor can be generated by three methods: thermal evaporation, chemical reduction, and Vapor-Liquid-Solid (VLS) method. In the thermal evaporation method, commercial ZnO powder is mixed with SnO2 and evaporated by heating the mixture at elevated temperature. In the chemical reduction method, zinc vapor, generated by the reduction of ZnO, is transferred to the growth zone, followed by reoxidation to ZnO. The VLS process, originally proposed in 1964, is the most commonly used process to synthesize single crystalline ZnO nanorods. In a typical process, catalytic droplets are deposited on the substrate and the gas mixtures, including Zn vapor and a mixture of CO/CO2, react at the catalyst-substrate interface, followed by nucleation and growth. Typical metal catalysts involve
gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
,
copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
, nickel, and tin. ZnO nanowires are grown epitaxially on the substrate and assemble into monolayer arrays. Metal-organic chemical vapor deposition ( MOCVD) has also been developed. No catalyst is involved in this process and the growth temperature is at 400 ~500 °C, i.e. considerably milder conditions compared to the traditional vapor growth method. Moreover, metal oxide nanorods (ZnO, CuO, Fe2O3, V2O5, others) can be simply made by heating initial metal in air in a thermal oxidation process. For example, to make a dense "carpet" of CuO nanorods it was found to be enough to heat Cu foil in air at 420 °C. Apart from these manufacturing schemes, ZnO nanorods and tubes can be fabricated by the combination of deep UV lithography, dry etch, and atomic layer deposition (ALD).


InGaN/GaN nanorods

InGaN/ GaN nanorod array light-emitting diodes can be manufactured with dry etching or focused ion beam etching techniques. Such LEDs emit polarized blue or green light Three-dimensional nanorod structures have a larger emitting surface, which results in better efficiency and light emission compared to planar LEDs. Ink-printed quantum dot nanorod LED (QNED) displays are being researched by Samsung, with InGaN nanorod LEDs replacing the organic OLED layer in QD-OLED displays.


Gold nanorods

The seed-mediated growth method is the most common and achieved method for synthesizing high-quality gold nanorods. A typical growth protocol involves the addition of gold nanospheres capped by cetyltrimethylammonium bromide (CTAB) or citrate, served as seeds, to the bulk HAuCl4 growth solution. The growth solution is obtained by the reduction of HAuCl4 with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB)
surfactant Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word ''surfactant'' is a Blend word, blend of "surface-active agent", coined in ...
and silver ions. Longer nanorods (up to an
aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...
of 25) can be obtained in the absence of silver nitrate by use of a three-step addition procedure. In this protocol, seeds are sequentially added to growth solution in order to control the rate of heterogeneous deposition and thereby the rate of crystal growth. The shortcoming of this method is the formation of gold nanospheres, which requires non-trivial separations and cleanings. In one modifications of this method sodium citrate is replaced with a stronger CTAB stabilizer in the nucleation and growth procedures. Raising the pH is another way to achieve high aspect ratio (> 25:1) nanorods with high yield (> 90%) at the cost of increased polydispersity. Another improvement is to introduce silver ions to the growth solution, which results in the nanorods of aspect ratios less than five in greater than 90% yield. Silver, of a lower reduction potential than gold, can be reduced on the surface of the rods to form a monolayer by underpotential deposition. Here, silver deposition competes with that of gold, thereby retarding the growth rate of specific crystal facets, allowing for one-directional growth and rod formation. Another shortcoming of this method is the high toxicity of CTAB. Polymers, such as
Polyethylene glycol Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum with many applications, from industrial manufacturing to medicine. PEG is also known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular wei ...
(PEG), Polyallylamine hydrochloride (PAH) coating; dietary fibers, such as chitosan; or biomolecules, such as phospholipids have been used to displace the CTAB out from the nanorod surface without affecting the stability has been reported.


Cation exchange

Cation exchange is a conventional but promising technique for new nanorod synthesis. Cation exchange transformations in nanorods are kinetically favorable and often shape-conserving. Compared to bulk crystal systems, the cation exchange of nanorods is million-times faster due to high surface area. Existing nanorods serve as templates to make a variety of nanorods that are not accessible in traditional wet-chemical synthesis. Furthermore, complexity can be added by partial transformation, making nanorod heterostructures.


See also

*
Nanowire file:[email protected], 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 ( ...
* Nanopillar * Aggregated diamond nanorod * Colloidal gold * nano


References


External links

{{Scholia
Nanorods show negative refraction in near-IR
(EE Times, December 5, 2005)

S. Ramanathan, S. Patibandla, S. Bandyopadhyay, J.D. Edwards, J. Anderson, J. Mater. Sci.: Mater. Electron 17, 651 (2006) Nanoparticles by morphology