How to Select NanomaterialsSilver Nanopow der. Image Credit: SkySpring Carbon nanotubes. Image Credit: RSCNanomaterials have features or particle sizes in the range of 1 to 100 nm. They can be natural or manmade and they have a wide variety of applicationsWhat are Nanomaterials?Nanomaterials are materials possessing one or more dimensional f eatures having a length on the order of a billionth of a meter, or 10-9, to less than 100 billionths of a meter. To illustrate this size, there are 25,400,000 nanometers in an inch and on a comparativ e scale, if a marble were a nanometer, than one meter would be the size of the earth.Nanotechnology DescriptionThe term nanomaterial includes all nanosized materials, including materials that can be engineered or f ound in nature. They are important because they exhibit unique properties because of the size of their features and scientists and engineers have learned how to manipulate and understand the relationship of their properties to size. The National Nanotechnology Initiative (NNI) website puts these ideas in a simple phrase, "At the nanoscale, the physical, chemical, and biological properties of materials differ in fundamental and valuable ways f rom the properties of indiv idual atoms and molecules or bulk matter."NanoScale. Image Credit: Recent discoveries in areas such as microscopy have giv en scientists and engineers new tools to observe and manipulate the phenomena that occur when matter is organized at the nanoscale level. The scanning tunneling mic roscope (STM) and the atomic force microscope (AFM) have made nanotechnology possible to enable scientists to utilize the unique physical, chemical, mechanical, and optical properties of materials that naturally occur at that scale. Nanomaterials fall under the f ield of nanotechnology and the events that occur at the nanoscale lev el are based on "quantum effects" and physical effects such as expanded surface area.How Are Nanomaterials Made?'Nanomaterials' is a term that includes all nanosized materials, including engineered nanoparticles, incidental nanoparticles, and nano-objects like those that exist in nature. Nanomaterials exist in nature in the form of biological materials or a byproduct of human activit ies. For instance, hemoglobin is 5.5 nanometers in diameter and biological ion channels can be as small as a few tenths of a nanometer. Natural nanomaterials such as smoke f rom fire and sea spray exist in the air around us. Manmade nanomaterial byproducts include automobile exhaust and welding fumes. Scientists have already copied the nanostructure of several natural items including the lotus leave to create water repellent surf aces beings used to make stain proof clothing and spider silk, which is naturally reinforced with Nanoscale crystals.Nanomaterials can also be engineered and are made through a process called nanomanufacturing. Nanomanufacturing has two basic approaches.Bottom-up builds products by building them up from atomic- and molecular-scale components. The process involves manipulation or sy nthetic methods of biochemistry in direct assembling subnanoscale building blocks, such as atomic molecular andsupramolecular elements into required nanoscale patterns. It can be a very time consuming process and is therefore better forbiomedical, chemical and physical sensors than large scale molecular electronics and computer parts. The fabricaion strategymust occur in parallel or in arrays to self-form groups of atoms f ast enough to produce useful structures of macroscopic size.Further research is being done to create self-assembling structures that will put themselves together and reduce the waste oftop-down approaches. Currently, the best bottom-up approach is nano-manipulation which allows for precise control over singleatoms and nanoscale particles for the formation of nanostructures.∙Top-down reduces large pieces of materials to the Nanoscale level. The process has evolv ed from lithographic techniques, requiring larger amounts of materials which can lead to waste from the discarding of excess material. Another difference from the bottom upapproach is that in the top-down approach, the parts or chips are both patterned and built in place so that no assembly step isneeded. It is a very useful process for the evolution of the electronics, computer, photonic, and microsystem industries.∙Combined bottom-up and top-down approaches will soon be the standard practice.Within the top-down and bottom-up categories of nanomanuf acturing, there are a growing number of new processes that enable nanomanufacturing. Among these are:∙Chemical vapor deposition (CVD) is a process in which chemicals react to produce very pure, high-performance f ilms. CVD inv olves flowing a precursor gas or gasses into a chamber containing one or more heated objects to be coated. A chemicalreaction occurs on or near the heated surfaces, resulting in the deposition of a thin film on the surface. CVD can be done with avariety of chemicals for a wide range of applications. The process can be enhanced with plasmas and ions to increase depositionrates and/or lower temperatures.Chemical Vapor Depositiono A dvantages include the conformability of the deposition; the film thickness is the same on all sides of the piece so products with odd s hapes can be properly covered. Other advantages of CVD are that a v ariety of materials can be deposited with high purity and relatively high deposition rates, and also requires less vacuum pressure.o Disadvantages for CVD include the properties of the precursors which need to be volatile at near-room temperature. The byproducts of the reactions and the precursors can be hazardous. The type of substrate is limited since the f ilms are deposited at higher temperatures and mechanical stress can occur if the materials have different thermal expansion coefficients.∙Molecular beam epitaxy (MBE) is a technique for epitaxial growth v ia the interaction of one or sev eral molecular or atomic beams that occur on a surface or heated substrate. Epitaxy means the arrangement of atoms on an order substrate. Three f actors def inethe MBE process; Thermal -energy molecular or atomic beams, substrate with elevated temperature, and high vacuum (10x 10-3 to10x10-9 Torr). The processes occurring during f ilm grow th include surf ace adsorption, surface dissociation and migrat ion, latticeincorporation and thermal desorption.o A dvantages include precise control of the beam f luxes and growth condition and compatibility with other high vacuum thin- f ilm processing methods. MBE is easy implementation of in situ diagnostic instruments and provides a cleangrowth environment for deposition.o Disadvantages of MBE are the sophisticated and expensive equipment needed, and the slow speed of the process.Another disadvantage is that it is difficult to control the multi-element rate.Molecular beam epitaxy (MBE) means creating a single crystal by building up orderly layers of atoms on top of asubstrate (base layer). Image Credit: ∙A tomic layer epitaxy, also called atomic layer deposition, is a process for depositing one-atom-thick layers on a surface. Films are deposited by a repetitive sequence of single layer deposition cycles composed of several gas-surface interactions that are allself-limiting. The crystal lattice structure achieved is thin, unif orm and aligned with the structure of the substrate.Atomic Layer Depositiono A dvantages of atomic layer epitaxy include the ability to accurately control the thickness of the layer. The process is simple and multilayer structures are easy to grow. Additional advantages include the wide range of film materials available, high density, and low impurity lev els; all of which are done at a lower temperature so sensitive substrates are not affected.o Disadvantages include the potential f or residue to be left behind from the precursors. The process is time consuming and usually only a fraction of a monolyer is deposited in one cycle. Also, several technologically important materials are too expensive to be deposited by the technique∙Dip pen lithography is a process in which the tip of an atomic force microscope is "dipped" into a chemical fluid and then used to "write" on a surface, like an old f ashioned ink pen onto paper. This is an effective technique for transporting molecules from the tipof an atomic force microscope (AFM) to substrates at resolutions comparable to those achieved with much more expensive andsophisticated lithographic methods.o A dvantages of dip pen lithography are that it is easy and inexpensiv e. Patterns for the experiments are easy to create and can be specific to an application since it can be applied to a large selection of substrates.o Disadvantages include the limiting f actors of the experiments such as solubility of the desired ink, the transfer and stability of the material within the water meniscus, and the adsorption of the material on the substrate surface. Inkscan be molecules, sol-gel materials, and biological species like proteinsSchematic diagram show ing how dip pen nanolithography w orks. Image Credit: ∙Nanoimprint lithography is a process for creating nanoscale f eatures by "stamping" or "printing" them onto a surface. A hard mold (mask) with a surf ace relief pattern is used to emboss a layer of resist. After heat and pressure are applied the mold is rev oked and the residual layer of resist is (dry) etched away to leav e behind a fully patterned resist.o A dvantages of this process include the ability to clean and reuse the mask which allows for imprint uniformity and def ect free f abrication. It is a simple process with simple machines that produce three-dimensional patterns on avariety of substrate materialso Disadvantages come f rom user error in overlaying the mask correctly, as well as from defects and debris on the template. Also, the higher the resolution the slower the throughput.Process of nanoimprint lithography. Image Credit: ∙Roll-to-roll processing (R2R) is a high-volume process to produce nanoscale electronic devices on a roll of ultrathin plastic or metal.The process is similar to nano-imprint lithography but rollers allow for a larger substrate to be patterned faster.o A dvantages include a lower substrate cost, steady state processing with high-throughput and high yield, lower cleanroom requirements and less expensiv e equipment. It has a high throughput and has been able to demonstrate100nm resolution as well as self-alignment.o Disadvantages of R2R include the limited equipment available because there is no previous generation equipment to model, and challenges with patterning and defect repair.Roll-to-roll process. Image Credit: Nitto Denko∙Self-assembly describes the process in which a group of components come together to form an ordered structure without outside direction. Self -assembly is the ultimate goal for many nanomaterial making processes because of the reduced chance for errorand reduction of waste.The properties exhibited by a nanomaterial are related to the composition, and process that the material was created by. Controlling the process parameters such as temperature and pressure as well as composition makes the material stronger, lighter, more durable, water-repellent,anti-reflectiv e, self-cleaning, ultraviolet-or-inf rared-resistant, antimicrobial, electrically conductive, etc.Product SelectionThe GlobalSpec SpecSearch database allow s industrial buyers to select nanomaterials by type of nanotechnology, material type, designspecif ications and application.Nanotechnology TypeTwo general constructions include,∙Single Walled (SWNT) materials which are constructed of a single plane of atoms. SWNT s hav e superior properties compared to doubled walled for certain applications∙Double Walled (DWNT) materials are constructed of two or more planes of atoms.Different types of nanomaterials and nanotechnology products are named for their individual shapes and dimensions.buckyballs Image Credit: NanotubesImage Credit: hochgeladen von Schwarzm amdots Image Credit:Nanotechnology NowNanogels ImageCredit:Nanodevices Image Credit: Nanofibers ImageNanowires ImageCredit: National Cancer InstituteNanopowders ImageNanoparticles ImageCredit: NanocatileversImage Credit:National Cancer InstituteNanoshells ImageCredit: National Cancer InstituteNanofilms ImageCredit: Material PropertiesGenerally, materials types include crystalline or amorphous properties. Nanomaterials represent materials from a structural perspective somewhere in between the two ty pes.∙Crystalline materials are composed of an orderly repeating array of atoms, molecules or ions. They come in two general forms;single crystalline, which have single atoms arranged periodically on a three dimensional lattice; and polycrystalline, which is aconsolidated assembly of small single crystals. This material has short and long range order meaning that the manner in whichatoms are arranged at any one location within a crystal is identical to the arrangement at any other location. Most metals arepolycrystalline and when they break the grains or crystallites can be observ ed randomly orientated within the material. The s urfacebetween grains is called the grain boundary.∙A morphous materials are non-crystalline. They have short range order but not long range order. This material is not composed of grains. A broken edge is observed at a smooth surface because they are considered a cooled liquid rather than a true solid.Polymers and ordinary glass are amorphous materials.Shape and DimensionNanomaterials are designed in a variety of shapes including particles, tubes, wires, films, flakes, or shells that have one or more nanometer-sized dimension. For example, carbon nanotubes have a diameter in the nanoscale, but can be several hundred nanometers long or even longer. Nanofilms or nanoplates have a thickness in the nanoscale, but their other two dimensions can be much larger.Particle and feature size is the diameter or width of nanomaterials and nanotechnology products. On consolidated nanomaterials or nanodevices, this is the diameter or width of the nanoscale feature or crystal. Size is an independent degree of freedom and can therefore be manipulated independent of composition, temperature and pressure to create materials that possess new properties. Scientists are try ing t o produce nanomaterials with tightly controlled size and size distribution so that the properties associated with size are observed and distinguishable. In order for the unique size-dependent properties to be utilized, the nanomaterials must be composed of monodisperse or nearly monodisperse nanoparticles. Often, specific surface f eatures are responsible for the unique properties of the material so it is critical t hat processing is controlled to yield size and the particular surface features responsible f or the materials unique charact eristics.The size of the particle or grain also influences properties of the grain boundary. As the grain size decreases the proportion of molecules or ions at the grain boundary increases. The rato of molecules or ions at the surface to the total in the grain is proportional to 1/r where r is the radius of the particle or grain. The size dependent properties that emerge in the nanometer length domain are in part a result of this increased ratio.Design SpecificationsImportant specif ications when selecting nanomaterials and nanotechnology products include,Specific Surface A rea (SSA)- SSA is measured in terms of mass per unit area and is easier to determine than crystal size. SSA can prov ide an indication of av erage particle size, but not particle size range or distribution. Nanoscale materials have far largersurface area then similar v olumes of larger-scale materials, meaning more surface is available f or interaction with other materialsaround them. T ypically, gas absorption techniques such as the Brunauer-Emmett-Teller absorption (BET) technique are used todetermine SSA.ApplicationNanomaterials and nanotechnology has a wide range of applications from baseball bats and tennis rackets to catalysts for ref ining cure oil and ultrasensitiv e detection and identif ication of biological and chemical toxins. The benefits of this technology come from the ability to tailor the structure of the material at the Nanoscale to achiev e specific properties. According to the National Nanotechnology Initiativ e (NNI), there already exist over 800 everyday commercial products that rely on Nanoscale materials processes.RisksNanomaterials have been linked to several risks associated with material hazard and exposure to the material. Care should be taken when handling nanomaterials. Safety precautions such as engineering controls, administrative controls, and personal protective equipment is recommended to avoid exposure to nanoscale materials.Industry StandardsAccording to the NNI, nanotechnology relies on standards through at least three concepts:1. Documentary standards define agreed-upon terminology or standard language f or a field of science, engineering, or technology; they are agreed-upon means for conducting measurements; agreed-upon performance characteristics of instruments or commercial products; and particularly, they are documented agreements on means to facilitate trade and commerce.2. Standards often refer to standard reference materials, materials that are certified by a national standards laboratory to have specified characteristics traceable to an international system of the fundamental system of physical units of measurement.3. Standards generally refer to the fundamental physical realization of the units of measurement defined in the International System of Units (SI). Around the world, there are numerous nanotechnology standards-setting groups that develop voluntary standards. Some of the leading standards setting organizations and their relevant nanotechnology committees are:∙International Standardization Organization(ISO) Technical Committee (T C) 229 on Nanotechnologies∙ASTM (f ormerly known as the American Society for Testing and Materials) International's Committee E56(Nanotechnology)∙International Electrotechnical Commission Technical Committee 113 (Nanotechnology Standardization for Electrical and Electronics Products and Systems)∙Institute of Electrical and Electronics Engineers' Nanotechnology CouncilResource sIntroduction of Nanoimprint LithographyMolecular Beam EpitaxyMov ing Roll to Roll Processing from the Lab to Manuf acturingNanofactsNanofiber NonwovensNanoshell particles: sy nthesis, properties and applicationsNanotechnology 101Park, Jong-Hee, and T. S. Sudarshan. Chem ical Vapor Deposition. Materials Park, OH: ASM International, 2001. Print.Understanding Nanotechnology。
