Nanotechnology

Nanotechnology is the use of matter on an atomic, molecular, or supramolecular level for industrial purposes. Nanotechnology's earliest application was in reference to the technical aspiration to manipulate atoms and molecules for the fabrication of macroscale products, better known as molecular nanotechnology. The National Nanotechnology Initiative set a more generalized definition of nanotechnology with parameters that state nanotechnology as the manipulation of matter with at least one dimension sized from 1 to 100 nanometers. Since the industry is primarily delineated and defined by size, this includes broad and various scientific and industrial uses, including surface science, organic chemistry, molecular biology, energy storage, semiconductor physics, molecular engineering, and microfabrication.

Nanotechnology and related products are generally manufactured by nano-structured materials and the tailoring of their properties. These materials include nano-powders, nanotubes, nanowires, and nanomembranes.

Nanomaterials are partially characterized by a tiny size, measured in nanometers, which is one millionth of a millimeter. Nano-sized particles can be created from a variety of products, such as carbon or minerals like silver, but nanomaterials by definition must have at least one dimension less than 100 nanometers. Most of these materials are too small to be seen with the naked eye. Materials engineered to be nanomaterials often take on unique or engineered optical, magnetic, electrical, and other properties. These properties can also offer new uses for electronics, medicine, and materials (such as cement or cloth where the use of nanomaterials can strengthen and reduce the weight of the final product). Common products such as sunscreens, cosmetics, sporting goods, stain-resistant clothing, tires, and electronics are manufactured using engineered nanomaterials. Further materials are being developed for diagnostics, imaging, drug delivery, and environmental remediation projects. The development of nanomaterials and related technologies is research heavy, and the larger nanomaterial and nanotechnology companies are often the larger producers of nanomaterial and nanotechnology research.

The top-down approach to nanofabrication works to miniaturize current technologies. These approaches are used for producing structures with long-range order and macroscopic connections. There is an expectation that top-down and bottom-up approaches will be integrated for new nanofabrication possibilities. The most common top-down approach to fabrication uses lithographic patterning techniques from short-wavelength optical sources. Often used in the fabrication of integrated circuits, the approach allows for patterning in parts and build in place with no assembly required.

Short-wavelength optical lithography techniques are being developed (with dimensions below 100 nanometers). These shorter-wavelength sources, such as extreme ultraviolet and x-ray, are being developed for smaller lithographic printing techniques that would offer the capability to achieve increasingly smaller features by depositing or removing thin layers.

Mechanical printing techniques, such as nanoscale imprinting, stamping, and molding, have been extended to nano dimensions (of about 20 to 40 nanometers). These techniques have been based on electron-beam lithography and the application of a stamp to create a pattern. In another approach, the stamp is used to mechanically press a pattern into a thin layer of material. And a variation is to make the relief pattern out of a photoresist on a silicon wafer by optical or electron-beam lithography and then pour a liquid precursor over the pattern and cure it. The resulting rubbery solid can in turn be used as a stamp. A distinction for the latter approach is the stamp is flexible and can be used to print nanoscale features on curved surfaces.

A bottom-up, or self-assembly, approach to nanofabrication uses chemical or physical forces operating at the nanoscale to assemble basic units into larger structures. Bottom-up approaches provide an important complement to top-down techniques. Inspiration for bottom-up approaches is derived from from biological systems, which researchers hope to replicate. Bottom-up approaches developed for producing nanoparticles include condensation of atomic vapors on surfaces to coalesce atoms in liquids. And liquid-phase techniques based on inverse micelles have been developed to produce size-selected nanoparticles of semiconductor, magnetic, and other materials.

An example of self-assembly that achieves limited control over formation and organization is the growth of quantum dots. Indium gallium arsenide (InGaAs) can be formed by growing thin layers of InGaAs on GaAs in a manner causing repulsive forces that results in the formation of isolated quantum dots. These dots can achieve a fairly uniform spacing.

Another approach to bottom-up nanofabrication, DNA-assisted approaches offer a method to assemble nanomaterials and possibly integrate hybrid heterogeneous parts into a single device. In this method, DNA-like recognition would direct attachments between objects in fluids and polymers with complementary DNA strands as adhesives, followed by more-permanent attachment approaches such as electrodeposition and metallization. This method has been explored as a possible way to self-assemble molecules for the crystal faces of compound semiconductors.

A proposed technique for bottom-up fabrication, especially in electrical and mechanical nanodevices, cold welding, or contact welding is a solid-state welding process without fusion or heating at the joint of two parts. The method was first proposed in the 1940s with two clean, flat surfaces of similar metal that would strongly adhere if brought into contact in a vacuum. This has shown potential in nanofabrication process, where cold welding has been demonstrated to weld together single-crystalline gold nanowires with diameters between 3 and 10 nm. The process has also been achieved within seconds by mechanical contact alone and under relatively low applied pressures. These welds have been shown to be nearly perfect, with the same crystal orientation, strength, and electrical conductivity as the rest of the nanowire. Welds have been achieved between gold and gold, gold and silver, and silver and silver, suggesting cold welding may be a generally applicable technique.

Functional approaches to nanomanufacturing work, in general, to control material properties by controlling the size of matter, rather than the chemical composition. Specifically, below a certain size, within the range of 1 to 100 nm, new physical properties emerge due to the spatial confinement of electronic states. This approach complements chemical strategies in the sense that properties that cannot be produced by conventional chemical reactions are accessible. This opens applications for nanotechnology in biology, medicine, environment, energy, communications, and computer technology. There are a variety of physical fabrication techniques, including wet-chemical and sol-gel processes to produce nanomaterials and related systems.

Another approach to functional nanomanufacturing is magnetic assembly for the synthesis of anisotropic superparamagnetic materials that have been designed for special-purpose nanoelectronic circuits. Magnetic assembly has also been used to develop nanochains and nanobundles for the development of electronic products with new properties. The use of magnetic assembly has led to superparamagnetic nanostructures with controlled aspect ratio, uniform size, and a well-defined shape, and express good colloidal stability. This has led to the ability to magnetically manipulate liquid and photonic crystals and offers possible applications in the treatment of cancer.

Biomimetic nanomaterials are artificially developed nanomaterials imitating properties of biomaterials or designed using biological principles. The biomimetic approach to nanomaterials and structures comes from the assumption that nature has created optimal living structures. An example is the "lotus effect" where lotus leaves resist wetting and dirt due to their micro- and nanostructured surfaces, and the study of these leaves has led to the development of waterproof paints and textiles. Another example is cobwebs, which have led to the development of polymer nanofibers with strength comparable to steel, able to withstand three times greater strain than steel wire with equal diameter.

Biomolecules are capable of self-assembly in regular structures and biomimetic approaches to nanomaterials work to use a similar self-assembling mechanism using biostructures such as matrices. This has been used to develop nanoconductors and nanotubes through the deposition of metal monolayers onto biopolymers. And techniques to imitate the complementarity of double-stranded DNA are being used to develop DNA-based nanomaterials. Ferritin, a protein that transports and stores iron in an organism, and forms nanocavities with an inner diameter of 8 nm, have been used to encapsulate iron oxide and cobalt oxide with a mean size of 6 nm.

Another approach to biomimetic nanofabrication employs growing nanoparticles of specific sizes inside bacteria or the biomass of plants such as wheat, oats, or alfalfa. Metal salts are added to the biological objects, and after biocatalytic reduction to metals these salts form nanoparticles. The salts have been added to the irrigation water to have them brought into the plants, and through this the nanoparticles are formed in the stems and other parts of the plants and can in turn be extracted.

As well, microorganisms are known to produce nanostructured mineral crystals and metallic nanoparticles with properties similar to chemically synthesized materials. This includes the formation of magnetic nanoparticles by magnetotactic bacteria, the production of silver nanoparticles within periplasmic space of pseudomonas stutzeri, and the formation of palladium nanoparticles using sulphate reducing bacteria in the presence of an exogenous electron donor. Actinomycetes, microorganisms that share characteristics with fungi and prokaryotes such as bacteria, are classified as prokaryotes but were originally classified as fungi. These actinomycetes were observed to synthesize gold nanoparticles extracellularly when exposed to gold ions under alkaline conditions. Further study has shown the use of elevated temperature conditions to be favorable for the formation of monodisperse particles, and in turn, there has been a use of actinomycetes for the intracellular synthesis of monodisperse gold nanoparticles.

Fungi have also been used for the biosynthesis of nanoparticles. In addition to monodispersity, nanoparticles with well-defined dimensions can be obtained using fungi. Compared to bacteria, fungi could be used as a source for the production of large amounts of nanoparticles, which is in part due to the higher secretion of proteins in fungi that creates higher nanoparticle formations. Isolated proteins from fungi cultures have also been used in nanoparticles production. Nanocrystalline zirconia was produced at room temperature by cationic proteins similar to silicatein secreted by certain fungals.

Yeast has been used for the synthesis of nanoparticles. In particular, gold nanoparticles have been synthesized intracellularly using the yeast fungi. The nanoparticles have been able to be manipulated through controlling parameters such as pH, temperature, gold concentration, and exposure time. A mycelium fungus, cladosporium cladosporioides, has been used to synthesize nanoparticles extracellularly; polysaccharides and organic acids released by the fungus are able to differentiate different crystal shapes and direct the growth of nanoparticles into extended spherical crystals. And fusarium oxysporum has been used to synthesize silver nanoparticles extracellularly. Studies have found a nitrate reductase to be responsible for the reduction of silver ions and the corresponding formation of silver nanoparticles. Other studies have found the reductases in fusarium oxysporum are probably necessary for the production of and reduction of silver nanoparticles.

Nanoelectronics is a subdivision of nanotechnology involving its use in electronic components. The interatomic interactions and quantum mechanical properties of materials used in nanoelectronics require more research to be understood better, but the extraordinarily small scale makes this challenging. As the dimensions of critical device elements approach atomic size, quantum tunnelling, and other quantum effects degrade, causing the functions of conventional semiconductor devices to become inoperative and creating the need for new conceptual solutions. Additionally, the smaller the electronic components become, the more difficult they are to manufacture.

Despite these challenges, a variety of nanoscale electronic devices have been created: devices with negative differential resistance, electrically-configurable switches, tunneling junctions, the carbon nanotube (CNT) transistor, and the unimolecular transistor among them. Certain devices have been connected to form circuits capable of performing logic and basic memory functions. Materials used in the manufacture of nanoelectronics include zero-dimensional materials (e.g. quantum dots); one-dimensional materials (e.g. nanowires); nanoclusters and nanocomposites; carbon-based materials (e.g. CNTs) fullerenes and graphene; and more. Other nanoelectronic devices include:

• Flexible energy storage devices
• Electronic textiles
• Multifunctional and responsive elastomers
• Artificial skin
• Flexible gas sensors
• Solar cells

Plastic C nanoelectronics is a distinct research area where materials science, chemistry, physics, nanotechnology, and engineering communities collaborate. Nanotechnology-based devices can have a wide variety of applications in physics, chemistry, biology, materials science, and medicine.

Nanomedicine is defined as the application of nanomaterials in the prevention, diagnosis, monitoring, control and treatment of diseases. It is recognized as a Key Enabling Technology in the European Union (EU), with the potential to provide innovative solutions aimed at addressing unmet medical needs. There are numerous concerns over the application of nanomaterials in medicine, for instance: the physicochemical properties of the nanoformulation can cause pharmacokinetic alterations, related to the absorption, distribution, metabolism, and excretion (ADME) of nanopharmaceuticals, the potential for uncontrollable crossing of biological barriers, or the persistence of toxic properties in the body and the environment.

Nanomaterials can be applied in nanomedicine in diagnosis (nanodiagnosis), controlled drug delivery (nanotherapy) and regenerative medicine. The optical, electrical, and magnetic properties of nanomaterials are tunable through quantum confinement. They can be engineered to varying sizes, shapes, chemical compositions, and surfaces, which makes them suitable for interacting with specific biological targets.

Researchers are developing multifunctional nanodevices capable of assisting in in vivo imaging, the identification of precancerous or cancerous cells, the release of drugs that target solely those cells, as well as in reporting on the effectiveness of treatment. Ideally, nanotechnology would enable the running of many diagnostic tests simultaneously, the tools for various tests to be incorporated into one small device, and make it possible for clinicians to perform tests without altering cells so that the cells may be reused.

Nanopumps rely on microfluidic MEMS (Micro-Electro-Mechanical System) technology to provide superior control of administered insulin doses and drug delivery control at the nanoliter level. These tiny pumps can be mounted on a disposable skin patch to supply insulin to the patient. One such device had been developed by Debiotech, a Swiss manufacturer of medical devices. Due to its low production cost the pump is suitable for applications in long-term treatment and as a disposable drug delivery system.

Tree-shaped synthetic molecules called dendrimers may also be used in drug delivery. Dendrimers can locate and eliminate specific malignant cells and deliver genes to cells without causing a negative immune response. A dendrimer can carry a molecule that recognizes cancer cells, a therapeutic agent to kill those cells, and a molecule that recognizes the signals of cell death. Researchers hope to tweak dendrimers so that their contents are released only in the presence of particular molecules associated with cancer.

It may take up to two decades for a new drug to enter the market after being discovered, during which numerous considerations should be addressed, such as whether there is sufficient skilled scientific and medical personnel that can dedicate ten to twenty years to a single project; whether the fundamental scientific premise is novel and secured with intellectual property protection, and whether the economic business plan is convincing enough to investors. Nanocrystals, liposomes and lipid-based, polymeric, protein-based, and metallic NPs are some of the most popular nanopharmaceuticals.

Nanocrystals are defined as crystals with at least one dimension of less than 100nm. Emend, Ostim, Rapamune, Vitoss, Ritalin, Abraxane, Avinza and TriCor are some of nanocrystalline pharmaceuticals that are currently available in the market. The table below shows the benefits and applications of six nanocrystalline pharmaceuticals.

Liposomal nanoformulated drugs are commercial DDSs (drug delivery systems) designed to neutralize the side effects of many conventional drugs. For instance, the liposomal formulation of doxorubicin (known by the commercial name of Doxil®) has made a substantial contribution to cancer treatment.

Polymeric NPs have many applications in drug and gene delivery, including the encapsulation of therapeutic agents. Pegylated drug and protein conjugates play an important part in the area of commercial nanopharmaceuticals. PEGylation is a process involving the conjugation of polyethylene glycol (PEG) chains to therapeutic proteins, peptides, or molecules. PEGylation results in the increase of the therapeutic protein’s mass, and as a result guards it from proteolytic enzymes and degradation. Additionally, the PEGylation process improves the pharmacokinetics of therapeutic proteins.

Nanomaterials can be used to create power sources for portable, flexible, and foldable electronics; electric transportation; large-scale energy storage, as well as in living environments and biomedical systems. Nanoparticles with different functionalities should be integrated in small architectures on micro- and nanoscales in order to overcome limitations imposed by high reactivity and chemical instability caused by the high surface area of nanomaterials.

Nanomaterials have the advantages of superior ionic transport and electronic conductivity over conventional battery and supercapacitor materials. In addition, they enable all intercalation sites available in the particle volume to be occupied, resulting in high specific capacities and rapid ion diffusion. Due to this, nanomaterial-based electrodes are able to withstand high currents and are viable for applications in high-power energy storage. However, the challenges related to the stability and manufacturing of nanomaterials limit their usefulness. As a result, nanomaterials are not widely employed in the manufacture of commercial devices, with the exception of carbon-nanotube additives and carbon coatings on silicon particles in lithium-ion battery electrodes.

There are in existence nanomaterials with varying chemical compositions and shapes, ranging from oxides, chalcogenides, and carbides to carbon and lithium alloys. Various particle morphologies, such as zero-dimensional (0D) nanoparticles and quantum dots; 1D nanowires, nanotubes, and nanobelts; 2D nanoflakes and nanosheets; and 3D porous nanonetworks. Nanoparticles can be combined with lithium to create wearable, structural energy storage technology and other energy storage solutions. The potential applications of lithium nanoparticles include the acceleration of charging and discharging of lithium ion batteries; research regarding their electrical, magnetic, optical, catalytic, dielectric, and biomedical properties; in polymers, nanofibers, nanowires, coatings, and textiles, and as alloys and catalysts.

Microfabrication is a process of constructing physical objects with dimensions in the micrometer to millimeter range. These objects can be comprised of miniature moving parts such as cantilevers and diaphragms, static structures such as flow channels and wells, chemically sensitive surfaces such as proteins and cells, and electrical devices such as resistors and transistors.

Microfabrication can be broken down into two distinct types: bulk micromachining and surface micromachining. In the former, the bulk of a substrate is used to build a device, and in the latter the device is built on the surface of the substrate. The applications of microfabricated device structures can be found in tools for molecular biology and biochemistry, cell biology, medical devices, and biosensors, and their functions may be significantly enhanced over conventional solutions.

Thin film is a vital part of the microfabrication process, which usually utilizes multiple thin films per product. In the case of electronic devices, the films employed may contain conductive metals. Optical devices may be microfabricated using reflective or transparent films to ensure clarity and visibility, and medical devices may use microbial growth-inhibiting chemical films.

Microfabricated structures are used in the manufacture of integrated circuits, semiconductors, microfluidic devices, flat panel displays, solar panel cells, sensors, fuel cells, and more. Unlike other methods, microfabrication can be used to create non-spherical, homogeneous monodisperse particles with asymmetrical shapes and custom size architectures. Microfabrication is a complex process with generally a high mask count. According to some reports, the fabrication of a typical memory chip requires 30 lithography, 10 oxidation, 20 etching, and 10 doping steps.

Photolithography, soft lithography, film deposition, etching, and bonding are the most significant microfabrication techniques.

Photolithography (or Optical Lithography) involves the transfer of geometric shapes on a mask to the surface of a silicon wafer through the processes of wafer cleaning, barrier layer formation, photoresist application, soft baking, mask alignment, exposure and development, and hard-baking. Throughout this process a photosensitive polymer is exposed to ultraviolet (UV) light through a mask, leaving a latent image in the polymer that can be controllably dissolved, in effect generating a desired pattern on the surface of an underlying substrate.

Few of the many underlying patterning techniques of soft lithography are microcontact printing (μCP), replica molding (REM), microtransfer molding, micromolding in capillary, solvent-assisted micromolding (SAMIM), phase-shifting edge lithography, nanotransfer printing, and nanoskiving. These patterning techniques involve printing, molding, and embossing with an elastomeric stamp.

In contrast to photolithography, soft lithography is suitable for the processing of a variety of elastomeric materials (i.e. materials that are mechanically soft). Soft lithography is compatible with polymers, gels, and organic monolayers. The most widely used material in soft lithography is PDMS (polydimethylsiloxane) on account of its low cost, mechanical flexibility and durability, low toxicity, biocompatibility, and more.

Film deposition is a microfabrication technique that consists of depositing micron-thick films on the surface of a substrate. These films can function as masking layers (protecting the base material from etching) or electrical components. The most commonly used materials in the production of films are plastics, silicon-containing compounds, metals, and biomolecules.

Etching entails the selective removal of materials from the surface of the microdevice by chemical or physical processes. If proceeding equally in all directions, etching is isotropic; if proceeding in a single specified direction, it is anisotropic. Etching can be subdivided into two other methods: wet etching and dry etching. Wet etching utilizes liquid chemicals, whereas dry etching is based on gaseous physico-chemical processes.

Bonding is a method of affixing substrates together with or without the use of intermediary layers. Tight seals or desired structures may be formed through reversible and irreversible bonding, of which there are numerous methods particular to the material worked with, including laser welding, ultrasonic welding, and adhesives.

Silicon is the most common material in microfabrication and is used in the manufacture of integrated circuits. It is known for its superior electrical and mechanical properties properties.

The use of glass in micromachining processes is not as common, but among some of its unique features is the advantage of optical transparency over silicon, making it suitable for applications where this is of benefit. Silica wafers and borosilicate wafers are two notable examples of glass-based micromachined products.

Since plastic is often the least expensive substrate material, microscale plastic devices can be viably produced in large volumes, making them ideal for use in disposable clinical applications.

Molecular engineering takes advantage of experimental, theoretical, and computational methods to study molecular properties and behaviors with the aim of discovering new materials, systems, and processes for specific purposes. It is an interdisciplinary field comprising aspects of chemical engineering, materials science, bioengineering, electrical engineering, physics, mechanical engineering, and chemistry.

Molecular engineering overlaps with nanotechnology in that both focus on materials on the nanoscale or smaller. Taking into account the highly fundamental nature of molecular interactions, the potential application areas are virtually limitless. Some of the early innovations of molecular engineering have appeared in the fields of immunotherapy, synthetic biology, and printable electronics.

Nanometrology is a branch of metrology concerned with the science of measurement at the nanoscale level. Atomic force microscopy, electron microscopy, and X-ray diffraction are some of the most commonly used techniques of nanometrology. Besides determining the dimensions of objects (shape, aspect ratio, and size distribution), this sub-discipline of metrology focuses on the determination of other physical properties, such as mechanical, magnetic, electrical, optical, chemical, and biological.