Nanoscience and nanotechnology
Nanoscience and nanotechnology are interdisciplinary fields, involving physics, chemistry, materials science, and mechanical, electrical and chemical engineering. At the nanoscale, the mechanical, electric, optical and magnetic properties of materials change, allowing the creation of new functional materials. Nanotechnology has a broad range of applications, from biomedical science to electronics.
Nanoscience, the science of objects or features with typical sizes of 1–100 nm, is one of the most important developments in decades. Miniaturization of electronic devices with elementary units smaller than 1 µm has revolutionized our daily lives. When matter is divided into such small objects, its mechanical, electric, optical and magnetic properties often change drastically. Accordingly, the relationship between nanoscience and nanotechnology has been largely symbiotic, with scientific discoveries leading to new technologies that in turn usher in new fundamental insights.
AN ALTERNATE WORLD
Many properties of solids change as their dimensions approach the nanoscale. A particle measuring 1 nm × 1 nm × 1 nm contains 64 atoms with only 8 on the inside: the remaining 56 atoms are at the surface. Therefore the properties of nanoparticles are dominated by surface atoms, which allows the creation of new properties simply by finely dispersing ordinary bulk materials. Insulators can become conductors, stable materials might become combustible, and relatively inert materials like gold might become efficient and selective catalysts when reduced to the size of a few nanometres.
Organic functional nanoparticles can be created using various different methods and have widespread applications including coatings, colorization, labelling of biological samples, drug delivery1 and gene transfection2. Inorganic nanoparticles are mechanically more stable and resilient to high temperatures, and so are preferred for some applications (Fig. 1).
Using inorganic nanoparticles to improve the mechanical, electrical and optical properties of organic polymers or inorganic materials leads to the creation of nanocomposites. For example, ductile ultra-high-strength steel (with strength significantly >1 GPa) is crucial for lightweight engineering, which translates to a lower carbon footprint by virtue of reduced material production and fuel consumption. Newly developed ductile steels have strengths of up to 1.5 GPa owing to the inclusion of nanoprecipitates of nickel, titanium and molybdenum (1–2% by weight), which are responsible for additional hardening during heat treatment.
Nanomaterials, which have nanoscale morphological features such as ‘islands’, rods (Fig. 2), or pores, have a large surface area and, as such, are desirable as a substrate for catalysts or for use in filters. Because the formation of defined nanostructures in organic molecules is well understood and can be easily adjusted, organic substances are often used as templates for inorganic nanoporous materials.
Nanoscale structures can be formed in two different ways: lithographic etching of single features (top down) and the self-assembly of small objects to larger architectures (bottom up; Fig. 3). With lithographic etching, individual structures can be created; however, it is technically demanding, expensive and typically limited to structures significantly bigger than 10 nm. Self-assembly is usually simple and many structures can be created at low cost; however, it does not make individual features.
One of the main applications of nano-technology — and therefore a driving force for nanoscience — is the electronics industry. Over the past few decades, the transistor has been continually miniaturized. Modern integrated circuits incorporate transistors with feature as small as 32 nm. Concurrently, data density on devices such as hard drives has increased to around 200 gigabytes per square inch, meaning that it requires an area approximating a square with sides barely 56 nm to store one bit of data. This makes circuits possible that can work faster with lower energy consumption3,4.
>> By understanding and manipulating matter at the nanoscale, new materials and tools can be created for a myriad of applications.
A particular feature of the nanoworld is quantum effects, for example electron tunnelling. A cyclist approaching a hill must convert chemical energy in his or her muscles into potential energy, which might be partly gained back as kinetic energy once he or she goes over the top of the hill. In quantum mechanics, a particle (for example, an electron) can act differently: for a finite hill size, there is always the possibility of arriving at the other side with the same energy intact, similar to if the particle travelled through the hill rather than over it. This property is at the heart of the scanning tunnelling microscope.
Chemistry will also benefit from nanoscience owing to a better understanding of the behaviour and properties of ever smaller amounts of liquids. This is essential when only tiny amounts of reactants are available, and could help control potentially toxic or explosive reactions. Attempts are underway to create a complete laboratory on a silicon wafer (a ‘lab on a chip’) to make it quicker and simpler to test for certain chemicals or biological samples, such as enzymatic analysis and DNA sequencing. However, these are still on the micrometre scale, at which the flow of liquid in a channel can be described using normal macroscopic hydro-dynamic equations; true nanofluidics is a technical and scientific challenge, as the behaviour of the liquid is qualitatively different. At this scale, the flow is affected by thermal fluctuations, long-range intermolecular interactions, the influence of the channel walls and the finite size of the molecules within the system5,6.
The field of supramolecular chemistry is concerned with the organization of molecular building blocks governed by weaker, non-covalent forces such as hydrogen bonding, hydrophobic and hydrophilic forces, van der Waals forces and electrostatic effects, and therefore functions at the nanometre scale. One exciting application of this branch of chemistry is organic or polymer electronics: the fabrication of electronic and opto-electronic devices using carbon-based rather than metallic materials, which are lighter, more flexible and less expensive.
There are big opportunities for tiny technologies. By understanding and manipulating matter at the nanoscale, new materials and tools can be created for a myriad of applications such as electronics, biomedical science and construction. In order to achieve this, several near-term challenges need to be addressed. These include improving methods of producing nanoscale objects, processing them and organizing them into complex arrangements. New methods of creating an interface between nano-objects and the macroscopic world, perhaps via electrical connections, can enhance our ability to control and benefit from them. Finally, there needs to be development of non-destructive methods to characterize the structure of nano-objects and their properties (including electrical, magnetic, thermal, mechanical and optical) in different environments.
Unlike bulk gold (Au), nanoclusters formed by Au atoms have catalytic properties; these are determined by their electronic and geometric structures. Using vibrational spectroscopy in the gas phase, the planar structure of Au7 and a pyramidal structure for Au20 were revealed by researchers from the Fritz Haber Institute in Germany. Even the reduction of the symmetry when a corner atom is cut from the tetrahedral Au20 cluster can be readily detected in the vibrational spectrum of Au19 (Gruene, P. et al. Science 321, 674, 2008).