Foundations

What nanotechnology is

Nanotechnology is defined by size rather than by subject. That makes it unusual among the sciences: a chemist, a physicist, a biologist and an electrical engineer can all be doing it, provided the structures they study or build are small enough.

The unit

The nanometre is one billionth of a metre. The Wikipedia article on the nanometre notes that the name joins the metric prefix nano-, from a Greek word for dwarf, to the metre, and that the unit was once called the millimicron because it is a thousandth of a micrometre. The same article offers yardsticks that make the scale easier to picture: a helium atom is about 0.06 nm across, a ribosome about 20 nm, and visible light has wavelengths from around 400 to 700 nm. A nanometre is therefore a few atoms wide, and most of the structures discussed on this site are too small to be seen with any ordinary light microscope.

A definition by scale

The definition most often quoted sets the working range at 1 to 100 nanometres. The Wikipedia overview of nanotechnology describes the field as the manipulation of matter with at least one dimension in that range, where surface area and quantum mechanical effects become important in describing a material's properties. The phrase "at least one dimension" matters. A sheet one atom thick but centimetres wide counts; so does a wire a few nanometres across but many micrometres long; so does a particle small in all three directions.

The same overview draws a distinction worth keeping. Nanoscience, it says, emphasises the pure science of the scale, while nanotechnology is the applied output, with its own methods such as nanolithography. It also notes that an earlier and narrower meaning of nanotechnology, the precise building of products atom by atom, is now usually called molecular nanotechnology. That older meaning is discussed on where the idea came from.

Public agencies use similar language. The United States National Nanotechnology Initiative frames its goal as understanding and controlling matter at the nanoscale, and describes itself as a coordinating effort across many federal departments and agencies rather than a single laboratory. The breadth of that coordination is itself a sign of how many disciplines the definition sweeps in.

Why size changes behaviour

If nanoscale objects behaved exactly like larger pieces of the same material, there would be no separate field. Two effects make the difference.

The first is surface. As an object shrinks, its surface area falls more slowly than its volume, so a growing share of its atoms sit at or near the surface. The Wikipedia article on the surface-area-to-volume ratio explains why that ratio governs processes that happen through a surface, such as diffusion and heat transfer. For a nanoparticle a few nanometres across, a large fraction of the atoms are surface atoms, with fewer neighbours and different bonding from those inside. Chemical reactivity, melting behaviour and the way the particle interacts with its surroundings all shift as a result. A simple thought experiment shows the trend: cut a cube into eight smaller cubes and the total volume is unchanged, but the exposed surface doubles; keep cutting, and surface quickly dominates.

The second is quantum confinement. Electrons in a solid behave as waves, and when the solid is only a few nanometres across, those waves are squeezed. Their allowed energies become more widely spaced, and optical and electrical properties begin to depend on size. The most visible example is the semiconductor quantum dot, whose colour changes as the crystal grows; see the quantum dots page. Metal particles show a related size dependence in the way they absorb light, described on the metal nanoparticles page.

Nanomaterials, natural and made

Many nanoscale materials are not new at all. Soot, clay minerals, volcanic ash and the pigments in certain stained glass all contain particles in the nanometre range. What changed in the late twentieth century was the ability to make such structures on purpose, to measure them one at a time, and to explain their properties. The Wikipedia article on nanomaterials notes that the International Organization for Standardization defines a nanomaterial as one with any external dimension, or internal or surface structure, in the range of approximately 1 to 100 nm, and distinguishes discrete nano-objects from nanostructured materials.

That distinction is useful when reading papers. A paper about nano-objects is usually about making, sorting and measuring individual particles, tubes, wires or sheets. A paper about nanostructured materials is usually about a bulk material, such as a porous solid, a composite or a patterned film, whose performance depends on structure at the nanoscale. Both are nanoscience; they simply ask different questions.

For short definitions of the terms used here, see the glossary. For the main families of materials, the carbon nanotubes, fullerenes and graphene pages are a natural next step.