Dec 9, 2003
Ball-bearings for the 21st century
Quantum dots will be to this century what ball-bearings were to the last - the unseen enablers of a new era of human invention. So says Paul O’Brien, head of chemistry at Manchester University in the UK and founder of a company called Nanoco that manufactures these nanometre-sized lumps of semiconductor. O’Brien believes that the firm’s manufacturing process can be used to make large quantities of quantum dots for a range of applications in medicine, security and electronics.
Quantum dots typically consist of thousands of atoms of an inorganic semiconductor in which pairs of electrons and holes can be created and confined. When these electron-hole pairs are excited with a beam of light, they re-emit light with a narrow spectrum that depends on the size of the crystal. So by carefully tweaking the size of a quantum dot through chemical synthesis, it can be made to emit light over a wide range of wavelengths.
The fact that quantum dots emit over a narrow spectrum makes them well suited to imaging, particularly for biological samples. Currently, biological molecules are imaged using naturally fluorescent molecules, such as organic dyes, with a different dye attached to each kind of molecule in a sample. But the dyes emit light over a broad range of wavelengths, which means that their spectra overlap and that consequently only about three different dyes can be used at the same time. With quantum dots, on the other hand, full-colour imaging is possible because large numbers of dots of different sizes can be excited by a light source with a single wavelength.
The wide range of colours that can be produced by quantum dots also means they have great potential in security. They could, for example, be hidden in bank notes or credit cards, producing a unique visible image when exposed to ultraviolet light. They could also be used in electronics applications such as data storage, light-emitting diodes, photovoltaic devices and flat-panel displays.
The quantum dots made by Nanoco consist of a core of semiconductor material, generally cadmium selenide, surrounded by an organic or inorganic coating. Many of the details of the company’s manufacturing process are secret, but O’Brien says that the chemistry involved is safer than that of many rival methods. The process involves incorporating the cadmium and selenium, which are highly toxic, in a stable compound known as a precursor molecule. This molecule is then decomposed in a solvent such as tri-noctylphosphine using a process pioneered by Mougeni Bawendi of the Massachusetts Institute of Technology and co-workers.
According to O’Brien, because Nanoco’s manufacturing process does not involve handling the extremely hazardous chemicals used by other companies, it can be more readily scaled up. He says that Nanoco’s technique also makes it easier to change the size of the dots’ core and shells, allowing the company to tailor the optical, electromagnetic and catalytic properties of its products.
Nanoco was spun out from Manchester University in December 2001 and received a SMART award worth £45 000 from the Department of Trade and Industry to fund its early development work. It can now produce multigram batches of quantum dots in the form of a powder, dissolved in a solvent or in wax for long-term storage, and sends out free samples to help generate business. “The fact that we could make production volumes of quantum dots met with some scepticism from certain nanotechnology applications developers around the world,” says O’Brien. “So we decided the best way to show we are for real was to offer them free evaluation samples.”
Nanoco has so far sold between £30 000 and £40 000 worth of quantum dots to industrial and academic labs this year. This may seem like small fry in comparison to the billions that are often talked about in connection with nanotechnology, but, as O’Brien points out, the firm is in a relatively healthy financial position. “The company has just two employees working out of the university labs, so we are just about profitable at the moment,” he says. “To increase sales much more we will have to get together with a third party. But we wish to expand so as to become the supplier of choice for both these materials and customized applications based on quantum dots.”
About the author
Edwin Cartlidge is news editor of Physics World.