Category: Evolutionary nanotechnology

It’s sobering to think that the hard disk drive is now 50 years old, and yet as a data storage technology it is still about 100 times more cost effective than its competitors. Yet its drawbacks are pretty apparent – hard drives are inherently slow and unreliable, and because of the mechanical nature of the device this is not likely to change fundamentally. Meanwhile various solid state memories, like SRAM, DRAM and flash memory, are very fast-growing market segments. Yet even with the continual shrinking of circuit dimensions, these technologies will not be able to match the raw storage capacity of hard drives. The search is on, then, for memory devices that have the capacity of hard drives but the robustness and speed of access of solid state devices. One candidate for such a device is the magnetic racetrack, invented by IBM’s Stuart Parkin. One needs to take Parkin’s views on magnetic data storage devices very seriously; as the inventor of the giant magnetoresistance based read head, it’s his work that has permitted the miniaturisation of hard disks that we see today and which makes possible devices like the video iPod.

I heard Parkin speak about his invention at last week’s Condensed Matter and Materials Physics meeting of the Institute of Physics, where he was one of the plenary lecturers. A version of this lecture, which Parkin recently gave at UCSB, can be downloaded from here – this includes some very helpful videos. Parkin’s invention is disclosed in this US patent; the basic idea is that data is stored in a pattern of magnetic domain walls in a nanowire of a magnetic material, which needs to be about 10 microns long and less than 100 nanometers wide. Rather than reading the pattern of magnetic domains by moving a read-head along the wire, in the magnetic racetrack the wire is held stationary and the magnetic domains are swept past a stationary read-head by applying a current. There’s a lot of fascinating physics in the way a current can move the domain walls, but the attraction of this arrangement from a practical point of view is that there are no moving parts, and the data density can be very high. Parkin envisages an array of these nanowires, each bent in a U-shape, with the bottom of the U held against a read head and a write head which are laid down by conventional planar silicon technology. It’s this compatibility with existing manufacturing technology that Parkin sees as a compelling advantage of his idea, as compared with other proposals for high density data storage devices depending on more exotic schemes using, for example, carbon nanotubes.

This review of “The Dance of Molecules : How Nanotechnology is Changing Our Lives”, by Ted Sargent, is published in today’s edition of Nature. The published version differs slightly from this unedited text, which is reproduced here by permission of Nature, and should not be further reproduced.

Every field needs its founding genius; the appropriately mythic figure for many in nanotechnology is Richard Feynman, on the strength of his 1960 lecture “There’s plenty of room at the bottom”. Yet this particular canonization is entirely retrospective; there’s little evidence that this lecture made much impact at the time, and Feynman rarely returned to the topic to develop his thoughts further. Perhaps a better candidate to be considered nanotechnology’s father figure is President Clinton, whose support of the USA’s National Nanotechnology Initiative converted overnight many industrious physicists, chemists and materials scientists into nanotechnologists. In this cynical (though popular) view, the idea of nanotechnology did not emerge naturally from its parent disciplines, but was imposed on the scientific community from outside. As a consequence, nanotechnology is a subject with an existential crisis – is there actually any firm core to this subject at all, any consensus as to what, at heart, defines nanotechnology?

This is the problematic territory that Sargent has tried to map out for the popular reader in “The Dance of Molecules”. How then, does Sargent deal with this tricky question of definition? “Nanotechnologists”, he says, “have as their goal to design and build matter to order, specified by a functional requirement”. This is fine, but it may leave followers of an earlier new discipline – Materials Science – puzzled, as this was their slogan too. He begins by maintaining the centrality of quantum mechanics, but really this is just an assertion of the centrality of chemistry. The title “The Dance of Molecules” might suggest the idea of Brownian motion, but this idea isn’t pursued. In the end he is forced to conclude that nanotechnology’s central theme isn’t scientific, but sociological – a new culture of interdisciplinarity that searches for convergence between increasingly atomized scientific fields.

There is one version of nanotechnology that does have clarity – K. Eric Drexler’s vision of scaled-down mechanical engineering. It is this revolutionary vision that underlies much of the popular image of nanotechnology, promoted through science fiction, films and computer games. Yet very few scientists take this version of nanotechnology seriously. This leaves a problem for popularizers who wish to reflect the scientific consensus. One can rebut these claims in detail, or one can simply dismiss them with appeals to the authority of distinguished scientists like the late Richard Smalley. Sargent takes a third course; he simply does not mention them. This seems to me to be the most unsatisfactory approach of all; if one thinks the Drexler vision is wrong, one should say so, otherwise the reading public, who are extensively exposed to these ideas, will be left very confused.

Lacking a strong science core, the book is written thematically, as a tour of application areas in health, environment and information. Quantum dots make frequent appearances, there’s quite a good description of molecular electronics, which is duly circumspect about the balance of potential and practical difficulty. The descriptions of bionanotechnology carry less conviction – the description of molecular motors seems particularly misleading. Many people will find the rather overwrought style irritating – perhaps the oddest of the many strange and strained metaphors and similes is his description of photolithography as being “like crop circles formed when the sun blazes through round partings in the English permacloud”.

Nanotechnology, above all an applied science, has been the subject of a possibly unprecedented push for early consideration of social, environmental and ethical impacts. Here the rhetoric is overwhelmingly positive, and the need for public engagement seen solely in terms of defusing possible opposition. We’re promised an end to cancer, the restoration of sight to the blind, and, via unconventional solar cells and the hydrogen economy, an end to our dependence on fossil fuels for energy. The possible downsides are largely limited to the potential toxicity of some nanoparticles. Even in military applications, the emphasis is on defensive applications and on the possibility that nanotechnology will make it much easier for the west to wage a “clean war”, in which combatants are easily distinguished from non-combatants. I don’t think you need to be a radical anti-technology activist to greet this claim with some scepticism.

The current difficulties of nanotechnology include its incompletely formed disciplinary identity and lack of clear definition, the overselling of its immediate potential economic and societal impacts, and its association with extreme utopian and dystopian futuristic visions. A good popular book could contribute to overcoming these difficulties by setting out a clear set of core scientific principles that underpin nanotechnology, making realistic claims for what applications and impacts will be possible on what timescale, and presenting a more sophisticated understanding of the relationships between science, the economy and society. This book does not fulfill this need.

The main driving force for developing plastic electronics – the use of semiconducting polymers to make logic devices, light emitting diodes and displays, and solar cells – is the hope that these materials can be processed very cheaply. Because these materials are soluble, devices can be made by processes like ink-jet printing or screen printing. Compared to standard silicon-based electronics, the performance of these devices is often not very good, but the fact that you won’t need the massive capital expenditure of a conventional silicon fab tilts the economics towards the plastic materials, at least for applications where cost is more important than performance. But Nature this week reports an interesting twist – a group from the Seiko-Epson labs in Japan report a new way of printing silicon directly from solution (see also the Epson press release here).

The method is based on using polysilane as a precursor material. Polysilane is essentially the silicon based analogue of the well-known polymer polyethylene, consisting of a long chain of silicon atoms, each of which has two hydrogen atoms attached. But unlike polyethylene, polysilane is both unstable and very difficult to work with, being insoluble in most common solvents. The Japanese group got over this problem by starting with a five-membered ring version of the polysilane molecule – cyclo-pentasilane (this is the silicon analogue of cyclopentane). They found that polysilane was soluble in solutions of this precursor, and these solutions could be ink-jet printed and converted into pure silicon layers by a simple heating step.

The silicon layers formed this way are amorphous, not crystalline, and their electronic properties are not very good compared to silicon films prepared by more conventional routes (though they are better than most polymer semiconductors). Plastic electronics still has some advantages over this new process, which requires temperatures too high for the use of plastic substrates. The printing step is also complicated by the need to exclude water and oxygen. Nonetheless, it’s an important step forward towards the development of low-cost electronics for applications like large area displays and cheap solar cells.

Today’s picture is a scanning electron micrograph of a hybrid structure in which organic light emitters are confined in a micropillar by a pair of inorganic multilayer mirrors. These hybrid organic/inorganic structures have interesting photonic properties that may have applications in quantum cryptography and quantum computing; this work comes from my colleagues in the physics department here at Sheffield in collaboration with some of our electrical engineers.

Image by Wen-Chang Hung, image post-treatment by Andy Eccleston.

This structure is made by laying down, by chemical vapour deposition, 12 pairs of alternate layers of silicon oxide and silicon nitride, each exactly one quarter of a light wavelength in thickness. This is coated by a 240 nm (half a wavelength) thick layer of the polymer polystyrene, doped with an organic dye called Lumogen red, which in turn is coated by another 12 pairs of layers, this time of tellurium oxide and lithium fluoride, thermally evaporated. The pillar is carved out of the resulting layer cake structure by using a focused ion beam.

The multilayers act as perfect mirrors. Imagine putting a light source in between two parallel mirrors – you’d get an infinite (if the mirrors are perfect) series of reflections of the light. In our situation the dye molecule is the light source; when it emits a single photon, that photon is going to interfere with its ghostly counterparts emitted from the reflections, which are all in phase with each other. This makes it a very efficient producer of single photons – potentially these could be used for quantum cryptography or quantum computing.

All of this has already been demonstrated using quantum dots – tiny particles of inorganic semiconductor – as the light emitter. What’s the advantage of using an organic dye instead? In these devices, photons are emitted when an electron and a hole annihilate. These electron-hole pairs – called excitons – are very weakly bound in ordinary semiconductors, which means that these devices only work at rather low temperatures, about 50 K. In organic molecules the charges distort the structure of the molecule itself, which means that the exciton is bound much more strongly and the device will work at room temperature. It goes without saying that this feature makes the possibility of an economically viable quantum computer seem much closer. To be fair, though, the organic materials have disadvantages, too – they are susceptible to being bleached by bright light.

The work is a collaboration between my colleagues in physics, Ali Adawi, Ashley Cadby, Daniele Sanvitto, Liam Connolly and Richard Dean, who are postdocs and grad students in the groups of David Lidzey, Mark Fox, and Maurice Skolnick. Device fabrication was done with the help of Wen-Chang Hung and Abbes Tahraoui, in Tony Cullis’s group in our Electronic and Electrical Engineering Department. It’s reported in the current edition of Advanced Materials here (subscription required).

Most research in Europe, in nanotechnology or any other field, is not funded by the European Union. Somewhere between 90% and 95% of research funding comes from national research agencies, working with their own procedures, to their own national priorities. This bothers some people, who see this as yet another example of the way in which Europe doesn’t get its act together and thus fails to live up to its potential. In research, the European Commission fears that, compared to rivals in the USA or the far east, European efforts suffer from fragmentation and duplication. Their solution is the concept of the “European Research Area”, in which different national funding agencies work to create a joint approach to funding, as well as doing what they can to ensure free movement of researchers and ideas across the continent. As part of this initiative, national research agencies have come together to form thematic networks. Nanoscience has such a network, and it is meeting this week in Amsterdam to finalise the details of a joint funding call on the theme of singly addressable nanoscale objects.

Another way of looking at the issue of the many different approaches used in funding nanoscience across Europe is that this gives us a laboratory of different approaches, a kind of controlled experiment in science funding models. Yesterday’s meeting was devoted to series of overviews of the national nanoscience landscape in each country. This was instructive and contrasting; among the large countries one had the German approach, with major groups across the country being supported with really substantial infrastructure. The French had most logical and comprehensive overall plan, while the talk describing the British effort (given by me) couldn’t entirely hide its ad-hoc and largely unplanned character. The presentations from smaller countries varied from really rather impressive displays of focused activities (from the Netherlands, Finland and Austria in particular), to more aspirational talks from countries like Portugal and Slovakia.

How do the European nations rank in nanoscience? The undisputed leader is clearly Germany, with France and the UK vying for second place. Readers of this blog will know that I’m suspicious of bibliometric measures, but some interesting data was shown showing France second and the UK third by total numbers of nanoscience papers, but with that order being reversed when only highly cited papers were considered. But the efforts of the rich, smaller European countries are very significant; these are countries with high per person GDP figures which typically spend a higher proportion of GDP on research than larger countries. They combine this with a very focused and targeted approach to the science they support. The Netherlands, in particular, looks very strong indeed in those areas that it has chosen to concentrate on.