Monthly Archives: September 2005

Eclipse

This coming Monday morning there is a solar eclipse, it isn’t a total eclipse of the sun, the last of those visible from the UK was in 1999, this is a partial eclipse. It starts 08:49, the point of maximum eclipse is a 10:01 and the moon leaves the solar disk at 11:18.

If you are lucky enough to be in the correct parts of Spain or North East Africa, then you’ll see around 95% of the sun vanish, those of us in the UK will have to be content with less. Here in London the best we’ll get is around 57%. Sky and telescope magazine has a useful chart showing just what you can expect to see depending on your location at the time of maximum eclipse.

If you do intend to watch the ellipse, weather permitting, please do not look at the sun either directly or though smoked glass or the like. There is very real risk of eye damage. Good information on suitable and unsuitable filters for viewing an eclipse are given by NASA.

Eclipses occur because of an interesting coincidence in astronomy, the Moon and Sun have very nearly the same apparent diameter as viewed from the Earth. This allows the moon to cover or partially cover the solar disk leaving the earth in shadow.

The orbit of the moon around the earth is elliptical, not exactly circular. Thus sometimes the moon is closer to the earth (perigee) and sometimes further from the earth (apogee) (note this isn’t the reason the moon sometimes looks large in the sky). So an eclipse occurring when the moon is at apogee occurs with the moon smaller than the sun as viewed from earth. This leads to a bright ring (annulus) of uncovered sun at the moment of maximum eclipse visible to viewers in the area of the antumbra shadow.

A total eclipse of the sun occurs when the moon is a perigee and hence appears larger in the sky. The moon’s apparent diameter is then large enough to cover the whole of the solar disk. Any viewers in the umbra part of the moon’s shadow will experience a total eclipse of the sun, while those in the in the penumbra part will see a partial eclipse of the sun.

The next total solar eclipse visible from the UK is in 2061, so book your holiday in Cornwall now….

What I do.

This post presents an overview of my work for the last 3 years. Essentially I’ve been developing new techniques for structuring materials on the sub milimeter scale by a combination of laser ablation, plasma chemistry and standard wet chemistry.

Silicon has been the wonder material of the late 20th century and looks set to continue well into the 21th, mono-crystalline silicon underpins much of the electronics now taken for granted and is also used in the developing Micro-Electro-Mechanical Systems (MEMS) technology. The structure and properties of bulk mono-crystalline silicon have been intensively studied since it was first produced.

The properties and applications of silicon at the meso- and nano-scales, however, have been less intensively exploited. Intensive research has demonstrated that the electronic and optical properties of nanoscale silicon structures change dramatically with respect to the bulk and these changes may lead to significant advances in devices and sensor. Furthermore, silicon mesostructures, which are already being used in the accelerometers that trigger automotive airbags, exhibit great promise for applications in, eg, antireflection coatings, microfluids and biomedical applications.

Chemically enhanced laser ablation of silicon provides a mechanism to quickly produce large numbers of near-identical meso-scale silicon structures which can be easily converted to nano-scale structures via standard wet chemistry procedures. Starting from a flat or patterned silicon substrate, pillars and cones will spontaneously form when the substrate is irradiated by a pulsed laser beam.

Applications of the structured surfaces.

The main body of my work is the production and characterisation of the structures I produce. I have provided samples to various groups interested in using them for various applications ranging from biological applications to electrode arrays, photonic structures and basic materials research. In this post I’ll concentrate on the biological applications.

Biological applications

The potential for localised delivery and subsequent activity of drugs [McAllister2000] and gene [Coulman2005] therapeutics within the skin and for fluid extraction for diagnostics [Torabi-pour2004] has stimulated research into cheap, painless ways to deliver the macro-molecules through the tough outer, stratum corneum (SC), layer of the skin as shown in fig. 1.


Figure 1: Schematic diagram showing how the microneedles should penetrate the Stratum Corneum allowing drug delivery to the Viable Epidermis, avoiding contacting the blood vessels and nerves of the Dermis layer. (Image courtesy of C. Allender, personal communication)

There is considerable interest in using silicon needles to puncture the SC producing microconduits though to the viable epidermis layer of the skin, through which macro-molecule treatments can be introduced.

The lithographic fabrication of microstructures on a silicon surface is a well established process, but requires a clean room, toxic chemicals or access to reactive ion etching equipment and producing large areas of needles is a time consuming process. This is in contrast to the relatively quick and low technology method we employ to produce large arrays of silicon pillars. As was seen in the previous chapter, wet etching of silicon pillars produced in SF6 leads to well defined, short, but sharp needles.

The structures formed by the chemically enhanced laser ablation of silicon may provide a cheap, easy way to produce large numbers of needles for experimentation. Silicon pillars covered with a layer of porous silicon may be ideal for the transfer of material into cells, the pillars acting as needles and the porous layer acting as a reservoir of the material to be injected.

There is still much work do be done in this area, there isn’t a whole lot more than can be done to alter the pillar shapes, so muuch of the future work will be lab trials investigating the usage of the pillars as needles.

A matter of scale (ice spikes and silicon pillars)

One of the great things about science is how interlinked everything is once you start looking deeply enough. One example of this came a couple of months back when I was reviewing the electron micrographs from the latest experiment.

My work involves the growing of meso-scale (about a 100th of a millimeter) pillars on a silicon wafer by laser irradiation. A complex combination of melting, ablation (explosive boiling) and redeposition of material happens at the silicon surface, with the net result that pillars grow from the silicon towards the laser. The pillars spend most of their growing time bathed in a hot (>5,000 degree C) plasma, this tends to smooth out the tips and sides of the pillars by a combination of melting and sputtering.

In this particular experimental run, I’d been varying the conditions under which I grow silicon pillars. What I saw was most unexpected; nano-scale (around a billionth of a metre in size) spikes sticking out of the top of almost every pillar I’d grown. You can see in the pictures below, that the pillars are around a twenty microns long, and the tiny spikes stick out vertically from the tips.

forrest of spikes spike closeup
Cross sectional view of silicon pillars showing nano-scale spikes, and a closeup of two pillars with spikes at their tips

After a bit of head scratching, it was realised that what we were seeing was just a much smaller scale version of the ice spike formation process.

If you’ve ever tried to make ice cubes with filtered or very pure water, you will probably have noticed that you sometimes get a spike or lump that grows upwards from the free surface of the ice. A very good example of one formed from a distilled water ice cube is shown below, it is about a hundred million times larger than the silicon spikes in the pictures above.

ice spike
Ice spike growing from an ordinary ice cube. Image taken from the excellent ice spike site at Caltech

Rather amazingly, given the wildly different growing conditions, the process by which both ice spikes and silicon spikes grow is essentially the same. In both cases you start with a liquid – water or molten silicon which cools and begins to solidify on the outer edges until there is just a tiny space that remains unfrozen.

Both water and silicon expand as they freeze, liquid water is forced out of the center of the ice cube tray, up through the hole. If the conditions are correct, then water forced out of the hole will freeze into a hollow spike. The water freezes around the top of the spike, adding to its length. The spike continues to grow until all the water has frozen or the hollow in the spike freezes shut.

What happens with the silicon spikes is that the last pulse from the laser melts the tips of the pillars. These cool relatively slowly once the laser shuts off, the outside solidifying first and increasing the pressure on the liquid trapped inside the tips. The liquid silicon escapes though a hole or crack, forming a tube with frozen sides and a liquid centre. This grows like the ice spike until it freezes solid. In the case of the silicon spikes, the growth happens in a matter of microseconds, compared to the many minutes that ice spikes take to grow.

While you may have some trouble duplicating my work in your kitchen, you should be able to produce ice spikes quite easily. All the details are on the Caltech ice spike site

Death of a pioneer

On Friday 16th September, Gordon Gould the inventor of the laser died. Gould claimed that the idea and principles for the laser came to him one afternoon in 1957. His lab book details his ideas on a page under the title “Some rough calculations on the feasibility of a LASER: Light Amplification by Stimulated Emission of Radiation”.

Gould invented two types of laser, the gas laser and the optically pumped laser, but failed to get a patent granted on the inventions until the intervention of a judge in 1987. This worked to his advantage, he was able to collect license fees on the patent at a time when the laser was popular, had the patent been granted in 1957, then it would have expired before the laser was in significant use and the revenue from the patent would have been much less.

The laser I use in my work is loosely based on Gould’s first attempts at a gas laser, but pretty much all high power industrial cuttings lasers are direct descendants of Gould’s invention.

Open House

Next weekend is the London Open House weekend. Saturday sees me visiting family in Tiptree for the family tree project, so I’m looking for somewhere local to visit on Sunday. That’s off now, so I might be able to do both days.

The search on the website is, however, pretty badly done and, in fact, not actually working at the moment. Now I seem to remember there was a tour of the Becontree estate and Valance House had an open day. I think this may be a bit too local. I know both the museam and the estate very well. I want to see what is open towards the Eastern end of the District Line – Hornchurch and Upminster, mainly because I can easily cycle there, and it is downhill all the way on the return journey.

I wonder just how many punters some of the events will attract. I really can’t see the estate tour being very well subscribed…