Posted tagged ‘Science’

SPM moves forward: Imaging the charge in a single-molecule.

01/03/2012

In experimental science, and I would imagine in other fields, there are often very clear trailblazers. The guys who seem to sneeze out high-impact publications that push the boundaries of what is possible, and usually with data so beautiful you could weep. The kind of guys who; after you have devised an amazing experiment and just begin to get the resources together for it, publish the same idea the next week with the sort of ease that makes you suspect they had just ‘knocked it out’ on a lazy Sunday afternoon.

In the field of Scanning Probe Microscopy (SPM) one such trailblazer is Leo Gross.

Scanning Probe Microscopy is the name given to a group of scientific instruments that can allow us to actually see the atoms on a material’s surface.

I’d like you to take a moment to consider that. We can actually make devices that allow us to see atoms. Until the beginning of the 20th century the very existence of atoms was doubted by the majority of the scientific community at the time. In the 1980’s, the first microscopes capable of seeing these elusive building blocks were invented. Now, thanks to Leo Gross, the technology moves forward again.

These microscopes do not work in the way expected by most people when you say the word “microscope”. It does not use light and there are no lenses. A common analogue to introduce people to the field is to consider an old-fashioned record player. Like the record player, a scanning probe microscope slides a sharp tip across the sample to “feel” the atoms and molecules upon it. I write “feel” in quotation marks as the actual mechanism of detecting the atoms differs between instruments and the exact needs of the experiment.

The two big boys of the STM world are the Scanning Tunnelling Microscope (STM) and the Atomic Force Microscope (AFM). Those interested should follow the links for the full details on how these devices work. For some time there was a little bit of smugness on the side of STM users as, their instrument was the first to allow atomic resolution and continued to do so reliably as a matter of course, whereas the AFM had to be worked at from its invention to yield its first pictures of atoms.

In 2009, Leo Gross spun that around, by taking picture of molecules that actually look like the diagrams you would draw in Chemistry class.  The details of this can be found in this IBM press release from the time and this Science paper. The key to this coup for AFM is that STM is, by its nature, an electrical measurement, a very-sensitive and glorified current detector. As such its ultimate resolution is limited by the electron states surrounding the molecule like a cloud. AFM does not have this limit, as its name suggests it works by “feeling” the various forcefields that surround the atoms in the molecules as so show them as they “are”. This difference is shown below in the image from the Science paper. A) Shows the chemical model of the pentacene molecule B) Shows the STM image and C) Shows the AFM image of the same molecule. Note the amazing similarity between A and C and the comparative “Fuzziness” of B.

Not content to leave it there:  Leo Gross’ group at IBM has been continuing to push the envelope as to what the AFM can do, now they have applied their uncanny ability to take amazing, high-resolution images to an AFM spin-off: Kelvin Probe Force Microscopy (KPFM). With this they have imaged the distribution of charges within a single-molecule. The story can be read for the public at the BBC and the full article is published in Nature Nano article for those with access

Kelvin-Probe uses the fact that when two metals are brought together a voltage/contact potential forms between them, due to the difference is workfunctions (how easy it is to pull the electrons out) of the metals. In KPFM, this voltage is formed between the sample and the tip of the probe which responds to the localised change. A feedback circuit is then used to apply its own voltage between tip and sample, countering the effect. The instrument’s record of how much voltage is used to cancel out the contact potential is recorded with sub-nanometer resolution and used to form an image.

To test the technique, Gross’ group studied a molecule called napthalocyanine, which has been shown to behave as a molecular switch. The molecule is shaped like a + sign, with four distinct arms and hydrogen atoms can be switched from one set of arms to another in a controlled way. In one state, the KPFM images show charge concentrated on two opposing arm (call them, North and South). After the switch is ‘pressed’ the images now show the charge concentrated on the other two arms ( East and West) having rotated by 90 degree around the molecule. The observed images matched very well with the distribution of charges predicted by theory (DFT calculations).

Still not satisfied, they went further by improving the resolution of their images by refining their AFM probe by picking up a single carbon monoxide molecule on the end of their tip to use as a much smaller, more refine and more precise probe in their experiment.

Long-blog post short, they can now see where the charge is gathered within on napthalocynanine molecule, which parts hold the positive charges and which the negative.  This knowledge is of great interest to scientist working on nanoelectronics devices. In such materials, especially those made from organic molecules, their function depends on the  separation of charge and the dynamics its storage and transport. In particular, these processes are critical to the operation of solar cells and their natural analogue the photosynthetic proteins in some plants and bacteria. The ability to actually see these effects is a major step-forward in both the development of SPM and future electronics.

Always, always, check the cables!

22/02/2012

The latest news from CERN is that the famous “Faster than light neutrinos” that filled both the scientific and popular press late last year, may be down to an experimental error after all.

The breaking news has been announced on Science Magazine’s “Science Express” board. I’ve put the link below and the note is worth reading for yourselves.

Science Express Post

However, in a nutshell, the big brew-ha-ha started when Scientists in the OPERA project sent neutrinos from the CERN lab in Geneva to a lab in Gran Sasso in Italy. Neutrinos are subatomic particles, which, due to their incredibly small mass and neutral charge, interact with practically nothing. They fly through space and whizz through the Earth and all of us with hardly any effect. They don’t even stop to wave. It was always thought that the travelled at speeds very close to but just under the speed of light , as the Special Theory of Relativity demands that nothing can travel any than light itself.

This claim, which has been tested and retested for nearly a century, was called into question when the OPERA scientists found that their neutrinos reached Gran Sasso 60 nanoseconds before (1 billionth of a minute) they should do travelling if light speed. These scientists knew that this would be a major upset to one of the fundamental theories of modern physics, and though scientists are always open-minded to new (reasonable) ideas, you don’t overturn Einstein unless you are really, really sure. So they repeated the experiment, over 3000 times, and after finding the same result, threw their findings out to the scientific community. They hoped, in a wonderful example of how science should be done, that a much larger number of eyes on the data and minds on the problem may see anything they overlooked.

Now the problem may have been spotted (may, I said, MAY, more data is needed). In a fashion haunting familiar to anyone working a in a lab the problem may be the result of a faulty cable. In this case, an optical fibre in the GPS system used to accurately track the distance between the two stations and keep the timing of the neutrinos flight. Seems data was passing through this cable faster than it was believed to be. How much faster, you say? Well, about 60 seconds, funnily enough.

Though such nanosecond precision in data transmission isn’t a requirement in my line of work, it just reinforces an important lab.life lesson. Always check the cables.

At the limit of Moore’s Law

22/02/2012

Stop to consider the amazing explosion of technology that has rapidly filled our lives in the last 50 years. Such the computer you’re reading this blog on, or perhaps you are viewing this page on a device smaller than your hand, or even the fact that these words can be read by anyone, from anywhere in the world.

Where did this all begin? What was the ‘seed’ that allowed all this technology to grow so fast and sprout up in every aspect of our lives?

I think a good argument could be made that the answer to this question is: The transistor.

A transistor is, basically, a tiny electrical switch. It stops and starts the flow of current along a circuit in response to series of electrical pulses.  These allows a computer’s to send a control signal to all of its various parts using this off/on flow of current to “speak” the 0/1 language of binary and perform operations by forming Logic Gates. In this way, the transistors form part of computer’s “nervous system” relaying signals from the CPU (its brain) to the various “organs” (the hard drive, the speakers etc.)

However, modern transistors are tiny things and the driving force behind the rapid increasing in computing power (while computers themselves shrink) is that scientists have found way to make smaller and smaller transistors allowing them to pack more and more onto the circuit boards in your PC.  Unfortunately, there is an rather obvious physical limit to how small you can make something: the atom. And it’s seems we have hit that limit with the latest reports in Nature Nano of a functioning Single Atom Transistor.

A common type of transistor is the Field Effect Transistor (FET), where the current flows between two electrodes called a Source and a Drain, and is controlled by a third called the Gate. The names are quite helpful as the operation of the FET can be described by a comparison to water. Imagine the Source to be a tap and the Drain, a plughole, and that the water needs to pass through length of rubber tubing dangling from the end of the tap into the sink. Turn on the tap and set the water going, then start to squeeze and release the tube rhythmically, stopping and releasing the flow. This is your Gate.

Of course in a real FET the water corresponds to the movement of charge carriers such as electrons or holes, the hose is a narrow channel of semiconductor whose size and shape is controlled by applying a voltage across it, restricting the flow of charge.

The single-atom transistor works in the same way. Michelle Simmons’ Group at the University of New South Wales first started with a hydrogen-passivated silicon surface. Using the tip of a Scanning Tunneling Microscope (STM) they carved away the hydrogen atoms in a “+ shaped” to form the Source, Drain and two Gate electrodes leaving a tiny gap in the centre. They then added phosphine (PH3) to the system, which binds to the newly bare silicon and ignores the hydrogen covered parts. This dopes the electrodes to make them more conducting.  In the tiny gap of the centre of the + shape, the author, managed to take three PH3 molecules on the surface and; breaking, swapping and reforming through some impressively simple thermal treatment, replaced a single silicon atom with one of Phosphorus. Measurement of the current between source and drain showed that it did indeed depend on the voltage applied across the gate electrodes, confirming the device was “transisting”.

I won’t go into further detail about the conduction measurements and the nitty gritty of energy levels and electrostatic potential calculations, however, I do hope to impress upon you the importance of transistors and the gravity of the work. Though single atoms have previously been shown to behave like transistors in the right circumstances, this is the first time a single atom transistor has been engineered i.e. the electrodes, the doping and the transistor atom itself have been deliberately and deterministically put in place with atomic precision. This is an amazing feat of nano-fabrication and represents the ultimate size limit of present computer hardware: It doesn’t get smaller than one atom.

As an extra bonus there are some signs that such a device can beyond the current level of computing. At low gate voltages, the conduction measurements along with calculations also show that the phosphorus atom retains its discrete quantum levels allowing for potential applications in Quantum Computing, logical operations and devices that make use of the “spookiness” of the quantum mechanics.

Of course this is a long step from use in an actual computer, let alone your laptops. The device can only be operated blow liquid nitrogen (<77 K) temperatures and the device fabrication makes use of ultra-high vacuum conditions. However, this is how science evolves into technology: someone finds out what is actually possible and then someone tries to make it work at a higher temperature or make the device a little bit more stable, inching closer and closer to practicality.

Not every scientific breakthrough makes it from the lab into your home, just as not every drug gets from the petri dish to your medicine cabinet or every species gets from an amoeba to the zoo. However, where there is enough passion and excitement, the Nerds will find the way.