When it comes to determining the physical surface shape of an unknown sample, scientists turn – without fail – to a technique known as Atomic Force Microscopy (AFM). The technique itself is easy to understand, although it requires sophisticated computers to actually make it work. Imagine that you’re blind, and are asked to describe the pattern of bumps on a piece of paper – maybe a piece of simple Braille, or the like. The obvious solution is to run your fingers over the paper in a set pattern – starting at one corner and going straight across, and then going back and doing another row, and so forth. Every time you come across a bump you announce it, and then carefully keep on going. By the time you reach the end of the sheet of paper, you know you’ve covered the entire thing (because you did it row by row), and you’re fairly confident you noted all of the bumps as they came along.
This is the exact same principle that is used by AFM, only (naturally) it’s slightly more sophisticated. A sample whose surface needs to be analyzed for bumps and valleys is inserted into the instrument, and a movable arm called a “cantilever” is manuevered into place over the sample, starting at one corner. At the very end of the cantilever, pointing towards the sample, is a microscopic pointed tip that is sharpened and made out of a very hard / resilient metal alloy. The point of the cantilever tip is extremely small, as it has to be smaller than the features its trying to measure, and some of the dips and bumps on the sample could be very tiny. The platform containing the sample is then moved very slowly in a stepwise fashion, controlled by computer-assisted actuators. As the tip of the cantilever makes contact with the sample, the cantilever (which has a bit of “bounce” to it) will either dip down to follow the topography of a valley, or rise up slightly if it encounters a bump.
This cantilevel displacement is monitored by the computer (usually by shining a laser on the backside of the cantilever and measuring the change in deflection). Therefore as the cantilevel scans back and forth over the surface, the series of valleys and bumps can be compiled and finally displayed on a computer monitor. However, while this method is quite sensitive towards the surface topography, it can’t really say anything about the nature of the samples material – if it’s rubbery, or hard, or soft, etc.
New results from Harvard chemists – published in the journal Nature: Nanotech – have now improved upon standard AFM technology to incorporate this new type of surface analysis. The crucial modification / innovation is that the Harvard researchers placed the cantilever tip not at the center of the lever (as is traditional), but rather they moved the tip to the corner of the cantilever. This means that the cantilever not only rises up and down in response to the surface roughness, but the cantilever can twist from side to side in response to the surface. A hard surface will not induce much twisting, while a soft surface will “drag” the cantilever and induce a stronger oscillation.
That such a simple modification can yield such a wealth of additional information is a fantastic discovery. Because the modification is so easy to incorporate, I have no doubt that other research groups that utilize AFM will quickly adopt this new technology.
The source of this article can be found at: http://www.nature.com/nnano/journal/v2/n8/full/nnano.2007.233.html