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Monday
Jan242011

It's a Small World After All

Optical microscopes are familiar instruments to visualize the world of small things, like cells and bacteria. The basic principle at work in a microscope is transmission/scattering of light through a material; and the use of lenses to magnify the image. Visible light however has an inherent size, a wavelength, that limits its resolution. Think of trying to probe a small hole in a wall by throwing a basketball through it: it won't work. You need to use perhaps a tennis ball instead. In this spirit, you need a probe with a smaller size to explore smaller things. Use of visible light in an optical microscope allows one to see things about the size of bacteria, about 1 millionth of a meter, or a micrometer (one can go down to around 0.2 micrometer if things are really pushed to the limit). The same principle however can be used to build an electron microscope: replace light photons with electrons, and lenses with magnets. The wavelength of electrons is typically shorter, and hence allows one to see smaller things; about one thousand times smaller than an optical microscope.

The state of the art of visualizing the world of small things was however achieved in 1981 with the invention of the scanning tunneling microscope. This device consists of a tiny needle hovering a few nanometers above the surface of the subject of interest. A voltage difference is applied between the tip of the needle and the surface of the subject. Because of quantum mechanics, electrons in the needle can "tunnel" through the empty gap between the needle and the surface! See previous post on quantum tunneling. This creates a measurable electrical current in the needle that one can use to infer the distance between the needle's tip and the surface. As the needle scans over the surface, it can then image the topography - bumps and ditches on the surface. The resolution of this device: a fraction of a nanometer! that's the size of an atom, ten thousand times smaller than a cell! This has allowed physicists to image for the first time the fuzzy quantum world of atoms and electrons. The device can also be used to manipulate atoms individually using the needle as a tweezer! Check out the accompanying videos and photos for an amazing visual tour of the quantum world.

The first video shows the workings of a traditional electron microscope. The second one that of a scanning electron microscope. The third video describes the quantum mechanical principles underlying the workings of a scanning tunneling microscope (STM). The final video shows a portable STM in action! The last picture shows the manipulation of atoms with an STM: iron atoms on the surface of copper. The ripples are actually the copper electrons...

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