Seeing atoms and molecules

Optical microscopes have been tremendously important in the development of science.  Unfortunately, their use is limited by the wavelength of visible light.  Using light with a 550-nm wavelength, for example, it is possible to barely resolve (distinguish between) objects that are 0.005 mm apart – good for looking at algae and bacteria, but not good for looking atoms and molecules.

The realization that electrons had wave properties led to the development of the electron microscope and a much higher degree of resolution.  Using electrons with a wavelength of 6.06  10-3 nm for example (a value much smaller than wavelengths of visible light) gives an improvement of about 105 in resolution.  With an electron beam instead of a light beam it is possible to distinguish objects separated by only about 50 nm.  Although this is in the range of dimensions of some large biomolecules, they can still only be seen as shapeless blobs.  The technique is still about an order of magnitude (ten times) too low in resolving power to get useful molecular information.

Because the resolving power of a microscope is related to the wavelength of the radiation used, it might seem logical that simply going to shorter and shorter wavelengths would ultimately make it possible to “see” individual atoms.  Unfortunately this is not so, and it is the Heisenberg uncertainty principle that gets in the way.  A shorter wavelength means a higher frequency and higher energy, and the energy of the radiation perturbs the particle being studied.  A simple picture is that the photon of radiation is so energetic that when it strikes the particle to be imaged it imparts sufficient energy to move it from its position.  In effect, with an accurate energy value of the photon, the position becomes imprecise.  So, this route to actually seeing atoms is doomed to failure.  A totally different approach is needed.

This new approach was developed in the early 1980s by Gerd Binning and Heinrich Rohrer, both received the Nobel Prize in physics in 1986 for their invention of the Scanning Tunneling Microscope (STM).  In the STM, a sharp needle-like probe is positioned within a few atomic diameters from a surface and a voltage applied to cause a current to flow between the probe and the surface.  The current is a function of the distance between the probe and surface.  As the probe moves horizontally along the surface, its height is controlled by an intricate feedback circuit.  The signal recorded defines the topography of the surface, a series of hills representing atoms, and valleys representing spaces between them.  Thus it is possible to “see” individual atoms.

								Piezoelectric probe 

												I

�h	The probe is made of piezoelectric material, which is a Pt-Ir alloy and is prepared so that it ends in a single atom.  Piezoelectric materials expand or contract when a voltage is applied to them, so applying a voltage to the probe causes it to move three-dimensionally.