Super-Hero Experiment #3: What Lies Beneath

atom.jpg What is the ultimate test of human control over the world as we know it, if it is not the manipulation and patterning of the fundamental units of matter: the atoms? Back in 1959, Caltech professor and physics great Richard Feynman said, “… I am not afraid to consider the final question as to whether, ultimately – in the great future – we can arrange the atoms the way we want; the very atoms, all the way down!” He predicted, in his famous speech There’s Plenty of Room at the Bottom, that scientists could expect to do very different and great things if equipped with the ability to manipulate matter on such a small scale, for atoms behave in manners very foreign to matter on the large scale, obeying instead the quirky laws of quantum mechanics. The question was: would anyone ever REALLY be able to manipulate a single atom, to ‘fiddle around’ with the very essence of matter?

800px-XeTube.jpg The pioneering Eigler-Schweizer experiment in 1990 answered with a powerful and inspiring YES. In a famous Nature article published that April, Donald Eigler and E.K. Schweizer reported the use of a device called a scanning tunneling microscope (STM) to position individual xenon atoms on a single-crystal smooth nickel surface. Eigler, a scientist at the IBM Research Division of the Almaden Research Center, demonstrated atomic precision manipulations with his STM device by spelling out I-B-M with atoms of xenon, the element you might know for its use in ‘neon’ signs. This experiment is now popularly viewed as the ‘Wright Brothers First-Flight’ of atomic scale manipulations and control. Eigler’s pioneering work in this area won him, along with many other awards, the 2010 Kavli Prize in Nanoscience, and a spot here in Super-Hero Experiments!

IBM.jpgThe Eigler-Schweizer experiment in 1990 demonstrated that the very fine tip on a scanning electron microscope could be used to drag an individual atom across an underlying surface, such as a flat sample of crystalline nickel. A scanning tunneling microscope is a device that employs a tiny, sharp metal tip to visualize structures far too small for the eyes to see. STM renders images that reflect the topography of a surface, like the humps and valleys of a mountain range, as well as its electronic structure. A tiny electrically charged tip is passed over a surface, and its trajectory is recorded, providing a ‘map’ of the surface. The distance between the tip and the surface, or the tip height, is constantly adjusted in order to maintain a constant tunneling current between the STM tip and the surface. An STM device is sensitive enough to detect single atoms residing on a smooth surface, which appear as bumps or ‘hills’ on an STM image.

STM.jpg Eigler recognized that the tip of a scanning tunneling microscope always exerts ‘a finite force on an adsorbate’, where the adsorbate is the atom or molecule of interest adsorbed onto the underlying surface. This finite force is composed of primarily Van der Waals and electrostatic components, weak chemical bonding interactions that exist between the adsorbed atom and the atom(s) on the very end of the STM tip. The overall magnitude and direction of the force that the STM tip exerts on the surface-bound atom can be tuned by adjusting the position and the voltage of the STM tip. This force tuning through STM tip adjustments is the principle behind Eigler and Schweizer’s atomic-scale manipulations.

The conditions of the Eigler-Schweizer experiment were such that the atoms remained bound to the underlying surface, in a mechanism known as the Sliding process. In this process, the bond between the manipulated atom and the underlying surface is never broken (Science 1991), but attractive forces between the atom of interest and the STM tip cause the atom to be dragged along with the moving tip under appropriate conditions. The atom is never completely removed from the surface under these conditions, because it generally ‘takes more force to pull at atom away from a surface than to move it along a surface’ (Nature 1990).

sliding process.jpg The Sliding process begins with operation of the STM in a ‘non-perturbative imaging mode’, with the STM tip sufficiently far from atoms on the surface not to affect their position (Figure – a). Once the desired atom is located, the STM tip is placed directly above the atom to be moved. The STM tip is then lowered toward the atom, thus increasing the tunnel current to a magnitude sufficient to cause interaction between the surface-bound atom and the atom(s) on the end of the STM tip (Figure – b). With the tip-atom interaction established, the tip may be moved under a ‘closed-loop’ electrical condition across the surface, dragging the desired atom with it (Figure – c). Finally, the tip is withdrawn upward by reducing the tunnel current back to the value appropriate for non-perturbative imaging (Figure – c). Under an ultra-high vacuum and extremely low temperatures, around 4° Kelvin, the atom is very stable once placed. Eigler even noted, during those first fateful scientific studies, that he could ‘perform experiments on a single atom for days at a time’ (Nature 1990).

IBM process.jpg By using the sliding process, Eigler and Schweizer were able to manipulate xenon atoms, one-by-one, into a pattern on a smooth nickel surface that spelled out IBM in 1.6 angstrom high ‘hills’ on a map. The xenon atoms were observed to ‘snap’ into place according to the ‘grid’-structure of the underlying nickel surface, where this repeating grid structure is characteristic of a crystal of nickel. In this snapping-into-place of xenon atoms moved across a nickel surface, we see an example of how scientists can find out more about the electronic properties of a surface with experiments of atom manipulation. By observing how atoms on a surface can be manipulated and moved, scientists can learn more about the rules of interaction between atoms of different elements and in different structures, making atomic manipulation an important tool for science.

The pioneering Eigler and his colleagues were the first to demonstrate that atomic manipulation was possible. Their original ‘Super-Hero’ experiment opened the floodgates to a whole new class of surface science, allowing researchers to create custom-made structures not normally available for study and investigation at the lab bench. It paved the way for additional means of manipulating and studying the world on the atomic scale, such as actually transferring atoms, molecules, or groups of atoms from surface to STM tip or from STM tip to surface (perpendicular processes). It led to revolutionary experiments that demonstrated such unbelievable phenomena quantum corral.jpg as quantum confinement of surface-state electrons in quantum corrals; a sort of trapping or ‘corralling’ of electrons inside of circular ring of atoms. A quantum corral has the appearance of and similar behavior to ripples in a ‘pond’ of electrons confined by an atomic ‘shore’. This and following discoveries permitted by atomic manipulation gave researchers richer tools for the investigation of quantum mechanical behavior.

Eigler and Schweizer also astutely predicted that their experiments might make “the prospect of atomic-scale logic circuits and other devices is a little less remote.” This become a fascinating reality, when Heinrich and colleagues under Eigler designed a system of molecular ‘dominoes’, composed of molecules that could be initiated to move in a cascade similar to toppling dominoes. Such molecular cascades were, as published in Science in 2002, arranged to serve as atomic-scale logic gates, essential elements of computing circuits! With this and many other examples of the impact that the original Eigler-Schweizer experiment had on surface science and quantum mechanics, there is no doubt: Eigler and Schweizer deserve Super-Hero status!

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• D. M. Eigler and E. K. Schweizer. Positing single atoms with a scanning tunneling microscope. Nature 344, 524-525 (1991)
• Joseph A. Stroscio and D. M. Eigler. Atomic and Molecular Manipulation with the Scanning Tunneling Microscope. Science 254, 1319-1326 (1990)
• A. J. Heinrich, C. P. Lutz, J. A. Gupta, D. M. Eigler. Molecule Cascades. Science 293, 1381-138 (2002)
• Nic Fleming. The Kavli Prize: Nanoscience Prize Explanatory Notes. The Norwegian Academy of Science and Letters (2010)

• Atom – Wiki Commons
• Xenon Sign – Wiki Commons
IBM Image – D.M. Eigler, E.K. Schweizer. Positioning single atoms with a scanning tunneling microscope. Nature 344, 524-526 (1990).
• Sliding Process – With license from, Nature 344, 524-525 (1991)
IBM ‘Spelling’ Process – With license from, Nature 344, 524-525 (1991)
• Quantum Corral Image – M.F. Crommie, C.P. Lutz, D.M. Eigler. Confinement of electrons to quantum corrals on a metal surface. Science 262, 218-220 (1993).

• Electronic Structure = information about how electrons behave and how they are organized around atoms and ions
• Van Der Waals = sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules
STROSCIO, J., & EIGLER, D. (1991). Atomic and Molecular Manipulation with the Scanning Tunneling Microscope Science, 254 (5036), 1319-1326 DOI: 10.1126/science.254.5036.1319