How Can you See What you Want to Make

The motors of biology and the transistors of microelectronics are invisible. Using the tip of a Scanning Tunneling Microscope (STM) as an assembly tool, allows the product to be imaged at the same time. In principle this is a way of making some kinds of nanostructures.

Figure 3.8, after [29] shows steps in the assembly of a ring of iron atoms on a 111 atomically clean surface of copper. In this case the iron atoms are not chemically bonded to the copper, but remain in fixed positions because the equilibrium thermal energy is extremely low in the ultra-high-vacuum cryogenic environment. The Fe atoms actually rest in fixed positions between nearest-neighbor Cu atoms on the 111 Cu surface. The tip of the STM is used to nudge the atoms gently from one of these depressions to a neighboring one, thus assembling the ring. The tip is not used to carry an Fe atom: if it were, the property of the tip for also providing an image would be seriously disturbed.

The interaction of the tip to move the Fe is likely to involve the divergent electric field from the tip, which can induce an electric dipole moment, p=aE, (3.7)

in the electron cloud of the atom. There is then a dipole interaction energy,

Because the £ field is stronger close to the tip, the dipole is pulled closer to the tip:

The strength of the force depends on the electric field approximately E = V/d, where V is the tip bias voltage and d is its spacing above the surface. Both Vand d can be adjusted through the controls of the STM. With the upward force suitably adjusted, it is found that the Fe atom will follow a horizontal displacement of the tip, thus moving the atom. Other forces may contribute here, possibly related to the tunneling current.

This situation therefore does allow a variable force of attraction between the tip and an atom. However, if the atom actually jumps onto the surface of the tip, then further control of that atom is lost.

This situation is a prototype for the imagined molecular assembler tip. It should be realized that STM tips in practice have radii very much larger than an atomic radius. So there is no way that a carried atom could be placed into a recessed position, because access would be blocked by the large radius of the tip. (Tips of smaller radius are found to vibrate excessively.)

Incidentally, the circular ripples, peaking at the center of the circle of Fe atoms are evidence for the wave nature of the electrons in the (111) surface of the Cu sam-

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Figure 3.8 Assembling a ring of 48 Fe atoms on a (111) Cu surface with an STM [29]. The diameter of the ring is 14.3 nm

Figure 3.8 Assembling a ring of 48 Fe atoms on a (111) Cu surface with an STM [29]. The diameter of the ring is 14.3 nm pie. These standing waves result from reflection of the electron waves from the barrier represented by the row of Fe atoms. This situation resembles the reflection of water waves from a solid surface such as a pier or the edge of a canal. The researchers [29] found that the spacing of the crests of the waves was in agreement with the nanophysics of electron waves which will be taken up in Chapter 4. From the nano-physics point of view these ripples represent electrons, acting independently, caught in a two-dimensional circular potential well.

A second example of assembly of a molecular scale object using an STM tip is shown in Figure 3.8 ([30] after Hopkinson, Lutz and Eigler). Here is a grouping of 8cesium and 8iodine atoms which have formed a molecule on the (111) surface of copper. It appears that the strong ionic bond of Cs and I has dominated the arrangement, illustrating that an STM tip may not dictate the individual locations of atoms in a structure that is being assembled.

Figure 3.9 Cesium and iodine on Cu 111 [30] This pattern represents a molecule on the copper surface which contains eight cesium and eight iodine atoms. This image illustrates that the assembled atoms may choose their own structure, beyond control of the tip, in this case driven by the strong ionic bond

In this situation one can speculate that the strongest interactions are between Cs and I ions, leading to eight molecules. These molecules are polar and probably arrange themselves in a structure dominated by the dipole-dipole forces between the Csl molecules.

In this case it not clear how strongly the molecules are attached to the copper surface.

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