4.1 Nanocars and the development of molecular manufacturing  (Page 6/7)

While the currently published nanocar is devoid of added functionality, the Tour Group is researching additionsto the chassis to facilitate transport and motility. Nanotrucks are a popular idea and simply refers to a modified nanocar that can carryobjects. A ‘bed’ could be synthesized into the chassis to carry substances ranging from metal ions to oxygen atoms. Of course, theobject being carried would most likely be specific to the nanotruck synthesized and the particular chemistry of its ‘bed’. Furthermore,bonding of a metal ion to the chassis of a nanocar would allow for interactions with an electric field, providing a mechanism forcontrolled motion. Along this vein, the entire chassis of the nanocar could be designed to maintain a dipole moment creating a favoredorientation in the presence of an electric field. Another form of motility that is being pursued is the addition of a light-drivensingle-directional molecular rotor. This type of rotor would ratchet forward through four isomeric states when stimulated by photons. TheTour group is looking to append such a motor to their chassis to create deliberate and controlled motion. Nanomachines that utilizethis deliberate and controlled motion will be the next milestone in molecular manufacturing.

The last of the three components of the nanocar to explore is the surface it operates on. Surface chemistryplays an undeniable role in the functionality of the nanocar, and must be taken into account in designing the system. First, a surface mustbe found where rolling, not sliding, dynamics are likely. Secondly, a surface must be chosen on which observations of the molecule are clearand measurable.

Gold was chosen as the operation surface because it was theorized to be optimal for the aforementionedconsiderations. As it turns out, gold does accommodate these factors well. It allows for a special type of chemical interaction betweenitself and C60, known as charge transfer bonding. While we will not go into the details of charge transfer bonding, it will be sufficient forthis textbook to consider the molecular interactions between the C60 and the gold surface as a type of van der walls forces or weakmolecular interaction. This type of interaction is optimal for two reasons. One, if the bond strengths were any weaker the C60 moleculewould simply slide like a bearing, as is the case with the C60 on graphite sheets. If the bond strengths were any stronger, the C60molecule would not move at all. Therefore, the appropriate strength of molecular interaction between the wheel and the surface must be foundto allow for rolling motion. Second, this interaction is temperature dependent and can be adjusted to control motion. Temperaturecontrolled bonding interactions allow for the observers to cool the system down to where the bond strengths are more effective to give theSTM time to resolve an image. Likewise, the system can be heated to allow for movement. Of course there is a range of possibletemperatures that the system is constrained by, namely the temperature of decomposition of the nanocar. Therefore, the surface must be ableto regulate motion while remaining within the available temperature range of the stable molecule. Each of these factors contributes to theease with which a molecule’s motion can be observed. In the next section, we focus on this subject as we discuss how the Tour Groupobserved nanocar 1.