NanoCars - How to build and drive them

Let's get scientific again!
This post is the long ago promised and pending follow-up post to my winning contribution to #suesas-sciencechallenge:
The NanoCar Race!
In this post I will now, as promised, take a closer look at the technical & chemical-physical background of nano-cars. Of course I will have to use published data only. Let's get into this:

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• What is a nano-car?

A nano-car is any type of molecule whose motion can be controlled in a very precise manner but without physically touching the entity. As soon as touching is involved it can not anymore be considered a "car" but rather as a wheelbarrow-like feature. This and other crucial definitions were strictly defined before the sophisticated race took place in April this year: Rules of the Race

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• How does nano cars look and work?

The structural appearance of a nano-car molecule and its' underlying steering mechanism are closely linked. Basically, there are two major options to induce controlled movement into single molecules. These are based either on electrostatic repulsion/ attraction or onto energetic excitation. Fundamentally, the molecule must have a net dipole for electrostatic interactions from which, in combination with the surrounding electrostatic field, a preferential direction of movement results. For energetic excitation the molecule must have a preferential direction through its own molecular geometry.
In this image the winning dipolar racer of the Rice-Graz Team can be seen:

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• Which setup allows to work on such an incredible scale?

In order to work with individual molecules it is necessary to allow investigations and manipulations in extremely small dimensions, hence enormours magnification. A scanning tunnel electron microscope (short STM) permits such work. In principle this device consists of a very sharp metal tip, which is scanned over a smooth metal surface with a distinct small gap (s) between the two of them. This gap is in the Å to nm range!
A voltage bias is applied, what leads to a very small, but measureable electron flux between the sample surface and the tip. This is called the tunneling current I. The tunneling current depends exponentially on the distance between the two metal entities. To move the tip across the surface piezo-electric materials P are used. By applying high-voltages to those piezos their extensions can be tuned on the right scale to move the tip very precisely for only a few Nanometers or even Ångstroms in the desired direction. To allow movement in all three dimensions there are three piezos: Px, Py and Pz.

Although extremely smooth and pure metal surfaces with highly precise cutting directions are used for these investigations, these surfaces also show irregularities and defects (B). Usually there are step edges (A), ie steps between two differently high atomic layers. As the tip crosses such features the control unit retracts the tip by changing the high-voltage applied to the Pz-piezo. Therefore preventing a crash of the tip into the surface, which would damage the whole setup. Eventually the actions performed by the control unit to maintain a constant tunneling current, while scanning across a surface, can be used to produce images of the investigated surface by using pseudocolor scales. Of course to work properly and very precisely it is necessary to cool the whole set-up down to roughly 4 K (equals -269 °C). This is then called Low-Temperature-STM (LT-STM) and as molecules are lying on the sample, they will freeze to the surface and thermal motion stops. This again allows a more precise work!
With this setup one can image and work on a scale so small, that a STM allows seeing, touching, moving and even "hearing" (spectroscopy) single molecules!
The STM is therefore lovingly referred to as a potent extension of the eyes, hands and ears of physicists and chemists.

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• How can individual nano-car molecules be moved?

Kowing about the power of a STM it is pretty straightforward to understand this now:
Step 1: The molecules have to be deposited onto the surface.
Step 2: The STM scans the surface, to find intact molecules.
Step 3: The STM tip is brought into close vicinity to the nano-car molecule.
Step 4: Any specific type of interaction (physical or non-physical) is induced.

ad Step 1: Usually a STM is housed in an ultra-high vacuum chamber to preclude any undesired contamination of the surface. In this clean environment the desired molecules are first evaporated and second allowed to adsorb on the sample surface.

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• How was the winning dipolar racer molecule driven?

As already mentioned above (and as also the name implies) the movement of the dipolar racer was induced by the interaction of a net-dipole and an applied electric field as shown in the following image:

Since the two well-established manipulation methods pushing (via Pauli-Repulsion) and pulling (via Van-der-Waals Interaction) were considered as "touching the molecule" - and therefore prohibited - the co-op team of the University of Graz and University of Rice went for electrostatic interaction. This newly investigated process was then routinely repeated during the race to maneuver the molecule on the given race track.

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• How was the winning dipolar racer molecule synthesized?

In order to be able to express my burning love for organic synthetic-chemistry I will give you a brief insight into the chosen synthetic route for the production of the so far
most efficient nano-car ever:

In the first step the amino group is protected and its' sterical requirements are increased. Therefore in the second step a standard aromatic nitration in para-position to the amid functionality can be performed. In the following step the wheels and axels are attached to the chassis by a Sonogashira-Coupling reaction. Deprotection and conversion of the primary to the tertiary amine are the last steps.

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• How did the other participant molecules of the race look like and how did they move?

The following table provides this information:

An "inelastic" propulsion mechanism means that the STM tip is placed above or at least very close to the molecule and the applied bias is increased. Therefore the amount and the energy of the injected electrons is raised, which in turn allows energetic excitation of the nano-car molecule. As it is in an excited state is can overcome the diffusion barrier and therefore can start jumping and moving around.

I hope you have enjoyed this scientific excursion into the background of nano-cars.
If there are still questions, feel free to ask them in the comments!
For more scientific insights follow me!

Best,
mountain.phil28

References
General Information is published in a Nature Nanotechnology Paper
Images #1 and #3 are taken from the Nature Nanotechnology Paper
Image #2 is self-designed, owned by myself
Synthetic Route Image #4 is published in a Supplementary
Image #5 is taken from a Nature Reviews Materials Paper

Further Links
Official NanoCar Race - Homepage
NanoCar Race - YouTube Channel