Nanocar Race Part 3: STM and AFM The Swiss Nanodragster has a size of about 1.5 nm (this is 100 000 times smaller than the diameter of a hair, diameter of a hair 0.15 mm compared to 1.5 nm). To see such a small molecule we need very precise tools. We use a special microscope to see the Nanocar. We use a Scanning Tunneling Microscope (STM) and Atomic Force Microscope (AFM) to image our Nanocar. STM and AFM are scanning probe techniques which means that we scan the sample with a sharp tip, because the resolution of optical microscopes is not good enough to see a single molecule. It is limited by the wavelength of the light. Our tip has the size of an atom. We scan the tip over the sample. If the tip is very close to the surface and a voltage is applied between tip and sample, there will be a small current because of the tunneling effect. In quantum mechanics it is possible for a small amount of electrons to walk through a wall! And we measure exactly these few electrons that pass the wall between the sample and the tip. In that way we don’t need to touch the sample which could for example destroy the molecule. Imagine a mini RC car would be our molecule. The minicar is approximately 10 cm long. Then we could say that we scan with a tip that is as huge as the Matterhorn over the minicar and try to get out an image. The tunneling current is strongly depending on the distance between tip and sample. You can only see it if the tip is very close. We use a a lot of electronics to create a feedback system that keeps the tunneling current constant. In this way we can control the distance between the tip and sample. Then we scan the tip line by line over the molecule. Since the molecule is higher than the surrounding substrate the distance between tip and molecule is lower and the current increases. The feedback system immediately reacts and retract the tip farer away from the molecule to keep the current constant. This movement is recorded by a computer. From this data we get a 3D image of the molecule. The Atomic Force Microscope (AFM) works in a different way. The atomic force is the force that acts between two atoms that come very close. In a simple picture you can think of the force between the first atom of the tip and the closest atom of the molecule. We use this force instead of the tunneling current to control the feedback system of the microscope. With the AFM you can see the chemical structure of the molecule and you can probe different properties of the molecule for example the charge distribution. Here in Basel you can find the first AFM ever built! This year it is the 30. anniversary of the AFM. Christoph Gerber, Professor in Basel, recently won the Kavli Price for the invention of the AFM and Ernst Meyer, our group leader, was the first PhD student who ever worked with an AFM.