Background: Scanning Tunneling Microscopy

A large part of QNS’s research is focused on scanning tunneling microscopy (STM). The STM is an amazing research tool because it combines the following capabilities in one tool:

1. Imaging of surface and atoms and molecules on surfaces with atomic-scale spatial resolution.
2. Measure energy-resolved (spectroscopic) information on single atoms and molecules.
3. Move atoms and molecules on surfaces and build nanostructures with exact control over the position of atoms.

Figure 1 shows an example of the fantastic imaging capabilities of STM. Here we were working on iron atoms on a two-layer thick film of insulating MgO, which was grown on a silver substrate. Each one of the mountains is a single iron atom, while each of the small bumps in a regular square pattern is a single oxygen atom of the top layer of MgO. This image was obtained with a very special tip in order to give this high spatial resolution.

Figure 1: Two iron atoms sitting on a surface of a thin film of magnesium-oxide (MgO). A special tip, functionalized with hydrogen, was used to obtain atomic resolution of the oxygen atoms of the MgO surface layer. Image size: (4 nm)2, image obtained with our “Alice” STM system.

Figure 2 illustrates the spectroscopic abilities of STM. Here we are looking at the spin properties of a single iron atom sitting on MgO. The conductance is flat around zero bias but has a step-wise change at +/-14mV. These steps correspond to spin excitation, where the tunneling electrons kick the Fe atom’s spin out of the ground state and into an excited state. Single-atom and single-molecule spectroscopy is a bread and butter type measurement at QNS.
Figure 2: Tunneling spectrum with the tip parked on Fe on MgO with a spin-polarized tip. Two steps can be seen at +/- 14 mV, which correspond to spin excitations of the Fe atom. Measured at 1.6 K and a magnetic field of 0.8 T with our “Eve” STM.