Figure 1: Atomic resolution
image of a graphite surface

Variable Temperature Scanning Tunnelling Microscope (VT-STM)

A scanning tunnelling microscope (STM) uses quantum mechanical tunnelling of an electrical current between a sharp metal tip and a conducting surface to enable us to image the surface with atomic resolution. An overview of the principle of operation is given in the link at the bottom of the page.

NPL has developed an Ultra-high vacuum (<1^-10 mbar or <1^-8 Pa), variable temperature STM capable of working from 5 K to 300 K. The full details of the system are given in the document CryoSTM Spec ( PDF 572 KB)

nano-Silicon
man

Other images can be found in our Image Gallery ( PDF 590 KB)

As well as imaging surfaces we are able to perform what is known as Scanning Tunnelling Spectroscopy (STS), whereby the change in the tunnel current is measured, at a single location, as the voltage between the STM tip and the sample is varied. Often this is done at very low temperatures (<10 K) in order to reduce the thermal noise on the measurements.

Figure 2: STS curve of graphite

Doing STS on surfaces, and indeed on single atoms or molecules located on surfaces, allows us to probe the electrical (and other) characteristics of the individual atoms or molecules. An STS curve of Graphite is shown in Figure2.  STS is extremely powerful as it can give detailed information as to the nature of individual atoms.

By using different tip material, such as magnetic tips, we are able to explore the magnetic behaviour of single magnetic atoms. This is often known as Spin-Polarised STM (SP-STM) as the tunnel current depends strongly on the relative spin of the electrons between the tip and the surface.

NPL is studying nanomagnetism using several techniques, one of which is SP-STM.

Figure 3: 3D image of iron
nanostructures on tungsten

Nanomagnetism is becoming an increasingly important technological area especially for the IT industry and it is vital that quantitative measurements are made on the nano-scale. SP-STM is able to show the magnetic behaviour of these nanostructures and relate this to their physical dimensions. Figure 3 shows as 3D image of iron nanostructures on tungsten.

The STM is also able to manipulate single atoms or molecules on surfaces (click on the link at the bottom of the page for some examples of atom manipulations at IBM) allowing a 'bottom-up' approach for the construction and study of nano-devices.

Figure 4: C₆₀ manipulation

In Figure 4 is a sequence showing our single molecule manipulations: C₆₀ carbon 'balls' were 'picked up' from the substrate (graphite) one by one using an STM tip. Each C₆₀ molecule consists of 60 C-atoms (hence the name) and its diameter is about 1 nm.

Accurate and meaningful measurements at the nanoscale, where often quantum effects are important, is in its infancy and STM, and its associated techniques, is a powerful tool for the study of these new and emerging devices.