# Scanning Tunnelling Microscope (STM)

• EIRE2003
In summary: Ok, so in summary, the STM is a spectroscopy tool that uses the tunnelling current to determine the density of states of a material.
EIRE2003
''The Scanning Tunnelling Microscope probes the density of states of a material''...

What does this actually mean?

The signal from an STM is proportional to the tunnelling current between the tip of the probe and the surface that is being scanned.
Now, tunnelling current is of course proportional to the tunnelling propability which in turn is determined by -among other things- the number of states that are allowed to tunnell for a specfic bias, i.e. it is proportional to the density of states.

EIRE2003 said:
''The Scanning Tunnelling Microscope probes the density of states of a material''...

What does this actually mean?

It means that the STM is doing "spectroscopy" rather than imaging. It scans something resembling an "I vs. V", at a particular location of a material, where I is the current, and V is the bias potential between the STM tip and the material. The density of states (assuming you know what this is), is proportional to the the derivative of this curve, i.e. dI/dV vs. V.

Zz.

Ok, am I right in suggesting that the 'density of states' is, how closely packed the energy levels in an atom are, or is it how closely packed the energy levels in the material as a whole? That is, the energy gap between the valence band and the conduction band?

Your first definition of density of states is the more correct one. The density of states determines how closely packed (in energy space) the energy levels in a quantum mechanical system are.

EIRE2003 said:
Ok, am I right in suggesting that the 'density of states' is, how closely packed the energy levels in an atom are, or is it how closely packed the energy levels in the material as a whole? That is, the energy gap between the valence band and the conduction band?

Er... DOS is dN/dE, where N is the number of states per unit volume.

You can't really consider the "energy levels in an atom" when you're doing a STM on a solid. I'm sure I've mentioned this many times. A solid doesn't behave the same way as isolated atoms. That's why "solid state physics" isn't the same field of study as "atomic/molecular physics". An atom's "DOS" is meaningless because an atom has discrete energy levels, and thus a dN/dE operation would be strange. The DOS you get from an STM of a solid, such as a superconductor, gives a continuous result!

Zz.

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## 1. What is a Scanning Tunnelling Microscope (STM)?

A Scanning Tunnelling Microscope (STM) is a scientific instrument used to image and manipulate materials at the atomic level. It works by scanning a sharp probe over the surface of a material, measuring the electrical current that flows between the probe and the material. This allows for the creation of high-resolution images of the material's surface, as well as the ability to manipulate individual atoms and molecules.

## 2. How does a Scanning Tunnelling Microscope (STM) work?

A Scanning Tunnelling Microscope (STM) works by using a sharp probe, usually made of tungsten, to scan over the surface of a material. The probe is positioned just a few nanometers above the surface and a small electrical bias is applied between the probe and the material. As the probe scans, it measures the flow of electrons (tunneling current) between the probe and the material. This information is then used to create an image of the material's surface.

## 3. What is the resolution of a Scanning Tunnelling Microscope (STM)?

The resolution of a Scanning Tunnelling Microscope (STM) is dependent on the size of the probe used. The smaller the probe, the higher the resolution. Currently, the best STMs have a resolution of around 0.1 nanometers, allowing for the visualization and manipulation of individual atoms and molecules.

## 4. What are the applications of a Scanning Tunnelling Microscope (STM)?

A Scanning Tunnelling Microscope (STM) has a wide range of applications in various fields of science, including materials science, nanotechnology, and biology. It is used to study the surface structure of materials, investigate the properties of nanoparticles, and observe biological molecules such as DNA and proteins at the atomic level. It also has potential applications in data storage, as it can be used to read and write information at the atomic scale.

## 5. What are the advantages of using a Scanning Tunnelling Microscope (STM)?

The main advantage of using a Scanning Tunnelling Microscope (STM) is its ability to image and manipulate materials at the atomic level. This allows for a deeper understanding of the properties and behavior of materials. Additionally, the images produced by an STM are of very high resolution, providing detailed information about the surface of a material. STMs also have the ability to operate in various environments, such as vacuum or liquid, making them versatile tools in scientific research.

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