Difference Between AFM and STM


Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) are two very efficient techniques for examining materials at the atomic and molecular levels. AFM works on the interaction forces between the tip and the sample surface. STM is a sort of microscopy that scans the surface of a sample using a conductive tip.

While AFM and STM have certain commonalities, they also have some substantial differences. Read this article to find out more about AFM and STM and how they are different from each other.

What is AFM?

AFM works on the interaction forces between the tip and the sample surface. The sharp tip of the cantilever is brought close to the sample surface, where it interacts with surface forces such as van der Waals forces, electrostatic forces, and magnetic forces. The cantilever's deflection as a result of these forces is measured using a laser beam reflected off the rear of the cantilever onto a position-sensitive photodetector.

AFM can produce a three-dimensional image of the sample surface with sub- nanometer resolution. The topography of the sample surface is mapped by scanning the tip in a raster pattern over the surface and measuring the deflection of the cantilever at each position. The collected data is utilised to generate a height map of the sample surface, revealing information about its shape, roughness, and surface features.

In addition to topographical information, AFM can provide other types of information about the sample surface. AFM can be used to map chemical properties such as the presence of certain functional groups on the surface by utilizing a functionalized tip. It can also measure the mechanical properties of the sample surface, such as elasticity, adhesion, and friction.

What is STM?

STM is a sort of microscopy that scans the surface of a sample using a conductive tip. Due to the quantum tunneling phenomenon, when the tip is brought very close to the sample surface, a small electric current can travel between the tip and the surface. The topography and other features of the sample surface can be determined by measuring this current.

STM works on the quantum mechanical tunneling effect, which states that electrons can pass through a potential barrier even if they lack the energy to overcome it. The STM tip is brought extremely close to the surface of the sample, and a small bias voltage is generated between the tip and the surface. This forms a potential barrier between the tip and the surface, through which electrons can tunnel if the distance between the tip and the surface is narrow enough.

A sensitive amplifier measures the current that passes between the tip and the surface as a result of the tunneling effect. The STM can produce an atomic- resolution three-dimensional image of the sample surface. The topography of the sample surface is mapped by rastering the tip over the surface and measuring the tunneling current at each position. The collected data is used to generate a height map of the sample surface, revealing information about its shape, roughness, and surface features.

In addition to topographical information, STM can provide information about the electrical properties of the sample surface. The local density of states (LDOS) of the sample surface can be estimated by measuring the tunnelling current as a function of the applied voltage. This can provide information regarding the sample surface's electronic structure and properties, such as the presence of electronic states around the Fermi level.

STM is a strong tool for investigating the physical and electrical properties of materials at the atomic and molecular levels. Its ability to provide atomic resolution and quantify the sample surface's local density of states makes it an effective tool in many fields of research.

Difference between AFM and STM

The following table highlights the major differences between AFM and STM −

Characteristics

AFM

STM

Principle

The AFM works on the interaction forces between the tip and the sample surface.

STM is a sort of microscopy that scans the surface of a sample using a conductive tip.

Stands for

Atomic Force Microscopy (AFM)

Scanning Tunneling Microscopy (STM)

Imaging Mode

Non-contact, tapping, and lateral force modes

Constant-height and constant-current modes

Imaging Capabilities

AFM can image both conductive and non- conductive samples.

STM only images conductive samples

Imaging Resolution

Sub-nanometer resolution in all modes

Atomic resolution

Sample Preparation

AFM requires minimal sample preparation.

STM samples must be conductive and clean.

Applications

It is used in materials science, biology, and chemistry.

It is used in materials science and physics.

Limitations

It is limited to imaging at room temperature and in air or liquid environments.

It is limited to conductive samples and requires a conductive substrate.

Conclusion

In conclusion, AFM can offer data on a sample's physical and mechanical properties, such as height, roughness, and elasticity. STM can provide data on a sample's electronic properties, such as conductivity and electronic structure.

The decision between the AFM and STM will be based on the properties of the sample and the type of information required.

Updated on: 14-Apr-2023

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