The Scanning Tunneling Microscope: Peering into Atomic Structure

What is a Scanning Tunneling Microscope

A scanning tunneling microscope (STM) is an instrument that allows scientists to visualize individual atoms on a surface. It operates by using a fine-tipped probe, called the needle or tip, which is brought close to the sample's surface. The STM measures the quantum mechanical tunneling current between the tip and the sample as they are scanned across each other.

The STM works based on the principle of quantum tunneling, where electrons can pass through a potential barrier even if their energy is lower than the height of the barrier.

How Tunnelling Current Works

As the tip approaches the sample surface, there exists a small probability that an electron from the tip will tunnel through the vacuum gap and enter the sample. This results in a current between the tip and the sample. The strength of this current is highly dependent on the distance between the tip and the surface, typically measured in angstroms (1 Å = 0.1 nm).

The exponential relationship between the tunnelling current and the distance can be described by the formula I ∝ exp(-αd), where I is the tunneling current, d is the distance between the tip and the sample, and α is a constant that depends on the properties of both the tip and the sample.

Feedback Loop for Constant Current

To maintain a consistent measurement, an STM uses a feedback loop to adjust the height of the tip relative to the surface. This ensures that the tunneling current remains constant during the scanning process. When the current deviates from its setpoint, the feedback system adjusts the position of the tip by moving it up or down until the desired current is restored.

This precise control allows the STM to map out the topography of surfaces with atomic resolution, providing detailed images that are crucial for understanding material properties at the nanoscale.

Atomic Lattice and Barrier Formula

The STM not only measures the tunneling current but also provides information about the atomic structure of the surface. By scanning the tip across the sample, scientists can create a map that shows the height variations corresponding to individual atoms or molecules.

The exact barrier formula used in STMs is derived from quantum mechanics and describes the probability of electron tunneling as a function of distance between the tip and the surface. This formula helps in understanding the electronic structure and bonding at the atomic scale.

Frequently asked questions

Try it live

Everything above runs in your browser — open Scanning Tunnelling Microscope: Tip Gap, Exponential Current and Atomic Lattice Scan and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.