STM

Scanning tunneling microscope (STM) is an instrument used to image surfaces with atomic-scale resolution. It operates by bringing a sharp, conductive tip very close to a sample surface and applying a bias voltage to allow electrons to tunnel between tip and sample. The tunneling current depends exponentially on the tip–sample separation, which enables high sensitivity to surface features at the atomic level.

In typical STM operation, a control loop adjusts the tip’s height to keep the tunneling current constant as the tip scans across the surface. The resulting vertical position data maps the topography of the surface with atomic resolution. In constant-height mode, the tip height is fixed while the current is measured to reveal variations in the local density of states or other electronic properties. Scanning tunneling spectroscopy (STS) extends STM by recording current versus voltage at a fixed tip position to probe the electronic structure of the material.

STM requires conductive samples and is commonly performed under ultra-high vacuum and at low temperatures to improve stability and resolution, though room-temperature and ambient-condition setups exist for certain applications. The technique has grown to include variants such as spin-polarized STM, which can probe magnetic properties, and combined modes that integrate STM with other characterization methods.

History and impact: the STM was invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zürich, leading to the Nobel Prize in Physics in 1986. It made it possible to image individual atoms and laid the groundwork for nanoscience and nanotechnology, enabling both fundamental studies of surface phenomena and experimental manipulation of atomic-scale structures.

Limitations include the need for clean, relatively flat conductive surfaces, sensitivity to vibrations and thermal drift, tip condition and geometry effects, and potential surface modification by the tip.