- The Scanning Tunneling Microscope (STM) is a powerful analytical instrument that enables scientists to image and manipulate surfaces at the atomic level.
- Invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich, the STM was the first device to provide real-space images of individual atoms on a surface, marking a revolution in nanoscience and surface physics. For this groundbreaking achievement, Binnig and Rohrer were awarded the Nobel Prize in Physics in 1986. The STM operates based on the principles of quantum tunneling, making it not only a marvel of engineering but also a direct application of quantum mechanics.
- At the core of an STM is an extremely sharp conductive tip—often made of tungsten or platinum-iridium—that is brought within a few angstroms (less than a nanometer) of a conductive or semiconductive sample surface. When a voltage is applied between the tip and the surface, quantum tunneling occurs: electrons tunnel through the vacuum gap between the tip and the sample. This tunneling current is exquisitely sensitive to the distance between the tip and the surface—changing exponentially with separation—which allows the STM to detect variations in surface topography at the atomic scale.
- To create an image, the STM scans the tip across the surface in a raster pattern using piezoelectric actuators, which allow for precise control of tip movement in the x, y, and z directions. In constant-current mode, the height of the tip is adjusted continuously to maintain a steady tunneling current, and the vertical movement of the tip is recorded to generate a topographic map of the surface. Alternatively, in constant-height mode, the tip height remains fixed while the current is recorded, useful for studying very flat surfaces at high speeds.
- One of the most remarkable capabilities of STM is its atomic resolution—the ability to image individual atoms and even intra-molecular structures. This makes it an indispensable tool in fields such as surface science, nanotechnology, materials science, and condensed matter physics. Researchers can use STM to study surface defects, atomic arrangements, electronic states, and chemical bonding characteristics with unprecedented detail.
- Beyond imaging, STMs can also be used for atomic manipulation. Scientists have demonstrated the ability to move individual atoms across a surface by precisely controlling the tip-sample interaction, enabling the construction of atomic-scale structures such as quantum corrals and logic gates—an essential step toward the development of molecular electronics and quantum computing devices. Additionally, Scanning Tunneling Spectroscopy (STS), an extension of STM, allows measurement of the electronic density of states at specific locations on a surface, providing insights into local electronic properties.
- However, STM has certain limitations. It requires ultraclean, smooth, and often conductive surfaces, limiting its use on insulating materials unless special techniques are used. It also typically operates under ultra-high vacuum (UHV) and low-temperature conditions to reduce noise and thermal drift, although ambient and liquid-environment STMs have been developed for biological and chemical applications.
