Scanning tunneling microscope

An STM brings an electrically conductive, very sharp tip close to a conducting sample without letting the two touch. With a voltage applied, electrons can tunnel across the narrow gap, producing a current. The current is highly sensitive to the tip–sample distance and to the electronic states available in the junction. The tip is scanned across the surface while instruments record the signal. Since the tunnel current changes steeply with distance, small vertical changes can be detected.

Piezoelectric actuators move the tip or sample in controlled steps. In constant-current operation, a feedback system adjusts the vertical position of the tip to keep the current near a chosen value; recording that movement produces an image. In constant-height operation, the tip height is held approximately fixed and current variations are recorded, which requires a sufficiently stable, relatively flat surface. The resulting contrast is not simply a photograph of surface atoms: it depends on both surface geometry and electronic structure, as well as the measurement settings.

Gerd Binnig and Heinrich Rohrer developed the scanning tunneling microscope at IBM's Zurich Research Laboratory; IBM dates its introduction to 1981. Their design earned them half of the 1986 Nobel Prize in Physics. The STM opened ways to investigate surfaces at atomic scales and became important in surface science, materials research and semiconductor studies. With carefully controlled tip–sample interactions, researchers have also used it to reposition individual atoms, demonstrating that the instrument can do more than image a surface.

The STM relies on an electrical tunneling junction, so the sample must provide suitable conductivity at the measurement surface; it is not a general-purpose instrument for imaging arbitrary insulating materials. Stable measurements also require careful control of tip position and unwanted motion. The exact image depends on the surface, tip, bias voltage, current setpoint and operating environment. STM images therefore need interpretation using the measurement conditions and, where possible, complementary methods. Specialized approaches can operate in different environments, but the technique is especially associated with clean, stable surfaces and precise instrumentation.