Scanning tunneling microscope

Scanning tunnelling microscopy (STM) is a form of scanning probe microscopy capable of imaging surfaces with atomic-scale resolution. Developed in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zürich, the technique revolutionised surface science by enabling direct observation and manipulation of individual atoms. Their achievement was recognised with the Nobel Prize in Physics in 1986. STM exploits the quantum mechanical phenomenon of electron tunnelling to sense the surface structure and electronic properties of conductive or semiconductive materials with sub-nanometre precision.

Historical Development

The conceptual and technological breakthrough achieved by Binnig and Rohrer built on earlier developments in electron physics and surface imaging. By combining ultra-sharp metallic tips, piezoelectric positioning systems, and sophisticated electronics, they created the first instrument capable of resolving atomic lattices on clean, well-prepared crystal surfaces. Early STM instruments operated in ultrahigh-vacuum environments at cryogenic temperatures to ensure surface cleanliness and mechanical stability. Subsequent generations of STMs expanded versatility to allow studies in air, liquids, magnetic fields, and across a broad temperature range extending beyond 1000 °C. This flexibility has allowed STM to become an essential tool in nanoscience, condensed-matter physics, surface chemistry, and catalysis research.

Principle of Operation

STM is based on quantum tunnelling, a quantum mechanical behaviour whereby electrons pass through a potential barrier that they cannot cross classically. When an atomically sharp metallic tip is brought within approximately 0.4–0.7 nm of a sample surface and a bias voltage is applied, electrons tunnel between the tip and the sample, generating a current highly sensitive to the separation distance. The tunnelling current depends exponentially on the tip–sample spacing and is also influenced by the local density of electronic states (LDOS) within the sample.
By raster scanning the tip across the surface and monitoring variations in tunnelling current, STM constructs an image that reflects a combination of surface topography and LDOS distribution. Because of the extremely strong distance dependence, STM routinely achieves lateral resolutions finer than 0.1 nm and vertical resolutions approaching 0.01 nm (10 pm), making atomic-scale imaging and manipulation possible.
A related technique, scanning tunnelling spectroscopy (STS), involves fixing the tip position and sweeping the bias voltage while measuring changes in tunnelling current. This produces a curve proportional to the LDOS as a function of electron energy and yields site-specific information on the electronic structure. STS can be used in high magnetic fields or in the presence of impurities to study electronic interactions, quasiparticle behaviour, and local spectroscopic signatures.

Imaging Modes and Data Acquisition

STM data are generally acquired using one of two imaging modes:

• Constant-current mode: A feedback system continuously adjusts the vertical (z-axis) position of the tip to maintain a fixed tunnelling current. The z-position required to preserve this current is recorded as the topographic signal. This mode is slower but stable, well-suited to uneven surfaces, and provides images that reflect both geometric height variations and LDOS differences.
• Constant-height mode: The tip height remains fixed during scanning, and variations in tunnelling current are recorded directly. This mode allows faster data collection but is usable only on atomically flat or very clean surfaces because the tip could collide with protrusions.

In both modes, the scanned area is typically a matrix ranging from 128 × 128 to over 1024 × 1024 points. The raw images are greyscale and often colour-enhanced during post-processing to highlight atomic sites, defects, or electronic modulations. High-speed STMs can acquire images at video rates, enabling the study of dynamic processes such as surface diffusion, adsorption, and reaction mechanisms.

Instrumentation and Components

A typical STM comprises several key components:

• Scanning tip: Usually made of tungsten or platinum–iridium wire, prepared by electrochemical etching or mechanical shearing to produce an atomically sharp apex. Tip quality critically determines resolution. During operation, tips may be conditioned in situ by voltage pulsing or by deliberately attaching surface atoms to refine the apex.
• Piezoelectric scanner: A tubular piezoelectric element, often made of lead zirconate titanate, controls precise movement in x, y, and z directions. Voltages applied to metallised quadrants of the tube induce picometre-scale expansions or contractions, allowing highly controlled tip positioning. Calibration tables are used to correct nonlinearities and crosstalk among axes.
• Coarse approach mechanism: Moves the tip towards the sample over larger distances before tunnelling begins. Coarse control is usually accomplished mechanically and monitored visually.
• Electronics and control systems: Include tunnelling current amplifiers, bias voltage sources, feedback controllers, and computer systems that synchronise scanning and data acquisition. Because tunnelling currents are typically in the sub-nanoampere range, amplification must occur close to the scanner.
• Vibration isolation: Essential due to extreme sensitivity of tunnelling currents to distance fluctuations. Isolation can be achieved using mechanical springs, gas springs, magnetic levitation, eddy-current damping, or dedicated acoustically and electromagnetically shielded rooms. High-stability STMs for spectroscopy often operate within anechoic, vibration-isolated environments.

Experimental Procedure

STM operation usually begins by preparing a clean, atomically defined sample surface, often a Miller-index-specified single crystal such as gold (e.g., Au(111)). The tip is positioned coarsely, then piezoelectrically driven towards the surface until tunnelling current is detected. Once tunnelling is established, scanning proceeds according to the chosen imaging mode. Parameters such as bias voltage, set-point current, and scan speed are adjusted to capture features ranging from atomic steps and surface reconstructions to adsorbed molecules.
STS measurements may be taken at specific surface sites by stabilising the tip, sweeping the bias voltage, and recording differential conductance. Such data reveal LDOS variations for defects, impurity atoms, or electronic phases.

Applications and Capabilities

STM has broad scientific and technological applications:

• Atomic manipulation: Individual atoms can be repositioned, enabling engineered nanostructures and demonstrations of atomic-scale logic or art.
• Surface science: Used to study surface reconstructions, adsorption processes, catalysis, and molecular self-assembly.
• Condensed-matter physics: Vital for investigating superconductivity, charge-density waves, spin textures, and quantum confinement.
• Nanotechnology and materials engineering: Supports design and characterisation of low-dimensional materials, thin films, and nanoscale devices.
• Chemical analysis: Through STS, STM can profile local electronic behaviour associated with chemical bonding, impurities, or reaction intermediates.