Scanning tunneling microscope

The STM relies on the quantum mechanical phenomenon of tunneling. When a sharp metal tip is brought within a few angstroms of a conductive sample, and a bias voltage is applied, electrons tunnel through the vacuum gap, producing a current that is exponentially sensitive to the tip-sample distance.¹ This exponential sensitivity gives the STM its atomic resolution.

Two main imaging modes are used: constant-current mode, where the tip height is adjusted to keep the current constant, and constant-height mode, where the tip is scanned at a fixed height and the current variations are recorded. Constant-current mode is more common for rough surfaces, while constant-height mode allows faster scanning.

An STM consists of a piezoelectric scanner that moves the tip with sub-angstrom precision, a vibration isolation system, and a feedback loop to control the tip-sample distance.² The tip is typically made of tungsten or platinum-iridium, sharpened to a single atom at its apex.

Vibration isolation is critical; early STMs used magnetic levitation or stacked metal plates with springs to dampen vibrations. Modern instruments often use eddy current damping and acoustic enclosures.

STMs are used in surface science to study atomic structure, adsorption, and surface reactions. They have been instrumental in imaging silicon surfaces, metal surfaces, and even molecules such as carbon nanotubes and DNA.³

Beyond imaging, STMs can manipulate individual atoms and molecules, enabling the construction of quantum corrals and the study of single-molecule electronics. They also serve as a tool for nanolithography and for measuring local electronic properties via scanning tunneling spectroscopy (STS).

While the STM is famous for imaging, it has several lesser-known facets:

• The first STM was built from a single piece of piezoelectric material, and the initial images of the Si(111) 7x7 reconstruction were a surprise because the surface was thought to be disordered.
• STM can be used in liquids and at low temperatures, enabling studies of electrochemical interfaces and superconductors.
• Spin-polarized STM uses a magnetic tip to image magnetic domains with atomic resolution.
• Inelastic electron tunneling spectroscopy (IETS) with an STM can identify vibrational modes of individual molecules.
• The STM inspired the development of the atomic force microscope (AFM), which can image non-conductive surfaces.
• Early STM images were often distorted by thermal drift and tip artifacts, leading to the development of sophisticated image correction algorithms.

Gerd Binnig and Heinrich Rohrer developed the STM at IBM Zurich, building on earlier work on tunneling in solid-state physics. They shared the 1986 Nobel Prize in Physics with Ernst Ruska, who invented the electron microscope.⁴

Other key contributors include Don Eigler, who first manipulated individual atoms with an STM in 1990, spelling out "IBM" with xenon atoms, and later researchers who developed low-temperature STMs for studying quantum phenomena.

Tunneling current

The electric current that flows through a potential barrier due to quantum tunneling.

Piezoelectric scanner

A device that changes shape when a voltage is applied, used for precise positioning.

Constant-current mode

An STM imaging mode where the tip height is adjusted to maintain a constant tunneling current.

Constant-height mode

An STM imaging mode where the tip is kept at a constant height and current variations are recorded.

Scanning tunneling spectroscopy (STS)

A technique that measures the local density of electronic states as a function of energy.