scanning tunneling microscope

scanning tunneling microscope. A scanning tunneling microscope is a powerful instrument used for imaging surfaces at the atomic level. It operates based on the quantum mechanical phenomenon of electron tunneling, where a sharp metallic tip is brought extremely close to a conductive sample. By maintaining a constant tunneling current through precise feedback, the instrument can map the topography of the surface with extraordinary resolution. This capability has revolutionized fields such as surface science, nanotechnology, and materials science.

Operating principle

The fundamental operating principle relies on the quantum tunneling effect. When a sharp conducting tip, typically made of tungsten or platinum-iridium, is positioned within a nanometer of a conductive sample, a bias voltage applied between them allows electrons to tunnel through the vacuum gap. The probability of this tunneling is exponentially dependent on the distance, making the measured current exquisitely sensitive to the tip-sample separation. This relationship is described by the work of physicists like Max Born and Robert Oppenheimer on quantum mechanics. By scanning the tip across the surface and using a feedback loop to maintain a constant current, the vertical movements of the tip directly trace the atomic-scale contours of the sample, effectively creating a topographical map.

Instrumentation

The core instrumentation requires exceptional mechanical stability and precision control. The heart of the system is a piezoelectric actuator, often made from materials like lead zirconate titanate, which can move the tip with sub-angstrom precision in three dimensions. The tip itself is mounted on a coarse approach mechanism, such as a louse walker or inertial slider, to initially bring it close to the sample. Vibration isolation is critical and is achieved using systems like spring suspension and eddy current damping. The entire apparatus is typically housed in an ultra-high vacuum chamber to prevent contamination, a standard also used in techniques like molecular beam epitaxy. Control electronics, including a high-gain feedback amplifier and computer interface, complete the system, with companies like Digital Instruments pioneering commercial versions.

Modes of operation

There are two primary modes of imaging. In constant current mode, the feedback loop adjusts the tip height to keep the tunneling current unchanged as it scans; the recorded height variations produce the standard topographical image. In constant height mode, the tip is held at a fixed vertical position while the changing tunneling current is recorded, allowing for faster scanning on atomically flat surfaces. Beyond imaging, the STM can operate in spectroscopic modes. Scanning tunneling spectroscopy involves modulating the bias voltage at a fixed location to measure the local density of electronic states, providing information akin to that from angle-resolved photoemission spectroscopy. It can also be used for atom manipulation, a technique famously demonstrated by Donald Eigler at IBM to spell "IBM" with xenon atoms on a nickel surface.

Applications

The applications of STM are vast and interdisciplinary. In fundamental physics, it has been indispensable for studying phenomena like charge density waves and the properties of high-temperature superconductors such as YBCO. In chemistry, it allows the direct observation of molecular adsorption and reactions on surfaces, complementing data from X-ray photoelectron spectroscopy. In materials science, it is used to characterize thin films, carbon nanotubes, and graphene. The field of nanotechnology heavily relies on STM for the characterization and manipulation of nanostructures. It has also been crucial in the development of spintronics and the study of topological insulators. Furthermore, it serves as a foundational tool in facilities like the National Institute of Standards and Technology.

History and development

The scanning tunneling microscope was invented in 1981 by Gerd Binnig and Heinrich Rohrer at the IBM Zurich Research Laboratory in Rüschlikon, Switzerland. Their work built upon earlier concepts in field emission and the theoretical understanding of tunneling. For this groundbreaking invention, which opened a new window into the atomic world, they were awarded the Nobel Prize in Physics in 1986, sharing it with Ernst Ruska, the inventor of the electron microscope. The early 1990s saw rapid commercialization and adaptation of the technology. Subsequent developments led to related techniques like the atomic force microscope, extending high-resolution imaging to non-conductive samples. The ongoing evolution of STM continues to push the boundaries of nanoscience, enabling discoveries in domains from quantum computing to catalysis.

Category:Microscopes Category:Scientific instruments Category:Nanotechnology