The scanning tunneling microscope (STM) is a scientific instrument that allows observation and interaction with matter at an incredibly small scale. Developed in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zürich, the STM allowed scientists to “see” individual atoms for the first time. This breakthrough earned them the Nobel Prize in Physics in 1986 and laid the groundwork for nanotechnology and atomic-scale research. The STM’s capabilities have transformed our understanding of surface science, enabling insights into the arrangement and behavior of atoms and molecules.
How It Works
The STM operates on the principle of quantum tunneling, where electrons can pass through a potential energy barrier without classically overcoming it. In the STM, this barrier is the tiny vacuum gap between a sharp, conductive tip and a conductive sample surface. A bias voltage is applied between the tip and the sample, causing electrons to “tunnel” across this gap and creating a measurable electrical current, the tunneling current.
The setup involves a conducting tip, often made of tungsten or platinum-iridium, brought within a few angstroms (0.2 to 0.4 nanometers) of a conductive sample. The tunneling current is highly sensitive to the distance between the tip and the sample, decreasing exponentially as the distance increases. This sensitivity allows for atomic-scale resolution.
A piezoelectric scanner controls the tip’s position in three dimensions (x, y, and z axes). This scanner, made from materials that expand or contract in response to applied voltage, allows for angstrom-level control of the tip’s movement. The STM operates in one of two modes: constant current or constant height.
In constant current mode, a feedback loop adjusts the tip’s height to maintain a constant tunneling current as it scans, with vertical adjustments recorded to form an image. For flat surfaces, the constant height mode can be used, where the tip-sample distance is fixed, and variations in the tunneling current are measured to form the image. To prevent contamination and ensure stable measurements, the STM often operates in an ultra-high vacuum environment. Variants exist for use in air or liquids.
Visualizing the Atomic World
The STM does not produce “pictures” in the conventional sense, but generates detailed maps of a surface. These maps are either topographical, showing the surface’s physical contours, or electronic density maps, illustrating electron distribution. The STM’s ability to resolve features smaller than 0.1 nanometers, with a depth resolution of about 0.01 nanometers, allows scientists to distinguish individual atoms and molecules. This atomic resolution provides a direct view of the atomic landscape of materials.
Information from STM images includes surface topography, atomic arrangements, and defects. Scientists can also probe the electronic properties of materials, such as the local density of electronic states. This is achieved by techniques like scanning tunneling spectroscopy, where the tip’s position is held constant, and the bias voltage is varied to record changes in current, allowing reconstruction of local electronic states. The data collected from the tip’s movement or current variations are processed by a computer to create visual representations that resemble atomic landscapes, offering a direct glimpse into the atomic structure of a surface.
Diverse Scientific Applications
The STM has contributed across various scientific disciplines, particularly in materials science, nanoscience, and semiconductor research. In materials science, it is used to study crystal structures, surface reconstruction, and the growth of thin films, providing insights into atomic-scale properties of materials, including surface roughness and defects. For example, it has documented the arrangement of individual atoms on metal surfaces like gold, platinum, nickel, and copper. The STM has also been employed to examine the absorption and diffusion of different species, such as oxygen, on surfaces.
In nanoscience, the STM characterizes nanoparticles and nanowires, facilitating the generation of nanostructures like quantum corrals. Its application extends to semiconductor research, where it helps examine electronic devices at the nanoscale and study the surface structure of materials like graphene. In chemistry, the STM is valuable for studying molecular adsorption and reactions on surfaces, allowing researchers to investigate reaction mechanisms at the molecular level. While primarily used for conductive materials, it can also image certain biological structures, such as DNA or proteins, when placed on conductive substrates.
Building with Atoms
Beyond its imaging capabilities, the STM is also used as a tool for atomic manipulation. The STM tip can push, pull, or pick up individual atoms and molecules on a surface. This capability enables “bottom-up” nanofabrication, where structures are built atom by atom, offering control over matter at the atomic scale.
A notable example of this atomic manipulation is the creation of the IBM logo in 1989, where individual xenon atoms were arranged on a nickel surface. This technique involves controlling tip-sample interactions, often by adjusting the tip’s distance to increase interaction with the target atom, then moving the tip laterally to reposition the atom. This ability to rearrange matter at the atomic scale has implications for future advancements in nanotechnology, including the design of custom materials and the exploration of quantum phenomena.