Scanning Probe Microscopy

Summary

Scanning probe microscopy is a methodology that allows direct observation of structures and properties at the atomic and molecular scales. The techniques are applicable to a wide variety of purposes, providing information that is otherwise inaccessible. Scanning probe microscopy is particularly appropriate to nanotechnology, permitting the direct construction of nanoscale objects atom-by-atom.

Definition and Basic Principles

Scanning probe microscopy is a methodology that interfaces the macroscale of human observation (1–10⁻² meters) to the atomic scale of the physical world, providing direct observation of features in the range of 100 micrometers to 10 picometers (10⁻⁴−10⁻¹¹ m). Though the upper resolution range for scanning probe microscopy is well within the range of other methods of microscopy, it is the lower range that is of most interest because it is at this range that direct observation of atomic and molecular-scale properties is possible.

The basic principles of scanning probe microscopy are founded on the quantum mechanical properties of atoms. The methods measure electronic properties (current, voltage, or “atomic force”) to observe the nature of surfaces and surface phenomena.

Quantum mechanics describes and defines the behavior of electrons in atoms. One of the rules of quantum behavior is that electrons are constrained to specific locales known as orbitals within the structure of an atom and are not allowed to exist at the boundaries of those locales. However, an observable property called quantum mechanical tunneling occurs, which permits electrons to move from one locale to another across the orbital boundaries. In scanning probe microscopy, the miniscule electrical current is measured due to quantum mechanical tunneling between the atoms at a surface and the atoms at the tip of an atomic-scale probe, which is a function of their relative positions. This provides a corresponding atomic-scale map of the surface structure.

Background and History

The scanning tunneling microscope (STM) was invented in 1982 by German physicist Gerd Binnig and Swiss physicist Heinrich Rohrer. Their device could be conceived of as a sort of quantum mechanical phonograph, in which an exceedingly sharp metallic needle scans a surface in a manner similar to the way a phonograph needle scans the groove of a vinyl phonograph record. The needle would ideally taper down to a single atom at the point, enabling atom-to-atom interaction at the surface. Sensitive digital-electronic measurement devices would measure the electronic tunneling current between the tip and the surface. The devices would then relay that data directly to a computer that would then correlate the values according to the relative dimensions and spatial relationships of the probe tip and the surface. The result would be displayed as an image having a resolution of atomic scale features.

In 1986, Binnig, along with American electrical engineer Calvin Quate and Swiss physicist Christoph Gerber, introduced the scanning force microscope (SFM), which is also known as the atomic force microscope, or AFM. This microscope was a variation on the STM that maintained a constant force between the scanning tip and the surface. This allowed any surface to be scanned, whereas the STM could only be used with electrically conductive surfaces. More recently, scanning near-field optical microscopy (SNOM) was developed, using measurement of short-range components of electromagnetic fields (a very small light source) between tip and surface to produce the equivalent of a photographic representation of the surface features.

How It Works

To appreciate the operation of scanning probe microscopy, it is necessary to understand the scale on which it operates. The unaided human eye can discern detail as small as approximately 0.1 millimeter. Optical microscopes can extend this to a resolution of about 0.0001 meter. Scanning electron microscopes can typically produce images with a resolution as fine as 10 micrometers (0.00001 meter). This is the range at which scanning probe microscopes only begin to work, and they typically provide information with a resolution of as little as 10 picometers (0.00000000001 meter).

At this scale, the operation is in the realm of quantum mechanical physics rather than classical physics, and the effects associated with that scale are very different from those that occur on a larger scale. The most important difference lies in what is meant by the word “surface.”

Quantum Mechanical Physics. On scales that are significantly larger than atomic and molecular diameters, a “surface” is solid matter, analogous to a smooth tabletop. At the atomic scale of quantum mechanics, however, there is no such thing as a hard surface in that sense.

Quantum mechanics describes the structure of atoms as a very small, dense nucleus of massive protons and neutrons, surrounded by a cloud of electrons that is 100,000 times greater in diameter than the nucleus. The electron cloud is therefore very diffused. The electrons in a neutral atom are equal in number to the protons contained in the nucleus, and are confined to specific three-dimensional regions, called orbitals, around the nucleus. and are allowed to have only very specific energies according to the orbitals they occupy. At this scale of operation, a scanning probe microscope measures the electromagnetic interaction of the electron clouds in the atoms of the probe tip and atoms of the surface being scanned.

Scanning Tunneling Microscopes (STMs). The STM operates by moving the atoms-wide point of the scanning tip across a metallic, and therefore electrically conducting, surface at a distance of less than one nanometer (10⁻⁹ meter). The device measures the magnitude of the “tunneling current” that arises between the probe tip and the surface atoms. This current is exceedingly small, and its measurement requires extremely sensitive digital sampling and amplification electronics, and computers to process the measured data according to the relative geometries of the tip and the surface. As the probe scans, the angle between the tip and the atomic surface changes, as does the distance between them. The tunneling current measurement thus has three-dimensional vector-field properties, and because the probe tip maintains a constant orientation, variations in the tunneling current are presumably caused by the three-dimensional shape of the atoms being scanned.

Atomic Force Microscopes (AFMs). The AFM operates in essentially the same manner as the STM, except that its function maintains a constant measured electrical force between the probe tip and the scanned atomic surface. In this function, the probe tip follows the shape of the atomic surfaces directly, rather than measuring a property difference that changes according to the shape of the surface. Several different modes of operation are available within this context, such as constant contact, noncontact, intermittent contact, lateral force, magnetic force, and thermal scanning. Each mode provides a different type of information about the surface atoms.

Scanning Near-Field Optical Microscopes (SNFOMs). The SNFOMs use an extremely small-point light source as the probe, rather than a physical tip. Measurement of the effect that the surface has on the light provides the image data. The technique can employ a broad range of wavelengths to investigate different properties of the atoms being scanned.

Applications and Products

The field of scanning probe microscopy is a high-technology research practice. Its direct applications are limited to the analytical study of surface phenomena and structures. The nature of the techniques of scanning probe microscopy makes it the method of choice for examination and study of surfaces that are not amenable to any other means of close examination, especially those of certain biological materials. Scanning probe microscopy, with its ability to probe and manipulate single atoms and so to form molecule-sized structures, is invaluable in chemistry and in the development of nanotechnology.

Surface Chemistry. Chemical reactions take place at the level of the outermost electronic orbitals of atoms, or at the electronic surface of the atoms involved. In catalyst-mediated reactions, the interaction among chemical species takes place on the surface of the catalyst material, where the atoms of the reactants interact electronically with the atoms of the catalytic material. This lowers the energy barriers that must be overcome for the reaction to occur, facilitating the desired reaction.

Scanning probe microscopy has a spectroscopy mode that directly measures electron energies in single atoms and of single atomic bonds between atoms in a molecule. The methodology provides a better understanding of the chemistry at surfaces, contributing to the development of new and improved chemical processes. Perhaps the most important aspect of this is the enhanced knowledge of the mechanisms of surface phenomena such as oxidation and chemical corrosion.

Electronic Materials and Integrated Circuits. Technology enables the construction of several million transistors on the surface of the small silicon chip that is the central processing unit (CPU) of a computer. Scanning probe microscopy enables the physical study of such structures in minute detail. It also enables the construction and study of transistor structures that are orders of magnitude smaller. The research in this field examines possible ways to construct integrated circuits and computers that exceed existing capabilities of production.

The available methods of production of integrated circuits have essentially reached the physical limit of their capabilities to construct viable semiconductor transistors. Atomic force microscopy is being used to investigate the construction of transistor-like structures based on quantum dots rather than on semiconductor junctions.

Another area of research is the construction of molecule-sized transistors made from graphene or carbon nanotubes and other materials. These technologies, when fully developed, will completely change the nature of computing by enabling the construction of a quantum computer, a device that could carry out in seconds calculations that existing computers would require possibly billions of years to complete.

Data storage would also be revolutionized by these innovations. In the 2010s, data storage reached capacities measured in terabits per square centimeter, with technological advances upping that capacity to tens of terabits by the 2020s. The ultimate binary data storage density would have each bit stored in the space of one atom, a density that can be envisioned only with scanning probe microscope technology.

Nanostructures and Nanodesign. Nanotechnology works at the nanometer scale of 10⁻⁹ meters. To appreciate this scale, imagine the length of one millimeter divided into one million segments, each of which would be one nanometer. The concept of nanotechnology is to produce physical machines constructed to that scale. Because scanning probe microscopy can manipulate single atoms, it can be used to construct nanoscale and even picoscale physical mechanisms. The latter are essentially individual molecules whose physical structures imitate those of much larger devices and mechanisms, such as gears.

In 2011, researchers at Tufts University reported the first successful construction of a working electric motor consisting of a single molecule using low-temperature scanning tunneling microscopy. As can be imagined, this is a complex field of research, because quantum effects play a significant role in the interoperability of such small devices.

One application of scanning probe microscopy that is of immediate importance is the study of friction and abrasion at the atomic level, which is where those processes take place. Atomic force microscopy can be used to scratch a material's surface, providing detailed information about how friction and abrasion actually work and what might be done to lessen or prevent those effects.

Biological Studies.Scanning electron microscopy (SEM) has been the workhorse of biological research since its invention, providing detailed images of minuscule biological structures. However, the technology has some practical limits because of the principles on which it functions. Many biological materials that are of interest cannot be studied in detail using SEM, but are amenable to study using scanning probe microscopy. The methods are useful in measuring the forces that exist among functional groups in biological and organic chemical structures. Scanning probe microscopy is frequently used to image biological materials such as cellular systems and internal cell structures, as well as biomolecules.

Social Context and Future Prospects

The efficiency and speed of scanning probe microscopes continue to improve as new tips are invented or as new mechanical systems, such as the microelectromechanical system, are integrated into the design. One such advancement occured in 2024, when scientists achieved precise control over the chirality of individual molecules through structural isomerization by employing low-temperature scanning tunneling microscopy and density functional theory calculations. This groundbreaking approach in nanochemistry advanced the meticulous fabrication of molecular systems molecule by molecule.

Despite important advancements, scanning probe microscopy is a field that will have very little direct social context because of the extremely small scale of its subject matter. However, the secondary effects of the knowledge and technology derived from research and development in this field could have a large social impact, primarily because of the economic benefits from the control of friction and from new technologies for integrated circuits and magnetic memory media. Any real predictions for the future prospects of the field of scanning probe microscopy are entirely speculative.

Bibliography

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