Scanning Tunneling Microscopy And Atomic Force Microscopy

Type of physical science: Condensed matter physics

Field of study: Surfaces

Scanning tunneling microscopes and the atomic force microscopes are part of a generation of apparatuses capable of imaging and manipulating matter at an atomic scale.

Overview

The scanning tunneling microscope (STM) and the atomic force microscope (AFM) are the first among a generation of apparatuses called scanning probe microscopes (SPMs), capable of visualizing and manipulating matter at an atomic scale. The first to be developed, the STM, is the fruit of collaborative effort between Heinrich Rohrer and Gerd Binnig of the International Business Machines Corporation (IBM) at its Zurich research laboratory. The AFM was the result of another collaborative effort, this time between Binnig and Christoph Gerber from IBM and Calvin F. Quate from Stanford University. As suggested by the name, scanning probe microscopes are equipped with a probe that is scanned back and forth in a raster fashion across the surface of the specimen. The differences between the various SPMs stem from the type of interactions that is allowed to occur between the sample and the probe. An image of the surface of a specimen is obtained through proper analysis of this interaction. All SPMs are equipped with a probe, a scanning mechanism, and an electronic control system.

The main component of an SPM is the probe, the size of which is crucial for resolution at the atomic scale. Ideally, the tip of the probe should be made up of either one single atom or a precisely defined geometry. Nevertheless, to the delight of the first developers, it was found that even relatively large tips can yield high-resolution images. In this case, atomic-scale protuberances at the surface of the large tip provide the actual single-atom tip needed for high resolution. In the probes used in the first SPMs, the sharpness of the tip was examined by a high-quality optical microscope. Even though dull at an atomic scale, tips selected with an optical microscope seem quite often to yield atomic-scale resolution. In the case of STMs, the probes used in the early models were obtained by manually etching or grinding one end of a thin tungsten wire, while the probes used in the first AFMs were obtained by simply shattering an industrial diamond and using some of the resulting shards. Since then, various methods for manufacturing single-atom tips have been developed. For example, one way to make a tungsten probe with only one atom at the tip is via electrochemical etching. A thin tungsten wire is dipped in an electrolyte solution and then subjected to a high voltage. The effect of the voltage is to dissolve layers from the surface of the wire until the desired thickness is obtained. Ultimately, the tip takes a pyramidal shape with a single atom at the top. Alternatively, some AFMs use tips microfabricated from silicon compounds by photolithographic techniques.

The second most important feature of SPMs is the precision with which the position of the probe relative to the sample is changed. To attain atomic-scale resolution, the position has to be changed with a three-dimensional precision of about one nanometer. This resolution is obtained by taking advantage of the ability of certain crystals, called piezoelectric crystals, to change shape in a reproducible fashion when subjected to a voltage. The extent of expansion or contraction of the crystal depends on the applied voltage and can be calibrated to yield the desired levels.

Commercial SPMs use a single device called a piezoelectric translator or a piezoelectric actuator. The translator is made with multiple layers of extremely thin piezoelectric ceramic material, and its motion is controlled with atomic-level precision by applying electricity to change the shape of the material. Usually, SPMs are provided with several interchangeable piezoelectric translators for use with different scan ranges. The electronic controls of an SPM perform three major functions: providing the necessary scanning voltages to the piezoelectric translator, monitoring the system used for to control and detect the interaction between the probe and the surface of the sample, and combining the information gathered about the position of the probe with that gathered from the probe-sample interaction. Eventually, the electronic controls translate all this information into a video picture of the sample's surface.

During the STM's imaging procedure, the tip of the probe is kept less than a nanometer from the sample's surface and a small voltage is applied between the sample and the probe. Under these conditions, the electron cloud of atoms at the end of the tip overlaps with the electron cloud of atoms at the surface of the specimen. With a thin tip, the overlapping occurs between clouds of only two atoms, one from the tip and one from the sample. This overlapping results in a measurable electrical current flowing through the vacuum or insulating material between the tip and the sample. Since there is no actual contact between sample and probe, the current is called tunneling current. It depends on the position of the sample relative to the tip, the composition of both the tip and the sample, and the voltage applied between the two. When the probe is moved laterally across the surface with the voltage between the probe and sample kept constant, the change in the distance between the probe and the surface caused by the atomic distribution along the surface results in a change in the tunneling current.

Two methods are used to translate the changes in tunneling current into an image of the surface. The first involves changing the height of the probe to keep the tunneling current constant; the voltage used to change the height is translated by a computer into an image of the surface. The second method scans the probe at a constant height above the sample, while the voltage across the probe and sample is changed to keep the tunneling current constant, and these changes in voltage are translated into an image of the surface. The constant-height mode allows faster scan rates, and although the depth resolution is reduced, the dependence of the tunneling current on the applied voltage at every position of the probe can be used to distinguish the various atoms composing the surface. The main limitation of the technique results from the fact that it is applicable only to conducting samples or to samples with some surface treatment.

AFMs differ from STMs in both the scanning process used and the types of interactions that are allowed to occur between the sample and the probe. AFMs include all microscopes capable of providing atomic-scale images of sample surfaces by measuring the forces acting between atoms in the sample and atoms in the probe tip. While the main features of all AFMs are the same, differences appear in the type of tip used and in the detection process. The probe is made of a spring-deflection sensor and a sharp tip mounted on a cantilever spring. The deflection of the cantilever spring resulting from the forces acting between atoms in the sample and atoms at the tip is monitored. The first cantilever springs used were merely small pieces of gold foil with tiny shards of diamond glued at one end to function as sharp tips. More advanced AFMs use integrated tip-cantilever springs microfabricated from silicon. The spring deflection is usually monitored optically through either beam deflection or interferometry. In the simpler of the two methods, the beam-deflection method, a laser beam is directed at the cantilever spring and the position of the reflected beam is detected. To scan the surface of a sample, because of the intricacy of the detection of the spring-cantilever deflections, the tip is usually held rigid and the sample is moved.

While diverse, the forces used by AFMs in imaging surfaces can be grouped in two major categories: short-range interatomic forces and longer-range force. The first type of force is encountered when the probe is made to practically touch the sample. The type and the size of the cantilever springs used allow for nondestructive contact forces of about a billionth of a newton (the newton is the unit used for force; a newton is about equal to the force one needs to hold an average-sized apple). The longer-range forces are encountered when the separation between the sample and the probe is between ten and one hundred nanometers. The various types of probe use various types of longer-range forces; common AFMs have taken advantage of electrostatic, magnetic, and van der Waals interactions. These forces are usually ten thousand times weaker than the contact-mode forces. Because of this difference, a more sensitive method for detecting their effect on the cantilever spring is needed. In one method, with the aid of an extra piezoelectric crystal, the cantilever spring is made to vibrate at its natural frequency. The varying forces acting on the cantilever spring affect the amplitude and rate of the oscillation. An image of the sample surface can then be obtained by measuring the changes in these oscillations. The main advantage offered by AFMs is their ability to image all types of samples.

Applications

Along with an unprecedented three-dimensional high resolution of surfaces, STMs and AFMs offer many advantages over previous microscopy techniques. First, they are compact, comparatively inexpensive, and relatively easy to manufacture and modify. Furthermore, SPMs are easy to use; most samples can be viewed as is, without any special preparation. Most important, SPMs provide more information about surfaces than any of the previous techniques. For example, they are capable of identifying the various atoms present at the surface of the sample and mapping the surface for various physical quantities such as electrical charge, temperature, and magnetic state.

The STM was the first apparatus capable of nondestructively imaging matter with an atomic-scale resolution and the first with the ability to identify the various atoms present at the surface of a sample. However, AFMs are capable of better resolution, making them more popular in nanotechnology applications. Additionally, by simply changing the type of probe used, AFMs can be made to image different physical characteristics of the surface. In spite of the fact that most industrial applications can be performed with either apparatus, their unique features allow them to play complementary roles.

In industry, SPMs have found applications in both design and quality control. In the manufacturing of diffraction-grating masters, for example, SPMs were used to guide the ruling machine cutting the grooves and to examine the quality of the final product; the combination of their lateral and vertical height resolution makes them ideal for visualizing the grooves in such a device. They were also used in both the design of magnetic recording heads and the manufacturing of the stampers used to dig holes through compact discs. SPMs have seen widespread use in the manufacture of electronic components.

One of the major accomplishments of the SPMs is in the field of nanolithography, or fabrication of structures at the nanometer scale. Prior to the development of SPMs, atomic-scale manipulation of matter was only the subject of theoretical stipulations. The breakthrough occurred around the end of 1989, when researchers at Rutgers University were able to dig a square hole of about 250 atoms across and 10 atoms deep. The hole was obtained by letting the STM probe sink into the surface of a metal-oxide crystal. A few months later, M. Eigler and Erhard K. Schweiser of IBM's Almaden Research Center were able to spell out their employer's three-letter acronym using 35 atoms of xenon. Soon after, it was shown that germanium atoms could be transferred from the tip to the surface of the sample and that a twenty-nanometer structure could be fabricated on a variety of semiconducting specimens.

There are various scanning-probe nanolithography techniques available. Most use specially designed AFMs, although STMs can be used for certain techniques, especially those involving the manipulation of individual atoms. One of the most basic techniques is nanoscratching, in which the tip of an AFM is used to remove material from the substance being worked on, often called a substrate. More elaborate techniques include dip-pen nanolithography (DPN), in which the tip of the AFM is used to deposit material on the substrate, and local oxidation nanolithography (LON), in which voltage is applied between the AFM tip and the substrate in a humid environment, causing water to condense between the tip and the substrate and induce an oxidation reaction in a precisely defined area.

SPMs are especially valuable for imaging organic matter. They were the first microscopes to offer high-resolution images nondestructively without the need of an extensive sample-preparation procedure. One feature unique to AFMs is their ability to image biological matter while still in water. Another important feature of SPMs is their ability to provide real-time images of biological processes. Compounds imaged by STMs include amino acids, proteins, DNA (deoxyribonucleic acid), and single cells.

The possibilities offered by SPMs in scientific research are limitless. By modifying the probes used for viewing and by imaging different surfaces, researchers are getting a better understanding of atomic interactions at sample surfaces, and this increased ability to test and manipulate matter has enabled scientists to study subjects that were not accessible before the development of SPMs. One of these new fields of study is nanotribology, the study of friction at an atomic scale.

Context

The development of the STM and the AFM was a culmination of about three centuries of progress in the field of microscopy. The first microscope, the optical microscope, was invented by Antoni van Leeuwenhoek at the end of the seventeenth century. In an optical microscope, a magnified image is obtained by directing light first onto the sample and then through a lens system. Optical microscopes can be used to view single cells, pathogenic agents, and bacteria. Their resolution is limited to about four hundred nanometers, however, half the wavelength of the light used for viewing. This limitation, called Abbe's barrier, results from the wave nature of light.

The next breakthrough in microscopy was the development of the transmission electron microscope, or TEM, during the early 1930s. Taking advantage of the wave properties of electrons, this type of microscope is able to resolve objects as small as 0.5 nanometer. The high resolution is obtained through the use of high-energy, short-wavelength electrons. Nevertheless, because of its reliance on the wave properties of electrons, the TEM remains subject to Abbe's barrier. Furthermore, the destructive power of the high-energy electron beams used in the TEM have limited its use to robust samples and samples requiring extensive surface treatment.

The limitation imposed by Abbe's barrier was overcome by the scanning electron microscope (SEM). The SEM operates by scanning back and forth over a sample with an electron beam. The resolution of the SEM is limited only by the size of the beam. SEMs came into wide use during the 1970s due to their ability to create three-dimensional surface relief impressions of specimens.

The first microscope to allow for a direct viewing of the atomic structure of surfaces was the field ion microscope (FIM). In this technique, helium ions are scattered off the surface of the sample, casting an imprint of the sample's surface on a fluorescent screen. The disadvantage of the technique is that it requires subjecting the sample to a high electric field. During the early 1970s, refinements of the FIM led to the Topografiner, a device with a sharp tip that scanned back and forth above the surface of the sample. The image is again obtained through field emission as a result of the high voltage applied between the tip and the sample. The resolution obtained by this device was limited to about four hundred nanometers.

The first among the SPMs to be developed, the STM is very similar to the Topografiner, using the same scanning technique and the same type of probe but differing in the interaction mechanism between the sample and the probe. The STM and later the AFM were the first microscopes to utilize nondestructive interactions at the atomic level between the sample and the probe. In addition, SPMs were the first tools to allow for the manipulation and control of matter at an atomic scale.

Principal terms

CANTILEVER SPRING: a flat piece of metal held rigidly at one end; when the other end is displaced from its equilibrium, it tends to oscillate up and down around the position of equilibrium

INTERFEROMETRY: a method used for measuring very small dimensions in which a beam of light is split in two and the two resulting beams are channeled through different paths, then compared

NATURAL FREQUENCY: one of the frequencies at which an object will oscillate when stimulated by some disturbance; it depends on the shape and the dimensions of the object

PHOTOLITHOGRAPHIC TECHNIQUE: a technique used in manufacturing semiconducting devices

PIEZOELECTRIC CRYSTALS: crystals that change shape in a reproducible manner when subjected to an electrical voltage or produce voltages in a reproducible manner when physically stressed

RASTER SCAN: a line-by-line scan of a surface, such as the scan the eyes perform when reading a page

TUNNELING CURRENT: the electrical current that flows between two conducting surfaces separated by a distance of a few nanometers; the current can be either through vacuum or through an insulator

VAN DER WAALS FORCES: an attractive force felt by atoms when they are a few nanometers apart

Bibliography

Bhushan, Bharat, ed. Scanning Probe Microscopy in Nanoscience and Nanotechnology. 3 vols. Heidelberg: Springer, 2010–13. Print.

Binnig, Gerd, and Heinrich Rohrer. "The Scanning Tunneling Microscope." Scientific American Aug. 1985: 50–56. Print. This article, written by the developers of the STM, is intended for the general reader.

Binnig, Gerd, and Heinrich Rohrer. "Scanning Tunneling Microscopy: From Birth to Adolescence." Review of Modern Physics 59.3 (1987): 615–25. Print. This is the Nobel Prize acceptance lecture, delivered jointly by Binnig and Rohrer on December 8, 1986. Covers the historic developments of the technique and provides an extensive bibliography.

Bonnell, Dawn A., and Sergei V. Kalinin, eds. Scanning Probe Microscopy for Energy Research. Hackensack: World Scientific, 2013. Print. World Scientific Ser. in Nanoscience and Nanotechnology 7.

Golovchenko, J. A. "The Tunneling Microscope: A New Look at the Atomic World." Science 232.4746 (1986): 48–53. Print. Without going into many details, the article gives a precise explanation of the concepts involved. Intended for the general reader.

Hansma, Paul K., et al. "Scanning Tunneling Microscopy and Atomic Force Microscopy: Application to Biology and Technology." Science 242.4876 (1988): 209–16. Provides a good, yet not easy, treatment of the biological applications of the technique and a description of the operation of the AFM.

Hansma, Paul K., and Jerry Tersoff. "Scanning Tunneling Microscopy." Journal of Applied Physics 61.2 (1987): R1–23. Print. This article is rich with technical information and includes an extensive bibliography.

Quate, Calvin F. "Vacuum Tunneling: A New Technique for Microscopy." Physics Today 39.8 (1986): 26–33. Print. The details and accuracy of the historic developments of the technique are impressive. Includes a copy of one of the pages of Binnig's laboratory notebook.

Rugar, Daniel, and Paul K. Hansma. "Atomic Force Microscopy." Physics Today 43.10 (1990): 23–30. Print. A comprehensive and simple description of the development and principles of AFM.

Schwarzschild, Bertram. "Physics Nobel Prize Awarded for Microscopies Old and New." Physics Today 40.1 (1987): 17–21. Print. A good description of the historical development of both the electron microscopy technique and the scanning tunneling microscopy technique.

Trefil, James. "Seeing Atoms: With the New Scanning-Probe Microscopes, It's Become Almost Routine." DISCOVER June 1990: 54–60. Print. Contains the simplest treatment of the principles involved and the advantages and limitations of the techniques.

Wickramasinghe, H. Kumar. "Scanned-Probe Microscopes." Scientific American Oct. 1989: 98–105. Print. For the reader interested in the impact of the development of the STM and in a description of the other probe microscopy techniques, this article provides the answers.

Yablon, Dalia G., ed. Scanning Probe Microscopy for Industrial Applications: Nanomechanical Characterization. Hoboken: Wiley, 2014. Print.

X-Ray and Electron Diffraction