What Is a Scanning Probe Microscope?
A scanning probe microscope (SPM) is a microscope that uses a needle-sharp probe to observe surface irregularities on the nanometer scale.
It is often used in a high vacuum to clean the sample surface, but can also be used in air. Recently, microscopes that can be used in liquid have also been developed.
There are various types of scanning probe microscopes, including scanning tunneling microscopes (STM) and atomic force microscopes (AFM). The latter of which was awarded the Nobel Prize in Physics in 1986 for its ability to capture individual atoms and for its significant contribution to the advancement of nanostructure science and technology.
Uses of the Scanning Probe Microscopes
Scanning probe microscopes are used to observe the surface conditions and measure the roughness of semiconductors, glass, liquid crystals, and other materials because they can observe surfaces at the nanometer level, which is extremely fine.
Specific targets for observation include the atomic arrangement of silicon single crystals and phenyl groups in organic compounds. It can also be used to observe and manipulate DNA in biological samples such as microorganisms, bacteria, and biomembranes.
The scanning probe microscopes are new microscopes developed in the 1980s, but its applications are expanding rapidly, with remarkable advances in atomic-level observation technology and the development of models that can measure friction, viscoelasticity, and surface potential. Measurement in liquids is also used in fields such as electrochemistry and biochemistry, enabling measurement of conditions closer to real environments.
Principle of Scanning Probe Microscopes
This section describes the principles of AFM and STM, two of the most commonly used scanning probe microscopes. The tip of a fine needle-like probe scans the surface of a sample to acquire image and position information. The probe is thin and scans at the atomic level, so it is not suitable for measuring samples with too much unevenness.
1. Scanning Tunneling Microscope (STM)
STM takes advantage of the fact that the strength of the tunneling current emitted from the tip of a metal probe toward the sample depends sensitively on the thickness of the insulator, the vacuum, and in between. It can accurately measure the local height of the sample surface with a high resolution (the shortest distance between two neighboring points) that allows us to resolve individual atoms on the surface of the material. The probe also allows the observation of atomically scaled unevenness patterns as the probe scans the sample surface.
The probe is made of tungsten or platinum with a pointed tip. When the probe and sample are brought close enough that their electron clouds overlap and a small bias voltage (a voltage used to define the DC operating point for small-signal amplification of an amplifier) is applied, a tunneling current flows due to the tunneling effect.
In STM, the tunneling current is kept constant by moving the metal probe horizontally (X, Y) across the sample surface and by feedback control of the distance between the probe and the sample (Z). Usually, the vertical movement is performed with a piezoelectric element that can control the distance with a precision smaller than the size of a single atom, and the interaction between single atoms is detected. Thus, STM has atomic resolution in three dimensions. A piezoelectric element is a passive device that utilizes the piezoelectric effect, a phenomenon in which a voltage is generated when pressure is applied.
2. Atomic Force Microscopy (AFM)
AFM measures the difference in microscopic atomic forces (weak cohesive forces between atoms that are not chemically bonded) between the probe and the sample surface and observes the surface by scanning it. A wide variety of models have been developed to measure frictional force, viscoelasticity, dielectric constant, and surface potential by applying AFM technology.
A probe attached to the end of a cantilever (cantilever) is brought into contact with the surface of a sample by a very small force. The distance (Z) between the probe and the sample is feedback-controlled to maintain a constant force (deflection) on the cantilever while scanning horizontally (X, Y) to obtain an image of the surface topography.
Other Information About Scanning Probe Microscopes
Types of probes
AFM and SPM, which are typical examples of scanning probe microscopes, both use probes, but they differ in type. Furthermore, there are many types of AFM alone, including materials and lengths, and it is important to select one that best suits the object to be measured.
In addition to the contact mode described in the principle, AFM also has a tapping mode, which is used to measure fragile organic samples and uses a dedicated probe. The probe is a consumable item and must be replaced by the user.