Scanning Probe Microscopy – Including Scanning Tunneling Microscopy and Atomic Force Microscopy – Principles and Applications

What is scanning probe microscopy (SPM)?

The evolution of nanotechnology to study and engineer nature at ever smaller scales has also driven the development of microscopy techniques to image and control nanoscale structures. One of the most widely used techniques to achieve this is scanning probe microscopy (SPM), where a probe is scanned over a surface to build a point-by-point image with atomic resolution. In contrast to classical light microscopy and electron beam microscopy, this type of microscopy reveals details far beyond the optical resolution limit (typically hundreds of nanometers¹) and also enables surface topography to be probed , which has led to a paradigm shift in the understanding of matter at the nanoscale.

The development of scanning probe microscopes started with the invention of the scanning tunneling microscope in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratories, winning them the Nobel prize in physics in 1986. This discovery seeded the development of a whole family of SPM techniques, which enable the probing of nanoscale electronic,^2,3,4 mechanical,^5,6 magnetic,^7,8 thermal^9,10and chemical¹¹ properties that are not readily detected by optical means. Beyond imaging surfaces at the nanoscale, these scanning probes can also be used to manipulate nanoscale matter, for instance to position individual atoms on a surface.¹² In this article, we will first focus on two of the most widely used variants of SPM, scanning tunneling microscopy (STM) and atomic force microscopy (AFM), followed by an overview of some more specialized variants of these techniques for particular applications.

Types of scanning probe microscope - Scanning tunneling microscopy (STM)

What is scanning tunneling microscopy?

STM uses a nanoscale probe to measure the topography and local electronic properties of a sample by scanning the probe over a surface. As the tip is scanned over the surface, a map of these properties can be built up with a resolution greatly exceeding that of an optical microscope, which allows truly nanoscale features to be visualized down to the level of single atoms.

How does a scanning tunneling microscope work?

STM makes use of the quantum mechanical phenomenon of electron tunneling from a sharp, conducting tip to a conducting surface at nanoscale proximity below 1 nm¹³ (Figure 1a). When a bias voltage is applied between the tip and the surface, tunneling enables a current to flow even when they are not in contact. This phenomenon arises from the wave-like quantum mechanical nature of electrons that yields a finite probability of an electron traversing the gap and thus a current, which is impossible in the classical picture.

The magnitude of the tunneling current has an exponential dependence on the separation, which makes it highly sensitive to the surface topography. At the same time, the current is also related to local electronic properties (density of states) of the surface and the probe itself, making STM sensitive to both height and electronic properties. The sharp tip ensures that the current flow is limited to an extremely small area and only a very small sample area is probed around a given location. By monitoring the tunneling current while scanning the tip over the surface, the surface topography and electronic properties can be mapped out (Figure 1b). A key aspect here is the use of high-precision piezoelectric positioners, which enable scanning and positioning of the tip above the surface with sub-nanometer resolution. Ultimately, the high resolution of STM is a result of the combination of the extreme separation sensitivity of the tunneling current with the high precision of tip positioning.

STM is typically performed in two different modes of operation, depending on the application: constant height mode and constant current mode (Figure 2). In constant height mode, the tip is scanned over the surface at a fixed separation and variations of the tunneling current are recorded, which are directly related to the surface topography. In constant current mode, a feedback loop is used to vary the tip height to keep the tunneling current constant as the tip is scanned across the surface, and the voltage applied to the piezoelectric height control is recorded. This mode is the slower of the two, as the probe height requires continuous readjustment. For a flat surface, this mode enables local electron density to be mapped out, while for a rough surface, the final image can have contributions from both the local electron density and topography.

Scanning tunneling microscope diagram

Figure 1: Operating principle of a scanning tunneling microscope. a) and b) A sharp, conducting tip is brought into close proximity to a conducting sample surface and a bias voltage is applied, resulting in a small tunneling current across the gap. This current is read out by a high gain amplifier. c) A piezoelectric scanning system is used to control the tip-surface separation and to scan the tip across the surface. The current amplitude can be used as a direct measure of tip-surface separation when the tip is kept at a constant height, enabling topographic imaging. Alternatively, a feedback loop can be used to maintain a constant current by varying the control voltage of the positioning system, which is recorded and yields a measure of topography and electron density across the sample.

Figure 2: STM scan modes. a) Constant current mode: the tip height is adjusted by a feedback loop such that the tunneling current remains constant, and the recorded tip height is directly related to the sample topography. b) Constant height mode: the tip is kept at a constant height as it is scanned over the sample and the tunneling current is recorded, which is related to the surface topography and electronic charge density.

Strengths, limitations and common problems

The key advantage of STM compared to other microscopy techniques is its extremely high resolution, which enables surface topography and electronic properties to be mapped out with far greater detail compared to e.g., optical microscopy techniques.

The main limitation of STM is that it requires an electrically conducting sample surface to work, which limits the type of materials that can be studied with it. Compared to optical and electron microscopy techniques, acquisition time and the size of the surface area that can be studied are also limited by the fact that the probe or sample itself needs to be scanned to build up an image, in contrast to direct optical imaging or fast, large-area scanning in scanning electron beam microscopy. In addition, for samples that are not atomically flat and also have inhomogeneous electronic properties, it can be difficult to distinguish the contributions of each to the final image, particularly in constant current mode.

A key problem in STM is the high sensitivity of the measurement to external factors such as mechanical vibrations and electronic noise. These can be detrimental by either inducing unwanted motion of the tip or adding noise to the inherently weak tunneling current signal, both of which result in artefacts that can render imaging impossible. For this reason, STMs are often large machines that need to be set up in vibration-free environments,¹³ operated under high vacuum and at cryogenic temperatures¹⁴ and use extremely low noise electronics for the highest performance. Taken together, this makes STM a costly technique that requires highly specialized conditions for optimum results, even though DIY enthusiasts have shown that it is also possible to build a basic but cost-effective STM at home.¹⁵

Applications of scanning tunneling microscopy

STM was originally conceived as an atomic resolution imaging technique but has had a profound impact over a wide spectrum of fundamental sciences. For instance, it allowed individual atoms of a surface to be seen for the first time¹⁶ (Figure 3 and 4) and the orbitals of a molecule to be visualized.¹⁷ In materials science, it provides new insights into the nanoscale properties of known materials and also enables the study of new nanoscale materials such as graphene¹⁸ and carbon nanotubes¹⁹, as well as assembling structures composed of individual atoms (Figure 5). In chemistry, STM allows how the surface roughness and electronic properties of catalysts²⁰ govern their performance to be understood. Although many biological samples are not electrically conducting, it has been shown that they can be coated with thin metal films,²¹ deposited on conducting substrates²² or scanned under humid conditions²³ so that they can be studied using STM.

Scanning tunneling microscope images

Figure 3: STM image of a gold surface. Individual atoms and their arrangement are directly visible. Dark bands correspond to pits in the surface, with an absence of atoms. Credit: Erwin Rossen.

Figure 4: STM image of a single cobalt atom on a copper surface. Credit: NIST, Joseph Stroscio et. al.

Figure 5: STM image of a quantum corral of cobalt atoms in an ellipse on copper, also assembled using an STM. Wave-like features emerge in the STM image due to the confinement of electrons in the corral. Credit: Joseph A. Stroscio Robert J. Celotta Steven R. Blankenship Frank M. Hess.

Types of scanning probe microscope - Atomic force microscopy (AFM)

What is atomic force microscopy?

AFM is a variation of scanning probe microscopy where a sharp tip is scanned across a surface to measure its nanoscale topography by probing interactions of the tip with the surface.

How does an atomic force microscope (AFM microscope) work?

An AFM works by optically measuring the deflection of a sharp tip on a flexible cantilever as it is scanned over the sample surface (Figure 6). This is done by shining a laser at the tip and detecting the light reflected from the tip using a photodetector, allowing for an extremely sensitive tip deflection measurement.

An AFM can be operated in different modes, corresponding to different types of interaction with the surface.

• In contact mode, the tip is brought into contact with the surface and scanned across it, such that tip deflection yields a direct measure of surface height.²⁴ In this mode, tip damage is quite frequent, and care must be taken that the tip does not get stuck to the surface.
   
• In dynamic mode,²⁵ the cantilever is driven to oscillate mechanically at its resonant frequency with an oscillation amplitude on the nm scale²⁶ due to the high tip stiffness, and is then positioned in close proximity with the surface. As the tip is scanned over the surface, its mechanical motion is modified by separation-dependent nanoscale forces without physical contact of the tip with the sample, which is advantageous for delicate surfaces. By continuously reading out the mechanical resonance frequency, amplitude and phase changes of the tip, a map of these interactions across the sample area can be made. This mechanical data contains information about the surface topography encoded in the tip frequency, as well as the material interfaces encoded in the mechanical phase.
   
• Tapping mode is an intermediate mode of operation where the tip is driven to oscillate at resonance with a relatively large amplitude such that it intermittently comes into contact with the surface. In this mode, at close proximity, the tip oscillation amplitude changes due to nanoscale interactions with the surface. The nano-positioning system is used to adjust the tip height dynamically to recover the original oscillation amplitude, which enables the force exerted on the tip by the surface to be mapped out and its topography retrieved. This mode is particularly suitable for fragile samples in liquids²⁷ or for performing AFM in ambient conditions, where condensation inevitably forms a liquid film on samples.

Apart from using different imaging modes, the tip may also be functionalized, e.g., by making it magnetic to allow mapping of a surface’s magnetic properties, or with molecules such as carbon monoxide to resolve individual molecular orbitals.²⁸

Atomic force microscope diagram

Figure 6: a) Schematic representation of an AFM. A cantilever with a sharp tip is positioned at nanoscale proximity to a surface and is scanned across it using a scanning sample stage. A laser is focused onto the cantilever and its reflection detected by a photodetector, enabling sensitive detection of small tip displacements from which the surface topography can be reconstructed. b) Scanning electron micrograph of an AFM cantilever with a sharp tip on its end. Credit: a) yashvant, b) Kristian Mølhave, both reproduced under the Creative Commons Attribution 2.5 Generic license.

Strengths, limitations and common problems

A key strength of AFM is that it can provide extremely high-resolution images of nanoscale samples, without requiring an electrically conducting sample as in STM. As the measurement process is purely mechanical and optical, AFMs are less sensitive to electrical noise than STM. This also enables them to be operated under different conditions, such as vacuum, cryogenic, liquid and even ambient conditions, which allows a large variety of samples to be studied.

At the same time, the detection of mechanical forces on the tip as a means of reading out topography makes AFM less precise and harder to interpret as multiple forces act on the tip with different distance scaling simultaneously,^13,26 compared e.g. to STM, which has a simpler current-distance relation. When using a functionalized AFM tip to measure magnetic, electrical or chemical surface properties, data interpretation can be even further complicated if the surface is not atomically flat.²⁸

Applications of atomic force microscopy

AFM was conceived by the same inventors as the STM and has become a common technique in different aspects of fundamental science, particularly in nanotechnology, materials science and biology. Typical applications include inspecting natural and artificial nanoscale structures, such as bacteria, nanocrystals, metal surfaces and atomically thin materials, as shown in Figure 7. The possibility of nanoscale imaging under ambient and liquid conditions is particularly interesting for nanoscale biology, where AFM enables the study of live cells²⁹ and the mechanics of cell membranes.³⁰

AFM images

Figure 7: AFM image examples. a) Fibrillar array of Cyanobacteria oscillatoria dried on glass. b) 3D image of nanocrystals on a substrate generated from AFM data. c) Thin metal film surface. d) Flake of atomically thin MoS₂ with two areas of different thickness on a polymer surface. Credit: a) Toby Kurk, reproduced under the Creative Commons Attribution-Share Alike 2.0 Generic license, b) and d) courtesy of the author, c) opposite.ps, reproduced under the Creative Commons Attribution-Share Alike 4.0 International license.

Types of scanning probe microscope - Scanning probe microscopy variants

Building on the wide success of STM and AFM for new insights into the nanoworld in a wide range of fields, multiple SPM techniques have been developed for specific purposes. In the following, we will give a short overview of some the most widely used ones.

Scanning kelvin probe microscopy (SKP)

Scanning kelvin probe microscopy (SKP) is a variation of AFM that enables the surface local electrical potential to be mapped out. Here, a difference in work function between the tip and surface results in an electrostatic force and a tip deflection that can be recorded. While this technique is particularly useful to study semiconducting devices³¹ such as solar cells as well as surface corrosion and coating properties³², its application to improve understanding of detailed mechanisms of cell transduction and reaction have also been proposed.³³

Scanning spreading resistance microscopy (SSRM)

Scanning spreading resistance microscopy (SSRM) is a scanning probe technique where an electrically conducting tip scans a biased sample surface to measure its electrical properties. In particular, it enables mapping of charge carrier density and reading out a sample’s conductance and resistance, for instance in semiconducting samples³⁴ (Figure 8). Here, a hard tip is often used to break through the oxide layer on the sample surface, and measurements may be performed in an inert atmosphere to reduce surface oxidation such that the intrinsic sample properties are measured.
 

Figure 8: SSRM. A biased, electrically conducting tip is scanned across a contacted sample and the current measured to map sample conductance and resistance.

Cold atom scanning probe microscopy (cold-atom SPM)

In cold atom scanning probe microscopy (cold-atom SPM), a trapped gas of ultracold atoms is used as a probe instead of a solid tip (Figure 9), and its motion in the trap is recorded. The cold-atom probe has the advantage of being orders of magnitude softer (lower spring constant) than a standard AFM tip, enabling sensitive measurement of tiny nanoscopic forces and extremely fragile samples, such as a freestanding carbon nanotube.³⁵ However, trapping ultracold atoms close to a surface at room temperature poses a serious technical challenge, and the effective tip apex size of the atomic is much larger than for a standard tip, resulting in reduced spatial resolution.
 

Figure 9: Cold-atom SPM uses a trapped, ultracold gas of atoms as a probe instead of a standard AFM tip, resulting in a strongly enhanced force sensitivity.

Scanning near-field optical microscopy (SNOM)

Scanning near-field optical microscopy (SNOM) enables a sample’s optical properties to be studied with resolution far beyond the diffraction limit to reveal structures much smaller than the optical wavelength used. In aperture-type SNOM, a sub-wavelength aperture such as a tapered optical fiber or hole in an AFM tip is illuminated to create an evanescent electromagnetic field tightly confined to the aperture. While in scattering-type SNOM, a metal-coated AFM is used (Figure 10). In both cases, the illuminated probe is scanned over the surface at nanoscale separation such that only a very small area of the surface is illuminated at one time, which allows the optical properties of the surface to be studied at extremely high resolution. SNOM is particularly useful for the study of biological samples³⁶ and photonic devices,³⁷ as well as nanomaterials such as graphene.³⁸
 

Figure 10: SNOM types. a) Aperture-type SNOM, using a sub-wavelength size aperture as a source of evanescent electromagnetic field, b) scattering-type SNOM, where far-field light is scattered off a sharp tip to excite a near-field evanescent field around it.

Conclusion

In this article, we have discussed SPM techniques with particular focus on the two most commonly used, STM and AFM, that have proven instrumental in atomic resolution surface analysis and revealed nanoscopic forces. We also considered variations of both techniques that provide deeper insights into a sample’s electrical, mechanical and optical properties at resolutions far beyond conventional approaches. Table 1 gives an overview of the typical applications, resolution and features of the techniques presented.

Table 1: Summary of SPM techniques.

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