AFM/SPM Principles
At the heart of the AFM is a delicate, ultraminiature point probe. A nano-finger. One that touches the surface of a sample in a way that can be controlled to remain minimally invasive to the sample. The rest of the instrument measures and controls this tactile interaction between the nano-finger and the sample surface. From the measurements we extract information about the sample surface, and with the control we modify the sample surface by design.
Figure 1 shows the AFM’s point probe. It is a sharp asperity at the end of an upside down pyramid or cone, called the AFM tip. This tip is typically made of silicon or silicon nitride. The important thing is that it terminates in a sharp asperity, which is where contact is made with the sample. The tip is either from the start integrated into, or later attached to, a microfabricated cantilever, whose mechanical compliance (spring constant) may be tailored to be smaller than the effective force constant that holds nearest neighbor atoms together in most solids. The combination of the tip and the cantilever is often referred to as the AFM’s probe. The material of the tip and the bulk of the cantilever may or may not be the same, again, usually silicon or silicon nitride. Another material besides silicon and silicon nitride used as an AFM tip is single crystal diamond, which is attached to the cantilever.
Among the instruments intimately associated with nanoscience and nanotechnology, none is more versatile and widely recognized, and none simpler in its core principles of operation than the Atomic Force Microscope (AFM). The modern day AFM has been made possible by the confluence of advances in several technological arenas: microfabrication, piezoelectric actuation, high-speed digital computing, electronics and optoelectronics, and noise control. The result is an instrument that not only gives us an intuitively comprehensible view of surfaces in three dimensional images with nanometer and Angstrom resolution, but also furnishes us with an interactive environment in which we can control and modify the world of molecules and atoms on the same surfaces.
The Power of Controlled, Sensitive, Tactile Interface with the Nanoscale world
To say that the AFM puts us in touch with the nanoscale world is hardly a cliché; quite the contrary. The AFM provides direct, precisely controlled tactile interface with the material world. Tactile interface is not unique to the AFM; in fact stylus profilers have offered the same several decades longer than the AFM. But the probe-sample forces are far smaller for an AFM than they are for a stylus profiler, and indeed for any other surface science instrument in which the probe contacts the sample. This is one reason the AFM is the only instrument that routinely gives us a clear view of the nano-scale structure of matter on samples ranging from hard metals to very soft polymers.
The AFM’s sensitive tactile interface is also enabling nanomanipulation of matter, including nanolithography and atomic and molecular rearrangement of surfaces, and measurement of intermolecular and intramolecular forces. All this is happening with profound implications for the advancement of nanotechnology and our molecular-knowledge-base of material science, with applications in industries as diverse as pharmaceuticals, aerospace, data storage, semiconductors, plastics and polymers, and biomaterials to name very few.
The AFM’s Resolution: The Probe, the Feedback System, and the Environment
Like its predecessor the Scanning Tunneling Microscope (STM), the AFM is best known for its amazing imaging resolution. High-resolution mapping of surface topography is still by far the biggest application of the AFM as measured by the number of images that the AFM users worldwide generate with their instruments. The AFM offers image resolution down to the atomic scale (Figure 2), although many users of the AFM rarely need this resolution, working instead mainly in the molecular or in the sub-micrometer regime.
Atomic scale AFM imaging is more routinely achievable in the vertical or Z dimension. Steps that separate individual atomic planes (terraces) are relatively easy to image with an AFM that offers sub-Angstrom level noise floor in the Z direction (Figure 3). Atomic resolution in the plane of the sample (or in the XY plane) often requires more sophisticated instrumentation and user experience, especially in the dynamic modes of AFM, which are described below.
To achieve its resolution, the AFM depends in part on the sharpness of the probe tip and the precision and quietness of its control system. The main parts of the control system are 1) a high-precision positioning actuator, usually of piezoelectric construction; 2) sophisticated control electronics, and 3) a highly sensitive position detection scheme, typically optical. (Figure 4).
Contrary to the widely quoted myth, the highest in-plane or XY imaging resolution of an AFM is often determined not by the sharpness of the AFM tip, but by the environment in which the sample is imaged, and by the noise level in the feedback system. Generating in-plane atomic resolution images with the AFM often depends much less on the tip than it does on what is covering the sample surface, and this is the reason atomic resolution AFM images are frequently obtained with more ease in high vacuum than they are in air (leaving aside the difficulties associated with evacuating and maintaining a high vacuum chamber). Similarly, if the feedback system is noisy, the sharpness of the tip matters very little when it comes to achieving atomic resolution, in any environment.
The vertical or Z resolution is completely independent of the sharpness of the tip. It depends on the noise in the system, and when feedback is enabled on the resolution limit of the actuating scheme (including the piezoelectric actuator itself) that moves the tip and the sample relative to each other in the vertical direction.
Dynamic Modes of AFM and Phase Imaging
The initial embodiment of the AFM had the tip in perpetual contact with the sample surface. This Contact-mode AFM is widely used still, but not as widely as the collection of dynamic modes. In dynamic modes, the tip executes small mechanical oscillations. Sometimes these oscillations bring about intermittent contact between the tip and the sample, once for each oscillation cycle. Sometimes there is no contact at all: the tip and sample approach contact, but diverge without contact, again once for each oscillation cycle (Figure 5). The dynamic modes of AFM, which include Magnetically-actuated mode (MAC mode AFM), Frequency-modulation AFM (FM-AFM), tapping mode AFM, and Torsion Mode AFM all offer methods of investigating the sample surface that expand the AFM’s applications far beyond topographic mapping.
Phase imaging is a major derivative mode of most dynamic-modes of AFM such as MAC mode and tapping mode. In phase imaging, the relative phase between the signal that mechanically drives the AFM cantilever and the signal at the detector is recorded pixel by pixel to create the image. This image often shows contrast across adjacent regions in the image which corresponds to difference in material properties of the regions. Frequently, the contrast in the phase image has no corresponding features in the topography of height image. (Figure 6a & b).
The underlying mechanism that gives rise to the contrast in phase images is not unique: it may be electrical, magnetic, viscoelastic, or chemical to name a few. Contrast in phase images is a consequence of the heterogeneous nature of the topmost layers of the sample including material adsorbed onto the sample from the environment. For this reason, phase images of the same area of a sample may look quite distinct in one environment versus another, e.g., air versus water.
Beyond Topography
The combination of the controlled tactile interface and the high resolution sets the AFM apart from other instruments used to characterize materials. In fact, it is precisely this combination that has made the AFM the instrument of choice for investigating a host of material properties on the scale of nanometers, chief among them the mechanical properties. (Figure 7a & b).
Functionalized AFM Tip
When the AFM tip is coated with a magnetic material, then the AFM can also measure and create maps of the magnetic structure and characteristics of the sample surface (Figure 8). When the tip is coated with a conductor or is itself made highly conducting (by doping the silicon tip) then a voltage applied to the tip can turn it into an electrical probe as well, making it possible to measure and create maps that correspond to the variation of the electrical properties of the sample surface. (In some cases, applying a voltage is not needed.) If a thermister (temperature-dependent resistor) is incorporated into the apex of the tip, or the tip is made temperature sensitive by any other means, then the AFM can also map thermal properties across the sample surface. If a particular chemical or biological species is coated over the tip, the tip becomes responsive to the complimentary chemical or biological agent on the sample surface.
In short, any method of making the tip sensitive to a particular property of the sample surface, will open a new window into the properties and characteristics of the sample. The list of methods based on functionalizing the AFM probe is continuously growing.
Beyond Images
Imaging and measurement of feature dimensions is no longer the only mainstay of AFM applications. Increasingly, the AFM is used to measure the forces between the tip and the sample surface at a given location on the sample surface. This is ushering in a rapidly expanding array of applications in which the mechanical properties of the sample at or immediately beneath its surface is measured locally, that is, with molecular in-plane resolution. Similarly, scientists routinely use the AFM to measure molecular forces between different parts of a nanoscale object with complex structure, say, a segment of a biological macromolecule. On many AFMs these forces can often be measured with picoNewton resolution. The entire fields of molecular biology and molecular chemistry are being transformed thanks to this uniquely powerful application that only AFM enables. (Figure 9).
