Atomic Force Microscope Operates at

Atomic force microscope operates at very close range and without a lens. There are several different types of atomic force microscope, and they all operate by measuring a local property, whether height, optical absorption, or magnetism, using a probe placed very close to the sample. This probe makes it possible to measure qualities over a small area, and an image of that area can be produced that resembles an image on a television screen, consisting of many rows or lines of information placed one above the other.

The size of the probe is what generally limits resolution, as opposed to a traditional microscope, where the limitation derives from diffraction effects. When brought close to the sample, the probe measures attractive or repulsive forces between the tip and the sample. The "contact" mode is also called the repulsive mode in which the instrument lightly touches a tip at the end of a leaf spring or "cantilever" to the sample.

A raster-scan drags the tip over the sample, and as this occurs, the detection apparatus measures the vertical deflection of the cantilever, indicating the local sample height. This means that in contact mode, the AFM measures hard-sphere repulsion forces between the tip and sample. The probe also works in noncontact mode to derive topographic images from measurements of attractive forces, and in this approach, the tip does not touch the sample. This device is able to produce a resolution of 10 pm.

It is superior to electron microscopes in that it can image samples in air and under liquids (Baselt paras. 2-4). The first such device was created "by meticulously gluing a tiny shard of diamond onto one end of a tiny strip of gold foil" (Hong-Qiang Li para. 1).

This was in the fall of 1985 as Gerd Binnig and Christoph Gerber used the cantilever to examine insulating surfaces, so that the small hook at the end of the cantilever pressed against the surface while the sample was scanned as the force between tip and sample was measured by tracking the deflection of the cantilever: This was done by monitoring the tunneling current to a second tip positioned above the cantilever. They could delineate lateral features as small as 300 A. The force microscope emerged in this way.

In fact, without the breakthrough in tip manufacture, the AFM probably would have remained a curiosity in many research groups. It was Albrecht, a fresh graduate student, who fabricated the first silicon microcantilever and measured the atomic structure of boron nitride. (Hong-Qiang Li para. 1) The tip-cantilever assembly today is usually microfabricated from Si or Si3N4. With further developments, the microcantilevers were perfected.

The development of the AFM is part of an ongoing process whereby scientists are trying to analyze smaller and smaller spaces, and the AFM offers many advantages: Scientists are thus gaining new knowledge about how matter operates and interacts at the atomic and molecular level. This means that they can now begin connecting different molecules to one another -- molecules that nature might never have been able to put together.

The result will be the creation of entirely new materials, such as a material 100 times stronger than steel but weighing only one-sixth as much. (Uldritch para. 11) Philip Ball emphasizes the importance of the AFM in molecular studies, noting that the AFM "allows researchers to probe the mechanical properties of molecules - how stiff or stretchy they are, for instance. A molecule can literally be grasped at one end by the AFM and pulled like a piece of elastic" (Ball 107). The AFM has its limitations a swell.

It is used in the analysis of proteins in medical research, but it cannot provide all the data needed: "The atomic force microscope has resolution sharp enough to see individual atoms but is unable to penetrate below the surface" (Dyson 44). The atomic force microscope has found many uses in different fields.

Brian Kooyman notes one use for the AFM in archaeological studies when he writes, The use of the Atomic Force Microscope has allowed Kimball and colleagues to produce textural analysis surface plots that allow them to assess the differences in polish in high and low areas of topography which is critical to success in differentiation of polishes.

(Kooyman 159) Ruth Kavenoff points out the use of the AFM in studying the genome, stating that the AFM "can visualize fine details like the two strands of the double helix in small segments of DNA, but they are not suited to DNA molecules as large as the bacterial chromosome" (Kavenoff 37). Another form of this microscope is called the scanning tunneling microscope (STM), which also provides pictures of atoms on or in surfaces.

Both types have been used for a variety of purposes, including "to solve processing and materials problems in a wide range of technologies affecting the electronics, telecommunications, biological, chemical, automotive, aerospace, and energy industries. The materials being investigated include thin and thick film coatings, ceramics, composites, glasses, synthetic and biological membranes, metals, polymers, and semiconductors. The AFM is being applied to studies of phenomena such as abrasion, adhesion, cleaning, corrosion, etching, friction, lubrication, plating, and polishing" ("What is an Atomic Force Microscope?" para. 4).

The AFM has a laser beam detection system to monitor the bending of the tip, and by this means a topographical image is generated. This imafge is third-dimensional and allows for the measurement of surface features and the generation of surface statistics. One company uses the AFM "to generate pore size distribution data for filtration membranes which is then used in process prediction and optimization. Different AFM imaging modes can be used to optimize the study of different surfaces increasing resolution or accessing further data.

Thus the non-contact AFM mode, as its name suggests, allows the imaging of soft easily damaged samples without contact" ("Atomic Force Microscopy" para. 2). Francesc Perez-Murano writes about the use of AFM in the process of nanolithography. This method has been used for two decades in order to define nanometer scale structures and devices: The most common method is based on applying a voltage between the AFM tip and the surface: the presence of humidity in the air induces local oxidation of the surface.

The resulting thin oxide layer forms itself into a nanostructure, or can serve as a mask for subsequent selective etching of the surface. (Perez-Murano para. 1). The author notes a new but related use of AFM for a process known as PMMA Polymethyl-methacrylate). The general ability of AFM to oxidize materials has been used to fabricate structures on many materials: Now we have applied the same technique to thin layers of PMMA. PMMA is particularly relevant to nanotechnology as it's used as a resist material for electron beam lithography.

The PMMA is locally exposed to a beam of electrons, changing its properties and making it solvent in a convenient developer. With AFM nanolithography, we have obtained at least the same resolution as with electron beam lithography systems, which are not as readily available as AFM in research laboratories. What's more, there is no need for a development process since the PMMA is directly eliminated. (Perez-Murano para.

2) Other methods have been used for scratching the PMMA by exerting a high force with the AFM tip, but this new approach involves a new mechanism responsible for eliminating the PMMA: "As confirmed by electrical measurements, the process involves an electrochemical reaction that causes the PMMA to dissolve" (Perez-Murano para. 3). The work is of interest both from a practical point-of-view - combination with electron beam lithography is already demonstrated - as well as for fundamental reasons - it identifies a new mechanism of surface modification.

Because AFM operates without damaging the material being analyzed, this method has been widely used for analyzing biological materials. In order to understand biological systems, their structure must be understood, and structural biology is the study of the structure and function of components of living systems. The AFM is a vital tool for analyzing the surface topography of native biomolecules at subnanometer resolution: Unlike X-ray crystallography and electron microscopy (EM), the AFM allows biomolecules to be imaged not only under physiological conditions, but also while biological processes are at work.

Because of the high signal-to-noise (S/N) ratio, the detailed topological information is not restricted to crystalline specimens. Hence single biomolecules without inherent symmetry can be directly monitored in their native environment. (Miller, Aebi, and Engel para. 1) The AFM can also offer data on the binding properties of biological systems, such as the specific interaction between two kinds of molecules.

This can be accomplished by binding one kind of molecule to the top of the cantilever and the other on the surface of the sample support: The adhesion force upon separation is then a measure of the binding strength. This method allowed the intermolecular forces between individual ligand-receptor pairs.. complementary DNA strands.. cell adhesion proteoglycans.. And the specific antigen-antibody interaction.. To be determined. (Miller, Aebi, and Engel para.

2) An advantage of the AFM is that it can analyze native tissue directly without prior dehydration, useful for such investigations as that of articular cartilage which has to be kept in physiological buffer to preserve its ultrastructure. Surface irregularities are often seen when using the scanning electron microscope, but these are absent using the AFM. One such analysis is described below: Occasionally, the cartilage surface exhibits local discontinuities where an underlying fibrous network is distinguishable.

Digestion of the cartilage surface with chondroitinase AC exposes this fibrous network more systematically so that the individual fibers are visualized with great clarity by AFM. When imaged at higher magnification, these distinct fibers exhibit a 60nm repeat, indicating that they are assembled from collagen fibrils. (Miller, Aebi, and Engel para. 4) The AFM has been shown to be valuable in similar analyses of biological materials and processes.

While AFM images also offer a view of the atomic detail of solids, the process is not useful for analyzing biomolecules such as proteins because they are designed to undergo conformational changes and form flexible supermolecular assemblies, meaning they are mechanically "soft" so that the surface cannot be probed for atomic detail. However, as Miller, Aebi, and Engel note, "state-of-the-art specimen preparation and instrumentation now allow the surface topography of native proteins to be imaged at subnanometer resolution" (para. 5).

A recent example of biological research using the AFM comes from Santa Barbara, California, where researchers used the AFM to discern unique properties of bone: Collagen, the most abundant protein in the human body -- serving as a structural component of a variety of tissues including bone, tendon and skin -- reveals special properties which allow it to "bounce back" when pulled or stressed in laboratory experiments. The AFM operates by tapping and pulling with a tiny needle. ("Bone Strength Probed by Scientists" para.

1) This research shows that the collagen in bone contains sacrificial bonds that rupture as the collagen is stretched, and these ruptures then heal. The purpose of these bonds is to provide a means for dissipating mechanical energy in collagen molecules (("Bone Strength Probed by Scientists" para. 2). AFMs constitute a subset within the larger group known as scanning-probe microscopes, which can utilize many different types of tips to measure electrical, mechanical, or magnetic properties. Tips exist that can perform simultaneous dimensional and electrical measurements.

It is when a scanning-probe microscope uses a tip that can discern properties at the atomic level that the instrument becomes an atomic-force microscope. Such devices can measure features within a few Angstroms and do so without harming the sample. As on scientist notes, "Manufacturers need to perform nondestructive measurements in all three dimensions to ensure their device geometries fall within ever-smaller tolerances" (Titus para. 4). Another company uses AFM to detect flaws in surface-acoustic-wave devices, defects that cannot be seen with an optical microscope.

The same company uses AFM to check the results of steps in photolithograpy (Titus paras 5-6). Kevin Kjoller considers how to measure the true resolving power of an atomic force microscope, noting first that resolution means the same as it does for an optical microscope, meaning the minimum distance between two adjacent objects that a microscope can identify as separate. Kjoller complains that most manufacturers substitute some meaningless term for resolution and ignore the reality.

Several issues need to be considered, such as the size of the probe and three types of limiting noise, electical, mechanical, and acoustic. Kjoller defines electtical noise as "the sum of the.