Microscopic Techniques And Its Usefulness For Studying Materials

Published Date: 02 Nov 2017

The science of microscopy has a history that spans more than 1900 years. Spread across countries and many different disciplines, it is widely accepted that the development of this branch of scientific analysis has a history rooted in the first century A.D. It was noticed by Romans, experimenting with clear glass of different sizes and shapes, through this they observed that objects would appear larger. (1) Breakthroughs were slow, with the only advancement being the use of a "reading stone" composed of a glass sphere that magnified when laid upon reading materials around 1000 A.D. It was only after the dark ages that, in 1590, two Dutch eyeglass makers took the first step toward the compound microscope. (2) It was noticed when making observations through multiple glass lenses, in sequence, held inside a tube that objects seemed greatly magnified. These tradesmen were Zaccharias Janssen and son Hans Janssen. A further advancement in microscopes was gained by the refinement of lenses using a new method of grinding and polishing in 1674 by Anton van Leeuwenhoek (1632-1723). Leeuwenhoek was able to see a magnification of 270 times with the combination of up to five hundred and fifty successive lenses combined with his advanced lens polishing techniques. These lenses were placed in a tube, similar to a telescope in order to magnify the objects of study. Major advancements in this field of science were slow and it was nearly two-hundred years before Carl Zeiss began making refinement to the microscopes he manufactured that significant progress was made. Zeiss hired Otto Schott to research the refinement of optical glass and later also Ernst Abbe to aid the manufacturing process. In 1872 the Abbe sine condition, a mathematical formula, was proposed that provided calculations for the maximum allowed resolution in microscopes possible with visible light (200nm). Later in 1903 Richard Zsigmondy, winner of a noble prize in chemistry 1925, developed the "ultra microscope" that allowed for the study of objects below that of visible light.

Progress in microscopy was limited to the optical lens microscope until 1931 when the electron microscope was invented. Ernst Ruska co-invented a microscope that used electrons sped up in a vacuum. Using these particles, which possess a much shorter wavelength than visible light, these scientists were able to see the diameters of individual atoms. However it was not until 1986 that Ernst Ruska received a Nobel Prize in Physics for his work on the electron microscope. The next major step in this direction was not made until was 1981 with the advent of the scanning tunneling microscope. Its creators Gerd Binning and Heinrich Rohrer were recognized for their achievements in this advancement with the Nobel Prize in Physics. This device was able to provide three-dimensional images even down to the atomic level. This was the first microscope to be able to analyze objects in three dimensions at this level of observation. This microscope, the STM, remains to this day one of the strongest to date.

The electron microscope and later the scanning tunneling microscope (STM) have greatly advanced our ability to study things at the atomic level with actual observation not just theory. Enabling researchers to further understand and observe things that only a generation before was purely theoretical. There have been many advances in this branch of microscopy that have been focused toward different ends by similar and alternate means.

In the case of nondestructive testing the scanning acoustic microscope (SAM) has made significant advancements. (3) These progressions were because of its specific sensitivity in the detection of both voids and de-laminations of a sub-micron thickness in material and product testing. The discovery of otherwise hidden defects shown in a layer by layer analysis sensitive to differences like: poor bonding, other matter inclusions, cracks, voids and other unplanned or unintended imperfections. (4) These problems are usually difficult to detect using more traditional x-ray radiography.

Another version of this type of microscopic detection is through the use of the C-mode scanning Acoustic Microscope (C-SAM). This technology is widely used in the testing of a wide range of material samples such as: metals, ceramics, composites and polymers materials used in varying industries. This device in particular has shown for years that it has a very relevant application in the study and analysis of integrated circuits and electronics. (4) This technique can be used to detect de-lamination, voids and cracking or crack propagation that can result in increased failure rates and corrosion susceptibility.

Nano-Technology has had a long-standing presence within human history whether most know it or not. From Its unknown use in art from in chalices to its use in stained glass windows all throughout the world, it has been with us all for a very long time. More than just in art it has contributed unintentionally to the advancement of ages of man as in the development of "Damascus" steel blades for swords with carbon nanotubes. (6) Other advances include the development of the field emission microscope by Erwin Muller in 1936 and the field ion microscope in 1951. These microscopes were the basis for another field of science with the term "molecular engineering" coined by Arthur von Hippel in 1956 which was applied to dielectrics and piezoelectrics. The term Nano-technology was first used in 1974 by Norio Taniguchi a professor at Tokyo Science University who described machining and manufacture of materials near or at the atomic scale. The Scanning Electron Microscope (SEM) has seen wide use in the field of Nano-technology. In 2006 the first Nano scale car was developed made of oligo center frame with alkyynyl axles and buckminsterfullerene wheels turning as it moved on a gold surface at 300 degrees Celsius.

Other recent technological studies have been made into the area of Nano crystals and Nano wires. The Nano crystals are being analyzed as a way to enhance the efficiency of solar cells as a power source. It is believed that through the use and application of Nano crystal technology in solar cells the overall limit of perceived efficiency can be raised. (7) Nano wires as seen through the scanning electron microscope, are being developed from various substances and used in the field of quantum computing, (8)

This branch of science is not without its politics and problems. An example of this was known as the SSC. The SSC, also nicknamed Desertron, was a supercollider that was in the process of being built near Waxahachie, Texas that would have been the world’s largest and most energetic. (9) In 1983 it had a planned energy output of 20 TeV per proton with a ring circumference of about 54 miles but that was updated to a proposed 40TeV later. In the end it was cancelled in 1993 because of the poor management skills of physicists and the Department of Energy combined with competition for funds with NASA and the international space station. This project that was scheduled to be completed in 1999 but was canceled due to projected cost increases. The SSC did not have that benefit of using some preexisting tunnels like during the construction of the LHC. The overwhelming cost estimate increase from 4 billion to an estimated 12 billion dollars contributed to its ultimate failure.

The optical microscope is also known as Light microscope which use visible light and lenses to magnify objects in a larger frame. This type of microscope has found implementation in different fields of science (microelectronics, geology, medicine and biology, as examples) and is very popular among non-professionals. Being able to observe an enlarged sample through your own eyes instead of a digital display is just more interesting to many. The optical microscope are very simple in uses and although they have a complex architectural design. In the modern world the optical microscope is concern to improve the resolution and make it easy so that one can observe it with naked eyes and make the idea of the objects. The improvements which need to consider are optics, cameras, lasers, filters, dyes, computing and molecular biology etc.If we see optical microscope it is related to imaging transmitted ,DIC,phase contrast, fluorescence.Also live cell techniques –dyes targeting organelles ,time lapse, spectral detection ,FRAP,FRET.To improve optical microscope performance we have to consider the resolution improving techniques."Zacharias Janssen was a Dutch spectacle maker credited with inventing the first compound microscope in 1590".This microscope has 3x to 9x magnifications power. The technique to get an image by using an optical microscope is to capture the image with normal light-sensitive cameras to produce a micrograph. At first those images were captured by photographic film but with the help of modern techniques it is now introduce more powerful and efficient CMOS and charge-coupled device (CCD) cameras. It allows capturing digital images with the help of CCD camera. Microscopes are instruments considered to manufacture extravagant visual or photographic images of objects too little to be seen with the bare eye. The microscope has to carry out three farm duties: construct a magnified image of the sample, take apart the particulars in the image, and provide the particulars visible to the human eye or camera. These groups of instruments consider not only multiple-lens (compound microscopes) designs among objectives and condensers, instead of very simple single lens instruments that are frequently hand-held, for example a loupe or magnifying glass. Now in terms of lens used in microscope there can be seen two types of optical microscope – they are simple microscope which use single lens and the other one is compound microscope which have multiple lenses. As we want to see an image in a large frame it is not sufficient to use just a single lens. For a clear view of an object we use the technique to use multiple lenses by following the principles of refraction and reflection of light. It can be implicit with the aid of a few straightforward rules regarding the geometry concerned in tracing light rays all the way through the lens. As far as resolution is concern magnification doesn’t limit resolution, at least not directly, numerical aperture does. But we know that if we increase magnification we can able to see smaller details. Magnification is actually about identical the resolution element to the detector whether it is human eyes or camera. As human eye has a limitation to see an object of certain resolution often given at 1 minute of arc depending on age and individuals. That corresponds to about 76 microns at a comfortable viewing distance 250 mm.As well cameras have a resolution limit forced by distance pixels, which is known as pixel pitch, it range from around 6 microns to 50 microns. The main concern of magnification is to make the image large enough so the resolved element can be seen by at least 2 or 3 detector elements, be they camera pixels or cones in the human eye. Microscopy allows people to realize that huge differences exist in the world we perceive versus the composition of the world we perceive. Although organic objects, tables and computers might appear to be solid or uniform structures, the true quantum world that composes these objects contains many stark differences than what is perceivable to the eye. The microscope is one of the keys to understanding the building blocks of the world around us.

Light sources for confocal microscopes can be divided into involving coherent

or having incoherent groups. LSCMs are related coherent. On the contrary, most of

the clinical con focal microscopes used in the clinic, for instance, the scanning-slit

confocal microscope used in the ophthalmology clinic uses halogen lamps as a form of

incoherent light source. With spatially incoherent illumination, the phase dealings

among fields at close to points are formly random.

Having coherent light sources have the significant possessions that the period distinction between any two points is stable with time. Examples of spatially

coherent light sources are lasers and arc types lamps with a small aperture that works as a

spatial filter. Besides that: temporal coherence. The laser with single frequency that have high temporal coherence. This term has an existence of a phase relationship among the fields with at certain stage of delay of T.In practical this lasers exhibit an exact phase affiliation for a constant time being, this is the coherence time. These lasers always have on its own wavelength which they can be tuned to an output for a small number of discrete wavelengths. There can be another way to employ these two or three lasers to present a broad collection of wavelengths. Many recent LSCMS exercise more than one laser as a source of light. This result the output of the laser enormously intense monochromatic which is one in color but in contracted form of frequencies, coherent and greatly collimated. This results a linear polarized of output, it can be oppressed in disparity interference contrast microscopy, microscope with polarized light and fluorescence polarization anisotropy. A high NA microscope can focus to a diffraction limited beam. The beam of this laser can be extended in diameter before it is adjusted with the confocal microscope. Naturally, this laser’s beam diameter is coupled to overfill the back aperture of the objective of this microscope. In case of TEMOO operation of a laser it forms a Gaussian beam in such situation of illumination pinhole aperture is not required at all, as the light shows to form from a point source at infinity. The beam of laser can be extended, with the help of high NA microscope purpose; a diffraction limited spot is formed at the focus. The number of photons absorbed per molecule of fluorophore per pulse scales with the average laser power squared in the process of two photon excitation. It is basically the resource of the experimental authentication of two photon excitation process. In the process of log- log plot of detected intensity versus power of laser where the slop of the plot is 2. Microscopy allows us to comprehend that enormous differences continue living in the world we recognize in opposition to the composition of the world we distinguish. Even though crude objects, tables and computers might appear to be solid or uniform structures, the proper quantum planet that composes these objects contains numerous stark differences than what is perceivable to the eye. The microscope is one of the keys to accepting the building blocks of the world around us.

Joe Johnston

Scanning electron Microscope.

The basic scanning electron microscope has been an invaluable tool to those in the field of materials studies since its discovery in the 1930's.(1) Its components work together to produce an image derived by the directing of electrons onto the surface of a carefully prepared specimen. The components culminate into a behemoth of a microscope with a vastly increased capacity over optical microscopes. The microscope needs a space of about the size of a picnic table to operate. Like many microscopes, it has a hefty list of advantages and disadvantages that makes it appropriate for use in specific situations. The microscope boasts an impressive level of magnification and can even develop three dimensional pictures of its specimens. To dive deeper in the scanning electron microscope its physical components and their functions must first be described.

As stated before the Scanning Electron Microscope basically takes an entire lab dedicated to the machines to accommodate its operation. Not to mention, a space for the careful preparation of many of its specimens. Many of the microscopes of this type are anywhere from three to four feet tall. The original SEM designed by Dr. Ardenne reached all the way from a lab counter top to the top of a ten foot tall ceiling.(2) The microscope itself is but a single component in the system. A computer with system specific programs is needed to operate the microscope and view the scanned images. The microscope tube sets on top of a reinforced box that holds the specimen and provides a vacuum for the specimen to be viewed in. This box also prevents harmful x-rays from leaving the confined space and injuring lab techs and damaging other valuable lab equipment.

Now that a description of how the equipment looks has been provided, the inner components and their functions can be discussed. The Scanning Electron Microscope, or SEM from this point forward, is actually a pretty simple concept and similar to the light microscope only in physical arrangement of components. An energy source like the light bulb in the case of a optical microscope, is needed to bounce waves off the image for our retinas or a monitor of some kind to see. The SEM uses an electron gun to produce this "viewing energy". An electron gun also known as a cathode ray tube (CRT) is used to emit the electrons needed for this to work. This electron gun is similar to the device used in older television sets. A tungsten element is often used due to its ability to withstand temperatures of three thousand plus degrees Celsius without melting.(3). When the metal is heated in the tube electrons are emitted through the tube and down into the first section of the microscope. The first part after the gun is called a focus lens or condenser lens. This lens is not glass like an optical microscope but it performs a similar task. It is a powerful magnet that applies a force on the electron and takes the scattered electron from the gun and focuses them into an organized beam. After this another focus lens intercepts the electron beam. This second lens does the same thing but even more so. Then, the beam is focused into a set of deflection coils. The deflection coils provide the scanning motion analogous with many of these types of microscopes. The coils are basically an electromagnet that moves the beam across the surface of the object being scanned. This process happens before an image is produced and obviously is happening at much faster rate than described here. The next step is what comes off of the sample.

With all these electrons being focused and smashed into the specimen an image is then produced. The image is produced again similarly to how an optical microscope works. Our eyes see the light reflected off of an object. In the case of the SEM all sorts of light rays and electrons are reflected, even harmful light rays such as x-rays. In this case it requires a special component is required to see and capture the many frequencies of electrons and light reflected off of the surface of the specimen. The first thing often reflected off the surface of the subject are secondary electrons. Secondary electrons are called secondary because they are generated in response to the initial electron being shot into the surface of the sample. Secondary electrons are the main means of image generation in an SEM. Of course this is only achievable with the help of a special receptor called a secondary electron detector. The secondary electron detector processes the information and with the help of the attached computer generates an image. Now, with this very basic understanding of how the components work we can now discuss what all this culminates into in terms of functionality.

Scanning Electron microscopes are very helpful in the field of materials science because of their great magnification capabilities. SEMs can magnify an image up to four hundred thousand times and have a resolving power of one nanometer. (4) In the materials field fractures, structures, and everything else that greatly affects the usability of a material in the engineering world, often, occurs on the atomic or Nano scale . For example, a special type of metal (Ti3Al) was studied and praised for its ability to withstand creep at elevated temperatures. But, the metal in question was very brittle. (5) The experimenters began using alloying elements like Molybdenum to increase the ductility at elevated temperatures. Before they could begin testing their newly alloyed metals they had to verify the A2 phase of the original material had changed to a two-phase structure called (A2+B). (5) The electron microscope verified the existence of the required microstructure and the scientist could proceed with the experiment. They were successful in increasing the ductility of their base material. Another great feature of the SEM is its ability to develop three-dimensional images of objects. One way this is achieved is by using advanced imaging technology like "inverse reconstruction using electron-material interactive models". (6) This is done with some advanced imaging technology. It takes a basic shape of the image and constructs a basic digital image to apply the SEM image to overlay on the model. It's less scientific but can give the viewer a great idea of what the object looks like if it were visible by our own eyes.

Scanning electron microscopes are great tools not only in the field of materials science but in the biological field as well. Metals are easy to use with SEMs because minimal surface preparation is needed. A special type of polishing needs done and that's about it. With biological samples the case is completely different. Samples that are hard like hair or something dry and rigid can withstand the forces of a vacuum and are prepared differently so the electrons will bounce off properly. However living tissue and biological samples are more intricate. They must be fixated so as not to be affected by the vacuum chamber. C. E. Jeffree describes the process: "Fixation is usually performed by incubation in a solution of a buffered chemical fixative, such as glutaraldehyde, sometimes in combination with formaldehyde and other fixatives, and optionally followed by post fixation with osmium tetroxide" (7). This process is complex and far more time consuming than simply polishing a surface.

Scanning electron microscopes are an amazing type of microscope. They have great capabilities as far as image construction and magnification. Their ability to generate a three dimensional images is very helpful in developing a good understanding of what is going on, on such a small scale. Some down sides to this type of microscope just as with any professional piece of scientific equipment is its price and its size. However with new technology they are getting smaller and almost can fit on a regular desktop or table top. Trained professionals must operate the equipment and they can pose dangers due to x-rays. With its many uses and ability to produce three-dimensional images the Scanning electron microscope has proven itself a very useful tool in the science of microscopy. Another facet of microscopic technology that has proven its usefulness is the acoustic microscope.

Acoustic

Acoustic Microscopy is a very useful form of nondestructive testing that is very valuable in industry for both biological and non-biological testing. There are three main forms of acoustic microscopes that are divided into two different types, reflection and transmission method microscopes. The two types of reflection microscopes are C-mode scanning acoustic microscopes and scanning acoustic microscopes. The other form is the scanning laser acoustic microscope which is primarily operated as a transmission method microscope but can also be setup as a reflection method microscope. It has also has had a large commercial impact on integrated circuits and identifying voids in them.

The oldest form of acoustic microscopy is scanning acoustic microscopy (SAM). SAM operates by creating a high frequency sound wave, typically 100-2000 MHz, and shooting it into a wide angle acoustic lens. This is then aimed at the sample with a large enough angle that wave propagation is eliminated in the sample.(Fig. 1b) Like the other two methods the sample needs to be submerged within an inert liquid, most commonly used is distilled water. However, this method can only produce an image of the surface of the material or an extremely small distance into the sample. Also it takes longer than some other methods at roughly 10 seconds to produce one frame. This is partly due to the fact that the transmitter is also the receiver meaning it has to constantly be flipped between transmitting and receiving.(2)

This slow imaging speed is made up for by the fact the SAM is the highest resolution of the three types. It can create an image that is almost as clear as an optical microscope. If you wish to get a resolution that is higher than an optical you need to make the object extremely cold. This is done by using liquid helium as the liquid the sample is immersed in lowering it to near absolute zero temperatures. The major drawback to this is that it can only be used in non-biological testing applications as any living thing would be destroyed at such low temperatures.(2)

Advancements in Scanning acoustic Microscopy eventually led to another form of reflection acoustic microscopy. This type is known as C-mode Scanning Acoustic Microscopes (C-SAM). C-SAM operates as a reflection style acoustic microscope instead of a transmission microscope which will be discussed further in the next section. A reflection acoustic microscope works by bouncing a sound wave from a transducer off a sample and back into a receiver. More specifically the C-SAM suspends a transducer over an inert fluid such as distilled water. It then shoots a high frequency pulse, typically 10-100MHz, through an acoustic lens, the fluid, and the sample. The echo is then measured and a picture of the sample including any voids, cracks, or other imperfections is formed. By recording the pulse at different time intervals the machine can form an image of a layer within the sample and not just the surface. This is done by using an electronic gate that will filter out all of the unnecessary pulses from other layers.(2)(Fig. 1c)

There are many uses for C-SAM’s in research today. One of the most popular is with semiconductors and integrated circuits specifically the packaging around them and the corrosion of them. However, C-SAM’s are extremely expensive and do take longer to produce an image then the other two main types of acoustic microscopy.

The last type of acoustic microscopy to be invented was scanning laser acoustic Microscopy (SLAM). SLAM is operated by shooting a high frequency sound wave, between 10-500 MHz, out of a transducer from the bottom of a sample. This wave is then measured by a rapid scanning laser beam that reads the reflected wave like an old record player. If the sample is not optically reflective then a reflective plastic needs to be placed over the sample so that the beam can be bounced back to the receiver. Due to the fact that the high frequency waves can’t travel through air the sample needs to be placed into an inert fluid.(2) When the sound wave encounters a void or other imperfections in the sample the sound wave will be dispersed, by the air pocket causing an inconsistency in the ripples.(1)(Fig 1a) Using this data from the ripples, a shadowgraph can be formed that shows the imperfections as a dark void on the graph.(2) Also because a mechanical wave is used instead of light, other properties of the material can be perceived. One area that is especially helpful to study is the elastic properties of a sample which allows stress concentrations to be seen.(3) When operating at a frequency of around 100 MHz a resolution of 50 µm in solid materials can be achieved.(1)

SLAM is the fastest of the three major types of acoustic microscopy operating at 30 frames per sec. This high scan rate allows a real-time image of the object to be created allowing a user to watch a defect form during testing. Also with the right monitor and software a color image of the sample can be produced.(2) A color image is usually produced when evaluating the elastic properties of a material as a different color can be assigned to each elastic number.(3) This real-time visual image is extremely helpful when trying to focus on or locate a specific area because visual references or landmarks on the material can be used. Another advantage is that they can accommodate a large test size with the right calibration up to a 10in. x 10in. plate.(2)

Acoustic microscopy has a variety of uses in today’s manufacturing and research. Although it is a very valuable tool it has remained a niche market due in part to its high cost and the fact that extremely high frequencies are needed to produce a clear image.(3) Also its ability to only go to the micro scale instead of the Nano scale holds it back for some applications. It is still used however in many applications that can benefit from nondestructive testing. One of these applications is on the examination of spot welds. Acoustic microscopy has been found to be very helpful in this application because you can see beneath the weld surface without having to cut into the specimen.(4) One exciting application of acoustic microscopy that could have a direct impact on our health and well-being is through biological testing. Right now acoustic microscopy is not used that often in a medical application but that is looking to change. Currently there is at least one doctor who is looking to apply it to the detection of tumors and lesions in real time. It can also be used to detect different properties of biological materials as well as scanning inside the body such as looking for clogged arteries. The largest benefit in using this for biological testing is that the cells can be left alive and there is no need for dying the cells or other preparation before testing.

One are part of industry that acoustic microscopy has had the largest influence on is in the study of defects in integrated circuits (IC). There are two main reasons that acoustic microscopes are favored in IC inspections. The first is that acoustic microscope can peer into an object and see what is hidden inside without destroying the object in the process. Also they are extremely sensitive to air pockets within an IC making detection of defects very simple. This early detection has allowed companies to detect when a machine is going bad and needs to be replaced sooner than they would have in the past. This has saved them countless dollars and helped retain customers due to a higher quality product. It can also lead to better designs as engineers are able to see where voids form and stress concentrators are located with certain shapes and manufacturing methods.(Fig. 2)

Acoustic microscopy is a form of research that has been around for decades but still has an exciting future ahead of it. Researchers are continuing to increase the resolution of images to better identify imperfections. Also modern technology is further reducing the cost of acoustic microscopes and allowing them to be used in more applications where it would not have been financially viable before. With the use of composites becoming more prevalent in modern manufacturing, acoustic microscopy is sure to play a role in looking at delaminating and internal voids in structure critical components.

Moheimin Khan

Scanning Probe Microscopy

The scanning probe microscope (SPM) is the most recent development in microscopy, and allows for the most detailed examination and resolution, down to the nanometer scale. SPMs utilize a small probe to scan a surface and determine its properties, and the two major types are the scanning tunneling microscope (STM) and atomic force microscope (AFM). Scanning probe microscopes started with the invention of the STM by G. Binnig and H. Rohrer in 1981 (6). The AFM came later in 1985 and the two won half of the Nobel Prize in Physics for their invention of the STM (5). Currently, the number of SPMs being used in experimentation is increasing because of the numerous ways to modify and change the tip, as well as study surfaces. SPM is a unique type of microscopy, as it does not rely on electrons or light, but uses a probe to create a three-dimensional image of the surface at the atomic level (1). The probe is a major part of an SPM, and it consists of a long cantilever and a tip with a radius in the nanometer range (6). The tip is scanned across the surface and becomes deflected as it interacts with the material. The motion of the probe is recorded by equipment and stored in a computer, where the image is formed (1). Unlike other techniques, SPM can produce high quality, three-dimensional images with resolutions as high as 0.1 nm (7).

The first important scanning probe technique is scanning tunneling microscopy. STM works by bringing the probe near the surface of the material being tested, and applying a potential difference (voltage) that is recognized and used to create an image of the area. The current between the wire and surface is measured and a feedback system records the information (6). The current is called the quantum tunneling current (QTC). QTC is a phenomenon of quantum mechanics where the electrons are able to pass through a solid surface because they are treated as waves. The probability of an electron being found on the opposite side can increase if the separation is small. Another part of the STM is the piezoelectric materials, which change their electrical properties when subjected to a pressure. Their function is to change the orientation of electric fields and voltage. In an STM, their function is to scan the tip of the probe. To increase accuracy, electronics and "a feedback loop" are used in the STM and this combination ensures that the tunneling current stays constant (5). The voltage through the piezoelectric can be adjusted by feedback to change the distance to the surface if no current is measured (6). The voltage also serves another purpose, which is to create the incredible three-dimensional STM images. During scanning, the voltage relationship between piezoelectrics on different axes is recorded and the full three-dimensional image is formed from incremental scans on the third axis. Data is shown as topographical or elevated maps, like potential wells and peaks (4). Though it is a great technology, there are both advantages and disadvantages to using the STM. One benefit is the accuracy of current monitoring due to the excellent tip control of the probe (5). In addition, the STM allows the surface of a material to be studied in depth, giving clear images of atomic structures (10). Unfortunately, there are limitations to the STM method. STM is usually used solely for conducting and semi-conducting materials because of the tunneling electrons (8). Also, it only provides information on surface details, not the inside.

There are many uses for the STM, and one important application is in the study of semi-conductors. Silicon, which is an important and widely used element in semiconductor applications, had been studied before using earlier electron methods. However, much was still unknown. Once the STM was invented, the structure of a type of silicon, Si(111) 7x7, was resolved to the atomic scale, providing detailed information on the arrangement of atoms in the unit cell (10). Along with computational methods, the STM allowed scientists to determine that the cause for the large structure was to reduce total surface energy by forming bonds (10). In addition, the STM was used to study the complex crystal structure of gallium arsenide (GaAs), as its different phases exhibit different properties and can be used in various electronic devices (10). The use of the microscope furthered knowledge on the surface structures of the semiconductors, so they could be better used in new electronics. Another important application of the STM is in atomic manipulation. Using the STM, molecules and even individual atoms can be positioned in a process known as nanofabrication (11). The STM is especially useful because it operates using the tunneling current, which can alter bonds between particles. Atoms can be moved horizontally by changing the voltage and current, which varies the force exerted by the probe, moving the atom; this is called lateral manipulation (11). A vertical manipulation can also be done by using the electric field to pick up the atom. The technique allows a manipulation of elements and different materials on the most fundamental level possible today. This ability to create and study structures at the atomic level has created a use for STM in future nanotechnology.

The second major technique of Scanning Probe Microscopy is atomic force microscopy. The AFM works by determining the force between atoms of the sample and tip of the probe by measuring the cantilever deflection (5). The force is variable, and depends on the sample, distance, and shape of the probe (4). The resulting information can be used to produce images similar to the STM. Many AFMs utilize laser deflection, where a laser is used to track the cantilever movement. It is reflected off the back of the lever, to a position detector which records the deflection (5). Each lever has a specific spring constant which determines its deflection under a certain amount of force, depending on the material of the lever (4). Force is calculated using Hooke's Law (F=-kx), where x is the vertical displacement of the lever and k is the spring constant. There are many different modes of operation for the AFM, making it versatile in the study of materials. In contact mode, the probe is scanned across the specimen with a constant force. As it is deflected due to variation in the surface, it is adjusted to the initial position (3). The tip maintains a constant displacement above the sample, and the necessary adjustments are recorded (5). The position of the scanner is used to create an image; resolutions can be less than 1 angstrom vertical and less than 1 nanometer horizontally (3). In dynamic force or tapping mode, the probe tip oscillates and periodically taps the sample. The lever is oscillated close to its natural (resonant) frequency and is placed close to the material. As the probe is repelled by interatomic forces, it taps the sample (5). This method gets rid of shear forces that occur in contact mode and is better suited for softer surfaces (7). The position of the tip and the amplitude of vibration are used to make a map of the area; resolutions can be less than 1 angstrom vertical and less than 50 angstroms horizontally (3). AFM is a highly advantageous technique and has a broad range of uses. AFM can image a variety of surfaces, including polymers, ceramics and composites, and biological material (5). Little or no preparation is necessary, and the environment that the sample is placed in can be air or a liquid (3). The large number of possible operating modes expands its range of application. Still, the AFM has some limitations. In some modes, the AFM scanning processes can be time-consuming and because of special detecting equipment, it is generally much more expensive than other forms of microscopy (11).

There are also many applications of AFM, one useful technique being the process of Nanolithography. Mechanical lithography is a procedure that uses a tool to create a pattern or deformation on a material. Because the AFM relies on small force measurements to work, the tip can be used as an extremely accurate machining tool, where the cutting depth is monitored by feedback (9). Different materials and probes have been tested, and the effect of machining on the tip has been observed. This led to the development of a durable, diamond probe that effectively manipulates the nanostructure of many materials, with machining depths around a few nanometers (9). Although the process is not efficient and can be very expensive, the ability of the AFM to accurately shape micro and nanostructures has furthered research and development in Nano science and future technology.

Some applications apply to both STM and AFM. One example is the use of SPM to examine deformation and crack growth in materials. The wear and possible failure of materials is always a concern, and often the starting points are hard to observe, especially in fatigue crack initiation. However, by using the STM, plastic deformation can be more easily studied. In one experiment, single crystal slip bands were easily visible using the STM and crack nucleation along with fatigue was distinct. This allowed for the study of shallow cracks due to high vertical resolution of the scanning tips (4). The AFM was also useful in fatigue and crack analyses. For example, a source of cracks was found in one experiment dealing with titanium samples, and the slip was measurable with high accuracy in the range of angstroms (4). In general, SPM is a great tool to examine deformation in materials. It allows the samples to be studied quickly, as the surface does not need to be polished. Thus, the microscopic effects of plastic deformation and even fracture can be easily studied. In addition, the three-dimensional operation and images provide valuable information. The SPM can be used to find the stress and strain fields close to a brittle crack. Its images can show the relation of material and its rate of transfer (12). Fatigue caused by the environment can be analyzed if a loading machine is used along with an SPM. Then, the impact of several factors on crack imitation and propagation can be constantly observed (12).

The Scanning Probe Microscope is a useful technique in the study of materials and their properties. As the newest of the microscopic techniques, SPM utilizes the newest scientific principles and technology to achieve results that further our understanding of materials all the way down to the atomic scales. The scanning transmission microscope and atomic force microscope are two of the most widely used SPM techniques, and they have many applications such as examination of small-scale deformations and cracks, even the manipulation of single atoms. The ability to study structures at nanometer level makes SPM an important tool in the developing fields of Nano science and nanotechnology, giving the microscopic technique a role in the future of engineering. The various modes of operation allow for countless methods to study a material, depending on the requirements, and many innovations will certainly be made in the future.