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All microscopes which provide images of atoms on or in the surface of any body is called atomic force microscope or the AFM, while all those microscopes which uses the principle of the atomic force microscopes are called as scanning probe microscope or the SPMâ„¢S.
These scanning probe microscopes work by measuring a local property such as height ,optical absorbtion magnetism etc ,by scanning over the sample by measuring the above property and providing the image on the television,with the probe or the tip placed very close to the sample.The resulting image obtained on the television screen consists of rows and columns of informationâ„¢s on above the other which have to be decoded for getting the image .The small probe sample seperation makes it possible to makes it possible to make measurements of and over a small area.
These microscopes have high accuracy to measure surfaces of size of nanometers (especially that of an atomic force microscope). These scanning probe microscopes have high resolution power in the order of picometers unlike the ordinary and traditional microscopes these scanning probe microscopes do not use or pocess any lens to show or measure the surface so the size of the probe rather than diffraction effects generally limit their resolution.
.These atomic force microscopes provide images of any surface in three dimensions
The below figure is of a atomic force microscope.
HISTORY OF ATOMIC FORCE MICROSCOPE
Ancestor of atomic force microscopes is the scanning tunneling microscopes or the STMâ„¢S.The difference between the scanning tunneling microscopes and the atomic force microscopes is that while the scanning tunneling microscopes provide images ,while simultaneously measuring the local property , the atomic force microscopes can measure and provide images of surfaces of any body ,be it organic or inorganic, metallic or non-metallic ,liquid or solid .
The history of atomic force microscopes begins in 1981 when the scanning tunneling microscope was first invented by G.BINNIG AND H.ROHRER who shared the second half of the noble prize in physics in 1986.It was in 1991 the concept of atomic force microscope and the two types of the atomic force microscopes one being the contact mode and the non contact modes of the atomic force microscope.
Atomic force microscopes is being used 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 investigating 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. The publications related to the AFM are growing speedily since its birth.
MODIFICATIONâ„¢S IN SCANNING PROBE MICROSCOPES(SPMâ„¢S)
Other measurements can be made using modifications of the SPM. These include
1) Variations in surface microfriction with a lateral force microscope (LFM)
2) Orientation of magnetic domains with a magnetic force microscope (MFM),
3) Differences in elastic modulii on the micro-scale with a force modulation microscope
A very recent adaptation of the SPM has been developed to probe differences differences in chemical forces across a surface at the molecular scale. This technique has been called the chemical force microscope (CFM). The Atomic foce microscopes and Scanning tunneling microscopes can also be used to do electrochemistry on the microscale.
4) TIP OF THE CANTILEVER
6) OPTICAL LEVER( If necessary)
7) FEEDBACK LOOP
9) A TELEVISION TO AQIURE THE IMAGE
HOW DOES AN AFM WORK Cantilever deflection detection by Laser beam deflection method
AFM operates by measuring attractive or repulsive forces between a tip and the sample. To achieve this most AFMs today use the optical lever, a device that achieves resolution comparable to an interferometer while remaining inexpensive and easy to use.
The optical lever operates by reflecting a laser beam off the cantilever.As stated above referring to the Figure 2 on the cantilever is the laser impinged when the cantilever with the sample tip scans the surface .When the tip moves over the surface cantilever deflects as such the laser undergoes an angular deflection of two fold and this angular deflection is captured by a photodectector which is position sensitive .This photodetector consists of two side by side photodiodes .the difference between the two photodetector signals indicate the position of the laser spot and thus the angular deflection of the cantilever.
Because the cantilever to detector distance generally measures thousands of time the length of the cantilever the optical lever greately magnifies the image or the motions of the tip.optical lever can theoriticallly obtain 2000 fold magnification .To aquire an image the microscope raster scans the probe over the sample while measuring the local property in question
Figure 3 Figure 4
TYPES OF ATOMIC FORCE MICROSCOPE
There are two types of atomic force microscopes or rather there are two modes of operation of an atomic force microscope
1) CONTACT MODE OF AFM
2) NON CONTACT MODE OF AFM
CONTACT MODE OF AFM
In the contact mode of the atomic force microscope or the dc mode the tip touches the surface while scanning the sample or a constant force is applied on the cantilever while scanning the surface .In the DC mode the cantilever should be soft enough to reduce the loading force as much as possible .Thus constant force is applied by a feed back network which is discussed later
NON CONTACT MODE OF AFM
In this sort of microscopy the tip does not touch the surface of the sample while scanning the surface ie the tip is vibrated in its resonant frequency vertically .in the AC mode different mechanical features are required such as they should pocess large spring constant ,high resonant frequency ,higher Q factor .In this the vibrating frequency of the tip changes or the resonant frequency of the tip changes when it touches the sample with high mechanical Q factor large changes occurs .This means that sensitivity is accomplished in the deflection sensingin an atomic foec microscope and that good regulation of a distance between a tip and sample is expected Both in the AC and the DC mode ,lateral resolution depends upon the tip surface .A sharper tip must be used to obtain higjer resolution images .The sharper tips are prepared by microfabrication technique
AFM CANTILEVERS HAVE HIGH FLEXIBILITY
usually the cantilevers have very high flexibility so as to sustain fluctuating.A high flexibility stylus exerts lower downward forces on the sample, resulting in less distortion and damage while scanning. For this reason AFM cantilevers generally have spring constants of about 0.1 N/m (figure 7).
It would take a very long time to image a surface by dragging a Slinky over it, because a Slinky cannot respond quickly as it passes over features. That is, a Slinky has a low
resonant frequency, but an AFM cantilever should have a high resonant frequency.
These should have high reflectivity as such they have a coating of gold over it
Schematic illustration of the meaning of "spring constant" as applied to cantilevers. Visualizing the cantilever as a coil spring, its spring constant k directly affects the downward force exerted on the sample.
The equation for the resonant frequency of a spring:
This shows that a cantilever can have both low spring constant and high resonant frequency if it has a small mass. Therefore AFM cantilevers tend to be very small. Commercial vendors manufacture almost all AFM cantilevers by microlithography processes similar to those used to make computer chips.
INEXPENSIVE AND REASONABLY SHARP TIPS BY MICROMACHINING
Force microscopists generally use one of three types of tip.
The "normal tip" (figure 8a; is a 3 Ã‚Âµm tall pyramid with ~30 nm end radius.)
The electron-beam-deposited (EBD) tip or "supertip" (figure 8b); improves on this with an electron-beam-induced deposit of carbonaceous material made by pointing a normal tip straight into the electron beam of a scanning electron microscope. The supertip offers a higher aspect ratio (it is long and thin, good for probing pits and crevices) and sometimes a better end radius than the normal tip.
Finally, Park Scientific Instruments offers the "Ultralever" (figure 8c), based on an improved microlithography process. Ultralevers offers a moderately high aspect ratio and on occasion a ~10 nm end radius.
Three types of AFM tip. (a) normal tip (3 Ã‚Âµm tall); (b) supertip; © Ultralever (also 3 Ã‚Âµm tall).
TUBE PIEZOCERAMICS POSITION THE TIP OR SAMPLE WITH HIGH RESOLUTION
Piezoelectric ceramics are a class of materials that expand or contract when in the presence of a voltage gradient or, conversely, create a voltage gradient when forced to expand or contract.This makes it possible for the three dimensional positioning of the samples to aquire 3D view of the object Piezoceramics make it possible to create three-dimensional positioning devices of arbitrarily high precision. Four electrodes cover the outer surface of the tube, while a single electrode covers the inner surface. Application of voltages to one or more of the electrodes causes the tube to bend or stretch, moving the sample in three dimensions most of the scanners are tube shaped piezo ceramics because they combine a single piece construction with high stability and large scan range using this brings high precision for atomin force microscopes(AFMâ„¢S).
Exploded view of a tube scanner. Applying a voltage to one of the four outer quadrants causes that quadrant to expand and the scanner to tilt away from it (XY movement). A corresponding negative voltage applied to the opposite quadrant doubles the XY range while preventing vertical motion. Applying a voltage to the inner electrode causes the entire tube to expand or contract (Z movement).
Exploded view of a tube scanner
AFMS USE FEEDBACK TO REGULATE THE FORCE ON THE SAMPLE
The AFM feedback loop. A compensation network monitors the cantilever deflection and keeps it constant by adjusting the height of the sample (or cantilever).
The presence of a feedback loop is one of the subtler differences between AFMs and older stylus-based instruments such as record players and stylus profilometers. The AFM not only measures the force on the sample but also regulates it, allowing acquisition of images at very low forces.
The feedback loop in above figure consists of the tube scanner that controls the height of the entire sample; the cantilever and optical lever, which measures the local height of the sample; and a feedback circuit that attempts to keep the cantilever deflection constant by adjusting the voltage applied to the scanner.
One point of interest: the faster the feedback loop can correct deviations of the cantilever deflection, the faster the AFM can acquire images; therefore, a well-constructed feedback loop is essential to microscope performance. AFM feedback loops tend to have a bandwidth of about 10 kHz, resulting in image acquisition times of about one minute.
ORDINARY STYLUS AND AN AFM
An ordinary styli profilometer has the following disadvantages
1) Calibration error
2) It can only image any sample in one dimension
3) Its accuracy and resolution is comparatively lesser than an atomic force microscope
4) It is difficult to drag the sliky over the sample
While for an ATOMIC FORCE MICROSCOPE following advantages prevail
1) No calibration error 2) Very high resolution and high precision 3) It can image the sample in 3 dimensions 4) A feedback network can also be provided in an atomic forc microscope to maintain a constant force on the cantilever
Figure below shows the ordinary styli profilometer and an AFM
ALTERNATE IMAGING MODES OF AFM
AFMs have two standard imaging modes
Almost all AFMs can measure sample topography in two ways: by recording the feedback output ("Z") or the cantilever deflection ("error"; see figure 6). The sum of these two signals always yields the actual topography, but given a well-adjusted feedback loop, the error signal should be negligible. As described below, AFMs may have alternative imaging modes in addition to these standard modes.
Optical lever AFMs can measure the friction between tip and sample
While topographic imaging uses the up-and-down deflection of the cantilever, friction imaging uses torsional deflection.
2.5 x 2.5 nm simultaneous topographic and friction image of highly oriented pyrolytic graphic (HOPG). The bumps represent the topographic atomic corrugation, while the coloring reflects the lateral forces on the tip. The scan direction was right to left.
Cross-sectional profile of friction data from above image showing stick-slip behavior.
If the scanner moves the sample perpendicular to the long axis of the cantilever friction between the tip and sample causes the cantilever to twist. A photodetector position-sensitive in two dimensions can distinguish the resulting left-and-right motion of the reflected laser beam from the up-and-down motion caused by topographic variations (Meyer and Amer, 1990).
Therefore, AFMs can measure tip-sample friction while imaging sample topography. Besides serving as an indicator of sample properties, friction (or "lateral force," or "lateral deflection") measurements provide valuable information about the tip-sample interaction.
Figure 13 shows a simultaneous friction and topography image of graphite atoms in which I have plotted the topography image as a three-dimensional project and implimentationion colored by the friction data. Each bump represents one carbon atom. As the tip moves from right to left, it bumps into an atom and gets stuck behind it. The scanner continues to move and lateral force builds up until the tip slips past the atom and sticks behind the next one. This "stick-slip" behavior creates a characteristic sawtooth waveform in the friction image (figure 14).
AFMS CAN MEASURE SAMPLE ELASTICITY
. AFMs can image sample elasticity by pressing the tip into the sample and measuring the resulting cantilever deflection.
1 x 1 Ã‚Âµm simultaneous topography (left) and elasticity (right) images of bovine serum albumen on silicon (sample prepared by Sie-Ting Wong of Abbott Laboratories).
AFM can also image the softness of a sample by pressing the cantilever into it at each point in a scan. The scanner raises the sample or lowers the cantilever by a preset amount, the "modulation amplitude" (usually 1-10 nm). In response, the cantilever deflects an amount dependent on the softness of the sample: the harder the sample, the more the cantilever deflects (figure 15).
Figure 16 shows an image of bovine serum albumen (BSA) on silicon. A number of bumps appear in the topography image, each presumably corresponding to a single BSA molecule. The elasticity image reveals that each of the bumps is soft relative to the silicon substrate, a reasonable result for protein molecules.
AFM AND BIOLOGY
Dull tips and tip-sample interaction forces prevent high-resolution imaging of biological structures
. Images 1, 50, and 100 of small collagen fibrils from a sequence of 100 images. Repetitive scanning of the same area progressively detaches the fibrils from the glass substrate, causing distortion in the direction of scanning, left-to-right and top-to-bottom.
The ability of AFM to image at atomic resolution, combined with its ability to image a wide variety of samples under a wide variety of conditions, has created a great deal of interest in applying it to the study of biological structures. Images have appeared in the literature showing DNA, single proteins, structures such as gap junctions, and living cells (for a review see Hoh and Hansma, 1992).
Unfortunately, AFM cannot image all samples at atomic resolution. The end radii of available tips confines atomic resolution to flat, periodic samples such as graphite. In addition, because biological structures are soft, the tip-sample interaction tends to distort or destroy them. Figure 12, for example, shows how forces exerted on small collagen fibrils tend to detach them from the substrate over a period of time, resulting in progressively greater distortion.
A number of companies are attempting to develop sharper tips, primarily by improved microfabrication processes. I have concentrated on investigating the tip-sample interaction with alternative imaging modes.
WHY AN AFM IN MARS
Three instruments located in a box called in the MECA(mars environmental compactibility assement containing an optical microscope an atomic force microscope and four wet chemical cells has been sent to mars .Two associated experiments ( patch plates electrometer ) are located on the robort arm of the lander which is ued to scoup up the Martian soil for the processing by the MECA instruments
Originally the part of the 2001 lander (cancelled in the year 2000 ) MECA now awaits a ride on the mars and the opportunity to complete its mission .based on the current mars architectures the earliest flight opportunity for MECA(or the MECA follow on will be in the year 2007)
MECA will determine the size and shapes of the particles and will perform experiments to learn about the abrasion adhesion and the electrostatic behaviour of the particles on the mars )
The optical microscope can image particles of about size varying from 1 millimeter to I micrometer .The AFM can image samples from 1 micrometer to about 1 nanometer this allows a closer look on the martian soil and exiting new insights can be expected
At theconclusion of the mission the MECA science team would recommend procedures to be followed by the NASA to ensure astronaut safety on Mars and provide recipes for simulating Martian soil on earth
PRINCIPLE OF AFM IMAGING
An AFM consists of a extremely sharp tip mounted or integrated on the end of the tiny cantilever spring which can be moved by a mechanical scnner over the surface to be observed .Every variation of the surface height varies the force acting on the tip and therefore varies the bending of the cantilever .The bending is measured by an integrated stress tensor at the base of the cantilever and recorded line by line in the electronic memory .The interaction between the tip and the sample can be compared with the good old turnable pick up,but the force in the AFM is about 1 million times smaller .The Mars â€œAFM is designed for a resolution of 10 nanometers in the image range of 10 micrometers)
Afm Chip With 8 Cantilevers
For redundancy the Mars Afm is equipped with 8 addressable sensors and cantilevers on one chip .Only one at a time is used for imaging .If the tip is worn or dirty it can be broken off by a command from the earth Figure 18
Afm System On Mars
For each experiment ajob list is sent from the earth via the orbiter to the lander computer on the mars .The following operations then run fully autonomously .First the Martian soil is scooped by the robort arm and loaded on the sample plate .Then the sample wheel is rotated 180 degrres and approached to within the scan range of the AFM.Afterwards the electronics moves the scan head and with the selected cantilever scans the image on the sample .Finally the images are stored on the lander computer and after a hard days work sent down to earth
The advancement of AFM in different fields is increasing day by day.One such field is CSAFM or the current sensing atomic force microscope which as the name suggests is used for sensing microcurrents at the same time give the topographic view of the surface .this operates in the same fashion as the ordinary atomic force microscopes do operate in the contact mode .Here the sharp tips used are coated with films of conducting materials .Few research have also been done in the nanomechanics of the cells .
In future it may be used for the examination of other planets also.The main disadvantage of it compared to its number of advantages is the sharp tip being used for topography which would damage the surfaces of smooth materials .some developments are also made in producing tips made of materials which would not damage the surface
1) ALBRACHET AND QUATE, Microfabrication of cantilever styli for atomic
2) WEISBORN ,ALBRACHET & QUATE,Forces in atomic force microscope in
air and water.
3) DAVID BASELT,Tip sample interactions in atomic force microscopyand its
TABLE OF CONTENTS
1.1 CONCEPT OF AFM 1.2 HISTORY OF AFM 1.3 MODIFICATIONS OF AFM
2) THEORY OF AFM
2.1 WORKING OF AFM 2.1.1 TYPES OF AFM 2.1.2 TYPES OF AFM
2.2 FEATURES OF CANTILEVERS 2.2.1 FLEXIBILITY OF CANTILEVERS 2.2.2 TIPS OF CANTILEVERS
2.3 TUBE PIEZOCERAMICS
2.4 FEEDBACK NETWORK
3) COMPARISION BETWEEN CONVENTIONAL
PROFILOMETER AND AFM
4 ) APPLICATIONS
4.1 ALTERNATE IMAGING MODES
4.1.1 MEASUREMENT OF FRICTION
4.2 ELASTICITY MEASUREMENT
4.3 AFM AND BIOLOGY
5) RECENT APPLICATIONS
5.1 AFM IN MARS
5.1.1 PRINCIPLE OF AFM IN MARS
5.1.2 WHIN IN MARS
6 ) CONCLUSIONS
GENERAL CONCEPT OF MICROSCOPY AND APPLICATIONS
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