Through the old ages imaging surfaces and analysing the morphology of assorted samples on a micro graduated table was a dominant and demanding challenge which scientists had to manage. Scaning investigation microscopy can bring forth a extremely exaggerated image of the surface or the majority of the sample by scanning the specimen utilizing a physical investigation. Atomic Force microscope ( AFM ) was originally developed in order to get the better of the restrictions of the scanning burrowing microscope ( STM ) .
In order to analyze and qualify a sample different techniques have to be carried into consequence. Due to the fact that human oculus every bit good as optical microscope can non be used to see dimensions at nano degree, other imaging techniques have to be obtained. Scaning investigation techniques brought a revolution in this field supplying a better image for a assortment of surfaces. Initially, scanning investigation microscopy ( SPM ) was introduced, but due to grounds that will be explained subsequently, atomic force microscopy ( AFM ) was besides founded. In this study the rules of operation of the atomic force microscopy, the methods, the advantages, the restrictions and the applications of this technique are presented.
More specific, the specimen is scanned utilizing a physical investigation and images of the surfaces are being formed. This surface image is being produced due to automatically traveling the investigation in a scan of the specimen and entering the interaction between the investigation and the surface as a map of place. The impressive and interesting factor of this technique is that it can make atomic declaration.
Atomic force microscopy ( AFM ) provides us with the ability to analyse other stuffs, like polymers, ceramic stuffs, proteins, biological samples making the image of nonconductive surfaces. In add-on, the inter-atomic forces between the sample and the tip can be measured while making digitally a topographical surface of the sample. Furthermore, atomic force microscopy is used to mensurate the thickness of a crystal growing bed or even to find the raggedness of a surface sample. In general, AFM can analyse and qualify samples at the atomic degree, with declaration runing from 0.1 nanometers to 1 nanometers.
Scaning Probe Microscopy ( SPM ) was founded in 1981 by Gerd Binnig and Heinrich Rohrer ( at IBM Zurich ) [ 1 ] . Furthermore, few old ages subsequently, in 1986 Nobel Prize in Physics was awarded to them for the innovation of the scanning burrowing microscope ( STM ) . Unfortunately, STM can merely image stuffs that can carry on a tunnelling current. In order to get the better of this restriction Atomic Force Microscope was invented by G. Binnig, Ch. Gerber and C. Quate at Stanford University. They attempted to paste a bantam crisp of diamond onto one terminal of a bantam strip of gilded oil.
How does it work?
Atomic force microscopy ‘s rule of operation is based on the measuring of the force between the investigation and the sample, which depends on their distance. A cantilever with a crisp tip at one terminal ( investigation ) is used to scan the sample. Recording the perpendicular place of the tip while is rastered across the sample a topographic image is build up by the computing machine. Cantilevers are normally made from Si ( Si ) or silicon nitride ( Si3N4 ) and scope from 100 to 200 I?m in length, 10 to 40 I?m in breadth and 0.3 to 2 I?m in thickness. [ 2 ]
Figure 1. Illustration of a tip scanning a sample surface. The image is reproduced from Rubens Bernardes-Filho & A ; Odilio Benedito Garrido de Assis
How are forces measured?
The cantilever acts as a spring and the sum of forces between the investigation and the sample is depended on the stiffness of the cantilever ( spring invariable ) and the distance between the investigation and the sample surface. These forces lead to a warp of the cantilever harmonizing to Hooke ‘s jurisprudence.
F = – kx
K = spring invariable
x= cantilever warp
Figure 2. A ) Spring word picture of cantilever B ) SEM image of a triangular SPM cantilever with investigation. Images are from MikroMasch.
Forces versus distance curve
The dominant interactions at short distances between the investigation and the sample in the AFM are Van der Waals interactions. However, there are besides other interactions such as capillary, electrostatic and magnetic, which are important in a farther distance from the surface. In methods like SPM ( Scaning Probe Microscopy ) these interactions play an of import function. [ 3 ]
When the investigation physical touches the surface ( contact manner ) , the investigation predominately experiences abhorrent Van der Waals forces. This has as a consequence the tip warp which was mentioned earlier. While the investigation moves off from the surface ( non-contact manner ) attractive Van der Waals forces are dominant. The dependance of the new wave dew Waals sing the distance between the tip and the sample is shown in Figure 3.
Figure 3. Plot of force as a map of probe-sample separation. Images are from Robert A. Wilson and Heather A. Bullen, Department of Chemistry, Northern Kentucky University, Highland Heights
Manners of operation
The basic construct of operation is shown at Figure 4. There are three primary manners of operation: contact manner, non-contact manner and tapping manner.
Figure 4. Principle of AFM
I. Contact manner is the first and more widely used manner of operation. It operates in the abhorrent government of the Van der Waals curve. The cantilever bends ( as shown in the image ) when the spring invariable of the cantilever is less than the surface of the sample. The force between the investigation and the sample is highly low ( a??10-9N ) and remains changeless by utilizing the feedback loops to keep a changeless cantilever warp. A few instruments operate in UHV but the bulk operate in ambient atmosphere, or in liquids. Furthermore, the stiffness of the cantilever should be less than the effectual spring changeless keeping atoms together due to the fact that the tip is in difficult contact with the surface. Most contact manner cantilevers have a spring invariable of & lt ; 1 N/m. [ 3 ] [ 4 ]
The advantages of the contact manner is that it is a fast scanning technique which can run good for unsmooth samples and can besides be used in clash analysis. In add-on, it is the lone manner where “ atomic declaration ” is possible. [ 3 ] However, because in this manner the investigation is in contact with the surface there is the possibility to damage or deform samples, particularly soft samples like biological tissues or polymers.
II. The non-contact manner uses attractive forces to interact the surface with the tip. A stiff cantilever is oscillated in the attractive government, intending that the tip is rather close to the sample but does non touch it ( as shown in the image ) . It oscillates above the liquid absorbed bed on the surface during scanning. [ 5 ] The force between the investigation and the sample is highly low ( a??10-12N ) . Measuring the alterations to the resonating frequence or the amplitude of the cantilever the surface topography can be measured. [ 4 ]
Figure 5. In the non-contact manner of operation a frequence displacement in the resonance extremum of the cantilever is induced. Picture is from the book, Atomic force microscopy: biomedical methods and applications. By Pier Carlo Braga and Davide Ricci
The non-contact manner has the benefit that it is a non-destructive manner of operation. Furthermore, the force exerted to the sample is really low ( a??10-12N ) and there no sidelong forces. The chiefly drawback is that it has by and large low declaration and in order to hold best imaging an extremist high vacuity ( UHN ) is needed. Furthermore, the scan velocity is lower than contact manner in order to avoid contact with the H2O bed. [ 3 ]
III. In the tapping manner or intermittent contact manner, the investigation lightly “ lights-outs ” on the surface of the sample during scanning and merely touches the sample at the underside of each oscillation. In most instances, the cantilever oscillates near its resonating frequence ( a??200 K Hz ) in order to better sensitiveness. [ 5 ] The cantilever oscillates closer to the sample in comparing to the non-contact manner. Stiff cantilevers and Si investigations are chiefly used for this manner because of the strong forces by the interactions with the thin H2O bed.
The most of import advantage of the tapping manner is that it allows high declaration of soft samples. It can be performed on both wet and dry sample surfaces. Besides, it eliminates sidelong forces such as retarding force. However, when imaging in liquids a slower velocity is needed in order to scan the sample.
Different operation manners should be chosen harmonizing to the features of the sample, since each manner has different advantages.
Today most AFMs use a optical maser beam warp system, introduced by Meyer and Amer in order to bring forth a topographic image. In an AFM contact manner foremost the AFM tip is brought manually near to the sample surface. Then the scanner makes a concluding accommodation in the distance between the tip and the sample harmonizing to a set-point determined by the user. Under ambient conditions, sample surfaces are covered by a bed of adsorbed gases dwelling chiefly of H2O vapour and N which is 10-30 monolayers thick.
Figure 6. Conventional diagram demoing the operation rules of the AFM in the contact manner. The image is from Cheryl R. Blanchard, Southwest Research Institute San Antonio
Now, the tip is in contact with the H2O bed and is scanned the sample under the action of the piezoelectric actuator ( either by traveling the tip relation to the sample or the other manner around ) . As shown in Figure 6, a optical maser beam aimed at the dorsum of the cantilever-tip reflects from the cantilever surface to a split photodiode. This sensor measures the bending of the cantilever during the scanning across the sample. A feedback cringle maintains changeless the separation between the tip and the sample by traveling the scanner in the omega way to keep the set-point warp. Finally, the fluctuation in the x-y plane and the distance the scanner moves in the omega way are used to bring forth a topographic image of the sample surface.
As I mentioned before the photodiode is divided into four parts, as shown in Figure 7. If the optical maser is displaced vertically along the places ( B-A, D-C ) a bending due to topography is produced, while if the motion is horizontal ( B-D, A-C ) a tortuosity because of the “ clash ” ( sidelong force ) is produced.
Figure 7. The scanned cantilever system. The image is reproduced from Prof. Nikos Frangis, Aristotle University, Thessaloniki, Greece
Tip and Cantilever
In the early old ages of AFM operation, the tips were made from crushed diamond atoms, which were manually glued on the cantilevers. Nowadays, investigations ( cantilevers with a crisp tip at one terminal ) are made from Silicon ( Si ) or Silicon nitride ( Si3N4 ) and are available commercially. In add-on, research workers consider C nanotubes as the following investigation stuff. Differentiations in cantilever ‘s length, stuff and form allow assorted spring invariables and resonating frequences. V- shaped cantilevers are more popular because they can supply a low mechanical opposition to perpendicular warp and high opposition to sidelong tortuosity. However, there are besides rectangular cantilevers ( Figure 8. ) A typical tip radius is about 10 nanometer, the spring invariable is between 0.1 to 100 N/m and the resonance frequence 5-500 kilohertz. [ 5 ]
Figure 8. Different investigations. Optical microscopy images of V-shaped cantilevers and rectangular cantilevers. Pictures are reproduced from DoITPoMS, Department of Materials Science and Metallurgy, University of Cambridge
Phase Imaging, besides referred to as stage sensing microscopy ( PDM ) is a powerful extension of Taping Mode Atomic Force Microscopy ( AFM ) that provides nanometer-scale information about surface construction which is frequently non revealed by other SPM techniques. In the stage manner imaging the stage slowdown between the signal that drives the cantilever oscillation and the cantilever oscillation end product signal are measured ( see Figure 8 ) . Changes in the stage slowdown can be correlated with specific stuff belongingss ( chiefly mechanical belongingss ) that consequence the interaction between the tip and the sample. The stage slowdown can be used to observe different stuff belongingss such as clash, adhesion and viscoelasticity. Phase imagination is really utile for polymer research and for look intoing the magnetic and electrical belongingss as in Electric Force Microscopy ( EFM ) and Magnetic Force Microscopy ( MFM ) .
Figure 8. Phase imaging. Picture is reproduced from K.L. Badcock and C.B. Chatterer
The tip of the AFM is used for mensurating forces, imaging surfaces and as a nanoscale tool for cutting or pull outing soft stuffs such as polymers and DNA.
AFM is a utile, interesting and uncovering imagination technique. In this point, images are presented in order to understand better and see some samples. To recognize the magnitude of this method dimensions are besides mentioned.
Figure 9. Tetraphenylporphyrin molecules deposited on vitreous silica, AFM image at ambient conditions ( MFP 3D, Asylum Research, CA ) . Contact manner, image size 20*20 Aµm2. Picture is reproduced from the Surface & A ; Plasma Technology. Research Group of the Institut fur Allgemeine Physik
Figure 10. Picture is reproduced from Colorado Advanced Photonics Technology Centre.
Figure 11. Some images that were obtained at EMTERC. Picture is reproduced from talk notes.
Advantages of AFM over other imaging techniques.
AFM vs. STM
AFM was developed in order to get the better of the restrictions of STM. While STM is by and large applicable merely to carry oning samples, AFM is applicable to both music directors and dielectrics. However, in some instances, the declaration of STM is better than AFM due to the exponential dependance of the burrowing current on the distance. [ 5 ]
AFM vs. SEM
SEM ( Scaning Electron Microscope ) .
Compare to SEM, AFM make measurings in three dimensions, x, Y, and omega supplying it with the advantage to obtain a 3-dimensional image of the surface without any readying and with low cost. In add-on, AFM does non necessitate a vacuity environment or any sample readying ; it can run in an ambient or liquid environment. Although the declaration of AFM is higher than SEM, the scan velocity is lower than SEM.
AFM vs. TEM
TEM ( Transmission Electron Microscopy )
AFM has the advantage that presents 3-dimensional images of a sample surface without any expensive sample readying. That is besides another benefit in comparing to TEM because more information about the construction is available.
AFM vs. optical microscope
AFM seems to hold a immense sum of advantages over the optical microscope. However, the lone restriction is by the radius of the tip.
Restrictions of AFM
Although AFM can run in assorted environments ( air, liquid, vacuity ) and can be used to analyze a broad assortment of samples ( music directors, dielectrics, semiconducting materials ) , there are some restrictions sing the atomic declaration. The radius of the tip and the acuteness of it can hold a great impact on the declaration.
Figure 12. Diagram of AFM cantilever tip interaction with a surface. Images are from Robert A. Wilson and Heather A. Bullen, Department of Chemistry, Northern Kentucky University, Highland Heights
As shown in Figure 12, while the tip scans across the sample, the sides of the tip make contact before the vertex and from that contact the feedback mechanism responds to the characteristic. This chiefly occurs when the characteristic is sharper than the tip. The phenomenon is known as tip whirl or tip imagination.
Another possible restriction of this technique is compaction of characteristics. Research workers are investigated the force per unit area of the tip over the sample. This compaction has a important impact in the instance where the samples are soft biological polymers such as DNA. Last but non least, the strong interaction with the sample, particularly when contact manner is performed, is another point that limits AFM possibilities and applications.
With the legion advantages that were discussed earlier AFM has significantly impacted the Fieldss of natural philosophies, chemical science, biological science and stuffs scientific discipline.
Potential applications of AFM:
Substrate raggedness analysis, mensurating the surface forces and the adhesion of thin-film and majority stuffs
Measures the elastic and fictile behavior and hardness via nanoindentation
Measures the country and the volume of the defect at a surface of different types of stuffs such as metals, crystals and ceramics
Ideal for analyzing cleft extension in surfaces due to the fact that AFM gives great contrast on level samples
Measures images of composite polymers with small or no sample readying
Visualises easy nanoparticles ; the size, the volume and the surface country of them
High declaration images of C nanotubes
Images integrated circuit french friess
Data storage media. Creates constructions with nanometer-sized dimensions
Measures bio-molecules such as DNA ( every bit long as they are straight attached to a surface )
Accurate metrological measurings on optically crystalline stuffs
Visualizes and characterizes paper coatings ( in the instance where paper coatings are comprised of nanoparticles )
Atomic force microscopy was introduced in 1986 as a method to analyze the surface of insulating samples. For building the sample ‘s topography the force that is present between the investigation and the sample has to be measured. This force depends on the distance between the investigation and the sample, the nature of the sample, the sample surface taint and the investigation geometry. In order to observe the supplanting of the cantilever a optical maser beam warp system is used. A optical maser is reflected from the dorsum of the cantilever and collected in a sensor. There are different manners of operation. At close contact the force is abhorrent piece at a larger separation the force is attractive. Harmonizing to the features of the sample different operation manners should be chosen. AFM leads us to the sensing of the atomic graduated table features on assorted insulating surfaces such as ceramic stuffs, polymers and biological samples. Furthermore, through the old ages AFM evolved into a promising and important instrument supplying new penetrations in the field of stuff scientific discipline, biological science and electrochemistry. Although there is a bound sing the operation of AFM, researcher attempt to get the better of it chiefly by manufacturing tips from different stuffs. The following measure is the usage of C nanotubes as tips.
[ 1 ] “ Scaning burrowing microscopy ” . G. Binnig, H. Rohrer ( 1986 ) . IBM Journal of Research and Development 30: 4.
[ 2 ] Park Scientific Instruments. A practical usher to scanning investigation microscopy ( 1997 )
[ 3 ] Atomic Force Microscopy, Basic Theory. Robert A. Wilson and Heather A. Bullen, Department of Chemistry, Northern Kentucky University, Highland Heights, KY 41099
[ 4 ] hypertext transfer protocol: //www.nanoscience.com/education/
[ 5 ] Lecture ‘s notes of the faculty: ENGT5129 Physical & A ; Electrical Measurements
[ 6 ] Atomic Force Microscopy. Cheryl R. Blanchard, Southwest Research Institute San Antonio