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Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM)

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Theory and Virtual Imaging and Analysis of Surfaces Using

Atomic Force Microscopy (AFM)

Aim of the Experiment:

The main objective of this experiment is helping students to be familiarized with the atomic force microscopy theory. Additionally, it aims to help them fully understood the performance of AFM instrument to see surfaces via using the Australian Microscopy and Microanalysis Research Facility (AMMRF) Myscope website.

Introduction:

Atomic Force Microscopy (AFM)

The atomic force microscope instrument is a type of scanning probe microscopes with a very high resolution. It works by scanning over a sample surface using a probe with a very sharp tip mounted on a cantilever (Eaton & West, 2010). As the tip comes to a close range with the surface of a sample, attractive force between the tip and the surface causes deflection of the cantilever towards the surface. Moving the tip even more closer to the surface, such that the tip and the surface are in contact, makes repulsive force to increasingly take over causing deflection of the cantilever away from the sample surface. These deflections are detected using a laser beam. As an incident beam is reflected off the flat surface of the cantilever, all deflections on the cantilever will result in slight changes in the path of the reflected beam. The reflected beam hits a position-sensitive photodiode which tracks and records the changes in the direction of the reflected beam. The photodiode is made of four segments (see figure 1) so as to indicate the position of the reflected laser spot on the photo-detector, thus, indicating the cantilever angular deflections (Ellis, 2016).

Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM)

Figure 1: Schematic diagram of an AFM and feedback electronics

To acquire an image, an AFM tip is passed over the area of interest by scanning the cantilever over the surface. As the tip passes over raised and lowered surface features, the cantilever deflections and the resulting changes in the path of the reflected beam is monitored and recorded by the photodiode. A feedback loop is used to maintain a constant laser position by controlling the height between the tip and the surface. A plot of the upward and downward movement as a function of the position of the tip produces a high resolution image of the sample surface topography. This mode of AFM operation is known as “contact” mode and it can damage soft or biological species. To avoid this problem, another mode, known as “tapping” mode is applied. In the tapping mode, the tip is made to vibrate rapidly up and down, while tapping the sample surface. However, there are smaller changes in force due to smaller changes in separation (see figure 2) in the tapping mode. Thus, the resolution is not as high as that in contact mode (Myscope, 2016).

Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 1

Figure 2: Forces curves in an AFM. The straight line is for a spring cantilever, while the normal Lennard-Jones curve is the force curve for interactions of atoms. The changes in the forces in AFM are shown by the net force curve.

Results and Discussion:

Obtaining virtual Atomic Force Microscopy images of two different surfaces that are scanned with two different modes. The first obtained virtual image was the scanning of Calibration grid surface image with contact mode by adjusting the scan size at the different values (0.5, 1, 5, 10, 20, 30, 50) microns in order to be able to see the different between different virtual images at different scan sizes values (see images 1-7 below).

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Image 3: The 50 micron scan size for the Calibration grid

Image 2: The 0.5 micron scan size for the Calibration grid

Image 1: The 1 micron scan size for the Calibration grid

Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 5Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 6Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 7

Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 8Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 9Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 10

Image 4: The 30 micron scan size for the Calibration grid

Image 6: The 20 micron scan size for the Calibration grid

Image 5: The 10 micron scan size for the Calibration grid

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Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 14

Image 7: The 5 micron scan size for the Calibration grid

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The second obtained virtual images were the scanning of Nanotubes surface images with tapping mode by adjusting the scan size at seven different values (0.1, 0.25, 0.5, 1, 2.5, 5, 20) microns (see images 8-14 below).

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Image 9: The 2.5 micron scan size for the Nanotube

Image 8: The 20 micron scan size for the Nanotube

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Image 11: The 0.5 micron scan size for the Nanotube

Image 10: The 1 micron scan size for the Nanotube

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Image 12: The 0.25 micron scan size for the Nanotube

Image 13: The 5 micron scan size for the Nanotube

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Image 14: The 0.1 micron scan size for the Nanotube

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First of all, before recording the images, the operating parameters such as scan rate and set point, should be adjusted in a very careful way in order to have good images of giving samples. One thing that is important to know is that these parameters can make an impact on each other after the adjustment process. Therefore, Care must be taken during the adjustment process (Ricci & Braga, 2008).

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: The 20 micron scan size for the Calibration grid image with a value around 200Hz of scan rate that would reduce the quality of the imageImage 15

: The 20 micron scan size for the Nanotube image with a value around 200Hz of scan rate that would reduce the quality of the imageImage 16

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In the above images (15, 16), the high vaules of scan rate affects the quality of the images. The reason is thatthe scan rate is responsible for controlling the rate at which the cantilever raster-scans across the sample area of interest (Step-by-step instruction for using the Digital Instruments Multimode AFM, 2016). In this experiment, the model value is 2.00 Hz while the range values are 1.00 Hz — 4.00 Hz. Therefore, when the scan rate is slow that would give images with good quality (Myscope, 2016). An appropriate selection of scanning rate ensures that the tip tracks all the features on a sample surface to produce a quality image.

Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 34Theory and Virtual Imaging and Analysis of Surfaces Using Atomic Force Microscopy (AFM) 35

: The 20 micron scan size for the Calibration grid image with a value around 9 Volts of set point that would reduce the quality of the imageImage 17

: The 20 micron scan size for the Nanotube image with a value around 145 mV of set point that would reduce the quality of the imageImage 18

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Adjusting the set point plays a significant role is improving the scanning performance. In the tapping mode, higher drive frequency means that there are low forces applied to the sample. Imaging in tapping mode gives better data when the set point is adjusted in a lower value than the cantilever’s resonance peak (Lowrie & Earles, 2008). In addition, often Soft samples in tapping mode need higher set point because that would help reducing the energy imparted to these soft samples whereas hard samples can be scanned easily by using contact mode (Lowrie & Earles, 2008). Minimising the set point amplitude would help in applying higher contact forces which leads to improving response time. A set value that is too high in contact mode will result in increased imaging force, which will damage the sample and subsequently, the tip itself.

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: The 20 micron scan size for the Nanotube image with a high value of integral gain~ 8, which would reduce the quality of the image.Image 20

: The 20 micron scan size for the Calibration grid image with a high value of integral gain ~7, which would reduce the quality of the image.Image 19

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Gains can determine the sensitivity of the feedback loop in regard to the variations in the tip’s amplitude of oscillation (Myscope, 2016). The model value of Proportional and Integral Gains are 0.400/ 4.00.Increasing gains too high reduces the quality of image due to introduction of high frequency excitations. However, the increment of Gains sometimes helps obtaining better images but only up to a point, above which high frequency noise is observed (Ricci & Braga, 2008).

Analysis of AFM data:

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AFM image of a Height Silicon surface that has carbon nanotubes and three of them were measured in this experiment.Image 21:

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AFM helps for looking on soft and rough surfaces on a nanometre level. In this experiment, three Carbon nanotubes on ‘Height Silicon surface’ were analysed. There are three tables of these nanotubes showing the average of ‘the height and the width’ and the standard deviations that were measured using Excel.

First nanotube:

Recording Times

Height (nm)

width (μm)

Standard Deviation

0.956056722

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: shows the average of the Height and the Width and the standard deviation in first nanotube of AFM of a Height Silicon surface.Table 1

0.009507891

Second Nanotube:

Recording Times

Height (nm)

width (μm)

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: shows the average of the Height and the Width and the standard deviation in second nanotube of AFM of a Height Silicon surface.Table 2

Standard Deviation

0.819515684

0.010243697

Third Nanotube:

Recording Times

Height (nm)

width (μm)

Standard Deviation

0.364977686

0.006356099

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: Shows the average of the Height and the Width and the standard deviation in second nanotube of AFM of a Height Silicon surface.Table 3

Conclusion:

The width and height of carbon nanotube specimens were measured using AFM in this experiment. The AFM instrument used to visualize surfaces using the Australian Microscopy and Microanalysis Research Facility (AMMRF) Myscope website provides a tool that can be used to measure local physical properties and observe other nanoscale features of these specimens. AFM visualization of carbon nanotubes has been achieved without physically interacting with the samples. The quantitative data obtained from the interaction with AMMRF provides a good understanding of the AFM technique as well as the physical properties of the materials under investigation.

Questions

Question 1:

As the AFM tip comes into contact with the surface atoms it experiencesattractive and then repulsive force.

When the tip approaches the surface, the distance between the tip and the surface will be reduced (approaches the attractive regime) as seen in figure 3. Attractive forces are due to instantaneous polarization of nearby atoms which lead to attractive interaction. As the tip moves very close to the surface, the force between the tip and the surface become increasingly repulsive (approaches the repulsive regime) as a result of exchange interactions caused by overlapping of electronic orbitals located at atomic distances.

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Figure 3: Force approach curve between the tip and the surface

Question 2:

The tip of the probe is closer to the surface in contact mode since it is dragged on the raised and lowered surface features without oscillation. In non-contact mode, the tip is made to oscillate up and down the samples surface without coming into contact with the sample surface. In fact, the contact mode is more sensitive towards surface characteristics because it passes over all the raised and lowered features to measure local properties such as friction, height and magnetism.

Question 3:

In contact mode, surface tension and/or electrostatic forces pull the probing tip towards the surface of the sample. The result of this feature is that the sample is likely to be damaged and hence, produce distorted image data. Thus, contact mode is highly influenced by adhesive and frictional forces. This is one disadvantage of contact mode compared to tapping mode (Eaton & West, 2010). In addition, in Contact mode, the lateral forces have the ability of distorting the image’s characteristics whereas in tapping mode the resolution of lateral is high. In fact, in most samples it ranges between 1-5 nm (Lowrie & Earles, 2008).

Question 4:

Some surfaces such as, mica or silicon are used assubstrates for Atomic Force Microscopy imaging because these substrates provide an atomically flat and smooth surface for specimen support. When a sample is supported on these surfaces, the sample molecules can easily be covered and detected by an AFM tip.

Question 5:

Tip convolution is an inherent and inevitable consequence that results from the radius of curvature of the AFM tip being larger than or similar to the width of the local surface feature being visualized. Thus, the greatest effect of probe convolution tends to be on features of smaller than or similar tip radii. Probing tips that are broader and less sharp enhance the effect of convolution. Tip convolution results in lower lateral resolution.

Question 6:

The type of artefact produced when imaging a surface with excessive force will be tip contamination and/or double tipping. Excessive force may scratch the surface of the material, carrying with it material debris that may accumulate on the tip. This will lead to a dull tip that will distort the image. Double tipping will occur if the tip is damaged due to hitting a hard surface with high force (Eaton & West, 2010).

Question 7:

(See the tables under analysis section)

The standard deviation (SD) might be a good measure of quantifying how varied the measurements in width and height are in relation to each set of data. A lower standard deviation is an indication that the measurements are closer to the expected value, while a higher SD is an indication that the measurements obtained are spread over a wide range of data points. Higher SD implies that the measurements are far away from the expected value, and therefore, less accurate.

Question 8

The average height and the diameter of the carbon nanotube cannot be considered equal, although they may be regarded as being closely related. The reason for this is that, the height measured depends on factors such as the adsorption distance between the nanotube and the substrate, and electronic properties of the nanotube and substrate causing difference in tunneling distance over the substrate and above the tube. The magnitude of these factors may not be precisely known (Lowrie & Earles, 2008).

References:

  1. AFM (Atomic Force Microscope) Instructions. (2016). Retrieved from http://www.nanoscience.ucf.edu/include/files/instructions/instructions-afm.pdf

  2. Eaton, P., & West, P. (2010). Atomic Force Microscopy. Oxford University Press.

  3. Ellis, A. (2016). Laboratory Manual for Nano2701/Nano8701 . In Structures and Characterization. .

  4. Lowrie, C., & Earles, S. (2008). Wavelet processing of Palladium Foil Images from Atomic Force Microscopy (AFM) measurements. In T. E, Nanotechnology (General) (pp. 1-4). Florida: The Electrochemical socity. Retrieved September 6, 2016, from https://books.google.com.au/books?id=Y5negnGMxo4C&pg=PA1&lpg=PA1&dq=high+scan+rate+will+reduce+the+AFM+image+quality&source=bl&ots=CsX8SVWw0B&sig=TDp_cHHPaeQzZrF9wTF2F8KwCXs&hl=ar&sa=X&ved=0ahUKEwiu7OGFq_vOAhWDHpQKHTfFDPEQ6AEIXDAH#v=onepage&q=high%20scan%

  5. Myscope. (2016). Retrieved from Australian Microscopy & Microanalysis Research Facility (AMMRF): http://www.ammrf.org.au/myscope/

  6. Ricci, D., & Braga, P. C. (2008). Recognizing and Avoiding Artifacts in AFM Imaging. In Methods in Molecular Biology, vol. 242: Atomic Force Microscopy: Biomedical Methods and Applications (pp. 25-36). Humana Press.

  7. Step-by-step instruction for using the Digital Instruments Multimode AFM. (2016). Retrieved from http://mcf.tamu.edu/docs/AFM_instructions.pdf