Analysis of Surfaces Using an Atomic Force Microscope - Lab Report Example

Name: Professor: Course: Date: Experiment: 2NANO7 Theory and Virtual Imaging and Analysis of Surfaces Using an Atomic Force Microscope (AFM) AIM: This lab experiment was performed with the aim of getting familiarized with the theory underlying the Atomic force microscope (AFM), and applying this technique in imaging of surfaces using a virtual AFM instrument at the Australian Microscopy and Microanalysis Research Facility (AMMRF) Myscope website. By applying this technique, a better understanding of how the instrument operates, including its advantages and limitations was a good gain of knowledge. INTRODUCTION: Atomic force microscopy (AFM) or scanning probe microscopy (SPM) is an imaging technique used to obtain images as well as other information from a sample at a very high resolution. The information obtained from the interaction of the probe and the sample surface can be a simple physical topography or diversified as measurements of chemical, magnetic or physical properties of the material [2]. As scanning of the probe is performed across the sample in a raster pattern to create a map of the property measured, these data are collected. AFM microscopic imaging reveals variation in the specific property measured, e.g. hardness, roughness, magnetic domains or height, over the area of the sample imaged [1]. AFMs have a wide range of applications including; biology, material science, data storage devices, semiconductors, polymers, and optics for measurement of nano-mechanical, nanoscale topography, nano-scale chemical mapping and nano-electrical. In this lab experiment, a Myscope virtual AFM was used to perform a practice scanning probe microscopy on two different surfaces. The first Calibration grid surface was scanned with the virtual instrument in contact mode while the second Nanotubes surface was scanned with the virtual AFM instrument in tapping mode. The virtual images of these tow surfaces were obtained at each scan size and under different feedback conditions. Three carbon nanotubes surface AFM image were selected and analyzed to determine the average width and height using the Nanoscope analysis software. PROCEDURE: Please refer to Manual for 2NANO7 “Theory and virtual imaging and analysis of surfaces using an atomic force microscope”, pages 67-71. RESULTS AND DISCUSSION: SPM Virtual Images The virtual SPM instrument at the AMMRF Myscope website was used to scan two surfaces; the Calibration grid in contact mode and the Nanotubes surface in tapping mode. The figures below show the different images obtained in the two modes at different scan size and feedback conditions. Figure 1(a): Optimal quality image obtained after adjusting the scan rate for Calibration grid surface at 20 micron scan size. Figure 1(b): Optimal quality image obtained after adjusting the scan rate for the nanotubes surface at 5 micron scan size. Figure 2(a): Optimal quality image obtained after adjusting the set point for Calibration surface grid at 20 micron scan size. Figure 2(b): Optimal quality image obtained after adjusting the set point for the nanotubes surface at 5 micron scan size. Figure 3(a): Optimal quality image obtained after adjusting the integral gain for Calibration grid surface at 20 micron scan size. Figure 3(b): Optimal quality image obtained after adjusting the integral gain for Calibration grid surface at 5 micron scan size. Figures 1, 2 &3 show the images that were obtained from the online virtual SPM after adjusting the feedback parameters (scan rate, set point and integral gain) to optimal quality images for Calibration grid surface and Nanotubes surface. The images shown below in figures 4(a) to 4(g) were obtained when the Calibration grid surface was scanned at different scan sizes, from 0.5 micron to 50 micron. Figure 4(a): Image obtained after adjusting the scan size to 0.5 micron for Calibration grid surface. Figure 4(b): Image obtained after adjusting the scan size to 1.0 micron for Calibration grid surface. Figure 4(c): Image obtained after adjusting the scan size to 5 micron for Calibration grid surface. Figure 4(d): Image obtained after adjusting the scan size to 10 micron for Calibration grid surface. Figure 4(e): Image obtained after adjusting the scan size to 20 micron for Calibration grid surface. Figure 4(f): Image obtained after adjusting the scan size to 30 micron for Calibration grid surface. Figure 4(g): Image obtained after adjusting the scan size to 50 micron for Calibration grid surface. The figures shown in the figures 5(a) to 5(b) represent the images that were obtained when nanotubes surface was scanned at different scan sizes. Figure 5(a): Image obtained after adjusting the scan size to 0.1 micron for nanotubes surface. Figure 5(b): Image obtained after adjusting the scan size to 0.5 micron for nanotubes surface. Figure 5(c): Image obtained after adjusting the scan size to 0.25 micron for nanotubes surface. Figure 5(d): Image obtained after adjusting the scan size to 1.0 micron for nanotubes surface. Figure 5(e): Image obtained after adjusting the scan size to 2.5 micron for nanotubes surface. Figure 5(f): Image obtained after adjusting the scan size to 5 micron for nanotubes surface. Figure 5(g): Image obtained after adjusting the scan size to 20 micron for nanotubes surface. On completion of training preparation, a certificate of successful completion of on-line test was issued. Figure 6: Certificate of Scanning Probe Microscope training preparation produced by AMMRF Myscope website on completion of the test for this lab. The AFM technique works by scanning an extremely sharp probe along the surface of the sample, maintaining the force between the surface and the probe at a carefully set low level [3]. The technique operates by a basic principle that a contact is maintained between a sharp tip, otherwise known as a probe and the surface of the sample by a feedback mechanism as it scans on the surface of the sample. The most commonly used probe is micro-fabricated silicon or silicon nitride cantilevers with sharp integrated tips. A typical probe radius vary from 5 to 20 nm. The three most common modes of operation of AFM used are contact mode, tapping mode (also known as Amplitude Modulation AFM, and non-contact mode, also known as Frequency Modulation AFM. In contact mode, the AFM instrument uses feedback to control the force on the surface of the sample, while in tapping mode, the cantilever maintains oscillation at near resonant frequency. Apart from measuring the force on the sample, it also regulates it, enabling image acquisition at very low forces. In both modes, the cantilever oscillates up and down by addition of a piezoelectric element [2]. ANALYSIS OF RESULTS QUESTION 1. As the tip comes into contact with the surface atoms it experiences forces. As a function of distance from the surface are these forces (a) repulsive and then attractive or (b) attractive and then repulsive. Why? Explain your answer in terms of the force-approach curve and the nature of the forces at a molecular level. When the tip moves closer to the sample surface, a number of forces operate to contribute to the motion of the cantilever. Typical dominant forces are coulombic and Van der Waals forces. Coulombic interactions are strong and short-range repulsive forces that originate from electrostatic repulsion by a cloud of electrons at the tip and sample. Van der Waals interactions are long-range attractive forces arising from temporary fluctuating dipoles. As the tip moves closer to the sample, attraction due to Van der Waal’s forces occur, followed by repulsion due to coulombic forces at small separations [5]. These interaction of forces results in the force-approach curve similar to the one shown in figure 7 below. Figure 7: Graph of force against distance for AFM QUESTION 2. AFM imaging can operate in a number of modes including contact or non-contact mode – can you explain the relative position of the tip with respect to the surface for these modes (i.e. is the tip closer to the surface in contact or noncontact mode). Which mode, contact or non-contact, would you expect to be more sensitive to the surface features and why?? The tip of the probe actually touches the sample and completely moves away from it in every cycle of oscillation in tapping mode. In non-contact AFM mode, the cantilever is always close to the sample, with a much smaller amplitude of oscillation. Non-contact AFM can be operated while almost touching the sample because the technique is very sensitive to smaller cantilever oscillations. Unlike the other techniques, non-contact AFM can be used to form true atomic resolution images. The tapping mode is normally the most stable mode in air and is commonly used than the other two modes. QUESTION 3. What is a disadvantage of contact mode compared to tapping mode? Contact mode causes a significant damage on the sample surface as well as on the tip due to the combination of lateral and normal forces compared to the lesser damage done by the method of tapping [4]. Also, there are more lateral forces at the sample-tip interaction in contact mode that may distort some of the features of an image. QUESTION 4. Why are surfaces such as mica or silicon typically used as substrates for AFM imaging? Mica is a layered material that is non-conductive and can be cleaved easily using a pin or a cellotape to provide clean and atomically flat surfaces. Mica can be highly charged at the surface because it is covered by a thin layer of water on exposure to ambient air [5]. This layer of water causes a continuous adhesion between the sample and the AFM tip. Silicon and mica also low roughness, a fact that makes them suitable for use in AFM. QUESTION 5. What is tip convolution? Explain how it affects the lateral resolution of AFM images. Tip convolution is an AFM inherent feature that distorts the width of the image as a result of sample and tip geometries, while the image height is not distorted. Convolution affects lateral resolution by profile broadening effect. QUESTION 6. Refer to the data you collected in section 6 of the Myscope Virtual AFM section Why does a scan rate that is too high reduce the image quality of an AFM image? The scan rate increases the velocity of the tip and reduces the quality factor of the cantilever, leading to formation of poor quality images. It can also damage the tip. Why does an integral gain value that is too high reduce the image quality of an AFM image? High integral gain value increases noise frequency in the scanned image, thus, reducing the image quality. Why does a set-point value that is too high in contact mode or too low in tapping mode reduce the image quality of an AFM image? High set-point value in contact mode increases the cantilever deflection, which reduces the quality of scan, resulting in a poor image. In tapping mode, the set point is the oscillation amplitude of the cantilever that controls the force on the tip [3]. A low set point value in tapping mode reduces the force applied, thus, reducing the image quality. What type of artefact is produced from imaging a surface with excessive force? Excessive force on the tip scratches and damages the sample surface, producing a blunt and poor quality image. Analysis of AFM from Nanoscope data QUESTION 7. Draw a table for each nanotube showing the average width, average height and standard deviation for each measurement. What might the standard deviation be a good measure of? The AFM image that was analyzed is shown below: Figure 8: Carbon nanotubes AFM image used in the analysis of Nanoscope data When three carbon nanotubes surface were analyzed to determine the average width and height using the Nanoscope analysis software, the results presented in the table below were obtained. Table 1: Analysis of AFM data showing the average height and width The standard deviation is a good measure of how close the measured values are close to the mean, and thus, the quality of scan linearity. From these results, the height has got a larger deviation compared to the width. QUESTION 8. (2) Let us assume that the carbon nanotube is an incompressible tube with a circular cross section. Based on the AFM data you analyzed would you consider the average height and diameter to be equivalent? If not then why might they not be equivalent? The average height is not equivalent to the average diameter. This is because the height is a function of different feedback parameters, while the width always appear with the same diameter. CONCLUSION: The AFM instrument produces a specimen’s image with a very high resolution by use of a mechanical probe. Piezoelectric elements are used to facilitate precise motion of electrons to enable accurate scanning. The AMMRF Myscope website provides SPM modules that make it easy to understand the principles of working of AFM. The virtual online AFM is an instructional primer and tool that simulates the real instrument. It can greatly assist a learner to understand how atomic force microscopy is used to produce high resolution images of specimens for scientific or industrial applications. It is also easier to learn to work with the real instrument after working with a virtual online AFM and Nanscope data analysis tool. In addition, it also provides an opportunity to understand the theories behind the instrument, and the principles of operation. References [1] Australian Microscopy and Nicroanalysis Research Facility. My Scope. n.d. website. 25 August 2015. [2] Bowen, W. Richard and Nidal Hilal. Atomic Force Microscopy in Process Engineering: An Introduction to AFM for Improved Processes and Products. United Kingdom: Butterworth-Heinemann, 2009. [3] Braga, Pier Carlo and Davide Ricci. Atomic Force Microscopy: Biomedical Methods and Applications. U.K, 2004. [4] Drelich, J. and Kash L. Mittal. Atomic Force Microscopy in Adhesion Studies. New York: CRC Press, 2005. [5] García, Ricardo. Amplitude Modulation Atomic Force Microscopy. London: John Wiley & Sons, 2011. Read More

PROCEDURE: Please refer to Manual for 2NANO7 “Theory and virtual imaging and analysis of surfaces using an atomic force microscope”, pages 67-71. RESULTS AND DISCUSSION: SPM Virtual Images The virtual SPM instrument at the AMMRF Myscope website was used to scan two surfaces; the Calibration grid in contact mode and the Nanotubes surface in tapping mode. The figures below show the different images obtained in the two modes at different scan size and feedback conditions. Figure 1(a): Optimal quality image obtained after adjusting the scan rate for Calibration grid surface at 20 micron scan size.

Figure 1(b): Optimal quality image obtained after adjusting the scan rate for the nanotubes surface at 5 micron scan size. Figure 2(a): Optimal quality image obtained after adjusting the set point for Calibration surface grid at 20 micron scan size. Figure 2(b): Optimal quality image obtained after adjusting the set point for the nanotubes surface at 5 micron scan size. Figure 3(a): Optimal quality image obtained after adjusting the integral gain for Calibration grid surface at 20 micron scan size.

Figure 3(b): Optimal quality image obtained after adjusting the integral gain for Calibration grid surface at 5 micron scan size. Figures 1, 2 &3 show the images that were obtained from the online virtual SPM after adjusting the feedback parameters (scan rate, set point and integral gain) to optimal quality images for Calibration grid surface and Nanotubes surface. The images shown below in figures 4(a) to 4(g) were obtained when the Calibration grid surface was scanned at different scan sizes, from 0.

5 micron to 50 micron. Figure 4(a): Image obtained after adjusting the scan size to 0.5 micron for Calibration grid surface. Figure 4(b): Image obtained after adjusting the scan size to 1.0 micron for Calibration grid surface. Figure 4(c): Image obtained after adjusting the scan size to 5 micron for Calibration grid surface. Figure 4(d): Image obtained after adjusting the scan size to 10 micron for Calibration grid surface. Figure 4(e): Image obtained after adjusting the scan size to 20 micron for Calibration grid surface.

Figure 4(f): Image obtained after adjusting the scan size to 30 micron for Calibration grid surface. Figure 4(g): Image obtained after adjusting the scan size to 50 micron for Calibration grid surface. The figures shown in the figures 5(a) to 5(b) represent the images that were obtained when nanotubes surface was scanned at different scan sizes. Figure 5(a): Image obtained after adjusting the scan size to 0.1 micron for nanotubes surface. Figure 5(b): Image obtained after adjusting the scan size to 0.

5 micron for nanotubes surface. Figure 5(c): Image obtained after adjusting the scan size to 0.25 micron for nanotubes surface. Figure 5(d): Image obtained after adjusting the scan size to 1.0 micron for nanotubes surface. Figure 5(e): Image obtained after adjusting the scan size to 2.5 micron for nanotubes surface. Figure 5(f): Image obtained after adjusting the scan size to 5 micron for nanotubes surface. Figure 5(g): Image obtained after adjusting the scan size to 20 micron for nanotubes surface.

On completion of training preparation, a certificate of successful completion of on-line test was issued. Figure 6: Certificate of Scanning Probe Microscope training preparation produced by AMMRF Myscope website on completion of the test for this lab. The AFM technique works by scanning an extremely sharp probe along the surface of the sample, maintaining the force between the surface and the probe at a carefully set low level [3]. The technique operates by a basic principle that a contact is maintained between a sharp tip, otherwise known as a probe and the surface of the sample by a feedback mechanism as it scans on the surface of the sample.

The most commonly used probe is micro-fabricated silicon or silicon nitride cantilevers with sharp integrated tips. A typical probe radius vary from 5 to 20 nm.

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