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Atomic Force Microscopy

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Sheng Tay

on 12 April 2013

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Transcript of Atomic Force Microscopy

Atomic Force Microscopy Introduction Experimental Results Discussion Working Principles of tapping mode Frequently Asked Questions The force between the tip and the sample surface is kept constant during scanning by maintaining a constant deflection of the tip The AFM works on a basis which involves the interaction force between the tip mounted on a cantilever and the sample surface which will cause a deflection of the cantilever. AFM is a type of scanning probe microscopy capable of very high resolution in the nanoscale. The AFM machine Laser Cantilever Sample Since the AFM relies on the principle of the interatomic interaction between the probing tip and sample surface, there are different force regimes for the operations of AFM. The cantilever positioned very close (< a few angstrom) The cantilever can experience both attractive and repulsive forces. Cantilever is oscillating near its resonant frequency with amplitude 100 to 200 nm. The cantilever is positioned at about 10 to 100 Angstroms with little contact or no contact with the sample surface. Cantilever is oscillating near its resonant frequency with amplitude <10 nm. In this experiment, we will focus mainly on the tapping mode. When the tip is brought into the proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke’s law.
The deflection is measured using a laser incident on the back of the cantilever. The reflected laser beam is then detected by a position sensitive photon-detector. When the cantilever bends, the position of the reflected spot will change. When the position of the reflected spot changes, it gives rise to a voltage change in the photon detector
The voltage change is then detected by the signal detector and a output signal will be given out to form the topography image of the sample. This method of using laser is called the optical detection. Experimental Procedure After calibration of the equipment, we conducted our scans on four different areas on the sample. We also conducted section analysis to find the dimensions of the patterns obtained. A darker color shade corresponds to a greater depth. We have a flattened 2D planar image of the selected region We select the area of interest (red arrows)on the 2D diagram by drawing a line across. This will give a cross sectional analysis. AFM also allows for a 3D image of our sample scan Source of error: During section analysis, we have to select the section of interest by drawing a line across the image manually, it could have caused some experimental error as the line could have been slanted.

Improvement: If we want a better determination of the width or depth of holes, it is recommended to repeat the measurements and get the average so as to improve on the accuracy of our results. The experiment is highly precise
Using AFM, we do not have to spend time preparing the sample. We can take measurements right after cleaning the sample off dirt. Even if the sample is still littered with some dirt, we can use the software to flatten the image. This procedure can average out the effects of dirt and impurities.
AFM allows us to see 2D and 3D images. Advantages Disadvantages Long duration for a scan
We have to manually draw a line across the image to get the measurements. This move introduces human error and causes inaccuracy to the readings. What other detection modes are there other than optical detection? A STM tip is placed very close to the back of the cantilever. There will be changes in the tunneling current between the tip and the cantilever as the cantilever oscillates. This change in the tunneling current can then be converted to topography image of the sample. STM detection Capacitive detection Changes in the capacitance can be employed to detect the deflection of cantilever. The back of the cantilever forms a parallel-plate capacitor with a parallel plate. The change in separation between the cantilever and the plate will cause changes to the capacitance. Experimental Details
Laser: 670nm wavelength
Sample: 5x5mm Silicon chip with Silicon Dioxide structure arrays Tan Ee Cheng (A0072794U)
Tay ShengYu (A0072549X) Can an AFM work on sample inside water? What is the difference between operating AFM in air and water? Yes, AFM can work on sample inside the water. Compared to using AFM in air, the optical detection method is preferred if AFM were to be used in water as the laser beam can pass through the medium. AFM is often used for surface imaging . Do you think it is possible to use AFM to create nano-scale patterns for nanofabrication? Yes. AFM tip can be for nanofabrication both physically or chemically. To physically modify the sample surface, load are applied. Load of around 1000~1500nN can be applied depending on the sample. The AFM tip can be used to physically scratch the surface of the sample, creating nano scale patterns. Other methods include the repositioning of the weakly bound nano particles. The shifting of the nano particles Another way to do nanofabrication is
to chemically modify the surface. This is called AFM tip-based nanografting.
1.Xintek Nanotechnology Innovation, 2006. Retrieved on 15 March 2013 at:

2.Nanotechnology Now, 2013. Retrieved on 15 March 2013 at:

3.Max Planck Institute, 2012. Retrieved on 15 March 2013 at:

4.AFM Contact and Non Contact, 2009. Retrieved on 15 March 2013 at:

5.Atomic Force Microscopy, 2013. Retrieved on 15 March 2013 at:

6.AGM Atomic Force Microscopy Laboratory, 2012. Retrieved on 15 March 2013 at:

7.Remote Access Microscopy, 2013. Retrieved on 15 March 2013 at:

8.Nanofabrication with Atomic Force Microscopy , 2004. Qian Tang, San-Qiang Shi,* and Limin Zhou
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon,
Hong Kong

9.Science at the Nanoscale: An Introductory Textbook(2010) pg 187 – 189. Chin Wee Song, Sow Chorng
Haur& Andrew T S Wee.

10:Olympus Micro Cantilever, 2013. Retrieved on 15 March 2013 at:

11.Tip selection for the resonance frequency / the force constant , 2012. Retrieved on 15 March 2013 at:

12.Digital Instruments Dimension 3100 Atomic Force Microscope system (Dimension 3000 appears very similar), 2007. Retrieved on 15 March 2013 at:
http://www.asmicro.com/Equipment/Dimension-3100-AFM.htm References In this lab session, we understand the working principles and experimental techniques of AFM tapping mode. We learnt that AFM is able to produce for us 2D and 3D images of our sample. We are also able to determine the width or depth (dimensional analysis) of any structure that exist on the surface.
One must remember to scan the image for different regions of the sample. For example, in our sample, we scanned four different regions of the sample and attained four different images.
Last but not least, to increase the accuracy of our results, one has to draw the analysis line multiple times to obtain an average value. Conclusion Department of Physics, National University of Singapore Width Depth Surface 1 Surface 2 Surface 4 Surface 3 Mainly used to study biological samples in aqueous medium A self-assembled monolayer (SAM) formed on a substrate is placed in a solution for nanografting.
As the AFM tip, coated with desired molecules, moves along the substrate, it locally removes the SAMs and exposes the substrate. This will allow the desired molecules on the AFM tip to chemisorb onto the exposed area. Sources of error Source of error: Peaks appeared periodically in our section analysis.

Reason: The cause of the peaks is not known. It is speculated that the peaks could be due to a systematic error in our equipments. Then, we are able to determine the hole width and depth by shifting the positions of the red arrows on the graph. Figure 1: AFM set-up (Source: 12) Figure 2: Cantilever tip (Source: 10) Figure 3: Cantilever tip (Source: 11) Figure 4: Van der Waals curve (Source: 4) Figure 6: AFM working principles (Source: 5) Figure 5: AFM working principles (Source: 7) Figure 7: AFM Set-Up (Source: 12) Figure 8: Sample Figure 9: STM detection mode in AFM (Source 9) Figure 10: Capacitive detection mode in AFM (Source 9) Figure 11: AFM operation in liquid (Source 6) Figure 12: Physical nanofabrication (Source 9) Figure 13: Chemical nanofabrication (Source 8)
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