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José Mc. Colón Santiago Kaitlyn Weiser

Physical and Chemical changes to a commercial Atomic Force Microscopy tip as a result of scanning/imaging. José Mc. Colón Santiago Kaitlyn Weiser. Advisor: Dr . Sriram Sundararajan Graduate Student: Chris Tourek. Agenda. Introduction to Atomic Force Microscopy Background Review

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José Mc. Colón Santiago Kaitlyn Weiser

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  1. Physical and Chemical changes to a commercial Atomic Force Microscopy tip as a result of scanning/imaging José Mc. Colón Santiago Kaitlyn Weiser Advisor: Dr. SriramSundararajan Graduate Student: Chris Tourek

  2. Agenda • Introduction to Atomic Force Microscopy • Background Review • Project Objective and Methods • Results so far… • Next Steps…

  3. Atomic Force Microscopy (AFM) • It was invented in 1986 by Calvin F. Quate and Christoph Gerber and follows the same principle as the SPM. • It is one of the foremost tools for imaging, measuring and manipulating matter at nanometer scale. • It allows direct measurement of surfaces contours or features with great accuracy and high resolution, as well as a variety of force measurements (adhesive, electrical, magnetic, molecular) etc. • How it works? • Cantilever, Piezoelectric elements • Modes • Contact Mode • Tapping Mode Joachim Loos, Advanced Materiales, 2005

  4. AFM Probes • Cantilevers • Ultra-sharp silicon tip (radii 10nm-50nm) • Cantilever Normal Stiffness (k)

  5. How it works?

  6. AFM Contact Mode • Contact Mode • Tip and sample are maintained in contact during scanning process. • Constant cantilever deflection and force is maintained in tip-sample interaction. • Deflection adjustment is displayed as height data. http://mee-inc.com/afm.html

  7. Motivation • Parameters, scanning condition and samples may play an important role in the physical and chemical changes that a commercial AFM probe may undergo and consequently affect the imaging results. • Physical changes to AFM tip after scanning have been observed, quantified and took into consideration when analyzing imaging results. • Chemical changes that a tip may undergo as a result of scanning a surface have not been measured.

  8. Background Review • “Resolution of AFM strongly depends on the geometry and chemical composition of the AFM tip.” • Physical Changes • Contact mode scanning induce physical changes (Plastic Deformation and wear). • It has been proved that these changes affect image quality and must be considered when interpreting results. M.L. Bloo; H. Haitjema; W.O. Pril, Measurement, 1999

  9. Background Review • “Presence of contaminant molecules on the AFM tip and cantilever can affect the measurement of tip-sample adhesion forces.” • Chemical changes • As received cantilevers show a high level of contamination and researchers are assuming that contamination is present in the AFM tip too. • Presence of silicon oils in cantilevers have been reported in most studies and it has been stated that is due to the packaging process. • Cleaning techniques, e.g., acid piranha solution and soaking in hexane have been developed to reduce the level of contamination in AFM cantilevers. • However, chemical changes to a AFM tip after scanning a surface have not been quantified. Yu-Shiu Lo; Neil D. Huefner; Winter S. Chan; Paul Dryden, Langmuir, 1999

  10. Objective • To quantify physical and chemical changes to a commercial AFM tip as a result of scanning/imaging. • Contact Mode • Different Samples (UHMWPE, Tantalum) • Normal Forces • Scanning time (distance)

  11. Tip Chemical composition evaluation • The Atom Probe Microscopy (APM) will be used to evaluate the AFM tip chemical composition after scanning. • Evaluation of the tip near apex region. SriramSundararajan; Christopher Tourek, Microssopy and Microanalysis, 2010

  12. Methods • Steps: • 2 nanoscience old tips were tested in a cooper sample for 10 minutes in contact mode. • 50 nN and 200 nN Normal Forces were applied.

  13. Tip Characterization • TGT1 V. Bykov; A. Gologanov; V. Shevyakov, Materials Science & Processing, 1998

  14. Calibrating Normal Stiffness • The normal stiffness of a cantilever is necessary to know because we need to know the accurate normal load used.

  15. How to Calibrate Normal Stiffness • By using Sader’s Method, we can calibrate the normal stiffness of the tip of our cantilever accurately. • The variables that are necessary in order to find the spring constant are the manufactures length (L) and width (b), evaluate the resonant frequency, and determine the quality factor of the cantilever (Q). John E. Sader; James W. M. Chon; Paul Mulvaney, Review of Scientific Instruments, 1999

  16. Need to know? • With these known factors, we are able to plug them into Sader’s Method and get a more accurate spring constant. (website) • In our experiments we will be using the website http://www.ampc.ms.unimelb.edu.au/afm/calibration.html

  17. Finding Frequency • When calculating frequency, we center the frequency curve and then our value is given to us in kHz.

  18. Proving need of Sader’s Method • In our practice experiments, we used two tips, labeled 1 and 2. The table below shows the spring constant range given by the manufacture and our results of the spring constant when using Sader’s Method (in N/m):

  19. The need of Sader’s method • The manufacture uses such a large range of possible spring constant’s that it is necessary to calibrate it for ourselves. • Now with the knowledge of an accurate spring constant, we can determine the normal load for an experiment and therefore saving our tips from possible destruction.

  20. Contact Mode Force Curves • Force Curves are important as they allow to control the amount of normal force applied during scanning. • http://www.ntmdt.com/spm-principles/view/force-distance-curves

  21. Contact Mode Force Curves • Plot of cantilever vertical deflection signal (V) vs. piezo movement (nm). • By taking the slope of the repulsive region the sensitivity (nm/V) is calculated and used as a conversion factor for the vertical deflection. • F=k*x • F=k(N/m)*sensitivity(nm/V)*∆V(V) • So if we know the sensitivity, normal stiffness, and the force we want to apply; we can calculate the change in voltage needed to reach that force.

  22. Tip 1 Force Curve • We ran a test experiment with two silicon AFM tips on a copper sample to certify the validity of the procedure under different loads.

  23. Tip 1 shape characterization • Before Scanning • After Scanning for 10 min with a normal force of 50 nN.

  24. Tip 1 shape characterization Before Scanning on Copper Sample Re=304.0231 nm After Scanning on Copper Sample Re=801.7164 nm Ry=229.7814 nm Ry=683.3484 nm ∆Ry=453.5670 nm Rx= 969.6825 nm Rx=449.1383 nm ∆Rx=520.5442 nm

  25. Next Steps • Monday July 11 • Set parameters and start a contact mode scanning with a silicon AFM commercial tip on a Ultra-high-molecular-weight polyethylene (UHMWPE) sample.

  26. AFM

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