Atomic Force Microscope and Magnetic Force Microscope Background Information

Transcription

1 Atomic Force Microscope and Magnetic Force Microscope Background Information Lego Building Instructions There are several places to find the building instructions for building the Lego models of atomic force microscopes (AFM) and magnetic force microscopes (MFM). Two of the sites are: We provide pictures here of our finished models. AFM Model Various views of the Lego AFM model shown with multiple sample tray. Page 1 of July 2009

2 Lego AFM Tip Views AFM Samples Shown are various arrangements of different materials. The top one is an amorphous arrangement, The bottom shows, from left to right, sodium chloried, close pack, sodium, and chlorine. Lego AFM sample of an amorphous solid. Page 2 of July 2009

3 Sodium Chlorine Sodium Chloride Close Pack Lego AFM multiple sample tray. The AFM samples can show a single material, different sizes of the material, or something resembling a crystal. Note that the Lego AFM can only operate in contact mode (Coulomb forces). Using the Excel Surface Plot feature, a contour similar to what is visible should be plotted. It is extremely difficult to convey that this is a field interaction. With the AFM simulation, students gain instrumentation expertise for a more realistic simulation with the MFM. MFM Model Various views of the Lego MFM model shown with sample tray. Page 3 of July 2009

4 Laser Glass Cover Slide Magnetic Tip Sample Tray Front View of MFM, Labeled MFM Samples Surface Magnets Non magnetized steel balls Lego MFM Samples, revealed. Page 4 of July 2009

5 Lego MFM Samples, sealed. When the samples are blocked from visual assessment, the students must rely on the field interaction. This is a more realistic simulation of the real instrument. Height adjustments of the lower pedestal holding the MFM may be necessary, depending on the sample. Background Information on AFM and MFM Operation There are many sources on how the AFM and MFM work. Below is a partial list of sources. Howland, Rebecca and Lisa Benatar, A Practical Guide to Scanning Probe Microscopy. Park Scientific Instruments, Page 5 of July 2009

6 Atomic Force Microscopy and Magnetic Force Microscopy Laboratory Activities The purpose of this experiment is to create pictorial representations of the data collected by a model AFM (and MFM) microscope. Advanced Set-up Determine if you want all of the laboratory stations to have the same variety of sample. Some instructors may consider a greater efficiency in this laboratory activity if each group has a different sample demonstrating differing characteristics of materials. For example, one sample may be large marbles, another, small ones, a third with the two interspersed such as in a sodium chloride arrangement. Other arrangements can be made such as face centered crystals, hexagonal packing, amorphous arrangements, grains and grain boundaries, etc. For a biological or chemical application, Lego crystals or molecules can be constructed for imaging. Extra batteries for the lasers should be on hand. Pre-Laboratory Discussion Begin the discussion by showing the students the model atomic force microscope (AFM). Ask them to suggest possible variables that will be important for describing the test surface being studied. Record all recommendations, without comment, except for clarification. Guide the discussion to measuring variations on position of the AFM tip. Demonstrate how the laser beam reflects off the slide glass and can be projected onto a piece of mounted graph or lined paper. It is best to have the paper mounted on the wall or some other immovable object. The students should devise their own way to determine the measurement of the deflection of the beam. NOTE: Please stress how to use the lasers safely. It is important that no laser light hits anyone s eyes. Explain to the students how to use the Excel surface plot feature if they are not already familiar with it. Consider each Lego spot in the Lego plane to be a coordinate that corresponds to a cell on the spreadsheet. It is important that the students do not use the samples apparent coordinate, but that of the Lego plane outside of the sample holder. They need to maintain constant increments of measurement in each of the X-Y positions, or a scaling Page 6 of July 2009

7 error will occur. Once the deflection data is collected, they should go to the graphing wizard and use the surface plot choice. Once the plot is drawn, the students may click on a corner and drag that corner to a new viewing angle. If you click on the legend, a menu comes up where you can change the scale interval for the colored contours. By clicking on axes, axis labels may be added. Laboratory Session Students will need to determine the equilibrium position on the graph paper. Many will use the bottom or top of the sheet as equilibrium. Others will do this calibration with the sample already in place. Let the students find their own way through the calibration process. It is important for the students to establish a scale, although arbitrary. If units were required, what units would be appropriate to use? Students should generate a plot of the surface of the sample. Post-Laboratory Discussion Ask how each group calibrated the graph paper for data collection. Include checks for repeatability (return to equilibrium) and determination of numeric scale. Was there significance to the horizontal distance between the AFM tip and the wall? Does this distance change the resolution of the experiment? How does it affect the data analysis? What is resolution? The concept of Coulomb forces between the tip and the materials needs to be drawn out. The connection of this model simulation to a real AFM should be made. (You may wish to postpone a discussion on topography versus electric field interaction until the laboratory activity on the MFM.) The significance of the peaks and valleys of their plots should be solicited from the presenters. Do the plots adequately represent the samples? What can be changed to improve the imaging? Ask how the tip geometry would change the results. Is the data density of measurements important? Is there a useful limit to the data density? Does the length of the cantilever arm affect the results? If we rotated the sample 90 o, would our results change? Why or why not? What if the sample were not composed of spheres? Page 7 of July 2009

8 Since the imaging is done through a field interaction between the tip and the sample, are we getting a true read of what the material looks like? How do you know? This discussion should lead into the next laboratory activity on the magnetic force microscope. Page 8 of July 2009

9 Magnetic Force Microscopy Laboratory The purpose of this experiment is to create pictorial representations of the data collected by a model MFM microscope. Advanced Set-up Determine if you want all of the laboratory stations to have the same variety of sample. Some instructors may consider a greater efficiency in this laboratory activity if each group has a different sample demonstrating differing characteristics of materials. For example, one sample may be magnets arranged with uniform strength and polarity orientation. Another may be with varying strength magnets (this can be accomplished with neodymium magnets), varying the polarity orientation (checkerboard or stripes), perhaps a magnet turned sideways, or an array of steel balls. It is important that the samples are sealed from view. Extra batteries for the lasers should be on hand. Pre-Laboratory Discussion Begin the discussion by showing the students the model magnetic force microscope (MFM). Ask the students to compare and contrast this device to the AFM. How are we able to tell the difference between topography and magnetic field? or How might we tell if we have a magnet or non-magnetized magnetic material? Let the students discuss how to approach this. You may wish to have some magnets and materials such as steel balls or paper clips for the students to interact with. This will help guide the students in developing a two-pass procedure. Laboratory Session Students will need to determine the equilibrium position on the graph paper. It is important for the students to establish a scale, although arbitrary. In this case, positive and negative displacements may be important, depending on how the sample is arranged. Students should generate two plots of the surface of the sample, one for each polarity of the probe tip. Page 9 of July 2009

10 Post-Laboratory Discussion Ask how each group calibrated the graph paper for data collection. Include checks for repeatability (return to equilibrium) and determination of numeric scale. The significance of the peaks and valleys of both plots should be solicited from the presenters. A comparison of plots should follow. Why would two passes be required? How did the magnetic characteristics of the tip affect the results? Ask how the tip geometry would change the results. Since the imaging is done through a field interaction between the tip and the sample, are we getting a true read of what the material looks like? How do you know? With what degree of confidence can the claims be supported? What does seeing an object mean? Is visible light necessary? Page 10 of July 2009

11 Instructions for Making Surface Plots with Excel Data Collection A uniform plane and coordinate system needs to be created for recording the data. This can be simply done with a sheet of graph paper or a Lego plane. Vertical lines can be labeled A, B, C, etc. while horizontal lines can be numbered 1, 2, 3, etc. This will correspond to the columns and rows designation for each cell in the Excel spreadsheet. Reference Point A1 Point J4 Reference Point A1 Point J4 For the AFM and MFM, create a scale for the projection point of the laser beam on the wall. A non-reflecting meter stick will work. Beginning at position A1, read the value on the scale and enter it into the A1 cell of the spreadsheet. Page 11 of July 2009

12 Continue along rows and columns until all values are entered. MFM data are being recorded here. Page 12 of July 2009

13 Plotting the Data From the Menu Bar select Insert Chart or select the Chart Wizard icon on the Tool Bar. The following dialog box, Chart Wizard Step 1 of 4 Chart Type, will come up. Select the Surface option and go with the default Chart sub-type. Then press Next>. Chart Wizard Step 2 of 4 Chart Source Data should come up. Press Next>. Page 13 of July 2009

14 Chart Wizard Step 3 of 4 Chart Options will open. You may label your chart in this dialogue box. Then press Next>. Chart Wizard Step 4 of 4 Chart Location will open. Select As new sheet and name the tab what you wish. You may want something like MFM Plot rather than the default Chart 1. Press Finish. Page 14 of July 2009

15 Your plot should open. You may wish to find a finer scale than the default in the Legend. Here the default is one color stripe for each 5 units of measure. Right click on the Legend and select Format Legend. Page 15 of July 2009

16 Select the Scale tab. You may wish to change the Major unit: to some more aesthetic value for your situation. In the example being used here, the Major unit scale was changed to 1.5. Press OK. Your plot should open. Page 16 of July 2009

17 We are almost done! Now for the exciting part. Roll the cursor to any corner of the graph and the cursor should change and show Corners. Click and drag the corner. You will be able to see the plot from many angles. If you wish to have your students print the plots, you may wish to suggest that they select the one or two views that best tell their story. Page 17 of July 2009