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1、1Procedures for Imaging of Standards at Atomic-Scale Resolution Using the Digital Instruments Multimode AFM Ortiz Nanomechanics LaboratoryMIT F01I. Sample Producers and Preparation Muscovite Mica is a yellowish, light-colored, transparent to translucent silicate (subclass : phyllosilicates) mineral
2、with the following chemistry : K2Al4Si6Al2O20(OH,F)4, Potassium aluminum silicate hydroxide fluoride. It is a hard, layered, crystalline (monoclinic 2/m) material that fractures along weak atomic planes (“cleavage“ planes), thus easily producing atomically flat surfaces of atoms having a regular lat
3、tice structure which are excellent for use in high-resolution AFM piezo calibration and as substrates for imaging of biological samples. Muscovite has a layered structure of aluminum silicate sheets weakly bonded together by layers of potassium ions. The potassium ions occupy large holes between 12
4、oxygen atoms, 6 from the layer above and 6 from the layer below; the resulting K-O ionic bonds are rather weak and easily broken. The cleavage sheets fairly flexible and elastic, hydrophilic, and negatively charged in water. Muscovite Mica has low iron content is a good electrical and thermal insula
5、tor. More detailed information on Muscovite Mica can be found here : http:/ (*http:/www.ill.fr/dif/3D-crystals/layers.html)Another typical AFM substrate is Highly Ordered Pyrolytic Graphite (HOPG) which is described in detail here : http:/ A few suppliers include the following companies : 1) Bioforc
6、e Lab (http:/) 2) Structure-Probe Inc. (*http:/ 3) Microscopy Mart / Pelco International (*http:/ Cleavage directions are detailed here : http:/ Attach to metal puck and then to piezo scanner cap.2II. Real Time Parameter Settings contact mode, air or fluid cantilever holder (it is typically much eas
7、ier to obtain images in water or buffer than in air) use stiffest spring constant cantilever; on DI V-shaped cantilever chips this is the shortest one with fattest legs use oxide-sharpened Si3N4 probe tips, e.g. Model NPS which has the following specifications (http:/www.di).com/products2/NewProbeGu
8、ide/ContactModeProbes.html :PARAMETERVALUE spring constant0.58 N/m nominal tip radius of curvature5-40nmcantilever leg length100m cantilever configurationV-shaped reflective coating Au shape of tipsquare pyramidal tip half angle35o use piezo scanner with smallest distance range available, A or EV sc
9、anner, E scanner should also work (the difference is the engagement mechanism). load sample, turn microscope and vibration table on, and then focus the laser as far out on the cantilever as possible to obtain the highest sensitivity let the system stabilize thermally for 30 minutes (with hood on) la
10、ser focusing (find maximum): the laser should never been switched off, i.e. turn the system on and off and all manipulations done with the laser onPANELPARAMETERSETTING Scan size1 m X offset0.00 nm Y offset0.00 nm Scan angle0.00 deg Scan rate61.00 Hz Number of samples512 Slow scan axisenabledScan Co
11、ntrolsZ limitbetween 55V-440V Integral gain0.001 Proportional gain0.00 Lookahead gain0.00Feedback ControlsSetpoint0 V AFM modeContactOther Controls Input attenuation1x Interleave ControlsInterleave modeDisabled Channel 1Data typeHeight* (*setting this parameter to deflection is typically easier) Hig
12、hpass filterOFF, 3-4 Lowpass filterOFF, 13III. Imaging Engage the surface. Make sure you are not false engaged (*see Section 10.10.1 of the DI AFM Manual). Reduce Scan Size to 12 nm. OR Engage with the Scan Size set to zero and slowly increase. Increase Scan Rate to 60Hz. Notice that if the Scan Rat
13、e is set much higher for atomic scale images to defeat some of the noise due to thermal drift. Adjust Integral Gain, Setpoint, Scan Rate, and Scan Angle to obtain a good image. Initially, the Setpoint should be kept as low as possible initially and then increased to obtain an image. The Z-center pos
14、ition should be close to 0V. The Scan Angle is known to have a huge effect, with optimal imaging conditions if the sample is rotated until the atoms are oriented vertically or when the fast scan axis is parallel to the a or b crystallographic axis. The Proportional Gain should stay at zero except fo
15、r large scan sizes (70% of the scanner range). The system determines the minimum value of the Integral Gain. If you start with a value less than the systems minimum, you wont get an image. Filters, you should be able to obtain an image with the filters off. In general, the filters should be set to off since filtering during data acquisition affects raw data and height values. If difficulty is experienced obtaining and image Withdraw and try a different location on the sample surface, then Engage again. See Secti
