MFM: AFM with magnetical probe magnetic force microscope, Measure magnetic domains combine with topography

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2 STM: scanning tunneling microscope tunneling of electrons between probe and surface, Resolve individual atoms, measure electrical, properties, induce photo luminesence. Only conducting samples. AFM: atomic force microscope Resolution limit ~STM, but more difficult to achieve. Any sample type. MFM: AFM with magnetical probe magnetic force microscope, Measure magnetic domains combine with topography In some cases can image individual Molecules - Å level resolution Many artifacts are possible! Some limitations on the types of samples that may be imaged. Relatively Inexpensive ($50K-$200K) & easy to learn difficult to master y x

3 SPM scanners are made from a piezoelectric material that expands and contracts proportionally to an applied voltage. Whether they expand or contract depends upon the polarity of the applied voltage. Digital Instruments scanners have AC voltage ranges of +220 to -220V. 0 V - V + V No applied voltage Contracted Extended In some versions, the piezo tube moves the sample relative to the tip. In other models, the sample is stationary while the scanner moves the tip. AC signals applied to conductive areas of the tube create piezo movement along the three major axes.

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11 *, *, photodiode laser AFM probe scans over the surface (in contact) probe piezo-element

12 *, *, The atomic force microscope measures topography with a force probe Cantilever touching a sample Optical lever Tube scanner measures 24 mm in diameter, while the cantilever is 100 µm long.

13 *, *, photodiode laser piezo x z y cantilever cantilever tip

14 *, *, AFMs use feedback to regulate the force on the sample The AFM feedback loop. A compensation network (which in AFM is a computer program) monitors the cantilever deflection and keeps it constant by adjusting the height of the sample (or cantilever).

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17 *, *, The presence of a feedback loop is one of the subtler differences between AFMs and older stylus-based instruments such as record players and stylus profilometers. The AFM not only measures the force on the sample but also regulates it, allowing acquisition of images at very low forces. The feedback loop (figure 6) consists of the tube scanner that controls the height of the entire sample; the cantilever and optical lever, which measures the local height of the sample; and a feedback circuit that attempts to keep the cantilever deflection constant by adjusting the voltage applied to the scanner. One point of interest: the faster the feedback loop can correct deviations of the cantilever deflection, the faster the AFM can acquire images; therefore, a well-constructed feedback loop is essential to microscope performance. AFM feedback loops tend to have a bandwidth of about 10 khz, resulting in image acquisition times of about one minute.

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19 *, SPM tip tipholder sample photodiode fluid in O-ring mirror laser beam fluid cell fluid out piezo translator motor control x,y,z piezo translator sample in air and in buffer solutions

20 &(*, ( AFM tip superhelical DNA plasmid path of AFM tip DNA double helix Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ Mg 2+ negatively charged mica surface

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24 In its repulsive "contact" mode, the instrument lightly touches a tip at the end of a leaf spring or "cantilever" to the sample. As a raster-scan drags the tip over the sample, some sort of detection apparatus measures the vertical deflection of the cantilever, which indicates the local sample height. Thus, in contact mode the AFM measures hard-sphere repulsion forces between the tip and sample. *, *, In noncontact mode, the AFM derives topographic images from measurements of attractive forces; the tip does not touch the sample. AFMs can achieve a resolution of 10 pm, and unlike electron microscopes, can image samples in air and under liquids.

25 2*, A tip is scanned across the sample while a feedback loop maintains a constant cantilever deflection (and force) The tip contacts the surface through the adsorbed fluid layer. Forces range from nano to micro N in ambient conditions and even lower (0.1 nn or less) in liquids.

26 3 *, A cantilever with attached tip is oscillated at its resonant frequency and scanned across the sample surface. A constant oscillation amplitude (and thus a constant tip-sample interaction) are maintained during scanning. Typical amplitudes are nm. Forces can be 200 pn or less The amplitude of the oscillations changes when the tip scans over bumps or depressions on a surface.

27 1 #*, The cantilever is oscillated slightly above its resonant frequency. Oscillations <10nm The tip does not touch the sample. Instead, it oscillates above the adsorbed fluid layer. A constant oscillation amplitude is maintained. The resonant frequency of the cantilever is decreased by the van der Waals forces which extend from 1-10nm above the adsorbed fluid layer. This in turn changes the amplitude of oscillation.

28 Contact Mode Advantages: High scan speeds The only mode that can obtain atomic resolution images Rough samples with extreme changes in topography can sometimes be scanned more easily Disadvantages: * &(4 &*, Lateral (shear) forces can distort features in the image The forces normal to the tip-sample interaction can be high in air due to capillary forces from the adsorbed fluid layer on the sample surface. The combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft samples (i.e. biological samples, polymers, silicon) due to scraping

29 * &(4 &*, Tapping Mode AFM Advantages: Higher lateral resolution on most samples (1 to 5nm) Lower forces and less damage to soft samples imaged in air Lateral forces are virtually eliminated so there is no scraping Disadvantages: Slightly lower scan speed than contact mode AFM

30 *, *, In principle, AFM resembles the record player as well as the stylus profilometer. However, AFM incorporates a number of refinements that enable it to achieve atomic-scale resolution: Sensitive detection Flexible cantilevers Sharp tips High-resolution tip-sample positioning Force feedback

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