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2. Atomic Force Microscop (AFM)

3. History and Definitions in Scanning Probe Microscopy (SPM) • History • Scanning Tunneling Microscope (STM) • Developed in 1982 by Binning, Rohrer, Gerber, and Weibel at IBM in Zurich, Switzerland. • Binning and Rohrer won the Nobel Prize in Physics for this invention in 1986. • Atomic Force Microscope (AFM) • Developed in 1986 by Binning, Quate, and Gerber as a collaboration between IBM and Stanford University. • Definitions • Scanning Probe Microscopy (SPM): Consists of a family of microscopy forms where a sharp probe is scanned across a surface • The two primary forms of SPM consist of: • Scanning Tunneling Microscopy (STM) • Atomic Force Microscopy (AFM) (also called Scanning Force Microscopy (SFM)) • There are 3 primary modes of AFM: • Contact Mode AFM • Non-contact Mode AFM • TappingMode™ AFM

4. The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. • The information is gathered by "feeling" the surface with a mechanical probe. • Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning. • The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. • Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes.

7. SPM tip tipholder sample piezo translator mirror laser beam photodiode motor control fluid cell fluid in fluid out O-ring sample x,y,z piezo translator Atomic Force Microscope in air and in buffer solutions

8. Basic SPM Components

9. Atomic Force Microscope laser photodiode cantilever z piezo cantilever tip x y

10. Contact mode AFM operates by scanning a tip attached to the end of a cantilever across the sample surface while monitoring the change in cantilever deflection with a split photodiode detector. • The tip contacts the surface through the adsorbed fluid layer on the sample surface. • A feedback loop maintains a constant deflection between the cantilever and the sample by vertically moving the scanner at each (x,y) data point to maintain a "setpoint" deflection. • By maintaining a constant cantilever deflection, the force between the tip and the sample remains constant. • The force is calculated from Hooke's Law: where • F = Force • k = spring constant • x = cantilever deflection. • Force constants usually range from 0.01 to 1.0 N/m • The distance the scanner moves vertically at each (x,y) data point is stored by the computer to form the topographic image of the sample surface. • Operation can take place in ambient and liquid environments.

11. Contact Mode AFM

12. Tapping Mode AFM operates by scanning a tip attached to the end of an oscillating cantilever across the sample surface. • The cantilever is oscillated at or near its resonance frequency with an amplitude ranging typically from 20nm to 100nm. The frequency of oscillation can be at or on either side of the resonant frequency. • The tip lightly “taps” on the sample surface during scanning, contacting the surface at the bottom of its swing. • The feedback loop maintains a constant oscillation amplitude. • The vertical position of the scanner at each (x,y) data point in order to maintain a constant "setpoint “ amplitude is stored by the computer to form the topographic image of the sample surface. • By maintaining a constant oscillation amplitude, a constant tip-sample interaction is maintained during imaging. • Operation can take place in ambient and liquid environments. In liquid, the oscillation need not be at the cantilever resonance.

13. TappingMode™ AFM

14. Contact Mode AFM • Advantages: • High scan speeds • Contact mode AFM is the only AFM technique which can obtain "atomic resolution" images. • Rough samples with extreme changes in vertical topography can sometimes be scanned more easily in contact mode. • Disadvantages: • Lateral (shear) forces can distort features in the image. • The forces normal to the tip-sample interaction can be high • Tapping Mode AFM • Advantages: • Higher resolution on most samples (1 nm to 5 nm). • Lower forces and less damage to soft samples imaged in air. • Disadvantages: • Slightly slower scan speed than contact mode AFM.

18. Advantages and disadvantages • The AFM has several advantages over the scanning electron microscope (SEM). Unlike the electron microscope which provides a two-dimensional projection or a two-dimensional image of a sample, the AFM provides a true three-dimensional surface profile. Additionally, samples viewed by AFM do not require any special treatments (such as metal/carbon coatings) that would irreversibly change or damage the sample. While an electron microscope needs an expensive vacuum environment for proper operation, most AFM modes can work perfectly well in ambient air or even a liquid environment. This makes it possible to study biological macromolecules and even living organisms. In principle, AFM can provide higher resolution than SEM. It has been shown to give true atomic resolution in ultra-high vacuum (UHV) and, more recently, in liquid environments.

19. A disadvantage of AFM compared with the scanning electron microscope (SEM) is the image size. The SEM can image an area on the order of millimetres by millimetres with a depth of field on the order of millimetres. The AFM can only image a maximum height on the order of 10-20 micrometres and a maximum scanning area of around 150 by 150 micrometres. • Another inconvenience is that an incorrect choice of tip for the required resolution can lead to image artifacts. Traditionally the AFM could not scan images as fast as an SEM, requiring several minutes for a typical scan, while a SEM is capable of scanning at near real-time

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