Pooria Gill PhD of Nanobiotechnology Assistant Professor at MAZUMS In The Name of Allah

Other Types of SPM Techniques Lateral Force Microscopy (LFM) Frictional forces measured by twisting or “sideways” forces on cantilever. Magnetic Force Microscopy (MFM) Magnetic tip detects magnetic fields/measures magnetic properties of the sample. Electrostatic Force Microscopy (EFM) Electrically charged Pt tip detects electric fields/measures dielectric and electrostatic properties of the sample Chemical Force Microscopy (CFM) Chemically functionalized tip can interact with molecules on the surface – giving info on bond strengths, etc. Near Field Scanning Optical Microscopy (NSOM) Optical technique in which a very small aperture is scanned very close to sample Probe is a quartz fiber pulled to a sharp point and coated with aluminum to give a sub-wavelength aperture (~100 nm)

Motivation Digitally image a topographical surface Determine the roughness of a surface sample or to measure the thickness of a crystal growth layer Image non-conducting surfaces such as proteins and DNA Study the dynamic behavior of living and fixed cells

History The Scanning Tunneling Microscope (STM) was invented by G. Binnig and H. Rohrer, for which they were awarded the Nobel Prize in 1984 A few years later, the first Atomic Force Microscope (AFM) was developed by G. Binnig, Ch. Gerber, and C. Quate at Stanford University by gluing a tiny shard of diamond onto one end of a tiny strip of gold foil Currently AFM is the most common form of scanning probe microscopy

Parts of AFM 1. Laser – deflected off cantilever 2. Mirror – reflects laser beam to photodetector 3. Photodetector – dual element photodiode that measures differences in light intensity and converts to voltage 4. Amplifier 5. Register 6. Sample 7. Probe – tip that scans sample made of Si 8. Cantilever – moves as scanned over sample and deflects laser beam

How the AFM Works The AFM brings a probe in close proximity to the surface The force is detected by the deflection of a spring, usually a cantilever (diving board) Forces between the probe tip and the sample are sensed to control the distance between the the tip and the sample. van der Waals force curve

Two Modes Repulsive (contact) At short probe-sample distances, the forces are repulsive Attractive Force (non-contact) At large probe-sample distances, the forces are attractive The AFM cantelever can be used to measure both attractive force mode and repulsive forces.

Contact Mode Measures repulsion between tip and sample Force of tip against sample remains constant Feedback regulation keeps cantilever deflection constant Voltage required indicates height of sample Problems: excessive tracking forces applied by probe to sample

Non-Contact Mode Measures attractive forces between tip and sample Tip doesn ’ t touch sample Van der Waals forces between tip and sample detected Problems: Can ’ t use with samples in fluid Used to analyze semiconductors Doesn ’ t degrade or interfere with sample- better for soft samples

Tapping (Intermittent-Contact) Mode Tip vertically oscillates between contacting sample surface and lifting of at frequency of 50,000 to 500,000 cycles/sec. Oscillation amplitude reduced as probe contacts surface due to loss of energy caused by tip contacting surface Advantages: overcomes problems associated with friction, adhesion, electrostatic forces More effective for larger scan sizes

Force Measurement The cantilever is designed with a very low spring constant (easy to bend) so it is very sensitive to force. The laser is focused to reflect off the cantilever and onto the sensor The position of the beam in the sensor measures the deflection of the cantilever and in turn the force between the tip and the sample.

Raster the Tip: Generating an Image The tip passes back and forth in a straight line across the sample (think old typewriter) In the typical imaging mode, the tip- sample force is held constant by adjusting the vertical position of the tip (feedback). A topographic image is built up by the computer by recording the vertical position as the tip is rastered across the sample. Scanning Tip Raster Motion

Constant-force scan vs. Constant-height scan Constant-force Advantages: Large vertical range Constant force (can be optimized to the minimum) Disadvantages: Requires feedback control Slow response Constant-height Advantages: Simple structure (no feedback control) Fast response Disadvantages: Limited vertical range (cantilever bending and detector dynamic range) Varied force

Tapping mode AFM Click on graph to play animation

Scanning the Sample Tip brought within nanometers of the sample (van der Waals)  Radius of tip limits the accuracy of analysis/ resolution  Stiffer cantilevers protect against sample damage because they deflect less in response to a small force  This means a more sensitive detection scheme is needed  measure change in resonance frequency and amplitude of oscillation Image courtesy of (

General Applications Materials Investigated: Thin and thick film coatings, ceramics, composites, glasses, synthetic and biological membranes, metals, polymers, and semiconductors. Used to study phenomena of: Abrasion, adhesion, cleaning, corrosion, etching, friction, lubricating, plating, and polishing. AFM can image surface of material in atomic resolution and also measure force at the nano-Newton scale.

What are the limitations of AFM? AFM imaging is not ideally sharp

Advantages and Disadvantages of AFM Easy sample preparation Accurate height information Works in vacuum, air, and liquids Living systems can be studied Limited vertical range Limited magnification range Data not independent of tip Tip or sample can be damaged

Topography Scanning Example of generated image upon scanning Pd thermally evaporated on Si

Surface Roughness Roughness typically measured as root mean squared (RMS)

Million Cantilever Wafer

Cantilever Gas Sensors (Noses)

AFM Tips 80 – 320  m 20  m 35  m 125  m

Intermolecular interactions Schematic of the force–extension characteristics of DNA: at 65 pN the molecule is overstretched to about 1.7 times its contour length, at 150 pN the double strand is separated into two single strands, one of which remains attached between tip and surface. MFP MFP is specially designed for force measurement purpose

Adhesion Force Imaging Height Adhesion Albumin Polystyrene Si PS pH 7 5 m

Adhesion and Hardness Imaging PLMA/PmMl 6 blend on Si imaged in water PLMA: poly (lauryl methacrylate) PmMl 6 : 2-methacryloyloxyethyl phosphorylcholine-co-lauryl methacrylate (1:6) 1  m HeightAdhesionStiffness Simultaneous Height, Adhesion and Stiffness maps are obtained with “Pulsed-Force” AFM technique.

Conclusions How AFM works Constant-height and constant-force scans (contact mode) Feedback control in constant-force mode Contact mode and tapping mode Force measurements with AFM Force curves: contact part to measure hardness and adhesion to measure intermolecular interactions Calibrations: Detector sensitivity (nm/V) = Inverse of contact slope on a hard surface => Convert the measured T-B signal (V) to cantilever deflection (nm) Spring constant (N/m) => Convert the cantilever deflection to force (N) [F=-kx]

The Future of Atomic Force Microscopy Sharper tips by improved microfabrication processes: tip – sample interaction tends to distort or destroy soft biological molecules Atomic or angstrom resolution images of live cell surfaces: development of more flexible cantilever springs and less damaging and nonsticky probes needed

References  Li, Hong-Qiang. “Atomic Force Microscopy”.  Baselt, David. “Atomic force microscopy”. afm.htmlhttp://stm2.nrl.navy.mil/how-afm/how- afm.html  Atomic Force Microscopy.  An Introduction to Atomic Force Microscopy  Basic Theory Atomic Force Microscopy (AFM)