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ATOMIC FORCE MICROSCOPY

PRESENTED BY VijAY S Khoiwal

BRANCHES OF MICROSCOPY There are three well-known branches of microscopy, Optical microscopy, Electron microscopy, and  scanning probe microscopy.

TYPES OF SCANNING PROBE MICROSCOPY Atomic Force Microscopy (AFM) Scanning Tunneling Microscopy (STM) Lateral Force Microscopy (LFM) Magnetic Force Microscopy (MFM) Electrostatic Force Microscopy (EFM) Chemical Force Microscopy (CFM) Near Field Scanning Optical Microscopy (NSOM)

CHARACTERISTICS OF COMMON MICROSCOPIC TECHNIQUES

SCANNING PROBE MICROSCOPY PRINCIPLE • Scans an extremely sharp tip (3-50 nm radius of curvature) across the object surface. • Tip mounted on a flexible cantilever, allowing it to follow the surface profile • Forces of interaction between the tip and the surface influence the movement of the cantilever. • These movements are detected by selective sensors.

PRINCIPLE OF AFM •The AFM brings a probe in close proximity to the surface •The force is detected by the deflection of a spring, usually a cantilever. •Forces between the probe tip and the sample are sensed to control the distance between the the tip and the sample.

Schematic of AFM operation

AFM TIP •Tips have a pyramidal geometry.

Three common types of AFM tip. (a) normal tip (3 µm tall); (b) supertip; (c) Ultralever (also 3 µm tall). Electron micrographs by Jean-Paul Revel, Caltech. Tips from Park Scientific Instruments; supertip made by Jean-Paul Revel.

AFM TIP One of the most important factors influencing the resolution which may be achieved with an AFM is the sharpness of the scanning tip. Best tips may have a radius of curvature of only around 5nm.

The tip is rounded off so the end radius is taken.

CANTILEVERS • AFM Cantilevers are specified by their width, length, and thickness. • These parameters determine important factors like resonance frequency and spring constant.

MODES IN AFM MODES OF OPERATION

FORCES OF INTERACTION

MODE

strong (repulsive) – constant force or constant distance

NON MODE

weak (attractive)-vibrating probe.

INTERMITTENT MODE/TAPPING MODE

strong (repulsive) - vibrating probe.

 Mode ▫ High resolution ▫ Damage to sample ▫ Can measure frictional forces Non- Mode ▫ Lower resolution ▫ No damage to sample Tapping Mode ▫ Better resolution ▫ Minimal damage to sample

a

b

c

(repulsive) Non- (attractive)

Tapping

Working mode • Sliding the probe tip across surface, heavily influenced by frictional and adhesive forces, which can damage samples and distort image data. • Sensing Van der Waals attractive forces between surface and probe tip held above surface (50 - 150 Å), low resolution and can also be hampered by the contaminant layer which can interfere with oscillation. • Tapping the surface with an oscillating probe tip, eliminates frictional forces by intermittently ing the surface and oscillating with sufficient amplitude to prevent the tip from being trapped by adhesive meniscus forces from the contaminant layer.

Sample Preparation for AFM Particle Characterization

METHOD OF SAMPLE PRREPARATION DEPENDS ON ENVIRONMENTAL MEDIA

Medium Imaging in air

Imaging in liquids Imaging of embedde d particles

Example Dry powder Carbon nano tube

Method substrate is chemically treated and dry, it is immediately ready for powder deposition diluted dispersant suspensions of carbon nanotubes are spin coated on a silicon wafer

Absorption on a polycationic treated surface or on an agarose coating then fixing agent is added.

Bio-particles in buffer

Soft and bio-materials Hard materials

A microtone is typically used to produce 0.5 micron or thinner slices Thin films can be spin coated on silicon or a glass substrate etching technique

AFM FOR NANOMATERIAL CHARACTERIZATION (cellulose nano crystals) SAMPLE PREPARATION TRAVERSE ELASTIC MODULUS STIFFNESS MEASUREMENT TOPOGRAPY TIP SURFACE ADHESION BENDING RIGIDITY

“Atomic Force Microscopy Characterization of Cellulose Nanocrystals “:Roya R. Lahiji,Xin Xu,Ronald Reifenberger, Arvind Raman,Alan Rudie, and Robert J. Moon*,: 2010 American Chemical Society

SAMPLE PREPARATION DEPOSITION ON FRESHLY PREPARED MICA SHEET

2% Wt CNC conc.

Washing with deionized water

Force displacement in AFM

I slope: Cantilever- CNC stiffness Modulus of CNC Elastic response

Pull off forces: Tip-surface interaction Adhesion Sufcace chemistry of CNC

Indentation

Approach Withdraw

At any applied force value, the corresponding difference in z-distance between the F(z) curves provides a measure of the CNC indentation at that force value d = cantilever bending +nano crystal

Indentation length

TRAVERSE ELASTIC MODULUS Theoretical value

Three-dimensional quartersymmetry model of the CNC-AFM indentation

Experimental value

Continued……. The resulting F(δ) curve can be fitted to physics-based models that predict the AFM tip-substrate mechanics, and the elastic modulus can be estimated by tuning the theoretical elastic modulus value to match the theoretical prediction with observed data.

Traverse elastic modulus: E= 18-50 mPa Mean = 35 mPa

STIFFNESS MEASUREMENT Keff (effective stiffness) = Kc(cantilever stiffness) +Ksample (stifness b/w tip and sample) The slope of the F(z) = dF/dz dF/dz

The effective stiffness contrast between mica and CNC can come due to meniscus effects caused by moisture condensing  geometry effects, material property effects

TOPOGRAPY

Observation: variable height (2-8nm) Length (100nm-300nm) RH influence: Not remarkable

Tip surface adhesion Observation: Measured in jumping mode No CNC height effect Edge effect Almost uniform along the length Affecting parameters:  Rh in adhesion due to meniscus effect of water  geometry also affects

Bending rigidity Measured in -mode AFM by pushing an AFM tip against a CNC By increasing the normal force into the 8-10 nN range, it was observed empirically that crystals A and B( 3-5 nN were insufficient to cause the relative motion ) The small radius of the bending curvature (i.e., large amount of bending over an ∼100 nm crystal length stiffening contribution of interhydrogen bonding between neighboring cellulose chains within the crystalline cellulose will resist local cellulose chain deformation resulting from an applied load.

dynamic-mode AFM topography images of two CNCs prior to and after manipulation (crystals A and B)

Indicating direction of force

APPLICATIONS Nanostructures Bucky balls and nanotubes. Surface of polymers Diffraction grating Integrated circuits.  pharmaceutical research, including drug polymorphic discrimination , determination of intrinsic dissolution rate of crystal planes, quantitation of adhesion between powders and particles and gelatine surfaces , For characterisation of nanoparticles

ADVANTAGES OF AFM • Does not require vacuum to operate. • Gives a 3D topographical representation of the surface of the sample. • Can give additional information about the physical characteristics of the material like co-efficients of friction. • Nanomanipulation.

Limitations:

Nonlinear behaviors in TM complicate the analysis of the data in of quantitative surface mechanical properties Indirect force modulation, generally realized by modulating either the height of the sample or that of the cantilever holder, induces shear stress of the due to the AFM configuration. Friction properties are thus mixed in the data leading to possible artifacts and make it difficult to realize true elastic imaging of surface Shear deformation or tip sliding may interfere in the measurements .

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