1. Introduction to Atomic Force Microscopy

2. AFM Background Invented by Binnig, Quate, and Gerber in 1986
Measures the interaction forces between the tip and surface (e.g. repulsive and attractive forces).
Vertical resolution: ~ 0.1 nm
Lateral resolution: 2-10 nm
Most common modes of AFM: Contact and tapping mode
Topography and other important information about the surface can be constructed into an image
Forces are measured in AFM and obey Hooke’s Law: F = k z
(k = cantilever spring constant; z = vertical deflection)
The force sensed by the AFM probe is calculated by multiplying the deflection of the cantilever by the cantilever’s spring constant.
Cantilevers are available with a variety of spring constants from roughly 0.01 to 100 Newtons/meter (or nanoNewtons/nanometer), providing a range of measurement capabilities for various surfaces.

3. Basic Operating Principle of AFMz

4. Basic Operating Principle of AFM
The beam from a diode laser is deflected off the back of an AFM cantilever to a quadrant photodetector. As the AFM tip is raster scanned across the surface, the up-down and left-right movement of the tip is monitored by the detector and converted into images of topography and friction, respectively.
photodiode
detector
diode
laser
laser light
tip
piezoceramic tube scanner
sample 4

5. AFM Tips SEM micrographs of: a square pyramidal Si3N4 tip
Most Cantilevers have a spring constant <Nm
SEM micrographs of:
a square pyramidal Si3N4 tip
a square pyramidal etched single crystal silicon tip
(c) a three-sided pyramidal natural diamond tip
(d) MWCNT attached to square pyrimidal silicon tip

6. Force Distance-Curves in Contact Mode AFM
Stage (i): The tip is far away from the surface and there is no interaction between tip and the surface. No cantilever deflection occurs during stage i.
Stage (ii): As the tip approaches the surface, the tip senses attractive forces causing the tip to “snap into contact” with the surface.
Stage (iii): As the tip keeps approaching the surface, long and short-range repulsive forces make the cantilever deflect.
Stage (iv): When the tip keeps approaching the surface, the cantilever bends resulting in a straight line appeared in cantilever deflection axis.
***As deflection reaches to maximum, the tip stops approaching and retracts from the surface.***
Stage (v): The retracting line appears because of the adhesive forces between the tip and the surface.
***Once the tip-surface separation is large enough to overcome long-range attractive interaction, the cantilever returns to zero deflection.***
tip-sample separation distance
repulsive regime
Solid line: tip approaching surface
Dashed line: tip retracting from surface

7. Contact Mode AFM
Tip remains in contact with the surface during scanning
Operates in the repulsive regime of the force curve
Tip is maintained at a constant force by moving the cantilever up and down as it scans
Topography and friction (i.e. lateral force) images are acquired simultaneously during scanning
Disadvantages: There exists large lateral forces on the sample as the tip is “dragged” over the surface. Problems with contact mode are caused by excessive forces applied by the probe to the sample.
Thus, possibility of damaging delicate samples.
Friction b/t two surfaces depends on the chemical and mechanical interaction b/t the surfaces.
Changes in chemical composition giving rise to friction are mapped w/ contact mode AFM.
As the tip scans the surface, changes in chemical composition can give rise to torsional twisting of the cantilever. This torsional twisting is proportional to the friction b/t the tip and the surface.

8. Tapping Mode AFM Also called “intermittent contact mode AFM”
Tip oscillates near its resonance frequency over the sample during scanning (gently “taps” the surface).
Lateral forces are dramatically reduced and is therefore useful for working with delicate or poorly immobilized specimens on the surface.
Topography (height) and phase (elasticity and stiffness properties) data are acquired simultaneously during scanning.
Phase imaging: measures the difference between the phase of the driving signal and the phase of output signal detected by the photodiode detector.
- A phase image maps out the differences in chemical composition, adhesion, and elasticity

9. Tapping Mode AFM

10. Tip Artifacts in AFM
Tip convolution: occurs when the radius of curvature of the tip is greater than the size of the feature that is imaged
- Protruding features can appear wide, and holes can appear narrower. Broader (less sharp) probes will enhance the effect.
Tip convolution effects on sample features

11. AFM-based Nanolithography Techniques

12. Alkanethiol SAM on gold Organosilane SAM on Si/SiO2
Structures of Alkanethiol and Organosilane Self-assembled Monolayers (SAMs)
Self-assembled monolayer: ordered molecular assemblies that spontaneously form by the adsorption of an active head group onto a solid surface
Alkanethiol SAM on gold
The macroscopic surface properties can be dramatically altered by changing the terminal head group. Such surface properties include wettability.
The van der Waals forces between these methylene carbons in alkanethiols provide a driving force to the self-assembly.
The properties of the head group define the surface properties of the assembled monolayer since it is the group that is present at the outer surface.
SAMs are an ideal model that can be used to study phenomena such as wetting, friction, adhesion, and biological interaction. SAMs facilitate studying surface phenomena and chemistry with unprecedented control.
Organosilane SAM on Si/SiO2

13. Structures of Alkanethiol SAMs on Au(111)
decanethiol (12 Å)
CH3(CH2)9SH
octadecanethiol (19 Å)
CH3(CH2)17SH
docosanethiol (23 Å)
CH3(CH2)21SH
hexanethiol (8 Å)
CH3(CH2)5SH
θ = 30o
Au(111)
Many applications in surface engineering require the creation of spatially defined chemistries at specific surface locations. For example, surface patterning could be used to create cell arrays where a specific cell type would adhere only to specific surface regions. This would enable the study of cell interactions with specific surface chemistries and allow comparison of the cellular behavior when presented with distinct surface chemical regions on the same substrate.

14. Nanoshaving Procedure

15. Nanoshaving Using AFM Topography image Cursor profile
Nanoshaved patterns produced within an octadecanethiol (ODT) SAM
-The topographic image displays twelve dark squares written into an ODT matrix in ethanol.
-The dark squares correspond to uncovered areas of Au(111). The areas of brighter contrast indicate taller features whereas the dark areas are shallower.

16. Nanolithography Techniques:
Nanografting
force
Acc. Chem. Res. 2000, 33, (S. Xu, G.-Y. Liu)
Depending on the choice of molecules, nanografting can generate patterns that are taller or shorter then the SAM matrix

17. Nanografting (cont’d)
(A) Cross-shaped pattern of 11-mercaptoundecanol (MUD) written into matrix of ODT.
(B) The simultaneously acquired frictional force image exhibits dark contrast for the MUD areas of the cross, which are terminated with hydroxyl groups. The surrounding methyl-terminated matrix areas of ODT exhibit lighter frictional contrast, clearly distinguishing the differences in surface chemistry after nanografting.
(C) The line profile indicates the nanostructure is 0.7 ± 0.3 nm shorter than the matrix SAM, in close agreement with the theoretical differences in thickness (ODT = 2.1 nm, MUD = 1.5 nm).

18. Nanolithography Techniques:
Biased-induced Lithography
Chem. Rev. 2003, 103, (C. Gorman)
Adv. Mater. 1999, 11, 55. (R. Maoz, S. R. Cohen, J. Sagiv)
Oxidation or replacement of surface molecules
When a elevated bias voltage is applied, an electric field is generated between a conductive tip and a conductive/semi-conductive sample
Surface molecules beneath the tip will become oxidized or replaced.
Operating conditions:
- Performed in air
- Tip is in direct contact with the sample during scanning

19. Nanolithography Techniques:
Dip-Pen Lithography
Science 1999, 283, 661. (C. Mirkin)
The AFM tip (pen) is coated with the molecules to be written (ink), and a clean substrate serves as the paper.
The molecular ink must have an affinity for both the AFM tip and for the substrate. For example, n-alkanethiols attach to silicon nitride AFM tips through physisorption (physical adsorption) and bind to surfaces of certain metals through chemisorption.
Molecules are transported to the surface via capillary diffusion through the nanoscopic water meniscus formed between the tip and sample in an ambient environment.
The transfer of the ink is dependent on the relative humidity, which affects the size of the water meniscus formed between the tip and the surface.
Tip is held in contact with the surface for certain time intervals.

20. Dip-Pen Lithography (cont’d)
Octadecanethiol (C18) and mercaptohexadecanoic acid (MHA) nanopatterns written on a gold substrate with DPN

23. An “approach curve” or “force-distance” curve displays the vertical cantilever bending vs. lever-sample displacement. This displacement is measured between the sample and the rigidly held rear end of the cantilever (as opposed to the front end with the tip which will bend in response to interaction forces).
(i) The lever and sample are initially far apart and no forces act.
(ii) As the lever is brought close to the sample, the tip senses attractive forces which cause the end of the lever to bend downward, thus signifying a negative (attractive) force.
(iii) The attractive force gradient exceeds the spring constant of the lever at this point, and this instability causes the tip to snap into contact with the sample.
(iv) The lever-sample displacement can continue to be reduced. Since this tip is in repulsive contact with the sample, the front end of the lever is pushed further and further upward. The force corresponds to the externally applied load.
(v) The motion is reversed. Adhesion between the tip and sample maintains the contact although there is now a negative (tensile) load.
(vi) Finally the tensile load overcomes the adhesion or pull-off force and the tip snaps out of contact with the sample.