Force Microscopy

Principle of Operation

Force Microscopy

• Basic Principle of Operation: detecting forces between a mass attached to a spring (cantilever), that feels some force when it is brought very close to the surface. Ideally the mass (tip) would not damage the surface.

• Sensor that responds to a force and a detector that measures it.

• The sensor-a cantilever beam with an effective spring constant k, moves in accordance with the forces acting on its tip

Force Microscopy

• Frequency of atoms vibration, , at room temperature ~ 1015 Hz

• The mass, m, of an atom ~ 10-30 kg The effective spring constant, k, between atoms is

k=2m1N/m

Materials Characterization

AFM -PrincipleAFM -Principle

AFM -PrincipleAFM -Principle

Materials Characterization

Courtesy Dr. Z. Barkai

Materials Characterization

AFM Images2. Carbon nanotube

3. Human chromosomes

TappingMode AFM image of single carbon-nanotube molecule on electrodes. These images represent an important breakthrough where we measured electronic transport through a single nanotube molecule for the first time. 530nm x 300nm scan courtesy C. Dekker and Sander Tans, Delft University of Technology, Department of Applied Physics and DIMES, The Netherlands.

Materials Characterization

Courtesy Dr. Z. Barkai

Materials Characterization

AFM Images1. Au (111)

High resolution scan of Au (111) surface, with reconstruction strips (inset)

hexagonal atomic structure. Scan size: 5nm; inset: 20 nm

Contact - Atomic Force Microscopy

All rights reserved @ Norbert

Hooke Law F = kx so x =1

k F = F + F T v c

F

Total Repulsive

Total Atractive

Fv

Fc

Laser

AFM -CantileversAFM -Cantilevers

kEwt

l

w

N

Tip radii A

3

3

4

4,

E - Young Module

width, l - lenght, t - thickness

Comercial cantilivers - SiO and Si

- 300

2 3

AFM -CantileversAFM -Cantilevers

Diamond-coated AFM tip FIB Sharp Tip

AFM -CantileversAFM -Cantilevers

Gold-coated Si3N4 Tip

Pyramidal, tetrahedral, or conical tips are the most

common tip shapes

AFM -CantileversAFM -Cantilevers

Depositing a Si3N4 layer on an etch pit in Si

Tips are broader then Si conical tips, harder, and thinner (stress in the film)

not suitable for non-contact (small thickness, small )

AFM -Resolution

AFM

• STM-single atom interaction

STM

AFM-several atoms on tip interact with

several atoms on surface

In contact, not necessarily a single atom

contact, radius of contact ~(Rd)1/2

(d-penetration depth, R-radius of tip)

AFM -Resolution

• Interaction of atom 1 : t=0 different from interaction of atom 3,2

• Each tip atom produces a signals with offset to each other

• Periodicity reproduced but no true atomic resolution