Fundamentals of Atomic Force Microscope (AFM)

Md Ataul MamunBSc. in EEE

Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh

Instructor: Dr. Nirmal Adhikari

Outline

• Introduction– Background– Motivation– Objectives

• Theory– Working principle– Operating modes

• Results and Analysis• Conclusion• Future Work

Introduction• AFM is one kind of scanning probe microscope

that possesses a very high resolution (on the order of fractions of nanometers)

• Operates by measuring force between its probe and the sample

• Can measure local properties, such as height, friction, magnetism with the probe

• Unlike the electron microscope, AFM provides a 3-D surface profileSource: https://en.wikipedia.org/wiki/Atomic-force_microscopy

Introduction

• In the field of solid state physics, it can be used for identification of atoms at a surface, and to find interactions between a specific atom and its neighboring atoms.

• Besides solid state physics, the AFM is applied in molecular engineering, polymer engineering, polymer chemistry etc.

• Due to its versatility, science and research students should know the working principle and applications of the AFM.

• AFM was invented by IBM Scientists in 1982• Improved AFM was invented and used by Gerd

Binnig et al. in 1980s which earned them noble prize in 1986

• The first commercially available AFM was introduced in 1989.

Background

Source: https://en.wikipedia.org/wiki/Atomic-force_microscopy

Objective• To learn AFM working principle, application, and

study of images

Motivation• Need to understand how to use AFM to study dye

monolayer on TiO2 surface

Theory• 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 with a tip radius of curvature on the order of nanometers.

• When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke’s law

F = -kx

Theory

Figure: AFM probe tip (on the order of nanometers) and cantilever

Source: https://en.wikipedia.org/wiki/Atomic-force_microscopy

Working principle

Figure: Working principle of AFM

Working principleFeedback loop:

AFM Modes of OperationAFM has 3 modes of operation • Contact mode

• Non contact mode

• Tapping mode (Tapping mode provides higher resolution with minimum sampledamage)

AFM Modes of Operation

Contact Mode:• Measures repulsion between tip and sample• Force of tip against sample remains constant (With Feedback)• Feedback regulation keeps cantilever deflection constant

Non Contact Mode:• Measures attractive forces between tip and sample• Tip doesn’t touch sample• Van der Waals forces between tip and sample detected• Doesn’t degrade or interfere with sample- better for soft samples

Tapping (Intermittent Contact) Mode:• Tip vertically oscillates 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• Tapping mode provides higher resolution with minimum

sample damage• More effective for larger scan sizes

AFM Modes of Operation

Results and Analysis

Conclusions

Future Work

• Characterize dye monolayer on TiO2 with AFM

• AFM has diverse applications in research areas• It is capable to produce 3-D images with high

resolution

Acknowledgements

• SDSU EE&CS Dept.• Dr. Qiquan Qiao• Dr. Nirmal Adhikari