Atomic Force Microscopy in Process Engineering

An Introduction to AFM for Improved Processes and Products

W. Richard Bowen, Nidal Hilal

• Publisher: Butterworth-Heinemann
• Year: 2009
• Print ISBN:
  9781856175173
• eBook ISBN:
  9780080949574
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

This is the first book to bring together both the basic theory and proven process engineering practice of AFM. It is presented in a way that is accessible and valuable to practising engineers as well as to those who are improving their AFM skills and knowledge, and to researchers who are developing new products and solutions using AFM. The book takes a rigorous and practical approach that ensures it is directly applicable to process engineering problems. Fundamentals and techniques are concisely described, while specific benefits for process engineering are clearly defined and illustrated. Key content includes: particle-particle, and particle-bubble interactions; characterization of membrane surfaces; the development of fouling resistant membranes; nanoscale pharmaceutical analysis; nanoengineering for cellular sensing; polymers on surfaces; micro and nanoscale rheometry. - Atomic force microscopy (AFM) is an important tool for process engineers and scientists as it enables improved processes and products - The only book dealing with the theory and practical applications of atomic force microscopy in process engineering - Provides best-practice guidance and experience on using AFM for process and product improvement

Chapter 1

Basic Principles of Atomic Force Microscopy

Daniel Johnson, Nidal Hilal and W. Richard Bowen

Publisher Summary

This chapter presents basic principles of operation of an atomic force microscope (AFM) that outlines the most common imaging modes and describes the acquisition of force distance measurements and techniques to calibrate cantilever spring constants. The AFM, also referred to as the scanning force microscope (SFM), is a part of a larger family of instruments termed as the scanning probe microscopes. These also include the scanning tunneling microscope (STM) and scanning near field optical microscope (SNOM), among others. The common factor in all SPM techniques is the use of a very sharp probe, which is scanned across a surface of interest, with the interactions between the probe and the surface being used to produce a very high resolution image of the sample, potentially to the subnanometer scale, depending upon the technique and sharpness of the probe tip. The AFM was first described as a new technique for imaging the topography of surfaces to a high resolution. It was created as a solution to the limitations of the STM, which was able to image only conductive samples in vacuum. The AFM has a number of advantages over electron microscope techniques, primarily its versatility in being able to take measurements in air or fluid environments rather than in high vacuum, which allows the imaging of polymeric and biological samples in their native state. It is highly adaptable with probes being able to be chemically fictionalized to allow quantitative measurement of interactions between many different types of materials—a technique often referred to as chemical force microscopy.

1.1 Introduction

The atomic force microscope (AFM), also referred to as the scanning force microscope (SFM), is part of a larger family of instruments termed the scanning probe microscopes (SPMs). These also include the scanning tunnelling microscope (STM) and scanning near field optical microscope (SNOM), amongst others. The common factor in all SPM techniques is the use of a very sharp probe, which is scanned across a surface of interest, with the interactions between the probe and the surface being used to produce a very high resolution image of the sample, potentially to the sub-nanometre scale, depending upon the technique and sharpness of the probe tip. In the case of the AFM the probe is a stylus which interacts directly with the surface, probing the repulsive and attractive forces which exist between the probe and the sample surface to produce a high-resolution three-dimensional topographic image of the surface.

The AFM was first described by [1]Binnig et al. as a new technique for imaging the topography of surfaces to a high resolution. It was created as a solution to the limitations of the STM, which was able to image only conductive samples in vacuum. Since then the AFM has enjoyed an increasingly ubiquitous role in the study of surface science, as both an imaging and surface characterisation technique, and also as a means of probing interaction forces between surfaces or molecules of interest by the application of force to these systems. The AFM has a number of advantages over electron microscope techniques, primarily its versatility in being able to take measurements in air or fluid environments rather than in high vacuum, which allows the imaging of polymeric and biological samples in their native state. In addition, it is highly adaptable with probes being able to be chemically functionalised to allow quantitative measurement of interactions between many different types of materials – a technique often referred to as chemical force microscopy.

At the core of an AFM instrument is a sharp probe mounted near to the end of a flexible microcantilever arm. By raster-scanning this probe across a surface of interest and simultaneously monitoring the deflection of this arm as it meets the topographic features present on the surface, a three-dimensional picture can be built up of the surface of the sample to a high resolution. Many different variations of this basic technique are currently used to image surfaces using the AFM, depending upon the properties of the sample and the information to be extracted from it. These variations include ‘static’ techniques such as contact mode, where the probe remains in constant contact with the sample, and ‘dynamic’ modes, where the cantilever may be oscillated, such as with the intermittent or non-contact modes. The forces of interaction between the probe and the sample may also be measured as a function of distance by the monitoring of the deflection of the cantilever, providing that the spring constant of the lever arm is sufficiently calibrated.

In this chapter the basic principles of operation of an AFM will be presented, outlining the most common imaging modes and describing the acquisition of force distance measurements and techniques to calibrate cantilever spring constants.

1.2 The Atomic Force Microscope

In Figure 1.1 the basic set-up of a typical AFM is shown. Cantilevers are commonly either V-shaped, as shown, or a rectangular, ‘diving board’ shaped. The cantilever has at its free end a sharp tip, which acts as the probe of interactions. This probe is most commonly in the form of a square-based pyramid or a cylindrical cone. A few examples of different configurations for levers and probes are shown in Figure 1.2. Commercially manufactured probes and cantilevers are predominantly of silicon nitride (the formula normally given for silicon nitride is Si₃N₄, although the precise stoichiometry may vary depending on the manufacturing process) or silicon (Si). Typically the upper surface of the cantilever, opposite to the tip, is coated with a thin reflective surface, usually of either gold (Au) or aluminium (Al).

Figure 1.1 Basic AFM set-up. A probe is mounted at the apex of a flexible Si or Si₃N₄ cantilever. The cantilever itself or the sample surface is mounted on a piezocrystal which allows the position of the probe to be moved in relation to the surface. Deflection of the cantilever is monitored by the change in the path of a beam of laser light deflected from the upper side of the end of the cantilever by a photodetector. As the tip is brought into contact with the sample surface, by the movement of the piezocrystal, its deflection is monitored. This deflection can then be used to calculate interaction forces between probe and sample.

Figure 1.2 Example of SEM images of different probes and cantilever types. A: pyramidal probe; B: conical high aspect ratio probe for high resolution imaging; C: two V-shaped cantilevers for contact mode imaging; D: chip with a series of beam-shaped levers of different lengths. In this case the levers are tipless to allow mounting of particles of interest for force measurements.

The probe is brought into and out of contact with the sample surface by the use of a piezocrystal upon which either the cantilever chip or the surface itself is mounted, depending upon the particular system being used (these two configurations are referred to as tip-scanning or surface-scanning, respectively). Movement in this direction is conventionally referred to as the z-axis. A beam of laser light is reflected from the reverse (uppermost) side of the cantilever onto a position-sensitive photode...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• About the Editors
• List of Contributors
• Chapter 1. Basic Principles of Atomic Force Microscopy
• Chapter 2. Measurement of Particle and Surface Interactions Using Force Microscopy
• Chapter 3. Quantification of Particle–Bubble Interactions Using Atomic Force Microscopy
• Chapter 4. Investigating Membranes and Membrane Processes with Atomic Force Microscopy
• Chapter 5. AFM and Development of (Bio)Fouling-Resistant Membranes
• Chapter 6. Nanoscale Analysis of Pharmaceuticals by Scanning Probe Microscopy
• Chapter 7. Micro/Nanoengineering and AFM for Cellular Sensing
• Chapter 8. Atomic Force Microscopy and Polymers on Surfaces
• Chapter 9. Application of Atomic Force Microscopy for the Study of Tensile and Microrheological Properties of Fluids
• Chapter 10. Future Prospects
• Index