{"id":278,"date":"2018-12-11T08:29:20","date_gmt":"2018-12-11T08:29:20","guid":{"rendered":"http:\/\/msp11.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=278"},"modified":"2018-12-11T09:18:14","modified_gmt":"2018-12-11T09:18:14","slug":"atomic-force-microscopy-ii","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp11\/chapter\/atomic-force-microscopy-ii\/","title":{"rendered":"Atomic Force Microscopy-II"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n\r\n

Learning Objectives<\/strong><\/p>\r\n

From this module students may get to know about the following<\/strong><\/p>\r\n

i.<\/strong> Review of Basics of AFM<\/em><\/p>\r\n

ii.<\/strong> Operating Modes of AFM<\/em><\/p>\r\n

iii.<\/strong> Various Kinds of Force Microscopes<\/em><\/p>\r\n \r\n

1. Introduction<\/strong><\/p>\r\n

Atomic force microscopy (AFM) includes various methods in for the interaction of probe and the specimen which helps to characterize various material properties. In this context AFM can characterize of mechanical, electrical, magnetic, and optical spectroscopic properties. In this article we explain in detail the different modes of operation in AFM and kinds of AFM systems.<\/p>\r\n \r\n

2. Modes of Operation in AFM<\/strong><\/p>\r\n

(1) Contact AFM:<\/strong> Works at a probe-surface separation of less than 0.5 nm.<\/p>\r\n

(2) Intermittent contact or tapping mode:<\/strong> Works at a probe-surface separation of 0.5-2 nm.<\/p>\r\n

(3) Non-contact AFM:<\/strong> Works at a probe-surface separation of 0.1-10 nm.<\/p>\r\n

The three modes are sketched in Figure 1 and the variations are tabulated in Table-1.<\/p>\r\n \r\n

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Figure 1. Different modes of operation in AFM<\/em><\/strong><\/p>\r\n \r\n\r\n \r\n

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Table 1. Various modes of AFM<\/em><\/strong><\/p>\r\n \r\n

2.1. Contact Mode AFM<\/strong><\/p>\r\n

In this mode, the AFM tip is gently in contact with the surface of sample, and records the small force between the probe and the surface. The cantilever is held at a distance less than few angstroms away from the surface of the sample. This mode operates in the repulsive regime of the Van-der Waals curve i.e. the force on the tip is repulsive. When the spring constant of cantilever is less compared to the surface, i.e., lower than effective spring constant that binds the atoms of the sample together, the cantilever bending occurs. With a constant cantilever deflection (using the feedback loops) the force between the probe and the sample is maintained constant and an image of the surface is obtained.<\/p>\r\n

In the contact AFM mode, the tip makes soft physical contact with the surface of the sample. Here, the deflection of the cantilever, (dx) is proportional to the force acting on the tip, via Hook\u2019s law, F = -k. x, where k is the spring constant of the cantilever. In contact-mode, the tip either scans at a constant small height above the surface or under the conditions of a constant force. In the constant height mode the height of the tip is fixed, whereas in the constant-force mode the deflection of the cantilever is fixed and the motion of the scanner in the direction of height (z) is recorded. By using contact-mode AFM, the atomic resolution images could be captured.<\/p>\r\n

For contact mode AFM imaging, it is necessary to have a cantilever soft enough to be deflected by very small forces and has a high enough resonant frequency to not be susceptible to vibrational instabilities. Generally, silicon nitride tips are used for contact mode.<\/p>\r\n \r\n

Advantages of Contact Mode AFM<\/strong><\/em><\/p>\r\n

a) High scanning speed<\/p>\r\n

b) Easier scanning for rough samples with abrupt changes in vertical topography.<\/p>\r\n

c) Used in friction analysis<\/p>\r\n \r\n

Disadvantages of Contact Mode AFM<\/em><\/strong><\/p>\r\n

a) Lateral forces can distort the image.<\/p>\r\n

b) Capillary forces from a fluid layer can cause large forces normal to the tip sample interaction.<\/p>\r\n

c) Combination of these forces reduces spatial resolution and can cause damage to soft samples. There are two contact scanning modes, as described below:<\/p>\r\n \r\n

2.1.1. Constant Height<\/strong><\/p>\r\n\r\n