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W. Richard Bowen and Nidal Hilal 4

W. Richard Bowen and Nidal Hilal 4

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oscillations. Detection of the shift in phase between the driving <strong>and</strong> oscillating<br />

frequencies allows the z-positioning of the cantilever to be adjusted<br />

to allow the cantilever to remain out of contact with the surface by the<br />

operation of a feedback loop [59]. Because the probe does not contact the<br />

surface in the repulsive regime, the area of interaction between the tip<br />

<strong>and</strong> the surface is minimised allowing potentially for greater surface resolution.<br />

As a result in this mode, it is imaging which is best able to achieve<br />

true atomic resolution, when examining a suitable surface under suitable<br />

conditions. However, in practice obtaining images of a high quality is a<br />

more daunting prospect than intermittent contact mode. When imaging<br />

in air all but the most hydrophobic regions of surfaces will have a significant<br />

water layer, which may be thicker than the range of the van der<br />

Waals forces being probed. This combined with the low oscillation amplitude<br />

will mean that the probe will be unable to detach from the water<br />

layer easily, degrading imaging resolution.<br />

.4.4 force volume Imaging<br />

1.4 IMAgINg MOdEs<br />

The force volume imaging mode is a combination of conventional<br />

imaging with the measurement of force distance curves (see Section 2.2<br />

for explanation of these measurements). For each pixel of the image, a<br />

force distance curve is obtained by bringing the probe into <strong>and</strong> then out<br />

of contact with the surface <strong>and</strong> simultaneously recording the deflection<br />

of the lever as a function of the z-directional translation, so that concurrently<br />

with obtaining a conventional topographic (height) image, information<br />

about how interaction forces between the probe <strong>and</strong> the sample<br />

vary with the sample topography is obtained. By plotting an image<br />

scaled to show the lowest force value for each pixel, an adhesion map of<br />

the surface can be obtained [60]. This is useful for highlighting different<br />

adhesive properties of different surface domains [60–62], which can be<br />

particularly useful if the probe itself carries a tailored chemical functionality<br />

[63, 64]. In addition plots of the relative elasticity of different surface<br />

domains can be produced, if working with sufficiently soft samples<br />

such as polymer layers [65] or surface immobilised cells [63]. Where force<br />

measurements are to be obtained between the probe <strong>and</strong> a relatively<br />

rough surface, a high variability between curves may be observed due<br />

to surface asperities, causing a great variation in the contact area from<br />

one point to another. If this is the case then the force volume mode is a<br />

useful way to obtain the large numbers of force curves needed over an<br />

area. As obtaining a large number of force curves at the same time is very<br />

memory intensive, force volume images are typically lower in resolution,<br />

in terms of the number of pixels in an image than the images obtained in<br />

other AFM modes.

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