Probe Microscopy Section (SPM)

Scanning microscopy
A. Scanning probe microscopy section (SPM)
Introduction
The atomic force microscope (AFM) is a scanning probe microscope with very-high-
resolution. AFM is used for imaging samples at nanoscale resolution. The AFM machine is
used for biomaterial characterization. Information about the AFM microscope will be used to
answer the following questions.
Questions
1. At approximately what distance would an AFM tip sense the repulsive
interatomic forces? (2 marks)
An AFM tip would detect the repulsive interatomic forces at small sample-tip
distances .Near the surface of a sample, a powerful repulsive force appears between
the sample particles and cantilever tip. The strong repellent forces arise due to sample-
tip interactions. When the repulsive force is prevalent, the tip and sample are
considered to be in contact ( Geng & Borodich et al., 2016).
2. It is relatively easy to make a cantilever with a spring constant low enough to sense
the forces between atoms, such as a cantilever made of aluminium foil that is 4 mm long
and 1 mm wide. However, AFM cantilever manufacturers do not make cantilevers that
large. They make them much smaller, typically 100 to 200 microns long and 20 to 50
microns wide. What is the reason for this? (4 marks)
The sharpness or bluntness of a tip significantly affects the side-way resolution of the AFM,
as the sample-tip interaction area depends on the tip radius. The radius of the apex of an
unused tapping mode tip is small to enhance resolution. Sharp tip achieves greater resolution
for the AFM image. AFM cantilever force sensing is based on the deflection of the cantilever

Scanning microscopy
beam. When the tip size is very large, the sample would be subjected to high pressing force
due to the cantilever deflection, thus causing sensing error (Zhao & Hu et al., 2013).
3. How is the force the AFM tip exerts on the sample surface related to its deflection or
bending? (1 marks)
The degree of cantilever deflection is based on the force exerted by AFM tip. The higher the
force, the greater the deflection ( Geng & Borodich et al., 2016).
4. What are the disadvantages of contact mode compared to tapping mode? (4 marks)
The cantilever tip drags across the sample in contact mode .Dragging of the cantilever causes
frictional and adhesive forces. Contact mode thus causes sample damage and image data
distortion. Tapping mode AFM prevents frictional damage to sample by intermittently making
contact with sample surface. Images by Tapping mode AFM show no alteration and better
resolution (Al-Rashdi & Hilal et al. ,2012).
5. What is kept constant during imaging in tapping mode? (1 mark)
Cantilever frequency or oscillation amplitude (Proksch & Yablon, 2012).
6. Figure 2 below Shows a typical resonance curve for an AFM tapping mode cantilever.

Scanning microscopy
a. Define what resonance frequency means in terms of an AFM cantilever (2 marks)
The cantilever resonance frequency is the frequency at which the oscillation amplitude
reaches maximum energy. Mathematically, cantilever resonance frequency is the square root
of its spring constant divided by its mass ( Tsou & Proksch et al., 2011).
b. Determine the approximate Q factor for the resonance curve in figure 2 (The
resonance curve is in an attached excel file called Figure 2) (4 marks)
Q = F0/F3Db ; F0=300
Q = 300/5×3=20Hz/E

Scanning microscopy
7. Why is the spring constant and resonant frequency for tapping mode probes much
larger, compared to contact mode probes when operating in air? (Hint: capillary layer)
(4 marks)
Van der Waals and capillary forces are involved during the application of AFM tapping mode
in an air tip-sample.Capillary force is due to water layout coating sample surface and the
cantilever tip.Repulsive force arise due to the contact between the tip and the sample.
Capillary force develops as the tip approaches the sample (Kavousi & Ebrahimi et al., 2011).
The capillary force is due to interactions between water films and the sample. As separation
reduces below a distance of the beginning of the capillary, a liqued column is established.
Retraction of the tip from the surface forms a meniscus and neck which breaks above a
critical distance ( Lauke & Heinrich et al. , 2014).
8. What information about the sample surface can force-distance curves potentially
give? (2 marks)
AFM topography images give information on overall surface morphology, can reveal defects,
and distinguish amorphous and crystalline phases, and identify nucleation and growth modes
(Liu & Wang, 2010).Additionally, images can be analysed to calculate areal surfaces
roughness parameters such as Sa (3D roughness average) and Sq (root mean square
roughness) or statistics on grain and domain size (Alsteens & Müller, 2013). Furthermore,
individual line sections can be used to determine step heights, film thickness, and even lattice
spacings (Waltman & Guo, 2012).
9. The company, called Bruker corporation, has developed an imaging mode called Peak
Force tapping. What are the advantages of this imaging mode compared to other
imaging modes? (3 marks)

Scanning microscopy
Tapping mode AFM does not drag tip on hence less surface alteration. Measurements with
tapping mode are achievable in ambient conditions. Tapping mode has high resolution of
samples than contact mode (Proksch & Walters et al., 2014).
10. What does calibration mean? Why is calibration such an important part of any
experiment? (3 marks)
The procedure of comparing reading on one piece of equipment or system, with another piece
of equipment that has been calibrated and referenced to a known set of parameters is called
calibration. The reference equipment should directly be in match with international standards
(Kelly & Sukhatme, 2011). Equipment calibration is conducted to check the accuracy and
quality of measurements recorded. Equipment tend to have drifted accuracy when measuring
specific parameters such as temperature and humidity ( Bolgen & Christiansen et al., 2010).
Calibrated equipment gives confident, reliable and repeatable result (Savescu & Laba, 2016).
11. In order to accurately measure quantitative forces using AFM what components of
the microscope must be calibrated? (4 marks)
AFM microscope has parts that need to be calibrated for accurate measurements of data
(Müller & Dufrene, 2010). The AFM components that are calibrated include:
i) cantilever sensor ii) Laser diode iii) Photodiode
iv) Sensitive photo-detector v) Probe tip vi) computer
12. Describe a technique to calibrate the cantilever deflection of an AFM cantilever.
Give advantages and disadvantages of the technique. (4 marks)

Scanning microscopy
Calibration of cantilever deflection of an AFM cantilever can be done by various techniques
such as thermal noise method, Static response techniques, Dynamic response method, the
piezolever method, e.t.c. The piezolever method is described below.
Piezolever method of calibrating AFM cantilever
Calibration designed apparatus are used to calibrate AFM cantilevers. Designed calibrated
apparatus is rested on the AFM stage. Optical microscope of the AFM locates the
piezolever.AFM cantilever is brought very close to the piezolever. The cantilever is aligned
above the piezolever (Kilpatrick & Thorén et al., 2016). Piezolever and cantilevers are brought
into contact using and AFM software. The voltage output from the piezolever is reset to zero.
This procedure is repeated until the AFM cantilever tip is in contact with the reverse side of
the piezolever tip. After a successful connection is made, the base of the piezolever is moved
up using the manual control of the AFM stage. The change in voltage is recorded. This
process is repeated until sufficient data are collected (Crawford & Bieske et al., 2012).
13. . Read the following paper carefully and answer the following
questions13a to 13c
(DOI: 10.1063/1.1144209, Authors: J. P. Cleveland, S. Manne, D. Bocek and P. K.
Hansma)
A non destructive method for determining the spring constant of cantilevers for
scanning force microscopy.
a. Describe the sources of error for this method? (4 marks)
The accuracy of force acting between a sample and an AFM cantilever is determined by the
spring constant of the AFM cantilever. Change in the resonant frequency of a cantilever is
measured by Cleveland method. Microspheres of known mass values are attached near the
free end of the cantilever. The errors encountered are due to deviations in cantilever shape,
presence of surface coating materials on a cantilever and substandard calibration methods.
Additionally, the error source lies with the estimation of the added mass, which depends