Biomedical Engineering Reference
In-Depth Information
18.4 Measurement of the Mechanical Properties of Nanostructures.........................................................326
18.4.1 Nanoscratch Testing ............................................................................................. 326
18.4.2 Nanohardness Test ............................................................................................... 326
18.5 Conclusions..................................................................................................................................327
References ............................................................................................................................................328
18.1 MEASUREMENT OF THE TOPOLOGY OF NANOSTRUCTURES
18.1.1 Field Emission Scanning Electron Microscope
Scanning electron microscopes (SEMs) have been used by researchers since 1935 to examine microm-
eter scale structures and more often recently for the examination of nanoscale structures [1-4] . This is
a versatile technique with which relatively large samples can be visualized, dimensional measurements
can be taken, and compositional analysis can be performed. The SEM works by initially firing primary
electrons at the sample to be imaged. Electrons are dislodged from the atoms at the surface of the
sample and are attracted to a positively charged detector grid. These electrons are known as secondary
electrons. When a set pattern of primary electron beam scanning is used over the surface, recording of
the secondary electrons allows the surface topology to be interpreted and displayed. Spatial resolution
within a given SEM depends on the primary electron beam spot size and the volume of material with
which the electrons interact. Under good conditions, such as high accelerating voltage (e.g., 30 kV),
well-aligned apertures, well-corrected astigmatism, small spot size (small probe current), and no sam-
ple charging, resolutions of 3 nm can typically be achieved. Conventionally, tungsten and carbon ele-
ments were used in SEMs. More recently to achieve longer cathode gun lives LaB 6 elements have been
adopted. Primary electrons that are bounced back off the surface are known as back-scattered electrons
(BSE). The energy of these electrons is directly related to the density of the atoms from which they are
repelled and therefore their recording allows the variation of surface composition to be visualized.
A field emission cathode in the electron gun of a SEM provides narrower probing beams result-
ing in both improved spatial resolution and less sample charging. Such systems are designated as
field emission SEMs (FESEMs). In order to achieve this increased electron focusing a different gun
design is required. In this design, electrons are expelled by applying a high electric field very close
to the filament tip. The size and proximity of the electric field to the electron reservoir in the filament
controls the degree to which electrons tunnel out of the reservoir. One type of field emission gun
commonly used is known as the Schottky in-lens thermal FESEM electron gun. Cold gun alternatives
are available for even finer FESEM resolution; however, these suffer rapid degradation and can there-
fore lead to expensive operation due to relatively frequent placement. The field emission guns have
higher stability, can allow higher current, and hence provide a smaller spot size. Under good oper-
ating conditions a typical FESEM resolution of 1 nm is achievable. Elements that add to improved
operation and FESEM resolution include designs with a beam booster to maintain high beam energy,
an electromagnetic multihole beam aperture changer, a magnetic field lens, and a beam path have
been designed to prevent electron beam crossover.
18.1.1.1 FESEM Case Studies
Researchers have investigated the potential beneficial effects of well-adhered TiO 2 nano layers on
dental implants for increased biocompatibility and have utilized FESEM to aid characterization of the
 
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