GENERIC METHODOLOGIES
2.3 ELECTRON MICROSCOPY
30 CHAPTER 2 GENERIC METHODOLOGIES FOR CHARACTERIZATION
Then, they talked about the two MFM operational modes which are static (DC) mode and dynamic (AC) mode and their respective equations. They also briefly introduced the equations for quantitative calculations of the magnetic potential energy and magnetic force acting on the tip.
Finally, they briefly summarized the most noted applications successfully performed by MFM in nanotechnology. These included: (1) new Fe3B nanowires (Fig. 2.12) and analysis to verify their util- ity as feasible magnetic composites, MFM tips, or magnetic recording matrices; (2) studies of mag- netotactic bacterium in which their cells mineralize magnetic nanoparticles covered by lipid–protein membrane. MFM has evolved from a purely scientific tool to a widely used micromagnetic imaging technique and that this will lead to comprehensive image displaying of various magnetic samples.
31 2.3 ELECTRON MICROSCOPy
which structural details (morphology) can be obtained. Electron microscopy is being used to observe a wide range of biological and inorganic specimens. Electron microscopy utilizes parallel beams of electrons that are accelerated by high voltages and focused through a series of electrostatic or magnetic lenses to illuminate the specimen and produce a magnified image (Martin-Palma & Lakhtakia, 2010).
Electron microscopy includes transmission electron microscopy (TEM) and scanning electron microscopy (SEM). High-resolution (HR) version in electron microscopy which refers to HRTEM and HRSEM are also existing methods, but these two will not be discussed in this chapter.
2.3.1 SCANNING ELECTRON MICROSCOPY
In an SEM, the electron beam is focused over the sample in a manner similar to that used in old- fashioned television sets with cathode-ray tubes. The number of backscattered electrons and/or the secondary electrons generated by the beam that emerge from the sample depends on the local composi- tion and topography of the sample. These electrons are collected by an electron detector, and an image is formed by plotting the detector signal as a function of the beam location. This technique has lower resolution than TEM, typically over 1 nm (Martin-Palma & Lakhtakia, 2010).
A diagram of SEM is shown in Fig. 2.13. The electron gun, which is on the top of the column, produces the electrons and accelerates them to an energy level of 0.1–30 keV. The diameter of electron beam produced by hairpin tungsten gun is too large to form a HR image. So, electromagnetic lenses and apertures are used to focus and define the electron beam and to form a small focused electron spot on the specimen. This process demagnifies the size of the electron source (~50 μm for a tungsten filament) down to the final required spot size (1–100 nm). A high-vacuum environment, which allows electron travel without scattering by the air, is needed. The specimen stage, electron beam scanning coils, signal detection, and processing system provide real-time observation and image recording of the specimen surface (Zhou & Wang, 2006).
FIGURE 2.12
Fe3b nanowire by (A) AFM, (b) MFM, and (C) MFM after reversion of tip magnetization.
From Hendrych, A., Kubinek, R., & Zhukov, A. V. (2007). The magnetic force microscopy and its capability for nanomagnetic studies—the short compendium. In Modern research and educational topics in microscopy.
32 CHAPTER 2 GENERIC METHODOLOGIES FOR CHARACTERIZATION
Srinivasan et al. (2007) characterized a series of nanoscale chemical patterning methods based on soft and hybrid nanolithographies using SEM with corroborating evidence from STM and lateral force microscopy. SEM was demonstrated and discussed as an analytical tool to image chemical patterns of molecules highly diluted within host self-assembled monolayer (SAM) and to distinguish regions of differential mass coverage in patterned SAMs.
The study has shown that the relative contrast of SAM patterns in scanning electron micrographs depends on the operating primary electron beam voltage, monolayer composition, and monolayer order, suggesting that secondary electron emission and scattering can be used to elucidate chemical patterns. Fig. 2.14 shows the specific images with operating conditions as mentioned in the figure caption.
SEM is used as an analytical tool for the qualitative evaluation of enhanced chemical patterning methods. In particular, the SEM was capable of distinguishing regions of differential mass coverage in patterned SAMs, as well as dilute chemical patterns (<5%) of isolated molecules. The current work also delineates the dependence of the contrast in SEM images of SAMs on operating conditions, which points to the as yet incomplete understanding of the secondary electron emission and scattering mecha- nisms of molecular overlayers on metal substrates. This indicates the convolution of many parameters, both intrinsic and extrinsic to SAMs, in determining SEM contrast and limits its current application in obtaining quantitative chemical information. However, the SEM is ideally suited to obtain qualita- tive analytical information such as pattern metrology, with spatial resolution, in chemical patterns of SAMs.
FIGURE 2.13
Schematic diagram of SEM.
From Steff. (2010, March). File:Schema MEB (en).svg. Retrieved 17.04.11, from Wikipedia: http://en.wikipedia.org/wiki/
File:Schema_MEB_%28en%29.svg.
33 2.3 ELECTRON MICROSCOPy
2.3.2 TRANSMISSION ELECTRON MICROSCOPY
In TEM, the electron beam travels through the sample and is condensed on a detector plate. A schematic diagram in Fig. 2.15 shows the instrumentation of TEM. Images are formed because different atoms inter- act with and absorb electrons to a different extent. Since electrons interact much more strongly with mat- ter than do X-rays or neutrons with comparable energies or wavelengths, the best results are obtained for sample thicknesses that are comparable to the mean free path of the electrons (the average distance trav- elled by the electrons between scattering events). The recommended thickness varies from a few dozen nanometers for samples containing light elements to tens or hundreds of nanometers for samples made of heavy elements. The resolving power of TEM is theoretically subatomic, although resolutions around 0.1 nm have been achieved in practice. Additionally, TEM allows researchers to generate diffraction pat- terns for determining the crystallographic structures of samples (Martin-Palma & Lakhtakia, 2010).
Yao et al. (2011) described a methodology based on hollow-cone dark-field (HCDF) TEM to study dislocation structures in both nano- and microcrystalline grains. The conventional approach based on a two-beam condition, which was commonly used to obtain weak-beam dark-field TEM images for dislocation structures, was very challenging to employ in study of nanocrystalline materials (especially when grains are less than 100 nm in diameter). Fig. 2.16A and B shows the ray diagram of HCDF-TEM.
The study conducted was on a trimodal Al metal–matrix composite (MMC) consisting of B4C particles, a nanocrystalline Al (NC-Al) phase, and a coarse-grained Al (CG-Al) phase that has been FIGURE 2.14
(A) Field emission scanning electron microcopy (FESEM) images acquired at 1 and 5 kV of microcontact printed regions of 11-mercaptoundecanoic acid (MUDA) that mirror the relief pattern on 10 μm × 10 μm posts on a stamp inked with a 25 mM MUDA (10 min stamp–substrate contact time). (b) FESEM images acquired at 1 and 5 kV of microcontact printed regions of 16-mercaptohexadecanoic acid (MHDA) that mirror the relief pattern on 10 μm × posts on a stamp inked with a 25 mM MHDA (15 s stamp–substrate contact time). The image contrast interchanged reversibly upon switching back and forth between 1 and 5 kV, demonstrating the contrast dependence of SAMs on the operating voltage of the SEM.
From Srinivasan, C., et al. (2007). Scanning electron microscopy of nanoscale chemical patterns. American Chemical Society, 191–201.
34 CHAPTER 2 GENERIC METHODOLOGIES FOR CHARACTERIZATION
FIGURE 2.15
Schematic diagram of TEM.
From Electron microscope, Wikipedia. https://en.wikipedia.org/wiki/Electron_microscope.
FIGURE 2.16
(A) The ray diagram of HCDF-TEM technique and (b) the diffraction plane configuration for HCDF-TEM imaging.
From Yao, B., Heinrich, H., Smith, C., Bergh, M. V., Cho, K., & Sohn, Y. H. (2011). Hollow-cone dark-field transmission electron microscopy for dislocation density characterization of trimodal Al composites. Micron, 29–35.
35 2.4 DIFFRACTION TECHNIQUES
reported to exhibit an extremely high strength and ductility. The technique, applied based on HCDF- TEM to examine the dislocation structure, was presented and subsequently employed to examine the dislocation density in trimodal Al MMCs that contain both nanocrystalline (NC-Al) and coarse-grain (CG-Al) AA5083 Al alloy.
The NC-Al phase in the consolidated and extruded sample contained a high density of dislocations ranging from 2 × 1015 m−2 to 1 × 1016 m−2. The dislocation density in the CG-Al phase even after hot stretch ratio (HSR) extrusion was measured to be relatively small, 4 × 1014 m−2. The authors have successfully conducted the examination where they suggest that the CG-Al phase can significantly enhance the ductility of the trimodal Al MMCs.