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==Introduction==

==Introduction==

[[file:v2M102Ch1Fg1.jpg|thumb|300px|{{figure number|1}}Schematic diagram of the GEMINI^&trade; electron column (a) and the electron beam path in the column (b).<ref name=Huangetal_2013>Huang, Jason, Timothy Cavanaugh, and Boaz Nur, 2013, An introduction to SEM operational principles and geologic applications for shale hydrocarbon reservoirs, ''in'' W. Camp, E. Diaz, and B. Wawak, eds., Electron Microscopy of Shale Hydrocarbon Reservoirs: [http://store.aapg.org/detail.aspx?id=1197 AAPG Memoir 102], 260 pp.</ref>]]

file:v2M102Ch1Fg1.jpg|{{figure number|1}}Schematic diagram of the GEMINI^&trade; electron column (a) and the electron beam path in the column (b).<ref name=Huangetal_2013>Huang, Jason, Timothy Cavanaugh, and Boaz Nur, 2013, An introduction to SEM operational principles and geologic applications for shale hydrocarbon reservoirs, ''in'' W. Camp, E. Diaz, and B. Wawak, eds., Electron Microscopy of Shale Hydrocarbon Reservoirs: [http://store.aapg.org/detail.aspx?id=1197 AAPG Memoir 102], 260 pp.</ref>

[[:file:v2M102Ch1Fg1.jpg|Figure 1a]] shows an example schematic diagram of a state-of-the-art electron column. The column shown here is the Zeiss GEMINI™ column. The ray path of the electrons is illustrated in Figure 1b. Specifically, electrons are emitted from a very fine heated filament with a high electrostatic extraction potential. The electrons are then immediately accelerated to a desired kinetic energy and propagate down the electron column. In the GEMINI column, a very simple two-lens system is employed to focus the electron beam. The upper lens, or the condenser lens, first demagnifies the emitted electrons to some extent. Next, some of the electrons are cut off by an aperture. The focusing strength of the condenser lens and the size of the aperture determine the beam current in the final spot. A final objective lens assembly then focuses the beam down to the desirable spot size. Depending on manufacturer design, the lenses used in the column can be magnetic or electrostatic (or a combination of the two types). In the case of the GEMINI column, the final objective lens is a combination of magnetic and electrostatic lenses. The main advantage of such a design is that there is virtually no magnetic field on the specimen that could distort the imaging if the specimen is magnetic.

[[:file:v2M102Ch1Fg1.jpg|Figure 1a]] shows an example schematic diagram of a state-of-the-art electron column. The column shown here is the Zeiss GEMINI™ column. The ray path of the electrons is illustrated in Figure 1b. Specifically, electrons are emitted from a very fine heated filament with a high electrostatic extraction potential. The electrons are then immediately accelerated to a desired kinetic energy and propagate down the electron column. In the GEMINI column, a very simple two-lens system is employed to focus the electron beam. The upper lens, or the condenser lens, first demagnifies the emitted electrons to some extent. Next, some of the electrons are cut off by an aperture. The focusing strength of the condenser lens and the size of the aperture determine the beam current in the final spot. A final objective lens assembly then focuses the beam down to the desirable spot size. Depending on manufacturer design, the lenses used in the column can be magnetic or electrostatic (or a combination of the two types). In the case of the GEMINI column, the final objective lens is a combination of magnetic and electrostatic lenses. The main advantage of such a design is that there is virtually no magnetic field on the specimen that could distort the imaging if the specimen is magnetic.

Since the first commercial instruments, many advances have been made, including development of a high-brightness electron gun allowing more electrons to be focused into a smaller beam spot, an improved electron optics design that can dramatically enhance imaging resolution, and a large portfolio of detectors that capture and separate different types of information generated by the primary electron beam. A more detailed description of a detection mechanism related to imaging contrast is elaborated in the following section.

Since the first commercial instruments, many advances have been made, including development of a high-brightness electron gun allowing more electrons to be focused into a smaller beam spot, an improved electron optics design that can dramatically enhance imaging resolution, and a large portfolio of detectors that capture and separate different types of information generated by the primary electron beam. A more detailed description of a detection mechanism related to imaging contrast is elaborated in the following section.

==SEM contract mechanisms==

==SEM contract mechanisms==

Photons with wavelengths in the ultraviolet, visible, and infrared light regions of the electromagnetic spectrum can also be excited by the primary electrons. This phenomenon is called cathodoluminescence (CL). The generation of CL is commonly associated with the presence of certain trace impurities in minerals and therefore is also widely used in geological applications. A few examples are illustrated next to demonstrate several common contrast mechanisms useful for the geological study of shales.

Photons with wavelengths in the ultraviolet, visible, and infrared light regions of the electromagnetic spectrum can also be excited by the primary electrons. This phenomenon is called cathodoluminescence (CL). The generation of CL is commonly associated with the presence of certain trace impurities in minerals and therefore is also widely used in geological applications. A few examples are illustrated next to demonstrate several common contrast mechanisms useful for the geological study of shales.

[[file:M102Ch1Fg3.jpg|thumb|300px|{{figure number|3}}A shale sample imaged using SE1 signal (left) and SE2 signal (right). Surface-specific information such as pore space and surface roughness is evident in the SE1 image. The SE2 image has more compositional influence, displaying organic matter (OM) bodies that are not evident in the SE1 image.<ref name=Huangetal_2013 />]]

file:M102Ch1Fg3.jpg|{{figure number|3}}A shale sample imaged using SE1 signal (left) and SE2 signal (right). Surface-specific information such as pore space and surface roughness is evident in the SE1 image. The SE2 image has more compositional influence, displaying organic matter (OM) bodies that are not evident in the SE1 image.<ref name=Huangetal_2013 />

file:M102Ch1Fg5.jpg|{{figure number|5}}A BSE2 image of gold (Au) nanoparticles showing crystallographic contrast.<ref name=Huangetal_2013 />]]

===SEM image examples===

===SEM image examples===

[[:file:M102Ch1Fg3.jpg|Figure 3]] shows an SE1 image of a shale sample. At low landing energy (less than 1 keV), the secondary electrons collected with the in-lens detector originate from the very surface of the sample. The SE1 signal is commonly used to image surface details at the highest resolution at the expense of compositional information. On the right side of Figure 3, the SE2 image of the same area as the SE1 image on the left shows the

[[:file:M102Ch1Fg3.jpg|Figure 3]] shows an SE1 image of a shale sample. At low landing energy (less than 1 keV), the secondary electrons collected with the in-lens detector originate from the very surface of the sample. The SE1 signal is commonly used to image surface details at the highest resolution at the expense of compositional information. On the right side of Figure 3, the SE2 image of the same area as the SE1 image on the left shows the

effects of higher landing energy and deeper specimen interaction, including more compositional and less topographical information.

effects of higher landing energy and deeper specimen interaction, including more compositional and less topographical information.

[[:file:M102Ch1Fg4.jpg|Figure 4]]a shows an SE2 image of a cross section of a shale sample that has been polished by argon-ion milling, a sample preparation technique that consists of using one or more beams of argon ions to gently polish the surface of a sample by sputtering away material, thus providing an extremely smooth surface for SEM investigation. The image was acquired with an Everhart-Thornley type secondary electron detector.

[[:file:M102Ch1Fg4.jpg|Figure 4]]a shows an SE2 image of a cross section of a shale sample that has been polished by argon-ion milling, a sample preparation technique that consists of using one or more beams of argon ions to gently polish the surface of a sample by sputtering away material, thus providing an extremely smooth surface for SEM investigation. The image was acquired with an Everhart-Thornley type secondary electron detector.

The same cross section was imaged with BSE1 with the Energy-selective backscatter (EsB) detector (Figure 4b). The image contrast (grayscale variation) reflects compositional variations (mean atomic number) of the sample. For example, the midgray represents silica matrix; the darker level represents organic matter. The brighter gray level reflects higher density carbonate phases, and the brightest gray level represents pyrite. Note the greater compositional contrast provided by the BS1 image (Figure 4b) over the SE2 image (Figure 4a). Although SE2 and BS1 images are capable of providing compositional information, auxiliary techniques, such as energy-dispersive x-ray spectrometry (EDS), are required to characterize elemental composition.

The same cross section was imaged with BSE1 with the Energy-selective backscatter (EsB) detector (Figure 4b). The image contrast (grayscale variation) reflects compositional variations (mean atomic number) of the sample. For example, the midgray represents silica matrix; the darker level represents organic matter. The brighter gray level reflects higher density carbonate phases, and the brightest gray level represents pyrite. Note the greater compositional contrast provided by the BS1 image (Figure 4b) over the SE2 image (Figure 4a). Although SE2 and BS1 images are capable of providing compositional information, auxiliary techniques, such as energy-dispersive x-ray spectrometry (EDS), are required to characterize elemental composition.

Compared to BSE1, for which the contrast is modulated by mean atomic number differences, BSE2 yield depends strongly on crystalline structures such as grain orientations. [[:file:M102Ch1Fg5.jpg|Figure 5]] shows a BSE2 image of gold (Au) nanoparticles. Contrast corresponding to different grains is revealed in the image despite all the chemically identical grains. Therefore, BSE2 electrons can be used to image crystallographic contrast in polycrystalline materials.

Compared to BSE1, for which the contrast is modulated by mean atomic number differences, BSE2 yield depends strongly on crystalline structures such as grain orientations. [[:file:M102Ch1Fg5.jpg|Figure 5]] shows a BSE2 image of gold (Au) nanoparticles. Contrast corresponding to different grains is revealed in the image despite all the chemically identical grains. Therefore, BSE2 electrons can be used to image crystallographic contrast in polycrystalline materials.

[[file:M102Ch1Fg6.jpg|thumb|300px|{{figure number|6}}(a) A BSE and CL image of a polished shale sample. Orange-hued quartz grains reflect low-grade metamorphic origin (slate); blue-hued quartz grains indicate higher grade metamorphism (phyllite-schist). (b) Detail of a large quartz grain in center of image (arrow) displays multiple generations of growth in CL; this distinction in zoning is not visible in SEM.<ref name=Huangetal_2013 />]]

file:M102Ch1Fg6.jpg|{{figure number|6}}(a) A BSE and CL image of a polished shale sample. Orange-hued quartz grains reflect low-grade metamorphic origin (slate); blue-hued quartz grains indicate higher grade metamorphism (phyllite-schist). (b) Detail of a large quartz grain in center of image (arrow) displays multiple generations of growth in CL; this distinction in zoning is not visible in SEM.<ref name=Huangetal_2013 />

Another complimentary technique is the detection of CL, in which certain materials will emit photons in the form of visible light as a result of interactions between specimen electrons and primary beam electrons. [[:file:M102Ch1Fg6.jpg|Figure 6]] shows an example of the effects of CL on a shale sample. The image was acquired with a dedicated CL detector. Variations in CL emission caused by mineral impurities could indicate provenance of individual quartz grains. Cathodoluminescence can also be used to differentiate between generations of quartz growth that are not distinguishable in SEM due to identical mean atomic number.

Another complimentary technique is the detection of CL, in which certain materials will emit photons in the form of visible light as a result of interactions between specimen electrons and primary beam electrons. [[:file:M102Ch1Fg6.jpg|Figure 6]] shows an example of the effects of CL on a shale sample. The image was acquired with a dedicated CL detector. Variations in CL emission caused by mineral impurities could indicate provenance of individual quartz grains. Cathodoluminescence can also be used to differentiate between generations of quartz growth that are not distinguishable in SEM due to identical mean atomic number.

==X-ray microanalysis==

==X-ray microanalysis==

==References==

==References==