AbstractPolarized light microscopy provides unique opportunities for analyzing the molecular order in heterogeneous systems, suchas living cells and tissues, without using exogenous dyes or labels. This article briefly discusses the theory of polarizedlight microscopy and elaborates on its practice using a traditional polarized light microscope and more specialized polarizationmicroscopes such as the LC-PolScope, Oosight, or Abrio. The microscope components specific to analyzing the polarization oflight, such as polarizer and compensator, are introduced, and quantitative techniques for measuring the birefringence of thespecimen point by point using a traditional polarizing microscope are discussed. The new LC-PolScope greatly improves theanalytic power of the technique, providing quantitative birefringence data simultaneously for every image point, thereby revealingmolecular order with unprecedented sensitivity and at the highest resolution of the light microscope.
Practical aspects discussedinclude the choice of optics, sample preparation, and combining polarized light with differential interference contrast andfluorescence microscopy. A glossary of polarization optical terms is also included to facilitate the discussion of observationsmade with a polarized light microscope. © 2013 Cold Spring Harbor Laboratory Press.
Ingham BSc (Hons), MSc, DipRMS, CEng, MInstNDT, EurGeol, CGeol, CSci, FGS, FRGS, MIAQP, in, 2013 Future developmentsOptical microscopes have now entered the digital age and modern microscopes tend to come with a computer attached. The major microscope manufacturers now offer options for automation of their optical microscopes with a range of functions being controlled via a touch-sensitive screen. Motors automatically move the objective, substage condenser, and the image projection system; all of which is optimized quite automatically for quality of illumination, resolution, and even focus. Combined with digital image capture systems and image analysis software we now have a real possibility of automated sample examination.
However, an experienced petrographer will always be required to supervise the process and check the results. Automation offers huge improvements in productivity and potentially significant reductions in the unit price of examinations. Automated modal analysis is likely to become a serious alternative for a number of standard tests that have traditionally been performed by cheaper (and arguably less accurate) chemical analysis methods. For example, the mix proportions of hardened concrete and mortars could routinely be determined by modal analysis instead of chemical analysis.Digital technology is changing the way that the findings of petrographic examinations are presented.
Digital image capture allows a series of sequential micrograph frames to be animated into movies to illustrate talks and websites ( Entwistle, 2003a). Digital movies demonstrating petrographic features of samples could be included within the report submitted to the client. The recent development of the birefringence imaging microscope offers the possibility of interesting applications for examination of geomaterials.
The equipment consists of a motorized rotating polarizer that is fitted below the sample stage and a digital camera that fits on the trinocular microscope head, both attached to a computer. As the motorized polarizer rotates, the camera collects birefringence data at a number (between five and fifty) of different positions. The data are then processed by special computer software to produce various types of false colour image. The birefringence microscope excels at detecting strain and defects within materials. Figures 11 and 12 show examples of false colour images obtained using the birefringence microscope. Existing applications include quality control of industrial diamonds, silicon carbide abrasives, and glass, study of decomposition of biomaterials, and mapping of collagen in heart valves.
Potential applications for geomaterials include investigation of rock microstructure (including the investigation of bowing marble panels), detection of alkali–silica reactive aggregates, and identification of flaws in a variety of construction products. Orientation (φ) image of a sandstone sample obtained using the birefringence microscope imaging system. The lines show the orientation of the extinction (slow axis orientation) for each of the quartz grains; ×150, 1mm across. (Courtesy of Oxford Cryosystems.)Electron microscopy is one of the main complementary techniques used in conjunction with optical microscopy hence developments in the field of electron microscopy concern the petrographer. The resolving power of the electron microscope is continually improving, with modern field emission scanning electron microscopy (FESEM) now providing magnifications of up to 550,000 times and resolutions down to 0.5 nm. One disadvantage of conventional electron microscopy is that the sample has to be viewed in a vacuum. Recent advances have allowed hydrated samples to be imaged using environmental scanning electron microscopy (ESEM) or alternatively, by soft X-ray transmission microscopy.
These methods allow ‘live’ examination and analysis of geomaterials undergoing reactions, for example, hydrating cement paste or carbonating lime. Also, the latest cryotransfer SEM allows sensitive hydrating specimens to be set in a stable state by quick-freezing, enabling previously impossible examinat ions. Advances in electron microscopy will continue to improve our understanding of geomaterials and reactions that they undergo.
Durrani, in, 2012 B Optical MicroscopeThe optical microscope can be used to analyze etched tracks in the size range of 0.5 μm to several hundreds of μm. Optical microscopes are composed of (1) eyepiece (one or two), (2) objective (one or more), (3) stage, (4) condenser (transmitted light and reflected light), and (5) stand.The accessaries should include: (1) standard ruler, used to calibrate the scales; (2) reticule, which can be put in the eyepiece for measurement of track or field size; (3) reticulate, which is used as a standard area for the measurement of track density (tracks/cm 2); (4) displacement transducer (one, two, or three), at least one for the measurement of depth ( z-axis). Another two are for x- and y-axis measurements. Dark-field microscopy can be used to increase the contrast of a transparent specimen.
By inserting a central stop before the condenser, some but not all of the light from the condenser is prevented from reaching the objective ( Fig. Only light scattered from the edges of the specimen is viewed. Thus, the specimen appears as a bright image against a dark background.
Dark-field microscopy is often used to visualize live specimens that have not been fixed or stained. For example, dark-field microscopy has been used to quantify the motility of bacteria and protozoa and to monitor the growth of bacterial microcolonies ( Korber et al., 1990 ). Although gross morphology can be delineated, internal details are not revealed. Murray and Robinow (1994) and Hoppert (2003) describe the nature of dark-field microscopy and its applications. Differential interference contrast (DIC) microscopy provides brightly colored, highly contrasting three-dimensional images of live specimens.
In DIC microscopy, the illuminating beam is split into two separate beams. One beam passes through the specimen, creating a phase difference between the sample beam and the second or reference beam. The two beams are then combined so that they interfere with each other. DIC can allow the detection of small changes in depth or elevation in the sample, thus giving the perception of a three-dimensional image ( Fig. Anisotropic light, light that depends on the angle of observation, originates from specimens that have asymmetry in their crystal lattice properties.
Anisotropy is observable in liquid and solid crystals; strained glasses; stressed plastic materials; crystallized resins and polymers; refracting surfaces; synthetic filaments; and biological fibers, cells, and tissues. Polarized light is light in one plane that can be used to examine anisotropy in sample materials. Polarization microscopy is traditionally used to determine the optical properties of soil minerals to aid in their identification ( Fig.
The optical anisotropy of individual crystals reflects the bonding patterns of units (e.g., molecules or elements) and usually involves differences in at least two crystallographic directions (at least two directions of polarized light). Multiple anisotropic crystals have optical characteristics above and beyond those of individual crystals. Anisotropy observed in a sample can provide more information about the sample than ordinary unpolarized light. For example, a result of light polarization is molecular birefringence. Molecular birefringence is manifested by long or flat molecules, especially polymeric macromolecules, and is particularly applicable in the examination of microbially produced extracellular polymers ( Fig.
In molecular birefringence, when polarized light encounters a series of atomic dipoles arranged in chains, as in long molecules, the strength of the dipoles causes the light to vibrate lengthwise along the chain, resulting in greater polar anisotropy at the poles. However, side chains on the molecules tend to reduce the strength of birefringence in the main chain of the molecule, resulting in less polar anisotropy. The patterns and strengths of anisotropy evident in a sample can give indications of the purity and elemental structure of the sample.
The resolution of a specimen's effects on polarized light depends on producing plane-polarized light with a polarizer and examining the effects with an analyzer. Two polarizers are used in polarization microscopy: a polarizer and an analyzer ( Fig. For transmitted light, the polarizer is placed between the light source and the substage condenser lens. The analyzer is placed between the objective and ocular lenses. When polarized light from the first polarizer vibrates in a direction that allows it to pass through the analyzer, the field of view in the microscope will be black as the polarizers are crossed with respect to their directions of vibration of light. Thus, in a polarizing microscope, contrast is the result of various interference phenomena throughout the sample.
Light interference or retardation at each point in a crystal results in contrast and color on a dark background. In accordance with the Michel-Levy interference spectra based on light retardation through varying sample thicknesses, light interference gives first order gray, high order white, and color to the sample image. Dark-field microscopy can be used to increase the contrast of a transparent specimen. By inserting a central stop before the condenser, some but not all of the light from the condenser is prevented from reaching the objective ( Figure 9.4). Only light that is scattered from the edges of the specimen is viewed. Thus, the specimen appears as a bright image against a dark background.
Dark-field microscopy is often used to visualize live specimens that have not been fixed or stained. For example, dark-field microscopy has been used to quantify the motility of bacteria and protozoa and to monitor the growth of bacterial microcolonies ( Korber et al., 1990). Although gross morphology can be delineated, internal details are not revealed. Murray and Robinow (1994) and Hoppert (2003) describe the nature of dark-field microscopy and its applications. Differential interference contrast (DIC) microscopy provides brightly colored, highly contrasting three-dimensional images of live specimens.
It all starts so innnocently and follows the story perfectly, but then faster than you can say Robert Downey Jnr suddenly were faced with the Mandarn, elemental warriors and a dragon. The year before Iron Man was released and the Marvel Universe truly began we had The Invincible Iron Man an origin story that went heavily off the comic rails. Call me old school but I hate when origins stories are changed and this is ludicrous. Needless to say I wasn't impressed, this Errol Flynn looking Tony Stark and the questionable animation surrounding him didn't make matters any better. Invincible iron man 594.
In DIC microscopy, the illuminating beam is split into two separate beams. One beam passes through the specimen, creating a phase difference between the sample beam and the second or reference beam. The two beams are then combined so that they interfere with each other. DIC can allow the detection of small changes in depth or elevation in the sample, thus giving the perception of a three-dimensional image ( Figure 9.7). Anisotropic light, light that depends on the angle of observation, originates from specimens that have asymmetry in their crystal lattice properties. Anisotropy is observable in liquid and solid crystals; stained glasses; stressed plastic materials; crystallized resins and polymers; refracting surfaces; synthetic filaments; and biological fibers, cells and tissues. Polarized light is light in one plane that can be used to examine anisotropy in sample materials.
Polarization microscopy is traditionally used to determine the optical properties of soil minerals to aid in their identification ( Figure 9.8). The optical anisotropy of individual crystals reflects the bonding patterns of units, e.g., molecules or elements, and usually involves differences in at least two crystallographic directions (at least two directions of polarized light). Multiple anisotropic crystals have optical characteristics above and beyond those of individual crystals.
Anisotropy observed in a sample can provide more information about the sample than ordinary unpolarized light. For example, a result of light polarization is molecular birefringence. Molecular birefringence is manifested by long or flat molecules, especially polymeric macromolecules, and is particularly applicable in the examination of microbially produced extracellular polymers ( Figure 9.9). In molecular birefringence, when polarized light encounters a series of atomic dipoles arranged in chains, as in long molecules, the strength of the dipoles causes the light to vibrate lengthwise along the chain, resulting in greater polar anisotropy at the poles.
However, side chains on the molecules tend to reduce the strength of birefringence in the main chain of the molecule, resulting in less polar anisotropy. The patterns and strengths of anisotropy evident in a sample can give indications of the purity and elemental structure of the sample. The resolution of a specimen’s effects on polarized light depends on producing plane polarized light with a polarizer and examining the effects with an analyzer. Two polarizers are used in polarization microscopy: a polarizer and an analyzer ( Figure 9.10). For transmitted light, the polarizer is placed between the light source and the substage condenser lens.
The analyzer is placed between the objective and ocular lenses. When polarized light from the first polarizer vibrates in a direction that allows it to pass through the analyzer, the field of view in the microscope will be black as the polarizers are crossed with respect to their directions of vibration of light. Thus, in a polarizing microscope, contrast is the result of various interference phenomena throughout the sample. Light interference or retardation at each point in a crystal results in contrast and color on a dark background. In accordance with the Michel-Levy interference spectra based on light retardation through varying sample thicknesses, light interference gives first order gray, high order white and color to the sample image.
Mohan Srinivasarao, in, 1989An optical microscope is perhaps one of the most used imaging instruments in a variety of disciplines to ‘see’ things at high magnification. One of the limitations of traditional optical microscopy techniques (far-field techniques) is its diffraction-limited resolution.
1 Despite the limited resolution, its widespread use as an imaging instrument is due to its simplicity and a unique variety of available contrast mechanisms. Examples of such contrast mechanisms are absorption, polarization, phase contrast, dark-field, and fluorescence, to name just a few.
Fluorescence microscopy in particular has revolutionized cell biology, allowing one to obtain images of cells and organelles in exquisite detail. Such advances have been made by combining lasers, electronic cameras, and digital image analysis. In view of the advantages provided by various contrast mechanisms and the ease of use, efforts to push the resolution (both spatial and axial) of the optical microscope well beyond that of the diffraction limit have produced a number of very useful techniques, such as confocal microscopy, 2 standing-wave fluorescence microscopy (SWFM) 3 (where one is able to visualize structure in 3D by optical sectioning of the object being viewed), solid immersion microscopy (SIM), 4 photon tunneling microscopy (PTM), 5, 6 and, at a higher spatial resolution, near-field optical microscopy (NSOM). 7–10 All of these techniques, by virtue of using light at visible wavelengths, provide information that is not accessible by other high-resolution techniques such as electron microscopy and the recent family of probe microscopes (scanning tunneling microscopy and force microscopy). 11–14 We will not dwell on such forms of microscopy and will refer the reader to excellent reviews and books that have been written. By means of an optical microscope were studied the principally carbonate interbeds of the ore horizon, practically not containing manganese oxides and overlapping carbonates (the so-called thick plates). In the studied rocks, organogenic residues were found throughout; however, their highest content coincides with the upper oxidized carbonate horizon of the ore strata.
Among these are present residues of foraminifera of the Nodosaridae and Hemigordiopsidae families (as determined by E.Ia. Leven, GIN RAS), as well as undetermined residues of large foraminifera-fusulinids, gastropods, and ostracods ( Figs. 3.106 and 3.107). Also present are oncolites with characteristic microorganogenic (algal) internal structure ( Fig. The results of microscope study under optical and scanning microscopes enable us to conclude that the rocks are represented by material of siltstone-pelitic and in part thin- to fine-sandstone dimensions, which to a significant degree have been subjected to recrystallization.
In the composition of the rocks, besides the aforementioned rare clasts of undetermined residues of small macrofauna (evidently gastropods and bivalves), microfauna (primarily foraminifera), and other organic residues (see Figs. 3.106 and 3.107), are also present numerous microbial structures ( Figs. 3.110 and 3.111A–E, G–K) representing mineralized residues of bacterial mats. Framboids of pyrite have been noted rarely ( Fig. Mineralized microbial remnants (A)–(E) and (G), (H) and segregations (framboids) of pyrite (F) in carbonate rocks, Ulutelyak deposit (SEM microphotographs taken by E.A. Zhegallo and E.Ya.
Oxides of manganese within the boundaries of the ore body represent the result of oxidized manganese-containing carbonates (manganese calcite and dolomite, possibly manganocalcite) under hypergenic conditions. Their distribution in the rock is uneven, due to the primary heterogeneity of the sediment as much by the organogenic (microbial, algal, and the like) nature of initial matrix (see Figs. 3.106, 3.108, and 3.112A–F), as its “microball” texture ( Fig. In many cases, alabandite is characterized by an analogous distribution in the rock (see Fig.
3.106F) (Gribov, 1972a). Manganiferous organogenic remnants and carbonate matrix of the Ulutelyak deposit. (A) ostracod test, sample 15/04 (crossed nicols); (B) ostracod test, sample 15/04 (parallel nicols); (C) algal texture, sample 8/04 (parallel nicols); (D) algal texture, sample 8/04 (parallel nicols); (E) algal texture, sample 12/04 (crossed nicols); (F) algal (?) texture, sample 15/04 (crossed nicols); (G) manganized cyanobacterial (?) matrix, sample 3/04 (crossed nicols); and (H) manganized cyanobacterial (?) matrix, sample 5/04 (crossed nicols). Figure 3.2(a) shows optical microscope images of the unstrained Si strips transferred onto the plastic host. A very high transfer rate is achieved with this flip-transfer technique.
The inset in Figure 3.2(a) shows a zoom-in image of the transferred strips. The gaps between strips stay very uniform and unchanged across the 1 mm length of the strips. This result demonstrates that this flip-transfer technique should be very attractive for device fabrication with regard to alignment considerations. The transfer technique provides some advantages over the printing technique demonstrated by Menard et al. First, no stamping or holding anchors are necessary, greatly reducing the complexity and possibly enhancing the yield of the transfer process. Second, in this transfer process, the bottom side of the active layer is exposed after transfer, providing an opportunity for manufacturing double-sided devices that other transfer techniques are unable to provide. However, it is further noted that in order to apply this anchor-free method, SOI source materials with a very thin BOX layer are needed.
Further, the current flip transfer cannot spread active materials to a large area, which can be easily done with a stamp-based transfer method. Optical microscope images. (a) As-transferred unstrained Si strips. The inset shows a magnified image. (b) Finished devices on unstrained Si. The 50 × 75 μm rectangles are 70-nm-thick Ti metal pads that are patterned on the Si strips, which run horizontally.
The distances between the edges of the metal pads are 10 μm, 5 μm and 3 μm, which correspond to different channel lengths. Measurements are made between neighboring metal pads. Because the metal pads cover three strips, the gate width is 60 μm. (c) Finished devices on strained Si; same conditions as in (b).In this section, the focus is on the strain effects on the devices’ static electrical properties, by making a comparison between the unstrained and strained Si TFTs.
Figure 3.2(b, c) shows images of finished devices on unstrained Si and strained Si, respectively. The distances between the edges of the rectangular metal pads shown in the figures are 3 μm, 5 μm and 10 μm, which correspond to different channel lengths. As shown in Figure 3.1, all these devices are made in the bottom gate form, although the fabrication of top-gated devices is also straightforward. The XRD test is performed on the Si/SiGe/Si sandwich strips before and after they are transferred to the flexible polymer host and when the TFT fabrication is complete. Strain sharing has already occurred before transfer, as explained previously. With these data, the crystalline quality can be verified after transfer, and the strain status of the Si/SiGe/Si membrane determined and compared at different steps.
Figure 3.3(a) shows the Si handle wafer diffraction peak from the SOI and the thin SiGe alloy along with thin Si layer peaks of the sandwich structure. Note that not all the area of the sample was patterned into strips, so the BOX is still present on some of the sample, providing an opportunity to compare diffraction peak shifts on the same sample. In the transfer step, only the fully undercut strips are transferred to the polymer host. Diffraction shows two (narrow) SiGe alloy peaks and two (broad) Si peaks (averaged lines were drawn for these two broad peaks to assist the observation), as expected. The pair at the lower angle corresponds to less compressively strained SiGe, due to strain sharing ( θ = 34.269°) on the free parts of the membrane, and to fully compressively strained SiGe alloy ( θ = 34.159°) (from the attached parts of the membrane). The peaks at the higher angle correspond to the very thin (hence broad) tensile strained Si films, again due to strain sharing in the free-standing parts of the membranes ( θ = 34.697°) and unstrained Si ( θ = 34.585°) (this peak coincides with the Si handle wafer peak). The angular shift in the Si peak is caused by the tensile strain induced by strain sharing, as known from transfers of membranes onto Si substrates 10.
The widths of the peaks reflect the thinness of the layers. The rigid 0.11° angular shift (indicated by arrows in Figure 3.3a) between the members of each pair reveals that all the strain relieved in the SiGe alloy layer is transferred to the Si layers without any inelastic relaxation. Figure 3.3(b) shows the XRD results on these released strained Si strips transferred onto the polymer host after complete TFT fabrication. Diffraction from unstrained Si strips transferred to a different piece of the same polymer host is also shown. As expected, only the Si thin-film peak is observed in the unstrained Si membrane, while the strained sample has a SiGe alloy peak at the lower Bragg angle and a Si thin-film peak at the higher angle. Thickness fringes on both samples can be clearly observed: they indicate that supreme crystalline quality is maintained throughout the transfer and the device fabrication. The existence of thickness fringes and the sharpness of diffraction peaks also verify that no or negligible numbers of dislocations are generated in the strain-sharing approach.
X-ray diffraction line scan (θ/2θ) results. (a) An average over released and unreleased portions of an Si/SiGe/Si membrane structure on the starting silicon-on-insulator (SOI) substrate. The arrows indicate a 0.11° angular shift of both the SiGe alloy peak and the Si thin-film peak for the released portions of the membrane relative to the unreleased portions, because of strain sharing in the released portion. The Si peak for the unreleased portion is at the same angle as the bulk-Si peak, as expected. The membrane peaks are broad because the layers are thin (averaged lines are drawn to assist the observation).
(b) Diffraction from a released, strain-relaxed Si/SiGe/Si membrane and an unstrained Si layer transferred onto polymer hosts, and after thin-film transistor (TFT) fabrication. A small systematic shift of the peaks relative to the relaxed, non-transferred membrane (a) is observed and is likely due to the flexibility of the polymer.There is no reference (such as the Si substrate Bragg peak) for the membranes transferred to the flexible polymer host. Bragg angles for membranes on the polymer may have slight, possibly random, shifts caused by the flexibility of the PET. The lines in Figure 3.3(a) extended to Figure 3.3(b) mark the positions of the Si and SiGe peaks of the strain-sharing (released) membrane. The corresponding peaks for the membranes on the polymer host are slightly, but rigidly, shifted, as indicated by the lines. The fact that the separation of the Si and SiGe peaks is maintained when the membrane is transferred to the PET confirms that the strain-sharing status remains identical throughout the entire dry transfer and TFT fabrication process.
The amount of tensile strain in Si layers on any plastic host can now be engineered in the same manner as in the strain-sharing technique: one can tune the SiGe/Si thickness ratio and the Ge fraction in the alloy layer. The strain in Si thin films due to strain sharing is calculated from the +0.11° angular shift, which gives the out-of-plane lattice constant of the strained Si layers. The in-plane strain is calculated by using ε ∥ = ε ⊥ ( 1 − v ) / 2 v, where v is the Poisson ratio for Si and is chosen as 0.277. The calculated strain in the Si layers is thus 0.362%, which agrees very well with the force-balance model in Mooney et al. Unstrained and strained Si TFTs on PET are electrically characterized in the same way: by using the coated ITO layer as the gate electrode, the SU-8 layer as the gate dielectric, and the metal pads on the surface as source and drain electrodes.
The measurements of TFT device characteristics are performed at room temperature with an HP4155B semiconductor parameter analyzer. The gate length ( L G) is determined by the gap between the source and drain metal pads and is varied from 3 μm to 50 μm. The gate width ( W G) is chosen as 60 μm. To compare the intrinsic properties between the unstrained and the strained Si TFTs, fresh devices of both types were electrically characterized.
Figure 3.4(a, b) shows representative output current–voltage ( I– V) curves of a 3 μm gate length TFT made on unstrained and strained Si active layers, respectively. N-type field-effect transistor (FET) characteristics are exhibited by both types of membrane. Since the active layers in both devices are undoped, the metal (Ti) contacts made for source and drain are Schottky contacts. For the strained Si active layer, a reduced Schottky barrier height may be present owing to the lowered conduction band energy (strain splits the conduction band and creates a lower energy Δ2 valley) in comparison to the unstrained Si. The lack of ‘full’ saturation in the strained Si TFT shown in Figure 3.4(b) is speculated to be due to the lower Schottky barrier height (equivalent to smaller parasitic source and drain resistance) of the strained Si TFTs. Figure 3.4(c, d) shows the comparison of drain current and transconductance between the unstrained and the strained Si TFTs with different gate lengths ( L G = 3, 10, 20 and 50 μm) at a low source–drain bias ( V DS) of 50 mV. The peak transconductance values for the unstrained and the strained Si TFTs were reached at around 12 V and 14 V, respectively ( Figure 3.4d).
Optical Microscope Principle Pdf Free
Note that even though the strained Si TFTs have lower threshold voltage (2.5 V) than the unstrained counterparts (5 V), both the drain current and the transconductance of the TFTs made on strained Si are much higher than those made on unstrained Si. The I ON/ I OFF ratio of the strained Si TFTs is more than 10 3 at V DS = 1.5 V. The field-effective mobility ( μ FE) in the linear region is extracted using the formula μ FE = L G g m/( W G C G V DS) 11, where L G and W G are the physical dimensions of the gate length and width, V DS = 50 mV, and g m is picked as the highest transconductance measured from the devices. The dielectric capacitance ( C G) is evaluated by measuring dummy patterns in the form of parallel capacitors and is 2.16 and 2.53 nF/cm 2 for unstrained Si and strained Si samples, respectively. The slight difference in the capacitance is attributed to the different final SU-8 thicknesses between these two samples and agrees well with the thickness measurement (1.8 μm and 1.4 μm for the unstrained and the strained samples). The extracted carrier mobility in TFTs made on both the unstrained and the strained Si with various gate lengths is shown in Figure 3.5.
The extracted decreasing mobility with shorter gate length is the evidence of an unoptimized source/drain parasitic resistance caused by the high Schottky barrier of metal contacts. The parasitic resistance has more influence on the intrinsic channel resistance when the gate length is reduced, which results in lower source-to-drain current ( I D) and thus lower extracted mobility 12. Extracted field-effect mobility ( μ FE) of unstrained and strained Si thin-film transistors (TFTs) on flexible polymer hosts.By comparing the strained and unstrained Si TFTs, it can be observed that the extracted mobility of strained Si TFTs shows remarkable enhancement relative to those made with unstrained Si.
The mobility enhancement is the result of two combined effects both caused by strain. The biaxial tensile strain splits the Si conduction band into two valleys that not only increase the electron mobility but also decrease the Schottky barrier height of the metal contacts made on the strained Si.
With the reduced Schottky barrier height, both thermionic emission and field emission are greatly enhanced, which results in higher source-to-drain current 13. Micro Raman is the adaptation of an optical microscope as a Raman sampling system. Delhaye and co-workers at the University of Lille are credited with the development of this technique. 237,238 A general review of the micro Raman technique can be found in ref. The system typically uses an optical microscope both to focus the laser excitation on the sample and to collect the scattered Raman signal.
The signal collected by the microscope is then optically matched to the double monochromator, and conventional scanning and photon counting are used to collect the signal. By switching mirrors it is also possible to image the sample on the microscope stage on a viewing screen or a video camera to help in positioning the beam focus. Combining the microscope with array detection is a natural extension and promises to minimize beam exposure times on the sample with a consequent reduction in sample damage. 240 There are many advantages to this approach, but it also introduces some additional complexity and ambiguity.
The advantages include resolution and sampling areas of the order of a square micron, low laser powers needed owing to the concentration of light by the microscope, imaging of the sample and area discrimination, smaller samples needed, and minimization of fluorescence from some sources. 241 The limitations include the short working distances defined by the microscope objective which limit sampling techniques, loss of polarization information owing to the large solid angle used to collect the Raman signal and high local heating effects. Applications to polymer spectroscopy are most evident for small samples such as single crystal lamella studies, polymer interfaces, phase separations, oriented material, composite systems and thin films. 105,154,241,242 Figure 20 shows oriented Raman spectra from a poly(ethylene terephthalate) fiber 20 μm in diameter. 154 Similarly, the spatial resolution has been used to study the transport of solvents in polymers. 243 Another aspect that is advantageous is the ability to discriminate spatially against fluorescence. Especially in crystalline polymers, where impurities are probably localized to surface and amorphous interfaces but still very difficult to remove chemically, using the microprobe it is possible to obtain Raman spectra that are of comparable quality to those obtained using the FT-Raman technique, which eliminates fluorescence interference.
The basic structure of TEM is similar to that of optical microscopes, replacing a light bulb with an electron gun, and glass lenses with magnetic lenses. Other distinctive features of TEMs compared to optical microscopes is the image-forming lens system, which consists basically of three lenses (objective, intermediate, and projection lenses) to cover the magnification from several tens to a few million times ( Fig. The magnification is selected by changing the focal length of the intermediate lens. The focal length of the objective lens is almost constant for a wide range of magnifications and adjusted only for focusing. As we cannot see electrons with bare eyes, the images are observed using a phosphor screen under the projection lens. A vacuum system is also necessary for the electrons to travel through the central axis of the microscope column. (2.9.4) δ = 1.2 C s λ 3 4Thus, a shorter wavelength (or a higher V acc) is still preferable for higher resolution.
If we use the wavelength of electrons at V acc = 200 kV ( λ = 0.00251 nm) and a C s value of 1 mm, which is typical of modern TEM, the resolution ( δ) reaches 0.42 nm. Though this value is about 150 times larger than the wavelength used, it is far smaller than the resolution of optical microscopes and a few tenths of that of the most advanced FE-SEM. For biological specimens, a lower V acc is useful because lower-energy electrons are scattered by organic matter more than higher-energy electrons, which results in the formation of images with higher contrast. (Note that an image with no contrast is meaningless even if the resolution is high!) However, for inorganic materials including clay minerals, such lower voltage is not advantageous. Generally, the maximum V acc is used for obtaining the highest resolution, as well as for maximum penetrating power.Besides the extremely high image resolution, the advantage of TEM analyses is the combination of imaging and diffractometry. If the specimen is irradiated with a parallel beam, diffracted electrons are converged by the objective lens to form a diffraction spot, or focused diffraction pattern, at the back-focal plane of the objective lens. If the focal length of the next imaging lens (intermediate lens) is adjusted to magnify this back-focal plane instead of the imaging plane of the objective lens, a magnified diffraction pattern appears on the phosphor screen ( Fig.
The area to form the diffraction pattern is confined by an aperture (selected-area aperture) located at the imaging plane of the objective lens (selected-area diffraction, SAD). Hence, the area of interest can be selected by observing a magnified image of the specimen and masking the outside of the area of interest in the image by the ‘shadow’ of the aperture.
However, there is a limitation for such confinement by the aperture because of the spherical aberration of the objective lens, and the minimum size of the confined area is roughly close to 100 nm in diameter. On the other hand, modern TEM have a function to make a fine probe just like SEM, using condenser lenses over the specimen. If the convergence angle of this probe is sufficiently small, a similar diffraction pattern to SAD can be obtained from the probing region with nanometre size.
This method is often called ‘nano-beam diffraction’. If the convergence angle is considerably large, each reflection in the diffraction pattern changes from a spot to a disc. Such diffraction patterns are called ‘convergent beam diffraction’ (CBD). The contrasts inside the discs are useful to obtain information on the specimens. For instance, we can determine the thickness of the specimen, symmetry (space group) of crystals, etc.
However, few applications of CBD to clay minerals have been reported. One main reason is that the convergent probe easily damages the beam-sensitive clay minerals.As magnified images are used to select the area for diffraction, diffraction is also used for imaging crystalline materials. To understand this, the origin of the contrast in TEM images should be explained. At the exit surface of the specimen, two types of electrons exist, namely, transmitted electrons, which do not change the direction, and scattered electrons. If the specimen is crystalline, the scattered electrons are confined to specific directions to form a diffraction pattern. TEM has an aperture (the objective aperture) at the back-focal plane of the objective lens where a focused diffraction pattern is always formed, as mentioned above ( Fig. By inserting the aperture during the observation of the diffraction pattern on the phosphor screen, diffraction spots can be observed, from which the images are formed.
If we select only the transmitted electrons at the centre spot, the resultant image is called a ‘bright-field (BF) image’. If we select certain diffraction spots (in case of non-crystalline material, a part of a halo ring formed by scattered electrons is selected), the image is called a ‘dark-field (DF) image’. The intensity of the transmitted electrons varies between particles or between positions inside a particle, due to the difference of compositions or simply due to the difference of the thickness. Note that such a contrast is formed also for non-crystalline specimens.
Moreover, the intensity of the transmitted electrons also changes if the specimen is crystalline, and intense diffraction occurs satisfying the Bragg condition or not, depending on the crystal orientation. Such contrast is called ‘diffraction contrast’. Diffraction contrast is easily identified by tilting the specimen during observation. By this contrast, we can identify, for instance, crystal domains or boundaries inside the specimen, crystal bending, and crystal defects such as dislocations (actually we see the strain area around the defects where the diffraction condition is changed). For the details of the diffraction contrast and their analysis, see the textbooks dedicated to TEM (e.g. Thomas and Goringe, 1979). Of course, the diffraction condition for the contrast can be optimized by observing the SAD pattern.
Figure 2.9.8. (A) Bright-field TEM image of lath-shaped, ( trans-vacant) 1M-illite, dispersed on a holey carbon film (microgrid).
(B) SAD pattern from the illite particle in (A). The pattern has a pseudo-hexagonal arrangement of hk0 reflections but its intensity distribution does not have a hexagonal symmetry. (C) Kinematical intensities for hk0 reflections calculated from the crystallographic parameters of 1M-illite. The intensities are normalized with the strongest reflection, whose intensity is set to 1000. Figure 2.9.9.
Bright-field image of cronstedtite, an Fe-bearing trioctahedral 1:1 clay mineral, observed slightly away from ± y i directions. The white bands near the centre and thin white lines correspond to the layers with different stacking sequences. Lattice fringes of 0.7 nm indicating the basal spacing also appear in the image because the ± 001 reflections were not excluded by the objective aperture.
(B) SAD pattern from the area in (A). The diffraction spots indicated by the arrows are from the white bands in (A) ( Kogure et al., 2001).