Scanning Electron Microscopy SEM

SEM is a useful technique for the analysis of plastics surfaces. Acmally, it is useful for any surface that survives in a vacuum. Almost all SEMs start by sputtering the surface with a thin layer of gold metal. If it is not already conductive, this makes the surface conductive, which is a requirement so, you are often not looking directly at the surface. It involves a finely collimated beam of electrons that sweeps across the surface of the analysis specimen. The beam is focused into a small probe that scans across the surface of a specimen. The beam interactions with the material result in the emission of electrons and photons as the electrons penetrate the surface. The emitted particles are collected with the appropriate detector to yield information about the surface. The final product of the electron beam collision with the surface topology of the sample is an image (Fig. 4.4). 

For example. Fig. 4.4 shows the result of topical SEM analysis of a drug-infused polymer coating on 

A helpful attachment to the SEM is the electron microprobe. An electron beam is focused on a sample surface, causing ionization to a depth of a few micrometers. Energies and wavelength of the emitted X-ray during the de-excitation cycle are characteristic of the elements present in the top layers of the sample. The result is not a true surface analysis, but the electron microprohe allows analysis of various spots of the sample surface.  

Samples suitable for SEM measurements include most solids which are stable under vacuum (metals, ceramics, polymers, minerals). Samples must be less than 2 cm in diameter. Non-conducting samples are usually coated with a thin layer of carbon or gold in order to prevent electrostatic charging. 

Elastically scattered electrons are collected by an annular detector and provide the elastic dark-field signal. A spectrometer deflects those electrons that have lost 

The SEM uses a beam of electrons to scan the surface of a sample and build a three-dimensional image of the specimen. Electrons are generated in the electron gun. The most commonly used is a tungsten-hairpin gun in which a tungsten 

An SEM system with the various detectors can provide high-resolution imaging, quantitative elemental analysis of the bulk material, and fast elemental mapping, as well as other benefits. 

Scanning electron microscopy is an important tool when examining the mode of wear of any sample. The surface of the sample is coated with a very thin layer (only several atoms thick) of a conductive material such as gold. The surface is scanned using a beam of electrons and the image magnified and recorded. 

Scanning electron microscopy is a major tool for checking the morphology of the substances. For natural polymers it is used to validate and confirm the changes in the structure that occurs after derivatization. Table 10.5 lists various changes in the morphology of polymers before and after derivization. 

Thakur et al. [68] studied the effect of carboxymethylation on the structure of amylopectin. The scanning electron micrographs of amylopectin and carboxymethyl amylopectin showed change in the structure of amylopectin after derivatization. Amylopectin particles are polyhedral in shape with granular surface, while carboxymethyl amylopectin particles are flaky with striated surface. 

Gum Particle shape Type of derivatization Particle shape Reference 

Amylopectin Polyhedral in shape with granular surface Carboxy- methylation flaky with striated surface [68] 

Gum kondagogu Polyhedral, with smooth surface Carboxy- methylation Polyhedral with rough surface [6] 

Type 1 - Transmission A fixed beam of light or electrons is transmitted through the thin specimen in the transmission mode of the optical microscope and in transmission electron 

Type 2 - Reflection A stationary beam is reflected off the specimen surface in the reflection mode of the optical microscopes or - for inorganic material only - in electron mirror microscopes here, bulk samples can be used. 

Type 3 - Scanning beam A focused beam (laser light or electron beam) is seanned aeross the speeimen, resulting in a refleeted beam from the surfaee (as in confocal laser scanning microscopy) or in secondary or backscattered electrons (BSE in scanning electron microscopes). 

Type 4 - A focused scanning beam is passed through the thin specimen (scanning transmission electron microscopes, STEMS). 

Type 5 - Scanning tip A mechanical tip is scanned across the specimen in order to make use of different physical properties in ATMs (or tunneling microscopes for conductive samples). 

In Scanning Electron Microscopy (SEM), a probe electron beam (typically a few hundred eV to a few lO s keV energy) is finely focused (down to 10 A capability in some instruments) and scanned over a solid surface. The interaction of the beam with the sample material generates a variety of responses, including fluorescence emission, which can be used for elemental analysis (see the EDS summary). 

The SEM is often the first or second (after an optical microscope) technique used to provide a magnified image of an area of the sample to be examined. It is often used in conjunction with an ancillary analytical technique, such as EDS, to provide elemental analysis capability to go with the imaging. 

Price et al. (1978) developed this technique for measurement ofthe interdiffusion of poly(vinyl chloride)-poly(8-caprolactone) at 70 °C. The major disadvantage of this technique is the radiation damage of the hydrophilic polymer during the SEM scan and the necessity of dehydrating the delivery device (am Ende, 1993). 

One of the useful methods to study gel structures is scanning electron microscopy (SEM). For ordinary observation in SEM, samples are dried. 

The structure of biomaterials deforms when dried. This is due to the surface tension of water during drying. In order to avoid this phenomenon, evaporation can be done at a critical point where there is no surface tension observed. Unfortunately, it is not practical because the critical point of water is 374°C at 22.06 MPa. In contrast, the critical point of carbon dioxide is 31.4°C at 7.375 MPa. 

The method involving the use of carbon dioxide was developed by Anderson and the procedure is as follows. First, the sample is immersed in ethanol and dehydrated. After the ethanol is replaced by isoamyl acetate, as this dissolves in both ethanol and liquid carbon dioxide, the sample is placed in a pressure vessel into which liquid carbon dioxide is introduced. Upon increasing the temperature to approximately 40° C while the container is closed, the pressure increases to about 12-13 MPa and exceeds the critical point. While maintaining the temperature and leaking carbon dioxide slowly, a dry sample is obtained [62]. By this method, the understanding of the microphysiology of microbes has advanced dramatically [63]. Currently available instruments include the HCPD-2 by Hitachi, the JCPD-5 by JASCO, the CPDO-30 from Balzers, and the 4770 by Parr. 

When critical point drying is performed to obtain dry polymer gels, the first treatment of ethanol itself changes the structure of gels. An appropriate combination of solvent that is suitable for critical point drying has not been established and further study is needed. 

Ordinary SEM is used at a pressure less than 10 Pa. However, samples with high water content, such as biomaterials and polymer gels, largely deviate from the original structure during the drying process under such vacuum. To solve this problem, the SEM that can be used at a low vacuum level has been developed [64]. In this SEM, a pressure differential is maintained by graded evacuation between the sample chamber and the 

6 Physical Characterization of Supercapacitor Materials 7.6.1 Scanning Electron Microscopy (SEM) 

Objective Lens Aperature Objective Lens Coil Scanning Coils and Stigmators 

Secondary Electron —h Detector Airlock Sample Stage 

For example, an SEM micrograph (Fig. 3.28) of poly(neutral red) film deposited on Pt foil shows that a microstmctured network of mass-interwoven fibers with diameters of 2-4 tim are formed The longest fiber is more than 0.4 mm [421], 

A transmission scanning electron microscope (TEM) is used to study thin layers (L 200 mn) see Fig. 4.5. 

XRD techniqnes are used to obtain information on the crystal stmcture [1,2,11,333, 421,422,424,425], The in sitn stndy of an electrode is also possible, i.e., following the changes as a fimction of potential. The X-ray absorption near edge stractirre (XANES) and extended X-ray absorption fine structure (EXAFS) techniques are also applied to study noncrystalline materials. 

X-ray ddfraction (XRD) studies provide information on the crystallinity of the polymer. For example, it was foimd by Manisankar et al. [333] that the copolymer of aniline and 4,4 -diaminodiphertyl sitUbne contains nanosized crystalline regiorts, especially in oxidized (doped) form. In Fig. 3.29 the relatively sharp peaks are related to the crystalline region (crystallite size 83 run), while the amorphous regions are represented by the broad low-intensity peaks. 

Nagano and Nishimoto [11] used this technique to study the degradation due to weathering of styrene-butadiene vulcanisates. The degradation of these vulcanisates subjected to outdoor exposure in the Arizona desert for a year was studied. The dynamic viscoelasticity of sample sheets 2 mm thick and sliced films 0.2 mm thick was measured. SEM images of the surface and sheet cross-sections were also taken to observe microscopic changes in the samples. 

SEM has also been used to study the structure of polyurethane foam-hydroxyapatite ceramics used in artificial bone materials [12] and for the surface analysis of rubber bonding [13]. 

In contrast to TEM, with typical sample thicknesses in the range of lOnm-lpm, sample depths for SEM often extend into the 10-50 mm range. As such, this technique is most often used to provide a topographic image of the sample surface. However, the electron beam is not confined to the top of the surface, but also interacts with lower depths of the sample. Consequently, SEM provides information regarding the species present at varying depths of the sample (Eigure 7.28)  

The least energetic emissions will not reach the surface from lower depths of the sample. For instance, Auger electrons that are emitted from deeper regions of the sample lose their energy through collisions with sample atoms before they reach the surface. As a result, AES is a very sensitive technique to probe the chemical composition of only the top 50-100 A (i.e., 15-30 monolayers). In comparison, the maximum escape depth of secondary electrons has been estimated as 5 nm in metals, and 50 nm in insulators. 

When the electrons impinge on the crystalline sample, they interact with individual lattice planes. When these interactions satisfy the Bragg condition, they exhibit backscattering diffraction and (due to the tilted sample) are directed toward a phosphor screen where the fluorescent pattern is detected by a CCD camera. The resulting pattern consists of a large number of intersecting bands, known as Kikuchi lines, which represent the unique crystallographic properties of the crystal 

The nonvacuum conditions within the sample chamber require a different type of detection system relative to conventional SEMs, referred to as a gaseous secondary 

Complete analysis of an adhesive bond necessarily involves establishing the topography of the adherend surface by electron microscopy. SEM not only provides a better depth of focus than optical microscopy (OM), but also higher resolution. Ledbury et comprehensively compared in Table 2 various microscopic techniques, including SEM, for adhesion studies. 

Selected Surface Characterization Methods Useful in Adhesion ) 

Specimen preparation Analytical instrument Regions analyzed Magnification X Comments 

Polish/etch OM Oxide/primer 1500 Weak in this area SEM/STEM preferred 

Polish/etch OM Primer/adhesive 1500 Good within magnification range 

The least energetic emissions will not reach the surface from lower depths of the sample. For instance, Auger electrons that are emitted from deeper regions of 

In addition to displaying the familiar bright-field images from secondary electron emission, BSE in a SEM may be used to determine the crystallography of (poly) 

Sample preparation for SEM analysis is trivial relative to TEM, with the sample simply deposited onto the top of an adhesive fastened to an aluminum stub/holder. Most often, conductive carbon tape is used to sequester the sample for FESEM, 

Specimen X drive arm Metal bellows —[ X drive motor 

The scanning electron microscope (SEM) forms an image by scanning a probe across the specimen, and in the SEM the probe is a focused electron beam. The probe interacts with a thin surface layer of the specimen, a few micrometers thick at most. The detected signal commonly used to form the TV-type image is the number of low energy secondary electrons emitted from the sample surface. Scanning electron microscopy is fully described in several texts [22-26], and its use with polymers has been reviewed by White and Thomas [27]. 

Imaging by scanning allows any radiation from the specimen, or any result of its interaction with the beam, to be used to form the image [31]. The appearance of the image will depend on the interaction involved and the detector and signal 

The samples, fibers and fractured composites surfaces were placed in a holder with the aid of carbon tape and subjected to metallic coating by gold to a thickness of 8 nm under an argon atmosphere using the Bal-Tec MED 020 metal coating equipment. The metallic samples were subjected to microscopic analysis in a LEO 440 SEM operating at 20 kW and using a secondary electron detector. 

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Otlier fonns of microscopy have been used to evaluate nanocrystals. Scanning electron microscopy (SEM), while having lower resolution tlian TEM, is able to image nanoparticles on bulk surfaces, for direct visualization of... 

The very high powers of magnification afforded by the electron microscope, either scanning electron microscopy (sem) or scanning transmission electron microscopy (stem), are used for identification of items such as wood species, in technological studies of ancient metals or ceramics, and especially in the study of deterioration processes taking place in various types of art objects. 

Occasionally, especially in the developmental phase of catalyst research, it is necessary to determine the oxidation state, exact location, and dispersion of various elements in the catalyst. Eor these studies, either transmission electron microscopy (TEM) or scanning electron microscopy (SEM) combined with various high vacuum x-ray, electron, and ion spectroscopies are used routinely. 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

The interface properties can usually be independently measured by a number of spectroscopic and surface analysis techniques such as secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), specular neutron reflection (SNR), forward recoil spectroscopy (FRES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), infrared (IR) and several other methods. Theoretical and computer simulation methods can also be used to evaluate H t). Thus, we assume for each interface that we have the ability to measure H t) at different times and that the function is well defined in terms of microscopic properties. 

When P[(St-NHCOCH3)-g-AAM] was hydrolyzed in the basic solution no PAAM was released. The scanning electron microscopy (SEM) micrograph of the copolymer shows that the hydrolyzed grafted beads are still covered with PAAMs with salient micrographs. The results reveal that AAM graft copolymerization is initiated by the nitrogen radical rather than any other radical. 

Further structural information is available from physical methods of surface analysis such as scanning electron microscopy (SEM), X-ray photoelectron or Auger electron spectroscopy (XPS), or secondary-ion mass spectrometry (SIMS), and transmission or reflectance IR and UV/VIS spectroscopy. The application of both electroanalytical and surface spectroscopic methods has been thoroughly reviewed and appropriate methods are given in most of the references of this chapter. 

Coacervation occurs in tropoelastin solutions and is a precursor event in the assembly of elastin nanofibrils [42]. This phenomenon is thought to be mainly due to the interaction between hydro-phobic domains of tropoelastin. In scanning electron microscopy (SEM) picmres, nanofibril stmc-tures are visible in coacervate solutions of elastin-based peptides [37,43]. Indeed, Wright et al. [44] describe the self-association characteristics of multidomain proteins containing near-identical peptide repeat motifs. They suggest that this form of self-assembly occurs via specific intermolecular association, based on the repetition of identical or near-identical amino acid sequences. This specificity is consistent with the principle that ordered molecular assembhes are usually more stable than disordered ones, and with the idea that native-like interactions may be generally more favorable than nonnative ones in protein aggregates. 

FIGURE 12.11 Scanning electron microscopy (SEM) photomicrographs of the tensile fracture surface of the ethylene-propylene-diene monomer (EPDM) rubber-melamine fiber composites. A, before ageing and B, after ageing at 150°C for 48 h. Test specimen is cut in tbe direction parallel to the milling direction. (From Rajeev, R.S., Bhowmick, A.K., De, S.K., Kao, G.J.P., and Bandyopadhyay, S., Polym. Compos., 23, 574, 2002. With permission.)... 

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