Electron microscopy has brought unprecedented capabilities to material scientists and engineers by allowing them to directly observe material structure at the nanoscale. An electron microscope illuminates the sample with electrons, accelerated to 300 – 30,000 eV (SEM) and 20,000-600,000 eV (TEM). This allows imaging with an ultra-high resolution of up to ~ 1.5 nm and ~ 0.1 nm for an SEM and TEM, respectively. In an SEM the incoming electron beam is condensed into a small beam, which is scanned over the object. An image is formed by the electrons that bounce off the surface of the specimen and are then collected by an appropriate detector. The observer mostly sees a picture of the surface of the sample, with little internal information. In contrast, a TEM produces an image that can often be approximated as a projection of the entire object (more precisely, the electron density of the object), including the surface and the internal structures. The incoming electron beam interacts with the sample as it passes through its thickness. TEM microscopes are equipped with advanced spectroscopic techniques including EELS and EDS that allow identification of the chemical composition of samples at the macro- and nanoscales, including segregation of atoms on surfaces, grain boundaries and point defects.
Fig. Examples of SEM and TEM micrographs: (a) SiC ribbons, (b) graphite and (c) nanotubes coating formed upon vacuum decomposition of SiC, (d, e) graphite and diamond nanoparticles; (f) bacteria spore
Adsorption has not only become a key separation process used in a variety of industrial applications but also a well accepted method to characterize specific surface area, volume and size of nano-pores in porous solids with interconnected porosity. This is commonly done by collecting and analyzing an adsorption isotherm, the amount of adsorbate adsorbed on (or desorbed from) a sorbent at constant temperature within a range of relative pressures. Some analytical methods including BJH (Barrett –Joyner-Halenda) or DH (Dollimore Heal) work best for solids with relatively large pores (> 5 nm), others including DA (Dubinin-Astakhov), HK (Horvath-Kawazoe), or SF (Saito-Foley) are most precise for strictly microporous (< 2 nm) solids, while relatively novel methods based on NLDFT (Non Local Density Functional Theory) and Monte Carlo simulations proved to be appropriate for solids having a wide distribution of pore sizes.
Fig. (a) Commercial porosity analyzer and typical pore size distributions of (b) microporous and (c) mesoporous carbon samples obtained using NLDFT analysis of Ar sorption isotherms collected at 77 K
Since vibrational information is very specific for the chemical bonds, Raman and Infrared spectroscopy allow quick and non-destructive analysis of gases, liquids and solids. Raman and Infrared spectroscopy signals may provide fingerprints by which molecules or crystal solids can be identified. When studying solids, Raman is more commonly used for phase or polytype detection, measurements of temperature, finding crystallographic orientation of crystalline samples, nanoparticle size determination (or diameter of carbon nanotubes), and analysis of defects and/or residual strain; while Infrared spectroscopy is more commonly used for the detection of surface functional groups. Vibrational spectroscopy reflects energy exchanges between the incident photons and the molecule. Raman effect takes place when light impinges upon sample and interacts with its electron cloud, exciting one of the electrons into a virtual state. The relaxation from this virtual state into a vibrational excited state generates Stokes Raman scattering. If the electron was initially in an excited vibrational energy state, then the relaxation from a virtual state into a ground state generates anti-Stokes Raman scattering. Surface-enhanced Raman scattering (SERS) allows local amplification of the Raman signal (up to ten billion times) through the interaction of plasmons of silver or gold nanoparticles and nearby molecules. In Infrared spectroscopy a beam of infrared light is passed through the sample and gets adsorbed at wavelengths close to IR active vibrational modes.
Fig. (a) Commercial Raman micro-spectrometer and typical Raman spectra of (b) diamond nanoparticles and (c) double walled carbon nanotubes
XPS and Auger spectroscopy are quantitative non-destructive techniques capable to measure elemental composition and electronic or chemical state of each element in the surface (1-10 nm). In other words, these techniques reveal what elements (and what amounts) are present within the surface layer and how these elements are bonded to each other. The techniques are routinely used to analyze semiconductors, metals, polymers, bio-materials and a variety of (nano-) composite materials in bulk or powder form. The XPS spectrum is recorded by illuminating a sample with an X-ray beam (commonly Al Ka), measuring the energy of the emitted electrons and calculating the binding energy of the sample elements according to Ebind = Eh? - Ee - F, where Eh? is the X-Ray energy, Ee is the kinetic energy of the emitted electrons, and F is the work function of the material. The spectrum is plotted as a number of detected electrons vs the binding energy. In Auger spectroscopy, a sample is illuminated by an electron beam of 2-50 keV and the energies of the electrons emitted from the sample are measured.
Fig. (a) Commercial X-Ray photoelectron spectrometer and (b) XPS spectra of a SiC surface before and after vacuum annealing at 1100 ºC, confirming removal of a native oxide.
Electrochemistry studies the reactions and processes that take place at the interface of an electron conductor (electrode) and an ionic conductor (electrolyte). The most demanding applications include analyses of novel materials for batteries, fuel cells and supercapacitors. In cyclic voltammetry (c-v), a voltage is varied at a constant rate across a sample cell up to a pre-defined limiting value, at which point the direction of the potential scan is reversed, and the same potential window is scanned in the opposite direction. The current response measured over this range of voltages reveals the reduction-oxidation properties of the cell electrode(s) and the electrode(s) capacitance. In addition, a c-v scan can provide information about the rate of charge transfer to (or within) the electrodes and the electrochemical stability of the electrode(s) or electrolyte. EIS is measured by applying a small AC sinusoidal potential at various frequencies to an electrochemical cell and measuring the current through the cell. The response of the cell to the excitation signal reflects the changes in both the amplitude and phase and thus the impedance of the cell can be expressed as a complex value consisting of a real and an imaginary part. The collected data are commonly presented as a Nyquist plot: the real component of the impedance vs. the imaginary component for the given range of frequencies. By fitting the obtained data into an appropriate model, EIS allows one to reveal the resistance of of the electrolyte, electrodes, contact resistance between electrodes and current collectors, and monitor various processes that contribute to the AC response of an electrochemical system.
Fig. (a) Commercial multi-channel electrochemical testing system, (b) glassware designed for three electrode measurements, (c) typical c-v curve and (d) Nyquist plot for a double layer capacitor cell.
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