Electron Microscopy General consideration Notes As we have studied before, LEED is a technique used to characterize the surface structure of crystalline materials by bombardment with a collimated beam of low energy electrons (20-200 eV). This instrument can be considered as Electron microscopy technique Notes eV (electron volt) is a unit of energy (like Joule) eV is the kinetic energy of bond-free electrons accelerating through an electric field having an applied voltage 1 volt. 1 eV = 1.602×10−19  J. Electron microscopy simply denotes to the analysis based on electrons characterization which obtained from the surface. These electrons can be: Scattered electrons due to electrons bombardment as in the case of the LEED Electrons emitted from the materials due to excitation using any kind of energy Sometimes, it is difficult to differentiate between the two kinds, however most of the instruments working in the energy 5-2000 eV can be considered as electron microcopic techniques. When the electron moves without any external facilities affecting its energy, we call the length of its path inside the material as mean-free-path.

1. Surface Plasmon Resonance When the electrons pass through a solid material, they lose some energy. We can detect the depth of the start point of the electron by comparative estimating the amount of energy losses. The electrons can lose the energy by many ways: 1. Plasmon scattering 2. Single-particle electron excitations involving valence electrons 3. Ionization of core electrons 1. Surface Plasmon Resonance The name plasmon derives from the physical plasma as a state of matter in which the atoms are ionized. At the lowest densities this means an ionized gas, or classical plasma; but densities are much higher in a metal, or quantum plasma, the atoms of a solid metal being in the form of ions. In both types of physical plasma, the frequency of plasma-wave oscillation is determined by the electronic density. In a quantum plasma the energy of the plasmon is its frequency multiplied by Planck's constant, a basic relationship of quantum mechanics.

The plasmon energy for most metals corresponds to that of an ultraviolet photon. However, for silver, gold, the alkali metals, and a few other materials, the plasmon energy is sufficiently low to correspond to that of a visible or near-ultraviolet photon. This means there is a possibility of exciting plasmons by light. If plasmons are confined upon a surface, optical effects can be easily observed. In this case, the quanta are called surface plasmons, and they have the bulk plasmon energy as an upper energy limit. Surface plasmons were first proposed to explain energy losses by electrons reflected from metal surfaces. Since then, numerous experiments have involved coupling photons to surface plasmons. Potential applications extend to new light sources, solar cells, holography, Raman spectroscopy, and microscopy. Surface plasmon depends 1. Nature of the materials 2. Radiative energy 3. Particles morphology

2. Single-particle electron excitations involving valence electrons Sometimes the energy of the electron beams is consumed to excite the valance electrons to move to higher 3. Ionization of core electrons When the energy became more high it makes ionization of the atoms. However, this case usually does not happen in the surface study. Therefore, the first two reasons are most important. Effect of the energy loses on the mean-free-path As discussed above, there are two main factors affecting the electrons energy; plasmon scattering and single-particle excitation. Theoretically, the relationship was estimated as shown in this figure. From this figure, we can conclude that Plasmon effects does not appear at low energy Low energy is required to investigate the surface After certain threshold; plasmon scattering has lowe effect

Electron attenuation length and surface specify The theoretical study depends on inelastic electron scattering Inelastic electron scattering means only the incident electrons are scattered and no electrons from the solid materials moved even inside the material. This hypothesis is not real in the real experiments. Accordingly, the scientists have used attenuation length term instead of the mean-free-path Attenuation length is the real distance the electrons pass through the material Also, this distance could be measured As shown in this figure, almost the same fashion was obtained which affirm the recommendation of using low energy to investigate the surface upon using electron microscopy

Types of surface techniques according to the incident and detected spices The types can be summarized in this table Electrons in/electrons out Electrons in/photons out Photons in/electrons out Photons in/photons out

X-ray photoelectron spectroscopy (XPS) Definition X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique from photon in/electrons out technique Simply the X-ray is used to emit the electrons from the atoms, every electron needs certain energy to emit from the atom, this energy is called Binding Energy. Therefore, XPS requires ultra high vacuum (UHV) conditions because

What are the XPS data? A typical XPS spectrum is a plot of the number of electrons detected (sometimes per unit time) (Y-axis, ordinate) versus the binding energy of the electrons detected (X-axis, abscissa). Each element produces a characteristic set of XPS peaks at characteristic binding energy values that directly identify each element that exist in or on the surface of the material being analyzed. These characteristic peaks correspond to the electron configuration of the electrons within the atoms, e.g., 1s, 2s, 2p, 3s, etc. The number of detected electrons in each of the characteristic peaks is directly related to the amount of element within the area (volume) irradiated. To generate atomic percentage values, each raw XPS signal must be corrected by dividing its signal intensity (number of electrons detected) by a "relative sensitivity factor" (RSF) and normalized over all of the elements detected. Why UHV system is very important in the XPS instruments? To count the number of electrons at each kinetic energy value, with the minimum of error, XPS must be performed under ultra-high vacuum (UHV) conditions because electron counting detectors in XPS instruments are typically one meter away from the material irradiated with X-rays.

XPS is used to analyze the surface… XPS is used to analyze the surface…. How about the electrons evolved from the bulk of the material It is important to note that XPS detects only those electrons that have actually escaped into the vacuum of the instrument. The photo-emitted electrons that have escaped into the vacuum of the instrument are those that originated from within the top 10 to 12 nm of the material. All of the deeper photo-emitted electrons, which were generated as the X-rays penetrated 1– 5 micrometers of the material, are either recaptured or trapped in various excited states within the material. For most applications, it is, in effect, a non-destructive technique that measures the surface chemistry of any material. Components of an XPS system 1. A source of X-rays 2. An ultra-high vacuum (UHV) stainless steel chamber with UHV pumps 3. An electron collection lens 4. An electron energy analyzer 5. Mu-metal magnetic field shielding 6. An electron detector system 7. A moderate vacuum sample introduction chamber 8. Sample mounts 9. A sample stage 10. A set of stage manipulators

Photon Sources Binding Energy Binding energy in the X axis can be calculated from the used photon energy which is known and the energy of the emitted electrons where Ebinding is the binding energy of the electron, Ephoton is the energy of the X-ray photons being used, Ekinetic is the kinetic energy of the electron as measured by the instrument and φ is the work function of the spectrometer (not the material). Therefore, Binding energy is the required energy to pull out the electron from the atom Photon Sources To obtain the X-ray which is used to analyze the sample in the XPS sytstem, high energy electrons are used to bombard a solid target . The emission from this target consists of characteristic line emissions associated with the filling of core holes by the incident electron beams. The optimum solid targets are Mg and Al Sometimes Na and Si are used. It is better to get monochromotic photon beam. In other words, a beam with a single wavelength

XPS as a core level spectroscopy XPS is also know Electron Spectroscopy for Chemical Analysis (ESCA) because most of the core electrons do have low binding energy (less than 1000 eV). Therefore, XPS can be used to analyze all surface Except (hydrogen and Hellium Qualitative analysis From the binding energy we can realize the metal and the emitted electrons as any electrons in any element has know binding energy as shown in this figure.

Quantitative analysis Generally, XPS is a surface analysis instrument. However, sometimes we can use it in the quantitative analysis for the pure material. The concentration of each element can be estimated according to the height of the obtained peak. Factors affecting the quantitative analysis accuracy 1. Signal-to-noise ratio 2. Peak intensity 3. Accuracy of relative sensitivity factors correction for electron transmission function 4. Surface volume homogeneity 5. Correction for energy dependency of electron mean free path 6. Degree of sample degradation due to analysis. If optimum conditions are used the accuracy (at the major peaks) may reach to 90-95 % However under normal conditions, the accuracy may reach to 80-90 %.

Important notes Uses of XPS XPS can be used to analyze the surface within a surface thickness 1 to 10 nm of the material being analyzed. XPS is also known as ESCA, an abbreviation for Electron Spectroscopy for Chemical Analysis. XPS detects all elements with an atomic number of 3 (lithium) and above. It cannot detect hydrogen (atomic weight = 1) or helium (atomic weight = 2) because the diameter of these orbitals is so small, reducing the catch probability to almost zero. Detection limits for most of the elements are in the parts per thousand range. Detections limits of parts per million (ppm) are possible, but require special conditions: concentration at top surface or very long collection time (overnight). XPS is routinely used to analyze inorganic compounds, metal alloys, semiconductors, polymers, elements, catalysts, glasses, ceramics, paints, papers, inks, woods, plant parts, make-up, teeth, bones, medical implants, bio-materials, viscous oils, glues, ion modified materials and many others. Uses of XPS elemental composition of the surface (top 1–10 nm usually) empirical formula of pure materials elements that contaminate a surface chemical or electronic state of each element in the surface uniformity of elemental composition across the top surface (or line profiling or mapping) uniformity of elemental composition as a function of ion beam etching (or depth profiling)

Degradation during analysis Depends on the sensitivity of the material to the wavelength of X-rays used, the total dose of the X-rays, the temperature of the surface and the level of the vacuum. Metals, alloys, ceramics and most glasses are not measurably degraded by either non-monochromatic or monochromatic X-rays. Some, but not all, polymers, catalysts, certain highly oxygenated compounds, various inorganic compounds and fine organics are degraded by either monochromatic or non-monochromatic X-ray sources. Non-monochromatic X-ray sources produce a significant amount of high energy X-rays (1– 15 keV of energy) which directly degrade the surface chemistry of various materials. Non-monochromatic X-ray sources also produce a significant amount of heat (100 to 200 °C) on the surface of the sample because the anode that produces the X-rays is typically only 1 to 5 cm (2 in) away from the sample. This level of heat, when combined with the X-rays, acts synergistically to increase the amount and rate of degradation for certain materials. Monochromatic X-ray sources, because they are far away (50– 100 cm) from the sample, do not produce any heat effects.