Introduction to Electron Spin Resonance EPR/ESR principle

Electron spin resonance (ESR), also known as electron paramagnetic resonance (EPR). Most of the electrons in a molecule are in pairs, and according to the Pauli exclusion principle, the electrons in each pair must be one spin up and one spin down, so the magnetic properties cancel each other out. Therefore, particles with unpaired electrons can show the phenomenon of magnetic resonance.

The principle of electron spin resonance is similar to that of nuclear magnetic resonance. However, in contrast, the sample is controlled in a fixed frequency microwave, and then the applied magnetic field is changed so that the difference in the electron energy order is the same as the microwave energy, and the unpaired electrons can move between the two energy orders. The net absorbed energy of the microwave is measured and converted to an ESR spectrum.

Electron spin resonance (ESR or EPR) is a powerful analytical method, mostly used to detect and analyze the properties of unpaired electrons in matter. The evaluation of ESR is becoming increasingly important as the state of the electrons in a substance has a great influence on its properties and functions. Whether the sample is solid, liquid or gas, many types of substances can be studied, from electronic materials to catalysts, biological samples.

Figure 1: Basic Configuration of ESR Hardware ESR Analysis Application Areas Electronic States such as Magnetic Materials and Semiconductors Semiconductors Lattice Defects and Impurities (Dopants) Electronic States of Glasses and Amorphous Materials Structural Tracking of Catalytic Reactions, Change of State of Charge Photocatalytic Reactivity and Mechanisms of Photochemical Reactions Polymer Polymers Polymers Polymerization Processes of Free Radicals (photopolymerization, graft polymerization) Polymers Resolutions (photolysis, pyrolysis, chemical decomposition) Reactive oxygen radicals are associated with aging of diseases in organisms Oxidative degradation of lipids (edible oils, petroleum, etc.) Detection of radiation-exposed foodstuffs Measurement of age and geological features of fossils using lattice defects ESR characterization Observation of the electronic behavior (kinetics) within molecules as well as methods to analyze phenomena by identifying their electronic environments ESR measurements provide information on the unpaired presence of electrons (number, type, ESR instruments offer non-destructive detection of a wide range of properties of the sample that can be measured in any phase (gas, liquid or solid).ESR has a wide range of applications and is commonly used in a variety of applications such as semiconductor and coating production lines, as well as in the clinical and medical fields, such as cancer diagnostics. It is also being actively used for basic pharmaceutical and agricultural research.

Figure 2: Hardware size diagram of FA100 and FA00 ESR in carbon materials Graphite is carbon in the shape of hexagonal crystals. Whereas graphene planar structure is tortoise shell shaped where the carbon to carbon bonding is covalent. In contrast the connections between these planar layers of carbon are weaker van der Waals forces (e.g. Figure 1). Within the planes there is a general electrical conductivity similar to metals, but between the planes semiconductor-like properties can be observed. This is why graphite is used as a raw material for many products, including electronic devices, automobiles, batteries, coatings, and more. In addition, adding dopants between graphene can improve the conductivity of the material or even develop its superconducting properties.

Figure 3: Basic structure of graphite Graphite and carbon fibers show ESR linear Dysonian absorption, a property only found in conducting materials. (Fig. 2) shows the ESR spectrum of a pencil core. The vertical asymmetric signal is observed because of the change in phase due to microwave influence. From the ratio of A to B (Fig. 2), the time required for the conduction electrons to pass through the plane can be obtained (4). The electronic structure of graphite can be analyzed based on the g-value and the linewidth (ΔH).

Figure 4: ESR spectra of a pencil lead (200°C) Afterwards, a pencil lead (4B) was placed in a test tube and its ESR spectra were measured while the temperature of the experiment was increased from -100 degrees to 200 degrees (using ES-DVT4). Graphite changed its line width on the ESR spectrum as the temperature changed (Figure 3). The characterization of graphite at low temperatures allows the signal position to show a shift in the g value and an increase in the linewidth, as well as the structure of the energy bands near the Fermi potential.

Figure 5: ESR spectra of a pencil core Conclusion ESR is an important method for assessing the solid-state properties of materials such as graphite, which provides information about the electronic structure. The existence of FullerenesC60 was confirmed in 1985(6) and it is antimagnetic, but produces unpaired electrons because it is easily oxidized or reduced to form free radicals. The electronic structure and solid-state properties of endohedral Fullerenes have been studied (7,8,9). The ESR of carbon fibers and multilayer carbon nanotubes is similar to that of polycrystalline graphite, and overlapping and different g-values have been observed due to conduction electrons and their defects (10).

1. G.Wagoner (1960): Spin Resonance of Charge Carriers in Graphite, Physical Review, 118, 647-653.

2. H. Ohya and J, Yamauchi(1989): Electron Spin Resonance－Micro Characterization of Material－, Kodansha Scientific, p289.

3. F. J. Dyson (1955): Electron Spin Resonance Absorption in Metals. II. Theory of Electron Diffusion and the Skin Effect, Physical Review, 98, 349–359.

4. G.Feher and A.Kip (1955): Electron Spin Resonance Absorption in Metals. I. Experimental, Physical Review, 98, 337-348.

5. J.W. McClure and Y.Yafet(1961): Proc. Of 5th Conferemce of Carbon, ed. S.Mrozowski,M.L. Studebaker, P.L.Jr.Walker, University Park, PA, Pergamon Press,p22(1963).

6. H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl & R. E. Smalley (1985): C60: Buckminsterfullerene, Nature, 318, 162-163.

7. H. Shinohara, Y. Saito(1996):Chemistry and Physics of Fullerene, The University of Nagoya Univ. Press, p302.

8. C.C Chancey, M.C.M. O’Brien (1997): The Jahn-Teller Effect in C60 and Other Icosahedral Complexes, Princeton University Press.

9. The Chemical Society of Japan (Ed) (1999):Chemistry of Fullerene―The Third Isotope of Carbon―,Quarterly Kagakusosetu, 43, Japan Scientific Societies Press.

10. J.B.Jones and L.S.Singer (1982): Electron spin resonance and the structure of carbon fibers, Carbon, 20, Issue 5, p379-385.