EPR in Material Science and Physics

Polymer degradation

HALS successfully prevents polymer photoxidation.
The degradation of polymers due to light exposure leads to discoloration of the polymer and a decrease in the mechanical properties (elasticity, toughness, etc). To prevent this decomposition, hindered amine light stabilizers (HALS) are added to the polymer. By monitoring the EPR signals of these light stabilizers, their effectiveness can be evaluated using the EMXnano.

Photo initiation and radical formation upon light irradiation
The EPR signal generated in the polymer during UV irradiation (left) is completely suppressed after addition of the HALS where only the HALS EPR spectrum is observed (right)

Polymer structure

EPR studies of polyelectrolyte multilayer (PEM) films using nitroxide spin labels.
Multilayers of polyelectrolytes (polymers bearing dissociated ionic groups) are formed by the alternating adsorption of oppositely charged polyelectrolytes, so called layer-by-layer technique. PEM films composed of strong polycation and weak polyanion that is usually spin labeled with free nitroxide (4-amino-TEMPO) are studied by EPR. The growth of the PEM films is monitored and quantitative EPR analysis provides information about each double layer.

Signal intensity of polycation/TEMPO-labeled polyanion multilayer films in contact with buffer solution of pH 4 in dependence on number of double layers NDL.

Paint properties

HALS EPR signal in paint indicating deterioration after UV exposure.
The main cause of paint film deterioration is the degradation of several components, including the binder and certain pigments. This is caused by the formation of free radicals from prolonged exposure to UV light (sunlight), moisture and freeze-thaw cycles. Free radicals are highly reactive and either form or breakdown chemical bonds in substances. In the case of paint durability on exposure, free radicals actually damage the film. This process is very similar to how skin ages. Skin contains free radicals that, when exposed to years of sunlight, will show signs of aging, including wrinkling, peeling, sun spots and overall dryness.

EPR spectra of HALS (hindered amine light stabilizers) detected in paint after UV exposure

Solar cells

Defects in amorphous silicon detected by EPR.
Silicon is the most common material for the production of solar cells in the photovoltaic industry either in mono- or polycrystalline form. Specific characterization of paramagnetic defects can be done by EPR to gain insight into how paramagnetic centers induced by degradation influence the efficiency of solar cell active layers. EPR studies on amorphous silicon photovoltaics demonstrated that a strong relationship exists between the presence of paramagnetic defects and the resulting charge collection efficiency in such material.

Light-induced defect in amorphous silicon detected by EPR due to breaking of weak Si-Si bonds

Transition metals

Determination of Mg coordination in wurtzite thin films
The transition-group, rare-earth and actinide ions are members of the 3d, 4d, 5d, 4f and 5f groups and are subject of a host of EPR investigations. One aspect that makes transition elements interesting subjects for study by EPR is their variable valence. For example, Zn1xMgxO complex is a versatile functional material for oxide semiconductors and the atomic arrangement in the bulk and at the interfaces determines important properties of the oxides. EPR is used to determine the Mg coordination in heteroepitaxial wurtzite Zn1xMgxO:Mn thin films.

Data courtesy Dr. Andreas Pöppl, Universitӓt Leipzig (J. Mater. Chem. C, 2015, 3, 11918)

Experimental and simulated EPR spectra at 297 K of Zn0.99Mg0.01O:Mn (pO2 = 0.016 mbar, cMn = 0.05%) thin film sample G5189 for B┴c (top) and BIIc (bottom). Asterisks indicate signals of Fe3+ and Cr3+ impurities in the sapphire substrate.