Controlled Dilution of Copper Spins in Phthalocyanine Thin Films for Electron Paramagnetic Resonance*

Electron paramagnetic resonance (EPR) provides a sensitive probe of hyperfine interactions in molecular solids. In copper phthalocyanine (CuPc), these interactions originate from localized copper spins, but concentrated spin systems often obscure fine details. To overcome this limitation, we demonstrate approaches for preparing diluted CuPc thin films by co-depositing metal-free phthalocyanine (H₂Pc) as a non-magnetic host matrix. Two complementary fabrication methods were employed to control the concentration of paramagnetic copper centers. The resulting films were characterized using x-ray diffraction (XRD) to assess crystallinity and atomic force microscopy to evaluate surface morphology.  The large relative XRD peak ratios are consistent with a large concentration of H₂Pc. We also discuss common challenges in employing diluted thin-film systems for high-resolution EPR investigations of hyperfine interactions in molecular semiconductors.