Growth, Vibrational, Optical, Mechanical and DFT Investigations of an Organic Nonlinear Optical Material – Phenylurea

In this work, combined experimental and computational studies of an organic nonlinear optical material, phenylurea (PU), are reported. Optical quality single crystal of PU, was successfully grown by the slow evaporation method. The unit cell parameters of the grown PU crystal belong to the monoclinic system and these belong to the non-centrosymmetric (NCS), P2(1), space group. Morphology of the crystal was indexed and it reveals its 14 distinct faces. Vibrational modes of PU functional groups were assigned successfully using FTIR and Raman spectrum. The grown crystal exhibits low cutoff wavelength at UV region with good optical transparency and blue wavelength emission. The preliminary measurements confirmed that second harmonic generation (SHG) activity (similar to 7.6 KDP and similar to 2.2 urea) with excellent laser induced threshold damage (LDT) having a value of 2.92 GW/cm(2) (similar to 1.95 urea). TG/DTA analysis shows that endothermic nature of grown crystal with thermal stability up to 154 degrees C. The Vickers micro hardness test confirms soft material behavior of PU crystal and it was further explored by crystal void percentage calculation. Electrical properties such as dielectric and photoconductivity measurements were carried out for grown crystal. Furthermore, density functional theory (DFT) studies such as Mulliken charge distribution, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP) map and first order hyperpolarizability (beta) were determined to probe the structure-property relationship. These results suggest that PU single crystal can be a better alternate for urea in NLO applications.