Influence of glutamic acid on the structural, optical, thermal, mechanical, dielectric, electrical, and NLO properties of potassium hydrogen phthalate crystals
摘要
This study reports, for the first time, the successful growth and comprehensive characterization of glutamic-acid (GA)-doped potassium hydrogen phthalate (KHP) single crystals, demonstrating tunable multifunctional properties through bio-amino acid incorporation. The novelty of this work lies in the simultaneous enhancement of optical transparency, dielectric performance, and mechanical strength achieved via molecular-level GA substitution without disturbing the parent lattice symmetry—an approach not previously explored for KHP. Single crystals of 1.0 mol% GA-doped KHP were grown by the slow evaporation technique. Single-crystal XRD confirmed the orthorhombic structure (space group Pca2₁) with slight lattice expansion (a = 9.612 Å, b = 13.338 Å, c = 6.468 Å) compared to pure KHP (a = 9.605 Å, b = 13.331 Å, c = 6.473 Å), verifying dopant incorporation. FTIR spectra displayed characteristic vibrational shifts (3310–3116 cm⁻1 for N–H stretching), confirming hydrogen-bond-mediated interactions. UV–Vis analysis revealed an absorption-edge blue shift from 350 to 340 nm, corresponding to an increased optical band gap from 3.54 to 3.65 eV, with overall transmittance exceeding 85% in the visible range. Thermogravimetric/DTA data established high thermal stability up to 300 °C, while Vickers microhardness measurements indicated a ~ 15% increase in hardness for the doped crystal. SEM images revealed smoother surfaces and uniform growth features, and EDAX confirmed compositional homogeneity with C: 32.10 wt%, O: 39.51 wt%, and K: 28.39 wt%. Electrical studies exhibited a rise in dielectric constant and AC conductivity with frequency; at 1 kHz, εᵣ increased from 4.8 ± 0.2 (pure) to 6.3 ± 0.3 (GA–KHP), and DC conductivity improved from 2.1 × 10⁻⁷ S cm⁻1 to 5.6 × 10⁻⁷ S cm⁻1, suggesting defect-assisted protonic conduction. Kurtz–Perry SHG tests yielded mean output voltages of 20 ± 1.4 mV for KHP, 12 ± 0.8 mV for GA–KHP, and 24 ± 1.7 mV for KDP, confirming retained non-centrosymmetry and an SHG efficiency of 0.50 ± 0.05 relative to KDP. Overall, GA doping substantially improves the optical, dielectric, and mechanical properties of KHP while maintaining high crystallinity, thermal stability, and chemical purity. These findings highlight the potential of amino-acid-assisted lattice engineering in developing sustainable semi-organic nonlinear optical and optoelectronic materials for next-generation photonic device applications.