Influence of L-Tryptophan on Growth and Optical Properties of PbS Nanocrystalline Thin Films

Abstract

The growth through the green chemical bath of PbS doped systematically with the biomolecule L-tryptophan led to growth of hybrid, inorganic-organic, nanocrystalline thin films onto glass slides at T~80°C. The thickness was found in the range of 230–140 nm. Morphological changes were analyzed using atomic force microscopy (AFM). FTIR (Fourier-transform infrared spectroscopy) spectra showed broad absorption bands located at ~3450 cm−1 attributed to stretching of the H2O molecules and two small absorption bands located at ~2285 cm−1 and ~2918 cm−1 along with a strong band at ~1385 cm−1 assigned to vibration modes corresponding to CO32− ions. In the patterns of X-ray diffraction (XRD), the cubic phase was identified in all the samples according to the angular positions 2θ~26.08°, 30.13°, 43.08°, 51.91°, 53.60°, 6251°, 68.98°, and 71.15°. Using the Scherrer formula on the XRD patterns, the grain size (GS) was determined; for the undoped sample, ~42 nm was found, whereas for the doped samples, ~42–22 nm was found. The electronic charge distribution of L-tryptophan was determined using the molecular electrostatic potential (MEP) to understand the decrease on the GS associated with the interaction of π electrons from conjugated rings and amino-acid functional groups. The absorbance spectra in doped films showed excitonic peaks at ~1.8–2.1 eV associated to a higher energy of the 1Sh → 1Sh and 1Ph → 1Pe electronic transitions. Through optical absorption, a shift for the band gap energy was observed from ~1.4 eV for the undoped sample and ~2.1–2.3 eV for the doped films, respectively. Such behaviour is generally associated with the GS decrease and the effect of quantum confinement; a simple model by calculating changes in Gibbs free energy (ΔG°) for growth of nanocrystals is presented.