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.
Highlights
PbS is a direct-band semiconductor with a relatively small band gap energy (Eg) ~0.41 eV at 300 K and can be used for fabrication on mid-infrared detectors, solar energy absorption devices, and optical switches [1]; it has been thoroughly investigated for its quantum confinement effect in which the wavelength dependency of optical absorption and emission process can be monitored over a wide range of energy [2]
As it is well-known, this semiconductor has been synthesized using various techniques, such as atomic layer deposition [5], SILAR [6], Journal of Nanomaterials hydrothermal synthesis [7], hot injection method [8], and chemical bath deposition [9]. This last technique has been used in the last decades for the synthesis of various semiconductor materials, and as such, the chemical bath (CB) approach is a green, simple, and cost-effective approach in which the material obtained is of good crystalline quality with acceptable stoichiometry and worthy of being applied for PbS synthesis
We propose that PbS nanocrystals are agglomerated around the L-tryptophan molecule generating nanocrystals; this phenomenon leads to the growth of PbS nanocrystals
Summary
PbS is a direct-band semiconductor with a relatively small band gap energy (Eg) ~0.41 eV at 300 K and can be used for fabrication on mid-infrared detectors, solar energy absorption devices, and optical switches [1]; it has been thoroughly investigated for its quantum confinement effect in which the wavelength dependency of optical absorption and emission process can be monitored over a wide range of energy [2]. Latest reports of doped-PbS nanocrystals with different monovalent and divalent ions and even with three inorganic ions which have been systematically incorporated in the crystal lattice showed the different reasons for investigating this semiconductor in depth according to the interesting changes observed [3, 4] As it is well-known, this semiconductor has been synthesized using various techniques, such as atomic layer deposition [5], SILAR [6], Journal of Nanomaterials hydrothermal synthesis [7], hot injection method [8], and chemical bath deposition [9]. This last technique has been used in the last decades for the synthesis of various semiconductor materials, and as such, the chemical bath (CB) approach is a green, simple, and cost-effective approach in which the material obtained is of good crystalline quality with acceptable stoichiometry and worthy of being applied for PbS synthesis.
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