Influence of number of pitches and substrate on the nanostructure and optical properties of ZnS helical sculptured thin films

• Chiral sculptured ZnS thin films with different pitches are produced.
• Optical spectra (both s- and p- polarization) of films on two types of substrates are compared.
• Results show a strong dependence on the structural void fraction and number of pitches.
• The reverse homogenization theory is used to calculate the complex refractive index.
• Band gap calculations showed that structures with larger grains and higher void fraction have smaller band gaps.

ZnS helical sculptured thin films with different number of pitches were produced on glass (microscope slides) and pre-deposited 7 nm ZnS on glass substrate. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) were used for structural analyses. Crystallographic structure of samples was obtained using x-ray diffraction (XRD) method which confirmed the formation of ZnS films on the substrates. Optical spectra of the samples were measured using a single beam spectrophotometer for both s- and p-polarized light and at different incident light angles. Optical results showed a strong dependence on the structural void fraction and number of pitches and, due to anisotropy of the helical structure, a dependence on the incident light angle. It is shown that by controlling the growth of these structures the optical spectra can be controlled.The reversed homogenization theory was used to calculate the complex refractive index of these structures and the percentage of the void fraction in the produced samples. This investigation showed that both real and imaginary parts of the refractive index are dependent on the structural void fraction; structures with larger grains and higher percentage of void fraction having larger real and imaginary parts of the refractive index. Band gap calculations showed that structures with larger grains and higher void fraction have smaller band gaps. A correlation is obtained between the band gap energies and the nano-strain developed in the structure of the produced films; band gap energy decreases with nano-strain.