Thickness-dependent plasma polymerized N-Vinyl-2-Pyrrolidone Thin Films: investigation of structural and optical properties

Plasma polymerized N-Vinyl-2-Pyrrolidone (PPNVP) thin films of different thicknesses are prepared onto glass substrates at room temperature under the pressure of 60 torr by capacitively coupled AC glow discharge plasma system.  Fourier Transform Infrared Spectroscopy (FTIR), Field-Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-Ray (EDX) Spectroscopy, and Ultraviolet-Visible (UV-Vis) Spectroscopy are used to obtain thickness-dependent structural, morphological, elemental, and optical properties of PPNVP thin films. The IR spectrum at higher thicknesses of these films indicated small but significant increases in the C-H stretching (2976-2982 cm^-1) and decreases in the C-H bending (1462-1460 cm^-1) regions. Moreover, the peaks of the hydroxyl group (3662 and 3656 cm^-1) are most important for thicker films, which give rise to very unique band profiles. According to FESEM, the polymeric structure of PPNVP thin films displays frequent cleavage-type fracture and indicates the presence of hydrophilic functional groups. The EDX results show that as the thickness of the film increased, the mass (%) of carbon and nitrogen increased, while the mass (%) of oxygen decreased. Based on the results of the UV-Visible spectroscopy, the optical absorbance of all the PPNVP thin films increases sharply in the UV region, peaks at around 300 nm, and then rapidly decreases up to around 350 nm. The direct (3.23-2.98 eV) and indirect (2.77-2.62 eV) band gaps get narrower as the thickness of a thin film increases, which is characterized by structural variation with film thickness.