Control of size of PbI2 nanocrystals using Langmuir–Blodgett films of n-octadecyl succinic acid

Lead iodide (PbI2) particulate films, composed of highly oriented nanocrystals have been generated in situ using an oriented inorganic extended solid within a deposited Langmuir–Blodgett film (LB film) of octadecyl lead succinate (ODSAPb). Solid (PbI2) was formed by the reaction of gaseous HI with ODSAPb LB film, which acts as the template for building the inorganic solid. Mixed LB systems with 1:1 mol ratio of ODSAPb with octadecyl amine (ODAM) were also used to form the PbI2. The films were characterized using attenuated total reflection FTIR (ATR-FTIR) to check whether HI had completely protonated the films. The observation of absorption features between 550 and 300 nm region in the UV-Visible spectra of these films strongly suggest sizes of PbI2 ranging between 2.0 and 3.0 nm. The optical band gap estimated from LB films of these two systems showed a marked increase from 0.36 eV for the pure ODSAPb template to 0.47 eV for the ODSAPb+ODAM mixed film. The LB films were examined by small angle XRD and atomic force microscopy (AFM) and indicated that there is a variation in the structural organization in the monolayers and in the size of PbI2 in the pure ODSAPb LB films as compared with the mixed films with ODAM. The average contraction in a axis and elongation in the c axis of the unit cell (a=b=3.9 Å and c=7.0 Å) for PbI2 in the template, compared with the values for PbI2 in bulk confirm the presence of the nanocrystals in the LB films. The increase in the band gap is also reflected in the increase in the c value of the unit cell parameter that changes from 6.3 to 7.0 Å in going from the pure ODSAPb matrix to mixed film of ODSAPb and ODAM.