Temperature induced size selective synthesis of hybrid Cds/pepsin nanocrystals and their photoluminescence investigation

There is growing interest in materials chemistry for taking advantage of the physical and chemical properties of biomolecules in the development of next generation nanoscale materials for opto-electronic applications. A biomimetic approach to materials synthesis offers the possibility of controlling size, shape, crystal structure, orientation, and organization. The great progress has been made in the control that can be exerted over optical materials synthesis using biomolecules (protein, nucleic acid)/mineral interfaces as templates for directed synthesis. We have synthesized the CdS nanocrystals using pepsin by biomimetic technique at four different set temperatures. X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) results showed that we are able to tune the size and distribution profile just by tuning the reaction (Rx) temperature and goes towards excitonic Bhor radius (2.5 nm) at low temperature (4 °C). The narrow absorption peak at 260 nm from Cd²⁺-pepsin complex dominates and indicates the size dispersion of the modified CdS nanoparticles are fairly monodisperse. Effective mass approximation (EMA) shows large blue-shift (~1 eV) in the band gap for the cubic phase from bulk hexagonal CdS. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra are dominated by a strong and narrow band-edge emission tunable in the blue region indicating a narrow size distribution. The reduction in PL efficiency is observed when the Rx temperature increases however no change in PLE spectra and temporal profiles of the band-edge PL is observed. At 4 °C, high emission efficiency with shift of PL spectrum in the violet region is observed for 1.7 nm size CdS quantum dots (QDs). Presence of pepsin has slowed the PL decay which is of the order of 100 μs.