2, Pohang Accelerator Laboratory, Pohang, , Korea (the Republic of)
Colloidal 4-ethynylstyryl and octyl cocapping silicon quantum dot (4-Es/Oct Si QD) and its spin-coated film were synthesized and fabricated at three different curing temperatures, 150, 250, and 350 °C under argon for 4 h. Thermal cross-linking of 4-ethynylstyryl terminated 4-Es/Oct Si QD during the curing process was confirmed by differential scanning calorimetry for the 4-Es/Oct Si QD powder and by Fourier transforms infrared spectroscopy for the Si QD thin films. The effect of thermal cross-linking of 4-ethynylstyryl capping groups on the electronic coupling between Si QDs in Si QD solids of thin film states was investigated by monitoring optical and electrical properties of the Si QD thin films at different curing temperatures. The optical bandgap values estimated from the extinction-coefficient graphs of 4-Es/Oct Si QD thin films are 2.83 eV, 2.7 eV, to 2.45 eV at the curing temperature of 150, 250, and 350 °C, respectively. Moreover, ultraviolet–visible absorbance and photoluminescence spectroscopy of the Si QD thin films showed a distinct extension into the longer wavelength. In addition, their current–voltage measurements showed an increase leakage current with increasing curing temperatures. The thermal cross-linking of 4-ethynylstyryl capping groups at curing temperatures of 250 and 350 °C and thermal decomposition of octyl capping groups at 350 °C decrease width and height of the energy barrier between the two QD neighbors, allow the expansion of the wavefunctions of individual Si QDs, and overlap with the neighboring QDs, thus strengthening electronic coupling between the Si QDs in QD solids, inducing significant changes in the valence band-edge, optical, and electrical properties.