Effect of annealing on the visible photoluminescence characteristics of octadecyltrichlorosilane monolayers on silica surfaces

We report the effect of annealing conditions, including annealing environment, annealing temperature, and annealing time, on the visible photoluminescence (PL) at similar to 450 nm from octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on silica surfaces. When the OTS SAMs are annealed at 200 degrees C in air, the PL and PL excitation peak wavelengths show considerable redshifts with increasing annealing time. We have also found that the annealing temperature and time affect the PL decay dynamics. The PL decay of the sample annealed at 150 degrees C for 2 h in air hardly depends on the measurement temperature. However, the PL decay of the samples annealed at 200 degrees C for more than 1 h starts to show the measurement temperature dependence, indicating that the thermally activated nonradiative decay is involved in the entire recombination processes. These annealing-dependent PL features can be interpreted in terms of the progress of thermal decomposition of alkyl chains in OTS SAMs during annealing and the concomitant changes in the microscopic and electronic structures of the relevant PL centers. Density functional theory calculations are also used to evaluate the changes in the energy levels of the emission centers during annealing.