Size-Dependent Photothermal Performance of Silicon Quantum Dots

Silicon quantum dots (Si QDs) have recently attracted attention in clinical imaging technology owing to their nontoxicity to living cells and tissues. Here, we investigate the size-dependent photothermal effect of hydrogen-terminated Si QDs, which provides a common surface for further functionalization of biocompatibility. Three samples of QDs with diameters of 2.2, 3.8, and 4.7 nm were prepared by a thermal disproportionation reaction of triethoxysilane hydrolyzed at pH 3 and subsequent hydrofluoric etching. The photothermal responses, which occur through the sequential absorption of photons under photoexcited conditions, are measured at increasing laser power using Raman spectroscopy. The photothermal effect, which is quantified by the Raman spectroscopic study, is size-dependent and enhanced for larger QDs. Hence, the photothermal heat released from the QDs might be controlled between room temperature and 275 °C. To investigate their practical use, we prepared QDs terminated with undecanoic acid monolayers, giving the solubility in water. As expected, we observed the size dependence of thermal conductivity properties on warming 2.5 mL water under light illumination. The temperature dependence of the photoluminescence spectra reveals the important role of nonradiative channels in the photothermal performance controlled by the QD size.