Here we report for the first time highly flexible quantum dot light-emitting diodes (QLEDs), in which a layer of red-emitting colloidal silicon quantum dots (SiQDs) works as the optically active component, by replacing a rigid glass substrate with a thin sheet of polyethylene terephthalate (PET). The enhanced optical performance for electroluminescence (EL) at room temperature in air is achieved by taking advantage of the inverted device structure. Our QLEDs do not exhibit parasitic EL emissions from the neighboring compositional layers or surface states of QDs over a wide range of driving voltages and do not exhibit a shift in the EL peak position as the operational voltage increases. Compared to the previous Si-QLEDs with a conventional device structure, our QLED has a longer device operational lifetime and a long-lived EQE value. The currently obtained brightness (∼5000 cd/m), the 3.1% external quantum efficiency (EQE), and a turn-on voltage as low as 3.5 V are sufficiently high to encourage further developments of Si-QLEDs.

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.

We report carboxy-terminated silicon quantum dots (SiQDs) that exhibit high solubility in water due to the high molecular coverage of surface monolayers, bright light emission with high photoluminescence quantum yields (PLQYs), long-term stability in the PL property for monitoring cells, less toxicity to the cells, and a high photothermal response. We prepared water-soluble SiQDs by the thermal hydrosilylation of 10-undecenoic acid on their hydrogenterminated surfaces, provided by the thermal disproportionation of triethoxysilane hydrolyzed at pH 3 and subsequent hydrofluoric etching. The 10-undecanoic acid-functionalized SiQDs (UA:SiQDs) showed long-term stability in hydrophilic solvents including ethanol and water (pH 7). We assess their interaction with live cells by means of cellular uptake, short-term toxicity, and, for the first time, long-term cytotoxicity. Results show that UA:SiQDs are potential candidates for theranostics, with their good optical properties enabling imaging for more than 18 days and a photothermal response having a 25.1% photothermal conversion efficiency together with the direct evidence of cell death by laser irradiation. UA:SiQDs have low cytotoxicity with full viability of up to 400 μg/mL for the short term and a 50% cell viability value after 14 days of incubation at a 50 μg/mL concentration.