Tunable, High Intensity Photoluminescence from Si/SiO₂ Core/Shell Quantum Dots via High-Pressure Water Vapor Annealing

As non-toxic, abundant, and low-cost luminophores, silicon quantum dots (Si QDs) can suit myriad applications, from luminescent devices to biological imaging. Nonthermal plasma-assisted decomposition of silane gas is an efficient, sustainable, and controllable method for synthesizing Si QDs. However, as-synthesized, Si QDs have a high dangling bond defect density and require additional passivation for widespread utilization. Liquid-based passivation methods, such as thermal hydrosilylation, can organically cap Si QDs but cannot prevent oxidation upon exposure to ambient air. Native oxidation can passivate the Si QDs through the formation of a silica (SiO₂) shell and ensure long-term air stability but occurs over at least one month. Therefore, we use high-pressure water vapor annealing (HWA) to quickly obtain Si/SiO₂core/shell quantum dots with tunable, high-intensity photoluminescence (PL) that results in long PL lifetimes and comparable PL quantum yields (PLQY).<br/><br/>HWA is a scalable method that was used to oxidize plasma-synthesized Si QDs with high-pressure steam within a few hours, including heating and cooling time. A high-pressure vessel with both pressure and temperature gauges was loaded with the Si QDs and a controlled amount of deionized water that tuned the pressure of the saturated steam. The vessel was heated between 250 and 300 °C using a thermal sand bath. After ten minutes at its equilibrated pressure, the vessel was vented to atmospheric levels. The vessel was then left to cool to room temperature. The dry Si/SiO₂ QDs were extracted and dispersed in toluene for characterization.<br/><br/>We first show the influence of additional hydrogen gas injection in synthesizing the Si QDs and demonstrate that contrary to previous reports, no additional gas injection in the plasma afterglow led to more stable silica shells. In varying the hydrogen gas flow rate from 0 to 100 sccm, the more hydrogen gas injection, the lower the intensity of the more thermally stable, network-structure silica peak in the measured Fourier-transform infrared (FTIR) spectra. The Si QDs synthesized without hydrogen gas had the highest PL intensity as well.<br/><br/>Then, we show that varying the applied pressure to oxidize Si QDs made using only argon and silane gas can tune the PLQY. We measure both steady-state and time-resolved PL (TRPL) to find that increasing the pressure from seven to 17 bar leads to blue-shifted PL and a 10% reduction in the PLQY. A stretched exponential function was used to fit the TRPL decay and extract lifetimes and dispersion factors. The lifetime also decreases with increasing pressure by 70 µs, but the dispersion factor increases from 0.81 to 0.87. When higher pressures are applied, the silica shells are also fully thermally relaxed. FTIR spectra show the presence of characteristic peaks for Si QDs oxidized in a humid environment. Furthermore, with increasing applied pressure, the peak wavenumber of the Si-O-Si bond increases as well. At an applied pressure of 17 bar, the peak wavenumber reaches 1080 cm^-1, which exceeds that of thermally grown silica.<br/><br/>Lastly, we report the influence of silica shell thickness. Thicker shells made using larger Si QDs oxidized with optimized HWA conditions led to a stable PLQY of over 40%. After three months of storage in room air, the PLQY plateaued at 35%. Si/SiO₂ QDs with thicker silica shells also had longer lifetimes (240 µs) and higher dispersion factors (0.85 – 0.9). Compared to other passivation methods, HWA can effectively passivate Si QDs within a much shorter time span and only using water. Using HWA, the PL of Si/SiO₂QDs can be tuned to meet various design requirements by controlling the applied pressure.