The Properties And Applications Of Silicon Quantum Dots

Unique optical property, great compatibility with microelectronics, good biocompatibility and high surface activity are the attributes of silicon quantum dots (Si QDs) that have been exploited in many fields, such as biological imaging, photoelectric devices and nano-catalysis. However, some problems seriously limit the realization of the full potential of Si QDs in various applications, containing the lack of the theoretical investigation, bad water dispersibility and delay of application research progress compared to other semiconductor QDs.In this work, we prepare Si QDs by using low-temperature plasma followed by surface modification. Both the photodetectors and light-emitting diodes (LEDs) based on Si QDs are prepared and further investigated. We also study the influence of twinning in Si QDs on the electrical structures and optical properties of Si QDs based on density functional theory using ab initio calculations. The main innovative results are displayed as follows:(1) We study the twinning of Si QDs in the frame work of density functional theory by representatively considering single-twinned and fivefold-twinned Si QDs. It is found that the formation of twinned Si QDs is thermodynamically possible and twinning enhances the stability of Si QDs. Both the quantum confinement effect and light absorption are weakened by twinning for Si QDs. The current results help to better understand the experimental work on twinned Si QDs and guide the tuning of Si-QD structures for desired properties.(2) A facile method to produce water-dispersible Si QDs is demonstrated. Based on the hydrosilylation of Si QDs, the self-assembly enabled by the amphiphilic polymer of F127 is employed to encapsulate hydrosilylated Si QDs in micelles. The resulting Si-QD micelles are well dispersed in water. The size of Si-QD micelles may be tuned in the range from 10 to 100 nm by adjusting the ratio of the number of Si QDs to that of F127 molecules. The PL QY of water-dispersible Si QDs (-24%) is finally comparable to that of hydrophobic Si QDs usually obtained with hydrosilylation. We have also found that the photostability of Si-QD micelles is excellent. The current results may help advance the technological use of freestanding Si QDs, especially in the area of bioimaging.(3) We fabricate Si-QD/graphene/Si photodetectors. It is found that Si QDs on the top of graphene cause the built-in potential of the graphene/Si Schottky junction to increase. They also significantly reduce the optical reflection of photodetectors. Both of the electrical and optical contributions of Si QDs enable us to demonstrate graphene/Si Schottky-junction photodetectors with excellent responsivity (≈0.495 AW-1), response speed (≤ 25 ns), and specific detectivity (≈7.4× 109 Jones). The responsivity peaking at≈877 nm is ideal for coupling with vertical-cavity surface emitting lasers (VCSELs) used in low-power integrated optoelectronic circuitry. The nearly unchanged low NEP values between 350 and 950 nm are very useful for broadband imaging.(4) We prepare hybrid Si-QD organic LED using Si-QD film as the light-emission layer. As the preliminary result, the LED emits stable and detectable red light after the quality of Si-QD film is improved.