Synthesis Of CuInS₂ Quantum Dots And Carbon Nanodots And Their Photoelectric Properties

Semiconductor quantum dots(QDs) have been widely applied in bioimaging and biosensing, photocatalysis, light-emitting diodes(LEDs), and photovoltaics(PVs), due to their size-tunable band gap, high photoluminescence(PL) quantum yield(QY), and solution processability. Recently, the power conversion efficiency of QD-based solar cells(QDSCs) has reached 5-8%. However, the intrinsic toxicity of Cd or Pb contained in those heavy-metal-based QDs potentially hinders our living environment and their commercialization. Cu In S2 QDs and carbon nanodots(CDs), owing to their low toxicity, have recently drawn significant attention. Ternary chalcopyrite Cu In S2 is one kind of ideal semiconductor materials for designing QD-PVs, due to low toxicity, high absorption coefficient, and direct band gap of 1.5 e V which matches well with the solar spectrum. CDs have been regarded as a viable alternative to organic dyes and heavy-metal-based QDs in many scientific fields, due to their good stability, low toxicity, superior resistance to photobleaching, and excellent biocompatibility. Despite all, the photoelectric properties of Cu In S2 QDs and CDs are less understood and the performances of PVs based on them are much lower than those based on QDs containing Cd or Pb. In addition, strong solid-state-aggregation-induced PL quenching of CDs severely limits its application in CD-LEDs. Therefore, the study of the photoelectric properties of Cu In S2 QDs and CDs and CD-based solid-state materials with high luminescence is very important to optimize the performances of photoelectric devices based on Cu In S2 QDs and CDs. Herein, We study the electron transfer(ET) processes from Cu In S2 QDs to Ti O2, the ET and charge separation processes between non-long-chain-molecule-functionlized CDs with visible intrinsic absorption and Ti O2, and efficient solid-state PL of CDs. The original works are organized as follows:1. We have synthesized Cu In S2/Cd S and Cu In S2/Zn S core/shell QDs with different core sizes and shell thicknesses by hot-injection method. The ET processes from Cu In S2 core/shell QDs to Ti O2 were investigated by time-resolved PL spectroscopy. After attaching Cu In S2 core/shell QDs to Ti O2, a significant shortening in the PL decays was clearly observed, indicating the ET from Cu In S2 core/shell QDs to Ti O2. The ET rates and efficiencies from Cu In S2/Cd S core/shell QDs to Ti O2 were superior to those of Cu In S2/Zn S core/shell QDs with the same core size and shell thickness, and an enhanced ET efficiency was surprisingly observed for 2.0 nm Cu In S2 core QDs after the growth of Cd S shell with appropriate thickness. On the basis of the experimental and theoretical analysis, the effects of band alignment and surface states on photoinduced ET processes from Cu In S2 core/shell QDs to Ti O2 electrodes were well explained, suggesting the rational design of well-performanced QD-PVs.2. We have synthesized the CDs with absorption band in the visible(CDs-V) and ultraviolet(CDs-U) region by microwave and thermal decomposition methods with the starting materials of citric acid and urea, respectively. The ET processes from non-long-chain-molecule-functionlized CDs with visible intrinsic absorption(CDs-V) to Ti O2 were investigated by time-resolved PL spectroscopy, and efficient photoinduced ET from CDs-V to Ti O2 with rate of 8.8×108 s-1 and efficiency of 91% was achieved in the CDs-V/Ti O2 composites. The CDs-V/Ti O2 composites exhibited excellent photocatalytic activity under visible light irradiation, superior to pure Ti O2 and CDs-U/Ti O2 composites, indicating that visible photoinduced electrons and holes in CDs-V/Ti O2 composites could be effectively separated. CD-sensitized Ti O2 solar cells were prepared. The IPCE curve of pure Ti O2 solar cells was almost zero in the visible region, and the IPCE curve of the CDs-V-sensitized Ti O2 solar cells was obviously enhanced in the visible region, demonstrating the sensitization effect of CDs-V on Ti O2. By comparing the IPCE spectrum of the CDs-V-sensitized Ti O2 solar cells with the absorption spectrum of CDs-V/Ti O2 composites, the visible photoinduced charge separation between CDs-V and Ti O2 should be efficient.3. We have synthesized the green(g-CDs) and blue(b-CDs) luminescent CDs by microwave and thermal decomposition methods with the starting materials of citric acid and urea, respectively, and provided an universal technique for preparing efficient CD-based phosphors. The CDs can be effectively adsorbed on the starch particle surfaces through hydrogen bonding, and effective dispersion of CDs on the surfaces of starch particles can suppress the non-radiative decay processes and PL quenching induced by aggregation of solid CDs, decelerating the non-radiative decay rate from 3.7×108 s-1 for the solid-state CDs to 0.43×108 s-1 and accelerating the radiative decay rate from 0×107 s-1 to 4.25×107 s-1. In addition, the starch matrix neither competed for absorbing excitation light nor absorbed the emission of CDs, leading to efficient PL emitting. As a result, the starch/CD phosphors with QY of ~50% were obtained. The starch/CD phosphors showed great potential in temperature sensors, phosphor-based LEDs, and patterning. The starch/CD phosphor-based LEDs emitted a cool white light with CIE coordinates of(0.26, 0.33) at an optimized current of 50 m A.