Synthesis And Applications Of Supra-(Carbon Nanodots) And Nanocrystals For Charge Transport Layers

Colloidal quantum dots(QDs), as an important member of semiconductor nanocrystals, exhibit very unique quantum confinement and surface effects,which make them have emission-wavelength tenability in applications, narrow emission linewidth, high photoluminesece quantum yield(PL QY), and good photostability. After functionalizing with ligands, QDs have demonstrated solution-processing, good compatibility with spin-coating and ink-printing technology, which can be considered as key materials for the next generation of electroluminescent flat-panel display and solid-state lightings. After steady progresses for developing QDs, various high PL QY QDs have been synthesized by researcher, including II-VI and III-V semiconductor based QDs exhibiting excitons related emission, Mn, Cu-doped QDs orginating from doping cations or radiative recombination centers related emission, and IV semiconductor based Si, C QDs with a controversial luminescent mechanism. With the advance in materials and device architectures as well as the research of the device physics, performances of light-emitting devices(LEDs) based on colloidal QD emitters, such as external quantum e?ciency(EQE), brightness, and device stability, have been pushed forward. Currently, solution-processed QD-LEDs based on Cd Se QDs exhibiting exciton emission have been developed by Chinese scientists, maximum EQE of up to 20.5 percent, maximum brightness of up to 42000 cd m-2 and operational lifetime of more than 100,000 hours. However, by contrast, eco-friendly C based QD-LEDs have a low maximum brightness of up to 90 cd m-2, an intangible electroluminescent mechanism, and a hopeless commercialization. Therefore, it is meaningful to improve the performances of cadmium-free C based QD-LEDs through optimizing the luminescence of C QDs as well as the charge transport layers. Based on the above considerations, the original works are listed as follows:1. We synthesized slightly reduced graphene oxide(GO) through ammonia reduction, which was used as hole injection layers of Cd Se QD-LEDs by solution-processing, and then investigated the performances of QD-LEDs through controlling the degree of reduction of GO. It was found that for the conventional Cd Se QD-LEDs spin-coating with isopachous reduced GO as hole injection layers, the performances of QD-LEDs were largely improved after the ammonia reduction, and the best device was fabricated with GO by 3h ammonia reduction. Combined UV-photoelectron spectroscopy(UPS), absorption spectroscopy(Abs) and atomic force microscopy(AFM) indicated that the conductivity of GO was enhanced, and the work function of GO was elevated after the reduction, which separately enhanced hole transport and inhibited hole injection. The best device was realized through a trade-off between hole transport and hole injection in the Cd Se QD-LED, which was fabricated with GO by 3 h ammonia reduction.2. We prepared Ti O2 and Zn O nano particles via low temperature solution precipitation, which were utilized as the electron transport layer(ETL) to deeply probe the electroluminescent mechanism of inverted Cd Se QD-LEDs. We found that doping with Cs CO3 in Zn O would enhance electron injection from ETL to QD layer, further improved the luminance and efficiency of QD-LEDs. Meanwhile, higher doping level would decrease the conductivity and increase the surface roughness of ETL, which was adverse to the properties of QD-LEDs. Moreover, Ti O2, as ETL, had the potential for further improving the efficiency of QD-LEDs by comparing the lifetime between QD layer and ETLs(Ti O2 and Zn O).3. We synthesized partially alkyl-chain-functionalized amphipathic carbon dots(CD-Ps), and obtained supra-carbon-nanodots(supra-CDs) through the self-assembly of CD-Ps, whose luminescence show weak emission in toluene and decompose when meeting water, resulting in strong photoluminescence. And supra-CDs/paper composite with the same water-induced enhanced photoluminescent behavior was achieved via coating paper with supra-CDs. With the supra-CDs/paper composite, we demonstrated water-jet printing luminescence patterns and mapping human sweat pore patterns by pressing a fingertip on the composite. Time-resolved PL spectroscopy, morphology characterization, fluorescence microscopy and NMR characterization unveiled the mechanism for the water triggered luminescent enhancement, and further demonstrated practical applications in optical information storage, forensic and healthcare. In addition, we fabricated novel QD-LEDs using full alkyl-chain-functionalized CDs(CD-Fs) as emitters, whose emission wavelengths were changed with the increase of the applied voltage.