Quantum-dot/Carbon Nanomaterial Composites: Controlled Fabrication, Characterization And Their Photoelectrical Property

In recent years, quantum-dot (QD)/carbon nanomaterial nanohybrids are of greatinterest for the development of next generation photovoltaic (PV) devices. PbSquantum dots (QDs) has a narrow band gap, large excitonic Bohr radius and canabsorb the light in near infrared range; however, the photoinduced electrons and holesare easy to recombine. In order to enhance the charge carrier separation and transport,many efforts have been made to combine QDs with carbon nanomaterials. Amongvarious carbon nanomaterials, the high surface area and unique electrical rendercarbon nanotubes and graphene are of great interest. In this context, we proposed toconstruct a nanocomposite material embracing QDs and carbon nanomaterials (carbonnanotube and graphene), the latter of which can effectively prevent the recombinationof photoinduced charge carriers in QDs.In this thesis, the binary PbS and ternary PbSxSe1-xsolid solution QDs withdifferent particle sizes and different morphologies were prepared by an organometallicmethod under different reaction conditions. Characterization on the synthesized QDswere carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM)and transmission electron microscopy (TEM), and the results showed thatnanoparticles were well crystallized in regular shapes with a monodispersion inparticle size. Furthermore, UV-Vis-NIR spectrophotometer and photoluminescence(PL) were employed for the study of photophysical properties. The results indicatedthat the synthesized QDs exhibited characteristic absorption in the near infraredregion, and the band gap energy could be tailored by the size control (quantizationeffect) and/or by the composition variation of QDs. For PbS QDs, the band gap couldbe tuned with the change of QD size by controlling the reaction conditions; while forthe solid solution QDs, the band gap could be modulated simply by varying thecomposition without changing the particle size.Next, a post-synthesis method was adopted to prepare the QDs/MWCNT andQDs/RGO nanocomposites, respectively, in which the QDs (PbS, PbSxSe1-x) werehybridized well with carbon nanomaterials (MWCNT, RGO), as evidenced by TEMobservation. Moreover, by varying the amount of quantum dots relative to carbonnanomaterials, different coverage of QDs on the surface of carbon nanomaterialscould be realized. UV-Vis-NIR spectrophotometer absorption spectra showed that theabsorption of nanocomposite was extended into the NIR wavelength region due to theinvolvement of QDs. PL measurements showed that compared to the pure QDs, boththe PL intensity and the fluorescence decay time of nanocomposite decreaseddramatically, implying apparent electron transfer from the QDs to the carbon nanomaterials. Accordingly, an apparently enhanced photoelectrical performance wasobserved in the nanocomposite composed of carbon nanomaterials and quantum dots.