Synthesis Of Dual Emitting Nanocrystals Based On Band Gap Engineering And Their Applications In WLED

The most interesting aspect of quantum dots (QDs) is the fact that they arebandgap-tunable materials by the size, and, as a result, their fluorescencecharacteristics can also be varied. When compared to organic dyes, QDs have severaloptical merits, such as a narrow, tunable, and symmetric emission spectrum andoutstanding photochemical stability. For this reason, QDs have found manyapplications in, for instance, light-emitting diodes (LEDs), photovoltaic devices, andbiolabeling.But during in depth study of nanocrystals application, especially in theestablishment of new photoelectric devices, there is an urge to achieve an exactknowledge regarding their band structures and electronic properties．To date, the mostcommon methods for the determination of the band structures parameters have beencyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), and X-rayabsorption spectroscopy (XAS). These techniques for determining the energy levels ofthe NCs generally involve sophisticated instruments and tedious sample preparationand/or complicated operation procedures. Thus, determination of the energy bandpositions of the NCs still remains a challenge. In addition, band gap engineering ofelectronic properties of the nanocrystals according to the band gap structureparameters to obtain unique properties and/or multiple functions materials as greaterusagevalue and realistic directive significance. In this thesis, the original works areorganized as follows:In the second chapt, we developed a dopant approach to determine HOMO and LUMO levels of the semiconductor NCs based on electronic spectroscopy methods.The experimental results were found to be in good agreement with those obtained bythe conventional CV method and theoretical calculations. Compared with that of thetraditional determination method, the energy level determined by the doping methodbased on electronic spectroscopy methods is more quickly and accurate and theexperimental conditions is moderate. Such an approach provides an alternative wayfor facile determination of the HOMO and LUMO levels of the semiconductor NCs,which is of great technical and fundamental importance in rational design of thedevices with the NCs as building blocks. In the third chapt, we applied the newmethod to determine energy levels of different NCs with different compositions, sizes,and shapes. We also studied the relationship between different nanocrystals with thesame band gap.In the forth chapt, according to the band gap structure parameters we haveobtained before, we designed a single component colloidal semiconductor nanocrystalphosphor consisting of a d-core and a q-well separated by a layer of ZnS, which havebeen well-characterized using a combination of optical and structural techniques. Bycontrol of the thickness of ZnS barriers, as-prepared d-core/q-well QDs presented dualemission bands, one of which being attributed to d-cores, and the other resulting fromq-wells. The dual emissive peaks are flexibly tunable in a range from the visibleregion to the near-infrared region by simple control of the core size and the shellthickness, respectively. Surface termination with ZnS was found to greatly improvequantum yields of samples, as well as better photochemical and thermal stabilities. AWLED lamp was fabricated using a commercial blue-LED chip combined with theoptimal phosphors as color converters. As-fabricated WLED showed not onlyimproved color rendering properties, compared to commercial ones, but a CRI valueof91and color temperatures of50005300K. These results indicate that theseas-prepared single-phase phosphors are promising as versatile light-emitting materialsfor various applications ranging from solid-state lighting to bioimaging, because ofthe flexibly tunable dual emission position in the Vis-to-NIR region.In the fifth chapt, to address the self-absorption, low PL QY and thermal sensitive issues,we have successfully prepared a single-component colloidal NCphosphor consisting of Cu:CdS–ZnSe–ZnS core–shell–shell NCs by the bottom-upmethod, which has been well characterized using a combination of optical andstructural techniques. As-prepared NCs showed dual emission bands, one of whichwas attributed to the Cu:CdS cores and the other resulted from the CdS–ZnSe type IIcore–shell structure. The dual emissive peaks were flexibly tunable ranging from thevisible to the NIR region by simple control of the core size and shell thickness,respectively. Surface termination with ZnS greatly improved the QYs of samples (byup to56%), and resulted in better photochemical and thermal stabilities. Thesesingle-phase phosphors showed nearly no self-absorption, which indicated that theywere an ideal broad band-emitting material for applications in white LEDs. A WLEDlamp was fabricated using a commercial blue LED chip combined with the optimalphosphors as color converters. As-fabricatedWLEDs showed improved colorrendering properties with a CRI of90and color temperature of5850K. These resultsindicated that these as-prepared singlephase phosphors were promising versatilelight-emitting materials for various applications ranging from solid-state lighting tobioimaging because of their flexibly tunable dual emission position in the visible toNIR region.