High-quality Blue-green Emitting Core/Shell Quantum Dots And Their Application In Light-Emitting Diodes

Semiconductor quantum dots (QDs) have narrow and stable fluorescence emissionwith tunable fluorescence peak position from ultraviolet (UV) to infrared (IR). The narrowemission and high quantum yields (QYs) of QDs make them well suited to biolabellingapplication because of the multiple colours emission with different QDs under a singleexcition light source. The excellent properties show great promise for use in the fields ofquantum dot light-emitting diodes (QD-LEDs), biology, solar cells and other prospects.However, these binary QD systems are still not very easy to deliver high quantum yield(QY) and stable blue-green emission between450nm and530nm due to the unstable fineparticles (the size of those quantum dots are too small which is less than2nm) are difficultto passivate by overcoating shells. However, the wavelength at450nm and530nm are ofparticular interest for the preparation of QD-based blue-green LEDs and as “must-have”emitters for next-generation displays and white solid-state lighting. Therefore, it is still veryimportant to exploit new synthesis methods since the efficient and controllable synthesis ofhigh quality blue-green QDs is still a challenge topic. Alloy quantum dots can achieveblue-green light emission and obtain the desired light emission not only by adjusting thesize but also by altering the component.(1) The optical properties of the core-shell quantum dots can be adjusted by changing thelight emitting wavelength of core nanocrystal, QY and the exciton lifetime. What ismore, core-shell quantum dots have low environmental sensitivity due to thehigh-activity core nanocrystals being isolated from the external environment by the shellmaterial epitaxial growth and have high resistance to photo bleaching; The shellmaterial can be passivated onto the surface of the nuclear nanocrystals effectively,which greatly improved the fluorescence quantum yield. But previous methods forsynthesizing quantum dots were involved using alkylphosphine (such astrioctylphosphine (TOP) and tributylphosphine (TBP)). It is well known that alkylphosphine and organometallic salts are hazardous, unstable, and expensivematerials and generally glove box is requested in the operation, which greatly limits thecommercial application of QDs. Therefore, the synthesis of stable blue-green QDswithout participating of TOP/TBP is an urgent problem to be solved. This paper takesthese questions as a starting point, synthesis of high-quality blue-green emitting alloyedquantum dots and core/shell quantum dots. The main results and conclusions are asfollows:High quality CdSSe quantum dots with blue-green emission have been preparedby tuning the composition with phosphine-free method. In order to achieve a highfluorescence quantum yield as well as high stability, we introduced a buffer layer ofCdS/ZnCdS into the middle of the CdSSe core nanocrystals and ZnS shell to reducelattice mismatch. This structure can effectively transit the lattice constant of the CdSSecore to the ZnS shell. As a result, this structure has been proved to be a successfulmethod to improve the fluorescence intensity as well as reduce the interfacial tensionand defects. The QY of the as-prepared blue-green emission CdSSe/CdS/ZnCdS/ZnScore/shell quantum dots can reach50-70%.(2) High quality ZnCdSSe quantum dots with450-550nm emision have been prepared bytuning the composition with phosphine-free method. Due to the small lattice mismatchbetween ZnCdSSe core quantum dots and the ZnS shell, ZnS was selected to directlyepitaxial grow on ZnCdSSe quantum dots. This approach not only simplifies theexperimental operation, but also greatly improves the fluorescence quantum yield whichcan reach50-75%with wavelength range of450-550nm. Subsequently theas-synthesized blue-green light-emitting core-shell quantum dots have been used forQD-LED device to test the electroluminescent properties. The blue QD-LEDs show anunprecedentedly high brightness over4700cd/m~2, which is the highest value everreported.