Efficient blue luminescence from colloidal CdSe quantum-dot quantum-well nanocrystals
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Efficient blue luminescence from colloidal CdSe quantum-dot quantum-well nanocrystals Y. Lu and X. A. Cao Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506, USA ABSTRACT CdS/CdSe/ZnS quantum dot quantum well (QDQW) nanocrystals were synthesized using the successive ion layer adsorption and reaction technique. CdSe QWs with a well width of 1.05 nm emitted blue light at 467 nm with a spectral full-width-at-half-maximum of ~30 nm. It was found that a 3-monolayer ZnS outer cladding layer can effectively passivate the QDQW structures, leading to a ~35% quantum yield (QY) of the QW photoluminescence. QDQW lightemitting diodes (LEDs) with blue QW electroluminescence (EL) were fabricated. The devices with an emitting layer comprising QDQWs embedded in a poly(N-vinylcarbazole) host were five times brighter than LEDs based on closely-packed QDQWs. However, the overall EL of the devices was dominated by interface state emission due to poor charge injection into the QDQWs. INTRODUCTION The size-tunable optical properties of colloidal quantum dots (QDs), along with their high luminescent efficiency, good stability and saturated color emission, make them particularly attractive as the active materials in light-emitting devices (LEDs) [1.2]. Group II-VI semiconductor QDs, particularly those based on CdSe(S), can be synthesized using wellestablished chemical colloidal processes, have been a subject of extensive research over the past decade [1]. To date, efficient red and green LEDs have been realized based on CdSe QDs with various dot sizes, but QD-LEDs emitting efficient blue light have yet to be developed due to a lack of appropriate QD materials [3,4]. In principle, blue light emission ~ 470 nm needed for flat panel display applications may be obtained from CdSe QDs smaller than 2 nm [5]. However, such small QDs with narrow size distributions and good quantum efficiencies are difficult to synthesize. Overcoating such small QDs with a wider bandgap passivating shell, which is necessary to improve the QD luminescent efficiency and stability, is also difficult. In addition, small QDs have a small absorption cross section, which leads to slow excitonic energy transfer into the QDs. All these factors make CdSe core QDs a poor choice for achieving efficient blue luminescence. The recent development of colloidal nanocrystals with diverse internal heterostructures provides an alternative means for tuning the optical properties of nanoparticles based on one particular material [6-10]. Among those unique nanocrystal heterostructures are quantum dotquantum wells (QDQWs), which are synthesized with a core of a large bandgap material followed by an intermediate shell of a smaller bandgap material and an outer shell of the same large bandgap material as the core. In analogy with planar QW structures, the intermediate shell acts as a QW, whereas the core and outer shell function as two cladding layers. Examples of QDQWs which have been studied include CdSe/HgS/CdSe [7], ZnS/CdS/ZnS [8],
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