Incorporating double-heterojunction in anisotropic semiconductor nanocrystals for novel optoelectronic applications
As semiconductor heterostructures play critical roles in today's electronics and optoelectronics, the introduction of active heterojunctions can impart new and improved capabilities that will enable the use of solution processable colloidal quantum dots (QDs) in future devices. Such heterojunctions incorporated into colloidal nanorods may be especially promising since the inherent shape anisotropy can provide additional benefits of directionality and accessibility in band structure engineering and assembly. Here, we develop double heterojunction nanorods where two distinct semiconductor materials with type II staggered band offset are both in contact with one smaller band gap quantum dot. The double heterojunction can provide independent control over the electron and hole injection/extraction processes while maintaining high photoluminescence yields and shape anisotropy. Developing new synthetic routes for the formation of ideal double heterostructures, we find that unexpected etching of nanocrystals by one of the most commonly used metal precursors, metal oleates. Especially, Zn-oleate is shown to etch CdS nanorods anisotropically, where the length decreases without a significant change in the diameter. Sodium oleate enhances the etch rate whereas oleic acid alone does not cause etching, indicating the importance of counter cation on the rate of oleate induced etching. Subsequent addition of Se precursors to the partially etched nanorod in Zn-oleate solution can lead to epitaxial growth of CdSe particles rather than the expected ZnSe growth despite an excess amount of Zn precursors being present. The composition of this epitaxial growth can be varied from CdSe to ZnSe depending on the amount of excess oleic acid or the reaction temperature. Similar tuning of composition can be observed when starting with collinear CdSe/CdS/CdSe rod/rod/rod heterostructures and spherical CdS (or CdSe/CdS core/shell) nanocrystals. Conversion of collinear rod/rod/rod structures to barbells and interesting rod growth from nearly spherical particles among other structures can also result due to the initial etching effect of metal oleates. These observations have important implications on our understanding of nanocrystal heterostructure synthesis and open up new routes to varying composition and morphology of these materials. One of the most interesting applications of the semiconductor nanocrystals is display device applications such as LEDs incorporating QDs (QD-LEDs) as the electroluminescent layer. Recent advances in QD-LEDs have led to efficiencies and brightness that rival the best organic LEDs. Nearly ideal internal quantum efficiency being achieved leaves light outcoupling as the only remaining means to improve external quantum efficiency (EQE) but that might require radically different device design and reoptimization. However, the current state-of-the-art QD-LEDs are based on spherical core/shell QDs, and the effects of shape and optical anisotropy remain essentially unexplored. Here, we demonstrate solution-processed, red-emitting double-heterojunction nanorod (DHNR)-LEDs with efficient hole transport exhibiting low threshold voltage and high brightness (76 000 cd/m2) and efficiencies (external quantum efficiency (EQE) = 12%, current efficiency = 27.5 cd/A, and power efficiency = 34.6 lm/W). EQE exceeding the expected upper limit of ∼8% (based on ∼20% light outcoupling and solution photoluminescence quantum yield of ∼40%) suggests shape anisotropy and directional band offsets designed into DHNRs play an important role in enhancing light outcoupling. With increasing demand for complex, multifunctional electronics in light-weight and compact format, a dual ability to convert electricity into different forms of energy/signal including mechanical, thermal, and radiant and, conversely, to generate electrical energy/signal from these other forms is an attractive feature. However, developing such a two-way electronic device that performs both roles reasonably well may be difficult since improving performance of one function may lead to degradation of the other. Such a challenge can be seen in LEDs that also function as photodetectors (PDs). For example, donor-acceptor blends often used for efficient charge separation in organic photovoltaics or PDs usually quench photoluminescence and therefore are unlikely to perform satisfactorily as electroluminescent materials in OLEDs. Separate single-function devices (e.g., separate LEDs and PDs) can be integrated into systems that can perform reversible processes, but the design requires twice as many devices and a more complex fabrication process. Here, we demonstrate highly efficient and fast light-responsive LEDs based on semiconductor nanocrystal quantum dots. In particular, anisotropic heterostructured nanorods are used as both light emitting and detecting materials. Band offsets designed into the nanorods can facilitate both charge recombination and separation and the anisotropic shape enhances light outcoupling and collection, ultimately allowing one device to function as both high performance LED and PD. Despite of the multifunctionality and fascinating applications of double heterostructures in nanocrystal system, in case of cadmium-based II-VI semiconductors, the presence of toxic elements has rendered their practical usage unlikely or at best difficult. We also develop Cd-free luminescent DHNRs with I-III-VI compound semiconductors. Based on the same etching and re-growing effect investigated with Cd-based materials, Facilitating those metal oleate in I-III-VI2 semiconductors enables to achieve the direct formation of heterostructures with CuInS2 and CuInSe2 on CuGaS2 nanorods, which has not been reported yet anywhere to the best of our knowledge. Interesting sawtooth structures and branch structures has been analyzed with high resolution TEM, STEM and EDS. After growing ZnSe or ZnS shell on the CuGaS2/CuInSe2 NRHs distinct PL and other interesting/useful optical properties, including large Stokes shift, charge transfer between components, and long lived excitons, were observed.
Double-Heterojunction Nanorods, Double Heterostructures, Anisotropic Semiconductor Nanocrystal, Quantum Dot, Quantum dot light emitting diode (QD-LEDs), QLED, Bidirectional Display, Cd-Free Quantum Dots
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign