Luminescent properties of semiconductor nanocrystals

Abstract

This thesis focuses on various aspects of nanocrystals (NCs) based on transition metal chalcogenide semiconductors (e.g. CdSe, CdTe, ZnSe). Different synthetic approaches have been developed for creating high quality monodisperse luminescent colloidal nanocrystals. The synthesis of these different nanosized crystals opened the opportunity to study and exploit the fascinating size-dependent physical and optical behavior of semiconductors in the nanosize regime. Detailed studies were performed on resonant energy transfer in host-guest systems between conjugated polymers and isotropic NCs, and between isotropic NCs and anisotropic NCs. The distinct optical properties make these luminescent NCs particularly interesting as the emissive component in lighting applications. These materials were therefore studied as the emissive component in thin film light-emitting diodes (LEDs). Some of the salient results are described in the following paragraphs. The thesis starts with a general introduction on the history and general properties of colloidal semiconductor nanocrystals in chapter 1. The physical principles of size quantization are briefly explained and followed by an introduction on the effects of the particle shape and composition on the optical and electronic properties. The application of nanocrystals in luminescent devices is discussed, setting the stage for the overall aim of the thesis. Chapter 2 describes a study of energy transfer in a host-guest system consisting of a blue-emitting poly(2,7-spirofluorene) (PSF) donor polymer and red-emitting CdSe/ZnS core-shell quantum dots as acceptor in solid films, using time-resolved optical spectroscopy, and in electroluminescent diodes. By introducing an electron transport layer in the LED the dominant pathway for quantum dot emission could be modified from energy transfer from the polymer host to direct electron-hole recombination on the quantum dot. This resulted in an increased device efficiency to 0.32 cd/A. The preparation of highly luminescent, anisotropic CdTe/CdSe colloidal heteronanocrystals is described in chapter 3. The reaction conditions used (low temperature, slow precursor addition, and surfactant composition) resulted in a tunable shape from prolate to branched CdTe/CdSe nanocrystals. Upon CdSe shell growth, the heteronanocrystals show a gradual evolution from Type-I (direct recombination holes and electrons in one material) to Type-II (indirect recombination of holes and electrons at the interface of two materials) optical behavior. These heteronanocrystals show a remarkably high photoluminescence quantum yield (up to 82%) and negligible thermally induced quenching up to temperatures as high as 373 K. Such high quantum yields and stability are unprecedented for Type-II nanocrystals. Chapter 4 shows a novel synthesis leading to highly luminescent CdTe nanocrystals using Li2[Cd4(SPh)10] clusters as a reactive Cd cluster compound at relatively low temperature, making it a safe precursor for the large scale synthesis of CdTe nanocrystals. The nanocrystals show high luminescent quantum yields up to 37% for branched CdTe nanostructures, and as high as 52% for CdTe/CdS core-shell heterostructures. CdTe/CdS nanocrystals were used to make LEDs in combination with organic layers for electron and hole injection. The devices show a maximum luminance efficiency of 0.35 cd/A. The first NC LEDs that emit linearly polarized light using macroscopically oriented quantum rods are described in chapter 5. In these devices a thin layer of quantum rods with an aspect ratio of 2.5, which were macroscopically oriented by a simple rubbing technique, has been used as an emitter. Devices were constructed by sandwiching the oriented quantum rods between two organic layers with electron and hole conducting properties to obtain improved injection and emission properties. In this way a polarized LED with an emission at 620 nm, luminance efficiency of 0.65 cd/A, and external a quantum efficiency of 0.49% was obtained. The intensity of the electroluminescent light polarized in the direction parallel to the long axis of the rods was 1.5 times higher than in the perpendicular direction. Excited state energy transfer from spherical green-emitting nanocrystals as donor to rod-shaped red-emitting nanocrystals as acceptor is demonstrated in chapter 6. For this purpose highly luminescent red-emitting core-shell CdSe/CdS quantum rods were synthesized and mixed with green-emitting core-shell CdSe/CdS quantum dots. For this donor-acceptor combination the Forster distance is less than 6.6 nm, which is close to sum of the diameters of the dots and rods. Hence, only quantum dots directly neighboring a quantum rod will efficiently participate in the energy transfer. A simple rubbing technique was used to uniaxially align the quantum rods dispersed in thin films of quantum dots. Such mixed films showed polarized red emission, while the excitation remained unpolarized. Highly luminescent colloidal narrow ZnSe:Mn doped nanowires are prepared in chapter 7, using preformed Li4[Zn10Se4(SPh)16] and Li2[Zn4(SPh)10] clusters together with elemental selenium and manganese stearate at moderate temperatures. The nanowires are highly crystalline and show a bright manganese photoluminescence. The wire diameter could be changed between 1 and 3 nm, resulting in aspect ratios above 80 for 2.5 nm wide nanowires. The emissive properties were further improved by the formation of a CdSe shell on the ZnSe surface, leading to colloidal nanowires with a luminescence quantum yield up to 40%. The reaction was tunable between spherical particles and anisotropic nanowire formation by changing the selenium content. Aligned ZnSe:Mn doped nanowires in a flow cell showed a weak polarized Mn emission with polarization perpendicular to the long axis of the nanowires. In conclusion, the work described in this thesis show several syntheses of highly luminescent semiconductor NCs, with high control over size, shape, and composition. The exciting and distinct optical properties of these NCs have been studied, in combination with the energy transfer between the NCs and polymers. This enabled the creation of both nanophosphors and LEDs, exhibiting the same exciting optical properties but then on a macroscopic scale.