Cation-Exchange Synthesis and Optical Properties of Anisotropic Semiconductor Nanocrystals


Student thesis: Doctoral Thesis

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Award date21 Aug 2023


Colloidal semiconductor nanocrystals, often denoted as quantum dots, are still sparking the interest of researchers worldwide while already being implemented into the most advanced products of the display industry. Their rise as promising materials was provided by narrow emission profiles, which can be easily tuned by the nanocrystal size because of quantum confinement. The size-dependent nature of quantum dot optical properties has led to development of a plethora of synthesis protocols that allows to fabricate monodisperse ensembles of nanocrystals with their luminescence fitting the modern standard Rec.2020 for ultra-high-definition television.

Apart from the size, the nanocrystal shape is yet another parameter which allows researchers to manipulate optical properties of quantum dots. Nanocrystals elongated in one direction, i.e. nanorods, lose quantum confinement in that particular direction, which comes with a number of benefits, such as enhanced carrier mobilities, increased absorption cross-section, and polarized emission along the long dimension of the nanorod. These properties offer potential for numerous applications that include optoelectronic devices such as light-emitting diodes (LEDs), displays, and lasers, through to roles as photocatalytic elements used in water splitting applications and as a basis in photoinitiators. However, only cadmium-based chalcogenide nanorods have seen the most refined development in terms of the degree of control of their size, shape and their performance parameters so far. Direct synthesis of other semiconductor compounds in the shape of anisotropic semiconductor nanocrystals is still lagging far behind the cadmium-based analogs. Particularly, near-infrared emitting nanorod systems are scarcely known, and their size distribution and emission properties are inferior to their spherical counterparts.

This thesis develops new and extends existing cation-exchange protocols for anisotropic near-infrared emitting nanorods based on copper-indium and mercury chalcogenides. After introducing photophysical phenomena associated with size- and shape-dependent properties of quantum dots and nanorods, the thesis compares direct synthesis methods of cadmium chalcogenides and other systems. Then, after highlighting the major challenges related to the synthesis of anisotropic non-cadmium based quantum dots, the cation-exchange approaches are discussed in the context of using Cd chalcogenide nanorods as templates, whose structure and size distribution remain intact throughout all consequent synthesis stages. The literature overview ends up by outlining major challenges in using cation-exchange for nanomaterials synthesis, particularly rather inferior optical properties of the final products due to the presence of defects and impurities.

In experimental parts, this thesis features the cation-exchange synthesis of CuInS2 quantum dots, characterized by a narrow size distribution and wurtzite structure, which were used as seeds for the synthesis of core/shell CuInS2/CdS nanorods. The wurtzite phase of CuInS2, which is hard to attain by direct synthetic means, is crucial to avoid formation of other competing morphologies, e.g. tetrapods. Changing the size of CuInS2 quantum dots and injection temperature is shown to be effective in tuning the dimensions of core/shell nanorods whose emission wavelength depended both on the core size and shell thickness. Time-dependent photoluminescence studies revealed slowing down of the recombination rate with increasing shell thickening, which ascribes the produced CuInS2/CdS nanorods to the quasi-type II heterostructure. Alloying of the CuInS2 cores with Zn did not perturb the shell growth process but enabled improvement of the photoluminescence quantum yield of the resulting core/shell CuInZnS2/CdS nanorods from 31% for Zn-free analog to 45%, alongside with adjustment of their emission wavelength into 700-900 nm spectral range. The size monodispersity and high photoluminescence quantum yield render CuInS2/CdS and CuInZnS2/CdS as some of the best anisotropic near-infrared emitters reported so far and confirms the effectiveness of the suggested cation-exchange synthetic strategy.

While the fabricated CuInS2/CdS nanorods possess less toxic elements in the core, yet they still contain a large amount of toxic cadmium in the whole structure. In the following chapter, the synthesis of CuInSe2/CuInS2 nanorods derived from CdSe/CdS ones is discussed with an emphasis on Cd-impurities originating from phosphonate ligands of the starting nanorods. Cadmium phosphonates were identified in the ligand shell of the Cu2-xSe/Cu2-xS nanorods, which are the main source of Cd contamination (~1.5-5.0 at.%) in the final composition of CuInSe2/CuInS2 after the final cation-exchange step. Ligand exchange with octylamine and trioctylphosphine oxide results in removal of phosphonates and reduces Cd content below 1%, while subsequent treatment with indium-phosphonate solution enhances the photoluminescence quantum yield of CuInSe2/CuInS2 nanorods from 20-30% up to 34-40% (depending on the core size), which renders them the brightest anisotropic ternary near-infrared luminophores reported so far. Transient absorption and time-resolved photoluminescence studies revealed quasi-type II band alignment in the obtained heterostructure, thus clarifying erroneous band alignment assignment cited in previous related literature. Moreover, CuInSe2/CuInS2 nanorods are shown to have the narrowest Cu+2 related emission, which is a combination of narrow size distribution of CuInSe2 cores attained by cation-exchange and proper surface passivation by anisotropic CuInS2 shell.

In the last chapter, the thesis presents a further expansion of the nanorod emission into the near-infrared spectral range by converting CdTe nanorods into HgTe nanorods using cation-exchange. The synthesis scheme utilizes the same two-step cation-exchange as it was developed for the CuInSe2/CuInS2 system yet introducing different ligand chemistry. Oleylamine is shown to alter the Cu content in HgTe nanorods, which in turn causes the emission blue-shift over the wide spectral range (from 2600 to 1500 nm). Copper content can be further reduced by using asymmetric dimethyl ethylene diamine, which is characterized by strong affinity towards Cu(I) and Cu(II) cations. The XRD studies confirmed formation of the wurtzite HgTe, whose optical properties were studied for the first time. Field effect transistors based on HgTe NRs demonstrated a high linear mobility of 0.04 cm2V-1s-1, and an ON/OFF ratio of 103. Infra-red photodetection characteristics, namely responsivity and specific detectivity reached 1 A/W, and up to 1010 Jones at 1340 nm. It was found that the carge transport characteristics of thin films of HgTe nanorods can be tuned by both surface passivation with different ligands and by varying the residual Cu content in HgTe nanorods, as well as the gate bias and operating tempera-ture of the devices.

In summary, thesis explores cation-exchange synthesis strategies for the fabrication of high-quality anisotropic semiconductor nanostructures, which are promising for a variety of application in near-infrared LEDs, bioimaging, and photodetectors.