Synthesis and optical studies of infrared emitting semiconductor nanocrystals

Abstract

Scientific interest in colloidal semiconductor nanocrystals (NCs), also called quantum dots (QDs), has grown tremendously in the last few decades, due to the large variety, combination of materials, and quantum confinement effects making them exhibit superior optical properties. The research in the field of narrow band gap, near-infrared (NIR) emitting materials is exploding with new synthetic methods, and applications. This thesis discusses our recent studies on NIR emitting materials including CdHgTe alloyed NCs, CdHgSe nanorods, aqueous and organic based HgTe NCs and PbS NCs. It is divided in 7 chapters. First chapter introduces general nanocrystal materials, quantum confinement effect, widely used synthetic approaches and possible anisotropic structures previously reported in literature. It covers the importance of infrared emitting QDs highlighting some of their applications. Further it reviews the progress of ion exchange in a variety of nanocrystal structures from the earliest accounts dating back over two decades ago to the present day. It introduces range of synthetic approaches most often used to carry out ion exchange, mainly focusing on cation replacement strategies, and then describe the rich variety of nanostructures these techniques can bring forth. It also describes some of the principles that are used to establish the relative ease of exchange and to systematically improve the process where the basic energetics is less favorable. To help further the understanding of the underlying fundamentals we have gathered together useful data from the literature on solubility, cation and anion hardness, ligand and solvent Lewis acid or base strengths for a wide range of chemical species generally used. The second chapter describes the general instrumental techniques used for characterization of nanomaterials. The third chapter discusses the scope of composition tuning on the optical properties of CdTe NCs (NCs) with incorporated Hg2+ ions, resulting in the alloy CdxHg(1-x)Te QDs. Generally, CdTe NCs emit in visible range of electromagnetic spectrum. However, on incorporation of Hg2+ ions it is possible to access the near infrared (IR) window, here demonstrated up to 1200nm, which is important for bio-labeling in tissue imaging, luminescent QD solar concentrators, photodetector and photovoltaic applications. The reaction kinetics of Hg2+ ion exchange reaction on CdTe QDs was studied. It highlights some important physical parameters for CdxHg(1-x)Te NCs, e.g. emission energies as a function of composition, Stokes shift, extinction coefficients, radiative lifetimes, quantum yields and rate constants. The variation of QD alloy band gap with the short wavelength intrinsic absorption coefficient which is linearly related to the effective alloy composition was indicative of a compositional gradient rather than a uniform distribution of both cations. In the initial stages of cation exchange for CdxHg1-xTe alloy QDs, the process has a marked effect - initially the photoluminescence quantum yield (PL QY) dropped by up to three orders of magnitude in some cases. However, on the longer timescales and more so where the amount of Hg2+ added was higher, both the PL QY and the PL lifetimes recovered (and in the latter case exceeded that of the CdTe starting material). The fourth chapter highlights the importance of crystal structure while choosing the systems for ion exchange processes. We have followed the partial ion exchange process of Hg2+ ions for Cd2+ ions in CdSe nanorods. Whilst CdSe nanorods, grown in organic solution have a hexagonal wurtzite structure, the limiting case for exchange, HgSe is more commonly encountered as a cubic zinc blende system. The exchange process was carried out at room temperature and pressure in an aqueous environment after phase transfer of the original CdSe nanorods. Consequently it was observed that under ambient conditions the exchange process terminated with an average composition of only Cd0.9Hg0.1Se. Following the changes during the process by optical spectroscopy and HAADF-STEM it was observed that the Hg2+ ions diffused into the rods to a point limited by stacking fault formation due to the different lattice structures of the two limiting cases of zinc blende and wurtzite. HAADF-STEM and EDS analyses also confirmed that the Hg substitution did not occur uniformly throughout individual nanorods, with Hg-poor and Hg-rich regions coexisting around the stacking faults. The formation of NIR emitting alloyed CdxHg1-xSe nanorods in an aqueous medium highlights the subtle dependence of the ion exchange process on the differences in the crystal structures of the two endpoint lattices. The fifth chapter discusses state of the art for aqueous colloidal HgTe QDs with PL emission from 900-1500 nm, important for telecommunication applications. We experimentally determined sizing curve for HgTe QDs and compared it with tight binding calculations, pseudo-potential calculations and others from literature. Following the Maxwell Garnett approach, intrinsic absorption co-efficient at 405 nm is calculated. Further, we estimated the molar extinction co-efficient at 405 nm highlighting its significance while determining concentration of QDs in solution. Lastly, we report quantum yield and radiative lifetime dependence on HgTe QDs sizes. The sixth chapter discusses HgTe QDs and PbS QDs synthesized via organic route. We synthesized HgTe QDs in DDT-oleylamine reaction mixture attempting to reach 3 μm emission range for methane gas sensing while fine tuning the reaction parameters. We covered the entire 900 nm to 3.3 μm emission window while highlighting some of the potential interfering factors e.g. solvents, ligands, cuvettes, black body radiations which need to be taken care of for further studies on this system. The second section of this chapter addresses PbS QDs grown in oleylamine- TOP reaction mixture with high quantum yield and exceptionally long radiative life times of the order of 1 μs. We studied the size dependence of decay times which is in good agreement with theoretical values for PbS QDs reported in literature. We calculated Hyang Rhys parameter, S, a measure of electron-phonon interaction strength for PbS QDs and compared it with high band gap CdTe QDs. The last chapter provides the conclusion and outlook for this thesis.

Date of Award	15 Jul 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Andrey ROGACH (Supervisor)