Thesis (M.Sc.)--York University, 2003. Graduate Programme in Mathematics. / Typescript. Includes bibliographical references (leaves 115-118). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pMQ99399

The synthesis of transition metal dichalcogenides (TMDs) are crucial to realization of their real-world applications in electronic, optoelectronic and chemical devices. However, the fabrication yield in terms of material quality, crystal size, defect density are poorly controlled. In this work, by employing the up-to-date stack-and-transfer and nano fabrication techniques, synthetic TMDs that obtained from different growth methods with various crystal qualities were studied. In most of the cases, better crystals with lower defect densities and larger crystal domain sizes are preferred. Self-flux method was developed to obtain better quality crystals comparing to the traditional chemical vapor transport, as characterized by lower defect densities. BN encapsulating graphene device platform was utilized and TMDs monolayers with different defect densities was inserted in between the BN/graphene interface, where intrinsic defects from the TMDs disturbed the electronic environment of graphene. With the better TMD crystal insertion, we obtain much better electrical performed device in terms of hysteresis, FWHM of Dirac peak and electron mobility. This device also showed advantage in quantum transport measurements . On the other hand, the presence of defects are not always undesired, especially when it comes to serve as electrocatalysts, in which most of the reactions take place at vacancy sites. However, similar to most of the MoS2 electronic devices, forming barrier-free metal semiconductor contact is the major challenge. We develop a platform that contact resistance could be monitored simultaneously with electrochemical activity. In this platform, the total device resistance is significantly reduced before electrochemical reaction happens while the intrinsic catalytic activity of the MoS₂ can be extracted. With this platform, we found the intrinsic catalytic activity of MoS₂ strongly correlated to H-coverage on its surface. By adding molecular mediator into electrolytes, H-coverage and the resulting HER activity was enhanced via “Catch and Release” mechanism. Molecular simulation was performed to support our experimental results.

Colloidal semiconductor nanocrystals, also known as quantum dots, are an extraordinary class of material, combining many of the most attractive properties of semiconductors with the practicality of solution chemistry. As such, they lie at a unique interface between inorganic chemistry, organic chemistry, solid-state physics, and colloidal chemistry. The rapid advance in knowledge of quantum dots over the past 30 years has largely been driven by interest in their fundamental physical properties and their broad applicability to challenges in nanoscience. However, much less attention has been paid to the chemistry underlying these features. In this dissertation, we discuss the state of nanocrystal chemistry and new insights we have unlocked by taking a bottom-up, chemistry-based approach to nanocrystal synthesis. We will cover these in a case-by-case fashion in the context of four chapters. Chapter 1 covers our CdTe nanocrystal synthesis surface chemistry studies with an eye toward CdTe photovoltaic technology, in which the role of CdTe surfaces is poorly understood. CdTe nanocrystals are traditionally a difficult material to synthesize, particularly with well-defined surface chemistry. In order to enable quantitative surface studies, we looked upstream and re-evaluated CdTe synthesis from the ground up. We identified a CdTe precursor largely overlooked since 1990, cadmium bis(phenyltellurolate) (Cd(TePh)2), and harnessed its excellent reactivity toward a synthesis of CdTe nanocrystals solely bound by cadmium carboxylate (Cd(O2CR)2) ligands. We then use this well-defined material to show that Cd(O2CR)2 ligands bind less tightly to CdTe nanocrystals than CdSe nanocrystals. This finding holds promise for the development of photovoltaics from colloidal CdTe feedstocks. Chapter 2 covers a tunable library of substituted thiourea precursors to metal sulfide nanocrystals. Controlling the size of nanocrystals produced in a given reaction is paramount to their use in opto-electronic devices, but the most widely used technique to control size is prematurely arresting crystal growth. We introduce a library of thiourea precursors whose organic substituents tune the rate of precursor conversion, which dictates the number of nanocrystals formed and the final nanocrystal size following complete precursor conversion. We use PbS as a model system to 1) demonstrate the concept of kinetically controlled nanocrystal size, 2) quantify substituent trends, and 3) optimize multigram scale syntheses. We then expand the thiourea methodology to a broad range of materials and nanocrystal morphologies. This work represents a paradigm shift that will greatly accelerate the pace of progress in nanocrystal science as it transitions from academia to a multibillion-dollar industry. Chapter 3 covers an analogously tunable library of substituted selenourea precursors, but focuses on the synthesis of PbSe nanocrystals. PbSe nanocrystal synthesis is notoriously low-yielding and poorly tunable, but the remarkable properties of PbSe nanocrystals in photovoltaics and electrical transport have driven interest in the material for decades. We develop a library of N,N,N’-trisubstituted selenourea precursors and leverage their fine conversion rate tunability to synthesize PbSe nanocrystals of many sizes in quantitative yields. Interestingly, the nanocrystals produced in this reaction are demonstrably less polydisperse than literature samples, exhibiting absorption linewidths approaching the single-particle limit. We quantify this narrowness using a transient absorption spectroscopy technique called spectral hole burning. Chapter 4 covers our efforts to dig deeper into nanocrystal nucleation and growth and use that new knowledge to develop luminescent downconverters ready for on-chip integration into LED lighting. By studying early time points in PbS and PbSe nanocrystal synthesis, we estimate solute concentrations, nucleation thresholds, and nanocrystal growth rates. In particular, we find that metal selenides and sulfides have very different nucleation and growth behavior, as well as that PbS nucleation is a surprisingly slow process. The lessons learned from these fundamental experiments have enabled us to rapidly develop red-emitting CdS/CdSe/CdS “spherical quantum well” emitters whose photoluminescence quantum yields are 90 – 95%.

Semiconductor nanocrystals have immense potential to make an impact in consumer products due to their narrow, tunable emission linewidths. One factor limiting their use is the ease and reproducibility of core/shell nanocrystal syntheses. This thesis aims to address this issue by providing chemical control over the formation of core/shell nanostructures by replacing engineering controls with kinetic controls. Chapter 1 contextualizes our study on nanoparticle synthesis with a brief discussion on the physics of quantum confinement and the importance of narrow size dispersities, core/shell band alignments, and low lattice mismatches and strain at core/shell nanocrystal interfaces. Next, the evolution of cadmium chalcogenide nanocrystal reagents is described, ranging from the original organometallic reagents used in the 1980s to modern approaches involving cadmium phosphonates and carboxylates. This is followed by a description of chalcogen precursors, highlighting the recent introduction of molecules whose well-controlled and tunable reaction rates allow for the size tuning of nanocrystals at 100% yield, and accompanying theories on nanocrystal nucleation. Chapter 2 covers work to expand the library of available sulfur precursors to a wider range of molecules relevant for the synthesis of cadmium sulfide nanocrystals. Using thioureas alone, only very fast or very slow precursor conversion rates can be accessed. This limits the accessible sizes of cadmium sulfide nanocrystals using a single hot injection of precursor at standardized reaction conditions. We observe that thiocarbonate and thiocarbamate precursors with varying electronic substituents allow access to intermediate precursor conversion rates and cadmium sulfide nanocrystal sizes. Interestingly, we note that these new precursor classes nucleate particles with higher monodispersity than ones synthesized from thioureas. These results indicate that in addition to precursor structure controlling precursor conversion rate, precursor structure additionally impacts nanocrystal monodispersity. Chapter 3 expands the library of sulfur and selenium precursors to include cyclic thiones and selenones which extends chemical control of precursor conversion kinetics to cover five orders of magnitude. This unprecedented breadth of rate control allows for the simultaneous hot injection of multiple precursors to generate core/shell or alloyed nanoparticles using precursor reactivity. Using this new synthetic strategy, we observe that kinetic control runs into several issues which we partially attribute to differences in cadmium sulfide and cadmium selenide critical concentrations and growth rates. Nevertheless, combined with a syringe pump shelling method, we are able to access core/shell and alloyed nanocrystals with photoluminescence quantum yields of 67-81%. Chapter 4 applies the concept of nanostructure control via precursor conversion kinetics to a better model system: two-dimensional nanoplatelets. Cadmium chalcogenide nanoplatelets are highly desirable materials due to their exceptionally narrow emission full width half max (FWHM) values which make them pure emitters relative to quantum dots or organic dyes. We synthesize 3 monolayer thick nanoplatelets whose lateral dimensions vary from 10 nm x 10 nm to 186 x 100 nm and demonstrate compositional control on the smallest platelet sizes with STEM EELS.

Semiconductor nanocrystals, otherwise known as Quantum dots (Q dots), are a new type of fluorophore that demonstrates many advantages over conventional organic fluorophores. These advantages offer the opportunity to improve known immunofluorescent methods and immunofluorescent biosensors for rapid and portable bacterial detection in water. The detection of the micro organism Escherichia coli O157:H7 by attenuation of a fluorophore’s signal in water was evaluated alone and in the presence of another bacterial species. A sandwich immunoassay with antibodies adhered to SU-8 as a conventional comparison to our novel attenuation detection was also conducted. The assays were tested for concentration determination as well as investigation for cross reactivity and interference from other bacteria and from partial target cells. In order to immobilize the capture antibodies on SU-8, an existing immobilization self-assembly monolayer (SAM) for glass was modified. The SAM was composed of a silane ((3-Mercaptopropyl) trimethoxysilane (MTS)) and hetero-bifunctional cross linker (N-γ-maleimidobutyryloxy succinimide ester (GMBS)) was utilized in this procedure. The SU-8 surface was activated using various acids baths and oxygenated plasma, and it was determined that the oxygenated plasma yielded the best surface attachment of antibodies. The use of direct fluorophore signal attenuation for detection of the target E. coli resulted in the lowest detectable population of 1x10¹ cfu/mL. It was not specific enough for quantitative assessment of target concentration, but could accurately differentiate between targeted and non-targeted species. The sandwich immunofluorescent detection on SU-8 attained the lowest detectable population of 1x10⁴ cfu/ml. The presence of Klebsiella pneumoniae in solution caused some interference with detection either from cross reactivity or binding site blocking. Partial target cells also caused interference with the detection of the target species, mainly by blocking binding sites so that differences in concentration were not discernable. The signal attenuation not only had a better lowest detectable population but also had less measurement error than the sandwich immunoassay on SU-8 which suffered from non-uniformed surface coverage by the antibodies.

Semiconductor devices are commonplace in every household. One application of semiconductors in particular, namely solid state lighting technology, is destined for a bright future. To this end, ZnO nanostructures have gained substantial interest in the research community, in part because of its requisite large direct band gap. Furthermore, the stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize controllable growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanorods, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy (steady state and time resolved) and X-ray diffraction (XRD). Furthermore, strong rectification in the ZnO/p-Si heterojunction is demonstrated. Nanorods have been successfully synthesized on silicon by a two-step process, involving the pre-coating of the substrate by a seed layer, followed by the chemical bath deposition of the nanorods. ZnO seed layers with particle sizes of about 5 nm are achieved by the thermal decomposition of zinc acetate dihydrate dissolved in ethanol. The effects of the seed layer density on the distribution, alignment and uniformity of subsequently grown nanorods were studied. The aspect ratio, orientation and distribution of nanorods are shown to be well controlled through adjusting the density of the ZnO nanoparticles pre-coated onto the substrates. It is shown that the seed layer is a prerequisite for the growth of well aligned ZnO nanorods on lattice mismatched Si substrate. The influence of various nanorod growth parameters on the morphology, optical and electrical properties of the nanorods were also systematically studied. These include the oxygen to zinc molar ratio, the pH of the growth solution, the concentration of the reactants, the growth temperature and growth time, different hydroxide precursors and the addition of surface passivating agents to the growth solution. By controlling these xii parameters different architectures of nanostructures, like spherical particles, well aligned nanorods, nanoflowers and thin films of different thicknesses are demonstrated. A possible growth mechanism for ZnO nanostructures in solution is proposed. XRD indicated that all the as-grown nanostructures produced above 45 C crystallize in the wurtzite structure and post growth annealing does not significantly enhance the crystalline quality of the material. In material grown at lower temperature, traces of zinc hydroxide were observed. The optical quality of the nanostructures was investigated using both steady-state PL and time-resolved (TR) PL from 4 K to room temperature. In the case of as-grown samples, both UV and defect related emissions have been observed for all nanostructures. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. The effect of annealing in different atmospheres was also investigated. Regardless of the annealing environment annealing at a temperature as low as 300 C enhances the UV emission and suppresses defect related deep level emission. However, annealing above 500 C is required to out-diffuse hydrogen, the presence of which is deduced from the I4 line in the low temperature PL spectra of ZnO. TRPL was utilized to investigate lifetime decay profiles of nanorods upon different post growth treatments. The bound exciton lifetime strongly depends on the post-growth annealing temperature: the PL decay time is much faster for as grown rods, confirming the domination of surface assisted recombination. In general, the PL analysis showed that the PL of nanorods have the same characteristics as that of bulk ZnO, except for the stronger contribution from surface related bound excitons in the former case. Surface adsorbed impurities causing depletion and band bending in the near surface region is implied from both time resolved and steady state PL. Finally, although strong rectification in the ZnO/p-Si heterojunction is illustrated, no electroluminescence has been achieved. This is explained in terms of the band offset between ZnO and Si and interfacial states. Different schemes are proposed to improve the performance of ZnO/Si heterojunction light emitting devices.

Properties of nanocrystals are extremely sensitive to their sizes when their sizes are smaller or of the order of the excitonic diameter due to the quantum confinement effect. The interest in this field has been concentrated basically in understanding the size-property relations of nanocrystals, for example, the pronounced variation in the bandgap of the material or the fluorescence emission properties, by tuning the sizes of the nanocrystals. Thus, the optical and electronic properties of semiconductor nanocrystals can be tailor-made to suit the needs of the specific application and hence is of immense importance. One of the major aspects necessary for the actual realization of the various applications is the ability to synthesize nanocrystals of the required size with a controlled size distribution. The growing demand to obtain such nanocrystals with the required size and controlled size distribution is met largely by the solution route synthesis of nanocrystals, that constitutes an important class of synthesis methods due to their ease of implementation and the high degree of flexibility. The main difficulty of this method is that the dependence of the average size and the size distribution of the generated particles on parameters of the reaction are not understood in detail and therefore, the optimal reaction conditions are arrived at essentially in an empirical and intuitive manner. From a fundamental point of view, understanding the growth kinetics of various nanocrystals can provide a deeper insight into the phenomena. The study of growth kinetics can be experimentally achieved by measuring the time evolution of diameter using several in-situ techniques like UV-absorption and small angle X-ray scattering. Having understood the mechanism of growth of nanocrystals, it is possible to obtain the required size of the nanocrystal using optimized synthesis conditions. The properties of these high quality nanocrystals can be further tuned by doping with a small percentage of magnetic ions. The optical and magnetic properties of these nanocrystals play an important role in the various technological applications. The first part of the thesis concentrates on the theoretical methods to study the electronic structure of semiconductor nanocrystals. The second part describes the studies performed on growth of various nanocrystal systems, both in the presence and absence of capping agents. The third part of the thesis describes the studies carried out on ZnO and doped ZnO nanocrystals, synthesized using optimal conditions that were obtained in the earlier part of the thesis. The thesis is divided into five chapters which are described below. Chapter 1 provides a brief overall perspective of various interesting properties of semiconductor nanocrystals, including various concepts relevant for the study of such systems. Chapter 2 describes experimental and theoretical methods used for the study of nanocrystals reported in this thesis. In Chapter 3 of this thesis, we report results of theoretical studies carried out on III-V and II-VI semiconductors using the tight-binding (TB) methodology. Chapter 4 presents the investigations on the growth kinetics of several nanocrystal systems. Chapter 5 presents experimental investigations carried out on undoped and various transition metal (TM) doped ZnO nanocrystals. In summary, we have performed electronic structure calculations on various nanocrystal systems, devised a novel method to obtain the size distribution from UV-absorption spectrum and studied the mechanism of growth in the presence and absence of capping agents in various II-VI semiconductors. Using the optimal conditions obtained from the growth studies, we prepare high quality ZnO nanocrystals of required size, both in free-standing and capped states and doped it with small percentages of various transition metals like Mn, Cu and Fe. We have then studied their optical and magnetic properties.

Electronic and optical properties of semiconducting nanocrystals, that can be engineered and manipulated by various ways like varying size, shape, composition, structure, has been a subject of intense research for more than last two decades. The size dependency of these properties in semiconductor nanocrystals is direct manifestation of the quantum confinement effect. Study of electronic and optical properties in smaller dimensions provides a platform to understand the evolution of fundamental bulk properties in the semiconductors, often leading to realization and exploration of entirely new and novel properties. Not only of fundamental interests, the semiconductor nanocrystals are also shown to have great technological implications in diverse areas. Besides size tunable properties, introduction of impurities, like transition metal ions, gives rise to new functionalities in the semicon-ductor nanocrystals. These materials, termed as doped semiconductor nanocrystals, have been the subject of great interest, mainly due to the their interesting optical properties. Among different transition metal doped semiconductor nanocrystals, manganese doped systems have drawn a lot on attention due to their certain advantages over other dopants. One of the major advantages of Mn doped semiconductor nanocrystals is that they do not suffer from the problem of self-absorption of emission, which quite often, is consid-ered detrimental in their undoped counterparts. The doped nanocrystals are known to produce a characteristic yellow-orange emission upon photoexcitation of the host that is relatively insensitive to the surface degradation of the host. This emission, originating from an atomic d-d transition of Mn2+ ions, has been a subject of extensive research in the recent past. In spite of the spin forbidden nature of the specific d-d transition, namely 6A1 −4 T1, these doped nanocrystals yield intense phosphorescence. However, one major drawback of utilizing this system for a wide range application has been the substantial inability of the community to tune the emission color of Mn-doped systems in spite of an intense effort over the years; the relative constancy of the emission color in these systems has been attributed to the essentially atomic nature of the optical transition involving localized Mn d levels. Interestingly, however, the Mn emission has a very broad spectral line-width in spite of its atomic-like origin. While the long (∼ 1 ms) emission life-time of the de-excitation process is well-studied and understood in terms of the spin and orbitally forbidden nature of the transition, there is little known concerning the process of energy transfer to the Mn from the host in the excitation step. In this thesis, we have studied the ultrafast dynamic processes involved in Mn emission and addressed the issues related to its tunability and spectral purity. Chapter 1 provides a brief introduction to the fundamental concepts relevant to the studies carried out in the subsequent chapters of this thesis. This chapter is started with a small preview of the nanomaterials in general, followed by a discussion on semiconducting nanomaterials, evolution of their electronic structure with dimensions and size as well as the effect of quantum confinement on their optical properties. As all the semiconducting nanomaterials studied in the thesis are synthesized via colloidal synthesis routes, a separate section is devoted on colloidal semiconducting nanomaterials, describing various ways of modifying or tuning their optical properties. This is followed by an introduction to the important class of materials “doped semiconductor nanocrystals”. With a general overview and brief history of these materials, we proceed to discuss about various aspects of manganese doped semiconductor nanocrystals in great details, highlighting the origin of the manganese emission and the associated carrier dynamics as well as different reported synthetic strategies to prepare these materials. The chapter is closed with the open questions related to manganese doped semiconductor nanocrystals and the scope of the present work. Chapter 2 describes different experimental and theoretical methods that have been employed to carry out different studies presented in the thesis. It includes common experimental techniques like UV-Vis absorption spectroscopy, steady-state and time-resolved photoluminescence spectroscopy used for optical measurements, X-ray diffraction, trans-mission electron microscopy and atomic absorption spectroscopy used for structural and elemental analysis. Experimental tools to perform special studies like transient absorption and single nanocrystal spectroscopy are also discussed. Finally, theoretical fitting method used to analyse various spectral data has been discussed briefly. Chapter 3 deals with the dynamic processes involved in the photoexcitation and emission in manganese doped semiconductor nanocrystals. For this study, Mn doped ZnCdS alloyed nanocrystal has been chosen as a model system. There are various radiative and nonrdiative recombination pathways of the photogenerated carriers and they often compete with each other. We have studied the dynamics of all possible pathways of carrier relaxation, viz. excitonic recombination, surface state emission and Mn d-d transition. The main highlight of this chapter is the determination of the time-scale to populate surface states and the Mn d-states after the photoexcitation of the host. Employing femtosecond pump-probe based transient absorption study we have shown that the Mn dopant states are populated within sub-picosecond of the host excitation, while it takes a few picoseconds to populate the surface states. Keeping in mind the typical life-time of the excitonic emission (∼ a few ns), the ultra-fast process of energy transfer from the host to the Mn ions explains why the presence of Mn dopant ions quenches the excitonic as well as the surface state emissions so efficiently. Chapter 4 presents a study of manganese emission in ZnS nanocrystals of different sizes. By varying the size of the ZnS host nanocrystal, we show that one can tune the Mn emission over a limited range. In particular, with a decrease in host size, the Mn emission has been observed to red-shift. We have attributed this shift in Mn emission to the change in the ratio of surface to bulk dopant ions with the variation of the host size, noting that the strength of the ligand field at the Mn site should depend on the position of the Mn ion relative to the surface due to a systematic lattice relaxation in such nanocrystals. The ligand field affects the emission wavelength directly by controlling the splitting of the t2 and e levels of Mn2+ ions. The surface dopant ions experience a strong ligand field due to distorted tetrahedral environment which leads to larger splitting of these t2 and e states. We further corroborated these results by performing doping concentration dependent emission and life-time studies. In Chapter 5 addresses two fundamental challenges related to manganese photolumines-cence, namely the lack of a substantial emission tunability and presence of a very broad spectral width (∼ 180-270 meV). The large spectral width is incompatible with atomic-like manganese 4T1 −6 A1 transition. On the other hand, if this emission is atomic in nature, it should be relatively unaffected by the nature of the host, though it can be manipulated to some extent as discussed in Chapter 3. The lack of Mn emission tunability and spectral purity together seriously limit the usefulness of Mn doped semiconductor nanocrystals. To understand why the Mn emission tunability range is very limited (typically 565-630 nm) and to understand the true nature of this emission, we carried out single nanocrystal imaging and spectroscopy on Mn doped ZnCdS alloyed nanocrystals. This study reveals that Mn emission, in fact, can vary over a much wider range (∼ 370 meV) and exhibits widths substantially lower (∼ 60-75 meV) than reported so far. We explained the occur-rence of Mn emission in this broad spectral range in terms of the possibility of a large number of symmetry inequivalent sites resulting from random substitution of Cd and Zn ions that leads to differing extent of ligand field contributions towards the splitting of Mn d-levels. The broad Mn emission observed in ensemble-averaged measurements is the result of contribution from Mn ions at different sites of varying ligand field strengths inside the NC. Chapter 6 presents a synthetic strategy to strain-engineer a nanocrystal host lattice for a controlled tuning of the ligand field effect of the doped Mn sites. It is realized synthesizing a strained quantum dot system with the structure ZnSe/CdSe/ZnSe. A larger lattice parameter of CdSe compared to that of ZnSe causes a strain field that is maximum near the interface, gradually decreasing towards the surface. We control the positioning of Mn dopant ions at different distances from the interface, thereby doping Mn at different predetermined strain fields. With the help of this strain engineering, we are able to tune Mn emission across the entire range of the visible spectrum. This strain induced tuning of Mn emission is accompanied by life-times that is dependent on the emission energy which has been explained in terms of perturbation effect on the Mn center due to the strain generated inside the quantum dot. The spectacular emission tuning has been explained by modelling the quantum dot system as an elastic continuum containing three distinct layers under hydrostatic pressure. From this modelling, we found that the strain is max-imum at the interface and decreases continuously as one goes away from the interface. We also show that the Mn emission maximum red shifts with increasing distance of the dopants from the maximum strained region. In summary, we have performed a study on the photophysical processes in manganese doped semiconductor nanocrystals. We have emphasized in understanding of different dynamic processes associated with the manganese emission and tried to understand the true nature of manganese emission in a nanocrystal. This study has brought out some new aspects of manganese emission and opened up possibilities to tune and control manganese emission by proper design of the host material.

Research activities towards the self-assembly of small organic molecules building blocks which lead to form supramolecular gel has increased extensively during the past two decades. The fundamental investigations of the morphological properties and the mechanical properties of these supramolecular gels are crucial for understanding gelation processes. Most supramolecular gelators were discovered by serendipity, but nowadays ratiional design of new gelators has become somewh at feasible. As a consequence, an increasing number of multi stimuli-responsive and functional molecular gels are reported, offering great prospects with myriads of applications includ ing drug delivery and smart materials as shown in scheme 1. Scheme 1 Part 2: Synthesis of semiconductor nanocrystals In the last two decades, the synthetic development of semiconductor col loidal nanocrystals has been extended from the adjustment of their size, shape, and composition of the particles at the molecular level. Such adjustments of nanocrystals at the molecula r level might open different fields of applications in materials and biological sciences. I n this chapter, the concept of the shape contr ol synthesis of colloidal nanocrystals with a narrow size distribution, and the synthesis of composition dependent alloy type mat erials are described (Scheme 2). Scheme 2 Chapter 2: Synthesis of luminescent semiconductor nanocrystals Part 1: Cadmium deoxycholate: a new and efficient precursor for high ly luminescent CdSe nanocrystals This part demonstrates the sy nthesis of Cadmium deoxycholate (CdDCh2), an efficient Cd-precursor for the synthesis of high quality, monodisperse, multi color emittting CdSe Scheme 3 nanocrystals, while maintaining their high photoluminescent quantum efficiency (Scheme 3). The high thermal stability of CdDCh2 (decomposition temperature: 332 °C) was utilized to achieve high injection and growth temperatures (∼300 °C) for the syntheesis of red emitting nanocrystals with a sharp f ull width at half maximum (FWHM) and multiple excitonic absorption features. We believe that CdDCh2 can be useful for the prreparation of other nanomaterials such as CdS, CdTe and CdSe@CdS core-shell QDs. Part 2: Ligand mediated exccited state carrier relaxation dynamics of Cd1-xZnxSe1-ySy NCs derived from bile salts Bile salts of Cadmium and Zinc provide a convenient and inexpensive single step synthetic route for highly photoluminescent and stable semiconductor nanocrystals (NCs). The high thermal stabilities of Cadmium and Zinc deoxycholates (CdDCh2 and ZnDCh2) allowed us to fine-tune the synthesis of the NCs at high temperatures while maintaining the monodispersity, crystallinity and reproducibility (Scheme 4). Organic capping agent induced lattice strain affects the excited Scheme 4 state relaxation processes of the NCs. The analysis of photoluminescence decay profiles revealed that the average lifettime decreased with the increasing lattice strain of the NCs. A kinetic stochastic model of photoexcited carrier relaxation dynamics of NCs was employed to estimate the values of the radiative recombination rates, the photoluminescence quenching rates and the non-radiative recombination rates of the NCs. These data showed that the non-radiative relaxation rates and the numbeer of surface trap states increased with the incrreasing lattice strain of the NCs. Such types of NCs can have great potential in nonlinear optics, photocatalysis and solar cells. Chapter 3: Synthesis of organic-inorganic hybrid materials Part 1: Hierarchical self-assembly of photoluminescent CdS nanoparticles into bile acid derived organogel: morphological and photophysical properties In this part a strategy towards integrating photoluminescent semiconductor nanoparticles into a bio-surfactant derived organoggel has been reported. A facially amphiphilic bile thiol was used for capping CdS nanoparticless (NPs) which were embedded in a gel derived from a new bile acid organogelator in order to furnish a soft hybrid material (Scheme 5). The presence of CdS NPs in a well-ordered 1D array on the organogel network was confirmed using microscopic Scheme 5 techniques. Photophysical stuudies of the gel–NP hybrid revealed resolved excitation and emission characteristics. Time resolved spectroscopic studies showed that the average lifetime value of the CdS NPs increased in the gel state compared to the sol phase. A kinetic model was utilized to obtain quantitative information about the different decay pathways of the photoexcited NPs in the sol and gel states. Part 2: A novel strategy towards designing a CdSe quantum dot–metallohydrogel composite material This section describes an efficiient method to disperse hydrophobic CdSe quaantum dots (QDs) in an aqueous phase using cetyltriimethylammonium bromide (CTAB) micelles without any surface ligand exchange. The water soluble QDs were then embedded in the 3D self-assembled fibrillar networks (SAFINs) of a hydrogel showing homogeneous dispersibility as eviidenced by Scheme 6 optical and electron microscopico techniques (Scheme 6). The photophyssical studies of the hydrogel–QD from composite are reported for the first time. These composite materials may have potential applications in biology, optoelectronics, sensors, non-linear optics and materials science. Part 3: Photophysical aspectts of self-assembled CdSe QD-organogel hyybrid and its thermoresponsive properties A luminescent hybrid gel was constructed by incorporating CdSe quantuum dots (QDs) in a facially amphiphilic bile acid derived dimeric urea organogel throough non-covalent interaction between ligands capped on QDs surface and hydrophobic pockets of the gel (Scheme 7). The optical transparency of the hybrid materials and the dirrectionalities of the QDs in the gel medium were confirmed by photophysical and microscopic studies. The detailed excited state dynamics of the QD–organogel hybrid has been reported for the first time with the help of lifetime analysis and a kinetic decay model, and thee data revealed that the average lifetime of the QDs decreased in the gel medium. The reversible thermoresponsive behavior of the QD doped organogel was investigated by steady-state fluorescence spectroscopy. W e believe that the results obtained herein provides a route to develop a thermoresponsive system for practical application, especially because of the spatial assembly between soft organic scaffolds and colloidal QDs. Scheme 7 Part 4: In-situ formation of luminescent CdSe QDs in a metallohydrogel: a strategy towards synthesis, isolation, storage and re-dispersion of the QDs A one step, in-situ, room temperature synthesis of yellow luminesce nt CdSe QD was achieved in a metallohydrog el derived from a facially amphiphilic bile salt, resulting in a QD-gel hybrid (Scheme 8). T he ordered self-assembly and homogeneous distribution of the CdSe QDs in the hydrogel network was observed from optical and electro n micrographs. The different excited state behav iors of the hybrid were revealed for the fir st time using time resolved spectroscopy. Ad ditionally, we described the successful isolation of the photoluminescent CdSe QDs from the gel followed by their re-dispersion in an organic solvent using suitable capping ligands. Scheme 8 Chapter 4: Facially a mphiphilic bile acid derived meta llohydrogel: an efficient template for th e enantioselective Diels-Alder reactio n An enantioselective Diels-Ald er reaction mediated by a facially amphiphilic bile acid derived metallogel scaffold has been a chieved (Scheme 9). Different hydrophobic domains present in Scheme 9 the gel appear to facilitate the enantioselective reaction. Various spectro scopic and electron microscopic techniques were employed to understand the possible reasons for the stereoselectivity in the gel. Subsequently, different counter anion s dependent rate accelerations and induced enantioselectivity in the ZnCh2 gel were studied in detail. These preliminary results of the non-covalent based supramolecular heterogeneous catalysis offer new possibilities for using metallogels as nanoreactors for different stereoselective reactions.

La perspective d'une société utilisant l'énergie de la lumière du soleil pour séparer la molécule d'eau en dihydrogène et en dioxygène, ces deux gaz servant de moyens de stockage et de vecteurs d'énergie, nécessite de nombreux développements. En particulier, il est nécessaire de choisir un matériau pouvant absorber la lumière et transférer son énergie aux charges électriques afin de générer un courant électrique. Parmi toutes les possibilités, ce mémoire étudie l'applicabilité des bâtonnets semiconducteurs de tailles nanométriques constitués d'un cœur de séléniure de cadmium et d'une coquille de sulfure de cadmium. Profitant des méthodes décrites ces dernières années et d'une méthodologie de fonctionnalisation, les objets obtenus présentent une grande monodispersité et peuvent être dispersés en milieu aqueux. Les propriétés photoélectrochimiques des nanobâtonnets sont explorées par microscopie électrochimique. Cette méthode permet de déterminer s'il y a un transfert de charge entre des molécules en solution et un substrat constitué des bâtonnets, et le cas échéant son sens. Ainsi les nanoparticules, soumises à une excitation lumineuse, transfèrent des électrons vers les molécules dans l'ensemble des cas explorés, révélant ainsi un caractère plus réducteur que la para-benzoquinone. Ce transfert est réalisé d'autant plus rapidement que le rapport entre la longueur et le diamètre des bâtonnets augmente, jusqu'à un optimum, mais aussi que la taille de la couche organique isolante les recouvrant diminue, comme l'ont révélé des suivis de réduction d'une sonde rédox moléculaire colorée, la résazurine. Ces charges ont été mises à profit pour fonctionnaliser les nanoparticules, au travers de la réduction d'un pont disulfure ou d'un sel d'or. Enfin des stratégies ont été explorées pour permettre aux particules de réaliser la réduction photosensibilisée de l'eau, au travers de la synthèse d'une cobaloxime, un catalyseur moléculaire, ou de la réduction de sels métalliques à propriété catalytique tels que le cobalt et le nickel. / The development of a society based on solar energy requires a way to store it. One possibility consists in water splitting that needs a material to collect and transform the energy contained in light beam in an electric charges movement. Among all possibility, we hereby explore the applicability of nanometers-sized semiconductor rods composed of a cadmium selenide core and a cadmium sulfide shell. Based on methods already developed and a new functionalization methodology, the obtained particles exhibit a high monodispersity and can be dispersed in water, a useful property for the final purpose. Their photo-electrochemical properties have been explored by electrochemical microscopy that allowed to determine whether there is charge transfer between mediators in solution and quantum rods deposited as substrate and its direction. It reveals that under light irradiation and in all cases herein experimented, they transfer electrons to the mediators, making them more reductive than para-benzoquinone. This transfer is fastened when the ratio between the length and the diameter of the rods increased until an optimum, but also when the width of the organic isolating shell decreases, as revealed by time-resolved reduction of resazurin, a colored rédox molecular probe. These charge transfer have been used to functionalize particles by reduction of a disulfide bridge or a gold salt. Finally, strategies have been explored to make these quantum rods able to photosensibilized water reduction through synthesis of a cobaloxime, a molecular catalyst, or metal salt reduction as cobalt and nickel known to exhibit catalytic activity.