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There is an ever-increasing desire to develop novel materials that could control the release of active compounds and increase their stability. Replacing petroleum-based synthetic polymers with sustainable materials has many advantages, such as reducing the dependence on fossil fuels, and diminishing environmental pollution. Recently, cellulose nanocrystal (CNC) obtained by acid hydrolysis of cellulose fibres has gained a lot of interest. The high mechanical strength, large and negatively charged surface area, and the presence of several hydroxyl groups that allow for modification with different functionalities make CNC an excellent candidate for various applications in the biomedical field. This thesis explores (i) the surface modification and characterization of modified CNC and (ii) the biomedical applications of these novel sustainable nanomaterials. In the first part, amine functionalized CNC was prepared. Ammonium hydroxide was reacted with epichlorohydrin (EPH) to produce 2-hydroxy-3-chloro propylamine (HCPA), which was then grafted to CNC using an etherification reaction. A series of reactions were carried out to determine the optimal conditions. The final product (CNC-NH2(T)) was dialyzed for one week. Further purification via centrifugation yielded the sediment (CNC-NH2(P)) and supernatant (POLY-NH2). The presence of amine groups was confirmed by FT-IR and the amine content was determined by potentiometric titration and elemental analysis. A high amine content of 2.2 and 0.6 mmol amine/g was achieved for CNC-NH2(T) and CNC-NH2(P), respectively. Zeta potential measurements confirmed the charge reversal of amine CNC from negative to positive when the pH was decreased from 10 to 3. TEM images showed similar structural properties of the nanocrystals along with some minor aggregation. This simple, yet effective synthesis method can be used for further conjugation as required for various biomedical applications. Moreover, the surface of CNC was modified with chitosan oligosaccharide (CSos). First, the primary alcohol groups of CNC were selectively oxidized to carboxyl groups using the catalyst, 2,2,6,6- tetramethylpiperidine-1-oxyl radical (TEMPO), and were then reacted with the amino groups of CSos via the carbodiimide reaction using N-hydroxysuccinimide (NHS) and 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC). The appearance of C=O peak in FT-IR spectrum of oxidized CNC (CNC-OX), accompanied by calculations based on potentiometric titration revealed that CNC was successfully oxidized with a degree of oxidation of 0.28. The grafting of CSos on oxidized CNC was confirmed by the following observations: (i) the reduction of the C=O peak in FT-IR of CNC-CSos and the appearance of new amide peaks; (ii) the significant reduction of the carbonyl peak at 175 ppm in the 13C NMR spectrum for CNC-CSos; (iii) a higher decomposition temperature in TGA of CNC-CSos; (iv) a positive zeta potential of CNC-CSos at acidic pH; and (v) a degree of substitution of 0.26, which is close to the DO (0.28), indicating that 90% of COOH groups on CNC-OX were involved in the formation of amide bonds with CSos. TEM and AFM studies also revealed a completely diff erent morphology for CNC-CSos. In the second part, the potential of exploiting CNCs as delivery carriers for two cationic model drugs, procaine hydrochloride (PrHy) and imipramine hydrochloride (IMI), were investigated. IMI displayed a higher binding to CNC derivatives compared to PrHy. Isothermal titration calorimetry (ITC), transmittance and zeta potential measurements were used to elucidate the complexation between model drugs and CNC samples. It was observed that the more dominant exothermic peak observed in the ITC isotherms leading to the formation of larger particle-drug complexes could explain the increased binding of IMI to CNC samples. Drug selective membranes were prepared for each model drug that displayed adequate stability and rapid responses. Different in vitro release profiles at varying pH conditions were observed due to the pH responsive properties of the systems. Both drugs were released rapidly from CNC samples due to the ion-exchange e ffect, and CNC-CSos displayed a more sustained release profile. Furthermore, the antioxidant properties of CNC samples and the potential of CNC-CSos as a carrier for the delivery of vitamin C was investigated. CNC-CSos/vitamin C complexes (CNCS/VC) were formed between CNC-CSos and vitamin C via ionic complexation using sodium tripolyphosphate (TPP). The complexation was confirmed via DSC and UV-Vis absorbance measurements. TEM images showed complexes with a size of approximately 1 micron. The encapsulation efficiency of vitamin C was higher (91%) at pH 5 compared to pH 3 (72%). The in vitro release of vitamin C from CNCS/VC complexes exhibited a sustained release of up to 3 weeks, with the released vitamin C displaying higher stability compared to a control vitamin C solution. Antioxidant activity and kinetics of various CNC samples were studied using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. CNC-CSos possessed a higher scavenging activity and faster antioxidant activity compared to its precursors, CNC-OX and CSos, and their physical mixture. Therefore, by loading vitamin C into CNC-CSos particles, a dynamic antioxidant system was produced. Vitamin C can be released over a prolonged time period displaying enhanced and sustained antioxidant properties since the carrier CNC-CSos also possesses antioxidant properties. As a result of this doctoral study, knowledge on the surface modification of CNC with amine groups and CSos have been advanced. The in vitro drug release and antioxidant studies suggest that systems comprising of CNC could be further explored as potential carriers in biomedical applications.

This dissertation presents new insight into the ability of small molecule passivated NCs to achieve intimate approach distances, despite being well passivated, while developing guiding principles in the area of ligand mediated microstructure control and the resulting macroscopic optical and electronic properties that close packing of high quality NCs enables. NC ligand coverage will be characterized quantitatively through thermogravimetric analysis (TGA), and qualitatively by photoluminescence and electroluminescence, in the case of functional devices; illustrating the importance of practitioner dependent control of ligand coverage through variations in the dispersion precipitation purification procedure. A unique examination of the relative contribution of energy and charge transfer in NC LEDs will demonstrate the ability to achieve charge transfer, at a level competitive with energy transfer, to well passivated NCs at various wt% loading in a polymer matrix. The observation of potential dependent recombination zones within an active layer further suggest novel, NC surface passivation mediated control of blend microstructure during solution processing towards the development of a bi-continuous network. Next, NC self-assembly and resulting microstructure dependent optical and electronic properties will be examined through electroluminescence and high-resolution transmission electron microscopy (TEM) micrographs of functional NC/polymer bulk heterojunction LEDs. The joint characterization of NC optical properties, and self-assembly microstructure provide a deeper understanding of the significant and inseparable effects of minimal changes in NC surface passivation on structure and function, and emphasize the potential to rely on strongly passivating ligands to control physical properties and processing parameters concurrently towards higher efficiency devices via low cost processing. Finally, micro-contact printing of blazed transmission gratings, using stable dispersions of core and core/shell NCs will be shown to produce close packed assemblies of NCs forming near-wavelength luminescent superstructures separated in space. We show the dominant contribution of a two-monolayer thick sharp interface CdS shell to the diffraction efficiency, and necessarily the refractive index, of the NCs, independent of core size. Utilization of these gratings as in-coupling elements at various positions within a device architecture are also examined. These new observations were achieved by unprecedented control of NC architecture during dispersion processing, while maintaining high luminescence, made possible by optimized NC surface passivation. These studies enable the formation of new LED architectures, and new optoelectronic devices based on angle resolved, monochromatic fluorescence from diffraction gratings prepared from simple solution processing approaches. Further, the novel observation of angle amplified interfering fluorescence from these features is argued to be a result of long range radiative coupling and superradiance enabled by the monodispersity and high-quality NC surface passivation described herein.

Doctor Scientiae - DSc / Semiconductor nanocrystals or quantum dots (QDs) are fluorescent nanometer-sized particles which have physical dimensions that are smaller than the excitonic Bohr radius, large surface area-to-volume ratios, broad absorption spectra and very large molar extinction coefficients. Biomedical applications of QDs are mainly based on II-VI QDs containing cadmium, such as CdSe/ZnS. These cadmium-based systems are associated with high toxicity due to cadmium. As a result, potential replacements of cadmium-based QDs in biological applications are needed. In this study, InP/ZnSe QDs were synthesized for the first time using a one-pot hot injection method. Furthermore, a growth-doping method was used for silver, cobalt and iron incorporation into the InP core. Water compatibility was achieved through ligand exchange with 3- mercaptopropionic acid. In vitro cytotoxicity and imaging/internalization of the as-prepared MP A-InP/ZnSe and MP A-capped CdTe/ZnS QDs were evaluated. InP/ZnSe QDs were successfully synthesized with ZnSe shell causing a 1.4 times reduction in trap-related emission.

Interactions between polysaccharide derivatives and rod-like cellulose nanocrystals were studied by light scattering. Two replicates of cellulose nanocrystals (DOE-2-12A and DOE-2-12B) were probed with polarized and depolarized dynamic light scattering. X-ray photoelectron spectroscopy experiments showed sulfate groups on cellulose nanocrystals. Decay rates from polarized dynamic light scattering experiments exhibited a significantly smaller angular dependence for both samples. However, DOE-2-12B showed a smaller angular dependence than DOE-2-12A. Lengths and diameters of DOE-2-12A and DOE-2-12B obtained by Broersma's formula were 229 " 19 and 19 " 7 nm and 240 " 18 and 22 " 6 nm, respectively. The resultant length and diameter of DOE-2-12B were comparable to those for cellulose whiskers obtained from cotton. Adsorption of pullulan 4-chlorocinnamate (P4CC03) onto cellulose nanocrystals (DOE-2-12B) was also studied by UV-Vis spectroscopy, zeta-potential measurements, and polarized and depolarized dynamic light scattering. UV-Vis spectroscopy of the P4CC03/water binary system and in situ light scattering showed UV crosslinking of pullulan 4-chlorocinnamate occurred in binary and ternary systems but led to different aggregation behavior in the two ternary systems: PreX where P4CC03 crosslinking occurred prior to the addition of cellulose nanocrystals and Rxn where cellulose nanocrystals were present during UV exposure. These studies showed P4CC03 adsorbed onto cellulose nanocrystals and UV induced crosslinking occurred at the surface of the cellulose nanocrystals. Zeta-potential measurements also showed that P4CC03 adsorbed onto cellulose nanocrystals. Finally, adsorption of 2-hydroxypropyltrimethylammonium xylans (HPMAXs) of degree of molar substitution MS = 0.34 onto rod-like cellulose nanocrystals (DOE-2-12Bs) were probed with zeta-potential measurements and polarized and depolarized dynamic light scattering. Zeta-potential changes of HPMAX/water, HPMAX/DOE-2-12B/water, and DOE-2-12B/water systems showed HPMAX adsorption onto DOE-2-12Bs. Intensity autocorrelation functions from Hv and Vv mode exhibited partial heterodyning. Decay time distributions of the binary and ternary systems showed that aggregates existed in the binary system but disappeared in the ternary system. These observations revealed that HPMAX adsorbed onto a fraction of the cellulose nanocrystals in the ternary system at low concentrations of HPMAX. Decreasing translational and rotational diffusion coefficients with increasing HPMAX concentration indicated HPMAX adsorption onto cellulose nanocrystals. A significant HPMAX concentration dependence of the ratio of rotational diffusion coefficient to translational diffusion coefficient showed strong adsorptive interactions between HPMAX and DOE-2-12B. These studies showed there were interactions between polysaccharides and cellulose nanocrystals even in very dilute solutions. Also, it was shown that probe diffusion studies with rod-like cellulose nanocrystals is a promising strategy for probing complicated polymer solutions. / Ph. D.

京都大学 / 0048 / 新制・課程博士 / 博士(農学) / 甲第19320号 / 農博第2141号 / 新制||農||1036(附属図書館) / 学位論文||H28||N4948(農学部図書室) / 32322 / 京都大学大学院農学研究科森林科学専攻 / (主査)教授 西尾 嘉之, 教授 木村 恒久, 教授 髙野 俊幸 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

A great deal of interest has been dawn on the colloidal chemistry based semiconductor nanocrysallites, also known as quantum dots (QDs). Because of the strong quantum confinement, quantum dots have unique size-dependent optical properties, which are much more superior to the conventional organic fluorescence materials. In addition, strong chemical resistant makes inorganic semiconductor QDs ideal candidate for next-generation of bio-labels and drug delivery vehicles. This report presents a user friendly approach to synthesize high quality biocompatible CdSe QDs in aqueous solution. Size of the dots can be controlled by adjusting the temperature, pH of the solution, and ratio of the precursors. A thin CdS layer was grown on CdSe QDs, forming a CdSe/CdS core/shell structure, to improve the photoluminescence. In order to use these QDs in-vivo, a more chemically robust coating, silica, was grown on the core/shell structure QD. The optical properties of the QDs were characterized by absorption and photoluminescence spectra. X-ray diffraction and transmission electron microscopy were conducted to verify the QDs composition and structure. / Ph. D.

Semiconductor nanocrystals with sizes comparable to the corresponding bulk excitonic diameter exhibit unique size-dependent electronic and optical properties resulting from quantum confinement effect. Such nanocrystals not only allow the study of evolution of bulk properties from the molecular limit providing important fundamental understandings, but also have great technological implications, leading to intense research over the past several years. Besides tuning the crystal size in the nm regime to obtain novel properties, an additional route to derive new functionalities has been to dope transition metal ions into a semiconductor host. Thus, transition metal doped nanocrystals are of great interest since it allows two independent ways to functionalize semiconductor materials, one via the tunability of properties by size variation and other due to properties of such dopants. Chapter 1 of the thesis provide a general introduction to the subject matters dealt in with this thesis, while the necessary methodologies have been discussed in chapter 2. Chapters 3 and 4 of this thesis deal with nanocrystal doping. Following suggestions in previous literatures that the doping of nanocrystal depends strongly upon the crystal structure of the synthesized host nanocrystal, we have studied the phase-transformation between the somewhat zinc-blende and the usual wurtzite structures for CdS and CdSe nanocrystals in chapter 5. In chapter 6 we have pointed out that a gradient structure is essential to achieve nearly ideal photoluminescence efficiency using heterostructured nanocrystals and also achieved strong two-photon absorptions, adding optical bifunctionality to these nanocrystals. Finally, in chapter 7, we establish different approaches to generate white-light using nanocrystals and their unique advantages, as a first step to realizing white light emitting devices. Chapter 1 provides a brief introduction to various interesting properties and concepts relevant for the studies carried out in the subsequent chapters of this thesis. The present status of the research in the field of semiconductor nanocrystals with an emphasis on synthesizing high quality nanocrystals, doping of nanocrystals and exciting optical properties exhibited by these nanocrystals has been discussed. We have discussed the existing theories and practices of colloidal synthesis that allow us to prepare high quality semiconductor nanocrystals with required size and very narrow size distribution. Optical properties, covering excitonic fine structure, photoluminescence, auger recombination and two-photon absorption have been discussed. We have described heterostructured nanocrystals of different types, particularly in the light of enhancing photoluminescence quantum yield. The difficulty in doping Mn2+ ion in semiconductor nanocrystals and the recent developments in this field have been addressed. Chapter 2 describes experimental and theoretical methodologies that have been employed to study different nanocrystal systems reported in this thesis. The topics covered in this chapter include UV-visible absorption spectroscopy, steady-state and time-resolved luminescence spectroscopy, X-ray diffraction, transmission electron microscopy, electron spin resonance spectroscopy, photoemission spectroscopy, two-photon absorption and least-squared-error fitting. Chapter 3 presents a detailed study of water soluble Mn2+-doped CdS nanocrystals synthesized using colloidal routes. Earlier efforts to dope Mn2+ ion into CdS nanocrystals and therefore, obtain the characteristic orange emission, have been largely impeded by the strong overlap of surface state emission of the host and Mn2+ d-emission. We are the first ones to obtain a distinct Mn2+ d-related emission at around 620 nm, well-separated from the surface state emission with its maximum near 508 nm. In spite of using very high (~30%) concentration of Mn2+ precursor, only ~1% Mn2+ was found in the final product, which is consistent with previous literatures, where Mn2+ doping in such nanocrystals was found to be extremely difficult. Most interestingly, present results establish that Mn2+ ion is found to be incorporated preferentially in the relatively larger sized nanocrystals compared to the smaller sized ones even within the narrow size distribution achieved for a specific reaction condition. We found that 55 oC is the optimum reaction temperature to synthesize Mn2+-doped CdS nanocrystals, at higher reaction temperatures, Mn2+ ions get annealed out of the substitutional sites, leading to a lower level of doping in spite of the formation of larger sized particles. Additionally, we could tune the color of the Mn2+ d- emission from red (620 nm) to yellow (580 nm) by increasing the reaction temperature from 55 oC to 130 oC. Another important aspect is that the synthesized nanocrystals readily dissolve in water without any perceptible effect on the Mn2+ d emission intensity. Chapter 4 discusses the outstanding problem that a semiconductor host in the bulk form can be doped to a large extent, while the same host in the nanocrystal form resist any appreciable level of doping. We first describe two independent models available in literatures to explain this baffling phenomenon. In one, it was suggested that the doping of Mn2+ ion in such nanoclusters is invariably an energetically unfavorable state, thus, Mn2+ ions get annealed out from the host nanocrystal and an increase in reaction temperature facilitate such annealing, a phenomenon known as self-purification. In the second model, it was suggested that the ease of initial adsorption of Mn2+ ions on specific surfaces of a growing nanocrystal, kinetically controls the extent of impurity doping. Specifically, it is easier to dope zinc-blende nanocrystals compared to their wurtzite counterpart. In contrast, the main claim of this chapter is neither crystal structure nor self-purification is as important in nanocrystal doping as lattice mismatch between the dopant and host lattice. To support this claim, we have doped Mn2+ ions into alloyed ZnxCd1-xS nanocrystals. Ionic radius of Mn2+ ion being in between those of Zn2+ and Cd2+ ions, the lattice mismatch between the host ZnxCd1-xS nanocrystal and MnS could be tuned in either side by tuning the composition “x”. It was gratifying to observe that there is an evident maximum of manganese content for Zn0.49Cd0.51S host nanocrystals that has no lattice mismatch with MnS, and the manganese content decreases systematically with increasing compressive as well as tensile lattice mismatches. Based on lattice parameter tuning, we could dope an extraordinarily higher amount of ~7.5% manganese for x = 0.49, at a reaction temperature as high as 310 oC and in a nanocrystal that exhibit wurtzite structure, which was previously suggested unfavorable for doping. These results prove our hypothesis that the strain fields generated because of the lattice mismatch between the dopant and host, are necessarily long range, much longer than typical nanocrystal dimensions and it tends to relieve itself by ejecting the dopant to the surface of nanocrystals, thus, resisting doping in such nanocrystals. High temperature synthesis, on the other hand, leads to a very high photoluminescence efficiency of ~25%. Chapter 5 deals with the phase-control of CdS and CdSe nanocrystals synthesized employing colloidal routes. CdS nanocrystals exhibit a very sensitive phase transformation from zinc-blende to wurtzite structure by increasing the reaction temperature from 280 to 310 oC, which is also accompanied by an increase in particle size from 6 to 6.8 nm, respectively. More importantly, just by changing the S precursor, it has been possible to change the crystal structure of the CdS nanocrystals at a given synthesis temperature of 310 oC. En route, we have synthesized >12 nm zinc-blende CdS nanocrystal, which is the largest one known in literature and that too employing the highest (310 oC) reaction temperature. Thus, our results contradict with the suggestions already in literatures that low reaction temperature and small crystal size favors zinc-blende structure. Also, we could tune crystal structure between zincblende and wurtzite at a given pressure of the reaction vessel and for a given solvent, just by changing the S-precursor, which is again in contradiction to previously made suggestions in literatures that high pressure or noncoordinating solvents favors the formation of zinc-blende nanocrystals. Instead, we believe that the surface energy might be crucial in stabilizing the usually rare zinc-blende structure for such nanocrystals. Chapter 6 is divided into two sections and deals with optically active heterostructured nanocrystals exhibiting high photoluminescence efficiency and strong two-photon absorption. In section-I, we probe the internal structure of extraordinarily luminescent (quantum yield = 85%) CdSeS nanocrystals making a somewhat unconventional use of Photoelectron spectroscopy, using the tunability of the photon energy from the third generation synchrotron radiation source as well as the traditional Mg Kα and Al Kα photon sources. CdSeS nanocrystals synthesized with Se:S precursor ratios 1:5 and 1:50, emitting red and green light have CdSe/CdSeS/CdS core/gradient-shell/shell and CdSeS/CdS gradient-core/shell structure, respectively. Gradient interface/core tunes the lattice parameters continuously between that of CdSe and CdS minimizing the interface related defects which in turn increases the photoluminescence efficiency even beyond that obtained from traditional core/shell nanocrystals, as evidenced by the nearly single exponential photoluminescence decay dynamics exhibited by these nanocrystals. Quantum mechanical calculations further show that a graded-core/shell structure leads to a remarkable spatial collapse and consequently a stronger overlap of the HOMO and LUMO wavefunctions towards the core region and thereby, making these luminescent beyond the traditional core/shell limit. In section-II, we have synthesized hetero-structured nanocrystals with CdSe rich core and CdS-ZnS hybrid shell using a simple single-step reaction. These nanocrystals exhibit a very rare example of an optically bi-functional material, simultaneously exhibiting high (~65%) photoluminescence efficiency and strong two-photon absorption cross-section of 1923 GM. Open-aperture z-scan technique was used to measure two-photon absorptions. Chapter 7 is divided into two sections and deals with the generation of white-light emitting nanophosphors. Section-I addresses the white-light emission from a blend of blue, green and red emitting CdSeS nanocrystals. Different shades of the emitted white-light were achieved by tailoring the composition of the blende. Chromaticity of the emitted light of a particular blend is independent of excitation wavelength. Section-II discusses a new approach to generate white-light by combining surface-state emission of nanocrystalline host and d-electron transitions from dopant centres, with an example of Mn2+-doped CdS nanocrystals. Relative contributions from both surface-state emission and Mn2+ d-emission can be tuned by controlling the dopant concentration to generate white lights of different shades. Similar to section-I, here again the chromaticity of the emitted light is independent of the excitation wavelength; but this approach offers additional advantages. Since the surface state emission as well as the Mn2+ d-emission are relatively less sensitive to a size variation compared to the band-edge emission, the chromaticity of the emitted light is not critically dependent on the particle size. Most importantly, these nanocrystals exhibit a huge stokes shift between the absorption and emission spectra resulting in a complete absence of the well-known self-absorption problem, thus, chromaticity of the white-light emitted by these nanocrystals remains unchanged both in dilute dispersion form as well as in solid state. Also there are two appendices in the thesis. Appendix A discusses the preparation of InP nanocrystals using a novel solvothermal route. Appendix B contains the equations explaining photoemission intensity ratios between Se and S (ISe/IS) for a model nanocrystal with a given internal structure.

The increasing demands on higher density, lower cost, higher speed, better endurance and longer retention has push flash memory technology, which is predominant and the driving force of the semiconductor nonvolatile memory market in recent years, to the position facing great challenges. However, the conventional flash memory technology using continuous highly doped polysilicon as floating gate, which is the most common in today’s commercial market, can't satisfy these demands, with the transistor size continuously scaling down beyond 32 nm. Nanocrystal floating gate flash memory and SONOS-type flash memory are considered among the most promising approaches to extend scalability and performance improvement for next generation flash memory. This dissertation addresses the issues that have big effects on nanocrystal floating gate flash memory and SONOS-type flash memory performances. New device structures and new material compatible to CMOS flow are proposed and demonstrated as potential solutions for further device performance improvement. First, the effect of nanocrystal-high k dielectric interface quality on nanocrystal flash memory performance is studied. By using germanium-silicon core-shell nanocrystals or ruthenium nanocrystals buried in HfO₂ as charge storage nodes, high interface quality has been achieved, leading to promising memory device performance. Next, another crucial challenge for nanocrystal flash memory on how to deposit uniformly distributed nanocrystal matrix in good shape and size control with high density is discussed. Using protein GroEL to obtain well ordered high density nanocrystal pattern, a flash memory device with Ni nanocrystals buried in HfO₂ is demonstrated. For this technique, the nanocrystal size is restricted to the GroEL's central cavity size and the density is limited by protein template. To overcome this limitation, a novel method using self-assembled Co-SiO₂ nanocrystals as charge storage nodes is demonstrated. Separated by thin SiO₂, these nanocrystals can form close packed form to achieve ultrahigh density. Finally, charge trapping layer band engineering is proposed for SONOS-type memory for better memory performance. By manipulating the pulse ratio of Hf and Al precursor during ALD deposition, the band diagram of Hf[subscript x]Al[subscript y]O charge trapping layer is optimized to have a Hf : Al ratio 3:1 at bottom and 1:3 at the top, leading to better trade-off between programming and retention for the of memory device. / text