Academic literature on the topic 'Photophysical Aspect -Semiconductor Nanocrystals'

Lead halide perovskites have been extensively studied to explore their photovoltaic properties. They offer a useful strategy for continuous tuning of the semiconductor bandgap. In addition to photovoltaic applications these lead halide perovskites offer rich photophysical properties with ability to induce electron and hole transfer at the semiconductor/electrolyte interface. The photoinduced electron transfer between CsPbBr3 quantum dots and methyl viologen shows electron transfer to be completed with 20 ps. The transient absorption spectroscopy and emission spectroscopy offers mechanistic and kinetic insights of the interfacial charge transfer processes. CsPbBr3 films cast from colloidal suspension can also be transformed into CsPbI3 via a halide exchange reaction upon exposure to a heated PbI2 solution (~70°C). The internal structure of hybrid CsPbBrxI3-x varies with increasing thickness of the exchanged film. The gradient structure thus allows us to probe the flow of the charge carriers within the film. The electron transfer properties that highlight photocatalytic properties of mixed halide nanocrystals will be discussed.

Abstract Perovskite semiconductor nanocrystals have emerged as a promising family of materials for optoelectronic applications including light-emitting diodes, lasers, light-to-electricity convertors and quantum light emitters. The performances of these devices are fundamentally dependent on different aspects of the excited-state dynamics in nanocrystals. Herein, we summarize the recent progress on the photoinduced carrier dynamics studied by a variety of time-resolved spectroscopic methods in perovskite nanocrystals. We review the dynamics of carrier generation, recombination and transport under different excitation densities and photon energies to show the pathways that underpin the photophysics for light-emitting diodes and solar cells. Then, we highlight the up-to-date spin dynamics and coherent exciton dynamics being manifested with the exciton fine levels in perovskite semiconductor nanocrystals which are essential for potential applications in quantum information technology. We also discuss the controversial results and the possible origins yet to be resolved. In-depth study toward a comprehensive picture of the excited-state dynamics in perovskite nanocrystals may provide the key knowledge of the device operation mechanism, enlighten the direction for device optimization and stimulate the adventure of new conceptual devices.

Colloidal semiconductor nanocrystals from CdSe show photoluminescence quenching via titration with porphyrin derivatives. This quenching is an indication of the formation of nanoassemblies via surface attachment of pyridyl linker groups. As a consequence of the complex formation, dynamic and/or static interactions between QD and porphyrins are induced. Quenching efficiencies depend critically on sample stability, temperature, solvent, and electronic properties of the porphyrins. In order to optimize photoinduced dynamic processes these parameters have to be under control.

The facile fabrication of free-floating organic nanocrystals (ONCs) was achieved via the kinetically controlled self-assembly of simple perylene diimide building blocks in aqueous medium. The ONCs have a thin rectangular shape, with an aspect ratio that is controlled by the content of the organic cosolvent (THF). The nanocrystals were characterized in solution by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. The ONCs retain their structure upon drying, as was evidenced by TEM and atom force microscopy. Photophysical studies, including femtosecond transient absorption spectroscopy, revealed a distinct influence of the ONC morphology on their photonic properties (excitation energy transfer was observed only in the high-aspect ONCs). Convenient control over the structure and function of organic nanocrystals can enhance their utility in new and developed technologies.

We present the methodologies for developing high-performance thermoelectric materials using nanostructured interfaces by reviewing our three studies and giving the new aspect of nanostructuring results. (1) Connected Si nanocrystals exhibited ultrasmall thermal conductivity. The drastic thermal conductivity reduction was brought by phonon confinement and phonon scattering. Here, we present discussion about the new aspect for phonon transport: not only nanocrystal size but also shape can contribute to thermal conductivity reduction. (2) Si films including Ge nanocrystals demonstrated that phonon and carrier conductions were independently controlled in the films, where carriers were easily transported through the interfaces between Si and Ge, while phonons could be effectively scattered at the interfaces. (3) Embedded-ZnO nanowire structure demonstrated the simultaneous realization of power factor increase and thermal conductivity reduction. The [Formula: see text] increase was caused by the interface-dominated carrier transport. The nanowire interfaces also worked as phonon scatterers, resulting in the thermal conductivity reduction.

Bioluminescence resonance energy transfer (BRET) is the non-radiative transfer of energy from a bioluminescent protein donor to a fluorophore acceptor. It shares all the formalism of Förster resonance energy transfer (FRET) but differs in one key aspect: that the excited donor here is produced by biochemical means and not by an external illumination. Often the choice of BRET source is the bioluminescent protein Renilla luciferase, which catalyzes the oxidation of a substrate, typically coelenterazine, producing an oxidized product in its electronic excited state that, in turn, couples with a proximal fluorophore resulting in a fluorescence emission from the acceptor. The acceptors pertinent to this discussion are semiconductor quantum dots (QDs), which offer some unrivalled photophysical properties. Amongst other advantages, the QD’s large Stokes shift is particularly advantageous as it allows easy and accurate deconstruction of acceptor signal, which is difficult to attain using organic dyes or fluorescent proteins. QD-BRET systems are gaining popularity in non-invasive bioimaging and as probes for biosensing as they don’t require external optical illumination, which dramatically improves the signal-to-noise ratio by avoiding background auto-fluorescence. Despite the additional advantages such systems offer, there are challenges lying ahead that need to be addressed before they are utilized for translational types of research.

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.

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.

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.

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 motivation of this thesis is to understand the physical properties of semiconductor quantum dots (QDs) and to get insight on the basic physics of charge separation in composites made from QDs with graphene oxide (GO)/organic semiconductors. The flexion phonon interactions is one of fundamental issues in solid state physics, which has a significant effect on both electrical and optical properties of solid state materials. This thesis investigates the physical properties of aqueous grown QDs through exciton-phonon coupling and non-radiative relaxation of excited carriers which have been carried out by temperature dependent photoluminescence spectroscopy. Several e orts have been made in order to understand the basic physics of photo induced charge separation in the hybrid systems made from QDs with graphene oxide and organic semiconductors. Investigations on the photoconductivity of the devices made from these hybrid composites have been carried out keeping the motive of its application in nanotechnology. This thesis work is presented in six chapters inclusive of summary and directions for future work. Chapter 1 discusses the background knowledge and information of the general properties of semiconductor nanostructures, QDs and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the exciton-phonon scattering and nonradiative relaxations of excited carriers in visible emitting cadmium telluride QDs with help of temperature and size dependent photoluminescence. Chapter 4 presents the investigations on time resolved photoluminescence dynamics and temperature dependent photoluminescence properties of near infrared (NIR) emitting mercury cadmium telluride (CdHgTe). Chapter 5 discusses the importance of NIR emitting silver sulphide (Ag2S) QDs and gives insight of nonradiative recombinations through defect/trap states. Chapter 6 investigates the excited state interactions between CdHgTe QDs and GO. Chapter 7 focuses on the understanding of basic physics of charge separation/transfer between poly (3hexylthiophene) and Ag2S QDs. Chapter 1: Semiconductor nanostructures have attracted significant scientific attention due to their fundamental physical properties and technological interests. Quasi zero dimensional nanocrystals or quantum dots (QDs) have shown unique optical and electrical properties compared to its bulk counterpart. These QDs show discrete energy levels due to the quantum confinement effect hence known as arti cial atoms. Large surface to volume ratio in these QDs is expected to play a crucial role in determing the photo-physical properties. Temperature dependent photoluminescence is a powerful tool for understanding the role of the large surface area on exciton recombination process in QDs. Inorganic QDs combined with different materials like graphene oxide or organic semiconductors forms an exciting class of synthetic materials which integrates the properties of organic and inorganic semiconductors. It is quite important to understand the basic physics of electronic interactions in these composites for its future application in many elds. Chapter 2: Synthesis of the inorganic QDs, graphene oxide, composites and fabrication of devices is an important and integral part of this thesis. Hydrothermal and three necked ask technique is adopted to get highly dispersible colloidal quantum dots in solvents. Synthesis of graphene oxide from graphite through oxidation and ultrasonication has been carried out to obtain homogenous dispersed graphene oxide in water. Structural properties have been studied by techniques like X ray diffraction, Raman spectroscopy, X ray photoelectron spectroscopy and high resolution transmission electron microscopy. Morphological properties are studied by atomic force microscopy and transmission electron microscopy. Optical properties are investigated by absorption spectroscopy, steady state and time resolved photoluminescence spectroscopy. Photoconductivity characteristics are analyzed to understand the basics of enhanced current in the various devices made from QDs composites. Chapter 3:Investigations on exciton phonon coupling and nonradiative relaxations in various sizes of visible light emitting cadmium telluride (CdTe) QDs size have been presented. Due to the large surface area, QDs are prone to have defect/trap states which can affect the exciton relaxation. Hence, understanding the role of such defect/trap states on photoluminescence is very essential for achieving the optimum optical properties. Temperature dependent (15 300 K) photoluminescence has been used to understand nonradiative relaxation of excited carriers. Thermally activated processes and multiple phonons scattering is thoroughly investigated to understand the quenching of photoluminescence with temperature. The strength of exciton-phonon coupling is investigated which determines the variation in energy bandgap of QDs with temperature. Role of exciton phonon scattering is also discussed to understand the basic physics of photoluminescence line width broadening in QDs. Chapter 4 and 5: This part of thesis focuses on the size and temperature pho-toluminescence properties of near infra red emitting ternary alloyed CdHgTe and Ag2S QDs. Near infrared emitting semiconductor quantum dots (QDs) have attracted significant scientific and technological interests due to their potential applications in the fields of photosensor, solar energy harvesting cells, telecommunication and biological tissue imaging etc. Structural and photophysical properties of CdHgTe QDs have been analyzed by high resolution transmission electron microscopy, X rayphotoelectron microscopy, photoluminescence decay kinetics and low temperature photoluminescence. Investigations on the nonradiative recombinations through trap/defects states and exciton phonon coupling are carried out in colloidal Ag 2S QDs which emits in the range of 1065 1260 nm. Particularly, the photoluminescence quenching mechanism with increasing temperature is analyzed in the presence of multiple nonradiative relaxation channels, where the excited carriers are thermally stimulated to the surface defect/trap states of QDs. Chapter 6 and 7: The aim of these chapters is to understand the basic physics of photo induced charge separation in the hybrid systems made from the inorganic QDs with graphene oxide and organic semiconductors. In chapter 6, CdHgTe QDs are decorated on graphene oxide sheets through physisorption. The excited state electronic interactions have been studied by optical and electrical characterizations in these CdHgTe QDs GO hybrid systems. In chapter 7, investigations are carried out for understanding the basic physics of charge separation in the composites of Ag2S QDs and poly (3hexylthiophene 2,5 diyl)(P3HT). These composites of inorganic organic materials are made by simple mixing with help of ultrasonication technique. Steady state and time resolved photoluminescence measurements are used as powerful technique to gain insight of energy/charge transfer process between P3HT and Ag2S QDs. Furthermore, investigations have been carried out on the photoconductivity of the devices made from these hybrid composites keeping the motive of its application in nanotechnology. Chapter 8: The conclusions of the work presented in this thesis are coherently summarized in this chapter. Thoughts and prospective for future directions are also summed up.

The motivation of this thesis is to understand the physical properties of semiconductor quantum dots (QDs) and to get insight on the basic physics of charge separation in composites made from QDs with graphene oxide (GO)/organic semiconductors. The flexion phonon interactions is one of fundamental issues in solid state physics, which has a significant effect on both electrical and optical properties of solid state materials. This thesis investigates the physical properties of aqueous grown QDs through exciton-phonon coupling and non-radiative relaxation of excited carriers which have been carried out by temperature dependent photoluminescence spectroscopy. Several e orts have been made in order to understand the basic physics of photo induced charge separation in the hybrid systems made from QDs with graphene oxide and organic semiconductors. Investigations on the photoconductivity of the devices made from these hybrid composites have been carried out keeping the motive of its application in nanotechnology. This thesis work is presented in six chapters inclusive of summary and directions for future work. Chapter 1 discusses the background knowledge and information of the general properties of semiconductor nanostructures, QDs and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the exciton-phonon scattering and nonradiative relaxations of excited carriers in visible emitting cadmium telluride QDs with help of temperature and size dependent photoluminescence. Chapter 4 presents the investigations on time resolved photoluminescence dynamics and temperature dependent photoluminescence properties of near infrared (NIR) emitting mercury cadmium telluride (CdHgTe). Chapter 5 discusses the importance of NIR emitting silver sulphide (Ag2S) QDs and gives insight of nonradiative recombinations through defect/trap states. Chapter 6 investigates the excited state interactions between CdHgTe QDs and GO. Chapter 7 focuses on the understanding of basic physics of charge separation/transfer between poly (3hexylthiophene) and Ag2S QDs. Chapter 1: Semiconductor nanostructures have attracted significant scientific attention due to their fundamental physical properties and technological interests. Quasi zero dimensional nanocrystals or quantum dots (QDs) have shown unique optical and electrical properties compared to its bulk counterpart. These QDs show discrete energy levels due to the quantum confinement effect hence known as arti cial atoms. Large surface to volume ratio in these QDs is expected to play a crucial role in determing the photo-physical properties. Temperature dependent photoluminescence is a powerful tool for understanding the role of the large surface area on exciton recombination process in QDs. Inorganic QDs combined with different materials like graphene oxide or organic semiconductors forms an exciting class of synthetic materials which integrates the properties of organic and inorganic semiconductors. It is quite important to understand the basic physics of electronic interactions in these composites for its future application in many elds. Chapter 2: Synthesis of the inorganic QDs, graphene oxide, composites and fabrication of devices is an important and integral part of this thesis. Hydrothermal and three necked ask technique is adopted to get highly dispersible colloidal quantum dots in solvents. Synthesis of graphene oxide from graphite through oxidation and ultrasonication has been carried out to obtain homogenous dispersed graphene oxide in water. Structural properties have been studied by techniques like X ray diffraction, Raman spectroscopy, X ray photoelectron spectroscopy and high resolution transmission electron microscopy. Morphological properties are studied by atomic force microscopy and transmission electron microscopy. Optical properties are investigated by absorption spectroscopy, steady state and time resolved photoluminescence spectroscopy. Photoconductivity characteristics are analyzed to understand the basics of enhanced current in the various devices made from QDs composites. Chapter 3:Investigations on exciton phonon coupling and nonradiative relaxations in various sizes of visible light emitting cadmium telluride (CdTe) QDs size have been presented. Due to the large surface area, QDs are prone to have defect/trap states which can affect the exciton relaxation. Hence, understanding the role of such defect/trap states on photoluminescence is very essential for achieving the optimum optical properties. Temperature dependent (15 300 K) photoluminescence has been used to understand nonradiative relaxation of excited carriers. Thermally activated processes and multiple phonons scattering is thoroughly investigated to understand the quenching of photoluminescence with temperature. The strength of exciton-phonon coupling is investigated which determines the variation in energy bandgap of QDs with temperature. Role of exciton phonon scattering is also discussed to understand the basic physics of photoluminescence line width broadening in QDs. Chapter 4 and 5: This part of thesis focuses on the size and temperature pho-toluminescence properties of near infra red emitting ternary alloyed CdHgTe and Ag2S QDs. Near infrared emitting semiconductor quantum dots (QDs) have attracted significant scientific and technological interests due to their potential applications in the fields of photosensor, solar energy harvesting cells, telecommunication and biological tissue imaging etc. Structural and photophysical properties of CdHgTe QDs have been analyzed by high resolution transmission electron microscopy, X rayphotoelectron microscopy, photoluminescence decay kinetics and low temperature photoluminescence. Investigations on the nonradiative recombinations through trap/defects states and exciton phonon coupling are carried out in colloidal Ag 2S QDs which emits in the range of 1065 1260 nm. Particularly, the photoluminescence quenching mechanism with increasing temperature is analyzed in the presence of multiple nonradiative relaxation channels, where the excited carriers are thermally stimulated to the surface defect/trap states of QDs. Chapter 6 and 7: The aim of these chapters is to understand the basic physics of photo induced charge separation in the hybrid systems made from the inorganic QDs with graphene oxide and organic semiconductors. In chapter 6, CdHgTe QDs are decorated on graphene oxide sheets through physisorption. The excited state electronic interactions have been studied by optical and electrical characterizations in these CdHgTe QDs GO hybrid systems. In chapter 7, investigations are carried out for understanding the basic physics of charge separation in the composites of Ag2S QDs and poly (3hexylthiophene 2,5 diyl)(P3HT). These composites of inorganic organic materials are made by simple mixing with help of ultrasonication technique. Steady state and time resolved photoluminescence measurements are used as powerful technique to gain insight of energy/charge transfer process between P3HT and Ag2S QDs. Furthermore, investigations have been carried out on the photoconductivity of the devices made from these hybrid composites keeping the motive of its application in nanotechnology. Chapter 8: The conclusions of the work presented in this thesis are coherently summarized in this chapter. Thoughts and prospective for future directions are also summed up.