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In this thesis, the strength of the LBIC system is enhanced in different aspects that includes its feasibility as a non-destructive characterization tool, the signal analysis and development of analytical solution to have better understanding on the defects and inhomogeneities in the quantum dot based hetero-structures for device applications, finally understanding its limits due to the size of the laser beam and interpretation of artefacts in the signal appearance due to the presence of co-devices. Chapter#1 provides the introduction and literature survey of the LBIC system. It covers the importance and area of application of the LBIC. Chapter#2 various tools and instrumentation are discussed briefly for the systems that are developed in the lab as well as standard tools utilised for the material characterization. A LBIC instrumentation a novel colloidal quantum dots (CQD) thin film deposition system is discussed. In the last part along with the standard characterization systems a software tool (semiconductor device simulator) is discussed, which is used to visualize and understand the LBIC profile that is obtained experimentally. Chapter#3 provides the information of colloidal synthesis of PbS and HgxCd1-xTe quantum dots. Device fabrication process is explained step by step for the following devices. p-n junction silicon diodes, PbS-CQD/Si hetero-structures, ITO/PbS-CQD/Al crossbar structures and HgCdTe-CQD/Si hetero-structures. Chapter#4 deals with the major constraints imposed on the LBIC due to the need of Ohmic contacts. To overcome this major limitation, in this work, the origin of the signal is studied with the remote contact geometry for silicon p-n junction devices. It was observed that the signals can be collected with the capacitively coupled remote contacts, where LBIC was ultimately demonstrated as contactless measurement tool without any compromise on the measurements and thus obtained physical parameters. The effect of finite laser beam size is also described, which was found to have effect on the actual dimensions measured with the LBIC images. LBIC utility is further enhanced with the Si/CQD based hetero-structure devices, which are the potential candidates in the evolving device technology to be utilized in various modular systems such as PDs and LED applications. Chapter#5 discusses the origin and possible mechanisms for lateral photo-voltage which is closely monitored in the PbS-CQD/Si hetero-junction device systems. Interestingly, it is observed that there are two different line profiles for n and p type Si substrates. Different mechanisms that give rise to this kind of profiles were found to be distinct and are related to the band alignment of the CQD/Si hetero-structure. It lead to the revelation of an interesting phenomenon and believed to be universally observed irrespective of the materials involved in the formation of hetero-junction. Simulations and experimental results are quite consistent and in agreement with each other, which confirm the underlying physical mechanism that connects the LBIC anomalies with the band alignment. Chapter#6 deals with the spatial variations in the transverse photocurrent in the PbS-CQD film which is studied as a function of applied bias. Analytical equation is setup for the photocurrent in the CQD film under applied bias with the help of available transport mechanism and equations from the literature. The spatial non-uniformity that exists in the photocurrent proved to be the result of spatial inhomoginities in the physical parameters. By correlating the spatial data to the analytical equation, it is shown that the inhomoginities can be predicted. This approach is important for the devices, where monolithic detectors are fabricated by depositing CQD film on Read-Out-Integrated-Circuit (ROIC), where the manifestation of non-uniformity can be understood and probably fixed. Chapter#7 HgCdTe CQD based devices are studied for the purpose of photo-detector applications in MWIR (3  5 μm) region. HgxCd1-xTe Colloidal quantum dots are technologically important due to their wide absorption range that covers different regions of the atmospheric window. HgxCd1-xTe are successfully synthesised, which covers the absorption edge up to ~6.25 m in the IR region. Absorption and photo-response studies are carried out on HgxCd1-xTe/Si hetero-junctions under incident IR radiation. It is observed that the band gap of the quantum dots can be tuned easily by controlling the growth time as a parameter, thus moulded HgxCd1-xTe CQD/Si hetero-structures were found to have good photo-response. Chapter#8 the summary and the future direction and scope of the work is discussed. This includes the interesting observations during this thesis work which are not reported here in details.

The assembly of nanostructures into “multi”-dimensional materials is one of the main topics occurring in nanoscience today. It is now possible to produce high quality nanostructures reproducibly but for their further application larger structures that are easier to handle are required. Nevertheless during their assembly their nanometer size and accompanying properties must be maintained. This challenge was addressed in this work. Lead chalcogenides have been chosen as an example system because they are expected to offer great opportunities as thermoelectric materials. Three different ways to achieve assemblies of lead chalcogenide nanostructures were used and the resulting structures characterized with respect to their potential application as thermoelectric material. The first means by which a “multi”-dimensional assembly of lead chalcogenide quantum dots can be produced is the formation of porous structures such as aerogels and xerogels. A procedure, where the addition of an initiator such as oxidizers or incident radiation is unnecessary, is introduced and the formation process studied by absorption spectroscopy. The time-consuming aggregation step could be significantly reduced by employing a slightly elevated temperature during gelation that does not lead to any observable differences within the resulting gel structures. After either supercritical or subcritical drying, highly porous monolithic gel structures can be achieved. During the gel formation the size and the shape of the particles changed and they were directly linked together. Nevertheless the resulting porous structures remain crystalline and size dependent effects of the optical properties could be shown. Gels produced from a mixture of PbS and PbSe QDs show a homogenous distribution of both materials but it is not clear to what extent they form an alloy. Although the particles are directly linked together the resulting porous structures possess a very high resistivity and so it was not possible to characterize the semiconductor aerogels with regard to their thermoelectric properties. To achieve an enhanced conductivity porous structures containing PbS and Au nanoparticles have been produced. As has been seen for the pure semiconductor gels the size of the PbS quantum dots has increased and elongated particles were formed. In contrast to the PbS QDs the Au nanoparticles did not change their size and shape and are unevenly distributed within the PbS network. Through the use of the gold nanoparticles the conductivity could be increased and although the conductivity is still quite small, it was possible to determine Seebeck coefficients near room temperature for a mixed semiconductor-metal gel. The second means by which QD solids could be formed was by the compaction of the QD building blocks into a material that is still nanostructured. Therefore the synthesis of PbS was optimized to achieve sufficient amounts of PbS quantum dots. The ligands used in the synthesis of the QDs unfortunately act as an insulating layer resulting in QD solids with resistivities as high as 2 Gigaohm. For this reason different surface modification strategies were introduced to minimize the interparticle distance and to increase the coupling between the QDs so as to increase the conductivity of the resulting quantum dot solids. One very promising method was the exchange of the initial ligands by shorter ones that can be destroyed at lower temperatures. By such heat treatments the resistivity could be decreased by up to six orders of magnitude. For the pressing of the quantum dots two different compaction methods (SPS and hydraulic pressing) were compared. While the grain growth within the SPS pressed samples is significantly higher the same densification can be achieved by a cold hydraulic pressing as well as by SPS. The densification could be further increased through the use of preheated PbS QDs due to the destruction of the ligands. Samples which had been surface modified with MPA and subsequently thermally treated show the best results with respect to their thermopower and resistivities. Nevertheless the conductivity of the QD solids is still too high for them to be used as efficient thermoelectric materials. The final assembly method does not involve QDs but instead with one dimensional nanowires. Therefore a synthesis was developed that enables the formation of PbS nanowires of different diameters and one that is easy up-scalable. By the use of a less reactive sulfur precursor and an additional surfactant the formation of nuclei is significantly retarded and within an annealing time of two hours nanowires can be formed presumably by an oriented attachment mechanism. Single crystalline nanowires with a diameter of 65-105 nm could be achieved with the longest axes of the nanowires being parallel to [100]. The resulting nanowires were used as building blocks for film formation on glass substrates by an easily implemented method that requires no special equipment. To characterize the films with a view to their possible application as a thermoelectric material, surface modifications of the films were performed to improve the charge transfer in the films and the Seebeck coefficients of the resulting films measured. Therefore the previous approach of using MPA was applied and a subsequent thermal treatment demonstrated very promising results. In addition an crosslinking ligand was used for surface treatment that leads to similar results as was observed for the thermally treated MPA approach. Both approaches lead to an order of magnitude decrease in the resistivity and due to the fewer grain boundaries present in the films composed of nanowires as compared to the QD assemblies the conductivity is significantly higher. The Seebeck coefficient measurements show that the thermal treatment only slightly affects the Seebeck coefficients. Therefore a significantly higher power factor could be achieved for the nanowire films than for the QD solids.

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We introduce new oxygen- and moisture-proof polymer matrixes based on polyisobutylene (PIB) and its block copolymer with styrene [poly(styrene-block-isobutylene-blockstyrene), PSt-b-PIB-b-PSt] for the encapsulation of colloidal semiconductor nanocrystals. In order to prepare transparent and processable composites, we developed a special procedure of nanocrystal surface engineering including ligand exchange of parental organic ligands to inorganic species followed by the attachment of specially designed short-chain PIB functionalized with an amino group. The latter provides excellent compatibility of the particles with the polymer matrixes. As colloidal nanocrystals, we chose CdSe nanoplatelets (NPLs) because they possess a large surface and thus are very sensitive to the environment, in particular in terms of their limited photostability. The encapsulation strategy is quite general and can be applied to a wide variety of semiconductor nanocrystals, as demonstrated on the example of PbS quantum dots. All obtained composites exhibited excellent photostability, being tested in a focus of a powerful white-light source, as well as exceptional chemical stability in a strongly acidic media. We compared these properties of the new composites with those of widely used polyacrylate-based materials, demonstrating the superiority of the former. The developed composites are of particular interest for application in optoelectronic devices, such as color-conversion light-emitting diodes, laser diodes, luminescent solar concentrators, etc.