Semiconductor Nanocrystals Based on Group IV Materials: Synthesis and Characterization towards Applications in Solar Cells

<p>Quantum dots have attracted a lot of interest in the past decade due to their physical and chemical properties. Quantum dots offer exciting possibilities for third generation photovoltaic devices. Light emitting quantum dots are stronger emitters than conventional organic dyes and are more resistant to degradation. This thesis focuses on the solution phase synthesis of semiconducting nanoparticles containing only easily available and relatively non-toxic materials, unlike cadmium containing nanoparticles. As an example, CdSe has been heavily studied for its outstanding optical properties. But the toxicity of cadmium encourage towards the use of other materials combining low toxicity with efficient emitting properties, such as silicon or germanium. We concentrate our research to silicon, germanium, tin, tin/germanium and Cu2ZnSnS4 (CZTS) nanoparticles. Tin based nanocrystals are poor emitters but have great potential as light harvesters in solar cells due to great semiconducting properties. The potential applications, crystal structures and properties of the target materials are described in Chapter 1. Chapter 2 details the characterization techniques used to define the nanoparticles synthesized in this research. Size and shape of the nanocrystals was evaluated using Transmission Electron Microscopy (TEM). The crystals structure was determined by X-ray diffraction (XRD) or Selected Area Electron Diffraction (SAED). The surface termination of quantum dots was assessed via Fourier Transform Infrared Spectroscopy (FTIR). Finally, the optical properties were determined using UV-Visible and photoluminescence spectroscopies. Silicon quantum dots (SiQDs) exhibit strong blue photoluminescence. The emission phenomenon of silicon nanostructures is still heavily debated in the literature. Chapter 3 looks into the origin of this fluorescence. The quantum dots were synthesized following a chemical reduction method in the presence of a surfactant. We evaluate the influence of the nanoparticle size variation on the optical properties. Then we explore the role of the passivation molecule on the surface of the silicon quantum dots on the light absorption and emission phenomena. The synthesis of CZTS nanoparticles via a solution phase process is described in Chapter 4. The aim of this research was the production of small monodisperse particles. We investigate the influence of the solvent environment in high temperature decomposition syntheses, followed by the study of a novel chemical reduction method for CZTS nanocrystals. Chapter 5 regroups the research conducted on germanium and tin quantum dots, as well as the study on germanium/tin alloy. Germanium quantum dots, strong light emitters, were characterized optically in this study. The semiconducting phase of tin has great physical properties but is unstable in an ambient environment. So far reported tin nanoparticles synthesized via a solution process display only the metallic structure of tin. Presenting similar structural properties, germanium is expected to stabilize the quantum dot configuration when alloyed to tin. In Chapter 6 are described three different collaborative projects towards the application of silicon quantum dots in solar cells. First silicon quantum dots were anchored to zinc oxide nanowires arrays. Then we investigated the optical properties of SiQDs blended in a matrix of block copolymers. The third project looks into the effect of SiQDs spread over the surface of a working silicon solar cell. Finally, the last chapter presents an overall conclusion and summarizes the main findings of this study. It also introduces perspectives for future work with concepts on how to overcome the problems encountered in this research and ideas towards concrete industrial application of quantum dots.</p>

Semiconductor Nanocrystals Based on Group IV Materials: Synthesis and Characterization towards Applications in Solar Cells

<p>Quantum dots have attracted a lot of interest in the past decade due to their physical and chemical properties. Quantum dots offer exciting possibilities for third generation photovoltaic devices. Light emitting quantum dots are stronger emitters than conventional organic dyes and are more resistant to degradation. This thesis focuses on the solution phase synthesis of semiconducting nanoparticles containing only easily available and relatively non-toxic materials, unlike cadmium containing nanoparticles. As an example, CdSe has been heavily studied for its outstanding optical properties. But the toxicity of cadmium encourage towards the use of other materials combining low toxicity with efficient emitting properties, such as silicon or germanium. We concentrate our research to silicon, germanium, tin, tin/germanium and Cu2ZnSnS4 (CZTS) nanoparticles. Tin based nanocrystals are poor emitters but have great potential as light harvesters in solar cells due to great semiconducting properties. The potential applications, crystal structures and properties of the target materials are described in Chapter 1. Chapter 2 details the characterization techniques used to define the nanoparticles synthesized in this research. Size and shape of the nanocrystals was evaluated using Transmission Electron Microscopy (TEM). The crystals structure was determined by X-ray diffraction (XRD) or Selected Area Electron Diffraction (SAED). The surface termination of quantum dots was assessed via Fourier Transform Infrared Spectroscopy (FTIR). Finally, the optical properties were determined using UV-Visible and photoluminescence spectroscopies. Silicon quantum dots (SiQDs) exhibit strong blue photoluminescence. The emission phenomenon of silicon nanostructures is still heavily debated in the literature. Chapter 3 looks into the origin of this fluorescence. The quantum dots were synthesized following a chemical reduction method in the presence of a surfactant. We evaluate the influence of the nanoparticle size variation on the optical properties. Then we explore the role of the passivation molecule on the surface of the silicon quantum dots on the light absorption and emission phenomena. The synthesis of CZTS nanoparticles via a solution phase process is described in Chapter 4. The aim of this research was the production of small monodisperse particles. We investigate the influence of the solvent environment in high temperature decomposition syntheses, followed by the study of a novel chemical reduction method for CZTS nanocrystals. Chapter 5 regroups the research conducted on germanium and tin quantum dots, as well as the study on germanium/tin alloy. Germanium quantum dots, strong light emitters, were characterized optically in this study. The semiconducting phase of tin has great physical properties but is unstable in an ambient environment. So far reported tin nanoparticles synthesized via a solution process display only the metallic structure of tin. Presenting similar structural properties, germanium is expected to stabilize the quantum dot configuration when alloyed to tin. In Chapter 6 are described three different collaborative projects towards the application of silicon quantum dots in solar cells. First silicon quantum dots were anchored to zinc oxide nanowires arrays. Then we investigated the optical properties of SiQDs blended in a matrix of block copolymers. The third project looks into the effect of SiQDs spread over the surface of a working silicon solar cell. Finally, the last chapter presents an overall conclusion and summarizes the main findings of this study. It also introduces perspectives for future work with concepts on how to overcome the problems encountered in this research and ideas towards concrete industrial application of quantum dots.</p>

Solar Energy as Renewable Energy for the Degradation of Pollutants using High Active Ag@TiO2/α-Fe2O3 Ternary Nanocomposite Photocatalyst

In this work, ternary Ag@TiO2/α-Fe2O3 nanocomposite were synthesized via solvothermal chemical reduction method using N,N-dimethylformamide (DMF) as solvent and reducing agent. The chemical procedure involves the use of only metals precursors without the need to use any other surfactants or capping agents. Physicochemical properties of the designed photocatalyst are found by means of various modern techniques. XRD data confirmed the high crystallinity of the obtained ternary nanocomposite. On the other hand, using TEM and HRTEM instruments, the shape and morphology of the Ag@TiO2/α-Fe2O3 nanocomposite were found to be spherical with an average particle size of 150 nm. The UV-Vis measurement shows that Ag@TiO2/α-Fe2O3 as photocatalyst exhibited good photo response in the visible region. The effect of preparation method and the performance of the designed photocatalyst were evaluated by photodegradation measurements of MB under visible light irradiation. We observed that the combination of metallic silver nanoparticles (AgNPs) and hematite iron oxide (α-Fe2O3) with titanium dioxide (TiO2) enhance the photocatalytic activity of the ternary Ag@TiO2/α-Fe2O3 photocatalyst compared to bare TiO2 suggesting its potential for many purification applications.

Batch Reactor vs. Microreactor System for Efficient AuNP Deposition on Actiavated Carbon Fibers

The process of noble metals ions recovery and the removal small fraction of nanoparticles from waste solution is an urgent topic not only from the economic but also ecology point of view. In this paper, the use of activated carbon fibers (ACF) as a “trap” for gold nanoparticles obtained by a chemical reduction method is described. The synthesized nanoparticles were stabilized either electrostatically or electrosterically and then deposited on carbon fibers or activated carbon fibers. Moreover, the deposition of metal on fibers was carried out in a batch reactor and a microreactor system. It is shown, that process carried out in the microreactor system is more efficient (95%) as compared to the batch reactor and allows for effective gold nanoparticles removal from the solution. Moreover, for similar conditions, the adsorption time of the AuNPs on ACF is shortened from 11 days for the process carried out in the batch reactor to 2.5 min in the microreactor system.

Using silver nanoparticles for the increase of YAG: Ce luminescence

In this study, silver nanoparticles with an average diameter of 50-70 nm were synthesized by the chemical reduction method. Subsequently, nanoparticles in different concentrations were introduced into the YAG: Ce luminescent powder synthesized by the method of two-stage coprecipitation into hexamine. The luminescence of samples with different contents of nanosilver was investigated. It was shown that the direct addition of nanosilver to YAG: Ce significantly impairs luminescence. Upon calcination at 900 °C, an increase in the luminescence of the YAG: Ce samples with silver nanoparticles was observed; however, the luminescence intensity was lower than that of the reference sample (without nanosilver). After calcination in an inert atmosphere at a temperature of 1550 °C, a significant increase in the luminescence intensity (of the order of 30-40 %) of the samples with the addition of a nanosilver was observed in comparison with the reference sample. Thus, silver nanoparticles can be successfully used to improve the YAG: Ce phosphors.

Highly Sensitive and Stable Copper-Based SERS Chips Prepared by a Chemical Reduction Method

Cu chips are cheaper than Ag and Au chips for practical SERS applications. However, copper substrates generally have weak SERS enhancement effects and poor stability. In the present work, Cu-based SERS chips with high sensitivity and stability were developed by a chemical reduction method. In the preparation process, Cu NPs were densely deposited onto fabric supports. The as-prepared Cu-coated fabric was hydrophobic with fairly good SERS performance. The Cu-coated fabric was able to be used as a SERS chip to detect crystal violet, and it exhibited an enhancement factor of 2.0 × 106 and gave a limit of detection (LOD) as low as 10–8 M. The hydrophobicity of the Cu membrane on the fabric is favorable to cleaning background interference signals and promoting the stability of Cu NPs to environment oxidation. However, this Cu SERS chip was still poor in its long-term stability. The SERS intensity on the chip was decreased to 18% of the original one after it was stored in air for 60 days. A simple introduction of Ag onto the clean Cu surface was achieved by a replacement reaction to further enhance the SERS performances of the Cu chips. The Ag-modified Cu chips showed an increase of the enhancement factor to 7.6 × 106 due to the plasmonic coupling between Cu and Ag in nanoscale, and decreased the LOD of CV to 10–11 M by three orders of magnitude. Owing to the additional protection of Ag shell, the SERS intensity of the Cu-Ag chip after a two-month storing maintained 80% of the original intensity. The Cu-Ag SERS chips were also applied to detect other organics, and showing wide linearity range and low LOD values for the quantitative detection.

Fabrication of zero-valent iron nanoparticles by green and chemical reduction methods: Application in the field of antibacterial activities for medicinal point of view

In the area of life sciences,iron nanoparticles (Fe NPs) have many applications. In this paper, the unique properties of iron nanoparticles as antimicrobials are studied. In this study, nanoparticles of iron have been fabricated by green and chemical reduction method. With the help of FESEM analysis and Zeta size analysis, the usual value of nanoparticles was found to be 10-30 nm in size. Furthermore, the prepared nanoparticles were examined for antibacterial perspective aligned with gram-positive and negative strains namely Staphylococcus aureus & Bacillus subtilis and Escherichia coli, using agar plate method and IC was also estimated using tube dilution assay.

Glycerol Valorization over ZrO2-Supported Copper Nanoparticles Catalysts Prepared by Chemical Reduction Method

Copper nanoparticles (NPs) and ZrO2-supported copper NPs (Cu NPs/ZrO2) were synthesized via a chemical reduction method applying different pH (4, 7 and 9) and evaluated in a glycerol dehydration reaction. Copper NPs were characterized with transmission electron microscopy (TEM) and UV–vis spectroscopy. Transmission electron microcopy (TEM) results revealed a homogeneous distribution of copper NPs. A hypsochromic shift was identified with UV–vis spectroscopy as the pH of the synthesis increased from pH = 4 to pH = 9. Zirconia-supported copper NPs catalysts were characterized using N2 physisorption, X-ray diffraction (XRD), TEM, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), temperature-programmed desorption of ammonia (NH3-TPD) and N2O chemisorption. The presence of ZrO2 in the chemical reduction method confirmed the dispersion of the copper nanoparticles. X-ray diffraction indicated only the presence of tetragonal zirconia patterns in the catalysts. XPS identified the Cu/Zr surface atomic ratio of the catalysts. TPR patterns showed two main peaks for the Cu NPS/ZrO2 pH = 9 catalyst; the first peak between 125 and 180 °C (region I) was ascribed to more dispersed copper species, and the second one between 180 and 250 °C (region II) was assigned to bulk CuO. The catalysts prepared at pH = 4 and pH = 7 only revealed reduction at lower temperatures (region I). Copper dispersion was determined by N2O chemisorption. With NH3-TPD it was found that Cu NPs/ZrO2 pH = 9 exhibited the highest total quantity of acidic sites and the highest apparent kinetic constant, with a value of 0.004 min−1. The different pH applied to the synthesis media of the copper nanoparticles determined the resultant copper dispersion on the ZrO2 support, providing active domains for glycerol conversion.