Browsing by Subject "Nanocrystals"
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Item Complex Refractive Index Modeling and Nanoscale Patterning of Solid-State Colloidal Quantum Dots for Nanophotonic Applications(2019-05) Dement, DanaThe small size of photoluminescent, nanocrystal quantum dots (QDs) leads to a variety of unique optical properties that are well-suited to many optoelectronic devices and nanophotonic studies. Here, we demonstrate techniques to further improve the design of solid-state QD structures. A problem for many applications is that predicting the optical behavior of QD solids is difficult because the complex refractive index of QD solids is a composite quantity that is dependent on size, ligand chain length, and the deposition process of the QDs. To address this problem, we show that the intrinsic refractive index of neat CdSe/CdS QDs can be extracted from solution-state absorption data. We then show how this information can be used with effective medium approximations to describe the effective refractive index of QD films associated with a variety of QD sizes and packing fractions. Our predictions are verified experimentally by spectroscopic ellipsometry. With our modeling tool, we can also understand packing variations between QD films and predict the absorption in solid-state QD structures, leading to significant savings in both time and materials. Using the same QD materials, we next address the need for accurate patterning of QD solids at the nanoscale. We have found that direct electron beam lithography is a straightforward patterning process that does not require ligand exchange and results in structures that retain bright photoluminescence. We demonstrate that feature sizes as narrow as 30 nm with many QD layers can be patterned. These structures can withstand sonication in a variety of solvents, show no distortion, and can be placed within 20 nm of their intended location nearly 100% of the time. Combining our nanofabrication technique with the ability to measure the refractive index of the QD pattern, we find that edge effects arising from the finite shape of the QD nanostructure lead to substantial absorption enhancement when compared to an equivalent volume region taken from a continuous QD film. Finally, we explore more complex structures by patterning QD arrays, multilayer QD structures, and QD disks inside plasmonic resonators. We believe that the work presented here lays important groundwork to improve the modeling of QD solids and reveals new ways QDs can be incorporated into devices and nanophotonic designs.Item Enhancing the Figure of Merit, ZT, of Silicon Germanium Nanocrystal Films by Synthesizing Dense Films using Nonthermal Plasma and Post Processing(2018-05) Mishra, SadhanaA thermoelectric material’s dimensionless figure of merit (ZT) determines the efficiency of conversion of heat into electricity of a thermoelectric generator (TEG). Increase in ZT, increases the efficiency of a thermoelectric generator. The figure of merit increases with increase in material’s electrical conductivity and Seebeck coefficient while decreases with an increase in thermal conductivity. Recently, the ZT of silicon germanium alloys have been increased by nanostructuring the bulk which lead to decrease in thermal conductivity by increase in phonon scattering due to nanoscale crystal grain sizes. Plasma synthesised doped silicon germanium nanocrystals have a narrow size distribution and are promising candidates as opposed to ball-milled nanopowder. Nanocrystals produced by plasma synthesis needs to be fabricated as thin films for microelectronic applications. Nanocrystal films synthesised, by rastering of substrates, in the nonthermal plasma reactor are very porous and hence have low electrical conductivity. To produce denser nanocrystal films, the plasma reactor was modified and post processes were introduced. Mixed-phase silicon films were produced by dual-plasma setup and these films were annealed to form fully nanoscrystalline silicon films. The mixed-phase and fully nanocrystalline silicon films were determined to have low porosity. These films were characterized for their crystallinity, thickness and the average crystal grain size. After characterization of the mixed-phase and nanocrystalline silicon films, the thermoelectric properties (Seebeck coefficient, thermal conductivity and electrical conductivity) were determined and hence the ZT. This ZT was determined with just silicon and thus low. Future work can be undertaken with mixed-phase film composed of doped silicon germanium alloy and to determine the thermoelectric properties.Item In-flight gas phase passivation of silicon nanocrystals for novel inorganic-silicon nanocrystal based electroluminescent devices.(2009-10) Liptak, Richard WilliamSilicon nanocrystals (SiNCs) have become a heavily researched material over the past several years. Researchers envision that this material can be used in many diverse applications such as electronic devices, non-toxic biological tags, optical devices such as LEDs, lasers or displays, thermoelectrics, and photovoltaic (PV) applications. For many of these proposed applications one needs to properly control the NC size and the surface chemistry via passivation. Current passivation techniques allow for the creation of highly efficient SiNC optical emitters, however the emission of these NCs are fixed in the red- NIR range. To resolve this issue several novel in-flight passivation techniques were investigated. A novel dual-plasma setup which allows for the in-flight passivation of SiNCs through a thermal or LPCVD based nitridation process was developed first. FTIR and XPS analysis were used to study the surface chemistry on of the nitride passivated NCs while TEM was used to investigate whether or not a “shell” was grown on the surface. PL measurements and thermal stability tests were performed on the nitride passivated NCs to gain a further understanding of the stability (in both air as well as other ambients) of the NCs and their surface chemistry. Tunable full color emission from SiNCs was developed for the dual-plasma reactor utilizing CF4 as both an etching and passivating source. F radicals generated in the etching plasma remove Si from the surface of the NC, while at the same time CF2 radicals lead to the formation of a fluorocarbon passivation layer on the NC surface. By controlling the parameters of the reactor (CF4 flow rate, power), the NC size and thus its color can be controlled. Red to green luminescence was observed from SiNCs and is believed to be due to the quantum confinement effect. The blue emission observed from the NCs is appears to be related to oxide related surface states. Despite the defects, high QY was observed from these CF4-etched NCs. The fluorocarbon passivation layer, although stable, prevents further functionalization of the NCs. To counteract this problem another silicon-based dry etch chemistry, SF6 was investigated. Full-color emission was observed from SF6 etched NCs, with QY 2X higher than that of CF4-etched NCs. A maximum QY of nearly 55% at 700 nm was observed after several weeks in air, comparable to that observed with alkyl passivation. The native oxidation of the bare oxidized and SF6-etched NCs were also studied. Results show that the NC oxidation follows the Cabrera-Mott mechanism for low temperature oxidation. Inorganic-NC based LED structures were then investigated. Fabrication processes for the inorganic hole and electron transport layers were developed by RF sputtering and atomic layer deposition (ALD). Thorough characterization was performed on the metal-oxide films (ZnO, TiO2, NiO) to verify their stoichiometry as well as study their optical and electrical properties. Novel inorganic-NC device structures were fabricated. Inorganic NC devices which use a metal-oxide HTL but no ETL, emit light, however their emission is so weak. The addition of an ETL increases the light output by a factor of 4, but the device reproducibility is poor. To improve efficiency two insulating matrix layers were investigated. In both cases, the film deposited on the top of the NC is rough, porous, discontinuous, and potentially full of traps – certainly not the ideal film for a device. Therefore, more work is needed, specifically on the NC layer to improve the structure of the as-deposited NC film, but efficient device structures appear to be possible.Item Inelastic scattering in STEM for studying structural and electronic properties of chalcogenide-based semiconductor nanocrystals(2013-09) Gunawan, Aloysius AndhikaTransmission electron microscopy (TEM) relies upon elastic and inelastic scattering signals to perform imaging and analysis of materials. TEM images typically contain contributions from both types of scattering. The ability to separate the contributions from elastic and inelastic processes individually through energy filter or electron energy loss spectroscopy (EELS) allows unique analysis that is otherwise unachievable. Two prominent types of inelastic scattering probed by EELS, namely plasmon and core-loss excitations, are useful for elucidating structural and electronic properties of chalcogenide-based semiconductor nanocrystals. The elastic scattering, however, is still a critical part of the analysis and used in conjunction with the separated inelastic scattering signals. The capability of TEM operated in scanning mode (STEM) to perform localized atomic length scale analysis also permits the understanding of the nanocrystals unattainable by other techniques. Despite the pivotal role of inelastic scatterings, their contributions for STEM imaging, particularly high-angle annular dark field STEM (HAADF-STEM), are not completely understood. This is not surprising since it is currently impossible to experimentally separate the inelastic signals contributing to HAADF-STEM images although images obtained under bright-field TEM mode can be analyzed separately from their scattering contributions using energy-filtering devices. In order to circumvent such problem, analysis based on simulation was done. The existing TEM image simulation algorithm called Multislice method, however, only accounts for elastic scattering. The existing Multislice algorithm was modified to incorporate (bulk or volume) plasmon inelastic scattering. The results were verified based on data from convergent-beam electron diffraction (CBED), electron energy loss spectroscopy (EELS), and HAADF-STEM imaging as well as comparison to experimental data. Dopant atoms are crucial factors which control optical, electronic, and also magnetic properties of semiconductors. Their location inside the materials has become more important with the miniaturization of devices. The precise determination of the position, however, poses a great challenge. Imaging using HAADF-STEM has proven adequate for locating heavy dopant atoms buried in relatively light matrix, particularly using aberration-corrected microscopes. The imaging method has been unsuccessful in detecting dopant atoms with similar atomic number as the matrix. Inelastic core-loss or inner-shell electronic excitations using EELS offer a unique solution when simultaneous imaging and EELS acquisitions are performed. The dopant atoms that are invisible in the images due to the small atomic number differences can be detected via spatial correlation with EELS core-loss data. Three types of samples with varying concentration of Mn dopant atoms in ZnSe nanocrystals were used to confirm such method. Precise locations of the dopant atoms on planes perpendicular to electron beam propagation could be determined although not all of the dopant atoms were detected due to limitations in experimental conditions.Another important type of chalcogenide-based nanocrystals is PbSe which is useful for solar cells. Colloidal method commonly used to synthesize the nanocrystals leave oleic acid capping ligands as surface passivation and size stabilizer. These ligands have critical roles in controlling electrical and optical properties of an individual nanocrystal and their assembly. Deemed insulating due to long chains of carbons, oleic acid is typically treated with short ligands such as hydrazines to decrease the inter-nanocrystal distances and improve electronic coupling among the neighboring nanocrystals. Despite its apparent insulating behavior, oleic acid was shown to exhibit surface plasmon coupling under certain circumstances. The geometric arrangement of the ligands was first investigated by HAADF-STEM imaging. Under air exposure, PbSe nanocyrstals easily oxidize to form oxide shells that are responsible for p-type doping by introducing surface acceptor states. At early oxidation stage (partial oxidation), prior to the formation of uniform oxide shells, the nanocrystals appear to form links between neighbors. Localized EELS analysis shows that these links are made of carbon based materials, most likely modified form of oleic acid ligands consisting of conjugated double bonds. Such modification occurred through oxidative dehydrogenation of the oleic acid ligands that is facilitated by the growing oxide shells on the surface of nanocrystals.Item Nonthemal plasma synthesis of indium phosphide nanocrystals and electrical properties of doped silicon nanocrystal films.(2010-02) Gresback, Ryan GerardThis thesis is concerning the plasma synthesis of semiconductor nanocrystals (NCs). Two systems of nanocrystals were studied, indium phosphide and doped silicon. A new method of synthesis of InP NCs is presented. It represents a new route for the synthesis of high quality compound semiconductor NCs. Additionally the electronic properties of doped silicon NCs were studied as a function of the doping concentration. Indium phosphide nanocrystals (InP NCs) were synthesized using a nonthermal plasma. The NCs were synthesized using a simple capacitively coupled plasma where the precursors are flowed through a 3/8” quartz tube with two outer ring electrodes. The size of the NCs was primarily controlled through the residence time of the NCs in the plasma. Residence times of 2-10 ms lead to particles with mean sizes between ~2.5-4 nm with size distributions less than 25% of the mean particle size. The mass yield for this system was found to be up to 40 mg/hr. When a ZnS shell was grown around the InP NCs, size-tunable emission from the blue-green to the red was observed. Quantum yields as high as 15% were observed with this synthesis route. This route allows for synthesis of free-standing NCs that can be easily manipulated with colloidal based techniques or included in devices without stabilizing ligands. The electrical conductivity of phosphorus doped Si NCs was studied as a function of the doping concentration. Doped Si NCs with mean sizes of 8-13 nm were spun cast onto a substrate with pre-deposited aluminum electrodes. The spin cast process produces films with zero to several monolayers of NCs. The conductivity of the films varies continuously from 10-11 S/cm for intrinsic NCs to 10-1 S/cm for highly doped NCs. These results indicate that the dopants are electrically active. The interpretation of these results means that the electronic properties of NCs can be tuned in a similar fashion as bulk semiconductors by introducing dopants. The ability to successfully dope NCs can have broad impact on the ability to form semiconductor devices.Item Simulation of dopant atom behavior in semiconducting nanocrystals(2015) Duncan, Samuel J. B.; Held, Jacob; Mkhoyan, K. AndreItem Synthesis and Doping of Silicon Nanocrystals for Versatile Nanocrystal Inks(2015-05) Kramer, NicolaasThe impact of nanotechnology on our society is getting larger every year. Electronics are becoming smaller and more powerful, the “Internet of Things” is all around us, and data generation is increasing exponentially. None of this would have been possible without the developments in nanotechnology. Crystalline semiconductor nanoparticles (nanocrystals) are one of the latest developments in the field of nanotechnology. This thesis addresses three important challenges for the transition of silicon nanocrys- tals from the lab bench to the marketplace: A better understanding of the nanocrystal synthesis was obtained, the electronic properties of the nanocrystals were characterized and tuned, and novel silicon nanocrystal inks were formed and applied using simple coating technologies. Plasma synthesis of nanocrystals has numerous advantages over traditional solution-based synthesis methods. While the formation of nanoparticles in low pressure nonthermal plasmas is well known, the heating mechanism leading to their crystallization is poorly understood. A combination of comprehensive plasma characterization with a nanoparticle heating model presented here reveals the underlying plasma physics leading to crystallization. The model predicts that the nanoparticles reach temperatures as high as 900 K in the plasma as a result of heating reactions on the nanoparticle sur- face. These temperatures are well above the gas temperature and sufficient for complete nanoparticle crystallization. Moving the field of plasma nanoparticle synthesis to atmospheric pressures is impor- tant for lowering its cost and making the process attractive for industrial applications. The heating and charging model for silicon nanoparticles was adapted in Chapter 3 to study plasmas maintained over a wide range of pressures (10 − 10^5 Pa). The model considers three collisionality regimes and determines the dominant contribution of each regime under various plasma conditions. Strong nanoparticle cooling at atmospheric pressures necessitates high plasma densities to reach temperatures required for crystallization of nanoparticles. Using experimentally determined plasma properties from the literature, the model estimates the nanoparticle temperature that is achieved during synthesis at atmospheric pressures. It was found that temperatures well above those required for crystallization can be achieved. Now that the synthesis of nanocrystals is understood, the second half of this thesis will focus on doping of the nanocrystals. The doping of semiconductor nanocrystals, which is vital for the optimization of nanocrystal-based devices, remains a challenge. Gas phase plasma approaches have been very successful in incorporating dopant atoms into nanocrystals by simply adding a dopant precursor during synthesis. However, little is known about the electronic activation of these dopants. This was investigated with field-effect transistor measurements using doped silicon nanocrystal films. It was found that, analogous to bulk silicon, boron and phosphorous electronically dope silicon nanocrystals. However, the dopant activation efficiency remains low as a result of self-purification of the dopants to the nanocrystal surface. Next the plasmonic properties of heavily doped silicon nanocrystals was explored. While the synthesis method was identical, the plasmonic behavior of phosphorus-doped and boron-doped nanocrystals was found the be significantly different. Phosphorus-doped nanocrystals exhibit a plasmon resonance immediately after synthesis, while boron-doped nanocrystals require a post-synthesis annealing or oxidation treatment. This is a result of the difference in dopant location. Phosphorus is more likely to be incorporated into the core of the nanocrystal, while the majority of boron is placed on the surface of the nanocrystal. The oxidized boron-doped particles exhibit stable plasmonic properties, and therefore this allows for the production of air-stable silicon-based plasmonic materials which is very interesting for certain applications. Finally the boron atoms were used to form a Lewis acidic nanocrystal surface chemistry allowing for the creation of ligand-less silicon nanocrystal solutions. This represents an immense step towards an abundant, non-toxic alternative to Pb and Cd-based nanocrystal technologies. The lack of long ligand chains enables the production of dense films with excellent electrical conductivity. This was demonstrated by forming uniform nanocrystal thin-films using simple and inexpensive spray coating techniques.Item Synthesis, deposition, and microstructure development of thin films formed by sulfidation and selenization of copper zinc tin sulfide nanocrystals(2014-08) Chernomordik, Boris DavidSignificant reduction in greenhouse gas emission and pollution associated with the global power demand can be accomplished by supplying tens-of-terawatts of power with solar cell technologies. No one solar cell material currently on the market is poised to meet this challenge due to issues such as manufacturing cost, material shortage, or material toxicity. For this reason, there is increasing interest in efficient light-absorbing materials that are comprised of abundant and non-toxic elements for thin film solar cell. Among these materials are copper zinc tin sulfide (Cu2ZnSnS4, or CZTS), copper zinc tin selenide (Cu2ZnSnSe4, or CZTSe), and copper zinc tin sulfoselenide alloys [Cu2ZnSn(SxSe1-x)4, or CZTSSe]. Laboratory power conversion efficiencies of CZTSSe-based solar cells have risen to almost 13% in less than three decades of research. Meeting the terawatt challenge will also require low cost fabrication. CZTSSe thin films from annealed colloidal nanocrystal coatings is an example of solution-based methods that can reduce manufacturing costs through advantages such as high throughput, high material utilization, and low capital expenses. The film microstructure and grain size affects the solar cell performance. To realize low cost commercial production and high efficiencies of CZTSSe-based solar cells, it is necessary to understand the fundamental factors that affect crystal growth and microstructure evolution during CZTSSe annealing. Cu2ZnSnS4 (CZTS) nanocrystals were synthesized via thermolysis of single-source cation and sulfur precursors copper, zinc and tin diethyldithiocarbamates. The average nanocrystal size could be tuned between 2 nm and 40 nm, by varying the synthesis temperature between 150 °C and 340 °C. The synthesis is rapid and is completed in less than 10 minutes. Characterization by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy confirm that the nanocrystals are nominally stoichiometric kesterite CZTS. The ~2 nm nanocrystals synthesized at 150 °C exhibit quantum confinement, with a band gap of 1.67 eV. Larger nanocrystals have the expected bulk CZTS band gap of 1.5 eV. Several micron thick films deposited by drop casting colloidal dispersions of ~40 nm CZTS nanocrystals were crack-free, while those cast using 5 nm nanocrystals had micron-scale cracks. We showed the applicability of these nanocrystal coatings for thin film solar cells by demonstrating a CZTS thin film solar cell using coatings annealed in a sulfur atmosphere. We conducted a systematic study of the factors controlling crystal growth and microstructure development during sulfidation annealing of films cast from colloidal dispersions of CZTS nanocrystals. The film microstructure is controlled by concurrent normal and abnormal grain growth. At 600 °C to 800 °C and low sulfur pressures (50 Torr), abnormal CZTS grains up to 10 µm in size grow on the surface of the CZTS nanocrystal film via transport of material from the nanocrystals to the abnormal grains. Meanwhile, the nanocrystals coarsen, sinter, and undergo normal grain growth. The driving force for abnormal grain growth is the reduction in total energy associated with the high surface area nanocrystals. The eventual coarsening of the CZTS nanocrystals reduces the driving force for abnormal crystal growth. Increasing the sulfur pressure by an order of magnitude to 500 Torr accelerates both normal and abnormal crystal growth though sufficient acceleration of the former eventually reduces the latter by reducing the driving force for abnormal grain growth. For example, at high temperatures (700-800 oC) and sulfur pressures (500 Torr) normal grains quickly grow to ~500 nm which significantly reduces abnormal grain growth. The use of soda lime glass as the substrate, instead of quartz, accelerates normal grain growth. Normal grains grow to ~500 nm at lower temperatures and sulfur pressures (i.e., 600 °C and 50 Torr) than those required to grow the same size grains on quartz (700 °C and 500 Torr). Moreover, carbon is removed by volatilization from films where normal crystal growth is fast. There are significant differences in the chemistry and in the thermodynamics involved during selenization and sulfidation of CZTS colloidal nanocrystal coatings to form CZTSSe or CZTS thin films, respectively. To understand these differences, the roles of vapor pressure, annealing temperature, and heating rate in the formation of different microstructures of CZTSSe films were investigated. Selenization produced a bi-layer microstructure where a large CZTSSe-crystal layer grew on top of a nanocrystalline carbon-rich bottom layer. Differences in the chemistry of carbon and selenium and that of carbon and sulfur account for this segregation of carbon during selenization. For example, CSe2 and CS2, both volatile species, may form as a result of chalcogen interactions with carbon during annealing. Unlike CS2, however, CSe2 may readily polymerize at room temperature and one atmosphere. Carbon segregation may be occurring only during selenization due to the formation of a Cu-Se polymer [i.e., (CSe2-x)] within the nanocrystal film. The (CSe2-x) inhibits sintering of nanocrystals in the bottom layer. Additionally, a fast heating rate results in temperature variations that lead to transient condensation of selenium on the film. This is observed only during selenization because the equilibrium vapor pressure of selenium is lower than that of sulfur. The presence of liquid selenium during sintering accelerates coarsening and densification of the normal crystal layer (no abnormal crystal layer) by liquid phase sintering. Carbon segregation does not occur where liquid selenium was present.