Cutting Edge Nanotechnology

The main purpose of this book is to describe important issues in various types of devices ranging from conventional transistors (opening chapters of the book) to molecular electronic devices whose fabrication and operation is discussed in the last few chapters of the book. As such, this book can serve as a guide for identifications of important areas of research in micro, nano and molecular electronics. We deeply acknowledge valuable contributions that each of the authors made in writing these excellent chapters

Optical and Mechanical Investigation of InAs /GaAs Quantum Dots Solar Cells and InAs Nanowires for the Application of Photovoltaic Device

Self-assembled quantum dots (QDs) and nanowires (NWs) are currently the subjects of extensive study due to their promising applications in optoelectronic devices. In order to enhance understanding of the short circuit current improvement in InAs/GaAs quantum dots solar cell (QDSC), the mechanisms of carrier escape by thermal activation and tunneling from InAs quantum dots (QDs) confinements in InAs/GaAs QDSCs are investigated. The fitted activation energy of electrons from temperature dependent photoluminescence (TDPL) is 114 meV. Using this fitted activation energy, calculated thermal escape time and tunneling time of electrons from the ground state of the QDs are 10-12 seconds and 10-6 seconds at 300K, respectively. These results indicate that at room temperature thermal escape is dominant for electrons escape from ground state. At low temperature (8K), tunneling mainly affects the electrons escape from ground state, since thermal energy cannot support electrons to overcome the fitted activation energy (barrier, 114 meV). In addition, in order to describe the new physics and achieve the final success in nanowire device for photovoltaic applications, the first step is to develop high-quality semiconductor nanowires on the selected substrate. Morphological and crystal structure characterizations were performed via SEM and TEM for InAs nanowire samples grown with and without Au seed on GaAs substrate using metal organic vapor phase expitaxy (MOVPE). Several major factors affect the NW growth in terms of shape, density, etc. For nanowire growth with Au seed, its growth direction mainly depends on the substrate, while its uniformity is initially related to the Au seed coverage. III/V ratio affects the NW aspect ratio (length/bottom width), ranged from 12.00 to 38.93. Increasing temperature accelerates the growth rate in both axial and radial directions. NWs grown without Au seed using a pattern mask show no tapering along the growth direction with an average diameter of 26 nm. All defects stop in the buffer layer when InAs nanowires grown with an Au seed, but a mix of ZB and WZ crystal phases were observed along the growth direction of nanowire. InAs NWs grown without Au seeds also show a mixture of different crystal phases along the growth direction. The diameter of InAs nanowire should be further reduced to 3-6 nm as to achieve PL response between 1000~1300 nm

Control of Electronic Coupling and Optical Properties in Quantum Dot Solids

In this thesis, we investigate the electronic coupling in quantum dot (QD) solids, optical anisotropies of nanowires (NWs) with diameters comparable to the wavelength of light, and the propagation of light in nanoribbon waveguides. In particular, we demonstrate a new mechanism to control the electronic coupling in QD solids thermomechanically, and how size controls the optical anisotropies in NWs. We firstly demonstrate that the electronic coupling in QD solids can be controlled by a new thermomechanical mechanism. This mechanism is realized by controlling the expansion and shrinkage of the interstitial material in the QD solids, which in turn controls the distance and distance-dependent electronic coupling between semiconductor nanocrystals (SNCs). Photoluminescence (PL) and TEM investigation demonstrate the tuning of the band gap emission in individual polycrystalline NWs and densely packed SNCs via this mechanism. At low temperature, temperature-induced blueshift in densely packed SNC film and redshift in polycrystalline NWs were realized. This is qualitatively different from bulk CdTe and isolated CdTe SNCs. The electronic coupling between the nearest SNCs for sub-nm distances agrees well with semiempirical calculations. Size dependence of optical anisotropies in NWs is demonstrated in this work. We found optical anisotropies in NWs with diameters comparable to the wavelength of light in the NW, i.e., beyond the electrostatic limit, are much lower than those of NWs in electrostatic limit. Finite-difference time domain calculations, with realistic parameters for the CdTe NWs, for excitation and PL anisotropy were carried out. It was found that the optical anisotropies of NWs display a strong size dependence when the NW is beyond the electrostatic limit. Changing the diameter allows tuning the polarization anisotropy from its maximum, predicted by the electrostatic limit, to zero. The optical anisotropies of a NW are determined by the diameter-wavelength ratio, the material dispersion, as well as the local refractive index of the surrounding. In addition, the optical anisotropies can be transferred into macroscopically aligned NW arrays, and the anisotropies of the NW arrays are determined by the optical anisotropies of isolated NWs, the disorder of the NWs in the film, the local environment and multiple scattering in the thick film. Furthermore, we show that self-assembled nanoribbons can serve as single-mode waveguides for the propagation of PL light. Calculations show that the minimum width needed for single-mode operation is approximately 150 nm, which agrees well with SEM measurements. The loss in the nanoribbon waveguides was quantitatively determined. Re-absorption was demonstrated in the nanoribbon waveguides to be a major contribution in the loss mechanism. Losses in the nanoribbon waveguide are on the same order of magnitude as for plasmon waveguides

Energy Transport and Conversion in Semiconductor Nanocrystal Solids

Solids constructed with single and multicomponent nanocrystal represent an exciting new form of condensed matter, as they can potentially capture not only the quantum features of the individual building blocks but also novel collective properties that arise from coupling of nanocrystal components. In this thesis, measurement and interpretation of temperature-dependent thermopower in semiconductor nanocrystal solids are used to elucidate the Fermi energy level and the density of state distribution. The physical understating of temperature dependence of thermopower is, in turn, utilized to develop a powerful tool with which to monitor doping in PbTe nanocrystal solids with different concentrations of Ag2Te nanocrystal dopants. Combining the temperature-dependent thermopower and electrical conductivity measurements provides a unique electronic spectroscopy tool with which to reveal the carrier distribution and dynamics in semiconductor nanocrystal solids

Development of High Efficiency III/V Photovoltaic Devices

Developments of photovoltaic (PV) devices are driven by increasing needs for economically competitive renewable energy conversion. To improve the efficiency of PV devices for outdoor applications, the concept of intermediate band solar cell (IBSC) has been proposed to boost the conversation efficiency to 63% under concentrated suns illumination, which requires two-step photon absorption (TSPA) dominates among other competing processes: carrier thermal escape, tunneling and recombination. To optimize the design of III-V QD-IBSCs, first, the effect of electric field on band structure and carrier dynamics and device performances were quantitative investigated via simulation and experiments. Second, to experimentally increase TSPA at room temperature, novel QD systems related QD-IBSCs were designed, fabricated and characterized. The InAs/Al0.3GaAs QD-IBSC shows high TSPA working temperature towards 110K, promising for a room temperature IBSC under concentrated sunlight. Alternative QD systems including GaSb/GaAs and type II InP/InGaP were also investigated via band structure simulations. Meanwhile, developments of PV devices under indoor low intensity light (0.1 µW/cm2-1 mW/cm2) illumination not only enable long lifetime radio-isotope based batteries, but also, more important for the daily life, have the potential to promote an emerging market of internet of things by efficiently powering wireless sensors. Single junction InGaP PV devices were optimized for low intensity light sources using via simulations and statistical control. To reduce the dark current and increase the absorption at longer wavelengths (\u3e550 nm), several parameters including doping and thickness were evaluated. The experimental results on the devices show higher conversion efficiencies than other commercial PVs under varied indoor light sources: 29% under 1µW/cm2 phosphor spectrum and over 30% efficiency under LEDs illumination. In addition, the work includes developments of InAs nanowires epi-growth for PV applications. Several marks for selective area growth were successfully made

Controlling the optical properties of colloidal lead halide perovskite nanocrystals by shape, size and dimensionality

In letzter Zeit gewinnen Metallhalogenid-Perowskit-Halbleitermaterialien zunehmend an Aufmerksamkeit, was auf ihre faszinierenden Eigenschaften und vielversprechenden optoelektronischen Anwendungen zurückzuführen ist. Die meisten der ersten Studien konzentrierten sich auf Volumenmaterial dieser Perowskite, während die aufkommenden kolloidalen Perowskit-Nanokristalle aufgrund ihrer einzigartigen Eigenschaften weiteres Interesse auf sich zogen. In dieser Arbeit werden die optischen Eigenschaften von verschiedenen kolloidalen Metallhalogenid-Perowskit-Nanokristallen untersucht und mit ihrer Form, Größe und Dimensionalität korreliert. Zum einen kann die Photolumineszenz von kolloidalen Metallhalogenid-Perowskit-Nanokristallen durch eine Verringerung ihrer Dimensionalität abgestimmt werden. Zweidimensionale Methylammonium-Bleihalogenid-Perowskit-Nanoplättchen wurden mit unterschiedlichen Dicken synthetisiert und deren Quanten-Größeneffekte werden hier quantitativ untersucht. Durch das Ersetzen des Methylammonium-Ions mit einem Cäsium-Ion, können komplett anorganische kolloidale Cäsium-Bleihalogenid-Perowskit-Nanokristalle erhalten werden und ihre optischen Eigenschaften werden durch die Änderung der chemischen Zusammensetzung sowie der Dimensionalität effektiv abgestimmt. Die Exzitonenbindungsenergie nimmt mit abnehmender Dicke von Nanokristallen zu. Darüber hinaus können die Photolumineszenz-Quantenausbeuten von kolloidalem Cäsium-Bleibromid-Perowskit-Nanoplättchen signifikant durch Zugabe einer PbBr2-Liganden-Lösung erhöht werden, welche die Oberflächendefekte repariert. Außerdem wurden andere Perowskit-Nanokristalle mit verschiedenen Formen, einschließlich von Cäsium-Bleibromid-Nanodrähten und Superkristallen, hergestellt und ihre optischen Eigenschaften untersucht. Es wurde festgestellt, dass Cäsium-Bleibromid-Nanodrähte durch eine orientierte Anlagerung von Nanowürfeln in kolloidalen Lösungen gebildet werden. Die Nanodrähte zeigen eine rotverschobene Photolumineszenz mit einer Polarisations-Anisotropie aufgrund ihrer länglichen Geometrie. Im Vergleich zu Nanowürfeln weisen die Nanodrähte auch eine viel geringere Photolumineszenz-Quantenausbeute auf, da der strahlungslose Zerfall durch die Ladungsträgermobilität entlang des Drahtes an Bedeutung gewinnt. Des Weiteren können sich Cäsium-Bleibromid-Nanowürfel in kolloidaler Lösung selbst anordnen und sogenannte Superkristalle bilden. Diese Superkristalle zeigen eine offensichtliche Rotverschiebung in der Photolumineszenz infolge einer interpartikulären elektronischen Kopplung durch den hinreichend kleinen Abstand zwischen den benachbarten Nanowürfeln. Inzwischen bleibt die hohe Photolumineszenz-Quantenausbeute der Nanowürfel-Untereinheiten in den Superkristallen erhalten. Zusammenfassend stellt diese Arbeit einen Einblick in die Dimensionalität abhängigen optische Eigenschaften von kolloidalem Bleihalogenid-Perowskit-Nanokristallen.Recently, metal halide perovskite semiconductor materials are gaining increasing attention owing to their fascinating properties and promising optoelectronic applications. Most of the initial studies focued on the bulk-like perovskite materials, while the emerging colloidal perovskite nanocrystals attract further interest due to their unique properties. In this thesis, the optical properties of various colloidal metal halide perovskite nanocrystals are explored and correlated with their shape, size and dimensionality. Firstly, the photoluminescence of colloidal metal halide perovskite nanocrystals can be tuned by decreasing their dimensionality. Two-dimensional methylammonium lead halide perovskite nanoplatelets with different thicknesses are synthesized and their quantum size effects are quantitatively investigated. By replacing the methylammonium ion with a cesium ion, all-inorganic colloidal cesium lead halide perovskite nanocrystals are obtained and their optical properties are effectively tuned by changing chemical composition as well as dimensionality. The exciton binding energy is found to increase with decreasing thickness of nanocrystals. In addition, the photoluminescence quantum yields of colloidal cesium lead bromide perovskite nanoplatelets can be significantly increased by adding PbBr2-ligand solution to repair the surface defects. Furthermore, other perovskite nanocrystals with different shapes including cesium lead bromide nanowires and supercrystals are prepared and their optical properties are investigated. Cesium lead bromide nanowires are found to be formed through an oriented attachment of nanocubes in colloidal solution. The nanowires show a redshifted photoluminescence with a polarization anisotropy due to their elongated anisotropic geometry. The nanowires also exhibit a much lower photoluminescence quantum yield compared to nanocubes due to nonradiative decay causued by charge carrier mobility along the wire. In addition, cesium lead bromide nanocubes can self-assembly into supercrystals in colloidal solution. The supercrystals show an obvious redshift in photoluminescence due to an interparticle electronic coupling enabled by the sufficiently small spacing between neighboring nanocubes. Meanwhile, high photoluminescence quantum yield of the nanocube subunits is retained in the supercrystals. In summary, this thesis provides an insight into dimensionality-dependent optical properties of colloidal lead halide perovskite nanocrystals