Necessity is the mother of all inventions. With Si hitting the speed bottleneck, newer materials to replace Si are being sought out. The ex-foliation based experiments on graphene by Geim and Novoselov at this point was perfect as many of its physical properties were fascinating from an electronics standpoint and hence it was very soon projected as a Si replacement for logic applications. In addition, graphene is also an attractive alternative to applications such as radio frequency devices, ultra-sensitive mass/chemical sensing, high-speed optoelectronics and transparent conductors for photo-voltaic applications. While the widespread success and utility of Si can be attributed to easy availability of source material and the ability to synthesize large areas of ultra high quality material, chemical vapor deposition (CVD) is the only available method to controllably produce large area monolayer graphene. CVD graphene is however polycrystalline and therefore defective. Hence, in order to promote graphene towards large-scale commercialization, it is necessary to be able to grow spatially homogeneous graphene with tailored defect densities. Transfer of atomic layers of graphene from the substrate on which it is grown, a Cu foil typically, on to an insulating substrate for electrical measurements is typically a major defect inducing step. Hence, a direct transfer-free fabrication of suspended device using graphene grown on thin films of electro-deposited Cu was attempted and successfully reported for the first time. Though it was shown that the fabrication process itself did not introduce any additional defects, the maximum obtained mobility on such fabricated structures was 5200 cm2/V·s. This value is lower than reported values in literature and thus improvements for electronic applications warranted further optimization. However, limitations on ability of electro-deposited Cu films (melting point of 1083 ◦C) to withstand high temperatures, 1000 ◦C, impeded further optimizations. Hence, growth on Cu foils was taken up. On Cu foil, we were able to identify the roles of the growth kinetics and system thermodynamics on the final quality of graphene. Specifically, by carefully altering the conditions during appropriate growth phases, we were able to obtain graphene films of tunable defect densities with motilities ranging from 200 - 20000 cm2/V·s. Using a host of characterization Techniques like electrical transport, Raman spectroscopic measurements, TEM imaging and water permeation studies, we find that the defect densities in graphene are largely concentrated at the boundaries, while the bulk of the graphene grain remains pristine. Further investigations revealed a thermodynamic correlation between the growth conditions and quality of the grain boundary in terms of defect density and structure. In addition to the influence of defects in graphene on charge mobility as seen before, their impact on the device contact resistance and charge transport hysteresis in graphene field effect transistors were also investigated. With a careful control on the film defect density, we were able to demonstrate devices with low contact resistance (1000 Ωµm ) and tunable hysteresis behavior. Finally, alternate substrates for graphene and its impact on the carrier densities were explored. Non-polar substrate SiO2 and polar substrates such AlN and AlGaN were chosen. On AlN, we obtained higher carrier mobility due to reduced phonon-electron scattering and a higher ’P’ doping behavior due to piezo-electric effects. Hence, to leverage the previous observation, novel FET device architecture with a HEMT based substrate using AlGaN was demonstrated.

The importance of infrared (IR) technology (for detection in the 3-5 μm and 8-14 μm atmospheric windows) has spread from military applications to civilian applications since World War II. The commercial IR detector market in these wavelength ranges is dominated by mercury cadmium telluride (MCT) alloys. The use of these alloys has, however, been faced with technological difficulties. One of the materials that have been tipped to be suitable to replace MCT is InAs/InxGa1-xSb strained layer superlattices (SLS’s). Atmospheric pressure metal-organic vapour phase epitaxy (MOVPE) has been used to grow InAs/GaSb strained layer superlattices (SLS’s) at 510 °C in this study. This is a starting point towards the development of MOVPE InAs/InxGa1-xSb SLS’s using the same system. Before the SLS’s could be attempted, the growth parameters for GaSb were optimised. Growth parameters for InAs were taken from reports on previous studies conducted using the same reactor. Initially, trimethylgallium, a source that has been used extensively in the same growth system for the growth of GaSb and InxGa1-xSb was intended to be used for gallium species. The high growth rates yielded by this source were too large for the growth of SLS structures, however. Thus, triethylgallium (rarely used for atmospheric pressure MOVPE) was utilized. GaSb layers (between 1 and 2 μm thick) were grown at two different temperatures (550 °C and 510 °C) with a varying V/III ratio. A V/III ratio of 1.5 was found to be optimal at 550 °C. However, the low incorporation efficiency of indium into GaSb at this temperature was inadequate to obtain InxGa1-xSb with an indium mole fraction (x) of around 0.3, which had previously been reported to be optimal for the performance of InAs/InxGa1-xSb SLS’s, due to the maximum splitting of the valence mini bands for this composition. The growth temperature was thus lowered to 510 °C. This resulted in an increase in the optimum V/III ratio to 1.75 for GaSb and yielded much higher incorporation efficiencies of indium in InxGa1-xSb. However, this lower growth temperature also produced poorer surface morphologies for both the binary and ternary layers, due to the reduced surface diffusion of the adsorbed species. An interface control study during the growth of InAs/GaSb SLS’s was subsequently conducted, by investigating the influence of different gas switching sequences on the interface type and quality. It was noted that the growth of SLS’s without any growth interruptions at the interfaces leads to tensile strained SLS’s (GaAs-like interfaces) with a rather large lattice mismatch. A 5 second flow of TMSb over the InAs surface and a flow of H2 over GaSb surface yielded compressively strained SLS’s. Flowing TMIn for 1 second and following by a flow of TMSb for 4 seconds over the GaSb surface, while flowing H2 for 5 seconds over the InAs surface, resulted in SLS’s with GaAs-like interfacial layers and a reduced lattice mismatch. Temperature gradients across the surface of the susceptor led to SLS’s with different structural quality. High resolution x-ray diffraction (HRXRD) was used to determine the thicknesses as well as the type of interfacial layers. The physical parameters of the SLS’s obtained from simulating the HRXRD spectra were comparable to the parameters obtained from cross sectional transmission electron microscopy (XTEM) images. The thicknesses of the layers and the interface type played a major role in determining the cut-off wavelength of the SLS’s.

No description available.

This thesis analyzes the chemical vapour deposition (CVD) growth of vertically aligned carbon nanotube (CNT) forests in order to understand how CNT forests grow, why they stop growing, and how to control the properties of the synthesized CNTs. In situ kinetics data of the growth of CNT forests are gathered by in situ optical microscopy. The overall morphology of the forests and the characteristics of the individual CNTs in the forests are investigated using scanning electron microscopy and Raman spectroscopy. The in situ data show that forest growth and termination are activated processes (with activation energies on the order of 1 eV), suggesting a possible chemical origin. The activation energy changes at a critical temperature for ethanol CVD (approximately 870°C). These activation energies and critical temperature are also seen in the temperature dependence of several important characteristics of the CNTs, including the defect density as determined by Raman spectroscopy. This observation is seen across several CVD processes and suggests a mechanism of defect healing. The CNT diameter also depends on the growth temperature. In this thesis, a thermodynamic model is proposed. This model predicts a temperature and pressure dependence of the CNT diameter from the thermodynamics of the synthesis reaction and the effect of strain on the enthalpy of formation of CNTs. The forest morphology suggests significant interaction between the constituent CNTs. These interactions may play a role in termination. The morphology, in particular a microscale rippling feature that is capable of diffracting light, suggest a non-uniform growth rate across the forest. A gas phase diffusion model predicts a non-uniform distribution of the source gas. This gas phase diffusion is suggested as a possible explanation for the non-uniform growth rate. The gas phase diffusion is important because growth by acetylene CVD is found to be very efficient (approximately 30% of the acetylene is converted to CNTs). It is seen that multiple mechanisms are active during CNT growth. The results of this thesis provide insight into both the basic understanding of the microscopic processes involved in CVD growth and how to control the properties of the synthesized CNTs.

In situ Raman spectroscopy was used to explore etching of carbon nanotubes as well as their growth from C60. The thesis is in three parts: (1) C60 seed particles were partially oxidized in air and were used to grow carbon nanotubes and other nanocarbon structures. Seed oxidization was characterized by monitoring the evolution of the Raman Ag(2) peak and the D band, and oxidation temperature was found to be critical to nanotube growth. (2) To further explore oxidation, carbon nanotubes were thermally oxidized in air at different temperatures, while the evolution of different Raman bands was tracked. Etching dynamics and band intensity evolution were tracked in situ. Notably, metallic species were found to etch much more rapidly than semiconducting species of similar diameter. (3) To confirm and expand on this, a novel, simultaneous two-laser Raman spectroscopy setup was used to track the thermal oxidation of carbon nanotubes in O2 and CO2 gases at different temperatures. Metallic species were resonant with one laser line, while semiconducting species were resonant with the other, so changes to sample metallicity could be tracked unambiguously in two separate spectra. Again, metals were found to etch more rapidly. In situ Raman spectroscopy can track the evolution of nanotubes in real time and provide insight into processing. In general, detailed process monitoring like this can help in the development of selective synthesis and processing.

Despite the tremendous application potentials of carbon nanotubes (CNTs) proposed by researchers in the last two decades, efficient experimental techniques and methods are still in need for controllable production of CNTs in large scale, and for conclusive characterizations of their properties in order to apply CNTs in high accuracy engineering. In this dissertation, horizontally well-aligned high quality single-walled carbon nanotubes (SWCNTs) have been successfully synthesized on St-cut quartz substrate by chemical vapor deposition (CVD). Effective radial moduli (Eradial) of these straight SWCNTs have been measured by using well-calibrated tapping mode and contact mode atomic force microscopy (AFM). It was found that the measured Eradial decreased from 57 to 9 GPa as the diameter of the SWCNTs increased from 0.92 to 1.91 nm. The experimental results were consistent with the recently reported theoretical simulation data. The method used in this mechanical property test can be easily applied to measure the mechanical properties of other low-dimension nanostructures, such as nanowires and nanodots. The characterized sample is also an ideal platform for electrochemical tests. The electrochemical activities of redox probes Fe(CN)63-/4-, Ru(NH3)63+, Ru(bpy)32+ and protein cytochrome c have been studied on these pristine thin films by using aligned SWCNTs as working electrodes. A simple and high performance electrochemical sensor was fabricated. Flow sensing capability of the device has been tested for detecting neurotransmitter dopamine at physiological conditions with the presence of Bovine serum albumin. Good sensitivity, fast response, high stability and anti-fouling capability were observed. Therefore, the fabricated sensor showed great potential for sensing applications in complicated solution.

In the context of the future hydrogen economy, effective production of hydrogen (H2) from readily available and sustainable resources is of crucial importance. Hydrogen generation via water splitting by solar energy or electricity has attracted great attention in recent years. In comparison with photocatalytic water-splitting directly using solar light, which is ideal but the relevant technologies are not yet mature, electrolysis of water with catalyst is more practical at the current stage. The Pt-group noble metals are the most effective electrocatalysts for hydrogen evolution reaction (HER) from water, but their high costs limit their applications. Due to the earth-abundance and low price, MoS2 is expected to be a good alternative of the Pt-group metals for HER. Plenty of researches have been conducted for improving the HER activities of MoS2 by optimizing its synthesis method. However, it remains challenging to prepare MoS2 catalysts with high and controllable activity, and more investigations are still needed to better understand the structure-performance correlation in this system. In this thesis, we report a new strategy for fabricating MoS2 eletrocatalysts which gives rise to much improved HER performance and allows us to tune the electrocatalytic activity by varying the preparation conditions. Specifically, we sulfurized molybdenum oxide on the surface of a Ti foil electrode via a facile chemical vapor deposition (CVD) method, and directly used the electrode for HER testing. Depending on the CVD temperature, the MoO2-MoS2 nanocomposites show different HER activities. Under the optimal synthesis condition (400ºC), the resulting catalyst exhibited excellent HER activity: an onset potential (overpotential) of 0.095 V versus RHE and the Tafel slope of 40 mv/dec. Such a performance exceeds those of most reported MoS2 based HER electrocatalysts. We demonstrated that the CVD temperature has significant influence on the catalysts in crystallinity degree, particle size and dispersion, morphology, and density of the edge sites etc., and these factors in turn determine the HER activity.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第22709号 / エネ博第406号 / 新制||エネ||78(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 坂口 浩司, 教授 松田 一成, 教授 野平 俊之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

The effects of methane (CH4), diborone (B2H6) and nitrogen (N2) concentrations on the structure and photoelectron emission properties of chemical vapor deposition (CVD) polycrystalline diamond films were studied. The diamond films were grown on single-crystal Si substrates using the hot-tungsten filament CVD technique. Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) were used to characterize the different forms of carbon in the films, and the fraction of sp3 carbon to sp3 plus sp2 carbon at the surface of the films, respectively. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the films. The photoelectron emission properties were determined by measuring the energy distributions of photoemitted electrons using ultraviolet photoelectron spectroscopy (UPS), and by measuring the photoelectric current as a function of incident photon energy.

Amorphous and nano- or micro- crystalline silicon thin films are intensively studied materials for photovoltaic applications. The films are used as intrinsic layer (absorber) in p-i-n solar cells. As opposed to crystalline silicon solar cells, the thin films contain about hundred times less silicon and can be deposited at much lower temperatures (typically around 200 0 C) which saves energy needed for production and makes it possible to use various low cost (even flexible) substrates. However, these films have a complex microstructure, which makes it difficult to measure and describe the electronic transport of the photogenerated carriers. Yet, the understanding of the structure and electronic properties of the material at nanoscale is essential on the way to improve the efficiency solar cells. One of the main aims of this work is the study of the structure and mechanical properties of the mixed phase silicon thin films of various thicknesses and structures. The key parameter of microcrystalline silicon is the crystallinity, i.e., the microcrys- talline volume fraction. It determines internal structure of the films which, in turn, decides about many other properties, including charge transport and mechanical sta- bility. Raman microspectroscopy is a fast and non-destructive method for probing the...