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Chemical Vapor Deposition of High-quality Graphene Films on Single-crystal Metal Foils
No full textDepartment of Chemistryope
Chemical Vapor Deposition of High Quality Graphene Films On Cu/Ni(111) Alloy Foils
Full text linkDepartment of Materials Science and EngineeringGraphene, a monolayer of sp2-bonded carbon atoms or one monolayer of graphite, has attracted intense attention in recent times due to its fascinating properties, such as excellent carrier mobility, good thermal conductivity, high mechanical strength and high optical transmittance. To date, chemical vapor deposition (CVD) has been verified to be the most promising method to synthesize large area graphene with high quality and low cost. Despite the remarkably rapid progress that has been achieved in this field during the past 10 years, there are still many problems or issues related to the fast growth of large area single-crystalline graphene and the controlled synthesis of bilayer and/or trilayer graphene that need to be addressed.
In this work, fast-growth of single crystal monolayer graphene by CVD has been achieved on ???home-made??? single crystal Cu/Ni(111) alloy foils over large area. Full coverage was achieved in 5 min or less for a particular range of composition (1.3 at.% to 8.6 at.% Ni), as compared to 60 min for a pure Cu(111) foil under identical growth conditions. These are the bulk atomic percentages of Ni, as a superstructure at the surface of these foils with stoichiometry Cu6Ni1 (for 1.3 to 7.8 bulk at.% Ni in the Cu/Ni(111) foil) was discovered by low energy electron diffraction (LEED). Complete large area monolayer graphene films so obtained, are either single crystal or close to single crystal, and include folded regions that are essentially parallel with each other and could originate from wrinkles that ???fell over??? to bind to the surface; these folds are separated by large, wrinkle free regions. The folds occur due to the buildup of interfacial compressive stress (and its release) during cooling of the foils from 1075 ??C to room temperature. Joining of well-aligned graphene islands (obtained by arresting the growth prior to full film coverage) was investigated with high magnification SEM and aberration-corrected high-resolution TEM as well as AFM, STM, and optical microscopy. Results show that many of the ???junction regions??? have folds and these arise from interfacial adhesion mechanics (the folds may originate from the buildup of compressive stress during cool-down, but these folds are different than those observed on the continuous graphene films???these folds in the joined islands occur due to ???weak links??? in terms of the interface mechanics).
In addition, we have synthesized very large-area, high quality bilayer and tri-layer graphene films by chemical vapor deposition; these films are almost entirely ???AB-stacked???. The number of layers of the graphene films was controlled by finely tuning the Ni concentration in the alloy foil. As a result, 95% area coverage of bilayer that is essentially 100% AB-stacked was achieved for samples of size 1.0 cm ?? 1.5 cm; and 60% area coverage of trilayer that is essentially 100% ???ABA-stacked??? over the same sample size. We have studied the stacking sequence of the as-prepared bilayer and multilayer graphene. Time-of-flight secondary ion mass spectrometry mapping, hydrogen etching with in situ scanning electron microscopy, and cross-sectional high-resolution transmission electron microscopy imaging show that the second-layer (the ???adlayer???; and thus, also the 3rd layer and so on) grows underneath the first layer, forming an ???inverted wedding cake??? structure.
Our work demonstrates that single crystal Cu/Ni(111) alloy foils can be made and used to prepare large-scale single crystal monolayer graphene and AB-stacked layer-tunable graphene films where all (or almost all) the AB-stacked regions have the same (single) crystal orientation. Graphene quality has been demonstrated through a combination of characterization methods and and graphene growth mechanism is discussed.clos
Anion radicals of fullerenes generated in photosensitized TiO2 or by quenching photoexcited C60BY Et3N (an EPR, VIS/NIR study)
No full textThe Kinetics of Dissolution of Single Crystal Diamond (100) and (110) in Nickel and Cobalt Films and Vapor-Liquid-Solid Growth of Graphene Ribbons on Single Crystal and Polycrystalline Cu Foils
Full text linkDepartment of Materials Science and EngineeringThe kinetics of dissolution of single crystal diamond (with surface orientation of 100 or 110, named as D(100) and D(110)) into nickel or cobalt films was measured, to the best of our knowledge, for the first time. This was possible through our discovery that at sufficiently high partial pressure of water vapor the rate determining step was breaking of C-C bonds at the diamond-metal interfaceat lower partial pressures of water vapor, the rate determining step was found to be removal of carbon from the surface of the metal, rather than C-C bond breaking at the diamond-metal interface. The rate of diffusion from the diamond-metal interface to the surface of the metal film was found to never be rate-limiting. We found that single crystal diamond with surface orientation 111 could not be dissolved in either cobalt (Co) or nickel (Ni) films in the temperature range we studied. The details of this study are provided in Chapter 3. Time-of-flight-secondary ion mass spectrometry depth profiles show a concentration gradient of C from a certain depth into the metal film surface down to the M???D(100) interface, and residual gas analyzer measurements show that the gas products formed in the presence of water vapor by reaction of C atoms diffusing to, and thus present at, the metal surface are CO and H2. As mentioned, we discovered two different regimes (we name them I and II) for the kinetics of dissolution of both D(100) and D(110), in which the rate-determining step was the removal of carbon atoms on the open metal surface (regime I, lower partial pressure of water vapor) or dissolution of diamond at the metal???diamond interface (regime II, higher partial pressure of water vapor) that yielded different Arrhenius parameters. We found that the rate of dissolution of diamond in Co was higher than that in Ni for both D(100) and D(110) and for both regimes I and II, and possible reasons are suggested. As mentioned, we also found that D(111) could not be dissolved at the Ni/D(111) and Co/D(111) interface in the presence of water vapor (over the same range of sample temperatures). The reaction paths for dissolution of C at the M???D(100) or M???D(110) interface and for removal of C from the free surfaces of Ni and Co were assessed through density functional theory modeling at 1273 K by colleagues Yongchul Kim and Prof. Geunsik Lee.
In Chapter 4, we describe the bottom-up direct growth of graphene ribbons catalyzed by deliberate introduction of silica particles onto a Cu(111) foil surface, and we ascribe their growth to a combination of vapor-liquid-solid (VLS) growth (longitudinal growth) and vapor-solid (VS) growth (lateral growth onto the already existing ribbon that extends longitudinally from the VLS growth). Micrometer-long single crystal graphene ribbons (tapered when grown above 900 ???, but uniform width when grown in the range 850 to 900 ???, as this latter temperature range is too low for VS growth) using silica particle seeds were synthesized on single crystal Cu(111) foil. We discovered that tapered and uniform-width graphene ribbons grew strictly along the Cu direction on Cu(111) and polycrystalline copper (Cu) foils. Silica particles on both the single crystal and polycrystalline Cu foils formed (semi-)molten Cu-Si-O droplets at growth temperatures, then catalyzed nucleation and drove the longitudinal growth of graphene ribbons. Longitudinal growth is likely by a vapor-liquid-solid (VLS) mechanism, but edge growth (above 900 ???) is due to catalytic activation of ethylene (C2H4) and attachment of C atoms or species (???vapor solid??? or VS growth) at the edges. We found that the taper angle is determined only by the growth temperature and that the growth rates were independent of the particle size. A surface-diffusion vapor-liquid-solid growth model thus seems most appropriate for rationalizing the longitudinal growth. According to our kinetics study, we found the activation enthalpy (1.73 ?? 0.03 eV) for longitudinal ribbon growth on Cu(111) from ethylene is lower than that for VS growth at the edges of the GRs (2.78 eV ?? 0.15 eV) and for the graphene island growth (2.85 ?? 0.07 eV) that occurs concurrently. (That is, the Cu(111) surface has both GRs and hexagonal graphene islands. The graphene islands nucleate and grow on the regions of the Cu(111) surface where there is not a silica particle.ope
Production of Rhombohedral Multilayer Graphene with the Goal of Making Diamane
Full text linkDepartment of ChemistryMultilayer graphene (MLG) is an outstanding anisotropic material whose electronic and mechanical properties are highly dependent on the stacking order of individual graphene monolayers. The most thermodynamically stable MLG configuration has a hexagonal crystal structure with an ABA???(Bernal) stacking sequence. Less energetically favorable and thus less abundant rhombohedral MLG with an ABC??? stacking order is of a great interest due to its remarkable properties: electronic (tunable energy band gap) and structural (the same order of carbon layers as in the cubic diamond along [111] direction). It remains extremely challenging to synthesize MLG of an ABC stacking sequence by using ???bottom-up??? approaches. To the best of our knowledge, there is only one report on the selective fabrication of ABC-stacked trilayer graphene by high-temperature annealing of n-type Si-terminated 6H-SiC(0001). The rhombohedral stacking order, however, has been reported to occur in natural graphite (up to 30%). Thus, in our attempt to make ABC-stacked MLG, we have chosen the ???top-down??? approach, namely, by cleaving natural graphite with a relatively high content of the rhombohedral (ABC???) phase.
Throughout our study, we used X-ray diffraction (XRD) characterization to detect and to quantify the content of the rhombohedral phase in natural graphite. Hexagonal and rhombohedral phases in exfoliated MLG flakes were distinguished and mapped by Raman spectroscopy. We also conducted chemical functionalization (fluorination) of exfoliated graphene flakes to test the possibility of chemically induced sp2 to sp3 phase transition of ABC-stacked MLG into ultrathin diamond-like film (diamane).clos
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Chemical vapor deposition graphene on polycrystalline copper foil
Full text linktextGraphene, a single atomic layer of sp²-bonded carbon, has been of significant interest to basic sciences and engineering. Among its unique properties are exceptional mechanical strength, from the strong carbon-carbon bond; high in-plane thermal conductivity; high carrier mobilities, since electrons and holes travel through graphene as mass-less Dirac fermions; and quantum effects (such as the quantum Hall effect), which can be observed at room temperature. In 2009, Li et al., of Professor Ruoff's research group at the University of Texas at Austin, published a seminal paper detailing the production of fairly high quality graphene grown on copper foils using chemical vapor deposition (CVD). The potential for scalability of graphene CVD processing is extremely attractive, and this is currently the most promising method for its commercial viability, particularly for transparent conductive electrodes (TCEs). Here, graphene-based TCEs are compared with TCEs made with multi-walled carbon nanotubes (MWCNTs). A novel technique to reduce the sheet resistance of MWCNT-based TCEs in half is described in detail. Even with these improvements, graphene-based TCEs outperform MWCNT-based TCEs. The decomposition of copper oxides at high temperatures in an oxygen deficient environment is characterized. The ability for the oxygen evolved from the copper foil during this decomposition to react with carbon on the surface of the copper substrate is verified. This phenomenon was used to develop a technique for getting clean pre-graphene growth copper substrates and allowing repeatable graphene nucleation results. A technique for growing large graphene domains inside a copper vapor trapping 'copper enclosure' is described. The quality of the graphene grown inside the copper enclosure is characterized and shown to be of very high quality. This technique can grow graphene domains over 0.5 mm across. Finally, a possible cause of graphene ad-layer growth on the copper surface is suggested. It is proposed that gas diffusing through the copper substrate at high temperature delaminates the graphene from the copper surface in some regions. This then allows carbon containing molecules to diffuse under the graphene and grow new graphene layers. The increased ad-layer growth in the presence of helium supports this.Materials Science and Engineerin
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WSe₂-based devices and oxide structures
No full texttextIn this work the transition metal dichalcogenide WSe₂ is exfoliated and characterized. It is shown from the electrical measurements that the material can display both p-type conduction as well as ambipolar characteristics. WSe₂ flakes were also oxidized through thermal and laser treatments to produce oxide structures. The structures and oxidation processes are characterized and described in this thesis.Mechanical Engineerin
Comprehensive evaluation of algal biofuel production: Experimental and target results
Full text linkWorldwide, algal biofuel research and development efforts have focused on
increasing the competitiveness of algal biofuels by increasing the energy and financial
return on investments, reducing water intensity and resource requirements, and increasing
algal productivity. In this study, analyses are presented in each of these areas—costs,
resource needs, and productivity—for two cases: (1) an Experimental Case, using mostly
measured data for a lab-scale system, and (2) a theorized Highly Productive Case that
represents an optimized commercial-scale production system, albeit one that relies on
full-price water, nutrients, and carbon dioxide. For both cases, the analysis described herein
concludes that the energy and financial return on investments are less than 1, the water
intensity is greater than that for conventional fuels, and the amounts of required resources at a meaningful scale of production amount to significant fractions of current consumption
(e.g., nitrogen). The analysis and presentation of results highlight critical areas for
advancement and innovation that must occur for sustainable and profitable algal biofuel
production can occur at a scale that yields significant petroleum displacement. To this end,
targets for energy consumption, production cost, water consumption, and nutrient
consumption are presented that would promote sustainable algal biofuel production.
Furthermore, this work demonstrates a procedure and method by which subsequent
advances in technology and biotechnology can be framed to track progress.Mechanical Engineerin
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