Browsing by Author "Araujo, Paulo T."

Now showing 1 - 7 of 7
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    Ambient-pressure CVD of graphene on low-index Ni surfaces using methane: A combined experimental and first-principles study
    (American Physical Society, 2018-07-23) Mafra, Daniela L.; Olmos-Asar, Jimena A.; Negreiros, Fabio R.; Reina, Alfonso; Kim, Ki Kang; Dresselhaus, Mildred S.; Kong, Jing; Mankey, Gary J.; Araujo, Paulo T.; Massachusetts Institute of Technology (MIT); Universidade Federal do ABC (UFABC); Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET); National University of Cordoba; Dongguk University; University of Alabama Tuscaloosa; Universidade Federal do Para
    The growth of large area single-layer graphene (1-LG) is studied using ambient pressure chemical vapor deposition on single-crystal Ni(111), Ni(110), and Ni(100). By varying both the furnace temperature in the range of 800-1100 degrees C and the gas flow through the growth chamber, uniform, high-quality 1-LG is obtained for Ni(111) and Ni(110) single crystals and for Ni(100) thin films. Surprisingly, only multilayer graphene growth could be obtained for single-crystal Ni(100). The experimental results are analyzed to determine the optimum combination of temperature and gas flow. Characterization with optical microscopy, Raman spectroscopy, and optical transmission support our findings. Density-functional theory calculations are performed to determine the energy barriers for diffusion, segregation, and adsorption, and model the kinetic pathways for formation of different carbon structures on the low-index surfaces of Ni.
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    Characterization and thermoelectric performance of polymer/inorganic hybrid films using spectroscopic and microscopic techniques
    (University of Alabama Libraries, 2019) Sutch, Tabitha Patricia; Szulczewski, Greg J.; University of Alabama Tuscaloosa
    The main goal of this dissertation was to investigate the thermoelectric performance of films of silver telluride (Ag2Te), bismuth sulfide (Bi2S3) and bismuth telluride (Bi2Te3) nanowires (NWs) dispersed in P(NDI2OD-T2). We hypothesize that the electrical properties of the films made with Ag2Te, Bi2S3, and Bi2Te3 will be n-type and behave like composite materials. Ag2Te was synthesized to make three different lengths of NWs. Powder x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS) confirmed that β-Ag2Te was synthesized with the anticipated stoichiometry. Scanning electron microscopy (SEM) of Ag2Te NWs in P(NDI2OD-T2) revealed that the longest Ag2Te NWs produced homogeneous composites whereas the shorter Ag2Te NWs phase-separated. Electrical conductivity and Seebeck coefficients for each composite film were determined and theoretical models were used to investigate charge-transport behavior. Seebeck coefficients confirmed that all composites were n-type. The longest Ag2Te NWs produced the highest electrical conductivities with parallel transport behavior and are promising to the field of thermoelectrics. Bi2S3 and Bi2Te3 NWs were synthesized, and their stoichiometry and structures were confirmed using XRD and EDS. SEM images of the NW/P(NDI2OD-T2) films revealed that both Bi2S3 and Bi2Te3 phase-separated. The electrical conductivity for each composite film was determined and theoretical models were applied. The electrical conductivity of the Bi2S3 and Bi2Te3 composites were slightly higher than pristine P(NDI2OD-T2), further indicating that the NWs phase-separated. Seebeck coefficients for both systems confirmed that the composites were n-type. Despite the various strategies for improving the film morphology, composites made with Bi2Te3 and Bi2S3 NWs did not produce promising results. The spin-dynamics of P(NDI2OD-T2) doped with cobaltocene were studied with continuous-wave electron paramagnetic resonance (EPR). Additionally, the electrical conductivity of cobaltocene doped P(NDI2OD-T2) films increased several orders of magnitude compared to pristine P(NDI2OD-T2). A sputter depth profile in conjunction with x-ray photoelectron spectroscopy was used to analyze how the dopant dispersed in the polymer matrix. Temperature-dependent pulsed EPR of two different doping concentrations suggested two different relaxation rates. Overall, this study investigated the temperature-dependent spin dynamics of cobaltocene doped P(NDI2OD-T2) films and lays the foundation for further investigations on n-doped polymer system.
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    Characterization of bismuth telluride two-dimensional nanosheets for thermoelectric applications
    (University of Alabama Libraries, 2015) Guo, Lingling; Wang, Hung-Ta; University of Alabama Tuscaloosa
    Solid-state thermoelectric devices are compact, scalable, quiet, and environmentally friendly, which are widely used as thermal engines or refrigerators. Bismuth telluride (Bi2Te3) and other V-VI group chalcogenides are known as one of the best thermoelectric materials specifically for applications in a temperature environment from room temperature to 300 ℃. Recently, the unique topological surface states were discovered in Bi2Te3 family materials, and these novel surface states are arisen from a strong spin-orbit coupling in topological insulators. Topological surface states are protected against time-reversal perturbations (i.e., non-magnetic impurities or surface defects), making the electronic transport essentially dissipation-less. Such unique transport behavior with zero energy loss provides new opportunities to enhance thermoelectric properties. Although the promise in thermoelectric properties of topological insulators have been shown in theoretical reports, there is a lack of experimental investigations for a better understanding of their basic properties. This research work focuses on the characterizations of fundamental properties of Bi2Te3 two-dimensional (2D) nanosheets. Samples were prepared via respective solvothermal synthesis and van der Waals epitaxy. The charged surface properties of Bi2Te3 2D nanosheets were investigated using kelvin probe force microscopy. The measured electrical potential difference between aminosilane self-assembled monolayer and Bi2Te3 nanosheet surfaces is found to be ∼650 mV, which is larger than that (∼400 mV) between the silicon oxide substrate and Bi2Te3 nanosheet surface. The elastic properties of Bi2Te3 2D nanosheets (i.e., Young’s modulus and prestress) were acquired by analyzing the thickness dependence of 2D nanosheet deformations creating by atomic force microscopy tips. The Young's modulus by fitting linear elastic behaviors of 26 samples is found only 11.7–25.7 GPa, significantly smaller than the bulk in-plane Young's modulus (50–55 GPa). Furthermore, the thermoelectric properties of Bi2Te3 2D nanosheets were characterized in the cryostat system at a temperature range of 20-400 K. The results reveal that electrical conductivity of 2D nanosheets decreases with increasing temperature and thickness, while the measured Seebeck coefficient does not show a strong thickness dependence and the value is smaller than bulk Bi2Te3. These fundamental properties would help improve the basic understanding of topological surface states towards practical applications.
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    Direct transfer of graphene onto flexible substrates
    (National Academy of the Sciences, 2013) Martins, Luiz G. P.; Song, Yi; Zeng, Tingying; Dresselhaus, Mildred S.; Kong, Jing; Araujo, Paulo T.; Universidade Federal de Minas Gerais; Massachusetts Institute of Technology (MIT); University of Alabama Tuscaloosa
    In this paper we explore the direct transfer via lamination of chemical vapor deposition graphene onto different flexible substrates. The transfer method investigated here is fast, simple, and does not require an intermediate transfer membrane, such as polymethylmethacrylate, which needs to be removed afterward. Various substrates of general interest in research and industry were studied in this work, including polytetrafluoroethylene filter membranes, PVC, cellulose nitrate/cellulose acetate filter membranes, polycarbonate, paraffin, polyethylene terephthalate, paper, and cloth. By comparing the properties of these substrates, two critical factors to ensure a successful transfer on bare substrates were identified: the substrate's hydrophobicity and good contact between the substrate and graphene. For substrates that do not satisfy those requirements, polymethylmethacrylate can be used as a surface modifier or glue to ensure successful transfer. Our results can be applied to facilitate current processes and open up directions for applications of chemical vapor deposition graphene on flexible substrates. A broad range of applications can be envisioned, including fabrication of graphene devices for opto/organic electronics, graphene membranes for gas/liquid separation, and ubiquitous electronics with graphene.
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    Mechanical and Thermal Properties of Linear Carbon Chains Encapsulated by Multi- and Double -Walled Carbon Nanotubes
    (University of Alabama Libraries, 2021) Sharma, Keshav; Araujo, Paulo T.; University of Alabama Tuscaloosa
    As a field of study, optics have been a critical to the development of material’s science. The interaction between light and matter is often non-destructive and non-invasive; making it powerful in the determination of materials’ compositions, and their properties at the electronic and vibrational levels. In the present work, we have broadened the use of light spectroscopy as a technique to determine, accurately, mechanical and thermal properties of molecular systems. More specifically, pressure (P) and temperature (T)-dependent Raman spectroscopy allowed us to access elusive mechanical (Young’s modulus (E), Grüneisen parameter (γ), and mechanical strain (ε)) and thermal properties (coefficient of thermal expansion (α), specific heat capacity (c_v), and thermal strain (ε_T)) of linear carbon chains (LCCs), which are one-atom thick linear carbon molecules. The results show that all these quantities follow universal relations that are solely dependent on P, T, and on the number of carbon atoms (N). In Appendix 01 we also describe how spectral derivative analysis combined with absorption and photo-luminescence spectroscopies allowed for unravelling elusive electronic and vibronic transition in free base 5,10,15,20-meso-tetra(pyridyl)-21H,23H-porphyrin (H2TPyP).
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    Raman spectroscopy of double- and triple-walled carbon nanotubes: fundamental, combination, and overtone modes
    (University of Alabama Libraries, 2020) Hue, Jia Wern; Araujo, Paulo T.; University of Alabama Tuscaloosa
    A single-walled carbon nanotube (SWNT) is a graphene sheet rolled up into a tube. Double-walled (DWNT) and triple-walled carbon nanotubes (TWNT) are two and three coaxial SWNTs respectively. Isolated species of DWNTs and TWNTs were only recently probed and it can be considered as a new branch in carbon nanotube science. Phonons, and the combination of phonons and overtones are fundamental to understanding optical processes, transport and thermoelectricity in carbon nanotubes. To study these phonons, resonance Raman spectroscopy is employed on DWNT and TWNT bundles, as well as isolated TWNTs. The phonon modes of interest to this dissertation are the radial breathing mode (RBM), the G-band, and the M-band. From the RBM, it is learnt that for inner tubes with a tube diameter less than 1.2 nm, the curvature effects are dominant. Evidence for intertube interactions was found, although for outer tubes the environmental effects dominate. There was also evidence for commensurate and incommensurate tubes. Studying the G-band allowed for possible chiral indices to be identified for the middle and outer tubes of isolated TWNTs. In addition, the frequency shifts of TWNT G-band frequencies relative to SWNT G-band frequencies were studied and compared to TWNT G-band frequency shifts. As for the M-bands, phonon mode assignments for peaks between 1680 cm-1 to 1850 cm-1 were attempted.
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    Stokes spectroscopy: the development of a novel method to acquire and interpret polarized emission spectra - applications to poly(3hexylthiophene and p(ndi2od-t2)
    (University of Alabama Libraries, 2018) Ulrich, Steven V.; Araujo, Paulo T.; University of Alabama Tuscaloosa
    Optics, as a field of study, has been a critical tool within the discipline of material science for over one hundred years. Through lights interaction with matter, researchers can determine information such as material composition, electronic and physical properties. This information is then used to guide research for specific applications. From the outside, it may seem as though optics as a field is complete; all possible experiments known and possible outcomes interpreted. However, one such property of light, specifically polarization, has proven difficult to measure and subsequently analyze in a meaningful way. Current techniques for measuring polarization information involve simple rotations of a linear polarizer, or analyzer, to get a loose understanding of an emitting sources polarization state. However, this technique and others like it are far from complete and much of the polarization information is still unavailable to researchers. One way to elucidate more polarization information is to implement a method proposed by Stokes in the late 1800s, in which four parameters are used to describe a sources intensity and polarization states. The goal of this work is to show how the addition of the Stokes technique to a typical spectroscopic setup, along with computational fitting, produces direct measurements of these polarization states. Further, we show the capabilities of these adaptations by applying the technique to two organic semi-conducting polymers, Poly(3-hexothilophene) and P(NDI2OD-T2). Doing so has allowed for further elucidation of material properties, including aggregate formation and energy transfer, which is typically unavailable for such materials at high temperature.