Process Characterization and Optimization of Roll-to-Roll Plasma Chemical Vapor Deposition for Graphene Growth

Large-scale production of graphene and other nanostructures remains a hindrance to their adoption in the semiconductor and materials manufacturing industries. The main purpose of this thesis is to develop an efficient and scalable technique for depositing graphene on various flexible substrates. Hence, a custom-built roll-to-roll capacitively coupled plasma chemical vapor system for deposition of graphene on flexible substrates is thoroughly described in this work. Graphene quality on Cu foil has been optimized for a roll-to-roll process using statistical optimization methods. Since graphene quality and uniformity depend on plasma input parameters, such as plasma power, gas pressure, and the gas mixture used, effects of input parameters have been explored to maximize graphene quality, as quantified by Raman spectroscopy using the ID/IG intensity ratio. Furthermore, in situ optical emission spectroscopy (OES) has been developed and utilized to determine the effects of several plasma species on graphene growth and quality. OES results demonstrate that graphene quality on Cu foil increases with CH radical emission; however, O and H atoms, C2 and CN radicals, and Ar+ ion all negatively correlate to graphene quality. Results aid in developing a conceptual model for a graphene growth mechanism that indicates the adverse impact of ion bombardment on graphene quality in the low-frequency capacitively coupled plasma. However, the existence of active carbon species in the plasma, such as CH radical, accelerates the growth process and leads to moderate-quality graphene deposition on Cu foil at web speeds reaching as high as 1 m/min. Nevertheless, graphene quality measured from Raman spectroscopy declines significantly with increased Cu foil velocity (web speed) in the roll-to-roll process, inducing a critical limitation in current production rates for roll-to-roll CVD nonmanufacturing techniques. With the aid of heat transfer modeling of the moving foil, we show that the graphene quality decrease is primarily due to Cu foil temperature decline with increased web speed. The Cu foil temperature distribution is determined both experimentally and numerically during roll-to-roll graphene growth as a function of web speed, plasma power and plasma length. The maximum Cu foil temperature in the plasma rises with increased plasma power due to increased heating from the plasma. However, the maximum Cu foil temperature decreases with increased web speed caused by higher heat advection by the moving foil. In addition, shortening the plasma slit (by decreasing the electrodes length) cools the Cu foil temperature and diminishes its temperature uniformity in the plasma region. Consequently, graphene crystallization, identified using Raman spectroscopy, improves with higher Cu foil temperatures. As a result, an optimum condition is defined by raising the plasma power, lowering the web speed and increasing the plasma region length, which consistently produces high-quality graphene on Cu foil. The throughput of graphene production can be increased by utilizing Ni foil as a substrate since carbon solubility in Ni is higher than in Cu. Thus, the effects of web speed and plasma power on Ni foil temperature distribution are evaluated during graphene deposition in the roll-to-roll process. Furthermore, the Ni foil cooling rate, which strongly affects carbon atom segregation from Ni after the growth process, is derived from the heat transfer model. Plasma power has negligible effects on the cooling rate, whereas the web speed has a significant impact on the cooling rate. Consequently, graphene has comparable quality at different plasma powers, whereas web speed controls graphene quality, particularly with regards to uniformity and thickness. Our work highlights the benefits of using Ni foil in a roll-to-roll process for graphene deposition at higher web speeds and lower substrate temperatures, rather than using Cu foil, which requires significantly more substrate heating. Plasma plays a crucial role in heating the foil for graphene deposition in the roll-to-roll process, without the need of a supplemental heating source. Thus, accurate measurement of the translational gas temperature in the plasma is vital, since gas temperature strongly influences the foil temperature distribution, which, in turn, affects graphene growth kinetics. Optical emission spectroscopy (OES) is used to measure the rotational temperatures of N2 + (B-X), CN (B-X) and H2 (d3Πu → a3Σg +), and to determine accurate translational gas temperatures. Power dissipation in the plasma is also measured to understand gas temperature variation for the experimental input conditions. Thus, the effects of plasma power, gas pressure and the addition of nitrogen (N2), oxygen (O2) and methane (CH4) gases on power dissipation and gas temperature in a hydrogen (H2) plasma are assessed. The rotational temperatures measured from the gas species have different values due to the non-equilibrium nature of the plasma. Of the gases measured, the rotational temperature of N2 + is most accurate in representing the translational gas temperature. These results improve the understanding and control of the thermochemical environment for carbon nanostructure growth in the plasma chemical vapor deposition processes. Graphene quality significantly depends on gas pressure since our plasma roll-to-roll system is sustained by a capacitively coupled plasma that operates in two modes, depending on the gas pressure and discharge gap. The modes are identified as alpha and gamma modes, and are sustained by volume ionization and secondary electron emission processes, respectively. Up to our knowledge, the presence of both modes at 80 kHz plasma frequency has not previously been reported. Thus, a detailed characterization of argon plasma is attempted to determine the underlying plasma physics of the low-frequency plasma. Due to strong ion bombardment on the electrodes, the gamma mode coexists with the alpha mode, resulting in a hybrid mode. The voltage square waveform is found to play an important role in sustaining this hybrid mode. The hybrid mode exists at low gas pressures of 5.5 and 9.5 mbar in the plasma set power ranges from 300 to 1100 W. However, the plasma at 13.8 mbar gas pressure transforms from hybrid to gamma mode when the plasma set power is beyond 750 W due to increased secondary electron emission processes. The emission spectra measured from optical emission spectroscopy reveal the presence of non-Ar species in the gamma mode, such as H, CH, and C2. These species are sputtered from the graphite electrodes by ion bombardment to produce secondary electrons that sustain the gamma discharge. Results show the possibility of sustaining the hybrid mode at a low plasma frequency using a tailored waveform. As a results of these plasma characterization tools, we report a continuous and rapid rollto- roll deposition of thin graphite film on Cu foil. The composition of the Ar/H2/CH4/N2/O2 plasma plays significant role in the successful direct growth of the thin graphite film on copper foil. Optical emission spectroscopy is used to characterize the plasma during graphite synthesis and show that the addition of N2 enhances the plasma reactivity, and O2 was found to increase the deposition rate of the graphite film. The film was characterized by Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The described large-scale graphite production can produce a graphite-Cugraphite structure or uniform thin graphite films for thermal management applications in electronics devices. Graphene growth optimization, substrate thermal analysis, and plasma characterizations are used to control graphene mass-production in a custom-built roll-to-roll plasma CVD system. These techniques are addressed to provide a route for nanomanufacturing of graphene and graphite on Cu and Ni foils. These methods aid in understanding the correlations between process conditions and graphene quality, as well as the interactions between the plasma and the substrate, to yield high-throughput production of high-quality graphene. The procedure outlined here can be applied to efficiently scale-up the production of other micro- and nanomaterials

Modeling of a Roll-to-roll Plasma CVD System for Graphene

Graphene is a 2D carbon material that has extraordinary physical properties relevant to many industrial applications such as electronics, oxidation barrier and biosensors. Roll-to-roll plasma chemical vapor deposition (CVD) has been developed to manufacture graphene at large scale. In a plasma CVD chamber, graphene is grown on a copper foil as it passes through a high-temperature plasma region. The temperatures of the gas and the copper foil play important roles in the growth of graphene. Consequently, there is a need to understand the temperature and gas velocity distributions in the system. The heat generated in the plasma creates a thermal field that enhances natural convection inside the system enclosure. The analysis of temperature and fluid flow of hydrogen was carried out numerically using FLUENT, a commercial computational fluid dynamics package. A three-dimensional model has been built including the heat source from the plasma, natural convection, radiation and simple gas reactions. The plasma is generated between two rectangular parallel plates whose major axis can be oriented either vertically or horizontally. The temperature and flow for the vertical plasma electrodes configuration exhibit higher values than the horizontal configuration due to increased interactions of the heated plates with the buoyancy-driven flow. Furthermore, the presence of the copper foil that is used as the substrate for graphene deposition decreases the temperature and velocity in the adjacent regions because the copper foil acts as a fin and impedes fluid flow, respectively. Finally, adding methane as a mixture with hydrogen increases the gas temperature in the plasma region due to the lower thermal conductivity of methane. The numerical results help in understanding the temperature and the flow in the roll-to-roll CVD plasma system that makes it suitable for modeling graphene production for the purpose of optimizing manufacturing process conditions

Process Characterization and Optimization of Roll-to-roll Plasma Chemical Vapor Deposition for Graphene Growth

The main purpose of this thesis is to develop an efficient and scalable technique for depositing graphene on various flexible substrates. Hence, a custom-built roll-to-roll capacitively coupled plasma chemical vapor system for deposition of graphene on flexible substrates is thoroughly described in this work. Graphene quality on Cu foil has been optimized for a roll-to-roll process using statistical optimization methods. Since graphene quality and uniformity depend on plasma input parameters, such as plasma power, gas pressure, and the gas mixture used, effects of input parameters have been explored to maximize graphene quality, as quantified by Raman spectroscopy using the I D/IG intensity ratio. Furthermore, in situ optical emission spectroscopy has been developed and utilized to determine the effects of several plasma species on graphene growth and quality. OES results demonstrate that graphene quality on Cu foil increases with CH radical emission; however, O and H atoms, C2 and CN radicals, and Ar + ion all negatively correlate to graphene quality. Results aid in developing a conceptual model for a graphene growth mechanism that indicates the adverse impact of ion bombardment on graphene quality in the low-frequency capacitively coupled plasma. However, the existence of active carbon species in the plasma, such as CH radical, accelerates the growth process and leads to moderate-quality graphene deposition on Cu foil at web speeds reaching as high as 1 m/min. Plasma plays a crucial role in heating the foil for graphene deposition in the roll-to-roll process, without the need of a supplemental heating source. Thus, accurate measurement of the translational gas temperature in the plasma is vital, since gas temperature strongly influences the foil temperature distribution, which, in turn, affects graphene growth kinetics. Optical emission spectroscopy (OES) is used to measure the rotational temperatures of N2+ (B-X), CN (B-X) and H2 (d3Πu → a3 Σg+), and to determine accurate translational gas temperatures. Power dissipation in the plasma is also measured to understand gas temperature variation for the experimental input conditions. Thus, the effects of plasma power, gas pressure and the addition of nitrogen, oxygen and methane gases on power dissipation and gas temperature in a hydrogen plasma are assessed. The rotational temperatures measured from the gas species have different values due to the non-equilibrium nature of the plasma. Graphene quality significantly depends on gas pressure since our plasma roll-to-roll system is sustained by a capacitively coupled plasma that operates in two modes, depending on the gas pressure and discharge gap. The modes are identified as alpha and gamma modes, and are sustained by volume ionization and secondary electron emission processes, respectively. Up to our knowledge, the presence of both modes at 80 kHz plasma frequency has not previously been reported. Thus, a detailed characterization of argon plasma is attempted to determine the underlying plasma physics of the low-frequency plasma. Due to strong ion bombardment on the electrodes, the gamma mode coexists with the alpha mode, resulting in a hybrid mode. The voltage square waveform is found to play an important role in sustaining this hybrid mode. The hybrid mode exists at low gas pressures of 5.5 and 9.5 mbar in the plasma set power ranges from 300 to 1100 W. However, the plasma at 13.8 mbar gas pressure transforms from hybrid to gamma mode when the plasma set power is beyond 750 W due to increased secondary electron emission processes. The emission spectra measured from optical emission spectroscopy reveal the presence of non-Ar species in the gamma mode, such as H, CH, and C2. These species are sputtered from the graphite electrodes by ion bombardment to produce secondary electrons that sustain the gamma discharge. Results show the possibility of sustaining the hybrid mode at a low plasma frequency using a tailored waveform. As a results of these plasma characterization tools, we report a continuous and rapid roll-to-roll deposition of thin graphite film on Cu foil. The composition of the Ar/H2/CH4/N2/O2 plasma plays significant role in the successful direct growth of the thin graphite film on copper foil. Optical emission spectroscopy is used to characterize the plasma during graphite synthesis and show that the addition of N 2 enhances the plasma reactivity, and O2 was found to increase the deposition rate of the graphite film. The film was characterized by Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The described large-scale graphite production can produce a graphite-Cu-graphite structure or uniform thin graphite films for thermal management applications in electronics devices. (Abstract shortened by ProQuest.

A Heat Transfer Model for Graphene Deposition on Ni and Cu Foils in a Roll-to-Roll Plasma Chemical Vapor Deposition System

Abstract High-throughput production is a major bottleneck for integration of graphene-based technologies in existing and future applications. Here, a semi-empirical heat transfer model is developed to optimize large-scale deposition of graphene on Ni and Cu foils in a roll-to-roll (R2R) plasma chemical vapor deposition (CVD) system. Temperature distributions in Ni and Cu foils during deposition are recorded with in situ temperature measurements using near-IR optical emission spectroscopy. The model indicates that foil movement significantly affects the temperature distribution and the cooling rate of the foil. Consequently, graphene growth on Cu is limited to lower web speeds for which the foil temperature is higher, and the residence time in the plasma is longer. On the other hand, graphene can be deposited on Ni at relatively higher web speeds due to moderately high diffusion rate of carbon in Ni and increased cooling rates up to 20 K/s with higher web speed. Critical limitations in the production rates of graphene using R2R CVD process exist due to significant effects of web speed on the temperature distribution of the substrate. The thermal analysis approach reported here is expected to aid in enhancing the throughput of graphene production in R2R CVD systems

Improvement of the Zienkiewicz–Zhu Error Recovery Technique Using a Patch Configuration

The Zienkiewicz–Zhu (ZZ) super-convergent patch recovery technique based on a node neighborhood patch configuration is used most widely for recovery of the stress field of a finite element analysis. In this study, an improved ZZ recovery technique using element neighborhood patch configuration is proposed. The improved recovery procedure is based on recovery of the stress field in the least-squares sense over an element patch that consists of the union of the elements surrounding the element under consideration. The proposed patch configuration provides more sampling points and improves the performance of the standard ZZ recovery technique. The effectiveness and reliability of the improved ZZ recovery approach is demonstrated through plane elastic and plastic plate problems. The problem domain is discretized with triangular and quadrilateral elements of different sizes. A comparison of the quality of error estimation using the ZZ recovery of derivative field and recovery of the displacement field using similar element neighborhood patch configurations is also presented. The numerical results show that the ZZ recovery technique and the displacement recovery technique, using a modified patch configuration, yield better results, convergence rate, and effectivity as compared with the standard ZZ super-convergent patch recovery technique. It is concluded that the improved ZZ recovery technique-based adaptive finite element analysis is very effective for converging a predefined accuracy with a significantly smaller number of degrees of freedom, especially in an elastic problem. It is also concluded that the improved ZZ recovery technique captures the plastic deformation problem solution errors more reliably than the standard ZZ recovery technique

Mesh Free Radial Point Interpolation Based Displacement Recovery Techniques for Elastic Finite Element Analysis

The study develops the displacement error recovery method in a mesh free environment for the finite element solution employing the radial point interpolation (RPI) technique. The RPI technique uses the radial basis functions (RBF), along with polynomials basis functions to interpolate the displacement fields in a node patch and recovers the error in displacement field. The global and local errors are quantified in both energy and L2 norms from the post-processed displacement field. The RPI technique considers multi-quadrics/gaussian/thin plate splint RBF in combination with linear basis function for displacement error recovery analysis. The elastic plate examples are analyzed to demonstrate the error convergence and effectivity of the RPI displacement recovery procedures employing mesh free and mesh dependent patches. The performance of a RPI-based error estimators is also compared with the mesh dependent least square based error estimator. The triangular and quadrilateral elements are used for the discretization of plates domains. It is verified that RBF with their shape parameters, choice of elements, and errors norms influence considerably on the RPI-based displacement error recovery of finite element solution. The numerical results show that the mesh free RPI-based displacement recovery technique is more effective and achieve target accuracy in adaptive analysis with the smaller number of elements as compared to mesh dependent RPI and mesh dependent least square. It is also concluded that proposed mesh free recovery technique may prove to be most suitable for error recovery and adaptive analysis of problems dealing with large domain changes and domain discontinuities

Process optimization of graphene growth in a roll-to-roll plasma CVD system

A systematic approach to mass-production of graphene and other 2D materials is essential for current and future technological applications. By combining a sequential statistical design of experiments with in-situ process monitoring, we demonstrate a method to optimize graphene growth on copper foil in a roll-to-roll rf plasma chemical vapor deposition system. Data-driven predictive models show that gas pressure, nitrogen, oxygen, and plasma power are the main process parameters affecting the quality of graphene. Furthermore, results from in-situ optical emission spectroscopy reveal a positive correlation of CH radical to high quality of graphene, whereas O and H atoms, Ar+ ion, and C2 and CN radicals negatively correlate to quality. This work demonstrates the deposition of graphene on copper foil at 1 m/min, a scale suitable for large-scale production. The techniques described here can be extended to other 2D materials and roll-to-roll manufacturing processes

A comprehensive experimental and modeling study of isobutene oxidation

Isobutene is an important intermediate in the pyrolysis and oxidation of higher-order branched alkanes, and it is also a component of commercial gasolines. To better understand its combustion characteristics, a series of ignition delay time (IDT) and laminar flame speed (LFS) measurements have been performed. In addition, flow reactor speciation data recorded for the pyrolysis and oxidation of isobutene is also reported. Predictions of an updated kinetic model described herein are compared with each of these data sets, as well as with existing jet-stirred reactor (JSR) species measurements.IDTs of isobutene oxidation were measured in four different shock tubes and in two rapid compression machines (RCMs) under conditions of relevance to practical combustors. The combination of shock tube and RCM data greatly expands the range of available validation data for isobutene oxidation models to pressures of 50 atm and temperatures in the range 666-1715 K. Isobutene flame speeds were measured experimentally at 1 atm and at unburned gas temperatures of 298-398 K over a wide range of equivalence ratios. For the flame speed results, there was good agreement between different facilities and the current model in the fuel-rich region. Ab initio chemical kinetics calculations were carried out to calculate rate constants for important reactions such as H-atom abstraction by hydroxyl and hydroperoxyl radicals and the decomposition of 2-methylallyl radicals.A comprehensive chemical kinetic mechanism has been developed to describe the combustion of isobutene and is validated by comparison to the presently considered experimental measurements. Important reactions, highlighted via flux and sensitivity analyses, include: (a) hydrogen atom abstraction from isobutene by hydroxyl and hydroperoxyl radicals, and molecular oxygen; (b) radical-radical recombination reactions, including 2-methylallyl radical self-recombination, the recombination of 2-methylallyl radicals with hydroperoxyl radicals; and the recombination of 2-methylallyl radicals with methyl radicals; (c) addition reactions, including hydrogen atom and hydroxyl radical addition to isobutene; and (d) 2-methylallyl radical decomposition reactions. The current mechanism accurately predicts the IDT and LFS measurements presented in this study, as well as the JSR and flow reactor speciation data already available in the literature.The differences in low-temperature chemistry between alkanes and alkenes are also highlighted. in this work. In normal alkanes, the fuel radical (R) over dot adds to molecular oxygen forming alkylperoxyl (R(O) over dot(2)) radicals followed by isomerization and chain branching reactions which promote low-temperature fuel reactivity. However, in alkenes, because of the relatively shallow well (similar to 20 kcal mol(-1)) for R(O) over dot(2) formation compared to similar to 35 kcal mol(-1) in alkanes, the (R) over dot+O-2 (sic) R(O) over dot(2) equilibrium lies more to the left favoring (R) over dot+O-2 rather than R(O) over dot(2) radical stabilization. Based on this work, and related studies of allylic systems, it is apparent that reactivity for alkene components at very low temperatures (1300 K), the reactivity is mainly governed by the competition between hydrogen abstractions by molecular oxygen and OH radicals. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.The work at NUI Galway was supported by Saudi Aramco under the FUELCOM program. Collaboration between NUI Galway and LRGP enters in the frame the COST Action CM1404.peer-reviewed2017-03-1

An experimental and modeling study of propene oxidation. Part 2: Ignition delay time and flame speed measurements

Journal articleExperimental data obtained in this study (Part II) complement the speciation data presented in Part I, but also offer a basis for extensive facility cross-comparisons for both experimental ignition delay time (IDT) and laminar flame speed (LFS) observables.To improve our understanding of the ignition characteristics of propene, a series of IDT experiments were performed in six different shock tubes and two rapid compression machines (RCMs) under conditions not previously studied. This work is the first of its kind to directly compare ignition in several different shock tubes over a wide range of conditions. For common nominal reaction conditions among these facilities, cross-comparison of shock tube IDTs suggests 20-30% reproducibility (2 sigma) for the IDT observable. The combination of shock tube and RCM data greatly expands the data available for validation of propene oxidation models to higher pressures (2-40 atm) and lower temperatures (750-1750 K).Propene flames were studied at pressures from 1 to 20 atm and unburned gas temperatures of 295-398 K for a range of equivalence ratios and dilutions in different facilities. The present propene-air LFS results at 1 atm were also compared to LFS measurements from the literature. With respect to initial reaction conditions, the present experimental LFS cross-comparison is not as comprehensive as the IDT comparison; however, it still suggests reproducibility limits for the LFS observable. For the LFS results, there was agreement between certain data sets and for certain equivalence ratios (mostly in the lean region), but the remaining discrepancies highlight the need to reduce uncertainties in laminar flame speed experiments amongst different groups and different methods. Moreover, this is the first study to investigate the burning rate characteristics of propene at elevated pressures (>5 atm).IDT and LFS measurements are compared to predictions of the chemical kinetic mechanism presented in Part I and good agreement is observed. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.The Rensselaer group was supported by the U.S. Air Force Office of Scientific Research (Grant No. FA9550-11-1-0261) with Dr. Chiping Li as technical monitor. Work at the University of Connecticut and at Princeton University was supported as part of the Combustion Energy Frontier Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, under Award Number DE-SC0001198. The work at Stanford University was supported by the Air Force Office of Scientific Research through AFOSR Grant No. FA9550-11-1-0217, under the AFRL Integrated Product Team, with Dr. Chiping Li as contract monitor. The work at NUI Galway was kindly supported by Saudi Aramco. The work of KAUST authors was supported by Saudi Aramco under the FUELCOM program.peer-reviewe