The development of graphene-based electronics may produce a new generation of electronic devices with enhanced performance over traditional materials. However, quality graphene electronics will require large-area, continuous graphene film produced by chemical vapor deposition (CVD), which is typically not stable when free-standing. Instead, CVD graphene may be coupled with polymer thin films to produce polymer/graphene laminates (PGLs), which show improved mechanical stability and good electrical conductivity over large areas. For example, single-layer graphene (SLG) has been coupled with polyetherimide (PEI) to produce thin film audio speakers with significantly enhanced energy efficiency compared to traditional speaker designs. However, the poor design and fabrication of PGLs may degrade the interfacial interactions of polymers with graphene and make the interface more susceptible to deformation. Interfacial weakening then limits their long-term reliability and performance in micro- and nano-electromechanical systems (MEMS/NEMS), in which materials are constantly subjected to dynamic loads. In this dissertation, we construct a framework for developing PGL thin films with controlled interfacial properties based on the rational design of polymer substrates and careful consideration of fabrication-induced defects. We evaluate this framework from several angles. First, the design of PEI/SLG film thickness is explored for controlling mechanical mixing and composite Young's modulus. Then, the role of thermal annealing during PEI/SLG fabrication is examined and its effect of mechanical mixing studied. Finally, we investigate mechanical fatigue in PEI/SLG thin films under dynamic loading with different pre-tensions.
The design of mechanical properties in PGLs is crucial for ensuring long-term stability and operational performance. The Young's modulus is an important mechanical parameter governed by mechanical mixing in PGLs, and better mechanical mixing is facilitated by improved stress transfer efficiency at the polymer/graphene interface. Thus, the control of mechanical mixing through PGL design is an important research topic for developing PGL thin films with good mechanical performance. To this end, we evaluated the design of PEI/SLG thin films with precisely controlled thicknesses to afford control over mechanical mixing and the composite Young's modulus. PEI concentrations of 3–6 wt% were used to spin-coat PEI/SLG films at precisely controlled thicknesses of ~200–1000 nm, which enabled the design of PEI/SLG thin films based on thickness control. The design of PEI/SLG film thickness afforded control over the volume fraction of SLG, which is related to the composite Young's modulus of the films via the well-known mixing rule. To validate this approach, we measured the composite Young's modulus of PEI/SLG films and modelled the relationship between modulus and film thickness (i.e., volume fraction) using the mixing rule. However, we found that linear regression analysis yielded an unexpectedly high effective Young's modulus of 1.12 ± 0.05 TPa for SLG. Further investigation revealed the larger-than-expected value was due to the gradual deterioration of mechanical mixing between SLG and PEI at film thicknesses > 250 nm. Overall, our results demonstrate that the control of mechanical mixing in PGL thin films is achievable through the physical design of film thickness, which fits well within the proposed framework for PGL development.
The fabrication of PGL materials is another important topic for controlling PGL properties. For example, the transfer process required for removing PGL thin films from a CVD substrate is known to introduce mechanical defects and electronic doping if improper processing conditions are used. However, little is known about the impact of thermal annealing on the properties of PGL materials. Normally, thermal annealing is thought to aid in the removal solvent and stress after polymer deposition on SLG, but we show that thermal annealing instead induces substantial structural damage and reduced film properties when poor conditions are used. Specifically, thermally annealing PEI/SLG in air past the Tg of PEI (~217 °C) caused widespread structural damage due to oxidation of the underlying Cu substrate. This conclusion was supported through an in-depth mechanistic investigation of annealing-induced deformation. First, changing the annealing atmosphere to high-purity nitrogen prevented widespread structural deformation from occurring during annealing, regardless of temperature. Second, the onset of film deformation during annealing in air was strongly associated with temperatures above the glass transition temperature of PEI. Finally, Arrhenius analysis yielded an activation energy of 159 kJ/mol for the deformation process, almost twice that associated with Cu oxidation and instead closer in scale to that of glass transitions in amorphous polymers. A physical failure mechanism was proposed based on the in-plane shrinkage of PEI during the glass-to-rubber transition caused by the release of internal stress induced by spin-coating. In-plane contraction of PEI would then cause compression of SLG as internal stress was released, which we confirmed from a significant blue-shift in the 2D band of SLG by ~30 cm-1 after annealing both with and without visible deformation. Importantly, we demonstrate that common secondary processing steps, like thermal annealing, will have critical effects on film properties if conditions are not properly controlled.
The mechanical fatigue and cycling stability of PGL thin films is another topic of interest for developing reliable PGLs with long operational lifetimes. Mechanical failure analysis might be used to evaluate failure mechanisms governing interfacial fatigue in PGL thin films. However, little work has been done understanding mechanical fatigue in PGLs. To this end, we explore the use of mechanical bulge testing for the evaluation of cycling fatigue in PGLs and construct a custom instrument, known as a bulge test apparatus (BTA), to perform fatigue measurements. In general, our BTA test platform provides a means of analyzing different fabrication/design conditions. To test its use, we prepared PEI/SLG thin films with different tensioning weights between 0 and 30 g. Then, the well-established methods for bulge testing were applied to the dynamic loading of PEI/SLG using the BTA. Specifically, the evolution of bulge-test-derived pre-stress in PEI/SLG was monitored as a function of cycle number and tensioning weight, which revealed large fluctuations in pre-stress with mechanical cycling up to ~200k cycles, which was unexpected for these films. To investigate the underlying mechanism, we further employed Raman spectroscopy, optical microscopy, and scanning electron microscopy (SEM) to determine the strain state of SLG before and after cycling and probe for structural changes. Raman measurements revealed that mechanical cycling induced a large redshift in the 2D and G peak positions of SLG by ~25.4 and ~10.1 cm-1, respectively, for 30 g-tensioned PEI/SLG, with similar shifts observed for 20 g-tensioned films. Optical and SEM images showed possible changes in the surface structure of SLG after cycling accompanying the shift in Raman characteristics. We discuss the possibility of interfacial failures involving in-plane slippage and out-of-plane buckling of SLG based on our results. If validated, the use of BTA may provide future insight regarding mechanical fatigue and failure at the PGL interface during dynamic loading. The practical relevance of methods for determining the influence of design and fabrication characteristics in PGLs will no doubt be invaluable for further development. Additional work in this area will be necessary to connect our analytical approach by BTA to underlying failure mechanisms in PGLs. / Doctor of Philosophy / Due to its record-breaking properties, graphene has tremendous promise for use in next-generation consumer electronics, like ultra-thin audio speakers and flexible displays. However, on its own, single-layer graphene (SLG) is not stable enough for practical uses. Therefore, in this dissertation, I construct a framework for developing polymer substrates to stabilize and enhance graphene based on the design and fabrication of polymer/graphene laminates (PGLs). To explore this framework, I design polyetherimide (PEI)/SLG thin films with controlled mechanical properties, evaluate fabrication-induced defects, and develop an analytical approach for measuring mechanical fatigue in these films.
The ability to precisely control thin film mechanics has important implications for the application of PGLs. However, it is challenging in PGL systems to enhance mechanical properties due to the limited amount of graphene that is used. Therefore, I show that the design of PEI/SLG film thickness can effectively control mechanical reinforcement, even at very low loadings of graphene. I further investigate the design of a controlled pre-tensioning in PEI/SLG and show that tensioning can be used to modify the mechanical response. However, using a custom mechanical testing platform, I also determine that mechanical fatigue may increase with pre-tensioning as well.
The impact of fabrication procedures on the properties of PGL thin films is another important aspect of PGL development that is often overlooked. Specifically, thermal annealing may introduce residual contamination and/or structural defects that reduce material properties. Therefore, I present a detailed study of thermal annealing-induced structural failures in PEI/SLG films, which shows that annealing at high temperature induces substrate oxidation and structure damage in the films. The results of this annealing study demonstrate the potential negative effects of fabrication on PGL development. Finally, this dissertation ends by discussing future directions in PGL design and fabrication that will need to be addressed in the coming years for further development of PGL thin film electronics.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/121082 |
| Date | 05 September 2024 |
| Creators | Croft, Zacary Lane |
| Contributors | Chemistry, Liu, Guoliang, Moore, Robert Bowen, Morris, Amanda, Esker, Alan R. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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