Tailoring Surface Forces to Control the Frictional Properties of Graphene

A pressing financial and environmental challenge is the impact of friction and wear on energy usage, economic costs, and greenhouse gas (GHG) emissions. Globally nearly ¼ of the world’s total energy is consumed at moving contacts, with 20% of that total used to overcome frictional forces. To combat the negative effects of friction and wear, thus mitigating economic spending on energy losses and reducing GHG emissions, new lubrication schemes need to be developed. Effective lubrication solutions will need to be compatible with a host of sliding conditions, including a diverse range of surface chemistries and structural features. One highly adaptable material capable of meeting this challenge is graphene, a two-dimensional sheet of carbon atoms with excellent electronic, optical, and thermal properties. Graphene additionally exhibits exceptional friction reducing and wear-resistant properties, although it is difficult to implement as a practical lubricant because its mechanical behavior strongly depends on its interactions with the top and bottom contacts within an interface. To effectively capitalize on the lubricating potential of graphene, a thorough investigation into the tribological responses of graphene in controlled sliding contacts is required. Towards this goal, this dissertation includes research into graphene that is in sliding contact with molecularly modified interfaces, dynamically oscillating on rough surfaces, and covalently immobilized to the supporting substrate. Adhesion and friction measurements on graphene with molecularly functionalized atomic force microscopy (AFM) tips demonstrated how both chemical functionality and shear strain can be used to tune the tribological response of the graphene-molecule sliding interface. Dynamic measurements of graphene on rough surfaces were exploited to examine the relationship between energy dissipation and friction at different frequencies. Pinning graphene to the supporting surface further showed how the physical properties of graphene can be manipulated at interfaces. By understanding how tailored adhesion, modulated out-of-plane forces, and localized pinning concertedly impact the tribological performance of graphene, the development of targeted lubricant technologies can take advantage of graphene’s sensitivity to different sliding conditions. Designer boundary lubrication schemes incorporating graphene can then play a central role in overcoming the challenges associated with energy losses at tribological contacts

Driving Surface Chemistry at the Nanometer Scale Using Localized Heat and Stress

Driving and measuring chemical reactions at the nanoscale is crucial for developing safer, more efficient, and environment-friendly reactors and for surface engineering. Quantitative understanding of surface chemical reactions in real operating environments is challenging due to resolution and environmental limitations of existing techniques. Here we report an atomic force microscope technique that can measure reaction kinetics driven at the nanoscale by multiphysical stimuli in an ambient environment. We demonstrate the technique by measuring local reduction of graphene oxide as a function of both temperature and force at the sliding contact. Kinetic parameters measured with this technique reveal alternative reaction pathways of graphene oxide reduction previously unexplored with bulk processing techniques. This technique can be extended to understand and precisely tailor the nanoscale surface chemistry of any two-dimensional material in response to a wide range of external, multiphysical stimuli

Metal-organic coordinated multilayer film formation: Quantitative analysis of composition and structure

Metal-organic coordinated multilayers are self-assembled thin films fabricated by alternating solution-phase deposition of bifunctional organic molecules and metal ions. The multilayer film composed of α,ω-mercaptoalkanoic acid and Cu (II) has been the focus of fundamental and applied research with its robust reproducibility and seemingly simple hierarchical architecture. However, internal structure and composition have not been unambiguously established. The composition of films up to thirty layers thick was investigated using Rutherford backscattering spectrometry and particle induced X-ray emission. Findings show these films are copper enriched, elucidating a 2:1 ratio for the ion to molecule complexation at the metal-organic interface. Results also reveal that these films have an average layer density similar to literature values established for a self-assembled monolayer, indicating a robust and stable structure. The surface structures of multilayer films have been characterized by contact angle goniometry, ellipsometry, and scanning probe microscopy. A morphological transition is observed as film thickness increases from the first few foundational layers to films containing five or more layers. Surface roughness analysis quantifies this evolution as the film initially increases in roughness before obtaining a lower roughness comparable to the underlying gold substrate. Quantitative analysis of topographical structure and internal composition for metal-organic coordinated multilayers as a function of number of deposited layers has implications for their incorporation in the fields of photonics and nanolithography

Plug-and-Play Approach for Preparing Chromatin Containing Site-Specific DNA Modifications: The Influence of Chromatin Structure on Base Excision Repair

The genomic DNA of eukaryotic cells exists in the form of chromatin, the structure of which controls the biochemical accessibility of the underlying DNA to effector proteins. In order to gain an in depth molecular understanding of how chromatin structure regulates DNA repair, detailed <i>in vitro</i> biochemical and biophysical studies are required. However, because of challenges associated with reconstituting nucleosome arrays containing site-specifically positioned DNA modifications, such studies have been limited to the use of mono- and dinucleosomes as model <i>in vitro</i> substrates, which are incapable of folding into native chromatin structures. To address this issue, we developed a straightforward and general approach for assembling chemically defined oligonucleosome arrays (i.e., designer chromatin) containing site-specifically modified DNA. Our method takes advantage of nicking endonucleases to excise short fragments of unmodified DNA, which are subsequently replaced with synthetic oligonucleotides containing the desired modification. Using this approach, we prepared several oligonucleosome substrates containing precisely positioned 2′-deoxyuridine (dU) residues and examined the efficiency of base excision repair (BER) within several distinct chromatin architectures. We show that, depending on the translational position of the lesion, the combined catalytic activities of uracil DNA glycosylase (UDG) and apurinic/apyrimidinic endonuclease 1 (APE1) can be either inhibited by as much as 20-fold or accelerated by more than 5-fold within compact chromatin (i.e., the 30 nm fiber) relative to naked DNA. Moreover, we demonstrate that digestion of dU by UDG/APE1 proceeds much more rapidly in mononucleosomes than in compacted nucleosome arrays, thereby providing the first direct evidence that internucleosome interactions play an important role in regulating BER within higher-order chromatin structures. Overall, this work highlights the value of performing detailed biochemical studies on precisely modified chromatin substrates <i>in vitro</i> and provides a robust platform for investigating DNA modifications in chromatin biology