Emil Sandoz-Rosado

Enhanced Graphene Mechanical Properties through Ultrasmooth Copper Growth Substrates

The combination of extraordinary strength and stiffness in conjunction with exceptional electronic and thermal properties in lightweight two-dimensional materials has propelled graphene research toward a wide array of applications including flexible electronics and functional structural components. Tailoring graphenes properties toward a selected application requires precise control of the atomic layer growth process, transfer, and postprocessing procedures. To date, the mechanical properties of graphene are largely controlled through postprocess defect engineering techniques. In this work, we demonstrate the role of varied catalytic surface morphologies on the tailorability of subsequent graphene film quality and breaking strength, providing a mechanism to tailor the physical, electrical, and mechanical properties at the growth stage. A new surface planarization methodology that results in over a 99% reduction in Cu surface roughness allows for smoothness parameters beyond that reported to date in literature and clearly demonstrates the role of Cu smoothness toward a decrease in the formation of bilayer graphene defects, altered domain sizes, monolayer graphene sheet resistance values down to 120 Ω/□ and a 78% improvement in breaking strength. The combined electrical and mechanical enhancements achieved through this methodology allows for the direct growth of application quality flexible transparent conductive films with monolayer graphene.

High strength films from oriented, hydrogen-bonded “graphamid” 2D polymer molecular ensembles

The linear polymer poly(p-phenylene terephthalamide), better known by its tradename Kevlar, is an icon of modern materials science due to its remarkable strength, stiffness, and environmental resistance. Here, we propose a new two-dimensional (2D) polymer, “graphamid”, that closely resembles Kevlar in chemical structure, but is mechanically advantaged by virtue of its 2D structure. Using atomistic calculations, we show that graphamid comprises covalently-bonded sheets bridged by a high population of strong intermolecular hydrogen bonds. Molecular and micromechanical calculations predict that these strong intermolecular interactions allow stiff, high strength (6–8 GPa), and tough films from ensembles of finite graphamid molecules. In contrast, traditional 2D materials like graphene have weak intermolecular interactions, leading to ensembles of low strength (0.1–0.5 GPa) and brittle fracture behavior. These results suggest that hydrogen-bonded 2D polymers like graphamid would be transformative in enabling scalable, lightweight, high performance polymer films of unprecedented mechanical performance.

Background data for modulus mapping high-performance polyethylene fiber morphologies

The data included here provides a basis for understanding “Interior morphology of high-performance polyethylene fibers revealed by modulus mapping” (K.E. Strawhecker, E.J. Sandoz-Rosado, T.A. Stockdale, E.D. Laird, 2016) [1], in specific: the multi-frequency (AMFM) atomic force microscopy technique and its application to ultra-high-molecular-weight Polyethylene (UHMWPE) fibers. Furthermore, the data suggests why the Hertzian contact mechanics model can be used within the framework of AMFM theory, simple harmonic oscillator theory, and contact mechanics. The framework is first laid out followed by data showing cantilever dynamics, force-distance spectra in AC mode, and force-distance in contact mode using Polystyrene reference and UHMWPE. Finally topography and frequency shift (stiffness) maps are presented to show the cases where elastic versus plastic deformation may have occurred.

The mechanical characterization of stacked, multilayer graphene cantilevers and plates

The mechanical properties of stacked graphene sheets with varying number of layers are examined. The stacked sheets are assembled by manually combining single layer CVD-grown graphene monolayers, resulting in a turbostratic multilayer graphene with irregular layer spacing greater than crystalline graphite. Due to the presence of multiple layers, the material is analyzed as a plate rather than a membrane. Bending stiffness is determined via the deflection of micron-scale cantilevers, prepared using focused ion beam milling, while in-plane tensile stiffness is characterized through center-loading of edge-supported circular specimens. Computational modeling and established analytical solutions are used to extract material and structural property information, and benchmark measured properties relative to complementary results from indentation tests. Stacked, few-layer CVD-grown graphene retains an in-plane elastic modulus of 350N/m/layer (corresponding to 1.04 TPa for an inter-layer spacing of 0.335nm), suggesting good load-sharing between stacked layers. Width-normalized bending stiffness was unmeasurable for cantilevers of 1 and 3 layers, while cantilevers of 5 and 10 layers had values of 11,100nN·nm and 1.3·106nN•nm respectively.

Mechanical Structure-Property Relationships for 2D Polymers Comprised of Nodes and Bridge Units

2D polymers have emerged as an infinitely-tailorable material with remarkable, tunable response and density-normalized mechanical properties far exceeding structural materials such as steel, high-performance fibers or reinforced composites. It is critical that the vast material design space of 2D polymers be mapped in order to achieve optimal mechanical performance, since hundreds of permutations of one class of 2D polymers known as covalent organic frameworks have already been synthesized in the decade since the introduction of these materials. To this end, this work establishes a general structure-property relationship for elastic modulus and strength for a common 2D polymer motif consisting of nodes linked by linear bridge polymer chains to form a two-dimensional network. The length of the bridge chains are parametrically varied to study the impact of chain compliance on stiffness and strength. The density-normalized isotropic strength of the graphene/polyethylene hybrid material known as graphylene begins at 0.015 GPa/kg·m3 (50% higher than that of perfect crystalline Kevlar®) and the density-normalized isotropic stiffness is 0.143 GPa/kg·m3 (31% higher than Kevlar®) and decreases non-monotonically with increasing bridge chain length. The mechanical response is mapped and correlated to the inherent molecular structure of these general 2D polymer as a framework for designing 2D polymer molecules for mechanical applications from the ground up.

Quantifying High-Performance Material Microstructure Using Nanomechanical Tools with Visual and Frequency Analysis

High-performance materials like ballistic fibers have remarkable mechanical properties owing to specific patterns of organization ranging from the molecular scale, to the micro scale and macro scale. Understanding these strategies for material organization is critical to improving the mechanical properties of these high-performance materials. In this work, atomic force microscopy (AFM) was used to detect changes in material composition at an extremely high resolution with transverse-stiffness scanning. New methods for direct quantification of material morphology were developed, and applied as an example to these AFM scans, although these methods can be applied to any spatially-resolved scans. These techniques were used to delineate between subtle morphological differences in commercial ultra-high-molecular-weight polyethylene (UHMWPE) fibers that have different processing conditions and mechanical properties as well as quantify morphology in commercial Kevlar®, a high-performance material with an entirely different organization strategy. Both frequency analysis and visual processing methods were used to systematically quantify the microstructure of the fiber samples in this study. These techniques are the first step in establishing structure-property relationships that can be used to inform synthesis and processing techniques to achieve desired morphologies, and thus superior mechanical performance.

Atomistic Simulation of a Two-Dimensional Polymer Tougher Than Graphene

A graphene/polyethylene hybrid 2D polymer, “graphylene”, exhibits a higher theoretical fracture toughness than graphene, while remaining 2× stiffer and 9× stronger than Kevlar®, per mass. Within the base structure of graphylene, the sp3-bonded polyethylene linkages provide compliance for ductile fracture, while the benzene rings contribute to high stiffness and strength. Combining stiff and compliant units to achieve enhanced mechanical performance demonstrates the potential of designing 2D materials at the molecular level.