Robert M. Elder

Stacking order dependent mechanical properties of graphene/MoS2 bilayer and trilayer heterostructures

Transition metal dichalcogenides (TMDC) such as molybdenum disulfide (MoS2) are two-dimensional materials that show promise for flexible electronics and piezoelectric applications, but their weak mechanical strength is a barrier to practical use. In this work, we perform nanoindentation simulations using atomistic molecular dynamics to study the mechanical properties of heterostructures formed by combining MoS2 with graphene. We consider both bi- and tri-layer heterostructures formed with MoS2 either supported or encapsulated by graphene. Mechanical properties, such as Youngs modulus, bending modulus, ultimate tensile strength, and fracture strain, are extracted from nanoindentation simulations and compared to the monolayer and homogeneous bilayer systems. We observed that the heterostructures, regardless of the stacking order, are mechanically more robust than the mono- and bi-layer MoS2, mainly due to the mechanical reinforcement provided by the graphene layer. The magnitudes of ultimate strength and f...

Graphene/hexagonal boron nitride heterostructures: Mechanical properties and fracture behavior from nanoindentation simulations

In-plane or vertically stacked heterostructures containing multiple 2D materials are promising for emerging applications, such as flexible electronics, piezoelectric sensors, and molecular separations. However, utilizing heterostructures requires a fundamental understanding of their mechanics, which is currently lacking. Here, we use reactive molecular dynamics to simulate nanoindentation of stacked hexagonal boron nitride (h-BN) and graphene structures, 2D materials with similar structures but differing electronic properties. We calculate the Youngs modulus, bending rigidity, ultimate strength, and the fracture strain of monolayers, homogeneous and heterogeneous bilayers, and alternating trilayers. Their mechanics are broadly similar, although graphene provides mild reinforcement to heterostructures. Further, we characterize the puncture created by nanoindentation, where we find that graphene allows smaller pores with a rougher fracture surface and more cleaved bonds than h-BN, which we attribute to differences in toughness. Our results demonstrate that these layered heterostructures maintain their mechanical robustness regardless of stacking order and provide insight into the influence of layer ordering in separation or passivation applications.In-plane or vertically stacked heterostructures containing multiple 2D materials are promising for emerging applications, such as flexible electronics, piezoelectric sensors, and molecular separations. However, utilizing heterostructures requires a fundamental understanding of their mechanics, which is currently lacking. Here, we use reactive molecular dynamics to simulate nanoindentation of stacked hexagonal boron nitride (h-BN) and graphene structures, 2D materials with similar structures but differing electronic properties. We calculate the Youngs modulus, bending rigidity, ultimate strength, and the fracture strain of monolayers, homogeneous and heterogeneous bilayers, and alternating trilayers. Their mechanics are broadly similar, although graphene provides mild reinforcement to heterostructures. Further, we characterize the puncture created by nanoindentation, where we find that graphene allows smaller pores with a rougher fracture surface and more cleaved bonds than h-BN, which we attribute to dif...

Ballistic Response of Polydicyclopentadiene vs. Epoxy Resins and Effects of Crosslinking

The ballistic performance of polydicyclopentadiene (pDCPD) was investigated and compared to two epoxy resins that a have similar glass transition temperature (Tg) to pDCPD. The ballistic performance of these materials (at an effective stain rate of 104–105 s−1) was characterized by determining the kinetic energy of the projectile where there is a 50 % probability that the projectile will penetrate a witness foil behind the sample (KE50). The ballistic performance of pDCPD showed a 300–400 % improvement over the structural epoxy resins. Typical, highly crosslinked epoxy networks become brittle at low temperatures, but pDCPD has a superior ballistic performance over a broad temperature range from (−55 to 75 °C), despite having a glass transition temperature of 142 °C, which characteristic of structural resins. pDCPD also exhibited a room temperature glassy storage modulus of 1.7 GPa, making pDCPD a potential structural resin that can overcome the structural vs. energy dissipation trade-off that commonly exists with some conventional crosslinked polymers. Quasi-static measurements of pDCPD when compared to epoxy resins suggested that the performance of pDCPD relates to higher fracture toughness and lower yield stress relative to typical epoxies, while molecular dynamics simulations comparing pDCPD to epoxy resins suggest that the performance of pDCPD is due to the lack of strong non-covalent interactions and the facile formation of nanoscale voids.

Mechanical properties of homogeneous and heterogeneous layered 2D materials

Transition metal dichalcogenides (TMDC) such as molybdenum disulfide (MoS2) are 2D materials that are promising for flexible electronics and piezoelectric applications, but their low mechanical strength limits practical use. In this work, we study the mechanical properties of heterostructures containing MoS2 and graphene, another 2D material with exceptional mechanical properties, using atomistic molecular dynamics simulations of nanoindentation. We consider bi- and tri-layer heterostructures where graphene either supports or encapsulates MoS2, and we compare to the monolayers and homogeneous bilayers. We extract mechanical properties (Youngs modulus) from nanoindentation simulations. All of the heterostructures have larger Youngs moduli than the mono- and bi-layer MoS2, demonstrating that graphene provides mechanical reinforcement regardless of layer stacking order. Our results demonstrate the potential of heterostructures to improve the mechanical properties of TMDC materials, which would increase their utility for device applications.