Elastic properties of a van der Waals solid: C60

Mechanical Properties of Cubene Crystals

The fullerene family, whose most popular members are the spherical C60 and C70 molecules, has recently added a new member, the cube-shaped carbon molecule C8 called a cubene. A molecular crystal based on fullerenes is called fullerite. In this work, based on relaxational molecular dynamics, two fullerites based on cubenes are described for the first time, one of which belongs to the cubic system, and the other to the triclinic system. Potential energy per atom, elastic constants, and mechanical stress components are calculated as functions of lattice strain. It has been established that the cubic cubene crystal is metastable, while the triclinic crystal is presumably the crystalline phase in the ground state (the potential energies per atom for these two structures are −0.0452 and −0.0480 eV, respectively).The cubic phase has a lower density than the monoclinic one (volumes per cubene are 101 and 97.7 Å3). The elastic constants for the monoclinic phase are approximately 4% higher than those for the cubic phase. The presented results are the first step in studying the physical and mechanical properties of C8 fullerite, which may have potential for hydrogen storage and other applications. In the future, the influence of temperature on the properties of cubenes will be analyzed.

Time-Domain Investigations of Coherent Phonons in van der Waals Thin Films

Coherent phonons can be launched in materials upon localized pulsed optical excitation, and be subsequently followed in time-domain, with a sub-picosecond resolution, using a time-delayed pulsed probe. This technique yields characterization of mechanical, optical, and electronic properties at the nanoscale, and is taken advantage of for investigations in material science, physics, chemistry, and biology. Here we review the use of this experimental method applied to the emerging field of homo- and heterostructures of van der Waals materials. Their unique structure corresponding to non-covalently stacked atomically thin layers allows for the study of original structural configurations, down to one-atom-thin films free of interface defect. The generation and relaxation of coherent optical phonons, as well as propagative and resonant breathing acoustic phonons, are comprehensively discussed. This approach opens new avenues for the in situ characterization of these novel materials, the observation and modulation of exotic phenomena, and advances in the field of acoustics microscopy.

Frustrated Layered Self-Assembly Induced Superlattice from Two-Dimensional Nanosheets

Here we reported a hierarchical self-assembly approach toward well-defined superlattices in supramolecular liquid crystals by fullerene-based sphere-cone block molecules. The fullerenes crystallize to form monolayer nanosheets intercalated by the attached soft hydrocarbon cones. The frustration caused by cross-sectional area mismatch between the spheres and the somewhat oversize cones leads to a unique lamellar superlattice whereby each stack of six pairs of alternating sphere-cone sublayers is followed by a cone double layer. While such areal mismatch problems in soft matter are usually solved by interface curvature, the lamellar superlattice solution is best suited to systems with rigid layers. Meanwhile, formation of the superlattice significantly improves the material's transient electron conductivity, with the maximum value being among the highest for π-conjugated organic materials. The design principle of solving steric frustration by forming a superlattice opens a new avenue toward self-assembled optoelectronic materials.

Increased elasticity and damping capacity of diamond-like carbon coatings by immobilized C60 fullerene clusters

Material loss and plastic deformation induced by frictional interactions at moving mechanical interfaces continue to be major issues responsible for efficiency and performance degradation of systems. Establishment of fully elastic interactions in the contact region without compromising the structural rigidity and integrity of materials represents a promising solution. In this study, we report on improving the elasticity, damping properties, ductility and wear resistance of diamond-like carbon (DLC) coatings through introducing an immobilized C60 cluster layer. The C60 clusters were immobilized using cysteamine (HS(CH2)2NH2) self-assembled monolayers (SAMs) attached to a pre-sputtered Au layer. A Ni adhesive layer was deposited onto plasma cleaned Si (100) substrates prior to Au, SAM-C60, and DLC deposition. Precise dynamic ultra nano-indentation tests indicated a drastic improvement in elasticity and damping capacity of the C60-DLC hybrid (Ni-Au-SAM/C60-DLC) multilayer coating compared to those of the C60-free (Ni-Au-DLC) multilayer. The behavior of the coatings under reciprocating contact conditions was evaluated. Quantification of the resistance of the coatings against wear and permanent deformation revealed a significant improvement in the wear rate from ∼3.38 × 10-8 to ∼5.14 × 10-10 mm3 N-1 mm-1 upon incorporation of the immobilized C60 clusters. The corresponding mechanisms were assessed through experiments and finite element (FE) simulations.

Molecular-Scale Understanding of Cohesion and Fracture in P3HT:Fullerene Blends

Quantifying cohesion and understanding fracture phenomena in thin-film electronic devices are necessary for improved materials design and processing criteria. For organic photovoltaics (OPVs), the cohesion of the photoactive layer portends its mechanical flexibility, reliability, and lifetime. Here, the molecular mechanism for the initiation of cohesive failure in bulk heterojunction (BHJ) OPV active layers derived from the semiconducting polymer poly(3-hexylthiophene) [P3HT] and two monosubstituted fullerenes is examined experimentally and through molecular-dynamics simulations. The results detail how, under identical conditions, cohesion significantly changes due to minor variations in the fullerene adduct functionality, an important materials consideration that needs to be taken into account across fields where soluble fullerene derivatives are used.

Solvation-Assisted Young’s Modulus Control of Single-Crystal Fullerene Nanowhiskers

Single-crystal nanowhiskers (NWs) composed of fullerene C70molecules were synthesized by the liquid-liquid interfacial precipitation method that usedm-xylene as a saturated solution of C70molecules. Bending behavior of the individual NWs was observed byin situtransmission electron microscopy equipped with nanonewton force measurements using an optical deflection method. The Young’s modulus of the NWs was estimated to be 0.3–1.9 GPa, which was 2–7% of the moduli of fullerene NWs with similar diameters synthesized using other solvents, that is, toluene and pyridine. The influence of the solvent used in the precipitation method on Young’s modulus is discussed.

He/Ar-atom scattering from molecular monolayers: C60/Pt(111) and graphene/Pt(111)

Supersonic He and Ar atomic beam scattering from C(60) and graphene monolayers adsorbed on a Pt(111) surface are demonstrated in order to obtain detailed insight into a gas-molecule collision that has not been studied in detail so far. The effective masses and phonon spectral densities of the monolayers seen by different projectiles are discussed based on classical models such as the hard cube model and the recently developed smooth surface model. Large effective masses are deduced for both the monolayers, suggesting collective effects of surface atoms in the single collision event. The effective Debye temperature of graphene was found to be similar to that reported in highly oriented pyrolytic graphite (HOPG), indicating that the graphene is decoupled well from the Pt substrate. A much smaller Debye-Waller factor was found for the C(60) layer, probably reflecting the strong C(60)-Pt(111) interaction.

Specific heat and thermal conductivity of solid fullerenes

Evidence is presented that the lattice vibrations of compacted C(60)/C(70) fullerite microcrystals consist predominantly of localized modes. Vibrational motions of the rigid molecules ("buckyballs") have been identified as well as their internal vibrations. Debye waves play only a relatively minor role, except below approximately 4 kelvin. By comparison with other crystalline materials, for these materials the Einstein model of the specific heat and thermal conductivity of solids, which is based on the assumption of atoms (in this case, buckyballs) vibrating with random phases, is in much better agreement with the measurements than the Debye model, which is based on collective excitations.