Free-standing graphene is inherently crumpled in the out-of-plane direction due to dynamic flexural phonons and static wrinkling. We explore the consequences of this crumpling on the effective mechanical constants of graphene. We develop a sensitive experimental approach to probe stretching of graphene membranes under low applied stress at cryogenic to room temperatures. We find that the in-plane stiffness of graphene is 20–100 N m−1 at room temperature, much smaller than 340 N m−1 (the value expected for flat graphene). Moreover, while the in-plane stiffness only increases moderately when the devices are cooled down to 10 K, it approaches 300 N m−1 when the aspect ratio of graphene membranes is increased. These results indicate that softening of graphene at temperatures <400 K is caused by static wrinkling, with only a small contribution due to flexural phonons. Together, these results explain the large variation in reported mechanical constants of graphene devices and pave the way towards controlling their mechanical properties.

Graphene with high carrier mobility µ is required both for graphene-based electronic devices and for the investigation of the fundamental properties of Dirac fermions. An attractive approach to increase the mobility is to place graphene in an environment with high static dielectric constant κ that would screen the electric field due to the charged impurities present near graphene's surface. Here we investigate the effect of the dielectric environment of graphene and study electrical transport in multi-terminal graphene devices suspended in liquids with κ ranging from 1.9 to 33. For non-polar liquids (κ < 5), we observe a rapid increase of µ(κ), with roomtemperature mobility reaching ~60,000 cm 2 Vs − 1 for devices in anisole (κ = 4.3). We associate this trend with dielectric screening of charged impurities adsorbed on graphene. We observe much lower mobility µ~20,000 cm 2 Vs − 1 for devices in polar liquids (κ ≥ 18) and explain it by additional scattering caused by ions present in such liquids.

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The search for optimal thermoelectric materials aims for structures in which the crystalline order is disrupted to lower the thermal conductivity without degradation of the electron conductivity. Here we report the synthesis and characterisation of ternary nanoparticles (two cations and one anion) that exhibit a new form of crystalline order: an uninterrupted, perfect, global Bravais lattice, in which the two cations exhibit a wide array of distinct ordering patterns within the cation sublattice, forming interlaced domains and phases. Partitioning into domains and phases is not unique; the corresponding boundaries have no structural defects or strain and entail no energy cost. We call this form of crystalline order 'interlaced crystals' and present the example of hexagonal CuInS 2 . Interlacing is possible in multi-cation tetrahedrally bonded compound with an average of two electrons per bond. Interlacing has minimal effect on electronic properties, but should strongly reduce phonon transport, making interlaced crystals attractive for thermoelectric applications.

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