The effect of intrinsic crumpling on the mechanics of free-standing graphene.

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.

Interlaced crystals having a perfect Bravais lattice and complex chemical order revealed by real-space crystallography.

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 CuInS2. 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.

Enhanced photoresponse in curled graphene ribbons.

Graphene has become one of the most promising materials for future optoelectronics due to its ultrahigh charge-carrier mobility, high light transmission, and universal absorbance in the near-infrared and visible spectral ranges. However, a zero band gap and ultrafast recombination of the photoexcited electron-hole pairs limit graphene's potential in photovoltaic generation. Recent studies have shown that hot carriers can enhance photovoltaic generation in graphene p-n junctions through the photothermoelectric effect (PTE). It is, therefore, desirable to synthesize graphene nanostructures with an intrinsic PTE-induced photocurrent response. Here we report a simple method to synthesize quasi-one dimensional (quasi-1D) curled graphene ribbons (CGRs) that generate a photocurrent response with two orders of magnitude enhancement. Scanning photocurrent and photoluminescence measurements reveal that the photocurrent response is primarily attributed to the PTE and that the infrared emission may arise from thermal radiation. These results offer a new way to fabricate graphene-based optoelectronics with an enhanced photoresponse.

Probing charge scattering mechanisms in suspended graphene by varying its dielectric environment.

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 room-temperature 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.