No abstract

High-resolution neutron diffraction has been used in conjunction with hydrogen/deuterium isotopic labeling to determine with unprecedented detail the structure of two archetypal aromatic liquids: benzene and toluene. We discover the nature of aromatic pi-pi interactions in the liquid state by constructing for the first time a full six-dimensional spatial and orientational picture of these systems. We find that in each case the nearest neighbor coordination shell contains approximately 12 molecules. Benzene is the more structured of the two liquids, showing, for example, a sharper nearest neighbor coordination peak in the radial distribution function. Superficially the first neighbor shells appear isotropic, but our multidimensional analysis shows that the local orientational order in these liquids is much more complex. At small molecular separations (<5 A) there is a preference for parallel pi-pi contacts in which the molecules are offset to mimic the interlayer structure of graphite. At larger separations (>5 A) the neighboring aromatic rings are predominantly perpendicular, with two H atoms per molecule directed toward the acceptor's pi orbitals. The so-called "anti-hydrogen-bond" configuration, proposed as the global minimum for the benzene dimer, occurs only as a saddle point in our data. The observed liquid structures are therefore fundamentally different than those deduced from the molecular dimer energy surfaces.

Phosphorene is a mono-elemental two-dimensional (2D) material with outstanding, highly directional properties and a thickness-tuneable band gap 1-8. Nanoribbons combine the flexibility and unidirectional properties of 1D nanomaterials, the high surface area of 2D nanomaterials and the electron-confinement and edge effects of both. Their structures can thus offer exceptional control over electronic bandstructure, lead to the emergence of novel phenomena and present unique architectures for applications 5,6,9-24. Motivated by phosphorene's intrinsically anisotropic structure, theoretical predictions of the extraordinary properties of phosphorene nanoribbons (PNRs) have been rapidly emerging in recent years 5,6,12-24. However to date, discrete PNRs have not been produced. Here we present a method for creating quantities of high quality, individual PNRs via ionic scissoring of macroscopic black phosphorus crystals. The top-down process results in stable liquid dispersions of PNRs with typical widths of 4 to 50 nm, predominantly single layer thickness, measured lengths up to 75 μm and aspect ratios of up to ~1000. The nanoribbons are atomically-flat single crystals, aligned exclusively in the zigzag crystallographic orientation. The ribbon widths are remarkably uniform along their entire length and they display extreme flexibility. These properties, in conjunction with the ease of downstream manipulation via liquidphase methods, now enable the search for predicted exotic states 6,12-14,17-19,21 and an array of applications where PNRs have been widely predicted to offer transformative advantages, ranging from thermoelectric devices to high-capacity fast-charging batteries and integrated high-speed electronic circuits 6,14-16,20,23,24. Phosphorene's anisotropic properties, including for electron, thermal and ionic transport, derive from its atomic structure where the atoms are arranged in corrugated sheets with two different P-P bond lengths (Fig. 1a) 1-8. Calculations predict that PNRs can possess enhanced characteristics compared with phosphorene and that their electronic structure, carrier mobilities and optical and mechanical properties can be tuned by varying the ribbon width, thickness, edge passivation, and by introducing strain or functionalization 6,12-14,20,22-24. Additionally, there have been numerous predictions of exotic effects in PNRs, including the spin-dependent Seebeck effect 17 , room temperature magnetism 6,21 , topological phase transitions 18 , large exciton splitting 14 and spin density waves 19. These results have led to suggestions of unique capabilities of PNRs in a number of applications such as thermoelectric devices 6,23 , photocatalytic water splitting 15 , solar cells 14 , batteries 6,24 , electronics 6,20,22 and quantum information technologies 14 .