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 .

No abstract

As synthesized, bulk singlewalled carbon nanotube (SWNT) samples are typically highly agglomerated and heterogeneous. However, their most promising applications require the isolation of individualized, purified nanotubes, often with specific optoelectronic characteristics. A wide range of dispersion and separation techniques have been developed, but the use of sonication or ultracentrifugation imposes severe limits on scalability and may introduce damage. Here, we demonstrate a new, intrinsically scalable method for SWNT dispersion and separation, using reductive treatment in sodium metal-ammonia solutions, optionally followed by selective dissolution in a polar aprotic organic solvent. In situ small-angle neutron scattering demonstrates the presence of dissolved, unbundled SWNTs in solution, at concentrations reaching at least 2 mg/mL; the ability to isolate individual nanotubes is confirmed by atomic force microscopy. Spectroscopy data suggest that the soluble fraction contains predominately large metallic nanotubes; a potential new mechanism for nanotube separation is proposed. In addition, the G/D ratios observed during the dissolution sequence, as a function of metal: carbon ratio, demonstrate a new purification method for removing carbonaceous impurities from pristine SWNTs, which avoids traditional, damaging, competitive oxidation reactions.

Graphene phonons are measured as a function of electron doping via the addition of potassium adatoms. In the low doping regime, the in-plane carbon G-peak hardens and narrows with increasing doping, analogous to the trend seen in graphene doped via the field-effect. At high dopings, beyond those accessible by the field-effect, the G-peak strongly softens and broadens. This is interpreted as a dynamic, non-adiabatic renormalization of the phonon self-energy. At dopings between the light and heavily doped regimes, we find a robust inhomogeneous phase where the potassium coverage is segregated into regions of high and low density. The phonon energies, linewidths and tunability are remarkably similar for 1-4 layer graphene, but significantly different to doped bulk graphite.PACS numbers: 63.22. Rc,78.67.Wj,81.05.ue Due to the intense scientific interest in graphene over the past few years, many of its basic properties have been determined. Now much of the effort in graphene research is devoted to tuning its properties in order to search for exotic physics and to extend and improve its potential for applications [1,2]. The properties of graphene can be tuned both by varying the number of layers in the graphene stack and via doping [2][3][4][5][6][7][8][9][10]. The current method of choice for doping graphene is via the electric field effect [3,4]. In this way the Fermi level can be controllably tuned to a maximum of E D =-0.3 eV away from the Dirac point (about 0.002 e − /C atom) giving carrier densities of ∼10 13 cm −2 . Similar levels of doping have also As the electronic structure is modified, so too is the electron-phonon interaction (EPI) [3,4,9,14,15]. A detailed understanding of this interaction is of great importance as it not only governs electronic transport, and hence the performance of graphene based electronic devices, but can also mediate exotic ground states such as superconductivity and charge density waves. At light doping levels a small (0.3%) hardening in the in-plane carbon phonon energies and narrowing in their linewidth have been reported [3][4][5][6][7][8][9][10]. This is due to a reduction in the electron-phonon scattering as the Kohn anomaly found in pure graphene at Γ is gradually removed to finite q [3, 4]. Here we extend the investigation of graphene phonons to higher dopings where we discover both a strong (3%) softening and significant linewidth broadening of the in-plane carbon phonons. We argue these effects are due to a novel, dynamic EPI arising from the 2D metallic nature of heavily doped graphene. In addition, we find that the tunability, phonons and EPI are remarkably similar for 1-4 layer doped graphene, but these systems exhibit significantly different behavior to doped bulk graphite.Graphene was prepared by micromechanical exfoliation of natural graphite onto an oxidized Si substrate (275 nm SiO 2 ) [16]. The substrate was then loaded into a sealed borosilicate tube with an optical window, evacuated and outgassed at 250 ○ C for 24 hours. An ingot of potassium metal was then ...

No abstract

No abstract