An unforeseen polymorph of coronene by the application of magnetic fields during crystal growth.

The continued development of novel drugs, proteins, and advanced materials strongly rely on our ability to self-assemble molecules in solids with the most suitable structure (polymorph) in order to exhibit desired functionalities. The search for new polymorphs remains a scientific challenge, that is at the core of crystal engineering and there has been a lack of effective solutions to this problem. Here we show that by crystallizing the polyaromatic hydrocarbon coronene in the presence of a magnetic field, a polymorph is formed in a ß-herringbone structure instead of the ubiquitous γ-herringbone structure, with a decrease of 35° in the herringbone nearest neighbour angle. The ß-herringbone polymorph is stable, preserves its structure under ambient conditions and as a result of the altered molecular packing of the crystals, exhibits significant changes to the optical and mechanical properties of the crystal.

Raman spectra of monolayer, few-layer, and bulk ReSe2: an anisotropic layered semiconductor.

Rhenium diselenide (ReSe2) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1̅. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe2 show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phonon modes of bulk and monolayer ReSe2 as well as the Raman tensors determining the scattering intensity of each mode. It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe2.

Encapsulated inorganic nanostructures: a route to sizable modulated, noncovalent, on-tube potentials in carbon nanotubes.

The large variety of hybrid carbon nanotube systems synthesized to date (e.g., by encapsulation, wrapping, or stacking) has provided a body of interactions with which to modify the host nanotubes to produce new functionalities and control their behavior. Each, however, has limitations: hybridization can strongly degrade desirable nanotube properties; noncovalent interactions with molecular systems are generally weak; and interlayer interactions in layered nanotubes are strongly dependent upon the precise stacking sequence. Here we show that the electrostatic/polarization interaction provides a generic route to designing unprecedented, sizable and highly modulated (1 eV range), noncovalent on-tube potentials via encapsulation of inorganic partially ionic phases where charge anisotropy is maximized. Focusing on silver iodide (AgI) nanowires inside single-walled carbon nanotubes, we exploit the polymorphism of AgI, which creates a variety of different charge distributions and, consequently, interactions of varying strength and symmetry. Combined ab initio calculations, high-resolution transmission electron microscopy, and scanning tunneling microscopy and spectroscopy are used to demonstrate symmetry breaking of the nanotube wave functions and novel electronic superstructure formation, which we then correlate with the modulated, noncovalent electrostatic/polarization potentials from the AgI filling. These on-tube potentials are markedly stronger than those due to other noncovalent interactions known in carbon nanotube systems and lead to significant redistribution of the wave function around the nanotube, with implications for conceptually new single-nanotube electronic devices and molecular assembly. Principles derived can translate more broadly to relating graphene systems, for designing/controlling potentials and superstructures.

Selected immobilization of individual nanoparticles by spot-exposure electron-beam-induced deposition.

The use of spot-exposure electron-beam-induced deposition (EBID) to immobilize targeted nanoparticles on a substrate is demonstrated, and investigated using experiment and simulation. Nanoparticles are secured in place through the build-up of carbonaceous material that forms in the region between a particle and substrate when an energetic electron beam is focused onto the particle and projected through to the substrate. Material build-up directly affects the strength of adhesion to the surface, and can be controlled through electron dosage and beam energy. By selectively immobilizing specific particles within surface agglomerations and removing the excess, we illustrate the potential for spot-exposure EBID as a new technique for nanofabrication.

Strategies for the immobilization of nanoparticles using electron beam induced deposition.

The mechanism of electron-beam-induced immobilization of nanoparticles on a substrate has been studied both experimentally and theoretically. Experiments have been performed for the case of 200-350 nm Co-Ni nanoparticles secured to a substrate using a 30 keV electron beam. Atomic force microscopy studies reveal that the fixing occurs due to the formation of a deposit beneath the nanoparticles, causing strong bonding to the substrate, even for a thin layer. Measurements of the lateral forces required to displace the immobilized nanoparticles have shown that a deposit layer of 0.5 nm results in a tenfold increase in the bonding strength. A comparison of measured profiles with the results of computer simulations clearly reveals that the major role in the formation of the deposit is played by low-energy electrons generated by energetic primary electrons in both the nanoparticles and substrate. It is also shown that the efficiency of bonding decreases with decreasing energy of primary electrons. Different strategies for electron-beam-induced immobilization of nanoparticles and optimization of the processes are discussed.