Welcome

Welcome to the research website of Chris Howard, Associate Professor in the Department of Physics & Astronomy at University College London.

We create new materials that have desirable functional properties or that exhibit interesting emergent phenomena and we study these materials with a wide range of experimental techniques. Outputs range from nano-textured electrodes for battery and fuel cells to the discovery of exotic electronic groundstates. A particular expertise of the group is the ability to controllably dope (add charge carriers to) materials, especially nanomaterials and layered materials, via the addition of guest ions. The doping can drastically modify the material's properties (e.g turning a semiconductor into a metal) and we have also shown can enable the thermodynamic dissolution of the materials in liquids, for example, to form true solutions of 2-dimensional materials.

Recent Highlights

Production of phosphorene nanoribbons

Watts et al. Nature (2019), 568, 216

Over the previous five years there have been over 100 publications predicting that nanoribbons made from the 2-dimensional material phosphorene would have extraordinary properties. These properties could offer transformative advantages in applications ranging from fast-charging batteries to flexible thermoelectric devices to nanoelectronics. There have also been predictions of spin desnity waves, tunable magnetism, half-metallicity and topological states. However, until this paper no one had made discrete phosphorene nanoribbons (PNRs), and there was no obvious modification of methods for producing graphene nanoribbons to phosphorene. We describe a novel method for creating quantities of high-quality, individual PNRs by ionic scissoring of macroscopic black phosphorus crystals, and characterise the ribbons in detail. The ribbons have typical widths of 4-50 nm, predominantly single-layer thickness, measured lengths of up to 75 μm and aspect ratios of up to 1,000. The nanoribbons are atomically flat single crystals, aligned exclusively in the zigzag crystallographic orientation with remarkably uniform widths along their entire lengths, and are extremely flexible. Gallery of ribbons.

Magnetic control of graphitic microparticles in aqueous solutions

Nguyen et al. Proc. Natl. Acad. Sci. (USA) (2019), 116(7), 2425

This paper, led by Dr Llorente Garcia also at UCL, presents the first ever magnetic transport of diamagnetic graphite microparticles in water solutions. Given the dominance of viscous drag forces at the microscale, moving a microparticle that is submerged in liquid is comparably as hard as moving a macroparticle within dense honey. While diamagnetism is a weak magnetic property, for graphite it can be exploited to manipulate graphite flakes in a liquid using magnets (see figure for set up). The contactless magnetic control of biocompatible micrographite, together with graphite's unique physical properties, opens up new possibilities for applications in sensing, analysis, synthesis, and diagnosis in chemistry, biology, medicine, and physics.

Charged Carbon Nanomaterials: Redox Chemistries of Fullerenes, Carbon Nanotubes, and Graphenes

Clancy et al. Chemical Reviews (2018) 118(16), 73638

Since the discovery of buckminsterfullerene, sp2-hybridised carbon nanomaterials (including fullerenes, carbon nanotubes, and graphene) have stimulated new science and technology across a huge range of fields. Charged carbon nanomaterials, formed via the addition of charge carriers (electrons or holes i.e. reduction or oxidation) to carbon nanomaterials, facilitate dissolution, purification, separation, chemical modification, and assembly of these functional materials. This review focuses on the fundamental structural forms: buckminsterfullerene, single-walled carbon nanotubes, and single-layer graphene, describing the generation of their respective charged nanocarbon species, their interactions with solvents, chemical reactivity, specific (opto)electronic properties, and emerging applications.

Single Crystal, Luminescent Carbon Nitride Nanosheets Formed by Spontaneous Dissolution

Miller et al. Nano Letters (2017), 17, 5891.

Bulk poly(triazine imide) (PTI)-based carbon nitrides are layered materials with a high degree of crystalline order. Here, we demonstrate that these semiconductors are spontaneously soluble in select polar aprotic solvents, that is, without any chemical or physical intervention. This thermodynamically driven dissolution process perfectly maintains the crystallographic form of the starting material (see figure for high resolution TEM of the entire PTI 2D nanocrystal), yielding solutions of defect-free, hexagonal 2D nanosheets. What's more, the PTI nanosheets are luminescent at wavelengths dependent on their degree of exfoliation.

Ionic Solutions of 2-dimensional Materials

Cullen et al. Nature Chemistry (2017), 9, 244

Strategies for forming liquid dispersions of nanomaterials typically focus on retarding reaggregation, for example via surface modification, as opposed to promoting the thermodynamically driven dissolution common for molecule-sized species. Here we demonstrate the true dissolution of a wide range of important 2D nanomaterials by forming layered material salts that spontaneously dissolve in polar solvents yielding ionic solutions of charged cations and negatively charged nanosheet anions. The benign dissolution advantageously maintains the morphology of the starting material, is stable against reaggregation and can achieve solutions containing exclusively individualized monolayers. We go on to show that the charge on the monolayers is reversible and can be harnessed to electroplate them with mircon precision over large areas. Our findings thus reveal a unique solution-like behaviour for 2D materials that enables their scalable production and controlled manipulation.

Electron Solvation and the Unique Liquid Structure of a Mixed-Amine Expanded Metal

Seel et al. Angew. Chem. Int. Ed (2017), 56(6), 1561

Alkali metals can dissolve in amines solvents to form solutions of metal ions and electrons. Metal-amine solutions thus provide a unique arena to tune the electron density from the extremes of electrolytic through to true metallic behavior. The existence and structure of a new class of concentrated metal-amine liquid, Li-NH3-MeNH2, is presented in which the mixed solvent produces a novel type of electron solvation and delocalization that is fundamentally different from either of the constituent systems.

Superconductivity in Ca-doped graphene laminates

Chapman et al.Sci. Rep. (2016), 6, 23254

In this work (performed with collobarotors including Nair and Grigorieva at Manchester UK) we created graphene and graphene hexagonal boron nitride "laminates" (from exfoliation and reassembly of their bulk layered counterparts) then doped with alalki metals to search for superconductivity. We found superconductivity in all Ca-doped graphene laminates at temperatures (Tc) between 4 and 6 K, with Tcs strongly dependent on the confinement of the Ca layer and the induced charge carrier concentration controlled by interleaving with hexagonal boron nitride layers, which also has a drastic effect on the metalicity of the doped material spectularly revealed by their colour change (see figure). By revealing the tunability of the superconducting response through doping and confinement of the metal layer, this work shows that achieving superconductivity in free-standing, metal decorated monolayer graphene is achievable with an optimum confinement of the metal layer and sufficient doping.



Superconducting graphene sheets in CaC6 enabled by phonon-mediated interband interactions

Yang et al. Nature Comms. (2014), 5, 3493

If one day monolayer graphene can be tuned to exhibit superconductivity (fascinatingly this is possible in 'twisted' bilayer graphene upon electrostatic doping) a good starting point is to understand the superconducting mechanism in intercalated graphite superconductors. This work, performed in collaboration with the Shen group at Stanford reports angle-resolved photoelectron spectroscopy measurements of the superconducting graphite intercalation compound CaC6 that distinctly resolve both its intercalant-derived interlayer band and its graphene-derived π* band. Our results indicate the opening of a superconducting gap in the π* band and reveal a substantial contribution to the total electron-phonon-coupling strength from the π*-interlayer interband interaction. Combined with theoretical predictions, these results provide a complete account for the superconducting mechanism in graphite intercalation compounds and lend support to the idea of realising superconducting graphene by creating an adatom superlattice.