Carrier density tuning in CuS nanoparticles and thin films by Zn doping via ion exchange.

Copper sulphide (covellite) nanoplatelets have recently emerged as a plasmonic platform in the near-infrared with ultrafast nonlinear optical properties. Here we demonstrate that the free-carrier density in CuS, which is an order of magnitude lower than in traditional plasmonic metals, can be further tuned by chemical doping. Using ion exchange to replace Cu with an increasing content of Zn in the nanoparticles, the free-hole density can be lowered, resulting in a long-wavelength shift of the localised plasmon resonances from 1250 nm to 1750 nm. The proposed approach provides new opportunities for tuning the plasmonic response of covellite nanocrystals as well as the carrier relaxation time which decreases for lower free-carrier densities.

Pillared Mo2TiC2 MXene for high-power and long-life lithium and sodium-ion batteries.

In this work, we apply an amine-assisted silica pillaring method to create the first example of a porous Mo2TiC2 MXene with nanoengineered interlayer distances. The pillared Mo2TiC2 has a surface area of 202 m2 g-1, which is among the highest reported for any MXene, and has a variable gallery height between 0.7 and 3 nm. The expanded interlayer distance leads to significantly enhanced cycling performance for Li-ion storage, with superior capacity, rate capably and cycling stability in comparison to the non-pillared analogue. The pillared Mo2TiC2 achieved a capacity over 1.7 times greater than multilayered MXene at 20 mA g-1 (≈320 mA h g-1) and 2.5 times higher at 1 A g-1 (≈150 mA h g-1). The fast-charging properties of pillared Mo2TiC2 are further demonstrated by outstanding stability even at 1 A g-1 (under 8 min charge time), retaining 80% of the initial capacity after 500 cycles. Furthermore, we use a combination of spectroscopic techniques (i.e. XPS, NMR and Raman) to show unambiguously that the charge storage mechanism of this MXene occurs by a conversion reaction through the formation of Li2O. This reaction increases by 2-fold the capacity beyond intercalation, and therefore, its understanding is crucial for further development of this family of materials. In addition, we also investigate for the first time the sodium storage properties of the pillared and non-pillared Mo2TiC2.

Mapping nanoscale dynamic properties of suspended and supported multi-layer graphene membranes via contact resonance and ultrasonic scanning probe microscopies.

Graphene's (GR) remarkable mechanical and electrical properties-such as its Young's modulus, low mass per unit area, natural atomic flatness and electrical conductance-would make it an ideal material for micro and nanoelectromechanical systems (MEMS and NEMS). However, the difficulty of attaching GR to supports, coupled with naturally occurring internal defects in a few layer GR can significantly adversely affect the performance of such devices. Here, we have used a combined contact resonance atomic force microscopy (CR-AFM) and ultrasonic force microscopy (UFM) approach to characterise and map with nanoscale spatial resolution GR membrane properties inaccessible to most conventional scanning probe characterisation techniques. Using a multi-layer GR plate (membrane) suspended over a round hole, we show that this combined approach allows access to the mechanical properties, internal structure and attachment geometry of the membrane providing information about both the supported and suspended regions of the system. We show that UFM allows the precise geometrical position of the supported membrane-substrate contact to be located and provides an indication of the local variation of its quality in the contact areas. At the same time, we show that by mapping the position sensitive frequency and phase response of CR-AFM response, one can reliably quantify the membrane stiffness, and image the defects in the suspended area of the membrane. The phase and amplitude of experimental CR-AFM measurements show excellent agreement with an analytical model accounting for the resonance of the combined CR-AFM probe-membrane system. The combination of UFM and CR-AFM provide a beneficial combination for the investigation of few-layer NEMS systems based on two dimensional materials.

Porous Silica-Pillared MXenes with Controllable Interlayer Distances for Long-Life Na-Ion Batteries.

MXenes are a recently discovered class of two-dimensional materials that have shown great potential as electrodes in electrochemical energy storage devices. Despite their promise in this area, MXenes can still suffer limitations in the form of restricted ion accessibility between the closely spaced multistacked MXene layers causing low capacities and poor cycle life. Pillaring, where a secondary species is inserted between layers, has been used to increase interlayer spacings in clays with great success but has had limited application in MXenes. We report a new amine-assisted pillaring methodology that successfully intercalates silica-based pillars between Ti3C2 layers. Using this technique, the interlayer spacing can be controlled with the choice of amine and calcination temperature, up to a maximum of 3.2 nm, the largest interlayer spacing reported for an MXene. Another effect of the pillaring is a dramatic increase in surface area, achieving BET surface areas of 235 m2 g-1, a sixty-fold increase over the unpillared material and the highest reported for MXenes using an intercalation-based method. The intercalation mechanism was revealed by different characterization techniques, allowing the surface chemistry to be optimized for the pillaring process. The porous MXene was tested for Na-ion battery applications and showed superior capacity, rate capability and remarkable stability compared with those of the nonpillared materials, retaining 98.5% capacity between the 50th and 100th cycles. These results demonstrate the applicability and promise of pillaring techniques applied to MXenes providing a new approach to optimizing their properties for a range of applications, including energy storage, conversion, catalysis, and gas separations.

Characterising porosity in platinum nanoparticles.

Accurately determining the morphology and hence the true surface areas of catalytic nanoparticles remains challenging. For many chemically synthesised nanoparticle suspensions conventional BET surface area measurements are often not feasible due to the large quantities of material required. For platinum, a paradigmatic catalyst, this issue is further complicated by the propensity of this metal to form porous aggregate structures comprised of smaller (ca. 2-5 nm) crystallites as opposed to continuous solid structures. This dendritic/porous particulate morphology leads to a large but poorly defined 'active' surface which is difficult to measure accurately. Here we compare, single nanoparticle electrochemistry with three dimensional (3D) electron tomography and quantitative 2D high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) analysis to yield insights into the porosity and chemically accessible surface area of a 30 nm diameter commercial Pt nanoparticle catalyst. Good quantitative agreement is found between 2D and 3D STEM-based measurements of the particle morphology, density and size distribution. Both 3D STEM tomography and single nanoparticle electrochemical measurements allow quantification of the surface area but the electrocatalytic surface area is found to be 2.8× larger than is measured in STEM; indicating the importance of the atomic scale roughness and structure (<2 nm) in contributing to the total catalytic surface area of the nanomaterial.