Engineering 2D Material Exciton Line Shape with Graphene/h-BN Encapsulation.

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS2, MoSe2, and WSe2 van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe2/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Tuning the 1T'/2H phases in WxMo1-xSe2 nanosheets.

Controlling materials' morphology, crystal phase and chemical composition at the atomic scale has become central in materials research. Wet chemistry approaches have great potential in directing the material crystallisation process to achieve tuneable chemical compositions as well as to target specific crystal phases. Herein, we report the compositional and crystal phase tuneability achieved in the quasi-binary WxMo1-xSe2 system with chemical and crystal phase mixing down to the atomic level. A series of WxMo1-xSe2 solid solutions in the form of nanoflowers with atomically thin petals were obtained via a direct colloidal reaction by systematically varying the ratios of transition metal precursors. We investigate the effect of selenium precursor on the morphology of the WxMo1-xSe2 material and show how using elemental selenium can enable the formation of larger and distinct nanoflowers. While the synthesised materials are compositionally homogeneous, they exhibit crystal phase heterogeneity with the co-existing domains of the 1T' and 2H crystal phases, and with evidence of MoSe2 in the metastable 1T' phase. We show at single atom level of resolution, that tungsten and molybdenum can be found in both the 1T' and 2H lattices. The formation of heterophase 1T'/2H WxMo1-xSe2 electrocatalysts allowed for a considerable improvement in the activity for the acidic hydrogen evolution reaction (HER) compared to pristine, 1T'-dominated, WSe2. This work can pave the way towards engineered functional nanomaterials where properties, such as electronic and catalytic, have to be controlled at the atomic scale.

Synthesis of mono- and few-layered n-type WSe2 from solid state inorganic precursors.

Tuning the charge transport properties of two-dimensional transition metal dichalcogenides (TMDs) is pivotal to their future device integration in post-silicon technologies. To date, co-doping of TMDs during growth still proves to be challenging, and the synthesis of doped WSe2, an otherwise ambipolar material, has been mainly limited to p-doping. Here, we demonstrate the synthesis of high-quality n-type monolayered WSe2 flakes using a solid-state precursor for Se, zinc selenide. n-Type transport has been reported with prime electron mobilities of up to 10 cm2 V-1 s-1. We also demonstrate the tuneability of doping to p-type transport with hole mobilities of 50 cm2 V-1 s-1 after annealing in air. n-Doping has been attributed to the presence of Zn adatoms on the WSe2 flakes as revealed by X-ray photoelectron spectroscopy (XPS), spatially resolved time of flight secondary ion mass spectroscopy (SIMS) and angular dark-field scanning transmission electron microscopy (AD-STEM) characterization of WSe2 flakes. Monolayer WSe2 flakes exhibit a sharp photoluminescence (PL) peak at room temperature and highly uniform emission across the entire flake area, indicating a high degree of crystallinity of the material. This work provides new insight into the synthesis of TMDs with charge carrier control, to pave the way towards post-silicon electronics.

Durable Zn-ion hybrid capacitors using 3D printed carbon composites.

Rechargeable Zn-ion hybrid capacitors (ZHCs) have gained considerable attention towards future energy storage applications owing to their non-flammable nature, high abundance of raw materials and remarkable energy storage performance. However, the uncontrolled growth of dendrites, interfacial corrosion of Zn anodes and limited mass loading of cathode materials, hinders their practical applicability. Herein, we demonstrate ZHCs with enhanced capacity and durability using a synergistic combination of a hybrid-ion electrolyte and a high-mass loading three-dimensionally (3D) printed graphene-carbon nanotube (Gr-C) cathode. The hybrid electrolyte composed of NaCl and ZnSO4, features higher ionic conductivity and lower pH compared with pristine ZnSO4, which enable uniform plating/stripping of Zn2+ ions on Zn anode, as demonstrated by in situ electrochemical and ex situ ToF-SIMs characterizations. Additionally, the multi-layered 3D Gr-C composite electrodes in ZHCs enable higher energy storage performance due to their porous architectures, high ion accessibility and dual-ion charge storage contributions. As a result, the 3D Gr-C//Zn cell unveiled a maximum capacity of 0.84 mA h cm-2 at 3 mA cm-2 with a high life cycle (78.7% at 20 mA cm-2) compared to the pristine electrolyte-based ZHCs (0.72 mA h cm-2 and 14.8%). The rapid rate measurements that we propose along with benchmarked energy density (0.87 mW h cm-2) and power density (31.7 mW cm-2) of hybrid electrolyte-based 3D Gr-C//Zn, pave the way for the development of dendrite-free and highly durable 3D energy storage devices.

Aqueous Inks of Pristine Graphene for 3D Printed Microsupercapacitors with High Capacitance.

Three-dimensional (3D) printing is gaining importance as a sustainable route for the fabrication of high-performance energy storage devices. It enables the streamlined manufacture of devices with programmable geometry at different length scales down to micron-sized dimensions. Miniaturized energy storage devices are fundamental components for on-chip technologies to enable energy autonomy. In this work, we demonstrate 3D printed microsupercapacitor electrodes from aqueous inks of pristine graphene without the need of high temperature processing and functional additives. With an intrinsic electrical conductivity of ∼1370 S m-1 and rationally designed architectures, the symmetric microsupercapacitors exhibit an exceptional areal capacitance of 1.57 F cm-2 at 2 mA cm-2 which is retained over 72% after repeated voltage holding tests. The areal power density (0.968 mW cm-2) and areal energy density (51.2 µWh cm-2) outperform the ones of previously reported carbon-based supercapacitors which have been either 3D or inkjet printed. Moreover, a current collector-free interdigitated microsupercapacitor combined with a gel electrolyte provides electrochemical performance approaching the one of devices with liquid-like ion transport properties. Our studies provide a sustainable and low-cost approach to fabricate efficient energy storage devices with programmable geometry.

Synthesis of emerging 2D layered magnetic materials.

van der Waals atomically thin magnetic materials have been recently discovered. They have attracted enormous attention as they present unique magnetic properties, holding potential to tailor spin-based device properties and enable next generation data storage and communication devices. To fully understand the magnetism in two-dimensions, the synthesis of 2D materials over large areas with precise thickness control has to be accomplished. Here, we review the recent advancements in the synthesis of these materials spanning from metal halides, transition metal dichalcogenides, metal phosphosulphides, to ternary metal tellurides. We initially discuss the emerging device concepts based on magnetic van der Waals materials including what has been achieved with graphene. We then review the state of the art of the synthesis of these materials and we discuss the potential routes to achieve the synthesis of wafer-scale atomically thin magnetic materials. We discuss the synthetic achievements in relation to the structural characteristics of the materials and we scrutinise the physical properties of the precursors in relation to the synthesis conditions. We highlight the challenges related to the synthesis of 2D magnets and we provide a perspective for possible advancement of available synthesis methods to respond to the need for scalable production and high materials quality.

Band-Structure Spin-Filtering in Vertical Spin Valves Based on Chemical Vapor Deposited WS2.

We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spectroscopy. The WS2 layers are then integrated in complete Co/Al2O3/WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of PCo/WS2 = 12%. Interestingly, for bi- and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.