Nina Christina Berner

The reduction of dimensionality has revealed exciting properties in layered 2D materials such as graphene and transition metal dichalcogenides (TMDs). In order to use these materials in functional devices, processes for reliable large scale synthesis, functionalization and integration must be developed. Here we present the thermally assisted conversion of various TMD layers. The sulfides and selenides of Mo and W have been produced on silicon chips. These films have been characterized thoroughly and electrically addressed. Furthermore, we report on CVD growth of MoS2 and WS2 monolayers in a microreactor setup. Besides transistor applications, TMDs can be used for other electronic applications such as chemical sensors and photodiodes. Integrating novel 2D materials with silicon technology may lead to significant advances toward a wide range of novel devices.

Covalently tethering photosensitizers to catalytically active 1T-MoS 2 surfaces holds great promise for the solar-driven hydrogen evolution reaction (HER). Herein we report the preparation of two new Ru II complex functionalized MoS 2 hybrids [Ru II (bpy) 2 (phen)]-MoS 2 and [Ru II (bpy) 2 (py)Cl]-MoS 2 . The influence of covalent functionalization of chemically exfoliated 1T-MoS 2 with coordinating ligands and Ru II complexes on the HER activity and photoelectrochemical performance of this dye-sensitized system was studied systematically. We find that the photoelectrochemical performance of this Ru II complex sensitized MoS 2 system is highly dependent on the surface extent of photosensitizers and the catalytic activity of functionalized MoS 2 . The latter was strongly affected by the number and the kind of functional groups. Our results underline the tunability of the photovoltage generation in this dye-sensitized MoS 2 system by manipulation of the surface functionalities, which provides a practical guidance for smart design of future dye-sensitized MoS 2 hydrogen production devices towards improved the photo-fuel conversion efficiency.

Alkaline water electrolysis produces H2 gas, which can be used as a fuel in H2/O2 fuel cells to generate power. The most energy intensive step in water electrolysis is the evolution of O2 due to the large anodic overpotential of the Oxygen Evolution Reaction (OER).1 Thus, understanding and optimizing electrocatalysts for OER remains one of the grand challenges for both physical electrochemistry and energy science. For the OER in alkaline media, the best performing electrocatalysts are thermally prepared RuO2 and IrO2, which exhibit the lowest OER overpotentials to date, but these oxides are expensive and somewhat unstable in alkaline media, rendering them impractical and uneconomical.1 First row Transition Metal Oxides (TMO), e.g. Mn, Ni, Co or Fe, show great promise as alternative materials for OER, as they exhibit low overpotentials and high stability at lower costs than those of RuO2 or IrO2. However, mechanistic studies of OER at TMO electrodes in alkaline media have been sparse and the nature of catalytic sites and the mechanism leading to O2 evolution are not well understood.2 In this work, pure and mixed Ni/Fe materials were electrochemically deposited on Ti supports to fabricate inexpensive electrocatalysts. Their potential as OER catalysts was elucidated in NaOH electrolyte with different amounts of Fe impurities; 1 ppb, 5 ppb and 102 ppb, as determined by Inductive Coupled Plasma (ICP) spectroscopy. The results indicate that the electrocatalytic activity of the materials depends on the ratio of Ni/Fe and the concentration of Fe impurities in the electrolyte. Most of the mixed catalysts show improved OER performances compared to the pure Ni and Fe oxide materials with respect to overpotential at 10 mA cm-2, Figure 1(a), Tafel slope values and Turnover Frequencies (TOF) numbers. Interestingly, the pure and mixed Ni/Fe materials in the NaOH electrolyte containing 5 ppb Fe impurities exhibited lower overpotentials at 10mA cm-2 compared to the same material in the NaOH containing 1 ppb and 100 ppb, Figure 1(a). This is thought to be due to the substitution of the Fe ions in the electrolyte for the Ni atoms in the material lattice, improving the OER performance.3 Pure and mixed manganese and ruthenium oxides were also examined in this work. The OER catalytic activity of pure manganese oxide compounds displaying overpotentials between 0.74 - 0.49 V at a current density of 10 mA cm-2. Furthermore, when combined with other compounds this overpotential value further decreases.4 However, mechanistic studies of the OER at thermally prepared DSA® type MnxOy electrodes in alkaline media have been sparse.2 Several of the mixed Mn/Ru electrode materials in this study were found to exhibit significantly improved OER activity and stability when compared with pure RuO2films, Figure 1(b), while lowering the cost of producing the catalyst.5 These Mn/Ru materials could therefore offer a competitive low-cost alternative to the already commercially available OER catalysts. The composition, morphology and structure of all the aforementioned materials are thoroughly characterised by X-Ray Photoelectron Spectroscopy (XPS), Raman spectroscopy and Scanning Electron Microscopy–Energy Dispersive X-Ray (SEM-EDX).  Finally, the Ni/Fe and Mn/Ru oxides will be a compared under cost and OER performance, to help identify the most economic and practical OER catalyst in this work.  Acknowledgements We would like to thank Science Foundation Ireland (SFI) under the Grant Number SFI/10/IN.1/I2969. References                 (1)           Lyons, M. E. G.; Doyle, R. L.; Fernandez, D.; Godwin, I. J.; Browne, M. P.; Rovetta, A. Electrochem. Commun. 2014, 45, 56-59.                 (2)           Fernández, J. L.; Gennero De Chialvo, M. R.; Chialvo, A. C. J. Appl. Electrochem. 2002, 32, 513-520.                 (3)           Klaus, S.; Louie, M. W.; Trotochaud, L.; Bell, A. T. The Journal of Physical Chemistry C 2015, 119, 18303-18316.                 (4)           Gao, M.-R.; Xu, Y.-F.; Jiang, J.; Zheng, Y.-R.; Yu, S.-H. J. Am. Chem. Soc. 2012, 134, 2930-2933.                 (5)           Browne, M. P.; Nolan, H.; Duesberg, G. S.; Colavita, P. E.; Lyons, M. E. G. ACS Catalysis 2016. Figure 1

Abstract The effect of adding chromium (Cr) to the copper catalyst for chemical vapour deposition growth of graphene is investigated. We observe a suppression of the formation of multilayer islands of graphene in the Cr-rich regions. This is shown with optical microscopy, scanning electron microscopy and scanning Raman spectroscopy. In addition, carbon isotope labelling is employed to elucidate the mechanism by which the formation of multilayer islands is minimised. The use of mixed catalysts is an important step in the optimisation of catalytic growth of graphene.

Covalently tethering photosensitizers to catalytically active 1T-MoS 2 surfaces holds great promise for the solar-driven hydrogen evolution reaction (HER). Herein we report the preparation of two new Ru II complex functionalized MoS 2 hybrids [Ru II (bpy) 2 (phen)]-MoS 2 and [Ru II (bpy) 2 (py)Cl]-MoS 2 . The influence of covalent functionalization of chemically exfoliated 1T-MoS 2 with coordinating ligands and Ru II complexes on the HER activity and photoelectrochemical performance of this dye-sensitized system was studied systematically. We find that the photoelectrochemical performance of this Ru II complex sensitized MoS 2 system is highly dependent on the surface extent of photosensitizers and the catalytic activity of functionalized MoS 2 . The latter was strongly affected by the number and the kind of functional groups. Our results underline the tunability of the photovoltage generation in this dye-sensitized MoS 2 system by manipulation of the surface functionalities, which provides a practical guidance for smart design of future dye-sensitized MoS 2 hydrogen production devices towards improved the photo-fuel conversion efficiency.

We report on 2H-2H'/1T phase conversion of MoS2 and MoSe2 polycrystalline films grown by thermally assisted conversion. The structural conversion of the transition metal dichalcogenides was successfully carried out by organolithium treatment on chip. As a result we obtained a new 2H-2H'/1T cophase system of the TMDs thin films which was verified by Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The conversion was successfully carried out on selected areas yielding a lateral heterostructure between the pristine 2H phase and the 2H'/1T cophase regions. Scanning electron microscopy and atomic force microscopy revealed changes in the surface morphology and work function of the cophase system in comparison to the pristine films, with a surprisingly sharp lateral interface region.

The optical nonlinearity of WS2, MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near infrared. The dependence of nonlinear absorption of the WS2 and MoS2 films on layer number and excitation wavelength was studied systematically. WS2 with 1~3 layers exhibits a giant two-photon absorption (TPA) coefficient. Saturation of TPA for WS2 with 1~3 layers and MoS2 with 25~27 layers was observed. The giant nonlinearity of WS2 and MoS2 is attributed to two dimensional confinement, a giant exciton effect and the band edge resonance of TPA.

Self-assembly of nanomaterials by wet chemistry methods is a suitable approach for the preparation of engineered structures with novel functionalities. In this work, we study the ability of long-chain amines to direct the growth of a layered nanomaterial, using [Re x Se y Cl z ] clusters as building blocks. The amines link to the clusters as ligands during the synthesis, directing the self-assembly due to their amphiphilic properties, which produces a platelet-shaped 2D material with sizes up to several μm in diameter and thicknesses in the range of 60–80 nm. This is, to the best of our knowledge, the first report on a one-step mild chemistry method for the preparation of 2D structures composed of alternate layers of self-assembled amines and sub-nm clusters of a rhenium chalcogenide. Furthermore, these materials can be used as a suitable source of clusters which then, conveniently released by a simple acid/base reaction, have been successfully incorporated to the surface of graphen...

Understanding the correlation between the physico-chemical properties of carbonaceous nanomaterials and how these properties impact on cells and subcelluar mechanisms is critical to their risk assessment and safe translation into engineered devices.

2D metal chalcogenide (MC) nanosheets (NS) have displayed high capacities as lithium-ion battery (LiB) anodes. Nevertheless, their complicated synthesis routes coupled with low electronic conductivity greatly limit them as promising LiB electrode material. Here, this work reports a facile single-walled carbon nanotube (SWCNT) percolating strategy for efficiently maximizing the electrochemical performances of gallium chalcogenide (GaX, X = S or Se). Multiscaled flexible GaX NS/SWCNT heterostructures with abundant voids for Li(+) diffusion are fabricated by embedding the liquid-exfoliated GaX NS matrix within a SWCNT-percolated network; the latter improves the electron transport and ion diffusion kinetics as well as maintains the mechanical flexibility. Consequently, high capacities (i.e., 838 mAh g(-1) per gallium (II) sulfide (GaS) NS/SWCNT mass and 1107 mAh g(-1) per GaS mass; the latter is close to the theoretical value) and good rate capabilities are achieved, which can be majorl...

The optical nonlinearity of WS2, MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near infrared. The dependence of nonlinear absorption of the WS2 and MoS2 films on layer number and excitation wavelength was studied systematically. WS2 with 1~3 layers exhibits a giant two-photon absorption (TPA) coefficient. Saturation of TPA for WS2 with 1~3 layers and MoS2 with 25~27 layers was observed. The giant nonlinearity of WS2 and MoS2 is attributed to two dimensional confinement, a giant exciton effect and the band edge resonance of TPA.

Two-dimensional layered transition metal dichalcogenides (TMDs) have attracted great interest owing to their unique properties and a wide array of potential applications. However, due to their inert nature, pristine TMDs are very challenging to functionalize. We demonstrate a general route to functionalize exfoliated 2H-MoS2 with cysteine. Critically, MoS2 was found to be facilitating the oxidation of the thiol cysteine to the disulfide cystine during functionalization. The resulting cystine was physisorbed on MoS2 rather than coordinated as a thiol (cysteine) filling S-vacancies in the 2H-MoS2 surface, as originally conceived. These observations were found to be true for other organic thiols and indeed other TMDs. Our findings suggest that functionalization of two-dimensional MoS2 using organic thiols may not yield covalently or datively tethered functionalities, rather, in this instance, they yield physisorbed disulfides that are easily removed.

This work describes silicon nanoparticle-based lithium-ion battery negative electrodes where multiple non-active electrode additives (usually carbon black and an inert polymer binder) are replaced with a single conductive binder; in this case the conducting polymer PEDOT:PSS. While enabling the production of well-mixed slurry-cast electrodes with high silicon content (up to 95 wt%), this combination eliminates the well-known occurrence of capacity losses due to physical separation of the silicon and traditional inorganic conductive additives during repeated lithiation/delithiation processes. Using an in situ secondary doping treatment of the PEDOT:PSS with small quantities of formic acid, electrodes containing 80 wt% SiNPs can be prepared with electrical conductivity as high as 4.2 S/cm. Even at the relatively high mass loading of 1 mg/cm2, this system demonstrated a first cycle lithiation capacity of 3685 mAh/g (based on the SiNP mass) and a first cycle efficiency of ~78%. After 10...

The optical nonlinearity of WS2 and MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near infrared range. The nonlinear absorption of few- and multi-layer WS2 and MoS2 films and their dependences on excitation wavelength were studied. WS2 with 1~3 layers exhibits a giant two-photon absorption (TPA) coefficient as high as (1.0±0.8)×10(4) cm/GW. TPA saturation was observed for the WS2 film with 1~3 layers and for the MoS2 film with 25~27 layers. The giant nonlinearity of WS2 and MoS2 films is attributed to a two-dimensional confinement, a giant exciton effect and the band edge resonance of TPA.

The non-covalent functionalisation of graphene is an attractive strategy to alter the surface chemistry of graphene without damaging its superior electrical and mechanical properties. Using the facile method of aqueous-phase functionalisation on large-scale CVD-grown graphene, we investigated the formation of different packing densities in self-assembled monolayers (SAMs) of perylene bisimide derivatives and related this to the amount of substrate contamination. We were able to directly observe wet-chemically deposited SAMs in scanning tunnelling microscopy (STM) on transferred CVD graphene and revealed that the densely packed perylene ad-layers adsorb with the conjugated π-system of the core perpendicular to the graphene substrate. This elucidation of the non-covalent functionalisation of graphene has major implications on controlling its surface chemistry and opens new pathways for adaptable functionalisation in ambient conditions and on the large scale.

This commmunication presents a study of atomic layer deposition of Al2O3 on transition metal dichalcogenide (TMD) two-dimensional films which is crucial for use of these promising materials for electronic applications. Deposition of Al2O3 on pristine chemical vapour deposited MoS2 and WS2 crystals is demonstrated. This deposition is dependent on the number of TMD layers as there is no deposition on pristine monolayers. In addition, we show that it is possible to reliably seed the deposition, even on the monolayer, using non-covalent functionalisation with perylene derivatives as anchor unit.

We present results of experiments to reproduce the bottom-up formation of covalently bonded molecular nanostructures from single molecular building blocks, previously demonstrated on various coinage metal surfaces, on a technologically more relevant semiconductor surface: Ge(001). Chlorine was established as the most stable passivation agent for this surface, successfully enabling diffusion of the organic molecular building blocks. Subsequent thermal activation of the intermolecular dehalogenation reactions on this surface resulted in the desired covalently connected molecules, however showing poor network quality when compared to those formed on noble metal substrates.

A highly efficient surface plasmon resonance (SPR) immunosensor is described using a functionalized single graphene layer on a thin gold film. The aim of this approach was two-fold: first, to amplify the SPR signal by growing graphene through chemical vapor deposition and, second, to control the immobilization of biotinylated cholera toxin antigen on copper coordinated nitrilotriacetic acid (NTA) using graphene as an ultrathin layer. The NTA groups were attached to graphene via pyrene derivatives implying π-π interactions. With this setup, an immunosensor for the specific antibody anticholera toxin with a detection limit of 4 pg mL(-1) was obtained. In parallel, NTA polypyrrole films of different thicknesses were electrogenerated on the gold sensing platform where the optimal electropolymerization conditions were determined. For this optimized polypyrrole-NTA setup, the simple presence of a graphene layer between the gold and polymer film led to a significant increase of the SPR sig...

Here we demonstrate significant improvements in the performance of supercapacitor electrodes based on 2D MnO2 nanoplatelets by the addition of carbon nanotubes. Electrodes based on MnO2 nanoplatelets do not display high areal capacitance because the electrical properties of such films are poor, limiting the transport of charge between redox sites and the external circuit. In addition, the mechanical strength is low, limiting the achievable electrode thickness, even in the presence of binders. By adding carbon nanotubes to the MnO2-based electrodes, we have increased the conductivity by up to 8 orders of magnitude, in line with percolation theory. The nanotube network facilitates charge transport, resulting in large increases in capacitance, especially at high rates, around 1 V/s. The increase in MnO2 specific capacitance scaled with nanotube content in a manner fully consistent with percolation theory. Importantly, the mechanical robustness was significantly enhanced, allowing the fabrication of electrodes that were 10 times thicker than could be achieved in MnO2-only films. This resulted in composite films with areal capacitances up to 40 times higher than could be achieved with MnO2-only electrodes.