73 results found
Suter TAM, Clancy AJ, Carrero NR, et al., 2021, Scalable sacrificial templating to increase porosity and platinum utilisation in graphene-based polymer electrolyte fuel cell electrodes, Nanomaterials, Vol: 11
Polymer electrolyte fuel cells hold great promise for a range of applications but require advances in durability for widespread commercial uptake. Corrosion of the carbon support is one of the main degradation pathways; hence, corrosion-resilient graphene has been widely suggested as an alternative to traditional carbon black. However, the performance of bulk graphene-based electrodes is typically lower than that of commercial carbon black due to their stacking effects. This article reports a simple, scalable and non-destructive method through which the pore structure and platinum utilisation of graphene-based membrane electrode assemblies can be significantly improved. Urea is incorporated into the catalyst ink before deposition, and is then simply removed from the catalyst layer after spraying by submerging the electrode in water. This additive hinders graphene restacking and increases porosity, resulting in a significant increase in Pt utilisation and current density. This technique does not require harsh template etching and it represents a pathway to significantly improve graphene-based electrodes by introducing hierarchical porosity using scalable liquid processes.
Tagliaferri S, Nagaraju G, Panagiotopoulos A, et al., 2021, Aqueous inks of pristine graphene for 3D printed microsupercapacitors with high capacitance., ACS Nano, ISSN: 1936-0851
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.
Och M, Martin M-B, Dlubak B, et al., 2021, Synthesis of emerging 2D layered magnetic materials., Nanoscale, Vol: 13, Pages: 2157-2180, ISSN: 2040-3364
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.
Tagliaferri S, Panagiotopoulos A, Mattevi C, 2021, Direct ink writing of energy materials, Materials Advance, Vol: 2, Pages: 540-563, ISSN: 2633-5409
3D printing is a promising technique for the sustainable fabrication of energy devices with arbitrary architectures. Extrusion-based 3D printing, called Direct Ink Writing, are increasingly used for the manufacturing of batteries, supercapacitors and catalytic systems. In order to obtain me-chanically stable and functional devices, inks formulation must meet stringent criteria for printability, that are usually expressed in terms of rheological properties. Inks are rheologically complex fluids, in which the electroactive materials are mixed with additives and solvents to form an extrudable and self-standing paste. The ink formulation process plays a key role in tuning the rheology and the functional properties of the printed device. In this review, inks formulation, rheological characteristics and device performance are critically discussed, providing insights into the rheology-printability and formulation- functional properties relationships. The main strategies that have been proposed to obtain printable inks from energy materials are reviewed. The role played by the different ink components to achieve the target rheology is contextualized and the integration of different inks into an all-printed device is discussed. Finally, an outlook on the future challenges and opportunities for the DYW of energy materials is provided with the view that general formulations which do necessitate thermal post processing could widen the opportunities of this manufacturing technique enabling the use for large scale production of energy devices.
Asaithambi A, Kozubek R, Prinz GM, et al., 2021, Laser- and Ion-Induced Defect Engineering in WS<inf>2</inf> Monolayers, Physica Status Solidi - Rapid Research Letters, Vol: 15, ISSN: 1862-6254
Tungsten disulfide is one of the prominent transition metal dichalcogenide materials, which shows a transition from an indirect to a direct bandgap as the layer thickness is reduced down to a monolayer. To use (Formula presented.) monolayers in devices, detailed knowledge about the luminescence properties regarding not only the excitonic but also the defect-induced contributions is needed. Herein, (Formula presented.) monolayers are irradiated with (Formula presented.) ions with different fluences to create different defect densities. Apart from the excitonic contributions, two additional emission bands are observed at low temperatures. These bands can be reduced or even suppressed, if the flakes are exposed to laser light with powers up to 1.5 mW. Increasing the temperature up to room temperature leads to recovery of this emission, so that the luminescence properties can be modified using laser excitation and temperature. The defect bands emerging after ion irradiation are attributed to vacancy defects together with physisorbed adsorbates at different defect sites.
Wakamura T, Wu NJ, Chepelianskii AD, et al., 2020, Spin-orbit-enhanced robustness of supercurrent in graphene/WS2Josephson junctions, Physical Review Letters, Vol: 125, ISSN: 0031-9007
We demonstrate the enhanced robustness of the supercurrent through graphene-based Josephson junctions in which strong spin-orbit interactions (SOIs) are induced. We compare the persistence of a supercurrent at high out-of-plane magnetic fields between Josephson junctions with graphene on hexagonal boron-nitride and graphene on WS2, where strong SOIs are induced via the proximity effect. We find that in the shortest junctions both systems display signatures of induced superconductivity, characterized by a suppressed differential resistance at a low current, in magnetic fields up to 1 T. In longer junctions, however, only graphene on WS2 exhibits induced superconductivity features in such high magnetic fields, and they even persist up to 7 T. We argue that these robust superconducting signatures arise from quasiballistic edge states stabilized by the strong SOIs induced in graphene by WS2.
Song W, Stein Scholtis E, sherrel P, et al., 2020, Electronic Structure Influences on the Formation of the Solid Electrolyte Interphase, Energy and Environmental Science, ISSN: 1754-5692
Weiss C, Carriere M, Fusco L, et al., 2020, Toward nanotechnology-enabled approaches against the COVID-19 pandemic, ACS Nano, Vol: 14, Pages: 6383-6406, ISSN: 1936-0851
The COVID-19 outbreak has fueled a global demand for effective diagnosis and treatment as well as mitigation of the spread of infection, all through large-scale approaches such as specific alternative antiviral methods and classical disinfection protocols. Based on an abundance of engineered materials identifiable by their useful physicochemical properties through versatile chemical functionalization, nanotechnology offers a number of approaches to cope with this emergency. Here, through a multidisciplinary Perspective encompassing diverse fields such as virology, biology, medicine, engineering, chemistry, materials science, and computational science, we outline how nanotechnology-based strategies can support the fight against COVID-19, as well as infectious diseases in general, including future pandemics. Considering what we know so far about the life cycle of the virus, we envision key steps where nanotechnology could counter the disease. First, nanoparticles (NPs) can offer alternative methods to classical disinfection protocols used in healthcare settings, thanks to their intrinsic antipathogenic properties and/or their ability to inactivate viruses, bacteria, fungi, or yeasts either photothermally or via photocatalysis-induced reactive oxygen species (ROS) generation. Nanotechnology tools to inactivate SARS-CoV-2 in patients could also be explored. In this case, nanomaterials could be used to deliver drugs to the pulmonary system to inhibit interaction between angiotensin-converting enzyme 2 (ACE2) receptors and viral S protein. Moreover, the concept of "nanoimmunity by design" can help us to design materials for immune modulation, either stimulating or suppressing the immune response, which would find applications in the context of vaccine development for SARS-CoV-2 or in counteracting the cytokine storm, respectively. In addition to disease prevention and therapeutic potential, nanotechnology has important roles in diagnostics, with potential to sup
Sokolikova MS, Mattevi C, 2020, Direct synthesis of metastable phases of 2D transition metal dichalcogenides, Chemical Society Reviews, Vol: 49, Pages: 3952-3980, ISSN: 0306-0012
The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T′) phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T′) phase. Bottom-up synthesis of materials in the 1T(1T′) phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T′) phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control.
Au H, Rubio N, Buckley DJ, et al., 2020, Thermal decomposition of ternary sodium graphite intercalation compounds, Chemistry: A European Journal, Vol: 26, Pages: 6545-6553, ISSN: 0947-6539
Graphite intercalation compounds (GICs) are often used to produce exfoliated or functionalised graphene related materials (GRMs) in a specific solvent. This study explores the formation of the Na-tetrahydrofuran (THF)-GIC and a new ternary system based on dimethylacetamide (DMAc). Detailed comparisons of in situ temperature dependent XRD with TGA-MS and Raman measurements reveal a series of dynamic transformations during heating. Surprisingly, the bulk of the intercalation compound is stable under ambient conditions, trapped between the graphene sheets. The heating process drives a reorganisation of the solvent and Na molecules, then an evaporation of the solvent; however, the solvent loss is arrested by restacking of the graphene layers, leading to trapped solvent bubbles. Eventually, the bubbles rupture, releasing the remaining solvent and creating expanded graphite. These trapped dopants may provide useful property enhancements, but also potentially confound measurements of grafting efficiency in liquid-phase covalent functionalization experiments on 2D materials.
Zatko V, Galbiati M, Dubois SM-M, et al., 2019, Band-structure spin-filtering in vertical spin valves based on chemical vapor deposited WS2., ACS Nano, Vol: 13, Pages: 14468-14476, ISSN: 1936-0851
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.
Chong JY, Wang B, Sherrell PC, et al., 2019, Fabrication of graphene‐covered micro‐tubes for process intensification, Advanced Engineering Materials, Vol: 21, Pages: 1-6, ISSN: 1438-1656
Graphene is known for its high surface‐area‐to‐mass ratio. However, for graphene to be used in engineering processes such as catalytic reactors or heat exchangers, high surface‐area‐to‐volume ratio is essential. Currently, graphene is only prepared in sheet form, which limits its surface‐area‐to‐volume ratio to around 200 m2 m−3. In this study, we propose and demonstrate a technique based on chemical vapour deposition (CVD) to realise graphene on a copper‐based micro‐tubular substrate to not only substantially increase its surface‐area‐to‐volume ratio to a value over 2000 m2 m−3, but also to eliminate maldistribution of flows commonly unavoidable in flat‐sheet configurations. Our approach uses a dual‐layer micro‐tubular substrate fabricated by a phase‐inversion facilitated co‐extrusion technique. In the substrate, a thin copper outer layer is employed to enable the CVD growth of graphene, and an inner Cu‐Fe layer is adopted to provide a strong mechanical support. Our study shows that this approach is feasible to produce graphene with a very high surface‐area‐to‐volume ratio for possible practical applications in catalytic reactors or heat exchangers, though problems such as the inter‐diffusion between the two metal layers and defects in graphene need to be further addressed. To the best of our knowledge, this study is the first attempt to prepare graphene with high surface‐area‐to‐volume ratio by a CVD route.
Sherrell PC, Palczynski P, Sokolikova MS, et al., 2019, Large-area CVD MoS2/WS2 heterojunctions as a photoelectrocatalyst for salt water oxidation, ACS Applied Energy Materials, Vol: 2, Pages: 5877-5882, ISSN: 2574-0962
Splitting salt water via sunlight into molecular oxygen and hydrogen for use as fuel or as an energy carrier is a clear pathway toward renewable energy. Monolayer MoS2 and WS2 are promising materials for the energetically demanding water oxidation reaction, absorbing ∼10% of incident light in the visible spectrum and possessing chemical stability and band edges more positive than the oxidation potential of water. A heterostructure of MoS2/WS2 forms a type-II heterojunction, supporting fast separation of the photogenerated charge carriers across the junction. Here, we show the role played by defects in determining the efficiency of the photon-driven oxidation process. By reducing the defects in this material system, it is possible to obtain an incident photon-to-current conversion efficiency (IPCE) of ∼1.6% and a visible-light-driven photocurrent density of 1.7 mA/cm2 for water oxidation. The efficiency is one order of magnitude higher than that of photoelectrocatalytic hydrogen reduction and water oxidation supported by liquid-phase exfoliated transition-metal dichalcogenides (TMDs). This result has been achieved with chemically vapor deposited (CVD) MoS2/WS2 heterojunctions, in the form of 100 μm large flakes assembled to form thin films. The large flakes sizes, compared to liquid-phase exfoliated materials (normally <5 μm), and thus the low edge flake density, and the flakes’ atomically sharp and clean interfaces between the flakes are responsible for reducing charge carrier recombination. These results show a general approach to the scalable synthesis of high-crystal-quality low-dimensional semiconductor photoelectrodes for solar energy conversion systems. It also shows the uniqueness of the CVD synthesis process of these materials, which can lead to high quality materials without the need of any postsynthesis treatments.
Wakamura T, Reale F, Palczynski P, et al., 2019, Spin-orbit interaction induced in graphene by transition metal dichalcogenides, Physical review B: Condensed matter and materials physics, Vol: 99, ISSN: 1098-0121
We report a systematic study on strong enhancement of spin-orbit interaction (SOI) in grapheneinduced by transition-metal dichalcogenides (TMDs). Low temperature magnetotoransport mea-surements of graphene proximitized to different TMDs (monolayer and bulk WSe2, WS2and mono-layer MoS2) all exhibit weak antilocalization peaks, a signature of strong SOI induced in graphene.The amplitudes of the induced SOI are different for different materials and thickness, and we findthat monolayer WSe2and WS2can induce much stronger SOI than bulk WSe2, WS2and mono-layer MoS2. The estimated spin-orbit (SO) scattering strength for graphene/monolayer WSe2andgraphene/monolayer WS2reachesª10 meV whereas for graphene/bulk WSe2, graphene/bulk WS2and graphene/monolayer MoS2it is around 1 meV or less. We also discuss the symmetry and typeof the induced SOI in detail, especially focusing on the identification of intrinsic (Kane-Mele) andvalley-Zeeman (VZ) SOI by determining the dominant spin relaxation mechanism. Our findingspave the way for realizing the quantum spin Hall (QSH) state in graphene.
Rider MS, Sokolikova M, Hanham SM, et al., 2019, Experimental signature of a topological quantum dot, Publisher: arXiv
Topological insulators (TIs) present a neoteric class of materials, whichsupport delocalised, conducting surface states despite an insulating bulk. Dueto their intriguing electronic properties, their optical properties havereceived relatively less attention. Even less well studied is their behaviourin the nanoregime, with most studies thus far focusing on bulk samples - inpart due to the technical challenges of synthesizing TI nanostructures. Westudy topological insulator nanoparticles (TINPs), for which quantum effectsdominate the behaviour of the surface states and quantum confinement results ina discretized Dirac cone, whose energy levels can be tuned with thenanoparticle size. The presence of these discretized energy levels in turnleads to a new electron-mediated phonon-light coupling in the THz range. Wepresent the experimental realisation of Bi$_2$Te$_3$ TINPs and strong evidenceof this new quantum phenomenon, remarkably observed at room temperature. Thissystem can be considered a topological quantum dot, with applications to roomtemperature THz quantum optics and quantum information technologies.
Sokolikova MS, Sherrell PC, Palczynski P, et al., 2019, Direct solution-phase synthesis of 1T’ WSe2 nanosheets, Nature Communications, Vol: 10, Pages: 1-8, ISSN: 2041-1723
Crystal phase control in layered transition metal dichalcogenides is central for exploiting their different electronic properties. Access to metastable crystal phases is limited as their direct synthesis is challenging, restricting the spectrum of reachable materials. Here, we demonstrate the solution phase synthesis of the metastable distorted octahedrally coordinated structure (1T’ phase) of WSe2 nanosheets. We design a kinetically-controlled regime of colloidal synthesis to enable the formation of the metastable phase. 1T’ WSe2 branched few-layered nanosheets are produced in high yield and in a reproducible and controlled manner. The 1T’ phase is fully convertible into the semiconducting 2H phase upon thermal annealing at 400 °C. The 1T’ WSe2 nanosheets demonstrate a metallic nature exhibited by an enhanced electrocatalytic activity for hydrogen evolution reaction as compared to the 2H WSe2 nanosheets and comparable to other 1T’ phases. This synthesis design can potentially be extended to different materials providing direct access of metastable phases.
Jia J, White ER, Clancy AJ, et al., 2018, Fast exfoliation and functionalisation of two-dimensional crystalline carbon nitride by framework charging, Angewandte Chemie, Vol: 57, Pages: 12656-12660, ISSN: 1521-3757
Two-dimensional (2D) layered graphitic carbon nitride (gCN) nanosheets offer intriguing electronic and chemical properties. However, the exfoliation and functionalisation of gCN for specific applications remain challenging. We report a scalable one-pot reductive method to produce solutions of single- and few-layer 2D gCN nanosheets with excellent stability in a high mass yield (35 %) from polytriazine imide. High-resolution imaging confirmed the intact crystalline structure and identified an AB stacking for gCN layers. The charge allows deliberate organic functionalisation of dissolved gCN, providing a general route to adjust their properties.
Sherrell PC, Sharda K, Grotta C, et al., 2018, Thickness dependant characterization of chemically exfoliated TiS2 nanosheets, ACS Omega, Vol: 3, Pages: 8655-8662, ISSN: 2470-1343
Monolayer TiS2 is the lightest member of the transition metal dichalcogenides family with promising application in energy storage and conversion systems. Use of TiS2 has been limited by the lack of rapid characterisation of layer number via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate layer number dependent Raman modes for TiS2. 1T-TiS2 presents two characteristics Raman active modes, A1g (out-of-plane) and Eg (in-plane). We identified a characteristic peak frequency shift of the Eg mode with the layer number and an unexplored Raman mode at 372 cm-1 whose intensity changes relative to the A1g mode with the thickness of TiS2 sheets. These two characteristic features of the Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS2. Further, we develop a method to produce oxidation-resistant inks of micron sized mono- and few-layered TiS2 nanosheets at concentrations up to 1 mg/mL .These TiS2 inks can be deposited to form thin films with controllable thickness and nanosheet density over cm2 areas. This opens up pathways for a wider utilization of exofliated TiS2 towards a range of applications.
Mattevi C, 2018, Electronic band structure of Two-DimensionalWS2/Graphene van derWaals Heterostructure, Physical Review B, ISSN: 2469-9950
Henck H, Ben Aziz Z, Pierucci D, et al., 2018, Electronic band structure of two-dimensional WS2/Graphene van der Waals heterostructures, Physical Review B, Vol: 97, Pages: 155421 – 1-155421 – 8, ISSN: 2469-9950
Combining single-layer two-dimensional semiconducting transition metal dichalcogenides (TMDs)with graphene layer in van der Waals heterostructuresoffers an intriguing means of controlling the electronic properties through these heterostructures. Here, we report the electronic and structural properties of transferred single layer WS2on epitaxial graphene using micro-Raman spectroscopy, angle-resolved photoemission spectroscopy measurements(ARPES)and Density Functional Theory(DFT)calculations. The results show good electronic properties as well as well-defined band arising from the strong splitting of the single layer WS2valence band at K points, with a maximum splitting of 0.44 eV.By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is closeto the calculations forsuspended single layer WS2. .Our results provide an important reference for future studies of electronic properties of WS2and its applications in valleytronic devices.
Wakamura T, Reale F, Palczynski P, et al., 2018, Strong anisotropic spin-orbit interaction induced in graphene by monolayer WS₂, Physical Review Letters, Vol: 120, ISSN: 0031-9007
We demonstrate strong anisotropic spin-orbit interaction (SOI) in graphene induced by monolayer WS₂. Direct comparison between graphene-monolayer WS2 and graphene-bulk WS₂ systems in magnetotransport measurements reveals that monolayer transition metal dichalcogenide can induce much stronger SOI than bulk. Detailed theoretical analysis of the weak antilocalization curves gives an estimated spin-orbit energy (Eso) higher than 10 meV. The symmetry of the induced SOI is also discussed, and the dominant z → −z symmetric SOI can only explain the experimental results. Spin relaxation by the Elliot-Yafet mechanism and anomalous resistance increase with temperature close to the Dirac point indicates Kane-Mele SOI induced in graphene.
Li K, Wang B, Chong JY, et al., 2017, Dynamic microstructure of graphene oxide membranes and the permeation flux, Journal of Membrane Science, Vol: 549, Pages: 385-392, ISSN: 0376-7388
Graphene oxide (GO) membranes have been reported to be a promising separation barrier that can retain small molecules and multi-valent salts because of the well-defined interlayer space between GO flakes. However, while some studies suggested fast liquid transport through the extremely tortuous transport path, contradictory observations (e.g. low permeation flux) have also been obtained. This paper revealed the dynamic microstructure of GO membranes, which affected the membrane performance significantly. We showed that all GO membranes prepared by varied methods and on different substrates presented a severe reduction in water permeability during filtration, due to the compaction of their original loose microstructure. The water flux could drop continuously from tens of LMH bar−1 to <0.1 LMH bar−1 after more than ten hours. This result demonstrated that the structure of GO membranes prepared by current approaches was far from the ideal laminar structure. The high permeability of GO membranes observed could be contributed by the disordered membrane microstructure. Therefore, the transport mechanisms assuming perfect laminar structure in GO membranes, and the fast transport hypothesis may not fully describe the water transport in GO membranes. Interestingly, the loosely packed microstructure of GO membranes was also found reversible depending on the storage conditions.
Reale F, Palczynski P, Amit I, et al., 2017, High-mobility and high-optical quality atomically thin WS2, Scientific Reports, Vol: 7, Pages: 1-10, ISSN: 2045-2322
The rise of atomically thin materials has the potential to enable a paradigm shift in modern technologies by introducing multi-functional materials in the semiconductor industry. To date the growth of high quality atomically thin semiconductors (e.g. WS2) is one of the most pressing challenges to unleash the potential of these materials and the growth of mono- or bi-layers with high crystal quality is yet to see its full realization. Here, we show that the novel use of molecular precursors in the controlled synthesis of mono- and bi-layer WS2 leads to superior material quality compared to the widely used direct sulfidization of WO3-based precursors. Record high room temperature charge carrier mobility up to 52 cm2/Vs and ultra-sharp photoluminescence linewidth of just 36 meV over submillimeter areas demonstrate that the quality of this material supersedes also that of naturally occurring materials. By exploiting surface diffusion kinetics of W and S species adsorbed onto a substrate, a deterministic layer thickness control has also been achieved promoting the design of scalable synthesis routes.
Bekaert J, Bignardi L, Aperis A, et al., 2017, Free surfaces recast superconductivity in few-monolayer MgB2: Combined first-principles and ARPES demonstration, Scientific Reports, Vol: 7, ISSN: 2045-2322
Two-dimensional materials are known to harbour properties very different from those of their bulk counterparts. Recent years have seen the rise of atomically thin superconductors, with a caveat that superconductivity is strongly depleted unless enhanced by specific substrates, intercalants or adatoms. Surprisingly, the role in superconductivity of electronic states originating from simple free surfaces of two-dimensional materials has remained elusive to date. Here, based on first-principles calculations, anisotropic Eliashberg theory, and angle-resolved photoemission spectroscopy (ARPES), we show that surface states in few-monolayer MgB2make a major contribution to the superconducting gap spectrum and density of states, clearly distinct from the widely known, bulk-like σ- and π-gaps. As a proof of principle, we predict and measure the gap opening on the magnesium-based surface band up to a critical temperature as high as ~30 K for merely six monolayers thick MgB2. These findings establish free surfaces as an unavoidable ingredient in understanding and further tailoring of superconductivity in atomically thin materials.
Pesci FM, Sokolikova MS, Grotta C, et al., 2017, MoS2/WS2 Heterojunction for Photoelectrochemical Water Oxidation, ACS Catalysis, Vol: 7, Pages: 4990-4998, ISSN: 2155-5435
Sokolikova MS, Sherrell PC, Bemmer VL, et al., 2017, Room-temperature growth of colloidal Bi2Te3 nanosheets, Chemical Communications, Vol: 53, Pages: 8026-8029, ISSN: 1364-548X
In this work, we report the colloidal synthesis of Bi2Te3 nanosheets with controlled thickness, morphology and crystallinity at temperatures as low as 20 °C. Grown at room temperature, Bi2Te3 exhibits two-dimensional morphology with thickness of 4 nm and lateral size of 200 nm. Upon increasing the temperature to 170 °C, the nanosheets demonstrate increased thickness of 16 nm and lateral dimensions of 600 nm where polycrystalline nanosheets (20 °C) are replaced by single crystal platelets (170 °C). Rapid synthesis of the material at moderately low temperatures with controllable morphology, crystallinity and consequently electrical and thermal properties can pave the way toward its large-scale production for practical applications.
Riley DJ, Song W, Lischner, et al., 2017, Tuning the Double Layer of Graphene Oxide through Phosphorus Doping for Enhanced Supercapacitance, ACS Energy Letters, Vol: 2, Pages: 1144-1149, ISSN: 2380-8195
The electrochemical double layer plays a fundamental role in energy storage applications. Control of the distribution of ions in the double layer at the atomistic scale offers routes to enhanced material functionality and device performance. Here we demonstrate how the addition of an element from the third row of the periodic table, phosphorus, to graphene oxide increases the measured capacitance and present density functional theory calculations that relate the enhanced charge storage to structural changes of the electrochemical double layer. Our results point to how rational design of materials at the atomistic scale can lead to improvements in their performance for energy storage.
Amit I, Octon TJ, Townsend NJ, et al., 2017, Role of charge traps in the performance of atomically-thin transistors, Advanced Materials, Vol: 29, ISSN: 1521-4095
Transient currents in atomically thin MoTe2 field-effect transistors (FETs) are measured during cycles of pulses through the gate electrode. The curves of the transient currents are analyzed in light of a newly proposed model for charge-trapping dynamics that renders a time-dependent change in the threshold voltage as the dominant effect on the channel hysteretic behavior over emission currents from the charge traps. The proposed model is expected to be instrumental in understanding the fundamental physics that governs the performance of atomically thin FETs and is applicable to the entire class of atomically thin-based devices. Hence, the model is vital to the intelligent design of fast and highly efficient optoelectronic devices.
Olowojoba GB, Kopsidas S, Eslava S, et al., 2017, A facile way to produce epoxy nanocomposites having excellent thermal conductivity with low contents of reduced graphene oxide, Journal of Materials Science, Vol: 52, Pages: 7323-7344, ISSN: 1573-4803
A well-dispersed phase of exfoliated graphene oxide (GO) nanosheets was initially prepared in water. This was concentrated by centrifugation and was mixed with a liquid epoxy resin. The remaining water was removed by evaporation, leaving a GO dispersion in epoxy resin. A stoichiometric amount of an anhydride curing agent was added to this epoxy-resin mixture containing the GO nanosheets, which was then cured at 90 °C for 1 hour followed by 160 °C for 2 hours. A second thermal treatment step of 200 °C for 30 minutes was then undertaken to reduce further the GO in-situ in the epoxy nanocomposite. An examination of the morphology of such nanocomposites containing reduced graphene oxide (rGO) revealed that a very good dispersion of rGO was achieved throughout the epoxy polymer. Various thermal and mechanical properties of the epoxy nanocomposites were measured and the most noteworthy finding was a remarkable increase in the thermal conductivity when relatively very low contents of rGO were present. For example, a value of 0.25 W/mK was measured at 30 °C for the nanocomposite with merely 0.06 weight percentage (wt%) of rGO present, which represents an increase of ~40% compared with that of the unmodified epoxy polymer. This value represents one of the largest increases in the thermal conductivity per wt% of added rGO yet reported. These observations have been attributed to the excellent dispersion of rGO achieved in these nanocomposites made via this facile production method. The present results show that it is now possible to tune the properties of an epoxy polymer with a simple and viable method of GO addition.
Mattevi C, 2016, Graphitic carbon nitride as a catalyst support in fuel cells and electrolyzers, Electrochimica Acta, Vol: 222, Pages: 44-57, ISSN: 1873-3859
Electrochemical power sources, such as polymer electrolyte membrane fuel cells (PEMFCs), require the use of precious metal catalysts which are deposited as nanoparticles onto supports in order to minimize their mass loading and therefore cost. State-of-the-art/commercial supports are based on forms of carbon black. However, carbon supports present disadvantages including corrosion in the operating fuel cell environment and loss of catalyst activity. Here we review recent work examining the potential of different varieties of graphitic carbon nitride (gCN) as catalyst supports, highlighting their likely benefits, as well as the challenges associated with their implementation. The performance of gCN and hybrid gCN-carbon materials as PEMFC electrodes is discussed, as well as their potential for use in alkaline systems and water electrolyzers. We illustrate the discussion with examples taken from our own recent studies.
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