Nanographene-Based Decoration as a Panchromatic Antenna for Metalloporphyrin Conjugates.

Porphyrins, a type of heterocyclic aromatic compounds consisting of tetrapyrroles connected by four substituted methine groups, are appealing building blocks for solar energy applications. However, their photosensitization capability is limited by their large optical energy gap, which results in a mismatch in absorption toward efficient harvesting of the solar spectrum. Porphyrin π-extension by edge-fusing with nanographenes can be employed for narrowing their optical energy gap from 2.35 to 1.08 eV, enabling the development of porphyrin-based panchromatic dyes with an optimized energy onset for solar energy conversion in dye-sensitized solar fuel and solar cell configurations. By combining time-dependent density functional theory with fs transient absorption spectroscopy, it is found that the primary singlets, which are delocalized across the entire aromatic part, are transferred into metal centred triplets in only 1.2 ps; and subsequently, relax toward ligand-delocalized triplets. This observation implies that the decoration of the porphyrin moiety with nanographenes, while having a large impact on the absorption onset of the novel dye, promotes the formation of a ligand-centred lowest triplet state of large spatial extension, potentially interesting for boosting interactions with electron scavengers. These results reveal a design strategy for broadening the applicability of porphyrin-based dyes in optoelectronics.

Cove-Edged Hexa-peri-hexabenzo-bis-peri-octacene: Molecular Conformations and Amplified Spontaneous Emission.

The bottom-up synthesis of an unprecedentedly large cove-edged nanographene, hexa-peri-hexabenzo-bis-peri-octacene (HBPO), is reported in this work. Chiral high-performance liquid chromatography and density functional theory (DFT) calculations revealed multiple conformations in solution. Two different molecular conformations, "waggling" and "butterfly", were found in crystals by X-ray crystallography, and the selectivity of conformations could be tuned by solvents. The optoelectronic properties of HBPO were investigated by UV/Vis absorption and fluorescence spectroscopies, cyclic voltammetry, and DFT calculations. The contorted geometry and branched alkyl groups suppress the aggregation of HBPO in solution, leading to a high fluorescence quantum yield of 79 %. The optical-gain properties were explored through transient absorption and amplified spontaneous emission spectroscopies, which enrich the choices of edge structures for potential applications in laser cavities.

Precise tuning of interlayer electronic coupling in layered conductive metal-organic frameworks.

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have attracted increasing interests for (opto)-electronics and spintronics. They generally consist of van der Waals stacked layers and exhibit layer-depended electronic properties. While considerable efforts have been made to regulate the charge transport within a layer, precise control of electronic coupling between layers has not yet been achieved. Herein, we report a strategy to precisely tune interlayer charge transport in 2D c-MOFs via side-chain induced control of the layer spacing. We design hexaiminotriindole ligands allowing programmed functionalization with tailored alkyl chains (HATI_CX, X = 1,3,4; X refers to the carbon numbers of the alkyl chains) for the synthesis of semiconducting Ni3(HATI_CX)2. The layer spacing of these MOFs can be precisely varied from 3.40 to 3.70 Å, leading to widened band gap, suppressed carrier mobilities, and significant improvement of the Seebeck coefficient. With this demonstration, we further achieve a record-high thermoelectric power factor of 68 ± 3 nW m-1 K-2 in Ni3(HATI_C3)2, superior to the reported holes-dominated MOFs.

Covalent modification of franckeite with maleimides: connecting molecules and van der Waals heterostructures.

The building of van der Waals heterostructures and the decoration of 2D materials with organic molecules share a common goal: to obtain ultrathin materials with tailored properties. Performing controlled chemistry on van der Waals heterostructures would add an extra level of complexity, providing a pathway towards 2D-2D-0D mixed-dimensional heterostructures. Here we show that thiol-ene-like "click" chemistry can be used to decorate franckeite, a naturally occurring van der Waals heterostructure with maleimide reagents. ATR-IR and NMR analyses corroborate the Michael addition mechanism via the formation of a S-C covalent bond, while Raman and HR-TEM show that the SnS2-PbS alternating structure of franckeite is preserved, and suggest that SnS2 reacts preferentially, which is confirmed through XPS. We illustrate how this methodology can be used to add functional molecular moieties by decorating franckeite with porphyrins. UV-vis-NIR spectroscopy confirms that the chromophore ground state remains operative, showing negligible ground-state interactions with the franckeite. Excited-state interactions across the hybrid interface are revealed. Time-resolved photoluminescence confirms the presence of excited-state deactivation in the linked porphyrin ascribed to energy transfer to the franckeite.

Percolation Effects in Electrolytically Gated WS2/Graphene Nano:Nano Composites.

Mixed networks of conducting and nonconducting nanoparticles show promise in a range of applications where fast charge transport is important. While the dependence of network conductivity on the loading level of conductive additive is well understood, little is known about the loading dependence of mobility and carrier density. This is particularly important as the addition of graphene might lead to increases in the mobility of semiconducting nanosheet network transistors. Here, we use electrolytic gating to investigate the transport properties of spray-coated composite networks of graphene and WS2 nanosheets. As the graphene loading is increased, we find that both conductivity and carrier density increase in line with the percolation theory with percolation thresholds (∼8 vol %) and exponents (∼2.5) consistent with previous reporting. Perhaps surprisingly, we find the mobility increases modestly from ∼0.1 cm2/V s (for a WS2 network) to ∼0.3 cm2/V s (for a graphene network) which we attribute to the similarity between WS2-WS2 and graphene-graphene junction resistances. In addition, we find both the transistor on- and off-currents to scale with loading according to the percolation theory, changing sharply at the percolation threshold. Through fitting, we show that only the current in the WS2 network changes significantly upon gating. As a result, the on-off ratio falls sharply at the percolation threshold from ∼104 to ∼2 at higher loading. Reflecting on these results, we conclude that the addition of graphene to a semiconducting network is not a viable strategy to improve transistor performance as it reduces the on:off ratio far more than it improves the mobility.

Liquid phase exfoliation of MoO2 nanosheets for lithium ion battery applications.

Molybdenum dioxide (MoO2) is a layered material which shows promise for a number of applications in the electrochemical energy storage arena. Mostly studied as a bulk layered material, MoO2 has not previously been exfoliated in large quantities. Here we demonstrate the liquid phase exfoliation of MoO2 in the solvent isopropanol, yielding reasonable amounts of good quality nanosheets. However, we found that, when dispersed in isopropanol under ambient conditions, MoO2 nanosheets are gradually oxidized to higher oxides such as MoO3 over a period of days. Conversely, if the nanosheets are processed into dried films immediately after exfoliation, and before oxidation has had a chance to progress, the nanosheets are relatively stable under ambient conditions, remaining unoxidised unless the films are heated. We also found that MoO2 nanosheets can be size selected by controlled centrifugation and show size-dependent optical properties. This allows us to propose spectroscopic metrics which allow concentration- and size-estimation from extinction spectra. Finally, we found that liquid-exfoliated MoO2 nanosheets could be used to produce lithium ion battery anodes with capacities of up to 1140 mA h g-1.

Solvent exfoliation stabilizes TiS2 nanosheets against oxidation, facilitating lithium storage applications.

Titanium disulfide is a promising material for a range of applications, including lithium-ion battery (LIB) anodes. However, its application potential has been severely hindered by the tendency of exfoliated TiS2 to rapidly oxidize under ambient conditions. Herein, we confirm that, although layered TiS2 powder can be exfoliated by sonication in aqueous surfactant solutions, the resultant nanosheets oxidise almost completely within hours. However, we find that upon performing the exfoliation in the solvent cyclohexyl-pyrrolidone (CHP), the oxidation is almost completely suppressed. TiS2 nanosheets dispersed in CHP and stored at 4 °C in an open atmosphere for 90 days remained up to 95% intact. In addition, CHP-exfoliated nanosheets did not show any evidence of oxidation for at least 30 days after being transformed into dry films even when stored under ambient conditions. This stability, probably a result of a residual CHP coating, allows TiS2 nanosheets to be deployed in applications. To demonstrate this, we prepared lithium ion battery anodes from nano : nano composites of TiS2 nanosheets mixed with carbon nanotubes. These anodes displayed reversible capacities (920 mA h g-1) close to the theoretical value and showed good rate performance and cycling capability.

Monolayer black phosphorus by sequential wet-chemical surface oxidation.

We report a straightforward chemical methodology for controlling the thickness of black phosphorus flakes down to the monolayer limit by layer-by-layer oxidation and thinning, using water as solubilizing agent. Moreover, the oxidation process can be stopped at will by two different passivation procedures, namely the non-covalent functionalization with perylene diimide chromophores, which prevents the photooxidation, or by using a protective ionic liquid layer. The obtained flakes preserve their electronic properties as demonstrated by fabricating a BP field-effect transistor (FET). This work paves the way for the preparation of BP devices with controlled thickness.

Electroconductive Biohybrid Collagen/Pristine Graphene Composite Biomaterials with Enhanced Biological Activity.

Electroconductive substrates are emerging as promising functional materials for biomedical applications. Here, the development of biohybrids of collagen and pristine graphene that effectively harness both the biofunctionality of the protein component and the increased stiffness and enhanced electrical conductivity (matching native cardiac tissue) obtainable with pristine graphene is reported. As well as improving substrate physical properties, the addition of pristine graphene also enhances human cardiac fibroblast growth while simultaneously inhibiting bacterial attachment (Staphylococcus aureus). When embryonic-stem-cell-derived cardiomyocytes (ESC-CMs) are cultured on the substrates, biohybrids containing 32 wt% graphene significantly increase metabolic activity and cross-striated sarcomeric structures, indicative of the improved substrate suitability. By then applying electrical stimulation to these conductive biohybrid substrates, an enhancement of the alignment and maturation of the ESC-CMs is achieved. While this in vitro work has clearly shown the potential of these materials to be translated for cardiac applications, it is proposed that these graphene-based biohybrid platforms have potential for a myriad of other applications-particularly in electrically sensitive tissues, such as neural and neural and musculoskeletal tissues.

Exciton and charge carrier dynamics in few-layer WS2.

Semiconducting transition metal dichalcogenides (TMDs) have been applied as the active layer in photodetectors and solar cells, displaying substantial charge photogeneration yields. However, their large exciton binding energy, which increases with decreasing thickness (number of layers), as well as the strong resonance peaks in the absorption spectra suggest that excitons are the primary photoexcited states. Detailed time-domain studies of the photoexcitation dynamics in TMDs exist mostly for MoS2. Here, we use femtosecond optical spectroscopy to study the exciton and charge dynamics following impulsive photoexcitation in few-layer WS2. We confirm excitons as the primary photoexcitation species and find that they dissociate into charge pairs with a time constant of about 1.3 ps. The better separation of the spectral features compared to MoS2 allows us to resolve a previously undetected process: these charges diffuse through the samples and get trapped at defects, such as flake edges or grain boundaries, causing an appreciable change of their transient absorption spectra. This finding opens the way to further studies of traps in TMD samples with different defect contents.

Production of Highly Monolayer Enriched Dispersions of Liquid-Exfoliated Nanosheets by Liquid Cascade Centrifugation.

While liquid exfoliation is a powerful technique to produce defect-free nanosheets in large quantities, its usefulness is limited by broad nanosheet thickness distributions and low monolayer contents. Here we demonstrate liquid processing techniques, based on iterative centrifugation cascades, which can be designed to achieve either highly efficient nanosheet size-selection and/or monolayer enrichment. The resultant size-selected dispersions were used to establish quantitative metrics to determine monolayer volume fraction, as well as mean nanosheet size and thickness, from standard spectroscopic measurements. Such metrics allowed us to design and optimize centrifugation cascades to enrich liquid exfoliated WS2 dispersions up to monolayer contents of 75%. Monolayer-rich dispersions show relatively bright photoluminescence with narrow line widths (<35 meV) indicating the high quality of the nanosheets. The enriched dispersions display extinction spectra with distinct features, which also allow the direct estimation of monolayer contents.