Search

Liquid-phase exfoliation (LPE) has been introduced as a versatile and scalable production method for two-dimensional (2D) materials. This method yields dispersions that allow for the fabrication of printable and flexible electronic devices. However, the fabrication of uniform and homogeneous films from LPE dispersions with a performance similar to that of bottom-up grown materials remains a challenge, as the film quality strongly influences the optical and electrical performance of devices. Furthermore, long-term stability remains a major challenge for all 2D material based applications. In this study, we report on highly conductive tiled network films made of platinum diselenide (PtSe2) flakes derived using a scalable LPE method. We characterized the homogeneous films in terms of morphology and electrical behavior. As an example of applicability, we produce a chemiresistive sensor structure with the PtSe2 films and show significant resistance changes upon periodic ammonia gas exposures, revealing a sub-0.1 part per million (ppm) detection limit (DL). More remarkably the devices are fully functional after 15 months, underlining the high stability of PtSe2 based devices.

Networks of solution-processed nanomaterials are becoming increasingly important across applications in electronics, sensing and energy storage/generation. Although the physical properties of these devices are often completely dominated by network morphology, the network structure itself remains difficult to interrogate. Here, we utilise FIB-SEM nanotomography to quantitatively characterise the morphology of nanostructured networks and their devices using nanometre-resolution 3D images. The influence of nanosheet/nanowire size on network structure in printed films of graphene, WS2 and silver nanosheets, as well as networks of silver nanowires, is investigated. We present a comprehensive toolkit to extract morphological characteristics including network porosity, tortuosity, specific surface area, pore dimensions and nanosheet orientation, which we link to network resistivity. By extending this technique to interrogate the structure and interfaces within vertical printed heterostacks, we demonstrate the potential of this technique for device characterisation and optimisation.

New luminogens for aggregation‐induced emission (AIE), which are characterized by a branched cross‐conjugated 2,6‐bis(1,2,2‐triarylvinyl)pyridine motif, have been synthesized exploiting the one‐pot Ti‐mediated tetraarylation of 2,6‐bis(arylethynyl)pyridines. Thin layer solid‐state emitters were prepared by spin‐coating of the luminogens, while AIE‐colloidal dispersions were investigated in terms of optical density and scattering behaviour. This has given insight into particle size distributions, time evolution of the aggregation and the influence of different functionalization patterns on the luminescence of molecular aggregates. In particular, a combination of extinction spectroscopy and dynamic light scattering is being proposed as a powerful method for investigating the dynamic aggregation process in AIE‐type colloids., Aggregate and shine! New luminogens for aggregation‐induced emission (AIE) have been synthesized exploiting the one‐pot Ti‐mediated tetraarylation of 2,6‐bis(arylethynyl)pyridines. Their aggregation‐related photophysics and viscochromicity have been studied in detail.

Originating from the hexagonal arrangement of magnetic ions in the presence of strong spin orbit coupling, α-RuCl3 is considered as model system for the Kitaev-Heisenberg model. While the magnetic properties of α-RuCl3 have been studied in bulk single crystals or micromechanically-exfoliated nanosheets, little is known about the nanosheets’ properties after exfoliation by techniques suitable for mass production such as liquid phase exfoliation (LPE). Here, we demonstrate sonication-assisted LPE on α-RuCl3 single crystals in an inert atmosphere. Coupled with centrifugation-based size selection techniques, the accessible size- and thickness range is quantified by statistical atomic force microscopy. Individual nanosheets obtained after centrifugation-based size selection are subjected to transmission electron microscopy to confirm their structural integrity after the exfoliation. The results are combined with bulk characterisation methods, including Raman and x-ray photoelectron spectroscopy, and powder diffraction experiments to evaluate the structural integrity of the nanosheets. We report changes of the magnetic properties of the nanomaterial with nanosheet size, as well as photospectroscopic metrics for the material concentration and average layer number. Finally, a quantitative analysis on environmental effects on the nanomaterial integrity is performed based on time and temperature dependent absorbance spectroscopy revealing a relatively slow decay (half-life of ∼2000 h at 20 °C), albeit with low activation energies of 6–20 kJ mol−1.

Iron oxide (Fe

2D polymer sheets with six positively charged pyrylium groups at each pore edge in a stacked single crystal can be transformed into a 2D polymer with six pyridines per pore by exposure to gaseous ammonia. This reaction furnishes still a crystalline material with tunable protonation degree at regular nano‐sized pores promising as separation membrane. The exfoliation is compared for both 2D polymers with the latter being superior. Its liquid phase exfoliation yields nanosheet dispersions, which can be size‐selected using centrifugation cascades. Monolayer contents of ≈30 % are achieved with ≈130 nm sized sheets in mg quantities, corresponding to tens of trillions of monolayers. Quantification of nanosheet sizes, layer number and mass shows that this exfoliation is comparable to graphite. Thus, we expect that recent advances in exfoliation of graphite or inorganic crystals (e.g. scale‐up, printing etc.) can be directly applied to this 2D polymer as well as to covalent organic frameworks., Angewandte Chemie. International Edition, 59 (14), ISSN:1433-7851, ISSN:1521-3773, ISSN:0570-0833

Reducing the size of a material, from a bulk solid to a nanomaterial, may lead to drastic changes of various properties including reactivity and optical properties. Chemical reactivity is often inc...

Liquid phase exfoliation is a commonly used method to produce 2D nanosheets from a range of layered crystals. However, such nanosheets display broad size and thickness distributions and correlations between area and thickness, issues that limit nanosheet application potential. To understand the factors controlling the exfoliation process, we have liquid-exfoliated 11 different layered materials, size-selecting each into fractions before using AFM to measure the nanosheet length, width, and thickness distributions for each fraction. The resultant data show a clear power-law scaling of nanosheet area with thickness for each material. We have developed a simple nonequilibrium thermodynamics-based model predicting that the power-law prefactor is proportional to both the ratios of in-plane-tearing/out-of-plane-peeling energies and in-plane/out-of-plane moduli. By comparing the experimental data with the modulus ratio calculated from first-principles, we find close agreement between experiment and theory. This supports our hypothesis that energy equipartition holds between nanosheet tearing and peeling during sonication-assisted exfoliation., Accepted Manuscript (30 pages) and Supporting Information (68 pages)

Alternative solutions need to be developed to overcome the growing problem of multi-drug resistant bacteria. This study explored the possibility of creating complexes of antibiotics with metal ions, thereby increasing their activity. Analytical techniques such as isothermal titration calorimetry and nuclear magnetic resonance were used to examine the structure and interactions between Cu(II), Ag(I) or Zn(II) and β-lactam antibiotics. The metal-β-lactam complexes were also tested for antimicrobial activity, by micro-broth dilution and disk diffusion methods, showing a synergistic increase in the activity of the drugs, and enzymatic inhibition assays confirming inhibition of β-lactamases responsible for resistance. The metal-antibiotic complex concept was proven to be successful with the activity of the drugs enhanced against β-lactamase-producing bacteria. The highest synergistic effects were observed for complexes formed with Ag(I).

Journal of materials chemistry / A 8(42), 22401 - 22415 (2020). doi:10.1039/D0TA07889A, The layered compound Ni$_{2}$P$_{2}$S$_{6}$ was electrochemically characterized for application as an anode material in sodium-ion batteries (SIBs). A high reversible capacity of 621 mA h g$^{���1}$ at 1 A g$^{���1}$ was achieved after 190 cycles. The investigation of the complex reaction mechanism of the conversion reaction was performed applying complementary techniques including X-ray powder diffraction, pair distribution function analysis, X-ray absorption spectroscopy, $^{19}$F/$^{23}$Na/$^{31}$P MAS NMR, TEM and nano-EDX. The results highlight that Na uptake for up to 5 Na per formula unit (f.u.) led to reduction of Ni$^{+2}$ to metallic Ni nanoparticles and concomitant formation of an intermediate compound Na$_4$P$_2$S$_6$. Increasing the Na content to 12 Na per f.u. generates nanocrystalline Na$_2$S, which is accompanied by the loss of the long-range order of the pristine sample. In the completely discharged state elemental Ni and Na$_2$S are present, but in contrast to literature reports, no evidence for the formation of NaxP phases was found. During the charge process, Ni$_3$S$_2$ is formed upon the release of ���11.7 Na per f.u., Published by RSC, London ��[u.a.]��

Because of their reduced dimensionality, two-dimensional materials show intriguing optical properties and strong light-matter interaction. In particular, group VI transition metal dichalcogenides have been extensively studied and proof-of-principle optical applications have been demonstrated. Most studies to date focus on individual mono- or bilayered micromechanically exfoliated samples, which often display significant variations between flakes. In this work, we study size-dependent optical properties of four group VI TMD materials: WS2, MoS2, WSe2, and MoSe2, each consisting of ensembles of nanosheets suspended in the liquid environment. Samples were produced by liquid-phase exfoliation and size-selected using cascade centrifugation with size and layer number distributions quantified by statistical atomic force microscopy. Differences in lateral size and layer number are reflected in systematic changes in the optical extinction and absorbance spectra, which we exploit to establish quantitative spectroscopic metrics to facilitate the measurement of nanosheet dimensions for each of the four materials. The lowest energy resonance, referred to as A-exciton, is analyzed in more detail. In all cases, an exponential red shift with increasing layer number is observed. Our experimental data, backed up with first-principle calculations, reveal that the magnitude of the shift is dependent on the molecular mass of the central metal atom (W, Mo), while the rate at which the peak shifts from monolayer to bulk depends on the band gap of the semiconductor.

Transition metal dichalcogenides (TMDSs) are a diverse source of semiconducting 2D materials interesting for a number of application areas. Group VI-TMDs are direct bandgap semiconductors in the monolayer form making them attractive in optoelectronics due the possibility of efficient light emission. Their optical properties are dominated by tightly bound excitons with typical binding energies of 100s of meV – significantly larger than in bulk materials or traditional inorganic semiconductors. The physics of these excitons is of both fundamental interest and importance for exploiting these properties in applications and has been explored by linear and nonlinear optical techniques typically on micromechanically cleaved nanosheets. In this talk I will review our progress on investigating the exciton physics of layered materials exfoliated in the liquid phase. Liquid phase exfoliation has become a popular technique to obtain nanosheets in the form of colloidally stable dispersions suitable for solution processing. It’s an extremely versatile technique that can be applied to a whole host of layered crystals. While dispersions of nanosheets are usually polydisperse containing broad distributions of lateral sizes and thicknesses, they can nonetheless be used to study fundamental physical properties once the distributions are quantified by statistical microscopy. The advantage of the technique is the broad applicability and characterisation of an ensemble eliminating potential variations across different nanosheets. In addition, environmental effects can be conveniently studied by exchanging the liquid environment.

Liquid exfoliation has become an important production technique to give access to large quantities of two-dimensional nanosheets in colloidal dispersion.[1] Importantly, this is a highly versatile technique that can be applied to numerous layered materials beyond graphene such as transition metal dichalcogenides, III-VI semiconductors, black phosphorus, layered oxides to name just a few. These can be cast into films and composites and have proven useful in a number of application areas. However, as produced dispersions contain a nanosheets of various sizes and thicknesses making post-exfoliation size selection extremely important to tap the full potential in the future. We have previously developed a universal and efficient size selection technique termed liquid cascade centrifugation.[2] Optimisation of this centrifugation procedure allowed us to produce monolayer-rich dispersions with monolayer volume fractions up to 75%. This was made possible because we have realised that nanosheet size and thickness, as well as monolayer content can be quantitatively extracted from for example optical extinction spectra due to edge and confinement effects greatly facilitating and accelerating the characterisation.[2,3] We now use this basic understanding and these improvements in sample quality to systematically investigate the effect of the chemical environment on the optical properties of the liquid-exfoliated nanosheets. For example, we show that the A-exciton energy depends on the layer number in a similar way for a number of different TMDs. Furthermore, dielectric screening from additives and solvents can be tracked via this A-exciton layer number dependence making this an ideal fingerprint for noncovalent functionalisation.[4] Last but not least, we show that the ability to dramatically increase the surface area of the layered materials on exfoliation can be used to explore new chemistry and to controllably functionalise either basal plane [5,6] or edge sites. [1] Bonaccorso, F., Bartolotta, A.; Coleman, J.N., Backes, C., Adv. Mater. 2016, 28, 6136-6166. [2] Backes, C.; Szydłowska, B.M.; … Coleman, J. N., ACS Nano 2016, 10, 1589-1601. [3] Backes, C.; Smith, R. J.; … Coleman, J. N., Nature Communications 2014, 5, 4576. [4] Vega-Mayoral, V.; Backes, C.; …. Coleman, J. N., Adv. Func. Mater. 2016, 7, 1028-1039. [5] Backes, C.; Berner, N. C.; …. McDonald, A., Angew. Ch. Int. Ed.. 2015, 54, 1-6. [6] Knirsch, K.; Berner, N. C.; …. Backes, C., ACS Nano 2015, 9, 6018-6030.

Group VI transition metal dichalcogenides (TMDs) are considered to be chemically widely inert, but recent reports point toward an oxidation of monolayered sheets in ambient conditions, due to defects. To date, the degradation of monolayered TMDs is only studied on individual, substrate-supported nanosheets with varying defect type and concentration, strain, and in an inhomogeneous environment. Here, degradation kinetics of WS2 nanosheet ensembles in the liquid phase are investigated through photoluminescence measurements, which selectively probe the monolayers. Monolayer-enriched WS2 dispersions are produced with varying lateral sizes in the two common surfactant stabilizers sodium cholate (SC) and sodium dodecyl sulfate (SDS). Well-defined degradation kinetics are observed, which enable the determination of activation energies of the degradation and decouple photoinduced and thermal degradation. The thermal degradation is slower than the photoinduced degradation and requires higher activation energy. Using SC as surfactant, it is sufficiently suppressed. The photoinduced degradation can be widely prevented through chemical passivation achieved through the addition of cysteine which, on the one hand, coordinates to defects on the nanosheets and, on the other hand, stabilizes oxides on the surface, which shield the nanosheets from further degradation.

The family of antiferromagnetic layered metal hexathiohypo diphosphates, M2P2S6 represents a versatile class of materials, particularly interesting for fundamental studies on magnetic properties in low dimensional structures, and yet exhibiting great potential for a broad variety of applications including catalysis, energy storage and conversion, and spintronics. In this work, three representatives of this family of 2D materials (M = Fe, Ni, and Mn) are exfoliated in the liquid phase under inert conditions and the nanosheet’s properties are studied in detail for different sizes of all three compounds. Centrifugation-based size selection is performed for this purpose. The exfoliability and structural integrity of the nanosheets is studied by statistical atomic force microscopy and transmission electron microscopy measurements. Further, we report size and thickness dependent optical properties and spectroscopic metrics for the average material dimensions in dispersion, as well as the nanomaterials’ magnetic response using a combination of cryo-Raman and superconducting quantum interference device measurements. Finally, the material stability is studied semi-quantitatively, using time and temperature dependent extinction and absorbance spectroscopy, enabling the determination of the materials’ half-life, portion of reacted substance and the macroscopic activation energy for the degradation.