Liquid phase exfoliation and size dependent properties of van der Waals crystals

Van der Waals crystals exhibit comparatively strong, typically covalent bonds in two dimensions and comparatively weak, typically non-covalent bonds between the two-dimensional lattice. This enables to separate individual two-dimensional layers of a van der Waals crystal which can be thinned down to atomic thickness in a process called exfoliation. The resulting nanosheets typically exhibit completely different properties compared to their corresponding bulk counterparts which can be exploited for various applications in advanced devices. Different methods have been presented for preparation of two-dimensional nanomaterials each with their respective up- and downsides. While some techniques can provide materials of highest quality, suitable for fundamental studies of inherent material properties, they typically lack scalability. Other methods focus on a high production rate of the nanomaterial, but introduce imperfections to the material due to the harsh conditions required. In recent years, exfoliation in the liquid phase has emerged to a widely used production technique due to the scalability and its wide applicability. While the industrial relevance of two-dimensional nanomaterials is somewhat linked to the quality of the material that can be prepared by high throughput methods, a deeper understanding of underlying fundamentals for the nanosheet preparation is required to improve state-of-the-art techniques. In the case of liquid-exfoliated nanosheets, this can be achieved by statistical studies of the nanomaterial dimensions that can be prepared and isolated by size selection techniques. In this work, sonication-assisted liquid phase exfoliation using different conditions and solvents and subsequent size selection was applied to a total of 17 different van der Waals crystals. The material dimensions of all fractions were quantified through statistical atomic force microscopy. The findings presented in this work demonstrate a fundamental correlation between the nanomaterial lateral size and thickness which is ascribed to equipartition of energy between processes of nanosheet delamination and tearing. This provides an experimental proxy to determine the ratio between the in-plane binding strength and the out-of-plane interlayer attraction. Isolation of different size-fractions of the same material and the knowledge over the nanomaterial dimensions in these fractions enables to study size-dependent changes of material properties in a quantitative manner. Measurements of optical properties on different sizes of dispersed nanosheets reveal systematic changes of the spectra with nanosheet size and enable to de-rive spectroscopic metrics for the size, thickness and concentration for different van der Waals nanomaterials, typically using extinction and absorbance spectroscopy. Structurally and compositionally different materials show similar changes in their optical response with changing material size which can be ascribed to a combination of confinement and dielectric screening effects, as well as changing contributions form scattering and electronically different material edges. Unifying principles across various materials were identified for the changes of the optical spectra with material dimensions. The knowledge of material dimensions and the understanding of the optical spectra enables to study the stability of different nanomaterial systems as function of time using optical spectroscopy such as extinction, absorbance or photoluminescence. A dependence of the speed and degree of the material decomposition on the storage temperature and the water content of the solvent is conveniently accessible for different material dimensions. The results presented within this work provide an advanced understanding of the exfoliation of layered crystals, unifying principles of optical properties as function of nanomaterial dimensions and proof-of-concept experiments for quantification of the material decomposition

Sonication-assisted liquid phase exfoliation of two-dimensional CrTe3 under inert conditions

Liquid phase exfoliation (LPE) has been used for the successful fabrication of nanosheets from a large number of van der Waals materials. While this allows to study fundamental changes of material properties’ associated with reduced dimensions, it also changes the chemistry of many materials due to a significant increase of the effective surface area, often accompanied with enhanced reactivity and accelerated oxidation. To prevent material decomposition, LPE and processing in inert atmosphere have been developed, which enables the preparation of pristine nanomaterials, and to systematically study compositional changes over time for different storage conditions. Here, we demonstrate the inert exfoliation of the oxidation-sensitive van der Waals crystal, CrTe3. The pristine nanomaterial was purified and size-selected by centrifugation, nanosheet dimensions in the fractions quantified by atomic force microscopy and studied by Raman, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and photo spectroscopic measurements. We find a dependence of the relative intensities of the CrTe3 Raman modes on the propagation direction of the incident light, which prevents a correlation of the Raman spectral profile to the nanosheet dimensions. XPS and EDX reveal that the contribution of surface oxides to the spectra is reduced after exfoliation compared to the bulk material. Further, the decomposition mechanism of the nanosheets was studied by time-dependent extinction measurements after water titration experiments to initially dry solvents, which suggest that water plays a significant role in the material decomposition

3D-imaging of Printed Nanostructured Networks using High-resolution FIB-SEM Nanotomography

Networks of solution-processed nanomaterials are important for multiple applications in electronics, sensing and energy storage/generation. While it is known that network morphology plays a dominant role in determining the physical properties of printed networks, it remains difficult to quantify network structure. Here, we utilise FIB-SEM nanotomography to characterise the morphology of nanostructured networks. Nanometer-resolution 3D-images were obtained from printed networks of graphene nanosheets of various sizes, as well as networks of WS2 nanosheets, silver nanosheets and silver nanowires. Important morphological characteristics, including network porosity, tortuosity, pore dimensions and nanosheet orientation were extracted and linked 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.Comment: 6 figure

Highly conductive and long-term stable films from liquid-phase exfoliated platinum diselenide

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

Quantifying the contribution of material and junction resistances in nano-networks

Networks of nanowires and nanosheets are important for many applications in printed electronics. However, the network conductivity and mobility are usually limited by the inter-particle junction resistance, a property that is challenging to minimise because it is difficult to measure. Here, we develop a simple model for conduction in networks of 1D or 2D nanomaterials, which allows us to extract junction and nanoparticle resistances from particle-size-dependent D.C. resistivity data of conducting and semiconducting materials. We find junction resistances in porous networks to scale with nanoparticle resistivity and vary from 5 Ohm for silver nanosheets to 25 GOhm for WS2 nanosheets. Moreover, our model allows junction and nanoparticle resistances to be extracted from A.C. impedance spectra of semiconducting networks. Impedance data links the high mobility (~7 cm2/Vs) of aligned networks of electrochemically exfoliated MoS2 nanosheets to low junction resistances of ~670 kOhm. Temperature-dependent impedance measurements allow us to quantitatively differentiate intra-nanosheet phonon-limited band-like transport from inter-nanosheet hopping for the first time.Comment: 5 figure

Enhancement of antibiotic-activity through complexation with metal ions - Combined ITC, NMR, enzymatic and biological studies

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)

Enriching and Quantifying Porous Single Layer 2D Polymers by Exfoliation of Chemically Modified van der Waals Crystals

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.ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

What happens structurally and chemically during sodium uptake and release by Ni2_{2}P2_{2}S6_{6} : a combined X-ray diffraction, X-ray absorption, pair distribution function and MAS NMR analysis

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