Inert Liquid Exfoliation and Langmuir-Type Thin Film Deposition of Semimetallic Metal Diborides.

Graphite is one of only a few layered materials that can be exfoliated into nanosheets with semimetallic properties, which limits the applications of nanosheet-based electrodes to material combinations compatible with the work function of graphene. It is therefore important to identify additional metallic or semimetallic two-dimensional (2D) nanomaterials that can be processed in solution for scalable fabrication of printed electronic devices. Metal diborides represent a family of layered non-van der Waals crystals with semimetallic properties for all nanosheet thicknesses. While previous reports show that the exfoliated nanomaterial is prone to oxidation, we demonstrate a readily accessible inert exfoliation process to produce quasi-2D nanoplatelets with intrinsic material properties. For this purpose, we demonstrate the exfoliation of three representative metal diborides (MgB2, CrB2, and ZrB2) under inert conditions. Nanomaterial is characterized using a combination of transmission electron microscopy, scanning electron microscopy, atomic force microscopy, IR, and UV-vis measurements, with only minimal oxidation indicated postprocessing. By depositing the pristine metal diboride nanoplatelets as thin films using a Langmuir-type deposition technique, the ohmic behavior of the networks is validated. Furthermore, the material decomposition is studied by using a combination of electrical and optical measurements after controlled exposure to ambient conditions. Finally, we report an efficient, low-cost approach for sample encapsulation to protect the nanomaterials from oxidation. This is used to demonstrate low-gauge factor strain sensors, confirming metal diboride nanosheets as a suitable alternative to graphene for electrode materials in printed electronics.

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.

Solution-Processed NiPS3 Thin Films from Liquid Exfoliated Inks with Long-Lived Spin-Entangled Excitons.

Antiferromagnets are promising materials for future opto-spintronic applications since they show spin dynamics in the THz range and no net magnetization. Recently, layered van der Waals (vdW) antiferromagnets have been reported, which combine low-dimensional excitonic properties with complex spin-structure. While various methods for the fabrication of vdW 2D crystals exist, formation of large area and continuous thin films is challenging because of either limited scalability, synthetic complexity, or low opto-spintronic quality of the final material. Here, we fabricate centimeter-scale thin films of the van der Waals 2D antiferromagnetic material NiPS3, which we prepare using a crystal ink made from liquid phase exfoliation (LPE). We perform statistical atomic force microscopy (AFM) and scanning electron microscopy (SEM) to characterize and control the lateral size and number of layers through this ink-based fabrication. Using ultrafast optical spectroscopy at cryogenic temperatures, we resolve the dynamics of photoexcited excitons. We find antiferromagnetic spin arrangement and spin-entangled Zhang-Rice multiplet excitons with lifetimes in the nanosecond range, as well as ultranarrow emission line widths, despite the disordered nature of our films. Thus, our findings demonstrate scalable thin-film fabrication of high-quality NiPS3, which is crucial for translating this 2D antiferromagnetic material into spintronic and nanoscale memory devices and further exploring its complex spin-light coupled states.

Improving the conductivity of graphite-based films by rapid laser annealing.

We present a method to anneal devices based on graphite films on paper and polycarbonate substrates. The devices are created using four different methods: spray-on films, graphite pencil-drawn films, liquid-phase exfoliated graphite films, and graphite powder abrasion-applied films. We characterize the optical properties of the films before and after laser annealing and report the two-terminal resistance of the devices for increased laser power density. We find the greatest improvement (16× reduction) in the resistance of spray-on film devices starting from 25.0 kΩ and reaching 1.6 kΩ at the highest annealing power densities. These improvements are attributed to local laser ablation of binders, stabilizers, and solvent residues left in the film after fabrication. This work highlights the utility of focused laser annealing for spray-on, drawn, printed, and abrasion fabricated films on substrates sensitive to heat/thermal treatments.

Comparative study of sonication-assisted liquid phase exfoliation of six layered coordination polymers.

Sonication-assisted liquid phase exfoliation was applied to six different layered coordination polymers (CPs) in aqueous surfactant solution. The resulting nanosheets were investigated for structural and compositional integrity and microscopic analysis gives insights into the relationship between the crystal structure of the materials and their exfoliability. Larger open pores seem to favour the production of nanosheets with higher aspect ratio of lateral size to thickness.

Understanding the Photoluminescence Quenching of Liquid Exfoliated WS2 Monolayers.

Monolayer transition metal dichalcogenides (TMDs) are being investigated as active materials in optoelectronic devices due to their strong excitonic effects. While mechanical exfoliation (ME) of monolayer TMDs is limited to small areas, these materials can also be exfoliated from their parent layered materials via high-volume liquid phase exfoliation (LPE). However, it is currently considered that LPE-synthesized materials show poor optoelectronic performance compared to ME materials, such as poor photoluminescence quantum efficiencies (PLQEs). Here we evaluate the photophysical properties of monolayer-enriched LPE WS2 dispersions via steady-state and time-resolved optical spectroscopy and benchmark these materials against untreated and chemically treated ME WS2 monolayers. We show that the LPE materials show features of high-quality semiconducting materials such as very small Stokes shift, smaller photoluminescence line widths, and longer exciton lifetimes than ME WS2. We reveal that the energy transfer between the direct-gap monolayers and in-direct gap few-layers in LPE WS2 dispersions is a major reason for their quenched PL. Our results suggest that LPE TMDs are not inherently highly defective and could have a high potential for optoelectronic device applications if improved strategies to purify the LPE materials and reduce aggregation could be implemented.

Liquid Phase Exfoliation of Rubrene Single Crystals into Nanorods and Nanobelts.

Liquid phase exfoliation (LPE) is a popular method to create dispersions of two-dimensional nanosheets from layered inorganic van der Waals crystals. Here, it is applied to orthorhombic and triclinic single crystals of the organic semiconductor rubrene with only noncovalent interactions (mainly π-π) between the molecules. Distinct nanorods and nanobelts of rubrene are formed, stabilized against aggregation in aqueous sodium cholate solution, and isolated by liquid cascade centrifugation. Selected-area electron diffraction and Raman spectroscopy confirm the crystallinity of the rubrene nanorods and nanobelts while the optical properties (absorbance, photoluminescence) of the dispersions are similar to rubrene solutions due to their randomized orientations. The formation of these stable crystalline rubrene nanostructures with only a few molecular layers by LPE confirms that noncovalent interactions in molecular crystals can be strong enough to enable mechanical exfoliation similar to inorganic layered materials.

Specifics of Thermal Transport in Graphene Composites: Effect of Lateral Dimensions of Graphene Fillers.

We report on the investigation of thermal transport in noncured silicone composites with graphene fillers of different lateral dimensions. Graphene fillers are comprised of few-layer graphene flakes with lateral sizes in the range from 400 to 1200 nm and the number of atomic planes from 1 to ∼100. The distribution of the lateral dimensions and thicknesses of graphene fillers has been determined via atomic force microscopy statistics. It was found that in the examined range of the lateral dimensions, the thermal conductivity of the composites increases with increasing size of the graphene fillers. The observed difference in thermal properties can be related to the average gray phonon mean free path in graphene, which has been estimated to be around ∼800 nm at room temperature. The thermal contact resistance of composites with graphene fillers of 1200 nm lateral dimensions was also smaller than that of composites with graphene fillers of 400 nm lateral dimensions. The effects of the filler loading fraction and the filler size on the thermal conductivity of the composites were rationalized within the Kanari model. The obtained results are important for the optimization of graphene fillers for applications in thermal interface materials for heat removal from high-power-density electronics.

The Role of Additives in Suppressing the Degradation of Liquid-Exfoliated WS2 Monolayers.

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.

Impact of Pretreatment of the Bulk Starting Material on the Efficiency of Liquid Phase Exfoliation of WS2.

Liquid phase exfoliation (LPE) is widely used to produce colloidal dispersions of nanomaterials, in particular two-dimensional nanosheets. The degree of exfoliation, i.e., the length to thickness aspect ratio was shown to be intrinsically limited by the ratio of in-plane to out-of-plane binding strength. In this work, we investigate whether simple pretreatment of the starting material can be used to change the in-plane to out-of-plane binding strength through mild intercalation to improve the sample quality in sonication-assisted LPE. Five different pretreatment conditions of WS2 were tested and the dispersions size-selected through centrifugation. From optical spectroscopy (extinction, Raman, photoluminescence), information on nanosheet dimension (average lateral size, layer number, monolayer size) and optical quality (relative photoluminescence quantum yield) was extracted. We find that the pretreatment has a minor impact on the length/thickness aspect ratio, but that photoluminescence quantum yield can be increased in particular using mild sonication conditions. We attribute this to the successful exfoliation of nanosheets with a lower degree of basal plane defectiveness. This work emphasizes the complexity of the exfoliation process and suggests that the role of defects has to be considered for a comprehensive picture.

Production of Quasi-2D Platelets of Nonlayered Iron Pyrite (FeS2) by Liquid-Phase Exfoliation for High Performance Battery Electrodes.

Over the past 15 years, two-dimensional (2D) materials have been studied and exploited for many applications. In many cases, 2D materials are formed by the exfoliation of layered crystals such as transition-metal disulfides. However, it has recently become clear that it is possible to exfoliate nonlayered materials so long as they have a nonisotropic bonding arrangement. Here, we report the synthesis of 2D-platelets from the earth-abundant, nonlayered metal sulfide, iron pyrite (FeS2), using liquid-phase exfoliation. The resultant 2D platelets exhibit the same crystal structure as bulk pyrite but are surface passivated with a density of 14 × 1018 groups/m2. They form stable suspensions in common solvents and can be size-selected and liquid processed. Although the platelets have relatively low aspect ratios (∼5), this is in line with the anisotropic cleavage energy of bulk FeS2. We observe size-dependent changes to optical properties leading to spectroscopic metrics that can be used to estimate the dimensions of platelets. These platelets can be used to produce lithium ion battery anodes with capacities approaching 1000 mAh/g.

Mechanochromic and Thermochromic Sensors Based on Graphene Infused Polymer Opals.

High quality opal-like photonic crystals containing graphene are fabricated using evaporation-driven self-assembly of soft polymer colloids. A miniscule amount of pristine graphene within a colloidal crystal lattice results in the formation of colloidal crystals with a strong angle-dependent structural color and a stop band that can be reversibly shifted across the visible spectrum. The crystals can be mechanically deformed or can reversibly change color as a function of their temperature, hence their sensitive mechanochromic and thermochromic response make them attractive candidates for a wide range of visual sensing applications. In particular, it is shown that the crystals are excellent candidates for visual strain sensors or integrated time-temperature indicators which act over large temperature windows. Given the versatility of these crystals, this method represents a simple, inexpensive, and scalable approach to produce multifunctional graphene infused synthetic opals and opens up exciting applications for novel solution-processable nanomaterial based photonics.

Synthesis and Exfoliation of a New Layered Mesoporous Zr-MOF Comprising Hexa- and Dodecanuclear Clusters as Well as a Small Organic Linker Molecule.

A new layered mesoporous Zr-MOF of composition [Zr30O20(OH)26(OAc)18L18] was synthesized by employing 5-acetamidoisophthalic acid (H2L) using acetic acid as the solvent. The new MOF, denoted as CAU-45, exhibits a honeycomb structure of stacked layers which comprise both hexa- and dodecanucelar zirconium clusters. Its structure was solved from submicrometer-sized crystals by continuous rotation electron diffraction (cRED). Liquid phase exfoliation and size selection were successfully performed on the material.

Mechanisms of Liquid-Phase Exfoliation for the Production of Graphene.

Liquid- phase exfoliation (LPE) is the principal method of producing two-dimensional (2D) materials such as graphene in large quantities with a good balance between quality and cost and is now widely adopted by both the academic and industrial sectors. The fragmentation and exfoliation mechanisms involved have usually been simply attributed to the force induced by ultrasound and the interaction with the solvent molecules. Nonetheless, little is known about how they actually occur, i.e., how thick and large graphite crystals can be exfoliated into thin and small graphene flakes. Here, we demonstrate that during ultrasonic LPE the transition from graphite flakes to graphene takes place in three distinct stages. First, sonication leads to the rupture of large flakes and the formation of kink band striations on the flake surfaces, primarily along zigzag directions. Second, cracks form along these striations, and together with intercalation of solvent, lead to the unzipping and peeling off of thin graphite strips that in the final stage are exfoliated into graphene. The findings will be of great value in the quest to optimize the lateral dimensions, thickness, and yield of graphene and other 2D materials in large-scale LPE for various applications.