Solution-Based Processing of Monodisperse Two-Dimensional Nanomaterials

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.

Microfluidics for Two-Dimensional Nanosheets: A Mini Review

Since the discovery of graphene, there has been increasing interest in two-dimensional (2D) materials. To realize practical applications of 2D materials, it is essential to isolate mono- or few-layered 2D nanosheets from unexfoliated counterparts. Liquid phase exfoliation (LPE) is the most common technique to produce atomically thin-layered 2D nanosheets. However, low production yield and prolonged process time remain key challenges. Recently, novel exfoliation processes based on microfluidics have been developed to achieve rapid and high yield production of few-layer 2D nanosheets. We review the primary types of microfluidic-based exfoliation techniques in terms of the underlying process mechanisms and the applications of the 2D nanosheets thus produced. The key challenges and future directions are discussed in the above context to delineate future research directions in this exciting area of materials processing.

Site-Selective Oxidation of Monolayered Liquid-Exfoliated WS2 by Shielding the Basal Plane through Adsorption of a Facial Amphiphile

In recent years, various functionalization strategies for transition‐metal dichalcogenides have been explored to tailor the properties of materials and to provide anchor points for the fabrication of hybrid structures. Herein, new insights into the role of the surfactant in functionalization reactions are described. Using the spontaneous reaction of WS₂ with chloroauric acid as a model reaction, the regioselective formation of gold nanoparticles on WS₂ is shown to be heavily dependent on the surfactant employed. A simple model is developed to explain the role of the chosen surfactant in this heterogeneous functionalization reaction. The surfactant coverage is identified as the crucial element that governs the dominant reaction pathway and therefore can severely alter the reaction outcome. This study shows the general importance of the surfactant choice and how detrimental or beneficial a certain surfactant can be to the desired functionalization.

Recent developments in carbon-based two-dimensional materials: synthesis and modification aspects for electrochemical sensors

This review (162 references) focuses on two-dimensional carbon materials, which include graphene as well as its allotropes varying in size, number of layers, and defects, for their application in electrochemical sensors. Many preparation methods are known to yield two-dimensional carbon materials which are often simply addressed as graphene, but which show huge variations in their physical and chemical properties and therefore on their sensing performance. The first section briefly reviews the most promising as well as the latest achievements in graphene synthesis based on growth and delamination techniques, such as chemical vapor deposition, liquid phase exfoliation via sonication or mechanical forces, as well as oxidative procedures ranging from chemical to electrochemical exfoliation. Two-dimensional carbon materials are highly attractive to be integrated in a wide field of sensing applications. Here, graphene is examined as recognition layer in electrochemical sensors like field-effect transistors, chemiresistors, impedance-based devices as well as voltammetric and amperometric sensors. The sensor performance is evaluated from the material's perspective of view and revealed the impact of structure and defects of the 2D carbon materials in different transducing technologies. It is concluded that the performance of 2D carbon-based sensors is strongly related to the preparation method in combination with the electrical transduction technique. Future perspectives address challenges to transfer 2D carbon-based sensors from the lab to the market. Graphical abstract Schematic overview from synthesis and modification of two-dimensional carbon materials to sensor application.

Neuromorphic nanoelectronic materials

Memristive and nanoionic devices have recently emerged as leading candidates for neuromorphic computing architectures. While top-down fabrication based on conventional bulk materials has enabled many early neuromorphic devices and circuits, bottom-up approaches based on low-dimensional nanomaterials have shown novel device functionality that often better mimics a biological neuron. In addition, the chemical, structural and compositional tunability of low-dimensional nanomaterials coupled with the permutational flexibility enabled by van der Waals heterostructures offers significant opportunities for artificial neural networks. In this Review, we present a critical survey of emerging neuromorphic devices and architectures enabled by quantum dots, metal nanoparticles, polymers, nanotubes, nanowires, two-dimensional layered materials and van der Waals heterojunctions with a particular emphasis on bio-inspired device responses that are uniquely enabled by low-dimensional topology, quantum confinement and interfaces. We also provide a forward-looking perspective on the opportunities and challenges of neuromorphic nanoelectronic materials in comparison with more mature technologies based on traditional bulk electronic materials.

Strain-Engineered Anisotropic Optical and Electrical Properties in 2D Chiral-Chain Tellurium

Atomically thin materials, leveraging their low-dimensional geometries and superior mechanical properties, are amenable to exquisite strain manipulation with a broad tunability inaccessible to bulk or thin-film materials. Such capability offers unexplored possibilities for probing intriguing physics and materials science in the 2D limit as well as enabling unprecedented device applications. Here, the strain-engineered anisotropic optical and electrical properties in solution-grown, sub-millimeter-size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral-chain lattice. The observed Raman spectra reveal anisotropic lattice vibrations under the corresponding straining conditions. The feasibility of using buckled 2D Te for ultrastretchable strain sensors with a high gauge factor (≈380) is further explored. 2D Te is an emerging material boasting attractive characteristics for electronics, sensors, quantum devices, and optoelectronics. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain-engineered functionalities.

Fluid-Assisted Sorted Assembly of Graphene on Polymer

The significant size distribution of as-synthesized nanomaterials presents a challenge for reproducable and reliable applications. In this paper, we report a fluidic-assisted sorted assembly method in which nanomaterial sorting and enhanced assembly can be achieved simultaneously. As a proof of concept, a two-dimensional (2D) graphene flake, with a large size variation, was chosen as the target nanomaterial system. This study synergizes a novel fluidic assembly design, suspending a rotating disk over a polydimethylsiloxane (PDMS) substrate, and a computational fluid dynamics (CFD) model using Ansys CFX to disclose the mechanism of sorted assembly. By controlling the rotating speed and the gap between the disk and the substrate, the flow field is altered. In contrast to centrifugal sorting, where larger particles move outward, in this study, the size of assembled graphene flake (average lateral size, X_c) reduces significantly from the center (X_c = 3 μm) to the edge of the disk (X_c = 2 μm). The particle sorting process is dictated by the fluid shear-stress, with higher shear-stress leading to smaller particles, while the assembly process is mainly dominated by the pressure field with higher pressure magnitude leading to better assembly. Near the edge of the disk, enhanced particle sorting is coupled with an enhanced assembly where a continuous graphene film with smaller X_c can be formed. To prove the potential application of this method, an ultrasensitive strain sensor with one of the lowest detection limits, 0.02%, is demonstrated. This research presents a novel route toward large-scale and cost-effective manufacturing of nanomaterial-based flexible electronics.

MXene Printing and Patterned Coating for Device Applications

As a thriving member of the 2D nanomaterials family, MXenes, i.e., transition metal carbides, nitrides, and carbonitrides, exhibit outstanding electrochemical, electronic, optical, and mechanical properties. They have been exploited in many applications including energy storage, electronics, optoelectronics, biomedicine, sensors, and catalysis. Compared to other 2D materials, MXenes possess a unique set of properties such as high metallic conductivity, excellent dispersion quality, negative surface charge, and hydrophilicity, making them particularly suitable as inks for printing applications. Printing and pre/post-patterned coating methods represent a whole range of simple, economically efficient, versatile, and eco-friendly manufacturing techniques for devices based on MXenes. Moreover, printing can allow for complex 3D architectures and multifunctionality that are highly required in various applications. By means of printing and patterned coating, the performance and application range of MXenes can be dramatically increased through careful patterning in three dimensions; thus, printing/coating is not only a device fabrication tool but also an enabling tool for new applications as well as for industrialization.

Engineering Two-dimensional Nanomaterials to Enable Structure-Activity Relationship Studies in Nanosafety Research

Emerging, two-dimensional engineered nanomaterials (2DNMs) possess unique and diverse physical and chemical properties, such as extreme aspect ratios, adjustable electronic properties as well as functional lattice defects and surface chemistry which underpin their interactions with biological systems. This perspective highlights the need for structure activity relationship (SAR) studies for key properties of emerging grapheme-related and inorganic 2DNMs upon prioritization based on their potential impact and trajectory for large-scale production and applications. Further, it is discussed how a synthesis platform of microbiologically sterile, size-sorted, "model" 2DNMs with precise structure would enable SAR toxicological studies and allow for the sustainable and safe translation of 2D nanotechnology to real-world applications.

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.

Applications of Two-Dimensional Nanomaterials in Breast Cancer Theranostics

Breast cancer is the leading cause of cancer-related mortality among women. Early stage diagnosis and treatment of this cancer are crucial to patients' survival. In addition, it is important to avoid severe side effects during the process of conventional treatments (surgery, chemotherapy, hormonal therapy, and targeted therapy) and increase the patients' quality of life. Over the past decade, nanomaterials of all kinds have shown excellent prospects in different aspects of oncology. Among them, two-dimensional (2D) nanomaterials are unique due to their physical and chemical properties. The functional variability of 2D nanomaterials stems from their large specific surface area as well as the diversity of composition, electronic configurations, interlayer forces, surface functionalities, and charges. In this review, the current status of 2D nanomaterials in breast cancer diagnosis and therapy is reviewed. In this respect, sensing of the tumor biomarkers, imaging, therapy, and theranostics are discussed. The ever-growing 2D nanomaterials are building blocks for the development of a myriad of nanotheranostics. Accordingly, there is the possibility to explore yet novel properties, biological effects, and oncological applications.

Controlled polymer crystal/two-dimensional material heterostructures for high-performance photoelectronic applications

The control of atomically thin two-dimensional (2D) crystal-based heterostructures wherein the interfaces of 2D nanomaterials are vertically stacked with other thin functional materials via van der Waals interactions is highly important for not only optimizing the excellent properties of 2D nanomaterials, but also for utilizing the functionality of the contact materials. In particular, when 2D nanomaterials are combined with soft polymeric components, the resulting photoelectronic devices are potentially scalable and mechanically flexible, allowing the development of a variety of prototype soft-electronic devices, such as solar cells, displays, photodetectors, and non-volatile memory devices. Diverse polymer/2D heterostructures are frequently employed, but the performance of the devices with heterostructures is limited, mainly because of the difficulty in controlling the molecular structures of the polymers on the 2D surface. Thus, understanding the crystal interactions of polymers on atomically flat and dangling-bond-free surfaces of 2D materials is essential for ensuring high performance. In this study, the recent progress made in the development of thin polymer films fabricated on the surfaces of various 2D nanomaterials for high-performance photoelectronic devices is comprehensively reviewed, with an emphasis on the control of the molecular and crystalline structures of the polymers on the 2D surface.

Organic–inorganic hybrid perovskite electronics

Organic–inorganic hybrid perovskite is a leading successor for the next generation of (opto)electronics.

Elucidating Charge Transport Mechanisms in Cellulose-Stabilized Graphene Inks

Solution-processed graphene inks that use ethyl cellulose as a polymer stabilizer are blade-coated into large-area thin films. Following blade-coating, the graphene thin films are cured to pyrolyze the cellulosic polymer, leaving behind an sp2-rich amorphous carbon residue that serves as a binder in addition to facilitating charge transport between graphene flakes. Systematic charge transport measurements, including temperature-dependent Hall effect and non-contact microwave resonant cavity characterization, reveal that the resulting electrically percolating graphene thin films possess high mobility (≈ 160 cm2 V-1 s-1), low energy gap, and thermally activated charge transport, which develop weak localization behavior at cryogenic temperatures.

Size-Dependent Control of Exciton-Polariton Interactions in WS2 Nanotubes

Multiwall WS₂ nanotubes (and fullerene-like nanoparticles thereof) are currently synthesized in large amounts, reproducibly. Other than showing interesting mechanical and tribological properties, which offer them a myriad of applications, they are recently shown to exhibit remarkable optical and electrical properties, including quasi-1D superconductivity, electroluminescence, and a strong bulk photovoltaic effect. Here, it is shown that, using a simple dispersion-fractionation technique, one can control the diameter of the nanotubes and move from pure excitonic to polaritonic features. While nanotubes of an average diameter >80 nm can support cavity modes and scatter light effectively via a strong coupling mechanism, the extinction of nanotubes with smaller diameter consists of pure absorption. The experimental work is complemented by finite-difference time-domain simulations, which shed new light on the cavity mode-exciton interaction in 2D materials. Furthermore, transient absorption experiments of the size-fractionated nanotubes fully confirm the steady-state observations. Moreover, it is shown that the tools developed here are useful for size control of the nanotubes, e.g., in manufacturing environment. The tunability of the light-matter interaction of such nanotubes offers them intriguing applications such as polaritonic devices, in photocatalysis, and for multispectral sensors.

Electrochemical exfoliation from an industrial ingot: ultrathin metallic bismuth nanosheets for excellent CO2 capture and electrocatalytic conversion

Formic acid (or formate) is a liquid fuel and chemical feedstock, and it is considered as one of the most useful value-added reductive products from electrochemical CO2 conversion. Green metallic Bi nanosheets are believed be a promising candidate for formic acid production in CO2 electroreduction. However, the complexity of their preparation with a low yield hinders their practical application on a large scale. Herein, we report that by using a cheap and commonly used industrial ingot, phase-pure two-dimensional bismuth nanosheets are fabricated on a large scale by a rapid electrochemical cathodic exfoliation method. In addition to featuring abundant active sites, the obtained Bi nanosheets possess exceptionally high adsorption capacity to CO2 compared to its bulk counterpart, resulting in remarkable enhancement in CO2 electroreduction with high selectivity toward formic acid over a wide range of negative potentials, high current density and satisfactory durability. This facile strategy opens a promising avenue for massive fabrication of metallic Bi nanosheets with excellent electrocatalytic performance for large-scale commercial utilization of CO2.

Electronic and optoelectronic applications of solution-processed two-dimensional materials

The isolation of graphene in 2004 has initiated much interest in two-dimensional (2D) materials. With decades of development, solution processing of 2D materials has becoming very promising due to its large-scale production capability, and it is therefore necessary to examine progress on solution-processed 2D materials and their applications. In this review, we highlight recent advances in the assembly of solution-processed 2D materials into thin films and the use of them for electronics and optoelectronics. We first present an overview about typical approaches to assemble solution-processed 2D materials into desired structures, including layer-by-layer assembly, Langmuir-Blodgett assembly, spin coating, electrophoretic deposition, inkjet printing, and vacuum filtration. Then, electronic and optoelectronic applications of such assembly films are presented, such as thin-film transistors, transparent conductive films, mechanical and chemical sensors, photodetectors and optoelectronic devices, as well as flexible and printed electronics. Finally, our perspectives on challenges and future opportunities in this important field are proposed.

2D-Organic Hybrid Heterostructures for Optoelectronic Applications

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low-cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D-organic heterostructures-from their synthesis and fabrication to their state-of-the-art optoelectronic applications-is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

Layered photochromic films stacked from spiropyran-modified montmorillonite nanosheets

Two-dimensional (2D) nanosheets are a class of fascinating host material that demonstrates a high specific surface area for the immobilization of functional molecules. Herein, we describe a layered photochromic film using montmorillonite 2D nanosheets immobilized with spiropyran units, which demonstrates a remarkable and reversible photochromic behavior. The synthesis of the layered photochromic film includes the intercalation and exfoliation of montmorillonite powders into 2D nanosheets using a spiropyran-modified surfactant and a subsequent vacuum filtration. The photochromic units of spiropyran-modified quaternary ammonium groups are immobilized on the surface of montmorillonite 2D nanosheets through an electrostatic interaction after exchanging with the native cations in montmorillonite during the intercalation and exfoliation. The photoisomerization of the spiropyran units between closed-ring spiropyran and open-ring merocyanine upon visible/UV irradiation contributes to the photochromic behavior of the layered film. The color contrast between the coloration and decoloration states of photochromic film is optimized by increasing the amount of spiropyran-modified cationic surfactant during the intercalation and exfoliation process. Our layered films with a visual photochromic behavior may promote their applications for optical data storage, optical switching and chemical sensing.

Fully Inkjet-Printed, Mechanically Flexible MoS2 Nanosheet Photodetectors

Solution-processed two-dimensional materials offer a scalable route toward next-generation printed devices. In this report, we demonstrate fully inkjet-printed photodetectors using molybdenum disulfide (MoS₂) nanosheets as the active material and graphene as the electrodes. Percolating films of semiconducting MoS₂ with high electrical conductivity are achieved with an ethyl cellulose-based ink formulation. Two classes of photodetectors are fabricated, including thermally annealed devices on glass with fast photoresponse of 150 μs and photonically annealed devices on flexible polyimide with high photoresponsivity exceeding 50 mA/W. The photonically annealed photodetector also reduces the curing time to milliseconds and maintains functionality over 500 bending cycles.

Using light, X-rays and electrons for evaluation of the nanostructure of layered materials

As a case study for the evaluation of the nanostructure of layered materials, we report on results of the comprehensive characterization of high-energy ball-milled layered molybdenum disulfide (2H-MoS₂) on different length scales. Analysis of X-ray powder diffraction patterns (XRPDs) including the Debye background at low scattering angles caused by uncorrelated single or few-layer MoS₂ slabs (full scattering model), yield much more precise data about the average stacking degree than routine XRPD evaluation, and an estimation of the amount of single layer material is possible. Reflections with super Lorentzian line shape can be satisfactorily modeled assuming different stacking sequences induced by the mechanical forces exerted during the high-energy ball-mill process. An advanced analysis of UV-Vis spectra to determine layer number and lateral crystallite size, which was recently developed for liquid exfoliation materials, is used for the first time, and the results demonstrate the universal applicability of the approach. The data obtained with this analysis support the main findings of evaluation of the XRPD data. Both methods clearly evidence that increasing the duration of high-energy ball-mill treatment leads to an increase of material with decreasing average stacking and a reduction of the lateral size of the slabs. Finally, high-resolution transmission electron microscopy enabled identification of defects which can hardly be detected in XRPDs or in UV-Vis spectra.

Abrupt Thermal Shock of (NH4)2Mo3S13 Leads to Ultrafast Synthesis of Porous Ensembles of MoS2 Nanocrystals for High Gain Photodetectors

Ultrafast synthesis of high-quality transition-metal dichalcogenide nanocrystals, such as molybdenum disulfide (MoS₂), is technologically relevant for large-scale production of electronic and optoelectronic devices. Here, we report a rapid solid-state synthesis route for MoS₂ using the chemically homogeneous molecular precursor, (NH₄)₂Mo₃S₁₃·H₂O, resulting in nanoparticles with estimated size down to 25 nm only in 10 s at 1000 °C. Despite the extreme nonequilibrium conditions, the resulting porous MoS₂ nanoparticles remain aggregated to preserve the form of the original rod shape bulk morphology of the molecular precursor. This ultrafast synthesis proceeds through the rapid decomposition of the precursor and rearrangement of Mo and S atoms coupled with simultaneous efficient release of massive gaseous species, to create nanoscale porosity in the resulting isomorphic pseudocrystals, which are composed of the MoS₂ nanoparticles. Despite the very rapid escape of massive amounts of NH₃, H₂O, H₂S, and S gases from the (NH₄)₂Mo₃S₁₃·H₂O mm sized crystals, they retain their original shape as they convert to MoS₂ rather than undergo explosive destruction from the rapid escape process of the gases. The obtained pseudocrystals are made of aggregated MoS₂ nanocrystals exhibit a Brunauer-Emmett-Teller surface area of ∼35 m²/g with an adsorption average pore width of ∼160 Å. The nanoporous MoS₂ crystals are solution processable by dispersing in ethanol and water and can be cast into large-area uniform composite films. Photodetectors fabricated from these films show more than 2 orders of magnitude higher conductivity (∼6.25 × 10^-6 S/cm) and photoconductive gain (20 mA/W) than previous reports of MoS₂ composite films. The optoelectronic properties of this nanoporous MoS₂ imply that the shallow defects that originate from the ultrafast synthesis act as sensitizing centers that increase the photocurrent gain via two-level recombination kinetics.

Contact and Noncontact Measurement of Electronic Transport in Individual 2D SnS Colloidal Semiconductor Nanocrystals

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. While much progress has been made toward the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products' inherent electronic transport properties. This is often due to their irregular morphology or small dimensions, which demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements. Here, we report the synthesis of nearly monodisperse 2D colloidal nanocrystals of semiconductor SnS and a thorough investigation of the intrinsic electronic transport properties of single crystals. We utilize a combination of multipoint contact probe measurements and ultrafast terahertz spectroscopy to determine the carrier concentration, carrier mobility, conductivity/resistivity, and majority carrier type of individual colloidal semiconductor nanocrystals. Employing this metrological approach, we compare the electronic properties extracted for distinct morphologies of 2D SnS and relate them to literature values. Our results indicate that the electronic transport of colloidal semiconductors may be tuned through prudent selection of the synthetic conditions. We find that these properties compare favorably to SnS grown using vapor deposition techniques, illustrating that colloidal solution synthesis is a promising route to scalable production of nanoscale 2D materials.

Silicon-Phosphorene Nanocavity-Enhanced Optical Emission at Telecommunications Wavelengths

Generating and amplifying light in silicon (Si) continues to attract significant attention due to the possibility of integrating optical and electronic components in a single material platform. Unfortunately, silicon is an indirect band gap material and therefore an inefficient emitter of light. With the rise of integrated photonics, the search for silicon-based light sources has evolved from a scientific quest to a major technological bottleneck for scalable, CMOS-compatible, light sources. Recently, emerging two-dimensional materials have opened the prospect of tailoring material properties based on atomic layers. Few-layer phosphorene, which is isolated through exfoliation from black phosphorus (BP), is a great candidate to partner with silicon due to its layer-tunable direct band gap in the near-infrared where silicon is transparent. Here we demonstrate a hybrid silicon optical emitter composed of few-layer phosphorene nanomaterial flakes coupled to silicon photonic crystal resonators. We show single-mode emission in the telecommunications band of 1.55 μm ( E_g = 0.8 eV) under continuous wave optical excitation at room temperature. The solution-processed few-layer BP flakes enable tunable emission across a broad range of wavelengths and the simultaneous creation of multiple devices. Our work highlights the versatility of the Si-BP material platform for creating optically active devices in integrated silicon chips.

A black phosphorus based synergistic antibacterial platform against drug resistant bacteria

In the fight against pathogenic bacteria, traditional antibiotic therapy is challenged by low efficiency and drug resistance. These drawbacks motivate the development of synergistic antibacterial therapy, but there is a lack of efficient synergistic platforms. Herein, with methicillin-resistant Staphylococcus aureus (MRSA) as a pathogenic bacterial model, we explored the potential of black phosphorus (BP) as a synergistic therapeutic platform for drug resistant bacterial infection. Acting as a substrate, reductant and stabilizer, BP nanosheets were decorated with Ag nanoparticles (NP) through an in situ growth strategy. The photothermal effect of the BP nanosheets allows Ag@BP nanohybrids to rapidly disrupt a bacterial membrane under near infrared (NIR) light irradiation. Moreover, the slowly released Ag⁺ elevates oxidative stress and sustainably suppresses bacterial proliferation for a long time. The combination of these two aspects endows the Ag@BP nanohybrids with synergistically enhanced antibacterial performance. Different from traditional antibiotics, the antibacterial effects of the Ag@BP nanohybrids are independent of the bacterial structure, which bypasses the issue of drug resistance. The in vivo studies show that the Ag@BP nanohybrids efficiently decrease the MRSA bacterial burden in mice and minimize infection associated tissue lesions. Besides, the excellent biocompatibility of the Ag@BP nanohybrids guarantees their biosafety for future clinical applications. Accordingly, this work demonstrates the potential of the BP nanosheets in the synergistic antibacterial therapy against drug resistant bacteria, and paves the way for developing 2D semiconductor based synergistic antibacterial nanodrugs.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.

Solution-Based Processing of Optoelectronically Active Indium Selenide

Layered indium selenide (InSe) presents unique properties for high-performance electronic and optoelectronic device applications. However, efforts to process InSe using traditional liquid phase exfoliation methods based on surfactant-assisted aqueous dispersions or organic solvents with high boiling points compromise electronic properties due to residual surface contamination and chemical degradation. Here, these limitations are overcome by utilizing a surfactant-free, low boiling point, deoxygenated cosolvent system. The resulting InSe flakes and thin films possess minimal processing residues and are structurally and chemically pristine. When employed in photodetectors, individual InSe nanosheets exhibit a maximum photoresponsivity of ≈5 × 10⁷ A W^-1 , which is the highest value of any solution-processed monolithic semiconductor to date. Furthermore, the surfactant-free cosolvent system not only stabilizes InSe dispersions but is also amenable to the assembly of electronically percolating InSe flake arrays without posttreatment, which enables the realization of ultrahigh performance thin-film photodetectors. This surfactant-free, deoxygenated cosolvent approach can be generalized to other layered materials, thereby presenting additional opportunities for solution-processed thin-film electronic and optoelectronic technologies.

Sorting Transition-Metal Dichalcogenide Nanotubes by Centrifugation

Tungsten disulfide (WS₂) nanotubes are cylindrical, multiwall nanotubes with various diameters and wall numbers. They can exhibit various unique properties depending on their structures and thus preparing samples with uniform structures is important for understanding their basic properties and applications. However, most synthesis methods have difficulty to prepare uniform samples, and thus, a purification method to extract nanotubes with a selected diameter and wall number must be developed. Here, we demonstrate a solution-based purification of WS₂ nanotubes using a surfactant solution. Stable dispersions of nanotubes were prepared using nonionic surfactants, which enabled us to sort the diameters and wall numbers of the nanotubes through a centrifugation process. By optimizing the conditions, we successfully obtained thin nanotubes with a mean diameter of 32 nm and mean wall number of 13 with relatively small distributions. Finally, we clarified the relationships between the structure and optical properties of the nanotubes.

Molecular chemistry approaches for tuning the properties of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) semiconductors, such as ultrathin layers of transition metal dichalcogenides (TMDs), offer a unique combination of electronic, optical and mechanical properties, and hold potential to enable a host of new device applications spanning from flexible/wearable (opto)electronics to energy-harvesting and sensing technologies. A critical requirement for developing practical and reliable electronic devices based on semiconducting TMDs consists in achieving a full control over their charge-carrier polarity and doping. Inconveniently, such a challenging task cannot be accomplished by means of well-established doping techniques (e.g. ion implantation and diffusion), which unavoidably damage the 2D crystals resulting in degraded device performances. Nowadays, a number of alternatives are being investigated, including various (supra)molecular chemistry approaches relying on the combination of 2D semiconductors with electroactive donor/acceptor molecules. As yet, a large variety of molecular systems have been utilized for functionalizing 2D TMDs via both covalent and non-covalent interactions. Such research endeavours enabled not only the tuning of the charge-carrier doping but also the engineering of the optical, electronic, magnetic, thermal and sensing properties of semiconducting TMDs for specific device applications. Here, we will review the most enlightening recent advancements in experimental (supra)molecular chemistry methods for tailoring the properties of atomically-thin TMDs - in the form of substrate-supported or solution-dispersed nanosheets - and we will discuss the opportunities and the challenges towards the realization of novel hybrid materials and devices based on 2D semiconductors and molecular systems.

Revealing the Effects of Electrode Crystallographic Orientation on Battery Electrochemistry via the Anisotropic Lithiation and Sodiation of ReS2

The crystallographic orientation of battery electrode materials can significantly impact electrochemical performance, such as rate capability and cycling stability. Among the layered transition metal dichalcogenides, rhenium disulfide (ReS₂) has the largest anisotropic ratio between the two main axes in addition to exceptionally weak interlayer coupling, which serves as an ideal system to observe and analyze anisotropy of electrochemical phenomena. Here, we report anisotropic lithiation and sodiation of exfoliated ReS₂ at atomic resolution using in situ transmission electron microscopy. These results reveal the role of crystallographic orientation and anisotropy on battery electrode electrochemistry. Complemented with density functional theory calculations, the lithiation of ReS₂ is found to begin with intercalation of Li-ions, followed by a conversion reaction that results in Re nanoparticles and Li₂S nanocrystals. The reaction speed is highly anisotropic, occurring faster along the in-plane ReS₂ layer than along the out-of-plane direction. Sodiation of ReS₂ is found to proceed similarly to lithiation, although the intercalation step is relatively quicker. Furthermore, the microstructure and morphology of the reaction products after lithiation/sodiation show clear anisotropy along the in-plane and out-of-plane directions. These results suggest that crystallographic orientation in highly anisotropic electrode materials can be exploited as a design parameter to improve battery electrochemical performance.

Size-Dependent Physical and Electrochemical Properties of Two-Dimensional MXene Flakes

Two-dimensional (2D) particles, including transition metal carbides (MXenes), often exhibit large lateral-size polydispersity in delaminated colloidal solutions. This heterogeneity results in challenges when conducting fundamental studies, such as investigating correlations between properties and the 2D flake size. To resolve this challenge, we have developed solution-processable techniques to control and sort 2D titanium carbide (Ti₃C₂T _x) MXene flakes after synthesis based on sonication and density gradient centrifugation, respectively. By tuning the sonication conditions, Ti₃C₂T _x flakes with varied lateral sizes, ranging from 0.1 to ∼5 μm, can be obtained. Furthermore, density gradient centrifugation was used to sort Ti₃C₂T _x flakes with different lateral sizes into more monodisperse fractions. These processing techniques allow for the characterization of size-dependent optical and electronic properties by measuring the absorption spectra and film conductivity, respectively. Additionally, by testing the material as electrochemical capacitor electrodes, we show the Ti₃C₂T _x flake-size dependence of electrochemical performance. Ti₃C₂T _x films made of flakes with lateral sizes of ∼1 μm showed the best capacitance of 290 F/g at 2 mV/s and rate performance with 200 F/g at 1000 mV/s. The work provides a general methodology which can be followed to control the size of MXenes  and other 2D materials for a variety of applications and fundamental size-dependent studies.

Assessing and Mitigating the Hazard Potential of Two-Dimensional Materials

The family of two-dimensional (2D) materials is comprised of a continually expanding palette of unique compositions and properties with potential applications in electronics, optoelectronics, energy capture and storage, catalysis, and nanomedicine. To accelerate the implementation of 2D materials in widely disseminated technologies, human health and environmental implications need to be addressed. While extensive research has focused on assessing the toxicity and environmental fate of graphene and related carbon nanomaterials, the potential hazards of other 2D materials have only recently begun to be explored. Herein, the toxicity and environmental fate of postcarbon 2D materials, such as transition metal dichalcogenides, hexagonal boron nitride, and black phosphorus, are reviewed as a function of their preparation methods and surface functionalization. Specifically, we delineate how the hazard potential of 2D materials is directly related to structural parameters and physicochemical properties and how experimental design is critical to the accurate elucidation of the underlying toxicological mechanisms. Finally, a multidisciplinary approach for streamlining the hazard assessment of emerging 2D materials is outlined, thereby providing a pathway for accelerating their safe use in a range of technologically relevant contexts.

A Novel Top-Down Synthesis of Ultrathin 2D Boron Nanosheets for Multimodal Imaging-Guided Cancer Therapy

Single atom nonmetal 2D nanomaterials have shown considerable potential in cancer nanomedicines, owing to their intriguing properties and biocompatibility. Herein, ultrathin boron nanosheets (B NSs) are prepared through a novel top-down approach by coupling thermal oxidation etching and liquid exfoliation technologies, with controlled nanoscale thickness. Based on the PEGylated B NSs, a new photonic drug delivery platform is developed, which exhibits multiple promising features for cancer therapy and imaging, including: i) efficient NIR-light-to-heat conversion with a high photothermal conversion efficiency of 42.5%, ii) high drug-loading capacity and triggered drug release by NIR light and moderate acidic pH, iii) strong accumulation at tumor sites, iv) multimodal imaging properties (photoacoustic, photothermal, and fluorescence imaging), and v) complete tumor ablation and excellent biocompatibility. As far as it is known, this is the first report on the top-down fabrication of ultrathin 2D B NSs by the combined thermal oxidation etching and liquid exfoliation, as well as their application as a multimodal imaging-guided drug delivery platform. The newly prepared B NSs are also expected to provide a robust and useful 2D nanoplatform for various biomedical applications.

Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures

The precise control in size/thickness, composition, crystal phases, doping, defects, and surface properties of two-dimensional (2D) layered transition metal chalcogenide (TMC) is important for the investigation of interwoven relationship between structures, functions, and practical applications. Of the multiple synthetic routes, solution-based top-down and bottom-up chemical methods have been uniquely important for their potential to control the size and composition at the molecular level in addition to their scalability, competitive production cost, and solution processability. Here, we introduce an overview of the recent advances in the solution-based preparation routes of 2D layered TMC nanostructures along with important scientific developments.

Coke-resistant defect-confined Ni-based nanosheet-like catalysts derived from halloysites for CO2 reforming of methane

In this study, halloysites, one of the most abundant clays, with hollow nanotube features were reconstructed by selectively etching silica from the outermost layer of the halloysites associated with unzipping the nanotubes to nanosheets via ball milling, and then, nickel nanoparticles were confined by the resulting defects in the nanosheets to boost charge transfer by a wet impregnation method. The obtained materials were developed as coke-resistant defect-confined Ni-based nanosheet-like catalysts for CO2 reforming of methane (CRM) for the first time. The as-prepared catalyst exhibited good coke and sintering resistance performance in CRM, and especially, there was almost no loss of activity even after a 20 h stability test due to the strong interaction between the Ni nanoparticles and the support. The present investigations may provide a new pathway for the design and application of highly coke-resistant CRM catalysts.

Functional inks and printing of two-dimensional materials

Graphene and related two-dimensional materials provide an ideal platform for next generation disruptive technologies and applications. Exploiting these solution-processed two-dimensional materials in printing can accelerate this development by allowing additive patterning on both rigid and conformable substrates for flexible device design and large-scale, high-speed, cost-effective manufacturing. In this review, we summarise the current progress on ink formulation of two-dimensional materials and the printable applications enabled by them. We also present our perspectives on their research and technological future prospects.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Layered Aggregation with Steric Effect: Morphology-Homogeneous Semiconductor MoS2 as an Alternative 2D Probe for Visual Immunoassay

Liquid-phase exfoliation routes unavoidably generate 2D nanostructures with inhomogeneous morphologies. Herein, thickness-dependent sorting of exfoliated nanostructures is achieved via a treatment of differential-zone centrifugation in the surfactant aqueous phase. With this approach, homogeneous MoS₂ nanosheets are obtained, and due to the intrinsic semiconducting characteristics, those 2D nanosheets are endowed with desired optical properties, rivaling classic gold nanoparticles in sensing applications. Furthermore, MoS₂ nanosheets with high uniformity and chemical inertness are coupled with proteins, exhibiting high performance in stability and anti-interferences for bioanalysis. As a consequence of aggregation-induced steric effect, distinguishing running shifts of antibody-anchored conjugates in gel electrophoresis are visually responsive to those specific antigens. This assay enables the easy and fast monitoring of tumor biomarkers just according to "naked-eye" identification of band location in electrophoresis results, which are presented by an alternative visual probe of 2D MoS₂ -protein conjugates. The developed visual immunoassay with the synergistic effect of gel electrophoresis techniques and 2D semiconductors pushes significant progress in "home-made" tests for disease early diagnosis.

One-step synthesis of ultrathin α-Co(OH)2nanomeshes and their high electrocatalytic activity toward the oxygen evolution reaction

The ultrathin α-Co(OH)₂nanomeshes were synthesized by an imidazole-directed route at room temperature, which exhibit excellent electrocatalytic activity for oxygen evolution reaction.

Multi-principal element transition metal dichalcogenides via reactive fusion of 3D-heterostructures

Mixed transition metal dichalcogenides have been prepared from binary precursors via mechanochemical generation of 3D-heterostructures and subsequent reactive fusion.

Preparation of 2D material dispersions and their applications

A comprehensive review on the exfoliation of layer materials into 2D materials, their assembly, and applications in electronics and energy.