Mild Covalent Functionalization of Transition Metal Dichalcogenides with Maleimides: A "Click" Reaction for 2H-MoS2 and WS2

Rapid and simple viral protein detection by functionalized 2D MoS2/graphene electrochemiluminescence aptasensor

In this work we present the development of an electrochemiluminescence aptasensor based on electrografting molybdenum disulphide nanosheets functionalized with diazonium salt (MoS2-N2+) upon screen-printed electrodes of graphene (SPEs GPH) for viral proteins detection. In brief, this aptasensor consists of SPEs GPH electrografted with MoS2-N2+ and modified with a thiolated aptamer, which can specifically recognize the target protein analyte. In this case, we have used SARS-CoV-2 spike protein as model protein. Electrochemiluminescence detection was performed by using the [Ru(bpy)3]2+/TPRA (tripropylamine) system, which allows the specific detection of the SARS-CoV-2 spike protein easily and rapidly with a detection limit of 9.74 fg/mL and a linear range from 32.5 fg/mL to 50.0 pg/mL. Moreover, the applicability of the aptasensor has been confirmed by the detection of the protein directly in human saliva samples. Comparing our device with a traditional saliva antigen test, our aptasensor can detect the spike protein even when the saliva antigen test gives a negative result.

Trifluoromethylation of 2D Transition Metal Dichalcogenides: A Mild Functionalization and Tunable p-Type Doping Method

Chemical modification is a powerful strategy for tuning the electronic properties of 2D semiconductors. Here we report the electrophilic trifluoromethylation of 2D WSe2 and MoS2 under mild conditions using the reagent trifluoromethyl thianthrenium triflate (TTT). Chemical characterization and density functional theory calculations reveal that the trifluoromethyl groups bind covalently to surface chalcogen atoms as well as oxygen substitution sites. Trifluoromethylation induces p-type doping in the underlying 2D material, enabling the modulation of charge transport and optical emission properties in WSe2. This work introduces a versatile and efficient method for tailoring the optical and electronic properties of 2D transition metal dichalcogenides.

Characterization of emerging 2D materials after chemical functionalization

The chemical modification of 2D materials has proven a powerful tool to fine tune their properties. With this motivation, the development of new reactions has moved extremely fast. The need for speed, together with the intrinsic heterogeneity of the samples, has sometimes led to permissiveness in the purification and characterization protocols. In this review, we present the main tools available for the chemical characterization of functionalized 2D materials, and the information that can be derived from each of them. We then describe examples of chemical modification of 2D materials other than graphene, focusing on the chemical description of the products. We have intentionally selected examples where an above-average characterization effort has been carried out, yet we find some cases where further information would have been welcome. Our aim is to bring together the toolbox of techniques and practical examples on how to use them, to serve as guidelines for the full characterization of covalently modified 2D materials.

Clicking beyond suspensions: understanding thiol-ene chemistry on solid-supported MoS2

Molecular functionalization of MoS2 has attracted a lot of attention due to its potential to afford fine-tuned hybrid materials that benefit from the power of synthetic chemistry and molecular design. Here, we report on the on-surface reaction of maleimides on bulk and molecular beam epitaxy grown single-layer MoS2, both in ambient conditions as well as ultrahigh vacuum using scanning probe microscopy.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

Directed morphology engineering of 2D MoS2 nanosheets to 1D nanoscrolls with enhanced hydrogen evolution and specific capacitance

1D-molybdenum disulfide (MoS2) nanoscrolls displayed enhanced electrochemical properties compared to 2D-MoS2 nanosheet counterparts. Rolling of nanosheets is the main fabrication route to nanoscrolls. However, owing to the conflict between chemical stability and multiple bending, the morphology transition from nanosheets to nanoscrolls is quite challenging. Herein we describe a reversible morphology transition from nanosheets to nanoscrolls by utilizing non-covalent interactions between MoS2 nanosheets and phenothiazine based organic dye. Interestingly, nanoscrolls can easily be opened back into nanosheets by destroying the non-covalent interactions with organic solvents. The prepared nanoscrolls exhibited enhanced electrochemical properties than nanosheets. Compared to nanosheets, nanoscrolls exhibited comparatively lower overpotential with a Tafel slope of 141 mV dec-1 and high specific capacitance of 1868 F g-1. Hydrogen evolution by the Volmer-Heyrovsky mechanism being superior for the nanoscrolls is envisaged by the relatively increased availability of Hads sites at MoS2 edges induced by scrolling. Whereas the high specific capacitance value of nanoscrolls is ascribed to the enhanced electrical double-layer capacitance mediated charge storage, which arises due to the synergistic effect of both scrolled structure and the electron-rich phenothiazine-based dye.

An emissive charge-transfer excited-state at the well-defined hetero-nanostructure interface of an organic conjugated molecule and two-dimensional inorganic nanosheet

Precise engineering of excited-state interactions between an organic conjugated molecule and a two-dimensional semiconducting inorganic nanosheet, specifically the manipulation of charge-transfer excited (CTE) states, still remains a challenge for state-of-the-art photochemistry. Herein, we report a long-lived, highly emissive CTE state at structurally well-defined hetero-nanostructure interfaces of photoactive pyrene and two-dimensional MoS2 nanosheets via an N-benzylsuccinimide bridge (Py-Bn-MoS2). Spectroscopic measurements reveal that no charge-transfer state is formed in the ground state, but the locally-excited (LE) state of pyrene in Py-Bn-MoS2 efficiently generates an unusual emissive CTE state. Theoretical studies elucidate the interaction of MoS2 vacant orbitals with the pyrene LE state to form a CTE state that shows a distinct solvent dependence of the emission energy. This is the first example of organic-inorganic 2D hetero-nanostructures displaying mixed luminescence properties by an accurate design of the bridge structure, and therefore represents an important step in their applications for energy conversion and optoelectronic devices and sensors.

Synthesis, Characterisation, and Functionalisation of Charged Two-Dimensional MoS2

The applications of exfoliated MoS2 are limited by its inert surface and poor interface. We have activated the surface of exfoliated 2H-MoS2 by reacting it with NaBH4 , forming an n-doped material as demonstrated by a negative zeta-potential value ζ=-25 mV and a 20 nm (0.05 eV) red-shift in its photoluminescence spectrum. The novel material's spectral properties were consistent with pristine 2H-MoS2 (as determined by HR-TEM, XPS, pXRD, DRIFT, TGA, and Raman spectroscopy). Importantly, it was readily dispersed in H2 O unlike 2H-MoS2 . Its dispersibility properties were explored for a variety of solvents and could be directly correlated with the relative permittivity of the respective solvents. The charged 2H-MoS2 reacted readily with an organo-iodide to deliver functionalized 2H-MoS2 . Our approach delivers aqueous dispersions of semiconducting 2H-MoS2 , without additives or chemical functionalities, and allows for controlled and facile functionalization of 2H-MoS2 opening multiple new avenues of semi-conducting MoS2 application.

Covalently Functionalized MoS2 Initiated Gelation of Hydrogels for Flexible Strain Sensing

Transition metal dichalcogenides (TMDs), with superior mechanical and electrical conductivity, are one of the most promising two-dimensional materials for creating a generation of intelligent and flexible electronic devices. However, due to the high van der Waals and electrostatic attraction, TMD nanomaterials tend to aggregate in dispersants to achieve a stable state, thus severely limiting their further applications. Surface chemical modification is a common strategy for improving the dispersity of TMD nanomaterials; however, there are still constraints such as limited functionalization methods, low grafting rate, and difficult practice application. Thus, it is challenging to develop innovative surface modification systems. Herein, we covalently modify an olefin molecule on surface-inert MoS₂, and the modified MoS₂ can be used as not only a catalyst for hydrogel polymerization, but also a cross-linker in the hydrogel network. Specifically, allyl is covalently grafted onto chemically exfoliated MoS₂, and this modified MoS₂ can be uniformly dispersed in polar solvents (such as acetone, N,N-dimethylformamide, and ethanol), remaining stable for more than 2 weeks. The allyl-modified MoS₂ can catalyze the polymerization of polyacrylamide hydrogel and then integrate in the network, which increases the tensile strength of the composite hydrogel. The flexible sensor based on the composite hydrogel exhibits an ideal operating range of 600% and a quick response time of 150 ms. At the same time, the flexible device can also track the massive axial stretching movements of human joints, making it a reliable option for the next wave of wearable sensing technology.

Patching sulfur vacancies: A versatile approach for achieving ultrasensitive gas sensors based on transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) garner significant attention for their potential to create high-performance gas sensors. Despite their favorable properties such as tunable bandgap, high carrier mobility, and large surface-to-volume ratio, the performance of TMDCs devices is compromised by sulfur vacancies, which reduce carrier mobility. To mitigate this issue, we propose a simple and universal approach for patching sulfur vacancies, wherein thiol groups are inserted to repair sulfur vacancies. The sulfur vacancy patching (SVP) approach is applied to fabricate a MoS₂-based gas sensor using mechanical exfoliation and all-dry transfer methods, and the resulting 4-nitrothiophenol (4NTP) repaired molybdenum disulfide (4NTP-MoS₂) is prepared via a sample solution process. Our results show that 4NTP-MoS₂ exhibits higher response (increased by 200 %) to ppb-level NO₂ with shorter response/recovery times (61/82 s) and better selectivity at 25 °C compared to pristine MoS₂. Notably, the limit of detection (LOD) toward NO₂ of 4NTP-MoS₂ is 10 ppb. Kelvin probe force microscopy (KPFM) and density functional theory (DFT) reveal that the improved gas sensing performance is mainly attributed to the 4NTP-induced n-doping effect on MoS₂ and the corresponding increment of surface absorption energy to NO₂. Additionally, our 4NTP-induced SVP approach is universal for enhancing gas sensing properties of other TMDCs, such as MoSe₂, WS₂, and WSe₂.

Direct Magnetic Evidence, Functionalization, and Low-Temperature Magneto-Electron Transport in Liquid-Phase Exfoliated FePS3

Magnetism and the existence of magnetic order in a material is determined by its dimensionality. In this regard, the recent emergence of magnetic layered van der Waals (vdW) materials provides a wide playground to explore the exotic magnetism arising in the two-dimensional (2D) limit. The magnetism of 2D flakes, especially antiferromagnetic ones, however, cannot be easily probed by conventional magnetometry techniques, being often replaced by indirect methods like Raman spectroscopy. Here, we make use of an alternative approach to provide direct magnetic evidence of few-layer vdW materials, including antiferromagnets. We take advantage of a surfactant-free, liquid-phase exfoliation (LPE) method to obtain thousands of few-layer FePS3 flakes that can be quenched in a solvent and measured in a conventional SQUID magnetometer. We show a direct magnetic evidence of the antiferromagnetic transition in FePS3 few-layer flakes, concomitant with a clear reduction of the Néel temperature with the flake thickness, in contrast with previous Raman reports. The quality of the LPE FePS3 flakes allows the study of electron transport down to cryogenic temperatures. The significant through-flake conductance is sensitive to the antiferromagnetic order transition. Besides, an additional rich spectra of electron transport excitations, including secondary magnetic transitions and potentially magnon-phonon hybrid states, appear at low temperatures. Finally, we show that the LPE is additionally a good starting point for the mass covalent functionalization of 2D magnetic materials with functional molecules. This technique is extensible to any vdW magnetic family.

Nanoscale Control of DNA-Linked MoS2-Quantum Dot Heterostructures

The ability to control the assembly of mixed-dimensional heterostructures with nanoscale control is key for the fabrication of novel nanohybrid systems with new functionalities, particularly for optoelectronics applications. Herein we report a strategy to control the assembly of heterostructures and tune their electronic coupling employing DNA as a linker. We functionalized MoS2 nanosheets (NSs) with biotin-terminated dsDNA employing three different chemical strategies, namely, thiol, maleimide, and aryl diazonium. This allowed us to then tether streptavidinated quantum dots (QDs) to the DNA functionalized MoS2 surface via biotin-avidin recognition. Nanoscale control over the separation between QDs and NSs was achieved by varying the number of base pairs (bp) constituting the DNA linker, between 10, 20, and 30 bp, corresponding to separations of 3.4, 6.8, and 13.6 nm, respectively. Spectroscopic data confirmed the successful functionalization, while atomic force and transmission electron microscopy were employed to image the nanohybrids. In solution steady-state and time-resolved photoluminescence demonstrated the electronic coupling between the two nanostructures, that in turn was observed to progressively scale as a function of DNA linker employed and hence distance between the two nanomoieties in the hybrids.

A "Click" Reaction to Engineer MoS2 Field-Effect Transistors with Low Contact Resistance

Two-dimensional (2D) materials with the atomically thin thickness have attracted great interest in the post-Moore's Law era because of their tremendous potential to continue transistor downscaling and offered advances in device performance at the atomic limit. However, the metal-semiconductor contact is the bottleneck in field-effect transistors (FETs) integrating 2D semiconductors as channel materials. A robust and tunable doping method at the source and drain region of 2D transistors to minimize the contact resistance is highly sought after. Here we report a stable carrier doping method via the mild covalent grafting of maleimides on the surface of 2D transition metal dichalcogenides. The chemisorbed interaction contributes to the efficient carrier doping without degrading the high-performance carrier transport. Density functional theory results further illustrate that the molecular functionalization leads to the mild hybridization and the negligible impact on the conduction bands of monolayer MoS₂, avoiding the random scattering from the dopants. Differently from reported molecular treatments, our strategy displays high thermal stability (above 300 °C) and it is compatible with micro/nano processing technology. The contact resistance of MoS₂ FETs can be greatly reduced by ∼12 times after molecular functionalization. The Schottky barrier of 44 meV is achieved on monolayer MoS₂ FETs, demonstrating efficient charge injection between metal and 2D semiconductor. The mild covalent functionalization of molecules on 2D semiconductors represents a powerful strategy to perform the carrier doping and the device optimization.

Molybdenum Diselenide and Tungsten Diselenide Interfacing Cobalt-Porphyrin for Electrocatalytic Hydrogen Evolution in Alkaline and Acidic Media

Easy and effective modification approaches for transition metal dichalcogenides are highly desired in order to make them active toward electrocatalysis. In this manner, we report functionalized molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) via metal-ligand coordination with pyridine rings for the subsequent covalent grafting of a cobalt-porphyrin. The new hybrid materials were tested towards an electrocatalytic hydrogen evolution reaction in both acidic and alkaline media and showed enhanced activity compared to intact MoSe₂ and WSe₂. Hybrids exhibited lower overpotential, easier reaction kinetics, higher conductivity, and excellent stability after 10,000 ongoing cycles in acidic and alkaline electrolytes compared to MoSe₂ and WSe₂. Markedly, MoSe₂-based hybrid material showed the best performance and marked a significantly low onset potential of -0.17 V vs RHE for acidic hydrogen evolution reaction. All in all, the ease and fast modification route provides a versatile functionalization procedure, extendable to other transition metal dichalcogenides, and can open new pathways for the realization of functional nanomaterials suitable in electrocatalysis.

Basal Plane Functionalization of Niobium Disulfide Nanosheets with Cyclopentadienyl Manganese(I) Dicarbonyl

Basal plane-functionalized NbS₂ nanosheets were obtained using in situ photolysis to generate the coordinatively unsaturated organometallic fragment cyclopentadienyl manganese(I) dicarbonyl (CpMn(CO)₂). Under UV irradiation, a labile carbonyl ligand dissociates from the tricarbonyl complex, creating an open coordination site for bonding between the Mn atom and the electron-rich sulfur atoms on the surface of the NbS₂ nanosheets. In contrast, no reaction is observed with 2H-MoS₂ nanosheets under the same reaction conditions. This difference in reactivity is consistent with the electronic structure calculations, which indicate stronger bonding of the organometallic fragment to electron-poor, metallic NbS₂ than to semiconducting, electron-rich MoS₂. X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, and powder X-ray diffraction (PXRD) were used to characterize the bonding between Mn and S atoms on the surface-functionalized nanosheets.

Synthesis and Structural Study of Amidrazone Derived Pyrrole-2,5-Dione Derivatives: Potential Anti-Inflammatory Agents

1H-pyrrole-2,5-dione derivatives are known for their wide range of pharmacological properties, including anti-inflammatory and antimicrobial activities. This study aimed to synthesize new 3,4-dimethyl-1H-pyrrole-2,5-dione derivatives 2a–2f in the reaction of N³-substituted amidrazones with 2,3-dimethylmaleic anhydride and evaluate their structural and biological properties. Compounds 2a–2f were studied by the ¹H-¹³C NMR two-dimensional techniques (HMQC, HMBC) and single-crystal X-ray diffraction (derivatives 2a and 2d). The anti-inflammatory activity of compounds 2a–2f was examined by both an anti-proliferative study and a production study on the inhibition of pro-inflammatory cytokines (IL-6 and TNF-α) in anti-CD3 antibody- or lipopolysaccharide-stimulated human peripheral blood mononuclear cell (PBMC) cultures. The antibacterial activity of compounds 2a–2f against Staphylococcus aureus, Enterococcus faecalis, Micrococcus luteus, Esherichia coli, Pseudomonas aeruginosa, Yersinia enterocolitica, Mycobacterium smegmatis and Nocardia corralina strains was determined using the broth microdilution method. Structural studies of 2a–2f revealed the presence of distinct Z and E stereoisomers in the solid state and the solution. All compounds significantly inhibited the proliferation of PBMCs in anti-CD3-stimulated cultures. The strongest effect was observed for derivatives 2a–2d. The strongest inhibition of pro-inflammatory cytokine production was observed for the most promising anti-inflammatory compound 2a.

Experimental and Theoretical Comparison of Potential-dependent Methylation on Chemically Exfoliated WS2 and MoS2

Reductant-activated functionalization is shown to enhance the methylation of chemically exfoliated MoS₂ (ceMoS₂) and ceWS₂ by introducing excess negative charge to facilitate a nucleophilic attack reaction. Relative to methylation in the absence of a reductant, the reaction produces a twofold increase in coverage of ceWS₂, from 25 to 52% coverage per WS₂. However, at every potential, the methyl coverage on ceWS₂ was ∼20% lower than that on ceMoS₂. We applied grand canonical density functional theory to show that at constant potential, more negative charge is present on 1T'-MoS₂ than on 1T'-WS₂, making methylation both thermodynamically and kinetically more favorable for 1T'-MoS₂ than 1T'-WS₂. This effect was moderated when the reactions were compared at constant charge, emphasizing the importance of comparing the reactivity of materials at nominally identical electrode potentials.

Chalcogen Bonding in the Molecular Dimers of WCh2 (Ch = S, Se, Te): On the Basic Understanding of the Local Interfacial and Interlayer Bonding Environment in 2D Layered Tungsten Dichalcogenides

Layered two-dimensional transition metal dichalcogenides and their heterostructures are of current interest, owing to the diversity of their applications in many areas of materials nanoscience and technologies. With this in mind, we have examined the three molecular dimers of the tungsten dichalcogenide series, (WCh₂)₂ (Ch = S, Se, Te), using density functional theory to provide insight into which interactions, and their specific characteristics, are responsible for the interfacial/interlayer region in the room temperature 2H phase of WCh₂ crystals. Our calculations at various levels of theory suggested that the Te···Te chalcogen bonding in (WTe₂)₂ is weak, whereas the Se···Se and S···S bonding interactions in (WSe₂)₂ and (WS₂)₂, respectively, are of the van der Waals type. The presence and character of Ch···Ch chalcogen bonding interactions in the dimers of (WCh₂)₂ are examined with a number of theoretical approaches and discussed, including charge-density-based approaches, such as the quantum theory of atoms in molecules, interaction region indicator, independent gradient model, and reduced density gradient non-covalent index approaches. The charge-density-based topological features are shown to be concordant with the results that originate from the extrema of potential on the electrostatic surfaces of WCh₂ monomers. A natural bond orbital analysis has enabled us to suggest a number of weak hyperconjugative charge transfer interactions between the interacting monomers that are responsible for the geometry of the (WCh₂)₂ dimers at equilibrium. In addition to other features, we demonstrate that there is no so-called van der Waals gap between the monolayers in two-dimensional layered transition metal tungsten dichalcogenides, which are gapless, and that the (WCh₂)₂ dimers may be prototypes for a basic understanding of the physical chemistry of the chemical bonding environments associated with the local interfacial/interlayer regions in layered 2H-WCh₂ nanoscale systems.

A fluorinated 2D magnetic coordination polymer

Herein we show the versatility of coordination chemistry to design and expand a family of 2D materials by incorporating F groups at the surface of the layers. Through the use of a prefuntionalized organic linker with F groups, it is possible to achieve a layered magnetic material based on Fe(II) centers that are chemically stable in open air, contrary to the known 2D inorganic magnetic materials. The high quality of the single crystals and their robustness allow to fabricate 2D molecular materials by micromechanical exfoliation, preserving the crystalline nature of these layers together with the desired functionalization.

Recent trends in covalent functionalization of 2D materials

Covalent functionalization of the surface is more crucial in 2D materials than in conventional bulk materials because of their atomic thinness, large surface-to-volume ratio, and uniform surface chemical potential. Because...

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

Covalent photofunctionalization and electronic repair of 2H-MoS2via nitrogen incorporation

A route towards covalent functionalization of chemically inert 2H-MoS₂ exploiting sulfur vacancies is explored by means of (TD)DFT and GW/BSE calculations. Functionalization via nitrogen incorporation at sulfur vacancies is shown to result in more stable covalent binding than via thiol incorporation. In this way, defective monolayer MoS₂ is repaired and the quasiparticle band structure as well as the remarkable optical properties of pristine MoS₂ are restored. Hence, defect-free functionalization with various molecules is possible. Our results for covalently attached azobenzene, as a prominent photo-switch, pave the way to create photoresponsive two-dimensional (2D) materials.

Covalent Patterning of 2D MoS2

The development of an efficient method to patterning 2D MoS₂ into a desired topographic structure is of particular importance to bridge the way towards the ultimate device. Herein, we demonstrate a patterning strategy by combining the electron beam lithography with the surface covalent functionalization. This strategy allows us to generate delicate MoS₂ ribbon patterns with a minimum feature size of 2 μm in a high throughput rate. The patterned monolayer MoS₂ domain consists of a spatially well‐defined heterophase homojunction and alternately distributed surface characteristics, which holds great interest for further exploration of MoS₂ based devices.

Fast Polymeric Functionalization Approach for the Covalent Coating of MoS2 Layers

We present the covalent coating of chemically exfoliated molybdenum disulfide (MoS₂) based on the polymerization of functional acryl molecules. The method relies on the efficient diazonium anchoring reaction to provoke the in situ radical polymerization and covalent adhesion of functional coatings. In particular, we successfully implement hydrophobicity on the exfoliated MoS₂ in a direct, fast, and quantitative synthetic approach. The covalent functionalization is proved by multiple techniques including X-ray photoelectron spectroscopy and TGA-MS. This approach represents a simple and general protocol to reach dense and homogeneous functional coatings on 2D materials.

Molecular Functionalization of 2H-Phase MoS2 Nanosheets via an Electrolytic Route for Enhanced Catalytic Performance

The molecular functionalization of two-dimensional MoS₂ is of practical relevance with a view to, for example, facilitating its liquid-phase processing or enhancing its performance in target applications. While derivatization of metallic 1T-phase MoS₂ nanosheets has been relatively well studied, progress involving their thermodynamically stable, 2H-phase counterpart has been more limited due to the lower chemical reactivity of the latter. Here, we report a simple electrolytic strategy to functionalize 2H-phase MoS₂ nanosheets with molecular groups derived from organoiodides. Upon cathodic treatment of a pre-expanded MoS₂ crystal in an electrolyte containing the organoiodide, water-dispersible nanosheets derivatized with acetic acid or aniline moieties (∼0.10 molecular groups inserted per surface sulfur atom) were obtained. Analysis of the functionalization process indicated it to be enabled by the external supply of electrons from the cathodic potential, although they could also be sourced from a proper reducing agent, as well as by the presence of intrinsic defects in the 2H-phase MoS₂ lattice (e.g., sulfur vacancies), where the molecular groups can bind. The acetic acid-functionalized nanosheets were tested as a non-noble metal-based catalyst for nitroarene and organic dye reduction, which is of practical utility in environmental remediation and chemical synthesis, and exhibited a markedly enhanced activity, surpassing that of other (1T- or 2H-phase) MoS₂ materials and most non-noble metal catalysts previously reported for this application. The reduction kinetics (reaction order) was seen to correlate with the net electric charge of the nitroarene/dye molecules, which was ascribed to the distinct abilities of the latter to diffuse to the catalyst surface. The functionalized MoS₂ catalyst also worked efficiently at realistic (i.e., high) reactant concentrations, as well as with binary and ternary mixtures of the reactants, and could be immobilized on a polymeric scaffold to expedite its manipulation and reuse.

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

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

Covalent Bisfunctionalization of Two-Dimensional Molybdenum Disulfide

Covalent functionalization of two‐dimensional molybdenum disulfide (2D MoS₂) holds great promise in developing robust organic‐MoS₂ hybrid structures. Herein, for the first time, we demonstrate an approach to building up a bisfunctionalized MoS₂ hybrid structure through successively reacting activated MoS₂ with alkyl iodide and aryl diazonium salts. This approach can be utilized to modify both colloidal and substrate supported MoS₂ nanosheets. We have discovered that compared to the adducts formed through the reactions of MoS₂ with diazonium salts, those formed through the reactions of MoS₂ with alkyl iodides display higher reactivity towards further reactions with electrophiles. We are convinced that our systematic study on the formation and reactivity of covalently functionalized MoS₂ hybrids will provide some practical guidance on multi‐angle tailoring of the properties of 2D MoS₂ for various potential applications.

Brain-Targeted Dual Site-Selective Functionalized Poly(β-Amino Esters) Delivery Platform for Nerve Regeneration

Brain injuries are devastating central nervous system diseases, resulting in cognitive, motor, and sensory dysfunctions. However, clinical therapeutic options are still limited for brain injuries, indicating an urgent need to investigate new therapies. Furthermore, the efficient delivery of therapeutics across the blood-brain barrier (BBB) to the brain is a serious problem. In this study, a facile strategy of dual site-selective functionalized (DSSF) poly(β-amino esters) was developed using bio-orthogonal chemistry for promoting brain nerve regeneration. Fluorescence colocalization studies demonstrated that these proton-sponge DSSF poly(β-amino esters) targeted mitochondria through electrostatic interactions. More importantly, this delivery system could effectively accumulate in the injured brain sites and accelerate the recovery of the injured brain. Finally, this DSSF poly(β-amino esters) platform may provide a new methodology for the construction of dual regioselective carriers in protein/peptide delivery and tissue engineering.

Unveiling the Photoinduced Electron-Donating Character of MoS2 in Covalently Linked Hybrids Featuring Perylenediimide

The covalent functionalization of MoS₂ with a perylenediimide (PDI) is reported and the study is accompanied by detailed characterization of the newly prepared MoS₂ -PDI hybrid material. Covalently functionalized MoS₂ interfacing organic photoactive species has shown electron and/or energy accepting, energy reflecting or bi-directional electron accepting features. Herein, a rationally designed PDI, unsubstituted at the perylene core to act as electron acceptor, forces MoS₂ to fully demonstrate for the first time its electron donor capabilities. The photophysical response of MoS₂ -PDI is visualized in an energy-level diagram, while femtosecond transient absorption studies disclose the formation of MoS₂ ^.+ -PDI^.- charge separated state. The tunable electronic properties of MoS₂ , as a result of covalently linking photoactive organic species with precise characteristics, unlock their potentiality and enable their application in light-harvesting and optoelectronic devices.

Covalent Cross-Linking of 2H-MoS2 Nanosheets

The combination of 2D materials opens a wide range of possibilities to create new-generation structures with multiple applications. Covalently cross-linked approaches are a ground-breaking strategy for the formation of homo or heterostructures made by design. However, the covalent assembly of transition metal dichalcogenides flakes is relatively underexplored. Here, a simple covalent cross-linking method to build 2H-MoS₂ -MoS₂ homostructures is described, using commercially available bismaleimides. These assemblies are mainly connected vertically, basal plane to basal plane, creating specific molecular sized spaces between MoS₂ sheets. Therefore, this straightforward approach gives access to the controlled connection of sulfide-based 2D materials.

Covalent functionalization of molybdenum disulfide by chemically activated diazonium salts

Covalent functionalization is one of the most efficient ways to tune the properties of layered materials in a highly controlled manner. However, molecular chemisorption on semiconducting transition metal dichalcogenides remains a delicate task due to the inertness of their surface. Here we perform covalent modification of bulk and single layer molybdenum disulfide (MoS₂) using chemical activation of diazonium salts. A high level of control over the grafting density and yield on MoS₂ basal plane can be achieved by this approach. Using scanning probe microscopies and X-ray photoelectron spectroscopy we prove the covalent functionalization of MoS₂.

Peptoid-directed assembly of CdSe nanoparticles

The high information content of proteins drives their hierarchical assembly and complex function, including the organization of inorganic nanomaterials. Peptoids offer an organic scaffold very similar to proteins, but with a wider solubility range and easily tunable side chains and functional groups to create a variety of self-assembling architectures with atomic precision. If we could harness this paradigm and understand the factors that govern how they direct nucleation and assembly of inorganic materials to design order within such materials, new dimensions of function and fundamental science would emerge. In this work, peptoid tubes and sheets were explored as platforms to assemble colloidal quantum dots (QDs) and clusters. We have successfully synthesized CdSe QDs with difunctionalized capping ligands containing both carboxylic acid and thiol groups and mixed them with maleimide containing peptoids, to create an assembly of the QDs on the peptoid surface via a covalent linkage. This conjugation was seen to be successful with peptoid tubes, sheets and CdSe QDs and clusters. The particles were seen to have a high preference for the peptoid surface but non-specific interactions with carboxylic acid groups on the peptoids limited control over QD density via maleimide conjugation. Replacing the carboxylic acid groups with methoxy ethers, however, allowed for control over QD density as a function of maleimide concentration. 1H NMR analysis demonstrated that binding of QDs to peptoids involved a subset of surface ligands bound through the carboxylate functional group, allowing the distal thiol to engage in a covalent linkage to the maleimide. Overall, we have shown the compatibility and control of CdSe-peptoid interactions via a covalent linkage with varying peptoid structures and CdSe particles to create complex hybrid structures.

DNA-driven dynamic assembly of MoS2 nanosheets

Controlling the assembly of molybdenum disulfide (MoS₂) layers into static and dynamic superstructures can impact on their use in optoelectronics, energy, and drug delivery. Toward this goal, we present a strategy to drive the assembly of MoS₂ layers via the hybridization of complementary DNA linkers. By functionalizing the MoS₂ surface with thiolated DNA, MoS₂ nanosheets were assembled into mulitlayered superstructures, and the complementary DNA strands were used as linkers. A disassembly process was triggered by the formation of an intramolecular i-motif structure at a cystosine-rich sequence in the DNA linker at acidic pH values. We tested the versatility of our approach by driving the disassembly of the MoS₂ superstructures through a different DNA-based mechanism, namely strand displacement. This study demonstrates how DNA can be employed to drive the static and dynamic assembly of MoS₂ nanosheets in aqueous solution.

Effect of temperature and exfoliation time on the properties of chemically exfoliated MoS2 nanosheets

A systematic investigation of the experimental conditions for the chemical exfoliation of MoS₂ using n-butyllithium as intercalating agent has been carried out to unravel the effect of reaction time and temperature for maximizing the percentage of monolayer thick-flakes and achieve a control over the content of metallic 1T vs. semiconductive 2H phases, thereby tuning the electrical properties of ultrathin MoS₂ few-layer thick films.

Recent advances on TMDCs for medical diagnosis

Transition metal dichalcogenides (TMDCs), such as MoS₂ and WS₂, have attracted much attention in biosensing and bioimaging due to its excellent stability, biocompatibility, high specific surface area, and wide varieties. In this review, we overviewed the application of TMDCs in biosensing and bioimaging. Firstly, the synthesis methods and surface functionalization methods of TMDCs were summarized. Secondly, according to the working mechanism, we classified and gave a detailed account of the latest research progress of TMDC-based biosensing for the detection of the enzyme, DNA, and other biological molecules. Then, we outlined the recent progress of applying TMDCs in bio-imaging, including fluorescence, X-ray computed tomographic, magnetic response imaging, photographic and multimodal imaging, respectively. Finally, we discussed the future challenges and development direction of the application of TMDCs in medical diagnosis. Also, we put forward our view on the opportunity of TMDCs in the big data of modern medical diagnosis.

Reversible Electron Doping of Layered Metal Hydroxide Nanoplates (M = Co, Ni) Using n-Butyllithium

Ambipolar doping of metal oxides is critical toward broadening the functionality of semiconducting oxides in electronic devices. Most metal oxides, however, show a strong preference for a single doping polarity due to the intrinsic stability of particular defects in an oxide lattice. In this work, we demonstrate that layered metal hydroxide nanomaterials of Co and Ni, which are intrinsically p-doped in their anhydrous rock salt form, can be n-doped using n-BuLi as a strong electron donor. A combination of X-ray characterization techniques reveal that hydroxide vacancy formation, Li⁺ adsorption, and varying degrees of electron delocalization are responsible for the stability of injected electrons. The doped electrons induce conductivity increases of 4-6 orders of magnitude relative to the undoped M(OH)₂. We anticipate that chemical electron doping of layered metal hydroxides may be a general strategy to increase carrier concentration and stability for n-doping of intrinsically p-type metal oxides.

Tunable Optical Transition in 2H-MoS2 via Direct Electrochemical Engineering of Vacancy Defects and Surface S-C Bonds

Although surface engineering has been regarded to be a great approach to modulate the optical and electrical properties of nanomaterials, the spontaneous covalent functionalization on semiconducting 2H-MoS₂ is a notoriously difficult process, while several reactions have been performed on metallic 1T-MoS₂. This limitation in functionalization is attributed to the difficulty of electron transfer from 2H-TMD to the reacting molecules due to its semiconducting property and neutral charge state. Unfortunately, this is an all too important prerequisite step toward creating chemically reactive radical species for surface functionalization reactions. Herein, an electrochemical approach was developed for facilitating direct surface functionalization of 2H-MoS₂ with 4-bromobenzene diazonium tetraborate (4-BBDT). Successful functionalization was characterized using various microscopic and spectroscopic analyses. During the course of investigating the change of optical transition seen for modified 2H-MoS₂ using photoluminescence measurement combined with theoretical calculations, our study uncovered that the controlling S-C bond and sulfur vacancy generation could tune the electronic structure of functionalized 2H-MoS₂.

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.

Exfoliation of 2D Materials for Energy and Environmental Applications

The fascinating properties of single-layer graphene isolated by mechanical exfoliation have inspired extensive research efforts toward two-dimensional (2D) materials. Layered compounds serve as precursors for atomically thin 2D materials (briefly, 2D nanomaterials) owing to their strong intraplane chemical bonding but weak interplane van der Waals interactions. There are newly emerging 2D materials beyond graphene, and it is becoming increasingly important to develop cost-effective, scalable methods for producing 2D nanomaterials with controlled microstructures and properties. The variety of developed synthetic techniques can be categorized into two classes: bottom-up and top-down approaches. Of top-down approaches, the exfoliation of bulk 2D materials into single or few layers is the most common. This review highlights chemical and physical exfoliation methods that allow for the production of 2D nanomaterials in large quantities. In addition, remarkable examples of utilizing exfoliated 2D nanomaterials in energy and environmental applications are introduced.

Controlled Covalent Functionalization of 2 H-MoS2 with Molecular or Polymeric Adlayers

Most air-stable 2D materials are relatively inert, which makes their chemical modification difficult. In particular, in the case of MoS₂ , the semiconducting 2 H-MoS₂ is much less reactive than its metallic counterpart, 1T-MoS₂ . As a consequence, there are hardly any reliable methods for the covalent modification of 2 H-MoS₂ . An ideal method for the chemical functionalization of such materials should be both mild, not requiring the introduction of a large number of defects, and versatile, allowing for the decoration with as many different functional groups as possible. Herein, a comprehensive study on the covalent functionalization of 2 H-MoS₂ with maleimides is presented. The use of a base (Et₃ N) leads to the in situ formation of a succinimide polymer layer, covalently connected to MoS₂ . In contrast, in the absence of base, functionalization stops at the molecular level. Moreover, the functionalization protocol is mild (occurs at room temperature), fast (nearly complete in 1 h), and very flexible (11 different solvents and 10 different maleimides tested). In practical terms, the procedures described here allow for the chemist to manipulate 2 H-MoS₂ in a very flexible way, decorating it with polymers or molecules, and with a wide range of functional groups for subsequent modification. Conceptually, the spurious formation of an organic polymer might be general to other methods of functionalization of 2D materials, where a large excess of molecular reagents is typically used.

Two-dimensional MoS2: a platform for constructing three-dimensional structures using RAFT polymerization

Controlled and straightforward functionalization are relevant strategies to obtain MoS₂ platforms with defined functionality and improved processability.

Simultaneous non-covalent bi-functionalization of 1T-MoS2 ruled by electrostatic interactions: towards multi-responsive materials

Dual functionalization of chemically exfoliated MoS₂ has been achieved by exploiting coulombic interactions among positively charged molecules and the negatively charged 2D flakes.

Tailoring the physicochemical properties of solution-processed transition metal dichalcogenides via molecular approaches

In this Feature Article we highlight the tremendous progress in solution-processed transition metal dichalcogenides and the molecular approaches employed to finely tune their physicochemical properties.