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

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.

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.

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.

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.

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.

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.

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.

A MoS2 platform and thionine-carbon nanodots for sensitive and selective detection of pathogens

This work focuses on the combination of molybdenum disulfide (MoS₂) and à la carte functionalized carbon nanodots (CNDs) for the development of DNA biosensors for selective and sensitive detection of pathogens. MoS₂ flakes prepared through liquid-phase exfoliation, serves as platform for thiolated DNA probe immobilization, while thionine functionalized carbon nanodots (Thi-CNDs) are used as electrochemical indicator of the hybridization event. Spectroscopic and electrochemical studies confirmed the interaction of Thi-CNDs with DNA. As an illustration of the pathogen biosensor functioning, DNA sequences from InIA gen of Listeria monocytogenes bacteria and open reading frame sequence (ORF1ab) of SARS-CoV-2 virus were detected and quantified with a detection limit of 67.0 fM and 1.01 pM, respectively. Given the paradigmatic selectivity of the DNA hybridization, this approach allows pathogen detection in the presence of other pathogens, demonstrated by the detection of Listeria monocytogenes in presence of Escherichia coli. We note that this design is in principle amenable to any pathogen for which the DNA has been sequenced, including other viruses and bacteria. As example of the application of the method in real samples it has been used to directly detect Listeria monocytogenes in cultures without any DNA Polymerase Chain Reaction (PCR) amplification process.

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.

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.

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.

Self-limiting covalent modification of carbon surfaces: diazonium chemistry with a twist

The chemistry of carbon surfaces has regained traction in recent years in view of its applicability towards covalent modification of a variety of (2D) materials. A general requisite is the formation of a dense and well-defined monolayer of aryl groups covalently bound to the surface. Given the use of reactive chemistries however, it is often not easy to achieve precise control over the monolayer growth while maintaining high grafting densities. Here we present a straightforward experimental protocol for the fabrication of well-defined covalent monolayers onto the surface of graphite. Using a combination of surface analytical tools, we demonstrate that the ascorbic acid mediated dediazoniation of aryldiazonium salts leads to self-limiting growth of monolayers with high grafting densities. The aryl radicals preferentially attach to the basal plane of the substrate and once the surface is covered with a covalent monolayer, the surface reaction does not proceed further to an appreciable extent. The layer thickness of the covalent films was measured using atomic force microscopy whereas the grafting efficiencies were assessed using Raman spectroscopy. The chemical composition of the grafted films was studied using X-ray photoelectron spectroscopy whereas scanning tunneling microscopy provided nanometer scale insight into the structure of the covalent films. Mechanistic aspects of the process are also discussed. The self-terminating chemistry described here is a new addition to the synthetic armory for covalent modification of materials and sets a strong foundation for achieving precise nanoscale control over the covalent functionalization process.

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.