Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) like molybdenum disulfide (MoS 2 ) are generating significant excitement due to their unique electronic, chemical, and optical properties. Covalent chemical functionalization represents a critical tool for tuning the properties of TMDCs for use in many applications. However, the chemical inertness of semiconducting TMDCs has thus far hindered the robust chemical functionalization of these materials. Previous reports have required harsh chemical treatments or converting TMDCs into metallic phases prior to covalent attachment. Here, we demonstrate the direct covalent functionalization of the basal planes of unmodified semiconducting MoS 2 using aryl diazonium salts without any pretreatments. Our approach preserves the semiconducting properties of MoS 2 , results in covalent C-S bonds, is applicable to MoS 2 derived from a range of different synthesis methods, and enables a range of different functional groups to be tethered directly to the MoS 2 surface. Using density functional theory calculations including van der Waals interactions and atomic-scale scanning probe microscopy studies, we demonstrate a novel reaction mechanism in which cooperative interactions enable the functionalization to propagate along the MoS 2 basal plane. The flexibility of this covalent chemistry employing the diverse aryl diazonium salt family is further exploited to tether active proteins to MoS 2 , suggesting future biological applications and demonstrating its use as a versatile and powerful chemical platform for enhancing the utility of semiconducting TMDCs.

Atomically thin transition-metal dichalcogenides (TMDs) have attracted considerable interest because of their unique combination of properties, including photoluminescence, high lubricity, flexibility, and catalytic activity. These unique properties suggest future uses for TMDs in medical applications such as orthodontics, endoscopy, and optogenetics. However, few studies thus far have investigated the biocompatibility of mechanically exfoliated and chemical vapor deposition (CVD)grown pristine two-dimensional TMDs. Here, we evaluate pristine molybdenum disulfide (MoS 2 ) and tungsten disulfide (WS 2 ) in a series of biocompatibility tests, including live−dead cell assays, reactive oxygen species (ROS) generation assays, and direct assessment of cellular morphology of TMD-exposed human epithelial kidney cells (HEK293f). Genotoxicity and genetic mutagenesis were also evaluated for these materials via the Ames Fluctuation test with the bacterial strain S. typhimurium TA100. Scanning electron microscopy of cultured HEK293f cells in direct contact with MoS 2 and WS 2 showed no impact on cell morphology. HEK293f cell viability, evaluated by both live−dead fluorescence labeling to detect acute toxicity and ROS to monitor for apoptosis, was unaffected by these materials. Exposure of bacterial cells to these TMDs failed to generate genetic mutation. Together, these findings demonstrate that neither mechanically exfoliated nor CVD-grown TMDs are deleterious to cellular viability or induce genetic defects. Thus, these TMDs appear biocompatible for future application in medical devices.

The metal diborides are a class of ceramic materials with crystal structures consisting of hexagonal sheets of boron atoms alternating with planes of metal atoms held together with mixed character ionic/covalent bonds. Many of the metal diborides are ultrahigh temperature ceramics like HfB2, TaB2, and ZrB2, which have melting points above 3000ºC, high mechanical hardness and strength at high temperatures, and high chemical resistance, while MgB2 is a superconductor with a transition temperature of 39 K. Here we demonstrate that this diverse family of non-van der Waals materials can be processed into stable dispersions of two-dimensional (2D) nanosheets using ultrasonication-assisted exfoliation. We generate 2D nanosheets of the metal diborides AlB2, CrB2, HfB2, MgB2, NbB2, TaB2, TiB2, and ZrB2, and use electron and scanning probe microscopies to characterize their structures, morphologies, and compositions. The exfoliated layers span up to micrometers in lateral dimension and reach thicknesses down to 2-3 nm, while retaining their hexagonal atomic structure and chemical composition. We exploit the convenient solution-phase dispersions of exfoliated CrB2 nanosheets to incorporate them directly into polymer composites. In contrast to the hard and brittle bulk CrB2, we find that CrB2 nanocomposites remain very flexible and simultaneously provide increases in the elastic modulus and the ultimate tensile strength of the polymer. The successful liquid-phase production of 2D metal diborides enables their processing using scalable low-temperature solution-phase methods, extending their use to previously unexplored applications, and reveals a new family of non-van der Waals materials that can be efficiently exfoliated into 2D forms.The metal diborides are a family of ceramic compounds with the general formula MB2, where M can be a variety of metals from Groups II, IVB and VB such as Hf, Cr, Ti, Zr, Mg, etc.The most common crystal structure for these materials is the AlB2 type, belonging to the P6/mmm space group symmetry, which has the boron atoms in covalently bound hexagonal sheets separated by layers of close-packed metal atoms, 1 as shown in Figure 1a. The interactions between the

Boron carbide (B4C) nanosheets were prepared using liquid phase exfoliation from bulk. Density functional theory showed how cleavage can occur along several different planes, stabilized rearranging the boron-rich cages into smaller ones.