Synthesis and properties of free-standing monolayer amorphous carbon.

Bulk amorphous materials have been studied extensively and are widely used, yet their atomic arrangement remains an open issue. Although they are generally believed to be Zachariasen continuous random networks1, recent experimental evidence favours the competing crystallite model in the case of amorphous silicon2-4. In two-dimensional materials, however,  the corresponding questions remain unanswered. Here we report the synthesis, by laser-assisted chemical vapour deposition5, of centimetre-scale, free-standing, continuous and stable monolayer amorphous carbon, topologically distinct from disordered graphene. Unlike in bulk materials, the structure of monolayer amorphous carbon can be determined by atomic-resolution imaging. Extensive characterization by Raman and X-ray spectroscopy and transmission electron microscopy reveals the complete absence of long-range periodicity and a threefold-coordinated structure with a wide distribution of bond lengths, bond angles, and five-, six-, seven- and eight-member rings. The ring distribution is not a Zachariasen continuous random network, but resembles the competing (nano)crystallite model6. We construct a corresponding model that enables density-functional-theory calculations of the properties of monolayer amorphous carbon, in accordance with observations. Direct measurements confirm that it is insulating, with resistivity values similar to those of boron nitride grown by chemical vapour deposition. Free-standing monolayer amorphous carbon is surprisingly stable and deforms to a high breaking strength, without crack propagation from the point of fracture. The excellent physical properties of this stable, free-standing monolayer amorphous carbon could prove useful for permeation and diffusion barriers in applications such as magnetic recording devices and flexible electronics.

Palladium-Catalyzed Carbonylations: Application in Complex Natural Product Total Synthesis and Recent Developments.

Carbon monoxide is a cheap and abundant C1 building block that can be readily incorporated into organic molecules to rapidly build structural complexity. In this Perspective, we outline several recent (since 2015) examples of palladium-catalyzed carbonylations in streamlining complex natural product total synthesis and highlight the strategic importance of these carbonylation reactions in the corresponding synthesis. The selected examples include spinosyn A, callyspongiolide, perseanol, schizozygane alkaloids, cephanolides, and bisdehydroneostemoninine and related stemona alkaloids. We also provide our perspective about the recent advancements and future developments of palladium-catalyzed carbonylations.

Nickel-Catalyzed Tandem Ueno-Stork Cyclization: Stereoselective 1,2-Dicarbofunctionalization of Cyclic Alkenes.

A nickel-catalyzed tandem Ueno-Stork cyclization is developed to enable stereoselective 1,2-dicarbofunctionalization of cyclic alkenes and efficiently build various bicyclic products. This new protocol does not involve any toxic or difficult-to-remove tin reagent and is scalable and amenable to build all-carbon quaternary centers.

Catalysis-Enabled Concise and Stereoselective Total Synthesis of the Tricyclic Prostaglandin D2 Metabolite Methyl Ester.

A concise and stereoselective total synthesis of the clinically relevant tricyclic prostaglandin D2 metabolite (tricyclic-PGDM) methyl ester in racemic form was accomplished in eight steps from a readily available known cyclopentene-diol derivative. The synthesis features a nickel-catalyzed Ueno-Stork-type dicarbofunctionalization to generate two consecutive stereocenters, a palladium-catalyzed carbonylative spirolactonization to build the core oxaspirolactone, and a Z-selective cross-metathesis to introduce the (Z)-3-butenoate side chain, a group challenging to introduce through traditional Wittig protocols and troublesome for the two previous total syntheses. A general Z-selective cross-metathesis protocol to construct (Z)-ß,γ-unsaturated esters was also developed that has broad functional group tolerance and high stereoselectivity. Additionally, our synthesis already accumulated 75 mg of valuable material for an 18 O-tricyclic-PGDM-based assay used in clinical settings for inflammation.

Electron-Beam-Induced Synthesis of Hexagonal 1 H-MoSe2 from Square ß-FeSe Decorated with Mo Adatoms.

Two-dimensional (2D) materials have generated interest in the scientific community because of the advanced electronic applications they might offer. Powerful electron beam microscopes have been used not only to evaluate the structures of these materials but also to manipulate them by forming vacancies, nanofragments, and nanowires or joining nanoislands together. In this work, we show that the electron beam in a scanning transmission electron microscope (STEM) can be used in yet another way: to mediate the synthesis of 2D 1 H-MoSe2 from Mo-decorated 2D ß-FeSe and simultaneously image the process on the atomic scale. This is quite remarkable given the different crystal structures of the reactant (square lattice ß-FeSe) and the product (hexagonal lattice 1 H-MoSe2). The feasibility of the transformation was first explored by theoretical calculations that predicted that the reaction is exothermic. Furthermore, a theoretical reaction path to forming a stable 1 H-MoSe2 nucleation kernel within pure ß-FeSe was found, demonstrating that the pertinent energy barriers are smaller than the energy supplied by the STEM electron beam.

Mono-, few-, and multiple layers of copper antimony sulfide (CuSbS2): a ternary layered sulfide.

Layered materials with controlled thickness down to monolayer are being intensively investigated for unraveling and harnessing their dimension-dependent properties. Copper antimony sulfide (CuSbS2) is a ternary layered semiconductor material that has been considered as an absorber material in thin film solar cells due to its optimal band gap (∼1.5 eV) with high absorption coefficient of over >10(4) cm(-1). We have for the first time developed solution-based approaches for the synthesis of mono-, few-, and multiple layers of CuSbS2. These include a colloidal bottom-up approach for the synthesis of CuSbS2 nanoplates with thicknesses from six layers to several layers, and a hybrid bottom-up-top-down approach for the formation of CuSbS2 mesobelts. The latter can be exfoliated by Li-ion intercalation and sonication to obtain layers down to monolayer thickness. Time-dependent TEM studies provide important insights into the growth mechanism of mesobelts. At the initial stage the nanoplates grow laterally to form nanosheets as the primary structure, followed by their folding and attachment through homoepitaxy to form prolate-like secondary structures. Eventually, these prolate-like structures form mesocrystals by oriented attachment crystal growth. The changes in optical properties with layer thickness down to monolayers have been studied. In order to understand the thickness-dependent optical and electrical properties, we have calculated the electronic structures of mono- and multiple layers (bulk) of CuSbS2 using the hybrid functional method (HSE 06). We find that the monolayers exhibit noticeably different properties from the multilayered or the bulk system, with a markedly increased band gap that is, however, compromised by the presence of localized surface states. These localized states are predominantly composed of energetically favorable Sb pz states, which break off from the rest of the Sb p states that would otherwise be at the top of the gap. The developed solution-based synthesis approaches are versatile and can likely be extended to other complex layered sulfides.

Taming PRMT5-adaptor protein interactions.

Protein arginine methyltransferase (PRMT)-5 is a prominent epigenetic regulator and therapeutic target. Recently, Krzyzanowski et al. identified stapled peptides that inhibit the interaction of PRMT5 with two of its adaptor proteins. This discovery creates opportunities for novel therapeutic development by selectively modulating PRMT5 activity.

A Degradable Difunctional Initiator for ATRP That Responds to Hydrogen Peroxide.

Mid-chain degradable polymers can be prepared by atom transfer radical polymerization from difunctional initiators that include triggers for the desired stimuli. While many difunctional initiators can respond to reducing conditions, procedures to prepare difunctional initiators that respond to oxidizing conditions are significantly less available in the literature. Here, a difunctional initiator incorporating an oxidizable boronic ester trigger was synthesized over four steps using simple and scalable procedures. Methyl methacrylate was polymerized by atom transfer radical polymerization using this initiator, and the polymerization kinetics were consistent with a controlled polymerization. The polymer synthesized using the difunctional initiator was found to decrease in molecular weight by 58% in the presence of hydrogen peroxide, while a control experiment using poly(methyl methacrylate) without a degradable linkage showed a much smaller decrease in molecular weight of only 9%. These observed molecular weight decreases were consistent with cleavage of the difunctional initiator via a quinone methide shift and hydrolysis of the methyl ester pendent groups in both polymers, and both polymers increased in polydispersity after oxidative degradation.

Discovery and Biological Characterization of PRMT5:MEP50 Protein-Protein Interaction Inhibitors.

Protein arginine methyltransferase 5 (PRMT5) is a master epigenetic regulator and an extensively validated therapeutic target in multiple cancers. Notably, PRMT5 is the only PRMT that requires an obligate cofactor, methylosome protein 50 (MEP50), to function. We developed compound 17, a novel small-molecule PRMT5:MEP50 protein-protein interaction (PPI) inhibitor, after initial virtual screen hit identification and analogue refinement. Molecular docking indicated that compound 17 targets PRMT5:MEP50 PPI by displacing the MEP50 W54 burial into a hydrophobic pocket of the PRMT5 TIM barrel. In vitro analysis indicates IC50 < 500 nM for prostate and lung cancer cells with selective, specific inhibition of PRMT5:MEP50 substrate methylation and target gene expression, and RNA-seq analysis suggests that compound 17 may dysregulate TGF-ß signaling. Compound 17 provides a proof of concept in targeting PRMT5:MEP50 PPI, as opposed to catalytic targeting, as a novel mechanism of action and supports further preclinical development of inhibitors in this class.

Synthesis of Co-Doped MoS2 Monolayers with Enhanced Valley Splitting.

Internal magnetic moments induced by magnetic dopants in MoS2 monolayers are shown to serve as a new means to engineer valley Zeeman splitting (VZS). Specifically, successful synthesis of monolayer MoS2 doped with the magnetic element Co is reported, and the magnitude of the valley splitting is engineered by manipulating the dopant concentration. Valley splittings of 3.9, 5.2, and 6.15 meV at 7 T in Co-doped MoS2 with Co concentrations of 0.8%, 1.7%, and 2.5%, respectively, are achieved as revealed by polarization-resolved photoluminescence (PL) spectroscopy. Atomic-resolution electron microscopy studies clearly identify the magnetic sites of Co substitution in the MoS2 lattice, forming two distinct types of configurations, namely isolated single dopants and tridopant clusters. Density functional theory (DFT) and model calculations reveal that the observed enhanced VZS arises from an internal magnetic field induced by the tridopant clusters, which couples to the spin, atomic orbital, and valley magnetic moment of carriers from the conduction and valence bands. The present study demonstrates a new method to control the valley pseudospin via magnetic dopants in layered semiconducting materials, paving the way toward magneto-optical and spintronic devices.

Directed Atom-by-Atom Assembly of Dopants in Silicon.

The ability to controllably position single atoms inside materials is key for the ultimate fabrication of devices with functionalities governed by atomic-scale properties. Single bismuth dopant atoms in silicon provide an ideal case study in view of proposals for single-dopant quantum bits. However, bismuth is the least soluble pnictogen in silicon, meaning that the dopant atoms tend to migrate out of position during sample growth. Here, we demonstrate epitaxial growth of thin silicon films doped with bismuth. We use atomic-resolution aberration-corrected imaging to view the as-grown dopant distribution and then to controllably position single dopants inside the film. Atomic-scale quantum-mechanical calculations corroborate the experimental findings. These results indicate that the scanning transmission electron microscope is of particular interest for assembling functional materials atom-by-atom because it offers both real-time monitoring and atom manipulation. We envision electron-beam manipulation of atoms inside materials as an achievable route to controllable assembly of structures of individual dopants.

Homo-endotaxial one-dimensional Si nanostructures.

One-dimensional (1D) nanostructures are highly sought after, both for their novel electronic properties as well as for their improved functionality. However, due to their nanoscale dimensions, these properties are significantly affected by the environment in which they are embedded. In this paper, we report on the creation of 1D homo-endotaxial Si nanostructures, i.e. 1D Si nanostructures with a lattice structure that is uniquely different from the Si diamond lattice in which they are embedded. We use scanning tunneling microscopy and spectroscopy, scanning transmission electron microscopy, density functional theory, and conductive atomic force microscopy to elucidate their formation and properties. Depending on kinetic constraints during growth, they can be prepared as endotaxial 1D Si nanostructures completely embedded in crystalline Si, or underneath a stripe of amorphous Si containing a large concentration of Bi atoms. These homo-endotaxial 1D Si nanostructures have the potential to be useful components in nanoelectronic devices based on the technologically mature Si platform.