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Over the past decade, there has beenmuch controversy regardingthe microscopic mechanism by which the π-electron-rich carbonnanomaterials such as graphene and carbon nanotubes can be dispersedin ionic liquids. Through a combination of a quantum mechanical calculationon the level of density functional theory, an extensive moleculardynamics study on the time scale of microseconds, and a kinetic analysisat the experimental time scale, we have demonstrated that collectivevan der Waals forces between ionic liquids and graphene are able todescribe both the short-ranged cation−π interaction andthe long-ranged dispersion interaction and this microscopic interactiondrives two graphene plates trapped in their metastable state whiletwo graphene plates easily self-assemble into graphite in water. [ABSTRACT FROM AUTHOR]

A consensus view in catalysis is that a higher density of catalytically active sites indicates a higher reaction rate. Using molecular dynamics simulations capable of mimicking the electrochemical formation of gas molecules, we herein demonstrate that this view is problematic for electrocatalytic gas production. Our simulation results show that a higher density of catalytic active sites does not necessarily indicate a higher reaction rate─a high density of active sites could lead to a reduction in the rate of reaction. Further analysis reveals that this abnormal phenomenon is ascribed to aggregation of the produced gas molecules near catalytic sites. This work challenges the consensus view and lays the groundwork for better developing gas-producing reaction electrocatalysts.

The low NH 3 yield rate is a grand challenge for electrocatalytic N 2 reduction to NH 3 . Herein, we report the first uranium single-atom catalyst (SAC) capable of catalyzing the electrochemical N 2 reduction reaction (NRR). The uranium SAC features a low limiting potential (<0.5 V) and near-zero free energy changes for N 2 adsorption and NH 3 desorption. The integration of these merits enables the uranium SAC to afford an unprecedentedly high NH 3 yield rate, 3-4 orders of magnitude higher than that of the Ru(0001) surface, which is widely recognized as an excellent NRR electrocatalyst. Further theoretical analysis reveals that the N 2 reduction catalyzed by the uranium SAC is synergistically regulated by the d and f electrons of atomic uranium. This work proposes a promising solution (that is, atomically dispersed uranium) to the daunting challenge associated with the low NH 3 yield rate, thus enabling the scalable production of NH 3 via electrochemical N 2 reduction.

The high electrical conductivity of 1T'-WTe 2 deserves particular attention and may show a high potential for hydrogen evolution reaction (HER) catalysis. However, the actual activity certainly does not match expectations, and the inferior HER activity is actually still ambiguous at the atomic level. Unraveling the underlying HER behaviors of 1T'-WTe 2 will give rise to a new family of HER catalysts. Our structural analysis reveals that the inferior activity could result from insufficient charge density around the Te site and blocked adsorption channel at the W site, which cause too weak hydrogen adsorption. Herein, we fabricated a single WTe 2 sheet-based electrocatalytic microdevice for directly extracting enhanced HER activity of doped electronegative F atoms. The overpotential at -10 mA cm -2 reduced to 0.27 V after F doping compared to 0.45 V for the original state. In situ electrochemical measurement and electrical tests on a single sheet indicate that doped F can regulate surface charge and hydrogen adsorption behavior. Furthermore, the theory simulation uncovers that the smaller atomic radius of F contributes to an empty coordination environment; meanwhile, strong electronegativity induces hydrogen adsorption. Thus, the Δ G H * at W sites around the doped F is as low as 0.18 eV. Synergistically modulating the charge properties and opening steric hindrance provides a new pathway to rationally construct electrocatalysts and beyond.

Recent discovery of intrinsic ferromagnetism in Fe 3 GeTe 2 (FGT) monolayer [Deng, Y.; Nature 2018, 563, 94-99; Fei, Z.; Nat. Mater. 2018, 17, 778-782] not only extended the family of two-dimensional (2D) magnetic materials but also stimulated further interest in the possibility to tune their magnetic properties without changing the chemical composition or introducing defects. By means of density functional theory computations, we explore strain effects on the magnetic properties of the FGT monolayer. We demonstrate that the ferromagnetism can be largely enhanced by the tensile strain in the FGT monolayer due to the competitive effects of direct exchange and superexchange interaction. The average magnetic moments of Fe atoms increase monotonically with an increase in biaxial strain from -5 to 5% in FGT monolayer. The intriguing variation of magnetic moments with strain in the FGT monolayer is related to the charge transfer induced by the changes in the bond lengths. Given the successful fabrication of the FGT monolayer, the strain-tunable ferromagnetism in the FGT monolayer can stimulate the experimental effort in this field. This work also suggests an effective route to control the magnetic properties of the FGT monolayer. The pronounced magnetic response toward the biaxial strain can be used to design the magnetomechanical coupling spintronics devices based on FGT.

Pentazole (HN 5 ) and its anion ( cyclo-N 5 - ) have been elusive for nearly a century because of the unstable N 5 ring. Recently, Zhang et al. reported the first synthesis and characterization of the pentazolate anion cyclo-N 5 - in (N 5 ) 6 (H 3 O) 3 (NH 4 ) 4 Cl salt at ambient conditions ( Science 2017, 355, 374 ). However, whether the cyclo-N 5 - in (N 5 ) 6 (H 3 O) 3 (NH 4 ) 4 Cl salt is protonated or not has been debated ( Huang and Xu, Science, 2018, 359, eaao3672 ; Jiang et al. Science, 2018, 359, aas8953 ). Herein, we employed ab initio molecular dynamics (AIMD) simulations, which can well present the dynamic behavior at realistic experimental conditions, to examine the potential protonated state of cyclo-N 5 - in both crystal and dimethyl sulfoxide (DMSO) solvent. Our simulations revealed that the protonation reaction of (N 5 ) 6 (H 3 O) 3 (NH 4 ) 4 Cl → (N 5 ) 5 (N 5 H)(H 2 O)(H 3 O) 2 (NH 4 ) 4 Cl is thermodynamically spontaneous according to Δ G < 0, and the small energy barrier of 12.6 kJ/mol is not enough to prevent the partial protonation of cyclo-N 5 - due to the temperature effect; consequently, both deprotonated and protonated cyclo-N 5 - exist in the crystal. In comparison, the DMSO solvent effect can remarkably reduce the difference of proton affinities among cyclo-N 5 - , H 2 O, and NH 3 , and the temperature effect can finally break these hydrogen bonds and lead to the deprotonated cyclo-N 5 - in DMSO solvent. Our AIMD simulations reconcile the recent controversy.

Monolayer chromium triiodide (CrI 3 ), as the thinnest ferromagnetic material demonstrated in experiment [ Huang et al. Nature 2017 , 546 , 270 ], opens up new opportunities for the application of two-dimensional (2D) materials in spintronic nanodevices. Atom-thick 2D materials with switchable electric polarization are now urgently needed for their rarity and important roles in nanoelectronics. Herein, we unveil that surface I vacancies not only enhance the intrinsic ferromagnetism of monolayer CrI 3 but also induce switchable electric polarization. I vacancies bring about an out-of-plane polarization without breaking the nonmetallic nature of CrI 3 . Meanwhile, the induced polarization can be reversed in a moderate energy barrier, arising from the unique porosity of CrI 3 that contributes to the switch of I vacancies between top and bottom surfaces. Engineering 2D switchable polarization through surface vacancies is also applicable to many other metal trihalides, which opens up a new and general way toward pursuing low-dimensional multifunctional nanodevices.

Ultrathin indium selenide (InSe), as a newly emerging two-dimensional material with high carrier mobility and a broad absorption spectrum, has been the focus of current research. However, the long-term environmental instability of atomically thin InSe seriously limits its practical applications. To develop an effective strategy to protect InSe, it is crucial to reveal the degradation mechanism at the atomic level. By employing density functional theory and ab initio molecular dynamics simulations, we provide an in-depth understanding of the oxidation mechanism of InSe. The defect-free InSe presents excellent stability against oxidation. Nevertheless, the Se vacancies on the surface can react with water and oxygen in air directly and activate the neighboring In-Se bonds on the basal plane for further oxidation, leading to complete degradation of InSe into oxidation products of In 2 O 3 and elemental Se. Furthermore, we propose an efficient strategy to repair the Se vacancies by thiol chemistry. In this way, the repaired surface can resist oxidation from oxygen and retain the original high electron mobility of pristine InSe simultaneously.

The chemical functionalization is proven to be an effective and controllable approach to modify the properties of black phosphorus (BP), and improve the air-stability of BP and its nanoelectronic applications [Nat. Chem., 2016, 8, 597]. However, covalent functionalization of BP and related properties are poorly understood. Here we present a theoretical investigation on the electronic structure and transport property of chemically modified BP. Our calculations reveal that the molecule modification generates a rather flat energy band within the bandgap, which leads to a reduced hole mobility of BP. Alternatively, we propose to use polymers bonded to BP surface, aiming at a balance between functionality and carrier mobility. The polymer-BP composites preserve both electron and hole mobility of pristine BP. Meanwhile, the stability of polymer-BP composites in ambient condition is enhanced as well.

Precise assembly of semiconductor heterojunctions is the key to realize many optoelectronic devices. By exploiting the strong and tunable van der Waals (vdW) forces between graphene and organic small molecules, we demonstrate layer-by-layer epitaxy of ultrathin organic semiconductors and heterostructures with unprecedented precision with well-defined number of layers and self-limited characteristics. We further demonstrate organic p-n heterojunctions with molecularly flat interface, which exhibit excellent rectifying behavior and photovoltaic responses. The self-limited organic molecular beam epitaxy (SLOMBE) is generically applicable for many layered small-molecule semiconductors and may lead to advanced organic optoelectronic devices beyond bulk heterojunctions.

van der Waals (vdW) epitaxy of ultrathin organic films on two-dimensional (2D) atomic crystals has become a sovereign area because of their unique advantages in organic electronic devices. However, the dynamic mechanism of the self-assembly remains elusive. Here, we visualize the nanoscale self-assembly of organic molecules on graphene and boron nitride monolayer from a disordered state to a 2D lattice via molecular dynamics simulation for the first time. It is revealed that the assembly toward 2D ordered structures is essentially the minimization of the molecule-molecule interaction, that is, the vdW interaction in nonpolar systems and the vdW and Coulomb interactions in polar systems that are the decisive factors for the formation of the 2D ordering. The role of the substrate is mainly governing the array orientation of the adsorbates. The mechanisms unveiled here are generally applicable to a broad class of organic thin films via vdW epitaxy.