Xiangfeng Duan

Molybdenum disulfide (MoS2) is an extensively studied two-dimensional layered semiconductor with interesting electronic and optical properties. Monolayer MoS2 features strong light–matter interactions due to its direct bandgap, whereas multilayer MoS2 is an indirect bandgap semiconductor and optically inactive. The molecular intercalation of MoS2 with organic cations offers a strategy to decouple the interlayer interaction, producing a bulk monolayer material, but is usually accompanied by a heavy electron doping effect that can diminish the intrinsic semiconductor properties or induce a phase transition. Here we report a chemical-dedoping strategy to tailor electron density in molecular-intercalated MoS2, thereby retaining monolayer semiconductor properties. By introducing a poly(vinylpyrrolidone)–bromine complex during the electrochemical intercalation process, we show that bulk monolayer MoS2 thin film …

The discovery of chiral-induced spin selectivity (CISS) offers a breakthrough in the ability to control spin orientation without the need for external magnetic fields, opening up exciting possibilities for innovative spintronic device designs. While numerous methods have been explored to introduce CISS into solid-state materials and devices, previous systems have often suffered from issues such as high inhomogeneity, low spin selectivity, limited stability, and challenges in creating robust spintronic devices. In this study, we introduce a novel class of chiral molecular intercalation superlattices (CMIS) as a stable and reliable platform for investigating CISS in solid-state materials. These CMIS structures were developed by intercalating chiral molecules into two-dimensional atomic layers. My utilizing these CMIS structures as spin-filtering layers, we achieved spin tunneling junctions with a tunnelling magnetoresistance …

Electrochemical hydrogenation of acetonitrile based on well-developed proton exchange membrane electrolyzers holds great promise for practical production of ethylamine. However, the local acidic condition of proton exchange membrane results in severe competitive proton reduction reaction and poor selection toward acetonitrile hydrogenation. Herein, we conduct a systematic study to screen various metallic catalysts and discover Pd/C exhibits a 43.8% ethylamine Faradaic efficiency at the current density of 200 mA cm−2 with a specific production rate of 2912.5 mmol g−1 h−1, which is about an order of magnitude higher than the other screened metal catalysts. Operando characterizations indicate the in-situ formed PdHx is the active centers for catalytic reaction and the adsorption strength of the *MeCH2NH2 intermediate dictates the catalytic selectivity. More importantly, the theoretical analysis reveals a …

The Tafel slope represents a critical kinetic parameter for mechanistic studies of electrochemical reactions, including the hydrogen evolution reaction (HER). Linear fitting of the polarization curve in a N2-saturated electrolyte is commonly used to determine Tafel slopes, which is, however, frequently plagued with inconsistencies. Our systematic studies reveal that the Tafel slopes derived from this approach are loading- and potential-dependent, and could substantially exceed the theoretical limits. Our analyses indicate that this discrepancy is largely attributed to the locally trapped HER-generated H2 in the catalyst layer. A non-negligible hydrogen oxidation reaction (HOR) current more prominently offsets the HER current at the smaller HER overpotential regime, resulting in an artificially smaller Tafel slope. On the other hand, at the higher overpotential where the HOR current becomes negligible, the locally trapped H …

Nonlinear optical processing of ambient natural light is highly desired for computational imaging and sensing. Strong optical nonlinear response under weak broadband incoherent light is essential for this purpose. By merging 2D transparent phototransistors (TPTs) with liquid crystal (LC) modulators, we create an optoelectronic neuron array that allows self-amplitude modulation of spatially incoherent light, achieving a large nonlinear contrast over a broad spectrum at orders-of-magnitude lower intensity than achievable in most optical nonlinear materials. We fabricated a 10,000-pixel array of optoelectronic neurons, and experimentally demonstrated an intelligent imaging system that instantly attenuates intense glares while retaining the weaker-intensity objects captured by a cellphone camera. This intelligent glare-reduction is important for various imaging applications, including autonomous driving, machine …

The sulfur reduction reaction (SRR) plays a central role in high-capacity lithium sulfur (Li-S) batteries. The SRR involves an intricate, 16-electron conversion process featuring multiple lithium polysulfide intermediates and reaction branches 1, 2, 3. Establishing the complex reaction network is essential for rational tailoring of the SRR for improved Li-S batteries, but represents a daunting challenge 4, 5, 6. Herein we systematically investigate the electrocatalytic SRR to decipher its network using the nitrogen, sulfur, dual-doped holey graphene framework as a model electrode to understand the role of electrocatalysts in acceleration of conversion kinetics. Combining cyclic voltammetry, in situ Raman spectroscopy and density functional theory calculations, we identify and directly profile the key intermediates (S 8, Li 2 S 8, Li 2 S 6, Li 2 S 4 and Li 2 S) at varying potentials and elucidate their conversion pathways. Li 2 S 4 …

The reduced dimensionality and interfacial effects in magnetic nanostructures open the feasibility to tailor magnetic ordering. Here, we report the synthesis of ultrathin metallic Co2Si nanoplates with a total thickness that is tunable to 2.2 nm. The interfacial magnetism coupled with the highly anisotropic nanoplate geometry leads to strong perpendicular magnetic anisotropy and robust hard ferromagnetism at room temperature, with a Curie temperature (TC) exceeding 950 K and a coercive field (HC) > 4.0 T at 3 K and 8750 Oe at 300 K. Theoretical calculations suggest that ferromagnetism originates from symmetry breaking and undercoordinated Co atoms at the Co2Si and SiO2 interface. With protection by the self-limiting intrinsic oxide, the interfacial ferromagnetism of the Co2Si nanoplates exhibits excellent environmental stability. The controllable growth of ambient stable Co2Si nanoplates as 2D hard …

Hybrid superlattices, composed of two-dimensional atomic crystals (2DACs) and self-assembled molecular layers, provide a platform for synergizing the rich physical properties of solid-state 2DACs with customizable molecular functionalities. This study introduces a modular electrostatic co-assembly approach for synthesizing 53 distinct hybrid superlattices, using various 2DACs and molecular building blocks. Mechanistic studies reveal that the electrostatic assembly is governed by charge balance and the equilibrium between electrostatic and van der Waals interactions. Leveraging the charge balance principle, we demonstrate a precise control over the number of molecular layers and interlayer spacing by modulating the molecular charge density. Our study establishes a universal approach to a library of layered hybrid superlattices, incorporating versatile molecular functionalities into solid-state 2DACs. It …