These authors contributed equally: Tingxin Li, Shengwei Jiang.Stacking order can significantly influence the physical properties of two-dimensional (2D) van der Waals materials 1 . The recent isolation of atomically thin magnetic materials 2-22 opens the door for control and design of magnetism via stacking order. Here we apply hydrostatic pressure up to 2 GPa to modify the stacking order in a prototype van der Waals magnetic insulator CrI3. We observe an irreversible interlayer antiferromagnetic (AF) to ferromagnetic (FM) transition in atomically thin CrI3 by magnetic circular dichroism and electron tunneling measurements. The effect is accompanied by a monoclinic to a rhombohedral stacking order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer AF coupling energy can be tuned up by nearly 100% by pressure. Our experiment reveals interlayer FM coupling, which is the established ground state in bulk CrI3, but never observed in native exfoliated thin films. The observed correlation between the magnetic ground state and the stacking order is in good agreement with first principles calculations 23-27 and suggests a route towards nanoscale magnetic textures by moiré engineering 28 .Intrinsic magnetism in 2D van der Waals materials has received growing attention 2-22 . Of particular interest is the thickness-dependent magnetic ground state in atomically thin CrI3. In these exfoliated thin films, the magnetic moments are aligned (in the out-of-plane direction) in each layer, but anti-aligned in adjacent layers 3,12-22 . They are FM (or AF) depending on whether there is (or isn't) an uncompensated layer. The relatively weak interlayer coupling compared to the intralayer coupling allows effective ways to control the interlayer magnetism, which have led to interesting spintronics applications including voltage switching 12-14 , spin filtering 16-20 and spin transistors 21 . The origin of interlayer AF coupling is, however, not well understood since interlayer FM order is the ground state in the bulk crystals. Recent ab initio calculations 23-27 and experiments 22,29,30 have suggested that stacking order could provide an explanation but a direct correlation between stacking order and interlayer magnetism is lacking.In bulk CrI3, the Cr atoms in each layer form a honeycomb structure, and each Cr atom is surrounded by six I atoms in an octahedral coordination (Fig. 1a). The bulk crystals undergo a structural phase transition from a monoclinic phase (space group C2/m) at room temperature to a

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As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide is the latest addition to the family of two-dimensional (2D) materials. However, realistic exploration of CrI-based devices and heterostructures is challenging due to its extreme instability under ambient conditions. Here, we present Raman characterization of CrI and demonstrate that the main degradation pathway of CrI is the photocatalytic substitution of iodine by water. While simple encapsulation by AlO, PMMA, and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing light exposure markedly improves stability, and CrI sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of the CrI/graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene.

The fabrication of highly active and robust hexagonal ruthenium oxide nanosheets for the electrocatalytic oxygen evolution reaction (OER) in an acidic environment is reported. The ruthenate nanosheets exhibit the best OER activity of all solution‐processed acid medium electrocatalysts reported to date, reaching 10 mA cm−2 at an overpotential of only ≈255 mV. The nanosheets also demonstrate robustness under harsh oxidizing conditions. Theoretical calculations give insights into the OER mechanism and reveal that the edges are the origin of the high OER activity of the nanosheets. Moreover, the post OER analyses indicate, apart from coarsening, no observable change in the morphology of the nanosheets or oxidation states of ruthenium during the electrocatalytic process. Therefore, the present investigation suggests that ruthenate nanosheets are a promising acid medium OER catalyst with application potential in proton exchange membrane electrolyzers and beyond.

Interaction between micrometer-long nanoantennas within an array considerably modifies the plasmonic resonant behaviour; for fundamental resonances in the infrared already at micrometer distances. In order to get systematic knowledge on the relationship between infrared plasmonic resonances and separation distances dx and dy in longitudinal and transverse direction, respectively, we experimentally studied the optical extinction spectra for rectangularly ordered lithographic gold nanorod arrays on silicon wafers. For small dy, strong broadening of resonances and strongly decreased values of far-field extinction are detected which come along with a decreased near-field intensity, as indicated by near-field amplitude maps of the interacting nanoantennas. In contrast, near-field interaction over small dx does only marginally broaden the resonance. Our findings set a path for optimum design of rectangular nanorod lattices for surface enhanced infrared spectroscopy.

The emergence of two-dimensional (2D) magnetic crystals and moiré engineering of van der Waals materials has opened the door for devising new magnetic ground states via competing interactions in moiré superlattices 1-9 . Although a suite of interesting phenomena, including multi-flavor magnetic states 10 , noncollinear magnetic states 10-13 , moiré magnon bands and magnon networks 14 , has been predicted in twisted bilayer magnetic crystals, nontrivial magnetic ground states have yet to be realized. Here, by utilizing the stacking-dependent interlayer exchange interactions in CrI3 (Ref. 15, 16 ), we demonstrate in small-twist-angle CrI3 bilayers a noncollinear magnetic ground state. It consists of antiferromagnetic (AF) and ferromagnetic (FM) domains and is a result of the competing interlayer AF coupling in the monoclinic stacking regions of the moiré superlattice and the energy cost for forming AF-FM domain walls. Above a critical twist angle of ~ 𝟑°, the noncollinear state transitions to a collinear FM ground state. We further show that the noncollinear magnetic state can be controlled by electrical gating through the doping-dependent interlayer AF interaction. Our results demonstrate the possibility of engineering new magnetic ground states in twisted bilayer magnetic crystals, as well as gate-voltage-controllable high-density magnetic memory storage.Moiré superlattices built on twisted bilayers of van der Waals materials have presented an exciting platform for studying correlated states of matter with unprecedented controllability [7][8][9] . In addition to graphene and transition metal dichalcogenide moiré materials 17 , recent theoretical studies have predicted the emergence of new magnetic ground states in twisted bilayers of 2D magnetic crystals [10][11][12][13][14] . These states are originated from the stacking-dependent interlayer exchange interactions in magnetic moiré superlattices. Two-dimensional CrI3 (similarly CrBr3 18 and CrCl3 19 ), in which stackingdependent interlayer magnetic ground states have been demonstrated by recent experiments 15, 16 , is a good candidate for exploring moiré magnetism.

The rational design of hydrogen evolution reaction (HER) electrocatalysts which are competitive with platinum is an outstanding challenge to make power-to-gas technologies economically viable. Here, we introduce the delafossites PdCrO 2 , PdCoO 2 and PtCoO 2 as a new family of electrocatalysts for the HER in acidic media. We show that in PdCoO 2 the inherently strained Pd metal sublattice acts as a pseudomorphic template for the growth of a strained (by +2.3%) Pd rich capping layer under reductive conditions. The surface modification continuously improves the electrocatalytic activity by simultaneously increasing the exchange current density j 0 from 2 to 5 mA/cm² geo and by reducing the Tafel slope down to 38 mV/decade, leading to overpotentials 10 < 15 mV for 10 mA/cm² geo , superior to bulk platinum. The greatly improved activity is attributed to the in-situ stabilization of a β-palladium hydride phase with drastically enhanced surface catalytic properties with respect to pure or nanostructured palladium. These findings illustrate how operando induced electrodissolution can be used as a top-down design concept for rational surface and property engineering through the strainstabilized formation of catalytically active phases.

Spin 1 2 honeycomb materials have gained substantial interest due to their exotic magnetism and possible application in quantum computing. However, in all current materials out-of-plane interactions are interfering with the in-plane order, hence a true 2D magnetic honeycomb system is still of demand. Here, we report the exfoliation of the magnetic semiconductor α-RuCl 3 into the first halide monolayers and the magnetic characterization of the spin 1 2 honeycomb arrangement of turbostratically stacked RuCl 3 monolayers. The exfoliation is based on a reductive lithiation/hydration approach, which gives rise to a loss of cooperative magnetism due to the disruption of the spin 1 2 state by electron injection into the layers. After an oxidative treatment, cooperative magnetism similar to the bulk is restored. The oxidized pellets of restacked single layers feature a magnetic transition at T N = 7 K in the in-plane direction, while the magnetic properties in the out-of-plane direction vastly differ from bulk α-RuCl 3 . The macro- Binary halide nanosheets have been predicted based on chemical intuition 3,4 or ab initio calculations. 7 Yet, no single layer halides have been synthesized so far, even though this class of compounds features an array of interesting electrical and magnetic properties.The magnetic semiconductor α-RuCl 3 is one such example. While it was investigated in the past as a host for intercalants 8,9 and as a lithium ion conductor, 10 current research focuses on its magnetic properties. Due to its layered honeycomb structure of spin 1 2 Ru 3+ centers in combination with spin orbit coupling (SOC), it is one of the few known materials featuring a zigzag antiferromagnetic (AF) ground state below a temperature of T N1 = 8 K. [11][12][13] In the zigzag order, the magnetic moments form ferromagnetic (FM) zigzag chains, whose magnetization direction is opposed to the neighboring chains within the plane. Additionally, there is a further magnetic phase transition observed at T N2 = 14 K. The origin of this transition is currently still under debate. This type of ordering was first observed in Na 2 IrO 3 14-16 and explained by the Kitaev-Heisenberg model, 17,18 which describes that a frustrated spin 1 2 honeycomb arrangement could lead to a variety of interesting spin structures. Based on the competition among the exchange interactions up to the third neighbor, the system could possibly be pushed into a quantum spin liquid regime by the manipulation of the competing interactions, thereby opening up applications in quantum computing. 17,19 Yet, the Na + ions in the interlayer space of Na 2 IrO 3 lead to disadvantageous interactions between the iridate layers, which interfere with theoretical predictions of a honeycomb arrange-2 ment of spin 1 2 magnetic arrays. 20 Eliminating the interlayer interaction could provide a route to manipulate the spin structure of real materials featuring a spin 1 2 honeycomb arrangement. In RuCl 3 , where no charged ions are in between the honeycomb layers, the interlayer in...