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Topologically protected magnon surface states are highly desirable as an ideal platform to engineer low-dissipation spintronics devices. However, theoretical prediction of topological magnons in strongly correlated materials proves to be challenging because the ab initio density functional theory calculations fail to reliably predict magnetic interactions in correlated materials. Here, we present a symmetry-based approach, which predicts topological magnons in magnetically ordered crystals, upon applying external perturbations such as magnetic/electric fields and/or mechanical strains. We apply this approach to carry out an efficient search for magnetic materials in the Bilbao Crystallographic Server, where, among 198 compounds with an over 300-K transition temperature, we identify 12 magnetic insulators that support room-temperature topological magnons. They feature Weyl magnons with surface magnon arcs and magnon axion insulators with either chiral surface or hinge magnon modes, offering a route to realize energy-efficient devices based on protected surface magnons.

Moire engineering1–3 of van der Waals magnetic materials4–9 can yield new magnetic ground states via competing interactions in moire superlattices10–13. Theory predicts a suite of interesting phenomena, including multiflavour magnetic states10, non-collinear magnetic states10–13, moire magnon bands and magnon networks14 in twisted bilayer magnetic crystals, but so far such non-trivial magnetic ground states have not emerged experimentally. Here, by utilizing the stacking-dependent interlayer exchange interactions in two-dimensional magnetic materials15–18, we demonstrate a coexisting ferromagnetic (FM) and antiferromagnetic (AF) ground state in small-twist-angle CrI3 bilayers. The FM–AF state transitions to a collinear FM ground state above a critical twist angle of about 3°. The coexisting FM and AF domains result from a competition between the interlayer AF coupling, which emerges in the monoclinic stacking regions of the moire superlattice, and the energy cost for forming FM–AF domain walls. Our observations are consistent with the emergence of a non-collinear magnetic ground state with FM and AF domains on the moire length scale10–13. We further employ the doping dependence of the interlayer AF interaction to control the FM–AF state by electrically gating a bilayer sample. These experiments highlight the potential to create complex magnetic ground states in twisted bilayer magnetic crystals, and may find application in future gate-voltage-controllable high-density magnetic memory storage. In moire superlattice van der Waals magnetic materials, competing interactions emerge and can stabilize new magnetic states. Here, stacking-dependent interlayer exchange interactions in small-twist-angle CrI3 bilayers yield an ordered ground state with coexisting ferromagnetic and antiferromagnetic regions.

Polytypism, the ability of materials to form crystal structures with different stacking sequences, occasionally causes materials with the same stoichiometry and similar local structures to have profoundly different properties. Herein, we discover a metastable 13-layer trigonal (13T) polytype of CaGaGe, a layered intermetallic phase comprised of [GaGe]2- honeycombs separated by Ca2+. 13T-CaGaGe is synthesized from arc-melting the elements, and its structure is elucidated via neutron powder diffraction. Air-stable 13T-CaGaGe has one misaligned [GaGe]2- layer for every 13 and transforms into the more stable 4-layer hexagonal (4H) CaGaGe polytype after annealing at 500 °C. Transition-metal-free 13T-CaGaGe shows remarkable activity in the catalytic hydrogenation of phenylacetylene to styrene and ethylbenzene, much higher than the 4H polytype. This work identifies the first 13-layer polytype for any crystal structure and further establishes the influence of polytypism on catalysis.

Transition-metal-free Zintl-Klemm phases have received little attention as heterogeneous catalysis. Here, we show that a large family of structurally and electronically similar layered Zintl-Klemm phases built from honeycomb layers of group 13 triel (Tr) or group 14 tetrel (Tt) networks separated by electropositive cations (A) and having a stoichiometry of ATr2 or ATrTt (A = Ca, Ba, Y, La, Eu; Tr = Ga, In; Tt = Si, Ge) exhibit varying degrees of activity for the hydrogenation of phenylacetylene to styrene and ethylbenzene at 51 bar H2 and 40-100 °C across a variety of solvents. The most active catalysts contain Ga with, formally, a half-filled pz orbital, and minimal bonding between neighboring Tr2 or TrTt layers. A 13-layer trigonal polytype of CaGaGe (13T-CaGaGe) was the most active, cyclable, and robust catalyst and under modest conditions (1 atm H2, 40 °C) had a surface specific activity (590 h-1) comparable to a commercial Lindlar's catalyst. Additionally, 13T-CaGaGe maintained 100% conversion of phenylacetylene to styrene at 51 bar H2, even after 5 months of air exposure. This work reveals the structural design elements that lead to particularly high catalytic activity in Zintl-Klemm phases, further establishing them as a promising materials platform for hydrogen-based heterogeneous catalysis.

Kagome magnets possess several novel nontrivial topological features owing to the strong correlation between topology and magnetism that extends to their applications in the field of thermoelectricity. Conventional thermoelectric (TE) devices use the Seebeck effect to convert heat into electrical energy. In contrast, transverse thermoelectric devices based on the Nernst effect are attracting recent attention due to their unique transverse geometry, which uses a single material to eliminate the need for a multitude of electrical connections compared to conventional TE devices. Here, a large anomalous transverse thermoelectric effect of ≈2 µV K

Most electronic materials exhibit a single dominant charge carrier type, either holes or electrons, along all crystallographic directions. However, there are a small number of compounds, mostly met...

Layered two-dimensional (2D) materials have attracted considerable interest for their exotic and anisotropic electronic behavior. One such material, Sn4As3, bears a resemblance in both structure and elemental composition to two other Sn- and As-containing layered materials that have recently demonstrated axis-dependent conduction polarity: NaSn2As2 and NaSnAs. Here, a new family of Pb-alloyed PbxSn4−xAs3 crystals was synthesized and the axis-dependent electronic and thermoelectric properties were evaluated. Up to one full equivalent of Pb could be alloyed into PbxSn4−xAs3 (0 < x < 1.06) before phase separation occurred. We establish the structural changes and the trends in the Raman spectra with increasing Pb substitution. These materials all exhibit metallic temperature-dependent resistivities and positive thermopowers along the in-plane and cross-plane directions. The absence of axis-dependent conduction polarity in these SnAs-layered materials is consistent with theoretical predictions, and illustrates that precise control over the atomic and electronic structure and doping is essential for realizing this phenomenon in new materials.

Stacking order can influence the physical properties of two-dimensional van der Waals materials1,2. Here we applied hydrostatic pressure up to 2 GPa to modify the stacking order in the van der Waals magnetic insulator CrI3. We observed an irreversible interlayer antiferromagnetic-to-ferromagnetic transition in atomically thin CrI3 by magnetic circular dichroism and electron tunnelling measurements. The effect was accompanied by a monoclinic-to-rhombohedral stacking-order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer antiferromagnetic coupling energy can be tuned up by nearly 100% with pressure. Our experiment reveals the interlayer ferromagnetic ground state, which is established in bulk CrI3 but not 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 calculations3–8 and suggests a route towards nanoscale magnetic textures by moire engineering3,9. Pressure-induced changes in the magnetic order of atomically thin van der Waals crystals are revealed and attributed to changes in the stacking arrangement.

Electronic materials generally exhibit a single isotropic majority carrier type, electrons or holes. Some superlattice1,2 and hexagonal3-5 materials exhibit opposite conduction polarities along in-plane and cross-plane directions due to multiple electron and hole bands. Here, we uncover a material genus with this behaviour that originates from the Fermi surface geometry of a single band. NaSn2As2, a layered metal, has such a Fermi surface. It displays in-plane electron and cross-plane hole conduction in thermopower and exactly the opposite polarity in the Hall effect. The small Nernst coefficient and magnetoresistance preclude multi-band transport. We label this direction-dependent carrier polarity in single-band systems 'goniopolarity'. We expect to find goniopolarity and the Fermi surface geometry that produces it in many metals and semiconductors whose electronic structure is at the boundary between two and three dimensions. Goniopolarity may enable future explorations of complex transport phenomena that lead to unprecedented device concepts.

Germanane, a van der Waals layered material predicted to have a direct band gap and high carrier mobility, is a promising two-dimensional material with applications in optoelectronics. The electronic properties of germanane have been well studied; however, experimentally measured properties are orders of magnitude lower than predicted values, potentially limiting future device applications. The structure of germanane contains an inherent disorder along the c-axis, resulting in a diffuse halo with hexagonal symmetry in the electron diffraction pattern. The origin of this disorder is not well understood, further limiting the device application of germanane. Here, we have used experimental and simulated electron diffraction patterns to show that this diffuse scattering arises from turbostratic disorder present in the germanane structure with rotational disorder as the main contribution. The maximum rotation angle in the examined germanane crystal is limited to three degrees. For larger angles, germanane would become unstable as DFT calculations show. DFT calculations also indicate that small angle rotations cause a change in charge distribution in Ge and H atoms, and thus should affect the electronic properties of germanane considerably. This study explains for the first time the origin of the c-axis disorder in this van der Waals structure and establishes computationally analyzed diffraction patterns as a tool to quantify turbostratic disorder.

Transverse thermoelectric generators (TTEGs) have the potential to overcome two challenging problems that limit the commercial application of classical thermoelectric generators (TEGs): the significant irreversible efficiency losses in the contacts and the thermal degradation of the contacts at the hot end. TTEGs are built from a single material that can generate a thermoelectric current in a direction that is orthogonal to a temperature gradient. They only require a single set of contacts that are displaced away from the hot end. Here, we experimentally explore the device design considerations for optimizing the performance of this far less explored class of thermal-to-electric generation devices. We fabricate Re4Si7 TTEGs and show that there is no measurable loss in efficiency with a single set of contacts that are midway between the hot and cold ends. We show that a new effect must be considered: the appearance of transverse thermal gradients that lead to significant differences between the performance of TTEGs implemented in isothermal and adiabatic geometries. The isothermal TTEG configuration leads to much higher device efficiencies. Overall, this work highlights how different the design of TTEGs is from that of conventional TEGs and the importance of controlling the device geometry for optimum waste heat recovery.

Moiré engineering

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. Although a suite of interesting phenomena, including multi-flavor magnetic states, noncollinear magnetic states, moiré magnon bands and magnon networks, 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, 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.

Made available in DSpace on 2021-06-25T10:51:07Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-01 New materials discovery is the driving force for progress in solid state physics and chemistry. Here we solve the crystal structure and comprehensively study physical properties of ZnVSb in the polycrystalline form. Synchrotron x-ray diffraction reveals that the compound attains a layered ZrSiS-type structure (P4/nmm, a = 4.09021(2) Å, c = 6.42027(4) Å). The unit cell is composed of a 2D vanadium network separated by Zn-Sb blocks that are slightly distorted from the ideal cubic arrangement. A considerable amount of vacancies were observed on the vanadium and antimony positions; the experimental composition is ZnV0.91Sb0.96. Low-temperature x-ray diffraction shows very subtle discontinuity in the lattice parameters around 175 K. Bonding V-V distance is below the critical separation of 2.97 Å known from the literature, which allows for V-V orbital overlap and subsequent metallic conductivity. From ab initio calculations, we found that ZnVSb is a pseudogap material with an expected dominant vanadium contribution to the density of states at the Fermi level. The energy difference between the antiferromagnetic and nonordered magnetic configurations is rather small (0.34 eV/f.u.). X-ray photoelectron spectroscopy fully corroborates the results of the band structure calculations. Magnetic susceptibility uncovered that, in ZnVSb, itinerant charge carriers coexist with a small, localized magnetic moment of ca. 0.25 μB. The itinerant electrons arise from the ordered part of the vanadium lattice. Fractional localization, in turn, was attributed to V atoms neighboring vacancies, which hinder full V-V orbital overlap. Furthermore, the susceptibility and electrical resistivity showed a large hysteresis between 120 K and 160 K. The effect is not sensitive to magnetic fields up to 9 T. Curie-Weiss fitting revealed that the amount of itinerant charge carriers in ZnVSb drops with decreasing temperature below 160 K, which is accompanied by a concurrent rise in the localized magnetic moment. This observation together with the overall shape of the hysteresis in the resistivity allows for suggesting a plausible origin of the effect as a charge-transfer metal-insulator transition. Ab initio phonon calculations show the formation of a collective phonon mode at 2.8 THz (134 K). The experimental heat capacity reflected this feature by a boson peak with Einstein temperature of 116 K. Analysis of the heat capacity with both an ab initio perspective and Debye-Einstein model revealed a sizable anharmonic contribution to heat capacity, in line with disordered nature of the material. Further investigation of the electron and phonon properties for ZnVSb is likely to provide valuable insight into the relation between structural disorder and the physical properties of transition-metal-bearing compounds. Physics Department Colorado School of Mines Institute of Low Temperature and Structure Research Polish Academy of Sciences University of Illinois at Urbana-Champaign Instituto de Física Teórica São Paulo State University (UNESP) University of California Santa Barbara Max Planck Institute for Chemical Physics of Solids Department of Chemistry and Biochemistry The Ohio State University National Synchrotron Radiation Research Center (NSRRC) Instituto de Física Teórica São Paulo State University (UNESP)

Spin-orbit torque (SOT)-driven deterministic control of the magnetic state of a ferromagnet with perpendicular magnetic anisotropy is key to next-generation spintronic applications including non-volatile, ultrafast and energy-efficient data-storage devices. However, field-free deterministic switching of perpendicular magnetization remains a challenge because it requires an out-of-plane antidamping torque, which is not allowed in conventional spin-source materials such as heavy metals and topological insulators due to the system's symmetry. The exploitation of low-crystal symmetries in emergent quantum materials offers a unique approach to achieve SOTs with unconventional forms. Here we report an experimental realization of field-free deterministic magnetic switching of a perpendicularly polarized van der Waals magnet employing an out-of-plane antidamping SOT generated in layered WTe

Two-dimensional van der Waals magnets with multiple functionalities are becoming increasingly important for emerging technologies in spintronics and valleytronics. Application of external pressure is one method to cleanly explore the underlying physical mechanisms of the intrinsic magnetism. In this paper, the magnetic, electronic, and structural properties of van der Waals-layered, Fe-deficient ${\mathrm{Fe}}_{3\ensuremath{-}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ are investigated. Magnetotransport measurements show a monotonic decrease in the Curie temperature $({T}_{C})$ and the magnetic moment with increasing pressure up to 13.9 GPa. The electrical resistance of ${\mathrm{Fe}}_{3\ensuremath{-}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ shows a change from metallic to a seemingly nonmetallic behavior with increasing pressure. High-pressure angle dispersive powder x-ray diffraction shows a monotonic compression of the unit cell and a reduction of the volume by $\ensuremath{\sim}25%$ with no evidence of structural phase changes up to 29.4(4) GPa. We suggest that the decrease in the ${T}_{C}$ due to pressure results from increased intralayer coupling and delocalization that leads to a change in the exchange interaction.

The graphane analogues of group 14 are a unique family of 2D materials due to the necessity of a terminal ligand for stability. Here we highlight how changing the surface ligand can lead to nonobvious interactions with other chemical species. We show using XRD, FTIR, and TGA that GeCH3 reversibly absorbs water into the van der Waals space, whereas GeH does not intercalate water. Molecular dynamics and density functional theory simulations predict that water datively interacts with the Ge-C ?* pocket on the Ge framework, resulting in local structural distortions. Surprisingly, these distortions give rise to an intense above band gap luminescence state of 1.87 eV, with an average lifetime of hundreds of picoseconds. This work opens potential applications for exploiting surface functionalization chemistry of 2D materials to create membrane and separation technologies.

The discovery of 2-dimensional (2D) materials, such as CrI3, that retain magnetic ordering at monolayer thickness has resulted in a surge of both pure and applied research in 2D magnetism. Here, we report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two additional features in the spectra that appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3., Thin samples CrI3 exhibit a phase transition under an applied magnetic field from layered antiferromagnetism to ferromagnetism. Here the authors observe an associated abrupt change in the magneto-Raman spectra, illustrating the sensitivity of Raman spectra to magnetic ordering.

Remarkable properties of two-dimensional (2D) layer magnetic materials, which include spin filtering in magnetic tunnel junctions and the gate control of magnetic states, were demonstrated recently1–12. Whereas these studies focused on static properties, dynamic magnetic properties, such as excitation and control of spin waves, remain elusive. Here we investigate spin-wave dynamics in antiferromagnetic CrI3 bilayers using an ultrafast optical pump/magneto-optical Kerr probe technique. Monolayer WSe2 with a strong excitonic resonance was introduced on CrI3 to enhance the optical excitation of spin waves. We identified subterahertz magnetic resonances under an in-plane magnetic field, from which the anisotropy and interlayer exchange fields were determined. We further showed tuning of the antiferromagnetic resonances by tens of gigahertz through electrostatic gating. Our results shed light on magnetic excitations and spin dynamics in 2D magnetic materials, and demonstrate their potential for applications in ultrafast data storage and processing. Gating dependent laser induced spin dynamics in an antiferromagnetic bilayer are observed and explained, with implications for future spintronic applications.

We discover the chemical composition of over 1000 materials that are likely to exhibit layered and 2D phases but have yet to be synthesized. This includes two materials our calculations indicate can exist in distinct structures with different band gaps, expanding the short list of 2D phase-change materials. Whereas databases of over 1000 layered materials have been reported, we provide the first full database of materials that are likely layered but are yet to be synthesized, providing a roadmap for the synthesis community. We accomplish this by combining physics with machine learning on experimentally obtained data and verify a subset of candidates using density functional theory. We find that our model performs five times better than practitioners in the field at identifying layered materials and is comparable to or better than professional solid-state chemists. Finally, we find that semisupervised learning can offer benefits for materials design where labels for some of the materials are unknown.

In exploring the suitability of tetraborylmethane precursors as C4− synthons, we have studied the limits of stepwise trimethylsilyl and trimethylgermyl substitutions on tetrakis (1,3-propanediolatoboryl)methane. In contrast to previous reports indicating minimal silylation of tetraborylmethanes, we have shown that organolithium-mediated deborylation-metalation methodologies readily proceed for up to two substitutions. This enables the synthesis of new borylmethane derivatives including (TMS)2CB(pg)2 and (TMG)2CB(pg)2 (pg = 1,3-propanediolate, TMS = trimethylsilyl, TMG = trimethylgermyl). While further base-activated deborylation attempts lead to protodeboronation, limited reactivity, or de-esterification of the boronate ester, increasing the degree of substitution imparts air- and water-stability to the boronate esters.

Inexpensive, transition metal-free intermetallic compounds have received almost no attention as heterogeneous catalysts. Here, we show that BaGa

Polytypism, or the ability for materials to crystallize with different stacking sequences, often leads to fundamentally different properties in families of two-dimensional materials. Here, we show that is possible to control the polytype of GeH, a representative two-dimensional material that is synthesized topotactically by first controlling the polytype sequence of the precursor Zintl phase. 1T, 2H, and 6R GeH can be prepared by the topotactic deintercalation of 1T EuGe2, 2H α-CaGe2, and 6R β-CaGe2, respectively. The 6R and 1T GeH polytypes exhibit remarkably similar properties and feature band gaps of 1.63 and 1.59 eV, respectively. However, the 2H CaGe2 precursor forms due to the incorporation of small amounts of In flux in the germanium lattice, which is retained when converted to GeH. Consequently, 2H GeH has a reduced band gap of 1.45 eV. Finally, temperature dependent diffraction of 6R GeH shows a negative coefficient of thermal expansion along the a-axis and a positive coefficient of thermal expan...

The sp3-hybridized group 14 graphane analogues are a unique family of 2D materials in which every atom requires a terminal ligand for stability. Consequently, the optical, electronic, and thermal properties of these materials can be manipulated via covalent chemistry. Herein, we review the methodologies for preparing these materials, and compare their functionalization densities to Si/Ge(111) surfaces and other covalently terminated 2D materials. We discuss how the electronic structure, optical properties, and thermal stability of the 2D framework can be broadly tuned with the ligand identity and framework element. We highlight their recent application in electronics, optoelectronics, photocatalysis, and batteries. Overall, these materials are an intriguing regime in materials design in which both surface functionalization and solid-state chemistry can be uniquely exploited to systematically design properties and phenomena.

Intermetallic Zintl phases whose structures consist of honeycomb layers of main group elements often serve as useful precursors for two-dimensional group 14 graphane derivatives. Here, we probe how the Raman spectra and band gaps evolve in these precursor intermetallic phases before they transform into their van der Waals counterparts. Through polarization-dependent Raman experiments on seven different group 14 and 15 Zintl phases with different stacking motifs, we show that the in-plane and out-of-plane Raman frequencies mostly depend on the reduced mass and the in-plane bond lengths of the elements constituting the honeycomb framework. From optical absorption measurements, we show that the honeycomb framework comprised of single elements is metallic, but when the framework is comprised of alternating elements in a BN-like fashion, such as in NaSnP, KSnAs and KSnSb, a band gap is opened up ranging from 0.54 eV for KSnSb to 0.95 eV for NaSnP. This study allows for the determination of the structure and stacking motifs for this emerging class of 2D layered metallic and semiconducting materials.

While the influence of alkyl chain length and headgroup size on self-assembly behaviour has been well-established for simple surfactants, the rational control over the pH- and concentration-dependent self-assembly behaviour in stimuli responsive peptides remains an elusive goal. Here, we show that different amphiphilic peptides can have similar self-assembly phase diagrams, providing the relative strengths of the attractive and repulsive forces are balanced. Using palmitoyl-YYAAEEEEK(DO3A:Gd)-NH(2) and palmitoyl-YAAEEEEK(DO3A:Gd)-NH(2) as controls, we show that reducing hydrophobic attractive forces through fewer methylene groups in the alkyl chain will lead to a similar self-assembly phase diagram as increasing the electrostatic repulsive forces via the addition of a glutamic acid residue. These changes allow creation of self-assembled MRI vehicles with slightly different micelle and nanofiber diameters but with minimal changes in the spin–lattice T(1) relaxivity. These findings reveal a powerful strategy to design self-assembled vehicles with different sizes but with similar self-assembly profiles.

Despite the immense interest in magnetic, topological, and magnetoelectronic phenomena in isolated layers of 2D materials and their vertically stacked heterostructures, there remains a dearth of 2D magnetic materials that can be readily exfoliated into single layers. Here, we synthesize EuSn2As2, the first exfoliatable, layered magnetic Zintl–Klemm phase that features van der Waals bonding between neighboring layers. In these crystals, Eu2+ 4f7 spins magnetically order at 24 K and are coupled ferromagnetically within each layer and antiferromagnetically, with some potential canting, across adjacent layers. Magnetic ground state DFT calculations and temperature-dependent transport measurements on single crystals together establish that EuSn2As2 is semimetallic. Finally, we show that this material can be thinned down to a few layers having steps corresponding to SnAsEuAsSn layers via micromechanical exfoliation. This work opens up further explorations into the design, synthesis, and layer-dependent magnetic properties of exfoliatable, lanthanide-based magnetic van der Waals Zintl phases.

The creation of 2D van der Waals materials with ferromagnetism above room temperature is an essential goal toward their practical utilization in spin-based applications. Recent studies suggest that intercalating lithium in exfoliated flakes of the ferromagnet Fe

Stacking order can influence the physical properties of two-dimensional van der Waals materials

We lay the foundation for determining the microscopic spin interactions in two-dimensional (2D) ferromagnets by combining angle-dependent ferromagnetic resonance (FMR) experiments on high quality CrI$_3$ single crystals with theoretical modeling based on symmetries. We discover that the Kitaev interaction is the strongest in this material with $K \sim -5.2$ meV, 25 times larger than the Heisenberg exchange $J \sim -0.2$ meV, and responsible for opening the $\sim$5 meV gap at the Dirac points in the spin-wave dispersion. Furthermore, we find that the symmetric off-diagonal anisotropy $\Gamma \sim -67.5$ $\mu$eV, though small, is crucial for opening a $\sim$0.3 meV gap in the magnon spectrum at the zone center and stabilizing ferromagnetism in the 2D limit. The high resolution of the FMR data further reveals a $\mu$eV-scale quadrupolar contribution to the $S=3/2$ magnetism. Our identification of the underlying exchange anisotropies opens paths toward 2D ferromagnets with higher $T_\text{C}$ as well as magnetically frustrated quantum spin liquids based on Kitaev physics., Comment: 5 pages, 4 figures

The covalent functionalization of 2D crystals is an emerging route for tailoring the electronic structure and generating novel phenomena. Understanding the influence of ligand chemistry will enable the rational tailoring of their properties. Through the synthesis of numerous ligand-functionalized germanane crystals, we establish the role of ligand size and electronegativity on functionalization density, framework structure, and electronic structure. Nearly uniform termination only occurs with small ligands. Ligands that are too sterically bulky will lead to partial hydrogen termination of the framework. With a homogeneous distribution of different ligands, the band gaps and Raman shifts are dictated by their relative stoichiometry in a pseudolinear fashion similar to Vegard’s law. Larger and more electronegative ligands expand the germanane framework, thereby lowering the band gap and Raman shift. Simply by changing the identity of the organic ligand, the band gap can be tuned by ∼15%, highlighting the po...

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties, enables the realization of unique physical phenomena in these few-atom thick building blocks and in proximity to other materials. Herein, using NaSn2As2 as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt2Pn2 (A = Group 1 or 2 element, Tt = Group 14 tetrel element and Pn = Group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman active modes of the bulk crystals via polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn2As2 samples are resistant towards oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy (ARPES), density functional theory (DFT), and transport on bulk and exfoliated samples, we show that NaSn2As2 is a highly conducting 2D semimetal, with resistivities on the order of 10-6 {\Omega} m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics.

The topotactic transformation of Zintl phases such as CaGe2 into organic-terminated germanium graphane analogues using haloalkanes is a powerful route for generating new 2D optoelectronic and spintronic building blocks. However, uniform ligand coverage is necessary for optimizing the properties and stability of these single-atom-thick frameworks. Here, we compare the effectiveness of different topochemical methods to maximize methyl-termination in GeCH3. We show that a previously developed CH3I/H2O phase transfer route produces a small percentage of partially oxidized germanane. The partially oxidized termination is readily removed upon HCl treatment, which leads to Ge–Cl termination, but rapidly reoxidizes after exposure to the ambient atmosphere. We then show that a one-pot route with CH3I in distilled CH3CN solvent and at least six equivalents of H2O results in no oxidation. The GeCH3 prepared from this one-pot route also has an increased −CH3/–H ratio of termination from ∼90:10 to ∼95:5, is air-stable...

The discovery of novel transition metal cubane clusters with different electron counts and topologies is of considerable importance due to the prevalence of this structure type in catalytic proteins. Here, we report the solvothermal synthesis and full characterization of [V7S8Cl2(en)8]Cl4 (en = ethylenediamine), the first early transition metal dicubane cluster. Electron paramagnetic resonance and magnetic susceptibility measurements show that the cluster has one unpaired electron localized on a single V4+, which according to DFT simulations is the central bridging vanadium. The remaining V3+ atoms feature significant metal–metal bonding.

As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium triiodide is the latest addition to the family of two-dimensional (2D) materials. However, realistic exploration of CrI3-based devices and heterostructures is challenging, due to its extreme instability under ambient conditions. Here we present Raman characterization of CrI3, and demonstrate that the main degradation pathway of CrI3 is the photocatalytic substitution of iodine by water. While simple encapsulation by Al2O3, PMMA and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing exposure of light markedly improves stability, and CrI3 sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of CrI3/graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene., 4 figures

While the ordering of amino acids in proteins and peptide-based materials is known to affect their folding patterns and supramolecular architectures, tailoring self-assembly behavior in stimuli responsive peptides by isomerizing a peptide sequence has not been extensively explored. Here, we show that changing the position of a single hydrophobic amino acid in short amphiphilic peptides can dramatically alter their pH-triggered self-assembly transitions. Using palmitoyl-IAAAEEEE-NH2 and palmitoyl-IAAAEEEEK(DO3A:Gd)-NH2 as controls, moving the Isoleucine away from the palmitoyl tail preferentially induces nanofiber formation over spherical micelles. Shifting the Isoleucine one residue away makes the transition pH more basic by 2 units. When in the third or fourth position, nanofibers are formed exclusively above 10 μM. We propose that moving the Isoleucine away from the tail enhances its ability to promote β-sheet formation instead of folding back into the palmitoyl core. These findings reveal a novel strategy for programming pH-triggered self-assembly by isomerizing a peptide sequence.

CONSPECTUS: The isolation of graphene has sparked a renaissance in the study of two-dimensional materials. This led to the discovery of new and unique phenomena such as extremely high carrier mobility, thermal conductivity, and mechanical strength not observed in the parent 3D structure. While the emergence of these phenomena has spurred widespread interest in graphene, the paradox between the high-mobility Fermi-Dirac electronic structure and the need for a sizable band gap has challenged its application in traditional semiconductor devices. While graphene is a fascinating and promising material, the limitation of its electronic structure has inspired researchers to explore other 2D materials beyond graphene. In this Account, we summarize our recent work on a new family of two-dimensional materials based on sp(3)-hybridized group IV elements. Ligand-terminated Si, Ge, and Sn graphane analogues are an emerging and unique class of two-dimensional materials that offer the potential to tailor the structure, stability, and properties. Compared with bulk Si and Ge, a direct and larger band gap is apparent in group IV graphane analogues depending on the surface ligand. These materials can be synthesized in gram-scale quantities and in thin films via the topotactic deintercalation of layered Zintl phase precursors. Few layers and single layers can be isolated via manual exfoliation and deintercalation of epitaxially grown Zintl phases on Si/Ge substrates. The presence of a fourth bond on the surface of the layers allows various surface ligand termination with different organic functional groups achieved via conventional soft chemical routes. In these single-atom thick materials, the electronic structure can be systematically controlled by varying the identities of the main group elements and by attaching different surface terminating ligands. In contrast to transition metal dichalcogenides, the weaker interlayer interaction allows the direct band gap single layer properties such as photoluminescence to be readily observable without the need to exfoliate down to single layers. Furthermore, these materials can be resilient to oxidation and thermal degradation, making them attractive candidates for next generation functional materials for electronic devices and beyond. This class of two-dimensional materials not only are promising building blocks for a variety of conventional semiconductor applications but also provide a pioneering platform to systematically and rationally control material properties using covalent chemistry. The stability and tunability of these versatile materials will push this system toward the forefront of two-dimensional research.