KMg4Bi3: A Narrow Band Gap Semiconductor with a Channel Structure.

The creation of new families of intermetallic or Zintl-phase compounds with high-spin orbit elements has attracted a considerable amount of interest due to the presence of unique electronic, magnetic, and topological phenomena in these materials. Here, we establish the synthesis and structural and electronic characterization of KMg4Bi3 single crystals having a new structure type. KMg4Bi3 crystallizes in space group Cmcm having unit cell parameters a = 4.7654(11) Å, b = 15.694(4) Å, and c = 13.4200(30) Å and features an edge-sharing MgBi4 tetrahedral framework that forms cage-like one-dimensional channels around K+ ions. Diffuse reflectance absorption measurements indicate that this material has a narrow band gap of 0.27 eV, which is in close agreement with the electronic structure calculations that predict it to be a trivial insulator. Electronic transport measurements from 80 to 380 K indicate this material behaves like a narrow band gap semiconductor doped to ∼1018 holes/cm-3, with thermopowers of ∼100 µV/K and appreciable magnetoresistance. Electronic structure calculations indicate this material is close to a topological phase transition and becomes a topological insulator when the lattice is uniformly expanded by 3.5%. Overall, this unique structure type expands the landscape of potential quantum materials.

All-optical switching of magnetization in atomically thin CrI3.

Control of magnetism has attracted interest in achieving low-power and high-speed applications such as magnetic data storage and spintronic devices. Two-dimensional magnets allow for control of magnetic properties using the electric field, electrostatic doping and strain. In two-dimensional atomically thin magnets, a non-volatile all-optical method would offer the distinct advantage of switching magnetic states without application of an external field. Here, we demonstrate such all-optical magnetization switching in the atomically thin ferromagnetic semiconductor, CrI3, triggered by circularly polarized light pulses. The magnetization switching behaviour strongly depends on the exciting photon energy and polarization, in correspondence with excitonic transitions in CrI3, indicating that the switching process is related to spin angular momentum transfer from photoexcited carriers to local magnetic moments. Such an all-optical magnetization switching should allow for further exploration of magneto-optical interactions and open up applications in high-speed and low-power spintronic devices.

Deterministic switching of a perpendicularly polarized magnet using unconventional spin-orbit torques in WTe2.

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 WTe2, a quantum material with a low-symmetry crystal structure. Our numerical simulations suggest that the out-of-plane antidamping torque in WTe2 is essential to explain the observed magnetization switching.

Dynamics of Two Distinct Exciton Populations in Methyl-Functionalized Germanane.

Methyl-substituted germanane is an emerging material that has been proposed for novel applications in optoelectronics, photoelectrocatalysis, and biosensors. It is a two-dimensional semiconductor with a strong above-gap fluorescence associated with water intercalation. Here, we use time-resolved photoluminescence spectroscopy to understand the mechanism causing this fluorescence. We show that it originates from two distinct exciton populations. Both populations recombine exponentially, accompanied by the thermally activated transfer of exciton population from the shorter- to the longer-lived type. The two exciton populations involve different electronic levels and couple to different phonons. The longer-lived type of exciton migrates within the disordered energy landscape of localized recombination centers. These outcomes shed light on the fundamental optical and electronic properties of functionalized germanane, enabling the groundwork for future applications in optoelectronics, light harvesting, and sensing.

Lucky Number 13: A 13-Layer Polytype of the Alkyne Hydrogenation Catalyst CaGaGe.

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.

The Chemical Design Principles for Axis-Dependent Conduction Polarity.

The recent discovery that specific materials can simultaneously exhibit n-type conduction and p-type conduction along different directions of the single crystal has the potential to impact a broad range of electronic and energy-harvesting technologies. Here, we establish the chemical design principles for creating materials with this behavior. First, we define the single-carrier and multicarrier mechanisms for axis-dependent conduction polarity and their identifying band structure fingerprints. We show using first-principles predictions that the AMX (A = Ca, Sr, Ba; M = Cu, Ag, Au; X = P, As, Sb) compounds consisting of MX honeycomb layers separated by A cations can exhibit p-type conduction in-plane and n-type conduction cross-plane via either mechanism depending on the doping level. We build up the band structure of BaCuAs using a molecular orbital approach to illustrate the structural origins of the two different mechanisms for axis-dependent conduction polarity. In total, this work shows this phenomenon can be quite prevalent in layered materials and reveals how to identify prospective materials.

The Fermi surface geometrical origin of axis-dependent conduction polarity in layered materials.

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.

Pressure-controlled interlayer magnetism in atomically thin CrI3.

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 moiré engineering3,9.

Fundamental Spin Interactions Underlying the Magnetic Anisotropy in the Kitaev Ferromagnet CrI_{3}.

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∼-5.2 meV, 25 times larger than the Heisenberg exchange J∼-0.2 meV, and responsible for opening the ∼5 meV gap at the Dirac points in the spin-wave dispersion. Furthermore, we find that the symmetric off-diagonal anisotropy Γ∼-67.5 µeV, though small, is crucial for opening a ∼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 µeV-scale quadrupolar contribution to the S=3/2 magnetism. Our identification of the underlying exchange anisotropies opens paths toward 2D ferromagnets with higher T_{C} as well as magnetically frustrated quantum spin liquids based on Kitaev physics.

Decomposition-Induced Room-Temperature Magnetism of the Na-Intercalated Layered Ferromagnet Fe3-xGeTe2.

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 Fe3-xGeTe2 induces a nonzero magnetization at T ∼ 300 K. However, the nanoscale nature of such experiments precludes precise observations of structural and chemical changes upon intercalation. Here, we report the preparation of sodium-intercalated NaFe2.78GeTe2 as well as the investigation into its structure and magnetic properties. Sodium readily intercalates into the van der Waals gap, as revealed by synchrotron X-ray diffraction. Concurrently, the Fe2.78GeTe2 layer becomes heavily charge doped and strained via chemical pressure, yet retains its structure and ferromagnetic transition temperature of ∼140 K. However, we observe the presence of a ferromagnetic amorphous iron germanide impurity over a wide range of synthetic conditions, leading to room-temperature magnetization. This work highlights the importance of strain and electronic control for manipulating the Curie temperature in 2D ferromagnets, while emphasizing the need for careful chemical analysis when exploring phenomena in exfoliated layers.

Transition Metal-Free Alkyne Hydrogenation Catalysis with BaGa2, a Hydrogen Absorbing Layered Zintl Phase.

Inexpensive, transition metal-free intermetallic compounds have received almost no attention as heterogeneous catalysts. Here, we show that BaGa2, a Zintl-Klemm compound composed of honeycomb sheets of Ga- anions separated by Ba2+ cations and known to react with H2 under moderate conditions to form a layered polyanionic hydride BaGa2H2, effectively catalyzes the hydrogenation of phenylacetylene into styrene and ethylbenzene under modest conditions (1-50 bar H2, 40-100 °C). Remarkably, the catalytic activity of BaGa2 (surface specific activities up to 8390 h-1) is on the same order of magnitude as commercial Pd-based catalysts. In contrast, BaGa2H2 shows negligible catalytic activity, thereby indicating that the unsaturated Ga- framework is necessary for phenylacetylene and styrene adsorption. These findings open up future explorations of utilizing and optimizing the long-term stability of transition metal-free intermetallic hydrogen absorbing compounds for hydrogen-based catalysis.

Covalent functionalization of two-dimensional group 14 graphane analogues.

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.

Covalently-controlled properties by design in group IV graphane analogues.

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.

Li intercalation into 1D TiS2(en) chains.

The intercalation of metal cations in 2D layered materials allows for the discovery of unique electronic, magnetic and correlated properties. We demonstrate that reversible Li intercalation is also achievable in the hybrid organic/inorganic dimensionally reduced 1D van der Waals solid TiS2(ethylenediamine). Upon intercalation, electrons are injected into the lattice as Ti(4+) is reduced to Ti(3+) leading to an order of magnitude decrease in electrical resistivity. This reversible intercalation process opens up new opportunities to fine-tune the physical properties in this emerging family of dimensionally reduced materials.

Programming pH-triggered self-assembly transitions via isomerization of peptide sequence.

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 ß-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.

Probing peptide amphiphile self-assembly in blood serum.

There has been recent interest in designing smart diagnostic or therapeutic self-assembling peptide or polymeric materials that can selectively undergo morphological transitions to accumulate at a disease site in response to specific stimuli. Developing approaches to probe these self-assembly transitions in environments that accurately amalgamate the diverse plethora of proteins, biomolecules, and salts of blood is essential for creating systems that function in vivo. Here, we have developed a fluorescence anisotropy approach to probe the pH-dependent self-assembly transition of peptide amphiphile (PA) molecules that transform from spherical micelles at pH 7.4 to nanofibers under more acidic pH's in blood serum. By mixing small concentrations of a Ru(bipy)3(2+)-tagged PA with a Gd(DO3A)-tagged PA having the same lipid-peptide sequence, we showed that the pH dependence of self-assembly is minimally affected and can be monitored in mouse blood serum. These PA vehicles can be designed to transition from spherical micelles to nanofibers in the pH range 7.0-7.4 in pure serum. In contrast to the typical notion of serum albumin absorbing isolated surfactant molecules and disrupting self-assembly, our experiments showed that albumin does not bind these anionic PAs and instead promotes nanofibers due to a molecular crowding effect. Finally, we created a medium that replicates the transition pH in serum to within 0.08 pH units and allows probing self-assembly behavior using conventional spectroscopic techniques without conflicting protein signals, thus simplifying the development pathway from test tube to in vivo experimentation for stimuli-responsive materials.

Synergizing a Large Ordinary Nernst Effect and Axis-Dependent Conduction Polarity in Flat Band KMgBi Crystals.

The exploration of quantum materials in which an applied thermo/electrical/magnetic field along one crystallographic direction produces an anisotropic response has led to unique functionalities. Along these lines, KMgBi is a layered, narrow gap semiconductor near a critical state between multiple Dirac phases due to the presence of a flat band near the Fermi level. The valence band is highly anisotropic with minimal cross-plane dispersion, which, in combination with an isotropic conduction band, enables axis-dependent conduction polarity. Thermopower and Hall measurements indicate dominant p-type conduction along the cross-plane direction, and n-type conduction along the in-plane direction, leading to a significant zero-field transverse thermoelectric response when the heat flux is at an angle to the principal crystallographic directions. Additionally, a large Ordinary Nernst effect (ONE) is observed with an applied field.  It arises from the ambipolar term in the Nernst effect, whereby the Lorentz force on electrons and holes makes them drift in opposite directions so that the resulting Nernst voltage becomes a function of the difference between their partial thermopowers, greatly enhancing the ONE. It is proven that axis-dependent polarity can synergistically enhance the ONE, in addition to leading to a zero-field transverse thermoelectric performance.

Quantum octets in high mobility pentagonal two-dimensional PdSe2.

Two-dimensional (2D) materials have drawn immense interests in scientific and technological communities, owing to their extraordinary properties and their tunability by gating, proximity, strain and external fields. For electronic applications, an ideal 2D material would have high mobility, air stability, sizable band gap, and be compatible with large scale synthesis. Here we demonstrate air stable field effect transistors using atomically thin few-layer PdSe2 sheets that are sandwiched between hexagonal BN (hBN), with large saturation current > 350 µA/µm, and high field effect mobilities of ~ 700 and 10,000 cm2/Vs at 300 K and 2 K, respectively. At low temperatures, magnetotransport studies reveal unique octets in quantum oscillations that persist at all densities, arising from 2-fold spin and 4-fold valley degeneracies, which can be broken by in-plane and out-of-plane magnetic fields toward quantum Hall spin and orbital ferromagnetism.

An efficient material search for room-temperature topological magnons.

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.

Axis dependent conduction polarity in the air-stable semiconductor, PdSe2.

Axis-dependent conduction polarity (ADCP) is a unique electronic phenomena in which the charge polarity of carrier conduction can differ from p-type to n-type depending on the direction of travel through the crystal. Most materials that exhibit ADCP are metals, and very few semiconducting materials exhibit this effect. Here, we establish that PdSe2, a ∼0.5 eV band gap semiconductor that is air- and water-stable, exhibits ADCP, through the growth and characterization of the transport properties of crystals with extrinsic p- and n-type doping levels of Ir and Sb, respectively, in the 1016-1018 cm-3 range. Electron doped PdSe2 exhibits p-type conduction in the cross-plane direction and n-type conduction along the in-plane directions above an onset temperature of 100-200 K that varies with doping level. Lightly p-doped samples show p-type thermopower in all directions at low temperatures, but above ∼360 K the in-plane thermopower turns negative. Density functional theory calculations indicate that the origin of ADCP arises from the complementary effective mass anisotropies in the valence and conduction bands in this material, which facilitate hole transport in the cross-plane direction, and electron transport along the in-plane directions. ADCP occurs at temperatures with sufficient thermal population of both carrier types to overcome the extrinsic doping levels to exploit the effective mass anisotropy. In total, the development of this stable semiconductor in which thermally or optically excited holes and electrons inherently migrate along different directions opens up numerous potential applications in a multitude of technologies.