Air-stable van der Waals PtTe2 conductors with high current-carrying capacity and strong spin-orbit interaction.

High-performance van der Waals (vdW) integrated electronics and spintronics require reliable current-carrying capacity. However, it is challenging to achieve high current density and air-stable performance using vdW metals owing to the fast electrical breakdown triggered by defects or oxidation. Here, we report that spin-orbit interacted synthetic PtTe2 layers exhibit significant electrical reliability and robustness in ambient air. The 4-nm-thick PtTe2 synthesized at a low temperature (∼400°C) shows intrinsic metallic transport behavior and a weak antilocalization effect attributed to the strong spin-orbit scattering. Remarkably, PtTe2 sustains a high current density approaching ≈31.5 MA cm-2, which is the highest value among electrical interconnect candidates under oxygen exposure. Electrical failure is caused by the Joule heating of PtTe2 rather than defect-induced electromigration, which was achievable by the native TeOx passivation. The high-quality growth of PtTe2 and the investigation of its transport behaviors lay out essential foundations for the development of emerging vdW spin-orbitronics.

Defect-gradient-induced Rashba effect in van der Waals PtSe2 layers.

Defect engineering is one of the key technologies in materials science, enriching the modern semiconductor industry and providing good test-beds for solid-state physics. While homogenous doping prevails in conventional defect engineering, various artificial defect distributions have been predicted to induce desired physical properties in host materials, especially associated with symmetry breakings. Here, we show layer-by-layer defect-gradients in two-dimensional PtSe2 films developed by selective plasma treatments, which break spatial inversion symmetry and give rise to the Rashba effect. Scanning transmission electron microscopy analyses reveal that Se vacancies extend down to 7 nm from the surface and Se/Pt ratio exhibits linear variation along the layers. The Rashba effect induced by broken inversion symmetry is demonstrated through the observations of nonreciprocal transport behaviors and first-principles density functional theory calculations. Our methodology paves the way for functional defect engineering that entangles spin and momentum of itinerant electrons for emerging electronic applications.

Catalyst- and Solvent-Free Synthesis of a Chemically Stable Aza-Bridged Bis(phenanthroline) Macrocycle-Linked Covalent Organic Framework.

Developing new linkage-based covalent organic frameworks (COFs) is one of the major topics in reticular chemistry. Electrically conductive COFs have enabled applications in energy storage and electrochemical catalysis, which are not feasible using insulating COFs. Despite significant advances, the construction of chemically stable conductive COFs by the formation of new linkages remains relatively unexplored and challenging. Here we report the solvent- and catalyst-free synthesis of a two-dimensional aza-bridged bis(phenanthroline) macrocycle-linked COF (ABBPM-COF) from the thermally induced poly-condensation of a tri-topic monomer and ammonia gas. The ABBPM-COF structure was elucidated using multiple techniques, including X-ray diffraction analysis combined with structural simulation, revealing its crystalline nature with an ABC stacking mode. Further experiments demonstrated its excellent chemical stability in acid/base solutions. Electrical-conductivity measurements showed that the insulating ABBPM-COF becomes a semiconducting material after exposure to iodine vapor.

Anisotropic Angstrom-Wide Conductive Channels in Black Phosphorus by Top-down Cu Intercalation.

Intercalation in black phosphorus (BP) can induce and modulate a variety of the properties including superconductivity like other two-dimensional (2D) materials. In this perspective, spatially controlled intercalation has the possibility to incorporate different properties into a single crystal of BP. We demonstrate anisotropic angstrom-wide (∼4.3 Å) Cu intercalation in BP, where Cu atoms are intercalated along a zigzag direction of BP because of its inherent anisotropy. With atomic structure, its microstructural effects, arising from the angstrom-wide Cu intercalation, were investigated and extended to relation with macrostructure. As the intercalation mechanism, it was revealed by in situ transmission electron microscopy and theoretical calculation that Cu atoms are intercalated through top-down direction of BP. The Cu intercalation anisotropically induces transition of angstrom-wide electronic channels from semiconductor to semimetal in BP. Our findings throw light on the fundamental relationship between microstructure changes and properties in intercalated BP, and tailoring anisotropic 2D materials at angstrom scale.

A scalable molecule-based magnetic thin film for spin-thermoelectric energy conversion.

Spin thermoelectrics, an emerging thermoelectric technology, offers energy harvesting from waste heat with potential advantages of scalability and energy conversion efficiency, thanks to orthogonal paths for heat and charge flow. However, magnetic insulators previously used for spin thermoelectrics pose challenges for scale-up due to high temperature processing and difficulty in large-area deposition. Here, we introduce a molecule-based magnetic film for spin thermoelectric applications because it entails versatile synthetic routes in addition to weak spin-lattice interaction and low thermal conductivity. Thin films of CrII[CrIII(CN)6], Prussian blue analogue, electrochemically deposited on Cr electrodes at room temperature show effective spin thermoelectricity. Moreover, the ferromagnetic resonance studies exhibit an extremely low Gilbert damping constant ~(2.4 ± 0.67) × 10-4, indicating low loss of heat-generated magnons. The demonstrated STE applications of a new class of magnet will pave the way for versatile recycling of ubiquitous waste heat.

Synthesis of a Copper 1,3,5-Triamino-2,4,6-benzenetriol Metal-Organic Framework.

We report the synthesis and characterization of a two-dimensional (2D) MX2Y2-type (M = metal, X, Y = N, S, O, and X ≠ Y) copper 1,3,5-triamino-2,4,6-benzenetriol metal-organic framework (Cu3(TABTO)2-MOF). The role of oxygen in the synthesis of this MOF was investigated. Copper metal is formed along with the MOF when the synthesis is done in argon as suggested by XRD. When the reaction was exposed to air with vigorous stirring, copper metal was not observed by XRD. However, if there is no stirring, then copper metal is formed, and we learned that this is because oxygen was not allowed to enter the solvent due to the formation of a MOF film at the air/water interface. For the sample synthesized in argon (Cu3(TABTO)2-Ar), an insulating Cu3(TABTO)2-Ar pellet (σ < 10-10 S cm-1) became a metallic conductor with an electrical conductivity of 0.78 S cm-1 at 300 K after exposure to iodine vapor. This work provides further insights into the role of oxygen in the synthesis of redox-active ligand-based MOFs, expands the family of 2D redox-active ligand-based electrically conductive MOFs, and offers more opportunities in sensing, photocatalytic, electronic, and energy-related applications.

Observation of spin-polarized Anderson state around charge neutral point in graphene with Fe-clusters.

The pristine graphene described with massless Dirac fermion could bear topological insulator state and ferromagnetism via the band structure engineering with various adatoms and proximity effects from heterostructures. In particular, topological Anderson insulator state was theoretically predicted in tight-binding honeycomb lattice with Anderson disorder term. Here, we introduced physi-absorbed Fe-clusters/adatoms on graphene to impose exchange interaction and random lattice disorder, and we observed Anderson insulator state accompanying with Kondo effect and field-induced conducting state upon applying the magnetic field at around a charge neutral point. Furthermore, the emergence of the double peak of resistivity at ν = 0 state indicates spin-splitted edge state with high effective exchange field (>70 T). These phenomena suggest the appearance of topological Anderson insulator state triggered by the induced exchange field and disorder.

Improved Efficiency of Perovskite Solar Cells Using a Nitrogen-Doped Graphene-Oxide-Treated Tin Oxide Layer.

Tin oxide (SnO2) is widely adopted as an electron transport layer in perovskite solar cells (PeSCs) because it has high electron mobility, excellent charge selective behavior owing to a large band gap of 3.76 eV, and low-temperature processibility. To achieve highly efficient SnO2-based PeSCs, it is necessary to control the oxygen vacancies in the SnO2 layer, since the electrical and optical properties vary depending on the oxidation state of Sn. This study demonstrates that the performance of PeSCs may be improved by using nitrogen-doped graphene oxide (NGO) as an oxidizing agent for SnO2. Since NGO changes the oxidation state of the Sn in SnO2 from Sn2+ to Sn4+, the oxygen vacancies in SnO2 can be reduced using NGO. Multiple devices are fabricated, and various techniques are used to assess their performance, including X-ray photoelectron spectroscopy, dark current analysis, and the dependence of the open-circuit voltage on light intensity. Compared with the average power conversion efficiency (PCE) of control devices, PeSCs with SnO2:NGO composite layers exhibit greater PCEs with less deviation. Therefore, the introduction of NGO in a SnO2 layer can be regarded as an effective method of controlling the oxidation state of SnO2 to improve the performance of PeSCs.

Partial Oxidation-Induced Electrical Conductivity and Paramagnetism in a Ni(II) Tetraaza[14]annulene-Linked Metal Organic Framework.

We report the synthesis and characterization of a two-dimensional (2D) conjugated Ni(II) tetraaza[14]annulene-linked metal organic framework (NiTAA-MOF) where NiTAA is a macrocyclic MN4 (M = metal, N = nitrogen) compound. The structure of NiTAA-MOF was elucidated by Fourier-transform infrared, X-ray photoemission, and X-ray diffraction spectroscopies, in combination with density functional theory (DFT) calculations. When chemically oxidized by iodine, the insulating bulk NiTAA-MOF (σ < 10-10 S/cm) exhibits an electrical conductivity of 0.01 S/cm at 300 K, demonstrating the vital role of ligand oxidation in the electrical conductivity of 2D MOFs. Magnetization measurements show that iodine-doped NiTAA-MOF is paramagnetic with weak antiferromagnetic coupling due to the presence of organic radicals of oxidized ligands and high-spin Ni(II) sites of the missing-linker defects. In addition to providing further insights into the origin of the induced electrical conductivity in 2D MOFs, both pristine and iodine-doped NiTAA-MOF synthesized in this work could find potential applications in areas such as catalase mimics, catalysis, energy storage, and dynamic nuclear polarization-nuclear magnetic resonance (DNP-NMR).

Gate-tunable giant nonreciprocal charge transport in noncentrosymmetric oxide interfaces.

A polar conductor, where inversion symmetry is broken, may exhibit directional propagation of itinerant electrons, i.e., the rightward and leftward currents differ from each other, when time-reversal symmetry is also broken. This potential rectification effect was shown to be very weak due to the fact that the kinetic energy is much higher than the energies associated with symmetry breaking, producing weak perturbations. Here we demonstrate the appearance of giant nonreciprocal charge transport in the conductive oxide interface, LaAlO3/SrTiO3, where the electrons are confined to two-dimensions with low Fermi energy. In addition, the Rashba spin-orbit interaction correlated with the sub-band hierarchy of this system enables a strongly tunable nonreciprocal response by applying a gate voltage. The observed behavior of directional response in LaAlO3/SrTiO3 is associated with comparable energy scales among kinetic energy, spin-orbit interaction, and magnetic field, which inspires a promising route to enhance nonreciprocal response and its functionalities in spin orbitronics.

Organic Radical-Linked Covalent Triazine Framework with Paramagnetic Behavior.

The production of multifunctional pure organic materials that combine different sizes of pores and a large number of electron spins is highly desirable due to their potential applications as polarizers for dynamic nuclear polarization-nuclear magnetic resonance and as catalysts and magnetic separation media. Here, we report a polychlorotriphenylmethyl radical-linked covalent triazine framework (PTMR-CTF). Two different sizes of micropores were established by N2 sorption and the presence of unpaired electrons (carbon radicals) by electron spin resonance and superconducting quantum interference device-vibrating sample magnetometer analyses. Magnetization measurements demonstrate that this material exhibits spin-half paramagnetism with a spin concentration of ∼2.63 × 1023 spins/mol. We also determined the microscopic origin of the magnetic moments in PTMR-CTF by investigating its spin density and electronic structure using density functional theory calculations.

Molecular Tunability of Magnetic Exchange Bias and Asymmetrical Magnetotransport in Metalloporphyrin/Co Hybrid Bilayers.

Individual molecular spins are promising quantum states for emerging computation technologies. The "on surface" configuration of molecules in proximity to a magnetic film allows control over the orientations of molecular spins and coupling between them. The stacking of planar molecular spins could favor antiferromagnetic interlayer couplings and lead to pinning of the magnetic underlayer via the exchange bias, which is extensively utilized in ultrafast and high-density spintronics. However, fundamental understanding of the molecular exchange bias and its operating features on a device has not been unveiled. Here, we showed tunable molecular exchange bias and its asymmetrical magnetotransport characteristics on a device by using the metalloporphyrin/cobalt hybrid films. A series of the distinctive molecular layers showcased a wide range of the interfacial exchange coupling and bias. The transport behaviors of the hybrid bilayer films revealed the molecular exchange bias effect on a fabricated device, representing asymmetric characteristics on anisotropic and angle-dependent magnetoresistances. Theoretical simulations demonstrated close correlations among the interfacial distance, magnetic interaction, and exchange bias. This study of the hybrid interfacial coupling and its impact on magnetic and magnetotransport behaviors will extend functionalities of molecular spinterfaces for emerging information technologies.

Solution-Processed Ferrimagnetic Insulator Thin Film for the Microelectronic Spin Seebeck Energy Conversion.

The longitudinal spin Seebeck effects with a ferro- or ferrimagnetic insulator provide a new architecture of a thermoelectric device that could significantly improve the energy conversion efficiency. Until now, epitaxial yttrium iron garnet (YIG) films grown on gadolinium gallium garnet (GGG) substrates by a pulsed laser deposition have been most widely used for spin thermoelectric energy conversion studies. In this work, we developed a simple route to obtain a highly uniform solution-processed YIG film and used it for the on-chip microelectronic spin Seebeck characterization. We improved the film roughness down to ∼0.2 nm because the extraction of thermally induced spin voltage relies on the interfacial quality. The on-chip microelectronic device has a dimension of 200 µm long and 20 µm wide. The solution-processed 20 nm thick YIG film with a 10 nm Pt film was used for the spin Seebeck energy converter. For a temperature difference of Δ T ≈ 0.036 K applied on the thin YIG film, the obtained Δ V ≈ 28 µV, which is equivalent to SLSSE ≈ 80.4 nV/K, is close to the typical reported values for thick epitaxial YIG films. The temperature and magnetic field-dependent behaviors of spin Seebeck effects in our YIG films suggest active magnon excitations through the noncoherent precession channel. The effective SSE generation with the solution-processed thin YIG film provides versatile applications of the spin thermoelectric energy conversion.

Tyrosine-mediated two-dimensional peptide assembly and its role as a bio-inspired catalytic scaffold.

In two-dimensional interfacial assemblies, there is an interplay between molecular ordering and interface geometry, which determines the final morphology and order of entire systems. Here we present the interfacial phenomenon of spontaneous facet formation in a water droplet driven by designed peptide assembly. The identified peptides can flatten the rounded top of a hemispherical droplet into a plane by forming a macroscopic two-dimensional crystal structure. Such ordering is driven by the folding geometry of the peptide, interactions of tyrosine and crosslinked stabilization by cysteine. We discover the key sequence motifs and folding structures and study their sequence-specific assembly. The well-ordered, densely packed, redox-active tyrosine units in the YYACAYY (H-Tyr-Tyr-Ala-Cys-Ala-Tyr-Tyr-OH) film can trigger or enhance chemical/electrochemical reactions, and can potentially serve as a platform to fabricate a molecularly tunable, self-repairable, flat peptide or hybrid film.