We report the synthesis of centimeter sized ultrathin GaN and InN. The synthesis relies on the ammonolysis of liquid metal derived two-dimensional (2D) oxide sheets that were squeeze-transferred onto desired substrates. Wurtzite GaN nanosheets featured typical thicknesses of 1.3 nm, an optical bandgap of 3.5 eV and a carrier mobility of 21.5 cm 2 V −1 s −1 , while the InN featured a thickness of 2.0 nm. The deposited nanosheets were highly crystalline, grew along the (001) direction and featured a thickness of only three unit cells. The method provides a scalable approach for the integration of 2D morphologies of industrially important semiconductors into emerging electronics and optical devices.

The predicted strong piezoelectricity for monolayers of group IV monochalcogenides, together with their inherent flexibility, makes them likely candidates for developing flexible nanogenerators. Within this group, SnS is a potential choice for such nanogenerators due to its favourable semiconducting properties. To date, access to large-area and highly crystalline monolayer SnS has been challenging due to the presence of strong inter-layer interactions by the lone-pair electrons of S. Here we report single crystal across-the-plane and large-area monolayer SnS synthesis using a liquid metal-based technique. The characterisations confirm the formation of atomically thin SnS with a remarkable carrier mobility of~35 cm 2 V −1 s −1 and piezoelectric coefficient of~26 pm V −1. Piezoelectric nanogenerators fabricated using the SnS monolayers demonstrate a peak output voltage of~150 mV at 0.7% strain. The stable and flexible monolayer SnS can be implemented into a variety of systems for efficient energy harvesting.

2D van der Waals materials exhibiting intrinsic magnetic order have attracted enormous interest in the last few years. [1-5] However, despite much progress in the control of their magnetic properties, for example through electrostatic gating or control of the stacking order, [6-9] little is known about the mechanisms governing fundamental magnetic processes in the ultrathin limit. For instance, the extensively studied materials CrI 3 (a semiconductor) and Fe 2 GeTe 3 (a metal) are soft ferromagnets in the bulk crystal form with a remanent magnetization far below the saturation magnetization (a few percent), [10,11] but surprisingly, they become hard ferromagnets when exfoliated to a few atomic layers, with a near squareshaped hysteresis and a large coercive field of H c ≈ 0.1−1 T. [1,12-14] Since hard ferromagnetic properties are crucial to applications, especially as a building block for van der Waals magnetic heterostructures, The recent isolation of 2D van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behavior of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here a widefield nitrogen-vacancy (NV) microscope is employed to directly image these processes in few-layer flakes of the magnetic semiconductor vanadium triiodide (VI 3). Complete and abrupt switching of most flakes is observed at fields H c ≈ 0.5-1 T (at 5 K) independent of thickness. The coercive field decreases as the temperature approaches the Curie temperature (T c ≈ 50 K); however, the switching remains abrupt. The initial magnetization process is then imaged, which reveals thickness-dependent domain wall depinning fields well below H c. These results point to ultrathin VI 3 being a nucleation-type hard ferromagnet, where the coercive field is set by the anisotropy-limited domain wall nucleation field. This work illustrates the power of widefield NV microscopy to investigate magnetization processes in van der Waals ferromagnets, which can be used to elucidate the origin of the hard ferromagnetic properties of other materials and explore field-and current-driven domain wall dynamics.

With no requirements for lattice matching, van der Waals (vdW) ferromagnetic materials are rapidly establishing themselves as effective building blocks for next-generation spintronic devices. We report a hitherto rarely seen antisymmetric magnetoresistance (MR) effect in vdW heterostructured Fe3GeTe2 (FGT)/graphite/FGT devices. Unlike conventional giant MR (GMR), which is characterized by two resistance states, the MR in these vdW heterostructures features distinct high-, intermediate-, and low-resistance states. This unique characteristic is suggestive of underlying physical mechanisms that differ from those observed before. After theoretical calculations, the three-resistance behavior was attributed to a spin momentum locking induced spin-polarized current at the graphite/FGT interface. Our work reveals that ferromagnetic heterostructures assembled from vdW materials can exhibit substantially different properties to those exhibited by similar heterostructures grown in vacuum. Hence, it highlights the potential for new physics and new spintronic applications to be discovered using vdW heterostructures.

The weak interlayer coupling in van der Waals (vdW) magnets has confined their application to two dimensional (2D) spintronic devices. Here, we demonstrate that the interlayer coupling in a vdW magnet Fe3GeTe2 (FGT) can be largely modulated by a protonic gate. With the increase of the protons intercalated among vdW layers, interlayer magnetic coupling increases.Due to the existence of antiferromagnetic layers in FGT nanoflakes, the increasing interlayer magnetic coupling induces exchange bias in protonated FGT nanoflakes. Most strikingly, a rarely seen zero-field cooled (ZFC) exchange bias with very large values (maximally up to 1.2 kOe) has been observed when higher positive voltages (Vg ≥ 4.36 V) are applied to the protonic gate, which clearly demonstrates that a strong interlayer coupling is realized by proton intercalation. Such strong interlayer coupling will enable a wider range of applications for vdW magnets. .

We report thickness-tuned electrical transport and Hall resistivity in highly anisotropic three-dimensional Dirac semimetal ZrTe 5 nanosheets. We find that when the thickness of the nanosheet is blow about 40 nm, the system takes a clear transition from topological semimetal with two bands carriers to a single band with conventional hole carriers. The resistivity peak temperature T * decreases systematically with decreasing thickness down to about 40 nm, then shifts up with the further decrease of the thickness. Analysis of the data below 40 nm indicates that the hole carriers completely dominate the transport in the entire temperature range, regardless of the temperature being below or above T * . By further tracking the carrier density, we find that the Fermi level shifts consecutively downward from conduction band to the valence band as decreasing the thickness. Our experiments unambiguously reveal a highly thickness-tuned transition of band topology in ZrTe 5 nanosheets.Zirconium pentatelluride ZrTe 5 , a fascinating new three-dimensional (3D) Dirac semimetal, has attracted considerable attention recently. It hosts not only rich exotic quantum phenomena related to the chiral Fermions in its highly anisotropic three-dimensional Dirac bands (1-3), but also its electronic structure, compared with other Dirac semimetals, such as Cd 3 As 2 , Na 3 Bi (4-6), presents extremely sensitivity to external perturbations such as magnetic fields, temperature, elastic tension or pressure (7-10). For example, the temperature T, according to the recent angle-resolved photoemission spectroscopy (ARPES) experiment (11), can induce a Lifshitz-type transition of electronic states from the hole band to electron band, leading to a resistance peak near the critical temperature T * . Indeed, the transport measurements on bulk ZrTe 5 have demonstrated clearly the changes from hole-dominated states above T * to electron dominated states below T * (9, 12). While the gapless topological Dirac semimetal phase has been demonstrated in bulk ZrTe 5 (1)(2)(3)13,14), the recent scanning tunneling microscopy (STM) surprisingly detected a bulk band gap with topological edge states at the surface step edge and, thus, indicated that single layered ZrTe 5 might be a two-dimensional topological insulator (15-17), which could host the quantum spin Hall effect (QSHE) (18,19). These contrast results indicated that the thickness, as an alternative way, may effectively tune the electronic structure in ZrTe 5 , though the mechanism has not been fully explored so far.In this letter, we study the transport properties in ZrTe 5 nanosheets with thickness down to 10 nm.We find that the Lifshitz transition temperature T * systematically shifts toward low temperatures as the thickness of the nanosheets decreases down to 40 nm, indicating the suppression of the electron carriers in the Dirac band. However, when the thickness is below 40 nm, a broad resistive peak shows up at high temperatures and moves up with further decrease of the thickness. Both longitudinal r...

Manipulating the exchange bias (EB) effect using an electronic gate is a significant goal in spintronics. The emergence of van der Waals (vdW) magnetic heterostructures has provided improved means to study interlayer magnetic coupling, but to date, these heterostructures have not exhibited electrical gate-controlled EB effects. Here, we report electrically controllable EB effects in a vdW heterostructure, FePS 3 -Fe 5 GeTe 2 . By applying a solid protonic gate, the EB effects were repeatably electrically tuned. The EB field reaches up to 23% of the coercivity and the blocking temperature ranges from 30 to 60 K under various gate-voltages. The proton intercalations not only tune the average magnetic exchange coupling but also change the antiferromagnetic configurations in the FePS 3 layer. These result in a dramatic modulation of the total interface exchange coupling and the resultant EB effects. The study is a significant step toward vdW heterostructure-based magnetic logic for future low-energy electronics. KEYWORDS: gate-tuned exchange bias effect, FePS 3 −Fe 5 GeTe 2 van der Waals heterostructures, interlayer magnetic coupling, proton intercalation

An amphoteric cellulose derivative, O‐carboxymethyl‐O‐2‐hydroxy‐3‐(trimethylammonio) propylcellulose (CM‐HTMAPC), was prepared by the etherification of O‐carboxymethylcellulose (CMC) with (3‐chloro‐2‐hydroxy‐n‐propyl)‐trimethylammonium chloride in a NaOH solution. Apparent molecular sizes of the amphoteric cellulose derivative in aqueous solutions of different NaCl concentrations under various pH conditions were investigated by gel permeation chromatography (GPC), and results were evaluated in terms of inter‐ and intramolecular ionic interactions. Relation between GPC and viscosity results was also discussed. A titration method to determine the apparent acidic dissociation constants of carboxymethyl substituents was developed and the effect of cationic substituents on apparent acidic dissociation constants was examined. ©1995 John Wiley & Sons, Inc.