Monolayer Limit Research Articles

Van der Waals electride: Toward intrinsic two-dimensional ferromagnetism of spin-polarized anionic electrons

Discoveries of two-dimensional (2D) magnetism originated from confined atomic layers in van der Waals (vdW) crystals provide an interesting arena for elucidating its fundamentals and enrich magneto-electric and quantum properties. However, a material that exhibits intrinsic 2D magnetism of interstitial electrons occupying layered space, as a root system of magnetic vdW crystals, remains obscure. In this work, 2D ferromagnetic vdW electride, [RECl]2+·2e− (RE = Y and La) is reported with perfectly isolated ferromagnetic 2D blocks encompassing quasi-atomic electron layers. The ferromagnetism of the vdW electride with Curie temperature of 100 K originates from the spin-polarized quasi-atomic electrons with a substantial moment up to ∼0.91 Bohr magneton, which behave as magnetic elements in paramagnetic lattice framework. Invariable ferromagnetism at the monolayer limit strongly supports the 2D ferromagnetism of quasi-atomic electrons. These findings expand the variety of 2D magnetic crystals, providing a promising platform to study the emergent magnetism of low-dimensional electron phases.

Two-Dimensional Magnets beyond the Monolayer Limit.

Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down to a single monolayer. However, the question is whether 2D magnetism extends beyond the monolayer limit, to chemical species formed by sparse but regular 2D arrays of magnetic atoms. Here we show that sub-monolayer superstructures of Eu atoms self-assembled on the silicon surface exhibit strong magnetic signals. Robust easy-plane magnetism is discovered in both one- and two-dimensionally ordered structures with Eu coverage of half monolayer and above. The emergence of 2D magnetism manifests itself by a strong dependence of the effective transition temperature on weak magnetic fields. The results constitute a versatile platform for miniaturization of 2D magnetic systems and seed an expandable class of atomically thin magnets for applications in information technologies.

Aluminum Oxide at the Monolayer Limit via Oxidant‐Free Plasma‐Assisted Atomic Layer Deposition on GaN

AbstractAtomic layer deposition (ALD) is an essential tool in semiconductor device fabrication that allows the growth of ultrathin and conformal films to precisely form heterostructures and tune interface properties. The self‐limiting nature of the chemical reactions during ALD provides excellent control over the layer thickness. However, in contrast to idealized growth models, it is challenging to create continuous monolayers by ALD because surface inhomogeneities and precursor steric interactions result in island growth. Thus, the ability to create closed monolayers by ALD would offer new opportunities for controlling interfacial charge and mass transport in semiconductor devices, as well as for tailoring surface chemistry. Here, encapsulation of c‐plane gallium nitride (GaN) with ultimately thin (≈3 Å) aluminum oxide (AlOx) is reported, which is enabled by the partial conversion of the GaN surface oxide into AlOx using sequential exposure to trimethylaluminum (TMA) and hydrogen plasma. Introduction of monolayer AlOx decreases the work function and enhances reactivity with phosphonic acids under standard conditions, which results in self‐assembled monolayers with densities approaching the theoretical limit. Given the high reactivity of TMA with surface oxides, the presented approach likely can be extended to other dielectrics and III–V‐based semiconductors, with relevance for applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.

Emerging perovskite monolayers.

The library of two-dimensional (2D) materials has been enriched over recent years with novel crystal architectures endowed with diverse exciting functionalities. Bulk perovskites, including metal-halide and oxide systems, provide access to a myriad of properties through molecular engineering. Their tunable electronic structure offers remarkable features from long carrier-diffusion lengths and high absorption coefficients in metal-halide perovskites to high-temperature superconductivity, magnetoresistance and ferroelectricity in oxide perovskites. Emboldened by the 2D materials research, perovskites down to the monolayer limit have recently emerged. Like other 2D species, perovskites with reduced dimensionality are expected to exhibit new physics and to herald next-generation multifunctional devices. In this Review, we critically assess the preliminary studies on the synthetic routes and inherent properties of monolayer perovskite materials. We also discuss how to exploit them for widespread applications and provide an outlook on the challenges and opportunities that lie ahead for this enticing class of 2D materials.

Ambient-Stable Two-Dimensional CrI3 via Organic-Inorganic Encapsulation.

Two-dimensional transitional metal halides have recently attracted significant attention due to their thickness-dependent and electrostatically tunable magnetic properties. However, this class of materials is highly reactive chemically, which leads to irreversible degradation and catastrophic dissolution within seconds in ambient conditions, severely limiting subsequent characterization, processing, and applications. Here, we impart long-term ambient stability to the prototypical transition metal halide CrI3 by assembling a noncovalent organic buffer layer, perylenetetracarboxylic dianhydride (PTCDA), which templates subsequent atomic layer deposition (ALD) of alumina. X-ray photoelectron spectroscopy demonstrates the necessity of the noncovalent organic buffer layer since the CrI3 undergoes deleterious surface reactions with the ALD precursors in the absence of PTCDA. This organic-inorganic encapsulation scheme preserves the long-range magnetic ordering in CrI3 down to the monolayer limit as confirmed by magneto-optical Kerr effect measurements. Furthermore, we demonstrate field-effect transistors, photodetectors, and optothermal measurements of CrI3 thermal conductivity in ambient conditions.

Magnetic order in XY-type antiferromagnetic monolayer CoPS3 revealed by Raman spectroscopy

Mermin-Wagner-Coleman theorem predicts no long-range magnetic order at finite temperature in the two-dimensional (2D) isotropic systems, but it does predict a quasi-long-range order with a divergent correlation length at the Kosterlitz-Thouless (KT) transition for planar magnets. As a representative of two-dimensional planar antiferromagnets, single-layer ${\mathrm{CoPS}}_{3}$ carries the promise of monolayer antiferromagnetic platforms for the ultimately thin spintronics. Here, with the aid of magnetostriction which is sensitive to the local magnetic order, we observe the signature phonon mode splitting of below ${T}_{\mathrm{KT}}$ in monolayer ${\mathrm{CoPS}}_{3}$. The two-magnon signal in the monolayer one manifests the associated magnetic transition. It indicates the quasi-long-range order in an exact 2D planar spin model below KT phase transition. The ratio ($J$\ensuremath{'}/$J$) between the interlayer and intralayer interactions, which characterizes the 2D behaviors, is evaluated to be around 0.03. Our results provide an efficient method to detect the quasi-long-range antiferromagnetic ordering in the two-dimensional magnets down to the monolayer limit.

Strain-Induced Bandgap Enhancement of InSe Ultrathin Films with Self-Formed Two-Dimensional Electron Gas.

Atomically thin indium selenide (InSe) is a representative two-dimensional (2D) family that have recently attracted extensive interest for their intriguing emerging physics and potential optoelectronic applications with high-performance. Here, by utilizing molecular beam epitaxy and scanning tunneling microscopy, we report a controlled synthesis of InSe thin films down to the monolayer limit and characterization of their electronic properties at atomic scale. Highly versatile growth conditions are developed to fabricate well crystalline InSe films, with a reversible and controllable phase transformation between InSe and In2Se3. The band gap size of InSe films, as enhanced by quantum confinement, increases with decreasing film thickness. Near various categories of lattice imperfections, the band gap becomes significantly enlarged, resulting in a type-I band alignments for lateral heterojunctions. Such band gap enhancement, as unveiled from our first-principles calculations, is ascribed to the local compressive strain imposed by the lattice imperfections. Moreover, InSe films host highly conductive 2D electron gas, manifesting prominent quasiparticle scattering signatures. The 2D electron gas is self-formed via substrate doping of electrons, which shift the Fermi level above the confinement-quantized conduction band. Our study identifies InSe ultrathin film as an appealing system for both fundamental research and potential applications in nanoelectrics and optoelectronics.

Magnetic coupling across the antiferromagnetic–antiferromagnetic interface

We investigate the magnetic coupling across the antiferromagnetic–antiferromagnetic (AFM–AFM) interface for the prototypical CoO-NiO bilayer system where the bulk Néel temperature (T N ) of NiO is higher than that of CoO. Using the temperature-dependent exchange-scattered electron intensities from the surface AFM lattice, the surface T N of CoO was estimated as a function of the CoO/NiO film thicknesses. Our results show that the surface T N of CoO layers is enhanced significantly from its bulk T N value and approaching the T N of the NiO layers, as the thickness of the CoO layers is reduced to the monolayer limit. Thus, thinner CoO layers are found to have higher T N than thicker layers on NiO, contrasting with the expected finite-size behavior. In addition to the short-range magnetic exchange coupling at the CoO-NiO interface, we observe the existence of a longer-range magnetic coupling across the interface, mediated by the magnetic correlations. Thus, the magnetic proximity effect is attributed to a combination of a short-range and a weaker long-range magnetic coupling, explaining the long AFM order propagation length in AFM–AFM superlattices and bilayers. Further, our results indicate a new approach to tune the AFM Néel temperature by varying the individual layer thickness of the bilayer system through the magnetic proximity effect.

Enhanced superconductivity in bilayer PtTe2 by alkali-metal intercalations

Layered platinum tellurium (PtTe2) was recently synthesized with controllable layer numbers down to a monolayer limit. Using ab initio calculations based on anisotropic Midgal-Eliashberg formalism, we show that by rubidium (Rb) intercalation, weak superconductivity in bilayer PtTe2 can be significantly boosted with superconducting Tc = 8 K in the presence of spin-orbit coupling (SOC). The intercalant on one hand mediates the interlayer coupling and serves as an electron donor, leading to large density of states at Fermi energy. On the other hand, it increases the mass-enhancement parameter with electron-phonon coupling strength comparable to that of Pt. The potassium intercalated bilayer PtTe2 has a comparable Tc to the case of Rb intercalation. The relatively high Tc with SOC combined with experimental accessible crystal structures suggest that these superconductors are promising platforms to study the novel quantum physics associated with two-dimensional superconductivity, such as the recently proposed type-II Ising superconductivity.

(Invited) Visualizing Carrier Transport and Recombination at Individual Defects within 2D Semiconductors

Two-dimensional (2D) semiconductors are attractive materials for a variety of electronic and photonic applications. This is due in large part to the ability to fabricate films of these materials with thicknesses approaching the monolayer limit and the emergent optical and electronic properties displayed at this extreme scale. Despite these fundamental advantages, active layers based on these materials exhibit a high density of edge/step terminations which are detrimental to carrier transport, limiting their utility in practical applications. Unambiguously evaluating the impact of different types of defects on carrier transport would help to guide the design of improved practical devices, but such information is impossible to generate using existing characterization techniques. Here, a novel mode of Scanning Electrochemical Cell Microscopy (SECCM) will be described which is capable of probing carrier transport across well-defined pathways within individual, well-defined 2D nanosheets. In this “Carrier Generation-Tip Collection” (CG-TC) mode, a focused light source is employed to locally generate carriers and an SECCM tip is used to collect carriers at another point within the same structure, allowing carrier transport within an individual nanosheet to be directly visualized. Analysis of CG-TC data obtained within pristine, defect-free regions of a material allows carrier diffusion lengths to be directly generated. CG-TC data obtained spanning step/edge defects then allows for recombination at individual, nanoscale defects to be rigorously evaluated. Quantitative studies applying CG-TC SECCM to visualize carrier transport within Transition Metal Dichalcogenide (TMD) materials will be presented to demonstrate the efficacy of this approach.

Multiple spin-orbit excitons in \u03b1-RuCl3 from bulk to atomically thin layers

The van der Waals Kitaev magnet α-RuCl3 has recently garnered considerable attention due to its possible realization of topological spin liquids. Combining Raman spectroscopy with numerical calculations, we report here the thickness dependence of electronic structure and ensuing low-energy excitations for exfoliated α-RuCl3. We observe two pronounced peaks at A1 = 249 meV and A2 = 454 meV, which are assigned to single and double spin-orbit (SO) excitons, respectively. Our numerical calculations support this interpretation by reproducing their spectral energy and shape with the electronic parameters: SO coupling λ = 140 meV, Hund’s coupling JH = 350 meV, and on-site Coulomb interaction U = 2.35 eV. The multiple SO excitons persist down to a single layer, whereas their peaks shift slightly to lower energy. For frequencies below 350 cm−1, both a magnetic continuum and phonons show noticeable thickness dependence. These results demonstrate that a SO entangled jeff = 1/2 picture remains valid in a monolayer limit despite the presence of lattice distortions.

Self-Hybridization and Tunable Magnon-Magnon Coupling in van der Waals Synthetic Magnets

Van der Waals magnets are uniquely positioned at the intersection between two-dimensional materials, antiferromagnetic spintronics, and magnonics. The interlayer exchange interaction in these materials enables antiferromagnetic resonances to be accessed at GHz frequencies. Consequently, these layered antiferromagnets are intriguing materials out of which quantum hybrid magnonic devices can be fashioned. Here, we use both a modified macrospin model and micromagnetic simulations to demonstrate a comprehensive antiferromagnetic resonance spectra in van der Waals magnets near the ultrathin (monolayer) limit. The number of optical and acoustic magnon modes, as well as the mode frequencies, are found to be exquisitely sensitive to the number of layers. We discover a self-hybridization effect where pairs of either optical or acoustic magnons are found to interact and self-hybridize through the dynamic exchange interaction. This leads to characteristic avoided energy level crossings in the energy spectra. Through simulations, we show that by electrically controlling the damping of surface layers within heterostructures both the strength and number of avoided energy level crossings in the magnon spectra can be controlled.

Ultranarrow TaS2 Nanoribbons.

Imposing additional confinement in two-dimensional (2D) materials yields further control over their electronic, optical, and topological properties. However, synthesis of ultranarrow nanoribbons (NRs) remains challenging, particularly for transition metal dichalcogenides (TMDs), and synthesizing TMD NRs narrower than 50 nm has remained elusive. Here, we report the vapor-phase synthesis of ultranarrow TaS2 NRs. The NRs are grown within carbon nanotubes, limiting their width and layer number, while stabilizing them against the environment. The NRs reach monolayer thickness and exhibit widths down to 2.5 nm. Atomic-resolution scanning transmission electron microscopy reveals the detailed atomic structure of the ultranarrow NRs and we observe a hitherto unseen atomic structure supermodulation of ordered defect arrays within the NRs. Density functional theory calculations show the presence of flat bands and boundary-localized states, and help identify the atomic configuration of the supermodulation. Nanotube-templated synthesis represents a unique, transferable, and broadly deployable route toward ultranarrow TMD NR growth.

Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order.

The discovery of intrinsic ferromagnetism in ultrathin two-dimensional van der Waals crystals opens up exciting prospects for exploring magnetism in the ultimate two-dimensional limit. Here, we show that environmentally stable CrSe2 nanosheets can be readily grown on a dangling-bond-free WSe2 substrate with systematically tunable thickness down to the monolayer limit. These CrSe2/WSe2 heterostructures display high-quality van der Waals interfaces with well-resolved moiré superlattices and ferromagnetic behaviour. We find no apparent change in surface roughness or magnetic properties after months of exposure in air. Our calculations suggest that charge transfer from the WSe2 substrate and interlayer coupling within CrSe2 play a critical role in the magnetic order in few-layer CrSe2 nanosheets. The highly controllable growth of environmentally stable CrSe2 nanosheets with tunable thickness defines a robust two-dimensional magnet for fundamental studies and potential applications in magnetoelectronic and spintronic devices.

Effect of Sulfurization Temperature on RF Sputtered MoS2 Thin Film

Molybdenum disulfide (MoS2) is one among the transition-metal dichalcogenide (TMD) family which exhibits exotic physical properties at their mono-layer limit. We report a facile way to fabricate stoichiometric, crystalline and star shaped MoS2 film. In this work, ultra-thin MoS2 films were fabricated by two step process (i) RF sputtering of MoS2 target followed by (ii) sulfurization to improve stoichiometry and crystallinity. In order to study the effect of sulfurization temperature on sputtered MoS2, sulfurization has been performed at five different temperatures - 700, 750, 775, 800 and 825°C. Surface morphology of as sputtered and sulfurized MoS2 films were characterized using optical and scanning electron microscopes. Crystallinity and layer thickness of the fabricated MoS2 films were estimated by using Laser Raman spectroscopy. These results confirm that as sputtered MoS2 films are discontinuous, amorphous in nature and it crystalizes into a layered structure during sulfurization at temperature ≥ 750°C. It was observed that at sulfurization temperature of 800°C, the nucleated crystallites well grown into a star shaped crystalline MoS2 with their thickness vary between 2 and 3 mono-layers. These star shapes can provide more surface area/edges that can be exploited to enhance the efficiency of gas sensors.

Transition of CrI2 from a two-dimensional network to one-dimensional chain at the monolayer limit.

Two-dimensional (2D) magnets show promising applications in spintronic devices and appeal increasing attention. CrI2, a counterpart of CrI3, is a magnetic van der Waals crystal. However, the structure of CrI2 at the monolayer limit is not well studied. Here, based on the density functional theory, we revealed the relationship between different phases of CrI2 monolayer and proposed a novel and stable chain structure. The one-dimensional (1D) CrI2 chain is a ferromagnetic semiconductor with robust electronic properties against twisting and tensile strain. Interestingly, the CrI2 chain exhibits superelasticity with a failure strain as large as 39%. In addition, both the magnetic moments on Cr atoms and the exchange energy increase with an increase in the tensile strain. Our results push magnetic ordering from 2D to 1D, which shows possible application prospects in magnetoelectric and spintronic devices.

Antimony oxide nanostructures in the monolayer limit: self-assembly of van der Waals-bonded molecular building blocks

Antimony oxide nanostructures have been identified as candidates for a range of electronic and optoelectronic applications. Here we demonstrate the growth of 2-dimensional antimony oxide nanostructures on various substrates, including highly oriented pyrolytic graphite (HOPG), MoS2 and α-Bi(110) nanoislands. Using scanning tunneling microscopy (STM) we show that the nanostructures formed are exclusively highly crystalline α-Sb2O3(111) monolayers with a lattice constant of 796 pm ± 7 pm. The nanostructures are triangular with lateral dimensions of up to ∼30 nm. Even though elemental antimony nanostructures are grown simultaneously mixed phases are not observed and both materials exhibit their own distinct growth modes. Moiré patterns are also observed and simulated, allowing confirmation of the atomic unit cell and an understanding of the orientation of the Sb2O3 structures with respect to the supporting materials. As in the bulk, the Sb2O3 nanostructures are formed from Sb4O6 molecules that are weakly interacting through van der Waals forces. This allows physical modification of the nanostructures with the STM tip. Scanning tunnelling spectroscopy reveals a wide band gap of at least 3.5 eV. Finally, we show that possible alternative structures that have unit cells comparable to those observed can be excluded based on our DFT calculations. The considered structures are a 2 × 2 reconstruction of β-Sb with one vacancy per unit cell and a van der Waals solid composed of Sb4 clusters. Previous reports have predominantly demonstrated Sb2O3 structures with much larger thicknesses.

Autocorrected off-axis holography of two-dimensional materials

The reduced dimensionality in two-dimensional materials leads a wealth of unusual properties, which are currently explored for both fundamental and applied sciences. In order to study the crystal structure, edge states, the formation of defects and grain boundaries, or the impact of adsorbates, high resolution microscopy techniques are indispensible. Here we report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations. Distinguished features of EH beyond conventional TEM imaging are the gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the 2D material. We demonstrate these features at the example of h-BN, at which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges.

Supported nanoparticle synthesis with Au bis-Ethylenediamine: The mechanism of adsorption onto oxides and carbons

Gold bis-ethylenediamine ([Au(en)2]3+ or AuED) is an effective precursor for the preparation of small Au nanoparticles on common catalyst supports. By studying the adsorption of AuED over a wide variety of supports with surveys of uptake versus time and pH, including XAS studies of solution and adsorbed species, we have demonstrated that an electrostatic mechanism accounts for the data much better than the previously postulated cation exchange mechanism. In the electrostatic mechanism 1) uptake-pH curves are volcano-shaped and are higher in amplitude and broader in pH the lower the PZC of the support, 2) AuED complexes retain at least one hydration sheath with diameter 13.3 Å (0.72 complex/nm2) and adsorb as outer sphere complexes, 3) the monolayer limit in aqueous solutions basified with NaOH is not determined by cation exchange stoichiometry but is a steric limit of close-packed, hydrated complexes. They adsorb at a much lower surface density than that of the surface hydroxyl groups and never electrically saturate the surface. 4) The change in AuED speciation at high pH is consistent with a change in the coulombic attraction of a 3+ versus a 2+ complex.Over carbon supports, adsorption is predominantly electrostatic, but an additional parallel mechanism is apparent in proportion to the graphitic nature of the support.Synthesized Au nanoparticles over the oxide and carbon materials was characterized by high sensitivity powder XRD, STEM imaging, and TPR. The resultant nanoparticles were generally smaller than other synthesis methods reported in the literature for silica, titania, alumina, and carbon. The loading limit of electrostatic adsorption is a surface density of about 1 μmol/m2. At loadings less than or equal to this, electrostatic adsorption provides a simple method with which to synthesize well dispersed Au nanoparticles.

Intrinsic Magnetic Order of Chemically Exfoliated 2D Ruddlesden-Popper Organic-Inorganic Halide Perovskite Ultrathin Films.

Thin film fabrication of 2D layered organic-inorganic hybrid perovskites (2D-OIHPs) for spintronic applications has been attempted using solution-based process like Langmuir-Blodgett technique. However, monolayer or few-layered 2D magnets are not yet realized, even though a wide spectrum of 2D Ruddlesden-Popper (RP) OIHPs are known as quasi-2D Heisenberg magnets in bulk compounds. Here, chemical exfoliation by solvent engineering is applied to successfully synthesize large-sized, few unit-cell-thick 2D RP-OIHPs. Comprehensive structural characterization reveals that binary co-solvents with high relative polarity in spin coating technique are the most effective among nine kinds of solvents. Above all, this enables few-layered 2D RP-OIHP ultrathin films sustaining their intrinsic magnetic order. It is found that XY-like magnetic anisotropy driven by Jahn-Teller effect responsible for ferromagnetism in seven-layered (C6 H5 CH2 CH2 NH3 )2 CuCl4 ultrathin films remains very robust, whereas Ising-like dipolar anisotropy responsible for canted antiferromagnetism in ten-layered (C6 H5 CH2 CH2 NH3 )2 MnCl4 ultrathin films is greatly reduced. It is expected that ferromagnetism even at monolayer limit should be possible by means of further sophisticated solvent engineering as long as Jahn-Teller effect is active. The chemical exfoliation using solvent engineering unambiguously can bring about a new breakthrough in the development of 2D RP-OIHP van der Waals magnets for ultrahigh energy-efficient spintronic, opto-spintronic devices.