Two-dimensional Van Der Waals Materials Research Articles

Interfacial Modulation of Spin-Orbit Torques Induced by Two-Dimensional van der Waals Material ZrSe3.

Two-dimensional van der Waals (2D vdW) materials are widely used in spin-orbit torque (SOT) devices. Recent studies have demonstrated the low crystal symmetry and large spin Hall conductivity of 2D vdW ZrSe3, indicating its potential applications in low-power SOT devices. Here, we study the interfacial contribution of SOTs and current-induced magnetization switching in the ZrSe3/Py (Ni80Fe20) and ZrSe3/Cu/Py heterostructures. SOT efficiencies of samples are detected by the spin-torque ferromagnetic resonance (ST-FMR), and out-of-plane damping-like torque (τB) is observed. The ratio between τB and the field-like torque (τA) decreases from 0.175 to 0.138 when inserting 1 nm Cu at the interface and then drops to 0.001 when the thickness of Cu intercalation is 2 nm, indicating that Cu intercalation inhibits the τB component of SOT. Moreover, the SOT efficiency is increased from 3.05 to 5.21, which may be attributed to the Cu intercalation being beneficial to improve the interface between Py and ZrSe3. Theoretical calculation has shown that the Cu spacer can change the conductivity of ZrSe3 from semiconductor to conductor, thereby decreasing the Schottky barrier and increasing the transmission efficiency of the spin current. Furthermore, magneto-optical Kerr effect (MOKE) microscopy is employed to verify the current-driven magnetization switching in these structures. In comparison to the ZrSe3/Py bilayer, the critical current density of ZrSe3/Cu/Py is reduced when inserting 1 nm Cu, demonstrating the higher SOT efficiency and lower power consumption in ZrSe3/Cu/Py structures.

Direct visualization of stacking-selective self-intercalation in epitaxial Nb1+xSe2 films

Two-dimensional (2D) van der Waals (vdW) materials offer rich tuning opportunities generated by different stacking configurations or by introducing intercalants into the vdW gaps. Current knowledge of the interplay between stacking polytypes and intercalation often relies on macroscopically averaged probes, which fail to pinpoint the exact atomic position and chemical state of the intercalants in real space. Here, by using atomic-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we visualize a stacking-selective self-intercalation phenomenon in thin films of the transition-metal dichalcogenide (TMDC) Nb1+xSe2. We observe robust contrasts between 180°-stacked layers with large amounts of Nb intercalants inside their vdW gaps and 0°-stacked layers with little detectable intercalants inside their vdW gaps, coexisting on the atomic scale. First-principles calculations suggest that the films lie at the boundary of a phase transition from 0° to 180° stacking when the intercalant concentration x exceeds ~0.25, which we could attain in our films due to specific kinetic pathways. Our results offer not only renewed mechanistic insights into stacking and intercalation, but also open up prospects for engineering the functionality of TMDCs via stacking-selective self-intercalation.

Realization of low potential barrier in MoS2/rGO heterojunction with enhanced electrical conductivity for thin film thermoelectric applications

Two-dimensional (2D) van der Waals materials in-plane anisotropy, caused by a low-symmetric lattice structure, has considerably increased their applications, particularly in thermoelectric. MoS2 and MoS2/reduced graphene oxide (rGO) thin films were grown on SiO2/Si substrate by atmospheric chemical vapor deposition technique to study the thermoelectric performance. Few layered MoS2 was confirmed by the vibrational analysis and the composition elements are confirmed by the x-ray photoelectron spectroscopy technique. The continuous grains lead to reduced phonon life time in A1g and low activation energy assists to enhance the electrical property. The MoS2/rGO has achieved the highest σ of 22 622 S m−1 at 315 K due to an electron-rich cloud around the electrons in S atoms near the adjacent layer of rGO.

Magnetism of MoS2 bilayers with intercalated and surface adsorbed Fe

Magnetic thin films composed of two-dimensional van der Waals materials will play an important role in future spintronics devices. Recently, Tuli et al. performed in-situ MOKE measurements of Fe deposited on an MoS2 substrate and found that a magnetic dead layer (MDL) exists up to approximately 2 Å (Tuli et al. (2021)). To understand the origin of the MDL, we investigate MoS2 bilayers with intercalated and surface adsorbed Fe using first-principles calculations. The results show that the Fe-intercalated bilayer MoS2 is more stable than the bilayer MoS2 with surface adsorbed Fe. The Fe-intercalated bilayer MoS2 is non-magnetic and behaves as the MDL. The systematic investigation of variation in the inter-layer distance between the intercalated Fe layer and surface of MoS2 layer reveals that the strong hybridization between Fe and S is the origin of the MDL. We also show that further intercalation of Fe makes the film magnetic. The results provide a deep understanding of the MDL in two-dimensional van der Waals magnetic materials.

Heavy fermions vs doped Mott physics in heterogeneous Ta-dichalcogenide bilayers

Controlling and understanding electron correlations in quantum matter is one of the most challenging tasks in materials engineering. In the past years a plethora of new puzzling correlated states have been found by carefully stacking and twisting two-dimensional van der Waals materials of different kind. Unique to these stacked structures is the emergence of correlated phases not foreseeable from the single layers alone. In Ta-dichalcogenide heterostructures made of a good metallic “1H”- and a Mott insulating “1T”-layer, recent reports have evidenced a cross-breed itinerant and localized nature of the electronic excitations, similar to what is typically found in heavy fermion systems. Here, we put forward a new interpretation based on first-principles calculations which indicates a sizeable charge transfer of electrons (0.4-0.6 e) from 1T to 1H layers at an elevated interlayer distance. We accurately quantify the strength of the interlayer hybridization which allows us to unambiguously determine that the system is much closer to a doped Mott insulator than to a heavy fermion scenario. Ta-based heterolayers provide therefore a new ground for quantum-materials engineering in the regime of heavily doped Mott insulators hybridized with metallic states at a van der Waals distance.

Sliding-mediated ferroelectric phase transition in CuInP2S6 under pressure

Interlayer stacking order has recently emerged as a unique degree of freedom to control crystal symmetry and physical properties in two-dimensional van der Waals (vdW) materials and heterostructures. By tuning the layer stacking pattern, symmetry-breaking and electric polarization can be created in otherwise non-polar crystals, whose polarization reversal depends on the interlayer sliding motion. Herein, we demonstrate that in a vdW layered ferroelectric, its existing polarization is closely coupled to the interlayer sliding driven by hydrostatic pressure. Through combined structural, electrical, vibrational characterizations, and theoretical calculations, we clearly map out the structural evolution of CuInP2S6 under pressure. A tendency toward a high polarization state is observed in the low-pressure region, followed by an interlayer-sliding-mediated phase transition from a monoclinic to a trigonal phase. Along the transformation pathway, the displacive-instable Cu ion serves as a pivot point that regulates the interlayer interaction in response to external pressure. The rich phase diagram of CuInP2S6, which is enabled by stacking orders, sheds light on the physics of vdW ferroelectricity and opens an alternative route to tailoring long-range order in vdW layered crystals.

Thickness-Tunable Zoology of Magnetic Spin Textures Observed in Fe5GeTe2.

The family of two-dimensional (2D) van der Waals (vdW) materials provides a playground for tuning structural and magnetic interactions to create a wide variety of spin textures. Of particular interest is the ferromagnetic compound Fe5GeTe2 that we show displays a range of complex spin textures as well as complex crystal structures. Here, using a high-brailliance laboratory X-ray source, we show that the majority (1 × 1) Fe5GeTe2 (FGT5) phase exhibits a structure that was previously considered as being centrosymmetric but rather lacks inversion symmetry. In addition, FGT5 exhibits a minority phase that exhibits a long-range ordered (√3 × √3)-R30° superstructure. This superstructure is highly interesting in that it is innately 2D without any lattice periodicity perpendicular to the vdW layers, and furthermore, the superstructure is a result of ordered Te vacancies in one of the topmost layers of the FGT5 sheets rather than being a result of vertical Fe ordering as earlier suggested. We show, from direct real-space magnetic imaging, evidence for three distinct magnetic ground states in lamellae of FGT5 that are stabilized with increasing lamella thickness, namely, a multidomain state, a stripe phase, and an unusual fractal state. In the stripe phase we also observe unconventional type-I and type-II bubbles where the spin texture in the central region of the bubbles is nonuniform, unlike conventional bubbles. In addition, we find a bobber or a cocoon-like spin texture in thick (∼170 μm) FGT5 that emerges from the fractal state in the presence of a magnetic field. Among all the 2D vdW magnets we have thus demonstrated that FGT5 hosts perhaps the richest variety of magnetic phases that, thereby, make it a highly interesting platform for the subtle tuning of magnetic interactions.

Ferroelectric polarization of graphene/h-BN bilayer of different stacking orders

In recent years, two-dimensional van der Waals materials including graphene and hexagonal boron nitride (h-BN) have attracted extensive interest from researchers of many fields. In this paper, the polarization properties of graphene/h-BN van der Waals heterostructures with different stacking orders are explored by first-principle calculations. Notably, the direction of graphene/h-BN heterostructure polarization can be flipped (from AB stacking to BA stacking) by changing its stacking method. According to differential charge density analysis, the interlayer charge redistribution is the main reason driving the polarization flip. Also, we find that the polarization increases with compression of the layers and tensile strain of the lattice. In addition, we investigated the effect of the twisting angle on the polarization. The results show that polarization flip occurs during interlayer rotation and, interestingly, the magnitude and direction of polarization remain constant over a range of twist angles. The results of this study provide insight into the polarization properties of graphene/h-BN heterostructures, which is important for their potential applications in electronic devices.

Deciphering the ultra-high plasticity in metal monochalcogenides.

The quest for electronic devices that offer flexibility, wearability, durability and high performance has spotlighted two-dimensional (2D) van der Waals materials as potential next-generation semiconductors. Especially noteworthy is indium selenide, which has demonstrated surprising ultra-high plasticity. To deepen our understanding of this unusual plasticity in 2D van der Waals materials and to explore inorganic plastic semiconductors, we have conducted in-depth experimental and theoretical investigations on metal monochalcogenides (MX) and transition metal dichalcogenides (MX2). We have discovered a general plastic deformation mode in MX, which is facilitated by the synergetic effect of phase transitions, interlayer gliding and micro-cracks. This is in contrast to crystals with strong atomic bonding, such as metals and ceramics, where plasticity is primarily driven by dislocations, twinning or grain boundaries. The enhancement of gliding barriers prevents macroscopic fractures through a pinning effect after changes in stacking order. The discovery of ultra-high plasticity and the phase transition mechanism in 2D MX materials holds significant potential for the design and development of high-performance inorganic plastic semiconductors.

Methods and optoelectronic device applications of semiconductor epitaxy assisted by two-dimensional van der Waals materials

ABSTRACT This review summarizes research activities on two-dimensional (2D) materials-assisted epitaxy of inorganic semiconductors and their optoelectronic device applications. We presented the overall research related to the growth of epitaxial semiconductor layers on 2D van der Waals materials and discussed various methods to perform controlled growth of semiconductor nanostructures and microstructures on 2D layers. The 2D layers’ benefits in semiconductor technology and their role in non-destructive micro-crystallographic analysis, integration of heterogeneous semiconductor devices, and the fabrication of detachable semiconductor devices. This is followed by an in-depth discussion on 2D materials-assisted growth and non-destructive transfer of random and regular semiconductor arrays and their applications in free-standing, flexible, individually addressable semiconductor optoelectronic devices, including micro display applications. The research covered in this review promotes novel applications of semiconductors grown on 2D materials and exploits ways to synergistically combine their functionalities.

Wafer scale growth of single crystal two-dimensional van der Waals materials.

Two-dimensional (2D) van der Waals (vdW) materials, including graphene, hexagonal boron nitride (hBN), and metal dichalcogenides (MCs), form the basis of modern electronics and optoelectronics due to their unique electronic structure, chemical activity, and mechanical strength. Despite many proof-of-concept demonstrations so far, to fully realize their large-scale practical applications, especially in devices, wafer-scale single crystal atomically thin highly uniform films are indispensable. In this minireview, we present an overview on the strategies and highlight recent significant advances toward the synthesis of wafer-scale single crystal graphene, hBN, and MC 2D thin films. Currently, there are five distinct routes to synthesize wafer-scale single crystal 2D vdW thin films: (i) nucleation-controlled growth by suppressing the nucleation density, (ii) unidirectional alignment of multiple epitaxial nuclei and their seamless coalescence, (iii) self-collimation of randomly oriented grains on a molten metal, (iv) surface diffusion and epitaxial self-planarization and (v) seed-mediated 2D vertical epitaxy. Finally, the challenges that need to be addressed in future studies have also been described.

Evidence of thickness-dependent surface-induced ferroelectricity in few-layer germanium sulfide obtained via scanning tunneling spectroscopy.

The discovery of ferroelectricity in two-dimensional van der Waals materials has sparked enormous interest from the scientific community, due to its possible applications in next-generation nanoelectronic devices, such as random-access memory devices, digital signal processors, and solar cells, among others. In the present study, we used vapor phase deposition to synthesize ultrathin germanium sulfide nano-flakes on a highly oriented pyrolytic graphite substrate. Nanostructures of variable thicknesses were characterized using scanning tunneling microscopy and spectroscopy. Tunneling currents under forward and backward biases were measured as a function of nano-flake thickness. Remarkably, we clearly observed a hysteresis pattern, which we attributed to surface ferroelectric behavior, consistent with the screening conditions of polarization charges. The effect increases as the number of layers is reduced. This experimental result may be directly applicable to miniaturized memory devices, given the two-dimensional nature of this effect.

Second-harmonic generation in 2D moiré superlattices composed of bilayer transition metal dichalcogenides.

Moiré superlattices (MSLs) in twisted two-dimensional van der Waals materials feature twist-angle-dependent crystal symmetry and strong optical nonlinearities. By adjusting the twist angle in bilayer van der Waals materials, the second-harmonic generation (SHG) can be controlled. Here, we focus on exploring the electronic and SHG properties of MSLs in 2D bilayer transition metal dichalcogenides (TMDs) with different twist angles through first-principles calculations. We constructed MSL structures of five TMD materials, including three single-phase materials (MoS2, WS2, and MoSe2) and two heterojunctions (MoS2/MoSe2 and MoS2/WS2) with twist angles of 9.4°, 13.2°, 21.8°, 32.2°, and 42.1° without lattice mismatch. Our findings demonstrate a consistent variation in the SHG susceptibility among different TMD MSLs as a response to twist-angle changes. The underlying reason for the twist-angle dependence of SHG is that the twist angle regulates the interlayer coupling strength, affecting the optical band gap of MSLs and subsequently tuning the SHG susceptibility. Through a comparison of the static SHG susceptibility values, we identified the twist angle of 9.4° as the configuration that yields the highest SHG susceptibility (e.g. 358.5 pm V-1 for the 9.4° MoSe2 MSL). This value is even twice that of the monolayer (173.3 pm V-1 for monolayer MoSe2) and AA'-stacked bilayer structures (139.8 pm V-1 for AA' MoSe2). This high SHG susceptibility is attributed to the strong interlayer coupling in the 9.4° MSL, which enhances the valence band energy (contributed by the antibonding orbitals of chalcogen-pz and transition metal-dz2) and consequently leads to a small optical band gap, thus improving the optical transitions. The findings of this study provide a straightforward way to improve the SHG performance of bilayer TMDs and also throw light on the sensitive relationship between the twist angle, band structure and SHG properties of TMD MSLs.

Ferroelectricity-enhanced potassium-ion storage in van der Waals layered CuInP2S6

We integrate ferroelectric principles into two-dimensional van der Waals materials to enhance both charge and potassium ion (K+) transport at the electrode interface. CuInP2S6 (CIPS) has a distinct room-temperature ferroelectric...

Single crystal growth and electrical transport of two-dimensional van der Waals antiferromagnetic Fe1.3Te

Two-dimensional (2D) van der Waals (vdW) materials with promising electrical, magnetic, optical, and mechanical properties have shown great potential for next-generation electronics and spintronics. Among them, vdW Fe1+xTe with varying...

Self-consistent phonon calculations and quartic anharmonic lattice thermal conductivity in 2D InS monolayer

Low lattice thermal conductivity (κl) is a crucial factor for higher figure-of merit and hence the efficiency of thermoelectric generators. There are several reports on intrinsically low κl values in two-dimensional (2D) van der Waals materials using density functional theory and molecular dynamics simulations. In general, phonon dispersions are studied at absolute zero temperature using the finite-displacement approach within harmonic approximations. In addition, the κlis calculated using the third-order cubic interatomic force constants (IFCs) by solving the Boltzmann transport equation for phonons. In these calculations, we use quartic IFCs to solve self-consistent phonon equations to obtain dynamical dispersion relations at a finite temperature of 300 K using finite-temperature Green’s function for Bosonic systems in 2D indium sulfide (InS) monolayer. The cubic and quartic IFCs are calculated using machine learning algorithms, namely, the ordinary least square fitting and the least absolute shrinkage and selection operator. It was found that there is a lowering of the renormalized anharmonic phonon frequencies in the dispersion relations at 300 K upon the inclusion of quartic IFCs and anharmonic terms in the case of the InS monolayer. Thus, the κlvalue is reduced to 0.6 W/mK as compared to 0.9 W/mK obtained using cubic IFCs.

Observation of the Superstructure in Fe5-xGeTe2 by X-ray Fluorescence Holography.

Fe5-xGeTe2 is a two-dimensional van der Waals material that exhibits ferromagnetic order with a high Curie temperature (TC) of around room temperature. In addition to TC, two magnetic transitions occur with decreasing temperature, and a charge-ordered state is observed at low temperatures. We employed Ge Kα X-ray fluorescence holography (XFH) for Fe5-xGeTe2 to directly investigate the local structure in the charge-ordered state, i.e., the superstructure. The Ge Kα XFH results revealed local atomic structures around the Ge atom, thus clarifying the simultaneous locations and arrangements of the Te, Fe, and Ge atoms. The atomic positions relative to the Ge atom are useful for understanding the coexistence of the ideal 1 × 1 structure and superstructure found in the charge-ordered state.

Broadband Photodetector Based on FePS3/WS2 van der Waals Type II Heterostructure.

In order to understand broadband photodetectors from ultraviolet-visible (UV-vis) to the near-infrared range, one needs to find novel two-dimensional (2D) van der Waals (vdW) materials with broadband optoelectronic performance. Transition metal phosphorus sulfides (TMPSs) have been reported as a new type of vdW material with generally broadband and p-type conductivity. Here, we report a high-performance and broadband photodetector consist of p-type FePS3 and n-type WS2 with a working range of 405-785 nm. The maximum values of responsivity and specific detectivity are 32.5 mA/W and 1.73 × 1012 jones at 405 nm and 2 V bias, which are better than those of its individual constituents and many other 2D vdW heterostructures. The high performance of the FePS3/WS2 photodetector is attributed to the built-in electric field in the FePS3/WS2 p-n heterostructure and type II band alignment. Present study demonstrates that the material family of TMPSs could be a promising platform for broadband photodetector applications.

The strain-induced transitions of the piezoelectric, pyroelectric, and electrocaloric properties of the CuInP2S6 films

Low-dimensional ferroelectrics, ferrielectrics, and antiferroelectrics are of urgent scientific interest due to their unusual polar, piezoelectric, electrocaloric, and pyroelectric properties. The strain engineering and strain control of the ferroelectric properties of layered two-dimensional van der Waals materials, such as CuInP2(S,Se)6 monolayers, thin films, and nanoflakes, are of fundamental interest and especially promising for their advanced applications in nanoscale nonvolatile memories, energy conversion and storage, nano-coolers, and sensors. Here, we study the polar, piezoelectric, electrocaloric, and pyroelectric properties of thin strained films of a ferrielectric CuInP2S6 covered by semiconducting electrodes and reveal an unusually strong effect of a mismatch strain on these properties. In particular, the sign of the mismatch strain and its magnitude determine the complicated behavior of piezoelectric, electrocaloric, and pyroelectric responses. The strain effect on these properties is opposite, i.e., “anomalous,” in comparison with many other ferroelectric films, for which the out-of-plane remanent polarization, piezoelectric, electrocaloric, and pyroelectric responses increase strongly for tensile strains and decrease or vanish for compressive strains.

Probing van der Waals magnetic surface and interface via circularly polarized X-rays

Advances in research of magnetic two-dimensional van der Waals (2D vdW) materials have opened up new opportunities in miniaturization of spintronic devices at the atomically thin limit. One major research thrust, which is the subject of this review, is that the magnetism of 2D vdW materials and their derived hetero-interface may be significantly affected by the local atomic geometry and environment. As a result, there is a pressing need for powerful advanced technique to characterize magnetic vdW surface and interface. Circularly polarized X-rays from synchrotron radiation light sources are key to this endeavor, as they can probe the microscopic magnetism of a specific element, for their atomic sensitivity, element-specificity, core-level excitation as well as capability of separation for the spin and orbit moments. We review a series of representative experimental achievements, exploiting circularly polarized X-rays, in (1) 2D vdW magnet, (2) magnetic metal/vdW interface, (3) molecule/vdW interface as well as (4) vdW/2D interface. On the basis of the review, advantages of probing magnetic vdW surface and interface by circularly polarized X-rays are presented, and challenges and opportunities are also discussed.