vdW Gap Research Articles

Lowering Contact Resistances of Two-Dimensional Semiconductors by Memristive Forming.

Two-dimensional semiconductors have great potential for beyond-silicon electronics. However, because of the lack of controllable doping methods, Fermi level pinning, and van der Waals (vdW) gaps at the metal-semiconductor interfaces, these devices exhibit high electrical contact resistances, restricting their practical applications. Here, we report a general contact-resistance-lowering strategy by constructing vertical metal-semiconductor-metal memristor structures at the contact regions and setting them into a nonvolatile low-resistance state through a memristive forming process. Through this, we reduce the contact resistances of MoS2 field-effect transistors (FETs) by at least one order of magnitude and improve the on-state current densities of MoTe2 FETs by about two orders of magnitude. We also demonstrate that this strategy is applicable to other two-dimensional semiconductors, including MoSe2, WS2, and WSe2, and a variety of contact metals, including Au, Cu, Ni, and Pd. The good stability and universality indicate the great potential for technological applications.

Recent Progress in 1D Contacts for 2D-Material-Based Devices.

Recent studies have intensively examined 2D materials (2DMs) as promising materials for use in future quantum devices due to their atomic thinness. However, a major limitation occurs when 2DMs are in contact with metals: a van der Waals (vdW) gap is generated at the 2DM-metal interfaces, which induces metal-induced gap states that are responsible for an uncontrollable Schottky barrier (SB), Fermi-level pinning (FLP), and high contact resistance (RC ), thereby substantially lowering the electronic mobility of 2DM-based devices. Here, vdW-gap-free 1D edge contact is reviewed for use in 2D devices with substantially suppressed carrier scattering of 2DMs with hexagonal boron nitride (hBN) encapsulation. The 1D contact further enables uniform carrier transport across multilayered 2DM channels, high-density transistor integration independent of scaling, and the fabrication of double-gate transistors suitable for demonstrating unique quantum phenomena of 2DMs. The existing 1D contact methods are reviewed first. As a promising technology toward the large-scale production of 2D devices, seamless lateral contacts are reviewed in detail. The electronic, optoelectronic, and quantum devices developed via 1D contacts are subsequently discussed. Finally, the challenges regarding the reliability of 1D contacts are addressed, followed by an outlook of 1D contact methods.

Strain Releasing of Flexible 2D Electronics through van der Waals Sliding Contact.

Two-dimensional (2D) materials have demonstrated promising potential for flexible electronics, owning to their atomic thin body thickness and dangling-bond-free surface. Here, we report a sliding contact device structure for efficient strain releasing. By fabricating a weakly coupled metal-2D junction with a van der Waals (vdW) gap in between, the applied strain could be effectively released through their interface sliding; hence minimized strain is transferred to the 2D lattice. Therefore, we observed stable device behavior with electrodes stretching over 110%, much higher than 2D devices using evaporated metal contacts. Furthermore, through multicycle straining-releasing measurements, we found the electrodes still form intimate contact with nearly constant contact resistance during sliding, confirming the optimization of device flexibility and electrical properties at the same time. Finally, we demonstrate this vdW sliding contact is a general device geometry and could be well-extended to various 2D or 3D bulk materials, leading to devices with much higher strain tolerance.

Stacking Faults and Intercalants in (Ta1–xTix)Se2 Revealed by Cross-Sectional Transmission Electron Microscopy

Structural defects in the layered compounds (Ta1–xTix)Se2 (0 ≤ x ≤ 1), characterized by anisotropy in electrical conductivity, have been studied by cross-sectional observation using chemically sensitive high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Intercalants were detected in the widest space between Se atoms across the van der Waals (vdW) gap, regardless of the Ti content (except TiSe2) or thermal history after the crystal growth. Binary TaSe2 with both 3R and 2H structures includes a high density of stacking faults, while Ti addition stabilizes the 1T structure with no stacking faults. We show that the chemical composition dominates the structural defects in the (Ta1–xTix)Se2 compounds. Substitution of Ta with Ti acts as a hole addition, which reduces the number of conduction electrons in the layer, while an intercalated atom in the vdW gap may not act as a donor of conduction electrons.

Observation of Néel-type skyrmions in acentric self-intercalated Cr1+δTe2

Transition-metal dichalcogenides intercalated with 3d-transition metals within the van der Waals (vdW) gaps have been the focus of intense investigations owing to their fascinating structural and magnetic properties. At certain concentrations the intercalated atoms form ordered superstructures that exhibit ferromagnetic or anti-ferromagnetic ordering. Here we show that the self-intercalated compound Cr1+δTe2 with δ ≈ 0.3 exhibits a new, so far unseen, three-dimensionally ordered (2×2×2) superstructure. Furthermore, high resolution X-ray diffraction reveals that there is an asymmetric occupation of the two inequivalent vdW gaps in the unit cell. The structure thus lacks inversion symmetry, which, thereby, allows for chiral non-collinear magnetic nanostructures. Indeed, Néel-type skyrmions are directly observed using Lorentz transmission electron microscopy. The skyrmions are stable within the accessible temperature range (100–200 K) as well as in zero magnetic field. The diameter of the Néel skyrmions increases with lamella thickness and varies with applied magnetic field, indicating the role of long-range dipole fields. Our studies show that self-intercalation in vdW materials is a novel route to the formation of synthetic non-collinear spin textures.

An Efficient Dopant for Introducing Magnetism into Topological Insulator Bi2Se3.

In this work, we obtained an effective way to introduce magnetism into topological insulators, and successfully fabricated single crystal C-Bi2Se3. The structural, electrical and magnetic properties of non-magnetic element X (B, C and N) doped at Bi, Se1, Se2 and VDW gap sites of Bi2Se3 were studied by the first principles. It is shown that the impurity bands formed inside the bulk inverted energy gap near the Fermi level with C doping Bi2Se3. Due to spin-polarized ferromagnetic coupling, the time inversion symmetry of Bi2Se3 is destroyed. Remarkably, C is the most effective dopant because of the magnetic moment produced by doping at all positions. The experiment confirmed that the remnant ferromagnetism Mr is related to the C concentration. Theoretical calculations and experiments confirmed that carbon-doped Bi2Se3 is ferromagnetic, which provides a plan for manipulating topological properties and exploring spintronic applications.

Self‐Intercalation Tunable Interlayer Exchange Coupling in a Synthetic van der Waals Antiferromagnet

AbstractOne of the most promising avenues in 2D materials research is the synthesis of antiferromagnets employing 2D van der Waals (vdW) magnets. However, it has proven challenging, due in part to the complicated fabrication process and undesired adsorbates as well as the significantly deteriorated ferromagnetism at atomic layers. Here, the engineering of the antiferromagnetic (AFM) interlayer exchange coupling between atomically thin yet ferromagnetic CrTe2 layers in an ultra‐high vacuum‐free 2D magnetic crystal, Cr5Te8 is reported. By self‐introducing interstitial Cr atoms in the vdW gaps, the emergent AFM ordering and the resultant giant magnetoresistance effect are induced. A large negative magnetoresistance (10%) with a plateau‐like feature is revealed, which is consistent with the AFM interlayer coupling between the adjacent CrTe2 main layers in a temperature window of 30 K below the Néel temperature. Notably, the AFM state has a relatively weak interlayer exchange coupling, allowing a switching between the interlayer AFM and ferromagnetic states at moderate magnetic fields. This work represents a new route to engineering low‐power devices that underpin the emerging spintronic technologies, and an ideal laboratory to study 2D magnetism.

Modulation Doping Enables Ultrahigh Power Factor and Thermoelectric ZT in n-Type Bi2 Te2.7 Se0.3.

Bismuth telluride‐based thermoelectric (TE) materials are historically recognized as the best p‐type (ZT = 1.8) TE materials at room temperature. However, the poor performance of n‐type (ZT≈1.0) counterparts seriously reduces the efficiency of the device. Such performance imbalance severely impedes its TE applications either in electrical generation or refrigeration. Here, a strategy to boost n‐type Bi2Te2.7Se0.3 crystals up to ZT = 1.42 near room temperature by a two‐stage process is reported, that is, step 1: stabilizing Seebeck coefficient by CuI doping; step 2: boosting power factor (PF) by synergistically optimizing phonon and carrier transport via thermal‐driven Cu intercalation in the van der Waals (vdW) gaps. Theoretical ab initio calculations disclose that these intercalated Cu atoms act as modulation doping and contribute conduction electrons of wavefunction spatially separated from the Cu atoms themselves, which simultaneously lead to large carrier concentration and high mobility. As a result, an ultra‐high PF ≈63.5 µW cm−1 K−2 at 300 K and a highest average ZT = 1.36 at 300–450 K are realized, which outperform all n‐type bismuth telluride materials ever reported. The work offers a new approach to improving n‐type layered TE materials.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects.

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out-of-plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Significantly enhanced interlayer ferromagnetic coupling in van der Waals Fe3GeTe2 bilayer by Be-ion intercalation

Two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) materials have recently received attention due to their potential applications in next-generation spintronic devices. However, the reduced dimensionality and weak interlayer vdW interaction seriously suppress the magnetic Curie temperature Tc, raising the concern with enhancing the interlayer FM coupling. It is argued that proper ion intercalation may enhance the interlayer coupling by establishing strong chemical bonding. In this work, this issue in a 2D vdW FM Fe3GeTe2 (FGT) bilayer as an example is addressed, and our first-principles calculations predict that beryllium (Be) can be a promising intercalant for such enhancement. It is revealed that the Be-ion migration in-between the vdW gap has only moderate energy barriers owing to its small ionic radius, suggesting the feasibility of reversible intercalation. Particularly, Be-ion intercalation can significantly enhance the interlayer FM coupling by reducing the interlayer distance. The strong bonding that pulls two FGT monolayers closer is ascribed to orbital hybridization between Be-ions and interfacial Te-FeI sites. Be-ion intercalation also contributes to electron doping via charge transfer, favoring the enhanced intralayer FM coupling. This work suggests an alternative scheme for reversibly controlled ferromagnetism enhancement in 2D vdW ferromagnets using ion intercalation.

Thermal transport and mixed valence in ZrTe3 doped with Hf and Se

Two-dimensional transition metal trichalcogenides (TMTCs) feature covalently bonded metal-chalcogen layers separated by the van der Waals (vdW) gap. Similar to transition metal dichalcogenides (TMDCs), TMTCs often host charge density waves (CDWs) and superconductivity, but unlike TMDCs, atomic chains in the crystal structure give rise to quasi one-dimensional (quasi 1D) conduction. ZrTe3 features the CDW below TCDW = 63 K and filamentary superconductivity below 2 K that can be enhanced by pressure or chemical substitution. Here, we report the presence of mixed valent Zr2+ and Zr4+ atoms in ZrTe3 crystals that are reduced by doping in ZrTe3−xSex and Zr1−yHfyTe3. Superconductivity is enhanced via disorder in Te2-Te3 atomic chains that are associated with CDW formation. Hf substitution on the Zr atomic site enhances TCDW due to unperturbed Te2-Te3 chain periodicity and enhanced electron-phonon coupling. Weak electronic correlations in ZrTe3−xSex are likely governed by the lattice contraction effects.

High-performance, self-powered flexible MoS2 photodetectors with asymmetric van der Waals gaps.

With an urgent demand for low-energy-consumption and wearable devices, it is desirable to find an easy, effective, and low-cost method to fabricate self-powered flexible photodetectors with simple configurations and high-performance. Self-powered photodetectors are normally fabricated based on either two different materials or the same material in contact with two different metal electrodes. Here, a flexible MoS2 photodetector with the same Au electrodes was fabricated on a polyethylene terephthalate (PET) substrate which exhibits self-powered properties. To our knowledge, its configuration is the simplest, and the fabrication process is easy to implement. At a bias of 0 V, the photodetector exhibits a high responsivity of 431 mA W-1, a short response/recovery time of 40 ms/40 ms, and excellent flexibility. Compared with those at a bias of 2 V, a dark current is sufficiently suppressed, and the response/recovery speed is significantly improved. It is found that the driving force of the self-powered photodetector is provided by the asymmetric Schottky barriers originating from the spontaneous generation of two van der Waals gaps with different widths. The asymmetric barriers exist stably at the interfaces between the 2D material and Au electrodes as further observed for ReS2 or GaSe flakes, which show the generality of asymmetric Schottky barriers between the 2D material and Au electrodes. The discovery here thus gives a new way to generate asymmetric Schottky barriers and develop high-performance self-powered photodetectors.

The interlayer coupling modulation of a g-C3N4/WTe2 heterostructure for solar cell applications.

Constructing van der Waals (vdW) heterostructures has been proved to be an excellent strategy to design or modulate the physical and chemical properties of 2D materials. Here, we investigated the electronic structures and solar cell performances of the g-C3N4/WTe2 heterostructure via first-principles calculations. It is highlighted that the g-C3N4/WTe2 heterostructure presents a type-II band edge alignment with a band gap of 1.24 eV and a corresponding visible light absorption coefficient of ∼106 cm−1 scale. Interestingly, the band gap of the g-C3N4/WTe2 heterostructure could increase to 1.44 eV by enlarging the vdW gap to harvest more visible light energy. It is worth noting that the decreased band alignment difference resulting from tuning the vdW gap, leads to a promotion of the power conversion efficiency up to 17.68%. This work may provide theoretical insights into g-C3N4/WTe2 heterostructure-based next-generation solar cells, as well as a guide for tuning properties of vdW heterostructures.

Research progress of novel properties in several van der Waals ferroelectric materials

Ferroelectric (FE) materials possess electrically switchable spontaneous polarizations, showing broad applications in various functional devices. For the miniaturization of electronic devices, two-dimensional (2D) van der Waals (vdW) ferroelectric materials and the corresponding bulk counterparts have aroused more interest of researchers. Recently, several kinds of 2D vdW ferroelectrics have been fabricated in experiment. These 2D vdW FEs, as well as their bulk counterparts, exhibit novel properties as demonstrated in experiment or predicted in theory. This paper is to review the recent progress of novel properties of several vdW ferroelectrics. In Section II, we introduce the unusual ferroelectric property—a uniaxial quadruple potential well for Cu displacements—enabled by the van der Waals gap in copper indium thiophosphate (CuInP<sub>2</sub>S<sub>6</sub>). The electric field drives the Cu atoms to unidirectionally cross the vdW gaps, which is distinctively different from dipole reorientation, resulting in an unusual phenomenon that the polarization of CuInP<sub>2</sub>S<sub>6</sub> aligns against the direction of the applied electric field. The potential energy landscape for Cu displacements is strongly influenced by strain, accounting for the origin of the negative piezoelectric coefficient and making CuInP<sub>2</sub>S<sub>6</sub> a rare example of a uniaxial multi-well ferroelectric. In Section III, we introduce the distinct geometric evolution mechanism of the newly reported M<sub>2</sub>Ge<sub>2</sub>Y<sub>6</sub> (M = metal, X = Si, Ge, Sn, Y = S, Sn, Te) monolayers and a high throughput screening of 2D ferroelectric candidates based on this mechanism. The ferroelectricity of M<sub>2</sub>Ge<sub>2</sub>Y<sub>6</sub> originates from the vertical displacement of Ge-dimer in the same direction driven by a soft phonon mode of the centrosymmetric configuration. Another centrosymmetric configuration is also dynamically stable but higher in energy than the ferroelectric phase. The metastable centrosymmetric phase of M<sub>2</sub>Ge<sub>2</sub>Y<sub>6</sub> monolayers allows a new two-step ferroelectric switching path and may induce novel domain behaviors. In Section IV, a new concept about constructing 2D ferroelectric QL-M<sub>2</sub>O<sub>3</sub>/graphene heterostructure to realize monolayer-based FE tunnel junctions or potentially graphene p-n junctions is reviewed. These findings provide new perspectives of the integration of graphene with monolayer FEs, as well as related functional devices. Finally, the challenge and prospect of vdW ferroelectrics are discussed, providing some perspective for the field of ferroelectrics.

Elastic Properties and Deformation Mechanisms in the van der Waals Single‐Crystalline Indium Selenide

The bulk van der Waals (vdW) single‐crystalline indium selenide (β‐InSe) displays exceptional plasticity in a layered crystalline form at both micro‐ and macroscale. However, the nanoscale origin of plasticity remains unclear. Herein, an atomic‐level study on the deformation mechanisms of InSe by using first‐principles calculations is reported. Remarkable anisotropic elasticity is observed in the vdW InSe layered crystal, and the stiffness is dramatically softened because of the vdW gap in the layered structure. The simulations capture the distinct fracture modes in the uniaxial tensile deformation, depending on the loading directions—brittle fracture in the [100] and [110] directions while ductile failure in the [001] direction. The InSe layered crystal structure exhibits superplastic deformability under uniaxial compression. Different transition pathways, including interlayer tangling, amorphization, and cross‐linking, are tracked along respective deformation directions. The unprecedented plasticity of InSe layered crystals can be attributed to the phase transition coupled with interlayer gliding and cross‐layer dislocation slipping. This study deepens our understanding of the deformation mechanisms of layered materials at the atomistic level and provides insights into tailoring material properties for low‐dimensional material design based on its deformation mechanisms.

Direct observation of trapped charges at ReSe2 and graphene heterojunctions

The van der Waals (vdW) heterojunction often reveals unexpected characteristics distinct from conventional junctions. We investigate an emergent interface phenomenon between monolayer ReSe2 and graphene via combined studies of scanning tunneling microscopy (STM) and density functional theory (DFT). When probing monolayer ReSe2 on graphene at bias voltages within the ReSe2 band gap (in-gap bias; −1.2 V to 0.5 V), strikingly, observed topograph appears just like ReSe2 as it shows precisely the same hexagonal periodicity of ReSe2 lattice instead of the underlying graphene lattice. To answer this puzzle, we examine the nature of charge redistribution at the confined vdW gap between ReSe2 and graphene. DFT calculations indicate that the electron accumulation arises right below the ReSe2 layer, while the electron depletion occurs from the graphene layer. This leads to an asymmetrically polarized two-dimensional layer of confined electron density (termed as trapped charges) in the vdW gap, which agrees well with the in-gap STM topograph. We also find that the accumulation of the trapped charge is enhanced strongly at the edge of ReSe2.

Bandgap Shrinkage and Charge Transfer in 2D Layered SnS2 Doped with V for Photocatalytic Efficiency Improvement.

Effects of electronic and atomic structures of V-doped 2D layered SnS2 are studied using X-ray spectroscopy for the development of photocatalytic/photovoltaic applications. Extended X-ray absorption fine structure measurements at V K-edge reveal the presence of VO and VS bonds which form the intercalation of tetrahedral OVS sites in the van der Waals (vdW) gap of SnS2 layers. X-ray absorption near-edge structure (XANES) reveals not only valence state of V dopant in SnS2 is ≈4+ but also the charge transfer (CT) from V to ligands, supported by V Lα,β resonant inelastic X-ray scattering. These results suggest V doping produces extra interlayer covalent interactions and additional conducting channels, which increase the electronic conductivity and CT. This gives rapid transport of photo-excited electrons and effective carrier separation in layered SnS2 . Additionally, valence-band photoemission spectra and S K-edge XANES indicate that the density of states near/at valence-band maximum is shifted to lower binding energy in V-doped SnS2 compare to pristine SnS2 and exhibits band gap shrinkage. These findings support first-principles density functional theory calculations of the interstitially tetrahedral OVS site intercalated in the vdW gap, highlighting the CT from V to ligands in V-doped SnS2 .

TaCo2Te2: An Air‐Stable, High Mobility Van der Waals Material with Probable Magnetic Order

AbstractVan der Waals (vdW) materials are an indispensable part of functional device technology due to their versatile physical properties and ease of exfoliating to the low‐dimensional limit. Among all the compounds investigated so far, the search for magnetic vdW materials has intensified in recent years, fueled by the realization of magnetism in 2D. However, metallic magnetic vdW systems are still uncommon. In addition, they rarely host high‐mobility charge carriers, which is an essential requirement for high‐speed electronic applications. Another shortcoming of 2D magnets is that they are highly air sensitive. Using chemical reasoning, TaCo2Te2 is introduced as an air‐stable, high‐mobility, magnetic vdW material. It has a layered structure, which consists of Peierls distorted Co chains and a large vdW gap between the layers. It is found that the bulk crystals can be easily exfoliated and the obtained thin flakes are robust to ambient conditions after 4 months of monitoring using an optical microscope. Signatures of canted antiferromagntic behavior are also observed at low‐temperature. TaCo2Te2 shows a metallic character and a large, nonsaturating, anisotropic magnetoresistance. Furthermore, the Hall data and quantum oscillation measurements reveal the presence of both electron‐ and hole‐type carriers and their high mobility.

Van der Waals Superstructure and Twisting in Self-Intercalated Magnet with Near Room-Temperature Perpendicular Ferromagnetism.

The emergence of van der Waals (vdW) magnets has created unprecedented opportunities to manipulate magnetism for advanced spintronics based upon all-vdW heterostructures. Among various vdW magnets, Cr1+δTe2 possesses high temperature ferromagnetism along with possible topological spin textures. As this system can support self-intercalation in the vdW gap, it is crucial to precisely pinpoint the exact intercalation to understand the intrinsic magnetism of the system. Here, we developed an iterative method to determine the self-intercalated structures and show evidence of vdW "superstructures" in individual Cr1+δTe2 nanoplates exhibiting magnetic behaviors distinct from bulk chromium tellurides. Among 26,332 possible configurations, we unambiguously identified the Cr-intercalated structure as 3-fold symmetry broken Cr1.5Te2 segmented by vdW gaps. Moreover, a twisted Cr-intercalated layered structure is observed. The spontaneous formation of twisted vdW "superstructures" not only provides insight into the diverse magnetic properties of intercalated vdW magnets but may also add complementary building blocks to vdW-based spintronics.

Bandgap Tuned WS2 Thin‐Film Photodetector by Strain Gradient in van der Waals Effective Homojunctions

AbstractVan der Waals (vdW) heterostructures (or heterojunctions) are formed by stacking two different 2D materials (e.g., graphene, h‐BN, or transition metal dichalcogenides) across vdW gaps. In a type‐II heterojunction, 2D semiconductors are aligned with staggered bandgaps, which can effectively separate electron and hole carriers, and enable promising high‐performance photovoltaics and photodetectors. Herein, an effective vdW‐homojunction is reported, formed by one 2D material (2H‐WS2) with vdW gap engineering leading to different electronic structures and type‐II junction formation. WS2 films are synthesized by W metal deposition and controlled sulfurization method leading to a nonuniform vdW gap strain in the film. The vdW strain gradients in multilayer WS2 films are confirmed by transmission electron microscopy analysis, and the modeling by density functional theory shows an effective type‐II homojunction formation via modulated bandgaps by the vdW gap strains. The superior performance of a broadband photodetector application is confirmed by photoluminescence and photocurrent experiments.