Van Der Waals Gap Research Articles

Small sized of anion doping effect on semiconducting single-walled carbon nanotube network for field-effect transistors

Carbon nanotubes have shown great promise for high-performance, large-area, solution processable field-effect transistors due to their exceptional charge transport properties. In this study, we utilize the spin-coating method to form networks from selectively sorted semiconducting single-walled carbon nanotubes (s-SWNTs), aiming for scalable electronic device fabrication. The one-dimensional nature of s-SWNTs, however, introduces significant roughness and charge trap sites, hindering charge transport due to the van der Waals gap (∼0.32 nm) between nanotubes. Addressing this, we explored the effects of anion doping on the spin-coated s-SWNT random network, with a focus on the influence of the small size of halogen anions (0.13–0.22 nm) on these electronic properties. Raman and ultraviolet–visible–near-infrared optical spectroscopy results indicate that smaller anions significantly enhance doping effects through strong non-covalent anion–π interactions, improving charge transport and carrier injection efficiency in s-SWNTs, especially for n-type operation. This improvement is inversely proportional to the size of the halogen anions, with the smallest anion (fluorine) effectively transitioning the electrical characteristics of the s-SWNT network from ambipolar to n-type by reducing both junction and contact resistances through anion doping, based on anion–π interaction.

Enhancement of superconductivity and structural phase transitions under pressure in FeTe0.89S0.11

The ternary iron chalcogenide, FeTe0.89S0.11 is a prominent member of the family of Fe-based superconductors with an ambient pressure Tc of around 8 K and a simple structure formed by layers of edge-sharing distorted tetrahedra separated by a van der Waals gap. Upon compression to 6 GPa, the Tc increases monotonically to 11 K. The superconductivity disappeared at about 6 GPa which was correlated to a softening of a Raman mode. High-resolution synchrotron X-ray diffraction shows that the FeTe0.89S0.11 started to transform from the monoclinic phase described by space group Cm to a disordered triclinic phase described by space group P1 at around 10 GPa (theoretical) - 14 GPa (experimental). Due to the phase transition the studied material transforms from a layered compound to a polymer, which is accompanied by a volume collapse of 4.65 %, indicating a discontinuous first-order phase transition. The disappearance of Raman modes, along with pressure independent resistivity, and ab initio calculations results support the structural and electronics phase transition. According to our results Fe(Te,S) may be useful for future high-field power applications being formed of relatively inexpensive and nontoxic elements.

Van der Waals gap engineering in 2D materials for energy storage and conversion

Van der Waals gap engineering in 2D materials for energy storage and conversion

Partial melting nature of phase-change memory Ge-Sb-Te superlattice uncovered by large-scale machine learning interatomic potential molecular dynamics

[GeTe]m-[Sb2Te3]n superlattice (GST-SL) is a promising material candidate to solve the critical problem of high-power consumption for phase-change memory technology. However, the switching mechanism is still under strong debate during the last decade. A key controversial question is that whether melting in GST-SL is possible. In this work, a large-scale machine learning interatomic potential (MLIP) molecular dynamics (MD) simulation with a one-order-of-magnitude larger atom number than that of conventional density functional theory (DFT) MD captures a unique partial melting behavior in GST-SL, where a sparse-nucleation-and-growth governed melting behavior is triggered by the flipping of atoms into van der Waals (vdW) gaps and forming cation-in-vdW-gap (CiV) defect as starting point, and then is slowly developed via propagating liquid-crystalline interfaces. In great contrast, the traditional melting of rock salt (RS)-GST occurs very fast due to instant homogeneous nucleation from high-density intrinsic vacancies distributed randomly in its cubic lattice. Therefore, the melting regions in GST-SL can be spatially localized in form of partial melting and are readily controlled. Moreover, the atomic distribution after melting in GST-SL is more chemical ordering than that in RS-GST. By the large-scale MLIP-MD, the present study provides a critical atomic insight into GST-SL phase-change behaviors that conventional DFT-MDs hardly achieve. This partial-melting nature in GST-SL helps explain the long-term-confused working principle of GST-SL-based phase-change memory, which will accelerate its application in the big-data era.

Low reset current mushroom cell phase‐change memory (PCM) using fiber‐textured homostructure GeSbTe on highly oriented seed layer

We report a low reset current 1T1R mushroom cell phase‐change memory (PCM) device that uses fiber‐textured homostructure GeSbTe (GST) grown on highly‐oriented TiTe2 seed layer. The homostructure device outperformed the industry standard device, that uses doped polycrystalline GST, on most figures of merit. The homostructure devices were also benchmarked against superlattice PCM devices with 10 periods of 5/5nm GST/Sb2Te3 grown on the TiTe2 seed layer, and were found to have same low reset current. We also observed by TEM that the alternating layers of GST/Sb2Te3 and TiTe2/Sb2Te3 in superlattice devices is intermixed in the switched region after the devices are cycled with reset/set pulses. Additionally, when the superlattice device is left in the set state the intermixed switched region crystallinity is textured and exhibits van der Waals gaps. The superlattice PCM devices require a precise layered structure that is hard to yield on a full wafer scale. In contrast, fiber‐textured homostructure PCM cells reported here are easily manufacturable, while providing similarly low reset current and low resistance drift which makes this device suitable for analog AI computation.This article is protected by copyright. All rights reserved.

High-temperature Néel skyrmions in Fe3GaTe2 stabilized by Fe intercalation into the van der Waals gap

Two-dimensional (2D) van der Waals (vdW) magnets that exhibit ferromagnetism at ambient temperature show great promise for spintronic applications. However, until now, only a few pristine or doped 2D magnets have demonstrated the ability to host non-collinear spin textures, thereby limiting their potential applications. Here we directly observe Néel-type skyrmions in the metallic vdW magnetic compound Fe3GaTe2 (FGaT) up to temperatures well above room temperature (≈340 K) in the absence of any external magnetic field. We show that the presence of defects in the structure of FGaT make its structure acentric and therefore compatible with hosting skyrmions that would otherwise not be possible. Indeed, in this regard it is very similar to the closely related compound Fe3GeTe2 (FGT), whose structure with the same space group P3m1 is also realized by defects. Interestingly, however, FGaT accommodates a significantly higher concentration of Fe within the vdW gaps, likely accounting for its enhanced Curie temperature (TC). In addition to the Néel skyrmions observed in the temperature range of 250–340 K, we also detect type-I and -II Bloch-type skyrmionic bubbles in the temperature range of 100–200 K due to an enhanced magnitude of dipole-dipole interactions relative to the Dzyaloshinskii-Moriya exchange interaction. Self-intercalation is thus a highly interesting property of vdW magnets that considerably modifies their fundamental properties.

Modulating charge density wave states in 1T-TaS[formula omitted] by self-intercalation: A DFT study

Complicated charge density wave (CDW) phases transitions in the transition-metal dichalcogenide (TMDs) 1T-TaS2 have attracted wide research interest owing to the accompanied complex electronic structure transition which has not been fully understood. Inspired by a recent experimental work (Zhao et al. (2020), reported the successful preparation of Ta atom self-intercalated 2H-TaS2), we employed first-principles methods to investigate Ta atom self-intercalated 1T-TaS2, exploring effects of self-intercalation on electronic structure, magnetism, and stability ofthe charge density wave (CDW) phase in 1T-TaS2. Our first-principle calculation shows that the intercalation of Ta atom into the van der Waals gap of 1T-TaS2 induce the phase transition from 1T phase to CDW phase, and the positions of intercalated Ta atom determine the distribution of David-star clusters. Accompanied by the structural phase transition, the Ta intercalated 1T-TaS2 also undergoes an electronic structure phase transition from non-magnetic metal to half-metal which is also different from conventional charge density wave’s insulating state. When appropriate holes are doped to the intercalation system, the CDW David-star cluster is gradually destroyed and eventually returns to its original 1T phase. Our studies enrich the phase diagram of TaS2 and highlight the effective manipulation of the CDW states via self-intercalation. This type of materials may have very promising applications in future electronic devices.

Structural evolution and thermoelectric performance in (GeTe)m(Sb2Te3)n compounds

Exploring the relationship between crystal structure and thermoelectric performance is a pivotal topic in the thermoelectric field. In this study, we have comprehensively investigated the correlation between the structural evolution of (GeTe)m(Sb2Te3)n pseudo-binary system and the thermoelectric properties. The proportion of van der Waals bonds increases with the rising Sb2Te3 content, resulting in an increase in the anisotropy of the electrical conductivity and a decrease in the average sound velocity. Additionally, the cation sites in the crystal lattice of these compounds exhibit a mixed occupancy of Ge/Sb atoms, although the cation sites adjacent to the van der Waals gaps are predominantly occupied by Sb atoms. The ultra-low lattice thermal conductivity of the GST124 and GST147 compounds is mainly attributed to the high concentration of van der Waals bonds and enhanced phonon scattering arising from Ge/Sb mixed cation occupancy and high density of defect structures. The high electrical conductivity combined with the low lattice thermal conductivity enables GST124 and GST147 compounds to achieve a maximum ZT value of 0.56 and 0.57, respectively. Higher thermoelectric performance can be achieved through optimization of the microstructure as well as the carrier concentration.

Spin disorder control of topological spin texture

Stabilization of topological spin textures in layered magnets has the potential to drive the development of advanced low-dimensional spintronics devices. However, achieving reliable and flexible manipulation of the topological spin textures beyond skyrmion in a two-dimensional magnet system remains challenging. Here, we demonstrate the introduction of magnetic iron atoms between the van der Waals gap of a layered magnet, Fe3GaTe2, to modify local anisotropic magnetic interactions. Consequently, we present direct observations of the order-disorder skyrmion lattices transition. In addition, non-trivial topological solitons, such as skyrmioniums and skyrmion bags, are realized at room temperature. Our work highlights the influence of random spin control of non-trivial topological spin textures.

Layered rare-earth stabilized Bi2O3: Organic amines intercalation under ambient conditions and their role as a solid adsorbent for iodine

Intercalation chemistry is pivotal in discovering the dimension-dependent properties of 2D materials and expanding their applications. Layered bismuth oxide with composition Bi0.775Ln0.225O1.5 (Ln = La, Pr, Nd, Sm, and Eu) has been demonstrated to intercalate both aliphatic and aromatic amines, resulting in the unit cell expansion along the c-axis. Aliphatic amines were present in a vertical orientation, whereas aromatic amines preferred a horizontal orientation. The intercalated amine concentrations ranged from 0.15 to 0.54 mole per mole of the host. Among the chosen amines, ethylenediamine intercalation enhanced the c-axis of Bi0.775Pr0.225O1.5 by 20 Å, justified by the high amounts of intercalant and intermolecular hydrogen bonding between them, resulting in a bilayer arrangement. The lamellar-plate-like morphology filled with intercalants and twinning collapse was noticed in the FESEM and SAED patterns after the intercalation process. The ethylenediamine intercalated Bi0.775Pr0.225O1.5 was further evaluated for iodine adsorption from non-aqueous solutions through chemisorption following pseudo-second-order kinetics. The X-ray diffraction peaks after adsorption suggested bilayer to monolayer transformation aided by chemical alterations within the host's van der Waals gaps. The N–I vibration mode, observed in the FTIR spectrum of the iodine-adsorbed sample, further augmented the interaction between the two during adsorption.

Lithium Ion Intercalation-Induced Metal-Insulator Transition in Inclined-Standing Grown 2D Non-Layered Cr2S3 Nanosheets.

Gate-controlled ionic intercalation in the van der Waals gap of 2D layered materials can induce novel phases and unlock new properties. However, this strategy is often unsuitable for densely packed 2D non-layered materials. The non-layered rhombohedral Cr2S3 is an intrinsic heterodimensional superlattice with alternating layers of 2D CrS2 and 0D Cr1/3. Here an innovative chemical vapor deposition method is reported, utilizing strategically modified metal precursors to initiate entirely new seed layers, yields ultrathin inclined-standing grown 2D Cr2S3 nanosheets with edge instead of face contact with substrate surfaces, enabling rapid all-dry transfer to other substrates while ensuring high crystal quality. The unconventional ordered vacancy channels within the 0D Cr1/3 layers, as revealed by cross-sectional scanning transmission electron microscope, permitting the insertion of Li+ ions. An unprecedented metal-insulator transition, with a resistance modulation of up to six orders of magnitude at 300 K, is observed in Cr2S3-based ionic field-effect transistors. Theoretical calculations corroborate the metallization induced by Li-ion intercalation. This work sheds light on the understanding of growth mechanism, structure-property correlation and highlights the diverse potential applications of 2D non-layered Cr2S3 superlattice.

Chemically tuned intermediate band states in atomically thin CuxGeSe/SnS quantum material for photovoltaic applications.

A new generation of quantum material derived from intercalating zerovalent atoms such as Cu into the intrinsic van der Waals gap at the interface of atomically thin two-dimensional GeSe/SnS heterostructure is designed, and their optoelectronic features are explored for next-generation photovoltaic applications. Advanced ab initio modeling reveals that many-body effects induce intermediate band (IB) states, with subband gaps (~0.78 and 1.26 electron volts) ideal for next-generation solar devices, which promise efficiency greater than the Shockley-Queisser limit of ~32%. The charge carriers across the heterojunction are both energetically and spontaneously spatially confined, reducing nonradiative recombination and boosting quantum efficiency. Using this IB material in a solar cell prototype enhances absorption and carrier generation in the near-infrared to visible light range. Tuning the active layer's thickness increases optical activity at wavelengths greater than 600 nm, achieving ~190% external quantum efficiency over a broad solar wavelength range, underscoring its potential in advanced photovoltaic technology.

Molecularly thin, two-dimensional all-organic perovskites.

Recently, the emergence of all-organic perovskites with three-dimensional (3D) structures has expanded the potential applications of perovskite materials. However, the synthesis and utilization of all-organic perovskites in 2D form remain largely unexplored because the design principle has not been developed. We present the successful synthesis of a metal-free 2D layered perovskite, denoted as the Choi-Loh van der Waals phase (CL-v phase), with the chemical formula A2B2X4, where A represents a larger-sized cation compared to B and X denotes an anion. The CL-v phase exhibits a van der Waals gap enabled by interlayer hydrogen bonding and can be exfoliated or grown as molecularly thin 2D organic crystals. The dielectric constants of the CL-v phase range from 4.8 to 5.5 and we demonstrate their potential as gate dielectrics for thin-film transistors.

First-Principles Prediction of High and Low Resistance States in Ta/h-BN/Ta Atomristor.

Two-dimensional (2D) materials have received significant attention for their potential use in next-generation electronics, particularly in nonvolatile memory and neuromorphic computing. This is due to their simple metal-insulator-metal (MIM) sandwiched structure, excellent switching performance, high-density capability, and low power consumption. In this work, using comprehensive material simulations and device modeling, the thinnest monolayer hexagonal boron nitride (h-BN) atomristor is studied by using a MIM configuration with Ta electrodes. Our first-principles calculations predicted both a high resistance state (HRS) and a low resistance state (LRS) in this device. We observed that the presence of van der Waals (vdW) gaps between the Ta electrodes and monolayer h-BN with a boron vacancy (VB) contributes to the HRS. The combination of metal electrode contact and the adsorption of Ta atoms onto a single VB defect (TaB) can alter the interface barrier between the electrode and dielectric layer, as well as create band gap states within the band gap of monolayer h-BN. These band gap states can shorten the effective tunneling path for electron transport from the left electrode to the right electrode, resulting in an increase in the current transmission coefficient of the LRS. This resistive switching mechanism in monolayer h-BN atomristors can serve as a theoretical reference for device design and optimization, making them promising for the development of atomristor technology with ultra-high integration density and ultra-low power consumption.

Ultralow Loading Copper-Intercalated MoO3 Nanobelts with High Activity against Antibiotic-Resistant Bacteria.

In recent years, the infection rate of antibiotic resistance has been increasing year by year, and the prevalence of super bacteria has posed a great threat to human health. Therefore, there is an urgent need to find new antibiotic alternatives with long-term inhibitory activity against a broad spectrum of bacteria and microorganisms in order to avoid the proliferation of more multidrug-resistant (MDR) bacteria. The presence of natural van der Waals (vdW) gaps in layered materials allows them to be easily inserted by different guest species, providing an attractive strategy for optimizing their physicochemical properties and applications. Here, we have successfully constructed a copper-intercalated α-MoO3 nanobelt based on nanoenzymes, which is antibacterial through the synergistic effect of multiple enzymes. Compared with α-MoO3, MoO3-x/Cu nanobelts with a copper loading capacity of 2.11% possess enhanced peroxidase (POD) catalytic activity and glutathione (GSH) depletion, indicating that copper intercalation significantly improves the catalytic performance of the nanoenzymes. The MoO3-x/Cu nanobelts are effective in inducing POD and oxidase (OXD) and catalase (CAT) activities in the presence of H2O2 and O2, which resulted in the generation of large amounts of reactive oxygen species (ROS), which were effective in bacterial killing. Interestingly, MoO3-x/Cu nanobelts can serve as glutathione oxidase (GSHOx)-like nanoenzymes, which can deplete GSH in bacteria and thus significantly improve the bactericidal effect. The multienzyme-catalyzed synergistic antimicrobial strategy shows excellent antimicrobial efficiency against β-lactamase-producing Escherichia coli (ESBL-E. coli) and methicillin-resistant Staphylococcus aureus (MRSA). MoO3-x/Cu exhibits excellent spectral bactericidal properties at very low concentrations (20 μg mL-1). Our work highlights the wide range of antibacterial and anti-infective biological applications of copper-intercalated MoO3-x/Cu nanobelt catalysts.

Tailoring Ultrafast Near-Band Gap Photoconductive Response in GeS by Zero-Valent Cu Intercalation.

Zero-valent intercalation of atomic metals into the van der Waals gap of layered materials can be used to tune their electronic, optical, thermal, and mechanical properties. Here, we report the impact of intercalating ∼3 atm percent of zero-valent copper into germanium sulfide (GeS). Advanced many-body calculations predict that copper introduces quasi-localized intermediate band states, and time-resolved THz spectroscopy studies demonstrate that those states have prominent effects on the photoconductivity of GeS. Cu-intercalated GeS exhibits a faster rise of transient photoconductivity and a shorter lifetime of optically injected carriers following near-gap excitation with 800 nm pulses. At the same time, Cu intercalation improves free carrier mobility from 1100 to 1300 cm2 V-1 s-1, which we attribute to the damping of acoustic phonons observed in Brillouin scattering and consequent reduction of phonon scattering.

Giant Deformation Induced Staggered-Layer Structure Promoting the Thermoelectric and Mechanical Performance in n-Type Bi2(Te, Se)3.

Bismuth telluride has long been recognized as the most promising near-room temperature thermoelectric material for commercial application; however, the thermoelectric performance for n-type Bi2(Te, Se)3-based alloys is far lagging behind that of its p-type counterpart. In this work, a giant hot deformation (GD) process is implemented in an optimized Bi2Te2.694Se0.3I0.006+3 wt%K2Bi8Se13 precursor and generates a unique staggered-layer structure. The staggered-layered structure, which is only observed in severely deformed crystals, exhibits a preferential scattering on heat-carrying phonons rather than charge-carrying electrons, thus resulting in an ultralow lattice thermal conductivity while retaining high-weight carrier mobility. Moreover, the staggered-layer structure is located adjacent to the van der Waals gap in Bi2(Te, Se)3 lattice and is able to strengthen the interaction between anion layers across the gap, leading to obviously improved compressive strength and Vickers hardness. Consequently, a high peak figure of merit ZT of ≈ 1.3 at 423 K, and an average ZT of ≈ 1.2 at 300-473 K can be achieved in GD sample. This study demonstrates that the GD process can successfully decouple the electrical and thermal transports with simultaneously enhanced mechanic performance.

Investigating structural, optical, and electron-transport properties of lithium intercalated few-layer MoS2 films: Unraveling the influence of disorder

Molybdenum disulfide is a promising candidate for various applications in electronics, optoelectronics, or alkali-ion batteries. The natural presence of the van der Waals gap allows intercalating alkali ions, such as lithium, into MoS2 films. Intercalation can modify the electronic structure as well as the electrical and optical properties. Here, we present a structural, optical, and electrical characterization of Li-intercalated few-layer MoS2 films. The intercalation was carried out by annealing MoS2 film in the presence of Li2S powder, serving as a lithium source. The initial MoS2 layers were prepared by pulsed laser deposition (PLD) and by sulfurization of 1 nm thick Mo film (TAC). The presence of lithium was confirmed by synchrotron-based x-ray Photoelectron Spectroscopy. The Raman spectroscopy, x-ray diffraction, and optical absorption measurements confirmed semiconducting behavior for all samples. All samples exhibited the thermally activated dependence of the electrical resistance, R, typical for the Efros–Shklovskii variable range hopping in a disordered semiconductor, ln R(T) ∝ (TES/T)1/2, where kBTES is the hopping activation energy. The PLD-grown MoS2 samples exhibited a relatively mild initial disorder primarily caused by grain boundaries. Lithium intercalation led to an increase in disorder, evident in the increase in kBTES and a substantial rise in electrical resistance. The TAC-grown undoped MoS2 sample already exhibited significant resistance, and the impact of Li intercalation on resistance was minimal. This observation was attributed to the fact that the TAC-grown MoS2 samples exhibit a perturbed stoichiometry (the S:Mo ratio ∼ 2.20), causing strong disorder even before Li intercalation. The electron doping caused by lithium, if any, was completely obscured by the effect of disorder.

Ion Transport Behavior in van der Waals Gaps of 2D Materials.

2D materials, with advantages of atomic thickness and novel physical/chemical characteristics, have emerged as the vital building blocks for advanced lamellar membranes which possess promising potential in energy storage, ion separation, and catalysis. When 2D materials are stacked together, the van der Waals (vdW) force generated between adjacent layered nanosheets induces the construction of an ordered lamellar membrane. By regulating the interlayer spacing down to the nanometer or even sub-nanometer scale, rapid and selective ion transport can be achieved through such vdW gaps. The further improvement and application of qualified 2D materials-based lamellar membranes (2DLMs) can be fulfilled by the rational design of nanochannels and the intelligent micro-environment regulation under different stimuli. Focusing on the newly emerging advances of 2DLMs, in this review, the common top-down and bottom-up synthesis approaches of 2D nanosheets and the design strategy of functional 2DLMs are briefly introduced. Two essential ion transport mechanisms within vdW gaps are also involved. Subsequently, the responsive 2DLMs based on different types of external stimuli and their unique applications in nanofluid transport, membrane-based filters, and energy storage are presented. Based on the above analysis, the existing challenges and future developing prospects of 2DLMs are further proposed.

Re-emerging magnetic order in correlated van der Waals antiferromagnet NiPS3.

Van der Waals (vdW) gap is a significant feature that distinguishes vdW magnets from traditional magnets. Manipulating the magnetic properties by changing the vdW gap has been hot topic in condensed matter research. Here we report a re-emerging magnetic order induced by pressure in a correlated vdW antiferromagnetic insulator NiPS3. It is found that the interlayer magnetoresistance (MR) nearly vanishes at the critical pressure where the crystal structure transforms fromC2/mphase to the slidingC2/mphase. On further compression within the slidingC2/mphase, a substantially enhanced MR emerges from low temperature associated with an insulator-to-metal transition, indicating a metallic antiferromagnetic phase. The enhanced re-emerging MR in slidingC2/mphase can be ascribed to the increasing magnetic interaction between neighboring layers due to the vdW gap narrowing. Our results provide important experimental clues for understanding the pressure effects on magnetism in correlated layered materials.