Strain engineering of van der Waals heterostructures

Electrical properties of disordered films of van der Waals semiconductor WS2 on paper

One of the primary objectives in contemporary electronics is to develop sensors that are not only scalable and cost-effective but also environmentally sustainable. To achieve this goal, numerous experiments have focused on incorporating nanomaterial-based films, which utilize nanoparticles or van der Waals materials, on paper substrates. In this article, we present a novel fabrication technique for producing dry-abraded van der Waals films on paper, demonstrating outstanding electrical characteristics. We assess the quality and uniformity of these films by conducting a spatial resistivity characterization on a 5 × 5 cm2 dry-abraded WS2 film with an average thickness of 25 μm. Employing transfer length measurements with varying channel length-to-width ratios, we extract critical parameters, including sheet resistance and contact resistance. Notably, our findings reveal a resistivity approximately one order of magnitude lower than previous reports. The film's inherent disorder manifests as an asymmetric distribution of resistance values for specific geometries. We explore how this behavior can be effectively modeled through a random resistance network (RRN), which can reproduce the experimentally observed resistance distribution. Finally, we investigate the response of these devices under applied uniaxial strain and apply the RRN model to gain a deeper understanding of this process.

Thick Does the Trick: Genesis of Ferroelectricity in 2D GeTe-Rich (GeTe)m (Sb2 Te3 )n Lamellae

The possibility to engineer (GeTe)m (Sb2 Te3 )n phase-change materials to co-host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. Here an experimental and theoretical study of epitaxial (GeTe)m (Sb2 Te3 )n with GeTe-rich composition is presented. These layered films feature a tunable distribution of (GeTe)m (Sb2 Te3 )1 blocks of different sizes. Breakthrough evidence of ferroelectric displacement in thick (GeTe)m (Sb2 Te3 )1 lamellae is provided. The density functional theory calculations suggest the formation of a tilted (GeTe)m slab sandwiched in GeTe-rich blocks. That is, the net ferroelectric polarization is confined almost in-plane, representing an unprecedented case between 2D and bulk ferroelectric materials. The ferroelectric behavior is confirmed by piezoresponse force microscopy and electroresistive measurements. The resilience of the quasi van der Waals character of the films, regardless of their composition, is also demonstrated. Hence, the material developed hereby gathers in a unique 2D platform the phase-change and ferroelectric switching properties, paving the way for the conception of innovative device architectures.

3D Strain Measurement of Heterostructures Using the Scanning Transmission Electron Microscopy Moiré Depth Sectioning Method

The mechanical properties of micro- and nanoscale materials directly determine the reliability of heterostructures, microstructures, and microdevices. Therefore, an accurate evaluation of the 3D strain field at the nanoscale is important. In this study, a scanning transmission electron microscopy (STEM) moiré depth sectioning method is proposed. By optimizing the scanning parameters of electron probes at different depths of the material, the sequence STEM moiré fringes (STEM-MFs) with a large field of view, which can be hundreds of nanometers obtained. Then, the 3D STEM moiré information constructed. To some extent, multi-scale 3D strain field measurements from nanometer to the submicrometer scale actualized. The 3D strain field near the heterostructure interface and single dislocation accurately measured by the developed method.

Highly Efficient Uniaxial In-Plane Stretching of a 2D Material via Ion Insertion

On-chip dynamic strain engineering requires efficient micro-actuators that can generate large in-plane strains. Inorganic electrochemical actuators are unique in that they are driven by low voltages (≈1 V) and produce considerable strains (≈1%). However, actuation speed and efficiency are limited by mass transport of ions. Minimizing the number of ions required to actuate is thus key to enabling useful "straintronic" devices. Here, it is shown that the electrochemical intercalation of exceptionally few lithium ions into WTe₂ causes large anisotropic in-plane strain: 5% in one in-plane direction and 0.1% in the other. This efficient stretching of the 2D WTe₂ layers contrasts to intercalation-induced strains in related materials which are predominantly in the out-of-plane direction. The unusual actuation of Li_x WTe₂ is linked to the formation of a newly discovered crystallographic phase, referred to as Td', with an exotic atomic arrangement. On-chip low-voltage (<0.2 V) control is demonstrated over the transition to the novel phase and its composition. Within the Td'-Li_{0.5- δ} WTe₂ phase, a uniaxial in-plane strain of 1.4% is achieved with a change of δ of only 0.075. This makes the in-plane chemical expansion coefficient of Td'-Li_0.5-δ WTe₂ far greater than of any other single-phase material, enabling fast and efficient planar electrochemical actuation.

Interface chemistry of two-dimensional heterostructures - fundamentals to applications

Two-dimensional heterostructures (2D HSs) have emerged as a new class of materials where dissimilar 2D materials are combined to synergise their advantages and alleviate shortcomings. Such a combination of dissimilar components into 2D HSs offers fascinating properties and intriguing functionalities attributed to the newly formed heterointerface of constituent components. Understanding the nature of the surface and the complex heterointerface of HSs at the atomic level is crucial for realising the desired properties, designing innovative 2D HSs, and ultimately unlocking their full potential for practical applications. Therefore, this review provides the recent progress in the field of 2D HSs with a focus on the discussion of the fundamentals and the chemistry of heterointerfaces based on van der Waals (vdW) and covalent interactions. It also explains the challenges associated with the scalable synthesis and introduces possible methodologies to produce large quantities with good control over the heterointerface. Subsequently, it highlights the specialised characterisation techniques to reveal the heterointerface formation, chemistry and nature. Afterwards, we give an overview of the role of 2D HSs in various emerging applications, particularly in high-power batteries, bifunctional catalysts, electronics, and sensors. In the end, we present conclusions with the possible solutions to the associated challenges with the heterointerfaces and potential opportunities that can be adopted for innovative applications.

Emerging field of few-layered intercalated 2D materials

The chemistry and physics of intercalated layered 2D materials (2DMs) are the focus of this review article. Special attention is given to intercalated bilayer and few-layer systems. Thereby, intercalated few-layers of graphene and transition metal dichalcogenides play the major role; however, also other intercalated 2DMs develop fascinating properties with thinning down. Here, we briefly introduce the historical background of intercalation and explain concepts, which become relevent with intercalating few-layers. Then, we describe various synthetic methods to yield intercalated 2DMs and focus next on current research directions, which are superconductivity, band gap tuning, magnetism, optical properties, energy storage and chemical reactions. We focus on major breakthroughs in all introduced sections and give an outlook to this emerging field of research.

Strain Relaxation in "2D/2D and 2D/3D Systems": Highly Textured Mica/Bi2Te3, Sb2Te3/Bi2Te3, and Bi2Te3/GeTe Heterostructures

Strain engineering as a method to control functional properties has seen in the last decades a surge of interest. Heterostructures comprising 2D-materials and containing van der Waals(-like) gaps were considered unsuitable for strain engineering. However, recent work on heterostructures based on Bi₂Te₃, Sb₂Te₃, and GeTe showed the potential of a different type of strain engineering due to long-range mutual straining. Still, a comprehensive understanding of the strain relaxation mechanism in these telluride heterostructures is lacking due to limitations of the earlier analyses performed. Here, we present a detailed study of strain in two-dimensional (2D/2D) and mixed dimensional (2D/3D) systems derived from mica/Bi₂Te₃, Sb₂Te₃/Bi₂Te₃, and Bi₂Te₃/GeTe heterostructures, respectively. We first clearly show the fast relaxation process in the mica/Bi₂Te₃ system where the strain was generally transferred and confined up to the second or third van der Waals block and then abruptly relaxed. Then we show, using three independent techniques, that the long-range exponentially decaying strain in GeTe and Sb₂Te₃ grown on the relaxed Bi₂Te₃ and Bi₂Te₃ on relaxed Sb₂Te₃ as directly observed at the growth surface is still present within these three different top layers a long time after growth. The observed behavior points at immediate strain relaxation by plastic deformation without any later relaxation and rules out an elastic (energy minimization) model as was proposed recently. Our work advances the understanding of strain tuning in textured heterostructures or superlattices governed by anisotropic bonding.

A first-principles study of the switching mechanism in GeTe/InSbTe superlattices

Interfacial Phase Change Memories (iPCMs) based on (GeTe)₂/Sb₂Te₃ superlattices have been proposed as an alternative candidate to conventional PCMs for the realization of memory devices with superior switching properties. The switching mechanism was proposed to involve a crystalline-to-crystalline structural transition associated with a rearrangement of the stacking sequence of the GeTe bilayers. Density functional theory (DFT) calculations showed that such rearrangement could be achieved by means of a two-step process with an activation barrier for the flipping of Ge and Te atoms which is sensitive to the biaxial strain acting on GeTe bilayers. Within this picture, strain-engineering of GeTe bilayers in the GeTe-chalcogenide superlattice can be exploited to further improve the iPCM switching performance. In this work, we study GeTe-InSbTe superlattices with different compositions by means of DFT, aiming at exploiting the large mismatch (3.8%) in the in-plane lattice parameter between GeTe and In₃SbTe₂ to reduce the activation barrier for the switching with respect to the (GeTe)₂-Sb₂Te₃ superlattice.

Thermal History-Dependent Current Relaxation in hBN/MoS2 van der Waals Dimers

Combining atomically thin layers of van der Waals (vdW) materials in a chosen vertical sequence is an emerging route to create devices with desired functionalities. While this method aims to exploit the individual properties of partnering layers, strong interlayer coupling can significantly alter their electronic and optical properties. Here we explored the impact of the vdW epitaxy on electrical transport in atomically thin molybdenum disulfide (MoS₂) when it forms a vdW dimer with crystalline films of hexagonal boron nitride (hBN). We observe a thermal history-dependent long-term (over ∼40 h) current relaxation in the overlap region of MoS₂/hBN heterostructures, which is absent in bare MoS₂ layers (or homoepitaxial MoS₂/MoS₂ dimers) on the same substrate. Concurrent relaxation in the low-frequency Raman modes in MoS₂ in the heterostructure region suggests a slow structural relaxation between trigonal and octahedral polymorphs of MoS₂ as a likely driving mechanism that also results in inhomogeneous charge distribution in the MoS₂ layer. Our experiment yields an aspect of vdW heteroepitaxy that can be generic to electrical devices with atomically thin transition-metal dichalcogenides.

Chalcogenides by Design: Functionality through Metavalent Bonding and Confinement

A unified picture of different application areas for incipient metals is presented. This unconventional material class includes several main-group chalcogenides, such as GeTe, PbTe, Sb₂ Te₃ , Bi₂ Se₃ , AgSbTe₂ and Ge₂ Sb₂ Te₅ . These compounds and related materials show a unique portfolio of physical properties. A novel map is discussed, which helps to explain these properties and separates the different fundamental bonding mechanisms (e.g., ionic, metallic, and covalent). The map also provides evidence for an unconventional, new bonding mechanism, coined metavalent bonding (MVB). Incipient metals, employing this bonding mechanism, also show a special bond breaking mechanism. MVB differs considerably from resonant bonding encountered in benzene or graphite. The concept of MVB is employed to explain the unique properties of materials utilizing it. Then, the link is made from fundamental insights to application-relevant properties, crucial for the use of these materials as thermoelectrics, phase change materials, topological insulators or as active photonic components. The close relationship of the materials' properties and their application potential provides optimization schemes for different applications. Finally, evidence will be presented that for metavalently bonded materials interesting effects arise in reduced dimensions. In particular, the consequences for the crystallization kinetics of thin films and nanoparticles will be discussed in detail.

Understanding the Structure and Properties of Sesqui-Chalcogenides (i.e., V2 VI3 or Pn2 Ch3 (Pn = Pnictogen, Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui-chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase-change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui-chalcogenides (Bi₂ Te₃ , Bi₂ Se₃ , Bi₂ S₃ , Sb₂ Te₃ , Sb₂ Se₃ , Sb₂ S₃ , and β-As₂ Te₃ ) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui-chalcogenides (Bi₂ Te₃ , Bi₂ Se₃ , Sb₂ Te₃ , and β-As₂ Te₃ ) show an unconventional bond-breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal-like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui-chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum-chemistry-based bonding analysis employing charge partitioning, which reveals that the four sesqui-chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.

Strain-based room-temperature non-volatile MoTe2 ferroelectric phase change transistor

The primary mechanism of operation of almost all transistors today relies on the electric-field effect in a semiconducting channel to tune its conductivity from the conducting 'on' state to a non-conducting 'off' state. As transistors continue to scale down to increase computational performance, physical limitations from nanoscale field-effect operation begin to cause undesirable current leakage, which is detrimental to the continued advancement of computing^1,2. Using a fundamentally different mechanism of operation, we show that through nanoscale strain engineering with thin films and ferroelectrics the transition metal dichalcogenide MoTe₂ can be reversibly switched with electric-field-induced strain between the 1T'-MoTe₂ (semimetallic) phase to a semiconducting MoTe₂ phase in a field-effect transistor geometry. This alternative mechanism for transistor switching sidesteps all the static and dynamic power consumption problems in conventional field-effect transistors^3,4. Using strain, we achieve large non-volatile changes in channel conductivity (G_on/G_off ≈ 10⁷ versus G_on/G_off ≈ 0.04 in the control device) at room temperature. Ferroelectric devices offer the potential to reach sub-nanosecond non-volatile strain switching at the attojoule/bit level^5-7, with immediate applications in ultrafast low-power non-volatile logic and memory⁸ while also transforming the landscape of computational architectures because conventional power, speed and volatility considerations for microelectronics may no longer exist.

Strain Relief during Ice Growth on a Hexagonal Template

Heterogeneous ice nucleation at solid surfaces impacts many areas of science, from environmental processes, such as precipitation, to microbiological systems and food processing, but the microscopic mechanisms underpinning nucleation remain unclear. Discussion of ice growth has often focused around the role of the surface in templating the structure of water, forcing the first layer to adopt the registry of the underlying substrate rather than that of ice. To grow a thick ice film, water in the first few ice layers must accommodate this strain, but understanding how this occurs requires detailed molecular-scale information that is lacking. Here we combine scanning tunneling microscopy, low-energy electron diffraction, and work-function measurements with electronic structure calculations to investigate the initial stages of ice growth on a Pt alloy surface, having a lattice spacing 6% larger than ice. Although the first layer of water forms a strictly commensurate hexagonal network, this behavior does not extend to the second layer. Instead, water forms a 2D structure containing extended defect rows made from face-sharing pentamer and octamer rings. The defect rows allow the majority of second-layer water to remain commensurate with the solid surface while compensating lateral strain by increasing the water density close to that of an ice surface. The observation of octamer-pentamer rows in ice films formed on several surfaces suggests that the octamer-pentamer defect motif acts as a flexible strain relief mechanism in thin ice films, providing a mechanism that is not available during the growth of strained films in other materials, such as semiconductors.

Thermal Transport Engineering in Graphdiyne and Graphdiyne Nanoribbons

Understanding the details of thermal transport in graphdiyne and its nanostructures would help to broaden their applications. On the basis of the molecular dynamics simulations and spectrally decomposed heat current analysis, we show that the high-frequency phonons in graphdiyne can be strongly hindered in nanoribbons because of the boundary scattering. The isotropic transport in graphdiyne can be switched to anisotropic along the armchair and zigzag directions. Adding side chains onto the nanoribbon edges further reduces the thermal conductivity (TC) along both armchair and zigzag directions thanks to the reduction of heat current carried by low-frequency modes, a mechanism that arises from the phonon resonances. The uniaxial tensile strain plays a different role in the TC of graphdiyne, armchair nanoribbons, and zigzag nanoribbons. Tensile strain causes the thermal conductivities of graphdiyne, and armchair nanoribbons increase first and then get reduced, whereas for zigzag nanoribbons, the TC decreases with strain first and reaches to a plateau. The different low-frequency phonon response on strain is the main reason for the different TC behavior. For graphdiyne and armchair nanoribbons, the low-frequency heat current is enhanced gradually first and then get reduced with the increase of strain, while that of zigzag nanoribbons decreases with strain and then increases slightly. The current studies could help us understand the phonon transport in graphdiyne and its nanoribbons, which is useful for their TC engineering.

Low temperature epitaxy of tungsten–telluride heterostructure films

Single-crystal like WTe₂ films are grown by exploiting van der Waals epitaxy at low temperatures, using pulsed laser deposition.

Unraveling the discrepancies in size dependence of hardness and thermal stability in crystalline/amorphous nanostructured multilayers: Cu/Cu-Ti vs. Cu/HfO2

Crystalline/amorphous interfaces (CAIs) confer outstanding mechanical properties on crystalline/amorphous nanostructured multilayers (C/ANMs), which are widely used in micro/nanodevices, because their unique interfacial structure possesses high strain compatibility. In this study, Cu/X (X = Cu-Ti, HfO2) C/ANMs with equal layer thicknesses (h) were comparatively investigated in terms of size-dependent hardness (H) and thermal stability to uncover the fundamental difference(s) between Cu/Cu-Ti and Cu/HfO2. It was found that both as-deposited Cu/Cu-Ti and Cu/HfO2 C/ANMs exhibited a maximum hardness at a critical thickness of h ∼30 nm, which was caused by a transition from confined dislocation gliding to dislocation transmission across the interface. Specifically, the Cu/Cu-Ti C/ANMs exhibited annealing hardening, whereas the Cu/HfO2 C/ANMs exhibited annealing softening associated with a minimum softening at h ∼ 30 nm, which was closely correlated with their thermal stability. In comparison with monolithic amorphous X thin films, the glassy X nanolayers in the present Cu/X C/ANMs exhibited reduced thermal stability and a trend that smaller sizes led to higher stability. The underlying mechanism of the size-dependent crystallization behavior of X nanolayers is discussed in terms of the constraining effects of the interface. These findings provide deep insights into the design of Cu/metallic-glass and Cu/ceramic-glass C/ANMs with optimal performance.