In-plane Tensile Strain Research Articles

Substrate-induced strain effect on magnetic and electrical transport properties of La0·67Ca0·33MnO3 thin films with varying orientation

This paper carries out a detailed study on magnetic and electrical transport properties of La0·67Ca0·33MnO3 (LCMO) thin films deposited on SrTiO3 substrates with different orientations, which provide varying degrees of in-plane tensile strain. As confirmed by the structure and morphology analyses, the homogeneous LCMO thin films with columnar grains are epitaxially grown along the orientations of the substrates, and the suffered tensile strain in the film is increasing from (100) LCMO to (111) LCMO. The obtained data suggest that the magnetic and electrical transport properties of the thin films are sensitive to the substrate-induced tensile strain. The resistivity, temperature coefficient of resistivity (TCR) and magnetoresistance (MR) of the thin films were in ascending order of (1 0 0) < (1 1 0) < (1 1 1), while the ferromagnetic-paramagnetic transition temperature (TC), magnetization and metal-insulator transition temperature (TIM) were in descending order of (1 0 0) > (1 1 0) > (1 1 1).

A theoretical study on tuning band gaps of monolayer and bilayer SnS2 and SnSe2 under external stimuli

Based on density functional theory, we systematically study the mechanical and electronic properties of monolayer and bilayer SnS2 and SnSe2. The electronic properties of these layers can be significantly tuned by applying in-plane strains and electric fields perpendicular to the sheets. The band gaps of monolayer SnS2 and SnSe2 slightly increase with the in-plane tensile strains, and they start to decrease after critical strains (5% for monolayer SnS2 and 7% for monolayer SnSe2). The band gaps of bilayer SnS2 and SnSe2 have a similar tendency to the monolayers with smaller critical strains (1% for bilayer SnS2 and 2% for bilayer SnSe2), which enables a semiconductor-to-metal transition at 10% strain for bilayer SnSe2. We also find that an external electric field perpendicular to bilayer SnS2 and SnSe2 modulates their electronic band gaps. Semiconductor-to-metal transitions are achieved at the electric fields of 0.27 V/Å for bilayer SnS2 and 0.13 V/Å for bilayer SnSe2.

Manipulation of the Electronic Transport Properties of Charge-Transfer Oxide Thin Films of NdNiO3 Using Static and Electric-Field-Controllable Dynamic Lattice Strain

Strongly correlated perovskite transition-metal oxides have received considerable attention due to their intriguing physical properties, including a metal-insulator transition that can be used in devices. Key questions remain, though, including whether in-plane tensile strain helps or hinders the transition. This study uses both static and dynamic lattice strain to manipulate the transport properties of NdNiO${}_{3}$ films systematically. The electric-field-tunable ferroelastic/piezoelectric strain approach can be used not only to obtain deeper insight into the intrinsic properties of nickelate films, but also as a simple, energy-efficient means to construct multistate resistive memory.

Strain effects on electronic and magnetic properties of the monolayer α-RuCl3: A first-principles and Monte Carlo study

The electronic and magnetic properties of a material can be altered by strain engineering. We elucidate the strain dependence of electronic and magnetic properties in α-RuCl3 monolayer by varying the biaxial in-plane tensile strain from 1% to 8%. The magnetic ground state of the α-RuCl3 monolayer evolves from antiferromagnetic zigzag (AFM-ZZ) configuration to ferromagnetic (FM) under a biaxial in-plane tensile strain higher than 2%. In a strain-free state, the FM configuration has a direct bandgap of 0.54 eV, and the AFM-ZZ configuration has an indirect bandgap of 0.73 eV. The energy bandgap of the α-RuCl3 monolayer undergoes a change by the variation of the tensile strain. Furthermore, a detailed Monte Carlo simulation has been implemented to investigate the magnetic properties of the considered system for varying values of tensile strain. Temperature dependencies of the thermodynamic quantities of interest as functions of strains display strong evidence supporting the first-principles calculations within density functional theory. Our Monte Carlo findings also suggest that the Curie temperature of the α-RuCl3 monolayer tends to get higher up to 20.11 K with a tensile strain 8%, which means that applying a strain leads to getting a more stable FM ground state. In addition, we find that magnetocrystalline anisotropy in the α-RuCl3 monolayer can be controlled by the applied strain.

Flexible Metal-Insulator Transitions Based on van der Waals Oxide Heterostructures.

Recently, flexible and wearable electronics are highly desirable because of their great potential in the next-generation information devices. In this work, we demonstrate the realization of the metal-insulator transition (MIT) effect in flexible rare-earth nickelate heterostructures. The NdNiO3 thin films are grown on lattice-mismatched mica substrates along the pseudocubic (111) direction via the van der Waals heteroepitaxy, in which the MIT behaviors are induced and modulated by carefully controlling the lattice strain and the ionic valence state with SrTiO3 and LaAlO3 buffering layers. Enhanced MIT properties with sharp transition and significant resistivity change between the metallic and the insulating states are achieved in the NdNiO3/LaAlO3/SrTiO3/mica heterostructures with appropriate in-plane tensile strain and suppressed concentration of Ni2+ ions. In addition, the proposed NdNiO3-based heterostructures exhibit excellent flexibility with reliable MIT characteristics not only in statically concave/convex bending but also in dynamically bending cycling up to 1000 times. The present work provides a platform to design and fabricate new flexible devices integrated with the MIT effect.

Phase transition in two-dimensional tellurene under mechanical strain modulation

We carry out computational simulations based on density functional theory (DFT) to investigate different phases of two-dimensional (2-D) tellurene. These phases are classified by their characteristic space groups and symmetry elements. Correlations of these phases to the bulk crystalline tellurium structure are also illustrated. Our specific interests include mechanical property calculations for different phases and the possible phase transitions between them. Simulation results show that these 2-D Te phases have very different elastic moduli due to their different atomic bonding and relaxed structures. Moreover, compression along the in-plane directions facilitates the α → β phase transition, while in-plane tensile strains always make the α-phase more stable than the β-phase. However, the energy difference between the two phases is comparable to or even much smaller than the thermal energy kT, depending on the in-plane strain direction. We find that further increase of the tensile strain along the chain direction beyond a critical value, ca. 12%, may lead to a possible α → γ phase transition. As the tensile strain is above 15%, the γ-phase will be more stable than the α-phase, accompanied by a further reduced transition energy barrier.

An interlayer/intralayer coupling mechanism for the thermal characteristics of polycrystalline few-layer graphene

In this paper, the dependence of thermal conductivities of polycrystalline few-layer graphene (PFG) on the layer number and in-plane strain is systematically studied using the reverse non-equilibrium molecular dynamics method. The thermal conductivities are shown to decrease with the increased layer number, but the dependency on the layer number is anomalously less than what has been observed in single-crystalline few-layer graphene. Through a detailed analysis of the in-plane and out-of-plane phonon density of states, it is found that such a weak dependence can be explained by an interlayer/intralayer coupling mechanism, which is sensitive to the presence of grain boundaries. It is also found that this coupling mechanism can also be greatly influenced by in-plane tension, which indicates that thermal conductivities of PFG can be manipulated using in-plane tensile strains in addition to grain boundaries. The present study provides fundamental understanding and valuable guidelines for the design of graphene based flexible devices for efficient thermal management.

Strain engineering the structures and electronic properties of Janus monolayer transition-metal dichalcogenides

We studied the structures and electronic properties of Janus transition-metal dichalcogenide monolayers MXY (M = Mo, W; X ≠ Y = S, Se, Te) by first-principles calculations. The results of the electronic band structures and the density of states reveal that all of the MXY monolayers show semiconducting characteristics. Particular attention has been focused on the bandgap engineering by applying in-plane biaxial compressive and tensile strain. It is observed that the bandgap values of the MXY monolayers decrease with the increase of strain degree under the tension and compression biaxial strain, and a semiconductor-to-metal transition can be undergone at a critical value of strain. The possibility of the tunable energy gap over a wide range makes MXY monolayers potential candidates for nanoelectronics and optoelectronics.

Thickness effects on the epitaxial strain states and phase transformations in (001)-VO2/TiO2 thin films

The thickness-dependent epitaxial strains and phase transformations of (001)-VO2/TiO2 thin films are investigated systematically in a wide thickness range (from 9 to 150 nm). Under a thickness of 18 nm, the tensile in-plane strain is maintained, owing to the good lattice and the symmetry matching between the VO2 thin film and the TiO2 substrate, but the compressive out-of-plane epitaxial strain is gradually relaxed. The epitaxial strains co-stabilize the rutile phase (R phase) in this thickness range. Beyond a thickness of 18 nm, the out-of-plane lattice c exhibits a sudden elongation and reaches the bulk level of 2.8528 Å at a thickness of 20 nm, which indicates a structural phase transition (SPT). A further increase of the film thickness results in another new phase (tetragonal-like or T-like) with lattice distortion, which maintains the tetragonal symmetry in the thickness range of 20 to 55 nm. From a thickness of 60 nm, the monoclinic phase (M1 phase) appears, which indicates another SPT from T-like to the monoclinic M1 phase. This SPT is more favorable energetically, owing to the assistance of the strain relaxation in the thicker films. Additionally, the metal-insulator transition temperature positively increases as a function of the out-of-plane strain. This result is consistent with the fact that the tensile strain along the cR axis (V-V atom chain) is conducive for the stabilized insulating phase. This work highlights strain engineering as a crucial avenue for manipulating the phase transformations and properties in the correlated electron system.

Design and material growth of AlGaN-channel two-dimensional-electron gas heterostructures employing strain-controlled quaternary AlGaInN barrier layers

In order to improve the thermal stability in AlGaN-channel two-dimensional electron gas (2DEG) heterostructures, we newly designed strain-controlled quaternary AlGaInN barrier layers, and then we grew them by metalorganic chemical vapor deposition. A 2DEG density as high as 2.5 × 1013 cm−2 and a good surface morphology were obtained for an (Al0.64Ga0.36)0.976In0.024N/Al0.19Ga0.81N heterostructure, in which in-plane tensile strain in the barrier layer was estimated to be 0.51%. It was also confirmed that the (Al0.64Ga0.36)0.976In0.024N/Al0.19Ga0.81N heterostructure showed an excellent thermal stability in its 2DEG properties even at high temperatures up to 900 °C. As a result of their thermal stability improvement, we were also able to confirm that the contact resistance was improved by applying high-temperature annealing.

Van der Waals Graphene Kirigami Heterostructure for Strain-Controlled Thermal Transparency.

Programming thermal transport across interfaces by engineering strain is of critical importance for exploring mechanical controllable and thermal manageable devices with multifunctionalities. Here, we report a van der Waals heterostructure that is composed of bilayer graphene kirigami with diverse layer cut patterns and assembly organizations and show that the thermal flow intensity across the van der Waals interfaces, named as thermal transparency, could be continuously regulated by applying an external in-plane tensile strain. The density of atomic interactions across the interfaces and the distribution of delocalized phonon modes in each graphene kirigami are elucidated to understand the underlying thermal transport mechanism and are also incorporated into a theoretical model for quantitative predictions of thermal conductance under mechanical strain. A proof-of-conceptual van der Waals graphene kirigami heterostructure by design is proposed and validated through extensive full-scale atomistic simulations on the feasibility and reliability of regulating the transparency ratio of thermal transport by mechanical strain, demonstrating its potential applications in thermal and electronic devices.

Domain structure transition from two to three dimensions in tensile strained (100)/(001)-oriented epitaxial tetragonal PZT film

Pb(Zr0.05Ti0.95)O3 thin films of 30 nm thickness were epitaxially grown at 600 °C on (001) KTaO3 single crystal substrates by metal organic chemical vapor deposition. Films consist of the mixture of the a/c-domain composed of the c-domain with out-of-plane polarization and the a-domain with in-plane polarization and the a1/a2-domain formed by the combination of a-domains whose in-plane polar axes are orthogonal to each other. Films above the Curie temperature (Tc) of 550 °C were found to be under the in-plane tensile strain and transformed to a1/a2-domains at Curie temperature when the temperature decreased. This a1/a2-domain further changed to the mixture of the a/c-domain and a1/a2-domain below 350 °C. This indicated that the two-dimensional domain structures of the a1/a2-domain changed to three dimensional ones below Tc. Although the a/c-domain was induced to relax the strain, the a1/a2-domain was still unrelaxed. This is different from the widely investigated in-plane compressive strained films that have a one to three dimensional domain structure.

Structure, magnetism and electrical transport in epitaxial La0.23Pr0.41Ca0.36MnO3 thin films: Consequences of film thickness

A systematic study of the consequences of in-plane tensile strain on the correlation between structure/microstructure and magneto-electrical properties of La0.23Pr0.41Ca0.36MnO3/SrTiO3 (001) thin films is performed. At lower film thickness (d≤27 nm) the dominant tensile strain supports layered morphology. At d=41 nm structural/microstructural crossover takes place and at d≥41 nm relaxation induced tilt and dilations of the lattice generate defects like dislocations, which in turn transforms the layer by layer growth into disordered brick type morphology. It appears that the relaxation even at higher thickness (∼81 nm) is partial only and the strain may have a self-sustained nature. The domination of the tensile strain suppresses the ferromagnetic-metallic phase due to the possible electronic reconstruction which could give rise to a magnetically disordered insulator ‘dead’ layer and a sizeable non-magnetic insulator state at d≤27 nm. In this thickness regime the possible selective orbital stabilization could also contribute to carrier localization. At d≥41 nm the severity of the impact of the interfacial electronic reconstruction is reduced with concomitant relaxation of the tensile strain which favors carrier delocalization and yields well defined metallic-ferromagnetic phase transitions. The hysteretic nature of the phase transitions reflects the thermal cycle dependent nature of the metallic-ferromagnetic and insulating antiferromagnetic phases in this strongly phase separated material.

Magnetic properties of bismuth-substituted neodymium iron garnet films on Gd3Ga5O12(100) substrates determined by ferromagnetic resonance measurements

Magnetic properties of Nd3−xBixFe5O12 (0 ≤ x ≤ 3) films prepared by metal organic decomposition on Gd3Ga5O12(100) substrates have been investigated. The Faraday rotation angle θF increased with bismuth concentration. The uniaxial anisotropy constant Ku, cubic magnetocrystalline anisotropy constant K1, gyromagnetic ratio γ, g-factor, and FMR line width ΔH were determined by ferromagnetic resonance measurements. Ku was positive and K1 was negative for all samples. Ku decreased from 9.83 × 104 to 2.05 × 104 erg/cm3 as x increased from 0 to 3. K1 was almost constant irrespective of x, and it took a value between −3.28 × 104 and −2.08 × 104 erg/cm3. γ, g-factor, and ΔH also decreased with increasing bismuth concentration. From the results of the investigation of the in-plane tensile strain applied to the films, the effect of strain on Ku is considered to be small.

Aligned-SWCNT film laminated nanocomposites: Role of the film on mechanical and electrical properties

In this study, well-aligned single-walled carbon nanotube (SWCNT) films were utilized as lamellae to prepare a multifunctional nanocomposite by a feasible ‘layer-by-layer’ preparation method. This structural design can not only dramatically reduce the agglomeration effect of SWCNTs, but also achieve high in-plane stiffness. It was found that the well-aligned SWCNT films contribute to the high stress-transfer between CNTs and present good wettability with epoxy matrix, thereby improving the mechanical enhancement. Interestingly, this laminated structure possesses distinct electrical behaviors under different loading conditions. The electrical resistance linearly increases with the increase of in-plane tensile strain but is insensitive to bending. This work could provide in-depth understanding on the mechanical and electrical properties of CNT-based laminated nanocomposites.

Phase diagram and physical properties of (110) oriented KNbO3 film

Based on the phenomenological Landau-Devonshire theory, the phase diagram and physical properties of (110) oriented KNbO3 film are investigated. The “misfit strain-misfit strain” phase diagram at room temperature is constructed in the frame of eighth-order thermodynamic potential. Three tetragonal phases and two orthorhombic phases are stable in phase diagram due to the strain anisotropy. The large in-plane compressive or tensile misfit strains favor the tetragonal phases. Moreover, both the tensile misfit strain u1 (or u2) and compressive strain u2 (or u1) induce the orthorhombic phases. Large piezoelectric coefficient is obtained in orthorhombic phases which have the small in-plane tensile misfit strains. It makes (110) oriented KNbO3 film suitable for applications in electromechanical devices.

Strain-Driven Mn-Reorganization in Overlithiated LixMn2O4 Epitaxial Thin-Film Electrodes

Lithium manganate LixMn2O4 (LMO) is a lithium ion cathode that suffers from the widely observed but poorly understood phenomenon of capacity loss due to Mn dissolution during electrochemical cycling. Here, operando X-ray reflectivity (low- and high-angle) is used to study the structure and morphology of epitaxial LMO (111) thin film cathodes undergoing lithium insertion and extraction to understand the inter-relationships between biaxial strain and Mn-dissolution. The initially strain-relieved LiMn2O4 films generate in-plane tensile and compressive strains for delithiated (x 1) charge states, respectively. The results reveal reversible Li insertion into LMO with no measurable Mn-loss for 0 1) reveals Mn loss from LMO along with dramatic changes in the intensity of the (111) Bragg peak that cannot be explained by Li stoichiometry. These results reveal a partially reversible site reorganization of Mn ions within the LMO fil...

Altering thermal transport by strained-layer epitaxy

Since strain changes the interatomic spacing of matter and alters electron and phonon dispersion, an applied strain can modify the thermal conductivity k of a material. We show how the strain induced by heteroepitaxy is a passive mechanism to change k in a thin film. Molecular dynamics simulations of the deposition and epitaxial growth of ZnTe thin films provide insights into the role of interfacial strain in the conductivity of a deposited film. ZnTe films grow strain-free on lattice-matched ZnTe substrates, but similar thin films grown on a lattice-mismatched CdTe substrate exhibit ∼6% biaxial in-plane tensile strain and ∼7% uniaxial out-of-plane compressive strain. In the T = 700 K–1100 K temperature range, the conductivities of strained ZnTe layers decrease to ∼60% of their unstrained values. The resulting understanding of dk/dT shows that strain engineering can be used to alter the performance of a thermal rectifier and also provides a framework for enhancing thermoelectric devices.

First-principles study of perpendicular magnetic anisotropy in ferrimagnetic D22-Mn3X (X = Ga, Ge) on MgO and SrTiO3

The magnetic anisotropy energy (MAE) of bulk D022-Mn3X (X = Ga, Ge), Mn3X/MgO, and Mn3X/STiO3(STO) heterostructures is calculated from first principles calculations. The main source of the large perpendicular magnetic anisotropy (PMA) of bulk Mn3X is identified as Mn atoms in the Mn-Mn layer. In the four heterostructures, the magnetic moment of interfacial Mn atoms was reversed when Mn3X was epitaxially grown on MgO and STO substrates. More importantly, a large in-plane tensile strain induced by lattice mismatch between Mn3X and MgO significantly changes the MAE, explaining the difficulty in experiments to obtain PMA in epitaxial Mn3X/MgO. Furthermore, interface and surface Mn atoms also help to enhance the PMA of Mn3X/STO (MgO) heterostructures due to dxy and dz2 states changing from occupied states in bulk Mn3X to unoccupied states in the interface (surface) Mn of the heterostructures. These results suggest that the PMA of manganese compound heterostructures can be produced by decreasing the lattice mismatch with substrates and will guide the search for ultrathin manganese compound films with high PMA epitaxially grown on substrates for the application of spintronic devices.

Understanding the anisotropic strain effects on lithium diffusion in graphite anodes: A first-principles study

The lithium diffusion in graphite anode, which is the most widely used commercial electrode material today, affects the charge/discharge performance of lithium-ion batteries. In this study, the anisotropic strain effects on lithium diffusion in graphite anodes are systematically investigated using first-principles calculations based on density functional theory (DFT) with van der Waals corrections. It is found that the effects of external applied strains along various directions of LixC6 (i.e., perpendicular or parallel to the basal planes of the graphite host) on lithium diffusivity are different. Along the direction perpendicular to the graphite planes, the tensile strain facilitates in-plane Li diffusion by reducing the energy barrier, and the compressive strain hinders in-plane Li diffusion by raising the energy barrier. In contrast, the in-plane biaxial tensile strain (parallel to the graphite planes) hinders in-plane Li diffusion, and the in-plane biaxial compressive strain facilitates in-plane Li diffusion. Furthermore, both in-plane and transverse shear strains slightly influence Li diffusion in graphite anodes. A discussion is presented to explain the anisotropic strain dependence of lithium diffusion. This research provides data for the continuum modelling of the electrodes in the lithium-ion batteries.