In-plane Uniaxial Strain Research Articles

Mechanical stimulation induces rapid fibroblast proliferation and accelerates the early maturation of human skin substitutes

The clinical treatment of large, full-thickness skin injuries with tissue-engineered autologous dermo–epidermal skin substitutes is an emerging alternative to split-thickness skin grafting. However, their production requires about one month of in vitro cell and tissue culture, which is a significant drawback for the treatment of patients with severe skin defects. With the aim to reduce the production time, we developed a new dynamic bioreactor setup that applies cyclic biaxial tension to collagen hydrogels for skin tissue engineering. By reliably controlling the time history of mechanical loading, the dynamic culturing results in a three-fold increase in collagen hydrogel stiffness and stimulates the embedded fibroblasts to enter the cell cycle. As a result, the number of fibroblasts is increased by 75% compared to under corresponding static culturing. Enhanced fibroblast proliferation promotes expression of dermal extracellular matrix proteins, keratinocyte proliferation, and the early establishment of the epidermis. The time required for early tissue maturation can therefore be reduced by one week. Analysis of the separate effects of cyclic loading, matrix stiffening, and interstitial fluid flow indicates that cyclic deformation is the dominant biophysical factor determining fibroblast proliferation, while tissue stiffening plays a lesser role. Local differences in the direction of deformation (in-plane equibiaxial vs. uniaxial strain) influence fibroblast orientation but not proliferation, nor the resulting tissue properties. Importantly, dynamic culturing does not activate fibroblast differentiation into myofibroblasts. The present work demonstrates that control of mechanobiological cues can be very effective in driving cell response toward a shorter production time for human skin substitutes.

Controlling pairing of \u03c0-conjugated electrons in 2D covalent organic radical frameworks via in-plane strain

Controlling the electronic states of molecules is a fundamental challenge for future sub-nanoscale device technologies. π-conjugated bi-radicals are very attractive systems in this respect as they possess two energetically close, but optically and magnetically distinct, electronic states: the open-shell antiferromagnetic/paramagnetic and the closed-shell quinoidal diamagnetic states. While it has been shown that it is possible to statically induce one electronic ground state or the other by chemical design, the external dynamical control of these states in a rapid and reproducible manner still awaits experimental realization. Here, via quantum chemical calculations, we demonstrate that in-plane uniaxial strain of 2D covalently linked arrays of radical units leads to smooth and reversible conformational changes at the molecular scale that, in turn, induce robust transitions between the two kinds of electronic distributions. Our results pave a general route towards the external control, and thus technological exploitation, of molecular-scale electronic states in organic 2D materials.

Optical properties of two-dimensional black phosphorus

Recently, black phosphorus (BP), an emerging layered two-dimensional (2D) material, has aroused much research interest. Distinguished from most of other 2D materials, BP is always a direct bandgap semiconductor regardless of the thickness, with the bandgap ranging from 0.3 eV (bulk) to 1.7 eV (monolayer), which is just fill the gap in the bandgap between graphene and transition metal dichalcogenides (TMDCs) in this frequency range. Besides, the BP exhibits many intriguing properties, such as high carrier mobility, highly tunable and anisotropic physical properties, which render the BP another star 2D material following graphene and TMDCs. In this review, we mainly focus on the advances in the optical properties of 2D BP, with the content covering the intrinsic optical properties and external perturbation effects on optical properties. Regarding the intrinsic optical properties, we introduce the anisotropic and layer-dependent optical absorption from interband transitions, the layer-dependent exciton binding energy and exciton absorption, visible-to-infrared photoluminescence, and stability of absorption and photoluminescence. As for external perturbation effects on optical properties, we introduce in-plane uniaxial and biaxial strain effects, gate-induced quantum confined Franz-Keldysh effect and Burstein-Moss effect. And finally we give a brief summary and outlook, pointing out some several interesting and important issues of BP, which need further studying urgently such as hyperbolic plasmons, intersubband transitions, optical properties in heterostructures and twist angle moiré superlattice and so on. This review gives an overview of the optical properties of BP and is expected to arouse the interest in further studying the BP.

Strain engineering the electronic and photocatalytic properties of WS2/blue phosphene van der Waals heterostructures

The effects of −8–8% in-plane uniaxial and biaxial strains on the electronic and photocatalytic activity of tungsten disulfide/blue phosphene (WS2/BlueP) are investigated within the framework of first-principles calculations.

Strain engineering the electronic and photocatalytic properties of g-C6N6/graphene heterostructures

The effects of in-plane uniaxial and biaxial strains on the electronic and photocatalytic properties of g-C6N6 and graphene (Gr) nanosheets, as well as g-C6N6/Gr heterostructure are investigated by first-principles. The g-C6N6 has a direct band gap of 3.28 eV and Gr shows metallicity. The most stable g-C6N6/Gr has the smallest interface formation energy of 0.074 eV with an opened band gap of 59.6 meV among three designed configurations. The absorption of g-C6N6/Gr is significantly enhanced, and the edge is red-shifted with respect to isolated g-C6N6 and Gr. The p-type Schottky barrier height (SBH) of g-C6N6/Gr is 3.17 eV. The uniaxial and biaxial strains can effectively adjust the strain energies, band gaps and work functions of g-C6N6 and Gr. However, the uniaxial strain on g-C6N6/Gr affects the band gap significantly, while the biaxial strain influences substantially on the interfacial distances, strain energies and work functions. Noticeably, the biaxial strains of -4% and -8% give rise to the direct-indirect band gap transition, moreover, the -4% strain make VBM and CBM of g-C6N6/Gr locate in the Gr and g-C6N6 layer, respectively, achieving effective separation of carriers. The Gibbs free energy of conducting hydrogen evolution reaction (HER) is 0.522 eV, 1.185 eV, 1.671 eV and 1.845 eV for -6%, -4%, -2% biaxial strained and ideal g-C6N6/Gr, respectively, indicating the biaxial strain can enhance the HER activity of g-C6N6/Gr. Moreover, the -4% biaxial strained g-C6N6/Gr has the strongest accumulation of carriers near the interfaces, which may be a better choice for enhancing photocatalytic activity. We hope that our results can provide new possibilities for manufacturing optoelectronic devices with superior performances.

Piezoplasmonics: Strain-induced tunability of plasmon resonance in AlAs quantum wells

We demonstrate tuning of a two-dimensional (2D) plasmon spectrum in modulation-doped AlAs quantum wells via the application of in-plane uniaxial strain. We show that the dramatic change in the plasma spectrum is caused by strain-induced redistribution of charge carriers between anisotropic Xx and Xy valleys. The discovered piezoplasmonic effect provides a tool to study the band structure of 2D systems. We use the piezoplasmonic effect to measure how the inter-valley energy splitting depends on the deformation. This dependency yields the AlAs deformation potential of E2=(5.6±0.3) eV.

A possible explanation for double-peak structure in strain dependence of critical current density in REBa2Cu3O coated conductors

REBa2Cu3O coated conductors (REBCO-CCs; RE: rare-earth element or Y) have garnered considerable attention in high-field magnet applications. However, the twin structure existing in the ab plane of REBCO complicates the dependence of critical current density on temperature, magnetic field, and uniaxial strain: . In particular, the mechanism for the appearance of anomalous double-peak structure in is unclear.In this study, to understand the complicated strain dependence of critical current density, we measured of REBCO-CCs with c-axis-correlated pins or with random pins as a function of in-plane uniaxial strain and magnetic field applied parallel to the c axis. In the case of (Y,Gd)BCO-CC with BaZrO3 nanorods, we only observed a mountain-shaped response of under the entire magnetic field. However, GdBCO-CC without artificial pinning centers, where random pinning is dominant, exhibited a valley-shaped response at low fields and then a mountain-shaped response at high fields. These results suggest that the variation in the pinning mechanism (correlated- or random pins) plays a crucial role in the absence/presence of double-peak structure in REBCO-CCs.We developed a phenomenological model for in which (i) strain dependence of superconducting properties in two domains of REBCO and (ii) ideal behavior of correlated- or random pinning are considered. Consequently, we observed that the valley-shaped response of can be realized in the case of ideal random pinning with limited parameters, while ideal correlated pinning always exhibits a mountain-shaped response of . Furthermore, a crossover from a valley-shaped response under low fields to a mountain-shaped response under high fields in the strain dependence of is qualitatively consistent with our observations. This qualitative agreement between our model and experimental data validates the efficacy of our model to provide a clear understanding of the strain dependence of in REBCO-CCs, including the anomalous double-peak structure.

Strain tunable band structure of a new 2D carbon allotrope C568

Recently, C568 has emerged as a new carbon allotrope, which shows semiconducting properties with a band gap around 1 eV and has attracted much attention. In this work, the external strain effects on the electronic properties of C568 have been studied theoretically through first-principle calculations. The numerical results show that while in-plane uniaxial and biaxial strains both reduces the band gap of C568 in case of tensile strain, their effects are quite different in the case of compressive strain. With increasing compressive uniaxial strain, the band gap of C568 first increases, and then dramatically decreases. In contrast, the application of compressive biaxial strain up to –10% only leads to a slight increase of band gap. Moreover, an indirect-to-direct gap transition can be realized under both types of compressive strain. The results also show that the optical anisotropy of C568 can be induced under uniaxial strain, while biaxial strain does not cause such an effect. These results indicate good strain tunability of the band structure of C568, which could be helpful for the design and optimization of C568-based nanodevices.

InTeI: a novel wide-bandgap 2D material with desirable stability and highly anisotropic carrier mobility.

Recently, stable 2D wide-bandgap semiconductors with excellent electronic and photoelectronic properties have attracted much scientific and technological interest. In this study, we predict a novel InTeI monolayer which has a wide bandgap of 2.735 eV and a anisotropic electron mobility as high as 12 137.80 cm2 V-1 s-1 based on first-principles calculations. With an exfoliating energy lower than that of monolayer phosphorene, it is feasible to synthesize the 2D InTeI monolayer through mechanical exfoliation from their 3D bulk crystals. Remarkably, the monolayer InTeI achieves the indirect-to-direct bandgap transition under a small in-plane uniaxial strain, while a quasi-direct bandgap can be achieved in the InTeI nanosheets with elevated thickness. The InTeI monolayer and nanosheets have suitable band alignments in the visible-light excitation region. In addition, our theoretical simulations determine that 2D InTeI materials exhibit more excellent oxidation resistance than black phosphorene. The results not only identify a novel class of 2D wide-bandgap semiconductors but also demonstrate their potential applications in nanoelectronics and optoelectronics.

Strain effects on the quantum capacitance of graphene nanoribbon devices

One of the most important characteristics of graphene-based nanodevices is the quantum capacitance (QC) of the channel which plays a dominant role in governing the device performance. Moreover, QC can be utilized to realize various types of high performance sensors and energy storage devices. In the present work, the effect of in-plane uniaxial strain on the QC of GNR-based devices is investigated utilizing a compact analytical model which accounts for several finite-size and edge effects. Carrier density and QC are calculated for the three distinct families of armchair GNRs. The strong modulation of QC by uniaxial strain is related to strain-induced changes in both bandgap and effective mass of GNRs. Our study could be helpful for designing GNR-based nanodevices in the Quantum Capacitance Limit.

Borophosphene: A New Anisotropic Dirac Cone Monolayer with a High Fermi Velocity and a Unique Self-Doping Feature.

Two-dimensional (2D) Dirac cone materials exhibit linear energy dispersion at the Fermi level, where the effective masses of carriers are very close to zero and the Fermi velocity is ultrahigh, only 2-3 orders of magnitude lower than the light velocity. Such Dirac cone materials have great promise in high-performance electronic devices. Herein, we have employed the genetic algorithm methods combined with first-principles calculations to propose a new 2D anisotropic Dirac cone material, an orthorhombic boron phosphide (BP) monolayer named borophosphene. Molecular dynamics simulation and phonon dispersion have been used to evaluate the dynamic and thermal stability of borophosphene. Because of the unique arrangements of B-B and P-P dimers, the mechanical and electronic properties are highly anisotropic. Of great interest is the fact that the Dirac cone of the borophosphene is robust, independent of in-plane biaxial and uniaxial strains, and can also be observed in its one-dimensional zigzag nanoribbons and armchair nanotubes. The Fermi velocities are ∼105 m/s, on the same order of magnitude as that of graphene. By using a tight-binding model, the origin of the Dirac cone of borophosphene is analyzed. Moreover, a unique feature of self-doping can be induced by the in-plane biaxial and uniaxial strains of borophosphene and the curvature effect of nanotubes, which is greatly beneficial for realizing high-speed carriers (holes). Our results suggest that the borophosphene holds great promise for high-performance electronic devices, which could promote experimental and theoretical studies for further exploring the potential applications of other 2D Dirac cone sheets.

High linear polarization of photoluminescence of GaInP epilayers at low temperature

Polarization of the photoluminescence (PL) emission depends on the crystal symmetry. The change of temperature results in elastic strain modification. Here we study temperature dependence of the linear polarization of the PL emission from the (001) surface of ordered GaInP epitaxial semiconductor alloys with high ordering parameters, and compare experimentally obtained PL polarization degree with that obtained on base of calculations. We explain high PL polarization observed at low temperature in terms of selection rules for band-to-band transitions derived from k·p theory. In the calculations we take into account changes of the alloy crystal symmetry due to atomic ordering, elastic biaxial and in-plane uniaxial strain in GaInP epilayer, and show how the crystal symmetry influences the PL polarization. Consideration of uniaxial, as well as biaxial strain provides an accurate description of the PL polarization as a function of temperature.

Anomalous rotation of the linearly polarized emission of bright excitons in strained WSe2 monolayers under high magnetic fields

Linearly polarized microphotoluminescence ($\ensuremath{\mu}$-PL) measurements of strained ${\mathrm{WSe}}_{2}$ monolayers in out-of-plane high magnetic fields are presented. At low temperature, a splitting of the bright exciton emission into two exciton components is observed, which is attributed to an in-plane uniaxial strain based on the full polarization dependence of the photoluminescence spectrum. High magnetic field measurements directly reveal a distinct evolution of the linear polarization and allows us to extract the valley coherence time constants (${T}_{s2}^{*}$) for both exciton components. For the high-energy transition of the exciton, the valley coherence time $\ensuremath{\simeq}0.45$ ps, closely matching ${T}_{s2}^{*}$ of an unstrained monolayer ($\ensuremath{\simeq}0.34$ ps). For the low-energy exciton, however, ${T}_{s2}^{*}$ is four times larger, $\ensuremath{\simeq}1.97$ ps. This valley coherence time observed here may be explored for future valleytronic applications.

Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+ Y Te Using a Spin-polarized Scanning Tunneling Microscope.

The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental techniques and methodologies. Here we use a novel home-built uniaxial-strain device integrated into our variable temperature scanning tunneling microscope that enables us to controllably manipulate in-plane uniaxial strain in samples and probe their electronic response at the atomic scale. Using scanning tunneling microscopy (STM) with spin-polarization techniques, we visualize antiferromagnetic (AFM) domains and their atomic structure in Fe1+yTe samples, the parent compound of iron-based superconductors, and demonstrate how these domains respond to applied uniaxial strain. We observe the bidirectional AFM domains in the unstrained sample, with an average domain size of ~50-150 nm, to transition into a single unidirectional domain under applied uniaxial strain. The findings presented here open a new direction to utilize a valuable tuning parameter in STM, as well as other spectroscopic techniques, both for tuning the electronic properties as for inducing symmetry breaking in quantum material systems.

Study of mechanical and electronic properties of single-layer FeB2

Recent experimental advances for the fabrication of various two dimensional (2D) materials introduced new structures with a wide prospect of applications. In this work, under in-plane uniaxial and biaxial strain, we employed first-principles density functional theory calculations to investigate the mechanical and electronic properties of Hexagonal Iron dibromide (FeB2) sheet. In mechanical section, we investigate deeply the mechanical analyzing of 2D structures to study elastic constants. Based on the results of our modelling, single layer FeB2 depending on the strain direction can yield remarkable elastic modulus in the range of 141.18 N/m for Young's modulus and also hold on ultimate stress up to 7.08 × 10−4 N/m, 7.96 × 10−4 N/m and 8.82 × 10−4 N/m at the corresponding strains 26.5%, 34% and 19.5% for uniaxial strain along x, y and biaxial strain, respectively. In this paper, second, third and fourth orders of elastic constants beside Young, Bulk and Shear's moduli and Poisson ratio of single layer of FeB2 have been determined, for the first time. By analyzing electronic band structure, we did not observe any band gaps in terms of strains. However, applying directional strain based on deformation matrices, bands get apart from Fermi level so that Dirac point has been vanished.

Plasmon modes in monolayer and double-layer black phosphorus under applied uniaxial strain

We study the effects of an applied in-plane uniaxial strain on the plasmon dispersions of monolayer, bilayer, and double-layer black phosphorus structures in the long-wavelength limit within the linear elasticity theory. In the low-energy limit, these effects can be modeled through the change in the curvature of the anisotropic energy band along the armchair and zigzag directions. We derive analytical relations of the plasmon modes under uniaxial strain and show that the direction of the applied strain is important. Moreover, we observe that along the armchair direction, the changes of the plasmon dispersion with strain are different and larger than those along the zigzag direction. Using the analytical relations of two-layer phosphorene systems, we found that the strain-dependent orientation factor of layers could be considered as a means to control the variations of the plasmon energy. Furthermore, our study shows that the plasmonic collective modes are more affected when the strain is applied equally to the layers compared to the case in which the strain is applied asymmetrically to the layers. We also calculate the effect of strain on the drag resistivity in a double-layer black phosphorus structure and obtain that the changes in the plasmonic excitations, due to an applied strain, are mainly responsible for the predicted results. This study can be readily extended to other anisotropic two-dimensional materials.

Ultrasensitive tunability of the direct bandgap of 2D InSe flakes via strain engineering

InSe, a member of the layered materials family, is a superior electronic and optical material which retains a direct bandgap feature from the bulk to atomically thin few-layers and high electronic mobility down to a single layer limit. We, for the first time, exploit strain to drastically modify the bandgap of two-dimensional (2D) InSe nanoflakes. We demonstrated that we could decrease the bandgap of a few-layer InSe flake by 160 meV through applying an in-plane uniaxial tensile strain to 1.06% and increase the bandgap by 79 meV through applying an in-plane uniaxial compressive strain to 0.62%, as evidenced by photoluminescence (PL) spectroscopy. The large reversible bandgap change of ~239 meV arises from a large bandgap change rate (bandgap strain coefficient) of few-layer InSe in response to strain, ~154 meV/% for uniaxial tensile strain and ~140 meV/% for uniaxial compressive strain, representing the most pronounced uniaxial strain-induced bandgap strain coefficient experimentally reported in 2D materials. We developed a theoretical understanding of the strain-induced bandgap change through first-principles DFT and GW calculations. We also confirmed the bandgap change by photoconductivity measurements using excitation light with different photon energies. The highly tunable bandgap of InSe in the infrared regime should enable a wide range of applications, including electro-mechanical, piezoelectric and optoelectronic devices.

Significant band gap induced by uniaxial strain in graphene/blue phosphorene bilayer

The effect of in-plane uniaxial strain on band structures and electronic properties of graphene/blue phosphorene (Gr/BPh) bilayer has been investigated by using first-principles calculations. The results show that the intrinsic electronic properties of graphene and blue phosphorene are preserved well in the unstrained bilayer. Upon the application of in-plane uniaxial strain, the band gap of the Gr/BPh bilayer can be easily opened by ∼120 meV near ±2%, and the value of band gap increases with increasing the strain value. A maximum gap value ∼240 meV is obtained around ±4% strain. By analyzing the projected density of states and charge density differences, we reveal that the opened band gap under strain is closely related to the change in electronic structure of Gr/BPh bilayer, which is greatly influenced by the electron redistribution between graphene and BPh layers as well as the orbital hybridizations between carbon and phosphorus atoms. Further study finds that the band gap can not be opened when the in-plane biaxial strain is applied to the Gr/BPh bilayer. The present work provides us an effective avenue to tune the electronic structures and band gap for Gr/BPh bilayer.

Strain-shear coupling in bilayer MoS2

Layered materials such as graphite and transition metal dichalcogenides have extremely anisotropic mechanical properties owing to orders of magnitude difference between in-plane and out-of-plane interatomic interaction strengths. Although effects of mechanical perturbations on either intralayer or interlayer interactions have been extensively investigated, mutual correlations between them have rarely been addressed. Here, we show that layered materials have an inevitable coupling between in-plane uniaxial strain and interlayer shear. Because of this, the uniaxial in-plane strain induces an anomalous splitting of the degenerate interlayer shear phonon modes such that the split shear mode along the tensile strain is not softened but hardened contrary to the case of intralayer phonon modes. We confirm the effect by measuring Raman shifts of shear modes of bilayer MoS2 under strain. Moreover, by analyzing the splitting, we obtain an unexplored off-diagonal elastic constant, demonstrating that Raman spectroscopy can determine almost all mechanical constants of layered materials.

In-Plane Uniaxial Strain in Black Phosphorus Enables the Identification of Crystalline Orientation.

Identification of the crystalline axis of anisotropic black phosphorus (BP) is important for investigating its physical properties, as well as for optical and electronic applications. Herein, it is showed that by applying in-plane uniaxial strain and measuring the changes of the Raman shifts, the crystalline axis of BP can be reliably determined. The strain effects on the Raman shifts are angle-dependent, and they can be expressed as a combination of the Raman responses under zigzag and armchair strain. Differing from previous polarized optical spectroscopic methods where the Raman intensity is analyzed, the proposed method uses the Raman frequency shift, which is less affected by laser polarization, excitation wavelength, the sample thickness, and the substrate. The effective strain applied on BP from the stretched substrate is estimated, and the results show that only 20 to 40% of the strain can be effectively transferred to BP flakes from a polyethylene terephthalate substrate. Our method provides not only an effective and robust approach to identify the crystalline orientation of layered BP, but it is also a model to extract additional information in strain-related studies. It can also be extended to other 2D anisotropic materials.