Small Uniaxial Strain Research Articles

Overlapping of linear optical spectra in metallic carbon nanotubes, Controlled by applied axial magnetic field and uniaxial strain

The excitons and absorption spectra of M11 and M22 transitions in metallic carbon nanotubes were calculated by the tight-binding model, under the combined effect of axial magnetic field and uniaxial strain. It is interesting to find that the separated absorption peaks induced by axial magnetic field will meet each other when the uniaxial strain is applied. At the same axial magnetic field, the first overlapping point is appeared for Mii−−2 and Mii+−1 exciton with small uniaxial strain, and the biggest uniaxial strain is needed for Mii−−1 and Mii+−2 exciton. When we change the magnitude of the axial magnetic field, the overlapping point displays different variation trend for four separate excitons. This overlapping phenomenon is also occurred for other metallic tubes: with bigger diameter, smaller uniaxial strain is needed. More important is that an useful tool is offered to regulate the optical properties of carbon nanotubes by applying simultaneously the axial magnetic field and uniaxial strain.

Piezomagnetic switching of the anomalous Hall effect in an antiferromagnet at room temperature

Piezomagnetism couples strain linearly to magnetic order, implying that it can produce and control magnetization. However, unlike magnetostriction, which couples magnetization quadratically to strain, it enables bidirectional control of a net magnetic moment. If this effect becomes large at room temperature, it may be technologically relevant, similar to its electric analogue, piezoelectricity. However, current studies of the piezomagnetic effect have been primarily restricted to antiferromagnetic insulators at cryogenic temperatures. Here we report the observation of large piezomagnetism in the antiferromagnetic Weyl semimetal Mn3Sn at room temperature. This material is known for its nearly magnetization-free anomalous Hall effect. We find that a small uniaxial strain on the order of 0.1% can control both the sign and size of the anomalous Hall effect. Our experiment and theory show that the piezomagnetism can control the anomalous Hall effect, which will be useful for spintronics applications. Control of magnetization is important for applications in spintronics. Now, the piezomagnetic effect allows strain to control the anomalous Hall effect in a metal at room temperature by rotating its antiferromagnetic order.

Emerging exotic properties of two-dimensional ternary tetrahexagonal BCN: Tunable anisotropic transport properties with huge excitonic effects for nanoelectronics and optoelectronics

A two-dimensional ternary tetrahexagonal BCN monolayer (th-BCN) is designed by applying Stone-Wales transformation to its pentagonal counterpart (p-BCN). Using first-principles calculations, we examine the dynamical, mechanical, thermal, and environment stability of th-BCN. The monolayer exhibits superior anisotropic electronic and optical properties compared to its counterparts (the domains of C and BN in the tetrahexagonal network and p-BCN). Direct band gap of 3.05 eV and excellent band edge alignment with the oxidation and reduction reactions of water heralds the use of th-BCN in photocatalytic water splitting. The band gap energy and band edge positions can be further adjusted by strain. The anisotropic high carrier mobility of th-BCN improves fast carrier separation and low recombination rate. The electronic transport direction can be easily controlled by strain engineering. For instance, one can rotate the preferred transport direction of hole by 90° with a very small uniaxial strain. Many-body effects beyond DFT reveal that the monolayer has strong light-harvesting capability (105cm−1) in the visible-ultraviolet region and exhibits strong excitonic effects with large excitonic binding energy of 0.98 eV. All these appealing properties make th-BCN a promising and tunable anisotropic 2D material for nanoelectronic and optoelectronic applications.

Superconductivity in the two-dimensional nonbenzenoid biphenylene sheet with Dirac cone

During the past decade, two-dimensional materials have attracted much attention in superconductivity due to their feasible physical properties and easy chemical modifications. Herein, we use a recently literature reported novel biphenylene sheet (BP sheet) for investigating superconductivity-related physical properties. The electronic states of BP sheet that appeared near the Fermi level are composed of p z orbital of carbon due to sp2 hybridization. Also, an anisotropic Dirac cone is formed just above the Fermi level by crossing two bands comprised of different carbon atoms. One of the two bands is quasi-flat thus leading to a peak of electronic density of states above the Fermi level. In addition, the rotational-vibration phonon mode of the six-membered carbon ring is strongly coupled with electrons. The electron-phonon coupling induces the superconductivity of 6.2 K in BP sheet. Furthermore, both small uniaxial strains and electronic doping can take the Dirac cone and high electronic density of state close to the Fermi level and further raise the superconducting critical temperature to 27.4 K and 21.5 K, respectively. The obtained result suggests that BP sheet with Dirac fermions and superconductivity can be a potential material for the development of future superconducting devices.

Two-Dimensional Janus Antimony Selenium Telluride with Large Rashba Spin Splitting and High Electron Mobility.

Janus two-dimensional materials with large Rashba spin splitting and high electron mobility are rarely reported but highly desired for nanoscale spintronics. Herein, using density functional theory calculations, we predicated Janus Sb2SexTe3–x (x = 1 or 2) monolayers simultaneously harboring these fascinating properties. The predicated monolayers are indirect semiconductors with great dynamical, thermal, and mechanical stability. The spin–orbital coupling (SOC) and the out-of-plane asymmetry lead to Rashba spin splitting at the conduction band minimum (CBM), which can be effectively tuned by the small uniaxial strain. The strong band dispersion at the CBM leads to small electron effective mass, consequently enabling a high electron mobility that reaches up to 6816.63 cm2 V–1 s–1. Moreover, Janus Sb2SexTe3–x monolayers possess great light absorption capability within the visible and infrared regions of solar light. Our findings highlight promising candidates for high-speed spintronic devices and may motivate more research efforts on carrier transport and SOC effects in Janus group V and VI monolayers.

Dirac Fermions in Graphene with Stacking Fault Induced Periodic Line Defects.

The exploration of carbon phases with intact massless Dirac fermions in the presence of defects is critical for practical applications to nanoelectronics. Here, we identify by first-principles calculations that the Dirac cones can exist in graphene with stacking fault (SF) induced periodic line defects. These structures are width (n)-dependent to graphene nanoribbon and are thus termed as (SF)n-graphene. The electronic properties reveal that the semimetallic features with Dirac cones occur in (SF)n-graphene with n = 3m + 1, where m is a positive integer, and then lead to a quasi-one-dimensional conducting channel. Importantly, it is found that the twisted Dirac cone in the (SF)4-graphene is tunable among type-I, type-II, and type-III through a small uniaxial strain. The further stability analysis shows that (SF)n-graphene is thermodynamic stable. Our findings provide an artificial avenue for exploring Dirac Ffermions in carbon-allotropic structures in the presence of defects.

Excitons in strain-induced one-dimensional moiré potentials at transition metal dichalcogenide heterojunctions.

The possibility of confining interlayer excitons in interfacial moiré patterns has recently gained attention as a strategy to form ordered arrays of zero-dimensional quantum emitters and topological superlattices in transition metal dichalcogenide heterostructures. Strain is expected to play an important role in the modulation of the moiré potential landscape, tuning the array of quantum dot-like zero-dimensional traps into parallel stripes of one-dimensional quantum wires. Here, we present real-space imaging of unstrained zero-dimensional and strain-induced one-dimensional moiré patterns along with photoluminescence measurements of the corresponding excitonic emission from WSe2/MoSe2 heterobilayers. Whereas excitons in zero-dimensional moiré traps display quantum emitter-like sharp photoluminescence peaks with circular polarization, the photoluminescence emission from excitons in one-dimensional moiré potentials shows linear polarization and two orders of magnitude higher intensity. These results establish strain engineering as an effective method to tailor moiré potentials and their optoelectronic response on demand.

Effect of uni-axial and bi-axial strains and vertical electric field on free standing buckled germanene

In this work, the effects of small uni-axial and bi-axial strains and vertical electric field on the band dispersion and dielectric response of germanene are studied by density functional theory. The results show that by applying the vertical electric field a small band gap at the K point is opened in germanene. The created band gap increases with electric field. The vertical electric field only affects the optical spectra in the low energy range below 1 eV. The band structure of germanene under small biaxial strain maintains the feature of bands near the K point without applying the strain, including a pair of two linear dispersive bands crossing at the Fermi level and the Dirac point position. The CBM at the Γ point moves downward as the biaxial strain increases. By applying the small uniaxial strain, a direct gap at K point is observed and the value of band gap increases with increasing of strain. Imaginary part of dielectric function shows that the inter-band transitions are red-shifted for uni- and bi-axial tensile strains and are blue shifted for uni- and bi-axial compressive strains. The predicted tunability of the band gap and dielectric response by applying the strain and electric field may leads to a wide range of applications of germanene in electronic and optoelectronic devices.

Monolayered Silicon and Germanium Monopnictide Semiconductors: Excellent Stability, High Absorbance, and Strain Engineering of Electronic Properties.

The discovery of stable two-dimensional (2D) semiconductors with exotic electronic properties is crucial to the future electronic technologies. Using the first-principles calculations, we predict the monolayered Silicon- and Germanium-monopnictides as a new class of semiconductors owning excellent dynamical and thermal stabilities, prominent anisotropy, and high possibility of experimental exfoliation. These semiconductors, including the monolayered SiP, SiAs, GeP, and GeAs, possess wide bandgaps of 2.08-2.64 eV obtained by hybrid functional calculation. Under small uniaxial strains (-2 to 3%), dramatic modulations of their band structures are observed, and furthermore, all the 2D monolayers (MLs) can be transformed between indirect and direct semiconductors. The monolayered GeAs and SiP exhibits extraordinary optical absorption in the range of visible and ultraviolet (UV) light spectra, respectively. The exfoliation energies of these monolayers are comparable to graphene, implying a strong probability of successful fabrication by mechanical exfoliation. These intriguing properties of the monolayered silicon- and germanium-monopnictides, combined with their highly stable structures, offer tremendous opportunities for electronic and optoelectronic devices working under UV-visible spectrum.

1D SbSeI, SbSI, and SbSBr With High Stability and Novel Properties for Microelectronic, Optoelectronic, and Thermoelectric Applications

AbstractMechanical exfoliation of 2D materials has triggered an explosive interest in low dimensional material research. We extend this idea to 1D van der‐Waals materials. Three 1D semiconductors (SbSeI, SbSI, and SbSBr) with high stability and novel electronic properties are discovered using first principles calculations. Both the dynamical and the thermal stability of these 1D materials are examined. We demonstrate that their nanowire thinner than 7 Å can be easily obtained by mechanical exfoliation, hydrothermal method, or sonochemical method. The bulk‐to‐1D transition results in dramatic changes in band gap, effective mass, and static dielectric constant due to quantum confinement, making 1D SbSeI a highly promising channel material for transistors with gate length shorter than 1 nm. Under small uniaxial strain, these materials are transformed from indirect into direct band gap semiconductors, paving the way for optoelectronic devices and mechanical sensors. Moreover, the thermoelectric performance of these materials is significantly improved over their bulk counterparts. These highly desirable properties render SbSeI, SbSI, and SbSBr promising 1D materials for applications in future microelectronics, optoelectronics, mechanical sensors, and thermoelectrics.

Two-dimensional Penta-BP5 Sheets: High-stability, Strain-tunable Electronic Structure and Excellent Mechanical Properties

Two-dimensional (2D) crystals exhibit unique and exceptional properties and show promise for various applications. In this work, we systematically studied the structures of a 2D boronphosphide (BP) monolayer with different stoichiometric ratios (BPx, x = 1, 2, 3, 4, 5, 6 and 7) and observed that each compound had a stable 2D structure with metallic or semiconducting electronic properties. Surprisingly, for the BP5 compounds, we discovered a rare penta-graphene-like 2D structure with a tetragonal lattice. This monolayer was a semiconductor with a quasi-direct band gap of 2.68 eV. More importantly, investigation of the strain effect revealed that small uniaxial strain can trigger the band gap of the penta-BP5 monolayer to transition from a quasi-direct to direct band gap, whereas moderate biaxial strain can cause the penta-BP5 to transform from a semiconductor into a metal, indicating the great potential of this material for nanoelectronic device applications based on strain-engineering techniques. The wide and tuneable band gap of monolayer penta-BP5 makes it more advantageous for high-frequency-response optoelectronic materials than the currently popular 2D systems, such as transition metal dichalcogenides and black phosphorus. These unique structural and electronic properties of 2D BP sheets make them promising for many potential applications in future nanodevices.

Electrical and lattice vibrational behaviors of graphene devices on flexible substrate under small mechanical strain

We present systematic experimental study on the electrical response and two-phonon Raman scattering mode of graphene under small uniaxial strain. The graphene, which was initially grown by chemical vapor deposition, was transferred to a transparent-flexible-polyethylene-terephthalate substrate. It was found that the electrical resistance increases as the stain is increased after a slight decrease in very small strain regimes of <0.20%. This is due to a relaxation of intrinsic ripples created during the transfer of the graphene to the polyethylene-terephthalate substrate. The gauge factor in the linear response regime was found to be about 22. Also, the 2D Raman bands of the strained graphene showed a distinct red-shift of −37 cm−1 per 1% strain for the 2D+ mode and −46 cm−1 per 1% strain for the 2D− mode. Finally, we determined the Gruneisen parameters of γ2Δ+ ∼ 2.05 and γ2Δ−∼ 2.55 for the phonons in free-standing graphene without a substrate. Our results provide electro-mechanical parameters for graphene-based flexible devices and show the potential of graphene for measuring strain in future flexible electronics.

Electronic structures for armchair-edge graphene nanoribbons under a small uniaxial strain

We theoretically investigate the electronic structures for armchair-edge graphene nanoribbons (AGNRs) under a small in-plane uniaxial strain along armchair (longitudinal) and zigzag (transversal) direction, respectively. We demonstrate that, by both the tight-binding calculation and first-principles study, the applying of a small asymmetrical strain results in variation of energy subband spacing, which opens a band gap for metallic AGNRs and modifies the band gaps for semiconducting AGNRs near the Fermi level. It is believed that these results are of importance in the band gap engineering and electromechanical applications of graphene-nanoribbon-based devices.

Effects of strain on electronic properties of graphene

We present first-principles calculations of electronic properties of graphene under uniaxial and isotropic strains, respectively. The semi-metallic nature is shown to persist up to a very large uniaxial strain of 30% except a very narrow strain range where a tiny energy gap opens. As the uniaxial strain increases along a certain direction, the Fermi velocity parallel to it decreases quickly and vanishes eventually, whereas the Fermi velocity perpendicular to it increases by as much as 25%. Thus, the low energy properties with small uniaxial strains can be described by the generalized Weyl's equation while massless and massive electrons coexist with large ones. The work function is also predicted to increase substantially as both the uniaxial and isotropic strain increases. Hence, the homogeneous strain in graphene can be regarded as the effective electronic scalar potential.

Multiscale Computer Simulation of Tensile and Compressive Strain in Polymer-Coated Silica Aerogels

AbstractWhile the low thermal conductivities of silica aerogels have made them of interest to the aerospace community as lightweight thermal insulation, the application of conformal polymer coatings to these gels increases their strength significantly, making them potentially useful as structural materials as well. In this work we perform multiscale computer simulations to investigate the tensile and compressive strain behavior of silica and polymer-coated silica aerogels. Aerogels are made up of clusters of interconnected particles of amorphous silica of less than bulk density. We simulate gel nanostructure using a Diffusion Limited Cluster Aggregation (DLCA) procedure, which produces aggregates that exhibit fractal dimensions similar to those observed in real aerogels. We have previously found that model gels obtained via DLCA exhibited stress-strain curves characteristic of the experimentally observed brittle failure. However, the strain energetics near the expected point of failure were not consistent with such failure. This shortcoming may be due to the fact that the DLCA process produces model gels that are lacking in closed-loop substructures, compared with real gels. Our model gels therefore contain an excess of dangling strands, which tend to unravel under tensile strain, producing non-brittle failure. To address this problem, we have incorporated a modification to the DLCA algorithm that specifically produces closed loops in the model gels. We obtain the strain energetics of interparticle connections via atomistic molecular statics, and abstract the collective energy of the atomic bonds into a Morse potential scaled to describe gel particle interactions. Polymer coatings are similarly described. We apply repeated small uniaxial strains to DLCA clusters, and allow relaxation of the center eighty percent of the cluster between strains. The simulations produce energetics and stress-strain curves for looped and nonlooped clusters, for a variety of densities and interaction parameters.

Anisotropic exchange splitting of excitons in (001)GaAs/Al0.3Ga0.7As superlattice studied by reflectance difference spectroscopy

Anisotropic exchange splitting (AES) is induced by the joint effects of the electron-hole exchange interaction and the symmetry reduction in quantum wells and quantum dots. A model has been developed to quantitatively obtain the electron-hole exchange energy and the hole-mixing energy of quantum wells and superlattices. In this model, the AES and the degree of polarization can both be obtained from the reflectance difference spectroscopy. Thus the electron-hole exchange energy and the hole-mixing energy can be completely separated and quantitatively deduced. By using this model, a (001)5 nm GaAs/7 nm Al0.3Ga0.7As superlattice sample subjected to [110] uniaxial strains has been investigated in detail. The n=1 heavy-hole (1H1E) exciton can be analyzed by this model. We find that the AES of quantum wells can be linearly tuned by the [110] uniaxial strains. The small uniaxial strains can only influence the hole-mixing interaction of quantum wells, but have almost no contribution to the electron-hole exchange interaction.

Transient Microstructure of Low Hard Segment Thermoplastic Polyurethane under Uniaxial Deformation

Microstructure evolution of a low hard segment (<10 mol %) thermoplastic polyurethane (LHS-TPU) has been followed by in-situ wide-angle X-ray (WAX) and small-angle X-ray scattering (SAX) with a focus on elucidating peculiar microstructural changes during uniaxial deformation (λ = 1−3.5). For the LHS-TPU, the hard segments, due to their low content and chemical structure, do not crystallize but form glassy regions that act as physical cross-links. Two types of soft segment crystallites are resolved upon elongation via DSC, SAX, and WAX experiments. Phase I consists of a small amount of initial crystallites (<2%) that function similar to conventional PU hard segment domains, deforming at small uniaxial strains (λ = 1−2) to a chevron-type morphology, which exhibit equatorial 4-point patterns in SAX. Phase II evolves at higher deformations (λ > 2) due to strain-induced crystallization. Phase II exhibits a conventional meridional 2-point pattern along the deformation direction with lamellar crystallites aligning in the plane normal to the deformation. WAX, SAX, and DSC confirm that both phases coexist over a small strain window (λ = 1.9−2.5), demonstrating the independent nature of the two crystalline phases. These findings indicate that the LHS-TPU in this study is similar to poly(butylene adipate) (PBA) in its morphological and structural behavior. This is further substantiated by NMR, which reveals that the LHS-TPU consists of 90% soft segments, which are identified as PBA via crystal structure analysis of a highly aligned fiber. The soft segments in the LHS-TPU dominate the morphology and the X-ray patterns upon deformation.

Local elastic constants in thin films of an fcc crystal.

In this work we present a formalism for the calculation of the local elastic constants in inhomogeneous systems based on a method of planes. Unlike previous work, this formalism does not require the partitioning of the system into a set of finite volumes over which average elastic constants are calculated. Results for the calculation of the local elastic constants of a nearest-neighbor Lennard-Jones fcc crystal in the bulk and in a thin film are presented. The local constants are calculated at exact planes of the (001) face of the crystal. The average elastic constants of the bulk system are also computed and are consistent with the local constants. Additionally we present the local stress profiles in the thin film when a small uniaxial strain is applied. The resulting stress profile compares favorably with the stress profile predicted via the local elastic constants. The surface melting of a model for argon for which experimental and simulation data are available is also studied within the framework of this formalism.

Influence of small uniaxial strains on the martensitic phase transition in alkali-metal systems

A neutron scattering investigation of the influence of a uniaxial elastic deformation on the martensitic phase transformation in the alkali-metal systems Li and ${\mathrm{Li}}_{10\mathrm{}\mathrm{at}.%}$Mg is described. The experimental results show that a small but symmetry-breaking elastic deformation of the bcc matrix drastically promotes the phase transformation. The deformation further induces a significant reduction of the otherwise large transformation hysteresis in a virgin crystal and leads to a memory effect; i.e., the hysteresis remains small when the applied elastic strain is released. Moreover, the applied deformation of about 0.1% of the lattice parameter induces a softening of the transverse acoustic ${\mathrm{TA}}_{1}$[110] phonon branch of several percent. The results are compatible with a description of martensitic nucleation in alkali metals as being triggered by strain fluctuations.

Strain-induced instabilities in hexagonal columnar phases

Structural instabilities in the hexagonal-packed cylinder phase of block copolymers subject to uniaxial tension orthogonal to the column axis are investigated theoretically. For small strains, a Landau elastic free energy density which is identical to that for hexagonal columnar liquid crystals can be used to determine the threshold and form of the instability, which can be of two types. In the first the distortion field is scalar and the columns or rods are modulated in thickness, while in the second the distortion field is a vector, resulting in saddle-splay curvature. A detailed theoretical analysis of the instability is presented for asymmetric diblock copolymers near the ordering transition. For these materials, the first-order phase transition to the hexagonal-packed rod phase is induced by composition fluctuations and belongs to the Brazovskii class. The coefficients in a series expansion of the Brazovskii free energy density are identified with the elastic constants in the elastic free energy density, enabling them to be calculated from a microscopic random phase approximation model. It is thus shown that small uniaxial strains should induce a transition from the hexagonal rod phase to a ``modulated rod'' structure above a critical strain.