Narrow Results

On the basis of the finite element approach, we systematically investigated the strain field distribution of conical-shaped InAs/GaAs self-organized quantum dot using the two-dimensional axis-symmetric model. The normal strain, the hydrostatic strain and the biaxial strain components along the center axis path of the quantum dots are analyzed. The dependence of these strain components on volume, height-over-base ratio and cap layer (covered by cap layer or uncovered quantum dot) is investigated for the quantum grown on the (001) substrate. The dependence of the carriers' confining potentials on the three circumstances discussed above is also calculated in the framework of eight-band k·p theory. The numerical results are in good agreement with the experimental data of published literature.

In this study, a system for non-contact in-situ measurement of strain during tensile test of thin films by using CCD camera with marking surface of specimen by black pen was implemented as a sensing device. To improve accuracy of measurement when CCD camera is used, this paper proposed a new method for measuring strain during tensile test of specimen with micrometer size. The size of pixel of CCD camera determines resolution of measurement, but the size of pixel can not satisfy the resolution required in tensile test of thin film because the extension of the specimen is very small during the tensile test. To increase resolution of measurement, the suggested method performs an accurate subpixel matching by applying 2nd order polynomial interpolation method to the conventional template matching. The algorithm was developed to calculate location of subpixel providing the best matching value by performing single dimensional polynomial interpolation from the results of pixel-based matching at a local region of image. The measurement resolution was less than 0.01 times of original pixel size. To verify the reliability of the system, the tensile test for the BeNi thin film was performed, which is widely used as a material in micro-probe tip. Tensile tests were performed and strains were measured using the proposed method and also the capacitance type displacement sensor for comparison. It is demonstrated that the new strain measurement system can effectively describe a behavior of materials after yield during the tensile test of the specimen at microscale with easy setup and better accuracy.

In this paper, we have proposed a step separate confinement heterostructure (SCH) based lasing nano-heterostructure In_0.90Ga_0.10As_0.59P_0.41/InP consisting of single quantum well (SQW) and investigated material gain theoretically within TE and TM polarization modes. In addition, the quasi Fermi levels in the conduction and valence bands along with other lasing characteristics like anti-guiding factor, refractive index change with carrier density and differential gain have also been investigated and reported. Moreover, the behavior of quasi Fermi levels in respective bands has also been correlated with the material gain. Strain dependent study on material gain and refractive index change has also been reported. Interestingly, strain has been reported to play a very important role in shifting the lasing wavelength of TE mode to TM mode. The results investigated in the work suggest that the proposed unstrained nano-heterostructure is very suitable as a source for optical fiber based communication systems due to its lasing wavelengths achieved at ~1.35 μm within TM mode, while ~1.40 μm within TE mode.

Electron group velocity for graphene under uniform strain is obtained analytically by using the tight-binding (TB) approximation. Such closed analytical expressions are useful in order to calculate the electronic, thermal and optical properties of strained graphene. These results allow to understand the behavior of electrons when graphene is subjected to strong strain and nonlinear corrections, for which the usual Dirac approach is no longer valid. Some particular cases of uniaxial and shear strain were analyzed. The evolution of the electron group velocity indicates a break-up of the trigonal warping symmetry, which is replaced by a warping consistent with the symmetry of the strained reciprocal lattice. To do this, analytical expressions for the shape of the first Brillouin zone (BZ) of the honeycomb strained reciprocal lattice are provided. Finally, the Fermi velocity becomes strongly anisotropic, i.e., for a strong pure shear strain (20% of the lattice parameter), the two inequivalent Dirac cones merge and the Fermi velocity is zero in one of the principal axis of deformation. We found that nonlinear terms are essential to describe the effects of deformation for electrons near or at the Fermi energy.

The effects of biaxial strain parallel to the (001) plane on the electronic structures and optical properties of Ge are calculated using the first-principles plane-wave pseudopotential method based on density functional theory. The screened-exchange local-density approximation function was used to obtain more reliable band structures, while strain was changed from −4% to $+$4%. The results show that the bandgap of Ge decreases with the increase of strain. Ge becomes a direct-bandgap semiconductor when the tensile strain reaches to 2%, which is in good agreement with the experimental results. The density of electron states of strained Ge becomes more localized. The tensile strain can increase the static dielectric constant distinctly, whereas the compressive strain can decrease the static dielectric constant slightly. The strain makes the absorption band edge move toward low energy. Both the tensile strain and compressive strain can significantly increase the reflectivity in the range from 7 eV to 14 eV. The tensile strain can decrease the optical conductivity, but the compressive strain can increase the optical conductivity significantly.

In this study, we propose a new bilayer-laminated magnetoelectric (ME) composite consisting of magnetostrictive Ni and Tb${}_{0.3}$Dy${}_{0.7}$Fe₂ (Terfenol-D) plates and piezoelectric Pb(Zr,Ti)O₃ (PZT) plate. The Ni–Terfenol–D-Ni/PZT composite is constructed and compared with the traditional Terfenol-D/PZT composite. The bias magnetic field and the electric field frequency dependences of the converse ME (CME) coefficient were investigated. It is shown that the Ni–Terfenol-D–Ni/PZT exhibits a large CME coefficient of 6.2 × 10${}^{-7}$ s/m at the electric field frequency of 42 kHz under a low bias magnetic field of 230 Oe, which results from the highly concentrated flux induced by Ni and the stress-interaction between Ni and magnetostrictive Terfenol-D.

The stability and electronic properties of the hexagonal, trigonal and rectangular cross-sectional GaP nanowires in wurtzite (WZ) phase are investigated using full potential linear augmented plane waves method. The rectangular cross-sectional nanowires are found more stable than the hexagonal and trigonal ones. The indirect bandgap structure of the nanowires is transformed into the direct bandgap one at a critical size connected to the geometry of the cross-section. The energy bandgap of the nanowires in the same cross-sectional group is enlarged by the quantum size effect. The effective carrier masses in the nanowires, calculated to be larger than those in bulk GaP, are found to slightly increase with the decrease in the size of the nanowires in the same cross-sectional groups. The mechanical strain effect on the electronic band structure is investigated for the rectangular GaP nanowires under the uniaxial and lateral strains. It is found that the indirect bandgap structures of the rectangular nanowires are transformed into the direct bandgap ones by the uniaxial high compression strains. It is also found that this transformation can be triggered by small uniaxial tensile and high lateral tensile strains in addition to the effect of size increase. The energy bandgap of the rectangular nanowires is determined to be narrowed by the uniaxial/lateral strains. It is obtained that the small rectangular nanowire is in the indirect bandgap structure for all the lateral strains and the larger one can be transformed into the direct bandgap structure more easily by the $x$-directional lateral tensile strains compared to the $y$-directional ones. The effective electron and hole masses are found to be reduced by the uniaxial highest tensile and compression strains of this work. It is determined that the lateral strains are not effective in making the electrons of the nanowires more mobile, but the $y$-directional lateral high tensile strains make the holes more mobile by reducing the effective hole mass in the small rectangular nanowire.

The effect of high-energy electron beam on the silicon carbide nanopowder’s structural parameters, strain and powder size was studied. The sample was irradiated with $\sim$2-MeV electron beam energy under different fluencies such as 1.13 ⋅ 10${}^{17}$, 1.89 ⋅ 10${}^{17}$, 2.79 ⋅ 10${}^{17}$ and 3.69 ⋅ 10${}^{17}$ cm${}^{-2}$ at the linear electronic accelerator. Initial and irradiated samples at various doses have been analyzed in the XRD. The nanostructural effects within FullProf are treated using the pseudo-Voigt profile function. The dependences of maximum strain and nanocrystallite size on irradiation dose were obtained and analyzed.

In this study, a molecular dynamics (MD) study has been performed on composition and temperature dependences of mechanical properties of CdTe${}_{1-x}$Se${}_x$ ($x=0.25$, 0.50 and 0.75) nanowires with a diameter of 6.93 nm. The simulation results show that CdTe${}_{0.75}$Se${}_{0.25}$ nanowire seems to be more ductile, whereas CdTe${}_{0.25}$Se${}_{0.75}$ nanowire seems to be more brittle at 1 K. Moreover, the temperature and composition exert significant effects on the mechanical properties of CdTeSe nanowires under stretching. We conclude that the dominancy of Se atoms yields a higher stability and strength at the lower temperature of 1 K, whilst it is the same for the nanowires with both higher Te and Se contents at the higher temperature of 300 K. The radial distribution functions (RDFs) have also been calculated for the CdTeSe nanowires based on the pair separation distance at 1 and 300 K.

Understanding the behavior of excess Ga is important for fabrication methods that employ the sputtering of GaSb-based materials. This is due to the comparatively low vapor pressure of Ga, which can result in GaSb becoming Ga-rich under experimental conditions. In this study, the growth and characterization of nonstoichiometric polycrystalline GaSb thin films with excess Ga, grown by RF magnetron sputtering, are reported. Ga content was adjusted by mixing N₂ in the Ar sputtering gas. The structural properties indicate that the grown thin films maintain the zinc blend structure of GaSb, and have an induced tensile strain along a direction parallel to the substrate. Excess Ga segregates towards the film surface and forms micro/nanoclusters. The internal tensile strain and the Ga cluster formation have little effect on the intrinsic properties of GaSb. These findings could lead to the fabrication of GaSb-based thin films using sputtering with excellent mass productivity.

The elastic and photocatalytic properties of multiferroic material InFeO₃ under strain are calculated through density functional theory. The calculated results indicate that the intrinsic InFeO₃ and the strained InFeO₃ meet the mechanical stability conditions and hold a relatively larger elastic coefficient than popular multiferroic material BiFeO₃. The calculated bandgap and band edge of InFeO₃ under tensile strain show that InFeO₃ could be a high-efficiency photocatalytic hydrogen production material. InFeO₃ under tensile strain holds the ability of photocatalytic water splitting to produce hydrogen with excellent ferroelectric, mechanical properties and absorption of visible light.

In this paper, we report the comparative study of some parameters of II–VI ternary alloy ZnCdTe and II–VI–O dilute oxide ZnCdTeO. The purpose of this comparative study is to establish both the ternary and quaternary alloys as superior materials for optoelectronic and solar cell applications in which the quaternary materials show more superiority than the ternary material. In this purpose, we take the data from the experiments previously done and published in renowned journals and books. The parameters of these alloys are mainly being calculated using Vegard’s law and interpolation method of those collected data. It was certainly demonstrated that the incorporation of O atoms produces a high bandgap ($\mathrm{\Delta }{E}_g$) reduction in host ZnCdTe (Zn${}_{1-x}$Cd_xTe) in comparison to the bandgap reduction in host ZnTe material with Cd incorporation. The bandgap of ZnCdTeO (Zn${}_{1-x}$Cd_xTe${}_{1-y}$O_y) was found to be reduced to 1.1357 at $x=y=0.5$ and the spin–orbit splitting energy (${\mathrm{\Delta }}_{\mathrm{\text{SO}}}$) value of ZnCdTeO was calculated to be 1.175eV for Cd concentration of 0.5mole and O concentration of 0.1mole both of which showed excellent results with the prospect of optoelectronic and solar cell applications. The constant rise in the spin–orbit curve signifies a very less internal carrier recombination which decreases the leakage current and augments the efficiency of solar cell. The lattice constants and strain calculation values give very good results and confirm the stability of the materials. Besides, the calculated band offsets values show that for ZnCdTeO, there is higher bandgap reduction than that of ZnCdTe. Moreover, ZnCdTeO covers a wide range of wavelength in the visible region starting from violet region at 393nm upto red region at 601nm. Both ZnCdTe and ZnCdTeO are found to have excellent applications in optoelectronic and solar cell devices though quaternary ZnCdTeO proves much supremacy over ternary ZnCdTe in all aspects of the properties.

In this work, strain and interfacial defect tailored electronic structures of h-BN/WSe₂ heterostructure are investigated systematically. The results show that the WSe₂/h-BN heterostructure is a direct bandgap semiconductor (1.211eV) with type-I band alignment compared with the isolated h-BN and WSe₂ monolayers. Applying the in-plane strain can well adjust the electronic structure of heterostructure, resulting in a transition from indirect to direct bandgap at the strain of −2% for the h-BN/WSe₂ heterostructures. The bandgap of h-BN/WSe₂ heterostructure monotonically increases at the compressive strains from −6% to −2%, whereas decreases at the tensile strains from 0% to 8%. In addition, introducing of vacancy defects and n- or p-type doping can effectively alter the band alignment of heterostructure. When the N and B vacancies or C doping are introduced in the h-BN layer, a significant transform from type-I to type-II band alignment is observed. These results suggest the h-BN/WSe₂ heterostructure becomes a good candidate for the application of optoelectronics and nanoelectronics devices.

The mixed-valence perovskite manganites attracted much attention because of their interesting electro-magnetic properties, and strain modulation on magnetic properties of perovskite manganites is worth exploring. In this paper, [La${}_{0.8}$Ca${}_{0.2}$MnO₃/La${}_{0.5}$Ca${}_{0.5}$MnO₃]${}_{20}$ superlattice films with different La${}_{0.8}$Ca${}_{0.2}$MnO₃ layer thicknesses are prepared by pulsed laser deposition (PLD). The crystal structures (lattice constants) of samples are measured by X-ray diffraction (XRD). The strain of the films is determined according to their crystal structures. The magnetic hysteresis loops (M-H loops) and magnetization versus temperature (M-T) curves of these superlattice films are measured by the physical property measurement system (PPMS). From the M-H loops, the coercive field (H${}_C)$ of the samples can be measured. From the M-T curves, the Curie temperature (T${}_C)$ of the samples can be obtained. All samples show a ferromagnetic to paramagnetic transition at T_C, and the [La${}_{0.8}$Ca${}_{0.2}$MnO₃/La${}_{0.5}$Ca${}_{0.5}$MnO₃]${}_{20}$ sample with the La${}_{0.8}$Ca${}_{0.2}$MnO₃ layer thicknesses of 72Å has an antiferromagnetic Néel transition at T_N. According to the strain state and magnetic phases, the magnetic properties are comprehensively analyzed. It is found that with the increase of ferromagnetic La${}_{0.8}$Ca${}_{0.2}$MnO₃ layer thickness, the ferromagnetic phase is increased, and the double exchange effect can also be enhanced, resulting in the increase of T_C. When the thickness of La${}_{0.8}$Ca${}_{0.2}$MnO₃ reaches 96Å, the uneven strain distribution in the superlattice can induce the reduction of ferromagnetic phase compared with antiferromagnetic phase (even antiferromagnetic phase starts to appear), and the double exchange effect can be weakened, and finally leading to the decrease of T_C. In addition, with increasing the La${}_{0.8}$Ca${}_{0.2}$MnO₃ layer thickness, H_C gradually decreases. The change of H_C is related to strain states and magnetic phase interactions in the samples. The analysis of the strain and magnetism can contribute to the understanding of perovskite manganite superlattice films.

In this work, the effects of magnetic proximity coupling on electronic structures, valley polarization and magneto-crystal anisotropy of VI₃/WSe₂ heterostructure are studied in detail based on density functional theory. The results show that the spin-valley polarization characters of WSe₂ monolayer can be induced by the proximity effect of ferromagnetic VI₃ monolayer. Based on the effective Hamiltonian $k\cdot p$ model, a large valley polarization of 8.7meV is observed, which is mainly contributed by the SOC effect and not magnetic exchange field. Meanwhile, the magneto-crystal anisotropy of VI₃ layer is also obviously altered from the [100] to [001] magnetic axis compared to the isolated VI₃ monolayer. The reduction in the interlayer space strengths the W–V atomic coupling, resulting in a significant increase in the spin and valley polarization. Compression strain shows weakening effect on the valley polarization, while the tensile strain largely enhances the valley polarization of VI₃/WSe₂ heterostructure, which can reach the maximum value of 29.1meV at the tensile strain of 8%. The V spin direction ($\theta$) has significant effect on valley polarization of WSe₂ layer, which is positively correlated with the magnetization strength of V atom in the vertical direction. Furthermore, constructing the VI₃/WSe₂/VI₃ sandwich heterostructure can induce a larger valley polarization, which can reach a maximum value of 32.4meV. These results indicate that the VI₃/WSe₂ heterostructure is a potential candidate for valleytronics.

We investigate the effects of biaxial tensile and compressive strains on the electronic structure of O-doped monolayer MoS₂ by density functional theory (DFT) in this paper. O-doped monolayer MoS₂ is an exothermic reaction. The doping of O leads to the transformation of the system from direct bandgap to indirect, and the bonding of Mo and O causes a large amount of charge transfer. The application of tensile strain leads to a decrease in the stability of the doped system, and the system always maintains the nature of indirect bandgap. The degree of interatomic charge transfer and bandgap value gradually decrease with the increase of tensile strain. The application of compression strain improves the stability of the doped system, and as the compressive strain increases, the bandgap of the doped system completes the indirect–direct–indirect transformation. The bandgap value shows a trend of increasing and then decreasing. Additionally, the degree of charge transfer between atoms is strengthened.

The effect of biaxial tensile deformation on the optoelectronic properties of Al-doped monolayers SnS₂ is explored using density functional theory. We found that the pure SnS₂ monolayer is an indirect bandgap semiconductor, and with Al doping it changes to a p-type semiconductor with a lowered bandgap value. After applying biaxial tensile deformation to the doped system, i.e., with the increase of tensile strain, bandgap value decreases. Analyzing the density of states, it is found that the conduction band in a doped system is primarily made of the Sn-5s orbital and the S-3p orbital, and the valence band is primarily made of Sn-5p orbital and S-3p orbital, with the majority of S-3p orbital. The conduction and valence bands change with the rise in tensile strain, and hence the gap in the density of states close to the Fermi energy level shrinks. It is also noticed that the absorption coefficient peaks and reflection peaks of the SnS₂ doped system subjected to biaxial tensile strain are blueshifted. The absorption coefficient and reflectance peaks of the doped system show a tendency to increase and then decrease with the increase of tensile strain.

La_0.5Ca_0.5MnO₃ (LCMO) thin films grown by pulsed laser deposition (PLD) and annealed at different temperatures were investigated by high angle X-ray diffraction, atomic force microscope (AFM), scanning electron microscope (SEM), and energy dispersive spectroscopy (SEM-EDS). The lattice parameters, surface morphology as well as the metal compositions of the films were obtained. It was found that the surface morphology of the films strongly depends on the annealing temperatures. The difference of the thermal expansion coefficients between the film and the substrate plays an important role in determining the morphology of the film surface. It induces an in-plane compressive stress in the LCMO films. The strains in the film can be relaxed by nanoscale grains and cracks.

Morphotropic phase boundary (MPB) compositions of (1-x)PMN-(x)PT system, where x=0.30, 0.32 and 0.34, were prepared in single perovskite phase by the B-site Columbite precursor method. Temperature dependence of dielectric behavior, measured at different frequencies, revealed the transformation of the system from relaxor to normal ferroelectric behavior and decrease in dielectric dispersion near transition temperature (T_c) with the increase in PT content. Appearance of an abnormal hump in the dielectric behavior of PMNT 68/32 composition confirms the MPB nature of the system. Remnant polarization, P_r was found to be ~ 23, 22 and 27 μC/cm², whereas coercive field, E_c was found to be ~ 3.3, 3.9 and 3.6 kV/cm for PMNT 70/30, 68/32 and 66/34 compositions respectively. Strain versus electric field (S-E) behavior revealed the piezoelectric nature of these compositions with a maximum strain ~ 0.75% at 31 kV/cm applied electric field in PMNT 68/32 composition. Temperature variation of the pyroelectric coefficient, P_i, confirms T_c, as obtained by dielectric measurements.

Theoretical calculation based on density function theory (DFT) and generalized gradient approximation (GGA) has been carried out in studying the magnetic properties of nitrogen-doped ZnO. The results show that ferromagnetism (FM) coupling between N atoms is more stable for the majority of 11 geometrically distinct configurations, and N atoms in ZnO have a clear clustering tendency. In addition, the formation and ionization energy of native defects in ZnO is analyzed and discussed. The effect of native defects on FM properties of nitrogen-doped ZnO has also been investigated. It is found that FM state is more favored than the AFM state in the presence of zinc vacancy or oxygen interstitial. In the paper, we also analyze strain effect on FM of nitrogen-doped ZnO.