Bauschinger and size effects in thin-film plasticity due to defect-energy of geometrical necessary dislocations

The Bauschinger and size effects in the thinfilm plasticity theory arising from the defect-energy of geometrically necessary dislocations （GNDs） are analytically investigated in this paper. Firstly, this defect-energy is deduced based on the elastic interactions of coupling dislocations （or pile-ups） moving on the closed neighboring slip plane. This energy is a quadratic function of the GNDs density, and includes an elastic interaction coefficient and an energetic length scale L. By incorporating it into the work- conjugate strain gradient plasticity theory of Gurtin, an energetic stress associated with this defect energy is obtained, which just plays the role of back stress in the kinematic hardening model. Then this back-stress hardening model is used to investigate the Bauschinger and size effects in the tension problem of single crystal Al films with passivation layers. The tension stress in the film shows a reverse dependence on the film thickness h. By comparing it with discrete-dislocation simulation results, the length scale L is determined, which is just several slip plane spacing, and accords well with our physical interpretation for the defect- energy. The Bauschinger effect after unloading is analyzed by combining this back-stress hardening model with a friction model. The effects of film thickness and pre-strain on the reversed plastic strain after unloading are quantified and qualitatively compared with experiment results.

Identifying the relationships between subsurface absorber defects and the characteristics of kesterite solar cells

Understanding the defect characteristics that occur near the space-charge regions(SCRs)of kesterite(CZTSSe)solar cells is important because the recombination loss at the CZTSSe/CdS interface is considered the main cause of their low efficiency.CZTSSe surfaces with different elemental compositions were formed without polishing(C00)and with polishing for 20 s(C20)and 60 s(C60).For C60,a specific region near the SCR was excessively Cu-rich and Zn-poor compared to C00 and C20.Various charged defects formed where the elemental variation was large.As the main deep acceptor defect energy level(E_(a2))near the SCR increased,the efficiency,open-circuit voltage deficit,and current density degraded,and this phenomenon was especially rapid for large E_(a2) values.As the E_(a2) near the SCR became deep,the carrier diffusion length decreased more for the CZTSSe solar cells with a low carrier mobility than for the CuInGaSe_(2)(CIGSe)solar cells.The large amplitude of the electrostatic potential fluctuation in the CZTSSe solar cells induced a high carrier recombination and a short carrier lifetime.Consequently,the properties of the CZTSSe solar cells were more strongly degraded by defects with deep energy levels near the SCR than those of the CIGSe solar cells.

Nonlinear change of ion-induced secondary electron emission in theκ-Al_(2)O_(3) surface charging from first-principle modelling

Secondary electron emission(SEE)induced by the positive ion is an essential physical process to influence the dynamics of gas discharge which relies on the specific surface material.Surface charging has a significant impact on the material properties,thereby affecting the SEE in the plasma-surface interactions.However,it does not attract enough attention in the previous studies.In this paper,SEE dependent on the charged surface of specific materials is described with the computational method combining a density functional theory(DFT)model from the first-principle theory and the theory of Auger neutralization.The effect ofκ-Al2O3 surface charge,as an example,on the ion-induced secondary electron emission coefficient(SEEC)is investigated by analyzing the defect energy level and band structure on the charged surface.Simulation results indicate that,with the surface charge from negative to positive,the SEEC of a part of low ionization energy ions(such as Ei=12.6 eV)increases first and then decreases,exhibiting a nonlinear changing trend.This is quite different from the monotonic decreasing tendency observed in the previous model which simplifies the electronic structure.This irregular increase of the SEEC can be attributed to the lower escaped probability of orbital energy.The results further illustrate that the excessive charge could cause the bottom of the conduction band close to the valence band,thus leading to the decrease of the orbital energy occupied by the excited electrons.The nonlinear change of SEEC demonstrates a more realistic situation of how the electronic structure of material surface influences the SEE process.This work provides an accurate method of calculating SEEC from specific materials,which is urgent in widespread physical scenarios sensitive to surface materials,such as increasingly growing practical applications concerning plasma-surface interactions.

Mechanical Properties of Simple s-p Metals, and Defect Energies from Electron Theory and from Interatomic Force Laws

The cleavage force F(z) needed to separate parallel atomic planes by a distance z is first discussed for simple s-p metals using density functional theory.For the s-p nearly free-electron metals the linearized Thomas-Fermi equation is solved self-consistently in the cases of (a) semi-infinite planes of jellium (i.e. smeared uniform positive ions) and (b) a semi-infinite cylinder of finite radius, cleaved by a plane perpendicular to its axis. In (a), the elastic region has the form F(z)=Az ∝ Zrs-11/2, where rs is the mean interelectronic distance in the jellium model. Size effects are then considered, with possible relevance to atomic force microscopy.Defect energies are treated, using both electron theory and pair force laws.

Synthesis of ZnO：Dy Nanopowder and Photoluminescence of Dy^3＋ in ZnO

A type of dysprosium-doped ZnO （ZnO：Dy） nanopowder was synthesized by high temperature calcinations. XRD was used to analyze the structure. Photoluminescence spectra were used to study the optical characteristic. PL of ZnO：Dy shows two different spectra which are broad band resulted from the defect of Dy in ZnO and sharp lines from the 4f→4f transition of isolated Dy^3 ＋ luminescence center. The emission and excitation spectra depend on the excitation wavelength and the concentration of Dy^3＋ . The broad bands with peaks at 600 and 760 nm are attributed to the recombination from an electron of the defect Dy in ZnO to a hole in VB.

UV-Vis and Photoluminescent Spectra of TiO_2 Films

TiO 2 films have been deposited on glass substrates using DC reactive magnetron sputtering at different oxygen partial pressures from 0.10Pa to 0.65Pa.The transmittance (UV vis) and photoluminescence (PL) spectra of the films were recorded.The results of the UV vis spectra show that the deposition rate of the films decreased at oxygen partial pressure P(O 2)≥0.15Pa,the band gap increased from 3.48eV to 3.68eV for direct transition and from 3.27eV to 3.34eV for indirect transition with increasing the oxygen partial pressure.The PL spectra show convincingly that the transtion for films was indirect,and there were some oxygen defect energy levels at the band gap of the films.With increasing the O 2 partial pressure,the defect energy levels decreased.For the films sputtered at 0.35 and 0.65Pa there were two defect energy levels at 2.63eV and 2.41eV,corresponding to 0.72eV and 0.94eV below the conduction band for a band gap of 3.35eV,respectively.For the films sputtered at 0.10Pa and 0.15Pa,there was an energy band formed between 3.12eV and 2.06eV,corresponding to 0.23eV and 1.29eV below the conduction band.

Study of High-Temperature Stability of Pyrochlore Zirconates

Based on defect chemistry theory and molecular dynamics,the defect formation energy and its relationship with the mechanism of pyrochlore-fluorite phase change were investigated,so as to reveal the underlying mechanism of high-temperature stability of pyrochlore zirconates.Results showed that with the rise of the atom mass of A,the defect formation energies decreased that meant the crystal structure tended to become more disordered.Noticeably,the first nearest cation antisite dominated the pyrochlore disorder transformation process.In addition,it was found that the diffusion of oxygen atoms was far higher than that of cations,and was increased with the temperature,thus also promoting the pyrochlore-fluorite transformation process.

Curvature and Size Effects on Reactivities of Mono-to Octa-vacancies in a(5,5) Single-walled Carbon Nanotube

Defect curvature was developed based on our previously proposed direction curvature theory. Defect curvature, as a universal criterion, was used to identify vacancy formation energies E_f of mono-vacancies to octa-vacancies in a（5,5） tube. An ab initio calculation results showed that E_f decreased with increasing the defect curvature K_（V_s）（s = 1~8）. The structures with removed carbon atoms along zigzag chain or the tubular axis were the most stable in each kind of Vs, because their corresponding K_（V_s） was the largest. In addition, local product structures disturbed the variation rule of E_f as K_（V_s）. There was an odd-even oscillation rule in the smallest E_f among each kind of Vs as the s value and vacancies V2, V4 and V6 were more stable. The stabilities of the related vacancy structures were confirmed by two dissociation processes.

Charge-sensitive deep level transient spectroscopy of helium-ion-irradiated silicon,as-irradiated and after thermal annealing

Electrically active defects in the phosphor-doped single-crystal silicon, induced by helium-ion irradiation under thermal annealing, have been investigated. Isothermal charge-sensitive deep-level transient spectroscopy was employed to study the activation energy and capture cross-section of helium-induced defects in silicon samples. It was shown that the activation energy levels produced by helium-ion irradiation first increased with increasing annealing temperature, with the maximum value of the activation energy occurring at 873 K, and reduced with further increase of the annealing temperature. The energy levels of defects in the samples annealed at 873 and 1073 K are found to be located near the mid-forbidden energy gap level so that they can act as thermally stable carrier recombination centres.

Mechanical properties,thermal conductivity and defect formation energies of samarium immobilization in Gd_(2)Zr_(2)O_(7):First-principles study and irradiation experiment

A density functional theory(DFT)study was employed to investigate the mechanical property,thermal conductivity,Debye temperature,electronic structure and defect chemistry of(Gd_(1-x)Sm_(x))_(2)Zr_(2)O_(7).All the(Gd_(1-x)Sm_(x))_(2)Zr_(2)O_(7) compounds exhibit an excellent structural and mechanical stability(Gd_(0.25)Sm_(0.75))_(2)Zr_(2)O_(7) has the lowest Young’s modulus of 213.7 GPa,the largest Possion’s ratio of 0.292,the lowest Debye temperature of 491.8 K and the lowest thermal conductivity.The calculated thermal conductivities of(Gd_(1-x)Sm_(x))_(2)Zr_(2)O_(7) are 1.17-1.21 W/(m·K)by the Clark’s model and 1.32-1.36 W/(m·K)by the Cahall’s model,respectively.The formation energies of O vacancies at 48f site are negative,which increase with the Sm content,however,the formation energies of O vacancies at 8b site are almost invariable.In addition,Sm partly occupying the Gd-site reduces distinctly the formation energies of defects such as A-site vacancies,cation antisite defects,anion Frenkel pairs of oxygen at 8b site and cation interstitials,which suggests that Sm-doped Gd_(2)Zr_(2)O_(7),especially equimolar GdSmZr_(2)O_(7),has a better irradiation tolerance.After the 16 MeVTa-ion irradiation at a fluence of 1×10^(14) or 2×10^(14) ions/cm^(2),the crystal structure of GdSmZr_(2)O_(7) transforms from pyrochlore to a defect fluorite without obvious amorphous phase.

Energy inefficiency diagnosis for Android applications: a literature review

Android applications are becoming increasingly powerful in recent years. While their functionality is still of paramount importance to users, the energy efficiency of these applications is also gaining more and more attention. Researchers have discovered various types of energy defects in Android applications, which could quickly drain the battery power of mobile devices. Such defects not only cause inconvenience to users, but also frustrate Android developers as diagnosing the energy inefficiency of a software product is a non-trivial task. In this work, we perform a literature review to understand the state of the art of energy inefficiency diagnosis for Android applications. We identified 55 research papers published in recent years and classified existing studies from four different perspectives, including power estimation method, hardware component, types of energy defects, and program analysis approach. We also did a cross-perspective analysis to summarize and compare our studied techniques. We hope that our review can help structure and unify the literature and shed light on future research, as well as drawing developers' attention to build energy-efficient Android applications.

First-principles Study of Point Defects in Stoichiometric and Non-stoichiometric Y_4Al_2O_9

By using the first-principles calculation, we studied the mechanisms of point defects in Y4AI209 （YAM）, a promising ternary oxide with excellent optical and thermal properties. It is found that the predominant native defect species is closely dependent on the chemical potentials of each constituent. In the case of O-rich condition, the oxygen interstitial has the very low defect formation energy, followed by the anti-site defects and AI vacancy; in the case of AI-rich condition, the oxygen vacancy yields the lowest defect formation energy, followed by the anti-site defects and AI interstitial. The present result shows that in all the possible chemical potential ranges, anti-site defects have relatively low defect formation energy and might exist in high concentration in YAM. Furthermore, AIy anti-site has relatively lower defect formation energy than the YAt anti- site throughout. The behaviors of defect complexes under non-stoichiometric condition, such as the AI203 or Y203 excess, are also investigated. The results provide helpful guide to optimize the experimental synthesizing of YAM.

Defect energetics and magnetic properties of 3d-transition-metal-doped topological crystalline insulator SnTe

The introduction of magnetism in SnTe-class topological crystalline insulators is a challenging subject with great importance in the quantum device applications. Based on the first-principles calculations, we have studied the defect energetics and magnetic properties of 3d transition-metal(TM)-doped SnTe. We find that the doped TM atoms prefer to stay in the neutral states and have comparatively high formation energies, suggesting that the uniform TMdoping in SnTe with a higher concentration will be difficult unless clustering. In the dilute doping regime, all the magnetic TMatoms are in the high-spin states, indicating that the spin splitting energy of 3d TM is stronger than the crystal splitting energy of the SnTe ligand. Importantly, Mn-doped SnTe has relatively low defect formation energy, largest local magnetic moment, and no defect levels in the bulk gap, suggesting that Mn is a promising magnetic dopant to realize the magnetic order for the theoretically-proposed large-Chern-number quantum anomalous Hall effect(QAHE) in SnTe.