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Two-dimensional germanium is considered a promising new channel material to replace silicon owing to its lower effective mass and larger electron–hole mobility. To investigate the transport characteristics of single-layer germanane transistors with gate lengths (Lg) below 5 nm, we utilize an ab initio quantum transport methodology. It was found that the n-type germanane transistors having Lg of 3 and 5 nm satisfy the International Technology Roadmap for Semiconductors (ITRS) requirements for the on-state current (Ion), effective delay time, and power-delay products of high-performance (HP) devices. Notably, by introducing a negative capacitive (NC) dielectric layer, the p-type germanane transistor having an Lg of 5 nm is almost able to meet the ITRS demands for HP devices. Despite reducing the gate length to 2 nm through the incorporation of the NC dielectric layer, the on-state currents for both n-type and p-type still satisfy approximately 80% of the ITRS standard. Therefore, monolayer germanane presents promising potential as a channel material in a sub-5 nm scale for HP applications. [ABSTRACT FROM AUTHOR]

An extension of a first-principle combined Monte Carlo method is proposed in this work to obtain the secondary electron emission characteristics of anisotropic crystal Al2O3. Unlike isotropic crystal Cu, density functional theory calculations reveal that the q-dependent energy loss function of Al2O3 in all directions is different. Therefore, an interpolation algorithm is introduced in the Monte Carlo method to determine the loss of energy and inelastic mean free path of electrons. The simulation results are in good agreement with experimental data. This method can be further used to simulate the secondary emission yield of other anisotropic crystal materials. [ABSTRACT FROM AUTHOR]

In our previous work, we synthesized a metal/2D material heterointerface consisting of L 1 0 -ordered iron–palladium (FePd) and graphene (Gr) called FePd(001)/Gr. This system has been explored by both experimental measurements and theoretical calculations. In this study, we focus on a heterojunction composed of FePd and multilayer graphene referred to as FePd(001)/ m -Gr/FePd(001), where m represents the number of graphene layers. We perform first-principles calculations to predict their spin-dependent transport properties. The quantitative calculations of spin-resolved conductance and magnetoresistance (MR) ratio (150 %– 200 %) suggest that the proposed structure can function as a magnetic tunnel junction in spintronics applications. We also find that an increase in m not only reduces conductance but also changes transport properties from the tunneling behavior to the graphite π -band-like behavior. Additionally, we investigate the spin-transfer torque-induced magnetization switching behavior of our junction structures using micromagnetic simulations. Furthermore, we examine the impact of lateral displacements (sliding) at the interface and find that the spin transport properties remain robust despite these changes; this is the advantage of two-dimensional material hetero-interfaces over traditional insulating barrier layers such as MgO. [ABSTRACT FROM AUTHOR]

Intermetallic compounds were applied widely in the fields of automotive and aerospace because of its excellent shape memory effect. In this work, the structural, mechanical, and thermodynamic properties of Ni–Ti intermetallic compounds are studied by first-principle calculation. The results show that Ti, NiTi3, NiTi2, NiTi, Ni4Ti3, Ni3Ti, Ni4Ti, and Ni are stable based on Born stable criterion. On the mechanical aspect, the bulk modulus of Ni–Ti intermetallic compounds increases with the rising electron density. Their ductility is ranked as follows: NiTi3 > Ni4Ti3 > NiTi2 > Ni4Ti > NiTi > Ni3Ti. And when the ratio of Ni and Ti is 3:1, it is the hardest. In addition, the Ni–Ti intermetallic compounds in this work are anisotropy, except NiTi2. On the thermodynamic aspect, the Debye temperature θD, melting point Tm, and minimum thermal conductivity kmin of Ni–Ti intermetallic compounds increase first and then decrease with the rising content of Ni. The θD and kmin of NiTi3 are the minimum among the Ni–Ti ICs mentioned above, and θD of Ni3Ti and kmin of NiTi are both the maximum. The melting point of Ni3Ti is 1940 K, it is the highest temperature. [ABSTRACT FROM AUTHOR]

This study presents a new model for point-defect modification in III-V-N alloys through first-principle calculations and several validation experiments conducted in our previous study, which explain the enhanced crystallinity of III-V-N alloys caused by proton irradiation and rapid thermal annealing (RTA). Validation experiments clarified that the conversion efficiency of the GaAsPN solar cell increased after proton irradiation followed by RTA, whereas that of the GaP solar cell decreased after the same process. Thus, the improved crystallinity of the GaAsPN alloy by this process is attributed to the decrease in nitrogen-related point defects in the crystal. The detailed annihilation mechanism of the nitrogen-related point defect was then studied using first-principle calculations demonstrating that the representative nitrogen-related point defects can change to a lower-energy state when a vacancy forms at its neighboring group V site, leading to the annihilation of the defects. It was concluded that vacancies created by proton irradiation enhance the annihilation of nitrogen-related point defects. [ABSTRACT FROM AUTHOR]

In this study, we discuss the observation of spin-wave interference generated by magnetic oscillators. We employ micromagnetic simulations for two coherent spin-Hall nanowire oscillators positioned nearby, horizontally or vertically. The two nanowires produce circular waves with short wavelengths on the order of 100 nm, which interfere with each other. In the horizontal configuration, the spin waves exhibit constructive and destructive fringes, indicating amplification or cancellation of the amplitudes, respectively. The synchronization of spin waves in the current geometry of the two nanowires is facilitated by the combination of dipolar fields and propagating spin waves. Additionally, the vertical alignment results in standing spin waves characterized by multiple antinodes and nodes. These observations are interpreted using a wave model that incorporates the superposition principle for each case. [ABSTRACT FROM AUTHOR]

We have theoretically explored the electronic and thermoelectric properties of the C2P4 monolayer with the interface of density functional theory and semi-classical transport theory. Our calculation shows a high Seebeck coefficient and low electronic thermal conductivity in the vicinity of zero chemical potential (μ = 0), resulting in a good power factor (PF) and a high figure of merit (ZT). More particularly, the electronic figure of merit (ZTe) exhibits two high peak values around μ = 0 due to the significant contribution of thermoelectric parameters. Furthermore, ZTe decreases by increasing the temperature, giving a peak value of 0.98 in the negative chemical potential (μ), whereas, for μ > 0, the peak value increases slightly with temperature. Additionally, the ZTe peak value is robust against ±10% of uni- and biaxial strains at room temperature. To make our calculation more realistic, we add phonon contributions to the thermal conductivity in pristine C2P4 and calculate the total ZT. We have found that phonon contribution dominates at low temperatures, and the ZT peak is reduced to 0.78. These optimal thermoelectric parameters of the C2P4 monolayer may be suitable for demonstrating the feasibility of a good thermoelectric material. [ABSTRACT FROM AUTHOR]

This paper reports for the first time an in-depth study based on first-principle calculations to unveil the underlying physics that governs the pressure induced structural phase transitions of Hg2Cl2 compound at room temperature. The phonon dispersion relations and phonon density of states have been critically explored for the tetragonal and orthorhombic phases of the compound to unveil the phonon modes associated with the phase transitions. The nature of the phase transition whether it is "displacive" or of "order–disorder" type has also been explored. We believe that the present study based on density functional theory and Born–Oppenheimer molecular dynamics calculations will help understand the underlying physics behind the above referred phase transitions and the anisotropic behavior of the compound, which in turn bears technologically significant relevance for its applications in optoelectronic devices, acousto-optic tunable filters, and in fiber-optic communication systems. [ABSTRACT FROM AUTHOR]

The analytical expressions for the cross-spectral density matrix elements of the partially coherent elegant Laguerre–Gaussian (eLG) vortex beam propagating through anisotropic turbulent plasma were derived based on the extended Huygens-Fresnel principle and used to study the changes in the polarization degree and the coherence degree of the partially coherent eLG vortex beam in the turbulent plasma. The numerical results show that the polarization degree of a partially coherent eLG vortex beam reaches a specific value (which is equal to the degree of polarization in the source plane) after a long propagation distance in the turbulent plasma. Moreover, this value is independent of the beam order, topological charge, correlation coefficient, wavelength of the source plane, anisotropy parameter, refractive index fluctuation variance, and outer and inner scales of the turbulent plasma. The results also show that with increasing distance, the coherence degree first decreases from unity and then oscillates around zero. However, this oscillation gradually disappears after traveling a long distance. Our results intuitively present the beam polarization and coherence properties through anisotropic hypersonic turbulence, which can be useful for optical communication in hypersonic turbulent environments. [ABSTRACT FROM AUTHOR]

The dynamic response of dielectric elastomers is widely used in many functional devices, but current research has neglected the effect of varying dielectric permittivity on their dynamic oscillations and stability. This paper studies the thin-walled dielectric balloon in which the stretch-dependent dielectric permittivity is considered. We obtain the dynamic equation of motion by Hamilton's principle. Based on the principle of no energy dissipation in conservative systems, we establish energy conservation at the maximum stretching position and at the initial moment, then we investigate the stability in the dynamic case. It is found that a stretch-related dielectric permittivity can increase the critical electric field of the balloon and can also change the mode of electric field instability and modulate the critical stretch value. In the dynamic case, the stretch-dependent permittivity increases the critical electric field by 4 % when the balloon is only subjected to electric force; moreover, it increases the critical stretch value by 316.68 % by changing the unstable mode from pull-in instability to snap-through instability. It is hoped that this work will provide new thinking in designing functional devices by using the dynamical response and stability of dielectric elastomers. [ABSTRACT FROM AUTHOR]

A new hexagonal phase of Si1−xGex alloys have been successfully synthesized through efforts in recent reports. Utilizing the combined first-principle calculations and special quasi-random model, we precisely investigated the structural, electronic, optical, and vibrational properties of hexagonal Si and Ge and disordered hexagonal Si1−xGex random alloys. We found a large negative deviation between the calculated lattice constants within the revised Perdew–Burke–Ernzerhof for solids functional and the linear fitting results. The electronic structures obtained by using the Tran–Blaha modified Becke–Johnson exchange potential confirm that hexagonal Si1−xGex (x > 0.625) alloys present direct bandgaps. Through solving the Bethe–Salpeter equation, the linear optical spectra of hexagonal Si and Ge are demonstrated. We reveal that the peaks of complex dielectric functions are redshifted with the addition of Ge atoms. Also, the real and imaginary parts exhibit strong anisotropy, which makes hexagonal Si1−xGex alloys potentially useful as nonlinear crystals. The transition is allowed in the infrared region for the hexagonal Si1−xGex (x > 0.625) alloys, and the linear optical spectra can be continuously tuned over a wide range of frequency with Ge addition in the infrared region. Furthermore, density-functional perturbation theory calculations were carried out to predict the off-resonance Raman activity. The results suggest that the vibrational modes of the Si–Si bond exhibit a strong dependency on the compositions, which provides a useful way to identify the most probable atomic configurations of hexagonal Si1–xGex alloys in future experiments. [ABSTRACT FROM AUTHOR]

An extension of a first-principle combined Monte Carlo method is proposed in this work to obtain the secondary electron emission characteristics of anisotropic crystal Al2O3. Unlike isotropic crystal Cu, density functional theory calculations reveal that the q-dependent energy loss function of Al2O3 in all directions is different. Therefore, an interpolation algorithm is introduced in the Monte Carlo method to determine the loss of energy and inelastic mean free path of electrons. The simulation results are in good agreement with experimental data. This method can be further used to simulate the secondary emission yield of other anisotropic crystal materials. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) materials provide an excellent platform for the study of the dimensional effect. The richer the types of 2D materials, the broader the unknown field we can explore. However, among the large number of 2D materials manufactured by humans, true single-crystalline (SC) atomically thin 2D metals are rare. The instability of SC 2D metal materials puts high demands on its fabrication process. By implementing molecular dynamics (MD) simulations, we proved that the SC biatomic-layer (BL) gallium film can be formed at the interface between two graphene layers. The Ga atoms deposited on the surface of the graphene on the copper substrate will spontaneously evolve into independent liquid nano-islands, and then cover the nano-island with a monolayer graphene. When the Ga nano-islands confined under the graphene layer are heated to 500 ° C, they will expand into a BL Ga film, and finally, the entire system is cooled to room temperature to obtain the SCBL Ga film. It is found that these nano-islands are in the liquid state at ∼ 400 ° C, but they undergo a phase transition and evolve into the solid state at ∼ 500 ° C. At the same time, the nano-islands also drop from 3D to 2D. In addition, the vertical heterostructure with moiré superstructure is formed between the SCBL Ga and the top layer graphene. The calculations of the electronic properties show that the Dirac conical point of the graphene in the heterostructure is shifted below the Fermi level, which proves that SCBL Ga is able to induce semimetallic to metallic conversion in graphene, indicating SCBL Ga can be used for metal contacts in 2D devices. [ABSTRACT FROM AUTHOR]

Layered thermoelectric (TE) materials have received widespread attention because of their inherent low lattice thermal conductivity and good TE properties. Recently, a new type of layered material Sb 2 Si 2 Te 6 polycrystalline was reported to show high power factor (∼ 10.6 μ W cm − 1 K − 2 at 579 K) and low thermal conductivity (0.69–0.74 W m − 1 K − 1 at 823 K), which is a kind of very promising TE material. Considering that layered materials are usually anisotropic, we use first principles combined with Boltzmann's transport equation to comprehensively predict the TE transport characteristics of single crystal p-type and n-type Sb 2 Si 2 Te 6 in this study. The results show that the TE properties of Sb 2 Si 2 Te 6 have obvious anisotropy. It has extremely low lattice thermal conductivity along the cross-plane direction. After the optimization of carrier concentration, the TE figure of merit of n-type Sb 2 Si 2 Te 6 is much higher than that of the p-type. N-type Sb 2 Si 2 Te 6 has good TE performance in the full temperature range [ Z T = 1.38 at 300 K, 3.54 at 500 K, 5.27 at 700 K (along the cross-plane direction); 1.97 at 300 K, 1.94 at 500 K, 3.08 at 700 K (along the in-plane direction)], and the optimal doping concentration at different temperatures is relatively close, showing its great potential in TE applications. [ABSTRACT FROM AUTHOR]

Crystal shape is a critical determinant of the physical and chemical properties of crystalline materials; hence, it is the challenge of controlling the crystal morphology in a wide range of scientific and technological applications. The morphology is related to the geometry of their exposed surfaces, which can be described by their surface energies. The surface properties of β-ZnMoO4 have not yet been well explored, either experimentally or theoretically. Thus, the first-principle calculation at the density functional theory level was carried out for different low-index surfaces of β-ZnMoO4, specifically (001), (010), (110), (011), (101), and (111), and the surface energy values (Esurf) were reported. The surface stability was found to be controlled by the undercoordinated [ Mo O n ... y V O x ] and [ Zn O n ... y V O x ] (n = 4 and 5; y = 1 and 2) clusters, i.e., their local coordination of Mo and Zn cations at the exposed surfaces, respectively, with the (111) surface being the most stable. A complete map of investigated β-ZnMoO4 morphologies was obtained using the Wulff construction and changing the values of the calculated energy surfaces. The final geometries from this map were compared with field emission-scanning electron microscopy images showing excellent agreement, prevising rectangular and hexagonal plates. Our findings will promote the use of facet engineering and might provide strategies to produce β-ZnMoO4-based materials for achieving morphology-dependent technological applications. [ABSTRACT FROM AUTHOR]

We analyze the main physical processes in quantum cascade lasers with a spatial separation between the region of photon radiation and longitudinal optical (LO) phonon emission, which facilitates the depopulation of the lower level of the optical transition. Our objective is to identify the reasons for the reduction of population inversion at low photon energy and explore methods to enhance it. The expression for population inversion is derived from an equation for a simplified density matrix. This approach allows us to consider the coherence of tunneling between different levels and comprehend its influence on transition probabilities in a straightforward manner. We have found out that the energy uncertainty principle is the fundamental factor limiting population inversion in terahertz lasers. By optimizing the tunneling matrix element between the two regions and the LO phonon emission time, it is possible to significantly increase the population inversion. The optimal value for the matrix element is smaller than its maximum possible value, while the optimal LO phonon emission time exceeds the time achieved during LO phonon resonant emission. [ABSTRACT FROM AUTHOR]

The estimation of the slowness and of the attenuation of pseudo surface acoustic waves (PSAWs) has been addressed by many authors, but still represents a challenge because of the intrinsic difficulty to provide a physically consistent representation of their characteristics. In this work, we propose an alternative approach to the popular use of complex slownesses along the propagation direction. We introduce in the standard harmonic model an optimal source term exactly compensating for the energy lost by leakage, by analogy with the problem of the reflection of a bulk acoustic wave on the surface. This leads us to the minimization of a function whose minimum is the PSAW slowness and whose minimum value gives the attenuation. The method is found to be stable and reliable compared to previously published data, without requiring any change in the partial wave selection rule, and further does not require one to consider waves with increasing energy in the depth of the substrate, which generally does not correspond to the observed phenomena. [ABSTRACT FROM AUTHOR]

The bulk acoustic wave (BAW) assisted gas sensor utilizes the BAW to raise the sensitivity of a gas sensor, which provides a new and universal physical strategy to greatly improve the sensitivity of gas sensors. However, the physical principle of this type of gas sensor has not been clarified yet. In this work, the physical principle of the BAW assisted gas sensor is investigated experimentally and theoretically, and the effects of sound pressure and acoustic streaming on the sensing process are directly verified. It indicates that the transfer of target gas molecules onto the sensing surface can be enhanced by sound pressure on the sensing surface, which results in a significant increase of both the sensing response and sensitivity. Also, it is found that the sensing surface can be cooled down by acoustic streaming, which causes a sensing response change opposite to the change direction caused by the sound pressure, and little change of the sensitivity. It is predicted and experimentally verified that when both acoustic streaming and sound pressure exist on the sensing surface, the sensing characteristics should be between those of the two extreme working modes in which there is only sound pressure or acoustic streaming on the sensing surface. [ABSTRACT FROM AUTHOR]

Correctly elucidating the strong electron correlations of vanadium dioxide (VO2) is of great significance to understand the physics of the metal–insulator transition (MIT) and develop potential applications of VO2. Standard density functional theory is believed to be inappropriate to describe the MIT of VO2. Herein, the recently developed GGA-1/2 quasiparticle approximation is employed to perform first-calculations on VO2. The electronic structures of the metallic and insulating phases of VO2 are well described. The GGA-1/2 calculations indicate that the preferential occupancy of the d// orbitals leads to strong Mott–Hubbard correlation, which induces the splitting of the d// orbitals and the MIT of VO2. The calculations on electron energy-loss function reveal that the satellite electronic energy-loss spectroscopy peak of metallic phase VO2 is resulting from the plasma resonance. This work demonstrates that the GGA-1/2 approach facilitates the electron correlation calculations of VO2 and suggests that the strong Coulomb correlation is necessary to trigger the MIT. [ABSTRACT FROM AUTHOR]

As the main way of underwater data transmission, acoustic communication is still limited by the low-level signal-to-noise ratio and channel capacity. The orbital angular momentum (OAM) based acoustic communication of acoustic vortex (AV) provides a new dimension to data transmission with an expanded channel capacity. Theoretical analyses and experimental measurements for the OAM communication of AV beams based on single-ring transceiver arrays are studied in air. Coaxial multi-OAM AV beams are generated with multiple topological charges encoded by the binary ASCII codes of various letters. The OAM modes of the AV beams are decoded with limited acoustic pressures detected by the single-ring receiver array around the vortex center based on the orthogonal property. It is proven that the channel capacity of the communication system can be increased effectively by the OAM modes of AVs, which are beneficial to data encryption and transmission without mutual interference of AVs of different orders. The favorable results provide theoretical bases and technical support to data transmission and OAM decoding for the OAM communication of AV beams using simplified single-ring transceiver arrays. [ABSTRACT FROM AUTHOR]

Despite a wide bandgap of 4.8 eV, Ga 2 O 3 has good electrical conductivity and thus has a wide range of potential applications. We previously reported that the bandgap of ϵ - Ga 2 O 3 is widened by Co-doping; here, we present a theoretical discussion of the changes in the electronic state induced by Co impurities. By comparing calculated and experimental absorptions, the experimentally observed optical bandgap was assigned to a transition from a bulk peak (1.0 eV below the valence band maximum) to the conduction band minimum. The photoabsorption of Ga 2 O 3 :Co is not readily explained simply in terms of the bandgap of Ga 2 O 3. However, the adjustable shift of the cutoff in its photoabsorption spectrum can be explained in terms of midgap impurity levels due to Co-doping, while the bandgap of Ga 2 O 3 was almost unchanged even when the Co-impurity concentration was high. In addition, the work function, which was determined experimentally by photoemission spectroscopy, increased with the content of Co impurities. This was attributed to a lowering of the Fermi level induced by Co-doping. [ABSTRACT FROM AUTHOR]

Using the maximum entropy principle, we present a general theory to describe ac and dc high-field transport in monolayer graphene within a dynamical context. Accordingly, we construct a closed set of hydrodynamic (HD) equations containing the same scattering mechanisms used in standard Monte Carlo (MC) approaches. The effects imputable to a linear band structure, the role of conductivity effective mass of carriers, and their connection with the coupling between the driving field and the dissipation phenomena are analyzed both qualitatively and quantitatively for different electron densities. The theoretical approach is validated by comparing HD results with existing MC simulations. [ABSTRACT FROM AUTHOR]

In the past, single bond energy of nanomaterials did not attract much attention, since many of their properties show a direct relation to cohesive energy. However, it is the single bond energy that determines the interaction between two atoms and even their bond lengths. Through introducing the bond number and the size-dependent cohesive energy model, the size-related single bond energy ɛ(N) of Cu clusters is resolved in this work, with the support of a thermodynamic method combined with first-principle calculation. It is found that the single bond is gradually strengthened as the size drops when compared with the bulk. Moreover, this enhanced bond strength is greatly important, especially in analyzing the Raman shift of semiconductor nanoparticles. [ABSTRACT FROM AUTHOR]

Organic semiconductor crystals exhibit unique optoelectronic characteristics such as high carrier mobility and laser oscillation. Nonetheless, it remains difficult thus far to achieve such potential activities on an optical device configuration mainly because of lack of knowledge of optical parameters such as refractive indices and their dispersion. Here, we demonstrate a simple but powerful design principle of high-performance organic single-crystal light-emitting devices. The method is based upon observation of emission-angle dependent spectra produced from an organic slab single crystal equipped with a one-dimensional diffraction grating. The emission spectra are characterized by sharply resolved lines whose locations are blueshifted or redshifted as a function of the emission angles. Detailed analysis of these emission lines has enabled us to solve equations of electromagnetic wave motion within and outside the slab crystal and to get solutions under appropriate boundary conditions. The design principle also allows us to relate crystal thicknesses and orders of both longitudinal and transverse modes of emission. Thus, we have been able to optimize the geometry of a slab organic crystal in an organic light-emitting device configuration. The relevant knowledge can directly be used for designing an organic laser either optically excited or electrically excited. [ABSTRACT FROM AUTHOR]

Using first principle calculations, we comprehend the electronic structure and optical constant of thallium lead halides TI3PbX5 (X = Cl, Br, I), which are viewed as potential materials for mid-infrared and near-infrared applications. The generalized gradient approximation function given by Perdew-Burke-Ernzerhof is used for geometry optimization and properties calculations. The calculated Mulliken charges are more positive for Pb than TI in the TI3PbCl5 system demonstrating a high electronic density contribution from the Pb-Cl bonds in contrast to the TI-Cl bonds. Similarly, the TI3PbCl5 configuration displays the smallest enthalpy value suggesting a more stable structure among different TI3PbX5 species studied in this work. The band gap value for TI3PbCl5 has been calculated as 3.52 eV, which decreases to 3.14 eV and 2.64 eV for the TI3PbBr5 and TI3PbI5, respectively. Additionally, the TI3PbI5 exhibits a high magnitude of real component of static dielectric function ε1(k) in contrast to TI3PbBr5 and TI3PbCl5. The optical absorption results display a redshift when TI3PbCl5 is doped with Br and I sequentially. These outcomes suggest that the concentration of halide elements can be adjusted to enhance the optical properties of TI3PbX5 for optoelectronic device applications. [ABSTRACT FROM AUTHOR]

In this work, the electronic structure and carrier mobility of single-walled boron arsenide nanotubes (BAsNTs) have been systematically studied by using Boltzmann transport equation with the relaxation time approximation. We found that the ionic characteristic of B–As bond results in the dipole shells in the optimized BAsNTs. It is predicted that both zigzag BAs nanotubes (ZNTs) and armchair BAs nanotubes are semiconductors, and the strong σ*–π* hybridization in small ZNTs leads to a rapid drop of bandgap with a decrease of radius. Interestingly, as the size (n) of the NTs decreases, the hole mobility (μh) of ZNTs has an evident 3p (p is an integer) oscillation but electron mobility (μe) basically falls down, which falls even faster when the radius gets smaller. Comparing the carrier mobility between BAsNTs and its unzipping nanoribbons, we found that rolling BAs nanoribbons (BAsNRs) into BAsNTs would increase the μe but decrease the μh. The different behavior of the carrier mobility in BAsNRs and BAsNTs results from their distinct bond features of edge states, which vary with different widths (for BAsNRs) or radii (for BAsNTs). [ABSTRACT FROM AUTHOR]

Experimental research has shown that much less charge injection occurs in polytetrafluoroethylene (PTFE) compared to polyethylene (PE). To clarify the mechanisms of charge injection from metals into polymer insulators, we comparatively studied charge injection in PE and PTFE oligomers using first-principles calculations. Two different models were studied: chemisorption (bonding) and physisorption (nonbonding). The results show that the electron injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface only in the case of chemisorption. The larger electron injection barrier of the metal/PTFE oligomer interface is mainly affected by the positive vacuum level shift of the metal/PTFE interface induced by electron transfer from the metal to PTFE along the chemical bonds. In the case of physisorption, the hole injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface. This is attributed to the larger ionization potential of PTFE compared to PE. The calculated results reasonably explain the experimental phenomena. The agreement between the experimental and calculated results verifies the rationality of our calculation models. The models used herein can likely be applied in other metal/polymer interfacial systems with acceptable accuracy. [ABSTRACT FROM AUTHOR]

Metal–insulator transitions in two dimensional materials represent a great opportunity for fast, low energy, and ultradense switching devices. Due to the small energy difference between its semimetallic and semiconducting crystal phases, phase transition in MoTe 2 can occur with an unprecedented small amount of external perturbations. In this work, we used the density functional theory to predict critical strain and electrostatic voltage required to control the phase transition of 3d and 4d metal doped MoTe 2. We found that small doping contents dramatically affect the relative energies of MoTe 2 crystal phases and can largely reduce the energy input to trigger the transition compared to the pristine case. Moreover, the kinetics corresponding to the phase transition in the proposed doped materials are several orders of magnitude faster than in MoTe 2. For example, we predict 6.3% Mn doped MoTe 2 to switch phase under 1.19 V gate voltage in less than 1 μ s with an input energy of 0.048 aJ / nm 3. Due to the presence of the dopant, the controlled change of phase is often complemented with a change in magnetic moment leading to multifunctional phase transition. [ABSTRACT FROM AUTHOR]

We report the results of ab initio calculations of a novel NiFeMnCr high entropy alloy (HEA) with potential applications as a high performance structural material. The bulk and defect property variations due to chemical disordering and magnetic frustration have been studied using both supercell and coherent potential approximation-based techniques. While magnetic frustration due to the presence of multiple 3d transition metals can severely affect the accuracy of vacancy formation energy in first-principles calculations, this effect should be suppressed at intermediate and high temperatures. An efficient approach to evaluate the chemical potential in HEA is constructed and implemented. Vacancy formation energies are computed based on the chemical potential. The statistical distribution of formation energies is weakly dependent upon the chemical identity of the vacancy. On the other hand, the calculated vacancy migration energies show that Fe is more likely to have a large migration barrier than Cr, Mn, or Ni. Finally, atomic-level stresses are computed. A qualitative model to explain the elemental segregation trend in HEA is built upon the atomic-level stress calculation results and provides a reasonable qualitative agreement with ion irradiation experimental data of a NiFeMnCr HEA. [ABSTRACT FROM AUTHOR]

We present a general principle of topography-directed (TD) inkjet printing for functional micro-tracks embedded in a flexible elastomer substrate. The essential features of the TD inkjet printing in a micro-structured substrate with periodic grooves and ridges are described in terms of the topographic parameters for the transformation from a single droplet to a filament or an edge-disjoint pattern of ink in the groove. Silver ink, being widely used for producing conductive wires by conventional inkjet printing, is utilized as a testbed in our study. The underlying mechanisms for the spreading and drying processes of ink drops under the topographic compartment can be understood in a two-dimensional parameter space of the aspect ratio of the groove and the contact angle of ink on the substrate. The wetting morphologies of ink droplets are described in an analytical model where the Laplace pressure and the mean curvature at the vapor/ink interface are taken into account. The first principle of the TD inkjet printing would be applicable for constructing a variety of functional micro-tracks with high pattern fidelity from different classes of solutions such as conducting polymers, organic semiconductors, and colloidal nanoparticles. [ABSTRACT FROM AUTHOR]

A novel Giant Magnetostrictive Actuator (GMA) experimental system with Fiber Bragg Grating (FBG) sensing technique and its modeling method based on data driven principle are proposed. The FBG sensors are adopted to gather the multi-physics fields' status data of GMA considering the strong nonlinearity of the Giant Magnetostrictive Material and GMA micro-actuated structure. The feedback features are obtained from the raw dynamic status data, which are preprocessed by data fill and abnormal value detection algorithms. Correspondingly the Least Squares Support Vector Machine method is utilized to realize GMA online nonlinear modeling with data driven principle. The model performance and its relative algorithms are experimentally evaluated. The model can regularly run in the frequency range from 10 to 1000 Hz and temperature range from 20 to 100°C with the minimum prediction error stable in the range from -1.2% to 1.1%. [ABSTRACT FROM AUTHOR]

Presents a study that analyzed non-Debye models and the superposition principle related to the dielectric response theory. Background on the superposition principle; Information on the Debye processes; Correlation between the dynamics of polarization and depolorization unit with the superposition principle.

Incorporating impurities in ZnO provide opportunities to manipulate its electronic and optical properties, which can be exploited for optoelectronic device applications. Among various elements doped in ZnO crystal structure, limited attempts have been accounted for the Sr–doped ZnO system. Further, no theoretical evidence has been reported so far to explore the Sr–doped ZnO frameworks. Here, we report first principle study for the pure and Sr-doped ZnO (Zn1-xSrxO) structure. We employed the Perdew–Burke–Ernzerhof exchange–correlation function parameters in generalized gradient approximations. In light of these estimations, we calculated the electronic band gap, density of states, and optical parameters, for example, absorption, dielectric functions, reflectivity, refractive index, and energy-loss. The studies suggested that Sr incorporation expanded the optical band gap of ZnO. In addition, the energy-loss significantly increased with Sr content which might be associated with an increase in the degree of disorder in the crystal lattice with Sr incorporation. Also, significant changes were seen in the optical properties of ZnO with Sr content in the low energy region. The theoretical results were likewise compared with the previously reported experimental data. [ABSTRACT FROM AUTHOR]

First-principles calculations of the thermal conductivity of two-dimensional materials have recently attracted a great deal of attention. The choice of the exchange-correlation (XC) and pseudopotential (PP) is a crucial step towards an accurate first-principles calculation using density functional theory (DFT). This work investigates the sensitivity of the intrinsic thermal conductivity and phonon properties of graphene to the choice of XC and PP in the temperature range of 300–550 K, using first-principles DFT simulations and an iterative solution of the Boltzmann transport equation. We consider six XC-PP combinations (LDA-NC, LDA-US, PBEsol-US, LDA-PAW, PBE-PAW, and PBEsol-PAW). Our results showed that the choice of XC-PP combination results in significant discrepancies, in the range of 5442–8677 W m − 1 K − 1 , among predicted thermal conductivities at room temperature. The LDA-NC and PBE-PAW combinations predicted the thermal conductivities in best agreement with available experimental data. The phonon properties revealed that these discrepancies are mainly due to variations in the prediction of phonon lifetimes and Grüneisen parameters from different XC-PP combinations. [ABSTRACT FROM AUTHOR]

The solar cell does not use a pn-junction to separate electrons from holes, but uses an undoped CdS layer that is p-type inverted when attached to a p-type collector and collects the holes while rejecting the backflow of electrons and thereby prevents junction leakage. The operation of the solar cell is determined by the minimum entropy principle of the cell and its external circuit that determines the electrochemical potential, i.e., the Fermi-level of the base electrode to the operating (maximum power point) voltage. It leaves the Fermi level of the metal electrode of the CdS unchanged, since CdS does not participate in the photo-emf. All photoelectric actions are generated by the holes excited from the light that causes the shift of the quasi-Fermi levels in the generator and supports the diffusion current in operating conditions. It is responsible for the measured solar maximum power current. The open circuit voltage (Voc) can approach its theoretical limit of the band gap of the collector at 0K and the cell increases the efficiency at AM1 to 21% for a thin-film CdS/CdTe that is given as an example here. However, a series resistance of the CdS forces a limitation of its thickness to preferably below 200 Å to avoid unnecessary reduction in efficiency or Voc. The operation of the CdS solar cell does not involve heated carriers. It is initiated by the field at the CdS/CdTe interface that exceeds 20 kV/cm that is sufficient to cause extraction of holes by the CdS that is inverted to become p-type. Here a strong doubly charged intrinsic donor can cause a negative differential conductivity that switches-on a high-field domain that is stabilized by the minimum entropy principle and permits an efficient transport of the holes from the CdTe to the base electrode. Experimental results of the band model of CdS/CdTe solar cells are given and show that the conduction bands are connected in the dark, where the electron current must be continuous, and the valence bands are connected with light where the hole currents are dominant and must be continuous through the junction. The major shifts of the bands in operating conditions are self-adjusting by a change in the junction dipole momentum. [ABSTRACT FROM AUTHOR]

In this study, we developed a first-principle-based full-dispersion Monte Carlo simulation method to study the anisotropic phonon transport in wurtzite GaN thin film. The input data of thermal properties in MC simulations were calculated based on the first-principle method. The anisotropy of thermal conductivity in bulk wurtzite GaN is found to be strengthened by isotopic scatterings and reduced temperature, and the anisotropy reaches 40.08% for natural bulk GaN at 100 K. With the GaN thin film thickness decreasing, the anisotropy of the out-of-plane thermal conductivity is heavily reduced due to both the ballistic transport and the less importance of the low-frequency phonons with anisotropic group velocities. On the contrary, it is observed that the in-plane thermal conductivity anisotropy of the GaN thin film is strengthened by reducing the film thickness. And the anisotropy reaches 35.63% when the natural GaN thin film thickness reduces to 50 nm at 300K with the degree of specularity being zero. The anisotropy is also improved by increasing the surface roughness of the GaN thin film. [ABSTRACT FROM AUTHOR]

In this work, we report the First-principle investigation and synthesis of MoS2@SnO2 heterostructure as high-performance humidity sensor by a two-step hydrothermal method. The first-principles calculations were performed to explain water molecule adsorption mechanism by applying density of state model to simulate the interaction between water molecule and sensing base material. The higher specific surface and the lower adsorption energy theoretically predicted the improvement on humidity sensing performance, which was confirmed by experiments testing. The MoS2@SnO2 heterostructure exhibited promoted humidity sensing characteristics on response time of 53 s and recovery time of 21 s, while switching the humidity between 11% relative humidity (RH) and 95% RH. The corresponding humidity sensing mechanisms of MoS2@SnO2 were elaborately interpreted. This work could bring forward a new design method on practical humidity sensing devices with an excellent stability and fast response by using MoS2@SnO2 heterostructure. [ABSTRACT FROM AUTHOR]

The process of p-type doping for GaN nanowires is investigated using calculations starting from first principles. The influence of different doping elements, sites, types, and concentrations is discussed. Results suggest that Mg is an optimal dopant when compared to Be and Zn due to its stronger stability, whereas Be atoms are more inclined to exist in the interspace of a nanowire. Interstitially-doped GaN nanowires show notable n-type conductivity, and thus, Be is not a suitable dopant, which is to be expected since systems with inner substitutional dopants are more favorable than those with surface substitutions. Both interstitial and substitutional doping affect the atomic structure near dopants and induce charge transfer between the dopants and adjacent atoms. By altering doping sites and concentrations, nanowire atomic structures remain nearly constant. Substitutional doping models show p-type conductivity, and Mg-doped nanowires with doping concentrations of 4% showing the strongest p-type conductivity. All doping configurations are direct bandgap semiconductors. This study is expected to direct the preparation of high-quality GaN nanowires. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel junctionless nanowire tunneling FET (JN-TFET) in which the source region is divided into an n+ as well as a p+ type region. We will show that this structure can provide a built-in n-p-n bipolar junction transistor (BJT) action in the on state of the device. In this regime, tunneling of electrons from the source valence band into the channel conduction band enhances the hole concentration in the p+ source region. Also, the potential in this region is increased, which drives a built-in BJT transistor by forward biasing the base-emitter junction. Thus, the BJT current adds up to the normal tunneling current in the JN-TFET. Owing to the sharp switching of the JN-TFET and the high BJT current gain, the overall performance of the device, herein called "BJN-TFET," is improved. On-state currents as high as 2.17×10-6 A/lm and subthreshold swings as low as ∼50 mV/dec at VDS=1V are achieved. [ABSTRACT FROM AUTHOR]

Murray's law states that the volumetric flow rate is proportional to the cube of the radius in a cylindrical channel optimized to require the minimum work to drive and maintain the fluid. However, application of this principle to the biomimetic design of micro/nano fabricated networks requires optimization of channels with arbitrary cross-sectional shape (not just circular) and smaller than is valid for Murray's original assumptions. We present a generalized law for symmetric branching that (a) is valid for any cross-sectional shape, providing that the shape is constant through the network; (b) is valid for slip flow and plug flow occurring at very small scales; and (c) is valid for networks with a constant depth, which is often a requirement for lab-on-a-chip fabrication procedures. By considering limits of the generalized law, we show that the optimum daughter-parent area ratio Γ, for symmetric branching into N daughter channels of any constant cross-sectional shape, is Γ = N-2/3 for large-scale channels, and Γ = N-4/5 for channels with a characteristic length scale much smaller than the slip length. Our analytical results are verified by comparison with a numerical optimization of a two-level network model based on flow rate data obtained from a variety of sources, including Navier-Stokes slip calculations, kinetic theory data, and stochastic particle simulations. [ABSTRACT FROM AUTHOR]

In spirit of the principle of least action, which means that when a perturbation is applied to a physical system, its reaction is such that it modifies its state to "agree" with the perturbation by "minimal" change of its initial state. In particular, the electron field emission should produce the minimum current consistent with boundary conditions. It can be found theoretically by solving corresponding equations using different techniques. We apply here the variational method for the current calculation, which can be quite effective even when involving a short set of trial functions. The approach to a better result can be monitored by the total current that should decrease when we on the right track. Here, we present only an illustration for simple geometries of devices with the electron flow. The development of these methods can be useful when the emitter and/or anode shapes make difficult the use of standard approaches. Though direct numerical calculations including particle-in-cell technique are very effective, but theoretical calculations can provide an important insight for understanding general features of flow formation and even sometimes be realized by simpler routines. [ABSTRACT FROM AUTHOR]

High temperature mechanical and thermodynamic properties of TiB2 are important to its applications as ultrahigh temperature ceramic, which were not well understood. In this study, the thermodynamic and mechanical properties of TiB2 were investigated by the combination of first principle and phonon dispersion calculations. The thermal expansion of TiB2 was anisotropic, αc/αa is nearly constant (1.46) from 300K to 1500 K, theoretically. The origination of this anisotropy is the anisotropic compressibility. The heat capacity at constant pressure was estimated from the theoretical entropy and fitted the experimental result quite well when higher-order anharmonic effects were considered. Theoretical isentropic elastic constants and mechanical properties were calculated and their temperature dependence agreed with the existed experiments. From room temperature to 1500 K, the theoretical slope is -0.0211 GPa·K-1, -0.0155 GPa·K-1, and -0.0384 GPa·K-1 for B, G, and E, respectively. Our theoretical results highlight the suitability of this method in predicting temperature dependent properties of ultrahigh temperature ceramics and show ability in selecting and designing of novel ultrahigh temperature ceramics. [ABSTRACT FROM AUTHOR]

Ionic conduction and relaxation for the cubic phase of three-dimensional CsPbCl3 perovskite with a mean crystal size of 500 nm, synthesized via a facile solution based method, have been investigated in wide temperature and frequency ranges by dielectric spectroscopic measurements. Dielectric data have been analyzed in terms of the complex impedance spectroscopy, AC conductivity and the complex electric modulus by using Maxwell–Wagner equivalent circuit model, universal power law, Havrilliak–Negami, and Kohlrausch–Williams–Watts models to explore the fundamental aspects of the ionic transport and relaxation mechanism in CsPbCl3 perovskite. Nyquist plots indicate the individual grain and grain boundary contributions to the total impedance. The temperature dependence of the DC conductivity and the relaxation time obtained from the analysis was observed to follow the Arrhenius behavior. The activation energy for the DC conductivity was found to be ∼0.25 eV, which was very close to that for the relaxation time. The scaling of the AC conductivity and the electric modulus spectra at different temperatures indicates the validity of the time-temperature superposition principle, i.e., common ionic conduction and relaxation mechanisms at different temperatures in CsPbCl3 perovskite. [ABSTRACT FROM AUTHOR]

The adsorption of several organo-functional groups (-NH2, -CH3, -COOH, -CHO, and -OH) and alanine on Li decorated carbon nanotubes (CNTs) are studied, based on the first-principle calculations. The calculated binding energies on Li-CNTs show obvious enhancement relative to the cases on pure CNTs, from about 0.3 eV to about 1.4 eV except -CH3, which is attributed to strong electrostatic dipole attraction between positive Li ion and polarized organo-functional groups by charge population analysis. It is interesting that the adsorption could be effectively adjusted under external electric field for the interaction with Li-group dipole. For the combinational contribution of charge redistribution and interaction of inherent electric dipole with external electric field, the adsorption of these organo-functional groups shows two discriminative variety trends. Finally, the adsorption of alanine including -NH2, -CH3, and -COOH groups is studied as an illustration to generalize above conclusions to organic macromolecule on Li decorated CNTs. [ABSTRACT FROM AUTHOR]

The K2NiF4-type structure La4CoNiO8 (LCNO), La4FeCoO8 (LFCO), and La4FeNiO8 (LFNO) are studied by using the first-principle electronic structure calculations. Our results indicate that the ground state of LCNO is a ferrimagnetism (FiM) with a large energy gap about 1.9 eV, LFCO and LFNO are antiferromagnetism with energy gaps about 1.3 and 1.4 eV, respectively. Their orthorhombic distortions, out-of-plane elongation, and tilting of octahedron are discussed. It is indicated that LFCO and LFNO have stronger crystal distortion than LCNO. Our calculations indicate that the in-plane magnetic exchange interaction of LCNO is much stronger than LFCO and LFNO, thus LCNO should have much higher magnetic ordering temperature than LFCO and LFNO. [ABSTRACT FROM AUTHOR]

1:2 B-site cation ordered Ba(Mg1/3Nb2/3)O3 ceramic was synthesized using conventional solid-state reaction at 1600 °C for 12 h. The structure parameters were obtained through Rietveld refinement of X-ray diffraction data. The Raman peak frequencies were obtained by Lorenz fitting on Raman spectrum. Four-parameter semiquantum model was used to fit the infrared (IR) reflectivity spectrum, and the fitted parameters were used to calculate the dielectric permittivity e and dielectric loss tand. A total of 9 active Raman and 16 active IR modes were obtained using first-principle calculations based on density functional theory with local density approximation. All of the vibrational modes were assigned and represented by linear combinations of the symmetry coordinates deduced using group theory analysis. The Raman mode with the highest frequency A1g (4) (789 cm-1) can be described as the breathing vibration of NbO6. The IR modes Eu (1) (149 cm-1) and A2u (2) (212 cm-1), which can be described as the twisting vibrations of Ba-MgO6/Ba-NbO6 on the a-b plane and the stretching vibrations of Ba-MgO6/Ba-NbO6 along the c direction, respectively, are the dominant contributing modes to e and tand. The dielectric property parameters obtained using IR spectrum fittings, first-principal calculations, and microwave measurements were compared. [ABSTRACT FROM AUTHOR]