High-throughput calculation screening for new silicon allotropes with monoclinic symmetry.

A total of 87 new monoclinic silicon allotropes are systematically scanned by a random strategy combined with group and graph theory and high-throughput calculations. The new allotropes include 13 with a direct or quasi-direct band gap and 12 with metallic characteristics, and the rest are indirect band gap semiconductors. More than 30 of these novel monoclinic Si allotropes show bulk moduli greater than or equal to 80 GPa, and three of them show even greater bulk moduli than diamond Si. Only two of the new Si allotropes show a greater shear modulus than diamond Si. The crystal structures, stability (elastic constants, phonon spectra), mechanical properties, electronic properties, effective carrier masses and optical properties of all 87 Si monoclinic allotropes are studied in detail. The electron effective masses ml of five of the new allotropes are smaller than that of diamond Si. All of these novel monoclinic Si allotropes show strong absorption in the visible spectral region. Taken together with their electronic band gap structures, this makes them promising materials for photovoltaic applications. These investigations greatly enrich the current knowledge of the structure and electronic properties of silicon allotropes.

Competitive adsorption of sulfamethoxazole and bisphenol A on magnetic biochar: Mechanism and site energy distribution.

Competitive adsorption and complementary adsorption between emerging pollutants has been observed in multiple studies. Investigation of the preference of pollutants for different types of adsorption sites can provide a supplementary perspective for understanding complementary adsorption. In this study, the simultaneous adsorption of two typical emerging pollutants, sulfamethoxazole (SMX) and bisphenol A (BPA), on magnetic biochar (MBC-1) was investigated. The results showed that the modification with ferric chloride optimized the surface properties of biochar (aromaticity, hydrophobicity, and oxygen-containing functional groups, etc.), and helped to remove SMX and BPA through various interactions. The equilibrium adsorption capacity of the two adsorbents was inhibited by competitive adsorption in the mixed solute systems, which was due to the same adsorption mechanism. When pH = 7, the SMX and BPA adsorption mainly involved pore filling, hydrophobic effect, π-π EDA, and hydrogen bonding. In addition, electrostatic force, surface coordination, and ion exchange have also been proven to be related to the adsorption of SMX and BPA. In the co-adsorption system, BPA's competitive advantage might be due to its superior hydrophobicity, charge property, and molecular diameter. In the competitive adsorption experiment, the total adsorption capacity (Qi) of the competitive solute exceeded the adsorption inhibition (△Qi) of the main solute, indicating that the two solutes occupied their preferred adsorption sites, which confirmed the complementary adsorption phenomenon. Complementary adsorption can be explained by the preference of SMX and BPA for different types of adsorption sites. BPA preferentially occupied high-energy sites in the co-adsorption system, such as π-π EDA interaction, ion exchange, and surface coordination. At the same time, SMX tended to be removed by hydrophobic interaction and hydrogen bonding.

Study on ESD Protection Circuit by TCAD Simulation and TLP Experiment.

The anti-ESD characteristic of the electronic system is paid more and more attention. Moreover, the on-chip electrostatic discharge (ESD) is necessary for integrated circuits to prevent ESD failures. In this paper, the mixed TCAD model of the ESD protection circuit is built and simulated, and the negative transmission line pulse (TLP) injection damage experiment is carried out on the CD4069UBC chip. The circuit model consists of three-dimensional shallow trench isolation (STI) diode TCAD models and a three-dimensional multi-gate Complementary Metal-Oxide-Semiconductor (CMOS) inverter TCAD model. Moreover, the TCAD modeling is based on a 0.25 µm technology node. Through the transient simulation of the electrothermal coupling, the electrical signal of the input and output nodes of the circuit and the distribution of the electrothermal parameters in the device are obtained. Moreover, by analyzing the simulation results, the physical phenomena and the mechanisms of interference and damage mechanism during TLP injection are explained. The location and type of diode damage in the TLP injection simulation results of the circuit model are consistent with the TLP experiment damage results. The proposed ESD protection circuit model and analysis method are beneficial to ESD robustness prediction and ESD soft damage analysis of IC.

Optimization of Electron Transport Pathway: A Novel Strategy to Solve the Photocorrosion of Ag-Based Photocatalysts.

Although Ag-containing photocatalysts exhibit excellent photocatalytic ability, they present great challenges owing to their photocorrosion and ease of reduction. Herein, an electron acceptor platform of Ag2O/La(OH)3/polyacrylonitrile (PAN) fiber was constructed using a heterojunction strategy and electrospinning technology to develop a novel photocatalytic membrane with a redesigned electron transport pathway. Computational and experimental results demonstrate that the optimized electron transport pathway included intercrystal electron transfer induced by the La-O bond between Ag2O and La(OH)3 as well as electron transfer between the catalyst crystal and electrophilic PAN membrane interface. In addition, the photocatalytic performance of the Ag2O/La(OH)3 membrane for tetracycline (TC) removal was still above 97% after five photocatalytic reaction cycles. Furthermore, the carrier life was greatly extended. Mechanistic study revealed that photogenerated holes on the Ag2O/La(OH)3 membrane were the main reactive species in TC degradation. Overall, this study proposes a novel electron transport pathway strategy that effectively solves the problems of photocatalyst photocorrosion and structural instability.

Comparison of cadmium adsorption by hydrochar and pyrochar derived from Napier grass.

Biochar (e.g. pyrochar and hydrochar) is considered a promising adsorbent for Cd removal from aqueous solution. Considering the vastly different physicochemical properties between pyrochar and hydrochar, the Cd2+ sorption capacity and mechanisms of pyrochars and hydrochars should be comparatively determined to guide the production and application of biochar. In this study, the hydrochars and pyrochars were prepared from Napier grass by hydrothermal carbonization (200 and 240 °C) and pyrolysis (300 and 500 °C), respectively, and the physicochemical properties and Cd2+ sorption performances of biochars were systematically determined. The pyrochars had higher pH and ash content as well as better stability, while the hydrochars showed more oxygen-containing functional groups (OFGs) and greater energy density. The pseudo second order kinetic model best fitted the Cd2+ sorption kinetics data of biochars, and the isotherm data of pyrochar and hydrochar were well described by Langmuir and Freundlich models, respectively. In comparison with hydrochar, the pyrochar exhibited better Cd2+ sorption capacity (up to 71.47 mg/g). With increasing production temperature, the Cd2+ sorption capacity of pyrochar elevated, while the reduction was found for hydrochar. The mineral interaction, complexation with surface OFGs, and coordination with π electron were considered the main mechanisms of Cd2+ removal by biochars. The minerals interaction and the complexation with OFGs was the dominant mechanism of Cd2+ removal by pyrochars and hydrochars, respectively. Therefore, the preparation technique and temperature have significant impacts on the sorption capacity and mechanisms of biochar, and pyrochar has better potential for Cd2+ removal than the congenetic hydrochar.

Low-energy Ga2O3 polymorphs with low electron effective masses.

We predict three Ga2O3 polymorphs with P21/c or Pnma symmetry. The formation energies of P21/c Ga2O3, Pnma-I Ga2O3, and Pnma-II Ga2O3 are 57 meV per atom, 51 meV per atom, and 23 meV per atom higher than that of ß-Ga2O3, respectively. All the polymorphs are shown to be dynamically and mechanically stable. P21/c Ga2O3 is a quasi-direct wide band gap semiconductor (3.83 eV), while Pnma-I Ga2O3 and Pnma-II Ga2O3 are direct wide band gap semiconductors (3.60 eV and 3.70 eV, respectively). Simulated X-ray diffraction patterns are provided for experimental confirmation of the predicted structures. The polymorphs turn out to provide low electron effective masses, which is of great benefit to high-power electronic devices.

Direct and quasi-direct band gap of novel Si-Ge alloys inP-3m1 phase.

This work investigates the crystal structure, stability, mechanical properties, electronic properties, effective masses, and optical properties of Si-Ge alloys in theP-3m1 phase. The elastic constants and phonon spectra proven that the Si-Ge alloys in theP-3m1 phase have mechanical and dynamic stability. The bulk modulus, shear modulus and Young's modulus of Si-Ge alloys in theP-3m1 phase decrease with the increase of Ge composition, and the three-dimensional diagram of Young's modulus and effective mass show that the mechanical and transport properties have anisotropy. The Si12Ge12in thehP24 phase is a quasi-direct band gap semiconductor material with a band gap of 1.081 eV, while the Si30-xGexalloy (x= 6, 12, 18, 24) in thehP30 phase are all direct band gap semiconductor materials with the band gaps of 0.541 eV, 0.430 eV, 0.561 eV, and 0.387 eV, respectively. ThehP30-Si6Ge24has a very small effective electron mass. ThehP24-Si12Ge12show excellent absorptive capacity in the visible and infrared region region. Based on this work, Si-Ge alloys in theP-3m1 phase are promising materials for photovoltaic applications.

Structural, Electronic, and Optical Properties of Hexagonal XC6 (X=N, P, As, and Sb) Monolayers.

Based on first-principles calculations, a novel family of two-dimensional (2D) IV-V compounds, XC6 (X=N, P, As and Sb), is proposed. These compounds exhibit excellent stability, as determined from the cohesive energies, phonon dispersion analysis, ab initio molecular dynamics (AIMD) simulations, and mechanical properties. In this type of structure, the carbon atom is sp2 hybridized, whereas the X (N, P, As and Sb) atom is nonplanar sp3 hybridized with one 2pz orbital filled with lone pair electrons. NC6 , PC6 , AsC6 and SbC6 monolayers are intrinsic indirect semiconductors with wide bandgaps of 2.02, 2.36, 2.77, and 2.85 eV (based on HSE06 calculations), respectively. After applying mechanical strain, PC6 , AsC6 and SbC6 monolayers can be transformed from indirect to direct semiconductors. The appropriate bandgaps and well-located band edge positions make XC6 monolayers potential materials for photocatalytic water splitting. XC6 family members also have high absorption coefficients (∼105  cm-1 ) in the ultraviolet region and higher electron mobilities (∼103  cm2  V-1 s-1 ) than many known 2D semiconductors.

Two-Dimensional Tetrahex-GeC2: A Material with Tunable Electronic and Optical Properties Combined with Ultrahigh Carrier Mobility.

Based on first-principles calculations, we propose a novel two-dimensional (2D) germanium carbide, tetrahex-GeC2, and determine its electronic and optical properties. Each Ge atom binds to four C atoms, in contrast to the known 2D hexagonal germanium carbides. Monolayer tetrahex-GeC2 possesses a narrow direct band gap of 0.89 eV, which can be effectively tuned by applying strain and increasing the thickness. Its electron mobility is extraordinarily high (9.5 × 104 cm2/(V s)), about 80 times that of monolayer black phosphorus. The optical absorption coefficient is ∼106 cm-1 in a wide spectral range from near-infrared to near-ultraviolet, comparable to perovskite solar cell materials. We obtain high cohesive energy (5.50 eV/atom), excellent stability, and small electron/hole effective mass (0.19/0.10 m0). Tetrahex-GeC2 turns out to be a very promising semiconductor for nanoelectronic, optoelectronic, and photovoltaic applications.

First-Principles Study on III-Nitride Polymorphs: AlN/GaN/InN in the Pmn21 Phase.

The structural, mechanical, and electronic properties, as well as stability, elastic anisotropy and effective mass of AlN/GaN/InN in the Pmn21 phase were determined using density functional theory (DFT). The phonon dispersion spectra and elastic constants certify the dynamic and mechanical stability at ambient pressure, and the relative enthalpies were lower than those of most proposed III-nitride polymorphs. The mechanical properties reveal that Pmn21-AlN and Pmn21-GaN possess a high Vickers hardness of 16.3 GPa and 12.8 GPa. Pmn21-AlN, Pmn21-GaN and Pmn21-InN are all direct semiconductor materials within the HSE06 hybrid functional, and their calculated energy band gaps are 5.17 eV, 2.77 eV and 0.47 eV, respectively. The calculated direct energy band gaps and mechanical properties of AlN/GaN/InN in the Pmn21 phase reveal that these three polymorphs may possess great potential for industrial applications in the future.

Penta-C20: A Superhard Direct Band Gap Carbon Allotrope Composed of Carbon Pentagon.

A metastable sp3-bonded carbon allotrope, Penta-C20, consisting entirely of carbon pentagons linked through bridge-like bonds, was proposed and studied in this work for the first time. Its structure, stability, and electronic and mechanical properties were investigated based on first-principles calculations. Penta-C20 is thermodynamically and mechanically stable, with equilibrium total energy of 0.718 and 0.184 eV/atom lower than those of the synthesized T-carbon and supercubane, respectively. Penta-C20 can also maintain dynamic stability under a high pressure of 100 GPa. Ab initio molecular dynamics (AIMD) simulations indicates that this new carbon allotrope can maintain thermal stability at 800 K. Its Young's modulus exhibits mechanical anisotropy. The calculated ideal tensile and shear strengths confirmed that Penta-C20 is a superhard material with a promising application prospect. Furthermore, Penta-C20 is a direct band gap carbon based semiconducting material with band gap of 2.89 eV.

Six novel carbon and silicon allotropes with their potential application in photovoltaic field.

By stacking up five novel cagelike structures, three novel three-dimensional (3D) sp 3 bonding networks, named hP24, hP30 and hP36, were predicted in this work for the first time. These three newly discovered structures have trigonal unit cell with the space groups of P-3m1, P-3m1 and P3m1, respectively. Using first-principle calculations, the physical properties, including structural, mechanical, electronic and optical properties of C and Si in hP24, hP30 and hP36 phases were systematically studied. All these newly discovered carbon and silicon allotropes were proven to be thermodynamically and mechanically stable. The wide indirect bandgap value in range of 3.89-4.03 eV suggests that C in hP24, hP30 and hP36 phases have the potential to be applied in high frequency and high power electronic devices. The direct bandgap value in range of 0.60-1.16 eV, the smaller electron and hole effective mass than diamond-Si, and the significantly better photon absorption characteristics than diamond-Si suggest that hP24-Si, hP30-Si and hP36-Si are likely to have better performance in photovoltaic applications than diamond-Si. hP24-Si also has the potential to be applied in infrared detectors.

First-Principles Study on Structural, Mechanical, Anisotropic, Electronic and Thermal Properties of III-Phosphides: XP (X = Al, Ga, or In) in the P6422 Phase.

The structural, mechanical, electronic, and thermal properties, as well as the stability and elastic anisotropy, of XP (X = Al, Ga, or In) in the P6422 phase were studied via density functional theory (DFT) in this work. P6422-XP (X = Al, Ga, or In) are dynamically and thermodynamically stable via phonon spectra and enthalpy. At 0 GPa, P6422-XP (X = Al, Ga, or In) are more rigid than F 4 ¯ 3 m-XP (X = Al, Ga, or In), of which P6422-XP (X = Al or Ga) are brittle and P6422-InP is ductile. In the same plane (except for (001)-plane), P6422-AlP and P6422-InP exhibit the smallest and the largest anisotropy, respectively, and P6422-XP (X = Al, Ga, or In) is isotropic in the (001)-plane. In addition, Al, Ga, In, and P bonds bring different electrical properties: P6422-InP exhibits a direct band gap (0.42 eV) with potential application for an infrared detector, whereas P6422-XP (X = Al or Ga) exhibit indirect band gap (1.55 eV and 0.86 eV). At high temperature (approaching the melting point), the theoretical minimum thermal conductivities of P6422-XP (X = Al, Ga, or In) are AlP (1.338 W∙m-1∙K-1) > GaP (1.058 W∙m-1∙K-1) > InP (0.669 W∙m-1∙K-1), and are larger than those of F 4 ¯ 3 m-XP (X = Al, Ga, or In). Thus, P6422-XP (X = Al, Ga, or In) have high potential application at high temperature.

Electronic, Mechanical and Elastic Anisotropy Properties of X-Diamondyne (X = Si, Ge).

The three-dimensional (3D) diamond-like semiconductor materials Si-diamondyne and Ge-diamondyne (also called SiC4 and GeC4) are studied utilizing density functional theory in this work, where the structural, elastic, electronic and mechanical anisotropy properties along with the minimum thermal conductivity are considered. SiC4 and GeC4 are semiconductor materials with direct band gaps and wide band gaps of 5.02 and 5.60 eV, respectively. The Debye temperatures of diamondyne, Si- and Ge-diamondyne are 422, 385 and 242 K, respectively, utilizing the empirical formula of the elastic modulus. Among these, Si-diamondyne has the largest mechanical anisotropy in the shear modulus and Young's modulus, and Diamond has the smallest mechanical anisotropy in the Young's modulus and shear modulus. The mechanical anisotropy in the Young's modulus and shear modulus of Si-diamondyne is more than three times that of diamond as determined by the characterization of the ratio of the maximum value to the minimum value. The minimum thermal conductivity values of Si- and Ge-diamondyne are 0.727 and 0.524 W cm-1 K-1, respectively, and thus, Si- and Ge-diamondyne may be used in the thermoelectric industry.

Physical properties of Si-Ge alloys in C2/m phase: a comprehensive investigation.

A new phase of C2/m Ge16 is first proposed in this paper. The structures and mechanical, anisotropic, electronic, transport and optical properties of Si-Ge alloys in the C2/m phase are studied using first principles calculations. All Ge16 and Si16-x Ge x alloys in the C2/m phase are proven to have mechanical and dynamic stability. By analyzing the three-dimensional (3D) perspective of the effective mass and Young's modulus, obvious anisotropies of transport and mechanical properties are found. Higher-resolution full band structures are obtained to determine the positions of the valence band maximum (VBM) and conduction band minimum (CBM). All materials have a higher photoelectron absorption than that of diamond Si. A high electronic mobility (16 527 cm2 V-1 s-1) and hole mobility (3033 cm2 V-1 s-1) are found in C2/m Si8Ge8 and Si4Ge12, respectively. Based on the large mobility and photoelectron absorption, the Si-Ge alloys in the C2/m phase are promising materials for electronics and optoelectronics applications.

III-Nitride Polymorphs: XN (X=Al, Ga, In) in the Pnma Phase.

The structural, mechanical, elastic anisotropy, and electronic properties, together with the stability, effective mass of holes and electrons for XN (X=Al, Ga, In) in the Pnma phase are investigated by using density functional theory calculations. The elastic constants and the phonon spectra all manifest III-nitride polymorphs: XN (X=Al, Ga, In) in the Pnma phase in this work are mechanically and dynamically stable at ambient pressure. Al atoms, Ga atoms, and In atoms lead to new electrical and band-gap properties: XN (X=Al, Ga, In) in the Pnma phase are all semiconductor materials with direct band gaps of 4.76 eV, 2.80 eV, and 0.66 eV, respectively, which present great application potentials in the new generation electronic devices such as ultraviolet detectors, visible light detectors, infrared detectors, violet-light diodes, and light-emitting diodes.

Structural, Mechanical, Anisotropic, and Thermal Properties of AlAs in oC12 and hP6 Phases under Pressure.

The structural, mechanical, anisotropic, and thermal properties of oC12-AlAs and hP6-AlAs under pressure have been investigated by employing first-principles calculations based on density functional theory. The elastic constants, bulk modulus, shear modulus, Young’s modulus, B/G ratio, and Poisson’s ratio for oC12-AlAs and hP6-AlAs have been systematically investigated. The results show that oC12-AlAs and hP6-AlAs are mechanically stable within the considered pressure. Through the study of lattice constants (a, b, and c) with pressure, we find that the incompressibility of oC12-AlAs and hP6-AlAs is the largest along the c-axis. At 0 GPa, the bulk modulus B of oC12-AlAs, hP6-AlAs, and diamond-AlAs are 76 GPa, 75 GPa, and 74 Gpa, respectively, indicating that oC12-AlAs and hP6-AlAs have a better capability of resistance to volume than diamond-AlAs. The pressure of transition from brittleness to ductility for oC12-AlAs and hP6-AlAs are 1.21 GPa and 2.11 GPa, respectively. The anisotropy of Young’s modulus shows that oC12-AlAs and hP6-AlAs have greater isotropy than diamond-AlAs. To obtain the thermodynamic properties of oC12-AlAs and hP6-AlAs, the sound velocities, Debye temperature, and minimum thermal conductivity at considered pressure were investigated systematically. At ambient pressure, oC12-AlAs (463 K) and hP6-AlAs (471 K) have a higher Debye temperature than diamond-AlAs (433 K). At T = 300 K, hP6-AlAs (0.822 W/cm·K−1) has the best thermal conductivity of the three phases, and oC12-AlAs (0.809 W/cm·K−1) is much close to diamond-AlAs (0.813 W/cm·K−1).