Prediction of novel semi-conducting two-dimensional MX2 phosphides and chalcogenides (M = Zn, Cd; X = P, S, Se) with 5-membered rings.

The discovery of novel two-dimensional (2D) materials is a significant obstacle for contemporary materials science. Research in the field of 2D materials has mainly focused on materials possessing 6-membered rings, high symmetry, and isotropic features. The examination of 2D materials presenting 5-membered rings, low symmetry and anisotropic characteristics properties has received scarce attention. In this study, we employed evolutionary algorithms and heuristic approaches combined with first-principles calculations to predict penta-MX2 structures (M = Zn, Cd; X = P, S, Se). All selected 2D penta-MX2 phases are dynamically, thermodynamically, mechanically, and thermally stable. Further discussion focuses on their structural, bonding, electronic and optoelectronic features. Our HSE06 calculations reveal that the penta-MP2, ZnPS, and MSSe structures are semiconductors with a band gap of 0.80-3.08 eV. Conversely, the 2D penta-MPSe (M = Zn, Cd) and CdPS phases are metallic. We additionally note that penta ß-ZnP2 and CdP2 display direct band gaps (1.39 eV and 1.18 eV, respectively), while the penta α-ZnP2, ZnPS, ZnSSe, α-CdSSe and ß-CdSSe possess indirect band gaps. Remarkably, 2D pentagonal MP2 (M = Zn, Cd), MSSe (M = Zn, Cd) and ZnPS 2D monolayers exhibit substantial optical absorption (>105 cm-1) throughout a broad range of the visible light spectra. Our results for crystal structure prediction expand the 2D penta-family of phosphides and chalcogenides, and demonstrate the potential of 2D penta-MX2 materials for optoelectronic applications.

Achieved negative differential resistance behavior of Si/B-substituted into a C6 chain sandwiched between capped carbon nanotube junctions.

Electronic transport properties of a pristine C6 chain and Si/B-substituted into the C6 chain sandwiched between two (5, 5) capped carbon nanotube electrodes were investigated through first-principles calculations based on non-equilibrium Green's functions (NEGF) conjugated with density functional theory (DFT). Si and B substitutions will affect the I-V curve of a pristine C6 chain. In the I-V characteristics, multi negative differential resistance (NDR) with large peak to valley ratio (PVR) and rectifying actions were observed. The NDR behavior originates from the joining and moving of conduction orbitals inside and outside of the bias window at a certain bias voltage. Furthermore, the assessment of transmission coefficient and distribution of molecular orbitals reveals that the rectifying performance is the result of the asymmetric distribution of the frontier molecular orbitals in the central region and their coupling with the electrodes. Multi NDR behavior of B substitution under very low bias voltage is a unique property of our proposed devices. Moreover, the CNT|C-(B-C)2-C|CNT molecular device shows a high PVR up to 31.8, which demonstrates that the proposed devices can be useful for molecular switching in nanoelectronic devices.

Response surface methodology optimizing the adsorptive removal of azithromycin using mesoporous silica SBA-15: Mechanism, thermodynamic, equilibrium, and kinetics modeling studies.

The objective of this research was to study an effective adsorbent for removing azithromycin (AZT) from industrial wastewater. AZT is an antibiotic used for many diseases remedy, but it is a pollutant to our environment; therefore, its residual should be removed from wastewater. The mesoporous SBA-15 silica as an efficient adsorbent was prepared by the hydrothermal method. The surface of mesoporous SBA-15 plays a significant role in the removal process; therefore, the characterization of the adsorbent was accomplished by several techniques. The batch system has been used, and the effect of four essential variables: pH (3-10), drug concentration (20-200 mg L-1), sorbent weight (0.2-2 g L-1), and temperature (20-40 °C) were investigated on the AZT removal efficiency by response surface methodology (RSM). The isotherm results were found to be in proper compliance with the isotherm model of Freundlich. In the kinetics part of this study, the experimental outcomes were fitted to the equation model of pseudo-second-order. The calculation of thermodynamic parameters shows that the removal process is spontaneous and endothermic. Upon the results, the vast surface area, the active functional groups, reusability, stability, and inexpensively make the mesoporous SBA-15 a suitable candidate for removal of AZT and similar antibiotics.

Prediction of beryllium clusters (Ben; n = 3-25) from first principles.

Evolutionary searches using the USPEX method (Universal Structure Predictor: Evolutionary Xtallography) combined with density functional theory (DFT) calculations were performed to obtain the global minimum structures of beryllium (Ben, n = 3-25) clusters. The thermodynamic stability, optoelectronic and photocatalytic properties as well as the nature of bonding are considered for the most stable clusters. It is found that the cluster with n = 15 is the transition point at which the configurations change from 3D hollow cages to filled cage structures (with an interior atom appearing in the structure). All the ground state structures are energetically favorable with negative binding energies, suggesting good synthetic feasibility for these structures. The calculated relative stabilities and electronic structure show that the Be4, Be10 and, Be17 clusters are the most stable structures and can be considered as superatoms. The electron configurations of Be4, Be10 and Be17 clusters with 8, 20 and 34 electrons are identified as 1S2 1P6, 1S2 1P6 1D10 2S2, 1S2 1P6 1D10 2S2 1F14, respectively. Theoretical simulations determined that all the ground state structures exhibit excellent thermal stability, where the upper-limit temperature that the structures can tolerate is 900 K. During AIMD simulation of O2 adsorption onto the Be17 cluster an interesting phenomenon was happening in which the pristine Be17 cluster becomes a new stable Be17O16 cluster. Based on ELF (electron localization function) analysis, it can be concluded that the Be-Be bonds in the small clusters are primarily of van der Waals type, while for the larger clusters, the bonds are of metallic nature. The Ben clusters show very strong absorption in the UV and visible regions with absorption coefficients larger than 105 cm-1, which suggests a wide range of potential advanced optoelectronics applications. The Be17 cluster has a suitable band alignment in the visible-light excitation region which will produce enhanced photocatalytic activities (making it a promising material for water splitting).

Prediction of unexpected B n P n structures: promising materials for non-linear optical devices and photocatalytic activities.

In the present work, a modern method of crystal structure prediction, namely USPEX conjugated with density functional theory (DFT) calculations, was used to predict the new stable structures of B n P n (n = 12, 24) clusters. Since B12N12 and B24N24 fullerenes have been synthesized experimentally, it motivated us to explore the structural prediction of B12P12 and B24P24 clusters. All new structures were predicted to be energetically favorable with negative binding energy in the range from -4.7 to -4.8 eV per atom, suggesting good experimental feasibility for the synthesis of these structures. Our search for the most stable structure of B n P n clusters led us to classify the predicted structures into two completely distinct structures such as α-B n P n and ß-B n P n phases. In α-B n P n , each phosphorus atom is doped into a boron atom, whereas B atoms form a B n unit. On the other hand, each boron atom in the ß-phase was bonded to a phosphorus atom to make a fullerene-like cage structure. Besides, theoretical simulations determined that α-B n P n structures, especially α-B24P24, show superior oxidation resistance and also, both α-B n P n and ß-B n P n exhibit better thermal stability; the upper limit temperature that structures can tolerance is 900 K. The electronic properties of new compounds illustrate a higher degree of absorption in the UV and visible-region with the absorption coefficient larger than 105 cm-1, which suggests a wide range of opportunities for advanced optoelectronic applications. The ß-B n P n phase has suitable band alignments in the visible-light excitation region, which will produce enhanced photocatalytic activities. On the other hand, α-B n P n structures with modest band gap exhibit large second hyperpolarizability, which are anticipated to have excellent potential as second-order non-linear optical (NLO) materials.

Copper halide diselenium: predicted two-dimensional materials with ultrahigh anisotropic carrier mobilities.

On the basis of first-principles calculations, we discuss a new class of two-dimensional materials-CuXSe2 (X = Cl, Br) nanocomposite monolayers and bilayers-whose bulk parent was experimentally reported in 1969. We show the monolayers are dynamically, mechanically and thermodynamically stable and have very small cleavage energies of ∼0.26 J m-2, suggesting their exfoliation is experimentally feasible. The monolayers are indirect-gap semiconductors with practically the same moderate band gaps of 1.74 eV and possess extremely anisotropic and very high carrier mobilities (e.g., their electron mobilities are 21 263.45 and 10 274.83 cm2 V-1 s-1 along the Y direction for CuClSe2 and CuBrSe2, respectively, while hole mobilities reach 2054.21 and 892.61 cm2 V-1 s-1 along the X direction). CuXSe2 bilayers are also indirect band gap semiconductors with slightly smaller band gaps of 1.54 and 1.59 eV, suggesting weak interlayer quantum confinement effects. Moreover, the monolayers exhibit high absorption coefficients (>105 cm-1) over a wide range of the visible light spectra. Their moderate band gaps, very high unidirectional electron and hole mobilities, and pronounced absorption coefficients indicate the proposed CuXSe2 (X = Cl, Br) nanocomposite monolayers hold significant promise for application in optoelectronic devices.

Evolutionary search for (M©B16)Q (M = Sc-Ni; Q = 0/-1) clusters: bowl/boat vs. tubular shape.

Herein, using universal structure predictor: evolutionary xtallography (USPEX) method, followed by density functional theory (DFT) calculations, we performed global searches for the most stable structures of (M©B16)Q (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni; Q = 0, -1) clusters. It was found that the obtained ground-state structures of (M©B16)Q clusters exhibited a distinct structural evolution as M changed from V to Ni: from bowl-shaped, to boat-shaped, to an M-centered tubular structure named wheel-shaped, to drum-shaped (the metal atom was adsorbed on top of the cross section of the B16 species). Our analysis shows that hyper-coordination and the size of the metal atom are two competing factors determining the relative stability and topological properties of the (M©B16)0/-1 clusters, resulting in unprecedented structures for Sc, Ti, and Ni-doped clusters. The calculated binding energies for these new configurations are even larger than those of the previously synthesized B16-1, (Mn©B16)-1, and (Co©B16)-1 clusters, indicating their very good stability and possible experimental synthesis. A net charge transfer from the metal atom to the boron moiety occurs for all clusters, indicating that electrostatic interactions play an important role in the stability of these materials. Finally, the Sc©M16 and Ti©B16 clusters exhibit not only excellent thermal stability but also large first hyper-polarizability. Hence, they are expected to be potential innovative candidates for excellent electro-optical materials.

Theoretical prediction of maximum capacity of C80 and Si80 fullerenes for noble gas storage.

In this paper, we try to demonstrate that how many helium, neon and argon atoms can be trapped into fullerene cages until the pressure becomes large enough to break the C80 and Si80 frameworks. The maximum number of helium, neon and argon atoms which can be encapsulated into C80 fullerene, is found with 46, 24 and 10 atoms respectively. Having investigated the mechanism of C80 opening, we found that if the number of helium and argon atoms reaches to 50 and 12 respectively, the C-C bonds of C80 are broken and the gas molecules escaped from the fullerene cage. The final optimization geometries of latter complexes are similar to the shopping cart. Therefore, this appearance is named as molecular cart. Moreover, the maximum capacity of Si80 fullerene for encapsulated noble gas atoms is found 95, 56 and 22 for helium, neon and argon atoms correspondingly. It is worth highlighting that the new phenomenon of trapping argon atoms into Si80 cage is observed, when a Si atom randomly added to the center of Ar19@Si80 structures. In this case, the Si-Si bonds of Si80 are broken and two argon atoms will escape from the cage. After that, the framework rebuilds its structure like the initial one. This phenomenon is introduced as molecular cesarean section. The estimated internal pressure of Ng atoms trapped into the fullerene cages is also investigated. Results show that the maximum calculated internal pressure is related to He46@C80 and He95@Si80 structures with 212.3 and 144.1GPa respectively.