Magnetism of single-walled silicon carbide nanotubes doped by boron,nitrogen and oxygen

We calculated, using spin polarized density functional theory, the electronic properties of zigzag (10,0) and armchair (6,6) semiconductor silicon carbide nanotubes (SiCNTs) doped once at the time with boron, nitrogen, and oxygen. We have looked at the two possible scenarios where the guest atom X (B, N, O), replaces the silicon X_Si, or the carbon atom X_C, in the unit cell. We found that in the case of one atom B @ SiCNT replacing a carbon atom position annotated by B_C exhibits a magnetic moment of 1 μ_B/cell in both zigzag and armchair nanotubes. Also, B replacing Si, (B_Si), induce a magnetic moment of 0.46 μ_B/cell in the zigzag (10,0) but no magnetic moment in armchair (6,6). For N substitution; (N_C) and (N_Si) each case induce a magnetic moment of 1 μ_B/cell in armchair (6,6), while N_Si give rise to 0.75 μ_B/cell in zigzag (10,0) and no magnetic moment for N_C. In contrast the case of O_C and O_Si did not produce any net magnetic moment in both zigzag and armchair geometries.     

Si-doped C3N monolayers as efficient single-atom catalysts for the reduction of N2O: a computational study

Density functional theory calculations are performed to probe reaction pathways of N₂O reduction by CO molecule catalysed over Si-doped C₃N (Si-C₃N) nanosheets. According to our results, a single Si atom can be stabilised above the C- or N-vacancy site of C₃N due to the formation of strong Si-N or Si-C covalent bonds. The reduction of N₂O over Si-C₃N is characterised as a two-step process. First, N₂O is dissociated to N₂ and an activated oxygen atom (O_ads) without an energy barrier. Then, the O_ads moiety is removed by CO molecule by overcoming negligible activation energy.     

Lithium absorption on single-walled boron nitride,aluminum nitride,silicon carbide and carbon nanotubes: A first-principles study

Using the DFT-B3LYP calculations we investigate the adsorption of Li atom on CNT, BNNT, AlNNT and SiCNT. We found that Li atom can be chemisorbed on zig-zag SiCNT with binding energy of −2.358 eV and charge transfer of 0.842 |e|, which are larger than the results of other nanotubes. The binding energy of Li on SiCNT is foun to be stronger than activation energy barrier indicating that Li metal could be well dispersed on SiCNTs. Furthermore, the average voltage caused by the lithium adsorption on SiCNT demonstrated that SiCNTs could exhibit as a stable anode similar to the lithium metal anode. The binding nature has been rationalized by analyzing the electronic structures. Our findings demonstrate that Li-BNNT, Li-SiCNT and Li-AlNNT systems exhibit spin polarized behaviors and can fascinating potential application in future spintronics. Also, Li-SiCNT system with rather small band gap might be a promising material for optical applications and active molecule in its environment.     

Theoretical study on the oxidation of zigzag silicon carbide nanotubes (SiCNTs) by singlet O2

Singlet O₂ produced upon photoexcitation is a very important oxidative reagent. The study on its reaction with nanotube might be useful not only to evaluate the stability of the nanotube upon air exposure and sunlight, but also to modify the properties of the nanotube. Considering the unique properties and wide applications of silicon carbide nanotube (SiCNT), in this paper, we performed extensive density functional theory (DFT) calculations to study the oxidation of a series of zigzag (n,0) SiCNTs (n=6 to 12) by singlet O₂. It is found that the reaction process contains two steps, namely, (i) [2+2] cycloaddition of a singlet O₂ to the Si–C bond, followed by (ii) the dissociation of the O–O bond, leading to the formation of an epoxide configuration with a highly exothermicity (>4.00 eV). Compared with pure SiCNT, the cycloaddition of singlet O₂ on tube leads to the decrease of the band gap, while the formation of the stable epoxy structure render band gap increase. Our results indicate that the SiCNT is more prone to be degraded after exposure to air and sunlight.     

Silicon carbide nanotubes (SiCNTs) serving for catalytic decomposition of toxic diazomethane (DAZM) gas: a DFT study

In the present study, the adsorption and decomposition of diazomethane (DAZM) on the surface of (6,0) zigzag silicon carbide nanotube (SiCNT) are investigated using density functional theory calculations. The geometry structures of the three stable configurations, adsorption energies and electronic properties of DAZM adsorption on the surface of SiCNT are investigated. It was found that the DAZM molecule is decomposed over the surface of (6,0) SiCNT with activation energy (E_act) of 0.523 eV. The curvature effect on the adsorption energies of the DAZM molecule is also considered by studying (5,0) and (7,0) SiCNTs. The results display that DAZM adsorption over smaller diameter of SiCNT is thermodynamically more favourable than larger one.     

Ab initio density functional theory investigation of Li-intercalated silicon carbide nanotube bundles

We present the results of ab initio density functional theory calculations on the energetic, and geometric and electronic structure of Li-intercalated (6,6) silicon carbide nanotube (SiCNT) bundles. Our results show that intercalation of lithium leads to the significant changes in the geometrical structure. The most prominent effect of Li intercalation on the electronic band structure is a shift of the Fermi energy which occurs as a result of charge transfer from lithium to the SiCNTs. All the Li-intercalated (6,6) SiCNT bundles are predicted to be metallic representing a substantial change in electronic properties relative to the undoped bundle, which is a wide band gap semiconductor. Both inside of the nanotube and the interstitial space are susceptible for intercalation. The present calculations suggest that the SiCNT bundle is a promising candidate for the anode material in battery applications.     

H2O2 adsorption on the BN and SiC nanotubes: A DFT study

We have performed a comparative density functional theory study on adsorption of hydrogen peroxide (H₂O₂) on the boron nitride and silicon carbide nanotubes (BNNT and SiCNT) in terms of energetic, geometric, and electronic properties. It has been found that the molecule is chemically adsorbed on both of the tubes so that its interaction with SiCNT (adsorption energy ∼−0.97 eV) is much stronger than that with BNNT (adsorption energy ∼−0.47 eV). The H₂O₂ adsorption on BNNT slightly decreases its work function, increasing the field electron emission from the BNNT surface while it may not affect that of the SiCNT. In addition, the adsorption process may increase the electrical conductivity of SiCNT while does not affect that of the BNNT, significantly. We believe that the SiCNT may be a potential candidate for detection of H₂O₂.     

Infrared and Raman Spectra for Amino Group and C˭O Stretching Modes in Biomolecule 5‐Aminouracil

Abstract

Assuming planar geometry of 5‐aminouracil and C_s point group symmetry, it has been possible to assign all the 36 (25a′+11a″) normal modes of vibration. The two NH bonds of the NH₂ group appear to be non-equivalent as the NH₂ stretching frequencies and do not satisfy the empirical relation proposed for the two equivalent NH bonds of the NH₂ group due to their involvement and interaction with one hydrogen bond and the other adjacent oxygen atom of the parent molecule. The planar and nonplanar bending modes due to the C₄?O₈ bond are expected with have lower magnitude compared with those due to the C₂?O₇ bond.     

Theory of coherent phenomena and fundamentals in nuclear resonant scattering

Muonic X-ray cascades in B, C, N, O and Ne following muonic atom formation in B₂H₆, CH₄, C₂H₆, C₄H₁₀, N₂, O₂ and Ne were investigated. The densities of the different target gases were low enough to prevent any contact of the atom or molecule on which the formation takes place with surrounding atoms or molecules during the cascade. Molecular effects of the capturing molecule are clearly seen. The observed transition yields could be reproduced by variation of only two parameters in a cascade calculation: the number of initially available electrons and the muon angular momentum distribution at the starting point of the calculation. By varying the number of electrons, the molecular effects could be described. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoluminescence of silicone oil treated by fluorocarbon plasma

We investigated photoluminescence characteristics of silicone oils treated by C2F6 and CHF3 plasma. The silicone oil treated by the C2F6 plasma emitted a white light mainly composed of 415 nm, 469 nm, and 554 nm emissions, while that treated by the CHF3 plasma emitted a pink light (415 nm). Fourier transformed infrared spectroscopy and Raman spectroscopy studies showed that the photoluminescence was correlated with the Si-C bond, the carbon-related defects and the oxygen vacancies. It was suggested that the light emitting at 554 nm was related to the Si-C bond and the carbon-related defects, while the pink emission at 415 nm was related to the oxygen vacancies.     

Ammonia adsorption on the C30B15N15 heterofullerene: DFT study of nuclear magnetic shielding and electric field gradient tensors of N and B nuclei

Ammonia adsorption on the external surface of C₃₀B₁₅N₁₅ heterofullerene was studied using density functional calculations. Three models of the ammonia-attached C₃₀B₁₅N₁₅ together with the perfect model were optimized at the B3LYP/6-31G^? level. The optimization process reveals that dramatic influences occurred for the geometrical structure of C₃₀B₁₅N₁₅ after ammonia adsorption; the B atom relaxes outwardly and consequently the heterofullerene distorts from the spherical form in the adsorption sites. The chemical shielding (CS) tensors and nuclear quadrupole coupling constants of B and N nuclei were calculated at the B3LYP/6-311G^** level. Our calculations reveal that the B atom is chemically bonded to NH₃ molecule. The B atom in the NH₃-attached form has the largest chemical shielding isotropic (CSI) value among the other boron nuclei. The C_Q parameters of B nuclei at the interaction sites are significantly decreased after ammonia adsorption.     

A DFT study on the formaldehyde (H2CO and (H2CO)2) monitoring using pristine B12N12 nanocluster

The interaction between formaldehyde monomer (H₂CO) as well as dimer ((H₂CO)₂) and pristine B₁₂N₁₂ nanocluster is investigated at B3LYP/6-311++G(d,p) level of theory. It is found that in contrary to the pristine boron nitride nanotube and nanosheet, formaldehyde adsorption induce considerable variation in the electronic properties of the B₁₂N₁₂ nanocluster. Also it is shown that the pristine B₁₂N₁₂ cluster could adsorb up to four monomer and three dimer of formaldehyde molecules in which the HOMO–LUMO gap decreased about 38–55%. Since the conductivity of the B₁₂N₁₂ nanocluster changes by the adsorption of formaldehyde molecules, the presence of this toxic gas could be detected. The Bader theory of atoms in molecules (AIM) is also applied to analyze the interaction of formaldehyde with nanocluster. It is suggested pristine B₁₂N₁₂ nanocluster could be a promising candidate for detecting formaldehyde molecule. The results indicate that B₁₂N₁₂ may be a promising chemical sensor for detection of formaldehyde molecule.     

Interaction of different types of nanocages (Al12N12, Al12P12, B12N12, Be12O12, Mg12O12, Si12C12 and C24) with HCN and ClCN: DFT,TD-DFT,QTAIM, and NBO calculations

In this work, the ability of different types of nanocages including Al₁₂N₁₂, Al₁₂P₁₂, Be₁₂O₁₂, B₁₂N₁₂, Si₁₂C₁₂, Mg₁₂O₁₂ and C₂₄ for the adsorption and detection of poisonous gases HCN and ClCN has been investigated, theoretically using the D3 dispersion corrected density functional theory (DFT-D3). The absorption spectra of HCN–nanocage and ClCN–nanocage complexes were calculated by the time-dependent density functional theory (TD-DFT) and compared with the calculated absorption spectrum of isolated nanocage to investigate the ability of nanocage for sensing of HCN and ClCN gases. It was found that the strongest interaction between HCN (ClCN) molecule and nanocage takes place when the molecule is adsorbed via its N atom on the surface of nanocage except for C₂₄. Also, it was shown that the Al₁₂N₁₂ is the best adsorbent for HCN and ClCN gases among the selected nanocages and Si₁₂C₁₂ is the best sensor for the detection of these gases using the electroconductivity and absorption spectroscopy techniques.     

Experimental estimation of the interaction parameter eO for ZnN

The use of dislocation measurements in Zinc single crystals reveals the presence of a yield point or flow stress corresponding to a Stages I–II (elastic to elastic-plastic) transition. This is commonly attributed to the presence of interstitials such as N. The mathematical models of Seeger and Dorn and Rajnak are used to evaluate certain microstructural parameters. These correlate favourably with experimental values. The excess work done over that to overcome the Peierls and frictional forces is then calculated per pair of pinning interstitials and equated to the energy of formation of a molecule of Zn₃N₂ which is believed to form. The results show that this assumption does lead to the stochiometry Zn₃N₂, thus verifying mathematical parameters from different equations using work-energy concepts. Use of the mechanical equivalent J also seems to indicate a favourable correlation for the stochiometry of Zn₃N₂, and for e_O as calculated calorimetrically.     

Tuning the electronic properties of single-walled SiC nanotubes by external electric field

The electronic properties of SiC nanotubes (SiCNTs) under external transverse electric field were investigated using density functional theory. The pristine SiCNTs were semiconductors with band-gaps of 2.03, 2.17 and 2.25 eV for (6,6), (8,8) and (10,10) SiCNTs, respectively. It was found the band gaps was reduced with the external transverse electric filed applied. The (8,8) and (10,10) SiCNTs changed from semiconductor to metals as the intensity of electric field reached 0.7 and 0.5 V/Å. The results indicate that the electronic properties of SiCNTs can be tuned by the transvers electric field with integrality of the nanotubes.     

First-principles study on the catalytic role of Ag in the oxygen adsorption of LaMnO3(0 0 1) surface

In the present paper, the catalytic role of Ag in the oxygen adsorption of LaMnO₃(0 0 1) surface has been theoretically investigated using first-principles calculations based on the density functional theory (DFT) and pseudopotential method. The O₂ adsorption energy is larger for the vertical adsorption and the covalent bond was formed between O₂ molecule and surface Mn. The calculation of electronic properties of interaction between Ag atom and LaMnO₃(0 0 1) surface demonstrates that the most stable position for Ag adsorption is hollow site. The O₂ adsorption energy dramatically increased from 0.298 eV to 1.108 eV due to Ag pre-adsorbed. It is Ag pre-adsorbed that facilitates O₂ adsorption on surface. The bond length and bond population of O₂ molecule indicate that Ag atom facilitates O₂ molecule dissociative adsorption. The Ag atom strengthens LaMnO₃(0 0 1) substrate activity and activity center was formed on surface, which enhances the electrocatalytic activity of LaMnO₃ as solid oxide fuel cells cathode material at low temperature.     

The Hartree-Fock exchange effect on the CO adsorption by the boron nitride nanocage

We studied the effect of Hartree-Fock (HF) exchange percentage of a density functional on the adsorption properties and electronic sensitivity of the B₁₂N₁₂ nanocluster to CO molecule. It was found that by an increase in the %HF, the LUMO level is nearly constant while the HOMO level is strongly stabilized, expanding the HOMO-LUMO gap (E_g). Also, the volume of the all structures decreased and the sensitivity of the B₁₂N₁₂ is slightly increased to CO molecule. For the pristine B₁₂N₁₂ cluster, the B66 and B64 bonds are about 1.43 and 1.49 Å at 10% HF, and 1.23 and 1.26 Å at 100% HF, respectively. The HF exchange between 10–20% may predict an accurate E_g for the B₁₂N₁₂ system. We concluded that functionals with a large %HF such as M06-HF, and M06-2X may significantly overestimate the E_g, and bond strength. We obtained a parabolic relationship between the %HF and the adsorption energy of CO molecule on the B₁₂N₁₂ cluster. Also, an increase in the %HF predicts a larger charge transfer from the CO molecule to the cage.     

Adsorption of H2S at Stone–Wales defects of graphene-like BC3: a computational study

We employed density functional theory to characterise H₂S adsorption, and dissociation on the pristine and Stone–Wales (SW) defected BC₃ graphenes. H₂S is predicted to be weakly adsorbed on the pristine graphene with the adsorption energy of about 7.11 kcal/mol. Two types of SW defects were generated by rotating a C–C bond (SW-CC) or a B–C bond (SW-BC) by about 90°. We predict that, in contrast to SW-BC, dehydrogenation of H₂S is energetically more favourable on the SW-CC compared to the associative adsorption. It is also found that SW-CC formation is more favourable than the formation of SW-BC. Molecular adsorption of H₂S on both of the SW defected sheets is more favourable than that on the pristine sheet. The preferable adsorption process on the SW-BC and SW-CC defected graphene sheets is via associative and dissociative mechanisms, respectively. Furthermore, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy gap of the SW-BC defected sheet is highly sensitive to the adsorption process which may be used for the detection of H₂S.     

Exohedral chemical functionalization of C48B6N6 with NH3: Binding energies and electronic structures of C48B6N6–(NH3)n=1−6

Theoretical study of exohedral chemical functionalization of C₄₈B₆N₆ with NH₃ molecules has been investigated using DFT. It was found that NH₃ molecule can be chemically adsorbed on boron sites of C₄₈B₆N₆, with a charge transfer from NH₃ to C₄₈B₆N₆. Adsorption energy and the quantity of electron charge transfer from latest adsorbed ammonia to C₄₈B₆N₆ decreased with increasing in the adsorbed NH₃ molecules. Despite the strong adsorption energies, electronic properties of C₄₈B₆N₆ is preserved after modification(s) with NH₃ molecule(s) and chemical modification of C₄₈B₆N₆ with NH₃ molecules can be viewed as some kind of safe modification.     

Frontier orbitals analysis and density-functional energetics for metal-substituted fullerene C58Fe2

The formation mechanism, geometric structures, and electronic properties of a metal-substituted fullerene C₅₈Fe₂ have been studied using frontier orbital theory (FOT) and density functional theory (DFT). FOT predicts that two Fe atoms prefer to substitute the two carbons of a [6,6] double bond of C₆₀ yielding a structure denoted as C₅₈Fe₂-3, which is different from the two equivalent substitution sites, i.e., the sites on the opposite of C₆₀ cage or in the nearest neighboring sites of a pentagonal ring for C₅₈X₂ (X=N and B), and also different from the cross sites of a hexagonal ring for C₅₈Si₂. Five possible structures of C₅₈Fe₂ are optimized using DFT to see whether FOT works. The DFT calculations support the prediction of FOT. The Mulliken charge of Fe atom in C₅₈Fe₂-3 shows that the two Fe atoms of C₅₈Fe₂-3 lose 0.70 electron to the carbons of the cage, and the net spin populations of Fe atom indicate that each Fe atom has 1.11 μ_B magnetic moments, while each of the four nearest neighboring carbons has magnetic moments. Thus, the two Fe atoms have ferromagnetic interaction with each other, and have weak antiferromagnetic interaction with their four nearest neighboring carbons, leaving 2.0 μ_B magnetic moments for the molecule.