Characterizations of B and N heteroatoms as substitutional doping on structure,stability, and aromaticity of novel heterofullerenes evolved from the smallest fullerene cage C20: a density functional theory perspective

The structural stabilities and electronic properties of C₂₀ fullerene and some its incorporated boron and nitrogen derivatives are probed at B3LYP/AUG‐cc‐pVTZ level of theory. According to density functional theory results, the topology of inserted B or N heteroatoms in [20]‐fullerene perturbs strongly the stability, energy, geometry, charge, polarity, nucleus‐independent chemical shifts, aromaticity, and highest‐occupied molecular orbital and lowest‐unoccupied molecular orbital (HOMO–LUMO) gap of the resulting heterofullerenes. Vibrational frequency (υ_min) calculations show that except N₁₀C₁₀, all other B_bN_nC_{20‐(b + n)} heterofullerenes with b, and n = 0, 4, 5, 8, and 10 are true minima. The calculated band gaps (?E_HOMO–LUMO) of B₈C₁₂, and N₈C₁₂ (2.86 eV), show them the most stable heterofullerenes against electronic excitations. While 10 B substituting in equatorial position increase the conductivity of B₁₀C₁₀ through decreasing its band gaps, 10 N doping in equatorial position enhance stability of N₁₀C₁₀ against electronic excitations via increasing its band gaps. High natural bond orbital and Mulliken charge transfer on the surfaces of B atoms, especially B₅N₅C₁₀with five B–N bonds in the equatorial position, provokes further investigation on its possible application for hydrogen storage. Nucleus‐independent chemical shift values show that B₅N₅C₁₀ is the most aromatic species. The calculated heat of atomization per carbon (ΔH_at/C) of B₈C₁₂ shows it the most thermodynamic stable heterofullerenes of each. Copyright © 2016 John Wiley & Sons, Ltd.     

Substituent effects on the 13C NMR chemical shifts of the imine carbon in N‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines

Long‐range electronic substituent effects were targeted using the substituent dependence of δ_C(C═N), and specific cross‐interactions were explored extendedly. A wide set of N‐(4‐X–benzylidene)‐4‐(4‐Y–styryl) anilines, p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y (X = NMe₂, OMe, Me, H, Cl, F, CN, or NO₂; Y = NMe₂, OMe, Me, H, Cl, or CN) were prepared for this study, and their ¹³C NMR chemical shifts δ_C(C═N) of C═N bonds were measured. The results show that both the inductive and resonance effects of the substituents Y on the δ_C(C═N) of p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y are less than those of the substituents Y in p‐X–C₆H₄CH═NC₆H₄–p‐Y. Moreover, the sensitivity of the electronic character of the C═N function to electron donation/electron withdrawal by the substituent X or Y attenuates as the length of the conjugated chain is elongated. It was confirmed that the substituent cross‐interaction is an important factor influencing δ_C(C═N), not only when both X and Y are varied but also when either X or Y is fixed. The long‐range transmission of the specific cross‐interaction effects on δ_C(C═N) decreases with increasing conjugated distance between X and Y. The results of this study suggest that there is a long‐range transmission of the substituent effects in p‐X–C₆H₄CH═NC₆H₄CH═CHC₆H₄–p‐Y. Copyright © 2012 John Wiley & Sons, Ltd.     

Substituent effect on structure,stability, and aromaticity of novel BnNmC20–(n+m) heterofullerenes

We are focusing our calculations on the structural stabilities and electronic properties of 26 novel B_nN_mC_20–(n+m) heterofullerenes, with n, m = 1 ? 5, at B3LYP/6‐311++G** and B3LYP/AUG‐cc‐pVTZ levels of theory. Vibrational frequency calculations on C₂₀ and its analogues show that except B₂N₂C₁₆ (1) and B₂N₂C₁₆ (2), all other heterofullerenes are true minima. The heats of atomization energies, binding energy, band gaps (ΔE_HOMO‐LUMO), aromaticity, nucleus‐independent chemical shifts, thermodynamic stability, kinetic stability against electronic excitation, binding energy as a stability criterion of different configurations, geometrical parameters, conformational structures, conductivity, charge transfer, and possibility for hydrogen storage of these heterofullerenes strongly depend on their number of heteroatoms, topology, filling patterns, and locations as well as “B‐site and N‐site attachments.” B₅N₅C₁₀ contains 5 alternating boron and nitrogen atoms in the equatorial position. It is predicted to be thermodynamically and kinetically the most stable against electron excites. Thus, it is energetically favorable and its electronic properties as well as stabilities make it perhaps a good candidate for an experimental investigation and testing verification.     

Annealing effects on the bonding structures, optical and mechanical properties for radio frequency reactive sputtered germanium carbide films

The effects of thermal annealing in vacuum on the bonding structures, optical and mechanical properties for germanium carbide (Ge_1−xC_x) thin films, deposited by radio frequency (RF) reactive sputtering of pure Ge(1 1 1) target in a CH₄/Ar mixture discharge, are investigated. We find that there are no significant changes in the bonding structure of the films annealed below 300 °C. The fraction of Ge-H bonds for the film annealed at temperatures (T_a) above 300 °C decreases, whereas that of C-H bonds show a decrease only when T_a exceeds 400 °C. The out-diffusion of hydrogen promotes the formation of Ge-C bonds at T_a above 400 °C and thus leads to a substantial increase in the compressive stress and hardness for the film. The refractive indices and optical gaps for Ge_1−xC_x films are almost constant against T_a, which can be ascribed to the unchanged ratios of Ge/C and sp²-C/sp³-C concentrations. Furthermore, we also find that the excellent optical transmission for an antireflection Ge_1−xC_x double-layer film on ZnS substrate is still maintained after annealing at 700 °C.     

Cyclacenes and short zigzag nanotubes with alternanting Ge―C bonds: theoretical impacts of Ge on the ground state,strain, and band gap

[6]_n Cyclacenes and short zigzag [6]_n carbon nanotubes (n = 5–10) have unstable singlet open‐shell (S_os) ground states. We have boosted their stability by implementing altering Ge―C bonds that acquire S_cs ground states with larger band gap (ΔE_LUMO–HOMO) at B3LYP and BPW91 levels of theory. Fascinatingly, homodesmic calculations indicated release of almost two folds of strain energy upon substitution of germanium for carbon. This may turn the green lights for synthesis of germanium–carbon cyclacenes and short zigzag nanotubes. Copyright © 2014 John Wiley & Sons, Ltd.     

Double scattering effect and its application in Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript> solid solution diagnosis

The effect of double scattering of Ar⁺ ions from the surface of C, Al, Si, Ti, Ge, and In targets is studied by the method of slow scattered ion spectroscopy. Based on this effect, a technique to estimate the cluster phase of germanium atoms in the Si_1−x Ge_x solid solution with a small (5–10%) content of germanium is suggested.     

Electronic structure,mechanical and optical properties of ternary semiconductors Si1-xGexC (X = 0, 0.25, 0.50, 0.75, 1)

The electronic structure of silicon carbide with increasing germanium content have been examined using first principles calculations based on density functional theory. The structural stability is analysed between two different phases, namely, cubic zinc blende and hexagonal phases. The zinc blende structure is found to be the stable one for all the Si_1-xGe_xC semiconducting carbides at normal pressure. Effect of substitution of Ge for Si in SiC on electronic and mechanical properties is studied. It is observed that cubic SiC is a semiconductor with the band gap value 1.243?eV. The band gap value of SiC is increased due to the substitution of Ge and the band gap values of Si _0.75 Ge _0.25 C, Si _0.50 Ge _0.50 C, Si _0.25 Ge _0.75 C and GeC are 1.322 eV, 1.413 eV, 1.574 eV and 1.657?eV respectively. As the pressure is increased, it is found that the energy gap gets decreased for Si_1-x Ge_xC (X?=?0, 0.25, 0.50, 0.75, 1). The elastic constants satisfy the Born – Huang elastic stability criteria. The bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also calculated and compared with the other available results.     

Lattice dynamics investigations of SiGe core-shell nanowires

We present results from lattice dynamics calculations on the phonon modes and specific heat of SiGe core-shell nanowires. The results show that phonon dispersion relation of SiGe nanowires consists of four acoustic branches. The frequency of the first optical mode at Γ point shifts to low frequency as the Ge concentration is increasing. There are three strong peaks in the spectra of density of states. The peaks at 9.0 THz and 15.0 THz can be attributed to the high frequency Ge-Ge and Si-Si bond vibration. The broad peak around 3.0 THz of pure silicon nanowire corresponds to the transverse acoustic branch of bulk silicon. Moreover, specific heat of SiGe nanowires increases (decreases) with the increase of the concentration x at low (high) temperature. The specific heat at 300 K can be fitted by C _V = x ² C _Ge + (1 − x)C _Si, where C _Ge and C _Si are specific heat of pure germanium and silicon nanowires respectively.     

Structural,electronic and magnetic properties of C59Ir,C58Ir2, and C69Ir heterofullerene nano-cages: first principles study

We studied the structural, electronic, and magnetic properties of C₅₉Ir, C₅₈Ir_2, and C₆₉Ir heterofullerenes by employing density functional theory and the generalized gradient approximation. There are six distinct isomers of C₅₈Ir₂ with high probability to form stable structures. The most stable structure of the C₆₉Ir heterofullerene was investigated by comparing the iridium binding energies at the different atomic sites on the D5h C₇₀ cage. There is a strong hybridization between the atomic orbitals of the iridium and those of the carbon atoms, leading to the spin quenching of the iridium atoms in the most stable C₅₈Ir₂ heterofullerene.     

Growth features at competing nucleation of quantum dots and nanopits in Si-Ge-C ternary system

The continuum elasticity model is applied to quantitatively investigate the growth features and nucleation mechanism of quantum dots (QDs), nanopits, and joint QDs-nanopits structures in SiGeC ternary systems. We determined the critical concentrations of carbon, x = 0.08 for Si_1?x C _x and y = 0.0033 for Si_1?x?y Ge _x C _y at the concentration of germanium x = 0.467, when the growth mode changes from, respectively, nucleation of QDs to formation of nanopits. We have shown that the increase in the carbon concentration in a SiC binary system and of germanium concentration in a SiGeC ternary system leads to a decrease in the critical size of QD. The free energy of mixing for Si_1?x?y Ge _x C _y ternary system was calculated and studied and its 3D sketch plotted. It is demonstrated that incorporation of carbon into Si and Ge up to background impurity concentration is energetically preferable, which explains the experimentally detected ??contamination?? of silicon and germanium crystals by carbon during the growth in the graphite crucible.     

Radiation-Induced Paramagnetic Centers in the Glasses Of CaO-Ga2O3-GeO2 System

The X-band (v ? 9.4 GHz) ESR spectra of the UV-, X-, and γ-irradiated glasses of CaO-Ga₂O₃-GeO₂ system with compositions similar to Ca₃Ga₂Ge₃O₁₂, Ca₃Ga₂Ge₄O₁₄, and Ca₃Ga₂O₆ crystals have been investigated at 300 and 77 K. It was shown that X- and γ-irradiation of the Ge-contained glasses induce simultaneously electron and hole paramagnetic centers, stable at room temperature. The UV-irradiation of Ge-contained glasses leads to the generation of stable centers of electron type, whereas the X- and γ-irradiation of glasses with Ca₃Ga₂O₆ composition induces stable hole centers, exclusively. The electron centers are assigned to E′ (Ge) centers with different local environments. ESR spectrum of hole centers is ascribed to O^? centers, localized on non-bridging oxygen of the glass network. The E′ (Ge) and O^? centers show high thermal stability in the CaO-Ga₂O₃-GeO₂ glass network. The obtained ESR spectra are described by spin Hamiltonian with g-factor of axial and rhombic symmetry for the E′ (Ge) and O^? centers, respectively. The nature, electron structure and some formation peculiarities of the radiation defects in CaO-Ga₂O₃-GeO₂ glasses are discussed in comparison with other glasses as well as Ca₃Ga₂Ge₃O₁₂ and Ca₃Ga₂Ge₄O₁₄ crystals.     

Self-assembling of C and Sn in Ge

Self-assembling of isoelectronic C and Sn impurities in Ge is predicted. The formation of the 1C4Sn tetrahedral cells is thermodynamically profitable in Ge-rich C_xSn_yGe_1−x−y (4x<y) alloys in the ultra dilute C impurity limit with 1×10^-8x1×10^-3. The concentrations of Sn atoms when all C atoms are surrounded only by Sn atoms are estimated for the lower molecular beam epitaxy, intermediate annealing and higher bulk crystallization temperatures. The origin of this phenomenon is a considerable decrease of the strain energy after self-assembling. The same self-assembling in Si is thermodynamically non-profitable due to the large cohesive energy of Si–C chemical bonds.     

Specific features of plastic relaxation of a metastable Ge x Si1 − x layer buried between a silicon substrate and a relaxed germanium layer

Heterostructures Ge/Ge _x Si_{1 ? x} /Si(001) grown by molecular beam epitaxy have been investigated using atomic scale high-resolution electron microscopy. A germanium film (with a thickness of 0.5–1.0 μm) grown at a temperature of 500°C is completely relaxed. An intermediate Ge_0.5Si_0.5 layer remains in a strained metastable state, even though its thickness is 2–4 times larger than the critical value for the introduction of 60° misfit dislocations. It is assumed that the Ge/GeSi interface is a barrier for the penetration of dislocations from a relaxed Ge layer into the GeSi layer. This barrier is overcome during annealing of the heterostructures for 30 min at a temperature of 700°C, after which dislocation networks having different degrees of ordering and consisting predominantly of edge misfit dislocations are observed in the Ge/GeSi and GeSi/Si(001) heteroboundaries.     

Theoretical study on C100 fullerenes and C96X4 (X=N,P, B,Si)

The geometrical structures and electronic properties of six fullerene isomers of C₁₀₀ were studied at the HF/6-31G^? and B3LYP/6-31G^? levels, respectively. The results of the fully optimized calculations show that three C₁₀₀ isomers 449:D₂, 425:C₁ and 442:C₂ are near isoenergetic isomers. The energies and properties of C₁₀₀ hexaanions were calculated. The C₁₀₀^6? (450:D₅) isomer is predicted to be the most stable isomer at the B3LYP/6-31G^? level, and the C₁₀₀^6? (449:D₂) isomer is 44.1 kcal/mol higher in energy. The heterofullerenes C₉₆X₄ (X=N, P, B, Si) formed from the initial C₁₀₀ (449:D₂) have also been investigated at the B3LYP/6-31G^? level. The HOMO–LUMO gaps and aromaticities show that the replacement of fullerene carbon atoms with four heteroatoms can enhance the electronic stabilization of C₁₀₀ (449:D₂).     

Influence of the germanium deposition rate on the growth and Photoluminescence of Ge(Si)/Si(001) self-assembled islands

The growth and photoluminescence of Ge(Si)/Si(001) self-assembled islands are investigated over a wide range of germanium deposition rates v_Ge = 0.1–0.75 Å/s at a constant growth temperature T _g = 600°C. Examination of the surface of the grown structures with an atomic force microscope revealed that, for all the germanium deposition rates used in the experiments, the dominant island species are dome islands. It is found that an increase in the deposition rate v_Ge leads to a decrease in the lateral size of the self-assembled islands and an increase in their surface density. The decrease in the lateral size is associated both with the increase in the germanium content in the self-assembled islands and with the increase in the fraction of the surface occupied by these islands. The observed shift in the position of the photoluminescence peak toward the low-energy range is also explained by the increase in the germanium content in the islands with an increase in the deposition rate v_Ge.     

Effect of germanium addition on the physical properties of Se–Te glassy semiconductors

The effect of germanium addition on the physical properties, i.e. density, molar volume, compactness, number of lone-pair electrons, average coordination number, heat of atomization, mean bond energy, cohesive energy and glass-transition temperature, of (Se₈₀Te₂₀)_{100? x} Ge _x (x = 0, 2, 4, 6) bulk glassy alloys was investigated. The density of the glassy alloys is found to decrease with increasing Ge content. The molar volume and compactness of the structure of the glass were determined from the measured density. The mean bond energy is proportional to the glass-transition temperature. The cohesive energy of the samples has been calculated using a chemical bond approach and is correlated with an increase in the optical energy gap with increase in the Ge content. The heat of atomization was also calculated and correlated with the optical energy gap. The glass-transition temperature has been estimated using different methods and is found to increase with an increase of Ge content.     

Silicon-germanium nanostructures with high germanium concentration

The impact of nucleation conditions on the quality of epitaxial layers of germanium and GeSi alloys containing a high Ge mole fraction grown on (100) silicon substrates using electron-beam epitaxy is considered. The Ge_xS_{1 -x}/Ge superlattices are grown on a Ge_ySi_{1 -y} (x > y) relaxed buffer layer. X-ray diffractometry, atomic force microscopy and Auger spectroscopy are the main techniques used to study the properties of the grown structures.     

EPR study of impurities in Ge-S glasses

In this paper the results of EPR investigation of Mn, Ag and Cu-doped Ge-S glasses are presented. In Ag and Cu-doped Ge₄₀S₆₀ glasses a new signal was found which is tightly associated with concentration of Ag and Cu. Some valuable information on the structure and the character of bonds in the glass-forming regions of Ge-S system were obtained. We have identified Ge [4 S] and Ge [2 Ge 2 S] tetrahedra as structural units in second glass-forming region of Ge-S system. In the first glass-forming region Ge [4 S] tetrahedra were identified as structural units, however, the nearest neighbours Ge-S distances are shorter and the character of bonds in Ge₂₅S₇₅ glass is much more covalent than in Ge₄₀S₆₀ one.     

Study of the component distribution in Si/GexSi1−x /Si heterostructures grown by molecular beam epitaxy

This paper reports on a study of the depth profile of components in GeSi heterostructures grown on low-temperature silicon (LTSi: T _gr ～ 350–400° C) and porous silicon by molecular-beam epitaxy. An excess Ge concentration was found by Auger electron spectroscopy depth profiling at the Ge_xSi_1?x /LTSi interface, which decreased in all samples subjected to annealing. The Ge diffusion activation energy was calculated to be E _a ≈ 1.6 eV in this case. An enhanced Ge concentration was also detected by x-ray photoelectron spectroscopy at the Si cap surface. Possible reasons for the surface enrichment of the silicon layer and of the Ge_xSi_1?x film interface by germanium are considered, and the relation between the component distribution and the structural features of plastically strain-relieved layers are discussed.     

Stable silicon‐centered localized singlet 1,3‐diradicals XSi(GeY2)2SiX: theoretical predictions

Some localized singlet 1,3‐σ‐diradicals, XSi(GeY₂)₂SiX, (X = H, CH₃, SiH₃, C(CH₃)₃, NH₂ for X = F; Y = H, CH₃, OH, NH₂, SiH₃ for X = H) are theoretically designed by the orbital phase theory, the density functional theory (DFT) calculations , the second order Møller–Plesset perturbation theory (MP2), and the complete active space self‐consistent field (CASSCF) methods. The silicon‐centered singlet diradicals are more stable than the lowest triplets and than the bicylic σ‐bonded isomers if the isomers exist. The most stable singlet diradicals are not the π‐type diradicals, but the σ‐type diradicals where the radicals interact with each other through the Si? Ge bonds in the four‐membered rings. Copyright © 2007 John Wiley & Sons, Ltd.