Molecular dynamics study of hydrogenated silicon clusters at high temperatures

This paper reports on a study of the stability of silicon clusters of intermediate size at a high temperature. The temperature dependence of the physicochemical properties of 60- and 73-atom silicon nanoparticles are investigated using the molecular dynamics method. The 73-atom particles have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene. They are surrounded by a ‘coat’ from 60 atoms of hydrogen. The nanoassembled particle at the presence of a hydrogen ‘coat’ has the most stable number (close to four) of Si–Si bonds per atom. The structure and kinetic properties of a hollow single-layer fullerene-structured Si₆₀ cluster are considered in the temperature range 10 K ≤ T ≤ 1760 K. Five series of calculations are conducted, with a simulation of several media inside and outside the Si₆₀ cluster, specifically, the vacuum and interior spaces filled with 30 and 60 hydrogen atoms with and without the exterior hydrogen environment of 60 atoms. Fullerene surrounded by a hydrogen ‘coat’ and containing 60 hydrogen atoms in the interior space has a higher stability. Such clusters have smaller self-diffusion coefficients at high temperatures. The fullerene stabilized with hydrogen is stable to the formation of linear atomic chains up to the temperatures 270–280 K.     

Identification of substitutional and interstitial Fe in 6H-SiC

Iron impurities on interstitial (Fe_i) and substitutional sites (Fe_S) in SiC have been detected by ⁵⁷Fe emission Mössbauer spectroscopy following implantation of radioactive ⁵⁷Mn⁺ parent ions. At temperatures <900 K two Fe_i species are found, assigned to quasi-tetrahedral interstitial sites surrounded by, respectively, four C (Fe_i,C) or Si atoms (Fe_i,Si). Above 900 K, the Fe_i,Si site is proposed to “transform” into the Fe_i,C site by a single Fe_i jump during the lifetime of the Mössbauer state (T _1/2?=?100 ns). Fe_i,C and substitutional Fe_S sites are stable up to >1,070 K.     

Structure and stability of a silicon cluster on sequential doping with carbon atoms

SiC is a highly stable material in bulk. On the other hand, alloys of silicon and carbon at nanoscale length are interesting from both technological as well fundamental view point and are being currently synthesized by various experimental groups (Truong et. al., 2015 [26]). In the present work, we identify a well-known silicon cluster viz., Si₁₀ and dope it sequentially with carbon atoms. The evolution of electronic structure (spin state and the structural properties) on doping, the charge redistribution and structural properties are analyzed. It is interesting to note that the ground state SiC clusters prefer to be in the lowest spin state. Further, it is seen that carbon atoms are the electron rich centres while silicon atoms are electron deficient in every SiC alloy cluster. The carbon–carbon bond lengths in alloy clusters are equivalent to those seen in fullerene molecules. Interestingly, the carbon atoms tend to aggregate together with silicon atoms surrounding them by donating the charge. As a consequence, very few Si–Si bonds are noted with increasing concentrations of C atoms in a SiC alloy. Physical and chemical stability of doped clusters is studied by carrying out finite temperature behaviour and adsorbing O₂ molecule on Si₉C and Si₈C₂ clusters, respectively.     

Mössbauer study of Fe in 3C-SiC following 57Mn implantation

Our investigations on substitutional and interstitial Fe in the group IV semiconductors, from ⁵⁷Fe Mössbauer measurements following ⁵⁷Mn implantation, have been continued with investigations in 3C-SiC. Mössbauer spectra were collected after implantation and measurement at temperatures from 300 to 905 K. Following comparison with Mössbauer parameters for Fe in Si, diamond and Ge, four Fe species are identified: two due to Fe in tetrahedral interstitial sites surrounded, respectively, by four C atoms (Fe_i.C) or four Si atoms (Fe_i,Si) and two to Fe in (or close to) defect free or implantation damaged substitutional sites. An annealing stage at 300–500 K is evident. Above 600 K the Fe_i,Si fraction decreases markedly, reaching close to zero intensity at 905 K. This is accompanied by a corresponding increase in the Fe_i,C fraction.     

Vacancy generation in silicon in the temperature range 100–633 K under electron irradiation

The processes of radiation defect formation in Si with 1 MeV electron irradiation in the temperature range 100–633 K have been investigated. It is established that the generation efficiency of vacancies λ_V increases with temperature, then starts to saturate at temperatures of 250 K and finally stays constant at T>300 K. It is shown that at high temperatures, the λ_V dependence can be caused by the additional scattering of “hot” interstitial atoms on acoustical and optical phonons, the numbers of which increase with the temperature. An explanation, based on the creation of quasi-molecule of “hot” interstitial and lattice atoms, is proposed.     

Tight-binding study of Si2Cn (n = 3 to 42) fullerene-like or nanodiamonds microclusters: are Si atoms isolated or adjacent?

We studied in tight-binding approximation involving sp^ν hybridization (ν=2,3), some Si₂C_n (n=3 to 42) microclusters. We then investigated, on one hand, fragments of fullerene-like structures (sp²), and on the other hand, nanodiamonds (sp³) of adamantane-type or a 44-atom nanodiamond (with 2 inner atoms which are assumed to play the role of bulk atoms). We compared the stabilities, i.e. the electronic energies of these clusters, according to the various positions of the 2 Si atoms. Results are very different in the two kinds of hybridization. Besides, they can be analysed according to two different points of view: either the clusters are considered as small particles with limited sizes, or they are assumed to be used as models in order to simulate the Si-atom behaviour in very larger systems. In sp² hybridization (fullerene-like geometries), the most stable isomer is always encountered when the 2 Si atoms build a Si₂ group, and this result holds for both viewpoints quoted above. Conversely, in sp³ hybridization (nanodiamonds), since Si atoms “prefer” sites having the minimum connectivity, they are never found in adjacent sites. We see that with a simple and fast computational method we can explain an experimental fact which is very interesting such as the relative position of two heteroatoms in the cluster. This enhances the generality and the fecondity in the tight binding approximation due essentially to the link between this model and the graph theory, link based on the topology of the clusters.     

Structure of self-implanted silicon annealed under enhanced hydrostatic pressure

Implantation of any ions at a sufficiently high dose and energy (E) into single-crystalline Si leads to the creation of amorphous Si (aSi), with damages peaking near the projected range (R _p) of implanted species. Enhanced hydrostatic pressure (HP) at a high temperature (HT) influences the recrystallization of aSi. The structure of self-implanted Czochralski silicon (Si⁺ dose, D=2×10¹⁶ cm^?2, E=150 keV, R _p=0.22 μm) processed for 5 h at 1400 or 1520 K under HPs up to 1.45 GPa was investigated by X-ray, secondary ion mass spectrometry and photoluminescence methods. The implantation of Si produces vacancies (V) and self-interstitials (Si_i). Vacancies and Si_is form complex defects at HT–HP, also with contaminants (e.g. oxygen, always present in Czochralski silicon). The mobility and recombination of V and Si_i as well as the kinetics of recrystallization are affected by HP, thus processing at HT–HP affects the recovery of aSi.     

Fullerene model of silicon nanofibers

A computational (quantum-chemical) experiment has been performed on constructing a silicon nanofiber by gas-phase deposition. A model of two-step polymerization has been proposed in which a fullerene Si₆₀ molecule serves as the main structural unit. The formation of oligomers of the molecule containing from three to eight molecular units explains the discrete values of the fiber width observed experimentally. The formation of a “rouleau” of oligomers leads to fiber growth in terms of length. It is shown that both steps are favorable in energy with the decisive superiority of a high-spin state, which poses the question of considering silicon nanofibers to be molecular magnets.     

A computational investigation of electronic structure as well as 19F and 29Si chemical shielding tensors in the fluorinated silicon fullerenes SinFn (n≤60)

Density functional theory (DFT) calculations are performed to investigate the electronic features of the structures of fluorinated polysilanes Si_nF_n (n=4, 6, 8, 10, 12, 20, 24, 28, 30, 32, 36, 50, and 60). Among all of these fluorinated polysilanes, Si₂₀F₂₀ has the highest binding energy and, thus, stability. The binding energy then shows a very slow (monotonically) decrease as the size of the fluorinated silicon fullerene n≥20 increases which can be related to an increase in fluorine–fluorine repulsion. Following an irregular pattern, the HOMO–LUMO energy gap strongly depends on the size of the cage. On the other hand, ²⁹Si CS parameters detect equivalent electronic environment for silicon atoms within Si_nH_n polysilanes with n≤20 while ²⁹Si NMR pattern indicates a few separated peaks for Si_nH_n polysilanes with n≥20. Seeking correlation between these peaks and local structures around silicon sites, Si_α, Si_β, Si_γ observed in these models shows that δ_iso(Si_γ)<δ_iso(Si_β) <δ_iso(Si_α). Obtaining similar values (458.8–478.7 ppm) of ¹⁹F calculated chemical shieldings for all the fluorinated polysilanes means the same tendency of the silicon atoms on the surfaces of all cages for contribution to chemical bonding with fluorine atoms.     

Theoretical study on the mono and multiply oxygenated Si60H60 fullerene

We have applied density functional theory (DFT) calculations to study the structures, stabilities, electronic and magnetic properties of mono and multiply oxygenated Si₆₀H₆₀ fullerenes (Si₆₀H_60–2nO_n, n = 1, 3, 6, 9, 10, 12, 18, 20, 21, 27 and 30). DFT results show that rearrangement between the closed [6,6] and [5,6] isomers of Si₆₀H₅₈O follows a two-step pathway involving an intermediate and two transition states. Preserving the C₃ symmetry in the cage structure, extra epoxidation of Si₆₀H₆₀ has been accomplished. Based on our results, formation energies per oxygen atom for the multiple additions of oxygen atoms on Si₆₀H₆₀ cage are positive (endothermic character), and increase with the increasing of the number of oxygen atoms. In general, the oxygenation of Si₆₀H₆₀ cage leads to an increase in the electrophilicity of the Si₆₀H_60–2nO_n oxides. The oxygenation of Si–Si bonds not only introduces a substantial broadening of the NMR pattern but also yield individual peaks, indicating different electrostatic environments of silicon nuclei in the Si₆₀H_60–2nO_n oxides.     

Formation of binary clusters by molecular ion irradiation

In the present study, we have observed silicon–carbon cluster ions (Si_nC_m⁺) emitted from a Si(1 0 0) surface under irradiation of reactive molecular ions, such as C₆F₅⁺, at 4 keV, 1 μA/cm². The cluster Si_n up to n=8 and “binary” cluster Si_nC up to n=6 are clearly detected for the C₆F₅⁺ irradiation. Stoichiometric clusters (Si_nC_m n=m) except SiC⁺ and other binary clusters which contain more than two carbon atoms (m≥2) were scarcely observed. The observed clusters show a yield alternation between odd and even n. The intensities of Si₄, Si₆ and Si₅C clusters are relatively higher than those of the neighboring clusters. In the case of Si₅C, it is considered that doped carbon atom acts as silicon atom. These results imply that the recombination through the nascent cluster emission and subsequent decomposition takes place during the cluster formation.     

Si–Si bond as a deep trap for electrons and holes in silicon nitride

A two-stage model of the capture of electrons and holes in traps in amorphous silicon nitride Si₃N₄ has been proposed. The electronic structure of a “Si–Si bond” intrinsic defect in Si₃N₄ has been calculated in the tight-binding approximation without fitting parameters. The properties of the Si–Si bond such as a giant cross section for capture of electrons and holes and a giant lifetime of trapped carriers have been explained. It has been shown that the Si–Si bond in the neutral state gives shallow levels near the bottom of the conduction band and the top of the valence band, which have a large cross section for capture. The capture of an electron or a hole on this bond is accompanied by the shift of shallow levels by 1.4–1.5 eV to the band gap owing to the polaron effect and a change in the localization region of valence electrons of atoms of the Si–Si bond. The calculations have been proposed with a new method for parameterizing the matrix elements of the tightbinding Hamiltonian taking into account a change in the localization region of valence electrons of an isolated atom incorporated into a solid.     

A density functional theoretic study of novel silicon-carbon fullerene-like nanostructures: Si40C20, Si60C20, Si36C24, and Si60C24

Fullerene-like silicon nanostructures with twenty and twenty-four carbon atoms on the surface of the Si₆₀ cage by substitution, as well as inside the cage at various orientations have been studied within the generalized gradient approximation to density functional theory. Full geometry optimizations have been performed without any symmetry constraints using the Gaussian 03 suite of programs and the LANL2DZ basis set. Thus, for the silicon atom, the Hay-Wadt pseudopotential with the associated basis set is used for the core electrons and the valence electrons, respectively. For the carbon atom, the Dunning/Huzinaga double zeta basis set is employed. Electronic and geometric properties of these nanostructures are presented and discussed in detail. Optimized silicon-carbon fullerene like nanostructures are found to have increased stability compared to the bare Si₆₀ cage and the stability depends on the number and the orientation of carbon atoms, as well as on the nature of silicon-carbon and carbon-carbon bonding.     

Si–Si optical phonon behavior in localized Si clusters of Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloy nanocrystals

Raman spectra acquired from Si _x Ge_1−x -nanocrystal-embedded SiO₂ films show dependence of the Si–Si optical phonon frequency on Si content. The frequency upshifts, and peak intensity increases as the silicon concentration increases. For a given Si content, the frequency remains unchanged with annealing temperature. Spectral analysis and density functional theory calculation reveal that the optical Si–Si phonon is related to the formation of localized Si clusters surrounded by Si/Ge atomic layers in the Si _x Ge_1−x nanocrystals and the intensity enhancement arises from the larger cluster size. The synergetic effect of surface tensile stress and phonon confinement determines the Si–Si optical phonon behavior.     

Electronic properties of assembled islands of hydrogen-saturated silicon clusters on Si(111)-(7 x 7) surfaces studied by scanning tunneling spectroscopy

We present results of scanning tunneling spectroscopy (STS) measurements of hydrogen-saturated silicon clusters islands formed on Si(111)-( 7×7) surfaces. Nanometer-size islands of Si₆H₁₂ with a height of 0.2-4 nm were assembled with a scanning tunneling microscope (STM) using a tip-to-sample voltage larger than 3 V. STS spectra of Si₆H₁₂ cluster islands show characteristic peaks originating in resonance tunneling through discrete states of the clusters. The peak positions change little with island height, while the peak width shows a tendency of narrowing for the tall islands. The peak narrowing is interpreted as increase of lifetime of electron trapped at the cluster states. The lifetime was as short as 10^-13 s resulting from interaction with the dangling bonds of surface atoms, which prevents charge accumulation at the cluster islands. Received 30 November 2000     

Formation of interfacial iron silicides on the oxidized silicon surface during solid-phase epitaxy

The early stages of iron silicide formation in the Fe/SiO _x /Si(100) ternary system during solid-phase epitaxy are studied by high-resolution (～100 meV) photoelectron spectroscopy using synchrotron radiation. The spectra of core and valence electrons taken after a number of isochronous heat treatments of the samples at 750°C are analyzed. It is found that the solid-phase reaction between Fe and Si atoms proceeds in the vicinity of the SiO _x /Si interface, which metal atoms reach when deposited on the sample surface at room temperature. Iron silicide starts forming at 60°C. Solid-phase synthesis is shown to proceed in two stages: the formation of the metastable FeSi interfacial phase with a CsCl-like structure and the formation of the stable β-FeSi₂ phase. During annealing, structural modification of the silicon oxide occurs, which shows up in the growth of the Si⁺⁴ peaks and attenuation of the Si⁺² peaks.     

Influence of screw extrusion on the atomic order in constructional steel

The atomic order in low-carbon steel strained by screw extrusion making the microstructure finer and changing the phase ratio in the material is studied by X-ray diffraction analysis. The steel is found to consist of multiscale structural fractions: a finely crystalline (300–600 Å) α-Fe matrix, nanodimensional (180–250 Å) Fe₃C and Fe₃Si or Fe₅Si₃ phases, and a fraction with randomly arranged atoms. A second-order order-disorder phase transition is revealed with order prevailing in the longitudinal (‖) section of the samples and disorder prevailing in their cross (⊥) section. Sets of “extended” and “compressed” planes are found to coexist in the structure for the first time, with extended planes prevailing in the (‖) section and compressed planes prevailing in the (⊥) one.     

Surface tensions of binary liquid alloys with strong chemical interactions

The statistical approach developed uses the concept of “surrounded atoms”. The strong chemical interactions existing between the atoms of the liquid alloys studied are expressed, to a great extent, by the energy of stabilization of “privileged surrounded atoms” (PSA) of the mixture. The model permits to show the effects of these PSA on the surface properties (particularly surface tension) of three systems: AlCu, FeSi and NiSi.     

Electronic structures of chemisorption systems on Si(111) surface I. Si(111)7 × 7/H,Cl

Electronic structures of chemisorption on Si(111)/H,C1 are investigated by the first principle DV-Xα cluster method. The calculations are carried out for chemisorption on different sites, based on the Si₁₃H₁₅ cluster, and the effect of surface vacancy and buckling on the electronic structure is examined in detail. The present calculation shows that the Si₁₃H₁₅ surface cluster reproduces very well the more sophisticated band calculation for the Si(111) surface. It is concluded that the vacancy model with chemisorbed atoms at appropriate sites is reasonable to interpret the observed UPS of Si(111) 7 × 7/H,C1. The charge transfer between the substrate atom and the adatom depends strongly both on the chemisorption sites and on the electronegativitv difference.     

Interaction of magic gold cluster with Si$_\mathsf{60}$ cage

First-principles studies are performed on Au₁₂W@Si₆₀ by using projector-augmented wave (PAW) method and generalized gradient approximation for the exchange-correlation energy. The geometry, electronic structure, orbital hybridization, and charge transfer are discussed. It is found that the magic Au₁₂W cluster interacts strongly with Si, thus stabilizes Si₆₀ cage structure. Meanwhile the metal cluster is dissociated when encapsulated in the Si₆₀ cage, and charges are transferred from the Si cage to the metal atoms.Received: 30 December 2003, Published online: 17 February 2004PACS: 61.48. + c Fullerenes and fullerene-related materials - 36.40.Cg Electronic and magnetic properties of clusters - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 71.20.Tx Fullerenes and related materials; intercalation compounds