Number Of Si Atoms Research Articles

The role of silicon on solute clustering and embrittlement in highly neutron-irradiated pressurized water reactor surveillance test specimens

• The role of Si content on irradiation embrittlement was clarified. • Solute atom clusters of highly neutron-irradiated Japanese pressurized water reactor surveillance test specimens were analyzed by atom probe tomography. • The cluster radius and number density decreased and increased, respectively, with increasing Si content, resulting in a constant volume fraction of SCs. • Increase in Si reduces the degree of irradiation embrittlement. This is consistent with the results of our previous study based on Bayesian statistical analysis. Solute clusters (SCs) formed in pressurized water reactor surveillance test specimens neutron-irradiated to a fluence of 1 × 10 20 n/cm 2 were analyzed via atom probe tomography to understand the effect of silicon on solute clustering and irradiation embrittlement of reactor pressure vessel steels. In high-Cu bearing materials, Cu atoms were aggregated at the center of cluster surrounded by the Mn, Ni, and Si atoms like a core-shell structure. In low-Cu bearing materials, Mn, Ni, and Si atoms formed cluster and these solutes were not comprised core-shell structure in SCs. While the number of Cu atoms in clusters was decreased with decreasing nominal Cu content, the number of Si atoms had clearly increased. The cluster radius ( r ) and number density ( N d ) decreased and increased with increasing nominal Si content, respectively. The shift in the reference temperature for nil-ductility transition ( ΔRT NDT ) showed a good correlation with the square root of volume fraction ( V f ) multiplied by ( V f × r ) . The negative relation between the nominal Si content and ΔRT NDT indicated that increasing of nominal Si content reduces the degree of embrittlement.

Influence of silane flow rate on the structural and optical properties of GaN nanowires with multiple-quantum-shells

In this study, we discuss the influence of SiH4 flow rates on the structural and optical properties of GaN nanowires (NWs) with multiple-quantum-shells (MQSs). To this end, we prepared two n-GaN core NW samples with different SiH4 flow rates. Subsequently, MQS active layers of the same structure were grown on each n-GaN core NW under identical growth conditions. The samples were characterized by scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray, and cathodoluminescence (CL) mapping. From the experimental results, we ascertained that a Si-rich layer was created between the sidewall of the NWs and MQSs, in which the number of Si atoms was mainly determined by the SiH4 flow rate. These Si atoms diffused into the MQSs during the growth, and significantly impacted the structural and optical properties, such as the shape and crystalline quality of the MQSs and NWs, and the CL intensity of the MQSs. On the basis of experimental results, we conclude that the SiH4 flow rate during the NW growth plays a critical role in the performance of an MQS-based optoelectronic semiconductor device.

Atomic and electronic structures of Si/Ge(100) interfaces studied by high-resolution photoelectron spectroscopy and scanning tunneling microscopy

The close similarity of silicon and germanium, isoelectronic group-IV elements, makes the integration of Ge layers on Si substrates suitable for technology development, but the atomic and electronic structures of ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ surfaces are still an open issue, in particular, for the alloy systems where Si is deposited on the Ge substrate. In this study, utilizing low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy using synchrotron radiation, we demonstrate that the formation mechanisms of the Si-on-Ge structures are controlled by two interface phenomena, namely Si indiffusion and Ge segregation on top of this surface. Employing these phenomena and controlling the Si quantity, one can synthesize the well-defined crystalline Ge-(2 \ifmmode\times\else\texttimes\fi{} 1)/${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$/Ge(100) stacks where the number of Si atoms at the host Ge lattice sites can be tuned. Using the obtained data on the atomic and electronic structures of such systems, we also propose a method for interface engineering of Ge/Si/Ge stacks with tailored properties as promising templates for growing the device junctions.

First-principles study of structural stability and lithium storage property of Si<sub><i>n</i></sub> clusters (<i>n</i> ≤ 6) adsorbed on graphene

Silicon/carbon composite is one of the most potential high-capacity anode materials for lithium-ion batteries. The interface state between silicon and carbon of silicon/carbon composite is an important factor affecting its electrochemical performance. In this paper, Si<sub><i>n</i></sub> (<i>n</i> ≤ 6) clusters with different numbers of Si atoms are constructed on graphene as a structural unit of carbon material. The geometric configuration, structure stability and electronic property of Si<sub><i>n</i></sub> clusters adsorbed on graphene (Si<sub><i>n</i></sub>/Gr) are studied by the first-principles method based on density functional theory (DFT). The results show that when the number of Si atoms <i>n</i> ≤ 4, the Si<sub><i>n</i></sub> clusters are preferentially adsorbed on graphene in a two-dimensional configuration parallel to graphene. When <i>n</i> ≥ 5, the Si<sub><i>n</i></sub> clusters are preferentially adsorbed on graphene in a three-dimensional configuration. With the increase of the number of Si atoms <i>n</i>, the thermodynamic stability of Si<sub><i>n</i></sub> clusters on graphene decreases significantly, the interface binding strength between Si<sub><i>n</i></sub> clusters and graphene decreases, and the charge transfer between Si<sub><i>n</i></sub> clusters and graphene becomes less. At the same time, the storage capacity of Li atoms in Si<sub><i>n</i></sub>/Gr complex is also studied. Li atoms are mainly stored on the graphene surface near Si<sub><i>n</i></sub> clusters and around Si<sub><i>n</i></sub> clusters. The complex synergistic effect of Si<sub><i>n</i></sub> clusters and graphene enhances the thermodynamic stability of Li adsorption. When <i>n</i> ≤ 4, storing two Li atoms is beneficial to improving the thermodynamic stability of <i>x</i>Li-Si<sub><i>n</i></sub>/Gr system, and the thermodynamic stability decreases with the increase of Li atom number. When <i>n</i> ≥ 5, the thermodynamic stability of <i>x</i>Li-Si<sub><i>n</i></sub>/Gr system decreases with the increase of Li atom number. In the <i>x</i>Li-Si<sub>5</sub>/Gr system, the C-C bond and Si-Si bond are mainly covalent bonds, while the Li-C bond and Li-Si bond are mainly ionic bonds with certain covalent properties.

Stable four‐membered cyclosilylenes at theoretical levels

AbstractWe aim to develop novel four‐membered cyclosilylenes that contain one (1 and 2), two (3), three (4), and four (5) numbers of Si atoms at the M06/6–311 + G*, B3LYP/aug‐cc‐PVTZ, and MP2/3–21G levels of theory. These structures show high stability and turn out as minima on their energy surfaces for showing no imaginary frequency. Structural and thermodynamic parameters, including bond length, divalent bond angle, singlet‐triplet energy gap (ΔES‐T), highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap (ΔEH‐L), proton affinity (PA), complexation energy (ΔECom) with CO molecule, nucleophilicity (N), chemical potential (μ), and natural charges, are assessed. The final order of singlet stability appears in the order 1 > 2 > 3 > 4 > 5 at three levels of theory.

Formation, geometric properties, and surface activities of nSi clusters (n = 1 − 4) doped graphene as metal-free catalyst

The formation processes, electronic, and catalytic properties of nSi (n = 1 − 4) atom-doped divacancy graphene (nSi-graphene) are discussed using density functional theory calculations. First, the formation mechanisms of nSi-graphene sheets are investigated in detail. According to the formation energies values, it is found that the tetrahedral 4Si cluster-anchored graphene has the least energy as compared with that of others. Second, the adsorption behaviors and electronic structures of adsorbed species on the 1Si-graphene and 4Si-graphene sheets are comparably analyzed. The adsorption of O2 molecule is more stable than that of the CO molecule; thus, the possible CO oxidation reactions on different nSi-graphene surfaces are investigated through Eley–Rideal. In the complete CO oxidation reactions, the formation process of CO3 complex on the 1Si-graphene sheet is the rate-controlling step, while the interaction between CO3 and CO on the 4Si-graphene has a relatively large energy barrier. This result illustrates that the different numbers of Si atoms can regulate the surface curvature and activities of graphene sheets, which provides a theoretical reference for designing the graphene-based metal-free catalyst in energy-related devices.

Low-temperature, high-growth-rate ALD of SiO2 using aminodisilane precursor

In the present study, SiO2 was deposited using the atomic layer deposition (ALD) with a 1,2-bis(diisopropylamino)disilane (BDIPADS) precursor. The use of this precursor exhibited a higher growth rate and lower initial growth temperature than the use of diisopropylaminosilane (DIPAS) did. The ALD reaction using BDIPADS produced SiO2 with excellent quality owing to the self-catalytic reaction between the amine ligand and O3; therefore, the SiO2 film has no impurities. When the growth temperature was increased gradually, the stoichiometry and density of SiO2 were improved also because the reaction between surface adsorbate species was nearly complete. ALD SiO2 exhibited a higher dielectric constant than the bulk SiO2 did, from the metal-oxide-semiconductor capacitor, because of the incorporated hydroxyl groups in the film. Furthermore, the etching characteristics were modulated by changing the growth temperature to ensure that the film can be used as a hard mask for lithography. From the DFT calculation, high reaction energy between the BDIPADS and Si–OH was observed. Moreover, the Si–Si cleavage results in the existence additional reaction sites, such as amine group, allowing low-temperature growth and enhanced productivity. Therefore, the number of Si atoms in a molecule affects the growth rate and initial growth temperature to ensure that BDIPADS is a highly excellent precursor for the SiO2 deposition; therefore, its use can lead to a remarkably high productivity.

Physical properties of silicene electrodes for Li-, Na-, Mg-, and K-ion batteries

The transition from lithium-ion batteries to sodium- and potassium-ion batteries will increase the power of electrochemical current sources and the rate of their charging. On the basis of the first principles of density functional theory and ab initio molecular dynamics simulations, the interaction of Li, Na, Mg, and K atoms with an autonomous silicene has been studied. The adsorption energies and the Si–Me (Me = Li, Na, Mg, K) bond lengths for different locations of the adsorbed metal atoms are calculated. The favorable adsorption site of Me on silicene nanosheet is identified and reported. In the approximation of the generalized gradient, the band structure of the “silicene/Me” systems is calculated. The metallic state of an autonomous metallized silicene can arise for various cases of adsorption of an alkali metal and when the ratio between the Mg and Si atoms in the system is 1:1. Metallization of the semiconductor does not occur when the number of adsorbed Mg atoms is less than the number of Si atoms.

Structural, bonding, and magnetic properties of Fen–xSix (n, x ⩽ 6) clusters: Theoretical investigation based on density functional theory

The structural, bonding, and magnetic properties of clusters of Fen, Sin, and Fen–xSix (n=2–6, x=1–6) optimized at the B3LYP/LanL2DZ level with symmetry constraints are determined. The effects of the substitution of Si in Fen on the properties of Fen are identified and discussed.The cohesion of the Fe cluster is nonmonotonically increased by the successive Si substitutions. In the mixed Fe–Si clusters, the Fe–Fe bond lengths are not affected by the Si substitutions and are similar to those in Fe clusters as well as the Fe–Si bond lengths in mixed clusters. Most of the Si–Si bond lengths in mixed clusters are similar to those in Si clusters, while a few Si–Si bond lengths characterized by the hybridization with high s-character are smaller than or equal to that in Si2. Several distances between neighboring atoms at equator in the mixed clusters are found to be remarkably large, and there is the lack of bonding between them. The average magnetic moments of mixed clusters decrease nonmonotonically with the number of Si atoms, and approach those of Si clusters. There are large amount of spin on Fe atoms and small amount of spin on Si atoms in the mixed clusters. Most of the local atomic moments on Si atoms align antiferromagnetically with respect to those on Fe atoms.The properties of Fe clusters with Si substitutions result from the electron transfer from Fe to its surrounding Si atoms. The electron transfer is closely connected with the properties of Fe–Si alloys.

Influence of Molecular Structure of POSS on Gas-Molecule Diffusion Coefficients Using Molecular Dynamic Simulation

Polymorphic structure of polyhedral oligomeric silsesquioxanes (POSS) derived from hydrolytic condensation of vinyltrimethoxysilane was validated by the spectral characterization, such as FTIR, UV-MALDI-TOF MS,etc. Discover module of Materials Stutio (MS) software which is a molecular dynamic simulations (MDS) program has been used to estimate the self-diffusion coefficients of small gas molecules in models of hybrid materials that validates the corresponding anticorrosion-experiment results. The models of 3D-amorphous cubic unit cells of different numbers of Si atom and hydroxyls generated during the hydrolytic condensation (T6, T7, T8, T9, T10,T8(OH)2and T8(OH)4cells), were employed to investigate self-diffusion coefficients by MDS for the N2, O2, Cl2, CO2, NO2, SO2and H2O molecules. The simulations results showed that all seven self-diffusion coefficients of N2, O2, Cl2, CO2, NO2, SO2and H2O in cells increased with the numbers of Si atoms and the generated hydroxyls. This increasing was discussed by the calculation, compared to the vinyltrimethoxysilane hybrid systems.

Influence of Molecular Structure on the Optical Property of POSS: a DFT Calculation Based Quantum Chemistry Calculation

Polymorphic structure of polyhedral oligomeric silsesquioxanes (POSS) derived from hydrolytic condensation of vinyltrimethoxysilane was validated by the spectral characterization, such as FTIR, UV-MALDI-TOF MS, etc. The models of 3D-amorphous cubic unit cells of different numbers of Si atom and hydroxyls generated during the hydrolytic condensation and the length of the organic side chain (T6, T7, T8, T9, T10, T8(OH)2and T8(OH)4cells), as models were established with a materials visualizer module of Material Studio(MS) software. The cells were optimized the configuration and calculated energy band structure and absorption coefficient (a) with Castep module of MS which is the quantum chemistry program based on density functional theory (DFT). The calculation results showed that the band gap was reduced with the number of Si atoms in POSS increased, but the presence of hydroxyl could make a significant reduction of them. The reduction of band gap would lower the frequency of the incident light that was absorbed by the POSS and heighten the wavelength of the absorbed incident light, which was proved by the calculation results of absorption coefficient. Such reduction was discussed in comparison with the vinyltri- methoxysilane hybrid systems.

Quantum chemical modelling of Si sub‐surface amorphisation due to incorporation of H atoms and its stabilisation by O atoms

AbstractHere we present the results of systematic simulating studies of atomic configurations in silicon “surface clusters” with different numbers (per cluster) of interstitial H and O atoms as impurities, obtained by semi‐empirical (PM3) and DFT quantum‐chemical methods. The aim of modeling was to study the near‐surface region Si amorphization caused by H‐incorporation (from H‐plasma, for example) and changes in the atomic structure of this amorphized sub‐surface region by additionally incorporated O‐atoms. A set of SixHy[HLOM] clusters was used in order to reproduce a local atomic structure of H‐ or H‐O‐containing complexes with different numbers of impurity atoms per cluster for the corresponding Si (111) and Si (100) near‐surface regions. Here x ‐ number of Si atoms, y ‐ number of H atoms which saturate dangling bonds at the cluster surface, L ‐ number of incorporated H atoms which create hydrogen‐containing defect complexes, M ‐ number of O atoms incorporated in hydrogen‐containing complexes. The optimization procedure, using the PM3 and DFT levels quantum chemical theory geometry optimisation of all Si clusters, allowed comparison of changes in the atomic structures of Si surface and sub‐surface regions with different configurations of Si‐H and Si‐H‐O impurity complexes (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structures and Electronic Properties of Si-Substituted Benzenes and Their Transition-Metal Complexes

Structural and electronic properties for a series of silicon-substituted benzenes (CnSimH6, where n = 0−6, m = 0−6, and n + m = 6) are studied through density functional theory calculations. Benzene is found to sustain its planarity up to two Si substitutions for all isomers. For three Si substitutions, only the 1,3,5-alternate structure (6) is planar, while for four Si substitution, only the 2,3,5,6 structure (10) is planar. Further Si substitution makes all the isomers for the rings nonplanar, which eventually leads to the fully puckered C3v structure for hexasilabenzene (13). The reorganization energies for these molecules are sufficiently low to be favorably utilized for hole conduction. All the molecules form very stable full-sandwich and half-sandwich complexes of the type η6-(CnSimH6)2Cr and η6-(CnSimH6)Cr(CO)3. The binding energies for these complexes increase with increase in the number of Si atoms in the rings. Strategies are proposed for experimental design of extended sheets of silicenes and m...

Photoluminescence of Si nanocrystals under selective excitation

Photoluminescence of Si nanocrystals passivated by different alkanes (hexane, octane, and tridecane) was studied at room temperature. It is shown that the emission band shape is not affected by the length of the carbon chain in the alkanes used for passivation. A pronounced fine structure of the photoluminescence band consisting of peaks separated by 150–160 meV was observed under resonant excitation. The structure is interpreted by predominant contribution from Si nanocrystal groups with particular stable size/shape existing in addition to the previously reported nanocrystals with “magic” numbers of Si atoms. The contribution of these stable nanocrystals is revealed using selective resonant photoexcitation to the higher energy states in the discrete energy spectrum of such nanocrystals.

Effect of composition modulation on the structural and optical properties of Si/Ge bilayers

We report structural as well as optical studies on Si/Ge bilayer structures having different individual layer thicknesses. The Raman spectrum of [Ge (5 nm)/Si (5 nm)] bilayer structure shows amorphous nature, while the [Si (5 nm)/Ge (5 nm)] bilayer structure shows a mixed nanocrystalline/amorphous behaviour of the layers. As the thickness of the individual layers increases to 10 nm, the introduction of large number of Si atoms at the interface results in reduction of Ge crystallization as well as higher intensity of interfacial SiGe alloy formation. This may be regarded as a consequence of the island growth induced surface roughening in the later case (i.e. in [Si (10 nm)/Ge (10 nm)] bilayer) as also revealed by corresponding atomic force microscopy (AFM) images. These results are also supported by Photoluminescence (PL) spectra recorded using two different photon energies of 300 and 488 nm along with the optical absorption measurements giving higher values of band gap as compared to their corresponding bulks, revealing the effect of quantum confinement in the deposited layers.

Ab initio investigation on the nonlinear optical properties of silicon clusters Sin (n=3–8)

First and second hyperpolarizabilities of small silicon clusters have been calculated using conventional ab initio methods systematically increasing the amount of electron correlation. Besides Si$_{5}$, upon successive addition of electron correlation in the MP2, MP3, MP4, CCSD, and CCSD(T) series, all clusters display the same behavior: i) the HF $γ _{//}$ values are the smallest, ii) the MP2 $γ _{//}$ values the largest, and iii) the latter values are good approximate to the reference CCSD(T) results because the overestimation is smaller than 10%. Contrary to the polarizability per Si atom, which decreases with the cluster size until reaching the bulk limit, the average second hyperpolarizability per Si atom presents a sawtooth behavior with maxima in $γ _{//}$ associated with even numbers of Si and minima with odd numbers of Si atoms.

Structures and stabilities of copper encapsulated within silicon nano-clusters: Cu@Si n ( n = 9–15)

Density functional electronic-structure calculations were performed for Cu@Si n ( n = 9–15) clusters. The lowest-energy endohedral structure and its stability for each Cu@Si n cluster were determined. The encapsulation of Cu within silicon clusters generates stable neutral Cu@Si n clusters. The binding energies and embedding energies of these clusters indicate that they are likely to be chemically stable. The relative cluster stabilities and other thermodynamic properties alternate with cluster size, with an apparent preference existing for clusters with an even number of Si atoms.

Arc Synthesis of Silicon‐Doped Heterofullerenes in Plasma at Atmospheric Pressure

For the first time, an arc synthesis at atmospheric pressure was carried out for obtaining silicon‐doped heterofullerenes C52Si8 and C62Si8. These molecules were detected in a pyridine extract from soot by mass spectrometry in amounts of no more than 1%. The abundance ratio of the heterofullerenes is the same as that for C60/C70 mixture. An explanation of the absence of heterofullerenes with lower number of Si atoms is given.

Growth, magic behavior, and electronic and vibrational properties of Cr-doped Si clusters

Silicon clusters doped with a Cr atom have been studied using ab initio plane-wave ultrasoft pseudopotential and Gaussian methods. The most stable structures and magic clusters have been determined for ${\mathrm{Si}}_{n\mathrm{Cr}}$ $(n=8--17)$ starting from many initial configurations. Our results show that for $n=8--11$ the number of Si atoms is not enough to surround a Cr atom fully and therefore the structures of these clusters are basket type in which the Cr atom has a bare part. A cage structure is formed for $n=12$. $\mathrm{Cr}@{\mathrm{Si}}_{12}$ and $\mathrm{Cr}@{\mathrm{Si}}_{15}$ show magic behavior. Among the charged clusters, anion of $\mathrm{Cr}@{\mathrm{Si}}_{12}$ and cation of $\mathrm{Cr}@{\mathrm{Si}}_{13}$ have high stability. The ionization potentials and electron affinities have been calculated. The dynamical stability of clusters is studied from vibrational calculations. The results of Raman activities and infrared intensities are presented for selected clusters. These can be used to identify the structures from experiments. The bonding nature in $\mathrm{Cr}@{\mathrm{Si}}_{n}$ clusters is found to change depending on the structure even when the cluster size is the same.

The carbosilane unit as a stable building block for liquid crystal design: a new class of ferroelectric switching banana-shaped mesogens.

New banana shaped liquid crystals with a carbosilane unit at one end were synthesised and depending on the number of Si-atoms either antiferroelectric (AF) or ferroelectric (FE) switching polar smectic C phases have been obtained.