Ion‐beam synthesis of 3C‐SiC surface layers on silicon

The attempt to grow 3C‐SiC thin films on silicon substrates has become an area of significant scientific interest, largely as a consequence of the impressive electrical properties that this polytype displays. In this paper, we have utilized low‐energy (20 keV) high‐fluence carbon implantation and a subsequent annealing step to form layers of 3C‐SiC directly on a silicon surface, and have investigated the effect of implantation fluence on the resultant materials properties. The quality of the Si/SiC interface is shown to be highly fluence‐dependent, with the formation of voids decreasing significantly with increased fluence. The conversion of carbon into 3C‐SiC is found to be most efficient at near‐stoichiometric concentrations, while at higher implantation fluences clusters of excess carbon are discovered to form within the silicon and to diffuse to the surface of the grown 3C‐SiC layer upon annealing. Copyright © 2011 John Wiley & Sons, Ltd.     

Hydrogen surface passivation of Si and Ge nanowires: A semiempirical approach

A semiempirical nearest‐neighbor tight‐binding approach, that reproduces the indirect band gaps of elemental semiconductors, has been applied to study the electronic and optical properties of Si and Ge nanowires (NWs). The calculations show that Si‐NWs keep the indirect bandgap whereas Ge‐NWs changes into the direct bandgap when the wire cross section becomes smaller. Also, the band gap enhancement of Si‐NWs showing to quantum confinement effects is generally larger than that of similar‐sized Ge‐NWs, confirming the larger quantum confinement effects in Si than in Ge when they are confined in two dimensions. Finally, the dependence of the imaginary part of the dielectric function on the quantum confinement within two different schemes: intra‐atomic and interatomic optical matrix elements are applied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2448–2454, 2010     

Nanostructuring at the surface of low‐energy lead‐implanted silicon by electron beam annealing

Low‐energy lead ion implantation and high‐temperature electron beam annealing were used to study the potential of producing Pb nanostructures on Si. Pb⁺ ions were implanted at high dose into p‐type (100) Si to the depth of 8.0 nm. The implanted samples were annealed under high vacuum conditions with an electron beam at 200–700 °C for 15 s. Rutherford Backscattering Spectrometry (RBS) shows rapid out‐diffusion of Pb atoms above 400 °C. However, some Pb atoms are still present in the near‐surface region after annealing the implanted samples at 700 °C. Lead nanostructures were found on samples annealed above 300 °C. Annealing the samples at 450 °C causes the formation of nanostructures as tall as 4.1 ± 0.1 nm. Many of these are arranged in ‘web‐like’ strings that extend over micrometer distances. Occasionally, much larger nano‐features (as wide as 500 nm in diameter, average height of 1.5 nm) appear in the centre of the strings. Annealing samples well above the melting point of lead results in randomly distributed small nanometer‐sized Si nano‐dots. Copyright © 2008 John Wiley & Sons, Ltd.     

From pure C(60) to silicon carbon fullerene-based nanotube: an ab initio study

The energetics, geometrical, and electronic properties of the silicon carbon fullerene-based materials, obtained from C(60) by replacing 12 carbon atoms of the C(60) cage with silicon atoms, are studied based on ab initio calculations. We have found that, of the two C(48)Si(12) isomers obtained, the one with the carbon atoms and the silicon atoms located in separated region, i.e., with a phase-separated structure is more stable. Fullerene-based C(36)Si(24) cluster, C(36)Si(24)-C(36)Si(24) dimer, and the nanotube constructed from the clusters are then studied. The calculations on the electronic properties of these silicon carbon fullerene-based nanomaterials demonstrate that the energy gaps are greatly modified and show a decreasing trend with increasing the size of the clusters. The silicon carbon fullerene-based nanotube has a narrow and direct energy band gap, implying that it is a narrow gap semiconductor and may be a promising candidate for optoelectronic devices.     

Structural and electronic properties of La‐doped CaTiO3 crystal

There is experimental and computational evidence that some important properties such as electrical conductivity and ferroelectricity in the CaTiO₃ crystal change according to the dopant states. Using an INDO quantum‐chemical computational method modified for crystal calculations we explore the stability of the La‐doped CaTiO₃ crystal in both phases, cubic and orthorhombic. The calculations are carried out by means of the supercell model based on the LUC (large unit cell) approach as it is implemented into the CLUSTERD computer code. The equilibrium geometry for impurity is found together with the crystalline lattice distortions. Atomic displacements and relaxation energies are analyzed in a comparative manner for the two crystallographic phases. A new effect of electron transfer from the local one‐electron energy level within the band‐gap to the conduction band is observed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Access to Novel Graphene‐Like Sheet of Hydroboron: First‐Principles Investigation

We designed a cyclic borane (B₆H₁₂) molecule with a benzene‐like structure, in which the six B atoms are located in the same plane. Three methods of B3LYP, MP2, and CCSD with the 6‐311++G** basis were used to investigate its structure, electronic property, and stability. Next, we calculated the stability and electronic property of three hydroboron derivatives with fused rings of B₁₀H₁₈, B₁₄H₂₄, and B₁₆H₂₆. Finally, we investigated three types of novel two‐dimensional infinite hydroboron sheets with diborane as a building block. The results of the phonon spectra ensure the dynamic stability of these predicted structures. Furthermore, the three types of hydroboron sheets are shown to have different band gap energies of less than 3.0 eV. Some investigations on the optical properties have also been performed. The predicted sheets are candidates for semiconductors, whose band gap energy can be tuned by the positions of the bridge hydrogen atoms in the sheets.     

First principles study on the structure and electronic properties of 2‐nitrimino‐1‐nitroimidazolidine

The properties of 2‐Nitrimino‐1‐nitroimidazolidine are calculated by using SIESTA code, which adopts the standard Kohn‐Sham self‐consistent density functional method in the local density approximation. The structures and electronic properties are analyzed, and the factors that affect the impact sensitivity are discussed based on the crystal structure, band energy, and projected density of state. The reason for the smaller impact sensitivity compared to RDX (hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine) is also explored from several respects such as the weakest bond dissociation energy in single molecule, and hydrogen bond, band gap in the crystal. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Temperature effects,energy shifts,and band entropies of Si semiconductors

Understanding the electronic properties of silicon semiconductors is important for the preparation of high-performance semiconductor materials. We calculated the band entropies, electronic structures, and bonding properties of a silicon semiconductor using density functional theory and the binding-energy and bond-charge model. The relationship between Si energy and temperature was studied using the tight binding (TB) approximation and bond-order-length-strength (BOLS) theory (BOLS-TB), with the Si (111) surface as an example. The specific binding energies and bonding properties of Si atoms in different surface atomic layers are discussed by analyzing the X-ray photoelectron spectra of the Si (111) surface at 953 and 1493 K. This study improves our understanding of how surface properties reflect local bonding states and deepens our understanding of how atomic-relaxation-derived Hamiltonian perturbations and temperature influence the binding energy of the surface region. It also contributes to the development of Si-based semiconductor materials by providing new ideas and methods.     

Understanding the synergistic effects,optical and electronic properties of ternary Fe/C/S‐doped TiO2 anatase within the DFT + U approach

Although TiO₂ is an efficient photocatalyst, its large band gap limits its photocatalytic activity only to the ultraviolet region. An experimentally synthesized ternary Fe/C/S‐doped TiO₂ anatase showed improved visible light photocatalytic activity. However, a theoretical study of the underlying mechanism of the enhanced photocatalytic activity and the interaction of ternary Fe/C/S‐doped TiO₂ has not yet been investigated. In this study, the defect formation energy, electronic structure and optical property of TiO₂ doped with Fe, C, and S are investigated in detail using the density functional theory + U method. The calculated band gap (3.21 eV) of TiO₂ anatase agree well with the experimental band gap (3.20 eV). The defect formation energy shows that the co‐ and ternary‐doped systems are thermodynamically favorable under oxygen‐rich condition. Compared to the undoped TiO₂, the absorption edge of the mono‐, co‐, and ternary‐doped TiO₂ is significantly enhanced in the visible light region. We have shown that ternary doping with C, S, and Fe induces a clean band structure without any impurity states. Moreover, the ternary Fe/C/S‐doped TiO₂ exhibit an enhanced photocatalytic activity, a smaller band gap and negative formation energy compared to the mono‐ and co‐doped systems. Moreover, the band edges of Fe/C/S‐doped TiO₂ align well with the redox potentials of water, which shows that the ternary Fe/C/S‐doped TiO₂ is promising photocatalysts to split water into hydrogen and oxygen. These findings rationalize the available experimental results and can assist the design of TiO₂‐based photocatalyst materials.     

Solar Fuels: Visible‐Light‐Driven Generation of Dihydrogen at p‐Type Silicon Electrocatalysed by Molybdenum Hydrides

We show that a robust molybdenum hydride system can sustain photoelectrocatalysis of a hydrogen evolution reaction at boron‐doped, hydrogen‐terminated, p‐type silicon. The photovoltage for the system is about 600–650 mV and the current densities, which can be sustained at the photocathode in non‐catalytic and catalytic regimes, are similar to those at a photoinert vitreous carbon electrode. The kinetics of electrocatalysed hydrogen evolution at the photocathode are also very similar to those measured at vitreous carbon—evidently visible light does not significantly perturb the catalytic mechanism. Importantly, we show that the doped (1–10 Ω cm) p‐type Si can function perfectly well in the dark as an ohmic conductor and this has allowed direct comparison of the cyclic voltammetric behaviour of the response of the system under dark and illuminated conditions at the same electrode. The p‐type Si we have employed optimally harvests light energy in the 600–700 nm region and with 37 mW cm^?2 illumination in this range; the light to electrochemical energy conversion is estimated to be 2.8 %. The current yield of hydrogen under broad tungsten halide lamp illumination at 90 mW cm^?2 is (91±5) % with a corresponding chemical yield of (98±5) %.     

Selective response of mesoporous silicon to adsorbants with nitro groups

We demonstrate that the electronic structure of mesoporous silicon is affected by adsorption of nitro-based explosive molecules in a compound-selective manner. This selective response is demonstrated by probing the adsorption of two nitro-based molecular explosives (trinitrotoluene and cyclotrimethylenetrinitramine) and a nonexplosive nitro-based aromatic molecule (nitrotoluene) on mesoporous silicon using soft X-ray spectroscopy. The Si atoms strongly interact with adsorbed molecules to form Si-O and Si-N bonds, as evident from the large shifts in emission energy present in the Si L(2,3) X-ray emission spectroscopy (XES) measurements. Furthermore, we find that the energy gap (band gap) of mesoporous silicon changes depending on the adsorbant, as estimated from the Si L(2,3) XES and 2p X-ray absorption spectroscopy (XAS) measurements. Our ab initio molecular dynamics calculations of model compounds suggest that these changes are due to spontaneous breaking of the nitro groups upon contacting surface Si atoms. This compound-selective change in electronic structure may provide a powerful tool for the detection and identification of trace quantities of airborne explosive molecules.     

乙炔和乙烯分子在Si(100)表面吸附的几何和电子结构的密度泛函研究

采用基于第一性原理的密度泛函理论和平板模型对Si(100)表面吸附乙炔和乙烯分子的构型稳定性以及电子结构进行系统研究. 结果表明: 无论是吸附乙炔还是乙烯分子, 当覆盖度为0.5 ML时, 最为稳定的吸附方式为dimerized模型; 当覆盖度增大到1.0 ML时, end-bridge模型为最稳定的吸附方式. 通过对各吸附模型的能带结构分析可知, 体系的带隙变化可以通过考察表层Si—Si二聚体中Si原子的配位环境来确定. 对于相同的吸附模型, 无论吸附分子是乙炔还是乙烯, 都具有非常相近的带隙. 吸附构型以及吸附分子的覆盖度对最小带隙及其来源有较大影响. 此外, 研究结果还表明, 杂化密度泛函方法更适合于描述Si(100)表面的电子结构, 尤其是对end-bridge吸附模型.     

Solid‐State NMR and DFT Combined for the Surface Study of Functionalized Silicon Nanoparticles

Silicon nanoparticles (NPs) serve a wide range of optical, electronic, and biological applications. Chemical grafting of various molecules to Si NPs can help to passivate their reactive surfaces, “fine‐tune” their properties, or even give them further interesting features. In this work, ¹H, ¹³C, and ²⁹Si solid‐state NMR spectroscopy has been combined with density functional theory calculations to study the surface chemistry of hydride‐terminated and alkyl‐functionalized Si NPs. This combination of techniques yields assignments for the observed chemical shifts, including the contributions resulting from different surface planes, and highlights the presence of physisorbed water. Resonances from near‐surface ¹³C nuclei were shown to be substantially broadened due to surface disorder and it is demonstrated that in an ambient environment hydride‐terminated Si NPs undergo fast back‐bond oxidation, whereas long‐chain alkyl‐functionalized Si NPs undergo slow oxidation. Furthermore, the combination of NMR spectroscopy and DFT calculations showed that the employed hydrosilylation reaction involves anti‐Markovnikov addition of the 1‐alkene to the surface of the Si NPs.     

Adsorption and Decomposition Mechanism of 1,1‐Diamino‐2,2‐dinitroethylene on Al(111) Surface by Periodic DFT Calculations

The adsorption of 1,1‐diamino‐2,2‐dinitroethylene (FOX‐7) molecule on the Al(111) surface was investigated by the generalized gradient approximation (GGA) of density functional theory (DFT). The calculations employ a supercell (4×4×2) slab model and three‐dimensional periodic boundary conditions. The strong attractive forces between oxygen and aluminum atoms induce the N? O bond breaking of the FOX‐7. Subsequently, the dissociated oxygen atoms and radical fragment of FOX‐7 oxidize the Al surface. The largest adsorption energy is ?940.5 kJ/mol. Most of charge transfer is 3.31e from the Al surface to the fragment of FOX‐7 molecule. We also investigated the adsorption and decomposition mechanism of FOX‐7 molecule on the Al(111) surface. The activation energy for the dissociation steps of P2 con?guration is as large as 428.8 kJ/mol, while activation energies of other con?gurations are much smaller, in range of 2.4 to 147.7 kJ/mol.     

Impact of Mn on the solution enthalpy of hydrogen in austenitic Fe‐Mn alloys: A first‐principles study

Hydrogen interstitials in austenitic Fe‐Mn alloys were studied using density‐functional theory to gain insights into the mechanisms of hydrogen embrittlement in high‐strength Mn steels. The investigations reveal that H atoms at octahedral interstitial sites prefer a local environment containing Mn atoms rather than Fe atoms. This phenomenon is closely examined combining total energy calculations and crystal orbital Hamilton population analysis. Contributions from various electronic phenomena such as elastic, chemical, and magnetic effects are characterized. The primary reason for the environmental preference is a volumetric effect, which causes a linear dependence on the number of nearest‐neighbour Mn atoms. A secondary electronic/magnetic effect explains the deviations from this linearity. © 2014 Wiley Periodicals, Inc.     

A first‐principles investigation of the hydrogen bond interaction and the sensitive characters in cis‐1,3,4,6‐tetranitrooctahydroimidazo‐[4,5‐d]imidazole

A theoretical study of structural and electronic properties of cis‐1,3,4,6‐tetranitrooctahydroimidazo‐[4,5‐d]imidazole (BCHMX) crystal is performed using density functional theory. The band structure, the total density of states, the atomic orbit projected density of states (PDOS) of C, N, O, and H, and Mulliken population analysis are discussed. The study by analyzing the PDOS shows that the structure of BCHMX crystal possesses C? H···O intra‐ and intermolecular hydrogen bonding. There are hydrogen bonds between H₃‐1s and O₅‐2p orbits, H₂‐1s and O₆‐2p orbits of intramolecules and between H₂‐1s and O₁‐2p orbits of intermolecules. The reasons for the smaller impact sensitivity compared with β‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane and 1,3,5‐trinitro‐1,3,5‐triazinane are also explored from the band gap in the crystal and the weakest bond dissociation energy in single molecule. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Tight‐binding molecular dynamics simulation of Si?H bond dissociation in silicon clusters

New model of Si? H bond dissociation is proposed and tested in the cluster Si₁₀H₁₆ by the simulation approach that combines classic molecular dynamics method and the self‐consistent tight‐binding electronic and total energy calculation one. It is shown that the monohydride Si? H bond is unstable with respect to silicon dangling bond and bend‐bridge Si? H? Si bond formation when this cluster traps the single positive charge and that hydrogen migrates through a path involving rather rotation around the Si? Si bond than the center of this bond (the bond‐centered position). These results can be useful for understanding hydrogen‐related phenomena at surfaces, interfaces, and internal voids of various hydrogenated silicon systems: electronic devices, silicon solar cells, and nanocrystalline and porous silicon. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 351–359, 2003     

X‐ray diffraction study of low‐energy carbon‐ion implanted Si(001)

In the search for Si‐ and C‐based crystalline phases in low‐energy ion implanted and electron‐beam annealed Si surface layers, X‐ray diffraction (XRD) measurements were performed at grazing incidence on samples of large SiC nanocrystals grown on a 90 nm thick Si layer containing C atoms. Diffraction patterns and reciprocal space maps did not reveal XRD patterns originating from the nanocrystals or the implanted layer, but did show that distortions of the Si crystal structure were introduced into the implanted layer. After annealing, the strain in the implanted layer is reduced, possibly by carbon atoms that have moved to locations close to dislocations and dislocation loops. This investigation underpins the growth theory of the SiC nanocrystals on Si, with carbon atoms migrating to form the nanostructures. Copyright © 2007 John Wiley & Sons, Ltd.     

Spatially Resolved Bottom‐Side Fluorination of Graphene by Two‐Dimensional Substrate Patterning

Patterned functionalization can, on the one hand, open the band gap of graphene and, on the other hand, program demanding designs on graphene. The functionalization technique is essential for graphene‐based nanoarchitectures. A new and highly efficient method was applied to obtain patterned functionalization on graphene by mild fluorination with spatially arranged AgF arrays on the structured substrate. Scanning Raman spectroscopy (SRS) and scanning electron microscopy coupled with energy‐dispersive X‐ray spectroscopy (SEM‐EDS) were used to characterize the functionalized materials. For the first time, chemical patterning on the bottom side of graphene was realized. The chemical nature of the patterned functionalization was determined to be the ditopic scenario with fluorine atoms occupying the bottom side and moieties, such as oxygen‐containing groups or hydrogen atoms, binding on the top side, which provides information about the mechanism of the fluorination process. Our strategy can be conceptually extended to pattern other functionalities by using other reactants. Bottom‐side patterned functionalization enables utilization of the top side of a material, thereby opening up the possibilities for applications in graphene‐based devices.