Silicon Sites Research Articles

Interaction of irradiation-induced point defects with transmutants (H, He, Li, Be, B, Mg, Al and P) in 3C-SiC ceramics

Cubic silicon carbide (3C-SiC) ceramics are one of the most promising candidates for structural materials in future fusion reactors. In a fusion environment, irradiation of high-energy neutrons induces a large number of point defects and many transmutants (H, He, Li, Be, B, Mg, Al and P). The interaction of irradiation-induced point defects with the transmutants affects the evolution of micro structure in 3C-SiC, however, the micro processes are still mysterious. In this work, we carry out systematical ab initio calculations concerning the interaction of irradiation-induced point defects with the transmutants in 3C-SiC. It is found that Li and Be atoms prefer to occupy the tetrahedral interstitial site surrounding by four carbon atoms. B atom is favorable to occupy the substitutional site of carbon, while Mg, Al, and P atoms prefer to occupy the substitutional site of silicon. Moreover, only B, Mg and Al pairs have positive binding energies, which suggests that B, Mg, and Al pairs tend to segregate and form clusters at substitutional sites. Vacancy and self-interstitial atom can act as trapping centers for transmutants, especially for Mg. The electron density and the volume difference are analyzed to understand the underlying reasons controlling the interaction of point defects with transmutants.

Enabling materials informatics for 29Si solid-state NMR of crystalline materials

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for obtaining precise information about the local bonding of materials, but difficult to interpret without a well-vetted dataset of reference spectra. The ability to predict NMR parameters and connect them to three-dimensional local environments is critical for understanding more complex, long-range interactions. New computational methods have revealed structural information available from 29Si solid-state NMR by generating computed reference spectra for solids. Such predictions are useful for the identification of new silicon-containing compounds, and serve as a starting point for determination of the local environments present in amorphous structures. In this study, we have used 42 silicon sites as a benchmarking set to compare experimentally reported 29Si solid-state NMR spectra with those computed by CASTEP-NMR and Vienna Ab Initio Simulation Program (VASP). Data-driven approaches enable us to identify the source of discrepancies across a range of experimental and computational results. The information from NMR (in the form of an NMR tensor) has been validated, and in some cases corrected, in an effort to catalog these for the local spectroscopy database infrastructure (LSDI), where over 10,000 29Si NMR tensors for crystalline materials have been computed. Knowledge of specific tensor values can serve as the basis for executing NMR experiments with precision, optimizing conditions to capture the elements accurately. The ability to predict and compare experimental observables from a wide range of structures can aid researchers in their chemical assignments and structure determination, since the computed values enables the extension beyond tables of typical chemical shift (or shielding) ranges.

Site-specificity reduction during Auger decay following Si:2p photoionization in Cl3SiSi(CH3)3 vapor: An interatomic-Coulombic-decay-like process

L23VV Auger decays caused by Si:2p core-level photoionization in Cl3SiSi(CH3)3 vapor have been studied by means of photoelectron Auger-electron coincidence spectroscopy. Upon the Si:2p photoionization, Cl3Si(CH2)nSi(CH3)3 (n = 0–2) shows two core-photoelectron emissions that are distinguishable by the silicon site. Cl3SiCH2CH2Si(CH3)3 (n = 2) and so forth, which have a long inter-site bridge, remember the initially core-ionized silicon-site and show site-specific fragmentation. In contrast, Cl3SiSi(CH3)3, which has direct SiSi bonding (n = 0), forgets the initially core-ionized silicon-site and shows negligible site-specificity in the final fragmentation stage. A reduction of the site specificity in Cl3SiSi(CH3)3 starts during a Si:L23VV Auger decay and its reduction process can be explained in terms of an interatomic-Coulombic-decay-like mechanism. Another site-specificity reduction, caused by inter-site energy randomization, occurs after every Si:L23VV Auger decay.

Zinc oxide and germanium dioxide adsorbed on silicon nanocage as anodes in lithium-ion battery and potassium-ion battery

In the current study, the Si48, ZnO-Si48, and GeO2-Si48 compounds as anodes in LIB and KIB are used and their performances through theoretical methods are investigated. Results indicated that top silicon site of Si48 is an acceptable position to ZnO and GeO2 adsorption. The Gad, EBG, q, and Vcell values of complexes of metal ions and studied nanocages are compared. Results indicated that ZnO and GeO2 can increase the Vcell of Si48 as anodes in LIB and KIB, significantly. Finally, Si48-GeO2 with high Vcell and q values as anodes in LIB and KIB is suggested to use in storage machines.

Effect of Different Aluminum Substitution Rates on the Structure of Tobermorite.

Tobermorite and Al-substituted tobermorite with varying Al/(Si + Al) molar ratios were synthesized via the static hydrothermal method. The effect of different Al substitution ratios on the structure of tobermorite was investigated by XRD, SEM–EDS, TG–DSC, FT–IR, 29Si, and 27Al MAS–NMR. The interplanar crystal spacing of (002) increased with the increment of the Al substitution ratio. The maximum ratio of the Al substitution for Si in the structure of tobermorite was about 15 at %, which was limited by the compositions of tobermorite and the electrostatic repulsive forces caused by Al substitution. The DSC curve of the tobermorite with an Al/(Al + Si) molar ratio of 0.15 showed a larger exothermic peak area at 850 °C, indicating that the Al-O-Si bond was formed in the structure of tobermorite and the Al participated in the recrystallization. The 29Si and 27Al MAS–NMR results confirmed that tetrahedral Al was introduced into the structure of the tobermorite. New silicon sites, Q2(1Al) and Q3(1Al), formed by the Al substitution for Si in bridging and non-bridging sites, were detected. Compared to the tobermorite without Al substitution, the adsorption capacity of Al-substituted tobermorite with Al/(Al + Si) = 0.15 to Na+ was improved by 44%.

Synthesis, Characterization, Hydrolytic Stability, Nickel(II) Chloride Complex and Anti-Parasitic Activity of Pyrene-Tethered Silatranes

The known silatrane chemistry with polycyclic aromatic hydrocarbons (PAHs) has not been extensively investigated. The current study features synthesis and characterization of silatranes bearing pyrene substituent at the apical silicon site starting from 1-pyrenecarboxaldehyde and N-(3-(trimethoxysilyl)propyl)ethane-1,2-diamine. The synthesized silatranes were subjected to time-dependent hydrolysis study to examine their hydrolytic stability. In addition, one of the silatrane has been used to entrap nickel chloride to obtain complex of 1:1 composition. Mass, IR spectroscopic data and theoretical study revealed about the chelate structure of the complex in which silatrane acts as a bidentate ligand. The protozoans are cause of numerous infective diseases and so the article also assesses the anti-parasitic potential of the prepared compounds. Both the silatranes showed significant activity against Giardia lamblia and Entamoeba histolytica and very low IC50 values of (6) against G. lamblia and (7) against E. histolytica indicate their potential to act as efficient drug in comparison to standard drug, metronidazole.

Interaction forces between colloidal starch and quartz and hematite particles in mineral flotation

In this study, we present theoretical and experimental data on the interaction forces between starch and particles of hematite and quartz as well as their implications for flotation and aggregation. The individual Hamaker constant for starch acting through vacuum, A11, was calculated, and its value, 6.27 × 10−20J, demonstrates the highly hydrophilic character of gelatinized starch. The Hamaker constants for quartz/water/starch and for hematite/water/starch, A132, were also calculated, and these values were 6.07 × 10−21J and 17.90 × 10−20J, respectively. It was confirmed that starch strongly adsorbs onto hematite, whereas on quartz, starch may adsorb onto only silicon surface sites. In fact, AFM images of quartz surfaces showed non-adsorbed areas or areas with a thin layer of starch that certainly do not interfere in subsequent adsorption by the collector (etheramine). The interaction energies between gelatinized starch and the hematite and quartz surfaces were estimated based on DLVO theory at pH 10.5 and a constant ionic strength (10−3 mol.L−1 NaCl). The repulsion electrostatic energy was higher for the quartz-starch system than for the interactions between hematite and starch. The calculated total energy of interaction between quartz and starch revealed that there is no possibility of aggregation for the individual colloids of starch and quartz particles. On the other hand, spontaneous adhesion between the colloids and hematite particles was confirmed by DLVO theory.

Ab initio analysis of nucleation reactions during tungsten atomic layer deposition on Si(100) and W(110) substrates

Novel insight into the mechanisms that govern nucleation during tungsten atomic layer deposition is presented through a detailed analysis using density functional theory. Using the calculated energetics, the authors suggest the most probable series of reactions that lead to monolayer formation on desired growth surfaces, Si(100) and W(110), during sequential doses of WF6 and SiH4. From this analysis, they conclude that a relatively high-energy barrier exists for initial nucleation of WF6 on a silicon substrate; therefore, the system is limited to physical adsorption and is only capable of accessing nucleation pathways once the reaction barrier is energetically accessible. During early doses of WF6, the initial silicon surface acts as the reductant. Results from this half-reaction provide support for the noncoalesced growth of initial W layers since nucleation is shown to require a 2:1 ratio of silicon to WF6. In addition, the release of H2 is significantly favored over HF production leading to the formation of fluorine-contaminated silicon sites; etching of these sites is heavily supported by the absence of fluorine observed in experimentally deposited films as well as the high volatility of silicon-subfluorides. In the second half-reaction, SiH4 plays the multipurpose role of stripping fluorine atoms from W, displacing any adsorbed hydrogen atoms, and depositing a silicon-hydride layer. Saturation of the previously formed W layer with silicon-hydrides is a crucial step in depositing the consecutive layer since these surface species act as the reductants in the succeeding dose of WF6. The SiH4 half-reaction reaches a limit when all fluorine atoms are removed as silicon-subfluorides (SiFxHy) and tungsten sites are terminated with silicon-hydrides. The WF6 dose reaches a limit in early doses when the reductant, i.e., the surface, becomes blocked due to the formation of a planar network of fluorine-containing tungsten intermediates and in later cycles when the reductant, i.e., adsorbed silicon-hydrides, is etched entirely from the surface. Overall, the calculated energetics indicate that WFxHy, SiFx, and H2 molecules are the most probable by-products released during the ALD process. Results from this work contribute significantly to the fundamental understanding of atomic layer growth of tungsten using silicon species as reducing agents and may be used as a template for analyzing novel ALD processes.

A spectroscopy and microscopy study of silicon nanoclusters grown on β-Si3N4(0 0 0 1)/Si(1 1 1) interface

Silicon has been grown on the (8 × 8)–reconstructed β-Si3N4(0 0 0 1) surface at 350 °C temperature. The pure Volmer-Weber growth mode has induced the formation of nano-sized silicon clusters randomly distributed on the surface. Synchrotron radiation photoelectron spectroscopy and scanning tunneling microscopy have been employed to study the system. A fit to the photoemission spectra, complemented by topographic information, has allowed us to assign each Si2p and N1s component to the different non equivalent sites of silicon and nitrogen atoms.

Application of weighted Voronoi diagrams to analyze nucleation sites of multicrystalline silicon ingots

Analysis of nucleation sites in multicrystalline silicon (mc-Si) grown by directional solidification is required for further grain refinement to reduce dislocation density. In this study, Voronoi diagrams were utilized to analyze nucleation sites of mc-Si grown by single-layer Si beads (SLSB)-seeding method. The grain distribution at the bottom of the ingot was almost reproduced by the weighted Voronoi diagram with a relaxation method to optimize the positions of generating points of the diagram, which correspond to the nucleation sites, and the difference of nucleation timing of each crystal grain. Comparison of the generating points with the optical image indicated that the nucleation started at the remarkably deep portions of the nucleation layer. Further grain refinement by SLSB-seeding method could be achieved by suppressing the formation of the remarkably deep portions of the nucleation layer.

(Invited) Deep-Level Analysis of Passivation of Transition Metal Impurities in Silicon

Experimental results on the interactions of transition metal (TM) impurities with hydrogen and on passivation by the displacement of the metals from interstitial to substitutional sites in crystalline silicon will be presented in this talk. Iron, titanium and vanadium from the 3d series, and Zr and Mo from the 4d series have been studied. The results have been obtained by means of deep level transient spectroscopy (DLTS), high resolution Laplace DLTS, minority carrier transient spectroscopy and secondary ion mass spectroscopy. The 3d metals are the most important TM series in relation to recombination in silicon. The most recombination active site is with the 3d metal in an interstitial position. It is found that in the presence of excess vacancies 3d TMs tend to move to substitutional sites and are quite stable in this position with no or much reduced recombination activity and much reduced diffusion. The experimental results on passivation of TMs with hydrogen show that in general the reactions of H atoms with TM impurities are much more effective in n-type material than in p-type Si. These findings can be easily explained considering charge states of the interacting species. Essentially the initial H-TM reaction is driven coulombically at equilibrium conditions. We have managed to promote reactions of hydrogen atoms with Fe, Mo and V impurities in p-type silicon by changing charge states of the interacting defects either by shifting the Fermi level upon the application of reverse bias to Schottky barrier diodes or by illumination of the contaminated and hydrogenated Si samples with the use of light emitting diodes with photon energies above the Si band gap energy. It has been found that the binding of hydrogen to the metal atoms, in all cases we have examined, is rather weak … of the order of 2 eV. This implies that the passivated fraction of the TMs resulting from the reaction of hydrogen with metals in bulk silicon is diminishingly small at device processing temperatures. Based on the results obtained we try to understand some of the previously published data on lifetime improvement in Si crystals by hydrogenation. In some of the commercially important cases, particularly those reporting on multi-crystalline slices, this is far from simple as other important effects such as enhanced diffusion, precipitation and gettering can take place during hydrogenation simultaneously with hydrogen-metal reactions. It appears that in some cases hydrogen-metal reactions does not seem to be the most important processes in enhancing the minority carrier lifetime of the silicon.

<i>Ab Initio</i> Theory of Si-Vacancy Quantum Bits in 4H and 6H-SiC

Point defects in wide band gap semiconductors have recently shown outstanding potential for implementing room temperature quantum bits and single photon emitters. These atomic scale tools can be used in various quantum information processing, sensing, and imaging applications. Silicon vacancy related photoluminescence centers in 4H, 6H, and 15R-SiC are among the most studied quantum bits that possess a particular spin-3/2 ground and excited state. The microscopic structures of these defects have been recently identified as isolated negatively charged silicon vacancy defects at the symmetrically non-equivalent silicon sites in SiC. Relying on this identification, here we carry out high precision ab initio simulations on negatively charged silicon vacancies in 4H and 6H-SiC and calculate the most important magneto-optical data, such as the zero-phonon photoluminescence energies, the zero-field-splitting, and the hyperfine tensors for the nearest and farther nuclear spins.

Orientation relationship between β-Si3N4 and Si in multicrystalline silicon ingots for PV applications

β-Si3N4 particles are thought to be nucleation sites for silicon in multicrystalline silicon (mc-Si) ingots for PV applications. The orientation relationship (OR) between Si and β-Si3N4 has been investigated by transmission electron microscopy (TEM). The following OR was found by analyzing diffraction patterns from a site specific sample prepared by focused ion beam (FIB)0001β-Si3N4‖11¯1Si4¯51¯0β-Si3N4‖011SiThe β-Si3N4 particle was identified as a likely nucleation site based on the grain boundaries (GB) extending from it. It was shown that these GBs were Σ3 boundaries. The nucleation process was discussed as a source for formation of twins in mc-Si.

Sequential Nano-Explosion Using Patterned Porous Silicon

This paper reports a novel method to study and control explosions of explosive porous silicon (porous silicon impregnated with an oxidizer). This is achieved by transferring an explosion from one site to another by adjusting size and distance between locally defined areas of porous silicon. Using standard microtechnology, porous silicon can be locally generated on a silicon wafer surface in such a way that the amount of exploding porous silicon is controlled by the layout. Such an approach allows a domino-like sequence of explosions on a single chip using only one manual ignition. Confinement and sequential explosions were studied for different dimensions of porous silicon areas with different shapes (circle, rectangle, and square) and separation distances between them. A critical separation distance between porous silicon sites is found in which sequential explosions have achieved. [2017-0260]

Operando Micro-Raman Study Revealing Enhanced Connectivity of Plasmonic Metals Decorated Silicon Anodes for Lithium-Ion Batteries

The efficient formulation of silicon based, high-loading electrode with good capacity retention and cycling stability remains challenging. To gain a better understanding of the ongoing processes and failure mechanisms occurring during battery performance, operando micro-Raman spectroscopy is helpful to map the active silicon sites. Herein, we present the investigation of the electrochemical performance of anodes composed of plasmonic metal (Ag and Au) decorated silicon, designed for enhancing Raman signal. Following the discovery that only a partial amount of the electroactive material undergoes lithiation in the first cycle, we show that the plasmonic metal tips can enhance the connectivity of the Si particles. The micro-Raman mapping of electroactive silicon material reveals how the plasmonic metals influence the distribution of silicon active sites during battery cycling. The ratio of electroactive Si is found to increase from Si to Si/Au and Si/Ag electrodes, and the results are explained in terms of ...

Lattice sites of ion-implanted Mn, Fe and Ni in 6H-SiC

Using radioactive isotopes produced at the CERN-ISOLDE facility, the lattice location of the implanted transition metal (TM) ions 56Mn, 59Fe and 65Ni in n-type single-crystalline hexagonal 6H-SiC was studied by means of the emission channeling technique. TM probes on carbon coordinated tetrahedral interstitial sites (TC) and on substitutional silicon sites (SSi,h+k) were identified. We tested for but found no indication that the TM distribution on SSi sites deviates from the statistical mixture of 1/3 hexagonal and 2/3 cubic sites present in the 6H crystal. The TM atoms partially disappear from TC positions during annealing at temperatures between 500 °C and 700 °C which is accompanied by an increase on SSi and random sites. From the temperature associated with these site changes, interstitial migration energies of 1.7–2.7 eV for Mn and Ni, and 2.3–3.2 eV for Fe were estimated. TM lattice locations are compared to previous results obtained in 3C-SiC using the same technique.

Hydrogen-abstraction reactions of fully hydrogenated silicon fullerene cages with the amino radical: a density functional study

We applied density functional theory calculations to study reactions NH2 •+ SinHn fullerenes (n = 4, 6, 8, 10, 20, 24, 30, 36, and 50). The reactions between SinHn fullerenes and NH2 • radical are obtained to be exothermic, proceeding through a hydrogen-bonded prereactive complex. The resemblance between the structures of the reactants and the transition states indicates that the transition states appear earlier for the considered exothermic reactions. The calculated ΔH0 298 and ΔETS are in good correlation with the spherical excesses φi, showing upward trend with increasing curvature on silicon sites. The reactions of Si20H20 and Si20F20 with NH2 • and NF2 • radicals were also investigated. The reaction between Si20H20 and NF2 • radical also proceeds through a hydrogen-bonded prereactive complex. However, this reaction is endothermic, and as compared with the reaction of NH2 •+ Si20H20, the prereactive complex has stronger hydrogen bonding which leads to higher activation barrier. The reaction of Si20F20 and NF2 • radical as well as NH2 • radical is obtained to be endothermic, having significantly higher barriers via “late” transition states, in comparison to the reaction of NH2 •+ Si20H20.

Synthesis and optical properties of P5+ co-doped Ba3SiO5:Eu2+ orange persistent phosphor

By co-doping the P5+ ions into Ba3SiO5:Eu2+, we synthesized new orange persistent phosphor using cellulose assisted liquid phase precursor (LPP) method. The emission spectra showed a typically broad orange emission band centered at 575 nm attributed to the 4f65d1 → 4f7 transition of Eu2+ ions under 400 nm excitation source. The persistent luminescence property of Eu2+ doped Ba3SiO5 orange phosphor was enhanced by co-doping non-equivalent phosphorus ions into silicon sites. This enhancement can be explained by formation of the trapping center due to the charge compenstation. Consequently, the afterglow decay exhibited an orange emission for more than 5 min after irradiation at under 365 nm for 1 min.

Characterization of the Bonds Developed between Calcium Silicate Hydrate and Polycarboxylate-Based Superplasticizers with Silyl Functionalities.

Major developments in concrete technology have been achieved with the use of polycarboxylate-based superplasticizers (PCEs) to improve the concrete rheology without increasing the mix water content. Currently, it is possible to control the fluidity of the fresh concrete and obtain stronger and more durable structures. Therefore, there is a strong incentive to understand the interactions between PCEs and cement hydrates at the atomic scale to design new customized functional PCEs according to the ever-increasing requirements of the concrete industry. Here, the bonding types generated between a PCE with silyl functionalities (PCE-Sil) and a synthetic calcium silicate hydrate (C-S-H) are analyzed using XRD, 29Si NMR spectroscopy, and synchrotron-based techniques, such as NEXAFS and EXAFS. The results indicated that the carboxylic groups present in PCE-Sil interact by a ligand-type bond with calcium, which modified not only the symmetry and coordination number of the calcium located at the surface of C-S-H but also the neighboring silicon atoms of the C-S-H. In addition, the silyl functionalities of the PCE-Sil generated covalent bonds through siloxane bridges between the silanol groups of PCE-Sil and the nonbonding oxygen located at the dimeric sites in C-S-H, forming new bridging silicon sites and subsequently increasing the silicate polymerization.

Ultrafast Emission Decay and Enhancement of Blinking of Single Quantum Dots in the Presence of Silicon and Metal/Metal Oxide Structures

We experimentally investigate the impact of concentration of colloidal quantum dots on their lifetime and blinking dynamics when they are situated near material layers consisting of gold, aluminum oxide, and amorphous silicon combinations. Our results show that in the presence of the silicon layer the gold not only does not suppress the blinking behavior but rather promotes it at much higher concentrations of the quantum dots. This process happens as the emission decay rate of the quantum dots is increased significantly. For an amorphous silicon thickness of ∼5 nm, single quantum dots near the specific combination of gold/amorphous silicon/aluminum oxide layers exhibit mostly bright state behavior. However, when the thickness of the amorphous silicon is increased, we find that the single quantum dots remain mostly in the dark state. The work presented here highlights the profound impact of defect sites in amorphous silicon on the interplay between plasmon field enhancement and Auger recombination. The res...