Si Nuclei Research Articles

Doped-SiCNT as a promising sensor for detection of CS2 molecule

ABSTRACTIn order to explore the novel sensors for detection of carbon disulfide (CS2) molecule, the electronic sensitivity of Pd or Ni–SiCNT to CS2 molecule is investigated using density functional theory and dispersion-corrected density functional theory methods. The adsorption energy, charge transfer, density of states and molecular frontier orbital of all systems are also analyzed. The adsorption energy values reveal that PdSi or NiSi–SiCNT weakly adsorbed the CS2 molecule and their electronic properties do not change by adsorbing a gas molecule. While replacing C atom with Ni and Pd atoms can enhance the adsorption energy. Moreover, it is found that the electronic properties of PdC or NiC–SiCNT are changed upon exposure to the CS2 molecule. And NiC–SiCNT has more sensitivity to CS2 when compared to other considered nanotubes. Therefore, NiC–SiCNT is more suitable for detection and CS2 adsorption than other investigated nanotubes. NBO analysis reveals that electrons transfer from the CS2 molecule to the nanotube. 13C and 29Si chemical shielding tensors are computed using the gauge-independent atomic orbital method. Nuclear magnetic resonance calculations reveal that the isotropy parameters at the sites of Si nuclei which are directly bonded to the impurity atoms undergo significant changes.

Bismuth Oxide Nanoparticles Partially Substituted with EuIII, MnIV, and SiIV: Structural, Spectroscopic, and Optical Findings.

Interest in nanostructured partially substituted bismuth oxides has been increasing over the last years. Research on new synthesis methods, properties, and possible uses for these oxides is needed. The objective of this paper is to synthesize β-Bi2O3, β-Bi2O3:Eu3+, β-Bi2O3:Mn4+, Bi12Bi0.8O19.2, Bi12Bi0.8O19.2/Li+, Bi12MnO20, and Bi12SiO20 nanoparticles and to investigate their structural, spectroscopic, and optical changes. Some of the causes that generated their properties are also discussed. These materials are important because the doping or partial substitution of bismuth oxide with these cations (Eu3+, Mn4+, and Si4+) modifies some properties such as optical absorption, reactivity toward CO2, among others. X-ray diffraction (in powders), high-resolution transmission electron microscopy, Fourier transform infrared (FTIR), resonance Raman scattering, diffuse reflectance, and solid-state magic-angle-spinning 29Si NMR were used for the characterization of the synthesized materials. We found that partial substitution of yellow Bi12Bi0.8O19.2 with Mn4+ and Si4+ changed the color to green and whitish, respectively. New bands in the Raman scattering and FTIR spectra of these oxides are deeply discussed. Raman scattering spectroscopy was a valuable and reliable technique to detect the Eu3+ and Mn4+ cations as dopants in the bismuth oxides. The 29Si chemical shift (δ) in Bi12SiO20 was -78.16 ppm, whereas in SiO2, it was around -110 ppm. This considerable shift in Bi12SiO20 occurred because of an increased shielding of the Si nucleus in the Si(O)4 tetrahedron. This shielding was provided by the low-electronegativity and highly polarizable Bi cations. The isovalent doping of β-Bi2O3 nanoparticles with Eu3+ enhanced their thermal stability over 400 °C. Variation in the optical absorption and reactivity toward the acidic CO2 molecule of the partially substituted bismuth oxides was explained on the basis of the optical basicity and ionic-covalent parameter concepts. Some possible uses for the synthesized oxides are suggested.

Interaction mechanisms and structural properties of MC19 (M = Si and Al) fullerenes with chlorophenylpiperazine isomers

Abstract A great deal of attention has been paid recently to the fullerene based complex systems for their possible applications as drug delivery vehicles and sensor applications. In the framework of this research, Si and Al-doped C20 fullerenes and physical interaction mechanisms of doped fullerenes with ortho, meta and para chlorophenylpiperazines were examined based on the computational density functional methods. Energetically stable complexes were searched with M06-2X and B3LYP functionals with 6-31G(d) basis set and some important geometrical structures including interatomic distances and dihedral angles were reported. All calculations were carried out in the gas phase and water media. It was observed that stabilities of the investigated systems were enhanced in the water phase calculations and upon adsorption besides the changes in NH stretching values, nuclear magnetic shielding tensors of 27 Al and 29 Si nuclei were considerably changed. In order to further analyse the electronic, physical and chemical properties, binding and band gap energies, electrophilicity indexes and chemical hardness values were also calculated.

Advantage of NMR and FTIR Spectroscopy to Determine Structure Role of CeO<sub>2</sub> in Complicated Borosilicate Glasses: New Approach

Microstructure of complicated glasses in the system 30Na2O-2Al2O3-25 SiO2-xCeO2 (43-x) B2O3, x changes from 0.5 to 20 mol% have been extensively studied. Structural determination of glasses containing high cerium oxide content (≥8 mol% CeO2) was carried out by 11B NMR and FTIR spectroscopy. On the other hand, 29Si MAS NMR experiment is hardly to be applied to glasses of CeO2 > 8 mol%. This is due to the paramagnetic action which is raised by cerium cations causing dilution or delaying in the resonance phenomenon. It is evidenced from NMR data that sodium oxide is high enough to modify the glass forming units which constitute the skeleton of the glass. Ceria is as well as silica and B2O3 all are acting as glass forming species. Decreasing of both fraction of boron tetrahedral units (N4) and chemical shift of silicon nuclei (δ) confirm the role of CeO2 as a glass former. On the other hand, fast decrease in N4 and chemical shift of Si nuclei with further increasing CeO2 contents (≥8 mol%) gives a clear evidence that the ability of cerium oxide to participate as a network former increases with increasing its content. New approach is applied to determine the fraction of CeO4 as a glass forming units. In this approach, we use the common advantage of 11B NMR and FTIR spectroscopy to obtain Ce4 fraction. The latter species cannot be determined from NMR spectroscopy, since very high relaxation time and magnetization of ceria cause intensive spectral broadening which prevent resonance spectra to be appeared.

All‐Solution‐Processed Random Si Nanopyramids for Excellent Light Trapping in Ultrathin Solar Cells

Si nanopyramids have been suggested as one of the most promising Si nanostructures to realize high‐efficient ultrathin solar cells or photodetectors due to their low surface area enhancement and outstanding ability to enhance light absorption. However, the present techniques to fabricate Si nanopyramids are either complex or expensive. In parallel, disordered nanostructures are believed to be extremely effective to realize broadband light trapping for solar cells. Here, a simple and cost‐effective method is presented to form random Si nanopyramids based on an all‐solution process, the mechanism behind which is the successful transfer of the generation site of bubbles from Si surface to the introduced Ag nanoparticles so that OH− can react with the entire Si surface to naturally form random and dense Si nucleus. For optical performance, it is experimentally demonstrated that the random Si nanopyramid textured ultrathin crystalline Si (c‐Si) can achieve light trapping approaching the Lambertian limit. Importantly, it is revealed, by numerical calculations, that random Si nanopyramids outperform periodic ones on broadband light absorption due to more excited optical resonance modes. The finding provides a new opportunity to improve the performance of ultrathin c‐Si solar cells with a simpler process and lower cost.

Quartet excitations and cluster spectra in light nuclei

The relation of quarteting and clustering in atomic nuclei is discussed based on symmetry-considerations. This connection enables us to predict a complete high-energy cluster spectrum from the description of the low-energy quartet part. As an example the $^{28}$Si nucleus is considered, including its well-established ground-state region, the recently proposed superdeformed band, and the high-lying molecular resonances.

A REDOR ssNMR Investigation of the Role of an N-Terminus Lysine in R5 Silica Recognition.

Diatoms are unicellular algae that construct cell walls called frustules by the precipitation of silica, using special proteins that order the silica into a wide variety of nanostructures. The diatom species Cylindrotheca fusiformis contains proteins called silaffins within its frustules, which are believed to assemble into supramolecular matrices that serve as both accelerators and templates for silica deposition. Studying the properties of these biosilicification proteins has allowed the design of new protein and peptide systems that generate customizable silica nanostructures, with potential generalization to other mineral systems. It is essential to understand the mechanisms of aggregation of the protein and its coprecipitation with silica. We continue previous investigations into the peptide R5, derived from silaffin protein sil1p, shown to independently catalyze the precipitation of silica nanospheres in vitro. We used the solid-state NMR technique 13C{29Si} and 15N{29Si} REDOR to investigate the structure and interactions of R5 in complex with coprecipitated silica. These experiments are sensitive to the strength of magnetic dipole-dipole interactions between the 13C nuclei in R5 and the 29Si nuclei in the silica and thus yield distance between parts of R5 and 29Si in silica. Our data show strong interactions and short internuclear distances of 3.74 ± 0.20 Å between 13C═O Lys3 and silica. On the other hand, the Cα and Cβ nuclei show little or no interaction with 29Si. This selective proximity between the K3 C═O and the silica supports a previously proposed mechanism of rapid silicification of the antimicrobial peptide KSL (KKVVFKVKFK) through an imidate intermediate. This study reports for the first time a direct interaction between the N-terminus of R5 and silica, leading us to believe that the N-terminus of R5 is a key component in the molecular recognition process and a major factor in silica morphogenesis.

Trivalent transition-metal cations and local structure in pyrope- and grossular-rich solid solutions investigated by 27Al and 29Si MAS NMR spectroscopy

Pyrope-rich garnet is an important “sink” for trivalent transition metals such as Cr 3+ , V 3+ , and Fe 3+ in Earth’s upper mantle. In order to obtain a better crystal-chemical understanding of transition metals in garnet and especially pyrope solid solutions, 27 Al and 29 Si MAS NMR spectra were collected on synthetic crystals of composition Mg 3 (Al 0.915, V 0.085 ) 2 Si 3 O 12 and Mg 3 (Al 0.98, Cr 0.02 ) 2 Si 3 O 12 , and compared with the 29 Si MAS NMR spectrum of a Fe 3+ -bearing grossular garnet [Ca 3 (Al 0.956 Fe 3+ 0.044 ) 2 Si 3 O 12 ]. The purpose of the study is to explore the nature of paramagnetic resonances, or their lack thereof, arising from interactions of Cr 3+ , V 3+ , and Fe 3+ with 27 Al and 29 Si nuclei. The 27 Al and 29 Si MAS NMR spectra of Cr 3+ -bearing pyrope show significant resonance broadening, similar to that of Fe 3+ -bearing grossular, which hinders any crystal-chemical interpretation. The 27 Al and 29 Si MAS NMR spectra of V 3+ -bearing pyrope, on the other hand, show several distinct and narrow paramagnetically shifted NMR resonances, even at relatively high concentrations of V 3+ . The various NMR resonances can be assigned to different local atomic configurations around AlO 6 or SiO 4 groups having zero, one, two or three next-nearest octahedrally coordinated V 3+ neighbors. The intensities of the experimental resonances were measured and compared to those calculated for the hypothetical case of statistically random mixing of V 3+ cations to investigate Al-V 3+ mixing behavior at the octahedral site. A simple analysis, based on the number of unpaired electrons for a given transition metal ( i.e ., Fe 2+,3+ , Cr 3+ , and V 3+ ) in pyrope, is made in an attempt to explain the marked differences in their behavior in 27 Al and 29 Si MAS NMR spectra.

Characterization of ozone-degraded composite of crosslinked polydimethylsiloxane with silica in water

Effect of ozone-charged water on degradation of silicone rubber, i.e., crosslinked polydimethylsiloxane/silica composite, was investigated by various analytical techniques, in which active species including active oxygen were generated by self-decomposition of ozone in water. The composite was less swollen with good solvent after degradation with ozone-charged water, whereas amount of extracted polydimethylsiloxane increased significantly, reflecting chain scission of the polymer. Hardness of the composite, measured with scanning probe microscope and hardness tester, was found to increase after degradation. The extracts were proved to be polydimethylsiloxane through FT-IR and GC-MS. The composite was characterized through DD/MAS and CP/MAS solid state NMR spectroscopies on 13C and 29Si nuclei. It was found that OSi(OH)(CH3) groups were made in polydimethylsiloxane due to oxidative reaction with hydroxyl radical and they were attracted to silica in the composite to result in the decrease in the swelling ratio and the increase in the hardness.

Using signal amplification by reversible exchange (SABRE) to hyperpolarise 119Sn and 29Si NMR nuclei

The hyperpolarisation of the 119Sn and 29Si nuclei in 5-(tributylstannyl)pyrimidine (ASn) and 5-(trimethylsilyl)pyrimidine (BSi) is achieved through their reaction with [IrCl(COD)(IMes)] (1a) or [IrCl(COD)(SIMes)] (1b) and parahydrogen via the SABRE process. 1a exhibits superior activity in both cases. The two inequivalent pyrimidine proton environments of ASn readily yielded signal enhancements totalling ∼2300-fold in its 1H NMR spectrum at a field strength of 9.4 T, with the corresponding 119Sn signal being 700 times stronger than normal. In contrast, BSi produced analogous 1H signal gains of ∼2400-fold and a 29Si signal that could be detected with a signal to noise ratio of 200 in a single scan. These sensitivity improvements allow NMR detection within seconds using micromole amounts of substrate and illustrate the analytical potential of this approach for high-sensitivity screening. Furthermore, after extended reaction times, a series of novel iridium trimers of general form [Ir(H)2Cl(NHC)(μ-pyrimidine-κN:κN')]3 precipitate from these solutions whose identity was confirmed crystallographically for BSi.

Synthesis and NMR analysis of 13C and 15N-labeled long-chain polyamines (LCPAs)

Site specific isotope labels in long-chain polyamines (LCPAs) for the NMR analysis of bioinspired silica nanocomposites.

Nucleon momentum distributions and elastic electron scattering from 19F, 25Mg, 27Al, and 29Si nuclei

The nucleon momentum distributions and elastic electron scattering form factors of the ground state for some odd 2s–1d shell nuclei, such as 19F, 25Mg, 27Al, and 29Si, have been investigated using the coherent density fluctuation model and expressed in terms of the fluctuation function (weight function) |f(x)|2. The fluctuation function has been related to the nucleon density distribution of the nuclei and determined from the theory. The property of the long-tail manner at high-momentum region of the nucleon momentum distribution has been obtained by theoretical fluctuation function. The calculated form factors F(q) of all nuclei under study are in very good agreement with those of experimental data throughout all values of momentum transfer q. It is concluded that the contributions of the quadrupole form factor F C2(q) in 25Mg and 27Al nuclei, which are characterized by the undeformed 2s–1d shell model, are necessary for getting a remarkable agreement between the theoretical and experimental form factors.

Broadband photoluminescence of hybrid Si/SiO x nanoparticles synthesized from silicon monoxide

Hybrid nanoparticles npSi/SiOx, having bright broadband photoluminescence (PL), have been synthesized using thermal disproportionation of silicon monoxide method and were used to derive sols and polymeric composites in the form of microdispersed PTFE particles and PBMA films. The peculiarities of behavior of both shape and intensity of inhomogeneously broadened PL band’s under laser excitation were determined for npSi/SiOx in sols and composites. Theese peculiarities are caused by relatively wide distribution of Si nuclei, quantum-confinement shifts in absorption (monocrystalline Si nucleus) and photoluminescence (oxygen deficient centers (npODC) in the amorphous SiOx shell) along with the effect of saturation of PL. Continuous laser exposure (at wavelength of 405 nm) of npSi/SiOx sols and composites causes the bleaching of PL. Kinetics of PL bleaching is individual for each sample type, however it always has a substantially nonexponential behavior and the bleaching rate falls two orders after ∼103 s of photoirradiation. The effect of postradiation restoration of the PL after the end of laser photoirradiation is revealed. Kinetics of restoration is essentially nonexponential, but individual for each sample type. Full restoration of PL band’s both intensity and shape for all samples requires no less than 15 h after the end of laser photoirradiation. Restoration completeness reveals the complete reversibility of PL bleaching in all npSi/SiOx based systems studied. A model is considered in which PL bleaching is caused by electron transfer from photoexcited npODC to a suitable trap and further posttreatment restoration of the PL caused by the tunneling recombination of electrons and ionized npODC+. The possibility of controlling the npSi/SiOx photosensitivity is discussed based on creating a specific shell around npSi/SiOx stimulating or, conversely, overwhelming the photoinduced charge separation.

Dynamic Structural Changes of SiO2 Supported Pt–Ni Bimetallic Catalysts over Redox Treatments Revealed by NMR and EPR

SiO2 supported Pt–Ni bimetallic catalysts with different nickel loadings were prepared and their structural changes after redox treatments were studied by XRD, NMR, and EPR. It is found that the paramagnetic Ni species are mainly located on the surface of silica lattice. The relaxation of detected 29Si nuclei in our samples is mainly governed by a spin-diffusion mechanism. The paramagnetic effects are reflected in the spin–lattice relaxation of Q4 species, with the oxidized samples presenting faster relaxation rates than the corresponding reduced ones. Meanwhile the Q3 species, which are in close contact with the paramagnetic nickel centers, are “spectrally invisible”. In reducing atmosphere Ni gradually diffuses into Pt NPs to form PtNi alloys. While under oxidization treatment, the alloyed Ni atoms migrate outward from the core of Pt NPs and are oxidized. The main EPR spectrum results from reduced nickel species, and the reduced samples show stronger EPR signal than the corresponding oxidized ones. Howe...

3D mesh calculations for covariant density functional theory

In contrast to non-relativistic approaches, 3D mesh calculations with a relativistic energy density functional have met with difficulties because of the challenges of variational collapse and fermion doubling. We overcome these difficulties by developing a novel method based on the ideas of the Wilson fermion as well as the variational principle for the inverse Hamiltonian. We demonstrate the applicability of this method by applying it to |$^{16}$|O, |$^{24}$|Mg, and |$^{28}$|Si nuclei, providing a detailed explanation of the formalism and verification of numerical implementation.

Study of multiplicity correlations in nucleus–nucleus interactions at high energy

In the present paper, some results on the correlations of the nucleus–nucleus interactions, at high energy, between different particle multiplicities are reported. The correlations between the multiplicities of the different charged particles emitted in the interactions of 22 Ne and 28 Si nuclei with emulsion at (4.1–4.5)A GeV/c have been studied. The correlations of the compound multiplicity nc, defined as the sum of both numbers of the shower particles ns and grey particles ng, have been investigated. The experimental data have been compared with the corresponding theoretical ones, calculated according to the modified cascade evaporation model (MCEM). An agreement has already been fairly obtained between the experimental values and the calculated ones. The dependence of the average compound multiplicity, on the numbers of shower, grey, black and heavy particles is obvious and the values of the slope have been found to be independent of the projectile nucleus. On the other hand, the variation of the average shower, grey, black and heavy particles is found to increase linearly with the compound particles. A strong correlation has been observed between the number of produced shower particles and the number of compound particles. Moreover, the value of the average compound multiplicity is found to increase with the increase of the projectile mass. Finally, an attempt has also been made to study the scaling of the compound multiplicity distribution showing that the compound multiplicity distribution is nearly consistent with the KNO scaling behavior.

Characterisation of magnesium potassium phosphate cements blended with fly ash and ground granulated blast furnace slag

Magnesium potassium phosphate cements (MKPCs), blended with 50wt.% fly ash (FA) or ground granulated blast furnace slag (GBFS) to reduce heat evolution, water demand and cost, were assessed using compressive strength, X-ray diffraction (XRD), scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) spectroscopy on 25Mg, 27Al, 29Si, 31P and 39K nuclei. We present the first definitive evidence that dissolution of the glassy aluminosilicate phases of both FA and GBFS occurred under the pH conditions of MKPC. In addition to the main binder phase, struvite-K, an amorphous orthophosphate phase was detected in FA/MKPC and GBFS/MKPC systems. It was postulated that an aluminium phosphate phase was formed, however, no significant Al–O–P interactions were identified. High-field NMR analysis of the GBFS/MKPC system indicated the potential formation of a potassium-aluminosilicate phase. This study demonstrates the need for further research on these binders, as both FA and GBFS are generally regarded as inert fillers within MKPC.

Structural analysis of zeolite beta through periodic ab initio simulations of XRD and [formula omitted] and [formula omitted] NMR spectra

For large systems containing hundreds of atoms in the unit cell, as usually is observed in the zeolitic materials, the use of periodic ab initio calculations is limited by the requirement of huge computation resources. In this work, by using the primitive unit cell of the zeolite beta crystal, which is 50% smaller than its conventional crystallographic unit cell, we are able to perform periodic ab initio calculations for this system. Contrary to cluster-type calculations, the correct chemical environment of the entire zeolite beta crystal is taken into account, allowing for an accurate determination of this XRD pattern and the NMR spectra of the 29Si and 17O nuclei. In particular, we present a complete characterization of the distribution of quadrupole coupling constants (Cq), asymmetry parameter (η), and isotropic chemical shifts for the δisoSi and the δisoO, which have not yet been investigated experimentally.

Interstitial silicon ions in rutileTiO2crystals

Electron paramagnetic resonance (EPR) is used to identify a new and unique photoactive silicon-related point defect in single crystals of rutile $\mathrm{Ti}{\mathrm{O}}_{2}$. The importance of this defect lies in its assignment to interstitial silicon ions and the unexpected establishment of silicon impurities as a major hole trap in $\mathrm{Ti}{\mathrm{O}}_{2}$. Principal $g$ values of this new $S=1/2$ center are 1.9159, 1.9377, and 1.9668 with principal axes along the $[\overline{1}10],[001]$, and $[110]$ directions, respectively. Hyperfine structure in the EPR spectrum shows the unpaired spin interacting equally with two Ti nuclei and unequally with two Si nuclei. These silicon ions are present in the $\mathrm{Ti}{\mathrm{O}}_{2}$ crystals as unintentional impurities. Principal values for the larger of the two Si hyperfine interactions are 91.4, 95.4, and 316.4 MHz with principal axes also along the $[\overline{1}10],[001]$, and $[110]$ directions. The model for the defect consists of two adjacent Si ions, one at a tetrahedral interstitial site and the other occupying a Ti site. Together, they form a neutral nonparamagnetic ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}\ensuremath{-}\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{0}$ complex. When a crystal is illuminated below 40 K with 442-nm laser light, holes are trapped by these silicon complexes and form paramagnetic ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}\ensuremath{-}\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defects, while electrons are trapped at oxygen vacancies. Thermal anneal results show that the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}\ensuremath{-}\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ EPR signal disappears in two steps, coinciding with the release of electrons from neutral oxygen vacancies and singly ionized oxygen vacancies. These released electrons recombine with the holes trapped at the silicon complexes.

Characterization, Luminescence, and Defect Centers of a Ce3+-Doped Li2Si2O5 Phosphor Prepared by a Solution Combustion Reaction

Li2Si2O5 doped with Ce was synthesized by solution combustion. The phosphor was characterized by powder x-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). Photoluminescence was investigated by excitation with UV radiation at 244 nm. An emission peak of the Ce3+ ion was observed at 400 nm. Gamma irradiated phosphor exhibits six thermoluminescence (TL) peaks at 110°C, 160°C, 190°C, 250°C, 320°C, and 370°C. Electron spin resonance (ESR) spectroscopy was used to study the defect centers induced in the phosphor by gamma irradiation and to identify the centers responsible for the TL process. The room-temperature ESR spectrum of the irradiated phosphor seemed to be a superposition of at least four distinct centers. One of the centers (center I) with principal g values $$g_{\vert\vert}=2.0307$$ and g ⊥ = 2.0040 was identified as an O2 − ion. Center II, with an isotropic g factor 2.0070, was identified as an F+-type center (singly ionized oxygen vacancy); this center correlates with the main low-temperature TL peak at 110°C. Center III was identified as a Ti3+ center and center IV, with a hyperfine interaction with a 29Si nucleus, as an intrinsic O−-type center with a neighboring silicon ion. A Ti4+ion, the precursor of the Ti3+ center, acts as a recombination center for the 110°C TL peak whereas a Ti3+ center relates to the main 250°C TL peak and is also the likely recombination center for this peak. An O− ion is associated with the 110°C TL peak as a possible recombination center. An additional defect center (V) was observed during thermal annealing experiments. This center (identified as an F+ center) seems to originate from an F center (oxygen vacancy with two electrons) and is not related to the any of the observed TL peaks.