Si Clusters Research Articles

In Situ Transmission Electron Microscopy Studies on Silicon Anodes in Lithium Ion Batteries

Extensive research for new energy storage materials has created a high demand for experimental techniques that can provide real-time, single-particle-level information on the dynamic electrochemical processes taking place at the electrode materials during battery charge/discharge cycles. In situ transmission electron microscopy (TEM) on lithium ion batteries has been offered exceptional opportunities for monitoring the dynamic processes of electrode materials during electrochemical reaction at both spatial and temporal resolution.In this talk, I will introduce in situ TEM studies on Si anodes which suffers the anomalous volumetric changes and fracture during lithiation process. Previously, the lithiation behaviour of a single Si particle has been explained in detail by simulation data and experimental observation. However, in real batteries, since lithiation occurs simultaneously in clusters of Si in a confined medium, understanding how the individual Si structures interact during lithiation in a closed space is necessary. In this regard, I demonstrated physical and mechanical interactions of swelling Si structures during lithiation using well-defined Si nanopillar pairs. Ex situ SEM and in situ TEM studies reveal that compressive stresses change the reaction kinetics so that preferential lithiation occurs on free surfaces when the pillars are mechanically clamped. Such mechanical interactions enhance the fracture resistance of lithiated Si by lessening the tensile stress concentrations in Si structures. This experiment reveals the surprising effects of nanostructure shape, size, and void space for lithiation and the results will contribute to improved design of Si structures at the electrode level for high-performance Li-ion batteries.

Mechanistic Variants in Gas-Phase Metal-Oxide Mediated Activation of Methane at Ambient Conditions.

The C-H bond activation of methane mediated by a prototypical heteronuclear metal-oxide cluster, [Al2Mg2O5](•+), was investigated by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in conjunction with high-level quantum mechanical calculations. Experimentally, hydrogen-atom abstraction from methane by the cluster ion [Al2Mg2O5](•+) takes place at ambient conditions. As to the mechanism, according to our computational findings, both the proton-coupled electron transfer (PCET) and the conventional hydrogen-atom transfer (HAT) are feasible and compete with each other. This is in distinct contrast to the [XYO2](+) (X, Y = Mg, Al, Si) cluster oxide ions which activate methane exclusively via the PCET route (Li, J.; Zhou, S.; Zhang, J.; Schlangen, M.; Weiske, T.; Usharani, D.; Shaik, S.; Schwarz, H. J. Am. Chem. Soc. 2016, 138, 7973-7981). The electronic origins of the mechanistically rather complex reactivity scenarios of the [Al2Mg2O5](•+)/CH4 couple were elucidated. For the PCET mechanism, in which the Lewis acid-base pair [Al(+)-O(-)] of the cluster acts as the active site, a clear correlation has been established between the nature of the transition state, the corresponding barrier height, the Lewis acidity-basicity of the [M(+)-O(-)] unit, as well as the bond order of the M(+)-O(-) bond. Also addressed is the role of the spin and charge distributions of a terminal oxygen radical site in the direct HAT route. The knowledge of the factors that control the reactivity of PCET and HAT pathways not only deepens our mechanistic understanding of metal-oxide mediated C-H bond activation but may also provide guidance for the rational design of catalysts.

Design strategy for controlled natural aging in Al–Mg–Si alloys

This study presents a design strategy for Al–Mg–Si alloys to control natural aging. Recently, trace addition of Sn was shown to suppress natural aging for up to two weeks, which was explained by the strong trapping of vacancies to Sn atoms. Here we explore the effect of solution treatment temperature, the combination of trace elements such as Sn and In, and the composition of main hardening elements Mg, Si and Cu on natural aging. The results are discussed based on the dissolvable amount of trace elements and their effect on diffusion retardation, and solute clustering mechanisms in Al–Mg–Si alloys. Thermodynamic calculations using the CALPHAD approach show that maximum retardation of natural aging is achievable at the highest trace element solubility, which exists at significantly different solution treatment temperatures for Sn or In. The effects of Mg, Si and Cu content on natural aging kinetics are interpreted via their influence on the Sn solubility and clustering mechanisms. It is proposed that Sn additions reduce the concentration of excess vacancies, which is most important for early Si clustering, and that the effect of Cu is comparable to the effect of Sn, but less pronounced. Based on the investigated parameter space, a design concept is proposed and an Al–Mg–Si alloy showing suppression of natural aging for >6 months and significant artificial aging potential is demonstrated.

Ostwald ripening in segregated SixN/SiyN multilayers

In this study, we investigate the microstructure and formation of segregated silicon nanocrystals (nc‐Si) from annealed Si‐rich SixN/SiyN multilayers. It is found the nc‐Si are separated and arranged in every sublayer with two different grain sizes. Through XPS and Raman analysis, it is discovered that Si atoms diffuse from the SiyN sublayers with smaller nc‐Si to the SixN sublayers with larger nc‐Si during annealing. Meanwhile, the grain size of SixN sublayers increase and that of SiyN sublayers decrease, compared to single layer sample. It is suggested that the atomic diffusion with grain size change phenomenon is ascribed to coalescence‐like growth followed by Ostwald ripening process, which eventually results in Si atoms transport from small Si clusters in SiyN sublayers to large Si cluster in SixN sublayers and forms the segregated nc‐Si multilayers. This work gives us better understanding of the nc‐Si growth and atomic diffusion in annealed Si‐rich multilayers.

First principle study of magnetic and electronic properties of single X (X = Al, Si) atom added to small carbon clusters (C n X, n = 2–10)

In this paper, the magnetic and electronic properties of single aluminum and silicon atom added to small carbon clusters (C n X; X = Al, Si; n = 2–10) are studied in the framework of generalized-gradient approximation using density functional theory. The calculations were performed for linear, two dimensional and three dimensional clusters based on full-potential local-orbital (FPLO) method. The total energies, HOMO–LUMO energy gap and total magnetic moments of the most stable structures are presented in this work. The calculations show that C n Si clusters have more stability compared to C n Al clusters. In addition, our magnetic calculations were shown that the C n Al isomers are magnetic objects whereas C n Si clusters are nonmagnetic objects.

Influence of short-term heat treatment on the microstructure and mechanical properties of EN AW-6060 T4 extrusion profiles—Part B

In the automotive industry aluminium and its corresponding semi-finished products contribute an essential part to the aim of weight reduction in car body structures. Aluminium alloys of the 6000 series with Mg and Si contents are preferred because of the possibility to increase strength by ageing processes. However, the cold formability in comparison to other materials like mild steels is quite low and due to this, complex parts are only producible at higher temperatures. Therefore, the so called Tailor Heat Treatment was developed to improve the cold formability of aluminium alloys. In this approach, a short-term heat treatment is conducted to achieve a local softening of the material due to dissolution of Mg and Si clusters (retrogression). This effect is used to improve the material flow, relief critical forming zones and enhance the overall formability of the material. Afterwards, strength can be increased again by ageing processes. However, up till now a holistic process understanding, taking into account all process parameters as well as a microstructural explanation is missing. Therefore, the focus of the fundamental investigations lies on connections between the mechanical properties and short-term heat treatment with industry-relevant heating rates as well as natural and artificial ageing process. Conclusively, the evolution of the mechanical properties with regard to the natural ageing process is compared with findings of DSC analysis, which were discussed in Part A. Based on these results, a process window is derived for the subsequent forming process and the final mechanical properties of the final part in dependency of the forming history as well as the artificial ageing process, are identified.

Chloride-based fast homoepitaxial growth of 4H-SiC films in a vertical hot-wall CVD**Project supported by the National High Technology R&D Program of China (No. 2014AA041402), the National Natural Science Foundation of China (Nos. 61474113, 61274007, 61574140), the Beijing Natural Science Foundation of China (Nos. 4132076, 4132074), the Program of State Grid Smart Grid Research Institute (No. SGRI-WD-71-14-004), and the Youth Innovation

Chloride-based fast homoepitaxial growth of 4H-SiC epilayers was performed on 4° off-axis 4H-SiC substrates in a home-made vertical hot-wall chemical vapor deposition (CVD) system using H2−SiH4−C2H4−HCl. The effect of the SiH4/H2 ratio and reactor pressure on the growth rate of 4H-SiC epilayers has been studied successively. The growth rate increase in proportion to the SiH4/H2 ratio and the influence mechanism of chlorine has been investigated. With the reactor pressure increasing from 40 to 100 Torr, the growth rate increased to 52 μm/hand then decreased to 47 μm/h, which is due to the joint effect of H2 and HCl etching as well as the formation of Si clusters at higher reactor pressure. The surface root mean square (RMS) roughness keeps around 1 nm with the growth rate increasing to 49 μm/h. The scanning electron microscope (SEM), Raman spectroscopy and X-ray diffraction (XRD) demonstrate that 96.7 μm thick 4H-SiC layers of good uniformity in thickness and doping with high crystal quality can be achieved. These results prove that chloride-based fast epitaxy is an advanced growth technique for 4H-SiC homoepitaxy.

Positron lifetime study of the formation of vacancy clusters and dislocations in quenched Al, Al–Mg and Al–Si alloys

The clustering kinetics in quenched pure Al, binary Al–Mg and binary Al–Si alloys were studied by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC) during natural ageing (NA). Shortly after quenching, positrons annihilate either in the bulk material or in vacancy-type defects such as mono-vacancies (in Al) and vacancy–solute complexes (in Al–Mg and Al–Si alloys). Upon NA, vacancy clusters of various sizes and number densities are formed. In Al, such clusters contain typically 3 vacancies. In Al–Mg and Al–Si alloys, complexes containing various vacancies and also solute atoms are formed. The presence of shallow positron traps was detected in temperature-dependent positron lifetime experiments. They were identified as quenched-in dislocations rather than Mg or Si clusters as no solute clustering signal during NA was observed in DSC runs of the binary Al–Mg and Al–Si alloys.

Methanol Activation Catalyzed by Small Earth-Alkali Mixed Silicon Clusters Sim–nMn with M = Be, Mg, Ca and m = 3–4, n = 0–1

The methanol activation pathways occurring on small pure and mixed silicon clusters Sim–nMn with M = Be, Mg, Ca and m = 3–4, n = 0–1 were investigated using quantum chemical computations (density functional theory B3LYP/aug-cc-pVTZ and coupled-cluster theory CCSD(T)/CBS extrapolated from energies with the aug-cc-pVnZ basis sets, n = D, T, Q) to examine their thermodynamic and kinetic feasibilities. In all cases considered, the cleavage of the O–H bond is favored over that of the C–H bond. The O–H bond cleavage in the presence of the singlet Si3 cluster is thermodynamically less preferred than on mixed Si2M clusters, even though it becomes more kinetically favored. Most importantly, the energy barriers for the O–H bond breaking on the singlet Si3, Si2Ca, and Si3Ca clusters are found to be lower than the previously reported results for metal clusters, catalytic metal surfaces, metal oxides, etc. The small mixed Si clusters thus appear to be good catalysts for methanol activation and most probably in other d...

First-Principles studies of silicon underpotential deposition on defective graphene and its relevance for lithium-ion battery materials

First-principles calculations are undertaken to analyze the properties of carbon-silicon hybrid materials consisting of silicon modified graphene and defective graphene to evaluate the stability of the structure and their interactions with lithium. Underpotential shifts are determined for the different structures on defective surfaces, showing that decoration of graphene defects with a small number of silicon atoms should occur at underpotentials. Nucleation overpotentials are also determined using a thermodynamic formalism, showing that the formation of nuclei should be hindered with respect to free standing Si clusters. These results analyzes the possibility of using underpotential deposition of silicon on graphene to obtain high capacity and cycling stable material for anodes of lithium batteries.

Silicon excess and thermal annealing effects on structural and optical properties of co-sputtered SRO films

In this work, the structural and optical properties of co-sputtered SRO (Silicon Rich Oxide) films are studied as a function of the annealing temperature and the change in the Si (Silicon) content. Si-excess of 4.4–16.9at.% were obtained by a variation of RF-power densities applied to Si target from 1.97 to 3.94W/cm2, respectively. The Si-excess calculated by XPS promotes the formation of Si clusters within SRO films where the size depends on the Si-excess quantity. After annealing, ultra-small oxidized silicon nanocrystals (Si-NCs) and agglomeration of Si-excess, promotes the possible creation of Si=O, centers like Eδ′ and ODC defects at the surface of Si-NCs. Therefore, two broad Photoluminescence (PL) bands are observed in the red (1.5–1.7eV) and blue (2–2.9eV) region. A high emission intensity in red region is observed in sample with 5.9at.% of Si-excess, which could be related with a complete activation of defects (Si=O) favored by the dominant average size of the Si-NCs (1.68nm). The density and average size of Si-NCs increase according to Si-excess, whereby a decrease in PL intensity is observed. Thus, a considerable Si-excess (>10at.%) is not necessary to obtain the maximum PL emission in red region.

A thermokinetic model for Mg–Si couple formation in Al–Mg–Si alloys

Mg–Si couples formed from atomic Mg and Si represent the first step in Mg–Si cluster formation in a dilute Al–Mg–Si system. Based on the thermodynamic extremal principle, a kinetic model for Mg–Si couple formation is developed. The model utilizes the trapping concept for the calculation of Gibbs energy of the non-equilibrium system and provides a generalized (multiplicative) form of the Oriani equation for description of the equilibrium state. The dissipation in the system accounts for diffusion of both Mg and Si atoms in the lattice.The model is compared with the classical Lidiard and Howard equilibrium theory. Some demonstrative examples are presented. Finally the model is applied to an experimentally studied system. Good quantitative agreement with quenching experiments is obtained, if, simultaneously, the impact of excess quenched-in vacancies and their gradual annihilation in the system, which has been already treated in a previous paper, are accounted for. The model is generally applicable for any couple (and pair) formation.

Development of Mo‐Ni‐Si‐B metallic glass with high thermal stability and H versus E ratios

We report a novel Mo‐Ni‐Si‐B metallic glass which can be solidified into fully amorphous state by melt-spinning process, with high crystallization onset temperature of over 1100K, extremely high Vickers hardness of 27.5±2.2GPa and relatively low Young's modulus of 364.3±6.6GPa. The dense cluster-packing model suggests that the addition of boron up to 10at.% can occupy vacant cluster-interstices of (Mo, Ni)-Si cluster arrays, which results in a more efficiently dense-packed cluster structure, destabilizes the formation of nanocrystalline phases, and systematically increases the glass-forming ability (GFA) in Mo‐Ni‐Si‐B alloys. The GFA parameters that do not directly rely on Tg, such as ΔT* and ε parameter, show greater reliability to evaluate GFA for Mo‐Ni‐Si‐B metallic glass exhibiting no clear Tg. The H/E and H2/(2E) ratios of the newly developed Mo‐Ni‐Si‐B metallic glass, which reflect wear resistance and resilience, exhibit the highest values among various hard ceramic materials as well as metallic glass-forming alloys developed up to now. These advantages of Mo‐Ni‐Si‐B metallic glass can be used more widely to form a high temperature wear-resistant coating layer on various substrates. Furthermore, the same idea might be used to form a metallic glass-nitride nanocomposite coating layer by reactive deposition in N2 ambient, with highly lubricative property and high wear-resistance, especially at high temperature.

Si-rich W silicide films composed of W-atom-encapsulated Si clusters deposited using gas-phase reactions of WF6 with SiH4.

We formed Si-rich W silicide films composed of Sin clusters, each of which encapsulates a W atom (WSi(n) clusters with 8 < n ≤ ∼ 12), by using a gas-phase reaction between WF6 and SiH4 in a hot-wall reactor. The hydrogenated WSi(n)H(x) clusters with reduced F concentration were synthesized in a heated gas phase and subsequently deposited on a substrate heated to 350-420 °C, where they dehydrogenated and coalesced into the film. Under a gas pressure of SiH4 high enough for the WSi(n)H(x) reactant to collide a sufficient number of times with SiH4 molecules before reaching the substrate, the resulting film was composed of WSi(n) clusters with a uniform n, which was determined by the gas temperature. The formed films were amorphous semiconductors with an optical gap of ∼0.8-1.5 eV and an electrical mobility gap of ∼0.05-0.12 eV, both of which increased as n increased from 8 to 12. We attribute this dependence to the reduction of randomness in the Si network as n increased, which decreased the densities of band tail states and localized states.

Raman characterization of damaged layers of 4H-SiC induced by scratching

Recent development of device fabrication of SiC is awaiting detailed study of the machining of the surfaces. We scratched 4H-SiC surfaces with a sliding microindenter made of a SiC chip, and characterized machining affected layers by micro-Raman spectroscopy. The results of the Raman measurement of the scratching grooves revealed that there were residual stress, defects, and stacking faults. Furthermore, with heavy scratching load, we found clusters of amorphous SiC, Si, amorphous carbon, and graphite in the scratching grooves. Analysis of the Raman spectra showed that SiC amorphization occurs first and surface graphitization (carbonization) is subsequently generated through the phase transformation of SiC. We expect that the Raman characterization of machined surfaces provides information on the machining mechanism for compound semiconductors.

Silicon nanocrystals embedded in oxide films grown by magnetron sputtering

This paper presents a comparison of the results that we obtained and reported over the last few years on the structural, optical and light emitting properties of Si-SiO2 and Si-Al2O3 films that were fabricated using a specific configuration of RF magnetron sputtering. In these films the Si volume fraction, x, varies along the film (which is typically 14 cm long) from a value of ~0.1 at one end to ~0.9 at the other end. For the films with x > 0.3, the formation of amorphous Si clusters was observed in as-deposited Si-SiO2 and Si-Al2O3 films. Si nanocrystals (Si-ncs) were generated by high-temperature annealing of the films in nitrogen atmosphere. We found that two processes can contribute to the Si-ncs formation: (i) the crystallization of the existing amorphous Si inclusions in the as-deposited films, and (ii) the thermally stimulated phase separation. Process (i) can be responsible for the independence of Si-ncs mean sizes on x in annealed films with x > 0.5. At the same time, difference in the structural and the light emitting properties of the two types of films was observed. For the samples of the same x, the Si-ncs embedded in the Al2O3 host were found to be larger than the Si-ncs in the SiO2 host. This phenomenon can be explained by the lower temperature required for phase separation in Si-Al2O3 or by the lower temperature of the crystallization of Si-ncs in alumina. The latter suggestion is supported by Raman scattering and electron paramagnetic resonance spectra. In contrast with the Si-SiO2, the Si-ncs embedded in Si-Al2O3 films were found to be under tensile stress. This effect was explained by the strains at the interfaces between the film and silica substrate as well as between the Si inclusions and the Al2O3 host. It was also shown that exciton recombination in Si-ncs is the dominant radiative channel in Si-SiO2 films, while the emission from the oxide defects dominates in Si-Al2O3 films. This can be due to the high number of non-radiative defects at Si-ncs/Al2O3 interface, which is confirmed by our electron paramagnetic resonance data and is consistent with our above suggestion of mechanical stresses in the Si-Al2O3 films.

Structural assignment, and electronic and magnetic properties of lanthanide metal doped silicon heptamers Si7M0/- with M = Pr, Gd and Ho.

The ground state geometries of neutral and anionic lanthanide-metal-doped silicon clusters Si7M0/- with M = Pr, Gd and Ho were determined by quantum chemical (DFT) computations and the previous experimental photoelectron spectra were assigned. The hybrid B3LYP functional is suitable for predicting the ground electronic states of these Si clusters and reproducing well their photoelectron spectra. All the most stable isomers are substitutive derivatives of the bicapped octahedral pure Si80/- clusters. The bicapped octahedral Si7M is generated by substituting one Si atom on a plane of the D4h octahedron by one M atom. Replacement of a Si atom on the C2 axis of another bicapped octahedron by a lanthanide metal atom, where two capping Si atoms are situated in front of opposite triangular face on the side of the central square, gives rise to the anionic Si7M-. The limited participation of f-electrons of the lanthanide metal atoms on the valence electronic structure and thereby in the bonding of Si7M0/- induces high magnetic moments of the doped clusters. As a consequence, not only Si7M0/- but also SinLn0/- clusters can be regarded as suitable building blocks for assembling silicon-based cluster magnetic materials.

Effects of gas flow rate on deposition rate and number of Si clusters incorporated into a-Si:H films

The suppression of cluster incorporation into a-Si:H films is the key to better film stability, because incorporated clusters contribute to the formation of SiH2 bonds and hence lead to light-induced degradation of the films. To deposit stable a-Si:H solar cells at a high deposition rate (DR), we studied the effects of the gas flow rate on DR and the number of Si clusters incorporated into a-Si:H films with discharge power as a parameter, using a multihollow discharge-plasma chemical vapor deposition method. We succeeded in depositing high-quality a-Si:H films with the incorporation of few clusters at DR of 0.1 nm/s. We also found that, under a low gas flow rate and a high discharge power, high-density clusters exist in plasma and hence DR is reduced as a result of radical loss to the clusters.

Global Materials Structure Search with Chemically Motivated Coordinates.

Identification of relevant reaction pathways in ever more complex composite materials and nanostructures poses a central challenge to computational materials discovery. Efficient global structure search, tailored to identify chemically relevant intermediates, could provide the necessary first-principles atomistic insight to enable a rational process design. In this work we modify a common feature of global geometry optimization schemes by employing automatically generated collective curvilinear coordinates. The similarity of these coordinates to molecular vibrations enhances the generation of chemically meaningful trial structures for covalently bound systems. In the application to hydrogenated Si clusters, we concomitantly observe a significantly increased efficiency in identifying low-energy structures and exploit it for an extensive sampling of potential products of silicon-cluster soft landing on Si(001) surfaces.

Irradiation-induced microchemical changes in highly irradiated 316 stainless steel

Cold-worked 316 stainless steel specimens irradiated to 74 dpa in a pressurized water reactor (PWR) were analyzed by atom probe tomography (APT) to extend knowledge of solute clusters and segregation at higher doses. The analyses confirmed that those clusters mainly enriched in Ni–Si or Ni–Si–Mn were formed at high number density. The clusters were divided into three types based on their size and Mn content; small Ni–Si clusters (3–4 nm in diameter), and large Ni–Si and Ni–Si–Mn clusters (8–10 nm in diameter). The total cluster number density was 7.7 × 1023 m−3. The fraction of large clusters was almost 1/10 of the total density. The average composition (in at%) for small clusters was: Fe, 54; Cr, 12; Mn, 1; Ni, 22; Si, 11; Mo, 1, and for large clusters it was: Fe, 44; Cr, 9; Mn, 2; Ni, 29; Si, 14; Mo,1. It was likely that some of the Ni–Si clusters correspond to γ′ phase precipitates while the Ni–Si–Mn clusters were precursors of G phase precipitates. The APT analyses at grain boundaries confirmed enrichment of Ni, Si, P and Cu and depletion of Fe, Cr, Mo and Mn. The segregation behavior was consistent with previous knowledge of radiation induced segregation.