Silicon Clusters Research Articles

Interlinkage of a siliconoid with a silsesquioxane: en route to a molecular model system for silicon monoxide

AbstractA new potential model system for silicon monoxide (SiO) is synthesized by the interlinkage of an unsaturated silicon cluster (siliconoid) with a polyhedral silsequioxane cage. Two derivatives with variable linker size are obtained by the corner‐capping reaction of Ph7T7(ONa)3 with a trichlorosilane featuring a remote benzylic chloride functionality and the subsequent nucleophilic substitution of the latter by the anionic hexasilabenzpolarene‐type Si6 siliconoid. Based on the recently proposed heterogeneous cluster model for the SiO structure, the stoichiometry between silicon and oxygen reiterates the suboxidic interface of the nano‐composite in a Si : O ratio of 14 : 12.

INVESTIGATION OF SILICON-HYDRIDE NANODOTPROPERTIES USING DENSITY FUNCTIONAL THEORY


 
 
 The work aims to elucidate the optical and structural properties of silicon hydride nanoclusters and its bulk via density functional calculations. The molecular geometry is fully optimized with the help of DFT method using Gaussian 09 and Gaussian View 05 Software. Bandgaps are calculated using two approaches; first one using HOMO-LUMO concept and the second one is extracted from UV-vis spectrum. The calculated bandgap of silicon hydride is 3.7 eV. XRD data has also been analyzed using VESTA software, which shows the presence of a silicon matrix that contains organized silicon clusters forming an amorphous silicon matrix. A comprehensive investigation of amorphous films by X-ray diffraction (XRD) shows that the strongest peak in XRD patterns occurs at approximately 30.95°. Using XRD data, average size of optimized silicon hydride nano-cluster has been found to be 158.5 nm. Apparently large value of the cluster size can be attributed to the partially amorphous nature of silicon hydride nanocluster. The partial amorphous nature of silicon hydride is further confirmed from the analysis of its Raman spectrum. An overestimation of size in this case is not ruled out. Raman spectrum is calculated to further confirm deviations in nano-crystalline behavior. We also investigate and reveal properties using IR-spectrum, UV-vis-spectrum, NMR spectrum, and HOMO-LUMO gap.
 
 

Active Buffer Matrix in Nanoparticle‐Based Silicon‐Rich Silicon Nitride Anodes Enables High Stability and Fast Charging of Lithium‐Ion Batteries

AbstractA very promising way to improve the stability of silicon in lithium‐ion battery (LIB) anodes is the use of nanostructured silicon‐rich silicon nitride (SiNx), known as a conversion‐type anode material. To investigate the conversion mechanism in this material in detail, SiN0.5 nanoparticles are synthesized and examined as LIB anodes using a combination of ex situ X‐ray photoelectron spectroscopy and solid‐state 7Li MAS NMR measurements. During the initial cycle, the conversion of SiN0.5 nanoparticles results in the formation of lithium silicides and a buffer matrix consisting of different lithium nitridosilicates and lithium nitride. These phases can be reversibly lithiated and contribute to the total reversible capacity of the silicon nitride active material. The structure of the material after conversion is best described by an amorphous solid solution. Further, it is shown that silicon‐rich silicon nitrides possess improved rate capability because of the higher ionic conductivity of the buffer matrix compared to pure silicon, and very fine dispersion of silicon clusters throughout the buffer matrix. Thus, unlike most conversion materials, the silicon‐rich silicon nitride exhibits an additional intrinsic active functionality of the buffer matrix that goes far beyond the mere reduction of electrolyte contact area and volume expansion.

(Invited) Controlling Void Space in Crumpled Graphene for High Stability Silicon Anodes

Silicon anodes are thought to soon replace graphite in next-generation Li-ion batteries due to silicon’s high capacity (3590 mAh/g for the Li15Si4 alloy at room temperature), availability and natural abundance. However, lithiation of silicon causes a large volume expansion (~300%) which can cause pulverization of the primary silicon particles, cracking and delamination of the bulk electrode and the formation of an unstable solid-electrolyte interface which must rebuild upon each cycle. Some combination of these effects leads to rapid anode failure via electrical isolation of active material and/or electrolyte depletion. To mitigate these challenges, clusters of silicon nanoparticles can be wrapped with flexible 2D-materials like graphene which can potentially act as a dimensionally and electrochemically stable, permeable barrier layers. This can be achieved in a scalable way via spray drying of aqueous dispersions of graphene oxide and silicon. In this talk, I will describe recent work from our group which aims to introduce a controlled amount of void space within the graphene protected silicon structures with the aim of engineering zero-strain silicon/carbon anode particles. In the absence of void space control, capillary forces acting during the spray drying process tightly wrap graphene around silicon clusters leaving little room for volume expansion. In one case, void space is introduced via incorporation of sacrificial polystyrene nanoparticles within the core by co-spray drying silicon, polystyrene and graphene oxide. In a second case, we do not use a sacrificial material but, instead, incorporate a responsive, cross-linked hydrogel within the core which can be expanded upon hydration to increase the volume of the graphene shell. When dehydrated, the gel shrinks to generate the required void space and acts as a Li-ion conducting, elastic binder within the core. In both cases, I will describe the systematic evolution of the crumpled graphene microstructure and its impact on anode performance in both half-cells and full Li-ion batteries.

Siliconoid Expansion by a Single Germanium Atom through Isolated Intermediates.

The growth of (semi‐)metal clusters is pivotal for nucleation processes in gaseous and condensed phases. We now report the isolation of intermediates during the expansion of a stable unsaturated silicon cluster (siliconoid) by a single germanium atom through a sequence of substitution, rearrangement and reduction. The reaction of ligato‐lithiated hexasilabenzpolarene LiSi6Tip5 (1Li⋅(thf)2, Tip=2,4,6‐triisopropylphenyl) with GeCl2⋅NHC (NHC=1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene) initially yields the product with exohedral germanium(II) functionality, which then inserts into an Si−Si bond of the Si6 scaffold. The concomitant transfer of the chloro functionality from germanium to an adjacent silicon preserves the electron‐precise nature of the formed endohedral germylene. Full incorporation of the germanium heteroatom to the Si6Ge cluster core is finally achieved either by reduction under loss of the coordinating NHC or directly by reaction of 1Li⋅(thf)2 with GeCl2⋅1,4‐dioxane.

Boron Silicon B2Si3qand B3Si2pClusters: The Smallest Aromatic Ribbons

The small binary boron silicon clusters B2Si3q with q going from -2 to +2 and B3Si2p with p varying from -3 to +1 were reinvestigated using quantum chemical methods. The thermodynamic stability of these smallest ribbon structures is governed by both Hückel and ribbon models for aromaticity. The more negative the cluster charge, the more ribbon character is shown. In contrast, the more positive the charge state, the more pronounced the Hückel character becomes. The ribbon aromaticity character can also be classified into ribbon aromatic, semiaromatic, antiaromatic, and triplet aromatic when the electron configuration of a ribbon structure is described as [...π2(n+1)σ2n], [...π2n+1σ2n], [...π2nσ2n], and [...π2n+1σ2n-1], respectively. Geometry optimizations of the B2Si3 lowest-energy structure by some density functional theory (DFT) functionals result in a nonplanar shape because it possesses an antiaromatic ribbon character. However, its π aromaticity assigned by the Hückel rule is stronger in such a way that several other DFT and coupled-cluster theory CCSD(T) calculations show that B2Si3 is indeed stable in a planar form (Cs). A new global equilibrium structure for the anion B2Si32-, which is a ribbon semiaromatic species, was identified. Some benchmark tests were also carried out to evaluate the performance of popular methods for the treatment of binary B-Si clusters. At odds with some previous studies, we found that with reference to the high accuracy CCSD(T)/CBS method, the hybrid TPSSh functional is reliable for a structure search, whereas the hybrid B3LYP functional is more suitable for simulations of some experimental spectroscopic results.

Kirkendall Effect in Twin-Roll Cast AA 3003 Aluminum Alloy

The formation of an inhomogeneous structure with central segregation occurs in commercial twin-roll cast AA 3003 aluminum alloy. The segregations form as manganese, iron, and silicon-rich channels spread in the rolling direction. Diffusion of silicon occurs during annealing at 450 °C, and the formation and growth of voids due to the Kirkendall effect occur. The evolution of pores studied by scanning electron microscopy and X-ray computed tomography shows that pores are formed near original pure silicon clusters. Their coalescence and the formation of large voids in the central zone of the strip occur at longer annealing times.

Silicon Cluster Arrays on the Monolayer of Hexagonal Boron Nitride on Ir(111)

Periodic structures of silicon are of interest in quantum-dot-based applications because of their unique optical and electronic properties. We report on the fabrication of stable quasi-ordered Si nanocluster arrays on the moiré of a hexagonal boron nitride (h-BN) monolayer on Ir(111). The h-BN monolayer promotes the growth of regular Si nanoclusters at 130 K and electronically decouples the clusters from the underlying metallic substrate. Using scanning tunneling microscopy and spectroscopy, we have investigated the cluster binding sites, their electronic structure, and their thermal stability. We find that the clusters display a size-dependent bandgap and that they are stable up to 577 K, after which cluster coalescence degrades the arrays.

Annealing-temperature-dependent evolution of hydrogen-related donor and its strong correlation with X-photoluminescence center in proton-irradiated silicon

We have investigated the formation and decay of hydrogen-related donors (HDs) and irradiation-induced intrinsic defects. N-type m:Cz and FZ silicon wafers, which were irradiated with 2 MeV protons and subsequently annealed at 100–600 °C, were analyzed using spreading resistance profiling and photoluminescence (PL). HDs formed at 260 °C and then disappeared in two stages at 400–440 and 500–540 °C. This decay behavior indicates the existence of two types of HDs with different thermal stabilities. PL measurements showed interstitial silicon clusters (W and X center), a carbon–oxygen complex (C center), and exciton lines bound to unknown shallow centers. The origin of the HDs was investigated based on the correlation of the formation and decay temperatures between HDs and irradiation-induced defects. The predominant defects at the early stage of annealing, such as the C and W centers, are ruled out as candidates for the core defects of HDs because annealing above 260 °C is indispensable for the HD formation. In contrast, the X center was found to be thermally generated above 200 °C and disappeared at 580 °C. The similarity of the formation and decay temperatures between the X and HD centers suggests that HDs are associated with the formation of the interstitial silicon-related defects attached to hydrogen. Our results suggest that controlling the formation of interstitial silicon-related defects is important for realizing desirable doping profiles with high accuracy and reproducibility for power devices. Annealing above 400 °C exclusively provides thermally more stable HDs, leading to the realization of more rugged power devices.

Evolution of Vibrational Spectra in the Manganese-Silicon Clusters Mn2Sin, n = 10, 12, and 13, and Cationic [Mn2Si13

A comparison of DFT-computed and measured infrared spectra reveals the ground state structures of a series of gas-phase silicon clusters containing a common Mn2 unit. Mn2Si12 and [Mn2Si13]+ are both axially symmetric, allowing for a clean separation of the vibrational modes into parallel (a1) and perpendicular (e1) components. Information about the Mn–Mn and Mn–Si bonding can be extracted by tracing the evolution of these modes as the cluster increases in size. In [Mn2Si13]+, where the antiprismatic core is capped on both hexagonal faces, a relatively simple spectrum emerges that reflects a pseudo-D6d geometry. In cases where the cluster is more polar, either because there is no capping atom in the lower face (Mn2Si12) or the capping atom is present but displaced off the principal axis (Mn2Si13), the spectra include additional features derived from vibrational modes that are forbidden in the parent antiprism.

Searching new structures of ruthenium-doped in small-sized silicon clusters: RuSin(n = 3–13) clusters

Searching new structures of ruthenium-doped in small-sized silicon clusters: RuSin(n = 3–13) clusters

Unsaturated amido-substituted six-vertex mixed silicon germanium clusters.

The synthesis of mixed silicon and germanium clusters SixGe6-x{N(SiMe3)Dipp}41-3 (x = 3.7, 3.1, 2.1) with amido-substituents and two unsubstituted germanium atoms was achieved in co-reductions using the tribromosilane {N(SiMe3)Dipp}SiBr3 and the tribromogermane {N(SiMe3)Dipp}GeBr3 in three different ratios. The ratio of Si and Ge used for the synthesis of 1-3 approximately corresponds to that in the final products. All compounds were characterized by single crystal X-ray diffraction and EI mass spectrometry. In addition, 1 and 2 were characterized by solid-state 29Si{1H} CP/MAS NMR spectroscopy and multinuclear NMR spectroscopy in solution.

Structures and electronic properties of VSin- (n = 14-20) clusters: a combined experimental and density functional theory study.

We present a systematic study of the structures and electronic properties of vanadium-doped silicon cluster anions, VSin- (n = 14-20), by combining photoelectron spectroscopy (PES) measurements and density functional theory (DFT) based theoretical calculations. High resolution PES of low temperature (10 K) clusters are acquired at a photon wavelength of 248 nm. Low-lying structures of VSi14-20- are obtained by a genetic algorithm based global minimum search code combined with DFT calculations. Excellent agreement is found between the measured PES and the simulated electron density of states of the putative ground-state structures. We conclude that clusters with sizes n = 14 and n = 15 prefer cage-like structures, with the encapsulated vanadium atom bonding with all silicon atoms, while a fullerene-like motif is more favorable for n ≥ 16. For the sizes n = 16 to 19, the structures consist of a V@Si14 with two, three, four, and five Si atoms on the surface of the cage. For n = 20 the structure consists of a V@Si15 with five Si atoms on the surface of the cage. VSi14- has the highest stability and stands out as a simultaneous closing of electronic and geometrical shells.

Anion photoelectron spectroscopy and density functional theory study of TM2Sin- (TM = V, Cr; n = 14-20) clusters.

We investigated the structural evolution and electronic properties of medium-sized silicon cluster anions doped with two transition metal atoms, TM2Sin- (TM = V, Cr; n = 14-20), by using mass-selective anion photoelectron spectroscopy combined with density functional theory (DFT) calculations. Putative ground state structures of these clusters were obtained by using a genetic algorithm coupled with the DFT calculations. It was found that the two TM atoms tend to form a TM-TM bond, which - except for V2Si19- - is shorter than the nearest neighbour distance in the crystalline state of the respective metals. The V2Sin- clusters with n = 14 to 17 exhibit structures based on a silicon hexagonal antiprism, while the larger ones exhibit more fullerene-like cage structures. Cr2Sin- clusters follow the same trend, although with a silicon hexagonal prism structure for n = 14 and 15, and the transition to fullerene-like structures occurring at n = 17. Among these clusters, TM2Si18- have the largest average binding energy and second order differences in energy, therefore the highest relative stability. All of the clusters possess total magnetic moment of 1 μB, but with very different contributions from the doped TM atoms. Especially in the Cr doped clusters there is a tendency towards an anitiferromagnetic arrangement of the magnetic moments of the two Cr atoms.

A strongly twisted SiSi bond with resemblance to a buckled dimer in an unexpected isomer of hexasilabenzene.

The reductive debromination of {N(SiMe3)Ph}SiBr31 with Rieke magnesium yields the six-vertex amido-substituted silicon cluster 2 with zwitterionic character that represents an unprecedented isomer of hexasilabenzene. The topology of Si1 and Si2 in 2 has bonding features of a highly twisted disilene and resembles that of a buckled dimer of Si(100)2 × 1 reconstructed surfaces. Cluster 2 forms the adducts 3 and 4 with NHCMe4 and DMAP, respectively. The NHC adduct 4 additionally coordinates to BH3 which affords the saturated cluster BH3NHCMe4Si6{N(SiMe3)Ph}6 (5). Furthermore, 2 undergoes addition with MeI and iodine to form the halogenated silicon clusters 6 and 7, respectively.

Simulation of initial stage of silicon cluster formation during post-annealing of memristive structures based on silicon oxide films subjected to Si+ implantation

The evolution of the SiO1.7 film defect structure caused by Si+ ion implantation and post-annealing at 1500K for 100 ns has been simulated by using two methods: Monte-Carlo and molecular dynamics. Clustering of silicon atoms and change of the near-structure close order are detected for the selected ion dose of 1·1017 cm-2. The results are discussed in relation to the problem of ion modification of silicon-oxide-based memristors and microscopic mechanisms of electroforming and resistive switching.

Preferential Growth Mode of Large-Sized Vacancy Clusters in Silicon: A Neural-Network Potential and First-Principles Study

An artificial-neural-network (ANN) interatomic potential trained with data from density-functional-theory (DFT) calculations is developed to reveal favorable modes of large-sized vacancy clusters i...

Alkali metals decorated silicon clusters (SinMn, n = 6, 10; M = Li, Na) as potential hydrogen storage materials: A DFT study

In this article, we have studied hydrogen storage properties of alkali-metal decorated silicon clusters (SinMn, n = 6, 10; M = Li, Na) using density functional theory (DFT). The electronic structure, stability and bonding properties of both bare and hydrogen adsorbed clusters are studied and verified using global reactivity descriptors and Quantum Theory Of Atoms in Molecules (QTAIM) study. No distortion in host clusters is observed upon adsorption. H2 bond lengths (0.74Å – 0.75Å), H − AM bond distances (2.134Å−3.108Å) and adsorption energy (0.059eV − 0.141eV) confirmed the adsorption process to be molecular and physisorptive in nature. Li sites in Si6Li6 and Si10Li10 can bind up to 18H2 and 40H2 molecules respectively resulting in a maximum gravimetric density of 14.7 wt% and 18.7 wt% respectively. Similarly, Na sites in Si6Na6 and Si10Na10 can adsorb up to 18H2 and 40H2 molecules resulting in a maximum gravimetric density of 10.6 wt% and 13.6 wt% respectively. The gravimetric densities for respective clusters thus obtained are quite higher than the target 5.5 wt% as set by US-DoE. In addition to this, ADMP-MD simulation reveal that all the host clusters adsorb molecular hydrogen reversibly and can undergo room temperature (300 K) desorption.

Growth pattern of doubly metal doped silicon clusters M2Sin with M2 = Mo2, Nb2, Ta2, W2, NbMo, TaW and n = 11–18. Formation of fused cages M2Si18

Geometries of doubly transition metal doped silicon clusters M2Sin, M2 = Mo2, Nb2, Ta2, W2, NbMo, TaW were determined by DFT computations. Geometries of M2Sin in the range from 11 to 18 Si atoms change from a tubular shape to a cage and then a fused cage. The M2Si18 sizes present us with a novel structural motif, the fused cage, for silicon clusters. Such fused cages arise from a fusion of both MSi10 and MSi12 prisms followed by addition of two Si atoms. Formation and filling of electron shells contribute to the high thermodynamic stability of M2Si18 fused clusters.

Homogenizing Silicon Domains in SiOx Anode during Cycling and Enhancing Battery Performance via Magnesium Doping.

SiOx (x ≈ 1) is one of the most promising anode materials for application in secondary lithium-ion batteries because of its high theoretical capacity. Despite this merit, SiOx has a poor initial Coulombic efficiency, which impedes its widespread use. To overcome this limitation, in this work, we successfully demonstrate a novel synthesis of Mg-doped SiOx via a mass-producible physical vapor deposition method. The solid-state reaction between Mg and SiOx produces Si and electrochemically inert magnesium silicate, thus increasing the initial Coulombic efficiency. The Mg doping concentration determines the phase of the magnesium silicate domains, the size of the Si domains, and the heterogeneity of these two domains. Detailed electron microscopy and synchrotron-based analysis revealed that the nanoscale homogeneity of magnesium silicates driven by cycling significantly affected the lifetime. We found that 8 wt % Mg is the most optimized concentration for enhanced cyclability because MgSiO3, which is the dominant magnesium silicate composition, can be homogeneously mixed with silicon clusters, preventing their aggregation during cycling and suppressing void formation.