Si Clusters Research Articles

Kinetics of large B clusters in crystalline and preamorphized silicon

We present an extended model for B clustering in crystalline or in preamorphized Si and with validity under conditions below and above the equilibrium solid solubility limit of B in Si. This model includes boron-interstitial clusters (BICs) with BnIm configurations—complexes with n B atoms and m Si interstitials—larger (n > 4), and eventually more stable, than those included in previous models. In crystalline Si, the formation and dissolution pathways into large BICs configurations require high B concentration and depend on the flux of Si interstitials. In the presence of high Si interstitial flux, large BICs with a relatively large number of interstitials (m ≥ n) are formed, dissolving under relatively low thermal budgets. On the contrary, for low Si interstitial flux large BICs with few interstitials (m ≪ n) can form, which are more stable than small BICs, and whose complete dissolution requires very intense thermal budgets. We have also investigated the kinetics of large BICs in preamorphized Si, both experimentally and theoretically. B was implanted at a high-dose into preamorphized Si, and the B precipitation was studied by transmission electron microscopy and by sheet resistance and Hall measurement techniques. A simplified model for B clustering and redistribution in amorphous Si is proposed, including the experimental value for the B diffusivity in amorphous Si and the energetics of BICs. Our model suggests that B2, B3I, B4I and B4I2 clusters are the most energetically favored configurations, with relative abundance depending on B concentration. After recrystallization, thermal anneals up to 1100 °C evidence that BICs evolve under very low flux of Si interstitials under the particular experimental conditions considered. Simulations indicate that for very high B concentrations and low Si interstitial flux a significant fraction of the initial small BICs evolves into larger and very stable BIC configurations that survive even after intense thermal budgets, as confirmed by energy filtered transmission electron microscopy analyses. The correlation between simulations and Hall measurements on these samples suggest that hole mobility is significantly degraded by the presence of a high concentration of BICs.

Effect of Incubation Coupled with Artificial Aging in T6 Heat Treatment of A356.2 Aluminum Casting Alloy

A356.2 is one of the most popular Al casting alloys for structural applications in automotive and aerospace sectors. Typically, the cast components from these alloys undergo a T6 heat treatment to significantly improve the mechanical properties. Solution heat treatments, incubation at room temperature, followed by artificial aging at an elevated temperature are the typical steps in a T6 heat treatment process. The incubation process (IP) is typically carried out between 4 to 12 hours on commercial foundry products and typically longer on aerospace castings. There has been a few studies on the mechanisms of precipitation reactions occurring during the incubation and natural aging processes, however all these are for the Al-Si-Mg wrought alloys which are markedly different in composition from Al-Si-Mg casting alloys such as the A356.2. A hypothesis for the sequence of precipitation reaction in the primary Al phase during incubation and artificial aging processes, and the quantified effect of these aging treatments on the resultant tensile properties of the cast components with A356.2 Al alloy is presented. The results show that depending on incubation time, the formation of individual clusters of Si and Mg, breakdown of the individual-clusters of Mg and formation of the co-clusters of Si and Mg (Mg5Si6) would take place. The formation of Mg1.8Si and the final stable Mg2Si phases occurs during artificial aging. The sequence of the precipitation reactions were formulated from micro-hardness data on the primary Al phase matrix, tensile properties and information from the prior-art in this field of study. Further, the time of incubation has a significant effect on the resultant mechanical properties because the precipitation reaction during artificial aging is directly affected by the duration of incubation time.

Analysis of band gap formation in graphene by Si impurities: Local bonding interaction rules

We report band gap formation in graphene by using bonding interactions with Si impurities in the form of Si cluster (Si n ). We demonstrate that neither the distortion in graphene nor the periodicity of the adsorption can lead to band gap opening. The calculated band gap for Si 2 is 0.83 eV at Dirac points and the effect of this Si–C interaction is maintained even when the cluster size is increased. However, there is a strong dependence of the size of the band gap on the size of the Si n . Analysis of the trend points to the change in the dispersion of the gap states due to the change in the Si–C bond.

Surface chemistry of the preferred (111) and (220) crystal oriented microcrystalline Si films by radio‐frequency plasma‐enhanced chemical vapor deposition

AbstractThe surface chemistry of the preferentially (111) and (220) crystal orientated chlorinated hydrogenated microcrystalline silicon (µc‐Si:H:Cl) films was studied using a rf PE‐CVD of a dichlorosilane (SiH2Cl2) and H2 mixture. The growing surface for the preferentially (220) crystal oriented µc‐Si:H:Cl films included much voids and dangling bonds, whereas the growing surface with the preferential (111) crystal orientation was chemically stable relatively. These findings suggest that the sticking process of deposition precursors and/or the reconstruction of Si clusters within the sub‐surface determine the preferential crystal orientation. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the stability of “non-magic” endohedrally doped Si clusters: A first-principles sampling study of M${\rm Si}_{16}^+$Si16+ (M = Ti,V,Cr)

Density-functional theory is used to study the geometric and electronic structure of cationic Si(16)(+) clusters with a Ti, V, or Cr dopant atom. Through unbiased global geometry optimization based on the basin-hopping approach, we confirm that a Frank-Kasper polyhedron, with the metal atom at the center, represents the ground-state isomer for all three systems. The endohedral cage geometry is thus stabilized even though only VSi(16)(+) achieves electronic shell closure within the prevalent spherical potential model. Our analysis of the electronic structure traces this diminished role of shell closure for the stabilization back to the adaptive capability of the metal-Si bonding, which is more the result of a complex hybridization than the originally proposed mere formal charge transfer. The resulting flexibility of the metal-Si bond can also help to stabilize "non-magic" cage-dopant combinations, which suggests that a wider range of materials may eventually be cast into this useful geometry for cluster-assembled materials.

The effect of geometry on cluster polarizability: Studies of sodium, copper, and silicon clusters at shape-transition sizes

The electronic properties of Na(16), Cu(16,) and Si(20-28) clusters were calculated using density functional theory with the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof. These clusters are special, as transitions in cluster shape occur at these sizes in the Na(n), Cu(n), and Si(n) cluster systems, respectively. Low-energy isomers that are comparable in stability, but possess distinctly different shapes, exist at each of these sizes, making these sets of isomers useful as probes of geometrical effects on cluster properties. Results for ionization potentials, electron affinities, and polarizabilities are shown to have a characteristic dependence on cluster shape. An analysis of the results reveals a close relationship between polarizability and cluster volume for all the isomers studied, despite the differences in cluster type and geometry. This relationship accounts for variations in polarizabilities among isomers of the same size, but different shapes, whereas previously published rules relating the polarizability to other cluster properties do not.

Direct Imaging of Two-State Dynamics on the Amorphous Silicon Surface

Amorphous silicon is an important material, amidst a debate whether or not it is a glass. We produce amorphous Si surfaces by ion bombardment and vapor growth, and image discrete Si clusters which hop by two-state dynamics at 295 K. Independent of surface preparation, these clusters have an average diameter of ∼5 atoms. Given prior results for metallic glasses, we suggest that this cluster size is a universal feature. The hopping activation free energy of 0.93±0.15 eV is rather small, in agreement with a previously untested surface glass model. Hydrogenation quenches the two-state dynamics, apparently by increasing surface crystallinity.

Annealing-induced evolution of optical properties of the multilayered nanoperiodic SiO x /ZrO2 system containing Si nanoclusters

The photoluminescence, infrared absorption, and Raman spectra of amorphous multilayered nanoperiodic a-SiOx/ZrO2 structures produced by vacuum evaporation and then annealed at different temperatures (500–1100°C) are studied. It is established that the evolution of the optical properties with increasing annealing temperature is controlled by sequential transformation of Si clusters formed in the SiOx layers from nonphase inclusions to amorphous clusters and then to nanocrystals. The finally formed nanocrystals are limited in sizes by the thickness of the initial SiOx layers and by chemical reactions with ZrO2.

New semiconducting silicides assembled from transition-metal-encapsulating Si clusters

We synthesized amorphous films composed of transition-metal-encapsulating Si clusters ( MSi n : M = Zr, Nb, Mo and W) by deposition of hydrogenated MSi n H x clusters onto solid substrates followed by annealing at 400–500 °C for dehydrogenation. The MSi n ( n = 7–20) cluster films are amorphous semiconductors with an optical gap > 0.4 eV and have larger electron and hole mobility than that of the hydrogenated amorphous Si (a-Si:H) film. In these films, while Si atoms form amorphous networks similar to those in a-Si:H films, the thermal stability is enhanced and the electronic disorder is reduced by the use of MSi n clusters as the unit structures. Structure modeling by ab initio calculations for MSi n films suggests that the encapsulated M atom works as a terminator of dangling bonds of the Si network.

Size and hydrogen saturation effects on third-order polarizabilities of Si clusters

We report the frequency-dependent third-order polarizabilities (γ) of a series of hydrogen-capped silicon clusters obtained using time-dependent density functional theory. We find that the magnitude of γ increases with a decrease in silicon cluster size, while the sign of γ at low input photon energy changes from positive to negative when the ratio of hydrogen saturation increases. At higher input photon energy, the sign of γ is determined mainly by cluster size. Our results rationalize several recent experiments on the optical properties of silicon nanostructures and are helpful for designing opto-electronic devices based on them.

Dopant Effects on Single and Multiple Excitons in Small Si Clusters: High-Level Ab Initio Calculations

Doping has been critical to the realization of current semiconductor devices, allowing for control and tuning of the electronic properties. Whereas doping is now common in bulk, its extension to spatially confined materials has not been easy. Recently, suitable synthetic methods have been developed, and it has become vital to understand the changes doping affects in spatially confined materials. For this purpose, we performed high-level, ab initio quantum-chemical calculations on small boron- and phosphorus-doped Si clusters, and obtained optical absorption spectra and excited states properties over a broad energy range, including single and multiple excitons. We find that doping blue-shifts the optical absorption spectra and moves the onset of multiexciton configurations to significantly higher energies. The latter factor can be very important in photovoltaic applications. The effect of doping on the cluster properties is similar to that of charging.

The influence of assisted ion beam bombardment on structure and electrical characteristics of HfSiO dielectric synthesized by DIBSD

We have investigated the influence of assisted ion beam bombardment on structure and electrical properties of HfSiO dielectrics deposited on Si (1 0 0) substrate by dual-ion beam sputtering deposition (DIBSD). The X-ray photoelectron spectroscopy (XPS) analysis indicates that assisted ion beam bombardment could suppress the formation of Si clusters and partial Si O bonds. The excellent electrical properties with maximum dielectric constant (18.6) and the smaller oxide-charge density (7.2 × 10 11 cm −2) and leakage current (2.8 × 10 −7 A/cm 2 at ( V fb−1) V) were obtained for HfSiO film by assisted ion beam bombardment at AIE = 100 eV, which provide a initial energy for the formation of film, activate the substrate surface atoms, enhance the polarization rate and improve the film surface compact and adhesion.

Molecular and Electronic Tuning of Si/Carbon Nanotube Hybrid System

In complex structures such as conjugated Si nanoparticles on carbon nanotubes (CNT), new properties arise out particle size control or nanoparticle–nanotube contact. In this work, the interaction of Si clusters (Sin, n=1–4) with metallic (5,5) single-wall carbon nanotube (SWCNT) is investigated using first principles calculations based on density functional theory, to show that electronic property of the Si/SWCNT system can change via Si–SWCNT conformation alone. Planar Si clusters on SWCNT exhibit the same metallic property regardless of size. However, once the planarity of Sin is lost, which happens when Si adsorption sites shifts towards the top and the Si–C bonds bend resulting to a buckled Sin configuration, a band gap of ∼0.32 eV in the entire Si/SWCNT system is introduced. The migration behavior of Si adatoms shows ease of cluster formation due to surface curvature. Such cluster formation can be accommodated by a distortion in the C–C bonds of CNT. Mechanism for the opening of the SWCNT band gap is presented.

First-principles calculations of structural and electronic properties of Ta-doped Si clusters, wires, and bulk systems

Recent experiments have shown that Ta@Si${}_{16}{}^{+}$ is a very stable cation from which it should be possible to create Si-based cluster assembled materials. In this paper we have studied, by means of first-principles spin-dependent generalized gradient approximation calculations, the structural and electronic properties of the following systems: (i) Ta@Si${}_{n}{}^{+}$ clusters in the range $n=14\ensuremath{-}18$; (ii) (Ta@Si${}_{16}$F)${}_{m}$ aggregates with sizes $m=1\ensuremath{-}8$ formed by Ta@Si${}_{16}$F molecules; (iii) infinite wires formed by stacking triangular (Ta@Si${}_{16}$F)${}_{3}$ aggregates twisted 60${}^{\ifmmode^\circ\else\textdegree\fi{}}$ to each other along the vertical axis; and (iv) the fcc phase of bulk Ta@Si${}_{16}$F. The minimum-energy Ta@Si${}_{16}{}^{+}$ cluster shows ${C}_{3v}$ symmetry, having 40 meV smaller total energy than a fullerenelike ${D}_{4d}$ isomer. However, the molecule Ta@Si${}_{16}$F formed with that ${D}_{4d}$ isomer is 40 meV more stable than that formed with the ${C}_{3v}$ one. We have optimized several [Ta@Si${}_{16}$F]${}_{n}$ aggregates ($n=1\ensuremath{-}8$) which contain the Ta@Si${}_{16}$ unit with ${D}_{4d}$ symmetry. The more bound (Ta@Si${}_{16}$F)${}_{6}$ aggregate is formed by stacking vertically two triangular (Ta@Si${}_{16}$F)${}_{3}$ aggregates which are twisted 60${}^{\ifmmode^\circ\else\textdegree\fi{}}$ to each other. The infinite wire formed with that (Ta@Si${}_{16}$F)${}_{6}$ aggregate as the unit cell has a cohesive energy 1.88 eV and a small highest occupied molecular orbital--lowest occupied molecular orbital gap. We have optimized also a metastable fcc bulk phase having the Ta@Si${}_{16}$F supermolecule as the unit cell. A Birch-Murnaghan fit to that phase produces a cohesive energy 0.84 eV at lattice constant 12.27 \AA{}, with bulk modulus 7.55 GPa and a phase stability to isotropic compression smaller than 0.75 GPa. That phase is nonmagnetic and shows a band gap of 0.20 eV. Using the values of hardness of Ta@Si${}_{16}$F molecules, we estimated a correction enhancement factor $~$3 to that small band gap. For that metastable solid we performed a 13.5-ps run of first-principles molecular dynamics annealing at 300 K and constant volume, and we found that the Ta@Si${}_{16}$F supermolecule in the fcc cell becomes severely distorted after the first 5 ps.

In situ study on the electronic structure of graphene grown on 6H–SiC [formula omitted] with synchrotron radiation photoelectron spectroscopy

Low energy electron diffraction (LEED) and synchrotron radiation photoelectron spectroscopy (SRPES) were used to study the synthesis process and detailed electronic structures of graphene produced by thermal decomposition of 6H–SiC (0 0 0 1¯) in ultrahigh vacuum (UHV). The LEED results showed that as annealing temperature increased, different reconstructions of 6H–SiC (0 0 0 1¯) appeared and the anisotropic graphene layers were produced finally. The results of C 1s core levels indicated that the component, which was assigned to graphene emerged and the interface interaction between graphene and the substrate was weak. The existence of Si clusters was confirmed by Si 2p core levels and perhaps these clusters led to rough epitaxial graphene surface. The results of valence band spectra showed that at high annealing temperature both of σ and π bonds existed in the grown graphene layers.

Si–Si optical phonon behavior in localized Si clusters of Si x Ge1−x alloy nanocrystals

Raman spectra acquired from Si x Ge1−x -nanocrystal-embedded SiO2 films show dependence of the Si–Si optical phonon frequency on Si content. The frequency upshifts, and peak intensity increases as the silicon concentration increases. For a given Si content, the frequency remains unchanged with annealing temperature. Spectral analysis and density functional theory calculation reveal that the optical Si–Si phonon is related to the formation of localized Si clusters surrounded by Si/Ge atomic layers in the Si x Ge1−x nanocrystals and the intensity enhancement arises from the larger cluster size. The synergetic effect of surface tensile stress and phonon confinement determines the Si–Si optical phonon behavior.

Nano-sized SiOx/C composite anode for lithium ion batteries

Nano-sized SiOx/C composite with core–shell structure is prepared by a modified Stöber method. After heat-treatment, the O/Si ratio in SiOx/C composite is near 1 and the core of SiOx presents a structure composing of amorphous Si clusters and ordered SiO2 domains. SiOx/C composite anode shows high specific capacity (ca. 800mAhg−1), excellent cycling stability, good rate-capability but low initial coulombic efficiency. Li2O and Li4SiO4 may generate in the initial lithiation process, which, combining with the carbon shell, can buffer the volume change caused by the alloying of Si with Li, and thereby improving the cycling stability of electrode. The nano feature of SiOx/C particle and the electronic conductive nature of carbon coating layer ensure the good rate-capability of SiOx/C electrode.

The spatial distribution of silicon NCs and erbium ion clusters by simultaneous high‐resolution energy filtered and Z‐contrast STEM and transmission electron tomography

AbstractImaging the spatial distribution of silicon nanocrystals (Si‐NCs) and erbium ion clusters can assist in the development of light‐emitting erbium‐doped silicon nanocomposites. We applied simultaneous energy‐filtered and high‐angle annular dark field imaging in the scanning transmission electron microscopy (STEM) mode; the former method images the Si NCs and the latter is sensitive to the high atomic number of the Er3+ ions. Both methods were performed simultaneously so as to avoid relative image drift. In Er‐doped Si nano‐composites, we find that the Er ions are not uniformly distributed in the matrix: instead, they form clusters that appear in projection images to decorate the boundaries of the Si‐NCs. This structure may develop during sample annealing, due to the low solubility of Er in the forming Si clusters. Although multi‐detector simultaneous imaging is a good way to avoid nanometer‐scale image drift that could otherwise conceal the nanoscale spatial relationship between Si‐NCs and Er clusters, an additional difficulty relates to the projection imaging methods. Essentially, it is hard to obtain spatial information parallel to the electron beam direction, so where there is apparent overlap we cannot ascertain whether an Er cluster (for example) lies above, below, or inside a given Si‐NC. Therefore, we also report on initial electron beam tomography investigations whose goal is ultimately to generate methods for obtaining the Er and Si cluster distribution in 3D. The results can lead to improved understanding of the sensitization mechanism and optimization for Er‐doped silicon nanocomposites; the clustered nature of the Er ions near the Si NCs is certainly undesirable and may explain the typically short Er3+ photoluminescence (PL) lifetimes (compared to Er‐doped silica) and the difficulty in reliably observing stimulated emission phenomena (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Retracted article: Measuring Si–C60 chemical forces via single molecule spectroscopy

We measure the short-range chemical force between a silicon-terminated tip and individual adsorbed C(60) molecules using frequency modulation atomic force microscopy. The interaction with an adsorbed fullerene is sufficiently strong to drive significant atomic rearrangement of tip structures.

Annealing Effects on Cathodoluminescence Properties of SiOx Films Deposited by Radio Frequency Sputtering

The effects of high-temperature thermal annealing on cathodoluminescence (CL) spectra in SiOx (0.9 ≤x ≤1.87) films prepared by radio-frequency sputtering are investigated. The CL intensities for the as-deposited films are weak but they increase after thermal annealing at 900 and 1100 °C. One of features in the CL spectra for the films annealed at 1100 °C is a peak at a photon energy of ∼2.7 eV with an asymmetric tail on the lower energy side. In order to analyze the spectral features, optical transition energies are calculated for Sin clusters with n = 2–5, embedded in a SiOx matrix, by ab initio molecular orbital calculation. In addition, the probabilities of formation are statistically estimated for those Si clusters under the assumption of a chemically ordered random network for the SiOx network. The comparison of the experimental results with the calculated transition energies and the statistics of the Si clusters suggests that a contribution of the Si2 clusters to the CL spectra are dominant, whereas those of the Sin clusters with n > 3 are considerably small.