Si Clusters Research Articles

Thermal stability of amorphous Si-rich W silicide films composed of W-atom-encapsulated Si clusters

We demonstrate the excellent thermal stability of an amorphous film composed of W-atom-encapsulated Si (WSin) clusters with n ≥ 8, formed by thermal deposition using WF6 and SiH4 gas sources. To determine how the structure of the constituting clusters affects the thermal stability of the film, we also prepared films containing unencapsulated WSin clusters in which the W atom was not fully encapsulated in the Sin cage, with n ≤ 7. The effect of annealing was investigated by Raman scattering and optical absorption measurements with repeated 10-min isochronal annealing in a N2 atmosphere at temperatures ranging from 500 to 1100 °C. The films, composed of WSin clusters completely encapsulating W atoms with a uniform composition of n = 12, remained in the same amorphous structure up to 1000 °C, although partial crystallization of Si began at 1100 °C. The stability decreased when the film contained unencapsulated WSin clusters, even with an average film composition of n ≤ 10; partial crystallizations of Si and WSi2 were observed after annealing at 800 °C. Density-functional theory calculations indicate that a structure assembled from three encapsulated WSi12 clusters preserves the bonding topology of the constituting clusters in which Si atoms are strongly bonded, accounting for the high thermal stability of the film composed of encapsulated WSin clusters.

Optimized interatomic potential for silicon and its application to thermal stability of silicene

An optimized interatomic potential has been constructed for silicon using a modified Tersoff model. The potential reproduces a wide range of properties of Si and improves over existing potentials with respect to point defect structures and energies, surface energies and reconstructions, thermal expansion, melting temperature and other properties. The proposed potential is compared with three other potentials from the literature. The potentials demonstrate reasonable agreement with first-principles binding energies of small Si clusters as well as single-layer and bilayer silicenes. The four potentials are used to evaluate the thermal stability of free-standing silicenes in the form of nano-ribbons, nano-flakes and nano-tubes. While single-layer silicene is mechanically stable at zero Kelvin, it is predicted to become unstable and collapse at room temperature. By contrast, the bilayer silicene demonstrates a larger bending rigidity and remains stable at and even above room temperature. The results suggest that bilayer silicene might exist in a free-standing form at ambient conditions.

Atomically manufactured nickel\u2013silicon quantum dots displaying robust resonant tunneling and negative differential resistance

Providing a spin-free host material in the development of quantum information technology has made silicon a very interesting and desirable material for qubit design. Much of the work and experimental progress has focused on isolated phosphorous atoms. In this article, we report on the exploration of Ni–Si clusters that are atomically manufactured via self-assembly from the bottom-up and behave as isolated quantum dots. These small quantum dot structures are probed at the atomic-scale with scanning tunneling microscopy and spectroscopy, revealing robust resonance through discrete quantized energy levels within the Ni–Si clusters. The resonance energy is reproducible and the peak spacing of the quantum dot structures increases as the number of atoms in the cluster decrease. Probing these quantum dot structures on degenerately doped silicon results in the observation of negative differential resistance in both I–V and dI/dV spectra. At higher surface coverage of nickel, a well-known √19 surface modification is observed and is essentially a tightly packed array of the clusters. Spatial conductance maps reveal variations in the local density of states that suggest the clusters are influencing the electronic properties of their neighbors. All of these results are extremely encouraging towards the utilization of metal modified silicon surfaces to advance or complement existing quantum information technology.

Transformation of amorphous alloy surface and thin film under impact of slow heavy ions

In this work thin foils of amorphous alloys VP800 (Fe73Si16B7Cu1Nb3) and VV8025X (Fe4Co66B14Cu1Nb2Mo1) maintained at the onset point temperature for primary crystallisation were irradiated with slow heavy ions (100–300keV Ar and Xe) at the fluence changing from 1010 to 1013ions/cm2. The preferential surface modification during ion implantation-sputtering was analysed with SRIM and PIXE. With the use of CEMS Fe and Fe(Si) clusters accompanied by Fe3Si and even Fe23B6 nanocrystals were found in the films irradiated at a lower fluence, whereas rather amorphous structure was found in surfaces more heavily implanted.

Polarized Raman spectra of BaSi2 epitaxial film grown by molecular beam epitaxy

The determination of Raman modes in the range between 280 and 500 cm−1 for a BaSi2(100) epitaxial film on a Si(001) substrate has been accomplished by polarized Raman measurements using a crystal rotation method. The six Raman lines from Si clusters in BaSi2 were observed in the range. In the analysis of their intensity changes as a function of crystal rotation angle based on the Raman tensors of the Si cluster, the six Raman internal modes of A1 (1), E (2), and F2 (3) were completely assigned to the observed Raman lines.

On the Analysis of Clustering in an Irradiated Low Alloy Reactor Pressure Vessel Steel Weld.

Radiation induced clustering affects the mechanical properties, that is the ductile to brittle transition temperature (DBTT), of reactor pressure vessel (RPV) steel of nuclear power plants. The combination of low Cu and high Ni used in some RPV welds is known to further enhance the DBTT shift during long time operation. In this study, RPV weld samples containing 0.04 at% Cu and 1.6 at% Ni were irradiated to 2.0 and 6.4×1023 n/m2 in the Halden test reactor. Atom probe tomography (APT) was applied to study clustering of Ni, Mn, Si, and Cu. As the clusters are in the nanometer-range, APT is a very suitable technique for this type of study. From APT analyses information about size distribution, number density, and composition of the clusters can be obtained. However, the quantification of these attributes is not trivial. The maximum separation method (MSM) has been used to characterize the clusters and a detailed study about the influence of the choice of MSM cluster parameters, primarily on the cluster number density, has been undertaken.

Density-functional study of the structures and properties of holmium-doped silicon clusters HoSi n (n = 3-9) and their anions.

The structures and properties of Ho-doped Si clusters, including their adiabatic electron affinities (AEAs), simulated photoelectron spectra (PESs), stabilities, magnetic moments, and charge-transfer characteristics, were systematically investigated using four density-functional methods. The results show that the double-hybrid functional (which includes an MP2 correlation component) can accurately predict the ground-state structure and properties of Ho-doped Si clusters. The ground-state structures of HoSi n (n = 3-9) are sextuplet electronic states. The structures of these Ho-doped Si clusters (aside from HoSi7) are substitutional. The ground-state structures of HoSi n- are quintuplet electronic states. Their predicted AEAs are in excellent agreement with the experimental ones. The mean absolute error in the theoretical AEAs of HoSi n (n = 4-9) is only 0.04eV. The simulated PESs for HoSi n- (n = 5-9) are in good agreement with the experimental PESs. Based on its simulated PES and theoretical AEA, we reassigned the experimental PES of HoSi4- and obtained an experimental AEA of 2.2 ± 0.1eV. The dissociation energies of Ho from HoSi n and HoSi n- (n = 3-9) were evaluated to test the relative stabilities of the clusters. HOMO-LUMO gap analysis indicated that doping the Si clusters with the rare-earth metal atom significantly increases their photochemical reactivity. Natural population analysis showed that the magnetic moments of HoSi n (n = 3-9) and their anions derive mainly from the Ho atom. It was also found that the magnetic moments of Ho in the HoSi n clusters are larger than the magnetic moment of an isolated Ho atom.

Multiple Exciton Generation in Si and Ge Nanocrystals: An ab Initio Comparative Study

We have simulated multiexciton generation (MEG) processes in Si and Ge nanocrystals, employing the equation of motion coupled cluster single and double as a high-level ab initio approach. Simulations, consistent with the experimental results reported so far, reveal the key role of the d-polarized valence component in the chosen basis set on the accuracy and reliability of the results. Moreover, the MEG thresholds calculated with def2SVP basis set are ∼8.23(8.07) eV for seven (eight)-atom Si clusters and ∼7.58(6.84) eV for similar Ge clusters. The normalized MEG thresholds of Ge nanocrystals are 8% smaller with respect to Si. Thus, in contrast to Si, they are more appealing to the optical device designers for enhancing the device quantum efficiency. Furthermore, the resemblance of the symmetry of the simulated seven-atom clusters to those of the experimentally domelike grown nanocrystals makes the behavior of their MEG quantum probability similar.

Suppression of silicon clusters using a grid mesh under DC bias

Si clusters generated during the plasma chemical vapor deposition (CVD) process have a great influence on the quality of the fabricated films. In particular, in hydrogenated amorphous silicon thin films (a-Si:H) used for thin film solar cells, Si clusters are mainly responsible for light-induced degradation. In this study, we investigated the amount of clusters incorporated into thin films using a quartz crystal microbalance (QCM) and specially designed cluster eliminating filters, and investigated the effect of the DC grid mesh in preventing cluster incorporation. Experimental results showed that as the applied voltage of the grid mesh, which is placed between the electrode and the QCM, decreased, the number of clusters incorporated into the film decreased. This is due to the electrostatic force from the grid mesh bias, and this method is expected to contribute to the fabrication of high-quality thin films by preventing Si cluster incorporation.

Nucleation and growth mechanisms of nano-scaled Si precipitates in Al-7Si supersaturated solid solution

A dense uniform distribution of nano-scaled Si precipitates has been achieved in Al-7Si alloy by high pressure solution treatment (HPST) and aging treatment. Precipitation behavior of Si phase was investigated using transmission electron microscopy (TEM). The results reveal that Si clusters initially precipitate from the Al(Si) solid solution, providing preferable nucleation sites for the equilibrium Si phase. The Si crystals exhibit spherical shape at first, and then grow rapidly parallel to {111}Si planes. The shortage of growth ledges on {111} interfaces leads to the inhibition of growth perpendicular to these planes, which results in Si triangles and platelets with high aspect ratio. An equation about the relationship between the relative free energy and the equilibrium ledge densities on Al-Si interfaces was proposed.

Atomistic Modelling of Si Nanoparticles Synthesis

Silicon remains the most important material for electronic technology. Presently, some efforts are focused on the use of Si nanoparticles—not only for saving material, but also for improving the efficiency of optical and electronic devices, for instance, in the case of solar cells coated with a film of Si nanoparticles. The synthesis by a bottom-up approach based on condensation from low temperature plasma is a promising technique for the massive production of such nanoparticles, but the knowledge of the basic processes occurring at the atomistic level is still very limited. In this perspective, numerical simulations can provide fundamental information of the nucleation and growth mechanisms ruling the bottom-up formation of Si nanoclusters. We propose to model the low temperature plasma by classical molecular dynamics by using the reactive force field (ReaxFF) proposed by van Duin, which can properly describe bond forming and breaking. In our approach, first-principles quantum calculations are used on a set of small Si clusters in order to collect all the necessary energetic and structural information to optimize the parameters of the reactive force-field for the present application. We describe in detail the procedure used for the determination of the force field and the following molecular dynamics simulations of model systems of Si gas at temperatures in the range 2000–3000 K. The results of the dynamics provide valuable information on nucleation rate, nanoparticle size distribution, and growth rate that are the basic quantities for developing a following mesoscale model.

A Study on the Aging Behavior of Al6061 Composites Reinforced with Y2O3 and TiC

The reinforcement particles play important roles in determining microstructural development and properties of Al6061 composites. In the present work, the aging behavior of Al6061 reinforced with Y2O3 and TiC particles produced via mechanical alloying are investigated. The results indicate that the peak-aged Al6061 alloy without reinforcement demonstrates the highest hardness, which corresponds to the formation of the Mg–Si precipitates. However, precipitation formation is not observed in the case of the Al6061 composites, which can be attributed to the fact that the Mg–Si clusters and GP zones are inhibited by the presence of the reinforcement particles. The solute elements segregate in the complex oxides or carbides and contribute to only a slight increase in hardness.

Improvement of Strength–Elongation Balance of Al–Mg–Si Sheet Alloy by Utilising Mg–Si Cluster and Its Proposed Mechanism

Improvement of Strength–Elongation Balance of Al–Mg–Si Sheet Alloy by Utilising Mg–Si Cluster and Its Proposed Mechanism

Insights into the Li Intercalation and SEI Formation on LiSi Nanoclusters

We report a first-principles atomic level assessment of the lithiation and reactivity of pre-lithiated Si clusters. Density functional theory formation energy calculations reveal that the pre-lithiated Li16Si16 cluster exposed to two different Li fluxes can store Li between the concentrations of Li2.5Si and Li3.5Si. This increase in storage capacity is attributed to the start of an amorphization process in the cluster, and more importantly these results show that the intercalation reaction can be controlled by the flux of the Li-ions. However, in a real battery, the lithiation of the anode occurs simultaneously to the electrode-electrolyte reactions. Here we simulate the solid-electrolyte interphase (SEI) formation and simultaneous lithiation of a Li16Si16 cluster in contact with two different electrolyte solutions: one with pure ethylene carbonate (EC), and another with a 0.6 M solution of LiPF6 in EC. Our ab initio molecular dynamics simulations show that the solvent and salt are decomposed leading to the initial stages of the SEI layer formation and large part of the added Li becomes part of the SEI. Interestingly, the pure EC solution results in lower storage capacity and higher reactivity, whereas the presence of the salt causes the opposite effect: higher lithiation and reduced reactivity.

FTIR and Raman study of rapid thermal annealing and oxidation effects on structural properties of silicon-rich SixC1-x thin films deposited by R.F co-sputtering

Si-rich silicon carbide (SixC1-x) thin films have been deposited by Radio Frequency (RF) co-sputtering. These films were deposited from a composite target consisting of crystalline silicon fragments regularly distributed on the surface of a pure graphite disc. The surface covered by Si fragments is about 65% of the total target surface. The deposited films were subjected to rapid (15min) thermal annealing, under inert atmosphere (Ar), at different temperatures varying from 700 to 1000°C. The influence of thermal annealing on structural properties of these films was investigated by Fourier Transform Infrared (FTIR) and Raman spectroscopy techniques. FTIR spectra obtained on the as deposited and annealed films show the presence of two mine absorption bands, which are assigned to vibrational modes of SiC and SiO bonds. The presence of SiO bonds is attributed to oxygen incorporation during the exposure of samples in the atmosphere before the analysis. Upon annealing, three significant changes occur in the SixC1-x films: a significant increase of both SiC and SiO bonds content, a decrease of the films thickness and a beginning of SiC crystallization after annealing at 1000°C. Raman spectra exhibit three bands which are assigned to SiSi, SiC and C:C vibration modes. Raman results suggest that C:C and SiSi bonds present inside the material are probably isolated or located in small clusters. Upon annealing, a crystallization of Si clusters and a graphitization of carbon clusters are observed.

Si clusters are more metallic than bulk Si.

Dipole polarizabilities were computed using density functional theory for silicon clusters over a broad range of sizes up to N = 147 atoms. The calculated total effective polarizabilities, which include contributions from permanent dipole moments of the clusters, are in very good agreement with recently measured values. We show that the permanent dipole contributions are most important for clusters in the intermediate size range and that the measured polarizabilities can be used to distinguish between energetically nearly degenerate cluster isomers at these sizes. We decompose the computed total polarizabilities α into the so-called dipole and charge transfer contributions, αp and αq, using a site-specific analysis. When the per-atom values of these quantities are plotted against N-1/3, clear linear trends emerge that can be extrapolated to the large size limit (N-1/3→0), resulting in a value for αN of 30.5 bohrs3/atom that is significantly larger than the per-atom polarizability of semiconducting bulk Si, 25.04 bohrs3/atom. This indicates that Si clusters possess a higher degree of metallicity than bulk Si, a conclusion that is consistent with the strong electrostatic screening of the cluster interiors made evident by the analysis of the calculated atomic polarizabilities.

Effect of straining graphene on nanopore creation using Si cluster bombardment: A reactive atomistic investigation

Graphene nanosheets have recently received a revival of interest as a new class of ultrathin, high-flux, and energy-efficient sieving membranes because of their unique two-dimensional and atomically thin structure, good flexibility, and outstanding mechanical properties. However, for practical applications of graphene for advanced water purification and desalination technologies, the creation of well controlled, high-density, and subnanometer diameter pores becomes a key factor. Here, we conduct reactive force-field molecular dynamics simulations to study the effect of external strain on nanopore creation in the suspended graphene by bombardment with Si clusters. Depending on the size and energy of the clusters, different kinds of topography were observed in the graphene sheet. In all the considered conditions, tensile strain results in the creation of nanopores with regular shape and smooth edges. On the contrary, compressive strain increases the elastic response of graphene to irradiation that leads to the formation of net-like defective structures with predominantly carbon atom chains. Our findings show the possibility of creating controlled nanopores in strained graphene by bombardment with Si clusters.

Microscopic mechanism for light-induced degradation of silicon solar cell in water vapor environment

Abstract Degradation is the key factor to determine the efficiency and stability of the silicon solar cells, which highly affects their cost and practical performance. Humidity has been well recognized to accelerate the degradation process of the silicon solar cells, but the microscopic mechanism of light-induced degradation in hydrogenated silicon devices at water vapor environment is still elusive. In this paper, combining time-dependent density functional theory with ab initio molecular dynamics (AIMD), we show the water cluster adsorption slightly more elongates the Si-Si bond under illumination compared to the vacuum, which induces electron localization to further greatly facilitate the oxygen molecule absorption and dissociation. This external degradation process is different from the intrinsic degradation mechanisms in hydrogenated silicon devices under light irradiation caused by formation of metastable hydrogen complexes. The transition-state calculations of degradation pathway show the oxidation reaction energy barrier of Si cluster in water vapor environment is lower than that in vacuum, which confirms AIMD observations. Our results not only shed new insight for understanding the microscopic mechanism of light-induced degradation under water vapor, but would also be beneficial for controlling the light-induced silicon degradation of solar cell in real application.

Unveiling the pentagonal nature of perfectly aligned single-and double-strand Si nano-ribbons on Ag(110)

Carbon and silicon pentagonal low-dimensional structures attract a great interest as they may lead to new exotic phenomena such as topologically protected phases or increased spin–orbit effects. However, no pure pentagonal phase has yet been realized for any of them. Here we unveil through extensive density functional theory calculations and scanning tunnelling microscope simulations, confronted to key experimental facts, the hidden pentagonal nature of single- and double-strand chiral Si nano-ribbons perfectly aligned on Ag(110) surfaces whose structure has remained elusive for over a decade. Our study reveals an unprecedented one-dimensional Si atomic arrangement solely comprising almost perfect alternating pentagons residing in the missing row troughs of the reconstructed surface. We additionally characterize the precursor structure of the nano-ribbons, which consists of a Si cluster (nano-dot) occupying a silver di-vacancy in a quasi-hexagonal configuration. The system thus materializes a paradigmatic shift from a silicene-like packing to a pentagonal one.

Threshold resistance switching in silicon-rich SiOx thin films**Project supported by the Open Project Program of Surface Physics Laboratory (National Key Laboratory) of Fudan University, China (Grant No. KF2015_02), the Open Project Program of National Laboratory for Infrared Physics, Chinese Academy of Sciences (Grant No. M201503), Zhejiang Provincial Science and Technology Key Innovation Team, China (Grant No. 2011R50012), and Zhejiang Provincial

Si-rich SiOx and amorphous Si clusters embedded in SiOx films were prepared by the radio-frequency magnetron cosputtering method and high-temperature annealing treatment. The threshold resistance switching behavior was achieved from the memory mode by continuous bias sweeping in all films, which was caused by the formation of clusters due to the local overheating under a large electric field. Besides, the I–V characteristics of the threshold switching showed a dependence on the annealing temperature and the SiOx thickness. In particular, formation and rupture of conduction paths is considered to be the switching mechanism for the 39 nm-SiOx film, while for the 78 nm-SiOx film, adjusting of the Schottky barrier height between insulator and semiconductor is more reasonable. This study demonstrates the importance of investigation of both switching modes in resistance random access memory.