Si Clusters Research Articles

Confined phase separation in SiOX nanometric thin layers

Phase separation in silicon-rich silica/silica multilayers was investigated using Atom Probe Tomography and Atomistic Kinetic Monte Carlo simulation. It is shown that the thickness of silicon-rich silicon oxide sublayers plays an important role during phase transformation. It determines the morphology of Si-rich phase formed after subsequent annealing, which is of prime interest for microelectronic and optoelectronic applications. Monte Carlo simulation reveals that the formation of isolated Si clusters can be achieved even in the case of spinodal decomposition and is directly related to the ratio between the spinodal wavelength and the sublayer thickness.

Nonequilibrium quantum chemical molecular dynamics simulations of C60 to SiC heterofullerene conversion

Nonequilibrium high-temperature quantum chemical molecular dynamics simulations based on the self-consistent-charge density-functional tight-binding (DFTB) method for the conversion of C60 to SiC fullerene by way of periodic Si atom supply are presented. Random supply of Si atoms on the surface of a perfect Ih-C60 buckminsterfullerene without simultaneous carbon atom removal merely leads to formation of an exohedrally adsorbed Si cluster during the entire length of our simulations via an Ostwald ripening process, whereas supply of Si atoms in combination with simultaneous carbon atom removal affords the formation of SiC fullerene structures up to a lower limit of 2:1 for the C:Si ratio. Our simulations demonstrate the importance of vacancy defects for atomic substitution-based approaches for heterofullerene cages, and hint at inherent difficulties of such approaches for the actual synthesis of hypothetical, idealized sp2-hybridized SiC nanostructures with a 1:1 ratio featuring fully alternating atomic structures and no Si–Si and C–C bonds.

Temporal coordination of sibilants in Polish onset clusters

In this study we employ the gestural syllable model to examine cluster-vowel timing in Polish sibilant initial (SI = {/∫m-, ∫p-, sp-, sk-/}) and sibilant final (SF = {/m∫-, p∫-, ps-, ks-/}) onset clusters. In this model, the timing of a complex onset is evaluated relative to a simplex onset and it is predicted that timing relations between onset and vowel should be invariant independently of onset complexity (Browman and Goldstein, 2000). Articulatory data of three speakers show that SI clusters conform to the predicted timing pattern in terms of a globally organized onset cluster relative to the vowel (“c-center”). This is compatible with previous findings for several languages. For SF clusters, however, there are considerable timing differences between complex and corresponding simplex onsets. This suggests that SF clusters are coordinated differently (and inconsistently) to the following vowel compared to SI clusters, as also reported previously for Romanian (Marin, 2013). We investigate to which extent this difference between SI and SF clusters is related to sibilants’ high coarticulatory resistance preventing a close C-V coordination. We will present an analysis of jaw movement data to consider possible effects of jaw position constraints on the temporal coordination of clusters.

Correlation between Volume Fraction of Silicon Clusters in Amorphous Silicon Films and Optical Emission Properties of Si* and SiH*

The volume fraction of silicon clusters in amorphous silicon (a-Si:H) films has been investigated using specially designed quartz crystal microbalances (QCMs) together with optical emission spectroscopy (OES). The optical emission intensities of Si* and SiH* and their intensity ratios are selected for comparison with the QCM results. We show that the volume fraction of silicon clusters strongly correlates with not only the electron temperature but also the SiH* intensity. This suggests that the ratios of Si*/SiH* and SiH* can be used to predict the volume fraction of Si clusters in a-Si:H films.

Comparative Investigation of Structural and Optical Properties of Si-Rich Oxide Films Fabricated by Magnetron Sputtering

RF magnetron sputtering of two separate silicon and oxide (SiO2 or Al2O3) targets in pure argon plasma was used for deposition of Six(SiO2)1-x and Six(Al2O3)1-x films with x=0.15-0.7 on long fused quarts substrate. The effect of post-fabrication treatments on structural and light emitting properties of the films with different x values was investigated by means of Raman scattering, electron paramagnetic resonance and X-ray diffraction as well as by photoluminescence (PL) methods. The formation of amorphous Si clusters upon deposition process was found for the both types of films. The annealing treatment at 1150°C during 30 min results in formation of Si nanocrystallites (Si-ncs). The latter were found to be larger in Six(Al2O3)1-x films than that in Six(SiO2)1-x counterparts with the same x values and are under tensile stresses. The investigation of photoluminescence properties of annealed films of both types revealed the appearance of visible-near infrared light emission. The Six(SiO2)1-x films demonstrated one broad PL band which peak position shifts gradually to from 1.4 eV to 1.8 eV with the x decrease. Contrary to this, for the Six(Al2O3)1-x films two overlapped PL bands were observed in the 1.4-2.4 eV spectral range with peak positions at ~2.1 eV and ~1.7 eV accompanied by near-infrared tail. Comparative analysis of PL spectra of both types samples showed that the main contribution to PL spectra of Six(SiO2)1-x films is given by exciton recombination in the Si-ncs whereas PL emission of Six(Al2O3)1-x films is caused mainly by carrier recombination either via defects in matrix or via electron states at the Si-ncs/matrix interface.

Phonon-induced pure-dephasing of luminescence, multiple exciton generation, and fission in silicon clusters

The size and temperature dependence of the pure-dephasing processes involved in luminescence, multiple exciton generation (MEG), and multiple exciton fission (MEF) are investigated for Sin clusters (n = 5-10, 15) using ab initio molecular dynamics and optical response function theory. The cluster bandgaps correlate with two types of binding energy, indicating that bandgaps can be used to characterize cluster stability. Ranging from 5 to 100 fs, the dephasing times are found to be longest for MEF and shortest for MEG, with luminescence falling in the middle range. Generally, the dephasing is fast, if the orbitals supporting the pair of states involved in the superpositions differ in energy, atomic localization, and number of nodes. The dephasing accelerates with temperature, because more phonon modes are activated, and lower frequency acoustic modes are able to explore the anhamonic part of the potential energy surface. The temperature dependence is stronger for larger clusters, since they possess a wider range of low-frequency anharmonic modes. Our research indicates that rapid dephasing in Si clusters favors generation of independent charge carriers from single and multiple excitons, making the clusters a promising material for photon energy conversion. The simulations of the dephasing processes reported in this work assist in understanding of the exciton evolution pathways in inorganic semiconductor clusters and other nanoscale materials.

Contrasting Genetic Structure among Populations of Two Amphidromous Fish Species (Sicydiinae) in the Central West Pacific

Both present-day and past processes can shape connectivity of populations. Pleistocene vicariant events and dispersal have shaped the present distribution and connectivity patterns of aquatic species in the Indo-Pacific region. In particular, the processes that have shaped distribution of amphidromous goby species still remain unknown. Previous studies show that phylogeographic breaks are observed between populations in the Indian and Pacific Oceans where the shallow Sunda shelf constituted a geographical barrier to dispersal, or that the large spans of open ocean that isolate the Hawaiian or Polynesian Islands are also barriers for amphidromous species even though they have great dispersal capacity. Here we assess past and present genetic structure of populations of two amphidromous fish (gobies of the Sicydiinae) that are widely distributed in the Central West Pacific and which have similar pelagic larval durations. We analysed sections of mitochondrial COI, Cytb and nuclear Rhodospine genes in individuals sampled from different locations across their entire known range. Similar to other Sicydiinae fish, intraspecific mtDNA genetic diversity was high for all species (haplotype diversity between 0.9–0.96). Spatial analyses of genetic variation in Sicyopus zosterophorum demonstrated strong isolation across the Torres Strait, which was a geologically intermittent land barrier linking Australia to Papua New Guinea. There was a clear genetic break between the northwestern and the southwestern clusters in Si. zosterophorum (φST = 0.67502 for COI) and coalescent analyses revealed that the two populations split at 306 Kyr BP (95% HPD 79–625 Kyr BP), which is consistent with a Pleistocene separation caused by the Torres Strait barrier. However, this geographical barrier did not seem to affect Sm. fehlmanni. Historical and demographic hypotheses are raised to explain the different patterns of population structure and distribution between these species. Strategies aiming to conserve amphidromous fish should consider the presence of cryptic evolutionary lineages to prevent stock depletion.

Room Temperature Deposition of Silicon Nanodot Clusters by Plasma-Enhanced Chemical Vapor Deposition

The formation of nanometer-scale (ns)-Si dots and clusters on p-GaN layers has been studied by controlling the early stage of growth during plasma-enhanced chemical vapor deposition (PECVD) at room temperature. We found that ns-Si dots and clusters formed on the p-GaN surface, indicating that growth was the Volmer-Weber mode. The deposition parameters such as radio frequency (RF) power and processing time mainly influenced the size of the ns-Si dots (40 nm-160 nm) and the density of the ns-Si dot clusters.

Molecular dynamics simulation of the role of hydrogenated Si clusters for fast rate mesoplasma epitaxy

The formation dynamics of nano-sized hydrogenated Si (Si : H) clusters and their interaction with the Si(1 0 0) substrate have been investigated with molecular dynamics simulation using the Tersoff potential. Several nm-sized clusters comprising H atoms are obtained during rapid cooling of atomic Si and H vapour mixtures. Upon surface impingement, the Si : H clusters display higher degrees of deformation and atomic self-ordering than the Si clusters formed with no H atom incorporation. This is due to the movement of the high potential H atoms toward the cluster surface during deformation, not to the local heat generation by the atomic H recombination within the cluster.

Electronic, magnetic and optical properties of Cu, Ag, Au-doped Si clusters

The structural, optical and magnetic properties of Cu, Ag, Au-doped Si7 Clusters have been systematically investigated using density functional theory calculations. The global optimized structures of Cu, Ag, Au-doped Si clusters are predicted to have a lower HOMO-LUMO gap and higher magnetic moment. M-doping (M = Cu, Ag, Au) in Si cluster widens a range of adsorption wavelength, especially Au-doping. The characteristics in electronic density of states (DOSs) show that C5v-Si6Cu has a big asymmetrical spin-up and spin-down. The average atomic moment is 0.428 mμB per atom for the Si6Cu cluster with C5v symmetry, while the average paramagnetic moment is 0.143 mμB per atom for other M-doped (M = Cu, Ag, Au) Si7 clusters.

A graph theoretical approach to fluctuating networks in glass-forming liquids: A molecular dynamics study with applications of the pebble game algorithm

The network structures fluctuating in space and time are studied in glass-forming liquids of silica (SiO2) and silicate (Mg2SiO4), by carrying out molecular dynamics (MD) simulations and then applying the graph theoretical algorithm of a "pebble game". The pebble game algorithm was developed in Thorpe and coworkers' studies on the percolation of rigidity to form amorphous solids and, although its exactness is proved only in two dimensions, it has been demonstrated to apply to three-dimensional networks. In liquid silica and silicates, the network connections are extended with amounts of network-forming Si, O atoms, and infinitely-percolating clusters emerge over an extensive range of compositions. The search along a network for free "pebbles", attached virtually to the constituent atoms to represent their degrees of freedom of motions, shows that one infinitely-percolating cluster contains some rigid clusters. These rigid clusters in liquid states fluctuate in space and time because of the alteration of connections at high temperatures, in contrast to those solidified below glass transition temperatures, and these fluctuations are responsible for the slow structural relaxations. The pebble game analyses thus give insights into the internal structures in infinitely-spanning networks, beyond surveys by the conventional theory of percolation, and reveal the behaviors of rigid clusters playing the dominant role in the slow structural relaxations in glass-forming liquids. We discuss these results especially by taking focus on varieties in network structures, such as the composition-dependent degrees of connectivity and structural transformation under high pressures.

Microscopic origin ofn-type behavior in Si-doped AlN

In contrast to a long held belief, it has been shown that $n$-type AlN can be achieved through Si-doping. This is unexplainable from the current theoretical understanding, a situation that hinders further progress in AlN-based ultraviolet (UV) technologies. From first-principles calculations, we find that $n$-type behavior arises under N-rich growth conditions due to high Si solubility and to the formation of $V$${}_{\mathrm{Al}}$-bound Si clusters. We show that metal-rich growth may lead to weak $n$-type behavior due to oxygen impurities binding and deactivating cation vacancies. We provide clues for designing production processes for $n$-type AlN as a base material for potential new UV sources.

Multidoping of Si Cages: High Spin States beyond the Single-Dopant Septet Limit

Density-functional theory based global geometry optimization is employed to systematically scrutinize the possibility of multi-doping of hydrogenated Si clusters in order to achieve high spin states beyond the septet limit of a single-atom dopant. While our unbiased configurational search reveals that the previously suggested Si18H12 double hexagonal prism structure is generally too small to accommodate two dopants in magnetized state, the larger Si24H24 cage turns out to be suitable for such applications. For dimer dopants M2+ = Cr2+, Mn2+ and CrMn+, the structural integrity of the host cage is conserved in the ground-state structure of corresponding M2+@Si24H24 aggregates, as is the unusually high spin state of the guest dopant, which in case of Cr2+ already exceeds the single-atom dopant septet limit by almost a factor of two. Moreover, the possibility of further increasing the cluster spin moment by encapsulating an even larger number of dopants into a suitably sized hydrogenated Si cage is illustrated for the example of a (CrMn+)2@Si28H28 aggregate with a total number of 18 unpaired electrons. These results strongly suggest multi-doping of Si clusters as a viable route to novel cluster-based materials for magneto-optic applications.

A Density Functional Study of the Gold Cages MAu16 (M = Si, Ge, and Sn)

Relativistic density functional calculations are performed to explore the promise of MAu16(M=Si, Ge, and Sn) clusters as magic clusters and building blocks in developing cluster-assembled materials. C1 and Cs, two isomers of SiAu16, GeAu16 and SnAu16 with M (Ge or Sn) at the center of the cage, named, respectively, as SiAu16—C1, SiAu16—Cs, GeAu16-center, and SnAu16-center, are calculated to be the most stable. The Au—M bond should have both ionic and covalent characteristics. Their static linear polarizabilities and first-order hyperpolarizabilities are found to be sensitive to the delocalization of the valence electrons of the M atom, as well as their structures and shapes.

Ab Initio Electronic Gaps of Ge Nanodots: The Role of Self-Energy Effects

Nanostructuring of a material leads to enormous effects on its excited state properties. This study, through the application of different state-of-the-art ab initio theoretical tools, investigates the effect of size on the electronic gap of germanium nanocrystals highlighting similarities and differences with respect to equivalent silicon nanostructures. We performed both GW and ΔSCF calculations for the determination of their electronic structure. While it is known that ΔSCF corrections to the Kohn–Sham gap vanish for extended systems, the two approaches were expected to be equivalent in the limit of small clusters. However, it has been recently found that for hydrogenated Si clusters the ΔSCF gaps are systematically smaller than the GW ones, while the opposite is true for Ag clusters. In this work we find that the GW gaps are larger than the ΔSCF ones for all the Ge dots, with the exception of the smallest one. Such crossing between the ΔSCF and the GW gap values was not expected and has never been obse...

Undoped and Nd3+ doped Si-based single layers and superlattices for photonic applications

This work presents the benefits of the superlattice approach to control light emission properties of materials with Si nanoclusters and rare‐earth ions. The undoped and Nd3+‐doped both Si‐rich‐SiO2 single layers and Si‐rich‐SiO2/SiO2 superlattices were grown by radio frequency magnetron sputtering. Their properties were investigated by means of spectroscopic ellipsometry, Fourier infrared transmission spectroscopy, transmission electron microscopy, and photoluminescence (PL) methods versus deposition conditions, annealing treatment, and superlattice design (doping and thickness of alternated sublayers). An intense Nd3+ emission from as‐deposited single layers and superlattices was observed. The lower annealing temperature (below 900 °C) of the single layers and superlattices favors the formation of amorphous Si clusters that act as effective sensitizers of rare‐earth ions. The highest Nd3+ PL intensity was achieved after a conventional annealing at about 600–800 °C in nitrogen flow for all samples. Crystallized Si‐nanoclusters were formed in Si‐rich‐SiO2 single layers upon annealing at 1000–1100 °C, whereas their formation in the superlattices occurred at higher temperatures (1100–1150 °C). The mechanism of Nd ions' excitation via energy transfer from Si‐nanoclusters and/or matrix defects, if any, is discussed.

The evolution of photoluminescence from Si cluster to nanocrystal in Si-rich oxide/SiO2 multilayer films

The evolution of photoluminescence (PL) from Si cluster to nanocrystal in Si-rich oxide (SRO)/SiO2 multilayer films is reported. Two PL bands can be obtained when the SRO layer thickness is no larger than 2 nm, and the high energy PL band (blue band) is related to the interband transition of Si cluster caused by quantum confinement effect (QCE), while the low energy band (red band) is related to the interface recombination. Both Si clusters and nanocrystals exist in the film when the SRO layer is increased to 3 nm, and the intense PL in the red band suggests that Si nanocrystals becomes the main light-emitting centers. The blue band disappears when the SRO layer is increased to 4 nm, and the PL mechanisms of the red band are QCE and interface defects in Si nanocrystals.

Si-rich Al2O3 films grown by RF magnetron sputtering: structural and photoluminescence properties versus annealing treatment

Silicon-rich Al2O3 films (Six(Al2O3)1−x) were co-sputtered from two separate silicon and alumina targets onto a long silicon oxide substrate. The effects of different annealing treatments on the structure and light emission of the films versus x were investigated by means of spectroscopic ellipsometry, X-ray diffraction, micro-Raman scattering, and micro-photoluminescence (PL) methods. The formation of amorphous Si clusters upon the deposition process was found for the films with x ≥ 0.38. The annealing treatment of the films at 1,050°C to 1,150°C results in formation of Si nanocrystallites (Si-ncs). It was observed that their size depends on the type of this treatment. The conventional annealing at 1,150°C for 30 min of the samples with x = 0.5 to 0.68 leads to the formation of Si-ncs with the mean size of about 14 nm, whereas rapid thermal annealing of similar samples at 1,050°C for 1 min showed the presence of Si-ncs with sizes of about 5 nm. Two main broad PL bands were observed in the 500- to 900-nm spectral range with peak positions at 575 to 600 nm and 700 to 750 nm accompanied by near-infrared tail. The low-temperature measurement revealed that the intensity of the main PL band did not change with cooling contrary to the behavior expected for quantum confined Si-ncs. Based on the analysis of PL spectrum, it is supposed that the near-infrared PL component originates from the exciton recombination in the Si-ncs. However, the most intense emission in the visible spectral range is due to either defects in matrix or electron states at the Si-nc/matrix interface.

Cluster structures observed in 40Ca from 12C+28Si scattering

We discuss evidences of strong cluster structures observed in nuclear reactions even at quite high excitation energies. The oscillatory angular distributions of the elastic scattering of the 12C+28Si system measured at 13 energies close to the Coulomb barrier were reproduced introducing in the scattering S-matrix additional Regge-pole form terms. These poles were interpreted as describing doorway states with 12C+28Si cluster structure in the composite nucleus 40Ca. The excitation energies of these states indicate that they can be part of a rotational band in 40Ca, together with similar states observed in the elastic scattering and α-transfer reactions of the 16O+24Mg system. We propose that the presently observed, largely deformed molecular band corresponds to the hyper-deformed band in 40Ca, which has been found in a new Nilsson-model-calculation, which predicts preference of the 12C + 28Si and 16O + 24Mg molecular structures, in accordance with our present results.

Direct visualization of reversible dynamics in a Si6 cluster embedded in a graphene pore

Clusters containing only a handful of atoms have been the subject of extensive theoretical and experimental studies, but their direct imaging has not been possible so far, with information about their structure provided mainly by theory. Here we report a direct atomically-resolved observation of a single Si₆ cluster trapped in a graphene nanopore. Furthermore, though electron-beam-induced irreversible atomic displacements have been reported before, here we report a sequence of images that show a reversible, oscillatory, conformational change: one of the Si atoms jumps back and forth between two different positions. Density-functional calculations show that the embedded cluster is exploring metastable configurations under the influence of the beam, providing direct information on the atomic-scale energy landscape. The capture of a Si cluster in a graphene nanopore suggests the possibility of patterning nanopores and assembling atomic clusters with a potential for applications.