Metastable Clusters Research Articles

Two dielectric contributions due to domain/cluster structure in the ferroelectrics with diffused phase transitions

The main dielectric contributions and the structure of dielectric spectrum of the ferroelectrics with diffused phase transitions are considered on the base of PbMg1/3Nb2/3O3 (PMN) crystals investigation. Dielectric spectra of the displacive, order-disorder and diffused phase transitions ferroelectrics are compared. Dielectric properties of the PMN are nontypical for displacive-type ferroelectrics and aren't defined mainly by soft phonon mode. The main polarization mechanisms are caused by the thermoactivated dynamics of the metastable polar phase clusters and polar - nonpolar phase boundaries.

Phonons and structure in nano-clusters: a molecular dynamics study for Al

The generalized phonon density of states for Al nano-clusters has been studied theoretically using molecular dynamics. In this way anharmonic effects could be fully considered. In order to investigate the structure of the clusters, we have also calculated the structure factor S(q) (Fourier transform of the pair correlation function). The calculations were done for systems consisting of 500 atoms and for several temperatures. Stable and metastable cluster states are observed: The clusters in the metastable state show a well defined peak in the structure factor S(q) which we interpret as superstructure. The appearance of the superstructure in the metastable state is accompanied by substantial changes in the phonon density of states.

Liquid secondary ion mass spectrometry of methyl glycosides of oligosaccharides using matrices containing carboxamides.

Intense cluster ions corresponding to proton-bound hetero-dimers of an amide molecule and an oligosaccharide molecule are observed in the liquid secondary ion mass spectra of methyl glycosides of oligoxylans if a solution of an aliphatic carboxamide in glycerol is used as the liquid matrix. These cluster ions are particularly abundant and persist for a long period if urea (U) or thiourea (TU) is used as the matrix additive. In these cases, cluster ions containing more than one molecule of U or TU and two oligosaccharide molecules are also observed. The intense signal due to the proton-bound hetero-dimer between U or TU and the oligosaccharide can be used with advantage for a molecular weight determination. The bonding interactions between a protonated saccharide molecule and a molecule U or TU in the proton-bound hetero-dimers are so strong that the urea molecules remain attached to the fragment ions during the decay of metastable cluster ions and even during collision-induced dissociation. Thus, the mass-analysed ion kinetic energy spectra of these proton-bound hetero-dimers are dominated by abundant cluster ions [Bn+U] and [Ym+U] arising from cleavage of the glycosidic bonds within the oligosaccharides. The collisionally-activated mass spectra of the proton-bound hetero-dimers additionally contain peaks of the free ions Bn and Ym. Therefore, these spectra clearly reflect the arrangement of the monosaccharide residues in the oligosaccharide and can be used conveniently for structural analysis.

Magnetic properties of 4d transition-metal clusters.

We analyze the stability of magnetic states obtained within the tight-binding model solved by molecular dynamics for cubo-octahedral (fcc) and icosahedral clusters of $3d$ (Ni and Co) and $4d$ (Pd, Rh, and Ru) transition metals. The magnetic states are stabilized by the Hund's rule exchange interaction between the $d$ orbitals. Taking realistic parameters deduced from band structure calculations and spectroscopic data, we obtained saturated ferromagnetism for ${\mathrm{Ni}}_{13}$, ${\mathrm{Co}}_{13}$, and ${\mathrm{Pd}}_{13}$ clusters, while ${\mathrm{Rh}}_{13}$ fcc and ${\mathrm{Ru}}_{13}$ clusters are partially magnetic, with an antiferromagnetic orientation of the moments on the central atoms in Ru clusters. Our results for $4d$ transition-metal clusters agree qualitatively with the recent experimental data and with other calculations. Metastable magnetic clusters were identified in several situations. We found a dependence of the equilibrium lattice constant on the magnetization in the considered clusters which may be explained by the character of wave functions in the cluster electronic structure.

Computer Simulation of Displacement Cascades in α-Zirconium

AbstractThe damage produced in α-zirconium at 100 K by displacement cascades with energy up to 20 keV has been investigated by MD simulations. In agreement with modelling of fcc and bcc metals, the defect production efficiency in zirconium is well below the NRT estimate. The number and size of clusters, both vacancy and interstitial, are increased by increasing PKA energy, and clusters containing up to 25 interstitials and 30 vacancies were formed by 20 keV cascades. Most interstitial clusters have dislocation character with perfect Burgers vectors of the form 1/3<1120>, but a few metastable clusters are formed and are persistent over the timescale of MD simulations. Collapse of the 30-vacancy cluster to a faulted loop on the prism plane was found to occur over a period of more than 100 ps

Atomic scale imaging of a 2D fluid-solid equilibrium for Cs and O coadsorbed on a Ru(0001) surface

A solid-fluid phase equilibrium has been studied by STM and LEED for coadsorbed Cs and O on a Ru(0001) surface. It is shown that a (√7 × √7)R19° Cs/O structure melts through a coexistance region into a 2D disordered phase upon rising the temperature. STM images recorded at 300 K reveal perfectly ordered islands coexisting with fluid areas which display also small ordered metastable clusters.

Trap-limited interstitial diffusion and enhanced boron clustering in silicon

Boron doped superlattices have been used to detect the diffusion of self-interstitials in Si. Interstitials were generated in the near-surface region by 40 keV Si implantation followed by diffusion at 670–790 °C. The interstitial diffusion profile at 670 °C is stationary for t≤1 h, demonstrating that the penetration depth of interstitials is limited by trapping. The concentration of traps is estimated to be ∼1017/cm3. For sufficiently long annealing times, interstitials diffuse beyond the trapping length with an effective trap-limited diffusivity ranging from ∼6×10−15 cm2/s at 670 °C to ∼1×10−12 cm2/s at 790 °C. The high interstitial supersaturation adjacent to the implant damage drives substitutional B into metastable clusters at concentrations below the B solid solubility limit.

Evaporative processes of sputtered alkali halide cluster ions

Metastable cluster ions produced by 8 keV atom sputtering of alkali halides have been measured. The dominant evaporative mechanism of the metastable ( MX) n M + cluster ions involves both monomer and dimer loss, and these processes have been examined using semi-empirical self-consistent-field molecular orbital methods to determine the geometry-optimized structures and decomposition energetics.

Superparamagnetic behavior and giant magnetoresistance in as-obtained Co-Ag metastable alloys

The correlation between the structure and the magnetic and magnetotransport properties of metastable alloys Co x Ag 100- x (14 ≤ x ≤ 62), obtained by magnetron sputtering, are investigated as function of the Co content, x. The as-obtained samples show superparamagnetic behavior at room temperature for almost all the concentration range, with blocking temperatures increasing with x. Such behavior is a consequence of the granular nature of the films, where the increase with the Co content of the mean grain diameters of the Co-rich metastable clusters responsible for the observed superparamagnetism, is the origin of the dependence of the blocking temperature with x. The fitting of the experimental data to the Langevin function has allowed an estimation of the magnetic grain size. Moreover, exchange anisotropy effects are detected for the higher values of x as a consequence of partial oxidation of the Co-rich particles. Finally, this granular structure generates the occurence of giant magnetoresistance (GMR) with maximum values around 40% and 20% at low and room temperature, respectively.

Synthesis of Unsaturated Os3W2 and Metastable Os4W Oxo-Ethylidyne Clusters by Solid-State Pyrolysis. Direct C-O Bond Cleavage of a Coordinated Ketenyl Ligand

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSynthesis of Unsaturated Os3W2 and Metastable Os4W Oxo-Ethylidyne Clusters by Solid-State Pyrolysis. Direct C-O Bond Cleavage of a Coordinated Ketenyl LigandJia-Huey Gong, Der-Kweng Hwang, Chyuan-Wu Tsay, Yun Chi, Shie-Ming Peng, and Gene-Hsiang LeeCite this: Organometallics 1994, 13, 5, 1720–1727Publication Date (Print):May 1, 1994Publication History Published online1 May 2002Published inissue 1 May 1994https://doi.org/10.1021/om00017a032RIGHTS & PERMISSIONSArticle Views65Altmetric-Citations14LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1009 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Krypton excimer emission from discharge excited rare gas clusters

We report the time-resolved observation of krypton second continuum excimer emission near 146 nm from a pulsed-discharge excited gas jet. The excimer emission reaches a peak intensity several millimeters from the nozzle and lasts up to several microseconds after the termination of the discharge. We report the temporal behavior of the excimer emission as a function of gas pressure, discharge energy, and position within the cathode-anode gap. The temporal and spatial characteristics of the excimer emission during the discharge are due to cluster excitation in the jet expansion region and is from a combination of the A 3∑+u and the B 1Σ+u states. A kinetic mechanism involving metastable atoms and clusters is described to explain the observed afterglow emission.

Using reflection time-of-flight mass spectrometer techniques to investigate cluster dynamics and bonding

Lase based time-of-flight mass spectrometer systems affixed with reflectrons are valuable tools for investigating cluster dynamics and reactions, spectroscopy and structures. Utilizing the reflectron time-of-flight mass spectrometer techniques, both decay fractions and kinetic energy releases of metastable cluster ions can be measured with high precision. By applying related theoretical models, the desired thermochemical values of metastable species can be deduced, which are otherwise very difficult to obtain. Several examples are discussed with attention focused on ammonia as a test case for hydrogen bond systems, and xenon for weaker van der Waals clusters. A brief overview of applications to investigating solvation effects on reactions and structures, delayed electron transfer and ionization through intracluster Penning ionization is also given.

Understanding and Controlling Transient Enhanced Dopant Diffusion in Silicon

AbstractImplanted B and P dopants in Si exhibit transient enhanced diffusion (TED) during initial annealing which arises from the excess interstitials generated by the implant. In order to study the mechanisms of TED, we have used B doping marker layers in Si to probe the injection of interstitials from near-surface, non-amorphizing Si implants during annealing. The in-diffusion of interstitials is limited by trapping at impurities and has an activation energy of -3.5 eV. Substitutional C is the dominant trapping center with a binding energy of 2-2.5 eV. The high interstitial supersaturation adjacent to the implant damage drives substitutional B into metastable clusters at concentrations below the B solid solubility limit. Transmission electron microscopy shows that the interstitials driving TED are emitted from {311} defect clusters in the damage region at a rate which also exhibits an activation energy of 3.6 eV. The population of excess interstitials is strongly reduced by incorporating substitutional C in Si to levels of ∼1019/cm3 prior to ion implantation. This provides a promising method for suppressing TED, thus enabling shallow junction formation in future Si devices through dopant implantation.

Adhesion of silver iodide molecules to gaseous metallic silver cluster cations

Abstract : The evaporative channels for the unimolecular dissociation of metastable AgXlY+ (X = 5-25; Y = 0-4) clusters made by fast atom bombardment of silver foil in the presence of CH3I vapor are observed in the first field free region of a double focussing mass spectrometer. We found three dominant neutral evaporative channels: Ag loss, Ag2 loss and AgI loss. Only Agl2I3+ is found to have an additional channel involving an (AgI)3 loss. Consistent with our previous studies of stable AgXlY+ clusters, we assume a structural formula of (AgX-Y)+(AgI)Y where the metallic part of the cluster conforms to Jellium model predictions. In comparing the relative evaporative loss of AgI from these metastable clusters, we observe: (a) evidence for significant AgI-AgI interaction for clusters whose metallic part has a ls2, IP2 or Is2,Ip4 Jellium configuration; (b) a near constant fractional loss of AgI as the number of AgI units in the parent cluster increases for clusters whose metallic part has a closed main Jellium shell; (c) a relative increase in the fraction loss of Agl as the number of AgI molecules in the parent cluster increases for clusters with an open shell configuration for the metallic part. These observations are discussed in terms of evaporative loss of AgI from structures in which the AgI molecules solvate the metallic part of the cluster. It is proposed that the heat of AgI evaporation is greatly determined by dipole/induced-dipole interactions between the permanent dipole of AgI and the polarizable delocalized electron density of the metallic part of the cluster.

Theoretical study of small Ar*n clusters in low-lying electronic states

Rydberg excited rare gas atom clusters are treated by the diatomics-in-ionic-systems (DIIS) method and by a simple model approach. According to the results of the calculations, metastable clusters Ar*n (n≥2) are not formed, most probably, from the lowest excited state (4s), but can be formed from higher excited states such as 4p. DIIS calculations of small Arn van der Waals clusters provide the excitation energy dependence on the number n of atoms. The model calculation shows that in the Ar13 van der Waals clusters the excited electron is pushed away from the inner cluster region to the outside region.

Formation of Fe 16N 2 and development of magnetic anisotropy in a FeN alloy

The effect of an applied stress on formation of Fe 16N 2 in a FeN alloy has been examined by use of a magnetic torque meter and a scanning electron microscope. Stress-assisted preferential nucleation of Fe 16N 2 and formation of metastable primary clusters have been detected by measuring magnetic anisotropy. It is found that uniaxial magnetization along the cube axis is not achieved in the early stage of precipitation, particularly during low temperature aging, but is achieved only after full aging at a higher temperature. On the basis of the analyses of magnetic torque curves, the possible structures of the primary clusters are proposed and compared with the microscopic observations of the precipitates.

Excitation and ionization of 4He clusters by electrons

Clusters are produced by expanding high pressure (P0≤20 bar), low temperature (T0≥5 K) helium gas through a 5 μm nozzle into a vacuum. The neutral beam time-of-flight distribution has three peaks which we associate with distinct groups of large and small clusters, and atoms. The beam is ionized by electron impact and the resulting time resolved charged fragment mass distribution reveals in addition to previously observed anomalies (‘‘magic numbers’’) a new strong He+4 signal at high source pressures and low temperatures. The dependence of the various charged and neutral metastable fragment currents on the bombarding electron energy reveals that each has a unique appearance potential. A comparison with the calculated energy required for an electron to create various electronic excitations in the interior of a large cluster indicates that the production and dynamical evolution of metastable 3S1 atomic and a 3Σ+u molecular excitations plays a significant role in the formation of charged fragments from large clusters, but that the production of detectable metastable cluster fragments apparently proceeds via decay of high lying excitonic states. The strong He+4 signal does not appear until the incident electron has about enough energy to create two metastable 3S1 excitations in a cluster. Thus we propose that this ion signal results from the recombination of a pair of a 3Σ+u molecular excitons in or on a large cluster, or possibly from the dynamical evolution of a metastable spin quartet bound hole-exciton pair.

Kinetic pathway in Stranski-Krastanov growth of Ge on Si(001).

The transition from 2D to 3D growth of Ge on Si(001) has been investigated with scanning tunneling microscopy. A metastable 3D cluster phase with well-defined structure and shape is found. The clusters have a {105} facet structure. Results suggest that these clusters define the kinetic path for formation of ``macroscopic'' Ge islands.

Dissociation dynamics: Measurements of decay fractions of metastable ammonia cluster ions

The decay fractions of metastable ammonia cluster ions which undergo unimolecular (evaporative) dissociation in a time window of 1–40 μs were measured by using a reflection time-of-flight mass spectrometer. Corrections concerning instrumental artifacts and ion trajectory of parents and daughters are made to imporve the precision of the measurements. The data are used to derive the Gspann parameter and heat capacity of clusters as described in evaporative ensemble model of metastable dissociation. Using the dissociation fractions measured in the present studies, in conjunction with kinetic energy release values previously measured in our laboratory, we apply Klots’ evaporative ensemble model to obtain binding energies of ammonia cluster ions (NH3)nH+, 4≤n≤17. The deduced binding energy values are found to be in very good agreement with both thermochemical data and Engelking’s modified statistical theory.

Computer simulation of cluster decay

Lennard-Jones metastable clusters consisting of 5 to 1030 atoms have been studied by molecular-dynamics simulation. Evaporation rate constants and caloric equations of state have been obtained in the microcanonical ensemble and with the use of RRK theory of unimolecular reactions and a local harmonicity approximation the canonical rate constants have been evaluted. The comparison with classical nucleation theory, specifically with the liquiddrop model, shows that the calculated equilibrium vapour pressure is significantly higher than theoretical one for small clusters, whereas the microcanonical results agree with theory.