Size-selected Silicon Clusters Research Articles

Higher Ionization Energies from Sequential Vacuum-Ultraviolet Multiphoton Ionization of Size-Selected Silicon Cluster Cations

The second to fifth ionization energies and the appearance sizes of the higher charge states of silicon clusters with 6–92 atoms are determined by sequential vacuum-ultraviolet multiphoton ionization of size-selected cluster ions in a cryogenic ion trap. The higher ionization energies are well described by the charging energy in a spherical droplet model. In contrast to the electron affinity and the first ionization energy of silicon clusters, the higher ionization energies seem to change less abruptly at the prolate-to-spherical shape transition.

2pcore-level binding energies of size-selected free silicon clusters: Chemical shifts and cluster structure

The 2$p$ core-level electron binding energies of size-selected silicon cluster ions have been determined from soft x-ray photoionization efficiency curves. Local chemical shifts and global charging energy contributions to the 2$p$ binding energy can be separated, because core-level and valence-band electron binding energies exhibit the same inverse radius dependence. The experimental 2$p$ binding energy distributions show characteristic size-specific patterns that are well reproduced by the corresponding electronic density of states obtained from density functional theory modeling. These results demonstrate that 2$p$ binding energies in silicon clusters are dominated by initial state effects, i.e., by the interaction with the local valence electron density, and can thus be used to corroborate structural assignments.

ChemInform Abstract: Silicon Clusters: Chemistry and Structure

The chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) have been studied with a number of different reagents using injected ion drift tube techniques. Both kinetic and equilibrium measurements have been performed as a function of temperature, and the influence of cluster annealing on chemical reactivity explored. Unlike metal clusters, where bulk behavior appears to be approached with around 30 atoms, large silicon clusters (n up to 70) are much less reactive than bulk silicon surfaces. These results suggest that the clusters in the size range examined here are not small crystals of bulk silicon, but have compact, high coordination number structures with few dangling bonds.

Low energy deposition of size-selected Si clusters onto graphite

Molecular Dynamics simulations have been performed to describe the deposition of size-selected silicon clusters onto a graphite surface. The cluster sizes range from N=9 to N=200 atoms per cluster, deposited with kinetic energies from E=0.5 eV to E=2.0 eV per atom. We find that the clusters remain mainly intact on top of the graphite substrate after deposition. The degree of cluster deformation, i.e., the shape of the cluster on the surface, can be controlled via the deposition energy, from the preservation of a 3D cluster morphology to surface wetting, i.e., formation of commensurate 2D islands.

Imaging size-selected silicon clusters with a low-temperature scanning tunneling microscope

Size-selected Si 30 and Si 39 clusters produced by a laser vaporization cluster source are deposited on the Ag(111) surface at room temperature and at liquid-nitrogen temperature respectively. Subsequently, the sample is transferred at low temperature (120 K) in a separate mobile ultrahigh vacuum chamber (vacuum-suitcase) from the cluster source to a low-temperature scanning tunneling microscope (STM). Soft landing of the supported clusters is indicated by the following observations: (i) atomic-resolution images taken at low bias voltages show transparent Si clusters and an unperturbed Ag(111) substrate; (ii) manipulation experiments on the supported clusters and subsequently taken atomic-resolution images show a defect-free Ag(111) surface. In spite of the fact that the clusters are mass-selected in the gas phase, a statistical analysis of the STM images indicates a finite size-distribution on the support. This finding is attributed to the presence of different isomers and/or cluster orientations on the surface.

Photoluminescence of silicon nanocrystallites: an astrophysical application

Photoluminescence (PL) measurements of size-selected silicon clusters thin films generated by laser pyrolysis of SiH 4 in a flow reactor and deposited at low energy after a mechanical velocity selection are presented. Several parameters have been investigated such as the mean size of the clusters, (in the range ∼3–5 nm) and the dispersion of their size distribution. Particular attention has been paid to the determination of the absolute value of the PL efficiency. Measurements show that such silicon nanocrystallites demonstrate high external PL efficiency (up to 12%) and strong dependence of the PL properties on the cluster size. Results have been applied to the physical and chemical interpretation of spectroscopic observations in many astronomical objects, of a broad, red emission band. This so called extended red emission (ERE) is attributed to the PL of an interstellar dust grain component when excited by the UV radiation emitted by a nearby star. Hence it has been shown that nanometer-size crystals of pure silicon are the best candidate materials as regard PL efficiency, peak wavelength and bandwidth. This demonstrates that size effects in interstellar grains play a dominant role in the understanding of this astrophysical phenomenon.

Improved one-phonon confinement model for an accurate size determination of silicon nanocrystals

In this article, we show how the well-known one-phonon confinement model can be improved to determine the diameter of silicon nanocrystalline spheres from the optical phonon wave-number shift, even using a physical-meaning weighting function. We show that the fundamental parameter is the knowledge of the phonon dispersion. The accuracy of our approach is supported by experimental data obtained by selective UV Raman scattering on nanocrystalline silicon thin films produced by size-selected silicon cluster beam deposition.

Optical properties of nanocrystalline silicon thin films produced by size-selected cluster beam deposition

Molecular beams of size-selected silicon clusters were used to grow nanocrystalline thin films. This technique allows the control of both average size and size dispersion of Si nanocrystals, and is then very useful to provide model materials for the study of the luminescence in silicon. We report results obtained by high-resolution electron microscopy, Raman spectrometry and photoluminescence spectroscopy.

Fragmentation dynamics of silicon cluster anions in collision with a silicon surface: contrast to aluminum cluster anions

The collision of a size-selected silicon cluster anon, SiN− (8 ⩽ N ⩽ 12), with a silicon surface was investigated by using a tandem time-of-flight mass spectrometer equipped with an ultrahigh-vacuum collision chamber. The size-selected parent cluster anion, SiN−, was fragmented dominantly into SiN− (n ≈ N/2) in the collision energy range of several electron volts per atom. The measured recoil velocity of the fragment anion from the surface revealed that about 40% of the collision energy is directly converted to the translation kinetic energies of the anion and its counter neutral product scattered from the surface. This conversion efficiency was much higher than that observed for AlN−, aluminum cluster anions, reported previously. This marked contrast indicates that AlN− interacts for a longer time with the surface and transmits its collision energy more efficiently to the surface than SiN−.

Raman spectra of size-selected silicon clusters and comparison with calculated structures

SMALL clusters of silicon, containing between 2 and 100 atoms, have been studied extensively both because of their intrinsic interest from the point of view of chemical structure and bonding and because of the potential technological applications of cluster-assembled materials1–3. Ab initio quantum-chemical calculations predict that very small clusters should have structures markedly different from that of the crystalline phase4–12. Experiments on two- (ref. 13), three- (ref. 14) and four-atom15 clusters have allowed some comparison with theoretical predictions, but structural information on larger clusters has been only indirect16. Here we report on structural studies of size-selected Si4 , Si6 and Si7 clusters prepared and isolated by low-energy deposition into a solid nitrogen matrix. Surface plasmon-polariton enhanced Raman spectroscopy17–20 yields well resolved vibrational spectra for each of these clusters in which the vibrational frequencies agree well with those predicted for optimized structures calculated by ab initio methods. We confirm that Si4 is a planar rhombus, and find that Si6 is a distorted octahedron and Si7 a pentagonal bipyramid.

Optical spectra of size-selected matrix-isolated silicon clusters

Size selected silicon clusters have been isolated in rare gas matrices and studied by optical absorption spectroscopy. The clusters were produced in a pulsed laser vaporization source, size selected with a quadrupole mass spectrometer and deposited at low energies into a cocondensed krypton matrix held at T<20 K. A comparison of the optical spectra of ten atom wide bands (Si25-Si35, Si35-Si45 and Si45-Si55) shows the general size evolution of the optical properties. Single cluster sizes have also been isolated and show somewhat sharper spectra than the bands. The measured spectra show similarities to spectra calculated using Mie theory and bulk optical constants. Cluster-cluster agglomeration was studied by evaporating the inert gas matrix. The results suggest that the clusters agglomerate into larger particles even under the mildest "soft landing" conditions.

Mobilities of silicon cluster ions: The reactivity of silicon sausages and spheres

The mobilities of size selected silicon cluster ions, Si+n (n=10–60), have been measured using injected ion drift tube techniques. Two families of isomers have been resolved by their different mobilities. From comparison of the measured mobilities with the predictions of a simple model, it appears that clusters larger than Si+10 follow a prolate growth sequence to give sausage-shaped geometries. A more spherical isomer appears for clusters with n&amp;gt;23, and this isomer completely dominates for unannealed clusters with n&amp;gt;35. Annealing converts the sausage-shaped isomer into the more spherical form for n&amp;gt;30. Activation energies for this ‘‘sausage-to-sphere’’ structural transition have been estimated for several cluster sizes and are ∼1.2–1.5 eV. We have examined the chemical reactivity of the sausages and spheres towards both C2H4 and O2. With C2H4 large differences in reactivity of the isomers were found, with the spherical isomer often being more reactive than the sausage form by more than an order of magnitude. With O2 the variations in reactivity were smaller. Despite the substantial differences in reactivity observed for the two isomers in the cluster size regime where both forms coexist, examination of a broader range of cluster sizes shows that there is not a systematic change in reactivity associated with the geometry change.

Silicon cluster ions: Evidence for a structural transition.

The mobilities of size-selected silicon cluster ions in helium have been measured using injected-ion drift-tube techniques. The results suggest that a major structural transition occurs for clusters with \ensuremath{\sim}27 atoms. Clusters with up to \ensuremath{\sim}27 atoms appear to follow a prolate growth sequence, resulting in geometries with an aspect ratio estimated to be \ensuremath{\sim}3. Larger clusters appear to have more spherical shapes. For annealed clusters this structural transition occurs over a narrow size range.

Nanosurface Chemistry on Size-Selected Silicon Clusters

Studies of the chemistry that occurs on the nanosurfaces of size-selected silicon clusters reveal a number of fascinating qualitative similarities to the behavior of bulk surfaces. However, silicon clusters containing up to 70 atoms appear to be much less reactive than bulk silicon surfaces. This unexpected result suggests that these large silicon clusters are not just small crystals of bulk silicon, but have much more compact geometric structures.

Interaction of silicon cluster ions with ammonia: Annealing, equilibria, high temperature kinetics, and saturation studies

The results of extensive studies of the chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) with ammonia are described. At room temperature all clusters react at close to the collision rate and collisional annealing of the clusters does not influence their reactivity. At temperatures slightly above room temperature (∼400 K) it is possible to establish an equilibrium. Binding energies of ammonia to the silicon clusters of ∼1 eV were determined from measurements of the equilibrium constants as a function of temperature. These small binding energies indicate that molecular adsorption occurs at close to room temperature. Saturation experiments reveal that ammonia only binds molecularly to a small number of sites on the clusters. In contrast, on bulk silicon surfaces at room temperature, rapid dissociative chemisorption occurs until all the surface dangling bonds are saturated. At temperatures above ∼470 K another process, probably dissociative chemisorption, becomes important. Absolute rate constants were measured for clusters with 30–70 atoms at a temperature of 700 K where the dissociative chemisorption process dominates. The sticking probabilities at this temperature are between 10−3 and 10−5, two to four orders of magnitude smaller than on bulk silicon at 700 K.

Silicon clusters: chemistry and structure

The chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) have been studied with a number of different reagents using injected ion drift tube techniques. Both kinetic and equilibrium measurements have been performed as a function of temperature, and the influence of cluster annealing on chemical reactivity explored. Unlike metal clusters, where bulk behavior appears to be approached with around 30 atoms, large silicon clusters (n up to 70) are much less reactive than bulk silicon surfaces. These results suggest that the clusters in the size range examined here are not small crystals of bulk silicon, but have compact, high coordination number structures with few dangling bonds.