Luminescence and Raman characterization of molecular and nanocrystalline silicon clusters (original) (raw)
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Structural and “in situ” vibrational study of luminescent cluster assembled silicon thin films
A Low Energy Cluster Beam Deposition apparatus is employed to produce cluster assembled silicon thin films (1 -500 nm thick) by using a laser vaporization source. The generated clusters are studied since their formation through time of flight mass spectra and the calculated size in the gas phase are compared with those of the deposited aggregates obtained through Dynamic Scanning Force Microscopy. The deposited material is also studied ''in situ'' by Raman and infrared spectroscopy. The spectra reveal that the as deposited clusters are hydrogenated with negligible amount of oxide. A comparison of the film properties before and after their air exposure shows that the exposition induces a consistent oxidation, leading to a near-infrared luminescent silicon nanoparticles surrounded by SiO x shells. D
The Journal of Chemical Physics, 1999
The structures and coalescence behavior of size-selected, matrix-isolated silicon clusters have been studied using surface-plasmon-polariton ͑SPP͒ enhanced Raman spectroscopy. The cluster ions were produced in a laser vaporization source, mass selected then deposited into a co-condensed matrix of Ar, Kr or N 2 on a liquid He cooled substrate. Raman spectra from monodisperse samples of the smaller clusters studied, Si 4 , Si 6 and Si 7 , show sharp, well-resolved, vibrations which are in good agreement with predictions based on ab initio calculations. From these comparisons we confirm that Si 4 is a planar rhombus, and assign Si 6 as a distorted octahedron and Si 7 as a pentagonal bypyramid. Si 5 depositions down to 5 eV did not reveal a measurable Raman spectrum under our experimental conditions. Evidence for cluster-cluster aggregation ͑or fragmentation͒ was observed under some conditions, even for a ''magic number'' cluster such as Si 6. The spectra of the aggregated small clusters were identical to those observed for directly deposited larger cluster ''bands,'' such as Si 25-35. The Raman spectra of the aggregated clusters bear some similarity to those of bulk amorphous silicon. Cluster-deposited thin films were prepared by sublimating the matrix material. Even under these ''soft landing'' conditions, changes in the Raman spectrum are observed with the thin films showing even greater similarity to amorphous silicon.
Structural, vibrational, and optical properties of silicon cluster assembled films
The Journal of Chemical Physics, 1998
Distributions of neutral Si-clusters centered around mean sizes of 50 and 200 atoms have been produced using a laser vaporization source and deposited on various substrates at room temperature in ultrahigh vacuum. The Si-cluster assembled films obtained, resulting from the random stacking of incident free nanosize clusters, were subsequently coated by appropriate protective layers before removing in air to perform ex situ infrared, visible, Raman, and photoluminescence spectrometry measurements, as well as transmission electron microscopy observations. The main characteristics of the cluster films are comparable to those observed for amorphous hydrogenated silicon and quite different to those of conventional nanoporous structures or clusters larger than 2-3 nm. The observed intense photoluminescence signal and band gap suggest the presence of a low number of dangling bonds probably due to surface reconstruction effects, connections between adjacent clusters, and oxygen contamination. As for the oxygen contamination, infrared and x-ray photoemission spectrometry measurements agree with the assumption of oxygen atoms trapped at the cluster surface. Finally, all the results on the vibrational and optical properties tend to confirm the failure of the classical confinement model in a diamond lattice to explain the behavior of such nanostructured materials with grain size typically in the nanometer range. The presence of five-membered rings characteristic of the Si-cluster structures in this size range with the subsequent rehybridization effects, as well as the connection process between adjacent clusters seem to be a track which is discussed for a better interpretation of the results.
Density-functional-based predictions of Raman and IR spectra for small Si clusters
Physical Review B, 1997
We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared ͑IR͒ intensities for all of the vibrational modes of several clusters ͑Si N with Nϭ3Ϫ8, 10, 13, 20, and 21͒ using the local density approximation ͑LDA͒. For the smaller clusters (Nϭ3Ϫ8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si 10 and for the lowest-energy structure of Si 13 found to date. For the static response, we compare our calculated polarizabilities for Nϭ10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems. ͓S0163-1829͑97͒01303-9͔
Surface and confinement effects on the optical and structural properties of silicon nanocrystals
Nanocrystals, and Organic and Hybrid Nanomaterials, 2003
In this work we investigate, by first-principles calculations, the structural, electronic and optical properties of: (1) oxygenated silicon-based nanoclusters of different sizes in regime of multiple oxidation at the surface, and (2) hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations within density functional theory.
Fluorescent silicon clusters and nanoparticles Klaus von Haeften
Fluorescent Silicon Clusters and Nanoparticles, 2016
The fluorescence of silicon clusters is reviewed. Atomic clusters of silicon have been at the focus of research for several decades because of the relevance of size effects for material properties, the importance of silicon in electronics and the potential applications in bio-medicine. To date numerous examples of nanostructured forms of fluorescent silicon have been reported. This article introduces the principles and underlying concepts relevant for fluorescence of nanostructured silicon such as excitation, energy relaxation, radiative and non-radiative decay pathways and surface passivation. Experimental methods for the production of silicon clusters are presented. The geometric and electronic properties are reviewed and the implications for the ability to emit fluorescence are discussed. Free and pure silicon clusters produced in molecular beams appear to have properties that are unfavourable for light emission. However, when passivated or embedded in a suitable host, they may emit fluorescence. The current available data show that both quantum confinement and localised transitions, often at the surface, are responsible for fluorescence. By building silicon clusters atom by atom, and by embedding them in shells atom by atom, new insights into the microscopic origins of fluorescence from nanoscale silicon can be expected.
Optical properties of nanocrystalline silicon embedded in SiO2
Science in China Series A: Mathematics, 1999
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.
Si/SiO 2 core shell clusters probed by Raman spectroscopy
European Physical Journal B, 2005
Using a pulsed microplasma source, clusters were produced through the ablation of a Si cathode and successive supersonic expansion. The Si cluster beam was deposited onto different substrates and the partial oxidation of the cluster surface avoided the growth of large agglomerates, preserving their nanocrystalline morphology. Micro-Raman spectroscopy was used for an accurate size diagnosis of the deposited nanoparticles. The size of the Si dots ranges between 2 and about 15 nm. The Si dots appear to have a Si oxide shell, as confirmed also by structural and compositional analysis through transmission electron microscopy and atomic force microscopy. Double Raman peaks were attributed to small Si agglomerates having a thin substoichiometric Si-O interface.
Surface and confinement effects on the optical and structural properties of silicon nanocrystals
2003
In this work we investigate, by first-principles calculations, the structural, electronic and optical properties of: (1) oxygenated silicon-based nanoclusters of different sizes in regime of multiple oxidation at the surface, and (2) hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations within density functional theory. In the first case we have varied systematically the number of Si=O bonds at the cluster surface and found a nonlinear reduction of the energy gap with the Si=O bond number. A saturation limit is reached, which allows us to provide a consistent interpretation of the photoluminescence (PL) redshift observed in oxidized porous silicon samples. Our results help also to explain some very recent findings on the single silicon quantum dot photoluminescence bandwidth. In the second case, after a preliminary study of the clusters stability, the properties of the ground and excited states have been compared varying the cluster dimensions from 1 to 29 Si atoms. Ab-initio calculations of the Stokes shift as a function of the cluster dimension will be presented. A structural model linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO2 matrix will be also suggested.