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Raman Spectroscopy of Nanomaterials: How Spectra Relate to Disorder, Particle Size and Mechanical Properties

Infrared (IR) spectroscopy is one of the most popular analytical techniques used to characterize sol-gel silica materials in their different stages. The method represents, in particular, a simple and versatile tool to investigate the microstructural evolution in gels and films, as a function of temperature and synthesis parameters. Several studies have shown that sol-gel IR absorption spectra exhibit, with respect to silica melt glass spectra, some specific features closely related to the peculiarities of sol-gel processing. Furthermore, because of the differences between silica bulk gels and films the spectra- microstructure correlation must be especially evaluated for thin films. IR spectroscopy has been used to evaluate residual porosity, Si-O-Si bonding rearrangements during drying and firing stages and to model the microstructure evolution during film processing. Some questions are still, however, arising around the interpretations of the IR spectra, in particular about the presence of cyclic species in the microstructure and disorder-induced vibrational modes. An attempt is made here to present an overview of the different relationships existing between IR spectra and microstructure of sol-gel silica films as they actually appear from current literature.

Infrared spectroscopy of sol–gel derived silica-based films: a spectra-microstructure overview

The origin of visible luminescencefrom “porous silicon”: A new interpretation

Despite their lack of long-range translational and orientational order, covalent amorphous solids can exhibit structural order over both short and medium length scales, the latter reaching to 20 A or so. Medium-range order is difficult to measure experimentally and to interpret unambiguously, but a variety of techniques have allowed several types of characteristic structural ordering to be identified and their origin elucidated.

Medium-range structural order in covalent amorphous solids

We take familiar inorganic oxide glasses and non-oxide glasses and the liquids from which they derive to review the current understanding of their atomic structure, ranging from the local environments of individual atoms to the long-range order which can cover many interatomic distances. The structural characteristics of important glasses and melts, like silicates, borates, alumino-silicates, halides and chalcogenides, are drawn from the results of recent spectroscopy and scattering experiments. The techniques include Nuclear Magnetic Resonance (NMR) and X-ray Absorption Fine Structure (XAFS), Neutron Scattering (NS) and Small- and Wide-angle X-ray Scattering measurements (SAXS/WAXS), and are often combined with computer simulation experiments in order to obtain detailed images of structure and diffusion in the glassy as well as in the molten state. We then review the current understanding of relaxation in glasses, liquids and polyamorphic states. This includes phenomenological models and theories of rela...

Inorganic glasses, glass-forming liquids and amorphizing solids

Vibrational decoupling of rings in amorphous solids

We review the experimental evidence and qualitative arguments for the existence of small highly regular rings of bonds in amorphous (a-)${\mathrm{SiO}}_{2}$ and selected other ${\mathit{AX}}_{2}$ tetrahedral glasses. The structure and vibrations of planar 3-rings and regular puckered 4-rings in a-${\mathrm{SiO}}_{2}$ are then modeled using Born central and noncentral forces. The vibrational coupling of these rings to the more disordered glass network is modeled by attaching a Bethe lattice at each connection. The calculated vibrational properties of the breathing modes of the rings are found to be quite consistent with the observed frequencies, linewidths, and isotope shifts of the sharp lines ${\mathit{D}}_{1}$ and ${\mathit{D}}_{2}$ seen in the Raman spectra of a-${\mathrm{SiO}}_{2}$. The results support the previous assignment of ${\mathit{D}}_{2}$ (606 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) to a planar 3-ring and ${\mathit{D}}_{1}$ (495 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) to a regular ring. Similar calculations for a-${\mathrm{GeO}}_{2}$ are consistent with the suggestion that this material contains a substantial concentration of nearly planar 3-rings. Our methods can be generalized to treat similar forms of intermediate range order in glasses having other network connectivities.

Regular ring dynamics in AX2 tetrahedral glasses.

A theoretical study of the electronic structure of amorphous $\mathrm{Si}{\mathrm{O}}_{x}$ is performed. The calculation is done using a realistic tight-binding Hamiltonian for a cluster-Bethe-lattice model. Results of the calculations are in good agreement with optical-absorption measurements for the entire range of concentrations and with photoemission spectra for $x=0,1, \mathrm{and} 2$. The nonlinear variation of the optical gap with concentration that takes place at $x=1.5$ is related to the break of Si chains and to the percolation threshold. Comparison with photoemission data shows that the Si-O-Si bond angle is \ensuremath{\sim} 125\ifmmode^\circ\else\textdegree\fi{} in agreement with chemical shift analysis. Our study indicates that the distribution is Si and O atoms in $a\ensuremath{-}\mathrm{S}\mathrm{i}{\mathrm{O}}_{x}$ is random with no O-O bonds. It is found that intermediate range order plays a fundamental role in the electronic structure of $a\ensuremath{-}\mathrm{S}\mathrm{i}{\mathrm{O}}_{x}$.

Theoretical study of the electronic structure of Si O x

We present an extensive study of the electronic properties of amorphous silicon nitride for a wide range of N concentrations. Local densities of states on Si and N for the ``ideal'' random network structure, defect states introduced into the semiconducting gap by both Si and N dangling bonds, the effect of wrong bonds (N\char22{}N bonds below stoichometry and Si\char22{}Si bonds above stoichiometry) on the local density of states, and, the energy dependence (from 0 to 10 eV) of the imaginary part of the dielectric function have been obtained as a function of N content. The main purpose has been the development of a theoretical model for amorphous compound that were both internally consistent and in overall agreement with experiment. We stress the following main results: (i) The semiconducting gap opens continuously from pure a-Si to the stoichiometric compound a-${\mathrm{Si}}_{3/7}$${\mathrm{N}}_{4/7}$. For low N content the gap widening is linear in x/(1-x) whereas, just before stoichiometry is reached, the gap opens very sharply to its maximum value. (ii)

Electronic structure, defect states, and optical absorption of amorphous Si 1 − x N x [0≤x/(1-x)≤2]

We study the equilibrium atomic configuration of atomic oxygen in silicon. We calculate the total energy of a cluster of silicon atoms before and after the incorporation of atomic oxygen. The energy is calculated for various atomic configurations to obtain the stable one. The equilibrium atomic distribution involves the separation of the silicon atoms forming a covalent bond to accommodate the oxygen atom. Analysis of the charge distribution reveals the break of the covalent silicon\char21{}silicon bond which is replaced by a Si\char22{}O\char22{}Si puckered bond. A careful analysis of the energy spectrum as well as of the charge-density matrix in the equilibrium configuration indicates the similarity between the new Si\char22{}O\char22{}Si bond formed and the basic chemical unit of ${\mathrm{SiO}}_{2}$. Analysis of the rotational and vibrational modes around the defect is in good agreement with infrared-absorption measurements.

E. Martínez

Papers

Vibrational decoupling of rings in amorphous solids

Regular ring dynamics in AX2 tetrahedral glasses.

Theoretical study of the electronic structure of Si O x

Electronic structure, defect states, and optical absorption of amorphous Si 1 − x N x [0≤x/(1-x)≤2]

Atomic oxygen in silicon: The formation of the Si-O-Si bond.