Silicon Backbone Research Articles

Syntheses of poly[(dimethyl)-co-(4,7,10,13-tetraoxatetradecylmethylsilanes)].An evaluation of the use of C8K versus Na in the preparation of polysilanes

To evaluate the use of C 8 K versus Na as the reducing agent In the preparation of polysilanes, the homogolymers poly(4,7,10,13-tetraoxatetradecylmethylsilanes) 1 and poly(dimethylsilane) 5 as well as the copolymers poly[(dimethyl)-co-(4,7,10,13-tetraoxatetradecylmethylsilanes)] 2-4 were synthesized from 4,7,10,13-tettaoxatetradecylmethyldichlorosilane (6) andier dimethyldichlonosilane (7). C 8 R in comparison to Na gave an increased yield and enhanced purity, when used with polar monomers, such as 6, that presumably intercalate into graphite. In the presence of increasing amounts of apolar 7 both a decrease in yield as well as purity is observed in the C 8 K based copolymerizations. In contrast, the type of monomer does not affect the results of the Na based copolymerizations. Whereas at ambient temperatures the introduction of dimethylsilane units in 2-4, causes minor changes in the polysilane optical properties, as compared to 1, at low temperatures differences are discernible both in solution and the solid state (thin film) in the case of 4 Na . Apperently, the incorparated dimethylsilane moieties reduce the mean free energy of defect formation E, i. e. at these low temperatures the silicon backbone adopts a mere extended conformation as reflected by the occurrence of as additional abrust trasition.

Nonionic Water-Soluble Polysilanes: Global Conformation in Solution and the Occurrence of Preferential Solvation

In aqueous solution UV-absorption and fluorescence emission spectroscopy reveal that the nonionic water-soluble polysilanes poly(4,7,10-trioxaundecylmethylsilane) (1) and poly(4,7,10,13-tetraoxatetradecylmethylsilane) (2) are present in a highly folded random coil. In contrast, in organic solvents the backbone adopts a more extended conformation. These changes in backbone conformation are accompanied by changes in the average backbone silicon segmental length L from 15 to 21−24. Concomitantly, the exciton coherence length also exhibits a 2-fold increase. The results obtained for 1 and 2 are compared to those of poly(4,7,10-trioxahexadecylmethylsilane) (3) and poly(4,7,10,13-tetraoxanonadecylmethylsilane) (4), which contain instead of a CH3, a C6H13 moiety as the side chain end group. A detailed analysis of aqueous/organic binary solvent mixtures of 2 reveals that already a low mole fraction of the organic solvent induces large shifts in the photophysical properties; i.e., preferential solvation occurs at a mole fraction of ca. 0.15. Hence upon addition of small amounts of organic solvents, the solute−solvent interactions, which in H2O prevent the silicon backbone from adopting a more extended conformation, are reduced, causing the backbone to be less distorted. This is supported by the increase in the quantum yield Φ in going from polar aqueous to apolar organic solvents.

Occurrence of Radical Cation Localization in Chemically Modified Poly(methylphenylsilane): Poly(methylphenyl-co-4-dimethylaminophenylmethylsilane)s and Poly(methylphenyl-co-4-bromophenylmethylsilane)s

Chemical modification of poly(methylphenylsilane) 1 using triflate chemistry gave two series of random copolymers with different composition ratios, viz., poly(methylphenyl-co-4-dimethylaminophenylmethylsilane)s 3−7 and poly(methylphenyl-co-4-bromophenylmethylsilane)s 8−9. The optical and electronic properties of the copolymers have been studied and are compared with those of the homopolymers 1 and poly(4-dimethylaminophenylmethylsilane) 2. Electrochemical measurements in THF/LiClO4 give strong evidence that, in marked contrast to the results observed for 1 and 2, the onset of oxidation (Vi) of 3−7 and 8−9, respectively, does not represent the silicon backbone valence band. Instead, in these copolymers, a mixture of decoupled electronic domains exists of which those possessing the lowest Vi govern the electrochemical response. As a consequence, the radical cations become confined, i.e. localization occurs. This interpretation is further supported by fluorescence emission spectroscopy; the presence of a confined exciton is indicated by the appearance of an additional broad band for the copolymers. Remarkably, copolymers with optical and electronic properties comparable to those of 3−7 are also accessible by (reversible) protonation of the 4-dimethylamino substituents of 2.

Thin polysilylene films. Their electronic and photoelectrical properties

Thin polysilylene films, their preparation, electronic and photoelectrical properties and chemical synthesis are discussed. Electronic properties are influenced by electron delocalization along the silicon backbone. Attached π-conjugated chromophores improve the photostability of the silicon main chain. Optical excitations lead to the formation of charge transfer states (ion-pairs), which under the influence of an external electric field can dissociate into free charge carriers.

Effects of structural defects on hole drift mobility in aryl-substituted polysilanes

Abstract The effects of a silicon-based structural defect on the hole transport properties of poly(methylphenylsilane) were studied by the DC time-of-flight technique. The defect was chemically introduced as silicon branching of the silicon backbone by copolymerizing methylphenyldichlorosilane and p-tolyltrichlorosilane with sodium. A good empirical relation was obtained between the hole drift mobility (μ E−0) at a zero electric field limit and the defect density. The value of μ E−0 decreased exponentially from 4 × 10−4 to 5 × 10−6cm2 V1 s−1 with an exponential increase in defect density from 0.0024 to 0.18. On the basis of Bassler's disorder formalism, hole transport in the polymers was quantitatively analysed, revealing the different contributions of diagonal and off-diagonal disorder with the defect density. The degree of fluctuation in the density of state mainly determined the hole drift mobility in specimens with fewer defects, while intersite hopping distance controlled the mobility in the polymers...

Fabrication of oriented silicon-based polymer thin film and its application to surface acoustic wave sensor

Polysilanes are silicon-based polymers, which consist of silicon backbone and organic substituents. Among a kind of polysilanes, poly(di-methyl silane) [Si(CH3)2]n has the most fundamental structure, and is now commercially available. In this paper, poly(di-methyl silane) thin films are fabricated by means of vacuum evaporation technique. Orientation of the silicon backbone is well controlled by changing growth rate or substrate temperature during the evaporation. Using these polymer films and LiNbO3 substrates, layered surface acoustic wave (SAW) devices are fabricated, and the characteristics of SAW's propagating along the layered structures are numerically calculated. As a result, it is found that the SAW with large shear horizontal (SH) component is available, and these layered structures are suitable for liquid sensor, which detects the change in the physical constants of liquid loaded on the device surface.

Perchloropolysilane: X-Ray Structure, Solid-State 29 Si NMR Spectroscopy, and Reactions of [SiCl2 ]n.

The silicon backbone is all-trans in the crystal structure of perchloropolysilane [SiCl2 ]n (see structure on the right). The Cl atoms can be substituted by nucleophiles to form polymers such as peralkoxy- and peraminopolysilanes.

Size-controlled percolation pathways for electrical conduction in porous silicon

Silicon shows photo- and electroluminescence at visible wavelengths when chemically etched into a microporous network of ‘wires’ several nanometres thick1. This raises the possibility of a silicon-based optoelectronic technology. The luminescence properties may be understood on the basis of the injection or creation of electrons and holes in the interconnected network of wires which recombine radiatively with high efficiency1,2. Elucidating the electron-transport mechanisms has been held back by several difficulties, particularly that of making stable, high-quality contacts to the porous material. Here we report experiments that probe the conduction process using photoemission stimulated by hard-ultraviolet/X-ray synchrotron radiation, obviating the need for good electrical contacts. We find that the conductivity of porous silicon films is temperature-dependent, and that the films become insulating at low temperatures. We suggest that these results may be understood in terms of a percolation process occurring through sites in the porous network in which conductivity is thermally activated, and we postulate that this activation may be the consequence of a Coulomb blockade effect3,4 in the nanoscale channels of the film. This is consistent with our observation of optical ‘unblocking’ of conducting pathways. These results imply that the size distribution of the nanowires in the silicon backbone plays a key role in determining the conduction properties, and that porous-silicon light-emitting diodes may use only a small (and the least efficient) fraction of the material. Improvements in electroluminescence efficiency may be possible by taking into account the percolative nature of the conduction process.

Effects of plasma irradiation on polysilane thin film

Effects of argon and/or deuterium plasma irradiation on the poly(di-methyl silane)-evaporated thin film have been examined by the X-ray diffraction, infrared absorption, optical absorption (200–800 nm), and electrical measurements. As a result, it is suggested that the surface of the film and some of the crystals become amorphous by the plasma irradiation, and the amorphous layer is expected to prevent the film from the oxidization. The polymers, whose silicon backbones are perpendicular to the substrate surface, are selectively damaged by the plasma irradiation. The reason is discussed on the basis of the crystal structure of poly(di-methyl silane) as well as the plasma dynamics.

Structure and optical properties of the planar silicon compounds polysilane and Wöhler siloxene

The two-dimensional silicon backbone structure of planar polysilane and W\"ohler siloxene is responsible for their exciting luminescing properties. We have prepared single crystals of siloxene by a topotactic reaction from crystalline CaSi${}_{2}$. The chemical composition was determined as [Si${}_{6}$H${}_{3}$(OH)${}_{3}$]${}_{n}.$ The x-ray crystal structure analysis identifies the so-called W\"ohler siloxene as 2D-poly[1,3,5-trihydroxocyclohexasilane]. Polysilane exhibits the same structural properties but with a chemical composition [Si${}_{6}$H${}_{6}$]${}_{n}.$ The optical properties (infrared transmission, photoluminescence, excitation spectroscopy) of these well-defined materials are presented. A heat treatment above $350\ifmmode^\circ\else\textdegree\fi{}$C in vacuum of W\"ohler siloxene results in a destruction of the planar ${}_{\ensuremath{\infty}}^{2}$ [Si${}^{\ensuremath{-}}$] structure by internal rearrangements, which is evidenced by the x-ray-diffraction pattern and characteristic changes in the optical spectra. The involvement of W\"ohler siloxene in the optical properties of porous Si is critically reviewed.

Soft X-ray emission and absorption spectroscopy for electronic structure analysis of cubic silicon clusters in Si K-shell threshold

Abstract Soft X-ray emission and absorption (SXEA) spectroscopy is presented as a possible method for electronic structure analysis. To demonstrate its feasibility, the electronic structure of silicon backbones in a cubic silicon cluster (octasilacubane) and its related clusters was analyzed by measuring the SXEA spectra in the Si K-shell threshold. Three discrete levels are observed in both Si3p occupied and unoccupied orbitals of octasilacubane, which may be caused by the large degree of degeneracy of the orbitals due to the high symmetry of the cubic silicon backbone structure. The measured narrow energy gap of 2.3 eV between the highest occupied and the lowest unoccupied orbitals of octasilacubane shows that Si3p σ-electrons in octasilacubane are more widely conjugated than those in the related clusters.

Characterization of poly(phenylsilsesquioxane) thin-film planar optical waveguides

Characterization results of thin-film poly(phenylsilsesquioxane) (PPSQ) planar-optical waveguides are presented. Results of absorption spectrum, refractive index, thermal stress effect, and waveguide loss measurements performed on 1-2 /spl mu/m PPSQ films indicate this silicon backbone polymer to be a strong potential candidate for optical waveguide integration within microelectronic systems. PPSQ films are shown to exhibit thermal stability with respect to volume, refractive index, and optical loss for temperatures up to 400/spl deg/C. The first TE mode of PPSQ planar optical waveguides between 1.32 and 1.72 /spl mu/m in thickness fabricated on 5-/spl mu/m HiPOX Si wafers exhibited optical loss of 0.16 dB/cm at 632.8 mn.

Unprecedented thermo‐ and ion‐responsive behavior of a non‐ionic water‐soluble polysilane

Polysilanes display remarkable optical and electronic properties that are affected by teh degree of σ‐conjugation, which depends on the conformation of the silicon backbone. The thermo‐ and ion‐responsive behavior of aqueous solutions of 1 (Figure) is described, including «inverse» abrupt termochromism and the tunability of the lower critical solution temperature (LCST) by addition of inorganic salts.magnified image

Double emulsions stabilized by macromolecular surfactants

Multiple emulsions are emulsions within emulsions, stabilized traditionally by monomeric emulsifiers both at the inner and outer interface. Double emulsions are thermodynamically unstable and fast coalescence, as well as fast release of markers and drugs, have been the main drawbacks of this technology. Polymeric synthetic emulsifiers, as well as natural macromolecules in combination with monomeric emulsifiers, have recently been studied and evaluated. The review brings new results on the use of amphiphilic proteins such as BSA and casein along with monomeric emulsifiers in relation to improved stability and release properties. Kinetic results are brought to demonstrate the role of BSA as the inner emulsifier and the outer emulsifier. Diffusion controlled mechanisms are suggested. Synthetic block copolymers based on silicon backbones and polyethylene oxide side-chains have also been studied. Double emulsions with relatively small droplets (5 μm) are formed and excellent stability (to storage, sheer and dilution) is obtained. Release data indicates that the newly designed double emulsions have longer shelf-life and very slow rates of release of markers. Such emulsions are good candidates for agricultural formulations.

Theoretical investigation of the origins of abrupt thermochromism in the polysilanes

Abstract Values of the conformation-dependent polymer-solvent interaction parameter VD have been calculated for some simple polysilane model compounds using quantum-mechanically derived molecular polarizabilities coupled with London's dispersion formula. The value VD = 0.97 kcal mol−1 obtained in the present study for the σ-conjugated polysilanes nearly equals the VD ≈ 1.1 kcal mol−1 estimated by Schweizer for the analogous π-conjugated carbon-backbone polymers, assuming similar physical conditions. The present results thus provide theoretical evidence in support of the pertinence of Schweizer's theory of abrupt thermochromism to the polysilanes. The conformational flexibility and versatility of the backbone is much greater for the polysilanes compared with most π-conjugated polymers, hence the conformational ‘defect energy’ ϵ of the former should be small or even negative. The combination of large VD (favouring the ‘ordered’ backbone) and small ϵ (favouring the ‘disordered’ backbone) predicted for the polysilanes would place the critical VD/ϵ parameter well in excess of the limiting value VD/ϵ ≥ 0.37 required for onset of abrupt thermochromism according the Schweizer. The (Austin Model 1) AM1-calculated change in polarizability Δα with respect to backbone rotation o was 0.63 A3 for the model polysilane but only 0.08 A3 for the analogous non-conjugated carbon-backbone model polymer. This substantial difference substantiates the existence of strong coupling between the molecular polarizability and the backbone conformation in the polysilanes. Only the longitudinal polarizability αxx contributes substantially to Δα, while the transverse polarizabilities αyy and αzz essentially combine with no net effect on Δα (i.e. Δαyy −Δαzz). The variation of the calculated band-gap energy Eg with α for corresponding values of o fits a second-order regression curve (correlation coefficient r = 0.98). As the silicon backbone approaches an all-trans conformation, the highest occupied molecular orbital (HOMO) energy increases while the lowest unoccupied molecular orbital (LUMO) energy decreases to reduce Eg. This increase in the HOMO energy is balanced by a nearly equal decrease in the HOMO-1 energy; likewise, the decrease in the LUMO energy is balanced by an increase in the LUMO+1 energy.

Preparation and optical properties of doubly-oriented poly-(di-methyl-silane) films

Doubly-oriented films of poly-(di-methyl-silane) (PDMS) were prepared by evaporation under controlled conditions on unidirectionally oriented poly(tetra-fluoro-ethylene) (PTFE) thin layer, which was mechanically deposited on fused quartz plate. It was confirmed by X-ray diffraction and polarized absorption measurements that the silicon backbone chains with all- trans conformation laid along the direction of PTFE orientation, and that a particular crystalline plane (110) was parallel to the PTFE layer. PDMS films with double orientation showed a new absorption which appeared only in the polarized spectrum parallel to the direction of chains, and in lower energy region, than the main peak at 4.1 eV.

Electronic structure of poly( p-(disilanylene)phenylene)

We present the geometrical and electronic structures of several isomers of poly( p-(disilanylene)phenylene), The structural analysis, performed at the 3-21G * level, shows that the isomers with the phenylene group perpendicular to the silicon backbone are the more stable conformations, displaying almost the same energy. The electronic properties, as obtained from the valence-effective Hamiltonian (VEH) band structure calculations, strongly depend on the disposition of the phenylene group into the polymeric backbone. The VEH predicts a wide and asymmetric absorption band in excellent agreement with UV experimental data.

Structure and Properties of Poly(n-pentyl-n-alkylsilanes). 2

The solid-state electronic properties and structures of poly(n-pentyl-n-hexylsilane) (C5-C6), poly(n-pentyl-n-octylsilane) (C5-C8), and poly(n-pentyl-n-dodecylsilane) (C5-C12) have been investigated using UV absorption, differential scanning calorimetry, and X-ray diffraction. At low temperatures, C5-C6 is semicrystalline with an all-trans backbone conformation. Upon heating, it transforms to the hexagonal, columnar, condis crystalline phase observed in many polysilanes. C5-C8 exhibits the hexagonal, columnar mesophase both above and below its first-order transition temperature of −32 °C. This type of behavior is found to be characteristic of poly(n-alkyl-n-alkyl‘silanes) with a side-chain asymmetry of two or three carbon atoms. The existence of first-order transitions and the observation of UV thermochromic red shifts upon cooling for these polymers which do not crystallize are most likely the result of polarization interactions between the σ-bonded silicon backbone and the alkyl side chains, which is consistent with the predictions of the model of Schweizer for conjugated polymers.

Charge Carrier Mobilities in Substituted Polysilylenes: Influence of Backbone Conformation

The results of a systematic study of charge migration in the crystalline, liquid-crystalline, and amorphous states of organo-substituted polysilylenes, using the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) method are summarized. A homologous series of symmetrically substituted poly(di-n-alkylsilylene)s with alkyl side chain lengths ranging from n-butyl to n-decyl has been studied, as well as a copolymer with branched substituents and two nonsymmetrically substituted amorphous polymers: poly(hexylmethylsilylene) and poly(methylphenylsilylene). The charge carrier mobilities of the dialkyl-substituted polysilylenes varied from 4 × 10-5 m2/(V s) for charge transport along the all-trans silicon backbone of poly(di-n-decylsilylene) to 2 × 10-7 m2/V s for the amorphous poly(hexylmethylsilylene). An increase of conformational disorder in the columnar mesophase as compared to the solid crystalline phase leads to an abrupt, reversible decrease in the charge carrier mobilities. A direct relationship between charge carrier mobilities and polysilylene backbone conformation has been established. The introduction of a phenyl group lowers the charge carrier mobility to 4 × 10-8 m2/(V s).

Charge carrier mobilities in liquid crystalline mesomorphic poly(DI‐n‐Alkylsilylene)s; influence of backbone conformation

AbstractThe charge transport properties of a series of symmetrically substituted mesomorphic poly(di‐n‐alkylsilylene)s are studied using the pulse‐radiolysis time resolved microwave conductivity (PR‐TRMC) technique. The observed conductivities for these polymers could be correlated with different backbone conformations present both in the crystalline solid phase and in the liquid crystalline mesophase. The transition from the solid phase to the mesophase is accompanied by a disordering of the silicon backbone that results in a decrease of the conductivity of up to two orders of magnitude. The charge carrier mobilities found varied from 5×10−5 m2 /Vs for the all‐trans conformation in the solid phase to 6×10−7 m2 /Vs for the disordered backbone conformation in the mesophase. The anisotropic radiation‐induced conductivity observed for aligned poly(di‐n‐hexylsilylene) samples demonstrate that charge carrier migration takes place preferentially in the direction of the polymer backbone.