Enhancement of fluorescence of porous silicon upon saturation by liquid crystal

The fluorescence of samples of porous silicon of various morphologies that are filled with a liquid crystal (LC), n-pentyl-n′-cyanobiphenyl (5CB), is studied. The fluorescence spectra of the sample, along with the long-wavelength band of porous silicon with a maximum in the range 627–667 nm, exhibit a short-wavelength band of 5CB with a maximum in the range 385–410 nm. The radiative relaxation times of porous silicon and 5CB lie in the micro- and nanosecond ranges, respectively. It is found that the filling of pores with 5CB enhances the fluorescence of porous silicon by two to three times. This enhancement is caused by non-radiative energy transfer from 5CB to the porous matrix as a result of efficient interactions between LC molecules and pore walls. Using IR spectroscopy, it is shown that the formation of hydrogen bonds between cyano groups of 5CB molecules and silanol groups of pore surface is the predominant type of these interactions. A transfer mechanism is suggested according to which excited associates of 5CB molecules transfer their energy via surface channels to excitons of porous silicon, enhancing its fluorescence.     

Low-temperature photoluminescence of 5CB liquid crystal

The photoluminescence spectra of the 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal have been investigated in detail at low temperatures 4.2-200 K, for the first time. The spectral data obtained are compared to the results of the luminescence study for the 5CB nematic phase at Т=300 K. The luminescence of the 5CB crystal state is characterized by emissions of both monomer and dimer structures. Besides, there are several energetically and conformationally non-equivalent types of monomers and dimers, and their amounts change with the temperature growth non-monotonously. The 5CB several crystal modifications, having different 5CB monomer and dimer conformers, have been found out below T=160 K. The 5CB crystal-crystal transition at Т=80 K is characterized with a total loss of the fine structure in the 5CB photoluminescence spectrum and a disappearance of the spectral band at 343 nm. At T=140 K, a new spectral band at 424 nm appears; it corresponds to the significantly overlapping 5CB dimers, its intensity grows under further heating. The present investigation gives a tool for the further characterization of the molecular alignments and changes in the 5CB molecular conformations, using the monomer and excimer fluorescence emissions as a probe. The conclusions made are confirmed by the IR-spectroscopy data, measured and analyzed for the 5CB in the same temperature region.     

Dependence of the threshold voltage in indium-phosphide pore formation on the electrolyte composition

Results of the experimental determination of the threshold voltage of pore formation for n-InP (100) crystals with a charge-carrier density of 2.3 × 10¹⁸ cm^?3 are presented. The threshold voltage of pore formation is shown to be a function of the electrolyte composition, in particular, of the acid concentration in the electrolyte solution. A 5% solution of hydrochloric acid is the most suitable etchant for obtaining high-quality porous indium phosphide films. It is possible to obtain a nanoporous layer that consists of pores 40 nm in diameter spaced by 5–10 nm. The porosity in this case is 45%.     

Self-aggregates formation of 3,4,9,10-tetra-(n-alkoxy-carbonyl)-perylenes in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films formed of some 3,4,9,10-tetra-(n-alkoxy-carbonyl)-perylenes and of their binary mixtures with the liquid crystal 4-octyl-4′-cyanobiphenyl (8CB) have been studied. Absorption and fluorescence studies were carried out. Both absorption and fluorescence spectra have revealed the formation of self-aggregates of dye molecules in monomolecular layers. Moreover, information about the molecular organization at the air-solid substrate interface has been obtained.     

Role of molecular interactions and end chain length on the photosensitivity of liquid crystalline alkyl cyanobiphenyl dimers – UV absorption-based approach through DFT calculations

In this paper, the photosensitivity of liquid crystalline alkyl cyanobiphenyl (nCB: n = 6, 7; n is the number of carbon atoms in the alkyl chain) dimers has been presented through density functional theory (DFT) calculations. The nCB structures have been optimized using the Becke, three-parameter, Lee–Yang–Parr hybrid functional with the 6-31+G (d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the DFT method. The excited states have been calculated via configuration interaction singles with the semi-empirical Hamiltonian Zerner intermediate neglect of differential overlap. The influence of molecular interactions and the end chain length on ultraviolet absorption spectral characteristics and the photosensitivity of the compounds has been discussed. These results offer a hint for the protection of various optical devices from the intense light induced damages, and to model photosensitivity.     

The molecular dynamics of nematic liquid crystal confined in porous membranes treated by different surfactants

The molecular dynamics of the well-known nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl geometrically restricted in Anopore and Synpor porous membranes with various pore structure and treated by different surfactants (namely decanoic acid and lecithin) is compared. In the Anopore membrane the chosen surfactants induce the homeotropic orientation of the molecules on the walls of the cylindrical pores and observed corresponding relaxation processes (librational modes) are practically the same. The dielectric measurements of lecithin treated Synpor membranes reveals the reorientation of the molecules from planar to homeotropic on the complex multilayer structure present in their volume. The dielectric strengths of the observed two molecular processes (δ-process and librational mode) are inversed in the ratio compared to the untreated membranes. The observed differences in molecular dynamics results from the orientation of the liquid crystal molecules in untreated and treated membranes and the structure of the membranes themselves.     

Relaxation processes of water confined to AlMCM-41 molecular sieves. Influence of the hydroxyl groups of the pore surface

A series of AlMCM-41 molecular sieves was prepared with constant composition (Si/Al = 14.7) and presumably same pore structure but different pore diameters (from 2.3 to 4.6 nm). The pore size distribution is narrow for each sample. The rotational fluctuations of water molecules confined inside the pores were investigated applying broadband dielectric spectroscopy (10⁻²–10⁷ Hz) over a large temperature interval (213–333K). A relaxation process, slower than that expected for bulk water, was observed which is assigned to water molecules forming a surface layer on the pore walls. The estimated relaxation time has an unusual non-monotonic temperature dependence, which is rationalized and modeled assuming two competing processes: rotational fluctuations of constrained water molecules and defect formation (Ryabov model). This paper focuses on the defects and notably the influence of the hydroxyl groups of the pore walls. The Ryabov model is fitted to the data and characteristic parameters are obtained. Their dependence on pore diameter is considered for the first time. The found results are compared with those obtained for other types of molecular sieves and related materials.     

Theoretical study on liquid crystal cyanobiphenyls: Phase stability and phase behavior

This article presents a theoretical study on liquid crystalline materials in homologous series of 4'-n-alkyl-4-cyanobiphenyl (nCB) with propyl (3CB), pentyl (5CB), and heptyl (7CB) groups. The atomic net charge and dipole moment components at each atomic center have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh–Schrodinger perturbation theory along with the multicentered-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a ‘6-exp’ potential function has been assumed for short-range interactions. Further, these interaction energy values have been used as input to calculate the translational entropy, and free energy of nCB (n=3, 5, and 7) molecules during the stacking, and in-plane interactions. The observed results have been correlated with the mesogenic behavior and phase stability based on the thermodynamic parameters introduced in this article. Further, an attempt has been made to elucidate the flexibility of a configuration at a particular temperature, which has a direct relation with phase transition property of the molecules.     

Photogeneration of electrons and holes in amorphous molecular semiconductors

This paper reports on the results of investigations into the photoconducting properties of amorphous molecular semiconductors based on films of two types: (i) poly(styrene) films doped with epoxypropylcarbazole (EPC) and a cationic polymethine dye (PD1) and (ii) poly(styrene) films doped with tetranitrofluorenone (TNF) and an anionic polymethine dye (PD2). Films of the first type possess p-type conductivity, whereas films of the second type exhibit n-type conductivity. It is found that, for films with n-type conductivity, unlike films with p-type conductivity, the activation energy of photogeneration of mobile charge carriers decreases with a decrease in the optical wavelength in the absorption range of the dyes. The possible mechanisms of the influence of the photoexcitation energy on the initial distance between charge carriers in electron-hole pairs are analyzed. The inference is made that, when the excess thermal energy of excited dye molecules dissipates at a low rate, the distance between the photogenerated electrons and photogeneration centers increases as compared to the distance between the photogenerated holes and photogeneration centers due to the electron-nucleus interaction.     

Strong n-type molecule as low bias negative differential resistance device predicted by first-principles study

A first-principles study of the transport properties of two thiolated pentacenes sandwiching ethyl is performed. The thiolated pentacene molecule shows strong n-type characteristics when contact Ag lead because of low work function about metal Ag. A strong negative differential resistance (NDR) effect with large peak-to-valley ratio of 758% is present under low bias. Our investigations indicate that strong n- or p-type molecules can be used as low bias molecular NDR devices and that the molecular NDR effect based on molecular-level leaving not on molecular-level crossing has no hysteresis.     

Photoluminescence of pentyl-cyanobiphenyl in liquid-crystal and solid-crystal states

The stationary and time-resolved photoluminescence spectra of n-pentyl-n′-cyanobiphenyl (5CB) are studied in a mesophase and solid-crystal state. The photoluminescence spectra of 5CB are determined by the molecular configuration and the intramolecular charge transfer in the excited state. It is shown that in the mesophase and solid-crystal state, 5CB exhibits along with monomer radiation at least two types of excimer radiation from different pre-dimer states.     

Förster resonance energy transfer in hybrid associates of colloidal Ag<Subscript>2</Subscript>S quantum dots with thionine molecules

Nonradiative resonance energy transfer in hydrophilic hybrid associates of thionine molecules (TH⁺) with colloidal Ag₂S quantum dots (QDs) with average diameter of 3.5 nm was studied. Photoluminescence spectra and its decay shown that for these systems the supplemental photosensitization of recombination luminescence of Ag₂S QDs (1200 nm) from the region of TH⁺ fluorescence (618 nm) is possible. It was found that the average lifetime of TH⁺ molecules luminescence is shortened during their association with Ag₂S QDs. Approximation of luminescence decay by stretched exponent with value of parameter β =?0.5 indicates on the inductive-resonance dipole-dipole (Förster) mechanism of nonradiative energy transfer (FRET). The efficiency of FRET was 0.29–0.41.     

Melting mechanism of monolayers adsorbed in cylindrical pores: An influence of the pore wall roughness

We have analyzed the mechanism of melting of layers adsorbed in cylindrical pores of porous materials. The goal was to understand the melting mechanism of simple fluids adsorbed in pores with heterogeneous wall surface. The studied system was a monolayer of methane molecules adsorbed in MCM-41 pore of diameter d = 4 nm. Both experimental (neutron scattering) and simulation (Monte Carlo) results proved extremely strong influence of the wall roughness on the melting mechanism. The most striking difference between melting on smooth and rough surfaces was in the temperatures of the transition. The transformation between solid-like and liquid-like monolayer phases adsorbed on a rough surface was observed in a very large temperature range and the solid like properties were observed even above the bulk methane melting temperature.     

An ionic force-field study of monomers, dimers and higher polymers in pentafluoride vapors

Pentafluoride compounds such as NbF₅ and TaF₅ have been reported in the literature to admit various states of polymerization coexisting with monomers in their vapor phase, in relative concentrations that vary with temperature and pressure. We construct a microscopic interionic force-field model for the molecular monomer of these compounds (including VF₅, SbF₅ and MoF₅ in addition to NbF₅ and TaF₅), the stable form of the monomer being in the shape of a D_3h trigonal bipyramid in all cases. The model emulates chemical bonds by allowing for electrical and short-range overlap polarizabilities of the fluorines, and is used to evaluate the structure and the stability of (MF₅)_n molecules with n running from 2 to 6. The dimer is formed by two distorted edge-sharing octahedral, while the trimer and the higher polymers can form rings of distorted corner-sharing octahedra. A chain-like configuration is also found for the trimer of NbF₅, which consists of a seven-fold coordinated Nb bonded to two distorted octahedra via edge sharing. Comparison of calculated vibrational frequencies and bond lengths with experimental data is made whenever possible. We find that there is a small net gain of energy in the formation of a dimer, while otherwise the static energy of the n-mer is very close to that of n separated monomers. High sensitivity of the state of molecular aggregation to the thermodynamic conditions of the vapor is clearly indicated by our calculations.     

The “corset effect” of spin-lattice relaxation in polymer melts confined in nanoporous media

Linear polyethylene oxides with molecular weightsM _w of 1665 and 10170 confined in pores with variable diameters in a solid methacrylate matrix were studied by proton field-cycling nuclear magnetic resonance relaxometry. The pore diameter was varied in the range of 9–57 nm. In all cases, the spin-lattice relaxation time shows a frequency dependence close toT ₁∞ v^3/4 in the range ofv=3·10^?1-2·10¹ MHz as predicted by the tube-reptation model. This protonT ₁ dispersion essentially reproduces that found in a previous deuteron study (R. Kimmich, R.-O. Seitter, U. Beginn, M. Möller, N. Fatkullin: Chem. Phys. Lett. 307, 147, 1999). As a feature particularly characteristic for reptation, this finding suggests that reptation is the dominating chain dynamics mechanism under pore confinement in the corresponding time range. The absolute values of the spin-lattice relaxation times indicate that the diameter of the effective tubes in which reptation occurs is much smaller than the pore diameters on the time scale of spin-lattice relaxation experimens. An estimation leads to a valued ^*～0.5 nm. The impenetrability of the solid pore walls, the uncrossability of polymer chains (“excluded volume”) and the low value of the compressibility in polymer melts create the “corset effect” which reduces the lateral motions of polymer chains to a microscopic scale of only a few tenths of a nanometer.     

Investigation of the closed porosity of functional ceramic materials by spin-echo small-angle neutron scattering

The closed porous structure in ceramic materials is investigated by spin-echo small-angle neutron scattering. A series of ceramic samples of oxygen–ion conductors based on bismuth molybdate with the general formula Bi_12.8 X _0.2Mo₅O_{34 ± δ} (X = Mg, Ba, Ca, Sr) is obtained by powder sintering for 6?45 h at a temperature close to the melting point. The samples are characterized by scanning electron microscopy and X-ray fluorescence analysis. It is found that they had a stoichiometric chemical composition, are singlephase, and contain clean pores between crystal grains. The pore size is determined by spin-echo small-angle neutron scattering and ranges from 2.2 to 3.5 μm. It is demonstrated that longer sintering times correspond to larger pores (the increase in their average diameter is as large as 30%). It is found that the studied materials lack a fractal pore structure.     

Influence of surface interactions on the dynamics of the glass former ortho-terphenyl confined in nanoporous silica

We present results on investigations of the dynamics of the glass forming ortho-terphenyl (oTP) confined in nanoporous silica. Calorimetry experiments showed that the glass transition temperature of the confined liquid, T_gconf, has a non-trivial pore size dependence and is strongly affected by surface interactions. Fluid-wall interactions introduce gradients of structural relaxation times in the pores. The molecules at the surface of the pores are slowed down compared to those at the center of the pores. We focus here on a pore diameter range (7 σ< d < 12 σ, where σ is the molecular diameter), where a large variety of dynamical behavior were observed. Depending on surface properties of the confined media, T _gconf may be smaller or larger than the bulk one. In a quite attractive matrix with a pore size of around 7 nm, the structural relaxation times gradient is important enough to allow the observation of two glass transitions for the same liquid. Effects of fluid wall interactions on the short time dynamics at high temperature were also investigated by quasielastic neutron scattering. The self and collective motions exhibit well above the bulk melting point the same dependence on fluid-wall interactions as at T_g.     

Growth of nanocrystalline CuIn3Se5 (OVC) thin films by ion exchange reactions at room temperature and their characterization as photo-absorbing layers

Nanocrystalline CuIn₃Se₅ thin films have been grown on ITO glass substrates using chemical ion exchange reactions with CdS, in alkaline medium at pH 11. The as-deposited films were annealed in air at 200 °C for 30 min and characterized using X-ray diffraction (XRD), transmission electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy to study the structural, compositional and morphological properties. The XRD patterns reveal the nanoparticles size to be of 18-20 nm diameter, while from the SEM images the nanoparticles size is estimated to be 20-30 nm. It is observed that the annealed films contain nanocrystallites connected with each other through grain boundaries, with grain size of about 100-125 nm and have an overall n-type electrical conductivity and higher photoconductivity. The current-voltage (I-V) characteristics (in dark and light) of these films indicated the formation of a Schottky like junction between the n-CuIn₃Se₅ (OVC) and CdS/ITO layers.     

Magnetic properties of Ni nanoparticles on microporous silica spheres

Ni nanoparticles (~32 nm particle diameter) have been synthesized on the walls of microporous (~1 nm pore diameter) silica spheres (~2.6 μm sphere diameter) and characterised magnetically to potentially produce a new class of core (silica micro-spheres)-shell (nanometallic)-type nanocomposite material. These magnetic nanocomposite materials display a characteristic increase in coercivity with reducing temperature. The average particle size has been used to calculate the anisotropy constant for the system, K. The discussion postulates the potential mechanisms contributing to the difference between the calculated K value and the magnetocrystalline anisotropy constant of bulk Ni. Various factors such as surface anisotropy and interparticle interactions are discussed as possible contributing factors to the anisotropy values calculated in the paper.     

Development of three-dimensional microstructure processing using macroporousn-type silicon

The development of a micromachining technique for processing arbitrary structures with high aspect ratios in bulk silicon is presented. It is based on utilizing standard microelectronic processes and electrochemical macropore formation onn-type silicon in electrolytes containing hydrofluoric acid. This pore-etching technique allows us to produce very regular pore arrays with pore diameters and distances in the micrometer range and pore lengths up to wafer thickness. Samples with prefabricated pore arrays which differ in pore spacing, pore diameter and geometry are used as substrates for a micromachining process. The pores will facilitate the anisotropic etch profile which is required for the desired high aspect ratios although an isotropic etch process is used. Very deep microstructures with steep pore walls and aspect ratios of 10–15 are produced with this technique. It is shown that smaller pore array dimensions improve microstructure resolution.