Surface Of Microcrystallites Research Articles

Fundamental transport properties in N-phenyl thin films

Conductivity of N-phenyl (N = 3,4) thin films (with thickness 2-15 µm) was investigated experimentally and theoretically using molecular dynamics simulations of microcrystallite (MC) occurrence and quantum chemical band structure calculations. At the first time, process of carrier kinetics is considered within a framework of self-consistent band structure approach. The external electric field was varied up to 106 V cm-1 using Al-Au electrodes with different voltage polarities. Depending on the film thickness and N, we have discovered the appearance of minima of effective activation energy. The observed effects are explained as a competition between the diffusion recombination of carriers from the trapping MC levels and the potential gradients on the MC surfaces. A good agreement between the performed theoretical simulations and experimental data is achieved.

Monolayers of ortho-H2, para-H2, para-D2 and normal-H2 adsorbed on NaCl(001) single crystal surfaces

Monolayers of pure ortho-H2, para-H2, para-D2 and normal-H2 adsorbed on NaCl(001) single crystal surfaces were investigated at temperatures T⩾10 K and pressures between 1×10−10 and 1×10−5 mbar by polarization Fourier transform infrared attenuated total internal reflection spectroscopy (PIRS-ATR). Single induced polarized infrared absorptions of the fundamental vibrational modes of pure (M=|1|) ortho-H2 and para-H2, as well as pure (M=|1|) para-D2 and, slightly shifted, in the mixture of normal-H2 were observed, allowing definite assignments of each. With respect to the gas phase values, the wave numbers were measured to be redshifted by 32.0 and 28.0 cm−1 for pure (M=|1|) ortho-H2 and para-H2. In close agreement with potential calculations, these absorptions were assigned to H2 molecules adsorbed on the cation sites of the large NaCl(001) terraces, while neither the absorptions associated with H2 adsorption above defects nor the absorptions of (M=0) H2 molecules above the anion sites, detected on microcrystallite surfaces at lower temperature, are observed under the conditions set. The present experiments corroborate the fact that the NaCl surface discriminates between the (M=|1|) ortho-, (M=0) ortho- and para- species. The lateral interactions have no influence on the stable sites and the discrimination, but they contribute by an additional shift of the peak frequencies of about 15%. Moreover, with polarized light, induced transition dipole moments perpendicular to the surface for ortho-H2, para-H2 and para-D2 were observed, while none parallel to the surface was detected. These results are in accordance with the calculations showing that the s-component of the polarization disappears due to free rotation of the H2 (D2) molecules in a plane parallel to the surface, while the p-component gives a maximum intensity. The integrated cross sections σ̄⊥ were measured and found to have the same order of magnitude: (6±3)×10−22 m/molecule for (M=|1|) ortho-H2 and (4±2)×10−22 m/molecule for para-H2 within the error limits; the theoretical value is 4.5×10−22 m/molecule for both species. Applying the extended Langmuir equation, the difference between the desorption energies (Edesortho−Edespara) of (M=|1|) ortho-H2 and para-H2 adsorbed on the (001) terraces of NaCl(film) was determined to be 0.6±0.2 kJ/mol, in agreement with the calculated value (0.8 kJ/mol). Assuming that the sticking coefficients of (M=|1|) ortho-H2 and para-H2 are not dependent on temperature between 17 and 30 K, the ratio sortho/spara of 0.06±0.03 was obtained.

Frenkel exciton scattering at microcrystallite surfaces caused by electric dipoles and monopoles

A theory of exciton scattering by electric charges located on microcrystallite surfaces is developed. The surface charges are assumed to be either electric monopoles or dipoles. When monopoles are present on microcrystallite surfaces, surface excitons are created in microcrystallites in which the sizes are larger than a critical value. On the other hand, when electric dipoles are present at microcrystallite surfaces, surface excitons are always created irrespective of the microcrystallite size. The theory is applied to explain the surplus exciton bandwidth observed in anthracene microcrystallites, which indicates the presence of surface scattering. The theory also predicts the presence of an additional and intense exciton absorption line which appears on the high-energy side of the host exciton absorption band.

Surface recombination model of visible luminescence in porous silicon

The most favourite candidate to explain visible photoluminescence (PL) of porous silicon (PS) is quantum confinement. It has been shown theoretically that decreasing size of silicon particles leads to the widening of the gap (physical confinement). According to calculations, red luminescence requires particle sizes that are too small to be directly observed in PS. On the other hand, chemical effects, other than etching, seem to affect the PL frequency. These effects indicate similarities to the PL of siloxene which is due to isolation of Si 6 rings by oxygen atoms (chemical confinement). Using self-consistent semi-empirical calculations, we have shown that the observed correlation between shift of the PL and Raman bands in PS can only be explained by chemical effects. Therefore, we suggest a surface recombination model of PL in PS, where the emission is determined primarily by physical confinement in features with nanometer dimensions on the surface of microcrystallites, while chemical substitutions tune the PL into the red. Empirical tight binding calculations on such particles show quantitative agreement with observed PL energies.

Photophysical Study of Frenkel Exciton States Using a Novel Microcrystallization Technique

A method determining the exciton bandwidth in aromatic microcrystallites is presented, and the behavior of Frenkel excitons in these substances is reviewed. The spectral bandwidth of excitation spectra of luminescence from microcrystallites is found to increase linearly with the microcrystallite size in anthracene and superlinearly in pyrene. A strong exciton scattering component at microcrystallite surfaces is found in anthracene microcrystallites. The k selection rule that is lost in small microcrystallites is recovered for large microcrystallites at a size of 134 Å in anthracene and 50 Å in pyrene. With these values, the surface scattering layer thickness is estimated to be 32 Å in anthracene, but nearly zero in pyrene microcrystallites. We suggest that the concept of the exciton migration in microcrystallites can be applied to biological systems where only 5 ∼ 10 molecules are arranged.

The Formation and Electrochemical Activity of Microporous Diamond Thin Film Electrodes in Concentrated KOH

The corrosion resistance and electrochemical activity of boron‐doped, polycrystalline diamond thin films were evaluated before and after potentiodynamic cycling between −0.7 and 0.7 V vs. Ag/AgCl in 15 weight percent KOH. The maximum anodic current densities at 0.7 V ranged from 0.3 to 6.0 mA/cm2. Two types of corrosion processes were observed. In one case, low‐quality films grown using a 2.9% C/H ratio developed circular pits (100 to 300 nm diam) located almost exclusively at the intercrystalline grain boundaries, creating a microporous film. For the most part, the diamond microcrystallites were morphologically unaffected by the polarization, but in a few areas, extreme pitting and smoothing of the microcrystallites occurred. In a second case, low‐quality films grown using a 1% C/H ratio experienced corrosion that advanced laterally across the diamond microcrystallite surfaces commencing at facet corners and step edges. High‐quality films grown using a 1.4% C/H ratio exhibited no signs of corrosion or morphological damage. A corrosion mechanism is proposed whereby the nondiamond carbon impurities exposed at the surface are preferentially etched away by anodic polarization. The electrochemical activity of the films, before and after cycling, was probed using cyclic and differential pulse voltammetry with . The morphological and surface chemical changes produced during the microporous film formation led to an increase in the oxidation and reduction peak currents by up to a factor of eight. The heterogeneous electron‐transfer rate constant, , decreased slightly by a factor of four after formation of the microporous film. It is supposed that the electron‐transfer reactions are not occurring exclusively through the nondiamond carbon impurity sites but rather that these impurities serve primarily an electronic role by supplying charge carriers to the film, thereby reducing resistivity.

Size-Dependent Exciton Densities of States and Exciton Bandwidths in Anthracene Microcrystallites

Applying a technique to determine the k=0 components in the exciton densities of sates (DOS) we found: (1) The DOS in anthracene microcrystallites appears as sharp bands followed by broad tails extended towards higher energies. (2) The exciton bandwidth which depends linearly on the size of microcrystallites where the diameter is smaller than 60 Å. (3) The bandwidth takes its maximum value 340 cm-1 for microcrystallite of 65 Å. (4) For larger microcrystallites, the bandwidth decreases asymptotically to 300 cm-1. (5) This width 300 cm-1 is equivalent to the bandwidth for bulk crystal. (6) Exciton scattering at the surface of microcrystallites is extremely effective.

Nature of the active sites in H-ZSM-11 zeolite modified with Zn(2+) and Ga(3+)

H-ZSM-11 zeolite modified with zinc and gallium by ion exchange was investigated using XRD, IR and TPD of ammonia. The modification of the material by zinc produced a lowering of the strong Bronsted acidic sites generating new and strong Lewis sites. Unlike zinc-zeolites, the gallium is localized preferentially on the outer surface of microcrystallites blocking a few Bronsted centers.

Localization of polyvalent cations in pentasil catalysts modified by metal oxides

Polyvalent cations in HZSM-5 zeolites were found to be preferentially localized at the outer surface of microcrystallites (for Me3+) or inside the channels of the zeolite structure (for Me2+). This difference was explained by both steric and electrostatic hindrances arising for the tervalent cations.

Study on the mechanism of oxide hydration and oxide pore closure during hydrothermal treatment of porous anodic Al 2O 3 films

Porous anodic Al 2O 3 films were investigated for their behaviour during hydrothermal treatment in H 2O at 100°C and a mechanism for the process of oxide hydration was proposed. The films were prepared galvanostatically in a 15% w/v H 2SO 4 bath at 20, 25 and 30°C and at 5, 15 and 35 mA cm −2. The resulting thicknesses were between 1.5 and 120 μm. It was revealed that the maximum amount of H 2O uptake, on prolonged hydration and retained by the oxides, is directly related to the pore void volume of initially produced dry films. Their ratio, m w,m/ v, is always close to 1 g cm −3 for all films having a mean thickness of ⪕ 75 μm. Additionally, dry or hydrated Al 2O 3 examined by XRD and ir spectroscopy techniques failed to reveal the presence of any crystalline form of Al 2O 3, AlOOH and Al(OH) 3. The absence of any crystalline hydrated forms is consistent with the dependence of m w,m on v supporting the hypothesis of a microcrystalline nature of films. An Al 2O 3 hydration mechanism was proposed including a “reaction” between H 2O and Al 2O 3 pore wall surface. It results in the addition of OH groups and chemisorbed molecular H 2O on microcrystallite surfaces effecting the separation of microcrystallites and physically adsorbed molecular H 2O on the hydroxylated/hydrated surfaces in the intercrystallite spaces causing the swelling of the moderately coherent hydrated oxide layer on pore walls. Ultimately, the process results in the depletion of Al 2O 3 at points where it has become emaciated, ie at the mouths of conical pores in the cases of the maximum limiting film thicknesses attained and in the production of a hydrated layer, on pore walls, of variable thickness sufficient to fill up pores along a distance from the pore base, depending on structural film features.

Thermal expansion of microcracks and grain boundaries in crystals (II). Thermal expansion and compression under pressure of intermicrocrystallite gaps in liquids

AbstractThe changes of melting points under pressure (melting curves) in terms of the microcrystallite model of liquids has been studied. These curves allowed the estimation of temperature dependencies of the scale factor n (or effective temperature) for microcrystallite surfaces for a representative group of liquids above the melting points. Further, by using the results of the author's previous works, general temperature relationship of the scale factor n for microcracks and gaps over a wide range of bulk temperature has been plotted. This relationship is characterized by the sharp jump at the melting point and is asymmetrical with respect to the melting temperature line. Solid‐liquid transitions from the standpoint of the microcrystallite model have been discussed.

The refinement of the temperature dependence of the scale factors n for microcrystallite surfaces in liquids

Crystal Research and TechnologyVolume 25, Issue 3 p. K49-K52 Short Note The refinement of the temperature dependence of the scale factors n for microcrystallite surfaces in liquids Dr. A. D. Vlasov, Dr. A. D. Vlasov All-Union Scientific Research Centre for, Study of Surface and Vacuum Properties, Novoslobodskaya 67/69, 103055 Moscow, USSRSearch for more papers by this author Dr. A. D. Vlasov, Dr. A. D. Vlasov All-Union Scientific Research Centre for, Study of Surface and Vacuum Properties, Novoslobodskaya 67/69, 103055 Moscow, USSRSearch for more papers by this author First published: March 1990 https://doi.org/10.1002/crat.2170250322AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Volume25, Issue3March 1990Pages K49-K52 RelatedInformation

GaAs Clusters in the Quantum Size Regime: Growth on High Surface Area Silica by Molecular Beam Epitaxy

Molecular beam epitaxy has been used to grow microcrystalline clusters of gallium arsenide (GaAs) in the size range from 2.5 to 60 nanometers on high-purity, amorphous silica supports. High-resolution transmission electron microscopy reveals that clusters as small as 3.5 nanometers have good crystalline order with a lattice constant equal to that of bulk GaAs. Study of the microcrystallite surfaces by x-ray photoelectron spectroscopy shows that they are covered with a shell (1.0 to 1.5 nanometers thick) of native oxides of gallium and arsenic (Ga(2)O(3) and As(2)O(3)), whose presence could explain the low luminescence efficiency of the clusters. Optical absorption spectra of the supported GaAs are consistent with the blue-shifted band edge expected for semiconductor microcrystallites in the quantum size regime.

Étude du dopage de couches minces d'oxyde cuivrique par spectrometrie de masse à étincelles

A reproducible method of doping thin films of copper oxide by alkali metals has been developed. The doped samples were analysed by spark mass spectrometry using the direct spark method. The results can be accounted for by the fixation of alkali metal atoms on the microcrystallite surfaces.