Crystalline Regime Research Articles

A neutron reflectometry study of the interface between poly(9,9-dioctylfluorene) and poly(methyl methacrylate)

Neutron reflectivity was used to study the interface between the semiconducting polymer poly(9,9-dioctylfluorene) (PFO) and the insulating polymer poly(methyl methacrylate) (PMMA). The PFO/PMMA interfacial width was measured in the nematic and crystalline phases of the PFO, both with the PMMA on top of the PFO and vice versa. These interfaces are broad compared to atomic length scales, with measured interfacial widths in the range from 10 to 20 Å. We found that the interfacial width was independent of both the chosen geometry and the thermal processing history. The equilibrium interfacial width only depended on temperature, with the width in the nematic phase of the PFO being broader than in the crystalline regime.

Structure of Fluorinated Side-Chain Smectic Copolymers: Role of the Copolymerization Statistics

Side-chain fluorinated copolymers present very specific wetting and tacking properties that are strongly related to their organization. We have studied the structure of these systems by X-ray scattering in the liquid crystalline regime, as a function of the ratio of fluorinated groups on one hand and of the copolymerization statistics on the other. The structures obtained are always highly ordered and exhibit a smectic B type organization, where the two types of pendant groups are able to crystallize independently. However, packing differences appear for copolymers with different chain statistics due to the existence of microphase separation between the side groups. The roles of these parameters on phase transitions, periodicity, and degree of crystallinity of the polymers are discussed. Their behavior with temperature, important for applications, is also studied.

Surface Charge Induced Reentrant Crystalline−Liquid Transition in Colloidal Systems: The Role of the Microion Disposition

The liquid → crystalline → liquid transition as a function of surface charge density (σ) for colloidal dispersions reported by Yamanaka, Yoshida, Koga, Ise, and Hashimoto [Phys. Rev. Lett. 1998, 80, 5806] is examined in detail by Brownian dynamics simulations of the microion distribution. In the absence of added electrolyte, two different distribution functions are determined: Gpc(r) centered at the center of the computation cell and gpc(r) centered at a selected macroion. The “thermal radii” rGtherm and rgtherm are determined for the respective distribution functions at the value exp(1), i.e., when the total electrostatic energy of the system, 〈Esys〉, is equal to the thermal energy kBT. The methods of juxtaposition of potential fields [Langmuir 1997, 13, 5849] and Brownian dynamics simulations are used to describe the disposition of the counterions, or an “orbital” picture of colloidal structures. It is suggested from this orbital model that the crystalline regime obtains when the thermal radii extend far from the parent macroions as this leads to maximum “sharing” of the counterions, and that the reentry transition occurs because of a drastic decrease in rgtherm with high surface charge densities.

Illumination Dependence of Microcrystalline PIN Diodes

ABSTRACTNumerical simulations of the current-voltage characteristics of PECVD-microcrystalline silicon based p-i-n diodes were performed to study the affect of defect density and mobility on solar cell performance. Depending on the combination of both parameters the ideality factor increases or decreases with applied forward bias. The reason is the variable contribution of volume recombination to the total diode current and space charge stored in defect states. The decrease in dark current with reduced hydrogen dilution can partly be attributed to a decrease in recombination centers by the same factor as predicted for midgap defect states by the analytic diode theory. Microcrystalline silicon solar cells deposited in the highly crystalline regime (high H-dilution) are limited by recombination of photogenerated carriers and high dark current. Both can be attributed to a large number of recombination centers. The fill factor of our state-of-theart solar cell is limited by the dark current for small illumination intensities, by series resistance for high illumination levels and by both at its maximum under AM1.5 illumination. Short-circuit current and open-circuit voltage pairs measured under intensities from 10-6 to 30 suns reveal a diode characteristic corresponding to an ideality factor of one at large forward bias.

Ultrahydrophobic Polymer Surfaces Prepared by Simultaneous Ablation of Polypropylene and Sputtering of Poly(tetrafluoroethylene) Using Radio Frequency Plasma

Ultrahydrophobic polypropylene surfaces were prepared by the simultaneous etching of polypropylene and etching/sputtering of poly(tetrafluoroethylene) (PTFE) using inductively coupled radio frequency argon plasma. The semicrystalline polypropylene surface is roughened due to the differential rates at which the crystalline and amorphous regimes ablate and is also fluorinated by the fluorocarbon plasma that results from the ablation/depolymerization of PTFE. The roughness of the polypropylene is controlled by the time of plasma etching. The presence of PTFE increases the rate of polypropylene roughening by reactive ion etching. The resulting roughened and fluorinated polypropylene surfaces were characterized by water contact angle, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and atomic force microscopy (AFM). Wettability was found to depend on the size scale and topology of the roughness. The most hydrophobic surfaces exhibited advancing and receding water contact angles of θA/θR = 172°/169°.

Colloidally induced smectitic fines migration: Existence of microquakes

AbstractThe phenomenon of colloidally induced fines migration is a challenge of both scientific and industrial importance. Its occurrence impacts permeability reduction and alteration of flow pathways in porous media, particulate contaminant migration in groundwater flow, and filtration. The release of smectitic fines is a threshold type of process resulting from discontinuous jumps, called microquakes in the interlayer spacing. There is a critical salt concentration at which these microquakes occur and produce fines migration in the porous media. The changes in the microstructure with decreasing salt concentration were analyzed using X‐ray diffraction. The transition between crystalline and osmotic swelling regime is also shown to depend on the type of cation. A mathematical model developed using colloidal principles predicts the swelling behavior of smectites in aqueous solutions. The model can predict the transition of swelling from crystalline to osmotic regimes and explain the effect of different cations on the transition.

No evidence for size-dependent icosahedral --> fcc structural transition in rare-gas clusters.

A structural model for a 3000-atom Ar cluster has been developed whose simulated electron-diffraction pattern is in excellent agreement with hitherto unexplained experimental results [J. Farges et al., J. Chem. Phys. 59, 3454 (1973)]. Although the cluster size is intermediate with respect to the icosahedral and fcc crystalline regimes, the model does not involve a structural transition. It consists of a persisting multiply twinned core with dominant fivefold symmetry, surrounded by a faulted shell with defects that stimulate 3D fcc single crystal growth and prevent hcp stacking.

Effects of Aluminate and Silicate on the Structure of Quaternary Ammonium Surfactant Aggregates

The effect of ionic aluminate and silicate species on the structure of surfactant aggregates used in the preparation of mesoporous aluminosilicates was investigated using dynamic light scattering and rheological techniques. Aqueous cetyltrimethylammonium chloride (CTAC) solutions saturated with sodium aluminate, sodium silicate, or tetramethylammonium silicate were studied over a wide range of CTAC concentrations (1.0−25.0 wt %), which included both spherical and rod-shaped micellar regimes. Sodium aluminate was observed to have no effect on the structure of CTAC micelles and so plays no role in aggregate formation in the prereaction stage of the production of aluminosilicates such as MCM-41. Silicate anions from sodium silicate and tetramethylammonium silicate strongly affected CTAC micelle formation, promoting the formation of flexible wormlike micelles at dilute concentrations in which elongated wormlike micelles are not entangled. The condensation of silicate ion with the tetramethylammonium cation in CTAC/tetramethylammonium silicate solution inhibited the condensation of silicate ion at the surface of the CTAC micelles. Results strongly suggest that, for the two silicate sources studied, flexible CTAC wormlike micelles surrounded by silicate oligomers are formed first in the preparation of aluminosilicate materials; the subsequent intermicellar condensation of silicate oligomers then leads to the gradual arrangement of the micelles into hexagonal liquid crystalline arrays. The effect of a chloride ion (at a constant molar Cl-/CTA+ ratio of 3.98) was also investigated using sodium chloride. Unlike the aluminate and the silicate ions, a transition between the rodlike and flexible wormlike micellar regions was observed. The semidilute regime, defined as the concentration range over which the collective diffusion coefficient is described by the power law model for single-chain polymer in good solvent, was observed at 11.0−21.6 wt %. The transition into the liquid crystalline regime was observed to begin at 21.6−25.0 wt %.

Effect of H on Si molecular-beam epitaxy

In Si crystal growth by molecular-beam epitaxy (MBE) at low temperatures there is known to be an epitaxial thickness: an initially crystalline regime before the deposited film becomes amorphous. The predominant impurity in MBE is hydrogen, but the role of background H in low-temperature MBE has not previously been assessed. Here the effect of deliberate dosing of the Si surface with atomic H during low-T growth is studied. The epitaxial thickness is shown to be sensitive to very small additional H fluxes (≊10−9 Torr, i.e., an increase in H only marginally above ambient). With further increases in dose rate, the epitaxial thickness decreases as hepi=h0−k(ln PH). Using secondary-ion-mass spectrometry data on the segregated H at the interface, we argue that breakdown in epitaxy is not caused directly by the surface concentration of adsorbed impurities. It is deduced that very small concentrations of H may influence the Si surface diffusion rate. The possible effect of background H adsorption on previous experiments on Si steps and surface diffusion is discussed.

Thermal energy of the crystalline one-component plasma from dynamical simulations.

Molecular-dynamics simulations employing 1024 particles and a high-accuracy spline approximation of the Ewald potential have been used to measure the excess internal energy of the classical one-component plasma in both the fluid and solid (bcc) phases over a wide range (1\ensuremath{\le}\ensuremath{\Gamma}\ensuremath{\le}2000) of the coupling parameter. The energy data for \ensuremath{\Gamma}\ensuremath{\le}300 are in excellent agreement with previous Monte Carlo calculations for this range, giving an independent corroboration of these calculations. Data in the crystalline regime 170\ensuremath{\le}\ensuremath{\Gamma}\ensuremath{\le}2000 are used to estimate the coefficients of the anharmonic energy component by least-squares fits. The first coefficient is in reasonable agreement with peturbation-theory predictions, although it is argued that determination of the coefficients in this manner is a rather ill-conditioned problem. The free-energy curves of the fluid and solid phases are found to cross at ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{m}}$\ensuremath{\approxeq}173, in agreement with recent estimates.

Spectroscopic studies of model polar stratospheric cloud films

Fourier transform infrared (FTIR) spectroscopy has been used to study nitric acid/ice films representative of type I polar stratospheric clouds (PSCs). These studies reveal that in addition to amorphous nitric acid/ice mixtures, there are three stable stoichiometric hydrates of nitric acid—nitric acid monohydrate (NAM), dihydrate (NAD) and trihydrate (NAT). Two distinct crystalline forms of the trihydrate were also observed. These two forms appear to differ in their concentration of crystalline defects, but not in their chemical composition. In addition to probing the composition of type I PSCs, we have also used FTIR spectroscopy to characterize laboratory surfaces on which measurements of heterogeneous reaction rates are performed. Our studies suggest that “water-rich NAT” is a two-phase system with separate ice and NAT crystalline regimes. Finally, we have used FTIR spectroscopy to determine the desorption kinetics for evaporation of model PSC films. Ice evaporation was found to follow zero-order desorption kinetics with a desorption barrier of 12±2 kcal/mol and a preexponential factor of 1030.5±1.5 molec/cm2-s.

Crystallization of trapped ions—A quantum approach

We present a quantum approach for a system of two or three ions in a Paul's trap. Under the simplifying assumption that the ionic repulsion is inverse square instead of Coulombic, we explicitly solve the time-dependent Schrödinger equation for this system. The quasi-periodic in time nature of the quasi-energy eigenstates provides a good description of both the “crystalline regime” and the “nonchaotic cloud regime” inside the trap. The characteristic size of the “two-ions crystal” in this quantum approach is compared to its value in the usual classical treatment.

Synthesis of metastable, semiconducting Ge-Sn alloys by pulsed UV laser crystallization

Thin microcrystalline films of the metastable semiconducting alloy Ge1−xSnx (x≊0.22) have been formed using excimer laser radiation to crystallize amorphous sputtered films on glass and semiconducting crystalline substrates. X-ray diffraction, electroreflectance, and Raman spectroscopy have been used to characterize the semiconducting material which is stable to at least 200 °C. The study demonstrates the possibility of extending earlier studies of amorphous Ge1−xSnx alloys into a crystalline regime with a direct band-gap variable with x from 0 up to about 0.5 eV. The crystallization technique is potentially applicable to the formation of other metastable semiconducting compounds of device potential.

Rheology of contcentrated suspensions of slender rods

The rheology og suspensions of slender rods is discussed for various concentration regimes (i.e. dilute, semi-dilute, concentrated and liquid crystalline regimes) on the basis of the general kinetic theory of Brownian particles.