An analytic model for the field dependence of charge mobility is developed within the long-range-correlated disorder model of a dipole glass. Release of a charge carrier from a deep state is considered as hopping drift and diffusion in a quasi-Coulomb potential well. The analytic results, containing only one numerical parameter obtained from independent simulation, are in good agreement with the fit to the Monte-Carlo simulations. The developed approach justifies applicability of the concept of the effective transport level for the modeling of organic materials with large molecular dipoles.
We present a simple model of the bimolecular charge carrier recombination in polar amorphous organic semiconductors in which the dominant part of the energetic disorder is provided by permanent dipoles and show that the recombination rate constant could be much smaller than the corresponding Langevin rate constant. The reason for the strong decrease of the rate constant is the long-range spatial correlation of the random energy landscape in amorphous dipolar materials; without spatial correlation, even strong disorder does not modify the Langevin rate constant. Our study shows that the signi ﬁ cant suppression of the bimolecular recombination could take place in homogeneous amorphous organic semiconductors and does not need large-scale inhomogeneity of the material.
The incorporation of Ca(2+) into smectite minerals is well-known to have a significant effect on the swelling behavior and mechanical properties of this environmentally and technologically important group of materials. Relative to common alkali cations such as Na+, K+, and Cs+, Ca(2+) has a larger charge/ionic radius ratio and thus interacts very differently with interlayer water molecules and the oxygens of the clay basal surface. Recent (2)H and (43)Ca NMR studies of the smectite mineral, hectorite, show that the molecular scale interlayer dynamics is quite different with Ca(2+) than with alkali cations. Classical molecular dynamics (MD) simulations presented here use a newly developed hectorite model with a disordered distribution of Li+/Mg(2+) substitutions in the octahedral sheet and provide new insight into the origin of the effects of Ca(2+) on the structure, dynamics, and energetics of smectite interlayers. The computed basal spacings and thermodynamic properties suggest the potential for formation of stable monolayer hydrates that have partial and complete water contents, a bilayer hydrate, and possible expansion to higher hydration states. The system hydration energies are comparable to those previously calculated for Caâ€“montmorillonite and are more negative than for Cs+ and N+ hectorite due to the higher hydration energy of Ca(2+). The coordination environments of Ca(2+) change significantly with increasing interlayer hydration, with the extent of coordination to basal oxygens decreasing as the number of interlayer molecules increases. On external (001) surfaces, the H2O molecules closest to the surface are adsorbed at the centers of ditrigonal cavities and bridge Ca(2+) to the surface. The Ca(2+) ions on the external surface are all in outer-sphere coordination with the basal oxygens of the surface, and the proximity-restricted region with a significant number of Ca(2+) is approximately 6 Å thick. Quantification of these interactions provides a basis for understanding intercalation of Ca(2+) by organic species and smectite minerals.
In situ XRD and NMR experiments combined with molecular dynamics simulations using the grand canonical ensemble (GCMD) show that cation size, charge and solvation energy play critical roles in determining the interlayer expansion of smectite clay minerals when exposed to dry supercritical CO2 under conditions relevant to the earthâ€™s upper crust, petroleum reservoirs, and geological CO2 sequestration conditions (323 K and 90 bar). The GCMD results show that the smectite mineral, hectorite, containing interlayer alkali and alkaline earth cations with relatively small ionic radii and high solvation and hydration energies (e.g., Li+, Na+ Mg2+, and Ca2+) does not intercalate dry CO2 and that the fully collapsed interlayer structure is the energetically most stable configuration. With increasing cation size and decreasing cation solvation energy, the energy barrier to CO2 intercalation decreases. With K+, Rb+, Cs+, Sr2+, and Ba2+ the monolayer structure is the stable configuration, and CO2 should spontaneously enter the interlayer. With Cs+ there is not even an energy barrier for CO2 intercalation, in agreement with the experimental XRD and NMR results that show clay layer expansion and CO2 incorporation. The number of intercalated CO2 molecules decreases with increasing size of the alkali cation but does not vary with ion size for the alkaline earth cations. 13C NMR spectroscopy and the GCMD simulations show that the average orientation of the intercalated CO2 molecules is with their O-C-O axes parallel to the basal clay surface and that they undergo a combination of rapid rotation about an axis perpendicular to the main molecular axis and wobbling motion with respect to the basal surface. Incorporation of CO2 in the interlayer decreases the coordination of Cs+ by the oxygen atoms of the basal surfaces, which is compensated by CO2 molecules entering their solvation shell, as predicted based on previously published NMR results. The GCMD simulations show that the strength of the interaction between the exchangeable cation and the clay structure dominates the intercalation energetics in dry scCO2. With relatively small cations, the cation-clay interactions outcompete cation solvation by CO2 molecules. The computed residence times for coordination among of interlayer species are consistent with the computed energetics.
The intercalation of H2O, CO2, and other fluid species in expandable clay minerals (smectites) may play a significant role in controlling the behavior of these species in geological C-sequestration and enhanced petroleum production and has been the subject of intensive study in recent years. This paper reports the results of a computational study of the effects of the properties of the charge balancing, exchangeable cations on H2O and CO2 intercalation in the smectite mineral, hectorite, in equilibrium with an H2O-saturated supercritical CO2 fluid under reservoir conditions using Grand Canonical Molecular Dynamics (GCMD) methods. The results show that the intercalation behavior is greatly different with cations with relatively low hydration energies and high affinities for CO2 (here Cs+) than with cations with higher hydration energies (here Ca2+). With Cs+, CO2 intercalation occurs in a 1-layer structure and does not require H2O intercalation, whereas with Ca2+ the presence of a sub-monolayer of H2O is required for CO2 intercalation. The computational results provide detailed structural, dynamical and energetic insight into the differences in intercalation behavior and are in excellent agreement with in situ experimental XRD, IR, quartz crystal microbalance, and NMR results for smectite materials obtained under reservoir conditions.
Наночастицы реализующие плазмонный нанолазер (SPASER), обладающий излучением со сверхузкой спектральной шириной линии, имеющие при этом малый размер и хорошую биосовместимость открывают новые перспективы в качестве потенциальных биологических зондов. К сожалению, за 10 лет, прошедших с момента первой демонстрации SPASER, не было проведено измерений, позволяющих прояснить, как конкретные компоненты конструкции SPASER наночастиц определяют их оптические характеристики. В настоящей работе теоретически и экспериментально рассматривается влияние толщины слоя усиления SPASER наночастиц и плотности образующих активный лосй SPASER наночастиц молекул красителя на характеристики генерации и фотостабильность SPASER наночастиц. Результаты показывают, что для резонатора с плазмонной наночастицей равной 16 нм достаточно слоя усиления 10–15 нм для минимизации порога генерации такого плазмонного нанолазера. Для этого типа системы наночастица-спазер минимальное количество эмиттеров красителя на частицу, называемое «порогом красителя», также имеет важное значение для генерации, помимо «порога накачки» мощности лазера. Более того, распределение эмиттера красителя внутри слоя усиления могло бы быть еще одним подходом к дальнейшему улучшению характеристик спазера. Полученные результаты дают нам возможность глубоко понять физическую сущность SPASER наночастиц и оптимизировать их работу для дальнейших применений в биологии.
Аннотация: разработана аналитическая модель, основанная на концепции транспортного уровня и эффективной температуры, для последовательного описания квази- и неравновесных режимов переноса в неполярных органических телах с гауссовским некоррелированным энергетическим расстройством. Полевые и температурные зависимости дрейфовой подвижности от неравновесного транспортного режима, относящиеся к эксперименту "времени полета", хорошо согласуются с результатами моделирования Монте-Карло в широком диапазоне полей и температур с использованием одного и того же набора параметров модели для обоих транспортных режимов.
Grand Canonical Molecular Dynamics (GCMD) simulations were performed to investigate the intercalation of CO2 and H2O molecules in the interlayers of the smectite clay, Na-hectorite, at temperatures and pressures relevant to petroleum reservoir and geological carbon sequestration conditions and in equilibrium with H2O-saturated CO2. The computed adsorption isotherms indicate that CO2 molecules enter the interlayer space of Na-hectorite only when it is hydrated with approximately three H2O molecules per unit cell. The computed immersion energies show that the bilayer hydrate structure (2WL) contains less CO2 than the monolayer structure (1WL) but that the 2WL hydrate is the most thermodynamically stable state, consistent with experimental results for a similar Na-montmorillonite smectite. Under all T and P conditions examined (323–368 K and 90–150 bar), the CO2 molecules are adsorbed at the midplane of clay interlayers for the 1WL structure and closer to one of the basal surfaces for the 2WL structure. Interlayer CO2 molecules are dynamically less restricted in the 2WL structures. The CO2 molecules are preferentially located near basal surface oxygen atoms and H2O molecules rather than in coordination with Na+ ions. Accounting for the orientation and flexibility of the structural −OH groups of the clay layer has a significant effect on the details of the computed structure and dynamics of H2O and CO2 molecules but does not affect the overall trends with changing basal spacing or the principal structural and dynamical conclusions. Temperature and pressure in the ranges examined have little effect on the principal structural and energetic conclusions, but the rates of dynamical processes increase with increasing temperature, as expected.
Adsorption and mobility of radioactive Cs+ isotopes in soil are among the most important factors affecting the long-term environmental footprint of nuclear accidents such as Chernobyl (1986) and Fukushima Daiichi (2011). In particular, Cs+ ions can be retained through their exchange with K+ naturally present in muscovite mica, one of the common soil mineral components. The ClayFF force field allowed us to realistically represent local inhomogeneities of the structure, composition, and charge on the muscovite (001) surface and to identify three structurally different types of adsorption sites. We performed molecular dynamics simulations of Cs+ and K+ adsorption at the hydrated muscovite surface and used quasi-one-dimensional site-specific potential of mean force calculations to quantify the energetics of ion exchange in this system for each individual site and for the entire muscovite surface on average. Irrespective of the type of adsorption site, both K+ and Cs+ cations are preferably adsorbed on the basal (001) muscovite surface at the centers of ditrigonal cavities as inner sphere surface complexes. The free energy difference between the most favorable and the least favorable surface sites for Cs+/K+ ion exchange amounts to 11.7 kJ/mol, with the most favorable sites occupying half of the surface and the least favorable type - 1/6 of the surface and the rest exhibiting an intermediate adsorption and ion exchange capacity. The simulation results are compared with available thermodynamic estimates based on recent X-ray reflectivity measurements.