Enantioseparation on a chiral nanostructured surface: Effect of molecular shape

In this paper we use the Monte Carlo simulation method to study adsorption of chiral molecules on a solid surface with periodic distribution of active sites. Namely, equilibrium adsorption of a racemic mixture of enantiomers represented by homonuclear tetramers is modeled on a square lattice with a chiral pattern of active sites. We consider two possible chiral structures of the tetramers which differ only by chain geometry but have equal adsorption energies. The effect of the chain geometry on the effectiveness of separation is assessed by comparing the corresponding adsorption selectivities obtained from the simulations. We present results of model calculations in which the parameters do not refer to any particular experimental system. These results indicate that the model chiral surface can, in general, adsorb preferentially the complementary enantiomer, regardless of its chain conformation. Specifically, it was shown that changes in the tetramer geometry, from S-shaped to -shaped, lead to marginal changes in the shape of both single component and mixed adsorption isotherms calculated for the enantioselective surface. In this context, the enantiomer separation on the surface proposed in this work was shown to be insensitive to molecular shape of the adsorbing species.     

Theoretical study of the influence of laser-induced defects on the adsorption of gases on solid surfaces

Laser treatment of a solid surface was modeled by applying an analytical theory as well as by using Monte Carlo simulations. The crystalline surface was assumed to be initially smooth and chemically uniform, that is free of impurities built into the structure. Creation of surface defects by a laser beam was assumed to have entirely random nature. In particular, the surface was assumed to have been scanned by the beam focused successively on randomly chosen points. In the course of the ablation process, the beam produces a pyramidal crater whose dimensions are proportional to the applied laser power. According to the assumed nature of the scanning procedure, the craters formed by the beam are allowed to overlap. The influence of the number of laser pulses and the crater dimensions on the structural and adsorptive properties of the surface were examined by analysis of the variation of the mean surface depth and the surface width. Changes in the adsorptive properties were also estimated by the calculation of the thermally programmed desorption (TPD) spectra of monomolecular adsorbates desorbed from laser-treated surfaces. Additionally, equilibrium adsorption isotherms were calculated for the obtained surfaces.     

The study on magnetite particles coated with bilayer surfactants

Magnetite particles were prepared by co-precipitation, then sodium oleic (SO) and sodium dodecyl benzene sulfonate (SDBS) were applied as inner and outer surfactants, respectively. IR and TG were used to study the surface adsorption of SO and SDBS on magnetite particles. The experimental results demonstrated that SO molecules were linked to the magnetite particles through chemical bond and SDBS coated on the surface of magnetite particles covered with SO by means of Van der Waals attraction. Furthermore, based on the adsorption isotherms of surfactants on the magnetite particles and the dependence of Zeta potential of particles on the surfactants concentrations, the adsorption mechanisms of these two surfactants on the magnetite particles were studied. The isotherm adsorption model for SO on magnetite particles showed excellent correlation to Langmuir type and the adsorption equation was (25 °C), while that for SDBS on magnetite particles coated with SO showed excellent consistence with Freundlich type and the adsorption equation was Γ = 0.32c^0.475 (25 °C). In addition, the results demonstrated that both SO and SDBS formed monolayer adsorption on the surface of magnetite particles.     

Atomic structure for Mg on InN(0001) surface

Structures of Mg adsorbed on InN(0001) surfaces are theoretically investigated by first-principle calculations. Of all the structures examined, the structure of R30° as caused by Mg adsorption at the Top sites with 1/3 monolayers coverage is most energetically favorable. Mg atoms may also substitute indium atoms, or accumulate at the voids inside InN film. The interstitial Mg defects may act as a potential source of compensation for the p-type behavior of Mg-doped InN at the surface.     

Modelling of phase transitions and reaction at CO adsorption on oxygen precovered Pd(1 1 1)

Using the interaction parameters up to the third neighbors and activated form of O and CO diffusion and their reaction, the model has been proposed for Monte-Carlo simulations describing the catalytic O + CO → CO₂ reaction and occurring phase transitions on Pd(1 1 1) surface. Upon adsorption of CO the pre-adsorbed oxygen transforms from p(2 × 2)_O phase into and phases in the limit of room and moderate temperatures, respectively. We demonstrate that the kinetic effects determine both the occurrence of the p(2 × 1)_O and disappearance of the phases at moderate and low temperatures, respectively. Using reaction rate as a fit parameter, we show that at room temperature the start of the reaction can be synchronized with the occurrence of phase.     

Behaviour of discontinuous gold films on SrTiO3 substrates under annealing

Morphological changes of thin, discontinuous gold films on SrTiO₃ substrates, resulting from evaporation in the temperature range of 1143-1278 K, have been investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). If the gold covered fraction of the surface is small, the evaporation kinetics can be related to the desorption of adatoms. Measuring the density of the gold beads and the time dependence of the effective thickness of the film as calculated from the diameter of the beads, the following parameters have been determined: the surface diffusion length of the gold adatoms, , the mass transfer surface diffusion coefficient, and the evaporation flux, .     

The coherent effect of chirped femtosecond laser pulses on the formation of ultracold molecules in a magneto-optical trap

We have investigated how the formation of ultracold molecules in a rubidium magneto-optical trap is affected by the application of positively-chirped femtosecond laser pulses. The application of femtosecond pulses resulted in a reduction, rather than an increase, in the rate of detection of ⁸⁵Rb₂ and ⁸⁷Rb₂ triplet ground-state molecules, and compared this to the effects of non-chirped pulses, continuous-wave light, and the formation rate when no additional light was applied. A dependence on the amount of chirp of the femtosecond laser pulses suggests that the reduction in the molecular signal is the result of a coherent effect, and provides a promising indication that coherent control could be used to engineer the production of ultracold molecules.     

GaN/LiNbO3 (0 0 0 1) interface formation calculated from first-principles

The stable adsorption sites for both Ga and N ions on the ideal and on the reconstructed LiNbO₃ (0 0 0 1) surface are determined by means of first-principle total energy calculations. A single N layer is found to be more strongly bound to the substrate than a single Ga layer. The adsorption of a GaN monolayer on the polar substrate within different orientations is then modeled. On the basis of our results, we propose a microscopic model for the GaN/LiNbO₃ interface. The GaN and LiNbO₃ (0 0 0 1) planes are parallel, but rotated by 30° each other, with in-plane epitaxial relationship [1 0 0]_GaN‖ [1 1  0]_LiNbO3. In this way the (0 0 0 1) plane lattice mismatch between GaN and LiNbO₃ is minimal and equal to 6.9% of the GaN lattice constant. The adsorbed GaN and the underlying LiNbO₃ substrate have parallel c-axes.     

Substantially low desorption barriers in recombinative desorption of deuterium from a Si(1 0 0) surface

We have studied desorption kinetics of deuterium molecules from a Si(1 0 0) surface by means of temperature-programmed desorption (TPD) spectra and isothermal desorptions.Three desorption components, denoted as β_1,A,β_1,B, and C, can be distinguished in semi-logarithmic plots of the TPD spectra.Their peak positions and intensities are strongly affected by the surface preparation methods employed, either with or without annealing to control the initial D coverage .Peak C appears at the leading edge of the TPD peak.It accounts for only about 5% of the TPD peak at and it diminishes rapidly with decreasing , vanishing at .In contrast, together the β_1,A and β_1,B peaks account for the whole TPD peak at any less than 1.0 ML. The maximum of the β_1,A peak is nearly constant at around the maximum temperature of the TPD peak.On the other hand, the β_1,B peak appears on the high-temperature side of the TPD peak and it systematically shifts to higher temperatures with decreasing .These results imply that first- and second-order kinetics are operating for the β_1,A and β_1,B desorptions, respectively.Isothermal desorption experiments confirm the above predicted kinetics for a limited region, namely .From the results for the rate curve analysis, the desorption barriers are evaluated to be 1.6 ± 0.1 eV and 1.8 ± 0.1 eV for the β_1,A and β_1,B desorptions, respectively.These values are substantially lower than the widely accepted value of ∼2.5 eV. To reproduce the measured TPD spectra we take the Arrhenius-type rate equation containing the first- and second-order rate terms for the β_1,A and β_1,B desorptions.The TPD spectra measured for can be reasonably fit with the proposed rate equation when the values given above for E_d,A and E_d,B are used. For , however, the TPD curves are not fit with the same values; rather, the best-fit curves require values for E_d,A and E_d,B larger than those given above. Combining the present kinetics results with those obtained by STM along with the studies, the β_1,A and β_1,B peaks may be attributed to desorption along the 2H path, while peak C may be attributed to desorption along the 4H path. The atomistic desorption mechanism as well as the energy relationship between the desorption barrier and isosteric heat of adsorption are discussed.     

A comparative study of the adsorption equilibrium of progesterone by a carbon black and a commercial activated carbon

In this paper the adsorption process of a natural steroid hormone (progesterone) by a carbon black and a commercial activated carbon has been studied. The corresponding equilibrium isotherms have been analyzed according to a previously proposed model which establishes a kinetic law satisfactorily fitting the C versus t isotherms. The analysis of the experimental data points out the existence of two well-defined sections in the equilibrium isotherms. A general equation including these two processes has been proposed, the global adsorption process being fitted to such equation. From the values of the kinetic equilibrium constant so obtained, values of standard average adsorption enthalpy () and entropy () have been calculated. Finally, information related to variations of differential adsorption enthalpy () and entropy () with the surface coverage fraction (θ) was obtained by using the corresponding Clausius-Clapeyron equations.     

Dynamics of NF3 in a condensed film on Au(1 1 1) as studied by electron-stimulated desorption

We report a low-temperature dynamics study of condensed layers of NF₃ on Au(1 1 1) by time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD), temperature-programmed desorption (TPD) and low-temperature scanning tunneling microscopy (LT-STM). Upon adsorption at 30 K, molecular NF₃ adsorption occurs first at the step edges and at minor terrace defect sites with the formation of 2D islands. Within the islands, NF₃ is adsorbed in an upright conformation via the nitrogen lone pair electrons projecting fluorine atoms away from the surface as judged by the presence of only a sharp F⁺ central beam in the ESDIAD pattern. At higher coverages, 3D islands start to populate the surface. Electron bombardment of a thick NF₃ (∼6 ML) layer adsorbed on the Au(1 1 1) surface leads to emission of F⁺, N⁺, NF⁺, and ions as observed in the TOF-ESD distribution. Upon heating to ∼37 K, a sudden decrease of the and ion yield, which is not related to thermal desorption, is observed which reflects the surface migration of NF₃ molecules, leading to local thinning of the film. The thinning process occurs at the temperature of onset of molecular rotations and self-diffusion in the bulk NF₃ crystal. In this process, some NF₃ molecules move closer to the surface which results in higher efficiency for ion neutralization by the underlying metal surface. In the TPD spectra, the monolayer desorption is observed to begin at ∼65 K, exhibiting zero-order kinetics with an activation energy of 21 kJ/mol.     

The structural formation of methylthiolate SAMs on Au(1 1 1) for short deposition times from solution

Scanning tunnelling microscopy was used to investigate the structural formation of methylthiolate self-assembled monolayers on Au(1 1 1). SAMs were prepared by exposing the gold crystal to an ethanol-dimethyldisulfide solution for immersion times of 12 min, 12 h, and 24 h. After preparation the formation of a () rect. striped phase was found. For this phase, the immersion time is the key parameter determining the size and number of ordered domains. Annealing induced a phase transition leading to large domains in a (3×4) structure. The annealing temperature determines whether a mixed phase of both structures or only the (3×4) structure are formed. We find no influence of the immersion time on the formation of the second structure. A structure model is presented for both phases on the basis on the same building block containing two methylthiolate molecules and a gold ad-atom.     

Combined experimental and theoretical investigation of the CO2 adsorption on LaMnO3+y perovskite oxide

The surface interaction of CO₂ with the perovskite-type oxide LaMnO_3+y has been investigated by means of density functional theory calculations and experimental measurements of adsorption isotherms in the temperature range 298-473 K. A (1 0 0) oriented slab of the cubic structure was used for modeling CO₂ adsorption. The reference unit cell contains alternating LaO⁺ layers and layers; one slab is LaO⁺-terminated and the opposite surface is terminated. A Freundlich isotherm fitted the experimental data satisfactorily. Analysis of the isosteric heat revealed an energetically heterogeneous character for the lanthanum manganite oxide surface, mainly due to the degree of heterogeneity of the adsorption center and due to the adsorbate-adsorbate lateral interactions. Considering theoretical calculations and thermodynamical approaches, two types of active sites were found to be responsible for irreversible and reversible adsorption of CO₂ as a function of surface coverage and O₂ treatment. Strong adsorption takes place on the surface containing La cations. The strongest adsorption is associated with surface oxygen vacancies, center. The next strongest adsorption, a flat adaptation of CO₂ molecules with respect to the surface sites, with a strong binding to a surface oxygen, leads to chemisorbed carbonate species. These adsorption modes are chiefly indicative of a high basic character of the lanthanum manganite oxide surface. Several cationic sites formed by lanthanum and manganese cations are able to weakly adsorb CO₂ molecules in perpendicular or bridged forms. The latter adsorption modes suggest a weak acidic character of the manganite adsorbent.     

Fluorescence and solvatochromism of a merocyanine dye with a high quadratic polarizability in solutions and polymer films

The influence of medium polarity on the spectroscopic and photophysical properties of 2-[(2E,4E)-6-(1,3,3-trimethyl-2,3-dihydro-1H-2-indolyliden)-2,4-hexadienyliden]malononitrile (THDM) in solutions and polymer matrices is studied at room temperature and 77 K under conditions of steady-state and pulsed laser excitation. A large bathochromic shift of the absorption spectra observed upon an increase in the solvent polarity is caused by a strong increase of the dipole moment in the ground state () on going to an excited Franck-Condon state (). Based on the solvatochromic data, the quadratic polarizability was calculated to be , which is close to the experimentally determined value . A strong narrowing of the fluorescence spectra in comparison with the absorption spectra at room temperature is observed upon an increase in the solvent polarity caused by a decrease in the bond length alternation parameter and by weakening of vibronic interactions in the singlet excited state. The mirror symmetry of the absorption and fluorescence spectra of THDM in ethanol at 77 K is explained by the increase of the electrostatic interactions between the solvent and merocyanine molecules. The dynamic fluorosolvatochromism of THDM in the picosecond range is caused by molecules reorientations of the polar environment occurring during a time period consistent with the dielectric relaxation time of these molecules.     

Equilibrium, kinetic and thermodynamic studies on the adsorption of the toxins of Bacillus thuringiensis subsp. kurstaki by clay minerals

The persistence of Bacillus thuringiensis (Bt) toxins in soil is further enhanced through association with soil particles. Such persistence may improve the effectiveness of controlling target pests, but impose a hazard to non-target organisms in soil ecosystems. In this study, the equilibrium adsorption of the Bt toxin by four clay minerals (montmorillonite, kaolinite, goethite, and silicon dioxide) was investigated, and the kinetic and thermodynamic parameters were calculated. The results showed that Bt toxin could be adsorbed easily by minerals, and the adsorption was much easier at low temperature than at high temperature at the initial concentration varying from 0 to 1000 mg L⁻¹. The adsorption fitted well to both Langmuir and Freundlich isotherm models, but the Freundlich equation was more suitable. The pseudo-second-order (PSO) was the best application model to describe the adsorption kinetic. The adsorption process appeared to be controlled by chemical process, and the intra-particle diffusion was not the only rate-controlling step. The negative standard free energy () values of the adsorption indicated that the adsorption of the Bt toxin by the minerals was spontaneous, and the changes of the standard enthalpy () showed that the adsorption of the Bt toxin by montmorillonite was endothermic while the adsorption by the other three minerals was exothermic.     

Etching and forward transfer of fused silica in solid-phase by femtosecond laser-induced solid etching (LISE)

We present a femtosecond laser-based technique for etching and forward transfer of bulk transparent materials in solid-phase. Femtosecond laser pulses with were focused through a fused silica block onto an absorbing thin film of Cr. A constraining Si wafer was pressed into tight contact with the Cr film to prevent lift-off of the film. A combination of the high temperature and pressure of the Cr, and compressive stress from the Si, resulted in etching of smooth features from the fused silica by cracking. Unlike in conventional ablative or chemical etching, the silica was removed from the bulk as single solid-phase pieces which could be collected on the Si. Using this so-called laser-induced solid etching (LISE) technique, 1-2 m deep pits and channels have been produced in the silica surface, and corresponding dots and lines deposited on the Si. The threshold fluence for etching was found to be with duration pulses. The morphology of the etched features are investigated as functions of fluence and exposure to multiple pulses.     

One-dimensional defects in iodine adlayers on Pt(1 0 0)

One-dimensional defect structures of closed-packed adlayers of iodine on Pt(1 0 0) were studied with scanning tunneling microscopy (STM). On the terraces of the Pt(1 0 0) surface we observed rotational domains with line defects running in [0 1 0] directions, in coexistence with nearly defect-free domains. In addition to these prevailing line defects (A-defects) with a local coverage lower than that of a defect-free surface, we report on much less frequently observed line defects with higher local coverages (B- and C-defects). The strong dependence of the concentration of these defects on the adsorption temperature is governed by the decrease of the overall iodine coverage with increasing temperature. Iodine adsorption at ∼1100 K leads to self-organization of A-defects in quasi-periodic arrangements. The relevance of these defects as important structural elements of commensurate superstructures of iodine on Pt(1 0 0) is stressed.     

Morphology and composition on Al surface irradiated by femtosecond laser pulses

We study the surface chemicals and structures of aluminum plates irradiated by scanning femtosecond laser pulses in air for a wide range of laser fluence from 0.38 to 33.6 J/cm². X-ray photoelectron spectroscopy and X-ray diffraction analyses indicate clearly that crystalline anorthic Al(OH)₃ is formed under femtosecond laser pulse irradiation. Besides aluminum hydroxide, crystalline Al₂O₃ is also found in the samples irradiated at high laser fluence. Field emission scanning electron microscopy demonstrates that the surfaces of the samples irradiated with low laser fluence are colloidal-like and that nanoparticles with a few nanometers in size are embedded in glue-like substances. For high laser fluence irradiated samples, the surfaces are highly porous and covered by nanoparticles with uniform size of less than 20 nm.     

Molecular depth profiling and imaging using cluster ion beams with femtosecond laser postionization

The emergence of cluster ion sources as viable SIMS probes has opened new possibilities for detection of neutral molecules by laser postionization. Previous studies have shown that with atomic bombardment multiphoton ionization using high-power femtosecond pulses leads to photofragmentation. The large amount of photofragmentation can be mostly attributed to high amounts of internal energy imparted to the sputtered molecules during the desorption process. Several pieces of preliminary data suggest that molecules subjected to cluster beam bombardment are desorbed with lower internal energies than those subjected to atomic beam bombardment. Lower energy molecules may then be less likely to photodissociate creating less photofragments in the laser postionization spectra. Here we present data taken from coronene films prepared by physical vapor deposition comparing a 40 keV ion source with a 20 keV Au⁺ ion source, which supports this hypothesis. Furthermore, the depth profiling capabilities of cluster beams may be combined with laser postionization to obtain molecular depth profiles by monitoring the neutral flux. In addition, imaging and depth profiling may be combined with atomic force microscopy (AFM) to provide three-dimensional molecular images.     

Photo-induced creation of nitrogen-related color centers in diamond nanocrystals under femtosecond illumination

Diamond nanocrystals deposited on a dielectric mirror at the focus of a microscope objective have been illuminated by femtosecond laser pulses. We have observed the photo-creation of color centers, under peak power corresponding to an intensity of about . In a nanocrystal initially containing a single Nitrogen Vacancy (NV) center, femtosecond illumination resulted in the transformation of this center into another one with different spectral features. These features are tentatively attributed to the neutral form NV⁰. This irreversible transformation goes together with the photocreation of other unstable color centers at the laser focus. Such behavior under femtosecond laser illumination place some limitations on the use of sub-picosecond pulses to trigger single photon emission from a single NV center in diamond nanocrystal.