KCl Surface Research Articles

Epitaxial Growth of Pentacene on Alkali Halide Surfaces Studied by Kelvin Probe Force Microscopy

In the field of molecular electronics, thin films of molecules adsorbed on insulating surfaces are used as the functional building blocks of electronic devices. Control of the structural and electronic properties of the thin films is required for reliably operating devices. Here, noncontact atomic force and Kelvin probe force microscopies have been used to investigate the growth and electrostatic landscape of pentacene on KBr(001) and KCl(001) surfaces. We have found that, together with molecular islands of upright standing pentacene, a new phase of tilted molecules appears near step edges on KBr. Local contact potential differences (LCPD) have been studied with both Kelvin experiments and density functional theory calculations. Our images reveal that differently oriented molecules display different LCPD and that their value is independent of the number of molecular layers. These results point to the formation of an interface dipole, which may be explained by a partial charge transfer from the pentacene to the surface. Moreover, the monitoring of the evolution of the pentacene islands shows that they are strongly affected by dewetting: Multilayers build up at the expense of monolayers, and in the Kelvin images, previously unknown line defects appear, which reveal the epitaxial growth of pentacene crystals.

Adsorption of PTCDA on NaCl(100) and KCl(100)

The adsorption of PTCDA on KCl and NaCl(100) surfaces has been investigated by means of first principles calculations. Besides a variety of adsorbate structures for single molecules and a monolayer of flat lying molecules on plain terraces, the influence of monoatomic steps and the different defect site at step edges has been studied in detail.

Finite size line broadening and superradiance of optical transitions in two dimensional long-range ordered molecular aggregates

The width and asymmetry of the line shape of the optical transition of a sample of two dimensional (2D) molecular J-aggregates was found to be related to a finite-size effect. The 2D aggregates were domains of the ordered monolayer of the fluorescent dye molecule 3,4,9,10-perylenetetracarboxylic acid dianhydride on a KCl(100) surface. Fluorescence and fluorescence excitation (FLE) spectra were measured as a function of temperature. The system shows a pronounced superradiant emission which yields additional information on the number of coherently coupled molecules participating in the emission. From calculations of the spectra within the tight binding model we find that the finite size of the 2D ordered domains of about N = 7 × 7 molecules, in combination with a Poissonian domain-size distribution, explains the line profile. Line broadening mechanisms due to site disorder or thermal effects--although not excludable straightaway--are not needed to explain the observed FLE line profile. This yields insight into the important, but so far not well understood, relation between the line profile and the size of ordered molecular aggregates.

Spectroscopy of isolated PTCDA molecules on the KCl(100) surface: Vibrational spectra and azimuthal orientation

Small amounts of the model molecule perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) were vacuum deposited on epitaxial KCl films on Ag(100). The use of a low substrate temperature (20 K) during deposition hampered molecular diffusion resulting in isolated monomers on the surface. Fluorescence and fluorescence excitation spectroscopy performed on these monomers yielded highly resolved spectra with narrow lines corresponding to individual vibronic modes. This high resolution in our spectra is caused by a very small inhomogeneous broadening due to well-defined adsorption sites of the molecule on the substrate. Indeed, by polarization dependent fluorescence spectroscopy we show that the flat-lying molecules exhibit a preferred azimuthal orientation on the surface, the long molecular axis being oriented along the [011] or the equivalent [011] direction of the substrate. Furthermore, the high resolution in the spectra allowed a detailed analysis of the vibronic modes. The vibrational modes of the adsorbed molecule are very similar to those of the free PTCDA molecule, but due to the presence of the substrate additional low energy modes which are relevant for the full understanding of the spectra couple to the transition.

Fluorescence spectroscopy of PTCDA molecules on the KCl(100) surface in the limit of low coverages: site selection and diffusion

We performed fluorescence (FL) and fluorescence excitation (FLE) spectroscopy on the model molecule perylene-3,4,9,10-tetracarboxyl acid dianhydride (PTCDA) for very low coverages (below 1% of a monolayer) on thin (100) oriented KCl films. Two different states of PTCDA molecules can be distinguished in the spectra: an initial state, which is observed directly after deposition of the molecules onto the cold sample at 20 K, and a final state, which is found after intensive optical excitation or thermal annealing of the sample. The spectrum of the final state is blue-shifted with respect to that of the initial state by 130 ± 15 cm(-1) and exhibits lines with significantly reduced widths. This can be explained by diffusion of molecules from initially populated terrace sites to energetically favoured step edge sites. Polarization dependent spectroscopy reveals the same azimuthal orientation of the molecules on both adsorption sites and leads to a model of the adsorption geometry of PTCDA at the KCl step sites. Our experiment demonstrates how optical spectroscopy can be used to investigate kinetic processes of fluorescent molecules on surfaces.

Step Density of Ionic-crystal Surfaces Derived by Observation of Intensity Distributions of Reflected Fast Protons under Grazing Incidence

Surface-channeling at grazing incidence of 550 keV protons on electron-bombarded KCl(001) and KBr(001) surfaces is investigated. In order to derive step density of the surfaces as a proportional function of electron-fluence lying in a range below 3×1015 cm−2, intensity distributions (scattering patterns) of reflected protons on a fluorescent screen are observed. With increasing the fluence, the scattering patterns shift to lower scattering angle. The peak-angles of the luminous intensity are compared with calculated results of computer simulations. For both KCl and KBr surfaces, the electron fluence of 1.0×1015 cm−2 is equivalent to the density of monolayer steps of 1.3×105 cm−1 piled upon in the simulation.

Substrate selected polymorphism of epitaxially aligned tetraphenyl-porphyrin thin films

Porphyrin molecules, of interest as versatile materials for organic electronics, are highly prone to formation of significantly different polymorphic phases. To elucidate the determinants for the specific polymorphic phase formed in thin films as well as for the arrangement of the molecules on a given substrate two different anisotropic substrate surfaces have been selected: KCl(100) and the oxygen reconstructed Cu(110) surface. We observe that the crystal structure of the thin films depends on the substrate, whereas the relative molecular orientations in both cases are similar. X-Ray and transmission electron diffraction of 30 nm thick tetraphenyl-porphyrin (H(2)TPP) and platinum tetraphenyl-porphyrin (PtTPP) thin films deposited on KCl(100) surfaces reveals that both kinds of molecules crystallize in a tetragonal polymorph with the (001) lattice planes, i.e. with their macrocycles, parallel to the substrate. Films deposited on the oxygen reconstructed Cu(110)-(2 × 1)O surface exhibit in contrast the triclinic polymorph even though molecules again align nearly parallel to the substrate surface as observed by LEED and X-ray diffraction. On both substrates we identify two driving forces for the epitaxial alignment of porphyrins: (i) molecules aligning with their macrocycles (nearly) parallel to the substrate surface and (ii) the porphyrin molecules forming a commensurate unit cell with the respective substrate. The polymorphic phase meeting both requirements is the most favorable to be formed on a given substrate and due to this structural flexibility in both cases well-ordered, epitaxially aligned porphyrin thin films are achieved.

Rainbow scattering under axial surface channeling from a KCl(001) surface

Fast He, Ne, Ar, and N atoms with projectile energies from 1 up to 60 keV are scattered under grazing polar angles of incidence from a flat and clean KCl(001) surface. For the scattering along low-index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. From classical trajectory calculations based on universal and individual pair as well as density functional theory (DFT) potentials, we obtained corresponding rainbow angles for comparison with the experimental data. Fair agreement was found for DFT and individual pair potentials calculated from Hartree-Fock wave functions.

Structure of CO2 Adsorbed on the KCl(100) Surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.

Preparation and structure of CuInSe2 thin films for solar cells at low substrate temperatures

CIS epitaxial films were grown on (001) KCl surface with PbS sublayer and on glassceramic at 400оС. Annealing of the (α + β)-CIS films on glass-ceramic, in two-step vacuum-arc plasma discharge at 550oC provided the formation of a homogeneous large-crystalline α- CIS

Elasticity, internal excitation, fragmentation, and charge transfer during grazing scattering of fast fullerenes from a KCl(001) surface

${\mathrm{C}}_{60}^{+}$ and ${\mathrm{C}}_{70}^{+}$ fullerenes with keV energies are scattered under grazing polar angles of incidence from an atomically clean and flat KCl(001) surface. For this model system of molecule surface interactions, the elastic properties of the fullerenes in front of the surface are studied by polar angular distributions. From the analysis of fragment spectra, the internal excitations of scattered molecules are deduced and excitation mechanisms are identified. Charge fractions indicate a kinematically induced neutralization of the fullerenes. Via an analysis of negatively charged fragments, the transition from a ``soft'' scattering event with intact outgoing fullerenes to postcollision multifragmentation is analyzed. The data are compared to three-dimensional molecular dynamics simulations based on empirical bond-order potentials.

Classical and quantum mechanical rainbow-scattering of fast He atoms from a KCl(0 0 1) surface

Fast He atoms with energies from 200 eV up to 16 keV are scattered under grazing polar angles of incidence from a flat and clean KCl(0 0 1) surface. For scattering along low-index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. From classical and semiclassical trajectory calculations based on individual Hartree–Fock pair and density functional theory (DFT) potentials, we obtain corresponding rainbow angles for comparison with the experimental data. The calculations were performed taking into account the rumpling of K and Cl in the topmost surface layer. Fair agreement with the experimental data is found for scattering along 〈1 0 0〉 for DFT as well as individual pair potentials calculated from Hartree–Fock wave functions.

Bubble attachment time and FTIR analysis of water structure in the flotation of sylvite, bischofite and carnallite

Water structure is the most important parameter that influences the flotation of soluble salts. In this paper bubble attachment time measurements and FTIR analyses were performed to investigate the effect of water structure on the flotation behavior of sylvite (KCl), bischofite (MgCl2·6H2O) and carnallite (KMgCl3·6H2O). The results from bubble attachment time measurements suggest that collector adsorption at the surface of KCl induces flotation with either the cationic collector, ODA, or anionic collector, SDS. In contrast bubble attachment did not occur for bischofite (MgCl2·6H2O) or carnallite (KMgCl3·6H2O). Results show that the surface charge is not a determining factor in the flotation of soluble salts.Further, the interaction between water molecules and the three chloride salts dissolved in aqueous solution were studied by measuring the shift in the hydrogen-bonding of water molecules. The results indicate that KCl is a structure breaker salt, while MgCl2·6H2O and KMgCl3·6H2O are structure maker salts.Viscosities for the brines of these three salts were determined. The results give additional evidence of differences in water structure and are in good agreement with the FTIR and bubble attachment results. The findings provide further evidence that water structure plays an important role in the flotation of soluble salts.

Direct evidence of the surface track potential

Angular and energy distributions of fragment protons dissociated from HeH${}^{+}$ during grazing-angle scattering from a KCl $(001)$ surface are measured. The surface of KCl $(001)$ is heated at ${180}^{\ifmmode^\circ\else\textdegree\fi{}}$C and the beam current is kept lower than 1 fA to prevent macroscopic surface charging. The angular distribution of the fragment protons shows a well-defined peak similarly to the grazing-angle scattering of atomic ions. The observed peak, however, is shifted from the specular angle toward larger scattering angles. The observed angular shift for the trailing proton is larger than that for the leading proton. These results clearly indicate that the motion of the fragment protons is affected by the surface track potential induced by the partner He ion.

Surface chemistry features in the flotation of KCl

Despite the successful industrial practice to recover KCl by flotation, the adsorption state of collectors at KCl salt surfaces is still not fully understood. In this paper, the flotation behavior of KCl and NaCl using different collectors and captive bubble contact angle measurements at KCl and NaCl crystal surfaces in their saturated solutions are reported. The influence of collector concentration and collector structure is considered. The results show that both cationic and anionic collectors produce a hydrophobic state at the structure breaking KCl salt surface, with finite contact angle values of between 30 degrees and 60 degrees. In contrast, the contact angle at the structure making NaCl salt surface is zero at all collector concentrations, suggesting that no collector adsorption occurs, a supposition that is supported by results from molecular dynamics simulation (MDS). Further, it appears that the presence of surface defects such as edges and corners promote the adsorption of collectors at the KCl surface, and that the flotation is achieved by bubble attachment at the edges/corners of KCl particles with the formation of aeroflocs. (C) 2009 Elsevier Ltd. All rights reserved.

Axial Surface-channeling of Fast Protons from an ESD-induced Stepped Surface of KBr(001)

To demonstrate experimental evidence caused by the lattice strain that was induced by the surface steps introduced by electronic stimulated desorption of ionic-crystals, the ion scattering experiment has been performed. Surface-channeling of protons incident with 5 mrad on an electron-irradiated KBr(001) surface is investigated using a 0.55 MeV beam of protons. The direction of the incident beam is adjusted along the <100> channeling condition and the channeling-dips of the scattering yields are observed. The electron dose-dependence of the minimum yields (χmin) and widths of the dips (φ1/2) are compared with those measured on a KCl(001) surface. For the results of the χmin and φ1/2, results of KBr and KCl show similar dependence on the irradiation dose. For the both surfaces, two small peaks appear in symmetric positions on the channeling-dips. Peak-separations between the two peaks and widths of the peaks increase with the irradiation dose. Although no difference of the lattice strain induced on the two surfaces is detected only by the present experimental results, slight difference between the results for the two surfaces is observed and it is expected to be related with experimental evidence to demonstrate the lattice strain.

Insight from First-Principles Calculations into the Interactions between Hydroxybenzoic Acids and Alkali Chloride Surfaces

Tuning of effective band gaps at insulator surfaces by adsorbed molecules is of fundamental interest but also technologically relevant for contact charging induced by adsorbed molecules like hydroxybenzoic acids. Our studies by density functional theory of the adsorption of benzoic acid (BA), salicylic acid (SA), and para-salicylic acid (p-SA) on perfect KCl and NaCl(100) surfaces as well as on polar and nonpolar step edges reveal the importance of polar defects in this context. We also found that the van der Waals dispersion contributes to about half of the adsorption energy in most cases. Therefore, the van der Waals interaction must be explicitly taken into account even for the interaction between molecules and wide band gap insulator surfaces, respective of the adsorption on flat or stepped surfaces. On the other hand, the adsorption geometry is still determined by short-range forces, that is, by the weak chemical bonds and electrostatic interactions. These short-range interactions are mostly mediated by resonant coupling between HOMO-1 and HOMO-2 molecular states and the valence band of the solid, which results in very similar adsorption energies of these molecules on the KCl and NaCl surfaces. The interaction strength is significantly modified only at defects with large dipole moments (e.g., pairs of polar steps) resulting in split-off edge states below the valence band. Resonant coupling to these states leads to an almost rigid down-shift of all molecular orbitals that efficiently reduces the effective band gap at the Surface. As a result, an effective band gap of 0.9 eV has been achieved oil the polar KCl [011] stepped Surface thanks to the relatively small HOMO-LUMO gap of SA.

Oscillatory Behavior of Thermoelectric Properties in p-PbTe Quantum Wells

The room-temperature dependences of the Seebeck coefficient, Hall coefficient, electrical conductivity, charge carrier mobility, and thermoelectric power factor were obtained as a function of thickness d (d = 8 nm to 400 nm) of PbTe epitaxial layers grown by thermal evaporation in vacuum of PbTe polycrystals doped with Na onto (100)KCl surfaces and covered with an Al 2 O 3 layer. Distinct oscillations in the d-dependences of the properties were observed and attributed to the size quantization of the energy spectra in PbTe layers. The experimental values of the oscillation period are in good agreement with the results of theoretical calculations using the effective-mass approximation and a model for a rectangular potential well with infinitely high walls. It follows from the obtained results that quantization of the energy spectrum in PbTe thin-film structures occurs not only for the electron gas but also for the hole gas.

Channeling effect in electronic spectra produced by grazing impact of fast protons on insulator surfaces

Electron emission due to grazing scattering of fast protons from LiF and KCl surfaces is studied under axial incidence conditions. The differential emission probability is calculated within a distorted-wave formalism, taking into account axial channeled trajectories. For different emission angles, electronic spectra for proton incidence along the two principal crystal axes ([100] and [110]) are compared with those co rresponding to an impact velocity in a random direction, finding effects associated with the channeling conditions.

Evolution of epitaxial order in para-sexiphenyl on KCl(1 0 0)

This work focuses on the structural and morphological studies of the evolution of epitaxially grown para-sexiphenyl (p-6P) thin films on freshly cleaved KCl(1 0 0) surfaces. The film growth is varied using growth times between 5 sec and 60 min and substrate temperatures in the range of 80–150 °C. The study is based on a combination of X-ray diffraction to derive structural information on the molecular scale, and atomic force as well as electron microscopy to obtain morphological information. The initial growth orientation of p-6P crystallites is identified to be ( 2 0 3 ¯ ) -oriented, which consists of edge-on molecules aligned parallel to 〈0 1 1〉KCl forming needles that are also parallel to these directions. Furthermore it is shown that surface step edges do not affect the needle growth, but those needles even overgrow step edges. The subsequent epitaxial growth turned out to be crucially dependent on the substrate temperature with respect to the quantitative ratios of material in different growth orientations, their in-plane alignment, the size of crystallites and the mosaicity of the films. It is shown that some crystallites start their growth using the initially grown needles as seeds, resulting in a different crystal orientation. For such crystallites a realignment of needles during their growth is identified, which is found to be strongly dependent on the growth temperature. Furthermore the heights of needles show distinct temperature dependence. In addition well-aligned terraced islands formed by standing molecules were observed. Finally a growth model for this system can be elucidated.