Single-molecular diffusivity and long jumps of large organic molecules: CoPc on Ag(100).

Energy dissipation and the transfer rate of adsorbed molecules do not only determine the rates of chemical reactions but are also a key factor that often dictates the growth of organic thin films. Here, we present a study of the surface dynamical motion of cobalt phthalocyanine (CoPc) on Ag(100) in reciprocal space based on the helium spin-echo technique in comparison with previous scanning tunnelling microscopy studies. It is found that the activation energy for lateral diffusion changes from 150 meV at 45-50 K to ≈100 meV at 250-350 K, and that the process goes from exclusively single jumps at low temperatures to predominantly long jumps at high temperatures. We thus illustrate that while the general diffusion mechanism remains similar, upon comparing the diffusion process over widely divergent time scales, indeed different jump distributions and a decrease of the effective diffusion barrier are found. Hence a precise molecular-level understanding of dynamical processes and thin film formation requires following the dynamics over the entire temperature scale relevant to the process. Furthermore, we determine the diffusion coefficient and the atomic-scale friction of CoPc and establish that the molecular motion on Ag(100) corresponds to a low friction scenario as a consequence of the additional molecular degrees of freedom.

Attenuation of Photoelectron Emission by a Single Organic Layer.

We report an in situ study of the thin-film growth of cobalt-phthalocyanine on Ag(100) surfaces using photoelectron emission microscopy (PEEM) and the Anderson method. Based on the Fowler-DuBridge theory, we were able to correlate the evolution of the mean electron yield acquired with PEEM for coverages up to two molecular layers of cobalt-phthalocyanine to the global work function changes measured with the Anderson method. For coverages above two monolayers, the transients measured with the Anderson method and those obtained with PEEM show different trends. We exploit this discrepancy to determine the inelastic mean free path of the low-energy electrons while passing through the third layer of CoPc.

Standard deviation of microscopy images used as indicator for growth stages.

Photoelectron emission microscopy (PEEM) and low energy electron microscopy (LEEM) can easily distinguish between organic molecules adsorbed in crystallites or in the wetting layers as well as the bare metal substrate due to their different electronic properties. Already before (and during) the condensation of such solid phases (2D islands or 3D crystallites), there is a dilute 2D gas phase. Such a 2D gas phase consists of molecules, which are highly mobile and diffuse across the surface. The individual molecules are too small to be resolved in PEEM/LEEM images. Here, we discuss, how image features below and above the resolution limit of a PEEM/LEEM affect the mean electron yield and its (normalized) standard deviation. We support our findings with two experimental examples: the deposition of cobalt phthalocyanine (CoPc) on Ag(100) and of perfluoro-pentacene on Ag(110). Our results demonstrate, how a spatial and temporal analysis of image series can be used to obtain information about molecular phases, which cannot be directly resolved in microscopy images.

Coexistence of nanowire-like hex and (1 × 1) phases in the topmost layer of Au(100) surface.

Scanning tunneling microscopy and density functional theory calculations were used to study the structure of the Au(100) surface after ion bombardment. The results indicate a development of two phases: the quasi-hexagonal (hex) and the (1 × 1) on the surface. A decrease in the number of surface atoms caused by ion bombardment leads to the development of a coexistence of phases inside the surface layer. Our experimental data and theoretical results rule out a scenario that the unreconstructed (1 × 1) domains under study represent a subsurface atomic layer revealed by ion sputtering. The hex phase is anisotropic, with the quantized width in accord with (6n + 1) formula. In certain conditions, nanowire-like structures consisting of seven atomic rows of alternating height are developed on the surface.

Phthalocyanine arrangements on Ag(100): From pure overlayers of CoPc and F16CuPc to bimolecular heterostructure.

We have utilized scanning tunneling microscopy (STM) and low energy electron diffraction to determine the structural properties of two types of metal-phthalocyanines (MPcs), i.e., cobalt-phthalocyanine (CoPc) and hexadecafluorinated copper-phthalocyanine (F16CuPc) on the Ag(100) surface. For coverage close to one monolayer, both systems form long-range ordered structures with square unit cells. The size and rotation of the unit cell with respect to the silver lattice depend on the chemical composition of MPc. Both types of molecules prefer adsorption with around a 30° angle between the molecular axis and the [011] silver direction. The CoPcs mainly arrange in a (5 × 5)R0 phase; however, two additional local arrangements, a 26 × 26 R 1 1 ○ and a (7 × 7)R0, were detected by STM. The F16CuPcs form a 29 × 29 R 2 2 ○ structure. The co-adsorption of CoPc and F16CuPc on the Ag(100) surface in a 1:1 ratio leads to the formation of a compositionally ordered chessboard-like 5 2 × 5 2 R 4 5 ○ structure. During filled states imaging, the different appearance of the central part of each MPc allows us to distinguish CoPcs from F16CuPcs. Regardless of the applied voltage polarity, the ligands of F16CuPcs appear brighter than the ligands of CoPcs.

Comparative assessment of calcitonin stimulation test using calcium gluconate and pentagastrin and the usefulness of procalcitonin basic and post-stimulation concentrations in the diagnosis of patients after surgery for medullary.

OBJECTIVES: The aim of the study was to compare the calcitonin (CT) stimulation tests with tests of calcium gluconate (CaG) and pentagastrin (PG), their tolerance and usefulness of PCT in the patients' diagnosis with active Medullary thyroid cancer (MCT) after thyroidectomy. METHODS: CT was marked in serum by the immunosorbent sandwich test. PCT was marked by the immunosorbent sandwich test, with the final reading of fluorenscence. PG was given intravenously at a dose of 0.5 mg/kg body weight for 10 seconds. CaG was also given by intravenous injection at a dose of 2.5 mg of elemental Ca/kg body weight at a rate of 5ml/min, for minimum 3 minutes. Blood was taken at the 0 minute, the 3 and 5 minute after getting the stimulating substances. RESULTS: The post-stimulation CT concentration in the 3 and 5 minute of the CaG test vs PG is significantly higher compared to the baseline. The maximal stimulation of the CT is in the 3 minute, but higher concentrations occurred using the CaG. CONCLUSION: The results of the study suggest a similar diagnostic value of the tests with CaG compared to the PG as stimulants. In the present study we noticed a trend of basic and post-stimulation concentrations of PCT to increase in the tests with PG and CaG which correspond with the elevated concentrations of CT.

Complex Surface Diffusion Mechanisms of Cobalt Phthalocyanine Molecules on Ag(100).

We used time-lapsed scanning tunneling microscopy between 43 and 50 K and density functional theory (DFT) to explore the basic surface diffusion steps of cobalt phthalocyanine (CoPc) molecules on the Ag(100) surface. We show that the CoPc molecules translate and rotate on the surface in the same temperature range. Both processes are associated with similar activation energies; however, the translation is more frequently observed. Our DFT calculations provide the activation energies for the translation of the CoPc molecule between the nearest hollow sites and the rotation at both the hollow and the bridge sites. The activation energies are only consistent with the experimental findings, if the surface diffusion mechanism involves a combined translational and rotational molecular motion. Additionally, two channels of motion are identified: the first provides only a channel for translation, while the second provides a channel for both the translation and the rotation. The existence of the two channels explains a higher rate for the translation determined in experiment.

Analysis of copper concentration in human serum by application of total reflection X-ray fluorescence method.

The chemotherapy and photon radiotherapy are the most often applied methods in treatment of the cancer diseases because of their effectiveness and high cure rates. Apart from eligible destruction of the tumour, one of the side effects of these treatment methods is possible modification of main and trace element concentration in different human tissues and fluids. In this paper, the copper (Cu) level in human serum was determined by total reflection X-ray fluorescence method in 142 chemotherapy patients and in 44 healthy persons being a control group. The Cu concentration in the chemotherapy group was found to be on the level 1.78 ± 0.909 mg/L, while in the control group, it was 1.08 ± 0.551 mg/L. Performed measurements allowed for calculation of the parameters of copper concentration distribution (mean value, standard deviation, median) for both analysed groups. The theoretical nature of the concentration distribution was tested and found as a log-normal distribution (control group) and a log-stable distribution (chemotherapy group). The copper concentration distributions for both studied group were statistically compared using Kolmogorov-Smirnov test, and the conclusion was that the distributions are statistically different. Serum Cu levels were significantly higher in the chemotherapy group than in the control group. Taking into account the results for the control group, the copper concentration reference quantile ranges in human serum were obtained. The values of the mean, median and other quantiles determined in this case can be applied in two-group comparison studies. The obtained results can be used as a diagnostic tool for chemotherapy patients.

Asymmetric one-dimensional random walks.

For one-dimensional surface diffusion in the presence of fields, movement of particles has to be considered as a random walk in which jumps to the right occur at a rate different from jumps to the left. Moments of the displacement distribution are worked out for such a one-dimensional walk to nearest-neighbor sites as well as by longer jumps to second nearest neighbors. The actual distribution of displacements, and how it changes as the asymmetry of the jump rates changes, is also examined, as this provides important information about the participating jump processes. We show that deriving the third moment gives a clear indication of the asymmetry in the random walk.

Atom movement on a dislocated surface.

The standard picture of growth at a screw dislocation assumes that the movement of adatoms on a dislocation loop is the same as on an ideal plane. We have examined this proposition by investigating the movement of a single tungsten adatom on a W(110) plane intersected by a screw dislocation. Surprisingly enough, adatom movement was entirely different than on a normal (110) plane: the overall diffusivity was higher, and the mobility varied with the location of the adatom relative to the dislocation core. This study demonstrates that surface transport is strongly affected in the vicinity of dislocations.

Long jump rates in surface diffusion: W on W(110).

We have reexamined the diffusion of W adatoms on W(110) using the field ion microscope. The diffusivity is in good agreement with previous results and reveals no unusual features, but from observations of the distribution of displacements, corrected for diffusion during temperature transients, we have for the first time been able to measure the temperature dependence of rates for nearest-neighbor and double jumps, as well as for vertical and horizontal transitions. Activation energies and frequency factors for all the long jumps are significantly larger than for single jumps, and a simple model is proposed to account for our observations.

Long jumps in surface diffusion.

Diffusion on metal surfaces is often viewed as the movement of atoms jumping between nearest-neighbor sites. However, statistical predictions suggest that at elevated temperatures longer jumps should become important. Now, with instruments such as the field ion microscope and the scanning tunneling microscope, which reveal individual adsorbed atoms, it has become possible to examine diffusion in some detail. Here we summarize the experimental efforts that have revealed the contributions of long jumps in various diffusion processes.