Yesim Darici

Self-assembly of aromatic thiols on Au(111)

We have studied the self-assembly of two aromatic thiols, 4-mercaptopyridine (4MPY ) and 4-hydroxythiophenol (4HTP), on Au(111) from aqueous solutions with in situ scanning tunneling microscopy and ex situ X-ray photoelectron and Auger electron spectroscopies. Upon adsorption of the molecules, islands of nanometer scale appear immediately on flat Au(111) terraces, which is in sharp contrast to the formation of depressions in the selfassembly of the n-alkanethiol monolayers. The islands increase both in size and number initially, and decrease slowly after reaching maximum values in about 15 min. The formation of the islands can be attributed to the Au atoms on the surface which become highly mobile due to the strong binding to the thiols. The decrease of the islands at the late stage is due to the fact that atoms in small islands diVuse into nearby big islands and surface step edges. Both 4MPY and 4HTP molecules are randomly distributed on the surface immediately after adsorption, but slowly arrange into domains of ordered structures. For 4MPY the ordered structure has lattice constants, a=5.1±0.3 A ˚ , b=

Surface passivation of GaAs with P2S5‐containing solutions

The surface chemistry of GaAs passivated with P2S5 solutions modified with sulfur and other sulfides has been studied with x‐ray photoelectron spectroscopy, room‐temperature photoluminescence, scanning Auger electron spectroscopy, and scanning electron microscopy. The degree of passivity achieved was measured by determining the intensity of room‐temperature photoluminescence before and after passivation. An increase in luminescence intensity of up to 23 times was observed after a treatment by solutions containing P2S5 and (NH4)2S buffered with sulfur due to reduced surface nonradiative carrier recombination. Phosphorus oxides and sulfur bound to the Ga and As species were present on the passivated surface. In addition, all the surfaces treated with sulfide solutions were covered to a variable extent with crystallized residues, and a treatment was devised to eliminate these residues while still retaining a passivated surface. Exposure of passivated surfaces to room‐temperature air increased the amount of g...

Leed measurements of Fe epitaxially grown on Cu(100)

Abstract Iron was epitaxially grown on a Cu(100) surface. Low energy electron diffraction (LEED) intensity versus energy curves were recorded for 1 and 10 layers of iron on Cu(100) at room temperature. A full dynamical analysis was performed using the renormalized forward scattering perturbation method. The surface Debye temperatures were determined to be 233 K for 1 ML Fe and 380 K for 10 layers of Fe. The value obtained for fcc iron was 550 K. A multiple relaxation approach was employed in analyzing the experimental data. The estimated interlayer spacings for the first and second layers were 1.78±0.02 A (first) and 1.81±0.02 A (second) for 1 ML Fe, and 1.81±0.02 A (first) and 1.78±0.02 A (second) for 10 layers of Fe on Cu(100). Auger electron spectroscopy was used to determine the thickness of the Fe films, and the LEED measurements indicate approximately a layer-by-layer growth until about 17 layers at room temperature. At higher temperatures there is evidence of iron diffusion or copper surface segregation.

Electron beam dissociation of CO and CO2 on ZnS thin films

The effects of electron bombardment in 10−6 Torr CO or CO2 upon the surface composition of ZnS thin films was determined with Auger electron spectroscopy. The ZnS surface composition was stable in 1×10−9 Torr residual vacuum or for CO and CO2 exposures without electron bombardment. With electron bombardment and CO both present, C was deposited and S decreased, but no O accumulated on the surface. This was interpreted as an electron beam stimulated surface reaction in which CO was dissociated by the electron beam CO2 was reformed leaving C on the surface. For CO2, both C and O were increased on the surface while S was reduced. This was interpreted as a surface reaction involving dissociation of CO2 with reformation of COx leaving both C and O on the surface. The data suggest that the surface dissociation of CO2 did not occur when the sample temperature was increased from 30 °C to 150–200 °C. At 300 °C, S diffused from the ZnS substrate to the surface. The consequences of these reactions on ZnS cathodolumin...

Mass transport through vertically aligned large diameter MWCNTs embedded in parylene

We have fabricated porous membranes using a parylene encapsulated vertically aligned forest of multi-walled carbon nanotubes (MWCNTs, about 7 nm inner diameter). The transport of charged particles in electrolyte through these membranes was studied by applying electric field and pressure. Under an electric field in the range of 4.4 × 10(4) V m(-1), electrophoresis instead of electroomosis is found to be the main mechanism for ion transport. Small molecules and 5 nm gold nanoparticles can be driven through the membranes by an electric field. However, small biomolecules, like DNA oligomers, cannot. Due to the weak electric driving force, the interactions between charged particles and the hydrophobic CNT inner surface play important roles in the transport, leading to enhanced selectivity for small molecules. Simple chemical modification on the CNT ends also induces an obvious effect on the translocation of single strand DNA oligomers and gold nanoparticles under a modest pressure (<294 Pa).

LEED measurements of one monolayer of iron epitaxially grown on Cu(111)

Abstract We report a LEED study of Cu(111) and one monolayer of iron epitaxially grown on Cu(111). A full dynamical analysis was performed using the renormalized forward scattering method. We studied the influence of the scattering potential employed on the quality of the LEED theoretical analysis. The analysis of the intensity versus energy for all the spots at room temperature gave a 2% contraction for the topmost layer of Cu(111) and 3% contraction for one monolayer of iron (in reference to Cu(111) bulk). We also detected Cu surface segregation at relative low temperatures (473 K) for iron epitaxially grown on Cu(111). Time and temperature play a significant role in the surface segregation process. Great care must be exercised when studying the electronic and magnetic properties of epitaxially grown fcc iron on Cu(111) when samples are prepared above room temperature.

Compensation origins in II-VI CZT materials

Abstract It is well known that II–VI CdTe, and CdZnTe (CZT) materials suffer from the presence of cadmium vacancies (V-Cd) and their complexes with impurities and defects, which lead to low resistivity and trapping. These defects are known generally as the A-centers, around 0.1–0.2 eV. In order to increase the resistivity, intentional and non intentional chemical and physical compensations are conducted; for CdTe, halogens (Cl, Br, I) or In are generally used, while for CZT, the compensation origins are still unknown. In this paper, we try to study the compensation by measuring deep levels and very shallow levels at temperatures as low as helium temperature by photoluminescence (PL) and photoinduced current transient spectroscopy (PICTS), and model the resistivity in order to clarify the origins of the compensation and the high resistivity in CZT materials.

Leed study of Fe epitaxially grown at 190°C on Cu(100)

Abstract We report a LEED study of iron deposited on Cu(100) at 190°C. Very sharp LEED patterns were obtained for high iron coverages. We observed evidence of an ill defined Fe/Cu interface for one monolayer of iron deposited at 190°C. The stability of the iron overlayers was tested as a function of time for various coverages. For one monolayer the intensity versus energy curve for the (00) beam shows time dependence at 190°C. For four layers, no significant changes were observed in the LEED spectra over a period of one hour. We measured for five layers of iron a top layer expansion of 2.8% relative to the bulk. The interplanar spacing for bulk fcc Fe at 190°C remains equal to the room value. In these experiments the samples were deposited at 190°C and kept at this temperature during the LEED measurements.

Structural studies of sulfur-passivated GaAs (100) surfaces with LEED and AFM

We present the results of Auger electron spectroscopy (AES ), low-energy electron diVraction (LEED) and atomic force microscopy (AFM ) analysis of sulfur-passivating layers on the GaAs (100) surface. The GaAs surfaces were passivated with both inorganic [(NH 4 ) 2 S x ] and organic [octadecylthiol (ODT )] S-based compounds. We prepared the inorganic sulfur-passivated GaAs(100) surfaces with a wet chemical treatment using (NH 4 ) 2 S solution. This was followed by thermal annealing of the treated sample in an ultrahigh vacuum ( UHV ). After ex-situ and in-situ treatments, the surface resulted in a (2◊1) LEED pattern. The LEED data (I‐E curves) were recorded and compared with dynamical LEED calculations for diVerent structural models for the sulfur-passivated GaAs (100) surface. The results showed that the sulfur-passivated (2◊1) surface structure is an arsenic‐sulfur dimer on a Ga-terminated substrate. The ex-situ AFM results also revealed a (2◊1) structure for the inorganic passivation and a very smooth surface for the organic ODT in the ethanol-treated sample.

A surface plasmon resonance study of the intermolecular interaction between Escherichia coli topoisomerase I and pBAD/Thio supercoiled plasmid DNA.

To date, the bacterial DNA topoisomerases are one of the major target biomolecules for the discovery of new antibacterial drugs. DNA topoisomerase regulates the topological state of DNA, which is very important for replication, transcription and recombination. The relaxation of negatively supercoiled DNA is catalyzed by bacterial DNA topoisomerase I (topoI) and this reaction requires Mg(2+). In this report, we first quantitatively studied the intermolecular interactions between Escherichia coli topoisomerase I (EctopoI) and pBAD/Thio supercoiled plasmid DNA using surface plasmon resonance (SPR) technique. The equilibrium dissociation constant (Kd) for EctopoI-pBAD/Thio interactions was determined to be about 8 nM. We then studied the effect of Mg(2+) on the catalysis of EctopoI-pBAD/Thio reaction. A slightly higher equilibrium dissociation constant (~15 nM) was obtained for Mg(2+) coordinated EctopoI (Mg(2+)EctopoI)-pBAD/Thio interactions. In addition, we observed a larger dissociation rate constant (kd) for Mg(2+)EctopoI-pBAD/Thio interactions (~0.043 s(-1)), compared to EctopoI-pBAD/Thio interactions (~0.017 s(-1)). These results suggest that enzyme turnover during plasmid DNA relaxation is enhanced due to the presence of Mg(2+) and furthers the understanding of importance of the Mg(2+) ion for bacterial topoisomerase I catalytic activity.