Published by American Chemical Society

Online ISSN: 1520-5827


Print ISSN: 0743-7463


Supramolecular Nanostructures of 1,3,5-Benzene-tricarboxylic Acid at Electrified Au(111)/0.05 M H 2 SO 4 Interfaces: An in Situ Scanning Tunneling Microscopy Study
  • Article

August 2005


100 Reads


B Han


L J Wan


Th Wandlowski
The potential-induced adsorption and self-assembly of 1,3,5-benzene-tricarboxylic acid (TMA) was investigated at the electrified Au(111)/0.05 M H2SO4 interface by in-situ scanning tunneling microscopy (STM) and surface enhanced infrared reflection absorption spectroscopy (SEIRAS) in combination with electrochemical techniques. Depending on the applied electric field, TMA forms five distinctly different, highly ordered supramolecular adlayers on Au(111) surfaces. We have elucidated their real-space structures at the molecular scale. In the potential range -0.25 V < E < 0.20 V, planar-oriented TMA molecules form a hexagonal open-ring honeycomb structure, Ia, a hydrogen-bonded ribbon-type phase, Ib, and a herringbone-type phase, Ic, stabilized by directional hydrogen bonding and weak substrate-adsorbate interactions. Interfacial water molecules are being replaced. In 0.20 V < or = E < 0.40 V, e.g., around the potential of zero charge, and at slightly higher coverages, a close-packed physisorbed adlayer of hydrogen-bonded TMA dimers, II, was observed. Further increase of the electrode potential to positive charge densities causes an orientation change from planar to upright. An initially disordered phase, IIIa, transforms into an ordered, stripelike chemisorbed adlayer, IIIb, of perpendicularly oriented TMA molecules. One carboxylate group per molecule is bound to the electrode surface, while the two other protonated carboxyl groups are directed toward the electrolyte and act as structure-determining components of a hydrogen-bonded two-dimensional ladder-type network. Structural transitions between the various types of ordered molecular adlayers are attributed to (hole) nucleation and growth processes.

Effect of Interfacial Ion Structuring on Range and Magnitude of Electric Double Layer, Hydration, and Adhesive Interactions between Mica Surfaces in 0.05–3 M Li + and Cs + Electrolyte Solutions

March 2014


255 Reads

Ions and water structuring at charged-solid/electrolyte interfaces and forces arising from interfacial structuring in solutions above 100 mM concentrations dominate structure and functionality in many physiological, geological and technological systems. In these concentrations electrolyte structuring occurs within the range of molecular dimensions. Here, we quantitatively measure and describe electric double layer (EDL) and adhesive interactions at mica-interfaces in aqueous CsCl and LiCl solutions with concentrations ranging from 50 mM to 3M. Complementary, using atomic force microscopy and surface forces apparatus experiments we characterize concentration-dependent stark differences in the inner and outer EDL force profiles, and discuss differences between the used methods. From 50 mM to 1 M concentrations, interactions forces measured in CsCl-solutions exhibit strong hydration repulsions, but no diffuse EDL-repulsions beyond the Stern layer. In confinement the weakly hydrated Cs+ ions condensate into the mica-lattice screening the entire surface charge within the Stern layer. In contrast, strongly hydrated Li+ ions only partially compensate the surface charge within the Stern layer, leading to the formation of a diffuse outer double layer with DLVO behaviour. Both, LiCl and CsCl solutions exhibit oscillatory ion-hydration forces at surface separations from 2.2 nm to 4-8 Å. Below 4-8 Å the force profiles are dominated in both cases by forces originating from water and/or ion confinement at the solid/electrolyte/solid interface. Adhesive minima and their location vary strongly with the electrolyte and its concentration due to specific ion-correlations across the interface, while dispersion forces between the surfaces are overpowered. Highly concentrated 3 M solutions exhibit solidification of the inner EDL structure and an unexpected formation of additional diffuse EDL forces with an increasing range, as recently measured in ionic liquids. Our results may have important implications for understanding and modelling of interaction forces present in static and dynamic systems under physiological and high salt conditions.

Direct Observation of Conformational Changes of β-Substituted Duodecithiophene on a Au(111)-(√3 × 22) Substrate Using in Situ Electrochemical STM in 0.1 M HClO 4

January 2010


70 Reads






The adsorption of hexahexylduodecithiophene (12T) on a Au(111) electrode was investigated by using cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM) in 0.10 M HClO(4). Potential control at 0.20 V (vs RHE) revealed adlayer structures of mostly folded and rarely angular (oblique) and extended conformations on a reconstructed Au(111)-(square root(3) x 22) surface. The angular and extended conformations predominate when the electrode potential is increased to 0.35 and 0.60 V. Folded structures are still evident, but dynamic STM studies showed unfolding of this conformation. With molecular STM imaging of 12T adlayers, we address the packing arrangement and conformational changes of 12T admolecules on the reconstructed Au(111) electrode surface.

Scanning Tunneling Microscopy of Sulfur and Benzenethiol Chemisorbed on Ru(0001) in 0.1 M HClO 4

June 2004


20 Reads

In situ scanning tunneling microscopy (STM) combined with linear sweep voltammetry was used to examine spatial structures of sulfur adatoms (SA) and benzenethiol (BT) molecules adsorbed on an ordered Ru(0001) electrode in 0.1 M HClO4. The Ru(0001) surface, prepared by mechanical polishing and electrochemical reduction at -1.5 V (vs RHE) in 0.1 M HClO4, contained atomically flat terraces with an average width of 20 nm. Cyclic voltammograms obtained with an as-prepared Ru(0001) electrode in 0.1 M HClO4 showed characteristics nearly identical to those of Ru(0001) treated in high vacuum. High-quality STM images were obtained for SA and BT to determine their spatial structures as a function of potential. The structure of the SA adlayer changed from (2 x mean square root of 3)rect to domain walls to (mean square root of 7 x mean square root of 7)R19.1 degrees and then to disordered as the potential was scanned from 0.3 to 0.6 V. In contrast, molecules of BT were arranged in (2 x mean square root of 3)rect between 0.1 and 0.4 V, while they were disordered at all other potentials. Adsorption of BT molecules was predominantly through the sulfur headgroup. Sulfur adatoms and adsorbed BT molecules were stable against anodic polarization up to 1.0 V (vs RHE). These two species were adsorbed so strongly that their desorption did not occur even at the onset potential for the reduction of water in 0.1 M KOH.

In situ STM observation of the CO adlayer on a Pt(110) electrode in 0.1 M HClO4 solution

June 2010


32 Reads

We have obtained the first in situ STM molecular image of a CO adlayer on a Pt(110)-(1 x 1) electrode surface in 0.1 M HClO(4) solution. The observed CO adlayer formed an ordered (2 x 1)-2CO structure at saturated coverage. The CO molecules were found to adsorb on top of each Pt surface atom; however, they were tilted with a zigzag configuration along the atomic rows because of the dipole-dipole repulsion of neighboring CO molecules. The high activity of the Pt(110) electrode for surface CO oxidation can be attributed to the low packing density of the adsorbed CO molecules as well as their tilted orientation.

Molecular Assembly and Electropolymerization of 3,4-Ethylenedioxythiophene on Au(111) Single Crystal Electrode as Probed by In Situ Electrochemical STM in 0.10 M HClO4

July 2010


26 Reads

We have used electrochemical scanning tunneling microscopy (EC-STM) to obtain molecular insights on the adlayer structures and electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) on a bare Au(111) single crystal electrode in 0.1 M HClO(4) solution. Cyclic voltammetric (CV) studies showed an increase in anodic current at 0.90 V with the oxidation of EDOT monomer occurring at E = 1.10 V (vs reversible hydrogen electrode). In situ STM revealed, for the first time, that EDOT molecules can spontaneously form organized adlayers on a bare Au(111) surface with 18 muM concentration of EDOT in aqueous solution. Molecularly resolved STM images of the EDOT adlayer showed two domains consisting of disordered and ordered structures with the formation of vacancy islands or "etch pits". Several EDOT structures were observed at +0.60 V, namely, (4 x 7), (5 x square root(37)), and (square root(7) x 3) with calculated coverages of 0.107, 0.114, and 0.111 ML, respectively. Electropolymerization was also carried out using in situ STM in 0.10 M HClO(4) under potential control.

Selective Synthesis of Zn1-xMnxSe Nanobelts and Nanotubes from [Zn1-xMnxSe](DETA)(0.5) Nanbelts in Solution (x=0-0.15) and Their EPR and Optical Properties

August 2010


51 Reads

Zn(1 - x)Mn(x)Se (x = 0-0.15) nanobelts and nanotubes can be synthesized via the removal of diethylenetriamine (DETA) in 1-octadecene (ODE) and ethylene glycol (EG), respectively, using [Zn(1 - x)Mn(x)Se](DETA)(0.5) nanobelts as a template. The as-prepared ZnSe nanobelts are single-crystalline and grown along the [001] direction, and the ZnSe nanotubes consist of nanoparticles assembled along the [001] direction. In addition, Mn(2+)-doped Zn(1 - x)Mn(x)Se (x = 0.05, 0.10, 0.15) nanotubes are prepared for the first time if doped [Zn(1 - x)Mn(x)Se](DETA)(0.5) nanobelts are used as the template. The formation process of Zn(1 - x)Mn(x)Se nanobelts and nanotubes has been studied, and a plausible mechanism is proposed. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectra of Zn(1 - x)Mn(x)Se nanobelts and Zn(1 - x)Mn(x)Se nanotubes have been investigated.

Large Area, Molecularly Smooth (0.2 nm rms) Gold Films for Surface Forces and Other Studies

August 2007


71 Reads

Using large-area (cm2) single-crystal mica sheets as the templating substrate, we have created correspondingly large template-stripped (TS) gold films (thickness 82 +/- 2 nm) that appear smooth to within 0.2 nm rms roughness over their entire area. These gold films, created without the use of any releasing solvent, are characterized using AFM, X-ray diffraction, multiple beam interferometric fringes of equal chromatic order (FECO), and contact angle measurements. Being molecularly smooth over large areas and (adjustably) semitransparent, these films are especially suitable for use in the surface force balance (SFB), as shown by measurements of the normal force (F) versus distance (D) profiles between such a flat gold surface and a bare mica surface in water. The F(D) profiles are in good agreement with DLVO theory down to molecular contact and indicate that the gold surface is negatively charged under water.

Size-dependent MRI relaxivity and dual imaging with Eu0.2Gd 0.8PO4·H2O Nanoparticles

May 2014


27 Reads

Three different sizes of Eu0.2Gd0.8PO4·H2O nanoparticles have been prepared to investigate the particle size influence on water proton relaxivity. Longitudinal relaxivity (r1) values increase for smaller particles, reaching as high as r1 = 6.13 mM(-1) s(-1) for a sample of 40 ± 4 nm particles, which, with a ratio of transverse/longitudinal relaxivity, r2/r1 = 1.27, are shown to be effective positive contrast agents. The correlation between relaxivity and the surface-to-volume ratio implies that access to surface Gd(3+) sites is the principal factor affecting relaxivity. On the other hand, although ionic molar relaxivity decreases for larger particles, the relaxivity per particle can be significantly greater. Gadolinium-based nanoparticles doped with fluorescent lanthanide elements have attracted attention for their dual-imaging abilities, combining magnetic resonance imaging (MRI) and fluorescence imaging agents. In both in vitro experiments with HeLa cells and in vivo experiments with C. elegans, strong red fluorescence is observed from Eu0.2Gd0.8PO4·H2O with high resolution, demonstrating the parallel use of the particles as fluorescence imaging agents.

Nanoplatelets of Sodium Hectorite Showing Aspect Ratios of ≈20 000 and Superior Purity

January 2013


125 Reads

Applying a combination of melt synthesis followed by long-term annealing a fluorohectorite is obtained which is unique in respect to homogeneity, purity and particle size. Counter intuitively, the hectorite undergoes a disorder to order transition upon swelling to the level of the bilayer hydrate. Alkylammonium exchanged samples show at any chain length only a single basal spacing corroborating a nicely homogenous layer charge density. Its intracrystalline reactivity improves greatly upon annealing making it capable to spontaneously and completely disintegrate into single clay lamellae of 1 nm. Realizing exceptional aspect ratios of around 20000 upon delamination, this synthetic clay will offer unprecedented potential as functional filler in highly transparent nanocomposites with superior gas barrier and mechanical properties.

Self-Assembly of a Hexagonal Boron Nitride Nanomesh on Ru(0001)

April 2007


270 Reads

Hexagonal boron nitride (h-BN) nanostructures were grown on Ru(0001), and are very similar to those previously reported on Rh(111). They show a highly regular 12 x 12 superstructure, comprising 2 nm wide apertures with a depth of about 0.1 nm. Valence band photoemission reveals two distinctly bonded h-BN species, and X-ray photoelectron spectroscopy indicates an h-BN monolayer film. The functionality of the h-BN/Ru(0001) nanomesh is demonstrated by using this structure for the assembly of gold nanoclusters.

Piperidine Adsorption on Hydrated ??-Alumina (0001) Surface Studied by Vibrational Sum Frequency Generation Spectroscopy

January 2005


115 Reads

The adsorption of piperidine vapor on the hydrated alumina (alpha-Al2O3, corundum) (0001) surface was investigated using vibrational broad bandwidth and scanning sum frequency generation (SFG) spectroscopy. The interfacial vibrational signature in the C-H stretching region of piperidine at the alumina (0001) surface is shown to be a sensitive spectroscopic probe revealing the adsorption mechanism. The neat piperidine surface, aqueous piperidine surface, and aqueous piperidium chloride surface were also investigated in the C-H stretching region by SFG to establish vibrational reference frequencies. After piperidine adsorption, piperidine vapor was removed and piperidine was found to be chemisorbed onto the alumina (0001) surface through protonation by surface hydroxyl groups. The O-H stretching region of the alumina surface before and after piperidine adsorption was also investigated, and the results revealed the decrease of the surface number density of alumina surface hydroxyl groups.

Density Functional Theory Study of Ethanol Decomposition on 3Ni/α-Al 2 O 3 (0001) Surface

October 2010


77 Reads

We have investigated the decomposition of ethanol (EtOH) on a 3Ni/α-Al₂O₃(0001) surface using periodic density functional theory calculations. A triangular Ni trimer doped on a 2 × 2 α-Al₂O₃(0001) surface was used to represent the 3Ni/α-Al₂O₃(0001) surface. We considered several possible pathways for EtOH decomposition over the 3Ni/α-Al₂O₃(0001) surface, including dehydrogenation and C-C bond cleavage. Our calculated results indicated that (i) the 3Ni/α-Al₂O₃(0001) surface possesses high activity to inhibit coke formation and (ii) the CH₂CH₂O((a)) → CH₂CHO((a)) + H((a)) reaction is the rate-determining step for the overall reaction [CH₃CH₂OH((a)) → CH(2(a)) + CO((a)) + 4 H((a))] with an energy barrier of 1.20 eV. One feasible channel leading to C-C bond cleavage is weakening of the C-C bond in the stable CH₂CO intermediate via transformation of the adsorbed structure to a metastable structure, thereby increasing the coordination number of the two C atoms to the Ni trimer. In addition, we also investigated the nature of the metal-ethanol bonding through scrutiny of density of states (DOS) and electron density difference contour plots. The DOS analysis allowed us to characterize the state interactions between ethanol and the surfaces; the electron density difference plots provide evidence that is consistent with the prediction from DOS analysis.

Growth Pattern of Ag n ( n = 1−8) Clusters on the α-Al 2 O 3 (0001) Surface: A First Principles Study

November 2010


19 Reads

We report an extensive first-principles study of the structure and electronic properties of Ag(n) (n = 1-8) clusters isolated in gas phase and deposited on the α-Al(2)O(3) surface. We have used the plane wave based pseudopotential method within the framework of density functional theory. The electron ion interaction has been described using projector augmented wave (PAW), and the spin-polarized GGA scheme was used for the exchange correlation energy. The results reveal that, albeit interacting with support alumina, the Ag atoms prefers to remain bonded together suggesting an island growth motif is preferred over wetting the surface. When compared the equilibrium structures of Ag clusters between free and on alumina substrate, a significant difference was observed starting from n = 7 onward. While Ag(7) forms a three-dimensional (3D) pentagonal bipyramid in the isolated gas phase, on alumina support it forms a planar hexagonal structure parallel to the surface plane. Moreover, the spin moment of the Ag(7) cluster was found to be fully quenched. This has been attributed to higher delocalization of electron density as the size of the cluster increases. Furthermore, a comparison of chemical bonding analysis through electronic density of state (EDOS) shows that the EDOS of the deposited Ag(n) cluster is significantly broader, which has been ascribed to the enhanced spd hybridization. On the basis of the energetics, it is found that the adsorption energy of Ag clusters on the α-Al(2)O(3) surface decreases with cluster size.

Modeling Adsorption of the Uranyl Dication on the Hydroxylated α-Al 2 O 3 (0001) Surface in an Aqueous Medium. Density Functional Study

March 2006


36 Reads

As a first step toward modeling the interaction of dissolved actinide contaminants with mineral surfaces, we studied low-coverage adsorption of aqueous uranyl, UO2(2+), on the hydroxylated alpha-Al2O3(0001) surface. We carried out density functional periodic slab model calculations and modeled solvation effects by explicit aqua ligands. We explored the formation of both inner- and outer-sphere complexes and estimated the corresponding adsorption energies. Effects of solvation were accounted for by explicit consideration of the first hydration shell of uranyl and by means of a posteriori corrections for long-range solvent effect. With energetics described at the GGA-PW91 level and under the assumption of a fully protonated ideal surface, we predict a weakly bound outer-sphere adsorption complex.

Stabilization and Acidic Dissolution Mechanism of Single-Crystalline ZnO(0001) Surfaces in Electrolytes Studied by In-Situ AFM Imaging and Ex-Situ LEED

June 2008


114 Reads

A combined approach of pH-dependent in-situ AFM topography and ex-situ LEED studies of the stability and dissolution of single-crystalline ZnO(0001)-Zn surfaces in aqueous media is presented. Hydroxide-stabilized and single-crystalline ZnO(0001)-Zn surfaces turned out to be stable within a wide pH range between 11 and 4 around the point of zero charge of pH PZC = 8.7 +/- 0.2. Hydroxide stabilization turned out to be a very effective stabilization mechanism for polar oxide surfaces in electrolyte solutions. The dissolution of the oxide surface started at an acidic pH level of 5.5 and occurred selectively at the pre-existing step edges, which consist of nonpolar surfaces. In comparison, the oxide dissolution along the ZnO(0001) direction proved to be effectively inhibited above a pH value of 3.8. On the basis of these microscopic observations, the mechanistic understanding of the acidic dissolution process of ZnO could be supported. Moreover, both the in-situ AFM and the ex-situ LEED studies showed that the stabilization mechanism of the ZnO(0001) surfaces changes in acidic electrolytes. At pH values below 3.8, the hydroxide-stabilized surface is destabilized by dissolution of the well-ordered radical3. radical3. R30 hydroxide ad-layer as proven by LEED. Restabilization occurs and leads to the formation of triangular nanoterraces with a specific edge termination. However, below pH 4 the surface structure of the crystal itself is ill-defined on the macroscopic scale because preferable etching along crystal defects as dislocations into the bulk oxide results in very deep hexagonal etching pits.

Enantiospecific Adsorption of Amino Acids on Hydroxylated Quartz (0001)

July 2009


88 Reads

Density functional theory calculations have been used to study the adsorption of glycine, alanine, serine, and cysteine on the hydroxylated (0001) surface of alpha-quartz. We found negligible differences in adsorption energies for the most stable minima of enantiomers of alanine on this surface. There are, however, measurable energy differences between the two enantiomers of both serine and cysteine in their most stable states. The source of this enantiospecificity is mainly the difference in the strength of hydrogen bonds between the surface and the two enantiomers. Our results provide initial information on how amino acids can exhibit enantiospecific adsorption on hydroxylated quartz surfaces.

Density Functional Theory Study of the Binding of Glycine, Proline, and Hydroxyproline to the Hydroxyapatite (0001) and (011̅0) Surfaces

June 2009


141 Reads

In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue and in synthetic biomaterials, we have investigated the interaction of three constituent amino acids of the collagen matrix with two major hydroxyapatite surfaces. We have employed electronic structure techniques based on the density functional theory to study a range of different binding modes of the amino acids glycine, proline, and hydroxyproline at the hydroxyapatite (0001) and (0110) surfaces. We have performed full geometry optimizations of the hydroxyapatite surfaces with adsorbed amino acid molecules to obtain the optimum substrate/adsorbate structures and interaction energies. The calculations show that the amino acids are capable of forming multiple interactions with surface species, particularly if they can bridge between two surface calcium ions. The binding energies range from 290 kJ mol(-1) for glycine on the (0001) surface to 610 kJ mol(-1) for hydroxyproline on the (0110) surface. The large adsorption energies are due to a wide range of interactions between the adsorbate and surface, including proton transfer from the adsorbates to surface OH or PO(4) groups. Hydroxyproline binds most strongly to the surfaces, but all three amino acids should be good sites for the nucleation and growth of the hydroxyapatite (0110) surface at the collagen matrix.

Adsorption of Organosilanes at a Zn-Terminated ZnO (0001) Surface: Molecular Dynamics Study

October 2006


64 Reads

Four different organosilanes (octyltrihydroxysilane, butyltrihydroxysilane, aminopropyltrihydroxysilane, and thiolpropyltrihydroxysilane) adsorbed at a reconstructed Zn-terminated polar ZnO (0001) surface are studied via constant temperature (298 K) molecular dynamics simulations. Both single adsorbed silane molecules as well as adsorbed silane layers are modeled, and the energy, distance, orientation, and alignment of these adsorbates are analyzed. The adsorbed silane molecules exhibit behavior depending on the chemical nature of their tail (nonpolar or polar) as well as on the silane concentration at the solid surface (single adsorption or silane layer). In contrast to the O-terminated ZnO surface studied previously, now adsorption can only occur at the vacancies of this reconstructed crystal surface, thus leading to an arched structure of the liquid phase near the crystal surface. Nevertheless, both nonpolar and polar single adsorbed silanes show a similar orientation and alignment at the surface (orthogonal in the former, parallel in the latter case) as for the O-terminated ZnO surface, although the interaction energy with the surface is considerably increased for nonpolar silanes while it is nearly unaffected for the polar ones. For adsorbed silanes within silane layers, the difference to single adsorbed silanes depends on the polarity of the tail: nonpolar silanes again show an orthogonal alignment, while polar silanes exhibit two different orientations at the solid surface-a head and a tail down configuration. This leads to two completely different but nevertheless stable orientations of these silanes at the Zn-terminated ZnO surface.

Effect of Substrate Surface Reconstruction on Interaction with Adsorbates: Pt on 6H-SiC(0001)

May 2010


50 Reads

Three reconstructed 6H-SiC(0001) surfaces, including a Si-rich 3 x 3 surface, a C-rich 6 square root(3) x 6 square root(3) surface, and a graphitized SiC surface, were used as substrates for the deposition of Pt overlayers. The interaction between Pt and the SiC(0001) surfaces was studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Pt reacts readily with the 3 x 3 surface to form platinum silicide even at room temperature. On the graphitized SiC surface, metal particles with low lateral dispersion form and keep on aggregating upon annealing. In contrast, homogeneously distributed small Pt nanoclusters were grown on the C-rich 6 square root(3) x 6 square root(3) surface. The unique nanomesh surface structure helps to stabilize the Pt nanoclusters until 800 degrees C. Above 1000 degrees C, Pt tends to diffuse into the subsurface region, forming the C/Pt silicide/SiC(0001) interface structure. The different surface electronic structures of the three Pt/SiC(0001) systems were discussed as well. The present data show that surface reconstruction provides an effective route to control the growth of metal overlayers and the formation of metal/substrate interfaces.

Growth and Analysis of Octadecylsiloxane Monolayers on Al 2 O 3 (0001)

August 2006


20 Reads

On solvent-cleaned and piranha-etched single-crystal Al2O3(0001) surfaces, uniform, robust, self-assembled monolayers of octadecylsiloxane (ODS) are formed by 48 h exposure to a solution containing octadecyltrichlorosilane (OTS) in an anhydrous atmosphere. X-ray photoelectron spectroscopy, atomic force microscopy, ellipsometry, and water contact angle measurements confirm the presence of a uniform, complete monolayer. Reducing the exposure time or omitting the piranha-etch leads to much less uniform coverage. The ODS monolayers are stable when stored in ambient atmospheres for month-long periods. Thermal desorption in a vacuum environment (10(-9) Torr) shows the ODS monolayer is thermally stable up to at least 420 K. When heated in 200 mTorr of flowing forming gas (N2-10% H2) for 1 h at 520 K, slow loss of ODS was indicated. A schematic model is proposed which involves island nucleation by covalent bonding of OTS to surface hydroxyl groups followed by growth through the addition of mobile ODS species.

Investigation of Formation Mechanism of Pt(111) Nanoparticle Layers Grown on Ru(0001) Core

February 2011


77 Reads

A layer growth mechanism of Pt-Ru bimetallic nanoparticles has been proposed with supporting experiments and calculations by density functional theory (DFT). Elongated Pt atoms on Ru nanoparticles were synthesized via a two-step route, and their structural details were obtained by high-resolution transmission electron microscopy. Because of the intrinsic mismatch of lattice spacing between the two elements, such an unusual growth was analyzed with the DFT simulations to explore the mystery of the growth mechanism. Pt atoms would rearrange the packing order and adjust the Pt-Pt atomic distance, and so do the Ru nanoparticles in order to achieve the optimal energy status of the bimetallic system. The resultant Pt(111) layers could stack on top of the Ru(0001) core more tightly by fitting the pockets left between the Ru atoms. The findings give insight into the formation mechanism of the nanosized Pt-Ru bimetallic catalyst and pave the way for designing bimetallic catalysts with tailored properties at the atomic level.

Indirect Molecular Epitaxy: Deposition of n-Alkane Thin Films on Au coated NaCl(001) and HOPG(0001) Surfaces.

May 2013


28 Reads

The epitaxial growth of organic molecules can lead to the formation of complex orientated morphologies. In previous work, we studied the kinetic and thermodynamic factors that drive the epitaxial growth of n-alkane thin films on HOPG(0001) and NaCl(001) by physical vapor deposition. A wide variety of morphologies are observed as a function of deposition conditions (substrate temperature, n-alkane chain length, etc). In the current study we examine how a modified substrate (Au deposited on a HOPG(0001) or NaCl(001) substrate) affects the epitaxial growth of n-C36H74 (50 nm thick) relative to the uncoated substrates. This "indirect epitaxy", in which the patterned attractive forces of the substrate are transferred through a thin metal film, can tailor the conditions for epitaxial growth. The observation of 4-fold symmetry for n-alkane growth on Au/NaCl(001) and 6-fold symmetry for n-alkane growth on Au/HOPG(0001) demonstrates indirect epitaxy over a wide range of substrate temperatures during deposition.

Epitaxial Growth of Single-Crystalline Al 2 O 3 Films on Cr 2 O 3 (0001)

May 2007


20 Reads

Thin, crystallographically oriented single-crystalline Al2O3 films can be grown epitaxially on Cr2O3(0001) by codeposition of Al vapor and O2 at a substrate temperature of 825 K. The properties and growth of these films were monitored by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Two routes of preparation were investigated: (i) stepwise growth by alternating deposition of Al at room temperature and subsequent exposure to O2 at elevated temperatures; (ii) codeposition of Al and O2 at T > 800 K. The first route was consistently found to result in the growth of a complex interfacial oxide followed by the growth of polycrystalline Al2O3. The second mode of preparation provided homogeneous and ordered, probably (0001)-oriented, films of Al2O3 that maintained a LEED pattern up to a thickness around 10 A. The surface sensitive Cr MVV Auger transition at 34 eV was completely attenuated once the Al2O3 layer had reached a thickness of 6 A, pointing to film homogeneity at an early stage. This was confirmed by the absence of a significant Cr signal in LEIS spectra.

Water Adsorption on the Stoichiometric (001) and (010) Surfaces of Hydroxyapatite: A Periodic B3LYP Study

February 2009


115 Reads

H2O adsorption on hexagonal hydroxyapatite (001) and (010) stoichiometric surfaces has been studied at B3LYP level with a localized Gaussian basis set of polarized double-zeta quality using the periodic CRYSTAL06 code. Because four Ca2+ cations are available at both surfaces, the considered H2O coverages span the 1/4<or=theta<or=5/4 range. The affinity of both HA surfaces for H2O is large: on the (001) surface, H2O adsorbs molecularly (binding energies BE approximately 80 kJ mol(-1) per adsorbed molecule), whereas it dissociates on the (010) surface, giving rise to new surface terminations (CaOwHw and POHw). The highly negative reaction energy for H2O dissociation (between -250 and -320 kJ mol(-1) per adsorbed H2O molecule) strongly suggests that the pristine (010) surface "as cut" from the hydroxyapatite bulk cannot survive in aqueous environment. Conversely, on the reacted surface, H2O adsorbs molecularly with BE similar to those computed for the (001) surface. The B3LYP BEs have been contrasted to the experimental water adsorption enthalpies measured by microcalorimetry on polycrystalline hydroxyapatite samples, showing a fairly good agreement and supporting the suggestion that H2O vapor adsorbs on the already reacted (010) crystalline faces. Harmonic B3LYP vibrational features of adsorbed H2O show, when compared to modes of the gas-phase H2O, a hypsochromic shift of the HOH bending mode (Deltadelta(HOH)=49 cm(-1)) and a bathochromic shift of the OH stretching modes larger than 1700 cm(-1) (Deltanu(OH)=427 cm(-1)), which are both in good agreement with literature experimental data.

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