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1.
Ultramicroscopy ; 162: 17-24, 2016 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-26720438

RESUMO

Helium Ion Microscopy is known for its surface sensitivity and high lateral resolution. Here, we present results of a Helium Ion Microscopy based investigation of a surface confined alloy of Ag on Pt(111). Based on a change of the work function of 25meV across the atomically flat terraces we can distinguish Pt rich from Pt poor areas and visualize the single atomic layer high steps between the terraces. Furthermore, dechanneling contrast has been utilized to measure the periodicity of the hcp/fcc pattern formed in the 2-3 layers thick Ag/Pt alloy film. A periodicity of 6.65nm along the ⟨112⟩ surface direction has been measured. In terms of crystallography a hcp domain is obtained through a lateral displacement of a part of the outermost layer by 1/√3 of a nearest neighbor spacing along ⟨112⟩. This periodicity is measured with atomic precision: coincidence between the Ag and the Pt lattices is observed for 23 Ag atoms on 24 Pt atoms. The findings are perfectly in line with results obtained with Low Energy Electron Microscopy and Phase Contrast Atomic Force Microscopy.

2.
J Chem Phys ; 140(23): 234705, 2014 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-24952558

RESUMO

We have investigated the interaction of oxygen with the Ag/Pt(111) surface alloy by thermal desorption spectroscopy (TDS). The surface alloy was formed during the deposition of sub-monolayer amounts of silver on Pt(111) at 800 K and subsequent cooling to 300 K. The low-temperature phase of the surface alloy is composed of nanometer-sized silver rich stripes, embedded within platinum-rich domains, which were characterized with spot profile analysis low energy electron diffraction. The TDS measurements show that oxygen adsorption is blocked on Ag sites: the saturation coverage of oxygen decreases with increasing Ag coverage. Also, the activation energy for desorption (Edes) decreases with Ag coverage. The analysis of the desorption spectra from clean Pt(111) shows a linear decay of Edes with oxygen coverage, which indicates repulsive interactions between the adsorbed oxygen atoms. In contrast, adsorption on alloyed Ag/Pt(111) leads to an attractive interaction between adsorbed oxygen atoms.

3.
Beilstein J Org Chem ; 9: 1698-704, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-24062830

RESUMO

Polystyrene sulfonate polymer brushes, grown on the interior of the microchannels in a microreactor, have been used for the anchoring of gallium as a Lewis acid catalyst. Initially, gallium-containing polymer brushes were grown on a flat silicon oxide surface and were characterized by FTIR, ellipsometry, and X-ray photoelectron spectroscopy (XPS). XPS revealed the presence of one gallium per 2-3 styrene sulfonate groups of the polymer brushes. The catalytic activity of the Lewis acid-functionalized brushes in a microreactor was demonstrated for the dehydration of oximes, using cinnamaldehyde oxime as a model substrate, and for the formation of oxazoles by ring closure of ortho-hydroxy oximes. The catalytic activity of the microreactor could be maintained by periodic reactivation by treatment with GaCl3.

4.
Nanoscale ; 4(15): 4712-8, 2012 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-22743701

RESUMO

Large-area (∼8000 mm(2)) Au nanogap plasmon resonator array substrates manufactured using maskless laser interference lithography (LIL) with high uniformity are presented. The periodically spaced subwavelength nanogap arrays are formed between adjacent nanopyramid (NPy) structures with precisely defined pitch and high length density (∼1 km cm(-2)), and are ideally suited as scattering sites for surface enhanced Raman scattering (SERS), as well as refractive index sensing. The two-dimensional grid arrangement of NPy structures renders the excitation of the plasmon resonators minimally dependent on the incident polarization. The SERS average enhancement factor (AEF) has been characterized using over 30 000 individual measurements of benzenethiol (BT) chemisorbed on the Au NPy surfaces. From the 1(a(1)), ß(CCC) + ν(CS) ring mode (1074 cm(-1)) of BT on surfaces with pitch λ(g) = 200 nm, AEF = 0.8 × 10(6) and for surfaces with λ(g) = 500 nm, AEF = 0.3 × 10(7) from over 99% of the imaged spots. Maximum AEFs > 10(8) have been measured in both cases.

5.
ACS Appl Mater Interfaces ; 4(2): 935-43, 2012 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-22235899

RESUMO

Exposure of a thin polymer film to a fluid can affect properties of the film such as the density and thickness. In particular in membrane technology, these changes can have important implications for membrane performance. Spectroscopic ellipsometry is a convenient technique for in situ studies of thin films, because of its noninvasive character and very high precision. The applicability of spectroscopic ellipsometry is usually limited to samples with well-defined interfacial regions, whereas in typical composite membranes, often substantial and irregular intrusion of the thin film into the pores of a support exists. In this work, we provide a detailed characterization of a polished porous alumina membrane support, using variable-angle spectroscopic ellipsometry in combination with atomic force microscopy and mercury porosimetry. Two Spectroscopic ellipsometry optical models are presented that can adequately describe the surface roughness of the support. These models consider the surface roughness as a distinct layer in which the porosity gradually increases toward the outer ambient interface. The first model considers the porosity profile to be linear; the second model assumes an exponential profile. It is shown that the models can be extended to account for a composite membrane geometry, by deposition of a thin polysulfone film onto the support. The developed method facilitates practicability for in situ spectroscopic ellipsometry studies of nonequilibrium systems, i.e., membranes under actual permeation conditions.

6.
Nano Lett ; 11(10): 4107-12, 2011 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-21848283

RESUMO

Using conducting probe atomic force microscopy (CAFM) we have investigated the electrical conduction properties of monolayer films of a pentathiophene derivative on a SiO(2)/Si-p+ substrate. By a combination of current-voltage spectroscopy and current imaging we show that lateral charge transport takes place in the plane of the monolayer via hole injection into the highest occupied molecular orbitals of the pentathiophene unit. Our CAFM data suggest that the conductivity is anisotropic relative to the crystalline directions of the molecular lattice.

7.
Phys Rev Lett ; 103(9): 096105, 2009 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-19792812

RESUMO

High resolution diffraction measurements reveal the emergence of anisotropic adatom islands during submonolayer homoepitaxial growth of Cu(001) at grazing incidence. This anisotropy is mimicked well in simulations only after the incorporation of attractive dipolar interactions between the surface and the atoms in the gas phase. The anisotropy of the islands depends markedly on the range of the attractive potential, which allows quantitative insight into the shape of the potential. The role of long- and short-ranged interactions is delineated.

8.
Opt Express ; 17(19): 16969-79, 2009 Sep 14.
Artigo em Inglês | MEDLINE | ID: mdl-19770915

RESUMO

In this paper, we detect and characterize the carbon contamination layers that are formed during the illumination of extreme ultraviolet (EUV) multilayer mirrors. The EUV induced carbon layers were characterized ex situ using spectroscopic ellipsometry (SE) and laser generated surface acoustic waves (LG-SAW). We show that both LG-SAW and SE are very sensitive for measuring carbon layers, even in the presence of the highly heterogeneous structure of the multilayer. SE has better overall sensitivity, with a detection limit of 0.2 nm, while LG-SAW has an estimated detection limit of 2 nm. In addition, SE reveals that the optical properties of the EUV induced carbon contamination layer are consistent with the presence of a hydrogenated, polymeric like carbon. On the other hand, LG-SAW reveals that the EUV induced carbon contamination layer has a low Young's modulus (<100 GPa), which means that the layer is mechanically soft. We compare the limits of detection and quantification of the two techniques and discuss their prospective for monitoring carbon contamination build up on EUV optics.

9.
Appl Opt ; 48(15): 2853-9, 2009 May 20.
Artigo em Inglês | MEDLINE | ID: mdl-19458734

RESUMO

Optical components required for infrared (IR) ellipsometry have distinctly worse characteristics compared to those available for the visible spectrum. The calibration of the optical components used is therefore essential for obtaining reliable results. Here a powerful method is outlined to calibrate simultaneously the polarization characteristics of a source and detector through the synchronous rotation of two polarizers. The performance of this method is to a large degree independent of the quality of (commercially available) polarizers. This renders this method robust and highly suitable for the IR range. Moreover, it is also inherently insensitive toward a nonlinear response of the detector. This enables us to use this method as the first step in the quantification of component imperfections.

10.
J Phys Condens Matter ; 21(22): 224002, 2009 Jun 03.
Artigo em Inglês | MEDLINE | ID: mdl-21715741

RESUMO

Ion sputtering at grazing incidence of the Cu(001) surface leads to the formation of a regular pattern of nanogrooves with a well defined separation distance between the grooves. The grooves are only two atomic layers deep for a low ion flux and their height remains the same independent of sputter time. The average separation distance of the nanogrooves is at least 5 nm and can be increased beyond 40 nm, depending on substrate temperature, fluence, the ion's mass and energy. Anneal experiments of a nanogroove pattern also show an increase in average nanogroove separation with anneal time. The increase of the average nanogroove separation with time is larger for nanogrooves created along a ⟨100⟩ azimuth compared to the ⟨110⟩ azimuth. The ⟨100⟩ oriented step edges show a high density of kinks, suggesting that detachment from kinks is the rate limiting step in the process that governs the periodicity. Also both adatoms and vacancies are involved in this process, while the grazing incident ion beam continuously creates new nanogrooves. The creation of new nanogrooves and the movement observed during annealing are used as ingredients for a description of the temporal behaviour of the average nanogroove periodicity.

11.
J Chem Phys ; 124(20): 204713, 2006 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-16774370

RESUMO

The optical properties of silver nanocrystal arrays are investigated using spectroscopic ellipsometry in combination with polarized reflection measurements. Analysis of the ellipsometry and reflectometry spectra in terms of the "thin island film" theory enables a transparent identification of the contribution of collective effects to the optical response. Negligible image charge effects imply that only dipole contributions have to be considered. The interactions between the hexagonally ordered silver nanocrystals give rise to an effective modification of the spherical response to oblate entities with different polarizabilities parallel and perpendicular to the substrate, expressed in terms of corresponding depolarization factors. The effect of nanocrystal ordering, nearest-neighbor distance, size distribution, surrounding ambient, and the optical properties of the single nanocrystals on the optical response are analyzed. The extent of plasmon resonance peak splitting as a function of surface coverage is discussed.

12.
Phys Rev Lett ; 93(8): 086103, 2004 Aug 20.
Artigo em Inglês | MEDLINE | ID: mdl-15447202

RESUMO

High-resolution LEED (low-energy electron diffraction) data of Cu(001) reveal an uniaxial in-plane lattice reconstruction by 1%. One-dimensional nanogrooves induced by ion bombardment involve the creation of steps that enable this reconstruction. This is the first verification of van der Merwe's prediction of step facilitated reconstruction. We confirm the predicted dependence on step orientation: <100> steps allow stress-relief and <110> steps do not, consistent with the known elastic anisotropy. Similar behavior is predicted for other nonreconstructed (001) surfaces of 3d and 4d metals.

13.
Phys Rev Lett ; 89(14): 146102, 2002 Sep 30.
Artigo em Inglês | MEDLINE | ID: mdl-12366056

RESUMO

Upon Ar+ ion erosion of a Cu(001) surface, we observed with HR-LEED faceted structures on the surface oriented preferentially along <100> at 200 K and along <110> at 330 K. These results are evaluated with a geometric rule, relating under kinetic conditions the orientation of the surviving steps to their advance rates. Application of this generic rule demonstrates that the <100> steps advance slowest. This reveals enhanced interlayer mass transport across these steps. We also show that a transition from kinetically favored step orientation into the thermodynamically favored one occurs at very short time scales even near room temperature.

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