Surface Science Research Papers - Academia.edu (original) (raw)
Surface science has evolved from being a sub-field of chemistry or physics, and has now established itself as an interdisciplinary topic. Knowledge has developed sufficiently that we can now understand catalysis from a surface science... more
Gold nanoparticles protected by a binary mixture of thiolate molecules have a ligand shell that can spontaneously separate into nanoscale domains. Complex morphologies arise in such ligand shells, including striped, patchy, and Janus... more
Gold nanoparticles protected by a binary mixture of thiolate molecules have a ligand shell that can spontaneously separate into nanoscale domains. Complex morphologies arise in such ligand shells, including striped, patchy, and Janus domains. Characterization of these morphologies remains a challenge. Scanning tunneling microscopy (STM) imaging has been one of the key approaches to determine these structures, yet the imaging of nanoparticles' surfaces faces difficulty stemming from steep surface curvature, complex molecular structures, and the possibility of imaging artifacts in the same size range. Images obtained to date have lacked molecular resolution, and only domains have been resolved. There is a clear need for images that resolve the molecular arrangement that leads to domain formation on the ligand shell of these particles. Herein we report an advance in the STM imaging of gold nanoparticles, revealing some of the molecules that constitute the domains in striped and Janus gold nanoparticles. We analyze the images to determine molecular arrangements on parts of the particles, highlight molecular "defects" present in the ligand shell, show persistence of the features across subsequent images, and observe the transition from quasimolecular to domain resolution. The ability to resolve single molecules in the ligand shell of nanoparticles could lead to a more comprehensive understanding of the role of the ligand structure in determining the properties of mixed-monolayer-protected gold nanoparticles.
It is well known that the existing calculations for helium scattering by solid surfaces, made on the first-order distorted-wave Born approximation, within the one-phonon approximation, disagree qualitatively with experiment at low... more
It is well known that the existing calculations for helium scattering by solid surfaces, made on the first-order distorted-wave Born approximation, within the one-phonon approximation, disagree qualitatively with experiment at low energies and temperatures. Generally, the calculations give sticking probabilities which are too low and which have the wrong trend with incident energy: further, the calculations are incorrect in the limit of zero incident energy. We investigate whether those calculations may be modified in order to correct them and to help them agree better with experiment. The modifications we make are, essentially, the introduction of reflection coefficients and the consideration of surface excitations (modes) different from those considered before.
The chemical bonding and geometric structure of acetylene adsorbed on Cu(1 1 0) is analyzed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) in combination with... more
The chemical bonding and geometric structure of acetylene adsorbed on Cu(1 1 0) is analyzed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) in combination with density functional theory (DFT) total energy geometry optimizations and spectral calculations. XPS reveals two different adsorbed species at liquid nitrogen temperature. The molecular alignment is deduced from angle-resolved XAS, revealing that in one site the molecules are aligned with the C-C axis along the [0 0 1] direction and in the other site with an average angle of 35°to the Cu rows. The position of the shape resonance is used to deduce a C-C bond length of 1.35 Å , which is close to the values obtained from the DFT geometry optimizations. XES reveals strong r-p mixing and new occupied states close to the Fermi level, originating from the out-of-plane p* orbital, which becomes occupied upon adsorption in agreement with the Dewar-Chatt-Duncanson model of the bonding.
The epitaxial growth and the structure of Cu on Si(111)7 × 7 deposited at high temperature (< 300–600°C) was investigated mainly by medium energy ion scattering (MEIS) and scanning tunneling microscopy (STM). The domain images whose... more
The epitaxial growth and the structure of Cu on Si(111)7 × 7 deposited at high temperature (< 300–600°C) was investigated mainly by medium energy ion scattering (MEIS) and scanning tunneling microscopy (STM). The domain images whose periodicity is about 5.5 ± 0.2 times of the Si bulk unit were observed at high sample bias (VS = 2.5 V). The periodicity
Techniques have been developed and implemented to gain insight into fundamental factors that affect the performance of electrodes in Li and Li-ion batteries and other energy storage devices. These include experimental strategies for... more
Techniques have been developed and implemented to gain insight into fundamental factors that affect the performance of electrodes in Li and Li-ion batteries and other energy storage devices. These include experimental strategies for monitoring the Raman scattering spectra of single microparticles of carbon and transition metal oxides as a function of their state of charge. Measurements were performed in electrolytes of direct relevance to Li and Li-Ion batteries both in the static and dynamic modes. In addition, novel strategies were devised for performing conventional experiments in ultrahigh vacuum environments under conditions which eliminate effects associated with presence of impurities, using ultrapure electrolytes, both of the polymeric and ionic liquid type that display no measurable vapor pressure. Also examined was the reactivity of conventional non aqueous solvent toward ultrapure Li films as monitored in ultrahigh vacuum with external reflection Fourier transform infrared spectroscopy. Also pursued were efforts toward developing applying Raman-scattering for monitoring the flow of charge of a real Li ion battery. Such time-resolved, spatially-resolved measurements are key to validating the results of theoretical simulations involving real electrode structures. A. PROGRESS MADE DURING THE PERIOD OF PERFORMANCE I. MICROPARTICLE CHARACTERIZATION a. Electrochemical characterization of lithiated transition metal oxide cathode particles in the absence of carbon, binders and other additives-A novel method is herein described for the electrochemical characterization of lithiated transition metal oxides in powder form as cathode materials for lithium ion batteries. The procedure involves application of sufficiently high pressures to a layer of oxide powder evenly dispersed on the surface of a Au foil so as to embed the particles into the soft metal substrate, thereby avoiding the use of carbon, binders and other additives. Implementation of this methodology for battery-grade LiMn 2 O 4 , LiCoO 2 and LiCo 0.15 Ni 0.85 O 2 particles supplied by various commercial sources yielded electrodes of excellent mechanical integrity displaying close to ideal cyclic voltammetric behavior in electrolytes of relevance to battery applications. The charge capacity of each of these compounds could be estimated quantitatively by dissolving the array of oxide particles in acid, once the electrochemical experiments were completed and then determining the amount of metal in the solutions by ICP-MS analysis. An analysis of voltammetric measurements as a function of scan rate yielded values of mean diffusion coefficients for Li in LiMn2O4 on the order of 5 x 10-10 cm 2 s-1. b. Electrochemical and in situ Raman studies of embedded carbon particle electrodes in nonaqueous liquid electrolytes-A method is described for the electrochemical and in situ Raman spectroscopy characterization of particles of graphite and other carbonaceous materials used as lithium intercalation battery anodes. This method relies on high-pressure embedding of such particles on thermally annealed Ni foils and therefore does not require binders or other additives that may interfere with the electrochemical and spectroscopic behavior. The results obtained with embedded KS-44 graphite and MCF28 carbon microfibers in 1 M LiClO 4 in 1:1 (by volume) ethylene carbonate/diethyl carbonate mixtures yielded cyclic voltammetric and in
A new simulation code named projectile surface interaction simulation (PSIS) has been used to study the extension of the collision cascades in sputtering induced by light projectiles. PSIS is based on the binary collision approach in a... more
A new simulation code named projectile surface interaction simulation (PSIS) has been used to study the extension of the collision cascades in sputtering induced by light projectiles. PSIS is based on the binary collision approach in a crystalline target. It is similar to the time independent version of MARLOWE but it takes advantage of the magic formula proposed by Biersack and used in the code TRIM. Results on the simulation of He on polycrystalline barium, con®rms that sputtering is produced mainly by direct emission or short-range cascades. However, a non-negligible fraction of particles are emitted far away from the projectile entrance point and from deep layers. Channeling of the projectile and of target atoms in the solid is proposed to explain these results.
State-resolved two-pulse correlation measurement is useful to study the time scale and mechanism of photo-induced desorption as well as electronic structures of admolecules before desorption. In photodesorption of nitric oxide molecules... more
State-resolved two-pulse correlation measurement is useful to study the time scale and mechanism of photo-induced desorption as well as electronic structures of admolecules before desorption. In photodesorption of nitric oxide molecules from Pt(1 1 1) at 250 K induced by 80 fs, 620 nm laser pulses, the yields and translational temperatures reached a maximum at zero delay time. In contrast, the rotational temperature of X ¼ 3=2 molecules reached a maximum at a delay time of AE1 ps, while that of X ¼ 1=2 molecules was enhanced at around zero delay. In addition, the ratio of total yields of X ¼ 1=2 molecules to that of X ¼ 3=2 molecules increased at short delay times. All of these results can be explained by the model of rotational heating, including spin-orbit (SO) coupling, where the difference in the correlation time was found to be due to the SO structure in the adsorbed phase. : S 0 0 3 9 -6 0 2 8 ( 0 2 ) 0 1 6 5 9 -X
Irradiation of highly dehydrated MgO by UV light in the presence of surface adsorbed hydrogen leads to the formation of particular types of surface colour centres indicated with F+ S (H ) (one-electron, paramagnetic) and F S (H ) (two... more
Irradiation of highly dehydrated MgO by UV light in the presence of surface adsorbed hydrogen leads to the formation of particular types of surface colour centres indicated with F+ S (H ) (one-electron, paramagnetic) and F S (H ) (two electrons, diamagnetic). F S centres are the surface counterparts of the well-known F colour centres formed in the bulk of ionic solids by high-energy irradiation or metal addition. In the particular case of F+ S (H ), the unpaired electron is in magnetic interaction with a nearby proton belonging to a hydroxyl group deriving from H 2 heterolytic dissociative chemisorption (H 2 H++H−) and consequent H+ stabilization on a surface oxide ion. The joint use of EPR, FT-IR and DR-UV-vis spectroscopies has allowed clarification of the mechanism of formation of these centres, which is based on the ionization of adsorbed hydryde groups by UV light and stabilization of the ionized electron into suitable positively charged surface electron traps. A fraction of these traps coincides with the site capable of stabilizing the hydride ion (in the form of bridged Mg 3 H ), which is built up by an array of three Mg2+ ions reproducing a (111) facelet of the oxide. The same site can be also seen as a O2− 3c vacancy (3-coordinated surface anionic vacancy). Ab-initio quantum chemical calculations confirm the proposed assignment, which goes beyond the original model of 5-coordinated surface anion vacancies at the flat (100) MgO plane, which is thus left in favour of a new model that describes the surface electron traps as being localized in less coordinated regions of the surface.
Available online xxxx Bimetallic catalysts have manifold technological applications; their reactivity can greatly exceed that of the original single metals. In this work, we investigate an ideal model-system consisting of a complete... more
Available online xxxx Bimetallic catalysts have manifold technological applications; their reactivity can greatly exceed that of the original single metals. In this work, we investigate an ideal model-system consisting of a complete monolayer of Pd underneath the surface of Au(111). First, we investigate the stability of this system, and then its interaction with hydrogen. We analyze in detail the energetics and the electronic interactions for the Volmer step of the hydrogen evolution reaction (HER), and the further absorption below the surface layer of gold. We combine Density Functional Theory based on computational techniques with the theory of electrocatalysis.
Development of bulk-type all-solid-state lithium-sulfur battery using LiBH4 electrolyte Appl. Phys. Lett. 105, 083901 (2014); 10.1063/1.4893666 Ultrahigh energy density Li-ion batteries based on cathodes of 1D metals with... more
Development of bulk-type all-solid-state lithium-sulfur battery using LiBH4 electrolyte Appl. Phys. Lett. 105, 083901 (2014); 10.1063/1.4893666 Ultrahigh energy density Li-ion batteries based on cathodes of 1D metals with -Li-N-B-N-repeating units in α-Li x BN2 (1 x 3) Novel spectro-electrochemical cell for in situ/operando observation of common composite electrode with liquid electrolyte by X-ray absorption spectroscopy in the tender X-ray region Rev. Sci. Instrum. 85, 084103 (2014); 10.1063/1.4891036 Ab initio study of EMIM-BF 4 crystal interaction with a Li (100) surface as a model for ionic liquid/Li interfaces in Li-ion batteries
The formation of two-component multilayer films on homogeneous solid surfaces is discussed using the mean-field theory and Monte Carlo' simulation. Adsorption systems exhibiting complete and incomplete mixing of components are discussed.... more
The formation of two-component multilayer films on homogeneous solid surfaces is discussed using the mean-field theory and Monte Carlo' simulation. Adsorption systems exhibiting complete and incomplete mixing of components are discussed. It is shown that the structure of adsorbed films depends strongly on the relative strength of the adsorbate-adsorbate and the adsorbate-substrate interactions for both components. In particular, it is demonstrated that the enhanced adsorption of one component due to adsorption of another gas, which is observed in monolayer films, occurs also in multilayer films. It is shown that in the systems considered in this work the displacement of preadsorbed film due to adsorption of the second component may occur via the desorption or via the transfer of molecules to the higher layers. The mechanism of the displacement is shown to depend strongly on the details of gas-gas and gas-solid interactions of both components.
Hydrogen evolution was observed from the noble-metal-free catalyst systems, comprising Rose Bengal, BF xbridged cobaloximes, and triethylamine, in an aqueous solution under irradiation of visible light. Two types of BF x -bridged... more
Hydrogen evolution was observed from the noble-metal-free catalyst systems, comprising Rose Bengal, BF xbridged cobaloximes, and triethylamine, in an aqueous solution under irradiation of visible light. Two types of BF x -bridged cobaloximessnamely, the annulated cobaloximes [Co(dmgBF 2 ) 2 (H 2 O) 2 ] (1, dmgBF 2 ) (difluoroboryl)dimethylglyoximate anion) and [Co(dpgBF 2 ) 2 (H 2 O) 2 ] (2, dpgBF 2 ) (difluoroboryl)diphenylglyoximate anion), and the clathrochelated cobaloximes [Co(dmg(BF) 2/3 ) 3 ](BF 4 ) (3) and [Co(dpg(BF) 2/3 ) 3 ](BF 4 ) (4)swere used as catalysts. Among the four cobalt complexes, complex 1 displayed the highest hydrogenevolving efficiency, with turnovers up to 327. Complexes 2 and 4 that bear the diphenylglyoximate ligands exhibited much lower efficiencies as compared with their analogues 1 and 3 that have the dimethylglyoximate ligands. The hydrogen-evolving efficiency of the annulated cobalt(II) complex 1 that contains two labile axial ligands is more than three times as high as that of the encapsulated cobalt(III) complex 3 that has a single macrobicyclic ligand. The different pathways for formation of the cobalt(I) species from these two types of cobaloximes are discussed on the basis of the results obtained from fluorescence and laser flash photolysis spectroscopic studies.
The interaction of 1,3-butadiene, 1-butene and 2-cis/trans-butenes on the Pt(1 1 1) and Pd(1 1 1) surfaces has been studied with density functional theory methods (DFT). The same most stable adsorption modes have been found on both metal... more
The interaction of 1,3-butadiene, 1-butene and 2-cis/trans-butenes on the Pt(1 1 1) and Pd(1 1 1) surfaces has been studied with density functional theory methods (DFT). The same most stable adsorption modes have been found on both metal surfaces with similar adsorption energies. For 1,3-butadiene the 1,2,3,4-tetra-r adsorption structure is shown to be the most stable one, in competition with a 1,4-metallacycle-type mode, which is only less stable by 10-12 kJ mol À1 . On Pt(1 1 1) these total energy calculations were combined with simulations of the vibrational spectra. This confirms that the 1,2,3,4-tetra-r adsorption is the most probable adsorption structure, but cannot exclude the 1,4metallacycle as a minority species. Although similar in type and energy, the adsorption on the Pd(1 1 1) surface shows a markedly different geometry, with a smaller molecular distortion upon adsorption. The most stable adsorption structure for the butene isomers is the di-r-mode. Similarly to the case of the 1,3-butadiene, the adsorption geometry is closer to the gas phase one on Pd than on Pt, hence explaining the different spectroscopic results, without the previously assumed requirement of a different binding mode. Moreover the present study has shown that the different selectivity observed on Pt(1 1 1) and Pd(1 1 1) for the hydrogenation reaction of butadiene cannot be satisfactory explained by the single comparison of the relative stabilities of 1,3-butadiene and 1-butene on these metals.
Formation of DNA complexes with Langmuir monolayers of cationic lipid octadecylamine (ODA) and new amphiphilic polycation poly-4-vinylpyridine with 16% cetylpyridinium groups (PVP-16) on the surface of native DNA aqueous solution with low... more
Formation of DNA complexes with Langmuir monolayers of cationic lipid octadecylamine (ODA) and new amphiphilic polycation poly-4-vinylpyridine with 16% cetylpyridinium groups (PVP-16) on the surface of native DNA aqueous solution with low ionic strength has been studied. AFM topographic images of DNA/ODA and DNA/PVP-16 complex Langmuir-Blodgett films deposited on the mica substrates were obtained. The complex structures and individual DNA molecules on the amphiphile monolayer surface were observed. The characteristic extended net-like structures and quasi-circular toroidal condensed conformations of the planar DNA complexes were formed in dependence on the polycationic amphiphile monolayer state during the DNA binding. The data obtained give evidence for the effectiveness of monolayer techniques for investigation the mechanisms of DNA complexation with amphiphilic mono-and polycations and demonstrate its perspectives for creation of supramolecular planar DNA-based selforganized nanostructures with nanoscale structural ordering.
The interaction of propene on Pt(1 1 1) has been studied using both slab and cluster models and density functional theory methods. The two most stable chemisorbed structures predicted by the slab calculations are equivalent geometries... more
The interaction of propene on Pt(1 1 1) has been studied using both slab and cluster models and density functional theory methods. The two most stable chemisorbed structures predicted by the slab calculations are equivalent geometries having both the C 1 -C 2 bond parallel to the surface with the centre of this bond above a bridge site. Hence, propene adsorbs on Pt(1 1 1) in a di-r coordination with the C@C double bond parallel to the surface on a short bridge site, in good agreement with the structure suggested from experiment. Results from cluster model calculations carried out for the most stable structure of adsorbed propene are in good agreement with the slab calculations. The binding energy of propene on Pt(1 1 1) is predicted to lie in the $0.4-0.5 eV range, in very good agreement with the experimental value.
Adsorption of benzene on oxygen rich and reduced SnO 2 surfaces is studied by employing density functional theory calculations, slab model and linear combination of atomic orbitals approach. Rather than preferential adsorption sites, it... more
Adsorption of benzene on oxygen rich and reduced SnO 2 surfaces is studied by employing density functional theory calculations, slab model and linear combination of atomic orbitals approach. Rather than preferential adsorption sites, it is found that the adsorption potential energy surface is flat at both surfaces. The bridging oxygen atoms on the stoichiometric surface induce both covalent and ionic bonding leading to weak chemisorptions, whereas bonding on the reduce surface is closer to physisorption. Deformation of the benzene adsorbate due to adsorption is negligible and only small opposite charge transfer is found explaining the differences between the two surfaces.
Potential applications of diamond-like carbon (DLC) coatings range from precision tools and biomedical implants to micro mechanical devices and engine components. Where uniform coatings are required on substrates with complex geometries,... more
Potential applications of diamond-like carbon (DLC) coatings range from precision tools and biomedical implants to micro mechanical devices and engine components. Where uniform coatings are required on substrates with complex geometries, plasma enhanced chemical vapour deposition (PECVD) is often a preferred deposition method. As a non-line of sight process, the geometry of the substrate is often considered negligible. For this reason analysis of PECVD coatings, such as amorphous carbon, has mostly been concerned with reactor deposition variables, such as bias voltage, pressure and gas ratios. Samples are therefore usually prepared and positioned to minimise the influence of other variables. By depositing nominally similar DLC films on silicon samples positioned horizontally and vertically on the reactor cathode plate it was possible to examine the variations in the coating characteristics and mechanical properties that occur due to the geometry of the substrate being coated. Topographic measurements and analysis of bonding structures revealed significant heterogeneity in the coatings. Electron microscopy showed variation in surface structure as well as thickness disparities of up to 50% in the vertical sample. Atomic force microscopy showed roughness, Ra, varied from 0.37 nm to 15.4 nm between samples. Raman spectroscopy highlighted variations in the sp2/sp3 bonding ratios whilst micro wear tests demonstrated how these variations reduce the critical load performance. These effects are explained in terms of the deposition mechanisms involved and are related to variation in deposition species and geometrical field enhancements within the deposition chamber. Improved understanding of these local variations will aid in the optimization of coatings for complex substrate geometries.
The structure of ultrathin NaCl films on Au(1 1 1) and on Au(11 12 12), as well as the one of bimolecular 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) and 1,4-bis-(2,4-diamino-1,3,5,-triazine)-benzene (BDATB) islands on NaCl films on... more
The structure of ultrathin NaCl films on Au(1 1 1) and on Au(11 12 12), as well as the one of bimolecular 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) and 1,4-bis-(2,4-diamino-1,3,5,-triazine)-benzene (BDATB) islands on NaCl films on both surfaces have been studied with a low-temperature scanning tunnelling microscope. We show that intermixed bimolecular assemblies based on selective threefold hydrogen-bonding (H-bonding), that have previously been observed on Au(1 1 1) and on Au(11 12 12), can also be stabilized on insulating NaCl films on Au, however, only if these films are grown on Au(11 12 12) and not on Au(1 1 1). The behaviour of the heterocomplex structures is found to be largely influenced by the structural properties of the underlying substrate and by the number of NaCl layers. On a partly NaCl-covered Au(1 1 1) surface, the excess of molecules after completion of the first layer on Au prefers to form a second molecular layer based on ordered heterocomplex structures rather than to adsorb on the NaCl islands. The use of a vicinal surface together with the strong cohesion characteristic of the NaCl film introduces smooth elastic deformations on the NaCl(0 0 1) plane. As a consequence, the periodically modified structure of the overlayer provides preferential binding sites and allows adsorption of two-dimensional molecular structures. In contrast to what is observed on Au(11 12 12), the molecular domains on the NaCl film do not follow the Au step directions, but the NaCl(0 0 1) high symmetry directions. Our results provide a strategy to increase the adsorption energy of flat molecules on insulating layers by choosing a vicinal metal substrate.
Adsorption of water on an ultra-thin alumina film grown epitaxially onto NiAl(1 1 0) has been investigated by temperature-programmed desorption, work function measurements, ultraviolet photoelectron spectroscopy, and highresolution X-ray... more
Adsorption of water on an ultra-thin alumina film grown epitaxially onto NiAl(1 1 0) has been investigated by temperature-programmed desorption, work function measurements, ultraviolet photoelectron spectroscopy, and highresolution X-ray photoelectron spectroscopy as a function of surface coverage at 100 K. At low coverages monomeric H 2 O species are adsorbed and it is proposed that the oxygen lone pair orbitals interact weakly via polarisation forces with the Al 3þ cations of the second layer. No indication for the dissociation of H 2 O and the formation of OH À species has been found. For intermediate coverages clustering of H 2 O molecules occurs and the growth of a three-dimensional ice layer is observed. Care has to be exercised in photoemission experiments of ice layers, since UV photon-induced dissociation of H 2 O in thicker ice layers is indicated.
Extensive research activity has been devoted to self-assembly of very small coherent islands. However, while island formation is commonly described by a widely used S-K growth scheme, more complex mechanisms based on competitive effects... more
Extensive research activity has been devoted to self-assembly of very small coherent islands. However, while island formation is commonly described by a widely used S-K growth scheme, more complex mechanisms based on competitive effects of kinetics and thermodynamics take place during the epitaxy of Si 1Àx Ge x on Si(0 0 1). The aim of this paper is to explain the formation and the evolution of Si 1Àx Ge x islands on Si(0 0 1). The paper presents a comprehensive investigation of the different growth modes of Si 1Àx Ge x films (with x varying from 0 to 1) on Si(0 0 1) and Si(1 1 1). The results are presented in the form of kinetic morphological growth diagrams of as-grown samples. Two and four growth regimes are distinguished on (1 1 1) and (0 0 1) respectively. These growth regimes correspond to different levels of relaxation. In particular the four regimes observed on Si(0 0 1) correspond to (i) no relaxation in regime I (2D layer), (ii) 15-20% relaxation in regime II (''huts'' islands with (1 0 5) facets), (iii) 20% and 50% relaxation in regime III (in ''huts'' and ''domes'' respectively) and (iv) 50% and 80% relaxation in regime IV (''domes'' with bimodal size distribution). Every growth regime characteristic of as-grown sample is also associated with a specific equilibrium steady state morphology which is obtained after long-term annealing of the as-grown samples. In the two first regimes (no or small strain relaxation) the equilibrium morphology of highly strained Si 1Àx Ge x deposits corresponds to (1 0 5) faceted islands. We show that these islands are stabilised by the compressive stress. As soon as strain is released, (1 0 5) facets disappear at the expense of the (1 1 3) and (1 1 1) facets and first-order transition occurs between ''huts'' and ''domes'' islands.
The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(111) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low energy electron microscopy, x-ray... more
The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(111) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low energy electron microscopy, x-ray absorption and high resolution core level spectroscopy. The study shows that a single boron nitride layer is formed on Pt(111), resulting in a coincidence structure. High resolution scanning tunneling microscopy (STM) images of the h-BN ultrathin film display only one of the atomic species in the unit cell. Probing the boron and nitrogen related local density of states by near edge x-ray adsorption fine structure measurements we conclude that the nitrogen sublattice is visible in STM images. The growth of the single hexagonal boron nitride layer by vapourized borazine in the pressure range of 1 × 10 −6 to 1 × 10 −8 at 800 • C is further studied by low energy electron microscopy, and reveals that the number of nucleation sites and the perfection of the growth is strongly pressure dependent. A model for the single, hexagonal, boron nitride layer on Pt(111)is proposed.
The interaction of a CO molecule with the Fe(100) surface is analyzed by the topology of the Laplacian of the electronic charge density −V2r. This analysis shows that the atomic graph of the top atoms of the Fe(100) surface is a cube that... more
The interaction of a CO molecule with the Fe(100) surface is analyzed by the topology of the Laplacian of the electronic charge density −V2r. This analysis shows that the atomic graph of the top atoms of the Fe(100) surface is a cube that exposes a face, F top , with a ''hole'' of charge or (3,+1) critical point of −V2r, whereas the corresponding graph for the second layer atoms is an octahedron that exhibits a vertex, V sec , protruding above the Fe(100) surface, with a ''peak'' of charge or (3,−3) critical point of −V2r. Atomic graphs determined for the CO molecule show that the carbon atom has a non-bonded vertex, C Vnb , and a torus, C torus , of charge depletion perpendicular to the C-O bond direction. As the CO molecule approaches the surface, electron transfer towards the tilted CO in turn induces two non-bonded critical points, O Vnb , on the O atom. In accordance with experimental and theoretical results, the topological theory of the Laplacian suggests that the preferred pathway for CO dissociation corresponds to the tilted orientation on the surface, in which the attractive interaction C torus -V sec is enhanced by the interaction of the two O Vnb with the F top on two Fe top atoms of the (100) surface.
We report UPS, work-function and LEED results on the interfacial reaction between evaporated Ta and Si(111)(7• surface under ultrahigh vacuum conditions. At room temperature, a disordered chemisorbed phase is formed at low coverage, 0 _<... more
We report UPS, work-function and LEED results on the interfacial reaction between evaporated Ta and Si(111)(7• surface under ultrahigh vacuum conditions. At room temperature, a disordered chemisorbed phase is formed at low coverage, 0 _< 1 monolayer (ML), and is characterized by an UPS Ta 5d peak shifted at about -1.2 eV as compared to bulk Ta. For 1 < 0 ~ 4 ML, although AES and XPS indicate a 1 Ta : 2 Si composition, the valence band spectra are still different from the TaSi 2 one. At higher coverage, the electronic structure of polycrystalline Ta is progressively recovered. Annealing of the deposits leads to silicide formation by interdiffusion and surface atomic ordering at temperatures from 650 ~ to 850 o C, depending on the thickness. All the deposits in the 0.8-100 ML range, including the smallest ones, are stable until -500 ~ and the silicide formation reaction begins only at higher temperature, reflecting an activation energy barrier. For 0 ~ 5 ML, clustering of the TaSi 2 phase occurs. For thicker deposits, continuous TaSi/overlayers are obtained. The electronic structure of this silicide is discussed in relation with existing models.
The structure, growth and stoichiometry of heteroepitaxial Pr 2 O 3 films on Si(1 1 1) were characterized by a combined RHEED, XRD, XPS and UPS study in view of future applications as a surface science model catalyst system. RHEED and XRD... more
The structure, growth and stoichiometry of heteroepitaxial Pr 2 O 3 films on Si(1 1 1) were characterized by a combined RHEED, XRD, XPS and UPS study in view of future applications as a surface science model catalyst system. RHEED and XRD confirm the growth of a (0 0 0 1) oriented hexagonal Pr 2 O 3 phase on Si(1 1 1), matching the surface symmetry by aligning the h1 0 1 0i oxide in-plane direction along the h0 1 1i Si azimuth. After an initial nucleation stage RHEED growth oscillation studies point to a Frank-van der Merwe growth mode up to a thickness of approximately 12 nm. XPS and UPS prove that the initial growth of the Pr 2 O 3 layer on Si up to $1 nm thickness is characterized by an interface reaction with Si. Nevertheless stoichiometric Pr 2 O 3 films of high crystalline quality form on top of these Pr-silicate containing interlayers.
The Auger electron spectroscopy study on chemistry of the 46.8°(111) twist grain boundary of an Fe-2.3%V alloy showed an extended phosphorus enrichment at temperatures in range of 500°C and 800°C. Simultaneously, slight but nearly... more
The Auger electron spectroscopy study on chemistry of the 46.8°(111) twist grain boundary of an Fe-2.3%V alloy showed an extended phosphorus enrichment at temperatures in range of 500°C and 800°C. Simultaneously, slight but nearly independent segregation of vanadium was also detected. The standard enthalpy and entropy of grain boundary segregation of phosphorus and vanadium were determined according to the Guttmann model of multicomponent interfacial segregation. Obtained data clearly show that this Σ = 19 coincidence boundary is special (i.e. low energy interface). The data also fit well with the predictive model of grain boundary segregation and confirm that phosphorus segregates interstitially at the grain boundary while vanadium substitutes iron atoms in the interface structure.
The formation and the growth of Sn films on CdTe(ll1) have been studied by hemispherically recorded ARXPS. The XPD patterns show that these films consist of cc-Sn. XPD and ARXPS depth profile analysis reveal the presence of an intermixing... more
The formation and the growth of Sn films on CdTe(ll1) have been studied by hemispherically recorded ARXPS. The XPD patterns show that these films consist of cc-Sn. XPD and ARXPS depth profile analysis reveal the presence of an intermixing zone at the interface which establishes during the initial stages of Film formation.
Low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and the normal incidence X-ray standing wave (NIXSW) technique have been used to determine the structure of the first surface phase formed on Cu{1 0 0} upon... more
Low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and the normal incidence X-ray standing wave (NIXSW) technique have been used to determine the structure of the first surface phase formed on Cu{1 0 0} upon deposition of 0.21 monolayers (ML) of tin. At this coverage the LEED images exhibits a modified p(2 · 2) structure with split fractional order diffraction spots. A double scattering LEED pattern simulation consistent with both the Sn surface coverage and the complex split beam LEED pattern suggests an ordered phase based on a p(2 · 2) structure with light anti-phase domain walls. Possible adsorption sites are identified from NIXSW data by triangulation of the distances between the Sn atoms and the (2 0 0) and (1 1 1) planes. The results indicate that Sn atoms adsorb in a fourfold site but does not distinguish between overlayer and substitutional sites. The STM data reveal small domains of p(2 · 2) structure formed by the Sn atoms being adsorbed in substitutional sites. These domains typically contain 16 Sn atoms arranged in a square 4 · 4 array, however larger and smaller rectangular domains are also observed with sides containing three, five and six Sn atoms. The width of the domain wall is found to vary with rows of one, three or five copper atoms separating the domains in both the [0 1 1] and [0 1 1 1] directions. The STM images indicate that the Cu atoms in the domain walls are vertically displaced from their expected positions. This is attributed to elastic strain due to the large metallic diameter of Sn (0.162 nm) relative to Cu (0.127 nm).
We report on the characteristics of metal-insulator-semiconductor (MIS) capacitors with aluminum nitride (AlN) as the insulator material. AlN has been grown on (1 1 1) Si by means of molecular beam epitaxy (MBE) and DC magnetron... more
We report on the characteristics of metal-insulator-semiconductor (MIS) capacitors with aluminum nitride (AlN) as the insulator material. AlN has been grown on (1 1 1) Si by means of molecular beam epitaxy (MBE) and DC magnetron sputtering (SPU). AlN layers have been characterized before and after dry thermal oxidation in O 2 . By analyzing changes in morphology and electrical properties, different oxidation mechanisms were identified, due to the crystalline quality difference of the AlN samples. In both cases, oxidation at 1000°C was beneficial for the electrical characteristics of the MIS structures, presumably due to passivation of atom vacancies. Although AlN was only partially oxidized, the flat-band voltage was reduced and the density of interface traps improved. Dominant conduction mechanism was Poole-Frenkel for the SPU sample, and changed to hopping after oxidation.
Epitaxial Ag ultra-thin films grown on Au(1 1 1) have been studied by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). In the submonolayer range, the spectroscopic measurements reveal the... more
Epitaxial Ag ultra-thin films grown on Au(1 1 1) have been studied by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). In the submonolayer range, the spectroscopic measurements reveal the existence of two parabolic dispersive surface Shockley bands associated with the Au substrate and the Ag islands, respectively. The surface state in the Ag region is shifted by about 160 meV towards the Fermi level. High resolution measurements on Au(1 1 1) reveal the splitting of each band in two spin-orbit contributions. A reduction of this spin-orbit splitting is observed for a 1 ML Ag film. Moreover, an annealing of the interface leads to an interdiffusion and to an additional shift of the surface state.
A periodic Density Functional Theory (DFT) study using Generalized Gradient Approximation (GGA) of the Ti deposition on a clean Si (100) surface was carried out. The results indicate that Ti adsorbs preferentially on two Si dimers forming... more
A periodic Density Functional Theory (DFT) study using Generalized Gradient Approximation (GGA) of the Ti deposition on a clean Si (100) surface was carried out. The results indicate that Ti adsorbs preferentially on two Si dimers forming polar covalent bonds with four Si atoms. The analysis of the Density of states (DOS) indicates that Ti 3d orbitals hybridize with the surface orbitals near the Fermi level and each Ti atom transfers one electron to the surface even at concentration of 6.8 × 10 14 Ti atom cm − 2 . At this concentration, a quite stable TiSi monolayer is formed and subsequent additions of Ti atoms would initiate metallic Ti growth on the TiSi interface.
... studies [14 and 17] have shown that water dissociatively adsorbed on TiO 2 (100) 1×1 surfaces at 300 K. Based on their results, for the defect-free surface, the O 1s peak broadening after water exposure is assumed to be solely due to... more
... studies [14 and 17] have shown that water dissociatively adsorbed on TiO 2 (100) 1×1 surfaces at 300 K. Based on their results, for the defect-free surface, the O 1s peak broadening after water exposure is assumed to be solely due to the hydroxyl species (OH) adsorbed on the ...
"A target of Fe-40 at.-%Al intermetallic alloy with ordered B2 structure was subjected to laser melting processing by a high energy XeCI excimer pulse (wavelength 308 nm, pulse length 120 ns) in low pressure air. The total thickness of... more
"A target of Fe-40 at.-%Al intermetallic alloy with ordered B2 structure was subjected to laser melting processing by a high energy XeCI excimer pulse (wavelength 308 nm, pulse length 120 ns) in low pressure air. The total thickness of the laser affected zone (LAZ) was ∼ 150 nm. The modified surface showed an increased roughness and the presence of cracking. The X-ray photoelectron spectroscopy (XPS) measurements revealed a strong enrichment in the aluminium concentration within the LAZ, as well as relatively high contents of oxygen and nitrogen incorporated in the near surface region. Both angle resolved and depth profile XPS analyses suggested that oxides, nitrides, and oxynitrides were mainly present within the LAZ as discrete particles in the metallic matrix rather than as surface layers. Significant surface hardness reduction was observed after laser treatment, which has been interpreted to be due to partial suppression of B2 lattice ordering. Electrochemical measurements in borate buffer solution showed a reduced anodic activity of the laser processed aluminide in the potential range between the open circuit potential and 0.1 V saturated calomel electrode, whereas at higher anodic overpotentials no substantial differences in behaviour were observed with respect to the untreated Fe–40Al surfaces. Such consequences of the excimer laser treatment may be explained by mechanisms involving aluminium enrichment and nitride formation processes inside the LAZ."
A core level and valence band photoemission study of thick 3C±SiC(1 1 1) and 3C±SiC(" 1 " 1 " 1) epilayers grown by sublimation epitaxy is reported. The as introduced samples show threefold 1 Â 1 low-energy electron diraction patterns.... more
A core level and valence band photoemission study of thick 3C±SiC(1 1 1) and 3C±SiC(" 1 " 1 " 1) epilayers grown by sublimation epitaxy is reported. The as introduced samples show threefold 1 Â 1 low-energy electron diraction patterns. For the Si face 3 p and 6 3 p reconstructed surfaces develop after in situ heating to 1100°C and 1300°C, respectively. For the C face a 3 Â 3 reconstruction form after heating to 980°C. A semiconducting behavior is observed for the 3 p and 3 Â 3 reconstructed surfaces while the 6 3 p reconstruction show a Fermi edge and thus a metallic-like behavior. The surface state on the 3 p surface is investigated and found to have K 1 symmetry and a total band width of 0.10 eV within the ®rst surface Brillouin zone. For the Si 2p and C 1s core levels binding energies and surface shifted components are extracted and compared to earlier reported results for 6H± and 4H±SiC.
We have carried out extensive ab initio calculations on the energetics of gold atoms deposited on graphite surfaces, using different geometries and approximations. We consistently find that gold adatoms easily diffuse over this surface... more
We have carried out extensive ab initio calculations on the energetics of gold atoms deposited on graphite surfaces, using different geometries and approximations. We consistently find that gold adatoms easily diffuse over this surface with a diffusion barrier of 0.05 eV. The adsorption energy is found to be more difficult to quantify within density functional theory. Combining our calculations with experimental values, we can estimate this adsorption energy to be of order 0.45 eV.
Silicon (Si) wettability is one of the important parameters in the development of Si-based biosensing and lab-on-chip devices. We report on UV laser induced hydrophobicity of Si (001) wafers immersed in methanol during the irradiation... more
Silicon (Si) wettability is one of the important parameters in the development of Si-based biosensing and lab-on-chip devices. We report on UV laser induced hydrophobicity of Si (001) wafers immersed in methanol during the irradiation with an ArF excimer laser. The irradiation with 800 pulses of the laser operating at 65 mJ/ cm 2 allowed to significantly increase the hydrophobicity of investigated samples as characterized by the static contact angle change from 77° to 103°. Owing to the irradiation with relatively low laser fluence, no measurable change in surface morphology of the irradiated samples has been observed with atomic force microscopy measurements. The nature of the hydrophobic surface of investigated samples is consistent with X-ray photoelectron spectroscopy analysis that indicates formation of Si–O–CH 3 bonds on the surface of the laser-irradiated material.
The load dependence of the Vickers microhardness on the as-grown (0 1 0) and (0 0 1), and cleaved (0 0 1) faces of cadmium tartrate pentahydrate (CTPH) single crystals has been investigated. The experimental results showed that, with an... more
The load dependence of the Vickers microhardness on the as-grown (0 1 0) and (0 0 1), and cleaved (0 0 1) faces of cadmium tartrate pentahydrate (CTPH) single crystals has been investigated. The experimental results showed that, with an increase in the applied load, the microhardness of the as-grown (0 1 0) and (0 0 1) faces decreases, while that of the unheated and heated (0 0 1) cleavage faces decreases first up to a load of 2.5 N and then increases. Analysis of the experimental results revealed that: (1) radial crack length, indentation size and applied indentation load are mutually related, and these dependences related with fracture mechanics are the basis of Meyer's empirical law, (2) with increasing indentation load, changes in the mechanism of development of indentation cracks from radial cracks to lateral cracks and surface chipping of the material, followed by predominantly surface chipping of the material are responsible for indentation size effect in CTPH crystals, (3) proportional specimen resistance model and Meyer's law not only explain the indentation size effect but also can be used to determine load-independent hardness H * , and (4) there is no direct relationship between microhardness and fracture toughness of different CTPH samples, while the values of load-independent hardness H * , and brittleness indices b and B of CTPH crystals increase linearly with the Meyer constant A. Procedures are given to determine load-independent hardness H * from the transition values of load and corresponding indentation size.
It is well known that the existing calculations for helium scattering by solid surfaces, made on the first-order distorted-wave Born approximation, within the one-phonon approximation, disagree qualitatively with experiment at low... more
It is well known that the existing calculations for helium scattering by solid surfaces, made on the first-order distorted-wave Born approximation, within the one-phonon approximation, disagree qualitatively with experiment at low energies and temperatures. Generally, the calculations give sticking probabilities which are too low and which have the wrong trend with incident energy: further, the calculations are incorrect in the limit of zero incident energy. We investigate whether those calculations may be modified in order to correct them and to help them agree better with experiment. The modifications we make are, essentially, the introduction of reflection coefficients and the consideration of surface excitations (modes) different from those considered before.
Nanocrystalline sulfated tin oxide with different sulfate contents (5-30 wt%) were prepared from hydroxylated tin oxide obtained by the precipitation method, followed by wet impregnation with SO 4 2− species using a sulfuric acid... more
Nanocrystalline sulfated tin oxide with different sulfate contents (5-30 wt%) were prepared from hydroxylated tin oxide obtained by the precipitation method, followed by wet impregnation with SO 4 2− species using a sulfuric acid solution. In samples calcined at 400, 500 and 600 • C the characterization of the solids was made by DTA, XRD and nitrogen adsorption. The strength and number of acid sites were determined by nonaqueous titration of n-butylamine in acetonitrile. Both Bronsted and Lewis acid sites were determined by FTIR spectra of pyridine adsorbed. Sulfated tin oxides were tested in the synthesis of 7-hydroxy-4-methyl coumarin via solvent free Pechmann reaction of resorcinol:ethylacetoacetate (molar ratio 1:2), at 120 • C. DTA measurements showed that SO 4 2− addition tends to slow down the formation of SnO 2 crystallites. XRD profiles showed that the sulfating inhibits the SnO 2 crystal growth. The SO 4 species remained strongly bonded at the SnO 2 surface stabilizing its crystallite size against sinter-ing and it acts as a structure porogen director mediating nanoparticle growth and assembly yielding a mesostructured form of SnO 2 with wormhole morphology and high thermal stability. The surface areas of the investigated samples were influenced with the sulphate content and calcination temperature. The acidity measurements showed that the total acidity increases with the rise of sulfate content up to 25 wt%. FTIR spectra of pyridine adsorbed on the catalysts showed the presence of both Bronsted and Lewis acid sites. The formation of 7-hydroxy-4-methyl coumarin increases with the increase of surface acidity showing a maximum when the sulfate content and calcination temperature were 25 wt% and 400 • C, respectively.
- by wafaa S . Abo El-Yazeed and +1
- •
- Materials Science, Surface Science, Catalysis, Nanocatalyst
Epitaxial iron oxide films are grown on Ag(1 1 1) by two methods. The oxide films are characterized using low-energy electron diffraction, X-ray photoelectron spectroscopy, and X-ray photoelectron diffraction. The first growth method is... more
Epitaxial iron oxide films are grown on Ag(1 1 1) by two methods. The oxide films are characterized using low-energy electron diffraction, X-ray photoelectron spectroscopy, and X-ray photoelectron diffraction. The first growth method is deposition of Fe films with thickness ranging from 1-10 monolayers on Ag(1 1 1) and oxidizing these films. The Fe film structure prior to oxidation is identified as poorly ordered bcc-Fe(1 1 0), and oxidation leads to the growth of poorly ordered FeO(1 1 1) films. The second method is sequential deposition of sub-monolayer Fe films (typically 60.5 ML) followed by oxidation. This procedure is repeated until the desired oxide film thickness is achieved. Iron oxide films grown by sequential deposition are identified as FeO(1 1 1) for oxide film thickness below $10 Å with growth of Fe 3 O 4 (1 1 1) for thicker films. Iron oxide films grown by the sequential deposition method have much better crystallographic order than those grown by oxidizing thicker iron films. Finally, the surface termination of the Fe 3 O 4 (1 1 1) films is investigated using X-ray photoelectron diffraction.
The physical significance of the difference between surface tension, g, and surface stress, fir =&lo + aujae,, with & the Kronecker delta, is investigated. It is concluded that the strain et, concerned is not the total strain, which may... more
The physical significance of the difference between surface tension, g, and surface stress, fir =&lo + aujae,, with & the Kronecker delta, is investigated. It is concluded that the strain et, concerned is not the total strain, which may be composed of elastic and plastic contributions, but only the elastic component of the strain. It follows that the value of the surface stress depends on experimental conditions and it is concluded that measurements of surface stress are possible from the effect of the hydrostatic pressure due to all curved surfaces as well as from the mechanical forces exerted by planar surfaces of crystals, amorphous substances and liquids when the strain rates are sufficiently large so as to inhibit plastic deformation, causing the strains to be elastic. Conversely measurements of surface tension via mechanical forces exerted by surfaces require sufficiently slow strain rates so that all strains utilized in the experiment be fully plastic.
Adsorption of the tripeptide, L-glutathione or GSH, was performed from vapour phase on Au(1 1 1) and monitored in situ by polarisation modulation reflection absorption infrared spectroscopy (PM-RAIRS). The surface was analysed by... more
Adsorption of the tripeptide, L-glutathione or GSH, was performed from vapour phase on Au(1 1 1) and monitored in situ by polarisation modulation reflection absorption infrared spectroscopy (PM-RAIRS). The surface was analysed by photoelectron spectroscopy (XPS) at two stages of adsorption, 5 or 50 min of exposure at P = 1 Â 10 À7 Pa; to do so, the sample was cooled down to 100 K before X-ray analysis; it was then warmed up to RT, and analysed again, in order to discriminate between strongly and weakly bound molecules. From the beginning of exposure and up to 55 min, the molecule is mainly adsorbed under a dimeric or cluster form, leading to an average coverage close to one monolayer. Compared to adsorption from aqueous phase that favours the anchoring of GSH molecules via the S atoms under a zwitterionic form, here one observes that part of the molecules bears neutral acid and amino groups and keeps their SH group intact. XPS data show the coexistence of unbound and Au-bound sulphur, the former type of species slightly decreasing upon warming the sample to RT. Heating to 500 K leads to an almost total desorption of the GSH layer.
The role of the silanization of silicon oxide (SiO x ) with different alkyl-silane molecules, in determining the morphology and structure of the overlying poly(3-hexylthiophene) (P3HT) layer has been studied by atomic force microscopy... more
The role of the silanization of silicon oxide (SiO x ) with different alkyl-silane molecules, in determining the morphology and structure of the overlying poly(3-hexylthiophene) (P3HT) layer has been studied by atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. Particular attention has been paid to the first thin layers close to the interface P3HT/SiO x . For each case the effect of the annealing temperature has been studied.
Bulk characteristics and deposition of gold nanoparticles on mica modified by (PAH) was studied byUV–visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM),dynamic light scattering (DLS) and... more
Bulk characteristics and deposition of gold nanoparticles on mica modified by (PAH) was studied byUV–visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM),dynamic light scattering (DLS) and atomic force microscopy (AFM). The size of gold nanoparticles (AuNPs)was 15 nm as determined by TEM and AFM. The electrophoretic mobilities and electrokinetic chargeof particles were quantitatively evaluated for a broad range of pH and ionic strength. Using Henry’smodel, it was calculated that the zeta potential of particles varied between − 50 mV and − 80 mV forpH 2 and pH 11, respectively (at 10−2M of NaCl). Measurements of nanoparticle deposition kineticswere performed under diffusion-controlled transport conditions using AFM and SEM imaging of particlemonolayers. The influence of the bulk suspension concentration was systematically studied. Additionally,the maximum coverage of particle monolayers, which monotonically increased with ionic strength, wasdetermined by SEM. The obtained data were in agreement with theoretical predictions derived from therandom sequential adsorption (RSA) model. It was also confirmed that by varying the bulk suspensionconcentration and ionic strength one can prepare homogeneous gold particle monolayers of controlledcoverage.
NbN superconductor and wide band gap AlN thin films were deposited using sputtering at room temperature. Study of the nitride interfaces are forerunner to the growth Josephson junctions that are considered able to work in the terahertz... more
NbN superconductor and wide band gap AlN thin films were deposited using sputtering at room temperature. Study of the nitride interfaces are forerunner to the growth Josephson junctions that are considered able to work in the terahertz frequency. We find that to be compatible with lithography technology and to have a high critical transition temperature, the substrate should not be overheated, and this means working in low power regime to limit the induced heating of the plasma. X-ray photoelectron spectroscopy and X-ray diffraction analysis were performed on samples deposited on crystalline, amorphous, flexible, and nanostructured substrates. The experimental results suggest us how to improve the deposition process in order to obtain the best nitride films as well as NbN/AlN/NbN trilayers for Josephson junction applications.
The connection between the way of development and appearance of mesa structures and typical hillocks developing during anisotropic etching of monocrystalline silicon in 5 M KOH saturated with isopropanol has been studied both... more
The connection between the way of development and appearance of mesa structures and typical hillocks developing during anisotropic etching of monocrystalline silicon in 5 M KOH saturated with isopropanol has been studied both theoretically and experimentally. A simple geometrical model of mesa formation was presented. Then, the process of hillock growth in result of mesa transformation caused by etch mask underetching was analyzed. At the second part of the paper, the model of hillocks formation was verified experimentally. For this purpose, a series of experiments with etching of mini-mesas through the masks with various patterns in the conditions favoring hillock formation was carried out. It was shown that at the final stage of the etching process the mesas transform into hillocks, bounded by the planes close to {1 1 1}. Probably at the first stage of hillocks formation, mini-mesas develop. Such stable mini-mesas, produced in the presence of random micro-masks, may transform into typical hillocks in effect of successive releasing of {1 1 1} mini-faces. It seems that the process of hillock formation is similar to mesa etching. The shapes of hillocks produced on the substrates with different from (0 0 1) crystallographic orientations have been also analyzed. The similarities of the hillocks and intentionally prepared mesas were underlined. It was stated that the morphologies of hillocks are strictly connected with crystallographic orientation of etched substrate. Different shapes of hillocks developing on the substrates with different crystallographic orientations, reflect the arrangement and inclination of {1 1 1} planes on considered substrate.