Publications (37)52.82 Total impact
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Article: Influence of metallic and dielectric nanowire arrays on the photoluminescence properties of P3HT thin films.
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ABSTRACT: The optical properties of organic semiconductor thin films deposited on nanostructured surfaces are investigated using time-resolved two-photon photoluminescence (PL) microscopy. The surfaces consist of parallel aligned metallic or dielectric nanowires forming well-defined arrays on glass substrates. Keeping the nanowire dimensions constant and varying only their spacing from 40 to 400 nm, we study the range of different types of nanowire-semiconductor interactions. For silver nanowires and spacings below 100 nm, the PL intensity and lifetime of P3HT and MDMO-PPV decrease rapidly due to the short-ranged metal-induced quenching that dominates the PL response with respect to a possible plasmonic enhancement of optical transition rates. In the case of P3HT however, we observe an additional longer-ranged reduction of non-radiative losses for both metallic and dielectric nanowires that is not observed for MDMO-PPV. Excitation polarization dependent measurements indicate that this reduction is due to self-assembly of the P3HT polymer chains along the nanowires. In conclusion, nanostructured surfaces, when fabricated across large areas, could be used to control film morphologies and to improve energy transport and collection efficiencies in P3HT-based solar cells.Nanotechnology 07/2012; 23(30):305402. · 3.98 Impact Factor -
Article: Bonding at copper–alumina interfaces established by different surface treatments: a critical review
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ABSTRACT: The present study summarizes the effects of processing conditions and substrate cleaning procedures on the structure, chemistry and bonding of Cu/(0001)Al2O3 interfaces as determined by advanced transmission electron microscopy techniques. The Cu/(0001)Al2O3 samples were prepared by molecular beam epitaxy (MBE) and solid-state diffusion bonding. Investigations of the MBE samples showed that the Al2O3 cleaning procedure alters the interfacial bonding. Metallic bonds occurred for an Ar+ ion sputtering and subsequent ultra-high vacuum (UHV) annealing treatment. Strongly ionic–covalent bonds were found for a wet chemical cleaning process followed by UHV annealing. The interfacial electronic structure did not reveal any significant changes compared to the bulk electronic structure for samples where the substrate surface was annealed in an oxygen-containing atmosphere after Ar+ ion pre-sputtering and UHV annealing. The results obtained at the solid-state diffusion-bonded Cu/Al2O3 samples indicated that the processing parameters such as temperature and load do not change the bonding behavior. Post-annealing of the solid-state diffusion-bonded Cu/Al2O3 samples in a well-defined oxygen partial pressure led to the formation of CuAlO2 at the interface between Cu and Al2O3, which improved the adhesion.Journal of Materials Science 04/2012; 41(16):5161-5168. · 2.02 Impact Factor -
Article: Quantitative Analysis of Layering and In-Plane Structural Ordering at an Alumina-Aluminum Solid-Liquid Interface, Acta Materialia
Acta Materialia 01/2011; 59(11):4378-4386. · 3.76 Impact Factor -
Chapter: Analytical TEM investigations of Pt/YSZ interfaces
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ABSTRACT: Metal-ceramic interfaces are of fundamental importance for a variety of industrial applications and are also of scientific interest. The physical and chemical properties of several technologically relevant nano-structural materials and devices are strongly affected and controlled by the presence of interfaces between the components. Already small amounts of impurities at the interface can dramatically change the properties of the system.12/2007: pages 369-370; -
Article: Interface structure and strain development during compression tests of Al2O3/Nb/Al2O3 sandwiches
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ABSTRACT: The interface structure of an Al2O3/Nb/Al2O3 sandwich produced by solid-state diffusion bonding was investigated in detail by various transmission electron microscopy (TEM) methods. The joint possessed at one interface a _boxclose_Nb (0001)_Al_2 O_3 {\hbox{(110)}}_{{{{\rm Nb}}}} {{ \,||\, (0001)}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} , $ {\hbox{[1{{$ {\hbox{[1{{, and on the other interface a $ {\hbox{(1{{$ {\hbox{(1{{ and $ {\hbox{[1{{$ {\hbox{[1{{ orientation relationship. At both interfaces, misfit dislocations formed to compensate the lattice mismatch as found by high-resolution transmission electron microscopy (HRTEM). Electron energy-loss near edge structure (ELNES) studies revealed that the interface is terminating with an Al layer resulting in Al–Nb bonds. Identical sandwiches were investigated on the meso- and macroscopic scale by performing compression tests and simultaneously monitoring the strain development at (001)Nb and _boxclose_boxclose0)_Nb (1{{\bar{{{1}}}}}0)_{{\rm Nb}} crystal faces. The full-field optical strain measurements (FFOM) revealed that the strain is localized at the interfaces when observed at the (001)Nb face while it is along the maximum shear directions of 36–54° inclined to the interface when observed at the (1[`1]0) (1{{\bar{{{1}}}}}0) face. The strain localization along a specific maximum shear direction results in the cleavage of Al2O3, always initiating from the interface possessing the (110)Nb ||(0001)Al2 O3 {\hbox{(110)}}_{{{{\rm Nb}}}} {{ \,||\, (0001)}}_{{{{\rm Al}}_{{{\rm 2}}} {{\rm O}}_{{{\rm 3}}} }} and $ {\hbox{[1{{$ {\hbox{[1{{ orientation relationship.Journal of Materials Science 11/2006; 41(23):7798-7807. · 2.02 Impact Factor -
Article: Ordered liquid aluminum at the interface with sapphire.
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ABSTRACT: Understanding the nature of solid-liquid interfaces is important for many processes of technological interest, such as solidification, liquid-phase epitaxial growth, wetting, liquid-phase joining, crystal growth, and lubrication. Recent studies have reported on indirect evidence of density fluctuations at solid-liquid interfaces on the basis of x-ray scattering methods that have been complemented by atomistic simulations. We provide evidence for ordering of liquid atoms adjacent to an interface with a crystal, based on real-time high-temperature observations of alumina-aluminum solid-liquid interfaces at the atomic-length scale. In addition, crystal growth of alumina into liquid aluminum, facilitated by interfacial transport of oxygen from the microscope column, was observed in situ with the use of high-resolution transmission electron microscopy.Science 11/2005; 310(5748):661-3. · 31.20 Impact Factor -
Article: Structures of BaF2-CaF2 heterolayers and their influences on ionic conductivity.
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ABSTRACT: Recently, artificial ion conductors have been prepared by growing epitaxial heterolayers consisting of BaF2-CaF2 using molecular beam epitaxy. The ionic conductivity of these heterolayers shows a strong dependence on the layer thickness [N. Sata, S. Eberman, K. Eberl, and J. Maier, Nature 408, 996 (2000)]. In this paper three such heterolayers with different spacings (sample A: 80 nm, sample B: 10 nm, sample C: 1 nm) are investigated by conventional transmission electron microscopy and high-resolution transmission electron microscopy. The spacings are chosen such that they fall into the three conductivity regimes observed in N. Sata et al. (l > 50 nm; 8 < l < 50 nm; l < 8 nm). In accordance with conductivity studies, the samples with spacings of 10 nm or greater (A,B) are epitaxial and continuous, whereas in the case of extremely small spacing (C) the continuity of the layers is destroyed by formation of a column-like structure. Analytical electron microscopy reveals that, instead of forming multilayers, Ca and Ba separate in different columns in sample C. The structure properties of sample A (large l) are quite ideal: Planar interfaces with regular arrays of misfit dislocations with their Burgers vectors on the interface are observed. In the case of sample B (medium l) the lattice misfit is accommodated, in addition, by wavy interfaces associated with dislocations characterized by a Burgers vector that makes a large angle to the interfaces. The (111) lattice spacing very close to the interfaces is markedly changed due to this novel relaxation mechanism in the multilayer. The influences of the crystallographic defects on the ionic conductivity are also discussed.The Journal of Chemical Physics 03/2004; 120(5):2375-81. · 3.33 Impact Factor -
Article: Successful application of spatial difference technique to electron energy-loss spectroscopy studies of Mo/SrTiO3 interfaces.
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ABSTRACT: The electron energy-loss near-edge structure (ELNES) of Mo/SrTiO3 interfaces has been studied using high spatial resolution electron energy-loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope. Thin films of Mo with a thickness of 50 nm were grown on (001)-orientated SrTiO3 surfaces by molecular beam epitaxy at 600 degrees C. High-resolution transmission electron microscopy revealed that the interfaces were atomically abrupt with the (110)Mo plane parallel to the substrate surface. Ti-L2,3 ( approximately 460 eV), O-K ( approximately 530 eV), Sr-L2,3 ( approximately 1950 eV) and Mo-L2,3 ( approximately 2500 eV) absorption edges were acquired by using the Gatan Enfina parallel EELS system with a CCD detector. The interface-specific components of the ELNES were extracted by employing the spatial difference method. The interfacial Ti-L2,3 edge shifted to lower energy values and the splitting due to crystal field became less pronounced compared to bulk SrTiO3, which indicated that the Ti atoms at the interface were in a reduced oxidation state and that the symmetry of the TiO6 octahedra was disturbed. No interfacial Sr-L2,3 edge was observed, which may demonstrate that Sr atoms do not participate in the interfacial bonding. An evident interface-specific O-K edge was found, which differs from that of the bulk in both position (0.8 +/- 0.2 eV positive shift) and shape. In addition, a positive shift (0.9 +/- 0.3 eV) occurred for the interfacial Mo-L2,3, revealing an oxidized state of Mo at the interface. Our results indicated that at the interface SrTiO3 was terminated with TiO2. The validity of the spatial difference technique is discussed and examined by introducing subchannel drift intentionally.Journal of Microscopy 05/2003; 210(Pt 1):94-101. · 1.63 Impact Factor -
Article: Advances in EELS spectroscopy by using new detector and new specimen preparation technologies.
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ABSTRACT: First results obtained with a Gatan UHV Enfina system, which was attached to a VG HB 501 UX dedicated STEM, are reported. The Enfina system is based on a CCD detector and offers, compared to the previously used photodiode array, a narrower point-spread function, higher sensitivity, and faster read-out capabilities. These improvements are demonstrated with electron energy-loss measurements on various oxides, such as Al2O3, TiO2 and SrTiO3. It is shown that a better energy resolution is achieved and that acquisition of high-energy absorption edges with a reasonable signal-to-noise ratio becomes possible. Furthermore, we report on the influence of the TEM specimen quality on the energy-loss spectra. Thin amorphous layers at the specimen surfaces, which are induced by ion-milling processes, can modify specific electron energy-loss near-edge structure features. We found that for the investigated ceramics the use of low-energy ion-milling systems is highly recommended, since the loss of energy-loss near-edge structure details by the presence of the amorphous layers is considerably reduced. This is especially true for very thin specimens.Journal of Microscopy 05/2003; 210(Pt 1):16-24. · 1.63 Impact Factor -
Article: Chemical reactions and morphological stability at the Cu/Al2O3 interface.
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ABSTRACT: The microstructures of diffusion-bonded Cu/(0001)Al2O3 bicrystals annealed at 1000 degrees C at oxygen partial pressures of 0.02 or 32 Pa have been studied with various microscopy techniques ranging from optical microscopy to high-resolution transmission electron microscopy. The studies revealed that for both oxygen partial pressures a 20-35 nm thick interfacial CuAlO2 layer formed, which crystallises in the rhombohedral structure. However, the CuAlO2 layer is not continuous, but interrupted by many pores. In the samples annealed in the higher oxygen partial pressure an additional reaction phase with a needle-like structure was observed. The needles are several millimetres long, approximately 10 microm wide and approximately 1 microm thick. They consist of CuAlO2 with alternating rhombohedral and hexagonal structures. Solid-state contact angle measurements were performed to derive values for the work of adhesion. The results show that the adhesion is twice as good for the annealed specimen compared to the as-bonded sample.Journal of Microscopy 11/2002; 208(Pt 1):11-7. · 1.63 Impact Factor -
Article: Electron energy-loss near-edge structure studies at the atomic level: reliability of the spatial difference technique.
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ABSTRACT: The spatial difference technique was applied to determine the interface-specific components of electron energy-loss near-edge structures and the results are in good agreement to those obtained by atomic column resolved measurements averaged over one atomic layer. In previous studies an experimental set-up had been chosen where the scanning areas, which are used to measure electron energy-loss spectra, were symmetrical in respect to the location of the investigated interfaces. In the present study, an asymmetric setting of the scanning areas was applied, which allows the interfacial signals to be determined directly. Comparing the results of the different measurements shows that the spatial difference technique is valid and can be used to obtain information about the electronic structure of interfaces where single atomic column resolved measurements are not yet possible.Journal of Microscopy 08/2002; 207(Pt 1):52-7. · 1.63 Impact Factor -
Article: Electron-energy-loss spectroscopy studies of Cu- alpha-Al2O3 interfaces grown by molecular beam epitaxy
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ABSTRACT: The electron-energy-loss near-edge structure (ELNES ) of a Cu-alpha-Al2O3 interface grown by molecular beam epitaxy has been studied using spatial difference electron-energy-loss spectroscopy in the scanning transmission electron microscope. Interpretation of the interface-specific Cu L2,3, Al L2,3 O K ELNES components implies the existence of Cu-O bonding at the interface together with the retention of a local octahedral oxygen coordination of aluminium atoms and hence their non-participation in the interface plane. There is significant evidence for interfacial charge transfer from the copper layer to the oxygen layer, resulting in the existence of one monolayer of copper at the interface nominally in the Cu + oxidation state. Furthermore, the ELNES studies reveal that the interfacial Cu-O bonds are of mixed ionic-covalent character. This becomes apparent when considering the O K ELNES where the presence of hybridized Cu 3d-O 2p states is indicated. The basal plane of alpha-Al2O3 at the interface is terminated by a layer of oxygen atoms which is directly bonded to the copper lattice. This chemical information was employed as an initial assumption in the subsequent simulation of experimental high-resolution transmission electron microscopy (HRTEM ) images of the same interface. A structural model for the atomistic arrangement at the interface was derived which exhibited a projected Cu-O distance of 0.2 nm. Since the O K ELNES provides information on medium-range order, the HRTEM-derived model of the interface was then used as input for the simulation of the experimental interface-specific O K ELNES component using multiple-scattering calculations. Excellent agreement between the theoretical and experimental ELNES results confirms the HRTEM model and rules out the possibility of an aluminium-terminated alpha-Al2O3 basal plane at the interface. This study attempts to demonstrate the synergism between HRTEM and spatially resolved electron-energy-loss spectroscopy measurements for structure determination and highlights the self-consistent aspect of such combined information.Philosophical Magazine A 07/1998; 78(2):439-465. -
Article: Interfacial Electronic Structure and Full Spectral Hamaker Constants of Si3N4 Intergranular Films from VUV and Sr-Veel Spectroscopy
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ABSTRACT: The interfacial electronic structure, presented as the interband transition strength Jcv(ω) of the interatomic bonds, can be determined by Kramers Kronig (KK) analysis of vacuum ultraviolet (VUV) reflectance or spatially resolved valence electron energy loss (SR-VEEL) spectra. For the wetted interfaces in Si3N4, equilibrium thin glass films are formed whose thickness is determined by a force balance between attractive and repulsive force terms. KK analysis of Jcv(ω) to yield ξ(ξ) for the phases present, permits the direct calculation of the configuration-dependent Hamaker constants for the attractive vdW forces from the interfacial electronic structure.Interband transition strengths and full spectral Hamaker constants for Si3N4 samples containing a SiYA1ON glass have been determined using SR-VEELS from grains and grain boundaries and compared with results from bulk VUV spectroscopy on separate samples of glass and nitride. The At2, Hamaker constant for Si3N4 with glass of the bulk composition is 8 zJ (zJ = 10−21 J) from the more established optical method. The EELS method permits the determination of vdW forces based upon actual local compositions and structure, which may differ noticeably from bulk standards. Current results show that full spectral Hamaker constants determined from VUV and SR-VEEL measurements of uniform bulk samples agree, but care must be taken in the single scattering and zero loss subtraction corrections, and more work is ongoing in this area. Still the results show that for the grain boundary films present in these polycrystalline Si3N4 samples the glass composition is of lower index of refraction. This can arise from increased oxygen content in the intergranular glass and leads to an increased value of the Hamaker constant (24 zJ) determined in situ from the SR-VEELS of a particular grain boundary film.MRS Proceedings. 12/1993; 357. -
Article: Characterization of chemical composition and electronic structure of Pt/YSZ interfaces by analytical transmission electron microscopy
Solid State Ionics, v.181, 1616-1622 (2010). -
Article: Synthesis of analytical and high-resolution transmission electron microscopy to determine the interface structure of Cu/Al2O3
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ABSTRACT: The structure of Cu/Al2O3 interfaces has been studied by high-resolution transmission electron microscopy (HRTEM) and analytical TEM. The use of electron energy-loss spectroscopy (EELS) in the determination of the local coordinations and bonding mechanisms at the interface is shown to be an essential extension for atomic structure evaluation by quantitative high-resolution TEM. A refined atomic model of the Cu/Al2O3 interface is then retrieved by iterative digital image matching. Multiple scattering calculations based on this model are vital to explain the interface specific components in the EELS O-K edge. The atomistic configuration at the interface is described by a structure model with CuO bonds across the interface.Ultramicroscopy. -
Article: Microstructural investigations on zirconium oxide–carbon nanotube composites synthesized by hydrothermal crystallization
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ABSTRACT: Zirconium oxide (ZrO2)/carbon nanotube composites were prepared by hydrothermal crystallization at 200 °C for 8 h of zirconium hydroxide [(Zr(OH)4nH2O; n=8–16] in the presence of carbon nanotubes. The hydrothermal crystallization of zirconium oxide, under autoclave conditions, occurred on the walls of the tubes. The process yielded a homogeneous composite powder consisting of microcrystalline ZrO2 and multi-walled nanotubes, hence representing an alternative route for the synthesis of ceramic–nanotube composites. Microstructural and qualitative characterizations were performed using different electron microscopy techniques.Carbon. -
Article: Electronic structure investigations of Ni and Cr films on (100)SrTiO3 substrates using electron energy-loss spectroscopy
Zeitschrift für Metallkunde, v.93, 362-371 (2002). -
Article: NanoTeflons: Structure and EELS characterization of fluorinated carbon nanotubes and nanofibers
Nano Letters, v.2, 491-496 (2002). -
Article: Layer and defect structures of Ba F2/CaF2 multilayers
Microscopy and Microanalysis, v.8, Supplement 2, 1164-1165 (2002). -
Article: Interfacial electronic structure of thin Cu films grown on Ar+- ion sputter-cleaned α-Al2O3 substrates
Journal of Materials Science & Technology, v.18, 117-120 (2002).
Top co-authors
Top Journals
Institutions
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2012
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The Ohio State University
Columbus, OH, USA
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2007
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Montanuniversität Leoben
- Department Physical Metallurgy and Materials Testing
Leoben, Styria, Austria
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2002–2003
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Max-Planck-Gesellschaft
- Max-Planck-Institute of Intelligent Systems
München, Bavaria, Germany
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