Showing papers by "Lukas M. Eng published in 2008"
TL;DR: A periodic modulation of the coupled plasmon resonance is observed for separations smaller than 1.5 microm and this interparticle coupling affects the scattering cross section in terms of spectral position and spectral width as well as the integral intensity of the Mie-scattered light.
Abstract: The spectral properties of two spherical metallic nanoparticles of 80 nm in diameter are examined with regard to the interparticle distance and relative polarization of the excitation light. One Au nanoparticle is attached to a scanning fiber probe and the second to a scanning substrate. This configuration allows three-dimensional and arbitrary manipulation of both distance and relative orientation with respect to the incident light polarization. As supported by numerical simulations, a periodic modulation of the coupled plasmon resonance is observed for separations smaller than 1.5 µm. This interparticle coupling affects the scattering cross section in terms of spectral position and spectral width as well as the integral intensity of the Mie-scattered light.
88 citations
TL;DR: The imaging of ferroelectric domains in BaTiO3 is demonstrated using an infrared-emitting free-electron laser as a tunable optical source for scattering scanning near-field optical microscopy and spectroscopy, showing that the orientation of the dielectric tensor with respect to the sample surface has a clear influence on the near- field signal.
Abstract: We demonstrate the imaging of ferroelectric domains in ${\mathrm{BaTiO}}_{3}$, using an infrared-emitting free-electron laser as a tunable optical source for scattering scanning near-field optical microscopy and spectroscopy. When the laser is tuned into the spectral vicinity of a phonon resonance, ferroelectric domains can be resolved due to the anisotropy of the dielectric properties of the material. Slight detuning of the wavelength gives rise to a contrast reversal clearly evidencing the resonant character of the excitation. The near-field domain contrast shows that the orientation of the dielectric tensor with respect to the sample surface has a clear influence on the near-field signal.
75 citations
TL;DR: In this article, the dependence on angle in incidence of extinction peak height and position was modeled using a multiple-multipoles (MMP) approach with different spectral features being clearly associated with the effective electric field distribution and coupling between individual nanorods.
Abstract: Silver nanorods have been grown by electrodeposition into thin film porous alumina templates (AAO). Optical transmission measurements using p-polarized incident white light shows clear plasmon resonance extinction peaks. We successfully model the dependence on angle in incidence of extinction peak height and position using a multiple–multipoles (MMP) approach with the different spectral features being clearly associated with the effective electric field distribution and coupling between individual nanorods.
75 citations
TL;DR: The effect of molecular orientation at metal contacts on interface properties was determined by examining the local work function of porphyrin on atomically smooth graphite using Kelvin force microscopy.
Abstract: The effect of molecular orientation at metal contacts on interface properties was determined by examining the local work function of porphyrin on atomically smooth graphite. The orientation was varied by self-assembly from the vapor phase, and the local potential was quantified by Kelvin force microscopy (scanning surface potential microscopy). When the porphyrin ring is oriented parallel to the substrate, the surface work function is 50 mV less than that of the highly ordered pyrolytic graphite; in contrast, when the porphyrin molecular plane is oriented perpendicular to the substrate, the surface work function is unchanged. The orientation dependence of the surface work function is determined by the geometric relationships between the delocalized charge densities in the molecule and substrate and possible interface bonding. The differences in interface properties with molecular configuration have important design implications to molecular electronic and organic electronic devices.
55 citations
TL;DR: The experimental results obtained with this nanoscale fabrication technique are well described by the so-called plasmon ruler equation and both the changes in particle diameter as well as in gap width can be characterized in-situ via far-field read-out of the optical properties of the dimers.
Abstract: We report on the in-situ controlled tuning of the particle gap in single pairs of gold nanodisks by photochemical metal deposition. The optically induced growth of nanodisk dimers fabricated by electron beam lithography leads to a decrease of the interparticle gap width down to 0 nm. Due to the increasing particle size and stronger plasmonic coupling, a smooth redshift of the localized surface plasmon (LSP) resonances is observed in such particle pairs during the growth process. The interparticle gap width, and hence the LSP resonance, can be tuned to any desired spectral position. The experimental results we obtain with this nanoscale fabrication technique are well described by the so-called plasmon ruler equation. Consequently, both the changes in particle diameter as well as in gap width can be characterized in-situ via far-field read-out of the optical properties of the dimers.
48 citations
TL;DR: In this article, the authors explored the use of STO as a buffer layer on piezoelectric pseudocubic Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) (PMN-PT) substrates, which serve to reversibly strain thin films.
Abstract: SrTiO3 (STO) is the most frequently used substrate material for complex oxide films. In this work, STO is explored as a buffer layer on piezoelectric pseudocubic Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) (PMN-PT) substrates, which serve to reversibly strain thin films. The STO buffer layer reduces the in-plane lattice parameter and allows for a better lattice matching to a broader range of thin film materials. STO films (30 nm) have been grown with epitaxial orientation on PMN-PT with an in-plane lattice parameter close to that of bulk STO. The substrate’s rhombohedral domain structure has been imaged by atomic force microscopy. The related ferroelectric domain structure has been investigated by piezoresponse force microscopy. Within a domain, STO grows with a rather low roughness (rms<0.2 nm). The transfer of the piezoelectric substrate strain to the STO film and its variation with an applied electric field are studied using x-ray diffraction. The strain dependence of the electrical resistance is measured for a fe...
29 citations
TL;DR: The confinement normal to the sample surface is demonstrated by making use of a sample-induced phonon polariton resonance in a ferroelectric lithium niobate sample.
Abstract: We report on the implementation of metal nanoparticles as probes for scattering and apertureless near-field optical investigations in the mid-infrared (mid-IR) spectral regime. At these wavelengths, an efficient electric-field confinement is necessary and achieved here through a gold metal nanoparticle of 80 nm in diameter (Au80-MNP) acting as the optical antenna. The Au80-MNP is attached to a standard AFM cantilever used as the spatial manipulator. When approached to a sample surface while being illuminated with an infrared beam, the Au80-MNP produces a considerably improved spatial confinement of the electric field compared to an ordinary scattering AFM tip. We demonstrate here the confinement normal to the sample surface by making use of a sample-induced phonon polariton resonance in a ferroelectric lithium niobate sample. Our experimental findings are in very good agreement with the quasistatic dipole model and show improved optical resolution via well-selected antenna particles.
21 citations
TL;DR: In this article, the authors report on the improved assembly and characterization of a small molecule organic field effect transistor (OFET) using self assembly molecules (SAM) and demonstrate that the resulting field mobilities of 10-5 cm2 V 1 s−1 reflect a high current density in their ultrathin but highly ordered structure.
Abstract: We report on the improved assembly and characterization of a small molecule organic field-effect transistor (OFET). Novel α,ω-dicyano substituted β,β′-dibutylquaterthiophene molecules (DCNDBQT) were synthesized and characterized by UV–Vis spectroscopy, differential scanning calorimetry, thermal gravimetric analysis and cyclic voltammetry. The ultra-thin organic film formation on TiO2 templates was effectively promoted through the specifically designed bifunctional self assembly molecules (SAM) 5-cyano-2-(butyl-4-phosphonic acid)-3-butylthiophene (CNBTPA). Excellent structural properties were found for up to 9 DCNDBQT molecule thick films prepared through UHV vacuum sublimation as investigated with UHV non-contact atomic force microscopy (nc-AFM) and X-ray diffraction. Both X-ray and nc-AFM data indicate that the DCNDBQT molecules form a well-ordered terraced structure exhibiting step heights of 1.5 nm to 2.0 nm layers. Hence, the DCNDBQTmolecules are linked to the functional SAM interface layer by H-bond interactions (see structure model) standing quasi perpendicular to the TiO2 template, and thus providing optimal orbital overlap neigh-bouring thiophene rings.
The vacuum sublimated DCNDBQT molecules form a closed packed and dense molecular layer that was used to construct and operate a nanoscopic OFET-structure. The resulting field mobilities of 10–5 cm2 V–1 s–1 reflect a high current density in our ultrathin but highly ordered structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
21 citations
TL;DR: These phases are investigated by Kelvin probe force microscopy in order to quantitatively deduce the electric interface barrier of the first monolayer and reveal that the binding behavior does not only affect the structural ordering but also the interface dipole formation, which leads to different work functions.
Abstract: Recently, we investigated the adsorption of octachloro zinc phthalocyanine (ZnPcCl8) on Ag(111) by scanning tunneling microscopy. Compared to the standard phthalocyanine, halogenated phthalocyanine molecules show a much more complex binding behavior, which results in the formation of three different structural phases. These phases follow from the ordering process with the formation of 8, 4 and 0 intermolecular hydrogen–halogen bonds (Abel et al 2006 ChemPhysChem 7 82). In the present work we investigate these phases by Kelvin probe force microscopy in order to quantitatively deduce the electric interface barrier of the first monolayer. Our measurements reveal that the binding behavior does not only affect the structural ordering but also the interface dipole formation, which leads to different work functions. The fact that we observe interface barriers of opposite signs between ordered and disordered molecular layers underlines the importance of exactly knowing the molecular arrangement at the interface when assembling organic molecule devices.
18 citations
TL;DR: In this paper, near-field optical observations of surface-plasmon-mediated light emission in electrically biased metal-insulator-semiconductor (MIS) tunnel junctions fabricated from Au and p-type Si with a native oxide layer as the tunneling barrier are reported.
Abstract: We report on near-field optical observations of surface-plasmon-mediated light emission in electrically biased metal-insulator-semiconductor (MIS) tunnel junctions fabricated from Au and p-type Si with a native oxide layer as the tunneling barrier. Our junctions exhibit stable broadband macroscopic light emission in the visible spectrum. Inspection of the optical near-field reveals highly localized hot spots due to local plasmon excitation and scattering, which are investigated with spectral distinction. Such MIS tunneling junctions are compatible with common complementary metal-oxide semiconductor technology and thus open up an interesting route toward the development of novel integrated optoelectronic and plasmonic devices.
7 citations
TL;DR: In this article, a row of parallely ordered and coupled molecular nanoshafts is shown to develop a shuttling transport of charges at finite temperature, without applying an external bias voltage, and a natura l diode effect allowing unidirectional charge transport along one field directi on while blocking the opposite direction.
Abstract: A row of parallely ordered and coupled molecular nanoshafts is shown to develop a shuttling transport of charges at finite temperature. The appearance of a cu rrent without applying an external bias voltage is reported as well as a natura l diode effect allowing unidirectional charge transport along one field directi on while blocking the opposite direction. The zero-bias voltage current appears above a threshold of initial thermal and/or dislocation energy.
TL;DR: In this article, a row of parallel ordered and coupled molecular nanoshafts is shown to develop a shuttling transport of charges at finite temperature, without applying an external bias voltage.
Abstract: A row of parallel ordered and coupled molecular nanoshafts is shown to develop a shuttling transport of charges at finite temperature. The appearance of a current without applying an external bias voltage is reported as well as a natural diode effect allowing unidirectional charge transport along one field direction while blocking the opposite direction. The zero-bias voltage current appears above a threshold of initial thermal and/or dislocation energy.
TL;DR: In this article, the surface effect on dielectric polarization and refractive indices of BaTiO 3 ultrathin films on single crystal substrates was investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole-dipole interaction.
Abstract: The surface effect on the dielectric polarization and refractive indices of BaTiO 3 ultrathin films on SrTiO 3 single crystal substrates is investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole–dipole interaction. The spontaneous polarization of the BaTiO 3 /SrTiO 3 film is reduced as its thickness decreases. However, an electronic polarization appears within the SrTiO 3 substrate in the neighborhood of the interface. This polarization, which vanishes far away from the interface into the SrTiO 3 bulk, is induced by the polarization of the BaTiO 3 film. Furthermore, we find the refractive index either for BaTiO 3 films or for SrTiO 3 substrates to be strongly reduced for light polarized perpendicular to the surface.