Katharina Al-Shamery

Impact of the Incorporation of Au Nanoparticles into Polymer/Fullerene Solar Cells †

The addition of small amounts of dodecylamine-capped Au nanoparticles into the active layer of organic bulk heterojunction solar cells consisting of poly(3-octylthiophene) (P3OT) and C(60) was recently suggested to have a positive impact on device performance due to improved electron transport. This issue was systematically further investigated in the present work. Different strategies to incorporate colloidally prepared Au nanoparticles with a narrow size distribution into organic solar cells with the more common donor/acceptor system consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) were pursued. Au nanoparticles were prepared with either P3HT or dodecylamine as ligands. Additionally, efforts were undertaken to incorporate nearly ligand-free Au nanoparticles into the system. Therefore, a procedure was successfully developed to remove the dodecylamine ligand shell by a postpreparative ligand exchange with pyridine, a much smaller molecule that can later partly be removed from solid films by annealing. However, for all types of nanoparticles studied here, the performance of the P3HT/PCBM solar cells was found to decrease with the Au particles as an additive to the active layer, meaning that adding Au nanoparticles is not a suitable strategy in the case of the P3HT/PCBM system. Possible reasons are discussed on the basis of detailed investigations of the structure, photophysics and charge transport in the system.

Organic Molecular Nanotechnology

A new route to bottom-up organic nanotechnology is presented. Molecular building blocks with specific optoelectronic properties are designed and grown via directed self-assembly arrays of morphologically controlled light-emitting organic nanofibers on template surfaces. The fibers can be easily transferred from the growth substrate to device platforms either as single entities or as ordered arrays. Due to the extraordinary flexibility in the design of their optoelectronic properties they serve as key elements in next-generation nanophotonic devices.

Dynamical studies of UV‐laser‐induced NO‐desorption from the polar NiO(111) versus the nonpolar NiO(100) surfaces

We have studied the UV‐laser‐induced desorption of NO adsorbed on an epitaxial film of NiO(111) grown on Ni(111). The desorbing molecules were detected state selectively via a resonance enhanced ionization technique [REMPI(1+1)] using the A 2Σ(v’=0,1,2)←X 2Π(v‘=0,1,2) transition as intermediate state. Our results are compared with our experiments on NO desorption from NiO(100). The similarities and differences of the results due to the different surface structure of the polar NiO(111) and the non polar NiO(100) are discussed. For both surfaces we observe bimodal velocity flux distributions independent of the rovibrational state. Due to a rotational temperature of about 400 K and a vibrational temperature of 1800 K thermal processes can be ruled out. The wavelength dependence of the desorption cross section strongly correlates with the electronic structure of the NiO indicating a surface mediated excitation process. The spin orientation in the NO molecules influences the life time of the excited state depe...

Water adsorption and growth of ice on epitaxial Fe3O4(111), FeO(111) and Fe2O3(biphase)

Deuterated water adsorption on epitaxially grown FeO(111), Fe3O4(111) and Fe2O3 (biphase) films was investigated in the range 110–320 K by infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) spectroscopy. At 110 K, a first water layer forms on Fe3O4(111) and Fe2O3 (biphase) before the second and higher layers develop. The first half layer on Fe3O4 adsorbs dissociatively. The second half layer develops features characteristic for hydrogen bonding and the formation of dimers is concluded. Also on Fe2O3(biphase), initial water adsorption is dissociative. A strongly bound minority species is observed. Heating to 169 K causes formation of ice clusters. On FeO(111) adsorption is molecular and weak. On all studied surfaces, thick ice layers grown at 110 K are amorphous. On Fe3O4(111) they transform at 170 K into hexagonal ice (IceH) while up to 10 L on FeO(111) remain amorphous. The mechanisms for adsorption and ice formation correlate with structure and termination of the different oxide surfaces.

Adsorption on oxide surfaces: structure and dynamics

Abstract In contrast to metal surfaces, oxide surfaces have only been studied rather recently with surface science methods. We report on the preparation and electron spectroscopic investigations of thin, well ordered surfaces of clean, adsorbate covered and modified oxide films. We identify surface excited states of a NiO(100) surface via electron energy loss spectroscopy in the regime of electronic excitations. Adsorption on well ordered terraces and on defects can be distinguished by choosing proper probe molecules. As a model system to study the structure and reactivity of an oxide supported ultrathin metal film we have deposited Pt onto a thin A1203(111) film grown on a NiAI(110) substrate. CO adsorption and low temperature dissociation in contrast to the bulk Pt has been observed.

Colloidal synthesis and structural control of PtSn bimetallic nanoparticles.

PtSn bimetallic nanoparticles with different particle sizes (1-9 nm), metal compositions (Sn content of 10-80 mol %), and organic capping agents (e.g., amine, thiol, carboxylic acid and polymer) were synthesized by colloidal chemistry methods. Transmission electron microscopy (TEM) measurements show that, depending on the particle size, the as-prepared bimetallic nanocrystals have quasi-spherical or faceted shapes. Energy-dispersive X-ray (EDX) analyses indicate that for all samples the signals of both Pt and Sn can be detected from single nanoparticles, confirming that the products are actually bimetallic but not only a physical mixture of pure Pt and Sn metal nanoparticles. X-ray diffraction (XRD) measurements were also conducted on the bimetallic particle systems. When compared with the diffraction patterns of monometallic Pt nanoparticles, the bimetallic samples show distinct shifts of the Bragg reflections to lower degrees, which gives clear proof of the alloying of Pt with Sn. However, a quantitative analysis of the lattice parameter shifts indicates that only part of the Sn atoms are incorporated into the alloy nanocrystals. This is consistent with X-ray photoelectron spectroscopy (XPS) measurements that reveal the segregation of Sn at the surfaces of the nanocrystals. Moreover, short PtSn bimetallic nanowires were synthesized by a seed-mediated growth method with amine-capped bimetallic particles as precursors. The resulting nanowires have an average width of 2.3 nm and lengths ranging from 5 to 20 nm.

Nanofibers from functionalized para-phenylene molecules

Tens to hundreds of micrometers long organic nanofibers have been generated from methoxy functionalized quaterphenylene molecules. The mutual alignment of the fibers is similar to that of previously reported nanofibers from para-hexaphenylene, and they emit intense, blue light centered at 400 nm with well resolved higher order vibronic peaks. The morphology is slightly different from that of para-hexaphenylene nanofibers, reflecting the different molecular structure. This study demonstrates that it is possible to generate organic nanofibers from artificially functionalized molecules and thus opens up the route to dedicated applications in new microdevices.

Rotational alignment in the UV-laser induced desorption of CO from Cr2O3(0001)

Abstract The rotational alignment of the laser-induced non-thermal desorption of CO adsorbed on an epitaxially grown film of Cr 2 O 3 (0001) has been studied using 193 nm (6.4 eV) laser light and a (1 + 1′)-REMPI technique. At low rotational quantum numbers J the molecules desorb like a ‘helicopter’ ( J - vector perpendicular to the surface) while at high J -values a ‘cartwheel’ like motion is preferred ( J - vector parallel to the surface). CO is adsorbed in a flat geometry as a single species. The resulting angular momentum alignment in the laser desorption is interpreted qualitatively as a sum of effects due to corrugation of the surface in the initial state and a change of the adsorption geometry in the intermediate state before desorption.

Two-dimensional state resolved imaging after UV-laser induced desorption: NO/NiO(111)

Abstract We present an experimental set-up which can image state selectively the two-dimensional angular distribution of neutrals or ions desorbing from a surface after UV-laser excitation together with the velocity flux distribution of a single rovibronic state in a single experiment. The method combines a state selective detection method like resonance enhanced multiphoton ionization (REMPI) with an imaging technique. This method is particularly fast in comparison to other one-dimensional methods and ideal for systems where the substrate is modified via irradiation with laser light. The normally used set-up in state resolved laser desorption experiments for two-dimensional imaging [1] is limited to look at the azimuthal distribution. We report on results of the determination of angular distributions of NO desorbing from NiO(111) after excitation with 193 nm (6.4 eV) laser light. The velocity flux distributions also obtained are compared to former one-dimensional experiments performed in our laboratory.

Ultraviolet-laser induced desorption of NO from the Cr2O3(0001) surface: Involvement of a precursor state?

NO molecules interact with the Cr2O3(0001) surface to form a chemisorption bond of 1.0 eV. At higher coverages an additional more weakly bound species appears in thermal desorption spectra with a binding energy of 0.35 eV. By infrared spectroscopy the weakly adsorbed species is identified to be an unusually strong bound NO-dimer exhibiting a weak feature at 1857 cm−1 beside the chemisorbate absorption band at 1794 cm−1. Laser induced desorption experiments performed at 6.4 eV are presented with main emphasis on the high coverage regime. The desorbing molecules are detected quantum state selectively using resonance enhanced multiphoton ionization. The desorbing molecules are strongly rotationally and vibrationally excited conform with a nonthermal excitation process. The velocity distributions of single rovibronic states of desorbing NO are bimodal and exhibit a strong coupling of rotation and translation. With increasing coverages an additional channel is observed appearing in the time-of-flight spectra o...