Joachim Brötz

Interface properties and band alignment of Cu2S/CdS thin film solar cells

The stoichiometry and electronic properties of bulk Cu2S thin films obtained by vacuum evaporation were investigated by optical spectroscopy, X-ray diffraction and photoemission spectroscopy. The Cu2S/CdS heterojunction interface has been prepared in situ and characterized by photoelectron spectroscopy (X-ray photoemission spectroscopy and ultraviolet photoelectron spectroscopy) after each growth step under ultra high vacuum conditions. The XPS core level spectra as well as valence band spectra of the substrate Cu2S and overlayer CdS were acquired after each step. From these measurements, a large overall band bending of 0.9 eV is observed. The valence band offset is determined to be ΔEVB=1.2 eV and the conduction band offset is ΔECB=0±0.1 eV.

Etched heavy ion tracks in polycarbonate as template for copper nanowires

Abstract 30 and 40 μm thick polycarbonate foils were irradiated with Au197 and Pb208 ions of kinetic energy 1–2 GeV with fluences between 106 and 109 ions/cm2. The latent tracks generated by the heavy ions were chemically etched providing membranes with cylindrical pores of diameters between 30 and 200 nm. These membranes have been used as templates for the creation of metallic nanowires of very high aspect ratio. A thin metal film deposited on one side of the membrane acted as cathode in the two-electrode electrochemical cell, while a copper cone served as anode. The wires were grown potentiostatically. The electrochemical process was monitored by registering chronoamperometric curves for different cathode overvoltages, temperatures and concentrations of the electrolyte. Under suitable conditions, single-crystalline needles were produced. The morphology and crystallinity of the copper nanowires were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction.

Reactive magnetron sputtering of Cu2O: Dependence on oxygen pressure and interface formation with indium tin oxide

Thin films of copper oxides were prepared by reactive magnetron sputtering and structural, morphological, chemical, and electronic properties were analyzed using x-ray diffraction, atomic force microscopy, in situ photoelectron spectroscopy, and electrical resistance measurements. The deposition conditions for preparation of Cu(I)-oxide (Cu2O) are identified. In addition, the interface formation between Cu2O and Sn-doped In2O3 (ITO) was studied by stepwise deposition of Cu2O onto ITO and vice versa. A type II (staggered) band alignment with a valence band offset ΔEVB = 2.1–2.6 eV depending on interface preparation is observed. The band alignment explains the nonrectifying behavior of p-Cu2O/n-ITO junctions, which have been investigated for thin film solar cells.

Electroless synthesis of platinum and platinum–ruthenium nanotubes and their application in methanol oxidation

A dependable and mild deposition procedure for the first electroless synthesis of platinum nanotubes in ion track etched polycarbonate was developed. It utilizes ethylenediamine to adjust the reactivity of Pt(IV) towards reduction and allows the controlled deposition of nanoscale platinum films, wires and tubes at room temperature. Single crystal structure determination proves the formation of Pt(II) as a side product next to elemental platinum. Highly polycrystalline nanostructures of 100 to 900 nm diameter and up to 30 μm length were obtained and characterized by TEM, SEM, EDS and XRD. The platinum nanotubes showed high activity in the electrooxidation of methanol in acidic environment. To illustrate the possibilities for synthesizing bimetallic nanotubes in the presented system, ruthenium was introduced by electroless and spontaneous deposition methods. The corresponding surface-normalized current densities are strongly dependent on the preparation method and can surpass commercial fuel cell catalysts, confirming the efficiency and flexibility of electroless metal plating in the preparation of nanomaterials.

Orientation dependent ionization potential of In2O3: A natural source for inhomogeneous barrier formation at electrode interfaces in organic electronics

The ionization potentials of In(2)O(3) films grown epitaxially by magnetron sputtering on Y-stabilized ZrO(2) substrates with (100) and (111) surface orientation are determined using photoelectron spectroscopy. Epitaxial growth is verified using x-ray diffraction. The observed ionization potentials, which directly affect the work functions, are in good agreement with ab initio calculations using density functional theory. While the (111) surface exhibits a stable surface termination with an ionization potential of ∼ 7.0 eV, the surface termination and the ionization potential of the (100) surface depend strongly on the oxygen chemical potential. With the given deposition conditions an ionization potential of ∼ 7.7 eV is obtained, which is attributed to a surface termination stabilized by oxygen dimers. This orientation dependence also explains the lower ionization potentials observed for In(2)O(3) compared to Sn-doped In(2)O(3) (ITO) (Klein et al 2009 Thin Solid Films 518 1197-203). Due to the orientation dependent ionization potential, a polycrystalline ITO film will exhibit a laterally varying work function, which results in an inhomogeneous charge injection into organic semiconductors when used as electrode material. The variation of work function will become even more pronounced when oxygen plasma or UV-ozone treatments are performed, as an oxidation of the surface is only possible for the (100) surface. The influence of the deposition technique on the formation of stable surface terminations is also discussed.

PVD of copper sulfide (Cu2S) for PIN-structured solar cells

Thin layers of chalcocite (Cu2S) have been deposited via physical vapour deposition using various pre- and post-treatment parameters. The electrical and morphological properties have been investigated by in situ XPS, SEM and XRD measurements. Calibrated photoluminescence experiments were performed to investigate the materials suitability as an absorber layer in thin-film solar cells. Measurements of annealed Cu2S layers on glass without any surface passivation showed an optical band gap of 1.25 eV as well as a splitting of the quasi-Fermi levels of 710 meV. This value exceeds the highest reported open-circuit voltage for Cu2S-based devices so far, which leads to the assumption that Cu2S has not been brought to its full potential yet. The band alignments for ZnO/Cu2S as well as Cu2S/Cu2O interfaces have been determined using in situ XPS interface experiments to suggest a novel device structure according to the favourable PIN-layout. First devices have been built, but show no efficiency due to shunting caused by the inferior morphology of the absorber layers.

Fabrication of porous rhodium nanotube catalysts by electroless plating

A versatile electroless plating procedure for the fabrication of rhodium nanomaterials was developed, leading to deposits consisting of loosely agglomerated metal nanoparticles. By using carbon black as the substrate, supported rhodium nanoparticle clusters were obtained. In combination with ion track etched polymer templates, the deposition protocol allowed the first direct synthesis of rhodium nanotubes. Polymer dissolution provided access to well defined, supportless and free-standing rhodium nanotubes of nearly cylindrical shape, 300 nm opening diameter, 28 μm length and 50 nm wall thickness. The characterization by SEM, TEM, EDS and XRD confirmed the purity of the deposit, displayed a small particle size of approximately 3 nm and revealed gaps in the range of a few nanometers between the rhodium particles. BET analysis verified the presence of pores of <5 nm. To evaluate the electrocatalytic potential of the rhodium nanotubes, they were applied in the amperometric detection of hydrogen peroxide. Compared to classical nanoparticle-based sensing concepts, improved performance parameters (sensitivity, detection limit, and linear range) could be achieved.

Long-range superconducting proximity effect in polycrystalline Co nanowires

We report experimental evidence of a long-range superconducting proximity effect in polycrystalline Co nanowires in contact with a superconducting W-based floating electrode (inducer). For electrical resistance measurements, voltage leads were connected to the Co nanowire on both sides of the superconducting inducer at a distance of 7.2 μm. We observed a 28% reduction of the nanowire resistance when sweeping the temperature below the inducers transition temperature Tc = 5.2 K. Our analysis of the resistance data shows that the superconducting proximity length in polycrystalline Co is as large as 1 μm at 2.4 K, attesting to a long-range proximity effect. Moreover, this long-range proximity effect is insusceptible to magnetic fields up to 11 T, which is indicative of spin-triplet pairing. Our results provide evidence that magnetic inhomogeneity of the ferromagnet enlarges the spatial extend of the spin-triplet superconducting proximity effect.

Anisotropic defect structure and transport properties of YBa2Cu3O7−δ films on vicinal SrTiO3(001)

The microstructure of YBa2Cu3O7−δ thin films grown on vicinal SrTiO3(001) has been studied as a function of the vicinal angle by x-ray diffraction using the two-dimensional q-scan technique. Our results reveal a strong correlation between the miscut of a SrTiO3(001) substrate and the anisotropic defect structure of the film. Furthermore, we observed an anisotropy of the corresponding critical current density up to 4.6 depending on the angle of miscut.

NiCo nanotubes plated on Pd seeds as a designed magnetically recollectable catalyst with high noble metal utilisation

Electroless plating of magnetic materials on catalytically active noble metal seeds is a powerful tool to design highly efficient recyclable catalysts. For the electroless plating procedure of metallic nanotubes in porous polymer templates, a sensitisation and activation process of the template is necessary. Therefore, metallic seeds are created on the surface of the polymer, which then enable the selective heterogeneous nucleation of a metal film on the templates surface. By choosing the metals for seeds and structures wisely, different functional materials can be purposefully combined. In this work magnetically recoverable catalysts were designed, which consist of magnetic NiCo nanotubes as carrier for catalytically active Pd seeds. The synthesised catalyst structures were thoroughly characterised by SEM, TEM, EDX, XRD, XPS, ICP-OES, VSM and then tested in the 4-nitrophenol reduction reaction, which was monitored by UV-Vis spectroscopy. After the reaction the structures were recycled and reused without a decrease in activity.