Ralf-Peter Blum

Growth of Al2O3 stripes in NiA(001)

Abstract Structure and growth mechanism of ultra-thin Al 2 O 3 films on NiAl(001) have been investigated with 180°-neutral impact collision ion scattering spectroscopy, high resolution spot profile analysis of low energy electron diffraction, and scanning tunnelling microscopy (STM). Ordered Al 2 O 3 films have been prepared by oxygen exposure at room temperature and subsequent annealing at T = 1200 K. The formation of epitaxial oxide films is unaffected by the initial composition or reconstruction of the substrate. The oxidized surface being covered by Al 2 O 3 appears to be microscopically rough, reversible stepped with step heights of about 3 A and consists of a network of elongated equally distributed oxide stripes along 〈100〉 and 〈010〉 directions of the NiAl(001) surface. The lateral anisotropy of the oxide is probably caused by the build up of internal stress in the growth process. After oxygen saturation exposure at room temperature and subsequent annealing, regions with rather regular arranged parallel oxide stripes show up in the STM images (mean width ≈ 27 A , period ≈ 54 A ). The corresponding low energy electron diffraction pattern shows the existence of orthogonal (2 × 1) antiphase domains with a (9 × 1) superstructure (period 26.02 A) close to the mean width of oxide stripes. Next to the oxide stripes, regrowing NiAl terraces and thin layers of amorphous Al 2 O 3 have been found.

Gold work function reduction by 2.2 eV with an air-stable molecular donor layer

Ultraviolet photoelectron spectroscopy was used to investigate neutral methyl viologen (1,1′-dimethyl-1H,1′H-[4,4′]bipyridinylidene, MV0) deposited on Au(111). As a result of molecule-to-metal electron transfer, the work function of Au(111) was decreased from 5.50to3.30eV. The energy levels of electron transport layers deposited on top of modified Au surfaces were shifted to higher binding energies compared to layers on pristine Au, and the electron injection barrier was reduced by 0.80eV for tris(8-hydroxyquinoline)aluminum (Alq3) and by 0.65eV for C60. The air-stable donor MV0 can thus be used to facilitate electron injection into organic semiconductors even from high work function metals.

Preparation-dependent surface composition and structure of NiAl(001): SPA-LEED and NICISS study

Abstract The surface composition and structure of NiAl(001) has been investigated by 180°-neutral impact collision ion scattering spectroscopy (NICISS) and high resolution spot profile analysis of low energy electron diffraction (SPA-LEED). Long time annealing below 500 K results in a p(1 × 1) LEED pattern. The surface can be described by a defect enriched Al terminated surface. Another p(1 × 1) LEED pattern appears after flashed annealing at 1400 K and rapid cooling at room temperature. The high temperature p(1 × 1) phase is however nearly perfectly Ni terminated. It has been found that the formation of the high temperature NiAl(001)-p(1 × 1) phase is closely related to roughening of the topmost Al layer by vacancy creation at high annealing temperatures. In the intermediate annealing temperature range about 800 K surface roughening results in an Al terminated missing row surface structure exposing a c(√2 × 3√2)R45° LEED superstructure.

Structure of Vanadium Oxide (V2O3) Grown on Cu3Au(100)

The epitaxial growth of vanadium oxide (V2O3) has been investigated by scanning tunneling microscopy (STM), low energy ion back-scattering (ISS) and low energy electron diffraction (LEED). Direct evaporation of vanadium onto metal surfaces (Cu, Au or Cu3Au) gives rise to massive surface alloying. The attempt to oxidize the vanadium film by consequent oxygen exposure leads to the formation of rough VOx films of poor quality and mixed valency. A new way of oxide formation has been developed by preoxidation of a Cu3Au substrate, which acts positively in two ways since it prevents completely alloy formation and also forces strong surface wetting of the vanadium oxide. As a result, two-dimensional layer growth of good quality has been achieved. Depending on the preoxygen content at Cu3Au(100), the amount of V deposition and annealing temperature, different epitaxial layers of vanadium oxides can be prepared. In particular, the surface structure of V2O3(0001) was investigated. The surface structure appears completely different from the half layer metal termination at Cr2O3(0001). Specifically, the full vanadium layer stabilized by one third of an oxygen layer located in pseudo bridge positions close to regular oxygen positions of a next layer. Close to defects the full vanadium layer appears also without oxygen stabilization.

Formation of vanadium oxide films on Cu3Au(100)

The initial growth of VO x has been investigated by low energy ion backscattering (NICISS) scanning tunnelling microscopy (STM), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Direct evaporation of vanadium onto the Cu 3 Au(100) substrate gives rise to massive surface alloying, consequent oxygen exposure leads to the formation of rough vanadium oxide films of poor quality. A better way has been developed by forming a thin oxygen layer at the clean Cu 3 Au substrate which acts positively in two ways: firstly, it prevents completely the alloy formation, secondly, a strong surface wetting of the vanadium oxide occurs resulting in two-dimensional layer growth of good quality. Depending on the pre-oxygen content at Cu 3 Au(100), the amount of V deposition and annealing temperature, different epitaxial layers of vanadium oxides can be prepared. Namely, three VO x species occur separately: an oxide with low oxygen content showing a quadratic crystallographic lattice, probably VO(100), V 2 O 3 (0001) with a hexagonal superlattice and finally, domains with a rectangular unit cell and VO 2 stoichiometry. As a consequence, oxygen treated Cu 3 Au(100) is ideally suited as a metal substrate for growing homogeneous 2D epitaxial vanadium metal oxides.

Organic heterojunctions: Contact-induced molecular reorientation, interface states, and charge re-distribution.

We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure – in the presence of Fermi-level pinning – at prototypical molecular heterojunctions comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy. For both layer sequences, we find that Fermi-level (EF) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes EF-pinned, however, at the interface towards the first molecular layer. This results in a charge transfer accompanied by a sheet charge density at the organic/organic interface. While molecules in the bulk of the films exhibit upright orientation, contact formation at the heterojunction results in an interfacial bilayer with lying and co-facial orientation. This interfacial layer is not EF-pinned, but provides for an additional density of states at the interface that is not present in the bulk. With reliable knowledge of the organic heterojunction’s electronic structure we can explain the poor performance of these in photovoltaic cells as well as their valuable function as charge generation layer in electronic devices.

Electronic Properties of Cu-Phthalocyanine/Fullerene Planar and Bulk Hetereojunctions on PEDOT:PSS

In this paper, we report UV and X-ray photoelectron spectroscopy studies on layered planar and mixed bulk heterojunctions of Cu-phtalocyanine (CuPc) and C60, a prototypical material pair for organic photovoltaic cells (OPVCs). The respective heterojunctions were formed on poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrates in order to achieve morphologies comparable to those in actual OPVCs. As a result of a CuPc-to-substrate electron transfer, the work function of pristine PEDOT:PSS is reduced from 5.4 to 4.4 eV. The deposition of C60 onto CuPc, however, leads to a work function increase of 0.2 eV. The codeposition of C60 and CuPc to form mixed bulk heterojunctions resulted in an effective anode work function of 4.6 eV. The energy offset between the highest occupied levels of CuPc and C60 was determined as 1.3 eV for both the layered planar and mixed bulk heterojunction. With reported values of the charge transport gap of C60, we estimate the upper limit of the open circuit voltage to be 1.0 eV for both types of heterojunctions. Our results demonstrate that the energy level offsets are independent of particular interface morphology in C60/CuPc heterojunctions grown on PEDOT:PSS, and that differences in device efficiency are due to other effects.

Orientation-Dependent Work-Function Modification Using Substituted Pyrene-Based Acceptors

The adsorption of molecular acceptors is a viable method for tuning the work function of metal electrodes. This, in turn, enables adjusting charge injection barriers between the electrode and organic semiconductors. Here, we demonstrate the potential of pyrene-tetraone (PyT) and its derivatives dibromopyrene-tetraone (Br-PyT) and dinitropyrene-tetraone (NO2-PyT) for modifying the electronic properties of Au(111) and Ag(111) surfaces. The systems are investigated by complementary theoretical and experimental approaches, including photoelectron spectroscopy, the X-ray standing wave technique, and density functional theory simulations. For some of the investigated interfaces the trends expected for Fermi-level pinning are observed, i.e., an increase of the metal work function along with increasing molecular electron affinity and the same work function for Au and Ag with monolayer acceptor coverage. Substantial deviations are, however, found for Br-PyT/Ag(111) and NO2-PyT/Ag(111), where in the latter case an adsorption-induced work function increase of as much as 1.6 eV is observed. This behavior is explained as arising from a face-on to edge-on reorientation of molecules in the monolayer. Our calculations show that for an edge-on orientation much larger work-function changes can be expected despite the prevalence of Fermi-level pinning. This is primarily ascribed to a change of the electron affinity of the adsorbate layer that results from a change of the molecular orientation. This work provides a comprehensive understanding of how changing the molecular electron affinity as well as the adsorbate structure impacts the electronic properties of electrodes.