ZnO nanocrystals distributed by spin-coating on SiO2/Si surfaces were annealed in UHV and studied in situ by synchrotron radiation based X-ray Photoelectron Spectroscopy. Changes in chemical composition and electronic structure of ZnO nanocrystal surfaces were found with increasing annealing temperatures. Annealing at 650 °C reduces the surface contaminant levels without any observed de-composition of ZnO. After annealing at 700 °C an initial de-composition of ZnO together with further reduction of contaminants was observed. As a result, 650 °C is found to be the optimal annealing temperature for thermal cleaning of ZnO nanocrystals. Chemical changes and induced point defect formation cause changes in the band structure of the ZnO/SiO2/Si system. An upward band bending of 0.7 eV on the surfaces of the ZnO nanocrystals was found after annealing at 300 °C. The bands on the surfaces of ZnO nanocrystals gradually bend downwards with increasing annealing temperatures. A downward band bending of 1.4 eV is the result after annealing at 750 °C for 1 h. This large downward band bending is explained as due to the change in balance of oxygen vacancies and zinc vacancies on the surfaces of ZnO nanocrystals.

Thermal cleaning in ultrahigh vacuum of ZnO nanocrystals distributed on SiO₂/Si surfaces has been studied using spectroscopic photoemission and low energy electron microscopy (SPELEEM). This study thus concern weakly bound ZnO nanocrystals covering only 5%–10% of the substrate. Chemical properties, crystallinity, and distribution of nanocrystals are used to correlate images acquired with the different techniques showing excellent correspondence. The nanocrystals are shown to be clean enough after thermal cleaning at 650 °C to be imaged by LEEM and x-ray PEEM as well as chemically analyzed by site selective x-ray photoelectron spectroscopy (μ-XPS). μ-XPS shows a sharp Zn 3d peak and resolve differences in O 1s states in oxides. The strong LEEM reflections together with the obtained chemical information indicates that the ZnO nanocrystals were thermally cleaned, but do not indicate any decomposition of the nanocrystals. μ-XPS was also used to determine the thickness of SiO₂ on Si. This article is the first to our knowledge where the versatile technique SPELEEM has been used to characterize ZnO nanocrystals.

Polymer/fullerene solar cells with three different device structures: A) diffuse bilayer, B) spontaneously formed multilayer, and C) vertically homogenous thin films, are fabricated. The photocurrent/voltage performance is compared and it is found that the self-stratified structure (B) yields the highest energy conversion efficiency

Polymer/fullerene solar cells with three different device structures: A) diffuse bilayer, B) spontaneously formed multilayer, and C) vertically homogenous thin films, are fabricated. The photocurrent/voltage performance is compared and it is found that the self-stratified structure (B) yields the highest energy conversion efficiency.

Characterization of nano-ZnO using SPELEEM and STMLeif Ericsson1, Kjell Magnusson1 and Alexei Zakharov21) Karlstad University, Sweden, 2) MAX-lab, Lund University, SwedenFuture functional applications of nanostructured ZnO, a wide bandgap semiconductor, include optical, electronic, optoelectronic, photovoltaic and piezoelectric devices. Much of the previous research has mainly concerned synthesis and growth of ZnO in different forms. In a previous survey, we have studied commercially available nanoparticles (Alfa Aesar GmbH, Germany) with an average size of 70 nm, in dispersions spin coated on SiO2/Si(001) surfaces. Using 2-10 mg/ml in ethanol and using ultrasound, we have prepared surfaces with a varying number of separated individual nanoparticles. These particles have been characterized with SEM and AFM, to reveal variations in morphology and size, and degree of agglomeration.In the present study, we have used Low-Energy Electron Microscopy (LEEM) and Photoemission Electron Microscopy (PEEM), the latter using synchrotron radiation, to further characterize the ZnO nanoparticles in terms of crystallinity, chemical state and electronic properties. The experiments have been done at MAX-lab, Lund, Sweden, at Beamline I311, using a system supplied by ELMITEC GmbH, Germany. In addition to microscopy, X-ray Photoelectron Spectroscopy from selected areas down to 1,3 micrometer, micro-XPS, has been used, yielding detailed spectroscopic information with this degree of lateral resolution.After a low-temperature anneal at 500 °C, LEEM images from 25 micrometer show strong (0,0)-reflections from separated areas of sizes down to less than 100 nm, in accordance with expectations for the size of the ZnO particles. Different electron energies show reflections from different grains due to orientation of the nanoparticles. Further annealing at 650 °C improves the LEEM image, yielding evidence for a fair degree of crystallinity and surface cleanliness in accordance with peak intensities from micro-XPS.The chemical state and electronic properties of both ZnO particles and SiO2/Si(001) substrate were studied with micro-XPS and PEEM, yielding well-resolved peaks from Zn 3d and Si 2p, which were used to create PEEM images with a perfect complementarity between Zn 3d and Si 2p based images. micro-XPS from ZnO nanoparticle agglomerates, ZnO free SiO2/Si(001) surface as well as mixed areas has been collected, showing differences in binding energies.For characterization of atomic structure and electronic structure with yet higher lateral resolution, Scanning Tunneling Microscopy and Spectroscopy, STM-STS, has been applied and preliminary result will be presented.

Controlled distribution of ZnO nanoparticles on Si/SiO2 surfacesIn recent years the properties and applications of ZnO nanomaterials has been extensively examined, partly due to the potential of ZnO as UV and visible light emitter and detector. Much of the previous work concern synthesis and growth of ZnO in different forms. In this survey, we have primarily studied commercially available nanoparticles and how they can be distributed on surfaces, aiming for future applications in e.g. photovoltaic devices. Later custom synthesised ZnO particles will be used in collaboration with Prof. Gunnar Westin at Uppsala University.ZnO nanoparticles, non-coated and organo-silane coated, with an average size of 70 nm from Alfa Aesar GmbH & Co KG, Germany, have been used in different dispersions for application on surfaces and the aim has been to achieve a controllable distribution of particles. In order to achieve this, different substrate preparations, solvents for dispersion and application methods have been used. Characterisation of the particle distribution has been done with Optical Microscopy (OM), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The chemical and the crystallographic properties of the particles have also been investigated using Auger Electron Spectroscopy (AES) and X-ray Diffraction (XRD).The substrates used are Si(001) with different preparations to create hydrophobic or hydrophilic surfaces. Nanoparticles were applied to the substrates using drop-coating and spin-coating. Several different solvents, including water, 1,2-PMA and chloroform, have been used for dispersions, yielding the possibility to use vapour pressure and solubility parameters to control the distribution. Separated particles are observed in all of the examined samples below a certain ZnO concentration. The amount of separated particles is dependent on the surface used, the solvent and the preparation procedure. A trend that can be followed from micro-scale to nano-scale is that smaller agglomerates correspond to more separated particles. Particles of different forms have been identified for later characterization.

We report on the effect of the substrate on the vertical phase separation in spin-coated thin films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-40,70-di-2-thienyl-20,10,30-benzothiadiazole] (APFO-3) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Compositional depth profiles of the films are measured by dynamic secondary ion mass spectrometry (SIMS).We found that changing the substrate from silicon to gold affects the composition profile near the substrate interface. This is caused by a specific interaction between the polymer (APFO-3) and the gold surface, as confirmed by X-ray photoelectron spectroscopy (XPS). The composition profile in the area away from the substrate interface, as well as the enrichment of the free surface with APFO-3, remain however unaffected by the choice of substrate. The vertical composition was also analysed for APFO-3:PCBM films spin-coated on indium tin oxide (ITO) coated with a thin layer of (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).