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Nanocomposites consisting of conductive polymers and functional nanoparticles have recently been employed in photodetectors and imagers. Here, we present a novel hybrid-organic photodetector (HPD) that was optimized for the detection of X-rays meeting the specific needs of medical imaging. Devices were fabricated using inorganic lead sulfide (PbS) nanocrystal (NC) quantum dots (QDs) and a blend of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Quantum dots convert X-rays directly into charge-carriers that then migrate through the organic blend to the contacting electrodes. The performance of such devices depends on the thickness and probably on the morphology of the active layer. We discuss the synthesis and characterization of the PbS quantum dots, their incorporation into a HPD, and the performance of the HPD in X-ray sensing. Scanning electron microscopy and transmission electron microscopy show the PbS-QD distribution in the organic matrix. We find a strong tendency of the QDs to phase-separate from the organic blend.

Medical X-ray imaging requires cost-effective and high-resolution flat-panel detectors for the energy range between 20 and 120 keV. Solution-processed photodetectors provide the opportunity to fabricate detectors with a large active area at low cost. Here, we present a disruptive approach that improves the resolution of such detectors by incorporating terbium-doped gadolinium oxysulfide scintillator particles into an organic photodetector matrix. The X-ray induced light emission from the scintillators is absorbed within hundreds of nanometres, which is negligible compared with the pixel size. Hence, optical crosstalk, a limiting factor in the resolution of scintillator-based X-ray detectors, is minimized. The concept is validated with a 256 × 256 pixel detector with a resolution of 4.75 lp mm−1 at a MTF = 0.2, significantly better than previous stacked scintillator-based flat-panel detectors. We achieved a resolution that proves the feasibility of solution-based detectors in medical applications. Time-resolved electrical characterization showed enhanced charge carrier mobility with increased scintillator filling, which is explained by morphological changes.

On self-assembled monolayer-covered Cu-Au substrates, localized volume shrinkage at initial dealloying sites leads to cracks within the attacked regions. It is started from well-controlled surface structures to gain fundamental insights in the driving mechanisms of localized corrosion and crack formation. Both the crack density and the crack morphology are critically dependent on surface orientation, crystallography, and inhibitor molecule species.

The structure and chemistry of thiol or selenol self-assembled organic monolayers have been frequently addressed due to the unique opportunities in functionalization of materials. Such organic films can also act as effective inhibition layers to mitigate oxidation or corrosion. Cu–Au alloy substrates covered by self-assembled monolayers show a different dealloying mechanism compared to bare surfaces. The organic surface layer inhibits dealloying of noble metal alloys by a suppression of surface diffusion at lower potentials but at higher applied potentials dealloying proceeds in localized regions due to passivity breakdown. We present an in situ atomic force microscopy study of a patterned thiol layer applied on Cu–Au alloy surfaces and further explore approaches to change the local composition of the surface layers by exchange of molecules. The pattern for the in situ experiment has been applied by micro-contact printing. This allows the study of corrosion protection with its dependence on different molecule densities at different sites. Low-density thiol areas surrounding the high-density patterns are completely protected and initiation of dealloying proceeds only along the areas with the lowest inhibitor concentration. Dealloying patterns are highly influenced and controlled by molecular thiol to selenol exchange and are also affected by introducing structural defects such as scratches or polishing defects.

Nanoporous gold (npg) produced by dealloying of Au alloys has been recently proposed for many different applications from catalysis to bio-sensors. Furthermore, Au alloys constitute an important model system in the context of corrosion, dealloying, and stress corrosion cracking. Adsorbed self-assembled monolayers of thiol molecules inhibit the dealloying reaction. On thiol-modified Cu-Au alloys dealloying finally proceeds in a localized reaction instead of a homogeneous process. Using ethanethiols as an example, we show here that thiols can be effectively removed by application of anodic oxidation. Homogeneous porosity forms then after an oxidation step. In-situ X-ray diffraction and crystal truncation rod analysis of ethanethiol-modified Au (001) confirms that a bare Au surface is recovered after a similar oxidation step. In consequence, a multi-step dealloying process including complete initial thiol-layers as well as partially or fully re-cleaned surfaces can produce different micro-cracks, providing larger scale openings, and homogeneous nanoporous Au in between. Such hierarchically structured surfaces may show superior applicability for reactions where mass transport into the porous layer plays a decisive role for the reaction rate or detection probability.

Flow field-flow fractionation is a powerful method for the analysis of nanoparticle size distributions, but its widespread use has been hampered by large analyte losses, especially of metal nanoparticles. Here, we report on the colloidal mechanisms underlying the losses. We systematically studied gold nanoparticles (AuNPs) during asymmetrical flow field-flow fractionation (AF4) by systematic variation of the particle properties and the eluent composition. Recoveries of AuNPs (core diameter 12 nm) stabilized by citrate or polyethylene glycol (PEG) at different ionic strengths were determined. We used online UV-vis detection and off-line elementary analysis to follow particle losses during full analysis runs, runs without cross-flow, and runs with parts of the instrument bypassed. The combination allowed us to calculate relative and absolute analyte losses at different stages of the analytic protocol. We found different loss mechanisms depending on the ligand. Citrate-stabilized particles degraded during analysis and suffered large losses (up to 74%). PEG-stabilized particles had smaller relative losses at moderate ionic strengths (1-20%) that depended on PEG length. Long PEGs at higher ionic strengths (≥5 mM) caused particle loss due to bridging adsorption at the membrane. Bulk agglomeration was not a relevant loss mechanism at low ionic strengths ≤5 mM for any of the studied particles. An unexpectedly large fraction of particles was lost at tubing and other internal surfaces. We propose that the colloidal mechanisms observed here are relevant loss mechanisms in many particle analysis protocols and discuss strategies to avoid them.

We report an electrochemical in situ X-ray diffraction (XRD) and ex situ atomic force microscopy (AFM) study on the influence of the halide additives bromide and iodide in comparison to chloride in 0.1 M H 2 SO 4 solution on the selective dissolution of Cu 3 Au (1 1 1). Our results suggest that the addition of these halides have a pronounced effect on the dealloying process and lead to peculiar changes in the resulting surface morphologies. For an additive of bromide we observed (similar to an earlier study with chloride) a sequence of specific Au surface layers with an initial ultra-thin stacking-inverted Au-rich layer, subsequent pure Au islands, and finally substrate oriented nanometer-scale ligaments (nanoporosity). These structures occur with a negative shift in the transformation potentials compared to pure sulfuric acid solution. With an additive of iodide no stable passive Au film is formed and a porous surface was observed at relatively short times and low potentials. A precipitation of copper iodide (CuI) visible in the ex situ AFM images leads to the formation of surface porosity with bimodal length scales.

During selective etching (dealloying) surface-sensitive X-ray diffraction employing Synchrotron light has been used to in-situ monitor the potential-controlled formation of Au-rich films on the surface of Cu3Au (111) in iodide-containing electrolytes. Similar to the case in pure sulfuric acid we observed a sequence of structural transformations starting from a well-prepared pristine surface to a porous film consisting of substrate-oriented Au ligaments. Also stacking-reversed ultrathin Au-rich films and Au islands form as intermediate steps but no passive-like behavior was observed in iodide-containing electrolytes, i.e. the surface quickly developed Au ligaments after reaching the Cu dissolution potential. At low overpotentials comparatively coarse Au islands point to a higher mobility of Au/electrolyte interfaces in iodide-containing solutions. At higher overpotentials and also with higher iodide concentrations an epitaxial Cu-iodide precipitate film showed an orientation relation of CuI (111) || CuAu (111) and two azimuthal domains of || and || . This partially dissolution-inhibiting bulk CuI layer is observed to produce a bimodal pore size instead of usually obtained homogeneous porosity. The X-ray data and supporting ex-situ AFM and SEM images show marked differences in the morphology and connectivity of the forming nanoporous Au layer. Precipitation layers are thus suggested to provide means for controlling the nanoporosity for applications of dealloyed films and surfaces.

On page 4877, F. U. Renner, A. Bashir, M. Valtiner, and co-workers describe a star-like dealloying corrosion morphology that appears during the localized attack of smooth well-prepared Cu-Au surfaces. The surfaces are initially protected by thiol or selenol inhibitior films. Localized dealloying of Cu-Au produces nanoporous gold under stress and crystallographic cracks - thereby opening a new approach combining surface science with nanoscale mechanical testing.

Dealloying is widely utilized but is a dangerous corrosion process as well. Here we report an atomistic picture of the initial stages of electrochemical dealloying of the model system Cu(3)Au (111). We illuminate the structural and chemical changes during the early stages of dissolution up to the critical potential, using a unique combination of advanced surface-analytical tools. Scanning tunneling microscopy images indicate an interlayer exchange of topmost surface atoms during initial dealloying, while scanning Auger-electron microscopy data clearly reveal that the surface is fully covered by a continuous Au-rich layer at an early stage. Initiating below this first layer a transformation from stacking-reversed toward substrate-oriented Au surface structures is observed close to the critical potential. We further use the observed structural transitions as a reference process to evaluate the mechanistic changes induced by a thiol-based model-inhibition layer applied to suppress surface diffusion. The initial ultrathin Au layer is stabilized with the intermediate island morphology completely suppressed, along an anodic shift of the breakdown potential. Thiol-modification induces a peculiar surface microstructure in the form of microcracks exhibiting a nanoporous core. On the basis of the presented atomic-scale observations, an interlayer exchange mechanism next to pure surface diffusion becomes obvious which may be controlling the layer thickness and its later change in orientation.

Crevice corrosion (CC) and localized corrosion of metals remains a serious concern for structural materials. Yet both, a real-time and in-situ visualization of (1) initiation of localized corrosion, and (2) corrosion within confined geometries, remains very challenging. In this contribution, we will discuss how multiple-beam interferometry (MBI) can be utilized to directly visualize corrosion processes in real-time and with Ångstrom resolution within well-defined confinement geometries[1]. Also, we will discuss how electrochemical AFM can be utilized to study localized phenomena such as dealloying on patterned surfaces[2]. We will particularly aim to discuss and describe techniques and electrochemical cells, that we recently developed and will discuss how they may become useful in the field of corrosion and localized corrosion. With MBI, we can detect and track active sites of aluminum corrosion in NaCl solution within confined geometries[3]. We find that CC of aluminum randomly initiates in the confined crevice mouth, where the distance between apposing surfaces is between 20-300 nm. We can directly track oxide dissolution/formation, and corrosion-rates as well as their retardation due to sodium vanadate inhibitors present in solution. Formation of a compacted oxide layer effectively inhibits CC in 5 mM NaCl solutions with 2.5 mM of added NaVO3, while inhibition rapidly breaks down at chloride concentrations above 50 mM. Breakdown of the inhibition-layers is initiated by rapid dissolution of the protective oxide within the confined zone. With our newly developed electrochemical AFM cell we can accurately track localized initiation sites of corrosion in model de-alloying experiments on patterned surfaces with nm resolution in x/y and atomic resolution in vertical directions[4]. The time resolution of our EC-AFM is currently limited to about 10 seconds per frame. In both parts of this contribution we will discuss experiments with model systems and we aim to specifically focus on technical hurdles and pros/cons of our techniques, including an outlook into what may be achieved in the future. References: [1] M. Valtiner, X. Banquy, K. Kristiansen, G. W. Greene, J. N. Israelachvili, Langmuir 2012, 28, 13080. [2] M. Valtiner, G. N. Ankah, A. Bashir, F. U. Renner, Review of Scientific Instruments 2011, 82, 023703. [3] B. R. Shrestha, Q. Hu, T. Baimpos, K. Kristiansen, J. N. Israelachvili, M. Valtiner, Journal of the Electrochemical Society 162(7), 2015, C327. [4] B. R. Shrestha, A. Bashir, M. Valtiner, F. U. Renner, Faraday Discussion - Corrosion 2015, in press.

Nanoporous gold (npg) produced by dealloying of Au alloys has been recently proposed for many different applications from catalysis to bio-sensors. Historically, Au alloys constitute an important model system in the context of corrosion, dealloying, and stress corrosion cracking. While electrochemical dealloying in sulfuric acid produces homogeneous nanoporosity a thiol-modified starting surface results in surface cracks showing a nanoporous core with very fine ligament size [1]. The intact surrounding area exhibits the knowm ultrathin Au layer on top of the alloy stabilized by the self-assembled thiol film [2]. Adsorbed self-assembled monolayers of thiol molecules inhibit thus the dealloying reaction. On thiol-modified Cu-Au alloys dealloying finally proceeds in a localized reaction instead of a homogeneous process. Volume shrinkage during dealloying has been reported for Ag-Au alloys [3]. The forming cracks within the nanoporous defect regions indicate a volume shrinkage and stress accumulation also for Cu-Au alloys. Thiols can be also removed from Au surfaces and thus hierarchical structures may be formed by combining different potential treatments [4]. The crystallographic orientation of the surface influences the number density of the formed cracks as well as the crack morphology. The crack density is on some grains of special surface orientations especially high, while others exhibit only few cracks. The morphology reveals a clearly crystallographic influence on the crack morphology. Especially on (001) and (111) oriented surfaces clear 4-fold (Fig.1) and 3-fold symmetries are observed, respectively. The crystallographic nature of the cracks formed under near-isotropic hydrostatic stress (Fig.2) is in contrast to uni-directional compression behavior, e.g. during nanoindentation [5]. With the possibility to control the initiation of dealloying by a controlled introduction of defects [6] in the self-assembled thiol film the here presented system offers an interesting approach to address the mechanical behavior of nanoporous material during a corrosion process. [1] A. Pareek, S. Borodin, A. Bashir, G. N. Ankah, G. A. Eckstein, M. Rohwerder, M. Stratmann, Y. Gründer, F. U. Renner, Initiation and Inhibition of dealloying of single crystalline Cu3Au (111) surfaces, J. Am. Chem. Soc. 133 (2011), 18264. [2] A. Pareek, G. N. Ankah, S. Cherevko, P. Ebbinghaus, K. J. J. Mayrhofer, A. Erbe, F. U. Renner, Effect of thiol self-assembled monolayers and plasma-polymer films on dealloying, RSC Advances 3 (2013), 6586. [3] S. Parida, D. Kramer, C. A. Volkert, H. Rosner, J. Erlebacher, J. Weissmuller, Phys. Rev. Lett. 97, 35504 (2006). [4] G. N. Ankah, A. Pareek, S. Cherevko, J. Zegenhagen, F. U. Renner, Hierarchical nanoporous films obtained by surface cracking on Cu-Au and ethanethiol on Au(001), Electrochimica Acta 2014. [5] A. Volkert, E. T. Lilleodden, D. Kramer and J. Weissmüller, Appl. Phys. Lett., 2006. [6] M. Valtiner, G. N. Ankah, A. Bashir and F. U. Renner, Atomic force microscope imaging and force measurement at electrified and actively corroding interfaces: Challenges and novel cell design, Rev. Sci. Instr. 82 (2011), 023703.

We report on the influence of chemical surface modification on the selective dissolution and dealloying of Cu3Au (111) in 0.1 M H2SO4. Cu–Au alloys serve as longstanding model alloys for dealloying and stress corrosion cracking. Employing well-defined hexadecanethiol, mixed-aminobenzenethiol, and plasma-polymerized hexamethyldisiloxane surface layers we obtain detailed atomic-scale insight in the stability of the surfaces. The initial structural evolution of the modified surfaces is tracked by in situ X-ray diffraction using synchrotron radiation. In comparison to the usual sequence of surface states on unmodified Cu3Au (111) the modified surfaces develop a thicker and more stable passive-like Au-rich film below the critical potential. In this regime we observe anodic shifts in the potentials of the surface structural transitions and the suppression of an otherwise observed island morphology. This extreme stability of the modified passive-like surfaces is further confirmed by ICP-MS coupled to a scanning micro-electrochemical flow cell and ex situ SEM. Above the critical potential, the presence of the inhibiting protective layers leads to a localized dealloying mechanism with an altered microstructure of the forming nanoporous film in the form of micro-cracks. Understanding the stability of alloy surfaces is a prerequisite for further applications, e.g. for lithography or sensors.

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We report the design of an improved electrochemical cell for atomic force microscope measurements in corrosive electrochemical environments. Our design improvements are guided by experimental requirements for studying corrosive reactions such as selective dissolution, dealloying, pitting corrosion, and∕or surface and interface forces at electrified interfaces. Our aim is to examine some of the limitations of typical electrochemical scanning probe microscopy (SPM) experiments and in particular to outline precautions and cell-design elements, which must necessarily be taken into account in order to obtain reliable experimental results. In particular, we discuss electrochemical requirements for typical electrochemical SPM experiments and introduce novel design features to avoid common issues such as crevice formations; we discuss the choice of electrodes and contaminations from ions of reference electrodes. We optimize the cell geometry and introduce standard samples for electrochemical AFM experiments. We have tested the novel design by performing force-distance spectroscopy as a function of the applied electrochemical potential between a bare gold electrode surface and a SAM-coated AFM tip. Topography imaging was tested by studying the well-known dealloying process of a Cu(3)Au(111) surface up to the critical potential. Our design improvements should be equally applicable to in situ electrochemical scanning tunneling microscope cells.

We report the onset of electrochemical doping and subsequent visible light emission at 5V and 360K from a planar light-emitting electrochemical cell with a 1mm interelectrode gap containing poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylenevinylene] (MEH-PPV), poly(ethylene oxide) (PEO), and XCF3SO3 (X=K,Li) as the active material. We rationalize the unprecedented low turn-on voltage of such wide-gap light-emitting electrochemical cells by demonstrating that the active material contains a mixture of crystalline PEO+XCF3SO3 domains and amorphous MEH-PPV domains at room temperature, but that the crystalline domains have melted at 360K resulting in a significant increase in the ionic conductivity.