Papers by Silke Christiansen
Scientific Reports, 2017
We show that the highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate... more We show that the highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) can successfully be applied as a hole selective front contact in silicon heterojunction (SHJ) solar cells. In combination with a superior electron selective heterojunction back contact based on amorphous silicon (a-Si), mono-crystalline n-type silicon (c-Si) solar cells reach power conversion efficiencies up to 14.8% and high open-circuit voltages exceeding 660 mV. Since in the PEDOT:PSS/c-Si/a-Si solar cell the inferior hybrid junction is determining the electrical device performance we are capable of assessing the recombination velocity (v I ) at the PEDOT:PSS/c-Si interface. An estimated v I of ~400 cm/s demonstrates, that while PEDOT:PSS shows an excellent selectivity on n-type c-Si, the passivation quality provided by the formation of a native oxide at the c-Si surface restricts the performance of the hybrid junction. Furthermore, by comparing the measured external qua...
arXiv (Cornell University), Nov 20, 2020
Nanoparticles occur in various environments as a consequence of man-made processes, which raises ... more Nanoparticles occur in various environments as a consequence of man-made processes, which raises concerns about their impact on the environment and human health. To allow for proper risk assessment, a precise and statistically relevant analysis of particle characteristics (such as e.g. size, shape and composition) is required that would greatly benefit from automated image analysis procedures. While deep learning shows impressive results in object detection tasks, its applicability is limited by the amount of representative, experimentally collected and manually annotated training data. Here, we present an elegant, flexible and versatile method to bypass this costly and tedious data acquisition process. We show that using a rendering software allows to generate realistic, synthetic training data to train a state-of-the art deep neural network. Using this approach, we derive a segmentation accuracy that is comparable to man-made annotations for toxicologically relevant metal-oxide nanoparticle ensembles which we chose as examples. Our study paves the way towards the use of deep learning for automated, high-throughput particle detection in a variety of imaging techniques such as microscopies and spectroscopies, for a wide variety of studies and applications, including the detection of plastic micro-and nanoparticles.
RSC Advances, 2018
Hollow mesoporous silica capsules (HMSC) are potential drug transport vehicles due to their bioco... more Hollow mesoporous silica capsules (HMSC) are potential drug transport vehicles due to their biocompatibility, high loading capacity and sufficient stability in biological milieu. Herein, we report the synthesis of ellipsoid-shaped HMSC (aspect ratio $2) performed using hematite particles as solid templates that were coated with a conformal silica shell through cross-condensation reactions. For obtaining hollow silica capsules, the iron oxide core was removed by acidic leaching. Gas sorption studies on HMSC revealed mesoscopic pores (main pore width $38Å) and a high surface area of 308.8 m 2 g À1. Cell uptake of dye-labeled HMSC was confirmed by incubating them with human cervical cancer (HeLa) cells and analyzing the internalization through confocal microscopy. The amphiphilic nature of HMSC for drug delivery applications was tested by loading antibiotic (ciprofloxacin) and anticancer (curcumin) compounds as model drugs for hydrophilic and hydrophobic therapeutics, respectively. The versatility of HMSC in transporting hydrophilic as well as hydrophobic drugs and a pH dependent drug release over several days under physiological conditions was demonstrated in both cases by UV-vis spectroscopy. Ciprofloxacin-loaded HMSC were additionally evaluated towards Gram negative (E. coli) bacteria and demonstrated their efficacy even at low concentrations (10 mg ml À1) in inhibiting complete bacterial growth over 18 hours.
arXiv (Cornell University), May 16, 2016
Hexagonally aligned, free-standing silicon nanowire (SiNW) arrays serve as photonic resonators wh... more Hexagonally aligned, free-standing silicon nanowire (SiNW) arrays serve as photonic resonators which, as compared to a silicon (Si) thin film, do not only absorb more visible (VIS) and nearinfrared (NIR) light, but also show an inherent photonic light concentration that enhances their performance as solar absorbers. Using numerical simulations we show, how light concentration is induced by high optical cross sections of the individual SiNWs but cannot be optimized independently of the SiNW array absorption. While an ideal spatial density exists, for which the SiNW array absorption for VIS and NIR wavelengths reaches a maximum, the spatial correlation of SiNWs in an array suppresses the formation of optical Mie modes responsible for light concentration. We show that different from SiNWs with straight sidewalls, arrays of inverted silicon nanocones (SiNCs) permit to avoid the mode suppression. In fact they give rise to an altered set of photonic modes which is induced by the spatial correlation of SiNCs in the array, and therefore show a higher degree of freedom to independently optimize light absorption and light concentration. Apart from explaining the good light absorbing and concentrating properties of SiNC arrays, the work justifies a revaluation of SiNW arrays as optical absorbers. Introduction: To overcome the problem of low light absorption in silicon (Si) thin films, anti-reflective and light trapping surface structures were implemented in advanced photovoltaic (PV) device concepts, and modern nanotechnology could further promote innovation in the development of elaborate light management strategies. Random scatterers, like pyramidal structures 1 , upright nanocones 2 , periodic 3 , quasi random 4,5 , or randomly textured structures 6 have been investigated and could reduce reflection and enhance light trapping of Si thin films towards the ray optics limit (Lambertian limit / Yablonovich limit) 7. In wavelength-scale photonic structures, like arrays of SiNWs 8 , photonic crystals 9 or Si nanospheres 10 classical ray optics is no longer valid and they offer a fundamentally different approach towards light trapping strategies. Here, light trapping in photonic modes can even exceed the Lambertian limit for certain resonant wavelengths 11. Photonic modes enhance the local density of states (LDOS) in the material, which permits to achieve higher PV device efficiencies caused by light concentration 11-14. Arrays of SiNWs were proposed to improve light absorption in Si in both different ways: while random multiple scattering in SiNW arrays enhances broadband light absorption 15-17 , resonant optical phenomena such as Mie scattering at individual SiNWs 18-20 constitute the optical response of the entire absorber, i.e. cause an overall wavelength-, angle-or polarization-selective photonic absorption enhancement 21-23. Recently, arrays of inverted SiNCs were demonstrated to even exceed the light trapping in SiNW arrays 24. Using the results of numerical simulations, this work presents strategies to optimize light absorption in free standing arrays of SiNWs and SiNCs and specifies the fundamental difference between the two
arXiv (Cornell University), Jan 19, 2017
We show that the highly conductive polymer poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)... more We show that the highly conductive polymer poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) can successfully be applied as a hole selective front contact in silicon heterojunction (SHJ) solar cells. In combination with a superior electron selective heterojunction back contact based on amorphous silicon (a-Si), mono-crystalline ntype silicon (c-Si) solar cells reach power conversion efficiencies up to 14.8% and high opencircuit voltages exceeding 660 mV. Since in the PEDOT:PSS/c-Si/a-Si solar cell the inferior hybrid junction is determining the electrical device performance we are capable of assessing the recombination velocity at the PEDOT:PSS/c-Si interface. An estimated of ~ 400 m/s demonstrates, that while PEDOT:PSS shows an excellent selectivity on n-type c-Si, the passivation quality provided by the formation of a native oxide at the c-Si surface restricts the performance of the hybrid junction. Furthermore, by comparing the measured external quantum efficiency with optical simulations, we quantify the losses due to parasitic absorption of PEDOT:PSS and reflection of the device layer stack. By pointing out ways to better passivate the hybrid interface and to increase the photocurrent we discuss the full potential of PEDOT:PSS as a front contact in SHJ solar cells.
Engineering reports, May 12, 2022
Through silicon vias (TSVs) are a key enabling technology for interconnection and realization of ... more Through silicon vias (TSVs) are a key enabling technology for interconnection and realization of complex three-dimensional integrated circuit (3D-IC) components. In order to perform failure analysis without the need of destructive sample preparation, x-ray microscopy (XRM) is a rising method of analyzing the internal structure of samples. However, there is still a lack of evaluated scan recipes or best practices regarding XRM parameter settings for the study of TSVs in the current state of literature. There is also an increased interest in automated machine learning and deep learning approaches for qualitative and quantitative inspection processes in recent years. Especially deep learning based object detection is a well-known methodology for fast detection and classification capable of working with large volumetric XRM datasets. Therefore, a combined XRM and deep learning object detection workflow for automatic micrometer accurate defect location on liner-TSVs was developed throughout this work. Two measurement setups including detailed information about the used parameters for either full IC device scan or detailed TSV scan were introduced. Both are able to depict delamination defects and finer structures in TSVs with either a low or high resolution. The combination of a 0.4× objective with a beam voltage of 40 kV proved to be a good combination for achieving optimal imaging contrast for the full-device scan. However, detailed TSV scans have demonstrated that the use of a 20× objective along with a beam voltage of 140 kV significantly improves image quality. A database with 30,000 objects was created for automated data analysis, so that a well-established object recognition method for automated defect analysis could be integrated into the process analysis. This RetinaNet-based object detection method achieves a very strong average precision of 0.94. It supports the detection of erroneous TSVs in both top view and side view, so that defects can be detected at different depths. Consequently, the proposed workflow can be used for failure analysis, quality control or process optimization in R&D environments.
arXiv (Cornell University), Aug 28, 2016
Multiexcitonic transitions and emission of several photons per excitation comprise a very attract... more Multiexcitonic transitions and emission of several photons per excitation comprise a very attractive feature of semiconductor quantum dots for optoelectronics applications. However, these higherorder radiative processes are usually quenched in colloidal quantum dots by Auger and other nonradiative decay channels. To increase the multiexcitonic quantum efficiency, several groups have explored plasmonic enhancement, so far with moderate results. By controlled positioning of individual quantum dots in the near field of gold nanocone antennas, we enhance the radiative decay rates of monoexcitons and biexcitons by 109 and 100 folds at quantum efficiencies of 60% and 70%, respectively, in very good agreement with the outcome of numerical calculations. We discuss the implications of our work for future fundamental and applied research in nano-optics. 2 Introduction The ability to modify the lifetime of an atomic state by simply placing the atom in different environments continues to fascinate physicists 1. Indeed, control of the optical properties of matter can also have exciting technological implications, e.g. in making brighter light emitting devices or more efficient lasers 2. Since the first theoretical proposal of Purcell in 1946 3 , a large body of experimental works has demonstrated that the spontaneous emission rate of an emitter can be modified close to surfaces 4 , in microresonators 2;5 , and in or close to nanostructures 6. Nevertheless, very large enhancement factors of several hundreds or thousands remain a great challenge. In particular, microcavity solutions, which require high quality factors, are not compatible with the
Energies
The outdoor performance monitoring of perovskite modules over 16 weeks is reported. Two different... more The outdoor performance monitoring of perovskite modules over 16 weeks is reported. Two different types of active perovskite layers were studied: one type contained formamidinium chloride (FACl) halide additives and the other contained no additives with the main purpose to investigate performance trends during the outdoor exposure of those type of devices. Long-term side-by-side outdoor testing of devices with and without halide additives was not implemented in the past and merits investigation in order to determine the impact of additives on perovskite performance and stability. Although the two types of modules displayed similar initial outdoor performance characteristics, their outdoor performance evolution differed. Different degradation rates between the modules with and without additives were obtained just after field installation. In particular, the modules with additives exhibited higher performance degradation under open-circuit loading conditions between current-voltage (I...
What determines accuracy of chemical identification when using microspectroscopy for the analysis of microplastics?
Chemosphere
Bone, 2018
Cement lines are known as thin peripheral boundaries of the osteons. With a thickness below 5 µm ... more Cement lines are known as thin peripheral boundaries of the osteons. With a thickness below 5 µm their composition of inorganic and organic compounds has been a matter of debate. Here, we hypothesized that cement lines become hypermineralized and their degree of mineralization is not constant but related to the tissue age of the osteon. Therefore, we analyzed the calcium content of osteons and their corresponding cement lines in a range of different tissue ages reflected by osteonal mineralization levels in cadaveric femoral cortical bone of both postmenopausal women with osteoporosis and bisphosphonate-treated cases. Quantitative backscattered electron imaging (qBEI) showed that cement lines are hypermineralized entities with consistently higher calcium content than their corresponding osteons (mean calcium content: 29.46 ± 0.80 vs. 26.62 ± 1.11 wt%; p<0.001). Micro-Raman spectroscopy complemented the qBEI data by showing a significantly higher phosphate/amide I ratio in the cement lines compared to the osteonal bone (8.78 ± 0.66 vs. 6.33 ± 0.58, p<0.001), which was both due to an increased phosphate peak and reduced amide I peak in cement lines. A clear positive correlation of cement line mineralization and the mineralization of the osteon was observed (r=0.839, p=0.003). However, the magnitude of the difference between cement line and osteonal calcium content decreased with increased osteonal calcium content (r=-0.709, p<0.001), suggesting diverging mineralization dynamics in these osseous entities. The number of mineralized osteocyte lacunae per osteon bone area correlated positively with both osteonal and cement line calcium content (p<0.01). The degree of mineralization of cement lines may represent another tissue age related phenomenon, given that it strongly relates to the osteonal mineralization level. Understanding of the cement lines' mineralization and their changes in aging and disease states is important for predicting crack propagation pathways and fracture resistance mechanisms in human cortical bone.
Chemical Communications, 2022
Chemical vapor deposited (CVD) amorphous tantalum-oxy nitride film on porous three-dimensional (3... more Chemical vapor deposited (CVD) amorphous tantalum-oxy nitride film on porous three-dimensional (3D) nickel foam (TaN x (O y)/NF) utilizing tantalum precursor, tris(diethylamino)(ethylimino)tantalum(V), ([Ta(NEt) (NEt 2) 3 ]) with preformed TaN bonds is reported as a potential selfsupported electrocatalyst for hydrogen evolution reaction (HER). The morphological analyses revealed the formation of thin film of core-shell structured TaN x (O y) coating (ca. 236 nm) on NF. In 0.5 M H 2 SO 4 , TaN x (O y)/NF exhibited enhanced HER activity with a low onset potential as compared to the bare NF (À50 mV vs. À166 mV). The TaN x (O y)/NF samples also displayed higher current density (À11.08 mA cm À2 vs. À3.36 mA cm À2 at 400 mV), lower Tafel slope (151 mV dec À1 vs. 179 mV dec À1) and lower charge transfer resistance exemplifying the advantage of TaN x (O y) coating towards enhanced HER performance. The enhanced HER catalytic activity is attributed to the synergistic effect between the amorphous TaN x (O y) film and the nickel foam.
Scientific Reports, May 17, 2021
The original version of this Article contained an error in Figure 2B, where an incorrect panel wa... more The original version of this Article contained an error in Figure 2B, where an incorrect panel was mistakenly included as panel (f). This panel has been removed, and cannot be replaced due to antibody issues affecting reproducibility. The original Figure 2 and accompanying legend appear below.
arXiv (Cornell University), Sep 9, 2015
We demonstrate a new approach for forming hybrid metal/carbonaceous nanostructures in a controlle... more We demonstrate a new approach for forming hybrid metal/carbonaceous nanostructures in a controlled direct laser planting process. Au-Ag nanoclusters in amorphous or crystalline carbonaceous matrices are formed with different morphology: nanoparticles, nanoflakes, and nanoflowers. In contrast to other generation techniques our approach is simple, involving only a single laser-induced process transforming supramolecular complexes dissolved in solvent such as acetone, acetophenone, or dichloroethane into hybrid nanostructures in the laser-affected area of the substrate. The morphology of the hybrid nanostructures can be steered by controlling the deposition parameters, the composition of the liquid phase and the type of substrate, amorphous or crystalline. The carbonaceous phase of the hybrid nanostructures consists of hydrogenated amorphous carbon in the case of nanoparticles and of crystalline orthorhombic graphite of nanoscale thickness in the case of flakes and flowers. To the best of our knowledge this is the first demonstration of the fabrication of orthorhombic graphite with metal nano inclusions. The remarkable quality and regularity of the micron-sized nanoscale thickness single crystal flakes allows for cutting high resolution nano scale structures, which in combination with the metallic nano inclusions offer much design freedom for creating novel devices for nano photonic applications. The encouraging properties of the nanomaterials with different composition, size and shape stimulate the development of efficient synthesis strategies aimed at fine-tuning the functionality.
Physical review, Jul 24, 2017
Cathodoluminescence spectroscopy is a key analysis technique in nanophotonics research and techno... more Cathodoluminescence spectroscopy is a key analysis technique in nanophotonics research and technology, yet many aspects of its fundamental excitation mechanisms are not well understood on the single-electron and single-photon level. Here, we determine the cathodoluminescence emission statistics of InGaN quantum wells embedded in GaN under 6-30-keV electron excitation and find that the light emission rate varies strongly from electron to electron. Strong photon bunching is observed for the InGaN quantum well emission at 2.77 eV due to the generation of multiple quantum well excitations by a single primary electron. The bunching effect, measured by the g (2) (t) autocorrelation function, decreases with increasing beam current in the range 3-350 pA. Under pulsed excitation (p = 2-100 ns; 0.13-6 electrons per pulse), the bunching effect strongly increases. A model based on Monte Carlo simulations is developed that assumes a fraction γ of the primary electrons generates electron-hole pairs that create multiple photons in the quantum wells. At a fixed primary electron energy (10 keV) the model explains all g (2) measurements for different beam currents and pulse durations using a single value for γ = 0.5. At lower energies, when electrons cause mostly near-surface excitations, γ is reduced (γ = 0.01 at 6 keV), which is explained by the presence of a AlGaN barrier layer that inhibits carrier diffusion to the buried quantum wells. The combination of g (2) measurements in pulsed and continuous mode with spectral analysis provides a powerful tool to study optoelectronic properties and may find application in many other optically active systems and devices.
Materials and Corrosion
In the current research, magnesium and its alloys have been intensively studied as resorbable imp... more In the current research, magnesium and its alloys have been intensively studied as resorbable implant materials. Magnesium materials combine their good mechanical properties with bioactivity, which make them interesting for guided bone regeneration and for the application as barrier membranes. In this study, the in vitro degradation behavior of thin magnesium films was investigated in cell medium and simulated body fluid. Three methods were applied to evaluate corrosion rates: measurements of (i) the gaseous volume evolved during immersion, (ii) volume change after immersion, and (iii) polarization curves. In this comparison, measurements of H2 development in Dulbecco's modified Eagle's medium showed to be the most appropriate method, exhibiting a corrosion rate of 0.5 mm·year−1. Observed oxide and carbon contamination have a high impact on controlled degradation, suggesting that surface treatment of thin foils is necessary. The bioactivity test showed positive results; more...
Journal of Materials Science, 2019
A novel single-step, laser-induced and solution-based process is presented for synthesizing compl... more A novel single-step, laser-induced and solution-based process is presented for synthesizing complex hybrid metal/carbon nanostructures. The process relies on simply illuminating the interface between a substrate and a liquid solution of the supramolecular complex [Au 13 Ag 12 (C 2 Ph) 20 (PPh 2 (C 6 H 4) 3 PPh 2) 3 ][PF 6 ] 5 (hereinafter abbreviated as SMC) with an unfocussed He-Cd laser having a wavelength of 325 nm and an intensity of I = 0.5 W/cm 2. The process results in hybrid nanostructures of well-controlled morphology: nanoparticles (NP) and 2D flakes, which may also grow jointly to form 3D morphologically complex multipetal 'flower-like' structures. At the atomic scale, the obtained metamaterials are complex in composition and structure, i.e., they contain bimetallic Au-Ag nanoclusters of diameter 3-5 nm incorporated inside a carbonaceous matrix. This matrix can be amorphous or crystalline, and the details of the compositional outcome can be controlled and steered by the laser deposition parameters. Au-Ag nanoclusters show plasmonic behavior including the enhancement of electromagnetic fields of visible light. This leads to the enhancement of Raman scattering by the Au-Ag nanoparticle ensemble within the carbonaceous matrix. This enables a 3D architecture for stimulating surface-enhanced Raman scattering (SERS).
Zenodo (CERN European Organization for Nuclear Research), Nov 24, 2020
Silver nanowire (AgNW) networks show excellent optical, electrical, and mechanical properties, wh... more Silver nanowire (AgNW) networks show excellent optical, electrical, and mechanical properties, which make them ideal candidates for transparent electrodes in flexible and stretchable devices. Various coating strategies and testing setups have been developed to further improve their stretchability and to evaluate their performance. Still, a comprehensive microscopic understanding of the relationship between mechanical and electrical failure is missing. In this work, the fundamental deformation modes of five-fold twinned AgNWs in anisotropic networks are studied by large-scale SEM straining tests that are directly correlated with corresponding changes in the resistance. A pronounced effect of the network anisotropy on the electrical performance is observed, which manifests itself in a one order of magnitude lower increase in resistance for networks strained perpendicular to the preferred wire orientation. Using a scale-bridging microscopy approach spanning from NW networks to single NWs to atomic-scale defects, we were able to identify three fundamental deformation modes of NWs, which together can explain this behavior: (i) correlated tensile fracture of NWs, (ii) kink formation due to compression of NWs in transverse direction, and (iii) NW bending caused by the interaction of NWs in the strained network. A key observation is the extreme deformability of AgNWs in compression. Considering HRTEM and MD simulations, this behavior can be attributed to specific defect processes in the five-fold twinned NW structure leading to the formation of NW kinks with grain boundaries combined with V-shaped surface reconstructions, both counteracting NW fracture. The detailed insights from this microscopic study can further improve fabrication and design strategies for transparent NW network electrodes.
Index matching at the nanoscale: light scattering by core–shell Si/SiOxnanowires
Nanotechnology, Sep 22, 2016
Silicon nanowires (SiNWs) show strong resonant wavelength enhancement in terms of absorption as w... more Silicon nanowires (SiNWs) show strong resonant wavelength enhancement in terms of absorption as well as scattering of light. However, in most optoelectronic device concepts the SiNWs should be surrounded by a contact layer. Ideally, such a layer can also act as an index matching layer which could nearly halve the strong reflectance of light by silicon. Our results show that this reduction can be overcome at the nanometer scale, i.e. SiNWs embedded in a silica (SiO x ) layer can not only maintain their high scattering cross sections but also their strong polarization dependent scattering. Such effects can be useful for light harvesting or optoelectronic applications. Moreover, we show that it is possible to optically determine the diameters of the embedded nanoscale silicon (Si) cores.
Optical and structural characterization of Si/SiGe heterostructures grown by RTCVD
Thin Solid Films, Jul 1, 2000
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Papers by Silke Christiansen