Fracture testing of free-standing thin films: drawbridge technique

A. Shafikov1*, R.W.E. van de Kruijs1, J. Benschop1,2, W.T.E. van den Beld1, Z.S. Houweling2, F. Bijkerk1

1) Industrial Focus Group XUV Optics, MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands

2) ASML Netherlands B.V., Veldhoven, The Netherlands


Fracture toughness is an important material parameter, which characterizes resistance to crack initiation and propagation. A number of methods exist for measuring fracture toughness at macro-scale, however, measuring fracture toughness of thin films has always been a challenge due to the necessity of very accurate alignment and control of the mechanical loads. In this work, we present a novel technique for tensile testing of freestanding ultrathin films. In this method, free-standing thin film specimen is actuated using a drawbridge structure fabricated in a silicon chip, which rises and pulls on the specimen when the chip is bent. The advantage of the method is in its’ simplicity and possibility to achieve nm-scale precision of displacement at the specimen by controlling the mm-range deflection of the chip.

Using this method, we studied fracture of 100 nm thick SiN films. The film was patterned to form double cantilever beam (DCB) test structure with a notch, acting as a pre-crack. Use of DCB geometry allowed to achieve stable, controlled propagation of the crack in the free-standing film upon gradual increase of mechanical load. Using finite element method, the total fracture energy Gc and the contributions from the two dominant fracture modes (opening and tearing) were evaluated. For the crack opening mode, fracture energy was measured to be GIc=6.7±0.3 J/m2, whereas for the tearing mode was about 3 times higher: GIIIC=19±2 J/m2, which suggests that additional energy dissipation occurs due to friction of the fracture surfaces.

Hypermethylated DNA is a cancer biomarker and can be detected in liquid biopsy samples like blood and urine. Upon the methylation of DNA, a methyl group is added to a cytosine base followed by a guanine base in the 5’-3’direction, the so-called CpG complex. Hypermethylated DNA based cancer diagnostics is not widely applied in the clinics nowadays due to the limited selectivity in the separation of hypermethylated DNA from non-methylated DNA. In this work we developed a hypermethylated DNA enrichment platform. The surface of the platform is coated with MBD2 proteins which function as receptors towards methylated CpGs. The binding strength of an MBD2 protein is a 2-fold higher towards a methylated CpG compared to a non-methylated CpG, thus enabling hypermethylated DNA enrichment. The hypermethylated DNA enrichment selectivity is directly dependent on the used MBD2 surface receptor density. At high MBD2 surface receptor densities both hypermethylated DNA and non-methylated DNA are enriched. On the other hand, using lower MBD2 surface receptor densities result into the enrichment of hypermethylated DNA only, which is due to the different binding strengths between an MBD2 protein and a (non-)methylated CpG. The hypermethylated DNA enrichment platform reduces the amount of co-enriched non-methylated DNA, thus enabling more selective cancer detection of hypermethylated DNA from liquid biopsy samples.

Single-Source Vapor Deposited Hybrid Halide Perovskites Thin Films for Photovoltaics.

Tatiana Soto-Montero, Suzana Kralj, Wiria Soltanpoor, Junia S. Solomon, Christoph Baeumer, Monica Morales-Masis*

Vapor-deposited halide perovskites are highly attractive for the fabrication of monolithic perovskite-silicon tandem solar cells using readily available textured Si bottom cells. However, vapor-deposited halide perovskites have been less explored as top cells as compared to solution-processed or two-step deposited perovskites. This study explores Pulsed Laser Deposition (PLD) as a single-source solvent-free room temperature vapor deposition technique for halide perovskites allowing conformal growth of thin films on texture substrates and the incorporation in heterostructure without damaging the underlayers. Specifically, we demonstrate that it is possible to fabricate double cation MA1-xFAxPbI3 thin films from a single solid target to achieve the desired stable photo-active cubic α-phase. During PLD, a mechanochemically synthesized 5 mm thick off-stoichiometric target is ablated pulse by pulse, creating a confined plasma plume that transfers the ablated species towards the substrate. Due to the different atomic masses of the target constituents (H, C, N, Pb, I), excess of MAI/ FAI compared to the PbI2 is incorporated to compensate for the losses of volatile compounds upon the laser ablation. Thus, the solid target composition and MA+ : FA+ ratio is critical to growing films with the desired stoichiometry. PLD-deposited films containing 30-60% of FA+ (based on XPS) showed the stabilization of the cubic α-phase as confirmed by XRD. Optical bandgaps tuning from 1.61 to 1.55 eV is demonstrated with tuning stoichiometry of the film determined by XPS. Furthermore, we investigated the critical role of the device contact layers ITO/SnO2/Fullerenes to aim for a columnar grain growth of films and its link to the solar cell performance achieving PCE > 12% for the first proof of concept solar cell double cation absorber by PLD. We demonstrate the potential of PLD as a solvent-free vapor-deposition technique to grow mixed-cation and multi-compound perovskites, opening the path for future developments of wide-bandgap perovskite thin-films with desired stoichiometry for monolithic tandem devices applications.

Alkaline-stable Poly(arylene piperidinium)-based Anion Exchange Membranes for Water Electrolysis

Xiuqin Wang and Rob G. H. Lammertink

Soft Matter, Fluidics and Interfaces, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, Enschede, 7522 NB, The Netherlands

The development of high-performance anion exchange membranes (AEMs) has been hampered by the "trade-off" issue of ionic conductivity and membrane swelling for water electrolysis. Crosslinking is a feasible method to handle this problem. Macromolecular crosslinkers have many reactive functional groups in the polymer chain, forming a much denser crosslinking network structure than the AEMs that use small molecular crosslinkers. Herein, partially functionalized polystyrene (FPVBC) was used as a macromolecular crosslinker to react with poly(aryl piperidine) (PAP) and finally fabricate novel crosslinked AEMs (C-FPVBC-x). The crosslinking polymer membranes with the ether-bond-free structure are beneficial to obtaining good overall membrane performance with high conductivity and a limited swelling ratio. Atomic force microscopy (AFM) results show that increasing the content of FPVBC facilitates the fabrication of well-developed micro-phase separation. The C-FPVBC-1.7 membrane demonstrates a maximum ionic conductivity of 40.15 mS cm-1 and a diffusion coefficient of 1.6110-9 cm2 s-1 at 30 °C. Besides, good alkaline stability is achieved for the C-FPVBC-1.7 AEM, where the conductivity only decreases by 6.94 % after alkaline treatment (1 M KOH, 50 °C for 1200 h). Furthermore, a water electrolyzer based on C-FPVBC-1.7 AEM exhibits a current density of 890 mA cm-2 at 2.4 V and 80 °C when using 1 M KOH aqueous solution as the electrolyte. Indicating the macromolecular crosslinked C-FPVBC-x AEMs possess a great potential for practical applications in water electrolysis.

Fig. 1 The chemical structure of the crosslinked AEMs.