We describe the mechanical properties of turbostratically graphitized carbon films obtained by carbon laser-patterning (CLaP) and their application as bending or mechanical pressure sensors. Stable conductive carbonized films were imprinted on a flexible polyethylene terephthalate (PET) substrate by laser-induced carbonization. After initial gentle bending, i.e. training, these sponge-like porous films show a quantitative and reversible change in resistance upon bending or application of pressure in normal loading direction. Maximum response values of ΔR/R0 = 388% upon positive bending (tensile stress) and −22.9% upon negative bending (compression) are implicit for their high sensitivity towards mechanical deformation. Normal mechanical loading in a range between 0 and 500 kPa causes a response between ΔR/R0 = 0 and −15%. The reversible increase or decrease in resistance is attributed to compression or tension of the turbostratically graphitized domains, respectively. This mechanism is supported by a detailed microstructural and chemical high-resolution transmission electron microscopic analysis of the cross-section of the laser-patterned carbon.
Fabricating electronic devices from natural, renewable resources has been a common goal in engineering and materials science for many years. In this regard, carbon-based coatings are of significance due to high availability of the raw materials and their environmental degradability. Carbonized materials and composites thereof have been proven as promising candidates for a wide range of future applications in flexible electronics, optoelectronics, energy storage or catalytic systems [1]. On the industrial scale, however, their application is inhibited by tedious and expensive preparation processes and a lack of control over the processing and material parameters. A promising tool to tackle that challenge is carbon laser-pattering (CLaP) allowing for the defined and site-selective synthesis of functional carbon-based materials for flexible on-chip applications [2]. Versatile inks, based on naturally occurring (molecular) starting materials are used to produce films, which are carbonized with a CO2-laser to obtain functional patterns of conductive porous carbon networks [3]. By chemical and physical fine-tuning of the laser patterned carbons (LP-C), we developed high-performance flexible resistive chemical and mechanical sensors. Their properties make them applicable in many fields e.g., robotics, bionics and even health monitoring. In this contribution, we present both, the material’s synthesis and tailored properties as well as in-depth microscopic and spectroscopic cross-sectional analyses of such coatings by advanced transmission electron microscopy, which provide unprecedented insights into the microstructure and local chemistry/bonding of laser-patterned carbon [4]. In that regard, we discuss the challenging cross-sectional preparation of such porous and sensitive structures by advanced ultramicrotomy. The gained insights in structure formation and local functionality pave the way for the utilization of alternative fast, large-scale carbonization methods such as rapid thermal annealing. [1] D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, M. C. Hersam, Chem. Soc. Rev. 2013, 42, 2824–2860. [2] R. Ye, D. K. James, J. M. Tour, Adv. Mater. 2019, 31, 1803621. [3] S. Delacroix, H. Wang, T. Heil, V. Strauss, Adv. Electron. Mater. 2020, 6, 2000463. [4] M. Hepp, H. Wang, K. Derr, S. Delacroix, S. Ronneberger, F. Loeffler, B. Butz, V. Strauss, NPJ Flex. Electron. 2021, accepted for publication
Complex devices like batteries, fuel cells or organic sensors and solar cells are commonly assembled from multiple functional components with different materials properties. To gain a fundamental understanding of the microstructure-property relations of such complex macroscopic structures, their degradation and failure mechanisms by complementary scale-bridging microscopic and spectroscopic techniques, it is desirable to prepare cross sections of entire devices or parts. Most challenging is the generation of electron transparent cross-sectional samples for investigation by (cryo) transmission electron microscopy (TEM); preparing TEM samples of devices consisting of different material classes may be utmost difficult. Therefore, TEM sample preparation is often preceded by the disassembly of a device down to individual components rendering an investigation of relations between the individual components impossible. (Cryo-)ultramicrotomy as well as plasma-FIB as advanced cross-sectioning techniques for challenging samples may solve those issues. Both are capable to generate ultra-thin, electron transparent cross sections of hundreds of micrometers in size being larger than the typical thickness of modern devices. In this contribution, we demonstrate the capabilities of modern (cryo-)ultramicrotomy in conjunction with advanced TEM to characterize such sensitive or reactive and thus challenging devices down to the atomic scale. Examples include battery parts, PEM fuel cells and novel flexible all-organic electronics. The achieved thickness of those cross sections is commonly of the order of a few 10 nm, which even allows for atomic-resolution imaging and advanced spectroscopy for microstructure and defect analyses as well as compositional and chemical-bond investigation, respectively. Hepp et al., Trained Laser-Patterned Carbon as High-Performance Mechanical Sensors, NPJ Flex. Electron. 6 (2022), 3, DOI: 10.1038/s41528-022-00136-0 Beaupain et al., Reaction of Li1.3Al0.3Ti1.7(PO4)3 and LiNi0.6Co0.2Mn0.2O2 in Co-sintered Composite Cathodes for Solid-State Batteries, ACS Appl. Mater. Inter. 13 (2021) 47488-47498, DOI: 10.1021/acsami.1c11750 Li et al., Atomic structure of sensitive battery materials and interfaces revealed by cryo-electron microscopy, Science 358 (2017), 506–510, DOI: 10.1126/science.aam6014 Figure 1
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