Open Access. © 2020 O-G Simionescu et al., published by De Gruyter. This work is licensed under the Creative Commons Attribu- tion 4.0 License Rev. Adv. Mater. Sci. 2020; 59:306ś313 Research Article Octavian-Gabriel Simionescu*, Cristina Pachiu, Octavian Ionescu, Niculae Dumbrăvescu, Octavian Buiu, Radu Cristian Popa, Andrei Avram, and Gheorghe Dinescu Nanocrystalline graphite thin layers for low-strain, high-sensitivity piezoresistive sensing https://doi.org/10.1515/rams-2020-0031 Received Jan 29, 2020; accepted May 22, 2020 Abstract: Bulk nanocrystalline graphite has been investi- gated as a possible candidate for piezoresistive sensors. The thin flms were grown using capacitively coupled plasma enhanced chemical vapor deposition and a tech- nological workfow for the transfer of the active material onto fexible substrates was established in order to use the material as a piezoresitive element. Preliminary electrical measurements under mechanical strain were performed in order to test the piezoresistive response of the material and promising GF values of 50 − 250 at 1% strain were ob- tained. Keywords: fexible sensors, flm transfer, gauge factor 1 Introduction Piezoresistive sensing relies on measuring the relative vari- ation of an electrical resistance ( ΔR R ) with respect to the applied mechanical strain ( ε = Δl l ) . The proportionality factor between these relative quantities represents the sen- sor sensitivity, termed as gauge factor (GF). Piezoresistive sensors are actively studied for a multitude integrated low- strain or high-strain sensing applications, such as: struc- *Corresponding Author: Octavian-Gabriel Simionescu: National Institute for Research and Development in Microtechnologies - IMT Bucharest, 126A Erou Iancu Nicolae Street, Voluntari city, Ilfov county, 077190, Romania; Faculty of Physics, University of Bucharest, 405 Atomistilor Street, Magurele city, Ilfov county, 077125, Romania; Email: octavian.simionescu@imt.ro Cristina Pachiu, Octavian Ionescu, Niculae Dumbrăvescu, Oc- tavian Buiu, Radu Cristian Popa, Andrei Avram: National In- stitute for Research and Development in Microtechnologies - IMT Bucharest, 126A Erou Iancu Nicolae Street, Voluntari city, Ilfov county, 077190, Romania Gheorghe Dinescu: National Institute for Laser, Plasma and Radi- ation Physics, 409 Atomistilor Street, Magurele city, Ilfov county, 077125, Romania; Faculty of Physics, University of Bucharest, 405 Atomistilor Street, Magurele city, Ilfov county, 077125, Romania tural health monitoring for various industries [1ś3]; pres- sure, inertial or cantilever MEMS [4]; as wearable elec- tronics for personalized health management by heartbeat, arterial pressure or respiratory rate monitoring [3, 5ś8]; touch/tactile sensors, motion detection and gesture regis- tration for man-machine interactions [3, 5ś7, 9ś11]. The common commercially available piezoresistive sensors are known as foil strain gauges and consist of a patterned metallic flm or a semiconductor bar exhibiting GFs around 2, or 100, respectively. Alternative materials and architec- tures are currently studied to improve sensitivity and sta- bility. Based on good and stable conductivity and favor- able chemistry and structure, graphene and other carbon- based composites and structures represent one of the most pursued paths. Graphene and graphene derived materials show great promise for strain sensing, usually based on a combina- tion between intrinsic, and inter-particle resistance varia- tion. Recent studies report GF values of: ~6.1 for low-strain sensing, using chemical vapor deposition (CVD) graphene transferred from a metal catalyst to polydimethylsilox- ane (PDMS) [12]; ~2 for strains up to 30% using buckled nanographene on PDMS [13]; ~11.4 for a graphene/epoxy composite [14]; ~1.9 for suspended graphene up to 3% strain [15]; from ~1143.5 to ~20.9 for 1 to 7 bilayers (BL) of graphene nanoplatelets (GNP), respectively, assembled layer by layer on a PDMS substrate at a strain of 5%, and ~301.61-29631 for a grid patterned 3 BLs of GNP at 10%- 25% strains [6]; ~457 for laser induced graphene (LIG) en- capsulated in a fexible polymer at strains up to 35% [7]; ~112 through direct laser writing (DLW) of porous graphitic structures on polyimide flms [3]; ~125 for inkjet printed graphene [16]; of ~20 for strains to 1% and of ~40 for strains higher than 1.5% in the case of LIG on Kapton flms [8]; and of ~2900 in the case of an individual single walled carbon nanotube (SWCNT) [17]. Although graphene and SWCNTs present extraordinary electrical conductivity, chemical stability and mechanical durability, they fall be- hind in regard to large-scale production and integration. In this context, large area directly grown and con- ventionally processable thin layers of nanocrystalline