Quasi in-situ observation of the elastic properties changes of the graphene–low-density polyethylene composites
Graphical Abstract
Introduction
Graphene, the first representative of 2D materials family is still extensively studied by the scientists [1], [2], [3]. It is related not only to its outstanding properties [4], [5], [6], [7], but also potential use in modern devices like van der Waals heterostructures [8], or elastic and transparent electronics [9], [10], [11].
Materials like polymers, textiles, and so on are used as a support [12], [13], [14], [15], [16] for elastic electronics. Due to the high quality material needed for electronics, needs of high scalability, and mass production, physical as well as chemical vapor deposition (PVD, CVD) processes has to be used [17] for graphene growth. But, these processes are incompatible for direct growth on elastic substrates (especially on polymers due to their low temperature of decomposition [18]). For that reason transfer processes are needed to investigate graphene onto elastic base. Graphene transfer can be done with [19], [20] or without [21], [22] additional support. After every technology step performed onto active layer, a material characterization has to be done. In case of 2D materials, Raman spectroscopy and scanning probe microscopy (SPM) related techniques are very helpful, e.g. conductive atomic force microscopy [23], [24], [25], magnetic force microscopy [26], [27], scanning thermal microscopy [28], [29], and Kelvin probe force microscopy (KPFM) [30], [31]. Combining both techniques it is possible to obtain information about electromechanical properties of investigated materials with nanometer resolution.
Y. Wang et al. using just Raman spectroscopy concluded, that graphene properties were almost no affected by various type of substrates [32]. They observed, that only silicon carbide (SiC) modified the Raman spectra of graphene. It was due to strain caused by the covalent bonding between SiC substrate and epitaxial graphene. This interaction has to be much higher than van der Waals forces between graphene and substrates like SiO2 or PDMS. R. Wang et al. stated contrary conclusion [33]. Using Raman spectroscopy and KPFM they observed graphene interactions with substrates like PMMA, SiO2, and SiO2 modified by silanes. They showed, that by the use of different substrates it is possible to modify doping level and work function of graphene. But, R. Wang et al. investigated smaller set of substrates than Y. Wang et al. Liu et al. observed graphene oxide–poly(3-hexylthiophene) (P3HT) nanofibers interactions combining KPFM and Raman spectroscopy [34], [35]. P3HT had different nanostructure controlled by solvent quality and concentration. Liu et al. concluded, that stronger interactions are present between nanofibers than between nanofibers and graphene oxide. Nevertheless, they used Si as a substrate, so they were unable to observe electromechanical interactions in their nanostructures.
Despite many works appeared about graphene composites and graphene deposited on polymers [36], [37], [38], to our knowledge there is lack of investigations showing electrical behavior of graphene on polymers subjected to mechanical elongation. Here, He et al. use stretching stage to tune the CPD of elongated graphene [39]. They obtained 0.161 eV change for 7% strain. This result was in the agreement with theoretical predictions [40], [41]. Nevertheless, the obtained results was too much localized (small scan area) and they observed relatively large CPD difference between the minimum and the maximum of the observed area.
In this paper we present the results of electromechanical studies of graphene–low-density polyethylene (LDPE) composites. Two types of graphene were transferred onto LDPE using PMMA support: graphene grown by physical vapor deposition on liquid metallic matrix (so called high strength metallurgical graphene, HSMG) and commercial chemical vapor deposited (CVD) graphene. Raman spectroscopy was used for preliminary characterization of graphene, whereas Kelvin probe force microscopy (KPFM) combined with electrical measurement setup and symmetrical stretching stage was used for observations of the electrical properties changes of elongated graphene. Maximum elongation of graphene–LDPE composites were 10%. Resistance of the HSMG graphene was changing from 130 kΩ up to 900 kΩ for HSMG graphene and from 100 kΩ up to 300 kΩ for CVD graphene. Observed resistance changes were compared with CPD recorded by KPFM. KPFM showed that resistance changes can be attributed rather to the structural discontinuities for the CVD graphene sample. In case of the HSMG graphene sample such behavior was not observed.
Section snippets
Samples preparation
Two distinct types of graphene were investigated. The first one was commercially available CVD graphene (Graphene Laboratories Inc.). The next one was HSMG produced using Kula et al. method [42]. In this method copper/nickel composite (substrate, 72% Cu, 28% Ni) was initially pre-heated above its melting temperature (1200–1250 °C) in argon protective atmosphere (constant pressure of 100 kPa). Substrate was kept at this temperature range for 1 min to decrease amount of defects which could influence
Raman spectroscopy
General state of graphene can be described taking into account D, G, and 2D modes frequencies, their FWHM and relative intensities [45], [46], [48]. Some derived modes like G* peak can be also observed. Raman spectra of benchmark graphene samples (CVD graphene/glass and HSMG graphene/glass) are presented in Fig. 2, while Raman modes parameters are summarized in Table 1.
Both CVD and HSMG samples, poses G and 2D characteristic graphene modes intensities in correct relation (the 2D peak has higher
Conclusion
We present comparative study of the CVD and HSMG graphene grown on copper and transferred onto LDPE. Both materials were stretched up to ∼10% elongation. Even if higher resistance change was observed on HSMG graphene (from approximately 200 kΩ to 900 kΩ compared with ∼100 kΩ to 300 kΩ for CVD material) it wasn’t observed linear, discontinuities-like features on CPD images during stretching. HSMG graphene work function wasn’t affected so much by strain in comparison with CVD graphene. Nevertheless,
Acknowledgments
This work was partially supported by the National Science Centre, Poland by the PRELUDIUM 9 programme (grant Nr 2015/17/N/ST7/03850) and Wrocław University of Science and Technology statutory grant (Nr 0401/0034/17).
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