Flexibility of Fluorinated Graphene-Based Materials

Abstract

The resistivity of different films and structures containing fluorinated graphene (FG) flakes and chemical vapor deposition (CVD)-grown graphene of various fluorination degrees under tensile and compressive strains due to bending deformations was studied. Graphene and multilayer graphene films grown by means of the chemical vapor deposition (CVD) method were transferred onto the flexible substrate by laminating and were subjected to fluorination. They demonstrated a weak fluorination degree (F/C lower 20%). Compressive strains led to a strong (one-two orders of magnitude) decrease in the resistivity in both cases, which was most likely connected with the formation of additional conductive paths through fluorinated graphene. Tensile strain up to 3% caused by the bending of both types of CVD-grown FG led to a constant value of the resistivity or to an irreversible increase in the resistivity under repeated strain cycles. FG films created from the suspension of the fluorinated graphene with a fluorination degree of 20-25%, after the exclusion of design details of the used structures, demonstrated a stable resistivity at least up to 2-3% of tensile and compressive strain. The scale of resistance changes R/R0 was found to be in the range of 14-28% with a different sign at the 10% tensile strain (bending radius 1 mm). In the case of the structures with the FG thin film printed on polyvinyl alcohol, a stable bipolar resistive switching was observed up to 6.5% of the tensile strain (bending radius was 2 mm). A further increase in strain (6.5-8%) leads to a decrease in ON/OFF current ratio from 5 down to 2 orders of magnitude. The current ratio decrease is connected with an increase under the tensile strain in distances between conductive agents (graphene islands and traps at the interface with polyvinyl alcohol) and thickness of fluorinated barriers within the active layer. The excellent performance of the crossbar memristor structures under tensile strain shows that the FG films and structures created from suspension are especially promising for flexible electronics.

Keywords: CVD-grown graphene; FG suspension; fluorinated graphene; resistive switching; resistivity; tensile and compressive strains.

Figure 1

(a,b) Atomic force microscopy (AFM) images of CVD-grown graphene films transferred to the SiO₂/Si and PET substrates (c) The Raman spectra for graphene on SiO₂/Si substrate. (d) Resistivity as a function of fluorination time for few (1–4) structures on PET and one structure on SiO₂/Si substrates for comparison. R_o for both graphene samples was ~900 Ohm/sq.

Figure 2

The scanning electron microscopy (SEM) (a) and AFM (b,c) images of CVD-grown multilayer graphene (MG) films transferred to the SiO2/Si (c) substrates and PET (b) with the use of a laminator. The AFM images were measured in the regime of the height (b) and the frictional forces (c). The MG thickness in pyramid tops was about 8-10 nm. (d) The Raman spectra for MG on SiO2/Si substrate measured in the two different points.

Figure 3

Changes in the resistivity under the fluorination process for the CVD-grown MG transferred onto (a) the SiO₂/Si and (b) PET substrates. In the case of (b), different curves correspond to the different tested structures.

Figure 4

Changes in the resistivity under the tensile (+) and compressive (-) strain due to the bending of (a) fluorinated graphene (the fluorination time was 10 min) and (b–d) fluorinated MG on PET. Initial resistances were (a) R₀ = 2.0 kOhm, (c) R₀ = 67 kOhm/sq, (d) R₀ = 10³ kOhm/sq. (a,b) The sequence of deformations and measurements is indicated by numbers. The fluorination time is given in (c,d) at the curves as a parameter. Inserts in (a) and (b) presents a sketchy image of the tested structures.

Figure 5

The optical image of (a) the structure created from the partially fluorinated graphene suspension with a thickness of about 30 nm on PET with Ag contacts and (b) shaped as two interdigital fingers. (c) Film resistance versus bending radius and strain at repeated measurements. Points 1 and 2 correspond to resistance before and after the bending. (d) Schematic representation of a film formed from flakes before and during bending.

Figure 6

(a) The vertical crossbar Ag/ PFG/PVA /Ag structures on the polyimide (PI) substrate. The schematic cross-section of the printed structure and image of the active layer surface is shown in the insets. (b) The current–voltage characteristics for the printed crossbar structure measured before the deformation.

Figure 7

(a) The dependence of the current in the low ON and high OFF resistance states on the tensile strain and the bending radius of the crossbar structure. After x-axis breaking, few values of the I_on and I_off currents for the structure after removal of the strain are given. The inset shows a photo image of the deformed structure; (b) The current–voltage characteristics for the printed crossbar structure measured at the different tensile strain and after deformation.

Figure 8

Schematic representation of the differences in the graphene fluorination process on the rigid and flexible substrates.