Graphene Oxide and Polymer Humidity Micro-Sensors Prepared by Carbon Beam Writing

Abstract

In this study, novel flexible micro-scale humidity sensors were directly fabricated in graphene oxide (GO) and polyimide (PI) using ion beam writing without any further modifications, and then successfully tested in an atmospheric chamber. Two low fluences (3.75 × 10¹⁴ cm^-2 and 5.625 × 10¹⁴ cm^-2) of carbon ions with an energy of 5 MeV were used, and structural changes in the irradiated materials were expected. The shape and structure of prepared micro-sensors were studied using scanning electron microscopy (SEM). The structural and compositional changes in the irradiated area were characterized using micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Rutherford back-scattering spectroscopy (RBS), energy-dispersive X-ray spectroscopy (EDS), and elastic recoil detection analysis (ERDA) spectroscopy. The sensing performance was tested at a relative humidity (RH) ranging from 5% to 60%, where the electrical conductivity of PI varied by three orders of magnitude, and the electrical capacitance of GO varied in the order of pico-farads. In addition, the PI sensor has proven long-term sensing stability in air. We demonstrated a novel method of ion micro-beam writing to prepare flexible micro-sensors that function over a wide range of humidity and have good sensitivity and great potential for widespread applications.

Keywords: carbon ion micro-beam writing; graphene oxide; humidity sensors; polymers.

Figure 1

(a) A schematic illustration of the maskless production of a micro-structure via ion-beam writing. (b) A schematic illustration of the sensory microstructure prepared by carbon ion-beam writing on the surfaces of GO, PET, PI, and PMMA.

Figure 2

An equivalent scheme (high-pass filter) as was used to measure the change in the capacitance of the prepared micro-structures with varying humidity.

Figure 3

The RBS experimental spectra (points) and SIMNRA simulated spectrum (line) for GO (a), PET (b), PI (c), and PMMA (d) and the experimental ERDA (points) and SIMNRA simulated (line) spectra for GO (e), PET (f), PI (g), and PMMA (h), all pristine and irradiated by 5.0 MeV carbon ions with fluences of 3.75 × 10¹⁴ cm⁻² and 5.625 × 10¹⁴ cm⁻².

Figure 4

The SEM images of the structures prepared using ion-beam writing on surfaces of GO (a), PET (b), PI (c), and PMMA (d). These samples were irradiated with a fluence 5.625 × 10¹⁴ cm⁻².

Figure 5

The EDS spectra of unaffected and irradiated foils of GO (a–c), PET (e–g), PI (h–j), and PMMA (k–m). The fluence 1800 nC/mm² equals 3.75 × 10¹⁴ cm⁻² and 2700 nC/mm² equals 5.625 × 10¹⁴ cm⁻².

Figure 6

The Raman spectra of pristine and irradiated GO (a), PET (b), PI (c), and PMMA (d). The fluence 1800 nC/mm² equals 3.75 × 10¹⁴ cm⁻² and 2700 nC/mm² equals 5.625 × 10¹⁴ cm⁻².

Figure 7

The deconvoluted C1s peaks from high-resolution XPS spectra of pristine and irradiated GO (a,e), PET (b,f), PI (c,g), PMMA (d,h). The irradiation was performed with fluence 5.625 × 10¹⁴ cm⁻².

Figure 8

The electric resistivity and frequency–voltage characteristics of prepared micro-structures depending on the relative humidity: (a) The sheet resistivity of GO microstructures, (b) the sheet resistivity of PET microstructures, (c) the sheet resistivity of PI microstructures, (d) the sheet resistivity of PMMA microstructures, (e) the voltage–frequency characteristic of the structure prepared on the GO surface with a fluence 5.625 × 10¹⁴ cm⁻², (f) the voltage–frequency characteristic of the structure prepared on the GO surface with a fluence of 5.625 × 10¹⁴ cm⁻² in logarithmic scale and compared with commercial ceramic capacitors.