Transfer-Free In Situ CCVD Grown Nanocrystalline Graphene for Sub-PPMV Ammonia Detection

Dennis Noll,Philipp Hönicke,Yves Kayser,Burkhard Beckhoff,Udo Schwalke,Stefan Wagner

doi:10.1149/2.0171807jss

Abstract

Reasonably-priced devices for the detection of toxic species in the atmosphere are critical for reasons of health. Previous research work shows the promising detection capabilities of graphene. Thus, we demonstrate the gaseous response of our nanocrystalline graphene field-effect transistors that can be fabricated hundredfold on a two inch substrate by our transfer-free in situ catalytic chemical vapor deposition process. By means of Raman spectroscopy and near edge X-ray absorption fine structure nanocrystallinity of the CCVD grown graphene films within the devices can be confirmed. Using a self-constructed vacuum probing station the sensitivity of the fabricated devices is extracted from dynamic electrical sampling measurements. With respect to ammonia it is found that the sensitivity is being higher than previously reported from other groups. Moreover, a comparable responsivity is achieved. A deeper understanding of the origin of the high sensitivity, which we attribute to the nanocrystallinity, is given by backgate input characteristics under varying ammonia concentration as well as from comparison with literature results on carbon nanotube gas sensors. Furthermore, the origin and influence of ammonia on the hysteresis of our nanocrystalline graphene field-effect transistors is discussed.

Highlights

For reasons of health protection the demand for environmental monitoring systems for the surveillance of harmful toxic gases and vapors in the ambient atmosphere has increased over the recent years
Studies have shown that the gaseous sensitivity of graphene devices seems to have their origin in residual surface adsorbates,[19] grain boundaries and defects in the graphene crystal structure[11,12,20] as well as defects in the underlying substrate.[21]. In this context we have investigated the gaseous sensitivity of our nanocrystalline graphene field-effect transistors, which are fabricated by means of a polymethyl methacrylate (PMMA)-enhanced transfer-free in situ catalytic chemical vapor deposition (CCVD) process.[22,23]
As the junctions among linked carbon nanotube (CNT) can be considered similar to grain boundaries we suggest that the exposition toward ammonia modifies the grain boundary potential barriers[46] of our nanocrystalline graphene, leading to the increased current flow and higher sensitivity of our nanocrystalline graphene field-effect transistors (ncGFETs)

Summary

Device Fabrication

Fabrication of our ncGFETs is done using a 2 silicon complementary metal-oxide-semiconductor (CMOS) process line, complemented with the CCVD process for the graphene deposition. By the use of an ultrasonic assisted heated solvent bath the lift-off photoresist and non-annealed PMMA is removed, leaving behind the dedicated PMMA-metal catalyst sites (see Fig. 1b). The substrates are annealed in nitrogen gas at a temperature of 900◦C to initiate metal grain growth and cluster formation.[22] the PMMA is pyrolyzed whereby the carbon becomes dissolved in the metal catalyst.[24] After 5 minutes of annealing methane (CH4) and Downloaded on 2018-08-30 to IP 134.130.184.83 address. Hydrogen (H2) are added into the CVD reactor for 10 minutes During this time the nanocrystalline graphene grows laterally extending over the borders of the metal catalysts connecting the dedicated catalyst sites.

Materials Characterization

Ammonia Detection

Findings

Conclusions