Electrical Characterization of Graphene Chemical Sensors



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Chemiresistive graphene sensors are promising for chemical sensing applications due to their simple device structure, high sensitivity, potential for miniaturization, low-cost, and fast response. The graphene films used in this work were grown on the Si face of semi-insulating, on-axis 6H-SiC substrates by Si sublimation at high temperature in a chemical vapor deposition reactor. The graphene films were used to fabricate devices with different geometries using oxygen plasma etching, followed by e-beam evaporation to form Ti/Au (10 nm/100 nm) contacts. The geometries included engineered defects of different sizes and shapes. The engineered defects of interest include 2D patterns of squares, stars, and circles, and 1D patterns of slots parallel and transverse to the contacts. The films were functionalized using N-ethylamino-4-azidotetrafluorobenzoate (TFPA-NH2) as a chemical linker, then zinc oxide nanoparticles (50 – 80 nm) were attached. In this study, first, the resistance and low frequency noise of devices were measured with and without ZnO nanoparticle functionalization. We find that, relative to pristine graphene devices, ZnO nanoparticle functionalization leads to reduced contact resistance but increased sheet resistance. In addition, in general, functionalization lowers 1/f current noise on most of the devices. The strongest correlations between noise and engineering defects, where normalized noise amplitude as a function of frequency f is described by a model of ANf-, are that  increases with graphene area and contact area, but decreases with device total perimeter, including internal features. We did not find evidence of a correlation between the scalar amplitude, AN, and the device channel geometries. In general, for a given device area, the least noise was observed on the least-etched device. In this work, the detection of decane, propyl benzene, thiophene and octanethiol by graphene-based sensors functionalized with and without ZnO has been extensively studied by modeling of devices with first-principles calculations based on density functional theory (DFT). The electronic properties of the pristine graphene and ZnO functionalized graphene were investigated in terms of the total density of states (TDOS) and projected density of states (PDOS). The simulation results showed that ZnO functionalized graphene provides a more energetically favorable surface for the adsorption of thiophene and octanethiol than the pristine graphene. In this work, the sensing properties of the devices were investigated for the decane, propyl benzene, thiophene, and octanethiol with and without ZnO nanoparticle functionalization. The ZnO functionalized devices showed the strongest response to thiophene and octanethiol vapor compared to the pristine graphene devices. Additionally, engineered defects onto graphene showed a marked improvement in sensitivity to thiophene and octane thiol vapors compared to unpatterned graphene. Furthermore, neither the pristine nor ZnO functionalized graphene sensors demonstrated significant response to decane or propyl benzene vapors. Based on these results, graphene sensor devices (with and without functionalization) were sensing chemicals that have sulfur atoms (S) rather than hydrocarbon molecules (C and H). There is no strong evidence of a correlation between the conductance and the device active (graphene) area or the contact area. On the other hand, we do see strong correlation and inverse correlation between the perimeter and conductance. As a result, engineered defects and functionalization with metal oxides significantly enhance the performance of graphene chemical vapor sensors. In high noise environments, acquiring sensor measurements accurately is difficult, especially when signals are weak compared to the noise levels. In these cases, linear filtering is not an appropriate technique for processing the signal and unique methods should be considered for extracting signal data. In this work, a simple and portable analog lock-in amplifier (LIA) circuit is designed for gas sensing. The LIA uses the phase detection (PD) technique to single out the data signal at a specific frequency. In PD-based techniques, signals at frequencies other than the reference signal are rejected and, therefore, do not influence the required signal data significantly. For the LIA, the reference sensor excitation is a 2 KHz 70 mVrms sinusoidal signal from the function generator, which is a part of the designed circuit. A low pass filter (LPF) after the PD circuit was then used to cancel high frequency noise and extract the absolute mean value of the data signal. In this work, PSPICE simulations were carried out using actual commercial IC models. This LIA circuit design enables one to acquire sensor measurements accurately in extremely noisy environments.



Chemical Sensors, Epitaxial Graphene, Functionalization, Gas Exposure, Graphene, Low Frequency Noise