Electrical and Low Frequency Noise Characterization of Graphene Chemical Sensor Devices Having Different Geometries

Abstract

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. In this work, we investigate the effect of (1) ZnO nanoparticle functionalization and (2) engineered defects onto graphene sensing channel on device resistance and low frequency electrical noise. The engineered defects of interest include 2D patterns of squares, stars, and circles and 1D patterns of slots parallel and transverse to the applied electric potential. The goal of this work is to determine which devices are best suited for chemical sensing applications. We find that, relative to pristine graphene devices, nanoparticle functionalization leads to reduced contact resistance but increased sheet resistance. In addition, functionalization lowers 1/f current noise on all but the uniform mesa device and the two devices with graphene strips parallel to carrier transport. The strongest correlations between noise and engineering defects, where normalized noise amplitude as a function of frequency f is described by a model of A_N/f^γ, are that γ 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, A_N, and the device channel geometries. In general, for a given device area, the least noise was observed on the least-etched device. These results will lead to an understanding of what features are needed to obtain the optimal device resistance and how to reduce the 1/f noise which will lead to improved sensor performance.

Keywords: 1/f noise; N-ethylamino-4-azidotetrafluorobenzoate (TFPA-NH2); ZnO nanoparticles; chemical sensor; contact resistance; device geometry; epitaxial graphene; functionalization; low frequency noise.

Figure 1

(a) Computer-aided design (CAD) schematic of the device designs studied here. Note that four die were printed on a chip (8 × 8 mm² area). The devices are classified into four groups based on graphene film patterning: (1) unpatterned (labeled “U”) and interdigitated group (labeled “I₁, I₂, I₃, I₄”); (2) patterned with horizontal slots (labeled “H₁, H₂”); (3) patterned with vertical slots (labeled “V₁, V₂”); (4) patterned with 2D patterns (labeled “MS, MC, ME₂, ME₇”). Detail of the 2D patterns is shown in the inset. Descriptions are provided in Table 1. (b) CAD schematic of the TLM structures. The graphene mesas are 20 µm wide, the 70 µm × 100 µm Ti/Au pads overlap the graphene by 5 µm, and the uncovered lengths are 30, 25, 20, 15, 14, 13, 12, 11, 10, 5, and 3 µm.

Figure 1

(a) Computer-aided design (CAD) schematic of the device designs studied here. Note that four die were printed on a chip (8 × 8 mm² area). The devices are classified into four groups based on graphene film patterning: (1) unpatterned (labeled “U”) and interdigitated group (labeled “I₁, I₂, I₃, I₄”); (2) patterned with horizontal slots (labeled “H₁, H₂”); (3) patterned with vertical slots (labeled “V₁, V₂”); (4) patterned with 2D patterns (labeled “MS, MC, ME₂, ME₇”). Detail of the 2D patterns is shown in the inset. Descriptions are provided in Table 1. (b) CAD schematic of the TLM structures. The graphene mesas are 20 µm wide, the 70 µm × 100 µm Ti/Au pads overlap the graphene by 5 µm, and the uncovered lengths are 30, 25, 20, 15, 14, 13, 12, 11, 10, 5, and 3 µm.

Figure 2

SR760 fast Fourier transform (FFT) spectrum analyzer noise measurement setup for graphene devices in a frequency range (a) from 0.24 Hz to 97.5 Hz and (b) from 0.001 Hz to 1 Hz at room temperature. A 3.3 kΩ wire wound resistor converted the induced current into a voltage for sampling either automatically by the SR760 or by an Agilent 34401A multimeter as triggered by an Agilent 33250A function generator.

Figure 3

Resistance of pristine and ZnO functionalized graphene as a function of distance between metal contacts as measured after fabrication. The dotted lines are the transfer length method (TLM) linear fits.

Figure 4

Sheet resistance, accounting for internal etched features, of different geometries on pristine and ZnO functionalized graphene as calculated from data extracted from TLM measurements and graphene features.

Figure 5

Normalized (S_V/V²) noise data, plotted vs. frequency and for each device: Pristine graphene devices are plotted with black triangles, functionalized devices are plotted with red circles. The four interdigitated devices of pristine graphene I₁–I₄ are also shown. A linear fit to a portion of the power spectrum, and the frequency range over which it was calculated, is shown for each data set. A representative 1/f line is also shown on each graph as a blue dashed line; the vertical placement is arbitrary, with no significance.

Figure 6

Noise scalar A_N plotted against (a) graphene area, (b) contact area, and (c) total perimeter for both pristine and ZnO functionalized graphene devices.

Figure 7

Frequency exponent γ plotted against (a) graphene area, (b) contact area, and (c) total perimeter for both pristine and ZnO functionalized graphene devices.