Fabrication of alcohol sensor using undoped and Al doped ZnO nanostructure film with polymer electrolyte gating

Abstract

We report the fabrication of two terminal and three terminal gas sensor using Al-doped ZnO nanostructured-films and polymer electrolyte gate dielectric on glass substrate using vacuum free chemical method. The Al doped ZnO films are characterized by UV-vis Spectrometer, SEM, EDX and XRD. The characterization results have revealed the polycrystalline structure of both undoped and doped ZnO; with loosely packed, porous, and spherical granny nanostructure with mean grain size 20-10 nm and bandgap of the films is within the range of 3.12-3.16 eV. The conductivity of the ZnO film is tuned by Al concentration and the maximum value of conductivity was observed in 3 % Al doped ZnO films. Similarly, the best performance index of TFT such as current ON/OFF ratio, high transconductance and low threshold voltage was observed in 3 % Al doping concentration. The ordinary (two-terminal) sensor and three-terminal (FET) sensors' responses towards three different concentrations 50, 250, 500 ppm of ethanol and methanol vapors have been studied. The sensitivity of the film is modulated by Al concentration and higher value of sensitivity was achieved at 3 % Al doped ZnO films. The use of polymer electrolyte enhanced the sensitivity of the device which is more effective in methanol vapor. The Response-Recovery time of the sensor is significantly improved in three terminal devices than the two terminal devices.

Keywords: Al doped ZnO; EDL gate dielectric; Field effect transistor sensor; Polymer electrolyte.

Fig. 1

Fabrication of FET as a sensor and setup for sensitivity measurement [12].

Fig. 2

Sensing Mechanism of ZnO/AZO thin Film.

Fig. 3

Sensing Mechanism of EDL gated TFT.

Fig. 4

Calculation and Comparison of Bandgap Energy of pristine [a] ZnO thin film and [b-f] various concentrations of Al-doped ZnO films using Absorbance.

Fig. 5

XRD pattern of undoped and A:ZnO thin films labelled in legends.

Fig. 6

XRD Pattern of ZnO and AZO corresponding to (100), (002) and (101).

Fig. 7

SEM Images of [a] Pristine ZnO thin film [12] and [b-f] 1 %–5 % Al-doped ZnO thin films respectively.

Fig. 8

[a] EDX analysis image of 3 % A:ZnO, [b] table representing wt. % of constituent elements in undoped and different concentration of Al-doped thin films, [c-h] SEM images of selected area of ZnO and different concentrations of A:ZnO thin films over which EDX analysis was done.

Fig. 9

Resistivity of pristine ZnO and various concentration of Al doped ZnO varying with temperature.

Fig. 10

Transfer Characteristics of ZnO and AZOs channeled, EDL gated TFTs.

Fig. 11

Drain Characteristics of 3 % Al-doped ZnO channeled, EDL gated TFT at various Gate-Source Voltages.

Fig. 12

Sensitivity of Bare and EDL layered Two-Terminal Sensors in response to [a] Ethanol and [b] Methanol Vapors.

Fig. 13

[a] Variation in Resistances of Bare and EDL Layered 3 % A:ZnO films with respect to time in response to Ethanol and Methanol Vapors [b] Bar chart for response and recovery time of Bare and EDL coated 3 % A:ZnO thin-film sensors.

Fig. 14

[a-c] Drain current, [d] Sensitivity of ZnO and AZOs channeled FETs before and after exposure to various concentrations of alcohol.

Fig. 15

Variation of Drain Current of 3 % A:ZnO channeled EDL dielectric polymer gated TFT in response to Ethanol and Methanol Vapors (500 ppm), and Bar chart showing Response and Recovery time of respective TFT embedded.