Thermal evaporation based V2O5 thin film for extended gate field effect transistor pH sensor

In this work, vanadium pentoxide (V2O5) thin film was synthesized on the corning glass substrate by using thermal evaporation method. For this purpose, 99.9% pure V2O5 powder was used that was evaporated from a tungsten boat. Subsequently, the prepared film was used as pH sensor in the extended gate field-effect transistor (EGFET) system. Structural properties, morphological features, sensing performance and linearity measurements of synthesized film were studied. The structural properties indicated that the V2O5 thin film has the preferred orientation along (001) plane. The pH-EGFET sensor based on V2O5 thin film was fabricated. The sensitivity and linearity of the device were determined in the pH range of 2–12 of the buffer solution. The sensitivity and linearity values were found to be 41.7 mV pH−1 and 83% respectively in the linear region. The results of present study show that the V2O5 can act as promising material for the EGFET PH sensor.


Introduction
The extended gate field-effect transistor pH sensing has been applied and developed in promising numerous fields such as environmental detections, biosensor application, clinical measurements, and analytical chemistry [1,2]. Since Bergveld [3] proposed the field effect transistor for ion sensitive (ISFET) in 1970, several studies have worked to enhance the detecting properties of the ISFET [4]. ISFET developed a various structure denoted to as extended gate field-effect transistor system, that is low-cost, flexible shape, simpler package, and long stability as compared to the ISFET [5]. Several metal-oxide based thin films have been utilized as a membrane of the pH sensor such as tin oxide [6], titanium oxide [7], and zinc oxide [8]. Among these, vanadium pentoxide (V 2 O 5 ) can be utilized for pH sensor device due to its excellent electrical, high conductivity, and stability. Guerra et al [9] synthesized V 2 O 5 film on glassy carbon as a pH-EGFET sensor using sol-gel method. This substrate is a non-graphitizing, carbon which combines glassy and ceramic properties with those of graphite. The pH sensor device showed linear response to the various buffer pH solutions. This film revealed a good linearity and sensitivity within the pH values 2-12 of sensitivity 58.1 mV pH −1 . The results shown that the pH 12 cannot be not utilized in the sensing measurement as the V 2 O 5 xerogel is unstable at pH 12, which was attributed to the production of met-avanadate in the solution [10]. Guidelli et al [11] evaluated V 2 O 5 /WO 3 film using sol-gel method on glassy carbon substrate. The V 2 O 5 /WO 3 gel was obtained from 0.1 M NaWO 3 (sodium tungstate) and 0.1 M NaVO 3 (sodium meta-vanadate) via using ion exchange technique. The acid solution was prepared via the filtrating procedure through a cationic ion-exchange resin at pH 3. The final solution was contained 5% WO 3 and 95% V 2 O 5 percentages, which was kept for two weeks at room temperature. The reaction was generated a viscous red V 2 O 5 /WO 3 gel. In order to the best response, V 2 O 5 was mixed with a molar concentration of 5% WO 3 , which was operated linearly performance even in case of the pH value up to pH 12.
The good results can be attributed to the layered structure of V 2 O 5 , which intercalates a large number of the organic and inorganic species. This produces a stable of the electronic properties which attributes to the surface diffusion of the ions (H + ) in the aquatic phase. Thin film membrane was tested as a pH sensitivity of 1.36 μA 1/2 /pH in saturation regime, and linear sensitivity of 68 mV pH −1 . Vieira et al [12] used V 2 O 5 nanostructures as pH-EGFET sensor application. The results indicated that the sensitivity of the sensor was close with the theoretical limit value 59.2 mV pH −1 as compared with other nanostructure membranes. Several of the researchers studied the sensing application of the V 2 O 5 thin film as pH-EGFET [9,11]. On the basis of previous studies, there is no relevant study published for V 2 O 5 /Corning glass thin films as pH-EGFET sensor by using thermal evaporation method. In this current study, V 2 O 5 thin film membrane was synthesized on the corning glass substrate using thermal evaporation technique. Afterward, the deposited thin film membrane used as a pH-EGFET sensor application. The results indicated good sensitivity and linearity, which relates to the sensing large areas of the thin film.

Experimental
2.1. V 2 O 5 thin film preparation V 2 O 5 thin film membrane was deposited on corning glass substrates in a vacuum chamber by using thermal evaporation technique. The source material V 2 O 5 (purity 99.997%) was evaporated from the tungsten small boat. The chamber pressure of the deposition method was maintained at 5×10 −6 mbar. The distance between the source material and the substrate was fixed at 35 cm. The thermal annealing process was achieved by using furnace system (Nabertherm model R 120/1000) with a changing the temperature annealing from 100°C to 500°C under air pressure conditions.

Characterization techniques
The structural properties were investigated through HR-x-ray X'Pert PRO MRD PW 3040 from the PANalytical x-ray diffractometer system equipped with Cu-K radiation (λ=1.54056 A°), which operates at 40 kV and 30 mA and Raman spectrometer system (Horiba Jobin Yuon HR 800UV, Edi-son, NJ, USA) with Ar+ as the excitation source operated at a wavelength of 514.55 nm (20 mW). The surface morphology was studied by using field emission scanning electron microscopy (FESEM) (model FEI Nova NanoSEM 450) equipped with energy dispersive x-ray spectrometer (EDX) for chemical components. The sensitivity measurements of V 2 O 5 thin film membrane was examined using Keithley (2400) high voltage source and MOSFET (HEF4007UBD) system.

pH-EGFET sensor fabrication
The pH-EGFET sensor based on V 2 O 5 thin film was fabricated. In this regard, the sample was connected with the membrane cavity to collect the reference electrode with the sample in different pH solutions. During the measurement process, the temperature was controlled to prevent the effects on the pH solutions and electrical properties of the MOSFET ((HEF4007UBD) system. Subsequently, the reference electrode was attached to the sensing area (1×1 cm 2 ) of the glass substrate. The measurement was performed using various pH solutions of range 2-12. The measurements of drain current to drain voltage (I DS -V DS ) and drain current to reference voltage (I DS -V RFF ) were made using two Keithley (2400), as revealed in the figure 1.

Structural characteristics of the V 2 O 5 thin film
The diffraction peaks of V 2 O 5 film prepared on corning glass substrate using thermal evaporation method are shown in figure 2. The results indicate the diffraction peaks of V 2 O 5 along various planes. The diffraction peak at 2θ=20.1°is associated to plane (001) with high intensity. The XRD peaks at (101), (110), (301), (011), (002), (020) reflections are indexed with the orthorhombic crystal structure. These peaks were matched to the standard list of the V 2 O 5 thin film (JCPDS Card No. 00-041-1426) [13]. After the films' annealing, the XRD peaks become sharp and larger in intensity due to improvement in the crystallinity of V 2 O 5 thin film [14]. These results are in agreement with previous studies such as Akl et al [15], Vijayakumar et al [16], Santos et al [17] and Kumar et al [18]. However, Guerra et al [9] found different results as compared with the current work whose detail has been presented in the introduction section. The crystallite size 'D' of V 2 O 5 thin film membrane along the high intensity peak (001) has been calculated using Debye Sherrer's formula [19]: Here, λ is the incident wave-length of the XRD radiation (1.5406 Å), β is represents to the full-width half maximum (FWHM), θ is the angle of the XRD diffraction. The crystallite size value along (001) peak has been found to be 26.5 nm.
The strain behaviour was calculated of the V 2 O 5 thin film along (001) peak using the relation: where c is represents to the lattice constant of the V 2 O 5 film,  c is the standard lattice constant, which estimates from the XRD measurements. The strain value has been found to be 0.228%. The value indicates the tensile behaviour of the V 2 O 5 thin film. Figure 3 indicates the Raman bands of the deposited V 2 O 5 thin film that were examined in the range 100-1000 cm −1 . The Raman peaks are associated to the orthorhombic structure of the V 2 O 5 thin film [20]. These result are also in agreement with the previous investigations [21]. The high intensity of the Raman spectra peak centred at B 3g mode (143 cm −1 ) can be related to the lattice structure vibrations. This peak is attributed to the vibration of V-O-V mode and this validates the orthorhombic structure of V 2 O 5 thin film [22]. The Raman peak at (195 cm −1 ) is associated to the layered structure [23]. The peak located at 992 cm −1 could be attributed to A g mode of terminal oxygen band (V=O). The observed peak at 695 cm −1 is associated to the stretching mode

Performance of V 2 O 5 thin film as pH sensor
The pH-EGFE sensor based on the V 2 O 5 thin film membrane is shown in figure 5. The pH sensitivity and linearity of the sensor were examined in various buffer solutions (pH=2, 4, 6, 8, 10, 12), with the sensing area of  These results reveal a good performance of V 2 O 5 film membrane as compared to the previous membranes prepared via different methods [9,11]. For the linear region, the sensitivity value of the pH sensor was evaluated from the line slope, which was 41.7 mV pH −1 and the linearity value was found to be 83%, respectively. The sensitivity of pH at saturation region of the V 2 O 5 film membrane was 40.82 μA 1/2 /pH and the linearity value was 99%, respectively. The fast sensor response of the membrane is commonly associated to the chemical reactions at the external surface layers. Since the area and shape of thin film was kept constant, the increment in the oxide volume indicates in the augmentation of the oxide layers number constituting the thin film and, consequently, the increase of deprotonation sites number in the bulk, which leads to fast sensor response. The results indicate that the V 2 O 5 membrane shows good performance and is stable at pH 12 as compare to that investigated in the past studies [11,25]. The sensitivity can be attributed to the conductivity of the thin film [11]. For this study, the pH sensitivity of V 2 O 5 membrane is related to the crystalline structure and effective sensing area, which was an important role in the improvement of sensitivity [26]. In addition, the sensing area and the sites numbers on the membrane surface show control pH sensing. Therefore, the pH sensing is dependent on site reactions of the external surface, which increases the surface potential of thin film membrane. This increases the aggregation of the electrical positive charges on the membrane surface and enhances the pH sensitivity. This indicates that the V 2 O 5 membrane synthesized on corning glass by thermal evaporation method is a good potential pH-EGFET material for application as a pH sensor with excellent linearity. The parameter (β) of the sensitivity can be measured by using equation below [27] where q is represents the charge of electrons, N s denotes to the density of the surface sites, K b and K a denotes to the basic and acidic equilibrium constants, k is defined as Boltzmann factor, C DL symbolizes the capacitance of double layer according to the Gouy-Chapman theory [28]. The mechanism of the surface charge has been described using the site-dissociation theory. The interaction theory between the membrane surface and connected electrolytes is assumed to the isolated reaction sites on the membrane surface, which is susceptible to dissociation process. The insulator surface consists of hydroxyl groups (MOH), which represents positively charged depending on the (H + ) ions concentration in the solution. After the solution matches the surface of the film, hydroxide groups (MOH) act as separate sites for chemical reactions in the thin film. This sensing mechanism starts with the accepting or donating of the proton (H + ) to form positive or negative ions in the solution, respectively. The pH of the solution activates the hydroxide ions to bind with the proton (MOH 2 + ) or discharge a proton (MO − ), which are the binding sites. The exchange reaction of proton is dependent on the concentration and distribution of the solution over the membrane surface, type of the surface sites and material, in addition to the point of zero charge (pH pzc ) (i.e. the point that the density of the surface charge is zero) [29]. The voltage of the surface potential between electrolyte solution interface and sensing surface is expressed as below following [30]: The mechanism of the reaction between membrane surface and electrolyte solution with the different pH values can be explained as, when pH>pH pzc then the hydroxide groups on the surface provide (OH + ), and when pH<pH pzc then the hydroxide gives (O − ). If pH=pH pzc then it indicates a neutral charge (OH), as given below [11]; - According to this mechanism, the surface of the membrane will be charged negative or positive voltage which conducts to the MOSFET system. When positive voltage is applied, the conduction channel will increase, leading to increased sensitivity. Conversely, the conduction channel will decrease, resulting in decreased of the sensitivity. The pH sensitivity value is considered acceptable as compared to the lower sensitivities, but can be enhanced by modifying numerous essential factors, such as the methods of preparation, substrate type, and measurement conditions.

Conclusion
V 2 O 5 thin film membrane as pH sensor was synthesized on the corning glass substrate by using thermal evaporation technique. The structural, morphological characteristics of thin film membrane were examined via using XRD diffraction, Raman spectroscopy, and FESEM system. The pH sensitivity and linearity of the membrane were studied using reference electrode and MOSFET (HEF4007UBD) instrument. The results indicated that the V 2 O 5 thin film exhibits a sensitivity and linearity in the linear region whose values are found to be 41.7 mV pH −1 and 83%, respectively. The sensitivity value and linearity in the saturation region are 40.82 μA 1/2 /pH, and 99%, respectively. These results can be assigned to the large surface area of the thin film which increases the number of the surface sites, resulting in an increase in the pH sensitivity.