Effects of drying temperature on tomato-based thin film as self-powered UV photodetector

Accepted Manuscript Full Length Article Effects of Drying Temperature on Tomato-Based Thin Film as Self-Powered UV Photodetector Myo Myo Thu, Atsunori Mastuda, Kuan Yew Cheong PII: DOI: Reference:

S0169-4332(18)30848-1 https://doi.org/10.1016/j.apsusc.2018.03.162 APSUSC 38910

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

5 September 2017 5 March 2018 21 March 2018

Please cite this article as: M. Myo Thu, A. Mastuda, K. Yew Cheong, Effects of Drying Temperature on TomatoBased Thin Film as Self-Powered UV Photodetector, Applied Surface Science (2018), doi: https://doi.org/10.1016/ j.apsusc.2018.03.162

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Effects of Drying Temperature on Tomato-Based Thin Film as Self-Powered UV Photodetector Myo Myo Thua , Atsunori Mastuda b and Kuan Yew Cheong *a E-mail: [email protected], [email protected], [email protected]. a

Electronic Materials Research Group, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia. b

Department of Electrical and Electronic Information Engineering, Graduate School of Engineering, Toyohashi University of Technology, Japan

Abstracts In this work, tomato thin-film is used as an active natural organic layer for UV photodetector. The effects of drying temperature (60 to 140⁰ C) on structural, chemical, electrical and UV sensing properties of tomato thin-film have been investigated. The photodetector consists of a glass substrate/tomato thin-film active layer/interdigitated aluminium electrode structure. As the drying temperature increases, surface and density of tomato thin-film is smoother and denser with thinner physical thickness. Chemical functional groups as a function of drying temperature is evaluated and correlated with the electrical property of thin film. A comparison between dark and UV (B and C) illumination with respect to the electrical property has been revealed and the observation has been linked to the active chemical compounds that controlling antioxidant activity in the tomato. By drying the tomato thin-film at 120oC, a self-powered (V = 0 V) photodetector that is able to selectively detecting UV-C can be obtained with external quantum efficiency () of 2.53x10-7%. While drying it at 140oC, the detector is better in detecting UV-B when operating at either 5 or -5 V with  of 7.7384x10-6% and 8.87x10-6%, respectively. The typical response time for raising and falling for all samples are less than 0.3 s.

Keywords : tomato, UV-B and UV-C radiation, drying temperature, UV-photodetector. 1. Introduction The Sun is the most powerful ultraviolet (UV) source, which is usually classified into three bands, UV-A (wavelength,  = 400–320 nm), UV-B ( = 320–280 nm) and UV-C ( = 200-280 nm). Only UV-A and partly UV-B are able to reach Earth surface. In order to utilize the remaining UV-B and C, the radiation source must be engineered out. There are needs to detect UV radiation for the used in various applications, such as biological and chemical sensors (detecting ozone, pollution level, organic compound and biological agents), flame detection (fire alarm, missile warning or combustion engine control), spatial optical communications (intra-and inter-satellite secured communications), emitter calibration (UV dosimetry and UV lithography), and astronomical studies [1-3] . Currently, the most widely used semiconductor-based photodetector materials is either from inorganic based such as silicon (Si) [4-10], wide and direct bandgap semiconductors such as SiC, III-nitrides compounds (AlN, GaN, InN), and selected II-V compounds [11-15] or organic based synthetic polymer [16-21]. In order to resolve issue of energy, researching on selfpowered UV photodetector with different materials has capture much attention [22 – 27]. Regardless whether the sensing materials are made of any of those materials, they are all sharing a common attribute that is inability to degrade in a short period of time after end of the photodetector’s lifetime. Therefore, disposable of the photodetector may create a huge environmental issue [28-30]. As a result, intensive afford has been initiated to search for materials that can be easily degraded with least toxicity aiming to reduce electronic waste [31,

32]. One of the solutions is to employ natural organic materials for this purpose that can be used for a designated period of time with acceptable performance. There are no reported work of using nature organic materials for UV photodetector application. However, there are some reports using nature organic materials, with minimum purification from its original source, to produce electronic devices such as memory, battery, transistor, etc [33-35]. Silk [35], carotenoid [36], β-carotene [37], aleo vera [38-41], leather [42] have been used as an active or passive material for the fabrication of biodegradability and environmental sustainable electronic devices. In this work, tomato as a natural source of organic material has been chosen to be used as an active material and converted to a solid thin film for the use of detecting UV radiation. Tomato is a member of Solanaceae family and it is considered as a kind of “Mediterranean diet”, which is strongly associated with its ability to reduce chronic degenerative diseases [43-46] due to its various pharmacologic properties [47-50] and rich in antioxidant compounds [51-54]. Various vitamins available in tomato are believe to be the source of antioxidant as the vitamins are activated by UV radiation, transferring of electrons that involves either releasing or trapping of electrons may occur. This electron transportation phenomenon can be modulated by applying an external power source and the detected current with respect to the intensity of UV radiation would serve as a basis in the photodetector of this work as well as justifying the usage of tomato as the active layer for this application. The importance of chemical composition affected by drying temperature may influence the performance of the photodetector will be systematically investigated. In order to use tomato as a solid-state UV photodetector, tomato juice must be extracted out from the fruit and the juice must then be formulated before it is deposited on a treated glass substrate and processed it into a solid and dried thin film. This natural material has yet been employed for this kind of work. Similarly, there are no related works that used other

natural organic materials for this purpose. The only available applications of tomato in electronic devices are for electronic nose and tongue [55] based on tomato the tomato to be used as a building block for any solid-state device application. Therefore, it is extremely important to process the tomato juice into a solid and dried thin film. However, up-to-date, there is also no literature related neither to the processing condition nor the property of thin-film tomato. Therefore, in this work, a systematic investigation has been performed in order to extract, formulate, and process the extracted tomato juice into a functional solid thin film with the objective of using it as a detecting layer of UV radiation with no (self-powered) or minimum operating power that is able to address issue of electronic waste.

2. 2.1

Materials and Methods Tomato juice preparation Extraction process of tomato juice consisted of four stages, namely washing,

homogenizing, centrifuging and vacuum filtering. The tomato fruits were bought from a local supermarket in Parit Buntar, Perak, Malaysia. A typical fresh tomato was between 38 and 56 g. The tomato was washed with distilled water in order to remove dirt. Then, it was cut into half for seeds removal before homogenized in a food blender (Philips HR2061). The homogenized juice were transferred into a centrifuged (Hettich Rotina 38) that operated at 3000 rpm for 30 min in order to remove any fibres from the juice by passing through a filter paper (Whatman No. 1) using a vacuum filter.

The extracted juice was collected in a plastic bottle and then stored in

refrigerate condition at 5oC for further usage. 2.2

Fabrication of test structure

A complete fabricated UV photodetector based on tomato thin-film as the test structure of this work is shown in Fig. 1(a) and its cross-sectional view is presented in Fig. 1(b). Laboratory microscope glass slides were used as the substrate of the fabrication of the test structure. The substrate was cut into a dimension of 1.5 cm x 1.5 cm using a diamond cutter and sequentially cleaned with acetone, ethanol, and deionized water in an ultrasonic bath for 30 min each. The cleaned substrate was then treated with 3 M of HCl (J.T. Baker) before the extracted tomato juice was spin-coated on top with 3000 rpm for 30 s. The spin-coated tomato layers were then dried in an oven at different temperature (60⁰ C, 80⁰ C, 100⁰ C, 120⁰ C and 140⁰ C) for 15 min. Finally, an array of Aluminium (Al) electrode was thermally evaporated on top of the dried tomato thin film using a turbo evaporator (K950X) through a shadow mask. The mask with interdigital electrode was having spacing of 300 µm. Al wire with a diameter of 1.0 mm was purchased from Alfa Aesar. The mask and Al wire were first cleaned in acetone ultrasonically for 30 min and dried before usage. 2.3

Characterization Effects of drying temperature on weight loss of the extracted tomato juice were examined

by thermogravimetric analysis (TGA) (Mettler Toledo STAR1 thermogravimetric analyzer) with heating rates of 2 and 10°C/min. Surface morphology of the dried tomato thin-film was investigated using a field-emission scanning electron microscope (FESEM) (S-36 Leica Cambridge Ltd. with Leo Supra 35 VP system). Surface topography of the dried tomato thin film was examined by an atomic force microscope (AFM) (Nanonavi SPA400-DFM, SII Nanotechnology). To identify chemical functional groups of the samples, Fourier Transform Infrared Spectroscopy (FTIR) (Perkin Elmer Spectrum 1) was used. Thickness and refractive index of the dried tomato thin-film was characterized by diffusive spectral reflectance technique

(MProbe 20). In addition, UV-Vis spectrophotometer (Cary 50-Varian) was used to determine the absorbance of the samples. To investigate the UV responds of the test structure, two different UV radiation sources (UV-B = 302 nm and UV-C = 254 nm) were used. The intensity of UV-B and C radiation sources were 0.0018 and 0.0013 A/cm2, respectively. The test structure was connected to an Agilent HP4156C Precision Semiconductor Parameter Analyzer (SPA) as shown in Fig. 1(b), and placed on a Lake Shore TTP4 cryogenic probe station. The current-voltage (I-V) and current-time (I-t) characteristics of the test structure with respect to the dark and UV conditions were measured. From the measurement, the current was then transformed into current density by dividing the current value with area of Al electrode (A = 9.45 x 10-2 cm2).

(a)

Al

Glass Tomato

Fig. 1. (a) Photo of a completed UV-photodetector with aluminium interdigital electrode deposited on tomato thin-film and (b) connection of test structure to a semiconductor parameter analyzer (SPA) controlled by a computer.

3. Results and discussion Figure 2 shows TGA result of extracted tomato juice heated at two different rates (2 and 10oC/min). There is no further weight loss beyond 95 and 115 oC for heating rate at 10 and 2oC/min, respectively. This temperature range (95 – 115oC) is considered as the transition

temperature for dehydration of tomato juice to occur as majority of the tomato juice is consists of water [56, 57]. In order to further understand the effects of drying temperature on the formation of solid thin film and UV sensing property, the drying temperatures were set between 60 to 140oC.

120

80

40

0 / min 2 C

60

0 / min 10 C

Weight (%)

100

20 0 20

40

60

80

100 120 140 160 180 200 220

Temperature (oC)

Fig. 2. Percentage weight losses as a function of temperature for extracted tomato juice heated at two different rates (2 and 10°C/min) measured by a thermogravimetric analyzer.

Surface morphology of tomato thin-film deposited on a glass substrate and dried at different temperatures was characterized by FESEM and the typical micrographs (10 KX) are shown in Fig. 3. For tomato thin-film dried lower than 140 oC, obvious white patches can be seen in the micrographs (Fig. 3 (a) to (d)). The density of the patches reduces as the drying

temperature is increased. The existing of the patches may be attributed to the incomplete solidification of the tomato thin-film. The temperature needs to solidify the layer can be correlated to the TGA result (Fig. 2), where a complete weight loss is observed beyond 95 or 115oC depending on the heating rate. Hence, the transition temperature (95 - 115oC) can be considered as the solidification temperature in which dehydration is completed. Therefore, below 95oC, it can be considered that the tomato layer is not fully dehydrated and it is revealed as the white patches in the micrographs. The patches are not observable at 140 oC as this temperature is far above-mentioned the transition temperature (95 - 115oC). In order to understand the effects of drying temperature on surface topography, atomic force microscopy has been performed. Figure 4 shows the three-dimensional views of surface topography of tomato thin-film dried at different temperatures. The root-mean square surface roughness with respect to the drying temperature is presented in Fig. 5. In general, the roughness reduces as the drying temperature increases. Approximately 86% reduction in roughness when the layer is dried from 60 to 140oC. The reduction in roughness may be attributed to the solidification process whereby the layer has been fully dried up (dehydrated) at higher temperature. This observation is in agreement with the results shown in FESEM (Fig. 3), whereby the patches appear in the micrographs may attribute to the surface roughness. As the tomato thin-film is being dried at higher temperature, it is expected that density of the thin film is increased as densification may happen when water is being dehydrated. This can be proven by the refractive index (n) of the thin-film dried at different temperature (Fig. 5). As the drying temperature increases, n value is also increased. The parameter n is directly correlated to the density of a measured material. In this work, a monochromatic light source with a wavelength of 302 nm was interacting with the tomato thin-film and the n value was extracted according to the procedure reported in Ref. [40].

The n value depends on the velocity (v) of light traveling in the thin film medium, i.e. the dried tomato layer, with n is inverse proportional to v. Therefore, if the thin film is denser, velocity of light traveling in the medium would be lower and hence the n value is higher. When the thin film is denser, it is also expected to have a thinner layer (Fig. 5) as the thin film was spin-coated using a similar rotational speed.

Fig. 3. FESEM micrograph (10 KX) of surface tomato thin-film dried at (a) 60⁰ C, (b) 80⁰ C, (c) 100⁰ C, (d) 120⁰ C and (e) 140⁰ C.

Fig. 4. Surface topography of three-dimensional view of tomato thin-film dried at (a) 60°C, (b) 80°C, (c) 100°C, (d) 120°C and (e) 140°C.

40 Refravtive index (n)

0.70

80

30 25

60

20 40

15 10

20

5 0

0

60

80

100

120

0.65 0.60 0.55

Refractive Index (n)

35

Thickness (nm)

Root Mean Square (RMS), Surface Roughness, (nm)

0.75

100 Thickness (nm)

RMS (nm)

0.50 0.45

140

Temperature (oC)

Fig. 5. Root-mean-square surface roughness, refractive index (measured at wavelength of 302 nm), and thickness of tomato thin-film as a function of drying temperature. Figure 6 presents the absorbance spectra measured by Fourier Transform Infrared (FTIR) Spectroscopy of tomato thin-film dried at different temperatures. The chemical functional groups of respective absorbance spectra is summarized in Table 1. In general, all samples show similar absorbance spectra at approximately same wavenumber except for their intensities. There are three main vitamin groups commonly reported in tomato [58], namely carotenoids (lycopens),

phenolic, and ascorbic acid (Vitamin C). These have been identified via FTIR result in this work. Two broad bands around 630.93 cm-1 and 3399.99 cm-1 are corresponding to -OH stretching and -OH bending, respectively [59]. These are associated to phenolic compounds, which is a kind of vitamin [60]. Most probably, the phenolic compounds are originated from Quercetin and Kaempferol in tomato [61]. Besides –OH group from phenolic compound, carbonyl functional group (C=O) is also being detected around 1844 cm-1 [62, 63]. This functional group may also associate to the other two vitamin groups, namely ascorbic acid and carotenoids [64]. There are another two absorbance bands that are located at approximately 1405 cm-1 (carboxylic acid, C=O) [65] and 1080 cm-1 (primary alcohol), which are assigned to ascorbic acid [66]. For carotenoids (lycopens), two absorbance bands are revealed in approximately 818 cm-1 (C-C stretching) and 630 cm-1 (CH- out of plane). Besides vitamin groups, lipids is another compound that can be found [60]. From FTIR result, absorption band at 2919.5 cm-1 is attributed to stretching mode of –C-CH3 and -CH2 and they may be originated from lipids of membrane in a crushed tomato [67]. According to the FTIR result (Fig. 6), absorbance intensities related to the three main vitamin groups and lipids are reduced when samples dried at 80oC if compared with at 60oC. Clifcorn and Peterson [68] experimentally shown that at temperature as high as 95 oC, ascorbic acid can be thermally degraded if compared to lower temperature. However, there is no literature reports on the changes of chemical compounds in tomato when heated beyond 95 oC. Therefore, in this research, an increasing trend in the three main vitamin groups and lipids has been demonstrated when the samples are dried from 100 up to 140 oC.

Table 1: Assignment of chemical functional groups of the respective absorbance spectra.

Referenc Chemical compound in tomato

Functional

60°C

80°C

100°C

120°C

140°C es

3420-3250cmPhenol

33399.

3399.9

3399.9

3399.9

3399.9

99

9

9

9

9

1060.4

1059.8

1080.0

1056.0

1080.0

3

9

3

4

3

850.67

850.77

905.34

856.89

818.7

1

[59]

(-OH) 1400-1440cm1 Carboxylic Ascrobi

aicd, (OH

c acid

bending and

[65, 69]

1063-1015cm-

Vitamin Group

1 (primary lcohols) 950-810cm-1 Lycope

C-C-stretch

ne

(Alcohols) (lycopene)

[64]

900-650cm-1, CH- out of plane(lycopenc

779.68

623.15

2919.5

2919.5

1869.6

818.62,

623.37

630.54

[64]

2919.5

2919.5

2919.5

[67]

1844.9

1844.9

1844.1

1844.9

2,

3,

3,

8,

,

1630.9

1630.9

1630.9

1634.1

1630.9

3

3

3

4

3

779.91

e), 2990-2850cm1 Lipid (C-CH3, CH2-) 1870-1650cmcarbonyl compound

1 (C=O),

[62, 63]

2.5

3399.99cm-1

100oC 120oC

60oC 80oC

140oC

Absorbance (a.u)

2.0

1.5

1.0

2919.5cm-1

1630.93cm-1 1080.03cm-1 1412.92 cm-1 1060.43cm-1 630.54cm-1

1844.91cm-1

779.91cm-1

1629.58cm-1 818.79cm-1

0.5

0.0

4000 3600 3200 2800 2400 2000 1600 1200 800

400

-1

Wavenumber (cm )

Fig. 6. FTIR absorbance spectrum of tomato at different drying temperatures. Optical property of tomato thin-film dried at different temperatures was measured by UV-Visible spectrophotometer and the absorbance spectra as a function of wavelength () is shown in Fig. 7. According to the result, a distinct absorbance spectrum is recorded in the range of  = 200 to 340 nm for all samples. The absorbance spectra cover the full range of UV-B ( = 280 – 315 nm) and partial of UV-A ( = 315 – 400 nm) and of UV-C ( = 180 – 280 nm). The results show that as drying temperature increases, intensity of the absorbance is reduced. As confirmed by previous literature, Quercetin and Kaempferol are the two main phenolic compounds available in tomato governing the absorption of UV in this particular wavelength [70] that enables effective scavenge of free radicals [71]. These two phenolic compounds are being confirmed by FTIR result (Fig. 6) in this work. The intensity of absorbance for two representative wavelengths from UV-B (302 nm) and C (254 nm) as a function of drying temperature is presented in Fig. 8. Therefore, these two wavelengths were used as the source to illuminate the tomato-based UV photodetector in this work. In general, the absorbance intensity

increases as the illumination wavelength reduces and the intensity is higher for the thin-film dried at 140oC if compared with samples dried at 120oC. This may be attributed to the higher concentration of phenolic compounds available in thin-film tomato dried at 140 than 120oC as being revealed in FTIR result (Fig. 6). When the drying temperature is lower than 120 oC, there is no significant differences in the absorbance intensity.

10.0 60o C 80o C 100o C 120o C 140o C

Absorbance (a.u)

8.0 6.0 4.0 2.0 0.0

200

250

300

350

400

450

500

550

Wavelength (nm)

Fig. 7. UV-Visible absorbance spectrum of tomato thin film dried at different temperatures.

9.0 254nm

Absorbance (a.u)

8.0

302nm

7.0 6.0 5.0 4.0 3.0 2.0 1.0 60

80

100

120

140

o

Temperature ( C)

Fig. 8. Intensity of absorbance for wavelength measured at 254 nm and 302 nm from UV-Visible spectrophotometer for tomato thin-film dried at different temperatures.

A test structure consists of tomato thin-film dried at different temperature and sandwiched between a glass substrate and array of Al interdigital electrodes (Fig.1(a)) was used to investigate electrical performance of the thin film (Fig. 1.(b)) and subsequently its UV sensing property. Initially, the test structures were measured in dark condition and the I-V responds was recorded and then transformed into current density (J) – voltage (V) for further analysis. The J-V outputs of test structures with tomato thin-film dried at three different temperatures are presented in Fig. 9. Drying temperature at 60 to 80oC were excluded in this set of experiment because the produced thin films were not fully dehydrated as being discussed in the previous paragraphs. The dark current density with respect to the applied voltage is compared with the current density when the test structures were illuminated with two different UV wavelengths (254 and 302 nm).

In general, there are changes in current density throughout the range of 5 V to -5 V. When the test structures are illuminated with a shorter wavelength (254 nm), current density with much lower value than in dark condition is recorded regardless of the drying temperature (Fig. 9). However, when the test structures are illuminated at higher wavelength (302 nm), the current density becomes higher than or comparable to the dark current density. The explanation of these observations are given below. Based on FTIR result (Fig. 6), vitamins and lipids are detectable in the samples. Within the compound of vitamins, as reported in literatures [58], carotenoids, phenolic and ascorbic acid are available [72]. It is believed that only phenolic and ascorbic acid are responding to UV in particular to UV-B [73] and C [71]. This respond is demonstrated in UV-visible spectrophotometry result (Fig. 7). When the test structure is illuminated with wavelengths of 254 and 302 nm, which are within the range of UV-C and B, respectively, free radial of vitamins (phenolic and/or ascorbic acid) are formed by releasing of electron and hydrogen. This phenomenon is part of an antioxidant activity being reported in tomato [74, 75]. In this work, when a voltage is applied to the test structure, the releasing of free electrons, due to antioxidant activity of the tomato being illuminated with UV radiation, may move according to its polarity. As a result, current is produced and measured. If this situation happens, the current density recorded under illumination of UV light would be higher than in dark condition. In contrast, the released free electrons can be trapped or scavenged by the free radial of vitamins. When this occurs, current density will be much lower than in dark condition. Therefore, there is a competition between (1) free radial formation with the releasing of free electron and (2) free electron scavenging activity by free radial when UV is illuminated on the test structure. By applying voltages, the flow of free electron will be affected depending on the competition

between the two processes that is affected by the wavelength being used. According to the J-V results in Fig. 9, UV-C (254 nm) with shorter wavelength and higher energy may produce more free radial of vitamins, if compared to its less energetic counterpart (302 nm). With higher amount of free radial, scavenging and trapping of free electrons would be more aggressive. Hence, less amount of free electrons can be moved to the opposite polarity when the voltage is applied. As a result, the current density is lower. In order to proof that tomato thin-film is governing the UV sensing effect, a control test structure without any tomato thin-film was tested. By only having the interdigital electrode that is directly sitting on top of the glass substrate, the JV responds are similar with and without illuminating with UV light regardless of the wavelength being used. This demonstrates that the responses of the photodetector is mainly attributed by the tomato thin-film.

10 -8

Current Density (A/cm )

10 -9

2

2

Current Density (A/cm )

10 -8

10 -10

10 -11

10 -12

302nm dark 254nm

(a) 60oC

10 -9

10 -10

10 -11

10 -12

dark 302nm 254nm

o

(b) 80 C

10 -13 -5

-4

-3

-2

-1

0

1

2

3

4

10 -13

5

-5

-4

-3

-2

-1

Voltage (V)

1

2

3

4

5

Voltage (V) 10 -7

2

Current Density (A/cm )

10 -8

10 -9

10 -10

10 -11

10 -12

Dark 302nm 254nm

(C) 100o C

-4

-3

-2

-1

0

1

2

3

4

10 -9

10 -10

10 -11

Dark 302nm 254nm

10 -12

5

-5

-4

-3

-2

-1

Voltage (V)

0

1

Voltage (V)

10 -8

2

-5

10 -8

(d) 120o C

10 -13

Current Density (A/cm )

Current Density (A/cm2)

0

10 -9

10 -10

10 -11 302nm Dark 254nm

o

(e) 140 C

10 -12 -5

-4

-3

-2

-1

0

1

Voltage (V)

2

3

4

5

2

3

4

5

Fig. 9. Current density – Voltage (J-V) characteristic of thin-film tomato based UV photodetector dried at (a) 100⁰ C, (b) 120⁰ C and (c) 140⁰ C. Performance of the tomato thin-film based UV photodetector was determined from the JV results. Four important figure of merits, namely responsivity (R), detectivity (D*), quantum efficiency (), and response time (), related to the performance of the photodetector have been extracted and calculated. Figure. 10 compares the R and D* values obtained from photodetector with tomato thin-film dried at 100, 120, and 140oC and measured at -5 V and 5 V that had been illuminated with UV radiation of 254 and 302 nm. The R and D* values were calculated based on equations [76]: (Eq. 1)

and )

(Eq. 2)

where J is photocurrent density, Pin is incident optical power, q is charge of electron, and J dark is current density in dark condition. R and D* is directly proportional according to Eq. (2). As drying temperature increases from 100 to 140oC, reduction trend of R and D* values are recorded by the photodetector measured under illumination of  = 254 nm; except for drying temperature at 120oC and measured at 5 V (Fig. 10 (b)). This exceptional case shows a slight increment in the R and D* values. However, when the photodetector measured by a longer wavelength (302 nm), a reversed trend is being observed. Figure 11 presents the R and D* values from photodetector measured at 0 V with tomato thin-film dried at 100, 120, and 140oC, indicating that the photodetector is responding even without any power supplied. This self-

powered photodetector is independent on the drying temperature and illuminating wavelength shows the same trend of R and D* with minimum values recorded at drying temperature of 120oC. In general, the values of calculated parameters measured at 5 and -5 V are relatively higher than those at V = 0 V (self-powered mode). This is because as a higher voltage is being applied (5 and -5 V vs. 0 V), a higher electrical field may be imposed on the free electrons. Hence, higher concentration of free electrons may be able to flow in the opposite polarity and contributes to higher current density. Therefore, the calculated parameters (R and D*) are higher. Figure 12 presents the percentage differences between R values obtained from the two wavelengths (254 and 302 nm) were calculated based on the results presented in Figs. 10 and 11. The percentage differences between R values of  = 254 nm and 302 nm (R %) are calculated based on the following equation:

ΔR% 

R 254nm  R 302nm 100 R 254nm

(Eq. 3)

where R254nm and R302nm are the responsivity obtained from  = 254 nm and 302 nm, respectively. When the percentage is positive, a higher responsivity of the photodetector with respect to the shorter (254 nm) wavelength is preferable. If the percentage is negative, one can conclude that the photodetector is having a higher responsivity to longer (302 nm) wavelength. The highest percentage difference (70.6%) in positive region is recorded in photodetector with tomato thin-film dried at 120oC and measured at 0 V with quantum efficiency () of 2.53x10-7% (Fig. 13), R of 0.0519x10-6 A/W, and D* of 0.7645x1021 Jones (Fig. 10). If compared with the figure of merits (R, D* and ) recorded from synthetic polymer based UV-photodetectors [16, 18, 19, 21, 77-83] the values obtained from tomato thin-film based UV-photodetector of this work are much lower values. This may be attributed to differences in processing technique,

operating voltage, detecting wavelength, and purity of the detection layer. With no external power supply (0 V) and yet the photodetector works with the highest responsivity, makes it as an energy saving and self-powered device. This indicates that the thin-film tomato dried at this specific temperature is applicable to detect UV-C (254 nm) that is generally used for germicidal purpose [84]. The increment of current density after illuminated by UV-C may be attributed to the higher generation rate of free electron in the tomato compounds as mentioned in the FTIR analysis as compared with the scavenging rate of the electron at this specific drying temperature. Due to Schottky contact between Al and dried tomato thin film that may induced localized electric field at V = 0 V (self-powered condition) [85], the abundant of free electron is being drifted towards the Al electrodes. When the thin film tomato is dried at higher temperature (140oC), it is selectively and sensitively detecting UV-B (302 nm) when the photodetector is operating either with 5 V (R = 1.8847x10-6 A/W, D* = 1.8319x1021 Jones,  = 7.74x10-6 %) or -5 V (R = 1.8704x10-6 A/W, D* = 1.5608x1021 Jones,  = 7.68x10-6%). The reason of having this observation may be due to the free and scavenging electron responses with respect to the free radical being created by different wavelength of UV radiation. The external quantum efficiency of photodetector with tomato thin-film dried at 120 and 140oC illuminated with 254 and 302 nm is compared and shown in Fig.13. In general, the efficiency is much lower when it is operated at 0 V if compared with 5 and -5 V regardless of the drying temperature and illumination wavelength. The efficiency is comparable for photodetector illuminated with both wavelengths when the tomato thin-film is dried at 120oC and operated at 5 and -5 V. However, the efficiency is lower when the photodetector is illuminated with shorter wavelength and tomato thin-film is dried at 140oC. Therefore, photodetector with tomato thinfilm dried at 120oC had been used for the subsequent investigation of the respond time related to

the photodetector. Since the efficiency is comparable between 5 and -5 V operated device, a 5-V biased had been chosen in the following investigation.

1.8 254nm R at -5V 302nm R at -5V

254nm D* at -5V 302nm D* at -5V

1.6

2.0

1.4

1.5 1.2

-6

1.0 2.0

1.0 2.0

(b) 0.5

254nm R at 5V

254nm D at 5V

302nm R at 5V

302nm D at 5V

0.8 1.8

1.5 0.0 100

120

140

Temperature (oC)

1.0

0.6 1.6

1.4 1.2

0.5 1.0

0.0

0.8 100

120

Temperature (oC)

140

21

(a)

Detectivity, D* (x10 Jones) Detectivity, D*(x1021Jones)

Responsivity, R (x10 A/W) Responsivity (R), (x10-6A/W)

2.5

Fig. 10. R and

D*

100,

120,

(a) -5 V and (b) light

of

254

25 0.08 20 0.06

15 10

0.04

5 0.02 0

photodetector thin-film dried

21

254nm D at 0V 302nm D at 0V

Detectivity, D* (x10 Jones)

at

tomato

30

25nm R at 0V 302nm R at 0V

-6

with

Responsivity, R,(x10 A/W)

0.10

obtained from

values

and 140oC with 5 V under UV and 302 nm.

0.00 100

120

140 o

Temperature ( C)

Fig. 11. R and D* values obtained from photodetector with tomato thin-film dried at 100, 120, and 140oC with 0 V under UV light of 254 and 302 nm.

Responsivity different percentage, (%)

150

(UV-C) 254 nm

100 0V 50 -5V

0V 5V

- 5V

5V

0 0V -50

(UV-B) 302nm

-5V 5V

-100 100

120

140

Temperature (oC)

Fig. 12. Percentage of responsivity difference of tomato thin-film based UV photodetector measured at different voltage and thin-film dried at different temperature. Positive and negative region in the figure shows a higher selectivity of UV-C and B, respectively, of a UV

External Quantum Effiency ( ), (%)

photodetector

10-4 140o(UV-C) C

254nm 10-5

5V

-5V 5V

-5V

10-6 0V

0V 10-7

10-8

120

140 o

Temperature ( C)

ieny ( ),

10 -4 UV-B (302 nm) 10 -5

5V

- 5V

5V

-5V

Fig. 13. External quantum efficiency value of tomato thin-film dried at 120oC and 140oC with the applied voltage 0 V under UV light of 254 and 302 nm.

In order to investigate the respond time of the photodetector, tomato thin-film dried at 120oC and measured at 5 V had been performed. The photodetector was illuminated with a 302nm UV light for a five periodic on and off cycle with an interval of 15 s each. The current density as a function of time (J-t) responding to the cycles is presented in Fig. 14. When the photodetector is exposed to UV light, the J value increases and reaches a relatively stable value before starting to decay. When the light is switched off, the current density decreases substantially and follows by an increment in current density until it is plateau off gradually. By consecutively switching on and off the UV light, the on-off ratio of current density are approximately the same. The extracted respond times for raising and falling (Fig. 15 (a) and (b)) for the five cycles are similar i.e. 0.34 s. It is comparable to the work reported by Han, et.al

(2009), in which a hybrid of TiO2 and (9,9-dihexylfluorene) (PFH) was used as a sensing material and the rise and fall response time are less than 0.30 s [86]. However, when comparing the times with other synthetic polymer based UV-photodetector, the values vary from a tens of microseconds to a few seconds depending on the materials, processing technique, operating voltage, and wavelength of detection [16, 18, 19, 77-83, 87]. The quick response and stability observed in this work enables the thin-film tomato been used as a UV photodetector at this specific wavelength.

Current Density (x10

-10

2

A/cm )

1.6 1.4 1.2

ON

ON

ON

ON

ON

ON

1.0 0.8 0.6 0.4

OFF OFF

0.2

OFF

OFF

OFF

OFF

0.0 0

15 30 45 60 75 90 105 120 135 150 165 180

Times (s)

Figure. 14. Responses current density charcteristic of tomato thin-film dried at 120oC as a function of time for 5 switching cycles of UV (wavelength of 302 nm) on and off and measured at 5 V.

Current Density (x10

-10

2

A/cm )

1.6 (a)

1.4 Tr =0.340 s (10% to 90 % )

1.2 1.0 0.8 0.6 0.4 0.2 0.0 15

16

17

Times (s)

Figure. 15(a). A representative raising time respond from Fig. 14.

-10

2

Current Density (x10 A/cm )

1.6 (b)

1.4 Tf=0.360 s (90% to 10 % )

1.2 1.0 0.8 0.6 0.4 0.2 0.0 30

31

Times (s)

32

Figure. 15 (b) A representative falling time respond from Fig. 14.

4.

Conclusion In this work, tomato juice has been extracted, formulated, and processed into a solid thin

film deposited on a glass substrate for the use of detecting UV radiation, in particular UV-B and UV-C. The effects of drying temperature to form a solid thin film on the structural, optical, chemical, and UV detecting properties have been investigated. As the drying temperature increased, a smoother, denser, and thinner tomato thin film can be produced. The change of chemical functional groups with respect to the drying temperature have been evaluated with the identification of three groups of vitamins and lipids. The electrical property of the dried tomato thin-film with respect to the chemical functional groups and antioxidant activity when illuminating under different UV wavelengths had been proposed. A self-powered (V = 0 V) UV photodetector that was able to detect UV-C can be produced by drying the tomato thin-film at 120oC with external quantum efficiency of 2.53x10-7%. When the drying temperature increased to 140oC, the photodetector operating at either 5 or -5 V was selectively detecting UV-B with external quantum efficiency of 7.74x10-6 % or 8.87x10-6%, respectively. Acknowledgement This work was supported by AUN/SEED Net Research Grant (No. 304/ PBAHAN.6050345), JICA and Universiti Sains Malaysia. References

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Effects of Drying Temperature on Tomato-Based Thin Film as Self-Powered UV Photodetector

Highlights:      

Tomato thin-film is used as an active natural organic layer for UV photodetector. Effects of drying temperature (60 to 140⁰C) on structural, chemical, optical, electrical and UV sensing properties have been investigated. Active chemical compounds that controlling antioxidant activity in tomato responsible to the UV sensing property. Tomato thin-film at 120oC, a self-powered (V = 0 V) photodetector that is able to selectively detecting UV-C, can be obtained. Drying the thin film at 140oC, the detector is better in detecting UV-B when operating at either 5 or -5 V. Typical response time for raising and falling for all samples are less than 0.3 s.