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Expeditious UV detection of tungstite (WO3·H2O) and tungsten oxide (WO3) decorated multiwall carbon nanotubes (MWCNT) based photodetector: ultrafast response and recovery time

  • Rahul Majumder
  • Soumalya Kundu
  • Ria Ghosh
  • Monalisa Pradhan
  • Dibyendu Ghosh
  • Shubham Roy
  • Subhadip Roy
  • Manish Pal ChowdhuryEmail author
Research Article
  • 153 Downloads
Part of the following topical collections:
  1. Materials Science: Materials for Modern Applications

Abstract

Tungstite (WO3·H2O) and tungsten oxide (WO3) decorated multi-walled carbon nanotube (MWCNT) based ultra-fast UV photodetectors were fabricated and characterized in this work. Tungstite was synthesized by wet chemical route and tungsten oxide was derived by calcinating tungstite at elevated temperatures (723 K and 1023 K). The crystal structures of tungstite and tungsten oxide were confirmed from X-ray diffraction analysis. The UV photo responsive behavior of the tungstite and tungsten oxide decorated MWCNT UV photodetectors were studied using 365 nm UV LED array in the intensity range 15–83 μW cm−2. The device showed good UV photoresponsivity with high external quantum efficiency (EQE). Tungstite decorated MWCNT UV photodetector exhibited maximum responsivity of 7.4 AW−1 along with EQE of 2550% under the incident UV intensity of 15 μW cm−2. Up to 500 Hz optical chopping frequency, the devices showed remarkable stability along with good repeatability of UV detection. Response and recovery time is recorded as low as 400 μs and 500 μs respectively for tungstite decorated UV photodetectors.

Keywords

MWCNT Tungstite Tungsten oxide UV photodetector Response time Recovery time 

1 Introduction

In the past decade, intensive research has focused on carbon nanotubes (CNT) due to its unique electrical, thermal and mechanical properties. CNTs are cylindrical tubes made of rolled-up sheet of single layer of carbon atoms (graphene) with typical diameter in nanometers. It can be synthesized by electrical arc discharge, laser ablation method and chemical vapor deposition (CVD) methods [1, 2, 3]. CNTs can be divided in three different categories depending upon the number of walls, single-walled CNT and multi-walled CNT (MWCNT). MWCNT has potential application in electrochemical devices, field emission devices, nano electronic devices, sensors [4, 5, 6] etc. Exposure to ultra-violet (UV) radiation can do serious damage to human and other living beings. From this point of view, UV photodetectors have an important role in health care. Even in commercial applications, such as space-to-space communications, flame sensing, pollution monitoring and water sterilization UV photodetectors are used extensively [7]. Though ZnO, GaN and In2O3 are used in UV photodetectors, there are few reports on tungsten oxide (WO3) as UV photodetectors [7, 8, 9, 10, 11]. Oxygen vacancies, oxygen absorption–desorption and electron–hole recombination phenomena near the surface of tungsten oxide contribute towards its photoconductive activity. In the present work, we report on fabrication and characterization of tungstite (WO3·H2O) and tungsten oxide decorated MWCNT based UV photodetectors. Tungstite and tungsten oxide were used as UV photoresponsive material and MWCNT network was exploited as the carrier of the photocurrent.

2 Materials and methods

2.1 Growth of MWCNT

MWCNT were grown on 300 nm oxidized silicon wafer using catalytic thermal chemical vapour deposition (CVD) technique. Oxidized silicon wafer was cleaned by using ultrasonication in acetone followed by vapour degreasing in hot iso-propanol. The catalyst solution was prepared by dissolving 50 mg ferric nitrate (Fe(NO3)2) in 10 ml ethanol. The solution was stirred with magnetic stirrer for 15 min at room temperature. A thin layer of catalyst seed layer was spin-coated on the oxidized silicon wafer at 1000 rpm for 60 s. The substrate was dried on a hot plate at 373 K for 5 min. The growth process was divided in three steps and carried out in a split open tube furnace. First step involved oxidation of the catalyst layer at 973 K in air for 10 min. In second step the catalyst layer was reduced at 1023 K in the continuous flow of hydrogen (H2) at 0.4 SLPM for 7 min. Finally, MWCNTs were grown on the reduced catalyst layer. After reduction, the chamber was flushed with argon (Ar) flow at 0.2 SLPM and maintained in Ar atmosphere until the growth temperature was increased to 1173 K. At this point flow of Ar, H2 and C2H2 were maintained at 0.6, 0.4 and 0.05 SLPM respectively. To reduce the presence of amorphous carbon, Ar gas was passed through water bubbler during growth at room temperature. The growth process was continued for 15 min at 1173 K. At the end, the chamber was taken out of the furnace and cooled to room temperature in 0.2 SLPM Ar flow.

2.2 Synthesis of tungstite and tungsten oxide

Tungsten oxide was prepared from sodium tungstate (Na2WO4) by solution based method as reported by Chai et al. [12]. 10 ml precursor solution of 0.15 M concentration was prepared in de-ionised water, followed by 30 min stirring with magnetic stirrer at 2000 rpm. 3.75 ml of nitric acid solution (3 M) was added drop-wise to the precursor solution under continuous stirring. Then the solution was stirred at 1500 rpm for 90 min before it was left for aging. Formations of tungstite and tungsten oxide are explained with the following reactions.
$${\text{Na}}_{2} {\text{WO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}} + 2{\text{HNO}}_{3} \to {\text{H}}_{2} {\text{WO}}_{4} + 2{\text{NaNO}}_{3} + 2{\text{H}}_{2} {\text{O}}$$
(1)
$${\text{H}}_{2} {\text{WO}}_{4} \to {\text{WO}}_{3} \cdot x{\text{H}}_{2} {\text{O}} + {\text{H}}_{2} {\text{O}}$$
(2)
$${\text{WO}}_{3} \cdot {\text{xH}}_{2} {\text{O}}\mathop \to \limits^{{773\,{\text{K}}/1023\,{\text{K}}}} {\text{WO}}_{3} + {\text{H}}_{2} {\text{O}}$$
(3)
WO3·xH2O was synthesized by oxidizing of Na2WO4 and the value of ‘x’ depends upon the degree of ionization of the solution. Adhikari et al. [13] proposed and confirmed the transformation of tungsten oxide from tungstite as explained in reaction 2 and 3 [13]. The solution was centrifuged at 5000 rpm for 10 min to separate the yellow precipitate (reaction 2). The precipitate was cleaned with de-ionized water and methanol repeatedly three times to remove residual chemicals. The precipitate was dried at 373 K and calcinated at 773 K and 1023 K to produce orthorhombic tungstite, monoclinic and mixed-phase tungsten oxide powder respectively. The three samples are noted as TO1, TO2 and TO3 respectively in the paper.

2.3 Characterization

Microstructural properties were investigated using FE-SEM (INSPECT F50, Netherland). Tungstite (TO1) and two tungsten oxide (TO2 and TO3) samples were characterised with Panalytical X-ray diffractometer (PW 3040/60). Optical properties are studied with Horiba T64000 Raman spectrometer (532 nm laser excitation at 2 mW/cm2 intensity). Kiethley A-221A-3 voltage source was used to apply bias voltage to the UV photodetectors and photo response current was measured using SRS 570 preamplifier and DAQ.

3 Result and discussion

3.1 Characterization of MWCNTs

FE-SEM image of the MWCNT grown by CVD technique is shown in Fig. 1a. Uniform growth of MWCNT network can be observed throughout the substrate. Raman spectroscopy data of the MWCNT display D and G peaks at 1350 cm−1 and 1590 cm−1, as shown in Fig. 1b. The intensity ratio (ID/IG < 100) of D and G peaks suggest growth of MWCNTs [14]. From Fig. 1b it can be observed that the ratio of intensity of D (ID) and G (IG) peaks is > 1, indicating the presence of defects in the MWCNTs [14].
Fig. 1

a FESEM image of MWCNT. b Raman spectra of MWCNT

3.2 Characterization of WO3·H2O and WO3

Surface morphology of the tungstite and tungsten oxide (TO1, TO2 and TO3) samples are shown in Fig. 2a–c. In the case of TO1, dried at 373 K, cuboid structures were observed. Similar cuboid particles are also observed in case of TO2. For the sample TO3, calcinated at 1023 K, large deformed particles formed due to agglomeration.
Fig. 2

FESEM images of a TO1, b TO2 and c TO3 powders. Scale bars correspond to 3 μm

X-ray diffraction (XRD) data of tungstite and tungsten oxide are shown in Fig. 3a. X-ray reflection from (020), (111) planes are observed at 16.45°, 25.57°, which are the prominent planes of tungstite (WO3·H2O). Three weak peaks of tungstite are also observed at 34.1°, 34.95°, 49.17° corresponds to (200), (002) and (042) planes respectively. The XRD data is in confirmation with the results reported by Miao et al. (JCPDS card no. 43-0679) [10]. The X-ray diffraction pattern indicates the formation of polycrystalline tungstite orthorhombic crystal, with good crystallinity along (002) and (111) planes. For sample TO2 predominant peak at 23.02° is observed along with several small peaks at 23.5°, 24.23°, 28.61° and 33.53°. TO2 is identified as the monoclinic phase of tungsten oxide (JCPDS no. 83-0951) [15]. The predominant peak at 23.02° is originated from the reflection from (002) planes whereas the weaker peaks represent (020), (200), (112) and (022) planes respectively. In sample TO3 all signature peaks of monoclinic are observed along with five smaller peaks at 26.48°, 35.33°, 47.17°, 48.56° and 55.68°. The peaks at 26.48°, 35.33° and 55.68° correspond to reflection from (120), (202) and (420) planes respectively. These three above mentioned peaks are observed for both monoclinic and orthorhombic WO3, but the peaks at 47.17° and 48.56° are the reflection from (004) and (104) planes of orthorhombic WO3 (JCPDS 20-1324) [16].
Fig. 3

a X-ray diffraction pattern and b Raman spectra of the three samples (TO1, TO2 and TO3)

Raman spectra of the three samples are shown in Fig. 3b. It the case of TO1, single broad peak and a single sharp peak are observed at 632 cm−1 and 944 cm−1 respectively. According to Ng et al. [17], transition metal oxides display sharp Raman peak in the range 950–1050 cm−1, which denotes the stretching of the metal–oxygen (W=O) double bond. For TO1, similar peak is observed at 944 cm−1, and the broad peaks at 632 cm−1 is due to the vibration of W‒O bond. The W‒O bond is weaker from the W=O, so the vibration is observed in the lower range. The peaks in between the range from 180 to 250 cm−1 occurs due to the stretching, bending or deformation of O‒W‒O as observed at 193 cm−1 and 230 cm−1 in TO1. In case of sample TO2 and TO3, the peaks at 713.8 cm−1 and 805.7 cm−1 are assigned to (O–W–O) stretching vibration whereas the peak at 271.6 cm−1 is due to the (W–O–W) deformation vibration [18, 19, 20]. The peaks are observed for both hexagonal and monoclinic WO3, but the peak 133.0 cm−1 correspond to the monoclinic WO3 [21].

3.3 UV photodetection characterization

The UV photodetector was fabricated by drawing silver (Ag) paste electrodes on the MWCNTs. Followed by drop casting of freshly sonicated TO1, TO2 and TO3 dispersed ethanol solutions in the area between two Ag electrodes. The schematic of the UV photodetector is shown in Fig. 4.
Fig. 4

Schematic of metal oxide decorated MWCNT UV photodetector

I–V characteristics of TO1 decorated MWCNT UV photodetector, at 1.0 V bias, is presented in Fig. 5a. The measurements were conducted in dark condition and under the UV illumination of 365 nm UV LED array at 15 μW cm−2 intensity. In Fig. 5b, the repeatability plot is shown for TO1 decorated UV photodetector. The repeatability measurement was performed under 83 μW cm−2 UV exposure and in dark, 5 s each. It is clearly seen from the plot that the UV photodetection is highly reproducible and the saturation of the photocurrent remained stable over the entire loop.
Fig. 5

a I–V characteristics of TO1 decorated MWCNT device under 15 μW cm−2 UV intensity at 1.0 V bias. b Time-resolved photocurrent of TO1 decorated MWCNT device in response to the on/off switching of 83 μW cm−2 UV

As shown in Fig. 6a, the photocurrent (Iph) of the UV detectors increased with UV intensity, varied from 15 to 83 μW cm−2. Photocurrent of a photodetector is measured from the difference between dark current (Idark) and light current (Ilight). Maximum photocurrent of 21.9 μA, 18.7 μA and 4.1 μA are obtained under UV illumination of 83 μWcm−2 for TO1, TO2 and TO3 decorated devices respectively. The measured photocurrents for the devices are much higher than the previously reported WO3 UV photodetectors [7, 8, 9]. In Fig. 6a, the photocurrent–UV intensity plot is fitted according to the following relation [22].
$$I_{ph} \propto P^{{{ \uptheta }}}$$
(4)
where P is the power of the incident UV and the response of the photocurrent to the corresponding UV light intensity is determined by \({ \uptheta }\). The response (\({ \uptheta }\)) of the TO1, TO2 and TO3 decorated devices are 0.28, 0.57 and 0.67 respectively. In case of TO1 decorated MWCNT device the response value is < 0.5, indicating presence of the traps at the interface [23]. Trapping and recombination of photo generated carriers and complex process of electron–hole generation yield the response (\({ \uptheta }\)) between 0.5 and 1, as obtained for the TO2 and TO3 decorated UV photodetectors [22]. The responsive behavior of different devices are shown in Fig. 6b. The responsivity (Ri) of the UV photodetector is measured from the photo current (Iph) and the illumination power over the effective UV detection area (Popt), using the following equation [24, 25],
$$R_{i} = \frac{{I_{ph} }}{{P_{opt} }}$$
(5)
For TO1 and TO2 decorated devices, maximum responsivities are 7.4 AW−1 and 4.1 AW−1 respectively. The responsivity of the devices is similar to the results reported by other researchers [7, 9]. As compared to the other two devices, TO3/MWCNT photodetector show lower responsivity of 0.6 AW−1.
Fig. 6

a Photocurrent response against the UV intensity (fitted with Iph \(\propto \text{P}^{{\uptheta}}\) ), b UV intensity dependent responsivity behavior and c Variation of EQE (%) along UV intensity (Fitted with power law) for TO1, TO2 and TO3 decorated MWCNT device

In Fig. 6c the photoconductive gain with respect to UV intensities is plotted for the three UV photodetectors. The ratio of photogenerated charge carriers against the number of absorbed photon by the photoactive martial is defined as photoconductive gain [26]. The photoconductive gain and external quantum efficiency (EQE) are expressed by the following equations,
$$Gain = \frac{{{\raise0.7ex\hbox{${I_{ph} }$} \!\mathord{\left/ {\vphantom {{I_{ph} } q}}\right.\kern-0pt} \!\lower0.7ex\hbox{$q$}}}}{{{\raise0.7ex\hbox{${P_{opt} }$} \!\mathord{\left/ {\vphantom {{P_{opt} } {h\nu }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${h\nu }$}}}} = \frac{{R_{i} }}{{{\raise0.7ex\hbox{$q$} \!\mathord{\left/ {\vphantom {q {h\nu }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${h\nu }$}}}}$$
(6)
$$EQE = \frac{{R_{i} }}{{{\raise0.7ex\hbox{$q$} \!\mathord{\left/ {\vphantom {q {h\nu }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${h\nu }$}}}} \times 100 \%$$
(7)
where q is the electronic charge, h is the Planck constant and \(\nu\) is the absorbed photon frequency. Maximum photoconductive gain of 25.5 (2550%), 13.97 (1397%) and 2.05 (205%) are calculated for the TO1, TO2 and TO3 decorated MWCNT UV photodetector respectively under the 83 μW cm−2 UV. The presence of traps and its contribution is explained by the power law fit in photocurrent and UV intensity plot shown in Fig. 6c. Under high UV light intensity, due to hole-trap saturation the devices showed lower EQE compared to the device under the low UV intensity.
The ability of a device to follow a fast varying optical signal is defined as the response speed of a photodetector. To measure the response speed of the UV photodetector, an optical chopper is used to switch the incident light on the device and the response was recorded using an oscilloscope. The photocurrent responses of the TO1 UV photodetector under different chopping frequencies, varied from 100 to 500 Hz, are shown in Fig. 7. Transient photocurrent measurement process of the UV photodetectors is described in the supplementary information. The device showed saturated ultrafast response under all the measured frequency. Even at the higher frequency of 500 Hz, the UV photodetectors showed remarkable reproducibility without any sign of cut-offs. Transient photoresponse, ultrasmall response time and recovery time for the devices are presented in Fig. 8. The response time (tr) is defined as the time taken to increase the photocurrent from 10% to 90% of the maximum photocurrent under the exposure of UV light and the time needed for the photocurrent to decay from 90% of the maximum value to the 10% is denoted as the recovery time (td) [7, 9].
Fig. 7

Ultra-fast response of TO1 decorated MWCNT photodetector at optical chopper frequency of a 100 Hz, b 200 Hz, c 300 Hz, d 400 Hz, e 500 Hz

Fig. 8

Response and recovery time of TO1, TO2 and TO3 decorated MWCNT UV photodetector

For TO1 decorated device the response and recovery time is calculated as low as 400 μs and 500 μs respectively which are much smaller than the values reported by Shao et al. [8]. They reported recovery and response time of 13 ms and 16 ms respectively for WO3 nanodisc/rGO UV photodetector devices [8]. Even compare to the other reported results of tungsten oxide UV photodetectors [7, 9], TO2 and TO3 devices had shown superior recovery time of 448 μs and 520 μs and response time of 504 μs and 540 μs respectively. A comparative study of response time, recovery time and other parameters is conducted between the present work and the previously reported for WO3, ZnO, InAs and TiO2 UV photodetectors, as shown in Table 1.
Table 1

Comparison of photoconduction parameters

Device

Photocurrent

Responsivity AW−1

EQE (%)

Rise/decay time

References

3D WO3 nanoshell

4.5 μA

5.1

1678

6.3 s/0.5 s

[7]

WO3 nanowire on carbon paper

0.6 μA

3 s/20 s

[9]

WO3 nanodisc/rGO

1.15 μA

6.4

13 ms/16 ms

[8]

WO3 polycrystal nanobelt

12 nA

2.6 × 105

8.1 × 107

[27]

Colloidal ZnO nanoparticle

~ 50 μA

61

< 0.1 s/1 s

[28]

TiO2 nanowire array

15.2 μA

0.17

60.7

[29]

InAs nanowire

~ 34.2 nA

4.4 × 103

[30]

WO3·H2O decorated MWCNT

21.9 μA

7.4

2550

400 μs/500 μs

This work

WO3 decorated MWCNT

18.9 μA

4.1

1397

448 μs/504 μs

This work

3.4 UV detection mechanism

UV activated charge generation mechanism of metal oxide (WO3 and WO3·H2O) is shown in Fig. 9a. In the dark environment the surface of the photoactive material absorb oxygen molecules by capturing free electrons from the metal oxide nanostructure, thereby creating a negatively charged oxygen ion layer over the surface [\(O_{2} \left( g \right) + e^{ - } \to O_{2}^{ - } \left( {ad} \right)]\). Due to incomplete oxidation and imperfect crystallization, surface oxygen vacancies are common in the metal oxide nano particles and these vacancy sites act as oxygen absorption centers [8]. Due to oxygen absorption a depletion region is formed along the surface of the metal oxide. Upon UV illumination electron–hole pairs are generated in the bulk of metal oxides. The UV generated carriers (holes) moved to the surface of the nano structure and recombine with the negatively charged oxygen ions \(\left[ {h^{ + } + O_{2}^{ - } \left( {ad} \right) \to O_{2} \left( g \right)} \right]\). The recombination increases the number free charges on the surface of photoactive material, thereby increasing the photocurrent in the metal oxide [8, 31, 32, 33].
Fig. 9

a Photoconduction mechanism of tungstite and tungsten oxide decorated MWCNT UV protodetector. b Energy band diagram of the photodetectors. c Schematic of carrier transport between photoactive particles and MWCNT

Ultrafast response of the tungstite and tungsten oxide decorated MWCNT UV photo detectors are explained in Fig. 9b, c. The electron affinity (EA) of the molecule, as determined by several process, such as, photodetachment threshold, charge transfer threshold, flame equilibrium etc., lies in the range 3.2–4.0 eV [8, 34]. Work function of MWCNT is reported to be ~ 4.53 eV [35]. The EA of the metal oxide is lower than the work function of MWCNT, so the UV generated free electrons move from the surface of the WO3 to the MWCNT very easily. In case of the conductivity and the carrier mobility, the MWCNT is much superior to metal oxide, resulting faster charge transportation through the device. Faster charge transport through the MWCNT contributes to the expeditious UV photodetection of the tungstite and tungsten oxide decorated MWCNT UV photodetectors.

4 Conclusion

Ultrafast UV photodetectors were fabricated using tungstite and tungsten oxide decorated MWCNTs. Orthorhombic tungstite, monoclinic and mixed phase tungsten oxide were synthesized by varying the calcination temperatures and verified from the XRD analysis and Raman spectroscopy. Under 15 μW cm−2 UV intensity, orthorhombic tungstate decorated MWCNT UV photodetector exhibited maximum responsivity of 7.4 AW−1, whereas monoclinic and mixed phase MWCNT decorated device showed 4.1 AW−1 and 0.6 AW−1 respectively. The UV photogenerated carrier transportation through the MWCNT contributed to the ultrafast response and recovery of the UV photodetectors. Even under the high frequency optical chopper the devices showed remarkable stability and repeatability. The response and recovery time is recorded as low as 400 μs and 500 μs for tungstite decorated MWCNT photodetector.

Notes

Acknowledgements

The authors would like to acknowledge Department of Science and Technology (DST), India (DST SERB Grant No.: EMR/2014/001001) for their financial support to the research.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

42452_2019_1799_MOESM1_ESM.docx (72 kb)
Supplementary material 1 (DOCX 71 kb)

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of PhysicsIIEST ShibpurHowrahIndia
  2. 2.Department of Materials ScienceIACSKolkataIndia
  3. 3.Department of PhysicsUniversity of JadavpurKolkataIndia
  4. 4.Department of Physical ScienceIISER KolkataMohanpurIndia

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