Expeditious UV detection of tungstite (WO3·H2O) and tungsten oxide (WO3) decorated multiwall carbon nanotubes (MWCNT) based photodetector: ultrafast response and recovery time
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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.
KeywordsMWCNT Tungstite Tungsten oxide UV photodetector Response time Recovery time
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 . 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
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
3.2 Characterization of WO3·H2O and WO3
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. , 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 .
3.3 UV photodetection characterization
Comparison of photoconduction parameters
3D WO3 nanoshell
6.3 s/0.5 s
WO3 nanowire on carbon paper
3 s/20 s
13 ms/16 ms
WO3 polycrystal nanobelt
2.6 × 105
8.1 × 107
Colloidal ZnO nanoparticle
~ 50 μA
< 0.1 s/1 s
TiO2 nanowire array
~ 34.2 nA
4.4 × 103
WO3·H2O decorated MWCNT
400 μs/500 μs
WO3 decorated MWCNT
448 μs/504 μs
3.4 UV detection mechanism
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 . 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.
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.
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.
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