Structural, optical and photoconductivity studies of ZnO bicones synthesized by seed-mediated method

Vacuum 168 (2019) 108856

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Structural, optical and photoconductivity studies of ZnO bicones synthesized by seed-mediated method

T

Rajkumar C. Department of Electronics and Communication, University of Allahabad, Allahabad, Uttar Pradesh, India-211002

A R T I C LE I N FO

A B S T R A C T

Keywords: Photoconductivity ZnO Seed PEG Bicones Vacuum

ZnO bicones (BC) have been synthesized by a seed-mediated method using polyethylene glycol (PEG) and triethylamine (TEA). As-synthesized ZnO BC has been characterized by X-ray diffraction method (XRD), UV–visible absorption spectroscopy, Photoluminescence spectroscopy (PL), Fourier Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Time-dependent Photoconductivity study. XRD pattern of ZnO BC shows hexagonal wurtzite phase structure. The UV–visible absorption spectrum of ZnO BC shows two absorption band edges at 302 nm and 370 nm due to its diameter and length. The PL spectrum of ZnO BC shows rich zinc interstitial defects due to the presence of an intense peak at 423 nm. The FTIR spectrum of ZnO BC confirms the presence of TEA and PEG. SEM and TEM images of ZnO nanostructures confirm the bicones structure as well as the size of the ZnO BC. Photoconductivity study shows that the time-dependent rise and decay curves of ZnO BC has very high photosensitivity in vacuum medium compared to its air medium due to the presence of hydrocarbon functional groups and oxygen deficiency atmosphere.

1. Introduction Zinc oxide (ZnO) is a wide bandgap (~3.3 eV) semiconductor material which is having a unique property such as high electron mobility, high exciton binding energy (~60 meV), good transparency and so on. In recent years, ZnO used for various optoelectronics applications which includes solar cells, laser diodes, light emitting diodes, photodetectors and so on [1–4]. Photoconductivity property used to find a better device for optoelectronics applications. Photoconductivity is the study of electrical conductivity of the materials due to absorption of electromagnetic radiations. The photoconductivity is not only depends on absorbance of electromagnetic radiations which is also on depends on temperature, applied field, wavelength of the light and intensity of the light [5–7]. In addition to this, the photoconductivity also depending on carrier density and complex process of carrier generation, trapping and recombination [8]. In photoconductivity, the time-dependent rise and decay curves of photocurrent are used to identify the trapping states and recombination states of the materials [9]. The rise and decay curves also used to identify the rise time, decay time, steadystate photocurrent, dark current and photosensitivity of the materials [10]. Recently, ZnO nanostructured materials with different morphologies are playing a significant role in various optoelectronics applications. Zinc oxide nanomaterials are having different types of

morphologies such as quantum dot, nanorods, nanoribbons, nanocones, nanoflower, nanobicone, nanobipyramids, nanowires, nanoplates, etc. [11–18]. Synthesis of zinc oxide nanostructures depends on precursors, capping agent, reducing agent and templates/matrices [16]. Rajkumar C et al. [10] synthesis ZnO nanospheres with rarely bicones with the help of zinc nitrate hexahydrate with triethylamine by co-precipitation method. Z. Guo et al. [19], G. Zhou et al. [20], D. Chateau et al. [21], A. Lombardi et al. [22] and H. Xu et al. [23] synthesis gold bicones/bipyramids by seed-mediated method, where seed was acted as a nucleation center. P. Srinivasan et al. [24] synthesis ZnO bicone/bipyramid with the help of Zinc acetate dihydrate and ethylene glycol/ polyethylene glycol by co-precipitation method, where the polyethylene glycol acts as a surfactant. M. Klaumünzer et al. [25] synthesis ZnO bicone with the help of zinc acetate dihydrate and polymer by coprecipitation method, where the polymer was acted as a matrix. In the co-precipitation method, mostly seed and polymers are playing an essential role for the growth of bicones, where seeds are acting as nucleation centers and polymers are acting as template or surfactant agents [26,27]. Co-precipitation is one of the simplest and cost-effective methods to prepare ZnO nanostructured materials. In the co-precipitation method, the seed was acted as nucleation centre to prepare bicones rapidly compared to other co-precipitation methods [27]. The electrical property of ZnO materials has been changed due to influence

E-mail address: [email protected]. https://doi.org/10.1016/j.vacuum.2019.108856 Received 6 June 2019; Received in revised form 5 August 2019; Accepted 6 August 2019 Available online 06 August 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig 1. Synthesis procedure of ZnO bicones.

ZnO [8,29,37].

of external atmosphere. Similarly, the photosensitivity of UV photodetectors had been changed due to presence of oxygen, water and other species on the surface of ZnO nanostructures. So, the relative humidity and vacuum also play an important role for photodetectors. In vacuum medium, the photoresponse of the ZnO nanostructures may be vary due to oxygen deficiency atmosphere. In vacuum medium, the loosely bounded oxygen molecules on the surface are pumped away to maintain oxygen deficient atmosphere. So, it will create high photoconductivity as compared to that in air medium [28,29]. In air medium, the loosely bounded oxygen molecules on the surface of metal oxides are desorbed under UV radiation. So, it will decrease the surface electron depletion region to enhance photoconductivity. But, in vacuum medium, the loosely bounded molecules are pumped away without UV radiation. So, this oxygen depletion state is continued as long as good vacuum is continued. According to literature, in good vacuum condition, the larger photoconductivity occurs due to two processes. One is fast as compared to that in air, another one is very slow that continues after the fast one is finished [28]. Q. Li et al. [30] reported that the decay process in vacuum medium also having two processes. One is fast and another one is very low compared to that in air. Even after one day, the decay process was not reached the same place as that in air. It was saying that in decay process, the desorbed molecules are pumped away in vacuum during UV illumination. But, the decay process was reached similar to that in air medium when oxygen concentration was included or increased. So, the lack of oxygen molecules will prevent the decay process in vacuum medium. Y. Shapira et al. [31] reported that when vacuum level was increased by stepwise, the rise time also increased by stepwise due to impurity carbon content present on the surface of ZnO which was confirmed by Auger analysis. Because, in vacuum medium, loosely bounded are pumped away. So, the oxygen molecules were chemisorbed with carbon content to form CO−2 ions. In vacuum medium, during band gap radiation, the photogenerated holes recombine with CO−2 to form physisorbed CO2 molecules, then physisorbed CO2 molecules desorbed from ZnO surface due to thermal energy created by band gap radiation as well as the effect of vacuum medium. Similarly, many authors have reported that the photoconductivity property of ZnO was varied due to the effect of vacuum medium (low to high) compared to that in air medium [32–36]. In this present work, ZnO bicones have been synthesized by a seed-mediated method with the help of ZnO seed, triethylamine and polyethylene glycol. The structural, optical, morphological and photoconductive properties of the synthesized ZnO bicones have been studied briefly and reported here. To the best of our knowledge, this is the first report wherein ZnO bicones have been prepared by a seed-mediated method with the help of ZnO seed, triethylamine and polyethylene glycol. In addition to this, photoconductivity of the prepared ZnO bicones has been investigated, where the direction of the applied field was normal to the direction of illumination. The prepared ZnO bicones in vacuum medium has highest photosensitivity as compared to other shapes of

2. Experimental 2.1. Materials and methods All the chemicals used in this experiment was commercially purchased and used without any further purification. In this work, a twoneck round-bottom flask (equipped with a condenser, thermo-controller, and magnetic stirring) was used to prepare ZnO seeds (nanoparticles) as per the method reported by Pacholski et al. [27]. In a typical synthesis of ZnO BC, 2.5 ml of triethylamine (N (CH2CH3)3) was added to 32.5 ml of deionized (DI) water and heated to 90 °C, then 5 ml of zinc nitrate hexahydrate (Zn (NO₃)₂•6H₂O) (0.25 M) was added by dropwise into the flask, next to that 5 ml of polyethylene glycol (H−(O−CH2−CH2)n−OH) (Mw = 8,000, 1 g dissolved in 10 ml water) was added by dropwise into the flask. Finally, the mixture was stirring continuously with 90 °C until the reaction was complete (2 h). Stirring and condensation were maintained the entire process. The entire process was shown in Fig. 1. In this process, the seed is acted as nucleation centre for rapid growth of bicones. The Zn(OH)2 precipitate created from Zinc Nitrate Hexahydrate which assists the seed to grow biconical shape. So, the nucleation happens quickly with the help of seed and growth happens with the help of Zn(OH)2 precipitate. 2.2. Characterization For characterization, the synthesized sample centrifuged at 3000 rpm for 30 min. Then the product was washed with acetone first followed by DI water several times. After that, the product dried at 60 °C in the hot air oven. At last, it was annealed at 400 °C and subjected to further characterization techniques. The crystal structure of the synthesized samples was carried out using X-ray diffraction (XRD) with Cu Kα radiation (Model: PW3040/60 X'pert PRO, Make: PANalytical Netherlands). Field emission electron microscopy (FESEM) and transmission electron microscopy (TEM) images were obtained using FESEM (FE-SEM 450 (FEI)) and TEM (The Tecnai G2 20 (FEI) STwin is a 200 KV), respectively. For SEM characterization technique, the ZnO sample was smeared on a small piece of adhesive carbon tape that was fixed on a brass stub. Then the sample was further subjected to gold coating using sputtering process with 10 mA of current for 10s. After that, the gold-coated sample was placed inside the chamber of SEM. Finally, the SEM shows the image by collecting the secondary electron/Back Scattered electrons from the samples. For TEM characterization, the samples were prepared by placing an extremely small amount of material suspended in ethanol. Then the solution was ultrasonicated to disperse the particles. Afterwards, a drop of the solution pipetted out and cast onto a 200-mesh carbon coated copper grid. Finally, the grid was dried before recording the micrographs. The 2

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Fig. 2. Experimental setup (RMM - RISH Multimeter, RL - 1MΩ and UV–visible Lamp - 300W) used in the measurement of photoresponse.

Seeds are supposed to provide nucleation centres around which bicones are formed. There are having no common coupling which may be due to the presence of polymer (PEG) acting as a surfactant, as shown in Fig. 6. The growth mechanism of ZnO bicones are shown in Fig. 6, where the seed is acting as nucleation center, and the polymer is acting as a capping agent. Similar kind of development of ZnO bicones had been reported by P. Srinivasan et al. [24]. Fig. 4a–b) Shows high-resolution TEM (HRTEM) image of ZnO sample, which reveals well-defined bicone-like morphology of the sample having good agreement with the FESEM results. The < 001 > direction that corresponds to the (002) plane of ZnO is parallel to the growth direction of the bicones. Similar kind of results had been reported by M. Klaumünzer et al. [25]. Fig. 4 d) shows lattice spacing was estimated as 0.24 nm which corresponds to the (101) plane in XRD of hexagonal ZnO bicones. Fig. 4 c) shows selected area electron diffraction (SAED) pattern of ZnO BC. The bright diffraction pattern of spots and circular ring pattern confirm the hexagonal wurtzite polycrystalline phase of ZnO. Particle size distributions of ZnO bicones are shown in Fig. 5. It shows that the diameters of the particles are in between ~80 and 240 nm and the lengths of the particles are in between ~150 and 450 nm.

UV–visible absorption spectrum of the samples was obtained at room temperature using a JASCO V650 Spectrometer. FTIR spectrum of the samples was obtained using Bruker Optic Gmbh, Germany, Model Tensor 27 spectrometer. The room temperature photoluminescence (PL) spectrum was obtained using VARIAN Cary Eclipse model spectrometer. 2.3. Measurement of UV–Visible photoresponse The time-dependent photoconductivity properties were investigated under UV–visible illumination (300W) with fixed bias voltage (20 V), i.e., rise and decay curves of ZnO BC photocurrent were studied at room temperature (34 °C) using a cell-type device as shown in Fig. 2. The cell was prepared by putting a thick layer of powdered sample in between two Cu electrodes, which is having a spacing of 1 mm and area of illumination is 0.25 cm2. Then, the powdered layer was pressed with a transparent glass plate. Afterwards, the prepared cell was mounted inside the dark chamber which was having a slit, where the light was allowed to fall over the cell. In this cell type device, the direction of illumination was normal to field across the electrodes. The UV–visible photo-response was measured using a Hg lamp of 300W as a photoexcitation source, and the current was measured using a digital multimeter (RISH Multi-18S) with RISH Multi SI-232 adapter. The rise and decay curves of photocurrent were recorded in air as well as in vacuum media (110–160 mm of Hg to create an oxygen-deficient atmosphere inside the glass chamber with the help of Tarson Rocky vac 300 vacuum pump). The load resistance RL (1 MΩ) was used to measure the current through fixed voltage power supply (20V) using the basic formula I=V/ R (Ampere).

3.2. Optical studies Fig. 7 a) shows the UV–visible absorption spectrum of ZnO BC. ZnO prepared without seed and polymer is found exhibiting one absorption edge [10] whereas ZnO with seed and polymer exhibits two absorption edges. Biconical morphology of ZnO may be attributed to two absorption edges. Similarly, P. R. Sajanlal et al. [38] had reported two absorption peaks in gold nanorods and had attributed it to the interaction of electromagnetic radiation with the length and width of the nanorods. Fig. 7 b) shows the room temperature photoluminescence spectrum of ZnO BC with an excitation wavelength of 371.06 nm. The strong violet band at 423 nm corresponds to transition from shallow zinc interstitial levels to the corresponding valence band [39]. The feeble blue band at 485 nm may be attributed to transition between zinc interstitial and zinc vacancy levels [40]. Two feeble green bands at 506 nm and 529 nm are supposed to involve a transition between the conduction band and oxygen vacancy [41] and that between zinc interstitial and singly ionized oxygen vacancy [10,42]. The strong violet emission is due to the presence of zinc rich environment during the synthesis process as zinc nitrate is being used as a precursor [43]. High concentration of zinc interstitials levels has been reported by Jorge Oliva et al. [43]. In the synthesis of ZnO quantum dots with zinc acetate used

3. Results and discussions 3.1. Structural and morphological studies Fig. 3 a) shows the XRD pattern of ZnO BC, which is indicating that the prepared ZnO BC has crystalline nature because the ZnO XRD pattern is matched with well-known hexagonal wurtzite-structure of ZnO (JCPDS NO. 01-089-0510). The average crystallite size of ZnO BC is ~27 nm. The crystallite size of each peak is shown in Table 1. From XRD measurement, it is also evident that the ZnO nanoparticles grow preferentially along c-axis. A similar observation has been reported by M. klaumunzer et al. [25]. Fig. 3 b) shows SEM images of ZnO BC. ZnO prepared without seed and polymer shows the formation of spherically shaped nanoparticles. 3

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Fig. 3. a) XRD pattern and b) SEM image of ZnO bicones.

adsorbed oxygen ion on the surface of ZnO nanoparticles. The initial fast rise in photocurrent under illumination may be ascribed to fast photogeneration of electron-hole pairs.

Table 1 Structural parameters of ZnO bicones. Position [2θ] (degree)

Planes

31.796 34.427 36.268 47.561 56.621 62.873 66.416 67.969 69.101 72.568 76.979

100 002 101 102 110 103 200 112 201 004 202

d (Å)

2.81 2.60 2.47 1.91 1.62 1.47 1.40 1.37 1.35 1.30 1.23

FWHM (degree)

Crystallite size (nm)

Average crystallite size (nm)

0.25 0.17 0.28 0.54 0.30 0.44 0.36 0.39 0.49 0.29 0.55

32.89 48.72 29.18 15.86 29.75 20.86 25.99 24.01 19.62 33.29 18.13

27.12

hν → h+ + e− The slow rise of photocurrent may be due to the slow desorption of oxygen molecules on the surface of ZnO nanostructures. The photogenerated holes recombine with the adsorbed oxygen ions on the surface, producing oxygen molecules; this reaction also eliminates the barrier which is near the surface of the nanostructure. This process is described by the following equation:

O2− + h+ → O2 + e− At the same time, the desorption of adsorbed oxygen ions on the surface of nanoparticles leaves captured electrons in the conduction band, thereby, increasing the sample conductivity and contributing to the photocurrent. When the UV–visible light is switched off, initially the current decreased very fast, which could be attributed to recombination of free electrons and holes. Later, the current decreased slowly, which could be attributed to the slow process of surface readsorption of oxygen molecules due to the presence of traps and recombination centres at the surface of ZnO. Fig. 8 b) Show the rise and decay curve of ZnO BC photocurrent in a vacuum medium. When the light is switched on, the current in vacuum medium shows some unusual pattern. At first, the photocurrent increases rapidly (region I in the transient response) followed by increases slowly over some exposure time (region II). After exhibiting lower photoconductivity, the current in vacuum increases rapidly (region III) and attains a maximum value until the light is switched off. The steadystate photocurrent in vacuum medium increases approximately three times higher than the air medium. In contrast to response in air, an extra section (region III) has been observed in vacuum medium where the photocurrent increases steeply dominating the whole growth pattern. When the light is switched off, the current decreases rapidly followed by a slow decrease due to minimum availability of oxygen molecules for readsorption process at the surface of ZnO BC and is consistent with other reports. The rise and decay curve show that the decay curve finished above the dark current that means it has positive photoconductivity. Q. Li et al. [32] suggested that the higher photoconductivity of ZnO in vacuum medium arises due to shorter desorption time of oxygen species under vacuum medium compared to that in air medium. On the other hand, J. Bao et al. [28] proposed that the higher photoconductivity in vacuum as a result of removal of lattice oxygen in a process that may be catalyzed by surface hydrocarbons. Fig. 7 b)

as a precursor. Fig. 6 c) shows the FTIR analysis of ZnO BC. The band observed at 3434 cm−1 is assigned to O-H stretching mode and N-H stretching mode due to the presence of hydroxyl group and an amine group [44–46]. The peaks between 3000 and 2000 are assigned to C-H stretching mode and CO2 groups [47]. The peak at 1633 cm-1 is assigned to the hydroxyl groups of chemisorbed and/or physisorbed H2O molecules on the surface of ZnO nanoparticles [48]. The band arises at 1473 cm−1 in ZnO BC are assigned to the presence of hydrocarbon groups, which is playing an important role in photoconductivity [49]. The band arises at 1025 cm−1 is assigned to O-H bending mode due to the presence of hydroxyl group [50]. The weak band arises at 512 cm−1 is assigned to the Zn-O bond [10,51–55]. 3.3. Photoconductivity study Fig. 8 shows the time-resolved rise and decay curves of photocurrent for ZnO BC with fixed bias voltage 20 V in air and vacuum media under UV–visible illumination with fixed photo-flux (7300 lux). Fig. 8 a) shows that after dark current stabilization when the light is switched on, the photocurrent initially increases rapidly followed by a slow rate of increase with time. In the dark, (before UV–visible illumination) oxygen molecules tend to adsorb onto the surface of ZnO by capturing free electrons and producing adsorbed oxygen ions as follows:

O2 (gas ) + e− → O2− Where O2 is the oxygen molecule, e− is the free electron, and O2− is the 4

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Fig. 4. a) TEM, b) HR-TEM, c) SAED pattern and d) lattice springes of ZnO bicones.

Fig. 5. Histogram representation of particle size distribution of a) Diameter and Length of ZnO bicones.

structures. In the vacuum medium, the seed and polymer increase the sensitivity of the ZnO material as compared to air medium due to the involving of more hydrocarbons in oxygen deficiency atmosphere. Rise and decay curves under periodic illumination (2 min) are shown in Fig. 9. The photosensitivity is defined as the ratio of the difference between steady-state photocurrent (Iph) and stabilized dark current (Idc ) to stabilized dark current (Idc ) [56]. From Fig. 7, the values of trap depth, photocurrent, dark current and photosensitivity are calculated

shows that the slow and prolonged rise in photocurrent under vacuum medium is due to "carbon-optical" reaction in the presence of surface hydrocarbon [8,28]. These surface hydrocarbons are created during the synthesis process with the help of triethylamine and polyethylene glycol as well as normal atmospheres. Because in normal atmosphere, CO2 also adsorb on the surface of ZnO to form CO− as well as CO−2 ions in the dark current period. So, in dark current, O2 as well as CO2 molecules adsorb on the surface of ZnO to reduce the conductivity of ZnO 5

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Fig. 6. Growth mechanism of ZnO bicones.

shapes. The seed and polymer help to prepare perfect bicone shaped ZnO without having any agglomeration due to good maintenance of stabilization, concentration and reaction temperatures. Otherwise, the ZnO may become a flower-like structure due to the high concentration and abnormal reaction temperature [57]. The high surface to volume ratio of ZnO BC may adsorb more number of oxygen and carbon dioxide molecules in the dark current period, but, it may be desorbed the adsorbed molecules during UV–visible illumination. In the vacuum

and reported in Table 2. From the discussion, it is concluded that the photosensitivity of ZnO in vacuum medium is very high as compared to air medium, which may be due to oxygen-deficient atmosphere and presence of hydrocarbon on the surface of ZnO. The prepared ZnO bicones in vacuum medium have the highest photosensitivity as compared to other ZnO shapes [8,29,37], which is shown in Table 3. Because, the prepared ZnO BC has high surface to volume ratio as well as perfect bicone shape without any agglomeration compared to other

Fig. 7. a) UV–visible absorption spectrum, b) PL spectrum and c) FTIR spectrum of ZnO bicones. 6

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Fig. 8. Rise and decay curves of photocurrent in a) air medium and b) vacuum medium.

Fig. 9. Rise and decay curves of ZnO under periodic (2 min) illumination in a) air as well as in b) vacuum media.

photosensitivity of ZnO in air and vacuum media [28]. Moreover, the prepared ZnO BC may be well suited for photodetector applications due to its good switching property under UV–visible illumination as shown in Fig. 9.

Table 2 Steady-state photocurrent (Ipc ) , stabilized dark current (Idc ) , Trap depth (eV) and Photosensitivity of ZnO bicones. Medium

Ipc (nA)

Idc (nA)

Trap depth (eV)

Photosensitivity (Ipc − Idc )/ Idc

Air Vacuum

373.80 1084.38

50.90 4.15

0.69 0.71

6.34 260.29

3.3.1. Trap depth determination “The decay portions of photocurrent after switching off the UV illumination have been used to calculate the trap depths (E). The curves for decay of photocurrent for all samples are obtained by the linear fitting of the photocurrent curve on a logarithmic scale. The fitting of decay portion of each sample is governed by bi-exponential law implying the distribution of traps of deferent natures situated in the band gap. Decay corresponding to different exponentials is given by the relation: I = IO exp (−pt ) , where IO is the current at the moment when light is switched off, I is the photocurrent at any instant of time and p is

medium, whole chemisorbed molecules (CO−2 ) are not pumped away, it may be desorbed during UV–visible illumination. Because, the chemisorbed molecules (CO−2 ) become physisorbed (CO2) molecules during UV–visible illumination. Later, the physisorbed molecules desorb from the surface of ZnO during UV–visible illumination [31]. Therefore, the large surface area plays an important role to increase the Table 3 Comparison of photosensitivity of ZnO with different shapes in vacuum medium. ZnO Shapes

Fixed Bias voltage (V)

Dark Current (nA)

Photocurrent (nA)

Photosensitivity (Ipc − Idc )/ Idc

Reference

nanoparticles nanorods irregular cuboids pyramid shaped microstructure Bicones

20 20 20 20 20

0.00625 0.002 0.018 100 4.15

0.1 0.08 0.128 2400 1084.38

15 39 6.11 23 260.29

[29] [29] [8] [37] Present work

7

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the probability of escape of an electron from the trap per second and its value is different for each exponential sections. The probability of an electron escaping from a trap is also given by the relation: p = Sexp (−E / kT ) . It is defined as the number per second that the quanta from crystal vibrations (phonons) attempt to eject the electrons from the traps multiplied by the probability of transition from trap to the conduction band and is of the order of 109 at room temperature”. The trap depths (E) relating to various exponentials are computed by above equations and is represented by

E = kTloge S − loge ⎛ ⎝

⎜

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loge (IO / I ) ⎞ t

⎟

⎠

where k is Boltzmann constant (1.381 × 10−23 J/K), T is the absolute temperature, and S is the frequency factor, i.e. the “attempt to escape frequency” [58]. The calculated values of an electron escaping from the traps and trap depths (trap ionization energies, eV) corresponding to the bi-exponential decay portions for ZnO BC are shown in Table 2. 4. Conclusions ZnO BC has been prepared by the seed-mediated method. In the seed-mediated method, seed acts as a nucleation center and polymer acts as a surfactant agent. As-prepared ZnO BC has been characterized by various characterization techniques. The absorption spectrum shows the two-absorption band edge, which is due to the effect of bicone morphology. The FTIR spectrum shows the presence of hydrocarbon functional group, which is playing the significant role to increase the photosensitivity in vacuum medium. The PL spectrum shows an intense peak at 423 nm that is due to the presence of rich zinc interstitial defects, which may be also involving to increase the photocurrent in prepared ZnO. XRD spectrum shows the hexagonal wurtzite structure and crystallite size of the ZnO BC. SEM and TEM images show the nanobicone structures of prepared ZnO material. TEM image also shows the size and internal structures of the ZnO BC. In photoconductivity, the photosensitivity of the prepared ZnO BC has more sensitivity in vacuum medium as compared to air medium due to involving of more hydrocarbon functional groups in oxygen deficiency atmosphere. The large surface to volume ratio helps to increase the photosensitivity of prepared ZnO BC. The prepared ZnO BC may be well suited for photodetector application due to its good switching property under UV–visible illumination. Acknowledgement I express my gratitude to Prof. Rajneesh Kumar Srivastava for his valuable suggestions. I also acknowledge the university grants commission (UGC) for providing fellowship and Materials Research Centre (MRC), MNIT Jaipur for providing characterization facilities. References [1] J. Gröttrup, V. Postica, D. Smazna, M. Hoppe, V. Kaidas, Y.K. Mishra, O. Lupan, R. Adelung, UV detection properties of hybrid ZnO tetrapod 3-D networks, Vacuum 146 (2017) 492–500, https://doi.org/10.1016/j.vacuum.2017.03.017. [2] V. Postica, I. Paulowicz, O. Lupan, F. Schütt, N. Wolff, A. Cojocaru, Y.K. Mishra, L. Kienle, R. Adelung, The effect of morphology and functionalization on UV detection properties of ZnO networked tetrapods and single nanowires, Vacuum 166 (2019) 393–398, https://doi.org/10.1016/j.vacuum.2018.11.046. [3] P. Manzhi, R. Kumari, M.B. Alam, G.R. Umapathy, R. Krishna, S. Ojha, R. Srivastava, O.P. Sinha, Mg-doped ZnO nanostructures for efficient organic light emitting diode, Vacuum 166 (2019) 370–376, https://doi.org/10.1016/j.vacuum. 2018.10.070. [4] M.I. Fathima, K.S.J. Wilson, Role of multilayer antireflective coating in ZnO based dye sensitized solar cell, Vacuum 165 (2019) 58–61, https://doi.org/10.1016/j. vacuum.2019.04.007. [5] M. Yang, C. Yan, Y. Ma, L. Li, C. Cen, Light induced non-volatile switching of superconductivity in single layer FeSe on SrTiO3 substrate, Nat. Commun. 10 (2019) 85, https://doi.org/10.1038/s41467-018-08024-w. [6] L. Wang, P. Chen, Y. Wang, G.-S. Liu, C. Liu, X. Xie, J. Li, B. Yang, Tape-Based photodetector: transfer process and persistent photoconductivity, ACS Appl. Mater.

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