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The effect of morphology and functionalization on UV detection properties of ZnO networked tetrapods and single nanowires
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The effect of morphology and functionalization on UV detection properties of ZnO networked tetrapods and single nanowires Vasile Posticaa, Ingo Paulowiczb,c, Oleg Lupana,b,∗, Fabian Schüttb, Niklas Wolffb, Ala Cojocaruc, Yogendra Kumar Mishrab, Lorenz Kienleb,∗∗, Rainer Adelungb,∗∗∗ a Department of Microelectronics and Biomedical Engineering, Center Nanotechnology and Nanosensors, Technical University of Moldova, 168 Stefan cel Mare Av., MD2004, Chisinau, Republic of Moldova b Institute for Materials Science, Faculty of Engineering, Kiel University, Kaiserstr. 2, D-24143, Kiel, Germany c Phi-Stone AG, Kaiserstr. 2, D-24143, Kiel, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Nanosensor Device UV photodetector CNT Individual nanostructures
Rapid detection and fast response of nanoelectronic devices based on semiconducting oxides is nowadays a modern and stringent subject of research. Device performances depend mainly on the morphologies of the metal oxide nanostructures. In the scope of this work, the influence of the structural morphology of three-dimensional (3-D) ZnO nano- and microstructured networks on the room temperature UV detection properties is studied in detail. We show that the formation of multiple potential barriers between the nanostructures, as well as the diameter of the nanostructures, which is in the same order of magnitude as the Debye length, strongly influence the UV sensing properties. Consequently, 3-D ZnO networks consisting of interconnected ultra-long wire-like tips (up to 10 μm) and with small wire diameters of 50–150 nm, demonstrated the highest UV sensing performances (UV response ratio of ∼3100 at 5 V applied bias voltage). Furthermore, we demonstrate the possibility of substantially increasing the UV sensing performances of individual ZnO nanowire (NW) (diameter of ∼50 nm) by surface functionalization with carbon nanotubes (CNTs), showing high response ratio (∼60–50 mW/cm2), as well as fast response (∼1 s) and recovery (∼1 s) times. The obtained results thus provide a platform with respect to the next generation of portable UV radiation detectors based on semiconducting oxide networks.
1. Introduction The necessity of UV photodetectors has increased during the last decades due to progressing formation of ozone holes [1,2]. Therefore, the monitoring of UV radiation plays an important role for human health by avoiding excessive exposure to UV radiation, which can lead to harmful effects on the skin and eyes [3–6]. Other applications of UV photodetectors are fire detection, UV imaging, spatial communications, missile tracking systems, etc. [3,7]. Due to its wide band gap (Eg = 3.4 eV), low cost, a wide variety of synthesis methods, and especially due to an incredible large variety of morphologies with high surface-to-volume ratio, ZnO is an ideal candidate for visible blind UV photodetectors [5,6,8]. High-performance UV photodetectors based on different types of ZnO morphologies have already been fabricated [9]. β-Ga2O3 is another interesting material which has also emerged as a
good candidate for fast-response solar-blind ultraviolet photodetectors [10]. Further increase of UV sensing performances can be achieved by surface functionalization with CNTs [3,11,12]. This strategy has also been used for other applications such as photocatalysis, photoelectrochemical and energy storage devices [13–15]. However, understanding the relationships between oxide morphologies, its structureproperty relationships, and practical applications is very important for large-scale synthesis of good functional nanomaterials for detectors and others. Recently, Nasiri et al. reported on portable visible-blind UV photodetectors based on novel ultraporous electron-depleted ZnO nanoparticle networks, which demonstrated high sensitivity and recorded high milliampere photocurrents even at very low ultraviolet light intensities [16]. Ning et al. reported a novel transparent and self-powered UV photodetector based on electrospun ZnO nanofiber arrays, with onoff ratios up to 104 at zero bias [17]. Also, Gedamu et al. demonstrated
∗ Corresponding author. Department of Microelectronics and Biomedical Engineering, Center Nanotechnology and Nanosensors, Technical University of Moldova, 168 Stefan cel Mare Av., MD-2004, Chisinau, Republic of Moldova. ∗∗ Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (O. Lupan), [email protected] (L. Kienle), [email protected] (R. Adelung).
https://doi.org/10.1016/j.vacuum.2018.11.046 Received 12 March 2018; Received in revised form 22 November 2018; Accepted 23 November 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Postica, V., Vacuum, https://doi.org/10.1016/j.vacuum.2018.11.046
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of 30% (RH) [18,23,24]. The UV response was defined as the ratio of current under UV illumination (IUV) and in the dark (Idark). For networks, the UV light power density (P) was set to 10 mW/cm−2. The responsivity (R) and the internal photoconductive gain (G) of the devices were calculated using Equations (1) and (2) [25,26]. These parameters are very important to evaluate the performances of UV photodetectors and to compare them with literature.
in the case of nanostructured networks that the morphology of ZnO nanoparticles is very important in order to achieve high UV sensing performances for UV sensing [18]. In this work it was demonstrated that 3-D tetrapodal structures of nano-ZnO have many advantages, such as their ultra-high porosity which is based on the 3-D assembly of the used ZnO tetrapods for network fabrication [18,19]. In this case, the diffusion of oxygen species is facilitated, even into the lowest layers of the free-standing material. This ensures the participation of the whole volume as sensing material in the UV detection mechanism [16]. Mishra et al. [19], Thepnurat et al. [20], and Alsultany et al. [21] also reported on excellent UV sensing properties of ZnO tetrapod networks, indicating that 3-D networks of ZnO nanostructures are excellent candidates for sensing applications [5,19]. Recently, the synthesis of different shaped ZnO tetrapods, i.e. different arm morphologies in the same process was demonstrated [22]. In order to find out the exact suitable morphology for fast and ultra-sensitive detection of UV light this process is ideal, as different morphologies can be produced in a simple and controlled manner. In the scope of the present work, the influence of ZnO tetrapod morphology on the UV sensing properties of highly porous ZnO 3-D networks is studied in detail. Furthermore, the UV sensing properties of individual ZnO NW were considerably improved by carbon nanotube (CNTs) surface functionalization, which was explained based on increased carrier separation at the ZnO/CNT interface and rapid charge transfer.
R=
G≅
Iph P ⋅S
qλ = η ⎛ ⎞G ⎝ hc ⎠
1 τμ V L2 e
(1) (2)
where Iph is the photocurrent, S is the area of the active layer, h is the Planck's constant, c is the speed of light, ƞ is the quantum efficiency, L is the distance between electrodes (100 μm), τ is the photocarrier lifetime, μe is the electron mobility, and V is the applied bias voltage. 3. Results and discussions The device structures for investigations of the sensing properties were fabricated using the procedure reported earlier [19,27,28], i.e., by simple contacting the free-standing 3-D ZnO networks on a glass substrate to pre-patterned Au/Cr pads using silver paste. The distance between Au/Cr pads in this investigation was ca. 100 μm. Previous studies demonstrated that a lower distance between electrical contacts is more preferable for higher UV response, responsivity, and rapidity of devices [19,29,30]. The current – voltage (I – V) characteristics of the fabricated devices are presented in Fig. 2a in the dark and Fig. 2b upon UV light exposure. All the devices showed a non-linear I – V characteristic due to the formation of energy potential barriers between the individual
2. Experimental section The synthesis process of ZnO nano- and microstructures is reported in previous works and is based on the thermal oxidation of Zn powder in a furnace [22]. During the same synthesis process, four different types of morphologies can be obtained: (i) tetrapods with large sheets on their legs (leg length of 3–8 μm and diameter of 250–500 nm), noted as ZnO-I (see Fig. 1a); (ii) nanowires with low content of sheets (diameter of 50–300 nm), noted as ZnO-II (see Fig. 1b); (iii) relatively big tetrapods (arm length of 10–30 μm and diameter of 1–3 μm), noted as ZnO-III (see Fig. 1c); (iv) relatively small tetrapods with complex arm structure (from base the leg diameter is constant up to an abrupt decrease in diameter, followed by a wire-like ultra-long segment), noted as ZnO-IV (see Fig. 1d). The detailed morphological, structural, chemical and gas sensing properties of samples were presented in a previous paper [22]. For TEM analysis a JEOL JEM2100 microscope (200 kV, LaB6 cathode) was used. Electron diffraction (ED) experiments were spatially restricted to a selected area of interest by an aperture. UV photodetection (λ = 365 nm) measurements were performed as described in our previous works in ambient air with a relative humidity
Fig. 1. SEM images of sample sets grown by flame transport synthesis FTS technique: (a) ZnO-I; (b) ZnO-II; (c) ZnO-III; and (d) ZnO-IV (inset: overview image with lower magnification).
Fig. 2. Current – voltage characteristics of devices fabricated using the synthesized samples: (a) in the dark; and (b) upon UV light exposure. 2
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Fig. 3. Dynamic UV light response at 5 V applied bias voltage for samples: (a) ZnO-I, (b) ZnO-II, (c) ZnO-III, and (d) ZnO-IV.
UV response at 5 V applied bias voltage for ZnO-I, ZnO-II, ZnO-III, and ZnO-IV sample sets is 620, 1250, 730, and 3100, respectively. By increasing the applied bias voltage, a decrease in the UV response for all samples was observed, which can be attributed to induced self-heating effects of ZnO nano- and microstructure networks under bias voltage [32]. This leads to an increased probability of photogenerated electronhole recombination [33]. The highest dependence of the UV response on the applied bias voltage was observed for sample sets ZnO-II and ZnO-IV, probably due to the lower diameter of the nano- and microstructures of these samples which leads to a higher self-heating effect [16].
ZnO nano- and microstructures in the 3-D network structures [19]. ZnO-IV samples showed higher dark (Idark) and UV (IUV) currents and are thus better suited for the integration in electronic devices and CMOS technology [31]. A high ratio of IUV/Idark is a good measure of the UV sensing properties, i.e. high UV response, responsivity and photoconductive gain. In the following the dynamic properties of different devices have been analyzed in detail. Fig. 3 shows the dynamic UV response of the fabricated devices. The calculated UV response (IUV/Idark) for all samples (with indication of error bars which represent the results from other samples fabricated using the same sample set of nanomaterial) is presented in Fig. 4a. The
Fig. 4. (a) UV response at room temperature (≈25 °C) for sample sets ZnO-I, -II, -III, and -IV at different applied bias voltages. (b) Calculated time constants for rising and decaying the photocurrent at 5 V applied bias voltage. (c) Responsivity and (d) photoconductive gain for sample sets ZnO-I, -II, -III, and -IV at different applied bias voltages.
3
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For each response bi-exponential fitting was applied (Equations (3) and (4)) in order to calculate the time constants of rising (τr1 – fast component and τr2 – slow component) and decaying photocurrent (τd1 – fast component and τd2 – slow component), i.e. the rapidity of devices [18,24], which are indicated in Figs. 3 and 4b. t
(
)
(
t
I (t ) = Idark + A1 1 − e− τr1 + A2 1 − e− τr 2 t
t
I (t ) = Idark + A3 e− τd1 + A 4 e− τd2
)
performances can be explained probably by low absorbance of UV light due to the presence of large area sheets on the arms of tetrapods, leading to higher backscattering of UV light [16]. Also, the presence of large area sheets leads to a lower concentration of the potential barriers through the networks, which is probably the more critical parameter [22]. The NW networks from sample set ZnO-II lead to the formation of less potential barriers [22]. Another important parameter in this case is the lower diameter of NW, which is comparable with the Debye length (for ZnO it is ≈ 20 nm [16]). It can explain the lower response time of the devices based on ZnO-II morphologies (see Fig. 3). It is well known that adsorption/photodesorption processes of oxygen species upon illumination with UV light are relatively slow processes, while the modulation of the potential barriers height is a faster process [35]. 3-D networks based on ZnO-III morphology are composed of larger tetrapods which lead to a lower concentration of potential barriers, i.e. a lower UV sensing performance (see Fig. 4). However, this leads to a higher influence of fast mechanisms on the UV sensing process, i.e. modulation of the potential barriers height, while the modulation of the conduction channel is considerably reduced due to the high diameter of the tetrapod arms (see Fig. 3d) [19,35]. This results in faster response times of such networks compared to networks with nanostructures having a lower diameter (ZnO-I and ZnO-II samples, see Fig. 4b). The highest UV sensing performances were obtained for samples having a ZnO-IV morphology (see Fig. 4). This can be explained based on the complex morphology of the tetrapod arms, i.e. presence of the long, wire-like tips of tetrapods with diameters comparable with the Debye length (50–150 nm). Thereby, numerous potential barriers throughout the networks are formed (see Fig. 1d) and thus give rise to the observed faster response times. In previous works the UV sensing properties of individual ZnO nanostructures, including NW, nanosheets and nanotetrapods were investigated, showing excellent performances [22]. However, in order to further increase the sensing properties, we propose to combine the morphologic aspect of individual ZnO micro- and nanostructures with functionalization with by CNTs. The detailed procedure of ZnO nanoand microstructures functionalization by CNTs was reported by a number of different studies [11,12,36]. Fig. 5 depicts the surface functionalization of ZnO microstructures with CNT networks from a solution containing 0.9 wt% CNTs [11,12,36]. The TEM bright field images show a dense network surrounding the ZnO microwires (see Fig. 5a and b), which are single crystalline with c-axis growth of the wurzite-type structure as indicated by the electron diffraction pattern given in Fig. 5c. Here we integrated an individual CNT-functionalized ZnO NW with a diameter of ∼50 nm at one end into the device using the procedure developed by Lupan et al. [11,34,37,38]. A SEM figure of the fabricated device is presented in Fig. 6a. The UV response of the device to different intensities of UV light is presented in Fig. 6b. The applied bias was 3 V. The UV response for the nanodevice to ∼200, ∼80, ∼50 and ∼10 mW/cm2 is ∼350, ∼118, ∼60 and ∼15, respectively. The calculated values for R at 200, ∼80 and ∼50 mW/cm2 are 1.17, 1.33,
(3) (4)
where A1, A2, A3, and A4 are positive constants [18,24]. The results show that devices fabricated using ZnO-III and ZnO-IV samples, i.e. based on tetrapodal ZnO nano- and microstructures, show a faster response compared to other samples, while samples based on ZnO nanowire networks showed the slowest response. Therefore, it can be concluded that ZnO tetrapod networks are more favorable for the fabrication of fast UV photodetectors, which was also demonstrated in our previous work [18]. However, to be more precise, the ZnO tetrapod networks with a composed morphology of arms are more attractive. It was demonstrated previously that the presence of needle-like tips of tetrapods with a very thin diameter lead to the formation of a higher number of potential barriers and thus to a higher modulation of the electron depletion region in these segments of the arm [19]. The calculated values for R and G at different applied bias voltages are presented in Fig. 4c and d, respectively. The increase in R due to an increase in the applied bias voltage is related to the increased photocurrent and G (see Equations (1) and (2)). The highest values were obtained for the ZnO-IV morphology with R ≈ 5.2 × 10−3, 16 × 10−3, and 58 × 10−3 A/W at 5, 10, and 20 V, respectively (see Fig. 4c). The internal photoconductive gain for all samples was lower than one, indicating no gain which can be related to the high active surface area and the relatively high UV light power density [25,26]. In the case of the ZnO-IV morphology, calculations show that G ≈ 1.5 × 10−3, 4.8 × 10−3, and 17 × 10−3 A/W at 5, 10, and 20 V, respectively (see Fig. 4d). In general, the high sensing performances of the networks are a result of the high porosity which allows for a better diffusion of the oxygen species [16,19]. It is well known that in the case of UV sensing, adsorption/photodesorption of oxygen species (O2 (g ) + e− → O2− (ads ) ) upon illumination with UV light play a major role [16,18,19,21]. As results, the low conductivity surface depletion layer (Lair) and potential barriers between nano- and microstructures (qVs1) are formed [16,18,19,21]. Upon illumination with UV light, electron-hole pairs are photogenerated [26]. While the photogenerated holes migrate to the surface to discharge the adsorbed oxygen species (photodesorption) by an electron-hole recombination process, the photogenerated electrons remain in the conduction channel (dUV) and lead to an improvement in the photocurrent, as well as to a decrease of the potential barrier height (qVs2) and narrowing of the electron depletion region (LUV) [34,35]. The detailed UV sensing mechanism of ZnO nano- and microstructure networks was presented in previous work [19]. In the case of the ZnO-I morphology the relatively low UV sensing
Fig. 5. (a,b) Bright field TEM images of ZnO microstructures decorated with a CNT network (0.9 wt% CNT). (c) Electron diffraction pattern for the [21-10] zone axis orientation. 4
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structures, consisting of different morphology types of ZnO, namely ZnO NW-T networks (ZnO-I), ZnO-T-nanosheets (ZnO-II), the common ZnO-T (ZnO-III), and ZnO-T with complex arm morphology (ZnO-IV), were investigated as UV photodetectors. The highest UV sensing performance is observed for the networks composed of ZnO-T with a complex arm morphology (from sample set ZnO-IV), showing the high UV response of ≈3100 at 5 V. This finding was explained by the formation of a higher number of potential barriers through the networks, compared to other samples, as well as the high porosity of the 3-D networks. Another characteristic of the investigated morphologies is the low, nanoscopic diameter (50–150 nm) of the long wire-like tips, which is comparable with the Debye length. This gives a more efficient modulation under adsorption/desorption of oxygen species. Therefore, the samples from ZnO-IV combine the two most important factors which are necessary for the fabrication of highly efficient and highly sensitive UV detectors. Additionally, in the case of individual ZnO NWs it was observed that surface functionalization with CNTs can efficiently increase the UV sensing performances. In summary, this study provides design rules for the fabrication of high-performance ZnO-based UV nano-photodetectors, including aspects of network morphology and surface functionalization. Acknowledgment O.L. acknowledges the Alexander von Humboldt Foundation for the research fellowship for experienced researchers (3-3MOL/1148833 STP) at the Institute for Materials Science, Kiel University, Germany. This research was funded in part by the German Research Foundation (DFG- Deutsche Forschungsgemeinschaft) under the schemes FOR 2093 (KI 1263/12-2 & AD 183/12-2) & SFB1261 (subprojects A5; and A6; KI1263/12-2, AD 1263/12-2 and AD 183/17-1). This research was in part supported by the Technical University of Moldova. References [1] G.L. Manney, M.L. Santee, M. Rex, N.J. Livesey, M.C. Pitts, P. Veefkind, E.R. Nash, I. Wohltmann, R. Lehmann, L. Froidevaux, L.R. Poole, M.R. Schoeberl, D.P. Haffner, J. Davies, V. Dorokhov, H. Gernandt, B. Johnson, R. Kivi, E. Kyrö, N. Larsen, P.F. Levelt, A. Makshtas, C.T. McElroy, H. Nakajima, M.C. Parrondo, D.W. Tarasick, P. von der Gathen, K.A. Walker, N.S. Zinoviev, Unprecedented Arctic ozone loss in 2011, Nature 478 (2011) 469. [2] J. Moan, Ozone holes and biological consequences, J. Photochem. Photobiol., B 9 (1991) 244–247. [3] E.S. Ates, S. Kucukyildiz, H.E. Unalan, Zinc oxide nanowire photodetectors with single-walled carbon nanotube thin-film electrodes, ACS Appl. Mater. Interfaces 4 (2012) 5142–5146. [4] O. Lupan, N. Wolff, V. Postica, T. Braniste, I. Paulowicz, V. Hrkac, Y.K. Mishra, I. Tiginyanu, L. Kienle, R. Adelung, Properties of a single SnO2:Zn2SnO4 – functionalized nanowire based nanosensor, Ceram. Int. 44 (2018) 4859–4867. [5] 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. [6] V. Postica, M. Hoppe, J. Gröttrup, P. Hayes, V. Röbisch, D. Smazna, R. Adelung, B. Viana, P. Aschehoug, T. Pauporté, O. Lupan, Morphology dependent UV photoresponse of Sn-doped ZnO microstructures, Solid State Sci. 71 (2017) 75–86. [7] H. Chen, K. Liu, L. Hu, A.A. Al-Ghamdi, X. Fang, New concept ultraviolet photodetectors, Mater. Today 18 (2015) 493–502. [8] Y.K. Mishra, R. Adelung, ZnO tetrapod materials for functional applications, Mater. Today 21 (2017) 631–651. [9] K. Liu, M. Sakurai, M. Aono, ZnO-based ultraviolet photodetectors, Sensors 10 (2010) 8604–8634. [10] H. Shen, Y. Yin, K. Tian, K. Baskaran, L. Duan, X. Zhao, A. Tiwari, Growth and characterization of β-Ga2O3 thin films by sol-gel method for fast-response solarblind ultraviolet photodetectors, J. Alloy. Comp. 766 (2018) 601–608. [11] O. Lupan, F. Schütt, V. Postica, D. Smazna, Y.K. Mishra, R. Adelung, Sensing performances of pure and hybridized carbon nanotubes-ZnO nanowire networks: a detailed study, Sci. Rep. 7 (2017) 14715. [12] V. Postica, F. Schütt, R. Adelung, O. Lupan, Schottky diode based on a single carbon–nanotube–ZnO hybrid tetrapod for selective sensing applications, Adv Mater Interfaces 4 (2017) 1700507. [13] W.-D. Zhang, L.-C. Jiang, J.-S. Ye, Photoelectrochemical study on charge transfer properties of ZnO nanowires promoted by carbon nanotubes, J. Phys. Chem. C 113 (2009) 16247–16253. [14] W.-D. Zhang, B. Xu, L.-C. Jiang, Functional hybrid materials based on carbon nanotubes and metal oxides, J. Mater. Chem. 20 (2010) 6383–6391.
Fig. 6. (a) SEM image of a device based on an individual CNT-functionalized ZnO nanowire with diameter of ∼50 nm at one end. In the inset is presented an image of the thinner end of the NW at higher magnification to show the presence of CNTs. (b) Dynamic UV response at room temperature (≈25 °C) to different intensities of UV light.
1.66 and 2.1 A/W, respectively, while the calculated values for G at 200, ∼80 and ∼50 mW/cm2 are 3.98, 4.52, 5.64 and 7.14, respectively. These values are much higher compared to values for networks of pure ZnO nano- and microstructures (see Fig. 4c and d). Also, we can conclude that photoconductive gain persists in our fabricated device, which can be explained based on the large surface-to-volume ratio of NW and short distance between electrodes (L), see Eq. (2) [26]. The biexponential fitting (Equations (3) and (4)) was also applied in order to calculate the time constants of rising and decaying photocurrent. For illumination with 50 mW/cm2 the following values were obtained: τr1 = 0.05 s, τr2 = 0.79 s, τd1 = 0.05 s, and τd2 = 0.22 s. The detailed UV sensing mechanism based on the energy band diagram was already reported in our previous papers [34,38,39]. However, in our case the highly improved performances of CNT-functionalized ZnO NW compared to a pristine one reported in our previous work [22] can be explained based on increased carrier separation at the ZnO/CNT interface, thus helping to enhance the performance of the photodetector [11,12,40,41]. More details on enhanced UV sensing properties of ZnO by surface functionalization with CNTs can be found in our previous work [11]. 4. Conclusions Four different 3-D nano- and microstructured ZnO network 5
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Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
The effect of morphology and functionalization on UV detection properties of ZnO networked tetrapods and single nanowires Vasile Posticaa, Ingo Paulowiczb,c, Oleg Lupana,b,∗, Fabian Schüttb, Niklas Wolffb, Ala Cojocaruc, Yogendra Kumar Mishrab, Lorenz Kienleb,∗∗, Rainer Adelungb,∗∗∗ a Department of Microelectronics and Biomedical Engineering, Center Nanotechnology and Nanosensors, Technical University of Moldova, 168 Stefan cel Mare Av., MD2004, Chisinau, Republic of Moldova b Institute for Materials Science, Faculty of Engineering, Kiel University, Kaiserstr. 2, D-24143, Kiel, Germany c Phi-Stone AG, Kaiserstr. 2, D-24143, Kiel, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Nanosensor Device UV photodetector CNT Individual nanostructures
Rapid detection and fast response of nanoelectronic devices based on semiconducting oxides is nowadays a modern and stringent subject of research. Device performances depend mainly on the morphologies of the metal oxide nanostructures. In the scope of this work, the influence of the structural morphology of three-dimensional (3-D) ZnO nano- and microstructured networks on the room temperature UV detection properties is studied in detail. We show that the formation of multiple potential barriers between the nanostructures, as well as the diameter of the nanostructures, which is in the same order of magnitude as the Debye length, strongly influence the UV sensing properties. Consequently, 3-D ZnO networks consisting of interconnected ultra-long wire-like tips (up to 10 μm) and with small wire diameters of 50–150 nm, demonstrated the highest UV sensing performances (UV response ratio of ∼3100 at 5 V applied bias voltage). Furthermore, we demonstrate the possibility of substantially increasing the UV sensing performances of individual ZnO nanowire (NW) (diameter of ∼50 nm) by surface functionalization with carbon nanotubes (CNTs), showing high response ratio (∼60–50 mW/cm2), as well as fast response (∼1 s) and recovery (∼1 s) times. The obtained results thus provide a platform with respect to the next generation of portable UV radiation detectors based on semiconducting oxide networks.
1. Introduction The necessity of UV photodetectors has increased during the last decades due to progressing formation of ozone holes [1,2]. Therefore, the monitoring of UV radiation plays an important role for human health by avoiding excessive exposure to UV radiation, which can lead to harmful effects on the skin and eyes [3–6]. Other applications of UV photodetectors are fire detection, UV imaging, spatial communications, missile tracking systems, etc. [3,7]. Due to its wide band gap (Eg = 3.4 eV), low cost, a wide variety of synthesis methods, and especially due to an incredible large variety of morphologies with high surface-to-volume ratio, ZnO is an ideal candidate for visible blind UV photodetectors [5,6,8]. High-performance UV photodetectors based on different types of ZnO morphologies have already been fabricated [9]. β-Ga2O3 is another interesting material which has also emerged as a
good candidate for fast-response solar-blind ultraviolet photodetectors [10]. Further increase of UV sensing performances can be achieved by surface functionalization with CNTs [3,11,12]. This strategy has also been used for other applications such as photocatalysis, photoelectrochemical and energy storage devices [13–15]. However, understanding the relationships between oxide morphologies, its structureproperty relationships, and practical applications is very important for large-scale synthesis of good functional nanomaterials for detectors and others. Recently, Nasiri et al. reported on portable visible-blind UV photodetectors based on novel ultraporous electron-depleted ZnO nanoparticle networks, which demonstrated high sensitivity and recorded high milliampere photocurrents even at very low ultraviolet light intensities [16]. Ning et al. reported a novel transparent and self-powered UV photodetector based on electrospun ZnO nanofiber arrays, with onoff ratios up to 104 at zero bias [17]. Also, Gedamu et al. demonstrated
∗ Corresponding author. Department of Microelectronics and Biomedical Engineering, Center Nanotechnology and Nanosensors, Technical University of Moldova, 168 Stefan cel Mare Av., MD-2004, Chisinau, Republic of Moldova. ∗∗ Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (O. Lupan), [email protected] (L. Kienle), [email protected] (R. Adelung).
https://doi.org/10.1016/j.vacuum.2018.11.046 Received 12 March 2018; Received in revised form 22 November 2018; Accepted 23 November 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Postica, V., Vacuum, https://doi.org/10.1016/j.vacuum.2018.11.046
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of 30% (RH) [18,23,24]. The UV response was defined as the ratio of current under UV illumination (IUV) and in the dark (Idark). For networks, the UV light power density (P) was set to 10 mW/cm−2. The responsivity (R) and the internal photoconductive gain (G) of the devices were calculated using Equations (1) and (2) [25,26]. These parameters are very important to evaluate the performances of UV photodetectors and to compare them with literature.
in the case of nanostructured networks that the morphology of ZnO nanoparticles is very important in order to achieve high UV sensing performances for UV sensing [18]. In this work it was demonstrated that 3-D tetrapodal structures of nano-ZnO have many advantages, such as their ultra-high porosity which is based on the 3-D assembly of the used ZnO tetrapods for network fabrication [18,19]. In this case, the diffusion of oxygen species is facilitated, even into the lowest layers of the free-standing material. This ensures the participation of the whole volume as sensing material in the UV detection mechanism [16]. Mishra et al. [19], Thepnurat et al. [20], and Alsultany et al. [21] also reported on excellent UV sensing properties of ZnO tetrapod networks, indicating that 3-D networks of ZnO nanostructures are excellent candidates for sensing applications [5,19]. Recently, the synthesis of different shaped ZnO tetrapods, i.e. different arm morphologies in the same process was demonstrated [22]. In order to find out the exact suitable morphology for fast and ultra-sensitive detection of UV light this process is ideal, as different morphologies can be produced in a simple and controlled manner. In the scope of the present work, the influence of ZnO tetrapod morphology on the UV sensing properties of highly porous ZnO 3-D networks is studied in detail. Furthermore, the UV sensing properties of individual ZnO NW were considerably improved by carbon nanotube (CNTs) surface functionalization, which was explained based on increased carrier separation at the ZnO/CNT interface and rapid charge transfer.
R=
G≅
Iph P ⋅S
qλ = η ⎛ ⎞G ⎝ hc ⎠
1 τμ V L2 e
(1) (2)
where Iph is the photocurrent, S is the area of the active layer, h is the Planck's constant, c is the speed of light, ƞ is the quantum efficiency, L is the distance between electrodes (100 μm), τ is the photocarrier lifetime, μe is the electron mobility, and V is the applied bias voltage. 3. Results and discussions The device structures for investigations of the sensing properties were fabricated using the procedure reported earlier [19,27,28], i.e., by simple contacting the free-standing 3-D ZnO networks on a glass substrate to pre-patterned Au/Cr pads using silver paste. The distance between Au/Cr pads in this investigation was ca. 100 μm. Previous studies demonstrated that a lower distance between electrical contacts is more preferable for higher UV response, responsivity, and rapidity of devices [19,29,30]. The current – voltage (I – V) characteristics of the fabricated devices are presented in Fig. 2a in the dark and Fig. 2b upon UV light exposure. All the devices showed a non-linear I – V characteristic due to the formation of energy potential barriers between the individual
2. Experimental section The synthesis process of ZnO nano- and microstructures is reported in previous works and is based on the thermal oxidation of Zn powder in a furnace [22]. During the same synthesis process, four different types of morphologies can be obtained: (i) tetrapods with large sheets on their legs (leg length of 3–8 μm and diameter of 250–500 nm), noted as ZnO-I (see Fig. 1a); (ii) nanowires with low content of sheets (diameter of 50–300 nm), noted as ZnO-II (see Fig. 1b); (iii) relatively big tetrapods (arm length of 10–30 μm and diameter of 1–3 μm), noted as ZnO-III (see Fig. 1c); (iv) relatively small tetrapods with complex arm structure (from base the leg diameter is constant up to an abrupt decrease in diameter, followed by a wire-like ultra-long segment), noted as ZnO-IV (see Fig. 1d). The detailed morphological, structural, chemical and gas sensing properties of samples were presented in a previous paper [22]. For TEM analysis a JEOL JEM2100 microscope (200 kV, LaB6 cathode) was used. Electron diffraction (ED) experiments were spatially restricted to a selected area of interest by an aperture. UV photodetection (λ = 365 nm) measurements were performed as described in our previous works in ambient air with a relative humidity
Fig. 1. SEM images of sample sets grown by flame transport synthesis FTS technique: (a) ZnO-I; (b) ZnO-II; (c) ZnO-III; and (d) ZnO-IV (inset: overview image with lower magnification).
Fig. 2. Current – voltage characteristics of devices fabricated using the synthesized samples: (a) in the dark; and (b) upon UV light exposure. 2
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Fig. 3. Dynamic UV light response at 5 V applied bias voltage for samples: (a) ZnO-I, (b) ZnO-II, (c) ZnO-III, and (d) ZnO-IV.
UV response at 5 V applied bias voltage for ZnO-I, ZnO-II, ZnO-III, and ZnO-IV sample sets is 620, 1250, 730, and 3100, respectively. By increasing the applied bias voltage, a decrease in the UV response for all samples was observed, which can be attributed to induced self-heating effects of ZnO nano- and microstructure networks under bias voltage [32]. This leads to an increased probability of photogenerated electronhole recombination [33]. The highest dependence of the UV response on the applied bias voltage was observed for sample sets ZnO-II and ZnO-IV, probably due to the lower diameter of the nano- and microstructures of these samples which leads to a higher self-heating effect [16].
ZnO nano- and microstructures in the 3-D network structures [19]. ZnO-IV samples showed higher dark (Idark) and UV (IUV) currents and are thus better suited for the integration in electronic devices and CMOS technology [31]. A high ratio of IUV/Idark is a good measure of the UV sensing properties, i.e. high UV response, responsivity and photoconductive gain. In the following the dynamic properties of different devices have been analyzed in detail. Fig. 3 shows the dynamic UV response of the fabricated devices. The calculated UV response (IUV/Idark) for all samples (with indication of error bars which represent the results from other samples fabricated using the same sample set of nanomaterial) is presented in Fig. 4a. The
Fig. 4. (a) UV response at room temperature (≈25 °C) for sample sets ZnO-I, -II, -III, and -IV at different applied bias voltages. (b) Calculated time constants for rising and decaying the photocurrent at 5 V applied bias voltage. (c) Responsivity and (d) photoconductive gain for sample sets ZnO-I, -II, -III, and -IV at different applied bias voltages.
3
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For each response bi-exponential fitting was applied (Equations (3) and (4)) in order to calculate the time constants of rising (τr1 – fast component and τr2 – slow component) and decaying photocurrent (τd1 – fast component and τd2 – slow component), i.e. the rapidity of devices [18,24], which are indicated in Figs. 3 and 4b. t
(
)
(
t
I (t ) = Idark + A1 1 − e− τr1 + A2 1 − e− τr 2 t
t
I (t ) = Idark + A3 e− τd1 + A 4 e− τd2
)
performances can be explained probably by low absorbance of UV light due to the presence of large area sheets on the arms of tetrapods, leading to higher backscattering of UV light [16]. Also, the presence of large area sheets leads to a lower concentration of the potential barriers through the networks, which is probably the more critical parameter [22]. The NW networks from sample set ZnO-II lead to the formation of less potential barriers [22]. Another important parameter in this case is the lower diameter of NW, which is comparable with the Debye length (for ZnO it is ≈ 20 nm [16]). It can explain the lower response time of the devices based on ZnO-II morphologies (see Fig. 3). It is well known that adsorption/photodesorption processes of oxygen species upon illumination with UV light are relatively slow processes, while the modulation of the potential barriers height is a faster process [35]. 3-D networks based on ZnO-III morphology are composed of larger tetrapods which lead to a lower concentration of potential barriers, i.e. a lower UV sensing performance (see Fig. 4). However, this leads to a higher influence of fast mechanisms on the UV sensing process, i.e. modulation of the potential barriers height, while the modulation of the conduction channel is considerably reduced due to the high diameter of the tetrapod arms (see Fig. 3d) [19,35]. This results in faster response times of such networks compared to networks with nanostructures having a lower diameter (ZnO-I and ZnO-II samples, see Fig. 4b). The highest UV sensing performances were obtained for samples having a ZnO-IV morphology (see Fig. 4). This can be explained based on the complex morphology of the tetrapod arms, i.e. presence of the long, wire-like tips of tetrapods with diameters comparable with the Debye length (50–150 nm). Thereby, numerous potential barriers throughout the networks are formed (see Fig. 1d) and thus give rise to the observed faster response times. In previous works the UV sensing properties of individual ZnO nanostructures, including NW, nanosheets and nanotetrapods were investigated, showing excellent performances [22]. However, in order to further increase the sensing properties, we propose to combine the morphologic aspect of individual ZnO micro- and nanostructures with functionalization with by CNTs. The detailed procedure of ZnO nanoand microstructures functionalization by CNTs was reported by a number of different studies [11,12,36]. Fig. 5 depicts the surface functionalization of ZnO microstructures with CNT networks from a solution containing 0.9 wt% CNTs [11,12,36]. The TEM bright field images show a dense network surrounding the ZnO microwires (see Fig. 5a and b), which are single crystalline with c-axis growth of the wurzite-type structure as indicated by the electron diffraction pattern given in Fig. 5c. Here we integrated an individual CNT-functionalized ZnO NW with a diameter of ∼50 nm at one end into the device using the procedure developed by Lupan et al. [11,34,37,38]. A SEM figure of the fabricated device is presented in Fig. 6a. The UV response of the device to different intensities of UV light is presented in Fig. 6b. The applied bias was 3 V. The UV response for the nanodevice to ∼200, ∼80, ∼50 and ∼10 mW/cm2 is ∼350, ∼118, ∼60 and ∼15, respectively. The calculated values for R at 200, ∼80 and ∼50 mW/cm2 are 1.17, 1.33,
(3) (4)
where A1, A2, A3, and A4 are positive constants [18,24]. The results show that devices fabricated using ZnO-III and ZnO-IV samples, i.e. based on tetrapodal ZnO nano- and microstructures, show a faster response compared to other samples, while samples based on ZnO nanowire networks showed the slowest response. Therefore, it can be concluded that ZnO tetrapod networks are more favorable for the fabrication of fast UV photodetectors, which was also demonstrated in our previous work [18]. However, to be more precise, the ZnO tetrapod networks with a composed morphology of arms are more attractive. It was demonstrated previously that the presence of needle-like tips of tetrapods with a very thin diameter lead to the formation of a higher number of potential barriers and thus to a higher modulation of the electron depletion region in these segments of the arm [19]. The calculated values for R and G at different applied bias voltages are presented in Fig. 4c and d, respectively. The increase in R due to an increase in the applied bias voltage is related to the increased photocurrent and G (see Equations (1) and (2)). The highest values were obtained for the ZnO-IV morphology with R ≈ 5.2 × 10−3, 16 × 10−3, and 58 × 10−3 A/W at 5, 10, and 20 V, respectively (see Fig. 4c). The internal photoconductive gain for all samples was lower than one, indicating no gain which can be related to the high active surface area and the relatively high UV light power density [25,26]. In the case of the ZnO-IV morphology, calculations show that G ≈ 1.5 × 10−3, 4.8 × 10−3, and 17 × 10−3 A/W at 5, 10, and 20 V, respectively (see Fig. 4d). In general, the high sensing performances of the networks are a result of the high porosity which allows for a better diffusion of the oxygen species [16,19]. It is well known that in the case of UV sensing, adsorption/photodesorption of oxygen species (O2 (g ) + e− → O2− (ads ) ) upon illumination with UV light play a major role [16,18,19,21]. As results, the low conductivity surface depletion layer (Lair) and potential barriers between nano- and microstructures (qVs1) are formed [16,18,19,21]. Upon illumination with UV light, electron-hole pairs are photogenerated [26]. While the photogenerated holes migrate to the surface to discharge the adsorbed oxygen species (photodesorption) by an electron-hole recombination process, the photogenerated electrons remain in the conduction channel (dUV) and lead to an improvement in the photocurrent, as well as to a decrease of the potential barrier height (qVs2) and narrowing of the electron depletion region (LUV) [34,35]. The detailed UV sensing mechanism of ZnO nano- and microstructure networks was presented in previous work [19]. In the case of the ZnO-I morphology the relatively low UV sensing
Fig. 5. (a,b) Bright field TEM images of ZnO microstructures decorated with a CNT network (0.9 wt% CNT). (c) Electron diffraction pattern for the [21-10] zone axis orientation. 4
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structures, consisting of different morphology types of ZnO, namely ZnO NW-T networks (ZnO-I), ZnO-T-nanosheets (ZnO-II), the common ZnO-T (ZnO-III), and ZnO-T with complex arm morphology (ZnO-IV), were investigated as UV photodetectors. The highest UV sensing performance is observed for the networks composed of ZnO-T with a complex arm morphology (from sample set ZnO-IV), showing the high UV response of ≈3100 at 5 V. This finding was explained by the formation of a higher number of potential barriers through the networks, compared to other samples, as well as the high porosity of the 3-D networks. Another characteristic of the investigated morphologies is the low, nanoscopic diameter (50–150 nm) of the long wire-like tips, which is comparable with the Debye length. This gives a more efficient modulation under adsorption/desorption of oxygen species. Therefore, the samples from ZnO-IV combine the two most important factors which are necessary for the fabrication of highly efficient and highly sensitive UV detectors. Additionally, in the case of individual ZnO NWs it was observed that surface functionalization with CNTs can efficiently increase the UV sensing performances. In summary, this study provides design rules for the fabrication of high-performance ZnO-based UV nano-photodetectors, including aspects of network morphology and surface functionalization. Acknowledgment O.L. acknowledges the Alexander von Humboldt Foundation for the research fellowship for experienced researchers (3-3MOL/1148833 STP) at the Institute for Materials Science, Kiel University, Germany. This research was funded in part by the German Research Foundation (DFG- Deutsche Forschungsgemeinschaft) under the schemes FOR 2093 (KI 1263/12-2 & AD 183/12-2) & SFB1261 (subprojects A5; and A6; KI1263/12-2, AD 1263/12-2 and AD 183/17-1). This research was in part supported by the Technical University of Moldova. References [1] G.L. Manney, M.L. Santee, M. Rex, N.J. Livesey, M.C. Pitts, P. Veefkind, E.R. Nash, I. Wohltmann, R. Lehmann, L. Froidevaux, L.R. Poole, M.R. Schoeberl, D.P. Haffner, J. Davies, V. Dorokhov, H. Gernandt, B. Johnson, R. Kivi, E. Kyrö, N. Larsen, P.F. Levelt, A. Makshtas, C.T. McElroy, H. Nakajima, M.C. Parrondo, D.W. Tarasick, P. von der Gathen, K.A. Walker, N.S. Zinoviev, Unprecedented Arctic ozone loss in 2011, Nature 478 (2011) 469. [2] J. Moan, Ozone holes and biological consequences, J. Photochem. Photobiol., B 9 (1991) 244–247. [3] E.S. Ates, S. Kucukyildiz, H.E. Unalan, Zinc oxide nanowire photodetectors with single-walled carbon nanotube thin-film electrodes, ACS Appl. Mater. Interfaces 4 (2012) 5142–5146. [4] O. Lupan, N. Wolff, V. Postica, T. Braniste, I. Paulowicz, V. Hrkac, Y.K. Mishra, I. Tiginyanu, L. Kienle, R. Adelung, Properties of a single SnO2:Zn2SnO4 – functionalized nanowire based nanosensor, Ceram. Int. 44 (2018) 4859–4867. [5] 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. [6] V. Postica, M. Hoppe, J. Gröttrup, P. Hayes, V. Röbisch, D. Smazna, R. Adelung, B. Viana, P. Aschehoug, T. Pauporté, O. Lupan, Morphology dependent UV photoresponse of Sn-doped ZnO microstructures, Solid State Sci. 71 (2017) 75–86. [7] H. Chen, K. Liu, L. Hu, A.A. Al-Ghamdi, X. Fang, New concept ultraviolet photodetectors, Mater. Today 18 (2015) 493–502. [8] Y.K. Mishra, R. Adelung, ZnO tetrapod materials for functional applications, Mater. Today 21 (2017) 631–651. [9] K. Liu, M. Sakurai, M. Aono, ZnO-based ultraviolet photodetectors, Sensors 10 (2010) 8604–8634. [10] H. Shen, Y. Yin, K. Tian, K. Baskaran, L. Duan, X. Zhao, A. Tiwari, Growth and characterization of β-Ga2O3 thin films by sol-gel method for fast-response solarblind ultraviolet photodetectors, J. Alloy. Comp. 766 (2018) 601–608. [11] O. Lupan, F. Schütt, V. Postica, D. Smazna, Y.K. Mishra, R. Adelung, Sensing performances of pure and hybridized carbon nanotubes-ZnO nanowire networks: a detailed study, Sci. Rep. 7 (2017) 14715. [12] V. Postica, F. Schütt, R. Adelung, O. Lupan, Schottky diode based on a single carbon–nanotube–ZnO hybrid tetrapod for selective sensing applications, Adv Mater Interfaces 4 (2017) 1700507. [13] W.-D. Zhang, L.-C. Jiang, J.-S. Ye, Photoelectrochemical study on charge transfer properties of ZnO nanowires promoted by carbon nanotubes, J. Phys. Chem. C 113 (2009) 16247–16253. [14] W.-D. Zhang, B. Xu, L.-C. Jiang, Functional hybrid materials based on carbon nanotubes and metal oxides, J. Mater. Chem. 20 (2010) 6383–6391.
Fig. 6. (a) SEM image of a device based on an individual CNT-functionalized ZnO nanowire with diameter of ∼50 nm at one end. In the inset is presented an image of the thinner end of the NW at higher magnification to show the presence of CNTs. (b) Dynamic UV response at room temperature (≈25 °C) to different intensities of UV light.
1.66 and 2.1 A/W, respectively, while the calculated values for G at 200, ∼80 and ∼50 mW/cm2 are 3.98, 4.52, 5.64 and 7.14, respectively. These values are much higher compared to values for networks of pure ZnO nano- and microstructures (see Fig. 4c and d). Also, we can conclude that photoconductive gain persists in our fabricated device, which can be explained based on the large surface-to-volume ratio of NW and short distance between electrodes (L), see Eq. (2) [26]. The biexponential fitting (Equations (3) and (4)) was also applied in order to calculate the time constants of rising and decaying photocurrent. For illumination with 50 mW/cm2 the following values were obtained: τr1 = 0.05 s, τr2 = 0.79 s, τd1 = 0.05 s, and τd2 = 0.22 s. The detailed UV sensing mechanism based on the energy band diagram was already reported in our previous papers [34,38,39]. However, in our case the highly improved performances of CNT-functionalized ZnO NW compared to a pristine one reported in our previous work [22] can be explained based on increased carrier separation at the ZnO/CNT interface, thus helping to enhance the performance of the photodetector [11,12,40,41]. More details on enhanced UV sensing properties of ZnO by surface functionalization with CNTs can be found in our previous work [11]. 4. Conclusions Four different 3-D nano- and microstructured ZnO network 5
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