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Ultraviolet photodetector on flexible polymer substrate based on nano zinc oxide and laser-induced selective metallization
Composites Science and Technology 190 (2020) 108045
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Ultraviolet photodetector on flexible polymer substrate based on nano zinc oxide and laser-induced selective metallization Huiyuan Zhang , Jihai Zhang , Gehong Su , Tao Zhou *, Aiming Zhang ** State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute, Sichuan University, Chengdu, 610065, China
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Flexible polymer A. Zinc oxide B. UV photodetector E. Selective metallization E. Laser activation
This study fabricated a sensitive and fast response nano zinc oxide (nano-ZnO) ultraviolet (UV) photodetector on flexible polymer substrate via laser direct structuring (LDS) technology. The polystyrene-block-poly-(ethylene-copropylene)-block-polystyrene copolymer/polypropylene (SEPS/PP) composites incorporated with laser sensitizer of copper hydroxyl phosphate [Cu2(OH)PO4] were designed as flexible LDS material. X-ray photoelectron spectroscopy analysis confirmed that partial Cu2þ in SEPS/PP composites was reduced to Cu0 (element copper), which could be used as a catalyst for selective metallization. As a result, the well-defined interdigitated copper electrode was successfully fabricated on polymer substrate. Moreover, the UV photodetector was fabricated through spin-coating nano-ZnO on the interdigitated copper electrode. Photocurrent analysis demonstrated that the obtained UV photodetector exhibited excellent sensitivity and stability. The rise and decay times of the UV photodetector were less than 1 and 0.25 s, respectively. This study provided a guideline to develop electronic devices on flexible polymer materials based on LDS technology.
1. Introduction In the past decade, laser-induced selective metallization was the topic of numerous investigations and widely used in communications, electronic equipment, medical equipment, and other fields [1–6]. Laser direct structuring (LDS) technology as a typical laser-induced selective metallization method has attracted increasing attention [7]. The main process of LDS technology includes the incorporation of laser-sensitive additives into the polymer matrix, selective laser activation in accor dance with the designed patterns, and metallization of the activation patterns by electroless plating. The conductive circuit and metallized pattern can be easily obtained on the surface of polymer substrates through LDS technology. Compared with other selective metallization methods, such as photolithography [8–10], ink printing [11–16], screen printing [17], and micro-contact printing [18], LDS technology has advantages of a mask-free, low-cost, time-saving, and flexible design. Moreover, LDS technology promotes the development of electronic components in the direction of simplification, refinement, and envi ronmental friendliness by integrating the advantages of laser and elec troless plating. According to the literature, many researchers have investigated LDS
technology. Yang et al. [19] prepared LDS materials based on thermo plastics and laser sensitizer of copper aluminate (CuAl2O4); they ob tained conductive copper patterns after selective laser activation and electroless copper plating. Zhou et al. [20] also fabricated LDS materials by adding copper nanoparticles to polycarbonate (PC). After laser irra diation, the copper nanoparticles, which can be used for initiating electroless copper plating, were exposed on the PC surface. Ratautas et al. [21] used polypropylene composites doped with multi-walled carbon nanotubes for LDS technology. They obtained copper patterns after laser activation followed by electroless plating. Yan et al. [22] reported polyurethane composites containing copper ethylene diamine tetra acetic acid (EDTA-Cu) laser-sensitive additive for LDS technology. Recently, Zhang et al. [1,23–25] systematically investigated the mech anism and application of LDS technology based on acryloni trile butadiene styrene substrate-contained laser sensitizer of organocopper copper hydroxyl phosphate [Cu2(OH)PO4], copper chromium oxide (CuO⋅Cr2O3), and antimony-doped tin oxide. However, few reports have discussed the fabrication of LDS materials based on the thermoplastic elastomer matrix, which is a good flexible substrate. In this study, the thermoplastic elastomer was based on polystyreneblock-poly-(ethylene-co-propylene)-block-polystyrene copolymer (SEPS)
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Zhou), [email protected] (A. Zhang). https://doi.org/10.1016/j.compscitech.2020.108045 Received 21 October 2019; Received in revised form 3 January 2020; Accepted 27 January 2020 Available online 28 January 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved.
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and polypropylene (PP). The SEPS/PP composite was first prepared using melt blending. According to our previous study [24], Cu2(OH)PO4 possesses strong absorption at 1064 nm NIR pulsed laser and can perform the photochemical reduction reaction. Therefore, the SEPS/PP composite incorporated with Cu2(OH)PO4 was designed as an LDS ma terial, which could be laser activated. This composite exhibited good flexibility, showing the good performance of laser-induced selective metallization. The distinct advantages of LDS technology based on the NIR laser are as follows. i) The NIR laser has specific cost-effective and practical advantages for commercial applications. ii) Computer-controlled laser activation can offer high-resolution selective activation without wet chemical pretreatment. iii) Selective metalliza tion is performed by simple and low-cost electroless plating. Compared with the copper electrode obtained by the laser writing process [26,27], the mechanical adhesion of the obtained copper on the polymer is stronger, and the conductivity is excellent and stable because of LDS technology. Here, nano-ZnO was used as an ultraviolet light-sensitive material since Mollow first observed the ultraviolet light response of ZnO thin film in the 1940s [28–31]. It is an important semiconductor material with a wide bandgap (3.37 eV) and high excitation binding energy (60 meV) [32]. The interdigitated copper electrode was fabri cated on the SEPS/PP/Cu2(OH)PO4 composite via LDS technology. The nano-ZnO film was then spin-coated on the interdigitated electrodes. The inorganic functional nano-ZnO membrane combined with metal lized elastomer surface was designed as a flexible UV photodetector responsive to 365 nm light. Herein, we introduced a simple method to fabricate a highly flexible UV sensor dependent on nano-ZnO film and LDS technology.
GQ10B, a pulse width of 100 ns) equipped with the graphic design software of EZCAD 2.0. The information on the laser setup and the method of determining the laser activation parameters are provided in Fig. S6 (Supporting Information). An interdigitated pattern on the sample surface was laser irradiated in a line-by-line manner (adjacent line: 30 μm) in an air atmosphere with scanning speed of 2000 mm/s, laser power of 8 W, and laser pulse frequency of 80 kHz. Finally, the laser-irradiated samples were ultrasonically cleaned in deionized water. 2.4. Electroless plating The metallization of copper was carried out in an electroless plating bath containing freshly prepared solution A, solution B, and deionized water mixture with a ratio of 1:2:3. During electroless plating, the electroless time and temperature were 15 min and 50 � C, respectively. Continuous gas agitation was used to maintain the uniformity of the electroless plating solution. An interdigitated copper electrode was ob tained on the SEPS/PP/Cu2(OH)PO4 composite surface. 2.5. Fabrication of UV photodetectors First, 5 g of PEG in 20 mL of deionized water was stirred for 30 min at ambient temperature to ensure that PEG was fully dissolved and to obtain a PEG aqueous solution. Second, 2.5 g of nano-ZnO powder was added into the PEG aqueous solution for further stirring for 1 h to gain a suspension, in which nano-ZnO was sufficiently dispersed. The suspen sion was spin-coated onto the composite with the interdigitated copper electrodes at 1000 rpm. Third, the composite coated with the suspension was placed in a vacuum oven for 4 h at 80 � C to allow nano-ZnO to form a film. In our experiment, PEG was used to adjust the viscosity of the nano-ZnO suspension and increase the adhesion between the nano-ZnO film and the composite with the interdigitated copper electrodes. Fig. 1 illustrates the schematic procedure for fabricating the UV photodetector based on LDS technology.
2. Experimental section 2.1. Materials SEPS (7311F) with 12 wt% polystyrene blocks was produced by Kuraray Co., Ltd. SEPS with the reputation of “green rubber” has excellent flexibility, and strong adhesion between the obtained copper layer and the SEPS substrate was easily achieved (Fig. S1 in Supporting Information). PP (T30S) was synthesized by Lanzhou Petrochemical Co., Ltd. Cu2(OH)PO4 was provided by Merck Chemicals Technology Co., Ltd. Electroless copper plating solution of A and B was purchased from Guangzhou Yi Shun Chemical Co., Ltd. Nano-ZnO (99.9%, 30 � 10 nm) was provided by Shanghai Macklin Biochemical Co., Ltd. The physical chemistry characterizations of nano-ZnO are presented in Figs. S2 and S3 (Supporting Information). Polyethylene glycol (PEG, Mw ¼ 20,000 g/ mol) was purchased from Tianjin Ruijinte Chemical Co., Ltd.
2.6. UV light response tests The prepared UV photodetector was irradiated with 365 nm ultra violet light. The signal of current came from interdigitated copper electrodes of the UV photodetector with a Keithley 2601B source meter (USA). The method was a two-point probe real-time measurement. The input voltage of the two-point measurement was set to 6 V, and the UV light intensities were 300, 600, 900, 1200, and 1500 mW/cm2. 2.7. Characterizations
2.2. Preparation of SEPS/PP/Cu2(OH)PO4 sample
The spectroscopy and morphology of nano-ZnO were analyzed by scanning electron microscopy (SEM; Hitachi S4800, 20 kV), trans mission electron microscopy (JEOL JEM-2011, 200 kV), and UV–visible infrared spectroscopy (UV-3600, Shimadzu). The crystal structures of nano-ZnO and the obtained copper patterns after electroless copper plating were analyzed through powder X-ray diffraction (XRD; Japan Rigaku D/max-2500). The composite sheet after laser irradiation was characterized by SEM (Hitachi S4800, 20 kV), X-ray photoelectron spectroscopy (XPS; Kratos XASAM 800), and contact angle measure ments (Germany, Krüss K100). The surface Raman spectra were measured on a micro-Raman spectrometer (Thermo Fisher Scientific). The confocal laser scanning microscopy (CLSM) topographical morphology of the sample was obtained by LSM 710, Zeiss. The hardness of the substrate based on SEPS with different PP contents was analyzed by an LX-A rubber durometer (Shanghai Liuling Instrument Factory). The adhesion property (Scotch tape test) between the polymer substrate and copper layer was analyzed according to the ASTM D3359 standard. Table S1 shows the classification of adhesion test results according to the ASTM D3359 standard (Supporting Information).
First, the SEPS/PP composite was prepared with the mass ratio of SEPS:PP ¼ 85:15 by melt blending at 220 � C through a twin screw extruder. The obtained pellets of the SEPS/PP composite incorporated with a laser sensitizer of Cu2(OH)PO4 (5 wt%) were prepared by melt blending at 220 � C using a twin screw extruder. The sheets of the SEPS/ PP/Cu2(OH)PO4 composite (65 � 65 � 3 mm3) were obtained by compression molding at 210 � C. As is known, adding PP can reduce the cost of composite based on SEPS. Figs. S4 and S5 (Supporting Informa tion) show that a copper layer of improved quality could be obtained on the surface of the substrates with a small amount of PP with minimal effect of flexibility. 2.3. Laser selective activation First, the sample sheets of the SEPS/PP/Cu2(OH)PO4 composite were subjected to ultrasonic removal of surface organic contaminants in NaOH solution (1.0 mol/L) for 10 min. Laser selective activation on the sample surface was conducted via optical fiber laser machining (MK2
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Fig. 1. Schematic of the UV photodetector’s fabrication through LDS technology.
Fig. 2. (a) Interdigitated pattern on SEPS/PP/Cu2(OH)PO4 after NIR pulsed laser activation; (b) SEM image, scale bars: 100 and 10 μm; (c) Photography of the interdigitated copper electrode after electroless copper plating; (d) SEM image, scale bars: 100 and 10 μm. 3
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3. Results and discussion
Fig. 3(a) presents the obtained copper layer with three characteristic peaks at 2θ ¼ 43.4� ,50.6� , and 74.2� , indicating that the high-quality copper layer was obtained on the SEPS/PP/Cu2(OH)PO4 composite. EDX analysis confirmed the elemental composition of the copper layer. Fig. 3(b) depicts the characteristic peaks of copper with two K peaks (8.9 and 8.0 keV) and one L peak (0.9 keV) in the EDX spectra. SEM was also used to study the thickness of the obtained copper layer. Fig. 3(c) shows that the copper layer had a thickness of approximately 1.8 μm. Fig. 3(d) shows the adhesion property of the copper layer analyzed through a Scotch tape test. The copper layer presented excellent adhesion strength with 5B level according to ASTM D3359, which entirely met the re quirements for industrial application. The microspores formed in laser activation allowed the copper layer to anchor into the polymer sub strates. Therefore, the adhesion property of the polymer substrate and the obtained copper layer was greatly enhanced.
3.1. Fabricating interdigitated copper electrode The preparation of the interdigitated copper pattern in our experi ment is schematically shown in Fig. 1. The interdigitated copper elec trodes were designed to contain 14 branches with a width of 2 mm. The gap between the two adjacent branches was 500 μm. Fig. S7 (Supporting Information) shows the details of the interdigitated electrodes. Fig. 2(a) shows the pattern of the interdigitated electrodes, which were laseractivated with a scanning speed of 2000 mm/s, laser power of 8 W, and laser frequency of 80 kHz. Fig. 2(b) shows the corresponding SEM images in the laser-activated area. We observed a series of regular microsized pores in the laser-activated area caused by high-energy laser irradiation. The typical diameter of these pores was around 4–8 μm. The substrate was selectively carbonized at the laser-irradiated areas because of the instantaneous high temperature generated by Cu2(OH) PO4 absorbing laser energy, resulting in the regular micro-sized pores and rough microscale structures on the surface of the SEPS/PP/Cu2(OH) PO4 composite. Fig. S8 (Supporting Information) shows that the wetta bility of the sample surface was reduced after laser activation because rough microscale structures formed on the surface of the SEPS/PP/ Cu2(OH)PO4 composite. Moreover, the micro-protuberance structures and several hierarchical mesh-porous structures in the microscale rough surface were the underlying reasons for the decreased wettability on the surface [33,34]. The topographic morphology could also be directly observed by the CLSM image in Fig. S9 (Supporting Information). Fig. 2 (c) shows that the interdigitated copper electrode was successfully ob tained after 15 min of electroless copper plating. The copper layer pre sented clear boundaries. Fig. 2(d) shows that the copper layer evenly covered the surface of the laser-activated area, and the resulting copper particles on the copper layer had a diameter of around 1 μm.
3.2. Mechanisms of laser activation and selective metallization For the LDS materials, the key to achieving high-quality and highresolution metal patterns lies in effective selective laser activation. XPS was carried out to further investigate the surface chemical composition changes on the laser-activated SEPS/PP/Cu2(OH)PO4 composites. The XPS pattern of Cu2(OH)PO4 without laser irradiation was also tested. In Fig. 4(a), for neat Cu2(OH)PO4 without laser irradi ation, the main peaks observed at 933.6 and 953.6 eV were attributed to the Cu2þ characteristic peaks of Cu 2p3/2 and Cu 2p1/2, respectively. For the SEPS/PP/Cu2(OH)PO4 composite after laser activation, good curve fitting with sum function (the asymmetric Lorentzian–Gaussian) was obtained in Fig. 4(a). Two new peaks were attributed to the Cu0 (element copper) characteristic peaks of Cu 2p3/2 and Cu 2p1/2 at 932.5 and 952.3 eV, respectively. Some of Cu2(OH)PO4 in the samples was reduced to Cu0 during laser irradiation, and Cu0 functioned as the
Fig. 3. Characterizations of the copper layer; (a) Copper layer XRD pattern (red) and reference Cu cubic structure (JCPDS card no. 04–0836) XRD peaks (blue bars); (b) EDX analysis and the corresponding SEM image, scale bar: 70 μm; (c) Cross-sectional SEM image, scale bar: 10 μm; (d) Scotch tape test of the obtained copper layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 4
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Fig. 4. (a) XPS patterns of Cu2(OH)PO4 and XPS curve fitting result (XPSPEAK v4.1) of SEPS/PP/Cu2(OH)PO4 composites after laser irradiation; (b) Raman spectra of the SEPS/PP/Cu2(OH)PO4 before (black curve) and after (red curve) laser activation. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
effective catalytic center for initiating electroless copper plating [1,23]. The chemical composition changes of SEPS/PP/Cu2(OH)PO4 before and after laser activation were also confirmed by the Raman spectra. Fig. 4(b) shows that Raman spectra clearly indicated a broad diffusion band at around 820–1750 cm 1 after laser activation. According to the literature, the broad band (1000–2000 cm 1) is designated to amor phous carbon [35]. Thus, Raman spectra revealed the carbonization on the SEPS/PP/Cu2(OH)PO4 composite surface. The generated amorphous carbon from carbonization on SEPS/PP/Cu2(OH)PO4 by laser irradia tion acted as a reducing agent, thereby providing an environment to reduce Cu2(OH)PO4 to Cu0 at high temperature. As described above, Cu0 and amorphous carbon are formed after laser activation. As we previously reported, the newly generated Cu0 on the composite surface is key to initiating selective metallization [1,23]. Cu0 on the surface acted as a catalytic site for copper deposition in electroless copper plating. The copper particles grew around the cata lytic sites, forming a thin copper layer on the SEPS/PP/Cu2(OH)PO4 composite.
photodetector is only the current. Herein, the sensing element fabricated on the SEPS/PP/Cu2(OH)PO4 was connected to an electronic circuit with input voltage of 6 V. The photocurrent signals were observed by computer data acquisition. Supplementary data related to this article can be found at https://do i.org/10.1016/j.compscitech.2020.108045. Repeatability and response speed are important parameters for evaluating the performance of a photodetector. To date, obtaining a photodetector with high repeatability and fast synchronous response remains a challenge. Fig. 5(c) shows the photocurrent of the fabricated photodetector during repetitive switching of UV irradiation or on/off switching. We observed no apparent degradation for an average photocurrent of ~200 μA over 100 s, indicating excellent photocurrent stability. Movie S2 demonstrates an application of the UV photodetector as a UV switch to control an LED bulb. Movies S3 and S4 show the application of UV photodetectors under different deformation as a UV switch to control an LED bulb. The supplied voltage was around 27 V. The UV photodetector was placed in a UV generator, which could pro duce ultraviolet light with wavelength and intensity of 365 nm and 1500 mW/cm2, respectively. The LED bulb turns on or off at the same time when the UV generator is turned on or off. We also confirmed the fast response capacity of the fabricated photodetector. In our experi ment, the rise time of the photodetector was defined from the initial current to increase to the peak value of 90%. The time from the peak value of 90% to the peak value of 10% was defined as the decay time of the photodetector. Fig. 5(d) shows that the rise and decay times were less than 1 and 0.25 s, respectively. Fig. 5(e) shows a comparison of the rise and decay times for ZnO-based photodetectors of 365 nm UV [28, 29,36–45]. Table S2 (Supporting Information) shows the specific values of response time. The results demonstrated that the obtained photode tector possessed good stability and fast response capacity, making it promising for large-area photodetector application. Supplementary data related to this article can be found at https://do i.org/10.1016/j.compscitech.2020.108045. In Fig. 5(f), the photocurrent change of the photodetector was measured under UV irradiation of 365 nm at different power intensities from 300 mW/cm2 to 1500 mW/cm2. The photocurrent increased gradually with increasing UV intensity, and this phenomenon may be due to a change in photon intensity from UV light. A high current of 220 μA was recorded at the UV intensity of 1500 mW/cm2, whereas the current was only 75 μA at the UV intensity of 300 mW/cm2. This result demonstrated that the fabricated photodetector could achieve high sensitivity performance for different UV signals. This application is attractive for distinguishing different UV intensities. Therefore, LDS technology combined with the nano-ZnO film is a low-cost, efficient
3.3. UV photodetector The highly sensitive photodetector was simply achieved through spin-coating a layer of nano-ZnO film on the interdigitated electrode. The photoresponse schematic of the nano-ZnO photodetector is exhibi ted in Fig. 5(a). Without UV light irradiation, oxygen in the environment was adsorbed on the defect sites in the nano-ZnO lattice and captured the electrons in nano-ZnO, which led to the formation of negative oxy gen molecules and the decrease in the electron carrier concentration in nano-ZnO. Therefore, the resistance in nano-ZnO was high, and the current in the circuit was low under no UV light irradiation. However, once nano-ZnO was exposed to UV light, the electron-hole pairs pro duced from nano-ZnO transferred to the surface quickly and were captured by the negative oxygen molecules on the surface. The oxygen molecules adsorbed on the surface were desorbed, and the electron carrier concentration in nano-ZnO increased rapidly and enhanced photocurrent and photoconductivity. Therefore, nano-ZnO could be used as a UV sensing material by collecting the current signal under UV irradiation. Fig. 5(b) presents the digital photograph of the as-prepared UV photodetector on the SEPS/PP/Cu2(OH)PO4 composite with good flexibility. The morphology of the nano-ZnO layer on the surface of the SEPS/PP/Cu2(OH)PO4 composite is shown in Fig. S10 (Supporting In formation). Movie S1 (Supporting Information) demonstrates the flexi bility of the UV photodetector in real time. In general, the sensing response signals are recorded by current, resistance, impedance, or capacitance in the sensors. However, the detection signal for the 5
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Fig. 5. (a) Photoresponse schematic of the nano-ZnO photodetector; (b) UV photodetector with good flexibility; (c) Photocurrent response of the UV photodetector upon 365 nm UV irradiation measured for UV-on and UV-off conditions at a UV intensity of 1500 mW/cm2; (d) Photocurrent rise and decay of UV photodetector measured; (e) Comparison of the rise and decay times for ZnO-based photodetectors of 365 nm UV [28,29,36–45]; (f) Change in the photocurrent toward different UV intensities.
method for producing UV photodetectors.
Declaration of competing interest
4. Conclusions
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
In summary, this study fabricated an interdigitated copper electrode on the SEPS/PP/Cu2(OH)PO4 composite with high flexibility via LDS technology. The selective metallization results demonstrated that the SEPS/PP/Cu2(OH)PO4 composite was an excellent LDS material with good flexibility. Furthermore, UV photodetector based on the nano-ZnO film on the interdigitated copper electrode was successfully fabricated through a simple spin-coating method. In the photocurrent measurement, the photodetector showed a different photoresponse capacity as a function of varying UV intensities. The rise and decay times as sensitivity parameters of the photodetector were less than 1 and 0.25 s, respectively, demonstrating that the photodetector exhibited good sensitivity, fast response, excellent repeatability, and stability. Therefore, this low-cost, high-efficiency LDS technology combined with the nano-ZnO film is promising for the largearea fabrication of photodetectors with excellent performance.
CRediT authorship contribution statement Huiyuan Zhang: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Jihai Zhang: Visualization, Investigation. Gehong Su: Formal analysis. Tao Zhou: Project admin istration, Supervision, Funding acquisition, Resources, Validation, Writing - review & editing. Aiming Zhang: Supervision. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51773126, 51473104), Outstanding Youth Foun dation of Sichuan Province (Grant No. 2017JQ0006), the National Key 6
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R&D Program of China (Grant No. 2017YFC1104800), and State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2018-209).
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compscitech.2020.108045. References [1] J. Zhang, J. Feng, L. Jia, H. Zhang, G. Zhang, S. Sun, T. Zhou, Laser-induced selective metallization on polymer substrates using organocopper for portable electronics, ACS Appl. Mater. Interfaces 11 (2019) 13714–13723. [2] P. Rytlewski, T. Bahners, F. Polewski, B. Gebert, J.S. Gutmann, N. Hartmann, U. Hagemann, K. Moraczewski, Laser-induced surface activation of biocomposites for electroless metallization, Surf. Coating. Technol. 311 (2017) 104–112. [3] J. Cai, C. Lv, A. Watanabe, Laser direct writing and selective metallization of metallic circuits for integrated wireless devices, ACS Appl. Mater. Interfaces 10 (2018) 915–924. [4] M. Gedvilas, K. Ratautas, E. Kacar, I. Stankeviciene, A. Jagminiene, E. Norkus, N. Li Pira, G. Raciukaitis, Colour-difference measurement method for evaluation of quality of electrolessly deposited copper on polymer after laser-induced selective activation, Sci. Rep. 6 (2016) 22963. [5] B.B. Xu, H. Xia, L.G. Niu, Y.L. Zhang, K. Sun, Q.D. Chen, Y. Xu, Z.Q. Lv, Z.H. Li, H. Misawa, H.B. Sun, Flexible nanowiring of metal on nonplanar substrates by femtosecond-laser-induced electroless plating, Small 6 (2010) 1762–1766. _ I. Stankevi�ciene, _ N. Li Pira, S. Sinopoli, [6] M. Gedvilas, K. Ratautas, A. Jagminiene, E. Kacar, E. Norkus, G. Ra�ciukaitis, Percolation effect of a Cu layer on a MWCNT/ PP nanocomposite substrate after laser direct structuring and autocatalytic plating, RSC Adv. 8 (2018) 30305–30309. [7] B.J.F. Bachy, Experimental investigation and optimization for the effective parameters in the laser direct structuring process, J. Laser Micro./Nanoeng. 10 (2015) 202–209. [8] J. Cai, C. Zhang, A. Khan, L. Wang, W.D. Li, Selective electroless metallization of micro- and nanopatterns via poly(dopamine) modification and palladium nanoparticle catalysis for flexible and stretchable electronic applications, ACS Appl. Mater. Interfaces 10 (2018) 28754–28763. [9] L. Zhao, D.Q. Chen, W.H. Hu, Patterning of metal films on arbitrary substrates by using polydopamine as a UV-sensitive catalytic layer for electroless deposition, Langmuir 32 (2016) 5285–5290. [10] S.J. Park, T.-J. Ko, J. Yoon, M.-W. Moon, K.H. Oh, J.H. Han, Copper circuit patterning on polymer using selective surface modification and electroless plating, Appl. Surf. Sci. 396 (2017) 1678–1684. [11] T. Yamada, K. Fukuhara, K. Matsuoka, H. Minemawari, J. Tsutsumi, N. Fukuda, K. Aoshima, S. Arai, Y. Makita, H. Kubo, T. Enomoto, T. Togashi, M. Kurihara, T. Hasegawa, Nanoparticle chemisorption printing technique for conductive silver patterning with submicron resolution, Nat. Commun. 7 (2016) 11402. [12] Y. Yong, M.T. Nguyen, T. Yonezawa, T. Asano, M. Matsubara, H. Tsukamoto, Y.C. Liao, T. Zhang, S. Isobe, Y. Nakagawa, Use of decomposable polymer-coated submicron Cu particles with effective additive for production of highly conductive Cu films at low sintering temperature, J. Mater. Chem. C 5 (2017) 1033–1041. [13] W. Li, H. Zhang, Y. Gao, J. Jiu, C.-F. Li, C. Chen, D. Hu, Y. Goya, Y. Wang, H. Koga, S. Nagao, K. Suganuma, Highly reliable and highly conductive submicron Cu particle patterns fabricated by low temperature heat-welding and subsequent flash light sinter-reinforcement, J. Mater. Chem. C 5 (2017) 1155–1164. [14] J.-J. Chen, G.-Q. Lin, Y. Wang, E. Sowade, R.R. Baumann, Z.-S. Feng, Fabrication of conductive copper patterns using reactive inkjet printing followed by two-step electroless plating, Appl. Surf. Sci. 396 (2017) 202–207. [15] Y. Farraj, M. Grouchko, S. Magdassi, Self-reduction of a copper complex MOD ink for inkjet printing conductive patterns on plastics, Chem. Commun. 51 (2015) 1587–1590. [16] Y.T. Hwang, W.H. Chung, Y.R. Jang, H.S. Kim, Intensive plasmonic flash light sintering of copper nanoinks using a band-pass light filter for highly electrically conductive electrodes in printed electronics, ACS Appl. Mater. Interfaces 8 (2016) 8591–8599. [17] J.Q. Xie, Y.Q. Ji, D.S. Mao, F.T. Zhang, X.Z. Fu, R. Sun, C.P. Wong, Sn-NanorodSupported Ag nanoparticles as efficient catalysts for electroless deposition of Cu conductive tracks, ACS Appl. Nano Mater. 1 (2018) 1531–1540. [18] S.-C. Huang, T.-C. Tsao, L.-J. Chen, Selective electroless copper plating on poly (ethylene terephthalate) surfaces by microcontact printing, J. Electrochem. Soc. 157 (2010) 222–227.
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Ultraviolet photodetector on flexible polymer substrate based on nano zinc oxide and laser-induced selective metallization Huiyuan Zhang , Jihai Zhang , Gehong Su , Tao Zhou *, Aiming Zhang ** State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute, Sichuan University, Chengdu, 610065, China
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Flexible polymer A. Zinc oxide B. UV photodetector E. Selective metallization E. Laser activation
This study fabricated a sensitive and fast response nano zinc oxide (nano-ZnO) ultraviolet (UV) photodetector on flexible polymer substrate via laser direct structuring (LDS) technology. The polystyrene-block-poly-(ethylene-copropylene)-block-polystyrene copolymer/polypropylene (SEPS/PP) composites incorporated with laser sensitizer of copper hydroxyl phosphate [Cu2(OH)PO4] were designed as flexible LDS material. X-ray photoelectron spectroscopy analysis confirmed that partial Cu2þ in SEPS/PP composites was reduced to Cu0 (element copper), which could be used as a catalyst for selective metallization. As a result, the well-defined interdigitated copper electrode was successfully fabricated on polymer substrate. Moreover, the UV photodetector was fabricated through spin-coating nano-ZnO on the interdigitated copper electrode. Photocurrent analysis demonstrated that the obtained UV photodetector exhibited excellent sensitivity and stability. The rise and decay times of the UV photodetector were less than 1 and 0.25 s, respectively. This study provided a guideline to develop electronic devices on flexible polymer materials based on LDS technology.
1. Introduction In the past decade, laser-induced selective metallization was the topic of numerous investigations and widely used in communications, electronic equipment, medical equipment, and other fields [1–6]. Laser direct structuring (LDS) technology as a typical laser-induced selective metallization method has attracted increasing attention [7]. The main process of LDS technology includes the incorporation of laser-sensitive additives into the polymer matrix, selective laser activation in accor dance with the designed patterns, and metallization of the activation patterns by electroless plating. The conductive circuit and metallized pattern can be easily obtained on the surface of polymer substrates through LDS technology. Compared with other selective metallization methods, such as photolithography [8–10], ink printing [11–16], screen printing [17], and micro-contact printing [18], LDS technology has advantages of a mask-free, low-cost, time-saving, and flexible design. Moreover, LDS technology promotes the development of electronic components in the direction of simplification, refinement, and envi ronmental friendliness by integrating the advantages of laser and elec troless plating. According to the literature, many researchers have investigated LDS
technology. Yang et al. [19] prepared LDS materials based on thermo plastics and laser sensitizer of copper aluminate (CuAl2O4); they ob tained conductive copper patterns after selective laser activation and electroless copper plating. Zhou et al. [20] also fabricated LDS materials by adding copper nanoparticles to polycarbonate (PC). After laser irra diation, the copper nanoparticles, which can be used for initiating electroless copper plating, were exposed on the PC surface. Ratautas et al. [21] used polypropylene composites doped with multi-walled carbon nanotubes for LDS technology. They obtained copper patterns after laser activation followed by electroless plating. Yan et al. [22] reported polyurethane composites containing copper ethylene diamine tetra acetic acid (EDTA-Cu) laser-sensitive additive for LDS technology. Recently, Zhang et al. [1,23–25] systematically investigated the mech anism and application of LDS technology based on acryloni trile butadiene styrene substrate-contained laser sensitizer of organocopper copper hydroxyl phosphate [Cu2(OH)PO4], copper chromium oxide (CuO⋅Cr2O3), and antimony-doped tin oxide. However, few reports have discussed the fabrication of LDS materials based on the thermoplastic elastomer matrix, which is a good flexible substrate. In this study, the thermoplastic elastomer was based on polystyreneblock-poly-(ethylene-co-propylene)-block-polystyrene copolymer (SEPS)
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Zhou), [email protected] (A. Zhang). https://doi.org/10.1016/j.compscitech.2020.108045 Received 21 October 2019; Received in revised form 3 January 2020; Accepted 27 January 2020 Available online 28 January 2020 0266-3538/© 2020 Elsevier Ltd. All rights reserved.
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and polypropylene (PP). The SEPS/PP composite was first prepared using melt blending. According to our previous study [24], Cu2(OH)PO4 possesses strong absorption at 1064 nm NIR pulsed laser and can perform the photochemical reduction reaction. Therefore, the SEPS/PP composite incorporated with Cu2(OH)PO4 was designed as an LDS ma terial, which could be laser activated. This composite exhibited good flexibility, showing the good performance of laser-induced selective metallization. The distinct advantages of LDS technology based on the NIR laser are as follows. i) The NIR laser has specific cost-effective and practical advantages for commercial applications. ii) Computer-controlled laser activation can offer high-resolution selective activation without wet chemical pretreatment. iii) Selective metalliza tion is performed by simple and low-cost electroless plating. Compared with the copper electrode obtained by the laser writing process [26,27], the mechanical adhesion of the obtained copper on the polymer is stronger, and the conductivity is excellent and stable because of LDS technology. Here, nano-ZnO was used as an ultraviolet light-sensitive material since Mollow first observed the ultraviolet light response of ZnO thin film in the 1940s [28–31]. It is an important semiconductor material with a wide bandgap (3.37 eV) and high excitation binding energy (60 meV) [32]. The interdigitated copper electrode was fabri cated on the SEPS/PP/Cu2(OH)PO4 composite via LDS technology. The nano-ZnO film was then spin-coated on the interdigitated electrodes. The inorganic functional nano-ZnO membrane combined with metal lized elastomer surface was designed as a flexible UV photodetector responsive to 365 nm light. Herein, we introduced a simple method to fabricate a highly flexible UV sensor dependent on nano-ZnO film and LDS technology.
GQ10B, a pulse width of 100 ns) equipped with the graphic design software of EZCAD 2.0. The information on the laser setup and the method of determining the laser activation parameters are provided in Fig. S6 (Supporting Information). An interdigitated pattern on the sample surface was laser irradiated in a line-by-line manner (adjacent line: 30 μm) in an air atmosphere with scanning speed of 2000 mm/s, laser power of 8 W, and laser pulse frequency of 80 kHz. Finally, the laser-irradiated samples were ultrasonically cleaned in deionized water. 2.4. Electroless plating The metallization of copper was carried out in an electroless plating bath containing freshly prepared solution A, solution B, and deionized water mixture with a ratio of 1:2:3. During electroless plating, the electroless time and temperature were 15 min and 50 � C, respectively. Continuous gas agitation was used to maintain the uniformity of the electroless plating solution. An interdigitated copper electrode was ob tained on the SEPS/PP/Cu2(OH)PO4 composite surface. 2.5. Fabrication of UV photodetectors First, 5 g of PEG in 20 mL of deionized water was stirred for 30 min at ambient temperature to ensure that PEG was fully dissolved and to obtain a PEG aqueous solution. Second, 2.5 g of nano-ZnO powder was added into the PEG aqueous solution for further stirring for 1 h to gain a suspension, in which nano-ZnO was sufficiently dispersed. The suspen sion was spin-coated onto the composite with the interdigitated copper electrodes at 1000 rpm. Third, the composite coated with the suspension was placed in a vacuum oven for 4 h at 80 � C to allow nano-ZnO to form a film. In our experiment, PEG was used to adjust the viscosity of the nano-ZnO suspension and increase the adhesion between the nano-ZnO film and the composite with the interdigitated copper electrodes. Fig. 1 illustrates the schematic procedure for fabricating the UV photodetector based on LDS technology.
2. Experimental section 2.1. Materials SEPS (7311F) with 12 wt% polystyrene blocks was produced by Kuraray Co., Ltd. SEPS with the reputation of “green rubber” has excellent flexibility, and strong adhesion between the obtained copper layer and the SEPS substrate was easily achieved (Fig. S1 in Supporting Information). PP (T30S) was synthesized by Lanzhou Petrochemical Co., Ltd. Cu2(OH)PO4 was provided by Merck Chemicals Technology Co., Ltd. Electroless copper plating solution of A and B was purchased from Guangzhou Yi Shun Chemical Co., Ltd. Nano-ZnO (99.9%, 30 � 10 nm) was provided by Shanghai Macklin Biochemical Co., Ltd. The physical chemistry characterizations of nano-ZnO are presented in Figs. S2 and S3 (Supporting Information). Polyethylene glycol (PEG, Mw ¼ 20,000 g/ mol) was purchased from Tianjin Ruijinte Chemical Co., Ltd.
2.6. UV light response tests The prepared UV photodetector was irradiated with 365 nm ultra violet light. The signal of current came from interdigitated copper electrodes of the UV photodetector with a Keithley 2601B source meter (USA). The method was a two-point probe real-time measurement. The input voltage of the two-point measurement was set to 6 V, and the UV light intensities were 300, 600, 900, 1200, and 1500 mW/cm2. 2.7. Characterizations
2.2. Preparation of SEPS/PP/Cu2(OH)PO4 sample
The spectroscopy and morphology of nano-ZnO were analyzed by scanning electron microscopy (SEM; Hitachi S4800, 20 kV), trans mission electron microscopy (JEOL JEM-2011, 200 kV), and UV–visible infrared spectroscopy (UV-3600, Shimadzu). The crystal structures of nano-ZnO and the obtained copper patterns after electroless copper plating were analyzed through powder X-ray diffraction (XRD; Japan Rigaku D/max-2500). The composite sheet after laser irradiation was characterized by SEM (Hitachi S4800, 20 kV), X-ray photoelectron spectroscopy (XPS; Kratos XASAM 800), and contact angle measure ments (Germany, Krüss K100). The surface Raman spectra were measured on a micro-Raman spectrometer (Thermo Fisher Scientific). The confocal laser scanning microscopy (CLSM) topographical morphology of the sample was obtained by LSM 710, Zeiss. The hardness of the substrate based on SEPS with different PP contents was analyzed by an LX-A rubber durometer (Shanghai Liuling Instrument Factory). The adhesion property (Scotch tape test) between the polymer substrate and copper layer was analyzed according to the ASTM D3359 standard. Table S1 shows the classification of adhesion test results according to the ASTM D3359 standard (Supporting Information).
First, the SEPS/PP composite was prepared with the mass ratio of SEPS:PP ¼ 85:15 by melt blending at 220 � C through a twin screw extruder. The obtained pellets of the SEPS/PP composite incorporated with a laser sensitizer of Cu2(OH)PO4 (5 wt%) were prepared by melt blending at 220 � C using a twin screw extruder. The sheets of the SEPS/ PP/Cu2(OH)PO4 composite (65 � 65 � 3 mm3) were obtained by compression molding at 210 � C. As is known, adding PP can reduce the cost of composite based on SEPS. Figs. S4 and S5 (Supporting Informa tion) show that a copper layer of improved quality could be obtained on the surface of the substrates with a small amount of PP with minimal effect of flexibility. 2.3. Laser selective activation First, the sample sheets of the SEPS/PP/Cu2(OH)PO4 composite were subjected to ultrasonic removal of surface organic contaminants in NaOH solution (1.0 mol/L) for 10 min. Laser selective activation on the sample surface was conducted via optical fiber laser machining (MK2
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Fig. 1. Schematic of the UV photodetector’s fabrication through LDS technology.
Fig. 2. (a) Interdigitated pattern on SEPS/PP/Cu2(OH)PO4 after NIR pulsed laser activation; (b) SEM image, scale bars: 100 and 10 μm; (c) Photography of the interdigitated copper electrode after electroless copper plating; (d) SEM image, scale bars: 100 and 10 μm. 3
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3. Results and discussion
Fig. 3(a) presents the obtained copper layer with three characteristic peaks at 2θ ¼ 43.4� ,50.6� , and 74.2� , indicating that the high-quality copper layer was obtained on the SEPS/PP/Cu2(OH)PO4 composite. EDX analysis confirmed the elemental composition of the copper layer. Fig. 3(b) depicts the characteristic peaks of copper with two K peaks (8.9 and 8.0 keV) and one L peak (0.9 keV) in the EDX spectra. SEM was also used to study the thickness of the obtained copper layer. Fig. 3(c) shows that the copper layer had a thickness of approximately 1.8 μm. Fig. 3(d) shows the adhesion property of the copper layer analyzed through a Scotch tape test. The copper layer presented excellent adhesion strength with 5B level according to ASTM D3359, which entirely met the re quirements for industrial application. The microspores formed in laser activation allowed the copper layer to anchor into the polymer sub strates. Therefore, the adhesion property of the polymer substrate and the obtained copper layer was greatly enhanced.
3.1. Fabricating interdigitated copper electrode The preparation of the interdigitated copper pattern in our experi ment is schematically shown in Fig. 1. The interdigitated copper elec trodes were designed to contain 14 branches with a width of 2 mm. The gap between the two adjacent branches was 500 μm. Fig. S7 (Supporting Information) shows the details of the interdigitated electrodes. Fig. 2(a) shows the pattern of the interdigitated electrodes, which were laseractivated with a scanning speed of 2000 mm/s, laser power of 8 W, and laser frequency of 80 kHz. Fig. 2(b) shows the corresponding SEM images in the laser-activated area. We observed a series of regular microsized pores in the laser-activated area caused by high-energy laser irradiation. The typical diameter of these pores was around 4–8 μm. The substrate was selectively carbonized at the laser-irradiated areas because of the instantaneous high temperature generated by Cu2(OH) PO4 absorbing laser energy, resulting in the regular micro-sized pores and rough microscale structures on the surface of the SEPS/PP/Cu2(OH) PO4 composite. Fig. S8 (Supporting Information) shows that the wetta bility of the sample surface was reduced after laser activation because rough microscale structures formed on the surface of the SEPS/PP/ Cu2(OH)PO4 composite. Moreover, the micro-protuberance structures and several hierarchical mesh-porous structures in the microscale rough surface were the underlying reasons for the decreased wettability on the surface [33,34]. The topographic morphology could also be directly observed by the CLSM image in Fig. S9 (Supporting Information). Fig. 2 (c) shows that the interdigitated copper electrode was successfully ob tained after 15 min of electroless copper plating. The copper layer pre sented clear boundaries. Fig. 2(d) shows that the copper layer evenly covered the surface of the laser-activated area, and the resulting copper particles on the copper layer had a diameter of around 1 μm.
3.2. Mechanisms of laser activation and selective metallization For the LDS materials, the key to achieving high-quality and highresolution metal patterns lies in effective selective laser activation. XPS was carried out to further investigate the surface chemical composition changes on the laser-activated SEPS/PP/Cu2(OH)PO4 composites. The XPS pattern of Cu2(OH)PO4 without laser irradiation was also tested. In Fig. 4(a), for neat Cu2(OH)PO4 without laser irradi ation, the main peaks observed at 933.6 and 953.6 eV were attributed to the Cu2þ characteristic peaks of Cu 2p3/2 and Cu 2p1/2, respectively. For the SEPS/PP/Cu2(OH)PO4 composite after laser activation, good curve fitting with sum function (the asymmetric Lorentzian–Gaussian) was obtained in Fig. 4(a). Two new peaks were attributed to the Cu0 (element copper) characteristic peaks of Cu 2p3/2 and Cu 2p1/2 at 932.5 and 952.3 eV, respectively. Some of Cu2(OH)PO4 in the samples was reduced to Cu0 during laser irradiation, and Cu0 functioned as the
Fig. 3. Characterizations of the copper layer; (a) Copper layer XRD pattern (red) and reference Cu cubic structure (JCPDS card no. 04–0836) XRD peaks (blue bars); (b) EDX analysis and the corresponding SEM image, scale bar: 70 μm; (c) Cross-sectional SEM image, scale bar: 10 μm; (d) Scotch tape test of the obtained copper layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 4
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Fig. 4. (a) XPS patterns of Cu2(OH)PO4 and XPS curve fitting result (XPSPEAK v4.1) of SEPS/PP/Cu2(OH)PO4 composites after laser irradiation; (b) Raman spectra of the SEPS/PP/Cu2(OH)PO4 before (black curve) and after (red curve) laser activation. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
effective catalytic center for initiating electroless copper plating [1,23]. The chemical composition changes of SEPS/PP/Cu2(OH)PO4 before and after laser activation were also confirmed by the Raman spectra. Fig. 4(b) shows that Raman spectra clearly indicated a broad diffusion band at around 820–1750 cm 1 after laser activation. According to the literature, the broad band (1000–2000 cm 1) is designated to amor phous carbon [35]. Thus, Raman spectra revealed the carbonization on the SEPS/PP/Cu2(OH)PO4 composite surface. The generated amorphous carbon from carbonization on SEPS/PP/Cu2(OH)PO4 by laser irradia tion acted as a reducing agent, thereby providing an environment to reduce Cu2(OH)PO4 to Cu0 at high temperature. As described above, Cu0 and amorphous carbon are formed after laser activation. As we previously reported, the newly generated Cu0 on the composite surface is key to initiating selective metallization [1,23]. Cu0 on the surface acted as a catalytic site for copper deposition in electroless copper plating. The copper particles grew around the cata lytic sites, forming a thin copper layer on the SEPS/PP/Cu2(OH)PO4 composite.
photodetector is only the current. Herein, the sensing element fabricated on the SEPS/PP/Cu2(OH)PO4 was connected to an electronic circuit with input voltage of 6 V. The photocurrent signals were observed by computer data acquisition. Supplementary data related to this article can be found at https://do i.org/10.1016/j.compscitech.2020.108045. Repeatability and response speed are important parameters for evaluating the performance of a photodetector. To date, obtaining a photodetector with high repeatability and fast synchronous response remains a challenge. Fig. 5(c) shows the photocurrent of the fabricated photodetector during repetitive switching of UV irradiation or on/off switching. We observed no apparent degradation for an average photocurrent of ~200 μA over 100 s, indicating excellent photocurrent stability. Movie S2 demonstrates an application of the UV photodetector as a UV switch to control an LED bulb. Movies S3 and S4 show the application of UV photodetectors under different deformation as a UV switch to control an LED bulb. The supplied voltage was around 27 V. The UV photodetector was placed in a UV generator, which could pro duce ultraviolet light with wavelength and intensity of 365 nm and 1500 mW/cm2, respectively. The LED bulb turns on or off at the same time when the UV generator is turned on or off. We also confirmed the fast response capacity of the fabricated photodetector. In our experi ment, the rise time of the photodetector was defined from the initial current to increase to the peak value of 90%. The time from the peak value of 90% to the peak value of 10% was defined as the decay time of the photodetector. Fig. 5(d) shows that the rise and decay times were less than 1 and 0.25 s, respectively. Fig. 5(e) shows a comparison of the rise and decay times for ZnO-based photodetectors of 365 nm UV [28, 29,36–45]. Table S2 (Supporting Information) shows the specific values of response time. The results demonstrated that the obtained photode tector possessed good stability and fast response capacity, making it promising for large-area photodetector application. Supplementary data related to this article can be found at https://do i.org/10.1016/j.compscitech.2020.108045. In Fig. 5(f), the photocurrent change of the photodetector was measured under UV irradiation of 365 nm at different power intensities from 300 mW/cm2 to 1500 mW/cm2. The photocurrent increased gradually with increasing UV intensity, and this phenomenon may be due to a change in photon intensity from UV light. A high current of 220 μA was recorded at the UV intensity of 1500 mW/cm2, whereas the current was only 75 μA at the UV intensity of 300 mW/cm2. This result demonstrated that the fabricated photodetector could achieve high sensitivity performance for different UV signals. This application is attractive for distinguishing different UV intensities. Therefore, LDS technology combined with the nano-ZnO film is a low-cost, efficient
3.3. UV photodetector The highly sensitive photodetector was simply achieved through spin-coating a layer of nano-ZnO film on the interdigitated electrode. The photoresponse schematic of the nano-ZnO photodetector is exhibi ted in Fig. 5(a). Without UV light irradiation, oxygen in the environment was adsorbed on the defect sites in the nano-ZnO lattice and captured the electrons in nano-ZnO, which led to the formation of negative oxy gen molecules and the decrease in the electron carrier concentration in nano-ZnO. Therefore, the resistance in nano-ZnO was high, and the current in the circuit was low under no UV light irradiation. However, once nano-ZnO was exposed to UV light, the electron-hole pairs pro duced from nano-ZnO transferred to the surface quickly and were captured by the negative oxygen molecules on the surface. The oxygen molecules adsorbed on the surface were desorbed, and the electron carrier concentration in nano-ZnO increased rapidly and enhanced photocurrent and photoconductivity. Therefore, nano-ZnO could be used as a UV sensing material by collecting the current signal under UV irradiation. Fig. 5(b) presents the digital photograph of the as-prepared UV photodetector on the SEPS/PP/Cu2(OH)PO4 composite with good flexibility. The morphology of the nano-ZnO layer on the surface of the SEPS/PP/Cu2(OH)PO4 composite is shown in Fig. S10 (Supporting In formation). Movie S1 (Supporting Information) demonstrates the flexi bility of the UV photodetector in real time. In general, the sensing response signals are recorded by current, resistance, impedance, or capacitance in the sensors. However, the detection signal for the 5
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Fig. 5. (a) Photoresponse schematic of the nano-ZnO photodetector; (b) UV photodetector with good flexibility; (c) Photocurrent response of the UV photodetector upon 365 nm UV irradiation measured for UV-on and UV-off conditions at a UV intensity of 1500 mW/cm2; (d) Photocurrent rise and decay of UV photodetector measured; (e) Comparison of the rise and decay times for ZnO-based photodetectors of 365 nm UV [28,29,36–45]; (f) Change in the photocurrent toward different UV intensities.
method for producing UV photodetectors.
Declaration of competing interest
4. Conclusions
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
In summary, this study fabricated an interdigitated copper electrode on the SEPS/PP/Cu2(OH)PO4 composite with high flexibility via LDS technology. The selective metallization results demonstrated that the SEPS/PP/Cu2(OH)PO4 composite was an excellent LDS material with good flexibility. Furthermore, UV photodetector based on the nano-ZnO film on the interdigitated copper electrode was successfully fabricated through a simple spin-coating method. In the photocurrent measurement, the photodetector showed a different photoresponse capacity as a function of varying UV intensities. The rise and decay times as sensitivity parameters of the photodetector were less than 1 and 0.25 s, respectively, demonstrating that the photodetector exhibited good sensitivity, fast response, excellent repeatability, and stability. Therefore, this low-cost, high-efficiency LDS technology combined with the nano-ZnO film is promising for the largearea fabrication of photodetectors with excellent performance.
CRediT authorship contribution statement Huiyuan Zhang: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Jihai Zhang: Visualization, Investigation. Gehong Su: Formal analysis. Tao Zhou: Project admin istration, Supervision, Funding acquisition, Resources, Validation, Writing - review & editing. Aiming Zhang: Supervision. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51773126, 51473104), Outstanding Youth Foun dation of Sichuan Province (Grant No. 2017JQ0006), the National Key 6
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R&D Program of China (Grant No. 2017YFC1104800), and State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2018-209).
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