ZnO-Decorated Carbon Nanotube Hybrids as Fillers Leading to

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ZnO-decorated CNT hybrids as fillers leading to reversible nonlinear I-V behavior of polymer composites for device protection Wenhu Yang, Jian Wang, Suibin Luo, Shuhui Yu, Haitao Huang, Rong Sun, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11492 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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ACS Applied Materials & Interfaces

ZnO-decorated CNT hybrids as fillers leading to reversible nonlinear I-V behavior of polymer composites for device protection Wenhu Yang, a Jian Wang, a,b Suibin Luo, a Shuhui Yu, a* Haitao Huang,c Rong Sun a* and Ching-Ping Wongd a

Guangdong Provincial Key Laboratory of Materials for High Density Electronic Packaging, Shenzhen

Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, 518055, E-mail: [email protected], [email protected] b

Department of Nano Science and Technology Institute, University of Science and Technology of

China, Suzhou, 215123,China. c

Department of Applied Physics and Materials Research Center, Hong Kong Polytechnic University,

Hung Hom, Kowloon, Hong Kong, China d

Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong, China.

KEYWORDS CNT-ZnO hybrids, Polymer composites, Voltage switchable dielectrics, Nonlinear I-V behavior, Device protection

ABSTRACT: Over-voltage protection is becoming increasingly important because of miniaturization and multifunction of electronic devices. Flexible, easily processible materials with nonlinear and reversible I-V behavior are highly desired. In this study, hybrid nanoparticles of ZnO-decorated CNT

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(CNT-ZnO) were synthesized via a sol-gel hydrothermal process employed into epoxy matrix to prepare composites. Microstructure analysis demonstrated that ZnO nanoparticles were well-bonded to the surface of CNT. The CNT-ZnO/epoxy composites exhibited nonlinear I-V behavior under increasingly applied voltage with a nonlinear coefficient of 5.01 (10 wt% filler loading). More importantly, the composites possessed excellent reversibility from dielectric to conductor and vise versa in the recycling of increase and decrease of applied electric field, in contrast to the poor recoverability of pure CNT filled epoxy. The mechanism of the nonlinear I-V behavior and reversibility was investigated and discussed. A simple circuit was fabricated, which well verified the protection function of the CNTZnO/polymer composites.

1. Introduction Low voltage switchable materials with high nonlinear current-voltage (I-V) coefficient are seriously concerned and highly desired to protect the microelectronic system from surge voltage and electrostatic discharge (ESD).1-3 It should be an insulator within an allowable voltage range and become conductive while the applied voltage is over a pre-defined threshold thus carrying large currents. Simultaneously, the current flowing to the component fixed in parallel will be reduced significantly and thus it is protected from over-voltage. Besides, the insulating and conducting states of the material should be repeatedly switched depending on the applied voltage. In the past years, conventional ZnO-based ceramic varistors have been used as surge or overvoltage protectors in various circuits.4 However, low capacitance density, high voltage threshold, poor mechanical properties and difficult processing of ZnObased ceramic varistors limit their applications in microelectronic devices based on printed circuit technology. 5-7 Polymeric composites filled with inorganic fillers have aroused much interest because of the growing demand for exploiting new-generation wearable electronic systems and increasingly miniaturized devices working at low voltages, which require flexibility and compatible processing ability with the art of printed circuit technology.8-12 Thus, polymer-based varistors have become a promising alternative to ACS Paragon Plus Environment

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the conventional inorganic semiconductor-based counterparts, which however, have been rarely investigated. In response to an increase of electric field, polymer composites containing conductive- or semiconductive fillers undergo rapid decrease in resistance, and the composites correspondingly switch from insulating to a conductive state.3, 13-15 However, the non-linear I-V behavior and mechanism of recovery in protecting circuits are yet to be unambiguously demonstrated. Carbon nanotube (CNT) is regarded as an ideal filler for fabricating high-performance polymer-based voltage switchable composites due to its excellent electrical, chemical, and mechanical properties. In our previous investigation on CNT filled epoxy, the I-V characteristics of the composites showed obvious nonlinear behavior.3 However, the recovery property of them is poor, which is undesired in practical applications. Hybrid composite particles composed of two kinds of nanoparticles, for example, ceramic supported metal or graphene supported metal hybrids, provide a strategy to control and refine the properties of polymeric composites effectively.12, 16-17 ZnO is an important wide-gap II-VI semiconductor (3.37 eV) that exhibits surge protective properties.6 In this article, CNT decorated with ZnO hybrid nanoparticles were prepared using a simple sol-gel hydrothermal process. The polymer composite filled with CNTZnO hybrids exhibited an excellent nonlinear behavior in the electric field. More importantly, the nonlinear voltage switchable characteristics of the composites showed good repeatability in the multiple measurement process. The mechanisms of these characteristics were deeply investigated and discussed. A protective circuit was designed and fabricated to verify the function of this CNT-ZnO/polymer composite. The results showed that the composite can effectively reduce the risk of device damage caused by a surge voltage, showing potential application in the increasingly high density circuits.

2. Experimental 2.1 Materials Zinc acetate (Zn(CH3COO)2•2H2O), Diethyleneglycol (DEG), Nitric acid, Sulfuric acid and ethanol were purchased from Aladdin Industrial Corporation. Multiwalled CNT with an external diameter of about 20-40 nm and approximately 10–20μm in length were from Shenzhen Nanotech Port. Epoxy resin 3 ACS Paragon Plus Environment


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(E-51) from the Wuxi Resin Factory was selected as the polymer matrix. 2-ethyl-4-methylimidazole (2E4MZ, supplied by Sinopharm Chemical Reagent Co., Ltd. ) was used as a curing agent. 2.2 Materials preparation Treatment of CNT A certain amount of CNT was introduced into the mixture of nitric acid and sulfuric acid (3:1 in volume) solution; the obtained CNT/acid mixture (1 g CNT/200 ml acid solution) was stirred for 4 h at 65 oC. Then the mixture was treated in an ice-water bath and neutralized with NaOH water solution. The black precipitation was collected by filtration and washed repeatedly with water and ethanol. The collected powder was dried in an oven. Synthesis of CNT-ZnO hybrids In a typical procedure, 2.2g Zn(CH3COO)2•2H2O was dissolved in 500 mL DEG, and then 20 mL deionized water was added to form solution. After that, the solution was magnetically stirred at 160–180 o

C for 5 min under stirring and then exposed in air for 48 h to get ZnO sol. Certain amount of purified

CNT was dispersed into the above ZnO sol and ultrasonicaed (40 kHz) for 30 min. Afterwards, the mixture was transferred into a Teflon-lined stainless steel autoclave. After reacting at 180 oC for 24h, the mixture was cooled to room temperature and the CNT-ZnO hybrid particles were obtained after centrifuging, washing and drying at 100 oC for at least 12 h. Preparation of CNT-ZnO/epoxy composite films In order to prepare the polymer composites, a certain amount of CNT-ZnO hybrids were added into butanone with stirring, followed by addition of epoxy resin. Then, curing agent and accelerant with a certain percentage were added. The mixture slurry was casted onto the Cu substrate using scraping method, and treated at 65 oC for 1 h to remove the solvent. Finally, the samples were cured at 160 oC for 2h. The weight percentage (wt%) of CNT-ZnO hybrids in the composite films ranges from 5% to 10 wt%. Meanwhile, the epoxy and CNT-epoxy composites were also prepared via the same procedure. The thickness of the film was around 20 µm. For measurements, silver paste was painted on top of the film as top electrode. ACS Paragon Plus Environment

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2.3 Characterization Scanning electron microscopy (SEM, FEI Nova NanoSEM450) and transmission electron microscopy (TEM, FEI Tecnai Spirit) were used to analyze the morphology of the CNT-ZnO hybrids and fractured surface of the composites. The crystal phase structures of the CNT and CNT-ZnO hybrids were characterized by X-ray diffraction. Raman microscope (Renishaw, Invia, 514.5 nm) was employed to detect the Raman scattering characteristics of the hybrids. The current-voltage (I–V) characteristics of the composites were measured with a probe system and a semiconductor parameter analyzer (Keithley 4200-SCS).

3. Results and discussion 3.1 Characterization results of CNT-ZnO hybrids Figure 1 (a) shows the surface morphology of the as-prepared CNT-ZnO hybrids. There are many ZnO nanoparticles attached to the CNT surface. Figure 1(b) presents a typical TEM image of one single CNT decorated with ZnO nanoparticles. The ZnO nanoparticles are strongly bonded to the CNT surface and the size of ZnO is in the range of 40-50nm.

Figure 1. Microstructure of ZnO decorated CNT, (a) SEM image and (b) TEM image.


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Figure 2. (a) The XRD patterns and (b) Raman spectra of CNT and CNT-ZnO hybrids. Figure 2(a) shows the XRD spectrum of the CNT and CNT-ZnO hybrids. The diffraction peaks from ZnO correspond to a hexagonal structure (JCPDS No. 36-1451). The broad peaks at 25.96◦ and 31.6◦ are attributed to the characteristic peak of CNT. The above results indicate that the obtained CNT-ZnO composites are composed of well crystallized hexagonal ZnO and CNT. Raman spectra for CNT and CNT-ZnO hybrids are presented in Figure 2(b). The results show that the Raman spectra of the CNT and CNT-ZnO hybrids show first order Raman absorptions for the disordered band (D) at 1352 cm cm-1 and the second band (G, sp2 carbon) at 1577 cm-1 of the hybrid. According to the previous reports18, the disorder band arises from defects in the CNT, such as the finite size of crystalline domains, bending in the nanotube, sp3-hybridized bonds, and some functional groups created by oxidation. The ID/IG of CNTs and CNT-ZnO hybirds are 1.02 and 0.93, respectively. It indicates that the degree of disorder in the walls of CNTs decreases while ZnO nanoparticles were successfully grown on the surface of CNT. Actually, the existence of C-H, C-O, C=O, and even O-C=O bonds are inevitable on the wall of purified CNT. The ZnO particles with oxygen vacancies can be easily anchored on the surface of CNT because of its coordinative affinity. As a result, the defects of CNTs were attenuated. There are no remarkable peaks of ZnO in the Raman spectra probably because of its nanoscale size.


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Figure 3. TGA curves for the CNT and CNT-ZnO hybrids measured at a heating rate of 10 °C/ min in air. The thermal gravimetric analysis (TGA) data of CNT and CNT-ZnO hybrids are presented in Figure 3. The initial weight loss of CNT-ZnO hybrids between 30 and 500 °C is due to the removal of adsorbed water and disintegration of amorphous carbon impurities. A significant weight loss is observed between 500 and 650 °C due to the decomposition of CNT. There is almost no obvious weight loss above 650 °C for CNT and CNT-ZnO hybrids. This reveals that the nanotubes have been completely oxidized. The final weight percentage (80%) represents the quantity of ZnO, while the weight loss between 450 and 650 °C represents the CNT content. So the weight ratio of CNT:ZnO in the CNT-ZnO hybrids is about 20:80. 3.2 Reversible nonlinear I-V characteristics of CNT-ZnO/polymer composites Figure 4 shows the I-V characteristic of various composites. As shown in Figure 4(a), the I-V measurement results of both CNT/epoxy and CNT-ZnO/epoxy composites exhibit Ohmic and nonOhmic behavior at low and high voltages, corresponding to region I and region II in Figure 4(b), respectively. For comparison, the I-V curves of ZnO/epoxy and ZnO/CNT/epoxy three-phase composite prepared by simple mixing of ZnO, CNT and epoxy are presented in Figure 4(c) and (d), respectively. The ZnO/epoxy composite exhibits nearly linear I-V behavior in the applied voltage range of 0~200 V. ACS Paragon Plus Environment

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For the ZnO/CNT/epoxy composite, it shows nonlinearity in the first test and the behavior is not repeatable in the second test.

Figure 4. I-V characteristics of (a) & (b) epoxy composites with various CNT and CNT-ZnO hybrids particles loading, (c) 10wt% ZnO/epoxy composite and (d) 1g ZnO/ 0.5g CNT/10g epoxy three-phase composite. The nonlinear coefficient α will follow the relationship: 19

 （log I 2  log I1 ) / (log V2  log V1 )

(1)

where I1 and I2 are the currents under the voltages of V1 and V2, respectively. The nonlinear coefficients of CNT/epoxy and CNT-ZnO/epoxy filled with varied filler loadings are shown in Figure 5. The α values for all composites are nearly at the same level in region I and no larger than 1.12. In region II, the α values of CNT/epoxy and CNT-ZnO/epoxy composites with 10wt% filler loading are about 5.08 and 5.01, respectively, which are much higher than that of region I. It is also noted that the α of CNT-


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ZnO/epoxy composites increases with the filler loading increasing from 5 wt% to 10 wt%. The above results indicate that both CNT-ZnO/epoxy and CNT/epoxy composites exhibit good nonlinear I-V character.

Figure 5. The nonlinear coefficients of CNT/epoxy and CNT-ZnO/epoxy filled with different filler loading. Figure 6 shows the current-voltage plots of cyclic voltage switching test for CNT-ZnO/epoxy and CNT/epoxy. As shown in Figure 6(a), the I-V curves of CNT-ZnO/epoxy composite (10wt% filler) exhibit good reversibility of the voltage switchable characteristic upto fifty measurements. The slight deviation of the first measurement should be caused by filamentary conductive paths which are formed between closely arranged ZnO nanoparticles. While for CNT/epoxy composite shown in Figure 6(b), the I-V plots show nonlinear during the initial measurement and present almost linear characteristic in the following test, which is similar with the behavior of ZnO/CNT/epoxy three-phase composite in Figure 4 (d), demonstrating that CNT and CNT/ZnO mixture cannot offer usable reliability. Microstructure analysis of CNT-ZnO/epoxy composites declare that the CNT-ZnO hybrids are nearly uniformly dispersed in the epoxy matrix, as shown in the inset of Figure 6(a).


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Figure 6. Current-voltage plots of cyclic voltage switching test for CNT-ZnO/epoxy and CNT/epoxy composites. The current compliance was set at 1 × 10-3A and the CNT-ZnO and CNT loadings are 10 wt%. Inset of (a): the SEM image of CNT-ZnO/epoxy composite. 3.3 Discussion on the mechanism of nonlinear I-V behavior and reversibility In summary, CNT-ZnO/epoxy nanocomposites exhibit stable voltage switching behavior for multiple scans, while the CNT/epoxy nanocomposite appears irreversible at the second scan. The mechanism of nonlinear I-V behavior is different for different types of inhomogeneous materials. Charge trapping,20 field-enhanced tunneling21 and filler-matrix charge transfer22 were used to interpret the nonlinear behavior in previous literatures. Specifically, in some polymer systems, filamentary conduction mechanism may also be responsible for the nonlinear I-V behavior.23 In the case of CNT-ZnO hybridized nanoparticles and epoxy composite, the mechanism of nonlinear I-V behavior can be interpreted by Figure 7. The schematic structure of CNT-ZnO hybridized nanoparticles is presented in Figure 7(a). The ZnO nanoparticles line up on the surface of CNTs. Based on the equipotential model, the electrons can hop from one ZnO to another and form an electric current under external electric field. In sequence, the electrons near the Fermi level of CNT can hop to the conduction band of ZnO and transport to the ZnO surface. Then these electrons form current through migration and hopping. Thus, the conductive paths in epoxy composites are composed of the CNTs-ZnO heterojunctions and ZnO-epoxy-ZnO units. ACS Paragon Plus Environment

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Figure 7. The (a) schematic of CNT-ZnO hybrids and (b) band diagram of CNT-ZnO hybrids The behavior of field emission of CNT-ZnO hybrid particles can be interpreted via a band diagram shown in Figure 7(b). Since the band gap of CNT is much narrower than that of ZnO, the CNT-ZnO can be considered as metal-semiconductor heterojunction, and there is a Schottky barrier at the interface of CNT and ZnO.24 Therefore, the electrons will inject from the Fermi level of CNT into the conduction band of ZnO through hopping and tunneling under an electric field, which can cause nonlinear conduction behavior. In Shang et al’s 25 study on the property of electric field-dependent conductivity of the CNT-introduced ZnO ceramics, nonlinear characteristics were also observed and the CNT-ZnO heterojunction was the main reason for nonlinearity through macroscopic quantum tunneling effect. The electronic hopping motion in the ZnO-epoxy-ZnO unit is similar to that in metal-insulator-metal cell composed of double-Schottky barrier structure. In our previous investigation on the doubleSchottky barrier structure, the hopping and tunneling current are derived from the electrons movement .3 Electrons with different energies are distributed in the conduction band of CNTs. The electrons at low energy level can pass through the barrier by the quantum tunneling effect, which forms a tunneling current. And the electrons with high energy can cross the barrier by hopping effect, which forms a hopping current. The tunneling current jt is defined as:


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jt   (V   V 3 )

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(2)

The hopping current jh can be expressed by jh =

V eV exp( ) R0 k0T

(3)

where both β and γ are constants. Combining the tunneling and hopping effects, the total current follow the relationship 3 j  jt  jh  V   V 3 

1 eV V exp( ) R0 k0T

(4)

Thus, the charge density will present a linear change at low applied voltages. However, the current varies rapidly under high voltages. Therefore, electronic tunneling and hopping occurring at double Schottky barriers of ZnO particles and the CNT-ZnO heterojunction are both responsible for the nonlinear I-V behavior of the composite. Moreover, possessing good repeatability of nonlinear characteristics is critical for this material to be applied in overvoltage protection.26-28 Figure 8 displays an illustration of various samples under continuous higher electric field sweep. In CNTs/epoxy composite, as presented in Figure 8(a), the filamentary conductive paths caused by the Joule heat will form between CNTs after incessantly high electric field sweeping.25, 29-33 As a result, the conductivity will increase significantly. In sequence, the CNT-epoxy-CNT unit becomes metal-conductor-metal unit and the double Schottky barriers gradually disappear and there is no obvious nonlinear effect under incessantly voltage sweeping. Figure 8(b) shows that some of the ZnO nanoparticles are bridged by a thin layer of epoxy in the CNT-ZnO/epoxy composites. Under a high enough electric field, filamentary conductive paths are formed between closely arranged ZnO nanoparticles due to Joule heating of the thin layer of epoxy, which results in the irreversible change of the I-V curves between the first cycle and the following ones (Figure 6a). After the first cycle, the above electron transport path becomes CNT-ZnO-ZnO-CNT, which is relatively stable for the following cycles.

As a result, the CNT-ZnO/epoxy composite possesses good

reversibility. ACS Paragon Plus Environment

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Figure 8. Illustration of the structure changes of the composites under higher electric field sweep: (a) CNTs/epoxy composites; (b) CNT-ZnO/epoxy composites. 3.4. Application of CNT-ZnO/epoxy composite in an ESD protection circuit A device was fabricated to verify the ESD protection of CNT-ZnO/epoxy composite with 10wt% loading, as illustrated in Figure 9(a). Figure 9(b) shows the equivalent circuit. R is the resistance of the CNT-ZnO/epoxy composite, R1 is the load resistance and R2 the resistance of the protected device. The value of R dramatically decreases with the increase of applied voltage as shown in Figure 9(c), indicating good protective effect of the composite for circuit or device. Figure 9(d) shows the relationship between output and input voltages of circuit with varied loading resistance. When there is no CNT-ZnO/epoxy layer, the voltage imposed on the device (Vo) increases linearly with the input voltage Vi. When there is the CNT-ZnO/epoxy VSD layer, only at low voltages the Vo shows linear increase with Vi (

ZnO-Decorated Carbon Nanotube Hybrids as Fillers Leading to

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