One-step synthesis of ZnO decorated CNT buckypaper composites and their optical and electrical properties

Materials Science and Engineering B 195 (2015) 38–44

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One-step synthesis of ZnO decorated CNT buckypaper composites and their optical and electrical properties J. Rodrigues a,∗ , D. Mata a , A. Pimentel b , D. Nunes b , R. Martins b , E. Fortunato b , A.J. Neves a , T. Monteiro a , F.M. Costa a a

Departamento de Física/I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa e CEMOP/UNINOVA, 2829-516 Caparica, Portugal b

a r t i c l e

i n f o

Article history: Received 16 October 2014 Received in revised form 10 January 2015 Accepted 28 January 2015 Available online 7 February 2015 Keywords: ZnO CNT Buckypaper Photoluminescence Electrical properties

a b s t r a c t ZnO/CNT composites were prepared using ZnO nanoparticles and tetrapods synthesized by the Laser Assisted Flow Deposition method. The co-operative behaviour between these two materials may give rise to the production of advanced functional materials with a wide range of applications in electronics and optoelectronics. Despite some degree of aggregation in the case of the nanoparticles, scanning electron microscopy images evidence that the produced ZnO structures are well dispersed in the CNT buckypapers. Independent of the ZnO morphology the samples resistivity was shown to be of the order of ∼10−1  cm while in the case of the electron mobility, the composite with tetrapods reveals a lower value than the ones obtained for the remaining samples. Well-structured ZnO luminescence was observed mainly in ultraviolet highlighting the high optical quality of the produced structures. The temperature dependence of the luminescence reveals a distinct trend for the composites with ZnO tetrapods and ZnO nanoparticles. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide is a wide bandgap semiconductor (Eg ∼ 3.4 eV) with important technological applications in several fields like optoelectronics, UV sensors, thin film transistors, energy conversion or biomedicine [1–8]. Among other characteristics, the large free exciton binding energy (∼60 meV), the low production cost and good thermal stability make the ZnO material an attractive metal oxide semiconductor. ZnO is a versatile functional material that can be grown in different morphologies (e.g. particles, rods, tetrapods, springs, helixes, belts) with sizes ranging from micro to nano scale [9–11]. Several techniques have been employed to grow this semiconductor like colloidal synthesis [12], chemical vapour deposition [13], molecular beam epitaxy [14], thermal evaporation [9], hydrothermal synthesis [15], among others. Due to its high vapour pressure and the fact that this material does not melt, instead decomposes into its atomic components at the temperature of 1977 ◦ C, at atmospheric pressure, the flux methods have been extensively studied [16]. Laser Assisted Flow Deposition (LAFD) proved to be a very

∗ Corresponding author. Tel.: +351 234 370 356; fax: +351 234 378 197. E-mail address: [email protected] (J. Rodrigues). http://dx.doi.org/10.1016/j.mseb.2015.01.009 0921-5107/© 2015 Elsevier B.V. All rights reserved.

efficient method to grow ZnO samples with high crystalline and optical quality [17–19]. This laser assisted synthesis method comprises the local heating of the ZnO precursor by a focused high power laser, and subsequent thermal decomposition of ZnO at its melting temperature. The generated gases are transferred to the low temperature regions, after the reaction of the zinc with the oxygen to form ZnO products. Different ZnO morphologies (microrods (MR), nanoparticles (NP) and tetrapods (TP)) can be obtained in the as-grown samples as a result from different kinetics/thermodynamics local conditions at different regions of the growth chamber [18,20]. ZnO potential for some applications can also be improved by forming composite structures. Therefore, the synergetic combination with other materials has been intensively investigated due to the possibility of the modulation of their properties [21]. The research on the modification of semiconductors with noble metal ions, for instance, as is the case of silver or gold, has gained a significant interest in different areas like photocatalysis, sensing, surface-enhanced Raman or biomedicine, showing very remarkable prospects [21–23]. For instance, one of the most important limitations of the ZnO nanostructures in photocaltalytic or photovoltaic applications is the high recombination rate of photoinduced charge carriers. One efficient method to overcome this problem is to deposit noble metals, as silver, on the surface of the semiconductor

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[24]. The photogenerated electrons are then transferred from the conduction band to the metal particle while the holes remain in the semiconductor surface, improving the charge separation. Carbon nanotubes (CNTs) show unique chemical, electrical and mechanical properties that have been explored in a wide range of technological applications at nanoscale [25]. As such, CNTs are auspicious materials for the development of functional nanocomposites [26]. CNTs can preserve their morphology and structure even when mixture with a high amount of other nanostructure due to their great mechanical properties. In this sense, different semiconductor nanostructures have been used to produce CNT composites by several methods [27]. CNT buckypaper is a self-supported membrane with entangled CNTs forming a flexible structure with chemical–physical stability and similar properties to those seen in nanotubes alone [28], offering the advantage of easy application as flexible electronic devices [29]. Constituting a functional component in various applications buckypapers can be used as permeable membranes, capacitors, electrodes for fuel cells, reinforcement in composite materials, among others [30–34]. This approach could be used as an effective method to create composites of CNTs with metal oxide nanoparticles with suitable properties to give rise new materials with tailoring properties [29,35]. ZnO appears as a good candidate to be incorporated in CNT buckypapers since this material enlarges the photon absorption region, increasing the photoconversion efficiency of the generated photocurrent [29]. Earlier works already proved that the UV-induced photoconductive behaviour and the photocurrent generation of ZnO/CNT structures revealed functional interaction between both phases giving a quantum efficiency of 1% in unbiased photoconductivity measurements [36]. The ZnO/CNT heterojunctions can have a charge transfer efficiency of up to 90%. Additionally, the ZnO intrinsic recombination of photoinduced electron–hole pairs is reduced, enhancing charge separation and transport properties [37]. Recently, our group envisaged fundamental studies on the synthesis and analysis of the physical properties of ZnO/CNT composites involving vertical aligned CNTs (VACNTs) decorated with ZnO nanostructures grown by LAFD [19]. As an extension of the preliminary work we now focus on the production of ZnO/CNT buckypapers composites to explore their fundamental properties for optical and electrical applications. For some applications, as it is the case of dye sensitized solar cells, during the device construction the introduction of a liquid electrolyte destroys the CNTs alignment compromising the integrity of the composite samples [38]. In this sense, using disordered CNTs in the form of buckypapers constitutes an advantage relative to the VACNTs. The combining semiconductor nature of the ZnO nanostructures with the excellent electric properties of CNTs in a flexible sample shows a great potential for application as an electrode for this type of solar cells. In this work, the morphological, structural, optical and electrical properties of the synthesized ZnO/CNT composites are analysed and discussed. Since in composites the particles size and morphology constitute important parameters for the resulting material properties, the effect of ZnO morphologies (TP and NP) on CNT buckypaper composites properties were studied.

2. Experimental The LAFD was performed on a modified laser floating zone (LFZ) growth chamber which comprises a 200 W CO2 laser (Spectron) coupled to a reflective optical set-up producing a circular crownshaped laser beam [39]. The laser beam was focused on the tip of the extruded cylindrical rods, previously prepared by mixing the ZnO powders (AnalaR, 99.7%) with polyvinyl alcohol (PVA,

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Fig. 1. Macroscopic and microscopic images of the produced buckypapers.

0.1 g ml−1 , Merck). The produced ZnO structures are collected in a sample holder attached to the upper spindle of the LFZ system, above the feed rod as previously described [17–20,40]. Functionalized multi-walled CNTs (NC3101), provided by Nanocyl supplier, were used to prepare the ZnO/CNT composites. The CNTs have an average length and diameter sizes of 1.5 ␮m and 9.5 nm, respectively, with a purity level of 95 wt.% and a 4 wt.% content of covalently bonded carboxylic groups ( COOH). ZnO/CNTs composites with a weight ratio of 4:1 were prepared in isopropyl alcohol (IPA, ≥99.8%, Sigma–Aldrich). The preparation procedure comprises essentially five steps: (1) CNT suspensions of 0.1 g l−1 start to be processed by high-speed shearing for 15 min (IKA T25Ultra-Turrax, working at 20,500 rpm) with a shearing force of 96 Pa to eliminate big CNTs agglomerates; (2) afterwards, the CNT suspension was sonicated (Selecta, working at 60 kHz, 200 W) for 60 min to yield mixtures of individualized CNTs and small sized agglomerates (<3 ␮m) [41]. At the same time, both the suspensions of ZnO TP and nanoparticles NP with a fixed concentration of 1 g l−1 each were sonicated in the same conditions of those of the CNTs suspension to de-agglomerate possible ZnO clusters. (3) Next, the suspensions were mixed together and sonicated during 15 min to promote interactions of individual ZnO particles with CNTs. (4) The ZnO/CNT suspension was then dropped into a cylindrical mould of 10 mm of diameter placed onto a 0.22 ␮m pore size filter (hydrophobic PTEF, Millipore) to produce the buckypapers of ZnO and CNTs by vacuum filtration [42]. This was accomplished by coupling a rotary vacuum pump to a filter-Büchner funnel-Kitasato flask setting. For each CNT membrane of 10 mm of diameter about 12 ml of suspension was used. (5) Finally, the membranes were dried in an oven at 80 ◦ C for 15 min. The final samples can be seen in Fig. 1. The composites thickness is around 100 ␮m for all the samples. The crystallinity of the ZnO/CNT composites was determined by X-ray diffraction (XRD) analysis using a PANalytical’s X’Pert PRO MRD X-ray diffractometer, with a monochromatic CuK␣ radiation ˚ XRD measurements were carried source (wavelength 1.540598 A). out from 20◦ to 60◦ (2), with a scanning step size of 0.016◦ .

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Fig. 2. SEM micrographs of LAFD produced ZnO crystals: (a) TP and (b) NP. A high amplification image of a tetrapod can be seen in the inset of (a).

The samples morphology and microstructure were characterized by scanning electron microscopy (SEM), using a Carl Zeiss AURIGA CrossBeam Workstation instrument equipped with an Oxford X-ray Energy Dispersive Spectrometer. Steady state photoluminescence (PL) was generated using the 325 nm light from a cw He–Cd laser and an excitation power density less than 0.6 W m−2 . The used wavelength allows exciting ZnO with above band gap photon energy. The samples were mounted in a cold finger of a closed-cycle helium cryostat and the sample temperature was controlled in a range from 14 K to room temperature (RT). The luminescence spectra were acquired using a dispersive system SPEX 1704 monochromator (1 m, 1200 grooves mm−1 ) fitted with a cooled Hamamatsu R928 photomultiplier tube. The electrical resistivity () and Hall mobility (H ) were determined from Hall effect measurements in the Van der Pauw geometry using silver contacts, with a Biorad model HL5500 at a constant magnetic field of 0.5 T. 3. Results and discussion Fig. 2 shows the SEM micrographs of the ZnO TP and NP grown by LAFD method. The TP exhibit four branches with a needle like shape. The thickness decreases from the central region to the tip of the branch, with dimensions between ∼500 nm and 2 ␮m in length. NP show a spherical-like morphology with diameters in the order of ∼100–200 nm, revealing some aggregation. Both structures (TP and NP) present good crystallinity, as evidenced by the XRD difratograms of the composite samples (Fig. 3). The XRD pattern of the sample containing only CNTs revealed the presence of one diffraction maxima at 25.8◦ corresponding to the (0 0 2) reflection and revealing an interlayer spacing of 0.34 nm, which is in good agreement with the literature [43]. The composites of CNT with ZnO TP and NP present similar diffraction patterns with maxima

Fig. 3. XRD diffractograms of the prepared samples.

that can be fully indexed to the hexagonal wurtzite ZnO structure with lattice constant of a = 0.33 nm and c = 0.52 nm in accordance with Ref. [44]. The micrographs of the three different types of composites prepared in this study are shown in Fig. 4: (a, b) buckypapers containing only COOH functionalized CNTs; a mixture of COOH functionalized CNTs with (c, d) ZnO TP and (e, f) ZnO NP. In the case of the samples prepared with TP a good dispersion in the composite was observed. It is worth noting that the TP maintained their shape during the preparation of the buckypaper, highlighting the mechanical properties of tetrapods produced by LAFD. Notwithstanding the aggressiveness of the composites production process, the excellent mechanical strength of tetrapods due to predominantly ionic bonds in ZnO enables to preserve the shape intact. On the other hand, NP tend to remain agglomerated during the solvent extraction step of the composite preparation process. Despite some aggregation observed in NP, the EDS analysis attested the homogeneous distribution of ZnO within the composites network, as shown in Fig. 5. The 14 K PL spectra for the ZnO TP and NP and their composites with CNTs are shown in Fig. 6. No luminescence is observed in the case of the CNTs alone, only when ZnO is incorporated. Either as nanostructures or embedded in the CNTs composites the ZnO luminescence is dominated by a well-structured near band edge (NBE) recombination and almost no visible emission was detected, attesting the high optical quality of the LAFD grow material. In the ZnO TP, NP and ZnO TP, NP/CNT composites the NBE emission is constituted by three main lines, one located at ∼3.36 eV due to the overlap of several donor-bound excitons (D0 X) transitions and the 3.31 eV line and its LO replica (at ∼3.24 eV). The inset depicts an enlargement of the high energy region of the PL spectra. In the case of the TP, after the incorporation in the CNT composite it was possible to de-convolute the high energy line into two components. The shoulder at ∼3.37 eV is likely due to the free exciton (FX) recombination. Noteworthy, in low dimensional structures, surface excitons (SX) have been observed and reported in the same region [45,46], so the presence of these luminescence centres should also be taken into account. The mentioned shoulder is also present in the TP samples without CNTs but with a lower definition, as can be better seen in Fig. 7, where higher resolution spectra are shown. For the TP samples the emission is dominated by the D0 X transitions, while for the NP samples the emission in the 3.31 eV region is the dominant one. The 3.31 eV line was reported before in different ZnO structures and has been associated with several phenomena like surface excitonic contribution or structural defects-related transition [47–50]. Previous studies in ZnO nanorods grown by the same method (to be published) suggest a strong correlation of the 3.31 eV emission line with the presence of surface states as identified by the influence of distinct plasma treatments on the recombination line intensity. As such, the incorporation of TP in the CNTs composite

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Fig. 4. SEM micrographs of (a and b) buckypaper composed only by CNTs, (c and d) ZnO TP/CNT and (e and f) ZnO NP/CNT composites.

Fig. 5. SEM images of (a) CNTs buckypaper, (c) ZnO TP/CNT and (g) ZnO NP/CNT; the respective EDS maps corresponding to C are presented in (b), (d) and (h), to Zn are presented in (e) and (i) and to O in (f) and (j).

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Fig. 6. Comparison between the 14 K PL spectra of the ZnO TP and NP and ZnO/CNT composites. The inset corresponds to an enlargement of the high energy region of the PL spectra.

associated with higher defect concentration on ZnO surface yielded an enhanced 3.31 eV line intensity. This fact constitutes another indication that the emission is affected by surface states. When incorporated in the CNTs composite, the surface of the TP is in direct contact with the functionalized CNTs which could lead to a higher concentration of electron–hole pairs in the surface of the TPs. The enhancement of the PL intensity in ZnO/CNT composites was previously reported in the literature [19,35] due to the generation of a higher concentration of optically active defects in the mentioned spectral region, namely improving surface and excitonic related recombination. The temperature dependent PL spectra of the NBE for the ZnO TP, NP and ZnO TP, NP/CNT composite samples are shown in Figs. 7 and 8, respectively. Additionally, the temperature dependence of the overall NBE integrated intensity in the 14 K – RT interval is also displayed. In the case of the TP a strong thermal quenching

a)

of the NBE is observed when the temperature raises from 14 K to RT following a similar behaviour of bulk ZnO [51]. The overall integrated intensity reveals that only approximately 10% of the emission observed at 14 K remains at RT. A similar trend was observed for the ZnO TP/CNT composite whereas a lower thermal quenching was identified in the 14 K–50 K region (Fig. 7). For the ZnO NP a strong decrease of the NBE emission with increasing temperature is also observed. At RT only ∼20% of the 14 K intensity is present. On the other hand, the ZnO NP/CNT composite reveals a higher PL thermal stability, showing that more than 50% of the 14 K luminescence intensity is still observed at RT (Fig. 8). The identified behaviour highlights the role of the feeding and de-excitation paths on both kinds on ZnO nanostructures and ZnO/CNT composites, revealing that for the ZnO NP/CNT a suppression of additional nonradiative paths results in higher thermal stability of the NBE recombination. Particularly, while in the case of TP nonradiative routes strongly influence the radiative recombination either with or without the CNTs composite, for the smaller ZnO structures embedded in the CNT the data evidence that the nonradiative channels, which compete with the ZnO luminescence, are much suppressed. Since the surface area is in the same order of magnitude for both structures the identified behaviour in the ZnO (NP, TP) and ZnO NP, TP/CNT is likely to be due to the differences in the samples morphology. The data suggest a different ability for the terminated surfaces of the nanostructures to spatial capture defects, which in turn are optically active. Electrical measurements revealed in plane resistivity values in the same order of magnitude for the CNTs and prepared ZnO/CNT composites (Table 1). A metallic behaviour was found for all the samples, as is expected for MWCNTs [42]. These results suggest that the addition of ZnO TP and NP to the CNTs does not change significantly the electrical resistivity, showing that the main contribution for this quantity comes from the CNTs. Highly dense CNT mats of individual tubes with an average length size of 1.5 ␮m can easily touch each other to form a high electrical conductive

b)

c)

Fig. 7. Temperature dependence of the PL emission for (a) ZnO TP and (b) ZnO TP/CNT composites. The spectra were obtained with 325 nm photon excitation and are vertically shifted for easier visualization. (c) Temperature dependence of the integrated intensity for the overall NBE emission.

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Fig. 8. Temperature dependence of the PL emission for (a) ZnO NP and (b) ZnO NP/CNT composites. The spectra were obtained with 325 nm photon excitation and are vertically shifted for clarification. (c) Integrated intensity as a function of temperature for the overall NBE emission.

Table 1 Electrical properties of the prepared composites obtained from Hall measurements.

In plane resistivity ( cm) Electron mobility (cm2 /V s) Carrier concentration (cm−3 )

CNTs

ZnO TP/CNT composites

ZnO NP/CNT composites

7.59 × 10−2 6.30 × 10−2 1.23 × 1021

8.01 × 10−2 2.15 × 10−2 3.67 × 1021

9.13 × 10−2 8.17 × 10−2 8.37 × 1020

3D-network where embedded semiconductive ZnO particles of shorter length sizes (typically between 100 and 500 nm) are not expected to affect negatively the electrical percolation of the CNT mat. The obtained values are consistent with those reported in the literature for CNTs buckypapers [26,42,52] with the dependence of the electrical conductivity mainly due to two factors: the conductivity of the nanotubes themselves and the ability of the charge carriers to tunnel between neighbouring nanotubes, which, in turn, depends on the number of contact points or conductive channels between the CNTs. If the CNTs network increase, more conductive pathways for the charge carriers are available, increasing the film conductivity [30]. Table 1 also evidences that distinct carrier mobilities occur for the CNTs and composites. Particularly, ZnO TP/CNT exhibits slightly low electron mobility. It is well established that the conductivity/resistivity depends on the carrier concentration and scattering time which in turn is dependent on backscattering carrier phenomena at surfaces and grain boundaries. The lowest mobility found for the ZnO TP/CNT indicates smaller carrier collision times in this composite. Since the composites resistivity is similar, a higher carrier concentration is expected for the ZnO TP/CNT sample as was experimentally confirmed and shown in Table 1. The results evidence that tailoring the electrical and optical properties of the ZnO/CNT composites is feasible by an appropriate choice of ZnO nanostructures.

4. Conclusions ZnO/CNT composites were prepared using ZnO nanoparticles and tetrapods synthesized by the Laser Assisted Flow Deposition method. SEM images evidence that the produced ZnO TP are well dispersed in the composite while the NP show some agglomerates. The morphology was kept for both structures before and after the composite preparation. The XRD data reveal high structural quality of the all ZnO structures. The samples resistivity was shown to be of the order of ∼10−1  cm for all set of samples with a carrier mobility dependent on the ZnO morphology. The lower mobility was found to occur for the ZnO TP/CNT composite where a high carrier concentration was measured. The samples luminescence is dominated by well-structured ultraviolet NBE recombination and almost no deep level emission was observed, revealing a high optical quality of the produced structures. In the ultraviolet region the free and donor bound exciton luminescence was identified as well as the 3.31 eV line. A ∼90% of the luminescence was extinguished between 14 K and the RT in the case of the ZnO TP/CNT composite while an higher thermal stability (∼50%) was found for the ZnO NP/CNT composite. In both cases embedding the ZnO NP and TP in the CNTs yields to an enhanced PL intensity with a smaller drop of the luminescence intensity in the composites compared with those observed in ZnO NP and ZnO TP. The obtained results suggest that a good combination of the ZnO optical and CNT electrical properties

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can be achieved by an appropriate choice and dispersion of the ZnO nanostructures which is of potential importance for technological applications. Acknowledgements The authors acknowledge the Portuguese Science and Technology Foundation (FCT) PTDC/CTM-NAN/2156/2012, PTDC/CTMNAN/117284/2010, RECI/FIS-NAN/0183/2012 (FCOMP-01-0124FEDER-027494), PEst-C/CTM/LA 25/2013-14, PEst-C/CTM/LA0011/ 2013, EXCL/CTM-NAN/0201/2012, PTDC/CTM/103465/2008, PTDC/ CTM-POL/1484/2012; EC: INVISIBLE (FP7 ERC AdG n◦ 228144), ORAMA CP-IP 246334-2, and APPLE (FP7-NMP-2010-SME/ 262782-2). J. Rodrigues thanks FCT for her PhD grant, SFRH/BD/ 76300/2011. D. Mata thanks CENTRO-07-ST24-FEDER-002030. A. Pimentel thanks FCT for BPD/76992/2011. References [1] E. Fortunato, P. Barquinha, A. Pimentel, A. Gonc¸alves, A. Marques, L. Pereira, R. Martins, Thin Solid Films 487 (2005) 205. ´ W.K. Chan, J. Appl. Phys. 103 (2008) [2] Y.F. Hsu, Y.Y. Xi, C.T. Yip, A.B. Djuriˇsic, 083114. [3] M. Boucharef, C. Di Bin, M.S. Boumaza, M. Colas, H.J. Snaith, B. Ratier, J. Bouclé, Nanotechnology 21 (2010) 205203. [4] L. Nie, L. Gao, X. Yan, T. Wang, Nanotechnology 18 (2007) 015101. [5] H. Zhang, B. Chen, H. Jiang, C. Wang, H. Wang, X. Wang, Biomaterials 32 (2011) 1906. [6] E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Gonc¸alves, A.J.S. Marques, L.M.N. Pereira, R.F.P. Martins, Adv. Mater. 17 (2005) 590. [7] P. Barquinha, E. Fortunato, A. Gonc¸alves, A. Pimentel, A. Marques, L. Pereira, R. Martins, Superlattices Microstruct. 39 (2006) 319. [8] Y. Liu, S. Li, X. Zhang, H. Liu, J. Qiu, Y. Li, K.L. Yeung, Inorg. Chem. Commun. 48 (2014) 77. [9] Z.L. Wang, Mater. Today 7 (2004) 26. [10] Y. Ding, Z.L. Wang, T. Sun, J. Qiu, Appl. Phys. Lett. 90 (2007) 153510. [11] T. Sun, J. Qiu, C. Liang, J. Phys. Chem. C 112 (2008) 715. [12] A.S. Pereira, M. Peres, M.J. Soares, E. Alves, A. Neves, T. Monteiro, T. Trindade, Nanotechnology 17 (2006) 834. [13] P.-C. Chang, Z. Fan, D. Wang, W.-Y. Tseng, W.-A. Chiou, J. Hong, J.G. Lu, Chem. Mater. 16 (2004) 5133. [14] A. Setiawan, Z. Vashaei, M.W. Cho, T. Yao, H. Kato, M. Sano, K. Miyamoto, I. Yonenaga, H.J. Ko, J. Appl. Phys. 96 (2004) 3763. [15] A. Pimentel, D. Nunes, P. Duarte, J. Rodrigues, F.M. Costa, T. Monteiro, R. Martins, E. Fortunato, J. Phys. Chem. C 118 (2014) 14629. [16] K. Takahashi, A. Yoshikawa, A. Sandhu, Wide Bandgap Semiconductors: Fundamental Properties and Modern Photonic and Electronic Devices, Springer, 2007. [17] J. Rodrigues, M.R.N. Soares, R.G. Carvalho, A.J.S. Fernandes, M.R. Correia, T. Monteiro, F.M. Costa, Thin Solid Films 520 (2012) 4717. [18] J. Rodrigues, M. Peres, M.R.N. Soares, A.J.S. Fernandes, N. Ferreira, M. Ferro, A.J. Neves, T. Monteiro, F.M. Costa, J. Nano Res. 18–19 (2012) 129. [19] J. Rodrigues, D. Mata, A.J.S. Fernandes, M.A. Neto, R.F. Silva, T. Monteiro, F.M. Costa, Acta Mater. 60 (2012) 5143.

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