Growth of Single-sided ZnO nanocombs/ML graphene Heterostructures

1 Introduction

Zero band gap graphene and its remarkable electronic and optical properties have fostered the fabrication of plenty of 2D–1D integrated semiconductor hybrid nanostructures (Biroju et al., 2014). In such a kind of graphene-based hybrid structure, graphene acts as the 2D substrate onto which several 1D semiconducting nanomaterials can be grown and superior properties may be expected. Graphene-based inorganic hybrid nanostructures offer additional functionality for realizing advanced nanoscale electronics and optoelectronics applications in photovoltaics, nanogenerators and field emission devices (Choi et al., 2010; Hwang et al., 2010). Owing to their unique electronic and optical properties, one dimensional ZnO nanostructures, with a wide band gap of 3.37 eV are considered to be important multifunctional building blocks for fabricating various nano-devices (Johnson et al., 2001; Arnold et al., 2003; Wan et al., 2004). It is significant material for ultraviolet and blue-light emitting devices (Nickel and Terukov, 2005). Recent studies have showed variety of ZnO nanostructures, such as nanorods, nanowires, nanotubes, nanoneedles, nanotetrapods, nanorings, nanocombs and so forth (Dai et al., 2003; Wang, 2004, 2007; Shen et al., 2006). Therefore new devices such as nano-generators, nano-lasers, and gas sensors could be designed from different ZnO nano-structures (Wang et al., 2006; Zhou et al., 2007; Liao et al., 2008; Qin et al., 2008a, 2008b). Due to its high surface area, unique mechanical and optical properties of ZnO nanocombs have been utilized in many applications such as biosensors, bioreactors, nano-cantilevers, ultraviolet laser arrays, and ultraviolet polarizers (Wang et al., 2003, 2006; Yan et al., 2003; Xu et al., 2006; Qin et al., 2008, 2008b). Thus, the ZnO nanocombs/2D graphene heterojunction structure would be highly desirable for next generation transparent, flexible electrical and optical devices, including flexible photovoltaics, displays, and light emitters.

So far, all ZnO nanocombs have been fabricated by using chemical vapor deposition (CVD) (Wang et al., 2003; Yan et al., 2003; Leung et al., 2004; Liu et al., 2004; Park et al., 2004; Xu et al., 2004; Liao et al., 2005; Pan et al., 2005; Huang et al., 2006; Lao et al., 2006; Lim et al., 2006; Shen et al., 2006; Xu et al., 2006; Zhang et al., 2006; Li et al., 2007; Song et al., 2007; Wang, 2007; Umar et al., 2008; Yang et al., 2008; Zha et al., 2008; Zhang et al., 2008; Kim et al., 2009; Lin et al., 2010; Chang et al., 2011; Choi et al., 2011; Kumar et al., 2011). Mostly used precursors in this process are Zn powders (Yan et al., 2003; Huang et al., 2006; Shen et al., 2006; Zhang et al., 2006; Li et al., 2007; Song et al., 2007; Umar et al., 2008), Zn foils (Zha et al., 2008), ZnO (Wang et al., 2003; Pan et al., 2005), and a mixture of ZnO and graphite powders (Leung et al., 2004; Park et al., 2004; Xu et al., 2004, 2006; Lao et al., 2006; Lim et al., 2006; Wang et al., 2006). Many studies reported on the synthesis of heterostructures, nanorods (Chang et al., 2011; Choi et al., 2011), nanowires (Kim et al., 2009; Lin et al., 2010; Kumar et al., 2011) ZnO/G, but until submitting this study no fabricated combs like nanostructures over graphitic substrate were published. This is may be due to low concentration of growth precursors inside the CVD chamber which is related to high temperature and long growth deposition time. These two reasons may etch SLG, FLG and MLG before the synthesis of ZnO nanostructures process starts. To avoid etching graphitic substrate surface and provide more growth precursors, substrate location and furnace temperature inside CVD chamber were crucial (Kahng et al., 2012). In this article we focus on the synthesis of single-side teethed ZnO nanocombs on multi-layer graphene (MLG), 10 layers, under low deposition temperature and without need of any catalyst. This is the first time ever to produce ZnO nanocombs over ML graphene. The formation of the nanocombs is investigated by analyzing the crystal morphology and optical properties using (SEM, scanning electron microscope), (EDX, energy dispersive X-ray spectroscopy), (XRD, X-ray diffraction), (RS, Raman spectrometer) and (PL, photoluminescence spectrometer).

2 Experimental setup

2.1

2.1 Wafer preparation

In this study, (MLG), 1–10 monolayers grown on Ni/SiO₂/Si disk with100 mm in diameter (MTI Co., item No. CVD GON-100D-US) were utilized as a substrate for the synthesis of ZnO nanocombs. The disk was cut into small pieces (2 cm × 1 cm) with characteristics shown in Fig. 1(a) and (b). Substrates were ultrasonically cleaned in successive baths of acetone and methanol for 5 min each, and then rinsed in distilled water and dried with gentle air blow.

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Figure 1 A schematic diagram of experiment setup for ZnO nanocombs synthesis process. (a) An inset drawing of graphene wafer utilized for ZnO nanocombs growth.

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3 Results and discussion

Two graphitic substrates A and B with 2 cm apart from each other were subject to gradual elevating temperature up to (600 ± 15 °C) and then kept for 20 min inside the horizontal furnace as illustrated in Fig. 1. The distribution of temperature degrees over two substrates surfaces, TA = 600 °C and TB, was measured during deposition time. Fig. 2 shows FESEM images of sample A surface at different magnifications. In Fig. 2(a) fractured thin film over the sample A surface can be seen. Elemental analysis of both (MLG) existence (spot 1) and disappears (spot 2) areas were taken and their related EDX spectra at spot (1) show elemental compositions of zinc, oxygen, nickel and carbon while EDX spectrum at spot (2) detected strong silicon signal and a weak intensity signal of zinc. The platinum signal in both spectrums comes from sample pre-sputtering to avoid charging effect caused by electric scanning beam and sample surface interactions. In Fig. 2(b) and (c) zinc rich ZnO big clusters are clearly seen on graphitic surface. The EDS spectrum in Fig. 2(a) demonstrates the purity of the sample. Mainly zinc and oxygen are detected with the atomic ratio of Zn to O 59.58:38.47 (1.5:1). Carbon with low percentage may come from graphene substrate or graphite-ZnO mixture due to Ar gas flushing at the beginning of each experiment, which will be verified in Raman spectrum discussion section. Full coverage of ZnO thin film over graphene substrate may explain low EDS percentage of carbon. Also, small platinum (Pt) signals on EDS spectrum can be related to the coating layer of Pt, which was used to reduce the charging occurring during FESEM imaging.

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Figure 2 FESEM images of (a) sample A surface with inset EDX spectra at two spots (1 and 2), (b) and (c) magnified images of ZnO thin layer and thick clusters grown on FLG/graphitic substrate.

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Due to high temperature (>600 °C), graphene layers (GL) staked to nickel start to be unstable and some areas of GL disappear, which agrees with Kahng et al. (2012) study, which determined the maximum temperature of ∼700 °C at which CVD-grown graphene films are almost stable (Kahng et al., 2012). Thermal stability of CVD grown MLG is necessary to establish good electrical contact between graphene and metal electrodes in electronic devices. Thermal annealing of graphene is also used to remove the supporting polymer layer applied during the transfer of CVD-synthesized graphene (Cheng et al., 2011; Nagareddy et al., 2011).

At T = 600 °C, MLG get damaged and cracks formed all over the MLG film, see small arrows in Fig. 2(a). We suggest that Ni catalyst began to form eutectic alloying at T = 460 °C with silicon, which is the middle layer of MLG substrate (Zhang et al., 2006; Beltran-Huarac et al., 2014). This alloy starts to attract more silicon atoms from Si substrate to form a spherical droplet by solid–liquid–solid (SLS) mechanism (Xing et al., 1999). Having different droplet sizes may crack the MLG and due to flowing of Ar gas inside the chamber, some MLG flakes may get removed from the substrate, see spot (2) in Fig. 2(a).

The temperature-distance curve over sample B surface is declined due to the furnace temperature reduction near the furnace right side end as shown in top of Fig. 3. The temperature was measured at a specific distance or locations apart from the hot zone or furnace center. A FESEM image taken at each location a, b, c, d, e and f, is shown in Fig. 3 respectively. Smooth and congested wire like nanostructures were obtained as illustrated in Fig. 3(a) and (b), nearest locations to hot zone. In Fig. 3(b) small combs like structure can be seen at the far end of ZnO nanowire, circled areas. High magnified images in Fig. 3(c) and (d) show nanowires with thick sided duds in one direction only or single-sided teeth structure. It can be noticed that these evolving nanocombs are far in distance from the sample surface. In Fig. 3(e) and (f) long single-sided teeth nanocombs were crowdedly seen. Each nanocomb usually consists of two parts: the comb ribbon with 5–60 μm length and 90–600 nm width, and one row of nanorods (the so-called teeth) growing along one side of the ribbon as in Fig. 3(e) and (f). The diameter of the teeth is about 30–100 nm, while their length is 100–700 nm depending on the position of the teeth. The length of both ribbons and tooth distribution diagrams is shown in Fig. 4(a) and (b) respectively. In general, the length of the teeth decreases gradually from the one end of the nanocomb ribbon to another as expected (Phan et al., 2011). It corresponds to the growth process and direction of the nanocomb, as will be discussed in the growth mechanism. These different ZnO morphologies are mainly occurred due to temperature fluctuations over the surface of same or different substrates and to the combustion of Zn vapor in the presence of oxygen. This work will only focus on the structural characterization and growth mechanism of nanocombs fabricated on sample B.

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Figure 3 FESEM images for sample B surface (a–b) ZnO nanowires at distance 0–5 mm from hot zone (600 °C), (c–d) ZnO nanocombs at 5–10 mm and (e–f) nanocombs at 10–15 mm.

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Figure 4 Distribution length of nanocombs (a) ribbons and (b) tooth.

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3.1

3.1 Growth mechanism

It appears that zinc rich thin film first deposited over MLG substrate and then it works as a buffer layer to grow nanowires and then nanocombs, which was found at the end of ultra-long nanowires going away from the MLG substrate as shown in Fig. 2(b) and (c). This indicates that the buffer layer was deposited before the growth process of nanocombs took place. This is because carbon (C) atoms of graphene directly bind to the Zn atoms of ZnO, thus Zn and O precursors can be widely absorbed onto the C atoms of graphene during the CVD process (Choi et al., 2011), see Fig. 5(a). Zn cluster is easily structured and grows over MLG substrate because Zn atoms produce one orderly atomic layer on last grapheme layer and they form orderly nucleation sites for the following crystal growth (Guo et al., 2011). It has been demonstrated that the Zn buffer layer played an important role in the improvement of crystal quality of ZnO films (Fu et al., 1998). Then, positive zinc starts to attract negative oxygen atoms in a self-catalyzed method and hence defined nucleated sites were developed. Here, ZnO nuclei grow increasingly and form clusters since the interaction of ZnO nuclei–ZnO precursor is stronger than that of the ZnO precursor–graphene (Fig. 5(b)). At these sites, ZnO stem grows along [ $2\bar{1}\bar{1}0$ ] .direction and again Zn clusters form Zn-terminated ZnO (0 0 0 1) polar chemically active surface. The preferred orientation of clusters is determined only by the preferred orientations of the initial nuclei at this stage (Kim et al., 2009). Therefore, unstable Zn-terminated surface could provide a proper site for Zn clustering or local enrichment of growing ZnO combs along [0 0 0 1] direction on the ZnO (0 0 0 1) surface (Qiang et al., 2009). When more and more Zn atoms deposited on (0 0 0 1) surface, they would accumulate to form more new Zn clusters, which would catalyze the growth of the teeth along (0 0 0 1) direction. Growth terminated when there are no more growth precursors are supplied over the MLG substrate.

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Figure 5 Schematic illustration of (a) graphene layer with Zn, O, ZnO growth precursors flowing over its surface (b) ZnO thin layer and 1D crystal growth.

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3.2

3.2 Structural and optical results

Fig. 6 reveals XRD patterns of the as-synthesized nanocombs with related crystal structure and phase purity of the ZnO nanostructures. The intense peaks of ZnO (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) prove the existence of wurtzite structure of ZnO. In addition, one weak zinc peak located at 2θ = 38.34° and strong peak of silicon at 2θ = 69.25°, which conforms with Zn and Si signals in the EDX spectrum for spot (2) shown in Fig. 2(b). Also, no peak is found corresponds to graphene, which is fully covered by grown zinc thin film and ZnO nanocombs.

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Figure 6 XRD patterns of sample B, where ZnO, Zn and Si crystal planes could be detected.

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We also studied the optical properties of resultant ZnO nanocombs by Raman scattering spectroscopy. Fig. 7(a) shows room-temperature spectrum in the wavenumber range of 100–3000 cm⁻¹ for an excitation wavelength of 514 nm for both sample A and B. To discuss the obtained spectrum of sample A, major Raman shift bands should be clarified. The major Raman features of graphene and graphite are the so called G band (∼1580 cm⁻¹) and 2D band (∼2670 cm⁻¹). The G band originates from in-plane vibration of sp² carbon atoms is a doubly degenerate (TO and LO) phonon mode (E_2g symmetry) at the Brillouin zone center (Phan et al., 2008). The 2D band originates from a two phonon double resonance Raman process (Djurisic and Leung, 2006). Here, we refer the band as 2D band because it is the second order overtone of the D band. It can be seen in Fig. 7(a) that the 2D band becomes broader and blue-shifted when the graphene thickness is large as in our multilayer graphene (Ferrari et al., 2006). The I_2D/I_G intensity ratio is about 1.7 and the FWHM of the 2D peak is about 79 cm⁻¹, which proves that the sample is predominantly composed of multilayer. In Fig. 7(a) sample B Raman scattering spectrum of sample (B) shows closed peaks at lower wavenumbers and hence, this spectrum was illustrated separately, as in Fig. 7(b), to identify each peak clearly.

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Figure 7 The corresponding room-temperature Raman spectra of (a) the as-synthesized sample A and grown ZnO nanocombs on sample B, (b) detail spectrum of Raman peaks taken from sample B for ZnO nanocombs and (c) room temperature photoluminescence (PL) of ZnO nanocombs grown over MLG compared to naked MLG.

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As it is known, the Wurtzite structure of ZnO belongs to the space group P6₃mc. At the point Γ in the Brillouin zone, there are phonon modes of Γ = A₁ + 2B₁ + E₁ + 2E₂ [55]. Among these, B₁(H) and B₁(L) modes are Raman inactive, E₂(H) and E₂(L) modes are non-polar active, and the two infrared-active modes of A₁ and E₁ are polar and have longitudinal (LO) and transversal optical (TO) components. The RS spectrum of the nanocombs shows conventional modes peaked at 331, 380, 409, 437 and 585 cm⁻¹, which are associated with processes of E₂(H)–E₂(L), A₁(TO), E₁(TO), E₂(H), and E₁(LO), respectively. There are no additional modes related to impurities. The results prove that the resultant ZnO nanocombs have good quality in the wurtzite structure.

Fig. 7(c) shows the measured room temperature PL spectra for the as-synthesized ZnO nanocombs over MLG substrate and MLG only samples. Here, two distinct peaks were observed. The first peak approximately at 353 nm for ZnO nanocombs was observed in the UV region and no peak is related to MLG. As reported, the dominant peaks at UV region are attributed to the near band edge emission (NBE) or recombination of free exciton (Rusli et al., 2012; Mahmood et al., 2013). The other ZnO nanocombs peaks in the visible region appear approximately at 416, 436, and 480 nm. The strong peak in the visible region, i.e., violet, blue and green emissions is associated with specific defects such as O vacancies and Zn interstitials and these defects are responsible for the recombination of the blue-green luminescence (Huang et al., 2001). This may be due to the shape transitions to the well-faceted hexagonal structure (Rusli et al., 2012). Low structural defects such as O vacancies and Zn interstitials may give sharper and stronger UV emission and weaker blue-green emissions.

4 Conclusion

We synthesized successfully ZnO single-sided nanocombs on multi-layers graphene using free catalyst CVD at 600 °C under atmospheric pressure. The obtained ZnO nanocombs are pure wurtzite polycrystalline structure. We suggest that the growth process of ZnO nanocombs is self-catalysis through the solid–vapor mechanism, starting from the (0 0 0 1)-Zn polar surface. Zinc buffer layer played an important role to grow ZnO nanocombs and to link these nanostructures to the graphene substrate. Additional assessments based on RS spectroscopy and PL revealed that the ZnO nanocombs had excellent crystal quality with interstitial defects.