Arabian Journal of Chemistry (2015) xxx, xxx–xxx
King Saud University
Arabian Journal of Chemistry
www.ksu.edu.sawww.sciencedirect.com
ORIGINAL ARTICLE
Growth of Single-sided ZnO nanocombs/ML
graphene Heterostructures
* Corresponding author. Tel.: +968 24142308; fax: +968 24414228.
E-mail address: [email protected] (M.S. Al-Ruqeishi).
Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
http://dx.doi.org/10.1016/j.arabjc.2015.06.0071878-5352 ª 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Al-Ruqeishi, M.S., Mohiuddin, T. Growth of Single-sided ZnO nanocombs/ML graphene Heterostructures. Arabian JoChemistry (2015), http://dx.doi.org/10.1016/j.arabjc.2015.06.007
Majid S. Al-Ruqeishi *, Tariq Mohiuddin
Department of Physics, College of Science, Sultan Qaboos University, P.O. Box 36, P.C. 123, Al-Khoudh, Oman
Received 3 March 2015; accepted 3 June 2015
KEYWORDS
ZnO nanocombs;
Graphene–semiconductor
hybrid;
Chemical vapor deposition;
Photoluminescence
Abstract We report catalyst-free growth of high-density single-sided ZnO nanocombs for the first
time on a multi-layer graphene (MLG). Structural analysis based on scanning electron microscope
reveals the nanocomb ribbon average diameter and length are about 90–600 nm and 5–60 lm,
respectively, while the diameter and length of the comb tooth are about 30–100 nm and 100–
700 nm respectively. In general, the length of the teeth decreases gradually from one end of the
nanocomb ribbon to another. ZnO crystal growth seems to involve two steps which are the forma-
tion of Zn buffer layer/graphene, which works as growth nucleation sites and long nanowires ends
with nanocombs structure. Raman and PL optical transitions prove the well-faceted hexagonal
structure of ZnO nanocombs as well as the existence of defects such as O vacancies and Zn inter-
stitials. Graphene-based inorganic hybrid nanostructures provide several potential applications in
optoelectronics and nanoscale electronics such as nanogenerators, photovoltaic devices, optical
devices, and photodetectors.ª 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is
an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Zero band gap graphene and its remarkable electronic andoptical 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 hybridstructure, graphene acts as the 2D substrate onto which several
1D semiconducting nanomaterials can be grown and superior
properties may be expected. Graphene-based inorganic hybridnanostructures offer additional functionality for realizingadvanced nanoscale electronics and optoelectronics applica-
tions in photovoltaics, nanogenerators and field emissiondevices (Choi et al., 2010; Hwang et al., 2010). Owing to theirunique electronic and optical properties, one dimensional ZnOnanostructures, with a wide band gap of 3.37 eV are consid-
ered to be important multifunctional building blocks for fabri-cating various nano-devices (Johnson et al., 2001; Arnoldet al., 2003; Wan et al., 2004). It is significant material for
ultraviolet and blue-light emitting devices (Nickel andTerukov, 2005). Recent studies have showed variety of ZnOnanostructures, such as nanorods, nanowires, nanotubes,
nanoneedles, nanotetrapods, nanorings, nanocombs and soforth (Dai et al., 2003; Wang, 2004, 2007; Shen et al., 2006).Therefore new devices such as nano-generators, nano-lasers,
urnal of
2 M.S. Al-Ruqeishi, T. Mohiuddin
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 nanocombshave 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, theZnO nanocombs/2D graphene heterojunction structure would
be highly desirable for next generation transparent, flexibleelectrical 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; Yanet 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; Shenet 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; Kimet al., 2009; Lin et al., 2010; Chang et al., 2011; Choi et al.,2011; Kumar et al., 2011). Mostly used precursors in this pro-
cess are Zn powders (Yan et al., 2003; Huang et al., 2006; Shenet 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 (Wanget al., 2003; Pan et al., 2005), and a mixture of ZnO and gra-
phite powders (Leung et al., 2004; Park et al., 2004; Xuet al., 2004, 2006; Lao et al., 2006; Lim et al., 2006; Wanget 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; Kumaret al., 2011) ZnO/G, but until submitting this study no fabri-
cated combs like nanostructures over graphitic substrate werepublished. This is may be due to low concentration of growthprecursors inside the CVD chamber which is related to high
temperature and long growth deposition time. These two rea-sons may etch SLG, FLG and MLG before the synthesis ofZnO nanostructures process starts. To avoid etching graphiticsubstrate surface and provide more growth precursors, sub-
strate location and furnace temperature inside CVD chamberwere crucial (Kahng et al., 2012). In this article we focus onthe synthesis of single-side teethed ZnO nanocombs on
multi-layer graphene (MLG), 10 layers, under low depositiontemperature and without need of any catalyst. This is the firsttime ever to produce ZnO nanocombs over ML graphene. The
formation of the nanocombs is investigated by analyzing thecrystal morphology and optical properties using (SEM, scan-ning electron microscope), (EDX, energy dispersive X-rayspectroscopy), (XRD, X-ray diffraction), (RS, Raman spec-
trometer) and (PL, photoluminescence spectrometer).
2. Experimental setup
2.1. Wafer preparation
In this study, (MLG), 1–10 monolayers grown on Ni/SiO2/Sidisk with100 mm in diameter (MTI Co., item No. CVDGON-100D-US) were utilized as a substrate for the
synthesis of ZnO nanocombs. The disk was cut intosmall pieces (2 cm · 1 cm) with characteristics shown inFig. 1(a) and (b). Substrates were ultrasonically cleaned in
Please cite this article in press as: Al-Ruqeishi, M.S., Mohiuddin, T. Growth of SinChemistry (2015), http://dx.doi.org/10.1016/j.arabjc.2015.06.007
successive baths of acetone and methanol for 5 min each,and then rinsed in distilled water and dried with gentle airblow.
2.2. ZnO nanocombs growth
Powders from Sigma Aldrich of graphite (99.99%, <45 lm)
and ZnO (99.9%, <5 lm), with mass ratio of (1:1) were mixedand an amount of 2 g was added each time into a combustionboat and used as source material. The source material was
loaded into the middle of a 2.4 cm-inner diameter quartz tube,which was placed at the center of a 45 cm long horizontal tubefurnace as shown in Fig. 1(c). The horizontal quartz tube was
connected to argon (99.999%) gas supply and a flow rate con-trol system at one end while the other end kept opened as it isshown in Fig. 1. Two substrates were placed 15 and 20 cmapart from the ZnO–graphite mixture inside the quartz tube,
near the open end. The Ar gas was then flushed inside thequartz tube to get rid of all other gases and kept at 10 sccm,standard centimeters cubic per minute. Furnace was switched
on and the temperature was raised up to (600 ± 15 �C) at aheating rate of 1.2 �C/s. The temperature is constant at themiddle area of the furnace but gradually decreases near its
edges, as illustrated in the temperature distribution curveshown in Fig. 1(c). Thermocouple was used to determine theexact temperature at each substrate surface during the process.At this temperature, samples were grown at different deposi-
tion time. After the source material was completely evapo-rated, the furnace was turned off and kept to cool down toroom temperature under the same Ar flow rate. The surface
of the samples was covered with a white–gray color thin layer.Then, samples were taken from the furnace for further analysisand characterizations.
2.3. Characterization
The obtained samples were then characterized by field emission
scanning electron microscopy (FESEM, JEOL JSM-7600F).The elemental composition of the samples was determined byenergy dispersive X-ray spectroscopy (EDX). The crystal struc-ture was identified using X-ray diffraction (XRD, X’Pert PRO,
Maximum generator output power: 3 kW with Cu Ka radia-tion). The optical characterization was done by micro-Ramanspectroscopy (Renishaw, RM1000-Invia) using 514 nm excita-
tion and Photoluminescence spectrometer (PerkinElmer LS55Fluorescence Spectrometer, pulsed Xenon with 3.8 eV excitedenergy).
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 horizontalfurnace 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 FESEMimages of sample A surface at different magnifications. InFig. 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 relatedEDX spectra at spot (1) show elemental compositions of zinc,
gle-sided ZnO nanocombs/ML graphene Heterostructures. Arabian Journal of
Graphite + ZnO mixture Ar
5cm 5 cm
Open End
T (ºC) Curve
(a) (b)
(c)
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.
1.8k
200x
45k
Spot (1)
Spot (2)
10 µm 100 nm
(a)
(b) (c)
100 µm
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.
Growth of Single-sided ZnO nanocombs/ML graphene Heterostructures 3
Please cite this article in press as: Al-Ruqeishi, M.S., Mohiuddin, T. Growth of Single-sided ZnO nanocombs/ML graphene Heterostructures. Arabian Journal ofChemistry (2015), http://dx.doi.org/10.1016/j.arabjc.2015.06.007
4 M.S. Al-Ruqeishi, T. Mohiuddin
oxygen, nickel and carbon while EDX spectrum at spot (2)detected strong silicon signal and a weak intensity signal ofzinc. The platinum signal in both spectrums comes from sam-
ple pre-sputtering to avoid charging effect caused by electricscanning beam and sample surface interactions. InFig. 2(b) and (c) zinc rich ZnO big clusters are clearly seen
on graphitic surface. The EDS spectrum in Fig. 2(a) demon-strates the purity of the sample. Mainly zinc and oxygen aredetected with the atomic ratio of Zn to O 59.58:38.47 (1.5:1).
Carbon with low percentage may come from graphene sub-strate or graphite-ZnO mixture due to Ar gas flushing at thebeginning of each experiment, which will be verified inRaman spectrum discussion section. Full coverage of ZnO thin
film over graphene substrate may explain low EDS percentageof carbon. Also, small platinum (Pt) signals on EDS spectrumcan be related to the coating layer of Pt, which was used to
reduce the charging occurring during FESEM imaging.Due to high temperature (>600 �C), graphene layers (GL)
staked to nickel start to be unstable and some areas of GL dis-
appear, which agrees with Kahng et al. (2012) study, whichdetermined the maximum temperature of �700 �C at whichCVD-grown graphene films are almost stable (Kahng et al.,
2012). Thermal stability of CVD grown MLG is necessary toestablish good electrical contact between graphene and metalelectrodes in electronic devices. Thermal annealing of grapheneis also used to remove the supporting polymer layer applied
during the transfer of CVD-synthesized graphene (Chenget 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 suggestthat Ni catalyst began to form eutectic alloying at T = 460 �Cwith silicon, which is the middle layer of MLG substrate
(Zhang et al., 2006; Beltran-Huarac et al., 2014). This alloystarts to attract more silicon atoms from Si substrate to forma spherical droplet by solid–liquid–solid (SLS) mechanism
(Xing et al., 1999). Having different droplet sizes may crackthe MLG and due to flowing of Ar gas inside the chamber,some MLG flakes may get removed from the substrate, seespot (2) in Fig. 2(a).
The temperature-distance curve over sample B surface isdeclined due to the furnace temperature reduction near the fur-nace right side end as shown in top of Fig. 3. The temperature
was measured at a specific distance or locations apart from thehot zone or furnace center. A FESEM image taken at eachlocation a, b, c, d, e and f, is shown in Fig. 3 respectively.
Smooth and congested wire like nanostructures were obtainedas illustrated in Fig. 3(a) and (b), nearest locations to hotzone. In Fig. 3(b) small combs like structure can be seen atthe far end of ZnO nanowire, circled areas. High magnified
images in Fig. 3(c) and (d) show nanowires with thick sidedduds in one direction only or single-sided teeth structure. Itcan be noticed that these evolving nanocombs are far in dis-
tance from the sample surface. In Fig. 3(e) and (f) longsingle-sided teeth nanocombs were crowdedly seen. Eachnanocomb usually consists of two parts: the comb ribbon with
5–60 lm length and 90–600 nm width, and one row of nanor-ods (the so-called teeth) growing along one side of the ribbonas 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 theposition of the teeth. The length of both ribbons and tooth dis-tribution diagrams is shown in Fig. 4(a) and (b) respectively.In general, the length of the teeth decreases gradually from the
Please cite this article in press as: Al-Ruqeishi, M.S., Mohiuddin, T. Growth of SinChemistry (2015), http://dx.doi.org/10.1016/j.arabjc.2015.06.007
one end of the nanocomb ribbon to another as expected (Phanet al., 2011). It corresponds to the growth process and direc-tion of the nanocomb, as will be discussed in the growth mech-
anism. These different ZnO morphologies are mainly occurreddue to temperature fluctuations over the surface of same or dif-ferent substrates and to the combustion of Zn vapor in the
presence of oxygen. This work will only focus on the structuralcharacterization and growth mechanism of nanocombs fabri-cated on sample B.
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 nanowiresand then nanocombs, which was found at the end of ultra-longnanowires going away from the MLG substrate as shown inFig. 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 tothe Zn atoms of ZnO, thus Zn and O precursors can be widely
absorbed onto the C atoms of graphene during the CVD pro-cess (Choi et al., 2011), see Fig. 5(a). Zn cluster is easily struc-tured and grows over MLG substrate because Zn atoms
produce one orderly atomic layer on last grapheme layer andthey form orderly nucleation sites for the following crystalgrowth (Guo et al., 2011). It has been demonstrated that theZn buffer layer played an important role in the improvement
of crystal quality of ZnO films (Fu et al., 1998). Then, positivezinc starts to attract negative oxygen atoms in a self-catalyzedmethod and hence defined nucleated sites were developed.
Here, ZnO nuclei grow increasingly and form clusters sincethe interaction of ZnO nuclei–ZnO precursor is stronger thanthat of the ZnO precursor–graphene (Fig. 5(b)). At these sites,
ZnO stem grows along [2110] .direction and again Zn clustersform Zn-terminated ZnO (0001) polar chemically active sur-
face. The preferred orientation of clusters is determined onlyby 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 enrich-ment of growing ZnO combs along [0001] direction on theZnO (0001) surface (Qiang et al., 2009). When more and more
Zn atoms deposited on (0001) surface, they would accumulateto form more new Zn clusters, which would catalyze thegrowth of the teeth along (0001) direction. Growth terminated
when there are no more growth precursors are supplied overthe MLG substrate.
3.2. Structural and optical results
Fig. 6 reveals XRD patterns of the as-synthesized nanocombswith related crystal structure and phase purity of the ZnOnanostructures. The intense peaks of ZnO (100), (002),
(101), (102), (110), (103) and (112) prove the existence ofwurtzite structure of ZnO. In addition, one weak zinc peaklocated at 2h = 38.34� and strong peak of silicon at
2h = 69.25�, which conforms with Zn and Si signals in theEDX spectrum for spot (2) shown in Fig. 2(b). Also, no peakis found corresponds to graphene, which is fully covered by
grown zinc thin film and ZnO nanocombs.We also studied the optical properties of resultant ZnO
nanocombs by Raman scattering spectroscopy. Fig. 7(a)
gle-sided ZnO nanocombs/ML graphene Heterostructures. Arabian Journal of
(f)
(d)(c)
(b)(a)
(e)
10 µm
1.5K
2K6K
1K
17 K
1.8 K
10 µm 1 µm
1K
1 µm
10 µm 10 µm
10 µm
Tem
pera
ture
(˚C
)
Distance (mm)
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.
Growth of Single-sided ZnO nanocombs/ML graphene Heterostructures 5
shows room-temperature spectrum in the wavenumber range
of 100–3000 cm�1 for an excitation wavelength of 514 nm forboth sample A and B. To discuss the obtained spectrum of
Please cite this article in press as: Al-Ruqeishi, M.S., Mohiuddin, T. Growth of SinChemistry (2015), http://dx.doi.org/10.1016/j.arabjc.2015.06.007
sample A, major Raman shift bands should be clarified. The
major Raman features of graphene and graphite are the socalled G band (�1580 cm�1) and 2D band (�2670 cm�1).
gle-sided ZnO nanocombs/ML graphene Heterostructures. Arabian Journal of
100 200 300 400 500 6000
5
10
15
20
25
Cou
nts
Nanocombs tooth length (nm)0 10 20 30 40 50 60
0
5
10
15
20C
ount
s
Nanocombs ribbon length (um)
(a) (b)
Figure 4 Distribution length of nanocombs (a) ribbons and (b) tooth.
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.
*(100)
(111)•
*(002)*(101)
+ (10
0 ) *(102) *(110)*(103)
*(112)
•(400)
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Inte
nsity
(a.u
.)
*ZnO Nanocombs• Si+ Zn
Figure 6 XRD patterns of sample B, where ZnO, Zn and Si
crystal planes could be detected.
6 M.S. Al-Ruqeishi, T. Mohiuddin
The G band originates from in-plane vibration of sp2 carbonatoms is a doubly degenerate (TO and LO) phonon mode(E2g symmetry) at the Brillouin zone center (Phan et al.,
Please cite this article in press as: Al-Ruqeishi, M.S., Mohiuddin, T. Growth of SinChemistry (2015), http://dx.doi.org/10.1016/j.arabjc.2015.06.007
2008). The 2D band originates from a two phonon double res-
onance Raman process (Djurisic and Leung, 2006). Here, werefer the band as 2D band because it is the second order over-tone of the D band. It can be seen in Fig. 7(a) that the 2D
band becomes broader and blue-shifted when the graphenethickness is large as in our multilayer graphene (Ferrariet al., 2006). The I2D/IG intensity ratio is about 1.7 and theFWHM of the 2D peak is about 79 cm�1, which proves that
the sample is predominantly composed of multilayer. InFig. 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 identifyeach peak clearly.
As it is known, the Wurtzite structure of ZnO belongs to
the space group P63mc. At the point C in the Brillouin zone,there are phonon modes of C = 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 twoinfrared-active modes of A1 and E1 are polar and have longi-tudinal (LO) and transversal optical (TO) components. TheRS spectrum of the nanocombs shows conventional modes
gle-sided ZnO nanocombs/ML graphene Heterostructures. Arabian Journal of
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.
Growth of Single-sided ZnO nanocombs/ML graphene Heterostructures 7
peaked at 331, 380, 409, 437 and 585 cm�1, which areassociated with processes of E2(H)–E2(L), A1(TO), E1(TO),
E2(H), and E1(LO), respectively. There are no additionalmodes related to impurities. The results prove that theresultant 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 substrateand MLG only samples. Here, two distinct peaks were
observed. The first peak approximately at 353 nm for ZnOnanocombs was observed in the UV region and no peak isrelated 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, and480 nm. The strong peak in the visible region, i.e., violet, blueand green emissions is associated with specific defects such as
O vacancies and Zn interstitials and these defects are responsi-ble for the recombination of the blue-green luminescence(Huang et al., 2001). This may be due to the shape transitionsto the well-faceted hexagonal structure (Rusli et al., 2012).
Low structural defects such as O vacancies and Zn interstitialsmay give sharper and stronger UV emission and weaker blue-green emissions.
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4. Conclusion
We synthesized successfully ZnO single-sided nanocombs on
multi-layers graphene using free catalyst CVD at 600 �C underatmospheric pressure. The obtained ZnO nanocombs are purewurtzite polycrystalline structure. We suggest that the growth
process of ZnO nanocombs is self-catalysis through the solid–vapor mechanism, starting from the (0001)-Zn polar surface.Zinc buffer layer played an important role to grow ZnO nano-combs and to link these nanostructures to the graphene sub-
strate. Additional assessments based on RS spectroscopy andPL revealed that the ZnO nanocombs had excellent crystalquality with interstitial defects.
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