Seed/catalyst-free growth of zinc oxide nanostructures on multilayer graphene by thermal evaporation
© Ahmad et al.; licensee Springer. 2014
Received: 26 December 2013
Accepted: 5 February 2014
Published: 18 February 2014
We report the seed/catalyst-free growth of ZnO on multilayer graphene by thermal evaporation of Zn in the presence of O2 gas. The effects of substrate temperatures were studied. The changes of morphologies were very significant where the grown ZnO structures show three different structures, i.e., nanoclusters, nanorods, and thin films at 600°C, 800°C, and 1,000°C, respectively. High-density vertically aligned ZnO nanorods comparable to other methods were obtained. A growth mechanism was proposed based on the obtained results. The ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics.
KeywordsGraphene Thermal evaporation Zinc oxide Nanostructure Hybrid integration
In recent years, strong attentions have been paid in the growth of semiconductor nanostructures on graphene[1–5] for electronic and optoelectronic applications. Nanostructures such as nanowires, nanorods, nanoneedles, nanosheets, and nanowalls can offer additional functionality to graphene for realizing advanced nanoscale applications in photovoltaics, nanogenerators, field emission devices, sensitive biological and chemical sensors, and efficient energy conversion and storage devices[6–8]. This is due to the superb properties of nanostructures such as high aspect ratio, extremely large surface-to-volume ratio, and high porosity[6–10]. Graphene has a great potential for novel electronic devices because of its extraordinary electrical, thermal, and mechanical properties, including carrier mobility exceeding 104 cm2/Vs and a thermal conductivity of 103 W/mK[11–14]. Therefore, with the excellent electrical and thermal characteristics of graphene layers, growing semiconductor nanostructures on graphene layers would enable their novel physical properties to be exploited in diverse sophisticated device applications. Graphene is a 2D hexagonal network of carbon atoms which is formed by making strong triangular σ-bonds of the sp2 hybridized orbitals. This bonding structure is similar to the (111) plane of zinc-blende structure and C plane of a hexagonal crystalline structure. With this regard, the growth of semiconductor nanostructures and thin films on graphene is feasible. Recently, there are several works on the growth and application of graphene/semiconductor nanocrystals that show desirable combinations of these properties not found in the individual components[15–20].
The 1D zinc oxide (ZnO) semiconducting nanostructures are considered to be important multifunctional building blocks for fabricating various nanodevices[21, 22]. Since graphene is an excellent conductor and transparent material, the hybrid structure of ZnO/graphene shall lead to several device applications not only on Si substrate but also on other insulating substrates such as transparent glass and transparent flexible plastic. Owing to the unique electronic and optical properties of ZnO nanostructures, such hybrid structure can be used for sensing devices[23–25], UV photodetector, solar cells, and light-emitting diodes. ZnO nanostructures have been synthesized by various physical and chemical growth techniques. These techniques include thermal evaporation[5, 29], hydrothermal[2, 3] and electrochemical deposition, and metal-organic vapor-phase epitaxy (MOVPE). In this paper, we report the seed/catalyst-free growth of ZnO structures on multilayer (ML) graphene by thermal evaporation. The dependence of substrate temperatures on the properties of grown structures was studied. Based on the obtained results, a growth mechanism was proposed.
Results and discussion
Density, diameter, length, thickness, and average aspect ratio of the grown ZnO structures
Diameter of nanorods/nanoneedles (nm)
Length of nanorods (nm)
Average aspect ratio
6.86 × 109
4 × 109
100 ± 10
1,000 ± 100
8 × 107
90 ± 20
4,000 ± 600
5 × 107
3,500 ± 500
1.2 × 108
The optical characteristics of ZnO were investigated using room temperature (RT) PL spectroscopy. Figure 3b shows the measured PL spectra for the samples grown at substrate temperatures of 600°C, 800°C, and 1,000°C. Here, two distinct peaks were observed. The first peak approximately at 383 nm for sample grown at 600°C and 382 nm for the samples grown at 800°C and 1,000°C were observed in the UV region. As reported, the dominant peaks at the UV region are attributed to the near-band edge emission (NBE) or recombination of free exciton[29, 31]. The peaks in the visible region appear approximately at 534, 561, and 525 nm for the samples grown at 600°C, 800°C, and 1,000°C, respectively. The strong peak in the visible region, i.e., green emission is associated with specific defects such as O vacancies and Zn interstitials and these defects are responsible for the recombination of the green luminescence[31, 32]. The highest peak intensity in UV emission and green emission was observed for the sample grown at 600°C. A small PL blueshift by 1 nm in the UV emission has been observed in the sample at 800°C. This may be due to the shape transitions to the well-faceted hexagonal structure. The intensity of green emission peak seems to decrease with the increase of temperature. It is well reported that the crystallinity of the grown structure by vapor-phase method improves with the increase of temperature. Low structural defects such as O vacancies and Zn interstitials may give sharper and stronger UV emission and weaker green emission. However, measurement of low-temperature PL is required to obtain more accurate and precise information about the crystallinity of the grown ZnO structures.
The effects of substrate temperatures on the morphological and optical properties of the grown ZnO on ML graphene were studied. Substrate temperatures seem to be a dominant parameter in determining the morphologies of ZnO structures since it is able to promote the breaking rates of C-C bonds of graphene. Based on the obtained results, the growth mechanism was proposed and discussed.
NFA thanks Malaysia-Japan International Institute of Technology for the scholarship. This work was funded by Nippon Sheet Glass Corp., Hitachi Foundation, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Malaysia Ministry of Science, Technology and Innovation, and Malaysia Ministry of Education.
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