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CONFIDENTIAL - AUTHOR SUBMITTED MANUSCRIPT NANO-105896.R2
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Catalyst Free Growth of ZnO Nanowires on Graphene and Graphene Oxide
and Its Enhanced Photoluminescence and Photoresponse
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Ravi K. Biroju1, Nikhil Tilak2, Gone Rajender2, S. Dhara3, P. K. Giri 1, 2*
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Centre for Nanotechnology and 2Department of Physics,
Indian Institute of Technology Guwahati, Guwahati 781039, India
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Surface and Nanoscience Division, Indira Gandhi Centre for Atomic Research,
Kalpakkam 603102, India.
ABSTRACT:
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We demonstrate the graphene mediated catalyst free growth of ZnO nanowires (NWs) on
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chemical vapor deposited (CVD) and chemically processed graphene buffer layers at a relatively
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low growth temperature (580°C) in presence and absence of ZnO seed layers. In case of CVD
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graphene covered with rapid thermal annealed (RTA) ZnO buffer layer, the growth of vertically
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aligned ZnO NWs takes place, while the direct growth on CVD graphene, chemically derived
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graphene (graphene oxide and graphene quantum dots) without ZnO seed layer resulted in
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randomly oriented sparse ZnO NWs. Growth mechanism was studied from high resolution
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transmission electron microscopy (HRTEM) and Raman spectroscopy of the hybrid structure.
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Further, we demonstrate strong UV, visible photoluminescence (PL) and enhanced photocurrent
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(PC) from the CVD graphene−ZnO NWs hybrids as compared to the ZnO NWs grown without
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the graphene buffer layer. The evolution of crystalinity in ZnO NWs grown with ZnO seed layer
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and graphene buffer layer are correlated with the Gaussian line shape of UV and visible PL. This
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is further supported by the strong Raman mode at 438 cm-1 significant for Wurtzite phase of the
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ZnO NWs grown on different graphene substrates. The effect of thickness of ZnO seed layers
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and the role of graphene buffer layers on the aligned growth of ZnO NWs and its enhanced PC
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are investigated systematically. Our results demonstrate the catalyst free growth and superior
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Corresponding Author email: [email protected]; Phone: +91 361 2582703
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performance of graphene−ZnO NW hybrid UV photodetectors as compared to the bare ZnO NW
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based photodetectors.
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KEYWORDS: Graphene−ZnO NW Hybrid; Photoluminescence; Rapid Thermal Annealing;
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Photoconductivity
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1. INTRODUCTION
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In recent years, graphene−semiconductor hybrid nanostructures have drawn enormous attention
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due to their potential applications in photovoltaics [1][2] nanogenerators [3], field emission
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devices [4], and efficient energy conversion and storage devices [5] etc.. However, the catalyst
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free growth of vertically aligned semiconductor NWs directly on different forms of graphene
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buffer layers is a challenging task. In this type of hetero structures, it could be possible to
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integrate the 2D graphene and 1D semiconducting NWs for Schottky junction devices with
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enhanced functionalities for ensuing optoelectronic applications. Among all, the graphene
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mediated growth of vertically aligned ZnO hybrid nanostructures has received much attention[6],
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since ZnO is a wide direct band gap (3.3 eV) semiconductor and has large exciton binding
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energy (60 meV) which can possess fast photo response upon illumination of UV light in the
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graphene based photo detectors (PDs) [7] and gas sensing capability. [8]
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Recently, vertically aligned growth of ZnO NRs/NWs by vapor liquid solid (VLS) and
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vapor solid (VS) mechanisms have been reported on single (SLG) and few layer (FLG) graphene
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in the presence and absence of gold (Au) catalyst by vapor phase growth.[3, 6, 9] Fabrication of
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vertically aligned ZnO NRs/NWs with high aspect ratio and extremely large surface−to−volume
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ratio, specifically on graphene substrates without the aid of metal catalyst is still challenging.
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With the excellent electrical, mechanical and thermal characteristics of graphene layers, growing
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ZnO nanostructures and thin films on graphene layers would enable their novel physical
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properties to be exploited in the diverse range of sophisticated device applications.[10]
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Therefore, several graphene−semiconductor nanocrystal hybrids have been successfully
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synthesized that show elegant combinations of properties not found in the individual
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components.[11] Recently, Q Xu et.al developed a metal−semiconductor−metal PD using
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hydrothermally grown ZnO NWs and observed the existence of surface plasmon resonance
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arising from the underlying graphene layer, which exhibited a UV to visible rejection ratio of ~
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4. [12] To the best of our knowledge, there are limited studies on the graphene mediated growth
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of vertically aligned, high density and high aspect ratio ZnO NWs on graphene buffer layers by
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thermal vapor deposition technique and the role of graphene layer in the enhanced
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photoluminescence (PL) and photoconductivity (PC) are least understood.[13] Further, there are
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very few reports on the growth of ZnO NWs on chemical vapor deposited (CVD)
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chemically derived graphene functional materials, such as graphene oxide (GO) and graphene
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quantum dots (GQDs), and the optical properties of such hybrid nanostructures for the hybrid
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photodetector applications are little explored.[14]
and
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In the present work, we demonstrate the catalyst free growth of aligned ZnO NWs on
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various substrates consisting of CVD graphene (GR), GO and GQDs with/without a thin ZnO
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seed layer on graphene. The structural quality of the as−grown graphene layer as well as the ZnO
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NWs grown on it is characterized by Raman spectroscopy along with the Raman mappings with
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514.5 nm laser excitation. The effect of ZnO seed layer and different graphene buffer layers on
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the structure of ZnO NWs was explored by high resolution transmission microscopy (HRTEM)
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and its optical properties were studied systematically by UV− visible absorption and PL
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spectroscopy. Further, enhanced UV PC was observed in the case of ZnO NWs grown on
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graphene/SiO2 substrate as compared to that grown without the graphene layer.
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2. EXPERIMENTAL
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2.1. Synthesis of CVD Graphene and Chemically Derived Graphene
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Single and few layer graphene samples were synthesized by a thermal CVD method and
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transferred on to the Si/SiO2 and quartz substrate by standard wet chemical method. The
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chemically derived graphene such as GO and GQDs are obtained from simple chemical
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exfoliation techniques. The full details of the experiment and characterization of these samples
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are described in the supporting information (see sections: SI1 to SI4).
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2.2. Deposition of High Quality ZnO Thin Films on Graphene
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High quality ZnO films with three different thicknesses: ~300 nm (code−Z1), ~100 nm
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(code−Z2) and ~10 nm (code−Z3), were deposited by RF magnetron sputtering on various
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graphene substrates for the growth of ZnO NWs. High purity ZnO sputter target (99.999%, Kurt
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J. Lesker, USA) was used as a source for the ZnO grain growth. Initially the chamber was
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evacuated to a base pressure of 6.7x10-6 mbar and during the sputtering it was maintained at
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1x10-2 mbar. The RF power was kept at 100 W. The substrates were heated to 200°C for better
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crystalinity and uniformity of the ZnO grains. Further, these ZnO thin films were subjected to
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rapid thermal annealing (RTA) treatment at 600°C in Ar gas ambient (flow rate of 200 standard
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cubic centimeters (SCCM)) atmosphere for 3 minutes using a commercial RTA system
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(MILA3000, Ulvac, Japan) in order to further improve the crystalline quality as well as for the
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removal of excess oxygen traps on the ZnO thin films on various substrates. Improvements in the
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crystalinity of ZnO grains and their phases were confirmed from XRD and HRTEM analyses
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(discussed later).
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2.3. Growth of ZnO NWs on Graphene Substrates
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Commercial nano sized activated Zn powder (purity ~ 99%, Aldrich) was taken as a source
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material in an alumina boat and placed at the center of a horizontal quartz tube kept inside a
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muffle furnace. The above prepared substrates were placed downstream ~ 5 cm away from the
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source material. Initially the quartz tube was pumped down to a pressure of ~ 10-3 mbar. In order
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to prevent the oxidation of the graphene layer, 300 SCCM of Ar gas was flushed into the
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chamber until it reached the set point with a rate of 18°C /min. When the furnace reached the
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desired temperature, 20 SCCM of O2 gas was introduced and gas pressure inside the chamber
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was maintained at 1.6 mbar for the growth time of 50 min. The source material temperature was
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maintained at ~ 600°C and the substrate temperature at ~ 580°C. After the completion of the
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reaction, the furnace was cooled down to room temperature. After the growth, the RTA is
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performed at 600°C in Ar atmosphere (300 SCCM) for 3 minutes to improve the crystalline
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quality of the as−grown ZnO NWs.
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2.4. Characterization Techniques
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The morphology and crystal structure of the as−grown samples were studied using electron
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microscopy tools such as field emission scanning electron microscope (FESEM, Sigma, Zeiss)
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and high resolution transmission electron microscope (HRTEM, JEM2100 operated at 200kV,
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JEOL). Micro Raman measurements were performed using a high resolution Raman
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spectrometer (inVia, Renishaw), with the excitation source 514.5 nm (Ar+ laser) and
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monochromator using 1800 gr.mm-1lines with a thermoelectric cooled CCD detector in the
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backscattering configuration to examine the crystalline quality and number of layers in the
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graphene. Raman mapping was carried out with10×10 µm2 frame size on the samples at 514.5
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nm laser excitation with a spatial resolution of 100 nm x 600 nm using a Streamline imaging
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facility for covering large area. Additionally, PL measurement was carried out at room
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temperature using 325 nm laser excitation with the same setup using a He−Cd laser source and
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monochromator using 2400 gr.mm-1 grating. Some of the PL measurements were performed
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using a 355 nm diode laser excitation in a commercial fluorimeter (AB2, Thermo Spectronic).
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The UV–vis–NIR absorption spectroscopy measurements were recorded using a commercial
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spectrophotometer (PerkinElmer UV Win Lab, UV-3101PC). PC measurements were performed
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with a micro probe station (ECOPIA EPS−500) connected to a source measure unit (Keithely
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2400, USA) for current−voltage (I-V) characteristics and 300W xenon lamp as a source to excite
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the sample. Note that the excitation wavelength was selected using a monochromator (Oriel
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Instruments, USA). The I−V setup was interfaced with a computer to collect the data using Lab
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Tracer 2.0 software.
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3. RESULTS AND DISCUSSION
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3.1. Raman Spectroscopy of Different Graphene Substrates
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Prior to the growth of ZnO NWs, the qualities of the GR, chemically processed GO and GQDs
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coated on SiO2 substrates were first characterized by Raman spectroscopy. The characteristic
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Raman spectra of as−grown GR, GO and GQDs are shown in Fig S1 (supporting information SI
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5). In addition, Raman mapping was performed on GR sample for the well−known D, G and 2D
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bands for the surface coverage and uniformity of graphene layer. Figure 1(a−c) represents the
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Raman mappings scanned in an area of 10x10 µm2, which shows a full coverage of SLG as
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evident from a sharp and prominent 2D peak at ~2700 cm-1. The graphitic G band at ~1595 cm-1
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signifies the sp2 hybridization of carbon atoms and assigned for the E2g (high) mode of in−plane
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C−C stretching vibration.[15] The ratio of intensities of 2D and G bands I(2D)/I(G) is ~1.00,
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which indicates the presence of SLG and FLG. The high intensity of the defect band D at ~1350
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cm-1 implies the presence of point and line defects in graphene. Some of the defects might have
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been introduced during the wet transfer process. The crystalline quality of different graphene
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materials was estimated from high resolution transmission electron microscopy (HRTEM) to
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support the Raman data. HRTEM images of GR, GO and GQDs are shown in the supporting
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information, Fig. S2. Corresponding SAED patterns are shown in the inset, which clearly show a
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hexagonal lattice pattern. Note that the multiple SAED spots in GO sample (see Fig. S2 (b)) are a
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signature of few layer GO.
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These graphene substrates are named according to the pretreatment conditions, as
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described in the Table 1.The surface morphology of ZnO NWs grown in each case was presented
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in Table 1. As discussed before, the GR, GO and GQDs ultra−thin films were prepared on SiO2
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substrates and ZnO films of thickness ~300nm (code−Z1), ~200nm (code−Z2) and ~10s nm
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(code−Z3) were deposited on various graphene buffer layers followed by RTA in an inert gas
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environment. The systematic study on the individual effects of graphene buffer layer, ZnO seed
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layer and ZnO buffer layers on the growth of ZnO NWs have been elucidated in the following
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sections. Further the growth mechanism, strong UV−visible PL and PC characteristics of various
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ZnO nanostructures on different graphene derivative materials as substrates are discussed below.
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3.2. Effect of ZnO Seed Layer
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The RTA of the ZnO thin film leads to formation of bigger grains of ZnO with improved
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crystallinity and the partial removal of defects in ZnO. These ZnO grains grown on the graphene
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layer play a crucial role in the vertical growth and alignment of the ZnO NWs. Due to similarity
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in hexagonal crystalline structure of graphene and ZnO, it leads to the epitaxial like growth of
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the ZnO NWs. Figure 2 represents the FESEM image of ZnO nanostructures grown on various
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graphene substrates coated with ZnO seed layers, as mentioned in the Table 1. Figure 2(a)
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illustrates the case of GRZ3, while Figs 2(b,c,d) illustrate the case of Z1Z3, GOZ3 and GQD,
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respectively. In the inset of the Fig. 2(a), the vertical growth of ZnO NWs on ultrathin ZnO
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buffer layer (Z3) coated GR substrate is clearly visible. On the other hand, dense bundles of
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randomly oriented ZnO NWs can be seen on GO and GQD substrates. This clearly indicates that
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GR layer below the Z3 film, which has some kind of epitaxial relation with the ZnO seed layer,
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promotes the growth of vertically aligned NWs, while the GO and GQD layers do not promote
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such kind of growth. Interestingly, the ZnO NWs grown on GRZ3 substrate show very strong
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UV PL as compared to the NWs grown on Z1Z3 substrate. Note that the visible PL is prominent
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in all the as−grown ZnO NWs on graphene samples (discussed later).The Z1Z3 and Z2Z3 layers
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show similar crystalline features after RTA treatment, which are consistent with the XRD results.
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Further HRTEM measurements were conducted on the GRZ3NW sample to understand
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the role of ZnO seed layer in the vertical growth of ZnO NWs. It was found that the growth of
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the NWs is initiated from the ZnO seed layer and no NW growth takes place directly on the GR
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layer. This may be due to fact that graphene has very low surface energy and is a very inert
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material to nucleate the ZnO on its surface, assuming a low defect density. However, GR layer
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helps to grow the ZnO NWs vertically on the ZnO seed. It is likely that due to the presence of
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graphene, no planar growth takes place for the ZnO NWs. In the present case, the vertical growth
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of NWs can be attributed to the presence of ZnO seed on graphene layer, based on the VS
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growth mechanism.[13] However, here we did not observe the growth of ZnO NWs with
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hexagonal facets, in contrast to our earlier work [6] where ZnO NWs were grown using gold as a
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catalyst and graphene as a seed layer. Thus, epitaxial growth is less efficient in the present case.
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Note that ZnO NWs growth on Z1Z3 is very dense without any vertical alignment (see Fig.
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2(b)).The possible explanation for this observation is that the thick ZnO layer covered with a thin
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ZnO layer, both have the same surface energy and the density of oxygen atoms is very high on
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these layers due to presence of oxygen interstitials.[6] These excess oxygen in the substrate help
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in the planar growth of the NWs [6] resulting into vertical alignment in the NWs. Interestingly,
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the direct growth of NWs on GR substrate without a ZnO layer (Z3) results in a growth of sparse
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ZnO NWs. This is evidenced from the FESEM image (see Fig. 3(a)). Thus, in a catalyst free
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growth, self catalytic seed layer is essential for the aligned growth of the NWs. Self−catalyzed
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growth of vertical ZnO NWs on various dielectric substrates have been reported earlier. Herein,
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self−catalyzed growth of ZnO NWs on graphene substrate has been demonstrated.
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Figure 3(b) represents the HRTEM image of the ZnO grains with an average diameter ~
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50 nm. Inset shows the lattice image of a spherical ZnO grain showing the polycrystalline lattice
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patterns of ZnO seeds. Further, HRTEM results obtained for the ZnO grains are in close
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agreement with the XRD results. Figure 3(c) shows the ZnO NW grown vertically on the ZnO
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seed and its corresponding SAED pattern showing the single crystal (002) planes. Figure 3(d)
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represents the magnified view of region I in the Fig. 3(c), which clearly depicts the nucleation of
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NW on the ZnO seed and vertical alignment onto (002) planes. The IFFT image in the inset
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(magnified view of region II) shows the lattice fringes for (002) planes with d−spacing 2.6 Å that
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strongly supports the Raman data (discussed later).
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The crystalline quality and the crystalline phases of RTA treated ZnO seed layers are
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assessed from the XRD pattern. Figure 4(a) represents the XRD pattern of Z1Z3 substrate after
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RTA treatment. The XRD pattern shows strong peaks at 2θ = 31.96°, , 34.66° and 36.44°
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corresponding to (100), (002) and (101) planes of ZnO. These grains act as the seeds for the
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growth of ZnO nanostructures during the vapor deposition process. This is evident from the
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TEM image of Fig.3(c) where the ZnO NW is grown directly on a ZnO grain. Thus, the ZnO
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buffer layer plays a crucial role in the growth of ZnO NWs. This can be confirmed by looking at
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the FESEM images of the ZnO NWs formed on the GR (Fig. 3(a)) and GRZ3 (Fig. 2(a))
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samples. GR substrate without a ZnO seed layer shows very sparse growth of ZnO NWs,
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whereas GRZ3 shows aligned growth of ZnO NWs. The diameters of the NWs are in the range ~
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20−30 nm (see Fig. 2(a)), dense coverage and is due to the huge number of overlapping ZnO
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grains.
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3.3. Effect of Various Buffer Layers
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3.3.1. GRZ3: In this case, a 10 nm ZnO seed layer is present over the graphene buffer layer for
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the growth of the ZnO NWs. The NWs grown on GRZ3 are much better aligned than those
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grown on Z1Z3 (Fig.2 (b)), GOZ3 (Fig. 2(c)) and GQDs (Fig. 2(d)). This implies that the
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graphene buffer layer assists the ZnO grains in the vertical alignment of the NWs. Interestingly,
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the lattice mismatch between the hexagonal ZnO crystal and graphene bond centered sites is very
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low.[16] This could lead to the epitaxial growth of ZnO NRs on graphene. Self−catalysed VLS
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growth of vertically aligned GaAs and InAs NWs on graphene substrates has been reported by
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Munshi et al.[16] and Hong et al.[17], respectively and it was suggested to be assisted by the
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strong van der Waals interactions. Our results are consistent with the above reports.
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3.3.2. Z1Z3: In this case, a 10 nm ZnO seed layer is present on the 300 nm ZnO layer, without
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the graphene layer. A dense and randomly oriented ZnO NWs growth takes place on Z1Z3 (see
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Fig. 2(b)). This can be attributed to the large number of overlapping grains on the ZnO thin film.
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The buffer layer Z3 seems to have negligible effect on the NW alignment. Once again, it implies
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that absence of graphene layer leads to random orientation of the ZnO NWs.
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3.3.3. GOZ3 and GQDs: Our results show that the GO and GQDs do not play any significance
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in the growth/alignment of the ZnO NWs as compared to the case of GR buffer layer. We believe
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that there are primarily two factors affecting the growth and orientation of the ZnO NWs: (i)
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non−uniform coverage of the GO layer on the substrate. (ii) High density of defects on the GO
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layer. This was clearly shown by the FESEM image in the supporting information, Fig. S3.
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During the growth, ZnO NWs may nucleate at the defect sites in GO and random growth takes
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place due to the absence of any lattice matching between GO and ZnO. Similar results were also
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observed for the case of GQDs substrate; due to the absence of the seed layer on the GQD,
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random growth takes place.
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3.4. Raman Spectroscopy of ZnO NWs
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Crystalline quality of ZnO NWs was probed by Raman spectroscopy on different graphene
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substrates based on the peak position, relative intensities and full width at half maxima
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(FWHM−∆ω) of the Raman spectra. Figure 4(b) illustrates the comparative Raman spectra of all
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the ZnO NWs samples. The Raman spectrum shows a peak at ~ 438 cm-1,which corresponds to
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the E2g (high) mode of ZnO NW significant for the Wurtzite phase. The position of E2g (high) and
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FWHM values have been calculated from the Gaussian peak fit that were labeled in the Fig. 4(b).
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Note that the FWHM of E2g (high) mode in the GRZ3NW is relatively lower than that of the
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NWs grown on other graphene substrates. This is consistent with the strong UV peak in the PL
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spectrum (discussed later) indicating a better crystallinity of the ZnO NWs grown in the
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GRZ3NW sample. Note that Z1Z3NW shows relatively lower FWHM of E2g (high) mode that of
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the GRZ3NW indicating better crystalinity in Z1Z3NW. This may be due to the contribution of
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thick ZnO buffer layer on the Raman spectrum. Thus, it can be concluded that the ZnO seed
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layer with GR buffer layer promotes better crystalinity. However, the Raman signal of graphene
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is not detected in the GRZ3NW samples, perhaps due to the dense coverage and long length of
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the NWs. To confirm the presence of graphene in these samples, we performed the Raman
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measurements on GRZ3 substrate (see supporting information, Fig. S4). E2g (high) in GRZ3
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reveals the presence of ZnO on the GRZ3 substrate as shown in Fig. S3 (a). Further, The Raman
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signatures of graphene (D, G & 2D bands) with reduced intensity of 2D band and redshift of the
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D & 2D bands (by ~ 10 cm-1) reveal that there is a strong interaction between graphene and ZnO
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grains. The Raman peak located at ~ 331 cm-1 in GOZ3NW corresponds to the 2E2 phonon.[18]
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A weak lower frequency mode at 236 cm-1 may be arising from the structural defects on the
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surface of the ZnO NWs.[18]
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3.5 Growth Mechanism
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Figure 5 (a) shows a schematic of the growth process and the morphology of the ZnO NWs
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grown on GRZ3 and Z1Z3 substrates, which is based on the experimental observations. When
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VS growth of ZnO NWs is performed simultaneously on both substrates, well aligned ZnO NWs
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were formed in the case of GRZ3 substrate, while dense and randomly oriented ZnO NWs were
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formed in the case of Z1Z3 substrate as noted before. FESEM image in each case is also
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included for comparison. Our results suggest that the presence of graphene promotes the vertical
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alignment in ZnO NWs. The mechanism behind this can be understood from Fig. 5(b) which
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shows the epitaxial relationship between ZnO in its hexagonal Wurtzite phase and bond centered
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sites of C−C bonds present in the sp2 hybridized graphene layer. The red shift (10 cm-1) of 2D
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band in the Raman spectra of GRZ3 substrate supports our assertion on the interaction between
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the GR and ZnO layers and the artificial lattice matching mentioned above. (See supporting
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information; Fig. S4)
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3.5. Optical Absorption Studies
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In order to assess the optical absorption in the ZnO nanostructures grown on graphene and ZnO
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buffer layer coated on quartz substrates, we have performed the UV−VIS absorption
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spectroscopy measurements. Figure 6(a) represents the absorption spectra of ZnO NWs grown
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on Z1Z3 and GRZ3 deposited on quartz substrates. The absorption spectra of GR and Z1Z3
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substrates are also shown for comparison. After ZnO vapor deposition on graphene, a very strong
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absorption peak at ~ 365 nm is clearly visible, which implies the growth of crystalline ZnO
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NWs. On the other hand, GR and Z1Z3 substrates show no significant UV absorption and a very
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weak absorption band at 373 nm, respectively. The UV absorption band intensity is significantly
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higher by a factor of 6 and 10 in GRZ3NW and Z1Z3NW, as compared to that of Z1Z3. The
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high UV absorption may be due to the large surface area and dense ZnO NWs array. The
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absorption data is consistent with the enhanced UV PL emission and PC from the graphene−ZnO
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NWs hybrids (discussed later). Note that there is an additional weak and broad absorption band
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identified in the visible region peaked at ~ 480 nm in Z1Z3NW sample, which is attributed to the
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oxygenated defect either oxygen vacancy (Vo) or oxygen interstitials (Oi)) states that were
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formed due to the physical vapor deposition (PVD) growth at relatively lower temperature.[6]
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3.6. Photoluminescence (PL) Studies
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UV and visible PL studies were conducted for the as−grown and RTA treated ZnO NWs samples
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by PL measurements with 325 and 355nm excitations (Supporting information, Fig. S5). Note
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that we have not observed any significant change in the UV and visible PL spectra after the RTA
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treatment of the ZnO NWs as compared to the untreated NWs. Hence, for further discussions of
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the PL data are presented for the RTA treated samples. For the reference, the PL spectra of
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as−grown ZnO NWs are described in the supporting information, Fig. S5. Figure 6(b) illustrates
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the PL evolution in ZnO NWs grown on all the substrates with 325 nm excitation. The PL
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spectra show a sharp UV emission peak (~ 380 nm) attributed to the near band edge (NBE)
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emission and a broad visible emission band centered at ~ 500 nm attributed to the transition
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between various surface defect energy levels in ZnO NWs. The most commonly observed
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defects that occur in the low temperature vapor phase growth of ZnO NWs are Vo or Oi and Zn
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interstitials (Zni) defects due to low formation energies.[6, 19] It is interesting to note that the
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ratio of integrated intensity of UV to visible PL is relatively high in both Z1Z3NW and
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GRZ3NW as compared to the other samples. The integrated intensities and the ratios of UV and
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visible PL for all the samples are shown in the Table T1 in the supporting information. This
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implies that the crystalline quality of the NWs grown on GRZ3 substrate is significantly high.
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Note that the density of the NWs is high in case of Z1Z3NW and the substrate ZnO thick layer
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may contribute to the UV and visible PL. Thus, randomly oriented ZnO NWs in Z1Z3NW may
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not possess high crystalline quality, though the observed PL intensity is comparable to that of the
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aligned NWs grown on the graphene layer. On the other hand, visible PL intensity is very high in
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GOZ3NW sample as compared to all other samples. This strong visible PL from GOZ3NW
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might be partly due to the GO, which may emit a blue light which can be considered as a second
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source of excitation. Note that our result is in contrast to that of Zeng et al., who reported the
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quenching of the visible PL due to the electron transfer between the excited ZnO and GO sheets.
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[14] In the present case, enhancement might be partly due to the visible PL from GO layers
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itself that contains oxygen functional groups and after the growth of ZnO NWs, concentration of
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oxygen is increased and supersaturated to plenty of oxygenated defects in the ZnO NWs during
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the PVD growth.
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Further, a comparative analysis was made on the PL data to understand the effect of ZnO
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seed layer (Z3), thickness of the ZnO buffer layer (Z1), graphene (GR) layer on the PL efficiency
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of the ZnO NWs. With two different laser excitations, the visible PL features are found to be
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similar. Presence of Z3 layer leads to a sharper and intense UV peak with a lower intensity
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visible band. Thus, the Z3 layer plays a crucial role in the catalyst free growth of ZnO NWs.
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Strong and broad band visible PL in all the samples (see Fig. 6(b)) is beneficial for displays and
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other light emitting applications.
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Figure7 shows the Gaussian fitting for the PL spectra of ZnO NWs grown on graphene
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and ZnO thin film substrates. Symbols represent the experimental data and solid lines correspond
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to the fitted data. Due to the dissimilar peak positions and asymmetric line shape, three PL peaks
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were fitted for the broad visible emission band in each case and sine peak for the UV emission
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peaked at 382 nm. As compared to the GRNW, the intensity of UV PL in Z1Z3NW and
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GRZ3NWis higher by a factor of 17 and 3, respectively (see Fig. 7(a) and (b)). Higher intensity
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UV PL in Z1Z3NW is most likely due to the contribution of the thick ZnO seed layer in the UV
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PL. On the other hand, in GRNW the integrated intensity of the fitted visible peaks at ~ 494, 532
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and 573 nm are higher as compared to Z1Z3NW and GRZ3NW,which signifies more number of
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oxygenated defects (Vo/Oi) in the ZnO NWs grown in absence of the Z3 layer (see Fig. 7(c)).
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Note that, the two visible PL peaks at ~ 530 and 580 nm are common in all the samples, which
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signifies the formation of neutral Oi defects.[19] Nevertheless, Z1Z3NW and GRZ3NW samples
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have strong UV emission and relatively lower intensity of visible emission. Note that in case of
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GO and GQDs coated substrates, the visible PL features are almost identical (see Fig. 6(b). The
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detailed peak parameters and possible assignments of the defect emissions are presented in the
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supporting information (Table T2).
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3.7. Photoconductivity (PC) Studies
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In order to evaluate the photoresponse of the GR−ZnO hybrids, we have performed the PC
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measurements at a fixed bias voltage (2V). For the PC measurements, Ag contact was deposited
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by thermal evaporation under high vacuum on top of the ZnO NWs surface by keeping a shadow
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mask with a channel width and length ~ 0.1 × 0.1 mm2. The thickness of the Ag layer is ~ 100
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nm. A schematic of the device configuration is shown in the Fig. 8(a). Figure 8 (b, c, d) represent
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the steady state dark current and photocurrent as a function of voltage in the GRZ3NW,
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Z1Z3NW and GOZ3NW, respectively, with UV (~ 365 nm) excitation. The corresponding time
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response of the photocurrent in each case is shown in the Fig. 8(e, f, g), respectively. Note that
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the PC of the ZnO NWs grown on GOZ3 and Z1Z3 layer is very less as compared to that of the
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NWs grown on the GRZ3 layer. At a bias voltage of 2 V, the PC in GRZ3NW is about 65 µA as
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compared to the PC of about 0.75 µA and 1.0 µA in Z1Z3NW and GOZ3NW, respectively. The
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high photocurrent in graphene case is probably resulting from the Schottky barrier formation
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between the bunch of vertically aligned ZnO NWs and graphene layer. In case of ZnO on GO
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films, it forms a heterojunction. Marginally higher PC in GOZ3NW as compared to Z1Z3NW
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might be due to the increased number of electron (e)−hole (h) pair generation and separation at
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the junction of GO and ZnO NWs interface, i.e. formation of excitons during the PC generation.
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Above certain bias voltage, the dark I−V curve is found to be nonlinear due to the presence of
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the traps/defects in each case. Due to the semiconductor-semiconductor heterojunction, the
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magnitude of the PC is about two orders of the magnitude lower in Z1Z3NW and GOZ3NW as
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compared to the case of GRZ3NW (see Fig. 8(c)) under the same bias condition. Thus, graphene
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layer enhances the sensitivity of the UV photocurrent of the ZnO layer on it. This is an
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interesting observation and it can be exploited in future optoelectronic devices. Note that the
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dark current is relatively high in GRZ3NW, which needs to be optimized for practical
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application. Less PC in case of GOZ3NW may be partly due to the disorder and more
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oxygenated functional groups present on GO, which is consistent with the PL data discussed
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earlier.
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Further, we have investigated the PC growth and decay behavior of ZnO NWs by fitting
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with bi−exponential function, following the report by Dhara et. al.[7] PC growth can be
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expressed as:
1 7
τ
τ
------------- (1)
8
Here I1, A1 and A2 are the constants. τ1 and τ2 are the time constants, which are calculated from
9
the fitting. For GRZ3NW, Z1Z3NW and GOZ3NW, the time constants of PC growth are found
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to be τ1 = 45.0s, τ2 = 3.7s, τ1 = τ2 = 1.6s and τ1 = 15.8s, τ2 = 0.1s respectively. PC decay is
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expressed as:
∞ 12
τ
τ
-------------- (2)
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Here A3 and A4 are the constants. τ1 and τ2 are calculated as 2.5s, 88.5s in the case of GRZ3NW,
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1.4s, 20.8s in Z1Z3NW and 9.8s, 37.9s in GOZ3NW, respectively. Note that the ∞
15
represents the photocurrent after long time, which is equal to the dark current. Thus, the time
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response of the photocurrent is relatively slow in GRZ3NW and GOZ3NW as compared to that
17
of Z1Z3NW, though GRZ3NW possesses higher sensitivity. The time response is mainly
18
controlled by the intrinsic defects in the ZnO layer. Further optimization of the growth
19
conditions are required for a faster response ZnO PD made on the graphene layer. Besides the
20
surface defects, graphene−ZnO hetero−interface may be partly responsible for the slow response
21
of the PD. More studies are underway to pinpoint the mechanism.
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4. CONCLUSION
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Catalyst free growth of ZnO NWs on different graphene buffer layers such as CVD graphene,
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GO and GQDs substrates in the presence/absence of ZnO seed layer was successfully
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demonstrated. Dense array of aligned ZnO NWs was formed in the case of RTA treated ZnO
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ultra−thin film on graphene, while the GO and GQDs substrates yield randomly oriented sparse
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ZnO NWs. HRTEM and Raman studies reveal the good crystalline quality of the ZnO NWs
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grown on graphene−ZnO buffer layer substrate in comparison with ZnO NWs grown on ZnO
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seed layer and GO−ZnO buffer layer substrates. A growth mechanism was proposed based on
8
the epitaxial relationship between ZnO and graphene as compared to that of the NWs grown
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without the graphene layer. The evolution of the UV and visible PL was studied and correlated
10
with effects of ZnO buffer layers on graphene substrates in the aligned growth of ZnO NWs with
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high crystalline quality. Highly enhanced PC was achieved in the case of ZnO NWs grown on
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graphene, which is consistent with the PL results. These results demonstrate the successful
13
fabrication and superior performance of ZnO NW−graphene hybrid UV PDs as compared to the
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bare ZnO NW based PDs. These hybrid nanostructures will be the building block for the next
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generation optoelectronic device.
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SUPPORTING INFORMATION
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Table T1: Ratio of the integrated intensity of UV PL to visible PL for the spectra recorded with
19
325 nm laser excitation. Table T2: Details of the PL peaks fitted with Gaussian line shape for the
20
near band edge (NBE) UV emission and defect related visible emissions for each sample. The
21
center of each peak (P1, P2, P3, P4 and P5) is denoted in nm unit. A12, A13 and A14 denote the
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ratio of integrated intensity of peak P1 to P2, P3 and P4, respectively.
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Fig. S1: Raman spectra of CVD graphene and chemically processed graphene (GO and GQDs);
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Fig. S2: Low resolution TEM images of (a) CVD graphene (GR), (b) GO, (c) GQDs, and
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corresponding SAED patterns (inset); Fig. S3: FESEM images of GO and ZnO NWs interface;
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Fig. S4: Raman spectra of Z3 and GRZ3 substrates: E2g high mode and Raman signatures of
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graphene; Fig. S5: UV−visible PL from as−grown ZnO NWs on graphene recorded with 355 nm
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laser excitation.
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ACKNOWLEDGEMENTS
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We thank Central Instruments Facility (CIF) for providing FESEM and micro Raman facilities.
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We acknowledge DST (No SR/55/NM−01/2005) for providing the HRTEM facility at IIT
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Guwahati. We thank A. K. Sivadasan, Indira Gandhi Centre for Atomic Research (IGCAR),
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Kalpakkam, for his help in the Raman mapping measurements. We also acknowledge Albert V.
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Tamashausky, Asbury Graphite Mills, USA, for providing high purity graphite flakes to
14
synthesize GO and GQDs. Dr. Shilpa Sharma and N. V. V. Subbarao are also duly
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acknowledged.
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TABLE CAPTIONS
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Table 1: The sample descriptions for the various graphene and ZnO coated buffer layer
substrates and morphology of the ZnO NWs in each case.
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Sl.
No.
Substrate
Code
Base Substrate
Sample
Substrate description
Morphology of the
ZnO NWs
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GR
Si/SiO2, Quartz
GRNW
Graphene layer only
Randomly orientated
sparse NWs
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GRZ3
Si/SiO2, Quartz GRZ3NW Graphene layer coated with 10 nm
ZnO seed layer
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Z1Z3
Si/SiO2, Quartz
Z1Z3NW
300 nm ZnO layer coated with 10
nm ZnO seed layer
Dense & randomly
orientated NWs
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Z2Z3
Si/SiO2
Z2Z3NW
100 nm ZnO layer coated with 10
nm ZnO seed layer
Dense & randomly
orientated NWs
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Z3
Si/SiO2,
Z3NW
10 nm ZnO seed layer only
Randomly orientated
NWs
6
GOZ3
Si/SiO2
GOZ3NW
10 nm ZnO seed layer on GO
Randomly orientated
NWs
8
GQD
Si/SiO2
GQDNW
Graphene quantum dots film,
annealed
Randomly orientated
ZnO NWs
NOTE: Z1: 300 nm ZnO film, Z2: 100 nm ZnO film and Z3: 10 nm ZnO film.
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Vertical NWs
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FIGURES:
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Figure 1. Spatial Raman mappings of CVD graphene: (a) D, (b) G and (c) 2D bands, which indicate the presence
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of BLG and FLG. Note that all the mappings are recorded with 10x10 µm2 area at 514.5 nm laser excitation.
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Figure 2. FESEM images of various ZnO nanostructures grown on different graphene-ZnO and ZnO
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buffer layer substrates: (a) GRZ3NW, (b) Z1Z3NW, (c) GOZ3NW, and (d) GQDNW.
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Figure 3.(a) FESEM image of ZnO NW grown directly on GR showing sparse and randomly oriented ZnO NWs.
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(b) TEM image of the ZnO grains on SiO2 with nearly uniform size (~ 40 nm), and inset shows the higher
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magnification TEM image of a spherical ZnO seed. (c) TEM image of a ZnO NW along with the ZnO seed layer
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in sample GRZ3NW and, the inset shows the corresponding SAED pattern showing (002) plane for the combined
layer. (d) Magnified TEM image of region I in (c), which depicts the nucleation of the ZnO NW on ZnO seed and
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its corresponding lattice fringes showing (002) planes of the ZnO NW with lattice spacing of 2.6Å.
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Figure 4. (a) XRD pattern of Z1Z3 substrate after RTA treatment. The prominent peaks correspond to the (100),
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(101) and (002) planes of Wurtzite ZnO. (b) Typical Raman spectra of ZnO NWs showing strong E2g (438 cm-1)
mode, which indicates the growth of crystalline ZnO NWs with wurtzite phase. Note that the peak positions and
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FWHM (∆ω) are denoted in cm-1 unit.
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Figure 5. (a) A schematic shows the growth mechanism of ZnO NWs on GRZ3 and Z1Z3 substrates. (b) The
epitaxial relationship between ZnO in its Hexagonal Wurtzite phase at the bond centered sites of sp2 hybridized
graphene layer.
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Figure 6. (a) Comparison of absorption spectra for ZnO NWs on graphene and ZnO coated quartz substrates. For
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comparison, spectra of GR and Z1Z3 prior to the growth of NWs are shown. The absorption peaks are denoted in
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nm unit. Note that the Z1Z3NW shows substantial absorption in the visible region, besides the strong UV
absorption. (b) Comparative PL characteristics of GRNW, GRZ3NW, GQDNW, GOZ3NW and Z1Z3NW after
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RTA treatment, measured with 325 nm laser excitation.
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Figure 7. Gaussian line shape fitting of the PL spectra: (a) Z1Z3NW, (b) GRZ3NW, and (c) GRNW. The
symbols represent the experimental data and solid lines correspond to the fitted data. The peak centers are
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denoted in nm unit.
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Figure 8. (a) A schematic of the Ag contact made on the ZnO NWs array for the photoconductivity (PC)
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measurement, illuminated with UV light (365 nm). (b−d) Dark current and photo current as a function of
voltage in GRZ3NW, Z1Z3NW and GOZ3NW respectively. (e−g) The time response of the respective
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photocurrent in GRZ3NW, Z1Z3NW and GOZ3NW at a fixed bias voltage (2V). The symbols represent the
experimental data and the solid lines correspond to the fitted data in each case.
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Graphical Image
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