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Highly transparent and conducting graphene-embedded ZnO films with enhanced photoluminescence fabricated by aerosol synthesis
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 085701 (http://iopscience.iop.org/0957-4484/25/8/085701) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.248.155.225 This content was downloaded on 05/02/2014 at 07:09
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Nanotechnology Nanotechnology 25 (2014) 085701 (7pp)
doi:10.1088/0957-4484/25/8/085701
Highly transparent and conducting graphene-embedded ZnO films with enhanced photoluminescence fabricated by aerosol synthesis Bob Jin Kwon1 , Jong-Young Kim1 , Soon-Mok Choi2 and Sung Jin An3 1
Icheon Branch, Korea Institute of Ceramic Engineering and Technology, 3321 Gyeongchung Daero, Sindun-myeon, Icheon-si, Gyeonggi-do, 467-843, Republic of Korea 2 School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Cheonan, 330-708, Republic of Korea 3 Department of Materials Science and Engineering, Kumoh Institute of Technology, 1 Yangho-dong, Gumi, Gyungbuk, 730-701, Republic of Korea E-mail: [email protected] and [email protected] Received 24 September 2013, revised 29 October 2013 Accepted for publication 11 November 2013 Published 4 February 2014
Abstract
Graphene/inorganic hybrid structures have attracted increasing attention in research aimed at producing advanced optoelectronic devices and sensors. Herein, we report on aerosol synthesis of new graphene-embedded zinc oxide (ZnO) films with higher optical transparency (>80% at visible wavelengths), improved electrical conductivity (>2 orders of magnitude, ∼20 k/), and enhanced photoluminescence (∼3 times), as compared to bare ZnO film. The ZnO/graphene composite films, in which reduced graphene oxide nanoplatelets (∼4 nm thick) are embedded in nanograined ZnO (∼50 nm in grain size), were fabricated from colloidal suspensions of graphene oxide with an aqueous zinc precursor. These new luminescent ZnO/graphene composites, with high optical transparency and improved electrical conductivity, are promising materials for use in optoelectronic devices. Keywords: graphene, ZnO, photoluminescence, electrical conductivity, transparency S Online supplementary data available from stacks.iop.org/Nano/25/085701/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
extraction efficiency and current injection [7–9]. Moreover, hybrids of semiconductors and graphene present interesting possibilities in diverse areas ranging across photovoltaics [10, 11], catalysis [12–15], and photodetectors [16, 17]. Zinc oxide (ZnO) has been a promising material in the development of exciton-based optoelectronic devices, such as light emitting diode and photovoltaic cells, because of its direct band gap of 3.3 eV at room temperature, and a large exciton binding energy of 60 meV [18–20]. Furthermore, graphenehybridized ZnO quantum dots (QD) appear promising for photovoltaics [10], photocatalysts [12, 13], gas sensors, and optical switching [16, 17], due to an effective charge-transfer
The exceptional mechanical, thermal, and electrical properties of graphene have prompted intensive research into a wide range of applications in physics [1, 2], chemistry [3], and materials science [4]. The fabrication of functional optoelectronic devices using graphene as a transparent electrode suggests the feasibility of an attractive device with flexibility, light weight, shock resistance, softness, and transparency [5, 6]. A number of graphene/inorganic hybrid structures in advanced light emitting diode (LED) devices have been reported because graphene nanoplatelets show attractive features such as enhanced light 0957-4484/14/085701+07$33.00
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c 2014 IOP Publishing Ltd
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Nanotechnology 25 (2014) 085701
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oxide (250 nm) coated Si wafer at 400 ◦ C. The G-O dispersion
process. However, in most cases of graphene-hybridized ZnO QD, quenching of emission has been reported, resulting from such interfacial charge transfer [10, 12, 13, 18]. Such decreases in emission yield suggest that an additional pathway for the disappearance of the charge carriers dominates because of the interactions between the excited semiconductor particles and the graphene sheets [12–17]. Recently, it was also reported that ZnO QD/graphene with a quasi-core–shell structure, fabricated by a spin-coating method, led to a reduced and broad ultraviolet photoemission with new graphene-related peaks, which was attributed to a band gap opening due to a bend structure of the graphene sheet encapsulating a ZnO QD [18]. The reduced PL of the core–shell-structured hybrid was assumed to be due to strain-induced band gap opening and interfacial charge transfer. Recently, mechanically exfoliated single-layer graphene sheets, transferred on top of ZnO film without chemical bonding, were reported to enhance the ultraviolet photoluminescence (PL) of the ZnO due to a resonant excitation of a graphene plasmon [21]. Low resistance and transparency are also crucial for improving the current injection and light extraction efficiency for the realization of solid-state lighting using light emitting diodes (LEDs). For these reasons, in the present work, we attempted to fabricate a transparent, conducting, and luminescent ZnO/graphene hybrid (composite) thin film using surface plasmon (SP) mediated photoluminescence. For the hybridization of ZnO with graphene, an aerosol synthetic method, namely ultrasonic-assisted spray pyrolysis (UASP), was applied. One of the advantages of UASP is that multiple components can be incorporated into an aerosol droplet to serve different purposes. By adding additional components to the starting graphene precursor, inorganic particles wrapped with graphene nanoplatelets can be obtained [22–24]. This aerosol synthetic route allows us to have a continuous mode of operation and readily scalable synthesis of large-area graphene-embedded thin film, or nanocomposite particles. The resulting ZnO/graphene thin film exhibits enhanced PL approximately 3 times greater than that of bare ZnO film. We have also shown that such PL enhancement can be achieved in a quasi-core–shell-type ZnO/graphene composite with reduced graphene oxide, which exhibits a larger PL enhancement than conventional reduced graphene oxide. Moreover, the high optical transmittance (>80%) and increased electrical conductivity (>2 orders of magnitude, ∼20 k/) of the ZnO/graphene composite films show that the present composites are promising as transparent, conducting, and luminescent materials for use in optoelectronic devices.
was prepared according to a modified Hummer’s method [25]. Raman spectra of the ZnO/graphene were collected using a Renishaw system 1000 Raman spectrometer with an Ar+ laser at 514 nm and a 1800 line mm−1 grating. The laser spot size was about 1 µm. The power of the laser was kept at 1.5 mW to avoid heating the samples. The spatially resolved Raman mapping data were achieved by Raman imaging with 1.5 µm steps in the X and Y directions. PL measurements were performed using the 325 nm line of a continuous-wave HeCd laser for excitation. Details of the PL measurements have been reported elsewhere [20]. The PL intensity was compared for the composite films with the same thickness (∼70 nm) and deposition condition (400 ◦ C, 6 h). 3. Result and discussion
Scheme 1 and figure 1 show schematics of the ultrasonicassisted spray pyrolysis method and the experimental apparatus, respectively. In the aerosol synthesis, as the water in the droplet evaporates, the graphene oxide (G-O) nanoplatelets migrate to the surface of the droplets (liquid–vapor interface) to form a shell [23, 26–28]. Chen et al observed that the multi-layer G-O surface films wrinkle under compressive stress during droplet shrinkage [23]. The monolayer G-O, which is suspended with second components in dilute aqueous phases, can be ultrasonically nebulized and dried/heated to produce nanoparticles that consist of graphene encapsulating the second component. They proposed that two factors govern this mechanism via MD simulation: (i) free energy reduction by interfacial adsorption of a G-O sheet, and (ii) slow diffusion of the high molecular weight sheets, which allows additional sheets to be scavenged by the interface as the droplet surface recedes during drying. In this work, it is expected that as the temperature and evaporation increase in the chamber, the dissolved Zn will consolidate into a ZnO particle in the droplet with capillary collapse of the graphene (G-O) shell, resulting in a nanosack-like morphology that encapsulates the ZnO particles. Because the ZnO grain size of ZnO is below 100 nm, the synthesized ZnO from Zn acetate precursor might be encapsulated in the droplet before deposition to a substrate, as expected from the previous work. After deposition, the G-O nanosheets were found to be present as crumpled layers between ZnO grains in the resulting films, as shown in the TEM image (figure 2(e)). At the same time, the G-O is partially reduced to reduced graphene oxide (rG-O) thermally [29] in the chamber at 400 ◦ C during deposition. The atomic ratios of carbon to oxygen (C 1s/O 1s) for the G-O and the G-O annealed at 400 ◦ C were 1.97 and 3.78, respectively, according to x-ray photoelectron spectroscopic (XPS) analysis results (figure S1 available at stacks.iop.org/Nano/25/085701/mmedia). According to the previous work, the C 1s/O 1s ratio was reported to be 6.8 and 11.36, from rapid thermal annealing (RTA) in Ar at 500 ◦ C and 1000 ◦ C, respectively [30]. The reduced nature of rG-O was confirmed by the decreased resistivity of the resulting rG-O/ZnO films (∼70 nm, 90–200 k/), as compared to that of the bare ZnO film (∼20 M/). When the film
2. Experiments
An aqueous dispersion of exfoliated graphene oxide (GO) [25] was nebulized with a Zn precursor to generate aerosol droplets and flowed through a preheated chamber. In a typical experiment, an aqueous Zn acetate solution (0.1 M) was directly added to a colloidal G-O dispersion with the surfactant of sodium dodecyl sulfate (1.0 wt%). The mixture was then nebulized to form aerosol droplets, which were blown with Ar carrier gas toward a preheated glass substrate, or a thermal 2
Nanotechnology 25 (2014) 085701
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Scheme 1. Scheme of the ZnO/graphene hybrid film obtained by ultrasonic-assisted spray pyrolysis (UASP). An aqueous dispersion of
graphene and Zn acetate solution was nebulized to generate aerosol droplets, which were flowed into a preheated chamber using Ar carrier gas. As the evaporation proceeded, the dissolved Zn consolidated as ZnO nanoparticles in the droplet with capillary collapse of the graphene (G-O) shell. In the preheated chamber (at 400 ◦ C), ZnO crystals (∼50 nm) encapsulated by the graphene were deposited on a substrate.
Figure 1. Illustration of ultrasonic-assisted spray pyrolysis (UASP).
was annealed at ∼500 ◦ C, the sheet resistance decreased to ∼20 k/. As a result, the ZnO/rG-O composite films were obtained on the glass and silicon substrates as shown in figure 2. Figure 2 shows optical, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) images of the ZnO/rG-O (5.0 wt%) composite film fabricated by UASP at 400 ◦ C. As shown in figures 2(a) and (b), a large-area, transparent ZnO/rG-O film was deposited on the glass and SiO2 /Si substrates. Figure 2(d) shows that the composite film was mainly composed of randomly oriented ZnO nanograins, and that their average grain size was ∼50 nm. The thickness of the ZnO/rG-O thin film was ∼70 nm (figure S2 available at stacks.iop.org/Nano/25/085701/mmedia). Figure 2(e) shows that a thin layer of rG-O (∼4 nm) is embedded in the ZnO nanograins. In the x-ray diffraction (XRD) patterns for the ZnO/rG-O composites (figure 3(a)), Bragg peaks due to (100), (002), (101), and (102) reflections for the ZnO were observed. The
positions of these peaks agree well with those of standard ZnO bulk (JCPDS No. 36-1451), but the relative intensity ratio was different from the ZnO bulk one. The (002) reflection was more intense than the (101) reflection for the ZnO and the composite films, which suggests that ZnO is preferentially grown along the (00l) direction. The peak intensity of the Bragg peaks due to ZnO was almost the same for the ZnO and the composite films, as shown in figure 3(a), which confirms that the crystallinity of the ZnO particle was not changed by the incorporation of graphene into the composite. Furthermore, we could not observe any discernible changes in TEM images (morphology, size; figure 2(e)) and XRD pattern (crystallite size, intensity; figure 3(a)) due to the incorporation of G-O. Therefore, we assume that the crystalline structure of ZnO/rG-O composites undergoes negligible changes upon the incorporation of G-O. Figure 3(b) presents typical Raman spectra of the ZnO/rGO composite films. The G and D modes are known to arise from first-order scattering of the E2g phonon of sp2 carbon atoms, and from a breathing mode of κ-point phonons of 3
Nanotechnology 25 (2014) 085701
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Figure 2. (a) A transparent ZnO/rG-O composite film on a glass substrate. (b) A large-area (80 mm × 80 mm) ZnO/rG-O film on a SiO2 /Si substrate. (c) A SEM image of ZnO/rG-O (5.0 wt%) obtained by UASP. (d) A bright-field TEM micrograph of the ZnO/rG-O thin film deposited on a thermal oxide layer. (e) A HR-TEM micrograph showing the rG-O thin layer (∼4 nm) embedded in nanograined ZnO.
Figure 3. (a) XRD patterns for ZnO/rG-O (2.0/5.0/7.0/10.0 wt%) films obtained by UASP. The Bragg peak intensities due to ZnO were almost invariant for the compounds. (b) Raman spectra for the ZnO/rG-O composite films on a SiO2 /Si substrate.
A1g symmetry, respectively [31]. In previous work on the ZnO quantum dot/graphene hybrid [18], the splitting of a doubly degenerate G peak into two sub-bands was observed and explained in terms of the strain induced by a bending of the graphene surrounding the ZnO quantum dots. However, no such feature was observed for the present composites. The Raman spectrum of the ZnO/rG-O films shows a D band around ∼1350 cm−1 , a G band around 1590 cm−1 , and a broad 2D band around 2800 cm−1 . The D band was detected at ∼1350 cm−1 , which is due to defects or edges [32]. The D, G, and 2D bands are shifted to lower wavenumbers for the ZnO/rG-O films and the D band peak intensity is decreased as compared to G-O precursor [33]; e.g., the G band is at ∼1600 cm−1 for the G-O precursor, but is at 1590 cm−1 for
the ZnO/rG-O film [36]. Also, the shift to lower wavenumber of the G band peak and the decreased D band peak intensity of the ZnO/rG-O films presumably result from the reduction of the G-O contained in the ZnO/rG-O composite film [37]. Figure 4 presents typical Raman spectra and spatial maps of the ZnO/rG-O composite film. As shown in figure 4, the Raman mapping analysis confirmed that, as the concentration of the G-O increased in the precursor, the surface coverage by the rG-O nanoplatelets gradually increased in the composite film. Figure 5(a) shows the PL spectra of typical ZnO/rG-O composite films and a bare ZnO film at room temperature. The excitonic PL peak intensity of the composites exhibits a dramatic increase with respect to that of the bare ZnO film. 4
Nanotechnology 25 (2014) 085701
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Figure 4. Spatial Raman maps (50 µm × 50 µm) of (a) ZnO/rG-O (2.0 wt%), (b) ZnO/rG-O (5.0 wt%), and (c) ZnO/rG-O (10.0 wt%) composite film for the G peak (∼1590 cm−1 ). As the concentration of G-O increased in the precursor, the surface coverage by the rG-O
nanoplatelets gradually increased in the composite film.
decreases to ∼90 k/square at 0.5 wt% G-O, in which the optical transmittance is >80% in the visible wavelength region (figure 5(c)). When the film was annealed at ∼500 ◦ C, the sheet resistance decreased to ∼20 k/. The present feature, between the conductivity and PL intensity, is also consistent with the previous results on CVD-grown graphene on ZnO [21]. In the previous result, PL enhancement was not observed for the CVD-grown graphene/ZnO structure, which is in contrast to the case of mechanically exfoliated graphene/ZnO structure with PL enhancement. This result was attributed to the lower conductivity of the CVD-grown graphene. The dominant PL peak of the composite exhibited almost the same position at 377 nm (3.26 eV) as that for the bare ZnO film, which agrees well with the known values for free excitonic emission with a band gap at room temperature. For the ZnO/rG-O composites, a new emission peak was observed at 409 nm (3.03 eV). In this stage, there are three possibilities for this new peak; (1) electronic transitions involving new unoccupied levels of rG-O; (2) blue emission from quantum sized rG-O embedded in the matrix [33, 34]; (3) a surface OH group in ZnO, which was reported in many oxides [40]. For (1), this is similar to the split level for the previously reported ZnO quantum dot/graphene hybrid [18]. Son et al showed that the strain introduced by curvature opens an electronic band gap (∼250 meV) in the graphene, and additional blue emission peaks result from a splitting of the lowest unoccupied orbitals (LUMO) of the graphene into three orbitals with distinct energy levels, according to density functional theory calculations. However, in this work, only a small peak (409 nm) due to the LUMO level was observed and no splitting of a doubly degenerate G peak into two sub-bands was observed in Raman spectra. For (2), our synthesized G-O has much larger lateral dimension than the reported rG-O quantum dot (5.0 wt%, which can be attributed to a blocking effect of multi-layered rG-O nanoplatelets. The dependence of the sheet resistance on the G-O concentration is also shown; it appears to be related to that of the PL intensity. (c) Optical transmittance for the present composite films on glass substrates. The transmittance for the composite is >80% in the visible wavelength region.
hydroxyl groups. The peaks due to deep level emission (DLE) were also observed at 479 nm (2.59 eV) and 537 nm (2.3 eV), implying increased defect sites in the ZnO/rG-O composite, as compared to bare ZnO [35]. Figure 5(c) shows optical spectra of the rG-O/ZnO composite films. The optical transmittance for the hybrids is >80% at visible wavelengths. This result indicates that the present ZnO/graphene composite will be useful for advanced optoelectronic devices as a transparent, conducting, and luminescent material.
optical transmittance and improved electrical conductivity of the rG-O/ZnO films show that the present composite films are transparent conductors. These results suggest that carefully controlled ZnO/graphene composite is promising for use as a transparent, conducting, and luminescent material in future optoelectronic applications. Acknowledgments
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0014415) and the Gyeongbuk Science & Technology Promotion Center (GBSP) Grant funded by the Korean government (MEST) (GBSP-001-111201-001).
4. Conclusion
In summary, a new graphene-embedded ZnO thin film with high transparency (>80%), improved conductivity (>2 orders of magnitude, ∼20 k/), and enhanced photoemission (∼3 times), compared to bare ZnO film, was fabricated by ultrasonic-assisted spray pyrolysis (UASP). The photoemission of the ZnO/rG-O (5.0 wt%) composite was 3 times greater than that of the bare ZnO film. The high
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Highly transparent and conducting graphene-embedded ZnO films with enhanced photoluminescence fabricated by aerosol synthesis
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 085701 (http://iopscience.iop.org/0957-4484/25/8/085701) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.248.155.225 This content was downloaded on 05/02/2014 at 07:09
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 085701 (7pp)
doi:10.1088/0957-4484/25/8/085701
Highly transparent and conducting graphene-embedded ZnO films with enhanced photoluminescence fabricated by aerosol synthesis Bob Jin Kwon1 , Jong-Young Kim1 , Soon-Mok Choi2 and Sung Jin An3 1
Icheon Branch, Korea Institute of Ceramic Engineering and Technology, 3321 Gyeongchung Daero, Sindun-myeon, Icheon-si, Gyeonggi-do, 467-843, Republic of Korea 2 School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Cheonan, 330-708, Republic of Korea 3 Department of Materials Science and Engineering, Kumoh Institute of Technology, 1 Yangho-dong, Gumi, Gyungbuk, 730-701, Republic of Korea E-mail: [email protected] and [email protected] Received 24 September 2013, revised 29 October 2013 Accepted for publication 11 November 2013 Published 4 February 2014
Abstract
Graphene/inorganic hybrid structures have attracted increasing attention in research aimed at producing advanced optoelectronic devices and sensors. Herein, we report on aerosol synthesis of new graphene-embedded zinc oxide (ZnO) films with higher optical transparency (>80% at visible wavelengths), improved electrical conductivity (>2 orders of magnitude, ∼20 k/), and enhanced photoluminescence (∼3 times), as compared to bare ZnO film. The ZnO/graphene composite films, in which reduced graphene oxide nanoplatelets (∼4 nm thick) are embedded in nanograined ZnO (∼50 nm in grain size), were fabricated from colloidal suspensions of graphene oxide with an aqueous zinc precursor. These new luminescent ZnO/graphene composites, with high optical transparency and improved electrical conductivity, are promising materials for use in optoelectronic devices. Keywords: graphene, ZnO, photoluminescence, electrical conductivity, transparency S Online supplementary data available from stacks.iop.org/Nano/25/085701/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
extraction efficiency and current injection [7–9]. Moreover, hybrids of semiconductors and graphene present interesting possibilities in diverse areas ranging across photovoltaics [10, 11], catalysis [12–15], and photodetectors [16, 17]. Zinc oxide (ZnO) has been a promising material in the development of exciton-based optoelectronic devices, such as light emitting diode and photovoltaic cells, because of its direct band gap of 3.3 eV at room temperature, and a large exciton binding energy of 60 meV [18–20]. Furthermore, graphenehybridized ZnO quantum dots (QD) appear promising for photovoltaics [10], photocatalysts [12, 13], gas sensors, and optical switching [16, 17], due to an effective charge-transfer
The exceptional mechanical, thermal, and electrical properties of graphene have prompted intensive research into a wide range of applications in physics [1, 2], chemistry [3], and materials science [4]. The fabrication of functional optoelectronic devices using graphene as a transparent electrode suggests the feasibility of an attractive device with flexibility, light weight, shock resistance, softness, and transparency [5, 6]. A number of graphene/inorganic hybrid structures in advanced light emitting diode (LED) devices have been reported because graphene nanoplatelets show attractive features such as enhanced light 0957-4484/14/085701+07$33.00
1
c 2014 IOP Publishing Ltd
Printed in the UK
Nanotechnology 25 (2014) 085701
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oxide (250 nm) coated Si wafer at 400 ◦ C. The G-O dispersion
process. However, in most cases of graphene-hybridized ZnO QD, quenching of emission has been reported, resulting from such interfacial charge transfer [10, 12, 13, 18]. Such decreases in emission yield suggest that an additional pathway for the disappearance of the charge carriers dominates because of the interactions between the excited semiconductor particles and the graphene sheets [12–17]. Recently, it was also reported that ZnO QD/graphene with a quasi-core–shell structure, fabricated by a spin-coating method, led to a reduced and broad ultraviolet photoemission with new graphene-related peaks, which was attributed to a band gap opening due to a bend structure of the graphene sheet encapsulating a ZnO QD [18]. The reduced PL of the core–shell-structured hybrid was assumed to be due to strain-induced band gap opening and interfacial charge transfer. Recently, mechanically exfoliated single-layer graphene sheets, transferred on top of ZnO film without chemical bonding, were reported to enhance the ultraviolet photoluminescence (PL) of the ZnO due to a resonant excitation of a graphene plasmon [21]. Low resistance and transparency are also crucial for improving the current injection and light extraction efficiency for the realization of solid-state lighting using light emitting diodes (LEDs). For these reasons, in the present work, we attempted to fabricate a transparent, conducting, and luminescent ZnO/graphene hybrid (composite) thin film using surface plasmon (SP) mediated photoluminescence. For the hybridization of ZnO with graphene, an aerosol synthetic method, namely ultrasonic-assisted spray pyrolysis (UASP), was applied. One of the advantages of UASP is that multiple components can be incorporated into an aerosol droplet to serve different purposes. By adding additional components to the starting graphene precursor, inorganic particles wrapped with graphene nanoplatelets can be obtained [22–24]. This aerosol synthetic route allows us to have a continuous mode of operation and readily scalable synthesis of large-area graphene-embedded thin film, or nanocomposite particles. The resulting ZnO/graphene thin film exhibits enhanced PL approximately 3 times greater than that of bare ZnO film. We have also shown that such PL enhancement can be achieved in a quasi-core–shell-type ZnO/graphene composite with reduced graphene oxide, which exhibits a larger PL enhancement than conventional reduced graphene oxide. Moreover, the high optical transmittance (>80%) and increased electrical conductivity (>2 orders of magnitude, ∼20 k/) of the ZnO/graphene composite films show that the present composites are promising as transparent, conducting, and luminescent materials for use in optoelectronic devices.
was prepared according to a modified Hummer’s method [25]. Raman spectra of the ZnO/graphene were collected using a Renishaw system 1000 Raman spectrometer with an Ar+ laser at 514 nm and a 1800 line mm−1 grating. The laser spot size was about 1 µm. The power of the laser was kept at 1.5 mW to avoid heating the samples. The spatially resolved Raman mapping data were achieved by Raman imaging with 1.5 µm steps in the X and Y directions. PL measurements were performed using the 325 nm line of a continuous-wave HeCd laser for excitation. Details of the PL measurements have been reported elsewhere [20]. The PL intensity was compared for the composite films with the same thickness (∼70 nm) and deposition condition (400 ◦ C, 6 h). 3. Result and discussion
Scheme 1 and figure 1 show schematics of the ultrasonicassisted spray pyrolysis method and the experimental apparatus, respectively. In the aerosol synthesis, as the water in the droplet evaporates, the graphene oxide (G-O) nanoplatelets migrate to the surface of the droplets (liquid–vapor interface) to form a shell [23, 26–28]. Chen et al observed that the multi-layer G-O surface films wrinkle under compressive stress during droplet shrinkage [23]. The monolayer G-O, which is suspended with second components in dilute aqueous phases, can be ultrasonically nebulized and dried/heated to produce nanoparticles that consist of graphene encapsulating the second component. They proposed that two factors govern this mechanism via MD simulation: (i) free energy reduction by interfacial adsorption of a G-O sheet, and (ii) slow diffusion of the high molecular weight sheets, which allows additional sheets to be scavenged by the interface as the droplet surface recedes during drying. In this work, it is expected that as the temperature and evaporation increase in the chamber, the dissolved Zn will consolidate into a ZnO particle in the droplet with capillary collapse of the graphene (G-O) shell, resulting in a nanosack-like morphology that encapsulates the ZnO particles. Because the ZnO grain size of ZnO is below 100 nm, the synthesized ZnO from Zn acetate precursor might be encapsulated in the droplet before deposition to a substrate, as expected from the previous work. After deposition, the G-O nanosheets were found to be present as crumpled layers between ZnO grains in the resulting films, as shown in the TEM image (figure 2(e)). At the same time, the G-O is partially reduced to reduced graphene oxide (rG-O) thermally [29] in the chamber at 400 ◦ C during deposition. The atomic ratios of carbon to oxygen (C 1s/O 1s) for the G-O and the G-O annealed at 400 ◦ C were 1.97 and 3.78, respectively, according to x-ray photoelectron spectroscopic (XPS) analysis results (figure S1 available at stacks.iop.org/Nano/25/085701/mmedia). According to the previous work, the C 1s/O 1s ratio was reported to be 6.8 and 11.36, from rapid thermal annealing (RTA) in Ar at 500 ◦ C and 1000 ◦ C, respectively [30]. The reduced nature of rG-O was confirmed by the decreased resistivity of the resulting rG-O/ZnO films (∼70 nm, 90–200 k/), as compared to that of the bare ZnO film (∼20 M/). When the film
2. Experiments
An aqueous dispersion of exfoliated graphene oxide (GO) [25] was nebulized with a Zn precursor to generate aerosol droplets and flowed through a preheated chamber. In a typical experiment, an aqueous Zn acetate solution (0.1 M) was directly added to a colloidal G-O dispersion with the surfactant of sodium dodecyl sulfate (1.0 wt%). The mixture was then nebulized to form aerosol droplets, which were blown with Ar carrier gas toward a preheated glass substrate, or a thermal 2
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Scheme 1. Scheme of the ZnO/graphene hybrid film obtained by ultrasonic-assisted spray pyrolysis (UASP). An aqueous dispersion of
graphene and Zn acetate solution was nebulized to generate aerosol droplets, which were flowed into a preheated chamber using Ar carrier gas. As the evaporation proceeded, the dissolved Zn consolidated as ZnO nanoparticles in the droplet with capillary collapse of the graphene (G-O) shell. In the preheated chamber (at 400 ◦ C), ZnO crystals (∼50 nm) encapsulated by the graphene were deposited on a substrate.
Figure 1. Illustration of ultrasonic-assisted spray pyrolysis (UASP).
was annealed at ∼500 ◦ C, the sheet resistance decreased to ∼20 k/. As a result, the ZnO/rG-O composite films were obtained on the glass and silicon substrates as shown in figure 2. Figure 2 shows optical, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) images of the ZnO/rG-O (5.0 wt%) composite film fabricated by UASP at 400 ◦ C. As shown in figures 2(a) and (b), a large-area, transparent ZnO/rG-O film was deposited on the glass and SiO2 /Si substrates. Figure 2(d) shows that the composite film was mainly composed of randomly oriented ZnO nanograins, and that their average grain size was ∼50 nm. The thickness of the ZnO/rG-O thin film was ∼70 nm (figure S2 available at stacks.iop.org/Nano/25/085701/mmedia). Figure 2(e) shows that a thin layer of rG-O (∼4 nm) is embedded in the ZnO nanograins. In the x-ray diffraction (XRD) patterns for the ZnO/rG-O composites (figure 3(a)), Bragg peaks due to (100), (002), (101), and (102) reflections for the ZnO were observed. The
positions of these peaks agree well with those of standard ZnO bulk (JCPDS No. 36-1451), but the relative intensity ratio was different from the ZnO bulk one. The (002) reflection was more intense than the (101) reflection for the ZnO and the composite films, which suggests that ZnO is preferentially grown along the (00l) direction. The peak intensity of the Bragg peaks due to ZnO was almost the same for the ZnO and the composite films, as shown in figure 3(a), which confirms that the crystallinity of the ZnO particle was not changed by the incorporation of graphene into the composite. Furthermore, we could not observe any discernible changes in TEM images (morphology, size; figure 2(e)) and XRD pattern (crystallite size, intensity; figure 3(a)) due to the incorporation of G-O. Therefore, we assume that the crystalline structure of ZnO/rG-O composites undergoes negligible changes upon the incorporation of G-O. Figure 3(b) presents typical Raman spectra of the ZnO/rGO composite films. The G and D modes are known to arise from first-order scattering of the E2g phonon of sp2 carbon atoms, and from a breathing mode of κ-point phonons of 3
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Figure 2. (a) A transparent ZnO/rG-O composite film on a glass substrate. (b) A large-area (80 mm × 80 mm) ZnO/rG-O film on a SiO2 /Si substrate. (c) A SEM image of ZnO/rG-O (5.0 wt%) obtained by UASP. (d) A bright-field TEM micrograph of the ZnO/rG-O thin film deposited on a thermal oxide layer. (e) A HR-TEM micrograph showing the rG-O thin layer (∼4 nm) embedded in nanograined ZnO.
Figure 3. (a) XRD patterns for ZnO/rG-O (2.0/5.0/7.0/10.0 wt%) films obtained by UASP. The Bragg peak intensities due to ZnO were almost invariant for the compounds. (b) Raman spectra for the ZnO/rG-O composite films on a SiO2 /Si substrate.
A1g symmetry, respectively [31]. In previous work on the ZnO quantum dot/graphene hybrid [18], the splitting of a doubly degenerate G peak into two sub-bands was observed and explained in terms of the strain induced by a bending of the graphene surrounding the ZnO quantum dots. However, no such feature was observed for the present composites. The Raman spectrum of the ZnO/rG-O films shows a D band around ∼1350 cm−1 , a G band around 1590 cm−1 , and a broad 2D band around 2800 cm−1 . The D band was detected at ∼1350 cm−1 , which is due to defects or edges [32]. The D, G, and 2D bands are shifted to lower wavenumbers for the ZnO/rG-O films and the D band peak intensity is decreased as compared to G-O precursor [33]; e.g., the G band is at ∼1600 cm−1 for the G-O precursor, but is at 1590 cm−1 for
the ZnO/rG-O film [36]. Also, the shift to lower wavenumber of the G band peak and the decreased D band peak intensity of the ZnO/rG-O films presumably result from the reduction of the G-O contained in the ZnO/rG-O composite film [37]. Figure 4 presents typical Raman spectra and spatial maps of the ZnO/rG-O composite film. As shown in figure 4, the Raman mapping analysis confirmed that, as the concentration of the G-O increased in the precursor, the surface coverage by the rG-O nanoplatelets gradually increased in the composite film. Figure 5(a) shows the PL spectra of typical ZnO/rG-O composite films and a bare ZnO film at room temperature. The excitonic PL peak intensity of the composites exhibits a dramatic increase with respect to that of the bare ZnO film. 4
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Figure 4. Spatial Raman maps (50 µm × 50 µm) of (a) ZnO/rG-O (2.0 wt%), (b) ZnO/rG-O (5.0 wt%), and (c) ZnO/rG-O (10.0 wt%) composite film for the G peak (∼1590 cm−1 ). As the concentration of G-O increased in the precursor, the surface coverage by the rG-O
nanoplatelets gradually increased in the composite film.
decreases to ∼90 k/square at 0.5 wt% G-O, in which the optical transmittance is >80% in the visible wavelength region (figure 5(c)). When the film was annealed at ∼500 ◦ C, the sheet resistance decreased to ∼20 k/. The present feature, between the conductivity and PL intensity, is also consistent with the previous results on CVD-grown graphene on ZnO [21]. In the previous result, PL enhancement was not observed for the CVD-grown graphene/ZnO structure, which is in contrast to the case of mechanically exfoliated graphene/ZnO structure with PL enhancement. This result was attributed to the lower conductivity of the CVD-grown graphene. The dominant PL peak of the composite exhibited almost the same position at 377 nm (3.26 eV) as that for the bare ZnO film, which agrees well with the known values for free excitonic emission with a band gap at room temperature. For the ZnO/rG-O composites, a new emission peak was observed at 409 nm (3.03 eV). In this stage, there are three possibilities for this new peak; (1) electronic transitions involving new unoccupied levels of rG-O; (2) blue emission from quantum sized rG-O embedded in the matrix [33, 34]; (3) a surface OH group in ZnO, which was reported in many oxides [40]. For (1), this is similar to the split level for the previously reported ZnO quantum dot/graphene hybrid [18]. Son et al showed that the strain introduced by curvature opens an electronic band gap (∼250 meV) in the graphene, and additional blue emission peaks result from a splitting of the lowest unoccupied orbitals (LUMO) of the graphene into three orbitals with distinct energy levels, according to density functional theory calculations. However, in this work, only a small peak (409 nm) due to the LUMO level was observed and no splitting of a doubly degenerate G peak into two sub-bands was observed in Raman spectra. For (2), our synthesized G-O has much larger lateral dimension than the reported rG-O quantum dot (5.0 wt%, which can be attributed to a blocking effect of multi-layered rG-O nanoplatelets. The dependence of the sheet resistance on the G-O concentration is also shown; it appears to be related to that of the PL intensity. (c) Optical transmittance for the present composite films on glass substrates. The transmittance for the composite is >80% in the visible wavelength region.
hydroxyl groups. The peaks due to deep level emission (DLE) were also observed at 479 nm (2.59 eV) and 537 nm (2.3 eV), implying increased defect sites in the ZnO/rG-O composite, as compared to bare ZnO [35]. Figure 5(c) shows optical spectra of the rG-O/ZnO composite films. The optical transmittance for the hybrids is >80% at visible wavelengths. This result indicates that the present ZnO/graphene composite will be useful for advanced optoelectronic devices as a transparent, conducting, and luminescent material.
optical transmittance and improved electrical conductivity of the rG-O/ZnO films show that the present composite films are transparent conductors. These results suggest that carefully controlled ZnO/graphene composite is promising for use as a transparent, conducting, and luminescent material in future optoelectronic applications. Acknowledgments
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0014415) and the Gyeongbuk Science & Technology Promotion Center (GBSP) Grant funded by the Korean government (MEST) (GBSP-001-111201-001).
4. Conclusion
In summary, a new graphene-embedded ZnO thin film with high transparency (>80%), improved conductivity (>2 orders of magnitude, ∼20 k/), and enhanced photoemission (∼3 times), compared to bare ZnO film, was fabricated by ultrasonic-assisted spray pyrolysis (UASP). The photoemission of the ZnO/rG-O (5.0 wt%) composite was 3 times greater than that of the bare ZnO film. The high
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