Structurally Nanocrystalline-Electrically Single Crystalline ZnO

Aug 19, 2014 - Structurally Nanocrystalline-Electrically Single Crystalline ZnO-. Reduced Graphene Oxide Composites. Woo Hyun Nam,. †,∇. Bo Bae Ki...
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Structurally Nanocrystalline-Electrically Single Crystalline ZnOReduced Graphene Oxide Composites Woo Hyun Nam,†,∇ Bo Bae Kim,§,∇ Seul Gi Seo,§ Young Soo Lim,*,§ Jong-Young Kim,∥ Won-Seon Seo,§ Won Kook Choi,⊥ Hyung-Ho Park,# and Jeong Yong Lee*,†,‡ †

Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Korea Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701, Korea § Energy and Environmental Division, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, Korea ∥ Icheon Branch, Korea Institute of Ceramic Engineering and Technology, Icheon 467-843, Korea ⊥ Interface Control Research Center, Future Convergence Research Division, Korea Institute of Science and Technology, Seoul 136-791, Korea # Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea ‡

S Supporting Information *

ABSTRACT: ZnO, a wide bandgap semiconductor, has attracted much attention due to its multifunctionality, such as transparent conducting oxide, light-emitting diode, photocatalyst, and so on. To improve its performances in the versatile applications, numerous hybrid strategies of ZnO with graphene have been attempted, and various synergistic effects have been achieved in the ZnO−graphene hybrid nanostructures. Here we report extraordinary charge transport behavior in Al-doped ZnO (AZO)-reduced graphene oxide (RGO) nanocomposites. Although the most challenging issue in semiconductor nanocomposites is their low mobilities, the AZO−RGO nanocomposites exhibit single crystal-like Hall mobility despite the large quantity of nanograin boundaries, which hinder the electron transport by the scattering with trapped charges. Because of the significantly weakened grain boundary barrier and the proper band alignment between the AZO and RGO, freely conducting electrons across the nanograin boundaries can be realized in the nanocomposites. This discovery of the structurally nanocrystallineelectrically single crystalline composite demonstrates a new route for enhancing the electrical properties in nanocomposites based on the hybrid strategy. KEYWORDS: ZnO, reduced graphene oxide, nanocomposite, charge transport, mobility remarkable mechanical strength (Young’s modulus ∼1.0 TPa), excellent thermal conductivity (∼5000 W m−1 K−1), and electrical properties (charge carrier mobility ∼20 000 cm2 V−1 s−1).26−28 Moreover, its high flexibility and impermeability to small molecules enabled in situ observations of the growth of colloidal nanoparticles in graphene liquid cells.29,30 Furthermore, graphene is an important building block in nanotechnology, so that hybrid strategy of graphene with ZnO has provided us with the opportunities for enhancing its intrinsic properties of ZnO in diverse manners. Synergistic effects in photocatalytic,31−33 electrochemical,34−36 and optical properties37−39 have been successfully demonstrated in ZnO− graphene hybrid nanostructures. Actually, we reported a novel photoemission effect from ZnO−graphene hybrid quantum dots in white-light-emitting diodes.39 Because most of its

B

ecause of its diverse attractions from the fundamental nature to the practical applications, ZnO has received exceptional attention among numerous semiconducting oxide materials.1−3 ZnO’s controllable electrical conductivity with high transparency has led to the considerable progresses in transparent electronics,4−8 and its wide band gap with a large exciton binding energy has stimulated the advent of oxidebased optoelectronic era.9−11 Furthermore, ZnO is not only an earth abundant energy material for photovoltaic,12−14 thermoelectric,15−18 and piezoelectric power generation19−21 but also a nontoxic photocatalyst in various environmental technologies.22−24 Therefore, numerous strategies have been attempted to design and fabricate ZnO nanostructures endowed with the suitability to each purpose of their applications. Graphene, a two-dimensional atomic layer of sp2-hybridized carbon arranged in a hexagonal lattice, has also attracted tremendous attention as one of the most important materials today.25 Graphene has a zero band gap as a result of its unique electronic structure, and it shows a large specific surface area (∼2630 m2 g−1), high optical transmittance (∼97.7%), © XXXX American Chemical Society

Received: May 15, 2014 Revised: July 28, 2014

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multifunctional properties of ZnO are critically influenced by the behavior of its electrons, understanding the behavior of electrons in the ZnO−graphene hybrid system is of great significance to expand the opportunities in relevant applications. Herein, we report single crystal-like charge transport properties in nanocomposites comprised of 2 mol % Aldoped ZnO (AZO) and reduced graphene oxide (RGO). Despite the large quantity of nanograin boundaries, the Hall mobilities in the nanocomposites were compatible to the values in ZnO single crystals and epitaxial thin films. The effects of the grain boundary scattering, which limits the mobility in ZnO nanocomposites, could not be observed due to the significantly weakened grain boundary barrier and the proper band alignment between RGO and ZnO. Our results not only show the way to the improvement in the electrical properties through the ZnO−graphene hybrid strategy but also demonstrate the significant role of graphene in the interface control of the hybrid nanocomposite for relevant applications. Structurally Nanocrystalline AZO−RGO Nanocomposites. The experimental procedure for the AZO−RGO nanocomposites is schematically illustrated in Figure 1. The Figure 2. Microstructural characterization of the AZO−RGO nanocomposites. Bright-field transmission electron microscope (TEM) micrographs of (a) the AZO nanoparticles and (b) the AZO−RGO hybrid powder. Geometric average particle size of the AZO nanoparticle was ∼90 nm. Black arrows in b indicate the RGO coated on the AZO nanoparticles. (c) A bright-field TEM micrograph of the AZO−2 wt % nanocomposite and (d) a high-resolution TEM micrograph taken from the box region in c. RGO at the grain boundaries in the AZO−RGO nanocomposite is marked by black arrows in c and d.

Figure S2−S4). The geometric average size of the AZO grains was ∼200 nm, around two times larger than that of the asprepared AZO nanoparticles (∼90 nm) because of inevitable grain growth during the SPS process. When we consolidated the same AZO nanoparticles (AZO−0 wt % nanocomposite) without the RGO in our previous report, the AZO nanocomposite exhibited almost the same average grain size and a similar relative density (∼94%).18 Furthermore, in both cases a tiny amount of ZnAl2O4 nanoprecipitates was produced during the SPS process regardless of the incorporation of the RGO (Supporting Information Figure S5).18 Therefore, the presence of the RGO does not considerably influence the microstructure of the AZO−RGO nanocomposites under this experimental condition. Electrically Single Crystalline AZO−RGO Nanocomposites. Figure 3 shows the room-temperature Hall mobilities and resistivities of the AZO−RGO nanocomposites in comparison with those in AZO nanocomposites,17,18 single crystals,40−43 and epitaxial thin films at room temperature.44−47 The mobilities of the AZO nanocomposite, which was prepared by the same AZO nanoparticles in our previous report, was ∼13 cm2 V−1 s−1,18 and it is far below the mobilities of single crystalline ZnO. Jood et al. also observed similar mobilities in their AZO nanocomposites as marked by a dashed box, indicating that grain boundary scattering is the limiting factor of the mobility in the nanocomposites.18 However, the electrical properties of the AZO nanocomposite were significantly enhanced by the incorporation of the RGO, and the mobility was quite compatible to those in single crystalline ZnO.

Figure 1. Schematic of the experimental procedure for the AZO− RGO nanocomposites. The experimental procedure for the AZO− RGO nanocomposites is schematically illustrated. (a) Preparation of AZO nanoparticles by a solution method graphene oxide (GO) by a modified Hummers method. (b) Coating and chemical reduction of the GO on the AZO nanoparticle and (c) resulting AZO−RGO hybrid powder. (d) Consolidation of the AZO−RGO hybrid powder by spark plasma sintering. (e) Characterization of the nanocomposites.

AZO−RGO nanocomposites were prepared through the consolidation of the AZO nanoparticles (Figure 2a) coated with the RGO (AZO−RGO hybrid powder, Figure 2b) using spark plasma sintering (SPS), and relative density of the nanocomposites was 90−91%. As marked by the arrows in Figure 2c (AZO−2 wt % RGO nanocomposite), the RGO was well dispersed at the grain boundaries in the nanocomposites without significant segregation. The corresponding interfacial structure of the AZO−2 wt % RGO nanocomposite can be more clearly seen in Figure 2d. The uniform coverage of the RGO in the nanocomposite was also evidenced by using spectroscopy mapping techniques (Supporting Information B

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the carrier concentration, Ni is the concentration of ionized impurities, and Fii(ξ) is a screening fuction.49 The μtotal values of single crystalline ZnO calculated using eq 1 are plotted as a function of carrier concentration (green line) in Figure 3a. The mobility values of 57.0 cm2 V−1 s−1 for AZO− 1 wt % RGO nanocomposite and 55.8 cm2 V−1 s−1 for AZO−2 wt % RGO nanocomposite lie very close to the values predicted by eq 1. In the absence of the grain boundary, the electron mobility in single crystalline ZnO is primarily determined by ionized impurity scattering at room temperature. In the case of polycrystalline ZnO, the electron transport is strongly hindered by the Schottky barrier arising from the trapped electrons at the grain boundaries,50,51 so that the high density of grain boundaries in the AZO nanocomposite should lead to a drastic increase in the resistivity of the AZO nanocomposite, as shown in Figure 3b. In this work, the electrons in the AZO−RGO nanocomposites move across the grain boundaries as freely as they are in the single crystalline ZnO and the mobility was not significantly affected by the grain size (Supporting Information Figure S7). Therefore, the introduction of the RGO into the AZO nanocomposite changes the electron transport behavior from nanocrystalline to single crystalline character. Temperature-Dependent Charge Transport Properties. To understand the charge transport in the AZO−RGO nanocomposites in detail, high-temperature Hall (HT-Hall) measurements were carried out. Figure 4 shows the temperature-dependent carrier concentrations and mobilities in the AZO−RGO nanocomposites in comparison with those of the

Figure 3. AZO−RGO nanocomposites exhibited single crystal-like charge transport behavior. (a) Mobilities and (b) resistivities of the AZO−RGO nanocomposites as a function of carrier concentration at room temperature. For comparison, room-temperature mobilities and resistivities of AZO nanocomposite,17,18 ZnO single crystals,40−43 and epitaxial thin films are also shown.44−47 Dashed box in panel a indicates the mobilities of the AZO nanocomposite without RGO. Green curves in panels a and b represent the calculated mobility and resistivity based on BHD model, respectively.

In single crystalline ZnO, the total mobility (μtotal) is determined by lattice vibration mobility (μlat) and ionized impurity mobility (μ ii ), and it can be expressed by Matthiessen’s rule μ lat μii μtotal = μ lat + μii (1) μlat is known to be 210 cm2 V−1 s−1 at room temperature,48 and μii can be calculated by Brooks−Herring−Dingle (BHD) model as reviewed by Ellmer μii =

3(εrε0)2 h3 n 1 Z2m*2e 3 Ni Fii(ξ)

(2)

ξ 1+ξ

(3)

where Fii = ln(1 + ξ) −

and ξ = (3π 2)1/3

εrε0h2n1/3 m*e 2

Figure 4. Temperature-dependent charge transport properties of the AZO−RGO nanocomposites. Temperature-dependent (a) carrier concentrations, (b) mobilities of the AZO−RGO nanocomposites in comparison with the AZO nanocomposite.18 The inset in b shows the temperature-dependent electrical conductivities of the nanocomposites.

(4)

Here, ε0 is permittivity of vacuum, εr is relative permittivity for ZnO, h is Plank constant, Z is the charge of the ionized impurities, m* is the effective mass, e is elementary charge, n is C

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AZO nanocomposite from our previous report.18 The carrier concentrations in the AZO nanocomposite and the AZO− RGO nanocomposites (Figure 4a) were not dependent on temperature, indicating that all the nanocomposites were degenerately doped regardless of the presence of the RGO. It should be noted that the carrier concentration (n ∼ 2.4 × 1020 cm−3) in the AZO−2 wt % RGO nanocomposite is around 3 times larger than that in the AZO nanocomposite (∼0.7 × 1020 cm−3), even though both the nanocomposites were prepared using the same AZO nanoparticles. Because the intrinsic carrier concentration in the RGO is far below the increment of the carrier concentration (∼1.4 × 1020 cm−3) in the AZO−RGO nanocomposites,52 it is obvious that the RGO in the nanocomposites is not the only source of the increased electron concentration. Although the origin of the increment of the carrier concentration is not yet fully understood, two possible reasons can be proposed. The first of these is related to Gupta’s ZnO grain boundary model.53 In polycrystalline ZnO, electrons trapped at the grain boundaries build up an electrostatic potential barrier known as Schottky barrier. These trapped electrons cannot contribute to the electrical conduction but cause scattering at the boundaries that limits the overall mobility. Therefore, it can be reasonably proposed that the trapped electrons at the defective interfaces of the AZO nanograins are released by the RGO. In this case, both the carrier concentration and mobility should increase together in the AZO−RGO nanocomposites due to the reduced or eliminated Schottky barriers. The next is related to the incorporation of carbon into the AZO lattice during the sintering process, and it has been reported that carbon can behave as a donor in ZnO crystal lattice.54 Figure 4b shows the temperature-dependent Hall mobilities of the AZO−RGO nanocomposites. The mobilities in AZO− RGO nanocomposites were considerably higher than that in the AZO nanocomposite in all temperature ranges, and they exhibited different temperature dependence. The mobilities in the AZO−RGO nanocomposites monotonically decreased with the increase in temperature, while that in the AZO nanocomposite exhibited positive temperature dependence, indicating the thermally activated process to overcome the Schottky barrier at the grain boundary.18 This result clearly indicates that the electron transport in the AZO−RGO nanocomposites is not limited by the grain boundary scattering. As a result, electrical conductivities in the AZO−RGO nanocomposites (inset in Figure 4b) are considerably enhanced compared to that in the AZO nanocomposite. Two possible models can be suggested as the origin of the enhanced mobility: one is due to the negligible Schottky barrier by the insertion of the RGO at the grain boundary, and the other is due to the dominant conduction through the high mobility path of the RGO, which will be found to be false later. Because the RGO can release the electrons from the defective regions of the grain boundaries as proposed in Figure 4a, it can lead to the lowering of the Schottky height. Furthermore, because the Dirac point of graphene (WRGO ∼ 4.46 eV)55 locates in the close proximity to the electron affinity of ZnO (χZnO ∼ 4.5 eV),2,56 and because all the nanocomposites are degenerately doped (EF > EC), the Fermi level in RGO should raise up to the EF, leading to the increase of the electron Fermi liquid as schematically drawn in Figure 5.57,58 Detailed calculation for the Fermi level is shown in Supporting Information Figure S8. In this case, electron transport across the grain boundaries could be free from the grain boundary

Figure 5. Band alignment of the AZO−RGO nanocomposites. A schematic band alignment of the AZO−RGO nanocomposites (χZnO, electron affinity of ZnO; W, Dirac point of RGO; EF, Fermi level; EC, conduction band of ZnO; EV, valence band of ZnO; EVac, vacuum level). ΔEF = EF − EC in AZO−1 wt % RGO nanocomposite and AZO−2 wt % RGO nanocomposite were determined to be 381 and 424 meV, respectively (Supporting Information Figure S8).

scattering. Therefore, the former is in a good agreement with the simultaneous increases both in the carrier concentration and mobility in AZO−RGO nanocomposites in Figure 4. This model can also be manifested by the I−V characteristics of the AZO and AZO−RGO nanocomposites in Supporting Information Figure S9. Trap-controlled space charge limited current, which typically originating from the Shottky barrier, was observed only in the AZO nanocomposite. On the other hand, AZO−RGO nanocomposites obviously exhibited the ohmic conduction, which indicating the freely conducting electrons across the grain boundaries. Dominant Conduction Path in AZO−RGO Nanocomposites. To elucidate the dominant conduction path in the AZO−RGO nanocomposites, the effective mass of electron at the Fermi level, that is, density of state (DOS) effective mass (md*) was determined as shown in Figure 6. In this experiment, the DOS effective mass in the AZO−RGO nanocomposites were estimated by using Pisarenko relation S=

2/3 8π 2k 2T *⎛ π ⎞ md ⎜ ⎟ 2 ⎝ 3n ⎠ 3eh

(5)

where S is Seebeck coefficient and k is Boltzmann constant. Basically, the Pisarenko relation is valid in parabolic band where the DOS effective mass is not dependent on the carrier concentration. However, the carrier concentration in each nanocomposite is independent of temperature as shown in Figure 4a, so that the Seebeck coefficient is linearly proportional to the constant DOS effective mass within the temperature range despite of the nonparabolic band in ZnO. From the temperature-dependent carrier concentration in Figure 3a and the Seebeck coefficient (inset in Figure 6a), we could determine the DOS effective mass from the slope of S × n2/3 versus T, as shown in Figure 6a. This approach has been successfully applied to evaluate the effective mass (∼0.33 me) in D

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sufficiently suppressed by introducing the graphene in the AZO nanocomposite. We believe that this discovery of the electrically single crystalline nanocomposite could encourage the extension of the hybrid nanocomposite technology to diverse practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, additional figures, and table. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.S.L.). *E-mail: [email protected] (J.Y.L.). Author Contributions

W.H.N., B.B.K., and S.G.S. performed the experiments and the measurements. W.H.N., B.B.K., Y.S.L., J.-Y.K., and W.-S.S. carried out detail data analysis. W.H.N. and Y.S.L. wrote the manuscript. W.K.C., H.-H.P., and J.Y.L. contributed with discussion and commented on the manuscript. All the authors shared ideas, contributed to the interpretation of the results. Author Contributions ∇

W.H.N. and B.B.K. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Institute for Basic Science (IBS) [IBS-R004-G3-2014-a00] and also supported by a grant from Korea Institute of Ceramic Engineering and Technology. Y.S.L. also acknowledges the support from the Nano·Material Technology Development Program (2011-0030147) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Republic of Korea.

Figure 6. Calculation of DOS effective mass of the AZO−RGO nanocomposites. (a) A plot of S × n2/3 versus T (cf. eq 5) for determining the DOS effective mass in the nanocomposites. The inset in a shows the temperature-dependent Seebeck coefficient of the AZO−RGO nanocomposites in comparison with the AZO nanocomposite.18 (b) DOS effective masses of the AZO−RGO nanocomposites and the AZO nanocomposite.18 The symbol (+) denotes the DOS effective masses in doped ZnO thin films reported by Kim et al.59

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REFERENCES

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dx.doi.org/10.1021/nl5018089 | Nano Lett. XXXX, XXX, XXX−XXX