www.advmat.de www.MaterialsViews.com
Yunfan Guo, Li Lin, Shuli Zhao, Bing Deng, Hongliang Chen, Bangjun Ma, Jinxiong Wu, Jianbo Yin, Zhongfan Liu,* and Hailin Peng* Transparent electrodes that both require high optical transmittance and low electrical resistance are essential components for numerous optoelectronic devices, such as touch screens, solar cells, and smart windows.[1,2] Over the past decades, indium tin oxide (ITO)[3] has been the prototypical material in the field of transparent electrode. However, the weakness of ITO, including brittle nature, scarce abundance limit, and low transmittance in infrared region may hinder its further use in the next generation of flexible electronic devices. Emerging nanomaterials, for example, carbon nanotubes,[4,5] graphene,[6,7] metal nanowires (NWs),[8,9] topological insulators,[10,11] and some hybrid nanostructures[12,13] are expected to be the strong competitors to ITO. Among these new alternatives, graphene with Dirac fermons is one of the most promising electrode materials owing to its excellent charge carrier mobility, high optical transmittance, and mechanical flexibility. However, typically obtained chemical vapor deposition (CVD) graphene is a polycrystalline film, consisting of many single-crystalline grains separated by synthesis-related grain boundaries, which adversely degrade graphene transport through the scattering of charge carriers.[14] In order to improve the performance of graphene-based transparent electrodes, a variety of efforts have been taken, including preparing graphene films with larger domains and fewer boundaries,[15,16] chemical doping by strong acid,[17] organic molecules,[18,19] or nanoparticles,[20] and constructing hybrid nanostructures with random 1D conductive materials, for example, metal NWs[21,22] or carbon nanotubes.[23,24] Even though the last two post-treatments have made a desirable progress for graphene-based transparent electrodes, some thorny problems such as chemical instability,[25] high junction resistance,[26,27] and light scattering[28,29] are urgent to be solved. As a new generation of Dirac materials, topological insulators (TIs) with insulating bulk gap and metallic surface states have garnered extensive attention in recent years.[30–32] In analogy to another Dirac material graphene, topological insulator nanostructures with the helical Dirac surface states have been theoretically predicted to be promising candidate for
Y. Guo, L. Lin, S. Zhao, B. Deng, H. Chen, B. Ma, J. Wu, J. Yin, Prof. Z. Liu, Prof. H. Peng Center for Nanochemistry Beijing Science and Engineering Center for Nanocarbons Beijing National Laboratory for Molecular Sciences College of Chemistry and Molecular Engineering Peking University Beijing 100871, P. R. China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201501912
Adv. Mater. 2015, 27, 4315–4321
efficient optoelectronic devices, such as infrared photodetectors, terahertz lasers, and transparent electrodes.[33] In contrast with traditional ITO, our previous experimental works have demonstrated that transparent electrodes based on topological insulator nanostructures exhibit outstanding broadband transmittance especially in near-infrared region,[10,11] together with the strong robustness against mechanical bending and chemical degradation. However, the electrical performance of topological electrodes has yet to be improved. Herein, we first experimentally realized 2D hybrid Dirac materials between topological insulator Bi2Se3 and graphene film for transparent electrode by a facile CVD method. Bi2Se3 nanoplates with high crystal quality and well morphology are preferentially grown along graphene boundaries on copper foil substrate, which can be used as a simple protocol to visualize graphene boundaries on a large scale.[34,35] The metallic surface states of Bi2Se3 may bridge graphene grain boundaries and facilitate the transport of charge carriers (Figure 1a,b). Our electrical measurements show that the sheet resistance of graphene boundary area was improved with the decoration of TI nanoplates, and the conductivity is comparable with other graphene-based hybrid nanostructures.[12,36] On the other hand, the electrical improvement in TI/graphene hybrid film does not incur significant degradation in transparency, which maintains a flat and high transmittance in broadband wavelength. Besides remarkable chemical stability, TI/graphene hybrid film has outstanding mechanical flexibility. Layered topological insulator Bi2Se3 possesses a rhombohedral crystal structure in the space group D53d (R-3m), with the lattice constant a = 4.140 Å, c = 28.636 Å. Each planar quintuple layer (QL, ≈1 nm in thickness) is ordered in Se–Bi–Se–Bi–Se sequence along the c-axis, connecting together by weak van der Waals interaction. Graphene, a single layer of sp2-bonded carbon atoms, has a honeycomb lattice structure with the lattice constant a = 2.46 Å, and the hexagonal periodicity of the (0001) surface d = 4.26 Å. Although the lattice mismatch is 2.9% between graphene and Bi2Se3, the use of van der Waals epitaxy may efficiently relax the lattice-matching condition[37] and facilitate the large-scale production of Bi2Se3/graphene hybrid nanostructures.[38–41] Importantly, defective graphene boundaries have enhanced chemical reactivity to easily absorb heteroatoms during van der Waals epitaxy.[42] Therefore, Bi2Se3 nanoplates can preferentially nucleate along the grain boundaries of graphene films in very low vapor super-saturation condition, and finally form “smart” conductive patches to bridge graphene grain boundary area (Figure S1, Supporting Information). Figure 1c shows a typical scanning electron microscopy (SEM) image of Bi2Se3/graphene hybrid nanostructures. Bi2Se3 nanoplates were successfully grown along graphene grain
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
2D Hybrid Nanostructured Dirac Materials for Broadband Transparent Electrodes
4315
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. Schematic diagram and morphology characterizations for TI/graphene hybrid electrodes. a) Schematic diagram to interpret electron current in polycrystalline graphene with boundaries. Both arrows indicate current flow. b) Schematic diagram to interpret Bi2Se3 nanoplates bridging graphene boundaries and making charge carriers to flow through. c,d) Typical scanning electron microscopy images of Bi2Se3/graphene hybrid nanostructures grown on copper foil substrates. Graphene boundaries can be visualized by the decoration of Bi2Se3 nanoplates. d) Hybrid films grown for different time. Scale bar: 2 µm. e) Raman spectra of Bi2Se3/graphene hybrid nanostructures transferred on SiO2/Si substrate using green (514 nm) laser excitation. Inset: typical optical microscopy image of Bi2Se3/graphene hybrid film transferred on SiO2/Si substrate. Scale bar: 50 µm. f) Photograph and SEM images of Bi2Se3/roll-to-roll graphene hybrid films on copper foil substrate. Scale bar: 20 µm. g) Photograph of a large-area and transparent Bi2Se3/ roll-to-roll graphene hybrid electrode on PET substrate. The outline of TI/graphene hybrid electrode on the PET substrate is rendered by red dashed lines. Scale bar: 2 cm.
boundaries and make graphene grains visualized. This method could be used as a simple protocol to image graphene grain boundaries on a large scale.[34,35] Note that the density of Bi2Se3 nanoplates can be adjusted by changing growth time, Figure 1d shows the SEM images of graphene decorated by Bi2Se3 nanoplates with growth time varying from 3 to 8 min. With longer growth time, more Bi2Se3 nanoplates nucleated on graphene boundaries and grown up to hundreds of nanometers in lateral size, making these “boundary lines” more continuous. Raman spectroscopy is a powerful tool to characterize both graphene and Bi2Se3. Figure 1e shows the Raman spectrum ranging from 80 to 3000 cm−1 at 514 nm laser excitation for Bi2Se3/graphene hybrid film on SiO2/Si substrate (inset of Figure 1e). At the low frequency region, two characteristic peaks of Bi2Se3 at 131 (Eg2) and 174 cm−1 (A1g2) correspond to the in-plane vibrational mode and the out-of-plane Se–Bi–Se– Bi–Se lattice vibrational mode, respectively, consistent with that of Bi2Se3 bulk crystal.[43] At higher frequency region, two typical graphene peaks at ≈1600 (G band) and ≈2705 cm−1 (2D-band) are observed, assigned to the stretching of C C bond and a second-order two-phonon process in sp2 carbon systems.[44] In addition, TI/graphene hybrid nanostructures can be further scaled up in batch production by using larger copper foil substrates or dynamic roll-to-roll process (Figure 1f). As shown in Figure 1g, with lamination transfer approach,[45] the large-area
4316
wileyonlinelibrary.com
hybrid films can be transferred onto polyethylene terephthalate (PET) plastic substrate directly, showing its high transparency. The quality and nucleation site of Bi2Se3 nanoplates on graphene films are crucial for electrical performance. Bi2Se3 nanoplates with metallic surface states may patch up graphene boundaries and reduce carrier scattering for higher conductivity. To this end, we used transmission electron microscopy (TEM) and selected area electron diffraction (SAED) to determine the structure and quality of TI/graphene hybrid electrodes. As shown in Figure 2a, graphene grains were recognized through the slight contrast from carbon support film, surrounded by Bi2Se3 nanoplates. From the close-up view, triangular and hexagonal nanoplates aligned along nearly identical orientations are observed (Figure 2b). Energy dispersive X-ray (EDX) analyses reveal that Bi2Se3 nanoplates grown on graphene remain the Bi:Se atomic ratio of 2:3, indicating stoichiometric Bi2Se3 free of detectable impurities during the experimental process (Figure S2, Supporting Information). The high-resolution TEM (HRTEM) image of Bi2Se3 nanoplate shows the expected hexagonal lattice with a lattice spacing of 2.1 Å (Figure 2c), agreeing well with the spacing of the (11–20) planes of layered Bi2Se3. The corresponding fast Fourier transformation (FFT) patterns verify the single crystalline nature of Bi2Se3 nanoplates (inset of Figure 2c). To further study the nucleation sites of Bi2Se3 nanoplates, extensive SAED patterns were captured with ≈200 nm
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 4315–4321
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, 27, 4315–4321
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
respectively (Figure S3b, Supporting Information). It is worth noting that the three middle patterns taken from the gap between Bi2Se3 nanoplates are consistent well with the patterns from two adjacent graphene grains, which indicated that Bi2Se3 nanoplates are predominantly deposited on graphene’s grain boundaries. In addition to highly crystalline structure, TI/graphene hybrid films have remarkable transmittance from the visible to near-infrared region. Both pristine graphene film and TI/graphene hybrid films were transferred onto transparent quartz substrates, showing good visible transparency (Figure 3a). For further evaluation, ultraviolet-visible-near infrared (UV–vis-NIR) spectroscopy of TI/ graphene hybrid films has been conducted (Figure 3b). The hybrid film grown for 3 min exhibits a flat and high transmittance of about 96.0% at the wavelength of 550 nm. With longer growth time, the corresponding T% slightly reduces to 95.2%, for the fact that the “boundary lines” are more continuous (Figure 3c). In comparison with monolayer graphene film, the introduction of Bi2Se3 nanoplates in hybrid films does not give rise to significant decrease in specular transmittance, which is only 1%–2% in the range of 400–2000 nm wavelength. On the other hand, the diffusive transmittance mode should not be ignored (insets of Figure 3e). Different from normal specular mode, diffuse transmittance not only collects the light coming out of the sample parallel to the incident light, but includes all forward scattered light by using an integrating sphere. As shown in Figure 3d, we measured diffusive transmittance for TI/ graphene hybrid films systematically. The difFigure 2. TEM studies for TI/graphene hybrid electrodes. a) Low-magnification TEM image ference between diffusive T% and specular of a graphene grain decorated by Bi2Se3 nanoplates. The graphene boundaries are rendered by white dashed lines. The white dashed arrow exhibits the direction along which SAEDs are T% is only about 1%, indicating the slight performed. The nine circles present positions where SAED patterns in d–f) are collected. Scale light scattering (Figure 3e). However, for Ag bar: 2 µm. b) Magnified TME image of the black square in (a). Scale bar: 200 nm. c) HRTEM NWs electrodes,[28,29] the difference in two image of Bi2Se3 nanoplates, showing the highly crystalline nature of the basal plane of Bi2Se3 transmittance modes can be up to ≈10%, nanoplates. Scale bar: 2 nm. Inset: corresponding fast Fourier transformation (FFT) pattern. which would be a thorny problem for its use (d)–(f) SAED patterns of specified spots in (a). The number labeled the angle along the arrow in displays and touch screens. pair in each panel. Scale bar: 5 1/nm. Experimental and theoretical studies reported that graphene grain boundaries would impede electrical transport.[46–48] To evaluate the influence aperture along the boundary of two adjacent graphene domains in Bi2Se3/graphene hybrid films. Figure 2d–f shows nine typical of Bi2Se3 on graphene boundary conductance, we performed in SAED patterns, taken from the orders labeled along the white situ electrical transport measurement. Figure 4a shows a repredashed arrow in Figure 2a. The first three patterns colored in sentative device fabricated on two adjacent graphene grains that blue cycles and last three ones in black together demonstrate coalesced at a single grain boundary. Multiple electrodes are patthe single-crystalline nature of two individual graphene grains. terned to allow simultaneous transport measurements both for The thickness of graphene samples was further verified to be the area with Bi2Se3 nanoplates and the one without. The elecmonolayer by analyzing line profile of diffraction patterns in trical measurement in Figure 4b shows both the current–voltage Figure S3a, Supporting Information. And the histogram of pat(I–V) curves are linear, indicative of the ohmic contact between tern orientation distributions from extensive SAED data of two graphene stripes and metal contact. The resistances extracted grains exhibited two pronouncing peaks at ≈14.2° and ≈50.1°, from the slopes of I–V curves are RG ≈ 7.5 kΩ sq−1 (R6,7),
4317
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. Spectroscopy characterization for TI/graphene hybrid electrodes. a) Photograph of a pristine graphene film, TI/graphene hybrid films with different growth time on quartz substrates, showing their good visible light transparency. b) Specular transmittance spectrum of graphene and TI/ graphene hybrid electrodes with different growth time over a broadband wavelength. c) Plot of specular transmittance of TI/graphene hybrid films versus growth time at the wavelength of 475, 550, and 850 nm, respectively. d) Diffusive and specular transmittances of ITO, CNT, Ag NWs, graphene/ Ag NW hybrid nanostructures and TI/graphene hybrid nanostructures on quartz substrates. e) The difference in diffusive transmittance and specular transmittance, indicating the light scattering by different materials. Inset: schematic diagrams of two different transmittance modes.
RBi2Se3/G ≈ 4.3 kΩ sq−1 (R2,3), respectively. Our results clearly show that Bi2Se3 nanoplates with the metallic surface states can bridge the boundary of coalesced graphene grains and thus improve the carrier transport. To evaluate the macroscopic conductivity, photolithography was used to fabricate electrode arrays on both pristine graphene films and TI/graphene hybrid films. Typical optical microscopy (OM) images are shown in the insets of Figure 4c, with chromium/gold (5/50 nm) as contact metal on SiO2/Si substrates. I–V curves for both films on a large scale keep linear and symmetric (Figure 4c). As illustrated in Figure 4d, the statistic sheet resistance (Rs) of hybrid films is obviously lower than that of graphene. Figure 4e shows the histograms of sheet resistance distributions for hybrid electrodes with different growth time. The average sheet resistance of TI/graphene hybrid electrodes grown for 3, 5, and 8 min turns out to be ≈510, 430, and 300 Ω sq−1, respectively. Compared with the conductivity of graphene film, the sheet resistance (Rs) of hybrid films decreased dramatically by one- to threefold. However, the rising conductivity comes at the expense of decreased transparency. Figure 4f exhibits the sheet resistance of TI/graphene hybrid films against optical transmittance at the wavelength of 475, 550, and 850 nm, respectively, suggesting a trade-off relationship between electrical conductivity and optical performance. At 550 nm, the T% for hybrid films with the best conductivity (300 Ω sq−1) can reach 95.2%, which is comparable with the 4318
wileyonlinelibrary.com
performance for graphene/carbon nanotube (CNT) hybrid films[12] and graphene/Ag NWs (low density) hybrid nanostructures[36] (Figure 4g). From this point of view, the metallic surface states of Bi2Se3 may bridge the grain boundaries of polycrystalline graphene, and enhance the electrical performance of pristine graphene films without incurring significant decrease in optical transmittance. In addition to high optical transmittance and low sheet resistance, chemical stability is an indispensable requirement for transparent electrodes in actual applications. According to the previous works,[3,49–51] the performance of transparent electrodes might be weakened by a variety of factors such as chemical degradation and UV light illumination. For example, acrylic acid is commonly used in the assembling of ITO-based touch screen for optically clear adhesives. Low concentration acrylic acid solution may leach out over time and cause serious corrosion of indium in ITO, which would lead to device failure during the assembling.[3] UV illumination, the critical challenge for many kinds of transparent electrodes should not be neglected. Usually, strong oxidants such as NO2,[50] SOCl2,[51] and TCNQF4[49] were used to substantially dope graphene and carbon nanotube electrodes for higher conductivity. However, these dopants are easily detached under UV illumination, which may result in conductance degradation or cause defect in graphene and CNTs electrodes.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 4315–4321
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 4. Electrical characterization for TI/graphene hybrid electrodes. a) SEM image of a device with multiple electrodes (numbered 1–8) contacting two graphene grains (indicated by red and yellow dashed lines) coalesced in a single boundary (indicated by white dashed line). Scale bar: 5 µm. Inset: magnified SEM image of the boundary areas with and without Bi2Se3 nanoplates. Scale bar: 1 µm. b) Source–drain current (I) versus voltage (V) characteristics for graphene boundaries with and without Bi2Se3 nanoplates. c) Current (I) versus voltage (V) characteristics for graphene boundaries with and without Bi2Se3 nanoplates measured on a large scale. Upper left inset: an OM image of a pristine graphene strip. Lower right inset: an OM image of a TI/graphene hybrid film strip. Scale bar: 50 µm. d) Histograms of sheet resistance distributions for graphene films and hybrid films in (c). e) Histograms of sheet resistance distributions for TI/graphene hybrid films with different growth time. f) Sheet resistance as a function of optical transmittance at the wavelength of 475, 550, and 850 nm for TI/graphene hybrid films with different growth time. g) Plot of sheet resistance versus transmittance for different transparent electrode materials.
In comparison with traditional electrode materials, the topologically protected metallic surface states of Bi2Se3 might enable Bi2Se3/graphene hybrid film high chemical stability and strong robustness to environmental perturbations, for example, chemical agents and UV light. We have made a series of contrast tests to demonstrate the outstanding chemical stability for TI/graphene hybrid films. First, we immersed Bi2Se3/graphene hybrid film into acrylic acid (2%, v/v) for 20, 40, 60, 120, and 240 s. As shown in Figure 5a and Figure S4 (Supporting Information), after acid treatment, the sheet resistance of hybrid film slightly changes from 430 Ω sq−1 to 510, 517, 523, 530, and 545 Ω sq−1, respectively. These observations indicate the TI/graphene hybrid electrode is much stable against chemical acid. In addition, we exposed the Bi2Se3/graphene hybrid electrode directly into UV light for 30, 60, 90, 150, and 210 s. As shown in Figure 5b and Figure S5 (Supporting Information), the corresponding sheet resistance of Bi2Se3/graphene hybrid film is around 620, 625, 627, 630, and 635 Ω sq−1 after UV treatment, respectively, which is a little bit higher than before (510 Ω sq−1). Therefore, due to the presence of nontrivial robust metallic surface states of topological insulator Bi2Se3 nanoplates, the outstanding chemical stability of Bi2Se3/graphene hybrid electrodes in different environments guarantees its wide use in practical applications.
Adv. Mater. 2015, 27, 4315–4321
Mechanical durability is a significant feature for flexible transparent electrodes. Owing to the natural frangibility, the predominant material ITO is highly brittle, which hampers its further use in the next-generation of flexible optoelectronic devices. Besides improved electrical conductivity, broadband high transmittance, and outstanding chemical stability, Bi2Se3/ graphene hybrid electrodes display excellent mechanical durability. Figure 5c shows the variation in the ratio of ΔR to R0 both for ITO/PET and Bi2Se3/graphene hybrid electrodes on PET substrate. For ITO, the ratio of ΔR to R0 reaches 70% in the first 600 bending cycles. And after 800 cycles, the ΔR to R0 ratio is up to 100%, indicating the broken of ITO film. By contrast, the resistance variations of Bi2Se3/graphene hybrid electrodes on PET substrate are always below 5% during the first 900 cycles, and slightly increase to 10% after 2000 cycles, indicative of the outstanding mechanical durability. Figure 5d illustrates the resistance change of hybrid electrodes for different bending radius. Compared with the obvious resistance increase for ITO electrodes on PET, the conductance variation for Bi2Se3/ graphene hybrid electrodes on the same substrates is unnoticeable with the bending radius down to 3 mm. Topological surface/edge states of Bi2Se3 protected by time-reversal symmetry could remain highly conductive path for electrical transport when encountered defects and mechanical dislocations.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
4319
www.advmat.de
COMMUNICATION
www.MaterialsViews.com were grown on graphene films (copper substrate) by a catalyst-free vapor transport deposition inside a 12 in. horizontal tube furnace with a 1 in. diameter quartz tube. The source material Bi2Se3 power (Alfa Aesar, purity 99.999%) was placed in the center of a small tube (460–500 °C) for evaporation. Obtained monolayer graphene films were placed downstream at certain locations in the tube. Ultrapure argon was used as carrier gas to transport hot Bi2Se3 vapor to cold graphene films. After the tube was pumped to a low pressure about 10 mTorr and flushed with the carrier gas repeatedly to remove air contamination, the growth of Bi2Se3/graphene hybrid nanostructures began. Typical growth conditions were a pressure of 150 Torr, source temperature of 490 °C, growth time of 3–10 min, gas flow rate of 500 sccm (standard cubic centimeters per minute), and growth temperature is below 280 °C. Characterization: Characterization was conducted by using optical microscopy (Olympus DX5r1 microscope), SEM (Hitachi S-4800, acceleration voltage 5–30 kV), TEM (FEI Tecnai F30; acceleration voltage, 300 kV) and UV–vis-IR (Perkin Elmer Lambda 950 spectrophotometer), micro-Raman spectroscopy (Horiba HR800 Raman system) with 514.5 nm (2.41 eV) laser line from an Figure 5. Chemical and mechanical durability of TI/graphene hybrid electrodes. a) Histogram Ar+ laser. Electrical measurements were carried out of sheet resistance distributions of Bi2Se3/graphene hybrid films before and after 20 s acrylic in a Micromanipulator 6200 probe station with a acid treatment. b) Histogram of sheet resistance distributions of Bi2Se3/graphene hybrid films Keithley 4200 semiconductor analyzer. before and after 30 s ultraviolet light (UV) treatment. c) Resistance change with respect to Device Fabrication: TI/graphene hybrid films bending cycles for Bi2Se3/graphene hybrid films on PET substrates compared with ITO film on were first transferred onto a silicon substrate PET. d) Resistance change of Bi2Se3/graphene hybrid films for different bending radius. with silicon oxide (300 nm) as dielectric layer. SEM was used to identify specific regions, while the single graphene boundary with and without In combination with the inherent flexibility of graphene films, Bi2Se3 nanoplates was preferred. Standard electron beam lithography robust Bi2Se3/graphene hybrid films hold promise for dissipa(EBL; STRATA DB 235, FEI) was performed to define micropatterns, and photolithography was carried out for the four-probe array devices tionless interconnects and novel flexible transparent electrodes. fabrication. Designed graphene strips and Bi2Se3/graphene hybrid In conclusion, we have presented the first experimental strips were shaped by plasma etching. Afterward, bilayer metal realization of high performance transparent electrode based on electrodes (5 nm Cr/50 nm Au) were deposited by thermo evaporation hybrid Dirac materials between Bi2Se3 nanoplate and graphene. (UNIVEX 300, Leybold Vacuum). At last, the device was lifted-off by hot In comparison with the conductivity of pristine graphene acetone and blow dried with nitrogen gas. films, the electrical performance for Bi2Se3/graphene hybrid Transport Measurement: A semiconductor analyzer (Keithley, SCS4200) was used to measure the four terminal electrical properties. electrodes increased dramatically by to threefold, presumably
because the metallic surface states of Bi2Se3 nanoplates could bridge graphene boundaries and thus improve the electrical conductivity of graphene. In addition, the Bi2Se3/graphene hybrid films have high broadband transmittance, outstanding chemical stability, and excellent mechanical durability. These exotic properties among topological insulator Bi2Se3/graphene hybrid electrodes suggest exciting prospects for future flexible optoelectronics, dissipationless interconnects, and novel electronic applications.
Experimental Section Synthesis of Bi2Se3/Graphene Hybrid Nanostructures: Two types of monolayer graphene samples were employed in this work: graphene films grown by normal chemical vapor deposition method on small pieces of laboratory copper foils (Alfa-Aesar stock #46365) and by dynamic rollto-roll process on large-area industrial electrolytic copper foils (Suzhou Fukuda Metal Co., LTD). CVD growth of graphene was carried out at temperatures up to 1000 °C by using a mixture of 10 sccm CH4 and 10 sccm H2 at a total pressure of 500 mTorr (30 Pa). Bi2Se3 nanoplates
4320
wileyonlinelibrary.com
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the National Basic Research Program of China (Grant Nos. 2014CB932500, 2011CB921904, and 2013CB932603), the National Natural Science Foundation of China (Grant Nos. 21173004, 21222303, 51121091, and 51362029), National Program for Support of Top-Notch Young Professionals, and Beijing Municipal Science & Technology Commission (No. Z131100003213016).
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: April 22, 2015 Revised: May 12, 2015 Published online: June 16, 2015
Adv. Mater. 2015, 27, 4315–4321
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, 27, 4315–4321
[26] E. C. Garnett, W. S. Cai, J. J. Cha, F. Mahmood, S. T. Connor, M. G. Christoforo, Y. Cui, M. D. McGehee, M. L. Brongersma, Nat. Mater. 2012, 11, 241. [27] M. S. Fuhrer, J. Nygard, L. Shih, M. Forero, Y. G. Yoon, M. S. C. Mazzoni, H. J. Choi, J. Ihm, S. G. Louie, A. Zettl, P. L. McEuen, Science 2000, 288, 494. [28] R. Y. Chen, S. R. Das, C. Jeong, M. R. Khan, D. B. Janes, M. A. Alam, Adv. Funct. Mater. 2013, 23, 5150. [29] L. B. Hu, H. S. Kim, J. Y. Lee, P. Peumans, Y. Cui, ACS Nano 2010, 4, 2955. [30] H. J. Zhang, C. X. Liu, X. L. Qi, X. Dai, Z. Fang, S. C. Zhang, Nat. Phys. 2009, 5, 438. [31] X. L. Qi, S. C. Zhang, Rev. Mod. Phys. 2011, 83, 1057. [32] M. Z. Hasan, C. L. Kane, Rev. Mod. Phys. 2010, 82, 3045. [33] X. A. Zhang, J. Wang, S. C. Zhang, Phys. Rev. B 2010, 82, 245170. [34] D. L. Duong, G. H. Han, S. M. Lee, F. Gunes, E. S. Kim, S. T. Kim, H. Kim, Q. H. Ta, K. P. So, S. J. Yoon, S. J. Chae, Y. W. Jo, M. H. Park, S. H. Chae, S. C. Lim, J. Y. Choi, Y. H. Lee, Nature 2012, 490, 235. [35] D. W. Kim, Y. H. Kim, H. S. Jeong, H. T. Jung, Nat. Nanotechnol. 2012, 7, 29. [36] H. O. Choi, D. W. Kim, S. J. Kim, S. B. Yang, H. T. Jung, Adv. Mater. 2014, 26, 4575. [37] W. H. Dang, H. L. Peng, H. Li, P. Wang, Z. F. Liu, Nano Lett. 2010, 10, 2870. [38] L. Z. Kou, F. M. Hu, B. H. Yan, T. Frauenheima, C. F. Chen, Nanoscale 2014, 6, 7474. [39] L. Z. Kou, F. M. Hu, B. H. Yan, T. Wehling, C. Felser, T. Frauenheim, C. F. Chen, Carbon 2015, 87, 418. [40] L. Z. Kou, B. H. Yan, F. M. Hu, S. C. Wu, T. O. Wehling, C. Felser, C. F. Chen, T. Frauenheim, Nano Lett. 2013, 13, 6251. [41] I. Popov, M. Mantega, A. Narayan, S. Sanvito, Phys. Rev. B 2014, 90, 035418. [42] K. Kim, H. B. R. Lee, R. W. Johnson, J. T. Tanskanen, N. Liu, M. G. Kim, C. Pang, C. Ahn, S. F. Bent, Z. N. Bao, Nat. Commun. 2014, 5, 781. [43] J. Zhang, Z. P. Peng, A. Soni, Y. Y. Zhao, Y. Xiong, B. Peng, J. B. Wang, M. S. Dresselhaus, Q. H. Xiong, Nano Lett. 2011, 11, 2407. [44] M. S. Dresselhaus, A. Jorio, M. Hofmann, G. Dresselhaus, R. Saito, Nano Lett. 2010, 10, 751. [45] L. G. P. Martins, Y. Song, T. Y. Zeng, M. S. Dresselhaus, J. Kong, P. T. Araujo, Proc. Natl. Acad. Sci. USA 2013, 110, 17762. [46] A. W. Cummings, D. L. Duong, V. L. Nguyen, D. V. Tuan, J. Kotakoski, J. E. B. Vargas, Y. H. Lee, S. Roche, Adv. Mater. 2014, 26, 5079. [47] O. V. Yazyev, Y. P. Chen, Nat. Nanotechnol. 2014, 9, 755. [48] Q. K. Yu, L. A. Jauregui, W. Wu, R. Colby, J. F. Tian, Z. H. Su, H. L. Cao, Z. H. Liu, D. Pandey, D. G. Wei, T. F. Chung, P. Peng, N. P. Guisinger, E. A. Stach, J. M. Bao, S. S. Pei, Y. P. Chen, Nat. Mater. 2011, 10, 443. [49] T. Takenobu, T. Kanbara, N. Akima, T. Takahashi, M. Shiraishi, K. Tsukagoshi, H. Kataura, Y. Aoyagi, Y. Iwasa, Adv. Mater. 2005, 17, 2430. [50] J. Kong, N. R. Franklin, C. W. Zhou, M. G. Chapline, S. Peng, K. J. Cho, H. J. Dai, Science 2000, 287, 622. [51] V. Skakalova, A. B. Kaiser, U. Dettlaff-Weglikowska, K. Hrncarikova, S. Roth, J. Phys. Chem. B 2005, 109, 7174.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
[1] S. P. Pang, Y. Hernandez, X. L. Feng, K. Mullen, Adv. Mater. 2011, 23, 2779. [2] C. G. Granqvist, A. Azens, A. Hjelm, L. Kullman, G. A. Niklasson, D. Ronnow, M. S. Mattsson, M. Veszelei, G. Vaivars, Sol. Energy 1998, 63, 199. [3] D. S. Hecht, L. B. Hu, G. Irvin, Adv. Mater. 2011, 23, 1482. [4] H. Z. Geng, K. K. Kim, K. P. So, Y. S. Lee, Y. Chang, Y. H. Lee, J. Am. Chem. Soc. 2007, 129, 7758. [5] J. T. Di, D. M. Hu, H. Y. Chen, Z. Z. Yong, M. H. Chen, Z. H. Feng, Y. T. Zhu, Q. W. Li, ACS Nano 2012, 6, 5457. [6] S. Bae, H. Kim, Y. Lee, X. F. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y. J. Kim, K. S. Kim, B. Ozyilmaz, J. H. Ahn, B. H. Hong, S. Iijima, Nat. Nanotechnol. 2010, 5, 574. [7] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature 2009, 457, 706. [8] P. C. Hsu, S. Wang, H. Wu, V. K. Narasimhan, D. S. Kong, H. R. Lee, Y. Cui, Nat. Commun. 2013, 4, 2522. [9] H. Wu, D. S. Kong, Z. C. Ruan, P. C. Hsu, S. Wang, Z. F. Yu, T. J. Carney, L. B. Hu, S. H. Fan, Y. Cui, Nat. Nanotechnol. 2013, 8, 421. [10] H. L. Peng, W. H. Dang, J. Cao, Y. L. Chen, W. Wu, W. S. Zheng, H. Li, Z. X. Shen, Z. F. Liu, Nat. Chem. 2012, 4, 281. [11] Y. F. Guo, M. Aisijiang, K. Zhang, W. Jiang, Y. L. Chen, W. S. Zheng, Z. H. Song, J. Cao, Z. F. Liu, H. L. Peng, Adv. Mater. 2013, 25, 5959. [12] S. H. Kim, W. Song, M. W. Jung, M. A. Kang, K. Kim, S. J. Chang, S. S. Lee, J. Lim, J. Hwang, S. Myung, K. S. An, Adv. Mater. 2014, 26, 4247. [13] B. W. An, B. G. Hyun, S.-Y. Kim, M. Kim, M.-S. Lee, K. Lee, J. B. Koo, H. Y. Chu, B.-S. Bae, J.-U. Park, Nano Lett. 2014, 14, 6322. [14] C. W. Jeong, P. Nair, M. Khan, M. Lundstrom, M. A. Alam, Nano Lett. 2011, 11, 5020. [15] Y. F. Hao, M. S. Bharathi, L. Wang, Y. Y. Liu, H. Chen, S. Nie, X. H. Wang, H. Chou, C. Tan, B. Fallahazad, H. Ramanarayan, C. W. Magnuson, E. Tutuc, B. I. Yakobson, K. F. McCarty, Y. W. Zhang, P. Kim, J. Hone, L. Colombo, R. S. Ruoff, Science 2013, 342, 720. [16] H. L. Zhou, W. J. Yu, L. X. Liu, R. Cheng, Y. Chen, X. Q. Huang, Y. Liu, Y. Wang, Y. Huang, X. F. Duan, Nat. Commun. 2013, 4, 2096. [17] A. Kasry, M. A. Kuroda, G. J. Martyna, G. S. Tulevski, A. A. Bol, ACS Nano 2010, 4, 3839. [18] Y. H. Lu, W. Chen, Y. P. Feng, P. M. He, J. Phys. Chem. B 2009, 113, 2. [19] W. Chen, S. Chen, D. C. Qi, X. Y. Gao, A. T. S. Wee, J. Am. Chem. Soc. 2007, 129, 10418. [20] F. Gunes, H. J. Shin, C. Biswas, G. H. Han, E. S. Kim, S. J. Chae, J. Y. Choi, Y. H. Lee, ACS Nano 2010, 4, 4595. [21] M. S. Lee, K. Lee, S. Y. Kim, H. Lee, J. Park, K. H. Choi, H. K. Kim, D. G. Kim, D. Y. Lee, S. Nam, J. U. Park, Nano Lett. 2013, 13, 2814. [22] I. N. Kholmanov, C. W. Magnuson, A. E. Aliev, H. F. Li, B. Zhang, J. W. Suk, L. L. Zhang, E. Peng, S. H. Mousavi, A. B. Khanikaev, R. Piner, G. Shvets, R. S. Ruoff, Nano Lett. 2012, 12, 5679. [23] J. H. Huang, J. H. Fang, C. C. Liu, C. W. Chu, ACS Nano 2011, 5, 6262. [24] Y. K. Kim, D. H. Min, Langmuir 2009, 25, 11302. [25] Y. H. Kim, C. Sachse, M. L. Machala, C. May, L. Muller-Meskamp, K. Leo, Adv. Funct. Mater. 2011, 21, 1076.
4321
Yunfan Guo, Li Lin, Shuli Zhao, Bing Deng, Hongliang Chen, Bangjun Ma, Jinxiong Wu, Jianbo Yin, Zhongfan Liu,* and Hailin Peng* Transparent electrodes that both require high optical transmittance and low electrical resistance are essential components for numerous optoelectronic devices, such as touch screens, solar cells, and smart windows.[1,2] Over the past decades, indium tin oxide (ITO)[3] has been the prototypical material in the field of transparent electrode. However, the weakness of ITO, including brittle nature, scarce abundance limit, and low transmittance in infrared region may hinder its further use in the next generation of flexible electronic devices. Emerging nanomaterials, for example, carbon nanotubes,[4,5] graphene,[6,7] metal nanowires (NWs),[8,9] topological insulators,[10,11] and some hybrid nanostructures[12,13] are expected to be the strong competitors to ITO. Among these new alternatives, graphene with Dirac fermons is one of the most promising electrode materials owing to its excellent charge carrier mobility, high optical transmittance, and mechanical flexibility. However, typically obtained chemical vapor deposition (CVD) graphene is a polycrystalline film, consisting of many single-crystalline grains separated by synthesis-related grain boundaries, which adversely degrade graphene transport through the scattering of charge carriers.[14] In order to improve the performance of graphene-based transparent electrodes, a variety of efforts have been taken, including preparing graphene films with larger domains and fewer boundaries,[15,16] chemical doping by strong acid,[17] organic molecules,[18,19] or nanoparticles,[20] and constructing hybrid nanostructures with random 1D conductive materials, for example, metal NWs[21,22] or carbon nanotubes.[23,24] Even though the last two post-treatments have made a desirable progress for graphene-based transparent electrodes, some thorny problems such as chemical instability,[25] high junction resistance,[26,27] and light scattering[28,29] are urgent to be solved. As a new generation of Dirac materials, topological insulators (TIs) with insulating bulk gap and metallic surface states have garnered extensive attention in recent years.[30–32] In analogy to another Dirac material graphene, topological insulator nanostructures with the helical Dirac surface states have been theoretically predicted to be promising candidate for
Y. Guo, L. Lin, S. Zhao, B. Deng, H. Chen, B. Ma, J. Wu, J. Yin, Prof. Z. Liu, Prof. H. Peng Center for Nanochemistry Beijing Science and Engineering Center for Nanocarbons Beijing National Laboratory for Molecular Sciences College of Chemistry and Molecular Engineering Peking University Beijing 100871, P. R. China E-mail: [email protected]; [email protected]
DOI: 10.1002/adma.201501912
Adv. Mater. 2015, 27, 4315–4321
efficient optoelectronic devices, such as infrared photodetectors, terahertz lasers, and transparent electrodes.[33] In contrast with traditional ITO, our previous experimental works have demonstrated that transparent electrodes based on topological insulator nanostructures exhibit outstanding broadband transmittance especially in near-infrared region,[10,11] together with the strong robustness against mechanical bending and chemical degradation. However, the electrical performance of topological electrodes has yet to be improved. Herein, we first experimentally realized 2D hybrid Dirac materials between topological insulator Bi2Se3 and graphene film for transparent electrode by a facile CVD method. Bi2Se3 nanoplates with high crystal quality and well morphology are preferentially grown along graphene boundaries on copper foil substrate, which can be used as a simple protocol to visualize graphene boundaries on a large scale.[34,35] The metallic surface states of Bi2Se3 may bridge graphene grain boundaries and facilitate the transport of charge carriers (Figure 1a,b). Our electrical measurements show that the sheet resistance of graphene boundary area was improved with the decoration of TI nanoplates, and the conductivity is comparable with other graphene-based hybrid nanostructures.[12,36] On the other hand, the electrical improvement in TI/graphene hybrid film does not incur significant degradation in transparency, which maintains a flat and high transmittance in broadband wavelength. Besides remarkable chemical stability, TI/graphene hybrid film has outstanding mechanical flexibility. Layered topological insulator Bi2Se3 possesses a rhombohedral crystal structure in the space group D53d (R-3m), with the lattice constant a = 4.140 Å, c = 28.636 Å. Each planar quintuple layer (QL, ≈1 nm in thickness) is ordered in Se–Bi–Se–Bi–Se sequence along the c-axis, connecting together by weak van der Waals interaction. Graphene, a single layer of sp2-bonded carbon atoms, has a honeycomb lattice structure with the lattice constant a = 2.46 Å, and the hexagonal periodicity of the (0001) surface d = 4.26 Å. Although the lattice mismatch is 2.9% between graphene and Bi2Se3, the use of van der Waals epitaxy may efficiently relax the lattice-matching condition[37] and facilitate the large-scale production of Bi2Se3/graphene hybrid nanostructures.[38–41] Importantly, defective graphene boundaries have enhanced chemical reactivity to easily absorb heteroatoms during van der Waals epitaxy.[42] Therefore, Bi2Se3 nanoplates can preferentially nucleate along the grain boundaries of graphene films in very low vapor super-saturation condition, and finally form “smart” conductive patches to bridge graphene grain boundary area (Figure S1, Supporting Information). Figure 1c shows a typical scanning electron microscopy (SEM) image of Bi2Se3/graphene hybrid nanostructures. Bi2Se3 nanoplates were successfully grown along graphene grain
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
2D Hybrid Nanostructured Dirac Materials for Broadband Transparent Electrodes
4315
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. Schematic diagram and morphology characterizations for TI/graphene hybrid electrodes. a) Schematic diagram to interpret electron current in polycrystalline graphene with boundaries. Both arrows indicate current flow. b) Schematic diagram to interpret Bi2Se3 nanoplates bridging graphene boundaries and making charge carriers to flow through. c,d) Typical scanning electron microscopy images of Bi2Se3/graphene hybrid nanostructures grown on copper foil substrates. Graphene boundaries can be visualized by the decoration of Bi2Se3 nanoplates. d) Hybrid films grown for different time. Scale bar: 2 µm. e) Raman spectra of Bi2Se3/graphene hybrid nanostructures transferred on SiO2/Si substrate using green (514 nm) laser excitation. Inset: typical optical microscopy image of Bi2Se3/graphene hybrid film transferred on SiO2/Si substrate. Scale bar: 50 µm. f) Photograph and SEM images of Bi2Se3/roll-to-roll graphene hybrid films on copper foil substrate. Scale bar: 20 µm. g) Photograph of a large-area and transparent Bi2Se3/ roll-to-roll graphene hybrid electrode on PET substrate. The outline of TI/graphene hybrid electrode on the PET substrate is rendered by red dashed lines. Scale bar: 2 cm.
boundaries and make graphene grains visualized. This method could be used as a simple protocol to image graphene grain boundaries on a large scale.[34,35] Note that the density of Bi2Se3 nanoplates can be adjusted by changing growth time, Figure 1d shows the SEM images of graphene decorated by Bi2Se3 nanoplates with growth time varying from 3 to 8 min. With longer growth time, more Bi2Se3 nanoplates nucleated on graphene boundaries and grown up to hundreds of nanometers in lateral size, making these “boundary lines” more continuous. Raman spectroscopy is a powerful tool to characterize both graphene and Bi2Se3. Figure 1e shows the Raman spectrum ranging from 80 to 3000 cm−1 at 514 nm laser excitation for Bi2Se3/graphene hybrid film on SiO2/Si substrate (inset of Figure 1e). At the low frequency region, two characteristic peaks of Bi2Se3 at 131 (Eg2) and 174 cm−1 (A1g2) correspond to the in-plane vibrational mode and the out-of-plane Se–Bi–Se– Bi–Se lattice vibrational mode, respectively, consistent with that of Bi2Se3 bulk crystal.[43] At higher frequency region, two typical graphene peaks at ≈1600 (G band) and ≈2705 cm−1 (2D-band) are observed, assigned to the stretching of C C bond and a second-order two-phonon process in sp2 carbon systems.[44] In addition, TI/graphene hybrid nanostructures can be further scaled up in batch production by using larger copper foil substrates or dynamic roll-to-roll process (Figure 1f). As shown in Figure 1g, with lamination transfer approach,[45] the large-area
4316
wileyonlinelibrary.com
hybrid films can be transferred onto polyethylene terephthalate (PET) plastic substrate directly, showing its high transparency. The quality and nucleation site of Bi2Se3 nanoplates on graphene films are crucial for electrical performance. Bi2Se3 nanoplates with metallic surface states may patch up graphene boundaries and reduce carrier scattering for higher conductivity. To this end, we used transmission electron microscopy (TEM) and selected area electron diffraction (SAED) to determine the structure and quality of TI/graphene hybrid electrodes. As shown in Figure 2a, graphene grains were recognized through the slight contrast from carbon support film, surrounded by Bi2Se3 nanoplates. From the close-up view, triangular and hexagonal nanoplates aligned along nearly identical orientations are observed (Figure 2b). Energy dispersive X-ray (EDX) analyses reveal that Bi2Se3 nanoplates grown on graphene remain the Bi:Se atomic ratio of 2:3, indicating stoichiometric Bi2Se3 free of detectable impurities during the experimental process (Figure S2, Supporting Information). The high-resolution TEM (HRTEM) image of Bi2Se3 nanoplate shows the expected hexagonal lattice with a lattice spacing of 2.1 Å (Figure 2c), agreeing well with the spacing of the (11–20) planes of layered Bi2Se3. The corresponding fast Fourier transformation (FFT) patterns verify the single crystalline nature of Bi2Se3 nanoplates (inset of Figure 2c). To further study the nucleation sites of Bi2Se3 nanoplates, extensive SAED patterns were captured with ≈200 nm
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 4315–4321
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, 27, 4315–4321
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
respectively (Figure S3b, Supporting Information). It is worth noting that the three middle patterns taken from the gap between Bi2Se3 nanoplates are consistent well with the patterns from two adjacent graphene grains, which indicated that Bi2Se3 nanoplates are predominantly deposited on graphene’s grain boundaries. In addition to highly crystalline structure, TI/graphene hybrid films have remarkable transmittance from the visible to near-infrared region. Both pristine graphene film and TI/graphene hybrid films were transferred onto transparent quartz substrates, showing good visible transparency (Figure 3a). For further evaluation, ultraviolet-visible-near infrared (UV–vis-NIR) spectroscopy of TI/ graphene hybrid films has been conducted (Figure 3b). The hybrid film grown for 3 min exhibits a flat and high transmittance of about 96.0% at the wavelength of 550 nm. With longer growth time, the corresponding T% slightly reduces to 95.2%, for the fact that the “boundary lines” are more continuous (Figure 3c). In comparison with monolayer graphene film, the introduction of Bi2Se3 nanoplates in hybrid films does not give rise to significant decrease in specular transmittance, which is only 1%–2% in the range of 400–2000 nm wavelength. On the other hand, the diffusive transmittance mode should not be ignored (insets of Figure 3e). Different from normal specular mode, diffuse transmittance not only collects the light coming out of the sample parallel to the incident light, but includes all forward scattered light by using an integrating sphere. As shown in Figure 3d, we measured diffusive transmittance for TI/ graphene hybrid films systematically. The difFigure 2. TEM studies for TI/graphene hybrid electrodes. a) Low-magnification TEM image ference between diffusive T% and specular of a graphene grain decorated by Bi2Se3 nanoplates. The graphene boundaries are rendered by white dashed lines. The white dashed arrow exhibits the direction along which SAEDs are T% is only about 1%, indicating the slight performed. The nine circles present positions where SAED patterns in d–f) are collected. Scale light scattering (Figure 3e). However, for Ag bar: 2 µm. b) Magnified TME image of the black square in (a). Scale bar: 200 nm. c) HRTEM NWs electrodes,[28,29] the difference in two image of Bi2Se3 nanoplates, showing the highly crystalline nature of the basal plane of Bi2Se3 transmittance modes can be up to ≈10%, nanoplates. Scale bar: 2 nm. Inset: corresponding fast Fourier transformation (FFT) pattern. which would be a thorny problem for its use (d)–(f) SAED patterns of specified spots in (a). The number labeled the angle along the arrow in displays and touch screens. pair in each panel. Scale bar: 5 1/nm. Experimental and theoretical studies reported that graphene grain boundaries would impede electrical transport.[46–48] To evaluate the influence aperture along the boundary of two adjacent graphene domains in Bi2Se3/graphene hybrid films. Figure 2d–f shows nine typical of Bi2Se3 on graphene boundary conductance, we performed in SAED patterns, taken from the orders labeled along the white situ electrical transport measurement. Figure 4a shows a repredashed arrow in Figure 2a. The first three patterns colored in sentative device fabricated on two adjacent graphene grains that blue cycles and last three ones in black together demonstrate coalesced at a single grain boundary. Multiple electrodes are patthe single-crystalline nature of two individual graphene grains. terned to allow simultaneous transport measurements both for The thickness of graphene samples was further verified to be the area with Bi2Se3 nanoplates and the one without. The elecmonolayer by analyzing line profile of diffraction patterns in trical measurement in Figure 4b shows both the current–voltage Figure S3a, Supporting Information. And the histogram of pat(I–V) curves are linear, indicative of the ohmic contact between tern orientation distributions from extensive SAED data of two graphene stripes and metal contact. The resistances extracted grains exhibited two pronouncing peaks at ≈14.2° and ≈50.1°, from the slopes of I–V curves are RG ≈ 7.5 kΩ sq−1 (R6,7),
4317
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. Spectroscopy characterization for TI/graphene hybrid electrodes. a) Photograph of a pristine graphene film, TI/graphene hybrid films with different growth time on quartz substrates, showing their good visible light transparency. b) Specular transmittance spectrum of graphene and TI/ graphene hybrid electrodes with different growth time over a broadband wavelength. c) Plot of specular transmittance of TI/graphene hybrid films versus growth time at the wavelength of 475, 550, and 850 nm, respectively. d) Diffusive and specular transmittances of ITO, CNT, Ag NWs, graphene/ Ag NW hybrid nanostructures and TI/graphene hybrid nanostructures on quartz substrates. e) The difference in diffusive transmittance and specular transmittance, indicating the light scattering by different materials. Inset: schematic diagrams of two different transmittance modes.
RBi2Se3/G ≈ 4.3 kΩ sq−1 (R2,3), respectively. Our results clearly show that Bi2Se3 nanoplates with the metallic surface states can bridge the boundary of coalesced graphene grains and thus improve the carrier transport. To evaluate the macroscopic conductivity, photolithography was used to fabricate electrode arrays on both pristine graphene films and TI/graphene hybrid films. Typical optical microscopy (OM) images are shown in the insets of Figure 4c, with chromium/gold (5/50 nm) as contact metal on SiO2/Si substrates. I–V curves for both films on a large scale keep linear and symmetric (Figure 4c). As illustrated in Figure 4d, the statistic sheet resistance (Rs) of hybrid films is obviously lower than that of graphene. Figure 4e shows the histograms of sheet resistance distributions for hybrid electrodes with different growth time. The average sheet resistance of TI/graphene hybrid electrodes grown for 3, 5, and 8 min turns out to be ≈510, 430, and 300 Ω sq−1, respectively. Compared with the conductivity of graphene film, the sheet resistance (Rs) of hybrid films decreased dramatically by one- to threefold. However, the rising conductivity comes at the expense of decreased transparency. Figure 4f exhibits the sheet resistance of TI/graphene hybrid films against optical transmittance at the wavelength of 475, 550, and 850 nm, respectively, suggesting a trade-off relationship between electrical conductivity and optical performance. At 550 nm, the T% for hybrid films with the best conductivity (300 Ω sq−1) can reach 95.2%, which is comparable with the 4318
wileyonlinelibrary.com
performance for graphene/carbon nanotube (CNT) hybrid films[12] and graphene/Ag NWs (low density) hybrid nanostructures[36] (Figure 4g). From this point of view, the metallic surface states of Bi2Se3 may bridge the grain boundaries of polycrystalline graphene, and enhance the electrical performance of pristine graphene films without incurring significant decrease in optical transmittance. In addition to high optical transmittance and low sheet resistance, chemical stability is an indispensable requirement for transparent electrodes in actual applications. According to the previous works,[3,49–51] the performance of transparent electrodes might be weakened by a variety of factors such as chemical degradation and UV light illumination. For example, acrylic acid is commonly used in the assembling of ITO-based touch screen for optically clear adhesives. Low concentration acrylic acid solution may leach out over time and cause serious corrosion of indium in ITO, which would lead to device failure during the assembling.[3] UV illumination, the critical challenge for many kinds of transparent electrodes should not be neglected. Usually, strong oxidants such as NO2,[50] SOCl2,[51] and TCNQF4[49] were used to substantially dope graphene and carbon nanotube electrodes for higher conductivity. However, these dopants are easily detached under UV illumination, which may result in conductance degradation or cause defect in graphene and CNTs electrodes.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, 27, 4315–4321
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 4. Electrical characterization for TI/graphene hybrid electrodes. a) SEM image of a device with multiple electrodes (numbered 1–8) contacting two graphene grains (indicated by red and yellow dashed lines) coalesced in a single boundary (indicated by white dashed line). Scale bar: 5 µm. Inset: magnified SEM image of the boundary areas with and without Bi2Se3 nanoplates. Scale bar: 1 µm. b) Source–drain current (I) versus voltage (V) characteristics for graphene boundaries with and without Bi2Se3 nanoplates. c) Current (I) versus voltage (V) characteristics for graphene boundaries with and without Bi2Se3 nanoplates measured on a large scale. Upper left inset: an OM image of a pristine graphene strip. Lower right inset: an OM image of a TI/graphene hybrid film strip. Scale bar: 50 µm. d) Histograms of sheet resistance distributions for graphene films and hybrid films in (c). e) Histograms of sheet resistance distributions for TI/graphene hybrid films with different growth time. f) Sheet resistance as a function of optical transmittance at the wavelength of 475, 550, and 850 nm for TI/graphene hybrid films with different growth time. g) Plot of sheet resistance versus transmittance for different transparent electrode materials.
In comparison with traditional electrode materials, the topologically protected metallic surface states of Bi2Se3 might enable Bi2Se3/graphene hybrid film high chemical stability and strong robustness to environmental perturbations, for example, chemical agents and UV light. We have made a series of contrast tests to demonstrate the outstanding chemical stability for TI/graphene hybrid films. First, we immersed Bi2Se3/graphene hybrid film into acrylic acid (2%, v/v) for 20, 40, 60, 120, and 240 s. As shown in Figure 5a and Figure S4 (Supporting Information), after acid treatment, the sheet resistance of hybrid film slightly changes from 430 Ω sq−1 to 510, 517, 523, 530, and 545 Ω sq−1, respectively. These observations indicate the TI/graphene hybrid electrode is much stable against chemical acid. In addition, we exposed the Bi2Se3/graphene hybrid electrode directly into UV light for 30, 60, 90, 150, and 210 s. As shown in Figure 5b and Figure S5 (Supporting Information), the corresponding sheet resistance of Bi2Se3/graphene hybrid film is around 620, 625, 627, 630, and 635 Ω sq−1 after UV treatment, respectively, which is a little bit higher than before (510 Ω sq−1). Therefore, due to the presence of nontrivial robust metallic surface states of topological insulator Bi2Se3 nanoplates, the outstanding chemical stability of Bi2Se3/graphene hybrid electrodes in different environments guarantees its wide use in practical applications.
Adv. Mater. 2015, 27, 4315–4321
Mechanical durability is a significant feature for flexible transparent electrodes. Owing to the natural frangibility, the predominant material ITO is highly brittle, which hampers its further use in the next-generation of flexible optoelectronic devices. Besides improved electrical conductivity, broadband high transmittance, and outstanding chemical stability, Bi2Se3/ graphene hybrid electrodes display excellent mechanical durability. Figure 5c shows the variation in the ratio of ΔR to R0 both for ITO/PET and Bi2Se3/graphene hybrid electrodes on PET substrate. For ITO, the ratio of ΔR to R0 reaches 70% in the first 600 bending cycles. And after 800 cycles, the ΔR to R0 ratio is up to 100%, indicating the broken of ITO film. By contrast, the resistance variations of Bi2Se3/graphene hybrid electrodes on PET substrate are always below 5% during the first 900 cycles, and slightly increase to 10% after 2000 cycles, indicative of the outstanding mechanical durability. Figure 5d illustrates the resistance change of hybrid electrodes for different bending radius. Compared with the obvious resistance increase for ITO electrodes on PET, the conductance variation for Bi2Se3/ graphene hybrid electrodes on the same substrates is unnoticeable with the bending radius down to 3 mm. Topological surface/edge states of Bi2Se3 protected by time-reversal symmetry could remain highly conductive path for electrical transport when encountered defects and mechanical dislocations.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
4319
www.advmat.de
COMMUNICATION
www.MaterialsViews.com were grown on graphene films (copper substrate) by a catalyst-free vapor transport deposition inside a 12 in. horizontal tube furnace with a 1 in. diameter quartz tube. The source material Bi2Se3 power (Alfa Aesar, purity 99.999%) was placed in the center of a small tube (460–500 °C) for evaporation. Obtained monolayer graphene films were placed downstream at certain locations in the tube. Ultrapure argon was used as carrier gas to transport hot Bi2Se3 vapor to cold graphene films. After the tube was pumped to a low pressure about 10 mTorr and flushed with the carrier gas repeatedly to remove air contamination, the growth of Bi2Se3/graphene hybrid nanostructures began. Typical growth conditions were a pressure of 150 Torr, source temperature of 490 °C, growth time of 3–10 min, gas flow rate of 500 sccm (standard cubic centimeters per minute), and growth temperature is below 280 °C. Characterization: Characterization was conducted by using optical microscopy (Olympus DX5r1 microscope), SEM (Hitachi S-4800, acceleration voltage 5–30 kV), TEM (FEI Tecnai F30; acceleration voltage, 300 kV) and UV–vis-IR (Perkin Elmer Lambda 950 spectrophotometer), micro-Raman spectroscopy (Horiba HR800 Raman system) with 514.5 nm (2.41 eV) laser line from an Figure 5. Chemical and mechanical durability of TI/graphene hybrid electrodes. a) Histogram Ar+ laser. Electrical measurements were carried out of sheet resistance distributions of Bi2Se3/graphene hybrid films before and after 20 s acrylic in a Micromanipulator 6200 probe station with a acid treatment. b) Histogram of sheet resistance distributions of Bi2Se3/graphene hybrid films Keithley 4200 semiconductor analyzer. before and after 30 s ultraviolet light (UV) treatment. c) Resistance change with respect to Device Fabrication: TI/graphene hybrid films bending cycles for Bi2Se3/graphene hybrid films on PET substrates compared with ITO film on were first transferred onto a silicon substrate PET. d) Resistance change of Bi2Se3/graphene hybrid films for different bending radius. with silicon oxide (300 nm) as dielectric layer. SEM was used to identify specific regions, while the single graphene boundary with and without In combination with the inherent flexibility of graphene films, Bi2Se3 nanoplates was preferred. Standard electron beam lithography robust Bi2Se3/graphene hybrid films hold promise for dissipa(EBL; STRATA DB 235, FEI) was performed to define micropatterns, and photolithography was carried out for the four-probe array devices tionless interconnects and novel flexible transparent electrodes. fabrication. Designed graphene strips and Bi2Se3/graphene hybrid In conclusion, we have presented the first experimental strips were shaped by plasma etching. Afterward, bilayer metal realization of high performance transparent electrode based on electrodes (5 nm Cr/50 nm Au) were deposited by thermo evaporation hybrid Dirac materials between Bi2Se3 nanoplate and graphene. (UNIVEX 300, Leybold Vacuum). At last, the device was lifted-off by hot In comparison with the conductivity of pristine graphene acetone and blow dried with nitrogen gas. films, the electrical performance for Bi2Se3/graphene hybrid Transport Measurement: A semiconductor analyzer (Keithley, SCS4200) was used to measure the four terminal electrical properties. electrodes increased dramatically by to threefold, presumably
because the metallic surface states of Bi2Se3 nanoplates could bridge graphene boundaries and thus improve the electrical conductivity of graphene. In addition, the Bi2Se3/graphene hybrid films have high broadband transmittance, outstanding chemical stability, and excellent mechanical durability. These exotic properties among topological insulator Bi2Se3/graphene hybrid electrodes suggest exciting prospects for future flexible optoelectronics, dissipationless interconnects, and novel electronic applications.
Experimental Section Synthesis of Bi2Se3/Graphene Hybrid Nanostructures: Two types of monolayer graphene samples were employed in this work: graphene films grown by normal chemical vapor deposition method on small pieces of laboratory copper foils (Alfa-Aesar stock #46365) and by dynamic rollto-roll process on large-area industrial electrolytic copper foils (Suzhou Fukuda Metal Co., LTD). CVD growth of graphene was carried out at temperatures up to 1000 °C by using a mixture of 10 sccm CH4 and 10 sccm H2 at a total pressure of 500 mTorr (30 Pa). Bi2Se3 nanoplates
4320
wileyonlinelibrary.com
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financially supported by the National Basic Research Program of China (Grant Nos. 2014CB932500, 2011CB921904, and 2013CB932603), the National Natural Science Foundation of China (Grant Nos. 21173004, 21222303, 51121091, and 51362029), National Program for Support of Top-Notch Young Professionals, and Beijing Municipal Science & Technology Commission (No. Z131100003213016).
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: April 22, 2015 Revised: May 12, 2015 Published online: June 16, 2015
Adv. Mater. 2015, 27, 4315–4321
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, 27, 4315–4321
[26] E. C. Garnett, W. S. Cai, J. J. Cha, F. Mahmood, S. T. Connor, M. G. Christoforo, Y. Cui, M. D. McGehee, M. L. Brongersma, Nat. Mater. 2012, 11, 241. [27] M. S. Fuhrer, J. Nygard, L. Shih, M. Forero, Y. G. Yoon, M. S. C. Mazzoni, H. J. Choi, J. Ihm, S. G. Louie, A. Zettl, P. L. McEuen, Science 2000, 288, 494. [28] R. Y. Chen, S. R. Das, C. Jeong, M. R. Khan, D. B. Janes, M. A. Alam, Adv. Funct. Mater. 2013, 23, 5150. [29] L. B. Hu, H. S. Kim, J. Y. Lee, P. Peumans, Y. Cui, ACS Nano 2010, 4, 2955. [30] H. J. Zhang, C. X. Liu, X. L. Qi, X. Dai, Z. Fang, S. C. Zhang, Nat. Phys. 2009, 5, 438. [31] X. L. Qi, S. C. Zhang, Rev. Mod. Phys. 2011, 83, 1057. [32] M. Z. Hasan, C. L. Kane, Rev. Mod. Phys. 2010, 82, 3045. [33] X. A. Zhang, J. Wang, S. C. Zhang, Phys. Rev. B 2010, 82, 245170. [34] D. L. Duong, G. H. Han, S. M. Lee, F. Gunes, E. S. Kim, S. T. Kim, H. Kim, Q. H. Ta, K. P. So, S. J. Yoon, S. J. Chae, Y. W. Jo, M. H. Park, S. H. Chae, S. C. Lim, J. Y. Choi, Y. H. Lee, Nature 2012, 490, 235. [35] D. W. Kim, Y. H. Kim, H. S. Jeong, H. T. Jung, Nat. Nanotechnol. 2012, 7, 29. [36] H. O. Choi, D. W. Kim, S. J. Kim, S. B. Yang, H. T. Jung, Adv. Mater. 2014, 26, 4575. [37] W. H. Dang, H. L. Peng, H. Li, P. Wang, Z. F. Liu, Nano Lett. 2010, 10, 2870. [38] L. Z. Kou, F. M. Hu, B. H. Yan, T. Frauenheima, C. F. Chen, Nanoscale 2014, 6, 7474. [39] L. Z. Kou, F. M. Hu, B. H. Yan, T. Wehling, C. Felser, T. Frauenheim, C. F. Chen, Carbon 2015, 87, 418. [40] L. Z. Kou, B. H. Yan, F. M. Hu, S. C. Wu, T. O. Wehling, C. Felser, C. F. Chen, T. Frauenheim, Nano Lett. 2013, 13, 6251. [41] I. Popov, M. Mantega, A. Narayan, S. Sanvito, Phys. Rev. B 2014, 90, 035418. [42] K. Kim, H. B. R. Lee, R. W. Johnson, J. T. Tanskanen, N. Liu, M. G. Kim, C. Pang, C. Ahn, S. F. Bent, Z. N. Bao, Nat. Commun. 2014, 5, 781. [43] J. Zhang, Z. P. Peng, A. Soni, Y. Y. Zhao, Y. Xiong, B. Peng, J. B. Wang, M. S. Dresselhaus, Q. H. Xiong, Nano Lett. 2011, 11, 2407. [44] M. S. Dresselhaus, A. Jorio, M. Hofmann, G. Dresselhaus, R. Saito, Nano Lett. 2010, 10, 751. [45] L. G. P. Martins, Y. Song, T. Y. Zeng, M. S. Dresselhaus, J. Kong, P. T. Araujo, Proc. Natl. Acad. Sci. USA 2013, 110, 17762. [46] A. W. Cummings, D. L. Duong, V. L. Nguyen, D. V. Tuan, J. Kotakoski, J. E. B. Vargas, Y. H. Lee, S. Roche, Adv. Mater. 2014, 26, 5079. [47] O. V. Yazyev, Y. P. Chen, Nat. Nanotechnol. 2014, 9, 755. [48] Q. K. Yu, L. A. Jauregui, W. Wu, R. Colby, J. F. Tian, Z. H. Su, H. L. Cao, Z. H. Liu, D. Pandey, D. G. Wei, T. F. Chung, P. Peng, N. P. Guisinger, E. A. Stach, J. M. Bao, S. S. Pei, Y. P. Chen, Nat. Mater. 2011, 10, 443. [49] T. Takenobu, T. Kanbara, N. Akima, T. Takahashi, M. Shiraishi, K. Tsukagoshi, H. Kataura, Y. Aoyagi, Y. Iwasa, Adv. Mater. 2005, 17, 2430. [50] J. Kong, N. R. Franklin, C. W. Zhou, M. G. Chapline, S. Peng, K. J. Cho, H. J. Dai, Science 2000, 287, 622. [51] V. Skakalova, A. B. Kaiser, U. Dettlaff-Weglikowska, K. Hrncarikova, S. Roth, J. Phys. Chem. B 2005, 109, 7174.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
[1] S. P. Pang, Y. Hernandez, X. L. Feng, K. Mullen, Adv. Mater. 2011, 23, 2779. [2] C. G. Granqvist, A. Azens, A. Hjelm, L. Kullman, G. A. Niklasson, D. Ronnow, M. S. Mattsson, M. Veszelei, G. Vaivars, Sol. Energy 1998, 63, 199. [3] D. S. Hecht, L. B. Hu, G. Irvin, Adv. Mater. 2011, 23, 1482. [4] H. Z. Geng, K. K. Kim, K. P. So, Y. S. Lee, Y. Chang, Y. H. Lee, J. Am. Chem. Soc. 2007, 129, 7758. [5] J. T. Di, D. M. Hu, H. Y. Chen, Z. Z. Yong, M. H. Chen, Z. H. Feng, Y. T. Zhu, Q. W. Li, ACS Nano 2012, 6, 5457. [6] S. Bae, H. Kim, Y. Lee, X. F. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y. J. Kim, K. S. Kim, B. Ozyilmaz, J. H. Ahn, B. H. Hong, S. Iijima, Nat. Nanotechnol. 2010, 5, 574. [7] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature 2009, 457, 706. [8] P. C. Hsu, S. Wang, H. Wu, V. K. Narasimhan, D. S. Kong, H. R. Lee, Y. Cui, Nat. Commun. 2013, 4, 2522. [9] H. Wu, D. S. Kong, Z. C. Ruan, P. C. Hsu, S. Wang, Z. F. Yu, T. J. Carney, L. B. Hu, S. H. Fan, Y. Cui, Nat. Nanotechnol. 2013, 8, 421. [10] H. L. Peng, W. H. Dang, J. Cao, Y. L. Chen, W. Wu, W. S. Zheng, H. Li, Z. X. Shen, Z. F. Liu, Nat. Chem. 2012, 4, 281. [11] Y. F. Guo, M. Aisijiang, K. Zhang, W. Jiang, Y. L. Chen, W. S. Zheng, Z. H. Song, J. Cao, Z. F. Liu, H. L. Peng, Adv. Mater. 2013, 25, 5959. [12] S. H. Kim, W. Song, M. W. Jung, M. A. Kang, K. Kim, S. J. Chang, S. S. Lee, J. Lim, J. Hwang, S. Myung, K. S. An, Adv. Mater. 2014, 26, 4247. [13] B. W. An, B. G. Hyun, S.-Y. Kim, M. Kim, M.-S. Lee, K. Lee, J. B. Koo, H. Y. Chu, B.-S. Bae, J.-U. Park, Nano Lett. 2014, 14, 6322. [14] C. W. Jeong, P. Nair, M. Khan, M. Lundstrom, M. A. Alam, Nano Lett. 2011, 11, 5020. [15] Y. F. Hao, M. S. Bharathi, L. Wang, Y. Y. Liu, H. Chen, S. Nie, X. H. Wang, H. Chou, C. Tan, B. Fallahazad, H. Ramanarayan, C. W. Magnuson, E. Tutuc, B. I. Yakobson, K. F. McCarty, Y. W. Zhang, P. Kim, J. Hone, L. Colombo, R. S. Ruoff, Science 2013, 342, 720. [16] H. L. Zhou, W. J. Yu, L. X. Liu, R. Cheng, Y. Chen, X. Q. Huang, Y. Liu, Y. Wang, Y. Huang, X. F. Duan, Nat. Commun. 2013, 4, 2096. [17] A. Kasry, M. A. Kuroda, G. J. Martyna, G. S. Tulevski, A. A. Bol, ACS Nano 2010, 4, 3839. [18] Y. H. Lu, W. Chen, Y. P. Feng, P. M. He, J. Phys. Chem. B 2009, 113, 2. [19] W. Chen, S. Chen, D. C. Qi, X. Y. Gao, A. T. S. Wee, J. Am. Chem. Soc. 2007, 129, 10418. [20] F. Gunes, H. J. Shin, C. Biswas, G. H. Han, E. S. Kim, S. J. Chae, J. Y. Choi, Y. H. Lee, ACS Nano 2010, 4, 4595. [21] M. S. Lee, K. Lee, S. Y. Kim, H. Lee, J. Park, K. H. Choi, H. K. Kim, D. G. Kim, D. Y. Lee, S. Nam, J. U. Park, Nano Lett. 2013, 13, 2814. [22] I. N. Kholmanov, C. W. Magnuson, A. E. Aliev, H. F. Li, B. Zhang, J. W. Suk, L. L. Zhang, E. Peng, S. H. Mousavi, A. B. Khanikaev, R. Piner, G. Shvets, R. S. Ruoff, Nano Lett. 2012, 12, 5679. [23] J. H. Huang, J. H. Fang, C. C. Liu, C. W. Chu, ACS Nano 2011, 5, 6262. [24] Y. K. Kim, D. H. Min, Langmuir 2009, 25, 11302. [25] Y. H. Kim, C. Sachse, M. L. Machala, C. May, L. Muller-Meskamp, K. Leo, Adv. Funct. Mater. 2011, 21, 1076.
4321
