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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Single-Crystal Antimonene Films Prepared by Molecular Beam Epitaxy: Selective Growth and Contact Resistance Reduction of the 2D Material Heterostructure Hsuan-An Chen,†,‡ Hsu Sun,‡,§ Chong-Rong Wu,†,‡ Yu-Xuan Wang,∥ Po-Hsiang Lee,⊥ Chun-Wei Pao,‡ and Shih-Yen Lin*,†,‡ †

Graduate Institute of Electronics Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan Research Center for Applied Sciences, Academia Sinica, No. 128, Sec. 2, Academia Rd., Taipei 11529, Taiwan § Institute of Lighting and Energy Photonics, College of Photonics, National Chiao-Tung University, No. 301, Gaofa 3rd Rd., Tainan City 71150, Taiwan ∥ Institute of Applied Mechanics, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan ⊥ Institute of Atomic and Molecular Sciences, Academia Sinica, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: Single-crystal antimonene flakes are observed on sapphire substrates after the postgrowth annealing procedure of amorphous antimony (Sb) droplets prepared by using molecular beam epitaxy at room temperature. The large wetting angles of the antimonene flakes to the sapphire substrate suggest that an alternate substrate should be adopted to obtain a continuous antimonene film. By using a bilayer MoS2/sapphire sample as the new substrate, a continuous and single-crystal antimonene film is obtained at a low growth temperature of 200 °C. The results are consistent with the theoretical prediction of the lower interface energy between antimonene and MoS2. The different interface energies of antimonene between sapphire and MoS2 surfaces lead to the selective growth of antimonene only atop MoS2 surfaces on a prepatterned MoS2/sapphire substrate. With similar sheet resistance to graphene, it is possible to use antimonene as the contact metal of 2D material devices. Compared with Au/Ti electrodes, a specific contact resistance reduction up to 3 orders of magnitude is observed by using the multilayer antimonene as the contact metal to MoS2. The lower contact resistance, the lower growth temperature, and the preferential growth to other 2D materials have made antimonene a promising candidate as the contact metal for 2D material devices. KEYWORDS: antimonene, conducting 2D materials, 2D material heterostructures, contact resistance, transistors

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electrodes have also been demonstrated.10,11 However, its limited field-effect mobility values have raised another concern for practical application. Recently, people have again moved their research focus to other group-V 2D materials such as phosphorene and arsenene.12 Phosphorene, also known as black phosphorus (bP), is expected to be of high mobility values and with a band-gap value around 1.75 eV.13,14 However, its device application is hindered by the rapid degradation of bP under the atmospheric condition.15 Therefore, another airstable 2D material antimonene has come into discussion.16,17 Bulk antimony has several allotropes. The most stable form was in a rhombohedral lattice structure which is called β-

INTRODUCTION The successful fabrication of graphene by using the mechanical exfoliation in 2004 gave birth to the research field of twodimensional (2D) materials.1 Huge effort has been devoted to the research of graphene for different device applications in the past few years.2−4 One most promising application for graphene is on electronic devices because of its high mobility values and the possibility to demonstrate its unique characteristics in a few atomic layers. However, due to its zero-band-gap nature, there is no off state for graphene transistors, which has limited the application of graphene in electronic devices.5,6 In this case, people have turned their attention to other 2D materials such as molybdenum disulfide (MoS2).7,8 With visible band-gap values, MoS2 transistors with high ON/OFF ratios have also been demonstrated.9 Theoretical works on MoS2 nanoribbons and thermal conduction across MoS2 and metal © XXXX American Chemical Society

Received: February 8, 2018 Accepted: April 13, 2018 Published: April 13, 2018 A

DOI: 10.1021/acsami.8b02394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) The Raman spectra and (b) the AFM images of Samples A and B and (c) the cross-sectional HRTEM image of Sample A.

antimonene.18 It consists of buckled hexagonal rings composed of antimony atoms staggered up and down through sp3 bonding with an indirect band gap (∼2.28 eV) when the material is thinned to monolayer.18 According to theoretical calculations, high carrier mobility (μe ∼ 630 and μh ∼ 1737 cm2·V−1·s−1) and good thermal conductivity are also predicated for antimonene.19 One most common approach to obtain new 2D materials is through the mechanical exfoliation. Different 2D materials such as graphene, MoS2, and bP can be easily obtained by using this method.1,9,15However, according to the theoretical calculation, the binding energy of β-antimonene is around 124 meV/atom, which is much larger than that of other 2D materials such as graphite (24 meV/atom), BN (26 meV/atom), and MoS2 (60 meV/atom).20 This phenomenon is resulted from a nonnegligible overlapping of lone pair orbitals from the neighboring layers. In this case, compared with other 2D materials, it would be very difficult to obtain antimonene flakes through the mechanical exfoliation.21 Therefore, several approaches such as chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and plasma immersion have been proposed to obtain epitaxially grown antimonene.22−24 However, most of results show only small flakes of antimonene growth. There is no direct evidence for the formation of largearea and single-crystal antimonene formation by using these methods at this stage. In this paper, single-crystal antimonene films can be grown on MoS2 surfaces at a low growth temperature of 200 °C by using molecular beam epitaxy (MBE). For blank sapphire substrates, only single-crystal antimonene flakes with large wetting angles can be obtained after the 300 °C postgrowth annealing procedure. The results are consistent with the theoretical prediction of lower interface energy between antimonene and MoS2. The selective growth of antimonene atop MoS2 surfaces is also demonstrated on a prepatterned MoS2/sapphire substrate. The significant specific contact resistance reduction of Au/antimonene electrodes on MoS2 surfaces suggests that improved performances can be

obtained for 2D material devices with antimonene as the contact metal.

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RESULTS AND DISCUSSION To investigate the influence of growth temperatures on antimonene, two samples with different growth temperatures are prepared by using MBE on sapphire substrates with the same growth duration 60 s. The growth temperatures of the two samples are room temperature (Sample A), and 190 °C (Sample B), respectively. The Raman spectra of the two samples are shown in Figure 1a. The characteristic Raman peaks Eg and A1g of antimonene are observed for Sample A, which suggest that β-antimonene is obtained for the sample.21 With further increasing growth temperature up to 190 °C as in the case of Sample B, no characteristic Raman peaks are observed for the sample, which may suggest that no antimonene is formed for the sample. The atomic force microscopy (AFM) images of the two samples are shown in Figure 1b. As shown in the figure, droplet structures are observed for Sample A, while a flat sapphire surface with no antimonene formation is observed for Sample B, which are consistent with the observation obtained from the Raman measurements. The results suggest that the sticking coefficient of Sb atoms would drop rapidly with increasing temperatures. In this case, there will be no sufficient Sb atoms on the sapphire surface for antimonene formation at a low growth temperature of 190 °C. To investigate the crystalline quality of Sample A, the cross-sectional high-resolution transmission electron microscopy (HRTEM) image of Sample A is shown in Figure 1c. As shown in the figure, no clear lattice structures are observed from the image. Amorphous Sb droplets instead of single-crystal antimonene are obtained for Sample A. The Raman peaks of Sample A actually come from tiny antimonene flakes embedded in the amorphous Sb droplets. To improve the crystalline quality of the Sb droplets, the other sample (Sample C) with the same growth conditions of Sample A is prepared. After MBE growth, the sample is annealed at 300 °C in a hot furnace under the nitrogen B


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Figure 2. (a) The Raman spectrum, (b) the AFM image, and (c) the cross-sectional HRTEM image of Sample C and (d) the magnified crosssectional HRTEM images of Samples A and C. The Raman spectrum of Sample A is also shown in (a) for comparison. (e) The relaxed semicoherent interface of antimonene and sapphire from DFT calculations.

environment for 10 min. The Raman spectrum of Sample C is shown in Figure 2a. The Raman spectrum of Sample A is also shown in the figure again for comparison. As shown in the figure, the same characteristic Raman peaks of β-antimonene are also observed for Sample C. Compared with Sample A, the same Raman peak positions of Sample C suggest that no significant film thickness thinning is observed for Sample C after the 300 °C annealing procedure.21 Also observed in the figure is the more intense Raman peak intensities of Sample C, which may imply an improved crystalline quality is obtained for the sample. The AFM image of Sample C is shown in Figure 2b. Compared with the droplet morphology of Sample A, layered clusters with triangular shapes are observed for Sample C. Similar with the more intense Raman peak intensities of Sample C, the results may indicate the formation of singlecrystal antimonene after the 300 °C annealing procedure. The cross-sectional HRTEM image of the Sample C is shown in Figure 2c. Different with the amorphous droplet observed for Sample A, polygonized single-crystal antimonene is obtained for Sample C. The magnified HRTEM images of Samples A and C near the sapphire substrate surfaces are shown in Figure 2d. With the amorphous structure observed for Sample A, a layered 2D crystal is observed for Sample C. The results have demonstrated that, by using the postgrowth annealing, higher temperatures can be adopted to improve the films’ crystalline quality and prevent the risk of Sb desorption with growth temperature higher than 190 °C in the MBE chamber. The other interesting phenomenon observed in Figure 2d is the same wetting angle of 127° for both amorphous Sb droplets (Sample A) and β-antimonene clusters (Sample C). Under the surface tension static equilibrium condition, the Young’s equation for the surface energies of the three interfaces of

free surface/sapphire, free surface/antimonene, and antimonene/sapphire is shown below γfree surface/sapphire = γantimonene/sapphire + γfree surface/antimonene × cos θ

(1)

where θ is the wetting angle and γ is the surface energies of the three interfaces.25 We carried out density functional theory (DFT) calculations to compute the surface/interfacial energies. The relaxed semicoherent interface of antimonene and sapphire is displayed in Figure 2e. The mismatch strain imposed on antimonene is +7.7% (i.e., the antimonene is subjected to tensile mismatch strain), which can potentially induce threedimensional growth mode to relax strain energy from lattice mismatch. Note that such mismatch strain looks large at the first glance; however, 2D materials usually can withstand large strains relative to bulk materials. For example, the ultimate tensile strain of MoS2 and silicene is around 20%.26,27 To estimate wetting angles, we must compute surface/interface energies of strained antimonene, sapphire, and antimonene− sapphire interfaces. As described in the Experiments Section, we employed the DFT-D2 method for the van der Waals interactions. The surface/interface energies of strained antimonene, sapphire, and antimonene−sapphire interface from the DFT calculations are 0.611, 5.175, and 5.441 J/m2, respectively. The wetting angle estimated using the Young’s equation (eq 1) is 115.8°, which is in good agreement with that measured from experiments. We also verified our calculations using the DFT-D3 method (see the Experiments section), and surface/interface energies from the DFT-D3 method are compiled in Table S1 in the Supporting Information. The wetting angle from the DFT-D3 method is 115.5°, which is almost identical with that from the DFT-D2 method. Hence, C


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ACS Applied Materials & Interfaces the antimonene film atop the sapphire substrate is subjected to a substantial amount of mismatch stress. In our previous publications, we have demonstrated that large-area and layer-number-controllable MoS2 can be grown directly on a graphene/sapphire substrate by using CVD and sulfurization of predeposited Mo films.28 Since there is no chemical bond between MoS2 and graphene, the van der Waals epitaxy of 2D materials onto other 2D materials does not seem to be hindered by the large lattice mismatch between the two materials. Therefore, it may be possible to grow large-area antimonene onto other 2D materials such as MoS2. To investigate this possibility, the other sample with bilayer MoS2 is prepared by sulfurizing a predeposited Mo film on a sapphire substrate.29 Similar with CVD-prepared MoS2, large-area growth and good layer number controllability can be achieved by using this method. In addition, selective growth on substrates is also achievable by using this growth technique.30 After MoS2 growth, the sample is transferred into the MBE chamber for antimonene growth at room temperature for 60 s. The sample is referred to as Sample D. The cross-sectional HRTEM image of Sample D is shown in Figure 3a. As shown in

Figure 4. (a) The cross-sectional HRTEM image, (b) the 2θ−θ XRD curve, and (c) the magnified cross-sectional HRTEM image of Sample E. (d) The relaxed semicoherent interface of antimonene and MoS2 from DFT calculations.

film thickness obtained from the HRTEM image. The results suggest that a single-crystal antimonene is obtained for Sample E. To further confirm this point, the magnified HRTEM image of Sample E is shown in Figure 4c. Layered antimonene with single crystal orientation (012) is observed for the sample, which is consistent with the results obtained from the XRD measurement. A layer separation of ∼0.4 nm observed for Sample E is also consistent with the predication for the layer separation of β-antimonene.21 We have also carried out DFTD2 calculations to examine the structure of the antimonene− MoS2 interface; see Figure 4d. Since the bilayer MoS2 was grown on the sapphire substrate, we imposed a +6.39% strain to the bilayer MoS2 to match the lattice of sapphire. As a result, the antimonene grown atop was subjected to a tensile mismatch strain of +3%. The surface energies of strained MoS2 and antimonene are 0.154 and 0.443 J/m2, and the interface energy of antimonene−MoS2 is 0.279 J/m2 from DFT-D2 calculations. The estimated wetting angle of antimonene on MoS2 is 106.4°, which is smaller relative to that of antimonene on sapphire. Furthermore, the interface energy between antimonene and MoS2 is much lower than that between antimonene and sapphire. The results indicate that it is more likely to grow large-area antimonene films on MoS2 surfaces, which is consistent with our experimental results of single-crystal antimonene film prepared by using MBE at 200 °C. Similar to antimonene/sapphire interfaces, we also verified our calculations using the DFT-D3 method. The surface/ interface energies of the antimonene/MoS2 system using the DFT-D3 method are compiled in Table S2 in the Supporting Information. The estimated wetting angle of antimonene/MoS2 from the DFT-D3 method is 106.5°, which is almost identical with the wetting angle estimated from the DFT-D2 method. To further investigate the preferential growth of antimonene on MoS2 surfaces, antimonene growth following the same growth conditions as Sample E is performed on a patterned MoS2/sapphire substrate. With the results of a single-crystal antimonene film on MoS2 surfaces of Sample E (growth temperature 200 °C) and no antimonene formation on sapphire surfaces of Sample B (growth temperature 190 °C), it is expected that antimonene would selectively grow only on

Figure 3. (a) The cross-sectional HRTEM image and (b) the magnified cross-sectional HRTEM image of Sample D. The top view and side view of β-antimonene lattice structures are also shown in (b).

the figure, a continuous film is observed on bilayer MoS2. To further investigate the crystal structure of the film, the magnified HRTEM image of Sample D is shown in Figure 3b. Two crystal domains with different orientations of βantimonene is observed in the figure. After careful examination, the left one is identical to the top view and the right one to the side view of β-antimonene. The top view and side view of βantimonene lattice structures are also shown in the figure for comparison. This indicates that a continuous polycrystalline antimonene film could be formed on MoS2 at room temperature. To further enhance the crystalline quality of the film, Sample E with a higher growth temperature of 200 °C and the same growth time 60 s is prepared. The cross-sectional HRTEM image of the sample is shown in Figure 4a. Similar to Sample D, a continuous film is observed for the sample. The thickness of the film is around 17 nm. Compared with the results of no antimonene formation of Sample B with 190 °C growth temperature, the observation of a continuous film for Sample E at 200 °C suggests that, on MoS2 surfaces, a higher sticking coefficient is obtained for Sb atoms. The 2θ−θ curve of Sample E measured by the X-ray diffraction system (XRD) is shown in Figure 4b. The observations of only Sb (012) and (024) peaks at 28.7° and 59.4° may suggest that an antimonene film with a single crystal orientation is obtained for Sample E.31 By using Kλ the Scherrer equation D = B cos θ , we can obtain the crystal size of the antimonene film to be 16.2 nm, where D is the crystal size, B is the full width at the half-maximum intensity of the peak at Sb (012), K is the Scherrer constant, and λ is the wavelength of incident X-ray.32 The derived value is close to the D


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Figure 5. (a) A schematic diagram showing the concept of selectively grown antimonene on a patterned MoS2/sapphire substrate. (b) The pictures taken under an optical microscope and the Raman spectrum before (left) and after (right) the antimonene growth. No antimonene Raman signals are observed on the sapphire surface. (c) The picture of the sample with selective antimonene growth on 40 × 25 μm2 MoS2 arrays patterned by using RIE on a quarter 2″ sapphire substrate. (d) The fabrication procedure of Au/antimonene electrodes with different separations on the MoS2/ sapphire substrate and (e) the resistance values versus electrode separations for Au/antimonene and Au/Ti electrodes.

MoS2 surfaces at a growth temperature 200 °C. A schematic diagram showing the concept of selectively grown antimonene on a patterned MoS2/sapphire substrate is shown in Figure 5a. With the low sticking coefficient of Sb atoms on sapphire surfaces, antimonene films will grow only atop MoS2 surfaces, which will lead to the selective growth of the antimonene/ MoS2 heterostructures on sapphire substrates. The pictures taken under an optical microscope and the Raman spectrum of the prepatterned MoS2/sapphire sample before and after the antimonene growth are shown in Figure 5b. As shown in the figure, antimonene films are observed only on MoS2 surfaces, which are consistent with the observation of the Raman spectrum that no antimonene signals are observed on sapphire surfaces. By using this method, we have demonstrated selective antimonene growth on 40 × 25 μm2 MoS2 arrays patterned by using reactive-ion etching (RIE) on a quarter 2″ sapphire substrate. The picture of the sample is shown in Figure 5c. The results have demonstrated that the lower interface energy on antimonene/MoS2 interfaces would lead to a complete film growth of antimonene only on MoS2 surfaces. The scalable and preferential growth of antimonene of MoS2 surfaces may also be applied to other 2D materials, which is advantageous for the establishment of antimonene/2D material heterostructures for practical applications. It has been demonstrated in previous publications that, by using graphene as the contact metals, a significant contact resistance reduction and enhanced device performances can be

observed for MoS2 transistors.33,34 Although a conclusive explanation is still unavailable at this stage, the results may indicate that, by using a conductive 2D material as the contact metal, the carriers may experience a barrier-free interface when external voltages are applied to the electrode. The sheet resistance of the antimonene film (Sample E) is 2.31 × 102 Ω/ sq, which is compatible to the value 5.7 × 102 Ω/sq reported for graphene grown directly on sapphire substrates.35 Therefore, it is possible that we can use the multilayer antimonene as the contact metal for other 2D materials. Compared with the necessary film transferring procedure of graphene, the similar deposition procedure to metals and low growth temperatures have made antimonene a more promising candidate for contact metal applications. To investigate this possibility, an antimonene film is grown on a bilayer MoS2/sapphire sample. The growth conditions are the same as Sample E. By using the standard photolithography, metal deposition, and chemical etching, Au/antimonene electrodes with different separations are fabricated on the MoS2/sapphire sample. The fabrication procedure is shown in Figure 5d. The resistance values versus electrode separations for Au/antimonene electrodes are shown in Figure 5e. The values for Au/Ti electrodes are also shown in the figure. Compared with Au/Ti electrodes, a resistance reduction up to 1 order of magnitude is observed for the Au/ antimonene electrodes. By using the transmission line method (TLM), the specific contact resistance values for the two different electrodes can be derived.34 They are 7.85 and 3.09 × E


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ACS Applied Materials & Interfaces 103 Ω·cm2 for Au/antimonene and Au/Ti electrodes, respectively. The specific contact resistance reduction up to 3 orders of magnitude has demonstrated that, by using a conducting 2D material such as multilayer antimonene as the contact metal, improved performances can be obtained for 2D material devices.

slabs of sapphire, MoS2, and antimonene with surface orientations identical with those of experiments were prepared and relaxed. The surface energies were computed using the following equation

γsurface =

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where Acell and Usurface are the cell area in the lateral dimension (i.e., normal to the surface orientation) and relaxed slab energies, and Ubulk is the relaxed energy of the same slab without an open surface. Note that the cell size in the lateral dimensions was adjusted for antimonene to be identical with those of sapphire and MoS2. For computing interface energies, the same antimonene, sapphire, and MoS2 slabs for computing surface energies were used. We created interfaces by gluing two slabs together and set the cell size along the Z directions equal to the sum of thickness of slabs in the bulk. The systems for computing interface energies are displayed in Figures 2e and 4d. Note that, because the systems were periodic in all directions, there were two interfaces in the simulation cells. The interface energy can be computed using the equation

CONCLUSIONS In conclusion, we have demonstrated that single-crystal antimonene films can be grown on MoS2/sapphire substrates at relatively low temperatures. For blank sapphire substrates, only single-crystal antimonene flakes with large wetting angles are obtained after the postgrowth annealing procedure. The results are consistent with the theoretical prediction of lower interface energy between antimonene and MoS2. The different interface energies of antimonene between sapphire and MoS2 surfaces lead to the selective growth of antimonene only atop MoS2 surfaces on a prepatterned MoS2/sapphire substrate. Besides the potential application in electronic devices for thin antimonene, the significant specific contact resistance reduction of Au/antimonene electrodes on MoS2 surfaces suggests that improved performances can be obtained for 2D material devices with antimonene as the contact metal. The preferential growth of antimonene on other 2D materials, the low growth temperature, and the significant specific contact resistance reduction have made this material a promising candidate for contact metals of 2D material devices.

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1 (Usurface − Ubulk) 2Acell

γinterface =

1 (Uinterface − U1,bulk − U2,bulk) 2Acell

where Uinterface is the energy of the relaxed system containing interfaces, and U1,bulk and U2,bulk were once again the energies of the slabs without surface and interfaces (i.e., in the bulk). Since recent studies suggested that the DFT-D3 method is more robust for the van der Waals interactions relative to the DFT-D2 method,43,44 we also carried out DFT-D3 calculations of surface/interface energies of antimonene/sapphire and antimonene/MoS2 from structures relaxed by the DFT-D2 to ensure the accuracy of calculations.

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EXPERIMENTS

For the molecular beam epitaxy (MBE) growth procedure, the samples are transferred into a customer-designed MBE system equipped with an SVTA Dual Filament effusion cell. To produce the antimony molecular beam, the cell temperature is set at 500 °C. Five samples are prepared under different conditions on different substrates. To investigate the material characteristics of antimonene, Raman spectra were measured by using a HORIBA JobinYvon HR800UV Raman spectroscopy system equipped with a 488 nm laser. To obtain the surface morphologies of the antimonene films, atomic-force microscopy (AFM) measurements were carried out with a BRUKER Dimension ICON AFM system. The cross-sectional HRTEM images are obtained by using a JEOL JEM-2800F TEM system operated at 200 kV. To derive the contact resistance of antimonene on MoS2 by using the TLM method, 100 × 200 μm2 Au/antimonene electrodes with different separations are fabricated on the MoS2/sapphire substrate.36 The fabrication procedure is as follows: (1) an antimonene film is grown on a bilayer MoS2/sapphire substrate under the same growth conditions of Sample E, (2) photoresist coating and window opening are done by using standard photolithography, (3) 100 nm Au deposition and following metal liftoff are performed to form electrodes, and (4) by using Au electrodes as the hard mask, unprotected antimonene can be chemically etched by using a 1 M NaOH aqueous solution. Following the similar procedure except for the chemical etching step, 100 nm Au/10 nm Ti electrodes with different separations are also fabricated on the other bilayer MoS2/ sapphire sample for comparison. The resistance values are extracted from the current−voltage curves measured by using a Keithley 2636B system. The density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) 5.4.437−39 with the project augment wave (PAW) pseudopotential,40 as well as the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional.41 The cutoff energy was 450 eV, and a 1 × 1 × 1 Monkhorst− Pack k-point mesh was employed. The convergence criteria were set to dE < 10−5 eV, where dE is the total energy difference between two steps. The DFT-D2 method was employed to incorporate van der Waals interactions between atoms.42 To compute the surface energies,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02394. Tables of the surface/interface energies and wetting angles of the antimonene/sapphire system and antimonene/MoS2 system computed using the DFT-3 method (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chun-Wei Pao: 0000-0003-0821-7856 Shih-Yen Lin: 0000-0001-7028-481X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by projects MOST 105-2221E-001-011-MY3 and MOST 106-2622-8-002-001 funded by the Ministry of Science and Technology, Taiwan, and in part by the iMATE project funded by Academia Sinica, Taiwan.

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REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306 (5696), 666−669. (2) Wang, H.; Yang, Y.; Liang, Y.; Robinson, J. T.; Li, Y.; Jackson, A.; Cui, Y.; Dai, H. Graphene-Wrapped Sulfur Particles as a Rechargeable Lithium−Sulfur Battery Cathode Material with High Capacity and Cycling Stability. Nano Lett. 2011, 11 (7), 2644−2647.

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