Topological insulator nanostructures for near-infrared transparent flexible electrodes Hailin Peng 1*, Wenhui Dang 1, Jie Cao 1, Yulin Chen 2,3, Di Wu 1, Wenshan Zheng 1, Hui Li 1, Zhi-Xun Shen 3,4, Zhongfan Liu 1 1 Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China. 2 Department of Physics and Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, UK 3 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA. 4 Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, CA 94305, USA. * To whom correspondence should be addressed. E-mail: hlpeng@pku.edu.cn. NATURE CHEMISTRY www.nature.com/naturechemistry 1
Figure 1S. (a) Thin sheet of flexible mica. (b) Ultraviolet, visible, and near-infrared (UVvis-NIR) spectra of thin sheets of muscovite mica (blue) and fluorphlogopite mica (red) showing transmittance in the UV-vis-NIR regime. NATURE CHEMISTRY www.nature.com/naturechemistry 2
Figure S2. Optical microscopy (OM) images and atomic force microscopy (AFM) images of the Bi 2 Se 3 nanosheets with difference thickness on mica substrate. (a, b, c) OM images in transmission mode for Bi 2 Se 3 nanosheets of ~ 10, 20 and 25 nm in thickness on mica, respectively. The cracked regions indicated by arrows have been made intentionally to demonstrate the optical contrast and determine the thickness of nanosheets. (d, e, f) AFM images of the nanosheets with thicknesses of ~ 10, 20 and 25 nm, respectively. (g, h, i) Line cuts taken across the crack edges of nanosheets reveal the thickness and surface roughness of nanosheets in (d, e, f). NATURE CHEMISTRY www.nature.com/naturechemistry 3
Figure S3. Estimation of 2D carrier densities from the topological surface state and bulk state based on ARPES measurements. The sheet resistance of a doped thick sample is mainly due to the bulk state for 10nm Bi 2 Se 3 thin film; but the surface state contribution increases when the thickness of the film decreases. For first order estimation, given that k f bulk = k f surface 0.1 Å -1 from the ARPES measurement of a doped 10-nm-thick sample, we can estimate that 2D carrier density from two (top and bottom) surface states is 2 π k 2 f /(2 π) 2 = 0.0016 Å -2 =1.6 10 13 cm -2, while the effective 2D density of bulk carrier roughly (4/3 π k 3 f (10nm))/(2 π) 3 = 0.0017 Å -2 =1.7 10 13 cm -2 ; thus the surface and bulk contributions are roughly the same. Furthermore, the surface contribution can be enhanced easily when the film gets thinner. Finally, even if the bulk carrier density is completely eliminated by moving E F into the bulk gap of the Bi 2 Se 3 film, the conduction can be maintained, due solely to the robust topological surface states. NATURE CHEMISTRY www.nature.com/naturechemistry 4
Figure S4. (a to d) SEM images of selected Bi 2 Se 3 nanoribbons with the thickness range of 20-50 nm, showing excellent mechanical flexibility. NATURE CHEMISTRY www.nature.com/naturechemistry 5
Figure S5. (a, b, c) TEM images of a ~12 nm thick Bi 2 Se 3 nanosheet, showing bending under electron beam bombardment. (d). High-resolution TEM image of the bent edge of the nanosheet revealing the thickness of the single-crystal nanosheet. NATURE CHEMISTRY www.nature.com/naturechemistry 6
Figure S6. Energy-dispersive X-ray spectra (EDX) collected from a Bi 2 Se 3 nanosheet transferred onto a copper grid. The acquired spectrum shows the atomic ratio of Bi:Se is 2:3, within the experimental error limit (~ 1 2 % uncertainty). Carbon and copper signals are from the TEM grid. NATURE CHEMISTRY www.nature.com/naturechemistry 7
Figure S7. (a) Raman spectrum of a Bi 2 Se 3 nanosheet grown on mica using green (514 nm) laser excitation. The characteristic peaks of rhombohedral Bi 2 Se 3 at 74, 132 and 174 cm -1 correspond to vibration modes A 1g 1, E g 2 and A 1g 2, respectively. (b) Optical image of the Bi 2 Se 3 nanosheet film. (c) Raman map of E g 2 band intensity over 7 µm by 12 µm area indicated by a white box in (b). NATURE CHEMISTRY www.nature.com/naturechemistry 8
Figure S8. (a) Relative humidity dependence of electrical resistance at room temperature for ~10 nm (blue) and 20 nm (red and green) thick nanosheet films of Bi 2 Se 3. The red and green traces were taken by increasing and decreasing the relative humidity, respectively. Inset shows the optical image of the 10 nm thick sample. (b) Temperature dependence of electrical resistance for a ~6 nm thick Bi 2 Se 3 nanosheet film measured in the range of 25~300 o C shows a typical metallic behavior. Inset is a typical optical image of the nanosheet film after heat treatment. NATURE CHEMISTRY www.nature.com/naturechemistry 9
Figure S9. (a) Variation in resistance with bend radius for a ~10 nm thick Bi 2 Se 3 sheet film and sputtered ITO films with various thicknesses on ~0.05 mm thick mica or PET substrates. (b) Resistance change over 1000 bending cycles with at a bending radius of 10 mm for ~10 nm thick Bi 2 Se 3 sheet on mica, ~10 nm thick ITO on mica and sputtered ITO on PET. We carried out the deposition of ITO films on the atomically thin mica flakes using a magnetron sputtering system (JGP560, SKY). The applied power was 48 W and the total gas pressure was 0.3 Pa (Ar 39 sccm, O 2 1 sccm). An ITO sputtering target (99.99% purity, In 2 O 3 /SnO 2 = 90: 10 wt%) was used. The film thicknesses were in the range of 10 100 nm determined by a surface profilometer and further confirmed by AFM measurements. Ref 1. Wu, H. et al. Low reflectivity and high flexibility of tin-doped indium oxide nanofiber transparent electrodes. J. Am. Chem. Soc. 133, 27-29 (2011). NATURE CHEMISTRY www.nature.com/naturechemistry 10