Performance at Wafer-Scale

Transcription

1 Supporting Information Self-Assembling VO 2 Nanonet with High Switching Performance at Wafer-Scale Jiasong Zhang, Haibo Jin*, Zhuo Chen, Maosheng Cao, Pengwan Chen, Yankun Dou, Yongjie Zhao, Jingbo Li* School of Materials Science and Engineering, Beijing Institute of Technology, Beijing , P.R. China *Corresponding Author (H. J.) hbjin@bit.edu.cn; (J.L.) lijb@bit.edu.cn. 1

2 1. Morphology and structure identification The low-magnification SEM image of the VO 2 nanonet (Figure S1a) shows uniform distribution of the nanorods on the 2-inch (001)-sapphire (Figure S1b). The structure of asgrown VO 2 nanonets was characterized by Raman and XRD, as shown in Fig. 1c and 1d. It is found the as-grown nanonets are of the monoclinic M phase and highly oriented with the crystallographic relation (020) M or (002) M //(001) S. The characteristic peaks of Raman pattern in Fig. 1c are in good consistence with the Raman results of VO 2 (M) reported by Donev et al.. 1 Above 45 o C, Raman peaks of the as-grown sample are submerged in noise due to high conductivity of the sample. Raman results of annealed samples show no large change from that of as-grown samples except the suppressed conductivity at high temperature (Fig. S1c). XPS measurements were performed to examine the oxidation states of V ions in samples (Fig. S1d). It is shown that the as-grown VO 2 contains partial V 3+ ions together with V 4+ ions, inferring the existence of oxygen vacancies. To replenish oxygen in as-grown samples, the short-time annealing process was employed. The as-grown samples were annealed in a short annealing furnace at 400 o C for 30 s under air atmosphere with a pressure of 10 4 Pa. The XPS results of annealed samples show that V 3+ ions disappear after the short-time annealing, confirming the oxygen vacancies were satisfactorily compensated by the short-time annealing. No additional XRD peaks (Fig. 1d) were observed after annealing. 2

3 Figure S1. Morphology and structure characterizations of VO 2 nanonets. a, SEM image of synthesized nanonets at low magnification. b, Photomacrograph of as-grown sample. c, Temperature-dependent Raman data of the annealed VO 2 nanonets, indicating no structural transition was induced by the short-time annealing. d, High-resolution XPS data of as-grown and short-time annealed samples, showing the as-grown films contain partial V 3+ ions together with V 4+ ions, however the V 3+ ions disappear after the short-time annealing. The position of the V 2p 3/2 core-level peak in the annealed sample is ~515.9 ev, close to the wellestablished value of ~515.8 ev for V 4+ in VO 2. 2 In the as-grown sample, the V 2p 3/2 core-level peak broadens and shifts toward lower binding energy by ~0.2 ev (Note: the binding energy of the V 2p 3/2 core level is ev for V 3+ in V 2 O 3 3 ), indicating a reduction in the oxidation state of V from V 4+ toward V 3+. 3

4 2. Horizontally epitaxial growth of VO 2 nanorods The growth directions of nanorods were determined by TEM (Fig. S2). The SAEDs indicate that the nanorods are well crystallized. Two growth directions were determined, i.e. [200](020) M and [002](020) M, as indexing of SAEDs in the insets of Fig. S2. The present two growth modes are reasonable, considering lattice mismatches between VO 2 and sapphire for different growth directions. The lattice mismatches are -4.46% (minus denotes the compressive strain) along the [200](020) M //< 1 10>(001) S direction, 2.11% along [002](020) M //<1 10>(001) S and 2.59% along [101](020) M //< 1 10>(001) S, respectively. 4 The smaller strain energy may serve as the compensational driving force for the [002](020) M growth. However, the growth direction [101](020) M which is equivalent to the [002](020) M was not observed by TEM observations. The [002](020) M and [101](020) M directions correspond to the same <011>{200} R direction in the high-temperature tetragonal structure (R phase). Since the growth was conducted at 230 o C, higher than the temperature of phase transition from low-temperature M phase to high-temperature R phase (~68 o C), the growth orientation of the nanorods is transformed from its high-temperature R phase. The lower mismatch strain along [002](020) M //<1 10>(001) S than that along [101](020) M //<1 10>(001) S makes the growth direction [002](020) M preferred. Therefore, the nanorods are found to epitaxially grow along two directions, [200](020) M and [002](020) M. 4

5 Figure S2. TEM images of as-grown nanorods. a, TEM image of a nanorod. The inset: SAED pattern of the [010] zone axis taken from the rod, indicating the rod grows along [200](020)M. This implies that the closest-packed V-V chain aligns along the growth direction of the nanorods. b, TEM image of another nanorod. The inset: SAED pattern of the [0 1 0] zone axis, indicating the growth direction of the rod is along [002](020)M with the V-V pair chain obliquely crossing the rod. 3. Details of the nucleation process 5

6 Figure S3. SEM images of the samples prepared at 230 C. a, 27min; b, 30min; c, 32min; d, 33min. Nuclei were observed on the (001)-sapphire after an incubation period of ~27 min. Some of the nuclei show regular hexagonal shape, indicating the formation of heteroepitaxial nuclei on the (001)-sapphire which has the threefold symmetry structure in-plane. In the nucleation process, new nuclei continuously form on substrate and primary large-size nuclei. At ~33 min, small nanorods grow from the secondary nuclei, while the primary large-size nuclei still exist. 4. Effects of orientations of substrates on the growth of VO 2 nanonets VO 2 nanostructures were grown on differently oriented sapphire substrates (Fig. S4). Differently oriented sapphire substrates lead to great difference of the morphology of VO 2. In fact, there are lattice-matching relations between (2 01) M and (100) S and between (100) M and (102) S. 5 The sample grown on (100)-sapphire shows a monodirectional array of nanorods in Fig. S4b with a preferred orientation of (2 01) M //(100) S as determined by XRD (Fig. S4e). This is because of the absence of multifold symmetric structure in (100) S. For the sample grown on (102)-sapphire, no regular growth was observed in Fig. S4c. In the lattice-matching relation of (100) M //(102) S the lattice mismatch is too large (up to 5%) to achieve the horizontally epitaxial growth of VO 2 on the (102)-sapphire. The VO 2 nanonet shows better electrical switching property than the VO 2 nanostructures grown on (100)-sapphire and (102)- sapphire as shown in Fig. S4g-i. 6

7 Figure S4. SEM images, XRD patterns and thermal hysteresis loops of resistance of VO2 nanostructures grown on differently oriented sapphire substrates. a, d, g, (001)sapphire. b, e, h, (100)-sapphire. c, f, i, (102)-sapphire. 5. Thermochromic properties of VO2 nanonets Figure S5. SEM image of VO2 on (001)-sapphires grown with the vanadyl oxalate concentration of 2.5 mmol/l at 230 oc for 4 h. 7

8 Table S1. Summary of VO 2 films and their thermochromic properties Samples T lum (LT/HT a ) T lum T sol (LT/HT) T sol Max T 2000nm (LT/HT) T 2000nm Ref. VO 2 nanonet 46.71%/46.15% 0.56% 59.5%/48.8% 10.7% 67.0% 92.6%/50.7% 41.9% this work VO 2 porous film 50.6%/49.4% 1.2% 58.9%/44.2% 14.7% 6 VO 2 porous film 43.3%/39.9% 3.4% 42.9%/28.8% 14.1% 54.6% 75.1%/24.1% 51.1% 7 VO 2 porous film 7.9% 70.2% 97.0%/61.4% 35.6% 8 TiO 2 /VO 2 /TiO %/54.7% 2.9% 46.1%/43.2% 2.9% 64.1% 54.4%/16.1% 38.3% 9 VO 2 /TiO %/44.8% 4.7% 52.2%/37.1% 15.1% 55.7% 78.7%/21.6% 57.1% 10 T vis b a) LT is the abbreviation of low temperature, denoting the properties are of the low-temperature M phase, and HT stands for the properties of the high-temperature R phase. b) Max T vis denotes the maximum of visible transmittance. The thermochromic properties reported in this work were calculated from the experimental optical data. The integrated luminous transmittance ( T lum, nm) and solar modulating ability (, nm) are two major performance parameters of smart windows, Tsol which were calculated from the following equations: 11, 12 T lum, sol = ϕ lum, sol ϕ ( λ) T( λ) dλ lum, sol ( λ) dλ T = T T lum, sol lum, sol( LT ) lum, sol( HT ) where T(λ) denotes transmittance at wavelength λ, ϕ (lum ) is the standard luminous efficiency function for the phototropic vision, and ϕ (sol) air mass 1.5 corresponding to the sun standing 37 above the horizon. 13 is the solar irradiance spectrum for (1) Donev, E. U.; Lopez, R.; Feldman, L. C.; Haglund, R. F. Confocal Raman Microscopy across the Metal- Insulator Transition of Single Vanadium Dioxide Nanoparticles. Nano lett. 2009, 9, (2) Silversmit, G.; Depla, D.; Poelman, H.; Marin, G. B.; De Gryse, R. Determination of the V2p XPS Binding Energies for Different Vanadium Oxidation States (V 5+ to V 0+ ). J. Electron. Spectrosc. 2004, 135,

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