Supporting Information for:

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1 Supporting Information for: A Full-Visible-Spectrum Invisibility Cloak for Mesoscopic Metal Wires Sang-Woo Kim, Byeong Wan An, Eunjin Cho, Byung Gwan Hyun, Yoon-Jong Moon, Sun- Kyung Kim, Jang-Ung Park The PDF file includes: Supporting Information Figures S1 S18 Legends for Supporting Information Movies S1 and S2 Other Supporting Information for this manuscript includes the following: Supporting Information Movies S1 and S2

2 Methods Fabrication of randomly networked Ag NFs. The randomly networked Ag NFs were fabricated using electrospinning, a simple technique for fabricating continuous long nanofibers with the suspension of Ag nanoparticles (Ag NPs) as ink for electrospinning process (NPK, Korea; average diameter of Ag nanoparticle: 40 ± 5 nm; solvent: ethylene glycol; concentration: 50 wt%). An electric field between the end of the nozzle and the ground of 7.6 kv/m was applied using electrospinning machine, and the inner and outer diameters of the nozzle were 0.33 and 0.64 mm, respectively. The electrospinning height was 15 cm. The environmental temperature and relative humidity were 17 and 4%, respectively. Ag NF diameters were controlled from 338 ± 33 nm to 1792 ± 167 nm by varying the flow rate of Ag NP ink from 0.2 ml/hr to 1.8 ml/hr, respectively. The electrospun nanofibers were annealed at 150 for 30 min in air (relative humidity: ~25%). The substrate used in the experiment was 1.5-mm-thick glass (Matsunami Glass Ind. Ltd, Japan; size: cm 2 ). Other details about the fabrication method of the electrospun metal NF network can be found elsewhere (1,3 5,14). As a transparent electrode, low-density Ag NF networks formed by electrospinning (flow rate: 0.2 ml/hr) exhibited a significantly low sheet resistance of 5.4 ± 0.9 per Ω sq with a transmittance of 96%. Fabrication of invisible Ag/Ag 2 O/NFs/IML. The Ag/Ag 2 O NFs/IML were prepared by the oxidation of Ag NFs and the coating of IML onto the oxidized Ag NFs sequentially. The oxidation process was conducted by using UVO treatment system (AC-6, AHTECH LTS, Korea). The random network of Ag NFs was placed at a distance of 4 cm from the mercury grid lamps for the exposure time of 10 mins. Then, an IML layer was coated onto the oxidized Ag NFs via the Meyer rod coating method. The IML solution (SOH 103H, ChangSung NanoTech., Korea) was dropped onto the oxidized Ag NFs, and then a Meyer rod (#7, RDspecialist Inc.) was pulled over the solution. During the Meyer rod coating process, the final film thickness was determined by the diameter of the used rods and the rod-to-rod spacing. The as-coated film was subsequently thermally annealed at 65 for 5 mins in the air. The thickness of an IML was typically 7 µm. Fabrication of random networked Ag NTs. Free-standing polymer fibers were produced by electrospinning a Polyvinylpyrrolidone (PVP) solution. PVP (Mw = 1,300,000 g/mol, Sigma- Aldrich) was dissolved in methanol at a concentration of 11 wt%. A voltage of 9.2 kv was applied to the nozzle (diameter: 0.64 mm) with the polymer solution flow rate of 0.3 ml/hr, and the free-standing PVP fibers were collected on an aluminum frame. Metal nanotroughs were prepared by thermal evaporation of Ag. A 100-nm-thickness Ag layer was deposited on the freestanding PVP fibers. The random network of Ag NTs was transferred to a glass substrate, and then PVP was removed in ethanol overnight in the air (relative humidity: ~25%). Other details about the fabrication method of the electrospun metal NT network can be found elsewhere

3 Fabrication of transparent electrode based heaters. To demonstrate the Ag/Ag 2 O NFs/IMR based heater, additional contact pads are required. PDMS shadow mask (sylgard-184, Sigma Aldrich) was used to prevent oxidation of both ends of AgNF random networks. After that, IML coating process was conducted in the Ag/Ag 2 O NFs region. Both ends of this transparent electrodes were connected to a voltage supplier after selectively forming additional silver epoxy (ELCOAT A-200, CANS, Japan) pads at the ends to act as contact pads. For the Ag NWs based heater, Ag NW solution (Sigma Aldrich, ; average diameter, and length of Ag NWs are 115 ± 2 nm and 40 ± 2 µm, respectively) was spin-coated at 500 rpm. for 30s onto a glass substrate. For the ITO based heater, a 120-nm-thick ITO film was deposited on a glass substrate using a radio frequency sputter system (SRN-120, SORONA, Korea). The average transmittance of Ag NWs and ITO films acquired at λ = 532 nm was ~94% for the visible spectrum. The sheet resistance of the Ag/Ag 2 O NFs/IMR, Ag NWs, and ITO film were 1.9, 15, and 60 Ω per cm 2, respectively. Cu wires were attached to both ends of each heater by using a silver epoxy (ELCOAT A-200, CANS, Japan), followed by epoxy curing for 10 mins at 25 C in air (relative humidity: ~ 25%). Heat spreading and defogging test. For the heat spreading test, Ag/Ag 2 O NFs/IML, Ag NWs, and 120-nm-thick ITO electrodes on polyimide (PI) film were prepared in the same manner as the heater fabrication steps. To minimize the heat transfer through a substrate, a thin and transparent PI film (Mitsubishi Corp., Japan; size: 5 50 mm 2 thickness: 25 µm, T g = 302 C) was used as the substrate for these three different heaters, instead of the thick glass (thickness: 1.5 mm). All heaters showed the same average optical transmittance of 94% for the visible spectrum. The one ends of the samples were attached horizontally on a hot plate (EC-1200NP, AS ONE Corp., Japan) by using a thermal grease (Samwon Chemical, Korea). Alternatively, the other ends of the samples were attached to a cooling pad. The temperature of hot plate was 140 as a hot spot, and the heat spreading was observed by thermal infrared camera. For the defogging test, Ag/Ag 2 O NFs/AR, Ag NWs, and ITO on glass substrate (Matsunami Glass Ind. Ltd, Japan) were prepared in the same manner as the heater fabrication methods. All of the electrodes were placed in a refrigerator for 60 mins to make the entire surface covered in frost. The size of the samples was mm 2 to match the automobile model. Surface, electrical, and optical characterizations of transparent electrodes. The surface morphology of the Ag NFs was observed by using a scanning electron microscope (S4600, Hitachi, Tokyo, Japan), an optical microscope (BX53, Olympus, Tokyo, Japan), a highresolution transmission electron microscope (HR-TEM) (JEM-2100F, JEOL, Tokyo, Japan). The atomic composition of the Ag NFs was analyzed by using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250XI, Thermo Fisher Scientific, USA). The four-point probe method and two-probe method were used for the sheet resistance and electrical resistance measurement by using a probe station with a Keithley 4200-SCS semiconductor parametric analyzer. To investigate the sheet resistance of Ag/Ag 2 O NF, 100-nm-thick gold pads were deposited by using thermal evaporation. The sheet resistance was estimated by using an electrical resistance change of each sample. The optical transmittance was measured by using an ultraviolet to near-infrared

4 spectroscopy system (Cary 5000 UV-Vis-NIR, Agilent, USA) with a diffuse reflectance accessory, and the transmittance of a glass substrate was used as a baseline. Thermal characterization of transparent electrodes. The DC bias was applied by using a power supply (Keithley 2260B-30-72, USA), and the temperature was measured by using a long wavelength infrared (LWIR) camera (T650sc, FLIR Systems, USA). The temperature distribution was analyzed using the FLIR Research IR software (Research IR Max, FLIR Systems). Measurement of scattering distributions. The scattering distributions were measured by a homebuilt far-field scanner equipped with a photodetector and an incident light source. A spectrometer (USB4000, Ocean Optics) was programmed to rotate along the azimuthal (φ) and polar (θ) directions with a step size of 2. A red- (λ = 660 nm)-, green- (λ = 532 nm), or blueemitting (λ = 450 nm) semiconductor laser (Three Color, Korea), or a broadband (λ = nm) supercontinuum laser (Super K Compact, NKT Photonics, USA) was used as an incident light source. For the measurement of the backward scattering distributions, the incident angle was set to be 60º with respect to a normal line. The ranges of the measured polar (θ) and azimuthal (φ) angles were set to be ±28º and ±40º, respectively, around the origin of (θ, φ) = (- 60º, 0º). For the measurement of the forward scattering distributions, the incident angle was set to be normal to the samples. The ranges of the polar and azimuthal angles were set to ±28º and ±180º, respectively, around the origin of (θ, φ) = (0º, 0º). Electromagnetic simulations. All the numerical simulations were performed by a homebuilt FDTD program. The FDTD simulations were used to obtain the scattering efficiencies for a variety of Ag/Ag 2 O NFs. To calculate the wavelength-resolved scattering cross section, electric and magnetic scattered fields were obtained by using the total-field scattered-field method (31), with a normally incident plane wave scanning the wavelength range between 450 and 700 nm in a step increase of 5 nm. Then, the obtained scattering cross section was divided by the total (Ag core + Ag 2 O shell) projected area to determine the scattering efficiency. The spatial resolution was set to be 5 nm for the in- plane axes. For the cylindrical axes, a periodic boundary condition was adopted.

5 Figure S1. Fabrication of Ag NFs by electrospinning process. (a) Plot of the diameter of Ag NFs vs. the solution flow rate used in the electrospinning process. (b) SEM images showing the randomly networked Ag NFs with different D s. Sale bars, 50 µm. (c) Magnified SEM images of single Ag NFs with D s. Scale bars, 1 µm. (d) Table summarizing the distribution parameters of randomly networked Ag NFs with different D s.

6 Figure S2. Material and optical characterization of an oxidized Ag shell. (a) XPS Spectrum of Ag 3d peaks of a bare Ag NFs film. (b) XPS Spectrum of Ag 3d peaks of an oxidized Ag NFs film with a 60-min UVO exposure. (c) Complex permittivity values of Ag 2 O and Ag films as a function of wavelength, adopted from reference (30) and (1), respectively.

7 Figure S3. Thickness controlled Ag 2 O shells by the duration of UVO exposure. (a) Schematic illustrations (upper) and SEM images (lower) of cross-sectional Ag/Ag 2 O NFs with an initial Ag NF diameter (D) of 400 nm with a variation of UVO exposure duration (0, 10, and 60 mins). (b) Plots of the Ag core diameter, Ag 2 O shell thickness, void thickness, and total (core + void + shell) diameter as a function of UVO exposure duration for an initial Ag NF diameter of 400 nm.

8 Figure S4. Camera images of randomly networked Ag NFs. (a,b) Schematic illustration (a) and photograph (b) of the camera imaging setup. (c) Photograph of a sample partly containing bare Ag NFs (left area) and Ag/Ag 2 O NFs/IML (right area), randomly dispersed on a glass substrate, in typical room light. Scale bar, 2 cm.

9 Figure S5. Effect of the thickness of an inner void shell. (a) Schematic of a Ag/void/Ag 2 O NF. An inner void shell is sandwiched between a Ag core and an outer Ag 2 O shell. (b) Simulated scattering efficiencies of a Ag/void/Ag 2 O NF with a variation of void thicknesses (d) for TE (left) and TM (right) polarized light. The diameter of the Ag core and the thickness of the outer Ag 2 O shell oxidation shell is 800 and 160 nm, respectively. The refractive index of the background is 1.5, which is identical to the IML used in the experiment.

10 Figure S6. Measured haze (λ = 550 nm) data for three different types of samples (bare Ag NFs, Ag/Ag 2 O NFs, and Ag/Ag 2 O NFs/IML) as a function of D. The oxidized samples have the same t value of 150 nm.

11 Figure S7. Fabrication of Ag NTs by electrospinning process. (a) Schematic illustrating the fabrication process of Ag nanotroughs (Ag NTs). (b d) Schematics and their matched SEM images of a Ag/Ag 2 O NT/IML (b), a Ag/Ag 2 O NT (c), and a bare Ag NT (d) with convex cross section. Scale bars, 200 nm.

12 Figure S8. Optical characterization of randomly networked Ag NTs. (a) Photographs of Ag/Ag 2 O NTs/IML with different diameter of Ag NTs on a glass substrate, taken in focused high-intensity light. Scale bars are 1 cm. (b,c) Measured transmittance (b) and haziness (c) of bare Ag NTs, Ag/Ag 2 O NTs, and Ag/Ag 2 O NTs/IML as a function of incident wavelength.

13 Figure S9. Quantification analysis of the measured backward scattering distributions. (a) Schematic illustrating the divergence angle (±θ) of scattered light. (b) Scattering intensity of bare Ag NFs, Ag/Ag 2 O NFs, and Ag/Ag 2 O NFs/IML as a function of divergence angle (±θ). The diameters (D) of Ag NFs are 300 (left), 750 (middle), and 1800 nm (right). All of the data are obtained from the measured backward distributions in Figures 2c 2e (λ = 450 nm). These angular scattering intensity plots are consitent with the simulated scattering efficiencies shown in Figure 3c.

14 Figure S10. Fabrication of micron-scale Ag/Ag 2 O NFs with a gradually variation of Ag 2 O shell thicknesses. (a) TEM images (upper) and SEM images (lower) of cross-sectional Ag/Ag 2 O NFs with an initial Ag NF diameter (D) of 1700 nm, after UVO exposure from 0 to 10 mins. Scale bars are 500 nm. (b) Plots of the diameter of Ag core, the thickness of Ag 2 O shell, the void thickness, and the total (core + void + shell) NF diameter as a function of UVO exposure duration.

15 Figure S11. Backward scattering distributions of micron-scale Ag/Ag 2 O NFs with a gradually variation of Ag 2 O shell thicknesses. Measured backward scattering distributions of the Ag/Ag 2 O NFs with an initial Ag NF diameter (D) of 1700 nm, as identically shown in Supporting Information Figure S8a. For these measurements, a green laser (λ = 532 nm) impinges on the samples with an incident angle of 60º.

16 Figure S12. Forward scattering distributions of Ag NFs with submicron- to micron-scale sizes. (a) Schematic of the measurement setup for forward scattering distribution. (b) Measured farfield distributions of bare Ag NFs, Ag/Ag 2 O NFs, and Ag/Ag 2 O NFs with IML with a variation of initial Ag NF diameters (D s), acquired at λ = 660 nm. For all of the oxidized samples, the Ag 2 O shell thickness is 160 nm.

17 Figure S13. Numerical scattering characterization of cloaked Ag/Ag 2 O NFs. (a) Surface plots presenting wavelength-dependent scattering efficiencies of a bare Ag NF, a Ag/Ag 2 O NF, and a Ag/Ag 2 O NF/IML with a variation of D s for TM polarized light. (b) Profiles of electric-field intensity of the structures with a Ag/Ag 2 O NF/IML (D = 800 nm) with a different t of 0, 20 or 100 nm, acquired at λ = 530 nm for TM polarized light. (c) Surface plots presenting wavelengthdependent scattering efficiencies of Ag/Ag 2 O NF/IML (D = 800 nm) with a variation of t s for TM polarized light.

18 Figure S14. Effect of an absorptive shell in cloaked Ag NFs. Surface plots presenting wavelength-dependent scattering efficiencies of core/shell Ag/dielectric NFs (D = 800 nm) with a variation of dielectric thicknesses (t s),for TE polarized light. The refractive index (n) and absorption coefficient (k) of (n, k) = (2.5, 0), (2.5, 0.1), of (2.5, 0.5) are imposed on the dielectric shell. The refractive index of the background is 1.5, which is identical to the IML used in the experiment.

19 Figure S15. Fabrication steps of the heater using cloaked Ag NFs.

20 Figure S16. Thermal reliability test of a Ag/Ag 2 O NFs/IML heater. Measured temporal change of the temperatures for a Ag/Ag 2 O NFs/IML heater, obtained by repeatedly applying a DC bias (6 V) for 50 cycles.

21 Figure S17. Comparison of the heating and cooling rates of NF-, NW-, and ITO-based heaters. (a) IR camera captured images of Ag/Ag 2 O NFs/IML, Ag NWs, and ITO heaters. Scale bars, 2 cm. (b) Measured heating and cooling rates of the same Ag/Ag 2 O NFs/IML, Ag NWs and ITO heaters as shown in (a).

22 Figure S18. Heat spreading characteristics of NF-, NW-, and ITO-based heaters. (a) Schematic illustration of a heat spreading test setup. All the samples are prepared on the 25-µm-thick polyimide substrate, attached horizontally on a hot plate. (b) IR camera captured images of the Ag/Ag 2 O NFs/IML, Ag NWs, and ITO films. Scale bar, 1 cm. (c) Measured spatial change of the temperatures for the same samples as shown in (b), as a function of the distance from the hot spot.

23 Supporting Information Movie S1. Clarity test of bare and cloaked Ag wires in high-intensity visible light. Supporting Information Movie S2. Defogging test of a cloaked-ag-wires-based defroster on an automotive windshield.