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Fast Adaptive Thermal Camouflage Based on Flexible VO2/Graphene/ CNT Thin Films Lin Xiao,*,† He Ma,‡ Junku Liu,† Wei Zhao,‡ Yi Jia,† Qiang Zhao,† Kai Liu,*,§ Yang Wu,‡ Yang Wei,*,‡ Shoushan Fan,‡ and Kaili Jiang‡ †

Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100094, China State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China § State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China ‡

S Supporting Information *

ABSTRACT: Adaptive camouflage in thermal imaging, a form of cloaking technology capable of blending naturally into the surrounding environment, has been a great challenge in the past decades. Emissivity engineering for thermal camouflage is regarded as a more promising way compared to merely temperature controlling that has to dissipate a large amount of excessive heat. However, practical devices with an active modulation of emissivity have yet to be well explored. In this letter we demonstrate an active cloaking device capable of efficient thermal radiance control, which consists of a vanadium dioxide (VO2) layer, with a negative differential thermal emissivity, coated on a graphene/carbon nanotube (CNT) thin film. A slight joule heating drastically changes the emissivity of the device, achieving rapid switchable thermal camouflage with a low power consumption and excellent reliability. It is believed that this device will find wide applications not only in artificial systems for infrared camouflage or cloaking but also in energy-saving smart windows and thermo-optical modulators. KEYWORDS: Vanadium dioxide, thermal camouflage, infrared, graphene, carbon nanotube

A

a low temperature monoclinic insulating phase to a high temperature rutile metallic phase, the optical conductivity of VO2 film changes abruptly, especially in the infrared region,12−14 which can be used in thermochromic applications as efficient smart windows15−17 and passive radiator.18 These promising properties make VO2 an ideal key material in adaptive thermal camouflage. Here we propose an adaptive thermal camouflage artificial system, which can blend into the surrounding background by electric heating to cheat thermal imaging cameras. The thermal camouflage system consists of a layer of free-standing nanothickness VO2/graphene/CNT (VGC) sandwich-like structure, which exhibits outstanding properties of fast switching, low power consumption and excellent reliability. The VGC sandwich-like thin film structure was prepared by directly growing VO2 film on graphene/CNT (GC) hybrid film (Figure 1a, see Methods). First, the fabrication of free-standing, strong GC thin film followed the general synthetic strategy reported in our previous work.19 Then the free-standing GC film was transferred on a quartz frame for further VO2

daptive camouflage is an ancient and amazing biological phenomenon seen in many animals. For example, octopus and chameleons change their skin color to match the surrounding background.1−4 Similar to this visual coloration, camouflage in infrared region is also a vitally important technique for the purpose of cloaking in thermal imaging, which attracts increasing attentions in lots of commercial and military applications.5−7 Considering the fact that thermal cameras can only detect and visualize thermal radiance pattern from targets, but may not directly reflect their real temperature,8 thermal cheating can be realized in two ways by modulating temperature or thermal emissivity. In most thermal camouflage situations, a hot target usually needs to be hidden into a relative cool background. Directly cooling the target is a feasible but not ideal approach,9 as excessive heat may still emit somewhere from the target. Instead of controlling temperature, engineering of emissivity provides an alternative and practical way for thermal camouflage. However, practical devices with an active modulation of emissivity have yet to be developed. Vanadium dioxide (VO2), which undergoes an insulator-tometal transition around 68 °C,10,11 has been demonstrated as a natural disordered metamaterial for emissivity engineering application, due to its large negative differential thermal emittance across the transition temperature.5 Transiting from © 2015 American Chemical Society

Received: October 8, 2015 Revised: November 18, 2015 Published: November 24, 2015 8365

DOI: 10.1021/acs.nanolett.5b04090 Nano Lett. 2015, 15, 8365−8370

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Figure 1. Fabrication and characterizations of VGC film. (a) A schematic fabrication process of VGC film. (b) Raman spectra of VGC film at room temperature (blue line) and 90 °C (red line). The characteristic vibration modes of insulating VO221,22 are given. (c) VGC film on a flexible insulating black tape, which is attached to a 12 mm-diameter steel rod. (d) SEM image of VGC film. The crossed-stacked CNT network beneath the VO2 layer is shown here. The scale bar in (d) indicates 10 μm.

Figure 2. Negative thermal emittance of the VGC film. (a) The schematic setup for measurement of thermal emissivity of the VGC film. (b−i) Thermal images recorded by the thermal camera during the temperature cycling. (j) The difference of IR temperature between the VGC film (TIR VGC) and the background (TIR B ). The emissivity in the camera is set to 0.95 in panels (b−j). (k) The temperature-dependent emissivity of the VGC film on black tape and the background coating. The arrows in panels (j) and (k) indicate the temperature cycling loop.

The free-standing VGC film can be transferred on any flexible surface for thermal cloaking. In our experiment, we attached the VGC film to a flexible frame made by a layer of insulation tape, which made it convenient for the film to be further attached to any surface. Figure 1c shows a VGC film on the tape adhered to a steel rod. The continuous, flat graphene layer in the VGC film with good mechanical property can fully fill the voids in the crossly stacked CNT film, guaranteeing a continuous and smooth VO2 layer (as shown in the SEM image in Figure 1d), which plays a crucial role to improve the thermochromic properties of the VGC film. As the VO2 layer was likely to be coaxially coated on the surface of CNTs (Supporting Information Figure S1a), VO2/CNT film without

deposition. Stoichiometric VOx film was deposited onto GC film by DC magnetron sputtering with a target of vanadium metal,15,20 and further annealed at 450 °C in low pressure O2 atmosphere to render VOx crystalline to near-stoichiometric VO2.14 The Raman spectrum (Figure 1b) of as-fabricated VGC film at room temperature displays peaks at 141(Ag), 194(Ag), 223(Ag), 261(Bg), 309(Ag), 337(Bg), 390(Ag), 500(Ag), and 617(Ag) cm−1, which confirm the characteristic vibration modes for the M1 insulating phase of VO2.21,22 These Raman characteristic peaks gradually vanish as the temperature rises due to transition of VO2 to its metallic phase above transition temperature. 8366

DOI: 10.1021/acs.nanolett.5b04090 Nano Lett. 2015, 15, 8365−8370

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Figure 3. Adaptive thermal camouflage demonstration for VGC-based device. (a) Thermal radiance from the background coating (black line) and free-standing VGC film (heating, red curve; cooling, blue curve). The VGC film is heated following two heating loops, i.e., A1−B1 and A2−H2−B2, to make the thermal radiance the same as that from the background coating. The arrows indicate the heating loops. The inset shows the structure of VGC-based device. (b−f) Thermal images of camouflage processes by direct (b,c) and hysteretic (d−f) heating strategies. (g) The evolution of IR temperature of the VGC film (A1−B1 and A2−H2−B2) and background coating (X1 and X2).

graphene layer exhibits a lower transmittance contrast because the voids on the film (Supporting Information Figure S1b) reduce the thermal camouflage effect. The dramatic change in complex refractive index of VO2 film12,23−25 across the phase transition makes the VGC film exhibit high optical contrast, especially in infrared region, for thermal camouflage applications. We used a thermal camera (Model PI160 Optris Inc., spectral range 7.5 to 13 μm) to measure temperature-dependent emissivity of VGC film, as well as to demonstrate its active thermal camouflage process. A freestanding VGC film (∼5 mm × 5 mm) was attached to a largearea black tape. The rest of the uncovered part of the tape was painted with thermal silver glue as background. As the experiment setup in Figure 2a shows, the thermal camera (emissivity was set to 0.95, except in the measurement of VGC film’s emissivity) points in normal direction at the sample to record the IR temperature. It is noteworthy that the VGC film appears to be hotter at low temperatures but cooler at high temperatures than the background during the temperature cycle (as shown in the thermal images in Figure 2b−i). IR temperature differences between the VGC film (TIR VGC) and the background (TIR B ) implies thermal emissivity differences between them at corresponding temperature (Figure 2j). In order to test the thermal emissivity of the sample, we employed a commonly used procedure to determine the unknown emissivity for IR thermometers in industry8 (see Methods). Briefly, integrated emissivity of the target was obtained by modifying emissivity in the thermal camera until IR temperature was equal to that measured by a thermocouple. Similar to most materials, the emissivity of the background is nearly independent of temperature (Figure 2k), while that of VGC film on black tape shows apparent negative differential emissivity around the transition temperature. The large tunability of emissivity (∼0.86 at 40 °C and ∼0.49 at 90 °C) makes the VGC film a promising material in active thermal camouflage. Any object above zero temperature emits thermal radiation, and its spectral radiance can be expressed as26

M ( λ , T ) = S ( λ , T ) ε (λ , T ) =

2πhc 2 1 ε (λ , T ) 5 hc / λkBT λ e −1

where λ is the spectroscopic wavelength, M(λ,T) and S(λ,T) are the spectral radiance from the object and a blackbody at the same temperature respectively, ε(λ,T) is the spectral emissivity of the surface, h is Planck’s constant, c is the speed of light in vacuum, and kB is the Boltzmann constant. Considering the sensing principle of thermal cameras, the electric signal U detected by an IR thermometer can be expressed as8 U = C[εSobj + (1 − ε)Samb − Scmr] = CM total

where C is specific constant of the IR thermometer, ε is the integrated emissivity of the object in the spectral range of the IR thermometer, and Sobj, Samb, and Scmr are integrated thermal radiances from blackbodies in the spectral range of the IR thermometer at the object, environment, and thermal camera temperature, respectively. If the total thermal radiance Mtotal from two objects are equal, thermal cameras cannot sense any differences. Based on the above analysis, we regulated the thermal radiance from VGC film, thanks to its large tunability of emissivity, according to the surrounding background for thermal camouflage. The good electrical conductivity of graphene/CNT substrate provides an efficient way to electrothermally drive VO2 phase transition. We fabricated a simple two-terminal current-controlled structure to demonstrate the thermal camouflage working principle (Figure 3a, inset). The structure consists of a layer of VGC film with Cu foils as electrodes and a layer of silver thermal glue (average emissivity ≈ 0.67 as shown in Figure 2k) as background coating. As the temperature of the thermal camera Tcmr is usually close to that of the environment Tamb in our experiments, the total thermal radiance Mtotal can be further simplified as M total ≈ ε(Sobj − Samb)

The calculated thermal radiances from the background coating and VGC film are given in Figure 3a (see details of theoretical calculations in Supporting Information). As the thermal radiances from VGC film followed different paths 8367

DOI: 10.1021/acs.nanolett.5b04090 Nano Lett. 2015, 15, 8365−8370

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Nano Letters during the heating and cooling processes due to the emissivity hysteresis of VGC film, we classified two different tunable temperature regions for thermal camouflage. The corresponding temperatures of intersections, where the tangent lines of heating and cooling curves of VGC film cross with that of the background, are determined as tb and ta in Figure 3a. The temperature ta determines the minimum temperature that VGC film can cloak activated by joule heating, while the temperature tb distinguishes the two different heating strategies, i.e., direct and hysteretic heating. Above tb, the thermal radiance from VGC film can be adapted to that of the background by direct heating. However, if the temperature is between ta and tb, VGC film needs to undergo a hysteretic heating strategy for thermal camouflage. In the following part, we conducted two experiments to demonstrate these two heating strategies, respectively. For direct heating strategy at temperature t1 = 65 °C, because thermal radiance from the VGC film (A1 in Figure 3a) is larger than that from the background (X1 in Figure 3a), VGC film appears to be hotter than the background as shown in Figure 3b. Due to the negative emissivity of the VGC film, the thermal radiance from VGC film decreases even when the film is locally heated to B1, where the radiance from VGC film is equal to that from the background (the corresponding thermal image is given in Figure 3c). As a result, the average IR temperature difference (ΔT1) between VGC film and the background decreases from 5.1 to 0.7 °C (Figure 3g and Supporting Information Movie 1). For hysteretic heating strategy at temperature t2 = 54 °C, the VGC film is heated from A2 to H2 (heating current 12 mA), then to B2 (heating current 6.5 mA), where the VGC film and background emit the same thermal radiance (A2, H2, B2 in Figure 3a). After that, the VGC film is blended into the background (Figure 3d−f and Supporting Information Movie 2). The corresponding IR temperature difference (ΔT2) decreases from 4.9 to 0.1 °C (Figure 3g). The thermal insulation and the extremely small heat capacity of the free-standing VGC film across the contacts allows the device to achieve faster response and lower power consumption. The power consumption for a complete phase transition of the VGF film at 10 mA is approximately 4.2 mW/ mm2 (Figure 4a), about two orders smaller than that of flexible thermochromic camouflage systems based on Si heater.27 The greatly reduced power consumption here is attributed to the extremely small heat capacity per unit area of the VGC film with nanothickness graphene/CNT substrate.19,28 We employed a photodetector with 0.35 ns rise time to measure the optical response of VGC film induced by electric pulses (Schematic setup is shown in Supporting Information Figure S2). The signals detected by the photodetector vary in response to the transmittance change of VGC film (Figure 4b). The corresponding off−on and on−off times (10% and 90% of the step height) are about 40.5 and 27 ms, respectively. The relatively rapid switching time here is superior to those of artificial thermochromic27 and electrochromic29−31 devices, and biological chromatophore organs32 (the comparison is shown in Table 1) due to the extremely small heat capacity of nanothickness VGC film. The rapid tunability of VGC film’s emissivity will be an outstanding property for adaptive thermal camouflage devices. The good reliability of VGC film is reflected by totally unchanged transmittances in metallic and insulating phases after over 100,000 cycles of full phase transition of VO2 driven by joule heating (Figure 4c), which is highly promising for future thermal camouflage applications.

Figure 4. Performance of VGC-based camouflage system. (a) Currentdependent optical transmission (black line) of VGC film at the wavelength of 1.5 μm and the corresponding power consumption (red dash line). (b) The transmitted response (black line) of VGC film induced by current pulses (blue line). (c) Reliability test of the VGC film over 100,000 on/off cycles in response to current pulses (duty cycle 1:1, cycle 2 Hz, amplitude current 10 mA).

Table 1. Comparison of Switching Times for Different Chromogenic Mechanisms mechanisms electrochromism

switching time [s]

systems WO3 with Ag-nanowire electrodes

PProdot-EtHx2 with Ag grid/ PEDOT electrodes

Viologen with ZnO-nanowire electrodes

biochrome thermochromism

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chromatophore organs of octopus flexible adaptive optoelectronic camouflage system VGC film thermal camouflage system

refs

∼1 (coloring) ∼4 (bleaching) 2.5 (coloring) 0.3 (bleaching) 0.17 (coloring) 0.142 (bleaching) 0.27

29

∼1

27

∼0.04

this work

30

31

32

DOI: 10.1021/acs.nanolett.5b04090 Nano Lett. 2015, 15, 8365−8370

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copper heating plate (80 cm × 80 cm × 3 mm), whose temperature was measured by a thermocouple (Pt100 temperature sensor) and controlled by a temperature controller (Model 332 temperature controller, Lake Shore Cryotronics Inc.). The integrated emissivity was obtained by changing the emissivity in the infrared camera until the IR temperature was the same as that of the thermocouple. Adaptive Thermal Camouflage Demonstration. A layer of free-standing VGC film with the black tape substrate was connected to copper foil as electrodes (Figure 3a Inset). Except the VGC film, the surfaces of copper electrodes and substrate were coated with a layer of silver thermal glue (integrated emissivity ∼0.67) as background. The VGC film was electrically heated by a programmed source meter (Agilent 2902A). The thermal images and videos were recorded with a thermal camera (PI160, Optris Inc.). Thermochromic Response Measurement for VGC Film. The schematic experimental setup of the thermochromic response measurement was shown in Supporting Information Figure S2. A free-standing VGC film was illuminated by a nearinfrared CW laser (wavelength 1554 nm). The VGC film was fed with square wave currents (duty cycle 1:1, cycle 2 Hz, amplitude current 10 mA) by a pulse generator (AgilentKeysight 8110A pulse generator). The transmitted light through the VGC film was detected by an InGaAs photodetector (wavelength range 850−1650 nm, 3 dB bandwidth 1 GHz, KG-PR-1G, Conquer Optical Technology Co.) and recorded by an oscilloscope (Tektronix TPS2012B).

Conclusions. We developed free-standing VGC films as emissivity tunable structures for adaptive thermal camouflage applications. With the application of conductive graphene/ CNT substrate, the thermal radiance from VGC films can be electrically regulated to the same as that from the background. The flexible VGC films can be adapted to various flexible substrates (such as textiles, plastics, and papers) with rapid switching response, low power consumption, and long-term stability. As the transition temperature of VO2 can be decreased continuously to room temperature by doping with transition metal elements,33−38 the work temperature of VGC-based thermal camouflage devices can be made to cover room temperature applications. The great simplicity of VGC-based device structure, coupled with its dramatic optical tunability, will find promising applications in optoelectronics, such as active modulated energy-saving smart windows and electrical induced thermo-optical modulators. Methods. Fabrication of Graphene/CNT Films. The fabrication process of graphene/CNT film can be found in our previous publication.19 The graphene was synthesized on Cu foil by low pressure thermal CVD method. The Cu foils were initially annealed at 1000 °C in 40 mTorr hydrogen atmosphere. After that, the graphene growth was carried out with flowing gas mixtures H2/CH4 = 2:35 sccm under 500 mTorr for 20 min. Superaligned CNT (SACNT) films were drawn from the SACNT array,39 and then crossly stacked on the graphene. After etching the Cu foil in 0.5 mol/L ferric chloride aqueous solution, graphene/CNT hybrid film was rinsed in deionized water several times and transferred to an quartz frame for further VO2 deposition. Synthesis of VO2 on Graphene/CNT or CNT Film. The synthesis process of VO2 film was similar to that in ref 15. The VOx layer was deposited onto the free-standing graphene/CNT film by a DC magnetron sputtering system with vanadium metal target (3 in., 99.95% purity). The sputtering was carried out with flowing gas mixtures (15 sccm Ar/O2 with O2 2% v/v, 35 sccm Ar) under 0.56 Pa for 20 min (DC power of 60 W) at room temperature. After VOx deposition, the VOx/graphene/ CNT was annealed in oxygen atmosphere (4 sccm O2, 4.5 Pa) at 450 °C for 10 min. The carbon-based film was partly oxidized during the high temperature annealing process in O2 flowing (as shown the Raman spectra in Supporting Information Figure S3 before and after annealing). The thickness of VO2 film was about 80 nm for the same synthesizing process on a quartz substrate. Physical Characterizations. SEM imaging was performed on a scanning electron microscope (FEI Nova NanoSEM 450), operating at 15 kV. The Raman spectra of the VGC film on Si wafer with 400 nm SiO2 was measured by a Raman spectroscopy (Jobin Yvon LabRAM HR800). The temperature of the sample was controlled by a temperature controller (Model 332 temperature controller, Lake Shore Cryotronics Inc.). Optical transmittance of the film was measured with a PerkinElmer-Lambda 950 (ultraviolet−visible−near-infrared) Spectrometer and a Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer. Integrated Emissivity Measurement. The integrated emissivity was measured by an thermal camera (PI160, Optris Inc.) following a standard procedure in industry. As shown in the experimental setup in Figure 2a, a layer of free-standing VGC film (5 mm × 5 mm) was attached to a layer of black tape (3M Scotch Black PVC Electrical Insulation Tape, 0.177 mm thick). The black tape with VGC film was in turn attached to a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04090. Optical transmittance of VO2/CNT film without graphene, experimental setup for thermochromic response measurement of VGC film, Raman spectra of graphene film before and after annealing in O2 flowing, and theoretical calculations (PDF) Adaptive thermal camouflage demonstrations for direct heating strategies (AVI) Adaptive thermal camouflage demonstrations for hysteretic heating strategies (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

L.X. and H.M. contributed equally to this work. L.X. and K.J. conceived the idea for the project. L.X., H.M., K.L., W.Z., and Y.W. prepared the VGC film and conducted the emissivity tests and thermal camouflage demonstration experiments. H.M. and J.L. conducted the Raman spectroscopy measurements. H.M. and Y.J. performed the SEM characterization. H.M., Y.J., and Q.Z. performed the optical transmittance and FTIR measurements. All the authors participated in discussions of the research and drafted the manuscript. Notes

The authors declare no competing financial interest. 8369

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ACKNOWLEDGMENTS The work was financially supported by the National Basic Research Program of China (2012CB932301), National Natural Science Foundation of China (51202012, 51202119, 51472142, and 51502337), and Chinese Postdoctoral Science Foundation (2014M550701 and 2015T80070).

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DOI: 10.1021/acs.nanolett.5b04090 Nano Lett. 2015, 15, 8365−8370