High-temperature differences in plasmonic broadband absorber on PET and Si substrates

The characteristics of a plasmonic resonator with a metal–dielectric–metal structure is influenced by the size, shape, and spacing of the nanostructure. The plasmonic resonators can be used in various applications such as color filters, light emitting diodes, photodetectors, and broadband absorbers. In particular, broadband absorbers are widely used in thermophotovoltaics and thermoelectrics. To achieve a higher photothermal conversion efficiency, it is important to provoke a larger temperature difference in the absorber. The absorption and thermal conductance of the absorber has a great impact on the temperature difference, but in order to further improve the temperature difference of the absorber, the thermal conductivity of the substrate should be considered carefully. In this study, we designed Cr/SiO2/Cr absorbers on different substrates, i.e., polyethylene terephthalate (PET) and silicon. Although their optical properties do not change significantly, the temperature difference of the absorber on the PET substrate is considerably higher than that on the Si substrate under laser illumination, i.e., 164 K for ΔTPET and 3.7 K for ΔTSi, respectively. This is attributed to the thermal conductance of the substrate materials, which is confirmed by the thermal relaxation time. Moreover, the Seebeck coefficient of graphene on the absorber, 9.8 μV/K, is obtained by photothermoelectrics. The proposed Cr/SiO2/Cr structure provides a clear scheme to achieve high performance in photothermoelectric devices.

www.nature.com/scientificreports/ thermoelectric applications. For instance, the thermoelectric conversion efficiency (η) at ΔT = 200 K (ZT = 1) is 8.2%, which is significantly higher than η = 4.8%, which is obtained at ΔT = 100 K 15 . The relationship between heat (Q) and temperature difference (ΔT) is expressed as the following relation: Q = C P m T , where C P and m are the specific heat and mass, respectively 16 . The heat flow ( − → Q ) is intimately related to the thermal conductivity ( κ ) based on the Fourier law: − → Q = −κ − → ∇ T , where − → ∇ T is the temperature gradient 17 . A large ΔT is achievable because the broadband MDM absorber has a low mass owing to the nanoscale thickness of the metal layers. However, for a high − → ∇ T , the thermal conductivity of the absorber should be carefully considered because the heat generated through light absorption in the top metal layer can be easily transferred to the substrate.
In this study, we demonstrate a Cr/SiO 2 /Cr absorber with polyethylene terephthalate (PET) and Si substrates to determine the effect of substrate thermal conductivity for efficient photothermal conversion. Regardless of the type of substrate, the simple Cr/SiO 2 /Cr layered structure absorbs light with high absorption (~ 97%), over visible wavelengths ranging from 450-800 nm 13 . However, the absorber on the PET with a low thermal conductivity, κ = 0.15 W/mK 18 , shows ~ 40 times higher temperature difference, compared to that on the Si substrate (bulk Si: κ = 40-150 W/mK 19,20 ). Furthermore, under the large temperature difference, we obtained the Seebeck coefficient of monolayer graphene on the absorber with the PET to be approximately 9.8 μV/K.

Results and discussion
Schematic and optical property of the absorber. Schematics of the Cr/SiO 2 /Cr absorber on the PET and Si substrates are presented in Fig. 1a. The Cr/SiO 2 /Cr absorber comprises a top Cr layer (3 nm), middle transparent SiO 2 layer (100 nm), and bottom reflection Cr layer (100 nm), with a SiO 2 layer (3 nm) on the top Cr layer to prevent oxidation 13 . The Cr and SiO 2 layers were evaporated using an electron beam evaporator on the PET and Si substrates. The measured reflectance spectra of the Cr/SiO 2 /Cr absorber on the PET and Si substrates indicate that the optical properties did not change significantly with the change in the substrate, as shown in Fig. 1b. Furthermore, using finite element method (FEM) simulations, the reflectance of the Cr/ SiO 2 /Cr structure was calculated. The difference between the measured and calculated reflectance was primarily caused by experimental errors, such as the thickness and roughness of the Cr and SiO 2 layers. Figure 1c illustrates the absorptance (A%), reflectance (R%), and transmittance (T%) of the Cr/SiO 2 /Cr absorber on the PET substrate. Based on the measured reflectance (R%) and transmittance (T%), the absorptance (A%) was obtained using the simple relation: A + R + T = 100 (%) 13,14 . The maximum absorptance of the Cr/SiO 2 /Cr absorber on the PET substrate was 92% near a wavelength of 870 nm, which response may show good absorptance performance in angled illumination as well 2,13 . Temperature of the absorber. The photothermal performances of the Cr/SiO 2 /Cr absorber on the PET substrate, Cr/SiO 2 /Cr absorber on the Si substrate, and bare PET and Si substrates were investigated using an infrared camera system (InfraScope) under laser illumination in the ambient atmosphere. Figure 2a-d show the thermal images under a 906-nm laser illumination with a laser power of 70 mW. The average temperatures of the white-square-marked areas in the thermal images differed vastly with respect to the substrates. The average temperature of the Cr/SiO 2 /Cr absorber on the PET substrate increased significantly from 26.7 °C to 190.7 °C (ΔT = 164 K), as compared with the Cr/SiO 2 /Cr absorber on the Si substrate (ΔT = 3.7 K), bare PET (ΔT = 0.3 K), and Si (ΔT = 0.9 K). In particular, as shown in Fig. 2e, the temperatures of the Cr/SiO 2 /Cr absorber on the PET substrate differed significantly in comparison to that on the Si substrate, even though they had identical structures with similar absorptance.
Photothermal performance. Figure 3a,b present the time-dependent temperature differences (ΔT) of the Cr/SiO 2 /Cr absorber on the PET and Si substrates, under different laser powers. The relaxation behavior of the temperature differences appeared during laser on and off. As shown in Fig. 3c, the thermal relaxation times of the absorbers were calculated via the fitting method, using the exponential rise (solid line) and decay (dashed line in Fig. 3c), ΔT = A exp(1 − t/τ), and ΔT = A exp(t/τ), where A and τ are a constant and the relaxation time, respectively 21 19,20 ). This difference in the thermal relaxation times of the absorbers indicates that the substrate affected the photothermal performance due to heat transfer. Figure 3d shows that the maximum temperature differences of the Cr/SiO 2 /Cr absorber on the PET and Si substrates are approximately 160 K and 4 K under a 906 nm laser illumination (70 mW, 10 s), respectively. As shown in Fig. 3e, the maximum temperature differences of the absorbers increase monotonously with increasing laser power. The maximum temperature difference of the Cr/SiO 2 /Cr absorber on the PET substrate was significantly higher than that of the Cr/SiO 2 /Cr absorber on the Si substrate.
Thermoelectricity in graphene. The Seebeck coefficient of single-layer graphene was measured using the photothermoelectric effect, which is based on the thermoelectric effect with a temperature gradient due to light absorption 23,24 . Figure 4a shows the optical and thermal images of the monolayer graphene on the Cr/SiO 2 /Cr absorber with a PET substrate, under laser illumination (906 nm, 70 mW, 10 s). The Cr/SiO 2 /Cr absorber was evaporated using an electron beam evaporator on a Au-coated PET substrate, as shown in Fig. 4a,b. Bulk indium was used to establish an electrical contact with the other electrode. Figure  . The I-V curve was obtained by the leakage current of the deposited SiO 2 layer (see Fig. 4d). The I-V curve exhibits a linear behavior under both dark and illumination conditions (906 nm, 70 mW, 10 s). The I-V curve is shifted under the illumination condition, as compared with that under the dark condition. Figure 4e shows that the temperature difference and open-circuit voltages (V oc ) depend linearly on the laser power. This behavior indicates that the V oc is primarily affected by the temperature difference, which is caused due to light absorption. The photovoltage due to the photothermoelectric effect is expressed as where S 1 and S 2 are the Seebeck coefficients of the materials, expressed as S = V/∇T 23,24 . Based on the measured voltage differences with respect to the temperature differences in Fig. 4f, the Seebeck coefficient of the device was 7.9 μV/K, which was obtained via the linear fitting method. As the layer composed of graphene and gold primarily contributed to the temperature gradients, the Seebeck coefficient of the graphene can be expressed as follows: S G (μV/K) = 7.9 (μV/K) + S Au (μV/K), where S Au has been reported to be approximately 1.9 μV/K near room temperature 28 . Therefore, the Seebeck coefficient of the graphene was approximately 9.8 μV/K, which is consistent with the values reported in literature (9 ~ 12 μV/K 29,30 ).

Conclusions
In summary, we demonstrated a Cr/SiO 2 /Cr broadband absorber on PET and Si substrates to compare the temperature differences caused by the difference in thermal conductivity. Based on the thermal images, the temperature differences were 160 K for the Cr/SiO 2 /Cr absorber on a PET substrate and 4 K for that on the Si substrate, under a 906-nm laser illumination with a laser power of 70 mW. The results of the thermal relaxation time indicated that the temperature differences were primarily affected by the thermal conductance of the substrates. The temperature differences of the Cr/SiO 2 /Cr absorber on the PET and Si substrates indicated the equal importance of thermal conductivity and absorbance to achieve high photothermal performance of the light absorber. Additionally, based on the temperature difference of the Cr/SiO 2 /Cr broadband absorber, the Seebeck coefficient of the monolayer graphene was found to be approximately 9.8 μV/K, which was obtained using the photothermoelectric effect. The result of the photothermoelectric effect indicated that the Cr/SiO 2 /Cr broadband absorber with the PET substrate can be used as an absorber in photothermoelectric devices including flexible applications, photothermal energy generators, and photothermal sensors.

Methods
Calculation. The reflection and transmission coefficients were calculated using the finite element method simulations. We used periodic boundary conditions with a unit cell of 100 nm along the x-and y-directions and perfectly matched layers at the boundaries in the z-direction. Plane waves were launched along the normal direction of incidence into the unit cell along the z-direction, and a power monitor was placed behind the source and the structure. The frequency-dependent complex refractive index of the material data, including Cr, SiO 2, and other materials used in the numerical simulations, was obtained from the data reported by Palik 31 .
Preparation of the absorber. The Cr and SiO 2 layers were simultaneously deposited on the PET and Si substrates, respectively, using an electron beam evaporator under high vacuum (~ 10 -7 torr). Single-layer graphene was grown via chemical vapor deposition and subsequently transferred onto the Cr/SiO 2 /Cr structure via wet transfer.
Measurements. The reflectance and transmittance were measured using a UV-VIS-NIR spectrophotometer (V-670, JASCO) with an integrating sphere from 400 to 1,500 nm at room temperature. The thermal images were captured using temperature measurement microscope systems (InfraScope III, Quantum Focus Instruments Corporation). The electrical measurements were performed using Agilent 4156C.