Thermal hysteresis measurement of the VO2 emissivity and its application in thermal rectification

Hysteresis loops in the emissivity of VO2 thin films grown on sapphire and silicon substrates by a pulsed laser deposition process are experimentally measured through the thermal-wave resonant cavity technique. Remarkable variations of about 43% are observed in the emissivity of both VO2 films, within their insulator-to-metal and metal-to-insulator transitions. It is shown that: i) The principal hysteresis width (maximum slope) in the VO2 emissivity of the VO2 + silicon sample is around 3 times higher (lower) than the corresponding one of the VO2 + sapphire sample. VO2 synthesized on silicon thus exhibits a wider principal hysteresis loop with slower MIT than VO2 on sapphire, as a result of the significant differences on the VO2 film microstructures induced by the silicon or sapphire substrates. ii) The hysteresis width along with the rate of change of the VO2 emissivity in a VO2 + substrate sample can be tuned with its secondary hysteresis loop. iii) VO2 samples can be used to build a radiative thermal diode able to operate with a rectification factor as high as 87%, when the temperature difference of its two terminals is around 17 °C. This record-breaking rectification constitutes the highest one reported in literature, for a relatively small temperature change of diode terminals.

where , = ) 0A ( C D 26 − 1) is the blackbody spectral radiance, ) = 1.48× 10 0)A Wm K , K = 1.43×10 0K m · K, and = 5.67×10 0Q Wm 0K K 0R . To determine the numerical value of ) , the reflection and transmission spectra of samples 1 and 2 have been measured for wavelengths between 2 µm and 22 µm, by means of a PerkinElmer Frontier FTIR spectrometer. The results are shown in Figure S1. Figure S1. Experimental transmittance and reflectance spectra of (a) sample 1 and (b) sample 2 at room temperature, as functions of the spectral wavelength. According to the ratio of emitted power , the main contributions to the integrant in Eq. (S1), and therefore to the average emissivity ) , come from wavelengths between 5 µm and 15 µm. This indicates that the spectral range of 2-22 µm is wide enough of calculate the integral in Eq.
(1) and thus to determine ) with a fairly good accuracy, specially for sample 1. The final results are, ) = 0.59 for sample 1 and ) = 0.52 for sample 2. Figure S2. Ratio of emitted power (REP) of sample 1 (black line) and sample 2 (blue line) at room temperature.

Effects of the deposition process, film thickness, and substrate on the VO 2 films
In a bottom-up approach, the characteristics of thin films with thicknesses up to 1 µm, are strongly influenced by the deposition process as well as by the nature and temperature of their substrates. For a crystallized substrate, the classical three primary deposition modes: Volmer-Weber (3D ad-atom clusters = islands), Frank-van der Merwe (2D ad-atom = layers) and Stranski-Krastanov (layer + island growth, a mixture of the first two) can be applied to epitaxially grows thin films at a crystal surface or interface. For the case of the PLD process used in this work, the material deposition is done out of thermodynamic equilibrium and in a strong directional way, which favours the appearance of islands (grains) and columnar growth.
The two substrates r-sapphire and silicon used in this work are different regarding the nature elements and the atomic structures. r-sapphire is a single crystal of Al 2 O 3 cut in a specific orientation (1102), with a very smooth surface (roughness peaks less than 0.3 nm). c-and rsapphire are ideal substrates for VO 2 growth due to the relatively small lattice mismatch. O the other hand, the Si(100) substrate constitutes a single crystal cut through the crystal exposing the (100) surface. An amorphous native oxide layer with a thickness of about ~5 nm covers the Si(100) surface.  Figure S3. SEM (1.5µm ´ 2.0µm) images obtained for four VO 2 thin films (top views) synthesized using identical PLD parameters. (a) and (c) stand for two VO 2 films deposited on a native-SiO 2 /Si substrate and show grains with sharp shapes. (b) and (d) represent two VO 2 films deposited on an r-sapphire substrate and display flat grains with an in-plane growth. The cracks observed in (d) are due to a defect of the platinum deposition during the metallization process used for obtaining the SEM images. The size of the grains is thus a function of VO 2 film thickness but their shape is determined by the substrate mainly.
According to Figs. S3 and S4, the substrates of r-sapphire and SiO 2 /Si lead to the formation of clusters of atoms (grains) with different shapes and distributions. The separated grains developed on SiO 2 /Si exhibit pyramidal, cubic, elongated, and sharp shapes with a disorganised distribution due essentially to the amorphous SiO 2 nanolayer. By contrast, the grains formed on r-sapphire present elongated and spherical-like shapes, and their sizes in the (xy) substrate plan are larger than the ones growth on SiO 2 /Si. Furthermore, the degree of connectivity of the grains on r-sapphire is higher than the one of the grains on SiO 2 /Si, as confirmed by the AFM images shown in Fig.3. The r-sapphire substrate tends to organize the grains and lead to low roughness (Sa=4.98 nm, Sq=6.18 nm, Sz=42.30 nm, ISO25178), while the SiO 2 /Si allows the growth of grains in the x, y and z directions, which leads to a relatively high roughness (Sa=12.30 nm, Sq=15.40 nm, Sz=105.00 nm, ISO25178) with a peak-to-peak distance (105 nm) comparable to the film thickness (100 nm). The SiO 2 amorphous layer is thus the source of the individual grains with random distribution shown in Figs. S3 and S4. Figure S4. AFM (2.0 µm ´ 2.0 µm) images obtained for two VO 2 thin films (100 nm) growth on substrates of (a) r-sapphire and (b) SiO 2 /Si. The average peak-to-peak distance (105.0 nm,) in (a) is longer than that (42.3 nm) in (b), which indicates that the roughness of the sample in (a) is higher than the one of the sample in (b).
Microstructures of the VO 2 +r-sapphire samples determined by SEM and AFM are consistent with the structure (atomic arrangement) provided by the XRD patterns shown in Fig. S5, where the VO 2 peaks are clearly visible. For the two thinner samples (100 nm and 200 nm in thicknesses), the VO 2 layers exhibit a mono-orientation characterized by (200) and (400) diffraction peaks, which is compatible with the grain shapes shown by both the SEM and AFM images in Figs. S3 and S4. An addition orientation (111) appears for the 400 nm-thick sample, which shows that as the film thickness increases, the substrate influence reduces and the material structure tends to depend more on the VO 2 intrinsic properties, as expected. Figure S5. XRD (q, 2q) patterns obtained for three VO 2 thin films synthesized on an rsapphire substrate with thicknesses of 100 nm, 200 nm, and 400 nm. The diffraction peaks (α, β, X) = ((200), (111), (400)) and (R 1 , R 2 , R 3 ) = ((012), (024), (036)) are associated to VO 2 and substrate, respectively. Figure S6 show that the XRD patterns obtained for VO 2 deposited on SiO 2 /Si are different than those shown in Fig. S5 for VO 2 deposited on r-sapphire. For the 100 nm-thick sample, there are not clear diffraction peaks, which indicates that the diffracting objects (grains) are too small and that their atomic order is weak. By contrast, the 300 nm-thick VO 2 film does present the diffraction peaks (011) and (022), which proves the mono-orientation in the sample. Figure S6. XRD (q, 2q) patterns obtained for two VO 2 thin films synthesized on a native-SiO 2 /Si substrate with thicknesses of 100 nm and 300 nm. The diffraction peaks (α, β) = ( (011), (022)), X = (301), and (S 1 , S 2 ) = ( (002), (004)) are associated to VO 2 , V 2 O 5 , and substrate, respectively.
SEM, AFM, and XRD images show thus significant differences between VO 2 films deposited on r-sapphire and SiO 2 /Si. The substrate generally drives the grained microstructure of VO 2 films with thickness smaller than 500 nm, while thicker films are expected to exhibit a reinforcement of the intrinsic VO 2 materials properties.